Host-Crystal Effects on the Mechanism of the π, π Phosphorescence. II. N-Heterocyclics

Abstract

The polarization of the π, π* phosphorescence emission of quinoxaline (1,4-diazanaphthalene) dissolved in EPA rigid glass as well as in durene host crystal has been measured and the two compared. The polarization characteristics of the different vibronic bands in the two hosts indicate that the emission originates from at least two mechanisms—a first-order mechanism that gives rise to Subspectrum I with the 0, 0 band as its origin, and a second-order, spin—orbit—vibronic mechanism that gives rise to Subspectrum II. When the host is changed from EPA to durene, the polarization of Subspectrum I in the quinoxaline framework seems to change from out of plane to long axis. Subspectrum II seems to be less sensitive to host-crystal effects than Subspectrum I. The similarity of these results to those obtained previously for naphthalene-d8 (Chaudhuri and El-Sayed) might indicate that the lone pair on the nitrogens in quinoxaline plays no specific role in explaining this phenomenon. This conclusion suggests that the most probable configuration for quinoxaline in durene is that in which the lone pairs on the nitrogen atoms of quinoxaline are far away from the methyl groups of durene. The observed durene host-crystal effects on the emission of quinoxaline can be explained in a similar manner to those given for naphthalene. It is proposed that the quinoxaline molecule in durene crystal is slightly nonplanar in its triplet state due to repulsive forces of the methyl hydrogens of durene and the hydrogen atoms of the quinoxaline molecule. The nonplanarity results in an enhancement of the spin—orbit interaction between singlet and triplet π, π* states and thus introduces long-axis polarization. Mixing between quinoxaline triplet state and durene-crystal states is an alternative mechanism and is briefly mentioned.