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.

Tuning of photocatalytic hydrogen production and photoinduced intramolecular electron transfer rates by regioselective bridging ligand substitution.

Artificial photosynthesis based on supramolecular photocatalysts offers the unique possibility to study the molecular processes underlying catalytic conversion of photons into chemical fuels in great detail and to tune the properties of the photocatalyst by alterations of the molecular framework. Herein we focus on both possibilities in studying the photocatalytic reduction of protons by derivatives of the well-known photocatalyst [(tbbpy)(2)Ru(tpphz)PdCl(2)](PF(6))(2) [4,4'-di-tert-butyl-2,2'-bipyridine (tbbpy), tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz)]. We report on a modified photocatalyst where the crucial bridging ligand tpphz is substituted by bromine and investigate the effect of the structural variation on the catalytic properties of the complex and its ultrafast intramolecular charge transfer behavior. It is found that structural modification stabilizes the phenanthroline-centered metal-to-ligand charge-transfer state on the tpphz moiety, thereby reducing the electron transfer gradient across the entire electron-relaying bridging ligand and at the same time accelerating nanosecond ground-state recovery. The same structural modifications cause an overall reduction of the catalytic activity of the complex. Thus, the results highlight the potential of small structural variations in the molecular framework of supramolecular catalysts in understanding the photoinduced charge-transfer processes and optimizing their catalytic performance.

Substitution-controlled ultrafast excited-state processes in Ru-dppz-derivatives.

Ru-dppz (dppz = dipyrido[3,2-a:2',3,3'-c]phenazine) complexes play an important role as environmentally sensitive luminescence sensors and building blocks for larger supramolecular compounds. Their photophysical properties are known to be highly sensitive to intermolecular solvent-solute interactions and solvent bulk-properties. Here, the synthesis and characterisation of a novel Ru-dppz derivative is reported. The potential of drastically tuning the photophysical properties of such complexes is exemplified, by introducing very simple structural modifications, namely bromine, into the dppz-ligand scaffold. The photophysics i.e. nature of excited states and the excited-state relaxation pathway of the various complexes has been investigated by means of electrochemical measurements, steady-state emission experiments and femtosecond time-resolved spectroscopy. It could be shown that the location of bromine substitution influences the relative energy between a luminescent and a non-luminescent metal-to-ligand charge-transfer state and therefore quenches or facilitates transitions between both. Hence it is illustrated that the luminescent properties and the underlying ultrafast excited-state dynamics of the complexes can be controlled by structural variations, i.e. by intramolecular interactions as opposed to changes in the intermolecular interactions.

Photophysics of an intramolecular hydrogen-evolving Ru-Pd photocatalyst.

Photoinduced electron-transfer processes within a precatalyst for intramolecular hydrogen evolution [(tbbpy)(2)Ru(tpphz)PdCl(2)](2+) (RuPd; tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'c:3'',2'',-h:2''',3'''-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)(2)Ru(tpphz)](2+) subunit Ru with that of the supramolecular catalyst RuPd, the individual electron-transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the molecular ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited-state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine-centered intraligand charge-transfer state, which in RuPd precedes formation of the Pd-reduced state. The photoreaction in the heterodinuclear supramolecular complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate-determining steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge-separated states.

Excited-state annihilation in a homodinuclear ruthenium complex.

Ultrafast excited-state annihilation in a homodinuclear ruthenium complex is observed. This coordination compound constitutes a model system for approaches towards artificial photosynthetic systems. The observation of pump-intensity dependent triplet-triplet annihilation highlights the importance of considering various loss mechanisms in the design of artificial photosynthetic assemblies.

Synthesis and characterization of regioselective substituted tetrapyridophenazine ligands and their Ru(II) complexes.

A series of novel regioselective substituted tpphz ligands and two novel mononuclear ruthenium complexes of the type [(tbbpy)(2)Ru(tpphzR(n))](PF(6))(2) (where tbbpy = 4,4'-di-tert.-butyl-2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine, with n = 2 and R represents the bromine substituents at different positions) have been synthesized. All compounds were completely characterized by NMR and MS spectroscopy, absorption and steady-state emission spectroscopy as well as emission lifetime and electrochemical measurements. Additionally the solid-state structures of the two isomers [(tbbpy)(2)Ru(Br(2)tpphz)](PF(6))(2) 6 and [(tbbpy)(2)Ru(tpphzBr(2))](PF(6))(2) 7 are presented and compared with the results of density-functional theory calculations (DFT). Furthermore calculated Raman spectra were obtained by means of DFT calculations and used to assign the vibrational modes of the measured off resonance Raman spectra. A clear influence caused by the electronic effects of the different type and position of the substituents of tpphz on the photophysical behavior was observed.