Anion−π Interaction-Directed Assembly of Polyoxometalate-Based Host–Guest Compounds and Its Contribution to Photochromism

Two rare copper-based host–guest crystalline compounds were synthesized with different photoresponsive reversible visible light photochromism as a result of blending of distinctively different functional components, naphthalenediimides (NDIs) tectons and polyoxometalates. A slight adjustment of the substituent of the diimide nitrogens, from rigid to semirigid ligands, will result in different modes of anion−π interactions, which eventually lead to different structures and different sensitivities to visible light. The unique anion−π interactions impart significant impacts on the structures of the compounds and, to some extent, the ability to gain or lose electrons of the NDIs of the compounds, which is evident from subtle changes in photochromic sensitivity and reduction potentials; if compounds bear more and stronger anion−π interactions, this might dramatically enhance the compounds’ stability and diminish the π-electron-deficient property of the core naphthalene ring of the NDIs.