Turning colour on and off using molecular disorder and proton transfer in multi-component molecular complexes

Six molecular complexes resulting from co-crystallisation experiments of 4-iodoaniline or the related 4-iodo-2-methylaniline with dinitrobenzoic acids are reported. The individual components are not coloured or very pale in colour, but when molecular complexes are formed combining the two, vividly red crystals are obtained when layered crystal structures prevail. Where proton transfer results in completely ionic co-components, colourless crystals are formed. Molecular disorder is also found to favour layered, coloured crystals. In the pair of thermochromic molecular complexes comprised of 4-iodo-2-methylaniline with 3,5-dinitrobenzoic acid, proton transfer from one molecule to the other is shown to result in a colour change from red to colourless.