Homofusion and heterofusion of triplet excitons in a ‘‘real’’ organic mixed crystal: Biphenyl host-naphthalene and pyrene guests system

The prompt and delayed emission spectra of a biphenyl host with naphthalene and pyrene both as guests mixed crystal have been studied. The fluorescence of naphthalene and pyrene and the excimer fluorescence of pyrene have been observed. The delayed emissions are observed from pyrene and naphthalene as delayed fluorescence of pyrene, phosphorescence of naphthalene, and pyrene. The investigations of excitation spectra of these emissions have shown to be very useful to understand the mechanism of delayed fluorescence generation. The heterogeneous defect-naphthalene and homogeneous naphthalene–naphthalene triplet–triplet annihilation are responsible for the production of delayed fluorescence of pyrene. The kinetic analyses from the decays and buildups have been made. The delayed luminescence decays at various temperatures have found to be nonexponential. However, in the long time region they are nearly exponential. Even in the long time region, the pyrene delayed fluorescence decays and buildups are nonexponential in the temperature range above 130 K. It has been established from the various Arrhenius plots that in this mixed crystal the temperature dependence of the phosphorescence lifetime is controlled by the thermal processes The kinetic scheme described satisfactorily explains the experimental results.