Theory of nonlinear optical properties of phenyl-substituted polyacetylenes

Abstract

In this paper we present a theoretical study of the third-order nonlinear optical properties of poly͑diphe-nyl͒polyacetylene ͑PDPA͒ pertaining to the third-harmonic-generation process. We study the aforesaid process in PDPA's using both the independent electron Hückel model, as well as correlated-electron Pariser-Parr-Pople ͑P-P-P͒ model. The P-P-P model based calculations were performed using various configuration-interaction ͑CI͒ methods such as the the multireference-singles-doubles CI and the quadruples-CI methods, and both the longitudinal and the transverse components of third-order susceptibilities were computed. The Hückel-model calculations were performed on oligo-PDPA's containing up to 50 repeat units, while correlated calculations were performed for oligomers containing up to ten unit cells. At all levels of theory, the material exhibits highly anisotropic nonlinear optical response, in keeping with its structural anisotropy. We argue that the aforesaid anisotropy can be divided over two natural energy scales: ͑a͒ the low-energy response is predominantly longitudinal and is qualitatively similar to that of polyenes, while ͑b͒ the high-energy response is mainly transverse, and is qualitatively similar to that of trans-stilbene.

Photoinduced absorption in disubstituted polyacetylenes: Comparison of theory with experiments

“…Since this conclusion is based upon calculations of various types ͑QCI, MRS-DCI͒, and on different oligomers ͑PDPA-5, PDPA-10͒, we conclude that these results are universally valid for PDPA's. Comparing these states to the ones contributing resonant peaks to the TPA and THG spectra of oligo-PDPA's calculated by us earlier, 16,17 we conclude that these states correspond to nothing but the mA g states of the corresponding oligo-PDPA. Recalling that the mA g is the state with a very strong dipole coupling to the 1B u state, which makes its presence prominent in both the TPA and THG spectra of several conjugated polymers, 13,21 it should not at all be surprising that it leads to an intense peak in their 1B u PA spectra as well.…”

“…The details of these CI-based many-body procedures have been presented in our earlier works. 10,16,17,20 Since the number of electrons in oligo-PDPA's is quite large because of the large unit cell, it is not possible to include all of the orbitals in the correlated calculations. Therefore, it is imperative to reduce the number of degrees of freedom by removing some orbitals from the many-body calculations.…”

“…The Ohno parametrization initially was carried out for small molecules, and, therefore, it is possible that the Coulomb parameters for the polymeric samples could be somewhat smaller due to interchain screening effects. 19 In various calculations performed on phenylenebased conjugated polymers including PDPA's, [9][10][11]16,17,20,21 we notice that "screened parameters" with U = 8.0 eV and i,i = 1.0, and i,j = 2.0, otherwise, proposed by Chandross and Mazumdar,19 lead to a much better agreement with the experiments, as compared to the "standard parameters" with U = 11.13 eV and i,j = 1.0, as proposed originally by Ohno. Therefore, in the present work we compare the results of our screened-parameter-based calculations with the experiments, while the results of our standard-parameter-based calculations are compared with similar calculations performed on corresponding polyenes.…”

“…We also compare the states visible in the PA spectra of the PDPA's with those in their TPA and the THG spectra computed by us recently. 16,17 The remainder of this paper is organized as follows. In Sec.…”

A Fortran 90 Hartree–Fock program for one-dimensional periodic π-conjugated systems using Pariser–Parr–Pople model

“…In recent years, our group and collaborators(author?) [28,29,30,31,32,33,34,35,36,37,38,39,40], along with numerous other groups(author?) [1,2,3,41,42,43,44,45,46], have used the P-P-P model to study the electronic structure and optical properties of conjugated molecules and oligomers.…”