The electronic properties of polyacetylene–polymethineimine block copolymers

Ionization energies (IE), electron affinities (EA) and electronic transition energies (Δ E(S 0–T 1) and Δ E(S 0–S 1)) of a series of polyacetylenes (PA), H(HCCH) n H ( n≤30), polymethineimines (PMI), H(HCN) n H, and their block copolymers (PA–PMI), H(HCCH) n/2 (HCN) n/2 H, were investigated using the MP2/6-31G(d) (IE, EA, Δ E(S 0–T 1)) and CIS/6-31G(d) (Δ E(S 0–T 1) and Δ E(S 0–S 1)) quantum mechanical methods. IEs and EAs of PA–PMI approached values found in PA and PMI, respectively, although small residual differences remained in the asymptotic limit. The results were explained on the basis of orbital visualization, calculated charge distributions and fragment molecular orbital (FMO) analysis, which revealed that, whereas the PA–PMI HOMO is localized in the PA block, the copolymer’s LUMO contains significant electron density in both blocks, resulting from interactions of the PA LUMO with several of the low-lying PMI fragment’s virtual orbitals. Electronic transition energies, Δ E(S 0–T 1) and Δ E(S 0–S 1), of the copolymer approached values found in pure PA, which is not consistent with the above picture. These results were explained on the basis of excited state orbital relaxation, in which the cation-like “hole”, localized in the PA block, exerts a greater pull on the electron than does the more electronegative PMI portion of the molecule.