Exciton dynamics probed by single molecule spectroscopy

Triplet exciton dynamics in isotopically mixed naphthalene crystals are inv estigated on a molecular scale using single terrylene molecules as local pr obes. Upon triplet excitation of the matrix, the resonance frequencies of t he probe molecules show reversible frequency excursions on the time scale o f seconds. The analysis based on localized and diffusing triplet excitation s indicates that the frequency fluctuations are induced by excitations migr ating in an energy funnel of trap states in the vicinity of the probe.