Mechanisms of external heavy atom effects on the lowest excited triplet states: Naphthalene and biphenyl <i>X</i> traps

Abstract

An external heavy atom effect was investigated on the phosphorescent states of naphthalene X traps and biphenyl X traps using a variety of heavy atom perturbers at liquid helium temperatures. Phosphorescence spectra, triplet lifetimes, sublevel properties, and detrapping rates were measured. The well-resolved phosphorescence spectra were obtained in almost all cases. The intensities of the vibronic bands were extremely weak in the heavy atom systems compared with those of the light atom systems. The triplet lifetimes range from 10 to 2300 ms in the naphthalene X traps and from 240 to 4200 ms in the biphenyl X traps. Selective enhancements were observed in the radiative decay rate constants of the triplet sublevels. Correlation between the intensities of phonon bands and the magnitudes of detrapping rates was found in the naphthalene X traps. From these results we conclude that: (i) The shortening of the triplet lifetimes is mostly due to the radiative enhancements of the T1→S0 transitions; (ii) the molecular states of the perturber, especially the lowest excited triplet states, are the most important perturbing states among the three possible states (atomic states, CT states, and molecular states); and (iii) the magnitudes of the intermolecular interactions are also enhanced in the heavy atom systems, which is partly due to those of electron–phonon interactions.