Os(bipy)(CN)4]2- and its relatives as components of polynuclear assemblies: structural and photophysical properties

A series of diimine-tetracyanoosmate anions [Os(diimine)(CN)4]2- [diimine=2,2'-bipyridine (bipy), 2,2'-bipyrimidine (bpym), 1,10-phenanthroline (phen), and 4,4'-tBu2-2,2'-bipyridine (tBu2bpy)] were prepared and isolated as their Na+ salts (water soluble) or PPN+ salts (soluble in organic solvents). Several examples were crystallographically characterized; the Na+ salts form a range of 1D, 2D, or 3D infinite coordination polymers via coordination of the cyanide groups to Na+ cations in either an end-on or a side-on manner. The [Os(diimine)(CN)4]2- anions are solvatochromic, showing three MLCT absorptions, which are considerably blue-shifted in water compared to organic solvents, in the same way as is well-known for the analogous [Ru(diimine)(CN)4]2- anions. Luminescence in the red region of the spectrum is very weak but (following the expected solvatochromic behavior) is higher energy and more intense in water. However, by exploiting the effect of metallochromism (ref 4), the emission from [Os(tBu2bpy)(CN)4]2- in MeCN can be very substantially boosted in energy, intensity, and lifetime in the presence of Lewis-acidic metal cations (Na+, Ba2+, Zn2+), which, in a relatively noncompetitive solvent, coordinate to the cyanide groups of [Os(tBu2bpy)(CN)4]2-. This has an effect similar in principle to hydrogen bonding of the cyanides to delta+ protons of water, but very much stronger, such that in the presence of Zn2+ ions in MeCN the 1MLCT and 3MLCT absorptions are blue-shifted by ca. 7000 cm(-1), and the luminescence moves from 970 nm (vanishingly weak) to 610 nm with a lifetime of 120 ns (dominant component). Thus, the binding of metal cations to the cyanides provides a mechanism to incorporate [Os(diimine)(CN)4]2- complexes into polynuclear assemblies and simultaneously increases their 3MLCT energy and lifetime to an extent that makes them comparable to much-stronger luminophores such as Ru(II)-polypyridines.     

Unusually strong space-charge-limited current in thin wires

The current-voltage characteristics of thin wires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanorods and demonstrate that the nonlinear behavior originates instead from space-charge-limited current. A theory of space-charge-limited current in thin wires corroborates the experiments and shows that poor screening in high-aspect ratio materials leads to a dramatic enhancement of space-charge limited current, resulting in new scaling in terms of the aspect ratio.     

Disparities between experimental and environmental conditions: Research steps toward making electrochemical water treatment a reality

Electrochemical water treatment is one of the key topics of environmental electrochemistry. Identifying electrocatalytic materials capable of electrogenerating high oxidant species in situ seems to have catalyzed researchers’ interest in these processes. While most studies have focused on ideal lab-made solutions, translation to higher technology readiness levels and commercialization requires reframing research questions in context of real water matrices. In this current opinion, we discuss disconnects that may occur when focusing on synthetic solution treatment rather than on real waters. Future research can fill the gaps identified herein, thus facilitating application of electrochemical water treatment technologies.     

Synthesis and characterization of sulfonated‐fluorinated,hydrophilic‐hydrophobic multiblock copolymers for proton exchange membranes

Hydrophilic/hydrophobic block copolymers as proton exchange membranes (PEMs) has become an emerging area of research in recent years. These copolymers were obtained through moderate temperature (～ 100 °C) coupling reactions, which minimize the ether‐ether interchanges between hydrophobic and hydrophilic telechelic oligomers via a nucleophilic aromatic substitution mechanism. The hydrophilic blocks were based on the nucleophilic step polymerization of 3,3′‐disulfonated, 4,4′‐dichlorodiphenyl sulfone with an excess 4,4′‐biphenol to afford phenoxide endgroups. The hydrophobic (fluorinated) blocks were largely based on decafluoro biphenyl (excess) and various bisphenols. The copolymers were obtained in high molecular weights and were solvent cast into tough membranes, which had nanophase separated hydrophilic and hydrophobic regions. The performance and structure‐property relationships of these materials were studied and compared to random copolymer systems. NMR results supported that the multiblock sequence had been achieved. They displayed superior proton conductivity, due to the ionic proton conducting channels formed through the self‐assembly of the sulfonated blocks. The nano‐phase separated morphologies of the copolymer membranes were studied and confirmed by atomic force microscopy. Through control of a variety of parameters, including ion exchange capacity and sequence lengths, performances as high, or even higher than those of the state‐of‐the‐art PEM, Nafion, were achieved. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1038–1051, 2009     

Communication: Bubbles, crystals, and laser-induced nucleation

Short intense laser pulses of visible and infrared light can dramatically accelerate crystal nucleation from transparent solutions; previous studies invoke mechanisms that are only applicable for nucleation of ordered phases or high dielectric phases. However, we show that similar laser pulses induce CO(2) bubble nucleation in carbonated water. Additionally, in water that is cosupersaturated with argon and glycine, argon bubbles escaping from the water can induce crystal nucleation without a laser. Our findings suggest a possible link between laser-induced nucleation of bubbles and crystals.     

High-level ab initio studies of unimolecular dissociation of the ground-state N3 radical

A comprehensive study of the unimolecular dissociation of the N(3) radical on the ground doublet and excited quartet potential energy surfaces has been carried out with multireference single and double excitation configuration interaction and second-order multireference perturbation methods. Two forms of the N(3) radical have been located in the linear and cyclic region of the lowest doublet potential energy surface with an isomerization barrier of 62.2 kcal/mol above the linear N(3). Three equivalent C(2v) minima of cyclic N(3) are connected by low barrier, meaning the molecule is free to undergo pseudorotation. The cyclic N(3) is metastable with respect to ground state products, N((4)S)+N(2), and dissociation must occur via intersystem crossing to a quartet potential energy surface. Minima on the seams of crossing between the doublet and quartet potential surfaces are found to lie substantially higher in energy than the cyclic N(3) minima. This strongly suggests that cyclic N(3) possesses a long collision-free lifetime even if formed with substantial internal excitation.     

Marangoni flow of Ag nanoparticles from the fluid-fluid interface

Fluid flow is observed when a volume of passivated Ag nanoparticles suspended in chloroform is mixed with a water/ethanol (v/v) mixture containing acidified 11-mercaptoundecanoic acid. Following mechanical agitation, Ag nanoparticles embedded in a film are driven from the organic-aqueous interface. A reddish-brown colored film, verified by transmission electron microscopy to contain uniformly dispersed Ag nanoparticles, is observed to spontaneously climb the interior surface of an ordinary, laboratory glass vial. This phenomenon is recorded by a digital video recorder, and a measurement of the distance traveled by the film front versus time is extracted. Surface (interfacial) tension gradients due to surfactant concentration, temperature, and electrostatic potential across immiscible fluids are known to drive interface motion; this well-known phenomenon is termed Marangoni flow or the Marangoni effect. Experimental results are presented that show the observed mass transfer is dependent on an acid surfactant concentration and on the volume fraction of water in the aqueous phase, consistent with fluid flow induced by interfacial tension gradients. In addition, an effective desorption rate constant for the Marangoni flow is measured in the range of approximately 0.01 to approximately 1 s(-1) from a fit to the relative film front distance traveled versus time data. The fit is based on a time-dependent expression for the surface (interface) excess for desorption kinetics. Such flow suggests that purposeful creation of interfacial tension gradients may aid in the transfer of 2- and 3-dimensional assemblies, made with nanostructures at the liquid-liquid interface, to solid surfaces.     

A roadmap to implementing metal-organic frameworks in electronic devices: challenges and critical directions

Metal-organic frameworks (MOFs) and related material classes are attracting considerable attention for applications such as gas storage, separations, and catalysis. In contrast, research focused on potential uses in electronic devices is in its infancy. Several sensing concepts in which the tailorable chemistry of MOFs is used to enhance sensitivity or provide chemical specificity have been demonstrated, but in only a few cases are MOFs an integral part of an actual device. The synthesis of a few electrically conducting MOFs and their known structural flexibility suggest that MOF-based electronic devices exploiting these properties could be constructed. It is clear, however, that new fabrication methods are required to take advantage of the unique properties of MOFs and extend their use to the realms of electronic circuitry. In this Concepts article, we describe the basic functional elements needed to fabricate electronic devices and summarize the current state of relevant MOF research, and then review recent work in which MOFs serve as active components in electronic devices. Finally, we propose a high-level roadmap for device-related MOF research, the objective of which is to stimulate thinking within the MOF community concerning the development these materials for applications including sensing, photonics, and microelectronics.     

UV Photodissociation Dynamics of HN3 in 190-248 nm

The H+N3 channel in the ultraviolet photodissociation of HN3 has been investigated from 190 nm to 248 nm using the high-n Rydberg H-atom time-of-°ight technique. Product translational energy distributions as well as product angular anisotropy parameters were determined for the H+N3 channel at di?erent photolysis wavelengths. N3 vibrational state distribution has also been derived from the product translational energy distribution at these wavelengths. Above photolysis wavelength 225 nm, HN3 predominantly dissociatethrough the repulsive state. Below 225 nm, a new slow channel starts to appear at 220 nm in addition to the existing channel. This channel is attributed to a ring closure dissociation channel to produce the cyclic N3 product. As photolysis energy increases, this new channel becomes more important.