The Cyclic Conversion Of PTAP On Ag Electrode Probed By SERS In Ionic Liquids

With the assistance of surface plasmon resonance(SPR), para-aminothiophenol(PATP) can be dimerized into 4, 4′-Dimercaptoazobenzene(DMAB). Reversely, the catalysate can also be reduced to PATP under the electrochemical control. The cyclec conversion was remarkably influenced by the interfacial environment, for example the crucial dependence on interfacial water. As a distinct ionic solvent, room temperature ionic liquids(RTILs) have provided a novel system for electrochemical study. Investigating the structures and states of water in the RTILs bulk and interface was the key to understand the properties of RTIL/H2 O mixtures and predict the reactivity of chemical process involving RTILs. The combination of high sensitive surface enhanced Raman spectroscopy(SERS) and electrochemical methods provided a powerful tool for probing the interfacial cyclic conversion of PATP. The main results in this paper are outlined as follows:1. The dependence on solvent of the conversion between PATP and DMAB was explored. By researching the conversion of PATP on a Ag electrode in different protonic(H2O, CH3OH) and aprotic(CH3CN, [BMIm]BF4) solvents, the essential of proton source in the electrochemical reduction of DMAB has been confirmed. In aprotic system, the conversion of DMAB to PATP on an electrode was depended on the presence of interfacial water.2. By means of model system of DMAB’s electrochemical reduction, the water states in [BMIm]BF4 with different water content were discussed. It was indicated that the trace water in [BMIm]BF4(xw = 0.01) mainly interacted with RTIL through hydrogen bonding. The water molecules were embedded in the bulk of RTIL so that cannot reach the reaction interface. When the water content was increased(xw ≥ 0.1), water molecules in the mixtures gradually self-aggregated into clusters to form continuous network, and stably exist in the interface.3. The influence of hydrophobicity of RTILs on the structure of concomitant water was studied. The electrochemical reduction degree of DMAB in [BMIm]BF4, [BMIm]PF6, and [BMIm]Tf2N with the same water content demonstrated that water molecules were more inclined to form clusters in hydrophobic RTILs, and the hydrophobic interfaces were more sensitive to the additional water.4. Based on the coadsorption of PATP and H2 O, the structure of interfacial water was directly investigated by SERS. The further analysis on the influence of electrode potential and water content manifested that: the intensity of the bands rested with PATP rather than the interfacial water content; the bands suffered first red-shift to a later blue-shift as the potential negatively moved, demonstrating that the configuration of H2 O was affected by the applied potential and the adsorbed methods of RTILs. The band frequency would decrease with the higher water content, indicating the formation of more hydrogen-bond between water molecules.