Electronic absorption, polarised excitation, and circular dichroism spectra of [5]-helicene (dibenzo[c,g]phenanthrene)

Abstract

The absorption and circular dichroism spectra of (+)-dibenzo[c,g]phenanthrene-9,10-dicarboxylic acid are reported, together with the polarised excitation and fluorescence spectra of the parent hydrocarbon, [5]-helicene (dibenzo[c,g]phenanthrene). The transition energies and dipole and rotational strengths of [5]-helicene have been calculated in the π–SCF approximation, using a general helix model and a model based on X-ray diffraction data for the molecular structure and a constant and a bond-length-dependent π-resonance integral. It is found that the theoretical spectroscopic quantities are not sensitive to the choice of structural model or of a constant or variable β-value, and that their relative values are in satisfactory agreement with experiment. It is concluded that (–)-[5]-helicene has the stereochemical form of a segment of a left-handed helix, and that the (+)-isomer of the derived 9,10-dicarboxylic acid has the enantiomorphic configuration.