An Ir(III) Complex Photosensitizer With Strong Visible Light Absorption for Photocatalytic CO2 Reduction

A cyclometalated iridium(III) complex having 2-(pyren-1-yl)-4-methylquinoline ligands [Ir(pyr)] has a strong absorption band in the visible region (ε444nm = 67,000 M−1 cm−1) but does not act as a photosensitizer for photochemical reduction reactions in the presence of triethylamine as an electron donor. Here, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H) was used instead of the amine, demonstrating that BI(OH)H efficiently quenched the excited state of Ir(pyr) and can undergo the photochemical carbon dioxide (CO2) reduction catalyzed by trans(Cl)-Ru(dmb)(CO)2Cl2 (dmb = 4,4′-dimethyl-2,2′-bipyridine, Ru) to produce formate as the main product. We also synthesized a binuclear complex combining Ir(pyr) and Ru via an ethylene bridge and investigated its photochemical CO2 reduction activity in the presence of BI(OH)H.


INTRODUCTION
Today, the consumption of fossil resources releases a tremendous amount of carbon dioxide (CO 2 ), which has had a serious impact on global climate change. The reduction in fossil resources in the future will induce shortages in both energy and carbon sources. To resolve these serious problems, the development of alternative energy systems that produce reduced volumes of CO 2 by using solar light as an energy source is desirable. To utilize a wider range of visible light from the sun, nature-inspired artificial Z-scheme systems have been developed by using semiconductors modified with metal complexes (Sato et al., 2011;Sekizawa et al., 2013;Kuriki et al., 2016Kuriki et al., , 2017Sahara et al., 2016;Kumagai et al., 2017). Some metal complex photocatalytic systems that consist of a photosensitizer (PS) and a catalyst (CAT) can selectively induce CO 2 reduction and suppress hydrogen (H 2 ) evolution. These systems require a sacrificial electron donor due to the relatively low oxidation power of the PS unit in the excited state (Yamazaki et al., 2015;Tamaki and Ishitani, 2017;Kuramochi et al., 2018a) Step-by-step excitation of both the semiconductor and the metal complex produces an electron with high reducing power and a hole with high oxidizing power, allowing for CO 2 reduction by weaker electron donors such as methanol (Figure 1; Sekizawa et al., 2013).
Ru(II) tris-diimine complexes [Ru(N ∧ N) 3 ] 2+ have been frequently used as the PS unit of supramolecular photocatalysts, which have strong absorption in the visible region, a long lifetime of the 3 MLCT excited state, and a stable one-electron reduced state. However, [Ru(N ∧ N) 3 ] 2+ has a problem in that one of the N ∧ N ligands is relatively easily released during the photocatalytic reaction to give [Ru(N ∧ N) 2 (Solvent) 2 ] 2+ -type complexes that work as catalysts for CO 2 reduction FIGURE 1 | The artificial Z-scheme system for CO 2 (carbon dioxide) reduction, consisting of a semiconductor (TaON) and a Ru(II) binuclear complex (RuRu') (Sekizawa et al., 2013).  (Lehn and Ziessel, 1990;Yamazaki et al., 2015;Kuramochi et al., 2018a). In addition, a PS unit with stronger absorption in the visible region compared to [Ru(N ∧ N) 3 ] 2+ should be more favorable for constructing new photocatalytic systems.
Herein, we report the successful use of Ir(pyr) as a PS for CO 2 reduction by using BI(OH)H as ED and trans(Cl)-Ru(dmb)(CO) 2 Cl 2 (Ru) as CAT. We also synthesized a supramolecular photocatalyst from Ir(pyr) (Ir(pyr)-Ru; Scheme 1) and investigated its photocatalytic activity for CO 2 reduction.

Photophysical and Electrochemical Properties
Fan et al. reported the photophysical and electrochemical properties of Ir(pyr) in dichloromethane or tetrahydrofuran (Fan et al., 2014). Since these solvents are less polar than the solvents suitable for CO 2 reduction, such as N,N-dimethylacetamide (DMA), we measured the photophysical and electrochemical properties of Ir(pyr) in DMA (Kuramochi et al., 2014). Figure 3 shows the ultraviolet-visible (UV-vis) absorption spectrum of Ir(pyr) in DMA, which shows a much stronger absorption in the visible region (ε 444nm = 67,000 M −1 cm −1 ) compared to [Ir(piq) 2 (dmb)] + and [Ru(dmb) 3 ] 2+ . According to the timedependent density functional theory (TD-DFT) calculation of the UV-vis spectrum of Ir(pyr) in DMA (Figure 4, red bars), the strong absorption band at 444 nm is due to the transitions from the highest occupied molecular orbital (HOMO)-1 to the lowest unoccupied molecular orbital (LUMO)+2 and from HOMO to LUMO+1, which correspond to the π − π * transitions of the pyrene moieties. The absorption at a wavelength >500 nm is assigned to the transition from HOMO-1 to LUMO and corresponds to the transition from the interligand transition from the dmb to the pyrene moieties and might include some     Kuramochi and Ishitani (2016). e The values were correlated by using conversion factor (−0.631 V) from NHE to Ag/AgNO 3 , see Fan et al. (2014) and Elgrishi et al. (2017). f Kuramochi et al. (2018b).
contribution from the singlet-triplet transitions, as described in the literature (Fan et al., 2014). The emission spectrum of Ir(pyr) in DMA is shown in Figure 4, and the Franck-Condon line-shape analysis ( Figure S1) gave a 0-0 band energy gap of 14,500 cm −1 for Ir(pyr), which is lower than that for [Ir(piq) 2 (dmb)] + (16,950 cm −1 ) (Kuramochi and Ishitani, 2016). The redox potentials of Ir(pyr) in DMA were obtained by cyclic voltammetry (CV; Figure S2) and differential pulse voltammetry (DPV) and are summarized in Table 1 together with those of [Ir(piq) 2 (dmb)] + and Ru (Kuramochi and Ishitani, 2016;Kuramochi et al., 2018b). The oxidation waves of the Ir complexes were measured in acetonitrile due to its widepotential window. The CV showed three each of reversible cathodic and irreversible anodic waves, indicating that Ir(pyr) is stable against reduction but relatively unstable to oxidation on the CV timescale. According to the DFT calculation, the LUMO mainly distributes across the dmb ligand of Ir(pyr) (Figure 4). Thus, it is expected that the first electron is injected into the dmb ligand, which benefits the electron transfer from the one-electron reduced species of Ir(pyr) to the Ru moiety. This property has been previously observed in [Ir(piq) 2 (dmb)] + (Kuramochi and Ishitani, 2016).

Emission Quenching by Electron Donors
The emission intensity of Ir(pyr)-Ru was similar to that of Ir(pyr), suggesting that oxidative quenching of the excited state of the Ir unit by the Ru unit does not proceed in Ir(pyr)-Ru. This is reasonable because the oxidative quenching process is endothermic; the oxidation potential of the excited state of Ir(pyr) is much more positive (−0.96 V; Table 1) than the reduction potential of Ru (−1.66 V).
Emission quenching of Ir(pyr) by 1-benzyl-1,4dihydronicotinamide (BNAH) was inefficient: Stern-Volmer constant (K SV ) = 13 M −1 in DMA. Assuming that the emission lifetime is 3.1 µs (Fan et al., 2014), the quenching rate constant (k q ) was estimated to be 4.2 × 10 6 M −1 s −1 . When a stronger electron donor, BI(OH)H (Hasegawa et al., 2006;Tamaki et al., 2015), was used, the emission of Ir(pyr) was more efficiently quenched ( Figure 5); K SV reached 3,000 M −1 in DMA/TEOA (5:1 v/v), and k q was 9.7 × 10 8 M −1 s −1 , which is close to the diffusion-controlled rate constant (Tamaki et al., 2013). The K SV of Ir(pyr)-Ru by BI(OH)H was 2,800 M −1 , which is similar to that of Ir(pyr). In previous work by Fan et al. (2014), Ir(pyr) did not work as a PS for H 2 evolution because the emission from Ir(pyr) was not quenched by TEA. This emission is not quenched by TEOA as well because TEOA has a similar oxidation potential to TEA. Conversely,

Photocatalytic CO 2 Reduction
DMA-TEOA (5:1 v/v) mixed solutions containing both Ir(pyr) and Ru or Ir(pyr)-Ru (0.05 mM) as the photocatalysts and BI(OH)H as the ED were irradiated at λ ex > 480 nm under a CO 2 atmosphere. In both cases, HCOOH was mainly detected with small amounts of CO and H 2 . Figure 6 shows the time profiles of product formation during the photocatalytic reaction. Blank experiments in the absence of the Ru catalyst produced trace amounts of CO (3.9 µmol) and formate (7.6 µmol) after 24 h. From the Stern-Volmer constants, the quenching efficiencies of the excited states of Ir(pyr) and Ir(pyr)-Ru by 0.1 M BI(OH)H were estimated as η q > 99%, indicating that the excited states of Ir(pyr) and Ir(pyr)-Ru were almost completely quenched by BI(OH)H under these reaction conditions. Figure 6 shows that Ir(pyr) does work as a PS for CO 2 reduction when using BI(OH)H. The time profiles for the mixture of Ir(pyr) and Ru showed a linear increase reaching a TON HCOOH = ∼2,000 during 24-h irradiation, indicating that Ir(pyr) has a high durability during photocatalytic CO 2 reduction. Although Ir(pyr)-Ru also worked as a photocatalyst for CO 2 reduction, it showed a lower activity compared to the mixture of Ir(pyr) and Ru. While the initial formation rates of the products were similar, the reaction stopped after just 5 h of irradiation in the case of Ir(pyr)-Ru. Because the reaction solution of Ir(pyr)-Ru was decolorized during the photocatalytic reaction, the low activity of Ir(pyr)-Ru would result from its low durability. The decoloration was also observed in irradiation experiments of [Ir(piq) 2 (dmb)] + and ED without the CAT because of hydrogenation of the ligands in [Ir(piq) 2 (dmb)] + (Kuramochi and Ishitani, 2016). Thus, it is also expected that the decoloration of Ir(pyr)-Ru is caused by hydrogenation of the Ir unit. In Ir(pyr)-Ru, the accumulated electron(s) might be stabilized because of electron hopping between the Ir and Ru units, which might be enhanced by the hydrogenation of the Ir unit. Figure 7 illustrates the time profiles of the products during the irradiation of CO 2 -saturated DMA/TEOA (5:1 v/v, 2.0 ml) solutions containing Ir(pyr) or [Ru(dmb) 3 ] 2+ as PS in the presence of trans-Ru(bpy)(CO) 2 Cl 2 (bpy = 2,2-bipyridine) and BI(OH)H as CAT and ED, respectively. The initial formation rate of HCOOH in the system using Ir(pyr) (Figure 7A) is slower than that using [Ru(dmb) 3 ] 2+ (Figure 7B), although Ir(pyr) has more intense absorption band at >480 nm than [Ru(dmb) 3 ] 2+ (Figure 3). In Figure 7, trans-Ru(bpy)(CO) 2 Cl 2 is used instead of Ru. Although trans-Ru(bpy)(CO) 2 Cl 2 has a much less negative reduction potential (−1.51 V vs. Ag/AgNO 3 ; Kuramochi et al., 2015) than Ru (−1.66 V vs. Ag/AgNO 3 ), the initial formation rate of HCOOH in the system using trans-Ru(bpy)(CO) 2 Cl 2 ( Figure 7A) is similar to that using Ru (Figure 6A), suggesting that the electron transfer process from the one-electron reduced Ir(pyr) to CAT is not the rate-determining step and does not significantly affect the reaction rate. Considering that the emission of Ir(pyr) is almost completely quenched, the slow initial formation rate of HCOOH in Ir(pyr) would result from the competitive back-electron transfer process soon after the electron transfer from BI(OH)H to the excited state of Ir(pyr) in the solvent cage (Kavarnos, 1993;Nakada et al., 2015).

CONCLUSION
Photocatalytic CO 2 reduction using Ir(pyr) as PS, which has a strong absorption in the visible region, proceeded efficiently for more than 1 day when a suitable electron donor, BI(OH)H, and Ru were used as CAT. A new supramolecular photocatalyst, Ir(pyr)-Ru, was successfully synthesized, which exhibited a similar reaction rate during the initial stage of CO 2 reduction to that of the mixed system, but the durability of Ir(pyr)-Ru was lower than that of the mixed system. While Ir(pyr) showed high durability in the mixed system, the initial formation rate of HCOOH tended to be slower than that of the catalytic system using [Ru(dmb) 3 ] 2+ as PS, which is possibly due to the faster back-electron transfer from the reduced Ir(pyr) to the oxidized BI(OH)H.

General Procedure
All chemicals and solvents were of commercial reagent quality and were used without further purification unless otherwise stated. DMA was dried over molecular sieves of size 4 Å and distilled under reduced pressure. TEOA was distilled under reduced pressure. Tetraethylammonium tetrafluoroborate was dried in vacuo at 100 • C overnight before use. [Ir(piq) 2 (BL)](PF 6 ) (Kuramochi and Ishitani, 2016), BNAH (Mauzerall and Westheimer, 1955), BI(OH)H (Hasegawa et al.,  2005; Zhu et al., 2008), Ru (Anderson et al., 1995), and BL (Sun et al., 1997) were synthesized according to literature procedures. 1 H NMR spectra were recorded on an AL400 NMR spectrometer. IR spectra were measured in dichloromethane on a JASCO FT/IR-610 spectrometer. Electrospray ionization-mass spectroscopy (ESI-MS) was undertaken using a SHIMADZU LCMS-2010A system with acetonitrile as a mobile phase. UVvis absorption spectra were recorded with a JASCO V-670 instrument. Emission spectra were measured at 25 • C under an Ar atmosphere using a JASCO FP-8600 spectrofluorometer with correlation for the detector sensitivity. Emission quenching experiments were performed in DMA or DMA/TEOA (5:1 v/v) solutions containing a complex and several different concentrations of BNAH or BI(OH)H.

Emission Spectral Fitting
Double-mode Franck-Condon band shape analysis was used to fit the emission spectra. The spectral fittings were carried out according to the following equation (Caspar et al., 1984) using the Wavemetrics Igor software.
exp −4log2 ṽ − E 00 + n 1ṽ1 + n 2ṽ2 v 1/2 2 I(ν) is the relative emission intensity at frequency ν. E 00 is the energy gap between the zeroth vibrational levels in the ground and excited states, n 1 and n 2 are the vibrational quantum numbers of the high-and low-frequency vibrational modes, respectively, S 1 and S 2 are the Huang-Rhys factors, and ν 1/2 is the half-width at half-maximum (fwhm) of the individual vibronic band. The 0-0 band energy gaps between the lowest excited state and the ground state were obtained from the emission spectral fitting ( Figure S1).

Ir(pyr)-Ru
An acetone/ethenol (1:2 v/v) mixed solution (6 ml) containing [Ru(CO) 2 Cl 2 ] n (9.9 mg, 4.3 × 10 −5 /n mol) was refluxed for 1 h, and then [Ir(pyr-mq) 2 (BL)](PF 6 ) (30 mg, 2.2 × 10 −5 mol) was added to it. The reaction mixture was heated to 60 • C and stirred for 4.5 h under Ar atmosphere. As the reaction proceeded, the starting red solution became a red suspension. The resulting solid was filtered and washed with ethanol. The solid was dissolved in dichloromethane (ca. 2 ml) and filtered to remove insoluble materials. The solution was evaporated to afford 30 mg (87%) of the titiled compound as a dark red solid. ESI-MS m/z:

Photocatalytic CO 2 Reduction
DMA-TEOA (2 ml; 5:1 v/v) solutions containing a mixture of PS and CAT or the supramolecular Ir(pyr)-Ru complex and BI(OH)H were bubbled with CO 2 for 30 min. Photo-irradiations were carried out in 11-mL Pyrex tubes (i.d. = 8 mm) with light at λ > 480 nm using a 500-W high-pressure Hg lamp combined with a K 2 CrO 4 solution filter (30% w/w, optical path length: 1 cm) using a merry-go-round apparatus. The reaction temperature was maintained at 25 • C using an IWAKI constant-temperature system (CTS-134A). The gaseous reaction products (CO and H 2 ) were quantified with GC-TCD (GL Science GC323), and the product (formate) in the solutions was analyzed with a capillary electrophoresis system (Otuka Electronics Co.CAPI-3300I).

Computational Methods
DFT calculations were carried out using the Gaussian 09 package of programs (Frisch et al., 2009). Each structure was fully optimized using the B3LYP functional using the 6-31G(d,p) basis set for all atoms except Ir and the standard doubleζ type LANL2DZ basis set with the effective core potential of Hay-Wadt for Ir. The calculation was carried out by using the polarizable continuum model (PCM) with default parameter for DMA. The stationary points were verified using the vibrational analysis.

AUTHOR CONTRIBUTIONS
YK concieved the research and conducted experiments. OI directed the project and co-wrote the paper.