Linear and nonlinear polarizabilities of polydiacetylene and polybutatriene chains: An ab initio coupled Hartree-Fock investigation

Ab initio Hartree-Fock 6-31G longitudinal polarizabilities and second hyperpolarizabilities of polydiacetylene and polybutatriene oligomers are computed and, then, extrapolated to the infinite polymer limit. Both the electronic and vibrational components are obtained with the latter evaluated in the double harmonic approximation. For static fields the electronic hyperpolarizability of the butatrienic form is almost 50 times larger than the acetylenic form; the ratio of the vibrational to the electronic component is 0.86 for the latter and 3.3 for the former. Thus, the two components are sensitive to the difference in bond length alternation (BLA) in different ways. Depending upon the particular nonlinear process it is shown that the vibrational hyperpolarizability may remain large even at optical frequencies. Three intense Raman-active k=Q modes dominate the vibrational component. At the level of theory used here these modes occur in the frequency range 1000-2500 cm^-1 and they consist of changes in the BLA as well as hydrogen wagging motions. For polydiacetylene, there is a 1:1 relation with the corresponding vibrations that make the most important hyperpolarizability contributions in polyacetylene and polyyne.