Supramolecular isomer-dependent photochromism and emission color tuning of bipyridinium salts

A study of the supramolecular isomer-dependent photoresponse and emission behaviors of a carboxybenzyl-substituted bipyridinium salt in the solid state is presented. Two supramolecular isomers (A1 and A2) are obtained by self-assembly with the same starting materials at different concentrations. The flexible conformation of the bipyridinium molecule results in different crystal packing geometries, which further influence and alter their photochromic behaviors owing to the crucial differences in the distance and orientation between the donor and acceptor moieties. Remarkably, the introduction of carboxyl groups into the bipyridinium molecule leads to intramolecular charge transfer (ICT) emissions in the visible region. A strong fluorescence emission can be detected in A1, and reversible on/off and multicolor (yellow, olivine and green) fluorescence switching, via photo-irradiation in the solid state, has been observed. Considering the easy chemical modification and the intriguing multicolor photoswitching properties, the present system shows great potential for use in solid photonic devices.