Photochromic N,C-Chelate 4-Coordinate Organoboron Compounds
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The objective of this thesis is to prepare new photochromic N,C-chelate four-coordinate organoboron compounds and investigate their photophysical and photochemical properties. Phenylpyridine-based organoboron compound (PhPyBMes2) was prepared and studied with NMR, X-ray, and UV-Vis. Experimental and theoretical results established that PhPyBMes2 is photochromic and the photoisomerization process involves a reversible intramolecular C-C bond formation/breaking. The structure of the dark colored isomer of PhPyBMes2 was established by 2D NMR experiments and computational studies. The discovery of the photochromic behavior of PhPyBMes2 led to the investigation of other phenylpyridine-based organoboron compounds. A series of substituted PhPyBMes2 compounds were synthesized with substituents on the phenyl ring or the pyridine ring of the N,C-chelate. Two other compounds were synthesized by substituting the mesityl rings in PhPyBMes2 with phenyl or pentaflurophenyl rings. The photophysical and photochemical properties of these compounds were studied by NMR, UV-Vis, and DFT/TD-DFT. The studies established that the steric hindrance imposed by the mesityl groups and the nature of substituents on the N,C-chelate have substantial effects on the photoisomerization process. New organoboron compounds based on N,C-chelates other than phenylpyridine were prepared and their photochemical and photophysical properties studied using NMR, UV-Vis, and DFT/TD-DFT. The phenyl group in the N,C-chelate was replaced by TMS-furan, TMS-thiophene, benzofuran, benzothiophene, and N-phenyl pyridylindole groups. The conducted studies show that these new compounds have photochromic properties similar to those of phenylpyridine-based compounds. Crystals of the dark isomer of the N-phenyl pyridylindole organoboron compound were successfully grown and the structure of the dark isomer was determined by X-ray diffraction. New organoboron compounds based on phenyl-benzoxazole and phenyl-benzothiazole have been also prepared and studied. The photoisomerization of these compounds is similar to the other N,C-chelate organoboron compounds but their thermal behavior is different.