In an age where next-generation all-optical circuitry and optical data storage are at the forefront of the telecommunications industry, the molecular engineering and design of new organic materials continues apace. Such materials are particularly attractive on account of their fast optical response times, and superior non-linear optical susceptibilities, relative to their traditional inorganic counterparts. While dipolar molecules dominate the field of organic non-linear optical (NLO) materials, octupolar molecules have the potential to produce far greater NLO effects; moreover, they have the capacity to produce 3-D sensitive NLO phenomena.

This PhD explores new classes of dipolar organic and octupolar organometallic materials, where computations have predicted them to serve with superior NLO properties. To this end, concerted experimental and theoretical data are employed to characterise the electronic structure of these materials and elucidate their NLO properties. Data for electronic structures in this thesis were secured via in-house and synchrotron-based X-ray diffraction experiments (by proxy), which the author employed for charge density analyses. Multipolar modelling of experimental charge densities of the subject NLO materials forms an integral part of this thesis. Topological analysis is applied to these electronic structures, using the quantum theory of atoms in molecules (QTAIM), from which the structural and chemical origins of their NLO properties are assessed. Complementary theoretical methods were also used in this work, including calculations undertaken via density functional theory, as well as the relatively new technique of X-ray constrained wave-function refinement, which especially complements multipolar modelling methods, providing direct corroboratory topological analysis. An array of complementary experimental and computational methods is employed to evaluate the NLO properties of these materials in the gas-, solution-, and solid state-phase. The organometallic complexes presented in this thesis were also synthesised by the author.

Chapter 1 of this work begins by presenting some of the main principles behind NLO phenomena, before providing a review of some of the most salient organic and organometallic NLO materials investigated, to date. Chapter 2 provides details pertaining to the experimental and computational methods used within this work to evaluate the molecular origins of the NLO properties of the materials investigated herein. Chapter 3 explores the molecular origins of the NLO properties of a new class of dipolar organic chromophores via structural analysis, experimental charge density analyses, hyper-Rayleigh scattering and density functional theory. Chapter 4 similarly investigates a new class of dipolar organic NLO chromophores via structural analysis, hyper-Rayleigh scattering and density functional theory. However, topological analysis herein was undertaken solely via the X-ray constrained wave-function fitting method, due to the absence of high-resolution X-ray diffraction data for experimental multipolar modelling. Chapter 5 investigates two ionic organic chromophores and the implications of their intermolecular interactions on their respective NLO responses by building up the ionic system using a ‘molecular lego’ approach. Chapters 6-7 detail investigations of newly identified octupolar NLO organometallic complexes, and feature several rare examples of charge-density studies of materials containing heavy elements, such as the transition metal, zinc, and bromine These heavy elements are particularly challenging even for state-of-the-art experimental and computational materials characterisation methods. Chapter 8 concludes this work, and identifies possible future directions for investigations of NLO materials for next-generation telecommunications.