Evidence for Delocalization between o-Carborane and Triphenylethylene via an Alkynyl Bridge

The pursuit of high solid-state fluorescence quantum yields (Φ) in o-carborane systems and the understanding of their luminescent mechanisms have long been sought targets but are constrained by a strong focus on carbon-vertex modification that relies on disubstitution to circumvent aggregation-caused quenching. This work challenges the convention by demonstrating that B9 boron-vertex monosubstitution, when coupled with a rational alkynyl-bridge conjugation, can achieve exceptional performance. We report two key synthetic breakthroughs that enable scalable and selective B9 functionalization, yielding the novel derivative B9-TriPE-yn-o-CB. This compound exhibits a high solid-state Φ of 83.5%─a value that is about 7.5-fold higher than other monosubstituted comparisons and surpasses most carbon-disubstituted analogues. We have employed UV-vis absorption spectroscopy, solution fluorescence emission spectroscopy at 77 K, solid-state fluorescence emission spectroscopy, single-crystal X-ray diffraction, and DFT calculations to explore the luminescent mechanisms and found that the insertion of an alkynyl bridge enhances the conjugation between TriPE and o-CB. Single-crystal X-ray diffraction reveals a critical shortening of the cage C-C bond (1.612 Å) and concerted B-B bond contractions, providing direct structural evidence for the suppression of nonradiative decay. DFT calculations corroborate efficient 3D (boron cage)-2D (chromophore) electron delocalization, in which π-electrons are conveyed into the o-CB cage via the alkynyl bridge in both ground and excited states. Our findings not only challenge the requirement for carbon-vertex disubstitution but also establish a 3D-2D conjugation design principle for activating the B9 boron vertex, thereby opening new avenues for high-performance boron cluster emitters.