Kinetic solvent effects on the reaction of an aromatic ketone π,π* triplet with phenol.: Rate-retarding and rate-accelerating effects of hydrogen-bond acceptor solvents

Quenching of the 2-benzoylthiophene pi,pi* triplet, (BT)-B-3*, by phenol yields the corresponding ketyl and phenoxyl radicals. Reaction rates were measured in 10 solvents having a range of hydrogen-bond acceptor strengths (beta(H)(2) values). There appear to be two mechanisms: (i) a bimolecular reaction of (BT)-B-3* with "free" (i.e., not H-bonded) phenol in which the (BT)-B-3* accepts both a proton and an electron from the phenol, the rate decreasing as beta(H)(2) increases; (ii) a trimolecular reaction of (BT)-B-3* with phenol that is H-bonded to a solvent molecule, PhO-H center dot center dot center dot S, in which the proton goes to the S and the electron to the (BT)-B-3*, the rate increasing as beta(H)(2) increases.