Xiaoyu Chen

Isolation and X-ray Crystal Structures of Triarylphosphine Radical Cations

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.

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.

Stable Tetraaryldiphosphine Radical Cation and Dication

Salts containing tetraaryldiphosphine radical cation 1(•+) and dication 1(2+) have been isolated and structurally characterized. Radical 1(•+) has a relaxed pyramidal geometry, while dication 1(2+) 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.

Reversible σ-Dimerizations of Persistent Organic 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.

Synthesis, Characterization, and Structures of a Persistent Aniline Radical Cation†

is regularly stacked along the b axis by C H···pintermolecular interactions. The interplanar distances arearound5.1 ,muchlargerthantheequilibriumvanderWaalsseparation of 3.4 , indicating that there are no intermolec-ular p–p interactions (Figure S6 in the Supporting Informa-tion). The phenyl ring of the TBAC

Synthesis, Crystal Structure, and Physical Property of Sterically Unprotected Thiophene/Phenylene Co‐Oligomer Radical Cations: A Conductive π–π Bonded Supermolecular meso‐Helix

Sterically unprotected thiophene/phenylene co-oligomer radical cation salts BPnT(·+) [Al(OR(F))(4)](-) (OR(F)=OC(CF(3))(3), n=1-3) have been successfully synthesized. These newly synthesized salts have been characterized by UV/Vis-NIR absorption and EPR spectroscopy, and single-crystal X-ray diffraction analysis. Their conductivity increases with chain length. The formed meso-helical stacking by cross-overlapping radical cations of BP2T(·+) is distinct from previously reported face-to-face overlaps of sterically protected (co-)oligomer radical cations.

Reply to Comments on “Synthesis, Characterization, and Structures of Persistent Aniline Radical Cation”†

We acknowledge the comments of Korth and Rathore et al. on our recent publication “Synthesis, Characterization, and Structures of Persistent Aniline Radical Cation” [1] and respond to the issues they raised: They criticize the characterization and structure determination of the aniline radical cation TBAC (TBA = 2,4,6-tri-tert-butylaniline), and are suspicious of its identification and related bond length changes with temperatures. Rathore and co-workers thought we had been misled by cyclic voltammetry experiments: this is not correct. Reversible oxidation waves are just an indication of the formation of radical cations but do not warrant prolonged stability. We thus stated in our paper: “Cyclic voltammetry of TBA in CH2Cl2 at room temperature with 0.1m Bu4NPF6 as a supporting electrolyte showed repeated well-defined reversible oxidation waves at E1/2 =+ 0.78 V versus Ag/Ag + (Figure S1, Supporting Information (SI)), indicating that the radical cation TBAC is stable under these conditions” (italics added). This only means that radical cation TBAC could form at the electrochemical time scale. Rathore et al. have obtained better absorption spectra (Figure 2B in Ref. [3]), which we recently reproduced independently. The very weak peak around 730 nm in the absorption spectrum previously was ignored by us. TBAC radical cation solution did slowly decompose but the radical cation still existed as its absorption peaks, typically the peak around 730 nm, were clearly observed even after 24 h. Figure 1 shows spectra reconstructed by superimposing the spectra displayed in Refs. [2,3], which include the absorption spectrum (shown as a gray line) of aliquots of the green reaction mixture (i.e. TBA + AgSbF6 in CH2Cl2), the experimental spectrum in acetonitrile (purple line), TD-UPBE0/ 6-31G(d)-computed excitation energies (on the UB3LYP/ CBSB7-optimized geometry, green line), and TD-UPBE0/631G(d)-computed excitation energies of aminyl radical TBA( H)C (on UB3LYP/CBSB7-optimized geometries, red line). Clearly the absorption spectrum of the green solution in CH2Cl2 is consistent with the experimental UV spectrum in CH3CN, and is supported by the TD-DFT calculation, but the spectrum differs from that for TBA( H)C. The UV/Vis spectra thus strongly support the formation of TBAC but not TBA( H)C. In fact we were prompted to isolate crystals of TBAC by its EPR spectrum, which strongly supported the formation and stability of the TBAC radical cation. Figure 2 shows simulated (Figure 2a) and experimental (Figure 2b) EPR spectra for TBAC. For comparison, we also added the simulated spectrum for neutral radical TBA( H)C (Figure 2c), using reported hyperfine coupling constants. All spectra were reproduced at the same magnetic field scales. Without a doubt, the experimental EPR spectrum for TBAC is in good agreement with simulated EPR spectrum, but does not fit for TBA( H)C. Note that the EPR sample was prepared by redissolving the green solid in the solvent, which Figure 1. Absorption spectra of TBAC and neutral aminyl radical TBA( H)C. The spectra were reconstructed by digitizing the spectra displayed in Refs. [2,3].