Four Chromic Compounds Constructed from Keggin Polyoxometalates and Viologens with Applications in Ultraviolet, Metal Ion Detection, and Ink-Free Printing

A combination of polyoxometalates and viologens to construct discoloration materials with charge transfer has become attractive presently. In this work, four kinds of POM–viologen color-changing materials were successfully synthesized, namely, [Cu(1,4-Mbby)2(HPMoVI10MoV2O40)] (1), [H2(1,4-Mbby)2]·(SiMo12O40) (2, 1,4-Mbby·Cl = (1-(4-methyl-benzyl)-[4,4']bipyridinyl-1-ium)chloridate), [H2(Bypy)2]·(SiMo12O40) (3), and [H(Bypy)]·(Bypy-2)1/2·(PMo12O40) (4, Bypy = 1-benzyl-4,4′-bipyridyinium chloride, Bypy-2 = 1,1′-dibenzyl-4,4′-bipyridine chloride). In compound 1, a [Cu(1,4-Mbby)2]4+ subunit caps on the Keggin anion to build a monosupporting cluster. Compounds 2–4 are discrete structures with dissociative POMs and viologens. All compounds 1–4 show good reversible photochromic properties under xenon lamp (300–400 nm) irradiation. Under the stimulation of current, compounds 1–4 also show reversible electrochromic properties within a certain range of applied voltage. The good color-changing properties of these compounds are due to the generation of free viologen radicals and charge transfer. Based on their color-changing properties, they can be applied to many fields, such as ultraviolet detectors, inkless printing, and visual detection of heavy metal ions. These practical applications are of positive significance for life and production.