Concomitant conformational dimorphism in 1,2-bis(9-anthryl)acetylene†
Previous literature reports have shown that single crystals of 1,2-bis(9-anthryl)acetylene (1), in which the molecule is essentially planar, can be fabricated as organic field effect transistors. We have identified a new conformational polymorph of 1 in which the two anthracene moieties are twisted out of plane by more than 60°. Occasionally, both the planar (1α) and twisted (1β) polymorphic forms crystallize out together representing a rare example of concomitant conformational polymorphism in a pure hydrocarbon. The two polymorphic forms also show distinctly different colours and morphologies – the planar form (1α) grows as long orange-red needles whereas the twisted form (1β) appears as yellow blocks. Furthermore, a variety of electronic structure-based calculations show that the planar molecule forms the more stable polymorph (1α), despite the conformation being close to the maximum in conformational energy. This may be attributed to 1α being stabilized by greater intermolecular dispersion forces as the planar conformation allows a denser packing. The new polymorph (1β) is a long-lived metastable structure, because of the marked difference in crystal packing. Since the conformation in 1β is more stable in isolation, and probably in solution state, this discovery raises questions about the control of crystallization of anthracene-based functional materials and the assumptions about conformational polymoprhism commonly made in crystal structure prediction studies.