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