Aggregation Induced Emission (AIE) based Fluorophore: Synthesis and Application as pH Sensor

Abstract

The classic pH-sensitive fluorophores can be effectively expanded from solo solution to both solution and solid states by introducing an AIE unit. Herein, we report two fluorescence Probes (TPASCNPy and TPESCNPy) based on the pyridine group for the pH sensor. Two organic fluorophores TPASCNPy and TPESCNPy were synthesized from 4-pyridine acetonitrile and thiophene building blocks containing TPA and TPE as AIE active moiety respectively. Both TPASCNPy and TPESCNPy display an ICT effect triggered by the D-π-A interaction between its AIE active moiety and pyridine units. However, both emit very faintly in solution, and they become extremely emissive in the aggregated state, representing the AIE phenomenon. In the case of TPASCNPy, its emission can be reversibly swapped between nom-emissive and emissive states by repeated protonation and deprotonation. In the case of TPESCNPy, its emission can be reversibly switched between red and yellow emission by frequent protonation and deprotonation. Due to this behavior, both Probes can function as chemosensors for detecting volatile organic chemicals with high acidity as well as fluorescent pH sensors in both solution and solid states. NMR spectroscopy analysis findings imply that the sensing mechanism is caused by a shift in the electron affinity of pyridinyl and TPA moiety. Thus, the selective recognition for acidic pH was confirmed by the completely non-emissive TPASCNPy and red fluorescence of TPESCNPy observed by fluorescence spectra.