Size-Matching Effect on Inorganic Nanosheets - American Chemical

Invited Feature Article pubs.acs.org/Langmuir

Size-Matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and Orientation of Molecular Adsorption as a Bottom-Up Methodology for Nanomaterials Shinsuke Takagi,*,†,‡ Tetsuya Shimada,† Yohei Ishida,†,§ Takuya Fujimura,†,§ Dai Masui,† Hiroshi Tachibana,† Miharu Eguchi,⊥ and Haruo Inoue*,† †

Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minamiohsawa 1-1, Hachiohji, Tokyo 192-0397, Japan ‡ PRESTO (Precursory Research for Embryonic Science and Technology), Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan § Japan Society for the Promotion of Science (DC1), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan ⊥ Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan S Supporting Information *

ABSTRACT: We have been investigating complexes composed of nanolayered materials with anionic charges such as clay nanosheets and dye molecules such as cationic porphyrins. It was found that the structure of dye assembly on the layered materials can be effectively controlled by the use of electrostatic host−guest interaction. The intermolecular distance, the molecular orientation angle, the segregation/ integration behavior, and the immobilization strength of the dyes can be controlled in the clay−dye complexes. The mechanism to control these structural factors has been discussed and was established as a size-matching effect. Unique photochemical reactions such as energy transfer through the use of this methodology have been examined. Almost 100% efficiency of the energy-transfer reaction was achieved in the clay−porphyrin complexes as a typical example for an artificial light-harvesting system. Control of the molecular orientation angle is found to be useful in regulating the energy-transfer efficiency and in preparing photofunctional materials exhibiting solvatochromic behavior. Through our study, clay minerals turned out to serve as protein-like media to control the molecular position, modify the properties of the molecule, and provide a unique environment for chemical reactions.

1. INTRODUCTION

molecular interactions are expected to play key roles. So far, many excellent structures of molecular assembly have been reported by the use of a self-assembly technique.1−4 Among many possible interactions such as host−host and host−guest interactions through van der Waals, hydrophobic/hydrophilic, hydrogen bonding, metal coordination, and electrostatic interactions, we have developed a novel methodology to control the molecular assembly structure by the use of host− guest interaction through electrostatic interactions. As a host material, clay minerals5−9 that are typical inorganic layered materials have been mainly used. Dye molecules such as porphyrin derivatives10,11 are used as functional dye guest molecules.12−27 In this article, the unique structure control of dye molecule assembly formed on the clay surface and their photochemical reactions and solvatochromic functions are reported.

In chemical reactions, intermolecular distances and the relative orientation angle play important roles. In fact, the control of the position of molecules is especially crucial in the living body. Mostly inspired by the living system, extensive studies on organic/inorganic hybrid compounds to exhibit functional behavior have been reported so far. Organic molecules generally tend to aggregate and/or segregate on inorganic surfaces, mainly because of the hydrophobic interaction and van der Waals interaction between the organic molecules. The formation of irregular aggregates may prevent us from realizing a useful reaction system, especially for photochemical reactions. The aggregation, such as H-aggregate formation, significantly decreases the excited-state lifetime of dye molecules. However, the segregation causes a decrease in reaction efficiency because of the decrease in neighboring probabilities of molecules. Only at the interface of segregated dyes they can react with each other. Thus, the control of assembly structure such as intermolecular distances and relative orientation should be especially crucial in designing a chemical reaction system where © 2013 American Chemical Society

Received: August 29, 2012 Revised: January 4, 2013 Published: January 5, 2013 2108

dx.doi.org/10.1021/la3034808 | Langmuir 2013, 29, 2108−2119

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Invited Feature Article

clay particles and a highly magnified view of the surface are shown in Figure 1b,c. As can be seen in Figure 1c, the surface of the clay particle is very flat, even at the atomic level over a wide area. The clay sheets stack to form large secondary particles in the powder state. Under appropriate conditions, the stacked clay sheets can swell or even exfoliate into a single nanosheet in solution. Under the present conditions, clay sheets completely exfoliate, when water is used as a solvent. The reversibility between the exfoliated and stacked state is a unique structural property of layered materials. Because saponite clay possesses anionic charges in the structure, cationic molecules can easily adsorb to form the organic/inorganic hybrid complex. Many researchers have been examining the form of the unique dye− clay hybrid. Although the hybrids can work as sensors, photofunctional materials, and photochemical reaction fields,5−9 more precise molecular-level control of the hybrid structure is needed. In this article, our attempt to control the molecular-level structure of the clay−dye hybrid and its unique properties are described.

Clay minerals are attractive multilayered inorganic materials that are characterized by (1) nanostructured flat sheets that can be formed by elements with a large Clarke number, (2) negatively or positively charged surfaces, (3) exfoliation or the stack ability of individual nanosheets depending on the surrounding conditions, (4) an interlayer space of whose volume can be reversibly changed, and (5) good optical transparency in visible region, when the particle size is small (ca. < 200 nm). The unit structure of saponite as one of the typical clay minerals is shown in Figure 1. Saponite clays

2. SIZE-MATCHING EFFECT IN COMPLEX FORMATION BETWEEN CLAYS AND DYES Because typical saponite clays possess anionic charges in each nanosheet, cationic guest molecules can form stable complexes by means of electrostatic interactions. Dye molecules having cationic moieties such as pyridinium or anilinium groups can form stable complexes with clays. Upon complex formation with the clay, the porphyrin molecule exhibits a relatively large red shift in the absorption spectrum.28−32 Red shifts of approximately 30 and 60 nm were observed for the exfoliated and stacked complexes in aqueous transparent dispersions, respectively. The unique effects of clay on the photochemical properties of dyes will be discussed later (section 4). It is known that organic dyes tend to form irregular aggregates in the clay complexes.33−38 We have confirmed that cationic stilbazolium, triphenylmethane, rhodamine, tetrazolium derivatives, and Safranine O easily form such aggregates on the clay surface. However, it has been found that some types of dyes can adsorb on the clay surface in a monomeric form up to 100% versus CEC without aggregation.30,31,39 This high-density adsorption structure without aggregation is very unique, especially for photochemical reactions. Because the monomeric

Figure 1. (a) Structure of saponite having one octahedral layer sandwiched by two tetrahedral layers, (b) AFM image of clay particles, and (c) AFM image of the surface of a clay particle and the ideal distribution of anionic charge.

possess layers consisting of a 2:1 pair of nanosheets with octahedral and tetrahedral microstructures. The typical chemical formula (saponite clay) is expressed as [(Si7.20Al0.80)(Mg5.97Al0.03)O20(OH)4]−0.77(Na0.77)+0.77. The isomorphous substitution of Si4+ by Al3+ in the tetrahedral layer produces anionic charges in the structure. The unit structure can be extended widely in two dimensions. In this case, the average intercharge distance on the clay surface is calculated to be 1.2 nm in the hexagonal array because the cation exchange capacity (CEC) is 0.997 meq g−1 and the theoretical surface area is 7.5 × 102 m2 g−1. An atomic force microscope (AFM) image of the

Figure 2. Absorption spectra of (a) saponite−cyanine and (b) saponite−TMPyP4+ complexes at various dye concentrations in an aqueous dispersion. The loading levels are 5, 10, 20, 40, 60, 80, 100% and 10, 20, 30, 40, 50, 60, 70, 80, 90% versus CEC for 3,3′-dimethyloxacarbocyanine and TMPyP4+, respectively. The λmax of TMPyP4+ without clay in solution is 420 nm, and that of TMPyP4+ on the clay surface is 450 nm. Thus, all TMPyP4+ molecule adsorb on the clay surface under the condition of b. 2109


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Figure 3. (a) Relationship between the maximum adsorption amount of porphyrins without aggregation and ΔL in the exfoliation state. (b) Image of the size-matching effect between TMPyP and clay: The good agreement between the intercharge distance in the guest molecule and that on the clay surface realizes the high density but without the aggregation structure of dyes assembly in the clay complexes. Lclay is the intercharge distance on the clay. Ldye is the intercharge distance in the dye molecule.

appropriate dye is used. Because the matching of intercharge distances of host and guest plays an important role, we termed this unique effect to suppress the aggregation behavior as the size-matching effect or the charge-matching effect, where the internegative charge distance within the adsorbing molecule matches well with that on the clay surface and thus the charge densities of both sides also match well (Figure 3b).30,31,39 It should be noted that the repulsive interactions between dye molecules on the clay surface also work and should be considered, especially for the dyes whose maximum adsorption ratio is below 100% versus CEC. In the case of tetracationic porphyrin−clay complexes, the average center-to-center intermolecular distance is calculated to be 2.4 nm under the saturated adsorption condition on the basis of a hexagonal array. It is expected that the host lattice could affect the arrangement of the guest molecule. This host lattice effect may modulate the intermolecular distance of guests. Because 2.4 nm is an average intermolecular distance, we should discuss the deviation or distribution of intermolecular distance. If the distribution of intermolecular distance is wide, then the spectral change due to the exciton coupling could be observed for the components whose intermolecular distances are short. Because such spectral changes were not observed at all (Figure 2b), the distribution of intermolecular distance should be pretty narrow in the porphyrin−clay complexes. This intermolecular distance is most interesting from the viewpoint of photochemical reactions because there is almost no interaction between the transition moments of porphyrin molecules. It surely avoids the drastic decrease in the excited-state lifetime if they cannot interact with each other in the excited state.

dye molecule can remain excited long enough in the complex, it is possible to construct efficient photochemical reaction systems in the clay−dye complexes. 40−44 The typical absorption spectra at various loading levels of cyanine and porphyrin dyes are shown in Figure 2. As can be seen, the cyanine dye exhibited apparent spectral changes as its loading level increased. This behavior indicates that the cyanine dye molecules form aggregates on the clay surface. However, tetrakis(N-methylpyridinium-4-yl)porphyrin (TMPyP4+) did not show any spectral change with increasing dye loadings. This indicates that TMPyP4+ does not aggregate even under the very high density condition of 100% adsorption versus CEC (1 molecule/5.0 nm2). To clarify the mechanism for the high-density adsorption in a monomeric form without aggregation behavior, we have examined many organic and inorganic dyes. The maximum adsorption amounts without aggregation of the examined dyes determined by the normalized absorption spectra and Lambert−Beer plot analyses are summarized in Table S1. [5,15-Phenyl-10,20-bis(1-methyl-pyridinium-4-yl)porphyrin (trans-H2DPyP2+), tetrakis(N,N,N-trimethylanilinium-4-yl)porphyrin (H2TMAP4+), tetrakis(1-methyl-pyridinium-4-yl) porphyrin(p-H2TMPyP4+), [5-phenyl-10,15,20-tris(1-methylpyridinium-4-yl)porphyrin (H2TriMPyP3+), and [5,10-phenyl15,20-bis(1-methyl-pyridiium-4-y1)porphyrin (cis-H2DPyP2+) showed adsorption to 100% versus CEC without aggregation. It is found out that there is a correlation between the maximum adsorption amount and the difference between the intercationic charge distance in the porphyrin molecule and the interanionic charge distance on the clay surface (ΔL), as can be seen in Figure 3a. A smaller |ΔL| means that the size of the interpositive charges of the porphyrins better matches that of the internegative charges on the clay surface to have the two opposite charges approach more closely; the electrostatic interaction between clay and dye becomes stronger. Thus, it is indicated that the stronger electrostatic interaction between the host and the guest prevents the aggregation of dyes that is induced by a much weaker guest−guest interaction. It is known that an aromatic compound tends to form aggregates because of π−π interactions. It turns out that two cationic parts are enough to prevent the formation of aggregates when an

3. STRUCTURE CONTROL OF DYES ASSEMBLY IN THE CLAY COMPLEXES 3.1. Intermolecular Distance Control between Porphyrins. As described in section 2, we expect to control the intermolecular distances by the use of a size-matching effect. To develop the size-matching effect as a unique technique for controlling the molecular alignment, we examined (i) the synthesis of clay minerals with different charge densities and (ii) their complex formation behavior with cationic porphyr2110


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Figure 4. (a) Relationship between the intercharge distance on the synthetic saponite and the average intermolecular distance at maximum adsorption on the basis of a hexagonal array of H2TMPyP4+ and H2TMAP4+. (b) Typical ideal images of the porphyrin adsorption structure on the synthetic saponite surface under saturated conditions: (left) on the clay with high charge density and (right) on the clay with low charge density. The intermolecular distance (indicated as an arrow) can be controlled depending on the clay charge density. Reprinted with permission from ref 45 (Copyright 2011, American Chemical Society).

Figure 5. (a) Structure of tetracationic, octacationic, and zwitterionic porphyrin. (b) Proposed adsorption structure of DPy2+DS2−PP, TMPyP4+, and TAEPyP8+ on the clay surface under the maximum adsorption condition. Reprinted with permission from ref 52 (copyright 2011 Elsevier).

ins.45 Saponite, which is one of the typical clay minerals, was selected in the present study. The typical formula of saponite is [(Si8 − xAlx)(Mg6 − yAly)O20(OH)4]−(x − y)Nax − y (x = 0.4−1.2, y = 0). The charge density of the clay should affect the adsorption behavior of guest molecules on the clay surface.46−51 We examined the synthesis of saponite, which has different charge densities in the range of x = 0.3−1.7. With respect to complex formation behavior with cationic porphyrins, it is expected that a larger (shorter) distance between anionic charges on the clay surface leads to a larger (shorter) intermolecular distance between adsorbed porphyrin molecules on the clay surface. The effect of the intercharge distance of the clay on the complex formation behavior with cationic porphyrins was examined. The maximum degrees of adsorption for H2TMPyP4+ and H2TMAP4+ were determined for all clays. The inter-cation distances in the molecule are 1.05 and 1.31 nm on the basis of AM1 calculations for H2TMPyP4+ and

H2TMAP4+, respectively. The calculated average intermolecular distances under the saturated adsorption condition for the clay with different intercharge distance are shown in Figure 4a. As can be seen, it turns out that the control of intermolecular distances between adjacent porphyrins under the saturated adsorption condition is possible by using the appropriate clay as shown in Figure 4b. The minimum intermolecular distance under the maximum adsorption condition depends upon the intramolecular charge distances of H2TMPyP4+(1.05 nm) and H2TMAP4+(1.31 nm). It is thus indicated that the high-density close packing of porphyrins on the clay surface takes place when the internegative charge distance on the clay matches well with the interpositive charge distance in the porphyrin molecule. When a redox-active species such as Fe2+/3+ is adopted as one of the components of clay minerals instead of the Si4+/Al3+ pair, it is expected that the average intercharge distance on the clay surface can be effectively controlled by the 2111


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degree of reduction/oxidation of the component, Fe2+/3+. In this case, the intermolecular distance between porphyrins can be controlled on the sole clay without preparing many types of clay. In self-assembly, the guest−guest interactions mostly play an important role in determining the assembly structure. However, in the size-matching effect, by the stronger host−guest interaction, an aggregation by guest−guest interaction is relatively suppressed. In other words, by utilizing the sizematching effect it is possible to realize the assembly structure where there is a gap with a desired distance between guest molecules. It should be noted that the homogeneous distribution of charges in the clay sheet is assumed in the present system, and the experimental results support it. However, if it is possible to control the distribution of charge, then we can design the molecular alignment to a more advanced level. What will happen if some specific guest−guest interactions are added to the size-matching effect? The effects of charges and structure in porphyrin molecules on their adsorption behavior on the clay surface were examined for the further precise control of the microstructure of the porphyrin assembly. 52,53 Tetrakis(1-(2-(trimethylammonio)ethyl)pyridinium-4-yl)-porphyrin octachloride (TAEPyP8+) with octacationic sites within the molecule and [5,10-bis(4sulfonatophenyl)-15,20-bis(1-methyl-pyridinium-4-yl)porphyrin (DPy2+DS2−PP) with dicationic and dianionic sites were examined as guest porphyrins and compared to TMPyP4+ (Figure 5a). From the viewpoint of size matching, each porphyrin has the same size of interpositive charge distance but has additional cationic sites within the molecule ((TAEPyP8+) or anionic sites (DPy2+DS2−PP). The maximum adsorption ratio without aggregation was determined for the three porphyrins on the saponite clay. Under the maximum adsorption condition, the average intermolecular distances in the complexes were calculated to be 2.4, 2.8, and 2.1 nm for TMPyP4+, TAEPyP8+, and DPy2+DS2‑PP on the basis of a hexagonal array, respectively (Figure 5b). It turns out that the molecular structure of porphyrin and additional charges within the molecules affect the intermolecular distances between porphyrin molecules on the clay surface. In the case of DPy2+DS2−PP, a slight red shift of λmax from 447 to 449 nm was observed just below the saturated adsorption condition. This indicates that DPy2+DS2−PP molecules interact weakly with each other, owing to the electrostatic attractive interaction between the cationic and anionic sites. The guest−guest interactions may modify the guest assembly structure in addition to the host−guest interactions for clay−TAEPyP8+ and clay−DPy2+DS2−PP complexes. Steric and electrostatic repulsion between the adsorbed molecules would enlarge the intermolecular distances for TAEPyP8+. Contrary to the case for DPy2+DS2−PP, electrostatic attractive interactions between the cationic sites and the anionic sites of the adjacent molecules seem to reduce the intermolecular distances. To gain insight into the fundamental photochemical property, the self-fluorescence quenching behavior of the complexes was examined. Fluorescence spectra of the clay complexes for TMPyP4+, TAEPyP8+, and DPy2+DS2−PP were examined at 5−90% versus the CEC of the clay.52 Very interestingly, no self-fluorescence quenching was observed at any density in the case of TMPyP4+ and TAEPyP8+. However, moderate self-fluorescence quenching was observed for DPy2+DS2−PP. These observations can be rationalized by the

difference in the average intermolecular distance. In the case of DPy2+DS2−PP, its short intermolecular distance may induce the self-fluorescence quenching due to electron transfer in its excited singlet state. In general, dye molecules suffer static and dynamic self-fluorescence quenching on the inorganic surfaces even at low densities of adsorption. In such case, it is very difficult to utilize those adsorbed dyes as sensitizers because of their short excited lifetimes. Thus, these porphyrins such as TMPyP4+ and TAEPyP8+ in the clay complexes are promising candidates in constructing efficient photochemical reaction systems on the inorganic surfaces. 3.2. Control of the Relative Orientation Angle of the Molecular Plane of Porphyrins against the Clay Surface. In addition to the intermolecular distance, the relative angle between molecules plays an important role for photochemical reactions. For example, Fö rster-type energy-transfer rate constant is governed by the orientation factor.54 The orientation of the porphyrin molecule on the clay surface was examined by the use of two types of porphyrins.55,56 A tetracationic porphyrin (tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP4+)) and a dicationic porphyrin ((cisbis(N-methylpyridinium-4-yl)-diphenylporphyrin (cisDPyP2+)) were used as the cationic porphyrins in this study. Two types of measurements were used to observe the orientational change behavior of porphyrins on the clay surface. First, the absorption maximum (λmax) of the porphyrin Soret band, which reflects the molecular structure of the porphyrin, was observed. Second, the orientational changes in the porphyrin were observed directly by polarized absorption spectroscopy by the use of an interface spectrometer equipped with a quartz waveguide system (System Instruments, SIS50BS). Because the absorbance is amplified dozens of times in the waveguide, even a monolayer of clay−porphyrin complexes on the quartz glass can be measured with sufficient absorbance. Samples for measurement were prepared as follows. The aqueous dispersion of the clay−porphyrin complex was cast on the quartz waveguide. The mostly monolayer formation was confirmed by AFM measurements (i.e., that the clay sheets are aligned on the quartz glass in a parallel (flat) orientation), as schematically depicted in the experimental setup (Figure 6a). Thus, by using polarized light, it is possible to estimate the molecular orientation angle with respect to the surface. The absorption spectra of TMPyP4+ adsorbed on the clay monolayer on the quartz waveguide glass with s- and ppolarized light in water are shown in Figure 6b. The absorbance with s-polarized light was much larger than that with ppolarized light. The polarized absorption experiment with cisDPyP2+ exhibited a similar result. These results clearly indicate that tetracationic and dicationic porphyrins adsorb on the clay surface with orientations that are nearly parallel to the clay surface in water. The tilt angle of the porphyrins was determined to be larger than 85° from the normal by a quantitative analysis of the p/s ratio of the absorbance. Solvent effects on the porphyrin orientation on the clay surface were further examined. Dioxane, which is miscible with water, was used as a typical nonpolar additive. The addition of dioxane induced significant spectral changes in the absorption spectra only for cis-DPyP2+ but not for TMPyP4+. The spectral change for cis-DPyP2+ by the addition of dioxane (0−90%) is shown in Figure 7a. A similar spectral shift was also observed when dimethylformamide (DMF) was added to the samples. The absorption spectra of the cis-DPyP2+-clay complex on the quartz waveguide 2112


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orientation angle of the molecular plane of porphyrin against the clay sheet was found to be controlled by the compositional change of surrounding solvent. When TMPyP4+ and cis-DPyP2+ coadsorb on the clay surface, we confirmed that it is possible to obtain a complex in which parallel adsorption (TMPyP4+) and vertical adsorption (cis-DPyP2+) against the clay surface coexist on the same clay surface by choosing the solvent composition. We have examined the factors that control the orientation angle of porphyrins thermodynamically. Because the orientation behavior correlates well with the hydrogen bonding parameter of the solvent and is governed by the entropy term, we concluded that hydrophobic interactions between the porphyrin ring and the clay surface play an important role.

4. UNIQUE PHOTOCHEMICAL BEHAVIOR OF DYES IN THE CLAY COMPLEXES The photochemical properties and behavior of dyes on the clay surface are sometimes quite unique. The λmax of tetracationic porphyrin shifts to longer wavelength by complex formation with clay. In the case of MgTMPyP4+, the λmax without clay is 449.5 nm, and the λmax values of MgTMPyP4+ on the clay surface and intercalated between clay sheets are 472.5 and 500 nm, respectively. By systematic experiments using the series of porphyrin derivatives, it was directly confirmed that the flattening of the porphyrin molecule is the dominant mechanism for the spectral shift on the surface of inorganic nanosheets.32,57,58 We have observed that many molecules undergo spectral shifts to longer wavelengths in similar ways. In the case of methylviologen, the fluorescence quantum yield is dramatically enhanced by complex formation with clay. The conformation change and coplanarization (flattening) of the two aromatic rings to suppress the intramolecular rotation between the two aromatic planes would be the main reasons for the fluorescence enhancement. In a similar way, we have confirmed that many dyes undergo a fluorescence enhancement. Especially from the viewpoint of photochemistry, the suppression of the self-fluorescence quenching behavior of dyes on the clay surface is important. As described before, in the case

Figure 6. (a) Experimental setup of the surface-interface spectrometer with a quartz waveguide. (b) Absorption spectra obtained with s- and p-polarized light for TMPyP4+ on the quartz waveguide in water. Reprinted with permission from ref 55 (copyright 2006, The Chemical Society of Japan).

covered with DMF were measured with s- and p-polarized light. The spectra obtained are shown in Figure 7b. With s-polarized light, a weaker absorption at 425 nm was observed. With ppolarized light, a stronger absorption at 425 nm and almost no absorption at 455 nm were observed. These results directly indicate that cis-DPyP2+ adsorbs on the clay surface in a nonparallel way under the present condition. However, the absorption spectrum of TMPyP4+ was not affected by the addition of dioxane or DMF. The orientation of TMPyP4+ and cis-DPyP2+ on the clay surface in water and water−dioxane or water−DMF can be illustrated as shown in Figure 7c. The

Figure 7. (a) Absorption spectra of the cis-DPyP2+−clay complex in water−dioxane (100:0 to 10:90 v/v). (b) Absorption spectra of cis-DPyP2+ with s- and p-polarized light on the quartz waveguide in DMF. (c) Schematic side views of the orientation of TMPyP4+ and cis-DPyP2+ on the clay surface with the change in solvent polarity. Reprinted with permission from ref 55 (copyright 2006, The Chemical Society of Japan). 2113


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of some porphyrins, the relative quantum yield of fluorescence on the clay surface does not depend on the adsorption density; there is no self-fluorescence quenching even under a very high density condition (1 molecule/5.0 nm2). These effects of clay on the photochemical properties of dyes could be very useful in constructing unique photochemical reaction systems. The examples of photochemical reactions in the clay−dye complexes will be described in the next section.

loading levels of porphyrins. The total concentration of mTMPyP(D) and p-TMPyP(A) was fixed to be 1.0 × 10−7 M. The porphyrin loading level versus cation exchange capacity (CEC) of the clay was adjusted from 0.05 to 90% by changing the concentration of the clay. The absorption spectra exhibited completely identical shapes at all loading levels of porphyrins. The fluorescence spectra change at various loading levels of porphyrin for 1/1 m-TMPyP(D)/p-TMPyP(A) is shown in Figure 8. The clear isoemissive points were observed as indicated by the arrows in Figure 8.

5. PHOTOCHEMICAL ENERGY TRANSFER IN THE CLAY−PORPHYRIN COMPLEXES TO AN ARTIFICIAL LIGHT-HARVESTING SYSTEM The intermolecular distance (r) sensitively affects photochemical reactions such as electron and energy transfer. The rate constant of electron transfer exponentially decreases against r, and that of energy transfer is proportional to r−6 in the case of Förster mechanism.54 In this specific case, the electron-transfer process is predominant at short r and the energy-transfer process is predominant at large r. Because the intermolecular distance is a crucial factor in chemical reactions, especially for photochemical reactions such as energy and electron transfer, the methodology controlling the intermolecular distance should be promising for developing unique photochemical reaction systems. Several dyes have been used for energy transfer on clay surfaces.51,59−66 We have been studying the excited energy-transfer reaction between adsorbed porphyrins on the clay surface.40−44 In previous work, the maximum quantum yield for the fluorescence energy transfer was ca. 35% between the Zn porphyrin (ZnTMAP) and the free-base porphyrin (H2TMPyP) on the clay surface.40 In our system, the quenching in an aggregation form (Figure S1a) was completely suppressed by the size-matching effect. Thus, the major factors lowering the excited singlet energy-transfer efficiency were assumed to be self-fluorescence quenching processes between porphyrins on the clay surface in nonaggregated form (Figure S1b) and/or the segregation structure of the two kinds of dyes (Figure S1c). Thus, the reaction conditions should be optimized in terms of the following aspects:42,67 (i) choosing the appropriate porphyrins in which the self-fluorescence quenching is negligible, (ii) choosing the porphyrins that do not form a segregation structure, and (iii) adjusting the reaction conditions such as the adsorption density and the donor− acceptor ratio to regulate the intermolecular distances. The self-fluorescence quenching behavior of the porphyrins on the clay surface was examined by recording the fluorescence spectra. We selected porphyrins p-TMPyP, m-TMPyP, oTMPyP, cis-DPyP, trans-DPyP, and tetrakis(1-methylpyridinium-4-yl) porphyrinatozinc (Zn(II)p-TMPyP) in terms of the difference in the intracharge distance in the molecule and the adsorption strength on the clay surface. Among the examined porphyrins, the self-fluorescence quenching of p-TMPyP and m-TMPyP on the clay surface was found to be almost negligible. Thus, these two porphyrins were selected for the examination of the energy-transfer experiments. It is assumed that one of the main factors in determining the self-quenching efficiency is the adsorption strength of the porphyrin molecules on the clay surface. The energy-transfer behavior was examined by both steadystate fluorescence measurements and time-resolved fluorescence measurements. The energy transfer from the excited singlet state of m-TMPyP (donor) to the ground state of pTMPyP (acceptor) on the clay surface was examined at various

Figure 8. Fluorescence spectra for m-TMPyP(D)/p-TMPyP(A)/clay complexes excited at 430 nm in aqueous solution. [m-TMPyP(D)] = [p-TMPyP(A)] = 5.0 × 10−8 M. The porphyrin loading levels were set at 0.05−90% vs CEC for the clay. The thick lines are individual fluorescence spectra of m-TMPyP(D)/clay and p-TMPyP(A)/clay complexes. The arrows indicate isoemissive points. Reprinted with permission from ref 42 (copyright 2011, American Chemical Society).

In Figure 9, the obtained values of energy-transfer efficiency (ηET) are plotted versus the loading levels of porphyrins. The

Figure 9. Energy-transfer efficiencies (ηET) at various loading levels and ratios of porphyrins. [m-TMPyP(D)] + [p-TMPyP(A)] = 1.0 × 10−7 M, [m-TMPyP(D)]/[p-TMPyP(A)] = 15/1 (■), 8/1 (▲), 3/1 (●), 1/1 (◊), 1/3 (△), 1/8 (○), 1/15 (□). The porphyrin loading levels were set at 0.05−90% vs CEC for the clay in aqueous solution. Reprinted with permission from ref 42 (copyright 2011, American Chemical Society).

energy-transfer efficiency increased as the loading level of porphyrin increased because the average intermolecular distance between porphyrins decreased. Amazingly, the maximum energy-transfer efficiency was ca. 100% when mTMPyP(D)/p-TMPyP(A) = 1/3, 1/8, and 1/15, and the loading levels of porphyrin were 80 and 90% versus CEC. The Förster-type energy-transfer rate constant is inversely proportional to the sixth power of the intermolecular distance. The 2114


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energy-transfer efficiency increased by increasing the ratio of pTMPyP on the clay surface because the number of energy acceptors (p-TMPyP) around the donor porphyrin molecules (m-TMPyP) contributes to the frequency factor of the energytransfer rate. From the results, the adsorption structure of two porphyrins on the clay surface can be discussed. Generally, two kinds of dyes segregate from each other when the dyes coadsorb on the clay surface.68 If the two kinds of porphyrins in this case had coadsorbed on a clay surface with a segregation structure (Figure 10a), then the energy-transfer efficiencies

would not have reached 100% because the donor porphyrins that are surrounded by donor porphyrins themselves could not transfer the excited energy to the acceptor porphyrin efficiently. Therefore, an ideal arrangement of the two kinds of coadsorbed porphyrins on the clay surface in the present system could be depicted in Figure 10b, as typical of the possible arrangements, where each donor molecule is surely surrounded by acceptor molecules. In the clay−porphyrin complexes, each dye molecule has monomeric adsorption onto the clay surface with little interaction between adsorbed porphyrins because of the size-matching effect. Thus, the integration (Figure 10b: a structure with repetition of the randomly adsorbed state) was realized in the present complexes. The time-resolved fluorescence spectra of m-TMPyP(D)/pTMPyP(A)/clay complexes were examined to elucidate the details of the energy-transfer reaction.42 In the experiment, the ratio of donor/acceptor porphyrin (m-TMPyP(D)/p-TMPyP(A)) was 1/3, and the total concentration of porphyrins was set to 90% loadings versus the CEC of the clay. Under such conditions, the energy-transfer efficiencies (ηET) determined with the steady-state fluorescence experiment was ca. 100%. The observed time-resolved fluorescence spectra for the mTMPyP(D)/p-TMPyP(A)/clay complex excited at 430 nm, that is, λmax of the donor porphyrin, are shown in Figure 11a. The change in spectral shape obviously indicates the energy transfer from excited donor porphyrin to acceptor porphyrin. For a 4.0−4.2 ns delay time, the obtained spectrum agreed well with that of p-TMPyP(A)/clay.

Figure 10. Possible ideal images of (a) segregation and (b) the integration adsorption structure of two kinds of porphyrins on the clay surface in the case of m-TMPyP(D)/p-TMPyP(A) = 1/3. Reprinted with permission from ref 42 (copyright 2011, American Chemical Society).

Figure 11. (a) Time-resolved fluorescence spectra for the m-TMPyP(D)/p-TMPyP(A)/clay complex 0.0−0.2, 1.0−1.2, 2.0−2.2, and 4.0−4.2 ns after excitation. The excitation wavelength was 430 nm. The sum of dye loading was set at 90% vs CEC for clay/m-TMPyP(D)/p-TMPyP(A) = 1/3 and [m-TMPyP(D)] + [p-TMPyP(A)] = 1.0 × 10−7 M. (b) Fluorescence decay profiles of the m-TMPyP(D)/p-TMPyP(A)/clay complex in the region of 635−655 nm corresponding to the fluorescence region for m-TMPyP(D). (c) Fluorescence decay profiles of m-TMPyP(D)/pTMPyP(A)/clay complex in the region of 770−790 nm corresponding to the fluorescence region from p-TMPyP(A). Sample conditions were 90% dye loadings vs CEC of the clay, m-TMPyP(D)/p-TMPyP(D) = 1/3, and [m-TMPyP(D)] + [p-TMPyP(A)] = 1.0 × 10−7 M. Reprinted with permission from ref 42 (copyright 2011, American Chemical Society). 2115


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photochemical oxygenation reaction toward the realization of artificial photosynthetic systems.

6. CLAY−PORPHYRIN COMPLEXES AS A UNIQUE MICROENVIRONMENT FOR CHEMICAL REACTIONS AND PHOTOFUNCTIONAL MATERIALS One of the most characteristic points of clay complexes is the reversible flexibility of the structure. In particular, the interlayer space provided by the clay sheets is reversibly expanding and shrinking to serve as the unique environment for chemical reactions. The solvatochromic behavior in transparent films composed of clay−porphyrin complexes was developed.70 Dicationic porphyrin (cis-DPyP2+) and synthetic saponite were used as a guest dye molecule and a host material, respectively. The typical transparent clay−porphyrin membrane was prepared as follows. An aqueous colloidal solution of clay and cis-DPyP was added to water with stirring. The obtained solution containing the clay−porphyrin complex was filtered with a poly-(tetrafluoroethylene) (PTFE) membrane filter with 0.1 μm pore size. The residual membrane on the PTFE filter could be transferred to the cover glass. The membrane exhibits high transparency in the visible and ultraviolet regions. The stacking number of clay sheet is 1800, and the loading level of porphyrin is 5.0% versus CEC for a typical clay membrane. The absorption spectrum and the X-ray diffraction (XRD) pattern of the clay−porphyrin film covered with DMF and under air are shown in Figure S2a,c, respectively. As can be seen, the reversible absorption spectral change accompanied by that of the XRD pattern was observed with the changes in the surrounding media. Two factors are simultaneously controlling to induce the orientation and spectral change in porphyrin in the film system. One is the swelling ability of the clay film, and the other is the orientation change ability of the porphyrin on the exfoliated clay. It is noteworthy that the clay−porphyrin film was very stable in all solvents used here, despite their swelling ability. The film was completely stable in water for over 300 days, but the film prepared by the same procedure without the cationic porphyrins was rather unstable. In the case of cis-DPyP2+ as a guest porphyrin, the color change of the film in various organic solvents was not visually as clear, and Sn(IV)TMPyP4+ afforded a more distinct color change in the membrane from green (in cyclohexane) to wine-red (in DMF), although the spectral changes in both cases were almost the same, as shown in Figure S3. We observed solvatochromic behavior in a clay−porphyrin membrane based on the structure change of the complex. As can be seen in Figure S2, the reversibly changeable space is provided by the clay sheets and porphyrin ring. Although this space is expandable in DMF and dioxane, it remains unchanged in cyclohexane. The coordination reaction in the space provided by a clay sheet and porphyrin ring was further examined. By using Zn as the central metal of porphyrin and pyridine derivatives as the coordinates, we have succeeded in the conjugation of nanospace control with a coordination reaction in the clay−porphyrin complex. In dichloromethane, the interlayer distance is 0.49 nm, and thus the interlayer space was almost closed (Figure S4a). In this case, the absorption spectrum of the Zn porphyrin was not affected by the addition of pyridine; that is, pyridine does not coordinate on Zn in this system (Figure S4b). The XRD patterns and absorption spectra of the clay−Zn porphyrin film in dioxane and under air are shown in Figure 13a. In dioxane, the interlayer distance is 0.67 nm, and thus the

Figure 12. Efficient excited energy-transfer reaction in clay−porphyrin complex. Reprinted with permission from ref 42 (copyright 2011, American Chemical Society).

The fluorescence decay analysis for the m-TMPyP(D)/pTMPyP-(A)/clay complex was carried out to estimate the energy-transfer rate constant (kET). In Figure 11b,c, the fluorescence decay profiles for the m-TMPyP(D)/p-TMPyP(A)/clay complex in the regions of (a) 635−655 and (b) 770− 790 nm are shown. The region of 635−655 nm mainly corresponds to the fluorescence region of m-TMPyP(D), and the region of 770−790 nm almost corresponds to that of pTMPyP(A). As shown in Figure 11b, the decay profile of mTMPyP(D)/p-TMPyP(A)/clay at 635−655 nm can be analyzed as a double-exponential decay. The time constants are calculated to be 0.4 ± 0.1 ns (τ1) and 5.6 ± 0.1 ns (τ2), respectively. τ2 agrees well with τacceptor, and the decay curve does not consist of the component with τdonor (7.4 ns). These results suggest that the short-lifetime component (τ1) is derived from the excited energy transfer from m-TMPyP(D) to adjacent p-TMPyP(A) on the clay surface. In the case of the decay curve of the m-TMPyP(D)/p-TMPyP(A)/clay complex at 770−790 nm (Figure 11c), the rise and decay components that can be analyzed as a double exponentials were obviously observed. The lifetimes are calculated to be 0.4 ± 0.1 ns (τ3) and 5.6 ± 0.1 ns (τ4) for the rise and decay components, respectively. The obtained time constant of the rise component (τ3) in the region of 770−790 nm completely agrees with that of the decay component (τ1) in the region of 635−655 nm. Thus, it can be concluded that τ1 and τ3 are the components derived from the energy-transfer reaction from m-TMPyP(D) to the adjacent p-TMPyP(A) on the clay surface. From the values of the short lifetime (τ1 or τ3 = 0.4 ± 0.1 ns) and the lifetime of the donor porphyrin (τdonor = 7.4 ± 0.1 ns), the energy-transfer rate constant kET can be obtained to be 2.4 ± 0.6 × 109 s−1. It is also interesting that kET could be analyzed as only one component. This strongly indicates that porphyrin molecules do not aggregate and that the intermolecular distance is uniform on the clay surface under the saturation conditions. The major factors for the quantitative energy-transfer reaction were (i) suppression of aggregation that shortens the excited lifetime, (ii) suppression of self-fluorescence quenching due to excited electron transfer,69 and (iii) realizing the integrated adsorption structure of the two kinds of dyes on the clay surface. These factors were all controlled by the sizematching effect. According to this idea, it is presumed that this efficient energy-transfer system can be extended to a wide range of dye, which fulfills the size-matching effect. Generally, almost 100% energy-transfer efficiency is a crucial requirement in constructing artificial light-harvesting systems (LHSs) involving multiple-step energy-transfer processes. Judging from the results, clay−porphyrin complexes are very promising candidates for photochemical reactions such as efficient artificial LHSs. We are trying to combine several kind of dyes to absorb all sunlight and add a photochemical reaction system such as a 2116


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Figure 13. (a) XRD patterns of clay−Zn porphyrin membrane in dioxane and under air. (b) Absorption spectra of the clay−Zn porphyrin membrane in dioxane. The concentration of pyridine is 0−0.4 M. (c) Synchronization between coordination space control and the coordination reaction.

system composed of the two porphyrins. By the combination of several dyes, the realization of an artificial light-harvesting system that can absorb sunlight over a wide range of wavelengths (380−700 nm) would be possible. The combination of an electron-transfer system and/or a photochemical reaction system and/or 3D structure formation to the present system would lead to the construction of an artificial photosynthetic system. As a summary, we have demonstrated that layered materials such as clay minerals have the following functionalities to (i) modify and adjust the photochemical properties of dye, (ii) control the molecular alignment, and (iii) serve as a unique flexible interlayer for chemical reactions. These functions are of course well exhibited by microenvironments provided by proteins in natural living systems. We believe that one of the most important common features of these materials is the flexibility of structure depending on the environment and/or guest molecules. Such flexibility plays an important role in the expression of a protein’s unique functionalities such as induced fit and an allosteric effect. As can be seen especially in section 6 in this article, such structural flexibility is also essential for the functionality of clay minerals as a unique microenvironment for chemical reactions. Layered materials such as clay minerals may be claimed to serve as proteinlike media in various chemical reactions.

interlayer space was expanded. In this case, the absorption spectrum of Zn porphyrin was affected by the addition of pyridine as shown in Figure 13b; that is, pyridine can coordinate on Zn in this system. These results indicate that the synchronization between the coordination space control and the coordination reaction is realized in the clay−Zn porphyrin system as shown in Figure 13c. We believe that such a flexible and reversible structure of clay complexes is very useful in preparing the unique chemical reaction field. As a preliminary result, we found that pyridine derivatives with bulky substituents cannot coordinate on Zn porphyrin and that pyridine and 4-methylpyridine coordinate easily. The space provided by the clay surfaces and the guest molecules can work as a stereoselective chemical reaction field.

7. SUMMARY AND PERSPECTIVE The unique functionalities of clay−dye complexes are described, mostly from the viewpoint of chemistry and photochemistry in the microenvironment of nanomaterials. We proposed a novel methodologya size-matching effect to control the structure of molecular assembly on an inorganic surface. By using clay and porphyrin dyes as the host and the guest, we succeeded in controlling the intermolecular distances and relative molecular orientation on the clay surface. One of the characteristic points of the stacked clay complex is its reversible flexibility. The space provided by the clay surface and guest dye molecule would serve as a unique environment for chemical reactions. For example, unique photoresponsible functional materials can be obtained by using layered materials. From the viewpoint of photochemistry, dyes in the clay complex sometimes exhibit very unique photochemical properties such as large absorption spectral shifts and fluorescence enhancement. By using these advantages, we established the novel artificial light-harvesting system based on the clay−dye system. Other advantages of clay complexes are their high stability and ease of further modification. For example, it is easy to add a third dye molecule to the described energy-transfer

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ASSOCIATED CONTENT

S Supporting Information *

Schematic images of adsorption structures of two kinds of dyes: (a) aggregation, (b) nonaggregation with integration, and (c) nonaggregation with segregation. The absorption spectra and XRD pattern of the clay−porphyrin film. Color change of the clay−porphyrin membrane. Maximum concentration of porphyrins for adsorption without aggregation on the clay surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. 2117


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(18) Theng, B. K. G. Formation and Properties of Clay−Polymer Complexes; Developments in Clay Science; Elsevier: Amsterdam, 2012; Vol. 4, Chapter 7, pp 201−241 (19) Liu, P. Polymer modified clay minerals: a review. Appl. Clay Sci. 2007, 38, 64−76. (20) Choy, J.-H.; Choi, S.-J.; Oh, J.-M.; Park, T. Clay minerals and layered double hydroxides for novel biological applications. Appl. Clay Sci. 2007, 36, 122−132. (21) Ruiz-Hitzky, E.; Aranda, P.; Darder, M.; Rytwo, G. Hybrid materials based on clays for environmental and biomedical applications. J. Mater. Chem. 2010, 20, 9306−9321. (22) Fukushima, Y. Organic/inorganic interactions in polymer/clay mineral hybrids. Clay Sci. 2005, 12−1, 79−82. (23) Okada, T.; Ide, Y.; Ogawa, M. Organic−inorganic hybrids based on ultrathin oxide layers: designed nanostructures for molecular recognition. Chem.−Asian J. 2012, 7, 1980−1992. (24) Ruiz-Hitzky, E.; Aranda, P.; Darder, M.; Ogawa, M. Hybrid and biohybrid silicate based materials: molecular vs. block-assembling bottom-up processes. Chem. Soc. Rev. 2011, 40, 801−828. (25) Chakraborty, C.; Dana, K.; Malik, S. Intercalation of perylenediimide dye into LDH clays: enhancement of photostability. J. Phys. Chem. C 2011, 115, 1996−2004. (26) Bizaia, N.; De Faria, E. H.; Ricci, G. P.; Calefi, P. S.; Nassar, E. J.; Castro, K. A. D. F.; Nakagaki, S.; Korili, S. A. Porphyrin-kaolinite as efficient catalyst for oxidation reactions. ACS Appl. Mater. Interfaces 2009, 1, 2667−2678. (27) Č eklovský, A.; Czimerová, A.; Lang, K.; Bujdák, J. Layered silicate films with photochemically active porphyrin cations. Pure Appl. Chem. 2009, 81, 1385−1396. (28) Chernia, Z.; Gill, D. Flattening of TMPyP adsorbed on laponite. Evidence in observed and calculated UV-vis spectra. Langmuir 1999, 15, 1625. (29) Kuykendall, V. G.; Thomas, J. K. Photophysical investigation of the degree of dispersion of aqueous colloidal clay. Langmuir 1990, 6, 1350. (30) Takagi, S.; Shimada, T.; Yui, T.; Inoue, H. High density adsorption of porphyrins onto clay layer without aggregation: characterization of smectite - cationic porphyrin complex. Chem. Lett. 2001, 30, 128−129. (31) Takagi, S.; Shimada, T.; Eguchi, M.; Yui, T.; Yoshida, H.; Tryk, D. A.; Inoue, H. High-density adsorption of cationic porphyrins on clay layer surfaces without aggregation: the size-matching effect. Langmuir 2002, 18, 2265−2272. (32) Ishida, Y.; Masui, D.; Shimada, T.; Tachibana, H.; Inoue, H.; Takagi, S. The mechanism of the porphyrin spectral shift on inorganic nanosheets: the molecular flattening induced by the strong host − guest interaction due to the “size-matching rule”. J. Phys. Chem. C 2012, 116, 7879−7885. (33) Bujdák, J.; Iyi, N.; Fujita, T. The aggregation of methylene blue in montmorillonite dispersions. Clay Miner. 2002, 37, 121−133. (34) Czímerová, A.; Bujdák, J.; Gáplovský, A. The aggregation of thionine and methylene blue dye in smectite dispersion. Colloids Surf., A 2004, 243, 89−96. (35) Miyamoto, N.; Kawai, R.; Kuroda, K.; Ogawa, M. Adsorption and aggregation of a cationic cyanine dye on layered clay minerals. Appl. Clay Sci. 2000, 16, 161−170. (36) Lu, L.; Jones, R. M.; McBranch, D.; Whitten, D. Surfaceenhanced superquenching of cyanine dyes as J-aggregates on laponite clay nanoparticles. Langmuir 2002, 18, 7706−7713. (37) Bhattacharjee, D.; Arshad Hussain, S.; Chakraborty, S.; Schoonheydt, R. A. Effect of nano-clay platelets on the J-aggregation of thiacyanine dye organized in Langmuir−Blodgett films: a spectroscopic investigation. Spectrochim. Acta, Part A 2010, 77, 232− 237. (38) Ogawa, M.; Kawai, R.; Kuroda, K. Adsorption and aggregation of a cationic cyanine dye on smectites. J. Phys. Chem. 1996, 100, 16218−16221. (39) Eguchi, M.; Takagi, S.; Tachibana, H.; Inoue, H. The Size matching rule in di-, tri-, and tetra-cationic charged porphyrin/

AUTHOR INFORMATION

Corresponding Author

*(S.T.) E-mail: [email protected]. (H.I.) E-mail: [email protected]. Tel: +81 42 677 2839. Fax: +81 42 677 2838. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partially supported by a Grant-in-Aid for Precursory Research for Embryonic Science and Technology (PRESTO) from the Japan Science and Technology Agency (JST) and JSPS Research Fellowship DC1 from the Japan Society for the Promotion of Science.

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