Integrated Supramolecular Systems: From Sensors to Switches

Integrated supramolecular systems with a receptor built in a photo- or electroactive unit have been reviewed with the focus on their particular electronic properties and different photochemical and electrochemical processes which make them suitable for cation sensing or switching. The fluoroionophores with an electron donating ionophore have been the most investigated and their initial weakness related to cation decoordination in the excited state. The small blue-shift of the fluorescence spectrum and the slight change of the emission quantum yield upon cation complexation, have now been overcome by a careful combination of several donor and acceptor units, which provide new low-lying excited states decoupled from the complexed ionophore and by using TICT probes where the electronic coupling between the D and A parts is too small to induce decoordination of the cation during the excited state lifetime. On the contrary the switching action requires that the binding ability of the ionophore be lowered or increased on a larger time scale. This has been done by electrochemical oxidation and by insertion of the ionophore into a photochromic system. Differences in binding ability of three to four orders of magnitude have been obtained and it is our belief that integrated supramolecular systems combining an ionophore and a photochromic moiety (photoionochromics) will be for cation switching as successfull as integrated fluoroionophores have been for sensing cations.