On the origin of long-lived emission in some charge-transfer crystals

Temperature dependence of emission spectra, decay times and intensity of emission, for molecular (1:1) crystals of charge-transfer (CT) complexes of tetrachloro- and tetrabromophthalic anhydride with penta- and hexamethylbenzene, have been investigated over a wide temperature range (1.7–300 K). A long-lived emission, observed in those crystals, has been identified as CT phosphorescence from CT triplet traps. No delayed emission, controlled by triplet—triplet annihilation (P-type), has been found. An explanation of observations connected with the fact that these complexes belong to the class where the lowest triplet state is of charge-transfer character is offered.