Varying the Electronic Structure of Surface-Bound Ruthenium(II) Polypyridyl Complexes

Combined Covalent and Supramolecular Polymerization to Reinforce Perylenebisimide Photosynthetic "Quantasomes"

PSII-inspired quantasomes have emerged as promising artificial photosystems evolving oxygen from water due to their integrated multi-chromophore asset, hierarchical architecture, and efficient light-harvesting capabilities. In this study, we adopt a combined covalent and supramolecular strategy by implementing a poly-styrene backbone that reinforces proximity and pairing between adjacent perylenebisimide (PBI) quantasome units. The covalent fixation of the quantasome network results in a significant enhancement of the photoelectrocatalytic performance on engineered IO-ITO photoanodes, with up to 290 % photocurrent increase (J up to 100 μA cm-2, λ >450 nm, applied bias <1.23 V vs RHE, F.E.O2 >80 %) compared to the non-polymerized analog. Moreover, the direct PBI-quantasome polymerization on the photoanode surface was performed by light irradiation of the radical initiator 2,2'-Azobis(2-methylpropionamidine), improving the photoelectrode robustness under high solar irradiance (>8 suns) and limiting the photocurrent loss (<20 %) at 1.52 V vs RHE compared to the non-polymerized system.

Predictable electronic tuning of FeII and RuII complexes via choice of azine: correlation of ligand pKa with Epa(MIII/II) of complex

Five new mononuclear ruthenium(II) tris-ligated complexes have been synthesised, varying through the choice of azine in the family of 3-azinyl-4-(4-methylphenyl)-5-phenyl-4H-1,2,4-triazole ligands (Lazine): [Ru(Lpyridine)](PF6)2 (1), [Ru(Lpyridazine)](PF6)2 (2), [Ru(L4-pyrimidine)](PF6)2 (3), [Ru(Lpyrazine)](PF6)2 (4), [Ru(L2-pyrimidine)](PF6)2 (5). Three of them, 1·2MeCN·Et2O, 3·2MeCN·Et2O and 4·2MeCN, have been structurally characterised, confirming the presence of the meridional isomer, as was previously reported for the FeII analogues. Cyclic voltammetry studies, in dry CH3CN vs. Ag/0.01 M AgNO3, show that all five RuII complexes undergo a reversible RuIII/RuII process, with the midpoint potential (Em) increasing from 0.87 to 1.18 V as the azine is changed: pyridine < pyridazine < 2-pyrimidine < 4-pyrimidine < pyrazine. A strong inverse linear correlation (R2 = 0.98) is found between the RuIII/RuII redox potential and the calculated HOMO orbital energies, which is consistent with the expectation that it is easier to oxidise (lower Em) a metal ion with a higher HOMO orbital energy. The same trend was reported earlier for the family of analogous FeII complexes, albeit at lower values of Em in all cases. In addition, the ionisation potentials of the RuII complexes, as well as those of the other group 8 analogues (FeII and OsII), showed a linear relationship with Epa. As the MIII/II redox potentials of a family of complexes has been previously reported to correlate with ligand pKa values, a computational protocol to calculate, in silico, the pKa of the Lazine family of ligands was developed. A strong linear relationship was found between the readily calculated pKa of the Lazine ligand and the Epa of the MII complex, for all three families of complexes (R2 = 0.98).

Ruthenium(II) complexes with phosphonate-substituted phenanthroline ligands as reusable photoredox catalysts

Ru(II) complexes with polypyridyl ligands (2,2'-bipyridine = bpy, 1,10-phenanthroline = phen) play a central role in the development of photocatalytic organic reactions. In this work, we synthesized four mixed-ligand [Ru(phen)(bpy)2]2+-type complexes (Ru-Pcat-A) bearing two phosphonate substituents P(O)(OH)(OR) (R = H, Et) attached to the phen core at positions 3,8 (Ru-3,8PH and Ru-3,8PHEt) and 4,7 (Ru-4,7PH and Ru-4,7PHEt) of the heterocycle in high yields (87-99%) and characterized them using spectral methods. Single crystal X-ray diffraction was employed to determine the coordination mode of the ditopic phen ligand in Ru-4,7PH. This complex exists as the neutral species and forms a 1D hydrogen-bonded framework in the crystals. The light absorption characteristics were found to be similar for all complexes prepared in this work. However, the emission maxima in aqueous solutions were significantly affected by the substitution of the heterocycle, ranging from 629 nm for Ru-4,7PH to 661 nm for Ru-3,8PHEt. The emission quantum yields in Ar-saturated deionized water showed a strong dependence on the substitution pattern of the phen ligand, with maximal values reaching approximately 0.11 for Ru-4,7PHEt and Ru-4,7PH, which is twice as high as that of the classical [Ru(bpy)3]2+ complex (Ru-bpy). The photocatalytic performance of Ru-Pcat-A was investigated using visible light photoredox catalytic transformations of tertiary amines. With Ru-Pcat-A, we achieved the phosphonylation of N-aryl-1,2,3,4-tetrahydroisoquinolines (THIQs) and cyanation of THIQs and N,N-dimethylaniline in methanol, while a mixture of nitromethane/methanol (1 : 1 v/v) proved to be the optimal solvent for conducting the nitromethylation of THIQs. In the majority of the studied reactions, Ru-4,7PHEt exhibited greater efficiency compared to Ru-bpy, and it could be easily separated from the products using water extraction and reused in the next catalytic cycle. We successfully performed seven consecutive nitromethylation and phosphonylation of N-phenyl-1,2,3,4-tetrahydroisoquinoline using the recycled homogeneous photoredox catalyst.

Surface-modified, dye-sensitized niobate nanosheets enabling an efficient solar-driven Z-scheme for overall water splitting

While dye-sensitized metal oxides are good candidates as H₂ evolution photocatalysts for solar-driven Z-scheme water splitting, their solar-to-hydrogen (STH) energy conversion efficiencies remain low because of uncontrolled charge recombination reactions. Here, we show that modification of Ru dye-sensitized, Pt-intercalated HCa₂Nb₃O₁₀ nanosheets (Ru/Pt/HCa₂Nb₃O₁₀) with both amorphous Al₂O₃ and poly(styrenesulfonate) (PSS) improves the STH efficiency of Z-scheme overall water splitting by a factor of ~100, when the nanosheets are used in combination with a WO₃-based O₂ evolution photocatalyst and an I₃^-/I^- redox mediator, relative to an analogous system that uses unmodified Ru/Pt/HCa₂Nb₃O₁₀. By using the optimized photocatalyst, PSS/Ru/Al₂O₃/Pt/HCa₂Nb₃O₁₀, a maximum STH of 0.12% and an apparent quantum yield of 4.1% at 420 nm were obtained, by far the highest among dye-sensitized water splitting systems and comparable to conventional semiconductor-based suspended particulate photocatalyst systems.

Ruthenium(II) complexes with phosphonate-substituted 1,10-phenanthroline ligands: synthesis, characterization and use in organic photocatalysis

Ru(II) complexes with polypyridyl ligands play a central role in the development of photocatalytic organic reactions. This work is aimed at the structural modification of such complexes to increase their...

Fluorescence Imaging and Photodynamic Inactivation of Bacteria Based on Cationic Cyclometalated Iridium(III) Complexes with Aggregation-Induced Emission Properties

Antibacterial photodynamic therapy (PDT) is one of the emerging methods for curbing multidrug-resistant bacterial infections. Effective fluorescent photosensitizers with dual functions of bacteria imaging and PDT applications are highly desirable. In this study, three cationic and heteroleptic cyclometalated Ir(III) complexes with the formula of [Ir(CˆN)₂ (NˆN)][PF₆ ] are prepared and characterized. These Ir(III) complexes named Ir(ppy)₂ bP, Ir(1-pq)₂ bP, and Ir(2-pq)₂ bP are comprised of three CˆN ligands (i.e., 2-phenylpyridine (ppy), 1-phenylisoquinoline (1-pq), and 2-phenylquinoline (2-pq)) and one NˆN bidentate co-ligand (bP). The photophysical characterizations demonstrate that these Ir(III) complexes are red-emitting, aggregation-induced emission active luminogens. The substitution of phenylpyridine with phenylquinoline isomers in the molecules greatly enhances their UV and visible-light absorbance as well as the photoinduced reactive oxygen species (ROS) generation ability. All three Ir(III) complexes can stain both Gram-positive and Gram-negative bacteria efficiently. Interestingly, even though Ir(1-pq)₂ bP and Ir(2-pq)₂ bP are constitutional isomers with very similar structures and similar ROS generation ability in buffer, the former eradicates bacteria much more effectively than the other through white light-irradiated photodynamic inactivation. This work will provide valuable information on the rational design of Ir(III) complexes for fluorescence imaging and efficient photodynamic inactivation of bacteria.

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

Investigation into the properties of a ruthenium(polypyridyl)-NHC compound

The asymmetric imidazolium salt [Ru(dmbpy)2(bpip)](PF6)3 (bpip = 1-benzyl-3-isopropyl-1H-imidazo[4,5-f][1,10]phenanthrolinium) was prepared by a four-step synthetic route. The isotopomer labelled with 13C at the C2 position of bpip was prepared from 1,10-phenanthroline-5,6-diamine and triethyl orthoformate-(formyl-13C). Deprotonation at low temperatures in acetonitrile using KO t Bu and KHMDS were monitored by NMR to investigate the formation of the free N-heterocyclic carbene.

Heterogeneous water oxidation photocatalysis based on periodic mesoporous organosilica immobilizing a tris(2,2'-bipyridine)ruthenium sensitizer

A periodic mesoporous organosilica (PMO) containing 2,2′-bipyridine groups (BPy-PMO) has been shown to possess a unique pore wall structure in which the 2,2′-bipyridine groups are densely and regularly packed. The surface 2,2′-bipyridine groups can function as chelating ligands for the formation of metal complexes, thus generating molecularly-defined catalytic sites that are exposed on the surface of the material. We here report the construction of a heterogeneous water oxidation photocatalyst by immobilizing several types of tris(2,2′-bipyridine)ruthenium complexes on BPy-PMO where they function as photosensitizers in conjunction with iridium oxide as a catalyst. The Ru complexes produced on BPy-PMO in this work were composed of three bipyridine ligands, including the BPy in the PMO framework and two X₂bpy, denoted herein as Ru(X)-BPy-PMO where X is H (2,2′-bipyridine), Me (4,4′-dimethyl-2,2′-bipyridine), t-Bu(4,4′-di-tert-butyl-2,2′-bipyridine) or CO₂Me (4,4′-dimethoxycarbonyl-2,2′-bipyridine). Efficient photocatalytic water oxidation was achieved by tuning the photochemical properties of the Ru complexes on the BPy-PMO through the incorporation of electron-donating or electron-withdrawing functionalities. The reaction turnover number based on the amount of the Ru complex was improved to 20, which is higher than values previously obtained from PMO systems acting as water oxidation photocatalysts.

Ruthenium complex photosensitizer fixed on 2,2′-bipyridine bridged-periodic mesoporous organosilica with iridium oxide exhibits an efficient photocatalytic water oxidation.

An Insulating Al2O3 Overlayer Prevents Lateral Hole Hopping Across Dye-Sensitized TiO2 Surfaces

Three chromophores of the general form [Ru(bpy')₂(4,4'-(PO₃H₂)₂-2,2'-bipyridine)]²⁺, where bpy' is 4,4'-(C(CH₃)₃)₂-2,2'-bipyridine (Ru(dtb)₂P); 4,4'-(CH₃O)₂-2,2'-bipyridine (Ru(OMe)₂P), and 2,2'-bipyridine (RuP) were anchored to mesoporous thin films of TiO₂ nanocrystallites at saturation surface coverages to investigate lateral self-exchange Ru^III/II intermolecular hole hopping in 0.1 M LiClO₄/CH₃CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E_1/2 (Ru^III/II) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant k_R extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO₂|Ru(OMe)₂P (k_et = 1.4 × 10⁶ s^-1) > TiO₂|RuP (7.1 × 10⁵ s^-1) > TiO₂|Ru(dtb)₂P (6.5 × 10⁴ s^-1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO₂|RuP and TiO₂|Ru(dtb)₂P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO₂(e^-)|Ru^III → TiO₂|Ru^II charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al₂O₃ on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.

Stabilization of Ruthenium(II) Polypyridyl Chromophores on Mesoporous TiO2 Electrodes: Surface Reductive Electropolymerization and Silane Chemistry

Stabilization is a critical issue in the long term operation of dye-sensitized photoelectrosynthesis cells (DSPECs) for water splitting or CO₂ reduction. The cells require a stable binding of the robust molecular chromophores, catalysts, and chromophore/catalyst assemblies on metal oxide semiconductor electrodes under the corresponding (photoelectro)chemical conditions. Here, an efficient stabilization strategy is presented based on functionalization of FTO|nanoTiO₂ (mesoporous, nanostructured TiO₂ deposited on fluorine-doped tin oxide (FTO) glass) electrodes with a vinylsilane followed by surface reductive electropolymerization of a vinyl-derivatized Ru(II) polypyridyl chromophore. The surface electropolymerization was dominated by a grafting-through mechanism, and rapidly completed within minutes. Chromophore surface coverages were controlled up to three equivalent monolayers by the number of electropolymerization cycles. The silane immobilization and cross-linked polymer network produced highly (photo)stabilized chromophore-grafted FTO|nanoTiO₂ electrodes. The electrodes showed significant improvements over structures based on atomic layer deposition and polymer dip-coating stabilization methods in a wide pH range from pH ≈ 1 to pH ≈ 12.5 under both dark and light conditions. Under illumination, with hydroquinone added as a sacrificial electron transfer donor, a photoresponse for sustained electron transfer mediation occurred for at least ∼20 h in a pH ≈ 7.5 phosphate buffer (0.1 M NaH₂PO₄/Na₂HPO₄, with 0.5 M NaClO₄). The overall procedure provides an efficient way to fabricate highly stabilized molecular assemblies on electrode surfaces with potential applications for DSPECs in solar fuels.

Mechanistic Study of Ruthenium-Catalyzed C-H Hydroxylation Reveals an Unexpected Pathway for Catalyst Arrest

We have recently disclosed [(dtbpy)₂RuCl₂] as an effective precatalyst for chemoselective C-H hydroxylation of C(sp³)-H bonds and have noted a marked disparity in reaction performance between 4,4'-di- tert-butyl-2,2'-bipyridine (dtbpy)- and 2,2'-bipyridine (bpy)-derived complexes. A desire to understand the origin of this difference and to further advance this catalytic method has motivated the comprehensive mechanistic investigation described herein. Details of this reaction have been unveiled through evaluation of ligand structure-activity relationships, electrochemical and kinetic studies, and pressurized sample infusion high-resolution mass spectrometry (PSI-MS). Salient findings from this investigation include the identification of more than one active oxidant and three disparate mechanisms for catalyst decomposition/arrest. Catalyst efficiency, as measured by turnover number, has a strong inverse correlation with the rate and extent of ligand dissociation, which is dependent on the identity of bipyridyl 4,4'-substituent groups. Dissociated bipyridyl ligand is oxidized to mono- and bis- N-oxide species under the reaction conditions, the former of which is found to act as a potent catalyst poison, yielding a catalytically inactive tris-ligated [Ru(dtbpy)₂(dtbpy N-oxide)]²⁺ complex. Insights gained through this work highlight the power of PSI-MS for studies of complex reaction processes and are guiding ongoing efforts to develop high-performance, next-generation catalyst systems for C-H hydroxylation.

Optimization of Photocatalyst Excited- and Ground-State Reduction Potentials for Dye-Sensitized HBr Splitting

Dye-sensitized bromide oxidation was investigated using a series of four ruthenium polypyridyl photocatalysts anchored to SnO₂/TiO₂ core/shell mesoporous thin films through 2,2'-bipyridine-4,4'-diphosphonic acid anchoring groups. The ground- and excited-state reduction potentials were tuned over 500 mV by the introduction of electron withdrawing groups in the 4 and 4' positions of the ancillary bipyridine ligands. Upon light excitation of the surface-bound photocatalysts, excited-state electron injection yielded an oxidized photocatalyst that was regenerated through bromide oxidation. High injection quantum yields (Φ_inj) and regeneration quantum yields (Φ_reg) were essential to obtain efficient bromide oxidation yet required a photocatalyst that is both a potent photoreductant and a strong oxidant after excited-state injection. The four photocatalysts utilized in this manuscript ranged from unity Φ_inj (1.0) and minimal Φ_reg (0.037) to minimal Φ_inj (0.09) and unity Φ_reg (1.0). The photocatalyst that displayed the highest overall dye-sensitized photoelectrosynthesis cell performances exhibited near unity Φ_reg (0.99), while a significant Φ_inj was still preserved (0.59). Thus, these results highlighted the delicate interplay between the ground- and excited-state reduction potentials of photocatalysts for dye-sensitized hydrobromic acid splitting.

Fundamental Factors Impacting the Stability of Phosphonate-Derivatized Ruthenium Polypyridyl Sensitizers Adsorbed on Metal Oxide Surfaces

A series of 18 ruthenium(II) polypyridyl complexes were synthesized and evaluated under electrochemically oxidative conditions, which generates the Ru(III) oxidation state and mimics the harsh conditions experienced during the kinetically limited regime that can occur in dye-sensitized solar cells (DSSCs) and dye-sensitized photo-electrosynthesis cells, to further develop fundamental insights into the factors governing molecular sensitizer surface stability in aqueous 0.1 M HClO₄. Both desorption and oxidatively induced ligand substitution were observed on planar fluorine-doped tin oxide (FTO) electrodes, with a dependence on the E_1/2 Ru(III/II) redox potential dictating the comparative ratios of the processes. Complexes such as RuP4OMe ( E_1/2 = 0.91 vs Ag/AgCl) displayed virtually only desorption, while complexes such as RuPbpz ( E_1/2 > 1.62 V vs Ag/AgCl) displayed only chemical decomposition. Comparing isomers of 4,4'- and 5,5'-disubstituted-2,2'-bipyridine ancillary ligands, a dramatic increase in the rate of desorption of the Ru(III) complexes was observed for the 5,5'-ligands. Nanoscopic indium-doped tin oxide thin films (nanoITO) were also sensitized and analyzed with cyclic voltammetry, UV-vis absorption spectroscopy, and X-ray photoelectron spectroscopy, allowing for further distinction of desorption versus ligand-substitution processes. Desorption loss to bulk solution associated with the planar surface of FTO is essentially non-existent on nanoITO, where both desorption and ligand substitution are shut down with RuP4OMe. These results revealed that minimizing time spent in the oxidized form, incorporating electron-donating groups, maximizing hydrophobicity, and minimizing molecular bulk near the adsorbed ligand are critical to optimizing the performance of ruthenium(II) polypyridyl complexes in dye-sensitized devices.

pH-Dependence of Binding Constants and Desorption Rates of Phosphonate- and Hydroxamate-Anchored [Ru(bpy)3]2+ on TiO2 and WO3

The binding constants and rate constants for desorption of the modified molecular dye [Ru(bpy)₃]²⁺ anchored by either phosphonate or hydroxamate on the bipyridine ligand to anatase TiO₂ and WO₃ have been measured. In aqueous media at pH 1-10, repulsive electrostatic interactions between the negatively charged anchor and the negatively charged surface govern phosphonate desorption under neutral and basic conditions for TiO₂ anatase due to the high acidity of phosphonic acid (p K_a,4 = 5.1). In contrast, the lower acidity of hydroxamate (p K_a,1 = 6.5, p K_a,2 = 9.1) leads to little change in adsorption/desorption properties as a function of pH from 1 to 7. The binding constant for hydroxamate is 10³ in water, independent of pH in this range. These results are true for WO₃ as well, but are not reported at pH > 4 due to its Arrhenius acidity. Kinetics for desorption as a function of pH are reported, with a proposed mechanism for phosphonate desorption at high pH being the electrostatic repulsion of negative charges between the surface and the anionic anchor. Further, the hydroxamic acid anchor itself is likely the site of quasi-reversible redox activity in [Ru(bpy)₂(2,2'-bpy-4,4'-(C(O)N(OH))₂)]²⁺, which does not lead to any measurable deterioration of the complex within 2 h of dark cyclic voltammogram scans in aqueous media. These results posit phosphonate as the preferred anchoring group under acidic conditions and hydroxamate for neutral/basic conditions.

Fluorescence sensing platform based on ruthenium(II) complexes as high 3S (sensitivity, specificity, speed) and "on-off-on" sensors for the miR-185 detection

Inspired by the enormous importance attributed to the biological function of miRNA, we pour our attention into the design and synthesis of four ruthenium(II) complexes and evaluate their applications as miR-185 detection agents by spectroscopic measurements. It was found that all complexes can form sensing platform for the detection of the complementary target miR-185 through the introduction of carboxyfluorescein (FAM) labeled single stranded DNA (P-DNA), giving the detection limits of 0.42nM for Ru 1, 0.28nM for Ru 2, 0.32nM for Ru 3, 0.85nM for Ru 4, all with instantaneous detection time in 1min. The results of the binding constant, fluorescence anisotropy (FA) and polyacrylamide gel electrophoresis experiments (PAGE) revealed that the ruthenium(II) complexes prefer to bind P-DNA other than hybrid duplexes DNA@RNA upon recognition, resulting in the detection of miR-185. These results provide useful suggestions in the new type of metal-based miRNA detection agents.

Inner Layer Control of Performance in a Dye-Sensitized Photoelectrosynthesis Cell

Interfacial charge transfer and core-shell structures play important roles in dye-sensitized photoelectrosynthesis cells (DSPEC) for water splitting into H₂ and O₂. An important element in the design of the photoanode in these devices is a core/shell structure which controls local electron transfer dynamics. Here, we introduce a new element, an internal layer of Al₂O₃ lying between the Sb:SnO₂/TiO₂ layers in a core/shell electrode which can improve photocurrents by up to 300%. In these structures, the results of photocurrent, transient absorption, and linear scan voltammetry measurements point to an important role for the Al₂O₃ layer in controlling internal electron transfer within the core/shell structure.

Correlating the para-Substituent Effects on Ru(II)-Polypyridine Photophysical Properties and on the Corresponding Hybrid P450 BM3 Enzymes Photocatalytic Activity

Ru(II)-diimine complexes covalently attached near the heme active site of P450 BM3 enzymes have been used to rapidly inject electrons and drive selective C-H functionalization upon visible light irradiation. Herein, we have generated a series of hybrid P450 BM3 enzymes containing a photosensitizer of general formula [Ru(4,4'-X₂bpy)₂(PhenA)]²⁺ where X = Cl, H, tBu, Me OPhe, OMe, or NMe₂, bpy = 2,2'-bipyridine, and PhenA = 5-acetamido-1,10-phenanthroline. We then probed the effect of electron-withdrawing and -donating groups at the para position of the 4,4'-X₂bpy ligands on the corresponding hybrid enzymes photocatalytic activity. A 3-fold improvement in initial reaction rate was noted when varying the substituent from Cl to tBu, however, the reaction rates decrease thereafter with the more electron donating groups. In order to rationalize those effects, we investigated the variation of the substituent on the photophysical properties of the corresponding [Ru(4,4'-X₂bpy)₂(bpy)]²⁺ model complexes. Several linear correlations were established between the E(III/II) potential, the MLCT emission, and absorption energies as well as the logarithm of the luminescence quenching rate vs the summative Brown-Okamoto parameter (Σσ_p+). Moreover, a downward curved Hammett plot is observed with the hybrid enzyme initial reaction rate revealing mechanistic details about the overall light-driven enzymatic process.

All-in-One Derivatized Tandem p+n-Silicon-SnO2/TiO2 Water Splitting Photoelectrochemical Cell

Mesoporous metal oxide film electrodes consisting of derivatized 5.5 μm thick SnO₂ films with an outer 4.3 nm shell of TiO₂ added by atomic layer deposition (ALD) have been investigated to explore unbiased water splitting on p, n, and p⁺n type silicon substrates. Modified electrodes were derivatized by addition of the water oxidation catalyst, [Ru(bda)(4-O(CH₂)₃PO₃H₂)-pyr)₂], 1, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate), and chromophore, [Ru(4,4'-PO₃H₂-bpy) (bpy)₂]²⁺, RuP²⁺, (bpy = 2,2'-bipyridine), which form 2:1 RuP²⁺/1 assemblies on the surface. At pH 5.7 in 0.1 M acetate buffer, these electrodes with a fluorine-doped tin oxide (FTO) back contact under ∼1 sun illumination (100 mW/cm²; white light source) perform efficient water oxidation with a photocurrent of 1.5 mA/cm² with an 88% Faradaic efficiency (FE) for O₂ production at an applied bias of 600 mV versus RHE ( ACS Energy Lett. , 2016 , 1 , 231 - 236 ). The SnO₂/TiO₂-chromophore-catalyst assembly was integrated with the Si electrodes by a thin layer of titanium followed by an amorphous TiO₂ (Ti/a-TiO₂) coating as an interconnect. In the integrated electrode, p⁺n-Si-Ti/a-TiO₂-SnO₂/TiO₂|-2RuP²⁺/1, the p⁺n-Si junction provided about 350 mV in added potential to the half cell. In photolysis experiments at pH 5.7 in 0.1 M acetate buffer, bias-free photocurrents approaching 100 μA/cm² were obtained for water splitting, 2H₂O → 2H₂ + O₂. The FE for water oxidation was 79% with a hydrogen efficiency of ∼100% at the Pt cathode.

Distant ultrafast energy transfer in a trimetallic {Ru-Ru-Cr} complex facilitated by hole delocalization

Multi-metallic complexes based on {Ru-Cr}, {Ru-Ru} and {Ru-Ru-Cr} fragments are investigated for their light-harvesting and long-range energy transfer properties. We report the synthesis and characterization of [Ru(tpy)(bpy)(μ-CN)Ru(py)₄Cl]²⁺ and [Ru(tpy)(bpy)(μ-CN)Ru(py)₄(μ-NC)Cr(CN)₅]. The intercalation of {Ru^II(py)₄} linked by cyanide bridges between {Ru(tpy)(bpy)} and {Cr(CN)₅} results in efficient, distant energy transfer followed by emission from the Cr moiety. Characterization of the energy transfer process based on photophysical and ultrafast time-resolved absorption suggests the delocalization of holes in the excited state, providing a pathway for energy transfer between the end moieties. The proposed mechanism opens the door to utilize this family of complexes as an appealing platform for the design of antenna compounds as the properties of the fragments could be tuned independently.

Aqueous Hydricity from Calculations of Reduction Potential and Acidity in Water

Hydricity, or hydride donating ability, is a thermodynamic value that helps define the reactivity of transition metal hydrides. To avoid some of the challenges of experimental hydricity measurements in water, a computational method for the determination of aqueous hydricity values has been developed. With a thermochemical cycle involving deprotonation of the metal hydride (pK_a), 2e^- oxidation of the metal (E°), and 2e^- reduction of the proton, hydricity values are provided along with other valuable thermodynamic information. The impact of empirical corrections (for example, calibrating reduction potentials with 2e^- organic versus 1e^- inorganic potentials) was assessed in the calculation of the reduction potentials, acidities, and hydricities of a series of iridium hydride complexes. Calculated hydricities are consistent with electronic trends and agree well with experimental values.

Photoelectrochemical hydrogen production in water using a layer-by-layer assembly of a Ru dye and Ni catalyst on NiO

Layer-by-layer assembly of a Ru dye and Ni catalyst on a p-type NiO photocathode enables photoelectrochemical H₂ generation in water.

Capture and conversion of sunlight into the storable energy carrier H₂ can be achieved through photoelectrochemical water splitting using light-absorbing cathodes and anodes bearing H₂ and O₂ evolving catalysts. Here, we report on the development of a dye-sensitised p-type nickel oxide (NiO) photocathode with a hexaphosphonated Ru(2,2′-bipyridine)₃ based dye (RuP3) and a tetraphosphonated molecular [Ni(P₂N₂)₂]²⁺ type proton reduction catalyst (NiP) for the photoreduction of aqueous protons to H₂. A layer-by-layer deposition approach was employed, using Zr⁴⁺ ions to link the phosphonate units in RuP3 and NiP in a supramolecular assembly on the NiO photocathode. This approach keeps the dye in close proximity to the catalyst and semiconductor surface, but spatially separates NiP from NiO for advantageous electron transfer dynamics. The NiO|RuP3–Zr⁴⁺–NiP electrodes generate higher photocurrents and are more stable than photocathodes with RuP3 and NiP co-immobilised on the NiO surface in the absence of Zr⁴⁺ cations linking dye and catalyst. The generation of H₂ with the NiO|RuP3–Zr⁴⁺–NiP hybrid electrode in pH 3 aqueous electrolyte solution during irradiation with a UV-filtered solar light simulator (λ > 400 nm, 100 mW cm^–2, AM1.5G) has been confirmed by gas chromatography at an underpotential of 300 mV (E_appl = +0.3 V vs. RHE), demonstrating the potential of these electrodes to store solar energy in the chemical bond of H₂.

Water splitting with polyoxometalate-treated photoanodes: enhancing performance through sensitizer design

Visible light driven water oxidation has been demonstrated at near-neutral pH using photoanodes based on nanoporous films of TiO₂, polyoxometalate (POM) water oxidation catalyst [{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂]^10- (1), and both known photosensitizer [Ru(bpy)₂(H₄dpbpy)]²⁺ (P2) and the novel crown ether functionalized dye [Ru(5-crownphen)₂(H₂dpbpy)](H₂2). Both triads, containing catalyst 1, and catalyst-free dyads, produce O₂ with high faradaic efficiencies (80 to 94%), but presence of catalyst enhances quantum yield by up to 190% (maximum 0.39%). New sensitizer H₂2 absorbs light more strongly than P2, and increases O₂ quantum yields by up to 270%. TiO₂-2 based photoelectrodes are also more stable to desorption of active species than TiO₂-P2: losses of catalyst 1 are halved when pH > TiO₂ point-of-zero charge (pzc), and losses of sensitizer reduced below the pzc (no catalyst is lost when pH < pzc). For the triads, quantum yields of O₂ are higher at pH 5.8 than at pH 7.2, opposing the trend observed for 1 under homogeneous conditions. This is ascribed to lower stability of the dye oxidized states at higher pH, and less efficient electron transfer to TiO₂, and is also consistent with the 4^th1-to-dye electron transfer limiting performance rather than catalyst TOF_max. Transient absorption reveals that TiO₂-2-1 has similar 1^st electron transfer dynamics to TiO₂-P2-1, with rapid (ps timescale) formation of long-lived TiO₂(e^-)-2-1(h⁺) charge separated states, and demonstrates that metallation of the crown ether groups (Na⁺/Mg²⁺) has little or no effect on electron transfer from 1 to 2. The most widely relevant findings of this study are therefore: (i) increased dye extinction coefficients and binding stability significantly improve performance in dye-sensitized water splitting systems; (ii) binding of POMs to electrode surfaces can be stabilized through use of recognition groups; (iii) the optimal homogeneous and TiO₂-bound operating pHs of a catalyst may not be the same; and (iv) dye-sensitized TiO₂ can oxidize water without a catalyst.