Polymer Chromophore-Catalyst Assembly for Solar ... - ACS Publications

Letter

Polymer Chromophore-Catalyst Assembly for Solar Fuel Generation Junlin Jiang, Benjamin D. Sherman, Yan Zhao, Ru He, Ion Ghiviriga, Leila Alibabaei, Thomas J. Meyer, Gyu Leem, and Kirk S. Schanze ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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ACS Applied Materials & Interfaces

Polymer Chromophore-Catalyst Assembly for Solar Fuel Generation Junlin Jiang,† Benjamin D. Sherman,≠ Yan Zhao,§ Ru He, † Ion Ghiviriga,† Leila Alibabaei,≠ Thomas J. Meyer,≠ Gyu Leem§* and Kirk S. Schanze§* †

Department of Chemistry and Center for Macromolecular Science and Engineering,

University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, United States §

Department of Chemistry, University of Texas at San Antonio, One UTSA Way, San

Antonio, TX 78249 ≠

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North

Carolina 27599-3290, United States

KEYWORDS: Water oxidation, photoanode, polymeric chromophore-water oxidation catalyst assembly, Layer-by-Layer, oxidation of alcohols

1 ACS Paragon Plus Environment


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ABSTRACT: A polystyrene-based chromophore-catalyst assembly (poly-2) has been synthesized and assembled at a mesoporous metal oxide photoanode. The assembly contains water oxidation catalyst centers based on [Ru(trpy)(phenq)2+] (Ru-Cat) and [Ru(bpy)3]2+ derivatives

(Ru-C)

as

chromophores

(trpy=

2,2′;6,2″-

terpyridine,

phenq

=

2-(quinol-8′-yl)-1,10-phenanthroline and bpy = 2,2'-bipyridine). The photophysical and electrochemical properties of the polychromophore-oxidation catalyst assembly were investigated in solution and at the surface of mesoporous metal oxide films. The Layer-by-Layer (LbL) method was utilized to construct multilayer films with cationic poly-2 and anionic poly(acrylic acid) (PAA) for light-driven photochemical oxidations. Photocurrent measurements of (PAA/poly-2)10 LbL films on mesoporous TiO2 demonstrate light-driven oxidation of phenol and benzyl alcohol in aqueous solution. Interestingly, illumination of (PAA/poly-2)5 films on a fluorine doped SnO2/TiO2 core/shell photoanode in aqueous solution give rise to an initial photocurrent (~ 18.5 µA-cm-2) that is in part ascribed to light driven water oxidation.

Dye sensitized photoelectrosynthesis cells (DSPECs) have been widely investigated towards the development of light driven water-splitting systems.1-3 In a DSPEC, absorption of light by a sensitizer (chromophore) deposited on the surface of a metal oxide semiconductor triggers a series of molecular and interfacial electron transfer events that drive water oxidation and solar fuel generation half reactions in the two separate electrode compartments of the DSPEC. Ruthenium complexes have been identified as one of the most active molecular water 2 ACS Paragon Plus Environment

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oxidation catalysts.4-7 In addition, to date, as a photosensitizer, the Ru(II) polypyridyl chromophore and its derivatives have been widely studied in systems for photocatalytic water oxidation because of properties such as strong light absorption, long excited state lifetimes, and good photo- and thermal stability.8-11 Adsorption of Ru(II) complexes on an oxide semiconductor surface establishes an interface capable of supporting light-driven charge separation. Mesoporous nanostructured TiO2 (nanoTiO2) and mesoporous core/shell SnO2/TiO2 surfaces supported on planar indium tin oxide (ITO) or fluorine doped tin oxide (FTO) substrates are among the most frequently used semiconductor electrodes for DSPEC applications.12 The spatial arrangement and relative stoichiometry of the water oxidation catalyst and the sensitizer on the semiconductor surface is crucial for the development of efficient light driven water splitting DSPEC devices. Several approaches have been explored to assemble catalysts and photosensitizers onto semiconductors, including

molecular

(IV)-phosphonate

co-deposition,13

bridges,14

layer-by-layer

molecular

covalent

(LbL) bonding,3

deposition and

with

Zr

cross-linked

electropolymerization via reductive vinyl coupling.15 LbL self-assembly of multilayers comprised of alternating charged polyelectrolytes allows facile fabrication of functional polyelectrolyte films on solid supports.16 We have previously reported the use of LbL polyelectrolyte self-assembly to fabricate chromophore-catalyst assemblies on mesoporous wide band gap metal oxide substrates for light driven water oxidation, where these assemblies consist of a polychromophore and a molecular water oxidation catalyst.17 Herein, we report the preparation of a polymeric chromophore–catalyst assembly, followed by its deposition onto a semiconductor via the LbL method to construct a DSPEC photoanode for water oxidation. The approach used to construct the polymer allows variation of the chromophore-catalyst ratio in the assembly. There are few previous reports concerning the preparation of polymer-based chromophore catalyst assemblies for light-driven water oxidation in which the chromophore-to-catalyst ratio can be controlled within the assembly.



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Scheme 1. Chemical structures of Ru-C, Ru-Cat, poly-1, poly-2 and poly-2-PF6.

The chemical structures used in the present study are shown in Scheme 1, which includes an alkyne functionalized [Ru(bpy)3]2+ derivative (bpy = 2,2'-bipyridine) as chromophore (Ru-C),

[Ru(phenq)(tpy)]2+

(tpy

=

2,2′;6,2″-

terpyridine,

phenq

=

2-(quinol-8′-yl)-1,10-phenanthroline) as oxidation catalyst (Ru-Cat), and polymer assemblies poly-1, poly-2, and poly-2-PF6. Ru-Cat was previously reported by Thummel and co-workers as a coordinately saturated polypyridine Ru center that is a competent water oxidation catalyst, albeit with a relatively low turnover frequency.18 The Ru-Cat complex was chosen for the present study because the lack of an aquo ligand in its coordination environment (in the resting state) allows it to be incorporated easily into the polymer synthesis, which involves several steps that are carried out in organic solvents that are incompatible with Ru-aquo complexes. In previous work, we applied nitroxide-mediated radical polymerization (NMP)-click methodology to construct well defined polystyrene scaffolds which can be used to assemble Ru-C chromophores (poly-1, Scheme 1).19 We previously reported energy/electron transfer 4 ACS Paragon Plus Environment

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and interfacial electron transfer at mesoporous TiO2 with poly-1.20, 21 On the basis of the observation that polymers with longer chains have a higher tendency for aggregation and give rise to lower surface coverage on mesoporous metal-oxide supports, the focus here was on polymers with a relatively low degree of polymerization (DP) of ~20.19 Ru-Cat and Ru-C (reaction ratio: 1:4) were attached to a polystyrene (Mn ~ 2,200, polydispersity index (PDI) ~ 1.2) via an azide-alkyne Huisgen cycloaddition reaction (poly-2, Scheme 1). Subsequent counterion exchange of poly-2 by treatment with ammonium hexafluorophosphate affords poly-2-PF6 (Scheme 1), which is soluble in acetonitrile. poly-2-PF6 was characterized in CD3CN solution by 1H NMR and infrared spectroscopy (see Figure S7). By integration of the characteristic 1H NMR resonances from the Ru-C and Ru-Cat units, the molecular ratio between Ru-Cat and Ru-C along the polymer chain was ~24:76, comparable with the 1:4 reaction ratio of the RuC to Ru-Cat used in the click reaction feed (see Supporting Information). Additionally, attachment of Ru-C and Ru-Cat to the polymer was supported by the absence of the azide stretch peak, which occurs at ~ 2050 cm-1 in the IR spectrum of the azide precursor (Figure S7b). The UV-visible absorption and emission spectra of poly-2, Ru-C and Ru-Cat were measured in a mixture of acetonitrile (50%) and methanol (50%) at room temperature and the spectra are shown in Figure 1. Ru-C features a strong ligand-based π → π* absorption with a maximum at λ ∼ 288 nm and a dπ(Ru) → π*(bpy) metal-to-ligand charge transfer (MLCT) band at λ ∼ 455 nm.19 Ru-Cat displays absorption features at 300 - 350 nm, and an MLCT absorption that is red-shifted by 25 nm compared to the MLCT absorption of Ru-C.18 The absorption of poly-2 at 310-365 nm and 485-600 nm appears as a composite of the absorption features of Ru-C and Ru-Cat. To confirm the fraction of Ru-C and Ru-Cat grafted to the polymer chains, the spectrum of poly-2 was simulated by using the component spectra and varying the ratio of Ru-Cat from 10 – 40% (Figure S8a). This procedure shows that the simulated absorption corresponding to a ratio of 30% Ru-Cat to 70% Ru-C is in good agreement with the experimental spectrum of poly-2 (Figure S8b).



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Figure 1.

UV-visible absorption and emission spectra for poly-2 (solid blue line), Ru-C

(dashed red line) and Ru-Cat (dotted black line). Excitation wavelength was 450 nm. All photophysical experiments were conducted in methanol/acetonitrile (v/v = 1/1).

The photoluminescence of Ru-C appears as a broad band with λmax ∼ 650 nm arising from the 3MLCT excited state; by contrast, Ru-Cat is essentially non-emissive. Importantly, the emission of poly-2 was markedly lower compared to that of Ru-C, with ~90% of the intensity (λ ∼ 650 nm) quenched. The significant emission quenching is interpreted as arising due to energy transport between chromophore sites along the polymer backbone by a hopping mechanism, followed by quenching via charge or energy transfer from the photoexcited chromophore to the catalyst center.22


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Figure 2.

Cyclic voltammetry measurements for Ru-C (a), Ru-Cat (b) and poly-2 (c) in

aqueous 0.1 M phosphate buffer solution (pH = 7), recorded at a scan rate of 50 mV/s with a carbon working electrode. Cyclic voltammetry measurements for ITO//(PAA/poly-2)5 (d, e) at different scan rates (20 mV/s, 50 mV/s and 100 mV/s) in 0.1 M HClO4 aqueous solution (pH = 1).

To probe their electrochemical properties, cyclic voltammetry of Ru-C, Ru-Cat, and poly-2 were studied in solution and/or as a film on a tin doped indium oxide (ITO) substrate (Figure 2, all potentials reported are vs. Ag/AgCl reference electrode.) In solution, Ru-C exhibits a reversible wave at 1.25 V due to the Ru(II/III) couple. The electrochemistry of Ru-Cat is more complex. First, the complex reveals a reversible anodic wave due to the Ru(II/III) couple at ~1.05 V. In addition, at potentials above 1.4 V Ru-Cat shows a significant anodic current (~60 µA, at 1.6 V). The enhanced current response is attributed to catalytic water oxidation.6 The onset of the catalytic wave occurs at ~1.3 V, and indicates that water oxidation occurs after formation of the Ru(IV) or Ru(V) state of the Ru-Cat. 7 ACS Paragon Plus Environment


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Polymeric assembly poly-2 contains both Ru-C and Ru-Cat sites, at an approximate ratio of 3:1, respectively. Therefore, we expect the polymer to display characteristics of both molecular units, perhaps with some differences due to interaction between the centers and modified due to their unequal loadings. Figure 2c shows the CV of poly-2 in solution; a single irreversible wave is observed at ~1.2 V, and then the catalytic anodic current is observed at potentials above 1.4 V. The 1.2 V wave is likely mainly due to the Ru-C(II/III) couple, but there may also be a contribution from Ru-Cat(II/III). Importantly, the fact that the Ru-C(II/III) wave is not reversible gives evidence for hole transfer from Ru(III)-C to Ru-Cat (eq. 1a), with the latter being regenerated by catalytic water oxidation (eq. 1b). Ru-C(III) + Ru-Cat(III) → Ru-C(II) + Ru-Cat(IV)

(1a)

Ru-Cat(IV) + H2O → → Ru-Cat(II) + 1/2O2 + 2H+

(1b)

To further understand the electrochemical properties of poly-2, LbL films were deposited onto an ITO electrode with poly(acrylic acid) (PAA) as an oppositely charged polyelectrolyte to afford an ITO//(PAA/poly-2)5 multilayer assembly (the 5 subscript means an LbL assembly consisting of 5 PAA/poly-2 bilayers). The resulting modified electrode was then probed by CV at scan rates of 20, 50, and 100 mV/s (Figure 2d and 2e). The CV of the multilayer assembly is qualitatively similar to that of poly-2 in solution, with subtle differences. First, there is a distinct pre-wave observed before the Ru-C(II/III) wave, which may be due to the Ru-Cat(II/III) couple. In addition, the catalytic current due to water oxidation is observed for potentials above 1.4 V. Close inspection of the Ru-C(II/III) wave at various scan rates (Figure 2e) reveals that it is reversible at 100 mV/s, but the wave becomes noticeably less reversible at lower scan rate. This behavior again suggests the involvement of Ru-C to Ru-Cat hole transfer in the LbL films (eq. 1a), albeit at a slower rate than observed for poly-2 in solution. To enable application of the chromophore-catalyst assembly at a photoanode, LbL films consisting of poly-2 and PAA were deposited onto mesoporous nanostructured TiO2 films on a fluorine

doped

tin

oxide

(FTO)

substrate

(FTO//nanoTiO2)

to

afford

FTO//nanoTiO2//(PAA/poly-2)10 (Scheme 2).23 As a control, the Ru-C loaded polymer (poly-1) was

also

deposited

onto

an

FTO//nanoTiO2 8

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substrate

affording

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FTO//nanoTiO2//(PAA/poly-1)10. The films were characterized by UV-visible absorption spectroscopy. The maximum visible absorbance of the poly-1 and poly-2 multilayer films is similar with OD ~ 0.12 at 475 nm (Figure S9). The FTO//nanoTiO2//(PAA/poly-2)10 exhibits enhanced

absorption

in

the

region

from

500

nm

to

550 nm

compared

to

FTO//nanoTiO2//(PAA/poly-1)10 due to the contribution of the Ru-Cat units in poly-2. The surface coverage of poly-1 and poly-2 on the nanoTiO2 films was calculated as described previously,24 with Γ ~ 6.9 x 10-9 mol-cm-2.

Scheme 2.

Schematic illustration for fabrication of poly-2 onto mesoporous substrates (e.g.

nanoTiO2 or SnO2/TiO2 core-shell structure atop FTO).

Light-driven phenol (PhOH) and benzyl alcohol (BnOH) oxidation experiments were performed to demonstrate the photocatalytic activity of poly-2. Phenol was chosen as the first target to study the organic oxidation performance for FTO//nanoTiO2//(PAA/poly-2)10, as it has been previously shown to be quite reactive with respect to oxidation by polypyridyl Ru oxidation catalysts.25 Phenol oxidation was measured by monitoring anodic photocurrent upon irradiation of the modified electrode under a 0.2 V applied bias and visible light, where the light source was a AM 1.5 with a 400 nm cutoff filter (λ > 400 nm, 100 mW-cm−2) (Scheme 2). A 400 nm long-pass filter was used to avoid direct band gap excitation of TiO2. Increasing [PhOH] results in a pronounced increase in the photocurrent from 4.5 to 11.5 µA-cm-2 (t = 30 s) 9 ACS Paragon Plus Environment


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at the poly-2 modified electrode (Figure 3a). Based on the literature,25 the enhanced photocurrent is attributed to oxidation of PhOH, catalyzed by the polymer assembly. In a control experiment the electrode modified with poly-1 exhibits only a slight increase photocurrent from 4.0 to 6.0 µA-cm-2 (t = 30 s) in the presence of PhOH (Figure S10a). The increased photocurrent at the poly-1 modified photoanode is likely due to one electron transfer from PhOH to RuIIIL3 units produced by charge injection into TiO2.25

Figure 3.

Photocurrent–time traces with 20 second light off/on cycles for

FTO//nanoTiO2//(PAA/poly-2)10 in PhOH solution (a) and FTO//nanoTiO2//(PAA/poly-2)10 in BnOH solution (b). The data were collected in aqueous sodium acetate buffer (20 mM, pH = 4.6) under illumination (1 sun, 100 mW-cm−2, 400 nm cutoff filter) with an applied bias of 0.2 V vs Ag/AgCl. Phenol was added at concentrations of 4, 8, and 12 mM, and BnOH was present in solution at 0.1 M.

Oxidation

of

benzyl

alcohol (BnOH) at FTO//nanoTiO2//(PAA/poly-2)10 and

FTO//nanoTiO2//(PAA/poly-1)10 was subsequently investigated and the results are shown in Figure 3b and Figure S10b. The photocurrent for both poly-2 and poly-1 films was observed to be ~ 4.2 and 3.8 µA-cm-2 (t = 30 s), respectively in the absence of BnOH, and it increased by approximately 115 % and 30 %, for poly-2 and poly-1, respectively, in the presence of 100 mM BnOH. The slight photocurrent enhancement at poly-1 film electrode again indicated oxidation of the organic substrate by the oxidized form of the chromophore, however to a lesser extent as when this process could be mediated through the presence of Ru-Cat at the electrode interface. 10 ACS Paragon Plus Environment

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Note that similar photocurrents were observed for the PhOH and BnOH substrates only when a substantially higher concentration of BnOH was added (100 mM vs. 4 mM). This result clearly arises due to the higher heterogeneous electron transfer rate for PhOH compared to BnOH.25, 26 In order to study photo-driven water oxidation, chromophore-catalyst assembly poly-2 was immobilized on a mesoporous sol-gel SnO2 film containing a TiO2 overlayer formed by atomic layer deposition (ALD). Previous studies have shown that the “core-shell” SnO2/TiO2 electrode enhances the performance of chromophore-catalyst modified photoanodes by suppressing charge recombination, allowing for improved catalytic turnover at the photoanode/electrolyte solution interface.17 Polymer assemblies poly-2 and poly-1 were deposited

onto

FTO//(SnO2/TiO2)

FTO//(SnO2/TiO2)//(PAA/poly-2)5,

substrate and

using

the

LbL

method

FTO//(SnO2/TiO2)//(PAA/poly-1)5.

to

afford

Figure

4

compares the photocurrent-time traces for the poly-1 (chromophore only) and poly-2 (chromophore-catalyst) modified electrodes immersed in aqueous KNO3/phosphate buffer solution. For the poly-2 photoanode, an initial photocurrent density of 18.5 µA-cm-2 was observed which gradually decreased to ~11.5 µA-cm-2 after multiple light-dark cycles. Under the same conditions, the poly-1 modified photoanode generated an initial photocurrent density of ~7.4 µA-cm-2 and finally stabilized at ~ 6.2 µA-cm-2 after three light on-off cycles, and the photocurrent density generated from bare FTO//(SnO2/TiO2) is negligible (Figure 4, Eappl = 0.2 V, 100 mW-cm−2, λ > 400 nm). The photocurrent-time dependence for the poly-2 photoanode was also tested under continuous illumination for 250 seconds (Figure S11, Eappl = 0.2 V, 100 mW-cm−2, λ > 400 nm). After a sharp decrease from the initial current of ~18 µA-cm-2 to ~12 µA-cm-2 during the first 45 seconds of illumination, the photocurrent remained relatively stable, declining only slightly to ~10 µA-cm-2.



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Figure 4.

Photocurrent–time traces with 20 second light off/on cycles for three anodes:

FTO//(SnO2/TiO2)//(PAA/poly-2)5 (solid red line), FTO//(SnO2/TiO2)//(PAA/poly-1)5 (dashed blue line), and bare FTO//(SnO2/TiO2) (dotted black line). The data were collected in 0.5 M KNO3 aqueous solution with 0.1 M phosphate buffer (pH = 7) under illumination (1 sun, 100 mW-cm−2, 400 nm cutoff filter) with an applied bias of 0.2 V vs Ag/AgCl.

While photocurrent is observed for both the poly-1 (chromophore) and poly-2 (chromophore-catalyst) modified electrodes when illuminated in aqueous KNO3/phosphate buffer solution, the current observed for the chromophore-catalyst assembly poly-2 is clearly enhanced. While it cannot be proved, it is likely that the photocurrent enhancement is due to light-driven water oxidation which is catalyzed by the Ru-Cat sites that are present in the poly-2 assembly film. The electrochemical experiments discussed above provide strong evidence that the catalyst is competent at water oxidation, and it is not at all surprising that water oxidation can be driven in a process as outlined schematically in Scheme 2. Although IPCE experiments have not been performed, it is evident from the relatively low photocurrent observed under 100 mW-cm-2 illumination that the overall photochemical quantum efficiency for photochemical water oxidation is comparatively low (i.e., 0.1 – 0.5%). The relatively low quantum yield is likely due to the fact that the turnover frequency of Ru-cat is low, and is not able to effectively compete with charge recombination. In addition, hindered ionic and/or charge-transport through the LbL films may negatively influence the quantum efficiency. 12 ACS Paragon Plus Environment

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Previous studies have demonstrated that Ru chromophore and oxidation catalyst assemblies can carry out the photochemical oxidation of organic alcohols,27, 28 and light driven water oxidation.29 Recently, we reported the use of polyelectrolyte LbL to construct interfaces featuring a polystyrene-based Ru polychromophore inner layer and outer Ru water oxidation catalyst layer for light-driven water oxidation.21 In the current study, instead of anchoring Ru catalyst molecules via electrostatic LbL self-assembly, we have first prepared a covalently linked Ru chromophore−catalyst polymeric assembly and then used this charged polymer to form a LbL surface film. In comparison with the previous multicomponent LbL approach, the LbL films studied here supported ~28 % higher photocurrent density after 250 s of continuous illumination. The improved photochemical performance of the covalently linked chromophore-catalyst polymer using the LbL strategy suggests improved charge injection and hole transfer to the catalyst by this polymeric surface assembly as compared to the system studied previously. In summary, a novel approach for synthesizing polymeric catalyst–chromophore assemblies with precise control of the ratio of catalyst:chromophore sites has been successfully developed. The chemical structure and catalyst:chromophore ratio were determined from 1H NMR, FTIR, and UV-visible absorption spectroscopies studies. The electrochemical properties of poly-2 showed characteristic features of the Ru(II) chromophore and catalyst subunits composing the polymer. The polymer assembly was deposited onto FTO//TiO2 and FTO//(SnO2/TiO2) semiconductor substrates using a LbL method to form a photoanode for use in a DSPEC. Systematic studies were carried out to understand the electrochemical properties, photo-catalytic activity, and stability of the photoanode interface. Based on photocurrent measurements, the polymer assembly deposited photoanode was capable of harvesting light energy to carry out the oxidation of organic substrates (PhOH and BnOH) and water. The photocurrent stability test also confirmed the perseverance of the polymer assembly during photo-catalytic operation. Future studies will explore the interfacial photodynamics between the polychromophore–catalyst assemblies and the semiconductor surface. 13 ACS Paragon Plus Environment


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ASSOCIATED CONTENT Supporting Information. Experimental procedures, 1H, FTIR,

and

UV-visible

spectra

of

13

C and 2-D NMR, mass spectra,

FTO//nanoTiO2//(PAA/poly-2)10

and

FTO//nanoTiO2//(PAA/poly-1)10 films results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This material is based on work supported solely as part of the UNC EFRC: Solar Fuels and Next Generation Photovoltaics, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001011. We thank Alex J. Burnett for his help in implementing FTO//(SnO2/TiO2)//(PAA/poly-2)5 and FTO//(SnO2/TiO2)//(PAA/poly-1)5.

REFERENCES: 1. Treadway, J. A.; Moss, J. A.; Meyer, T. J., Visible Region Photooxidation on TiO2 with A Chromophore-Catalyst Molecular Assembly. Inorg. Chem. 1999, 38, 4386-4387. 14 ACS Paragon Plus Environment

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2. Alibabaei, L.; Sherman, B. D.; Norris, M. R.; Brennaman, M. K.; Meyer, T. J., Visible Photoelectrochemical Water Splitting Into H2 and O2 in A Dye-Sensitized Photoelectrosynthesis Cell. Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 5899-5902. 3. Alibabaei, L.; Brennaman, M. K.; Norris, M. R.; Kalanyan, B.; Song, W.; Losego, M. D.; Concepcion, J. J.; Binstead, R. A.; Parsons, G. N.; Meyer, T. J., Solar Water Splitting in A Molecular Photoelectrochemical Cell. Proc. Natl. Acad. Sci. U.S.A. 2013, 110, 20008-20013. 4. Zong, R.; Thummel, R. P., A New Family of Ru Complexes for Water Oxidation. J. Am.

Chem. Soc. 2005, 127, 12802-12803. 5. Sens, C.; Romero, I.; Rodriguez, M.; Llobet, A.; Parella, T.; Benet-Buchholz, J., A New Ru Complex Capable of Catalytically Oxidizing Water to Molecular Dioxygen. J. Am.

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29. Sherman, B. D.; Xie, Y.; Sheridan, M. V.; Wang, D. G.; Shaffer, D. W.; Meyer, T. J.; Concepcion, J. J., Light-Driven Water Splitting by a Covalently Linked Ruthenium-Based Chromophore-Catalyst Assembly. ACS Energy Lett. 2017, 2, 124-128.

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