Ruthenium Picolinate Complex as a Redox Photosensitizer With Wide-Band Absorption

Ruthenium(II) picolinate complex, [Ru(dmb)₂(pic)]⁺ (Ru(pic); dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N^∧N)₃]²⁺ (N^∧N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λ_abs < 670 nm) and stronger reduction ability in a one-electron reduced state ($E_{1/2}^{\text{red}}$ = −1.86 V vs. Ag/AgNO₃), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO₂ to CO under red-light irradiation (λ_ex > 600 nm). TON_CO reached 235 and Φ_CO was 8.0%. Under these conditions, [Ru(dmb)₃]²⁺ (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λ_ex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOF_CO = 6.7 min⁻¹) and larger TON_CO (2347) than Ru(dmb) (TOF_CO = 3.6 min⁻¹; TON_CO = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption.

Introduction

Redox photosensitizers, which absorb visible light and facilitate the electron transfer process, play a key role in various photochemical reactions, such as CO₂ reduction (Takeda et al., 2017; Tamaki and Ishitani, 2017), water oxidation (Fukuzumi et al., 2016), hydrogen evolution (Schulz et al., 2012), and organic synthesis (Prier et al., 2013). Effective photosensitizers should be endowed with three important properties, including (1) visible-light absorption, (2) a long lifetime in the excited state to initiate the electron transfer process, and (3) reducing and/or oxidizing power that is strong enough to donate electrons or holes to the catalyst. In particular, the utilization of visible-light over a wider range of wavelengths is important both to utilize sunlight efficiently and avoid the internal filter effect and side reactions that are commonly caused by the light-absorption of catalysts and/or electron donor/acceptor. Ru(II) complexes coordinated with three diimine ligands, [Ru(N^∧N)₃]²⁺ (N^∧N = diimine ligand) are the most widely used redox photosensitizers in various photochemical redox reactions because these types of complexes exhibit strong absorption in the visible-light region and have a long lifetime in their triplet metal-to-ligand charge-transfer (³MLCT) excited states (Juris et al., 1988; Thompson et al., 2013).

However, one of the disadvantages of [Ru(N^∧N)₃]²⁺-type photosensitizers is the limited access to the wavelength region of visible light, e.g., λ_abs < 560 nm in the cases of N^∧N = 2,2′-bipyridine (bpy) and 4,4′-dimethyl-2,2′-bipyridine (dmb), and these complexes cannot utilize visible light having lower energy (λ > 560 nm). To overcome this, ligand-modified Ru(II) photosensitizers have been reported. For example, Ru(II) complexes have an extended π-system for photodynamic therapy (Zhang et al., 2017) and multinuclear Ru(II) complexes by conjugated bridging ligand are used for hydrogen evolution (Tsuji et al., 2018). However, these modifications lower the reducing power of photosensitizers and limit the choice of catalyst especially for the reduction of CO₂. On the other hand, we have reported an osmium(II) analog, i.e., [Os(N^∧N)₃]²⁺, which could function as a redox photosensitizer utilizing a much wider wavelength range of visible light (λ_abs < 700 nm) due to its singlet-to-triplet direct excitation (S-T absorption) and drive photocatalytic CO₂ reduction by red-light irradiation (λ_ex > 620 nm) in the combination with rhenium(I) catalyst unit (Tamaki et al., 2013b), whereas the high toxicity of Os^VIIIO₄ inhibits the wider application of osmium complexes.

Therefore, we developed a novel ruthenium(II) redox photosensitizer that can utilize a wider wavelength range of visible light than [Ru(N^∧N)₃]²⁺. In the photocatalytic system for CO₂ reduction, a photosensitizer mediates an electron from a sacrificial electron donor to a catalyst. Since the positive shift of the LUMO level of redox photosensitizer should limit the choice of a catalyst for reducing CO₂, for the expansion of the useable wavelength range, we try to decrease the energy-gap between HOMO and LUMO by the negative shift of the HOMO level, while maintaining the LUMO level. We introduced anionic electron-donating picolinate instead of a diimine ligand into a ruthenium complex (Norrby et al., 1997; Couchman et al., 1998). [Ru(dmb)₂(pic)]⁺ (Ru(pic); Hpic = picolinic acid) was synthesized, and we investigated its photophysical properties and functions as a redox photosensitizer using [Ru(dmb)₃]²⁺ (Ru(dmb)) as a reference redox photosensitizer and Re(dmb)(CO)₃Br (Re) as a catalyst for the reduction of CO₂ (Hawecker et al., 1983; Gholamkhass et al., 2005; Tamaki et al., 2016). Chart 1 shows structures and abbreviations of the metal complexes used.

CHART 1

Chart 1. Structures and abbreviations of complexes used.

Results and Discussion

Figure 1 displays UV-vis absorption spectra of Ru(pic), Ru(dmb), and Re measured in N,N-dimethylacetamide (DMA). Ru(pic) exhibited a broad singlet MLCT absorption band at λ_abs = 450–640 nm, with molar absorptivity at an absorption maximum (λ_max = 498 nm) of 1.04 × 10⁴ M⁻¹cm⁻¹, which was red-shifted in wavelength compared to that of Ru(dmb) (λ_abs = 420–550 nm). The absorption band attributed to the π-π* transition of dmb ligands was observed at 294 nm. According to this result, Ru(pic) have the potential to utilize visible light over a wider range of wavelengths (λ_abs < 670 nm) than Ru(dmb) (λ_abs < 560 nm). This expected red-shift of the MLCT band should be induced by the stronger electron-donating ability of the picolinate ligand to negatively shift the energy level of HOMO.

FIGURE 1

Figure 1. UV-vis absorption spectra of Ru(pic) (green line), Ru(dmb) (red line), and Re (broken line) measured in a DMA solution.

Ru(pic) exhibited phosphorescence from its ³MLCT excited state (Figure 2) with a quantum yield of Φ_em = 0.8% and a lifetime of τ_em = 66 ns. Emission spectrum of Ru(pic) (λ_em = 734 nm) was also red-shifted compared to that of Ru(dmb) (λ_em = 638 nm). The quantum yield and lifetime of Ru(pic) were smaller and shorter than those of Ru(dmb) (Φ_em = 9.1%, τ_em = 741 ns) due to the 12-times faster non-radiative deactivation process (Ru(pic): k_nr = 1.5 × 10⁷ s⁻¹; Ru(dmb): k_nr = 1.2 × 10⁶ s⁻¹), which is a reasonable behavior from energy-gap law. Table 1 summarizes photophysical properties of Ru(pic) along with those of Ru(dmb) and Re.

FIGURE 2

Figure 2. Normalized emission spectra of Ru(pic) (green line) and Ru(dmb) (red line) measured in a DMA solution. The excitation wavelength was 480 nm.

TABLE 1

Table 1. Photophysical properties of Ru(pic), Ru(dmb), and Re.^a.

Figure 3 shows the cyclic voltammograms of Ru(pic) and Ru(dmb) and their redox potentials are summarized in Table 2 along with that of Re. Ru(pic) displayed two reversible reduction waves and a reversible oxidation wave, which are attributable to the subsequent reduction of two dmb ligands and the oxidation couple of Ru^III/II, respectively. Both the first reduction ($E_{1/2}^{\text{red}}$ = −1.86 V vs. Ag/AgNO₃) and oxidation ($E_{1/2}^{0\text{x}}$ = 0.41 V) waves were observed at more negative potentials than those of Ru(dmb) ($E_{1/2}^{\text{red}}$ = −1.74 V and $E_{1/2}^{0\text{x}}$ = 0.77 V), which should be induced by the stronger electron-donating ability of the picolinate ligand. The stronger reducing power of one-electron reduced species (OERS) of Ru(pic) ($E_{1/2}^{\text{red}}$ = −1.86 V) facilitates an increase in the number of choices of applicable catalyst because the electron transfer from OERS of Ru(pic) to a catalyst must occur during photocatalysis in the case of reductive quenching mechanisms. When using Ru(pic) as a photosensitizer and Re as a catalyst, the electron transfer process from OERS of Ru(pic) to Re ($E_{1/2}^{\text{red}}$ = −1.76 V) occurs exothermically.

FIGURE 3

Figure 3. Cyclic voltammograms of Ru(pic) and Ru(dmb) measured in a DMA solution containing Et₄NBF₄ (0.1 M) as a supporting electrolyte with a Ag/AgNO₃ (10 mM) reference electrode.

TABLE 2

Table 2. Electrochemical properties of the metal complexes in DMA^a.

These results indicated that Ru(pic) had some advantages with respect to its function as a redox photosensitizer compared with Ru(dmb), including its wider wavelength range of visible-light absorption and stronger reducing power of OERS, which is effective in the electron transfer to the catalyst. However, certain unfavorable properties were also observed, i.e., a shorter lifetime (τ_em = 66 ns) and weaker oxidizing power in its excited state (ΔE = E(Ru(dmb)*/Ru(dmb)⁻)–E(Ru(pic)*/Ru(pic)⁻) = 0.28–(−0.11) = 0.39 V). In the reductive quenching process, an excited photosensitizer accepts an electron from a sacrificial electron donor. Weaker oxidation power in the excited state of a photosensitizer should decrease the driving force of this electron transfer process. In addition, since this process competes with the radiative and non-radiative deactivation processes from the excited state of a photosensitizer by itself, the shorter lifetime results in less opportunity of the reductive quenching process to occur. To evaluate whether reductive quenching occurs, the emission intensity from Ru(pic) was compared in the presence of five different concentrations of a sacrificial electron donor, 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) (Tamaki et al., 2013a; Hasegawa et al., 2015) in DMA-triethanoamine (TEOA; 5:1 v/v). As shown in Figure 4, the emission intensities from the ³MLCT excited state of Ru(pic) decreased at higher concentrations of BIH, which indicated that the excited Ru(pic) was quenched by BIH. The quenching rate constant was determined to be k_q = 1.7 × 10⁸ M⁻¹s⁻¹ from the Stern-Volmer plot (Figure S1) and the lifetime of the emission (τ_em = 66 ns), which was 8-times slower than that of Ru(dmb) (k_q = 1.4 × 10⁹ M⁻¹s⁻¹) as expected from the weaker oxidizing power in the ³MLCT excited state of Ru(pic). In the photocatalytic reaction condition, i.e., [BIH] = 0.2 M, 69% of the excited Ru(pic) was estimated to be quenched by BIH, which should be enough to initiate a photocatalytic reaction.

FIGURE 4

Figure 4. Emission spectra of Ru(pic) in Ar-saturated DMA-TEOA (5:1 v/v) containing five different concentrations of BIH (0–24 mm).

To clarify the produced species as a result of the quenching of excited Ru(pic) by BIH, UV-vis absorption spectral change was observed during photo-irradiation of Ru(pic) in the presence of BIH (Figure 5). Irradiation by light at λ_ex = 480 nm caused spectral changes and new absorption bands appeared at λ_abs = 420 and 547 nm. The shape of differential absorption spectra before and after irradiation (Figure 5B) were quite similar to that of OERS of Ru(pic) obtained by electrochemical spectroscopy (Figure S2). These results indicate that the reductive quenching of the ³MLCT excited state of Ru(pic) by BIH proceeded successfully to give OERS of Ru(pic) (Equation 1) and Ru(pic) can be expected to function as a redox photosensitizer over the wide-range absorption of visible light.

FIGURE 5

Figure 5. UV-vis (A) absorption and (B) differential absorption spectral change of a DMA-TEOA (5:1 v/v, 4 mL) solution containing Ru(pic) (0.1 mM) and BIH (0.2 M) during irradiation using light at λ_ex = 480 nm (0–8 min at 1-min intervals). The incident light intensity was 5.0 × 10⁻⁹ einstein·s⁻¹. Blue and red lines represent spectra at 0 and 8-min irradiation, respectively.

The results of photocatalytic reactions for the reduction of CO₂ are summarized in Table 3. In a typical run of photocatalytic reactions, a mixed solution of DMA-TEOA (5:1 v/v) containing Ru(pic) (50 μM), Re (50 μM), and BIH (0.2 M) as a sacrificial electron donor was irradiated under a CO₂ atmosphere using light at λ_ex > 620 nm. CO production proceeded linearly and selectively and the turnover number for CO production (TON_CO) was 235 after 36 h of irradiation (Figure 6A). The quantum yield for CO formation (Φ_CO) was determined to be Φ_CO = 8% using λ_ex = 600-nm light (light intensity: 6.0 × 10⁻⁹ einstein·s⁻¹). By contrast, when using Ru(dmb) as a redox photosensitizer instead of Ru(pic), no photocatalysis proceeded (Figure 6A) because Ru(dmb) does not absorb lower-energy light at λ_ex > 620 nm (Figure 1). To compare the function as a redox photosensitizer, the photocatalytic reactions were also conducted under photo-irradiation condition, where both Ru(pic) and Ru(dmb) absorb incident light (λ_ex > 480 nm). In this condition, both systems photocatalytically produced CO with high selectivity. Figure 6B shows the time course of photocatalytic CO production using light at λ_ex > 500 nm, and the system using Ru(pic) formed CO faster (TOF_CO = 6.7 min⁻¹) than Ru(dmb) (TOF_CO = 3.6 min⁻¹) in the initial stage of photocatalysis. TON_CO reached 2347 and 2100 after 36 h of irradiation using Ru(pic) and Ru(dmb), respectively. The values of Φ_CO using light at λ_ex = 480 nm (light intensity: 6.0 × 10⁻⁹ einstein·s⁻¹) were 10% and 44% in the cases using Ru(pic) and Ru(dmb), respectively. The Ru(pic) system demonstrated similar Φ_CO values in both irradiation conditions (λ_ex = 600 and 480 nm). These results indicated that Ru(pic) has a clear advantage of a wider wavelength range of utilizable visible light compared to Ru(dmb), even for the photocatalytic condition of λ_ex > 480 nm. Since Ru(pic) displays larger molar absorptivity in the λ_abs > 480-nm region and a wider wavelength range than Ru(dmb) (Figure 1), Ru(pic) absorbs a much larger number of photons at λ_ex > 480-nm, which leads to a faster TOF_CO and larger TON_CO, even though the quantum yields for CO production were lower.

TABLE 3

Table 3. Photocatalytic properties using the mixed system of the Ru(II) photosensitizer and Re^a.

FIGURE 6

Figure 6. Photocatalytic production of CO as a function of irradiation time using Ru(pic) () or Ru(dmb) () as a photosensitizer: CO₂-saturated DMA-TEOA (5:1 v/v, 2 mL) solutions containing Ru(II) photosensitizer (50 μM), Re (50 μM), and BIH (0.2 M) were irradiated at (A) λ_ex > 620 nm or (B) λ_ex > 500 nm.

The quantitative analyses of BIH and its oxidized compound during photocatalysis were conducted in the system using 0.1 M of BIH to simplify the HPLC analyses. As the only oxidized compound of BIH, two-electron oxidized and deprotonated BIH (BI⁺) was observed (Equation 2).

Figure 7 shows the change in the amounts of both BIH and BI⁺ during photocatalytic reaction along with the amount of CO produced. The amount of produced BI⁺ was fairly similar to that of CO. For example, after 20 h of irradiation, 205 μmol of BI⁺ and 203 μmol of CO formed. CO is the two-electron reduced compound of CO₂, and BIH supplies two electrons per molecule to give BI⁺ as a oxidized form. These results clearly indicate that BIH acted as a two-electron donor in the photocatalytic reactions using Ru(pic) as a redox photosensitizer (Equation 3).

FIGURE 7

Figure 7. Photocatalytic production of CO () and BI⁺ () and consumption of BIH (♦): CO₂-saturated DMA-TEOA (5:1 v/v) solutions containing Ru(pic) (50 μM), Re (50 μM), and BIH (0.1 M) were irradiated at λ_ex > 500 nm.

The reaction mechanisms of the photocatalytic reactions using Ru(pic) and Re were investigated. Since Re does not absorb light at λ_ex > 460 nm, as shown in Figure 1, Ru(pic) should absorb the irradiated photon selectively under photocatalytic reaction conditions, i.e., λ_ex > 600 nm or > 480 nm. The photon absorption by Ru(pic) gives its OERS via the reductive quenching process of its ³MLCT excited state by BIH, as described above (Equation 1). The reducing power of OERS of Ru(pic) ($E_{1/2}^{\text{red}}$ = −1.86 V) is strong enough to trigger electron transfer to Re ($E_{1/2}^{\text{red}}$ = −1.76 V), which functions as a catalyst for the reduction of CO₂. The process of two-electron supply using BIH has already been reported in the photocatalytic reaction system using a Ru(II)-Re(I) supramolecular photocatalyst (Tamaki et al., 2013a). The initial process of the photocatalysis is also a photoinduced electron transfer from BIH to the Ru(II) tris-diimine type photosensitizer unit, forming OERS of the photosensitizer unit and one-electron oxidized BIH (BIH·⁺). BIH·⁺ is rapidly deprotonated by TEOA to give BI·. TEOA functioned only as a base, but not as a sacrificial electron donor to quench the excited photosensitizer unit. BI· has a strong reducing power ($E_{1/2}^{\text{red}}$ = −1.95 V) (Zhu et al., 2008) enough to provide one more electron to the supramolecular photocatalyst to be converted to BI⁺. In other words, BIH works as a two-electron donor by one-photon excitation of the photocatalyst via the ECE mechanism. Similar processes should also proceed in the photocatalytic system using Ru(pic) and Re because both Ru(pic) ($E_{1/2}^{\text{red}}$ = −1.86 V) and Re ($E_{1/2}^{\text{red}}$ = −1.76 V) have a lower reduction potential than BI· ($E_{1/2}^{\text{red}}$ = −1.95 V). Based on this investigation, the electron-supply processes of BIH are presumed, as depicted in Equation 4.

Photocatalysis using Ru(pic) displayed an advantages of a wider wavelength region of visible-light absorption, which achieved both red-light driven CO₂ reduction (λ_ex > 620 nm) and faster CO production than the system using Ru(dmb) (λ_ex > 500 nm), whereas the quantum yield for CO formation using Ru(pic) (Φ_CO = 10%) was 1/4 the value when Ru(dmb) (Φ_CO = 44%) was used. The main reason for smaller Φ_CO should be the smaller quantum yield of one-electron reduction (Φ_OERS) of Ru(pic). Φ_OERS of Ru(pic) using light at λ_ex = 480 nm (light intensity: 5.0 × 10⁻⁹ einstein·s⁻¹) was determined to be 8.3%, which was 1/8 that of Ru(dmb) (Φ_OERS = 66%). The elementary processes of one-electron reduction of Ru(pic) is displayed in Scheme 1. The reductive quenching of the ³MLCT excited state of Ru(pic) by BIH gives an ion pair, [Ru(pic)⁻···BIH·⁺]. If the ion pair dissociate, free OERS and BIH·⁺ are obtained. The charge-recombination processes from the ion pair or by the re-collision of OERS of Ru(pic) and BIH·⁺ should form Ru(pic) and BIH. The differences in properties between Ru(pic) and Ru(dmb), i.e., the cationic valence and the reducing power of OERS, should affect each elementary process and consequently the quantum yield for one-electron reduction. Since OERS of Ru(dmb) is a monovalent cation, the ion pair with BIH·⁺ involves cationic repulsion, which should accelerate the dissociation process. On the other hand, OERS of Ru(pic) is zero-valent, which provides no repulsion between BIH·⁺, and therefore, the dissociation process should become slower when using Ru(pic) (smaller k_esc). In addition, since the reducing power of OERS of Ru(pic) ($E_{1/2}^{\text{red}}$ = −1.86 V) is stronger than that of Ru(dmb) ($E_{1/2}^{\text{red}}$ = −1.74 V), the driving forces for the charge-recombination processes become larger when Ru(pic) is used (larger k_rec1, k_rec2). Consequently, the smaller Φ_OERS using Ru(pic) should be induced by the slower dissociation process of the ion pair and the faster charge-recombination processes. The quantitative analyses of the factors controlling Φ_OERS of photosensitizing complexes are in progress and will be reported elsewhere.

SCHEME 1

Scheme 1. The one-electron reduction processes of Ru(pic).

In the photocatalytic reaction conditions, the electron-consuming process for CO₂ reduction via the electron transfer to Re (the broken box in Scheme 1) will compete against the charge-recombination by the re-collision of OERS and BIH·⁺. Therefore, since Φ_OERSs were determined in the absence of Re, Φ_CO (10%) was larger than the expected value from half of Φ_OERS (8.3/2 = 4.2%), which was derived from the fact that the reduction of CO₂ to CO is a two-electron reduction process. Higher reduction potential of Ru(pic) should operate in favor of the electron transfer to Re. Therefore, the ratio of quantum yields for CO₂ reduction between using Ru(pic) and Ru(dmb), i.e., Φ_CO(Ru(pic))/Φ_CO(Ru(dmb)) = 10/44 = 0.23, became larger than that for one-electron reduction (Φ_OERS(Ru(pic))/Φ_OERS(Ru(dmb)) = 8.3/66 = 0.13). In other words, Ru(pic) has another advantage of faster electron transfer to Re in the photocatalysis.

Experiments

General Procedures

¹H NMR spectra were measured using a JEOL ECA400II (400 MHz) system in solutions of acetone-d₆. The residual protons of acetone-d₆ were used as an internal standard for measurements. Electrospray ionization-mass spectroscopy (ESI-MS) was performed using a Shimadzu LCMS-2010A system with acetonitrile as the mobile phase. UV-vis absorption spectra were measured with a JASCO V-565 spectrophotometer. Emission spectra were measured using a Horiba Fluorolog-3-21 spectrofluorometer equipped with a NIR-PMT R5509-43 near infrared detector. A Horiba FluoroCube time-correlated single-photon counting system was used to obtain emission lifetimes. The excitation light source was a NanoLED-515L pulse lamp (510 nm). A HAMAMATSU absolute PL quantum yield spectrometer C9920-02 was used to determine emission quantum yields. The samples were degassed by Ar-bubbling of solutions for 30 min prior to measuring emissions. Emission quenching experiments were performed on solutions containing the complexes and five different concentrations of BIH. The quenching rate constants k_q were calculated from linear Stern-Volmer plots for the emission from the ³MLCT excited state of the photosensitizing complexes and their lifetimes. The redox potentials of the complexes were measured in an Ar-saturated DMA solution containing Et₄NBF₄ (0.1 M) as a supporting electrolyte using cyclic voltammetric techniques performed with an ALS CHI-720Dx electrochemical analyzer with a glassy carbon disk working electrode (3 mm diameter), a Ag/AgNO₃ (10 mM) reference electrode, and a Pt counter electrode. The supporting electrolyte was dried under vacuum at 100°C for 1 day prior to use. The scan rate was 200 mV·s⁻¹.

Photocatalytic Reactions

Photocatalytic reactions were performed in DMA–TEOA (5:1 v/v) solutions containing the photosensitizer (50 μM), Re (50 μM), and BIH (0.2 M). After the solution was purged with CO₂ for 20 min, the solution was irradiated. For TON measurements, the mixed solution (2 mL) in an 11 mL test tube (i.d. 8 mm) was irradiated in a merry-go-round apparatus using λ_ex > 620 nm light from a halogen lamp equipped with a Rhodamin B (0.2% w/v, d = 1 cm) solution filter or λ_ex > 500 nm light from a high-pressure Hg lamp equipped with a uranyl glass and a K₂CrO₄ (30% w/w, d = 1 cm) solution filter. During irradiation, the temperature of the solution was maintained at 25°C using an EYELA CTP-1000 constant-temperature system. For quantum yield measurements, the mixed solution in a quartz cubic cell (11 mL, light pass length: 1 cm) was irradiated in a Shimadzu photoreaction quantum yield evaluation system QYM-01 using 600 nm or 480 nm light from a 300 W Xe lamp equipped with a 600 nm or 480 nm (FWHM: 10 nm) bandpass filters. The temperature of the solution was controlled during irradiation at 25 ± 0.1°C using an IWAKI CTS-134A constant-temperature system. The gaseous products of photocatalysis, i.e., CO and H₂, were analyzed by GC-TCD (GL science GC323). A capillary electrophoresis system (Agilent 7100) was used to analyze HCOOH. HPLC analyses for BIH and BI⁺ were conducted using a JASCO 880-PU pump, a Develosil ODS-UG-5 column (250 × 4.6 mm), a JASCO 880–51 degasser, and a JASCO UV-2070 detector. The column temperature was maintained at 30°C using a JASCO 860-CO oven. The mobile phase was a 6:4 (v/v) mixture of acetonitrile and a NaOH–KH₂PO₄ buffer solution (50 mM, pH 7) with a flow rate of 0.5 mL·min⁻¹.

Electrochemical Spectroscopy

Electrochemical spectroscopy to determine the molar absorptivity of OERS was performed using a JASCO PU-980 pump and an EC Frontier flow-type electrolysis cell VF-2 equipped with a carbon felt working electrode (18 mm diameter), a Ag/AgNO₃ (10 mM) reference electrode, and a Pt wire counter electrode in an Ar-saturated acetonitrile solution of Ru(pic) (0.5 mM) and Et₄NBF₄ (0.1 M) as a supporting electrolyte. Applied potential was controlled using an ALS CHI-720Dx electrochemical analyzer and UV-vis absorption spectra were measured using a Photal MCPD-9800 spectrometer (Otsuka Electronics) and a flow-type transmission cell (light pass length: 1.5 mm) (Ishitani et al., 1994).

Quantum Yields for One-Electron Reduction of Photosensitizers

A 4-mL DMA–TEOA (5:1 v/v) solution of the photosesnsitizer (0.1 mM) and BIH (0.2 M) in a quartz cubic cell (light pass length: 1 cm) was purged with Ar for 20 min, and then irradiated with the 500-W Xe lamp combined with a 480-nm (FWHM = 10 nm) bandpass filter (Asahi Spectra Co.), ND filter, and a 5-cm-long H₂O solution filter. UV–vis absorption spectral changes during irradiation were measured using a Photal MCPD-9800 spectrometer (Otsuka Electronics). The light intensity was determined as 5.0 × 10⁻⁹ einstein·s⁻¹ using a K₃Fe(C₂O₄)₃ actinometer.(Hatchard and Parker, 1956) The amount of OERS of Ru(pic) was calculated using the molar absorption coefficient of OERS (500–700 nm) obtained by electrochemical spectroscopy.

Materials

DMA was dried over molecular sieves 4A, distilled under reduced pressure (~10 mmHg) and used in a week. TEOA was distilled under reduced pressure (<1 mmHg) and used in a month. Both solvents were kept under Ar in the dark. All other reagents were of reagent-grade quality and used without further purification.

Synthesis

Ru(dmb) (Sullivan et al., 1978), Re (Morimoto et al., 2013), and BIH (Hasegawa et al., 2005; Zhu et al., 2008) were prepared according to the methods reported in the literatures. Ru(pic) was synthesized using a method similar to the synthesis of [Ru(bpy)₂(pic)](PF₆) (bpy = 2,2′-bipyridine) (Norrby et al., 1997; Couchman et al., 1998), except for using dmb instead of bpy. [Ru(dmb)₂(pic)](PF₆) (Ru(pic)): ¹H NMR (acetone-d₆) δ/ppm: 8.81 (d, J = 5.6 Hz, 1H), 8.65 (s, 1H), 8.63 (s, 1H) 8.60 (s, 1H), 8.55 (s, 1H), 8.14 (dd, J = 5.6, 0.8 Hz, 1H), 8.03 (dd, J = 6.4, 2.4 Hz, 1H), 7.94 (d, J = 5.6 Hz, 1H), 7.91 (d, J = 5.6 Hz, 1H), 7.77 (d, J = 5.6 Hz, 1H), 7.70 (dd, J = 5.6, 0.8 Hz, 1H), 7.64 (d, J = 5.6 Hz, 1H), 7.50 (dd, J = 6.4, 2.4 Hz, 1H), 7.43 (dd, J = 5.6, 1.2 Hz, 1H), 7.26 (dd, J = 5.6, 1.2 Hz, 1H), 7.21 (dd, J = 5.6, 1.2 Hz, 1H), 2.67 (s, 3H), 2.58 (s, 3H), 2.55 (s, 3H), 2.49 (s, 3H). ESI-MS (in acetonitrile) m/z: 592 ([M–PF${}_6^{-}$]⁺). Anal. calcd for C₃₀H₂₈F₆N₅O₂PRu·H₂O: C, 47.75; H, 4.01; N, 9.28. Found: C, 47.72; H, 3.75; N, 9.40.

Conclusion

Ruthenium(II) picolinate complex, Ru(pic), successfully functioned as a redox photosensitizer with a much wider wavelength range of visible-light absorption (λ_abs < 670 nm) compared with a fairly typical Ru(dmb) (λ_abs < 560 nm). The system using Ru(pic) as a photosensitizer and Re as a catalyst photocatalyzed the reduction of CO₂ to CO by red-light irradiation (λ_ex > 620 nm). TON_CO reached 235 and Φ_CO was 8.0%. Even in the irradiation conditions where Ru(dmb) also absorbed light, i.e., λ_ex > 500 nm, the system using Ru(pic) demonstrated faster CO formation (TOF_CO = 6.7 min⁻¹) and larger TON_CO (2347) than that using Ru(dmb) (TOF_CO = 3.6 min⁻¹, TON_CO = 2100).

Author Contributions

KT, DS, YY, and YT performed all experiments. YU and OI designed this project. YT wrote the manuscript.

Funding

This work was supported by JSPS KAKENHI Grant Numbers JP17H06440 in Scientific Research on Innovative Areas Innovations for Light-Energy Conversion (I4LEC), JP18K14238, JP17K14526.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2019.00327/full#supplementary-material

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