"Molecular Photochemistry - how to
study mechanisms of photochemical
reactions ?"
Bronislaw Marciniak
Faculty of Chemistry, Adam Mickiewicz University,
Poznan, Poland
2012/2013 - lecture 2
Contents
1. Introduction and basic principles
(physical and chemical properties of molecules in the excited
states, Jablonski diagram, time scale of physical and chemical
events, definition of terms used in photochemistry).
2. Qualitative investigation of photoreaction mechanisms steady-state and time resolved methods
(analysis of stable products and short-lived reactive intermediates,
identification of the excited states responsible for photochemical
reactions).
3. Quantitative methods
(quantum yields, rate constants, lifetimes, kinetic of quenching,
experimental problems, e.g. inner filter effects).
Contents cont.
4. Laser flash photolysis in the study of photochemical
reaction mechanisms (10–3 – 10–12s).
5. Examples illustrating the investigation of photoreaction
mechanisms:
- sensitized photooxidation of sulfur (II)-containing organic
compounds,
- photoinduced electron transfer and energy transfer processes,
- sensitized photoreduction of 1,3-diketonates of Cu(II),
- photochemistry of 1,3,5,-trithianes in solution.
2. Qualitative investigation of photoreaction
mechanisms - steady-state and time
resolved methods
- analysis of stable products
- identification of short-lived reactive
intermediates
- identification of the excited states
responsible for photochemical reactions
Jablonski diagram
S2
IC
S1
T2
IC
ISC
+Q
T1
R
A
S0
F
IC
+Q
Ph
ISC
R
Scheme of photochemical reaction
A
h
A*
I
B+C
Intermediates
Stable
products
- analysis of stable products
- identification of short-lived reactive intermediates
- identification of the excited states responsible for
photochemical reactions
Norrish type II Photoreaction
Ph
Ph
O
H
Ph
O*
H
h
O*
H
ISC
R R'
R R'
R R'
(K)
(1K)
(3K)
(1)
Ph

CH2=CRR' + PhCOCH3
Ph
OH
(2)
OH
R'

R R'
(D)
R
(3)
PhCOCH2CH2CHRR'
Analysis of stable products
A
h
A*
I
B+C
1. Preparative irradiations
2. Product analysis: GC, HPLC, TLC, GCMS, LCMS,
spectroscopic methods etc.
3. Separation of products from the reaction mixture:
- prepartative GC, HPLC, TLC,
- column chromatography
- other methods
4. Identification of separated products:
spectroscopic methods: IR, NMR, UV-Vis, Fl, MS,
elemental analysis etc.
Note:
Separated products can be used as reference samples in the
quantitative analysis
Analysis of stable products - example
Norrish type II photoreaction of valerophenone (0.1 mol/dm3)
in methanol
irr > 300 nm
C6H5COCH2CH2CH2CH3
h
C6H5COCH3 + CH2=CHCH3 + cyclobutanol derivative
0
5
10
15
czas retencji
Retention
time[min]
[min]
(1:20)
(1:20)
CH2 =CHCH 3
sygnał detektora
signal
Detector's
(1:200) PhCOCH3
(1:200) Internal
standard standard
wewnętrzny
CH 3 OH
(1:2000) PhCOCH 2 CH 2 CH2 CH3
Ph
Ph
OH
OH
CH 3
CH 3
//
20
Norrish type II Photoreaction
Ph
Ph
O
H
Ph
O*
H
h
O*
H
ISC
R R'
R R'
R R'
(K)
(1K)
(3K)
(1)
Ph

CH2=CRR' + PhCOCH3
Ph
OH
(2)
OH
R'

R R'
(D)
R
(3)
PhCOCH2CH2CHRR'
Photochemistry of Valerophenone in methanol GC MS results
acetophenone C6H5C(O)CH3
m/e (relative intensity):
121(3,4), 120(M+,41), 106(8), 105(100), 78(9), 77(83), 51(30), 50(11)
1-phenyl-2-methylcyclobutanol (izomer trans)
m/e (relative intensity):
162(M+,3), 135(9), 134(33), 133(25), 120(100), 105(76), 91(15),
78(43), 77,(42), 51(18)
1-phenyl-2-methylcyclobutanol (izomer cis)
m/e (relative intensity):
135(8), 134(7), 133(12), 120(100), 105(56), 91(10), 78(40), 77(36),
51(12).
Steady-state irradiation systems
6
7
1
2
3
45
1- excitation source, 2- diaphragm, 3- thermal filter (cell with H2O),
4- lens, 5- light filter, 6- merry-goround system
A
h
A*
I
B+C
Identification of short-lived reactive intermediates
1. Spectroscopic methods - flash photolysis
- UV-Vis absorption and emission
- IR
- NMR (CIDNP)
- EPR
2. Chemical methods
3. Kinetic methods
ns laser flash photolysis
Z
K
Laser
M
P
st art
R
C
Benzophenone- (Phenylthio)acetic
Tetrabutylammonium Salt
O
BP +
S CH2 C O
Sovent: CH3CN
N
1 s
Absorbance
0.04
Absorbance
0.04
0.02
0.00
0.0
-7
2.0x10
-7
4.0x10
-7
6.0x10
12 s
time [s]
0.02
45 s
110 s
0.00
150 s
400
600
800
wavelength [nm]
Fig. Transient absorption spectra of intermediates following the quenching
of benzophenone triplet by Ph-S-CH2-COO-N+(C4H9)4 (0.01M).
Inset: kinetic trace at 710 nm.
Absorbance
0.06
0.04
710 nm
0.02
520 nm
0.00
0.02
after 1 s
200 400 600 800
Time [ns]
0.04
0.04
Absorbance
Absorbance
0
0.02
710 nm
0.00
0
50
100
Time [s]
150
after 150 s
0.00
400
500
600
700
800
Wavelength [nm]
Fig. Transient absorption spectra following triplet quenching of BP (2 mM) by
C6H5-S-CH2-COO-N+R4 (10 mM) after 1 s and 150 s delays after the flash in
MeCN solution. Insets: kinetic traces on the nanosecond and microsecond time scales
Reaction scheme
R
R
R
R
N
N
R
R
O

C O
C
R
R
O
O
CH2

C

O
CH2
R
CH2
C
S
S
O
S
R
C

N
R
O

R
BP
PTAAS
(Hofmann
elimination)
R
C
R
C
OH

N
R
R

O
+ H+
R
C
OH

R
N
R
R
CO2
System studied
Sensitizers
4-Carboxybenzophenone
(CB)
Benzophenone
(BP)
O
O
COO
Sulfur-Containing Organic Compounds
(Quenchers):
carboxylic acids
thioethers
R S R
alcohols
R S (CH2)n OH
trithianes
H
R
S
S
R
H
S
R
+
amino acids
H
R S (CH2)n COO-
NH3
R S (CH2)n CH COONH2
R S (CH2)n CH COO-
Sulfur-Containing Organic Compounds
(Quenchers):
methionine derivatives
methionine-containing
di-, tripeptides and
polypeptides
NH R1
CH3 S (CH2)2 CH C R2
O
e.g. Met-Gly, Gly-Met
Met-Met, Met-Met-Met
Met-Met-Ala
Met-Gly-Met
Met-Enkephalin
Motivations
• Oxidative stress
– Alzheimer’s disease
– Biological aging
• Basic issues
– Neighboring-group effects
– Details of oxidative scheme
Our Traditional Scheme
3
CB*
+
[ CB-
>S
>S ]
  
kesc
kbt
CB +
kCH
CB- +
>S
CBH
+
CH2-S-CH2-

or CH3-S-C H-
>S
Reference Spectra of CB
4-Carboxybenzophenone Transients
25000
Triplet
Radical anion
20000
Ketyl radical
15000
10000
5000
0
400
500
600
Wavelength (nm)
700
Intermediates
3(CB)*
(540 nm)
CB
(660 nm)
CBH
(570 nm)
S+
S+
S
S
S O
+
(480 nm)
S
S
+
N
S
+
(380 nm)
(410 nm)
S
+
CB + C6H5-S-CH2-COOH in aqueous solution
0.16
 = 660 nm
0.06
A
125 ns
1.25 s
12.5 s
50 s
0.03
0.00
0.12
50
100
150
A
0
0.08
0.04
0.00
400
500
600
700
800
Wavelength (nm)
Fig. Transient absorption spectra following laser flash photolysis recorded at four
different delay times. Benzophenone ([CB = 2 mM) and (phenylthio)acetic acid
([C6H5-S-CH2-COOH] = 20 mM) in Ar-saturated aqueous solutions pH = 7.5.
Inset: kinetic trace at  = 660 nm
A
h
A*
I
B+C
Identification of short-lived reactive intermediates
1. Spectroscopic methods - flash photolysis
- UV-Vis absorption and emission
- IR
- NMR (CIDNP)
- EPR
2. Chemical methods
3. Kinetic methods
Identification of short-lived reactive intermediates
2. Chemical methods - chemical trapping
A
h
A*
R
+Z
RZ
stable product
Scavenger (Z) of free radicals:
- does not absorb excitation light
- selectively react with R with a large rate
- does not react with A, A* and RZ
- does not affect the mechanism of RZ formation
- form RZ easy to detect.
Typical scavengers: O2, alkenes, RNO, I2
2. Chemical methods - example
Y.L. Chow, G. Buono-Core, J. Am. Chem. Soc. 108, 1234, (1986)
„Role of the Acetylacetonyl Radical in the Sensitized Photoreduction
of Bis( acetylacetonato)copper( II)”
Spin trapping of acetylacetonyl radicals (acac ):

(CH3)3C-NO + CH(COCH3)2

(CH3)3C-N-CH(COCH3)2
O
EPR spectrum of the benzophenone-sensitized
photoreduction of Cu(acac)2, in the presence
of 2-nitroso-2-methylpropane measured after
two-minute irradiation of a methylene chloride
solution of Cu(acac)2 (1mM), 2-nitrozo-2methylpropane (2 mM), and benzophenone
(5 mM), hyperfine splitting constants:
aN = 1.363 mT, aH = 0.315 mT
and g = 2.0062.
B [mT]
2. Chemical methods - example
Y.L. Chow, G. Buono-Core, J. Am. Chem. Soc. 108, 1234, (1986)
„Role of the Acetylacetonyl Radical in the Sensitized Photoreduction
of Bis( acetylacetonato)copper( II)”
Trapping of acac with alkenes:
C C
+ CH(COCH3)2
addition
products
RZ product analysis: GCMS and NMR, IR
Conclusion: acac was proved to be the reactive intermediate in the
sensitized photoreduction of Cu(acac)2.
Different Actions of Scavengers
• Direct capture of free radicals.
• Repair of damage caused by radicals.
• This second mechanism is important for the repair of
damage by free radicals in biological systems.
A
h
A*
I
B+C
Identification of short-lived reactive intermediates
1. Spectroscopic methods - flash photolysis
- UV-Vis absorption and emission
- IR
- NMR (CIDNP)
- EPR
2. Chemical methods
3. Kinetic methods
3. Kinetic methods
Example (N.J. Turro, Modern Molecular Photochemistry, p. 261,
„Involvement of T1 (n,*) of benzophenone as the chemically reactive
agent in the photoreduction of benzophenone by benzydrol”
Ph2CO + Ph2CHOH
h
OH OH
Ph-C
C-Ph
Ph Ph
B
BH2
B  B*
B*  B
B* + BH2  BH
2BH  BH-BH
B* + Q  B + Q*
BH-BH
aIa
kd[B*]
kr[B*][BH2]
kp[BH]2
kq[B*][Q]
3. Kinetic methods
Example (N.J. Turro, Modern Molecular Photochemistry, p. 261,
„Involvement of T1 (n,*) of benzophenone as the chemically reactive
agent in the photoreduction of benzophenone by benzydrol”
1
1
kd
1


B
a
ak r [ BH 2 ]
k q [Q]
1
1
kd



B
a
ak r [ BH 2 ] ak r [ BH 2 ]
Experiments:
kd / kr = 0.05 M
kq / kr = 500
Taking kq = 1x109 M-1s -1
kd  105 s -1
  10 s
Conclusion:
T1 (n,*) of benzophenone is the reactive state.
A
h
A*
I
B+C
Kinetic methods in the study of the mechanism of photochemical reactions
Procedure:
- assumption of the kinetic scheme
- appropriate equations should be derived, e.g. dependence of R vs. [A] or
[Q]
- experiments, rate constants determnation and the interpretation of the
results
Kinetic methods are so-called indirect methods and must confirmed by
direct methods.
A
h
A*
I
B+C
Determination of the reactive state in a photoreaction:
1. Direct methods ( A, F, P, EPR)
2. Indirect methods (sensitization and quenching)
If the photoreaction is wavelenght- independent, the involvement of
upper excited states can be neglected.
Question: S1 or/and T1
triplet
sensitization
S1
ISC
PS
h
S0
T1
PT
triplet
quenching
Direct
PS
PT
S1
T1
QS
Photosensitized
QT
S0
Experiment (result)
Reactive state
(conclusion)
1. Only S1 quenched, reaction inhibited
None
2. Only T1 quenched, reaction inhibited
T1
3. Only T1 quenched, reaction uninhibited
S1
4. Only T1 sensitized, reaction does not occur
S1
5. Only T1 sensitized, reaction occurs
T1 or S1 + T1
Experimental Methods for Detection of Intermediates
and Excited States [Turro]
Reactive
intermediate
Direct
methods
Indirect methods
S1
F, A
CIDNP, KINETICS, PRODUCTS
T1
P, A, EPR
CIDNP, KINETICS, PRODUCTS
R3C +
A, F, P
MI, CHEM, PRODUCTS
R3C -
A, F, P
MI, CHEM, PRODUCTS
R3C •
A, F, EPR
MI, CHEM, PRODUCTS
Biradical
A, F, P, EPR
CIDNP, MI, CHEM, PRODUCTS
Scheme of photochemical reaction
A
h
A*
I
B+C
Intermediates
Stable
products
- analysis of stable products
- identification of short-lived reactive intermediates
- identification of the excited states responsible for
photochemical reactions
A
h
A*
I
B+C
Kinetic methods in the study of the mechanism of
photochemical reactions
Procedure:
- assumption of the kinetic scheme
- appropriate equations should be derived, e.g. dependence of R vs. [A] or
[Q]
- experiments, rate constats determnation and the interpretation of the
results
Kinetic methods are so-called indirect methods and must confirmed by
direct methods.
Example:
Photochemistry of Phenyl Alkyl Ketones in the Presence of PPh3
PhCOR + PPh3 + CH3OH
h
 > 330 nm
Ph3PO + PhCHR
OCH3
where: R = CH3, (CH2)2CH3, (CH2)3CH3, (CH2)2CH(CH3)2, (or Ph)
Norrish type II photoreaction
Ph
Ph
O
H
Ph
O*
H
h
O*
H
ISC
R R'
R R'
R R'
(K)
(1K)
(3K)
(1)
Ph

CH2=CRR' + PhCOCH3
Ph
OH
(2)
OH
R'

PPh3
R R'
(D)
PPh3
R
(3)
PhCOCH2CH2CHRR'
Kinetic scheme
K
3K
h

ISC = 1.0

1K
k2
 K +  + H -abstraction
3K
CH3OH
k3
3B

3K
4
+ PPh3 
k
k
5
[K-PPh3] 
CH3OH
k6
[K-PPh3] 
k7
[K-PPh3]
PhCH(OCH3)R + Ph3PO
K + PPh3
3B

3B
8

AP + olefina
3B
9

3B
3K
K
k
k
CB
k
10
+ PPh3 
K + PPh3
0
 1 (k4 T k10  B )[ PPh3 ]  k4 T k10  B [ PPh3 ]2

0
 1 (k4 T k10  B )[ PPh3 ]

1
1
1
 
 e  k4 T [ PPh3 ]
0,  - quantum yields of acetopnenone (AP) in the absence and presence of PPh3
(AP , CB, K)
e - quantum yield of ether (PhCH(OCH3)R (or Ph3PO)
T = 1 / (k2 +k3)
B = 1 / (k7 +k8 +k9)
 = k5 / (k5 +k6) = emax
0
5
10
15
czas retencji
Retention
time[min]
[min]
(1:20)
(1:20)
CH2 =CHCH 3
sygnał detektora
signal
Detector's
(1:200) PhCOCH3
(1:200) Internal
standard standard
wewnętrzny
CH 3 OH
(1:2000) PhCOCH 2 CH 2 CH2 CH3
Ph
Ph
OH
OH
CH 3
CH 3
//
20
Stern-Volmer plot for the valerophenone photolysis
in the presence of PPh3
2.6
2.4
valerophenone (0.1 M)
2.2
2.0
1.8
0
 /
1.6
1.4
1.2
1.0
0.8
0.00
slope = k4T + k10 B = (34+-3) M
0.01
0.02
[PPh3], M
0.03
-1
0.04
Reciprocal of e vs reciprocal of [PPh3]
50
valerophenone (0.1 M)
45
1/e
40
35
30
-1
intercept/slope = k4T = (30+-5) M
25
20
30
40
50
60
70
80
-1
1/[PPh3], M
90
100
110
Table
Summary of kinetic data
Keton
k4T + k10B k4T
T
B
k410-9
k1010-8
[M-1]
[M-1]
[ns]
[ns]
PhCOCH2CH2CH3
8712
9930
70
110
1.4
<2
PhCOCH2CH2CH2CH3
343
305
16
102
1.9
<1
9.41.6
103
97
2.1
< 0.3
914
250
-
1.0
-
PhCOCH2CH2CH2(CH3)2
PhCOCH3
4.7
200
[M-1s-1] [M-1s-1]
Conclusions
• Application of Stern-Volmer relation for AP, CB,
and K leads to the same values of rate constants
• k4 = 2 109 M-1s -1
(for all aromatic ketones used)
• k10 << k4
(reaction of biradical with PPh3 can be neglected)
•
emax =  =
k5
k5 +k6
= 0.08
8 % - chemical quenching (reaction)
92 % - physical quenching
Norrish type II photoreaction
Ph
Ph
O
H
Ph
O*
H
h
O*
H
ISC
R R'
R R'
R R'
(K)
(1K)
(3K)
(1)
Ph

CH2=CRR' + PhCOCH3
Ph
OH
(2)
OH
R'

PPh3
R R'
(D)
PPh3
R
(3)
PhCOCH2CH2CHRR'