| Summary: | The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended <i>closo-ortho</i>-carboranyl luminophores is known to be affected by structural fluctuations and molecular geometry, but investigation of this relationship has been in progress to date. In this study, four naphthyl-based bis-<i>o</i>-carboranyl compounds, in which hydrogen (<b>15CH</b> and <b>26CH</b>) or trimethysilyl groups (<b>15CS</b> and <b>26CS</b>) were appended at the <i>o</i>-carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, <b>15CH</b> and <b>26CH</b> distinctly exhibited a dual emissive pattern in rigid states (in solution at 77 K and in films), attributed to locally excited (LE) and ICT-based emission, while <b>15CS</b> and <b>26CS</b> showed strong ICT-based greenish emission. Intriguingly, the molecular structures of the four compounds, analyzed by single X-ray crystallography, showed that the C-C bond axis of the <i>o</i>-carborane cage in the trimethysilyl group-appended compounds <b>15CS</b> and <b>26CS</b> were more orthogonal to the plane of the appended naphthyl group than those in <b>15CH</b> and <b>26CH</b>. These features indicate that <b>15CS</b> and <b>26CS</b> present an efficient ICT transition based on strong <i>exo</i>-π-interaction, resulting in a higher quantum efficiency (Φ<sub>em</sub>) for ICT-based radiative decay than those of <b>15CH</b> and <b>26CH</b>. Moreover, the <b>26CS</b> structure revealed most orthogonal geometry, resulting in the highest Φ<sub>em</sub> and lowest <i>k</i><sub>nr</sub> values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended <i>o</i>-carboranyl luminophores could be achieved by the formation of a specific geometry between the <i>o</i>-carborane cage and the aromatic plane.
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