Electronic structure of merocyanine form of indolinespirobenzopyrans determined by electronic spectroscopy and dielectric measurements

Abstract

The permanent dipole moments of the merocyanine form of four indolinespirobenzopyrans in the ground (µ_g) and first excited states (µ_e) were determined by electronic spectroscopy and dielectric measurements. The high value of µ_g(11.8–13.3 D) indicates strong intramolecular charge transfer from nitrogen to oxygen atoms and supports the zwitterionic structure of the merocyanine form. The values of µ_e are much less (3.5–5.1 D) in comparision with µ_g, showing that the electronic structure of the merocyanine form in the first excited singlet state is closer to a non-polar quinoid type structure.