Chemical and Photophysical Behaviour of π-Extended Tropyliums

Non-benzenoid annulenes have long fascinated organic chemists, due to their chemical properties, aromaticity, and photophysical behaviour. When incorporated into a polycyclic aromatic framework, such non-hexagonal rings can give rise to nonplanar structures with modified optical properties and improved solubility compared to planar, defect-free graphene. Among non-benzenoid annulenes, the tropylium cation has been of special interest, due to its unique blend of reactivity (owing to its positive charge) and stability (a result of its aromaticity). Indeed, this cation has found wide utility as a versatile ligand , stimulus-responsive dye , and an organocatalyst. Yet, reports of polycyclic aromatics featuring this heptagonal annulene are sparse. This Thesis employs tropylium and its neutral, nonaromatic homologue, cycloheptatriene (Figure 1) as key structural motifs within an extended π-framework to gain fundamental insights into the electronic and optical properties of non-benzenoid and charged polycyclic aromatics. We find that judicious engineering of strain into the framework of sterically overcrowded tropyliums can cause its aromaticity to rupture, forming an “aromatic-to-nonaromatic” equilibrium at room temperature. Moreover, modifying the conjugation length in a series of cycloheptatriene-rotors was found to vastly alter their photoluminescence properties, allowing for new modes of chemical reactivity. We also report our synthetic forays toward a highly warped redox-active warped nanographene, as well as preliminary findings on the excited-state proton transfer dynamics in a series of hydroxybenzotropyliums. 1. T. Murahashi, M. Fujimoto, M. A. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki and H. Kurosawa, Science, 2006, 313, 1104–1107 2. U. P. N. Tran, G. Oss, D. P. Pace, J. Ho and T. V. Nguyen, Chem. Sci., 2018, 9, 5145–5151. 3. D. J. M. Lyons, R. D. Crocker and T. V. Nguyen, Chem.—Eur. J., 2018, 24, 10959–10965. 4. P. K. Saha, A. Mallick, A. T. Turley, A. N. Bismillah, A. Danos, A. P. Monkman, A.- J. Avestro, D. S. Yufit and P. R. McGonigal, Nature Chem., 2023 15, 516–525. 5. A. T. Turley, P. K. Saha, A. Danos, Aisha N. Bismillah, A. P. Monkman, D. S. Yufit, B. F. E. Curchod, M. K. Etherington, and Paul R. McGonigal, Angew. Chem. Int. Ed., 2022, 61, e202202193

Structure and atomic dynamics in condensed matter under pressure and Li-ion battery materials

The main goal of this research was to apply first-principles electronic structure calculations to investigate atomic motions in several condensed materials. This thesis consists of five separate but related topics that are classified into two main categories: structure of materials under pressure and Li ion dynamics in lithium battery materials. The atomic structure of liquid gallium was investigated in order to resolve a controversy about an anomalous structural feature observed in the x-ray and neutron scattering patterns. We explored the pressure effect when modifying the liquid structure close to the solid-liquid melting line. The atomic trajectories obtained from first-principles molecular dynamics (FPMD) calculations were examined. The results clarified the local structure of liquid gallium and explained the origin of a peculiar feature observed in the measured static structure factor. We also studied the structure of a recently discovered phase-IV of solid hydrogen over a broad pressure range near room temperature. The results revealed novel structural dynamics of hydrogen under extreme pressure. Unprecedented large amplitude fluxional atomic dynamics were observed. The results helped to elucidate the complex vibrational spectra of this highly-compressed solid. The atomic dynamics of Li ions in cathode, anode, and electrolyte materials - the three main components of a lithium ion battery - were also studied. On LiFePO4, a promising cathode material, we found that in addition to the commonly accepted one-dimensional diffusion along the Li channels in the crystal structure, a second but less obvious multi-step Li migration through the formation of Li-Fe antisites was identified. This discovery confirms the two-dimensional Li diffusion model reported in several Li conductivity measurements and illustrates the importance of the distribution of intrinsic defects in the enhancement of Li transport ability. The possibility of using type-II clathrate Si136 as an anode material was investigated. It was found that lithiated Si-clathrates are intrinsic metals and their crystal structures are very stable. Calculations revealed the charge and discharge voltages are very low and almost independent of the Li concentrations, an ideal property for an anode material. Significantly, migration pathways for Li ions diffusing through the cavities of the clathrate structures were found to be rather complex. Finally, the feasibility of a family of Li3PS4 crystalline and nanoporous cluster phases were studied for application as solid electrolytes. It was found that the ionic conductivity in the nanocluster is much higher than in crystalline phases. It is anticipated that the knowledge gained in the study of battery materials will assist in future design of new materials with improved battery charge and discharge performance

Solid -State NMR of High Spin Nuclei With Large Quadrupolar Coupling Constants.

The goal of this study is to develop a method of characterizing the aluminum sites in solid aluminum containing systems that are difficult to determine by normal nuclear magnetic resonance spectroscopy, NMR, or X-ray crystallography. Specifically, the aim is to characterize the aluminum sites in the industrial cocatalyst methylaluminoxane, MAO. The structure of MAO, found in both solid and gel form, has not been well determined. First, an automated 27Al (spin 5/2) single-crystal NMR experiment was used to determine the experimental electric field gradient, EFG, of andalusite, a well-studied mineral with large quadrupolar interactions. The experimental results were compared to ab initio calculated values. All aluminum sites in the unit cell could be assigned, even the crystallographically equivalent but magnetically inequivalent sites. Next, a variable-temperature frequency-stepped NMR experiment was developed for use with powder samples. The method was tested on a niobium (spin 9/2) complex and validated with a spinning side-band analysis and X-ray crystal determination. From the frequency-stepped NMR spectra of four imidoalanes, containing ring or cage structures, the 27Al Cq and eta values were determined. The experimental results compared well to ab initio calculations and connections could be made between the corresponding ring angles or distances and values for Cq and eta. Finally, variable-temperature solid-state 27Al NMR spectra of MAO were acquired from -80° to 130°C. An analysis shows that pristine MAO has a highly rigid structure. Slight exposure to air causes the MAO molecules to become more mobile with no detectible -OCH3 groups in the 13C[1H] CPMAS spectrum. The 27Al frequency-stepped NMR spectrum was used to determine a four-coordinate aluminum resonance with a Cq of ∼26 MHz and eta ∼ 1. This study thus proposes a highly rigid polymeric structure of MAO containing groups of two six-membered ring dimers connected by agostic Al-H-CH2 bonding all with four-coordinate aluminum sites. On slight exposure to air the agostic bond is replaced with oxygen and individual cage structures are formed which have more mobility than the polymeric structure and potential for four-, five- and six-coordinate aluminum sites

Molecules in Superfluid Helium Nanodroplets

This open access book covers recent advances in experiments using the ultra-cold, very weakly perturbing superfluid environment provided by helium nanodroplets for high resolution spectroscopic, structural and dynamic studies of molecules and synthetic clusters. The recent infra-red, UV-Vis studies of radicals, molecules, clusters, ions and biomolecules, as well as laser dynamical and laser orientational studies, are reviewed. The Coulomb explosion studies of the uniquely quantum structures of small helium clusters, X-ray imaging of large droplets and electron diffraction of embedded molecules are also described. Particular emphasis is given to the synthesis and detection of new species by mass spectrometry and deposition electron microscopy

Frame Transformation Relations and Symmetry Analysis of Fluxional Symmetric Rotor Dimers

symmetr

Polyphospholyl ligands as building blocks for the formation of polymeric and spherical assemblies

This thesis is concerned with polyphospholyl ligands as building blocks for the formation of polymeric and spherical assemblies. Within the scope of this work, the coordination chemistry of 1,2,4-triphosphaferrocenes, 1,2,4-triphospholyl anions and pentaphosphaferrocenes towards Cu(I) halides is investigated. Thereby, the first two substance classes preferably tend to the build-up of novel one-, two- and three-dimensional polymers bearing rather uncommon or even unprecedented structural motifs. On the other hand, pentaphosphaferrocenes are capable of the template-directed synthesis of discrete, nano-sized supramolecules. Within a comprehensive study on the template requirements a series of small molecules could be encapsulated in these spheres for the first time. In addition, hitherto unknown host complexes were obtained which all differ in size, charge and topology. Besides the coordination chemistry, also novel complexes and salts were synthesized and characterized, which can now be used as building blocks in supramolecular chemistry

Reports to the President

A compilation of annual reports for the 1985-1986 academic year, including a report from the President of the Massachusetts Institute of Technology, as well as reports from the academic and administrative units of the Institute. The reports outline the year's goals, accomplishments, honors and awards, and future plans