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 $$\mathcal{H}\mathrm{=B}\mathbf{H}·\mathbf{g}·\mathbf{S}{\mathrm{+DS}}_z^2{\mathrm{+E(S}}_x^2{\mathrm{-S}}_y^2\mathrm{),}$$ in which S = 1. $$\begin{array}{cc}\text{D/hc=±0.1007±0.0003\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}& \text{E/hc=∓0.0182±0.0002\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}\\ {\mathrm{\hspace{0.17em}\hspace{0.17em}g}}_{\mathrm{xx}}\text{=2.0047±0.0005}& {\mathrm{\hspace{0.17em}\hspace{0.17em}g}}_{\mathrm{yy}}\text{=2.0035±0.0005}\end{array}$$ $$g_{\mathrm{zz}}\text{=2.0019±0.0005.}$$ 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.