Theoretical investigations of optical polarizations in chromophoric aggregates and nonlocal charge distributions in clusters

Abstract

Materials exhibiting large linear and nonlinear optical polarizations are a subject of great interest due to their many potential applications. In this thesis, I have investigated the factors that control such polarizations in the molecular and supramolecular levels. The main emphasis has been to understand the role of weak intermolecular forces like the dipole-dipole, hydrogen-bonding and -stacking interactions, in controlling the overall polarization responses in such class of materials. Another major theme that has been considered in the thesis is the modeling of the structures and optical properties of various metallic clusters. We have devised strategies to stabilize various metallic clusters through the inorganic route. The thesis is divided into eight chapters. The first chapter provides a brief introduction to linear and nonlinear polarizations and their various potential applications. The design of molecular and supramolecular materials possessing high laser damage thresholds with high dielectric constants and fast laser response time have been discussed. Various strategies are discussed for enhancement of the polarization responses in macromolecular aggregates like organic crystals and thin-filM.S. structures. A number of computational techniques are also outlined for the calculation of vi the static and dynamic electric field induced linear and non-linear response functions. In the second chapter, a theory is developed based on dipole- dipole interactions to determine the excitation spectra of multichromophoric aggregates in various orientations of the monomers. Numerical calculations are performed on dimers of D- -A systeM.S. like paranitroaniline and their derivatives in various modes of arrangements to quantify the proposed analytical theory. We predict that the head-to-tail arrangement of the dipoles in the aggregate leads to the maximum enhancement in the second harmonic responses ( ). Additional H-bonding interactions between the monomers further increases the polarization responses. The third chapter aiM.S. at providing a quantitative estimation of the role of dipolar and H-bonding interactions in controlling the polarization responses in molecular aggregates. These two forces have been optimized for the (HX)n aggregates (X=F, Cl and Br). It is found that for the strong H-bonded clusters like the (HF)n, planar cyclic rings are formed leading to very small . However, for H-bonds of intermediate strengths, non-centric structures are formed with appreciable value. Similar conclusions are also derived from the calculation of for the linear chains of (HF)n aggregates. The main inference from this chapter is that the H-bonding in the intermediate energy scales with appropriate directionality will lead to cooperative enhancement in . The fourth chapter deals with the conformational orientations of dipolar molecules that are connected through alkane chains which result in confined geometries. SysteM.S. like calix[3]arenes provide a nice example to study the role of dipolar frustration in the odd-membered chromophoric aggregates. The calculations performed on calix[3]arenes suggest that, while the -value decreases monotonically with increase in the cone-angle of the all-parallel calix[3]arenes, it increases with increase in the cone-angle for the frustrated geometries. Molecular structures as retrieved from the structural database support our conclusions with cone-angle as the unique parameter. The fifth chapter discusses the variation in for the dipolar aggregates which are connected by flexible spacers. The specific dipolar orientations are considered for oxo-bridged paranitroaniline dimers (PNA-O-PNA) for a quantitative estimation of dipolar interactions. We suggest molecular systeM.S. where the maximum polarization responses can be attained by ’conformationally locking’ the dimers through C-C bridges. Additionally, the origin of the odd-even oscillations in the second harmonic generation responses in alkyl bridged di-chromophores are also discussed. A simple theory based on the conformational flexibility of the alkane chains is provided to explain the oscillations in for these systeM.S.. It is shown that for the dipoles connected by even spacers, there is a cancellation of the dipole moment together with , due to the staggered conformation of alkane chains. However, when the number of spacers are odd, the dipoles have an eclipsed conformation which leads to addition of the dipole vectors with appreciable . The sixth chapter compares and contrasts the conventional - conjugated systeM.S. with the all-metal molecular systeM.S. like Al4Li4 for their nonlinear optical responses. It is shown that the all-metal clusters exhibit polarizations that are orders of magnitude higher than their organic counterparts of similar sizes. This arises primarily due to the poor - separation in the all-metal molecules which remarkably reduces the optical gap for the all-metal Al4Li4 systeM.S.. The strong charge-transfer from the alkali metals to the Al4 ring further enhances the transition dipole moment. The seventh chapter provides a methodology for separating the and energies in the ground state structures for clusters. Through this method, we are able to assign the overall aromaticity/antiaromaticity within all-metal systeM.S.. The method is also utilized to study the electron delocalizations in a -only clusters like the (Li)n systeM.S.. The (Li)n clusters exhibit oddeven oscillations in their binding energies as a consequence of frustration and pairing up of the electrons for the odd and even membered (Li)n clusters, respectively. A simple Heisenberg-spin Hamiltonian qualitatively explains the odd-even oscillations in magnetic binding energies. In the final chapter, strategies are proposed for the possible synthesis of all-metal antiaromatic compounds through the organometallic route like complexations of transition metal ions. Complexation of the 4 Al4Li4 clusters with low-valent transition metals like Fe(0) and Ni(0) facilitates metal-toligand charge transfer leading to an addition of two extra -electrons to the Al4Li4 rings and making it aromatic. Substitution reactions are also proposed within the conventional sandwich complexes wherein the organic molecules can be replaced by the all-metal systeM.S.. We find that, while for the halfsandwich complexes, (Al4M4)Fe(CO)3 (M=Li, Na and K), direct substitutions are highly exothermic, for the full-sandwich complexes, (Al4M4)2-Ni, the substitution reaction proceeds through a hybrid organic-inorganic intermediate, (Al4M4)Ni(C4H4) which makes it suitable for synthesis.