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9,9'-Spirobifluorene-based o -carboranyl compounds C1 and C2 were prepared and fully characterized by multinuclear nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. The solid-state structure of C1 was also determined by single-crystal X-ray diffractometry. The two carboranyl compounds display major absorption bands that are assigned to π - π * transitions involving their spirobifluorene groups, as well as weak intramolecular charge-transfer (ICT) transitions between the o -carboranes and their spirobifluorene groups. While C1 only exhibited high-energy emissions (λ em = ca. 350 nm) in THF at 298 K due to locally excited (LE) states assignable to π - π * 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, C2 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 0 ) and first-excited (S 1 ) 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 o -carborane cages in C1 and C2 clearly reveal that the o -carborane cage in C1 rotates more freely than that in C2 . 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 o -carborane cage.

Photophysical Properties of Spirobifluorene-Based o-Carboranyl Compounds Altered by Structurally Rotating the Carborane Cages