Aggregated Structures of Rhodamine 6G Intercalated in a Fluor

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Aggregated Structures of Rhodamine 6G Intercalated in a Fluor-Taeniolite Thin Film Ryo Sasai,*,† Taketoshi Fujita,‡ Nobuo Iyi,‡ Hideaki Itoh,† and Katsuhiko Takagi§ Research Center for Advanced Waste and Emission Management (ResCWE), Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan, Advanced Materials Laboratory (AML), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, and Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received February 20, 2002. In Final Form: June 14, 2002 To clarify the structure and species of rhodamine 6G (R6G) aggregates formed in taeniolite (TN) interlayer spaces, the oriented thin films of R6G-TN hybrid materials were prepared and analyzed. XRD investigations of these films indicated that the R6G molecules were intercalated in the TN interlayers and that the TN layers were parallel to the glass plate in the thin films. Most of the intercalated R6G molecules were assumed to form various kinds of H-type aggregates in the TN interlayer spaces, since the R6G-TN hybrid films exhibited no emission spectrum by excitation at λ ) 530 nm. The peak-deconvolution results of the UV/vis absorption spectra of the hybrid films indicated that there was one kind of H-type R6G dimer in the thin film. Moreover, polarized UV/vis spectroscopy revealed the existence of high-order aggregates in addition to these dimers. The high-order aggregates, but not the dimers, were found to be aligned perpendicularly to the TN surface in the TN interlayer spaces.

Introduction Clay minerals such as montmorillonite, saponite, hectorite, and taeniolite, which are negatively charged multilayered materials, possess two-dimensional spaces in which various ionic or polar organic molecules can be accommodated by ion-exchange reactions.1-8 It is wellknown that these intercalated guest organic molecules form various types of self-assembled structures or aggregates such as lamella, interdigitated monolayers, or H- or J-aggregates by their interaction with the highly negatively charged densities on the clay layer surface.1-8 Many researchers have investigated the synthesis and characterization of these functional organic-clay hybrid materials for their unique and useful properties in the development of intercalated functional organic compounds.7-22 However, the structures of these intercalated * To whom correspondence should be addressed. Tel: +81-52-789-5851. Fax: +81-52-789-5834. E-mail: rsasai@ rescwe.nagoya-u.ac.jp. † Research Center for Advanced Waste and Emission Management (ResCWE), Nagoya University. ‡ National Institute for Materials Science (NIMS). § Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya University. (1) Grim, R. E. Clay Mineralogy; McGraw-Hill: New York, 1953. (2) Whitingham, M. S.; Jacobson, A. J. Intercalation Chemistry; Academic Press: New York, 1982. (3) Alberti, G.; Bein, T. Comprehensive Supramolecular Chemistry; Pergamon: Oxford, U.K., 1996; Vol. 7. (4) Mortland, M. M.; Fripiat, J. J.; Chaussidon, J.; Oytterhoeven, J. J. Phys. Chem. 1963, 67, 248. (5) Farmer, V. C.; Mortaland, M. M. J. Chem. Soc. A 1966, 344. (6) Weiss, A. Clay Clay Miner. 1963, 10, 191. (7) Kikan Kagaku Sosetsu No. 21, Microporous Crystals; edited by the Chemical Society of Japan; Japan Scientific Societies Press: Tokyo, 1994. (8) Kikan Kagaku Sosetsu No. 42, Muki-yuuki Nano Fukugoutai Busshitsu, edited by the Chemical Society of Japan; Japan Scientific Societies Press: Tokyo, 1999. (9) Takagi, K.: Kurematsu, T.; Sawaki, Y. J. Chem. Soc., Perkin Trans. 2 1991, 1517. (10) Ogawa, M.; FujII, K.; Kuroda, K.; Kato, C. Mater. Res. Soc. Symp. Proc. 1991, 233, 89. (11) Ogawa, M.; Ishikawa, A. J. Mater. Chem. 1998, 8, 463.

organic compounds in clay interlayer spaces remain to be clarified in detail. To clarify the structures of such intercalated compounds formed in clay interlayer spaces, the properties and structure of the dye aggregates in the clay have been studied by various experimental methods.23-44 Lo´pez et al. have investigated the aggregated structures of rhodamine dyes in clay minerals. In this study, their electronic absorption and emission spectra were revealed to be variable, depending on the formation of dimers, trimers, and higher-order aggregates, by analyzing the spectroscopic data of these dye molecules in the clay layer surface.23-29 Two different associated states of the rhodamine dyes could be assumed, based on the dimer (12) Seki, T.; Ichimura, K. Macromolecules 1990, 23, 31. (13) Sasai, R.; Ogiso, H.; Shindachi, I.; Shichi, T.; Takagi, K. Mol. Cryst. Liq. Cryst. 2000, 56, 6979. (14) Sasai, R.; Ogiso, H.; Shindachi, I.; Shichi, T.; Takagi, K. Tetrahedron 2000, 645, 39. (15) Sasai, R.; Itoh, H.; Shindachi, I.; Shichi, T.; Takagi, K. Chem. Mater. 2001, 13, 2012. (16) Cooper, S.; Dutta, P. K. J. Phys. Chem. 1990, 94, 114. (17) Ogawa, M.; Takahashi, M.; Kuroda, K. Chem. Mater. 1994, 6, 715. (18) Sasai, R.; Shichi, T.; Gekko, K.; Takagi, K. Bull. Chem. Soc. Jpn. 2000, 73, 1925. (19) Ogawa, M.; Takahashi, M.; Kato, C.; Kuroda, M. J. Mater. Chem. 1994, 4, 519. (20) Ogawa, M.; Handa, T.; Kuroda, K.; Kato, C.; Tani, T. J. Phys. Chem. 1992, 96, 8116. (21) Sakoda, K.; Kiminami, T. Chem. Phys. Lett. 1993, 216, 270. (22) Fujita, T.; Iyi, N.; Klapyta, Z. Mater. Res. Bull. 1998, 33, 1693. (23) Tapia Este´vez, M. J.; Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I.; Schoonheydt, R. A. Clay Miner. 1994, 29, 105. (24) Tapia Este´vez, M. J.; Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. J. Colloid Interface Sci. 1994, 162, 412. (25) Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Trends Chem. Phys. 1996, 4, 191. (26) Lo´pez Arbeloa, F.; Tapia Este´vez, M. J.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Clay Miner. 1997, 32, 97. (27) Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I.; Costela, A.; Garcı´a-Moreno, I.; Figuera, J. M.; Amat-Guerri, F.; Sastre, R. J. Lumin. 1997, 75, 309. (28) Lo´pez Arbeloa, F.; Herra´n Martı´nez, J. M.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Langmuir 1998, 14, 4566. (29) Chaudhuri, R.; Lo´pez Arbeloa, F.; Lo´pez Arbeloa, I. Langmuir 2000, 16, 1285.

10.1021/la020183y CCC: $22.00 © 2002 American Chemical Society Published on Web 07/27/2002

Aggregated Structures of Rhodamine 6G

exciton absorptions and monomer fluorescence emissions. The aggregation behavior was found to be variable in different kinds of clay hosts, such as Laponite, hectorite, montmorillonite, and sepiolite, with different swelling and stacking properties. Fujita et al. have suggested that the R6G molecules in taeniolite interlayers are oriented with its longest xanthene ring axis perpendicular to the layer surface, as observed by the one-dimensional Fourier analysis of the powder X-ray diffraction profiles.30 Yariv et al. have also reported the metachromasy of crystal violet dye and triphenylmethane dye, as well as their intercalation into various kinds of clay in aqueous suspensions.31,32 An analysis of the results showed the adsorption structure of crystal violet to vary depending on a change in the amount of the crystal violet dye based on the ion-exchange capacity of the clay. A possible structure for the adsorbed/ intercalated triphenylmethane dyes aligned on the clay layer surface in a dilute aqueous suspension has been proposed by Yamaoka et al. on the basis of results obtained by electric linear dichroism measurements.33 In previous papers, we have described the relationship between the regiospecific photodimerization of the stilbenecarboxylate and the alignments of the stilbazolium salt intercalated in clay interlayers.18,34-36 The intercalation behavior and aggregation of methylene blue, a cationic thionine dye, has also been reported on clay minerals in an aqueous suspension.37-43 In these studies, the aligned aggregates of the dye molecules intercalated/adsorbed on the clay dispersed in a medium were reported by analysis of the spectroscopic data which indicated the cointercalation of the solvent molecules with the dye molecules. It is intriguing to determine the aggregated structure of such intercalated dye molecules in solid films when considering the many potential applications of such functional guests in clay minerals. Recently, we have analyzed the almost parallel orientation of cationic functional dyes intercalated in montmorillonite thin films on a glass plate by means of an UV/vis polarized spectroscopic method.13-15,44 In this study, the structure and species of a typical xanthene dye aggregate, rhodamine 6G (R6G), intercalated in taeniolite (TN) interlayer spaces as R6G-TN hybrid thin films have been reported. Experimental Section Preparation of Rhodamine 6G-Taeniolite Hybrid Thin Films. Li-taeniolite (LiTN: LiMg2LiSi4O10F2) was prepared by exchanging the sodium ions of synthetic Na fluor-taeniolite (NaMg2LiSi4O10F2, Topy Industries) with lithium ions by a method described in previous papers.30 The cation-exchange capacity (CEC) was determined to be 1.57 ( 0.09 mequiv/g by (30) Fujita, T.; Iyi, N.; Kosugi, T.; Ando, A.; Deguchi, T.; Sota, T. Clays Clay Miner. 1997, 45, 77. (31) Yariv, S.; Nasser, A. J. Chem. Soc., Faraday Trans. 1990, 86, 1593. (32) Yariv, S.; Gosh, D. K.; Hepler, L. G. J. Chem. Soc., Faraday Trans. 1991, 87, 1201. (33) Yamaoka, K.; Sasai, R. J. Colloid Interface Sci. 2000, 225, 82. (34) Sasai, R.; Takagi, K. New Ceram. 1998, 11, 40. (35) Sasai, R.; Shin’ya N.; Shichi, T.; Takagi, K.; Gekko, K. Langmuir 1999, 15, 413. (36) Yamaoka, K.; Sasai, R.; Takata, N. Colloids Surf., A 2000, 175, 23. (37) Fornili, S. L.; Sgroi, G.; Izzo, V. J. Chem. Soc., Faraday Trans. 1 1981, 77, 3049. (38) Cenens, J.; Schoonheydt, R. A. Clays Clay Miner. 1988, 36, 214. (39) Sunwar, C. B.; Bose, H. J. Colloid Interface Sci. 1990, 136, 54. (40) Schoonheydt, R. A.; Heughebaert, L. Clay Miner. 1992, 27, 91. (41) Aznar, A. J.; Casal, B.; Ruiz Hitzky, E.; Lo´pez Arbeloa, F.; Lo´pez Arbeloa, I.; Alvarez, A. Clay Miner. 1992, 23, 205. (42) Bujda´k, J.; Komadel, P. J. Phys. Chem. B 1997, 101, 9065. (43) Bujda´k, J.; Janek, M.; Madejova´, J.; Komael, P. J. Chem. Soc., Faraday Trans. 1998, 94, 3487. (44) Sonobe, K.; Kikuta, K.; Takagi, K. Chem. Mater. 1999, 11, 1089.

Langmuir, Vol. 18, No. 17, 2002 6579 Scheme 1. Structural Formula of Rhodamine 6G Moleculesa

a The arrow (M) indicates the optical transition moment of the rhodamine 6G molecule.

the ammonium acetate adsorption method. Laser grade rhodamine 6G (R6G: cf. Scheme 1), which was purchased from Lambda Physik, was used without further purification. The LiTN suspension was prepared by dispersing 2 g of the LiTN powder in 100 mL of deionized water and then spin-coating on a quartz glass substrate at a spinning rate of ca. 3000-4000 rpm. The R6G-TN hybridization was carried out by immersing the LiTN thin films in a 5 g/L R6G aqueous solution. The intercalation degree of the R6G molecules was controlled by the cationexchange capacity (CEC) of the clay, suggesting that the intercalation occurs by a cation exchange of the dye cations and Li+. The intercalation reaction was performed in a decomposition vessel under autogene pressure of the reaction temperature, with a rapid-quench type hydrothermal apparatus operating at 100 MPa. The exchange temperature was adjusted at 100 °C for 12 h. The obtained R6G-TN hybrid thin films were kept at 50 °C overnight for drying. Characterization. X-ray diffraction analysis of the R6GTN hybrid thin films was carried out at room temperature with a RINT 1200 diffractometer (Rigaku) using a monochromatized Cu KR radiation source. The absorption spectra were recorded in the region of 350-700 nm on a JASCO V-550 spectrophotometer. The polarized UV/vis absorption spectra were also recorded on this spectrophotometer with a JASCO polarizer unit attachment (model RSH-452). The emission spectra were measured on a JASCO FP-750 spectrofluorometer. For identification of the R6G-TN composites, CHN elemental analysis was conducted using a Perkin-Elmer 2400 II CHN element analyzer. The specific surface area of the LiTN thin films was estimated by the multipoint BET method using a Beckman Coulter Omnisorp 360 gas sorption analyzer.

Results and Discussion Characterization of the R6G-TN Hybrid Thin Films. The amount of R6G dyes included in the hybrid thin films was estimated from the value of the carbon weight fraction obtained from CHN analysis. The observed CHN values of C ) 21.90 ( 1.10, H ) 2.40 ( 0.09, and N ) 2.22 ( 0.20 wt % indicate that the R6G molecules was calculated to be ca. 30% of the cation sites in the TN interlayer spaces. But, the intercalation sites for TN was calculated to be ca. 71%, which is obtained from a CEC of 1.57 ( 0.09 mequiv/g, as determined by the ammonium acetate adsorption method. Therefore, it can be concluded that the actual adsorption degree of the R6G molecules is ca. 43 mol % of the CEC. The area/unit charge of TN was calculated to be about 0.24 nm2 using structure parameters for KTN shown by Toraya et al.45 An R6G molecule occupied ca. 0.4 nm2 in area in the case of perpendicular orientation to the TN surface. From comparison with these area, this intercalation rate (ca. 43 mol %) could be considered as the saturated amount. (45) Toraya, H.; Iwai, S.; Marumo, F.; Hirao, M. Z. Kristallogr. 1977, 146, 73.

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Figure 2. UV/vis absorption and emission spectra of rhodamine 6G in an aqueous solution (broken line) and the rhodamine 6G-taeniolite hybrid in the thin film (solid line). Excitation is at 530 nm. Figure 1. X-ray profiles of the Li-taeniolite thin film (a) and the rhodamine 6G-taeniolite hybrid thin film (b). These films were made by the spin-coating method.

The specific surface area of the LiTN thin film was evaluated to be 8.0 m2/g by the BET method. The R6G occupied area is estimated at 0.48 nm2/unit cation exchangeable site in the external surface on the basis of the chemical formulas obtained from ICP analysis and the results of crystal structure analysis by Toraya et al.45 Here, the contribution of the R6G molecules adsorbed on the external surface is assumed to be almost negligible when supposing that the present specific surface area is attributed to the external specific surface area of LiTN in the thin films. This is due to external cation-exchangeable sites of only ca. 1% out of the total cation exchangeable sites. The XRD profiles of both the LiTN (a) and R6G-TN hybrid (b) thin films are shown in Figure 1. These show only 00l diffractions with narrow half-bandwidths, suggesting that the TN layer sheets are regularly oriented parallel to the quartz glass plate. These results indicate that the R6G molecules are intercalated in the TN interlayer spaces judging from the fact that the basal spacings d001 of the TN layers increased from 1.52 to 2.35 nm. The gallery height h of the layer space was calculated to be 1.39 nm by subtracting the thickness of the TN layer sheet 0.96 nm from the d001 value, which was in good agreement with the long-axis length of the R6G molecule (ca. 1.4 nm); i.e., the R6G molecules were oriented almost perpendicularly to the TN surface in the interlayer spaces as pointed out by Fujita et al.30 The obtained thin film calculated was ca. 60 nm on the basis of the weight of the films and the densities of the R6G-TN hybrid materials. The molar concentration of the R6G molecules included in the hybrid thin film could, therefore, be calculated as 1.04 mol/dm3 from the thickness of the thin film. The weight percentage of the intercalated R6G molecules (ca. 31.3 wt %) was obtained from CHN analysis. Absorption and Emission Spectra of the R6G-TN Hybrid Thin Films. Figure 2 shows the absorption and emission spectra of the R6G molecules in aqueous solution in the absence of TN (dotted lines) and in the hybrid thin films (solid lines). The absorption spectrum of R6G

molecules in aqueous solution (11.0 µmol/dm3) has a peak at λ ) ca. 530 nm and a shoulder at around λ ) ca. 500 nm, which originated from R6G monomers. In the R6GTN hybrid thin films, decrease of peak intensity at ca. 530 nm and broadening of the main absorption peak are observed. This result indicates that R6G molecules in hybrid thin films can be considered to be in the average of the several states of the intercalated R6G molecules. In the aqueous solution without TN, it is well-known that the intensity of main absorption peaks at 530 nm decreases with an increase in molar concentration, and the shoulder around 500 nm grows and is observed as a perspicuous peak with an increase in molar concentration. This behavior is explained as follows: the H-type species of R6G molecules, such as dimers, trimers, and higher-order aggregates, are formed with increasing molar concentration of R6G. Therefore, the reason for decrease in peak intensity and broadening of the absorption peak could be considered as that the R6G molecules form the same H-type species in the TN interlayers. The R6G in aqueous solution without TN exhibited a strong emission at around λ ) 550 nm arising from the monomer species excited at 530 nm; however, no emission was observed in the R6G-TN hybrid thin films, even when intercalated R6G molecules were excited at 530 nm. There are some possibilities to explain why this emission quenching of R6G molecules occurs in TN interlayers. These possible reasons are as follows: (1) All intercalated R6G molecules form H-type species, which are known as nonfluorescence species. (2) Fluorescence of monomers, which exist in TN interlayers, is quenched by H-type species partially formed. (3) Fluorescence of R6G molecules is quenched by the interaction between R6G molecule and cation-exchangeable site on TN surface. However, to clarify the reason for emission quenching, further detailed investigation will be necessary. To identify how the H-type species of the R6G molecules were included in the hybrid thin films, the absorption spectrum of the R6G-TN hybrid thin film (cf. solid line in Figure 2) was separated into partial absorption bands on the basis of the following assumptions: (1) The absorption spectrum is expressed by the sum of the partial absorption bands, approximately given as Gaussian distribution curves. (2) The contribution of the R6G monomers to the absorption spectrum of the hybrid thin

Aggregated Structures of Rhodamine 6G

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Figure 4. Schematic representation of the intermolecular configuration of the H-type dimers with angle β.

Figure 3. Deconvoluted absorption spectra of the rhodamine 6G-taeniolite hybrid thin film. The partial absorption bands (broken lines) involved in the spectrum were numbered 1-6 from the right. The combined spectra of these partial bands are shown with a solid line.

Table 1. Spectroscopic and Structural Parameters of the H-type Dimer of the R6G Molecules Intercalated in the TN Interlayers λ1 (nm)

λ2 (nm)

U (cm-1)

β (deg)

r (nm)

584

501

-1425

29

0.71

from the absorption maxima and is given as film is assumed to be negligible on the basis of the emission spectrum and a consideration of the specific external surface area of the clay. On the basis of these assumptions, we carried out a deconvolution of absorption spectrum to the partial absorption bands by applying the nonlinear least-mean method (Levenberg-Marquardt algorithm) of peak-fitting program packed in Microcal Origin (Microcal Software Inc.) to the measured absorption spectrum plotted against wavenumber. Peak position, half-value width, and peak height of each partial absorption bands were used as parameters. Initial position and the number of partial absorption band were estimated from the position, and the number of minima appeared in the second-differential curve of measured spectrum.46 The separated partial absorption peaks are shown in Figure 3, where the main absorption of the intercalated R6G molecules consist of three partial absorption bands with absorption maxima at 584 (1), 535 (2), and 501 (3) nm, respectively. As described above, it is well-known that absorption spectrum of the concentrated R6G aqueous solution possesses the absorption peaks around 500 and 600 nm by formation of H-type species. Moreover, Lo´pez et al. reported that almost the same absorption bands were observed in the case of R6G-clay aqueous suspension, too.23-29 Therefore, two partial absorption bands observed at 584 and 501 nm in this study can be ascribed to one kind of H-type dimer with the sandwich structures. This result indicates that the intercalated R6G molecules form at least only one type of H-type dimer in the TN interlayers. A partial absorption band at 535 nm was at the same peak position as the absorption peaks of monomer R6G molecules in dilute aqueous solution. Therefore, it is speculated that this partial absorption band is attached to the intercalated R6G monomers slightly existing in the TN interlayers and surface. Table 1 summarizes the spectroscopic and structural parameters of an H-type dimer calculated on the basis of an exciton theory.47 Here, the parameters U and β in Table 1 are explained by the following equations. The interaction energy is calculated (46) Sasai, R.; Shichi, T.; Gekko, K.; Takagi, K. Bull. Chem. Soc. Jpn. 2000, 73, 1925. (47) Kasha, M. Radiat. Res. 1963, 20, 55.

U ) (ν1 - ν2)/2

(1)

where ν1 and ν2 are the wavenumbers of the absorption maxima of the dimers, respectively. The angle between the polarization axes of R6G, β, is given as

tan2(β/2) ) A1/A2

(2)

by using the areas of the dimer bands, A1 and A2, respectively. From these data, the sandwich structures could be depicted, as shown in Figure 4, for a set of dimers formed in the R6G-TN hybrid thin film. The H-type dimer possessed a twist angle of 29° as a dihedral angle between the parallel-arranged two xanthene rings of the R6G molecules. If such dimers are supposed to represent a sandwich-type model, as in homogeneous media, the intermolecular distance (r) in the dimer can be estimated by using the data in Table 1.

U ) -|M|2 cos β/r3

(3)

where |M| is the transition moment of the monomer, as calculated from a previous paper by Lo´pez et al.24 As a result, an r value of 0.71 nm was obtained for the H-type dimer. When this dimer is assumed to be formed in the TN interlayers, the values do not fit the gallery space (h) of the R6G-TN hybrid, as the r values are rather smaller than h. If one takes into account the above considerations, the xanthene planes of the intercalated R6G dimers are assumed to be almost perpendicular to the TN surface. UV/Vis Polarized Absorption Spectra of the R6GTN Hybrid Thin Films. When the direction of the optical electric vector is parallel to the optical transition moment, the molecules can absorb light the most efficiently. Thus, if the optical transition moments of the chromophore in a molecule are regularly aligned in the oriented thin films, the absorption spectra should change with the change in the incident angle when a linear polarized light is used as the incident light. The tilt angle (ΘT) of the xanthene ring plane of the R6G of the hybrid thin film can, therefore, be estimated by UV/vis polarized spectroscopy. The TN layer sheets are assumed to be oriented parallel to the

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Figure 5. Schematic illustration of the polarized absorption measurements. The sample film was set parallel to the xy plane and rotates around the x axis. The molecular axis of the aligned compounds was abbreviated as M.A., the optical transition moment was abbreviated as M, the angle of the molecular axis to the optical transition moment was abbreviated as θ, the tilt angles of the molecular axis were abbreviated as γ, and the rotational angle around the x axis was abbreviated as R.

glass plate. Figure 5 shows a schematic illustration of the UV/vis polarized absorption spectroscopic measurements.48 Absorbances, Ax and Ay, of the two orthogonal polarized lights were measured with a change in the rotational angle R of the substrate around the x axis. Here, Ax and Ay represent the absorbance for the horizontal and vertical polarized light against the x axis, respectively. Figure 6 shows the absorption spectra of the R6G-TN hybrid thin films by the x- and y-polarized incident light at various angles R. Little change in the absorption spectra could be observed in the case of the x-polarized incident light with the change in the R values, while the intensity of a new sharp absorption that appeared at λ ) 456 nm increased with an increase in the R values by the y-polarized light incident into the R6G-TN hybrid oriented thin films. Such interesting behavior clearly indicates that the R6G-TN hybrid thin films include an H-type species which is sensitive to the y-polarized incident light. Since the absorption in the range of visible light is caused by the optical transition moment parallel to the long molecular axis of the xanthene ring (cf. Scheme 1), it is concluded that the H-type species sensitive to the y-polarized light may be perpendicularly aligned with the glass plate, i.e., the TN interlayer surface. This species has an absorption maximum at a shorter wavelength than that for the H-type dimer. This indicates that the H-type species of a higher-order aggregate than the dimers are sensitive to y-polarized light. The dichroic ratios, R, at a given wavelength can be calculated from the two kinds of absorption spectra shown in Figure 6. Angle γ, defined as the angle between the molecular axis and the normal line of the glass plate, can be estimated by the dependence of the R values on the incident angle R.44,48,49 Here, angle γ can be expressed by the following equation:

Figure 6. UV/vis polarized absorption spectra of the rhodamine 6G-taeniolite hybrid thin films. Linear x- (a) and y-polarized (b) light was used as the incident light.

transition moment. In the case of R6G molecules, the optical transition moment of the absorption in the visible range is along the long molecular axis; i.e., θ ) 0°. Therefore, eq 4 can be written as follows:

R ) [2 sin2 R - (3 sin2 R - 1) sin2 γ]/sin2 γ

(5)

(3 sin2 R - 1)(3 cos2 θ - 1)sin2 γ}/{2 sin2 θ + (2 - 3 sin2 θ) sin2 γ} (4)

Figure 7 shows the correlations between the dichroic ratios, R, and the incident angles, R, in the R6G-TN hybrid thin film at λ ) 456 (O) and 535 (0) nm. The R values at an absorption maximum (λ ) 456 nm) of the higher-order H-type aggregates depended on the R values with a minimum at R ) 0°. These results indicate that the intercalated R6G molecules form higher-order H-type aggregates and are regularly aligned on the TN interlayer surfaces. Angle γ was estimated to be 10.6° by fitting eq 5 to the correlation between R and R. The tilt angle ΘT was then evaluated to be 79.4° as the angle between the molecular axis and the TN layer surface. Moreover, the gallery height h is determined to be 1.38 nm on the basis of both the ΘT value and molecular length (1.40 nm), which are in good agreement with results obtained from the XRD studies. The R6G molecules in the higher-order H-type aggregates in the TN interlayer spaces are inclined 79.4° toward the TN layer surface and form an interdigitated structure aligned within the TN interlayer spaces (cf. Figure 8). The R values at λ ) 535 nm of the H-type dimer

Here Ax and Ay are the absorbances of the two perpendicular polarized incident lights at a given wavelength and θ denotes the angles of the molecular axis to the optical

(48) Fukuda, K.; Kato, T.; Nakahara, H.; Shibasaki, Y. ChohakumakuBunnsi Soshikimaku-no-Kagaku; Kodansya: Tokyo, 1993. (49) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VHC: New York, 1986.

R ) Ay/Ax ) {2[sin2 θ + sin2 R(3 cos2 θ - 1)] -

Aggregated Structures of Rhodamine 6G

Figure 7. Dependence of the dichroic ratios, R, of rhodamine 6G-taeniolite against the incident angles in the hybrid oriented thin film at 456 (O) and 535 (0) nm. Symbols denote the experimental data with the solid line determined in accordance with eq 5.

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Figure 9. Schematic features of intercalation state of the rhodamine 6G molecules in the taeniolite interlayer spaces.

interlayer spaces did not form any regularly aligned aggregates in the R6G-TN hybrid thin films. This species, an H-type dimer, possibly exists at random in the remaining spaces between the higher-order H-type aggregates (cf. Figure 9). Conclusions By analysis of the UV/vis nonpolarized and polarized spectroscopic data, the aggregated R6G molecules intercalated in the R6G-TN thin film could be quantitatively determined. In the present hybrid thin films, R6G molecules were mostly intercalated in the TN interlayers to form two kinds of H-type species of the intercalated R6G molecules, i.e., a dimer and a higher-order aggregate. UV/vis polarized spectroscopic data showed that this dimer exists in the TN interlayer spaces with a random orientation. The higher-order R6G aggregates in the TN interlayers were found to form an interdigitated structure with a tilt angle of 79.4° against the TN layer surface. Figure 8. Schematic model of the higher-order aggregates of the intercalated rhodamine 6G molecules in the taeniolite interlayer spaces.

remained constant at about 1.0 irrespective of the R values. The R6G molecules of the H-type dimer in the TN

Acknowledgment. The authors express thanks to Prof. F. Lope´z Arbeloa of the Universidad del Paı´s Vasco (Bilbao, Spain) for discussions and suggestions on the aggregation of rhodamine 6G (N.I.). LA020183Y