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 pi-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.