Exploring the polymorphism of crystalline pentacene

Starting from each of the five complete X-ray structures published so far for crystalline pentacene, we have computed the structures of minimum potential energy, and obtained two local minima of the potential energy, i.e., two different "inherent structures" of mechanical equilibrium. This behavior, which has been found to be independent of the details of the potential model, indicates that there are at least two different single crystal polymorphs of pentacene. One of the two polymorphs corresponds to the structure originally determined by Campbell et al..[Acta Cryst. 14 (1961) 705]. The other polymorph corresponds to the structure found in all more recent measurements. The calculations predict significant differences between the corresponding Raman spectra of the lattice phonons, which we have checked experimentally, confirming the existence of two polymorphs. The correct identity of the samples, initially assigned only by matching experimental and calculated spectra, has been verified directly with X-ray diffraction measurements. Finally, we have obtained theoretical information on the global stability of the minima by systematically sampling the potential surface of crystalline pentacene, and found that the two polymorphs correspond to the two deepest minima. Further deep minima with layered structures, which might correspond to the thin film polymorphs found to grow on substrates, are also predicted.