The nature of 6,6′-bis(triphenylamine) substituted BINOL as chromophoric and fluorogenic hybrid chemosensor for selective fluoride detection

6,6′-Bis(triphenylamine)-1,1′-binaphthyl-2,2′-diol (1) is highly fluorescent in visible region. On treatment with a variety of anions, the UV–vis absorption as well as the fluorescent behavior of 1 in CH2Cl2 is substantially changed. Among the tetrabutylammonium (TBA) salts we tested, including F−, OH− (from TBAOH in MeOH), Cl−, Br−, I−, OAc−, HSO4−, TsO− (Et4N salt), and H2PO4−, F− was found to be the most effective fluorescence quencher. Quantitative fluorescence analysis of the titration data revealed that the anions are classified into two categories: (1) Simple mono-anions, such as F−(logβ2F=11.00±0.08), OH−(logβ2OH=9.05±0.01), and Cl−(logβ2=9.96±0.70) that follow the stoichiometry of 1:2, indicating the formation of (1·X2)2− complexes; (2) Oxo-anions, such as OAc−(logβ1OAc=5.13±0.012), H2PO−4(logβ1HP=4.87±0.03), and TsO− (using tetraethylammonium salt, (logβ1OTs=3.36±0.02) that show the stoichiometry of 1:1, indicating the formation of (1·X)− complexes. The complexation behaviors were further confirmed by 1H NMR spectroscopy. In the co-crystal prepared from 1,1-binaphthyl-2,2′-diol (BINOL) and Et4NF, F− and BINOL are assembled to form a linear polymeric array, with a dimeric group of BINOL/BINOL anions linked as the side-chains. Intramolecular hydrogen bond was observed within the BINOL anion. This provides structural insights about the BINOL–fluoride complex formation.

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