Vibronic Spin–Orbit Interactions in Heteroaromatic Molecules. II. Phosphorescence of Quinoxaline and Other Diazanaphthalenes

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Abstract
Solvent effects on phosphorescence and its polarization characteristics indicate that the two major spin–orbit coupling mechanisms through which the 3B2(ππ*) state of quinoxaline acquires dipole‐allowed character via nπ* states are 1B1(nπ*) ↔  lim so3B2(ππ*) and 1A1(ππ*) ↔  lim b1vib1B1(nπ*) ↔  lim so3B2(ππ*). The deuterium isotope effect on the emission characteristic and the intensity of vibronic bands of the n→π* absorption suggest that the perturbing vibration(s) is an out‐of‐plane C–H bending mode(s).