Coherent one-dimensional exciton transport and impurity scattering

A model is developed which describes the transport of coherent one-dimensional excitons in molecular crystals containing a low concentration impurity having excited state energy above that of the host crystal’s exciton band. An expression is obtained for the exciton mean-square displacement and the exciton flux. The model is then employed to determine the effect of impurity scattering on an experimental observable, the time dependence of trap emission after impulse optical excitation of a one-dimensional crystal. The resulting formula is used to calculate, without adjustable parameters, the time-dependent intensity of phosphorescent trap emission from crystals of 1,2,4,5-tetrachlorobenzene (TCB) containing the naturally occurring scattering impurity, hd-TCB, at 1.25 °K. The results are compared to experiment and found to be in good agreement. Although the question remains open, the results are consistent with a coherent mode of triplet exciton transport in TCB and indicate the importance of naturally occurring isotopic impurity scattering in this type of system.