Abstract:

We synthesized a 4,4'-bipyridine polyoxomolybdate ((4,4'-bipyridine)Mo7O22·H2O) single crystal supramolecular compound by a hydrothermal method and characterized its composition, thermostability, structure, spectra and electron properties by elemental analysis, thermal gravimetric and differential thermal analysis (TG-DTA), powder X-ray diffraction (XRD), single crystal X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), and electron spin resonance (ESR). Experimental results show that the molecular formula of the supramolecular compound is C10H12Mo7N2O23 and it does not decompose below 320 ℃ in air. It belongs to the monoclinic system(space group P2/n) with a=1.22561(19) nm, b=0.55222(9) nm, c=1.8385(3) nm, β=103.221(2)°, V=1.2113(3) nm3, Z=2 and Dc=3.289 g·cm-3. The final statistics based on F2 are GOF=0.982, R1=0.0228 and wR2=0.0557 for I >2σ(I). The single crystal supramolecular compound consists of protonated 4,4'-bipyridine cations and polyoxomolybdate [Mo7O22]2- anions as well as lattice water. They form a two dimensional (2D) network through hydrogen bonds, electrostatic attraction, and intermolecular forces. The samples show photochromism under UV irradiation and thermochromism under annealing conditions. XRD and FTIR verify that before and after the color changes, the crystal structure of the samples is almost unchanged except for distortion. ESR spectra indicate that a difference exists between the thermochromic and photochromic mechanisms. This supramolecular compound with thermochromic and photochromic behaviors can serve as a reference model for a chromic mechanism and may have potential application in fields such as sensors and photosensitive materials.

Key words: Supramolecular compound, Polyoxometalate, Photochromism, Thermochromism, Bipyridine