TRIPLET EXCIMER FORMATION OF TRIPHENYLAMINE AND RELATED CHROMOPHORES IN POLYSTYRENE FILMS

The phosphorescence of triphenylamine (TPA) and of the (N,N-diphenylamino)phenylmethyl ester of 2-methylpropanoic acid (DAPM) was investigated in polystyrene films at temperatures from 20 to 230 K. Their phosphorescence spectra at low temperatures are similar to those recorded in rigid glassy solvents at 77 K, but as the temperature is raised, a new phosphorescence band at 470 nm grows in and the low-temperature band gradually disappears. The new emission is due to an excimeric triplet since it is similar to one found earlier from the polymeric analogue of DAPM and is also similar to the totally excimeric phosphorescence of crystalline TPA at 77 K. Rates of excimer formation and dissociation were determined as a function of temperature as were the corresponding activation energies. Triplet exciton migration is not the rate-limiting step in excimer formation. The activation energies for dissociation of the triplet excimers were found to depend upon the concentration of the amine. For DAPM the evidence suggests that excimers formed in dilute samples are in a strained configuration which relaxes slowly compared to the excimer lifetime.