HIGH-PRESSURE STUDIES OF A HYDROGEN-TRANSFER PHOTOREACTION IN A CRYSTALLINE SOLID - ACRIDINE FLUORENE

We report the effect of pressure on the photoinitiated hydrogen transf er reaction in a mix crystal of acridine in fluorene. Upon excitation to its triplet state, the aza-atom of acridine abstracts a hydrogen at om from the methylene bridge of fluorene. The reaction rate constant f or both the hydro- and deutero- (methylenic) species has been measured as a function of pressure. For both species, the room temperature tra nsfer rate increases exponentially with increasing pressure. Furthermo re, 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 smal ler slope. In contrast, the D-transfer rate remains too small to affec t 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 r egime is observed at low temperature. This limiting low temperature ra te increases with pressure and reflects the tunneling rate from the vi brational ground state. At ambient pressure it was too small to be acc essible to measurements. High pressure allows the direct determination of this ground state tunneling rate. The implications of our observat ions in the tunneling mechanism are discussed.