Chiral Cyclic [n]Spirobifluorenylenes: Carbon Nanorings Consisting of Helically Arranged Quaterphenyl Rods Illustrating Partial Units of Woven Patterns

Chiral cyclic [n]spirobifluorenylenes consisting of helically arranged quaterphenyl rods, illustrating partial units of woven patterns, were designed and synthesized as a new family of carbon nanorings. The synthesis was accomplished by the Ni(0)-mediated Yamamoto-coupling of chiral spirobifluorene building blocks. The structures of the cyclic 3- and 4-mers were determined by X-ray crystallographic analysis. These carbon nanorings exhibited a strong violet colored emission with high quantum yields in solution (95%, 93%, and 94% for 3-, 4-, and 5-mer, respectively). Other spectroscopic properties, including their chiroptical properties, were also investigated. The gvalues for circularly polarized luminescence were found to be in the order of 10-3, where that of the 4-mer showed a relatively higher value 8.5 x 10-3. Characteristic spiroconjugation induced by multiple (≧ 3) bifluorenyl units, for example the even-odd effect of the number of units in the matching of the sign of the orbitals, were also revealed by DFT calculations.