Synthesis, structures, and solid-state photoresponsive color change behavior of boronium complexes bearing a pyridine-imine, diimine, or pyridine-ketone bidentate ligand

9-Borabicyclo[3.3.1]nonane-based boronium triflates bearing a N-substituted 2-pyridylmethanimine, N,N′-dialkylethane-1,2-diimine, or 2-arylcarbonylpyridine ligand were synthesized. Their tetracoordinate boron structures were determined using ¹¹B NMR spectra and X-ray crystallography. The pyridine-imine complexes exhibited solid-state photoresponsive color changes upon UV irradiation, which indicated that boronium complexes without a bipyridine moiety also have photoresponsive capabilities. Combination of TD-DFT calculations and measurements of UV-vis absorption and fluorescence properties, diffuse reflectance spectra, and ESR spectra provided suggestions on the determining factor of the photoresponsive color change capabilities and structures of the photoproducts.