Photophysics and Photochemistry of an Asymmetrically Substituted Diazene: A Suitable Cage Effect Probe

Abstract

The photophysics and photochemistry of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene were 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 effect studies in liquids, supercritical fluids, and compressed gases. The biphenyl moiety acts as an antenna efficiently transferring electronic energy to the dissociative (1)n,pi 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 determine a lifetime of 0.7 ps for the buildup of 1-biphenyl-4-yl-1-methyl-ethyl radicals (BME center dot). The sum of in-cage reaction rate constants of BME center dot radicals by combination and disproportionation is 5 x 10(10) s(-1). The free radical quantum yield in solution is 0.21 (phi(BME center dot)) in n-hexane at room temperature, whereas 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 center dot radicals. Transient IR measurements show CO stretching bands of the excited (3)pi,pi and (1)n,pi states but no dissociation up to 0.5 ns. A fluorescence lifetime of I ns for this ketone is consistent with this observation. By transient actinometry and kinetic decays in the microsecond time range, we measured epsilon(BME center dot) = (2.3 +/- 0.2) x 10(4) M-1 cm(-1) at 325 nm and a second-order rate constant of 5.9 x 10(9) M(-1)s(-1) for the consumption of BME center dot radicals.