A Diphenylamine-Functionalized [5]Helicene as a Solvatochromic Fluorescent Sensor for Methanol: Synthesis, Photophysics, and Computational Analysis

Abstract

A novel diphenylamine- and cyano-substituted [5]helicene derivative (M111) was rationally designed, synthesized, and characterized as a fluorescence-based sensor for methanol detection. The molecule features a rigid π-conjugated helical backbone with a donor–acceptor (D–A) architecture, enabling efficient intramolecular charge transfer (ICT) in the excited state. Photophysical studies in solvents of varying polarities revealed pronounced solvatochromic behavior, including bathochromic shifts and fluorescence quenching in polar media, particularly methanol. In dichloromethane–methanol mixtures, M111 exhibited a highly linear fluorescence response with a low detection limit of 4.5% (v/v), demonstrating excellent sensitivity. To elucidate the underlying electronic mechanisms, ground- and excited-state properties were investigated by using DFT and TD-DFT calculations with CAM-B3LYP, M06–2X, and ωB97XD functionals. The computation demonstrates that both general dielectric stabilization and specific hydrogen-bonding interactions between methanol and the cyano acceptor sites contribute to red-shifted emission. The combination of experiment and computational results establishes M111 as a promising helicene-based platform for developing selective and real-time fluorescence sensors for methanol and related polar solvents.