ARTICLE pubs.acs.org/Langmuir
Novel Methodology To Control the Adsorption Structure of Cationic Porphyrins on the Clay Surface Using the “Size-Matching Rule” Tsuyoshi Egawa,† Hajime Watanabe,† Takuya Fujimura,† Yohei Ishida,†,‡ Masafumi Yamato,† Dai Masui,† Tetsuya Shimada,† Hiroshi Tachibana,† Hirohisa Yoshida,† Haruo Inoue,† and Shinsuke Takagi*,†,§ †
Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397 Japan ‡ Japan Society for the Promotion of Science (DC1), Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan § PRESTO (Precursory Research for Embryonic Science and Technology), Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama, Japan
bS Supporting Information ABSTRACT: Saponite-type clays that have different cation exchange capacities were successfully synthesized by hydrothermal synthesis. The structure and properties were analyzed by X-ray diffraction, X-ray fluorescence, 27Al NMR, FT-IR, thermogravimetric and differential thermal analysis, atomic force microscopy, and cation exchange capacity measurement. The intercharge distances on the synthetic saponite (SS) surfaces were calculated to be 0.81.9 nm on the basis of a hexagonal array. The complex formation behavior between SS and cationic porphyrins was examined. It turns out that the average intermolecular distance between porphyrin molecules on the SS surface can be controlled, depending on the charge density of the SS. In the case of tetrakis(1-methylpyridinium-4-yl)porphyrin (H2TMPyP4+), the average intermolecular distances on the SS surface can be controlled from 2.3 to 3.0 nm on the basis of a hexagonal array. It was also found that absorption maxima of porphyrins depend on the charge density of the SS. The adsorption behavior of porphyrin on the SS surface can be rationally understood by the previously reported “size-matching rule”. This methodology using hostguest interaction can realize a unique adsorption structure control of the porphyrin molecule on the SS surface, where the gap distance between guest porphyrin molecules is rather large. These findings will be highly valuable to construct photochemical reaction systems such as energy transfer in the complexes.
’ INTRODUCTION The control of molecular alignment and orientation is one of the most important techniques for developing science and technology. Especially in the field of chemistry and physics, nanotechnology attracts much attention to achieve a breakthrough in molecular alignment controls. Many chemists have been challenged to construct materials where the structure can be controlled on the molecular level by the bottom-up strategy. Recently, a self-assembly technique1 has been widely utilized as the bottom-up technique. Many excellent works using the selfassembly technique have been reported in recent years.24 In the case of the self-assembly technique, the interaction between guest molecules plays an important role to determine the molecular alignment and orientation. We have advocated the novel technique “size-matching rule”, which controls the molecular alignment and orientation without complicated procedures using clay minerals as host materials.513 The host (clay minerals)guest (molecules) interaction is crucial in our work, while the guestguest interaction is important in the typical selfassembly technique. Since typical clay minerals have anionic charges on the surfaces, cationic organic molecules can adsorb on r 2011 American Chemical Society
the surface by electrostatic interactions.1422 We have been studying synthetic saponite (SS)cationic porphyrin complexes and found that the porphyrin molecules do not aggregate on the SS surface up to about 100% adsorption versus the cation exchange capacity (CEC) of the SS.510 Since a size-matching of the distance between anionic sites on the SS and the distance between cationic groups of the dye molecule is a crucial factor for realizing nonaggregated dye assemblies, we termed this effect the “size-matching rule”. In the case of typical synthetic clay minerals such as a saponite, the average intercharge distance is 1.2 nm on the basis of a hexagonal array. Thus, the average intermolecular distance (center to center) of tetracationic porphyrin can be fixed to 2.4 mm, when the dye loading is 100% vs CEC.5,6 This intermolecular distance is suitable for photochemical reactions, since there is no interaction between porphyrins in the ground state which leads to the drastic decrease of the excited-state lifetime, while they can interact with each other in the excited Received: June 15, 2011 Revised: July 19, 2011 Published: July 20, 2011 10722
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Langmuir state. Like this, high density without aggregation (interaction in the ground state) structure was successfully realized in SSporphyrin complexes. By using this unique structure control technique, we have examined photochemical reactions such as an artificial light-harvesting-type energy transfer between two kinds of porphyrin on the SS surface.1113 If it is possible to control the intercharge distance on the SS surface, precise control of the intermolecular distance on the SS surface is expected. Since the intermolecular distance sensitively affects photochemical reactions such as electron and energy transfers, the precise control of the intermolecular distance can contribute to the construction and development of efficient photochemical reaction systems. In this paper, to develop the size-matching rule as a unique technique for controlling the molecular alignment and orientation, we examined (i) the synthesis of clay minerals with different charge densities and (ii) their complex formation behavior with cationic porphyrins. Saponite, which is one of the typical clay minerals and is studied as a host material for organicinorganic nanocomposites, was selected in the present study. According to the classification of clay minerals, [(Si8xAlx)(Mg6yAly)O20(OH)4](xy)Naxy (x = 0.41.2, y = 0) is defined as saponite.15 Thus, we examined the synthesis of saponite which has different charge densities in the range of x = 0.31.7. In the complex formation behavior with cationic porphyrins, it is expected that a larger (shorter) distance between anionic charges on the SS surface leads to a larger (shorter) intermolecular distance between adsorbed porphyrin molecules on the SS surface. Since the charge density of SS affects the adsorption behavior of guest molecules, the charge density effects23 were examined by using (i) different types of clay minerals such as montmorillonite, hectrite, and saponite,2426 (ii) reduced charge montmorillonite,23,24,2729 and (iii) chemically synthesized clay.14,3034 In the case of using (i) different types of clay minerals, the origin of charges in the clay structure is not unified. Furthermore, montmorillonite, which is a natural clay mineral, contains iron and is colored. To discuss the details of complex formation behavior, (iii) chemically synthesized clay mineral is favored, especially from the viewpoint of photochemistry. Thus, saponite clays with various charge densities were synthesized in the present study to examine the adsorption structure of porphyrin dyes.
’ EXPERIMENTAL SECTION Materials. Sodium silicate solution (Na2SiO3 solution no. 3, 28% SiO2, 9% Na2O) was purchased from Kishida Chemical. Nitric acid 1.42 (70%, GR), magnesium chloride hexahydrate (>99.0%, GR), and aluminum(III) chloride hexahydrate (>98.0%, GR) were purchased from Kanto Chemical. Ammonia solution (28%, GR) and sodium hydroxide (97%, GR) were purchased from Nacalai Tesque. Water was deionized with an ORGANO BB-5A system (PF filter 2 + G-10 column). The synthetic saponite, Sumecton SA [(Si7.2Al0.8)(Mg5.97Al0.03)O20(OH)4]0.77, was received from Kunimine Industries. It was purified by repeated decantation from water and washed with ethanol. From the surface area of 750 m2 g1 (theoretical)11 and the CEC of 99.7 mequiv/100 g, the average area per anionic site is calculated to be 1.25 nm2.6 Thus, the average distance between the neighboring anionic sites can be calculated to be 1.20 nm on the basis of a hexagonal array. Tetrakis(1-methylpyridinium-4-yl)porphyrin (H2TMPyP4+) and tetrakis(N,N,N-trimethylanilinium-4-yl) porphyrin (H2TMAP4+) were purchased from Frontier Scientific (Figure 1). The purity of porphyrins was checked by 1H NMR (JEOL, 270 MHz).
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Figure 1. Structures of H2TMPyP4+ (left) and H2TMAP4+ (right). The intercation distance in the molecule is 1.05 and 1.31 nm based on AM1 calculations for H2TMPyP4+ and H2TMAP4+, respectively.
Analysis. An MMS-200 autoclave (OM Lab-Tech) was used for the hydrothermal synthesis. Centrifugation was conducted with a Hitachi himac CR20GII (Hitachi Koki). The X-ray diffraction (XRD) pattern was measured with an MPX-18 (MAC Science). A Shimadzu TG/DTA DTG-60H was used for thermogravimetric measurement. A Jasco FT/ IR-410 was used for IR measurement. In atomic force microscopy (AFM) measurement, SPI4000 and SPA300HV systems (SII NanoTechnology Inc.) were used. The AFM sample was prepared as follows. A 10 μL suspension of the SS (30 mg L1) was cast on a mica substrate, and the sample obtained was dried at room temperature. 27Al MAS NMR was measured by a JNM-EX270WB FT NMR system (JEOL). [Al(H2O)6]Cl3 was used as the standard for chemical shift. A wavelengthdispersive X-ray fluorescence (WDXRF) spectrometer (a Rigaku ZSX 100e) was used for the elemental analysis for SS. Calibration curves (fundamental parameter method) for Si, Mg, Al, Na, and O atoms were prepared by using NaCl, NaBr, Mg, MgO, Al, Al2O3, SiO2, JB-2, JP-1, JMn-1, and JH-1 (geological standards provided by Advanced Industrial Science and Technology (AIST)) and Sumecton SA. Absorption spectra were obtained on Shimadzu UV-3100 and UV-3600 UVvis spectrophotometers. The CEC of SS was measured by the calcium chloride method as described in the literature.35 Each SS was treated with 1 M calcium chloride (CaCl2) aqueous solution. A precise amount of each sample was added to a centrifuge tube with 10 mL of 1 M CaCl2. The suspension was well shaken, left overnight, and then centrifuged. The clear liquid was discarded, and the CaCl2-saturation procedure was repeated five times by using the fresh 10 mL of CaCl2 solution each time. The obtained sample was treated with 1 M sodium nitrate aqueous solution, and the suspension was centrifuged. The treatment by sodium nitrate solution and the centrifugation were repeated 5 times. The amount of Ca2+ ion in the obtained supernatant, which corresponds to the CEC of the SS, was titrated with 0.01 M ethylenediamine-N,N,N0 ,N0 -tetraacetic acid (EDTA) solution. Preparation Methods for SSPorphyrin Complexes. Absorption spectra of SSporphyrin complexes were observed as described below. Exfoliated SSporphyrin complex was typically prepared by mixing the aqueous SS transparent colloid solution and the respective aqueous porphyrin solution under stirring. The concentration of SS was 8.0 106 equiv L1 (= 8.0 mg L1 in the case of Sumecton SA). The loading levels of porphyrin vs CEC of the SS in the complex were controlled by changing the concentration of porphyrin.
’ RESULTS AND DISCUSSION Hydrothermal Synthesis of Saponites with Different Charge Densities. The synthesis of saponites that have different
charge densities was examined by hydrothermal synthesis according to previous papers.3645 The general formula of saponite is expressed as [(Si8xAlx)(Mg6yAly)O20(OH)4](xy)Naxy, where x = 0.41.2 and y = 0. In the formula, x y determines the CEC of the SS. Because y is very small under the typical synthetic 10723
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Table 1. Synthetic Conditions for the Saponites, Composition of Raw Materials, and Calculated Ratio of Each Element mole ratiob synthetic saponitea
mass of Na2SiO3 solution/g
mass of MgCl2 3 6H2O/g
mass of AlCl3 3 6H2O/g
NaOH/g
SS1(0.8)
13.46
12.62
4.26
0.83
6.27
6.15
1.73
2.07
SS2(0.9)
14.39
12.67
3.21
0.6
6.7
6.17
1.3
1.5
Si
Mg
Al
Na
SS3(0.95)
14.66
12.63
2.89
0.51
6.83
6.15
1.17
1.25
SS4(1.0)
14.8
12.44
2.74
0.53
6.89
6.14
1.11
1.33
SS5(1.2)
15.45
12.44
1.97
0.39
7.2
6.14
0.8
0.98
SS6(1.4)
15.96
12.45
1.4
0.25
7.43
6.06
0.57
0.62
SS7(1.6)
16.25
12.62
1.06
0.26
7.57
6.15
0.43
0.65
SS8(1.8)
16.44
12.63
0.84
0.19
7.66
6.15
0.34
0.47
a
The intercharge distance (nm) on the SS surface based on a hexagonal array calculated from the composition of raw materials is given in parentheses. b Si + Al is normalized to be 8.
conditions, x, means the replacement ratio of Si by Al in the tetrahedral layer is the major factor to determine the CEC of the SS. The formula of Sumecton SA (SSA), which is a typical commercially available synthetic saponite, is [(Si7.20Al0.80)(Mg5.97Al0.03)O20 (OH)4]0.77(Na0.49Mg0.14)+0.77. We synthesized saponites under various synthetic conditions by changing the ratio of Si, Al, and Mg in the raw materials. The value of x was set in the range of 0.341.73. The amount of raw materials and calculated ratio of each element are summarized in Table 1. The typical synthetic procedure for the saponite (x = 0.80) was as follows. Solution A was prepared as follows: 15.45 g of Na2SiO3 solution was diluted by the addition of 80 mL of deionized water, and 5 mL of nitric acid solution was added to the diluted Na2SiO3 solution. Solution B was prepared as follows: 12.44 g of MgCl2 3 6H2O and 1.97 g of AlCl3 3 6H2O were dissolved in 20 mL of deionized water. Solutions A and B were combined, and the obtained solution was added to 52 mL of ammonia solution under continuous stirring within approximately 3 min. The formed precipitate was filtered with a glass filter (type 25G3) and washed with deionized water repeatedly. A 5 mL aqueous solution of NaOH (1.55 M) was added to the collected residue. The obtained slurry was kept at 573 K and 8.5 MPa in the autoclave for about 6 h. The saponite component was collected from the reaction mixture by 3 days of hydraulic elutriation and a centrifugal separation of the supernatant (18 000 rpm, 5 h). The yields of SSs were around 5070%, which were independent of the value of x as shown in Table 1. The SS samples, where x is 1.73, 1.30, 1.17, 1.11, 0.80, 0.57, 0.43, and 0.34, were characterized by XRD, thermogravimetry/differential thermal analysis (TG/DTA), IR, AFM, X-ray fluorescence (XRF), and CEC measurement. In the case of SS1SS4, the samples contained an analcime (NaAlSi2O6) as an impurity at 15.7°, 25.8°, and 30.4° in the XRD patterns. The samples were further purified by hydraulic elutriation for 2 days. After purification, all samples exhibited specific XRD patterns as a typical saponite such as Sumecton SA without any impurities. XRD patterns of each SS after purification are shown in Figure 2. The rather broad peak pattern of (001) indicates that the particle size is very small. The particle size data obtained by AFM measurement are described later. In the TG/DTA profiles for all samples, the endothermic peak was observed at 393 and 1073 K due to the dehydration, and the exothermic peak was observed at around 1081 K due to the phase transition from a saponite to an enstatite. The weight loss at
Figure 2. XRD patterns for SS and SSA.
5731173 K showed good agreement with the hydroxyl group weight in the formula. All observed peaks in TG/DTA for SS coincide well with a pure saponite. IR measurement showed peaks at 660 cm1 (νMgO), 3650 cm1 (νOH), 3440 and 1650 cm1 (δH2O), and 1015 cm1 (νSiO) for all samples. These peaks coincide with those observed for SS. Especially, the peak at 660 cm1 indicates that the SS is not a two-octahedraltype SS but a three-octahedral-type SS. AFM images of SS were observed on a mica substrate. The typical AFM image (in the case of SS4) is shown in Figure 3. Exfoliated clay plates which have around 1.21.4 nm thickness were clearly observed by AFM for all SSs. These observations indicate that all synthesized materials are layered materials, and each sheet is exfoliated in aqueous solution. The theoretical thickness of the SS sheet is 0.96 nm. Since the SS sheet is covered with a few layers of water at ambient humidity, the observed thickness of 1.21.4 nm is reasonable. The diameter is around 20100 nm for all samples, which is almost the same as that of SSA. The amount of Al in the SS structure does not affect the diameter of the particle. 27 Al MAS NMR was measured for SS. The chemical shifts of the Al atom in the tetrahedral sheet (AlIV) and the octahedral sheet (AlVI) are different from each other. Thus, the distribution of Al atom in the SS sheet can be determined by measuring 27Al MAS NMR. The typical 27Al MAS NMR spectra, in the case of 10724
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Figure 3. (a) Typical AFM image of SS particles (SS4). (b) Cross section view at indicated line in image (a).
Figure 4. Typical 27Al MAS NMR spectra of synthetic saponites, SS1, SS4, SS6, and SS8. [Al(H2O)6]Cl3 was used as the standard for chemical shift.
SS1, SS4, SS6, and SS8, are shown in Figure 4. In the spectra, the peak due to AlIV at around 60 ppm is much larger than that due to AlVI at around 5 ppm. These observations indicate that most of the Al atoms locate not at the octahedral sheet but at the surface tetrahedral sheet. XRF measurements were conducted to determine the chemical composition of the synthesized samples. The obtained chemical composition, the CEC, and the calculated intercharge distance on the basis of a hexagonal array are summarized in Table 2. The calculation procedure is described in the Experimental Section and previous papers.6,11 The CEC values determined by the saturation method35 are also shown (the details of the saturation method are described in the Experimental Section and the literature35). The CEC values calculated from the XRF data are apparently different from those obtained by the saturation method, especially for SS1 and SS2. We believe the calculated CEC values from the XRF data are reliable. Since SS1 and SS2 showed poor dispensability during the CEC measurement, perfect cation exchange seems to be difficult. Thus, the CEC values obtained by the saturation method seem to be smaller than the actual values. Good agreement of the calculated intercharge distance by the use of XRF data and the expected one from the composition of raw materials also
supports that the XRF data are reliable. After all, judging from 27 Al MAS NMR and XRF analysis, we have succeeded to synthesize a variety of saponites ([(Si8xAlx)(Mg6yAly)O20(OH)4](xy)) with different charge densities, where most part of the charge originates in the surface tetrahedral layer. The calculated intercharge distance on the SS surface was 0.83 1.92 nm on the basis of a hexagonal array. It should be noted that the CEC values determined by XRF only indicate the permanent charges without the variable charges. Since the saponite possesses relatively high permanent charges, the variable charges are negligible under the near-neutral condition. The SS samples are named as SS1(0.83), where the value in parentheses is the intercharge distance (nm) on the SS surface calculated from the XRF data. Complex Formation Behaviors of Synthetic Saponite with Cationic Porphyrins. The effect of the intercharge distance of the SS on the complex formation behavior with cationic porphyrins was examined. Cationic porphyrins such as H2TMPyP4+ and H2TMAP4+ can adsorb as flat monolayers, without discernible aggregation up to 100% adsorption loadings vs CEC of the SSA.510 As we have reported, a matching of distances between the negatively charged sites on the SSA surface and that between the positively charged sites in the porphyrin molecule is an important factor to achieve a high-density adsorption without aggregation. The absorption spectrum of the SS6(1.45)H2TMPyP4+ complex was observed at various dye loadings as shown in Figure 5. The two components were obviously observed for H2TMPyP4+. The long-wavelength absorption at 453 nm is the component of the adsorbed porphyrins on the SS surface, and the short-wavelength absorption at 421 nm is the component of the nonadsorbed porphyrins in a bulk solution. The λmax shift of the porphyrin upon adsorption on the SS6(1.45) surfaces has been revealed to be mainly dependent on the flattening of the meso-substituents with respect to the porphyrin rings.6,46,47 The linearity of the plot in the inset of Figure 5 was observed below 116% vs CEC of SS6(1.45). Thus, the porphyrin molecules do not aggregate at any dye loading, and the maximum adsorption rates are 116% in this condition. In the same manner, the maximum degrees of adsorption for H2TMPyP4+ and H2TMAP4+ were determined for all SSs as shown in Figure 6. In the case of SS1(0.83), SS2(0.92), SS3(1.00), SS4(1.04), and SS8(1.92)H2TMAP4+ complexes, the absorption spectra exhibited a small spectral change depending on the increase of dye loading below each maximum degree of adsorption, due to the aggregation of porphyrins on the SS surface. The vertical axis in Figure 6, which is the maximum degree of adsorption, is a function of the CEC of SS. Since it is difficult to 10725
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Table 2. Chemical Formulas and the Intercharge Distances of Synthetic Saponites
a
[(Si8xAlx)(Mg6yAly)O20(OH)4](xy), composition
CEC/mequiv g1 calculated from the
intercharge distance/nm calculated from
determined by XRF
composition determined by
the composition determined by
synthetic saponitea
x
y
xy
XRF
saturation method
XRF
saturation method
SS1(0.8)
1.61
0.05
1.56
1.95
1.3
0.83
1.04
SS2(0.9)
1.32
0
1.32
1.68
1.19
0.92
1.1
SS3(0.95)
1.11
0
1.11
1.42
1.2
1
1.09
SS4(1.0)
1.03
0
1.03
1.32
1.14
1.04
1.12
SS5(1.2)
0.79
0.01
0.78
1
0.98
1.19
1.20
SSA
0.80
0.03
0.77
0.99
0.99
1.20
1.20
SS6(1.4) SS7(1.6)
0.56 0.45
0.03 0
0.53 0.45
0.69 0.59
0.71 0.68
1.45 1.57
1.42 1.46
SS8(1.8)
0.33
0.03
0.3
0.39
0.56
1.92
1.61
The intercharge distance (nm) on the SS surface based on a hexagonal array calculated from the composition of raw materials is given in parentheses.
Figure 5. Absorption spectra of SS6(1.45)H2TMPyP4+ complexes at various dye loadings up to 180% vs CEC in aqueous solution. Inset: Absorbanceconcentration plots for the H2TMPyP4+ complex at Soret bands (421 nm ([) and 452 nm (9)) in aqueous solution. The concentration of SS was 8.0 106 equiv L1.
Figure 7. Relationship between the intercharge distances on synthetic saponite and the maximum adsorption densities of H2TMPyP4+ (b) and H2TMAP4+ (2) (nm2).
Figure 6. Relationship between the intercharge distances on synthetic saponite and the maximum degree of adsorption vs CEC/% of H2TMPyP4+ (b) and H2TMAP4+ (2).
Figure 8. Relationship between the intercharge distance on the synthetic saponite and the average intermolecular distance at maximum adsorption on the basis of a hexagonal array (nm) of H2TMPyP4+ (b) and H2TMAP4+ (2).
image the actual adsorption density of a porphyrin molecule on the SS surface, the conversion from “the degree of adsorption vs CEC” to “the adsorption density of molecules”, which is independent of the CEC value of the SS, was conducted to clearly
understand the porphyrin adsorption behavior on the SS surface. The relationship between the charge density of the SS and the maximum adsorption density of molecules (nm2) of porphyrin dyes is shown in Figure 7. 10726
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Figure 9. Typical supposed images of the porphyrin adsorption structure on the synthetic saponite surface: (left) on the SS with high charge density, (right) on the SS with low charge density. The intermolecular distance (indicated as an arrow) can be controlled depending on the SS charge density.
Two important features were observed: (i) It turns out that the maximum adsorption density of porphyrin molecules depends on the charge density of the SS both for H2TMPyP4+ and H2TMAP4+. In other words, using the appropriate SS can control the intermolecular distances between adjacent porphyrins at the saturated condition as shown in Figure 8. (ii) The SS charge density, which afforded the maximum adsorption density of porphyrins, was not the same for H2TMPyP4+ and H2TMAP4+. It seemed that the maximum adsorption density depends on the intramolecular charge distance of H2TMPyP4+ and H2TMAP4+. The intramolecular charge distances of H2TMPyP4+ and H2TMAP4+ were calculated to be 1.05 and 1.31 nm by AM1 methods, respectively.8 Judging from Figure 7, the SS charge densities, which afford the maximum adsorption density of porphyrins, are around 1.0 and 1.2 nm for H2TMPyP4+ and H2TMAP4+, respectively. These relationships indicate that the high-density close packing of porphyrins on the SS surface takes place when the intercharge distance on the SS and the intramolecular charge distance in the porphyrin molecule match well. So far, we propose the size-matching rule only for the combination of SSA whose intercharge distance is 1.2 nm and various organic dyes.510 The obtained results here indicate that the sizematching rule can be expanded for a wide variety of combination between SS hosts and organic dyes. In the case of SS1(0.83) and SS8(1.92), incomplete exfoliation of SS sheets would be the dominant reason for the extremely low maximum adsorption rates. Since the appropriate charge density provides the exfoliation ability of the synthetic saponite, very low (SS1(0.83)) and high (SS8(1.92)) charge density saponites exhibit incomplete exfoliation. The turbulences of SS1 and SS8 in the absorption spectra were apparently stronger than those of other samples as shown in Figure S5, Supporting Information. Typical supposed images of the porphyrin adsorption structures on the SS surface are shown in Figure 9. It turns out that the distances between porphyrin molecules on the SS surface can be controlled from 2.3 to 3.0 nm for H2TMPyP4+ and from 2.5 to 2.9 nm for H2TMAP4+ on the basis of a hexagonal array. Since the intermolecular distance is a crucial factor for chemical reactions, especially for photochemical reactions such as energy transfer and electron transfer, we believe that the present intermolecular control methodology is promising to develop unique photochemical reaction systems on the inorganic surface. The λmax of porphyrin in absorption spectra includes important information on the porphyrin adsorption behavior. The red shift of the porphyrin Soret band is induced by coplanarization of
Figure 10. Relationship between the intercharge distance on the SS surface and the λmax of H2TMPyP4+ (b) and H2TMAP4+ (2) in the SS complexes.
the porphyrin ring and the peripheral aromatic ring on the flat SS surface.6,46,47 Therefore, the stronger adsorption should induce the larger red shift of the Soret band in the SS complex. The obtained λmax values of H2TMPyP4+ and H2TMAP4+ in SS complexes are shown in Figure 10. Since SS1(0.83), SS2(0.92), SS3(1.00), SS4(1.04), and SS8(1.92)H2TMAP4+ complexes exhibited a small shift of λmax depending on the porphyrin concentrations due to aggregation, these λmax values are not included in Figure 10. As a result, the λmax in the complex depends on the intercharge distance on the SS for each porphyrin. When the intercharge distances on the SS are around 1.03 and 1.33 nm, the red shifts are maximum for H2TMPyP4+ and H2TMAP4+, respectively. Since the intercharge distances are matched well with the intramolecular charge distance in H2TMPyP4+ and H2TMAP4+ (1.05 and 1.31 nm), it is revealed that the size-matching combination of the SS and the porphyrin induces the maximum red shift in the complex. Since three methyl groups in H2TMAP4+ surround the ammonium atom, the bulkiness around the ammonium group should reduce the flattening of the porphyrin molecule on the SS surface. Thus, the red shift in H2TMAP4+ (from 412 to 427.5 nm) is small compared to that in H2TMPyP4+ (from 421 to 455 nm). It should be noted that the larger red shift in H2TMPyP4+ also depends on the structure of the meso-substituents. Since the electron-withdrawing ability of the methylpyridinium group in H2TMPyP4+ is stronger than that of the trimethylanilinium group in H2TMAP4+, a larger red shift is induced in H2TMPyP4+. 10727
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Langmuir These observations strongly indicate that the size-matching rule is important not only for the adsorption structure, but also for the photochemical properties of dyes on the SS surface.
’ CONCLUSION Saponite-type clays that have different cation-exchangeable capacities were successfully synthesized by hydrothermal synthesis. The intercharge distance on the SS surface affected the adsorption behavior of cationic porphyrins. Not only the maximum adsorption amount of porphyrin on the SS surface but also the strength of the interaction between the SS sheet and the guest porphyrin molecule is governed by the size-matching rule. The intermolecular distance between porphyrin molecules on the SS surface at saturated adsorption conditions can be controlled by choosing the appropriate SS. The distances between porphyrin molecules on the SS surface were from 2.3 to 3.0 nm for H2TMPyP4+ and from 2.5 to 2.9 nm for H2TMAP4+ on the basis of a hexagonal array. These precise controls of intermolecular distances are promising to construct a photochemical reaction system such as an artificial light-harvesting system. The methodology for controlling the intermolecular distances using the size-matching rule should be noted as a unique novel method, compared to conventional self-assembly techniques. In the case of typical self-assembly techniques, van der Waals interaction between guest molecules is necessary to make a specific adsorption structure. Therefore, the large gap distance between guest molecules is hardly achieved. Using the “sizematching technique” can achieve the unique adsorption structure, where guest molecules locate with a large gap distance. ’ ASSOCIATED CONTENT
bS
Supporting Information. Typical structure of saponite, data of TG-DTA and FT-IR, absorption spectra and absorption concentration plot of H2TMAP4+, and absorption spectra of the aqueous SS suspension. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
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[email protected]. Phone: +81 42 677 2839. Fax: +81 42 677 2838.
’ ACKNOWLEDGMENT This work has been partly 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 Fellow DC1 from the Japan Society for the Promotion of Science. ’ REFERENCES (1) Whitesides, M. G.; Grzybowski, B. Science 2002, 295, 2418– 2421. (2) Tien, J.; Terfort, A.; Whitesides, M. G. Langmuir 1997, 13, 5349–5355. (3) Yoshimoto, S.; Honda, Y.; Ito, O.; Itaya, K. J. Am. Chem. Soc. 2008, 130, 1085–1092. (4) Sakaguchi, H.; Matsumura, H.; Gong, H.; Abouelwafa, A. Science 2005, 310, 1002–1006.
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