Oxidative cyclization of N-acylhydrazones. Development of highly selective turn-on fluorescent chemodosimeters for Cu2+

Abstract

A series of N-acylhydrazones were synthesised and found to be “turn-on” fluorescent chemodosimeters for Cu²⁺. Among the tested transition metal ions such as Cu²⁺, Pb²⁺, Zn²⁺, Cd²⁺, Hg²⁺, and Ni²⁺, a prominent fluorescence enhancement of up to 1000-fold was only observed for Cu²⁺ in acetonitrile (CH₃CN). This was indicated by an onset of unprecedented structured emission. Detailed experiments established that the highly Cu²⁺ selective fluorescence enhancement resulted from an oxidative cyclization by Cu²⁺of the originally nonfluorescent N-acylhydrazones into highly fluorescent rigid 1,3,4-oxadiazoles, n-dope type blocks in optoelectronic materials. The chemodosimeters can be applied to sense Cu²⁺ at nM levels in CH₃CN and sub-µM levels in neutral aqueous environments, despite a slower response in the latter case. It is expected that these redox-based chemodosimeters might be of general applicability.