Ruthenium Photosensitizers Research Articles

Proton-Coupled Electron Transfer in a Series of Ruthenium-Linked Tyrosines with Internal Bases: Evaluation of a Tunneling Model for Experimental Temperature-Dependent Kinetics

Photoinitiated proton-coupled electron transfer (PCET) kinetics has been investigated in a series of four modified tyrosines linked to a ruthenium photosensitizer in acetonitrile, with each tyrosine bearing an internal hydrogen bond to a covalently linked pyridine or benzimidazole base. After correcting for differences in driving force, it is found that the intrinsic PCET rate constant still varies by 2 orders of magnitude. The differences in rates, as well as the magnitude of the kinetic isotope effect (KIE = kH/kD), both generally correlate with DFT calculated proton donor-acceptor distances. An Arrhenius analysis of temperature dependent data shows that the difference in reactivity arises primarily from differences in activation energies. We use this kinetic data to evaluate a commonly employed theoretical model for proton tunneling which includes a harmonic distribution of proton donor-acceptor distances due to vibrational motions of the molecule. Applying this model to the experimental data yields the conclusion that donor-acceptor compression is more facile in the compounds with shorter PT distance; however, this is contrary to independent calculations for the same compounds. This discrepancy is likely because the assumption in the model of Morse-shaped proton potential energy surfaces is inappropriate for (strongly) hydrogen-bonded systems. These results question the general applicability of this model. The results also suggest that a correlation of rate vs proton tunneling distance for the series of compounds is complicated by a concomitant variation of other relevant parameters.

Kinetics of Photocatalytic Water Oxidation at Liposomes: Membrane Anchoring Stabilizes the Photosensitizer

A tris(bipyridine)ruthenium(II) photosensitizer (PS) was anchored to the lipid bilayer of liposomes together with Ru-, Co-, or Ir-based water-oxidation catalysts in order to study the effect of liposomes on photocatalytic water oxidation in the presence of Na2S2O8. The Ru-based and Co-based systems both showed O2 production upon light irradiation, whereas the Ir-based system did not. Membrane anchoring caused a large shift in the quantum yield of oxidative quenching of the photosensitizer excited state by peroxodisulfate, which decreased from 180% in homogeneous solution to 7.3% at the surface of liposomes. For the Ru-based system the electron-transfer rate between the photosensitizer PS+ and the water-oxidation catalyst was increased relative to oxidative quenching. Consequently, the rate-limiting step of the photocatalytic water oxidation reaction at liposomes was oxidative quenching, whereas previous work showed that in homogeneous solution it is the reduction of the oxidized photosensitizer PS+ by the...

The role of ruthenium photosensitizers in the degradation of phenazopyridine with TiO2 electrospun fibers

Abstract There has been increased interest in titanium dioxide (TiO2) fibers as a photocatalyst for the degradation of persistent organic and biopharmaceutical toxins in the environment. The photocatalytic efficiency of TiO2 fibers is typically limited to UV irradiation due to its semiconductor bandgap. TiO2 fibers have been prepared and adsorbed the visible photosensitizer cis-dichlorobis(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium (II) (Ru(dcbpyH2)2Cl2) on the surface in order to absorb from 300 to 800 nm in the visible spectrum and drive photocatalysis. This dye fiber material has been applied to the degradation of 2,6-pyridinediamine,3-(phenylazo) monohydrochloride or phenazopyridine (PAP) as a model biopharmaceutical waste. The Ru(dcbpyH2)2Cl2–sensitized fibers degrade PAP under UV and visible irradiation following a first order reaction rate. The rate constants for PAP degradation in visible light when using bare TiO2 fibers and dye-sensitized TiO2 fibers were 0.014 ± 0.001 min−1 and 0.012 ± 0.001 min−1 respectively, showing little change. Under UV irradiation, the rate decreased from 0.032 ± 0.003 min−1 and 0.012 ± 0.002 min−1. These results suggest that the major pathway for the degradation of PAP may occur through the valence band.

A novel ruthenium sensitizer with –OMe substituted phenyl-terpyridine ligand for dye sensitized solar cells

A new ruthenium photosensitizer [Ru(triOMePhtpy)(bpydca)(NCS)]PF6 (1) (triOMePhtpy=4′-(3,4,5-triOMe)Phenyl-2,2′:6′,2″-terpyridine; bpydca=4,4′-dicarboxy-2,2′-bipyridine) has been synthesized and characterized by IR, UV–Vis, NMR and Mass spectroscopy. The molecule shows a moderately intense MLCT band at 511nm. Along with the intraligand transitions in the UV region, the molecule shows several high energy MLCT bands which has been nicely manifested by the TDDFT calculation. DFT optimized structure shows a distorted octahedral geometry around central ruthenium atom, where the phenyl ring attached to the 4′ position of terpyridine moiety makes a torsion angle of 34.09° with the central pyridine ring. pKa of the –COOH functionalities was determined spectrophotometrically by monitoring the variation of MLCT band position with pH and found to be ∼3.98. Photovoltaic measurements of the present dye in a typical DSSC configuration was found to exhibit Jsc=5.10mAcm−2, FF=66.02%, Voc=0.68V and η=4.62% at an optimized intensity of 50mWcm−2.

Gold catalysis with less fuss

In an advance that simplifies gold redox catalysis, a research team led by A. Stephen K. Hashmi of Heidelberg University, in Germany, has discovered a means for carrying out visible-light-mediated reactions without the need for an auxiliary oxidant or a ruthenium or iridium photosensitizer. The additional reagents are typically needed to promote oxidation of gold(I) to gold(III) as part of the catalytic cycle. The Hashmi group determined that a phosphine gold(I) chloride catalyst can function as its own photosensitizer when irradiated with visible light. When paired with an aryldiazonium salt, the gold(I) species is oxidized to a gold(III) species that can complete the difunctionalization of alkynes to make α-aryl ketones (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201511487). What’s more, by using a P,N-bidentate ligand, the researchers were able to isolate and obtain the X-ray crystal structure of the gold(III) species (shown above). Chemists have debated whether gold(III) intermediates are involved

Visible-light sensitized photocatalytic hydrogen generation using a dual emissive heterodinuclear cyclometalated iridium(III)/ruthenium(II) complex

The synthesis, spectroscopic and electrochemical properties of the heterodinuclear iridium(III)/ruthenium(II) photosensitizer Ir-BBI-Ru, i.e. [(ppy)2Ir-(tmBBI)-Ru(tbbpy)2][PF6] (ppy = 2-phenylpyridine; tmBBI = 5,5′,6,6′-tetramethyl-2,2′-bibenzimidazole; tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine) are presented. Its absorbance lies well within the visible light spectrum, showing absorption features typical for dinuclear ruthenium(II) bibenzimidazole complexes. The complex exhibits characteristic emission properties of both metal centers, indicative of an incomplete energy transfer. The application of Ir-BBI-Ru as visible-light absorbing photosensitizer in the field of photocatalytic water reduction is demonstrated by considerable hydrogen generation with a Co(II) catalyst and triethanolamine as sacrificial donor.

New strategy to incorporate nano-particle sized water oxidation catalyst into dye-sensitized photoelectrochemical cell for water splitting

In order to develop a new strategy to deposit nano-particle sized water oxidation catalyst based on earth abundant element to the photoanode in a photoelectrochemical cell for water splitting, Co3O4 as water oxidation catalyst was prepared and subsequently modified by 3-aminopropyltriethoxysilane. The amino functionalized Co3O4 catalyst was carefully characterized and then integrated to the ruthenium dye sensitized photoelectrode through fast Schiff base reaction. Cyclic voltammetry experiments in the dark confirmed that the modified Co3O4 catalyst was still active toward water oxidation, which could be initiated by oxidation of the ruthenium photosensitizer. Under visible light irradiation, incorporation of the modified Co3O4 catalyst resulted in dramatic enhancement of the transient photocurrent density for the photoanode, which was 8 times higher than that of without Co3O4 catalyst.

Detecting and Preventing the Formation of Photosensitizer‐Catalyst Colloids in Homogeneous Light‐Driven Water Oxidation

Combining cationic ruthenium photosensitizers (RuPS) with anionic polyoxometalate water oxidation catalysts (POM-WOCs) is the standard approach for light-driven POM-based water oxidation. Here, we show that colloid formation by electrostatic aggregation of a molecular photosensitizer {e.g., [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine)} and a POM-WOC {e.g., [Co4(H2O)2(α-PW9O34)2]10–} significantly affects catalytically relevant system parameters. A facile, quantitative procedure for colloid detection using syringe filtration and UV/Vis spectroscopy is presented, and we illustrate that photosensitizer-POM colloid formation is a general phenomenon under typical WOC conditions and is observed for a range of photosensitizers and POMs. It is further demonstrated that for some systems an increase in the ionic strength of the solution prevents colloid formation. Significant changes in the electronic interactions between RuPS and POM-WOC under colloidal and homogeneous conditions are reported, thus highlighting the need for fast and reliable colloid identification. In addition, the study raises awareness about colloid formation in homogeneous solar-energy conversion schemes driven by two or more ionic species.

Ruthenium(II) Photosensitizers with Electron‐Rich Diarylamino‐Functionalized 2,2′‐Bipyridines and Their Application in Dye‐Sensitized Solar Cells

AbstractNew ruthenium(II) photosensitizers [Ru(dcbpy)(L)(NCS)2] (dcbpy = 4,4′‐dicarboxylic acid‐2,2′‐bipyridine; L = 4,4′‐bis{di[4‐(N,N′‐dimethylamino)phenyl]amino}‐2,2′‐bipyridine (1), 4,4′‐bis[di(4‐methoxyphenyl)amino]‐2,2′‐bipyridine (2), and 4,4′‐bis[di(4‐tolyl)amino]‐2,2′‐bipyridine (3)) were prepared and characterized and their application in dye‐sensitized solar cells is presented. The optical absorption of these photosensitizers gives a peak at around 540 nm, which is very similar to that of the standard N719. The maximum incident photon‐to‐current conversion efficiency (IPCE) of 80.6 % was obtained for 3, which corresponded to a power conversion efficiency (η) of 5.68 % under standard air mass (AM) 1.5 sunlight (versus N719 at 6.76 %). Molecular cosensitization of 3 with an organic dye, QS‐DPP‐I, yielded higher η values up to 6 % relative to the cells based on individual photosensitizers, and the corresponding IPCE can reach 93.6 % at 549 nm. A preliminary stability test of the devices was also conducted.

Optimizing the photovoltaic performance of thiocyanate-free ruthenium photosensitizers by structural modification of C^N cyclometalating ligand in dye-sensitized solar cells

Five new thiocyanate-free ruthenium(II) complexes with different electron-donating functionalized cyclometalating ligands C^N were synthesized, characterized and applied as photosensitizers in dye-sensitized solar cells (DSSCs). Their photophysical, electrochemical, thermal and photovoltaic properties have been investigated and density functional theory (DFT) calculations have been carried out on these dyes. These dyes exhibit good thermal stability with the onset decomposition temperature at 5% weight-loss (Td) of around 330°C. The DSSC device using the Ru(II) dye with the 9-tolylcarbazole chromophore exhibited the highest power conversion efficiency (η) up to 3.39%, with a short-circuit photocurrent density (Jsc) of 8.06mAcm−2, an open-circuit photovoltage (Voc) of 0.62V and a high fill factor (ff) of 0.68 under illumination of an AM 1.5 solar cell simulator.

Examining Photoinduced Energy Transfer in Pseudomonas aeruginosa Azurin

Pseudomonas aeruginosa azurin has been an important model system for investigating fundamental electron transfer (EleT) in proteins. Early pioneering studies used ruthenium photosensitizers to induce EleT in azurin and this experimental data continues to be used to develop theories for EleT mediated through a protein matrix. In this study we show that putative EleT rates in the P. aeruginosa azurin model system, measured via photoinduced methods, can also be explained by an alternate energy transfer (EngT) mechanism. Investigation of EngT in azurin, conducted in this study, isolates and resolves confounding phenomena--i.e., zinc contamination and excited state emission--that can lead to erroneous kinetic assignments. Here we employ two azurin photosensitizer systems, the previously reported Ru(2,2'-bipyridine)2(imidazole) and an unreported phototrigger, Ru(bpy)2(phen-IA), Ru(2,2'-bipyridine)2(5-iodoacetamido-1,10-phenanthroline), that has a longer lifetime, to better resolve convoluted kinetic observations and allow us to draw clear distinctions between photoinduced EngT and EleT. Extensive metal analysis, in addition to electrochemical and photochemical (photoinduced transfer) measurements, suggests Zn-metalated azurin contamination can result in a biexponential reaction, which can be mistaken for EleT. Namely, upon photoinduction, the observed slow phase is exclusively the contribution from Zn-metalated azurin, not EleT, whereas the fast phase is the result of EngT between the photosensitizer and the Cu-site, rather than simple excited-state decay of the phototrigger.

Assembling model tris(bipyridine)ruthenium(ii) photosensitizers into ordered monolayers in the presence of the polyoxometallate anion [Co4(H2O)2(α-PW9O34)2]10−

Model ruthenium(ii) photosensitizers with long alkyl chains are assembled into ordered monolayers on water with or without an anionic polyoxometallate. LB films on mica substrates are visualized using AFM.

Conformational Dynamics of a Fast Folding Cytochrome Captured by Time-Resolved Spectroscopy

We probe intrachain contact dynamics in unfolded cytochrome cb562 by monitoring heme quenching of excited ruthenium photosensitizers covalently bound to residues along the polypeptide. Tertiary contact formation kinetics provide insight into the upper

Intrachain Contact Dynamics in Unfolded Cytochrome cb562

We have investigated intrachain contact dynamics in unfolded cytochrome cb562 by monitoring heme quenching of excited ruthenium photosensitizers covalently bound to residues along the polypeptide. Intrachain diffusion for chemically denatured proteins proceeds on the microsecond time scale with an upper limit of 0.1 μs. The rate constants exhibit a power-law dependence on the number of peptide bonds between the heme and Ru complex. The power-law exponent of -1.5 is consistent with theoretical models for freely jointed Gaussian chains, but its magnitude is smaller than that reported for several synthetic polypeptides. Contact formation within a stable loop was examined in a His63-heme ligated form of the protein under denaturing conditions. Loop formation accelerated contact kinetics for the Ru66 labeling site, owing to reduction in the length of the peptide separating redox sites. For other labeling sites within the stable loop, quenching rates were modestly reduced compared to the open chain polymer.

Artificial Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation

Human society faces a fundamental challenge as energy consumption is projected to increase due to population and economic growth as fossil fuel resources decrease. Therefore the transition to alternative and sustainable energy sources is of the utmost importance. The conversion of solar energy into chemical energy, by splitting H2O to generate molecular O2 and H2, could contribute to solving the global energy problem. Developing such a system will require the combination of several complicated processes, such as light-harvesting, charge separation, electron transfer, H2O oxidation, and reduction of the generated protons. The primary processes of charge separation and catalysis, which occur in the natural photosynthetic machinery, provide us with an excellent blueprint for the design of such systems. This Account describes our efforts to construct supramolecular assemblies capable of carrying out photoinduced electron transfer and to develop artificial water oxidation catalysts (WOCs). Early work in our group focused on linking a ruthenium chromophore to a manganese-based oxidation catalyst. When we incorporated a tyrosine unit into these supramolecular assemblies, we could observe fast intramolecular electron transfer from the manganese centers, via the tyrosine moiety, to the photooxidized ruthenium center, which clearly resembles the processes occurring in the natural system. Although we demonstrated multi-electron transfer in our artificial systems, the bottleneck proved to be the stability of the WOCs. Researchers have developed a number of WOCs, but the majority can only catalyze H2O oxidation in the presence of strong oxidants such as Ce(IV), which is difficult to generate photochemically. By contrast, illumination of ruthenium(II) photosensitizers in the presence of a sacrificial acceptor generates [Ru(bpy)3](3+)-type oxidants. Their oxidation potentials are significantly lower than that of Ce(IV), but our group recently showed that incorporating negatively charged groups into the ligand backbone could decrease the oxidation potential of the catalysts and, at the same time, decrease the potential for H2O oxidation. This permitted us to develop both ruthenium- and manganese-based WOCs that can operate under neutral conditions, driven by the mild oxidant [Ru(bpy)3](3+). Many hurdles to the development of viable systems for the production of solar fuels remain. However, the combination of important features from the natural photosynthetic machinery and novel artificial components adds insights into the complicated catalytic processes that are involved in splitting H2O.

Ruthenium(II) Photosensitizers of Tridentate Click‐Derived Cyclometalating Ligands: A Joint Experimental and Computational Study

A systematic series of heteroleptic bis(tridentate)ruthenium(II) complexes of click-derived 1,3-bis(1,2,3-triazol-4-yl)benzene N^C^N-coordinating ligands was synthesized, analyzed by single crystal X-ray diffraction, investigated photophysically and electrochemically, and studied by computational methods. The presented comprehensive characterization allows a more detailed understanding of the radiationless deactivation mechanisms. Furthermore, we provide a fully optimized synthesis and systematic variations towards redox-matched, broadly and intensely absorbing, cyclometalated ruthenium(II) complexes. Most of them show a weak room-temperature emission and a prolonged excited-state lifetime. They display a broad absorption up to 700 nm and high molar extinction coefficients up to 20 000 M(-1)cm(-1) of the metal-to-ligand charge transfer bands, resulting in a black color. Thus, the complexes reveal great potential for dye-sensitized solar-cell applications.

Ru-based donor–acceptor photosensitizer that retards charge recombination in a p-type dye-sensitized solar cell

We report on the synthesis and characterization of a donor-acceptor ruthenium polypyridyl complex as a photosensitizer for p-type dye-sensitized solar cells (DSSCs). The electrochemical, photophysical, and photovoltaic performance of two ruthenium-based photosensitizers were tested in NiO-based DSSCs; bis-(2,2'-bipyridine-4,4'-dicarboxylic acid)(2)N-(1,10-phenanthroline)-4-nitronaphthalene-1,8-dicarboximide ruthenium(II), ([Ru(dcb)(2)(NMI-phen)](PF(6))(2)) and tris-(2,2'-bipyridine-4,4'-dicarboxylic acid)(3) ruthenium(ii), [(Ru(dcb)(3))Cl(2)]. The presence of an electron-accepting group, 4-nitronaphthalene-1,8-dicarboximide (NMI), attached to the phenanthroline of [Ru(dcb)(2)(NMI-phen)](2+) resulted in long-lived charge separation between reduced [Ru(dcb)(2)(NMI-phen)](2+) and NiO valence band holes; 10-50 μs. In the reduced state for [Ru(dcb)(2)(NMI-phen)](2+), the electron localized on the distal NMI group. In tests with I(3)(-)/I(-) and Co(4,4'-di-tert-butyl-bipyridine)(3)(2+/3+) electrolytes, [Ru(dcb)(2)(NMI-phen)](2+) outperformed [Ru(dcb)(3)](2+) in solar cell efficiency in devices. A record APCE (25%) was achieved for a ruthenium photosensitizer in a p-type DSSC. Insights on photosensitizer regeneration kinetics are included.

Theoretical Study on the Electronic Excitations of a Porphyrin-Polypyridyl Ruthenium(II) Photosensitizer

In this work, we investigated the UV-vis spectra of the [Ru(bipy)(2)(MPyTPP)Cl](+) (MPyTPP = 5-pyridyl-15,20,25-triphenylporphyrin) complex and its related species [Ru(bipy)(2)(py)Cl](+) and MPyTPP, by using time-dependent density functional theory and a set of functionals (B3LYP, M05, MPWB1K, and PBE0) in chloroform with the basis set 6-31++G(d,p) for nonmetal atoms and the pseudopotential LANL2DZ for Ru. Practically no geometrical changes are observed in the Ru environment when py ligand is replaced by MPyTPP. This replacement favors the electronic redistribution from bipy ligands to Ru, and from the metal to MPyTPP ligand, as indicated by NBO analysis. We found that M05 functional predicts very well the UV-vis spectra, as it shows a low deviation with respect to the experimental data, with a maximum error of 0.19 eV (11 nm). M05 theoretical electronic spectrum of [Ru(bipy)(2)(MPyTPP)Cl](+) complex indicates that the presence of the Ru complex does not alter Q porphyrin bands, while charge transfer bands from Ru to bipy and porphyrin ligands mixes up in the region close to the porphyrin Soret band. Theoretical analysis allows the decomposition of this broad experimental band into specific ones identifying the Soret band and new metal to ligand charge transfers toward porphyrin at 425 and 478 nm, which were not possible in none of the moieties MPyTPP and [Ru(bipy)(2)(Py)Cl](+) complex. In the UV region, the most intense intraligand band of bipy ligands becomes slightly blue-shifted both in the experimental and in the theoretical spectrum of [Ru(bipy)(2)(MPyTPP)Cl](+) complex compared to that in [Ru(bipy)(2)(py)Cl](+) complex. Some of the bands of [Ru(bipy)(2)(MPyTPP)Cl](+) showed in this theoretical study may have practical applications. That is the case for the band at 478 nm, with potential use in PDT, and those more energetic at 348 and 329 nm, which could help in the cleavage mechanism of DNA performed by this ruthenium complex.

Photocatalytic hydrogen evolution by a diiron hydrogenase model based on a peptide fragment of cytochrome c556 with an attached diiron carbonyl cluster and an attached ruthenium photosensitizer

It is of particular interest to mimic the process of intramolecular electron relay at the active site of [FeFe]-hydrogenase in order to understand the mechanism of the catalytic activity of H2 evolution. We have recently focused on using the native CXXCH peptide sequence of the C-terminal segment of cytochrome c556 as a platform which holds a diiron carbonyl cluster via two cysteines and have attached a ruthenium photosensitizer via a histidine. The modified peptide with the two metal moieties is found to act as the photocatalyst for H2 evolution with a turnover number of ~9 over 2h at pH 8.5 in the presence of ascorbate as a sacrificial reagent.

Structures and spectroscopic properties of ruthenium phenanthroline solar-cell sensitizers: A computational study

The density functional theory (DFT) was applied to fully optimize the ground- and excited-state structures of six ruthenium photosensitizers with 0–4 carboxyls coordinating phenanthroline ligands. The electronic spectra in solution were calculated by the time-dependent DFT (TD-DFT) method. The results showed that all the carboxylic sensitizers are good candidates for dye-sensitized solar cell because of their intense and wide absorption bands in the visible region. Varying carboxyl groups of the sensitizers slightly changes the absorption spectra, but the number and substituted position of these anchors strongly improve the electron-accepted ability of their situated phenanthrolines.