Second-harmonic generation and electronic structure of polymethineimines: a theoretical study

Abstract

The second-harmonic-generation (SHG) coefficients and properties of low-lying electronic states in polymethineimines (---CH=N---)_x up to x = 5 are calculated exactly within the PPP (Pariser-Parr-Pople) Hamiltonian. The sp² hybridized nitrogen-atom parameters for the PPP model are taken from the literature, tested, and found to be satisfactory on pyridine, pyrazine, pyrimidine and s-triazine for which experimental optical gaps are known. The polymethineimines show an optical gap comparable to that of polyenes and the extrapolated single-strand gap for the infinite polymer is 2.67 eV, slightly smaller than that obtained for polyacetylene. The SHG coefficients in polymethineimines are comparable to that of the isoelectronic push-pull polyenes with moderately strong electron donors and acceptors. These molecules show a strong length dependence of the SHG coefficient which varies as L^3.04. These results are also valid for the mixed-stack donor-acceptor systems at half-filling.