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

Abstract

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 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.

Understanding Ir(III) Photocatalyst Structure–Activity Relationships: A Highly Parallelized Study of Light-Driven Metal Reduction Processes

“…This decrease could be attributed to lowering Φ hv stabilizing the HOMO hypsochromically shifts the absorption spectraor less charge separation in mixed excited states, therefore facilitating back-electron transfer in the encounter complex. As the excited state becomes 3 LC dominated (transiting from type II to type IV), the HSOMO and LSOMO are spatially localized exclusively on the C^N ligand. As the oxidation of the amine (D) is more likely to occur as it approaches this ligand, the spatial proximity between the HSOMO and D + formed after quenching could result in more electronic orbital overlap, resulting in higher rates of k −q .…”

“…Given this spatial distribution, visible light transitions are found to be metal-ligand-to-ligand charge transfer (MLLCT, dπ C^N → π* N^N ). 64 These transitions typically populate the 1 MLLCT state, where rapid rates of intersystem crossing and internal conversion subsequently yield the 3 MLLCT state with high efficiencies. 65 If the π* orbitals localized on the cyclometalating ligand are sufficiently stabilized, internal conversion from the 3 MLLCT state to a ligand centered triplet state ( 3 LC) will occur.…”

“…65 If the π* orbitals localized on the cyclometalating ligand are sufficiently stabilized, internal conversion from the 3 MLLCT state to a ligand centered triplet state ( 3 LC) will occur. 70 Prolonged excited state lifetimes are observed for 3 LC states (≥2 μs), attributed to less metal character in the excited state, while 3 MLLCT excited states are generally shorter lived.…”

“…LC states are well characterized through emission spectral profiles depicting vibronic resolution, as T 1 → S 0 is dominated by π* C^N → dπ C^N transitions, contrasting the structureless emission spectra of charge-separated 3 MLLCT states. 70 To investigate how ligand structure controls these photophysical properties, we developed an in situ synthetic protocol to afford 1440 structurally diverse [Ir(C^N) 2 (N^N)] + complexes assembled from 60 cyclometalating ligands and 24 ancillary ligands (the same ligands are used in this work).…”

“…Differentiation between these four types was determined by the fitting parameters of the spectral profile with Gaussian functions, primarily by the standard deviation (σ). We classified type I as pure 3 MLLCT (σ = 0.169 eV) and type IV as pure 3 LC (σ = 0.057 eV), while types II and III are mixed electronic configurations where either 3 MLLCT (type II) or 3 LC (type III) dominates (Figure 3A). Vibronic substructure is clearly visible in both type III and type IV excited states, but the change in relative peak ratios between Em 0−0 and Em 0−1 indicates less nuclear reorganization in type IV following excitation.…”