Molecular Engineering of Dipolar and Octupolar Non-Linear Optical Materials for Next-Generation Telecommunications

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.EPSR

Molecular Magnetism Modeling with Applications in Spin Crossover Compounds

Molecular magnetic materials have become flourishing fields for research and technological developments due to their novel behavior compared to classical magnetic materials. Molecular magnetism modeling has reached a certain degree of maturity, although several experimental findings are still open problems. This chapter is aimed at providing a general introduction to physical modeling in molecular materials with a special emphasis placed on spin crossover compounds. This presentation includes Ising-type models and their generalizations, such as Wajnflasz and Pick, Bousseksou et al., Zimmermann and König, Sorai and Seki, and Nasser et al., along with their applications to the characterization of phase transition, hysteresis behavior, and thermal relaxations in spin crossover compounds. Recent experimental findings are explained in this context and the relevance of theoretical results for technological applications is also discussed

Molecular Structure and Modeling of Water-Air and Ice-Air Interfaces Monitored by Sum-Frequency Generation.

From a glass of water to glaciers in Antarctica, water-air and ice-air interfaces are abundant on Earth. Molecular-level structure and dynamics at these interfaces are key for understanding many chemical/physical/atmospheric processes including the slipperiness of ice surfaces, the surface tension of water, and evaporation/sublimation of water. Sum-frequency generation (SFG) spectroscopy is a powerful tool to probe the molecular-level structure of these interfaces because SFG can specifically probe the topmost interfacial water molecules separately from the bulk and is sensitive to molecular conformation. Nevertheless, experimental SFG has several limitations. For example, SFG cannot provide information on the depth of the interface and how the orientation of the molecules varies with distance from the surface. By combining the SFG spectroscopy with simulation techniques, one can directly compare the experimental data with the simulated SFG spectra, allowing us to unveil the molecular-level structure of water-air and ice-air interfaces. Here, we present an overview of the different simulation protocols available for SFG spectra calculations. We systematically compare the SFG spectra computed with different approaches, revealing the advantages and disadvantages of the different methods. Furthermore, we account for the findings through combined SFG experiments and simulations and provide future challenges for SFG experiments and simulations at different aqueous interfaces

Mechanisms of light energy harvesting in dendrimers and hyperbranched polymers

Since their earliest synthesis, much interest has arisen in the use of dendritic and structurally allied forms of polymer for light energy harvesting, especially as organic adjuncts for solar energy devices. With the facility to accommodate a proliferation of antenna chromophores, such materials can capture and channel light energy with a high degree of efficiency, each polymer unit potentially delivering the energy of one photon-or more, when optical nonlinearity is involved. To ensure the highest efficiency of operation, it is essential to understand the processes responsible for photon capture and channelling of the resulting electronic excitation. Highlighting the latest theoretical advances, this paper reviews the principal mechanisms, which prove to involve a complex interplay of structural, spectroscopic and electrodynamic properties. Designing materials with the capacity to capture and control light energy facilitates applications that now extend from solar energy to medical photonics. © 2011 by the authors; licensee MDPI, Basel, Switzerland

Summaries of FY 1997 Research in the Chemical Sciences

The objective of this program is to expand, through support of basic research, knowledge of various areas of chemistry, physics and chemical engineering with a goal of contributing to new or improved processes for developing and using domestic energy resources in an efficient and environmentally sound manner. Each team of the Division of Chemical Sciences, Fundamental Interactions and Molecular Processes, is divided into programs that cover the various disciplines. Disciplinary areas where research is supported include atomic, molecular, and optical physics; physical, inorganic, and organic chemistry; chemical energy, chemical physics; photochemistry; radiation chemistry; analytical chemistry; separations science; heavy element chemistry; chemical engineering sciences; and advanced battery research. However, traditional disciplinary boundaries should not be considered barriers, and multi-disciplinary efforts are encouraged. In addition, the program supports several major scientific user facilities. The following summaries describe the programs

Proceedings of the Thirteenth International Conference on Time-Resolved Vibrational Spectroscopy

The thirteenth meeting in a long-standing series of “Time-Resolved Vibrational Spectroscopy” (TRVS) conferences was held May 19th to 25th at the Kardinal Döpfner Haus in Freising, Germany, organized by the two Munich Universities - Ludwig-Maximilians-Universität and Technische Universität München. This international conference continues the illustrious tradition of the original in 1982, which took place in Lake Placid, NY. The series of meetings was initiated by leading, world-renowned experts in the field of ultrafast laser spectroscopy, and is still guided by its founder, Prof. George Atkinson (University of Arizona and Science and Technology Advisor to the Secretary of State). In its current format, the conference contributes to traditional areas of time resolved vibrational spectroscopies including infrared, Raman and related laser methods. It combines them with the most recent developments to gain new information for research and novel technical applications. The scientific program addressed basic science, applied research and advancing novel commercial applications. The thirteenth conference on Time Resolved Vibrational Spectroscopy promoted science in the areas of physics, chemistry and biology with a strong focus on biochemistry and material science. Vibrational spectra are molecule- and bond-specific. Thus, time-resolved vibrational studies provide detailed structural and kinetic information about primary dynamical processes on the picometer length scale. From this perspective, the goal of achieving a complete understanding of complex chemical and physical processes on the molecular level is well pursued by the recent progress in experimental and theoretical vibrational studies. These proceedings collect research papers presented at the TRVS XIII in Freising, German

Quantum Confined Noble Metal Monolayer Protected Clusters Investigated Using Linear and Nonlinear Microscopy

Elemental quantum confined nanocluster systems were previously demonstrated to have unusual optical, electronic, catalytic and magnetic properties suggesting to classify them as a new form of matter. Optical investigations in solution phase ensembles using monolayer protected nanoclusters (MPCs) allowed the community to experimentally confirm that the metal-to-insulator transition in gold occurs at ~300 gold atoms. However, investigations of single nanoclusters using optical microscopy and spectroscopy to determine effects of quantum confinement in MPCs were not reported until now. In my dissertation work, I interrogated isolated single quantum confined Au25 MPCs on a solid substrate. My observations made on isolated and aggregated MPCs on solid using two-photon excited fluorescence (TPEF) near-field scanning optical microscopy (NSOM) revealed that their native quantum confinement effects manifest primarily when they are isolated from aggregates and solution ensembles. This is consistent with the picture of narrowing of the density of states (DOS) when the quantum clusters are removed from aggregates and studied in isolation on solid. Also, it agrees with the enhancement expected for volume-normalized oscillator strengths (f12/V) of electronic transitions in the presence of quantum size effects. In order to obtain isolated single nanoclusters on solid, I devised a procedure where I synthesized MPCs, isolated them in solution phase and then deposited isolated single nanoclusters on solid substrate with ~160 nm average inter-nanocluster distances. Scanning transmission electron microscopy (STEM) confirmed the isolation of single nanoclusters on solid. The investigations of isolated MPCs on solid using aperture-based TPEF NSOM elicit ~30 nm point resolution which is ~5-fold better than the typical confocal point resolution. Also, my findings on possible local field enhancement for MPCs suggest the potential to use isolated MPCs, MPC arrays, meshes or lattices to obtain significantly enhanced TPEF properties that can be used in molecular computing, bioimaging,sensing, and data storage applications. On a separate investigation, I explored materials that can increase the theoretical efficiency limit of organic photovoltaics (OPV) via ntramolecular singlet exciton fission (iSEF). I interrogated a quinoidal bithiophene molecule in solution that revealed highly efficient ultrafast iSEF with ~180% singlet-to-triplet conversion efficiency. Our finding of iSEF in a small molecule invigorates theoretical and experimental investigations of small molecule iSEF materials to make highly efficient solar cells.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135860/1/neranga_1.pd

Elucidating the Electronic Origins of Intermolecular Forces in Crystalline Solids

It is not possible to study almost any physical system without considering intermolecular forces (IMFs), no matter how insignificant they may appear relative to other energetic factors. Countless studies have shown that IMFs are responsible for governing a wide variety of physical properties, but often the atomic-origins of such interactions elude experimental detection. A considerable amount of work throughout the course of this research was therefore placed on using quantum mechanical simulations, specifically density functional theory (DFT), to calculate the electronic properties of solid-materials. The goal of these calculations was a better understanding of the precise origins of interatomic energies, down to the single-electron level. Furthermore, experimental X-ray diffraction and terahertz spectroscopy were both utilized because they are able to broadly probe the potential energy surfaces of molecular crystals, enhancing the theoretical data. Combining DFT calculations with experimental measurements enabled in-depth studies into the nature of specific non-covalent interactions, with results that were often unexpected based on conventional descriptions of IMFs. Overall, this work represents a significant advancement in understanding how subtle changes in characteristics like orbital occupation or electron density can have profound effects on bulk properties, highlighting the fragile relationship that exists between the numerous energetic parameters occurring within condensed phase systems