To explore structure-activity relationships with respect to light-harvesting behavior, a family of neutral iridium complexes [Ir(ppy)2(LR)] 1-4 (where ppy = 2-phenylpyridine, and NN = 2-(1H-pyrrol-2-yl)pyridine and its functionalized derivatives) were designed and synthesized. The structural modifications in metal complexes are accomplished through the attributions of electron-donating CH3 in 2, OCH3 in 3, and electron-withdrawing CF3 in 4. The structural analysis displays that the pyridylpyrrole acts as one-negative charged bidentated ligand to chelate the iridium center. The electrochemical and photophysical properties of these complexes were systematically studied. The neutral 1-4 as well as the ionic structurally analogous [Ir(ppy)2(bpy)](PF6) (5) were utilized as PSs in photocatalytic hydrogen generation from water with [Co(bpy)3](PF6)2 as catalyst and triethanolamine (TEOA) as electron sacrificial agent in the presence of salt LiCl. Complex 1 maintains activity for more than 144 h under irradiation, and the total turnover number is up to 1768. The electrochemical properties and the quenching reaction indicate the H2 generation by neutral complexes 1-4 is involved exclusively in the oxidative quenching process.

Neutral Cyclometalated Ir(III) Complexes with Pyridylpyrrole Ligand for Photocatalytic Hydrogen Generation from Water.

Long-Lived Triplet Excited-State Bichromophoric Iridium Photocatalysts for Controlled Photo-Mediated Atom-Transfer Radical Polymerization

Photopolymerization offers a unique opportunity to convert liquid monomers to polymers using light as the activation source. Recently, major efforts have been devoted to developing visible light photo-initiating systems, and the search for new dyes that can be incorporated into photocurable resins and polymerize a resin within a few seconds is still ongoing. With the aim of exploring a maximum of structures to reach this goal, quinoxaline has been identified to be a promising scaffold for the design of UV-centered and visible light photo-initiating systems. In this review, an overview of the different quinoxaline-based dyes will be given. In order to evidence the interest in these structures, comparisons with reference systems will be given.

/pdf/recent-advances-on-quinoxaline-based-photoinitiators-of-8hjoo8sk.pdf

Recent Advances on Quinoxaline-Based Photoinitiators of Polymerization

The development of stable, Earth-abundant, and high-activity molecular water oxidation catalysts under acidic and neutral conditions remains a great challenge. Here, the use of N-heterocyclic carbene (NHC)-based iron(III) complex 1 {[phenyl(tris(3-methylimidazol-1-ylidene))borate]2Fe(III)}+ as a catalyst for water oxidation under acidic and neutral conditions was investigated. Two iron(II) carbene complexes, 2 {[2,6-bis(3-methylimidazolium-1-yl)pyridine]2Fe}2+ and 3 {[2,6-bis(3-methylimidazolium-1-yl)pyridine-4-carboxylic acid]2Fe}2+, were also used for comparison. A series of experiments demonstrate that complex 1 has excellent performance in terms of both catalytic activity and stability. In addition, the faradaic efficiency and turnover frequency (TOF) reach 95.0% and 2.8 s-1, respectively. An overpotential of ca. 490 mV is obtained at pH 1.5. Density functional theory (DFT) calculations indicate that dehydrogenation is the potential-determining step (PDS) in water oxidation. Complex 1 has a lower free energy barrier in this process than 2 and 3. High-valent Fe species are further proven in 1 by spectroelectrochemical measurements, which are crucial in promoting water oxidation. This study is expected to contribute to the development of homogeneous water oxidation catalysis under acidic and neutral conditions.

Iron-N-heterocyclic carbene complexes as efficient electrocatalysts for water oxidation under acidic conditions.

Developing photosensitizers with high durability is desirable to boost the practical application of photocatalytic hydrogen evolution. Herein, referring to the successful strategy in the field of light-emitting electrochemical cells, the reported Ir(III) complex with intramolecular [Formula: see text] interaction, Ir2, is used as a photosensitizer to explore its durability. Photocatalytic hydrogen evolution experiment exhibits that the durability of Ir2 is significantly improved with the duration of 39 h, which is three times longer than that of the classical Ir(III) complex Ir1 (ca. 13 h) under the same condition. As revealed by theoretical calculation, the incorporation of intramolecular [Formula: see text] interaction inhibits the rupture of metal–ligand bond in the excited state, thereby reducing the possibility of complex degradation. This is a novel approach to achieve a durable Ir(III) photosensitizer, which stimulates new molecular engineering endeavors. The finding proves the applicability of molecular design strategy in the field of light-emitting electrochemical cells to the photocatalytic hydrogen evolution system, thus boosting the cross integration of different disciplines. 

Enhanced durability of cyclometalated Ir(III) photosensitizer through intramolecular π-π interaction for robust photocatalytic hydrogen evolution

The development of novel iridium(III) complexes has continued as an important area of research owing to their highly tunable photophysical properties and versatile applications. In this report, three heteroleptic dimesitylboron-containing iridium(III) complexes, [Ir(p-B-ppy)2 (N^N)]+ {p-B-ppy=2-(4-dimesitylborylphenyl)pyridine; N^N=dipyrido[3,2-a:2',3'-c]phenazine (dppz) (1), dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) (2), and 1,10-phenanthroline (phen) (3)}, were prepared and fully characterized electrochemically, photophysically, and computationally. Altering the conjugated length of the N^N ligands allowed us to tailor the photophysical properties of these complexes, especially their luminescence wavelength, which could be adjusted from λ=583 to 631 nm in CH2 Cl2 . All three complexes were evaluated as visible-light-absorbing sensitizers for the photogeneration of hydrogen from water and as photocatalysts for the photopolymerization of methyl methacrylate. The results showed that all of them were active in both photochemical reactions. High activity for the photosensitizer (over 1158 turnover numbers with 1) was observed, and the system generated hydrogen even after 20 h. Additionally, poly(methyl methacrylate) with a relatively narrow molecular-weight distribution was obtained if an initiator (i.e., ethyl α-bromophenylacetate) was used. The living character of the photoinduced polymerization was confirmed on the basis of successful chain-extension experiments.

Visible-Light-Driven Hydrogen Production and Polymerization using Triarylboron-Functionalized Iridium(III) Complexes.

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Visible-Light-Driven Hydrogen Production and Polymerization using Triarylboron-Functionalized Iridium(III) Complexes.