AB INITIO STATIC POLARIZABILITY AND FIRST HYPERPOLARIZABILITY OF MODEL POLYMETHINEIMINE CHAINS .2. EFFECTS OF CONFORMATION AND OF SUBSTITUTION BY DONOR/ACCEPTOR END GROUPS

摘要

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 (alpha(L)) and first hyperpolarizability (beta(L)). For the unsubstituted polymer the differences in beta(L) per unit cell among the four conformations examined-trans-transoid (TT), cis-transoid (CT), trans-cisoid (TC) and helical (HEL)-are large. beta(L)/N is positive for the CT polymer, in contrast with the other conformers and with mu(L)/N, which is negative in all cases. The TC polymer has the largest eta(L)/N despite having a relatively small alpha(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 beta(L)/N for small/medium N. In the case of the TT and, particularly, the CT conformer this leads to a maximum in eta(L)/N vs N at N=4-6, which is substantially greater in magnitude than the one at N = infinity. The latter is unaffected by substitution. We compare the [beta(L)(N)/N](max) obtained at intermediate N for substituted PMI chains with those calculated for linear polyenes and other pi-conjugated oligomers. The shape of all the beta(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. (C) 1997 American Institute of Physics. [References: 76]