Effects of impurity scattering and transport topology on exciton migration and trapping: An experimental study of quasi-one-dimensional molecular crystals

A series of experiments in which the time resolved triplet x-trap emission from single crystals of 1,2,4,5-tetrachlorobenzene (TCB) at 1.35°K is presented for various concentrations of the doped-in scattering impurity, d ₂-TCB. It is demonstrated that exciton-impurity scattering is the dominant process affecting macroscopic exciton transport and trapping. The time-dependent trapping rate is found to be proportional to the inverse square root of the scattering impurity concentration in agreement with theoretical prediction. This implies that transport is close to strictly one-dimensional. Excellent agreement between the data and a model involving microscopically incoherent transport is found, but the data also shows generally good agreement with a model employing microscopically coherent transport. From the concentration dependence and time-dependent trapping curves, an upper bound of ∼5×10³ sec^-1 can be placed on the frequency of multidimensional steps between one-dimensional chains. Transport is macroscopically diffusive. The basic parameters characterizing long range exciton migration and trapping are obtained.