Supramolecular control of photochromism in a β-cyclodextrin/Schiff base system

The inclusion complex of a salicylaldehyde Schiff base (anil) in β-cyclodextrin (βCD) is reported for the first time. UV-vis spectroscopic and fluorescence studies show that N-(1-adamantyl)salicylaldimine (ASA), which is thermochromic as a free crystalline compound, becomes photochromic upon encapsulation into βCD. Characterisation of the complex has been performed by NMR spectroscopy in aqueous solution and by UV and fluorescence spectroscopy and powder X-ray diffraction in the crystalline state. The NMR studies indicate host:guest ratio of 1:1 and prove that only the adamantyl moiety of the guest is enclosed into the host cavity, whereas the hydroxyphenyl part is outside. Trials to grow single crystals of the complex produced a novel βCD lattice, which can be rationalized by applying the inclusion mode suggested by NMR spectroscopy. βCD in the crystal lattice seems to act as a medium allowing the cis to trans isomerisation of ASA, a volume-demanding process necessary for the expression of photochromism in this class of molecules. The study demonstrates that βCD nanocavities can lead to supramolecular control of the photochemical behaviour of appropriately designed anils, a property with many potential applications.