Abstract
We report a zero‐field optically detected magnetic resonance study under high pressure for triplet pyrimidine in benzene, pyrazine in benzene and in p‐dichlorobenzene, and s‐tetramethylpyrazine in durene. Generally, the pressure sensitivity of the zero‐field splitting (ZFS) parameter D, ∂D/∂P, for these compounds is much higher than that for quinoxaline. This is rationalized in terms of a smaller π‐electron cloud in the monocyclics than in quinoxaline. For pyrazine and pyrimidine, the 3nπ* nature of the lowest triplet leads to a larger spin–orbit contribution to the pressure shift. We observed a larger change in the ZFS parameter E for pyrazine in benzene than in dichlorobenzene. This is explained by the difference in crystalline packing between the two host lattices. There is a large change in D and a multiplet splitting under high pressure for tetramethylpyrazine. These are ascribed to the presence of a pseudo‐Jahn–Teller interaction in this molecule.