Decoherence of excitons in multichromophore systems:Thermal line broadening and destruction of superradiant emission

We study the temperature-dependent dephasing rate of excitons in chains of chromophores, accounting for scattering on static disorder as well as acoustic phonons in the host matrix. From this we find a power-law temperature dependence of the absorption linewidth, in excellent quantitative agreement with experiments on dye aggregates. We also propose a relation between the linewidth and the exciton coherence length imposed by the phonons. The results indicate that the much debated steep rise of the fluorescence lifetime of pseudoisocyanine aggregates above 40 K results from the fact that this coherence length drops below the localization length imposed by static disorder.