Iridium(III) Bis-Pyridine-2-Sulfonamide Complexes as Efficient and Durable Catalysts for Homogeneous Water Oxidation.

A family of tetradentate bis(pyridine-2-sulfonamide) (bpsa) compounds was synthesized as a ligand platform for designing resilient and electronically tunable catalysts capable of performing water oxidation catalysis and other processes in highly oxidizing environments. These wrap-around ligands were coordinated to Ir(III) octahedrally, forming an anionic complex with chloride ions bound to the two remaining coordination sites. NMR spectroscopy documented that the more rigid ligand frameworks-[Ir(bpsa-Cy)Cl2](-) and [Ir(bpsa-Ph)Cl2](-)-produced C1-symmetric complexes, while the complex with the more flexible ethylene linker in [Ir(bpsa-en)Cl2](-) displays C2 symmetry. Their electronic structure was explored with DFT calculations and cyclic voltammetry in nonaqueous environments, which unveiled highly reversible Ir(III)/Ir(IV) redox processes and more complex, irreversible reduction chemistry. Addition of water to the electrolyte revealed the ability of these complexes to catalyze the water oxidation reaction efficiently. Electrochemical quartz crystal microbalance studies confirmed that a molecular species is responsible for the observed electrocatalytic behavior and ruled out the formation of active IrOx. The electrochemical studies were complemented by work on chemically driven water oxidation, where the catalytic activity of the iridium complexes was studied upon exposure to ceric ammonium nitrate, a strong, one-electron oxidant. Variation of the catalyst concentrations helped to illuminate the kinetics of these water oxidation processes and highlighted the robustness of these systems. Stable performance for over 10 days with thousands of catalyst turnovers was observed with the C1-symmetric catalysts. Dynamic light scattering experiments ascertained that a molecular species is responsible for the catalytic activity and excluded the formation of IrOx particles.

Toward quantifying the electrostatic transduction mechanism in carbon nanotube molecular sensors.

Despite the great promise of carbon nanotube field-effect transistors (CNT FETs) for applications in chemical and biochemical detection, a quantitative understanding of sensor responses is lacking. To explore the role of electrostatics in sensor transduction, experiments were conducted with a set of highly similar compounds designed to adsorb onto the CNT FET via a pyrene linker group and take on a set of known charge states under ambient conditions. Acidic and basic species were observed to induce threshold voltage shifts of opposite sign, consistent with gating of the CNT FET by local charges due to protonation or deprotonation of the pyrene compounds by interfacial water. The magnitude of the gate voltage shift was controlled by the distance between the charged group and the CNT. Additionally, functionalization with an uncharged pyrene compound showed a threshold shift ascribed to its molecular dipole moment. This work illustrates a method for producing CNT FETs with controlled values of the turnoff gate voltage, and more generally, these results will inform the development of quantitative models for the response of CNT FET chemical and biochemical sensors.

Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces.

Molecular hydrogen (H(2)) is derived from the hydrothermal alteration of olivine-rich planetary crust. Abiotic and biotic processes consume H(2) to produce methane (CH(4)); however, the extent of either process is unknown. Here, we assess the temporal dependence and limit of abiotic CH(4) related to the presence and formation of mineral catalysts during olivine hydrolysis (i.e., serpentinization) at 200 °C and 0.03 gigapascal. Results indicate that the rate of CH(4) production increases to a maximum value related to magnetite catalyzation. By identifying the dynamics of CH(4) production, we kinetically model how the H(2) to CH(4) ratio may be used to assess the origin of CH(4) in deep subsurface serpentinization systems on Earth and Mars. Based on our model and available field data, low H(2)/CH(4) ratios (less than approximately 40) indicate that life is likely present and active.

Novel polyaromatic-terminated transition metal complexes for the functionalization of carbon surfaces.

In order to investigate the process of noncovalent adsorption on glassy carbon surfaces, two terpyridine ligands 4-pyren-1-yl-N-[5-([2,2';6',2'']terpyridin-4'-yloxy)-pentyl]-butyramide (tpy~py) and N-[5-([2,2';6',2'']terpyridin-4'-yloxy)-pentyl]-2-naphthamide (tpy~nap) as well as the homoleptic cobalt(II) complexes of these ligands (Co(tpy~py)(2)(PF(6))(2) and Co(tpy~nap)(2)(PF(6))(2)) were synthesized. Electrochemical measurements in solution were used to characterize the transport behavior of these complexes and to verify that the polyaromatic portion of each ligand did not dramatically influence the electronic properties of the transition metal complex. The adsorption of the cobalt complexes above on glassy carbon electrode surfaces was then examined using cyclic voltammetry and was found to be well described by Langmuir or Frumkin isotherms. The free energy of adsorption for Co(tpy~py)(2)(PF(6))(2) was considerably larger than that for Co(tpy~nap)(2)(PF(6))(2): -41 versus -30 kJ/mol.

Electrochemical analysis of single-walled carbon nanotubes functionalized with pyrene-pendant transition metal complexes.

The noncovalent functionalization of single-walled carbon nanotubes (SWNTs) is important in the development of advanced materials and nanoelectronic sensors and devices. A cobalt-terpyridine transition metal complex with pendant pyrene moieties has been shown to successfully functionalize SWNTs via noncovalent pi-pi stacking interactions. Cyclic voltammetry at SWNT coated platinum electrodes has been utilized to investigate the process of surface modification and provides conclusive evidence of robust surface functionalization. The electrochemical methodology for examining surface functionalization of SWNTs described herein is generalizable to any redox-active system and provides a simple and powerful means for in situ examination of processes occurring at the surface of nanostructured materials.

Discovery and high-throughput screening of heteroleptic iridium complexes for photoinduced hydrogen production.

The catalytic process of photoinduced hydrogen generation via the reduction of water has been investigated. The use of parallel synthetic techniques has facilitated the synthesis of a 32 member library of heteroleptic iridium complexes that was screened, using high-throughput photophysical techniques, to identify six potential photosensitizers for use in catalytic photoinduced hydrogen production. A Pd/Ni thin film hydrogen selective sensor allowed for rapid quantification of hydrogen produced via illumination of aqueous systems of the photosensitizer, tris(2,2'-dipyridyl)dichlorocobalt ([Co(bpy)(3)]Cl(2)), and triethanolamine (a sacrificial reductant) with ultra-bright light emitting diodes (LEDs). The use of an 8-well parallel photoreactor expedited the investigation of the hydrogen evolution process and facilitated mechanistic studies. All six compounds investigated produced considerably more hydrogen than commonly utilized photosensitizers and had relative quantum efficiencies of hydrogen production up to 37 times greater than that of Ru(bpy)(3)(2+).

Iron(II) and copper(I) coordination polymers: electrochromic materials with and without chiroptical properties.

The electrochemical and optical properties of films prepared from two different Fe(II) coordination polymers (TPT[Fe(II)TPT](n)(PF(6))(2)(n) (TPT = terpyridine-phenyl-terpyridine) and CTPCT[Fe(II)CTPCT](n)(PF(6))(2)(n) (CTPCT = chiral terpyridine-phenyl-chiral terpyridine)) and a coordination polymer based on Cu(I) metal centers (PDP[Cu(I)PDP](n)(BF(4))(n)) (PDP = phenanthroline-dodecane-phenanthroline) have been studied. The oxidation of a PDP[Cu(I)PDP](n)(BF(4))(n) film coated on an indium-tin oxide (ITO) electrode by stepping the potential from 0.0 to +1.4 V vs Ag/AgCl led not only to the complete bleaching of the absorption in the visible region of the spectrum within 5 min but also to a redox-induced dissociation and dissolution of the polymer. The reverse reaction of binding and reassembling the polymer at the electrode surface, upon stepping the potential back to 0.0 V, occurred with a rate which was at least 1 order of a magnitude slower. In contrast, the bis(2,2':6',2' '-terpyridine)iron(II)-based redox polymers TPT[Fe(II)TPT](n)(PF(6))(2)(n) and CTPCT[Fe(II)CTPCT](n)(PF(6))(2)(n), during similar spectroelectrochemical experiments, not only exhibited a dramatically enhanced switching rate but also displayed symmetric switching kinetics. The films did not show signs of deterioration over 150 switching cycles. Additionally, in an effort to assemble an electrochromic device with chiroptical properties, the electrochromism of films generated from the enantiomerically pure CTPCT[Fe(II)CTPCT](n)(PF(6))(2)(n) polymer was studied through circular dichroism.

Coulomb blockade and the Kondo effect in single-atom transistors.

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems. Experiments to date have examined a multitude of molecules conducting in parallel, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions or scanning probes as well as three-terminal single-molecule transistors made from carbon nanotubes, C(60) molecules, and conjugated molecules diluted in a less-conducting molecular layer. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.