A Reversible Redox Reaction in a Keggin Polyoxometalate Crystal Driven by Visible Light: A Programmable Solid‐State Photochromic Switch

The colourless crystals of (PPh 4 ) 3 [PW 12 O 40 ]⋅3 C 3 H 7 NO ( 1) are converted to the dark blue crystals of {(PPh 4 ) 3 [PW 12 O 40 ]⋅3C 3 H 7 NO} 0.85 {(PPh 4 ) 3 (C 3 H 7 NO) +. [PW V W VI 11 O 40 ] − ⋅2C 3 H 7 NO)} 0.15 ( 2 ) upon irradiation with visible light in an interesting single crystal to single crystal transformation. This photochromic conversion is accompanied by the reduction of concerned Keggin anion from {PW VI 12 } to {PW V W VI 11 }. This redox conversion is characterized by various spectroscopic techniques including single crystal X‐ray diffraction studies. The photochromic properties of compound 1 can be controlled reversibly through the dimethylformamide (DMF) molecule as a function of temperature and proton exposure in a gas‐solid reaction. The present work can be described as a new concept of programmable photochromism with the formation of photochromic pockets in crystalline 1 host (solid state), wherein a solvent can be plugged at a time to show light induced coloration.