Second-order nonlinear optical properties of eight-membered centrosymmetric cyclic borasiloxanes

Abstract

In an efficient 2+2 cyclocondensation reaction, electron-withdrawing arylboronic acids with diphenylsilanediol were converted to eight-membered cyclic borasiloxanes [(RPhBO)(PhSiO)]₂ [R = F (1), 2,4-F (2), CF₃ (3), CN (4), NO₂ (5)]. All these compounds were characterized by elemental analysis, FT-IR, and NMR (¹H, ¹³C, ¹⁹F, ²⁹Si and ¹¹B) and structurally confirmed by single-crystal X-ray diffraction studies. Compounds 1–5 crystallized in the centrosymmetric space group, revealing an eight-membered ring (B₂O₄Si₂) configuration with organic substitutions, which occupied axial and equatorial positions. Compounds 1 and 2 feature 2D network hydrogen bonding whereas compound 3 has a solid-state 3D supramolecular architecture. The other two compounds 4 and 5 exhibit only C–H⋯π interactions which lead to a 1D polymeric structure and these intermolecular interactions influence crystal packing. Photophysical properties were studied by UV-visible and fluorescence spectroscopic techniques. In addition, the optical band gaps (E_g) of compounds 1–5 were determined by diffuse reflectance spectra and compound 5 has a lower band gap value than the others, due to the strong electron-withdrawing nitro group in compound 5. The thermal behaviour of compounds 4 and 5 has been investigated using thermogravimetric analysis and differential thermal analysis, both compounds being stable up to 250 °C. The nonlinear optical response of the crystalline powdered borasiloxanes by Q-switched Nd-YAG laser and second harmonic generation (SHG) efficiency of borasiloxanes 4 and 5 were found very similar to those of potassium dihydrogen phosphate. The SHG efficiency of the centrosymmetric borasiloxanes mainly arises from the distorted silicon atom and non-covalent interactions which preclude the dipoles in the antiparallel arrangement in crystal packing. Further, optical and nonlinear properties of the borasiloxanes were investigated by density functional theory calculations.