π-Electron absorption and circular dichroism spectra of [6]- and [7]-helicene

The absorption and c.d. spectra of [7]-helicene are reported, together with a re-examination of the electronic spectrum of [6]-helicene. The transition energies and dipole and rotational strengths of [6]- and [7]-helicene have been evaluated in the π–SCF approximation, and theoretical absorption and c.d. spectra are derived on the assumption that the absorption bands have Gaussian forms. It is found that the degree of correspondence between the experimental and theoretical spectra declines with increasing molecular size over the series [5]-, [6]-, and [7]-helicene, and the significance of σ–π interaction and of π-electron exchange between the nearest-neighbour non-bonded carbon atoms in the terminal rings of the higher helicenes are discussed. It is concluded that the (+)-isomers of [4]-, [5]-, [6]-, and [7]-helicene have a right-handed helical configuration.