With the inspiration from an oxygen evolving complex (OEC) in Photosystem II (PSII), a mononuclear Ru(II) complex with a tetradentate ligand containing two carboxylate groups has been synthesized and structurally characterized. This Ru(II) complex showed efficient catalytic properties toward water oxidation by the chemical oxidant cerium(IV) ammonium nitrate. During the process of catalytic water oxidation, Ru(III) and Ru(IV) species have been successfully isolated as intermediates. To our surprise, X-ray crystallography together with HR-MS revealed that the Ru(IV) species is a seven-coordinate Ru(IV) dimer complex containing a [HOHOH](-) bridging ligand. This bridging ligand has a short O...O distance and is hydrogen bonded to two water molecules. The discovery of this very uncommon seven-coordinate Ru(IV) dimer together with a hydrogen bonding network may contribute to a deeper understanding of the mechanism for catalytic water oxidation. It will also provide new possibilities for the design of more efficient catalysts for water oxidation, which is the key step for solar energy conversion into hydrogen by light-driven water splitting, the ultimate challenge in artificial photosynthesis.

The coupling of electron and proton transfer is an important controlling factor in radical proteins, such as photosystem II, ribinucleotide reductase, cytochrome oxidases, and DNA photolyase. This was investigated in model complexes in which a tyrosine or tryptophan residue was oxidized by a laser-flash generated trisbipyridine-Ru(III) moiety in an intramolecular, proton-coupled electron transfer (PCET) reaction. The PCET was found to proceed in a competition between a stepwise reaction, in which electron transfer is followed by deprotonation of the amino acid radical (ETPT), and a concerted reaction, in which both the electron and proton are transferred in a single reaction step (CEP). Moreover, we found that we could analyze the kinetic data for PCET by Marcus' theory for electron transfer. By altering the solution pH, the strength of the Ru(III) oxidant, or the identity of the amino acid, we could induce a switch between the two mechanisms and obtain quantitative data for the parameters that control which one will dominate. The characteristic pH-dependence of the CEP rate (M. Sjodin et al. J. Am. Chem. Soc. 2000, 122, 3932) reflects the pH-dependence of the driving force caused by proton release to the bulk. For the pH-independent ETPT on the other hand, the driving force of the rate-determining ET step is pH-independent and smaller. On the other hand, temperature-dependent data showed that the reorganization energy was higher for CEP, while the pre-exponential factors showed no significant difference between the mechanisms. Thus, the opposing effect of the differences in driving force and reorganization energy determines which of the mechanisms will dominate. Our results show that a concerted mechanism is in general quite likely and provides a low-barrier reaction pathway for weakly exoergonic reactions. In addition, the kinetic isotope effect was much higher for CEP (kH/kD > 10) than for ETPT (kH/kD = 2), consistent with significant changes along the proton reaction coordinate in the rate-determining step of CEP.

Conventional cancer chemotherapy is often associated with toxicity issues. Thus, new drug delivery systems (DDSs) are developed as alternatives owing to their potential to selectively target affected cells while sparing normal tissues. Among them, noninvasive and biocompatible mesoporous silica nanoparticle (MSN)-based targeted DDSs have developed rapidly. In particular, controlled gatekeepers capping the pore entrances of MSNs play prominent and crucial roles in achieving specific drug release and avoiding premature leakage in the delivery process before the target is reached, and perfect gatekeepers can only be removed under specific internal or external stimuli, such as pH, redox potential, temperature, biomolecules, light, magnetic field and ultrasound, or a combination of these stimuli, which is significant for precise therapeutic treatments and potential applications in human bodies. Thus, the main focus of this review is to highlight the most recent progress on the design of various controlled MSN gatekeepers to achieve 'zero premature release' drug delivery. The diverse gatekeepers are categorised into the following kinds according to their types and characteristics: (1) polymers; (2) inorganic nanomaterials; (3) host-guest assemblies; and (4) biomacromolecules. This review will offer a broad palette of opportunities for researchers with interests including nanomaterial fabrication and modification, targeted drug delivery and stimuli-responsive drug release.

Discovery of an efficient catalyst bearing low overpotential toward water oxidation is a key step for light-driven water splitting into dioxygen and dihydrogen. A mononuclear ruthenium complex, Ru(II)L(pic)(2) (1) (H(2)L = 2,2'-bipyridine-6,6'-dicarboxylic acid; pic = 4-picoline), was found capable of oxidizing water eletrochemically at a relatively low potential and promoting light-driven water oxidation using a three-component system composed of a photosensitizer, sacrificial electron acceptor, and complex 1. The detailed electrochemical properties of 1 were studied, and the onset potentials of the electrochemically catalytic curves in pH 7.0 and pH 1.0 solutions are 1.0 and 1.5 V, respectively. The low catalytic potential of 1 under neutral conditions allows the use of [Ru(bpy)(3)](2+) and even [Ru(dmbpy)(3)](2+) as a photosensitizer for photochemical water oxidation. Two different sacrificial electron acceptors, [Co(NH(3))(5)Cl]Cl(2) and Na(2)S(2)O(8), were used to generate the oxidized state of ruthenium tris(2,2'-bipyridyl) photosensitizers. In addition, a two-hour photolysis of 1 in a pH 7.0 phosphate buffer did not lead to obvious degradation, indicating the good photostability of our catalyst. However, under conditions of light-driven water oxidation, the catalyst deactivates quickly. In both solution and the solid state under aerobic conditions, complex 1 gradually decomposed via oxidative degradation of its ligands, and two of the decomposed products, sp(3) C-H bond oxidized Ru complexes, were identified. The capability of oxidizing the sp(3) C-H bond implies the presence of a highly oxidizing Ru species, which might also cause the final degradation of the catalyst.

Practical-efficiency catalysis of the water oxidation process (2 H 2 O!O 2 + 4 e À + 4 H + ) is the highly sought element of emerging artificial photosynthetic energy-conversion technology. [1,2] While oxygen evolution in naturally occurring photosynthesis, which supports nearly all existing life forms, relies on a sophisticated Mn-based complex, [1a, 3] the majority of artificial molecular water-oxidation catalysts (WOCs) are Ru-based complexes with relatively simple polypyridyl ligands. [4, 5] Recently emerged evidence in favor of alternative O 2 -evolving mechanisms for Ru-catalyzed water oxidation, [5] such as solvent water nucleophilic attack (WNA) and direct O À O coupling via interaction of two M-O units (I2M), [5a, 6-8] demonstrated dramatic mechanistic consequences of different ligand designs, which are yet to be fully rationalized. Understanding of intricate ligand-dependent preferences for one mechanism over the other is necessary for further progress to be made. [5, 7] Unfortunately, vast structural differences between Ru-bound ligands of WOCs which operate by the WNA or I2M mechanism hampers determinations of ligand influence on catalytic pathways.

A photoelectrochemical device with a molecular Ru catalyst assembled via pH-modified Nafion on a dye-sensitized nanostructured TiO(2) film as anode and a Pt foil as cathode has been successfully demonstrated to split water into O(2) and H(2) driven by visible light.

This perspective article reports the most significant advances in the field of water oxidation-from molecular water oxidation catalysts (WOCs) to photoelectrochemical cells. Different series of catalysts that can be applied in visible light-driven water oxidation catalysis are discussed in details and several key aspects of their catalytic mechanisms are introduced. In order to construct a water oxidation electrode from molecular catalysts, proper immobilization methods have to be employed. Herein, we present one section about how to attach catalysts onto an electrode/material surface. Finally, the state of the art photoelectrochemical cells that achieve visible light-driven water splitting are described.

Here splits the sun: A dinuclear ruthenium complex has been synthesized and employed to catalyze the homogeneous water oxidation (see picture; purple Ru, green Cl, blue N, red O). An exceptionally high turnover number was observed both for chemical (CeIV as the oxidant) and light‐driven ([Ru(bpy)3]2+‐type photosensitizers) water splitting.