[Ru^V(NCN-Me)(bpy)(=O)]³⁺ mediated efficient photo-driven water oxidation

Abstract

The [Ru(NCN-Me)(bpy)H₂O](PF₆)₂ complex (1) (where NCN-Me is the neutral N-methyl-3,5-di(2-pyridyl)pyridinium iodide after deprotonation of the C–H bond) acts as an active catalyst for the visible light driven oxidation of water, when employed with [Ru(bpy)₃]²⁺ as the photosensitizer and Na₂S₂O₈ as the sacrificial electron acceptor in a phosphate buffer solution at pH 6.5, in the presence of a visible light emitting diode (LED; visible light 10.2 mW cm⁻²) at 20 °C. The system exhibits very high activity, achieving 130 turnovers with respect to catalyst and an initial turnover rate TOF of 0.10 s⁻¹. The controlled experiments conducted for the photo-oxidation of water indicate that the catalytic system is dependent upon the following: (i) a visible light source, (ii) photosensitizer, (iii) catalyst and, (iv) pH. During the photocatalysis of water oxidation for 2 h in a phosphate buffer solution at pH 6.5, there was no degradation of the catalyst 1, which indicates the good photo-stability of the complex. The NCN-Me coordinates as a neutral ligand after deprotonation of the central C–H bond; generating the negatively charged carbon center favors a reduction in the redox potential of the complex, which considerably lowers the overpotential to 100 mV at pH 6.5 for the oxidation of water.

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Introduction

The energy demand causes a great bottleneck for the society and the day-by-day escalation in demand cannot be fulfilled by fossil-based nonrenewable energy sources. Almost ninety percent of energy is derived from the non-renewable fossil-based fuels and the combustion of these fuels has caused the environmental disorder, which is quite challenging. Environmental scientists are in search of suitable, novel, environmentally friendly alternative energy sources that can be utilized to overcome this problem, which is a formidable task for researchers. Solar energy and water are the most abundant alternative energy resources for the development of non-fossil based fuel.¹

Green plants utilize solar energy to convert water and CO₂ into chemical energy. During this process, two moles of water are oxidized to form one mole of O₂, four protons and four electrons in the presence of sunlight by the oxygen-evolving center (OEC) present in the photosystem II.^1,2 The Gibbs free energy (ΔG₀) for the overall water-splitting reaction (1) is +237.2 kJ mol⁻¹, which is thermodynamically an uphill reaction.² However, an efficient system has not yet been developed for the photo-oxidation of water.

2H₂O → O₂ + 4H⁺ + 4e⁻ 0.82 V vs. NHE (pH = 7) (1)

This has attracted the attention of researchers for the development of molecular water oxidation catalysts (WOCs) that oxidize water to release O₂, protons and electrons in the presence of an artificial light source, as a pathway for natural photosynthesis.³

Various dinuclear and mononuclear Ru complexes containing neutral N-donor atoms have been reported for photocatalytic water oxidation in the presence of [Ru(bpy)₃]²⁺ as photosensitizer and Na₂S₂O₈ as a sacrificial electron acceptor.⁴ Some interesting dinuclear ruthenium complexes were reported by Sun et al., containing two carboxylate groups in the bridging ligand.^4a–c The presence of the carboxylate group decreased the redox potential and resulted in very low overpotential. They also synthesized various types of other dinuclear complexes containing negatively charged bridging ligands. A couple of dinuclear ruthenium catalysts having the dpp⁻ type (2,9-di-(pyrid-20-yl)-1,10-phenanthroline) organic bridging ligand for photochemical water oxidation were reported by Thummel and group.^4f–g

A series of mononuclear Ru aqua complex catalysts have been reported for the photo-oxidation of water.⁵ Llobet et al. have reported the Ru aqua complex containing neutral N-donor ligands (Hbpp and H3p (2-(5-phenyl-1H-pyrazol-3-yl) pyridine)), which are highly active towards photocatalytic water oxidation in the presence of [Ru(bpy)₃]²⁺ as photosensitizer and [Co(Cl)(NH₃)₅]²⁺ as a sacrificial electron acceptor.^5f Sun and group reported another mononuclear Ru complex [Ru(bda)(isoq)] (H₂bda = 2,2′-bipyridine-6,6′-dicarboxylic acid), whose activity towards chemical water oxidation was comparable with the activity of the OEC.^5j Several other types of mononuclear catalysts with negatively charged ligands have also been reported by Sun and his group (i.e. [Ru(pda)(pic)₃], [Ru(bda)(pic)₂]). Due to the strong electron-donating ability of the negatively charged ligand, it reduced the redox potential.^5k

The reported carbene based Ru complex [Ru(tpy)(Mebim-py)(OH₂)]²⁺ (Mebim-py = 3-methyl-1-pyridylbenzimidazol-2-ylidene) acts as an impressive rate enhancement catalyst towards chemical water oxidation.^6a Tanaka et al. have reported the cyclometalated trans-[Ru(tpy)(PAD)(OH₂)]⁺ complex which undergoes photoisomeric geometrical transformation to the cis-[Ru(tpy)(PAD)(OH₂)]⁺ (PAD = 2-(pyrid-2′-yl)acridine) isomer upon irradiation of visible light which act as an efficient catalytic for water oxidation under chemical and electrochemical conditions.^6b ​Photocatalytic water oxidation by NHC or remote-NHC have not yet been reported. For a better catalyst, (i) low overpotential and (ii) stability of the catalyst are the key factors. Sun et al. have already explored the strong electron-donating ability of the negatively charged ligand, which favors reducing the redox potential.^5k This has caused us to consider the possibilities of photo-oxidation of water using [Ru(NCN-Me)(bpy)H₂O](PF₆)₂ (1) as the photocatalyst, which possesses a negative carbene type coordination. The oxidation of [Ru^IV=O]²⁺ to [Ru^V=O]³⁺ was observed at 0.95 V vs. NHE, where NCN-Me is the neutral N-methyl-3,5-di(2-pyridyl)pyridinium iodide after deprotonation of the C–H bond.

In our previous study, we reported the mononuclear Ru complex [Ru(NCN-Me)(bpy)H₂O](PF₆)₂, which shows very high catalytic activity to oxidize the PADHH to PADH⁺; however, a significant catalytic activity towards chemical water oxidation (TON = 2.5 out of 25) could not be observed using Ce^IV as an oxidant.⁷ In the case of chemical water oxidation, the [Ru^VI=O]⁴⁺ species was responsible for water oxidation and was highly unstable, quickly converting into the stable [Ru^V=O]³⁺species under highly acidic conditions (pH 1.6). In buffered conditions (pH = 6.5), an elevated catalytic current was observed at 1.02 V vs. NHE due to the active species [Ru^V=O]³⁺. The observed overpotential for this system at pH = 6.5 is 100 mV. Herein, we report that [Ru(NCN-Me)(bpy)H₂O](PF₆)₂ shows very high photocatalytic activity towards photo-oxidation of water, with a TON of 130 and initial TOF 0.10 s⁻¹ in the presence of photosensitizer and sacrificial electron acceptor.

Results and discussion

Synthesis

The preparation of the complex [Ru(NCN-Me)(bpy)H₂O](PF₆)₂ has been reported elsewhere.^7,8 Briefly, the de-iodination of [Ru^II(NCN-Me)(bpy)(I)](PF₆) by AgPF₆ in an acetone water mixture generates the aqua complex [Ru^II(NCN-Me)(bpy)(OH₂)](PF₆)₂ (1), wherein the NCN-Me coordinates as a neutral ligand after deprotonation of the central C–H bond. The crystallographic structure and characteristic properties have already been depicted in our previous report.⁷

Electrochemistry

The previously reported Pourbaix diagram has been modified and replotted vs. NHE in Fig. 1.⁷ The 1e⁻ oxidation of [Ru^II(OH₂)]²⁺ to [Ru^III(OH)₂]³⁺ takes place at 0.59 V vs. NHE for pH < 3. Between pH 3 to 10.5, proton coupled, one electron transfer generates the conversion of [Ru^II(OH₂)]²⁺ to [Ru^III(OH)]²⁺, where the observed slope is a slope of 60 mV pH⁻¹. The pK_a values for [Ru^II(OH₂)]²⁺ ⇆ [Ru^II(OH)]⁺ and [Ru^III(OH₂)]³⁺ ⇆ [Ru^III(OH)]²⁺ were 10.5 and 3, respectively. A slope of 60 mV per pH was observed between pH 3 to 10.5, separated by a potential of 200 mV and parallel to the [Ru^II(OH₂)]²⁺/[Ru^III(OH)]²⁺, and was attributed to the one proton coupled, one electron transfer between [Ru^III(OH)]²⁺ and [Ru^IV(=O)]²⁺. Two proton and one electron transfer was observed between [Ru^III(OH₂)]³⁺ and [Ru^IV(=O)]²⁺ within the pH range 0 to 3, and the slope was 120 mV pH⁻¹. One electron transfer occurs between [Ru^IV(=O)]²⁺ and [Ru^V(=O)]³⁺ and also between [Ru^V(=O)]³⁺ and [Ru^VI(O)]⁴⁺, within the pH range of 6 to 14 at a constant potential of 0.95 V and 1.27 V vs. NHE, respectively. The black straight line at potential 1.26 V vs. NHE within the pH range 0 to 14 shows the 1e⁻ transfer between [Ru(bpy)₃]²⁺ and [Ru(bpy)₃]³⁺.

Fig. 1 Pourbaix diagram of catalyst 1. Black line at 1.26 V vs. NHE indicates redox behavior of [Ru(bpy)₃]²⁺.

The stability of the [Ru(bpy)₃]³⁺ species is very low and it is usually reduced back to [Ru(bpy)₃]²⁺ at neutral pH or near to neutral pH. The observed oxidation potential of [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ is 1.26 V vs. NHE. From the Pourbaix diagram, the oxidation of [Ru^IV=O]²⁺ to [Ru^V=O]³⁺ was found at 0.95 V vs. NHE. The oxidation potential of [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ and the [Ru^V=O]³⁺/[Ru^VI=O]⁴⁺ are almost the same, therefore the formation of the [Ru^VI=O]⁴⁺ species by [Ru(bpy)₃]³⁺ is not possible, so the photocatalytic water oxidation could not be triggered by the [Ru^VI=O]⁴⁺. However, [Ru(bpy)₃]³⁺ can easily oxidize [Ru^IV=O]²⁺ to [Ru^V=O]³⁺, since the potential for [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺ is high enough, compared to the [Ru^IV=O]²⁺/[Ru^V=O]³⁺ couple. From the electrochemical study, it was observed that the catalyst shows a strong catalytic current at potential 1.02 V vs. NHE at pH 6.5 for water oxidation, which indicates that the active species involved is [Ru^V=O]³⁺ (Fig. 2). Therefore, for photocatalytic water oxidation, [Ru(bpy)₃]²⁺ can act as a better photosensitizer.

Fig. 2 Cyclic voltammogram and DPV of 1 (0.5 mM) and the background in a phosphate buffer (pH 6.5) solution.

Photocatalysis

The photocatalytic water oxidation was performed by considering a three component system containing photosensitizer [Ru(bpy)₃]²⁺, a sacrificial electron acceptor (Na₂S₂O₈) and catalyst 1 in phosphate buffer (pH 6.5) with a visible light emitting diode (LED; visible light 10.2 mW cm⁻²) at 20 °C. Whenever the sacrificial electron acceptor was changed to cobalt(III) pentaamine chloride, there was almost no oxygen evolution noticed in the system. The evolved O₂ was monitored by 320 TESTO Flue Gas Analyzer with an oxygen sensor probe and quantified by gas chromatograph. Quantum yield determination was carried out for the catalyst on the basis of number of water molecules oxidized per photon per O₂ molecule. After the irradiation of light, the photosensitizer [Ru(bpy)₃]²⁺ undergoes excitation to form the [Ru(bpy)₃*]²⁺, which undergoes oxidative quenching by S₂O₈²⁻ generating [Ru(bpy)₃]³⁺ and SO₄^*− radicals. At the same time, a thermal reaction between the oxidant SO₄^*− and [Ru(bpy)₃]²⁺ also generates [Ru(bpy)₃]³⁺. The overall reaction in the [Ru(bpy)₃]²⁺/S₂O₈²⁻ system is depicted in eqn (2)–(5).

2[Ru(bpy)₃]²⁺ + hν → 2[Ru(bpy)₃*]²⁺ (2)

2[Ru(bpy)₃*]²⁺ + 2S₂O₈²⁻ → 2[Ru(bpy)₃]³⁺ + 2SO₄^*− + 2SO₄²⁻ (3)

2[Ru(bpy)₃]²⁺ + 2SO₄^*− + hν → 2[Ru(bpy)₃]³⁺ + 2SO₄²⁻ (4)

4[Ru(bpy)₃]²⁺ + 2S₂O₈²⁻ + 2hν → 4[Ru(bpy)₃]³⁺ + 4SO₄²⁻ (5)

This process is accompanied by the oxidation of two equivalents of H₂O to O₂, carried out by [Ru(bpy)₃]³⁺ in the presence of visible light and WOC. The reaction conditions of the photocatalytic water oxidation was optimized from two phosphate buffer solutions containing two different pH values (6.5 and 7), however, at pH 6.5 a higher rate of O₂ evolution was observed than at pH 7. This was expected due to the lower stability of [Ru(bpy)₃]³⁺, i.e. due to the decomposition of [Ru(bpy)₃]³⁺ through OH⁻ attack on the bipyridine ring at higher pH.⁹ Hence, this optimized pH solution was used for all the experiments. Various controlled experiments were performed to substantiate that the H₂O oxidation was catalyzed by the catalyst (1). The catalytic conditions were as follows:

(a) Without photo sensitizer: only in the presence of the catalyst and Na₂S₂O₈, a negligible amount of O₂ evolution takes place, which reveals that in the three component system for photocatalytic water oxidation, the presence of photosensitizer plays a crucial role (Fig. 3).

Fig. 3 Photochemical oxygen evolution in phosphate buffer (pH = 6.47, 2 mL) solutions of (a) Na₂S₂O₈ (1 × 10⁻² M) and catalyst [Ru(NCN-Me)(bpy)(OH₂)](PF₆)₂ (1) (5 × 10⁻⁶ M), black curve; (b) Na₂S₂O₈ (1 × 10⁻² M) [Ru(bpy)₃]Cl₂ (1 × 10⁻³ M), red curve; (c) Na₂S₂O₈ (1 × 10⁻² M), [Ru(bpy)₃]Cl₂ (1 × 10⁻³ M), catalyst [Ru(NCN-Me)(bpy)(OH₂)](PF₆)₂ (1) (5 × 10⁻⁶ M), green curve.

(b) Without catalyst: the photocatalytic system containing only the photosensitizer and Na₂S₂O₈, shows a minor evolution of O₂ at a very slow rate compared to that in the presence of all three components (like photosensitizer, catalyst and Na₂S₂O₈ (Fig. 3)).

(c) No oxygen was evolved under dark conditions when complex 1 was added into the deoxygenated phosphate buffer (pH 6.5) solution containing Na₂S₂O₈ and [Ru(bpy)₃]Cl₂.

(d) Irradiation of the system containing [Ru(bpy)₃]Cl₂ (1 × 10⁻³ M), catalyst 1 (5 × 10⁻⁶ M) and Na₂S₂O₈ (1 × 10⁻² M) results in the evolution of 1.3 μmol of O₂, due to photo-oxidation of water after 2 h with a TON of 130 and initial TOF of 0.10 s⁻¹ (up to 1500 s). In the presence of the photosensitizer, there is some O₂ evolution (Fig. 3), therefore, it was subtracted from each experiment carried out in the presence of the catalyst 1, and further calculations were carried out accordingly.

(e) The photocatalytic effect on turning the light source “on and off” is displayed in Fig. S3.† The slope of the curve during the “initial on” time is higher than the following “on” periods, which is expected due to the slight decomposition of the photosensitizer during the “off” time period.

With increasing concentration of 1, the rate of O₂ evolution increases. The initial rates up to 1500 s of illumination were considered for the kinetic effect on the photocatalysis. It was found to be first order with respect to the catalyst concentration of 1 (Fig. 4), with the rate of 2.8 × 10⁻⁵ μmol s⁻¹. (Fig. S5†). When the catalyst concentration was fixed at 5 μM and the concentration of [Ru(bpy)₃]Cl₂ was varied (Fig. S6†), the rate of evolution of O₂ also increased with increasing concentration of [Ru(bpy)₃]Cl₂. The rate of the O₂ evolution was found to be first order dependent with respect to the photosensitizer concentration (K_obs = 9.8 × 10⁻⁵ μmol s⁻¹) (Fig. S7†).

Fig. 4 O₂ evolution at pH 6.5 in phosphate buffer containing Na₂S₂O₈ (1 × 10⁻² M), [Ru(bpy)₃]Cl₂ (1 × 10⁻³ M) with various concentrations of 1.

The quantum yield for the photochemical oxidation of water was determined for the catalyst, considering the number of water molecules oxidized per photon per O₂ molecule. The calculated quantum efficiency Φ of the photo-oxidation of water was 18% with respect to the complex. It was calculated using eqn (6), where ΔG_P is the Gibbs free energy for the photochemical reaction generating the O₂ product, K_P is the rate (mol s⁻¹) of generation of O₂, E_S is the incident intensity of irradiation (W m⁻²) and A is the irradiated area (m²).¹⁰

It was observed that the actual pH of the system decreases from 6.5 to 4.3 after irradiation, when the evolution of O₂ is saturated (Fig. 5). The cyclic voltammogram of the complex also reveals that the catalytic activity depends on the acidity of the solution. On increasing the acidity of the solution, the catalytic curve shifts towards the positive direction. Thus, the deactivation of the reaction system may be due to the increasing acidity of the solution, since the destabilization of [Ru(bpy)₃]³⁺ occurs. For the justification of the above assumption, the pH of the system was adjusted to the initial value with the addition of saturated KOH solution, and again irradiated with the same visible light under inert conditions. The evolution of O₂ took place after the second run; however, the pH of the system dropped to 4.8 (Fig. 5). This indicates that the deactivation of the system is due to the increasing acidity of the solution, and the amount of O₂ evolved was decreased during the second irradiation.

Fig. 5 O₂ evolution in phosphate buffer (2 mL, pH = 6.5) containing Na₂S₂O₈ (1 × 10⁻² M), [Ru(bpy)₃]Cl₂ (1 × 10⁻³ M), catalyst 1 (10 × 10⁻⁶ M), and its successive reactivation in a photochemical water oxidation system by alkalization.

Photo-stability of the photosensitizer

The decomposition of the photosensitizer was determined using UV-vis spectroscopy, considering two different reaction mixtures containing photosensitizer ([Ru(bpy)₃]²⁺) and Na₂S₂O₈ in the absence or presence of catalyst (Fig. S9†). It was noticed from the UV-vis-spectra after irradiation for 30 min, that the rate of decomposition of the photosensitizer was higher in the absence of catalyst than in the presence of the catalyst. This suggests that the catalyst preserves the photosensitizer from decomposition, by reducing [Ru(bpy)₃]³⁺ to [Ru(bpy)₃]²⁺. The [Ru(bpy)₃]³⁺ species plays a most important role in the photochemical water oxidation and is decomposed in the absence of catalyst, without any source present to reduce [Ru(bpy)₃]³⁺ to [Ru(bpy)₃]²⁺ in order to preserve it.

Photo-stability of the complex

The stability of the complex in the presence of light is a key factor in the photo-oxidation of water. The photostability of the complex was studied using optical spectra, by irradiating the catalyst (20 × 10⁻⁶ M) in phosphate buffer at pH 6.5 for a time span of 3 h. After irradiation of the catalyst solution with visible light for 3 h, there was no significant spectral change. This suggests that there was no photo-degradation of the catalyst in the presence of the visible light source (Fig. S10†).

Plausible mechanism

The possible mechanism for the photochemical water oxidation is proposed in Scheme 1. As inferred from the cyclic voltammogram of 1 at pH 6.5, an anodic current following the oxidation potential of the [Ru^IV=O]²⁺/[Ru^V=O]³⁺ couple increases. This enhancement of catalytic current in a single direction is indicative of electrocatalysis of water-oxidation, where the active species is [Ru^V=O]³⁺ at pH 6.5. It is expected that [Ru^V=O]³⁺ generates [Ru^III–O–OH]²⁺ in the presence of water. The [Ru^IV–OO]²⁺ species is formed upon 1e⁻ reduction of [Ru^III–O–OH]²⁺. This system releases one O₂ molecule, from the water attack on the [Ru^IV–OO]²⁺. The entire catalytic process involves 4e⁻ oxidation of catalyst 1 to release 1 mol of O₂. From the Pourbaix plot of [Ru(bpy)₃]²⁺/[Ru(bpy)₃]³⁺, the redox potential appears at 1.26 V vs. NHE, which reveals that the [Ru(bpy)₃]³⁺ species can easily generate [Ru^V=O]³⁺. During the photochemical reactions (vide supra) depicted in eqn (5), the 4 equivalents of [Ru(bpy)₃]³⁺ generated by 4 equivalents of [Ru(bpy)₃]²⁺ are actively involved in the 4e⁻ oxidation of catalyst 1. The released four electrons are further utilized for the reduction of 4[Ru(bpy)₃]³⁺ to 4[Ru(bpy)₃]²⁺ in the catalytic cyclic process.

Scheme 1 The proposed catalytic cycle for photo-oxidation of water by 1.

Comparison with chemical water oxidation

In our previous report, significant catalytic activity towards chemical water oxidation (TON = 2.5 out of 25) by the mononuclear Ru complex [Ru(NCN-Me)(bpy)H₂O](PF₆)₂ was not observed in presence of Ce^IV as an oxidant.⁷ During chemical water oxidation, the [Ru^VI=O]⁴⁺ species was responsible for water oxidation and was highly unstable, and quickly converted into the stable [Ru^V=O]³⁺species under highly acidic conditions (pH 1.6). However, in buffered conditions (pH = 6.5) an elevated catalytic current at 1.02 V vs. NHE was observed due to the active species [Ru^V=O]³⁺. The observed overpotential for this system at pH = 6.5 is 100 mV. Since, at pH 6.5 CAN does not behave as an oxidant, due to conversion into the corresponding hydroxide, it was necessary to carry out the photochemical water oxidation using the photosensitizer [Ru(bpy)₃]Cl₂ and the sacrificial electron acceptor Na₂S₂O₈. The complex [Ru(NCN-Me)(bpy)H₂O](PF₆)₂ shows very high photocatalytic activity towards photocatalytic water oxidation, with a TON of 130 and initial TOF of 0.10 s⁻¹.

Conclusion

[Ru(NCN-Me)(bpy)(OH₂)](PF₆)₂ was utilized as a photocatalyst for water oxidation in a phosphate buffer solution at pH 6.5. The catalytic effects conducted under various controlled experiments indicate that the photo-oxidation of water is dependent upon the following: (i) visible light source, (ii) photosensitizer, (iii) catalyst and, (iv) pH. Under dark conditions, no O₂ evolution was noticed in this system. In the presence of visible light, the photocatalytic water oxidation occurred in a three component system with catalyst 1, photosensitizer [Ru(bpy)₃]²⁺ and the sacrificial electron acceptor Na₂S₂O₈. Without the photosensitizer, the reaction mixture containing catalyst and the sacrificial electron acceptor does not show any photocatalytic O₂ evolution. However, some amount of O₂ evolution is detected from the system containing the photosensitizer and sacrificial electron acceptor. Sun et al. already proposed that the catalytic system containing the strong electron-donating ability of the negatively charged ligands favor reducing the redox potential. The same concept is also applicable for the catalyst reported in this work, where the NCN-Me coordinates as a neutral ligand after deprotonation of the central C–H bond, generating the negatively charged carbon center. In this remote-NHC type complex, the lowering of the redox potential facilitates a considerably lower overpotential of 100 mV at pH 6.5 for the oxidation of water. To the best of our knowledge, based on a literature survey, the photo-oxidation by the carbene coordinated ruthenium complex was not reported before. The photo-stability of the catalyst and reasonable TONs and TOFs is also a major advantage in this system, compared to the reported photocatalysts.^4,5 This work could help to develop and modify the synthetic strategies to design suitable negatively coordinating cyclometalated ruthenium complexes to lower the overpotential for water oxidation, since in a catalytic path the role of overpotential is a very important factor.

Experimental

Materials and methods

Materials. All the manipulations were carried out using standard Schlenk techniques under nitrogen atmosphere. Solvents like diethyl ether, dichloromethane, acetonitrile, acetone, ethanol and hexane were dried, degassed and stored under nitrogen atmosphere prior to use. The photosensitizer [Ru(bpy)₃]Cl₂ and the sacrificial electron acceptor Na₂S₂O₈ were purchased from Sigma Aldrich. The complex [Ru(NCN-Me)(bpy)(OH₂)](PF₆)₂ was synthesized according to the reported procedure.⁷

Electrochemistry. Cyclic voltammetry was performed in phosphate buffer solution of pH 6.5, using a CHI model 1140 electrochemical analyzer, a glassy carbon electrode (diameter 3 mm, freshly polished) as the working electrode and a platinum wire as the counter-electrode. The electrolyte used was phosphate buffer solution and the reference electrode was a saturated calomel electrode (SCE).

O₂ evolution. The photochemical water oxidation was carried out in a jacketed glass reaction vessel of volume 10 mL, equipped with a standard taper joint. This cell was connected to a water circulating system to maintain the temperature of 20 °C. A 320 TESTO Flue Gas Analyzer with oxygen sensor probe was connected to the reaction vessel filled with 5 μM catalyst ([Ru(NCN-Me)(bpy)(OH₂)](PF₆)₂(1)), 1 mM [Ru(bpy)₃]²⁺ and 0.01 M Na₂S₂O₈ dissolved in phosphate buffer (2 mL, pH = 6.5). The vessel was then sealed with a rubber septum and was degassed by purging with N₂. The reaction system was then irradiated with NIKILITE visible LED light with an intensity of 10.2 mW cm⁻² under stirring conditions. The production of O₂ as a function of time was monitored using the 320 TESTO Flue Gas Analyzer. The O₂ evolution was studied by changing the concentration of the catalyst (1), as well as the concentration of [Ru(bpy)₃]²⁺. The quantification of O₂ evolution was carried out using a gas chromatograph with argon as the carrier gas.

Photostability test. The photo degradation of the catalyst 1 in phosphate buffer (pH 6.5) was studied by using an Agilent technologies Cary 8454 UV-vis spectrophotometer. In order to analyze the photo-stability of the photosensitizer, two different reaction mixtures were prepared in phosphate buffer (2 mL, pH – 6.5), one contained only photosensitizer (1 mM) and Na₂S₂O₈ (0.1 M), the other had the photosensitizer (1 mM), Na₂S₂O₈ (0.1 M) and the catalyst (5 μM). From these mixtures, 150 μL were taken in two different 25 mL beakers and diluted to 3 mL using the same phosphate buffer. The UV-vis spectra of the two diluted solutions were measured, after which, both the diluted reaction mixtures were irradiated with visible light for 30 min and again the absorption spectra was measured.

Acknowledgements

This study is supported by the Department of Science and Technology (DST) under Grant SB-FT/CS-016/2014, supporting S.K.P. Indian School of Mines (ISM), Dhanbad is acknowledged for the partial supporting by the Grant (FRS (45)/2013–2014/AC). JP, KM are grateful to Indian School of Mines(ISM) for institute fellowships. SKP acknowledges CRF ISM, Department of Chemical Engineering, Department of Environmental Science and Engineering, and Department of Applied Chemistry (ISM Dhanbad) for the analytical facilities.

References

1. (a) Energy Production and Storage – Inorganic Chemical Strategies for a Warming World, Encyclopedia of Inorganic Chemistry, ed. R. H. Crabtree, John Wiley & Sons, Inc., 2nd edn, 2010, pp. 73–87; (b) G. Renger, Energy Transfer and Trapping in Photosystem II, in The Photosystems: Structure, Function and Molecular Biology, ed. J. Barber, Elsevier Science Publishers, Amsterdam, The Netherlands, 1992, p. 45; (c) L. M. C. Barter, D. R. Klug and R. van Grondelle, Energy Trapping and Equilibration: A Balance of Regulation and Efficiency, in Photosystem II: The Light-driven Water: Plastoquinone Oxidoreductase, ed. T. J. Wydrzynski and K. Satoh, Springer, Dordrecht, The Netherlands, 2005, p. 491; (d) J. N. Siedow and D. A. Day, Respiration and Photorespiration, in Biochemistry and Molecular Biology of Plants, ed. B. B. Buchanan, W. Gruissem and R. L. Jones, American Society of Plant Physiology, Rockville, MD, 2000, p. 676.
2. (a) Molecular Water Oxidation Catalysts: A Key Topic for New Sustainable Energy Conversion Schemes, ed. A. Llobet, John Wiley & Sons, 1st edn, 2014; (b) H. Schiller and H. Dau, J. Photochem. Photobiol., B, 2000, 55, 138; (c) Oxygenic Photosynthesis: The Light Reactions, ed. D. R. Ort and C. F. Yocum, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1996.
3. (a) L. I. Kristhalik, Biochim. Biophys. Acta, 1986, 849, 162; (b) L. I. Kristhalik, Bioelectrochem. Bioenerg., 1990, 23, 249.
4. (a) Y. Jiang, F. Li, B. Zhang, X. Li, X. Wang, F. Huang and L. Sun, Angew. Chem., Int. Ed., 2013, 52, 3398–3401; (b) Y. Xu, L. Duan, L. Tong, B. Åkermark and L. Sun, Chem. Commun., 2010, 6506–6508; (c) Y. Xu, T. Åkermark, V. Gyollai, D. Zou, L. Eriksson, L. Duan, R. Zhang, B. Åkermark and L. Sun, Inorg. Chem., 2009, 47, 2717–2719; (d) J. Mola, C. Dinoi, X. Sala, M. Rodriguez, I. Romero, T. Parella, X. Fontrodona and A. Llobet, Dalton Trans., 2011, 40, 3640–3646; (e) C. Sens, I. Romero, M. Rodriguez, A. Llobet, T. Parella and J. Benet-Buchholz, J. Am. Chem. Soc., 2004, 126, 7798–7799; (f) R. Zong and R. P. Thummel, J. Am. Chem. Soc., 2005, 127, 12802–12803; (g) Z. Deng, H.-W. Tseng, R. D. Zong and R. Thummel, Inorg. Chem., 2008, 47, 1835.
5. (a) J. J. Concepcion, J. W. Jurss, M. R. Norris, Z. Chen, J. L. Templeton and T. J. Meyer, Inorg. Chem., 2010, 49, 1277–1279; (b) J. J. Concepcion, J. W. Jurss, J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc., 2008, 130, 16462–16463; (c) J. J. Concepcion, M.-K. Tsai, J. T. Muckerman and T. J. Meyer, J. Am. Chem. Soc., 2010, 132, 1545–1557; (d) N. Kaveevivitchai, R. Chitta, R. Zong, M. E. Ojaimi and R. P. Thummel, J. Am. Chem. Soc., 2012, 134, 10721–10724; (e) H.-W. Tseng, R. Zong, J. T. Muckerman and R. Thummel, Inorg. Chem., 2008, 48, 11763–11773; (f) S. Roeser, P. Farrás, F. Bozoglian, M. Martínez-Belmonte, J. Benet-Buchholz and A. Llobet, ChemSusChem, 2011, 4, 197–207; (g) R. Cao, W. Lai and P. Du, Energy Environ. Sci., 2012, 5, 8134–8157; (h) M. D. Kärkäs, T. Åkermark, E. V. Johnston, S. R. Karim, T. M. Laine, B.-L. Lee, T. Åkermark, T. Privalov and B. Åkermark, Angew. Chem., Int. Ed., 2012, 51, 11589–11593; (i) B. Radaram, J. A. Ivie, W. M. Singh, R. M. Grudzien, J. H. Reibenspies, C. E. Webster and X. Zhao, Inorg. Chem., 2011, 50, 10564–10571; (j) L. Wang, L. Duan, L. Tong and L. Sun, J. Catal., 2013, 306, 129–132; (k) L. Duan, Y. Xu, M. Gorlov, L. Tong, S. Andersson and L. Sun, Chem.–Eur. J., 2010, 16, 4659–4668.
6. (a) Z. Chen, J. J. Concepcion and T. J. Meyer, Dalton Trans., 2011, 40, 3789–3792; (b) S. K. Padhi, R. Fukuda, M. Ehara and K. Tanaka, Inorg. Chem., 2012, 51, 5386–5392.
7. J. Patel, K. Majee, E. Ahamed, K. Tanaka and S. K. Padhi, Dalton Trans., 2015, 44, 920–923.
8. (a) T. Koizumi, T. Tomon and K. Tanaka, Bull. Chem. Soc. Jpn., 2003, 76, 1969–1975; (b) M. Toyama, K. Inoue, S. Iwamatsu and N. Nagao, Bull. Chem. Soc. Jpn., 2006, 79, 1525–1534.
9. P. K. Ghosh, B. S. Brunschwig, M. Chou, C. Creutz and N. Sutin, J. Am. Chem. Soc., 1984, 106, 4772.
10. J. R. Bolton, S. J. Strickler and J. S. Connolly, Nature, 1985, 316, 495–500.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12074a

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