Abstract
We report the effect of pressure on the photoinitiated hydrogen transfer reaction in a mixed crystal of acridine in fluorene. Upon excitation to its triplet state, the aza‐atom of acridine abstracts a hydrogen atom from the methylene bridge of fluorene. The reaction rate constant for both the hydro‐ and deutero‐ (methylenic) species has been measured as a function of pressure. For both species, the room temperature transfer rate increases exponentially with increasing pressure. Furthermore, the rate of change is the same for the two isotopic species. At 77 K, the H‐transfer rate again increases exponentially, but with a smaller slope. In contrast, the D‐transfer rate remains too small to affect the precursor triplet lifetime which hardly changes over a pressure range of 36 kbar. The H‐transfer rate was also measured as a function of temperature for a few higher pressures. A temperature independent regime is observed at low temperature. This limiting low temperature rate increases with pressure and reflects the tunneling rate from the vibrational ground state. At ambient pressure it was too small to be accessible to measurements. High pressure allows the direct determination of this ground state tunneling rate. The implications of our observations in the tunneling mechanism are discussed.