Single Pulse Two-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 τ = 27 ± 1 μs is observed. Addition of triethylamine results in quenching of the benzophenone phosphorescence. Quenching rate constants KQ have been determined from emission intensifies and from experimental decay rate constants. The values KQ = (2.6 ± 0.3) × 109 M-1s-1 and KQ = (4.0 ± 0.5) × 109 M-1s-1, respectively, are obtained. On the addition of 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).