Naphthyl‐ and Quinoline‐Appended o‐Carboranyl Luminophores: Intramolecular Charge Transfer‐Based Radiative Decay Controlled by Structural Geometry around C−C Bond Axis

Naphthyl‐ and quinoline‐appended o‐carboranyl compounds, NCB and QCB, respectively, were prepared and characterized by multinuclear nuclear magnetic resonance spectroscopy, elemental analysis, and single crystal X‐ray diffraction. Although both the compounds were non‐emissive in the solution state at 298 K, they were photoluminescent in the rigid state (in THF at 77 K and film state) in the region 450–550 nm. Theoretical calculation of the optimized structure in the S1 state suggested that the low‐energy emissive bands for NCB and QCB were attributed to intramolecular charge transfer (ICT) transition. Intriguingly, the C−C bond axis of the o‐carborane in NCB in the solid state was more orthogonal to the plane of the appended aromatic group than that in QCB, indicating relatively high delocalization between the o‐carborane and aromatic moieties of NCB. The quantum efficiency and radiative decay constant of the ICT‐based emission of NCB in the film state were much higher than those of QCB. These findings imply that the structural geometry around the C−C bond axis of the o‐carborane is a decisive factor in accelerating the ICT‐based radiative decay in the o‐carbonyl luminophores in the rigid state. Photophysical properties of naphthyl‐ and quinolone‐appended closo‐o‐carboranyl dyad compounds were investigated. Significant differences in the quantum efficiency and radiative decay constant for the ICT‐based emission of the two compounds verified that the structural geometry around the o‐carborane unit was a decisive factor in controlling the efficiency of the ICT‐based radiative decay in o‐carboranyl luminophores.