Triplet exciton lifetime under pressure in "pure anthracene"

The triplet exciton lifetime in "pure" anthracene crystals is found to decrease by as much as a factor of 3 at room temperature under pressures up to 6.4 kbar and in a reversible fashion. Temperature measurement under pressure from 200 to 400 K show that the effect is due to a state ∼0.3 eV below the exciton band. Experiments with tetracene doped anthracene revealed that trapping by tetracene is increased by a factor of 2 over the same pressure range, indicative of enhanced diffusion. A comparison with the "pure" anthracene effect reveals that pressure enhanced diffusion is insufficient for explaining this effect; the cross section for capture by the 0.3 eV state must change by more than a factor of 4 in going from 1 atm to 6.4 kbar. A triplet excimer is invoked for explaining our results, consistent with earlier work. The pressure dependence of the mean capture cross section for this species is determined.