Single configuration interaction study on conjugated betainic chromophores based on DFT optimized geometries

The structure of a series of heterocyclic betaines was calculated by methods of density functional theory (DFT). The charge distribution and bond characteristics of these compounds were analyzed by Weinhold’s natural bond orbital analysis (NBO) and by natural resonance theory (NRT). In order to probe the aromatic character of the ring fragments, Schleyer’s nucleus-independent chemical shifts (NICSs) were calculated by GIAO-RHF. Ab initio single configuration interaction calculations (SCI) correctly predict intense π→π* transitions at low energies, but the transition energies of the color bands are overestimated. Torsion around the interfragmental bond increases the charge separation between the molecular fragments and the dipole moment. The molecular fragments become increasingly aromatic. The absorption wavelengths increase on torsion while the oscillator strengths decrease.