Artificial Photosynthesis Model: Photochemical Reaction System with


Dec 27, 2018 - Cite this:ACS Omega 2018, 3, 12, 18563-18571 ..... (21,23) Clay Minerals (saponite): Sumecton SA was received from Kunimine Industries ...
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Article Cite This: ACS Omega 2018, 3, 18563−18571

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Artificial Photosynthesis Model: Photochemical Reaction System with Efficient Light-Harvesting Function on Inorganic Nanosheets Takamasa Tsukamoto,*,†,‡,∥ Tetsuya Shimada,§ and Shinsuke Takagi*,§ †

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Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ‡ Japan Society for the Promotion of Science (JSPS/PD), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan § Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1, Minami-ohsawa, Hachiohji-shi, Tokyo 192-0397, Japan S Supporting Information *

ABSTRACT: In natural photosynthesis system, its complicated photofunctions are achieved with high efficiency through precise arrangements of dye molecules in proteins. However, it is difficult to imitate such reaction systems artificially because of the complexity of the protein structures. As the way to approach this issue, we suggest the selfassembling behavior of photofunctional dyes on inorganic nanosheets. In this study, photochemical reaction system with a light-harvesting function was newly constructed on a clay nanosheet as an artificial photosynthesis system model by using a metalloporphyrin as a photocatalyst and a subporphyrin as a photoanntena. Under the condition of their co-adsorption on the nanosheet, efficient energy transfer from the subporphyrin to the metalloporphyrin of up to 98% was achieved in the case of donor/acceptor ratio of 1:1. By utilizing such dye−clay complexes, the metalloporphyrin photocatalyst could catalyze the photochemical conversion of cyclohexene by the excitation of both the subporphyrin photoantenna and itself. This lightharvesting system enabled the photocatalytic reaction to use a wider range of visible region without any energy loss because of suppression of unexpected other deactivation processes by precise arrangement of dyes in contrast to general co-adsorption systems. These results would be useful in constructing various types of artificial photosynthesis systems using self-assembling behavior.



INTRODUCTION Natural plants have two general phases in their photosynthesis system: (i) excited energy transfer in light-harvesting (LH) system and (ii) photoinduced electron transfer reaction in reaction center (RC).1,2 These complicated photofunctions are achieved by precise arrangements of functional molecules in steric structures of proteins. This natural reaction system is, however, too difficult to imitate artificially because of the complexity of the protein structures. Therefore, we have focused on a clay nanosheet as one of ubiquitous inorganic host materials. We have investigated the self-assembling behaviors and arrangement structures of guest dye molecules adsorbed on the nanosheet surface with the aim to construct an artificial photosynthesis system model. The clay minerals are negatively charged multilayered aluminosilicates in general.3 Because of their cation-exchange capacity (CEC), interlayer counter mineral cations can be exchanged with other organic guest molecules easily, and photofunctional applications such as energy transfers or electron-transfer reactions have been investigated using dye guest molecules by many researchers.4−10 Especially clay aqueous dispersion, where swellable clay such as saponite is exfoliated as single nanosheets, is beneficial for optical applications because of © 2018 American Chemical Society

its transparency in the UV−visible range. However, dye molecules adsorbed on or introduced within inorganic host materials including clay tend to decline their photoactivities due to unexpected aggregation behavior.11−13 It was recently reported that for some multicationic molecules, the aggregation can be suppressed on the clay nanosheet even under high-density adsorption conditions when intramolecular positive-charge distances in guest molecules and average negative-charge distances on the clay surface coincide well each other (size-matching effect).14 As an application study of this effect, a precisely controlled energy transfer between two or three dye molecules has been accomplished,15,16 in contrast to reports of other host materials.17,18 Additionally, through adsorption on or intercalation into the clay nanosheet surfaces/interfaces, photoactivities of dye molecules such as fluorescence quantum yield or excited lifetime tend to be kept or enhanced due to fixation of molecular structure.19−23 These findings are preferred for developing photofunctional materials. Received: September 30, 2018 Accepted: December 12, 2018 Published: December 27, 2018 18563

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nonaggregate arrangement of the photocatalyst by the sizematching effect. In this study, utilizing this knowledge about dye−clay complexes, we investigated photocatalytic reaction with lightharvesting system as a novel artificial photosynthesis system model (Figure 2a). Precisely designed +3-charged boron(III) subporphyrin (1-LH) is chosen as an energy donor for the light-harvesting system and +3-charged antimony(V) porphyrin (2-RC) are chosen as an energy acceptor and a photocatalyst for the photoreaction system (Figure 2b).21,23

On the other hand, photochemical oxygenation reactions of alkenes sensitized by metalloporphyrins with water as both electron and oxygen atom donor in water−acetonitrile solution have been reported by several groups including ours.24−26 Especially, antimony(V) porphyrins can catalyze the photochemical reaction efficiently due to a central metal with large electronegativity and valency.26 A proposed reaction mechanism of this oxygenation reaction sensitized by antimony(V) porphyrin using hexachloroplatinate(IV) anion ([PtIVCl6]2−) as an electron acceptor and cyclohexene (C6H10) as a substrate is shown in Figure 1. Not only oxygenated but also chlorinated species of the alkenes are produced in the presence of chloride anion.



RESULTS AND DISCUSSION Molecular Designs of 1-LH and 2-RC. By utilizing subporphyrin skeleton, 1-LH as a photoanntena is designed to have absorption spectra in the wavelength region where 2-RC cannot have absorption, and it allows us to use a wider range of visible light compared to our previous studies. Additionally, the overlap of emission of 1-LH and the absorption of 2-RC was also suitably designed to enhance the energy-transfer efficiency (described later). 2-RC as a photocatalyst is designed to catalyze the photochemical conversion of cyclohexene with high efficiency by utilizing antimony(V) porphyrin derivative. Both 1-LH and 2-RC have three methylpyridinium units, and their substitution positions fulfill the size-matching effect for precise adsorption on the clay nanosheet surface. The CEC of saponite clay is ca. 1.0 × 10−3 equiv g−1 and its structure and stoichiometric formula are shown in Figure S1 in the Supporting Information. The aqueous dispersion of saponite is substantially transparent in the UV−visible range because of its small particle size (<∼100 nm). The average negative charge distance on the saponite clay surface is estimated to be ca. 1.2 nm14 (Figure 2c). On the other hand, the intramolecular positive charge distances in 1-LH and 2-RC molecules are calculated to be 1.1 nm and 1.1 and 0.7 nm (pyridinium to pyridinium and pyridinium to metal), respectively, according to DFT calculations28 (Figures 2c and S2).

Figure 1. Proposed reaction scheme of photochemical oxygenation and chlorination of cyclohexene using antimony(V) porphyrin as a sensitizer.

Recently, the photochemical reaction by using a cationic metalloporphyrin−anionic clay complex as a sensitizer was also achieved.27 During the photoreaction, the porphyrin−clay complex was more stabilized than that without the clay because of steric and electric protection of the activated porphyrin radical from an attack of anion by the clay surface. Additionally, this complex could catalyze the reaction well even under high porphyrin adsorption conditions due to

Figure 2. Artificial photosynthesis system model of the present work. (a) The photochemical reaction system with light-harvesting system on the clay surface. (b) The molecular structures of 1-LH and 2-RC and (c) the distances between their cationic parts and anionic ones on the saponite clay surface. 18564

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Figure 3. Photochemical properties of dye molecules. (a) The absorption (solid line) and fluorescence (dash line, excited at 400 nm) spectra of 1LH and 2-RC on the clay surface in water−acetonitrile (30:70 (v/v)). The loading level was at 10% versus CEC of clay. [1-LH] = 3.0 × 10−7 M, [2-RC] = 1.0 × 10−7 M. (b) The fluorescence decay profile and fluorescence lifetimes (τf) for 1-LH and 2-RC on the clay surface in water− acetonitrile (30:70 (v/v)). The dye loadings on the clay surface were 0.2% versus CEC. [1-LH] = [2-RC] = 8 × 10−8 M. (c−e) The fluorescence intensity of dye molecules on the clay surface at various dye loadings in water−acetonitrile (30:70 (v/v)). [1-LH] = [2-RC] = 2.0 × 10−7 M.

Table 1. Fluorescence and Energy Transfer Parameters of Dye Molecules on the Clay Surface in Water−Acetonitrile (30:70 (v/v))a fluorescence parameters compound 1-LH 2-RC

Φf 0.67 0.034

τf/10−9 s 6.8 1.4

energy transfer parameters

kf/107 s−1

kd/107 s−1

9.9 2.4

4.9 69

J/M−1 cm3 −12

2.3 × 10

kET/s−1 8.7 × 1010

Φf is the fluorescence quantum yield (10% vs CEC). τf is the fluorescence lifetime (0.2% vs CEC). kf and kd are radiative and nonradiative deactivation rate constants. J is the spectral overlap integral between the fluorescence spectrum of donor and the absorption spectrum of acceptor. kET is the theoretical energy transfer rate constant. a

6.8 and 1.4 × 10−9 s, respectively, according to time-resolved fluorescence decay profiles (Figure 3b). The fact that both the decay curves can be fitted as single exponentials indicated that these molecules behave as monomers even on the clay. Their radiative and nonradiative rate constants (kf and kd) were estimated according to eqs 1 and 2.29

Photochemical Properties of 1-LH and 2-RC on the Clay Surface. First, the photochemical properties of photoanntena (1-LH) and photocatalyst (2-RC) were investigated. Absorption and fluorescence spectra of 1-LH and 2-RC on the clay surface were observed in water−acetonitrile (30:70 (v/v)) (Figure 3a). For the absorption spectra, 1-LH had peaks at 375 and 460 nm and 2-RC had them at 420, 550, and 590 nm. Aggregation behaviors were suppressed for both the dye molecules, judging from the spectral shapes at this adsorption density (10% vs CEC). This condition is beneficial for constructing artificial light-harvesting system because the photoantenna 1-LH was covering the wavelength region where the photocatalyst 2-RC cannot absorb. For the fluorescence spectra, a peak of 1-LH was observed at 545 nm and that of 2-RC was at 600 and 650 nm. The absorption spectra of the energy acceptor 2-RC and the fluorescence spectra of the energy donor 1-LH have a large overlap suitable for energy transfer (described later). Fluorescence quantum yields of 1-LH and 2-RC (ΦfD and ΦfA) without any selfquenching behaviors (describe later) were 0.67 and 0.034 and their fluorescence lifetimes (τfD and τfA) were estimated to be

Φf = τf =

kf k f + kd 1 k f + kd

(1)

(2)

Their radiative rate constants (kfD and kfA) were calculated to be 9.9 and 2.4 × 107 s−1, respectively. Also, their nonradiative constants (kdD and kdA) were calculated to be 4.9 and 69 × 107 s−1, respectively. The kdA value was an order of magnitude greater than other rate constants because an intersystem crossing rate constant would be great because of internal heavy atom effect by antimony. The theoretical Förster resonance energy transfer rate constant (kET) value was estimated to be 8.7 × 1010 s−1 in the Experimental section. The theoretical kET 18565

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Figure 4. Energy transfer from 1-LH to 2-RC on the clay surface. (a) The fluorescence spectra for co-adsorbed donor and acceptor at various dye loadings in water−acetonitrile (30:70 (v/v)). The dash lines are the individual fluorescence spectra of 1-LH and 2-RC at 10% versus CEC of clay. [1-LH] = [2-RC] = 2.0 × 10−7 M ([1-LH]/[2-RC] = 1:1, excited at 390 nm). [1-LH] = 2.7 × 10−7 M, [2-RC] = 2.0 × 10−7 M ([1-LH]/[2-RC] = 4:3, excited at 390 nm). [1-LH] = 3.0 × 10−7 M, [2-RC] = 1.0 × 10−7 M ([1-LH]/[2-RC] = 3:1, excited at 400 nm). (b) The energy-transfer efficiencies at various dye loadings and ratios. [1-LH]/[2-RC] = 1:1 (■), 4:3 (●), 3:1 (⧫). (c) The co-adsorption arrangements of the energy donor and acceptor.

value in this system would be suitable for proceeding of energy transfer because it is large enough compared with the radiative and nonradiative deactivation rate constants of donor. These parameters for fluorescence and energy transfer are summarized in Table 1. Self-quenching behaviors of 1-LH and 2-RC on the clay surface were investigated in water−acetonitrile (30:70 (v/v)) (Figures 3c−e and S3). The self-quenching is expected to take place due to collisional reactions such as an electron transfer reaction between dye molecules in excited state and ground state. This behavior lets dye molecules lose their own excited singlet lifetimes, leading to a decrease in the energy-transfer efficiency. Thus, it is preferred that the self-quenching behavior is suppressed for the construction of efficient energy-transfer systems. Maximum loading levels of 1-LH23 and 2-RC21 without the aggregation behavior were 85 and 75% versus CEC of the clay, respectively. In the case of 1-LH, the selfquenching behavior was completely suppressed even at high dye loadings (Figure 3c). The result would be because of the molecular structure of 1-LH where all of three intramolecular distances fulfill the size-matching effect due to an equilateraltriangular array. On the other hand, for 2-RC, the selfquenching behavior was slightly observed (Figure 3d). However, this behavior was suppressed under a co-adsorption condition with 1-LH (Figure 3e). The result was expected to be due to a well-ordered adsorption condition induced by 1LH fulfilling the size-matching effect strictly. The fact that the excited energy of 2-RC can be kept by co-adsorption with 1LH is beneficial for applying 2-RC as a photocatalyst to photochemical reaction systems on the clay surface. Energy Transfer from 1-LH to 2-RC on the Clay Surface. In this section, we focus on the light-harvesting performance of the photoanntena (1-LH). Singlet−singlet

energy transfer from 1-LH to 2-RC on the clay surface was investigated in water−acetonitrile (30:70 (v/v)). Molar ratios of 1-LH to 2-RC were modulated as D/A = 1:1, 4:3, and 3:1. Total loading levels of the two types of dye molecules were set at 5−70% versus CEC by controlling the clay concentrations. Fluorescence spectra of 1-LH and 2-RC were observed by exciting at their B-bands. For all samples, the fluorescence intensity of 1-LH at 545 nm decreased and those of 2-RC at 600 and 650 nm increased (Figure 4a). Their mixed fluorescence spectra can be fitted well as superposition of individual reference spectra of 1-LH and 2-RC. These results suggest that energy transfer from excited 1-LH to 2-RC took place. The energy-transfer efficiency (ηET) was calculated according to eq 3. F DA (3) FD DA where ηET is the energy-transfer efficiency, F is the fluorescence intensity of the donor in the energy-transfer sample, and FD is the fluorescence intensity of the donor in individual reference sample of the donor. The ηET values at various dye loadings and ratios are shown in Figure 4b. The maximum energy-transfer efficiencies were 98, 96, and 90% in the case of [1-LH]/[2-RC] = 1:1, 4:3, and 3:1 at 70% versus CEC, respectively. For time-resolved fluorescence spectra at 60% versus CEC of the clay, fast decay and rise components in emitting regions of the donor and acceptor were observed as τ = ca. 6 × 10−11 s (Figure S4). This result also indicated the occurrence of energy transfer. An order of experimental kET value was estimated to be 2 × 1010 s−1 and almost agreed with the theoretical one mentioned above. The ηET values tended to be greater with an increase in dye loadings for all ratios of the donor and acceptor. It would be because of a decrease in the ηET = 1 −

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center-to-center distance between the donor and the acceptor (R) because of an increase in the adsorption density on the clay surface. And, the ηET values also tended to be greater with a decrease in the donor to acceptor ratios at all dye loadings. This result would be because of an increase in the number of acceptor molecules adjacent to the donor molecule. The fact that the ηET value reached almost 100% indicates that the two types of dye molecules co-adsorb on the clay surface randomly (Figure 4c). As a result, we succeeded in constructing an efficient energy-transfer system using 1-LH and 2-RC on the clay surface without aggregation and segregation. Photochemical Reaction Sensitized by 2-RC on the Clay Surface. In this section, we discuss the catalytic ability of the photocatalyst (2-RC). The photochemical oxygenation and chlorination reaction of cyclohexene sensitized by 2-RC on the clay surface was examined by irradiating visible light (λmax = 550 nm) to the Q-band. A decrease in the absorption spectra of [PtIVCl6]2− (250−350 nm) indicates that the photochemical reaction proceeded. Under a condition of 15% dye loading versus CEC of the clay, during photoirradiation, a decomposition of 2-RC was observed moderately during 160 min of irradiation (Figure 5a). It has been reported that the decomposition of porphyrin is due to an addition or substitution of chloride anion (Cl−) to porphyrin cation radical.27 The quantum yield (ΦeTA) for an electron transfer from excited 2-RC to [PtIVCl6]2− was estimated to be 10% from irradiation photon intensity and the decreased amount of [PtIVCl6]2− absorption at 2 min irradiation as mentioned in the Method section. The ΦeTA value is rough because the scattering of clay was superimposed slightly on the absorption spectra of [PtIVCl6]2−. Total concentration of photochemical oxygenated and chlorinated products of cyclohexene observed by gas chromatography−mass spectrometry (GC−MS) was 3.5 × 10−4 M for 160 min of irradiation (Table 2iii). 2-RC would have efficient catalytic ability because its turnover number (TONA), which is the molar number of oxidative products of cyclohexene until 1 mol photocatalyst is decomposed, was ca. 130. On the other hand, the photochemical reaction cannot proceed well (total product was 4.7 × 10−5 M (Table 2ii)) by irradiation with visible light (λmax = 460 nm) because of the bare absorption of 2-RC at 460 nm (Figure S5). Additionally, 1-LH has almost no catalytic ability (Table 2i) because the quantum yield (ΦeTD) for the electron transfer from 1-LH to [PtIVCl6]2− was estimated to be 0.5% (Figure S6). TON of 1-LH (TOND) was no more than 17. Connection of the Light-Harvesting and Photocatalytic Functions on the Clay Surface. Finally, we investigated the photochemical reaction sensitized by the photocatalyst (2-RC) excited by energy transfer from the photoanntena (1-LH) on the clay surface. The photochemical oxygenation and chlorination reaction of cyclohexene sensitized by 2-RC co-adsorbed with 1-LH on the clay surface was examined by irradiating visible light (λmax = 460 and 550 nm) to the Q-bands of the donor and acceptor, respectively. As a result, an efficient photochemical reaction proceeded by exciting both 1-LH and 2-RC. Unlike the photochemical reaction system with only 2-RC, under a condition of 60% versus CEC of the clay and [1-LH]/[2-RC] = 3:1, during the photoirradiation, the decomposition of 2-RC was completely suppressed by 60 min irradiation at both 460 and 550 nm. On the other hand, the decomposition of 1-LH was observed barely (Figure 5b,c). The quantum yields for the electron transfer to [PtIVCl6]2− were estimated for 2 min irradiation at

Figure 5. Absorption spectral change of the photoreaction mixture. (a) Photochemical reaction sensitized by 2-RC. The irradiated wavelength was 550 nm. The loading level was 15% versus CEC of clay. [2-RC] = 1.0 × 10−5 M. Irradiation time (λmax = 550 nm) was 0, 1, 2, 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, and 160 min. (b, c) Photochemical reaction sensitized by 2RC under the co-adsorption condition with 1-LH. The loading level was 60% versus CEC of clay. [1-LH]/[2-RC] = 3:1. [1-LH] = 3.0 × 10−5 M. [2-RC] = 1.0 × 10−5 M. (b) The irradiated wavelength was 460 nm. Irradiation time (λmax = 460 nm) was 0, 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, and 60 min. (c) The irradiated wavelength was 550 nm. Irradiation time (λmax = 550 nm) was 0, 1, 2, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, and 80 min. [cyclohexene] = 1.0 × 10−1 M. [PtIVCl62−] = 5.0 × 10−4 M. [clay] = 2.0 × 10−4 equiv L−1. The solvent was water−acetonitrile (30:70 (v/v)).

460 nm (ΦeTD*A). The ΦeTD*A was 9%. This value was similar to the ΦeTA value (10%) and different from the ΦeTD value (0.5%). This result indicates that the energy transfer from 1LH to 2-RC proceeded and the photoinduced electron-transfer reaction took place on 2-RC molecule despite the excitation of 1-LH. On the other hand, the quantum yields for irradiation at 550 nm (ΦeTDA*) was the same as ΦeTA value (10%). The result that the ΦeTD*A value was 0.9 times as the ΦeTDA* value can be rationalized by the fact that the energy-transfer efficiency (ηET) was 90% under this condition. This result suggested that the photocatalytic ability of 2-RC was maintained even under the co-adsorption condition with 1LH due to the size-matching effect. Total photochemical oxygenated and chlorinated product concentration was 3.2 × 10−4 M for 60 min (Table 2iv) and 3.4 × 10−4 M for 80 min 18567

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Table 2. Products of Photochemical Oxygenation and Chlorination Reaction of Cyclohexene on the Clay Surfacea

[1-LH] = 3.0 × 10−5 M, [2-RC] = 1.0 × 10−5 M, [PtIVCl62−] = 5.0 × 10−4 M, [cyclohexene] = 1.0 × 10−1 M, [clay] = 2.0 × 10−4 equiv L−1. ΦeT is the photoinduced electron-transfer quantum yield from excited sensitizer to [PtIVCl62−] estimated at 2 min irradiation. TON is the molar number of products until 1 mol photocatalyst is decomposed. Visible lights (460 and 550 nm) were irradiated for 1-LH and 2-RC, respectively. The solvent was 5 mL water−acetonitrile (30:70 (v/v)). a

Figure 6. Proposed reaction scheme for the photochemical oxygenation and chlorination of cyclohexene sensitized by 2-RC excited by the energy transfer from 1-LH.



CONCLUSIONS The photochemical oxygenation and chlorination reaction of cyclohexene sensitized by 2-RC with the energy transfer system consisting of 1-LH on the clay surface was investigated. The aggregation and self-fluorescence quenching behaviors were not observed for both molecules on the clay surface even at high dye loading levels. Because of these simple adsorption behaviors (no aggregation and no segregation), the efficient energy transfer up to 98% from 1-LH to 2-RC on the clay surface was succeeded. In the case of dye−clay complex where both molecules are co-adsorbed, the efficient photocatalytic reaction sensitized by 2-RC followed by the excitation of both photoantenna and photocatalyst directly originated from this energy-transfer system. Additionally, in the co-adsorbed sample, the decomposition of 2-RC was completely suppressed during the photoreaction in contrast to the sample without 1LH. As a result, we succeeded in providing light-harvesting function with a photocatalytic reaction without any deactivation using a self-assembly of precisely designed dyes on the clay surface, and demonstrated the advantage of organic− inorganic hybrid compounds for the development of advanced photofunctional materials such as the artificial photosynthetic system.

irradiation (Table 2v). These values were same as that without energy donor. The TON values of 2-RC (TONA) excited at both 460 and 550 nm were larger than that without 1-LH and could not be determined because the decomposition of 2-RC was completely suppressed. From this result, it was suggested that 2-RC co-adsorbed with 1-LH was more stable in the photochemical reaction than that without the energy donor. On the other hand, 1-LH was decomposed barely and those of 1-LH (TOND) were about 170. The stabilization of 2-RC is expected to be due to an active species transfer reaction from 2-RC cation radical to 1-LH because the decomposition of porphyrin is due to an attack of chloride anion (Cl−) on porphyrin cation radical30 (Figure 6) (discussed in the Supporting Information section, Figures S7−S9). Quantum yield for the electron transfer to 2-RC cation radical from 1LH was estimated to be ca. 100% according to the complete suppression of porphyrin decomposition. A proposed reaction scheme for this photochemical reaction system is shown in Figure 6. These result indicate that precise self-assembly of dye molecules on the clay surface enables the construction of highly effective photofunctions such as light-harvesting (LH) system and reaction center (RC) like natural plants. Few studies have been reported on such self-assembling systems composed of both energy-transfer and electron-transfer processes.31 Additionally, the photocatalyst was protected from the attack of Cl− anion by the photoantenna. This phenomenon is similar to the mechanism of RC protection from photodamage by antenna dyes in plants.



EXPERIMENTAL SECTION Materials. 1-LH and 2-RC were synthesized according to the established routes and the details are described in our previous paper.21,23 Clay Minerals (saponite): Sumecton SA was received from Kunimine Industries Co., Ltd. Hexachloroplatinate(IV) disodium salt (denoted as Na2[PtIVCl6]) was purchased from Aldrich. Cyclohexene was 18568

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clay. Monochromatic lights (λmax = 460 and 550 nm) were irradiated to the reaction mixture for excitations of the energy donor and acceptor, respectively. Calculation of Theoretical Förster Resonance EnergyTransfer Rate Constant. A theoretical Förster resonance energy transfer rate constant is estimated according to eq 4.32

purchased from Nacalai Tesque, Inc. and purified by short alumina column just before use. Instruments. UV−visible absorption spectra were obtained on Shimadzu UV-3150 spectrophotometer. Emission spectra were recorded on Jasco FP-6600 spectrofluorometer. Timeresolved fluorescence signals were measured by Hamamatsu Photonics C4780 system based on a streak detector. An Nd3+ YAG laser with an optical parametric generator (EKSPLA PL2210JE + PG-432, FWHM 25 ps, 1 kHz) was used for excitation. Monochromatic light through filters (Edmund Optics Inc.) from a 500 W Xe arc lamp (USHIO 500-DKO) was used for photochemical reactions. An interference filter was used for monochromatic lights (λmax = 460 and 550 nm). Irradiation photon intensity was measured by ADCMT 8230 optical power meter. The reaction products were analyzed by Shimadzu QP-2010 GC−MS spectrometer. Sample Preparation for Absorption, Fluorescence, and Time-Resolved Fluorescence Spectra. The dye molecules were added to water−acetonitrile (30:70 (v/v)) solution. Concentrations of 1-LH and 2-RC were 3.0 and 1.0 × 10−7 M for absorption and fluorescence spectra, respectively. Then, the clay aqueous dispersion was added to sample solution under stirring. Loading levels versus cation-exchange capacity (CEC) of the clay were set at 10% by controlling the concentration of the clay. Fluorescence quantum yields of the dye molecules were determined using dihydroxo(5,10,15,20tetraphenylporphyrinato) antimony(V) chloride as a standard (Φf = 0.052).21 Concentrations of the dyes were 8.0 × 10−8 M, and their loading levels were set at 0.2% versus CEC for timeresolved fluorescence spectra. Sample Preparation for Energy Transfer in [1-LH + 2RC]−Clay Complex. 1-LH as an energy donor and 2-RC as an energy acceptor were added to water−acetonitrile (30:70 (v/v)) solution by molar ratio [1-LH]/[2-RC] = 1:1, 4:3, and 3:1. Concentrations of the two compounds mixture were 4.0, 4.7, and 4.0 × 10−7 M at 1:1, 4:3, and 3:1, respectively. Then, the clay aqueous dispersion was added to the sample solution under stirring. Loading levels versus CEC of the clay were controlled by the concentration of the clay. Photochemical Reaction Using [2-RC]−Clay Complex. Concentrations of 2-RC as a sensitizer, cyclohexene as a substrate, and Na2[PtIVCl6] as an electron acceptor were 1.0 × 10−5, 1.0 × 10−1, and 5.0 × 10−4 M, respectively. Solvent was 5 mL water−acetonitrile (30:70 (v/v)). Loading level of 2-RC was 15% versus CEC of the clay in 2.0 × 10−4 equiv L−1 clay dispersion. The sample solution was added in a 1 × 1 × 4 cm3 quartz cell sealed by septum cap. Oxygen in the sample solution was removed by nitrogen bubbling for 30 min in the dark at room temperature. Monochromatic light (λmax = 550 nm) was irradiated to the stirred sample through 4 cm light path. The photochemical reaction was monitored with UV− visible spectrometer through 1 cm light path. The reaction mixture after the photochemical reaction was vacuum-distilled, and products were analyzed by GC−MS spectrometer. Photochemical Reaction Using [1-LH + 2-RC]−Clay Complex. Concentration of 1-LH as an energy donor was 3.0 × 10−4 M. The concentrations of 2-RC, the substrate, and the electron acceptor, the volumes of solvent and optical cell, and the methods of optical measurement and product analysis were same as those in the investigation without the energy donor described above. Molar ratios of 1-LH to 2-RC were modulated as [1-LH]/[2-RC] = 3:1. Total loading level of 1-LH (45%) and 2-RC (15%) was 60% versus CEC of the

kET =

9000 ln 10κ 2k fD 128π 5n 4NR6

J

(4)

where κ is the orientation parameter (assuming κ2 = 0.53 calculated from orientation angles between donor and acceptor according to waveguide dichroic absorption spectra in Figure S10),33 kfD is the radiative deactivation rate constant of donor (kfD = 9.9 × 107 s−1), n is the refractive index of the bulk medium (n = 1.34 in water−acetonitrile (30:70 (v/v))), N is the Avogadro constant, R is the average center-to-center distance between donor and acceptor (assuming R = 2.7 nm at 60% vs CEC), and J is the spectral overlap integral between the fluorescence spectrum of donor and the absorption spectrum of acceptor (Figure S11). The J value was calculated to be 2.3 × 10−12 M−1 cm3 according to eq 5.34 D A

J=

∫ Fν ε4

dν ̅ (5) ̅ D where ν̅ is the wavenumber, F is the fraction of the total fluorescence intensity of donor, and εA is the extinction coefficient of acceptor. Calculation of Electron Transfer Quantum Yield in Photochemical Reaction. A quantum yield (ΦeT) for an electron transfer from an excited dye molecule to [PtIVCl6]2− was estimated according to eq 6. ΦeT =

hcNν ̅ V ΔC P Δt

(6)

where h is the Planck’s constant, c is the velocity of light, N is the Avogadro’s constant, ν̅ is the irradiation wavenumber (ν̅ = 21700 or 18200 cm−1 (λmax = 460 or 550 nm)), V is the volume of sample solution (V = 5 mL), ΔC is the decrease in concentration of [PtIVCl6]2−, P is the irradiation photon intensity, and Δt is the irradiation time.



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02594. Structure of synthetic saponite; optimized structures of 1-LH and 2-RC obtained by DFT calculations; fluorescence spectral change of 1-LH and 2-RC at various dye loadings; fluorescence decay profiles of [1LH + 2-RC]−clay complex; absorption spectral change of the photoreaction mixture under various reaction conditions; waveguide dichroic absorption spectra of 2RC; extinction coefficient of 2-RC and the fraction of the total fluorescence intensity of 1-LH; discussion of active species transfer from 2-RC to 1-LH (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.T.). *E-mail: [email protected] (S.T.). 18569

DOI: 10.1021/acsomega.8b02594 ACS Omega 2018, 3, 18563−18571

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Article

ORCID

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Takamasa Tsukamoto: 0000-0003-0017-7183 Shinsuke Takagi: 0000-0001-7013-4942 Present Address ∥

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan (T.T.).

Author Contributions

T.T. initiated the present work, and T.S. and S.T. directed it. T.T. conducted the synthesis of the complexes, and a series of photochemical measurements. T.S. measured and analyzed time-resolved fluorescence spectra. T.T., T.S., and S.T. wrote the manuscript. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas “All Nippon Artificial Photosynthesis Project for Living Earth (AnApple)” grant (No. 25107521), a Grant-in-Aid for Scientific Research (B) (No. 24350100) from the JSPS and a Grant-in-Aid for JSPS Fellows (No. 2603441).



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DOI: 10.1021/acsomega.8b02594 ACS Omega 2018, 3, 18563−18571