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Cage effect
Conformational effect
2
Photochemistry of dibenzyl ketone as an exemplar of cage effect
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Definition of cage effect
4
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
monomers
Schematic representation of a
guest@micelle complex
G
G
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5
GC traces of product
distributions upon irradiation in
solution and in HDTCl micelle
O
Me
h!
BA
O
Me
+ MeMe
+
AA AB BB
A comparison between solution and micellar irradiations
6
In micellar solutions the % cage depends on the surfactant
concentration
Cage effect dramatically increases at a certain concentration of surfactant
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7
Sulfate surfactants
CH3(CH2)nOSO3Na
Bigger micelles,
more hydrophobic cage.
slower exit to water
In micellar solutions the % cage depends on the the cage size
Reactive radicals escape
from smaller cages more
easily.
8
In micellar solutions the % cage depends on the guest structure
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9
Cage effect depends on the exit rate.
For a given guest the
rate of exit decreases
with increasing micelle
size.
For a given guest the rate
of exit decreases with the
hydrophobicity of the guest.
10
Nuclear isotope effect on triplet radical pairs
(b) The degree of
13C=O in DBK
increases with
photolysis (90%
conversion)
(a) Initial sample of
13C=O enriched DBK
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11
Origin of nuclear isotope effect
12
R P
R *R I P
R
1
*R 3*
R
3
I
1
I P
h!
General Photochemical Paradigm
h!
h!
S0 S1 T1
3
RP 1
RP P
ISC ISC
O
BA
1*
O
BA
3* O
A B
3
O
A B
1
O
BA
1
hν
P
Understanding nuclear isotope effect
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13
S=1(triplet)
α
β
αβ
S=0 (singlet)
MS=0
!"#"!
α
α
β
β α
β
MS=1 MS=-1 MS=0
Ms1
Ms1
Ms1
Ms2
Ms2
Ms2
!! !! !"+"!
Spin ½ particles
(electrons, 1H, 13C)
Spin 1 particles (two coupled electrons, two
coupled protons)
14
T S T S
z
S2
!
Hi
z
S1
"
Hi
S1 and S2
coupled, Hi
uncoupled
S2 becomes
uncoupled from S2 and
becomes coupled to Hi
S1
"
S2
!
z
HiS1
"
S2
!
Zero field
HZ
Vector representation of triplet-singlet conversions: intersystem
crossing (ISC)
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15
Mechanisms of crossing from T to S (intersystem crossing, ISC)
Torques on spins
16
ACO B ACO | B ACO | BACO B ACO B
• • • • • • • • • •
3 1
3
ACO-B A-B
"KNIT""SNIP"
CAGED TRIPLET
GEMINATE PAIR
GENERATED
SOLVENT SEPARATED
TRIPLET GEMINATE
PAIR
"ENCOUNTER COMPLEX":
ISC OCCURS WHEN PAIR
IS SOLVENT SEPARATED
3
HYPERFINE
INTERACTION
CAUSES TWIST
Vector representation of triplet-singlet conversions in I(RP) and
I(BR): intersystem crossing (ISC)
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17
The effect of electron-nuclear hyperfine coupling on T-S conversion
T+ T0 T-
S
e-n hfc
T+ T0 T- S
Triplets coupled to nuclei with spin will cross to the singlets faster
than triplets coupled to nuclei without spins.
Triplet radical pairs coupled to 13C will cross to singlets faster than
triplets coupled to 12C
Result: Separation of 13C radical pairs from 12C radical pairs
18
PhCH2CCH2Ph
O
h!
T1S1 PhCH2C
O
CH2Ph
3
12
C
escape13
C CAGE
PhCH2CCH2Ph
O
DBK
PhCH2C
O
CH2Ph
1
b
PhCH2CH2
O
CH3 PhCH2CH2
CH2
PMAP
ENRICHED IN 13
C
FREE RADICALS
PhCH2CH2Ph + CO
IMPOVISHED IN 13
C
O
PhCH2C
O
CH2Ph
Cage effect can be utilized for isotope enrichment
The competion is between cage escape and hyperfine induced ISC
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19
Effect of an applied magnetic field on the T splitting
T+ T0 T- S
!!
"! + !"
""
T"!
# !"
T0
T+
S
T levels split apart, T0 has the same energy as S
Hz
Only T0 → S ISC allowed
20
S S
T
T+
T-
T0
Free
Radicals
Free
Radicals
Cage
Reaction
Cage
Reaction
Zero Magnetic Field High Magnetic Field
Nuclear isotope effect will
increase cage reactions
Magnetic field effect will
decrease cage reactions
Spin chemistry of radical pairs in supramolecular systems
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21
The effect of external magnetic field on the cage effect
The cage effect decreases. More exit from host cage.
Initial 90% photolysis
at 0 applied mag. field
90% photolysis
at 15,000 G applied
mag. field
Isotope enrichment decreases in presence of applied magnetic field
48% 13C 55% 13C62% 13C
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23
Octaacid as a Cage
13.73 Å
5.46 Å
11.36 Å
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
O OO O
O O
OHO
24
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25
O O
H2C H2CCH2– CO
h!
AA
A
The primary radical pair prefers to rotate than
decarbonylate
Medium RAA1+RAA2 AA p-RP
Hexane -- >99 --
Octa acid 10 34 56
Relative product distribution (%)
O O
RAA1 RAA2
O
ISC
p-RP
O
o-RP
26
O
hυ
Ohυ
O
Free space needed for rotation
O
O
Lack of free space
O
h!, ISCR
R
+
O
R
CH3
O
X
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27
O
h!, ISCR
R
+
O
R
CH3
49 51
0 100
28
Structure of MFI zeolites
5.3 Å x 13.3 Å 6.6 Å x 11.5 Å
Molecular diameter
allows diffusion into
the internal surface
Molecular diameter
does not allow
diffusion into
the internal surface
ca 5.5 Å
9Å
4.5Å5.4Å
20Å
6.7 Å
I
Elliptical 10-membered ring
Circular 10-membered ring
20 Ä
ca 5.4 Å
20 Å
ca 5.5 Å
O CH3
O
CH3
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29
(oACOB)ex
oAexCOBin
(B-B)in
BinCOAex
oAex Bin Bin oAex
. . . .
(oAoA)ex
hv
(oA)ex+(B)ex
. .
2 hv
(oACOB)ex
(oA)ex+(B)ex
. .
Models for oACOB Photolysis on MFI Zeolites (Ketones in holes, left. Ketones on surface, right)
(BB)ex(oAoA)ex (oAB)ex+ +
(a) (c)
2 = oACOB 3 = pACOB
CH3
O
O
H3C
oAexCOBin BinCOoAex
oAexCOBin
oAexCOBin
BinCOoAex
BinCOoAex
(pACOB)in
Model for pACOB Photolysis on MFI Zeolites
(pA B)in (pAB)in
. .
(A)
(B)
hv
Ketone completely inside:
100 % cage
Ketone half inside,half
outside: -100 % cage!!!
para ketone fits into
the internal surface
O CH3
ortho ketone does not
fit into the internal
surface
O
CH3
30
Molecular vs. supramolecular radical-radical combination
A B
O
A B
A A
A B
B B
A A
B B
h!
A + B
Diffusion in a supercage
(Geminate Combination)
Diffusion in a
Molecular Solvent
(Random FR Combination)
Diffusion in a
Supramolecular sieve
(Selective FR Combination)
CE = -100%
2
CH3
O
CE = 100%
CE = 0%
O
H3C
oMeDBK = oACOB pMeDBK = pACOB
-CO
30
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32
O O ~ 60o rotation
O
O
~ 180o rotation
h!
no rotation
LiX
NaX
KX
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33
An schematic of supramolecular conformational control of a
photoreaction with two competing paths: *R → I(BR) + *R → I(RP)
34
Controlling the competition between Type I and Type II
products by controlling the *R →
I(BR) of the Type II process
Type II
Inhibited by
preorganization
Type II
Accelerated by
preorganization
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O
R
R
O
h! O
O
+
R
R
Type I
O
R
O
R
RR
-CO
OH
R" OH
R'
O
R'
Type II
h!
36
-C3H7 35 4 50 11
-C8H17 - - 10 90
R
O O
R
h!
R
O
+
R
+
O
+
(ppm)
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37
Supramolecular mechanistic rationalization of the micellar
effect: Preorganization of the conformation of *R
Polar group
favored near
water interface
*R → I(BR)*R → I(RP)
*R → I(BR)
38
Exemplar of micellar control of ratio of Type I and
Type products
I(RP) Free radical
products
I(BR) products
I(RP) favored by fast diffusional separation
I(BR) favored by preorganization
and enhanced cage effect in micelles