Delayed luminescence of organic mixed crystals. Guest concentration effect in the p-terphenyl host-pyrene guest system

An effect of guest concentration on various prompt and delayed emissions from a p-terphenyl host-pyrene guest mixed crystal is studied. The presence of an adventitious defect site is ascertained. The experimental results suggest that both the defects and the guests are distributed in a random manner in the host lattice and that the free singlet excitons are trapped at monomeric and nascent excimeric pyrene sites with equal efficiency. The defect phosphorescence decay is nonexponential at low guest concentration but becomes exponential at higher concentrations. Second order analysis of the decays at various concentrations confirm that at low guest concentration pyrene delayed fluorescence originates from homofusion of defect triplet excitons whereas heterofusion of defect-guest triplet excitons predominates at higher concentrations