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

Molecular Systems Design & Engineering Pub Date: 2022-06-24 DOI: 10.1039/D2ME00075J

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.

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