Paramagnetic resonance of phosphorescent tetramethylpyrazine

Electron paramagnetic resonance spectra have been obtained of tetramethylpyrazine in its lowest (ππ*) triplet state in several rigid matrices. Some unusual observations were made : 1. (1) In a durene crystal host the in-plane zero-field principal axes are not parallel to the molecular symmetry axes.2. (2) In rigid glass solution the zero-field parameters are strongly dependent on solvent polarity. Both effects are discussed in terms of pseudo Jahn-Teller interaction in conjunction with a crystal-field influence. In a durene matrix the zero-field parameters are The relatively small value of , as compared with that of the corresponding aromatic hydrocarbon, probably results from vibronic mixing of the lowest 3ππ* with the near-by 3nπ* state. Hyperfine structure was observed during measurements at 1·95 K ; it is in agreement with the ππ* character of the lowest triplet state and is due to coupling between the unpaired electrons and protons in the non-rotating methyl groups.