Using Colour to Control Conformation in a Chemical System Containing Multiple Tricyanofuran Photoacids

Colour vision relies on selective, reversible isomerisation by visible light of a mixture of retinyl chromophores in photoreceptor cells. Synthetic molecular mimics of this wavelength-dependent induction of function are rare, despite the attractiveness of controlling chemical processes solely by the wavelength of incident light. Here, we report a colour-responsive chemical system composed of a cationic receptor complex, two competing chiral anionic ligands and two metastable photoacids with contrasting absorption properties. Tricyanofuran photoacids were synthesised with absorption maxima of varying wavelengths across the whole visible spectrum. Protons released by the photoacids upon selective irradiation reversibly mask the more basic receptor-bound ligand, leading to ligand exchange that can be observed as a shift in the circular dichroism (CD) spectrum of the reporter complex. A ~90 nm separation between the absorbance maxima of the photoacids allowed each to be selectively photoisomerised in the presence of the other. The concentration of released protons, and hence the magnitude of the shift in CD response, were controlled by changing the wavelength of the incident visible light. Different output behaviours (OR/AND logic gates and wavelength detection) were programmed into the system by varying the relative proportions of the photoacids.