Near-Infrared Colorimetric and Fluorescent Cu²⁺ Sensors Based on Indoline–Benzothiadiazole Derivatives via Formation of Radical Cations

Abstract

The donor–acceptor system of indoline–benzothiadiazole is established as the novel and reactive platform for generating amine radical cations with the interaction of Cu2+, which has been successfully exploited as the building block to be highly sensitive and selective near infrared (NIR) colorimetric and fluorescent Cu2+ sensors. Upon the addition of Cu2+, an instantaneous red shift of absorption spectra as well as the quenched NIR fluorescence of the substrates is observed. The feasibility and validity of the radical cation generation are confirmed by cyclic voltammetry and electron paramagnetic resonance spectra. Moreover, the introduction of an aldehyde group extends the electron spin density and changes the charge distribution. Our system demonstrates the large scope and diversity in terms of activation mechanism, response time, and property control in the design of Cu2+ sensors.