Modification of Porphyrin Dye in Hybrid Catalyst - ACS Publications

Research Article Cite This: ACS Catal. 2018, 8, 1018−1030

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Development of a Lower Energy Photosensitizer for Photocatalytic CO2 Reduction: Modification of Porphyrin Dye in Hybrid Catalyst System Dong-Il Won, Jong-Su Lee, Qiankai Ba, Yang-Jin Cho, Ha-Yeon Cheong, Sunghan Choi, Chul Hoon Kim,* Ho-Jin Son,* Chyongjin Pac, and Sang Ook Kang* Department of Advanced Materials Chemistry, Korea University, Sejong 30019, Korea S Supporting Information *

ABSTRACT: A series of Zn−porphyrin dyes was prepared and anchored onto a TiO2 surface to complete a dye-sensitized photocatalyst system, Zn−porphyrin-|TiO2|-Cat, and tested as lower energy photosensitizers for photocatalytic CO2 reduction. Three major synthetic modifications were performed on the Zn−porphyrin dye to obtain a lower energy sensitization and improve the catalyst lifetime. We found that incorporating acetylene and linear hexyl groups into the Zn−porphyrin core allowed facile lower energy sensitization, and the addition of the cyanophosphonic acid as an anchoring group gave the long-term dye stability on the TiO2 surface. Under irradiation with red light of >550 nm and a light intensity of 207 mW/cm2, the hybrid ZnPCNPA catalyst showed a TONRe of ∼800 over an extended time period of 90 h. The photocatalytic activities of porphyrin hybrids differ greatly with the binding strength of the anchoring groups of dye and spectral range of the irradiated light and its intensity. KEYWORDS: Porphyrin antenna, CO2 to CO conversion, TiO2−porphyrin hybrid systems, heterogeneous catalysis, photocatalysis

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INTRODUCTION Much effort has been exerted to design and fabricate molecular assemblies to realize an efficient artificial photosynthesis system (APS).1 Nature operates the best light-harvesting system, which is known as the “Photosystem II”, carries serial chlorophyll units, and arranges them in a coaxial manner to ensure efficient energy transfer.1−4 As such, the structurally related porphyrin molecule is an obvious choice for an APS. Porphyrin bears an intense low-energy Q-band whose absorption reaches 600−700 nm, and the corresponding energy/electron transfer process has been well documented. Indeed, porphyrin dyes have been successfully applied in dye-sensitized solar cells (DSSCs), resulting in a great improvement in the solar-to-electron conversion efficiency due to lower energy light (650−800 nm) harvesting that can be attributed to the enhancement in efficiency.5−11 However, one notable drawback of the use of porphyrin dye is its photoinstability. Under natural photosynthetic conditions it degrades by itself, albeit slowly.12 During natural photosynthesis, nonphotochemical quenching (NPQ) regulates photo© XXXX American Chemical Society

degradation and provides further protection from excessive photon flux.13−15 Considering the proposal for the photoprotection protocol given by the Z-scheme,16 lower energy sensitization may well be applicable to relieve photobleaching encountered in porphyrin dye. However, despite the importance of a low energy utilization, the use of redabsorption antenna like porphyrin derivatives has been less explored in the photocatalytic system for CO2 reduction due to their limitations most likely caused by photobleaching and short-lived charge separation (Chart 1A)17−23 when compared to the mostly used Ru/Ir−polypyridyl complex.24 Introduction of porphyrin dyes to the APS has been attempted by several research groups including Perutz,17−19 Ishitani,20 Tamiaki,20 and Inoue,21,22 and each group reported homogeneous porphyrinic dyad models linked to the tricarbonyl rhenium/ manganese complex for photocatalytic CO2 reduction. Our Received: August 31, 2017 Revised: December 13, 2017

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DOI: 10.1021/acscatal.7b02961 ACS Catal. 2018, 8, 1018−1030

Research Article

ACS Catalysis

Chart 1. Schematic Representation of the Approaches to Photocatalytic CO2 Reduction, Showing Electron Pathways between a Light-Absorbing Porphyrin Dye and Molecular Re(I) Reduction Catalyst (ReC)a

a

Direct electron transfer (ET) between two components with or without connection (A) and ET between three components mediated by the TiO2 semiconductor (B).

Scheme 1. Sequential Energy/Electron Transfer Process in a Dye-Sensitized Hybrid System from Lower Energy Porphyrin Photosensitizers (ZnPs) to TiO2 (molecular immobilizer and electron mediator) and Re(I) Reduction Catalyst (ReC)

sumption of unstable excited and/or oxidized states is expected to slow down a photodegradation of photosensitizer (PS) during photolysis. As an example of a successful DSP, Reisner and co-workers demonstrated that a hybrid system constructed by the covalent anchoring of both a visible-light-absorbing PS and a reduction catalyst for hydrogen evolution on TiO2 particles exhibited high catalytic performance with good durability in photocatalysis28,32−34 even if their hybrid system was targeted for water reduction. The strategy described in Scheme 1 has enabled the buildup of an efficient charge-separating triad system with the following on-demand multielectron transfer toward the rhenium-based CO2 reduction catalyst (ReC), (bpy)ReI(CO)3Cl (bpy = 2,2′bipyridine),35−37 via TiO2 semiconductor. Herein, we present an efficient and sustainable photocatalytic system that utilizes a porphyrin photosensitizer and modifies its end-on functional group onto the energy-controlled catalytic particles (TiO2|-Cat) not only to achieve high solar to electron conversion efficiency but also to correlate the dye structure and photostability relationship on the surface of TiO2.

motivation of the use of porphyrin dye is not just for the development of photosensitizer itself but for lower energy photosensitization with modification of porphyrin dye. To this end, a series of porphyrin supramolecules was to be designed for the purpose of durable, efficient performance in CO2 reduction using homogeneous transition metal complexes as electrocatalyst.24−27 Recently, the emerging approach of dye-sensitized photocatalysis (DSP) has received much attention because of the facile tunability, the improvement of durability, and no limitation of solvent selection based on the immobilization of components on semiconductor particles.28 On the basis of conventional DSSCs, the TiO2 semiconductor, which is mostly used as a scaffold and as an electron collector/mediator in DSP, can be a direct solution to solve the low charge separation efficiencies by the short excited-state lifetime of porphyrin dye (τem = ∼2 ns) (Chart 1B). Such good charge separation is based on a fast electron injection from the excited-singlet dye (1Dye*) into the conduction band (CB) of TiO2. The TiO2 electrons could be further transported to the adjacent reduction catalyst and used for CO2 reduction with the multielectron reduction chemical processes.29−31 The recovery of dye can be rapidly completed by reduction of the radical cation (dye•+) with sacrificial electron donor (SED). Such smooth con-

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EXPERIMENTAL SECTION Synthesis and Characterization. Porphyrin dye series were synthesized following the synthetic procedure (Scheme 2 1019


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ACS Catalysis Scheme 2. Synthetic Routes of the Zn−Porphyrin Antenna (ZnPs)

solution was degassed for 15 min. Then the mixture was stirred overnight at 60 °C. After cooling to room temperature, the solvent was evaporated under reduced pressure. The residue was purified by silica gel chromatography using dichloromethane/methanol (5:1 v/v) as eluent to give ZnPCNPE as a dark green solid. Yield 30% (0.18 g). 1H NMR (400 MHz, CDCl3) δ 9.65 (d, J = 4.56 Hz, 2H), 9.62 (d, J = 4.56 Hz, 2H), 8.89 (d, J = 4.54 Hz, 2H), 8.84 (d, J = 4.58 Hz, 2H), 8.03 (m, 2H), 7.87 (d, J = 8.68 Hz, 2H), 7.72 (t, J = 8.16 Hz, 4H), 7.62− 7.52 (m, 8H), 7.47−7.33 (m, 11H), 7.25−7.22 (m, 5H), 7.12 (t, J = 7.34 Hz, 2H), 7.02 (d, J = 8.40 Hz, 2H), 4.22 (br, 4H), 3.81 (t, J = 6.03 Hz, 8H), 1.48−1.28 (m, 6H), 1.05−0.68 (m, 8H), 0.50−0.16 (m, 36H). 13C NMR (101 MHz, CDCl3) δ 160.0, 151.8, 151.5, 150.9, 150.7, 147.4, 138.5, 135.0, 133.0, 132.7, 132.3, 132.2, 132.1, 132.0, 132.0, 131.8, 131.0, 130.8, 130.2, 130.0, 130.0, 129.8, 129.6, 128.7, 128.5, 126.3, 125.4, 125.2, 123.7, 123.4, 122.9, 121.0, 115.9, 105.4, 68.7, 63.5, 30.9, 28.7, 25.0, 22.2, 16.2, 13.7. MS (MALDI-TOF) calcd for C91H95N6O7PZn: 1478.6291. Found: 1478.6310 [M]+. ZnPCNPA. To a solution of ZnPCNPE (0.052 g, 0.035 mmol) in distilled dichloromethane (3 mL) and triethylamine (0.05 mL), bromotrimethylsilane (0.05 mL, 0.35 mmol) was added dropwise after degassing for 15 min. Then the mixture was stirred for 4 h at 40 °C. After cooling to room temperature, MeOH was added into the reaction solutions. The solvent was evaporated under reduced pressure. The residue was dissolved in a minimal amount of dichloromethane/hexane (1:4 v/v) and crystallized at 0 °C to give ZnPCNPA as a dark green solid. Yield 48% (0.024 g). 1H NMR (400 MHz, CDCl3) δ 9.66−9.45 (m, 4H), 8.92−8.64 (m, 4H), 8.03 (d, J = 7.95 Hz, 2H), 7.97 (d, J = 8.15 Hz, 2H), 7.89 (s, 1H), 7.87−7.80 (m, 3H), 7.70 (t, J = 8.58 Hz, 3H), 7.53 (s, 1H), 7.35−7.20 (m, 3H), 7.12 (t, J = 6.90 Hz, 4H), 7.01−6.99 (m, 6H), 5.61 (br, 2H), 3.85 (t, J = 6.45 Hz, 8H), 1.53−0.22 (m, 44H). MS (MALDI-TOF) calcd for C87H87N6O7PZn: 1422.5665. Found: 1422.2909 [M]+.

and Scheme S1 in SI) from [5,15-bis-ethynyl-10,20-bis[2,6di(n-hexoxy)phenyl]porphinato]zinc (1), 4-iodo-N,N-diphenylbenzenamine (2), 4-iodobenzoic acid (3), 2-cyano-3-(4iodophenyl)acrylic acid (4), 2-cyano-3-(4-iodophenyl)vinylphosphonate (5), and ZnPCNCA, which were synthesized according to literature procedures.38−42 fac-[Re(4,4′-bis(diethoxyphosphorylmethyl)-2,2′-bipyridine)(CO)3Cl] (ReC) was synthesized according to the literature procedure.29 ZnPCA. To a stirring, mixed solution of 1 (0.40 g, 0.41 mmol), 2 (0.17 g, 0.47 mmol), and 3 (0.12 g, 0.47 mmol) in toluene (80 mL) and triethylamine (15 mL), triphenylphosphine (0.40 g, 1.5 mmol), CuI (0.08 g, 0.42 mmol), and Pd2(dba)3 (0.20 g, 0.022 mmol) were added after the mixed solution was degassed for 15 min. Then the mixture was stirred overnight at 60 °C. After cooling to room temperature, the solvent was evaporated under reduced pressure. The residue was purified by silica gel chromatography using dichloromethane/methanol (30:1 v/v) as eluent to give ZnPCA as a dark green solid. Yield 35% (0.19 g). 1H NMR (400 MHz, CDCl3) δ 9.55−9.53 (m, 4H), 8.83−8.73 (m, 4H), 8.25 (d, J = 6.82 Hz, 1H), 7.99 (d, J = 5.90 Hz, 1H), 7.79−7.73 (m, 2H), 7.68−7.60 (m, 2H), 7.32−7.23 (m, 6H), 7.19−7.12 (m, 6H), 7.08−7.02 (m, 2H), 7.02−6.94 (m, 4H), 3.87−3.78 (m, 8H), 0.93−0.82 (m, 8H), 0.60−0.25 (m, 36H). 13C NMR (101 MHz, CDCl3) δ 160.1, 152.0, 151.6, 150.8, 150.7, 147.4, 145.0, 135.9, 135.6, 134.5, 132.6, 132.3, 132.0, 131.1, 131.0, 130.9, 130.7, 130.5, 130.3, 130.3, 130.2, 129.6, 127.2, 127.1, 126.5, 125.2, 123.7, 122.8, 121.0, 118.7, 115.7, 105.4, 68.8, 31.0, 28.8, 25.1, 22.2, 13.7. MS (MALDI-TOF) calcd for C85H85N5O6Zn: 1335.5791. Found: 1335.8843 [M]+. ZnPCNPE. To a stirring, mixed solution of 1 (0.40 g, 0.41 mmol), 2 (0.17 g, 0.47 mmol), and 5 (0.14 g, 0.47 mmol) in toluene (80 mL) and triethylamine (15 mL), triphenylphosphine (0.40 g, 1.5 mmol), CuI (0.08 g, 0.42 mmol), and Pd2(dba)3 (0.20 g, 0.022 mmol) were added after the mixed 1020


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ACS Catalysis

Apparent Quantum Yield Measurement. The apparent quantum yield Φ(CO) for CO production was determined for the ZnP-|TiO2|-ReC suspensions, a band-pass filter (420−450 nm) was used to isolate the 436 nm light from the emission light of a high-pressure mercury lamp (1000 W, model 6171, Newport Corp.), and the incident light flux was determined by using a 0.2 M ferrioxalate actinometer solution.43 The apparent quantum yield (AQY) of CO formation for the hybrid system in the presence of 2.5 vol % water was determined in a linear time−conversion region. As defined in eq 1, the measured AQY is not the real quantum yield with the scientific definition but represents a relative estimate for the utilization efficiency of photogenerated electrons into CO formation with respect to the incident light intensity because the number of photons absorbed by the sensitizer cannot be exactly determined. The relatively low AQY appears to arise, at least in part, from poor light harvesting by the dye due to extensive light scattering in the particle dispersion system.

Preparation of ZnP-Sensitized TiO2 Film and TiO2Free Sample for Optical Spectroscopy. A transparent TiO2 layer on the ITO or glass plate was prepared by doctor blade printing commercially available TiO2 paste (average 20 nm, Dyesol, DSL-90-T) and then dried for 2 h at 25 °C. The TiO2 electrodes were gradually heated under flowing air at 325 °C for 15 min, at 375 °C for 15 min, at 450 °C for 15 min, and at 500 °C for 15 min. The resulting slides were sensitized by submerging the film in a 0.3 mM solution of ZnPCNPA for 2 h. The sensitized films were rinsed with THF or CH3CN and dried under a stream of N2 gas. The TiO2-free ZnPCNPA solid film for noninjection reference was prepared using the dropcasting method with either a large excess of chenodeoxycholic acid (CDCA) (>1000 equiv), polystyrene, or poly(methyl methacrylate) (PMMA) polymer. In the case of TiO2-free sample with CDCA, ZnPCNPA (1 μmol) is dissolved in the dichloromethane solution in the presence of 1 M CDCA. For polystyrene and PMMA polymer, 1 μmol of ZnPCNPA is dissolved in dichloromethane solution with 10 wt % PMMA or polystyrene. Each of the prepared solutions is drop casted on glass substrate and oven dried at 80 °C for 6 h. Preparation of ZnP-Sensitized TiO2 Catalyst. Commercially available Hombikat UV-100 (TiO2) was thoroughly washed with distilled water, ultrasonically treated in water, separated by centrifugation, and then dried in an oven under N2. The TiO2 particles (25 mg) were stirred overnight in THF solution (ZnPCA and ZnPCNCA) and dichloromethane solution (ZnPCNPA) of ZnP (1.87 μmol) and then subjected to centrifugation. The collected solids were washed with the solvent and then dried in an oven under N2. The ZnPdeposited TiO2 powders (25 mg) were dispersed into CH3CN/ tert-butanol of ReC (0.25 μmol) and allowed to stand overnight under stirring. The photocatalysts (ZnPs-|TiO2|-ReC) were separated by centrifugation, washed with adsorption solution, dried in an oven (70 °C), and stored under N2 in the dark. UV−Vis absorption spectroscopy confirmed that each supernatant separated after centrifugation of the ZnP- and ReCtreated suspensions shows negligible absorption of ZnP or ReC (Figure S9). Photocatalyzed CO2 Reduction. Suspensions of ZnP-| TiO2|-ReC particles (10 mg with 0.75 μmol of ZnP and 0.1 μmol of ReC) in 3 mL of N,N-dimethylformamide (DMF) containing BIH (0.1 M) were placed in a quartz cell (1 cm pass length; 6.0 mL total volume) or pyrex cell (1.4 cm pass length; 5.9 mL total volume), bubbled with CO2 for 30 min, sealed with a septum, and then irradiated using a Xenon lamp (450 W, model 66924, Newport corporation) or LED lamp (Cree. 60 W) while stirring. For the Xenon lamp, the incident light (λ > 420 nm) was obtained by passing the light from the Xenon lamp through a water layer of a 10 cm path length and a glass light filter. Note that there exists a little deviation among the modulated light intensities (adjusted with increasing the intensity of the original light (109 mW/cm2) from 2 to 4 times) due to the reduction of lamp intensity with an increase of usage time and position sensitivity by distance error between the reactor holder and the lamp every photoreaction. Consequently, the actual intensities of irradiated light were determined as follows: 109, 207, and 414 mW/cm2. The amounts of CO evolved in the overhead space of the cell were determined by gas chromatography (HP6890A GC equipped with a TCD detector) using a SUPELCO Carboxen 1010 PLOT Fused Silica Capillary column.

AQY =

2 × amount of CO generated per unit time number of incident photons per unit time

(1)

Adsorption and Desorption Experiment. A 5 mg amount of TiO2 particles was added to the prepared 0.74 μmol of ZnPCA and ZnPCNCA solution in 2 mL of THF and 0.74 μmol of ZnPCNPA solution in dichloromethane, and three test vials were stirred for different time periods at room temperature. Then each suspension was subjected to centrifugation. The separated solution phase was diluted to one fourth (ZnPCA andZnPCNCA) or one tenth (ZnPCNPA). The amount of adsorbed ZnPs was quantified through UV−Vis absorption comparison of the diluted solution. For the desorption experiment, the suspensions of ZnP(0.37 μmol)-| TiO2|-ReC(0.05 μmol) particles (5 mg) in 3 mL of DMF containing BIH (0.1 M) were placed in a 5 mL Ar-purged vial, stirred for a fixed time, and then subjected to centrifugation. After the filtered solution was diluted to one tenth, the amount of desorbed porphyrin antenna was quantified by UV−Vis absorption spectroscopy (Figure S30 in SI). Picosecond Time-Resolved Fluorescence Measurements. Picosecond time-resolved fluorescence measurements were made using a commercial picosecond fluorescence lifetime measurement system (Hamamatsu streak camera, C11200). The light source was a commercial optical parametric amplifier (TOPAS-prime, Light-Conversion) seeded with a commercial regenerative amplifier system (Spitfire-Ace, Spectra-Physics) operating at 1 kHz. The center wavelength and pulse energy were adjusted to 400 nm and about 1 μJ, respectively. The output was spectrally filtered by using a pair of prisms. A singlet lens was used to focus the excitation beam to the sample, and the photoluminescence was collected in a back-scattering geometry using a parabolic mirror. The emission was sent to a monochromator and detected with the streak camera. Magic angle detection was used to avoid the effect of polarization. Widths (fwhm) of the instrumental response function (IRF) were about 180 ps in the 10 ns time window. All data were acquired in single photon counting mode using the Hamamatsu U8167 software.

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RESULTS AND DISCUSSION Materials. Scheme 1 shows a molecular array of the Zn− porphyrin photosensitizer and Re(I) molecular catalyst on the TiO2 semiconductor that represents the overall energy or 1021


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ACS Catalysis electron flow, eventually leading to CO2 to CO conversion. For lower energy photosensitization and post binding to TiO2, organic functional groups were introduced to the Zn− porphyrin dye. First, to improve the lower energy-harvesting capability in the B/Q-band, two acetylene units were engaged to the 5 and 15 positions of the Zn−porphyrin.7−9,42,44 To each side of the acetylene units, phenyl amine and ethenyl cyanide groups were attached to complete electron-donor and -acceptor dyes, respectively. Second, linear hexyl spacers were put in at the 10 and 20 positions of the Zn−porphyrin to avoid π−π stacking between the porphyrin dyes on the surface of TiO2, which resulted in inefficient light harvesting and electron injection. 5−7 Third, three different anchoring groups (−COOH, −CNCOOH, −CNPO3H2) were brought to the end-on ethenyl cyanide group to test their binding capability at the TiO2. The desired Zn−porphyrin dyes (ZnPs) were successfully prepared by modifying synthetic protocols reported by Hupp and co-workers (Scheme 2).38−41 The ethynyl Zn−porphyrin core was synthesized according to a previously published procedure.38 The subsequent asymmetric substitution gave the desired ZnPCA and ZnPCNCA upon Sonogashira cross-coupling reaction with 1 equiv of triphenylamine derivative (2) and 1 equiv of acidic anchoring groups (3 and 4) in the presence of Pd2(dba)3. The products were isolated in moderate yield (29− 47%). The porphyrin dye with phosphonic anchoring group was added with one additional hydrolysis procedure. The ZnPCNPE was synthesized in the presence of the diethyl phosphite derivative (5), followed by hydrolysis of the corresponding phosphonic ester using triethylamine and bromotrimethylsilane to produce ZnPCNPA in moderate yield (48 ± 6%). All products were confirmed by MALDI-TOF mass spectrometry and 1H/13C NMR spectroscopy. The detailed synthetic procedures and characterization of all porphyrin derivatives are given in the Experimental Section. Similarly, the Re(I) complex was functionalized with two methyl phosphonic acid groups to give fac-[Re(4,4′-bis(diethoxyphosphorylmethyl)-2,2′-bipyridine)(CO) 3 Cl] (ReC),29,45 which needs to be immobilized onto the TiO2 surface. The TiO2 semiconductor powder (UV100 Hombikat, Huntsman) was used to immobilize the prepared ZnPs and ReC on the ground not only to facilitate facile electron transport but also to adjust energies between the two photofunctional units, ZnPs and ReC.46−51 The attachment of both functionalities was confirmed via diffuse-reflectance spectroscopy (DRS), IR absorption spectra, and secondary ion mass spectrometry (SIMS) of the treated particles (Figure 1, Figure S8, and Figure S23−26 in SI). Photophysical Properties. To investigate the electronic properties of ZnPs and obtain the energy levels of the singlet excited state (S1 and S2), the UV−Vis absorption and fluorescence spectra of porphyrin dyes and the anchored dyes were measured in DMF and after fixation of transparent mesoporous TiO2 film on FTO substrate, respectively (Table 1 and Figure 1). All porphyrin dyes exhibit typical porphyrin absorption features with a strong B-band in the range 400−500 nm and moderate Q-band in the range 650−700 nm. Compared to ZnPCA, the absorption bands of ZnPCNCA and ZnPCNPA are slightly red shifted due to effective intramolecular charge transfer derived from the electron-withdrawing ability and π-elongation of the cyanoacrylic moiety. In three porphyrin dyes, absorption bands on mesoporous TiO2 film are slightly blue shifted and broadened. The Q-bands of ZnPCNCA and

Figure 1. UV−Vis absorption spectra of 1.65 μM ZnPs in DMF. Inset shows diffuse-reflectance spectra (DRS) of ZnPs-|TiO2|-ReC powders.

ZnPCNPA-|TiO2 films are more blue shifted than those of ZnPCA-|TiO2 film, suggesting that the environments created by the porphyrin derivatives of ZnPCNCA and ZnPCNPA on the mesoporous TiO2 surface differ from that of ZnPCA.52 The zero−zero excitation energies (E0−0) were determined from the crossing point of the normalized absorption and emission spectra (Figure S18), and the values are summarized in Table 1. Slightly smaller values of ZnPCNCA and ZnPCNPA (E0−0 = 1.82 and 1.81 eV) are in accordance with their relatively red-shifted absorption spectra. Excitation at 460 nm results in a red emission for all porphyrin derivatives centered at approximately 679 nm (±4 nm) with good mirror symmetry with the lowest energy absorption bands. Small Stokes shifts were observed for all porphyrins (Δv = 155−217 cm−1), indicating less reorganization of the porphyrins’ dipole moment in the excited state as reported in previous asymmetric porphyrin dyes.53 Electrochemical Properties. The energy levels of the ZnPs were determined via cyclic voltammetry (CV) to estimate the possibility of electron injection from the ZnPs to the CB of TiO2 (Table 2). From the oxidation and reduction onset or E1/2 potential in CVs, each S1 and S2 energy level of ZnPs was estimated to be about −1.25/−1.78, − 1.26/−1.80, and −1.15/ −1.55 V (versus SCE) for ZnPCA, ZnPCNCA, and ZnPCNPA, respectively (see Table 2). Note that ZnPCNPA showed the relatively lower S1 and S2 energy levels compared to ZnPCA and ZnPCNCA. A similar trend is also observed in the DFT calculation of the donor−acceptor ZnPs prepared: the LUMOs of ZnPCNCA (−2.72 eV) and ZnPCNPA (−2.63 eV) have a lower energy than ZnPCA (−2.39 eV) (see Table 2 and Figure S22). BIH, 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole, has been used as a sacrificial electron donor in this study. The recovery efficiencies for the reduction of ZnP•+ with BIH (ΔGreg) were estimated. The redox potential of E1/2ox (0.25 V vs SCE) allows it to favorably regenerate all oxidized porphyrin dyes (ZnP•+) (ΔGreg = −0.31 to −0.41 eV). The excited-state oxidation potential (E*ox) of ZnPs associated with oxidative quenching was estimated from the addition of E0−0 to the E1/2ox of ZnPs. A more positive E*ox of −0.95 V was obtained for ZnPCNPA compared to ZnPCA and ZnPCNCA (E*ox = ∼−1.05 V). However, in all cases, the excited-state energy levels of the ZnPs are lower than the apparent flat-band potential (Vfb) of TiO2 (−1.50 V versus SCE upon 3% (v/v) H2O).29 Under such energy alignment, it can be presumed that the electron injection might be endergonic in photocatalytic reaction of the hybrid. 1022


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ACS Catalysis Table 1. Photophysical Properties of ZnPs Compounds ZnPCA ZnPCNCA ZnPCNPA a

λmax,S [ε, × 104]a

λmax,Q [ε, × 104]a

λmax,S [nm]b

λmax,Q [nm]b

λmax,S [nm]c

λmax,Q [nm]c

λem [nm]

Stokes shift [cm−1]

460(18.9) 463(27.4) 462(16.5)

667(4.28) 673(6.73) 672(3.92)

443 443 443

645 652 650

462 462 460

654 657 652

674 683 681

155 217 196

Extinction coefficient: ε [cm−1 M−1]. bDRS: diffuse-reflectance spectroscopy. cZnPs-|TiO2 film.

Table 2. Electrochemical Properties of Components Used in This Study (V vs SCE)a oxidation [V]

reduction [V]

compound

E1/2ox

E1/2red1

E1/2red2

Eelec [eV]

E0−0 [eV]

Ecal [eV]

E*ox [V]b

E*red [V]c

ΔGreg [eV]d

ZnPCA ZnPCNCA ZnPCNPA RePE BIH

0.77 0.78 0.86

−1.25 −1.26 −1.15 −1.43

−1.78 −1.80 −1.55 −1.76

2.02 2.04 2.01

1.86 1.82 1.81

2.16 1.94 2.01

−1.09 −1.04 −0.95

0.61 0.56 0.66

−0.36 −0.31 −0.41

0.25

In DMF. bOxidation potential in the excited singlet state; E*ox = Eox − E0−0. cReduction potential in the excited singlet state; E*red = Ered + E0−0. d The recovery efficiencies by the reduction of ZnP•+ with BIH (SED) are quantitatively compared with the relative driving force for dye regeneration (ΔGreg), which is calculated from the excited-state reduction potentials of 1Dye* (Ered*) and the oxidation potential of BIH (E1/2ox): ΔGreg = E1/2ox(BIH) − E*red (ZnPs). a

Table 3. Results of Visible-Light-Driven CO2 Reduction with the Porphyrin-Based Hybrid TiO2 Catalyst in Different Conditionsa entry

system

wavelength (nm)

light intensity (mW/cm2)

CO (μmol)

CO (TONRe)/@tirr (h)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

ZnPCA-|TiO2|-ReC ZnPCA-|TiO2|-ReC ZnPCA-|TiO2|-ReC ZnPCA-|TiO2|-ReC ZnPCA-|TiO2|-ReC ZnPCA-|TiO2|-ReC ZnPCNCA-|TiO2|-ReC ZnPCNCA-|TiO2|-ReC ZnPCNCA-|TiO2|-ReC ZnPCNCA-|TiO2|-ReC ZnPCNCA-|TiO2|-ReC ZnPCNCA-|TiO2|-ReC ZnPCNPA-|TiO2|-ReC ZnPCNPA-|TiO2|-ReC ZnPCNPA-|TiO2|-ReC ZnPCNPA-|TiO2|-ReC ZnPCNPA-|TiO2|-ReC ZnPCNPA-|TiO2|-ReC ZnPCNCA-|TiO2 TiO2|-ReC ZnPCNCA-|TiO2|-ReCc ZnPCNCA-|ZrO2|-ReC ZnPCNCA-|TiO2|-ReCe ZnPCNCA + ReCf porphyrin−Re(I) dyadg

>420 >550 >420 >550 >420 >550 >420 >550 >420 >550 >420 >550 >420 >550 >420 >550 >420 >550 >420 >420 >420 >420 under dark >420 >520

109 109 207 207 414 414 109 109 207 207 414 414 109 109 207 207 414 414 207 207 207 207 n.d.c 207

60 102 81 103 79 91 60 82 57 69 57 75 56 22 76 82 50 56 n.d.d n.d.d n.d.d 3 n.d.c 83 180

600/34 1015/56 806/28 1028/42 793/11 909/11 594/35 823/62 565/29 690/71 564/22 752/22 580/108 215/108 756/78 820/90 500/37 557/37

32/25 55/10f 360/6g

a The following standard conditions were employed: 0.1 M BIH and 10 mg of TiO2 are commonly used as sacrificial electron donor and immobilizer of 0.75 μmol of ZnPs and 0.1 μmol of ReC, respectively. The total volume of the solution or suspension was 3 mL. All photocatalytic reactions are performed in CO2-saturated DMF containing 2.5 vol % H2O with the light intensity variation of the Xenon lamp (450 W). b@tirr indicates the time when the catalytic efficiencies are leveled off. cAr bubbling (no CO2 atmosphere). dNot detected. eUnder dark condition. fZnPCNCA (0.1 mM) + ReC (0.1 mM) in the presence of 0.1 M BIH irradiated at >420 nm. gTaken from ref 19.

Photocatalytic CO2 Reduction. With the intense lowenergy absorption and defined energy levels of porphyrin derivatives (ZnPs) in mind, we examined the performance of porphyrin-sensitized catalytic particles for photocatalytic CO2 to CO conversion. The TiO2 hybrid particles were preloaded with Zn−porphyrinic sensitizers, ZnPCA, ZnPCNCA, ZnPCNPA, chemisorbed by 0.1 μmol of ReC, and photolyzed under two

different photolytic conditions: one with full-spectrum irradiation, 420 nm long pass, and the other with selective red light irradiation, 550 nm long-pass. BIH was used as a sacrificial electron donor.54 Photocatalysis was performed in a heterogeneous manner with which 10 mg of hybrid powders, BIH (0.1 M), and 3 mL of CO2-saturated DMF were mixed together. Gas chromatography (GC) was used to quantify the 1023


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Figure 2. Time period for CO formation for (A) ZnPCA-|TiO2|-ReC (□ at λ > 550 nm; ■ at λ > 420 nm), (B) ZnPCNCA-|TiO2|-ReC (Δ at λ > 550 nm; ▲ at λ > 420 nm) and homogeneous ZnPCNCA + ReC (● at λ > 420 nm), (C) ZnPCNPA-|TiO2|-ReC (○ at λ > 550 nm; ● at λ > 420 nm), and (D) overlapped plots of CO formation for three ZnPs-|TiO2|-ReC hybrids in the presence of 0.1 M BIH under light at an intensity of 207 mW/cm2 (xenon lamp).

respectively, in a linear time−conversion region. A 13C isotope tracer experiment for the porphyrin hybrids (ZnPCNCA-|TiO2|ReC) was undertaken by 13C NMR (DMF-d7) and GC-MS. As shown in Figure S33, prior to irradiation, a strong signal indicating 13CO2 was detected at 125.7 ppm in a 13C NMR spectrum of a DMF-d7 solvent containing 10 mg of ZnPCNCA-| TiO 2 |-ReC dispersion and 0.1 M BIH under a 13 CO 2 atmosphere. Irradiation at λ > 400 nm for 10 h led to both a decrease in the 13CO2 peak and the appearance of a sharp signal at 185.2 ppm, which was attributed to the successive 13CO2to-13CO conversion. The origin of the carbon atoms in the produced CO is further confirmed by the 13C isotope abundance (>90%) analyzed by GC-MS (see Figure S34). As shown in Figure 2 and Table 3, the photocatalytic activities increase in the order of ZnPCA > ZnPCNCA > ZnPCNPA but are not persistent for a longer period of time and are reversed in the opposite manner, ZnPCA < ZnPCNCA < ZnPCNPA, showing a direct anchoring group dependency in the photochemical CO2 reduction. It is reasonable that such a higher binding affinity to metal oxides would lead to the inhibited photobleaching behavior during photocatalysis. Both ZnPCA and ZnPCNCA revealed the relatively higher desorption amount and rate (Figure S30). The detached porphyrin molecules experience a significant photobleaching behavior under continuous irradiation (Figure 4). In contrast, the ZnPCNPA bearing the strong binding ability of the phosphonic acid group56−59 revealed the highly suppressed desoprtion from TiO2. These results are consistent with the delayed leveling-off

CO product, and H2 was the only byproduct that was detected, at an amount of less than 4% of the total CO produced. Highperformance liquid chromatography (HPLC) was used to analyze the liquid-phase contents, but formic acid and oxalic acid, which are believed to be other byproducts from the CO2 reduction, were not detected. The control experiments confirmed that the codeposition of both ZnPs and ReC on TiO2 semiconductor is essential for the efficient production of CO because ZnPs-|TiO2 or TiO2|-ReC without fixation of either ReC or ZnPs reveals almost no efficiency at the catalytic reduction of CO2 to CO (see entries 19 and 20 in Table 3). The much lower CO2 to CO conversion efficiency for ZnPs-|ZrO2|-ReC relative to ZnPs-|TiO2|-ReC shows that the electron transfer pathway from ZnPs to ReC through the TiO2 semiconductor is predominant compared to ZrO2 given that the CB of ZrO2, which is about 1.3 eV higher than TiO2,55 is energetically much higher than 1ZnP*. This result, in turn, indicates the low possibility of direct electron transfer between ZnP and ReC anchored on a semiconductor surface (see entry 22 in Table 3). The IR absorption bands of the CO ligands of the ReC remain almost unchanged after photolysis, indicating that the Re(I) catalyst works persistently in the CO2-to-CO conversion step after its immobilization onto the TiO2 surface (Figure S32). The apparent quantum yield (AQY) of CO formation at 436 nm for the ternary hybrids (ZnPs-|TiO2|-ReC) in the presence of 2.5 vol % water was determined to be (3.21 ± 0.07) × 10−2, (2.82 ± 0.04) × 10−2, and (1.42 ± 0.02) × 10−2 for ZnPCA, ZnPCNCA, and ZnPCNPA, 1024


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Figure 3. Formation of CO in 9 cycle repetitions under lower energy irradiation (>500 nm) using a LED lamp (60 W, Cree Inc.) after CO2 rebubbling for 30 min in the dark; 10 mg of ZnPCNPA(0.75 μmol)-|TiO2|-ReC(0.1 μmol) in 3 mL of CO2-saturated DMF/H2O mixture solvent (3 vol % water) containing 0.1 M BIH.

Figure 4. (A) Duration for the CO formation of ZnPCA-|TiO2|-ReC (■ at >420 nm; ● at >550 nm) in the presence of 3 vol % water with a light intensity of 109 mW/cm2 and a photographic image (inset, left) of the reaction vessels after 35 h of photoreaction. (B) Relative UV−Vis absorbance and photographic images (inset, right) of the reaction solution filtered after photoreaction, indicating the magnitude of photobleaching of ZnPCA present in the DMF reaction solution.

behavior of the homogeneous systems appears at an early stage (6−10 h), considerably faster than those of our ternary system, the TONRe of our ternary system is about 2-fold higher than that of a homogeneous linked porphyrin−Re dyad system19 and significantly higher than that of a homogeneous separation system (Porphyrin + Re(I) catalyst) (see entries 24 and 25 in Table 3).61 The lower performance of the homogeneous system can be understood as a result of unwanted intermolecular interaction between long-lived reactive species, thus leading to the degradation of original components.62−65 The above TONRe values obtained from irradiation at λ > 550 nm are comparable to or even better than those at λ > 420 nm including blue-green light. We conducted repetitive irradiation experiments to verify the durability of the porphyrin-sensitized hybrids. As shown in the TONRe plots for ZnPCNPA-|TiO2|-ReC under lower energy irradiation (Figure 3), no leveling-off behavior was observed in each cycle, and the CO2 to CO conversion efficiency increased slightly with extended cycles from a TONRe of 62 in the second cycle to a TONRe of 160 in the ninth cycle, finally reaching the total TONRe of 878 over a time period of 95 h. Effect of Different Wavelength and Light Intensity of Light Source in Photocatalysis. Since π-conjugated organic molecules photodegraded rapidly upon receiving higher energy photonic flux in visible light (400−500 nm),66,67 our ZnPs

tendency which means extension of the photocatalysis lifetime. Among ZnPs-based hybrid systems, ZnPCA showed the best activity under lower energy irradiation (λ > 550 nm), giving a high TONRe of >1028 during 42 h (TONRe = molar ratio of CO formed/ReC used) (Figure 2A), while the ZnPCNPA-based catalytic system revealed the best longevity irrespective of the wavelength control, showing no leveling-off tendency for >90 h to give a TONRe of ∼800 (Figure 2C). The hybrid system with ZnPCA outperformed that with ZnPCNPA but only survived less than 42 h. The high performance of ZnPCA at an early stage can be interpreted as a result of an efficient electron injection facilitated with the favorable electronic delocalization in sp2hybridization of the carboxylic acid.60 Compared to ZnPCA and ZnPCNCA, the relatively lower photocatalytic efficiency of ZnPCNPA with the induction period (∼5 h) can be explained in terms of low electron injection driving force derived from the electron delocalization hindered by sp3-hybridized phosphonic acid group60 and the relatively lower reduction potential of ZnPCNPA (Table 2). As previously demonstrated,29,45,50 the immobilization of multicomponents on TiO2 is a potential tactic for building a robust photocatalytic system consisting of molecular photosensitizer and catalyst. Its supremacy is apparent when the performance is compared with the corresponding homogeneous systems reported previously. While the leveling-off 1025


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Figure 5. (A) Time period of CO formation for ZnPCNPA-|TiO2|-ReC hybrid (● at λ > 550 nm (207 mW/cm2) and ■ at 420 > λ > 550 nm (207 mW/cm2)) in the presence of 3 vol % water with photographic images of the reaction vessels with two different wavelength ranges. (B) Plots of the CO production versus time for ZnPCNPA-|TiO2|-ReC hybrid by irradiation at >550 nm with different light intensities (109, 207, and 414 mW/cm2).

Natural photosynthetic organisms vent excess light energy in their antenna systems to regulate the energy and prevent damage to the organism,12−14 so the intensity of the light energy must be considered as another key factor affecting the durability of the hybrid photocatalysts. Along this line, the photoreactions were tested under intensity-modulated irradiation at 109, 207, and 414 mW/cm2. There is an obvious tradeoff between improving the injection efficiency (ηinj) from PS* to the CB of TiO2 and accelerating the photobleaching behavior of fixed porphyrin dyes when the intensity of irradiated light increases. As illustrated by the red squares in Figure 5B, under a higher light intensity of 414 mW/cm2, activity leveled off at a TONRe of 350 (90 h to reach TONRe of 215 and 820, respectively, with no appreciable leveling-off tendency (Figure 5B). It is highly probable that the excess excitation of porphyrin sensitizers accelerates the photobleaching of the sensitizer during the photoreaction. We have thus concluded that the intensity of the irradiated light is also responsible for the lifetime of the hybrid photocatalysts in addition to the lower light sensitization. Regarding the temperature factor on variation of light intensity, we now think that the slight rise of reactor temperature (5 °C rise with increasing light intensity from 109 to 414 mW/cm2) has almost no impact on the overall photocatalytic conversion given the rise of reactor temperature is small and the elevated temperature condition also activates the quenching pathways involving the charge recombination. On the basis of the above-experimental results and discussion, a proposed photocatalytic cycle would follow the overall reaction sequence shown in Scheme 3: upon lower energy sensitization, an excited electron injects from 1ZnP* to the CB of TiO2 via oxidative quenching (process 1). Then TiO2 shuttles the electron (process 2) to the ReC site (process 3) concomitantly. At the ReC center, proton-coupled two-electron reduction of CO2 takes place to give CO (process 4) following the monomeric mechanism(s)69−71 rather than dimer formation (CO2-bridged dimer complex, LReCOOReL),64,72 as suggested in our previous ternary system because the Re(I) molecular catalyst is anchored on the TiO2 surface with very

follow a similar photonic response. Indeed, in situ UV−Vis measurements of the detached ZnPs varying with time and incident photonic wavelength substantiate the degree of photodegradation. The photographic image in the inset of Figure 4A shows that after 35 h the initial intense green ZnPCA|TiO2|-ReC particles have become pale yellow with a >420 nm filter condition, while the particles with >550 nm remain strongly colored. A comparison of the absorbance of ZnPCA (Figure 4B) shows that the lower energy irradiation effectively slows photobleaching. These results hold important implications for the earlier leveling-off tendency observed in the above ZnPCA-based photocatalysis under >420 nm (see the levelingoff properties of Figure 2A). However, we found that almost one-half of the initially anchored dyes, in the cases of ZnPCA and ZnPCNCA, were detached from TiO2 in DMF solvent (Figures S29 and S30), implying two electron injection pathways operate: one with chemical anchoring and the other through collision. A blue shift in the B/Q-band peaks observed after photolysis can be interpreted in terms of the saturation of acetylene and the double bond in the bridging and acrylic acid part, respectively, by hydrogenation in the presence of BIH and light, as reported by Ishitani et al.68 To cement the wavelength dependency of the ZnPCNPA hybrids, we further carried out photolysis under two modulated wavelength ranges (420 < λ < 550 nm versus λ > 550 nm) but all of the light intensities being matched by irradiance of ∼207 mW/cm2 irrespective of different wavelength. As shown in Figure 5A, a discernible leveling-off tendency of TONRe with an involvement of higher energy irradiation (420 < λ < 550 nm) is commonly observed in all photolysis experiments. Note that the measured efficiencies are not real, quantitatively defined values because the number of photons (in different wavelengths) absorbed by porphyrin dyes cannot be properly determined in the modulated wavelength range, but rather, these measurements stand for a relative measure of the CO formation efficiency. These trends are consistent with the photobleaching behavior studied in the above UV−Vis absorption experiments (Figure 4). The early leveling off of the behavior with blue light is also dominant in ZnPCA- and ZnPCNCA-based hybrids (Figure S36). 1026


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film was prepared with a large excess of CDCA (>1000 equiv), polystyrene, or PMMA polymer to avoid data complexity with a fast quenching process that would be derived from the aggregated state of porphyrin derivatives in film.77 As shown in Figure 6A and 6B, the fast decay component of about 140 fs and the corresponding rise component were resolved at the B-band (490 nm) and Q-band (650 nm) emissions, respectively. For control of TiO2-free ZnPCNPA solid films and solution, the lifetime of the Q-band emission was >1 ns (Figure S38 and Figure 6A). Meanwhile, the fast internal conversion (140 fs) and the subsequent fast deactivation (∼1.1 ps) were resolved at the Q-band emission of ZnPCNPA-|TiO2 film, indicating that the fast injection process between ZnP and TiO2 nanoparticle indeed occurs. It is reasonable to assume that such efficient electron injection is due not only to the strong electronic coupling between the Q-band of ZnP and the CB of TiO2 but also to the existence of various shallow surfacetrapping sites and surface bands which are 0.3−1.0 eV below the CB of TiO2.78−82 Huber et al. reported a similar fast electron injection into the surface trap state, where the CB edge lies ∼1 eV above the S1 state of the dye.55 In addition, the emission quenching of 490 nm (taken by B-band excitation) by addition of TiO2 particles supports the possibility of electron injection from S2 of ZnP to the CB of TiO2 apart from the injection through Q-band excitation (Figure S39).78 The overall photophysical reaction pathways between ZnP and TiO2 particles are summarized in Figure 6C.

Scheme 3. Possible Photocatalytic Sequence of CO2 to CO Conversion of the Ternary Hybrid System (ZnPs-|TiO2|ReC) via Oxidative Quenching

low coverage.29 The overall cycle is completed after the recovery of ZnP by the reduction of ZnP•+ with BIH (process 5). For the interfacial electron transfer dynamics between TiO2 and Re(I) catalyst, Reisner and Hammestrom et al. previously reported the slow electron transfer dynamics (ca. microseconds to ca. milliseconds) in TiO2/Re(I) complex by means of IR and transient absorption (TA) spectroscopy.30,73 Therefore, it is expected that an electron transfer from TiO2(e−) to the anchored ReC proceeds with slow electron transfer dynamics (ca. a few hundreds of microseconds on average). The chemical conversion process from CO2 to CO at the Re(I) catalytic site is highly feasible to do with the TiO2(e−), which might be long lived74−76 enough to be transferred to the adjacent Re(I) catalytic site ready for the first/second reduction step in chemical processes (Scheme 3). The on-demand electron supply from TiO2(e−) to ReC is thought to be a main reason for the persistent, efficient photocatalytic behavior of this hybrid system. Optical Spectroscopy. To confirm the electron injection from the photoexcited porphyrin dye (1ZnP*) to TiO2 particles, time-resolved fluorescence (TRF) measurements for photoelectrodes (ZnPCNPA-|TiO2 and TiO2-free ZnPCNPA film) on a sapphire glass window and ZnPCNPA dissolved in DMF were performed. In this experiment, the TiO2-free ZnPCNPA solid film was used as noninjecting reference. The reference

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CONCLUSION In summary, we found that Zn−porphyrin dye serves as an ideal molecular platform to develop lower energy lightsensitized photocatalysis with addition of suitable organic groups to the dye, resulting in a desirable photocatalysis lifetime that has not been witnessed before. The immobilization of the porphyrin molecule on Re(I)-fixed semiconductors allows each porphyrin molecule to follow an energy/electron transfer

Figure 6. Time-resolved fluorescence signals of ZnPCNPA dissolved in DMF (A) and ZnPCNPA-|TiO2 mesoporous film on optical window (B). Proposed energetics and interfacial processes occurring in the ZnP-sensitized TiO2 system (C). 1027


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2014R1A6A1030732), the Functional Districts of the Science Belt support program, Ministry of Science, ICT and Future Planning (2015K000287), Next Generation Carbon Upcycling Project of Climate Change Response Project funded by MSIT of the Republic of Korea (NRF-2017M1A2A2046738), the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A3B03934670 and NRF2016R1D1A3B03936414), and the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2009-0082580).

process with its efficient charge separation and injection during photolysis. Under selective red-light irradiation with appropriate intensity (207 mW/cm2), the ZnPCA-|TiO2|-ReC catalyst demonstrated the most efficient CO2 to CO reduction activity for an extended period of time, reaching to a TONRe of ∼1000 for >42 h. Most of all, the hybrid ZnPCNPA catalyst recorded a TONRe of ∼800 over a 90 h period, proving to be the most practical one among the dye-modified systems; we believe lower energy sensitization was effective, and our catalyst performances are among the top contenders in the photocatalytic CO2 reduction system known to date. Several conditions are to be met for the lower energy photosensitization including the anchoring ability of dye on TiO2 particle, the spectral range of the irradiated light, and its intensity. Within the variation of those conditions, a structure− performance relationship has been established to guide the optimum dye design and reaction conditions for long-term catalytic performance. Further optimization in the performance of an alternate red-light-active antenna with a stronger, persistent adsorbing anchoring group as well as on extending the Re(I) complex with other types of molecular reduction catalysts is currently underway. While this CO2 reduction hybrid system is built to work only in the presence of a sacrificial electron donor, it can be widely applied for the bioinspired two-step photoexcitation Z-scheme system using a shuttle redox mediator (IO3−/I−, I3−/I−, Fe2+/Fe3+, or RGO (reduced graphene oxide)) for charge recombination between the reduction part and the oxidation part.16,83−86 We anticipate that the current porphyrin-based hybrid will be an efficient reduction part applicable to a future Z-scheme approach using water as the electron source with water as the oxidation catalyst.

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ABBREVIATIONS ZnPs, porphyrin dyes (ZnPCA, ZnPCNCA, ZnPCNPA); ReC, (4,4′bis(methylphosphonic acid)-2,2′-bipyridine)-Re I(CO)3Cl; RePE, Re(4,4′-Y2-2,2′-bipyridine)(CO)3Cl (Y = CH2PO(OC2H5)2); BIH, 1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b02961. Characterization data of ZnPs (1H/13C NMR, MALDITOF, and TOF-SIMS mass spectra), results of photophysical (UV−Vis absorption and photoluminescence spectra) and electrochemical data (cyclic voltammogram) of ZnPs, plots of CO formation versus time and FT-IR spectra for hybrid catalysts, 13C isotopic labeled CO2 NMR/GC-MS experiments, DFT calculation of ZnPs, Mott−Schottky data, and GC spectrum of gas in the reaction vessel (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ho-Jin Son: 0000-0003-2069-1235 Sang Ook Kang: 0000-0002-3911-7818 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF1028


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