Phosphine Oxide-Induced Strong Emission and Multistimuli-Responsive Fluorochromism of <i>o</i>-Carborane-Based Luminogens

A series of <i>o</i>-carborane-based triads, functionalized at the carbon vertices with diphenylphosphine groups (phosphine or phosphine oxide) as auxiliary groups and polycyclic aromatic moieties (naphthyl, phenanthryl, or pyrenyl) as donors, were successfully synthesized. The synergistic effects of phosphine and aromatic groups on the optoelectronic properties of <i>o</i>-carborane-based luminescent molecules were studied. All of these compounds exhibit both locally excited (LE) emission and intramolecular charge transfer (ICT) emission, with the LE/ICT ratio and the occurrence of multiple emissions significantly influenced by the valence state of phosphorus and the steric hindrance of the aromatic rings. Notably, these compounds demonstrate enhanced emission in the solid or aggregated states and possess stimulus-responsive properties related to solvent and temperature changes. X-ray analysis revealed that stronger intermolecular interactions contribute to the higher quantum yield of the oxide.