CARBOCATIONIC FLUORESCENCE AND ITS EFFICIENT ELECTRON-TRANSFER QUENCHING

The absorption and emission spectral properties of several carbocations have been examined in acidified acetonitrile and Nafion matrices. Xanthenium and thioxanthenium carbocations are characterized by moderately strong yellow-to-orange fluorescence (tau(F) = 1-30 ns). In contrast, triphenylmethyl carbocation is very weakly emitting. The fluorescence of xanthenium and thioxanthenium carbocations is readily quenched by aromatics by a predominantly charge-transfer mechanism; the quenching rate constants are in the limit of diffusion control. Electron-transfer-derived radical cations (from aromatic quenchers) and radicals (from carbocations) are observed in the course of nanosecond laser flash photolysis. The yields of net electron transfer are, however, low (less-than-or-equal-to 0.02), suggesting efficient back electron transfer in the radical cation/radical pairs in singlet configuration.