Paramagnetic Resonance of the Phosphorescent States of Quinoline and Isoquinoline

Paramagnetic resonance measurements are reported in solid solutions of quinoline, and of isoquinoline in durene under irradiation with ultraviolet light at 77°K. The fine structure of the magnetic resonance spectrum observed at 9.2 Gc/sec may be described by the spin Hamiltonian $\mathcal{H}\mathrm{=\beta }\mathbf{H·g·S}{\mathrm{+DS}}_z^2{\mathrm{+E(S}}_x^2{\mathrm{-S}}_y^2)$ , with S = 1. D, E, and the principal values of g are reported for each molecule. The principal axis systems of the zero‐field tensors D are found to be displaced from the molecular axis system of durene in the host crystal in both the quinoline and isoquinoline doped crystals. The D axis systems are found to be rotated about an axis perpendicular to the molecular plane of durene. The principal axis systems of D and g are found to be the same within experimental error for the quinoline phosphorescent state. Hyperfine structure is observed in both molecules, and is assigned to coupling with protons in the 1, 4, 5, and 8 ring positions in quinoline, and in the 1, 4, 5, and 8 ring positions in isoquinoline. The normalized spin density at these positions is approximately equal to 0.23 in both molecules, assuming that the hyperfine coupling tensors for doublet‐state π radicals are applicable to triplet‐state molecules. The phosphorescent states of quinoline and isoquinoline are interpreted to result from π—π^* excitations. The decay constants of the isomers were found to be 0.80±0.02 and 0.95±0.1 sec, respectively. Preliminary measurements on the phosphorescent state of cinnoline are consistent with the interpretation of this as an n—π^* triplet state.