MESOLYTIC SCISSION OF C-C BONDS AS A PROBE FOR PHOTOINDUCED ELECTRON-TRANSFER REACTIONS OF QUINONES

Photoinduced electron transfer reactions of chlorinated benzoquinones are investigated using bibenzylic donors that undergo rapid fragmentat ion upon oxidation. The fragmentation rates and the quantum yields are used to probe the dynamics of back-electron transfer (BET) in two typ es of radical ion pairs. The triplet ion pairs formed by interception of excited state quinones give products with high quantum yields. The singlet ion pairs formed by irradiation of the charge-transfer (CT) co mplexes between the quinones and the donors undergo reactions with sig nificantly lower efficiency. The advantage of the first method (triple t quenching) over the CT-irradiation depends on the energetics of BET. It is large for reactions with relatively small Delta G(et) for BET a nd it decreases for reactions with more negative Delta G(bet). The ind irectly obtained rates of BET are in excellent agreement with literatu re data for similar, but unreactive systems, and the rates of C-C bond scission in radical cations generated in these systems are consistent with the thermodynamics of these processes.