Delayed emission from an organic mixed crystal: Decay and rise characteristics of phosphorescence from a biphenyl guest-carbazole host mixed crystal

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Abstract
The decay and buildup characteristics of phosphorescence of biphenyl guest-carbazole host mixed crystal have been investigated in the temperature range 15–110°K. The phosphorescence decay and rise are nearly exponential in the temperature range investigated. It has been found that the phosphorescence rise time of biphenyl guest is less than its decay time. This difference between rise and decay time in the ``real'' mixed crystal arises from heterogeneous triplet-triplet annihilation involving a guest triplet and a defect triplet. The delayed fluorescence of annihilative origin has not, however, been observed owing to a very small fraction of the triplets involved in annihilation. The temperature dependence of the guest phosphorescence lifetime is an intermolecular process and is controlled by the thermally excited process T1D → T*1D → T1H → T1G. A kinetic scheme for guest-impurity heterogeneous triplet-triplet annihilation is discussed. The thermal energy gap obtained from various Arrhenius plots relevant to the kinetic scheme is in good agreement with the spectroscopic energy difference T1H − T1D.