SINGLET-STATE PHOTOCHEMISTRY OF DIBENZYL KETONE AND ITS O-TOLYL DERIVATIVES

The singlet state contribution of alpha-cleavage in the photochemistry of dibenzyl ketone (DBK) and o-tolylmethyl benzyl ketone (TBK) was investigated. The fitting of a master Stern-Volmer equation for the simultaneous quenching of the S1 and T1 states of DBK and TBK leads to the conclusion that in both ketones alpha-cleavage occurs in the singlet state, but to a lesser (20%-30%) extent than in the triplet. Comparison of the relative fluorescence and alpha-cleavage quantum yields of DBK and TBK is also consistent with the presence of singlet state alpha-cleavage in both ketones. The introduction of a methyl group in the ortho position of the phenyl ring in TBK introduces a second singlet state ketone reaction, delta-hydrogen-atom abstraction. This reaction is faster than alpha-cleavage in the S1 state, but proceeds with a low quantum efficiency. A delta-hydrogen-atom-induced internal conversion is proposed to be a major mechanism for the deactivation of the S1 state of TBK. This deactivation of S1 serves to reduce the efficiency of intersystem crossing and competes effectively with alpha-cleavage in S1. For TBK, o,o'-dimethyl TBK and mesitylmethyl benzyl ketone, which all show fast singlet state hydrogen-atom abstraction, the quantum yields of fluorescence and alpha-cleavage decrease. On the basis of the assumption of a certain (20%) participation of singlet state alpha-cleavage and fast delta-hydrogen-atom abstraction, a general mechanism and rate constants of each process were estimated for DBK and TBK.