Synthesis of arylhydrazone-based molecular switches using aryldiazonium silica sulfate nanocomposites and analysis of their isomerization

Abstract A fast and efficient method for the synthesis of a series of arylhydrazones by reacting aryldiazonium silica sulfate nanocomposites with malononitrile, ethyl acetoacetate and dimedone is reported. All reactions were carried out in water at room temperature and the corresponding products were obtained in 77–87% yields. The existence of two kinds of intramolecular hydrogen bonds in the arylhydrazones synthesized using ethyl acetoacetate enables these compounds to be switched by rotation about the hydrazone C N bond, which leads to a reversible isomerization between their E and Z configurations. This switching can be controlled by changing the polarity of the solvents. The E/Z ratio of each synthesized compound was studied in CHCl3 and DMSO. The ratios of the E/Z were calculated using 1H NMR data in both CDCl3 and DMSO‑d6, and used to calculate ΔG° for the E-Z isomerization of each synthesized compound in these two solvents. By changing the solvent from CHCl3 to DMSO, the E/Z ratios decreased. The results demonstrated that the ΔG° values for the formation of Z isomers were more negative than those of the E isomers in DMSO. This is why Z isomers are more stable than E isomers in DMSO. The results of density functional theory (DFT) calculations at B3LYP/6–311++G (d,p) level of theory, were in agreement with the experiments and confirmed the increased stability of Z isomers in DMSO. In most cases, DFT calculation in CDCl3 and DMSO indicate that the dipole moments of the Z isomers are significantly higher than those of the E isomers. Finally, the effect of temperature on the E-Z isomerization was studied using dynamic 1H NMR. The findings demonstrated that temperature does not have any significant effect on the E-Z isomerization in CDCl3 and DMSO‑d6.