The dynamics of one-dimensional excitons in liquids

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Abstract
The properties of excitons in one‐dimensional molecular aggregates, dissolved at room temperature in a liquid, were studied by means of femtosecond nonlinear optical experiments. Both the one‐exciton band (i.e., Frenkel‐excitons) and multiexciton bands contribute to the observed nonlinear optical response. The rapid motions in the liquid lead to ultrafast perturbations of the molecular energy levels. This localizes the excitons on limited sections of the chains of aggregated molecules. Ultrafast frequency‐resolved pump–probe spectroscopy on the lowest two exciton bands was employed to determine the delocalization length of the optical excitations. The kinetics of the exciton populations was measured by ultrafast grating scattering experiments and time‐resolved single photon counting. A model is described in which the multiexciton bands act as doorway states in the exciton–exciton annihilation process. These bands thereby determine the population decay of the Frenkel excitons at high excitation densities. Room temperature photon echo experiments show that stochastic perturbations of the exciton transition frequencies occur on two distinct time scales. In particular the slow components of the fluctuations are affected by motional narrowing, associated with the exciton delocalization length. It is therefore argued that the optical dephasing of excitons is directly related to the spatial extent of the excitation on the aggregate chain.