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.
Bibliographical noteFunding Information:
We gratefully acknowledge support from the UK Engineering and Physical Sciences Research Council (EPSRC) (grant no. EP/K004956/1 and EP/P007821/1). JMS is grateful to the University of Manchester for the support of a Presidential Fellowship. Calculations were performed on the UK National Archer HPC facility, accessed through membership of the UK Materials Chemistry Consortium and funded by the EPSRC (EP/ L000202), and on the SiSu supercomputer at the IT Center for Science (CSC), Finland, via the Partnership for Advanced Computing in Europe (PRACE) project 13DECI0317/IsoSwitch.
© The Royal Society of Chemistry.