Electronic structure calculations on helicenes. Concerning the chirality of helically twisted aromatic systems

CNDO/S calculations including configuration interaction of up to 300 singly excited states have been performed on the MMP2 and PM3 optimized geometries of [n]-helicenes, with n from 3 to 9. Oscillator and rotatory strengths have been calculated for all states down to 200 nm, with generally good agreement in the long wavelength regions. The computed data are analyzed on the basis of a four-orbital model yielding two forbidden L and two allowed B states. The latter ones including signs correlate with two high intensity absorptions of opposite signs observed in the spectra of all helicenes except the smallest one. The description in terms of tangential and radial transition dipoles provides a physical picture of the excited states. It is shown that the -,+ pattern of these bands is conclusive evidence for the M-helical absolute configuration of these compounds.