A vibronic model Hamiltonian suitable for the study of the optical properties in the near-IR-visible of partially localized systems is presented and discussed. The problem of how to deal with localization in a symmetric species is examined in view of understanding the origin of the observed Stark spectra. It is shown that a weak perturbation that breaks the symmetry of the Hamiltonian, such as that due to interaction with the solvent, allows charge localization in the ground state, as well as provides for a good prediction of the observed Stark spectra.