Geometric and electronic structure of zwitterionic push-pull polyenes for nonlinear optics

We present a study of the chemical structure of highly polar push-pull conjugated molecules of interest for nonlinear optics, in which purely inductive ammonium/borate substituents, R₃N⁺/BR₃^-, are attached to a trans-polyene chain of variable length either directly or via phenyl groups. The performance of ab initio Hartree-Fock and Møller-Plesset second order (MP2) methods is compared with that of the hybrid density functional theory (DFT) and semiempirical (AM1) Hartree-Fock and configuration interaction methods. Taking into account electron correlation at the MP2 level is important to obtain reliable molecular geometries; the hybrid DFT (BH and HLYP functional) results are found to be in close agreement with the MP2 results. Some geometries obtained with the semiempirical methods are in qualitative disagreement with the MP2/6-31G results. A chemical rationalization of the optimized structures and the nature of the substituent effect is proposed.