Polyoxometalate-Zr-Organic Cage Hybrid Ionic Salts Exhibiting Rapid Photochromic Behavior

Two hybrid ionic salts, {[Cp3Zr3μ3-O(μ2-OH)3]2(C6H10O4)3}2[PMo12]·Cl·xSolvent (ZrAAD-PMo12) and {[Cp3Zr3μ3-O(μ2-OH)3]2(C6H10O4)3}2[SiMo12]·xSolvent (ZrAAD-SiMo12), were successfully synthesized via the liquid–liquid diffusion method. They were assembled from presynthesized adipic acid-directed capsular zirconium-based metal–organic cages (ZrAAD) and Keggin-type heteropolymolybdates ([PMo12O40]3– and [SiMo12O40]4–), respectively. Under visible light irradiation, the two hybrid salts exhibited fast and reversible photochromic behavior. Solid-state UV–vis spectra revealed new Mo5+ characteristic peaks at 740 nm (ZrAAD-PMo12) and 725 nm (ZrAAD-SiMo12) upon light exposure, corresponding to reduction of Mo6+. Additionally, the EPR results further confirmed the presence of Mo5+ ions after light irradiation. Furthermore, the plausible photochromic mechanism was proposed.