GUEST-IMPURITY HETEROGENEOUS TRIPLET-TRIPLET ANNIHILATION IN ORGANIC MIXED CRYSTALS - BENZ[F]INDAN GUEST IN BIPHENYL AND IN FLUORENE HOSTS

Delayed fluorescence has been observed in mixed crystals with benz[f]indan guest in biphenyl host and also in fluorene host. At 77°K, the delayed fluorescence intensity shows a second-order dependence on the phosphorescence intensity, indicating the biexcitonic origin of the delayed fluorescence observed. Temperature dependence of intensity and of decay and growth behavior of delayed fluorescence and phosphorescence is similar to that generally observed in mixed-crystal systems where T1G<T1H< S1G<S1H- The thermal energy gap obtained from various Arrhenius plots, however, do not give the spectroscopic energy gap in either case; instead a lower activation energy has been obtained. Evidence is presented that an unknown impurity causes the delayed fluorescence in these cases. Kinetics for such guest-impurity heterogeneous triplet-triplet annihilation is discussed.