The effect of side chain substituents on third-order optical nonlinearity of conjugated polymers: a theoretical study

We present a detailed investigation concerning the influence of phenyl side groups, adjacent to a carbon backbone, on the third-order nonlinear optical susceptibility χ(3)(ω) of conjugated polymers. A theoretical calculation of χ(3)(ω is carried out within the simple two-band model of the electronic structure of the polymer based on a tight-binding approximation. Our treatment takes into consideration two sources of optical nonlinearity: the interband transitions of π-electrons and combined (interband—intraband) ones. The influence of the substituent groups upon the electronic structure of the polymer is modeled by changing the Coulomb integral on the sites of the main chain to which the side groups are attached. We adopt the Genkin—Mednis approach to derive analytical expressions for χ(3)(ω) responsible for the third-harmonic generation, the intensity-dependent index of refraction and the two-photon absorption. The spectra χ(3)(ω) are evaluated numerically for all these effects and the resonance structure of the spectra is analyzed depending on the distance between the side groups and their coupling strength with the backbone. It is found that the interaction between the side groups and the backbone results in a change of the sign of χ(3) (0) from positive (which is predicted for polyacetylene) to negative and leads to an enhancement of the χ(3)(0) coefficient with increasing side-group separation along the backbone. The resonance behavior of χ(3)(ω) in the transparency region of the polymer also reveals remarkable differences from that observed experimentally in polyacetylene. Our results show that the optimal positioning of the substituent groups along the backbone may be promising for engineering new polymer materials with significantly enhanced nonlinear optical properties.