Effects of a sulfur substituent on the behavior and generation mode of aroyloxyl radicals in the photolysis of diaroyl peroxides

Article abstract of DOI:10.1016/S0009-2614(99)01147-1

Continuous light irradiation of bis(p-methylthiobenzoyl) peroxide (p-MeSBPO) at 300 nm in acetonitrile gave only p-methylthiobenzoic acid as a product. On pulsed laser photolysis the absorption spectrum due to intermediate p-methylthiobenzoyloxyl radicals was observed in the region 400-800 nm, and the radical yield was significantly decreased in the presence of 1,3-pentadiene or oxygen as a triplet quencher. The results indicate that p-MeSBPO is decomposed from the excited triplet state and the radicals exclusively undergo hydrogen-atom abstraction from the solvent. The origin of the effect is discussed on the basis of experimental and theoretical results.

Full text of DOI:10.1016/S0009-2614(99)01147-1

3 December 1999
Ž
.
Chemical Physics Letters 314 1999 261–266
www.elsevier.nlrlocatercplett
Effects of a sulfur substituent on the behavior and generation
mode of aroyloxyl radicals in the photolysis of diaroyl peroxides
)
Ji-ichiro Hashimoto, Katsunori Segawa, Hirochika Sakuragi
Department of Chemistry, UniÕersity of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
Received 2 August 1999; in final form 23 September 1999
Abstract
Ž
.
Ž
.
Continuous light irradiation of bis p-methylthiobenzoyl peroxide p-MeSBPO at 300 nm in acetonitrile gave only
p-methylthiobenzoic acid as a product. On pulsed laser photolysis the absorption spectrum due to intermediate p-methyl-
thiobenzoyloxyl radicals was observed in the region 400–800 nm, and the radical yield was significantly decreased in the
presence of 1,3-pentadiene or oxygen as a triplet quencher. The results indicate that p-MeSBPO is decomposed from the
excited triplet state and the radicals exclusively undergo hydrogen-atom abstraction from the solvent. The origin of the effect
is discussed on the basis of experimental and theoretical results. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction
less reactive compared to other benzoyloxyl radicals
Ž
.
studied so far, and bis p-methylthiobenzoyl perox-
ide was found to decompose from the excited triplet
state not only on acetophenone sensitization but also
on direct irradiation. In connection with these find-
ings, the effect of the methylthio group, in particular,
on the intersystem crossing of aromatic compounds
has been studied in the present work.
Photolysis of diaroyl peroxides, such as dibenzoyl
peroxides, yields the corresponding aroyloxyl radi-
cals, which may decarboxylate to give aryl radicals,
abstract a hydrogen atom from substrates, or add to
olefins and aromatic rings. Laser flash photolysis
w
x
w
x
1–10 and time-resolved ESR 11,12 have been
applied as direct methods to elucidate the reactivity
of aroyloxyl radicals. In the course of our investiga-
w
x
tion 13–15 into structure–reactivity relationships of
substituted benzoyloxyl radicals, we found that a
substituent methylthio group affected the behavior of
the benzoyloxyl radicals and the mode of photolysis
of the peroxide precursor. Thus, p-methylthio-
benzoyloxyl radicals were found to be stabilized and
2. Experimental and molecular orbital calcula-
tions
Proton and carbon-13 NMR spectra were recorded
Ž
on a Varian Gemini-200 200 and 50 MHz, respec-
.
Ž
tively or a JEOL FX-270 270 and 67.5 MHz,
respectively spectrometer. UV absorption and fluo-
.
rescence spectra were measured on a JASCO Ubest
55 or a Shimadzu 1600 spectrophotometer and a
Hitachi F-4000 spectrofluorimeter, respectively.
)
Corresponding author. Fax: q81-298-53-6503; e-mail:
sakuragi@staff.chem.tsukuba.ac.jp
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 99 01147-1

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)
262
J. Hashimoto et al.rChemical Physics Letters 314 1999 261–266
Ž
.
w
x
Acetonitrile Dojin, spectrograde was used as
photolysis have been given elsewhere 19 . The tem-
perature of the sample cells was controlled with a
thermostated cell holder and a circulating water bath
received as a solvent; 1- and 2-methylthionaphtha-
lenes were prepared by methylation of the corre-
sponding thiols with dimethyl sulfate.
bis p-Methylthiobenzoyl peroxide p-MeSBPO
was prepared at y708C from the corresponding ben-
zoyl chloride and sodium peroxide by a standard
Ž
.
Haake F3-K .
U
Ž
.
Ž
.
Ž
.
Molecular orbital calculations UHFr6-31G of
the potential energy curves for decarboxylation of
some benzoyloxyl radicals were carried out using the
w
x
Ž
.
w
x
Ž
.
procedure 16 ; mp 103–1058C; UV CH3CN : lmax
Gaussian 98 program 20 . For calculations PM3-CI
of excitation energies of peroxides, the WinMOPAC
1
Ž
.
.
Ž
.
Ž
304 nm ´ 42 000 ; H NMR CDCl3 : d 2.54 s,
Ž
.
w
x
6H, CH3 , 7.31 d, Js8.6 Hz, 4H, phenyl H , 7.96
V2.0 program 21 was used. The PM3-CI calcula-
tions include complete active space configurations
constructed from the two highest occupied and the
two lowest unoccupied MO’s of the ground configu-
rations.
13
Ž
.
Ž
.
d, Js8.6 Hz, 4H, phenyl H ; C NMR CDCl3 :
14.7, 121.3, 125.1, 130.0, 147.7, 163.0; found: C,
56.95; H, 4.45; S, 19.21%; calculated for
C16 H14O4S2: C, 57.47; H, 4.22; S, 19.18%.
Ž
.
Ž
.
1- p-Methylthiobenzoyloxy -2 1 H -pyridone
Ž
.
p-MeSBPy was prepared by the reaction of p-
methylthiobenzoyl chloride with 1-hydroxy-2-pyri-
3. Results and discussion
done in methylene chloride; mp 103–1058C, UV
CH3CN : lmax 304 nm ´ 21 000 ; 1H NMR
Ž
.
Ž
.
3.1. BehaÕior of p-methylthiobenzoyloxyl radicals
Ž
.
Ž
.
Ž
CDCl3 : d 2.64 s, 3H, CH3 , 6.33 td, Js6.6, 1.7
.
Ž
Ž
.
Ž
Hz, 1H, pyridyl H , 6.87 dd, Js8.9, 1.3 Hz, 1H,
bis p-Methylthiobenzoyl peroxide p-MeSBPO,
.
Ž
.
pyridyl H , 7.39–7.56 m, 4H, pyridyl and phenyl
Scheme 1 was photolyzed by continuous light irra-
H , 8.16 d, Js8.3 Hz, 2H, phenyl H ; 13C NMR
y3
.
Ž
.
Ž
diation at 300 nm at a low concentration 1.0=10
mol dmy3 in acetonitrile under argon at room tem-
Ž
.
.
CDCl3 : 14.6, 105.1, 121.3, 123.0, 125.1, 130.7,
Ž
135.6, 139.3, 148.5, 157.3, 162.7; found: C, 59.83;
H, 4.31; N, 5.28; S, 12.27%; calcd. for C13 H11NO3S:
C, 59.76; H, 4.24; N, 5.36; S, 12.27%.
perature to give only p-methylthiobenzoic acid 1.74
molrmol peroxide as a product; formation of nei-
.
ther thioanisole nor p-methylthiophenyl p-methyl-
A solution of p-MeSBPO in acetonitrile was
thiobenzoate, a geminate product, was observed. In
Ž
Ž
irradiated with 300-nm light Rayonet RPR-3000
contrast, a similar photolysis of bis p-methoxy-
y3
benzoyl peroxide 1.3=10 mol dmy3 gave rise
.
.
Ž
.
lamps , and the products were analyzed qualitatively
Ž
.
and quantitatively by HPLC and GLC. The quantum
yield was determined by irradiating p-MeSBPO in
acetonitrile with 302-nm light from a JASCO SM-5
super-monochro-irradiator equipped with a 300-W
xenon lamp. Consumption of the peroxide was moni-
tored by HPLC, and the number of light quanta was
to the corresponding acid 0.70 molrmol peroxide ,
anisole 0.39 , and p-methoxyphenyl p-methoxy-
Ž
.
Ž
.
benzoate 0.06 . The quantum yield for photolysis of
p-MeSBPO was determined as 0.26 for continuous
light irradiation at 302 nm in acetonitrile. This value
is in the same range as those for the polycyclic
aromatic hydrocarbon-sensitized singlet decomposi-
Ž
.
Ž
.
determined by potassium tris oxalato ferrate III
w
x
w
x
actinometry 17 .
tion of dibenzoyl peroxide 22 .
Fluorescence quantum yields were determined in
Ž
acetonitrile using naphthalene as a standard ff s
. w
x
0.21 in ethanol 18 . Fluorescence lifetimes were
measured by the single photon counting technique
Ž
.
Horiba NAES 110 time-resolved fluorophotometer .
Laser-flash photolyses were performed using an
Ž
excimer laser Lambda Physik LPX-100, XeCl, 308
.
.
nm, 10-ns fwhm, 70 mJrpulse and a pulsed xenon
arc Wacom KXL-151, 150 W as a monitoring light
Ž
source. The details of the apparatus for laser-flash
Scheme 1.

(
)
J. Hashimoto et al.rChemical Physics Letters 314 1999 261–266
263
y4
Ž
Pulsed laser excitation of p-MeSBPO 1.2=10
mol dmy3 at 308 nm under argon in acetonitrile at
.
room temperature exhibited transient absorption
bands at 400–600 and 600–800 nm, as shown in
Fig. 1. Both bands exhibited identical decay time
constants of 6.5 ms, as monitored at 530 and 750 nm,
and were reasonably assigned to p-MeSC6 H4CO2 P
radicals by comparison with the spectra of other
w
x
benzoyloxyl radicals 1–10 . Another band at 300–
400 nm may also be attributed to those radicals. An
identical transient absorption spectrum was observed
Ž
.
on laser photolysis of 1- p-methylthiobenzoyloxy -
Fig. 2. Transient absorption spectra observed on 308 nm laser
p-MeSBPy, 4.6 = 10y4 mol
Ž
2 1H -pyridone
Ž
.
.
y4
excitation of p-MeSBPy 4.6=10
mol dmy3 in acetonitrile at
.
Ž
dmy3 in acetonitrile Fig. 2, Scheme 1 . The ab-
sorption bands around 400–800 nm showed an iden-
tical lifetime of 6.5 ms. In addition, a band due to
Ž
.
ambient temperature under argon.
vents. UHFr6-31GU calculations were performed to
estimate the difference in bond dissociation energies
between p-MeSC6 H4CO2 P and C6 H5CO2 P, both
being assumed to have Cs symmetry. The calcula-
tions suggested that both radicals had potential en-
ergy curves exhibiting the same shape and that the
w
2-pyridyloxyl radicals appeared around 390 nm 23–
x
25 . The activation energy for the disappearance of
p-MeSC6 H4CO2 P was determined to be 13 kJ moly1
y1
Ž
Ž
.
.
log Ars
s7.5 by monitoring the decay rate of
transient absorption from p-MeSBPO at 760 nm in
Ž
the temperature range 5 to 488C under argon in
bond dissociation energy of p-MeSC6 H4CO2 P
y1
.
acetonitrile. This value is close to the activation
p-MeSC6 H4 PqCO2 was 12 kJ mol larger than
y1
Ž
.
energy 17 kJ mol
for intramolecular hydrogen-
that of C6 H5CO2 P. Another method, UB3LYPr6-
31GU , was used to elucidate the potential energy
curves; preliminary results showed that the trends
were nearly identical, although the difference in the
bond dissociation energy was rather small.
atom abstraction in o-methylbenzoyloxyl radicals
w
x
2,14 .
The product distribution together with the above
results indicates that p-MeSC6 H4CO2 P exclusively
undergoes hydrogen-atom abstraction from the sol-
vent acetonitrile. The reluctance to decarboxylation
may be due to contributions from canonical struc-
tures of intramolecular charge-transfer arising from
the electron-donating methylthio group in polar sol-
The stability of p-MeSC6 H4CO2 P is also re-
flected by its reactivity with various substrates and
so, bimolecular rate constants, k, for the reaction
with the substrates were calculated from the pseudo-
first-order decays of the transient absorptions: kexptl
w
x
sk0 qk substrate , where k0 refers to first-order
reactions by which the aroyloxyl radical decays at
zero substrate concentration. In Table 1 the bimolec-
ular rate constants for three kinds of benzoyloxyl
radicals are compared. The p-MeSC6 H4CO2 P radi-
cal is not only less reactive than C6 H5CO2 P but also
p-MeOC6 H4CO2 P with regard to hydrogen-atom
Ž
.
abstraction e.g., toluene, cyclohexane as well as
Ž
.
addition to olefins cyclohexadiene, pentadiene and
aromatic rings benzene .
Ž
.
(
3.2. Mode of photochemical decomposition of bis p-
)
methylthiobenzoyl peroxide
Fig. 1. Transient absorption spectra observed following 308 nm
y4
laser excitation of p-MeSBPO 1.2=10
mol dmy3 in aceto-
.
The methylthio group also affects the mode of
Ž
photochemical decomposition of the peroxide; p-
nitrile at ambient temperature under argon.

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264
J. Hashimoto et al.rChemical Physics Letters 314 1999 261–266
Table 1
Rate constants for reactions of p-methylthiobenzoyloxyl radicals with various substrates in acetonitrile at room temperaturea
Substrate
kr107 dm3 moly1 sy1
p-MeSC6 H4CO2 P
0.021
p-MeOC6 H4CO2 P
C6 H5CO2 P
Toluene
0.22
0.18b
0.064
0.050b
120
4.7
Benzene
-0.001
14
1.4
130
5.2
1,3-Cyclohexadiene
Cyclohexene
Cyclohexane
1,3-Pentadiene
88b
0.032
0.014
0.020
2.9
2.0b
0.039
0.019b
3.5
3.7
46
a Laser flash photolyses were performed with p-MeSBPO, p-MeOBPO and BPO in concentrations of 3.5=10y4 , 5.7=10y4 and
7.1=10y2 mol dmy3, respectively, under argon in acetonitrile. Substrate concentrations were varied between two and ten times, in the
range of 10y3 –10y1 mol dmy3, depending on the combinations of peroxide and substrate. The decay of benzoyloxyl radicals was
monitored at 760 nm in all cases.
b
w x
Ref. 7 .
w
x
MeSBPO is decomposed from the excited triplet
state, as revealed by the effects of triplet quenchers
and sensitizers. Furthermore, the detection of no
geminate product is characteristic of the triplet
mechanism for decomposition of dibenzoyl perox-
ides. Thus, the radical yield from laser photolysis of
p-MeSBPO was significantly decreased by the addi-
tion of 1,3-pentadiene as a triplet quencher. The
excited singlet state 27 . In the singlet cases, 1,3-
pentadiene did not reduce the radical yields, although
the disappearance of the radicals was accelerated by
Ž
the diene. Actually, in photolyses of bis o-meth-
o-MeOBPO and bis p-methoxy-
benzoyl peroxides p-MeOBPO as well as bis p-
.
Ž
. Ž
oxybenzoyl
.
Ž
.
Ž
.
Ž
.
iodobenzoyl and bis o-bromobenzoyl peroxides in-
cluding heavy atoms, the addition of 1,3-pentadiene
Stern–Volmer plot for radical yield exhibits a straight
did not reduce the radical yields at all. The spin–orbit
3
y1
line with a slope KSV of 26 dm moly1. Molecular
coupling constant of a sulfur atom z 365 cm
is
and
Ž
.
Ž
.
y1
Ž
.
oxygen also efficiently quenched the radical forma-
much smaller than those of iodine 5069 cm
tion due to peroxide decomposition KSV s51 dm3
bromine atoms 2460 cm , and is even smaller
y1
Ž
Ž
.
moly1 , though the radical lifetime was not affected
.
Ž
y1 . w
than that of a chlorine atom 587 cm 28 . Conse-
x
at all. Assuming nearly diffusion-controlled rate con-
w
x
stants for these quenching processes 26 leads to a
triplet lifetime of several nanoseconds. This is in
agreement with the spectral rise which has almost
the same time constant as the instrumental response
Ž
.
of the laser system 20 ns . Laser photolysis of
p-MeSBPO in the presence of acetophenone as a
triplet sensitizer exhibited the same transient absorp-
tion spectrum as did the direct photolysis. Similarly,
m-MeSBPO was decomposed from the triplet state
as indicated by the effects of triplet quenchers in
laser flash photolyses.
Involvement of the triplet state in the direct pho-
tolysis is in marked contrast to the cases of other
diaroyl peroxides, which are decomposed from the
Fig. 3. Absorption spectra of o-MeOBPO, p-MeOBPO and p-
MeSBPO in acetonitrile.

(
)
J. Hashimoto et al.rChemical Physics Letters 314 1999 261–266
265
Table 2
Ž
.
Ž .
MO calculations PM3-CI indicate that i ex-
cited singlet states, S2 and S1, of p-MeSBPO are
f60 kJ moly1 lower than those of p-MeOBPO,
Ž
.
Ž
.
Fluorescence lifetimes tf and quantum yields ff of methoxy-
and methylthio-substituted benzenes and naphthalenes in aceto-
nitrile under argon
Ž .
respectively, ii the excited triplet states, T2 and T1,
Compound
Anisole
tf
7.5a
ff
of p-MeSBPO are 15–20 kJ moly1 lower than those
0.23
Ž .
of p-MeOBPO, respectively, and iii T1 is located
0.24a
0.013
0.63
approximately 40 kJ moly1 below S1 in p-MeSBPO
but approximately 80 kJ moly1 below S1 in p-
MeOBPO. These results suggest that the sulfur atoms
introduced into the aromatic rings accelerate the
intersystem crossing of the chromophores by lower-
ing the excitation energy levels to result in smaller
S–T splitting. The triplet state of p-MeSBPO pro-
vides sufficient energy to cleave the weak peroxide
bond, though the two-bond fission does not occur.
Thioanisole
1-Methoxynaphthalene
0.11
13.4
0.53b
0.053
0.38
1-Methylthionaphthalene
2-Methoxynaphthalene
0.12
12.5
12.8c
2.3
0.56c
0.039
2-Methylthionaphthalene
a
b
c
w
x
w
x
w x
Ref. 29 . Ref. 18 . Ref. 30 .
quently, the heavy atom effect is not the cause of the
triplet mechanism for p-MeSBPO decomposition.
As shown in Fig. 3, p-MeSBPO exhibited a
strong UV absorption band around 300 nm in aceto-
nitrile, while p-MeOBPO showed a similar absorp-
tion band around 270 nm and had a tiny absorbance
due to the peroxide bond at 308 nm in the same
solvent. The 308-nm light corresponds to the absorp-
tion band of p-MeSBPO due to p,pU excitation;
however, in p-MeOBPO this light directly excites
the n,sU band of the peroxide bond. The difference
in the excitation mode might affect the multiplicity
in which the peroxides are decomposed. However,
o-MeOBPO, which was decomposed from the ex-
cited singlet state, exhibited a p,pU absorption band
4. Conclusion
Introduction of a methylthio group into the phenyl
ring of dibenzoyl peroxide switches the multiplicity
responsible for photochemical decomposition of di-
aroyl peroxides from the excited singlet state to the
triplet state. The effect of the methylthio group in
accelerating intersystem crossing is also observed in
aromatic compounds such as benzenes and naph-
thalenes. This type of sulfur effect is attributed to
reduction of the S-T gap due to lowering of the
singlet and triplet energy levels, but is not attributed
to the heavy atom effect. The methylthio group
introduced reduces the reactivity of benzoyloxyl rad-
icals by an electronic effect.
Ž
.
around 300 nm lmax 298 nm, ´ 11 400 . MeOBPO’s
were decomposed in the excited singlet state, irre-
spective of the excitation modes.
Fluorescence spectra of thioanisole and 1- and
2-methylthionaphthalenes appeared at wavelengths
30–50 nm longer than those of anisole and 1- and
2-methoxynaphthalenes, respectively. Fluorescence
quantum yields and singlet lifetimes of thioanisole
and the methylthionaphthalenes were found to be
much lower and shorter, respectively, than those of
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