Electron Paramagnetic Resonance Studies of Photoselected Triplet Molecules. I. Phenoxazine

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Abstract
Paramagnetic resonance of the phosphorescent triplet state was carried out for phenoxazine molecules in rigid glasses at 77°K. The line shapes, in Δm=2 and Δm=1 regions, depend upon the incident light polarization. With complementary results from mixed crystals of diphenylphenoxazine, it was possible to relate the principal‐axis system of the tensor describing the zero‐field splitting to the molecular‐axis system. The zero‐field energy levels of the triplet state are of the same order of magnitude as those of aromatic molecules. Selective excitation of the families of molecules corresponding to stationary values of the resonance field showed that: (1) the resonant triplet state is mainly populated via one singlet—singlet transition S1←S0, (2) the dipolar electric transition moment of this excitation is approximately parallel to the nitrogen—oxygen axis of the phenoxazine molecule. Different aspects of photoselection of magnetoselected phosphorescent molecules are discussed.