Charlotte L. Stern

Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.

Post-Synthesis Alkoxide Formation Within Metal#Organic Framework Materials: A Strategy for Incorporating Highly Coordinatively Unsaturated Metal Ions

A new noncatenated metal-organic framework containing pendant alcohol functionalities was synthesized. The alcohols were then post-synthetically converted to either lithium or magnesium alkoxides, with the incorporated metals anchored far from nodes or carboxylate functionalities. The metal alkoxide sites can be obtained stoichiometrically while maintaining the permanent porosity and large surface area of the parent hydroxylated material. The incorporated metal ions are found to induce an unusual pattern of binding energetics for H(2): isosteric heats of adsorption increase, rather than decrease, with increasing H(2) loading. Additionally, at 1 atm and 77 K, uptake (at least with low Li(+) loading) is increased by two hydrogen molecules per Li(+).

Alignment of acentric MoO3F33− anions in a polar material: (Ag3MoO3F3)(Ag3MoO4)Cl

(Ag 3 MoO 3 F 3 )(Ag 3 MoO 4 )Cl was synthesized by hydra(solvato)thermal methods and characterized by single-crystal X-ray diffraction (P3m1. No. 156, Z = 1, a = 7.4488(6) A, c = 5.9190(7) A). The transparent colorless crystals are comprised of chains of distorted fac-MoO 3 F 3- 3 octahedra and MoO 2- 4 tetrahedra anions, as suggested by the formulas Ag 3 MoO 3 F 3 and Ag 3 MoO + 4 , and are connected through Ag + cations in a polar alignment along the c-axis. One Cl anion per formula unit serves as a charge balance and connects the two types of chains in a staggered fashion, offset by ∼1/2 x c. In MoO 2- 4 the Mo atom displaces towards a single oxide vertex, and in MoO 3 F 3- 3 , the Mo displaces towards the three oxide ligands. The ordered oxide-fluoride ligands on the MoO 3 F 3- 3 anion is important to prevent local inversion centers, while the polar organization is directed by the Cl - anion and interchain dipole-dipole interactions. The dipole moments of MoO 3 F 3- 3 and MoO 2- 4 align in the negative c-axis direction, to give a polar structure with no cancellation of the individual moments. The direction and magnitude of the dipole moments for MoO 3 F 3- 3 and MoO 2- 4 were calculated from bond valence analyses and are 6.1 and 1.9 debye (10 -18 esu cm) respectively, compared to 4.4 debye for polar NbO 6 octahedra in LiNbO 3 , and 4.5 debye for polar TiO 6 octahedra in KTiOPO 4 (KTP).

Crystal Structure of a Silyl Cation with No Coordination to Anion and Distant Coordination to Solvent

The crystal structure of a stable silyl cation, triethylsilylium, in the form of its tetrakis (pentafluorophenyl)borate salt [Et3Si+ (C6F5)4B-] (Et, ethyl) shows no coordination between cation and anion. The closest silicon-fluorine distance is greater than 4 angstroms. A toluene solvent molecule is close enough to cause some deviations from planarity at the silicon, but the silicon-toluene distance is well beyond the sum of the silicon and carbon covalent radii. The toluene molecule is essentially planar and undistorted, as expected if little or no positive charge has been transferred from silicon to toluene.

Kinetic separation of propene and propane in metal-organic frameworks: Controlling diffusion rates in plate-shaped crystals via tuning of pore apertures and crystallite aspect ratios

A series of isostructural, noncatenated, zinc-pillared-paddlewheel metal-organic framework materials has been synthesized from 1,2,4,5-tetrakis(carboxyphenyl)benzene and trans-1,2-dipyridylethene struts. Substantial kinetic selectivity in the adsorption of propene over propane can be observed, depending on the pore apertures and the rectangular-plate morphology of the crystals.

A coordination chemistry dichotomy for icosahedral carborane-based ligands

Although the majority of ligands in modern chemistry take advantage of carbon-based substituent effects to tune the sterics and electronics of coordinating moieties, we describe here how icosahedral carboranes-boron-rich clusters-can influence metal-ligand interactions. Using a series of phosphine-thioether chelating ligands featuring meta- or ortho-carboranes grafted on the sulfur atom, we were able to tune the lability of the platinum-sulfur interaction of platinum(II)-thioether complexes. Experimental observations, supported by computational work, show that icosahedral carboranes can act either as strong electron-withdrawing ligands or electron-donating moieties (similar to aryl- or alkyl-based groups, respectively), depending on which atom of the carborane cage is attached to the thioether moiety. These and similar results with carborane-selenol derivatives suggest that, in contrast to carbon-based ligands, icosahedral carboranes exhibit a significant dichotomy in their coordination chemistry, and can be used as a versatile class of electronically tunable building blocks for various ligand platforms.

Ultrafast photodriven intramolecular electron transfer from a zinc porphyrin to a readily reduced diiron hydrogenase model complex.

Diiron complexes modeled on the active site of the [FeFe] hydrogenases having the general formula [Fe(2)(mu-R)(CO)(6-n)(L)(n)], where commonly R = alkyl or aryl dithiolate and L = CO, CN(-), or PR(3), are a promising class of catalysts for use in photodriven H(2) production. However, many of these catalysts are difficult to photoreduce using chromophores that absorb visible light. Here we report the synthesis and spectroscopic characterization of a naphthalene-4,5-dicarboximide-1,8-dithiolate diiron complex [NMI-Fe(2)S(2)(CO)(6), 1] and a covalently linked, fixed-distance zinc 5,10,15-tri-n-pentyl-20-phenylporphyrin-NMI-Fe(2)S(2)(CO)(6) donor-acceptor dyad (2). The electron-withdrawing nature of the NMI group makes the diiron complex among the most easily reduced hydrogenase mimics reported to date (-0.74 V vs SCE). In the presence of triflic acid, the cyclic voltammogram of 1 showed an increase in current at the first reduction wave at -0.78 V and a new reduction wave at -1.4 V. As the acid concentration was increased, the current at -0.78 V remained constant while the current at -1.4 V increased significantly, which is consistent with a catalytic proton reduction process. Selective photoexcitation of the Zn porphyrin in 2 with 553 nm, 110 fs laser pulses in both toluene and CH(2)Cl(2) yielded transient absorption spectra showing a distinct peak at 616 nm, which has been assigned to [NMI-Fe(2)S(2)(CO)(6)](-*) on the basis of spectroelectrochemical measurements on 1. The 616 nm peak was used to monitor the charge separation (CS) and charge recombination (CR) dynamics of 2, which yielded tau(CS) = 12 +/- 1 ps and tau(CR) = 3.0 +/- 0.2 ns in toluene and tau(CS) = 24 +/- 1 ps and tau(CR) = 57 +/- 1 ps in CH(2)Cl(2). Photoexcitation of the disulfide precursor to 2 in both toluene and CH(2)Cl(2) produced only the singlet and triplet excited states of the Zn porphyrin, showing that electron transfer is favorable only when the diiron complex is present. Photoexcitation of 2 in the presence of trifluoroacetic acid was shown to generate H(2).

Turning on catalysis: incorporation of a hydrogen-bond-donating squaramide moiety into a Zr metal-organic framework.

Herein, we demonstrate that the incorporation of an acidic hydrogen-bond-donating squaramide moiety into a porous UiO-67 metal-organic framework (MOF) derivative leads to dramatic acceleration of the biorelevant Friedel-Crafts reaction between indole and β-nitrostyrene. In comparison, it is shown that free squaramide derivatives, not incorporated into MOF architectures, have no catalytic activity. Additionally, using the UiO-67 template, we were able to perform a direct comparison of catalytic activity with that of the less acidic urea-based analogue. This is the first demonstration of the functionalization of a heterogeneous framework with an acidic squaramide derivative.

A Radically Configurable Six-State Compound

Radically Organic Metals such as manganese are relatively stable over a wide range of oxidation states. In contrast, purely organic compounds are rarely susceptible to incremental addition or removal of electrons without accompanying fragmentation or coupling reactions. Barnes et al. (p. 429; see the Perspective by Benniston) report a catenane (a compound comprising interlocked rings) in which the topological structure stabilizes six different states that successively differ by the presence or absence of one or two electrons in the framework. The hepta-oxidized state proved remarkably resilient to oxygen exposure. An interlocked-rings topology stabilizes a wide range of collective oxidation states in a metal-free organic compound. [Also see Perspective by Benniston] Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.

Electronic tuning of nickel-based bis(dicarbollide) redox shuttles in dye-sensitized solar cells

Rational design of a new series of boron-functionalized Ni{sup III}/Ni{sup IV}–bis(dicarbollide) clusters results in a family of robust and tunable redox shuttles (see diagram; EDG and EWG denote electron-donating and -withdrawing groups, respectively). This offers a means to rationally control the redox properties in dye-sensitized solar cells (DSCs), leading to exceptionally high open-circuit voltages.