A new hydrogen evolving cobalt catalyst 1 based on a pentapyridine ligand has been synthesized and characterized. Its photocatalytic activity in the presence of a Ru(bpy)3(2+) sensitizer and ascorbic acid as a sacrificial electron donor has been screened in purely buffered aqueous solutions showing TONs and TOFs strongly dependent on both catalyst concentration and pH with the best results obtained at 50 μM 1 and at pH 4 (TON = 187, TOF = 8.1 min(-1)). The photochemical mechanism, as revealed by flash photolysis, involves reaction of the excited sensitizer with ascorbic acid to yield Ru(bpy)3(+) as a primary photo-generated reductant, capable of electron transfer to 1 with a remarkable rate (bimolecular rate constant k = 5.7 (±0.7) × 10(9) M(-1) s(-1)). For hydrogen generation, two one-electron photochemical reduction steps of 1 are needed along with hydride formation and protonation. Under the experimental conditions used, hydrogen evolution is mainly limited by partial decomposition of both the sensitizer and the catalyst. Moreover, accumulation of the oxidation product of the ascorbic acid donor, dehydroascorbic acid, is observed to strongly decrease the hydrogen production yield. As shown by flash photolysis, this species is capable of quenching the reduced ruthenium species (k = 4.4 (±0.5) × 10(7) M(-1) s(-1)) thus competing with electron transfer to the catalyst.

Photochemical hydrogen evolution studies aimed at evaluating new molecular catalysts have usually exploited Ru(bpy)3(2+) (where bpy = 2,2'-bipyridine) as the reference photosensitizer, thanks to its suitable optical and redox properties. In principle, an additional improvement of the photocatalytic performances can be achieved also by a careful adjustment of the photophysical and/or electrochemical characteristics of the ruthenium-based sensitizer. Herein we describe homogeneous molecular systems for photocatalytic hydrogen evolution composed of a series of ruthenium polypyridine complexes as the photosensitizers (), a cobaloxime catalyst, and ascorbic acid as the sacrificial electron donor. Suitable functionalizations of the 4,4' positions of bipyridine ligands have been addressed in order to modify the redox properties of the chromophores rather than their optical ones. A careful and detailed kinetic characterization of the relevant processes at the basis of hydrogen evolving photocatalysis has been addressed to rationalize the observed behavior. The results show that the ruthenium complex involving two 2,2'-bipyridines and one 4,4'-dimethyl-2,2'-bipyridine () may outperform the standard Ru(bpy)3(2+) (), combining the right balance of structural and redox properties, thus posing as an alternative benchmark photosensitizer for the study of new hydrogen evolving catalysts.

Hydrogen production from water splitting is nowadays recognized as a target, fundamental reaction for the production of clean fuels. Indeed, tremendous efforts have been devoted towards the research of suitable catalysts capable of performing this reaction. With respect to heterogeneous systems, molecular catalysts such as metal complexes are amenable to chemical functionalization in order to fine tune the catalytic properties. In this paper a new class of tris(2-pyridylmethyl)-amine (TPMA) cobalt(ii) complexes (CoL0-4) has been synthesized and employed as hydrogen evolving catalysts under photochemical conditions taking advantage of Ru(bpy)3(2+) (where bpy is 2,2'-bipyridine) as a light-harvesting sensitizer and ascorbic acid as a sacrificial electron donor. Tuning of the photocatalytic activity has been attempted through the introduction of different substituents at the catalyst periphery rather than through a direct chemical modification of the chelating TPMA ligand. The results show that CoL0-4 behave as competent hydrogen evolving catalysts (HECs), although the effects played by the different substituents on the catalysis are relatively modest. Possible reasons supporting the observed behavior as well as possible improvements of the aforementioned tuning approach are discussed.

Inspired by natural photosynthesis, the aim of light‐driven water splitting is to produce renewable fuels by exploiting solar radiation. Sustained hydrogen production is desirable in such systems, and the oxidation of water to oxygen is currently recognized as the bottleneck of the entire process. Therefore, solutions for this difficult task retain a fundamental interest. In this paper, we present a bioinspired, three‐component system for water oxidation that comprises a tetracationic porphyrin ZnII complex as the photosensitizer, a tetraruthenium water‐oxidation catalyst, and sodium persulfate as the electron acceptor. An in‐depth photophysical study reveals the photogeneration of a pentacation radical of the porphyrin (quantum yield up to Φ = 1.01) upon oxidative quenching of the triplet excited state by persulfate. Electron transfer from the water‐oxidation catalyst to the pentacation radical (hole scavenging) is slow (bimolecular rate constant, k < 4 × 107 M–1 s–1), and this is likely the main reason for the low efficiency of the system in photocatalytic tests for water oxidation. Perspectives for improvements of the system and for the development of a light‐activated device for water splitting are discussed.