Theoretical investigations on cooperative phenomena in a few molecular and macromolecular systems.

Abstract

In the present thesis, we have attempted to elucidate the nature of dipolar interactions, hydrogen bonding and charge transfer in various technologically promising molecules and their polymorphs and between certain large systeM.S. like carbon nanotubes (CNTs) and metal clusters, focusing particularly on the structural variation, electrical and electronic properties of such species. Through various structural modifications, we have tried to modulate the extent of charge transfer by tweaking the strength of hydrogen bonding and other weak interactions like dipole-dipole interactions in these systeM.S.. Our findings support that such interactions are particularly important in modulating carrier mobilities in, and linear and non-linear optical responses of these classes of systeM.S.. We also find that Coulombic charge transfer between metal clusters and CNT in the metal- CNT composites result in semiconductor to metal transition. The thesis is divided into five chapters: The first chapter provides a brief introduction to non linear optics (NLO), the theory of Davydov Splitting and carrier (electron and hole) mobilities from the standpoint of Marcus’ theory and Quantum-Classical equations. Computational methodologies together with mathematical expressions for the optical and transport properties discussed in the preceding chapters have also been illustrated in detail. In the second chapter, we investigate the effect of intramolecular dipolar interactions in governing the NLO responses of free base porphyrin. Free base porphyrin has four pyrrole rings arranged in a coplanar arrangement in such a fashion that the dipole moments of the pyrrole rings trans to each other cancel, resulting in a net zero dipole moment of the overall molecule. On the other hand, for N-confused porphyrin, a synthetic molecule, one of the pyrrole rings is such that its N atom is located outside the macrocyclic cavity. For this molecule, the dipole moment of the pyrrole ring trans to the one confused, no longer cancels, resulting in a non-zero ground state dipole moment. We describe in this chapter how systematically distorting one pyrrole ring out of the coSynopsis 2 planar condition results in a non-zero dipole moment of the molecule, together with its NLO responses. In the third chapter, we consider co-operative phenomena in Magnesium complexes of structurally similar molecules like Bacteriochlorin, Chlorin and Porphyrin to understand the nature of long range dipole-dipole interactions, and how such phenomena affect the light absorption of oligomers of such systeM.S.. We find that monomers of these molecules in the Light Harvesting Complexes (LHCs) are not planar, the Mg atom being around 0.4 - 0.6 Å out of the plane of the porphyrinoid structural moiety. We find that the cross sections of optical absorption are larger for the non-planar conformations than the corresponding planar conformations of all the systeM.S.. We also estimate the differences in light absorption cross-sections of the oligomers of these systeM.S., particularly considering relative geometrical orientations similar to those found in the LHCs. In the fourth chapter, we consider linear hydrogen bonded chains of urea and thiourea and their derivatives to understand the effect of the varying nature of bonding on the NLO responses and charge mobilities in such linear chains. While molecules of Urea, N,N’-Dimethyl Urea, Thiourea and N,N’-Dimethyl Thiourea are held in their respective linear chains by hydrogen bonding, N,N,N’,N’ Tetramethyl Urea and N,N,N’,N’ Tetramethyl Thiourea are held together in linear chains by dipolar interactions. We attempt to elucidate the contribution of electrostatic forces and mixing of low-energy states in the intermolecular interactions in these systeM.S.. We find that the linear polarizability (α) increases linearly with increasing chain length; however the first order non-linear optical response (β) shows an oscillatory behavior. We elucidate such behavior of β from a simple two-state picture. Finally, in the fifth chapter, we consider the nature of interactions between metallic and semiconducting CNTs and metal clusters in elucidating the semiconductormetal transitions of these composites. We consider the noble metals Au and Pt, and present our results from calculations on various clusters and chains of the metals physisorbed onto CNTs. We find that the semiconducting CNTs upon interaction with the physisorbed metal clusters become metallic. Metallic CNTs, however, fail to show transition to semiconductors upon adsorption of metal clusters onto them.