Stimuli-Responsive Viologen-Based Coordination Supramolecular Isomers Modulated by π-Stacking Interactions

Supramolecular isomerism provides a useful platform for correlating molecular packing with physicochemical properties, yet packing-induced isomers remain uncommon because coordination modes and intermolecular interactions are difficult to control. Herein, regulation of intermolecular π-stacking between monodentate, highly conjugated PhSQ ligands affords a pair of viologen-based copper(II) isomers, CuCl2(PhSQ)2 (1 and 2; PhSQ = N-(4-sulfophenyl)─4,4′-bipyridinium). Single-crystal X-ray diffraction reveals that 1 forms a supramolecular organic framework through π-stacking interactions, whereas 2 adopts a densely packed architecture composed of A-B-style supramolecular chains. Isomer 1 exhibits pronounced photochromic and thermochromic responses, while 2 shows no discernible color change under comparable stimuli. Isomer 1 also displays higher structural stability, stronger light and thermal responsiveness, significantly enhanced absorbance, and a photothermal conversion efficiency of 31.58%, compared with 25.03% for 2. These contrasting properties are attributed to differences in the types and strengths of intermolecular weak interactions, especially the more abundant π-stacking interactions in 1, highlighting the importance of packing modes in the rational design of stimuli-responsive supramolecular materials.