Localizing the initial excitation - A case study on NiO photocathodes using Ruthenium dipyridophenazine complexes as sensitizers

We report on the localization of the initially excited electronic state within the molecular framework of a series of [Ru(bpy)₂dppz]²⁺ derivatives (bpy:2,2'-bipyridine, dppz: dipyrido-phenazine) as sensitizers in NiO based photocathodes. The introduction of conjugated linkers with phenylene and triazole moieties in the bpy ligand sphere separates the NiO surface from the metal center and hence is considered to stabilize the charge separated state, which results from light-driven hole injection. However, introduction of the conjugated linkers also alters the localization of the excess electron density in the excited state within the ligand sphere and impacts the extent to which the charge-separated state is formed. The study emphasizes that tuning the ligand with the lowest-energy π* orbital distal or proximal to the NiO surface significantly affects the initial charge-separation and the solar cell performance. The stability of the charge-separated state correlates with the observed photocurrents in dye-sensitized solar cells. Furthermore, the study challenges the widely accepted concept that the introduction of extended anchoring groups, i.e. increasing Ru - NiO distance, stabilizes the charge-separated state and suppresses charge recombination at the metal-oxide molecule interface.

Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production

The light-induced processes occurring in two dye-catalyst assemblies for light-driven hydrogen production were investigated by ultrafast transient absorption spectroscopy. These dyads consist of a push-pull organic dye based on a cyclopenta[1,2-b:5,4-b’]dithiophene (CPDT) bridge, covalently linked to two different H2-evolving cobalt catalysts. Whatever the nature of the latter, photoinduced intramolecular electron transfer from the excited state of the dye to the catalytic center was never observed. Instead, and in sharp contrast to the reference dye, a fast intersystem crossing (ISC) populates a long-lived triplet excited state, which in turn non-radiatively decays to the ground state. This study thus shows how the interplay of different structures in a dye-catalyst assembly can lead to unexpected excited state behavior and might open up new possibilities in the area of organic triplet sensitizers. More importantly, a reductive quenching mechanism with an external electron donor must be considered to drive hydrogen production with these dye-catalyst assemblies.

Tailored protective groups for surface immobilization of ruthenium dyes

McKenna reaction conditions are applied to the [Ru(4,4'-(CH₂PO₃Et₂)₂(bpy)](PF₆)₂ model chromophore in order to obtain [Ru(4,4'-(CH₂PO₃TMS₂)₂(bpy)](Br_2-x)(PF₆)_x (x = 0-2) (2) by replacing the alkyl moieties of the phosphonates with TMS groups (TMS = trimethylsilyl). The model complex is immobilized onto both NiO powder and NiO electrodes on FTO (fluorine doped tin oxide) using organic solvents. The stability of surface binding in aqueous media and the DSC performance of 2 are tested and compared to those of a conventional dye of structure [Ru(4,4'-(CH₂PO₃TBA₂)₂(bpy)](PF₆)₂ (1) (TBA = tetrabutyl ammonium). This is the first example of a ruthenium based chromophore with a phosphonic acid silyl-ester being directly immobilized onto a NiO surface. In addition, complex 2 exhibits superior stability towards desorption in aqueous media and at the same time showing improved DSC performance and stability in acetonitrile and a slightly higher dye loading on the electrode surface compared to 1.

Production of solar chemicals: gaining selectivity with hybrid molecule/semiconductor assemblies

Research on the production of solar fuels and chemicals has rocketed over the past decade, with a wide variety of systems proposed to harvest solar energy and drive chemical reactions. In this Feature Article we have focused on hybrid molecule/semiconductor assemblies in both powder and supported materials, summarising recent systems and highlighting the enormous possibilities offered by such assemblies to carry out highly demanding chemical reactions with industrial impact. Of relevance is the higher selectivity obtained in visible light-driven organic transformations when using molecular catalysts compared to photocatalytic materials.

Ultrafast Energy Transfer in Dinuclear Complexes with Bridging 1,10-Phenanthroline-5,6-Dithiolate

We report herein the preparation and characterization of dinuclear complexes with the bridging ligand 1,10-phenanthroline-5,6-dithiolate (phendt^2-) bearing Ru(bpy)₂ or Ir(ppy)₂ at the diimine moiety and Ni(dppe), Ni(dppf), CoCp, RhCp*, and Ru( p-Me-ⁱPr-benzene) at the dithiolate unit. In comparison with the mononuclear precursors used in the synthesis, all dinuclear complexes were characterized by absorption and photoluminescence spectroscopy as well as cyclic voltammetry. Because of the beneficial spectral and electrochemical properties of the Ir/Co complex for a light-driven charge separation, this complex was investigated in detail by time-resolved luminescence {nanosecond (ns)-resolution} and transient absorption spectroscopy {femtosecond (fs)-resolution}. All measurements supported by DFT calculations show that the observed effective luminescence quenching by the dithiolate coordinated metal is caused by an ultrafast singlet-singlet Dexter energy transfer.

Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine-Sensitized NiO Photocathodes

Understanding the structural and electronic factors governing the efficiency of dye-sensitized NiO photocathodes is essential to optimize solar fuel production in photoelectrochemical cells (PECs). For these purpose, three different ruthenium dyes, bearing either two or four methylphosphonate anchoring groups and either a bipyridine or a dipyridophenazine ancillary ligand, were synthesized and grafted onto NiO films. These photoelectrodes were fully characterized by XPS, ToF-SIMS, UV-vis absorption, time-resolved emission and femtosecond transient absorption spectroscopies. Increasing the number of anchoring groups from two to four proved beneficial for the grafting efficiency. No significant modification of the electronic properties compared to the parent photosensitizer was observed, in accordance with the non-conjugated nature of the grafted linker. The photoelectrochemical activity of the dye-sensitized NiO electrodes was assessed in fully aqueous medium in the presence of an irreversible electron acceptor and photocurrents reaching 190 μA.cm^-2 were recorded. The transient absorption study revealed the presence of two charge recombination pathways for each of the sensitizers and evidenced a stabilized charge separated state in the dppz derivative, supporting its superior photoelectrochemical activity.

Titania modification with a ruthenium(ii) complex and gold nanoparticles for photocatalytic degradation of organic compounds

Interaction between two kinds of titania modifiers, i.e., a ruthenium complex and gold nanoparticles, influenced the resultant properties and photocatalytic activities of hybrid photocatalysts under UV and/or vis light irradiation.

Synthesis and characterization of ruthenium and rhenium dyes with phosphonate anchoring groups

The synthesis and characterization of new Ru and Re complexes with elongated phosphonate substituents are presented, enabling their immobilization on electrode surfaces.

Photocatalytic proton reduction with ruthenium and cobalt complexes immobilized on fumed reversed-phase silica

Heterogeneous photocatalytic hydrogen production with a non-covalently immobilized molecular ruthenium based photosensitizer (PS) and a cobalt polypyridyl based water reducing catalyst (WRC) is reported. PS and WRC were derivatized with C₁₈-alkyl chains and immobilized by adsorption on hydrophobic fumed silica. The resulting loaded support was suspended in water with anionic or cationic surfactants and subjected to heterogeneous photocatalytic H₂ production with ascorbate as sacrificial electron donor (SED). No leaching was observed under catalytic conditions, thus catalysis was truly heterogeneous. The catalytic performance of immobilized PS and WRC clearly exceeded that of homogeneous catalysis at low concentrations. At high concentration, diffusion and light limitation lead to lower reaction rates, but the same stability as for homogeneous reactions was still achieved. WRC concentration variations indicated a relatively high stability (up to 1300 H₂/Co) and mobility of amphiphilic catalysts on the hydrophobic silica surface. Comparison of fumed silica with porous and non-porous silica showed, that a high BET surface area along with a good accessibility from the reaction media are crucial for catalytic performance. Mechanistic investigations by transient absorption spectroscopy displayed reductive quenching of excited PS by ascorbate followed by on particle electron transfer to WRC as reaction pathway. Particles with additional cationic surfactants exhibited a significantly higher catalytic performance as compared to anionic surfactants. Non-covalent anchoring of correspondingly derivatized WRCs or PSs to reversed-phase silica offers a rapid and versatile transition from homogeneous to heterogeneous molecular proton reduction.

Supramolecular bimetallic assemblies for photocatalytic hydrogen generation from water

A series of supramolecular assemblies of the type [Ru(L-L)₂(L′-L)MX₂)]ⁿ⁺ are reported where L-L is 2,2′-bipyridine (bipy), 4,4′-di-tetra-butyl-bipyridine (tbbipy) or 4,4′-diethoxycarbonyl-2,2′-bipyridine (dceb), L-L′ is tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′′,3′′′-j]phenazine (tpphz), 2,2′:5′,2′′-terpyridine (2,5-bpp), 2,2′:6′,2′′-terpyridine, (2,6-bpp), 2,5-di(pyridine-2-yl)pyrazine (2,5-dpp) or 2,3-di(pyridine-2-yl)pyrazine (2,3-dpp), and MX₂ is PdCl₂, PtCl₂ or PtI₂. The photocatalytic behaviour with respect to hydrogen generation of these compounds and their ultrafast photophysical properties are discussed as a function of the nature of the peripheral ligands, the bridging ligands and the catalytic centre. The results obtained show how differences in the chemical composition of the photocatalysts can affect intramolecular photoinduced electron transfer processes and the overall photocatalytic efficiency.