Multi‐stimuli‐responsive chromic switching self‐assembled with polyoxometalate and copper‐viologen frameworks

Abstract Chromic materials with multi‐stimuli responses have received tremendous attention from industry and academia for their potential applications in smart windows, data storage, military camouflage, molecular switches, modulators, etc. To develop multi‐stimuli‐responsive color‐changing materials with high sensitivity and high stability, a polyoxometalate‐based copper‐viologen framework, formulated as Cu 2 (H 2 O) 2 (L) 2 [PW 12 O 40 ][OH − ]·2H 2 O ( HNU‐97, H 2 L 2+ = 1,1′‐(1,4‐phenylenebis(methylene))bis(4‐carboxypyridinium)), was prepared by a mild one‐step self‐assembly strategy. Structural analysis shows that HNU‐97 presents a supramolecular structure formed by a one‐dimensional copper‐viologen framework and polyoxometalates through rich hydrogen bonds, anion‐π interactions, etc. Discoloration studies show that HNU‐97 exhibits reversible and rapid response characteristics to various external stimuli, such as 10 s to UV radiation, 5 min to heating, and 5 min to ammonium/amine vapors. This mechanism reveals that different external excitation sources lead to different preferential transfer directions of electrons, resulting in different discoloration phenomena. To construct stimuli‐chromic materials with good mechanical properties, we successfully introduced HNU‐97 into the poly(vinyl alcohol) (PVA)/glycerol (Gly) hydrogel to obtain a composite HNU‐97@PVA/Gly . Remarkably, HNU‐97@PVA/Gly combines the merits of each component and can achieve significant response discoloration behavior under both UV irradiation and ammonia atmosphere. This work will provide an idea for the construction of multiple stimuli‐responsive materials.