"Molecular Photochemistry - how to
study mechanisms of photochemical
reactions ?"
Bronislaw Marciniak
Faculty of Chemistry, Adam Mickiewicz University,
Poznan, Poland
2012/2013 - lecture 3
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.
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
2. Quantitative methods
- quantum yields,
- rate constants,
- lifetimes,
- kinetic of quenching,
- experimental problems, e.g. inner filter effects
Definition of terms used in photochemistry
Quantum yields 
h
For a photochemical reaction A  B
differential quantum yield:
d [x]
 x  dt
Ia
A 
-
d [A ]
dt
Ia
B 
d [ B]
dt
Ia
Kinetic scheme
rate
h
A(S0)  A(S1)
Ia (einstein dm-3 s-1)
A(S1)  A(S0) + hf
kf [A(S1)]
A(S1)  A(S0) + heat
kIC [A(S1)]
A(S1)  A(T1)
kISC [A(S1)]
A(S1)  B + C
kr [A(S1)]
A(S1) + Q  quenching
kq [A(S1)] [Q]
A(T1)  A(S0) + hp
kp [A(T1)]
A(T1)  A(S0) + heat
k'ISC [A(T1)]
A(T1)  B' + C'
k'r [A(T1)]
A(T1) + Q  quenching
k'q [A(T1)] [Q]
Steady-state approximation :
Ia = (kf + kIC + kISC + kr + kq[Q]) [ A(S1)] = [A(S1)]/S
Fluorescence quantum yield:
f = kf [ A(S1)] / Ia
 f = kf S
IC = kIC S
ISC = kISC S
For photochemical reaction from S1:
R = kr [ A(S1)] / Ia
 A =  B = kr S
Phosphorescence quantum yield:
p = kp[ A(T1)] / Ia
p = ISCkpT
For photochemical reaction from T1:
'R = k'r [ A(T1)] / Ia
'A = 'B = ISC k'r T
Quantum yield measurement
Chemical actinometry:
- Uranyl Oxalate Actinometry
H2C2O4
hv

UO2+2
H2O + CO2 + CO
R = 0.602 (for 254 nm)
R = 0.561 (for 313 nm)
- Benzophenone-Benzhydrol Actinometry
(C6H5)2CO + (C6H5)2CHOH  (C6H5)2C(OH) C(OH) (C6H5)2
R = 0.68 (for 0.1M BP and 0.1M benzhydrol in benzene)
- 2-Hexanone Actinometry (Norrish Type II)
acetone = 0.22 (for 313 nm)
Typical dependence of quantum yield vs Iat
A
a
B)
b
Ia t
Quantum yield of intermediates
Laser flash photolysis:
I = st Ap st /  Ast p
Ap and  Ast transient absorbances for intermediate and actinometer
p and st molar absorption coefficents of intermediate and actinometer
st quantum yield of actinometer (using benzophenone equal to ISC= 1)
A(ex) for irradiated solution = A(ex) for actinometer
Rate constants
kr = R /S from S1
k'r = 'R / (ISC T) from T1
S and T from direct measurement (laser flash
photolysis)
Kinetic of quenching
rate
h
A(S0)  A(S1)
Ia (einstein dm-3 s-1)
A(S1)  A(S0) + hf
kf [A(S1)]
A(S1)  A(S0) + heat
kIC [A(S1)]
A(S1)  A(T1)
kISC [A(S1)]
A(S1)  B + C
kr [A(S1)]
A(S1) + Q  quenching
kq [A(S1)] [Q]
A(T1)  A(S0) + hp
kp [A(T1)]
A(T1)  A(S0) + heat
k'ISC [A(T1)]
A(T1)  B' + C'
k'r [A(T1)]
A(T1) + Q  quenching
k'q [A(T1)] [Q]
Stern-Volmer equation

0
f
f
for S1
 1  kq S0 [Q]
0R
 1  kq S0 [Q]
R
S0
 1  kq S0 [Q]
S
1
1
 0  kq [Q]
S
S
kobs  k 0 + kq [Q]
S0 
S 
1
k f + k ISC + k IC + kr
1
k f + k ISC + k IC + kr + kq [Q]
Stern-Volmer equation

0
p
p
for T1
 1  k q'  0T [Q]
'0R
 1  kq' 0T [Q]
'R
0T
 1  kq' 0T [Q]
T
1
1
 0  kq' [Q]
T
T
k obs  k0 + k 'q [Q]
0T 
1
'
k p + k ISC
+ kr'
T 
1
'
k p + k ISC
+ kr' + kq' [Q]
Quenching of 3CB* by Met-Gly in aqueous solutions at pH = 6.8
kobs
1
 0  kq' [Q]
T
kq = (2.14  0.08)  109 M-1 s -1
Quenching Rate Constants (109 M-1 s-1) for quenching
of CB triplet state
Triplet Quenchers
pH neutral
pH basic
Thiaproline
2.1
2.6
Methionine
2.5
2.3
Alanine
0.0005
0.18
S-(Carboxymethyl)cysteine
0.81
0.75
Met-Gly
2.1
2.3
L-Met-L-Met
2.9
1.8
Gly-Gly-Met
1.8
1.9
Met-Enkephalin
1.9
1.8
Rate constants of the order of 109 M-1 s-1
indicative of electron transfer
Methionine
+
H3N
C COO
H2N
( C H2 )2
C COO
( C H2) 2
S
S
CH3
CH3
pK a = 9.06
Traditional Scheme
3
CB*
+
[ CB-
>S
>S ]
  
kesc
kbt
CB +
kCH
CB- +
>S
CBH
+
CH2-S-CH2-

or CH3-S-C H-
>S
Definition of terms used in photochemistry
2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry 79, 293–465
Inner-filter effects
Term used in two different ways:
(1) During an irradiation experiment, absorption of incident
radiation by a species other than the intended primary
absorber is also described as an inner-filter effect.
Definition of terms used in photochemistry
2007 IUPAC, S. E. Braslavsky, Pure and Applied Chemistry 79, 293–465
Inner-filter effects
(2) In an emission experiment, it refers to
(a) an apparent decrease in emission quantum yield at
high concentration of the emitter due to strong
absorption of the excitation light
(b) an apparent decrease in emission quantum yield
and/or distortion of bandshape as a result of
reabsorption of emitted radiation (particularly severe
for emitters with small Stokes shift).
Ia [einstein dm-3 s-1]
h
A 

IaA  I0 1 -10-ε AcA l

h
A + Q 
ε AcA
-(ε c  ε c ) l
IaA(Q) 
I0 1 - 10 A A Q Q
ε A c A  ε QcQ


I aA(Q)
ε AcA

Q(A)
Ia
ε QcQ
- (ε c  ε c ) l
I aA(Q)
I 0 (1 - 10 A A Q Q )
ε AcA

A
Ia
ε A c A  ε QcQ
I 0 (1 - 10-ε AcA l )
Corrections for inner filter effect (1)
(for the absoprtion of incident light by Q)
ε A c A  ε QcQ
A, corr
A(Q), obs
Ia
 Ia

ε AcA
I
corr
f
I
obs
f
 1 - 10- ε AcA l 


 1 - 10-(ε AcA ε QcQ ) l 


ε A c A  ε Q cQ  1 - 10- ε Ac A l


 1 - 10-(ε AcA ε QcQ ) l
ε AcA





Corrections for inner filter effect (2)
(for reabsorption of fluorescence of A by Q)
Icorr
f
obs
Iobs
I
 f  -fε Q[Q] l'
TQ
10
Changes of fluorescence spectra of benzene with various Cu(acac)2 concentrations
Changes of fluorescence spectra of benzene with various Cu(acac)2 concentrations
without correction
with correction
Stern-Volmer plot for the quenching of benzene fluorescence by Cu(acac)2
Experimental setups for measuring fluorescence spectra
0
I f/If
3
2
-1
slope = kqS = (1060 +- 20) M
taking S = 29.5 ns
1
10
-1 -1
kq = 3.6 x 10 M s
0
0.0
0.5
1.0
1.5
2.0
2.5
[Cu(acac)2], mM
Stern Volmer plot for quenching of benzene fluorescence by Cu(acac)2
- front-face technique (ex=250 nm, f=278 nm)