Migration of singlet and triplet excitons in charge-transfer crystals

Single crystals of some charge-transfer (CT) complexes were studied by optical techniques, within the temperature range 4.2-300 K, to gather information relevant to the dimensionality and self-traping of singlet and triplet excitons. Mobile singlet excitons were observed above 100 K when the self-trapping energy was overcome. Activation energy of the mobility of triplet excitons was found to be dependent on their CT character. This dependence confirms the self-trapping origin of the low temperature traps. 1D hopping model describes motion of triplet excitons whereas 3D model referres to migration of singlet excitons. Mobility of singlet and triplet excitons is considerably slowed down in the TCNB-DMN crystal, most probably because of orientational disorder of the donor molecules which leads to a dispersive-like character of exciton motion.