MIGRATION AND ANNIHILATION OF TRIPLET EXCITATION-ENERGY IN THE CHARGE-TRANSFER ANTHRACENE-TETRACHLOROPHTHALIC ANHYDRIDE CRYSTAL

The emission spectra of the charge-transfer crystal of anthracene-tetrachlorophtalic anhydride were studied in the 1. 7-300 K temperature range. Delayed fluorescence was observed over the whole temperature range. At temperatures below approximately 40 K it results from heterogeneous (mobile exciton-trapped exciton) triplet-triplet annihilation. At higher temperatures it is due to homogeneous triplet-triplet annihilation. The existence of mobile excitons is also proved by the character of the ESR lines which are very narrow Lorentzians. The phosphorescence, observed only in the range 1. 7-40 K, originates from at least two different kinds of traps with energies approximately 6 cm** minus **1 and approximately 140 cm** minus **1, respectively. The vibrational structure of the phosphorescence is identical to that of anthracene, with 0-0 band blue-shifted by approximately 400 cm** minus **1. A kinetic model of trapping and detrapping of triplet excitons as well as the nature of the traps are discussed.