Polarization of the π*→π and π*→n Phosphorescence Spectra of N-Heterocyclics

Phosphorescence polarization measurements have been performed by the method of photoselection. Since this technique requires a knowledge of the polarization of at least two perpendicularly polarized absorption bands, the method can be fully exploited in the case of the N-heterocyclics where the (n, π*) and (π, π*) absorptions are perpendicular to each other. The results on pyrazine (1,4-diazabenzene) show that the π*→n emission is polarized along the N···N axis, in agreement with previous work. The polarization is uniform throughout the 600-cm—1ν6(ag) progression and does not alternate as does the intensity and the vibrational frequency. This behavior might arise from a resonance-type interaction which produces a progression with mixed Franck—Condon characteristics. Quinoxaline (1,4-diazanaphthalene) is found to have a π*→π phosphorescence spectrum which is highly polarized out of the plane. Approximate calculations show that the (n, π*) singlet state in quinoxaline is at least as important as the (σ, π*) states in mixing with the emitting (π, π*) triplet state. The general conclusion is reached that in nitrogen heterocyclics when the phosphorescent state is of the (n, π*) type, the mixing singlet is of the (π, π*) type, or in-plane Rydberg state, whereas if the emitting triplet state is of the (π, π*) type, the mixing singlet might be of the (n, π*), (σ, π*) and/or perpendicularly polarized Rydberg state.