title: A mechanophysical phase transition provides a dramatic example of colour polymorphism: the tribochromism of a substituted tri(methylene)tetrahydrofuran-2-one.
abstract: BACKGROUND
  Derivatives of fulgides have been shown to have interesting photochromic properties. We have synthesised a number of such derivatives and have found, in some cases, that crystals can be made to change colour on crushing, a phenomenon we have termed "tribochromism". We have studied a number of derivatives by X-ray crystallography, to see if the colour is linked to molecular structure or crystal packing, or both, and our structural results have been supported by calculation of molecular and lattice energies.
  
  
  RESULTS
  A number of 5-dicyanomethylene-4-diphenylmethylene-3-disubstitutedmethylene-tetrahydrofuran-2-one compounds have been prepared and structurally characterised. The compounds are obtained as yellow or dark red crystals, or, in one case, both. In two cases where yellow crystals were obtained, we found that crushing the crystals gave a deep red powder. Structure determinations, including those of the one compound which gave both coloured forms, depending on crystallisation conditions, showed that the yellow crystals contained molecules in which the structure comprised a folded conformation at the diphenylmethylene site, whilst the red crystals contained molecules in a twisted conformation at this site. Lattice energy and molecular conformation energies were calculated for all molecules, and showed that the conformational energy of the molecule in structure IIIa (yellow) is marginally higher, and the conformation thus less stable, than that of the molecule in structure IIIb (red). However, the van der Waals energy for crystal structure IIIa, is slightly stronger than that of structure IIIb - which may be viewed as a hint of a metastable packing preference for IIIa, overcome by the contribution of a more stabilising Coulomb energy to the overall more favourable lattice energy of structure IIIb.
  
  
  CONCLUSIONS
  Our studies have shown that the crystal colour is correlated with one of two molecular conformations which are different in energy, but that the less stable conformation can be stabilised by its host crystal lattice. Graphical abstractGraphical representation of the structural and colour change in the tribochromic compound (III).
authors.affiliation: Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah, 21589 Saudi Arabia.
authors.forename: Abdullah M
authors.initials: AM
authors.lastname: Asiri
authors.affiliation: School of Chemistry, Cardiff University, Cardiff, Wales CF10 3AT UK.
authors.forename: Harry G
authors.initials: HG
authors.lastname: Heller
authors.affiliation: Chemistry, Faculty of Natural and Life Sciences, University of Southampton, Southampton, SO17 1BJ UK.
authors.forename: David S
authors.initials: DS
authors.lastname: Hughes
authors.affiliation: Chemistry, Faculty of Natural and Life Sciences, University of Southampton, Southampton, SO17 1BJ UK ; Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, 21588 Saudi Arabia.
authors.forename: Michael B
authors.initials: MB
authors.lastname: Hursthouse
authors.affiliation: School of Life Sciences, University of Bradford, Richmond Road, Bradford, BD7 1DP UK.
authors.forename: John
authors.initials: J
authors.lastname: Kendrick
authors.affiliation: School of Life Sciences, University of Bradford, Richmond Road, Bradford, BD7 1DP UK.
authors.forename: Frank Jj
authors.initials: FJ
authors.lastname: Leusen
authors.affiliation: Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, Cittadella Universitaria, Monserrato, CA I-09042 Italy.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Montis
keywords: Colour polymorphism; Crystal structure analysis; Molecular and lattice energy calculations; Photochromism; Tribochromism