Photophysics and photochemistry of an asymmetrically substituted diazene: an adequate cage effect probe

HOIJEMBERG, PABLO ARIEL; ZERBS, JOCHEN; REICHARDT, CHRISTIAN; SCHWARZER, DIRK; CHESTA, CARLOS A.; SCHROEDER, JÖRG; ARAMENDIA, PEDRO FRANCISCO

Cubatão, San Pablo

Encuentro; IX Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2008

CEPEMA-USP

The photophysics and photochemistry of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene was thoroughly studied by means of steady state photolysis and 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,pi* state centered on the diazo moiety. By picosecond experiments irradiating at the biphenyl and at the diazo 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·). In cage reaction rate of BME· radicals by combination and disproportionation is 5 1010 s-1. Free radical quantum yield in solution approaches 0.3 in hexane at room temperature, while dissociation quantum yield approaches 50%. The symmetric ketone: 2,4-bis-biphenyl-4-yl-2,4-dimethyl-pentan-2-one was used as reference compound for the production and reaction of BME· 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 1 ns is in line with this observation. By transient actinometry and kinetic decays in the microsecond time range we were able to measure epsilon(BME·) = 1.7 ± 0.2 10^4 M^-1cm^-1 at 325 nm and a second order rate constant of 4.5 10^9 M^-1s^-1 for the consumption of BME· radicals.