ELECTRON-TRANSFER REACTIONS BETWEEN EXCITED DIARYLMETHYL AND TRIARYLMETHYL CARBOCATIONS AND AROMATIC DONORS

The excited singlet states of a variety of diarylmethyl and triarylmethyl carbocations were characterized by fluorescence spectroscopy. The dibenzosuberenyl, xanthenyl and 9-phenyl-xanthenyl cations are strongly fluorescent (PHI = 0.1-0.3) and have fluorescence lifetimes in the 30-40 ns range in 2,2,2-trifluoroethanol. The fluorescence of each of these cations is efficiently quenched by a variety of substituted aromatics (e.g. cumene, anisole, toluene) with rate constants in excess of 10(9) M-1 s-1. There is a correlation between the observed rate constants and the oxidation potential of the aromatic quencher which suggests that an electron transfer process occurs to generate a dibenzosuberenyl radical/silane radical cation pair. This hypothesis is confirmed by irradiation of the dibenzosuberenyl cation in the presence of benzyltrimethylsilane. This reaction produces 5-benzyldibenzocycloheptene, which is formed by addition of the benzyl radical produced via cleavage of the silane radical cation to either the dibenzosuberenyl radical or cation. Quenching studies suggest that the efficiency of product formation is controlled by the competition between cage escape and back electron transfer for the initial geminate radical/radical ion pair.