Theoretical investigation on bridged triarylamine helicenes: UV/visible and circular dichroism spectra

The UV/visible and CD spectra of bridged triarylamine helicenes have been simulated using the TDDFT/B3LYP/6-31+G(d) method. They reproduce well the experimental spectra. For M-helicenes, the Cotton effect associated with the lowest-energy transition is negative for the smallest helicenes and becomes positive for the largest one, as a result of the variations of the angle between the electric and magnetic transition dipole moments. Since these species are stable upon oxidation, the UV/visible and CD spectra of their radical cations have been simulated. They show low-energy absorption and CD bands, figuring out enhancement of the first hyperpolarizability.

The Cotton effect of the lowest-energy transition band in bridged triarylamine helicenes is small because the electric and magnetic transition dipole moments are quasi perpendicular.