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

초록

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 S-1 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.

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