Jillian L. Dempsey

Synthesis and photophysical characterization of porphyrin and porphyrin–Ru(II) polypyridyl chromophore–catalyst assemblies on mesoporous metal oxides

A layer-by-layer procedure has been used to prepare chromophore–catalyst assemblies consisting of phosphonate-derivatized porphyrin chromophores and a phosphonate-derivatized Ru(II) water oxidation catalyst on the surfaces of SnO2 and TiO2 mesoporous, nanoparticle films. In the procedure, initial surface binding of the phosphonate-derivatized porphyrin is followed in sequence by reaction with ZrOCl2 and then with the phosphonate-derivatized water oxidation catalyst [RuII(2,6-bis-(1-methylbenzimidazole-2-yl)pyridine)(2,2′-bipyridine-4,4′-hydroxymethylphosphonate)(H2O)]2+, [RuII(Mebimpy)(4,4′-(PO(OH)2–CH2)2-bpy)(OH2)]2+. Fluorescence from both the free base and Zn(II) porphyrin derivatives on SnO2 is quenched; substantial emission quenching of the Zn(II) porphyrin occurs on TiO2. Transient absorption difference spectra provide direct evidence for appearance of the porphyrin radical cation on SnO2via excited-state electron injection. For the chromophore–catalyst assembly on SnO2, transient absorption difference spectra demonstrate rapid intra-assembly electron transfer oxidation of the catalyst following excitation and injection by the porphyrin chromophore.

Theoretical modeling of low-energy electronic absorption bands in reduced cobaloximes

The reduced Co(I) states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we analyze the low-energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task.

Growth and Post-Deposition Treatments of SrTiO3 Films for Dye-Sensitized Photoelectrosynthesis Cell Applications.

Sensitized SrTiO3 films were evaluated as potential photoanodes for dye-sensitized photoelectrosynthesis cells (DSPECs). The SrTiO3 films were grown via pulsed laser deposition (PLD) on a transparent conducting oxide (fluorine-doped tin oxide, FTO) substrate, annealed, and then loaded with zinc(II) 5,10,15-tris(mesityl)-20-[(dihydroxyphosphoryl)phenyl] porphyrin (MPZnP). When paired with a platinum wire counter electrode and an Ag/AgCl reference electrode these sensitized films exhibited photocurrent densities on the order of 350 nA/cm(2) under 0 V applied bias conditions versus a normal hydrogen electrode (NHE) and 75 mW/cm(2) illumination at a wavelength of 445 nm. The conditions of the post-deposition annealing step-namely, a high-temperature reducing atmosphere-proved to be the most important growth parameters for increasing photocurrent in these electrodes.

Ligand steals spotlight from metal to orchestrate hydrogen production

In comparison to green plants, we humans severely underuse the sun’s energy. Although the high costs of solar photovoltaics have been an early barrier to widespread utilization, the lack of efficient solar energy storage mechanisms has greatly hindered the adoption of this sustainable energy resource. The development of sunlight-to-fuel technologies, with the goal of mimicking the process of photosynthesis on an industrial scale, has been a major and growing global research endeavor over the last decade (1, 2). Key to these technologies is the transformation of abundant but energy-poor feedstocks (like water and carbon dioxide) into energy-rich fuels (like hydrogen and methanol). Transition metal catalysts help orchestrate the proton-coupled electron transfer processes that underpin fuel synthesis, such as the production of hydrogen (3–6). Transition metal hydride intermediates are almost always invoked as key species during hydrogen evolution, with the metal at the center of the reactivity docking both protons and electrons. Reporting in PNAS, Solis et al. (7) now show that the ligand can play a role similar to the metal center, with a C–H bond in a phlorin reacting like a metal hydride to release hydrogen.

Bathochromic Shifts in Rhenium Carbonyl Dyes Induced through Destabilization of Occupied Orbitals

A series of rhenium diimine carbonyl complexes was prepared and characterized in order to examine the influence of axial ligands on electronic structure. Systematic substitution of the axial carbonyl and acetonitrile ligands of [Re(deeb)(CO)3(NCCH3)]+ (deeb = 4,4-diethylester-2,2-bipyridine) with trimethylphosphine and chloride, respectively, gives rise to red-shifted absorbance features. These bathochromic shifts result from destabilization of the occupied d-orbitals involved in metal-to-ligand charge-transfer transitions. Time-Dependent Density Functional Theory identified the orbitals involved in each transition and provided support for the changes in orbital energies induced by ligand substitution.

Switching between Stepwise and Concerted Proton-Coupled Electron Transfer Pathways in Tungsten Hydride Activation

Catalytic processes to generate (or oxidize) fuels such as hydrogen are underpinned by multiple proton-coupled electron transfer (PCET) steps that are associated with the formation or activation of metal-hydride bonds. Fully understanding the detailed PCET mechanisms of metal hydride transformations holds promise for the rational design of energy-efficient catalysis. Here we investigate the detailed PCET mechanisms for the activation of the transition metal hydride complex CpW(CO)2(PMe3)H (Cp = cyclopentadienyl) using stopped-flow rapid mixing coupled with time-resolved optical spectroscopy. We reveal that all three limiting PCET pathways can be accessed by changing the free energy for elementary proton, electron, and proton-electron transfers through the choice of base and oxidant, with the concerted pathway occurring exclusively as a secondary parallel route. Through detailed kinetics analysis, we define free energy relationships for the kinetics of elementary reaction steps, which provide insight into the factors influencing reaction mechanism. Rate constants for proton transfer processes in the limiting stepwise pathways reveal a large reorganization energy associated with protonation/deprotonation of the metal center (λ = 1.59 eV) and suggest that sluggish proton transfer kinetics hinder access to a concerted route. Rate constants for concerted PCET indicate that the concerted routes are asynchronous. Additionally, through quantification of the relative contributions of parallel stepwise and concerted mechanisms toward net product formation, the influence of various reaction parameters on reactivity are identified. This work underscores the importance of understanding the PCET mechanism for controlling metal hydride reactivity, which could lead to superior catalyst design for fuel production and oxidation.

Enhanced Performance in PbS Quantum Dots Solar Cells via Pulsed Laser Deposited ZnO Layer

Laser deposition of ZnO under different O2 pressures controls the performance of the n-type semiconductor in PbS quantum dots solar cells. Detailed characterization shows the mechanism behind it and allows to obtain important device improvement.

When Electrochemistry Met Methane: Rapid Catalyst Oxidation Fuels Hydrocarbon Functionalization

An electrochemical strategy for rapid generation of the highly reactive species necessary for C−H bond functionalization may enable improved technology for methane conversion.