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

Abstract

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^—1[ν₆(a_g)] 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.