Vibronic contribution to static NLO properties: exact results for the DA dimer

The standard 2-state model for push–pull polyenes is extended to include electron–phonon coupling. Exact adiabatic results are obtained for the vibronic contributions to static non-linear-optical polarizabilities. These contributions are very large. The Herzberg–Teller expansion offers a well-controlled approximation and naturally relates these vibronic contributions to vibrational intensities. The model parameters can be extracted from experiment: the analysis of available experimental data and the comparison with results of quantum chemistry calculations prove that the proposed model is very simple yet very accurate. The important role of electron–phonon coupling in linear and non-linear optical properties of other classes of compounds is also discussed.