CF3 H-bonding locked aromatic stacking of picric acid with mechanofluorochromic fluorophores: highly selective reusable sensor and rewritable fluorescence platform

Abstract

Tailoring the molecular structure of organic fluorescent molecules is an important tool used to generate functional materials with the desired attributes for optical, opto-electronic and bio-imaging applications. Herein, we have prepared CF₃/CH₃ functionalized mechanofluorochromic molecules (Cz-4-CF₃ and Cz-4-CH₃) and demonstrated functional group-dependent picric acid (PA) sensing and stimuli-responsive fluorescence switching. Cz-4-CF₃ and Cz-4-CH₃ exhibited strong fluorescence in solution as well as solid state. Cz-4-CF₃ showed mechanical force and heating-induced reversible fluorescence switching between two fluorescence states, whereas Cz-4-CH₃ displayed off–on fluorescence switching. Cz-4-CF₃ required scratching and annealing to revert to the initial-state fluorescence from melt state, but annealing alone transformed Cz-4-CH₃ to the initial state. Cz-4-CF₃ showed highly selective fluorescence sensing of picric acid (PA) among nitroaromatic compounds (NACs), including dinitrophenol (limit of detection = 51.4 nM). Dual-state fluorescent Cz-4-CF₃ was used to fabricate thin-film (filter paper and PVA/PMMA composite thin film) fluorescent sensors for PA in aqueous medium. Cz-4-CF₃–PVA and free standing Cz-4-CF₃–PMMA thin films showed highly selective fluorescence quenching upon immersion in aqueous PA solution (10⁻⁹ to 10⁻² M). Importantly, the fluorescence was fully recovered upon dipping into pure water, and the film was reused for several cycles of PA sensing. Cz-4-CF₃-coated filter paper also showed clear quenching of fluorescence in PA solution (10⁻⁷ M) that could also be regenerated upon immersion in water. NMR studies indicated a possible interaction between CF₃ and the hydroxyl group. Single-crystal structural analysis of Cz-4-CF₃–PA co-crystals confirmed strong H-bonding between CF₃ and the hydroxyl group of PA, which facilitated face-to-face aromatic π-stacking between carbazole and PA aromatic units and led to fluorescence quenching by charge transfer. Computational studies further support the charge transfer from Cz-4-CF₃ to PA. Highly selective fluorescence sensing of PA by other fluorophores (Cz-3-CF₃, TPA-4-CF₃ and TPA-3-CF₃) with CF₃ substitution elucidated the broader scope of CF₃ functionality for developing a PA sensor in aqueous medium. The thin-film fluorescence sensing, recyclability and contact mode detection of PA with high selectivity in aqueous medium demonstrate the practical utility for Cz-4-CF₃ for developing onsite detection of PA.