Mastering the production of solar fuels by artificial photosynthesis would be a considerable feat, either by water splitting into hydrogen and oxygen or reduction of CO(2) to methanol or hydrocarbons: 2H(2)O + 4hnu --> O(2) + 2H(2); 2H(2)O + CO(2) + 8hnu --> 2O(2) + CH(4). It is notable that water oxidation to dioxygen is a key half-reaction in both. In principle, these solar fuel reactions can be coupled to light absorption in molecular assemblies, nanostructured arrays, or photoelectrochemical cells (PECs) by a modular approach. 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 water oxidation mechanism for the "blue" Ru dimer cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(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. As a catalyst, the blue dimer is limited by competitive side reactions involving anation, but we have shown that its rate of water oxidation can be greatly enhanced by electron transfer mediators such as Ru(bpy)(2)(bpz)(2+) (bpz is 2,2'-bipyrazine) in solution or Ru(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)(bpy)(2+) on ITO (ITO/Sn) or FTO (SnO(2)/F) electrodes. In this Account, we describe a general reactivity toward water oxidation in a class of molecules 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). The first two were in the series Ru(tpy)(bpm)(OH(2))(2+) and Ru(tpy)(bpz)(OH(2))(2+) (bpm is 2,2'- bipyrimidine; tpy is 2,2':6',2''-terpyridine), which undergo hundreds of turnovers without decomposition with Ce(IV) as oxidant. Detailed mechanistic studies and DFT calculations have revealed a stepwise mechanism: initial 2e(-)/2H(+) oxidation, to Ru(IV)=O(2+), 1e(-) oxidation to Ru(V)=(3+), nucleophilic H(2)O attack to give Ru(III)-OOH(2+), further oxidation to Ru(IV)(O(2))(2+), and, finally, oxygen loss, which is in competition with further oxidation of Ru(IV)(O(2))(2+) to Ru(V)(O(2))(3+), which loses O(2) rapidly. An extended family of 10-15 catalysts based on Mebimpy (Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine), tpy, and heterocyclic carbene ligands all appear to share a common mechanism. The osmium complex Os(tpy)(bpy)(OH(2))(2+) also functions as a water oxidation catalyst. Mechanistic experiments have revealed additional pathways for water oxidation one involving Cl(-) catalysis and another, rate enhancement of O-O bond formation by concerted atom-proton transfer (APT). Surface-bound [(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)Ru(II)(bpm)Ru(II)(Mebimpy)(OH(2))](4+) and its tpy analog are impressive electrocatalysts for water oxidation, undergoing thousands of turnovers without loss of catalytic activity. These catalysts were designed for use in dye-sensitized solar cell configurations on TiO(2) to provide oxidative equivalents by molecular excitation and excited-state electron injection. Transient absorption measurements on TiO(2)-[(4,4'((HO)(2)P(O)CH(2))(2)bpy)(2)Ru(II)(bpm)Ru(II)(Mebimpy)(OH(2))](4+), (TiO(2)-Ru(II)-Ru(II)OH(2)) and its tpy analog have provided direct insight into the interfacial and intramolecular electron transfer events that occur following excitation. With added hydroquinone in a PEC configuration, APCE (absorbed-photon-to-current-efficiency) values of 4-5% are obtained for dehydrogenation of hydroquinone, H(2)Q + 2hnu --> Q + H(2). In more recent experiments, we are using the same PEC configuration to investigate water splitting.
"[Ru(bpy) 3 ] 2+ is a well-studied transition metal lumophore complex. The robustness and versatility of ruthenium(II)polypyridyl complexes in general is reflected by the myriad of applications in which they have been reported as the central component—from oxygen sensors (Liu et al 2008) and luminescent nano-architectures (Jebb et al 2007) to dye sensitized solar cells (Hagfeldt and Grätzel 2000) and water-splitting applications (Concepcion et al 2009). The photophysical properties of the system have been fully studied in depth (Balzani et al 1996), and it is an attractive luminescent reporter group due to its respectable quantum yield in aerated water and the long lifetime of its luminescent 3 MLCT state compared to organic fluorophores, usually hundreds of nanoseconds, where MLCT refers to metal-toligand charge transfer where an electron from a mainly metalcentred molecular orbital is promoted into a higher energy ligand-centred molecular orbital. "
[Show abstract][Hide abstract] ABSTRACT: Luminescence and imaging studies of 500 nm diameter colloidal silica stained with the
transition metal complex [Ru(bpy)3Cl2], [Ru(bpy)3⊂SiNP], have been detailed and suggest
that such particles are ideal for particle tracking velocimetry (PTV) or particle imaging
velocimetry (PIV) for analysis of fluid flow in microchannels. Silica particles were synthesized
using a modification to the St¨ober synthesis to cage the transition metal complex within the
core of the nanoscale particles. The particles [Ru(bpy)3⊂SiNP] exhibit luminescence at
620 nm, characteristic of the caged [Ru(bpy)3]2+ species with a lifetime of 790 ns upon
excitation at 450 nm. A collection of the luminescence spectra from the images of the particles
in a microchannel have the same profile as the spectra collected from solutions of
[Ru(bpy)3⊂SiNP], confirming that the luminescence images are attributed to
[Ru(bpy)3]2+ luminescence. PIV and PTV measurements from image sequences give flow
velocities that match well with the theoretical velocity profile for a rectangular-sided
microchannel of 100 μm depth.
"Fe(IV). This trend is also found to be valid for the heavier congeners: an AB mechanism is demonstrated for Ru V O (Concepcion et al., 2008; Romain et al., 2009), Ru VI O (Romain et al., 2009), or Ir V O (Hull et al., 2009; Vilella et al., 2011) and an RC mechanism for Ru IV O (Romain et al., 2009; Wasylenko et al., 2010). Additional factors such as hydrogen bonding interactions or base-assisted reactions can also play a vital role in the O-O bond formation step. "
[Show abstract][Hide abstract] ABSTRACT: O-O bond formation is one of the key reactions that ensure life on earth. Dioxygen is produced in photo-system II, as well as in chlorite dismutase. The reaction mechanisms occurring in the enzyme active sites are con-troversially discussed – although their structures have been resolved with less unambiguity. Artificial molecular catalysts have been developed in the last years to obtain vital insights into the O-O bond formation step. This review put together the scarce literature on the topic that helped in understanding the key steps in the O-O bond formation reactions mediated by high-valent oxo complexes of the first-row transition metals.
"Oxygen generation through photocatalytic water splitting under visible light radiation is a challenging process. In the recent five years, revolutionary developments in photoelectrochemical water splitting using Mn-oxo complexes and Co-based molecular catalysts (Cady et al. 2008; Dismukes et al. 2009) as well as Ru-and Ir-based compounds (Concepcion et al. 2009; Sala et al. 2009) associated with dye-sensitized semiconductors (Woodhouse and Parkinson 2008; Youngblood et al. 2009) have been made. In particular, the developed Mn/Nafion, Mn/TiO 2 , Mn/WO 3 , Co/Fe 2 O 3 , Co/ZnO systems may be extended to heterostructures of a variety of semiconductors. "
[Show abstract][Hide abstract] ABSTRACT: Manganese is presented in the form of a Mn4CaO5 cluster in photosystem II and is one of the most important cofactors in oxygenic photosynthesis. The Mn4CaO5 cluster of photosystem II located in the thylakoid membranes catalyzes a light-driven water-splitting reaction to achieve energy storage on the large scale at room temperature and neutral pH in green plants, algae, and cyanobacteria. The Mn-mediated water splitting reaction uses the chemical bond rearrangement to transform the energy-deficient water molecule to energy-rich oxygen and hydrogen molecules for energy storage. Great progress and breakthroughs in illustrating the structure and mechanism of water oxidation in photosystem II have been made using the combination of modern molecular genetics and sophiscated biophysical techniques in the past decade. In particular, the three-dimensional structure of photosystem II with oxygen-evolving activity has been determined at an atomic level in 2011, which provides a complete picture with the specific position of each atom in the Mn4CaO5 cluster and interaction between the each of atoms with its own amino acid ligand in the protein complexes. These progresses have significantly enhanced our understanding of the mechanisms of water splitting in natural photosynthesis and offered a unique opportunity for transforming solar energy into our energy system to solve the global energy crisis. To mimic the water oxidation of photosystem II oxygen evolving complex, appealing Mn-containing catalytic materials were discovered. In this chapter, the structural and mechanistic models of manganese in natural photosynthetic systems and the applications of manganese-containing catalytic materials via artificial photosynthesis in green renewable energy production will be summarized and evaluated. The manganese-based systems include Mn-oxo mix valence compound, Mn-oxo cubic compound, calcium manganese materials, Mn-oxo tetramer/Nafion, Mn-oxo dimer/titanium oxide, and Mn-oxo oligomer/tungsten oxide and show a compelling working principle by combing the active catalysts in water splitting with Nafion or semiconductor hetero-nanostructures for effective solar energy harnessing.
Manganese: Chemical Properties, Medicinal Uses and Environmental Effects, Edited by Shu Tian Gan and Hui Rong Kong, 02/2012: chapter 2: pages 27-50; Nova Science Publishers.
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