Direct photophysical evidence for quenching of the triplet excited state of 2,4,6-triphenyl(thia)pyrylium salts by 2,3-diaryloxetanes

The thiapyrylium salt 1b is an efficient electron-transfer photosensitizer in the preparative irradiation of trans, trans-2,3 -diphenyl-4-methyloxetane (2a). Previously, the reaction has been assumed to occur through the triplet excited state of 1b, which has a very high intersystem crossing yield (phi(ISC) = 0.97). In the present report, direct evidence for triplet quenching in the oxidative cycloreversion of the 2,3-diaryloxetanes is provided. The rate constants k(q)(T-1) and the free energy changes of the electron transfer DeltaG(ET)(T-1) have been determined for the reaction between 1b and oxetanes 2a-d and 2a'. The process has been found to be exergonic in all cases except for 2d; this agrees with the fact that no photoproduct was obtained in the preparative irradiation of 1b in the presence of 2d. Good correlation between k(q)(T-1) and DeltaG(ET)(T-1) was observed, confirming the involvement of the excited triplet state in the reaction mechanism.