"Molecular Photochemistry - how to
study mechanisms of photochemical
reactions ?"
Bronislaw Marciniak
Faculty of Chemistry, Adam Mickiewicz University,
Poznan, Poland
2012/2013 - lecture 6
5. Examples illustrating the investigation
of photoreaction mechanisms:
- sensitized photooxidation of sulfur (II)-containing organic compounds
example III
Our Traditional Scheme
3
CB*
+
[ CB-
>S
>S ]
  
kesc
kbt
CB +
kCH
CB- +
>S
CBH
+
CH2-S-CH2-

or CH3-S-C H-
>S
Sensitized Photooxidation
of (Phenylthio)acetic Acid
System studied
O
C
+
Solvent: CH3CN
O
S CH22 C OH
0.04
 = 540 nm
0.02
A
0.05
50 - 100 ns
1 - 1.5 s
10 - 12 s
140 - 160 s
0.01
0.00
0
50
100
150
Time [s]
A
0.03
0.02
0.01
0.00
400
500
600
700
Wavelength  [nm]
Fig. Transient absorption spectra following laser flash photolysis recorded at four different
delays time benzophenone ([BP] = 2  10–3 M) and (phenylthio)acetic acid
([C6H5-S-CH2-COOH] = 2  10-2 M) in Ar-saturated acetonitrile.
Inset: kinetic trace at  = 540 nm
7000
BP
3
6000
4000
-1
-1
 [dm mol cm ]
5000
C6H5-S
3
3000
BPH
2000
1000
0
350
400
450
500
550
600
650
700
Wavelength  [nm]
Fig. Reference spectra of intermediates (BPH, 3BP*): (i) ketyl radical BPH in acetonitrile,
(ii) triplet state of benzophenone 3BP* in acetonitrile, and (iii) phenylthiyl radical C6H5-S
in water (from pulse radiolysis)
Table 1a. Quenching rate constants of benzophenone triplet state
by (phenylthio)acetic acid (kq) and quantum yields for formation
of intermediates, disappearance of benzophenone (BP),
and formation of CO2 (CO2)
~
b
b–
results for tetrabutylammonium salt
Our Traditional Scheme
3
CB*
+
[ CB-
>S
>S ]
  
kesc
kbt
CB +
kCH
CB- +
>S
CBH 
+
CH2-S-CH2-

or CH3-S-C H-
>S
COOH
3
CH2
*
BP
+
S
Ph
COOH
...
BP
CH2
S
Ph
kbt
COOH
ksep
 = 0.33
kH1
 =0
kH2
 = 0.28
 = 0.39
CH2
BP
not observed
S
COOH
Ph
CH
BPH
S
CH2
BPH
S
Ph
CO2
 = 0.28
Ph
Ph-SH
Ph-S
Ph-S-S-Ph
 = 0.30
Ph-S-CH3
Ph-S-CH2-CH2-S-Ph
COOH
3
CH2
*
BP
+
S
Ph
COOH
...
BP
CH2
S
Ph
kbt
COOH
ksep
 = 0.33
kH1
 =0
kH2
 = 0.28
 = 0.39
CH2
BP
not observed
S
COOH
Ph
CH
BPH
S
CH2
BPH
S
Ph
CO2
 = 0.28
Ph
Ph-SH
Ph-S
Ph-S-S-Ph
 = 0.30
Ph-S-CH3
Ph-S-CH2-CH2-S-Ph
COOH
3
CH2
*
BP
+
S
Ph
COOH
...
BP
CH2
S
Ph
kbt
COOH
ksep
 = 0.33
kH1
 =0
kH2
 = 0.28
 = 0.39
CH2
BP
not observed
S
COOH
Ph
CH
BPH
S
CH2
BPH
S
Ph
CO2
 = 0.28
Ph
Ph-SH
Ph-S
Ph-S-S-Ph
 = 0.30
Ph-S-CH3
Ph-S-CH2-CH2-S-Ph
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
Table 1b. Quenching rate constants of benzophenone triplet state
by (phenylthio)acetic acid (kq) and quantum yields for formation
of intermediates, disappearance of benzophenone (BP),
and formation of CO2 (CO2)
~
b
b–
results for tetrabutylammonium salt
Our Traditional Scheme
3
CB*
+
[ CB-
>S
>S ]
  
kesc
kbt
CB +
kCH
CB- +
>S
CBH 
+
CH2-S-CH2-

or CH3-S-C H-
>S
N
N
O
C O

C
O
N
O
C
CH2
O
O
(b)
CH2
S
C
O
C
OH
S

CH2
OH

N


N
(2a)
C
PTA AS
and/or
AS
C
J. Am. Chem. Soc., 125, 11182 (2003)

O
(1)
(E2 Hof mann elimination)
C
S
O
(2b)
OH

N
CO2
O
 CH
S
(a)
C
C
System studied
OOC
O
C
+
Solvent: H2O
O
S CH22 C O
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
Spectral Resolutions
Composite fit
Radical anion
Ketyl radical
PhSCH2 radical
Data
0.08
0.07
0.06
-
(CB ) = 0.97
(CBH) < 0.10
(PhSCH2) ~ 0.93
0.05
A
0.04
0.03
0.02
0.01
0.00
400
500
600
Wavelength (nm)
700
800
Table 1c. Quenching rate constants of benzophenone triplet state
by (phenylthio)acetic acid (kq) and quantum yields for formation
of intermediates, disappearance of benzophenone (BP),
and formation of CO2 (CO2)
~
b
b–
results for tetrabutylammonium salt
Our Traditional Scheme
3
CB*
+
[ CB-
>S
>S ]
  
kesc
kbt
CB +
kCH
CB- +
>S
CBH 
+
CH2-S-CH2-

or CH3-S-C H-
>S
O
Scheme
3CB *
CH2
S
C6H5
+
O

CB - . . .
O
-
O
CH2
CB + S
C6H5

-
O
CH2
S
C6H5

kH
kbet
-
O
ksep
O-
O

CH

+ CBH
S
C6H5   0.10
O

CB - +

S
 = 0.97
OCH2
C6H5
 ~0

Products
( C6H5SCH3,
C6H5SCH2CH2SC6H5 )
CH2
+ CO2
S
C6H5
 = 0.92
 ~ 0.93
Conclusions:
Photochemical pathways (primary and
secondary reactions) for the sensitized oxidation
of phenylthioacetic acid depend on
its ionization form (solvent used)
and the presence of associated counter cations
(tetraalkylammonium salt)
Application of Photooxidation
of Sulfur-Containing Organic
Compounds in Free Radical
Polymerization
Reaction scheme
3
CB*
+
[ CB-
>S
>S ]
  
kesc
kbt
CB +
kCH
CB- +
>S
CBH

+

CH2-S-CH2-
>S
- CO2
or CH3-S-CH-
R"•
R' •
Systems studied
BP + C6H5-S-CH2-COO–N+R4
(R = n-butyl, n-propyl, etyl, metyl)
BP + C6H5-S-CH2-COOH
monomer:
O
O C CH CH2
CH2
O
H2C HC C O CH2 C C2H5
CH2
O C CH CH2
O
2-Ethyl-2-(hydroxymethyl)-1,3-propanediol triacrylate
(TMPTA)
Solvent: CH3CN
Reaction scheme
R
R
R
N
+ monomer
N
R
R
O

C O
Polymerization
R
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
Flow of heat [a.u.]
1.0
BP
BP + C6H5-S-CH2-COOH, [0,1M]
0.5
BP + C6H5-S-CH2-COO–N+(C4H9)4
BP + C6H5-S-CH2-COO–N+(C3H7)4
BP + C6H5-S-CH2-COO–N+(C2H5)4
BP + C6H5-S-CH2-COO–N+(CH3)4
BP + C6H5-S-CH2-COOH
0.0
20
40
Time [s]
Photopolymerization kinetic traces
60
80
Polymerization rates (Rp), quantum yield of polymerization (p) and
quantum yield of CO2 (CO2)
Rp [mol/s]
 CO 2
p
BP
23,4
–
400
BP +
S CH2 COOH
52,9a
28.4b
0,53
910a
480b
BP +
S CH2 COO N(CH3)4
50,1
0,42
850
BP +
S CH2 COO N(C2H5)4
76,0
0,65
1300
BP +
S CH2 COO N(C3H7)4
74,5
0,62
1270
BP +
S CH2 COO N(C4H9)4
73,8
0,67
1260
Układy fotoinicjujące
a
concentration of acid 0.1 mol/dm3
b concntration of acid 0.01 mol/dm3
Rate of polymerization [ mol/s]
80
60
40
Rp  k p [M ]
20
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
 R I A
kt
0.7
CO2
Plot of polymerization rate (Rp) vs. square root of the CO2 quantum
yield
Conclusions
• BP + C6H5-S-CH2-COO–N+R4 (R = n-butyl, n-propyl,
and ethyl) were shown to be effective co-initiators of
free-radical photopolymerizations.
• A linear correlation was found for the polymerization
rates vs. the square root of the CO2 quantum yields,
and this indicates that the C6H5SCH2 radicals are
responsible for the initiation step of the
polymerizations.
• Application of the laser flash photolysis and steadystate photochemical methods allowed led to
description of the mechanism of free radical
polymerization.