Photophysical and Photochemical Properties of N, C-Chelate Organoboron Compounds and Their Pt(II) Complexes
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The impact of two constitutional isomers, 2-(4-BMes2-Ph)-pyridine (p-B-ppy) and 5-BMes2-2-ph-pyridine (p-ppy-B), as N,C-chelate ligands on the structures, stabilities, electronic and photophysical properties, and Lewis acidities of Pt(II) complexes has been investigated. Six Pt(II) complexes, Pt(p-B-ppy)Ph(DMSO), Pt(p-B-ppy)Ph(Py), [Pt(p-B-ppy)Ph]2(4,4’-bipy), Pt(p-ppy-B)Ph(DMSO), Pt(p-ppy-B)Ph(Py), and [Pt(p-ppy-B)Ph]2(4,4’-bipy), have been synthesized and fully characterized. The Lewis acidity of the complexes was examined by fluoride titration experiments using UV-Vis, phosphorescence, and NMR spectroscopic methods, establishing that the p-ppy-B complexes have stronger binding constants while the p-B-ppy complexes have a much lower affinity toward F-. A diboron compound with both 3-coordinate boron and 4-coordinate boron centers, (5-BMes2-2-ph-py)BMes2 (B2ppy) has been synthesized, which is luminescent but have a high sensitivity toward light. UV and ambient light cause this compound to isomerize via the formation of a C−C bond between a mesityl and the phenyl group, accompanied by a drastic color change from yellow to dark olive green. The structure of the dark color species was established by 2D NMR experiments and geometry optimization by DFT calculations. The dark color species can thermally reverse back to (5-BMes2-2-ph-py)BMes2 via the breaking of a C−C bond. The N, C-chelate ligand was found to play a key role in promoting this unusual and reversible photo–thermal isomerization process on a tetrahedral boron center. The impact of Pt(II) on the photoisomeration of four-coordinate boron center was also studied. The free ligand four-coordinate organoboron derivative B-ppy-ppy behaved in the same way as B2ppy. The photoisomeration process in the corresponding Pt(II) coupled complex (B-ppy-ppy)PtPh(t-Bu-py) is nearly completely deactivated, which may be attributed to either the low-lying 3MLCT excited state through which the excess energy in excited state was dissipated as phosphorescence or the greater π conjugation which can stabilize the excited state.