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

Abstract

The colourless crystals of (PPh ) [PW O ]⋅3 C H NO (1) are converted to the dark blue crystals of {(PPh ) [PW O ]⋅3C H NO} {(PPh ) (C H NO) [PW W O ] ⋅2C H NO)} (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 } to {PW W }. 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.

Solvent mediated reversible solid state photochromism of {Mo72Fe30} Keplerate

“…[39][40][41][42] In addition, we have also recently reported the solvent driven programmable photochromic behaviour of POMs. 43 Our study thoroughly investigates the visible light induced photoactivity of an ionic Keggin complex ((PPh 4 ) 3 [PW 12 O 40 ]•3DMF) as a function of the DMF solvent held within its crystal lattice unlike previous examples of OAC-POM and OSC-POM. The ability to control the photoactivity of the complex by the removal and the subsequent incorporation of the DMF solvent within its crystal lattice makes the concept/model of solvent driven solid state photochromism highly flexible and unique.…”

A Two-Dimensional Metal-Organic-Framework Formed From a Cobalt(II) Ion and a Bifunctional Ligand Exhibiting Thermochromic Behavior

“…Generally, the chromism is associated with the reversible color change of the materials in response to the transformations occur in the corresponding materials. Depending on the external stimuli, which causes the color change in these materials, such as, light, temperature, pressure, and solvent molecules, they are categorized as photochromic (Pardo et al, 2011;Tandekar et al, 2018), thermochromic (Lim et al, 2018;Liu and Li, 2019), piezochromic , and solvatochromic materials (Lu et al, 2011;Mehlana et al, 2013), respectively. The investigation of the underlying mechanism for such sort of chromic behavior reveals that the color change is associated to the one of the following reasons: (i) charge transfer (CT)/electron transfer between the organic ligands [such as, 4,4-bipyridinium (Toma et al, 2015;Zhang et al, 2016), vialogen based ligands (Wan et al, 2015;Hu et al, 2017)] and the metal which changes their absorption properties, (ii) disruption of the interaction between the solvent molecules and the compounds which leads to the alterations in the crystal packing/supramolecular interactions (such as, hydrogen bonding, and π···π interactions), and (iii) coordination geometry transformations (Kundu et al, 2014;Burneo et al, 2015;Thapa et al, 2018) due to the loss of metal bound solvent molecules/rearrangement of the coordinating atoms around the metal centers.…”

Crystal Structure and Reversible Photochromism of Pb(II)‐N,N‐Dimethylformamide Modified Keggin‐Type Polyoxometalates

“…It is worth mentioning here that POMs, as inorganic materials, have also been applied in the study of solid‐state photochromic materials . POMs usually form photochromic systems by virtue of existence of organic cations . Two common POM‐based photochromic materials usually contain organoammonium (OA) and organosulfonium (OS) cations .…”

“…Interestingly, the coloration mechanism of these two kinds of materials is different. Yamase's team confirmed that the photochromism of OA‐POM is based on intermolecular electron transfer, which is related to proton transfer from OA to POM anion . For example, when compound [NH 3 Pr i ] 6 [Mo 8 O 26 (OH) 2 ]·2H 2 O is irradiated by UV light, the electron on terminal oxygen atom (O d ) of [Mo 8 O 26 (OH) 2 ] 6− is transferred to Mo(VI) site and forms Mo(V) species, while the H atom of [NH 3 Pr i ] 6+ is transferred to the μ 3 bridging oxygen atom (O c ) site in [Mo 8 O 26 (OH) 2 ] 6− ; thus the compound of [NH 3 Pr i ] 6 [Mo 8 O 26 (OH) 2 ]·2H 2 O is changed to its coloring state .…”

“…For example, when compound [NH 3 Pr i ] 6 [Mo 8 O 26 (OH) 2 ]·2H 2 O is irradiated by UV light, the electron on terminal oxygen atom (O d ) of [Mo 8 O 26 (OH) 2 ] 6− is transferred to Mo(VI) site and forms Mo(V) species, while the H atom of [NH 3 Pr i ] 6+ is transferred to the μ 3 bridging oxygen atom (O c ) site in [Mo 8 O 26 (OH) 2 ] 6− ; thus the compound of [NH 3 Pr i ] 6 [Mo 8 O 26 (OH) 2 ]·2H 2 O is changed to its coloring state . Unlike OA‐POM, the photochromism of OS‐POM does not involve the proton transfer process . Dessapt and Mialane found that under ultraviolet irradiation, the electron on O d of POM is transferred to Mo atom.…”