Heavy-atom effects on the free radical yield and the triplet yield of the fluorescence quenching were studied in acetonitrile by using 9,10-dicyanoanthracene as the electron-accepting fluorescer and a series of para-halogenated anis贸le (I), aniline (II), and A./V-dimethylaniline (III) as the electron-donating quenchers. increases as the atomic number of the halogen substituent increases for all the systems, whereas 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 spin and the locally excited triplet state for the systems II and III.
The fluorescence quenching mechanism in a highly exothermic region of the Rehm-Weller relationship is shown to be a long-distance electron transfer for producing the geminate radical ion pair with fluorescer radical cation in an electronically excited state and quencher radical anion in the ground state.
The free enthalpy dependence of the free-radical yield aR of the electron-transfer (ET) fluorescence quenching was studied in acetonitrile by using anthracenecarbonitriles as the electron-accepting fluorescer and 1,4-diphenyl-1,3-butadienes as the electron-donating quencher. eR decreases, passes through a minimum, increases with increase of AGf, the free enthalpy change involved in the actual ET process, and then suddenly falls when AGf goes beyond -0.25 eV. Switchover of the quenching mechanism was suggested for the ET fluorescence quenching: The radical pairs are exclusively produced by the full ET in the encounter state between the fluorescer and the quencher when AGf is smaller than -0.4 eV, but in contrast through the partial ET, Le., the exciplex formation as the primary quenching products when AGf is larger than -0.4 eV.
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