Quantum chemical investigation of linear and nonlinear optical polarizabilities in organic molecular aggregates and inorganic clusters

Abstract

In the present thesis, we have tried to elucidate the proper parameters upon which the nonlinear optical properties of materials depend. The thesis is divided into five chapters. The first chapter provides a brief introduction to Nonlinear optics and its various applications. In the next chapter we have developed a model considering the orientation effects of the molecules in an assembly and discuss briefly how such different orientations can lead to changes in the optical properties of the molecular assemblies. We have also outlined the methods for stablilization of such assemblies due to Hydrogen - bonding interactions. In the third chapter we consider a series of push-pull type organic chromophores and consider their various dipolar orientations and suggest the best orientation for maximizing their NLO responses based on the theory developed in chapter 2. In the fourth chapter, we consider a series of bridged push-pull chromophores where orientation of the dipoles are restricted by the degrees of freedom (bonds connecting the push-pull moieties) and try to understand the factors that govern their behaviors in an electric field. We also suggest modifications to the experimental systems that will maximize their optical polarizations. Finally, in the fifth chapter, we extend our dipole interaction theory to finite-sized metallic clusters. These hetero-atomic metal clusters are interesting because of the ease by which they can be polarized due to out-of-plane charge transfer contrary to TT- conjugated organic polymers which are polarized due to delocalization of the vr-electrons. We compare and contrast the NLO properties of the conventional organic 7r-conjugated molecules to that of these metal clusters of similar sizes and geometries. We show that the NLO properties for these metal-clusters are orders of magnitude higher than their organic counterparts. We conjecture that these metal-clusters are good candidates for next generation NLO materials.