The triplet lifetime dependence on vibrational energy for gas-phase carbonyl aromatic compounds: benzophenone and fluorenone

Abstract

The dependences of the intensities and decay rates of CO2 laser induced delayed fluorescence on vibrational energy and bath gas pressure were used to study the dominant relaxation pathways in the lowest triplet state, T1, over a wide range of vibrational energy Evib. With this in view, the excess energy dependences of nonradiative relaxation processes rates in the T1 state, such as intersystem crossing from the T1 to ground S0 states, reversible intersystem crossing between excited S1 and T1 states and intermolecular vibrational relaxation were compared. It was found that under vibrational equilibrium at high vibrational temperature (Tvib ≫ T) the experimentally observed nonexponential dependence of the gas-phase triplet decay time on Evib reflected the excess vibrational energy (in the T1) dependence of the intersystem crossing rate from T1 to S0 that was well approximated by an Arrhenius' activated type rate equation in contrast to exponential decay for isolated molecules.