| Summary: | 9,9′-Spirobifluorene-based <i>o</i>-carboranyl compounds <b>C1</b> and <b>C2</b> were prepared and fully characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The solid-state structure of <b>C1</b> was also determined by single-crystal X-ray diffractometry. The two carboranyl compounds display major absorption bands that are assigned to <i>π</i>−<i>π</i>* transitions involving their spirobifluorene groups, as well as weak intramolecular charge-transfer (ICT) transitions between the <i>o</i>-carboranes and their spirobifluorene groups. While <b>C1</b> only exhibited high-energy emissions (λ<sub>em</sub> = ca. 350 nm) in THF at 298 K due to locally excited (LE) states assignable to <i>π</i>−<i>π</i>* transitions involving the spirobifluorene group alone, a remarkable emission in the low-energy region was observed in the rigid state, such as in THF at 77 K or the film state. Furthermore, <b>C2</b> displays intense dual emissive patterns in both high- and low-energy regions in all states. Electronic transitions that were calculated by time-dependent-DFT (TD-DFT) for each compound based on ground (S<sub>0</sub>) and first-excited (S<sub>1</sub>) state optimized structures clearly verify that the low-energy emissions are due to ICT-based radiative decays. Calculated energy barriers that are based on the relative energies associated with changes in the dihedral angle around the <i>o</i>-carborane cages in <b>C1</b> and <b>C2</b> clearly reveal that the <i>o</i>-carborane cage in <b>C1</b> rotates more freely than that in <b>C2</b>. All of the molecular features indicate that ICT-based radiative decay is only available to the rigid state in the absence of structural fluctuations, in particular the free-rotation of the <i>o</i>-carborane cage.
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