Humidity-Dependent Reversible Aggregation of Rhodamine 6G Dye

Jul 27, 2004 - The spectroscopic behavior of rhodamine 6G (R6G) dye intercalated in layered hexaniobate K4Nb6O17 was investigated. R6G cations were ...
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Langmuir 2004, 20, 7583-7588

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Humidity-Dependent Reversible Aggregation of Rhodamine 6G Dye Immobilized within Layered Niobate K4Nb6O17 Ryota Shinozaki† and Teruyuki Nakato*,‡,§ Laboratory of Environmentally Benign Materials Chemistry, Faculty of Agriculture, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu-shi, Tokyo 183-8509, Japan, Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan, and PRESTO, Japan Science and Technology Corporation, 4-1-8 Hon-cho, Kawaguchi-shi, Saitama 332-0012, Japan Received March 12, 2004. In Final Form: June 9, 2004 The spectroscopic behavior of rhodamine 6G (R6G) dye intercalated in layered hexaniobate K4Nb6O17 was investigated. R6G cations were intercalated into the niobate through displacement of preintercalated alkylammonium ions. Powder X-ray diffraction and elemental analysis indicated that the dye molecules were densely accommodated in the interlayer spaces of niobate. The spectroscopic behavior of intercalated R6G was characterized by humidity-dependent aggregation at room temperature. The dye molecules were present dominantly as monomers under humid conditions (93% relative humidity (RH)), while they formed dimers under relatively dry conditions (20% RH). The aggregation-deaggregation of dye occurred reversibly depending on the humidity. The reversible aggregation was not accompanied by a large alteration of the interlayer structure of the sample, because only a small amount of water was adsorbed/desorbed with a small change in the basal spacing of the intercalation compound during the humidity change.

Introduction Intercalation of photofunctional molecules into various layered inorganic materials has attracted considerable attention from the viewpoints of both constructing novel integrated photofunctional systems and probing the microenvironments of two-dimensional nanoarrays provided by the inorganic lattices.1 The photoactive molecules may exhibit unusual photophysical and photochemical behavior in the interlayer spaces based on interactions between the intercalated guest and the inorganic host, or the guest molecules themselves. In addition, the confined molecules show peculiar spectroscopic behavior that reflects the interlayer microenvironments. Many dye molecules have been investigated as such functional moieties and/or spectroscopic probes within various inorganic matrixes. Among inorganic layered materials, layered niobates and titanates are distinguished by their photoactive characters exemplified by photocatalytic activities2 from other inorganic matrixes such as clay minerals and layered double hydroxides. Thus, intercalation compounds of niobates and titanates may exhibit peculiar photofunctions. In fact, one of the authors has clarified that intercalation compounds of niobates and titanates with * To whom correspondence should be addressed. Phone/fax: +8142-388-7344. E-mail: [email protected]. † Laboratory of Environmentally Benign Materials Chemistry, Tokyo University of Agriculture and Technology. ‡ Graduate School of Bio-Applications and Systems Engineering (BASE), Tokyo University of Agriculture and Technology. § PRESTO, Japan Science and Technology Corporation. (1) (a) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399. (b) Shichi, T.; Takagi, K. J. Photochem. Photobiol., C 2000, 1, 113. (2) (a) Domen, K.; Kudo, A.; Shibata, M.; Tanaka, A.; Maruya, K.; Onishi, T. J. Chem. Soc., Chem. Commun. 1986, 1706. (b) Kudo, A.; Tanaka, A.; Domen, K.; Maruya, K.; Aika, K.; Onishi, T. J. Catal. 1988, 111, 67. (c) Kudo, A.; Sayama, K.; Tanaka, A.; Asakura, K.; Domen, K.; Maruya, K.; Onishi, T. J. Catal. 1989, 120, 337.

viologen undergo photoinduced host-guest electron transfer3 and that a tris-bipyridyl ruthenium complex shows peculiar spectroscopic behavior within the layered oxides.4 Along this line, photofunctions of the niobates and titanates intercalated with thiazine,5 cyanine,6,7 porphyrin,8 and phthalocyanine9 dyes have been investigated. These studies have revealed that the intercalation compounds of layered niobates and titanates with photofunctional dyes can exhibit unusual photochemical and spectroscopic behavior. However, variation of the functional guest molecules is rather limited compared with dye-clay systems, and fine-tuning of the interlayer microstructures is not well developed. As an example of the precise control of interlayer microstructure, we report here the aggregation behavior of rhodamine 6G (R6G+, Chart 1a), a cationic rhodamine dye, intercalated in the interlayer space of hexaniobate K4Nb6O17 (Chart 1b)10 in which the aggregation state of the dye is reversibly controlled by the atmospheric humidity. Rhodamine dyes have been utilized as typical (3) Nakato, T.; Kuroda, K.; Kato, C. Chem. Mater. 1992, 4, 128. (4) Nakato, T.; Kusunoki, K.; Yoshizawa, K.; Kuroda, K.; Kaneko, M. J. Phys. Chem. 1995, 99, 17896. (5) (a) Nakato, T.; Iwata, Y.; Kuroda, K.; Kato, C. J. Inclusion Phenom. 1992, 13, 249. (b) Kaito, R.; Kuroda, K.; Ogawa, M. J. Phys. Chem. B 2003, 107, 4043. (6) (a) Miyamoto, N.; Kuroda, K.; Ogawa, M. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2000, 341, 259. (b) Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Mater. Chem. 2004, 14, 165. (7) Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Phys. Chem. B 2004, 108, 4268. (8) (a) Nakato, T.; Iwata, Y.; Kuroda, K.; Kaneko, M.; Kato, C. J. Chem. Soc., Dalton Trans. 1993, 1405. (b) Bizeto, M. A.; De Faria, D. L. A.; Constantino, V. R. L. J. Mater. Sci. Lett. 1999, 18, 643. (c) Yamaguchi, Y.; Yui, T.; Takagi, S.; Shimada, T.; Inoue, H. Chem. Lett. 2001, 644. (d) Tong, Z.; Shichi, T.; Oshika, K.; Takagi, K. Chem. Lett. 2002, 876. (9) Kaito, R.; Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Mater. Chem. 2002, 12, 3463. (10) (a) Gasperin, M.; Le Bihan, M. T. J. Solid State Chem. 1980, 33, 89. (b) Gasperin, M.; Le Bihan, M. T. J. Solid State Chem. 1982, 43, 346.

10.1021/la049354k CCC: $27.50 © 2004 American Chemical Society Published on Web 07/27/2004

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Chart 1. Schematic Structures of (a) Rhodamine 6G and (b) K4Nb6O17a

Shinozaki and Nakato

with R6G+ because of the aggregation of incorporated dye molecules.14-21 Lopez Arbeloa et al. have reported that R6G+ molecules form aggregates on clay layers and that the type of aggregates depends on the charge distribution and stacking regularity of the clay layers.16-18 Sasai et al. have also observed the aggregation of R6G+ molecules within oriented clay films; the dye cations tend to form aggregates in the clay minerals that have a high layer charge density.19-21 Hence, we can obtain invaluable information for understanding the microenvironments of the interlayer spaces of niobate from the aggregation behavior of the intercalated rhodamine dye. We show in the present paper that the anionic niobate lattice, characterized by a high charge density, densely confines the cationic R6G+ molecules whose aggregation state varies with cointercalation of only a small amount of water molecules. Reversible aggregation upon an external stimulus such as humidity has not been investigated systematically for dyes intercalated in inorganic layered compounds; an exception is azobenzene incorporated in clay minerals, the dye which has been assumed to change its aggregation state upon photoisomerization.25 Experimental Section

a K4Nb6O17 has two structurally different types of alternate interlayer spaces (interlayers I and II), and K+ ions of interlayer I are located in “more open” cavities than those of interlayer II to exhibit high ion-exchangeability.

spectroscopic probes of various heterogeneous media.11-21 There are a few investigations of reversible humiditydependent aggregation of rhodamine dyes within some organic polymer matrixes, for example, Nafion (a perfluorosulfonate cation-exchangeable polymer)22,23 and gellatin.24 On the other hand, aggregation of dye in the interlayer spaces has been reported for various inorganic layered materials because the host lattices densely accommodate the dye molecules in many cases. In fact, characteristic spectroscopic properties have been observed for various intercalation compounds of inorganic oxides (11) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (12) del Monte, F.; Mackenzie, J. D.; Levy, D. Langmuir 2000, 16, 7377. (13) (a) Kobayashi, Y.; Imai, Y.; Kurokawa, Y. J. Mater. Sci. Lett. 1988, 7, 1148. (b) Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I. J. Colloid Interface Sci. 1997, 187, 105. (14) Grauer, Z.; Avnir, D.; Yariv, S. Can. J. Chem. 1984, 62, 1889. (15) Endo, T.; Nakada, N.; Sato, T.; Shimada, M. J. Phys. Chem. Solids 1988, 49, 1423. (16) (a) Tapia Estevez, M. J.; Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I. Langmuir 1993, 9, 3629. (b) Tapia Estevez, M. J.; Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I. J. Colloid Interface Sci. 1994, 162, 412. (c) Tapia Estevez, M. J.; Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I.; Schoonheydt, R. A. Clay Miner. 1994, 29, 105. (d) Lopez Arbeloa, F.; Tapia Estevez, M. J.; Lopez Arbeloa, T.; Lopez Arbeloa, I. Langmuir 1995, 11, 3211. (17) Tapia Estevez, M. J.; Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I. J. Colloid Interface Sci. 1995, 171, 439. (18) Lopez Arbeloa, F.; Tapia Estevez, M. J.; Lopez Arbeloa, T.; Lopez Arbeloa, I. Clay Miner. 1997, 32, 97. (19) Fujita, T.; Iyi, N.; Kosugi, T.; Ando, A.; Deguchi, T.; Sota, T. Clays Clay Miner. 1997, 45, 77. (20) (a) Sasai, R.; Fujita, T.; Iyi, N.; Itoh, H.; Takagi, K. Langmuir 2002, 18, 6578. (b) Bujdak, J.; Iyi, N.; Kaneko, Y.; Czimerova, A.; Sasai, R. Phys. Chem. Chem. Phys. 2003, 5, 4680. (21) Iyi, N.; Sasai, R.; Fujita, T.; Deguchi, T.; Sota, T.; Lopez Arbeloa, F.; Kitamura, K. Appl. Clay Sci. 2002, 22, 125. (22) Zhu, C.; Bright, F. V.; Wyatt, W. A.; Hieftie, G. M. J. Electrochem. Soc. 1989, 136, 567. (23) Litwiler, K. S.; Kluczynski, P. M.; Bright, F. V. Anal. Chem. 1991, 63, 797. (24) Choi, M. M. F.; Tse, O. L. Anal. Chim. Acta 1999, 378, 127.

Materials. K4Nb6O17‚3H2O was prepared by mixing K2CO3 and Nb2O5 (Wako Pure Chemical Ltd.) in a molar ratio of 2.1:3.0 and subsequently heating it at 1100 °C for 10 h in air, according to the method in the literature,26 and was confirmed by powder X-ray diffraction (XRD). Chloride salts of propylammonium (PA+) and hexylammonium (HA+) (Tokyo Kasei Co.) were used as received. A chloride salt of R6G+ (Sigma Chemical Co.) was used after recrystallization in ethanol. Intercalation of the Rhodamine 6G Dye into K4Nb6O17. R6G+ was intercalated into the niobate through the displacement of preintercalated guest species. K4Nb6O17 was treated with PA+ or HA+ and then with R6G+. K4Nb6O17 was allowed to react with a 1 mol dm-3 aqueous solution of PA+ or HA+ at room temperature for 90 min (for PA+) or at 333 K for 3 weeks (for HA+). The atomic ratio of N (of PA+ or HA+) to K was 10:1. The product was washed with water until the washing became Cl--free and then was dried under ambient conditions. The niobate treated with PA+ or HA+ was then allowed to react with a 0.03 mol dm-3 aqueous solution of R6G+ at 353 K for 3 weeks. The R6G+ solution was adjusted to a pH of 6 with alkylamines corresponding to the ammonium species involved in the precursor. The molar ratio of R6G+/ alkylammonium was 5. The reaction was carried out in the dark in order to avoid photodegradation of R6G+. The product was washed with methanol, water, and finally acetone and then dried under ambient conditions. We designate the final products as R6G-PA-Nb6O17 and R6G-HA-Nb6O17 in order to distinguish the intermediate alkylammonium ions. Analyses. All the samples were characterized with powder X-ray diffraction (XRD, MAC Science MX Labo diffractometer with monochromatic Cu KR radiation), thermogravimetry (TG, Seiko Instruments TG/DTA 6300), and IR spectroscopy (Jasco Valor-III spectrometer with a KBr disk technique). The TG measurements were carried out under a flow of air. The compositions of the samples were determined by using wavedispersive X-ray fluorescence (XRF, Rigaku RIX-3000) and CHN analyses. Bead samples were prepared by fusing with Li2B4O7 for the XRF measurements. The spectroscopic properties of the intercalated R6G were investigated with diffuse reflectance visible and fluorescence spectroscopies. The reflectance spectra were recorded on a Jasco U-best 55 spectrophotometer equipped with an integrating sphere. To avoid saturation of absorption, the samples were diluted with R-Al2O3 powders. The fluorescence spectra were measured with a Jasco FP-6500 spectrofluorometer. Since the samples contained large amounts of R6G+, undiluted powdery (25) Ogawa, M.; Ishii, T.; Miyamoto, N.; Kuroda, K. Adv. Mater. 2001, 13, 1107. (26) Nassau, K.; Shiever, J. W.; Bernstein, J. L. J. Electrochem. Soc. 1969, 116, 348.

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Figure 1. Powder XRD patterns of (a) K4Nb6O17‚3H2O, (b) PA-Nb6O17 intercalation compound, (c) R6G-PA-Nb6O17 intercalation compound, (d) HA-Nb6O17 intercalation compound, and (e) R6G-HA-Nb6O17 intercalation compound. samples gave considerably weak and red-shifted (∼100 nm) apparent emission peaks due to reabsorption; thus, the samples were diluted with R-Al2O3 until the spectral shape and position became unchanged.27 All the diluted samples were kept in a desiccator of 93% relative humidity (RH) at 293 K (containing a saturated Na2SO4 solution) or 20% RH at 293 K (containing a saturated CH3COOK solution) for 1 week prior to the spectroscopic measurements.

Results Synthesis of the Intercalation Compounds. We successfully prepared intercalation compounds of K4Nb6O17 with R6G+ through the displacement of preintercalated alkylammonium ions, although direct reactions of the niobate with R6G+ hardly yielded intercalation compounds. Two alkylammonium ions, PA+ and HA+, gave intercalation compounds with different interlayer structures: one contained PA+ in every other interlayer space,28 and the other accommodated HA+ within all the interlayer spaces.29 This is due to the peculiar layered structure of K4Nb6O17, consisting of two types of alternate interlayer spaces with different exchangeabilities of the interlayer K+ ions.10,30 The intercalation compounds with R6G+ retained these interlayer structures. The R6G-PA-Nb6O17 sample yielded by the stepwise reaction of K4Nb6O17 with PA+ and R6G+ was an intercalation compound where R6G+ ions were accommodated in every other interlayer space. The XRD pattern of the product of K4Nb6O17 with PA+ (Figure 1b) is assigned to the PA+-KxNb6O17 intercalation compound incorporating PA+ ions into every other interlayer space.28 The gallery height was estimated as 0.54 nm by subtracting the basal spacing of anhydrous K4Nb6O17 (1.64 nm) from that of the (27) The sample/Al2O3 weight ratio for the measurement of emission spectra was around 1:1000. When the ratio increased to about 1:20, 1:3, and 1:0, the apparent emission maximum red-shifted from that observed for the enough diluted sample by about 20, 90, and 100 nm, respectively, and the apparent intensity (of the raw spectrum) decreased to 1/30, 1/60, and 1/60, respectively. The rather large spectral shift of ∼100 nm would be rationalized not only by the usual reabsorption observed in concentrated liquid samples but also by some additional optical effects characteristic of the powdery solids, such as scattering and refraction. (28) Nakato, T.; Miyamoto, N.; Harada, A.; Ushiki, H. Langmuir 2003, 19, 3157. (29) Nakato, T.; Sakamoto, D.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1992, 65, 322. (30) Kinomura, N.; Kumada, N.; Muto, F. J. Chem. Soc., Dalton Trans. 1985, 2349.

sample

composition

PA-Nb6O17 HA-Nb6O17 R6G-PA-Nb6O17 R6G-HA-Nb6O17

PA0.9H0.9K2.2Nb6O17 HA3.1H0.2K0.7Nb6O17 R6G0.4PA0.2H2.2K1.2Nb6O17 R6G0.7HA0.6H2.3K0.4Nb6O17

intercalation compound (2.18 nm).29,31 The reaction of PA+intercalated niobate with R6G+ gave the product, whose XRD pattern (Figure 1c) exhibits a gallery height of 1.58 ()3.22-1.64) nm if the organic cations are assumed to be present in every other interlayer space, as for the case of the PA+-KxNb6O17 intercalation compound. Since the size of a R6G+ molecule is estimated as 1.4 nm (by Chem3D software) in the longitudinal direction, the gallery height of the product is large enough to accommodate R6G+ molecules in a nearly perpendicular orientation within the interlayer spaces. The R6G-HA-Nb6O17 sample obtained by the successive reaction of K4Nb6O17 with HA+ and R6G+ also incorporated the dye, but the guest species was intercalated in all the interlayer spaces. XRD patterns show that the product with HA+ gives a basal spacing of 2.04 nm, corresponding to a gallery height of 1.22 nm for the fully intercalated interlayer structure (Figure 1d).29 The sample after the reaction with R6G+ gives a similar diffractogram (Figure 1e) with a somewhat increased basal spacing of 2.19 nm; the gallery height is estimated as 1.37 nm, and thus, R6G+ ions can be intercalated. Spectroscopic measurements supported the intercalation of R6G+ ions. Both the products yielded after the reaction with R6G+ exhibited strong red color due to the dye, and the visible diffuse reflectance spectra indicate characteristic absorption bands assignable to R6G+ ions, as described later in detail. Also, the IR spectra of these samples (Supporting Information Figure S1) exhibit many definite absorption bands of R6G+, for example, 1727 cm-1, CdO stretching in the carbonyl-phenyl group; 1523, 1549, and 1625 cm-1, in-plane vibration of the xanthene ring; 1331 cm-1, aryl C-N stretching.19,21 Elemental analyses indicated dense accommodation of the dye. Table 1 shows the compositions of the R6G+intercalated niobates determined by XRF and CHN analysis. Since the C/N atomic ratios of samples determined with CHN analysis were lower than the theoretical ratio of R6G+, indicating that the intermediate alkylammonium ions were not completely displaced but instead cointercalated in the intercalation compounds with R6G+, the organic contents were divided into the amounts of alkylammonium and R6G+ according to the C/N ratios. We calculated the interlayer volume occupied by a R6G+ molecule. The interlayer volume per a negative charge of the [Nb6O17]4- layers is estimated as (a × c × ∆d)/2, where a and c are lattice parameters of K4Nb6O1710 and ∆d is the gallery height of the intercalation compound: 1.58 and 1.39 nm for R6G-PA-Nb6O17 and R6G-HA-Nb6O17, respectively. The molecular volumes of PA+ and HA+ are calculated as 0.07 and 0.12 nm3, respectively, by using Chem3D software. From these values and compositions of the samples, we obtain the interlayer volumes occupied by a R6G+ molecule as 1.01 nm3 for both the intercalation (31) If we assume that PA+ ions are intercalated in every other interlayer space, the gallery height is estimated by subtracting the d020 spacing corresponding to the sum of the two interlayer spacings of reactant (1.64 nm for anhydrous K4Nb6O17) from the basal spacing of the product. In contrast, the gallery height is given by the difference between the basal spacing of the product and the d040 spacing of anhydrous K4Nb6O17 (0.82 nm) if the guest molecules are intercalated into all of the interlayer spaces of niobate.

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Figure 2. Visible diffuse reflectance spectra of (a) aqueous R6G+ solution (10-6 mol dm-3), (b) R6G-PA-Nb6O17 intercalation compound stored for 1 week in a desiccator of 93% RH after preparation, (c) sample b after standing for 1 week after being transferred to a desiccator of 20% RH, (d) sample c after standing for 1 week after being transferred to a desiccator of 93% RH, (e) R6G-HA-Nb6O17 intercalation compound stored for 1 week in a desiccator of 93% RH after preparation, (f) sample e after standing for 1 week after being transferred to a desiccator of 20% RH, and (g) sample f after standing for 1 week after being transferred to a desiccator of 93% RH.

compounds, by assuming a uniform distribution of the R6G+ molecules. These are close to the volume of a R6G+ molecule occupying the interlayer space of taeniolite clay (1.14 nm3),32 where the dye is intercalated to the maximum extent. Hence, we conclude that the amounts of R6G+ intercalated in the niobate reach an upper limit, although the molecular volume of R6G+ is calculated as 0.41 nm3 by Chem3D software; the saturated amount would be sterically determined.21 Spectroscopic Properties of the Intercalated Rhodamine Dye. Both the niobates intercalated with R6G+ exhibited characteristic spectral bands due to the dye in their visible diffuse reflectance and emission spectra. Figure 2 shows the visible spectra of the intercalation compounds and that of an aqueous R6G+ solution. The spectral shape of both the intercalation compounds stored under 93% RH (Figure 2b and e) is similar to that of R6G+ in water (Figure 2a) where the dye molecules are present as monomers,14,16,17,23,33-35 while the absorption maximum (521 nm) is slightly blue-shifted from that for the aqueous R6G+ solution (526 nm). On the other hand, the emission spectra shown in Figure 3 manifest that the intercalation compounds (Figure 3b and e) and a diluted aqueous R6G+ solution (Figure 3a) give emission bands assignable to the monomeric R6G+ species11,12,14,17,22-24,35,36 (32) Calculated from the data in ref 21. (33) (a) Deshpande, A. V.; Namdas, E. B. Chem. Phys. Lett. 1996, 263, 449. (b) Otsuki, S.; Adachi, K. Polym. J. 1995, 27, 655. (34) Lopez Arbeloa, F.; LLona Gonzalez, I.; Ruiz Ojeda, P.; Lopez Arbeloa, I. J. Chem. Soc., Faraday Trans. 2 1982, 78, 989. (35) Mialocq, J. C.; Hebert, P.; Armand, X.; Bonneau, R.; Morand, J. P. J. Photochem. Photobiol., A 1991, 56, 323. (36) Li, Y.; Ding, L.-J.; Gong, Y.-K.; Nakashima, K. J. Photochem. Photobiol., A 2004, 161, 125.

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Figure 3. Steady-state emission spectra of (a) aqueous R6G+ solution (10-6 mol dm-3), (b) R6G-PA-Nb6O17 intercalation compound stored for 1 week in a desiccator of 93% RH after preparation, (c) sample b after standing for 1 week after being transferred to a desiccator of 20% RH, (d) sample c after standing for 1 week after being transferred to a desiccator of 93% RH, (e) R6G-HA-Nb6O17 intercalation compound stored for 1 week in a desiccator of 93% RH after preparation, (f) sample e after standing for 1 week after being transferred to a desiccator of 20% RH, and (g) sample f after standing for 1 week after being transferred to a desiccator of 93% RH. Each of the spectra in sets b-d or e-f is recorded with the same intensity scale.

with similar shapes; the luminescence wavelength of both the intercalation compounds (548-549 nm) is shorter than that of the aqueous R6G+ solution (552 nm). We note that the difference in cointercalated alkylammonium ions did not affect the spectral profiles of R6G+. We observed spectral changes for the intercalated R6G+ ions depending on the humidity. Figure 2 also shows the diffuse reflectance spectra of the R6G+-intercalated niobates kept under different relative humidities. The samples were stored under 93% RH at first and then transferred to a desiccator of 20% RH. Parts c and f of Figure 2 exhibit a split in two absorption bands at around 500 and 527 nm for both R6G-PA-Nb6O17 and R6GHA-Nb6O17 samples by reducing the atmospheric humidity, for instance, when they were kept under 20% RH for 1 week. The appearance of the bands at shorter wavelengths indicates the formation of R6G+ aggregates under low humidity. R6G+ molecules are known to be partly aggregated when they are dissolved in concentrated solutions34 or confined densely in solid matrixes such as clay minerals14,16-18,20,21 to give absorption bands at around 450-500 nm. Judging from the absorption wavelength, the aggregated species observed in the niobate is assigned to H-type dimers but not to other ones such as J-type dimers and higher order H-aggregates. Another spectral change observed for the samples under 20% RH is a slight red-shift of the monomer band, which appears at 527528 nm. The emission spectra of the intercalated R6G+ ions are also altered with the humidity, as shown in Figure 3. Relatively weak emission at slightly red-shifted positions is observed for the samples kept under 20% RH (Figure 3c and f). This result is in harmony with that

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the interlayer spaces of niobate containing the saturated amounts of R6G+ do not additionally incorporate large amounts of water molecules. Discussion

Figure 4. TG curves of R6G-PA-Nb6O17 intercalation compound stored under (a) 93% RH and (b) 20% RH.

observed for the visible spectra if R6G+ ions form nonfluorescent H-type dimers. These humidity-dependent spectral changes were reversible. The spectroscopic features observed for both R6G-PA-Nb6O17 and R6G-HA-Nb6O17 at the initial conditions of 93% RH were fully restored, when the samples after standing in the dried desiccator (20% RH) were transferred to the humid one (93% RH), as evidenced by visible diffuse reflectance (Figure 2d and g) and emission (Figure 3d and g) spectra. The diffuse reflectance spectra are identical with those of the initial samples stored under 93% RH, indicating dissociation of the aggregates with increasing humidity. The emission intensity also recovered under the high humidity. When the intercalation compounds were transferred again to the conditions of 20% RH, both the diffuse reflectance and emission spectra became identical with those observed for the first 20% RH conditions (data not shown). Effect of the Humidity on the Water Content of the Intercalation Compounds. The humidity change between 93 and 20% RH caused adsorption-desorption of small amounts of water in the R6G+-intercalated niobates. Figure 4 compares TG curves of R6G-PANb6O17 stored under 93 and 20% RH. While the two curves indicate a similar temperature dependence, the sample kept under the more humidified conditions exhibits a larger mass loss below 423 K. This observation reveals that the R6G+-intercalated samples adsorb certain amounts of water when they are exposed to highly humid atmospheres. The TG data yield the water content of R6GPA-Nb6O17 as 5.2 and 4.5 molecules per [Nb6O17]4- unit for the samples kept under 93 and 20% RH, respectively. These values correspond to 13.0 and 11.3 molecules of water per R6G+ molecule for the samples humidified and dried, respectively. Also, R6G-HA-Nb6O17 contains 4.0 and 2.5 molecules of water per [Nb6O17]4- unit, or 5.7 and 3.6 molecules per R6G+ molecule, under 93 and 20% RH, respectively. These data indicate that only small amounts, that is, about two molecules per R6G+ molecule of water are adsorbed/desorbed during the humidity changes. The XRD patterns of the intercalation compounds showed a small alteration of the basal spacing with the humidity, being in accordance with the TG results. The basal spacings of both R6G-PA-Nb6O17 and R6G-HANb6O17 decreased by 0.08 nm when the samples were transferred from the conditions of 93% RH to those of 20% RH. However, the diffraction profiles were little altered; thus, the interlayer structure of both the R6G+-intercalated niobates was almost unchanged with the humidity. These observations are in harmony with the TG results showing that the amounts of water adsorbed/desorbed during the humidity changes are small. We presume that

The spectroscopic behavior of R6G+ dye is known to reflect the polarity of microenvironments, and the present results clarify the rather high polarity of the interlayer spaces of K4Nb6O17. The monomer absorption band of R6G+ shifts to shorter wavelengths in polar media.11,14,22,35,37 The absorption maxima of the monomeric R6G+ intercalated in the niobate appear at wavelengths shorter than that in water.14,16-18 This is in contrast with the spectroscopic behavior of R6G+ immobilized within clay minerals where the absorption band appears at the wavelength red-shifted from that in water. The polar interlayer microenvironments of niobate are rationalized by the large charge density of the anionic oxide layers; the less polar microenvironment within the clays will be explained by the low layer charge density of the clay layers and the presence of hydrophobic siloxane networks on the layer surfaces. The emission spectra of the intercalation compounds represented by the blue-shifted fluorescence maxima coincide with this assumption because the fluorescence maximum of R6G+ has been reported to shift toward a shorter wavelength in polar media.36 Some other factors can also affect the spectroscopic behavior of intercalated R6G+, although we suppose that the high polarity of the interlayer arrays of K4Nb6O17 is a dominant cause of the spectral shift. Hydrogen-bonding interactions between the dye and the surrounding species is a possible reason for the blue-shift of R6G+ absorption. The absorption maximum of R6G+ monomers dissolved in ethanol shifts to a shorter wavelength by addition of water, and it approaches the absorption wavelength in water with an increase in the water content.38 However, the absorption wavelength of R6G+ in the layered niobate is shorter than that of the aqueous R6G+ solution. This spectral shift is not explained by the usual hydrogenbonding interactions observed in water; nevertheless, peculiar hydrogen bonding between the dye and the intercalated water cannot be excluded if we assume that confinement in the interlayer spaces gives unusual states of water molecules. On the other hand, our results indicate that the cointercalated alkylammonium ions little affect the interlayer microenvironments. Although the alkylammonium ions can modify the interlayer spaces to be hydrophobic, we do not observe spectroscopic evidence, such as a red-shift of the absorption band, for hydrophobic microenvironments. The reversible aggregation of R6G+ induced by humidity characterizes the behavior of the dye molecules intercalated in K4Nb6O17. Deaggregation of R6G+ by the insertion of a small amount of water into the interlayer spaces is unusual, because the dye molecules have been reported to form aggregates in various solid media like clay minerals when they are present densely. In addition, the small alteration of the basal spacing with humidity suggests that the arrangement of intercalated R6G+ molecules does not greatly change during the aggregation-deaggregation process. Since K4Nb6O17 is known as a wide-gap oxide semiconductor, interactions between the niobate layers and the intercalated dye molecules can affect the spectroscopic (37) Hinckley, D. A.; Seybold, P. G.; Borris, D. P. Spectrochim. Acta, Part A 1986, 42, 747. (38) Lopez Arbeloa, F.; Lopez Arbeloa, T.; Gil Lage, E.; Lopez Arbeloa, I.; De Schryver, F. C. J. Photochem. Photobiol., A 1991, 56, 313.

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behavior. In fact, host-guest energy and/or electron transfer has been reported to occur for intercalation compounds of semiconducting oxides with dyes.4,7,9 Similar interactions are possible for the present system. However, the alteration of the spectroscopic behavior dependent on humidity should not be attributed to variations of the host-guest interactions, because the interlayer microenvironments other than aggregation-deaggregation are modified little with humidity, as evidenced by XRD and TG. A few papers show reversible aggregation of R6G+ in polymer matrixes. R6G+ molecules are aggregated under dry conditions in Nafion membranes.22,23 This behavior has been attributed to the coordination of water to dye molecules to prevent aggregation and/or high polarity of the dried polymer matrix; however, the polarity of the interlayer microenvironments of niobate should not be altered greatly because the humidity change induces small differences in the water contents and basal spacings of the intercalation compounds. A similar humidity dependence of the aggregation state has been reported for a cationic flavylium dye incorporated into sugar gel films; the dye molecules are aggregated under dry conditions.39 In consequence, we ascribe the aggregation-deaggregation of R6G+ to an alteration of the intermonomeric interactions in the dimer caused by the coordination of water molecules. Lopez Arbeloa et al. have reported that R6G+ dimers in water are thermodynamically stabilized by the formation of intermolecular hydrogen bonds between the oxygen atom on the xanthene ring and the carboxylic group of the R6G+ molecules.34 If the R6G+ molecules are aggregated through hydrogen bonds in the interlayer space of the niobate where diffusion of the dye molecules is rather restricted, the incorporation of about two water molecules per R6G+ molecule can greatly affect the intermolecular interaction to dissociate the R6G+ aggregates. The incorporated water molecules can migrate among the confined R6G+ ions to form new hydrogen bonds between the dye and the water with breakage of the dyedye hydrogen bonds. The presence of such a certain waterdye interaction is supported by the fact that the addition of a fixed amount of water (two molecules per R6G+ molecule) causes deaggregation irrespective of the type of intercalation compounds (R6G-PA-Nb6O17 or R6G-HANb6O17). Moreover, the aggregation-deaggregation process would not largely alter the arrangement of the densely (39) Matsushima, R.; Ogiue, A.; Kohno, Y. Chem. Lett. 2002, 436.

Shinozaki and Nakato

intercalated R6G+ molecules, being in harmony with the small variation of the basal spacing during the humidity change. Humidity-induced alteration of hydrogen-bonding interactions has also been reported for a monolayer film of an amphiphilic azobenzene derivative having a urea headgroup.40 In this film, hydrogen bonds form between the urea headgroup and the quartz substrate under conditions of low humidity, while they are broken at high humidities because water molecules are adsorbed to block the urea-substrate interaction. Conclusions R6G molecules are densely intercalated into the interlayer spaces of layered niobate K4Nb6O17 through the displacement of preintercalated alkylammonium ions. The characteristic behavior of the intercalated dye molecules is their reversible aggregation induced by adsorption-desorption of only small amounts of water molecules, the property which may be utilized to create, for example, vapor sensors. We also emphasize that the humidityinduced reversible aggregation investigated in the present study occurring under ambient conditions does not require a large energy for changing the aggregation states. Since dyes immobilized in solid matrixes often exhibit unusual photochemical and electrochemical behavior based on their peculiar molecular arrangement, the humidity-dependent aggregation will provide a novel method of tuning the conditions of functional dyes intercalated in inorganic layered materials. +

Acknowledgment. We thank Professor Sohzoh Suzuki (Tokyo University of Agriculture and Technology), Dr. Shigetaka Yakabe (PRESTO, Japan Science and Technology Corporation), and Professor Katsuaki Konishi (Graduate School of Environmental Earth Science, Hokkaido University) for their help in the elemental analyses. This work was partly supported by a Grant-in-Aid for Young Scientists (No. 14750661) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Supporting Information Available: IR spectra of K4Nb6O17 and the intercalation compounds with alkylammonium ions and R6G+ (Supporting Information Figure S1, PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA049354K (40) Seki, T.; Fukuchi, T.; Ichimura, K. Langmuir 2000, 16, 3564.