Helical Pitch Dependent Optical Properties of π-Extended Aza[5]helicene Radical Cations

We prepared a series of pi-extended aza[5]helicenes with different substituents at internal positions. The structures determined by X-ray diffraction analysis revealed that the steric bulkiness of substituents affected the helical pitch of aza[5]-helicenes. Their UV/vis absorption spectra showed little dependence on structural distortion. However, the absorption spectra of oxidized species were considerably dependent on internal substituents. Finally, theoretical calculations confirmed that the intramolecular through-space interaction caused a hypochromic shift of the lowest energy bands for the oxidized species.