Viologen Monolayer on the Clay

Article pubs.acs.org/JPCC

Microstructures of the Porphyrin/Viologen Monolayer on the Clay Surface: Segregation or Integration? Saki Konno,† Takuya Fujimura,†,‡ Yuta Otani,† Tetsuya Shimada,† Haruo Inoue,† and Shinsuke Takagi*,†,§ †

Department of Applied Chemistry, Graduate Course of Urban Environment 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 332-0012, Japan S Supporting Information *

ABSTRACT: Microstructures of the porphyrin/viologen monolayer on an anionic clay surface (synthetic saponite) were investigated by the observation of photochemical behavior of porphyrin. Fluorescence behaviors of porphyrin−viologen− clay complexes were observed by steady state and time-resolved fluorescence spectroscopy. Although fluorescence of porphyrin was effectively quenched by coadsorbed viologen on the clay surface, a part of the fluorescence of porphyrin was not quenched and remained even at high loading level of viologen. According to time-resolved fluorescence measurement, the decay profile of excited singlet porphyrin can be analyzed by two- or three-component fitting for porphyrin−viologen−clay complexes. These results indicate that porphyrin and viologen adsorb with island (segregation) structure on the clay surface. The size of the island formed by porphyrin was quantitatively estimated for two kinds of porphyrins. It turned out that the porphyrin molecular structure affects the size of the island. It has been believed that electron transfer on a clay surface is inefficient due to segregation of dyes between clay sheets (A. J. Bard et al., J. Phys. Chem. 1984, 88, 5519). Our results indicate that segregation behavior sensitively depends on the structure of the dye, and it is possible to construct an efficient electron transfer system on the clay surface.

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transfer reaction in clay complexes.9,19,20 It has been believed that electron transfer in the system of clay minerals is inefficient, due to the aggregation and segregation behavior of dyes on the clay surface. When Ru(bpy)32+ and methylviologen (MV2+) were used as an electron donor and an electron acceptor, respectively, the efficiency of fluorescence quenching of the excited state of Ru(bpy)32+ by coadsorbed MV2+ was very low.9 It was supposed that the reason for inefficient electron transfer was that Ru(bpy)32+ and MV2+ were located in different sheets of the clay interlayers (intersheet segregation, Figure 2(a)).9 The interaction between guest molecules may cause the integration adsorption structure. Segregation of dyes drastically reduces the efficiency of the intermolecular photochemical reactions since the molecules are separated from each other between clay sheets or on the clay surfaces. We expect that the segregation behavior of dyes should not always occur, and intrasheet segregation (Figure 2(b)) or integration (Figure 2(c)) of two kinds of dyes on the clay surface would happen by choosing the appropriate dyes.

INTRODUCTION Photoinduced electron transfer reactions in heterogeneous systems have been studied over the past decades to construct efficient photochemical reaction systems, such as an artificial photosynthetic system with nanostructured environments.1−3 Various host materials such as zeolites, layered materials, vesicles, and micelles have been examined to construct a photochemical electron transfer reaction system.4−14 These materials have a possibility to act as a chemical reaction field to set the molecules at the appropriate position to achieve efficient electron transfer and retard back electron transfer. Clay minerals15−18 (Figure 1) can be used as unique hosts for photochemical reactions since their multilayered structure provides unique quasi-two-dimentional spaces with a very flat surface. However, few studies have been reported on electron

Received: July 29, 2014 Revised: August 11, 2014 Published: August 18, 2014

Figure 1. Structure of synthetic clay. © 2014 American Chemical Society

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dx.doi.org/10.1021/jp5076274 | J. Phys. Chem. C 2014, 118, 20504−20510

The Journal of Physical Chemistry C

Article

Industries Co., Ltd. It was purified by repeated decantation from water and washed with ethanol. From the surface area of 750 m2 g−1 (theoretical)33 and the cationic exchange capacity (CEC) of 99.7 mequiv/100 g, the average area per anionic site is calculated to be 1.25 nm2. Thus, the average distance between the neighboring anionic sites is estimated to be 1.20 nm on the basis of a hexagonal array. Water was deionized just before use with an ORGANO BB-5A system (PF filter × 2+G10 column). Tetrakis(1-methylpyridinium-4-yl) porphyrin (H2TMPyP4+) and zinc tetrakis(trimethylanilinium-4-yl) porphyrin (ZnTMAP4+) were purchased from Aldrich. Their counterions (tosylate ion) were exchanged with chloride by an ion exchange column (ORGANO AMBERLITE IRA400JCL). The purity of porphyrins was confirmed by 1H NMR (JEOL (270 MHz)). 1′,1-Bis(2,4-dinitrophenyl)-4′,4-bipyridinium dichloride (DNPV2+) was purchased from Tokyo Chemical Industry and was used as received. Analysis. Absorption spectra were measured with a Shimadzu UV-3600 UV−visible spectrophotometer. Fluorescence spectra were measured with a JASCO FP-6500 spectrofluorometer. The fluorescence spectra of the clay− H2TMPyP4+−DNPV2+ complex and the clay−ZnTMAP4+− DNPV2+ complex were measured excited at 450 and 429 nm, respectively. TG/DTA measurements were carried out with a Shimadzu DTG-60H to determine the water content of the dyes. Time-resolved fluorescence signals were measured by a Hamamatsu Photonics C4780 system based on a streak detector. An Nd3+ YAG laser with an OPG (EKSPLA PL2210JE + PG-432, 429, and 450 nm, fwhm 25 ps, 1 kHz) was used for excitation. Sample Preparation. Exfoliated clay−dye complexes were prepared as follows. Porphyrin aqueous solution was mixed with DNPV2+ aqueous solution under stirring. Then, the obtained solution was mixed with aqueous clay solution by a stopped flow mixer (Applied Photophysics, RX, 2000). In absorption measurements, the concentration of clay (40 mg L−1) was fixed, and the loading levels of DNPV2+ vs CEC of the clay in the complex were adjusted by changing the concentration of DNPV2+. In fluorescence measurements, the concentrations of porphyrin (5.0 × 10−7 M (5.0% vs CEC)) and the clay (40 mg L−1) were fixed, and the loading levels of DNPV2+ were changed from 1.0% to 90% vs CEC by adjusting the concentration of DNPV2+. The concentration of porphyrin was constant for all samples to maintain the absorbance of porphyrin in fluorescence measurements. Under these conditions, solutions are completely transparent in the visible region. In time-resolved fluorescence measurement experiments, the concentration of clay was 40 mg L−1, and the loading level of porphyrin and DNPV2+ was 5.0 × 10−7 M (5.0% vs CEC) and 1.8 × 10−5 M (90% vs CEC), respectively.

Figure 2. Image of (a) intersheet segregation, (b) intrasheet segregation, and (c) integration of two kinds of dyes on the clay surface.

In the present paper, we examined the fluorescence quenching behavior due to electron transfer reaction between excited singlet porphyrins and 1′,1-bis(2,4-dinitrophenyl)-4′,4bipyridinium (DNPV2+, Figure 3) on the clay surface. Both

Figure 3. Structure of (a) cationic porphyrins (H2TMPyP4+ and ZnTMAP4+) and (b) DNPV2+.

cationic molecules fulfill the “size-matching rule”, and thus the aggregation behavior, that makes photochemical behavior complicated, is suppressed as described below. So far, we have reported unique complexes composed of cationic porphyrins and clay minerals.21−30 We have successfully realized unique clay−porphyrin complexes in which the porphyrin molecules adsorb on the clay surface without aggregation even at 100% adsorption vs CEC,21 although dye molecules tend to aggregate on the clay surface.15−18,31 The formation of these complexes was rationalized by a size matching of distances between the charged sites on the porphyrin and corresponding distances on the clay surface. We have termed this the “size-matching rule” or “intercharge distance matching rule”.21−23,30 The image for the sizematching rule is shown in Figure S1 (Supporting Information). Here, aggregation is defined as the adsorption structure which induces the absorption spectral change through the interactions between transition dipole moments in adjacent molecules. The aggregation of dye molecules such as H-aggregates significantly decreases their excited state lifetimes and makes it difficult to perform photochemical reactions. We have also found that selfquenching was completely suppressed in the case of some specific porphyrins.32 The systems which fulfill the sizematching rule enable us to examine the research on photoinduced electron transfer since dye molecules adsorb on the clay surface without aggregation. In the present paper, the fluorescence quenching behavior of cationic porphyrins by DNPV2+ was examined by steady state fluorescence and timeresolved fluorescence measurement to discuss the adsorption structure of molecules on the clay surface. Tetrakis(1methylpyridinium-4-yl) porphyrin (H2TMPyP4+) and zinc tetrakis(trimethylanilinium-4-yl) porphyrin (ZnTMAP4+) were used as cationic porphyrin that can suffer fluorescence quenching by DNPV2+.

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RESULTS AND DISCUSSION Adsorption Behavior of Porphyrin and DNPV2+ on the Clay Surface. The suppression of dye aggregation on the clay surface is a crucial problem to construct an efficient electron transfer system, as mentioned in the Introduction. We have already shown that in the case of some specific porphyrins aggregation is completely suppressed below the loading level of 100% by the size-matching effect.21−23,30 Porphyrin molecules, such as H2TMPyP4+ and ZnTMAP4+, do not aggregate on the clay surface up to 100% adsorption vs CEC. DNPV2+ was used as an electron acceptor since electron transfer between the excited singlet porphyrin and DNPV2+ is estimated to be

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EXPERIMENTAL SECTION Chemicals. Sumecton SA (Figure 2, a synthetic saponaite, Na0.77Mg5.97Al0.83Si7.20O20(OH)4) was received from Kunimine 20505



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experiments. Fluorescence spectra of the clay−H2TMPyP4+− DNPV2+ and clay−ZnTMAP4+−DNPV2+ complexes are shown in Figure 6 and Figure 7, respectively. The concentration of

sufficiently exergonic.34 We examined the adsorption behavior of DNPV2+ molecules on the clay surface by the observation of absorption spectra at various loading levels of DNPV2+. The absorption spectra of DNPV2+ on the exfoliated clay at various loading levels in aqueous solution are shown in Figure 4. The

Figure 6. Fluorescence spectra of the clay−H2TMPyP4+−DNPV2+ complex excited at 450 nm in aqueous solution. [H2TMPyP4+] = 5.0 × 10−7 M (5.0% vs CEC), [clay] = 40 mg L−1 in water. The DNPV2+ loading levels were 1.0−90% vs CEC.

Figure 4. Absorption spectra of the clay−DNPV2+ complex at various DNPV2+ concentrations up to 300% vs CEC in aqueous solution ([clay] = 1.6 × 10−5 eq L−1).

λmax of DNPV2+ without clay and on clay is 265 and 267 nm, respectively. Judging from the normalized absorption spectra (not shown), the spectral shape of DNPV2+ stayed completely the same below the loading level of 80% vs CEC. Above the loading level of 80%, free molecules in aqueous solution appeared. The Lambert−Beer plot also supports the estimation of the maximum loading level of DNPV2+ without aggregation (Figure 5). Thus, it turned out that DNPV2+ molecules adsorb

Figure 7. Fluorescence spectra of the clay−ZnTMAP4+−DNPV2+ complex excited at 429 nm in aqueous solution. [ZnTMAP4+] = 5.0 × 10−7 M (5.0% vs CEC), [clay] = 40 mg L−1 in water. The DNPV2+ loading levels were 1.0−90% vs CEC.

DNPV2+ was changed from 1.0 to 90% vs CEC of the clay. As can be seen, apparent fluorescence quenching of porphyrins by the addition of DNPV2+ was observed for both cases, even at low loading level of DNPV2+. It is reported that the fluorescence quenching of Ru(bpy)32+ by MV2+ was inefficient, due to the segregation behavior.9 These results indicate that a perfect intersheet segregation does not occur in the present system. In Figure 8, the normalized fluorescence intensities are plotted versus the concentration of DNPV2+. Although the

Figure 5. Lambert−Beer plots for the clay−DNPV2+ complex at 270 nm (▲) and 315 nm (■) in aqueous solution ([clay] = 1.6 × 10−5 eq L−1).

on the clay surface up to 80% vs CEC without aggregation and desorption. The distance between cationic sites in the DNPV2+ molecule was estimated to be 0.7 nm by AM1 calculation, while the average distance between anionic sites on the clay surface was calculated to be 1.2 nm.35 The size-matching effect seems to moderately effect the adsorption behavior of DNPV2+ and suppress the aggregation of DNPV2+ on the clay surface. In the present condition, it turned out that both porphyrin and DNPV2+ molecules do not aggregate even at high density conditions on the clay surface. These conditions are suitable for analyzing fluorescence quenching behavior. Fluorescence Quenching Behavior of Porphyrins by DNPV2+ on the Clay Surface. The fluorescence quenching behavior of excited singlet porphyrin by DNPV2+ coadsorbed on the clay surface was examined by steady state fluorescence measurements. Two types of porphyrin (H2TMPyP4+ and ZnTMAP4+, Figure 3(a)) were used as dyes in these

Figure 8. Normalized fluorescence intensity at various DNPV2+ loading levels. [H2TMPyP4+] = [ZnTMAP4+] = 5.0 × 10−7 M, [clay] = 40 mg L−1 in water. The DNPV2+ loading levels were 1.0− 90% vs CEC. 20506



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Figure 9. Distribution images of the clay−porphyrin−DNPV2+ complex. The loading level of porphyrin is 5% vs CEC of the clay. (a) Integration type, (b-1) segregation type for H2TMPyP4+, and (b-2) segregation type for ZnTMAP4+.

the time-resolved fluorescence measurement system was carried out to distinguish the model shown in Figure 9(a) and (b). Time-Resolved Fluorescence Measurement of Porphyrin−DNPV2+−Clay Complexes. We measured the excited singlet lifetime of porphyrin and estimated fluorescence quenching rates on the clay surface by analyzing the porphyrin fluorescence decay curve obtained by the time-resolved fluorescence measurement system. The reports on timeresolved fluorescence in the clay complexes are very few since the decay behavior of the excited molecule on the clay surface is too complicated in general, mainly due to aggregation. Perfect suppression of aggregation due to the size-matching effect makes it possible to analyze time-resolved fluorescence behavior simply in our experiment. The fluorescence decay curves for clay−H2TMPyP4+−DNPV2+ and clay−ZnTMAP4+− DNPV2+ complexes are shown in Figures 10 and 11, respectively. The fluorescence decay curves for clay−porphyrin complexes can be analyzed with a single-exponential fitting (τ0). On the other hand, those for clay−porphyrin−DNPV2+

porphyrin fluorescence was effectively quenched even at low loading level of DNPV2+, the fluorescence intensity of porphyrin became constant at high loading level of DNPV2+. The remaining fluorescence ratio at 90% loading level of DNPV2+ was 54% for H2TMPyP4+ and 13% for ZnTMAP4+, respectively. From these results, the adsorption distribution structure of porphyrin and DNPV2+ on the clay surface can be discussed. Fluorescence quenching of porphyrins by the addition of DNPV2+ was observed, even at low loading level of DNPV2+. When DNPV2+ loading level is 20%, the average intermolecular distance is calculated to be 5.4 nm on the basis of a uniform hexagonal array. An efficient electron transfer is impossible at such a far intermolecular distance. This indicates that the adsorption distribution of DNPV2+ molecules is not uniform and that DNPV2+ molecules adsorb around porphyrin. On the other hand, all fluorescence from the excited porphyrin was not quenched even when the loading level of DNPV2+ is 90% vs CEC of the clay. There are two possible explanations for these observations. The first explanation is that the electron transfer rate is insufficient to achieve efficient fluorescence quenching, even when DNPV2+ molecules surround porphyrin. The second one is that a part of the porphyrin molecules are surrounded by porphyrin itself, and inner porphyrin cannot suffer fluorescence quenching. Thus, it is possible to propose two adsorption distribution structures as shown in Figure 9. Image (a) is for integration type, and (b) and (c) are for segregation type. The fluorescence quenching efficiency of the clay−ZnTMAP4+−DNPV2+ complex was higher than that of the clay−H2TMPyP4+−DNPV2+ complex. Considering that electron transfer between excited singlet ZnTMAP4+ and DNPV2+ is more exergonic than that between excited singlet H2TMPyP4+ and DNPV2+, the adsorption distribution structure shown in Figure 9(a) seems to be possible. On the other hand, Figure 9(b) is the image for segregation type. Because the remaining fluorescence ratio at 90% loading level of DNPV2+ was 54% for H2TMPyP4+ and 13% for ZnTMAP4+, the number of molecules forming the island should be around 44 and 7, respectively, on the assumption that porphyrin neighboring to DNPV2+ is completely quenched. When the number of molecules forming the island is 44 (Figure 9(b-1)), the number of the outer circumference and inner molecule is ca. 20 and 24, respectively. In this case, the ratio of the inner molecule is 55% which agrees well with the remaining ratio of fluorescence (54%). However, it is impossible to determine the adsorption distribution structure by using the data obtained by steady state fluorescence measurement. A further detailed experiment using

Figure 10. Fluorescence decay profile of the clay−H2TMPyP4+− DNPV2+ complex in aqueous solution. [H2TMPyP4+] = 5.0 × 10−7 M (5.0% vs CEC), [DNPV2+] = 1.8 × 10−5 M (90% vs CEC), and [clay] = 40 mg L−1 in aqueous solution. The excitation wavelength was 450 nm. 20507



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porphyrin−DNPV2+ should affect the island size of porphyrin. The bulky substituent (ammonium group) in the ZnTMAP4+ molecule would prevent the effective packing from forming a larger island. To understand the factors to determine the island size, a more detailed experiment for many varieties of adsorbing molecules including the thermodynamic observations of segregation is necessary. Electron transfer rate can be obtained from the values of τ0 and τi on the basis of eq 3. 1 τ0 = k f + k nr (1) τi =

1 k f + k nr + keT

keT =

(2)

1 1 − τi τ0

(3)

where τ0 and τi are the excited singlet lifetimes in the absence and presence of an electron acceptor, respectively. kf and knr are the rate constants for the radiative and nonradiative decay processes. keT is an electron transfer rate constant. Calculated electron rate constants are shown in Table 2. ZnTMAP

Figure 11. Fluorescence decay profile of the clay−ZnTMAP4+− DNPV2+ complex in aqueous solution. [ZnTMAP4+] = 5.0 × 10−7 M (5.0% vs CEC), [DNPV2+] = 1.8 × 10−5 M (90% vs CEC), and [clay] = 40 mg L−1 in aqueous solution. The excitation wavelength was 429 nm.

Table 2. Electron Transfer Rates on the Clay Surface for Clay−Porphyrin−DNPV2+ Complexes

complexes required a double- or triple-exponential decay fitting. Estimated lifetimes for each complex are summarized in Table 1. ai in Table 1 are the pre-exponential factors for τ i components. The observation of two or three excited singlet lifetimes components for clay−porphyrin−DNPV2+ complexes strongly indicated that there are two or three kinds of porphyrins in different situations on the clay surface. Since the values of τ1 were nearly consistent with the values of τ0, τ1 can be attributed to the porphyrin, which does not neighbor to DNPV2+ molecules. The short lifetime component can be attributed to the excited lifetimes of porphyrin, which neighbors to DNPV2+ molecules and suffers fluorescence quenching by DNPV2+. These results clearly eliminate the possibility of the structure shown in Figure 9(a), and we can conclude that porphyrin molecules make an island surrounded by DNPV2+ as shown in Figure 9(b) and (c). Judging from the values of preexponential factors, in the case of H2TMPyP4+, 58% of H2TMPyP4+ did not suffer fluorescence quenching, while in the case of ZnTMAP4+, 25% of ZnTMAP4+ did not suffer fluorescence quenching. By using these values and supposing the hexagonal-like shape of the island, the numbers of porphyrin molecules forming the island (n) are ca. 52 (nH2TMPyP) and 12 (nZnTMAP) for H2TMPyP4+ and ZnTMAP4+, respectively.38 The island size of H2TMPyP4+ was larger than that of ZnTMAP4+. The interaction between porphyrins and

donor

k1/ns−1

k2/ns−1

k3/ns−1

H2TMPyP ZnTMAP

0.014 0.38

0.49 4.2

49

afforded larger keT compared to H2TMPyP, as theoretically expected.34 Because the redox potential of porphyrin on the clay surface cannot be measured, quantitative discussion is not possible. The electron transfer rates were smaller than we expected in the present system, especially for H2TMPyP. It is speculated that the unique effect of the very flat clay surface on the reorganization energy of the molecule in the electron transfer process is working. The reorganization energy for the electron transfer process may be relatively large because the structure of the molecule adsorbing on the clay surface is fixed.

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CONCLUSIONS Fluorescence quenching behaviors of adsorbed porphyrin by DNPV2+ on the clay surface were examined by steady state and time-resolved fluorescence measurement. Fluorescence quenching rates were quantitatively analyzed by time-resolved fluorescence measurement. Although it is believed that intermolecular photochemical processes such as an electron transfer reaction is inefficient in the clay system, significant fluorescence quenching due to electron transfer was observed

Table 1. Fluorescence Lifetimes for Porphyrins on the Clay Surface in Aqueous Solution donor

acceptor

τ0/nsa

H2TMPyP H2TMPyP ZnTMAP ZnTMAP

DNPV DNPV

5.6

τ1/nsb

τ2/nsb

5.2

1.5

0.67

0.19

τ3/nsb

a1c

a2c

0.58

0.42

0.25

0.10

a3c

0.9 0.02

0.66

a [clay] = 500 mg L−1, [H2TMPyP4+] = [ZnTMAP4+] = 8.4 × 10−6 M (6.7% vs CEC) in aqueous solution. b[clay] = 40 mg L−1, [H2TMPyP4+] = [ZnTMAP4+] = 5.0 × 10−7 M (5.0% vs CEC), [DNPV2+] = 1.8 × 10−5 M (90% vs CEC) in aqueous solution. The excitation wavelength is 450 and 429 nm for H2TMPyP4+ and ZnTMAP4+, respectively. ca1, a2, and a3 are the pre-exponential factors of τ1, τ2, and τ3 components.

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for the porphyrin−DNPV2+−clay system. In addition, we found that porphyrin molecules form an island (segregation) structure on the clay surface, and the island size depends on the molecular structure of porphyrin. This indicates that there is a possibility to control segregation behavior by adjusting the guest molecule on the clay surface. Because segregation behavior is a crucial factor for intermolecular photochemical reactions, the findings in this study expand the possibilities to construct efficient photochemical reaction systems on the inorganic surface.

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(9) Ghosh, P. K.; Bard, A. J. Photochemistry of Tris(2,2′bipyridyl)(ruthenium(II) in Colloidal Clay Suspensions. J. Phys. Chem. 1984, 88, 5519−5526. (10) Ranjit, K. T.; Kevan, L. Photoreduction of Methyl Viologen in Zeolite X. J. Phys. Chem. B 2002, 106, 1104−1109. (11) Xiang, B.; Kevan, L. Effects of Chloromethanes on the Photoionization of Methylphenothiazine in Silica Gels at Room Temperature. Langmuir 1995, 11, 860−863. (12) Sung-Suh, H. M.; Kevan, L. Photoionization of Metalloporphyrins in Silica Gel. J. Phys. Chem. Soc., Faraday Trans. 1998, 94, 1417−1420. (13) Lukac, S.; Harbour, J. R. Association of Methyl Viologen and Its Cation Radical with Dihexadecyl Phosphate Vesicles. J. Am. Chem. Soc. 1983, 105, 4248−4250. (14) Colaneri, M. J.; Kevan, L.; Schmehl, R. An Electron Spin Resonance Study of Charge Separation in Frozen Sodium Dodecyl Sulfate Micellar Solutions Containing Tris(2,2′-bipyridine)ruthenium(II) Complexes and Alkylmethylviologens. J. Phys. Chem. 1989, 93, 397−401. (15) Ogawa, M.; Ide, Y.; Okada, T. Organic-inorganic Hybrids Based on Ultrathin Oxide Layers - Designed Nanostructures for Molecular Recognition. Chem.Asian J. 2010, 7, 1980−1992. (16) Takagi, K.; Shichi, T. Clay Minerals as Photochemical Reaction Fields. J. Photochem. Rev. 2000, 1, 112−130. (17) Takagi, S.; Eguchi, M.; Tryk, D. A.; Inoue, H. Porphyrin Photochemistry in Inorganic/Organic Hybrid Materials: Clays, Layered Semiconductors, Nanotubes, and Mesoporous Materials. J. Photochem. Photobiol. C: Photochem. Rev. 2006, 7, 104−126. (18) Thomas, J. K. Physical Aspects of Photochemistry and Radiation Chemistry of Molecules Adsorbed on Silica, .Gamma.-Alumina, Zeolites, and clays. Chem. Rev. 1993, 93, 301−320. (19) Thomas, J. K.; Liu, X.; Iu, K. K. Studies of Surface Properties of Clay Laponite using Pyrene as a Photophysical Probe Molecule. 2. Photoinduced Electron Transfer. Langmuir 1992, 8, 539−545. (20) Schoonheydt, R. A.; De Pauw, P.; Villers, D.; De Schriver, F. C. Luminescence of Tris(2,2′-bipyridine)ruthenium(II) in Aqueous Clay Minerals Suspensions. J. Phys. Chem. 1984, 88, 5113−5118. (21) 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. (22) 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. (23) Eguchi, M.; Takagi, S.; Tachibana, H.; Inoue, H. The Size Matching rule in Di-, Tri-, and Tetra-Cationic Charged Porphyrin/ Synthetic Clay Complexes: Effect of the Inter-Charge Distance and the Number of Charged Sites. J. Phys. Chem. Solid 2004, 65, 403−407. (24) Takagi, S.; Eguchi, M.; Yui, T.; Inoue, H. Photochemical Electron Transfer Reactions in Clay-Porphyrin Complexes. Clay Sci. 2006, 12, 82−87. (25) Takagi, S.; Tryk, D. A.; Inoue, H. Photochemical Energy Transfer of Cationic Porphyrin Complexes on Clay Surface. J. Phys. Chem. B 2002, 106, 5455−5460. (26) Eguchi, M.; Takagi, S.; Inoue, H. The Orientation Control of Dicationic Porphyrins on Clay Surfaces by Solvent Polarity. Chem. Lett. 2006, 35, 14−15. (27) Fujimura, T.; Shimada, T.; Hamatani, S.; Onodera, S.; Sasai, R.; Inoue, H.; Takagi, S. High Density Intercalation of Porphyrin into Transparent Clay Membrane without Aggregation. Langmuir 2013, 29, 5060−5065. (28) Takagi, S.; Eguchi, M.; Inoue, H. Light Harvesting Energy Transfer and Subsequent Electron Transfer of Cationic Porphyrin Complexes on Clay Surfaces. Langmuir 2006, 22, 1406−1408. (29) Eguchi, M.; Tachibana, H.; Takagi, S.; Tryk, D. A.; Inoue, H. Dichroic Measurements on Dicationic and Tetracationic Porphyrins on Clay Surfaces with Visible Light Attenuated Total Reflectance. Bull. Chem. Soc. Jpn. 2007, 80, 1350−1356.

ASSOCIATED CONTENT

S Supporting Information *

Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been partly supported by a Grant-in-Aid for Scientific Research (B), a Grant-in-Aid for Scientific Research on Innovative Areas (No. 25107521), a Grant-in-Aid for Precursory Research for Embryonic Science and Technology (PRESTO) from Japan Science and Technology Agency (JST), and by Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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(30) Takagi, S.; Shimada, T.; Ishida, Y.; Fujimura, T.; Masui, D.; Tachibana, H.; Eguchi, M.; Inoue, H. Size-Matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and Orientation of Molecular Adsorption as a Bottom-Up Methodology for Nanomaterials. Langmuir 2013, 29, 2108−2119. (31) Bujdak, J.; Iyi, N.; Fujita, T. The Aggregation of Methylene Blue in Montmorillonite Dispersions. Clay Miner. 2002, 37, 121−133. (32) Ishida, Y.; Shimada, T.; Tachibana, H.; Inoue, H.; Takagi, S. Regulation of the Collisional Self-quenching of Fluorescence in Clay/ Porphyrin Complex by the Strong Host-guest Interaction. J. Phys. Chem. A 2012, 116, 12065−12072. (33) The surface area measured with the N2 gas adsorption method is 162 m2 g−1. This value was measured for stacked clay minerals in the solid state. Under our experimental conditions, the theoretical specific surface area (750 m2 g−1) is suitable. (34) The oxidation potentials of H2TMPyP4+ and ZnTMAP4+ are 1.30V36 and 0.95V37 vs NHE (E0(D+/D)), respectively. The excited singlet state energies are around 2.0 eV (ΔG00). The reduction potential of DNPV2+ is 0.02 V vs NHE (E0(A/A−)) in aqueous solution. According to these values, ΔG0 for the electron transfer process is calculated to be −0.7 and −1.1 V for H2TMPyP4+ and ZnTMAP4+, respectively. (35) The average surface area per negative site is 1.25 nm2. The mean distance between anionic sites is calculated to be 1.2 nm on the basis of a hexagonal array. (36) Kalyanasundaram, K.; Neumann-Spallart, M. Photophysical and Redox Properties of Water-Soluble Porphyrins in Aqueous Media. J. Phys. Chem. 1982, 86, 5163−5169. (37) Kalyanasundaram, K. Photochemistry and Sensitized Evolution of Hydrogen from Water Using Water-Soluble Cationic Porphyrins. Tetrakis(trimethylaminophenyl)porphyrinatozinc and Its Free Base. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1365−1374. (38) Typical diameter of a clay sheet is around 50 nm. Thus, one clay sheet possesses around 2000 anionic charges (= 50 × 50/1.25). 5% loadings vs CEC correspond to 100 anionic charges and 25 porphyrin molecules for each clay sheet. Thus the maximum n is 25, on the assumption that the porphyrin molecule uniformly distributed to each clay sheet. Because the estimated nH2TMPyP is apparently larger than 25, intersheet segregation has to take place in the case of H2TMPyP4+.

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Viologen Monolayer on the Clay

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