Theoretical study on the alkyl chain length impact of azobenzene-based photoresponsive ionic liquids

The light-modulated isomerization and aggregation behavior of ionic liquids (ILs) in aqueous solutions holds fundamental and technological significance. Although several azobenzene-based photoresponsive ILs have been synthesized, there is still a lack of understanding regarding the aggregation mechanism, regularity of the alkyl chain length, and the position of the azobenzene (cis- and trans-) in these photoresponsive ILs. To elucidate the structure-property relationship of photoresponsive ILs, four types of azobenzene groups photosensitive ILs ([AzoC_nDMEA]Br, n = 2,4,6,10) in both trans- and cis- configurations were investigated by density functional theory (DFT) calculations. We investigated the geometric properties of cations, H-bonds interactions of ionic pairs, microstructures of clusters, and the interactions between ILs and water molecules. It was found that the molecular volume of cis- is smaller than that of trans- cation structures. Despite multiple H-bonds between the anions and the ammonium group of cations, longer alkyl chains weaken anion-cation interactions. The interaction energies of trans- n[AzoC₂DMEA]Br (1 ≤ n ≤ 4) clusters are stronger than those of cis-. Moreover, the interaction energy between trans-structures of photoresponsive ILs and water molecules is smaller than that of cis- structures based on the DFT calculations. The interaction energies per water molecule in the ILs-water clusters tend to saturation as the number of water molecules increases. The electrostatic interaction plays a crucial role in the stabilization of ILs and water systems. The structure-property relationship of photoresponsive ILs including the regularity of the alkyl chain length and the azobenzene position as well as the microscopic interaction mechanism of ILs and ILs-water clusters had been studied from theoretical calculation perspective. This work can contribute to an in-depth understanding of the microcosmic interactions of azobenzene-based photoresponsive ILs and aid in designing them in a "task-specific" way.