D–π–A type ferrocene-substituted azobenzene photochromic switches: synthesis, structures, and electrochemical and photoisomerization studies

A series of D–π–A type azobenzenes with a ferrocenyl/ferrocenylethynyl electron donor at the para/meta-position have been synthesized and characterized by electrochemistry and UV-vis spectroscopy. Compared to the D–π type meta-ferrocenyl/ferrocenylethynyl azobenzenes, the photoisomerization properties of D–π–A type meta-ferrocenyl/ferrocenylethynyl azobenzene are significantly improved. The photoisomerization properties of D–π–A type meta-ferrocenyl azobenzene are significantly different from those of D–π–A type para-ferrocenyl azobenzene, due to the electronic communication influenced by the substitution position of the redox center on the azobenzene moiety. Among the derivatives, compound 17 exhibits the highest amount of the cis form (97%) in the photostationary state (PSS) upon UV light (365 nm) irradiation. Compounds 13 and 15–17 exhibit excellent fatigue resistance and reversibility upon several reversible isomerization cycles.