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

Cubatao, San Pablo, Brasil

Conferencia; IX ELAFOT (Encuentro Latino Americano de Fotoquímica y Fotobiología); 2008

The photophysics and photochemistry of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazenewas thoroughly studied by means of steady state photolysis and laser flash photolysis from thepicosecond to the microsecond time domain. The compound has favorable features as a radicalphotoinitiator and as a probe for cage effects studies in liquids, supercritical fluids, andcompressed gases. The biphenyl moiety acts as an antenna, efficiently transferring electronicenergy to the dissociative 1n,π* state centered on the diazo moiety. By picosecond experimentsirradiating at the biphenyl and at the diazo centered transitions we were able to demonstrate thisfact as well as to determine a lifetime of 0.7 ps for the build-up of 1-biphenyl-4-yl-1-methylethylradicals (BME·). In cage reaction rate of BME· radicals by combination anddisproportionation is 5 1010 s-1. Free radical quantum yield in solution approaches 0.3 in hexaneat 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 theproduction and reaction of BME· radicals. Transient IR measurements show CO stretching bandsof the excited 3π,π* and 1n,π* states but no dissociation up to 0.5 ns. A fluorescence lifetime of 1ns is in line with this observation. By transient actinometry and kinetic decays in themicrosecond time range we were able to measure εBME· = 1.7 ± 0.2 104 M-1cm-1 at 325 nm and asecond order rate constant of 4.5 109 M-1s-1 for the consumption of BME· radicals.