SINGLE-PULSE 2-PHOTON LASER-ASSISTED REDUCTION OF BENZOPHENONE BY TRIETHYLAMINE TO FORM EXCITED BENZOPHENONE KETYL RADICAL

The pulsed laser photolysis at 308 nm of the benzophenone-triethylamin e system in acetonitrile solution has been studied at room temperature . In the absence of triethylamine, phosphorescence emission of triplet benzophenone centred at 460 nm and with a lifetime of tau = 27 +/- 1 mu s is observed. Addition of triethylamine results in quenching of th e benzophenone phosphorescence. Quenching rate constants k(Q) have bee n determined from emission intensities and from experimental decay rat e constants. The values (2.6 +/- 0.3) x 10(9) M(-1)s(-1) and k(Q) = (4 .0 +/- 0.5) x 10(9) M(-1)s(-1), respectively, are obtained. On the add ition triethylamine, benzophenone phosphorescence diminishes and simul taneously a new emission with a fast decay rate and centred at 575 nm develops. This signal has been identified as emission from the excited benzophenone ketyl radical. A single pulse two-photon absorption mech anism responsible of the emission at 575 nm has been identified and a kinetic model based on this mechanism has been developed. The model gi ves a satisfactory quantitative account of both our experimental resul ts and those reported in the literature (quantum yield of radical form ation and behaviour of the system as photoinitiator).