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 (λabs < 670 nm) and stronger reduction ability in a one-electron reduced state (E1/2red = −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 (λex > 600 nm). TONCO reached 235 and ΦCO 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 (λex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOFCO = 6.7 min−1) and larger TONCO (2347) than Ru(dmb) (TOFCO = 3.6 min−1; TONCO = 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 CO 2 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 ( 3 MLCT) 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 CO 2 . 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 CO 2 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 VIII O 4 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 CO 2 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 CO 2 , for the expansion of the useable wavelength range, we try to decrease the energygap 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) 3 Br (Re) as a catalyst for the reduction of CO 2 (Hawecker et al., 1983;Gholamkhass et al., 2005;Tamaki et al., 2016). Chart 1 shows structures and abbreviations of the metal complexes used. 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 × 10 4 M −1 cm −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.

RESULTS AND DISCUSSION
Ru(pic) exhibited phosphorescence from its 3 MLCT excited state (Figure 2) with a quantum yield of 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) ( em = 9.1%, τ em = 741 ns) due to the 12-times faster non-radiative deactivation process (Ru(pic): k nr = 1.5 × 10 7 s −1 ; Ru(dmb): k nr = 1.2 × 10 6 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 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 red 1/2 = −1.86 V vs. Ag/AgNO 3 ) and oxidation (E 0x 1/2 = 0.41 V) waves were observed at more negative potentials than those of Ru(dmb) (E red 1/2 = −1.74 V and E 0x 1/2 = 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 red 1/2 = −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 red 1/2 = −1.76 V) occurs exothermically.
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 visiblelight 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 ( E = 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 3 MLCT 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 × 10 8 M −1 s −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 × 10 9 M −1 s −1 ) as expected from the weaker oxidizing power in the 3 MLCT 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.
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   Complex Measured in a DMA solution containing the complex (0.5 mM) and Et 4 NBF 4 (0.1 M) with a scan rate of 200 mV·s −1 under an Ar atmosphere. b Redox potentials of the photosensitizers (PS) in their excited states were calculated from E 0x 1/2 -E 00 and E red 1/2 + E 00 , respectively. of the 3 MLCT 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.
The results of photocatalytic reactions for the reduction of CO 2 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 CO 2 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 ( CO ) was determined to be 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 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 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.
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 CO 2 , 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).
The reaction mechanisms of the photocatalytic reactions using Ru(pic) and Re were investigated. Since Re does not absorb  ]. g λex = 600 nm (light intensity: 6.0 × 10 −9 einstein·s −1 ). h λex = 480 nm (light intensity: 6.0 × 10 −9 einstein·s −1 ). i Quenching rate constants for emission from Ru(II) photosensitizers by BIH obtained from the slopes of Stern-Volmer plots and lifetimes of excited states. j Quenching fractions of emission from Ru(II) photosensitizers by 0.2 M of BIH calculated as 0.2kqτem/(1 + 0.2kqτem). k Quantum yield for one-electron reduction of the photosensitizer using light at λex = 480 nm (light intensity: 5.0 × 10 −9 einstein·s −1 ).  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 3 MLCT excited state by BIH, as described above (Equation 1). The reducing power of OERS of Ru(pic) (E red 1/2 = −1.86 V) is strong enough to trigger electron transfer to Re (E red 1/2 = −1.76 V), which functions as a catalyst for the reduction of CO 2 . 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 red 1/2 = −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 red 1/2 = −1.86 V) and Re (E red 1/2 = −1.76 V) have a lower reduction potential than BI· (E red 1/2 = −1.95 V). Based on this investigation, the electronsupply 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 CO 2 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) ( CO = 10%) was 1/4 the value when Ru(dmb) ( CO = 44%) was used. The main reason for smaller CO should be the smaller quantum yield of one-electron reduction ( OERS ) of Ru(pic). 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) ( OERS = 66%). The elementary processes of one-electron reduction of Ru(pic) is displayed in Scheme 1. The reductive quenching of the 3 MLCT 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 red 1/2 = −1.86 V) is stronger than that of Ru(dmb) (E red 1/2 = −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 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 OERS of photosensitizing complexes are in progress and will be reported elsewhere.
In the photocatalytic reaction conditions, the electronconsuming process for CO 2 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 OERS s were determined in the absence of Re, CO (10%) was larger than the expected value from half of OERS (8.3/2 = 4.2%), which was derived from the fact that the reduction of CO 2 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 CO 2 reduction between using Ru(pic) and Ru(dmb), i.e., CO (Ru(pic))/ CO (Ru(dmb)) = 10/44 = 0.23, became larger than that for one-electron reduction ( OERS (Ru(pic))/ 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.

General Procedures
1 H NMR spectra were measured using a JEOL ECA400II (400 MHz) system in solutions of acetone-d 6 . The residual SCHEME 1 | The one-electron reduction processes of Ru(pic).
protons of acetone-d 6 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 3 MLCT excited state of the photosensitizing complexes and their lifetimes. The redox potentials of the complexes were measured in an Ar-saturated DMA solution containing Et 4 NBF 4 (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/AgNO 3 (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 CO 2 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 K 2 CrO 4 (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 constanttemperature system. The gaseous products of photocatalysis, i.e., CO and H 2 , 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-KH 2 PO 4 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/AgNO 3 (10 mM) reference electrode, and a Pt wire counter electrode in an Ar-saturated acetonitrile solution of Ru(pic) (0.5 mM) and Et 4 NBF 4 (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 H 2 O 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 K 3 Fe(C 2 O 4 ) 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.
The system using Ru(pic) as a photosensitizer and Re as a catalyst photocatalyzed the reduction of CO 2 to CO by red-light irradiation (λ ex > 620 nm). TON CO reached 235 and 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.