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Using microwave induced phosphorescence techniques we determined the rate constants for populating and decay of the lowest triplet state of tetramethylpyrazine (TMP) in a durene crystal. The dynamic behaviour of the spin states Ty, and Tz, corresponding to the in-plane spin axes (principal axes of the dipolar coupling tensor) y' and z' proves to be identical : ky, r = kz, r (radiative decay), ku,=kv (absolute decay rates) and Pu,=Pv (populating rates). This result supports the suggestion of de Groot et al., based on E.S.R. experiments, that for the phosphorescent state of TMP in the durene host the in-plane spin axes y' and z' have undergone an appreciable (~45 ~ rotation with respect to the in-plane molecular axes y and z.

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