Triplet state formation efficiencies of aromatic hydrocarbons in solution

The triplet state formation efficiencies,фT, of ten aromatic hydrocarbons have been deter­mined in ethanol and, in some cases, in other solvents. Fluorescence yields, фF, for the various hydrocarbon-solvent systems were measured where literature values were not available. With the exception of coronene the sum of фF + фT was found to be unity within experi­mental error, which indicates little or no internal conversion from the first excited singlet state of each hydrocarbon. Rate constants for intersystem crossing,kIS and the ratio kIS/kF, where kF is the radiative transition probability, have been calculated for each hydrocarbon in ethanol. The ratio varies much less than do the separate rate constants and the significance of this is discussed in terms of possible selection rules for spin forbidden non-radiative transitions.