Delayed fluorescence in a pyrene-doped naphthalene crystal: homofusion and heterofusion of triplet excitons

Prompt and delayed emissions from a naphthalene crystal doped with pyrene have been investigated in the temperature range 77-300 K. It is shown that delayed fluorescence (DF) in this crystal originates from heterofusion and homofusion of two different types of trap triplets at low temperature (T<130 K) and from homofusion of pyrene triplets at high temperature (T>200 K). Pyrene concentration effects on DF suggest that free triplet excitons are trapped more efficiently at monomeric pyrene sites than at nascent excimeric pyrene sites