Deuterium effect on the pressure coefficient of the tunneling rate in the acridine-fluorene solid-state photoreactive system

We report the pressure effect on the intermolecular deuterium transfer tunneling rate in the acridine-doped fluorene crystal at 77, 150, and 200 K. Similar to the hydrogen transfer, the tunneling rate is exponentially enhanced by pressure. The pressure slope for this exponential enhancement, however, is found to be more temperature dependent for deuterium than for hydrogen tunneling. The ratio of the pressure coefficients for H and D stands at 2.6 at 77 K, gradually decreasing with increasing temperature until it becomes essentially unity at room temperature. An intuitive model based on the mass dependence of the tunneling distance is presented to rationalize these observations.