Phosphorescence of Quinoxaline-Durene Mixed Crystal — Analysis of Fine Structures and Decay Characteristics

Abstract

The phosphorescence spectrum of quinoxaline doped in durene crystal is observed at 4.2°K. The spectrum is composed of a number of sharp bands, many of which being interpreted as arising from the coupling with the ground state molecular vibrations of quinoxaline. Associated with the 0,0 and vibronic bands three weak bands were observed which are separated from the vibronic band by 19, 30 and 52 cm−1. The nature of these three bands is not well understood, but it is certain that they are not due to the lattice vibrations of the durene host. The decay of the phosphorescence is investigated at fifteen different temperatures between 4.21 and 1.59°K. Above 2.41°K the decay is single exponential with a lifetime of 0.246 sec. Below 2.19°K, however, the decay is expressed as a sum of two exponential decays whose lifetimes are 0.757 and 0.093 sec. The shorter one is approximately one third of the lifetime at 4.21°K. The change of the decay characteristics at a temperature between 2.19 and 2.41°K is interpreted to be associated with the spin alignment. In terms of these data, together with the polarization ratio observed by Chaudhuri and El-Sayed, radiative and radiationless rate constants are estimated for each of the three magnetic subcomponents of the triplet state. The experimental results and their interpretation are partly in disagreement with the work by de Groot, Hesselmann and van der Waals, and the source of the discrepancy is analyzed.