The dynamics of triplet excitons in naphthalene crystals are investigated using single terrylene molecules as local probes. Triplet excitations in the host matrix are generated by Xe-lamp illumination. The triplet excitations induce shifts in the resonance frequencies of the individual probe molecules. These excitation-induced frequency shifts fluctuate and relax on a time scale of seconds subsequent to the matrix excitation. For the description of the frequency shifts two models are applied. One is based on localized excitations and is similar to the sudden-jump model. The other is based on mobile excitations that are harvested by an energy funnel of trap states in the vicinity of the probe molecule. The analysis indicates that the frequency fluctuations result from triplet excitations migrating within the funnel.
