High-Density Adsorption of Cationic Porphyrins on Clay Layer

Langmuir 2002, 18, 2265-2272

2265

High-Density Adsorption of Cationic Porphyrins on Clay Layer Surfaces without Aggregation: The Size-Matching Effect Shinsuke Takagi,* Tetsuya Shimada, Miharu Eguchi, Tatsuto Yui, Hirohisa Yoshida, Donald A. Tryk, and Haruo Inoue*,† Department of Applied Chemistry, Graduate Course of Engineering, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397 Japan Received October 8, 2001. In Final Form: November 29, 2001 Several inorganic-organic hybrid complexes were synthesized from a synthetic clay (Sumecton SA) and cationic porphyrins (+4 charge). In the clay-porphyrin complexes, the λmax values of the Soret bands of the porphyrins were shifted to longer wavelengths compared to those in water. Two types of complexes were formed depending on the preparation method. One is assigned to a complex in which the porphyrin molecules are adsorbed on the external surfaces of the dispersed clay layers (type b complexes). The other is assigned to a complex in which the porphyrin molecules are intercalated within the stacked clay layers (type c complexes). The aqueous solutions of both types of complexes do not scatter light in the UV-visible wavelength region. Surprisingly, the porphyrin molecules were found to adsorb on the clay sheets as densely packed monolayers with controlled intermolecular gap distance. In type b, the porphyrins are adsorbed as flat monolayers, without discernible aggregation, that precisely neutralize the negative charges of the clay surface. According to fluorescence lifetime measurements, the adsorbed porphyrin molecules have sufficiently long lifetimes to be used as sensitizers. The fluorescence lifetimes of tetrakis (N, N, N-trimethyl-anilinium-4-yl) porphyrin were found to be 4.1 ns in type b complexes and 3.2 ns in type c, while that in water is 9.3 ns. We report here a novel method in which highly dense yet controllable structures without aggregation can be produced as adsorbed layers on clay surfaces for the first time. We propose that the mechanism for this extraordinary monolayer adsorption could be a precise matching of distances between the negatively charged sites on the clay sheets and that between the positively charged sites in the porphyrin molecule. We have termed this the “size-matching effect.”

Introduction Clay minerals are well known as multilayered inorganic materials that provide quasi-two-dimensional spaces, which, because of their well-defined dimensions, should be interesting from the viewpoint of microenvironments for chemical reactions.1-5 Intercalation of organic molecules into clay layer structure can afford a very interesting inorganic-organic hybrid compounds.3-5 Recently, synthetic clay minerals have attracted increasing interest, especially for their application to photochemical reactions.1-10 Because of the photochemical interest, complexes between clay minerals and dyes are expected to provide new functions and versatility. Although many different types of dye molecules have been utilized in modifying the unique chemical reaction environment provided by clay minerals, these molecules tend to aggregate on the clay surface or in the interlayer spaces.3-5 * To whom correspondence should be addressed. † CREST, JST (Japan Science and Technology). (1) Thomas, J. K. Acc. Chem. Res. 1988, 21, 275. (2) Thomas, J. K. Chem. Rev. 1993, 93, 301. (3) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (4) Takagi, K.; Shichi, T. In Solid State and Surface Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 2000; Vol. 5, p 31. (5) Takagi, K.; Shichi, T. J. Photochem. Photobiol., C 2000, 1, 112. (6) Takagi, K.; Usami, H.; Fukuya, H.; Sawaki, Y. J. Chem. Soc., Chem. Commun. 1989, 1174. (7) Usami, H.; Takagi, K.; Sawaki, Y. J. Chem. Soc., Perkin Trans. 2 1990, 1723. (8) Usami, H.; Takagi, K.; Sawaki, Y. J. Chem. Soc., Faraday Trans. 2 1992, 88, 77. (9) Usami, H.; Takagi, K.; Sawaki, Y. Bull. Chem. Soc. Jpn. 1991, 64, 3395. (10) Takagi, K.; Shichi, T.; Usami, H.; Sawaki, Y. J. Am. Chem. Soc. 1993, 115, 4339.

Since aggregation in general decreases the excited-state lifetimes of dyes drastically, the control of aggregation on the solid surface is very important to induce photochemical reactions. The conformational control of dye adsorption on clay surfaces has been considered difficult in the past, and the reports of photochemical reactions in clay minerals have been confined mostly to reactions between molecules that are attached to the surface and in close proximity, as in an adsorbed aggregate.6-10 In the present work, Sumecton SA (SSA) was used; it is an artificially synthesized cation-exchangeable clay (synthetic saponite). SSA is colorless and highly pure and has good dispersibility in water. Since aqueous solutions of SSA are transparent in the UV-visible range, they are well suited for photochemical reactions. Porphyrin analogues have various useful characteristics as functional dyes.11-14 Zinc and free base porphyrin derivatives were used as dyes because of their well-known properties and usefulness as sensitizers.11,12 The adsorption of porphyrins deposited on clay layers15-32 and the bathochromic shifts of the porphyrin Soret band have been reported17,18 in the (11) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 1999. (12) Takagi, S.; Inoue, H. In Multimetallic and Macromolecular Inorganic Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 1999; Vol. 6, p 215. (13) Takagi, S.; Morimoto, H.; Shiragami, T.; Inoue, H. Res. Chem. Intermed. 2000, 23, 171. (14) Takagi, S.; Suzuki, M.; Shiragami, T.; Inoue, H. J. Am. Chem. Soc. 1997, 119, 8712. (15) Ukrainczyk, L.; Chibwe, M.; Pinnavaia, T. J.; Boyd, S. A. Environ. Sci. Technol. 1995, 29, 439. (16) Chibwe, M.; Ukrainczyk, L.; Boyd, S. A.; Pinnavaia, T. J. J. Mol. Catal., A 1996, 113, 249. (17) Chernia, Z.; Gill, D. Langmuir 1999, 15, 1625. (18) Kuykendall, V. G.; Thomas, J. K. Langmuir 1990, 6, 1350.

10.1021/la011524v CCC: $22.00 © 2002 American Chemical Society Published on Web 02/12/2002

2266

Langmuir, Vol. 18, No. 6, 2002

Takagi et al.

Figure 1. Structures of (left) Sumecton SA and (right) cationic porphyrins examined in the present work.

study of clay-porphyrin complexes. However, the dependence of the adsorption behavior on porphyrin concentration, and particularly the control of aggregation phenomena of porphyrin molecules on clay sheets, has received little attention. In the present study, we have examined the concentration dependence in detail and have identified conditions under which aggregation is negligible. To this end, we have carefully compared the distances between the charged sites on the clay sheets and between those within the porphyrin molecules. Thus, we were able to ensure that the ratios were very close to unity.33 In addition, we have employed relatively low concentrations of clay in water to suppress interactions between clay sheets and thus to control the stacking process. Under these conditions, the adsorption properties of porphyrins on clay surfaces and the photochemical properties of the complexes were studied. Experimental Section Chemicals. Sumecton SA was received from Kunimine Industries Co., Ltd. and was used without further purification. Water was deionized just before use by use of an ORGANO BB5A system (PF filter × 2 + G-10 column). The conductivity of the water used was below 0.02 µS cm-1. Tetrakis(1-methyl-pyridinium-4-yl) porphyrin (H2TMPyP 1) and tetrakis(N, N, N-trimethyl-anilinium-4-yl) porphyrin (H2TMAP 3) were used as the +4-charged porphyrins and were purchased from Aldrich; the counterion (tosylate anion) was exchanged with chloride by use of an ion exchange column (ORGANO AMBERLITE IRA400JCL). The corresponding Zn complexes (ZnTMPyP 2 and ZnTMAP 4) were synthesized according to a method reported in the literature.34 Analysis. Absorption spectra were measured with a Shimadzu UV -2400 spectrophotometer. For samples which had absorbances larger than 2.0, a cell with a short path length (5 mm or 3 mm) was used. Fluorescence spectra were measured with a Hitachi F-4010 instrument. In steady-state absorption and fluorescence measurements, a poly(methyl methacrylate) cell (19) Dias, P. M.; De Faria, D. L. A.; Constantino, V. R. L. J. Inclusion Phenom. Macrocyclic Chem. 2000, 38, 251. (20) Onaka, M.; Shinoda, T.; Aichi, K.; Suzuki, K.; Izumi, Y. Mol. Cryst. Liq. Cryst. 1996, 277, 149. (21) Carrado, K. A.; Wasserman, S. R. Chem. Mater. 1996, 8, 219. (22) Bedioui, F. Coord. Chem. Rev. 1995, 144, 69. (23) Carrado, K. A.; Forman, J. E.; Botto, R. E.; Winans, R. E. Chem. Mater. 1993, 5, 472. (24) Carrado, K. A.; Tjiyagarajan, P.; Winans, R. E.; Botto, R. E. Inorg. Chem. 1991, 30, 794. (25) Carrado, K. A.; Winans, R. E. Chem. Mater. 1990, 2, 328. (26) Giannelis, E. P. Chem. Mater. 1990, 2, 627. (27) Kamayama, H.; Suzuki, H.; Amano, A. Chem. Lett. 1988, 1117. (28) Ukrainczyk, L.; Chibwe, M.; Pinnavaia, T. J.; Boyd, S. A. J. Phys. Chem. 1994, 98, 2668. (29) Bonnet, S.; Forano, C.; De Roy, A.; Besse, J. P. Chem. Mater. 1996, 8, 1965. (30) Perez-Bernal, M. E.; Ruano-Casero, R.; Pinnavaia, T. J. Catal. Lett. 1991, 11, 55. (31) Tagaya, H.; Ogata, A.; Kuwahara, T.; Ogata, S.; Karasu, M.; Kadokawa, J.; Chiba, K. Microporous Mater. 1996, 7, 151. (32) Park, I. Y.; Kuroda, K.; Kato, C. Chem. Lett. 1989, 2057. (33) Preliminary results were reported in Takagi, S.; Shimada, T.; Yui, T.; Inoue, H. Chem. Lett., 2001, 128. (34) Maier, J. P.; Thommen, F. J. Chem. Soc., Faraday Trans. 2 1981, 77, 845.

was used to avoid adsorption of porphyrin onto the cell wall, when only an aqueous solution of porphyrin was measured. XRD analysis was carried out with a MPX-18 (MAC Science Co., Ltd.) The fluorescence decay was measured using a picosecond fluorescence lifetime measurement system under photon-counting conditions (Hamamatsu, C4334 streak scope, connected with CHROMEX 250IS polychromator) with an EKSPLA PV-401 optical parametric generator (424-480 nm, 25-ps fwhm, >100 µJ, 5 Hz) pumped by the third harmonic radiation of a Nd3+-YAG laser, EKSPLA PL2143b (355 nm, 25-ps fwhm, 15 mJ). The laser flux was reduced with neutral density filters to avoid multiphoton absorption processes and nonlinear effects. Sample Preparation. Clay-porphyrin complexes were prepared as follows. Type a Samples. Aqueous porphyrin solutions (1-4) were prepared at appropriate concentrations as reference samples for the clay-porphyrin complex. Type b Complexes. Type b samples were prepared by mixing aqueous SSA solution and the respective aqueous porphyrin solutions. The SSA concentration was 500 mg L-1 unless otherwise noted. The concentration of anionic sites in solution was calculated to be 5.0 × 10-4 mol L-1 from the cation exchange capacity. Type c Complexes. Type c samples were prepared by repeating three times a freeze (liquid N2)-thaw cycle with type b complexes. Type c samples were found to have properties different from those of type b, as described later. Type d Complexes. Type d samples were prepared by filtration of aqueous solutions of type b complexes with a PTFE (poly(tetrafluoroethylene) resin) membrane filter (0.5-µm or 0.1µm pore size).

Results and Discussion Estimation of Intercharge Distances for Clay and Porphyrins. The characteristics of Sumecton SA are known in detail. The stoichiometric formula is [(Si7.20Al0.80)(Mg5.97Al0.03)O20 (OH)4]-0.77‚(Na0.49Mg0.14)+0.77, the surface area is 750 m2 g-1, and the cationic exchange capacity (CEC) is 99.7 meq/100 g. On the basis of these values, the average area per anionic site is calculated to be 1.25 nm2, and thus the average distance between anionic sites on the clay surface is estimated to be 1.12 nm, on the basis of the assumption of a square array. On the basis of PM3 calculations, the free base porphyrins H2TMPyP (1) and H2TMAP (3) and their corresponding zinc complexes ZnTMPyP (2) and ZnTMAP (4) (see Figure 1) were estimated to have the following distances between adjacent cationic sites: 1.09 nm for 1 and 2 and 1.35 nm for 3 and 4. Absorption Spectra. The aqueous SSA solutions were essentially transparent in the UV-visible region under the present experimental conditions. The UV-visible spectra of the porphyrin samples of type a (1a, 2a, 3a, and 4a), type b (1b, 2b, 3b, and 4b), and type c (1c, 2c, 3c, and 4c) are shown in Figure 2. The absorption spectra for each type of sample, that is, types a, b, and c, for a given porphyrin exhibited apparent differences from each other for all four porphyrins examined (1-4). In general, type b complexes showed spectral shifts of the Soret band to longer wavelength

High-Density Adsorption of Cationic Porphyrins

Langmuir, Vol. 18, No. 6, 2002 2267

Figure 2. Absorption spectra of cationic porphyrins with clay and without clay in the Soret band region (top: 1a, 1b, 1c, 2a, 2b, and 2c; bottom: 3a, 3b, 3c, 4a, 4b, and 4c). [SSA] ) 500 mg L-1, [porphyrin] ) 1.0 × 10-6 M (0.8% vs CEC). Table 1. λmax of the Soret Bands for the Four Porphyrins under Various Conditions λmax/nm porphyrin

a

b

c

1 2 3 4

420.5 433.5 411.5 420.0

451.5 462.5 424.0 428.5

484.5 486.0 435.0 432.5

compared to type a, and those for type c complexes were found at longer wavelengths compared to those of type b. Fluorescence spectra for each of the complexes showed similar bathochromic shifts. Absorption λmax values of the porphyrin Soret bands for each sample are summarized in Table 1. Thomas et al.18 have reported similar behavior in terms of absorption spectra for Laponite-TMPyP complexes. According to their report,18 and confirmed by our experimental results described below, type b complexes can be assigned to a species in which the porphyrin molecules are adsorbed on both surfaces of the clay layers, which are well dispersed in solution, and those of type c can be assigned to one in which these molecules are intercalated into the interlayer space of clay. Many explanations for the bathochromic shift of the porphyrin Soret band have been proposed in the past.17,18 For example, either protonation of the porphyrin ring nitrogens or an aggregation of porphyrin molecules on the clay surface could induce a bathochromic shift of the porphyrin Soret band. Thomas et al.18 interpreted the spectral change associated with TMPyP as a flattening of the porphyrin molecule on the clay sheet, that is, the four cationic methylpyridinium moieties become parallel to the porphyrin ring. Recently, Chernia et al.17 also reported on the basis of PM3 calculations that the bathochromic shifts of the porphyrin Soret band depend on the degree of flattening. This flattening involves an approach to coplanarity of the mesosubstituents, for example, pyridinium moieties, with respect to the porphyrin ring, which leads to enhanced π-conjugation and electron withdrawing effect. Our results were fully consistent with this interpretation.

The fact that not only free base porphyrins but also zinc porphyrins showed spectral changes due to interactions with clay indicates that protonation of ring nitrogens could not be the reason for the bathochromic shift. Furthermore, we carried out a direct experiment to check the effect of protonation. Addition of HCl (0.1 M) to free base samples of types b and c induced bathochromic shifts of the porphyrin Soret band (3b: 424 nm f 434 nm, 3c: 435 nm f 435.5 nm), accompanied by increases in the molar absorptivity, which can be interpreted as protonation of the ring nitrogens. The spectral change caused by addition of H+ was reversibly recovered by addition of aqueous KOH. These observations clearly indicate that the original spectral change of the porphyrin adsorbed on clay is not caused by protonation of porphyrin ring nitrogens. As described later, variation of the porphyrin concentration over a wide range (0.8∼96% vs CEC) did not affect the shape of the absorption spectra. Moreover, the excited singlet lifetimes of types b and c complexes did not diminish significantly compared to those of type a as described later. These observations also indicate that porphyrin aggregation could not be the reason for the bathochromic shifts observed for types b and c. As can be seen in Table 1, the magnitudes of the spectral shifts of the Soret band for TMAP (3 and 4) were smaller than those for TMPyP (1 and 2). Since the moieties surrounding the cationic site in TMAP are bulkier than those in TMPyP, it is more difficult for the TMAP molecule to lie flat. This effect of bulkiness of the porphyrin on the degree of spectral shift is consistent with the interpretation that the bathochromic shifts of the porphyrin Soret band depend on the degree of flattening, as proposed by Chernia et al.17 XRD Analysis. Estimation of the interlayer distances for type d complexes was carried out by means of X-ray diffraction (XRD) measurements. Since no porphyrin was detected in the filtrate during the preparation procedures, all of the porphyrin molecules were assumed to be adsorbed on the clay surface. Since the reflectance spectra for the type d samples were similar to those for type c, the structures should resemble each other. The interlayer distances obtained for the type d samples at porphyrin loading level (48% vs CEC) were increased compared to that for the unmodified clay. (1d (0.42 nm), 2d (0.47 nm), 3d (0.51 nm), and 4d (0.54 nm) vs unmodified SSA(0.40 nm)) Detection of a diffraction peak in the XRD measurements (not shown) confirms the layer structure of type d complexes. Increases of the interlayer distances indicate an intercalation of porphyrin molecules into the clay interlayer space. In TMAP (3 and 4), which has larger substituents than TMPyP (1 and 2), larger increases of the interlayer distances were observed. The orientation of the porphyrin molecules in the clay layer can be estimated by XRD analysis to some extent. The interlayer distances shown indicate that the orientations of the porphyrin molecules should be almost parallel to the clay layer. If the porphyrin molecules take on an orientation vertical to the clay layer, the interlayer distance should be around 1.8 nm. Type c samples, which exhibit absorption spectra similar to those of type d samples, should have analogous structures. More detailed estimation of the structures of the clay-porphyrin complexes is described later. The Size-Matching Effect and the Effect of Porphyrin Concentration on the Absorption Spectra. Porphyrin concentration effects on absorption spectra were examined to clarify the structures and configurations for type b. If the clay sheets are stacked, the area of external surface available for the formation of the complex is limited, and the absorption spectra associated with the

2268


Figure 3. Absorption spectra of SSA-porphyrin complex 1b at various porphyrin concentrations up to 140% vs CEC.

Figure 4. Absorption spectra of SSA-porphyrin complex 3b at various porphyrin concentrations up to 128% vs CEC.

Figure 5. Absorbance-concentration plots for porphyrin 1 type b complex at Soret (451.5 nm) and Q-band (542 nm) in aqueous solution.

porphyrin should change at some saturation loading level. After saturation of type b complex formation, either types a or c may appear. Absorption spectra of complexes 1b and 3b at various porphyrin loading levels are shown in Figures 3 and 4. In the present experiment, the definition of porphyrin loading level (solution concentration) versus CEC is based on the number of charges on the porphyrin. Because these porphyrins (1-4) have four cationic sites per molecule, 100% versus CEC means that the number of porphyrin molecules is 25% versus the number of anionic sites on SSA, and thus the porphyrins neutralize all of the negative charges on the clay surface. It is surprising that the shapes of the absorption spectra were consistently the same at every porphyrin loading level below 100% versus CEC, as shown in Figures 3 and 4. However, the spectral shape changed at loading levels above 100% versus CEC. Judging

Takagi et al.

Figure 6. Absorbance-concentration plots for porphyrin 3 type b complex at Soret (424 nm) and Q-band (524 nm) in aqueous solution.

from the difference spectra (1b (140%)-1b (100%) and 3b (128%)-3b (96%)), the spectral change can be assigned to species a for both 1 and 3, that is, the free molecule in aqueous solution for concentrations in excess of 100% versus CEC. Absorbance versus concentration plots obtained for the wavelengths of the Soret and Q-bands for complexes 1b and 3b are shown in Figures 5 and 6, respectively. The linearities of these plots, that is, the high degree of adherence to Beer’s Law, for both Soret and Q-bands were retained to a remarkable extent for every porphyrin loading level below 100% versus CEC. From these plots, the molar absorptivities for the respective Soret and Q-bands were obtained: 1.6 × 105 mol dm-3 cm-1 at 451.5 nm and 1.3 × 104 mol dm-3 cm-1 at 542.0 nm for complex 1b; and 2.0 × 105 mol dm-3 cm-1 at 424.0 nm and 8.7 × 103 mol dm-3 cm-1 at 524.0 nm for complex 3b. It was thus concluded that aggregation is completely inhibited and 3 exists as a monolayer, in which the porphyrin is adsorbed in a flat orientation, for every loading level in the SSA-porphyrin complex. Because these results indicate that all of the porphyrin molecules can adsorb on the clay surface even at a nearly 100% loading level, the SSA sheets must exist as single, well-dispersed sheets or loosely associated layers with very large interlayer distances in water. Therefore, the porphyrin molecules appear to be able to adsorb on the clay layers at high density, neutralizing all of the negative charges on the clay surface, with negligible aggregation. In general, the aggregation of dye molecules, which is induced by π-π and hydrophobic interactions, tends to be accelerated on clay surfaces3-5 and it has thus far been difficult to find ways to suppress this aggregation. The essentially complete suppression of aggregation of porphyrin molecules on a clay surface even at very high porphyrin loading levels (∼100% vs CEC) is unique. These findings constitute the first example of monolayer adsorption on clay without aggregation. On the basis of the results described thus far, we propose the following reasons for the extraordinary monolayertype adsorption of porphyrins on clay. First, there appears to be excellent matching between the intercharge distances on the porphyrins and on the clay sheets. As already indicated, the distances between cationic sites in the porphyrin molecules were estimated to be 1.09 nm for TMPyP and 1.35 nm for TMAP, compared to the average distance between anionic sites on the clay surface, which was calculated to be 1.12 nm. We believe that this matching of distances is crucial in achieving the observed highdensity, flat monolayer adsorption (Figure 7). Further measurements are in progress in our laboratory that are


Langmuir, Vol. 18, No. 6, 2002 2269

Figure 7. Schematic view of SSA-porphyrin type b complexes. Left: side view; right: top view.

designed to test the present hypothesis, with a variety of additional types of clays and dyes. These results will be reported in due course. Second, the possibility that the adsorption involves up to four relatively strong electrostatic interactions per porphyrin molecule should also be quite important. Two possible arrangements of porphyrin molecules on the clay surface are proposed (Figure 7). Since arrangement a yields larger gap distance (0.71 nm) between porphyrins than does arrangement b (0.64 nm), a may be preferred. The precise arrangement of the porphyrins is currently being examined by means of AFM. Monolayer adsorption of porphyrins and phthalocyanines on TiO2,35 SiO2,36 and metal surfaces37-42 has been reported. Control of the packing of the molecules is difficult, the distances being determined typically by the van der Waals size of the molecule in a given orientation. Thus, the center-to-center distances reported in the literature are relatively short (ca. 1.2-1.8 nm)35-42 and significant electronic interactions are unavoidable. Here, we succeeded in controlling the gap distance between porphyrins by means of the size-matching effect on the clay. Porphyrin concentration effects on the absorption spectra of type c complexes were also examined. Absorption spectra for complex 1c and 3c at various porphyrin loading levels are shown in Figures 8 and 9. In contrast to results for complex b, the absorption spectra changed at low porphyrin loading levels (∼8% vs CEC). This result indicates that both monomer and aggregated porphyrin coexist above 8% loading versus CEC. Absorbanceconcentration plots for complex 1c and 3c are shown in Figures 10 and 11, respectively. The adherence to Beer’s Law for the Soret band was retained only at low porphyrin loading levels, that is, below ca. 8% versus CEC (1.0 × 10-5 M). In the type c complexes, in which the clay sheets are stacked, the possible interaction of porphyrin molecules that are adsorbed on adjacent clay sheets could induce the spectral changes. From the absorbance-concentration plots (Figures 10 and 11), the molar absorptivities for the Soret bands were found (35) Cherian, S.; Wamser, C. C. J. Phys. Chem. B 2000, 104, 3624. (36) Bos, M. A.; Werkhoven, T. M.; Kleijn, J. M. Langmuir 1996, 12, 3980. (37) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197. (38) Gimzewski, J. K.; Joachim, C. Science 1999. 283, 1683. (39) Hipps, K. W.; Lu, X.; Wang, X. D. Mazur, U. J. Phys. Chem. 1996, 100, 11207. (40) Barlow, D. E.; Hipps, K. W. J. Phys. Chem. B 2000, 104, 5993. (41) Hipps, K. W.; Barloe, D. E.; Mazur, U. J. Phys. Chem. B 2000, 104, 2444. (42) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607.

Figure 8. Absorption spectra of SSA-porphyrin complex 1c at various porphyrin concentrations up to 100% vs CEC. For samples that had absorbances larger than 2.0, a short path length cell (5 mm or 3 mm) was used and the appropriate corrections were applied (× 2.0 for the 5-mm cell; × 3.3 for the 3-mm cell).

Figure 9. Absorption spectra of SSA-porphyrin complex 3c at various porphyrin concentrations up to 49% vs CEC.

to be 1.3 × 105 mol dm-3 cm-1 at 484.0 nm for complex 1c and 1.6 × 105 mol dm-3 cm-1 at 435.0 nm for complex 3c. On the basis of the absorption spectra and XRD profiles, the structures for types b and c complexes are proposed in Figure 12. In the type b complexes, the clay sheets appear to be dispersed at least to the extent at which the porphyrin molecules on different clay sheets do not interact significantly with each other. Therefore, SSA in aqueous solution must exist as dispersed single sheets or very loosely associated layers with large interlayer distances. Taking

2270


Figure 10. Absorbance-concentration plots of SSA-porphyrin complex 1c at Soret band (484.5 nm) at various porphyrin concentrations. For samples with absorbances larger than 2.0, a short path length cell (5 mm) was used and the appropriate correction factor applied (× 2.0).

Figure 11. Absorbance-concentration plots of SSA-porphyrin complex 3c at Soret band (435 nm) at porphyrin concentrations in the range from 0 to 2 × 10-5M.

Figure 12. Possible structures of SSA-cationic porphyrin type b and c complexes.

into consideration the reported XRD measurements43,44 and attractive Coulombic interactions between the clay sheets, a loosely associated layer structure seems reasonable for type b complexes. Although detailed prediction of the orientations of the porphyrin molecules on the clay sheet is difficult, a certain minimum degree of ordering is expected. Otherwise, the porphyrin molecules would tend to aggregate on the clay sheet at high porphyrin loading levels. In the type c complexes, larger bathochromic shifts of the Soret band (compared to a) than those for type b could be caused by a stronger flattening effect on the porphyrin molecules. Significant flattening of the porphyrin molecules could be induced if the clay layers are stacked rigidly. (43) Norrish, K. Discuss. Faraday Soc. 1954, 18, 120. (44) Fukushima, Y. Clays Clay Miner. 1984, 32, 320.

Takagi et al.

Figure 13. Time course of the conversion from type b to c complexes for porphyrins 1, 2, 3, and 4. [SSA] ) 500 mg L-1 and the porphyrin loading level was 6.7% vs CEC.

The Equilibrium between b and c Forms of the Complexes. The approaches to equilibrium between b and c forms of the complexes were examined by following the time dependences of the absorption spectra. Although the type b complexes were relatively stable, the absorption spectra change gradually at 296 ( 1 K in water. Judging from the spectra, the spectral change can readily be identified as the conversion from b to c. Since the conversion from b to c proceeded spontaneously, either slowly with the passage of time or more rapidly with repeated freeze-thaw cycles, the b form can be considered to be a metastable state and the c form can be considered to be more stable under the present experimental conditions. The time dependences of the conversion from b to c were observed for each of the four porphyrins at 296 ( 1 K. The conversion percentages were determined by calculation from the absorbance changes at λmax of the c forms for 1, 2, and 3 and those at λmax of complex 4b, as indicated in the vertical axes of Figure 13. The conversion rates for TMPyP (1 and 2) were apparently faster than those for TMAP (3 and 4), as seen in Figure 13. The bulkiness of the porphyrin meso-groups affects the conversion between the b and c forms significantly. Because the adsorbed porphyrin molecules become sandwiched between the clay layers, that is, intercalated, in the c form, the effect of the degree of flatness of the porphyrin molecule on the conversion is reasonable. More detailed measurements were carried out for porphyrin 4. Specifically, the effects of the SSA and porphyrin concentrations on the conversion percentages are shown in Figures 14 and 15. When the SSA concentration was maintained at 167 mg L-1, lower porphyrin concentrations yielded a higher degree of conversion from b to c. Thus, a higher-density layer of adsorbed porphyrin on the clay surface is seen to suppress the stacking of the clay layers. In addition, we have examined the effect of higher porphyrin loading levels on the conversion of b to c. Judging from the results given in Figure 14, the conversion of b to c would be expected to be very slow for high porphyrin loading levels. This is understandable because the bulkiness of porphyrin molecule would suppress the change of b to c. This is due to neutralization of charges on clay sheet by cationic porphyrin. When the concentration of 4 was fixed at 0.38% versus CEC, lower SSA concentrations exhibited higher degrees of conversion from b to c (Figure 15). These observations indicate the possibility of controlling the equilibrium between b and c by changing the experimental conditions. The time dependence of the formation of different types of ag-


Langmuir, Vol. 18, No. 6, 2002 2271

Figure 16. Fluorescence decay curves of type b complexes for each of the four porphyrins excited at the λmax values of the respective Soret bands. Figure 14. Time course of the conversion from 4b to 4c at various porphyrin concentrations ([4] ) 0.38, 1.15, and 2.30% vs CEC, [SSA] ) 167 mg L-1).

Figure 17. Fluorescence decay curves of type c complexes for each of the four porphyrins excited at the λmax values of the respective Soret bands.

Figure 15. Time course of the conversion from 4b to 4c at various SSA concentrations ([SSA] ) 167, 500, and 1000 mg L-1, [4] ) 0.38% vs CEC).

gregated species (e.g., single molecule a bimolecular adduct a higher aggregates) during the adsorption process has been reported in previous studies on clay-dye complexes.45,46 In the present work, no such change in the type of adsorbed species was observed other than the conversion from b to c. Another factor that controls the equilibrium is the nature of the solvent, for example, the presence of a nonaqueous solvent component such as acetonitrile. When the SSA concentration was 500 mg L-1, the equilibrium percentage of the b form of the complex after repeated freeze-thaw cycles was found to be significantly affected by the acetonitrile content. Even though the equilibrium percentage of b was below the detection limit for solvent compositions MeCN/H2O < 1/5, the percentage increased as the MeCN content increased. When the composition was 67% MeCN, the b form was found to be ca. 80%. Moreover, the addition of KCl to the solution was found to accelerate the conversion from b to c. Thus, it is clearly possible to control the equilibrium by changing conditions such as the solvent composition and the ionic strength. These findings are also interesting from the viewpoint of the characterization of the clay structure itself. The equilibrium between stacked and nonstacked forms of the clay sheets should exist under a variety of conditions, regardless of the existence of porphyrin. Our results suggest that cationic porphyrins may be an excellent (45) Neumann, M. G.; Schmitt, C. C.; Gessner, F. J. Colloid Interface Sci. 1996, 177, 495. (46) Jacobs, K. Y.; Schoonheydt, R. A. J. Colloid Interface Sci. 1999, 220, 103.

probe18 with which to characterize the properties of clay minerals, as well as perhaps other layered materials. The Excited Singlet Lifetime of Porphyrins with Clay. Even though the structures of clay-porphyrin complexes have been studied in a fair amount of detail, the photochemical and photophysical properties of the porphyrin in the complex have thus far not been subjected to study. We measured the excited singlet lifetime of porphyrin in the complex as a basic photophysical property. The excited singlet lifetime of a dye adsorbed on clay should be expected to be very short because of aggregation, but the results showed this not to be true. To obtain the singlet lifetimes of the type b and c complexes (SSA (500 mg L-1), porphyrin (8.4 × 10-6 M (6.7% vs CEC))), the fluorescence decay curves were measured, as shown in Figures 16 and 17, respectively. Although the decay behavior for some of the complexes included a short lifetime component,47 most of the curves could be analyzed with a single-exponential fitting. Estimated lifetimes for each of the complexes are presented in Table 2. For all four of the porphyrins examined, the singlet lifetimes decreased in the order a > b > c. Free base porphyrins exhibited longer singlet lifetimes than the zinc counterparts. The lifetimes for the type b and c complexes are sufficient to undergo photochemical reactions. As far as we are aware, this is the first example of the measurement of an excited singlet lifetime for a clay-porphyrin complex. SSA-porphyrin complexes should be quite promising for fundamental research on photochemical reactions such as energy transfer and electron transfer in a well-defined environment because of both their relatively long excited-state lifetimes and their optical transparency. The study of such photochemical reactions of SSAporphyrin complexes is now in progress. (47) In the lifetime measurement experiment, a rather high loading level of porphyrin was used to obtain sufficient emission from the excited porphyrin. Therefore, complex c in particular may include a small amount of aggregated porphyrin, which would account for the somewhat short lifetime.

2272


Takagi et al.

Table 2. Flurorescence Lifetimes for the Four Porphyrins in Aqueous Solution without Clay and in Clay-Porphyrin Complexes porphyrin

conditions

fluorescence lifetime (ns)

H2TMPyP

aqueous solution without clay (1a) externally adsorbed on clay (1b) intercalated in clay (1c)

5.1 3.8, 0.7 3.2, 0.2

ZnTMPyP


1.3 0.6, 0.2 0.5

H2TMAP


9.3 4.1 3.2

ZnTMAP


1.8 0.7 0.4, 0.1

Conclusions Two types of complexes formed in solution between the synthetic anionic clay SSA and cationic porphyrins were examined. The first is one in which the SSA exists as single, dispersed sheets or loosely associated layers and in which the porphyrin adsorbs on the clay surface (type b complexes); the second is one in which the SSA layers are stacked, and the porphyrin molecules are intercalated in the interlayer space (type c complexes). For the first time, it was found that the porphyrins adsorb as highdensity monolayers, apparently in a flat orientation, on the clay layer surface, with negligible aggregation in the type b complexes. On the basis of fluorescence lifetime measurements, clearly both types of complexes have singlet lifetimes sufficiently long to undergo photochemical reactions.

The intercharge distances for the porphyrin and clay were quite well matched in the present work. We believe that this matching of distances (“size-matching effect”) is a key factor in making the observed monolayer adsorption possible. By adjusting the relationship between the charge density of the clay, which controls the intercharge distance and the intercharge distance for the guest molecule, it should be possible to favor this type of complex. Up to the present, there have been only limited applications of claydye complexes to photochemical reactions because the dye molecules tend to aggregate on the clay surface. Our present findings may expand the possibilities for the preparation and utilization of clay-dye complexes. The application of high-density monolayer-type clay-porphyrin complexes in artificial light-harvesting systems could be promising. LA011524V

High-Density Adsorption of Cationic Porphyrins on Clay Layer

Recommend Documents