Kinetics of the delayed luminescence decay of 1,2-benzanthracene in polystyrene films

Abstract

Phosphorescence, delayed fluorescence (DF) and delayed excimer fluorescence (DEF) have been observed from thin films of 1,2-benzanthracence in polystyrene at ambient temperatures. No prompt excimer fluorescence is observed. At long modulation times of the excitation pulses, the phosphorescence decay is exponential with an average decay time of 225 ms. Under similar conditions, the DF decay is also exponential but τ DF < 1 2 τ phos. The DEF lifetime approaches that of the phosphorescence as the modulation time is increased. The DF intensity is found to depend on the square of the excitation intensity but both DEF and phosphorescence intensities demonstrate a linear dependence. The intensity ratios I DF/ I phos and I DEF/ I phos depend approximately on the square of the solute concentration quite apart from the dependence which these ratios have on the intensity of absorbed light. It is proposed that both DF and DEF arise from the interaction of mobile triplet excitons with molecular pair species previously excited to the triplet level by energy transfer. For DF to occur, it is assumed that the pair species are simply any two solute molecules with intermolecular separation distances less than some critical value. For DEF to occur, the pair species must not only be close together but also must have the correct relative orientation.