Multi-component crystallisation routes to tuneable optical properties

The application of multi-component crystallisation as a route to tuneable and switchable colour properties is investigated. Molecular complexes of haloanilines with nitro- and dinitro-substituted benzoic acid derivatives are presented, which enable determination of the crucial structural properties required for colour in these systems, in addition to demonstrating possible approaches to achieving tuneable and temperature-dependent colour. The findings offer a framework for future design and development of this class of functional materials. Two possible mechanisms for thermochromism have been established in the series of molecular complexes. The first mechanism is disorder-facilitated structural rearrangement; molecular disorder prevails in many of the molecular complexes, due to competition between donor and acceptor sites, and inherently results in an inefficient crystal packing in the structures. In some of the molecular complexes, the increased crystal space surrounding the molecules facilitates a structural rearrangement, which is accompanied by a change in colour, and allows the phase transition to occur in a single-crystal to single-crystal manner. The thermochromic transition temperature is tuneable through modification of the haloaniline molecule, including the reversibility of the transition. The second thermochromic mechanism is proton transfer, between the haloaniline and dinitrobenzoic acid components in metastable systems; the phase transition occurs with a distinct colour change but does not occur in a single-crystal to single-crystal manner. Similarly to the disorder-facilitated thermochromism, the transition temperature is tuneable through exchange of the halogen atom. One molecular complex combines the two thermochromic mechanisms, with the crystal space generated through disorder allowing proton transfer to occur with preservation of the single crystal. Proton transfer in the series of molecular complexes is key to the generation of both colour and switchable colour, with the transfer of a proton acting as a colour switch; a salt-cocrystal continuum region has been established for the series, based on ΔpKa values. The colour in the solid-state is induced through formation of a mixed stack arrangement of aromatic electron-donor and electron-acceptor molecules, which themselves are colourless or pale-coloured. Where at least one of the molecular components is in their neutral form, colour prevails, ranging between red and yellow; a number of possible stoichiometries and stacking types of the donor and acceptor have been observed. Tuneable colour has been achieved through simple modifications of the molecular components, through exchange of the halogen substituent or introduction of a methyl group, for example.