Living in the salt-cocrystal continuum: indecisive organic complexes with thermochromic behaviour

Charlotte L. Jones, Jonathan M. Skelton, Stephen C. Parker, Paul R. Raithby, Aron Walsh, Chick C. Wilson, Lynne H. Thomas

Research output: Contribution to journalArticlepeer-review

28 Citations (Scopus)


A family of multicomponent haloaniline/3,5-dinitrobenzoic acid molecular crystals with striking red-to-colourless temperature-induced thermochromism is identified and characterised. Four thermochromic pairs of 1 : 1 neutral cocrystals and ionic salts are identified which, unusually, grow concomitantly under the same conditions. The coloured cocrystals are found to be metastable, kinetically trapped during crystallisation, and convert via proton transfer to the more stable salt forms on heating. The colour of the neutral form and the temperature of the transition can be tuned through the halogen and by chemical substitution on the aniline component. From structural characterisation and first-principles modelling, we elucidate the origin of the metastability of the cocrystals and link structural changes through the phase transition to the striking visible colour change. By deliberately exploiting the uncertainty of the salt-cocrystal continuum, where the small pKa difference between components enables significant solid-state structural rearrangements induced by proton transfer, this work highlights a novel design paradigm for engineering new organic thermochromics with tailored physical properties.

Original languageEnglish
Pages (from-to)1626-1634
Number of pages9
Issue number10
Publication statusPublished - 2019

Bibliographical note

Publisher Copyright:
© The Royal Society of Chemistry.

All Science Journal Classification (ASJC) codes

  • General Chemistry
  • General Materials Science
  • Condensed Matter Physics


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