Chiroptical absorption and emission properties of chiral triarylboranes

Due to their unique emission properties, chiral triarylboranes are of particular interest for a wide range of optical applications. Rigid chiral propeller-like superstructures can be established by incorporation of bulky substituents that slow down or even prevent the rotation along the B-C bond. In the present study, we focus on subtle influences on the preferential propeller handedness and elucidate the conformational preferences of two chiral triarylboranes, which possess only small chiral ortho-(2-butoxy)-phenyl substituents as the source of stereochemical bias. Notably, the conformational analysis at the B3LYP and B3LYP-d3bj levels of theory suggested quite different preferences, predicting a stronger bias towards a certain handedness when considering dispersion corrections. Utilizing vibrational and electronic circular dichroism spectroscopy, it is shown that the small substituents could indeed induce only a small preference of propeller handedness. The spectra analysis further confirmed that the B3LYP-based conformational distributions provided much better agreements with the experimental band shapes than those obtained from B3LYP-d3bj. The deviations are reasoned to arise from a too strong emphasis on dispersive C-H⋯π interactions, i.e., a problem related to the predicted conformational distributions, and not to the quality of the single conformer VCD or ECD spectra. For further characterization, the fluorescence and circular polarized luminescence (CPL) spectra were recorded and a weak CPL activity could be observed for both triarylboranes.