Spectral Diffusion and Triplet Exciton Localization in an Organic Glass

Phosphorescence spectra of glassy 2‐bromonaphthalene films prepared by vapor deposition at 4.2 K are recorded under site‐selective excitation. The Stockes shift between absorption and emission origins gradually vanishes as the excitation energy is scanned across the inhomogeneouslybroadened profile of the T 1 ← S 0 OO transitionindicating a continuous decrease of the exciton diffusivity with exciton energy. The effect is rationalized in terms of stochastic hopping transport across a Gaussian density of states. A Monte Carlo computer simulation provides the basis for quantitative data interpretation. A criterion for excitonimmobilization is established.