Synthesis and characterisation of azobenzene-bridged cationic–cationic and neutral–cationic electrochromic materials

• Azobenzene-bridged bipyridinium and ester-pyridinium materials are synthesized.
• Azobenzene-bridged bipyridinium materials exhibit reversible electrochemical properties.
• Azobenzene-bridged diester materials display the best electrochromic properties.
• Pyridine-containing materials show longer response time than ester-containing materials.

Azobenzene-bridged cationic–neutral and cationic–cationic were synthesized, characterized and compared with azobenzene-bridged neutral–neutral materials. Azobenzene-bridged cationic–cationic and neutral–neutral materials exhibited well-defined and reversible redox couples during redox processes, good electrochromic behavior, and appropriate contrast ratios in the visible regions. However, azobenzene-bridged cationic–neutral materials exhibited a quasi-reversible electrochemical process. The electrochromic stability of these compounds is in the order: azobenzene-bridged neutral–neutral materials > azobenzene-bridged cationic–cationic materials > azobenzene-bridged cationic–neutral materials. Electrochromic devices based on azobenzene-bridged neutral–neutral materials achieved maximum optical contrast, fast response time, and high coloration efficiency. The substituents of azobenzene-bridged cationic–cationic materials and the anions of azobenzene-bridged cationic–neutral materials did not significantly influence their electrochemical properties and response time, but slightly affected the maximum optical contrast, change in optical density, and coloration efficiency.

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