Unsymmetric Molecular Hinges Showing Intramolecular Charge Transfer Excitation: Strong Photoluminescence Properties in the Red to Near-Infrared Region Exhibited by Tri-BF2 Complexes

Rationally designed unsymmetric tri-BF2 red to near-infrared (NIR) emitters were first reported in this study via a facile one-pot synthetic approach. The single-crystal structures and density functional theory calculations suggest their tunable conformation change around the bipyrrole part and unsymmetric structures in which three boron atoms are coordinated with the existing four different bidentate binding pockets, leaving two of the uncoordinated nitrogens in the pyrrole part. These novel tri-BF2 emitters in the red to NIR region demonstrated ca. 30% of photoluminescence quantum yield (ΦPL) in toluene at room temperature and highly intense fluorescence at 77 K (up to 89% of ΦPL in 2-MeTHF), which is attributed to increased molecular flexibility and enhanced intramolecular charge transfer interactions resulting from symmetry breaking. The photophysical modulation in multi-BF2 complexes was achieved by the coordinated number of BF2 units, providing a step forward to understanding and guiding the design of the most efficient versatile emissive organoboron compounds.