Triplet exciton dynamics in isotopically mixed naphthalene crystals are investigated on a molecular scale using single terrylene molecules as local probes. Upon triplet excitation of the matrix, the resonance frequencies of the probe molecules show reversible frequency excursions on the time scale of seconds. The analysis based on localized and diffusing triplet excitations indicates that the frequency fluctuations are induced by excitations migrating in an energy funnel of trap states in the vicinity of the probe.

• Received 27 August 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.2195