Abstract
We report optically detected magnetic resonance (ODMR) experiments at pressures up to 40 kbar for p‐benzoquinone (BQ) in dibromobenzene (DBB) and for a deep x‐trap in a BQ‐d4 neat crystal. It is known from previous works that the phosphorescent triplet state (3B1g) of these systems is vibronically coupled to a nearby 3Au state, such that the former exhibits a double‐minimum potential well. The zero‐field splitting (ZFS) parameter D of both systems changes by several gigahertz over our pressure range. The greatest slope occurs at zero pressure, and the rate of change slows down exponentially at higher pressure. In addition, we observed a phase transition in BQ‐d4 crystals at 6 kbar, which causes a huge step‐discontinuity in both D and E. In contrast to the enormous change in D, the E‐value is not sensitive to pressure except for the step‐change during the phase transition. Triplet state kinetics and phosphorescence spectra are used to establish the existence of an in‐plane distortion at least in the high‐pressure phase. A model is proposed to interpret these results based on the pressure‐dependent vibronic interaction, leading to a reduction of the spin–orbit coupling contribution to the ZFS.