ESR Study of the Phosphorescent State of Benzo[f]quinoline

Electron spin resonance absorption has been observed in a single-crystal solid solution of benzo[f]quinoline in biphenyl under irradiation at 120 K with light from a high-pressure mercury lamp. The fine structure of the resonance spectrum observed at 9.25 GHz can be described by the spin HamiltonianH=βH·g·S+D[S_z^2-\frac13S(S+1)]+E(S_x^2-S_y^2) in which S=1, D=±0.09926(18) cm−1, E=\mp0.04781(6) cm−1, gxx=2.0024(6), gyy=2.0018(4), and gzz=2.0018(4); the z axis is normal to the molecular plane, the x and y axes being the in-plane fine-structure principal axes. The principal x axis deviates by ±4° from the biphenyl long axis and the principal z axis by 7° from the biphenyl out-of-plane axis. The phosphorescence spectra and lifetimes have been measured both at 77 K and at 4.2 K. Marked doublet splittings were observed in the phosphorescence bands at 4.2 K, suggesting a large guest-host interaction.