1
V. Ramamurthy (murthy)
Department of Chemistry
University of Miami
Coral Gables, FL
Email: murthy1@miami.edu
Supramolecular Photochemistry
Controlling Photochemical Reactions Through
Weak Interactions and Confinement
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4
References
“Supramolecular Photochemistry: The Control of Organic
Photochemistry and Photophysics Through Intramlecular Interactions”
Chapter 13 in “Modern Molecular Photochemistry of Organic
Molecules”, N. J. Turro, V. Ramamurthy and J. C. Scaiano, 2010.
“Reaction Control by Molecular Recognition – A Survey from the
Photochemical Perspective” by C. Yang, C. Ke, Y. Liu and Y. Inoue,
Chapter 1 in “Molecular Encapsulation: Organic Reactions in
Constrained Systems”, U. H. Brinker and J. -L. Mieusset, 2010.
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Thermal Chemistry
Photochemistry
What is the difference?
• Mode of activation
• Selectivity in activation
• Energy distribution
6
• Transition state connects a single reactant to a single product and it is
a saddle point along the reaction course.
• Collisions are a reservoir of continuous energy (~ 0.6 kcal/mol per
impact).
• Collisions can add or remove energy from a system.
• Concerned with a single surface.
Visualization of Thermal Reactions
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Visualization of Thermal Reactions
8
Visualization Photochemical Reactions
Two surfaces are involved
Adiabatic
Diabatic
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9
400 nm 700 nm500 nm
71.5 kcal/mol 57.2 kcal/mol 40.8 kcal/mol
Ultraviolet Region
Chemical Bonds of
DNA and Proteins
Damaged
Infrared Region
Chemical Bonds Energy
too low to make or break
chemical bonds.
X-Rays
0.1 nm
300,000 kcal/mol
Microwaves
1,000,000 nm
0.03 kcal/mol
Huge energies
per photon.
Tiny energies
per photon.
Themal energies
at room temperature
ca 1 kcal/mole
Light and Energy Scales
10
Photochemistry consists of two parts
• Photochemical
• Photophysical
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Points to Remember
• Electronic Configuration of States, nπ*; ππ*
• Spin Configuration of States (S and T)
• Singlet-Triplet Gap (ΔE (S-T)
• Rules of Intersystem Crossing (El-Sayed’s
Rule)
• Absorption and Emission
• Fluorescence and Phosphorescence
• Radiative and Radiationless Transitions
• Kasha’s Rule
Electronic and Spin Configuration of States
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13
Singlet-Triplet Gap and Intersystem Crossing
El-Sayed’s Rule
Stokes Shift
Absorption and Emission
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12
8
4
0
x10
6
700600500400300200
x500
Fluorescence and Phosphorescence
16
Photochemists’ handy horoscope of a molecule
Jablonski diagram: Radiative and Radiationless Transitions
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17
Photophysics: Excimer Emission
1.5
1.0
0.5
0.0
650600550500450400350
wavelength (nm)
in methylcyclohexane solution
10 mM
1 mM
0.1 mM
0.01 mM
7.5 mM
5 mM
2.5 mM
normalizedfluorescenceintensity
18
Pyrene as an exemplar of excimer formation
hν
*
*
*
- hν
+
*
+
Excimer
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TICT Emission
20
Kasha’s Rule
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Examples of Common Organic Chromophores
O O
O
O O O O
Carbonyls
Olefins
Enones
Aromatics
22
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23
Olefins
(!!*)
Geometric Isomerization
Hydrogen atom
transfer
Proton transfer
Electron transfer
Di-!-methane
(Zimmerman) rearrangement
Addition to C=C bond
(non-concerted)
Carbonyls
(n!*)
"-Cleavage
#-Cleavage
Pyramidalization
Pericyclic reactions: Sigmatropic shifts
Electrocylisations
Cycloadditions
Photochemistry: Common Photoreactions
24
O
CH3CO + C(CH3)3
h!
Products
I
h!
+ I Products
O
h!
O
Products
"-Cleavage
I h!
+ I Products
Photochemistry: Primary Photoreactions (1)
X X
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O
Ph
Ph
O
Ph
Ph
h!
Products
O
h!
O Products
H CH2OH
H CH2OH
h!
Products
"-Cleavage
Ph O
O
h!
Ph O
O
Products
O
O
h! O
O
Products
XX
Photochemistry: Primary Photoreactions (2)
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X X
H
R
O OH
2OH+ Products
CH3OH
h!
Sens.
C6H5
+
C6H5
H
Products
O
OH
h!
+
OH
OH
+
CH2OH+
h!
Products
Hydrogen Abstraction
Photochemistry: Primary Photoreactions (3)
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27
Ar2CO + R2NCH3 Ar2CO + R2NCH3
h!
Products
PhCH + Me2NCH2RCHPh PhCH + Me2NCH2RCHPh Products
Products
h!
N
H
+
N
H
+
O
h!
Me2NCH2R+
O
Me2NCH2R Products+
h!
Electron Transfer
X X
N
Photochemistry: Primary Photoreactions (4)
28
X X
X X
Addition to C=C bond, triplet, non concerted
O O
+
h!
Products
+
h!
Products
Sens
O
O
O
+
h! Products+
Cl
Cl
h!
+
Cl
Cl
Products
Sens
Photochemistry: Primary Photoreactions (5)
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I1
P1
P2
P3
R R*
hυ
I2
P4
P5
P6
Photochemistry often yields multiple products
• Nature of the excited state, nπ* and ππ*
• Nature of the spin state, S1 and T1
• Level of the excited state, S1 and S2; T1 and T2
Controlling Photochemical Reactions
Through Conventional Means
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31
Ph
O
Ph
O O
Ph
Ph
O
Ph
O O
Ph
Benzene 100 0
Acetonitrile 58 42
Formamide 38 62
h! + + ++
H
n"* ""*
S1
S2
n!*
!!* Product A
Product B
Reactant
Nature of the excited state, nπ* and ππ* control
through solvents
32
O
O
O
O
O
O
O
O
O
O
O
O
O
O
++
None
In presence of BF3
20 71 10
0 10 90
h!
n"* ""#
S1
S2
n!*
!!* Product A
Product B
Reactant
Nature of the excited state, nπ* and ππ* control through
additives
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33
h!
Sens.
h!
Direct
+
CH2
h!
Direct
h!
Sens.
C6H5 h!
C6H5
OCH3
CH3OH
C6H5
CH2OH
h!
Sens.
O
O
O
h!
sensitized
h!
Spin state control through sensitization
Spin control
T1
S1 Product A
Product B
Reactant
Slow
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Cyclohexane 4.97
n-Butyl chloride 2.37
n-Propyl bromide 0.41
Ethyl iodide 0.25
(10% mole %)
Cis/trans dimerSolvent
100 : 0
T1 : 1 : 9
S1 :
h!
Spin state control through heavy atom effect
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35
S1
S2Product A
Product B
Reactant
T1
T2
Product D
Product C
Kasha's rule and large energy gaps
Large energy gap and violation of Kasha’s Rule
36
S1
S2 Product A
Product B
Reactant
Large
energy
gap
S
O
+
+
O
SH
O
HS
O
OSH
O
+
Controlling nature of reactive state with wavelength of irradiation
Reaction from upper excited singlet states
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Controlling nature of reactive state with wavelength of
irradiation: Reaction from upper excited triplet states
S1
Large
energy
gap
T1
Tn
2 photons
77 K
2 photons
77 K
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• Electronic barrier: Electronic configuration (nπ* vs. ππ*)
• Spin barrier: Spin configuration (S1 vs. T1)
• Enthalpic barrier: Presence of activation energy
• Entropic barrier: Changes in conformational, rotational and
translational freedom
• Competition: Radiative, radiationless and other reactive
modes
Controlling Photochemical Reactions
ΔF# = ΔH#–TΔS#
Φp = kr/Σk
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Overcoming the enthalpic barrier with temperature
control
R P1P2
R*
P1*P2*
Energy barrier
Smaller or no
energy barrier
40
+ + +
+
N
-78 o
h!
Room temp.
h!
Temperature o
C
22 30 45 6 11 8
0 30 34 3 5 31
20 5 - 75- 35 -
78 10 - - - 90
N
4 1 2 3 5
N N
1 2 3 4
5 1
Overcoming the enthalpic barrier by controlling temperature of
photoreaction
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II11
P1
P2
P3
R
hυ
II22
P4
P5
P6
R*
42
h!
O
Me H
Ph
H.
.
O
Me H
Ph
H.
.
O
Me H
Ph
H H2C C
Me
Ph
OH++
H
Ph Me
O
(CH3)3COH
O
H
Benzene
O
Me
H
Ph
H
" in ter-butanol: 1.0
" in benzene: 0.4
Cutting cut down competition with choice of solvent
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+ (A*D A+
D+
)
A+
D+ A + D
O O
Ph Ph
O
D
Hexane
O
O
Ph
Ph
A*
O
Acetonitrile
! in hexane 0.5
! in acetonitrile 0.001
Cutting cut down competition with choice of solvent
I1
P1
P2
P3
hυ
I2
P4
P5
P6
R
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45
h!
254 nm
h!
313 nm
OH
O O
O
254 nm
313 nm
Controlling chemistry with wavelength of excitation
46
Me
OH
EtO
O
O
O
CH3
C
HO
H
OEt
O O
H3C
C
H
OH
EtO
O
+
O
CH3
C
HO
H
OEt
O O
H3C
C
H
OH
EtO
O
Hexanes 1 : 1
CH3OH 1 : 5
h!
Me
OSiEt3
EtO
O
O
O
CH3
C
Et3SiO
H
OEt
O O
H3C
C
H
OSiEt3
EtO
O
O
H3C
C
H
OH
EtO
O
h!/Hexane
100%
does not exist
Conformational control through choice of solvent
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Gas phase
Solution
(solvent + solute)
Reaction more selective
Why reactions in biological media are highly selective
compared to gas phase and solution?
Are there any other media with some of the features of
biological media?
Protein
Medium Matters
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How do a biological media enforce selectivity?
✹ by restricting the rotational and translational motions
✹ by pre-organizing the reactants
✹ by controlling the extent and the location of free space
within a reaction cavity
Photoactive
yellow protein
Highly selective geometric isomerization occurs within a protein medium
BacteriorhodopsinRhodopsin
Green fluorescent
protein
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Controlling Photochemical
Reactions Through Weak
Interactions and Confinement