Paramagnetic Resonance Measurements of the Phosphorescent State of Quinoxaline

Paramagnetic resonance absorption has been observed in a single-crystal solid solution of quinoxaline in durene when irradiated at 77°K with uv light from an AH-6 mercury-arc source. The fine structure of the resonance spectrum observed at 9.8 Gc/sec may be described by the spin Hamiltonian H=BH·g·S+DSz2+E(Sx2−Sy2),in which S = 1. D/hc=±0.1007±0.0003 cm−1E/hc=∓0.0182±0.0002 cm−1 gxx=2.0047±0.0005 gyy=2.0035±0.0005gzz=2.0019±0.0005.The fine structure is practically identical to that of the naphthalene triplet state observed by Hutchison and Mangum. The observed hyperfine structure is assigned to a coupling with the 5, 8 protons, and the two N14 nuclei. The angular dependence is in qualitative agreement with the tensor for C–H proton hyperfine interaction for doublet-state aromatic radicals measured by Cole, Heller, and McConnell, as well as the tensor for aromatic N14 interaction derived from experiments by Weissman. With the assumption that hyperfine interaction in a triplet state may be described by the same tensors, normalized π-electron spin densities are approximately 0.28 for the 5, 8 carbon atoms, and 0.1 for the N14 atoms. Satellite lines which are split from the primary hyperfine lines by the proton resonance frequency are observed and interpreted as simultaneous electron-spin nuclear-spin transitions. The interactions leading to these transitions are assumed to be primarily due to hyperfine coupling with protons whose unresolved first-order interaction is much smaller than the proton-resonance frequency.