Effect of bond length alternation and of bond length alternation oscillations on the molecular nonlinear optical response of push-pull polyenes

Quantum chemical ab initio calculations have been performed on a model push-pull polyene. In these calculations the mean bond length alternation of the chain has been constrained to specific discrete values so as to yield various structures ranging from a polyenelike geometry to a zwitterionic geometry. It is shown that this purely geometric distortion of the nuclei induces relevant changes of the electronic properties such as molecular dipole moment, molecular polarizabilities, and hyperpolarizabilities. Also, simulated vibrational spectra are heavily affected by changes in geometry. By changing the molecular environment (solvent effect) the theoretically predicted molecular nonlinear optical behavior parallels the trends observed experimentally, thus showing the key role played by the bond length alternation parameter. In Appendix A the derivative relationship among hyperpolarizabilities at various orders as a function of the bond length alternation parameter is discussed.