Boronate derivatives of damnacanthal: Synthesis, characterization, optical properties and theoretical calculations

Abstract

The natural anthraquinone Damnacanthal (1) was isolated from the roots of Morinda panamensis and used as a building block in the construction of five fluorescent boronate derivatives (4a–4e). The novel compounds were obtained in moderate yields by the condensation of 1, anthranilic acid (2) and diverse phenylboronic acids (3a–3e) in acetonitrile. Boronate derivatives were fully characterized by 1H, 13C, 11B and 2D nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), ultraviolet–visible spectroscopy (UV–vis), and high-resolution mass spectrometry (HR-MS). The photophysical properties were evaluated in tetrahydrofuran revealing that the optical band gap (Egopt) values increase from 2.54 eV for 4a to 2.65 eV for 4d. The maximum emission wavelength and fluorescence quantum yield (ΦF) of the boron complexes were found to be sensitive to the substituents on the phenyl group attached to the boron atom. The resulting derivatives 4a–4c presented emission bands at approximately 522 nm (ΦF = 0.41, 0.13 and 0.37%, respectively); while derivatives 4d and 4e presented broad emission bands with maxima at 676 nm (ΦF = 6.27%) and 614 nm (ΦF = 0.78%), respectively. The molecular geometry, electronic properties and intramolecular non-covalent interactions of the new molecules were evaluated using density functional theory (DFT) and quantum theory of atoms in molecules (QTAIM) methods. The value of the energy gap between the HOMO and LUMO increases from 2.89 eV for 4b, 2.96 eV for 4c, 3.04 eV for 4e, 3.08 eV for 4a to 3.32 eV for 4d. Furthermore, compound 4d showed a higher rotational energy barrier (3.33 kcal/mol) at the dihedral angle N1/B1/C24/C25 than its analogs, due to the F∙∙∙π and F∙∙∙O interactions.