Electron Paramagnetic Resonance Absorption in Oriented Triphenylene in Its Phosphorescent State at Low Magnetic Fields

Abstract

Electron paramagnetic resonance absorption in oriented triphenylene in its lowest triplet state has been investigated in static magnetic fields below 50 G at 77°K. The orienting medium was melt‐grown single crystals of symmetric dodecahydrotriphenylene, the structure of which has been briefly investigated and compared with that of vapor‐grown crystals. The lowest triplet state of triphenylene in this host has been shown to possess lower than threefold symmetry. Taking the normal to the mean molecular plane of triphenylene to be the $C$ axis and the mean molecular plane to be the $\mathrm{AB}$ plane, the results have been described by the spin Hamiltonian $\mathcal{H}\mathrm{\hspace{0.17em}=\hspace{0.17em}g\hspace{0.17em}|\hspace{0.17em}\beta \hspace{0.17em}|\hspace{0.17em}}\mathrm{H}·\mathrm{S}\mathrm{\hspace{0.17em}+\hspace{0.17em}}\mathrm{S}\mathrm{·T·}\mathrm{S}$ , where $\text{S\hspace{0.17em}=\hspace{0.17em}1}$ , $\text{g\hspace{0.17em}=\hspace{0.17em}2.003}$ (isotropic), and, at 77°K, ${\mathrm{|\hspace{0.17em}T}}_{\mathrm{CC}}\text{\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}hc\hspace{0.17em}≡\hspace{0.17em}|\hspace{0.17em}D\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}hc\hspace{0.17em}=\hspace{0.17em}0.133526}{\mathrm{cm}}^{\text{−1}}$ , $\text{σ{|\hspace{0.17em}D\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}hc}\hspace{0.17em}=\hspace{0.17em}0.000048}{\mathrm{cm}}^{\text{−1}}$ , ${\mathrm{|\hspace{0.17em}T}}_{\mathrm{AA}}{\mathrm{\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}hc\hspace{0.17em}=\hspace{0.17em}|\hspace{0.17em}T}}_{\mathrm{BB}}\text{\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}hc\hspace{0.17em}≡\hspace{0.17em}|\hspace{0.17em}E\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}hc\hspace{0.17em}=\hspace{0.17em}0.000564}{\mathrm{cm}}^{\text{−1}}$ , $\text{σ{|\hspace{0.17em}E\hspace{0.17em}|\hspace{0.17em}/\hspace{0.17em}hc}\hspace{0.17em}=\hspace{0.17em}0.000048}{\mathrm{cm}}^{\text{−1}}$ . The triplet‐state lifetime and the angular dependence of the resonance absorption linewidths have also been studied at 77°K, and the temperature dependence of the zero‐field splitting parameters has been determined in the temperature range ∼ 67°–∼ 90°K. For this host, the departure of the symmetry of the triphenylene triplet state from threefold symmetry has been found to decrease with decreasing temperature, in contrast to the behavior previously reported for distorted triplet states of other canonically threefold symmetric molecules in various environments. Preliminary exploration of a photochemical reaction induced in the triphenylene–dodecahydrotriphenylene mixed crystal by broad‐band ultraviolet irradiation is described.