HEAVY-ATOM EFFECTS ON THE EXCITED SINGLET-STATE ELECTRON-TRANSFER REACTION

Heavy-atom effects on the free radical yield PHI-R and the triplet yield PHI-T of the fluorescence quenching were studied in acetonitrile by using 9,10-dicyanoanthracene as the electron-accepting fluorescer and a series of para-halogenated anisol (I), aniline (II), and N,N-dimethylaniline (III) as the electron-donating quenchers. PHI-T increases as the atomic number of the halogen substituent increases for all the systems, whereas PHI-R decreases for the system I and does not change for the systems II and III. 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 I, and in terms of the spin-orbit coupling between the geminate radical pair state with singlet and the locally excited triplet state for the systems II and III.