Intramolecular vibrational relaxation: Effects on electronic nonradiative relaxation rates

Abstract

The effects of intramolecular vibrational relaxation (IVR) on radiationless transition rates are investigated. IVR has previously been suggested as an explanation of the observed saturation of the decay rate in tetracene S1→S0 internal conversion and in toluene T1→S0 intersystem crossing at high vibrational energies. Explicit calculations for transition rates under weak coupling in the statistical limit are presented which demonstrate that when IVR is operative a slow exponential increase of the nonradiative decay rate with vibrational energy is expected, in contrast to the experimentally observed rapid initial increase followed by saturation at higher vibrational energies. Speculations of alternative decay mechanisms are presented, and new, simple approximate formulas are derived for the relative nonradiative decay rates of single vibronic levels.