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The electronic structure of neutral and singly charged conjugated molecular clusters is investigated by means of quantum-chemical calculations. We first assess the influence of interchain interactions on the nature of the singly charged species (polarons) in organic conjugated polymers. In a two- chain model aggregate, the polaron is found to be delocalized over the two conjugated chains for short interchain separation. Such a delocalization strongly affects the geometric and electronic relaxation phenomena induced by charge injection, which in turn lead to a dramatic spectral redistribution of the linear absorption cross section. We then consider pentacene clusters built from the experimental crystal structure and compute the HOMO and LUMO bandwidths, which are decisive parameters for charge transport in the limiting case of band-like motion (i.e., complete delocalization of the excess charge over a large number of interacting molecules). Very large bandwidths are obtained, in agreement with the remarkable electron and hole charge-carrier mobilities reported recently for ultrahigh purity pentacene single crystals.

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