Primary photochemical process of thiobenzophenone as studied by laser flash photolysis

Article abstract of DOI:10.1246/cl.2009.1086

Self-quenching of the triplet excited state of thiobenzophenone (TBP) was suppressed in a viscous alcoholic solution, an SDS micellar solution and an ionic liquid of TMPA TFSA. In those solutions, the photochemical primary process of TBP could be studied

Full text of DOI:10.1246/cl.2009.1086

1086
Chemistry Letters Vol.38, No.11 (2009)
Primary Photochemical Process of Thiobenzophenone as Studied by Laser Flash Photolysis
Miyuki Tanaka, Tomoaki Yago, and Masanobu Wakasaꢀ
Department of Chemistry, Graduate School of Science and Engineering, Saitama University,
255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570
(Received September 4, 2009; CL-090811; E-mail: mwakasa@chem.saitama-u.ac.jp)
Self-quenching of the triplet excited state of thiobenzophe-
none (TBP) was suppressed in a viscous alcoholic solution, an
SDS micellar solution and an ionic liquid of TMPA TFSA. In
those solutions, the photochemical primary process of TBP could
be studied by ns laser flash photolysis. Thiobenzophenone ketyl
and phenylsulfanyl radicals were found to be generated from
hydrogen abstraction of benzenethiol by TBP. The rate constant
for the reaction was observed to be 1:1 ꢁ 107 sꢂ1 molꢂ1 dm3.
was observed to be as short as 77 ns (k ¼ 4:2 ꢁ 109
s
ꢂ1 molꢂ1 dm3). Such a short lifetime is due to the diffusion-con-
trolled self-quenching reaction of 3TBPꢀ. Thus, we conclude that
3
the self-quenching of TBPꢀ is suppressed in the viscous alco-
holic solution, SDS micellar solution and the ionic liquid, and
the photochemical primary process of TBP can be studied in
those solutions.
3
To clarify the reactivity of TBPꢀ, reaction with benzene-
thiol (PhSH, 0{100 ꢁ 10ꢂ3 mol dmꢂ3) was studied by laser flash
photolysis. Upon irradiation of a 1/25 mixture of i-BuOH/
c-HexOH containing TBP and PhSH (5 ꢁ 10ꢂ3 mol dmꢂ3), a
Benzophenone (BP) is the most commonly used molecule in
photochemical studies1–3 and its primary photophysical and pho-
tochemical processes have been well investigated by direct
measurement of intermediates with the aid of ns laser flash pho-
tolysis2,3 and time-resolved ESR.4 However, the primary process
of thiobenzophenone (TBP, Ph2C=S), which is a sulfur ana-
logue of BP, has scarcely been studied.5,6 de Mayo reported on
the laser flash photolysis of TBP in cyclohexane and observed
a short-lived transient absorption at 400–600 nm (ꢀmax ¼ 475
nm).5 The absorption was tentatively assigned to the triplet–
triplet (T–T) absorption of TBP. Das carried out more quantita-
tive studies of the triplet excited state of TBP (ꢀmax ¼ 400 and
515 nm in benzene).6 Since the triplet excited state of TBP dis-
appears by fast self-quenching due to the large spin–orbital cou-
pling of sulfur, to the best of our knowledge, there has been no
detailed study of its reactivity. Moreover, TBP contains a heavy
sulfur atom. Thus, its photochemical primary process is attrac-
tive to compare with that of BP.
3
strong T–T absorption band of TBPꢀ was observed at 515 nm.
The decay of the T–T absorption accelerated with increasing
concentration of PhSH. The PhSH concentration dependence
of the 3TBPꢀ decay rate constant (k) observed at 515 nm is
shown in Figure 1. A good linear relationship was observed be-
tween k and PhSH concentration. From the slope of the plots, the
rate constant for the reaction between 3TBPꢀ and PhSH was de-
termined to be 1:1 ꢁ 107 sꢂ1 molꢂ1 dm3. This value is ten times
smaller than that observed for the similar reaction between 3BPꢀ
and PhSH (1:2 ꢁ 108 sꢂ1 molꢂ1 dm3) although the viscosity is
slightly higher (37.3 cP) than ꢀthat of the present study.10 Such
3
a small rate constant of TBP may be due to the lower triplet
3
energy of TBPꢀ (166 kJ molꢂ1) than that of benzophenone
(290 kJ molꢂ1).
The transient absorption spectra observed for the reaction
between TBP and PhSH (1 ꢁ 10ꢂ1 mol dmꢂ3) at delay times of
0.1, 0.5, 1.0, and 1.5 ms after laser irradiation are shown in
Figure 2. The transient absorption spectrum observed at a delay
time of 0.1 ms after laser irradiation has three absorption peaks at
380, 490, and 515 nm. The AðtÞ curves observed at 490 and
515 nm showed single exponential decay, while the AðtÞ curve
observed at 380 nm had a fast decay component and an almost
In this letter, we study the laser flash photolysis of TBP in a
viscous alcoholic solution [1/25 (v/v) mixture of 2-methyl-1-
propanol (i-BuOH) and cyclohexanol (c-HexOH), 49.7 cP], a
sodium dodecyl sulfate (SDS, 1:60 ꢁ 10ꢂ1 mol dmꢂ3) micellar
solution, and an ionic liquid of N,N,N-trimethyl-N-propylammo-
nium bis(trifluoromethanesulfonyl)amide (TMPA TFSA,
72.6 cP). The ns laser flash photolysis apparatus was essentially
the same as an apparatus described elsewhere.7 The third har-
monic (355 nm) of a Nd:YAG laser was used as an excitation
light source. TBP was synthesized as described in the litera-
ture.8,9 The concentration of TBP in the employed solution
3.0
2.5
2.0
1.5
1.0
was 2:0 ꢁ 10ꢂ3 mol dmꢂ3
.
Laser flash photolysis was performed at 296 K. Time pro-
files of the transient absorption, AðtÞ, were measured at 515 nm
in the presence of TBP only, because the triplet excited state
of TBP shows the T–T absorption around 515 nm.6
1
3
TBP þ hꢁð355 nmÞ ! TBPꢀ ! TBPꢀ
ð1Þ
0
20
40
60
80
100
Here, 1TBPꢀ and 3TBPꢀ represent the singlet and triplet excited
states of TBP, respectively. The lifetimes of 3TBPꢀ observed in
each solution were as follows: 662 ns (in 1/25 mixture of i-
BuOH/c-HexOH), 821 ns (in SDS) and 559 ns (in TMPA
TFSA). On the other hand, the lifetime of 3TBPꢀ in benzene
[PhSH] / 10-3 mol dm-3
Figure 1. PhSH concentration dependence of the 3TBPꢀ decay
rate constant (k) observed in a 1/25 mixture of i-BuOH/
c-HexOH at 515 nm.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.11 (2009)
1087
ꢃ
ꢃ
Here, TBPH and PhS represent thiobenzophenone ketyl and
phenylsulfanyl radicals, respectively. These results indicate that
the observed rate constant of 1:1 ꢁ 107 sꢂ1 molꢂ1 dm3 should be
0.10
due to the hydrogen abstraction of TBPꢀ from PhSH.
3
0.08
0.06
0.04
0.02
0.00
In the present study, the self-quenching of 3TBPꢀ could suc-
cessfully be suppressed in a viscous alcoholic solution of i-
BuOH/c-HexOH, a SDS micellar solution, and an ionic liquid
of TMPA TFSA. The reasons for suppressing the self-quenching
of 3TBPꢀ may be as follows: high viscosity of the alcoholic solu-
tion,10 cage effects with the high microviscosity of SDS micellar
solution,14–17 and cage effects with the low microviscosity of
ionic liquid.7,18,19 To elucidate the mechanism for suppressing
the self-quenching, it is desirable to clarify the reaction field
as a nanoscale structure. Along this way, further studies with
the MFE probe (magnetic field effect as a probe of reaction field)
are now in progress.
350
400
450
500
550
600
Wavelength / nm
This work was partially supported by a Grant-in-Aid for Sci-
entific Research (Nos. 20081005 and 2003002) in the Priority
Areas ‘‘Science of Ionic Liquids’’ (No. 452) and ‘‘High Field
Spin Science in 100 T’’ (No. 451) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT) of Japan.
Figure 2. Transient absorption spectra observed for the reac-
tion of TBP and PhSH (1 ꢁ 10ꢂ1 mol dmꢂ3) at delay times of
0.1 ( ), 0.5 ( ), 1.0 ( ), and 1.5 ms ( ) after laser irradiation.
0.14
0.12
0.10
0.08
0.06
0.04
References and Notes
1
For example: N. J. Turro, Modern Molecular Photochem-
istry, Benjamin/Cummings, Menlo Park, California, 1978;
N. J. Turro, V. Ramamurthy, J. C. Scaiano, Principles of
Molecular Photochemistry, University Science Books,
Sausalito, California, 2009.
2
3
J. C. Scaiano, J. Photochem. 1974, 2, 81.
H. Shizuka, M. Yamaji, Bull. Chem. Soc. Jpn. 2000, 73, 267,
and references therein.
[PhSH] = 100 mM
0 mM
0.02
0.00
4
5
6
H. Hayashi, Y. Sakaguchi, H. Murai, Y. J. I’Haya, J. Phys.
Chem. 1986, 90, 4403.
D. R. Kemp, P. de Mayo, J. Chem. Soc., Chem. Comm. 1972,
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C. V. Kumar, L. Qin, P. K. Das, J. Chem. Soc., Faraday
Trans. 2 1984, 80, 783.
-0.02
0
1
2
3
4
5
6
7
Time / µs
7
8
M. Wakasa, J. Phys. Chem. B 2007, 111, 9434.
V. Polshettiwar, M. P. Kaushik, Tetrahedron Lett. 2004, 45,
6255.
Figure 3. The AðtÞ curves observed at 380 nm in the absence
and presence of PhSH (1 ꢁ 10ꢂ1 mol dmꢂ3).
constant one. From the previous reports,5,6 the peaks observed at
490 and 515 nm can safely be assigned to the T–T absorption of
3TBPꢀ. The AðtÞ curve observed at 380 nm is shown in Figure 3.
Since the decay of the fast component was accelerated in the
presence of PhSH and the lifetime was observed to be the same
as that at 515 nm, the fast component could be assigned to the
T–T absorption of 3TBPꢀ. The almost constant value can be as-
signed to the transient absorption of thiobenzophenone ketyl rad-
ical for the following reasons: (1) the yields of this component
increased with increasing concentration of PhSH, (2) the similar
ketyl radical of benzophenone ketyl radical has a transient ab-
sorption band around 360 nm,11 and (3) the product analysis of
the photochemical reaction of TBP in THF suggests the forma-
tion of thiobenzophenone ketyl radical.12 Moreover, a weak tran-
sient absorption band for phenylsulfanyl radical was observed at
450 nm.13 From these results, we can safely describe the present
photochemical reaction of TBP with PhSH as follows:
9
T. J. Curphey, J. Org. Chem. 2002, 67, 6461.
10 A. Hamasaki, T. Yago, M. Wakasa, J. Phys. Chem. B 2008,
112, 14185.
11 Y. Sakaguchi, H. Hayashi, S. Nagakura, J. Phys. Chem.
1982, 86, 3177.
12 N. Kito, A. Ohno, Bull. Chem. Soc. Jpn. 1973, 46, 2487.
13 G. Jeschke, M. Wakasa, Y. Sakaguchi, H. Hayashi, J. Phys.
Chem. 1994, 98, 4069.
14 J. C. Scaiano, E. B. Abuin, L. C. Stewart, J. Am. Chem. Soc.
1982, 104, 5673.
15 M. Gratzel, J. K. Thomas, J. Am. Chem. Soc. 1973, 95, 6885.
16 N. J. Turro, M. Aikawa, A. Yekta, J. Am. Chem. Soc. 1979,
101, 772.
17 Y. Fujiwara, Y. Taga, T. Tomonari, Y. Akimoto, T. Aoki, Y.
Tanimoto, Bull. Chem. Soc. Jpn. 2001, 74, 237.
18 A. Hamasaki, T. Yago, T. Takamasu, G. Kido, M. Wakasa,
J. Phys. Chem. B 2008, 112, 3375.
¨
19 M. Wakasa, T. Yago, A. Hamasaki, J. Phys. Chem. B 2009,
113, 10559.
3
ꢀ
ꢃ
ꢃ
TBP þ PhSH ! TBPH þ PhS
ð2Þ
Published on the web (Advance View) October 17, 2009; doi:10.1246/cl.2009.1086