Ferrocene-appended boronated ester: effect of cyanovinylene group on the nonlinear optical properties and colorimetric detection of fluoride ion

Abstract

A ferrocene-conjugated boronated ester with a cyanovinylene unit L2 has been synthesized by a Suzuki–Miyaura borylation reaction and characterized by analytical and spectroscopic techniques. Further, the L2 structure was confirmed by single-crystal X-ray diffraction studies; it crystalizes in a monoclinic crystal system with a P21/n centrosymmetric space group and exhibits various non-covalent interactions in the crystal packing. The solvatochromic studies revealed no considerable changes for receptor L1 and L2 because of the lack of significant changes in the dipole moment at the ground and excited states; these results coincided with the density functional theory (DFT) and time-dependent DFT calculations (B3LYP/6-31+G** level of theory). The redox potentials were studied using the cyclic voltammetric method, and results show that the half wave potential of L2 was higher than that of the parent ferrocene due to the presence of the electron-withdrawing cyanovinylene group in the receptor L2. The second-order nonlinear optical properties of L1 and L2 were measured using the Kurtz and Perry powder method; L2 shows 1.52 times higher efficiency than L1 owing to the presence of the strong electron-withdrawing cyanovinylene group and various non-covalent interactions in the crystal packing. Furthermore, NLO-active L1 and L2 were utilized for colorimetric anion sensing using various tetrabutylammonium (TAB) salts; no considerable changes were observed in the receptor L1, but the receptor L2 shows selectivity and sensitivity towards F⁻ ion due to the presence of the cyanovinylene unit, which increases the acidity of the boron atom, and it formed a complex, L2–F⁻. The stoichiometric ratio was confirmed using the Job's plot, which showed 1 : 1 complex formation, and the limit of detection was found to be 9.5 μM. The formation of the L2–F⁻ complex was further confirmed through ¹¹B and ¹⁹F NMR titrations and ESI-mass spectrometry. The optical, nonlinear optical and sensing studies were further supported by quantum chemical (DFT/TD-DFT) calculations.