Fluorescence Quenching Efficiency and Sensing Mechanism of Donor–Acceptor-Type Nitroaromatic Compounds Using Poly(3-thiophene ethoxide)-CTAB Complex

Abstract

A surfactant-assisted strategy was employed to enhance the photoluminescence properties of polythiophene for the detection of nitroaromatic compounds (NACs). Poly(3-thiophene ethanol) (P3TE) was synthesized via chemical oxidative polymerization and subsequently complexed with cetyltrimethylammonium bromide (CTAB) to form a stable and soluble P3TE-CTAB complex. Structural characterization using Fourier transform infrared spectra, proton nuclear magnetic resonance spectra, powder X-ray diffraction, X-ray photoelectron spectroscopy, and electron microscopy confirmed the successful complexation and revealed improved structural ordering and morphological transformation from flake-like aggregates to cylindrical coil-like nanostructures. The P3TE-CTAB complex exhibited a stable yellow emission in dimethyl sulfoxide with a quantum yield of 39.5% and a large Stokes shift of 132 nm. The fluorescence sensing behavior toward 12 structurally different nitroaromatic compounds, including nitroanilines, nitrophenols, and nitrotoluenes, was systematically investigated. Fluorescence quenching among para-substituted nitro compounds follows the order para-nitroaniline > para-nitrophenol > para-nitrotoluene with Stern–Volmer constants 9.1 ± 0.9 × 103 M–1, 2.9 ± 0.2 × 103 M–1, and 5.3 ± 0.2 M–1, respectively. The quenching efficiency strongly depended on type of donor (−NH2, −OH, −CH3) and its position with respect to acceptor (−NO2) groups in the analyte. Fluorescence lifetime, dynamic light scattering, and field emission scanning electron microscopy studies demonstrated that nitroaromatic analytes promote aggregation of the P3TE-CTAB complex without significant shortening of the excited-state lifetime. These results suggest that fluorescence quenching is predominantly governed by aggregation-induced static quenching, while photoinduced electron transfer (PET) and the inner filter effect (IFE) provide additional contributions. Correlation of quenching efficiency with DFT-computed molecular hyperpolarizability, LUMO energy, and spectral overlap established that analyte hyperpolarizability is the primary factor governing polymer aggregation and fluorescence quenching. Real-sample analysis further demonstrated the practical applicability of the P3TE-CTAB system for nitroaromatic detection.