Tunable photo/thermochromic properties of Cd(II)-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)(H2O)(2)]<middle dot>2H(2)O<middle dot>EtOH}(n) (1) and {[Cd(p-BDC)(CPB)(H2O)]<middle dot>(L)<middle dot>DMF}(n) (2) (p-H2BDC = 1,4-benzene-dicarboxylate, HCPB<middle dot>Cl = 1-(4-carboxyphenyl)-4,4 '-bipyridinium<middle dot>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.