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

To explore structure-activity relationships with respect to light-harvesting behavior, a family of neutral iridium complexes [Ir(ppy)₂(L_R)] 1-4 (where ppy = 2-phenylpyridine, and N̂N = 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.

Two Excited State Collaboration of Heteroleptic Ir(III)-Coumarin Complexes for H2 Evolution Dye-Sensitized Photocatalysts

Interfacial electron injection from a photoexcited surface-immobilized dye to a semiconductor substrate is a key reaction for dye-sensitized photocatalysts. We previously reported that the molecular orientation of heteroleptic Ir(III) photosensitizer on the TiO2 nanoparticle surface was important for efficient interfacial electron injection. In this work, to overcome the weak light absorption ability of heteroleptic Ir(III) photosensitizer and to improve the photoinduced charge-separation efficiency at the dye–semiconductor interface, we synthesized two heteroleptic Ir(III) complexes with different coumarin dyes, [Ir(C6)2(H4CPbpy)]Cl and [Ir(C30)2(H4CPbpy)]Cl [Ir-CX; X = 6 or 30; HC6 = 3-(2-enzothiazolyl)-7-(diethylamino)coumarin, HC30 = 3-(2-N-methylbenzimidazolyl)-7-N,N-diethylaminocoumarin, H4CPbpy = 4,4′-bis(methylphosphonic acid)-2,2′-bipyridine], as the cyclometalated ligands and immobilized them on the surface of Pt-cocatalyst-loaded TiO2 nanoparticles. Ultraviolet-visible absorption and emission spectroscopy revealed that the singlet ligand-centered (1LC) absorption and triplet 3LC emission bands of Ir-C30 occurred at shorter wavelengths than those of Ir-C6, while time-dependent density-functional-theory data suggested that the ligand-to-ligand charge transfer (LLCT) excited states of the two complexes were comparable. The photocatalytic H2 evolution activity of the Ir-C6-sensitized Pt-TiO2 nanoparticles (Ir-C6@Pt-TiO2) under visible light irradiation (λ > 420 nm) was higher than that of Ir-C30@Pt-TiO2. In contrast, their activities were comparable under irradiation with monochromatic light (λ = 450 ± 10 nm), which is absorbed comparably by both Ir-CX complexes. These results suggest that the internal conversion from the higher-lying LC state to the LLCT state effectively occurs in both Ir-CX complexes to trigger electron injection to TiO2.

Ultrafast excited state dynamics of iridium(III) complexes and their changes upon immobilisation onto titanium dioxide layers

Time-resolved spectroscopy was applied to investigate the excited state dynamics of two heteroleptic Ir(III) complexes with the general formula [Ir(C^N)2(N^N)](+), where C^N and N^N represent different cyclometalating and diimine ligands, respectively. The excited state relaxation is influenced by the ligand substitution as well as the light polarisation. Vibrational relaxation occurs in the sub-ps timescale and interligand charge transfer results in polarisation dependent signal dynamics with a time constant of about 30 ps. Electron injection from the iridium dye to TiO2 is analysed with respect to potential applications in solar energy conversion.

The effect of photocatalyst excited state lifetime on the rate of photoredox catalysis

Four different iridium(iii) polypyridyl complexes with varying excited state lifetimes are used as photocatalysts to study the effect of excited state lifetime on the rate of a prototypical photoredox-catalyzed reaction, the trifluoromethylation of quinoline.

Photostable ester-substituted bis-cyclometalated cationic iridium(iii) complexes for continuous monitoring of oxygen

Bis-cyclometalated cationic iridium(iii) complexes bearing an ester group possess long luminescence lifetimes, good oxygen-sensitivity and excellent photostability.