Polymorphism Versus Thermochromism: Interrelation of Color and Conformation in Overcrowded Bistricyclic Aromatic Enes

The nature of the thermochromic form of overcrowded bistricyclic aromatic enes (BAEs) has been controversial for a century. We report the single‐crystal X‐ray structure analysis of the deep‐purple and yellow polymorphs of 9‐(2,7‐dimethyl‐9H‐fluoren‐9‐ylidene)‐9H‐xanthene (11), which revealed the molecules in a twisted and a folded conformation, respectively. Therefore, the deeply colored thermochromic form B of BAEs is identified as having a twisted conformation and the ambient‐temperature form A as having a folded conformation. This relationship between the color and the conformation is further supported by the X‐ray structures of the deep‐purple crystals of the twisted 9‐(9H‐fluoren‐9‐ylidene)‐9H‐xanthene (10), and of the yellow crystals of the folded 9‐(11H‐benzo[b]fluoren‐11‐ylidene)‐9H‐xanthene (12). Based on this conclusive crystallographic evidence, eleven previously proposed rationales of thermochromism in BAEs are refuted. In the twisted structures, the tricyclic moieties are nearly planar and the central double bond is elongated to 1.40 Å and twisted by 42°. In the folded structures, the xanthylidene moieties are folded by 45° and the fluorenylidene moieties by 18–20°. Factors stabilizing the twisted and folded conformations are discussed, including twisting of formal single or double bonds, intramolecular overcrowding, and the significance of a dipolar aromatic “xanthenylium‐fluorenide” push–pull structure. The structure of the thermochromic ambient‐ and high‐temperature forms of the yellow and purple crystals of fluorenylidenexanthenes were elucidated by X‐ray structure analysis. The controversial, deeply colored thermochromic form of bistricyclic aromatic enes (BAEs) has finally been identified as having a twisted conformation, whereas the ambient‐temperature form has a folded conformation.