Lineshapes of molecular aggregates, exchange narrowing and intersite correlation

The influence of intersite correlation of inhomogeneities on the lineshape of exciton spectra is investigated. Starting point is a perturbation treatment, valid for strong intermolecular coupling, which is applied to one-dimensional aggregates. Analytical expressions for the linewidth are acquired for cyclic aggregates and compared with values from linear aggregates. The linewidth increases with increasing intersite correlation and exhibits an N−1/2 dependence for long aggregates with N molecular units. For intermediate intermolecular coupling the N-value derived from the present model relates to a coherence length of the exciton.