Relationship between the Molecular Geometry and the Radiative Efficiency in Naphthyl-Based Bis-Ortho-Carboranyl Luminophores.

The efficiency of intramolecular charge transfer (ICT)-based emission on π-aromatic-group-appended closo-ortho -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- o -carboranyl compounds, in which hydrogen ( 15CH and 26CH ) or trimethysilyl groups ( 15CS and 26CS ) were appended at the o -carborane cage, were synthesized and fully characterized. All the compounds barely displayed an emissive trace in solution at 298 K; however, 15CH and 26CH 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 15CS and 26CS 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 o -carborane cage in the trimethysilyl group-appended compounds 15CS and 26CS were more orthogonal to the plane of the appended naphthyl group than those in 15CH and 26CH . These features indicate that 15CS and 26CS present an efficient ICT transition based on strong exo -π-interaction, resulting in a higher quantum efficiency (Φ em ) for ICT-based radiative decay than those of 15CH and 26CH . Moreover, the 26CS structure revealed most orthogonal geometry, resulting in the highest Φ em and lowest k nr values for the ICT-based emission. Consequently, all the findings verified that efficient ICT-based radiative decay of aromatic group-appended o -carboranyl luminophores could be achieved by the formation of a specific geometry between the o -carborane cage and the aromatic plane.