Molecular Orientation of Rhodamine Dyes on Surfaces of Layered

Feb 17, 2005 - Binder, H.; Kohlstrunk, B.; Brenn, U.; Schwieger, W.; Klose, G. Colloid Polym. Sci. 1998, 276, 1098. VimondLaboudigue, A.; Prost, R. Cl...
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J. Phys. Chem. B 2005, 109, 4608-4615

Molecular Orientation of Rhodamine Dyes on Surfaces of Layered Silicates Juraj Bujda´ k* and Nobuo Iyi National Institute for Materials Science, Namiki 1-1, Tsukuba Ibaraki 305-0044, Japan ReceiVed: July 7, 2004; In Final Form: NoVember 23, 2004

Films of the layered silicates fluorohectorite (FH) and saponite (Sap) with various rhodamine dyes were prepared. The dyes with acidic as well as large hydrophobic groups in their molecule were not adsorbed on the surface of FH, which was interpreted in terms of high charge density on the surface of this silicate. All adsorbed dyes formed similar forms, such as isolated cations and H-type molecular aggregates, which were characterized by different spectral properties. Polarized ultraviolet-visible (UV-vis) spectroscopy was used for the characterization of the molecular orientation of dye chromophores on the silicate surface. The isolated dye cations and species, which absorbed light at the low energy part of the spectra, were only slightly tilted with respect to the plane of the silicate surface. The cations forming H-aggregates and absorbing light at low wavelengths were oriented in a nearly perpendicular fashion. The nearly perpendicular orientation was observed as a strong increase of dichroic ratio with film tilting. The orientation of the cations in H-aggregates depends partially on the structure of the dye molecule, namely, on the type of amino group (primary, secondary, or tertiary) in the dye molecule. The type of amino groups probably plays a role in the suitable orientation of dye cations for effective electrostatic interaction between the cations and the negatively charged siloxane surface. X-ray powder diffraction could not distinguish dye phases of dye monomers and molecular aggregates.

1. Introduction In recent years, there has been an extensive effort for the development of optofunctional materials, in which dye molecules are embedded in solid host compounds. For the functionality of such materials, various conditions and parameters are crucial, for example, the control of the chemical state, the distribution and orientation of chromophores on a molecular level, and the formation of molecular assemblies. The extent and types of formed molecular assemblies dramatically influence the optical properties of chromophores. H-aggregates, with a sandwich-type intermolecular association between molecules, absorb light of higher energies. They quench fluorescence, so their presence degrades the lasing ability of laser dye systems.1 J-aggregates are characterized by a head-to-tail intermolecular association. The formation of the J-aggregates is less frequent and much related to the geometry of the dye molecule. These species absorb light of lower energies and emit fluorescence.2 Both films and colloids of rhodamine (R) dyes embedded and adsorbed on a solid substrate have been investigated, for example, R dyes absorbed in organic polymers2 and silicas3, adsorbed on the surfaces of glass,4 intercalated in layered inorganic hosts,5 and adsorbed in the cavities of molecular sieves.6 R molecules form aggregates at the solid/liquid interface, which is generally controlled by the concentration of the dye and properties of both the substrate7,8 and dyes.9 The optical properties of R dyes in colloids of layered silicates, such as clays and micas, have been extensively studied.10,11 The cations of R dyes form H-dimers and larger aggregates in clay colloids.11 In some cases, the formation of J-dimers and larger J-aggregates has also been reported.12,13 The formation of variable dye species * Corresponding author. E-mail: [email protected]. Phone: 00421-259410 459 and 00421-2-59410 444. Permanent address: Institute of Inorganic Chemistry, Slovak Academy of Sciences, Bratislava, SK-845 36, Slovak Republic.

in clay colloids provides valuable information on the properties of colloidal clay particles, such as swelling and particle size.14 Recently, the effect of the layer charge distribution of the layered silicate montmorillonite on the formation of rhodamine 6G (R6G) aggregates was proved.7,8 The amount of dye aggregates increased with the layer charge. A strong relation of the rhodamine 6G (R6G) fluorescent properties in clay colloids to its aggregation state was found.8 A random orientation of dye cations is expected in solutions but unlikely in solids. A certain degree of anisotropy or preferential orientation occurs for dye cations adsorbed on solid surfaces, although, in some cases, more dye species are present, which are consequently characterized by distinct structural varieties.15 In recent years, there have been several attempts to characterize the orientation of R dye cations adsorbed on various inorganic substrates. Fourier analysis X-ray diffraction and polarized infrared spectroscopy measurements were used for the determination of the molecular orientation of R6G in the swelling mica fluor-taeniolite.16 The xanthene dye cations orient with their longest axis almost perpendicular to the host surface. Further studies, mostly applying spectroscopy methods using polarized light and second harmonic generation measurements, revealed the formation of almost perpendicularly oriented cations of this dye on similar substrates.17-19 The perpendicular orientation is, however, restricted to higher-order aggregates of the H type. H-dimers and monomeric cations are, in most cases, only slightly tilted with respect to the silicate plane.20 Similar results were also reported for other R dyes and the reaction systems with other solid substrates.18 Recently, we found that the amount of the perpendicularly oriented cations can be controlled by the substrate charge. Consequently, the formation of perpendicularly oriented species may not occur at the surface of any silicate specimen but is dependent on the surface charge properties.7 The effect of the dye concentration on the molecular orientation was proved in another study.15 The vertical orienta-

10.1021/jp0470039 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/17/2005

Molecular Orientation of Rhodamine Dyes tion of dye cations, which form the H-aggregates, seems to be a universal phenomenon, because similar molecular orientations and their structural as well as spectral features were observed for the structurally different dye oxazine 4 (Ox4).21,22 On the other hand, very few tilted methylene blue (MB) chromophores were detected by polarized spectroscopy in similar layered silicate films. This is in contrast to the relatively structural similarity of thiazine and oxazine dyes, represented by MB and Ox4, respectively. We assume that the particular structural features of the dye molecule (cation) itself should significantly affect the dye molecule orientation on the surface. No systematic research concerning the structural effects of dye molecules on their molecular orientation on layered silicate surfaces has been done. The objective of this work was to prepare oriented films of two representative layered silicates, saponite (Sap) and fluorohectorite (FH), with embedded R dyes. The films were characterized by X-ray diffraction (XRD), and the molecular orientation was studied using polarized ultraviolet-visible (UV-vis) spectroscopy. The spectra were obtained at different angles between the incident light and a film normal for each film using x- and y-polarized light. Such a series of measurements enabled the statistically accurate calculation of the orientation angles of embedded dye molecules. The orientation angles were separately determined for each dye species, by calculation of the dichroic ratios for each measured wavelength. 2. Experimental Section Materials. Two layered silicates, both of the trioctahedral type, were used: Li+-saturated synthetic fluorohectorite (FH; Corning) and natural saponite (Sap; SapCa-1).23 The former represents the layered silicate with a high charge density; its structure and properties are similar to the expandable mica taeniolite.24 FH was used as obtained without any further purification. Sap was purified by suspension sedimentation in order to separate mineral admixtures. The purity of Sap was verified by XRD and infrared spectroscopy (not shown). The sodium form of saponite (Sap) was prepared by the ion-exchange reaction of SapCa-1, performed as a repeated saturation with NaCl solution (1 mol dm-3). The cation-exchange capacities (CEC) of FH and Sap were 1.50 and 0.95 mmol g-1, respectively.23 Six representative R dyes with high purities (for application in lasers) were purchased from Lambda Physik and used without further purification. The dyes include the three cationic dyes rhodamine 6G/benzoic acid, 2-[6-(ethylamino)-3-(ethylimino)2,7-dimethyl-3H-xanthen-9-yl]-ethyl ester, monohydrochloride/, rhodamine 3B (R3B)/benzoic acid, 2-[6-(diethylamino)-3-(diethylimino)-3H-xanthen-9-yl]-ethyl ester, monohydrochloride/, and rhodamine 123 (R123)/benzoic acid, 2-(6-amino-3-imino3H-xanthen-9-yl)-methyl ester monochloride/ and the three dyes, which may form zwitterions due to the presence of an acidic carboxyl group, rhodamine B (RB)/2-[6-(diethylamino)-3-(diethylimino)-3H-xanthen-9-yl] benzoic acid, rhodamine 19 (R19)/ benzoic acid, 2-[6-(ethylamino)-3-(ethylimino)-2,7-dimethyl-3Hxanthen-9-yl], perchlorate/, and rhodamine 560 (R560)/o-(6amino-3-imino-3H-xanthen-9-yl)-benzoic acid. The structural formulas of the dyes are shown in Figure 1. The UV-vis spectra of the compounds were identical to those published by the company. Some basic data for the characterization of the spectra of the dyes used are listed in Table 1. R dyes were stable in aqueous, ethanol, and mixed water/ethanol solutions; that is, no changes with time were observed in their solution spectra.12,25-29

J. Phys. Chem. B, Vol. 109, No. 10, 2005 4609

Figure 1. Structural formulas of rhodamine dye cations and zwitterions.

A few drops of layered silicate suspensions were deposited on quartz slides by a spin coating method in order to obtain films with the preferential orientation of silicate particles parallel to the slide surface. An ion-exchange reaction with dye cations was performed in an ethanol/aqueous solution with a volume ratio of 1:1 and a dye molar concentration of 2 × 10-2 mol dm-3. The quartz plates with the silicate films were immersed in the dye solution and heated at 60 °C for 2 h. Due to the large excess of dye, the concentration of the dye solutions did not change significantly after the ion-exchange reaction. Films with adsorbed dyes were washed in deionized water in order to remove the excess dye solution. No irreversible adsorption of the dyes on quartz plates without clay films was observed. The UV-vis spectra were measured using a V-550 UV-vis spectrophotometer (Jasco Co., Ltd.) at room temperature. The polarized UV-vis absorption spectra were recorded using a Jasco polarizer. A series of spectra was measured using both horizontally and vertically polarized light, that is, the light polarized in the directions of the x- and y-axes, respectively. The quartz slide with the dye/clay film sample was rotated around the x-axis at angle R, changing the orientation of the slides with respect to the y-axis only.7 Hence, a dichroism should be only observed in the vertically polarized spectra. The horizontally polarized spectra were also measured, because they were necessary for the calculations of the dichroic ratio (R) values. The R Value, which is defined as shown in eq 1, can be experimentally determined as the ratio of absorbance for the y-polarized light (Ay) to that for the x-polarized light (Ax):

R(λi) )

Ay(λi) Ax(λi)

(1)

The R values were calculated for each measured point in the range of wavelengths from 250 to 900 nm. In general, the dichroic ratio (R) reflects the angle between the light direction and the transition moment. For setup of our measurements, it depends on the film tilt (angle R) and the

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Bujda´k and Iyi

TABLE 1: Spectral Characteristics of Rhodamine Dyes in Solutions and Silicate Filmsa spectral data of films main band: λmax, Θ dichroic band: λmax, Θ

dye

spectral data for the dye solutions: λmax, (), reference, solvent

R19 RB R560

528 nm, (109 000 dm3 mol-1 cm-1), 25, ethanol 552 nm, (107 000 dm3 mol-1 cm-1), 26, ethanol 535 nm, (89 900 dm3 mol-1 cm-1), 27, ethanol

R3B

559 nm, (94 000 dm3 mol-1 cm-1), 12, water

R123

512 nm, (85 200 dm3 mol-1 cm-1), 28, ethanol

R6G

530 nm, (105 000 dm3 mol-1 cm-1), 29, ethanol

536 nm, 26.6° (Sap) 575 nm, 29.3° (Sap) 516 nm, 29.6° (Sap) 519 nm, 24.1° (FH) 574 nm, 29.8° (Sap) 574 nm, 24.9° (FH) 516 nm, 29.5° (Sap) 525 nm, 25.0° (FH) 532 nm, 30.7° (Sap) 536 nm, 25.9° (FH)

465 nm, 54.0° (Sap) 460 nm, 42.9° (Sap) 459 nm, 50.0° (FH) 500 nm, 47.3° (Sap) 500 nm, 42.9° (FH) 475 nm, 38.8° (Sap) 454 nm, 55.6° (FH) 467 nm, 61.8° (Sap) 465 nm, 67.1° (FH)

a The spectral characteristics of dye solutions include data from the literature. λmax values of dyes in films of layered silicate were determined by second derivative spectroscopy. Angles (Θ) (see eq 3) at λmax are shown. For the RB/Sap Film, no band was resolved. R19, rhodamine 19; RB, rhodamine B; R560, rhodamine 560; R3B, rhodamine 3B; R123, rhodamine 123; R6G, rhodamine 6G; Sap, saponite; FH, fluorohectorite.

orientation of the transition moment with respect to the surface normal (angle γ): 20

R)

2 sin2 R - (3 sin2 R - 1) sin2 γ sin2 γ

(2)

For symmetric xanthene dyes, the transition moment is parallel to the long molecular axis of the xanthene ring.21 The angle between the long molecular axis and the clay surface can be expressed by

Θ ) 90° - γ

(3)

The XRD measurements were recorded using a RINT-1200 powder X-ray diffractometer (Rigaku Co., Ltd.) with Ni-filtered Cu KR radiation (λCu KR ) 0.154 18 nm) at a scanning speed of 2θ ) 2° min-1. 3. Results and Discussion We were not able to prepare the FH films with the dyes R19 and RB. The FH films did not adsorb these dyes in significant amounts, which was insufficient for the determination of the molecular orientation. This phenomenon can be interpreted in the following way: (1) Due to the presence of acidic carboxyl groups (Figure 1), R19 and RB may form zwitterions in aqueous solutions; that is, their molecules bear both positive and negative charges. In FH dispersions, the chemical equilibrium is shifted in favor of the zwitterionic molecules, due to the basic properties of the silicate. The high basicity of FH is due to the partial hydrolysis of the octahedral sheets from the edges of the layers. Indeed, the octahedral sheets are composed of MgO4F2-n(OH)n and LiO4F2-n(OH)n octahedrons, which, in aqueous suspensions, react as Bronsted bases with water molecules and form OHions. The basic character of the material has been experimentally confirmed.30 The zwitterionic character of the R dyes may contribute to the repulsive forces between the silicate surface and dye molecules. The high density of the negative charge on the FH surface can play a significant role and contribute to the repulsion forces. On the other hand, a relatively high fraction of the surface of Sap is not charged. This is due to the lower charge density, that is, fewer negative charge centers per unit surface area, as qualitatively expressed by the CEC value. Moreover, the charge of FH is due to substitutions in the octahedral sheets homogeneously spread over a larger surface area. The charge on the Sap surface is located at the less basal oxygen atoms due to substitutions and charge location mainly in the tetrahedral sheets.31 The presence of a noncharged surface can be an

important factor for the adsorption of the zwitterionic dyes R19 and RB on Sap. (2) R19 and RB are relatively hydrophobic due to the presence of hydrophobic groups in their molecules. The RB molecule contains four ethyl groups bound to nitrogen atoms. The R19 molecule contains two ethyl and two methyl groups bound to the nitrogen and xantene ring carbon atoms, respectively (Figure 1). The presence of hydrophobic groups on the R molecules could contribute to the low adsorption of these dyes on the highly charged surface of FH. The low affinity of neutral molecules to the FH surface has been experimentally observed.32 (3) Interestingly, R560 was adsorbed on the surface of FH in sufficient amounts, even though its molecule bears an acidic carboxyl group. This phenomenon can be explained by the polar character of this dye, that is, by the absence of hydrophobic alkyl groups. The adsorption of R3B, structurally similar to RB, can be interpreted in a similar way; R3B, although formed by hydrophobic cations, does not contain an acidic group. In summary, the lack of adsorption on FH films was observed only for such R dyes whose molecules contain both hydrophobic and acidic groups. Linearly Polarized Spectroscopy. The main bands in the spectra recorded without changing the angle between the film and light beam were mostly assigned to the dye monomers (Figure 2). The absorption maxima were red-shifted with respect to those recorded for the dye solutions, which can be explained by the effect of a more polar environment at the silicate surface.33 The largest shift was observed for RB (+23 nm), which might also be attributed to the formation of small amounts of the J-aggregates or J-dimers.19,34 The shift to lower wavelengths upon the dye adsorption was observed only for the case of the R560 spectra, being -19 and -16 nm for Sap and FH, respectively. The main bands in the R560/silicate spectra are probably complex, also including transitions related to the H-dimers and/or larger aggregates. The shifts to negative values indicate a significant portion of these species in the films with this dye. It should be pointed out that R560 is a structurally simple dye with a flat shape and without bulky hydrophobic groups. This fact is quite important for dye cation aggregation, which leads to distinct changes in the spectral properties.9 Although the presence of hydrophobic groups may lead to an increase in the extent of molecular aggregation, it also induces the increase in the distance between the transition moments of neighboring molecules in the body of the aggregate. Consequently, the presence of bulky groups reduces the coupling between transition moments, thus minimizing subsequent spectral changes.9

Molecular Orientation of Rhodamine Dyes

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Figure 2. (a) Polarized UV-vis spectra of rhodamine dye/saponite films: (A) rhodamine 19; (B) rhodamine B; (C) rhodamine 560. (b) Polarized UV-vis spectra of rhodamine dye/saponite films: (A) rhodamine 3B; (B) rhodamine 123; (C) rhodamine 6G. (c) Polarized UV-vis spectra of rhodamine dye/fluorohectorite films: (A) rhodamine 560; (B) rhodamine 3B; (C) rhodamine 123; (D) rhodamine 6G. The spectra were measured using y-polarized light. The films were tilted around the x-axis at angles of R ) 0-45° with respect to the y-axis. The arrows indicate changes in the spectra with increasing R.

The absorption of light and transition to the excited state depends on the orientation of the electric component of electromagnetic radiation with respect to the orientation of the transition moment of the chromophore. The spectra measured using x-polarized light did not significantly change with the film tilt, because the orientation of the film did not change with respect to the x-axis. The absorption slightly increased for all wavelengths with the film tilting (not shown) due to elongation of the spectral path. In the case of spectra measured using y-polarized light, the angle between an electric vector of light and the surface normal significantly changed. Consequently, the spectra significantly changed with the increasing tilt of the film when using y-polarized light (Figure 2). Interestingly, we observed in principle similar changes for all the measured films: (1) An increase in the absorbance with increasing R was observed for the species absorbing light in a narrow, high energy part of the spectra. The phenomenon of absorbance that increased with increasing R is called positive dichroism (PD). (2) Weakening of the light absorption was observed for the main band and species absorbing in the low energy part of the spectra. The PD indicates the presence of species with a transition moment tilted nearly perpendicular to the surface of the film. The dichroic ratio (R) increases with the tilting of the transition moment but is also proportional to the fraction amount of chromophores with the tilted transition moments absorbing light at the same wavelength. The strongest PD was observed for a

R6G/FH film. One should note that the dichroic species of R6G in the FH film were undetectable in the spectrum recorded at the angle R ) 0° but were clearly detected at 465 nm for the higher inclination angles (R). At the film angle R ) 45°, a dichroic band reached the intensity of the main peak centered at 532 nm. The dichroic band was significantly shifted to higher energies with respect to that assigned to the isolated dye cations. The dichroic band was assigned to the H-aggregates. The PD occurred for all the films in the high energy part of the spectra; however, in most cases, it was relatively weak. A moderate PD was also observed for the R6G/Sap film at the same wavelengths as observed for a FH film. Also, for the films containing other R dyes, PD was assigned to the molecular aggregates of the H type, which absorbed light at wavelengths significantly shorter with respect to the peaks assigned to monomers. An opposite trend was observed for the bands related to dye cation monomers and species absorbing light in the lower energy part of the spectra. With a film tilt, the absorbance values measured using y-polarized light slightly decreased. The effect was rather small but systematically observed for all the film specimens. It indicated a parallel or slightly tilted orientation of transition moments with respect to the surface plane. Such an orientation was identified for monomers and H-dimers. However, the species absorbing at the lowest energies may include also J-dimers and/or J-aggregates. The presence of more phases of dye cations in the interlayer spaces of silicates is not surprising. It has been predicted for the RB/layered silicate

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Figure 3. Relationship between orientation angle (Θ) and wavelength of absorbed light for the silicate films with adsorbed (a) rhodamine 6G, rhodamine 123, and rhodamine 3B, and (b) rhodamine 560, rhodamine B, and rhodamine 19. The angles were calculated for films with fluorohectorite (cross) and saponite (circle). They are compared with spectra of dye/saponite films, to more easily identify the angles and to estimate amounts for respective dye species. The spectra were measured at a tilting angle of R ) 45° and using x- (solid) and y-polarized (dashed) light.

intercalation compound using molecular calculations and also experimentally confirmed.35,36 Dye Cation Orientation. The dichroic spectra do not provide quantitative data on the orientation of the transition moments. For the calculation of the dye orientation, the dichroic ratio (R) values were calculated according to eq 1. The calculations were performed for all the measured wavelengths and film tilt angles (R). The least-squares method using the data of R(λi, Rj) (where λi ) 250-900 nm and Rj ) 0-45°) was used for the calculation of γ, which is the angle between the vector of the transition moment and the normal of the surface plane. The calculation was performed on the basis of the functional relation between R, R, and γ in eq 2. The angles between the silicate surface and the long axis of the xanthene ring (Θ) were calculated using eq 3. Figure 3 shows the relationship between the values of Θ and the light wavelength. The figure includes the spectra of the respective dyes embedded in the Sap films, so that one could identify the orientation angle (Θ) for the particular dye forms (monomers and aggregates) and estimate their amount. The precision of the orientation determination is low in the low absorbance regions due to the large effects of noise and interference of the baseline. Figure 3 confirms the trends observed in the polarized spectra (Figure 2). The dye cations,

which absorb light at higher wavelengths, were only slightly tilted with respect to the silicate surface plane. In most cases, the orientation angles (Θ) approached zero at the edge of the low energy part of the spectra. The species absorbing light at the highest wavelengths probably included also J-aggregates. The J-aggregate assemblies are characterized by absorption at significantly lower energies with respect to that of isolated cations. Structurally, they are formed by a head-to-tail intermolecular association. This type of association is favored if the cations are oriented at low angles or lying parallel to the surface.37 Each spectrum is probably composed of several bands, and their number reflects a variety of dye species adsorbed on the different environments. The bands likely overlap each other. The orientation angle determined by polarized spectroscopy is not an angle for one homogeneous dye species but is likely a superposition of the partial contributions from more than one species, such as the H- and J-aggregates, or dimers, and monomers. If perpendicularly oriented dye cations in the H-aggregates absorb light at low wavelengths and are characterized by a very broad band, they may also partially contribute to the resulting angle at medium wavelengths. On the other hand, the angles at the highest wavelengths are not significantly

Molecular Orientation of Rhodamine Dyes affected by the influence of the perpendicularly oriented molecules. This explains why the orientation angles continually increase with the energy of absorbed light and are never constant in any spectral range. The orientation angle at the wavelengths of maximal absorption did not vary much with dye structure. As mentioned above, the main bands represent light absorption by dye monomers together with the H- and possibly J-dimers and do not include much light absorption by the larger-sized H-aggregates. The orientation angle varied from 24 to 31°, and there was no clear relation of this angle to the dye structure. Interestingly, for the pairs of films prepared from FH and Sap, the orientation angle of the main band was always higher for the R dye/Sap films (Table 1). The dye species absorbing light at low wavelengths clearly exhibited a nearly perpendicular orientation of transition moments, characterized by an angle (Θ) above 40°. This is in agreement with the PD observed for the spectra measured using y-polarized light. The orientation angle of the nearly perpendicularly oriented transition moment was related to the dye structure and decreased in the order R6G, R19, R560 ∼ R123 ∼ R3B, RB. The highest angles were observed for dye cations with secondary amino groups (R6G and R19), approaching values above 60°. Unfortunately, we could not prepare a film of FH containing a sufficient amount of the R19 dye. Due to the charge effect on the dye aggregation,7,8 one would expect higher angles (Θ) for the hypothetical R19/FH reaction system than those observed for the R19/Sap film. Medium angles just below 60° were measured for the films with embedded R560 and R123 dyes. The angles were always higher for the films composed of the FH substrate. The dyes R560 and R123 are characterized by primary amino groups in their molecules. Interestingly, the angles for the R3B/Sap film were also medium, but those of R3B/FH are of lower values. The latter case resembles the reaction system characterized by the lowest molecular tilt, that is, the RB/Sap film. Both RB and R3B are dyes with molecules having tertiary amino groups with ethyl groups bound on the amino nitrogen atoms. Interestingly, the orientation of the dye cations forming dye aggregates does not probably relate much to the presence of an acidic group but rather depends on the structural features related to the amino groups in the molecule. Amino groups bear a partial positive charge and interact with the negatively charged surface of silicates. The H-bond interaction cannot be neglected as well. The interaction of amino groups with surface oxygen atoms depends on the type of amino group itself. For the electrostatic interaction, one of the most fundamental criterions is a close contact between negatively and positively charged atoms. For different types of amino groups, the steric orientation and angle between the surface and N(amine)-C(ring) bond would be different. The orientation of a molecule would fit such an arrangement in order to optimize the electrostatic interaction between the molecule and the charged surface.38 A different interaction would then also affect the orientation of the entire dye molecule/cation at the silicate surface. Interestingly, in the previous studies, we observed that oxazine 4, a cationic dye with a secondary-type amino group, exhibited an almost perpendicular orientation on the surface of the expandable mica, taeniolite. The orientation angles of oxazine 4 were similar to those of R6G.21,22,39 On the other hand, almost no PD was observed for the H-aggregates of MB, with a cation bearing tertiary amino groups. Highly tilted species were hardly detected for MB cations, and, moreover, only for the films of silicates with high charge densities.40

J. Phys. Chem. B, Vol. 109, No. 10, 2005 4613

Figure 4. X-ray diffraction patterns of (a) saponite and (b) fluorohectorite films before and after rhodamine dye intercalation.

X-ray Diffraction Patterns. Figure 4 shows the X-ray diffraction (XRD) pattern of the dye/silicate films. The films were the same as those used for the spectral measurements. For the Sap films (Figure 4a), the XRD patterns indicate the formation of relatively homogeneous phases characterized by a basal spacing between 2.05 and 1.77 nm. The basal spacing values of the dye/silicate films are significantly larger that that of the pristine Sap (1.38 nm). The basal spacing generally depends on the size and orientation of the intercalated dye cations. However, the resulting value can be influenced by the fraction of space occupied by the guest molecules and that filled with the remaining inorganic cations and water, which had not been exchanged and/or desorbed during the dye intercalation. An incomplete ion exchange for RB intercalation on montmorillonite has just been recently reported.36 For the dependence on the reaction conditions used, the amount of nonexchanged inorganic cations represented a 5-25% fraction of the sum of the dye and inorganic cations. Interestingly, the values of the basal spacings of RB/montmorillonite organoclays reflected very sensitively the amount ratios of the intercalated dye and nonexchanged inorganic ions.36 In other words, the basal

4614 J. Phys. Chem. B, Vol. 109, No. 10, 2005 spacings are very sensitively affected by the extent of the dye intercalation. In our case, the values of the basal spacings roughly correlate with the size of the cations and decrease in the order R6G (2.05 nm) ∼ R3B (2.04 nm), R19 (1.99 nm), RB (1.92 nm), R560 (1.86 nm), and R123 (1.77 nm). A more heterogeneous phase in a R19/Sap film is indicated by the broader diffraction band of low intensity. It can be explained in terms of the lower amount of R19 adsorbed on the silicate surface, which is also indicated by the lower absorbance values in the R19/Sap film spectrum compared to the other spectra (Figure 2a). A less-ordered structure was observed for the R dye/FH films (Figure 4b). This was probably due to a lower adsorption and/ or intercalation of the dye cations into this silicate, which is also indicated by the low absorbance values of the films’ spectra (Figure 2b). Indeed, the absorbance values in the dye spectra of the FH films were less than one-third of those recorded for the same dyes adsorbed into Sap films. The only explanation is a very incomplete saturation of the surface and interlayer spaces of FH by the dye molecules. Consequently, the phases of the intercalated and nonintercalated interlayer spaces and interstratified structures were present. It is in agreement with a reported study36 stating that an incomplete exchange of inorganic cations for RB led to the presence of more phases of intercalated dye in the layered silicate. These phases were detected by two bands in diffraction patterns related to the basal spacing of 1.87 and 2.07-2.32 nm.36 The lower degree of intercalation also observed in our work for FH is in contrast to the higher cationexchange capacity of this silicate.23 One possible explanation of the low dye adsorptions is the partially limited expandability of FH. Indeed, vermiculite-like properties of FH have also been observed in other studies.23,24 These properties are probably related to the high layer charge.41 The broadness of some peaks does not enable us to determine the diffraction angle which would represent the dominant arrangement of dye molecules. For example, two peaks in the XRD patterns were found for both the R6G/FH and R3B/FH films. Higher basal spacings for R6G/FH and R3B/FH were centered at about 1.8 and 1.6 nm, respectively, although a higher baseline at lower angles indicates a slight diffraction related to even larger basal spacing. An XRD pattern of the R123/FH film is characterized by a broad, asymmetric band. Only the R560/FH specimen gave a single and well-resolved diffraction band, centered at about 1.63 nm. This is probably due to the low absorption of R dyes on the FH surface, as the basal spacing values were always lower from those determined for the films of Sap. 4. Conclusions (1) The adsorption of R dyes on the surface of layered silicates depends on the properties of both the dye and the silicate. We could not prepare the films combining the silicate of high negative charge (FH) with the dyes with acidic as well as large hydrophobic groups in their molecule. However, rhodamine dyes bearing a positive charge and containing either hydrophobic or acidic groups, or none of these, were readily adsorbed on the silicate surface. (2) The formation of phases with a non-homogeneous molecular orientation of R dyes on the surface of layered silicates was observed using polarized spectroscopy. The isolated dye cations were only slightly tilted, and the tilting of dye species, which absorbed light of lowest energies, approached even lower angles. We assume that these species might include traces of the J-type molecular assemblies. Strongly tilted and almost perpendicular orientations of the cations, which formed

Bujda´k and Iyi the H-aggregates, were proven. This was observed for all dyes, independent of their molecular structure. (3) The orientation of the dye H-aggregates on a silicate surface does not depend on the presence of an acidic group or the hydrophobic character of the compound. It is rather partially controlled by the type of amino group (primary, secondary or tertiary), which probably plays a role in the suitable orientation of the molecule for effective electrostatic interaction with oxygen atoms on the siloxane surface. (4) X-ray diffraction patterns do not detect phases or reflect the orientation of the dye cations as was observed by the polarized spectra. The phases of the H-aggregates with nearly perpendicular cations and sites occupied by slightly tilted isolated cations were not distinguished by the X-ray diffraction measurements. This can be interpreted in terms of a very heterogeneous distribution of dye species on a silicate surface. One has to consider that fractions of perpendicularly oriented dye cations could have been very small. The X-ray diffraction revealed incomplete and non-homogeneous occupation of the FH surface by rhodamine dye cations. Acknowledgment. This study was supported by JSPS fellowship funds (Grant No. S-01550) for the promotion of international cooperation from the Japan Science and Technology Corporation (JST), for which we are deeply indebted. Financial support from the Slovak Grant Agency for Science VEGA (Grant No. 2/3102/23) is also acknowledged. References and Notes (1) Del Monte, F.; Ferrer, M. L.; Levy, D. Langmuir 2001, 17, 4812. (2) Vogel, R.; Harvey, M.; Edwards, G.; Meredith, P.; Heckenberg, N.; Trau, M.; Rubinsztein-Dunlop, H. Macromolecules 2002, 35, 2036. (3) Ferreira, L. F. V.; Lemos, M. J.; Reis, M. J.; do Rego, A. M. B. Langmuir 2000, 16, 5673. Gruzdkov, Y. A.; Parmon, V. N. J. Chem. Soc., Faraday Trans. 1993, 89, 4017. Meech, S. R.; Yoshihara, K. Photochem. Photobiol. 1991, 53, 627. Blonski, S. Chem. Phys. Lett. 1991, 184, 229. Morgenthaler, M. J. E.; Meech, S. R. J. Phys. Chem. 1996, 100, 3323. (4) Rao, K. D.; Sharma, K. K. Opt. Commun. 1995, 119, 132. (5) Stathatos, E.; Lianos, P.; Couris, S. Appl. Phys. Lett. 1999, 75, 319. (6) Bockstette, M.; Wohrle, D.; Braun, I.; Schulz-Ekloff, G. Microporous Mesoporous Mater. 1998, 23, 83. Hoppe, R.; Ortlam, A.; Rathousky, J.; Schulz-Ekloff, G.; Zukal, A. Microporous Mesoporous Mater. 1997, 8, 267. (7) Bujda´k, J.; Iyi, N.; Kaneko, Y.; Czı´merova´, A.; Sasai R. Phys. Chem. Chem. Phys. 2003, 5, 4680. (8) Bujda´k, J.; Iyi, N.; Sasai, R. J. Phys. Chem. B 2004, 108, 4470. (9) Hachisako, H.; Yamazaki, T.; Ihara, H.; Hirayama, Ch.; Yamada, K. J. Chem. Soc., Perkin Trans. 2 1994, 7, 1681. (10) Endo, T.; Nakada, N.; Sato, T.; Shimada, M. J. Phys. Chem. Solids 1988, 49, 1423. Grauer, Z.; Avnir, D.; Yariv, S. Can. J. Chem. 1984, 62, 1889. (11) Lo´pez Arbeloa, F.; Herra´n Martı´nez, J. M.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Langmuir 1998, 14, 4566. (12) Chaudhuri, R.; Lo´pez Arbeloa, F.; Lo´pez Arbeloa, I. Langmuir 2000, 16, 1285. (13) Lo´pez Arbeloa, F.; Martı´nez Martı´nez, V.; Banuelos Prieto, J.; Lo´pez Arbeloa, F. Langmuir 2002, 18, 2658. (14) Lo´pez Arbeloa, F.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Trends Chem. Phys. 1996, 4, 191. Lo´pez Arbeloa, F.; Tapia Este´vez, M. J.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Clay Miner. 1997, 32, 97. (15) Kikteva, T.; Star, D.; Zhao, Z.; Baisley, T. L.; Leach, G. W. J. Phys. Chem. B 1999, 103, 1124. (16) Fujita, T.; Iyi, N.; Kosugi, T.; Ando, A.; Deguchi, T.; Sota, T. Clays Clay Miner. 1997, 45, 77. (17) Elking, M. D.; He, G.; Xu, Z. J. Chem. Phys. 1996, 105, 565. (18) Baba, R.; Ishibashi, K.; Sato, O.; Hashimoto, K.; Fujishima, A. Denki Kagaku 1993, 61, 1030. Wang, D.; Wan, L. J.; Wang, C.; Bai, C. L. J. Phys. Chem. B 2002, 106, 4223. Simpson, G. J.; Rowlen, K. L. Anal. Chem. 2000, 72, 3407. Yamaguchi, A.; Uchida, T.; Teramae, N.; Kaneta, H. Anal. Sci. 1997, 13, 85. Belovolova, L. V.; Maslyanitsin, I. A.; Savranskii, V. V.; Shigorin, V. D. KVantoVaya Elektronika 1996, 23, 553. (19) Su, G. J.; Yin, S. X.; Wan, L. J.; Zhao, J. C.; Bai, C. L. Surf. Sci. 2004, 551, 204.

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