Ab initio investigation of doping-enhanced electronic and vibrational second hyperpolarizability of polyacetylene chains.

The effect of charging on the longitudinal second hyperpolarizability of polyacetylene (PA) chains containing up to nearly 70 carbon atoms has been investigated ab initio by characterizing chains with and without an explicit alkali atom (Li, Na, K) as dopant. Whereas charging dramatically enhances the static electronic and vibrational hyperpolarizabilities, γ[sup e][sub L](0) and γ[sup v][sub L], of an isolated chain at intermediate chain lengths, the presence of an alkali atom counterion substantially reduces this effect. As the size of the alkali atom increases, most properties, including the hyperpolarizabilities, approach those of the isolated chain. Detailed analysis shows that the behavior of γ[sup e][sub L](0) is most simply explained in terms of a reduced electrostatic pinning potential due to increased distance between chain and counterion. At all chain lengths studied γ[sup e][sub L](0) of PA is enhanced by alkali doping. For chains containing 50 carbon atoms (N[sub C] = 50), the increase due to K doping is about 9 x 10[sup 7] a.u., which more than doubles the value for an undoped chain of similar length. The normalized quantity γ[sup e][sub L](0)/N[sub C] exhibits a maximum for the isolated soliton (at about N[sub C] = 61) that is over four times that of the infinite undoped (and unbent) chain. When the alkali dopant is taken into account this maximum diminishes considerably and shifts to larger N[sub C] than we have considered. In comparison with the maximum for the undoped species (at N[sub C] = ∞) there is a small enhancement of γ[sup e][sub L](0)/N[sub C] for K doping, but none for either Li or Na doping at the coupled-perturbed Hartree—Fock (CPHF)/6-31G level of theory. Intermediate length isolated chains bearing a charged soliton show order of magnitude increases in γ[sup v] for the degenerate four-wave mixing (DFWM) and, especially, electric field-induced second harmonic generation (dc-SHG)... [ABSTRACT FROM AUTHOR]