J . Org. Chem. 1999, 64, 1285-1290
1285
P h otoch em istr y of Nitr oben zen eth iol. Selective Gen er a tion of th e
Th io Ra d ica l a n d Th ion e Tr ip let Sta te a s a F u n ction of Solven t
P ola r ity
Maksudul M. Alam and Osamu Ito*
Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980-77, J apan
Received October 23, 1998
Transient absorption spectra of p-nitrobenzenethiol (NBSH) have been measured by the nanosecond
•
laser flash photolysis method. In nonpolar and less polar solvents, nitrobenzenethio radical (NBS )
was predominantly formed by the homolytic fission of the S-H bond of NBSH. In protic polar
solvents, on the other hand, formation of the triplet state of deprotonated nitrobenzenethione
3
-
[
(NBT )*], which was confirmed by triplet quenching experiments with O
observed. Quantum yield of intersystem crossing (Φ ) and the lowest triplet energy (ET1) of (NBT )*
were evaluated to be 0.36 and 58 kcal/mol, respectively, in ethanol. In aprotic polar solvents such
2
and â-carotene, was
3
-
T
3
-
•
as acetonitrile, both (NBT )* and NBS were produced in the same time. Electron transfer occurs
from donors to (NBT )* and to NBS . For an electron acceptor, the electron-transfer reaction takes
place from only (NBT )* and not from NBS . Thus, NBSH can be used as a selective generator of
the thione triplet and thio radical by changing the properties, such as polarity and protic/aprotic
character, of the solvents used.
3
-
•
3
-
•
In tr od u ction
other hand, p-nitrobenzenethiol (NBSH) exhibits an acid/
base equilibrium (Scheme 1) in polar solvents and in
Photochemistry of aromatic thiols and disulfides has
been extensively studied.1 The sulfur compounds are
known to be an effective source of sulfur-centered free
radicals and ion radicals, which are important intermedi-
ates in photochemical reactions and also in kinetic
investigations.2 Flash photolysis methods using lasers
and Xe lamps as excitation sources have been employed
as attractive techniques to detect reactive intermediates
protic solutions because of the low pK
a
, ranging from 5.6
of NBSH. It has been demonstrated that the
NBT ) resonance structure makes a larger contribution
-8
9-11
to 4.7,
-
(
-
than does the thiolate form (NBS ) in protic polar
solvents because of the high electron-withdrawing ability
-5
11
2
of the NO group (Scheme 1).
Although a photochemical investigation of NBSH to
•
produce the thio radical (NBS ) has been reported in
such as free radicals and ion radicals of the sulfur
2
nonpolar solvent, a detailed study in polar solvents and
compounds.1
-3,5-8
Arylthiols with electron-donating and
and Cl
in protic solutions has not been reported. It would be
anticipated from the equilibrium above that multiple
reactive species were produced as a function of the
electron-withdrawing substituents such as NH
2
are known to produce S-centered radicals by direct
transient observations.1
-3
-
-
fractions of NBSH and NBT (or NBS ) in polar and in
protic/aprotic solvents.9
-11
Recently, the importance of
In the ground state, the substituted benzenethiols are
more likely to exist as the nondissociated thiol in various
thione photochemistry in chemical and biological view-
values.9
-11
On the
solvents because of their high pK
a
points has been recognized for various thiol-thione
tautomers.1
2,13
In particular, the participation of triplet
states of thiones in various chemical and biochemical
(
1) (a) Oswald, A. A.; Griesbaum, K.; Hudson, B. E. J . Org. Chem.
reactions has been frequently pointed out.1
2,13
1
963, 28, 2361. (b) Thyrion, F. C. J . Phys. Chem. 1973, 77, 1478. (c)
Takakura, T.; Tagami, M.; Okuyama, M.; Kamada, H. J . Spectrosc.
Soc. J pn. 1975, 24, 282.
In the present study, we have investigated the photo-
chemistry of NBSH, including the influences of the
nature of solvents (polar/nonpolar and protic/aprotic) on
its photochemistry. The transient intermediates produced
by the different forms of NBSH have been identified and
spectroscopically characterized. In addition, we reported
their reactivities, such as energy-transfer and electron-
transfer reactions toward various substrates.
(2) (a) Ito, O.; Matsuda, M. J . Am. Chem. Soc. 1979, 101, 1815. (b)
Ito, O.; Matsuda, M. J . Am. Chem. Soc. 1979, 101, 5732. (c) Ito, O.;
Matsuda, M. J . Am. Chem. Soc. 1981, 103, 5871. (d) Ito, O.; Matsuda,
M. J . Am. Chem. Soc. 1982, 104, 1815.
(
(
3) Burkey, T. J .; Griller, D. J . Am. Chem. Soc. 1985, 107, 246.
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(
b) Newcomb, M.; Kaplan, J . Tetrahedron Lett. 1987, 28, 1615.
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Bohne, C.; Boch, R.; Scaiano, J . C. J . Org. Chem. 1990, 55, 5414.
(
(
(
6) Alam, M. M.; Watanabe, A.; Ito, O. J . Org. Chem. 1995, 60, 3440.
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Soc. 1995, 117, 9699. (b) Aveline, B. M.; Kochevar, I. E.; Redmond, R.
W. J . Am. Chem. Soc. 1996, 118, 10113.
Commercially available p-nitrobenzenethiol (NBSH), p,p-
dinitrobenzene disulfide (NBDS), 3,3′,5,5′-tetramethylbenzi-
(8) (a) Alam, M. M.; Watanabe, A.; Ito, O. Photochem. Photobiol.
1
996, 63, 53. (b) Alam, M. M.; Konami, H.; Watanabe, A.; Ito, O. J .
Chem. Soc., Perkin Trans. 2 1996, 263. (c) Alam, M. M.; Watanabe,
A.; Ito, O. Int. J . Chem. Kinet. 1996, 28, 405.
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(b) Maciejewski, A.; Steer, R. P. Chem. Rev. 1993, 93, 67. (c) Lapinski,
L.; Prusinowska, D.; Nowak, M. J .; Bretner, M.; Felczak, F.; Maes, G.
Adamiwciz, L. Spectrochim. Acta, Part A 1996, 52, 645.
(13) (a) Alam, M. M.; Fujitsuka, M.; Watanabe, A.; Ito, O. J . Phys.
Chem. A 1998, 102, 1338. (b) Alam, M. M.; Fujitsuka, M.; Watanabe,
A.; Ito, O. J . Chem. Soc., Perkin Trans. 2 1998, 817.
(
9) Clare, B. W.; Cook, D.; Ko, E. C. F.; Mac, Y. C.; Parker, A. J . J .
Am. Chem. Soc. 1966, 88, 1911.
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1973, 1969.
11) Bordwell, F. G.; Hughes, D. L. J . Org. Chem. 1982, 47, 3224.
(
2
(
1
0.1021/jo982133x CCC: $18.00 © 1999 American Chemical Society
Published on Web 02/04/1999