Radiative and nonradiative transitions in the first excited singlet state of symmetrical methyl-substituted acetones

Abstract

Radiative and nonradiative rates of symmetrically substituted, methylated acetones have been measured in the gas phase. Observed radiative lifetimes are compared to calculated radiative lifetimes obtained from the Strickler−Berg (SB) expression. The calculated values are greater by a factor of 8−9, and an empirical procedure by which the true radiative lifetime can be estimated from the SB expression and the geometry−sensitive correction parameter is given. The rates of nonradiative transitions are reduced appreciably by the α, α′ substitution of methyl groups to the C−C−CO−C−C skeleton. The steric crowding introduced by methyl group substitution is considered to be responsible for suppressing the vibrational motion of the promoting modes. In most cases, the S₁ → T₁ intersystem crossing predominates over the other nonradiative transitions.