Photoelectrocatalytic response of tris(2,2'-bipyridyl)ruthenium(II)-modified mesoporous silica film on ITO electrode with methyl viologen quencher and EDTA donor

Mahmoud Rahal, Neus Vilà, Christelle Despas, Malik Sebbat, Akshay Silswal, Gediminas Jonusauskas, Nathan Mcclenaghan, Emmanuel Oheix, Bénédicte Lebeau, Morgan Cormier, et al.

► To cite this version:

Mahmoud Rahal, Neus Vilà, Christelle Despas, Malik Sebbat, Akshay Silswal, et al. Photoelectrocat- alytic response of tris(2,2'-bipyridyl)ruthenium(II)-modified mesoporous silica film on ITO electrode with methyl viologen quencher and EDTA donor. Journal of Solid State Electrochemistry, 2025, 10.1007/s10008-025-06254-9. hal-04998378

HAL Id: hal-04998378

https://uha.hal.science/hal-04998378v1

Submitted on 20 Mar 2025

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Photoelectrocatalytic response of tris(2,2'-bipyridyl)ruthenium(II)-
modified mesoporous silica film on ITO electrode with methyl viologen
quencher and EDTA donor

Mahmoud Rahal,ª Neus Vilà,ª Christelle Despas,ª Malik Sebbat,ᵇ Akshay Silswal,ᶜ
Gediminas Jonusauskas,ᵈ Nathan McClenaghan,ᶜ Emmanuel Oheix,ᵉ,>f Bénédicte
Lebeau,ᵉ,f Morgan Cormier,ᵇ Jean-Philippe Goddard,ᵇ,* Alain Walcariusª,\*

<footnote>ª Université de Lorraine, CNRS, Laboratoire de Chimie Physique et Microbiologie pour les Matériaux et l'Environnement (LCPME), Nancy F-54000, France</footnote>

<footnote>ᵇ Université de Haute-Alsace (UHA), Université de Strasbourg, CNRS, Laboratoire d'Innovation Moléculaire et Application (LIMA) UMR 7042, 68100 Mulhouse, France</footnote>

<footnote>ᶜ Université de Bordeaux, CNRS, Institut des Sciences des Moléculaires (ISM) UMR 5255, 33405 Talence, France</footnote>

<footnote>ᵈ Université de Bordeaux, CNRS, Laboratoire Ondes et Matière d'Aquitaine (LOMA) UMR 5798, 33405 Talence, France</footnote>

<footnote>ᵉ Université de Haute-Alsace (UHA), CNRS, IS2M UMR 7361, 68100 Mulhouse, France</footnote>

<footnote>f Université de Strasbourg, 67000 Strasbourg, France</footnote>

<footnote>* Corresponding authors: Alain Walcarius, alain.walcarius@univ-lorraine.fr, LCPME, 405 rue de Vandoeuvre, 54600 Villers-les-Nancy, France; Jean-Philippe Goddard, jean-philippe.goddard@uha.fr, LIMA, 3 rue des Frères Lumière, 68093 Mulhouse, France.</footnote>
**Abstract**

In the present paper, we investigate the photoelectrochemical behaviour of a tris(bipyridine) derivative (denoted [Ru(bpy)₂(bpy')]²⁺) covalently immobilized in the nanochannels of a vertically aligned mesoporous silica film (VMSF) generated by electrochemically assisted self-assembly (EASA) onto a transparent indium-tin oxide (ITO) electrode, in solutions containing a quencher (methyl viologen, MV²⁺) and a model substrate partner (ethylenediaminetetraacetate, EDTA). The optimal film loading was first determined according to its solid-state photophysical properties. Then, the electrochemically accessible [Ru(bpy)₂(bpy')]²⁺ moieties were exploited in the presence of the MV²⁺ quencher alone in solution (aerated medium) to demonstrate the possible electrochemical regeneration of the photocatalyst immobilized in VMSF on ITO. This was achieved on the basis of experiments carried out by hydrodynamic amperometry at constant potential under successive sequential illumination (at 400 nm) and dark periods. Finally, the functionalized film was applied to the photoelectro catalytic oxidation of EDTA in an oxygen-free solution containing also MV²⁺ species, thanks to the effective electrochemical regeneration of the quencher, as also evidenced by photocurrent measurements as a function of time with successive switching of light from ‘off’ to ‘on’ and vice versa. Such photoelectro catalytic transformation of EDTA is possible at a much lower potential (-0.1 V) than that observed for its direct electrocatalytic oxidation by the same [Ru(bpy)₂(bpy')]²⁺-functionalized film operating without light (+1.2 V).

**Keywords**

Oriented mesoporous silica thin film; ruthenium tris(bipyridine); photocatalyst; methyl viologen; EDTA; electrochemical regeneration.
# 1. Introduction

The use of organic photoredox catalysts, combining both photochemical and electrochemical activity, is a rapidly growing field in the current decade due to their interest in developing sustainable synthetic approaches as alternatives to traditional methods [1-3]. Rather than competing, photocatalysis and electrocatalysis can even offer some synergy [4], which contributes to the emerging and exciting field of organic photoelectrochemistry [5]. Transition metal complexes, such as ruthenium polypyridyl complexes, are particularly interesting because they make it possible to carry out photoredox catalysis based on the absorption of light in the visible range [6-8]. For example, illuminating $[Ru(bpy)_3]^{2+}$ (bpy = 2,2'-bipyridine) at 452 nm (i.e., a wavelength allowing maximum absorption of light by $[Ru(bpy)_3]^{2+}$), leads to the formation of a stable and long-lived excited triplet Metal-to-Ligand Charge Transfer (MLCT) state (denoted $*[Ru(bpy)_3]^{2+}$) [6,8]. Such intermediates can be engaged in single-electron transfer reactions as effective oxidative or reductive species that are likely to trigger photooxidation or photoreduction processes in the presence of appropriate oxidative or reductive quenchers [8]. However, the need to regenerate the photocatalyst or the quencher usually involves the use of large quantities of additional reagents, which may either be part of the catalytic cycle itself or may only be sacrificial reagents which may thus generate undesirable side products in the medium [9,10]. A way to circumvent this drawback is electrochemistry [11], and the proof-of-concept of electrochemical regeneration of solution-phase photocatalyst $([Ru(bpy)_3]^{2+})$ and quencher (methyl viologen, $MV^{2+}$) was recently reported [12].

For several reasons (cost efficiency, reusability, facile separation) and practical uses, redox photocatalysts have to be immobilized on solid supports [13,14]. Of course, when immobilized, the redox photocatalysts must keep their photochemical/photocatalytic and electrochemical activity, be accessible to the illumination and to external reagents (quencher, substrate partner), be stable and likely to be regenerated. Various strategies and supports have been developed for heterogenization of molecular catalysts, mainly based on adsorption or covalent grafting [14,15]. For the specific case of ruthenium(II) tris(bipyridine) derivatives, one can cite (i) their adsorption, ion exchange or encapsulation in porous materials such as clays, zeolites or metal-organic frameworks [14], (ii) their attachment to graphene oxide via adsorption, ion-pair association or covalent bonding [16-18], (iii) the formation of cross-linked polymeric networks bearing covalently-attached ruthenium(II) tris(bipyridine) moieties [19,20], or (iv) their adsorption or covalent bonding to silica or mesoporous silica particles [21-23]. Silica materials are indeed attractive supports because: i) they can be easily manufactured
with on-demand structure and porosity, ii) one can tune their functionalization level, iii) their rich organosilane-based chemistry offers different routes for covalent bonding which ensures a strong linkage between the support and the catalyst, iv) the immobilized photocatalysts keep their photophysical properties, and v) the silica matrix may also contribute to improve the photostability of [Ru(bpy)₃]²⁺ species [21-26]. If such systems are indeed very promising for photochemical applications, they cannot be applied as such in photoelectrochemistry because of the insulating nature of the silica matrix. Nevertheless, it is now well-established that silica-based materials can be used for electrochemical purposes when appropriately confined to electrode surfaces [27,28], the most practical systems being designed as thin films on electrodes [29], which can even be obtained by electrochemically-assisted deposition [30]. An ideal configuration is provided by highly ordered and oriented mesoporous silica films made of nanochannels vertically aligned onto the electrode surface, as those prepared by electrochemically-assisted self-assembly (EASA) [31-33], as they ensure fast transport processes between the solution and the electrode [33-35]. Their pore length and diameter can be tuned according to various strategies [32, 33, 35-38] and they can be functionalized by covalent bonding of redox-active derivatives [39,40], which remains electrochemically accessible thanks to effective long-range charge transfer by electron hopping through the isolating nanochannels [41,42]. Examples are also available for polypyridyl-iron complexes which kept both their optical and electrochemical properties once immobilized [43-45]. Dealing with polypyridyl complexes of ruthenium, [Ru(bpy)₃]²⁺ can indeed be accommodated in such vertically aligned mesoporous silica films via favourable electrostatic interactions [46-49], but such weakly bound species may suffer from significant desorption upon use of the film electrode in solution [50,51].

In a very recent paper [52], we described the preparation of vertically aligned mesoporous silica films (VMSF) functionalized by covalent bonding of [Ru(bpy)₂(bpy')]²⁺ (with bpy = 2,2'-bipyridine and bpy' = a derivatized 4,4'-dimethyl-2,2'-bipyridine unit) coated on an indium-tin oxide (ITO) electrode, exhibiting an electrochemical response with a much better operational stability than VMSF simply doped with weakly bound [Ru(bpy)₃]²⁺ species, with promising luminescence and photocatalytic properties. In the present work, we report the characterization of the solid-state photophysical properties of [Ru(bpy)₂(bpy')]²⁺-bearing VMSF depending on their functionalization level, and we investigate their photoelectrocatalytic response in solution containing methyl viologen (MV²⁺) as quencher and EDTA as electron donor in view of possible application to the electrochemical regeneration of MV²⁺ in relation to photoredox
catalysis. We demonstrate that the photoelectrochemical oxidation of EDTA by the photocatalyst in the presence of MV²⁺ is feasible at much lower overpotential than its direct electrocatalytic oxidation using only an ITO electrode covered with VMSF carrying [Ru(bpy)₂(bpy')]²⁺ (without MV²⁺ quencher).

## 2. Material and methods

### 2.1. Reagents

All reagents used for the preparation of [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF on ITO electrode [52] are specified in Supplementary Information (section 1.1). The solutions used for photoelectrochemical experiments were typically composed of 1,1'-dimethyl-4,4'-bipyridinium dichloride (methyl viologen, MV²⁺, 98%, Merck), ethylenediaminetetraacetic acid disodium salt (EDTA, 99%, Prolabo) and sodium nitrate (NaNO₃, 99%, Merck), which were used as received. High-purity water (18.2 MΩ cm) obtained from a Purelab Option-Q from ELGA was used to prepare all solutions.

### 2.2. Film electrode preparation

[Ru(bpy)₂(bpy')]²⁺-functionalized VMSF were prepared on ITO electrode according to a two-step procedure (please see details in Ref. [52]). Briefly, azide-functionalized VMSFs were first prepared by EASA using a sol solution containing a mixture of precursors (namely tetraethoxysilane (TEOS, 98%, Thermo Scientific) and 3-azidopropyltriethoxysilane (AzPTES, prepared as in [53]), which were co-condensed in the presence of cetyltrimethylammonium bromide (CTAB, 99%, Sigma Aldrich) acting as structure-directing agent, upon the application of a cathodic potential of -1.3 V for 20 s to an ITO electrode (surface resistivity 8–12 Ω, Delta Technologies) immersed in a hydrolysed sol solution (200 mM [TEOS+AzPTES, at molar ratios ranging from 2.5/97.5 to 7.5/92.5 AzPTES/TEOS)] and 64 mM CTAB) in hydroalcoholic medium (1:1, water:ethanol, v/v). Then, after removing the surfactant template from the films (by immersion in 0.1 M HCl in ethanol for 15 min), the obtained azide-bearing film electrodes were functionalized with [Ru(bpy)₂(bpy')]²⁺ by click coupling (Scheme 1) using an alkynyl-bearing [Ru(bpy)₂(bpy')]²⁺ reagent (I) typically prepared by reaction of 4-[(2-propyn-1-yloxy)4'-methyl-2,2'-bypiridine (synthesized as in [23,54,55]) and dichlorobis(bipyridine)ruthenium(II) (Ru(bpy)₂Cl₂, 97%, Sigma Aldrich), see detail in Supplementary Information (section 1.2). This functionalization was achieved by allowing the electrodes to react for 36 h in the dark in dimethylformamide:water (7:3, v/v) medium
containing the alkynyl-bearing $[Ru(bpy)_2(bpy')]^{2+}$ compound (20 mg in 20 mL), copper(II) acetate ($Cu(CH_3COO)_2 \cdot H_2O$, 14 mg, 98%, Sigma Aldrich) and ascorbic acid (70 mg, 100%, Prolabo). The possible unreacted species and remaining traces of the catalyst were removed by rinsing the films successively with dimethylformamide (DMF, 99.8%, Sigma Aldrich), 0.1 M of sodium diethyldithiocarbamate ($[C_2H_5)_2NCSSNa \cdot 3H_2O$, 99%, Sigma Aldrich) in ethanol, and acetonitrile (for 10 min each). The mesostructure of the obtained films is exactly the same as reported in our previous paper [52], so no further characterization is provided here.

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Schematic illustration of the derivation of the azide-functionalized VMSF material by click coupling with an alkynyl-bearing $[Ru(bpy)_2(bpy')]^{2+}$ compound (I).</figcaption></figure>

**2.3. Apparatus and experimental procedures**

The model PGSTAT 101 A AutoLab potentiostat, monitored by the NOVA software (Metrohm), was used for the generation of VMSF by EASA and for the electrochemical characterization of the obtained films. A three-electrode electrochemical cell was employed for that purpose, including the film-modified ITO working electrode (0.50 cm² area), an Ag/AgCl reference electrode (Metrohm) and a platinum rod counter electrode. For film synthesis by EASA, the ITO electrode was placed at the bottom of a home-made one-compartment cell (as in Ref. [41]). $[Ru(bpy)_2(bpy')]^{2+}$-functionalized VMSFs were first characterized by cyclic voltammetry (CV) in aqueous medium (+0.1 M $NaNO_3$). Then, the photoelectrochemical measurements were carried out in aqueous medium (0.1 M $NaNO_3$ electrolyte) containing also
MV²⁺, alone or with EDTA, at selected concentrations (see text and figure legends) using a homemade cell (see photographs in Supplementary Information (section 2, Fig. S1). A constant potential of -0.1 V (vs. Ag/AgCl) was applied to the working electrode and photocurrents were recorded under illumination of the [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF at 400 nm using the Lightning Cure LC8 Spot Light-source apparatus (Hamamatsu) at a power of 4.5 W and a distance of 5 cm. The experiments were carried out at room temperature (20°C) without thermostatic bath so that the electrode could become heated under prolonged illumination.

Photoluminescence and absolute quantum yields, as well as photostability tests (see section 3 of Supplementary Information), were determined using a Horiba Fluorolog-3 fitted with a Hamamatsu R928 PMT and equipped with an integrating sphere. The absolute quantum yield (Φ) is calculated using the expression: $Φ = [P_c-(1-A)P_b]/(A.L_a)$ where $A=1-(L_c/L_b)$ where $L_a$ represents the total amount of excitation light with an empty sphere, $L_b$ and $P_b$ are the total amount of detected excitation light and emitted light, respectively, with the sample in the integrating sphere but off the excitation beam [56]. $L_c$ and $P_c$ represent the amount of detected excitation light and emitted light, respectively, with the excitation beam hitting the sample in the integrating sphere. For measurement of lifetime, a tuneable nanosecond laser (5 ns, 135 mJ @355 nm from Nd:YAG amplified laser pumping OPO, 10 Hz, Ekspla model NT342B-10-WW); produced tuneable excitation pulses in the range 410-2300 nm. A spectrograph analysed light signals (Princeton Instruments Acton model SP2300) coupled with a high dynamic range streak camera (Hamamatsu C7700). The bi-/triexponential fitting function was employed by the iterative deconvolution method using Origin software for the fitting of lifetime decays. The following fitting function was used $I(t) = I(0)\sum\alpha_i e^{(-t/\tau_i)}$ where $I(t)$ and $I(0)$ are the fluorescent intensity at time t and 0 respectively, $\tau_i$ are the contributing amplitude and its corresponding lifetime.

# 3. Results and discussion

## 3.1. Photophysical characterization of [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF

The [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF investigated here were prepared from three different AzPTES/TEOS ratio (i.e., 2.5/97.5, 5/95 and 7.5/92.5), thus containing increasing contents of redox photocatalyst [52]. As reported in our previous work [52], their UV-visible spectra revealed two absorption bands (one broad centered at ~305 nm and assigned to the π-π* transition of bipyridyl ligand [57], and another one at ca. 445 nm characteristic of the metal-
to-ligand charge transfer, MLCT [57]), in agreement with the observation made for [Ru(bpy)₃]²⁺ accommodated into mesoporous silica particles via electrostatic interactions [24].

A more detailed investigation of the photophysical properties of [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF was performed here for further insight into the excited state properties of VMSF. Exciting the films in their dry state at a wavelength of 450 nm resulted in the emission spectra illustrated in Figure 1. The corrected steady-state emission spectra of the [Ru(bpy)₂(bpy')]²⁺-functionalized films on ITO support revealed an emission maximum at 610 nm, similar to the red emission of the complex in solution. Quantum yield (Q.Y.) calculations yielded values of 0.5%, 1.2%, and 0.6% for materials prepared using AzPTES/TEOS ratios of 2.5/97.5, 5/95, and 7.5/92.5, respectively (see Table 1). An optimal complex loading appeared at the 5/95 ratio, the quantum yield seeming to decrease at higher loading, suggesting that additional quenching occurs at higher complex concentrations.

After continuous irradiation for 1 hour, the [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF films prepared with AzPTES/TEOS ratios of 2.5/97.5, 5/95, and 7.5/92.5 retained 87%, 83%, and 83% of their initial fluorescence intensity, respectively (see Fig. S2A-C in section 3 of Supplementary Information). These films demonstrate significant photostability, comparable to the ruthenium complex in solution (with matched emission intensity), despite the static molecular environment in the films compared to the dynamic behavior of molecules in solution (see Fig. S2D in section 3 of Supplementary Information).

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Emission spectra of [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF (prepared from AzPTES/TEOS ratio of 2.5/97.5, 5/95, and 7.5/92.5), recorded at an excitation wavelength of 450 nm in absence of solvent.</figcaption></figure>
<table><caption>**Table 1.** Absolute quantum yield and lifetime data of [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF prepared from AzPTES/TEOS ratio of 2.5/97.5, 5/95, and 7.5/92.5. (Time resolved decay fitted with two and three exponential fits with lifetime (τ) where α gives fractional contributions).</caption><thead><tr><th>[Ru(bpy)₂(bpy')']²⁺-functionalized VMSF prepared from AzPTES/TEOS ratio</th><th>Q.Y. (%)</th><th>α₁</th><th>τ₁ (μs)</th><th>α₂</th><th>τ₂ (μs)</th><th>α₃</th><th>τ₃ (μs)</th></tr></thead><tbody><tr><td>2.5/97.5</td><td>0.5</td><td>0.84</td><td>0.594</td><td>0.16</td><td>1.33</td><td>-</td><td>-</td></tr><tr><td>5/95</td><td>1.2</td><td>0.59</td><td>0.629</td><td>0.19</td><td>1.35</td><td>0.22</td><td>0.149</td></tr><tr><td>7.5/92.5</td><td>0.6</td><td>0.08</td><td>0.589</td><td>0.01</td><td>1.39</td><td>0.91</td><td>0.159</td></tr></tbody></table>

Figure 2 illustrates the results obtained from lifetime measurement of the [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF prepared with differing AzPTES/TEOS ratios. They showed rather complicated excited-state kinetic decays, which reflects the differing potential local microenvironments (Table 1). Longer lifetime components may result from enhanced oxygen shielding and/or structural rigidity. While the 2.5/97.5 ratio could be satisfactorily fit to a bi-exponential model with a major 594 ns and a minor 1.3 µs component, the higher ratios required three components to be successfully fit and an increasingly dominant short (circa 150 ns) component, suggesting a different and perhaps less uniform arrangement.

<figure><img src="image_4.png" /></figure>
**Figure 2.** Emission of [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF prepared from AzPTES/TEOS ratio of (A) 2.5/97.5, (B) 5/95, and (C) 7.5/92.5 (λexc = 450 nm). Emission recorded on streak camera at 10 µs scale. Respective kinetics of [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF prepared from AzPTES/TEOS ratio of (D) 2.5/97.5, (E) 5/95, and (F) 7.5/92.5 fitted with bi- (D) and triexponential (E & F) decay fit.

**3.2. Photoelectrochemical response of [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF on ITO**

From the above results, it appears that [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF prepared from AzPTES/TEOS ratio of 5/95 should be the optimal one for photoelectrochemical applications, so further experiments will be carried out with this type of film. As shown in Figure 3A, this is also confirmed by preliminary photoelectrochemical experiments evidencing the most intense photocurrents for the film bearing 5% of the photocatalyst (from measurements performed in an aerated aqueous electrolyte medium, where only dissolved oxygen is expected to have a role in the quenching of the excited photocatalyst [58]), in spite of larger [Ru(bpy)₂(bpy')']²⁺ content in VMSF prepared from an AzPTES/TEOS ratio of 7.5/92.5 [52]. As the silica mesostructure is known to be stable at elevated temperature (i.e., under calcination conditions [32], one can reasonably assume that it is also stable over prolonged illumination. Note that the immobilized sensitizer could suffer from heating during illumination, but quite stable photoelectrochemical response can be observed after 50 successive cycles of measurements (see Fig. S3 in Supplementary Information (section 4)). Even if they are covalently bonded to an insulating silica matrix, these [Ru(bpy)₂(bpy')']²⁺ moieties are indeed electrochemically accessible (as demonstrated for other redox groups immobilized in such oriented silica films [41-45]), and they exhibit a reversible electrochemical response, quite stable upon continuous voltammetric cycling (i.e., 86% of the signal maintained after 150 cycles, see Figure 3B). This is much better than [Ru(bpy)₃]²⁺-impregnated mesoporous silica film (i.e., only 50% of the signal intensity was kept after 15 cycles and peak currents dropped to 20% after 150 cycles, see inset in Fig. 3B) due to significant leaching of the probe in the solution. It is thus interesting to evaluate the photoelectrochemical response of such durably immobilized photocatalyst in VMSF.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** (A) Hydrodynamic amperograms recorded in a stirred aqueous solution containing 0.1 M $NaNO_3$, using ITO electrodes modified with $[Ru(bpy)_2(bpy')]^{2+}$-functionalized VMSF (prepared from AzPTES/TEOS ratios of 2.5/97.5, 5/95 and 7.5/92.5, respectively), by applying a potential of -0.1 V and irradiating at 400 nm for 5 s every 10 s period. (B) 150 successive cyclic voltammograms recorded continuously in an aerated 0.1 M $NaNO_3$ electrolyte solution at a potential scan rate of 100 mV s⁻¹, using an ITO electrode modified with $[Ru(bpy)_2(bpy')]^{2+}$-functionalized VMSF (prepared from AzPTES/TEOS ratio of 5/95); the inset represents the same experiment performed with a bare mesoporous silica film previously impregnated with $[Ru(bpy)_3]^{2+}$ species (1 h soaking in 1 mM $[Ru(bpy)_3]Cl_2$).</figcaption></figure>

Typical photoelectrochemical responses of $[Ru(bpy)_2(bpy')]^{2+}$-functionalized VMSF on ITO electrode immersed in $MV^{2+}$ solutions are depicted in Figure 4. The main reaction involves the photochemical generation of an excited state of the photocatalyst in the film ($*[Ru(bpy)_2(bpy')]^{2+}$) and its subsequent reaction with the quencher in solution ($MV^{2+}$) to form $[Ru(bpy)_2(bpy')]^{3+}$ and $MV^{•+}$ (eq. 1), which must be respectively reduced and oxidized afterwards to regenerate both the photocatalyst and the quencher.

$$*[Ru(bpy)_2(bpy')]^{2+} + MV^{2+} \rightarrow [Ru(bpy)_2(bpy')]^{3+} + MV^{•+} \quad (1)$$

For the sake of simplicity, $[Ru(bpy)_2(bpy')]^{2+}$ and $[Ru(bpy)_2(bpy')]^{3+}$ will be respectively written $[Ru]^{2+}$ and $[Ru]^{3+}$ in the following. Almost no net photocurrent was observed in aerated medium because the quencher cannot be regenerated and both products resulting from the photochemical reaction (i.e., $MV^{•+}$ and $[Ru]^{3+}$) are simultaneously and respectively oxidized and reduced at the ITO electrode biased at -0.1 V, leading to a net current close to zero as shown (see red curve in Fig. 4A). By contrast, significant cathodic photocurrents can be
obtained in the presence of oxygen in the medium, stable upon successive steps of switching
light from ‘off’ to ‘on’ (see green curve in Fig. 4A), as a result of effective regeneration of the
quencher via the well-established oxidation of MV•+ by O2 [59]. This reaction path (see bottom
right of Fig. 4A) involving the photoexcitation of [Ru]2+ into *[Ru]2+, the chemical oxidation
of *[Ru]2+ into [Ru]3+ by reaction with MV2+ (thanks to concomitant reaction of MV•+ with O2
to re-form the MV2+ quencher), and the electrochemical reduction of [Ru]3+ back into [Ru]2+,
was previously demonstrated for [Ru(bpy)3]2+ in solution [12] and the present results confirm
that this is also possible for the [Ru(bpy)2(bpy’)]2+ covalently immobilized in VMSF coated
onto an ITO electrode. The process is really catalytic as quite low MV2+ quencher
concentrations are likely to generate quite large photocurrents (e.g., 0.4 µA photocurrent for
[MV2+] = 10-5 M), which increased and tended to level off with increasing the amount of MV2+
in solution (see Fig. 4B). Increasing MV2+ concentration by three orders of magnitude (i.e.,
from 10-5 M to 10-2 M) resulted in less than a 5-fold increase in the photocurrents obtained (i.e.,
1.8 µA compared to 0.4 µA, as shown in Fig. 4B). Note that the electrochemical regeneration
of the ground state [Ru(bpy)2(bpy’)]2+ after oxidative quenching is possible in a wide potential
window (typically ranging from -0.5 to +0.4 V [12]), but an optimal potential of -0.1 V was
applied here to avoid any undesirable reactions that could be encountered at larger
overpotentials involving redox processes of intermediates (such as O2- or H2O2, for instance)
likely to be formed in the medium.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** (A) Hydrodynamic amperograms recorded in a stirred aqueous solution containing 10 mM MV<sup>2+</sup> (+0.1 M NaNO<sub>3</sub>), using an ITO electrode modified with [Ru(bpy)<sub>2</sub>(bpy')]<sup>2+</sup>- functionalized VMSF (prepared from AzPTES/TEOS ratio of 5/95), by applying a potential of -0.1 V and irradiating at 400 nm for 5 s every 10 s period. Schemes illustrating the expected reactions likely to occur in both deaerated and aerated media are shown on the right part of the figure. (B) Hydrodynamic amperograms (one cycle of irradiation for 5 s at 400 nm) recorded in the same stirred aqueous solution as in (A) in the presence of MV<sup>2+</sup> at various concentrations (from 0.01 to 10 mM). The insert represents the variation of photocurrents as a function of the MV<sup>2+</sup> concentration.</figcaption></figure>

Stronger photoelectrocatalytic responses were observed in the presence of an electron
donor (EDTA) in the medium in addition to MV<sup>2+</sup>, as illustrated in Figure 5. In this case, one
needed to operate in anaerobic conditions (to avoid any involvement of molecular oxygen in
the MV<sup>2+</sup>/MV•+ redox couple [59] or its possible role in the direct quenching of bipyridine
ruthenium(II) complexes [58]), the MV²⁺ quencher regeneration being ensured by the electrochemical oxidation of MV·⁺ at the electrode surface. This reaction is likely facilitated by the ability of VMSF to accumulate MV²⁺ due to favourable electrostatic interactions [60]. As shown, large anodic photocurrents were recorded, which were reproducible during nine sequences of consecutive light and dark cycles, with intensities increasing with the EDTA concentration in solution (see parts A&B in Fig. 5). They originate from the electrochemical oxidation of MV·⁺ species produced by the quenching of the excited *[Ru]²⁺ in the film by MV²⁺ (eq. 1), generating [Ru]³⁺ moieties that are likely to oxidize EDTA while regenerating [Ru]²⁺ [61] (see scheme in part D of Fig. 5). The response time was very fast (i.e., 90% of the maximum photocurrents are reached within the first 0.25 s of illumination), which can be explained by the rapid charge transfer events and mass transport processes in such functionalized silica membranes consisting of nanochannels oriented vertically with respect to the underlying electrode support [33-35,41,42]. Note that one cannot extract more quantitative kinetic information from the electrochemical data as one does not know accurately the composition of the pore channels and it can evolve during the reaction (i.e., accumulation of MV²⁺ [60]). The intensity of photocurrents, plotted as a function of the EDTA concentration (Fig. 5C), were however dramatically dependent on the quencher concentration in the medium (i.e., 1 or 5 mM MV²⁺). This is confirmed by measurements made on the basis of the same EDTA concentration but in a medium containing various concentrations of MV²⁺. As illustrated in Figure 6, the photocurrents associated to the photoelectro catalytic oxidation of 10 mM EDTA were found to increase proportionally to the MV²⁺ concentration. This may appear surprising on the basis of the photoelectrochemical mechanism expected for the system [Ru(bpy)₃]²⁺-MV²⁺-EDTA (Fig. 5D) [12,61-63], indicating a catalytic role of the MV²⁺ redox mediator being continuously regenerated electrochemically. The explanation has to be found in the fact that this electrochemical regeneration happens at the electrode/solution interface where current intensities are governed by the concentration gradient of the electroactive species, being thus limited by the MV²⁺ concentration in solution. So, if one wishes to transform high concentrations of the substrate partner, one would need to add the MV²⁺ quencher at a concentration as high as that of EDTA in order to prevent the electrochemical regeneration from becoming the rate-determining step of the photoelectro catalytic process. Obviously, the presence of MV²⁺ in the medium is mandatory to allow the photoelectro catalytic transformation of the EDTA substrate (Fig. 5D), which is confirmed by a control experiment indicating no effect of EDTA on the photoelectrochemical response of the [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF-ITO electrode in deaerated medium (see Fig. S4A in Supplementary Information,
section 5), even if some photocurrents can exist (independently on the presence of EDTA or not) due to possible contribution of water photolysis [64,65]. A second control experiment performed in the presence of oxygen in the medium (see Fig. S4B in Supplementary Information, section 5) revealed the existence of some cathodic photocurrents originating from the quenching of *[Ru]²⁺ by O₂ [58], but they were almost unaffected by the addition of EDTA in the medium because such O₂ quencher cannot be regenerated electrochemically and EDTA, due to its negative charge, is not likely to cross the negatively-charged silica membrane, contrary to MV²⁺ cations.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** (A,B) Hydrodynamic amperograms recorded in a stirred aqueous solution containing (A) 5 mM or (B) 1 mM MV²⁺ (+0.1 M NaNO₃) and EDTA at various concentrations (from 0.02 to 5 mM in (A) and from 0.1 to 10 mM in (B)), using the same [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF on ITO electrode as in figure 4, by applying a potential of -0.1 V and irradiating at 400 nm for 5 s every 10 s period. (C) Variation of photocurrents as a function of the EDTA concentration, data from (A,B). (D) Scheme illustrating the reactions involved.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** (A) Hydrodynamic amperograms recorded in a stirred aqueous solution containing 10 mM EDTA and MV<sup>2+</sup> at various concentrations (from 0.01 to 10 mM, with 0.1 M NaNO<sub>3</sub> as electrolyte), using the same [Ru(bpy)<sub>2</sub>(bpy')]<sup>2+</sup>-functionalized VMSF on ITO electrode as in figures 4 & 5, by applying a potential of -0.1 V and irradiating at 400 nm for 5 s every 10 s period. (B) Corresponding variation of photocurrents as a function of the MV<sup>2+</sup> concentration.</figcaption></figure>

The driving force of the photoelectrocatalytic process is the oxidation of EDTA by [Ru]<sup>3+</sup> species [65-67] generated during the photochemical event (quenching of *[Ru]<sup>2+</sup> by MV<sup>2+</sup>). But it should be noted that the oxidation of EDTA can also be carried out by electrocatalysis, exploiting the [Ru(bpy)<sub>2</sub>(bpy')]<sup>2+</sup>-functionalized VMSF as long-range charge transfer mediator, as illustrated in Figure 7. Although not yet described to the best of our knowledge, such mediated oxidation of EDTA appears to be similar as that reported for the electrocatalytic oxidation of tri-*n*-propylamine by [Ru(bpy)<sub>3</sub>]<sup>2+</sup>-doped silica materials [68,69]. It is also consistent with the electrocatalytic oxidation of bromide species by [Ru(bpy)<sub>3</sub>]<sup>2+</sup> in solution [70]. As shown here, the addition of EDTA to the medium resulted in a significant increase in the intensity of the anodic peak at +1.2 V (with concomitant disappearance of the cathodic one), and the anodic currents increased in direct proportion to the EDTA concentration (Fig. 7). This is consistent with the mediated electrocatalytic scheme described hereafter (eq. 2 & eq. 3):

$$[\text{Ru(bpy)}_2(\text{bpy'})]^2+ - e^- \rightarrow [\text{Ru(bpy)}_2(\text{bpy'})]^3+ \quad (2)$$

$$[\text{Ru(bpy)}_2(\text{bpy'})]^3+ + \text{EDTA} \rightarrow [\text{Ru(bpy)}_2(\text{bpy'})]^2+ + \text{EDTA}^{+}_{\text{ox}} \quad (3)$$

Overall, EDTA can be oxidized either photoelectrocatalytically (Fig. 5) or electrocatalytically (Fig. 7), but the first approach involving the photochemical generation of an excited triplet
Metal-to-Ligand Charge Transfer state (*Ru[(bpy)₂(bpy')']²⁺) which decreases dramatically the overpotential from +1.2 V to -0.1 V.

<figure><img src="image_9.png" /></figure>

**Figure 7.** Cyclic voltammograms recorded in blank aqueous electrolyte solution (containing only 0.1 M NaNO₃) at a potential scan rate of 20 mV s⁻¹, using an ITO electrode modified with [Ru(bpy)₂(bpy')']²⁺-functionalized VMSF (prepared from AzPTES/TEOS ratio of 5/95), respectively in the absence and the presence of increasing concentrations of EDTA (from 0.3 to 5.1 mM). The insert shows an enlargement of the response without EDTA.
## 4. Conclusions

The present investigation of [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF demonstrates that the covalently-attached photocatalyst retains its photophysical properties and that such films deposited onto an ITO electrode are promising for photoelectrocatalytic applications. From a preliminary photophysical characterization, it was shown that materials prepared using an AzPTES/TEOS ratio of 5/95 resulted in an optimal complex loading leading to the highest quantum yield for the excited state *[Ru(bpy)₂(bpy')]²⁺*. Then, based on this film composition and using a methyl viologen quencher, alone or in the presence of EDTA as electron donor, we pointed out the interest of electrochemistry to regenerate either the immobilized photocatalyst or the quencher. When operating in the absence of EDTA, the system is operating in the reduction mode (i.e., O₂ reduction with the aid of the MV²⁺/MV·⁺ redox couple) and the oxidized [Ru(bpy)₂(bpy')]³⁺ moieties can be reduced electrochemically to regenerate the [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF. In the presence of EDTA (and absence of oxygen), the system is operating in the oxidation mode (i.e., EDTA oxidation by the produced [Ru(bpy)₂(bpy')]³⁺ moieties) and the electrochemical oxidation of intermediate MV·⁺ species allows efficient regeneration of the MV²⁺ quencher. Interestingly, this photoelectrocatalytic approach enables an oxidation of EDTA at much lower potential values than those required for its electrocatalytic oxidation by [Ru(bpy)₂(bpy')]²⁺-functionalized VMSF without illumination.

## Acknowledgements

The authors acknowledge financial supports from the French "Agence Nationale de la Recherche" (public grant N° ANR-21-CE07-044, PHOTOMATON2) and from the "Région Grand-Est" ("Jeune Chercheur" project ELECTROCAT).

## Competing interest statement

The authors have no competing interests to declare.
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