Photo-driven water oxidation performed by supramolecular photocatalysts made of Ru(II) photosensitizers and catalysts.

Two new supramolecular photocatalysts made of covalently linked Ru(II) polypyridine chromophore subunits ([Ru(bpy)3]2+-type species; bpy = 2,2'-bipyridine) and [RuL(pic)2] (L = 2,2'-bipyridine-6,6'-dicarboxylic acid; pic = 4-picoline) water oxidation catalyst subunits have been prepared. The new species, 1 and 2, contain chromophore and catalyst subunits in the molecular ratios 1:1 and 1:2, respectively. The model chromophore species [Ru(bpy)2(L1)]2+ (RuP1; L1=4-[2-(4-pyridyl)-2-hydroxyethyl]-4-methyl-2,2'-bipyridine) and [Ru(bpy)2(L2)]2+ (RuP2; L2 = 4,4'-bis[2-(4-pyridyl)-2-hydroxyethyl]-2,2'-bipyridine) have also been prepared. The absorption spectra, oxidation behavior, and luminescent properties of 1 and 2 have been studied, and the results indicate that each subunit largely maintains its own properties in the supramolecular species. However, the luminescence of the chromophore subunits is significantly quenched in 1 and 2 in comparison with the luminescence of the respective model species. Both 1 and 2 exhibit catalytic water oxidation in the presence of cerium ammonium nitrate, exhibiting an I2M mechanism, with a better efficiency than the known catalyst [RuL(pic)2] under the same experimental conditions. Upon light irradiation, in the presence of persulfate as a sacrificial acceptor agent, 1 and 2 are more efficient photocatalysts than a system made of separated [Ru(bpy)3]2+ and [RuL(pic)2] species, highlighting the advantage of using multicomponent, supramolecular species with respect to isolated species. The O-O bond formation step is I2M, even in the photo-driven process. The photocatalytic process of 2 is more efficient than that of 1, with the turnover frequency reaching a value of 1.2 s-1. A possible reason could be an increased local concentration of catalytic subunits in the needed bimolecular assembly required for the I2M mechanism in 2 with respect to 1, a consequence of the presence of two catalytic subunits in each multicomponent species 2.

Self-assembled systems for artificial photosynthesis.

The last few decades have seen an impressive development in molecular-based artificial photosynthesis, thanks to the design of integrated light-harvesting antennae, charge separation systems, and catalysts for water oxidation or hydrogen production based on covalently linked subunits. However, in recent years, self-assembly and spontaneous aggregation of components emerged - sometimes also through serendipity - for the preparation of multicomponent systems aimed to perform the basic processes needed for artificial photosynthesis. Here we critically discuss some key articles that have recently shown the potential of self-assembly for artificial photosynthesis, ranging from self-assembly of antennae and charge separation systems to integrated antenna/catalyst assemblies, to planned co-localization of various components into restricted environments. It is evident that self-assembly can generate emerging properties with respect to the non-aggregated species, and such emerging properties can be quite convenient for designing efficient photocatalytic systems.

Photoinduced Water Oxidation in Chitosan Nanostructures Containing Covalently Linked RuII Chromophores and Encapsulated Iridium Oxide Nanoparticles.

The luminophore Ru(bpy)2 (dcbpy)2+ (bpy=2,2'-bipyridine; dcbpy=4,4'-dicarboxy-2,2'-bipyridine) is covalently linked to a chitosan polymer; crosslinking by tripolyphosphate produced Ru-decorated chitosan fibers (NS-RuCh), with a 20 : 1 ratio between chitosan repeating units and RuII chromophores. The properties of the RuII compound are unperturbed by the chitosan structure, with NS-RuCh exhibiting the typical metal-to-ligand charge-transfer (MLCT) absorption and emission bands of RuII complexes. When crosslinks are made in the presence of IrO2 nanoparticles, such species are encapsulated within the nanofibers, thus generating the IrO2 ⊂NS-RuCh system, in which both RuII photosensitizers and IrO2 water oxidation catalysts are within the nanofiber structures. NS-RuCh and IrO2 ⊂NS-RuCh have been characterized by dynamic light scattering, scanning electronic microscopy, and energy-dispersive X-ray analysis, which indicated a 2 : 1 ratio between RuII chromophores and IrO2 species. Photochemical water oxidation has been investigated by using IrO2 ⊂NS-RuCh as the chromophore/catalyst assembly and persulfate anions as the sacrificial species: photochemical water oxidation yields O2 with a quantum yield (Φ) of 0.21, definitely higher than the Φ obtained with a similar solution containing separated Ru(bpy)3 2+ and IrO2 nanoparticles (0.05) or with respect to that obtained when using NS-RuCh and "free" IrO2 nanoparticles (0.10). A fast hole-scavenging process (rate constant, 7×104  s-1 ) involving the oxidized photosensitizer and the IrO2 catalyst within the IrO2 ⊂NS-RuCh system is behind the improved photochemical quantum yield of IrO2 ⊂NS-RuCh.

Carbohydrates and Charges on Oligo(phenylenethynylenes): Towards the Design of Cancer Bullets.

Two new tetralkylammonium-OPEs, bearing one or two positively charged groups directly linked to the aromatic residues and two ß-d-glucopyranose terminations, were synthesized. Their peculiar structural features, joining the biologically relevant sugar moieties, flat aromatic cores and positive charges, make these luminescent dyes soluble in aqueous media and able to strongly interact with DNA. As a result of UV/Vis spectral variations, DNA melting temperature measures, viscometric titrations and induced CD, we propose a partial insertion of the OPEs aromatic core into the helix, stabilized by glucose H-bonding with the groups accessible from the grooves. This interaction leads to the quenching of the OPE luminescence due to guanine reduction. The biocompatibility of the monocationic OPE with healthy and cancer cells, and the reduction of proliferation in HEp-2 cancer cells induced by the dicationic one, make this class of compounds promising for future biological applications.

Multichromophoric hybrid species made of perylene bisimide derivatives and Ru(ii) and Os(ii) polypyridine subunits.

Herein, the synthesis and the photophysical and redox properties of a new perylene bisimide (PBI) species (L), bearing two 1,10-phenanthroline (phen) ligands at the two imide positions of the PBI, and its dinuclear Ru(ii) and Os(ii) complexes, [(bpy)2Ru(µ-L)Ru(bpy)2](PF6)4 (Ru2; bpy = 2,2'-bipyridine) and [(Me2-bpy)2Os(µ-L)Os(Me2-bpy)2](PF6)4 (Os2; Me2-bpy = (4,4'-dimethyl)-2,2'-bipyridine), are reported. The absorption spectra of the compounds are dominated by the structured bands of the PBI subunit due to the lowest-energy spin-allowed π-π* transition. The spin-allowed MLCT transitions in Ru2 and Os2 are inferred by the absorption at 350-470 nm, where the PBI absorption is negligible. The absorption band extends towards the red region for Os2 due to the spin-forbidden MLCT transitions, intensified by the heavy osmium center. The reduction processes of the compounds are dominated by two successive mono-electronic PBI-based processes, which in the metal complexes are slightly shifted compared to the free ligand. On oxidation, both metal complexes undergo an apparent bi-electronic process (at 1.31 V vs. SCE for Ru2 and 0.77 V for Os2), attributed to the simultaneous one-electron oxidation of the two weakly-interacting metal centers. In Ru2 and Os2, the intense fluorescence of L subunit (λmax, 535 nm; τ, 4.3 ns; Φ, 0.91) is fully quenched, mainly by photoinduced electron transfer from the metal centers, on the ps timescale (time constant, 11 ps in Ru2 and 3 ps in Os2). Such photoinduced electron transfer leads to the formation of a charge-separated state, which directly decays to the ground state in about 70 ps in Os2, but produces the triplet π-π* state of the PBI subunit in 35 ps in Ru2. The results provide information on the excited-state processes of the hybrid species combining two dominant classes of chromophore/luminophore species, the PBI and the metal polypyridine complexes, and can be used for future design on new hybrid species with made-to-order properties.

Water oxidation catalysis upon evolution of molecular Co(III) cubanes in aqueous media.

The increasing global energy demand has stimulated great recent efforts in investigating new solutions for artificial photosynthesis, a potential source of clean and renewable solar fuel. In particular, according to the generally accepted modular approach aimed at optimising separately the different compartments of the entire process, many studies have focused on the development of catalytic systems for water oxidation to oxygen. While in recent years there have been many reports on new catalytic systems, the mechanism and the active intermediates operating the catalysis have been less investigated. Well-defined, molecular catalysts, constituted by transition metals stabilised by a suitable ligand pool, could help in solving this aspect. However, in some cases molecular species have been shown to evolve to active metal oxides that constitute the other side of this catalysis dichotomy. In this paper, we address the evolution of tetracobalt(III) cubanes, stabilised by a pyridine/acetate ligand pool, to active species that perform water oxidation to oxygen. Primary evolution of the cubane in aqueous solution is likely initiated by removal of an acetate bridge, opening the coordination sphere of the cobalt centres. This cobalt derivative, where the pristine ligands still impact on the reactivity, shows enhanced electron transfer rates to Ru(bpy)3(3+) (hole scavenging) within a photocatalytic cycle with Ru(bpy)3(2+) as the photosensitiser and S2O8(2-) as the electron sink. A more accentuated evolution occurs under continuous irradiation, where Electron Paramagnetic Resonance (EPR) spectroscopy reveals the formation of Co(ii) intermediates, likely contributing to the catalytic process that evolves oxygen. All together, these results confirm the relevant effect of molecular species, in particular in fostering the rate of the electron transfer processes involved in light activated cycles, pivotal in the design of a photoactive device.

The use of a vanadium species as a catalyst in photoinduced water oxidation.

The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(V(IV)5V(V)1)O7(OCH3)12](-) species, 1. Photoinduced water oxidation catalyzed by 1, in the presence of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and Na2S2O8, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru(bpy)3(3+) and 1 occurs with a bimolecular rate constant of 2.5 × 10(8) M(-1) s(-1). The time-resolved formation of the oxidized molecular catalyst 1(+) in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes.

Red-emitting [Ru(bpy)2(N-N)]2+ photosensitizers: emission from a ruthenium(II) to 2,2'-bipyridine (3)MLCT state in the presence of neutral ancillary "super donor" ligands.

The synthesis and characterization of a novel family of [Ru(II)(bpy)2(N-N)](PF6)2 (bpy = 2,2'-bypyridine) complexes are reported, where N-N = pyridine/pyrimidine/pyrazine functionalized in different positions with the electron-donating bicyclic hexahydropyrimidopyrimidine (hpp) unit. A series of bidentate ligands 1a-5a were synthesized in good to high yields (55-96%). The corresponding complexes 1b, 2b, and 5b were prepared in n-butanol, while complexes 3b and 4b were prepared in a mixture of n-butanol and water (1/1, v/v) in modest to good yields (23-76%). Both ligand and complex structures were fully characterized by a variety of techniques, including X-ray crystallography. In cyclic voltammetric studies, all the complexes exhibit a Ru(III/II) couple, which is ∼500 mV less positive than the Ru(III/II) couple in Ru(bpy)3(2+). The (1)MLCT and (3)MLCT states of all of the complexes (530-560 nm/732-745 nm) are shifted bathochromically in comparison to those of Ru(bpy)3(2+) (450 nm/620 nm). These values are in good agreement with DFT and TD-DFT calculations.

Photocatalytic water oxidation: tuning light-induced electron transfer by molecular Co4O4 cores.

Isostructural cubane-shaped catalysts [Co(III)(4)(µ-O)(4)(µ-CH(3)COO)(4)(p-NC(5)H(4)X)(4)], 1-X (X = H, Me, t-Bu, OMe, Br, COOMe, CN), enable water oxidation under dark and illuminated conditions, where the primary step of photoinduced electron transfer obeys to Hammett linear free energy relationship behavior. Ligand design and catalyst optimization are instrumental for sustained O(2) productivity with quantum efficiency up to 80% at λ > 400 nm, thus opening a new perspective for in vitro molecular photosynthesis.

Synthesis and Photophysics Characterization of Boronic Styril and Distyryl BODIPYs for Water-Based Dye-Sensitized Solar Cells.

In this study, two boronic acid BODIPYs are obtained through a microwave-assisted Knoevenagel reaction. The aim is to use them for the first time as dyes in a photosensitized solar cell (DSSC) to mimic chlorophyll photosynthesis, harvesting solar light and converting it into electricity. The microwave-assisted Knoevenagel reaction is a straightforward approach to extending the molecular conjugation of the dye and is applied for the first time to synthesize BODIPY's boronic acid derivatives. These derivatives have proved to be very useful for covalent deposition on titania. This work studies the photo-physical and electrochemical properties. Moreover, the photovoltaic performances of these two new dyes as sensitizers for DSSC are discussed. Experimental data show that both dyes exhibit photosensitizing activities in acetonitrile and water. In particular, in all the experiments, distyryl BODIPY was more efficient than styryl BODIPY. In this study, demonstrating the use of a natural component as a water-based electrolyte for boronic BODIPY sensitizers, we open new possibilities for the development of water-based solar cells.

Changing the role of 2,2'-bipyridine from secondary ligand to protagonist in [Ru(bpy)2(N-N)]2+ complexes: low-energy, red emission from a ruthenium(II)-to-2,2'-bipyridine 3MLCT state.

Two new bidentate ligands (1 and 2) with bicyclic guanidine moieties were synthesized and attached to a Ru(II)(bpy)(2) core (bpy = 2,2'-bipyridine) to afford complexes 3 and 4, which were characterized by spectroscopic and electrochemical methods. Complex 4 was further characterized by X-ray crystallography. In cyclic voltammetric studies, both complexes show a Ru(II/III) couple, which is 500 mV less positive than the Ru(II/III) couple of Ru(bpy)(3)(2+). The (1)MLCT and (3)MLCT states of 3 (560 nm/745 nm) and 4 (550 nm/740 nm) are significantly red-shifted with respect to Ru(bpy)(3)(2+) (440 nm/620 nm). Compounds 3 and 4 exhibit emission from a Ru(II)-to-bpy (3)MLCT state, which is rarely the emitting state at λ > 700 nm in [Ru(bpy)(2)(N-N)](2+) complexes.

Ru(ii) water oxidation catalysts with 2,3-bis(2-pyridyl)pyrazine and tris(pyrazolyl)methane ligands: assembly of photo-active and catalytically active subunits in a dinuclear structure.

Two mononuclear Ru(ii) complexes, i.e. [RuCl(κ3N-terpy)(κ2N-dpp)]PF6 ([1]PF6; terpy = 2,2':6',2''-terpyridine; dpp = 2,3-bis(2'-pyridyl-pyrazine) and [RuCl(κ3N-tpm)(κ2N-dpp)]Cl ([2]Cl; tpm = tris(1-pyrazolyl)methane), and one dinuclear complex, i.e. [Ru2Cl(κ3N-tpm)(µ-κ2N:κ2N-dpp)Ru(κ2N-bpy)2][PF6]3 ([3][PF6]3; bpy = 2,2'-bipyridine), have been synthesized and their water oxidation catalytic properties have been investigated. A combined DFT and experimental (35Cl NMR and conductivity measurements) study aimed to elucidate the nature of [1]+ and [2]+ in aqueous solution has also been performed, indicating that one water molecule is allowed to enter the first coordination sphere of [2]+ in the ground state, replacing one tpm nitrogen. Conversely, in the case of [1]+, water coordination, assumed to be needed for the water oxidation process, presumably occurs following the oxidation of the metal. For all complexes, a catalytic wave has been detected in acetonitrile/water 1 : 1 (v/v) solution in the range 1.4-1.7 V vs. SCE. In all cases, water oxidation (investigated at pH < 8) takes place initially via a proton-coupled two-electron, two-proton process with the formation of an Ru(iv)[double bond, length as m-dash]O moiety, followed by one electron oxidation and water nucleophilic attack. The TON and TOF values (within the range of 16-33 and 1.3-2.2 h-1, respectively) of the complexes are higher than those of the benchmark [Ru(LLL)(LL)(OH2)]2+-type species (LLL and LL are tridentate and bidentate polypyridine ligands, respectively), which is [Ru(terpy)(bpm)(OH2)]2+.

Synthesis, characterization, absorption spectra, and luminescence properties of multinuclear species made of Ru(II) and Ir(III) chromophores.

A series of new mixed-metal Ru(II)-Ir(III) trinuclear complexes have been prepared and characterized, together with their mononuclear parents and a series of closely related dinuclear and trinuclear homometallic Ru(II) and Ir(III) species, and their absorption spectra and luminescence properties (both at 77 K in rigid matrix and at room temperature in fluid solution) have been studied. The absorption spectra and luminescence properties of the Ru(II) species and subunits are dominated by metal-to-ligand charge-transfer (MLCT) transitions and excited states, whereas ligand centered (LC) transitions and excited states govern the spectroscopic and photophysical properties of most of the Ir(III) species here studied, with MLCT states playing an important role when cyclometalated Ir(III) subunits are present. Each metal-based subunit retains in the multinuclear arrays its own spectroscopic properties, but in the case of the mixed Ru-Ir species an efficient, additional decay channel is opened for the excited states involving the Ir-centered subunits, that is, photoinduced energy transfer to the lower-lying MLCT state(s) involving the Ru centers.

Publisher Correction: Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation.

In the version of this Article originally published, in the graphical abstract the y-axis units of the plot read 'mA cm-2', but should have read 'µA cm-2'. Additionally, an erroneous gap appeared in the red trace. These errors have now been corrected.

Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation.

The oxygen in Earth's atmosphere is there primarily because of water oxidation performed by photosynthetic organisms using solar light and one specialized protein complex, photosystem II (PSII). High-resolution imaging of the PSII 'core' complex shows the ideal co-localization of multi-chromophore light-harvesting antennas with the functional reaction centre. Man-made systems are still far from replicating the complexity of PSII, as the majority of PSII mimetics have been limited to photocatalytic dyads based on a 1:1 ratio of a light absorber, generally a Ru-polypyridine complex, with a water oxidation catalyst. Here we report the self-assembly of multi-perylene-bisimide chromophores (PBI) shaped to function by interaction with a polyoxometalate water-oxidation catalyst (Ru4POM). The resulting [PBI]5Ru4POM complex shows a robust amphiphilic structure and dynamic aggregation into large two-dimensional paracrystalline domains, a redshifted light-harvesting efficiency of >40% and favourable exciton accumulation, with a peak quantum efficiency using 'green' photons (λ > 500 nm). The modularity of the building blocks and the simplicity of the non-covalent chemistry offer opportunities for innovation in artificial photosynthesis.

A functionalized, ethynyl-decorated, tetracobalt(III) cubane molecular catalyst for photoinduced water oxidation.

A new tetracobalt(III) cubane 1, carrying functionalized peripheral ethynyl groups, was prepared. Cubane 1 catalyses photoinduced water oxidation, indicating that the ethynyl groups do not negatively affect the catalytic properties of the Co cubane assembly. In contrast, the quantum yield for water oxidation (0.36) is significantly increased with respect to the prototype, simplest species.

Light-driven water oxidation with a molecular tetra-cobalt(III) cubane cluster.

Photoinduced water oxidation to molecular oxygen takes place in systems made of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as the photosensitizer, [Co4O4(O2CMe)4(py)4] (py = pyridine) as the molecular catalyst and Na2S2O8 as the sacrificial electron acceptor. The photochemical quantum yield of the process reaches the outstanding value of 30% and depends on pH and catalyst concentration. Transient absorption spectroscopy experiments aimed to clarify the first events of the photocatalytic process are also reported.

Photo-induced water oxidation with tetra-nuclear ruthenium sensitizer and catalyst: a unique 4 x 4 ruthenium interplay triggering high efficiency with low-energy visible light.

The combined use of a tetranuclear Ru(II) dendrimeric photosensitizer (1) and of a tretraruthenium substituted polyoxotungstate (2) as the catalyst enables photo-induced water oxidation at 550 nm producing O(2) with an outstanding quantum yield of 0.30.

Photoinduced water oxidation sensitized by a tetranuclear Ru(II) dendrimer.

A multimetallic ruthenium(II) dendrimer is used for the first time to photosensitize dioxygen production from water by IrO2 nanoparticles; the system is more efficient than an analogous system based on the more commonly used [Ru(bpy)3]2+-type photosensitizers, in particular for the ability of the dendrimer to take advantage of the red portion of the solar spectrum.