S-Containing Double Helicenes Featuring Robust Radical Cations

This study introduces two S-containing double helicenes capable of forming robust radical cations. These compounds were synthesized via the Scholl reaction, which was modulated to an intermediate stage by reducing the amount of oxidant, resulting in a sulfur-containing [7]helicene. X-ray crystallography analysis reveals that both S-containing double helicenes exhibit twisted conformations consistent with the global minima identified through density functional theory (DFT) calculations. Upon chemical oxidation with nitrosonium, these double helicenes produced π-conjugated radical cations, whose radical nature was confirmed using electron paramagnetic resonance spectroscopy. These radical cations remained stable in solution when exposed to air for several weeks, as monitored with absorption spectroscopy. One of the double helicenes forms π-stacked columns in the crystals of both neutral molecule and radical cation salt. The radical cation hexafluoroantimonate salt exhibits a room-temperature conductivity of up to 0.16 S cm^-1, attributed to the significant π-π interactions observed in its crystal structure. This study indicates the potential of double helical π-skeletons as a versatile platform for developing resilient π-conjugated radical cations without compromising π-π interactions, with potential applications in organic electronics.