The Synthesis Of Azobenzene-based Ionic Liquids And The Study Of Their Photoresponsive Performance

Due to their unique properties, ionic liquids(ILs) are attracting extensive attention of scientists and have found numerous applications in industrial processes as a benign alternative to conventional volatile organic solvents in recent years. ILs are composed solely of ions with numerous combinations of anions and cations. The particular interest is their highly tunable properties by varying the chemical structures of the ions comprised. Thus, by rational design of cation and anion, ILs with desired characteristics can be created for a particular application. In recent years, although some photoresponsive ILs have been reported in the literatures, the type of the photoresponsive ILs and their systematic study are very limited. Therefore, further investigation is necessary for their physicochemical properties and light responsive performance. In this thesis, the photoresponsive feature of ILs has been improved by introducing azo phenyl group structure, which could be photoisomerized under UV irradiate, into anions of ILs. Thermal properties, photoresponsive performance and solution conductivity modulation of these ILs have been studied in some details. The main contents of the work are as follows.1. In this work, 12 photoresponsive azobenzene-based ILs were designed and synthesized by a simple anion exchange procedure. This procedure has the advantages of less reaction step, rapid reaction speed, improved yield of ILs, and contribution to environmental protection and the like. These azo-based ILs include 3-methyl-1-alkyl-3H-imidazolium4-Phenyl-azo-benzoate [CnH2n+1mim][PBA](n=8,10,12), 3-methyl-1-alkyl-3H-imidazolium 2-(4-Hydroxy-phenylazo)-benzoate [CnH2n+1mim] [HPBA](n=8,10,12), 3-methyl-1-alkyl-3H-imidazolium 3-(4-Dimethylamino-phenylazo)-benzoate [CnH2n+1mim][MMR](n=8,10,12), and 3-methyl-1-alkyl-3H-imidazolium 2-(4-Dimethylamino-phenylazo)-benzoate([CnH2n+1mim][MR])(n=8,10,12). These ILs were characterized by NMR, DSC and TGA techniques, and the factors affecting the melting point and thermal stability of them such as side chain of the cation and the chemical structure of anion were also investigated.2. The UV-vis absorption spectrum of these azo-based ILs in different molecular solvents were studied in detail.By comparison of the changes in the UV-vis absorption spectra before and after the ultraviolet irradiation, these ILs were found to have good photoresponsive characteristics. Factors affecting their maximum absorption wavelength such as alkyl side length of the cations, structure of the anions and properties of the solvent were examined. It was shown that when the illumination time was increased, the isomerization efficiency of these ILs was increased until the equilibrium of the light reaction. The reaction equilibrium time decreased in the order: [CnH2n+1mim][PBA] > [CnH2n+1mim][HPBA] > [CnH2n+1mim][MMR] > [CnH2n+1mim][MR]. Solvent properties had a great influence on the extent of light photoisomerization, and the reaction rate and the conversion efficiency in dimethyl sulfoxide were always highest. In addition, the reversible transformation from cis to trans structures was observed when heating the illuminated ILs.3. Conductivities of azo-based ILs in four kinds of organic slovents(DMSO, acetonitril, acetone and tetrahydrofuran) were determined at 25.0°C. It was found that conductivity of these ILs solutions decreased with the increase of carbon chain length of the cations. The type of the anions and the nature of the solvent had a great effect on the conductivity of the ILs solutions, conductivities of the ILs in DMSO decreased in the order: [CnH2n+1mim][MMR] > [CnH2n+1mim][MR] > [CnH2n+1mim] [HPBA] > [CnH2n+1mim] [PBA], and the order of the conductivity in different solvents was: acetone > acetonitrile > DMSO > THF. Photoresponsive conductivities for dilution solutions of the azo-based ILs in the organic solvents and in the water-MDSO mixtures had also been determined. The results showed that after 3h irradiation, the conductivity change of [C8H17mim][PBA] in DMSO, THF, acetone and acetonitrile were 385%, 177%, 7.9%, 1.5%, respectively. The corresponding conductivity change of [C8H17mim][HPBA] was, respectively, 239%, 51%, 6.9% and 4.4%, and their conductivity change decreased with the increase of water content in the mixed sovents.