CHAPTER
13
Supramolecular
Organic Photochemistry:
The Control of Organic
Photochemistry and Photophysics
through Intermolecular Interactions
13.1 The Current and Emerging Paradigm
of Supramolecular Organic Chemistry
Supramolecular chemistry has evolved from molecular chemistry over the last half
century as the science of “chemistry beyond the molecule.” Supramolecular organic
chemistry can be described in a number of ways, such as: (1) chemistry of molecular
complexes; (2) chemistry of noncovalent bonds; (3) chemistry of the intermolecular
bond; and (4) chemistry of molecular recognition. These are appealing, but somewhat
fuzzy, descriptions of “chemistry beyond the molecule.”1−3
Nevertheless, they
allow the chemist latitude in a wide range of concepts that are useful for deciding
what is supramolecular and what is not. Rather than trying to pin down a precise
definition of supramolecular organic chemistry, let us try to describe its fundamental
intellectual foundation: Supramolecular organic chemistry is based on the concept that
whereas organic chemistry is generally interpreted in terms of individual molecular
structures and/or dynamics for which covalent bonding is the dominant feature determining
molecular structure, there are situations where introduction of more than one
interacting molecular structure is required to understand the chemistry under consideration.
To a photochemist, the comparative ideas of molecular and supramolecular
structures are analogous to the ideas of ground-state and excited-state potential energy
surfaces. In this analogy, organic photochemistry is chemistry beyond the groundstate
surface: A ground-state potential energy surface determines the ground-state
chemistry and properties of a ground-state organic molecule (R), but an electronically
excited-state surface determines the photochemical and photophysical properties of
the electronically excited state (∗
R). Knowledge of the ground-state surface is insufficient
to understand the photochemistry of organic molecules. When a single molecular
structure is insufficient to describe the ground-state or excited-state chemistry of a
molecule under investigation, this is a signature that the transition from molecular
organic photochemistry to supramolecular organic photochemistry has occurred. In 925
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926 Chapter 13 Supramolecular Organic Photochemistry
Section 4.38, examples of supramolecular systems, exciplexes and excimers, were
described. These are simple, yet definitive examples of electronically excited “supermolecules”
for which a complex of two molecules is required to understand the
photochemistry and photophysics of a system.
All of the features of modern molecular organic photochemistry may be readily
transferred and incorporated into the developing paradigm of supramolecular organic
photochemistry.4,5
One must merge the “molecular” aspects of the organic photochemistry
paradigm with those of supramolecular organic chemistry.
First, let us consider the essential and critical features of a supramolecular organic
chemical system. We start by viewing a supramolecular system as “self-similar” to a
molecular system in terms of structure and dynamics, and then add features that might
be required to develop the paradigm of supramolecular organic photochemistry. The
key intellectual unit of the molecular system is the strong and directional covalent
chemical bond that holds the atoms of a molecule together. The paradigm of the
covalent bond ties together the critical features of structure and reactivity of organic
molecules. In this paradigm, intermolecular and noncovalent interactions are generally
considered to be so weak, nondirectional, and nonspecific that they play only a
secondary role in determining structure and reactivity in systems under study. As the
intermolecular interactions or noncovalent interactions between molecules become
increasingly selective and stronger, the system begins to transform from one that can
be well described as molecular to one that must be described as supramolecular. In a
truly supramolecular system, the basic features become difficult to understand simply
on the basis of molecular chemistry. The contrast between monomer (molecular) and
excimer (supramolecular) emissions (Section 4.38) is such an example. Indeed, for
many supramolecular systems, the basic intuition derived from molecular chemistry
will fail even at the qualitative level!
In summary, the term “supramolecular chemistry” invokes a chemistry beyond the
molecule and emphasizes noncovalent intermolecular bonds between two or more
molecules rather than emphasizing a single molecule and the covalent bonds that
hold it together. In the same way that a molecule can be defined as an assembly of
atoms that is held together by strong molecular covalent bonds, a “supermolecule” or
“supramolecular assembly” can be defined as a complex of two or more molecules that
are held together by weak intermolecular noncovalent bonds.6,7
We are immediately
confronted by the meaning of “weak” bonds. A useful benchmark for the “relative
weakness” of a bond is a comparison of the strength of a bond to the average energy
of collisional impacts between molecules. Around room temperature (∼ 300 K,
27◦
C) this energy is on the order of 1kcal mol−1
(∼ kT ). Thermal energy, kT is the
energy available from the thermal motion of colliding particles at a particular temperature
(k = Boltzmann constant and T = temperature in Kelvin). With this benchmark
in mind, bonds with an energy on the order of 5 kcal mol−1
will be rapidly broken
with molecular collisions on the order of kT . For example (Section 8.4), if we take
5 kcal mol−1
as the activation energy for a bimolecular reaction (typical A factor in an
Arrhenius equation, of 108
s−1
), the rate of bond breaking will be on the order of 105
–
106
s−1
. In other words, by the criterion of kT , “weak” bonds, will only last to the
order of 10−5
–10−6
s or less! Individual noncovalent bonds are always weak and will
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Section 13.1 Paradigm of Supramolecular Organic Chemistry 927
Cation---π bond
π---π bond Charge-transfer
bond
ED = electron donor
EA = electron acceptor
van der Waals
bond
Hydrogen bond CH---π bond
H
ED
EA
H
C
X
O Z
Y+
Figure 13.1 Examples of noncovalent bonds between molecules. The
symbol --- indicates a weak intermolecular bond.
generally possess strengths < 5 kcal mol−1
. Examples of commonly encountered noncovalent
bonds are shown in Fig. 13.1: cation---π bond, hydrogen bonds, C H---π
bonds, π---π bonds, van der Waals interactions (also termed dispersion interactions),
and charge-transfer (CT) interactions.8−12
We can compare noncovalent dynamics to
the time required to break a “very weak” covalent bond of 30 kcal mol−1
, whose rate
would be ∼ 10−8
s−1
(a lifetime of ∼ 50,000 h, or ∼ 6 years)! However, as we will
emphasize throughout this chapter, although most individual noncovalent bonds may
be weak relative to kT , when a significant number of noncovalent bonds operate together
for a supermolecule, the overall net bonding, and therefore the lifetime of the
supramolecular complex, can be significant.
Now, we consider a general strategy for translating the very successful paradigm
of molecular structures and dynamics into a successful paradigm for supramolecular
structures and dynamics. In describing the structure of molecules, chemists start
with molecular composition (numbers and kinds of atoms), then move to the level
of molecular constitution (how the atoms are connected by covalent bonds), then to
molecular configuration (how the atoms around a given atom are arranged in space),
and finally to molecular conformation (the possible shapes of a molecule resulting
from plausible orientations of groups of atoms). Molecular dynamics is the study of
the change of one or more of these molecular structural features with time. Similarly,
in describing the structure of supramolecular systems, chemists consider supramolecular
composition (numbers and kinds of molecules that make up the supermolecule),
supramolecular constitution (how the molecules of the supermolecule are connected
by noncovalent bonds), supramolecular configuration (how the molecules that make
up the supermolecule are arranged with respect to each other around a given molecule
in space), and supramolecular conformation (a specific set of plausible orientations
for all of the molecules of the supermolecule in space). Supramolecular dynamics is
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928 Chapter 13 Supramolecular Organic Photochemistry
the study of the change of one or more of these supramolecular structural features
with time.
Most of the intermolecular noncovalent bonds in Fig. 13.1 can be conveniently
classified in general terms as resulting from classical polarization forces or quantum
mechanically induced dispersion forces. Polarization forces arise from dipole moments
that are induced in molecules by nearby permanent electrically charged poles
(cation---π interaction in Fig. 13.1) or by nearby permanent dipoles (e.g., hydrogen
bond in Fig. 13.1).9,10
Dispersion forces are universal and are due to instantaneous
variations in the electron clouds (a quantum mechanical idea) that cause fluctuating
dipoles in a molecule. These fluctuating dipoles in turn induce dipoles in surrounding
molecules (e.g., CH---π, π---π, van der Waals interactions in Fig. 13.1). Both polarization
and dispersion forces contribute additively to weak noncovalent intermolecular
bonds. Dispersion forces common to both polar and nonpolar molecules are general
and among the most important types of forces contributing to the formation of noncovalent
bonds. Finally, quantum mechanical interactions for which charge is transferred
from a HO to a LU can be significant only when the HO is a good electron donor and
the LU is a good electron acceptor (Section 6.21). We note that interactions can be either
attractive (bonding) or repulsive (antibonding) for noncovalent interactions, just
as they are for covalent and ionic interactions. Thus, an interaction per se does not
guarantee net bonding.
This chapter deals with supramolecular organic photochemistry, the science in
which intermolecular noncovalent bonding between two or more molecules is responsible
for the failure of a single molecular structure to provide an understanding of
experimental characteristics of photochemical processes along the pathway from an
electronically excited state ∗
R to a reactive intermediate I, and finally to an isolated
product P. In particular, we are concerned with the supramolecular photochemistry of
a special class of supermolecules: guest@host complexes where the @ symbol indicates
a noncovalent complex between a guest and a host. Therefore, first we consider
the paradigm of guest@host complexes and then develop a paradigm for the photochemistry
of these supermolecules.
13.2 A Paradigm of Supramolecular Organic Chemistry:
guest@host Complexes
Molecular biology has been at the center of some of the great intellectual revolutions
of the twentieth and twenty first centuries. Chemistry has been an important contributor
to this revolution and supramolecular organic chemistry provides the promise of
reducing the enormous complexity of biological systems to manageable levels from
knowledge of individual molecular structure and interactions between these molecular
structures; the latter knowledge is at the heart of supramolecular chemistry. Many
important biological processes operate on the basis of a common chemical event that
can be termed molecular recognition, the highly selective binding of a molecule or
molecular fragment by another molecule to form a complex through one or more weak
noncovalent bonds between the molecules in guest@host complex.13−16
It might be
said that molecular recognition is an expression of molecular sociology that describes
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Section 13.2 guest@host Complexes 929
the causes (attractions and repulsions) of how and why molecules behave and organize
themselves into certain structures and exhibit certain structural dynamics in the
presence of one or more molecules. Understanding the basis of molecular recognition
is a major concern of supramolecular chemistry. While molecular chemistry was
successful in achieving selectivity with aggregates of atoms possessing a relatively
small number of strong covalent bonds, the supramolecular chemistry of biological
systems achieves selectivity by large numbers of relatively weak noncovalent interactions
between organic molecules in aqueous media (Section 13.6). The very unusual
properties of water come into play in determining noncovalent interactions, such as
hydrophobic bonding, and will be of special importance for the understanding of
supramolecular organic chemistry in aqueous media.17−20
Although there is currently no available paradigm for supramolecular chemistry
that is as powerful as the one for molecular chemistry, we will describe some general
features of the developing paradigm of supramolecular chemistry that will be sufficient
for us to understand the qualitative aspects of supramolecular photochemistry. Again,
it is important to stress that an important feature of supramolecular systems is that
in general a supermolecule is held together as an assembly by a number of weak
noncovalent intermolecular bonds. Although individual noncovalent bonds may be
weak, the summation of a large number of weak bonds in a guest@host complex can
lead to a strong overall bonding of the components of the supermolecule. The reason
for the strong bonding is that at any instant a certain number of these weak bonds
will be in place and contribute to the overall stability of the supermolecule. However,
because of the weak nature of the bonds, supermolecules may be constantly in flux of
bond making and breaking processes that allows for the rich diversity and complexity
of supramolecular systems, and especially of biological systems.
Let us consider in more detail the meaning of the symbol guest@host used to define
the supermolecule. The distinction between guest and host is clearest when the
guest is a relatively small molecule that is partially or completely surrounded by a
relatively large molecular host. However, in this chapter we will see that the guest
and host molecules also could be of a similar size. The formation of a guest@host
supermolecule is called “complexation,” a term that preserves (1) the concept of relatively
labile and weak noncovalent bonds and (2) the concept that individual molecular
components of the supermolecule substantially retain their individual molecular properties
to a considerable extent. Every noncovalent bond that can be formed by a host
to a guest can be considered as a bonding valence. In this sense, a host is polyvalent
or multivalent and capable of making a multitude of weak bonds to a guest at any
instant (e.g., see Fig. 13.2). Complexation is a form of noncovalent intermolecular
bonding between the guest and host (i.e., a reciprocal and complementary molecular
recognition of the noncovalent valences of the guest by the host and vice versa).
Thus, “bond strength” of a guest@host complex refers to the average summation of
all instantaneously weak noncovalent bonds between the guest and host. We recognize
that, contrary to the situation for covalently bonded molecules, the instantaneous
bond strength and number of noncovalent bonds of a guest@host complex may vary
considerably with time. The latter characteristic is responsible for the flexibility and
selective reaction chemistry of many guest@host complexes.
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930 Chapter 13 Supramolecular Organic Photochemistry
A host with four
valences to interact
with a guest
A guest with four
complementary
valences
A guest@host complex
bound with four
noncovalent bonds
Figure 13.2 Schematic representation of the polyvalent noncovalent bonding of a guest to
a host. The “Y” shape represents a valence of the host and the triangle shape represents a
complementary valence of the guest.
13.3 Toward a Paradigm for Supramolecular
Organic Photochemistry
A comparison of the paradigms of molecular organic photochemistry and supramolecular
organic photochemistry is shown in Scheme 13.1. We saw throughout the text how
the sequence of structures, such as those shown in Scheme 13.1a, works very well as
a starting point for analyzing the mechanisms of molecular organic photochemical
reactions that follow the pathway, R + hν → ∗
R → I → P. An analogous elementary
paradigm for supramolecular organic photochemistry is shown in Scheme 13.1b.
The essential chemical difference between Scheme 13.1a and b is the inclusion of a
“schematic circle” around each of the structures of Scheme 13.1b. The circle represents
any hypothetical host that forms a guest@host complex for each of the species
R, ∗
R, I, and P. Thus, an overall supramolecular photochemical reaction may be represented
by the sequence: R@host + hν → ∗
R@host → I@host → P@host where
the circle of Scheme 13.1b represents the host. The molecular photochemistry of
Scheme 13.1a can be understood as an extension of a “solvent cage” model (Section
7.37);21,22
the implication of the circle of Scheme 13.1b is that the understanding
of supramolecular photochemistry requires a guest@host complex model for which
the circle represents a specific host that binds the guest through noncovalent bonds and
that these weak bonds control the pathways followed in a supramolecular photochemical
reaction. Note that in general the supramolecular structure of ∗
R is the same as that
of R as the result of the Franck–Condon principle, which demands that the geometries
of R@host and ∗
R@host are identical at the instant of creation of ∗
R@host.
This chapter describes examples of the guest@host complexation influence on
both the primary supramolecular photochemical process, ∗
R@host → I@host, and
the secondary thermal processes, I@host → P@host. In addition, supramolecular
photophysical processes, ∗
R@host → R@host (+ heat or light), will be described.
We will see that the ability to manipulate the course of photochemical reactions by
photolyzing R@host complexes rather than R in a solvent cage expands considerably
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Section 13.3 Toward a Paradigm for Supramolecular Organic Photochemistry 931
(b) R I P
hν
*R
(a) R I P
hν
*R
Scheme 13.1 (a) Paradigm of molecular organic photochemistry
in ordinary organic solvents. (b) Paradigm of supramolecular
organic photochemistry of guest@host complexes. The circle
around R, ∗
R, I, and P represents a supercage of a host that causes
the overall photochemistry to occur differently than it does in an
ordinary solvent cage.
the possible selective control of products from photochemistry of organic molecules
and also the selective control of available mechanisms for investigation.
In Scheme 13.1a, a solvent cage made up of small molecules is understood to exist
about R, ∗
R, I, and P along the photochemical reaction pathway. Indeed, this solvent
cage may be considered as the simplest supramolecular host. The chemical behavior
of R, ∗
R, I, and P in a solvent cage (e.g., guest@[solvent cage]) is a useful benchmark
for comparison with the chemical behavior of supramolecular guest@host complexes.
In this case, [solvent cage] represents the host in solution. It is important to note that
interactions between guest and solvent molecules are very weak, random, and not
directional. When the chemistry of a guest complexed to a host differs significantly
from that expected from a solvent cage, we can term the host as a “supercage” that is
responsible for the supramolecular behavior.
In general, there is more than one product (P) produced in a photochemical
reaction as the result of competing primary (∗
R → I1 + I2 + . . .) and secondary
(I → P1 + P2 + . . .) processes. Let us say, for simplicity, that only two products (P1
and P2) are formed for the same ∗
R and we would like to form one product selectively,
either P1 or P2. Our goal is to employ supramolecular effects by simply putting a
circle of Scheme 13.1 around ∗
R to direct the ∗
R to either P1 or P2, not both. Now, we
develop a strategy to produce P1 or P2 selectively through supramolecular control as
follows. Scheme 13.2a shows an example of the possible influence of supramolecular
interactions on the partitioning of two photophysical processes. Scheme 13.2b shows
an example of the possible influence of supramolecular interactions on the partitioning
of a photophyscial and photochemical process. Conceptually and conveniently we can
divide the competition between two photochemical reactions into three categories
(Scheme 13.2): (1) ∗
R yielding P1 and P2 via independent intermediates (I1 and I2,
Scheme 13.2c); (2) ∗
R giving P1 and P2 via funnels F1 and F2 (Scheme 13.2d), and
(3) ∗
R yielding P1 and P2 via a single primary intermediate (I, Scheme 13.2e). In
addition, in the I → P secondary thermal processes for which I is a radical pair (RP),
it is important to consider how supercages can influence competition between the
Igem → Pgem step and the Igem → IFR → PFR (free radical = FR) steps (Scheme 13.2f),
since the geminate products (Pgem) and free radical products (PFR) will generally be
different. Next to each general example in Scheme 13.2 is a specific chemical example.
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(a) *R
R
(Photophysicalradiationless
process)
(Photophysicalradiative
process)
Deactivation
Emission
+ Δ
R + hν
*
S*
(b) *R
P1 (Photochemical)
(Photophysical)
Dimer
Monomer
or excimer
fluorescence
R
(c) *R
I1
I2
I1
I2
Type I products
Type II products
P1 (P1)
O
O
H
(P2)
C6H5
C6H5
P2
(d) *R
F1
F2
P1
P2
(e) *R
(f) *R
I
I
P1
P1
P2
P1
P2
P1
P2
+ hν
OH
O
C6H5
C6H5
*
*O
O
H
C6H5
C6H5
*O
*O
H
C6H5
HO
HO
OH +
C6H5
C6H5 Ar
O
C6H5
C6H5CH2CH2Ar
C6H5CH2CH2C6H5 + C6H5CH2CH2Ar + ArCH2CH2Ar
C6H5
CH2
Ar
CH2
C6H5
CH2
Ar
CH2
H2C
Ar
C6H5
C6H5
I(gem)
P2
I(FR)
Igem IFR
Scheme 13.2 Common competing primary photophysical and photochemical processes from ∗
R and I
with a specific example. Each arrow shown branching from ∗
R or I has an associated first- or second-order
rate constant. Supramolecular effects can control the relative magnitudes of the branching rates.
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Section 13.4 Control of Activation Parameters and Competitive Reaction Rates 933
In the following sections, we will show how the molecular photochemistry and
photophysics of guest@[solvent cage] systems, described in the earlier chapters, can
be transformed into a description of the supramolecular photochemistry and photophysics
of guest@host systems. Examples are discussed in the following sections. For
the sake of easy conceptualization, the influence of the supramolecular effect on photoreactions
is discussed under four sections, the influence of complexation to a host on
R, ∗
R, I, and P, respectively. As a concrete exemplar of a guest@host system of very
wide scope, we will discuss guest@enzymes, which serve as an inspiration for the
synthetic supramolecular guest@host complexes. Since enzymes operate in aqueous
environments, first we will describe the photochemistry of guest@host complexes in
aqueous media. In addition, we will consider examples in crystals and porous solids
where a remarkable influence of the host has been observed. These selected systems
serve as exemplars for a large number of supramolecular complexes.
13.4 An Enzyme as an Exemplar Supramolecular Host
for guest@host Complexes. Control of Activation Parameters
and Competitive Reaction Rates through Supramolecular Effects
Perhaps, the quintessential exemplar for the schematic host circle in Scheme 13.1b is
the active site of an enzyme. Let us analyze the general and familiar mechanism of
how a host enzyme transforms a small guest molecule (called a substrate by biologists)
selectively into a targeted product molecule. We can then employ these mechanistic
features to generate a general paradigm based on intuition gained from the enzyme
model for all synthetic supramolecular guest@host complexes.
The first step in enzyme action is the molecular recognition of the guest substrate
by the host enzyme (or the reciprocal molecular recognition of the host by
the substrate).6,13
Molecular recognition may be defined as the selective noncovalent
binding of the substrate to the host to form a guest@host complex. After binding
to the enzyme, the guest is transported to the active site of the enzyme (a reaction cavity
built within the enzyme host structure). While bound to the active site, the guest
is chemically transformed with extraordinary selectivity and at a remarkable rate to
a desired, biologically functional product. The chemistry of the guest is determined
entirely by the structure and dynamics of the guest@enzyme complex. By analogy,
the chemistry of the guest in any guest@host complex is determined by the structure
of the supramolecular complex and cannot be understood in terms of the guest
molecular structure alone or the chemistry of the guest molecule in ordinary solvents.
The chemistry performed on the substrate in a guest@enzyme complex is exquisite
in terms of rate, chemioselectivity, regioselectivity, and stereoselectivity, all accomplished
on (hydrophobic) organic molecules in an aqueous (hydrophilic) environment!
One of the holy grails of supramolecular chemistry is the emulation of the extraordinary
chemical selectivity of small guest molecule reactions in a guest@host complex
in the chemist’s laboratory. In developing a supermolecular organic photochemistry
paradigm, we will be replacing large enzymes with small molecular hosts. A strategy
to finding the holy grail is to mimic some of the structural and dynamic features
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934 Chapter 13 Supramolecular Organic Photochemistry
of guest@enzyme complexes in simpler organic guest@host complexes. This strategy
is termed biomimetic chemistry. Molecular recognition in supramolecular organic
chemistry means binding with the purpose of product control. Describing and understanding
supramolecular control of organic photochemistry is a goal of this chapter.
Now, let us review some of the key supramolecular features of the
substrate@enzyme complex that can be mimicked in simpler synthetic organic molecular
guest@host complex: (1) preorganization of the guest in a host cavity whose size
is on the order of a small organic molecule in dimension (∼ 5–20 ˚A diameter); (2) constraint
and control of the translational and rotational motions of the guest; (3) control
of the extent, shape, and location of “free space” available to the guest; (4) preorganization
of host chemical functionality so that it can operate on the guest and
achieve the desired chemical selectivity on the reactions of the guest. Preorganization
by complexation of the guest and host inhibits the freedom of motion of the guest
molecule and correspondingly decreases the entropy ( S) of the guest. In the term
G = H − T S, a negative value of S increases the free energy (i.e., makes the
complexation thermodynamically less favorable, Section 13.8). Thus, the decrease in
S associated with preorganization (complexation) must be compensated by a decrease
in H, if the guest@host complex is to have an overall negative free energy and
be stable. This opposing interplay between entropy and enthalpy, known as enthalpy–
entropy compensation, is achieved through a number of weak interactions between
the guest and the host. Nature has skillfully managed to balance opposing tendencies
between H and S through the billions of years of supramolecular evolution of
biological systems. When dealing with guest@host complexes in aqueous solutions
it is also important to consider the entropic changes in solvent water in addition to
those of individual host and guest molecules. Indeed, the so-called hydrophobic effect
appears to be dominated by entropic changes in the water structure upon formation
of certain guest@host complexes in aqueous solution.
Now we return to the issue of using supramolecular effects in selectively controlling
the formation of either P1 or P2 (Scheme 13.2; Fig. 13.3).23−25
In a completely
general way, a high chemical selectivity for P1 or P2 can be interpreted in terms of
a simple free energy diagram for which, hypothetically, a reactant (R) possesses a
lower free energy of activation ( G‡
) to a desired product (say, P1) than to a second
undesired product (say, P2). For simplicity, let us consider the energy diagram in
Fig. 13.3 showing three different scenarios: (a) a molecular system for which the free
energy barriers to P1 and to P2 are of comparable heights ( G‡
1 = G‡
2), resulting in
comparable rates of reaction from R to P1 and P2 and in no selectivity in formation
of P1 or P2 from R; (b) a supramolecular system for which the barrier leading to P1 is
more or less the same as the molecular system, but with a significantly reduced barrier
leading to P2( G‡
1 > G‡
2), resulting in a faster relative rate and high selectivity
for P2 formation; (c) a supramolecular system for which the barrier leading to P1 is
more or less the same as the molecular system, but the barrier leading to P2 significantly
increased G‡
1 < G‡
2. This increase results in a faster relative rate and high
selectivity for formation of P1. Scenario (b) represents a case of selectivity based on
supramolecular catalysis (selective reaction acceleration) of formation of P1 and scenario
(c) represents a case of selectivity based on supramolecular inhibition (selective
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Section 13.4 Control of Activation Parameters and Competitive Reaction Rates 935
ΔG‡
ΔG‡
1
P1
P1 (50%)
(a)
P2 (50%)
P2
R
Acceleration
Inhibition
R
ΔG‡
2
ΔG‡
cat
ΔG‡
P1
P1
(b)
P2
P2
R
R
ΔG‡
inh
ΔG‡
P1
P1
(c)
P2
P2
R
R
Figure 13.3 Schematic representation of (a) a nonselective chemical
reaction R → P1 + P2 for which G‡
1 = G‡
2; (b) a selective reaction
R → P2 for which G‡
1 > G‡
2 through catalysis; (c) a selective
reaction R → P1 for which G‡
1 < G‡
2. (Catalysis = cat and
inhibition = inh.)
reaction deceleration) of formation of P2. The energy barriers shown in Fig. 13.3a
can be manipulated by supramolecular control of rate controlling features, such as
precomplexation, collision frequency, orientations, distance of separation, and conformational
preferences. In each case, the relative free energy of activation for the
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936 Chapter 13 Supramolecular Organic Photochemistry
*O
*R
*R
H
C6H5
C6H5
O
O
I1(RP)
H
C6H5
Type I
products
(P1)
Type I
products
(P1)
Type II
products
(P2)
C6H5
O
I1(RP)
I2(BR)
O
O
H
C6H5
C6H5
HO
O
C6H5
C6H5
*O
O
H C6H5
C6H5
C6H5
ΔG‡
ΔG‡
1
I1 I2
*R
Type I Type II
= ΔG‡
2
Scheme 13.3 A hypothetical situation for which the Type I and Type II molecular reactions possess
comparable energies of activation ( G‡
1 = G‡
2).
formation of the desired product is lowered relative to the free energy for formation
of the undesired product by either catalysis (Fig 13.3b) or inhibition (Fig 13.3c).23
As specific photochemical exemplars of the situations in Fig. 13.3, let us consider a
competition between Type I α-cleavage and Type II intramolecular hydrogen atom abstraction
(Scheme 13.3) from an excited state (∗
R). Both are unimolecular processes.
However, the Type II reaction produces an I(BR) and has a stringent requirement
that the γ -hydrogen required for abstraction be available through a conformation that
places the C H bond at the proper orientation relative to the half-filled n-orbital of
the n,π∗
state of ∗
R. On the other hand, the Type I reaction produces an I(RP) and
only requires the overlap of the bond connecting the carbonyl carbon to the α-carbon
with the half-filled n-orbital of the n,π∗
state of ∗
R. The latter primary photochemical
process does not depend significantly on the conformation of the ∗
R side chain. Since
the conformational equilibrium is rapid in solution one can assume both processes
would occur with comparable efficiency. The situation shown in Scheme 13.3 can be
considered in terms of the modified energy diagram shown in Fig. 13.4. Here P1 and
P2 of the primary photochemical step are in fact intermediates I1 and I2. Consider the
energy diagram in Scheme 13.3 and compare it with that in Fig. 13.3a, where the situation
shown is such that the energy of activation for both Type I, ∗
R → I(RP), and
Type II, ∗
R → I(BR), processes are essentially the same, so that the rate of formation
of Type I and Type II products are comparable, (i.e., G‡
1 = G‡
2 and kI ∼ kII).
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Section 13.4 Control of Activation Parameters and Competitive Reaction Rates 937
*R
Acceleration
Inhibition
Type I
products
(P1)
Type II
products
(P2)
I1(RP)
I2(BR)
O
O
H
C6H5
C6H5
HO
O
C6H5
C6H5
*O
O
H
C6H5
C6H5
ΔG‡
ΔG‡
1
I1 I2
*R
(a) Supramolecular acceleration of Type II reaction
> ΔG‡
2
*R
Type I
products
(P1)
Type II
products
(P2)
I1(RP)
I2(BR)
HO
O
C6H5
C6H5
ΔG‡
ΔG‡
1
I1 I2
*R
(b) Supramolecular inhbition of Type II reaction
< ΔG‡
2
O
H
C6H5
C6H5
O
*O
H
C6H5
C6H5
O
Figure 13.4 Effect of supramolecular conformational on Type I and Type II reactions. (a) The Type II
reaction is catalyzed so that G‡
1 > G‡
2; (b) The Type II reaction is inhibited so that G‡
1 < G‡
2.
Thus, a strategy for favoring Type I over Type II products is to design ∗
R@host
complexes for which the conformation of the side chain places the γ -hydrogen at a
distance far removed from the carbonyl oxygen in the n,π∗
state of ∗
R. In Fig. 13.4a
(cf. Fig. 13.3), the supramolecular guest@host complex is imagined to favor, through
preorganization, a conformation that brings the γ -H into the proximity of the norbital
and “catalyzes supramolecularly” the rate of the ∗
R → I(BR) process without
significantly affecting the rate of the ∗
R → I(RP) process. This favors the formation of
the I(BR) and Type II products (P2) over the formation of I(RP) and the Type I products
(P1). In Fig. 13.4b (cf. Fig. 13.3), the supramolecular host is imagined to favor, through
preorganization, a conformation that places γ -hydrogen at a distant location from the
n-orbital and “inhibits supramolecularly” the rate of the ∗
R → I(BR) process. Such an
inhibition favors the formation of the RP and Type I products (P1) over the formation
of the BR and the Type II products (P2).
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938 Chapter 13 Supramolecular Organic Photochemistry
13.5 Extending Some of the Key Structural and Dynamic
Features of guest@enzyme Complex to Organic
guest@host Complexes. The Host Reaction Cavity Concept
A guest@host chemical reaction is viewed in the same mechanistic terms as reactions
of the guest in ordinary molecular solvents. However, in the mechanism of guest@host
reactions, the reactions of the guest can be controlled to a certain extent by the host,
as we have seen in the examples in the previous section. The host control of the
guest chemistry depends critically on the chemical structure of the host “cavity” that
binds the guest, the dynamics of the guest in the cavity, and the dynamics of guest
complexation (the rates of entry and exit of the guest from the guest@host complex).
The cavity structure of the host is represented in a general and nonspecific manner by
the circle around the guest in Scheme 13.1. The critical features of the cavity are related
to its ability to preorganize the guest in a manner that either catalyzes or inhibits certain
reaction pathways (Figs. 13.3 and 13.4). These structural features can be geometric,
chemical, or physical. For example, size and shape are geometric features of a cavity.
Another equally important geometric feature is the size and shape of portals that allow
access to the host cavity. The fluidity and flexibility that a guest experiences in a cavity
are physical features of the cavity. The critical chemical features of the cavity are the
existence of chemically active groups that can operate on the chemistry of the guest
in the guest@host complex.
We can compare the cavity of a supramolecular host to the cage around a solute in
an isotropic solution of a small molecular solvent (e.g., benzene, acetonitrile, water).
The solvent cage is very fluid and flexible. Consequently, its dimensions, size, and
shape change with time and are therefore not well defined. Solvent molecules can be
easily displaced, so that size matching of the reactant, products, and the reaction cavity
generally requires very small activation energy on the order of kT . For example, larger
molecules can use their thermal energy to just “push solvent” around so that the fluid
solvent cage easily adjusts to “fit” around the molecule in the solvent cage. On the
other hand, when the reaction cavity of a host possesses a well-defined molecularly
rigid boundary, size and shape matching with the guest will become important and
may even become the main factor controlling the feasibility of a reaction. A micelle
resembles a solvent cage in that a hydrophobic guest can be considered to be “imbibed”
in an oil droplet that is surrounded by water.
For example, consider the effect of size and shape dimensions on product formation
for a molecularly inflexible (rigid) host cavity. In considering the space available in
the host’s cavity, the notion of “free space” is a useful concept (Fig. 13.5).26
Whether
a particular reaction is catalyzed or inhibited in such a cavity will depend on how
the possible intermediates (I1 and I2) and products (P1 and P2) fit within the available
space (including free space) provided by the host’s cavity for occupancy by the guest
reactant. It is important to note that geometrical features of free space are measured in
terms of free volume. Free space, which describes size and shape, has more meaning
than a free volume; for example, the free space could be chiral or achiral. There is
free space between two molecules when the distance between them is greater than the
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Section 13.5 The Host Reaction Cavity Concept 939
R*
P2
P2 is too large to fit within the
reaction cavity (abundant free
space, but the product shape
does not fit)
P1
P1 fits within the reaction cavity
Free space
(unshaded area)
Allowed
Forbidden
Reaction
cavity
Figure 13.5 Schematic representation of the free space available in a guest@host complex
and its effect on product formation. The shape of P1 “allows” the reaction to proceed without
supramolecular steric hindrance. The shape of P2 “forbids” the reaction to proceed due to steric
hindrance.
sum of their van der Waals diameters. Thus, the reaction cavity of a host is defined
as the sum of space occupied by the guest in terms of its van der Waals size plus any
available free space surrounding it within a host. A certain amount of free space is
always present around a molecule in a guest@host assembly. The extent of available
free space in a guest@host complex depends on the size and shapes of the host and
guest.
The host reaction cavity concept emphasizes the size and shape changes that
occur as the reactant guest is transformed into the product and how this phenomenon
is commensurate or not with the available space of the reaction cavity.27−31
For
example, the reaction cavity, particularly as applied to guest@host complexes in water,
possesses the following features:
1. A reaction cavity is a space in the host with noncovalent valences that can bind
to a guest (Fig. 13.2).
2. The binding of the guest to the reaction cavity reduces the diffusional and
rotational mobility of the guest molecules and provides a boundary (e.g., a
hydrophilic–hydrophobic boundary in an aqueous solvent) across which the
guest molecules (R, ∗
R, I, and P) may not cross without overcoming an energy
barrier.
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940 Chapter 13 Supramolecular Organic Photochemistry
R
(a) (b) (c)
P1 P2 P2
R
P1 P1
R
P2
Figure 13.6 Examples of the effects of reaction cavity of a guest@host complex
on product formation. (a) A fluid host reaction space that can easily adjust
to surround a guest (e.g., solvent cage, micelles). (b) A relatively rigid cavity
of a solution guest@host complex surrounded by solvent (e.g., cyclodextrins,
zeolites). (c) A rigid cavity of a crystal guest@host cavity. In (b) and (c) P2 is
disfavored because of strong supramolecular steric hindrance.
3. The free space within a reaction cavity relative to the size and shape of the
guest is an important parameter: The complementary guest and host shape,
size, location, directionality, and dynamics control in large part the extent to
which the host can influence a photoreaction (Fig. 13.5).
4. The size, shape, fluidity, flexibility, and rigidity of reaction cavities vary among
various supramolecular assemblies (Fig. 13.6).
5. When the atoms–molecules constituting the walls of the reaction cavity are
stationary and relatively rigid (possess time-independent positions on the time
scale of the guest reaction, e.g., solids or crystals), the space necessary to allow
the conversion of a guest molecule to its photoproducts must be built into the
reaction cavity (e.g., zeolites). On the other hand, in systems where the walls of
the host are relatively flexible (e.g., micelles), the space may adjust during the
course of a reaction. For such media, the space of a reaction cavity is modified
by structural fluctuations of the medium and cannot be readily represented by
static molecular models (Fig. 13.6).
6. The reaction cavity may contain or be associated with specific functional
groups or atoms that may interact strongly (attractively or repulsively) with
guest molecules, the transition state or the intermediates as the guest proceeds
to products (Fig. 13.2). Such specific interactions may lead to unique product
selectivity and enhance or decrease the relative rates of primary and secondary
processes and the quantum yields for reactions (Fig. 13.7).
7. The functional groups may exist as a coguest (CG) in the reaction cavity
or within the exterior boundary of the cavity, but bound to the boundary by
noncovalent bonds. In Fig. 13.8, CG could represent a molecule or an ion.
Weak and directional bonds between CG and the guest (R, R∗
, and I) can
significantly impact the course of photoreactions (Fig. 13.8).
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Section 13.6 Supercages, Cavitands and Capsules 941
*R
Weak interactions preserved
Weak interactions destroyed
P2
P1
I2
I1
Figure 13.7 Schematic representation of favorable bonding interactions preserved during
the formation of P1, but destroyed during the formation of P2.
R(a) *R---CG I---CG P---CG
R
CG CG CG CG
(b) *R I P
Figure 13.8 Schematic representation of two supramolecular preorganizations that
can control the course of a photoreaction. (a) A coguest (CG) resides in the same
cavity as ∗
R. (b) A coguest resides outside the cavity, but is bound to it by noncovalent
attractions.
13.6 Some Exemplar Organic Hosts for Aqueous
Solution Supramolecular Photochemistry:
Supercages, Cavitands, and Capsules
From the descriptions of supramolecular reactions presented in Sections 13.4 and
13.5, one may conclude that selectivity in photochemical reactions could be achieved
if a “reaction cavity” of a host resembling that of an enzyme could be synthesized in
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942 Chapter 13 Supramolecular Organic Photochemistry
Table 13.1 Nomenclature, Characteristics, and Cartoon Representations of
Various Hosts in Solution
Definition Schematic Structure
Name of Host (Refers to the Host) (Large Spheres Represent Solvent)
Micelle An assembly of small molecules, held together
by noncovalent bonding that can completely
surround a small guest molecule. Examples:
sodium dodecyl sulfate (SDS), hexadecyl
trimethyl ammonium chloride (HDTCl)
(Sections 13.16 and 13.21)
Cavitand A molecule possessing a structurally intrinsic
permanent cavity that can contain a small
guest molecule by partially surrounding
it. Examples: cyclodextrins (CDs) and
cucurbiturils (CBs) (Sections 13.10 and 13.16)
Capsule An assembly of two cavitands that can
structurally organize noncovalently to
completely surround a small molecule.
Examples: CD and octa acid (OA) (Sections
13.10, 13.11, 13.14, and 13.16)
Carcerand and
Hemicarcerand
A molecule possessing a structurally intrinsic
permanent cavity that can completely
surround, and thereby incarcerate, a small
guest molecule. The cavity may possess
portals through which guests may enter or
exit the cavity. If the guest is unable to exit or
enter through the portals during the relevant
photochemical process the host is considered
a carcerand. If the guest is able to exit or
enter through the portals during the relevant
photochemical process the host is considered a
hemicarcerand. Examples: Cram’s carcerand,
supercages in faujasite (FAU) and pentasil
(MFI) zeolites (Sections 13.10–13.12, 13.14,
13.20, and 13.22)
the laboratory. Although the reaction cavity provided by an enzyme has not been duplicated
in the laboratory, the reaction cavities more confined, ordered, and organized
than the one offered by an isotropic solution of a small solvent molecules have nevertheless
been exploited to achieve selectivity in photochemical reactions mimicking
the thermal reactions of enzymes.32−45
Hosts and guest@host complexes in aqueous
solution that we discuss in this chapter are summarized in Tables 13.1 and 13.2. The
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Section 13.6 Supercages, Cavitands and Capsules 943
Table 13.2 Nomenclature, Characteristics and Cartoon Representations of
Various guest@host Complexes in Solution
Name of guest@host Definition Schematic Structure
Complex (Refers to Solution) (Large Spheres Represent Solvent)
Micelleplex Guest@micelle complex. Examples:
DBK@SDS (DBK = dibenzyl
ketone), cylopentenone@deconate
G
Cavitandplex Guest@cavitand complex. Examples:
cinnamic acid@CB
G
Capsuleplex
(Hemicapsulplex)
Guest@capsule complex.
Examples: DBK@OA2,
anthracene2@OA2
G
Hemicarceplex Guest@carcerand complex (portals
are large enough for guests
to exit (ex) or enter the host
cavity). kex > kphoto Examples:
biacetyl@Cram’s carcerand
G
Carceplex Guest@carcerand complex (portals
are too small for guest to
exit or enter the host cavity).
kex < kphoto Examples: cyclobutadiene@Cram’s
carcerand
G
names in the tables are arbitrary and intended only to be useful by suggesting certain
supramolecular features of the host or the guest@host complexes without getting
entangled in nomenclature. Many more hosts have been exploited, but the examples
presented here amply illustrate the phenomena. For simplicity and convenience, the
guest@host complexes can be classified into two groups: (1) complexes that exist as
stable species in homogeneous solvents, of which water is the most important, and
(2) complexes that exist as stable species in the solid state. In this section, we consider
guest@host complexes in aqueous solution and will focus on a few selected systems.
First, we describe the properties of several exemplar hosts that have been employed
widely and have shown considerable scope in supramolecular photochemistry. Then
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944 Chapter 13 Supramolecular Organic Photochemistry
we go on to show how the molecular photochemistry and photophysics of organic
molecules are controlled by the structure and dynamics of the guest@host complex.
Furthermore, we consider two families of supramolecular host supercages in solution:
(1) supramolecular assemblies of small molecules that aggregate in solution
to form a host for one or more guest molecules and (2) a large single molecule (or an
assembly of two or several large molecules) with a cavity capable of binding to one
or more guest molecules. Schematic representations of these two families are shown
in Tables 13.1 and 13.2. For aqueous solvents, it is important to always keep in mind
that water as a solvent has some very special properties because of its cohesive nature
due to hydrogen bonds and to the strength and organization of these hydrogen bonds.
In the same way that simply looking at a molecular structure is insufficient to understand
supramolecular effects, considering the supramolecular guest@host structure
alone may also be insufficient to understand chemistry in water. In Tables 13.1 and
13.2, the spheres represent solvent molecules and G represents a guest molecule in
a guest@host complex. We use the term “supercage” to distinguish the characteristics
of the space that surrounds a guest in guest@host complex with respect to that
in a small molecule solvent. The cage surrounding the guest in an organic solvent
is fluid (solvent cage). In micelles, though flexible, the supercage is more restrictive
than in organic solvents. In solids, such as crystals and zeolites, the supercage is
rigid and more restrictive than in micelles. Table 13.2 shows schematically a variety
of guest@host complexes in which photoreactions have been conducted in solution;
these complexes provide the potential for different extents of selectivity for the photochemistry
or photophysics of ∗
R. Names are provided to characterize structures in
a manner that suggests the dominant features of the host as a molecular container and
of the guest@host complexes that are formed.
As an illustration of polymolecular assembly of small molecules that form a
supramolecular host, we will consider as an exemplar the micelles (Table 13.1)
formed in water with SDS molecules shown in Fig. 13.9. The molecular structure
of SDS (Fig. 13.9) consists of a hydrophobic hydrocarbon “tail,” (CH2)11CH3,
and a hydrophilic negatively charged “head” group, (SO−
4 ) typical of compounds
called surfactants.46,47
In aqueous solution, near a critical concentration [8 × 10−3
M;
critical micelle concentration (CMC)], spontaneous SDS molecules abruptly and
spontaneously aggregate to form micelles. The roughly spherical structure of the
micelle is composed of ∼ 60 SDS molecules with a diameter of ∼ 2–3 nm (20–30 ˚A).
The “core” of the micelle is a hydrophobic supercage that can absorb hydrophobic
organic molecules that are not soluble in bulk aqueous solution.
A micelle can be conceptualized as a special type of solvent cage, consisting of
a self-assembly of molecules that behave as a fluid “supercage” and can serve as a
host to hydrophobic organic molecules. In this case, the host “cavity” is considered
to be a liquid hydrophobic space in water created by the micelle and is represented as
a circle (Scheme 13.1) with several surfactant structures included to signify that the
circle represents the supercage of a micelle (Fig. 13.9). An important feature of an SDS
micelle is that it is fluid-like. As a fluid, its shape and size are not fixed, but can expand
by adding more SDS molecules as the supercage absorbs and solubilizes hydrophobic
organic molecules. In a truly supramolecular fashion, the hydrophobic organic guest
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Section 13.6 Supercages, Cavitands and Capsules 945
SDS
O– Na+
O
O OS
Micelle
SDS Molecule
in water
2–3 nm (20–30 Å)
in diameter
Normal H2O
Ordered H2O
SO4
SO4
SO4 SO4
SO4
Figure 13.9 Schematic representation of an SDS molecule surrounded by water
(left) and an SDS micelle surrounded by water (middle) and SDS molecules
forming a spherical micelle.
molecule in the guest@SDS micelle complex can contribute to the stability of the
hydrophobic supercage of the SDS micelle through noncovalent interactions with
the surfactant molecules of the micelle. The guest@SDS complex behaves as an
independent supramolecular unit for which the properties of the guest and host operate
together and cooperatively. Micelles of structure, as described above (Fig. 13.9),
are formed by a number of surfactant molecules, (cetyltrimethyl ammonium halide,
sodium dodecanoate, sodium cholate, etc.).
An included hydrophobic organic molecule in a guest@SDS micelle complex, although
in a bulk aqueous medium, is solubilized as a guest in a hydrophobic cavity
provided by the micelle. However, significant quantitative and qualitative differences
between a cage of nonviscous solvent and the supercage of an SDS micelle exist.
For example, a guest diffuses out of a solvent cage of a nonviscous solvent in a few
picoseconds; in a bulk solvent, the guest’s translational motion is unlimited and its
rotational motion is relatively unhindered. The time that an organic molecule in a
guest@SDS complex spends in an SDS micelle depends on the hydrophobicity of the
guest molecule. The more hydrophobic the molecule, the longer it stays in the micellar
cavity. Alternatively, a hydrophobic organic molecule prefers to stay in the hydrophobic
core of the micelle and is slow to exit to the aqueous phase. Rather than the few
picoseconds dwelling time, an organic molecule, depending on its hydrophobic character,
can reside in a micelle supercage for many thousands or millions of picoseconds!
This greater residence time, as seen in Section 13.21, can have significant effects on
the photochemistry of geminate radical pairs (Scheme 13.2f), produced by the Type I
photoreaction in SDS and other related micellar structures.
Now, let us consider a different kind of host that is termed a “cavitand” (Table 13.1).
A cavitand is defined as a single molecule with an intrinsic concave cavity that
persists in solution. The concave cavity of a cavitand possesses polyvalences suitable
for noncovalent binding of one or more guests. Thus, a cavitand may be viewed
as a molecular container with a preorganized, structurally enforced, and permanent
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946 Chapter 13 Supramolecular Organic Photochemistry
concave internal surface that serves as a molecular free space for binding one or more
guest molecules whose convex size and shape complement the concave shape of the
cavitand. We will use the term cavitand in a very general sense to apply to molecules
possessing a bowl-shaped concave cavity (with one or two openings) whose size and
shape allow it to be a host to small organic molecules (Table 13.1). Cavitands are
held together by strong, directional covalent bonds. As a result, they can only undergo
minor conformational changes upon binding to guests. Thus, cavitands are flexible to a
minor extent and possess relatively “hard” and solid-like reaction cavities. As a result,
the size and shape of the reaction cavity of a cavitand does not change significantly
during the binding of a guest or during the course of a reaction, in contrast to the
soft and liquid-like cavities of micelles. A cavitand may be represented schematically
as a bowl-shaped cavity or a cylinder with two openings, as shown in Table 13.1.
A complex of a guest with a cavitand is known as cavitandplex, where the guest is
partially exposed to solvent water and partially in the bowl of the cavitand host.
There are several water-soluble cavitands that are commonly used as reaction
hosts for controlling supramolecular photoreactions.48,49
Of these, we use CDs, CBs,
and OA that bear very similar geometric structural features and we use them as
exemplars for bowl-shaped water-soluble hosts (Figs. 13.10 and 13.11). There are
several commonly available cyclodextrins (CDs) (known as α, β, and γ ) whose major
cavity opening termed portal (Fig. 13.11) varies from ∼ 5 ˚A (α-CD) to ∼ 6.4 ˚A
(β-CD) to ∼ 8.3 ˚A (γ -CD).50,51
The size of the portal determines the size of molecules
COOH O OO
OO
O O
O
O O
HO
HO
HO
HO
HO
HO
HO OH
OH
OH
OH
OH
OH
OH
OH
O O
O
COOH COOH
COOH
HOOC
HOOC
HOOC
COOH
or
carton
representation
Octa acid (OA) β–Cyclodextrin (CD)Cucurbit[8]uril (CB)
O
O
O
Figure 13.10 Schematic bowl-shaped representations of CD, CB, and OA (n = number). Note the polar
groups at the portals of the hydrophobic cavities of the cavitands.
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Section 13.6 Supercages, Cavitands and Capsules 947
N
N
N
OH
OH
OH
OA
CD
CB
OHO O O
O
O
O
O
O
OOO
O
OOOO
O
O O
O
O
O
O
O O
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OH
OHHO
HO
HO
HO
HO
HO
HO
HO
O
O
O O O O
H H HH
HH H H
HO
HO
HO HO
O
10 Å
8 Å
9.1 Å
10 Å
Width
Width
Width
α-CD: 5.7 Å
β-CD: 7.8 Å
γ-CD: 9.5 Å
Width
CB[6]: 3.9 Å
CB[7]: 5.4 Å
CB[8]: 6.9 Å
5.5 Å
n
O
O
N
Figure 13.11 Structures and dimensions of CB, CD, and OA. Dimensions include van der Walls radius.
that can enter and be adsorbed in the cavity. Cucurbiturils (CBs) with different sizes are
easily synthesized.52,53
Cucurbituril portals (Fig. 13.10) are lined with polar carbonyl
groups, whereas those of CDs are lined with polar hydroxyl groups. Similar to CDs,
CBs, e.g., CB[6], CB[7], and CB[8], respectively, also exist in various sizes of the
cavity. Octa acid, which has a structure similar to CD and CB, has four COOH groups
at each of the top and bottom portals of ∼ 10 ˚A and 5.5 ˚A, respectively.54
All three
hosts are water soluble and capable of solubilizing in their hydrophobic cavities,
hydrophobic organic molecules that are insoluble in water. Because the water only
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948 Chapter 13 Supramolecular Organic Photochemistry
“sees” the hydrophilic exterior of these cavitands, hydrophobic molecules may be
solubilized as guest@cavitand complexes in aqueous solution.
In aqueous solutions, the guests in the 1:1 guest@CD, guest@CB, or guest@OA
complexes are surrounded partially by the cavity of the host and partially by the
solvent water; such complexes are termed cavitandplexes (Table 13.2). In some cases,
a 1:2 guest@(CD)2 or 2:2 [guest]2@(CD)2 complex may form where the guest is
encapsulated by the two bowl-shaped molecules of CD aligned one above the other
by their wider portals and the complex itself is completely surrounded by water. Under
such circumstances, the two cavitands are said to form a “capsule” that surrounds the
guest(s). The entire 1:2 guest@(CD)2 or 2:2 [guest]2@(CD)2 complexes is termed
a capsuleplex. In general, OA with COOH at the portals prefers to form a closed
capsuleplex rather than a cavitandplex. Dipolar repulsion between the carbonyl groups
at the portals strongly inhibits capsule formation in CBs. Depending on the dynamics
of the capsule, the capsuleplex could have the properties of either hemicarceplex or
carceplex. If the guest is able to escape during the time scale of photoreaction, it would
be termed a hemicarceplex; if not, it would be termed a carceplex (Section 13.8). These
terms are intended to be a qualitative descriptive, and therefore useful to categorize
and characterize the qualitative features of a wide range of supramolecular host and
guest@host complexes.
Enzymes operate in an aqueous environment where hydrophobic bonding (driven
mostly by entropic factors, e.g., such as release of cavity-held water molecules and
gain of freedom for water molecules surrounding the guest) plays an important role
in encouraging the water insoluble substrate to bind to the hydrophobic pockets of an
enzyme. A similar effect operates during complexation of organic guests with micellar,
capsule, and cavitand hosts in water (Fig. 13.9). Upon entry of the guest into the host
cavity, weak intermolecular forces, such as van der Waals, hydrogen bonding, C H--π
and π---π interactions (Fig. 13.1) in addition to the hydrophobic bonding hold
the guest in.
As with all supramolecular systems, the above guest@host complexes are held
together only by weak noncovalent intermolecular forces; hence, at equilibrium they
are labile in nature and within a certain time scale undergo reversible dissociation.
When dissociation occurs, the guest is very weakly bound and can rapidly escape
into the bulk aqueous media. However, if the two halves of the host cavitands are
covalently linked, a carcerand Table 13.1 composed of a single molecule is created.
In such a carcerand, there is internal free space available for occupancy by guests that
can pass through the portals provided by the covalent linkages. Such examples are
discussed in Section 13.22.
13.7 Some Exemplar Hosts of Supramolecular Photochemistry
in the Solid State: Crystals and Porous Solids
The concept of a molecular host for small organic molecules in solution can be extended
to solid hosts. On first consideration, it may seem that solids would not be
particularly suitable as supramolecular hosts, as one might envision very little free
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Section 13.7 Crystals and Porous Solids 949
GG G G
(b)(a)
Scheme 13.4 Schematic representation of guest@host
complexesinsolids:(a)guest@zeoliteand(b)guest@crystal.
In the latter, each molecule can be viewed as a guest,
surrounded completely by other molecules of the same
structure that serve as a rigid “supercage.”
space, causing crystals to be too rigid for the molecular motion required to make and
break bonds, especially in bimolecular reactions. However, many organic crystals
and porous inorganic solids have been shown to be particularly useful hosts for controlling
a range of supramolecular photochemical and photophysical processes.55−59
Scheme 13.4 shows schematically the structure guest@host complexes in crystalline
porous solids (a) and in organic crystals (b). We will consider zeolites as an exemplar
of inorganic supramolecular porous hosts. Several exemplars of organic crystal hosts
will also be described.
Silica (SiO2) is a well-known porous inorganic solid capable of adsorbing and
noncovalently bonding to organic molecules (as in chromatography). Although silica
is a macroscopic solid, it possesses a huge internal porous void space capable of
adsorbing organic molecules. The walls of the pores are made of very strong and rigid
O Si O bonds; the oxygen atoms are tetrahedrally arranged around each Si atom.
Zeolites are crystalline materials that are related to silica where an aluminum atom
is substituted for a fraction of the tetrahedral sites of pure silica.55
The frameworks
of these porous solids thus obtained contain well-defined and uniform empty pores,
channels, and cages that can serve as cavities or supercages for organic molecules of
the appropriate size and shape. Substitution of trivalent aluminum ions for a fraction
of the tetravalent silicon ions at lattice positions results in a network of void space that
bears a net negative charge for each aluminum atom. Each of these negative charges
in turn must be compensated by a positive counterion (Li+
, Na+
, K+
, etc.). The latter
counterions are mobile and may occupy various exchange sites depending on their
radius, charge, and degree of hydration. They can be replaced, to varying degrees, by
exchange with other cations. Many organic molecules whose sizes are on the order
∼ 5–10 ˚A can be accommodated in the intracrystalline cavities of zeolites.
We describe examples of the supramolecular effects of two families of crystalline
zeolites, the so-called FAU and MFI families (Fig. 13.12), on the photochemistry of
organic molecules. The topological structure of the FAU family of zeolite consists
of an interconnected three-dimensional (3D) network of relatively large spherical
cavities termed supercages (diameter of ∼ 13 ˚A; Fig. 13.12a). Each supercage is
connected to four other supercages through 8 ˚A portals that are tetrahedrally arranged
about the center of the supercage. The MFI family of zeolites also has a 3D porous
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950 Chapter 13 Supramolecular Organic Photochemistry
13 Å
8 Å
Portal
(a) (b)
Portal
Supercage
9 Å
5.5 Å
Faujasites (FAU)
(X and Y zeolites)
Pentasils (MFI)
(ZSM-5, ZSM-11 zeolites)
Figure 13.12 Schematic structures of the internal portals of (a) FAU
and (b) MFI zeolites.
structure (Fig. 13.12b). The supercages of FAU are essentially directly connected
to one another; in the case of the MFI, channels ∼ 5.5 ˚A long exist between the
supercages. The intersection of these channels form supercages whose diameter is ∼ 9
˚A. In summary, both the FAU and MFI zeolites possess portals leading to supercages,
with the FAU portals and supercages being somewhat larger than those of MFI. An
important feature of zeolites as host is that guest molecules can only access the
supercages of the void space by squeezing through the portals leading to the internal
surface. Thus, although the supercage of a FAU zeolite possesses a diameter of ∼ 13
˚A, the portal leading to the supercage is only ∼ 8 ˚A in diameter. A similar situation
is found in MFI zeolites as well, where the diameter of the cage and the portals are 9
and 5.5 ˚A, respectively.
Since zeolites are crystalline materials, the supercages are organized in a regular
repeating pattern of supercages in a macroscopically solid material (Scheme 13.4).
An important difference between the two families of zeolites is that in contrast to the
FAU family, the MFI family possess very little aluminum in their framework, and
consequently have very few charge compensating cations in their void space. Thus,
the FAU can be considered to have very polar and hydrophilic supercages filled with
cations whereas the pentasils have nonpolar and hydrophobic supercages with very
few cations.
The cations occupying the internal supercages of the FAU zeolite take up some of
the free space of the supercage, and therefore provide steric hindrance to guests and
species undergoing reaction in the supercage. We will see examples of how the steric
effects of cations in a FAU zeolite provide outstanding examples of supramolecular
steric effects on the course of photochemical reactions of guest@faujasite complexes.
It is easy to understand that zeolites, porous solids with internal empty supercages,
can bind a guest molecule to form guest@zeolite complexes (Scheme 13.4a). But not
all solids are porous. For example, most organic crystals are nonporous solids that
form by a regular repeated pattern of molecules adjacent to one another and bonded
by noncovalent interactions. In organic crystals, however, a collection of molecules
called the unit cell is repeated periodically in exactly the same arrangement over and
over throughout the entire crystal. In organic crystals, molecules are arranged in an
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Section 13.8 Hemicarceplexes and Carceplexes 951
orderly, geometric, 3D structure (Scheme 13.4b). Unlike other guest@host systems
discussed above, the guest and host molecules are the same molecule in a pure crystal.
The most important characteristic of crystals is their molecular periodicity and rigidity.
In organic crystals, each molecule may be considered as a guest that is surrounded
by a host (made up of the same molecule) consisting of a cage of regularly spaced
molecules surrounding it. This rigid, solid guest@crystal complex is structurally
analogous to, but at the opposite extreme of, the flexible, liquid solvent cage. We
will discuss the roles of these features in controlling photoreactions in a crystal.
13.8 The Role of Time Scale and Dynamics in Supramolecular
Organic Photochemistry. The Transient and Persistent
Supramolecular Complex Concept. Hemicarceplexes
and Carceplexes
Supramolecular complexes are formed through association of host (represented by
a circle in Scheme 13.5) and guest (represented as G in Scheme 13.5) molecules
that are held by weak intermolecular forces. In an overall photochemical reaction
G can be a reactant (R), an electronically excited state (∗
R), an intermediate (I), or a
product (P). The strength of these guest@host complexes are measured in terms of
an equilibrium constant (Keq) defined in Scheme 13.5. This constant measures the
binding strength of guest–host complex.60
As the structures of R, R∗
, I, and P differ,
their equilibrium constants of complexes with the host are not expected to be identical.
To avoid complications from uncomplexed R, R∗
, I, and P, one needs to establish what
fraction of the G molecules are complexed to the host and what fraction is dissolved
in the bulk solvent. The value of Keq for the complex depends on the kinetics of
complex formation and dissociation (Scheme 13.5). The bimolecular formation rate
of the complex is equal to kentry[G][O] and the unimolecular rate of dissociation of the
complex is kexit[G@H]. Since formation of a guest@host complex is a bimolecular
process the fraction in the guest@host complex can be controlled by the concentration
of host molecules. For example, a large excess of the host will favor the formation of
the complex. Once the R@host is excited, ∗
R@host is formed and photochemical–
photophysical processes are initiated. The extent of the supramolecular host influence
kentry [G][●]
—
kexit [●]
kentry
kexit
keq =G+ G
G
kentry [G][H]
—
kexit [G@H]
kentry
kexit
keq =G@HH + G
Scheme 13.5 Complexation and decomplexation of
guest@host complexes. The circle represents a host.
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952 Chapter 13 Supramolecular Organic Photochemistry
on the photochemical–photophysical processes depends on the equilibrium constants
of R∗
, I, and P with the host; for the supramolecular influence to be fully effective, the
rates of each step in the photochemical reactions and photophysical processes must
be faster than the exit rate of G from the complex. In this section, we discuss this
phenomenon in terms of the time scale of photochemical–photophysical processes
compared to the rate of exit of the guest from the host.61
The time scale for common photochemical and photophysical processes spans over
12 orders of magnitude, that is, from ∼ 10−12
s (vibrational, electronic relaxation,
ultrafast photoreactions) to ∼ 10 s (long-lived phosphorescence). The dynamics of
the assembly and disassembly of a guest@host complex can also span over a wide
temporal range of many orders of magnitude. Even the dynamics of host structures can
span many orders of magnitude. For example, the supercages of zeolites are essentially
infinitely long lived on any achievable time scale since the disruption of the supercage
requires breaking strong covalent bonds. On the other extreme, the lifetime of an
individual micelle may be a few milliseconds or shorter. We say that the supercage
of a zeolite is persistent and the supercage of a micelle is transient on conventional
time scales of laboratory measurements (minutes or longer). The terms persistent and
transient are relative and we need to refer to a benchmark time scale of interest in a
particular experiment.
In photochemistry, the benchmark time scales are the lifetimes of ∗
R and I,
species whose inherent dynamics will remove them from the system by chemical
reaction or photophysics. Thus, the pertinent benchmark lifetime of an ∗
R@host
or I@host complex is the lifetime of either the host or the guest (in this case the
guest is either ∗
R or I). For simplicity of presentation of the photochemistry of
the supramolecular complexes, we will assume that the host structure is persistent
throughout the photochemical steps; that is, the host structure does not change during
the lifetime of the reaction. However, we cannot in general assume that the time
scale of dissociation of an ∗
R@host or I@host complex will be long relative to the
photochemical events of interest; the structure of an ∗
R@host or I@host complex may
not be constant during the lifetime of the reaction. For example, the exit rate of ∗
R
(or I) from an ∗
R@micelle (or I@micelle) complex may be fast or slow relative to
the photochemistry or photophysics of ∗
R (or I) in the complex. Thus, ∗
R@micelle
(I@micelle) may be persistent or transient relative to the rates of photochemistry or
photophysics.
From the above discussion, we can see that an ∗
R@host complex has meaning
as a supramolecular photochemical object during a certain time scale, namely, the
lifetime of the ∗
R@host complex itself relative to the time scale of the photochemistry
or photophysics of ∗
R in the complex. We will apply the term “carceplex” to ∗
R@host
or I@host complexes for which the guest is persistent; that is, it is completely
“incarcerated” by the host during the lifetime of a photochemical or photophysical
process under investigation (Table 13.2). We will apply the term “hemicarceplex”
to ∗
R@host or I@host complexes for which the guest is transient; that is, ∗
R exits
from the host during the lifetime of a photochemical or photophysical process under
investigation (Table 13.2).
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Section 13.8 Hemicarceplexes and Carceplexes 953
*RR I1 I2 I3
P3P1 P2
Free
(molecular)
Carceplex
(supramolecular)
Hemicarceplex
Increasing time
Scheme 13.6 Schematic of the time-dependent conversion of a supramolecular carceplex
(full circle) to a supramolecular hemicarceplex (dotted circle) to a molecular system (no
circle).
As carceplexes, ∗
R@host and I@host see a single host environment during the
photochemical events of interest. In hemicarceplexes, ∗
R@host and I@host witness a
range of environments during the photochemical events of interest. These differences
in host environments between carceplexes and hemicarceplexes are illustrated below.
Since the lifetime of photochemical and photophysical processes vary over many
orders of magnitude, the notions of a persistent carceplex and transient hemicarceplex
(partially caged) are relative and not absolute. Accordingly, we modify the circle
representation of the host (Scheme 13.1) to accommodate the time-dependent carceplex
and hemicarceplex situation: A closed circle represents a persistent carceplex and
a dotted circle represents a hemicarceplex (Scheme 13.6). The absence of any type
of circle represents guests in the bulk solvent. For example, ∗
R(S1)@micelle may
be persistent (rate of fluorescence faster than the rate of exit from micelle), whereas
∗
R(T1)@micelle may be transient (∗
R(T1) may exit the micelle and react essentially
only when present in solvent).
As an example of the carceplex and hemicarceplex paradigm Scheme 13.6 shows
a hypothetical situation for a system that starts with a persistent ∗
R@host carceplex to
produce a persistent I1@host complex that leads to two products: P1 and a secondary
transient I2@host complex. The latter transient hemicarceplex is visualized to be
able to produce a product P2 or escape to the solvent and produce a third product in
the solvent, I3 → P3. A chemical exemplar of this possibility is discussed in Section
13.21.
Thus, we can see that the rate of exit of a guest from a guest@host complex is
an important parameter in determining whether a photochemical or photophysical
process is best described as molecular, supramolecular, or a combination of the two.
An extreme example of how the definition of supramolecular depends on the lifetime
of ∗
R can be seen by considering a photochemical reactions that take place on the
picosecond or shorter time scale. For such a reaction, an ordinary solvent cage behaves
as a carcerand since ∗
R does not leave the host during its lifetime.
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954 Chapter 13 Supramolecular Organic Photochemistry
13.9 Supramolecular Control of Photochemical
and Photophysical Processes: General Principles
What do we mean by a “supramolecular control” of a photochemical reaction? To
answer this question, we will integrate the fundamental and familiar paradigm of
molecular organic photochemistry (e.g., the R + hν → ∗
R → I → P sequence) with
the paradigm of supramolecular organic chemistry (guest@host chemistry).
It will be convenient to consider supramolecular control of unimolecular and
bimolecular reactions separately. In both cases, the notion of preorganization of the
guest(s) by the host will be a critical factor.
For unimolecular reactions:
1. Host (and coguest) enforced proximity of functionalities that actively inhibit or
accelerate photophysical processes from ∗
R or I. Examples: heavy atom effect
in micelles, cyclodextrins, and zeolites. (See Sections 13.10 and 13.13.)
2. Preorganization of the guest molecular structure in a guest@host complex
through host enforced conformation of the guest that either inhibits or
accelerates one of the competing reaction pathways from ∗
R or I (or both).
Examples: Type I versus Type II from ∗
R and fragmentation versus cyclization
of I(BR). (See Sections 13.14.)
3. Host enforced constraint of the translational or rotational motions of ∗
R or
I(RP). Examples: control of reactions of geminate RP. (See Sections 13.20 and
13.21.)
4. Influence of the host (and coguest) on the cavity-free space that is accessible to
∗
R and I. For example, creation of chiral space that can lead to supramolecular
diasteriomeric relationships for I(RP). (See Sections 13.15 and 13.19.)
5. Host and coguest enforced supramolecular steric effects that control the free
space available to ∗
R and I(RP) in the host cavity. Examples: cage effect on
I(RP) for DBK (see Section 13.21).
For bimolecular processes the most common supramolecular effect is host enforced
spatial location and relative orientation in space of guest and coguest in the
host’s cavity that leads to one of the following:
1. An enforced spatial proximity of guest and coguest that leads to a high
“effective” local concentration of the coguest that is accessible to ∗
R or to I.
Example: excimer formation (see Section 13.11).
2. An enforced spatial separation of guest and coguest that causes protection of
∗
R or I from reagents that are external to the host cavity. Examples: protection
of ∗
R from quenching by O2 and other quenchers, energy transfer (ET), and
by electron transfer (et) through walls of host cavity inhibition of I(RP) from
radical–radical reactions, and making transient species persistent: radicals,
cyclobutadiene (see Sections 13.10 and 13.12).
3. An enforced spatial proximity and selective orientation of ∗
R or I with respect
to coguest. Example: [2 + 2] cycloadditions (see Sections 13.16–13.18).
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Section 13.10 Enhancement of Room Temperature Phosphorescence 955
13.10 Supramolecular Control of Unimolecular Photophysical
Processes by Preorganization of guest@host Complexes:
Enhancement of Room Temperature Phosphorescence
Phosphorescence from aromatic hydrocarbons AH (e.g., naphthalene, phenanthrene)
is not observed in fluid solutions for several reasons: (1) The rate constant (kP) for
phosphorescence from ∗
R(T1) of aromatic hydrocarbons is extremely small, typically
on the order of 1s−1
or less; (2) the ∗
R(T1) state of aromatic hydrocarbons is quenched
by impurities (Q) at close to the rate of diffusion; (3) the quantum yield of formation
of ∗
R(T1) from ∗
R(S1) for some aromatic hydrocarbons (AH) is low. Impurities (Q)
represent any quencher present in the system that competes with phosphorescence,
with molecular oxygen from air being the most common and ubiquitous quencher
(Section 14.1). Now, we employ a supramolecular preorganization strategy to rationally
design guest@host complexes that will exhibit strong phosphorescence from
AH guests in solution at room temperature.
Scheme 13.7 shows a general situation for the steps leading to phosphorescence
from ∗
R(T1). In order to observe a significant quantum yield of phosphorescence from
an AH, the following conditions must hold: kST > k1 and kP > k2[Q]. A supramolecular
strategy for the observation of room temperature phosphorescence would involve
designing a AH@host system that makes kST > k1 and kP > k2[Q] (Scheme 13.8).
In Chapter 4, we saw that for molecular systems heavy atoms can enhance the
rate of both kST and kP. In addition, incarceration of ∗
R(T1) in an ∗
R(T1)@host will
inhibit bimolecular quenching by species in the bulk solution. Thus, an ∗
R(T1)@host
complex that preorganizes heavy atoms into the proximity of ∗
R(S1) and ∗
R(T1)
is a candidate for enhancing the observation of phosphorescence of AH at room
temperature. Cyclodextrins, micelles, and zeolites are examples of potential hosts that
can preorganize heavy atoms into proximity with the ∗
R(S1) and ∗
R(T1) of an AH. In
the case of micelles, the Na+
ions of SDS can be readily exchanged for heavy cations,
such as Tl+
. Thus, a AH@TlDS (thalium dodecyl sulfate) micelle will bring the ∗
R(S1)
and ∗
R(T1) of an AH into proximity of a heavy Tl+
ion (atomic number Z = 81). This
supramolecular feature will increase both kST and kP. In addition, the incarceration
of ∗
R(S1) and ∗
R(T1) in the micelle supercage will inhibit bimolecular quenching of
∗
R(T1) by any potential quenchers in the aqueous phase, especially oxygen. In CDs,
the preorganization involves bringing a molecule posessing a heavy atom as a coguest
into the same cavity as the AH guest.
*R(S1)
not *R(T1)
kST
k1
not hνP
k2 [Q]
*R(T1) S0 + hνP
kP
Scheme 13.7 Schematic representation of the kinetics
of competing pathways on the way to phosphorescence.
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956 Chapter 13 Supramolecular Organic Photochemistry
*R(S1)
(a)
not *R(T1)
kST
k1
*R(T1)
Heavy atom effect
mainly on kST so that
kST > k1
k2 [Q]
*R(T1)
(b)
not *R(T1)
kP
R(S0) + hνP
Heavy atom effect
mainly on kP so that
kP > k2 [Q]
Scheme 13.8 Strategy for the supramolecular control of room
temperature phosphorescence.
SDS
TIDS
500
Wavelength (nm)
300Na+
700
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
OH H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
OH H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
O
H H
OH H
O
Tl+
Intensity
Phosphorescence
Fluorescence
Figure 13.13 Luminescence of naphthalene@SDS and naphthalene@TlDS.
This preorganization strategy has proved successful in a number of cases. An
example for naphthalene adsorbed in micelles is shown in Fig. 13.13.62,63
Although
no phosphorescence is observed with naphthalene in aqueous solution, when it is
included within a TlDS micelle, a strong phosphorescence ensues.
Cyclodextrins have also proven to be excellent hosts to observe phosphorescence
at room temperature when heavy atom containing coguests are absorbed together
with an AH. For example, when CDs are used as hosts and brominated coguests
(e.g., dibromomethane, 2-bromoethanol) are present, phosphorescence from aromatic
molecules, such as phenanthrene, can be readily observed at room temperature in
aqueous solutions (Fig. 13.14).64
The same strategy as described above for micelle and CD hosts of AH were also
used with porous solid FAU zeolites. The supercages of FAU zeolites contain a large
number of exchangeable cations. These cations can be considered as coguests that can
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Section 13.10 Enhancement of Room Temperature Phosphorescence 957
487.5
Wavelength (nm)
Phenanthrene
Solvent
molecules
PhenanthrenePhosphorescence
Fluorescence
CH2Br2
Intensity 650.0325.0
Figure 13.14 Luminescence of phenanthrene@CD in solution in the absence (solid line) and
presence of CH2Br2 (dashed line). In the latter case, a phenanthrene/CH2Br2@CD complex is
formed. Cartoon representations of phenanthrene in solution and as a CD complex are shown
on the left and right, respectively.
interact with incarcerated guest molecules in the supercages. By careful choice of the
cations, one can control the photochemistry of guest (∗
R and I) that takes place inside
the supercage. For example, phosphorescence at room temperature has been observed
from a number of AH in a series of heavy atom exchanged AH@zeolites.65
One of the
most important FAUs is the MX zeolite, where M refers to the exchangeable cations
of the zeolite and X refers to high Al content in the framework of the zeolite. For
example, FAU with M = Na+
is commonly available. The Na+
may be exchanged
for monovalent heavy ions, such as Rb+
, Cs+
, or Tl+
, to produce RbX, CsX, or
TlX. Naphthalene in fluid solution is strongly fluorescent, but nonphosphorescent.
When naphthalene is incarcerated in the supercages of LiX, intense fluorescence
and only weak phosphorescence are observed from naphthalene@LiX. However,
naphthalene@CsX and naphthalene@TlX exhibit intense phosphorescence and very
little fluorescence (Fig. 13.15). The correlation of phosphorescence intensity with
cation mass (and more fundamentally with atomic number, Z) clearly suggests the
Intensity
700
LiX
LiX CsX
TlX
Naphthalene
Cation, Li+
, Cs+
, Tl+
600500
Wavelength (nm)
400300
CsX TlX
PhosphorescenceFluorescence
Figure 13.15 Luminescence of naphthalene in FAU zeolites MX as a function of M.
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958 Chapter 13 Supramolecular Organic Photochemistry
1,n-diphenylpolyene
Cation Tl+
TlX
700 900 1100
Wavelength (nm)
Intensity
1300 1500
Phosphorescence
Figure 13.16 Phosphorescence of 1,n-diphenylpolyenes@TlX zeolites (n = number).
external heavy atom perturbation strongly increases as one goes from Li (Z = 3),
Cs (Z = 55) through Tl (Z = 81).
Polyenes do not phosphoresce significantly, even at low temperature in rigid media,
because even if ∗
R(T1) is produced, it undergoes rapid twisting motions about C C
bonds (Section 10.5). Hence, the observation of phosphorescence from polyenes
(e.g., trans-stilbene and 1,6-all-trans-diphenylhexatriene) incarcerated within Tl+
exchanged zeolites is remarkable (Fig. 13.16). The observed phosphorescence is the
result of the enforced close proximity between the heavy atom cations and the olefin
within a zeolite. In this case, a combination of the heavy atom effect on kST and kP, and
the constrained twisting motion (e.g., cis–trans isomerization) lead to rapid ∗
R(T1)
deactivation. Intersystem crossing and phosphorescence rates have to compete with
other processes, such as cis–trans isomerization that deplete the S1 and T1 states.
Zeolites lower the normally efficient cis–trans isomerization process from both S1
and T1 states through supramolecular steric hindrance and the heavy cations enhance
the kST and kP (see Scheme 13.8).
The final example in this category of supramolecular enhancement of phosphorescence
deals with phosphorescence of thioketones, molecules analogous to ketones
except for the replacement of oxygen by sulfur (Chapter 15). Although these
molecules have high values of kST and kP, they do not phosphoresce in solution at room
temperature, due to quenching by oxygen and by ground-state thioketone molecules
(self-quenching) at diffusion controlled rates. No measurable phosphorescence could
be observed at a concentration > 10−6
M. Once again, the supramolecular approach
helps solve this problem.66
For example, although thiocamphor (Fig. 13.17) shows
no phosphorescence in aqueous solution (10−4
M), it phosphoresces intensely when
encapsulated as a 2:2 complex with OA in water. Though the effective concentration
within the complex is 0.35 M, no self-quenching occurs. This phenomenon is
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Section 13.11 Enhancement of Excimer Formation of ∗
R 959
*S
S
500
S
550 600
Wavelength (nm)
650 700 850
*S
Solvent molecules
Intensity
Figure 13.17 Phosphorescence of thiocamphor@OA in water (solid line) and perfluorodimethylcyclohexane
solvent (dashed line) at room tempearture. Cartoon representation
of thiocamphor in solution and in an OA capsule are shown on the left and right, respectively.
explained as the result of supramolecular preorganization of the two incarcerated
thioketones into a relative orientation such that the two C S groups are situated at
a significant distance from one another (Fig. 13.17) and are unable to reorient to the
quenching configuration required during the lifetime of ∗
R. Thus self-quenching is
eliminated by host enforced preorganization that occurs as a result of guest@host complexation.
Furthermore, the thione is also protected by an OA capsule from quenching
by oxygen.
13.11 Supramolecular Control of Bimolecular Photophysical
Processes by Preorganization of guest@host Complexes:
Enhancement of Excimer Formation of ∗
R
In Section 4.38, we learned that excimer formation and emission are observed for
certain AHs, such as pyrene. However, in homogeneous solvents other AHs, such as
anthracene, do not exhibit significant EX emission even at very high concentration;
in the specific case of anthracene, the reason is anthracene undergoes efficient [4 + 4]
photocycloaddition that competes favorably with EX emission (Section 12.8). However,
the supramolecular preorganization with a coguest strategy provides a means
for designing systems that will allow the observation of EX formation for molecules,
such as anthracene, that do not exhibit EX emission in homogeneous solvents.
Anthracene and the cavitand OA form a 2:2 capsule, [anthracene]2@[OA]2, soluble
in aqueous solution.67
Figure 13.18 shows a schematic of the structure for this complex.
Supramolecular preorganization of two anthracenes in [anthracene]2@[OA]2
compel these anthracene molecules to be organized in an excellent geometry for
excimer formation. Indeed, a strong excimer fluorescence is observed upon photoexcitation
of [anthracene]2@[OA]2 (Fig. 13.18). But we may now ask, why does the
[4 + 4] photocycloaddition reaction not occur in the complex? There are at least two
possible supramolecular answers: (1) The orientation in the complex may place the
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960 Chapter 13 Supramolecular Organic Photochemistry
400
Monomer
Excimer
450 500
Wavelength (nm)
Intensity
550 600
Solvent
molecules
(b)
(a)
Figure 13.18 Left: (a) Monomer emission from anthracene in aqueous solution. (b) Excimer
emission of anthracene in a [anthracene]2@[OA]2 complex. Cartoons on either side of the
emission spectra represent possible arrangements of two anthracene molecules in solution
(left) and in a 2:2 capsuleplex.
two anthracenes so that the 9,10-positions of the anthracenes are in close enough proximity
for excimer formation, but are too far apart for bond formation, and (2) the final
product formation is inhibited because it has a geometry that is larger than the space
available in the host cavity (Fig. 13.6) causing a barrier to the [4 + 4] reaction. The
latter would be an example of a supramolecular steric hindfance to product formation.
Thus, application of supramolecular preorganization strategies provides a means for
observing the excimer of anthracene, not observed in solutions in which anthracene
is highly soluble, to be observed easily in aqueous solution!
The above preorganization strategy extends to solid-state hosts as well. For example,
anthracene exhibits excimer emission when adsorbed onto dry NaY zeolite
(FAU type zeolite), but monomer emission on coadsorption of water by the zeolite
(Fig. 13.19).68
The interpretation for these results is analogous to the one reported
above in solution. In anthracene@(dry)NaY assemblies, cation---π interaction
(Fig. 13.19) favors aggregation of two (or more) anthracene molecules in the ground
state leading to excimer emission. The importance of cation---π interaction becomes
obvious when the zeolites are hydrated and anthracene@(wet)NaY complexes are
formed. When sodium ions are hydrated, as would be the case in zeolites impregnated
with water, only monomer emission is observed. As indicated in Fig. 13.19, within dry
NaY, cation---π interaction attracts two anthracene molecules closeby (favoring excimer
formation). When the cation is hydrated with water, the cation---π interaction is
replaced by a stonger H2O---Na+
noncovalent bond and the two anthracene molecules
separate from one another inhibiting excimer formation and favoring monomer emis-
sion.
The final example in this category relates to pyrene excimer emission. In spite
of pyrene’s tendency to form an excimer, at very low concentration (< 10−6
M), the
rate of excimer formation does not compete with the other processes that deactivate
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Section 13.11 Enhancement of Excimer Formation of ∗
R 961
300 400 500
Wavelength (nm)
Absorptionintensity
600
378
420
Absorption
Absorption
Excimer
fluorescence
Monomer fluorescence
444
393
700
+
= water
Na+
(a)
(c)
(b)
Fluorescenceintensity
Figure 13.19 (a) Absorption and emission spectrum of anthracene@NaY in a dry sample.
(b) Absorption and emission spectrum of anthracene@NaY in a sample to which water
was added. (c) Schematic of the alignment of two anthracene molecules within dry and
hydrated NaY zeolites; water = smaller open circles and cation = larger shaded circle.
S1. However, even when the bulk concentation is < 10−6
M, the effective “local concentration”
can be increased to values > 10−1
M with the help of supramolecular
guest@host complexation. Such effective concentrations can be achieved with the
cavitand CD (Fig. 13.20) and with micelles. Both β-CD (1:1 complex) and γ -CD
(2:1 complex) solubilize hydrophobic pyrene in water. However, only the 2:1 complex
with γ -CD brings two such molecules within van der Waals distance (Fig. 13.20)
and enhances EX emission.69
Thus, EX formation from pyrene even at a bulk concentration
of 10−6
M could be observed provided γ -CD is present in solution. The local
concentration within the reaction cavity of γ -CD is > 10−1
M.
Wavelength (nm)
Pyrene
Solvent
molecules
2 Pyrenes
Excimer fluorescence
Monomer fluorescence
350 450 550
Intensity
Figure 13.20 Luminescence spectrum of an aqueous solution of pyrene in the presence
(dashed line) and absence (solid line) of γ -CD. Cartoon representations of pyrene in
solution (left) and within a CD cavity (right).
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962 Chapter 13 Supramolecular Organic Photochemistry
13.12 Supramolecular Control of Triplet–Triplet Energy
Transfer through the Walls of a Carcerand Host
In Sections 7.4–7.10, we learned that ET and et between molecules can occur over
distances of 10 ˚A or greater through solvent molecules or through a molecular spacer
separating the donor and acceptor molecules. We defined a carcerand as a host that
completely surrounds its guest (Table 13.1). Biacetyl (1) has been incarcerated as
a guest in “Cram’s carcerand” (2) to form a 1@2 complex (Fig. 13.21). The et
and ET transfer from electronically excited 1(T1)@2 to energy acceptors and electron
donors in the solution surrounding the complex has been investigated.70−72
The carcerand serves as a “molecular host spacer” that prevents the HO and LU
orbitals of 1 from overlapping directly with the orbitals of an ET or et partner.
Nevertheless, both ET and et from 1(T1)@2 to energy and electron partners have
been observed in CH2Cl2 solution; the latter processes occur, however, with considerably
smaller rate constants than those observed in nonviscous solvents in which
direct collisions between the 1(T1) and the ET or et partners occur. For example
(Table 13.3), both ET from 1(T1) to pyrene and et from diphenylaniline to 1(T1)
occur with a rate close to that of diffusion (∼ 5 × 109
M−1
s−1
) in CH2Cl2 solution.
However, for the 1(T1)@2 complex, the rate constant of the ET drops by more
than three orders of magnitude to ∼ 1 × 106
M−1
s−1
and the rate constant of the
et drops by more than five orders of magnitude to ∼ 3 × 104
M−1
s−1
, respectively.
H2C H2C H2C H2C
CH2CH2
CH2CH2
CH2
CH2
CH2
CH2 CH2
CH2
CH2 CH3
CH2
H2C
H2CH3C
H3C
H3C
H2CH2CH2CH2C
H Cram’s carcerand
host (2)
Cartoon
representation
of 1@Cram’s carcerand
Guest biacetyl (1)O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OO
O
O
O
HHH
HH H H
Figure 13.21 Biacetyl (1) incarcerated in Cram’s carcerand (2). The insert to the right
is a schematic of the 1@Cram’s carcerand (2).
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Section 13.12 Triplet–Triplet Energy Transfer through the Walls of a Carcerand Host 963
Table 13.3 Comparison of the Rate Constants for ET and et Reactions
of 1(T1) as a Free Molecule in Solution and as 1(T1)@2 in
Solution
Energy Donor Energy Acceptor Product Product from
or Electron or Electron from 1 Due to Py and Amine Rate
Acceptor Donora
ET or et Due to ET or et (M−1
s−1
)
1(T1) (ET) Py(S0) 1(S0) Py(T1) ∼ 5 × 109
1(T1)@2 (ET) Py(S0) 1(S0)@2 Py(T1) ∼ 1× 106
1(T1) (et) (C6H5)2NH(S0) (1•−
) [(C6H5)2NH]•+
(S0) ∼ 5 × 109
1(T1)@2 (et) (C6H5)2NH(S0) (1•−
)@2 [(C6H5)2NH]•+
(S0) ∼ 3 × 104
a. Py = pyrene.
These numbers suggest that, although the host molecular cage prevents direct van
der Waal’s overlap of the electron clouds of 1(T1) and the electron clouds of either
pyrene or diphenylaniline, sufficient orbital overlap occurs. This result may be due
to either a superexchange overlap mechanism or through the overlap of very weak
portions of the wave functions that extend (tunnel) in space beyond the van der
Waals’ size of the guest and energy and electron partners. Although ET and et can
quench 1(T1) through the walls of the host carcerand, chemical reactions, such as
hydrogen atom abstraction, cannot quench 1(T1) at a measurable rate. For example,
phenols, such as resorcinol, which can quench 1(T1) through hydrogen atom abstraction
at close to the rate of diffusion, cannot quench 1(T1)@2 at a measurable
rate.
Electron transfer across supramolecular walls is not unique to the above guest–host
combination. For example, trans-4,4’-dimethyl stilbene (DMS), a neutral hydrophobic
molecule, forms a DMS@OA2 capsule.73
On the other hand, OA (Fig. 13.11)
does not encapsulate cationic methylviologen (MV), a good electron acceptor. However,
the latter is attracted to the exterior of the OA through Coulombic cation–anion
(COO−
) interaction. In spite of physical separation between the electron donor DMS
and acceptor MV by the molecular walls of OA, the fluorescence of DMS@OA2
is quenched by MV through et from the ∗
R(S1) state of DMS to MV (Fig. 13.22).
The fact that et can occur through the walls of OA could be inferred by the use of a
supramolecular technique. Methylviologen has an excellent affinity for the cavitand
host CB[7] (Figs. 13.11 and 13.22). Thus, upon addition of CB[7], MV readily forms
a MV@CB[7] complex (Fig. 13.22). In aqueous solution, the donor DMS remains as
a DMS@OA2 complex and the acceptor MV as an MV@CB[7] complex. Because of
individual incarcerations, the donor and acceptor molecules are separated by double
wall of the two capsules. Interestingly, once the MV is separated from the walls of OA
via supramolecular complexation with CB[7], the fluorescence of DMS is recovered.
This example illustrates the extent of possible manipulations with supramolecular
assemblies.
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964 Chapter 13 Supramolecular Organic Photochemistry
340 360
(a)
(b)
(c)
380 400
DMS@OA2
MV
420
Wavelength (nm)
440 460 480 500
N+
e–
+N
N+
e–
+
N
Intensity
Figure 13.22 Quenching the fluorescence of DMS@OA2 ([DMS]= 6 × 10−7
M, [OA]
= 1.2 × 10−6
M, λex = 320 nm) by MV (6 × 10−6
M) and recovery of fluorescence upon
addition of CB[7]. (a) Emission from DMS@OA2; (b) emission from DMS@OA2 in the
presence of MV; (c) emission from DMS@OA2 in the presence of MV@CB[7].
13.13 Supramolecular Control of Unimolecular Photochemical
Processes by Preorganization in guest@host Complexes:
Supramolecular Selectivity of the Reactive State
In some cases, different products, P1 and P2, originate from ∗
R(S1) and ∗
R(T1), respectively
(Scheme 13.9). One such example is provided in Scheme 13.10. Dibenzobarrelene
(3) upon direct excitation yields product 4, while upon triplet sensitization
(acetone) it yields product 5. Product 4 originates from ∗
R(S1) and product 5 originates
from ∗
R(T1). Now, let us consider how to use supramolecular preorganization to
control which of two primary photochemical processes occurs. In Section 13.10, we
saw that preorganization of heavy atoms as coguests in a guest@host complex will
accelerate spin forbidden transitions of ∗
R. Thus, the heavy atom effect can also be
used to enhance ∗
R(S1) → ∗
R(T1) intersystem crossing (ISC) and thus control product
distributions in photochemical reactions when one reaction occurs from ∗
R(S1)
and the other from ∗
R(T1). In the absence of the heavy atom effect, ∗
R(S1) leads to P1
and in the presence of the heavy atom effect, ∗
R(S1) leads to P2.
As an example of supramolecular control of selection of P1 and P2 by photolysis
of 3 through the strategy of Scheme 13.9, the coguest heavy cation effect was used to
fine-tune product distributions of 4 and 5 within a zeolite.74
For example, irradiation of
dibenzobarrelene included in zeolite KY (Z of K = 19) gives 4 as the major product,
whereas photolysis in TlY (Z of Tl = 81) yields 5 as nearly the exclusive product
(Scheme 13.10). The coguest cation Tl+
accelerates the S1 → T1 intersystem crossing
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Section 13.14 Supramolecular Selectivity of the ∗
R → I Processes 965
hν
hν
kST
kr×n (S1) kr×n (T1)
kr×n (S1) kr×n (T1)
R *R(T1)*R(S1)
kST
R *R(T1)*R(S1)
P2P1
Scheme 13.9 Schematic representation of the path from
∗
R to products P1 and P2 for a molecular (top) and a
supramolecular (bottom) system.
hν
+
43
Acetonitrile
Acetone
KY
TlY
77
—
75
1
23
100
25
99
5
Scheme 13.10 Supramolecular heavy atom control of 4 and 5 formation during
the photolysis of dibenzobarralene 3.
to a rate faster than reaction from S1. Once Tl is formed it produces 5 selectively. The
lighter cation K+
has little effect on ISC and the product distribution is similar to that
in acetonitrile proving the role of the heavy cation.
13.14 Supramolecular Control of Unimolecular Photochemical
Processes by Preorganization of guest@host Complexes:
Supramolecular Selectivity of the ∗
R → I Processes
On occasion, there may be two (or more) competing primary photochemical reactions
proceeding from a single ∗
R. Scheme 13.2c shows schematically the situation where
one primary photochemical reaction produces an intermediate (I1) that leads to a product
(P1) and a second primary photochemical reaction produces a second intermediate
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966 Chapter 13 Supramolecular Organic Photochemistry
(I2) that leads to a different second product (P2). As an exemplar of such a possibility,
we present the photochemistry of benzoin alkyl ethers that undergo competing Type I
and Type II primary processes (Scheme 13.11). The Type I reaction from ∗
R is expected
to be essentially independent of the conformation of the ketone. On the other
hand, the Type II reaction is strongly conformation dependent because the alkyl chain
for the γ -H to be abstracted must be close to the cabonyl group during the lifetime
of ∗
R.
+
+ + +
+
O
Conformer 6A
Conformer 6B
Type II
Type I
H5C6
O
H5C6
C6H5
OCH3
O
H5C6
C6H5
OR
OR
OR
OR
O
O
H5C6 H5C6
C6H5
O
O
H5C6
C6H5
C6H5
O
H5C6
C6H5
OCH3
OCH3
OCH3
OCH3
O
H5C6
O
O
Benzene
Cetyltrimethyl ammonium
chloride micelle
R = CH3 (6)
R = CH3 (6)
X = H
8 9 10
10
9 8 11
11
39%
8%
—
—
—
45%
23%
7%
—
52%
X = CH2–(CH2)5–CH3
—
—
—
—
Benzene
Cetyltrimethyl ammonium
chloride micelle
R = CH2–(CH2)6–CH3 (7)
49%
36%
H5C6
C6H5
CH3
H5C6
C6H5
OH
OH
O
H5C6
H5C6
C6H5
C6H5
CH2
O
X
H5C6
C6H5
O
Scheme 13.11 Comparison of the products from the Type I versus Type II reactions of benzoin alkyl ethers
in homogeneous solution and in micellar solution.
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Section 13.14 Supramolecular Selectivity of the ∗
R → I Processes 967
In homogeneous solutions, the Norrish Type I reaction is the only photoreaction
that occurs for the benzoin alkyl ethers shown in (Scheme 13.11). The Norrish Type II
reaction, although plausible, is not observed, due to the unfavorable conformation of
∗
R for this reaction in solution. Incorporation of benzoin alkyl ethers in a micellar medium
suppresses the Type I pathway via the cage effect (for details see Sections 13.21
and 8.42).75
Under such circumstances, the normally less competitive Type II reaction
becomes more competitive for certain benzoin alkyl ethers and is the major
pathway for benzoin methyl ether (6) (Scheme 13.11) in cetyltrimethyl ammonium
bromide and cetyltrimethyl ammonium chloride micelles. This result is explained as
the consequence of conformational control of the guest by the interface of the host
micelle (Fig. 13.23). Of the two conformations 6A and 6B shown in Fig. 13.23b and
Scheme 13.11, the conformer 6B at the micellar interface (probably due to the preference
for the polar carbonyl and methoxy groups to be in the aqueous phase) leads to
the Norrish Type II products. Benzoin octyl ether (7) failed to give the Norrish Type II
products and afforded only the para-substituted benzophenone (rearrangement product
11) via the Norrish Type I reaction followed by radical–radical coupling in the
same micelle media (Scheme 13.11). For 7, the long alkyl chain prefers to remain
in the hydrocarbon interior leading to predominantly conformer 7A at the micellar
interface (Fig. 13.23c). Under such conditions, the alkyl chain is unfavorably placed
relative to the oxygen atom of the n,π∗
state for the excited ketone to abstract the
γ -hydrogen.
Conformational control of the competition between Type I and Type II reactions
can also be achieved by incarceration of ∗
R in OA (shown schematically in
Fig. 13.24).76
α-Alkyl dibenzyl ketones, similar to α-alkyl benzoin ethers discussed
above, can undergo both Type I and Type II primary photoreactions. Independent
of the chain length, the two α-alkyl dibenzyl ketones (α-propyl and α-octyl) shown
in Scheme 13.12 upon irradiation in hexane solution yield predominantly the Type I
product; the length of the alkyl chain has no influence on the distribution of products in
the homogeneous solvent hexane. On the other hand, upon irradiation these ketones incarcerated
within the OA as a guest@(OA)2 complex in water the product distribution
is different. While 12@(OA)2 yields predominantly the Type I products, 13@(OA)2
yields predominately Type II products (Scheme 13.12). In the latter case, the minor
Type II products in hexane are the major products within the confined reaction cavity
of OA.
This change in reactivity pattern is attributable to the supramolecular control by
the host cavity through conformational preorganization. Nuclear magnetic resonance
(NMR) analysis confirms that for 13 the alkyl chain is preorganized favorably for
facile γ -hydrogen abstraction, while for 12 the abstractable γ -hydrogen on the alkyl
chain is spatially remote from the carbonyl chromophore.
It is interesting to note that the reactivity pattern for α-alkyl benzoin
ethers@micelles (Scheme 13.11), is contrary to what is observed in the α-alkyl dibenzyl
ketones@(OA)2 complex. In micelles, the interface between the hydrophobic core
and the aqueous phase plays a major role in controlling the conformation of guests in
guest@micelle complexes. For example, ketones, such as α-octyl benzoin ether, in a
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968 Chapter 13 Supramolecular Organic Photochemistry
O
H
Molecular
conformational
control
(steric)
Conformer 6A Conformer 6B
*R(A) *R(B) I(BR)
I(BR)
I(RP)
I(RP)
(a)
(b)
(c)
OCH3
CH3
CH2
O OO
HO
H
O
H
OCH3
Supramolecular
conformational
control
Supramolecular
conformational
control
Conformer 6A Conformer 6B (preferred)
I(BR)
CH3
OO
H
O
O OO OH
OCH3
H2O H2O
Conform 7A (preferred) Conform 7B
O
O OO OO
H
H
O OO O
O OO O
O OO O
H2O
O OO O
OH
O
H5C6
C6H5
CH2
O O
H
O
O
CH5C6
C6H5
Figure 13.23 Schematic interpretation of the conformational control of the Type I versus Type II reactions
of benzoin alkyl ethers. (a) In solution, conformational equilibrium controls the products distribution.
(b) Within a micelle, benzoin methyl ether is favored to undergo Type II reaction. (c) Within a micelle, the
Type II reaction of benzoin octyl ether is forbidden.
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Section 13.14 Supramolecular Selectivity of the ∗
R → I Processes 969
O
Type II not allowed
(a)
Type II allowed
(b)
OH
H
H
H
Figure 13.24 Schematic of the supramolecular
conformational control of Type I versus Type II
reactions from ∗
R. If the host preorganizes the
conformation (a), the Type I reaction is favored. If
the host preorganizes the conformation (b), Type II
reaction is favored.
Type II
Type I
Type I
products
Type II
products
Substrate Medium Type 1/Type II Products Ratio
12 Hexane
OA/H2O
80 : 20
89 : 11
13 Hexane
OA/H2O
85 : 15
10 : 90
Occurs independent
of conformation
Occurs from a
specific conformation
12 R = n-C3H7
13 R = n-C8H17
R
O
OH
R
R′
O
hν
hν
Scheme 13.12 Alkyl chain length dependent conformational control on product distributions in α-alkyl
dibenzyl ketones.
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970 Chapter 13 Supramolecular Organic Photochemistry
micellar medium adopt a conformation such that the polar carbonyl group is placed at
the interface and the nonpolar octyl chain deep in the hydrophobic core (Fig. 13.23c).
On the other hand, the preferred conformation of α-octyl dibenzyl ketone@OA is
one in which the octyl chain is closer to the carbonyl, favoring the Type II reaction
(Fig. 13.24). Such exquisite control on product selectivity could not be obtained without
supramolecular control through preorganization of the conformation of the guest
by the host.
13.15 Supramolecular Chiral Effects on Two Competing
Primary Processes of ∗
R Involving Biradical Intermediates:
Preorganization in guest@host Assemblies
So far, we dealt with examples where a single ∗
R gave two different intermediates
through two different primary photoreactions or two ∗
R (S1 and T1) underwent different
primary photochemical processes. Now, we consider examples where the reaction
products (P1 and P2) from ∗
R are stereoisomers (Scheme 13.13). Thus, final products
are either enantiomers (when the reactant is achiral) or diastereomers (when the reactant
contains a chiral auxiliary; the products here are not mirror images). Stereocontrol
of photochemical reactions that are difficult to achieve in homogeneous solution may
be achievable with supramolecular effects.
For example, a remarkable feature of the photolysis of ketones in crystals is the
possibility of selective formation of one enantiomer from achiral molecules. The basis
for this enantiomeric selectivity requires an overall chirality of the crystal. Thus, the
chirality of the crystal structure as a chiral “host” is propagated down to the level
of the molecular “guest” during a photochemical process. As a specific example, we
consider the formation of two enantiomeric biradicals from a γ -hydrogen abstraction
reaction involving an achiral molecular guest in a chiral crystal host.
One example of such enatiomerically selective intramolecular γ -hydrogen abstraction
reaction in the case of ketone 14 is shown in Scheme 13.14.77,78
This achiral
molecule possesses two prochiral γ -hydrogens (A and B). Prochiral hydrogens are
those when replaced give chiral products that are enantiomers. In this particular example,
the C H bond would be replaced by a C C bond. In an achiral environment of
a homogeneous solution, each of these prochiral hydrogen is abstracted by the excited
P1 (mirror image of P2)
*R
I1
P2 (mirror image of P1)I2
Scheme 13.13 Possibility of formation of enantiomeric
products from ∗
R via the same primary
photoreaction involving different intermediates.
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Section 13.15 Preorganization in guest@host Assemblies 971
O
COOH
H
14 14
14 15 Salt 16
17A 17B
17
CH3
H
γ-Hydrogen
prochiral B
γ-Hydrogen
γ-Hydrogen
prochiral A
A
Abstracted
B
Abstracted
Mirror images
O
COOH
H
H
CH3
Enantiomeric excess = ee =
% Enantiomer 17A – % Enantiomer 17B
————————————————————
% Enantiomer 17A + % Enantiomer 17B
CH3
H2N
C6H5
H
O
COO–
H
CH3
H
COOCH3
Crystals
CH2N2 workup
ee 80%
HO
CH3
O
COOH
H
CH3
H
HO
COOH
CH3
H
HO
COOH
CH3
H
H
+
CH3
H3N
C6H5
hν
Scheme 13.14 An example of chiral induction in the crystalline state. Chiral crystals yield enantiomerically
enriched products.
carbonyl with exactly equal rates leading to a 1:1 racemic mixture of the two optical
isomers for the chiral cyclobutanol product. Thus, in solution the two pathways
have identical activation free energy barriers. However, when acid (14) and a chiral
amine (15) are crystallized, a crystalline chiral salt (16) is formed. When salt 16
is irradiated, one of the two possible cyclobutanol products (17) is formed in high
enantiomeric excess (ee; see Scheme 13.14). As expected, in solution both prochiral
γ -hydrogen atoms are abstracted with equal efficiency. However, in the crystal there
is a clear preference for only one of the prochiral γ -hydrogen atoms. The main effect
of the host molecules surrounding ∗
R is the preorganization of the reactants in a single
chiral conformation that enhances abstraction of only one of the two possible prochiral
hydrogens. The non-identical two prochiral hydrogens in the chiral crystal have
different energy barriers for abstraction. Indeed, the X-ray crystal structure shown for
the salt (16) in Fig. 13.25 clearly shows that one of the two prochiral hydrogens is
closer.
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972 Chapter 13 Supramolecular Organic Photochemistry
2.6 Å
3.6 Å
hν
Figure 13.25 X-ray crystal structure of salt 16. Note that the two prochiral hydrogens
are at different distances from the carbonyl group. The excited carbonyl would prefer to
abstract the closer hydrogen (2.6 vs. 3.6 ˚A).
13.16 Supramolecular Effects on Bimolecular Primary
Processes: Preorganization through Orientational Effects
in guest/coguest@host Supramolecular Assemblies
In Section 13.11, we saw how supramolecular preorganization of a guest and coguest
in the cavity of a host can lead to enhancement of EX formation. Supramolecular
preorganization can also be employed to control the products of bimolecular primary
photochemical reactions from ∗
R. As an example, we describe the control of reaction
regioselectivity on [2 + 2] photocyloadditions.
The irradiation of 3-n-butylcyclopentenone (18) in homogeneous solution leads to
a “head-to-tail” [2 + 2] photocyclodimer (19) as the major product (Scheme 13.15a).
However, irradiation of this ketone incarcerated in a micelle leads to the “head-tohead”
[2 + 2] cycloadduct (20) as the major product.79
This disparity in product
selectivity results from preorientation in the micelle of the ketone in a head-to-head
fashion (Fig. 13.26). Upon absorption of a photon, 3-n-butylcyclopentenone molecule
adds to an adjacent 3-n-butylcyclopentenone molecule before the relative orientation
changes. In homogeneous solution, the head-to-tail orientation is preferred by the
more favorable head-to-tail interactions of two dipoles of the molecule. In the micelle
supercage, this favorable interaction is weakened by the strong dielectric constant at
the micelle–water boundary. Head-to-head orientation is favored by the attraction of
the negative end of the molecular dipole into the aqueous portion of the boundary
and the hydrophobic attraction of the alkyl chain into the hydrophobic portion of the
micelle supercage.
Upon irradiation, 18 adds to heptenyl acetate (21) in organic solvents to give a
single adduct 23 (Scheme 13.15b) via a 1,4-BR intermediate (see Section 11.15).
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Section 13.16 Preorganization in guest/coguest@host Supramolecular Assemblies 973
O
O
(a) +
O
18 19 20
n-Bu
n-Bu
n-Bu
O O
n-Bu n-Bu
C6H6
C6H12
KDC Micelle
91%
96%
2%
9%
4%
98%
hν
O
OAc
O
OAc
(b) ++
18 21 22 23
n-Bu n-Bu
CH3OH
C6H12
KDC Micelle
C5H11 C5H11
O
OAc
n-Bu
C5H11
0%
0%
70%
100%
100%
30%
hν
Scheme 13.15 Products of (a) photodimerization and (b) cross-cycloaddition
in organic solvents and micelles. Product distribution in micelles is controlled
by molecular preorganization at the micellar–water interface (see Fig. 13.26).
(Potassium dodecanoate = KDC; n-Bu = n-butyl).
O O –
K+
O
CO O O
CH3
O
–
K
+
O
C
O
–
K +
OC
O –
K+
O
C
O
Figure 13.26 Preorganization of cyclopentenone and vinyl acetate molecules
at the micellar–water interface.
However, irradiation of these two molecules incarcerated in a potassium decanoate
micelle yields a mixture of two adducts (22 and 23), with 22 as the major product.
This finding is consistent with the notion that a micellar interface helps to orient the
reactant molecules (Fig. 13.26).80
A higher yield of cross-adducts over dimers was
achieved with 30 mol excess of the vinyl acetate. Similar to the example shown in
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974 Chapter 13 Supramolecular Organic Photochemistry
Scheme 13.11 and Fig. 13.23, the long alkyl chain of the olefin helps to anchor the
olefin at the micelle–water interface in the geometry shown in Fig. 13.26.
The above two examples illustrate how the interface of a micelle can preorient
molecules and thus reverse the observed photobehavior of molecules in homogeneous
solvents.
Photoselectivity based on preorganization of guest molecular orientation can
also be achieved within water-soluble carceplexes composed of CB and OA hosts
(Fig. 13.11). Two examples of [2 + 2] cyclodimerizations are provided in Schemes
13.16 and 13.17. The [2 + 2] photodimerization of trans-cinnamic acid (24) could
result in the formation of four different stereoisomeric dimers (26–29), differing in
the orientation of the carboxylic acid and phenyl groups. However, irradiation of 24
in water produces cis-cinnamic acid (25) exclusively. Irradiation of the supramolecular
complex of cucurbituril 24@CB[8] results in the mirror symmetric dimer (26)
shown in Scheme 13.16.81
This product is formed to the exclusion of the several other
possible stereoisomeric [2 + 2] dimers. This result illustrates the remarkable ability
of the host CB[8] to preorient two cinnamic acid molecules in a mirror symmetric
hν
hν
COOH
CB[8]
COOH
H2O
X = CO2H
24 25
COOHHOOC
Not formedFormed
26
H5C6 C6H5
X
π
X
π
X
πππ
π
X
X
X
X
π
X
π
COOH
HOOC
29
H5C6
C6H5COOHHOOC
27
H5C6
C6H5 COOH
HOOC
28
H5C6
C6H5
Scheme 13.16 Photodimerization of cinnamic acid included in CB[8].
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Section 13.16 Preorganization in guest/coguest@host Supramolecular Assemblies 975
hν
Acetonitrile
34333231
Anti HTAnti HHSyn HTSyn HH
(a)
(b)
6
8
4
12
84
96
5
5
—
74
3
—
Direct
Triplet sensitized
Electron-transfer sensitized
30
++
hν
34 34
+
Scheme 13.17 (a) Photodimerization of indene under various conditions in solution. (b) Expected
orientation of indene molecules wihin OA capsule.
fashion (Scheme 13.16). This selective preorganization is facilitated by weak forces,
such as the hydrophobic effect (which drives the incarceration of two trans-cinnamic
acid molecules in the host cavity) and π---π interactions between the phenyl groups,
which facilitates the favored mirror symmetry of the two incarcerated guests. The
tight fit within the organic host results in an enhanced barrier inhibiting geometric
isomerization and preorganization of reactive molecules favors a single dimer.
The polar head group and nonpolar tail of the olefins in the above examples
facilitated their orientation at the hydrophobic–hydrophilic interface. Control of photodimerization
of olefins lacking these features could nevertheless be achieved within
the OA capsule.82
As shown in Scheme 13.17 upon direct excitation, triplet and et
sensitizations the anti-head-tail dimer (anti HT; 34) was formed in < 10% yield from
indene (30). But when indene2@OA2 was irradiated in aqueous solution, 34 was
produced as the sole product. The structure of the preferred dimer results from the
preorganization of the two indene molecules in the host capsule. Lack of free space
precludes reorientation of the indene to produce dimers with the structure found in
solution photolysis.
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976 Chapter 13 Supramolecular Organic Photochemistry
13.17 Supramolecular Effects on ∗
R in the Solid State:
Preorganization through Conformational and Orientational
Control in the Solid State
There are many examples of supramolecular preorganization for control of [2 + 2]
photodimerizations in the crystalline solid state.58,83,84
As mentioned in Section 13.16,
irradiation of trans-cinnamic acid (24) in solution yields cis-cinnamic acid (25) and
[2 + 2]photodimerization does not occur. On the other hand, irradiation of crystalline
24 leads exclusively to [2 + 2] photodimerization (Scheme 13.18). The crystalline
state can be considered as a hard and rigid “host” for all of its constituent molecules.
Thus, it “arrests” substantial molecular motion, such as the cis–trans isomerization,
by increasing the barrier for this process in the excited state ∗
R. More importantly,
preorganization in the crystal can favor the photodimerization to a single [2 + 2] photodimer.
Let us see how the crystalline state favors a single photodimer. Cinnamic
acids are observed to crystallize in three polymorphic forms, termed α, β, and γ
forms, each showing a distinct photochemical behavior, as determined by the structural
relationship of nearby cinnamic acid molecules in the polymorphic form. In the
α-crystalline form, the double bond of a molecule in one stack overlaps with that of a
centrosymmetrically related molecule in an adjacent stack that are within ∼ 4.2 ˚A (a
plausible distance for bond formation in the solid state) (Fig. 13.27). Crystals of this
type, upon irradiation, produce the centrosymmetric dimer (29). In the β-type packing,
the molecules are separated by a short repeat distance of ∼ 4 ˚A (Fig. 13.27). In
this form, neighboring molecules up the stack are translationally equivalent and show
considerable face-to-face overlap of the C C bonds that are involved in the [2 + 2]
photodimerization. All cinnamic acids that crystallize in this structure react photochemically
to give products of the same stereochemistry, namely, mirror symmetric
dimer 26. In the γ -type structure, adjacent molecules are offset so that the potentially
reactive double bonds do not overlap, and furthermore the distance between them is
large (∼ 5 ˚A). Crystals of this type are photostable toward [2 + 2] dimerization.
COOH
COOH
COOH
COOH
Crystal
Solution
No reaction
α-Form
Double-bond separation: 3.6–4.1 Å
Nearest-neighbor relation:
Centric
β-Form
Double-bond separation: 3.6–4.1 Å
Nearest-neighbor relation:
Translation
γ-Form
Double-bond separation: 4.7–5.1 Å
Nearest-neighbor relation:
Translation
24
25
29
Ar
Ar
Ar
Ar
hν
hν
COOH
COOH
26
Ar
Ar
Scheme 13.18 Photoreactivity of crystalline cinnamic acids in three polymorphic forms.
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Section 13.18 Supramolecular Effects on ∗
R: Templated Photodimerization in the Solid State 977
4.01 Å
β-Packing α-Packing
3.66 Å
Figure 13.27 Packing arrangements of cinnamic acid in α- and β-forms. Olefins are within a reactive
distance (∼ 4 ˚A) in both crystalline forms and each one would give different isomers.
13.18 Supramolecular Effects on ∗
R: Templated
Photodimerization in the Solid State
The analogy with the supramolecular complex in solution and the crystalline state
is closer for certain two-component crystals (also known as mixed crystals) where
one molecule in the crystal lattice serves as an orienting or “templating” host for a
second molecule that is photoactive. The term templating has been coined for the
action by which the host molecule enforces the orientation of the photoactive guest
(Scheme 13.19) in a mixed guest@host crystal.85
The strategy of the templation
No dimerization
Preorganization with a
templating small molecule
Preorganized for
dimerization
Templating molecule
Y
Y
X
X
Y
X
Y
X
Y
X
Y
X
hν
hν
Scheme 13.19 Schematic representation of the principle of templation for dimerization
of olefins. The template helps to preorganize olefins toward formation of a
single dimer after photoexcitation.
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978 Chapter 13 Supramolecular Organic Photochemistry
N
N
N N
Crystal
(13.1)
(13.2)
(13.3)
Solution
Solid
No reaction
35
3536 36
3537 37
hν
hν
O H N
S
S
O H N
O H N
O H N
OHN
OHN
OHN
OHN
Solid
hνN H
H
N
N
N
N
H
H
H
H
H
HN
N
N
S
SN H
H
N
N
N
N
H
H
H
H
H
HN
N
N
Scheme 13.20 Templated photodimerization of olefins in the crystalline state. Hydrogen bonding between
the template and the olefin directs the dimerization process.
approach to photochemistry of mixed crystals, while not of general scope, works out
remarkably well in selected cases. In this strategy, a template molecule is chosen such
that the predictable packing of the template–host molecules in the crystalline state
will enable the potentially reactive guest molecules to pack in a manner that would
facilitate selective [2 + 2] photodimerization.
As an example of the templating effect, consider the photochemistry of trans-
1,2-bis(4-pyridyl) ethylene (35) upon irradiation in solution and in templated mixed
crystals (Scheme 13.20, Eq. 13.1).86
Olefin 35, like cinnamic acid (24), undergoes only
cis–trans isomerization in solution (Eq. 13.1). Furthermore, irradiation of crystals of
35 does not yield any dimeric products or cis–trans isomerization. Based on the crystal
structure of 35, absence of dimerization upon irradiation of crystals is not surprising.
Olefin 35 crystallizes in a layered structure in which olefins of neighboring molecules
are separated by > 6.5 ˚A. However, two adjacent C C groups, 35, can be engineered
to pack within 4.2 ˚A with the help of templates, such as 1,3-dihydroxybenzene (36)86
and thiourea (37).87
Irradiation of mixed crystals of 36 and 35 (Eq. 13.2) or 37 and
35 (Eq. 13.3) results in the formation of a single photodimer in quantitative yield!
In these cases, hydrogen bonding between the template hosts (36 or 37) and the
guest (35) preorganizes C C bonds of the reacting olefins parallel to each other and
within ∼ 4.2 ˚A, thus facilitating the dimerization process in the crystalline state, to
yield a single [2 + 2] photodimer (Eqs. 13.2 and 13.3). The key feature that makes
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Section 13.19 Preorganization in guest@host Assemblies 979
these templates work in the solid state is that they have the hydrogen-bonding centers
separated by ∼ 4 ˚A, which holds the olefin (35) at either end of the molecule. As
expected, the same templates will not work for olefins that lack the hydrogen-bonding
ability. Also, these templates are only effective in the solid state and do not have the
ability to align 35 for dimerization in solution.
13.19 Supramolecular Chiral Effects on ∗
R in Concerted
Reactions and Reactions Involving Funnels: Preorganization
in guest@host Assemblies
Note that in Sections 6.9 and 6.12 pericyclic photoreactions and cis–trans isomerization
were predicted to be concerted or to proceed through funnels and that a distinction
between the two mechanisms was generally not possible through experiments.
As a result, we will not attempt to distinguish the supramolecular effects on concerted
reactions versus funnels, but will consider both of them to respond in a similar
fashion to supramolecular preorganization effects. For simplicity, we will use the funnel
notation ∗
R → F → P to describe the effects. In addition, we will consider only
stereoselectivity in pericyclic reactions, which generally proceed via singlet states involving
∗
R(S1) → F steps (Section 6.22). In particular, we will describe the use of
supramolecular effects to control the enantiomeric or diastereomeric selectivity of selected
photoreactions (Scheme 13.21), a subtle and challenging general problem in
organic synthesis.88
As a specific exemplar of Scheme 13.21 for which the host forms the chiral environment,
consider the electrocyclic ring closure of the tropolone (38, Scheme 13.22,
Eq. 13.4).89
There two “allowed” disrotatory motions are possible from ∗
R, which are
equally probable in a symmetric environment. Therefore, a racemic mixture of the
ring-closed cyclobutenes (39) is expected (Eq. 13.4). Molecule 40 forms a complex
with β-cyclodextrin (40@β-CD). Since β-CD is a chiral host, the complex 40@βCD
is chiral. This result means that the two disrotatory motions, which are completely
equivalent in an achiral environment (i.e., in the absence of CD), are different to
some extent in the chiral environment of the host. Depending on the difference in
the free energies of activation for the two disrotatory motions of *40@β-CD, one of
the two enantiomers 41 will be formed in an ee. In this case, the value of ee is 33%
(Scheme 13.22, Eq. 13.5).
As a specific exemplar of Scheme 13.21 for which a chiral coguest forms a chiral
environment in the cavity of a host, consider the exemplar of Eq. 13.6 for tropolone
(42) in the zeolite NaY. An ee of 78% is found in product 43 upon photolysis of the
tropolone ether (42) in the achiral FAU zeolite NaY when a chiral coguest, ephedrine,
is incarcerated in the same supercage as 42 to form a 42/ephedrine@NaY. In this case,
the inclusion of ephedrine in the supercage causes the free space in the supercage to
become a chiral space. Thus, the photochemistry of the 42/ephedrine@NaY complex
leads to the formation of a considerable excess of one of the enantiomeric products, 43.
As expected, chiral inductors do not give any ee in the photoproducts in methylene
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980 Chapter 13 Supramolecular Organic Photochemistry
P1
(Enantiomer of P2 if R is achiral;
diastereomer if R has a chiral auxiliary)
F1
P2
(Enantiomer of P1 if R is achiral;
diastereomer if R has a chiral auxiliary)
F2
*R
Scheme 13.21 Schematic representation of two pathways for photochemical
reactions going through two funnels that lead to different
products.
(13.4)
(13.5)
(13.6)
39A
38
40 40A
43A42 43B
40B
Out In
Enantiomer A
ee = 33%
ee = 78%
39B
Enantiomer B
O
O O
O
O
O
R2
hν hν
hν
O
O
R2
O R2
O
O
O
CH2C6H5
C6H5
β-CD
hν
NaY/Ephedrine
O R2
O
O R2
O
O
O
C6H5
H
O
CH2C6H5
+
+
O
CH2C6H5
O C6H5
H
O
Scheme 13.22 Photocyclization of achiral tropolones gives chiral products. Chiral induction is observed
in supramolecular assemblies, but not in solution.
chloride solution where supramolecular preorganization through guest@host complexes
is absent.42
In the examples above, a chiral inductor or a chiral host was used to achieve
chiral induction in the photoproduct. A common technique used in ground-state
organic chemistry to achieve enantioselectivity or diastereoselectvity is to use a chiral
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Section 13.20 Mobility Control on I@host Assemblies 981
44
45
Diastereomer 1
Diastereomeric excess = de =
% Diastereomer 1 – % Diastereomer 2
————————————————————
% Diastereomer 1 + % Diastereomer 2
O
O
O
O
H3C
CH3CN, de = 0%
OA, de = 57%
CH3
hν
CH3CN, de = 10%
NaY Zeolite, de = 57%
hν
O
O
O O
CH3
O O
Diastereomer 2
O
CH3
CH3
O
NH
N
H
Scheme 13.23 Photocyclization of tropolones appended with chiral auxiliaries. A chiral
auxiliary has a better influence within a supramolecular structure than in solution.
auxiliary, a covalently linked chiral inductor, to induce chiralilty in the products.
Generally, this method does not work well for photochemical reactions in solution.
However, by a host enforced preorganization technique that creates a close proximity
between the reactive center and the chiral auxiliary, chiral induction in photoproducts
has been achieved.42
Two examples, 44 in which the host is an OA capsule and 45
in which the host is a NaY supercage, are presented in Scheme 13.23.90,91
In these
cases, the chiral auxiliary has no influence in solution and the diastereomeric excess
(de) is small (< 10%) while in a supramolecular environment it is able to bring about
diastereoselectivity in the photoproducts to the extent of 88%. Cation---π interaction
in zeolites and confined space in an OA capsule probably play an important role in
this unusual phenomenon.
13.20 Supramolecular Effects on Reaction Intermediates I:
Mobility Control on I@host Assemblies
This section deals with examples where the supramolecular effect is used to control
reactions for which a single primary photochemical process produces a single
intermediate (I) that leads to multiple products (Scheme 13.2e). For example, photolysis
of naphthyl ester (46) gives a geminate singlet radical pair A via β-cleavage
(Scheme 13.24).92
This radical pair, as shown in Scheme 13.24, results in at least
seven products (47–53). Can this reaction be made selective to the extent that only
products from the initial primary geminate singlet radical pair, 47 and 48, are formed?
To achieve this, first we need to restrict the translational mobility of the partners of
the radical pair. We also need to keep the radical pairs close to each other to enable
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982 Chapter 13 Supramolecular Organic Photochemistry
O
O
O*1
O
O
O
OH
X
X
X
X
X =
X
+
5150
OH
OH
OH
C6H5
C6H5
C6H5
H5C6
+
+
+
49
47
46
48
Radical pair B OH
–CO
52 53
hν
O
C6H5
Radical pair A
O
C6H5
O
O
C6H5
Scheme 13.24 Photo-Fries reaction of a naphthyl ester.
faster radical–radical recombination than decarbonylation (to produce 49–53) to favor
formation of 47 and 48. A supercage rather than a solvent cage is needed to keep the
radical pair together for a longer time and meet the requirements for selectivity.
Thus, in principle, a micelle, a cavitand (CD), a hemicarcerand (OA), or solid
host, such as zeolite, should reduce the observed products from nine to two. Photolysis
of supramolecular complexes of guest 46 with any of the above hosts has proved
this reasoning to be correct by only producing 47 and 48.92−97
Thus, the incarceration
effectively restricts translational motion to favor formation of 47 and 48. To
further enhance the selectivity of the reaction, the system must reduce the rotational
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Section 13.20 Mobility Control on I@host Assemblies 983
O
O
O
1
2
3 4
O
(a)
(b)
(c)
Product of
least motion
Product of
most motion
(d)
Least motion pathway
Most motion pathway
OH
OH
47
48
C6H5
C6H5
C6H5
Scheme 13.25 Two pathways requiring different extents of motion yield different products
during photo-Fries reaction of a naphthyl ester.
mobilities of the radical pair A. Migration of the acyl radical to the 2-position of the
naphthoxy radical requires much less relative rotational motion than migration to the
4-position (Scheme 13.25). Thus, if the rotational mobility of the acyl radical pair is
severely restricted, a single product (47) is possible and was achieved with 46@NaY
supercages and 46@(OA)2 complexes. The rotational mobility is reduced in NaY by
the cation Na+
---radical interaction and the acyl radical migration to the 4-position
of the naphthoxy radical forbidden by the lack of free space in the OA capsule.
The above exemplar of the supramolecular control to favor products 47 and 48
during photo-Fries reaction results from a series of steps involves only singlet states:
∗
R(S1) → 1
I(RP) → 1
P. The 1
I(RP) → 1
P step is spin allowed and occurs efficiently
when the pair is constrained from diffusional separation. The situation is quite different
when a 3
I(RP) is involved. As an exemplar of the latter (Scheme 13.26), we consider
DBK (54), which upon photolysis in solution yields the only product, diphenyl
ethane (DPE, 55) via an α-cleavage process similar to the one described above. In this
case, rearrangement similar to the one that occurred in naphthyl ester (46) to yield oRP
(56) and p-RP (57) could compete with the decarbonylation process; however, in
homogeneous solution decarbonylation is essentially complete. This variation has its
origin in the different spin characteristics of the reactive intermediates (I): Photoreaction
of 54 occurs from the triplet state, while that of 46 occurs from the excited
singlet state. In 54, the radical pair 3
I(RP) is generated by the cleavage process in
∗
R(T1); therefore, ISC to singlet radical pair, 1
I(RP), must precede the coupling. In
solution, the rate of ISC 3
I(RP) → 1
I(RP) is much slower than the rate of diffusion
of 3
I(RP) out of the solvent cage. As a result, the rearrangement products 56 and 57
are formed as very minor products (∼ 0.2%). Therefore, to control the reaction toward
formation of the rearrangement products, first we must provide a supercage for
the primary radical pair. This reasoning indeed works to a certain extent even in the
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984 Chapter 13 Supramolecular Organic Photochemistry
O O
56
o-RP
Rearrangement
Rearrangement
Radical pair C
Radical pair D
–CO
Decarbonylation
H2C
O
57
p-RP
O
hν
H2CCH2
55
DPE
54
DBK
Scheme 13.26 Possible products during Norrish Type I reaction of DBK.
relatively fluid supercages of micelles: Irradiation of 54@SDSmicelle yields 5.5% of
the p-RP (57).98
Since micelles are fluid and flexible structures and offer much less
restriction than carcerands and zeolites, we would expect the yield of rearrangement
products to increase with the more rigid carcerands and zeolites as hosts.
Indeed, a significant enhancement of the restriction of mobility of the I(RP)
occurs in the more rigid supercages of zeolites. For example, photolysis of 54@NaX
gave 56 and 57 in 40% yield, a dramatic increase from that in solution (0.2%) and
micelle (5.5%).99−101
The reaction within a zeolite occurs in the small and rigid
supercage (Fig. 13.12), where the complexation to cations reduces the translational
and rotational mobility of the primary radical pair C. The data in Table 13.4 clearly
show that with increasing size of the exchangeable cation the following changes in
product distributions occur: (1) The extent of primary radical pair coupling products
relative to decarbonylation product increases and (2) the ratio of the products from
ortho-coupling to para-coupling increases. These results are consistent with a simple
supramolecular steric effect in the reduction of the free space (Fig. 13.28) available
to the primary, (C6H5CH2CO• •
CH2C6H5), and secondary, (decarbonylated) radical
pairs (C6H5CH•
2
•
CH2C6H5), produced by photolysis of 54 (Scheme 13.26). Observed
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Section 13.20 Mobility Control on I@host Assemblies 985
Table 13.4 Product Distributions from the Photolysis of 54@MX,
54@OA2, and 54@micelle
Medium DPE (55) (%) o-RP (56) (%) p-RP (57) (%) [56 + 57]/55
LiX Zeolite 80 5 15 20/80
NaX Zeolite 60 15 25 40/60
KX Zeolite 40 40 20 60/40
NaX/C6H6 Zeolite 5 60 35 95/5
OA Capsule 51 49 49/51
SDS Micelle 95 5.5 5.5/94.5
Water 100 0.2 0.2/99.8
LiX NaX KX
Na+
Benzene
NaX with benzene
Figure 13.28 Schematic representation of how the free space within the
supercage varies with the size of the cation. Also, the free space could be
controlled with adsorption of organic molecules, such as benzene.
selectivity for ortho-coupling product 56 over para-coupling product 57 is similar to
the observations in the case of naphthyl ester (46) (Schemes 13.24 and 13.25) and its
origin once again is related to the restricted rotation in a supercage with very little
free space. On further reduction of the free space within the supercage by adsorption
of benzene as a coguest in a 54/C6H6@NaX complex (Fig. 13.28), rearrangement
products 56 and 57 form in 95% yield!100−103
From the above examples in porous solid zeolites, it is clear that for supermolecular
complexes a decrease in the free space of the reaction cavity favors rearrangement
products of I(RP). This trend is also achieved in aqueous solution. For example, when
the capsule 54@OA2 was irradiated in water, p-RP (57) was obtained in 49% yield,
a significant improvement from 0.2% in water.104,105
Now, we consider a 1,4-BR intermediate, I(BR), yielding products via two processes
(P1 and P2). As an exemplar of I(BR) consider the 1,4-BR derived via Type II
reaction of carbonyl compounds (Scheme 13.27). As shown in Scheme 13.27, the 1,4BR
derived via the γ -hydrogen abstraction can undergo two processes (P1 = coupling
and P2 = disproportionation) that yield different products. The intermediate I(BR), although
initially formed in a cisoid geometry, can adopt a transoid geometry through
conformational rearrangement. The reactivities of these conformationally distinct
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986 Chapter 13 Supramolecular Organic Photochemistry
C
R
R
R
O OH
OH
+
HO
P1
P2
H
Favorable conformation
for Types II and I
Cisoid I(BR)
Favorable conformation
for Type I only
C
R
O
H
R
hν
OH
Transoid I(BR)
R
Scheme 13.27 Intermediates and possible products during Norrish Type II reaction
of a ketone with an abstractable γ -hydrogen. Conformational effects emphasized.
rotomers differ (Section 9.18). The cisoid rotomer can plausibly yield both cyclobutanol
and olefins via cyclization and disproportionation; on the other hand, the transoid
rotomer can give only olefins via cleavage. The scenario here is similar to the benzoin
ether photochemistry where the two conformations of R and ∗
R underwent different
primary photoreactions (Types I and II) (Section 13.14 and Fig. 13.4). In the current
example, it is the two conformations of I (1,4-BR) undergoing different reactions. Can
the same reasoning used in Fig. 13.4 be used here to control the chemistry of I(BR)?
In solution, generally the transoid BR conformation is preferred and the disproportionation
products (P2) are formed in larger amounts. How can we increase the amount
of cyclobutanol (cyclization product) with supramolecular techniques? The essential
preservation of the 1,4-BR intermediate in the cisoid conformation is possible in a
rigid reaction cavity with little free space for the BR to rotate to the transoid conformer.
This preservation of the 1,4-BR intermediate was achieved within NaX, NaY,
and ZSM-5 zeolites and β-cyclodextrin.106,107
Two such examples, butyrophenone
(58) and valerophenone (59), are shown in Scheme 13.28. Irradiation of these ketones
as complexes in these reaction media gave cyclobutanol in large amounts. In these
supramolecular assemblies, most likely the cisoid I(BR) cyclized without relaxing to
the more stable transoid I(BR). In principle, cisoid I(BR) can also disproportionate,
which is likely when the cyclization product does not fit within the reaction cavity. For
example, only cleavage products result upon irradiation of 58@ZSM-5 and 59@ZSM-
5. It is very likely that within the smaller cavity of ZSM, the cisoid I(BR) was forced
to undergo only the cleavage as the cyclobutanol (product of cyclization) would not fit
in the channels of ZSM-5 (Fig. 13.12). This would represent a supramolecular steric
effect on the formation of the cyclobuanol.
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Section 13.21 Time-Dependent Supramolecular Effects on Reaction Intermediates (I) 987
C
RO
+ + HO
C6H5
C6H5 C6H5
H
hν
O
Disproportionation
R = H
R = CH3
Butyrophenone (58) Methanol
NaX
NaY
ZSM-5
β-Cyclodextrin–water
Ketone Medium
Ratio of the Products of
Disproportionation–Cyclization
13.6
2.7
3.2
73.0
3.8
3.5
1.2
1.1
>100.0
2.9
Methanol
NaX
NaY
ZSM-5
β-Cyclodextrin–water
Valerophenone (59)
Cyclization
Scheme 13.28 Products of disproportionation and cyclization controlled by the shape and
size of the reaction cavity.
13.21 Time-Dependent Supramolecular
Effects on Reaction Intermediates (I)
The examples discussed in Sections 13.14–13.20 were concerned with supramolecular
effects that control the primary photochemical ∗
R → I and secondary thermal I → P
processes, where the R, ∗
R, I, and P remained within the supramolecular host during
the entire reaction. In the time scale of the overall reaction (∗
R → P), the reaction cavity
served as a carcerand and R, ∗
R, I, and P did not exit the reaction cavity. In other
words, the guest@host complex remained a time-independent carceplex. This situation
may not apply when the lifetime of an excited state (∗
R) or an intermediate (I) is
fairly long and exit of the guest from the host becomes important. Now, we describe
supramolecular effects on reactions where some fraction of the reaction intermediates
exit the reaction cavity before yielding the final products. Under such conditions,
the prediction of the observed products must take into consideration the time the
reaction intermediates spend in the different environments, that is, in a carcerand,
hemicarcerand, or solvent cage (Section 13.8). For example, consider the carceplex–
hemicarceplex assignments for ∗
R@micelle for an SDS micelle for R = naphthalene:
The lifetimes of ∗
R(S1), ∗
R(T1), and an SDS micelle are τS ∼ 0.1μs, τT ∼ 1000 μs
and τMicelle ∼ 10 μs, respectively. Thus, since τS < τMicelle, ∗
R(S1)@SDS the micelle
is considered a carceplex; however, since τT > τMicelle, ∗
R(T1)@SDS micelle is considered
a hemicarceplex. The consequences of this distinction are considerable. For
example, for the carciplex ∗
R(S1)@SDS micelle, ∗
R(S1) is strongly protected from
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988 Chapter 13 Supramolecular Organic Photochemistry
O
O
O
AA AB BB
61 62 63
A
A B
CO + A + B
Cage effect (%) = × 100
[AB] – ([AA] + [BB])
——————————
([AB] + [AB] + [BB])
+ +
B
hν
60
C
H3C
H3C H3C
CH3
Scheme 13.29 Possible products during Norrish Type I reaction of the unsymmetrical
4-methyl DBK.
bimolecular quenching by oxygen during its entire lifetime, whereas for the hemicarceplex
∗
R(T1)@SDS micelle, ∗
R(T1) is only protected while it is incarcerated in the
host. When ∗
R(T1) exits the host, strong quenching by the oxygen in the bulk aqueous
phase can occur. Under conditions where the quenching by oxygen is faster than the
return of ∗
R(T1) to a micelle, the rate of exit from the micelle determines the lifetime
of ∗
R(T1).
In this section, we illustrate this feature with the photochemistry of an unsymmetrical
DBK 60 represented as ACOB as an exemplar (Scheme 13.29)108,109
In homogeneous solution, all unsymmetrical DBKs (ACOB, where A and B are
different groups) undergo photolysis to produce a primary ACO• •
B triplet geminate
radical pair, 3
I(RP)gem, that decarbonylates to A•
+ B•
. The latter produce AA + AB +
BB coupling products in a statistical 1:2:1 molar ratio through free radical–radical
combinations (Scheme 13.29). In other words, the cage effect for these radical–
radical reactions in nonviscous homogeneous solution is zero. A priori, we can
imagine that depending on the type of supercage in which the primary product of
α-cleavage, 3
I(RP)gem, is produced, the geminate radical–radical coupling products
ACOB, ACOB’ (B’ is an isomeric group of B; ACOB and ACOB’ are two molecules
with the same molecular formula, but with different structures (see 60 and 64 in
Scheme 13.30) or AB could be selectively formed for appropriately selected hosts.
For example, in a very rigid and tight reaction cavity re-formation of the original
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Section 13.21 Time-Dependent Supramolecular Effects on Reaction Intermediates (I) 989
O
60
62
64
RP-I1
RP-I2
Carceplex all the way
A
AB
B
kCO
krearr
O
hνH3C
*O
H3C
O
H3C
H2C
H3C
H3C
H2CCH2
CH3
Scheme 13.30 Photochemical behavior of 4-methyl dibenzyl ketone in a carceplex environment. No
radicals escape. Only AB and rearranged products are obtained.
ACO-B bond, which is broken, might be the only pathway allowed if translational and
rotational motions are strongly inhibited. This process could be the case, for example,
in tightly packed crystals.
As the host cavity becomes more flexible, the separation of the B•
radical from
the ACO•
radical will occur, and recombination to form regioisomers (ACOB ) of
the initial ACOB is possible. If the host allows considerable separation of the B•
and ACO•
radicals, but provides a strong boundary preventing them from becoming
free radicals, decarbonylation may occur to produce a secondary geminate pair of A•
and B•
radicals. As discussed in Section 13.20, this occurs in zeolites and in an OA
capsule where the rearrangement product is formed in a significant amount (∼ 50%)
and after decarbonylation only AB is obtained. In these cases, both the primary and
secondary radical pairs are incarcerated during their entire lifetimes. As illustrated in
Scheme 13.30, in these hosts R, ∗
R, I, and P remain within the same reaction cavity
until the end of the reaction. In other words, the products of the reaction could be
predicted assuming the guest@host complex is a carceplex during the entire course
of reaction ∗
R → P.
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990 Chapter 13 Supramolecular Organic Photochemistry
O
60
64
62
I1 I2
Carceplex (supramolecular) Hemicarceplex
hν
A
AB
–CO
B
O
61 62 63
AA + AB + BB
H3C
*O
H3C
O
H3C
H2C
H3C
H3C
H2CCH2
CH3
I3
Free (molecular)
A B
H3C
H2CCH2
Scheme 13.31 Photochemical behavior of 4-methyl dibenzyl ketone in a hemicarceplex environment.
Some of the radicals escape. A mixture of AA, AB, and BB and rearranged products are obtained.
However, if the host structure is fluid and has a weak boundary for incarcerating
the guest, a fraction of the radicals A and B exit the A/B@host complex and become
free radicals. These free radicals undergo random radical–radical reactions resulting
in the formation of AA + AB + BB. An exemplar of this situation is provided by the
60@SDS micelle, as illustrated in Scheme 13.31.
The photochemistry of 60@micelle was discussed in some detail in Sections 8.41
and 8.42, as well as in Section 13.20. Examples of supramolecular effects on the
I(RP)@micelle → P@micelle were discussed in connection with magnetic effects on
the products of photoreactions that involve an ∗
R(T1)@micelle → 3
I(RP)@micelle
process. In particular, magnetic effects on the cage effect for radical–radical reactions
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Section 13.21 Time-Dependent Supramolecular Effects on Reaction Intermediates (I) 991
in micellar systems were described. Therefore, here we will only emphasize the
supramolecular control of possible products from the I → P step.
Photolysis of 60 in nonviscous solvents results in a 1:2:1 mixture of products
AA, BB, and AB (Scheme 13.29). However, photolysis of the 60@micelle (e.g.,
HDTCl) gives AB as the product in a yield higher than that expected for a random
1:2:1 coupling of the free radicals A•
+ B•
.110−113
This significant change in product
distribution favoring the formation of AB is due to the restriction of the mobility for
the geminate radical pair provided by the host micellar structure. This change only
occurs at or above the critical micelle concentration (the minimum concentration
at which micelles are formed). The predominant formation of AB in a micelle is
consistent with the entrapment and restriction of translational motion of the geminate
radical pair in a small hydrophobic reaction cavity. If all the coupling between A•
and B•
radicals occurred within the micelle, the cage effect should have been 100%
to yield only AB. The fact that this is not the case suggests that some fraction of the
radicals A•
and B•
escape the micellar supercage. As illustrated in Scheme 13.31,
the coupling reaction possibly occured both within the micelle and in the aqueous
exterior. Therefore, with respect to the fraction of escaping A•
and B•
radicals, the
supramolecular complex is considered a hemicarceplex. Thus, although the reaction
starts as an ∗
R@carcerand → I@carcerand, as a function of time, some fraction of
the intermediate I@carcerand has become I@hemicarcerand. The extent to which the
complex is a hemicarceplex depends on the relative rate of exit for the radical pair from
the micelle to the aqueous exterior with respect to rate of coupling for the geminate A•
and B•
radicals. For example, for a carceplex the exit rate is expected to be much lower
than the entrance rate while for the hemicarceplex both could be almost the same or
the exit rate could be faster (Scheme 13.32; Section 13.8). As indicated above, just
because R forms a carceplex there is no guarantee that ∗
R, I, and P would form a
careceplex with a given host.
The rate of exit for the radical pair depends on both the hydrophobicity of the radicals
and the hydrophobicity of the micelle supercage. It is predicted and found that
the cage effect increases with the number of methyl groups (increasing hydrophobicity)
in the radical pair A•
and B•
. For example, the cage effect within the hexadecyl
trimethyl ammonium chloride micelle for DBK is 31%, whereas the cage effect for
4,4’-di-tert-butyl dibenzyl ketone is 95%. The results of photolysis of 60 in micelles
kentry
kexit
G + G
kentry
kexit
G + G
Carceplex:
Hemicarceplex: kentry ~ kexit
kentry >> kexit
kentry < kexit
Scheme 13.32 Dynamics of exit and entrance of a guest from a
hemicarceplex and carceplex. The full circle represents a carceplex
and the dashed circle represents a hemicarceplex.
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992 Chapter 13 Supramolecular Organic Photochemistry
100
90
80
70
60
50
40
30
20
10
0
%Cageeffect
CH2
O
CH2 A–A + A–B + B–BC
6 8 10 12 14 16
Chain length
A
O
BC
CH3
hν
Figure 13.29 Dependence of the cage effect on the micellar size
that is controlled by the alkyl chain length of the detergent.
of alkyl chain length between 6 and 14 (shown in Fig. 13.29) suggests an inversely
proportional relationship between rate of radical escape from a micellar cage and the
size of the micelle. The larger the micelle, the greater the hydrophobicity of the host
and the greater the retention of the geminate radical pair. The estimated exit rates for
the benzyl radical (C6H5CH•
2) from micelles of different sizes (sodium decyl sulfate:
2.7 × 106
s−1
; SDS: 1.8 × 106
s−1
, and sodium tetradecyl sulfate: 1.2 × 106
s−1
) correlate
well with the observed trend in the cage effect; that is, a larger micelle has a
greater cage effect and because of its greater hydrophobicity slows the benzyl radical
escape accordingly.
To summarize, the extent to which the geminate radicals (A•
and B•
) can escape
the micellar reaction cavity depends on both the size of the reaction cavity (micellar
size) and the hydrophobicity of the RP. The more hydrophobic the radical, the slower
its ability to venture outside the micellar reaction cavity. The larger the micelle, the
lesser the radical pair’s escape of the reaction cavity.114,115
From the above presentation, it should be clear that consideration of the
guest@host complex as carceplex or hemicarceplex depends on both the guest and
the host, as well as on the time scales of the steps along the pathway ∗
R → P. For example,
hexadecyl trimethyl ammonium chloride micelle is a carceplex for photolysis
of 4,4’-di-tert-butyl dibenzyl ketone (cage effect ∼100%), but a hemicarceplex for
DBK (cage effect ∼30%). Thus, while analyzing photoreactions in a supramolecular
assembly, the nature of the reaction cavity varies, depending on the lifetime of guests,
reactions, intermediates, and the cavity.
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Section 13.22 Stabilization of Reactive Product Molecules (P) 993
13.22 Supramolecular Effects on Products (P@carcerand):
Stabilization of Reactive Product Molecules (P)
Preorganization of the reactant molecule(s) and continued influence of the host on R,
∗
R, I, and P are essential for achieving selectivity in a supramolecular photochemical
process. In a homogenous solvent, a primary photoproduct (P) could sometimes
be reactive enough to transform itself to a different product. Similarly, a reactive
intermediate (I), as the name implies, would further react to yield a stable product, (P).
Supramolecular hosts have been used to “tame” the reactivity of unstable molecules
and reactive intermediates (Scheme 13.33). Thus, the supramolecular host could
prolong the life of a highly reactive primary photoproduct (P), such as cyclobutadiene
and primary reactive species (I), such as radicals, carbenes, carbocations, and ion
radicals. Examples of such stabilization are highlighted in this and the following
sections. As illustrated in Schemes 13.33 and 13.34, extended incarceration of R,
∗
R, I, and P is crucial to prolonging the life of reactive molecules and intermediates.
Photochemical reactions are often capable of producing highly energy-rich and
transient intermediates (I) and reactive products (P). For example, molecules, such
as cyclobutadiene, benzyne, benzocyclopropanone, and 1,2,4,6-cycloheptatetraene
(Scheme 13.34), generally have a fleeting existence in solution at room temperature,
as they often dimerize and/or react with oxygen. In contrast, when genertaed within a
supramolecualr assembly in solution, these molecules are stable for hours to days at
room temperature. A class of supramolecular carcerand hosts originally synthesized
by Cram is very useful for this purpose (for one such example see Fig. 13.21).116
Variations of the original Cram’s hosts were also used, but for simplicity we represent
all of them as a generic cage, (see Scheme 13.34).
As for cyclobutadiene, self-dimerization of the highly strained intermediate made
characterization in fluid solutions impossible. However, upon irradiation of
α-pyrone@carcerand, cyclobutadiene is generated as the product (Eq. 13.7).117
Cyclobutadiene,
very reactive toward self-dimerization, when generated and trapped
within the carcerand under oxygen-free conditions is indefinitely stable at room
temperature.
R *R
I2 P2
P1
I1
R *R I P1 P2
Scheme 13.33 Schematics of stabilization for reactive intermediates
and highly reactive products through supramolecular architectures.
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994 Chapter 13 Supramolecular Organic Photochemistry
N
O HO HO
HH
N
O
O
(13.7)
(13.8)
(13.9)
(13.10)
O
O
O
hν
hν
hν
hν
hν
Scheme 13.34 Highly reactive molecules made stable with Cram’s carcerand.
Benzocyclobutenedione serves as a precursor for both benzocyclopropanone and
benzyne (Eq. 13.8). Photolysis (> 400 nm) of benzocyclobutenedione@carcerand
yields the highly strained benzocyclopropanone (Eq. 13.8). This molecule, normally
unstable at room temperature, once incarcerated, can even be subjected to X-ray
crystallographic studies.118,119
Benzyne produced by irradiation (> 300 nm) of benzocyclopropanone@carcerand
through elimination of carbon monoxide, stable only
at cryogenic temperature in inert matrices (< 10 K, < −263◦
C), is stable for several
hours at −78◦
C within the confined space of a carcerand.
Usually, highly strained 1,2,4,6-cycloheptatetraene (Eq. 13.9) is characterized
by generating it by photolysis of a diazo precursor at 15 K (−258◦
C) in an Ar
matrix. It was found to dimerize at higher temperatures when the matrix melted. As
shown in Eq. 13.9, upon triplet sensitization, phenyl diazirine@carcerand yielded the
1,2,4,6-cycloheptatetratriene via rearrangement of phenyl carbene and the 1,2,4,6cycloheptatetraene@carcerand
was stable at room temperature as long it remained
incarcerated.
A final example in this category is the stabilization of acetophenone enol produced
by Type II reaction of butyrophenone (Eq. 13.10). Acetophenone enol, when free in
solution, even in the absence of acid or base, readily tautomerizes to the keto form.
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Section 13.23 Making Transient Intermediates (I) Persistent through Incarceration 995
However, when generated by photolysis of butyrophenone@carcerand it was found
to be stable for at least 3 days even in the presence of trifluoro acetic acid.120
13.23 Supramolecular Effects on Reactive Intermediates
(I@carcerand): Making Transient Intermediates (I) Persistent
through Incarceration
Quite often the reactive intermediate I is a reactive carbon-centered free radical. Due
to the diffusion controlled radical–radical reactions, the lifetimes of these radicals
in isotropic solutions are on the order of microseconds or less. For example, benzyl
radicals, when generated in isotropic solutions, by photolysis of DBKs undergo
radical–radical combination reactions in times on the order of a few microseconds.
Thus, these reactive carbon-centered radicals are transient reactive intermediates
in solution. Dibenzyl ketones form complexes with solid cyclodextrins and zeolites.
Carbon-centered radicals generated by photolysis of DBK@cyclodextrins or
DBK@zeolites can have lifetimes of days.121−124
For example, photolysis of α,α’dimethyl
dibenzyl ketone included in β-cyclodextrin generates a phenylethyl radical
that when trapped in solid cyclodextrin is stable for 3 days or longer (Scheme 13.35,
Eq. 13.11). Similarly, the diphenylmethyl radical generated in the channels of
ZSM-5 zeolite by photolysis of tetraphenyl acetone persists for weeks (Eq. 13.12).
These examples show how the supramolecular inhibition of diffusion arrests radical–
radical reactions to the extent that the transient free radicals in solution become
persistent in radical@carcerand complexes.
In this context, note that radical cations of stilbene and other polyenes produced
by photolysis could be stabilized for months in the channels of ZSM-5 zeolite. 1,nDiphenylalkene
cation radicals, which have a nanosecond lifetime in solution, are
stable for months in the channels of ZSM-5 (Eq. 13.13)125,126
Similarly, organic
carbocations remain unreactive for days within zeolites. One such example is the
diphenylmethyl carbocation generated during photolysis of the diphenylmethyl radical
trapped in ZSM-5 zeolite (Eq. 13.14).124,127
The final example in this category is the stabilization of fluorophenoxy carbene@carcerand.
This carbene intermediate, a normally transient species, persists
for days when generated by photolysis of 3-fluoro-3-phenoxydiazirine@Cram’s host.
Incarceration prevents carbene–carbene dimerization and reaction with water and
oxygen (Eq. 13.15).128
From the examples above (Schemes 13.34 and 13.35), it should be clear that the
concept of time scale and transience of reactive intermediates within supramolecular
assemblies is different from that in isotropic solution. Intermediates normally
considered transient, short-lived species persist within the above assemblies. Characterization
of radicals and other transient species (radical ions, carbocations, etc.)
requiring time-resolved laser techniques in solution can be carried with simpler methods
when their reactivity is arrested by incarceration by a host. In supramolecular
assemblies, intermediates and products, which based on solution behavior would be
defined as highly reactive, become “tame” and have lifetimes several orders longer
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996 Chapter 13 Supramolecular Organic Photochemistry
N N
Cram’s carcerand Stable for days
Stable for days
O F
CH2Cl2
(13.12)
(13.11)
(13.13)
(13.14)
(13.15)
O hν
hν
hν
F
ZSM-5 Zeolite
Stable for weeksZSM-5 Zeolite
O
O
ZSM-5 Zeolite
β-Cyclodextrin solid
Stable for months
+
+
Stable for weeks
Scheme 13.35 Examples of incarcerated reactive intermediates in zeolites, cyclodextrins,
and Cram’s carcerand.
than that in solution. Supramolecular assemblies stabilize the products by restricting
their mobility and accessibility by other reactants.
13.24 Summary
Chemistry involves the mastery of an ever increasing universe of complexity. The
chemist has mastered atoms, assemblies of a single nucleus and orbiting electrons held
together through electrostatic interactions, and preorganized electronic configurations
TurroB second pages 2009/10/12 12:28 p. 996 (chap13) Windfall Software, PCA ZzTEX 13.9
References 997
that define the structure and control the reactivity of atoms. Proceeding to molecules,
assemblies of two or more atoms, the chemist has mastered the molecular structure
through an understanding of the covalent bond. The natural extension of molecular
structure is the “supermolecule,” an assembly of two or more molecules whose
structure and control requires an understanding of intermolecular noncovalent bonds.
Supermolecules are the building blocks of supramolecular chemistry. Although the
level of complexity is much greater for supramolecular systems than for molecular
systems, the principles described in this chapter allow a means of guiding expectations
and classifications for a merging of supramolecular chemistry and photochemistry.
Supramolecular chemistry is a work in progress concerned with mastering the rules
of noncovalent bonds as they apply to the understanding and control of photochemical
reactions.
Using the guest@host complex as a paradigm, this chapter describes how the
journey from the solvent cage to the supramolecular cavity provides a systematic
and useful means of understanding and controlling the chemistry of ∗
R@host and
I@host complexes, the key reactive intermediates of the fundamental paradigm for
organic photochemistry. In the next decades, chemists will seek the development of a
powerful paradigm of the noncovalent, intermolecular bond and apply this knowledge
for the ever increasing complex structures ranging from materials sciences to chemical
biology. Such knowledge will open new horizons for the photochemist to continue to
harness and exploit the supramolecular features of ∗
R@host and I@host complexes.
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