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An Artificial Photocatalytic System Using Polydiacetylene-(-NHphen)Ru(bpy)2 for Cofactor Regeneration and CO2 Reduction Jinheung Kim, Soojin Kim, Songyi Lee, Tikum Florence Anjong, Ha Yoon Jang, Ji-Yeong Kim, Chiho Lee, Sungnam Park, Hye Jin Lee, and Juyoung Yoon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08532 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 7, 2016

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The Journal of Physical Chemistry

An Artificial Photocatalytic System Using Polydiacetylene-(NH-phen)Ru(bpy)2 for Cofactor Regeneration and CO2 Reduction Soojin Kim,1 Songyi Lee,1 Tikum Florence Anjong,1 Ha Yoon Jang,1 Ji-Yeong Kim,1 Chiho Lee,3 Sungnam Park,3 Hye Jin Lee,2* Juyoung Yoon,1* Jinheung Kim1*

1

Department of Chemistry and Nano Science,

Ewha Womans University, Seoul 120-750, Korea 2

Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, 80 Daehakro, Buk-gu, Daegu, 41566, Republic of Korea 3

Department of Chemistry, Korea University, Seoul, 136-701, Korea

Mailing Address: Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea. Phone: +82-2-3277-4453 Fax: +82-2-3277-3419 E-mail: [email protected] (H. J. Lee); [email protected] (J. Yoon); [email protected] (J. Kim)

1

ACS Paragon Plus Environment


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ABSTRACT: For the practical use of a photo-bioreactor for artificial photosynthesis, efficient visible light-absorbing materials have to link reduction and oxidation catalysts for an efficient energy flow. As a step toward this goal of an NADH regeneration system and enzymatic production of solar fuels from CO2, we report the synthesis of a new polydiacetylene compound that is covalently connected with [Ru(phen-NH2)(bpy)2]2+ (bpy = 2,2’-bipyridine, phen = 1,10phenonthroline-5-amine). The [(bpy)2Ru(phen-NH-)]-polydiacetylene absorbed a wide range of visible light because of the presence of two chromophores, the Ru complex and polydiacetylene. The polyacetylene backbone was converted from blue to red by conformational changes under the catalytic reaction conditions in a buffer solution. The electron transfer from the photoexcited [Ru(phen)(bpy)2]2+ to the polydiacetylene backbone was observed. In a visible light-driven photocatalytic NAD+ reduction by (cyclopentadienyl)Rh(bpy)(H2O)2+ with [(bpy)2Ru(phen-NH)]-polydiacetylene, NADH was regenerated, and the reactivity using Ru(bpy)2(phen-NH)polydiacetylene was enhanced relative to control experiments using only [Ru(phen)(bpy)2]2+ or polydiacetylene. The consecutive carbon dioxide reduction coupled with formate dehydrogenase was carried out to utilize the in-situ photoregenerated NADH catalytically. The catalytic condition using [(bpy)2Ru(phen-NH-)]-polydiacetylene also showed much higher reactivity than the controls.

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INTRODUCTION Recent high prices and limited supplies of fossil fuels have provided the impetus for the present worldwide effort to find alternative sources of energy. The exploitation of solar energy is an especially attractive option because it is abundant and environmentally clean. Because solar radiation is not available at all times, practical methods to store solar energy as chemical bonds for later use should be developed. Natural photosynthesis uses sunlight to convert CO2 and water into carbohydrates by photoregenerating small biomolecules, such as NAD(P)H and ATP. Therefore, CO2 is widely recognized as a clean solar energy storage material.1 A desirable strategy for solar energy storage is to convert CO2 to other useful high-energy chemical substances. Ubiquitous in all living systems, 1,4-dihydronicotinamide adenine dinucleotide (NADH) broadly serves as a biological electron transporter. Various NADH-dependent enzymes carry out necessary reactions in biological systems using the energy stored in NADH. Therefore, NADH regeneration using solar energy and aqueous protons attracts great research interest because of its close relationship with artificial photosynthesis. Useful and valuable chemicals can be produced by enzymes connected with NADH photoregeneration systems.2,3 For example, formate dehydrogenase, formaldehyde dehydrogenase, and methanol dehydrogenase, which are NADHdependent enzymes, can convert CO2 into formic acid, formaldehyde, and methanol, respectively. In the development of photochemical reaction systems to store solar energy, proper lightcollecting species need to harvest as many photons as possible in the solar energy wavelength region, have a long excited-state lifetime, and have high enough efficiency to transfer excited electrons to the catalyst. Because about half of the total solar light reaching the earth is in the visible region, the use of light-harvesting units to collect visible light has attracted significant 3



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interest.4 Various organic and inorganic chromophores with characteristic absorption bands in the visible region, such as organic compounds, metal complexes, conjugated polymers, and quantum dot nanoparticles, have been used as light-absorbing units in photochemical reaction systems.5-8 Supramolecular systems of polymer and carbon nanomaterials coupled with various visible light collectors such as phthalocyanine and porphyrin have allowed considerable advances in the area of light-induced electron transfer chemistry and light harvesting. Tris(2,2’-bipyridyl)ruthenium(II) complex (Ru(bpy)32+) has attracted considerable attention for several decades because of its potential applications as a photosensitizer that promotes the visible-light-induced splitting of H2O into H2 and O2.9-11 Such Ru(bpy) complexes have been covalently or coordinatively coupled with a hydrogen evolving catalyst for water splitting.12-14 However, absorption of Ru(bpy)32+ is limited in the visible region below 500 nm, and most photocatalytic systems containing Ru(bpy)32+ have shown lower quantum efficiencies than other comparable organic chromophores. To collect visible-light photons in NADH regeneration systems, researchers have used quantum dots, graphene, and conjugated polymers coupled with other molecular chromophores, including porphyrin and BODIPY, to increase quantum efficiency.3,15 More examples of efficient photocollectors for NADH regeneration and CO2 conversion are needed to develop effective photovoltaic devices that can convert solar energy into chemical energy. Polydiacetylenes (PDAs), a relatively new class of colored conjugated polymers, are intriguing materials that exhibit interesting electronic and optical properties because of extended π-electron delocalization along their backbones. PDAs have been used for various applications, such as photovoltaic cells, light-emitting diodes, and chemical sensors.16-19 PDAs can be prepared by UV irradiation from self-assembled diacetylene monomers, which are usually blue. 4


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These blue PDAs can show a distinct color change to red PDAs and have been developed for various PDA-based colorimetric and fluorescent sensors.20-23 However, PDAs have never been applied as a visible light collector and used in photocatalytic reaction systems. Recently, another conjugated polymer was used to incorporate an Rh complex and carry out NADH regeneration, and the conjugated polymer regenerated 26% of the NADH in 26 hr.24 Herein, we report a new visible light collector that shows a wide absorption in the visible region, composed of a PDA backbone coupled with a (phen)Ru(bpy)2 chromophore. A unique, homogeneous photocatalytic system containing the ruthenium complex–coupled polymer and a molecular Rh catalyst showed adequately high efficiency to produce enzymatically active NADH from NAD+ in aqueous solution. Also, this system coupled with formate dehydrogenase (FDH) showed a high reactivity in transforming CO2 into formic acid.

METHODS Materials and Instrumentation. All reagents purchased from Aldrich were used without further purification. Water was purified with a MilliQ purification system. [Cp*Rh(bpy)Cl]2+ was prepared according to previously published methods.25 The UV-vis absorption spectra were acquired with an HP spectrophotometer. Emission spectra were collected on a Perkin-Elmer LS 55 luminescence spectrophotometer. DA-NH-phen and DA-[(-NH-phen)Ru(bpy)2]Cl2 (2). The diacetylene monomer containing phenanthroline

(DA-NH-phen)

was

prepared

from

commercially

available

10,12-

pentacosadiynoic acid (1) by formation of an amide bond with 1,10-phenonthroline-5-amine. To a solution containing 1 (1.33 mmol) in 20 mL of methylene chloride, oxalyl chloride (4.1 mmol) 5



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was added dropwise at room temperature. The resulting solution was stirred at room temperature under N2. After 1 hour, catalytic amount of DMF was added to the solution, and the resulting solution was stirred for additional 3~4 hr. After evaporating the solvent in vacuo, the residue was redissolved in 10 ml THF. The resulting solution was added dropwise to the solution containing 1,10-phenonthroline-5-amine (1.7 mmol) dissolved in 5 ml THF and 5 ml pyridine in an ice bath. This mixture was allowed to stir overnight at room temperature under N2. The solvent was transferred to distilled water to afford a crude product that was purified via silica column chromatography (CHCl3:CH3OH = 100:1 - 20:1) to give 0.61 g (83.1%) of the desired monomer product, DA-NH-phen. C37H49N3O, calculated C, 80.54, H, 8.95, N, 7.62 %; found C, 80.72, H, 8.88, N, 7.51 %. 1H NMR (300 MHz, CDCl3) (Figure S1): (ppm) 0.82-0.86 (t, 3H), 1.21- 1.60 (m, 28H), 1.72-1.83 (m, 4H), 2.22-2.25 (m, 4H), 2.34-2.57 (t, 2H), 7.45-7.55 (s, 2H), 7.64-7.67 (m, 2H), 8.00-8.01 (t, 1H), 8.19 (t, 1H), 8.39-8.392 (t, 2H). 13C-NMR (300 MHz,CDCl3) (Figure S2): (ppm) 173.07, 149.77, 149.51, 146.13, 144.16, 135.79, 130.84, 130.47, 128.13, 124.44, 123.36, 122.53, 120.28, 77.74, 77.11, 76.69, 65.36, 65.36, 65.21, 37.14, 31.91, 29.64, 29.63, 29.61, 29.48, 29.35, 29.27, 29.18, 29.10, 28.92, 28.86, 28.75, 28.34, 28.25, 25.70, 22.69, 19.20, 14.15. FAB HRMS m/z = 552.3950 [M + H]+, calc. for C37H49N3O+ = 552.3909. A mixture of N-(1,10-phenanthrolin-5-yl)pentacosa-10,12-diynamide (DA-NH-phen, 1.1 mmol) and Ru(bpy)2Cl2 (1.1 mmol) in ethanol was refluxed for 48 hr. After the solution had been filtered to remove insoluble materials, the filtrate was evaporated to a total volume of 5-10 ml. The solution was gradually added to diethyl ether with stirring. The resulting precipitate was collected by suction filtration and air dried to give 98 mg of 2 (88.7%). Calculated C, 66.07, H, 6.32, N, 9.46 %; found C, 65.89, H, 6.42, N, 9.29 %. 1H NMR (300 MHz, CDCl3) (Figure S3): 0.82-0.86 (t, 3H), 1.28-1.31 (m, 29H), 1.79-1.98 (m, 2H), 2.05-2.3 (m, 10H) 2.80-2.91 (t, 2H), 6


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7.21-7.27 (s, 2H), 7.75-7.94 (m, 4H), 8.07 (d, 2H), 8.23-8.34 (m, 3H), 8.58 (s, 3H), 8.60 (d, 2H), 8.70-8.81 (m, 4H), 9.65 (d, 1H), 11.59 (s, 1H). 13C-NMR (300 MHz,CDCl3) (Figure S4): (ppm) 175.17, 157.26, 157.19, 157.09, 151,89, 151.59, 151.50, 151.31, 151.05, 150.50, 147.81, 145.02, 138.82, 138.59, 136.33, 135.60, 135.14, 131.04, 128.48, 128.34, 128.05, 127.83, 126.56, 126.07, 125.53, 119.80, 65.50, 65.49, 37.17, 32.11, 29.85, 29.83, 29.81, 29.68, 29.65, 29.54, 29.30, 29.22, 29.11, 29.08, 28.60, 28.58, 25.95, 22.89, 19.42, 14.34. FAB HRMS m/z = 966.4378 [M H]+, calc. for C37H49N3O+ = 966.4328. Preparation of [Ru(bpy)2(phen)]2+. [Ru(bpy)2(phen)]Cl2 was previously reported as a trihydrated salt26 but has been prepared as a hydrate, [Ru(bpy)2(phen)]Cl2⋅7H2O, by the following method. A mixture of cis-RuCl2(bpy)2⋅2H2O (0.5 mmol) and 1,10-phenanthroline (0.55 mmol) in ethanol (25 ml) was refluxed overnight. After the solution was filtered to remove insoluble materials, the filtrate was reduced by evaporation to about 5 ml. The solution was gradually added to diethyl ether (80 ml) with stirring. The deposited orange precipitate was collected by suction filtration and air dried (yield 80%). The compound was recrystallized from hot water before use. 1H NMR (300 MHz, CDCl3) (Figure S5): 0.82-0.86 (t, 3H), 1.28-1.31 (m, 29H), 1.79-1.98 (m, 2H), 2.05-2.3 (m, 10H) 2.80-2.91 (t, 2H), 7.21-7.27 (s, 2H), 7.75-7.94 (m, 4H), 8.07 (d, 2H), 8.23-8.34 (m, 3H), 8.58 (s, 3H), 8.60 (d, 2H), 8.70-8.81 (m, 4H), 9.65 (d, 1H), 11.59 (s, 1H). Electrochemical Measurements. Cyclic voltammetry was carried out using a 630C multi-potentiostat (CH Instruments TX, USA). A Pt coiled counter electrode and Ag/AgCl reference electrode were used in all electrochemical measurements. A freshly cleaned ITO electrode was used for each experiment, and a background scan of buffer alone was collected for each electrode and subtracted from subsequent scans. CV was performed in a 50 mM phosphate 7



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buffer solution prepared with triply-distilled water. The solution was purged with Ar gas for 15 min before each measurement, and Ar gas flowed over the solution during the experiments. Emission Lifetimes and Transient Absorption (TA) Experiments. The laser source for sample excitation was a picosecond diode laser operating at a wavelength of 467 nm at 10 MHz (Picoquant). A dichroic mirror and a 488 nm long pass filter (Semrock) were mounted to collect the emission and to eliminate the excitation wavelength. With a fast detector (Hamamatsu R3809) and a TCSPC board (Becker-Hickl SPC-830), the instrument response function (IRF) of the system is typically about 150 ps. To avoid local bleaching in the polymer, the sample was continuously scanned at a rate of 1 Hz in the area of 100 µm × 100 µm, during the measurements. The phosphorescence signals were obtained using an automated motorized monochromator. Phosphorescence decay profiles were analyzed using a triexponential decay model (OriginPro 8.0, OriginLab). For TA experiments, a visible pump pulse at 450 nm was used to excite P1 and Ru(phen)(bpy)22+ and subsequent electronic relaxation was monitored by using a white light continuum. To check undesired photodegradation during the experiments, UV-vis spectra of the sample solutions were obtained before and after the TA experiments and no significant degradation was observed. Photoelectrochemical Measurement. Photoelectrochemical measurements were carried out in a three-armed cell consisting of reference (Ag/AgCl, BASI, MF-2063 RE-5), working (P1 photoanode), and platinum wire (part no.: CHI115) counter electrodes using an electrochemical analyzer (CHI Instruments 1100A). A 0.1 M NaCl solution containing 0.4 M TEOA was used as a redox couple/electrolyte. Newport solar simulator (69911) was used as a light source. Light intensity was obtained via a VSLI standard incorporating Oriel P/N 91150 V. 8


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Photocatalytic NADH Regeneration. The typical photochemical regeneration of NADH was carried out in the presence of P1, 3, NAD+, and TEOA in phosphate buffer (pH 5.0 – 9.1) at room temperature. Photoreactions were performed in a 3 mL glass cuvette equipped with a small magnetic stirrer and a stopper. Samples (1 ml) containing all of the components were transferred into the cuvette and then illuminated. Deaerated samples were prepared by repeated evacuation followed by Ar flushing. A 450 W Arc Xe lamp equipped with a 420 nm cutoff filter was used. The production of NADH was determined by UV absorption at 340 nm (ε = 6220 cm-1M-1) and HPLC. The photoproduct NADH was isolated by HPLC (Youngin Instrument, Korea, Acme 9000,) with an Inertsil C18 column (ODS-3V, 4.6 I.D. x 150 mm). HPLC conditions were as follows: the mobile phase was 0.085% phosphoric acid; the flow rate was 1.0 ml/min; the monitor wavelength was 210 nm; the retention times of NAD+ and NADH were 18.7 and 18.2 min, respectively. Artificial Photosynthesis of Formic Acid from CO2. The photosynthesis of formic acid from CO2 occurred in a quartz reactor under an inert atmosphere at room temperature. A 450 W xenon lamp with a 420 nm cut-off filter was used as a light source. A typical reaction was performed with P1, NAD+, 3, and FDH (3 units) in sodium phosphate buffer (0.1 M, pH 7.0) with TEOA under CO2 (flow rate: 0.5 mL/min). After bubbling with CO2 for 30 min without light, the reactor was exposed to visible light. The amount of formic acid was quantitated by GC (7890A, Agilent Technologies).

RESULTS AND DISCUSSION The

diacetylene

monomer

bonded

covalently

with

phenanthroline,

N-(1,10-

phenanthrolin-5-yl)pentacosa-10,12-diynamide (DA-NH-phen), was synthesized via a coupling 9



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reaction of 10,12-pentacosadiynoic acid (1) and 1,10-phenonthroline-5-amine. Then, Ru(bpy)2(phen-NH-)-attached diacetylene monomer (2) was prepared by complexation of DANH-phen and Ru(bpy)2Cl2 (Figure 1). P1 was prepared by photo-irradiating a mixture of 1 and 2 (7:1) in a buffer solution, and P2 was prepared by irradiating 2 alone (Experimental Section). The average molecular weights of P1 and P2, obtained by gel permeation chromatography, were 50480 and 42730, respectively. The molecular weight of P2 corresponds to about 114 molecules of 1. Assuming that P1 is also composed of 114 monomers of 1 and 2 (7:1 ratio), the calculated molecular weight of P1 (50770) is close to that obtained experimentally. Therefore, the number of 2 molecules incorporated in a single strand of P1 is calculated to be 14. The Ru content of P1 was measured by ICP-MS analysis and the 3373.9 ppb Ru was obtained. The concentration of Ru in P1 solution prepared for the following experiments was calculated the Ru value obtained by ICP-MS. Although 1 had no absorption band in the visible region, 2 showed absorption at 460 nm, derived from the Ru(phen)(bpy)2 moiety in water (Figure 2a). The aqueous solution of P1 appeared blue and had additional bands at 545, 590, and 640 nm derived from the PDA backbone, in addition to the absorption band of the Ru moiety. These additional absorption bands in a longer-wavelength region will be able to collect more photons in the visible region. The blue form of P1 turned red in pH 7.0 phosphate buffer within 30 min, after which the red form was quite stable. The 590 and 640 nm bands observed in the blue form of P1 diminished rapidly in the red form in phosphate buffer. Also, the blue form was stable in water, indicating that Naphosphate salt affected the blue-to-red transition. Such blue-to-red transitions of various PDA molecules have been reported in ionic interactions with PDA.20-23 Then, the MLCT emission band at 600 nm derived from the Ru(bpy)2(phen-NH-)2+ 10


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moiety of 2 and P1 was compared under the same concentration of Ru to understand how the emission intensity is related to the absorption spectral changes shown in Figure 2a. The emission intensity of 2 was much higher than those of the blue and red forms of P1 (Figure 2b). The intensity of red P1 was lower than that of blue P1. The low emissions of blue and red P1 relative to 2 indicates that the excited state of the Ru moiety was quenched by the PDA backbone of P1, and the red form was quenched more efficiently under the same conditions. This conclusion is consistent with our finding that the emission of Ru(phen)(bpy)22+ was similarly quenched upon addition of P2 (Figure S6). Additional information about the quenching of the Ru moiety by the PDA backbone of P1 was obtained by monitoring the emission lifetimes of Ru(phen)(bpy)22+ and P1. The lifetime of the Ru moiety of P1 in photoluminescence (PL) experiments served as a good probe for studying the kinetics of the electron transfer between the Ru moiety and the PDA backbone. The two curves clearly demonstrated that the emission lifetime of P1 decreased significantly in comparison to that of Ru(phen)(bpy)22+ (Figure S7). Next, a cyclic voltammogram (CV) of P1 was obtained in phosphate buffer to compare with that of Ru(phen)(bpy)22+. Under the same Ru concentration, the oxidative current of P1 appeared at ~1.1 V due to the Ru(II)-to-Ru(III) transition (ITO vs. Ag/AgCl) but was much lower than that of Ru(phen)(bpy)22+ (Figure 3). The lower current was believed to be derived from a low diffusion coefficient of P1 in aqueous solution. In the CV of P1, the reductive current was very low relative to the oxidative current. It seems that the Ru(III) species generated through the oxidation scan was reduced back to Ru(II) by electron transfer from the PDA backbone. Indeed, the CV of Ru(phen)(bpy)22+ in the presence of P2 showed a similarly low current in the reduction scan (Figure S8). The photochemical reduction of NAD+ was performed using P1, [Cp*Rh(bpy)(H2O)]2+ 11



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(3; Cp* = pentamethylcyclopentadienyl, bpy = 2,2’-bipyridine), and triethanolamine (TEOA) as a sacrificial electron donor in phosphate buffer (pH = 7.0) at ambient temperature. After a 6-h irradiation using a 420 nm cutoff filter, 0.073 mM NADH (37%) was observed (Figure 4a). In a control experiment that used Ru(phen)(bpy)22+ instead of P1 in the photochemical reaction, less NADH (0.023 mM) was obtained. These results demonstrate that the PDA backbone of P1 has a significant role in the reduction of 3 and acts as an additional light-absorbing and electron transfer agent in the photoregeneration of NADH. The visible light-driven energy flow occurred from P1 to NAD+ via 3 at the expense of TEOA. When the Rh catalyst or TEOA were excluded from the system, no NADH was formed. The photoreduction of NAD+ using P2 and 3 in another control experiment afforded 9%, demonstrating that P2 has some efficacy in collecting photons and transferring excited electrons to 3, but not as much as P1. Researchers recently reported an Rh-coordinated conjugated polymer that regenerated 21% NADH in NAD+ photoreduction after 26 h.24 The influence of solution pH on the rate of NADH formation using P1 and 3 was also investigated. Figure 4b shows the conversion yields of NADH obtained as the pH of the phosphate buffer solution varied. At pH 7.0, the maximum yield for NADH generation was observed; a quite good yield was also observed at 8.0. However, at pH 5.0, 6.0, and 9.0, the yields were much lower than that at pH 7.0, demonstrating that NADH generation was prominently inhibited at low and high proton concentrations. The decrease in the rate of NADH formation at an acidic pH derives from the protonation of TEOA, which is transformed into a less effective electron donor. Similarly low reactivity at pH levels below 7.0 have also been observed in other reports using TEOA for the photoregeneration of NADH by Rh catalysts.27 Transient photocurrent was measured on a photoelectrochemical test cell fabricated by 12


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drop-casting P1 in ethanol (0.08 g/ml) onto fluorine-doped tin oxide glass. Figure 5 shows the photocurrent response versus time for P1 and P2. When P1 was used as an electrode, a photocurrent response under visible light irradiation was reversibly generated and was much higher than that with P2. The photocurrent response of P1 reproducibly increased under each irradiation. Finally, the enzymatic reduction of CO2 to formic acid was examined using formate dehydrogenase (FDH) to use the NADH produced in this photocatalytic system. The amount of formic acid was quantitated by GC. Upon irradiation of a CO2-saturated solution of P1, NAD+, and 3 in the presence of FDH (3 units), 50 µM·h-1 of formic acid was observed (Figure 6). The production of formic acid by P1 + 3 for 6 h was 270 µmol, while Ru(phen)(bpy)23+ + 3 and P2 + 3 were 140 and 50 µmol, respectively. Control experiments performed in the absence of P1, 3, or FDH showed no significant formation of formic acid. These data clearly reveal the high efficiency of P1 over Ru(phen)(bpy)23+ and P2.

Scheme 1. Artificial photosynthesis of NADH and formic acid by PDA-based photocatalytic system under visible light.

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Based on all of our described observations, the electron of the photoexcited Ru moiety in P1 is directly transferred to the PDA backbone and then to 3, which is supported by the luminescence quenching and photocatalytic NADH regeneration results (Figure 6). Because 9 % NADH was regenerated with P1 and 3, the electron transfer from the photo-excited P2 to the Rh(III) complex should occur. More NADH was regenerated using P1 with 3, indicating the significant role of the PDA-Ru(II) dyad (Scheme 1). The electron transfer to the PDA backbone could result in relatively stable charge separation because P1 afforded much higher efficiency in the NADH regeneration by 3 than by Ru(phen)(bpy)23+ or P2. In order to understand electron transfer from the Ru(II) photosensitizer to PDA, the molecular orbital energies of Ru(phen)(bpy)22+ and P2 were calculated directly from electrochemical results calibrated with ferrocene (Figure S9). Based on the data shown in Figure S6, S8, and S9, The Gibbs free energy can be reasonably estimated from the respective electrochemical potentials of Ru(phen)(bpy)22+ or P2 and the emission energy of Ru(phen)(bpy)22+, shown in the equation, ∆G°ET = -e{[Eox (Ru) – Ered (PDA)]- EMLCT (Ru*)} = -e{[1.1 – (-0.6)]-2.8} = -1.1 eV. The energy difference between the energy of the triplet MLCT excited state of the Ru(II) complex and the LUMO level of the PDA backbone in P1 is a driving force for the electron transfer. We have measured time-resolved photoluminescence as shown Figure S7 and have found that phosphorescence is significantly quenched in P1. But both electron transfer and energy transfer can quench phosphorescence. Therefore, only the time-resolved PL results cannot be used to directly distinguish two processes. To understand the relaxation dynamics of the electronically excited P1 further, transient absorption (TA) experiments with P1 and 14


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Ru(phen)(bpy)22+ excited by a pump pulse at 450 nm were carried out. This allows us to measure how fast the ground state hole of Ru complex is filled. TA signal from P2 is almost negligibly small and the electronic excitation of PDA moiety in P1 can be negligible (Figure S10). TA signal measured with P1 remains for a long time and its decaying profile is quite similar to that of [Ru(phen)(bpy)2]2+. The 1.1 ns component obtained by fitting to the time-resolved PL signal is plotted for comparison. The decay of the TA signal of P1 was analyzed by comparing with the simulation of the 1.1 ns decaying component of P1 obtained by the photoluminescence experiments (Figure S7 and S10). If the energy transfer occurs from Ru complex to PDA in P1, the TA signal should decay on the same time scale as the time-resolved PL signal does. If the electron transfer occurs from Ru complex to PDA in P1, the TA signal should decay much more slowly than the time-resolved PL signal. The TA signal of P1 decayed much more slowly which indicates an electron transfer process. Therefore, our time-resolved PL and TA experimental results lead to conclude that the photoinduced electron transfer should occur from Ru complex to PDA in P1. The reduction of 3 by the electron transfer from the photoexcited P1 affords a (η4Cp*H)Rh(III) species responsible for transferring its hydride to NAD+. Crystal structures of such (η4-Cp*H)Rh(III) complexes were recently reported, and these species are responsible for NAD+ reduction and hydrogen production.28,29 The oxidized polymer backbone or Ru moiety of P1 is regenerated by the electron supply from TEOA. Evidence of an electron transfer from TEOA to Ru(III) was obtained from the CV of Ru(phen)(bpy)22+ in the presence of TEOA in 0.1 M phosphate buffer (Figure S11). The catalytic oxidation currents were observed because of the reduction of Ru(phen)(bpy)23+ by TEOA.

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CONCLUSIONS The foregoing results demonstrate the Ru complex-tethered PDA to be an active photocollector for the visible light-driven photoregeneration of NADH using an Rh catalyst in aqueous solution. The photoexcited electrons of the Ru(phen)(bpy)23+ moiety of P1 are directly transferred to the PDA and to 3, but the electrons seem to be mostly transferred to the PDA, according to the NADH regeneration results. Consecutive conversion of CO2 to formic acid also occurred in connection with the photocatalytic system in the presence of FDH. Future work will focus on synthesizing variants of the polymer complex and incorporating a catalyst to develop photocatalytic systems with more efficient NADH production and CO2 conversion.

Acknowledgements This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) which was granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (20153030031720, to H. Lee) and a grant from the National Creative Research Initiative Programs of the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2012R1A3A2048814, to J. Yoon).

Supporting Information Available: Emission spectra, time-resolved emission decay profiles, cyclic voltammograms, 1H-NMR spectrum, and 13C-NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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O OH

1

O N

N H

N Ru

N N

N

N N Ru N N N

O

OH

O

N 2 N

N N N Ru N N N

OH NH OH OH O O O

O

OH

O

OH O NH

O

OH

O

OH

UV irradiation

P1 O

OH

O

OH OH OH OH O O O

O

OH

O

OH O OH

O

OH

O

OH

UV irradiation

P2

Figure 1. Polydiacetylene-(-NH-phen)Ru(bpy)2 (P1) prepared from a mixture of two monomers (1 : 2 = 7 : 1) and polydiacetylene polymer (P2) prepared using only 1 under UV light irradiation.

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(a)

Absorbance

1.2

blue P1 0.8

red P1 0.4

2 0.0 400

600

800

Wavelength (nm)

(b)

400

2

PL Intensity (a.u.)

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300

200

100

blue P1 red P1

0 500

600

700

Wavelength (nm)

Figure 2. (a) UV-vis absorption spectra of 25 µM 2 (dotted) and the blue (dashed) and red (solid) forms of P1 in 0.2 M phosphate buffer (pH 7.0). Inset, photographs of the blue and red forms of P1. (b) Fluorescence spectra of 50 µM 2 (dotted) and the blue (dashed) and red (solid) forms of P1 in 0.2 M phosphate buffer (pH = 7.0). 22


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0.4

Current (µA)

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0.0

P1 -0.4

2+

-0.8

Ru(bpy)2(phen) 1.2

0.8

0.4

0.0

Potential (V)

Figure 3. Cyclic voltammograms of Ru(phen)(bpy)32+ (dashed) and P1 (solid) in 100 mM phosphate buffer (scan rate = 25 mV/sec). WE, ITO; RE, Ag/AgCl.

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(a)

[NADH] (mM)

0.08

0.06

0.04

0.02

0.00 0

2

4

6

Time (h)

(b)

40

30

Yield, %

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20

10

0 5

6

7

8

9

pH value

Figure 4. (a) Photocatalytic activities in visible-light driven NADH photoregeneration. (●) P1 + 3, (■) Ru(phen)(bpy)23+ + 3, (○) P2 + 3, (□) 3, (△) P1. Reaction conditions: 25 μM P1 (or P2), 25 μM 3, 0.2 mM NAD+, and 0.4 M TEOA for 6 h irradiation in 75 mM phosphate buffer (pH = 7.0). (b) Photocatalytic activities of P1 with 3 in NADH regeneration at different pH. 24


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8 on 2

Photocurrent (µA/cm )

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off

6

P1 4

P2 2

0 0

120

240

360

480

600

Time (min)

Figure 5. Photocurrent response of FTO/P1 and FTO/P2 electrodes under illumination (1 sun) (three electrodes: platinum counter electrode, Ag/AgCl reference electrode) in 0.1 M NaCl aqueous solution containing 0.1 M TEOA.

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300 HCOOH Produced (µmol)

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250 200 150 100 50 0 0

3

6

Time (h)

Figure 6. Visible-light driven photocatalytic activities of P1 in artificial photosynthesis of formic acid from CO2. (●) P1 + 3, (■) Ru(phen)(bpy)23+ + 3, (▲) P2 + 3, (□) 3, (△) P1. Reaction conditions: 25 μM P1 (or P2), 25 μM 3, 0.2 mM NAD+, FDH (3 unit), and 0.4 M TEOA for 6 h irradiation in 75 mM phosphate buffer (pH = 7.0) under continuous flow of CO2 (0.5 mL/min).

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Ru(bpy) - ACS Publications - American Chemical Society

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