Structural traps for singlet excitons in doped anthracene single crystals and their photochemical significance

Abstract

The fluorescence emission from anthracene crystals doped with both acridine and carbazole has been investigated and compared with that emanating from “pure” crystals and crystals containing structural imperfections. The spectra of carbazole-doped crystals consist of the anthracene excitonic emission and a “defect” spectrum with a progression based on a broad origin at ∼275 cm^–1. The intensity of this defect emission increases with increasing carbazole concentration and decreases with increasing temperature. Quite different behaviour is observed with acridine-doped crystals in that at no concentration is the intrinsic anthracene emission observed. In such crystals the emission is structured with a broad origin, which varies both in half width and position with acridine concentration, corresponding to defect anthracene emission. In both cases traps for the singlet exciton arise as a result of the introduction of the smallest quantity of the impurities into anthracene, and may be considered as perturbed anthracene molecules at internal surfaces where segregation of the impurity occurs. The observation that segregation of the impurity occurs at concentrations of acridine and carbazole in anthracene less than 100 p.p.m. is consistent with recent autoradiographic studies. The traps introduced by structural imperfections, viz dislocations, and by these chemical impurities are similar.

The presence of a second impurity in carbazole and 9-cyanoanthracene doped crystals (2-hydroxyanthracene) at low concentrations may be identified from characteristic progressions based on two origins (24 356 cm^–1 and 24 166 cm^–1). Spectral changes accompanying photodimerisation indicate that both sites participate in the reaction to form a mixed dimer and that this competes with the formation of di-p-anthracene from anthracene molecules located at defects.