CONICET | Buscador de Institutos y Recursos Humanos

The photophysics and photochemistry of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene was thoroughly studied by laser flash photolysis from the picosecond to the microsecond time domain. The compound has favorable features as a radical photoinitiator and as a probe for cage effects studies in liquids, supercritical fluids, and compressed gases. The biphenyl moiety acts as an antenna efficiently transferring electronic energy to the dissociative 1n,p* state centered on the azo moiety. By picosecond experiments irradiating at the biphenyl and at the azo centered transitions we were able to demonstrate this fact as well as to determine a lifetime of 0.7 ps for the build-up of 1-biphenyl-4-yl-1-methyl-ethyl radicals (BME·). The sum of in cage reaction rate constants of BME· radicals by combination and disproportionation is 5 1010 s-1. The free radical quantum yield in solution is 0.21 (fBME) in hexane at room temperature, while the dissociation quantum yield approaches 50 %. The symmetric ketone: 2,4-bis-biphenyl-4-yl-2,4-dimethyl-pentan-2-one was used as a reference compound for the production and reaction of BME· radicals. Transient IR measurements show CO stretching bands of the excited 3p,p* and 1n,p* states but no dissociation up to 0.5 ns. A fluorescence lifetime of 1 ns for this ketone is consistent with this observation. By transient actinometry and kinetic decays in the microsecond time range we measured eBME· = 2.3 ± 0.2 104 M-1cm-1 at 325 nm and a second order rate constant of 5.8 109 M-1s-1 for the consumption of BME· radicals.