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

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 non fluorescent 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-mu M 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.