Search

Thick overlayers of TiO2 (4—143 nm) are deposited on Si, GaAs, and GaP electrodes by atomic layer deposition.

Splitting water to hydrogen and oxygen is a promising approach for storing energy from intermittent renewables, such as solar power. Efficient, scalable solar-driven electrolysis devices require active electrocatalysts made from earth-abundant elements. In this mini-review, we discuss recent investigations of homogeneous and heterogeneous hydrogen evolution electrocatalysts, with emphasis on our own work on cobalt and iron complexes and nickel-molybdenum alloys.

Nickel and cobalt complexes of 14-membered tetraazamacrocycles mediate electron transfer and produce CO in the photochemical reduction of CO 2 . Two integrated systems for the CO 2 reduction are discussed: (1) photophysical properties of bifunctional supramolecules, Ru(bpy) 2 (bpy-L 10 M) 4+ (bpy = 2,2'-bipyridine, bpy-L 10 = 1-(2,2'-bipyridin-6-ylmethyl)-1, 4,8,11-tetraazacyclotetradecane, M = H 2 , Ni) and its limitation as a photocatalyst; (2) advantages of multimolecular system with p -terphenyl as a photosensitizer, a cobalt macrocycle as an electron mediator, and a tertiary amine as a sacrificial electron donor.

The use of electroabsorption spectroscopy to determine the dipole-moment changes that occur in the metal-to-ligand, ligand-to-metal, and metal-to-metal charge-transfer transitions in mononuclear and binuclear transition metal complexes is reviewed. The ground-excited state dipole-moment differences are much smaller than expected for the transfer of unit electronic charge between the donor and acceptor centers. The results are discussed in terms of a model in which two factors, electron delocalization and polarization of the acceptor, donor or bridging ligand electrons in response to the changed charge on the metal centers, are considered to be primarily responsible for the relatively small dipole-moment changes. The implications of the results for electronic coupling elements and reorganization energies are also discussed.

Natural photosynthesis uses sunlight to drive the conversion of energy-poor molecules (H2O, CO2) to energy-rich ones (O2, (CH2O)n). Scientists are working hard to develop efficient artificial photosynthetic systems toward the “Holy Grail” of solar-driven water splitting. High on the list of challenges is the discovery of molecules that efficiently catalyze the reduction of protons to H2. In this Account, we report on one promising class of molecules: cobalt complexes with diglyoxime ligands (cobaloximes). Chemical, electrochemical, and photochemical methods all have been utilized to explore proton reduction catalysis by cobaloxime complexes. Reduction of a CoII-diglyoxime generates a CoI species that reacts with a proton source to produce a CoIII-hydride. Then, in a homolytic pathway, two CoIII-hydrides react in a bimolecular step to eliminate H2. Alternatively, in a heterolytic pathway, protonation of the CoIII-hydride produces H2 and CoIII. A thermodynamic analysis of H2 evolution pathways sheds new lig...

In the Robin and Day classification, mixed-valence systems are characterized as Class I, II or III depending on the strength of the electronic interaction between the oxidized and reduced sites, ranging from essentially zero (Class I), to moderate (Class II), to very strong electronic coupling (Class III). The properties of Class I systems are essentially those of the separate sites. Class II systems possess new optical and electronic properties in addition to those of the separate sites. However, the interaction between the sites is sufficiently weak that Class II systems are valence trapped or charge localized and can the be described by a double-well potential. In Class III systems the interaction of the donor and acceptor sites is so great that two separate minima are no longer discernible and the energy surface features a single minimum. The electron is delocalized and the system has its own unique properties. The Robin and Day classification has enjoyed considerable success and most of the redox systems studied to date are readily assigned to Class II. However the situation becomes much more complicated when the system shows borderline Class II/III behavior. Such “almost delocalized” mixed-valence systems are difficult to characterize. In this article spectral band shapes and intensities are calculated utilizing increasingly complex models including two to four states. Free-energy surfaces are constructed for harmonic diabatic surfaces and characterized as a function of increasing electronic coupling to simulate the Class II to III transition. The properties of the charge-transfer absorption bands predicted for borderline mixed-valence systems are compared with experimental data. The treatment is restricted to symmetrical (ΔG0 = 0) systems.

The nuclear barrier to electron transfer arises from the need for reorganization of intramolecular and solvent internuclear distances prior to electron t ransfer. For reactions with relatively small driving force ("normal" free-energy region) the nuclear factors and rates increase as intrinsic inner-shell and outer-shell barriers decrease; this is illustrated by data for transition metal complexes in their ground electronic states. By contrast, in the inverted free-energy region, rates and nuclear factors decrease with d ecreasing "intrinsic" barriers; this is illustrated by data for the decay of charge-transfer excited states. Several approaches to the evaluation of the outer-shell barrier are explored in an investigation of the distance dependence of the nuclear factor in intramolecular electron-transfe r processes. One-electron-tra nsfer reactions are at the heart of biological energy transduction and many electrochemical, photochemical, and thermal reaction sequences. For electron transfer