Abstract
Polyoxometalates (POMs) are important inorganic photochromic materials to be potentially applied in photo-induced switch, energy storage, and even the detection of light. However, due to the limited sensitivity of POMs, it is difficult to realize the photochromic response to weak visible light. In this paper, by the coordination of solvated Pb(II), a new structure-defined chain-like polyoxomolybdate complex of [(Pb(DMF)4)3(P2Mo18O62)2]n (Pb3Mo18, DMF = dimethylformamide) has been demonstrated by a facile solvent-diffusion approach. By virtue of interactions between Pb(DMF)4 and polyoxoanions, Pb3Mo18 shows an ultrasensitive photochromic response to weak visible lights and forms the reduced 'heteropoly blue' species through ligand-to-metal charge transfer (LMCT) process. A new mechanism is firstly proposed here that the 6s orbital lone electron pair on Pb(II) can effectively stabilize the generated hole of oxygen atoms as a result of O→Mo charge transfer. Through the proposed mechanism, the LMCT barrier is drastically lowered and allows the coloration to be occurred even upon weak visible light. Also, because the conductivity of Pb3Mo18 enhances with the increase of reduction extent, its electrochemical impedance signals are proportionally response to irradiation intensity. Especially, for the first time, the polyoxomolybdate composite can be used to detect weak visible light, in which the optical signal can be converted into electrical signal output. Moreover, Pb3Mo18 can be drip-coated on the surface of the screen printed chip electrode, which is facile to the detection of light by portable devices compatible with computers, mobile phones and other electronic equipment. This work not only highlights a new approach to the molecular design of photochromic POMs by the coordination of metal ions with the effect of inert electron pair, but also lays a foundation to extend the application of POMs as light signal sensors.
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