Voltammetric Recognition of Cis (Z) and Trans (E) Isomers of Azobenzene and Azocrown Ethers

Abstract

An electroanalytical approach allowing the recognition of isomers of azo compounds and the determination of their ratio in the solution is introduced. Adsorptive preaccumulation of the title compounds at the electrode surface retains on the electrode the ratio of isomers present in the solution. Voltammetric reduction of the adsorbed species allows recognition of the cis (Z) and trans (E) forms of the azo-crowns because their reduction potentials in alkaline solutions are different. In the region below pH 10 fast isomerization follows the first electron transfer, hence both forms appear to be reduced at the same potential. At pH>10 interactions of the large crown radical anion with alkali metal cations slow down the isomerization reaction at the electrode surface and a separate reduction peak for each isomer is seen on the voltammogram. The adsorptive voltammetric method provides a simple and useful way of monitoring the progress of isomerization reactions in the solution. Comparison of azobenzenes, azodibenzo- and azotribenzo-crown ethers revealed the largest stability of the latter against Z-to-E isomerization in aqueous solutions.

Key Words:

• Monolayers
• crown ethers
• adsorptive stripping voltammetry