1
1
Solution
(solvent + solute)
Increasing selectivity
Medium Matters
Rhodopsin
Gas phase
How do biological media enforce selectivity?
2
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
2
3
Water Soluble Hosts as Confined Media
CucurbiturilsCyclodextrins Pd Nano Cage Calixarenes
SDS / CTACSDS / CTAC NaChNaCh // NaDChNaDCh OctaOcta acidacidDendrimers
4
Are there any other media with some of the features of
biological media?
How can we achieve such a high level of selectivity in
photochemical reactions in a laboratory?
•Ability to solubilize substrates in water
• Weak interactions
• Confinement
3
5
Cartoons of micelle structure
C12 C16
SDS CTABCore (2-3 nm)
Stern Layer
(up to a few A)
Gouy-Chapman Layer
(up to several hundred A)
Water molecule
SO3
-
Na+
N
+
Br-
6
Why do micelles form at all?
4
7
The critical micelle concentration phenomenon: Sudden break in
properties near a certain concentration of surfactant
Monomers only Monomers plus
micelles
8
N
H
H
2 SBD
6 SBD
70 Å29 Å
8 Å
N-CH2-CH2-CO-NH-CH2-CH2-N
Dendrimers: Macromolecules as micelles
5
9
generation surface groups diameter
(Å)
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
6
12
24
48
96
192
384
768
27.9
36.2
48.3
66.1
87.9
103.9
126.8
147.3
separation of the
surface groups
(Å)
12.4
12.8
12.7
12.6
11.5
10.3
9.8
7.7
16
32
64
128
256
512
1024
surface groups
8
Generations of dendrimers
10
Water soluble organic hosts: Cyclodextrins
D-glucopyranoside units
connected into a cycle via
1,4-glycosidic linkages
5 Å
9 Å
7 Å
6
11
12
~ 5 Å ~ 9 Åˇ
~ 12 Åˇ
Cyclodextrins as supramolecular hosts
7
13
Water soluble organic hosts: Cucurbiturils
 Easily prepared by the
condensation of glycoluril
in acidic medium.
 Hexamer [CB6] known since
early 1900’s, first characterized
in 1981.
 Kim and coworkers pioneered the
synthesis and isolation of the
higher CBs [n = 7, 8, 10] in 2000.
NN
N N
O
O
n
14
8
15
CB[6]b
!"CDa
CB[7]b
#"CDa
CB[8]b
$"CDa
a (Ao
) b (Ao
) c (Ao
) d (Ao
) Volume (Ao3
)
14.4 5.8 3.9 9.1 164
5.2 4.7 8.0 174
16.0 7.3 5.4 9.1 279
6.4 6.0 8.0 262
17.5 8.8 6.9 9.1 479
8.3 7.5 8.0 472
14.6
15.4
17.4
b
a
b
c
d
c
Cavity
a) Szejtli, J. Chem. Rev.,1998, 98 (5), 1743 -1754,
b) Lee, J. W., Samal, S.; Kim, K., Acc. Chem. Res. (2003), 36(8), 621-630
a
d
16
11.363 A0
5.461 A0
13.729 A0
7.58 A0
Water soluble organic host: Octa Acid
O
OH
OO O OO O
H H HH
O O
H HH
O OO
O
O O
H
O O
OH HOOH HO
HO OH
HO
O OO O
O O
O
9.07 A0
Soluble in sodium tetraborate buffer solution (10mM)-pH > 9
9
17
Octa acid (OA), Cucubituril (CB) and Cyclodextrin (CD)
A comparison
18
18.7 Aº
13.1 Aº
Pd
Pd
Pd
Pd Pd
P
d
Pd =
H2
N
N
H2
Pd
N
N
N
NN
N
H2
N
N
H2
Pd
NO2
NO2
+
M6L4 Octahedral cage
6
4
Water soluble inorganic host: Fijita’s Pd host
10
19
Cronstedt discovered “boiling stones” which he called “zeolites” from the Greek:
zeo (boil) and lithos (stone).
Baron Cronstedt 1722-1765 A zeolite, as found in Nature
Discovery of zeolites
20
Zeolites: Sythetic
200 nm1 µm 2.5 µ
m
2 µm
Zeolite-A ZSM-5 Zeolite-X or Y Zeolite-L
0.4 nm
0.5 nm
0.7 nm 0.7 nm
More than 65% of the earth’s crust consists of 3D crystalline
polyaluminosilicates (3D-CPAS): feldspar, zeolite, and ultramarine.
Zeolite is a class of 3D-CPAS having nanochannels and
nanocavities.
11
21
22
12
23
Characteristics of Faujasites (Zeolites)
Mx(AlO2)x (SiO2)y.ZH2O
• Microporous solid
• Large surface area
• Well defined channels/cages
• Si/Al ratio = 2.4
• Type I - 4 cations /supercage
• Type II- 4 cations /supercage
24
Nomenclature of elementary supramolecular hosts
13
25
The enzyme guest@host paradigm
G G+
Host Guest Guest@Host
We’ll be using this paradigm to
discuss supramolecular systems
26
C12 C16
SDS CTABCore (2-3 nm)
Stern Layer
(up to a few A)
Gouy-Chapman Layer
(up to several hundred A)
Water molecule
SO3
-
Na+
N
+
BrSurfactant
monomersSchematic representation of a
guest@micelle complex
G
G
14
27
Nomenclature of elementary supramolecular
guest@host complexes
28
Container Store
15
29
Container Store
30
H2O
hydrophobic functionality hydrophilic functionality
Water soluble polymer
CucurbiturilsCyclodextrins Pd Nano Cage Calixarenes
SDS / CTACSDS / CTAC NaCh / NaDChNaCh / NaDCh Octa acidOcta acidDendrimers
CrystalsCrystals Zeolites
Supramolecular Containers
16
31
Guest
Boundary
Reaction cavity
Free Space
Supramolecular Containers
32
Role of Weak Interactions
Na+
Cation---π π---π
H
C–H---π
Hydrogen bond
O
X
Y H
Z
van der Waals
!+
!"
eA
eD
Charge transfer
17
33
Molecular and Supramolecular Organic Photochemistry
R represents a guest molecule. The circle represents a host molecule.
Molecular organic photochemistry
Supramolecular organic photochemistry
34
The beginnings of supramolecular organic chemistry: Cram, Lehn, Pedersen
18
35
The “circle” (host) can accelerate (b)
or inhibit (c) the rate of a reaction
Energy diagram representation of supramolecular control of a reaction
The top reaction (a) is indiscriminate
since the activation energies for R
going to P1 or P2 are identical
36
An exemplar of supramolecular control of a
photoreaction with two competing paths
19
37
Acceleration and inhibition of the Type II photoreaction
20
39
40
Hydrogen bond
O
X
Y H
Z
Pre-organization Through Weak Interactions