Electronic interactions and thermal disorder in molecular crystals containing cofacial pentacene units

We computed the intermolecular electronic coupling and the band structure of three pentacene derivatives that stack cofacially in one or two dimensions. We rationalize the results building a map of the coupling between HOMOs and LUMOs of isolated pentacene molecules as a function of the relative molecular orientation and finding the position on such map of the actual molecular pairs. The apparently chaotic dependence of the intermolecular coupling from the crystal structure is explained, and directions for the design of materials with improved electric transport properties are given. We run molecular dynamics simulations to explore the variation of the intermolecular coupling due to the thermal motions. It is shown that, even in the crystal phase, the limited conformational space explored at room temperature is large enough to produce consistent variation in the intermolecular coupling. The consequences of this finding on the practicability of the transport properties of organic materials are analyzed.