Photodissociation of tetraphenylhydrazine in compressed liquid solvents

Abstract

The viscosity dependence of the photodissociation of tetraphenylhydrazine has been investigated in n-alkanes at pressures of 1–4500 bar by pump–probe absorption spectroscopy and time-correlated single photon counting. Transient absorption lifetimes recorded at 650 nm and fluorescence lifetimes agree within experimental error, suggesting that the dynamics of tetraphenylhydrazine is determined by a single kinetic process which we attribute to the dissociation to diphenylaminyl radicals. In contrast to the decay times in n-alkanes at 1 bar, the pressure-dependent rate constants show a strong viscosity dependence indicating that a barrier-crossing process which is in the high damping limit of the Kramers–Smoluchowski theory controls the dissociation reaction. There is evidence for an additional activationless reaction channel leading to diphenylaminyl radicals for η→∞ when the barrier-crossing process is suppressed.