Reactivity-Based Detection of Copper(II) Ion in Water: Oxidative Cyclization of Azoaromatics as Fluorescence Turn-On Signaling Mechanism

An oxidative cyclization reaction transforms nonemissive
azoanilines
into highly fluorescent benzotriazoles. We have found that introduction
of multiple electron-donating amino groups onto a simple o-(phenylazo)­aniline platform dramatically accelerates its conversion
to the emissive polycyclic product. Notably, this chemistry can be
effected by μM-level concentrations of copper­(II) ion in water
(pH = 6–8) at room temperature to elicit >80-fold enhancement
in the green emission at λem = 530 nm. Comparative
kinetic and electrochemical studies on a series of structural analogues
have established that the accelerated reaction rates correlate directly
with a systematic cathodic shift in the oxidation onset potential
of the azo precursors. In addition, single-crystal X-ray crystallographic
analysis on the most reactive derivative revealed the presence of
a five-membered ring intramolecular hydrogen-bonding network. An enhanced
contribution of the quinoid-type resonance in such conformation apparently
facilitates the mechanistically required proton transfer step, which,
in conjunction with electron transfer at lower oxidation potential,
contributes to a rapid cyclization reaction triggered by copper­(II)
ion in water.