Photoelectrocatalytic Dioxygen Reduction Based on a Novel Thiophene-Functionalized Tricarbonylchloro(1,10-phenanthroline)rhenium(I)

A novel Re (I) complex of [Re(CO)3Cl(L)], {L = 2-([2,2’-bithiophen]-5-yl)-1-phenyl-1H-imidazo [4,5-f][1,10]phenanthroline}, was synthesized, and its optical (UV–Visible absorption and emission spectroscopy), cyclovoltammetric and photoelectrochemical oxygen reduction properties were studied. The geometric and electronic properties were also investigated by density functional theory calculations. It was found that the ITO electrode coated with drop-casted [Re(CO)3Cl(L)] film exhibited cathodic photocurrent generation characteristics. The illuminated film exhibited a maximum cathodic photocurrent up to 30.4 μA/cm2 with an illumination intensity of 100 mW/cm2 white light at a bias potential of −0.4 V vs. SCE in O2-saturated electrolyte solution, which was reduced by 5.1-fold when thoroughly deoxygenated electrolyte solution was used, signaling that the electrode performed well on the photoelectrochemical oxygen reduction. The photo-electrocatalytic hydrogen peroxide production was proved with a maximum H2O2 concentration of 6.39 μM during 5 h of the photoelectrocatalytic process. This work would guide the construction of more efficient rhenium-based photo(electro)catalytic molecular systems for O2 sensing, hydrogen peroxide production and other types of photoelectrochemical energy conversion and storage.


Introduction
The most severe challenges facing humanity in the 21st century may be energy shortages due to our dependence on limited fossil fuel resources [1][2][3]. To address this challenge, there has been significant scientific interest in finding alternative ways to harvest, convert and store energy. Considering the abundant and renewable nature of sunlight, the direct conversion of it into energy stored in chemical bonds is believed to be a particularly promising way to overcome the problem. Recently, the photoelectrocatalytic production of H 2 O 2 has attracted increasing interest in terms of energy and environmental contexts [4][5][6][7][8][9].
It is well known that photo(electro)catalytic performance could be affected by visiblelight absorption, charge separation and interface chemical reaction [7]. In addition, it was reported that an increase in π-conjugation could extend the absorption band into

Fabrication of the Modified Electrode
Indium-Tin oxide (ITO)-coated glass substrates used for the drop-casted films were treated as shown below. First, the ITO substrates were ultrasonicated in detergent for 20 min and then in de-ionized water for 5 min. Second, the thus-treated ITO substrates were soaked in a mixture of 25% NH 3 •H 2 O-30% H 2 O 2 -deionized water (v/v/v, 1/1/5) and boiled at 70 • C for 20 min. Finally, the ITO substrates were thoroughly rinsed with copious deionized water and ethanol and then dried in a blast oven for further use. The ITO electrodes coated with [Re(CO) 3 Cl(L)] were prepared according to the reported method [36] by drop-casting dichloromethane solutions of [Re(CO) 3 Cl(L)] (1.38 µM, if concentrations otherwise stated) onto pre-treated ITO substrates by a microsyringe and drying in an air atmosphere.

Optical, Electrochemical and Photoelectrochemical (PEC) Experiments
Based on the method described by Demas et al. [38], the emission quantum yield (Φ u ) of [Re(CO) 3 Cl(L)] was derived according to Equation (1) by using an air-equilibrated water solution of [Ru(bpy) 3 ] 2+ (bpy = 2,2 -bipyridine) as the standard sample (Φ s = 0.028 [39]): where the subscripts u and s refer to unknown and standard samples, respectively, I is integrated emission intensities, A (<0.1) is the solution absorbances at the excitation wavelength and n denotes the refractive indices of the solvents. Cyclic voltammetry and PEC measurements were completed at room temperature using a CHI601 electrochemical analyzer. Cyclic voltammetry experiments were performed employing an ITO, an Ag wire and a Pt disk as the working electrode, the pseudo-reference electrode and the counter electrode, respectively, in a CH 2 Cl 2 solution of [Re(CO) 3 Cl(L)] with 0.1 M Bu 4 NPF 6 as the supporting electrolyte. The PEC measurements were performed using a [Re(CO) 3 Cl(L)] drop-casted ITO electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and the Pt plate electrode as the counter electrode, which were immersed in an Na 2 SO 4 aqueous solution (0.1 M). The working electrode that was placed at a position of an approximately 15 cm distance between the film surface and a 500 W xenon lamp (Changtuo Co. Ltd., Beijing, China) was irradiated with an effective area of 0.28 cm 2 by a white light produced from the xenon lamp light source equipped with an infrared cut-off filter (730 nm > λ > 325 nm). The light intensities were measured using an ST-900M photometer (Photoelectric Instrument Factory, Beijing Normal University). The uncertainties for the photocurrents/photocurrent densities reported were estimated to be less than 15%. The H 2 O 2 produced in PEC cells was quantified by means of the spectrophotometric molybdenum-triiodide method [40].

Computational Details
The ground state geometry optimization of the [Re(CO) 3 Cl(L)] monomer and its dimer was performed with the Gaussian 09 [41], using a density functional theory (DFT)-based method with the hybrid B3LYP functional [42,43]. In addition, the LanL2DZ basis set was adopted for the complexed rhenium metal [44,45], while the 6-31G(d) basis set was applied for C, H, N, Cl and O [46,47]. The optimized structure was confirmed through vibration analysis, and the frontier orbital distribution was explained using Löwdin population analysis [48].

Synthesis and Characterization
L and its Re(I) complex of [Re(CO) 3 Cl(L)] were synthesized according to the routes shown in Scheme 1. The ligand L was synthesized according to a modified method we published before [23]. [Re(CO) 3 Cl(L)] was synthesized as previously reported [49] by reacting  5 Cl] and L in toluene, and it was easily purified by a simple washing with toluene, followed by silica gel column chromatography with dichloromethane-ethyl acetate (v/v, 4:1) as an eluent. [Re(CO) 3 Cl(L)] was characterized by elemental analysis and 1 H NMR (Supporting Information, Figure S1), infrared ( Figure S2) and mass spectroscopies. The experimental values of the C, H, N elemental analysis match their theoretical values. The integrated areas of the proton resonance peaks in the 1 H NMR spectra of both L and [Re(CO) 3 Cl(L)] correspond to 16 protons, which are consistent with the theoretical prediction. The chemical shifts of the protons adjacent to two coordinating nitrogen atoms of 1,10-phenanthroline in [Re(CO) 3 Cl(L)] were observed to experience more evident shifts relative to those in L. In addition, the corresponding protons at the other positions of L and [Re(CO) 3 Cl(L)] did not show significant differences in their chemical shifts. The positive ion electrospray ionization mass spectrum of the complex showed a strong molecular ion peak centered at m/z = 766.995, which is attributable to [M + H + ] + , with a calculated m/z value of 766.991. The IR spectrum ( Figure S2) showed three strong absorption peaks at 2016, 1922 and 1887 cm −1 , which were attributed to the stretching vibrations of three carbonyl groups.
shown in Scheme 1. The ligand L was synthesized according to a modified method we published before [23]. [Re(CO)3Cl(L)] was synthesized as previously reported [49] by reacting equimolar [Re(CO)5Cl] and L in toluene, and it was easily purified by a simple washing with toluene, followed by silica gel column chromatography with dichloromethane-ethyl acetate (V/V, 4:1) as an eluent. [Re(CO)3Cl(L)] was characterized by elemental analysis and 1 H NMR (Supporting Information, Figure S1), infrared ( Figure S2) and mass spectroscopies. The experimental values of the C, H, N elemental analysis match their theoretical values. The integrated areas of the proton resonance peaks in the 1 H NMR spectra of both L and [Re(CO)3Cl(L)] correspond to 16 protons, which are consistent with the theoretical prediction. The chemical shifts of the protons adjacent to two coordinating nitrogen atoms of 1,10-phenanthroline in [Re(CO)3Cl(L)] were observed to experience more evident shifts relative to those in L. In addition, the corresponding protons at the other positions of L and [Re(CO)3Cl(L)] did not show significant differences in their chemical shifts. The positive ion electrospray ionization mass spectrum of the complex showed a strong molecular ion peak centered at m/z = 766.995, which is attributable to [M + H + ] + , with a calculated m/z value of 766.991. The IR spectrum ( Figure S2) showed three strong absorption peaks at 2016, 1922 and 1887 cm −1 , which were attributed to the stretching vibrations of three carbonyl groups.

Optical Properties
The UV-vis absorption and photoluminescence spectra of L and [Re(CO)3Cl(L)] in dichloromethane (DCM) and the drop-casted film on the indium-tin oxide (ITO)-coated glass substrate are compared in Figure 1. As shown in Figure 1a, [Re(CO)3Cl(L)] exhibited a metal-to-ligand charge-transfer (MLCT) absorption shoulder band [50,51] centered at ~450 nm and an intraligand π-π* absorption band centered at 380 nm, which is bathochromically shifted by 10 nm as compared to the π-π* absorption band at 370 nm observed for L [23]. In addition, the most prominent characteristic of the film is the appearance of a new absorption band centered at 600 nm, in addition to the feature that the π-π* absorption is red-shifted and significantly broadened with respect to the spectrum of [Re(CO)3Cl(L)] in DCM, indicating the formation of J-aggregates in the film [52]. Based on the onset absorption at 488 nm for [Re(CO)3Cl(L)] in DCM, the optical gap of the complex was roughly estimated to be 2.54 eV.
The photoluminescence excitation and emission spectra of [Re(CO)3Cl(L)] in DCM are shown in Figure 1b. The excitation spectrum (λem = 500 nm) showed a distinct band at 400 nm with a feature close to that of the UV-vis absorption spectrum, indicating that the photoluminescence originated from the excitation of [Re(CO)3Cl(L)] rather than impurity. Upon excitation at 400 nm, [Re(CO)3Cl(L)] in DCM displayed a broad and structureless

Optical Properties
The UV-vis absorption and photoluminescence spectra of L and [Re(CO) 3 Cl(L)] in dichloromethane (DCM) and the drop-casted film on the indium-tin oxide (ITO)-coated glass substrate are compared in Figure 1. As shown in Figure 1a, [Re(CO) 3 Cl(L)] exhibited a metal-to-ligand charge-transfer (MLCT) absorption shoulder band [50,51] centered at 450 nm and an intraligand π-π* absorption band centered at 380 nm, which is bathochromically shifted by 10 nm as compared to the π-π* absorption band at 370 nm observed for L [23]. In addition, the most prominent characteristic of the film is the appearance of a new absorption band centered at 600 nm, in addition to the feature that the π-π* absorption is red-shifted and significantly broadened with respect to the spectrum of [Re(CO) 3 Cl(L)] in DCM, indicating the formation of J-aggregates in the film [52]. Based on the onset absorption at 488 nm for [Re(CO) 3 Cl(L)] in DCM, the optical gap of the complex was roughly estimated to be 2.54 eV.
The photoluminescence excitation and emission spectra of [Re(CO) 3 Cl(L)] in DCM are shown in Figure 1b. The excitation spectrum (λ em = 500 nm) showed a distinct band at 400 nm with a feature close to that of the UV-vis absorption spectrum, indicating that the photoluminescence originated from the excitation of [Re(CO) 3 Cl(L)] rather than impurity. Upon excitation at 400 nm, [Re(CO) 3 Cl(L)] in DCM displayed a broad and structureless emission band at 500 nm, which could be ascribed to the luminescence from the 3 MLCT excited state [53][54][55]. Molecules 2023, 28, x FOR PEER REVIEW 5 of 16 emission band at 500 nm, which could be ascribed to the luminescence from the 3 MLCT excited state [53][54][55]. In order to further shed light on the aggregation behavior of [Re(CO)3Cl(L)], we explored the concentration-dependent UV-vis absorption and photoluminescence spectra. As shown in Figure 2, upon increasing the concentrations of [Re(CO)3Cl(L)] from 1.38 to 10 μM, a new shoulder at ~450 nm evolved, the values of full width at the half of the maximum became typically higher and the molar extinction coefficient ε increased with increasing concentrations up to a maximum ε of 5.24 M −1 cm −1 at a concentration of ~10 μM (see Figure 2c). This ε value was little affected when concentrations further increased. The above observation revealed the successive formation of aggregates in the solutions with concentrations between 1.38 and 10 μM.  In order to further shed light on the aggregation behavior of [Re(CO) 3 Cl(L)], we explored the concentration-dependent UV-vis absorption and photoluminescence spectra. As shown in Figure 2, upon increasing the concentrations of [Re(CO) 3 Cl(L)] from 1.38 to 10 µM, a new shoulder at~450 nm evolved, the values of full width at the half of the maximum became typically higher and the molar extinction coefficient ε increased with increasing concentrations up to a maximum ε of 5.24 M −1 cm −1 at a concentration of~10 µM (see Figure 2c). This ε value was little affected when concentrations further increased. The above observation revealed the successive formation of aggregates in the solutions with concentrations between 1.38 and 10 µM. emission band at 500 nm, which could be ascribed to the luminescence from the 3 MLCT excited state [53][54][55]. In order to further shed light on the aggregation behavior of [Re(CO)3Cl(L)], we explored the concentration-dependent UV-vis absorption and photoluminescence spectra. As shown in Figure 2, upon increasing the concentrations of [Re(CO)3Cl(L)] from 1.38 to 10 μM, a new shoulder at ~450 nm evolved, the values of full width at the half of the maximum became typically higher and the molar extinction coefficient ε increased with increasing concentrations up to a maximum ε of 5.24 M −1 cm −1 at a concentration of ~10 μM (see Figure 2c). This ε value was little affected when concentrations further increased. The above observation revealed the successive formation of aggregates in the solutions with concentrations between 1.38 and 10 μM.  To confirm the results obtained by UV-vis absorption spectroscopy, concentrationdependent excitation and emission spectra of [Re(CO) 3 Cl(L)] in DCM were also measured. As clearly shown in Figure 3a, the excitation spectra show that the excitation intensities of [Re(CO) 3 Cl(L)] continually increased by increasing the solution concentrations. Moreover, significant red-shifts were observed for the excitation maxima in the low-energy region. As seen in Figure 3b, emission spectra also displayed significant changes with the concentration variations, namely, although the emission intensities were observed to increase with increasing concentrations, a new blue-shifted emission peak centered at 454 nm with substantially reduced emission quantum yields from 0.57% for 1.38 µM to 0.10% for 34.5 µM (see Figure 3c) evolved, revealing strong electronic coupling between [Re(CO) 3 Cl(L)] chromophores due to the aggregation formation. This conclusion is in close agreement with that from the absorption spectral analysis. The poorly emissive characteristic of the aggregation state of [Re(CO) 3 Cl(L)] could be a common aggregation behavior, which elicited a rapid non-radiative transition, thus leading to strong emission decay or quenching. To confirm the results obtained by UV-vis absorption spectroscopy, concentrationdependent excitation and emission spectra of [Re(CO)3Cl(L)] in DCM were also measured. As clearly shown in Figure 3a, the excitation spectra show that the excitation intensities of [Re(CO)3Cl(L)] continually increased by increasing the solution concentrations. Moreover, significant red-shifts were observed for the excitation maxima in the low-energy region. As seen in Figure 3b, emission spectra also displayed significant changes with the concentration variations, namely, although the emission intensities were observed to increase with increasing concentrations, a new blue-shifted emission peak centered at 454 nm with substantially reduced emission quantum yields from 0.57% for 1.38 μM to 0.10% for 34.5 μM (see Figure 3c) evolved, revealing strong electronic coupling between [Re(CO)3Cl(L)] chromophores due to the aggregation formation. This conclusion is in close agreement with that from the absorption spectral analysis. The poorly emissive characteristic of the aggregation state of [Re(CO)3Cl(L)] could be a common aggregation behavior, which elicited a rapid non-radiative transition, thus leading to strong emission decay or quenching.

DFT Calculation
Enhancement in the separation of photo-generated electron-hole pairs has been reported to have a great effect on improving the photocatalytic performance of organic semiconductors [55][56][57]. To gain deep insights into the electronic structures and enhanced photoelectrocatalytic activities of [Re(CO)3Cl(L)], DFT calculations were carried out. The optimized structures ( Figure S3 Table S2, along with those abstracted from the crystal structure previously reported for an analogous Re complex [57]. As listed in Table S2, the computed geometric structures were proved to be rather reliable, as evidenced by the comparison of the optimized structural data of [Re(CO)3Cl(L)] with those of an analogous Re complex [57]. It is worth noting

DFT Calculation
Enhancement in the separation of photo-generated electron-hole pairs has been reported to have a great effect on improving the photocatalytic performance of organic semiconductors [55][56][57]. To gain deep insights into the electronic structures and enhanced photoelectrocatalytic activities of [Re(CO) 3 Cl(L)], DFT calculations were carried out. The optimized structures ( Figure S3 Table S2, along with those abstracted from the crystal structure previously reported for an analogous Re complex [57]. As listed in Table S2, the computed geometric structures were proved to be rather reliable, as evidenced by the comparison of the optimized structural data of [Re(CO) 3 Cl(L)] with those of an analogous Re complex [57]. It is worth noting that [Re(CO) 3 Cl(L)] has a more pronounced delocalization of π-electron density than the parent complex of [Re(CO) 3 Cl(phen)]. As expected and previously reported [56,58], the introduction of the bithiophene group to [Re(CO) 3 Cl(phen)] can extend π-conjugation, which could promote the separation of photo-generated charges and enhance the light absorption into the visible spectrum, contributing to enhanced photoelectrocatalytic activity.
For the distribution of frontier molecular orbitals in the monomer and dimer of [Re(CO) 3 Cl(L)] (see Figures S4 and S5), the highest occupied molecular orbitals (HOMOs) are located over the d-orbital of the Re(I) center, proving the fact that the frontier orbital transitions occur mainly from the tricarbonyl rhenium(I) chloride moiety [59], but HOMO is only distributed on one rhenium center for the dimer. Moreover, the lowest unoccupied molecular orbital (LUMO) of the monomer is mainly distributed on the phenanthroline, and there is a partial distribution on the fused imidazole and thiophene rings. On the contrary, its thiophene rings markedly contribute to the LUMO of the dimer. As seen in Obviously, the E g of the dimer became narrower, which was beneficial for the light absorption of photocatalysts and the effective separation of the electron-hole pairs existing in the dimer, further improving photocatalytic/photoelectrocatalytic properties [57,58,60]. This speculation will be supported by the following PEC study.
Molecules 2023, 28, x FOR PEER REVIEW 7 o parent complex of [Re(CO)3Cl(phen)]. As expected and previously reported [56,58], introduction of the bithiophene group to [Re(CO)3Cl(phen)] can extend π-conjugat which could promote the separation of photo-generated charges and enhance the li absorption into the visible spectrum, contributing to enhanced photoelectrocatalytic tivity.
For the distribution of frontier molecular orbitals in the monomer and dime [Re(CO)3Cl(L)] (see Figures S4 and S5), the highest occupied molecular orbitals (HOM are located over the d-orbital of the Re(I) center, proving the fact that the frontier orb transitions occur mainly from the tricarbonyl rhenium(I) chloride moiety [59], but HOM is only distributed on one rhenium center for the dimer. Moreover, the lowest unoccup molecular orbital (LUMO) of the monomer is mainly distributed on the phenanthrol and there is a partial distribution on the fused imidazole and thiophene rings. On contrary, its thiophene rings markedly contribute to the LUMO of the dimer. As seen Obviously, the Eg of the dimer became narrower, which was beneficial for the li absorption of photocatalysts and the effective separation of the electron-hole pairs ex ing in the dimer, further improving photocatalytic/photoelectrocatalytic proper [57,58,60]. This speculation will be supported by the following PEC study.

Electrochemical Properties
The electrochemical properties of [Re(CO)3Cl(L)] were studied by using cyclic v ammetry, and corresponding cyclic voltammograms (CVs) are shown in Figure

Electrochemical Properties
The electrochemical properties of [Re(CO) 3 Cl(L)] were studied by using cyclic voltammetry, and corresponding cyclic voltammograms (CVs) are shown in Figure 5a. Sweeping to negative potentials at 0.1 V s −1 yielded two reduction peaks with peak potentials at −0.95 and −1.46 V vs. Ag. The first reduction pointed convincingly to the oxidation of [Re(CO) 3 Cl(L)] aggregates, which caused the buildup of excess electrolytes in the polymer matrix and the subsequent discharge of these species [61,62]. The second reduction peak 2 was found to be at −1.46 vs. Ag, which is quasi-reversible and ascribed to be a ligand-based reduction with the added electron residing on L, furnishing [Re(CO) 3 Cl(L) . ] − , and it may undergo dimerization by building an Re 0 -Re 0 bond [63,64], as illustrated in following Equation (2): a previous electrochemical study, the first oxidation peak may originate from metal-centered Re(I/II) oxidations [65][66][67], while the second oxidation process was believed to involve the thiophene moieties, inducing their polymerization on the electrode surface [16,68]. Figures 5c,d present potential scan rate-dependent CVs of [Re(CO)3Cl(L)] in nitrogen-saturated DCM solution on positive sweep and negative potential sweep, respectively, with scanning rates ranging from 0.1 V s −1 to 0.5 V s −1 . In all cases, the mentioned maximum oxidation peak current and reduction peak current stayed fully irreversible or reversible with no significant changes when the scan rates were altered. More importantly, the maximum oxidation and reduction peak currents were directly proportional to the square root of the scan rates with R 2 > 0.96 (Figure 5b). This result revealed that a freely diffusing species in the solution obeyed the Randles-Sevcik Equation (3) [3,9,17].
Here, imax is the maximum oxidation peak current or reduction peak current, n is the number of electrons involved in the redox reaction, D represents the diffusion coefficient of the complex, v is the critical scan rates (V s −1 ) and C stands for the concentration of the involved redox species.  When the potentials were scanned to positive potentials, the complex exhibited two ill-defined and irreversible oxidation peaks at +0.2 and +0.83 V, respectively. According to a previous electrochemical study, the first oxidation peak may originate from metal-centered Re(I/II) oxidations [65][66][67], while the second oxidation process was believed to involve the thiophene moieties, inducing their polymerization on the electrode surface [16,68]. Figure 5c,d present potential scan rate-dependent CVs of [Re(CO) 3 Cl(L)] in nitrogensaturated DCM solution on positive sweep and negative potential sweep, respectively, with scanning rates ranging from 0.1 V s −1 to 0.5 V s −1 . In all cases, the mentioned maximum oxidation peak current and reduction peak current stayed fully irreversible or reversible with no significant changes when the scan rates were altered. More importantly, the maximum oxidation and reduction peak currents were directly proportional to the square root of the scan rates with R 2 > 0.96 (Figure 5b). This result revealed that a freely diffusing species in the solution obeyed the Randles-Sevcik Equation (3) [3,9,17].
Here, i max is the maximum oxidation peak current or reduction peak current, n is the number of electrons involved in the redox reaction, D represents the diffusion coefficient of the complex, v is the critical scan rates (V s −1 ) and C stands for the concentration of the involved redox species.

PEC Properties
The photoelectrocatalysis of thin films is one of the most promising techniques for converting solar energy into electrical energy and chemical energy [69][70][71]. It is necessary to take into account a perfect balance between light absorption and charge transport from the active site to the supporting electrode for highly efficient PEC activity. Therefore, the thickness of the film is particularly important for the photoelectrocatalytic performance [15,72]. Thus, we prepared ITO electrodes drop-casted with [Re(CO) 3 Cl(L)] of varying thicknesses. Six different thicknesses of films were prepared by varying the DCM solution concentrations of [Re(CO) 3 Cl(L)] from 1 to 6 mg mL −1 (see Figure 6a). As shown in Figure 6b, all the films with varying thicknesses showed prompt and significant photocurrent responses to the on-off light illumination, implying the good PEC activity of the films. Additionally, the photocurrent saturation point was found to be reached for the film prepared from 4 mg/mL solution. On the contrary, when the casting solution concentrations were greater than or less than 4 mg/mL, the generated photocurrent decreased significantly. Therefore, the film prepared from a [Re(CO) 3 Cl(L)] concentration of 4 mg/mL was used for the following PEC studies. To further study the photocurrent polarity of the [Re(CO) 3 Cl(L)] film, we measured the photocurrent responses of the film with varying applied bias potentials from −0.4 to +1.0 V vs. SCE in N 2 degassed 0.1 M Na 2 SO 4 aqueous solution. As displayed in Figure 7a, photocurrents decreased evidently when the applied bias increased from −0.4 to +0.4 V vs. SCE, demonstrating that the film generated the cathodic photocurrents, and [Re(CO) 3 Cl(L)] acted as a typical p-type semiconductor. When the bias potentials further increased, photocurrents switched to weakly anodic, indicating that the film was of weak rectifying behavior. Moreover, when the photocurrent density of the film attained 0 µA/cm 2 , a bias potential of 0.2 V needed to be applied, implying that the open-circuit photovoltage of the film was +0.2 V. A maximum photocurrent (photocurrent density) of 6.0 µA (21.4 µA/cm 2 ) was observed for the film biased at −0.4 V vs. SCE and immersed in air-equilibrated 0.1 M Na 2 SO 4 solution, which outperformed what we previously reported for an Re(I) complex-based drop-casted film [15]. As shown in Figure 7b, the photocurrent (photocurrent density) was raised to 8.5 µA (30.4 µA/cm 2 ) for [Re(CO) 3 Cl(L)]-coated ITO electrode biased at −0.4 V vs. SCE in oxygen saturated electrolyte solution, which is 6.1 times the photocurrent (photocurrent density) of 1.4 µA (5.0 µA/cm 2 ) observed for that in the nitrogen-saturated electrolyte solution. These results proved that the photocurrents observed were cathodic. It is worth noting that the photocurrent (photocurrent density) observed at zero bias potential was found to be 1.6 µA (5.7 µA/cm 2 ), which compares favorably with those for most of the previously reported thin films [73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90]. Thus, we conclude that the film we studied here may be a highly promising candidate for applications in PEC oxygen reduction or sensing.

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10 of 1 observed were cathodic. It is worth noting that the photocurrent (photocurrent density observed at zero bias potential was found to be 1.6 μA (5.7 μA/cm 2 ), which compares fa vorably with those for most of the previously reported thin films [73][74][75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90]. Thus, we con clude that the film we studied here may be a highly promising candidate for application in PEC oxygen reduction or sensing.  In term of this, PEC experiments adding an electron donor of hydroquinone (H 2 Q) to the electrolyte solution were performed. As seen in Figure 7c, photocurrents were observed to decrease when the H 2 Q concentrations increased, further supporting the generation of the cathodic photocurrents. The result may be interpreted in light of the fact that the addition of the electron donor and acceptor to the electrolyte can inhibit or promote the directional electron movement from the ITO electrode to the electrolyte. Furthermore, we implemented a PEC oxygen reduction experiment with oxygen molecules in the electrolyte as electron acceptors. Photocathodes for the reduction of O 2 to H 2 O 2 have attracted increasing interest because of their potential advantages in terms of cost and safety [5,6,91]. Therefore, we examined whether the [Re(CO) 3 Cl(L)] film electrode could generate H 2 O 2 in the above-mentioned PEC cell with an O 2 -equilibrated Na 2 SO 4 aqueous solution under a bias potential of −0.4 V vs. SCE. The PEC production of H 2 O 2 was confirmed using the spectrophotometric molybdenum-triiodide method [92] (see Figure 8). Figure 8a presents changes in the UV-vis absorption spectra of the molybdenum-iodide indicator solution, in which small aliquots of the PEC cell solutions were successively added. During 5 h photoelectrocatalysis, an absorption band with λ max at 350 nm was detected, and the band intensities were found to increase with an increasing photoelectrocatalysis duration, indicating the formation of H 2 O 2 in the PEC cell solution. The H 2 O 2 concentrations were quantified according to the working curves, as shown in the Figure S8 insets. As shown in Figure 8b, a maximum H 2 O 2 concentration of 6.39 µM was obtained after 5 h photoelectrocatalysis. Notably, the H 2 O 2 concentration almost linearly increased over 5 h, implying that the film was robust against peeling off the electrode surface for at least 5 h. It is also important to note from Figure S9 that the UV-vis absorption spectra for [Re(CO) 3 Cl(L)] based ITO electrode before and after 5 h photoelectrocatalysis did not show obvious changes, indicating that [Re(CO) 3 Cl(L)] was of good PEC stability. The H 2 O 2 production by photoelectrocatalysis, electrocatalysis and photocatalysis was also comparatively studied under similar experimental conditions. As shown in Figure 9, all three methods showed a positive hydrogen peroxide production, but the PEC production of H 2 O 2 was much more efficient than the addition of the photocatalysis and the electrocatalysis, indicating that photoand electro-catalysis synergistically enhanced the production of H 2 O 2 . The above results indicate that [Re(CO) 3 Cl(L)]-based film electrode has the advantages of simple production, high stability and a low cost and serves as an interesting parent complex for developing more efficient photoelectrocatalysts for O 2 reduction or sensing or photoelectrocatalytic electrodes for H 2 O 2 production.

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indicating the formation of H2O2 in the PEC cell solution. The H2O2 concentrations were quantified according to the working curves, as shown in the Figure S8 insets. As shown in Figure 8b, a maximum H2O2 concentration of 6.39 μM was obtained after 5 h photoelectrocatalysis. Notably, the H2O2 concentration almost linearly increased over 5 h, implying that the film was robust against peeling off the electrode surface for at least 5 h. It is also important to note from Figure S9 that the UV-vis absorption spectra for [Re(CO)3Cl(L)] based ITO electrode before and after 5 h photoelectrocatalysis did not show obvious changes, indicating that [Re(CO)3Cl(L)] was of good PEC stability. The H2O2 production by photoelectrocatalysis, electrocatalysis and photocatalysis was also comparatively studied under similar experimental conditions. As shown in Figure 9, all three methods showed a positive hydrogen peroxide production, but the PEC production of H2O2 was much more efficient than the addition of the photocatalysis and the electrocatalysis, indicating that photo-and electro-catalysis synergistically enhanced the production of H2O2. The above results indicate that [Re(CO)3Cl(L)]-based film electrode has the advantages of simple production, high stability and a low cost and serves as an interesting parent complex for developing more efficient photoelectrocatalysts for O2 reduction or sensing or photoelectrocatalytic electrodes for H2O2 production.    Based on previous reports [1,93] and our current results [23,94], the mechan cathodic photocurrent generation and H2O2 synthesis is postulated as follows (Figu light irradiation of the rhenium complex excited the electron from HOMO into LU Based on previous reports [1,93] and our current results [23,94], the mechanism of cathodic photocurrent generation and H 2 O 2 synthesis is postulated as follows ( Figure 10 Molecules 2023, 28, x FOR PEER REVIEW 12 form the excited state complex, which further transferred the excited electrons to elec acceptors (O2) in the electrolyte solution; the oxidized [Re(CO)3Cl(L)] was regenerate accepting the electron from the conduction band of ITO. In this case, the donor presen the electrolyte would be certainly unfavorable for the cathode photocurrent genera The combination of photoelectrochemically generated O2⦁ − with protons in the electro solution yielded H2O2 (see Equations (4) and (5)).

Conclusions
In summary, a novel Re(I)-phenanthroline complex with a bithiophene group, [Re(CO) 3

Conclusions
In summary, a novel Re(I) complex with a bithiophene grou throline derivative ligand, Re(CO)3ClL, is synthesized and demon ous aggregation behavior in DCM solutions at high concentration films. The experimental results and DFT calculations indicated t bithiophene group resulted in enhanced conjugation. The PEC cell casted film modified ITO electrode as working electrode under con tial of −0.4 V vs. SCE, and N2and O2-saturated electrolyte solution cathodic photocurrent density of 5.0 and 30.4 μA/cm 2 , respective the PECs possessed an intriguing photoelectrocatalytic O2 reduct portantly, the photoelectrocatalytic O2 reduction product was evid a maximum [H2O2] of 6.39 μM was achieved after 5 h of photoelect would guide the design of more efficient Re complex-based film tions in PEC O2 sensing and H2O2 production, related energy stora electrochemiluminescence [94].

Conclusions
In summary, a novel Re(I) complex with a bithiophene grou throline derivative ligand, Re(CO)3ClL, is synthesized and demon ous aggregation behavior in DCM solutions at high concentration films. The experimental results and DFT calculations indicated t bithiophene group resulted in enhanced conjugation. The PEC cell casted film modified ITO electrode as working electrode under con tial of −0.4 V vs. SCE, and N2and O2-saturated electrolyte solution cathodic photocurrent density of 5.0 and 30.4 μA/cm 2 , respective the PECs possessed an intriguing photoelectrocatalytic O2 reduct portantly, the photoelectrocatalytic O2 reduction product was evi a maximum [H2O2] of 6.39 μM was achieved after 5 h of photoelec would guide the design of more efficient Re complex-based film tions in PEC O2 sensing and H2O2 production, related energy stora electrochemiluminescence [94].

Conclusions
In summary, a novel Re(I) complex with a bithiophene group-grafted 1,10-phenanthroline derivative ligand, Re(CO)3ClL, is synthesized and demonstrated to exhibit obvious aggregation behavior in DCM solutions at high concentrations and in its drop-casted films. The experimental results and DFT calculations indicated that introduction of the bithiophene group resulted in enhanced conjugation. The PEC cell with Re complex dropcasted film modified ITO electrode as working electrode under conditions of a bias potential of −0.4 V vs. SCE, and N2and O2-saturated electrolyte solution, exhibited a maximum cathodic photocurrent density of 5.0 and 30.4 μA/cm 2 , respectively, demonstrating that the PECs possessed an intriguing photoelectrocatalytic O2 reduction property. More importantly, the photoelectrocatalytic O2 reduction product was evidenced to be H2O2, and a maximum [H2O2] of 6.39 μM was achieved after 5 h of photoelectrocatalysis. The results would guide the design of more efficient Re complex-based film electrodes for applications in PEC O2 sensing and H2O2 production, related energy storage and conversion and electrochemiluminescence [94].
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1. Figure 3 ClL-monomer (b) and Re(CO) 3 ClLdimer (c) from DFT calculations. Cl green, Re cyan, O red, C gray, N blue, S yellow, H white; Figure S4: The distribution of MOs of Re(CO) 3 ClL-monomer. Cl green, Re cyan, O red, C gray, N blue, S yellow, H white; Figure S5: The distribution of MOs of Re(CO) 3 ClL-dimer. Cl green, Re cyan, O red, C gray, N blue, S yellow, H white; Figure S6: Partial molecular orbital energy levels of Re(CO) 3 ClL-monomer calculated by DFT; Figure S7: Partial molecular orbital energy levels of Re(CO) 3 ClL-dimer calculated by DFT; Figure S8 Figure S9: Normalized UV-Vis absorption spectra of a CH 2 Cl 2 solution (blue line), and drop-coated film of Re(CO) 3 ClL before (black line) and after (red line) photoelectrocatalysis for 5 h; Table S1: Computational energy levels of Re(CO) 3 ClL-monomer; Table S2: Comparison of computational selected bond lengths (Å), bond angles ( • ), and dihedral angels ( • ) of Re(CO) 3 ClLwith the atomic labelling scheme (left) and the molecular structure (right) shown below this table using the DFT-B3LYP at the LanL2DZ level with those of crystal structure of fac-[ReBr(CO) 3 (L 3 )], the Ref crystal, which was reported in text Ref. [57].

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.