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Title:Microporous materials fabricated from discrete molecular cages
Author(s):Moneypenny, II, Timothy Patrick
Director of Research:Moore, Jeffrey S.
Doctoral Committee Chair(s):Moore, Jeffrey S.
Doctoral Committee Member(s):Suslick, Kenneth S.; Cheng, Jianjun; Fout, Alison R.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):porous materials
porous molecular solids
organic cage
alkyne metathesis
dynamic covalent chemistry
Abstract:In the context of materials science, an interesting relationship exists between the properties of solid materials and the existence of void spaces within them. In fact, whether the presence of voids is desired or not tends to depend on one’s perception of the effects that voids induce. In densified materials, for example, the presence of voids can be detrimental to structural integrity. Thus, such materials that contain voids are considered defective. On the other hand, when voids are desirable, their presence in certain materials is essential to material behavior. In zeolites, for example, the size, shape, and connectivity of void spaces regulate catalytic activity. In reality, however, and at some finite length scale, all real materials contain intrinsic void space; a consequence of the imperfect packing arrangements of atoms. Thus, it is necessary not only to elucidate what effects voids have on the properties of materials, but also to investigate methods that provide control of void features within solid materials. While all materials possess intrinsic voids, the ability to introduce intentional voids in solids presents multiple difficulties. The statement “nature abhors a vacuum” is a familiar quip that reflects this challenge of designing open pore spaces in solid materials, as porous frameworks with open void spaces are often higher in energy relative to their more dense structural counterparts. Nonetheless, during the last few decades, technology has advanced such that scientists have significant control over the size, shape, and position of voids within solids. Materials such as zeolites, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs) all demonstrate the profound ability to position pores of various shapes and sizes with molecular precision in a solid framework. This control over pore design has led to significant materials applications for porous materials including adsorption, catalysis, and molecular separation. Despite the successes of porous networks such as zeolites, MOFs, and COFs, there remains a need for greater molecular diversity and tunable microenvironments that are precise in molecular design. Moreover, there is a need for fundamental understanding of the relationship between characteristics of voids derived from molecular species and the behavior these entities exhibit within solid materials. Herein, we test the hypothesis that discrete molecular cages with non-collapsible pores are building blocks for porous solids by preparing molecular cages via alkyne metathesis. We demonstrate that molecular pores can be rationally synthesized from tritopic organic precursors in a single step and assembled in the solid state to afford permanently porous materials. Featuring organic synthesis and modular packing, our methodology provides molecular control for the fabrication of functional porous materials with precise microenvironments. First, a non-intuitive precursor design principle for synthesizing molecular cages via alkyne metathesis is described. By subjecting a series of precursors with varying bite angles to AM, it is experimentally demonstrated that the product distribution and convergence towards product formation is strongly dependent on precursor bite angle. Furthermore, it was discovered that precursors with the ideal tetrahedron bite angle (60º) do not afford the most efficient pathway to the product. These results lend credence to the underlying systemic issues facing the synthesis of 3D architectures via dynamic covalent chemistry, where variations in precursor geometry lead to significant deviation of product distributions away from discrete products. Next, a systematic study of the effects of molecular shape-persistence on the porosity of molecular solids is discussed. Three molecular cages synthesized via alkyne metathesis and post-synthetic modifications were designed to provide controlled, stepwise adjustments in molecular shape-persistence. Experimental measurements of nitrogen adsorption taken from rapidly and slowly crystallized solids of each cage demonstrated a trend in porosity that correlated with shape-persistence. Molecular dynamic simulations that modeled cage motion corroborated the trend seen in the experimental data and emphasized that shape-persistence governs the microporosity of these materials. Our integrated synthetic and computational approach demonstrates that the microporosity of this class of molecular solids can be controlled through fine-tuning at both the atomic and microscales. Lastly, the fabrication and characterization of a novel solid-state lithium electrolyte nanocomposite derived from a porous molecular cage is discussed. A solid-liquid electrolyte nanocomposite (SLEN) fabricated from an electrolyte system and a porous organic cage exhibits ionic conductivity on the order of 1 x 10-3 S cm-1. With an experimentally measured activation barrier of 0.16 eV, this composite is characterized as a superionic conductor. Furthermore, the SLEN displays excellent oxidative stability up to 4.7 V vs. Li/Li+. This simple three-component system enables the rational design of electrolytes from tunable, discrete molecular architectures that possess intrinsic void space.
Issue Date:2018-07-02
Rights Information:Copyright 2018 Timothy Patrick Moneypenny II
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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