Tetrahedral Silicon-Centered Dibenzoylmethanatoboron Difluorides: Synthesis, Crystal Structure, and Photophysical...

Four tetrahedral silicon-centered derivatives of dibenzoylmethanatoboron difluoride (DBMBF₂ ) were synthesized and characterized. Their structural and optical features both in solution and the solid state were investigated by using X-ray crystallography, steady-state and time-dependent spectroscopy, and DFT-based calculations. In dilute solutions, the molar absorption coefficient increases from 40500 to 175200 M^-1  cm^-1 as the number of DBMBF₂ fragments in a molecule increases from one to four, while, in contrast, the nonradiative rate constant of fluorescence decay decreases from 0.49 to 0.34. In the solid state, absorption and emission spectra depend on the degree of crystallinity and microcrystal size. The tris-DBMBF₂ derivative forms fully overlapping dimeric structures that exhibit excimer-like fluorescence, which is accurately predicted by the quantum-chemical calculations. The mono-DBMBF₂ derivative exhibits fully reverse mechanofluorochromic behavior.