Michael E. Foster

Synthesis, Characterization, and Computational Studies of Cycloparaphenylene Dimers

Two novel arene-bridged cycloparaphenylene dimers (1 and 2) were prepared using a functionalized precursor, bromo-substituted macrocycle 7. The preferred conformations of these dimeric structures were evaluated computationally in the solid state, as well as in the gas and solution phases. In the solid state, the trans configuration of 1 is preferred by 34 kcal/mol due to the denser crystal packing structure that is achieved. In contrast, in the gas phase and in solution, the cis conformation is favored by 7 kcal/mol (dimer 1) and 10 kcal/mol (dimer 2), with a cis to trans activation barrier of 20 kcal/mol. The stabilization seen in the cis conformations is attributed to the increased van der Waals interactions between the two cycloparaphenylene rings. These calculations indicate that the cis conformation is accessible in solution, which is promising for future efforts toward the synthesis of short carbon nanotubes (CNTs) via cycloparaphenylene monomers. In addition, the optoelectronic properties of these dimeric cycloparaphenylenes were characterized both experimentally and computationally for the first time.

Thin Film Thermoelectric Metal–Organic Framework with High Seebeck Coefficient and Low Thermal Conductivity

Abstract : A new thermoelectric material with high Seebeck coefficient and low thermal conductivity is demonstrated based on an electrically conducting metal-organic framework (MOF) using the guest at MOF concept. This demonstration opens a new avenue for the future development of thermoelectric materials.

Novel metal–organic framework linkers for light harvesting applications

Metal–organic frameworks (MOFs) are composed of organic linkers and coordinating metals that self-assemble to form a crystalline material with tunable nanoporosity. Their synthetic modularity and inherent long-range order create opportunities for use as new functional electronic materials. Using quantum mechanical computational methodologies we propose novel conjugated organic linkers that are capable of forming the same one-dimensional infinite metal-oxide secondary building units (SBUs) as the well-known IRMOF-74. This structural arrangement allows for the formation of a continuous π–π stacking network that should enable charge transport in fashion analogous to organic semiconductors. The structural and electronic properties (fundamental and optical gaps) of the isolated proposed linkers were modeled using a non-empirically tuned long-range corrected functional that leads to significantly improved results compared with experimental benchmarks. In addition, periodic hybrid density functional calculations were employed to model the extended MOF systems. Our results demonstrate how the electronic properties of MOFs can be readily modified to have favorable orbital alignments with known electron acceptors that should facilitate charge transfer. The predicted properties are in good agreement with experiment (i.e. UV-Vis absorption spectra), demonstrating the power of this computational approach for MOF design.

Energy and charge transfer by donor–acceptor pairs confined in a metal–organic framework: a spectroscopic and computational investigation

Molecular organization of donor–acceptor pairs within a metal–organic framework (MOF) offers a new approach to improving energy and charge transfer at donor–acceptor interfaces. Here, the photo-physical effects of infiltrating MOF-177 (ZnO4(BTB)2; BTB = 1,3,5-benzenetribenzoate) with α,ω-dihexylsexithiophene (DH6T) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), representing well-established polymeric and molecular materials used in organic photovoltaics, were probed using UV-visible absorption and luminescence spectroscopies combined with first-principles electronic structure calculations. The energetics of guest molecule infiltration were determined by constructing potential energy curves from self-consistent charge density-functional tight-binding (SCC-DFTB) calculations. These reveal that infiltration is energetically favored and that DH6T and PCBM are strongly bound to MOF-177 by 55 kcal mol−1 and 57 kcal mol−1, respectively. Solution-phase infiltration with PCBM achieved a 22 wt% loading, comparable to those in bulk heterojunction solar cells, but without evidence of phase segregation. DH6T loadings were very light (maximum of ∼1 molecule per 11 unit cells), but this was sufficient to produce significant quenching of the MOF-177 photoluminescence (PL). The coincident appearance of DH6T PL demonstrates that efficient Forster resonance energy transfer (FRET) from the MOF-177 linkers to DH6T occurs. These results show that the MOF is a multifunctional host that not only confines and stabilizes guest molecules, but also plays an active role, serving as a photon antenna that harvests light not efficiently absorbed by a donor molecule (DH6T in this case) and transferring it to guest acceptor molecules. Finally, time-dependent density functional theory (TDDFT) predicts the existence of linker-to-PCBM charge transfer states, suggesting that photoconductivity might be achievable in an appropriately designed guest@MOF system.

Results for aliovalent doping of CeBr3 with Ca2

Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr3) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca2+) was used as a dopant to strengthen CeBr3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca2+ dopant were grown. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca2+-doped CeBr3 exhibited little or no change in the peak fluorescence emission for 371 nm optical excitation for CeBr3. The structural, electronic, and optical properties of CeBr3 crystals were studied using the density functional theory within the generalized gradient approximation. The calculated lattice parameters are in good agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr3, including the dielectric function, were calculated.Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr3) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca2+) was used as a dopant to strengthen CeBr3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca2+ dopant were grown, i.e., 1.9% of the CeBr3 molecules were replaced by CaBr2 molecules, to match our target replacement of 1 out of 54 cerium atoms be replaced by a calcium atom. Precisely the mixture was composed of 2.26 g of CaBr2 added to 222.14 g of CeBr3. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca2+-doped CeBr3 exhibited little or no change in the...

Transforming MOFs for Energy Applications Using the Guest@MOF Concept

As the world transitions from fossil fuels to clean energy sources in the coming decades, many technological challenges will require chemists and material scientists to develop new materials for applications related to energy conversion, storage, and efficiency. Because of their unprecedented adaptability, metal-organic frameworks (MOFs) will factor strongly in this portfolio. By utilizing the broad synthetic toolkit provided by the fields of organic and inorganic chemistry, MOF pores can be customized to suit a particular application. Of particular importance is the ability to tune the strength of the interaction between the MOF pores and guest molecules. By cleverly controlling these MOF-guest interactions, the chemist may impart new function into the Guest@MOF materials otherwise lacking in vacant MOF. Herein, we highlight the concept of the Guest@MOF as it relates to our efforts to develop these materials for energy-related applicatons. Our work in the areas of H2 and noble gas storage, hydrogenolysis of biomass, light-harvesting, and conductive materials will be discussed. Of relevance to light-harvesting applications, we report for the first time a postsynthetic modification strategy for increasing the loading of a light-sensitive electron-donor molecule in the pores of a functionalized MIL-101 structure. Through the demonstrated versatility of these approaches, we show that, by treating guest molecules as integral design elements for new MOF constructs, MOF science can have a significant impact on the advancement of clean energy technologies.

Oxygen solubility and transport in Li–air battery electrolytes: establishing criteria and strategies for electrolyte design

Li–air or Li–oxygen batteries promise significantly higher energies than existing commercial battery technologies, yet their development has been hindered by a lack of suitable electrolytes. In this article, we evaluate the physical properties of varied electrolyte compositions to form generalized criteria for electrolyte design. We show that oxygen transport through non-aqueous electrolytes has a critical impact on the discharge rate and capacity of Li–air batteries. Through experiments and molecular dynamics simulations, we highlight that the choice of salt species and concentration have an outsized influence on oxygen solubility, while solvent choice is the major influence on oxygen diffusivity. The stability of superoxide reaction intermediates, key to the oxygen reduction mechanism, is also affected by variations in salt concentration and the choice of solvent. The importance of reactant transport is confirmed through Li–air cell discharge, which demonstrates good agreement between the observed and calculated mass transport-limited currents. These results showcase the impact of electrolyte composition on transport in metal–air batteries and provide guiding principles and simulation-based tools for future electrolyte design.

Solution-processable donor-acceptor polymers with modular electronic properties and very narrow bandgaps.

Bridgehead imine-substituted cyclopentadithiophene structural units, in combination with highly electronegative acceptors that exhibit progressively delocalized π-systems, afford donor-acceptor (DA) conjugated polymers with broad absorption profiles that span technologically relevant wavelength (λ) ranges from 0.7 < λ < 3.2 μm. A joint theoretical and experimental study demonstrates that the presence of the cross-conjugated substituent at the donor bridgehead position results in the capability to fine-tune structural and electronic properties so as to achieve very narrow optical bandgaps (Eg (opt) < 0.5 eV). This strategy affords modular DA copolymers with broad- and long-wavelength light absorption in the infrared and materials with some of the narrowest bandgaps reported to date.

Computational investigation of the role of counterions and reorganization energy in a switchable bistable [2]rotaxane.

Switchable bistable [2]rotaxanes, such as those of the Stoddart-Heath-type, show promise for the development of molecular electronic devices and functional prototypes have been demonstrated. Herein, one such switchable rotaxane system is studied computationally at the AM1-FS1 and DFT levels of theory. The results show that the computationally efficient AM1-FS1 method, (efficient relative to DFT) is capable of reliably predicting properties such as binding site preference and coconformational relative stabilities as well as the barrier to isomerization between the different coconformational states. These properties play important roles in the functionality of rotaxane-based molecular electronic devices. In addition, the role of the counterions is assessed from a computational standpoint. The results reveal that inclusion of counterions is not as significant as has been previously suggested. Finally, the reorganization energy associated with oxidation/reduction of the complex is studied. This provides a possible link to the origin of the observed conductivity difference between the two coconformational states, the property upon which device functionality is based.

Theoretical Study of Binding Site Preference in [2]Rotaxanes.

Rotaxanes that can be switched between co-conformations by some external stimulus are of interest because the switching mechanism might be used to create molecular devices capable of producing useful work. Probably the most common approach to create a switchable rotaxane is to start with a rotaxane where the ring interacts more strongly with one of two possible binding sites along the shaft and then apply an external stimulus that weakens the binding interaction between the ring and the shaft at this site, thereby changing the binding site preference. We have investigated binding site preference in two rotaxanes and two pseudorotaxanes with electronic structure calculations at several levels of theory. To gain insight into the origins of the intercomponent binding, empirical approximations were applied to estimate the electrostatic and dispersion contributions. Dispersion has been thought to make an important contribution to the intercomponent interaction in the presence of π-π stacking interactions between the components, but the role of dispersion interaction has been a controversial issue because many computational methods neglect this interaction. For example, AM1 semiempirical calculations neglect dispersion but often predict correct co-conformational preferences. This suggests that inclusion of the dispersion interaction is required for correct quantitative, but not qualitative, description of the intercomponent binding, a result that is supported by the analytic partitioning of the binding interactions. The origins of this result are investigated.