Amplification of Dissymmetry Factors in π-Extended [7]- and [9]Helicenes

pi-Extended helicenes constitute an important class of polycyclic aromatic hydrocarbons with intrinsic chirality. Herein, we report the syntheses of pi-extended [7]helicene 4 and pi-extended [9]helicene 6 through regioselective cyclodehydrogenation in high yields, where a prefusion strategy plays a key role in preventing undesirable aryl rearrangements. The unique helical structures are unambiguously confirmed by X-ray crystal structure analysis. Compared to the parent pristine [7]helicene and [9]helicene, these novel pi-extended helicenes display significantly improved photophysical properties, with a quantum yield of 0.41 for 6. After optical resolution by chiral high-performance liquid chromatography, the chiroptical properties of enantiomers 4-P/M and 6-P/M are investigated, revealing that the small variation in helical length from [7] to [9] can cause an approximately 10-fold increase in the dissymmetry factors. The circularly polarized luminescence brightness of 6 reaches 12.6 M-1 cm(-1) as one of the highest among carbohelicenes.

Automated summary

In this study, pi-Extended [7]helicene and pi-Extended [9]helicene were successfully synthesized with improved photophysical properties and high circularly polarized luminescence brightness. The research also revealed that a small variation in helical length can cause an increase in dissymmetry factors.