Ab initio static polarizability and first hyperpolarizability of model polymethineimine chains. II. Effects of conformation and of substitution by donor/acceptor end groups

An ab initio coupled Hartree-Fock study has been carried out on polymethineimine (PMI) oligomers with an increasing number of unit cells (up to N = 20) to determine the effect of conformational differences, and of end-capping by a strong donor-acceptor pair, on the static electronic longitudinal polarizability (α_L) and first hyperpolarizability (β_L). For the unsubstituted polymer the differences in β_L per unit cell among the four conformations examined - trans-transoïd (TT), cis-transoïd (CT), trans-cisoïd (TC) and helical (HEL) - are large, β_L/N is positive for the CT polymer, in contrast with the other conformers and with μ_L/N, which is negative in all cases. The TC polymer has the largest |β_L|/N despite having a relatively small α_L/N. Substitution of a strong acceptor (NO₂) at the carbon end of an oligomer and a strong donor (NH₂) at the nitrogen end results in a large increase of β_L/N for small/medium N. In the case of the TT and, particularly, the CT conformer this leads to a maximum in |β_L|/N vs N at N=4-6, which is substantially greater in magnitude than the one at N=∞. The latter is unaffected by substitution. We compare the [β_L(N)/N]_max obtained at intermediate N for substituted PMI chains with those calculated for linear polyenes and other π-conjugated oligomers. The shape of all the β_L/N vs N curves obtained is fit to a function which yields parameters characterizing the delocalization along the backbone, the unit cell asymmetry, and the asymmetry of the chain ends.