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

✍ Scribed by I.D.L. Albert; Puspendu K. Das; S. Ramasesha

• Publisher: Elsevier Science
• Year: 1991
• Tongue: English
• Weight: 581 KB
• Volume: 176
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(91)90157-5

✦ Synopsis

The second-harmonic-generation (SHG) coefficients and properties of low-lying electronic states in polymethineimines (-CH=N-), up to x= 5 are calculated exactly withm the PPP (Pariser-Parr-Pople)

Hamiltonian. The sp2 hybridized nitrogenatom 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 gapcomparable to that of polyenes and the extrapolated smgle-strand gap for the infinite polymer is 2.67 eV, slightly smaller than that obtained for polyacetylene. The SHG coefficients in polymethineimines arecomparable to that ofthe 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'.04. These results are also valid for the mixed-stack donor-acceptor systems at half-filling.