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Hoijemberg PA, Zerbs J, Reichardt C, Schwarzer D, Chesta CA, Schroeder J, Aramendia PF: Photophysics and photochemistry of an asymmetrically substituted diazene: a suitable cage effect probe. J Phys Chem A. 2009 May 14;113(19):5531-9.
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*). The sum of in-cage reaction rate constants of BME* radicals by combination and disproportionation is 5 x 10 (10) s (-1). The free radical quantum yield in solution is 0.21 (phi (BME*)) 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) pi,pi* and (1) n,pi* 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 epsilon (BME*) = (2.3 +/- 0.2) x 10 (4) M (-1) cm (-1) at 325 nm and a second-order rate constant of 5.8 x 10 (9) M (-1) s (-1) for the consumption of BME* radicals. |
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