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Hendon, Christopher H.,Walsh, Aron,Dincă,, Mircea American Chemical Society 2016 Inorganic Chemistry Vol.55 No.15
<P>The development of conductive metal-organic frameworks is challenging owing to poor electronic communication between metal clusters and the organic ligands that bridge them. One route to overcoming this bottleneck is to extend the inorganic dimensionality, while using the organic components to provide chemical functionality. Using density functional theory methods, we demonstrate how the properties of the alkaline-earth oxides SrO and BaO are ttansformed upon formation of porous solids with organic oxygen sources (acetate and trifluorocetate). The electron affinity is significantly enhanced in the hybrid materials, while the ionization potential can be tuned over a large range with the polarity of the organic moiety. Furthermore, because of their high-vacuum fraction, these materials have dielectric, properties suitable for low-kappa applications.</P>
Absorbate-Induced Piezochromism in a Porous Molecular Crystal
Hendon, Christopher H.,Wittering, Kate E.,Chen, Teng-Hao,Kaveevivitchai, Watchareeya,Popov, Ilya,Butler, Keith T.,Wilson, Chick C.,Cruickshank, Dyanne L.,Miljanić,, Ognjen Š,.,Walsh, Aron American Chemical Society 2015 Nano letters Vol.15 No.3
<P/><P>Atmospherically stable porous frameworks and materials are interesting for heterogeneous solid–gas applications. One motivation is the direct and selective uptake of pollutant/hazardous gases, where the material produces a measurable response in the presence of the analyte. In this report, we present a combined experimental and theoretical rationalization for the piezochromic response of a robust and porous molecular crystal built from an extensively fluorinated trispyrazole. The electronic response of the material is directly determined by analyte uptake, which provokes a subtle lattice contraction and an observable bathochromic shift in the optical absorption onset. Selectivity for fluorinated absorbates is demonstrated, and toluene is also found to crystallize within the pore. Furthermore, we demonstrate the application of electronic structure calculations to predict a physicochemical response, providing the foundations for the design of electronically tunable porous solids with the chemical properties required for development of novel gas-uptake media.</P>
Hendon, Christopher H.,Yang, Ruo Xi,Burton, Lee A.,Walsh, Aron The Royal Society of Chemistry 2015 Journal of materials chemistry. A, Materials for e Vol.3 No.17
<▼1><P>The incorporation of tetrafluoroborate and hexafluorophosphate in halide perovskites is found to result in substantial band gap widening.</P></▼1><▼2><P>Halide perovskites have attracted attention for light-to-electricity conversion in solar cells due to their favorable optoelectronic properties. In particular, the replacement of the A cation by an isovalent molecule has proven highly successful. We explore the substitution of the X anion, producing polyanion perovskites based on hexafluorophosphate and tetrafluoroborate. Starting from CsPbI3, the effect of partial and complete substitution is investigated using relativistic electronic structure calculations. BF4<SUP>−</SUP> results in a larger perturbation to the electronic structure than PF6<SUP>−</SUP>; however, both localise the band edge states, and the end member compounds are predicted to be wide band gap dielectrics.</P></▼2>
Electroactive Nanoporous Metal Oxides and Chalcogenides by Chemical Design
Hendon, Christopher H.,Butler, Keith T.,Ganose, Alex M.,Romá,n-Leshkov, Yuriy,Scanlon, David O.,Ozin, Geoffrey A.,Walsh, Aron American Chemical Society 2017 Chemistry of materials Vol.29 No.8
<P/><P>The archetypal silica- and aluminosilicate-based zeolite-type materials are renowned for wide-ranging applications in heterogeneous catalysis, gas-separation and ion-exchange. Their compositional space can be expanded to include nanoporous metal chalcogenides, exemplified by germanium and tin sulfides and selenides. By comparison with the properties of bulk metal dichalcogenides and their 2D derivatives, these open-framework analogues may be viewed as three-dimensional semiconductors filled with nanometer voids. Applications exist in a range of molecule size and shape discriminating devices. However, what is the electronic structure of nanoporous metal chalcogenides? Herein, materials modeling is used to describe the properties of a homologous series of nanoporous metal chalcogenides denoted np-MX<SUB>2</SUB>, where M = Si, Ge, Sn, Pb, and X = O, S, Se, Te, with Sodalite, LTA and aluminum chromium phosphate-1 structure types. Depending on the choice of metal and anion their properties can be tuned from insulators to semiconductors to metals with additional modification achieved through doping, solid solutions, and inclusion (with fullerene, quantum dots, and hole transport materials). These systems form the basis of a new branch of semiconductor nanochemistry in three dimensions.</P>
Is iron unique in promoting electrical conductivity in MOFs?
Sun, L.,Hendon, C.,Park, S.,Tulchinsky, Y.,Wan, R.,Wang, F.,Walsh, A.,Dinca, M. Royal Society of Chemistry 2017 Chemical Science Vol.8 No.6
<P>Identifying the metal ions that optimize charge transport and charge density in metal-organic frameworks is critical for systematic improvements in the electrical conductivity in these materials. In this work, we measure the electrical conductivity and activation energy for twenty different MOFs pertaining to four distinct structural families: M-2(DOBDC)(DMF)(2) (M = Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+); H4DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid; DMF = N, N-dimethylformamide), M-2(DSBDC)(DMF)(2) (M = Mn2+, Fe2+; H4DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid), M2Cl2(BTDD)(DMF)(2) (M = Mn2+, Fe2+, Co2+, Ni2+; H2BTDD = bis(1H-1,2,3-triazolo[4,5-b],[40,50-i]dibenzo[1,4]dioxin), and M(1,2,3-triazolate)(2) (M = Mg2+, Mn2+, Fe2+, Co2+, Cu2+, Zn2+, Cd2+). This comprehensive study allows us to single-out iron as the metal ion that leads to the best electrical properties. The iron-based MOFs exhibit at least five orders of magnitude higher electrical conductivity and significantly smaller charge activation energies across all different MOF families studied here and stand out materials made from all other metal ions considered here. We attribute the unique electrical properties of iron-based MOFs to the high-energy valence electrons of Fe2+ and the Fe3+/2+ mixed valency. These results reveal that incorporating Fe2+ in the charge transport pathways of MOFs and introducing mixed valency are valuable strategies for improving electrical conductivity in this important class of porous materials.</P>
Sun, Lei,Hendon, Christopher H.,Minier, Mikael A.,Walsh, Aron,Dincă,, Mircea American Chemical Society 2015 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.137 No.19
<P/><P>Reaction of FeCl<SUB>2</SUB> and H<SUB>4</SUB>DSBDC (2,5-disulfhydrylbenzene-1,4-dicarboxylic acid) leads to the formation of Fe<SUB>2</SUB>(D<B>S</B>BDC), an analogue of M<SUB>2</SUB>(D<B>O</B>BDC) (MOF-74, DOBDC<SUP>4–</SUP> = 2,5-dihydroxybenzene-1,4-dicarboxylate). The bulk electrical conductivity values of both Fe<SUB>2</SUB>(D<B>S</B>BDC) and Fe<SUB>2</SUB>(D<B>O</B>BDC) are ∼6 orders of magnitude higher than those of the Mn<SUP>2+</SUP> analogues, Mn<SUB>2</SUB>(DEBDC) (E = O, S). Because the metals are of the same formal oxidation state, the increase in conductivity is attributed to the loosely bound Fe<SUP>2+</SUP> β-spin electron. These results provide important insight for the rational design of conductive metal–organic frameworks, highlighting in particular the advantages of iron for synthesizing such materials.</P>
Park, Sarah S.,Hendon, Christopher H.,Fielding, Alistair J.,Walsh, Aron,O’Keeffe, Michael,Dincă,, Mircea American Chemical Society 2017 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.139 No.10
<P>The structure-directing role of the inorganic secondary building unit (SBU) is key for determining the topology of metal organic frameworks (MOFs). Here we show that organic building units relying on strong Ir interactions that are energetically competitive with the formation of common inorganic SBUs can also play a role in defining the topology. We demonstrate the importance of the organic SBU in the formation of Mg2H6(H3O)(TTFTB)(3) (MIT-25), a mesoporous MOF with the new ssp topology. A delocalized electronic hole is critical in the stabilization of the TTF triad organic SBUs and exemplifies a design principle for future MOF synthesis.</P>