Published in JACS, researchers in the Papanikolas and Waters groups, in collaboration with members of the Meyer group at Carolina Chemistry and the Papoian Group at the University of Maryland, describe how solid-phase peptide synthesis has been applied to the preparation of phosphonate-derivatized oligoproline assemblies containing two different RuII polypyridyl chromophores coupled via "click" chemistry.
In water or methanol the assembly adopts the polyproline II (PPII) helical structure, which brings the chromophores into close contact. Excitation of the assembly is followed by rapid, efficient intra-assembly energy transfer to the inner RuII. The oligoproline/click chemistry approach holds great promise for the preparation of interfacial assemblies for energy conversion based on a family of assemblies having controlled compositions and distances between key functional groups.
Researchers in the Meyer Group, in collaboration with colleagues from UNC's Department of Physics and Astronomy, RTI International, and Rutgers University, used orthorhombic Nb2O5 nanocrystalline films functionalized with [Ru(bpy)2(4,4′-(PO3H2)2bpy)]2+ as the photoanode in dye-sensitized photoelectrosynthesis cells (DSPEC) for hydrogen generation. As published in the journal Chemistry of Materials, they undertook a set of experiments to establish key properties—conduction band, trap state distribution, interfacial electron transfer dynamics, and DSPEC efficiency, to develop a general protocol for future semiconductor evaluation and for comparison with other wide-band-gap semiconductors.
The investigators found that, for a T-phase orthorhombic Nb2O5 nanocrystalline film, the conduction band potential is slightly positive (<0.1 eV), relative to that for anatase TiO2. Anatase TiO2 has a wide distribution of trap states including deep trap and band-tail trap states. Orthorhombic Nb2O5 is dominated by shallow band-tail trap states. Trap state distributions, conduction band energies, and interfacial barriers appear to contribute to a slower back electron transfer rate, lower injection yield on the nanosecond time scale, and a lower open-circuit voltage (Voc) for orthorhombic Nb2O5, compared to anatase TiO2. In an operating DSPEC, with the ethylenediaminetetraacetic tetra-anion (EDTA4–) added as a reductive scavenger, H2 quantum yield and photostability measurements show that Nb2O5 is comparable, but not superior, to TiO2.
Metal-organic frameworks, MOFs, represent a new class of structurally ordered hybrid materials whose properties can be fine-tuned at the molecular level to suit many applications. In particular, recent works from the Lin and Meyer groups have demonstrated rapid energy migration over long distances and efficient electron transfer quenching at the interfaces of emitting MOFs.
MOFs with triplet metal-to-ligand charge transfer excited-states offer a promising scaffold for amplified quenching, a signal gain as a result of interactions between a sensing material and analytes accompanied by rapid energy migration. A remarkable example of MOF-based amplified quenching has been published in JACS using Ru(II)-bpy based MOFs and cationic quenchers. The MOF surface is partially terminated with carboxylate groups that have strong non-covalent interactions with cationic quenchers and lead to quenching enhancements of up to 7000-fold compared to a model complex. This work points to the potential of designing MOF-based sensors for sensitive and selective sensing of many analytes via amplified quenching.
Arey Distinguished Professor of Chemistry Thomas Meyer has been awarded the 2012 Porter Medal. This distinction is awarded every two years to the scientist who, in the opinion of the European Photochemistry Association, the Inter-American Photochemistry Society, and the Asian and Oceanian Photochemistry Association, has contributed most to the science of photochemistry with particular emphasis on more physical aspects, reflecting George Porter's own interests.
The simultaneous, concerted transfer of electrons and protons-electron-proton transfer (EPT)-is an important mechanism utilized in chemistry and biology to avoid high energy intermediates. There are many examples of thermally activated EPT in ground-state reactions and in excited states following photoexcitation and thermal relaxation. In a collaborative work, published in PNAS, members of the Forbes, Moran, Meyer, and Papanikolas groups report application of ultrafast excitation with absorption and Raman monitoring to detect a photochemically driven EPT process (photo-EPT). In this process, both electrons and protons are transferred during the absorption of a photon. Photo-EPT is induced by intramolecular charge-transfer (ICT) excitation of hydrogen-bonded-base adducts with either a coumarin dye or 4-nitro-4'-biphenylphenol. Femtosecond transient absorption spectral measurements following ICT excitation reveal the appearance of two spectroscopically distinct states having different dynamical signatures.
One of these states corresponds to a conventional ICT excited state in which the transferring H+ is initially associated with the proton donor. Proton transfer to the base (B) then occurs on the picosecond time scale. The other state is an ICT-EPT photoproduct. Upon excitation it forms initially in the nuclear configuration of the ground state by application of the Franck-Condon principle. However, due to the change in electronic configuration induced by the transition, excitation is accompanied by proton transfer with the protonated base formed with a highly elongated H+-B bond. Coherent Raman spectroscopy confirms the presence of a vibrational mode corresponding to the protonated base in the optically prepared state.
Understanding of dye-sensitized solar cells (DSSCs) has evolved significantly by application of both transient and steady-state photocurrent measurements and theory. These devices are based on nanostructured oxide semiconductors with appropriate band energies. Photocurrents and potentials are induced by molecular excitation, injection, and intra-film diffusional electron transfer. They provide a basis for a family of photovoltaic devices which have reached solar conversion efficiencies in excess of 10%.
Scientists in the Meyer Group report in the Journal of Physical Chemistry C, a related, detailed study on H2 evolution in dye-sensitized photolectrosynthesis cells (DSPECs) based on [Ru(bpy)2(4,4'-(PO3H2)2bpy)]2+ as the sensitizer on TiO2 with added reductive scavengers TEOA and EDTA. The goal was to utilize a combination of laser flash photolysis, transient photocurrent, and steady-state photocurrent measurements to explore the underlying details of hydrogen evolution in a DSPEC configuration including factors that dictate cell efficiency.
Research published in Inorganic Chemistry by the Meyer Group, reports the results of a preliminary study on the oxidation of benzyl alcohol.
The results are notable in demonstrating a significant rate enhancement, and in identifying multiple pathways for alcohol oxidation, including a novel base-assisted pathway that appears to involve concerted hydride proton transfer, HPT.
Composite structures of Ru(bpy)2(4,4'-(PO3H2)2bpy)2+ surface bound to nanocrystalline TiO2 with an overlayer of Ru(bpy)32+ ion exchanged into Nafion, FTO|nanoTiO2 -[Ru(bpy)2 (4,4'-(PO3H2)2bpy)]2+/Nafion,Ru(bpy)32+ (FTO = fluorine-doped tin oxide), have been prepared and characterized by the Meyer Group, as published in the Journal of Physical Chemistry B.
Steady-state emission and time-resolved lifetime measurements demonstrate that energy transfer occurs from Nafion,Ru(bpy)32+* to adsorbed Ru(bpy)2 (4,4'-(PO3H2)2bpy)2+ with an efficiency of ~0.49. Energy transfer sensitizes photoinjection by the adsorbed metal-to-ligand charge transfer (MLCT) excited state by an "antenna effect."
In a collaborative work involving the Lin, Papanikolas and Meyer Groups, isomorphous metal−organic frameworks, MOFs, were designed and synthesized to study the classic Ru to Os energy transfer process that has potential applications in light-harvesting with supramolecular assemblies. Published in JACS, the researchers show how the crystalline nature of the MOFs allows precise determination of the distances between metal centers by X-ray diffraction, thereby facilitating the study of the Ru→Os energy transfer process.
The mixed-metal MOFs with 0.3, 0.6, 1.4, and 2.6 mol % Os doping were also synthesized in order to study the energy transfer dynamics with a two-photon excitation at 850 nm. The Ru lifetime at 620 nm decreases from 171 ns in the pure Ru MOF to 29 ns in the sample with 2.6 mol % Os doping. In the mixed-metal samples, energy transfer was observed with an initial growth in Os emission corresponding with the rate of decay of the Ru excited state. These results demonstrate rapid, efficient energy migration and long distance transfer in isomorphous MOFs.
Researchers in the Meyer Group have shown single-electron activation of multielectron catalysis to be viable in catalytic water oxidation with stepwise proton-coupled electron transfer, leading to high-energy catalytic precursors. For the blue dimer, cis,cis-[(bpy)2 (H2 O)RuIIIORuIII(H2O)(bpy)2]4+, the first well-defined molecular catalyst for water oxidation, stepwise 4e-/4H+ oxidation occurs to give the reactive precursor [(O)RuVORuV(O)]4+.
This key intermediate is kinetically inaccessible at an unmodified metal oxide surface, where the only available redox pathway is electron transfer. The Meyer Group reports a remarkable surface activation of indium−tin oxide (In2O3:Sn) electrodes toward catalytic water oxidation by the blue dimer at electrodes derivatized by surface phosphonate binding of [Ru(4,4'-((HO)2P(O)CH2)2bpy)2(bpy)] 2+. Surface binding dramatically improves the rate of surface oxidation of the blue dimer and induces water oxidation catalysis.
As published in JACS, notable progress has been made recently in the Meyer Group in identifying single-site catalysts for water oxidation including detailed elucidation of mechanism. For applications in electrocatalysis or photoelectrocatalysis, transferring solution reactivity to conducting or semiconductor solution interfaces is important to accelerate rates and minimize catalyst in a device configuration.
Electrocatalytic water oxidation occurs through the use of the phosphonate-derivatized single-site catalyst which functions on conducting and semiconductor oxide surfaces, retains the solution mechanism on the surface, and provides a basis for sustained, electrocatalytic water oxidation over a range of pH values.
Water oxidation is a key reaction in natural photosynthesis and in many schemes for artificial photosynthesis. Although metal complexes capable of oxidizing water based on Ru, Mn, and Ir are known, a significant question is whether or not dimeric or higher order structures are required for water oxidation. Researchers in the Meyer and Templeton Groups report in JACS on single-site catalytic water oxidation by the monomeric complexes [Ru(tpy)(bpm)(OH2)]2+ and [Ru(tpy)(bpz)(OH2)]2+ (tpy is 2,2':6',2"-terpyridine; bpm is 2,2'-bipyrimidine; bpz is 2,2'-bipyrazine) by a well-defined mechanism involving RuV=O.
These results are important in establishing detailed mechanistic insight into water oxidation at a single ruthenium site and in paving the way toward a family of robust water oxidation catalysts. For more information about Artifical Photosynthesis and other energy solutions for the future, visit Professor Meyer's faculty page.
The sun provides about 10,000 times our current daily energy needs and is the ultimate solution to the world's need for renewable, environmentally friendly energy. However, to be practical, utilization of solar energy requires energy storage on massive scales, far greater than any existing technology. Photosynthesis provides a role model in this regard, with the only practical approach consisting of an "Artifical Photosynthesis" strategy with "solar fuels" as the product. Solar fuels are high-energy molecules like carbohydrates or hydrogen with the energy of the sun stored in chemical bonds. Target reactions are water splitting into hydrogen and oxygen and light-driven reduction of CO2 to CO or other reduced forms of carbon.
At the U.S. DOE-funded Energy Frontier Research Center here at the Department of Chemistry, the goal is to generate solar fuels using dye-sensitized photoelectrosynthesis cells, DSPEC. Molecules do most of the work in this approach, absorbing light, transferring electrons, and catalyzing reactions. The DSPEC design also benefits from a modular approach, allowing the separate parts to be synthesized, evaluated, and improved in an iterative manner. Using an integrated team-based structure, we are making real progress in translating our concept of the DSPEC into a viable prototype. Many challenges lie ahead but there is light shining at the end of a very long tunnel.
Photoinduced formation, separation, and buildup of multiple redox equivalents are an integral part of cycles for producing solar fuels in dye-sensitized photoelectrosynthesis cells (DSPECs). Published in JACS, researchers in the Meyer Group have investigated the excitation wavelength-dependent electron injection, intra-assembly electron transfer, and pH-dependent back electron transfer on TiO2 for the molecular assembly [((PO3H2-CH2)-bpy)2Rua(bpy-NH-CO-trpy)Rub(bpy)(OH2)]4+ ([TiO2–RuaII–RubII–OH2]4+; ((PO3H2-CH2)2-bpy = ([2,2′-bipyridine]-4,4′-diylbis(methylene))diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2′:6′,2″-terpyridin]-4′-yl)-N-((4′-methyl-[2,2′-bipyridin]-4-yl)methyl) benzamide); bpy = 2,2′-bipyridine).
This assembly combines a light-harvesting chromophore and a water oxidation catalyst linked by a synthetically flexible saturated bridge designed to enable long-lived charge-separated states. Following excitation of the chromophore, rapid electron injection into TiO2 and intra-assembly electron transfer occur on the subnanosecond time scale followed by microsecond–millisecond back electron transfer from the semiconductor to the oxidized catalyst, [TiO2(e-)–RuaII–RubIII–OH2]4+→[TiO2–RuaII–RubII–OH2]4+.
The first designed molecular catalyst for water oxidation is the Ruthenium "blue dimer." Although there is experimental evidence for extensive electronic coupling across the µ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled RuIII ions across the µ-oxo bridge. Researchers in the Meyer Group, in collaboration with the Templeton Group, and Los Alamos National Laboratory, present the results of a comprehensive experimental investigation, combined with DFT calculations, in Inorganic Chemistry.
The purpose of their investigation is to summarize the properties of the blue dimer and emphasize the role of strong electronic coupling between the nominally RuIII ions across the micro: µ-oxo bridge.Their experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound.
Dr. Zuofeng Chen, a postdoctoral research fellow in the Meyer Group and with the UNC Energy Frontier Research Center, has been selected to receive a Postdoctoral Award for Research Excellence for the 2011-2012 academic year. This recognition is awarded by the Office of Postdoctoral Affairs at UNC, whose review committee members were very impressed with Dr. Chen's research accomplishments and his potential to become an outstanding researcher and scholar in his field.
Dr. Chen's research focuses on solar energy. Sunlight is the largest carbon-neutral, environment-friendly energy source and one of the major routes for tapping into this abundant energy source is using sunlight to make chemical/solar fuels. This can be accomplished by artificial photosynthesis, such as the splitting of water into H2 and O2 or the conversion of CO2 into methane, CH4, or methanol, CH3OH. Dr. Chen currently works on the study of water oxidation, carbon dioxide reduction, and surface proton coupled electron transfer. His research in these fields is groundbreaking and has received international recognition.
In celebration of the 50th Anniversary of Inorganic Chemistry, the journal is presenting "Voices of Inorganic Chemistry," a series of interviews featuring leaders in the field who have helped make inorganic chemistry what it is today. In the video below, Carolina Chemistry Arey Distinguished Professor Thomas Meyer is interviewed by Richard Eisenberg, the journal's Editor-in-Chief.
Considered a world leader in electron transfer chemistry and solar energy conversion by artificial photosynthesis, professor Meyer is the recipient of major ACS awards in inorganic chemistry and a member of the National Academy of Sciences. He was part of the first team to give evidence of electron transfer quenching in photochemical reactions, an elementary step that is essential to converting sunlight into stored chemical energy. In this interview, professor Meyer discusses how his interests were developed working with legendary chemist Henry Taube as a graduate student and what he views as major challenges in science going forward.
In an article in Inorganic Chemistry, the Meyer Group reports on dramatic rate enhancements observed for the oxidation of phenols, including tyrosine, at indium−tin oxide electrodes modified by the addition of the electron-transfer relays, with clear evidence for the importance of proton-coupled electron transfer and concerted electron−proton transfer.
The observations made are striking in demonstrating the impact of surface-bound electron-transfer relays ITO-RuII and ITO-OsII on interfacial electron transfer. They also provide clear evidence for an important role for interfacial EPT at modified surfaces with potentially important implications for analysis and electrocatalysis at derivatized oxide surfaces.
Published in JACS, researchers in the Meyer Group elaborate on how the complex [Ru(Mebimpy)(bpy)(OH2)]2+ [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2′-bipyridine] and its 4,4′-(PO3H2CH2)2bpy derivative on oxide electrodes are water oxidation catalysts in propylene carbonate and 2,2,2-trifluoroethanol (TFE) to which water has been added as a limiting reagent.
The rate of water oxidation is greatly enhanced relative to that with water as the solvent and occurs by a pathway that is first-order in H2O; an additional pathway that is first-order in acetate appears when TFE is used as the solvent.
Researchers in the Meyer Group report in Inorganic Chemistry the preparation and characterization of optically transparent nanoITO films derivatized with surface-bound chromophores and molecular catalysts at levels comparable to those of nanoTiO2.
In contrast to nanoTiO2, surface derivatives on nanoITO undergo facile interfacial electron transfer allowing for rapid, reversible, potential controlled color changes, direct spectral, rather than current, monitoring of voltammograms, and multilayer catalysis of water oxidation.
Published in Dalton Transactions, members of the Meyer Group show how electrocatalytic water oxidation occurs on high surface area, nanocrystalline ITO (nanoITO) surface-derivatized by phosphonate-binding of the catalyst [Ru(Mebimpy)(4,4′-((HO)2OPCH2)2bpy)(OH2)]2+ (Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy is 2,2′-bipyridine).
With nanoITO, spectral data can be acquired on electrochemically generated intermediates and voltammograms monitored spectrophotometrically.
Mastering the production of solar fuels by artificial photosynthesis will be a considerable feat. In other words, use light to either split water into hydrogen and oxygen or reduce CO2 to methanol or hydrocarbons: 2H2O + 4hv → O2 + 2H2; 2H2O + CO2 + 8hv → 2O2 + CH4. The modular approach uses light absorption, electron transfer in excited states, directed long range electron transfer and proton transfer, both driven by free energy gradients, combined with proton coupled electron transfer (PCET) and single electron activation of multielectron catalysis. Until recently, a lack of molecular catalysts, especially for water oxidation, has limited progress in this area. Analysis of the water oxidation mechanism for the "blue" Ru dimer cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+ (bpy is 2,2'-bipyridine) has opened a new, general approach to single site catalysts both in solution and on electrode surfaces.
The Meyer Group has developed a class of molecules with a general reactivity toward water oxidation whose properties can be "tuned" systematically by synthetic variations based on mechanistic insight. These molecules catalyze water oxidation driven either electrochemically or by Ce(IV). Researchers in the Meyer Group are working to incorporate these catalysts into a functioning photoelectrochemical cell to produce solar fuels.
In a collaboration between researchers in the Papanikolas Group and the Meyer Group, results of CW and lifetime emission studies have been used to demonstrate facile intra-strand energy transfer in the derivatized polystyrene polymer
[PS-4-CH2CH2NHC(O)-(RuII(4,4'-(CONEt2)2bpy)2)17
(OsII(bpy)2))3](PF6)40 in four rigid media: frozen 5:4 (v:v) propionitrile:butyronitrile solutions at 77 K, polymethyl-methacrylate (PMMA) and polyethylene glycol-dimethacrylate (PEG-DMA) films, and silica xerogel monoliths at room temperature.
Continued rapid energy transfer in rigid media is in contrast to electron transfer which is inhibited. This can be explained by energy transfer theory and is due to a decrease in the energy transfer barrier because of the frozen nature of the medium. The abbreviation used for the polymer defines the chemical link to the polystyrene backbone and gives the extent of loading out of 20 available sites. This was an important observation since one goal of the work with polymers was to use them as light absorbing antenna in artificial photosynthesis applications. As assemblies, the multi-site polymers were massive light absorbers but at isolated, electronically weakly coupled sites. In order to use the excited state energy for energy conversion at a remote site requires facile intra-strand energy migration and transfer on the lifetime of the polymer-bound excited states.