Evaluation of 1H-NMR Spectroscopy-Based Quantification Methods of the Supramolecular Aggregation of a Molecular Photosensitizer†.

The supramolecular dimerization of a ruthenium polypyridyl precursor of a well-developed family of hydrogen-evolving photocatalysts via π-π interactions of the polyheteroaromatic bridging ligand was quantified with concentration-dependent 1 H-NMR spectroscopy. The data sets were analyzed with different calculation and fit methods. A comparison between the results of direct calculation and linear and nonlinear approaches showed that the application of a global nonlinear fit procedure yields the best results. The presented methods are also applicable for dimerization processes in the solution of other molecular moieties.

Active repair of a dinuclear photocatalyst for visible-light-driven hydrogen production.

The molecular apparatus behind biological photosynthesis retains its long-term functionality through enzymatic repair. However, bioinspired molecular devices designed for artificial photosynthesis, consisting of a photocentre, a bridging ligand and a catalytic centre, can become unstable and break down when their individual modules are structurally compromised, halting their overall functionality and operation. Here we report the active repair of such an artificial photosynthetic molecular device, leading to complete recovery of catalytic activity. We have identified the hydrogenation of the bridging ligand, which inhibits the light-driven electron transfer between the photocentre and catalytic centre, as the deactivation mechanism. As a means of repair, we used the light-driven generation of singlet oxygen, catalysed by the photocentre, to enable the oxidative dehydrogenation of the bridging unit, which leads to the restoration of photocatalytic hydrogen formation.

Pump-Probe Fragmentation Action Spectroscopy: A Powerful Tool to Unravel Light-Induced Processes in Molecular Photocatalysts.

We present a proof of concept that ultrafast dynamics combined with photochemical stability information of molecular photocatalysts can be acquired by electrospray ionization mass spectrometry combined with time-resolved femtosecond laser spectroscopy in an ion trap. This pump-probe "fragmentation action spectroscopy" gives straightforward access to information that usually requires high purity compounds and great experimental efforts. Results of gas-phase studies on the electronic dynamics of two supramolecular photocatalysts compare well to previous findings in solution and give further evidence for a directed electron transfer, a key process for photocatalytic hydrogen generation.

Optimization of hydrogen-evolving photochemical molecular devices.

A molecular photocatalyst consisting of a Ru(II) photocenter, a tetrapyridophenazine bridging ligand, and a PtX2 (X=Cl or I) moiety as the catalytic center functions as a stable system for light-driven hydrogen production. The catalytic activity of this photochemical molecular device (PMD) is significantly enhanced by exchanging the terminal chlorides at the Pt center for iodide ligands. Ultrafast transient absorption spectroscopy shows that the intramolecular photophysics are not affected by this change. Additionally, the general catalytic behavior, that is, instant hydrogen formation, a constant turnover frequency, and stability are maintained. Unlike as observed for the Pd analogue, the presence of excess halide does not affect the hydrogen generation capacity of the PMD. The highly improved catalytic efficiency is explained by an increased electron density at the Pt catalytic center, this is confirmed by DFT studies.

Towards hydrogen evolution initiated by LED light: 2-(1H-1,2,3-Triazol-4-yl)pyridine-containing polymers as photocatalyst.

Two- and three-component polymethacrylates, featuring a 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine-based metal complex as photosensitizer, a viologen-type electron mediator, and a triethylene glycol methyl ether as solubilizing part are synthesized by statistical reversible addition-fragmentation chain transfer (RAFT) radical polymerization allowing the construction of well-defined copolymers. Thereby, heteroleptic ruthenium(II) and iridium(III) complexes serve as charged photosensitizers. In hydrogen evolution experiments, as proof-of-concept, triethylamine is utilized as a sacrificial donor and colloidal platinum as hydrogen evolving catalyst. The macromolecules bearing heteroleptic iridium(III) complexes of the general formula [Ir(ppy)2 (trzpy)]PF6 (ppy: 2-phenylpyridine; trzpy: 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine) and [Ir(btac)2 (trzpy)]PF6 (btac: 3-(2-benzothiazolyl)-7-(diethylamino)coumarin) are photocatalytically active producing molecular hydrogen in water upon illumination at 470 nm. By changing the cyclometalating ligand from ppy to btac, the photocatalytic performance of the copolymer as reflected in the turnover number increases by two orders of magnitude.

Palladium versus platinum: the metal in the catalytic center of a molecular photocatalyst determines the mechanism of the hydrogen production with visible light.

To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.

Supramolecular activation of a molecular photocatalyst.

The effects of the planar aromatic organic molecules anthracene and pyrene on the catalytic performance of the intramolecular hydrogen evolving photocatalyst [Ru(tbbpy)2(tpphz)PdCl2](PF6)2 functioning as a photocatalytic dyad have been studied. (1)H-NMR studies on [Ru(tbbpy)2(tpphz)PdCl2](PF6)2 and [Ru(tbbpy)2(tpphz)](PF6)2 show a pronounced interaction of pyrene with the ruthenium complexes due to π-π-interactions. The solid state structure of [Ru(tbbpy)2(tpphz)PdCl2]2[Mo8O24] shows a pronounced π-π-stacking of the polyaromatic ligands. In addition, dimerization constants for the complexes and association constants between the complexes and pyrene were determined. Studies on the photocatalytic hydrogen production show a decreased induction phase and increased turn over frequencies during the initial phase of the catalysis in the presence of anthracene and pyrene utilising the catalyst [Ru(tbbpy)2(tpphz)PdCl2](PF6)2 irrespective of the nature of the polycyclic aromatic hydrocarbon.

The role of bridging ligand in hydrogen generation by photocatalytic Ru/Pd assemblies.

The synthesis and characterisation of two terpyridine based ruthenium/palladium heteronuclear compounds are presented. The photocatalytic behaviour of the Ru/Pd complex containing the linear 2,2':5',2''-terpyridine bridge (1a) and its analogue the non-linear 2,2':6',2''-terpyridine bridge (2a) are compared together with the respective mononuclear complexes 1 and 2. Irradiation of 1a with visible light (e.g., 470 nm) results in the photocatalytic generation of dihydrogen gas. Photocatalysis was not observed with complex 2a by contrast. A comparison with the photocatalytic behaviour of the precursors 1 and 2 indicates, that while for 1a the photocatalysis is an intramolecular process, for the mononuclear precursors it is intermolecular. The photophysical and electrochemical properties of the mono- and heterobinuclear compounds are compared. Raman spectroscopy and DFT calculations indicate that there are substantial differences in the nature of the lowest energy (3)MLCT states of 1a and 2a, from which the contrasting photocatalytic activities of the complexes can be understood.