Decay of the triplet state I. First-order processes in solution

The decay of the trip let states of naphthalene, 2-bromonaphthalene, anthracene, 1,5-dichloroanthracene and 9-bromophenanthrene have been studied in isopentane solution over the temperature range 22 to —196 °C and in propylene glycol solution from 54 to —80 °C, corresponding to a variation in viscosity by a factor of 108. An experimental arrangement for flash photolysis investigations at low temperatures is described. When allowance is made for discontinuities in the viscosity-temperature relations all our measurements are accounted for in terms of two regimes of trip let decay: a high viscosity regime where rate is nearly independent of temperature and viscosity and the mechanism is truly unimolecular and a low viscosity regime w here diffusion-controlled quenching processes are operative.