Two Multistimuli-Responsive POM/Viologen Hybrids: From Inkless Printing to Smart Hydrogels and Electronic Skin

In this paper, two Anderson-type polyoxometalate/viologen-based hybrids were hydrothermally synthesized, {Zn[H(1,4-pmbpy)]₂(TeMo₆O₂₄)}[H₂(1,4-pmbpy)](TeMo₆O₂₄)·3H₂O (1) and {Co[H(1,4-pmbpy)]₂(TeMo₆O₂₄)}[H₂(1,4-pmbpy)](TeMo₆O₂₄)·4H₂O (2). Both compounds exhibited rapid, multistimuli-responsive color changes. 1 and 2 showed rapid photochromism under a xenon lamp irradiation, changing from yellow to green within 3 and 5 min, respectively. Meanwhile, compound 2 showed reversible thermochromism. Mechanistic studies (EPR, XPS) reveal that the color change originates from electron transfer from POM oxygen donors to viologen bipyridine nitrogen acceptors, generating viologen radicals. Both compounds selectively detect NH₃ and ethylenediamine by size effects. Integration into polyacrylamide/sodium alginate (PAAm/SA) hydrogels yielded gradient photochromism, tunable by crystal concentration, irradiation time, and intensity, along with ultrafast response (2 s), high toughness, and strain-sensitive conductivity. The hydrogel precisely monitors human motions (finger and wrist bending) and facial expressions, demonstrating promise for electronic skin. Moreover, compound 1-based coated filter paper has been successfully utilized in erasable, inkless printing. This synergistic strategy advances stimuli-responsive materials for optical and flexible electronics.