No description available.

Luminescent metal organic frameworks (MOFs) are promising new materials with applications as sensors, photocatalysts, and other luminescent devices. Although MOFs retain the chemical and physical properties of their constituents, the properties of the MOF are often altered from those of its building blocks, making rational design and synthesis difficult. Anthracene is a polyaromatic hydrocarbon whose photophysical properties have been found to be easily tuned through structural modifications. The tunability of anthracene makes it an ideal candidate for use in luminescent devices, such as photoprobes and organic light emitting diodes. MOFs designed with π conjugated molecules like anthracene ligands possess similar photophysical properties such as absorption and fluorescence in the UV and visible spectrum. In hopes of better understanding how the photophysical properties of the organic ligand is altered upon incorporation into a MOF, the spectroscopic properties of anthracenedicarboxylic acids were studied before and after integration into zinc based MOFs. Steady state and time resolved measurements were performed on three anthracenedicarboxylic acids: 9,10-anthracenedicarboxylic acid, 2,6-anthracendicarboxylic acid, and 1,4-anthracenedicarboxylic acid. The position of the carboxylic acid groups on anthracene was found to effect the position and structure of the absorption and emission spectra. The difference in the spectra is attributed to the perturbation by the acid groups on certain electronic transitions with dipole moments across two of the three axes of anthracene. The position of the acid groups had different effects on the fluorescence quantum yields and lifetimes of the three anthracenic acids studied. Two of the linkers were synthesized into MOFs through a solvothermal reaction with zinc nitrate, to form PCN-13, from 9,10-anthracenedicarboxylic acid, and [Zn(C₁₆H₈O₄)(H₂O)]_n, from 2,6-anthracenedicarboxylic acid. The luminescent properties of the two MOFs were studied and compared to those of the free based linker. Incorporation of the luminescent anthracenedicarboxylic acids into Zn based MOFs were found to either increase or decrease the luminescent properties of the ligands. / Master of Science

Flexible electronics have garnered much interest over the past several decades. Hybrid organic-inorganic materials, such as metal-organic frameworks, offer a unique opportunity to encompass the effective electronic properties of the inorganic material and the flexible nature of the organic with the potential of enhancing other desirable properties, such as the contributing multiferroicity. Using a first principles approach, the goal of this thesis is to serve as a guide for identifying potential ferroelectric and multiferroic metal-organic frameworks. This is done through a screening method of metal-organic frameworks based on their geometry; certain symmetry operators cannot be present in a ferroelectric material. We report the theoretical spontaneous polarization for several dozens of MOFs in which ferroelectricity has not previously been tested, and further we discuss the likelihood that these materials could be engineered to have either increased polarization or added ferromagnetism, the latter of which would lead to multiferroicity.

A growing field of research has evolved around the design and synthesis of a variety of porous metal-organic framework (MOF) materials. Some of the most promising areas for which these materials are potentially useful candidates include gas-separation, heterogeneous catalysis, and gas-storage, and all of these applications involve placing the MOF under pressure. There is clearly a need to understand the structural response of MOFs to applied pressure. Nevertheless, hitherto there are very few published investigations dedicated to determining the behaviour of porous hybrid materials under pressure. Through the use of high-pressure single-crystal X-ray diffraction studies, a series of MOF materials have been studied. Here we present the effect of pressure on a series of MOFs. In chapter 2, the effect of pressure on the prototypical MOF called MOF-5 was studied experimentally from ambient pressure to 3.2 GPa. Here, application of pressure was driven by the hydrostatic medium being forced into the pores of the MOF, which altered the mechanical properties of MOF-5, in particular, medium inclusion delayed the onset of amorphization. Complementary computational analysis was also performed to elucidate further the effect of medium inclusion on compressive behaviour. Detailed structural data was also collected as a function of pressure on the MOF Cu-btc. Application of pressure caused solvent to be squeezed into the pores (like MOF-5) until a phase transition occurred, driven by the sudden compression and expansion of equatorial and axial Cu–O bonds. High-pressure post-synthetic modification of a MOF is reported for the first time. On application of pressure of 0.2 GPa to the Cu-based MOF called STAM-1, a ligand exchange reaction takes place resulting in a change in pore size, shape, and hydrophilicity of the resulting pores. Here, we also demonstrate the ability to force hydrophilic molecules into hydrophobic pores using pressure, counteracting the hydrophobic effect. A high-pressure combined experimental and computational study has been carried to probe the effect of pressure on ‘breathing’ mechanisms in a zeolitic imidazolate framework (or ZIF) called ZIF-8. The penetration of guest molecules and the accommodation of pressure are shown to be inextricably linked to the rotation of methylimidazolate groups in the structure. Finally, the application of pressure to the MOF Sc₂BDC₃ and the nitro functionalized derivative Sc₂(NO₂-BDC)₃ was also studied. Here, the effect of chemical modification of the organic ligand, whilst maintaining framework topology, has been investigated as it pertains to compressibility. Directionality of compression is observed and this is rationalized with respect to the framework topology and medium inclusion/exclusion.

The use of Raman spectroscopy in ultra high vacuum to assess structure and reactivity at the interface of tris-(8-hydroxyquinoline) aluminum (Alq3) with vapordeposited metals is presented. Understanding the structure of the interface between electron transport layer materials such as Alq3 and low work function metals such as Al, Mg and Ca is vital for engineering organic light emitting diodes with high efficiency and low driving voltage. Reactivity at the interface of Al, Mg and Ca with Alq₃ thin films is examined with Raman spectroscopy along with the non-reactive Ag/Alq₃ interface for comparison. Additionally, the effect of a thin LiF barrier layer on reactivity at the Al/Alq₃ and Mg/Alq₃ interfaces is also examined. Raman spectroscopy of post-deposited Ag on Alq3 films confirms preservation of the Alq₃ structure along with evolution of simple surface enhancement of Alq₃ spectral intensities. Changes in key vibrational modes of Alq₃ upon Ag deposition are consistent with weak interaction of Ag with the conjugated ring of the ligand. In contrast, vapor deposition of Al onto Alq₃ films results in the appearance of new Raman modes linked to the formation of an Al-Alq₃ adduct. Additionally, Raman modes associated with graphitic carbon are also noted for the Al/Alq₃ interface and are attributed to partial degradation of the organic film. The Raman spectral results for deposition of Mg onto Alq3 films also indicate formation of a complex interfacial region composed primarily of Mg-Alq₃ adducts and small-grained amorphous or nanocrystalline graphite. Raman spectroscopy of the Ca/Alq₃ interface is also indicative of formation of a Ca-Alq₃ complex; however, the graphitic carbon in this system is noted to be more disordered, sp³-type carbon compared to that observed for Al/Alq₃ and Mg/Alq₃. Examination of the Al/LiF/Alq₃ and Mg/LiF/Alq₃ interfaces illustrates that 5 Å-thick LiF layers partially block reaction chemistry between the metal and organic, while 10 Å thick LiF films completely eliminates reactivity at these interfaces. Implications of the presence of chemical species observed at these metal/organic interfaces on charge transport in devices are also discussed.

No description available.

Metal-organic frameworks (or coordination polymers) are a recently-identified class of porous polymeric materials, consisting of metal ions or clusters linked together by organic bridging ligands. The major advantage of MOFs over other traditional materials, such as zeolites or activated carbons, is that their synthesis methods have provided an extensive class of crystalline materials with high stability, tunable metrics, and organic functionality. The ability to modify the physical environment of the pores and cavities within MOFs allow tuning of the interactions with guest species, and serves as a route to tailor the chemical stability and/or reactivity of the frameworks for specific applications. The classical way to incorporate functional groups into a MOF is the modification of the organic precursor with specific substituents before synthesizing the MOF itself; we call this approach pre-functionalization method. Functionalization of organic precursors is the initial and necessary step to obtaining functionalized isostructural MOFs and also provides the possibility for the post-synthetic modification of MOFs. However, in some cases, the functional groups may interfere with MOF synthesis and alter the topology of desired MOF. The goal of this proposed research is to explore the possibilities of metal-organic frameworks (MOFs) as novel porous structures, to study the effect of functional groups on the topologies and adsorption behavior of MOFs, and to understand how the synthesis conditions affect the phase purity and the in-situ reaction of ligands.

Adsorption in porous materials plays a significant role in industrial separation processes. Here, the host-guest interaction and the pore shape influence the distribution of products. Metal-organic frameworks (MOFs) are promising materials for separation purposes as their diversity due to their building block synthesis from metal corners and organic linker gives rise to a wide range of porous structures. The selectivity differs from MOF to MOF as the size and shapes of their pores are tuneable by altering the organic linkers and thus changing the host-guest interactions in the pores. Using mainly molecular simulation techniques, this work focuses on three types of separations using MOFs. Firstly, the experimental incorporation of calix[4]arenes in MOFs as a linker to create additional adsorption sites is investigated. For a mixture of methane and hydrogen, it is shown that in the calix[4]arene-based MOFs, methane is adsorbed preferentially over hydrogen with much higher selectivities compared to other MOFs in the literature. Remarkably, it was shown that extra voids created by calix[4]arene-based linkers, were accessible to only hydrogen molecules. Secondly, the strong correlation between different pore sizes and shapes in MOFs and their capabilities to separate xylene isomers were investigated for a number of MOFs. Finally, the underlying molecular mechanism of enantioseparation behaviour in a homochiral MOF for a number of chiral diols is presented. The simulation results showed good agreement with experimental enantioselectivity values. It was observed that high enantioselectivity occurs only at high loadings and when a perfect match in terms of size and shape exists between the pore size and the adsorbates. Ultimately, the information obtained from molecular simulations will further our understanding of how network topology, pore size and shape in MOFs influence their performance as selective adsorbents for desired applications.

Thesis advisor: Chia-Kuang (Frank) Tsung / The core-shell hybrid structure is the simplest motif of two-component systems which consists of an inner core coated by an outer shell. Core-shell composite materials are attractive for their biomedical, electronic and catalytic applications in which interface between core and shell is critical for various functionalities. However, it is still challenging to study the exact role that interface plays during the formation of the core-shell structures and in the properties of the resulted materials. By studying the formation mechanism of a well interface controlled one-to-one metal nanoparticle (NP)@zeolite imidazolate framework-8 (ZIF-8) core-shell material, we found that the dissociation of capping agents on the NP surface results in direct contact between NP and ZIF-8, which is essential for the formation of core-shell structure. And the amount of capping agents on the NP surface has a significant effect to the crystallinity and stability of ZIF-8 coating shell. Guided by our understanding to the interface, one-to-one NP@UiO-66 core-shell structure has also been achieved for the first time. We believe that our research will help the development of rational design and synthesis of core-shell structures, particularly in those requiring good interface controls. / Thesis (MS) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

This thesis explores the use of computational chemistry to aid in the design of metal-organic frameworks (MOFs) and other materials. A focus is placed on finding exceptional materials to be used for removing CO2 from fossil fuel burning power plants, with other avenues like vehicular methane storage and landfill gas separation being explored as well. These applications are under the umbrella of carbon capture and storage (CCS) which aims to reduce carbon emissions through selective sequestration. We utilize high-throughput screenings, as well as machine learning assisted discovery, to identify ideal candidate materials using a holistic approach instead of relying on conventional gas adsorption properties. The development of ideal materials for CCS requires all aspects of a material to be considered, which can be time-consuming. A large portion of this work has been with high-throughput, or machine learning assisted discovery of ideal candidates for CCS applications. The chapters of this thesis are connected by the goal of finding ideal materials for CCS. They are primarily arranged in increasing complexity of how this research can be done, from using high-throughput screenings with more simple metrics, up to multi-scale machine learning optimization of pressure swing adsorption systems. The work is not presented chronologically, but in a way to tell the best story. Work was done by first applying high-throughput computational screening on a set of experimentally realized MOFs for vehicular methane storage, post-combustion carbon capture, and landfill gas separation. Whenever possible, physically motivated figures of merits were used to give a better ranking and consideration of the materials. From this work, we were able to determine what the realistic limits might be for current MOFs. The work was continued by looking at carbon-based materials (primarily carbon nanoscrolls) for post-combustion carbon capture and vehicular methane storage. The carbon-based materials were found to outperform MOFs; however, further studies are needed to verify the results. Next, we looked at ways to improve the high-throughput screening methodology. One problem area was in the charge calculation, which could lead to unrealistic gas adsorption results. Using the split-charge equilibration method, we developed a robust way to calculate the partial atomic charges that were more accurate than its quick calculation counterparts. This led to gas adsorption properties which more closely mimicked the results determined from time-consuming quantum mechanically derived charges. Simplistic process optimization was then applied to nearly ~3500 experimental structures. To the best of our knowledge, this is the first time that any process optimization has been applied to more than 10s of materials for a study. The process optimization was done by evaluating the desorption at various pressures and choosing the value which gave the lowest energetic cost. It was found that a material synthesized by our collaborators, IISERP-MOF2, was the single best experimentally realized material for post-combustion carbon capture. What made this an interesting result is that by conventional metrics IISERP-MOF2 does not appear to be outstanding. Next, functionalized versions of MOFs were tested in a high-throughput manner, and some of those structures were found to outperform IISERP-MOF2. Although high-throughput computational screenings can be used to determine high-performance materials, it would be impossible to test all functionalized versions of some MOFs, let alone all MOFs. Functionalized MOFs are noteworthy because MOFs are highly tuneable through functionalization and can be made into ideal materials for a given application. We developed a genetic algorithm which, given a base structure and a target parameter, would be able to find the ideal functionalization to optimize the parameter while testing only a small fraction of all structures. In some cases, the CO2 adsorption was found to more than quadruple when functionalized. A better understanding of how materials perform in a PSA system was achieved by performing multi-scale optimizations. Experimentally realized MOFs were tested using atomistic simulations to derive gas adsorption properties. After passing through a few sensible filters, they were then screened using macro-scale pressure swing adsorption simulators, which model how gas separation may occur at a power plant. Using another genetic algorithm, the conditions that the pressure swing adsorption system runs at was optimized for over 200 materials. To the best of our knowledge, this is the highest amount of materials that have had been optimized for process conditions. IISERP-MOF2 was found to perform the best based on many relevant metrics, such as the energetic cost and how much CO2 was captured. It was also found that conventional metrics were unable to be used to predict a material’s pressure swing adsorption performance.