Theoretical Investigation Of Nonlinear Optical Responses In Conjugated Polymer Chains

We investigated nonlinear optical (NLO) responses in organic conjugated linear chain such as polydiacetylene (PDA) and polyene (PA) at the ab initio levels. Both the finite-field (F.F) method and recently proposed method based on the local atomic charge derivatives are employed with the aim to identify the origins and the length dependence of NLO response. We discussed the growth of the first and the third order polarizability of both PDA and PA show saturation behavior at long chain-length, and search the differences between the two polymers.Through the calculation of polarizability for PDA and PA, we found that for the first and the third order polarizability of PDA in short chain, the growth index achieves saturation quickly compared to PA, and then becomes the linearity to grow. In the long chain-length limit, after the growth of PDA saturates and tends to become linear growth, the growth rate of PDA is lower than PA. Thus, the first and the third order polarizability of PDA are both smaller than PA. The local charge derivative picture gives a qualitative account for the saturation behavior. We found the increment of the first and the third polarizability includes the polarization of the charges on the double or triple bond in the central region and the expansion of the region near the chain-end. We also identify the terminal parts of the chain as the main contributor for the nonlinearity. The polarization of the charges in the chain end of PDA is smaller than PA, so PDA has smaller the first and the third order polarizability than PA. PDA has single, double and triple bond, which structure is more complex than PA. The presences of the two pairs of special electron which are at both triple bond at both ends, increase its bond ends of the electronic polarization, and also affects on electronic polarization on the double bond.We also discussed the effect of substituted on PDA and PA. The results showed that the effect of the substituted group is localized, and that the increment of the first and the third order polarizability due to the substituted groups rapidly increases initially with increasing length, but the increase becomes slower and slower, and finally reaches a constant value. The substitution causes bond-length distortions near chain end. This distortions stem from the re-distribution of electron charge densities, which alter the ionic-electronic repulsions and thus bond lengths. Third order polarizability is more sensitivity to the electron state. Thus, third order polarizability has larger changes under the substitution. Substitution also causes more affection on PA than PDA. It revealed that the different combinations of substituents results in profoundly different NLO polarizability for short chain-length cases. However, the effects by the presence of substitute become additive in the long chain-length limit.