Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Titanium dioxide-based nanomaterials for photocatalytic fuel generations.

There is increasing interest in developing artificial systems that can mimic natural photosynthesis to directly harvest and convert solar energy into usable or storable energy resources. Photocatalysis, in which solar photons are used to drive redox reactions to produce chemical fuel, is the central process to achieve this goal. Despite significant efforts to date, a practically viable photocatalyst with sufficient efficiency, stability and low cost is yet to be demonstrated. It is often difficult to simultaneously achieve these different performance metrics with a single material component. The heterogeneous photocatalysts with multiple integrated functional components could combine the advantages of different components to overcome the drawbacks of single component photocatalysts. A wide range of heterostructures, including metal/semiconductor, semiconductor/semiconductor, molecule/semiconductor and multi-heteronanostructures, have been explored for improved photocatalysts by increasing the light absorption, promoting the charge separation and transportation, enhancing the redox catalytic activity and prolonging the functional life-time. The present review gives a concise overview of heterogeneous photocatalysts with a focus on the relationship between the structural architecture and the photocatalytic activity and stability.

Progress, challenge and perspective of heterogeneous photocatalysts

The development of cyclometalated Ir(III) complexes has enabled important breakthroughs in electroluminescence because such complexes permit the efficient population of triplet excited states that give rise to luminescent transitions. The triplet states of Ir(III) complexes are advantageous over those of other transition metal complexes in that their electronic transitions and charge-transfer characteristics are tunable over wide ranges. These favorable properties suggest that Ir(III) complexes have significant potential in a variety of photofunctions other than electroluminescence. In this critical review, we describe recent photonic applications of novel Ir(III) complexes. Ir(III) complexes have been shown to affect the exciton statistics in the active layers of organic photovoltaic cells, thereby improving the photon-to-current conversion efficiencies. Nonlinear optical applications that take advantage of the strong charge-transfer properties of triplet transitions are also discussed. The tunability of the electrochemical potentials facilitates the development of efficient photocatalysis in the context of water photolysis or organic syntheses. The photoredox reactivities of Ir(III) complexes have been employed in studies of charge migration along DNA chains. The photoinduced cytotoxicity of Ir(III) complexes on live cells suggests that the complexes may be useful in photodynamic therapy. Potential biological applications of the complexes include phosphorescence labeling and sensing. Intriguing platforms based on cyclometalated Ir(III) complexes potentially provide novel protein tagging and ratiometric detection. We envision that future research into the photofunctionality of Ir(III) complexes will provide important breakthroughs in a variety of photonic applications.

Photofunctional triplet excited states of cyclometalated Ir(III) complexes: beyond electroluminescence

Shortly after their discovery at the end of the 80s, stable singlet carbenes have been recognized as excellent ligands for transition metal based catalysts, and as organo-catalysts in their own right. At the end of the 2000s, it has been shown that they can coordinate main group elements in their zero oxidation state, and even activate small molecules. This review covers examples in the literature dealing with the most recent application of stable singlet carbenes, namely their use to stabilize radicals and radical ions that are otherwise unstable.

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Carbene-stabilized main group radicals and radical ions

Frustrated Lewis pairs have been used to activate a variety of small molecules. In this review we focus on the recent chemistry of FLPs with CO2, CO, N2O, NO and SO2. While FLP capture of these small molecule is achieved in all of these cases, subsequent applications of the products include stoichiometric and catalytic reductions of CO2, C–O bond scission of CO and use of FLP–NO radicals in polymerization.

Frustrated Lewis pair chemistry of carbon, nitrogen and sulfur oxides

Salts containing triarylphosphine radical cations 1•+ and 2•+ have been isolated and characterized by electron paramagnetic resonance (EPR) and UV–vis absorption spectroscopy as well as single-crystal X-ray diffraction. Radical 1•+ exhibits a relaxed pyramidal geometry, while radical 2•+ becomes fully planar. EPR studies and theoretical calculations showed that the introduction of bulky aryl groups leads to enhanced p character of the singly occupied molecular orbital, and the radicals become less pyramidalized or fully flattened.

Isolation and X-ray Crystal Structures of Triarylphosphine Radical Cations

Salts containing tetraaryldiphosphine radical cation 1•+ and dication 12+ have been isolated and structurally characterized. Radical 1•+ has a relaxed pyramidal geometry, while dication 12+ prefers a planar, olefin-like geometry with a two-electron π bond. The alteration of the geometries of the tetraaryldiphosphine upon oxidation is rationalized by the nature of the bonding. The EPR spectrum showed that the spin density of radical 1•+ is mainly localized on phosphorus atoms, which is supported by theoretical calculation.

Stable tetraaryldiphosphine radical cation and dication.

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.

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

Salts that contain radical cations of benzidine (BZ), 3,3',5,5'-tetramethylbenzidine (TMB), 2,2',6,6'-tetraisopropylbenzidine (TPB), and 4,4'-terphenyldiamine (DATP) have been isolated with weakly coordinating anions [Al(OR(F))(4)](-) (OR(F) = OC(CF(3))(3)) or SbF(6)(-). They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of benzidine or its alkyl-substituted derivatives in CH(2)Cl(2). The salts were characterized by UV absorption and EPR spectroscopy as well as by their single-crystal X-ray structures. Variable-temperature UV/Vis absorption spectra of BZ(·)(+)[Al(OR(F))(4)](-) and TMB(·)(+)[Al(OR(F))(4)](-) in acetonitrile indicate an equilibrium between monomeric free radical cations and a radical-cation dimer. In contrast, the absorption spectrum of TPB(·)(+)SbF(6)(-) in acetonitrile indicates that the oxidation of TPB only resulted in a monomeric radical cation. Single-crystal X-ray diffraction studies show that in the solid state BZ and its methylation derivative (TMB) form radical-cation π dimers upon oxidation, whereas that modified with isopropyl groups (TPB) becomes a monomeric free radical cation. By increasing the chain length, π stacks of π dimers are obtained for the radical cation of DATP. The single-crystal conductivity measurements show that monomerized or π-dimerized radicals (BZ(·)(+), TMB(·)(+), and TPB(·)(+)) are nonconductive, whereas the π-stacked radical (DATP(·)(+)) is conductive. A conduction mechanism between chains through π stacks is proposed.

From Monomers to π Stacks, from Nonconductive to Conductive: Syntheses, Characterization, and Crystal Structures of Benzidine Radical Cations

A class of well-defined reversible σ-dimerizations of 9,10-dialkoxyanthracene radical cations are presented. Yellow crystals of the σ-dimerized dication dissociate to purple solutions of monomeric radical cations in solution. The identity and stability of radical cations were unequivocally confirmed, providing evidence for reversible σ-dimerizations of persistent radical cations of aromatic systems.

Xiaoyu Chen

Papers

Isolation and X-ray Crystal Structures of Triarylphosphine Radical Cations

Stable tetraaryldiphosphine radical cation and dication.

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

From Monomers to π Stacks, from Nonconductive to Conductive: Syntheses, Characterization, and Crystal Structures of Benzidine Radical Cations

Reversible σ‐Dimerizations of Persistent Organic Radical Cations