Investigation of the appropriateness of sensitized luminescence to determine exciton motion parameters in pure molecular crystals.

Abstract

A wealth of experimental data has been collected over the years regarding the sensitized luminescence of molecular crystals. Indeed, such observations have played a primary role in attempts to characterize the dynamical aspects of exciton transport in these materials. Nonetheless, as has been noted previously, serious questions of interpretation remain concerning the relationship between primary experimental observables and microscopic parameters which govern exciton transport and capture. In the past these questions have led to an uncertainty, not always acknowledged, in the values of exciton diffusion constants reported in the literature. On the other hand, careful analysis of some recent experiments suggests that this uncertainty is much larger than is commonly assumed, and calls into question the utility of sensitized luminescence experiments as a quantitative probe of exciton transport. In this paper we carefully review these questions and show how they arise in a quantitative analysis of time-dependent sensitized luminescence in naphthalene, anthracene, and 1,2,4,5-tetrachlorobenzene crystals. This work, in conjunction with similar analyses of exciton-annihilation and transient-grating experiments, leads us to conclude that sensitized luminescence in its present form is useful as a probe of the capture process but not of exciton motion in pure crystals unless it is aided by independent additional experiments. We propose and discuss such experiments which might assist in disentangling effects arising from the capture process from those associated with exciton motion.