Structure and Photochromic Properties of a Histidine Salt of 12-Silicotungstate Supramolecular Compound

We synthesized a novel histidine salt of 12-silicotungstate supramolecular compound ((HisH(2))(2) SiW12O40 center dot 6H(2)O) by a solution method. Its composition, thermostability, and structure were characterized by elemental analysis, thermal gravimetric and differential thermal analysis (TG-DTA), and single crystal X-ray diffraction, respectively. The experimental results show that the supramolecular compound has a composition of C12H34N6O50SiW12 and it is stable under 135 degrees C in air. It belongs to a monoclinic system (space group C2/c) with a=2.44005(18) nm, b=1.29788(10) nm, c=1.86898(14) nm, beta=124.0380(10)degrees, V=4.9048(6) nm(3), Z=4, and D-c=4.465 g . cm(-3). The final statistics based on F-2 are goodness-of-fit (GOF) = 1.268, R-1=0.0344 and wR(2)=0.0851 for (I>2 sigma(I)). X-ray diffraction analysis further reveals that this supramolecular compound mainly consists of polyoxometalate anions [SiW12O40](4-), protonated histidine cations [HisH(2)](2+) and lattice water. A three-dimensional (3D) network is formed through hydrogen bonding. The resulting sample shows photochromism under UV irradiation. We propose a possible photochromic mechanism by an analysis of the chromic sample's electron spin resonance (ESR) spectrum.