Observation of the set of three sublevel phosphorescence spectra of quinoxaline. Analysis of radiative mechanisms

A method to observe phosphorescence spectra for the three triplet sublevels is proposed, and is applied to quinoxaline in a single crystal of naphthalene-d8. The method consists of measurements of the time resolved phosphorescence spectra at temperatures where spin polarization is achieved and under the influence of the microwave field to saturate the magnetic transition between the spin sublevels. The method is best applied to the cases where total decay rate constants are small for two sublevels. The Tz spectrum is identical to the known thermally averaged spectra observed at 4 K, whereas the Tx and Ty spectra are markedly different from the Tz spectrum in the spectral distributions. The relative radiative rate constants as well as the total decay rate constants are obtained by the method of microwave induced delayed phosphorescence for some selected vibronic bands. On the basis of the observed spectra and the kinetic data, mechanisms in which vibronic bands gain intensity are discussed. Finally, from the qualitative analysis of the experimental results in conjunction with theoretical considerations, the magnitudes of the matrix elements with respect to spin–orbit coupling are estimated. In the matrix element over molecular orbitals the contribution of one center terms dominates. The sum of two center and three center terms contributes about 10%, and the sum of three center terms contributes about 3% of the matrix element.