Experimental and Computational Studies of New Nonlinear Optical Materials

Abstract

Organic materials with nonlinear optical properties have various technological and scienti c applications. This work, had as main purpose the development and structural characterization of these new substances. Compared to dipolar molecules, the octupolar molecules proved to be substantially more advantageous for obtaining materials with high nonlinear optical response. Focusing on octopular compounds, like guanidine salts and its derivatives, and on thiocyanuric acid, some new salts were synthesized and the structural characterization was performed using the single crystal X-ray di raction. The salts that present noncentrosymmetric structures, were studied experimentally with the Kurtz-Perry powder method for determination of the second order susceptibility. For this materials the polarizabilities and hyperpolarizabilities of microscopic units were calculated with various computational methods (Hartree-Fock and DFT) using as starting point the geometries obtained experimentally. It was used the oriented gas model to calculate the second-order susceptibilities from the microscopic optical properties, for crystals made of microscopic units with no special symmetry. Two di erent local eld corrections were used in the oriented gas model. The rst correction was Lorenz-Lorentz correction for spherical cavity and the other an extension of the Onsager's reaction eld developed by Wortmann and Bishop. Comparing with the experimental results, the second correction describe better the nonlinear e ects. Nonlinear optical properties are intrinsically connected with the electronic distribution of each compound, so it is important to understand its behavior and the necessary conditions in order to improve the nonlinear optical response in materials and consequently their applicability. The electronic density of two structures with guanidine derivatives were studied starting from the low temperature X-ray di raction and applying a multipolar re nement. This allowed us to obtain several properties of the electronic density. This can be studied from a topological point of view, allowing us the obtention of several properties of the electronic density and of the inter and intra-molecular interactions.