Triplet exciton caging in two dimensions

It is proposed that the bimolecular process of triplet exciton fusion to form singlet excitons can be enhanced by reducing the size of the domain in which the triplet exciton pair is free to move. These small domains, or exciton cages, are much more effective when the host material is highly anisotropic, and the triplet excitons are constrained to move in one or two dimensions. In the present case, the host material is tetracene crystal in which triplet exciton diffusion is essentially two dimensional. The exciton caging is produced by introducing relatively high concentrations of a guest molecule that can reflect the triplet exciton rather than trap it; in the present case, 2,3-benzocarbazole (BC) is used. Polycrystalline mixtures of BC in tetracene were prepared in the mol fraction range 0%-50%. In tetracene, the mode of decay of the first excited singlet state is by fission into two neighboring triplet excitons that can undergo geminate recombination. The introduction of BC is found to increase the tetracene fluorescence lifetime at room temperature from 100 psec at 0% BC to 360 psec at 50% BC, while the lifetime at 77°K is relatively constant. In addition, the characteristic increase in the tetracene fluorescence quantum efficiency with increasing excitation intensity is found to diminish as the BC concentration is increased. The lifetime and the relative fluorescence efficiency experiments are interpreted in part by invoking the concept of exciton caging, in which the geminate recombination of the original triplet exciton fission pair is enhanced by the presence of exciton reflectors that reduce the size of the domain in which the excitons can freely move. A computer simulation is made of the fission and fusion process in an ideal mixed crystal containing randomly distributed exciton reflectors, and the response of the model agrees well with the observed results.