38
J. Phys. Chem. 1991, 95, 38-42
respectively, for the apex-to-apex, edge-to-edge, and face-to-face approaches), we estimate the following Boltzmann-weighted populations at room temperature (quantities given refer to % contributions from dZ2and dXLyl,respectively): 75, 25 (apex-toapex): 35, 65 (edge-to-edge); 50, 50 (face-to-face, where the eg
degeneracy is maintained). These populations are then used to average the hi:: values (in the rms sense). In section IIIC, we continue with the effective one-electron Hamiltonian notation, hi:!, but with the understanding that these matrix elements are evaluated in terms of the H(,:2g elements, as described above.
Heavy-Atom Effects on the Excited Singlet State Electron-Transfer Reaction Koichi Kikuchi,* Masato Hoshi,+Taeko Niwa, Yasutake Takahashi, and Tsutomu Miyashi Department of Chemistry, Faculty of Science, Tohoku University, Aramaki Aoba, Aoba- ku, Sendai 980, Japan (Received: April 5, 1990; In Final Form: July 2, 1990)
Heavy-atom effectson the free radical yield @R and the triplet yield GT of the fluorescence quenching were studied in acetonitrile by using 9,lO-dicyanoanthracene as the electron-accepting fluorescer and a series of para-halogenated anisole (I), aniline ( I I ) , and N,N-dimethylaniline (111) as the electron-donating quenchers. @T increases as the atomic number of the halogen substituent increases for all the systems, whereas @R decreases for the system I and does not change for the systems 11 and 111. These heavy-atom effects are interpreted in terms of the spin-orbit coupling between the singlet exciplex and the locally excited triplet state for the system 1, and in terms of the spin-orbit coupling between the geminate radical pair state with singlet spin and the locally excited triplet state for the systems I1 and 111.
Introduction Heavy-atom effects on the photoinduced electron-transfer (ET) reaction have been extensively studied for the triplet-state ET reaction, where the free-radical yield extremely decreases on substituting a heavy atom into the triplet quencher.l** In a previous work3 we demonstrated that such heavy-atom effect is caused by the spin-orbit coupling between the geminate or encountered radical pair state with triplet spin and the singlet ground state, when the members of the radical pair contact with each other before the diffusive separation of the radical pair into free radicals. In this sense, an electronic interaction between the members of the radical pair might lead to the display of characteristics similar to those of a singlet exciplex. In the case of the excited singlet state ET reaction, the triplet In molecules may be produced in addition to the radical a highly polar solvent such as acetonitrile, it has been indicated that the ET fluorescence quenching occurs at a long distance (-7 A) of the flouorescer and the q ~ e n c h e r Thus . ~ it has been supposed that the ET fluorescence quenching in acetonitrile is caused by a weak electronic exchange interaction between the fluorescer and the quencher, and hence the primary quenching product is the geminate radical pair with singlet spin, the members of which are separated by solvent molecules. This type of ET may be called as the full ET or the direct ET. This supposition has recently been confirmed by the studies on the free enthalpy dependence of the free radical aRof the ET fluorescence quenching:,' because aR has satisfactorily been interpreted with a semiclassical ET theory8-Io which assumes a weak electronic exchange interaction between the electron donor and acceptor. If the energy of a radical pair is higher than the energy of a locally excited triplet state, and if there is a spin-orbit coupling between the geminate radical pair state and the locally excited triplet state when the members of the geminate radical pair contact with each other, the triplet formation is expected to be enhanced by substituting a heavy atom into the quencher. The fluorescence of an electron donor-acceptor system may also be quenched by the partial ET, Le., the exciplex formation. Therefore, the fluorescence may be competitively quenched by the full and partial ET according to the general reaction scheme given by Weller." The former is predominant in a nonpolar solvent whcreas the latter in a polar solvent. When the exciplex 'Present address: Biological Laboratory, Kao Co.,2606 Akabane Ichikawamachi, Haga-gun, Tochigi 321-34, Japan.
0022-365419 112095-0038$02.50/0
is formed in a highly polar solvent such as acetonitrile, it may rapidly dissociate into the geminate radical pair without giving the exciplex fluorescence. Even if the exciplex fluorescence cannot be observed, therefore, it has to be kept in mind that there are two possible pathways for the excited singlet state ET reaction in a polar solvent which are very hard to discriminate. In fact Iwa et al.'* have already suggested the competitive formation of the geminate radical pairs and the exciplex in the fluorescence quenching in methanol. It is noted that the difference between the exciplex state and the contact ion pair state has clearly been depicted by FO11 et aI.l3 In this work we study the fluorescence quenching of 9,lO-dicyanoanthracene (DCA) with para-halogenated anisole (X-AS), aniline (X-AL), and N,N-dimethylaniline (X-DMA) in acetonitrile. It is shown that the heavy-atom effects on the free-radical yield aR and the triplet yield aTof fluorescence quenching are useful to discriminate whether the quenching is due to the partial ET (the exciplex formation) or the full ET, and that aT is extremely increased by the spin-orbit coupling between the exciplex and the locally excited triplet state, but slightly by the spin-orbit coupling between the geminate radical pair state with singlet spin and the locally excited triplet state. Experimental Section Materials. Aniline (H-AL), N,N-dimethylaniline (H-AL) (Nakarai), anisole (H-AS), 4-chloroanisole (CI-AS), and 4(1) Steiner, U.; Winter, G . Chem. Phys. Lett. 1978, 55, 364. (21 Ulrich. T.: Steiner. U. E.: Foll. R. E. J . Phvs. Chem. 1983. 87. 1873.
(3) Kikuchi, K.; Hoshi, M.; Abe, E.; Kokubun: H. J . Photochem. Photobioi., A: Chem. 1988, 45, 1. (4) Leonhardt, H.; Weller, A. 2.Phys. Chem. (Munich) 1961, 29, 277. (5) Nibbe, H.; Rehm, D.; Weller, A. Ber. Bunsen-Ges. Phys. Chem. 1968, 7-, 2 -7 5 7.
(6) Kikuchi, K.; Takahashi, Y.; Koike, K.; Wakamatsu, K.; Ikeda, H.; Miyashi, T. Z.Phys. Chem. (Munich), in press. (7) (a) Could, I. R.; Ege, D.;Mattes, S. L.; Farid, S. J . Am. Chem. SOC. 1987, 109, 3794. (b) Could, 1. R.; Moser, J. E.; Ege, D.;Farid, S. J . Am. Chem. SOC.1988, 110, 1991. (c) Could, I. R.; Ege, D.;Moser, J. E.; Farid, S. J . Am. Chem. SOC.1990, 112, 4290. (8) Ulstrup, J.; Jortner, J. J . Chem. Phys. 1975, 63, 4358. (9) Efrima, S.;Bixon, M. Chem. Phys. 1976, 13, 447. (10) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J . Am. Chem. Soc. 1984, 106, 5057. (11) Weller, A. Z . Phys. Chem. (Munich)1982, 130, 129. (12) Iwa, P.; Steiner, U. E.: Voneimann. E.: Kramer. H. E. A. J . Phvs. Chem. 1982, 86, 1277. (13) Foil, R. E.; Kramer, H. E. A.; Steiner, U. E. J . Phys. Chem. 1990, 94, 2476.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991 39
Heavy-Atom Effects on Free-Radical Yield
TABLE I: Yields for Triplet (aT)and Free Radical ( i p R ) Produced with Fluorescence Quenching, Energy of Radical Pair (-AGb), and Rate Constants for Fluorescence Quenching ( k J , Free-Radical Recombination ( k M ) , and Geminate Radical Recombinations To Yield Triplet-State (k,) and Ground-State Molecules (kb) qucnchcr @T @R k., IO9 M-l s-l -AG,, eV k,,, IO9 M-l s-I k,,, IO9 s-l kb. IOio s-I H-AS CI-AS Br-AS I-AS H-AL CI-AL Br-A L I-AL H-DMA CI-DMA Br-DMA I-DMA
0.023 0.10 0.49 0.56
