A Multichromic Cd(II) Coordination Polymer Based on a Cyanovilogen Ligand: Photoluminescence, Inkless and Erasable Printing, and Anti-Counterfeiting QR

A new multichromic Cd(II) coordination polymer (CP), LLU-201, was synthesized solvothermally using CdBr2·4H2O, biphenyl-3,3',5,5'-tetracarboxylic acid (H4BTA), and the cyanoviologen ligand 1,1'-bis(3-cyanobenzyl)-[4,4'-bipyridine]-1,1'-dibromo (BCNBPY·2Br). Structural analysis reveals a three-dimensional framework. LLU-201 undergoes rapid, reversible photochromism and thermochromism while retaining its crystalline structure. Under 365 nm irradiation, bright yellow crystals turn dark blue within 5 s, while heating from 90 to150°C produces a green state. UVvis spectroscopy, EPR measurements, and DFT calculations indicate that both transformations arise from electron transfer from the carboxylate oxygen atoms of H4BTA to the cyanoviologen acceptors, generating viologen radicals. LLU-201 also exhibits irradiation-dependent luminescence quenching. Its fast, stable, and reversible photoresponse enables inkless printing that self-erases after approximately 7.5 h in darkness and QR codes activated by brief UV exposure. These properties highlight the potential of LLU-201 for rewritable information storage and optical anti-counterfeiting applications.