Redox-Assisted Self-Assembly of π-Conjugated Chromophores Provides Function-Enhanced Superstructures
Kaixuan Liu, Adam Levy, Chuan Liu, Jean-Hubert Oliver
Department of Chemistry, Cox Science Center, 1301 Memorial Drive, University of Miami, Florida 33124
While nature has mastered the art of engineering non-equilibrium structures that enable energy capture, conversion, and storage with unrivaled efficiency, this level of structural control over synthetic materials with dimensions spanning the nano- to mesoscale remains elusive. Consequently, current technology exploits equilibrium-based strategies to construct organic electronic materials for which molecular interactions and macroscopic organization are neither structurally nor electronically configured for maximum efficiency. Redox-assisted self-assembly of water-soluble perylene diimide will be presented as a new tool to access out-of-equilibrium intermediates through which to navigate the aggregation free energy landscapes and engineer supramolecular assemblies kinetically trapped in local energy minimum. Investigating the electronic properties of these off-equilibrium superstructures using ground-state electronic absorption spectroscopy indicates a 30% enhancement of the exciton bandwidth when compared to equilibrium-constructed architectures. Such modification of nanoscale-object electronic properties exclusively provided by redox-assisted self-assembly originates from a reconfiguration of the superstructure conformation. Examination of the solid-state morphology of assemblies produced through electronically perturbed out-of-equilibrium intermediates reveals complex hierarchical architectures that feature micrometer-long anisotropic domains. To conclude, a preliminary molecular road map to regulate redox-assisted self-assembly will be introduced. The ability to modulate nanoscale-object electronic structure, used in conjunction with facile hierarchical organization offers exceptional promises for the development of optoelectronic materials.
Evidence of Ferrimagnetism in II-VI Dilute Magnetic Quantum Dots
Jasleen K. Bindra1,2, Lavrenty Gutsev1, Sebastian A. Stoian1,2, Johan van Tol2, Kedar Singh3, Geoffrey F. Strouse1, Naresh S Dalal1,2*
1Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, USA
2National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, USA
3School of Physical Sciences, Jawaharlal Nehru University, New Delhi 110067, India
4Department of Physics, Institute of Science, Banaras Hindu University, Varanasi 221005, India
The onset of spin frustration with increasing iron incorporation into wurtzite ZnSe is computationally predicted and experimentally tested. Density functional theory (DFT) models indicate magnetic spin frustration arises due to formation of local Fe-Fe spin lattices within the ZnSe lattice. Comparison of the DFT-GGA spin models to the experimentally prepared 1.8 nm Fe0.1Zn0.9Se QD reveal the predicted spin frustration within the QD is observed by comparison to field and temperature dependent Mossbauer and magnetic susceptibility measurements. The use of computational methods that can anticipate the onset of desirable properties apriori in QDs prior to the synthetic preparation will enhance the toolset of the experimentalist when searching for specific magnetic properties.
Polymer coated lanthanide based nanoparticles as potential PARACEST MRI contrast agents
Pratik Roy, Daniel R. Talham
Department of Chemistry, University of Florida
Lanthanide based Paramagnetic Chemical Exchange Saturation Transfer (PARACEST) agents offer an alternative to T1 and T2 contrast agents. These systems rely on dynamic chemical exchange of labile protons with bulk water and lead to a decrease in magnetization giving negative contrast in MRI. The major advantage is that one can turn ‘on’ and ‘off’ the image contrast by the application of an external radiofrequency pulse. All published examples of PARACEST agents are single molecule systems, although there are potential advantages to particle-based systems. The interaction of double-hydrophilic block copolymers, containing a poly (acrylic acid) block with europium ions in water leads to the spontaneous formation of polymer-coated nanoparticles, with an average diameter near 20 nm. Saturation transfer experiments with these assemblies show a significant decrease in intensity of the bulk water signal, suggesting it’s potential application as a PARACEST agent.
Supermolecular Building Layer (SBL) Approach to MOFs for Biomedical Applications
Jarrod F. Eubank, Christian Beauchemin, Eric Alonso, and Juan Garcia
Florida Southern College
The necessity for materials with targeted functions for various applications has driven scientists to pursue rational synthetic strategies toward design. The relatively recent development of a class of inorganic-organic hybrid materials known as metal-organic frameworks (MOFs) has significantly aided in the advancement of modern synthetic strategies toward functional materials due to their ease of construction and exceptional tenability. Efforts began with the use of simple molecular building blocks (MBBs, resulting in geometric moieties having multi-connectedness (i.e., greater than 2-connected)) and advanced to more hierarchically complex, and thereby more selective, design strategies like the supermolecular building block (SBB) and, more recent, supermolecular building layer (SBL) techniques. These exclusive approaches have opened avenues to materials with unprecedented or previously difficult to attain properties, such as pores lined with free carboxyl groups. Here, we focus on the SBL approach to target materials for biomedical applications. Preliminary work indicates the ability to design and utilize carboxyl-functionalized MOFs for drug delivery applications. In addition, the construction of MOFs via the incorporation of bioactive molecules suggests potential for additional biomedical applications.
Fe3Se4 Phase Stabilized by Transition Metal Doping
Luis A. Saucedo, David Hardy, Geoffrey F. Strouse, Michael Shatruk
Florida State University
With a growing interest in rare-earth free magnetic materials, Fe3Se4 is a promising material, being composed of earth-abundant elements and showing ferrimagnetic ordering at 340 K. In a common synthesis of this material, elemental selenium is used as a precursor, resulting in long reaction times and the presence of ferrimagnetic Fe7Se8 or superconducting FeSe impurity. Herein we show a synthetic route using molecular precursors, which allow not only for shorter reaction times, but also for stabilization of the Fe3Se4 phase by doping with small concentrations of 3d transition metals (e.g., V, Cr, Mn). The synthesis, characterization and magnetic studies of these materials will be discussed.
Revisiting Bond Breaking and Making in the Solid State: Where are the Electrons?
Xiaoyan Tan,1 Alexander A. Yaroslavtsev,2 Vincent Yannello, Zachary P. Tener,1 Daniel Haskel,3 Francois Guillou,4 Andrey Rogalev,4 Michael Shatruk1
1 Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306, USA
2 European XFEL GmbH, 22869 Schenefeld, Germany
3 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA
4 European Synchrotron Radiation Facility, 71 Avenue des Martyrs, CS40220, Grenoble, France
More than 30 years ago, a seminar paper by Hoffmann and Zheng  addressed the chemical aspects of first-order phase transitions in ThCr2Si2-type structures. They suggested that the breaking and making of the X–X bonds between the [T2X2] layers (X = group 13-16 element; T = transition metal), separated by layers of alkali, alkali-earth, or rare-earth metal atoms (A), correlates with the filling of the electronic d-band of the transition metal. Such correlations have been the focus of many studies on structural, electronic, and magnetic phase transitions in this class of materials.
The structural phase transitions in the ThCr2Si2-type materials involve the collapse or expansion of the structure along the tetragonal c axis, i.e. in the direction parallel to the interlayer X–X bonds. Despite many examples of such transitions being known, the direct experimental assessment of changes in the electron density redistribution between the A and [T2X2] layers upon the formation and breaking of the X–X bonding interactions is largely lacking. Earlier studies by us  and other groups  have revealed fascinating pressure-induced transitions in EuCo2Pn2 (Pn = P, As) which are accompanied by the change in the Eu oxidation state and the transition from the localized (4f) magnetism to itinerant (3d) magnetism. In the present contribution, we demonstrate that the changes in the electron concentration in the [Co2Pn2] layer defy the formal electron-counting rules that are often used for Zintl-like phases. X-ray absorption measurements offer the direct insight into the changes in the Eu oxidation state and magnetism and the associated redistribution of the electron density in the [Co2Pn2] layer.
The research reported in this presentation has been supported by the National Science Foundation under Award DMR-1507233.
1. Hoffman, R.; Zheng, C. J. Phys. Chem. 1985, 89, 4175–4181.
2. Tan, X.; Fabbris, G.; Haskel, D.; Yaroslavtsev, A. A.; Cao, H.; Thompson, C. M.; Kovnir, K.; Menushenkov, A. P.; Chernikov, R. V.; Garlea, V. O.; Shatruk, M. J. Am. Chem. Soc. 2016, 138, 2724-2731.
3. Chefki, M.; Abd-Elmeguid, M. M.; Micklitz, H.; Huhnt, C.; Schlabitz, W.; Reehuis, M.; Jeitschko, W. Phys. Rev. Lett. 1998, 80, 802-805.
Tuning the Selectivity Between C2H2 and CO2 in Molecular Porous Materials
Tony Pham,1 Katherine A. Forrest,1 Kai-Jie Chen,2 Douglas M. Franz,1 Michael J. Zaworotko,2 and Brian Space1
1.) Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, CHE205, Tampa, FL 33620-5250, United States
2.) Department of Chemical & Environmental Sciences, University of Limerick, Limerick, Republic of Ireland
Experimental and theoretical C2H2 and CO2 adsorption studies were performed in MPM-1-Cl and MPM-1-TIFSIX, two robust molecular porous materials (MPMs) with the empirical formula [Cu2(adenine)4Cl2]Cl2 and [Cu2(adenine)4(TiF6)2], respectively. Both MPMs consist of a hydrogen-bonding network that is self-assembled by dinuclear copper–adenine paddlewheel complexes. The difference between the two structures is simply the axial ligand. Experimental measurements revealed that MPM-1-Cl displays higher low-pressure uptake, isosteric heat of adsorption (Qst), and selectivity for C2H2 than CO2, whereas the opposite is true for MPM-1-TIFSIX. Dynamic breakthrough experiments and modeling studies confirmed that MPM-1-Cl is more selective toward C2H2, while MPM-1-TIFSIX is more selective toward CO2. Molecular simulations revealed that both MPMs display a greater preference for C2H2 within their large channels. However, MPM-1-TIFSIX contains a small accessible channel in the structure that is not present in MPM-1-Cl; this channel represents the primary binding site in the material. Calculation of the classical MPM–adsorbate interaction energies and diffusion barriers about the primary binding site in MPM-1-TIFSIX revealed that this site is more favorable for CO2 than C2H2, thus explaining why the selectivity in this MPM is inverted relative to MPM-1-Cl.
Molecular dynamics of metal-organic framework [(CH3)2NH2]Mn(HCOO)3 near magnetic and ferroelectric transitions using 1H, 55Mn NMR.
2. Rhea Reyes2
3. John Haddock1
4. Arneil Reyes3
5. Naresh Dalal1
1. Florida State University
2. New York University
3. National High Magnetic Field Laboratory
Dimethylammonium Manganese Formate (DMMnF) is of interest as a multiferroic metal-organic framework. For the first time, 1H and 55Mn NMR are used to study the known ferroelectric transition at TFE~184K and antiferromagnetic (AF) transition at TN ~8.5K. A broad and strong zero-field 55Mn NMR signal was found below TN, with a calculated 8T internal field due to ordered Mn moments. This signal is suppressed above 0.3T, consistent with the known spin-flop transition. Dramatic changes in 1H spectra correlate to the magnetic ordering with a critical exponent of β =1/4, in contrast to the mean field prediction’s value of 0.5. The spin-lattice relaxation T1 recovery exhibits a double exponential behavior, with the long and short components differing by 3 orders of magnitude. The long T1 component dips near 150K and 8.5K, while the short component exhibits a huge enhancement approaching the critical regime followed by an exponential decay at low temperatures. This behavior is suggestive of an opening of a spin gap, Δ ~ 4.45K, about half of TN. Implications of these results will be discussed.
Tunable Solid State Fluorescence in Isoreticular Metal Organic Frameworks
University of Central Florida
In this work highly stable zirconium based metal organic frameworks isoreticular to UiO-66 were prepared utilizing highly fluorescent links. These links allow for the systematic control over the emissive profile of the prepared material. This study shows how the tailoring of organic linkers with specific properties can be incorporated into a MOF in order to produce tunable properties. Three organic linkers were synthesized with Blue, Green, and Orange fluorescence to prepare solid solutions with properties similar to those observed in solution. This tunability allows for complete control of the emission profile as well as the temperature of the emitted white light. This careful design of organic linkers provides a strategy that can give insight into the photophysical manipulation of MOF monomers and their projected properties inside the MOF.
High Refractive Index Polymer Composites
Florida State University
Hybrid organic/inorganic thermoset polymers synthesized through the thiol-ene coupling reaction have proved to produce materials with a high refractive index (n) making them potentially suitable for optical applications. A series of hard transparent polymer composites were made from tetravinylsilane (TVS) and 1.3-Benzenedithiol (BDTH) with varying concentrations of zirconium oxo-cluster Zr6(OH)4O4(OMc)12 (ZOC) incorporated. The resulting polymers exhibited a significant improvement in the refractive index relative to the parent polymer, TVS and BDTH (n = 1.693). This improvement reaches a maximum value of 1.719 at a loading of 2.5 wt.% ZOC and then decreased as the concentration of ZOC in the polymer matrix increased. The refractive index of ZOC itself was determined to be 1.539 by the Becke Line method which is well below the observed refractive index of the polymer composites. The trend in the refractive index of the composites as a function of ZOC loading and the high refractive index achieved (1.719), despite the low inherent refractive index of the ZOC itself, was found to be due to changes in the bulk density of the composites. Finally, the glass transition temperatures (Tg) of the polymers seemed to decrease as the mole fraction of ZOC increases, resulting in a compromise of its hardness. Solid state Si29 NMR and RAMAN spectra show evidence that a significant amount of unreacted vinyl groups are left over when high concentration of ZOC is present in the polymer matrix.
Synthesis of High Refractive Index Lens Materials
Yue Su, Albert Stiegman
Florida State University
High refractive index polymer materials are synthesized from with the self-initiating thiol-ene coupling reaction between the trivinylphosphine derivatives and selected dithiols. The presence of highly polarizable elements of P, S and Se in P=S/P=Se bonds in the vinyl monomers and the presence of aromatic structures and S in thiols generates polymers exhibiting high refractive indices ranging from 1.64 to 1.75. The polymers also have Abbe Numbers from 22.73 to 37.18 depending on the composition. After undergoing a slow graduated curing process that reached a final temperature of 120, hard, transparent optical materials were obtained. Solid-state 31P NMR was performed to characterize the the degree of cross-linking in the thiol-ene polymer. DSC and DMA are used to measure Tg and Young’s modulus respectively. The transmittance over visible range of the spectrum was carried out by UV-vis spectroscopy and other physical properties such as the density were measured.
The pH Effect in Seed-Mediated Growth of Gold Nanorods
Gang Chen, Reese Gallagher, Xing Zhang
Department of Chemistry, University of Central Florida
Although various gold nanorods (AuNRs) have been produced with different aspect ratio, current synthesis methods through seed-mediated growth are far from ideal, sharing the same drawbacks, such as low yield of gold conversion (~20%), poor shape uniformity and reproducibility, due to a lack of understanding of the reaction mechanism. While the mechanism of the anisotropic growth of AuNRs is not clear yet, the experimental detail in the literature show that the final products depend on the rate of chemical reduction of metal ion. Basically, the reduction rate depends on both the reactivity and concentration of the reactants (gold salts and ascorbic acid (AA)). The concentration of AA is commonly utilized to control the reduction rate. For example, to decrease the reduction rate, the ratio of AA to gold ion has been optimized to be 1.1 which is far below the stoichiometric ratio (1.5), giving extremely poor yield of gold conversion (~20%). In this research, to achieve a high gold conversion yield, we keep the stoichiometric ratio of AA to gold ion (1.5) but tuning the reduction rate by reactivity. As a polyol compound, the reactivity of AA depends on the pH of the system. At stoichiometric ratio (1.5), the optimal pH range for AA has been discovered to get prefect AuNRs with improved uniformity, reproducibility and gold conversion yield (> 80%). The gold conversion yield can be improved further at higher ratio of AA to gold ion than the stoichiometry. With the same idea, the PI has extended the reducing agent to other polyol compounds, such as phenol, hydroquinone, catechol, resorcinol, and phloroglucinol.