Large vibronic contributions to nlo properties of conjugated systems

Recent exact results on the adiabatic DA dimer, as a model for static NLO properties of push-pull polyenes, are briefly reviewed. Emphasis is put on the non-linearity induced by the electron-phonon (e-ph) coupling and on the resulting huge amplification of non-linear responses. The Herzberg-Teller approach to e-ph coupling in the DA dimer is discussed as an approximate scheme to calculate vibronic contributions to static NLO, and to relate these quantities to vibrational spectra. The same approach is adopted to investigate vibronic contributions to static NLO properties in symmetric conjugated molecules and polymers, in terms of a simple three-state model.