Yong-Jun Yuan

Visible‐Light‐Driven H2 Generation from Water and CO2 Conversion by Using a Zwitterionic Cyclometalated Iridium(III) Complex

Concerns over global warming and energy demand have motivated academic research towards the increasing utilization of solar energy. In practice, solar-energy conversion is yet a challenging and important subject for light-driven hydrogen evolution from water and reduction of carbon dioxide into chemical energy stored in the form of fuel. To achieve this long-standing goal, solar-light-driven, electrontransfer reactions, accomplished by means of molecularbased photosensitizers (PSs) and sensitizer-semiconductors, have displayed the most promising result, because of their importance for understanding of solar-energy harvesting and artificial photosynthesis. Even though photosynthetic systems with considerable activity have been created, the solarto-fuel conversion efficiency by visible light still remains a rather difficult challenge. In the search for better lightdriven systems, which usually suffer from a potential thermodynamic limit, the key component is photoactive materials that are capable of capturing photon energy and result in efficient generation of a long-lived, charge-separated state and facilitate the extremely complex multielectron reduction of substrates at low overpotentials. Then use of transitionmetal complexes is still an extremely attractive strategy, because tuning of the photophysical and electrochemical properties can be systematically achieved through synthetic modification. Most of the photochemical systems for hydrogen generation proceed efficiently in aqueous media with a high proportion of organic solvent, such as acetonitrile and acetone, as cosolvents, because of the complexity of the multielectron processes and the insolubility of PSs in water. An increasing amount of water in the water/organic solvent mixture causes a substantial decrease of catalytic activity for hydrogen formation. Organic solvents with high dielectric constants provide a higher solubility of the PS in homogeneous systems and also offers a beneficial action for reducing the internal charge of the PS. However, the use of aqueous media has been steadily gaining importance to minimize potential environmental impacts and simplify the systems. Despite improvements, water reduction by visible light is still less active in pure water without the organic cosolvents and only a few aqueous homogeneous systems have been described to date. These urgent aspects inspired us to focus on design and development of synthetic complexes with improved photophysical properties for a typical innovative process and a better understanding of the solar-light-driven reaction that is involved. Herein we describe the formation of a new heteroleptic iridium complex [IrACHTUNGTRENNUNG(4-CF3bt)2ACHTUNGTRENNUNG(Hbpdc)] (1) (where 4CF3bt= (4-trifluoromethyl)-2-phenylbenzothiazole and H2bpdc= 2,2’-bipyridine-4,4’-dicarboxylate) and its activity towards highly efficient H2 generation from water and clean conversion of CO2 under visible-light irradiation. By employing the ancillary H2bpdc (N^N) and 4-CF3bt (C^N) ligands, complex 1 was readily available in a satisfying yield by a general two-step, bridge-splitting pathway. The N^N ligand with carboxyl substituents may assist, not only in imparting water solubility of the complexes because of the presence of an acid–base equilibria, but also in anchoring on nanocrystalline TiO2 photoanodes for an efficient and directional electron transport. In addition, we chose 2-phenylbenzothiazole (bt) as a suitable subunit in the potential photoactive compounds based primarily on the attractive electron-demanding capabilities and photochemical stabilities. Modification of the bt species by attachment of a trifluoromethyl moiety often minimizes self-quenching and improve the charge-transfer properties of the corresponding complex; this is an essential prerequisite for certain photochemical applications. As a consequence, the assembly of the d-metal iridium ACHTUNGTRENNUNG(III) with the combination of the bt and bdpc species offers great potential for interesting light-harvesting processes in water. The desired zwitterionic complex 1 was fully characterized by conventional spectroscopic and analytical methods (see the Supporting Information). Furthermore, the molecular structure in the solid state was confirmed by a single-crystal X-ray study. A pair of cyclometalated C^N ligands and a chelating N^N ligand is oriented in a distorted octahedral coordination geometry around the central iridium atom, as shown in Figure 1. The trans-orientated Ir N ACHTUNGTRENNUNG(Hbpdc) distances of 2.1041(2) and 2.1267(2) are found to be significantly longer than those observed in the cyclometalated ligand [a] Y.-J. Yuan, Dr. Z.-T. Yu, X.-Y. Chen, J.-Y. Zhang, Prof. Dr. Z.-G. Zou Eco-Materials and Renewable Energy Research Center National Laboratory of Solid State Microstructures Department of Materials Science and Engineering Nanjing University, NO. 22, Hankou Road, Nanjing Jiangsu 210093 (P.R. China) Fax: (+86) 25-8368-6632 E-mail : yuzt@nju.edu.cn zgzou@nju.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102147.

Hydrogen photogeneration promoted by efficient electron transfer from iridium sensitizers to colloidal MoS2 catalysts.

We report the utilization of colloidal MoS2 nanoparticles (NPs) for multicomponent photocatalytic water reduction systems in cooperation with a series of cyclometalated Ir(III) sensitizers. The effects of the particle size and particle dispersion of MoS2 NPs catalyst, reaction solvent and the concentration of the components on hydrogen evolution efficiency were investigated. The MoS2 NPs exhibited higher catalytic performance than did other commonly used water reduction catalysts under identical experiment conditions. The introduction of the carboxylate anchoring groups in the iridium complexes allows the species to be favorably chem-adsorbed onto the MoS2 NPs surface to increase the electron transfer, resulting in enhancement of hydrogen evolution relative to the non-attached systems. The highest apparent quantum yield, which was as high as 12.4%, for hydrogen evolution, was obtained (λ = 400 nm).

Direct assembly of new cobalt(III)-lanthanide(III) heterometallic frameworks with NaCl-like topology

Through hydrothermal treatment, a series of new 3D noninterpenetrating cobalt(III)-lanthanide(III) heterometallic coordination assemblies bearing 2,2′-bipyridine-4,4′-dicarboxylate (H2bpdc) ligands, CoLn2(bpdc)3(NO3)3(H2O)6·3.5H2O (Ln = Gd (2), Eu (3), Sm (4), La (5)), have been obtained which may be regarded as being derived from Co2(Hbpdc)2(bpdc)2(H2O)2·2H2O (1) with one-dimensional (1D) zigzag chains. The results of single-crystal X-ray analysis and the X-ray photoelectron spectroscopy study show that each Co center in all of the compounds is located in six-coordinate octahedral surroundings, and exhibits the trivalent oxidation state. Compounds 2–5 possess isostructural 3D NaCl-type frameworks containing Ln2O16 building blocks, exhibiting square pores with dimensions of ca. 8 × 8 A2 along the [010] direction, which are occupied by nitrate anions and non-coordinated water molecules. The frameworks of 2–5 are stable up to ca. 300 °C followed by removal of the coordinated and uncoordinated water molecules, as is evident from the associated weight loss. The magnetic and gas adsorption properties for the Co(III)-Gd(III) system have been investigated.

One-dimensional assembly of TiO2 nanoparticles toward enhancing light harvesting and electron transport for application in dye-sensitized solar cells

One-dimensional (1D) assembly of TiO2 nanoparticles (NPs) was successfully achieved by an improved electrospinning technique. Electrospinning precursor solution was prepared with conventional TiO2 NPs and polyvinylpyrrolidone dispersed in ethanol. The newly developed 1D assembly of nanoparticles (AS-NP) has been introduced into the photoanode in dye-sensitized solar cells (DSSCs). Compared to the traditional disordered stacking of TiO2 NPs, the AS-NPs bring in faster charge transport and longer electron lifetime, as well as a higher light scattering ability (especially in the wavelength range from 500 nm to 650 nm). It is exhibited that the AS-NPs can enhance electron transport and light scattering, while retaining the merits of NP morphology. Consequently, the efficiency of a cell based on an AS-NP/NP bilayer photoanode could be improved by about 15% in comparison to a reference cell made of absolute TiO2 NPs.

Energy Transfer in a Hybrid IrIII Carbene–PtII Acetylide Assembly for Efficient Hydrogen Production

A new heterometallic supramolecular complex, consisting of an iridium carbene-based unit appended to a platinum terpyridine acetylide unit, representing a new Ir(III) -Pt(II) structural motif, was designed and developed to act as an active species for photocatalytic hydrogen production. The results also suggested that a light-harvesting process is essential to realize the solar-to-fuel conversion in an artificial system as illustrated in the natural photosynthetic system.