Polyoxometalate–viologen hybrids are reviewed, including structural design, competing electron transfer process, and new knowledge on photochromic mechanisms and their applications.

Herein, a multifunctional photochromic two-dimensional metal–organic framework (PMOF), [Zn(bcbpy)_0.5(pma)_0.5(H₂O)]·3H₂O (1), was synthesized (H₂bcbpy·2Cl = 1,1′-bis(3-carboxylatobenzyl)-(4,4′-bipyridinium) dichloride, H₄pma = pyromellitic acid). It showed an efficient ultraviolet light detection ability with an obvious color change from colorless to blue and fast response to an ultraviolet intensity of as low as 0.001 mW cm⁻² in the narrow-band UV region. Furthermore, based on the Lewis acidic site and redox ability of the bipyridinium ligand, compound 1 showed amine-selective sensing upon contact with different amine vapors (NH₃, ethylamine, n-propylamine, n-butylamine, diethylamine and triethylamine) and was also deposited on paper for use as portable test strips. The finely powdered sample could also be deposited on paper by coating it with a solution of ethanol, and the paper was used as an inkless and erasable print medium, which could be reprinted at least five times.

Increasing interest in chromic materials is due to the growing demand. However, most chromic materials exhibit color changes in response to only one stimulus, but there are multiple stimuli in nature. Therefore, the construction of multistimuli responsive chromic materials still faces great challenges. Herein, a new multi-stimuli responsive coordination polymer [Zn₂(2,3-PDC)₂CV·(H₂O)₂]·H₂O (1) (2,3-PDC = 2,3-pyridine dicarboxylic acid, CV = N,N′-4,4′-bipyridiniodipropionate) has been successfully synthesized, which exhibits photochromism under 300 W xenon lamp irradiation accompanied by an obvious color change from colorless to light blue. Meanwhile, compound 1 displays excellent thermochromic properties with a color change from colorless to light yellow when heated at 106 °C. The product of thermochromism is named 1T and the loss of free water improves the photoresponsive properties of 1T. Moreover, the compound can show differentiable detection of amines because of the electron-deficient nature of the viologen. Finally, 1 shows excellent electrochromic properties and turns from colorless to purple at E = −3 V. In conclusion, compound 1 exhibits multi-chromic behaviors in response to light, heat, amines and electricity, which are prominent in viologen based coordination polymers.

The framework exhibits photochromism and high sensitivity of amines and Cr₂O₇²⁻, and it also can be used as test strips.

The “reversible” photochromism of polyoxomolybdate–viologen hybrids can be achieved without proton transfer, and the weak electron-withdrawing ability of viologen favors photoreduction of Mo(vi).