The triplet lifetime dependence on vibrational energy for gas-phase carbonylaromatic 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 E_vib. 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 (T_vib ≫ T) the experimentally observed nonexponential dependence of the gas-phase triplet decay time on E_vib 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.