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,π* state centered on the azo moiety. By picosecond experiments irradiating at the biphenyland 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·). 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 (φBME·) 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· radicals. Transient IR measurements show CO stretching bands of the excited 3 π,π* and 1 n,π* 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 εBME· ) (2.3 ( 0.2) × 104 M-1 cm-1 at 325 nm and a second-order rate constant of 5.8 × 109 M-1 s-1 for the consumption of BME· radicals.