Evolution of iridium-based molecular catalysts during water oxidation with ceric ammonium nitrate.

Organometallic iridium complexes have been reported as water oxidation catalysts (WOCs) in the presence of ceric ammonium nitrate (CAN). One challenge for all WOCs regardless of the metal used is stability. Here we provide evidence for extensive modification of many Ir-based WOCs even after exposure to only 5 or 15 equiv of Ce(IV) (whereas typically 100-10000 equiv are employed during WOC testing). We also show formation of Ir-rich nanoparticles (likely IrO(x)) even in the first 20 min of reaction, associated with a Ce matrix. A combination of UV-vis and NMR spectroscopy, scanning transmission electron microscopy, and powder X-ray diffraction is used. Even simple IrCl(3) is an excellent catalyst. Our results point to the pitfalls of studying Ir WOCs using CAN.

NIR luminescence intensities increase linearly with proportion of polar thiolate ligands in protecting monolayers of Au38 and Au140 quantum dots.

The near-infrared photoluminescence of monolayer-protected Au38 and Au140 clusters (MPCs) is intensified with exchange of nonpolar ligands by more polar thiolate ligands. The effect is general and includes as more polar in-coming ligands: thiophenolates with a variety of p-substituents; alkanethiolates omega-terminated by alcohol, acid, or quaternary ammonium groups; and thio-amino acids. Remarkably, place exchanges of the initial phenylethanethiolates on Au38 MPCs by p-substituted thiophenolates and thio-amino acids and of hexanethiolates on Au140 MPCs by omega-quaternary ammonium terminated undecylthiolates result in increases in the near-infrared (NIR) luminescence intensities that are linear with the number of new polar ligands. The increased intensities are systematically larger for thiophenolate ligands having more electron-withdrawing substituents. Analogous effects on intensities are observed in the NIR emission of Au140 MPCs upon place exchange of alkanethiolates with thiolates having short connecting alkanethiolate chains to quaternary ammonium and to omega-carboxylic acid termini, and with oxidative charging of the Au cores. The observations are consistent with sensitivity of the luminescence mechanism to any factor that enhances the electronic polarization of the bonds between the Au core atoms and their thiolate ligands. The luminescence is discussed in terms of a surface electronic excitation, as opposed to a core volume excitation.

Dynamic and static quenching of fluorescence by 1-4 nm diameter gold monolayer-protected clusters.

How the efficiency of molecular quenching by Au nanoparticles depends on nanoparticle size is reported for (a) dynamic (collisional) quenching of four different fluorophores by three Au nanoparticles having similar protective layers but differing core diameters (1.1, 1.6, and 2.0 nm) and (b) static quenching in the electrostatic association between [Ru(bpy)3]2+ and five tiopronin-protected Au nanoparticles having core diameters from 1.3 to 3.9 nm. The quenching constants systematically increase with core size. In (a), the dynamic constants scale with the molar absorbance coefficients of the nanoparticles, showing the essentially of the absorbance/emission spectral overlap, and the associated nanoparticle core density of electronic states, in energy-transfer quenching. In (b), the fluorescence of the Au nanoparticle itself was enhanced by energy transfer from the [Ru(bpy)3]2+ fluorophore.

Ligand effects on optical properties of CdSe nanocrystals.

We report effects of various organic and inorganic ligands on optical properties of CdSe nanocrystals (NCs) by changes in their photoluminescence and absorbance spectra. Surface ligand loss occurring during dilution and purification of solutions of CdSe NCs leads to a decrease of photoluminescence intensity. The complex of trioctylphosphine with Se atoms on the surface of CdSe NCs is found responsible for the trap emission band that is red-shifted relative to the photoluminescence band edge.

Functional Monolayers with Coordinatively Embedded Metalloporphyrins.

Perpendicularly oriented iron porphyrins are absorbed onto a gold surface when interconnected long-chain diimidazolyl groups coordinate axially to the metal center from either side of the ring plane (see schematic representation). The stacking of the rings is simultaneously prevented. The monolayers have been characterized structurally and electrochemically.

Bulky adamantanethiolate and cyclohexanethiolate ligands favor smaller gold nanoparticles with altered discrete sizes.

Use of bulky ligands (BLs) in the synthesis of metal nanoparticles (NPs) gives smaller core sizes, sharpens the size distribution, and alters the discrete sizes. For BLs, the highly curved surface of small NPs may facilitate growth, but as the size increases and the surface flattens, NP growth may terminate when the ligand monolayer blocks BLs from transporting metal atoms to the NP core. Batches of thiolate-stabilized Au NPs were synthesized using equimolar amounts of 1-adamantanethiol (AdSH), cyclohexanethiol (CySH), or n-hexanethiol (C6SH). The bulky CyS- and AdS-stabilized NPs have smaller, more monodisperse sizes than the C6S-stabilized NPs. As the bulkiness increases, the near-infrared luminescence intensity increases, which is characteristic of small Au NPs. Four new discrete sizes were measured by MALDI-TOF mass spectrometry, Au(30)(SAd)(18), Au(39)(SAd)(23), Au(65)(SCy)(30), and Au(67)(SCy)(30). No Au(25)(SAd)(18) was observed, which suggests that this structure would be too sterically crowded. Use of BLs may also lead to the discovery of new discrete sizes in other systems.

Thin-film solid-state electroluminescent devices based on tris(2,2'-bipyridine)ruthenium(II) complexes.

The behavior of light-emitting electrochemical cells (LEC) based on solid films ( approximately 100 nm) of tris(2,2'-bipyridine)ruthenium(II) between an ITO anode and a Ga-In cathode was investigated. The response times were strongly influenced by the nature of the counterion: small anions (BF(4)(-) and ClO(4)(-)) led to relatively fast transients, while large anions (PF(6)(-), AsF(6)(-)) produced a slow time-response. From comparative experiments of cells prepared and tested in a glovebox to those in ambient, mobility of the anions in these films appears to be related to the presence of traces of water from atmospheric moisture. An electrochemical model is proposed to describe the behavior of these LECs. The simulation results agreed well with experimental transients of current and light emission as a function of time and show that the charge injection is asymmetric at the two electrodes. At a small bias, electrons are the major carriers, while for a larger bias the conduction becomes bipolar.

Transmission surface-plasmon resonance (T-SPR) measurements for monitoring adsorption on ultrathin gold island films.

Evaporation of ultrathin (1.3-10 nm nominal thickness) gold films onto quartz or mica leads to the formation of a layer of rather uniform gold islands on the transparent support. The morphology of ultrathin gold island films of various thicknesses was studied by using atomic force microscopy (AFM) and scanning electron microscopy (SEM) imaging. The surface plasmon (SP) absorption characteristic of such films is highly sensitive to the surrounding medium, with the plasmon band changing in intensity and wavelength upon binding of various molecules to the surface. The binding process can be monitored quantitatively by measuring the changes in the gold SP absorption, by using transmission UV/Vis spectroscopy. The method, termed transmission surface plasmon resonance (T-SPR) spectroscopy, is shown to be applicable to both chemically and physically adsorbed molecules, in liquid or gas phase, with measurements carried out either ex situ or in situ (real-time measurements) using a variety of molecular probes. Binding to a preformed molecular layer on the Au surface produces a similar response, suggesting the possible use of T-SPR for selective sensing. The sensitivity of T-SPR spectroscopy in detecting molecular binding to the gold depends strongly on the film preparation conditions, and may be comparable to that obtained in surface plasmon resonance (SPR) sensing.

Stability of thin-film solid-state electroluminescent devices based on tris(2,2'-bipyridine)ruthenium(II) complexes.

The factors affecting the operating life of the light-emitting electrochemical cells (LECs) based on films of tris(2,2'-bipyridine)ruthenium(II) both in sandwich (using an ITO anode and a Ga:Sn cathode) and planar (using interdigitated electrode arrays (IDAs)) configurations were investigated. Stability of these devices is greatly improved when they are produced and operated under drybox conditions. The proposed mechanism of the LEC degradation involves formation of a quencher in a small fraction of tris(2,2'-bipyridine)ruthenium(II) film adjacent to the cathode, where light generation occurs, as follows from the observed electroluminescence profile in the LECs constructed on IDAs, showing that the charge injection in such devices is highly asymmetric, favoring hole injection. Bis(2,2'-bipyridine)diaquoruthenium(II) is presumed to be the quencher responsible for the device degradation. A microscopic study of photo- and electroluminescence profiles of planar light-emitting electrochemical cells was shown as a useful approach for studies of charge carrier injection into organic films.