Spirobifluorene‐Based o‐Carboranyl Compounds: Insights into the Rotational Effect of Carborane Cages on Photoluminescence

Abstract 9,9′‐Spirobifluorene‐based closo ‐ o ‐carboranyl ( SFC1 and SFC2 ) compounds and their nido ‐derivatives ( nido ‐ SFC1 and nido ‐ SFC2 ) were prepared and characterized. The two closo ‐compounds displayed major absorption bands assignable to π–π* transitions involving the spirobifluorene group, as well as weak intramolecular charge‐transfer (ICT) transitions between the o ‐carboranes and their spirobifluorene moieties. The nido ‐compounds exhibited slightly blueshifted absorption bands resulting from the absence of the ICT transitions corresponding to the o ‐carborane moieties due to the anionic character of the nido ‐ o ‐carboranes. While SFC1 exhibited only high‐energy emissions in THF at 298 K (only from locally excited (LE) states assignable to π–π* transitions on the spirobifluorene group), remarkable emissions in the low‐energy region were observed in the rigid state such as in THF at 77 K and in the film state. SFC2 displayed intense emissions in the low‐energy region in all states. The fact that neither of the nido ‐derivatives of SFC1 and SFC2 exhibited low‐energy emissions and the TD‐DFT calculation results of each closo ‐compound clearly verified that the low‐energy emission was based on ICT‐based radiative decay. The conformational barriers from each relative energy calculation upon changing the dihedral angles around the o ‐carborane cages for both compounds confirmed that the rotation of the o ‐carborane cages and terminal phenyl rings for SFC1 is freer than that for SFC2 .