Synthesis and characterization of a chiral spirobifluorene cyclic hexamer

A new chiral macrocycle 1-[6], consisting of six chiral spirobifluorene units linked in a cyclic arrangement, was successfully synthesized via a homo-coupling of the corresponding acyclic trimer. Notably, this chiral cyclic hexamer exhibited flexibility in solution, and both low-temperature nuclear magnetic resonance (NMR) experiments and density functional theory (DFT) calculations suggest that its stable structure is not the hexagonal D6-symmetric structure but rather the figure-eight-shaped D2-symmetric structure. In this D2-symmetric structure, the opposing bifluorenyl units are π-stacked, and it is suggested that this π-stacked region undergoes rapid dissociation and reformation at room temperature. The HOMO is distributed over the π-stacked region, with a HOMO-LUMO gap of 4.02 eV. The macrocycle exhibited strong violet fluorescence. The absorption dissymmetry factor |gabs| was 7.4 × 10−3, which is larger compared to the series of chiral smaller macrocycles 1-[n] (n = 3 to 5). Additionally, the CPL efficiency, indicated by a BCPL value of 189 M−1 cm−1, is relatively high among chiral organic luminescent molecules.