Tunable photo/thermochromic properties of Cd()-viologen coordination polymers modulated by coordination modes for flexible imaging films and anti-counterfeiting

Two photochromic Cd( ii )-CPs were obtained based on the viologen ligand using different synthetic routes, named {[Cd 4 ( p -BDC) 4 (CPB) 2 (H 2 O) 2 ]·2H 2 O·EtOH} n ( 1 ) and {[Cd( p -BDC)(CPB)(H 2 O)]·(L)·DMF} n ( 2 ) ( p -H 2 BDC = 1,4-benzene-dicarboxylate, HCPB·Cl = 1-(4-carboxyphenyl)-4,4′-bipyridinium·Cl, L = 2,4-dinitrochlorobenzene, and DMF = N , N -dimethylformamide), respectively. Due to different coordination modes, the two Cd( ii )-CPs show different structures. Compound 1 exhibits a three-dimensional (3D) framework with bimetallic nodes, while compound 2 displays a 2-fold interpenetrated (4,4) net topology. Notably, the two Cd( ii )-CPs exhibit substantial disparities in photo/thermochromism, which can be attributed to variations in donor-acceptor (D-A) distances arising from structural differences. Compound 1 showed visually sensitive photo- and thermochromic behavior due to multi-pathway electron transfer and short D-A distances, which is relatively rare in electron-transfer type photochromic systems. In contrast, 2 only demonstrates insensitive photochromic behavior, with a slight deepening of the color observed after 2 hours of UV light, which is due to the mono-pathway electron transfer and long D-A distance. Moreover, we first combined Cd( ii )-viologen CPs with polydimethylsiloxane (PDMS) to prepare a 1 @PDMS flexible UV imaging film. 1 @PDMS exhibits excellent bendability and stretchability and maintains good photochromic properties after 100 bending cycles. To demonstrate the rapid color response and distinct color contrast of 1 , its application in anti-counterfeiting is also demonstrated. Two Cd( ii )-viologen frameworks modulated by coordination modes with tunable photo/thermochromic properties for flexible imaging films and anti-counterfeiting.