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-triethylamine 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 the benzophenone phosphorescence. Quenching rate constants k(Q) have been determined from emission intensities and from experimental decay rate 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 addition triethylamine, benzophenone phosphorescence diminishes and simultaneously 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 mechanism responsible of the emission at 575 nm has been identified and a kinetic model based on this mechanism has been developed. The model gives a satisfactory quantitative account of both our experimental results and those reported in the literature (quantum yield of radical formation and behaviour of the system as photoinitiator).