Phosphorescence Mechanisms in Naphthalene N-Heterocyclics

Abstract

The polarization of the phosphorescence spectra of quinoline, isoquinoline, and quinoxaline is measured at 4.2 and 77°K in biphenyl and durene hosts. The results for quinoline and isoquinoline resemble those obtained previously for naphthalene and quinoxaline. The apparent anamolous polarization in the durene is also observed for these molecules. A discussion of the “apparent” change is briefly given. As in quinoxaline, a new spectrum not observed for naphthalene is found to originate from mechanism(s) involving vibronic interaction. A tentative assignment of the vibrations responsible for the appearance of the new spectra is made. A theoretical discussion is given of the possible mechanisms responsible for the appearance of the vibronically induced part of the spectrum. A comparison of theoretical predictions with observed results leads to the following conclusions: (1) There is some indication that, as in pyrazine, the contamination of the ground state with triplet character of the 3 (n − , π*) state in quinoxaline is not negligible. But unlike pyrazine, the effect of this mixing appears only in the vibronic second‐order spectrum (and not in the first‐order spectrum as it does in pyrazine). This is due to the fact that the emitting state in quinoxaline is of the (π, π*) type. (2) In the second‐order schemes, as well as the first order, the important spin–orbit interaction takes place between the symmetric and antisymmetric states with respect to reflection in the molecular planes. (3) Qualitative estimates show that the presence of low energy n, π* states can account for: (a) the difference between the total radiative power of N‐heterocyclics and their parent hydrocarbons; and (b) the preferential enhancement of the vibronic part of the spectrum in N‐heterocyclics over that existing in aromatic hydrocarbons.