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Polymorphism Versus Thermochromism: Interrelation of Color and Conformation in Overcrowded Bistricyclic Aromatic Enes
Author(s) -
Biedermann P. Ulrich,
Stezowski John J.,
Agranat Israel
Publication year - 2006
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200501118
Subject(s) - xanthene , thermochromism , intramolecular force , crystallography , chemistry , crystal structure , molecule , diacetylene , stereochemistry , photochemistry , organic chemistry , polymerization , polymer
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‐9 H ‐fluoren‐9‐ylidene)‐9 H ‐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‐(9 H ‐fluoren‐9‐ylidene)‐9 H ‐xanthene ( 10 ), and of the yellow crystals of the folded 9‐(11 H ‐benzo[ b ]fluoren‐11‐ylidene)‐9 H ‐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.