A quadruple helicene with a rubicene core: synthesis, structural analyses and properties

We report the synthesis of a quadruple helicene with a rubicene core R1 by a Scholl reaction. Among the 10 stereoisomers including 4 pairs of enantiomers and 2 meso isomers, only 2 pairs of enantiomers and 1 meso isomer have been isolated. The sample structures were unambiguously determined by X-ray crystallography to be (P,P)6-(P,P)5/(M,M)6-(M,M)5-R1-A, which has a propeller-shaped structure, and (M,M)6-(P,P)5/(P,P)6-(M,M)5-R1-B and (M,P)6-(P,M)5-R1-C, which have saddle-shaped structures. The chiral resolutions of R1 were carried out by chiral HPLC, revealing two pairs of chiral stereoisomers (P,P)6-(P, P)5/(P,P)6-(M,M)5, (M,M)6-(P,P)5/(M,M)6-(M,M)5 as well as a meso isomer (M,P)6-(P,M)5, which were further characterized by CD spectroscopy and time-dependent density functional theory (TD-DFT) calculations. Surprisingly, the UV-vis absorption and emission spectra of these resolved stereoisomers and unresolved R1 were almost identical. In addition, the chemical oxidation of R1 led to the formation of radical cations and dications at room temperature.