Noniterative density functional response approach: application to nonlinear optical properties of p-nitroaniline and its methyl-substituted derivatives.

We report the effect of substitution, position of the substituent, and the symmetry on the nonlinear optical properties ofp-nitroanline (PNA) and its derivatives using our implementation of the noniterative approximation of couple-perturbed Kohn-Sham (CPKS) equation in the deMon2k. Dipole moment, static polarizability, and first hyperpolarizability of these!-conjugated donor-acceptor organic derivatives of PNA and its methyl- substituted analogs are calculated using our method at different exchange correlation functionals, namely, BP86, BPW91, and BLYP, using 6-31++G** basis set. A comparison of results obtained by our method with those obtained by MP2 (finite-field perturbation) method is presented in this paper. The effect of optical gap on charge transfer and subsequently on polarizabilities has been illustrated. Introduction The area of nonlinear optics (NLO) continues to be in the limelight since past few decades. There has been tremendous advancement in the field of NLO materials and the experimental techniques available to study them. Progress in the development of many such technologically important processes in areas of optical information processing, telecommunications, integrated optics, optical computers, laser technology, etc., is based on understanding of molecular properties of the constituent materials. A change in the environment of a molecule has an effect on its structural and physical properties. The effect of a weak external perturbation on the electronic distribution of the molecule is thus reflected in its response properties. Response electric properties, in particular, have been studied for a wide variety of atoms and molecules in the past using different approaches. Materials with high second-order responses are considered important for NLO applications. Organic molecules with delocalized!electrons have attracted a lot of attention as they exhibit particularly large nonlinear responses. 1 The microscopic structure-property relationship for such molecules may lead to discovery of improved NLO characteristics in materials, thus, facilitating the design of new molecules for potential NLO applications. 2 This could be done through study of response electric properties, namely, polarizability and hyperpolarizability, of the molecules using computational methods. Electron-correlation effects play a crucial role in the deter- mination of these properties. NLO properties of a variety of push-pull phenylenes have been studied extensively. 3,4 The sensitivity of the NLO properties to the conjugation length, donor and acceptor substitutions, and effects due to symmetry in the molecule are well-known. 5 Over the period,p-nitroaniline (PNA) has been identified as a prototype system to understand the large nonlinear responses exhibited due to the presence of donor-acceptor moieties which lead to charge transfer in the PNA molecule. There are number of studies available for the PNA molecule. 6-13 Bulat et al. 14 studied the PNA molecule as well as the effect of chain length on electric properties of aliphatic push-pull compounds at different levels of theory and discussed the effect of electron correlation on the calculations. Determination of first and second order polarizability, in particular, plays an important role in understanding the response of molecule to an external weak electric perturbation. Electric response properties 15-22 has been studied extensively usingab initiomethods 22-24 as well as density functional theory (DFT) for both atoms and molecules. DFT among other theoretical methods has been used to study electric properties of such materials. The Hartree-Fock theory like computational scaling, while accounting for electron-correlation effects, makes DFT an obvious choice for large scale computations.