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 (αL) and first hyperpolarizability (βL). For the unsubstituted polymer the differences in βL per unit cell among the four conformations examined—trans-transoı̈d (TT), cis-transoı̈d (CT), trans-cisoı̈d (TC) and helical (HEL)—are large. βL/N is positive for the CT polymer, in contrast with the other conformers and with μL/N, which is negative in all cases. The TC polymer has the largest |βL|/N despite having a relatively small αL/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 βL/N for small/medium N. In the case of the TT and, particularly, the CT conformer this leads to a maximum in |βL|/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 [βL(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 βL/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.