Lap

pubs.acs.org/Langmuir © 2009 American Chemical Society

On the Arrangements of R6G Molecules in Organophilic C12TMA/Lap Clay Films for Low Dye Loadings Sandra Salleres, Fernando L
opez Arbeloa,* Virginia Martı´ nez Martı´ nez, Teresa Arbeloa, and I~nigo L
opez Arbeloa Departamento de Quı´mica Fı´sica, Universidad del Paı´s Vasco UPV/EHU, Apartado 644, 48080-Bilbao, Spain Received July 6, 2009. Revised Manuscript Received September 1, 2009 Absorption and fluorescence spectroscopies with linearly polarized light are applied to characterize the adsorbed species of rhodamine 6G (R6G) laser dye in ordered organophilic laponite (Lap) clay films for low dye loadings. The organophilic character of the clay was controlled by the number of organic surfactant dodecyl-trimethylammonium (C12TMA) cations intercalated into the interlayer space of the clay. Experimental results suggest that for moderate to high surfactant contents (>70% of the total cation exchange capacity, CEC, of the clay) the accessibility of the interlayer space for R6G molecules is reduced. In these cases, the first stage in the adsorption of R6G molecules is at the external surface, where dye molecules can self-associate even for very low dye loading (around 0.1%CEC), probably for the limitation of the external surface area. The presence of a nonfluorescent H-type aggregate with a short-displaced coplanar structure and a fluorescent oblique head-to-tail J aggregate is reported. The inclined structure of this J aggregate is stabilized by the surfactant molecules at the external surface.

Introduction Surface modifications of clay minerals have received much attention in the last few decades because they can improve the design of new materials with promising applications.1,2 Organoclays are essential to developing polymer nanocomposites,3-5 and they are widely used as adsorbents of organic pollutants, as rheological control agents in paints, in cosmetics, and so forth.6-9 Several routes can be employed to modify the surface of the natural clays, but ion exchange with surfactant alkylammonium cations is the most common method of preparing organoclays.10-13 The intercalation of the surfactant molecules in smectite-type clay minerals changes the typical hydrophilic character of natural clay minerals to an organophilic environment, enhancing the adsorption ability for nonionic or apolar molecules. Indeed, organoclays have been largely *Corresponding author. Phone: +34 94 601 5971. Fax: +34 94 601 3500. E-mail: [email protected]. (1) Betega de Paiva, L.; Morales, A. R.; Valenzuela Dı´ az, F. R. Appl. Clay Sci. 2008, 42, 8–24. (2) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 95, 399–438. (3) Chowdhury, S. R. Polym. Int. 2008, 57, 1326–1332. (4) Kelarakis, A.; Kyunghwan, Y. Eur. Polym. J. 2008, 44, 3941–3945. (5) Markarian, J. Plast. Addit. Compd. 2005, 4, 18–25. (6) Lee, J. F.; Mortland, M. M.; Chiou, C. T.; Kile, D. E.; Boyd, S. A. Clays Clay Miner. 1990, 38, 113–120. (7) Zhao, H.; Vance, G. F. Water Res. 1998, 32, 3710–3716. (8) Patel, H. A.; Somani, R. S.; Bajaj, H. C.; Jasra, R. V. Bull. Mater. Sci. 2006, 29, 133–145. (9) Kemnetz, S. J.; Still, A. L.; Cody, C. A.; Schwindt, R. J. Coat. Technol. 1989, 61, 47–55. (10) Xi, Y.; Ding, Z.; He, H.; Frost, R. L. J. Colloid Interface Sci. 2004, 277, 116– 120. (11) Vaia, R. A.; Teukolsky, R. K.; Giannelis, E. P. Chem. Mater. 1994, 6, 1017– 1022. (12) Zhu, J.; He, H.; Zhu, L.; Wen, X.; Deng, F. J. Colloid Interface Sci. 2005, 286, 239–244. (13) Hongping, G.; Ray, F.; Jianxi, Z. Spectrochim. Acta, Part A 2004, 60, 2853– 2859. (14) Salleres, S.; L
opez Arbeloa, F.; Martı´ nez, V.; Arbeloa, T.; L
opez Arbeloa, I. J. Phys. Chem. C 2009, 113, 965–970. (15) Salleres, S.; L
opez Arbeloa, F.; Martı´ nez, V.; Arbeloa, T.; L
opez Arbeloa, I. J. Colloid Interface Sci. 2008, 321, 212–219. (16) Salleres, S.; L
opez Arbeloa, F.; Martı´ nez, V.; Corc
ostequi, C.; L
opez Arbeloa, I. Mater. Chem. Phys. 2009, 116, 550–556.

930 DOI: 10.1021/la902414n

used as supporting host materials for embedding photofunctional molecules.14-20 In this way, the organoclay acts as an organic-inorganic hybrid solid matrix for the incorporation of dye molecules with potential applications in the design of new photonic or photoelectronic devices and in nonlinear optics. Most of these applications require macroscopic arrangements of the photoactive molecules in the host material, and many efforts have been focused in the past few years on producing oriented guest architectures.1,21-27 The adsorption of rhodamine 6G (R6G) dye, probably the most used laser dye, in unmodified laponite (Lap) clay favors the formation of H-type aggregates for moderate to high loadings of the dye.28-31 These H-type aggregates are nonfluorescent or weakly fluorescent and act as efficient quenchers for the emission of monomers, drastically reducing by more than 3 orders of (17) Sasai, R.; Itoh, T.; Ohmori, W.; Itoh, H.; Kusunoki, M. J. Phys. Chem. C 2009, 113, 415–421. (18) Lucia, L. A.; Yui, T.; Sasai, R.; Takagi, S.; Takagi, K.; Yoshida, H.; Whitten, D. G.; Inoue, H. J. Phys. Chem. B 2003, 107, 3789–3707. (19) Bujdak, J.; Iyi, N. Chem. Mater. 2006, 18, 2618–2624. (20) Matsuoka, R.; Yui, T.; Sasai, R.; Takagi, K.; Inoue, H. Mol. Cryst. Liq. Cryst. 2000, 341, 333–338. (21) L
opez Arbeloa, F.; Martı´ nez Martı´ nez, V. J. Photochem. Photobiol., A 2006, 181, 44–49. (22) L
opez Arbeloa, F.; Martı´ nez Martı´ nez, V. Chem. Mater. 2006, 18, 1407– 1417. (23) Martı´ nez Martı´ nez, V.; Salleres, S.; Ba~nuelos, J.; L
opez Arbeloa, F. J. Fluoresc. 2006, 17, 233–240. (24) F. L
opez Arbeloa, F.; Martı´ nez Martı´ nez, V.; Arbeloa, T.; L
opez Arbeloa, I. J. Photochem. Photobiol., C 2007, 8, 85–105. (25) Bujdak, J.; Iyi, N.; Kaneko, Y.; Czı´ merov
a, A.; Sasai, R. Phys. Chem. Chem. Phys. 2003, 54, 680–4685. (26) Sasai, R.; Fujita, T.; Iyi, N.; Itoh, H.; Takagi, K. Langmuir 2002, 18, 6578– 6583. (27) Sonobe, K.; Kikuta, K.; Takagi, K. Chem. Mater. 1999, 11, 1089–1093. (28) Martı´ nez Martı´ nez, V.; L
opez Arbeloa, F.; Ba~nuelos Prieto, J.; Arbeloa L
opez, T.; L
opez Arbeloa, I. Langmuir 2004, 20, 5709–5717. (29) Martı´ nez Martı´ nez, V.; L
opez Arbeloa, F.; Ba~nuelos Prieto, J.; Arbeloa L
opez, T.; L
opez Arbeloa, I. J. Phys. Chem. B 2004, 108, 20030–20037. (30) Martı´ nez Martı´ nez, V.; L
opez Arbeloa, F.; Ba~nuelos Prieto, J.; L
opez Arbeloa, I. Chem. Mater. 2005, 17, 4134–4141. (31) Martı´ nez Martı´ nez, V.; L
opez Arbeloa, F.; Ba~nuelos Prieto, J.; L
opez Arbeloa, I. J. Phys. Chem. B. 2005, 109, 7443–7450. (32) L
opez Arbeloa, F.; Ruiz Ojeda, P.; L
opez Arbeloa, I. Chem. Phys. Lett. 1988, 148, 253–258.

Published on Web 09/18/2009

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Scheme 1. Experimental Setup to Record the Polarized Absorption (Left) and Fluorescence (Right) Spectra as a Function of the Twisted Angle δ of the Sample around Its Vertical Axis

magnitude the fluorescence capacity of R6G/Lap systems.32,33 Recently, we have demonstrated that the intercalation of surfactant dodecyl-trimethyl-ammonium (C12TMA) cations in Lap clay not only increases the fluorescence ability of R6G monomers,14-16 with maximal fluorescence efficiency for moderate to high C12TMA systems (in the 70-120% CEC, percentage of the total cation exchange capacity of the clay), but also reduces the tendency of R6G to self-associate. Indeed, the dimerization constant KD of R6G in Lap films with an initial moderate C12TMA content of 70% CEC (KD =0.00616) is nearly 1 order of magnitude lower than that of pure Lap films (KD =0.04329). Besides, the presence of surfactant favors the formation of fluorescent J-type aggregates against weakly fluorescent H-type aggregates. These behaviors are similar to those reported in the liquid state, for instance, when water is changed by ethanol as a solvent,32,33 and they are ascribed to modifications of the environmental character from hydrophilic to hydrophobic ambience. Therefore, the incorporation of surfactant molecules in clay systems is a good strategy for improving the general fluorescence capacity of dyes intercalated into the interlayer space of clay systems or any other host material. It was previously demonstrated that the elaboration of supported Lap films by the spin-coating technique is a good method for designing clay films with a macroscopic organization of stacked layers in a parallel arrangement.22-24 Such a disposition creates a well-defined 2D environment for the encapsulation of dye molecules with a macroscopic ordered arrangement.21-23,30 Fortunately, the parallel stacking of the clay layers is maintained after the intercalation of surfactant molecules.14 For this reason, the posterior incorporation of R6G monomers (reported in clay films with very low dye loading) leads to the spontaneous macroscopic orientation of the dye molecules with a preferential inclination with respect to the normal to the films. Thus, R6G monomers adopt an orientation angle of around 62
with respect to the layer normal for a pure Lap film or for an organophilic Lap (organoLap) film with low C12TMA content (around 30% CEC).14,16 However, by increasing the surfactant content, the monomers are disposed toward the perpendicular of the Lap films, with preferential orientations of up to 45
for Lap films with high C12TMA contents (>120% CEC). These angles were reported for experimental data obtained at the maximal intensity of the monomer absorption and fluorescence bands. However, there is some experimental evidence (spectral shifts, bandwidth and shape, fluorescence efficiencies and lifetimes, and dichroic ratios) that could suggest the presence of different R6G species in these organoLap films, even for a very low dye concentration (33) L
opez Arbeloa, F.; Urrecha Aguirresacona, I.; L
opez Arbeloa, I. Chem. Phys. 1989, 130, 371–378.

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(around 0.1% CEC).14 The aim of the present work is to gain insight into the characterization of R6G species adsorbed for a very low dye loading (around 0.1% CEC) in organophilic C12TMA/Lap films with different surfactant contents by absorption and fluorescence spectroscopy with linearly polarized light.

Experimental Section The sodium form of laponite B (Lap) clay was supplied by Laporte Industries Ltd. and was used as received. This synthetic clay mineral is characterized by its high chemical purity and small particle size (70% CEC, OL3-OL5). This last observation would suggest the presence of at least two molecular species of R6G in Lap films for a very low R6G loading and moderate to high C12TMA content. Langmuir 2010, 26(2), 930–937

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Article Table 1. Orientation Angle ψ (Estimated Error (1
) of the Transition Dipole Moment of R6G with Respect to the Clay Layer Normal for Several Organophilic C12TMA/Lap Films Obtained by Absorption and Fluorescence with Linearly Polarized Light at Different Analysis Wavelengthsa absorption (deg) sample

fluorescence (deg)

505 nm 525 nm 550 nm 560 nm 590 nm 640 nm

R6G/Lap 62 R6G/OL1 62 R6G/OL2 58 R6G/OL3 42 48 41 R6G/OL4 39 45 34 R6G/OL5 41 46 36 R6G/OL (calcined) 41 46 39 a Data obtained in pure Lap21,30 and in a included for comparison.

60 60 58 47 46 46 40 calcined

44 35 41 33 39 33 39 33 film are also

Figure 2. Absorption dichroic ratio of R6G adsorbed in the OL3 organophilic Lap film as a function of the twisting angle δ. The inset represents the fitting of eq 1 at three representative wavelengths: (a) 505, (b) 525, and (c) 550 nm.

The aim of the present work is to gain insight into the evolution of the absorption and emission dichroic ratios of R6G for a very low loading in organophilic Lap films as a function of the surfactant content. Figure 1 shows the evolution of the absorption band with incident H- and V-polarized light with the twisting angle δ of R6G in the moderate C12TMA content (70% CEC) OL3 organoLap film. The V-polarized absorption band does not show any important evolution with the twisting angle (Figure 1B). This is a consequence of the isotropic behavior of the dye molecules in the plane of the clay layers. Indeed, all orientations of adsorbed R6G molecules in the clay plane are equally probable.30 This isotropic behavior confirms the ordered parallel stacking of the Lap particles in the supported films, preserving the isotropic behavior of the individual clay layers in the macroscopic arrangement. However, the global absorbance of the H-polarized absorption band increases with the twisting angle of the film (Figure 1A), together with a bathochromic shift of the main absorption band. This result contrasts with those previously reported for R6G at very low dye loading in a pure Lap film30 and in the OL1 organoLap film with a low C12TMA content of 30% CEC.14 In those cases, the absorbance of the H-polarized absorption band decreased by increasing the twisting angle δ without any appreciable displacement of the main absorption band. These different behaviors indicate that the preferential orientation of R6G molecules in Lap films depends on the surfactant content. The evolution of the absorption dichroic ratio (DHV(ab), obtained from the ratio of the H-polarized over the V-polarized absorption spectra, DHV(ab)
AH/AV) with the twisting angle δ of the R6G/OL3 film is shown in Figure 2. Depending on the absorption wavelength,a different evolution of DHV(ab) versus δ can be reported. These results suggest the presence of several R6G entities in the organoLap film with moderate C12TMA content, even for the very low dye loading as in the present sample (∼ 0.1% CEC). This was not the case for R6G in the pure Lap film30 and for the low C12TMA content (30% CEC) OL1 film,14 where the dichroic ratio did not show any appreciable dependence on the absorption wavelengths (see Figures 2 and 5 in refs 30 and 14, respectively, for further details), indicating the presence of a unique R6G species. This absorbing species was assigned to R6G monomers not only because of the very low dye concentration in the films but also because of the shape of the absorption band obtained with unpolarized light. Langmuir 2010, 26(2), 930–937

Figure 3. Absorption dichroic ratio of the R6G/OL4 (120% CEC) organophilic Lap film showing a more prominent evolution in the three different spectral regions: (a) 505 nm, (b) 525 nm, and (c) 550 nm (inset).

From the evolution of the dichroic ratio with the twisting angle for the R6G/OL3 film (Figure 2), at least three absorption regions can be considered: at around 525 nm; close to the monomer absorption maximum, with a relative low increase in the DHV(ab) value by increasing the twisting angle δ; and at around 505 and 550 nm, on both sides of the monomer absorption peak, with a relatively prominent increase in DHV(ab) with the twisting angle δ. These two last absorption regions are generally identified with H-(505 nm) and J-type (550 nm) absorption bands of R6G aggregates, although the presence of this kind of R6G species in the present system as a result of the very low concentration of the dye is surprising. According to eq 1, the orientation angle ψ of the transition moment of these R6G entities with respect to the normal of the clay films can be estimated from the slope of the linear relationship of DHV(ab) versus sin2 δ. The results for the three representative absorption wavelengths of the R6G/OL3 film are presented in Figure 2. The good correlation (with correlation coefficients r > 0.998) observed in these linear representations confirms that the different absorbing R6G species are actually adsorbed with a preferential orientation with respect the normal to the film. From the slope of these linear relationships, orientation angles of 42
(505 nm), 48
(525 nm), and 41
(550 nm) are obtained for the R6G/OL3 films (Table 1). It should be noted that these values are obtained directly from the experimental data without separating the contributions of individual components in the recorded absorption spectra. For instance, the evolution of the dichroic DOI: 10.1021/la902414n

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Figure 4. Corrected (A) H- and (B) V-polarized emission spectra after excitation at 495 nm with unpolarized light of the R6G/OL3 film as a function of the twisting angle δ of the sample around its vertical axis. Scheme 2. Possible Arrangements of Monomers in the Clay Layer to Reach Different Dimersa

a

(A) Twisted sandwich dimer, (B) displaced coplanar dimer, (C) “lain” oblique head-to-tail dimer, and (D) “inclined” oblique head-to-tail dimer. The orientations of the transition moments with respect to the clay layer (wide arrows) are also included.

ratio at the monomer absorption maximum (around 525 nm) could be affected by the corresponding evolutions at the other representative absorption wavelengths (around 505 and 550 nm) (Figure 2). In any case, the changes induced by the C12TMA loading are evident considering that the R6G monomers are disposed toward the clay layers with an orientation angle of ψ ≈ 62
for pure Lap and low surfactant Lap films (i.e., OL1 and OL2) (Table 1). The wavelength dependence of the R6G absorption dichroic ratio observed in the moderate organophilic OL3 film is confirmed from experimental results obtained for highly organophilic C12TMA/Lap films, with surfactant contents of 120% CEC (OL4, Figure 3) and 170% CEC (OL5, Figure SI1 in Supporting Information). Even in these latter cases, the evolutions of DHV(ab) with twisting angles δ at 505 and mainly at 550 nm are more prominent. In fact, the orientation angles, obtained from the linear correlation of DHV(ab) versus sin2 δ for the OL4 film are (inset of Figure 3) 39
(505 nm), 45
(525 nm), and 34
(550 nm) (Table 1). Similar results are obtained for the most organophilic OL5 film (inset of Figure SI1), with orientation angles of 41
(505 nm), 46
(525 nm), and 36
(550 nm). All of these results clearly demonstrate that the orientation of the R6G species with respect to the clay normal decreases by increasing the content of surfactant C12TMA molecules in the interlayer space of Lap films: from a ψ angle of 62
for R6G monomers in a pure Lap film or a low C12TMA content Lap film (120% CEC, OL4, and OL5) (Table 1). All of these results contrast with those reported for Sasai et al.17 in which a parallel disposition of the R6G molecules with respect to the clay layers of CnTMA (with n = 12 and 16) Lap films was proposed from XRD data analysis: the interlayer space of CnTMA/Lap films with a surfactant content of up to 170% 934 DOI: 10.1021/la902414n

CEC did not increase after the incorporation of a small number (0.5% CEC) of R6G molecules. However, the application of the XRD technique to evaluate the inclination angle of guest molecules in a clay system has been previously questioned, mainly in those systems in which the macroscopic recorded interlayer space could not strongly affected by the incorporated guest molecules. Indeed, the recorded XRD data of organophilic CnTMA/Lap films with very high surfactant contents will not be affected by the presence of small amounts of R6G molecules, independently of the orientation of the intercalated dye molecules. Two absorption H- and J-type bands for dye aggregations could be ascribed to two different aggregates (i.e., a shortdisplaced θ > 54.7
coplanar H-type and a long-displaced θ < 54.7
coplanar J-type aggregate, shown Scheme 2B) or to an aggregate with two absorption bands (i.e., a twisted sandwichlike aggregate, shown in Scheme 2A, or an oblique headto-tail aggregate, shown in Scheme 2C), as is proposed by exciton theory.36,37 In the later cases, the transition dipole moments associated with both bands (shown in Scheme 2 by wide arrows) should be orthogonal to each other, but their orientations with respect to the film normal would depend on the molecular arrangement of the aggregates in the clay layers. For instance, sandwich dimers in which the chromophoric rings of the monomers are disposed parallel to the clay layers would induce transition dipole moments oriented within the clay plane without any out-of-plane component. However, the transition dipole moments of oblique head-to-tail aggregates would present complementary angles with respect to the clay layers. Because the orientation angles οbtained at the H-type (ψ ≈ 40
at 505 nm, (36) McRae, E. G.; Kasha, M. Physical Process in Radiation Biology; Academic Press: New York, 1964. (37) Kasha, M.; Rawls, H. R.; El-Bayouni, M. A. Pure Appl. Chem. 1965, 11, 371–392.

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Figure 5. Evolution of the emission dichroic ratio of the R6G/ OL3 sample as a function of the twisting angle of the film. The inset shows the fitting at the three representative wavelengths: (a) 560 nm, (b) 590 nm, and (c) 640 nm.

Table 1), and at the J-type (ψ ≈ 37
at 550 nm) absorption bands in the present work are not orthogonal each other, both absorption bands should therefore be ascribed to two different aggregates. The nature of these two absorbing species is discussed below. For deeper insight into the adsorbed R6G species with very low dye content in organoLap films with moderate to high C12TMA content (>70% CEC), fluorescence spectra with polarized light are now analyzed. Figure 4 shows the evolution of the fluorescence spectra of the R6G/OL3 film (70% CEC surfactant content) with H and V polarization after excitation with unpolarized light at 495 nm. The intensity of the V-polarized emission spectra augments by twisting the film around its vertical axis, without any noticeable change in the shape of the fluorescence band (Figure 4B). These results are similar to those observed for very low R6G loading in a pure Lap film23 and in the low surfactant content (30% CEC) OL1 film.14 However, the intensity of the Hpolarized emission spectra of the R6G/OL3 film drastically increases with the twisting angle δ for emission wavelengths longer than the monomer fluorescence band centered at 560 nm (Figure 4A), shifting the emission band to lower energies. This evolution contrasts with that reported for R6G in a pure Lap film and a low organophilic OL1 film, where a progressive decrease in the fluorescence intensity in the global fluorescence band was observed. (See Figures 5 and 6 in refs 20 and 14, respectively, for further details.) The present evolution is better confirmed for organoLap films with high C12TMA contents. (See Figure SI2 and SI3 for OL4 and OL5 films, respectively.) The isoemissive point at around 540 nm for H-polarized emission spectra is due to the residual emission of a fluorescent impurity in C12TMA. This was experimentally confirmed by means of an organoLap film without R6G dye, which presents residual emission at around 520 nm for the broad excitation and emission slits used in the present work. This residual emission was corrected in the evaluation of the fluorescence dichroic ratio. The fluorescence dichroic ratio (evaluated from the ratio between the fluorescence intensity for H- and V-emission polarizations, DHV(fl)
IH/IV) increases with the twisting angle δ for the moderate to high organophilic Lap films. (See Figure 5 for the specific case of the R6G/OL3 film and Figures SI2 and SI3 for the R6G/OL4 and R6G/OL5 films, respectively.) The extension of this evolution, however, depends on the emission wavelength. These results confirm the presence of different fluorescent R6G Langmuir 2010, 26(2), 930–937

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species in moderate to high organophilic Lap films, which are characterized above by polarized absorption spectroscopy. According to eq 2, DHV(fl) values present good linear correlations with cos2(22.5 þ δ) for the three representative emission wavelengths, as shown in the inset of Figure 5 for the specific case of the R6G/OL3 film, from which the orientation angles ψ of the emitting species can obtained: 47
(560 nm), 44
(590 nm), and 35
(640 nm). The angles derived for other organoLap films are included in Table 1, providing orientation angles of 46
(560 nm), 39-41
(590 nm), and 33
(640 nm) for the highest C12TMA contents (>120% CEC) of Lap films (OL4 and OL5 samples). The orientation angles reported from the fluorescence data agree with those obtained from absorption with polarized light (Table 1). For instance, the orientation angle ψ ≈ 35
obtained for the absorption J-band at 550 nm for high organophilic Lap films (>120% CEC, OL4, and OL5) is consistent with the value ψ= 35
obtained from the fluorescence band at 640 nm for these films (Table 1). The orientation angles evaluated for R6G monomers in organoLap films by absorption and emission polarization are also coincident (Table 1). Taking into account previous results on the adsorption of R6G in hydrophilic pure Lap29,31 and organophilic C12TMA/Lap films,16 the presence of absorption bands on both sides of the main absorption band of R6G monomers and new emitting species at longer wavelengths in moderate to high C12TMA content Lap films can be assigned to the presence of R6G aggregates, even for the very low dye concentration used in the present work. These surprising results can be explained as follow. Because R6G is more soluble in an organic solvent such as ethanol than in water, one should expect a higher tendency of R6G to be adsorbed in organophilic C12TMA/Lap films than in hydrophilic pure Lap films. This was experimentally observed for low C12TMA content Lap films (70% CEC), the opposite effect was observed, decreasing the R6G absorbance by increasing the surfactant concentration in the order of OL3 > OL4 > OL5. Consequently, there is an optimal C12TMA content for the maximal adsorption of R6G. Probably for low to moderate surfactant content Lap films, there is enough free space to adsorb dye molecules and the organophilic environment in the interlayer space favors the intercalation of the dye. However, for high surfactant content Lap films, the free space to accommodate dye molecules in the interlayer space of the clay is drastically reduced and dye molecules would probably be heterogeneously adsorbed in the most accessible adsorption surfaces of the clay particles as a cluster, leading to dye aggregation. To corroborate this interpretation, we have proceeded to collapse the accessibility of the interlayer space of the Lap particle by the calcination of Lap films in a oven at 550
C for 2 h. The posterior adsorption of C12TMA surfactant and R6G dye molecules does not shift the 001 DRX peak to lower diffraction angles, indicating the inaccessibility of the interlayer space for organic compounds in the calcined film. Therefore, C12TMA and R6G molecules are adsorbed at the external surface of the calcined film. Figure 6 (bottom) shows the H-polarized absorption (left) and fluorescence (right) spectra of R6G adsorbed in a calcined Lap film with surfactant molecules. The evolution of these spectra with the twisting angle δ is qualitatively similar to that observed for noncalcined organoLap films with high C12TMA content: an increase in the absorbance for the global absorption band with a bathochromic shift by increasing the twisting angle and DOI: 10.1021/la902414n

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Figure 6. Evolution of the corrected H-polarized absorption (bottom, left) and fluorescence (bottom, right) spectra as a function of the twisting angle of the film for R6G molecules coadsorbed with the C12TMA molecules at the external surface of a calcined pure Lap film. The upper panels represent the corresponding dichroic ratios.

augmentation/diminution of the emission intensity at longer/ shorter wavelengths with an isoemissive point at around 540 nm. The absorption dichroic ratio illustrates the three different evolution regions, at 505, 525, and 550 nm, with the twisted angle (Figure 6, top left), whereas the fluorescence dichroic ratio shows the isoemissive point at around 540 nm (Figure 6, top right). The orientation angles derived from the corresponding linear representation of DHV(ab) versus sin2 δ or DHV(fl) versus cos2(22.5 þ δ) (inset of Figure 6, top) are included in Table 1. These orientation angles are similar to those obtained for noncalcined R6G/OL4 and R6G/OL5 films; consequently, we can conclude that the anisotropic behaviors of R6G in organoLap films with high C12TMA content (>70% CEC) are mainly a consequence of externally adsorbed dye species. Because in solid films the available external surface of Lap clays should be very low, dye molecules should be forced to be adsorbed close to each other, favoring dye aggregation. Time-resolved fluorescence spectroscopy confirms the existence of fluorescent J-type aggregates of R6G in moderate to high organophilic OL3-OL5 films with very low dye content. Indeed, it was previously reported that fluorescent J-aggregates of R6G in pure Lap films are characterized by a longer lifetime (τ > 10 ns) than that of unquenched R6G monomers (τ ≈ 4 ns).31 Fluorescence decay curves of R6G in the moderate OL3 films are analyzed as up to three exponential decays with short (τ1 ≈ 0.5 ns), moderate (τ2 ≈ 2.5 ns), and long (τ3 ≈ in the 7-13 s range) fluorescence lifetimes. (See Tables SI2 and SI3 in the Supporting Information for further details.) The longer lifetime is observed to be dependent on the emission wavelengths: with values from τ3= 7-13 ns at 550 nm to τ3 = 13-21 ns at 590 nm. These results clearly show that the R6G species in OL films emitting at longer wavelengths (i.e., J-type aggregates) have an associated long fluorescence lifetime. Indeed, the intensity average lifetime of 936 DOI: 10.1021/la902414n

the R6G/OL3 films changes with the emission wavelength from Æτæia=5.9 ns at 550 nm to Æτæia=9.1 ns at 590 nm (Table 2). Similar results are obtained for R6G in OL4 and OL5 and in the calcined C12TMA/Lap films, suggesting that the longer-wavelength-emitting species of R6G at the external surface of organoLap films can be assigned to J-type aggregates. The transition dipole moment of these aggregates should be orientated with an angle of ψ ≈ 35
with respect to the layer normal (Table 1). Three different geometrical arrangements of monomers can provide J-type fluorescent dyes (Scheme 2),36,37 oblique head-totail aggregates (Scheme 2C,D), and long-displaced θ < 54.7
coplanar displaced aggregates (Scheme 2B). Short-displaced coplanar aggregates are not fluorescent and induce H-type absorption bands, for instance, the band observed at 505 nm in the absorption spectra of R6G in OL3-OL5 and calcined C12TMA/Lap films. However, long-displaced coplanar aggregates are potentially fluorescent and induce J-type absorption and emission bands. When these long-displaced coplanar aggregates are adsorbed in clay layers, they require orientation angles of ψ (= 90-θ) > 35.3
with respect to the layer normal. This limiting angle is that observed from the J-type fluorescent band at around 640 nm for R6G in OL4 and OL5 and calcined C12TMA/ Lap films (Table 1), but at this angle the emission wavelength of the J aggregates should be similar to that of monomers.36,37 Consequently, the long-wavelength J emission with an orientation angle of ψ ≈ 35
with respect to the normal layer should be assigned to oblique head-to-tail aggregates. Two arrangements of monomers in the clay layer can provide oblique head-to-tail dimers: the so-called “lain” oblique head-totail dimer, in which the second monomer unit is folded toward the clay layer (Scheme 2C), and the so-called “inclined” oblique headto-tail dimer, in which the second monomer unit is folded out of the layer plane (Scheme 2D). Both aggregates would imply Langmuir 2010, 26(2), 930–937

Salleres et al.

Article

Table 2. Intensity Average Lifetimes Obtained at 550 and 590 nm for R6G in the organoLap Samples with Very Low Dye Loadinga sample

Æτæia 550 nm (ns)

Lap OL1 OL2 OL3 OL4 OL5 a Lifetimes of the pure comparison.

Æτæia 590 nm (ns)

Scheme 3. Inclined Head-to-Tail Dimer Adsorbed at the External Surface of the Clay Layer Stabilized by the Coadsorbed C12TMA Surfactant

4.2 4.5 4.8 5.9 5.2 7.3 5.9 9.1 5.7 7.4 5.6 7.3 R6G/Lap sample are also included for

orthogonal transition dipole moments but with different orientations with respect to the clay layer. In the former dimer, the transition moment of the J-type absorption and fluorescence bands is oriented parallel to the clay plane whereas the second transition moment (that responsible for the H-type absorption band) is disposed perpendicular to the layer plane. Such an aggregate cannot explain the present results. However, the inclined head-to-tail dimer is the only aggregate that can explain J-type absorption and fluorescence bands with an orientation angle of ψ < 54.7
, as is reported in the present work. This inclined head-to-tail dimer is observed only for R6G at the external surface of Lap films with surfactant molecules. We have determined that this aggregate is not formed at the external surfaces of hydrophilic pure Lap films (calcined Lap films without any posterior incorporation of surfactant molecules). Indeed, the anisotropic behavior of R6G in this calcined Lap films (data not shown) is totally different from that shown in Figure 6 for R6G in the calcined C12TMA/Lap film. Consequently, inclined oblique head-to-tail aggregates of R6G are reported only at the external surfaces of Lap surfaces with coadsorbed surfactant molecules. Taking into account the geometrical disposition of the second monomer unit with respect to the clay surface, it is difficult to give a physical interpretation for the formation of such kind of aggregates. A tentative explanation is that at the external surface of clay layers (where there are no geometrical restrictions as in the case of the restricted interlayer space) the adsorbed amphiphilic molecules can adopt the most stable molecular arrangement, for instance, that in which the cationic heads are close to the clay surface and the long alkyl chains oriented out of the surface. Under these conditions, R6G molecules self-associate to reach the most stable configuration, i.e. that of an inclined oblique head-totail dimer sandwiched between two surfactant molecules, as illustrated in Scheme 3. Indeed, the presence of surfactant molecules at the external surfaces of Lap films could drastically reduce the free surface area of Lap layers available for R6G molecules, avoiding the formation of lain head-to-tail dimers along the clay surface. Hydrophobic forces can also play an important role because the geometry of R6G aggregates in liquid solution show an important dependence on the nature of the

Langmuir 2010, 26(2), 930–937

solvent: head-to-tail aggregates of R6G have been reported in liquid ethanol, whereas in water R6G self-associates as coplanar sandwichlike dimers.32,33

Conclusions Absorption and fluorescence spectroscopy with linearly polarized light can be used not only to evaluate the orientation of absorbing and emitting dye species adsorbed in ordered clay films, or any other organized 2D framework, but also to characterize adsorbed species mainly those species adsorbed with a preferential orientation toward the perpendicular of the clay layer. High surfactant loadings can collapse the interlayer space of the clay to adsorb fluorescent cationic dyes such as R6G, at least for low dye loadings as in the present work. Consequently, dye molecules would be adsorbed at the most accessible external surfaces of the clay particles. The presence of surfactant at these surfaces would favor the formation of fluorescent inclined head-to-tail aggregates of R6G, probably stabilized by the disposition of the surfactant molecules at the external surface, and nonfluorescent shortdisplaced coplanar aggregates. Acknowledgment. The University of the Basque Country UPV/EHU is thanked for financial support (research project GIU06/80). S.S. also thanks the UPV/EHU for a fellowship. Supporting Information Available: Synthesis conditions in the elaboration of the five organoLap films and fluorescence lifetimes and preexponential factors of R6G in the five OL films with very low dye loading. Evolution of the absorption dichroic ratio of the R6G/OL5 sample and the fluorescence anisotropy of the R6G/OL4 and R6G/OL5 films. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la902414n

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