Spectral diffusion of individual pentacene molecules in P-terphenyl crystal: Stochastic theoretical model and analysis of experimental data

We propose a microscopic theoretical model to explain recent experiments involving the spectral diffusion of individual pentacene molecules embedded in p-terphenyl crystal. The experimental spectral diffusion trajectories are analyzed in terms of three stochastic characterizations: The time autocorrelation of transition frequency fluctuations, the time-dependent distribution of spectral jumps, and the equilibrium distribution of frequencies. The observed spectral diffusion is well described by our model, and we determine detailed quantitative information about the localized excitations that are responsible for the fluctuations in the pentacene transition frequency. We find that the spectral diffusion of the pentacene transition is caused by the reorientation of the central phenyl ring in the p-terphenyl molecule, and that this reorientation can only occur at a wall between domains of different central phenyl ring ordering. Furthermore, we find that only those pentacene molecules that reside within a few lattice spacings of these domain walls exhibit spectral diffusion.