Radical Cations of Dithienothiophenes
J. Phys. Chem. A, Vol. 101, No. 6, 1997 1061
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appeared at wavelengths less than 350 nm. The new band can
be attributed to the triplet excited state of DNB.22 The result
indicates that (2,3-DTT)* is quenched by energy transfer to
T
DNB. From the dependence of the decay rate constant of
T(2,3-DTT)* on DNB concentration, the bimolecular rate
constant for the energy transfer was estimated to be 8.8 × 109
M-1 s-1. As for cyclohexane solution of 3,4-DTT and DNB,
quenching of T(3,4-DTT)* by energy transfer was also observed
and the rate constants was 1.1 × 1010 M-1 s-1
.
(5) Wintgens, V.; Valat, P.; Garnier, F. J. Phys. Chem. 1994, 98, 228.
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Conclusion
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In the present study, we showed that the radical cations of
DTTs were generated efficiently by photoirradiation in the
presence of appropriate electron acceptors in polar solvent. The
reactions occurred at the diffusion-controlled rate from both the
singlet and triplet excited states of DTT when DNB was used
as an acceptor (∆G0 for the reaction <-80 kJ mol-1). Since
electron transfer from the singlet excited state competes with
intersystem crossing, it only becomes evident when the con-
centration of DNB is higher than 10-2 M. The contribution of
electron transfer from the singlet excited state also became large
than for DNB when CCl4 was used as an acceptor. In this case
∆G0 for the reaction from the triplet excited state was also less
negative. In cyclohexane, the triplet excited state of DTT was
deactivated by energy transfer to DNB rather than electron
transfer.
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Elements; Marcel Dekker: New York, 1980; Vol. 14.
Acknowledgment. We are grateful to Professor Tadamasa
Shida, Kyoto University, for absorption spectra measurements
of γ-irradiated samples. The present work is partly funded by
the Grant-in Aid on Priority-Area-Research on Photoreaction
Dynamics (No. 08218207) from the Ministry of Education,
Science, Sports and Culture.
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References and Notes
Amsterdam, 1988.
(1) For example: (a) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F.
Synth. Met. 1990, 39, 243. (b) Casper, J. V.; Ramamurthy, V.; Corbin, D.
R. J. Am. Chem. Soc. 1991, 113, 600. (c) Oeter, D.; Ziegler, Ch.; Go¨pel,
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(2) For example: (a) Hotta, S.; Waragai, K. J. Mater. Chem. 1991, 1,
835. (b) Egelhaaf, H.-J.; Ba¨uerle, P.; Rauer, K.; Hoffmann, V.; Oelkrug,
D. Synth. Met. 1993, 61, 143. (c) Garcia, P.; Pernaut, J. M.; Hapiot, P.;
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(20) The ꢀ value of DNB was estimated from electron-transfer reaction
between DNB and N,N,N′,N′-tetramethyl-1,4-phenylenediamine. ꢀ of the
tetramethylphenylenediamine radical cation was reported by Hausser and
Murrell (J. Chem. Phys. 1957, 27, 500).
(21) Fujitsuka, M.; Sato, T.; Segawa, H.; Shimidzu, T. Chem. Lett. 1995,
99.
(22) In the laser flash photolysis of the cyclohexane solution containing
DNB only, a similar transient absorption band appeared at wavelengths
less than 350 nm. Since the absorption band was readily quenched by
oxygen, it can be assigned to the triplet excited state of DNB.