Evidence for Delocalization between <i>o</i> -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.