Water-soluble fluorescent 4-amino-N-aryl-1,8- naphthalimide logic gates for saccharides and metal cations

Abstract

Twelve fluorescent logic gates were designed and synthesised based on a “fluorophore-spacer-receptor” model (for compounds 5-10) and a “receptor1-spacer- fluorophore-spacer-receptor2” model (for compounds 1-4, 11 and 12). The 12 compounds were built around a naphthalimide fluorophore, having aryl moieties at the imide position, including phenyl (1 and 5), 1,2-dimethoxyphenyl (2 and 6), benzo-15- crown-5 (3), benzo-18-crown-6 (4 and 7), ortho-phenylboronic acid pinacol ester (8 and 11), ortho-phenylboronic acid (9), and meta-phenylboronic acid (10 and 12). Substituents at the 4-position of the naphthalimide included piperazine (1-4, 11 and 12), azetidine (5-7), and chloro (8-10). The compounds were studied by means of UV-visible absorption spectroscopy and steady-state fluorescence spectroscopy. Compounds 1-12 were studied in terms of proton concentration and solvent polarity. Compounds 3, 4, and 7 were also studied in the presence of cations, including sodium and barium(II), whilst compounds 8-12 were studied in the presence of three monosaccharides; glucose, fructose, and galactose. Compounds 1-7 were studied in water, water/methanol mixtures, and methanol, whilst compounds 8-12 were studied in phosphate-buffered saline (PBS), PBS/methanol mixtures, and methanol. Compounds 1, 11, and 12 function as one-input proton-driven YES gates, displaying stronger fluorescence in methanol. Compounds 2, 5 and 6 function as one-input proton-driven PASS 1 gates which are brighter in methanol. Compound 7 functions as a two-input proton- and cation-driven PASS 1 gate, being brightest also in methanol. Compound 3 behaves as a three-input AND gate with respect to protons, sodium cations, and solvent polarity. Compound 4 behaves as a three-input AND-INHIBIT-OR combinatorial gate with respect to protons, barium(II) cations, and solvent polarity. Compound 8 behaves as a three-input OR1,2-INHIBIT3 gate with respect to protons, saccharides, and solvent polarity, compound 9 behaves as a three-input AND gate with respect to protons, fructose, and solvent polarity, and compound 10 functions as a two-input proton-enabled solvent polarity-disabled INHIBIT gate. Solvent polarity was shown to be a very important factor when considering the fluorescence responses and logic of 1-12. This discovery opens up the possibility of revisiting existing compounds and modifying their logic behaviour by simply tuning the solvent polarity in which they are studied, thus allowing for more potential applications.