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

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 B11(nπ*) ↔ lim soB23(ππ*) and A11(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).