Preparation, Photophysical Characterization, and Modeling of LDS722

Article pubs.acs.org/Langmuir

Preparation, Photophysical Characterization, and Modeling of LDS722/Laponite 2D-Ordered Hybrid Films Nerea Epelde-Elezcano, Eduardo Duque-Redondo, Virginia Martínez-Martínez,* Hegoi Manzano,* and Iñigo López-Arbeloa Molecular Spectroscopy Laboratory, University of the Basque Country UPV/EHU, Apartado 644, 48080 Bilbao, Spain ABSTRACT: A novel hybrid material with promising optical properties for nonlinear optical applications is presented, as formed by LDS 722 organic dye confined in Laponite clay. Thin films of the hybrid material with different dye loadings have been prepared. The film thickness, the dye and water content, and the clay swelling due to guest molecule incorporation have been characterized. Then, the photophysical properties of the thin films have been studied in detail using experimental methods and molecular simulation. As the dye load increases, the hybrid films present a hypsochromic shift in absorption and a bathochromic shift in emission. The former is attributed to the increasing strength of solvation of the dye donor group, while the latter is ascribed to a switch from an intramolecular to an intermolecular charge-transfer process as the dye load increases. The LDS 722 molecules are preferentially oriented in the host clay almost in parallel to the platelet surfaces, inducing macroscopic order that makes the material responsive to polarized light.

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INTRODUCTION The incorporation of dyes into an ordered inorganic host is a good tactic for achieving materials with improved optical properties.1−11 Many benefits arise from the synergy between the inorganic host and organic guests: the host matrix provides thermal, chemical, and photoprotection to the guest molecule, a nanostructured template that can induce the preferential orientation of the dye molecules, and a confined environment that can improve their photophysical properties by the reduction of internal motions and/or increasing molecular planarity. In this sense, layered-structure materials are important matrices in designing highly ordered bidimensional films.6,12 In particular, clay minerals, characterized by their ionic exchange capacity and expansible interlayer spaces, have been broadly used as hosts in organic/inorganic composite materials. Dye/clay ordered systems in the solid state are easily obtained.13 On the one hand, very transparent clays films with a parallel disposition of the clay layers are successfully elaborated by spin coating. On the other hand, after the intercalation process, the cationic guests can spontaneously accommodate a preferential orientation with respect to the plane of the clay films. Generally, the self-organization of the guest molecules is mainly controlled by the balance between clay−guest and guest−guest interactions. In other words, the final arrangement will depend on the type of clay (different number, distance, and location of the layer negative charge), the molecular structure of the dye guest, and the dye loading.6,14−16 As a consequence of the molecular alignment in a macroscale domain, the film could present an anisotropic photoresponse © 2014 American Chemical Society

with respect to the plane of the linearly polarized light, which is interesting due to the associated nonlinear optical processes. In particular, this hybrid material might give rise to third-order NLO properties due to the restriction of molecular orientation from a 3D random orientation in the bulk to a 2D random orientation in the clay.17−19 For the practical implementation of the material, a high optical density and/or fluorescence efficiency is desired as well. However, very limited cases have been successful.17,20−23 There are a number of reasons that these materials may fail: (1) the high tendency of dye molecules to self-associate in the adsorbed state, even at low loadings, due to a high local concentration at the surfaces, which drastically decreases the fluorescence emission;24−29 (2) the rigid environment of the clay surface, which could induce a distortion of the dye structure;30 and (3) the dye’s tendency to intercalate within the clay with its dipole transition moments in a centrosymmetric distribution. In this work, the incorporation of LDS 722 dye into the interlayer space of Laponite clay films is studied (Figure 1). Laponite clay is chosen as a host material because it offers films with high optical transparency (required for most of the optoelectronic applications) and a high capacity to adsorb cationic dyes. On the other hand, LDS 722 is selected as a guest because it is a positively charge dye with a high molecular hyperpolarizability due to its push−pull flexible structure (amino donor group on one end and ethyl-pyridinium acceptor group on the other end). However, it crystallizes in a Received: May 29, 2014 Revised: July 23, 2014 Published: August 4, 2014 10112

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EXPERIMENTAL AND COMPUTATIONAL METHODS

Materials. The sodium form of Laponite (Lap) clay was supplied by Laporte Industries Ltd. This clay is characterized by its high purity, and it was used as received. Its cation-exchange capacity (CEC) is 73.3 mequiv/100 g.34 Laser-grade styryl 722 dye (LDS 722) was purchased from Exciton and was used without further purification. Dye/Clay Films. Supported thin solid films of Lap were prepared by the spin-coating technique (BLE spinner, model Delta 10). A few drops of a well-dispersed 1.5% w/w suspension in water were extended on a glass plate, and the clay films were obtained after two consecutive spinning steps: the first at 500 rpm during 30 s and the second at 2500 rpm during 60 s. After the spinning process, the clay films were dried in an oven at 60 °C overnight. The incorporation of dye molecules into Lap clays was conducted by the immersion of the films in LDS 722 aqueous/ethanol solution (v/v of 50/50%). The dye loading was modified by changing the immersion time (from 5 min to 48 h) and the concentration of the dye solution (10−5 to 10−4 M). After the adsorption of the dye, the films were rinsed with ethanol and water and were dried in an oven at 40 °C overnight. Instrumental. The interlayer space of the elaborated dye−clay films was determined by X-ray diffraction (Philips, model PW 1710). Elemental CHN analysis (Euro EA 3000 series elemental analyzer) was used to quantify the amount of dye incorporated. Thermogravimetric analysis TGA (Mettler Toledo TGA/SDTA 851E) was used to estimate the water content. The thickness of the films was obtained by profilometry (Alpha-step D100) at the center of the thin films. Absorption and fluorescence spectra of LDS 722-Lap films were recorded by means of a UV/vis spectrophotometer (Varian, model Cary 4E) and a fluorimeter (SPEX, model Fluorolog 3-22) equipped with double monochromators in the excitation and emission channels, respectively. All absorption spectra were registered in transmittance mode from 350 to 750 nm. The dispersion of incident light by the clay films was corrected by placing a clay film without dye in the reference beam. The emission spectra were recorded from 520 to 840 nm, after excitation at 470 nm, in the front face mode by detecting the emitted light at 22.5° with respect to the incident beam. Absorption spectra with linearly polarized light were recorded using a Glam-Thompson polarizer in the incident (absorption) beams. The anisotropy study was performed by recording the absorption spectra for horizontal (X axis, AX) and vertical (Y axis, AY) polarized light with respect to the incident beam (Z axis) for different orientations of the sample with respect to the incident light by twisting the supported film around its Y axis at different δ angles, as is described in ref 35. The dichroic ratio (DX,Y), obtained from the relation between both spectra (DX,Y = AX/AY), was evaluated for δ angles from 0 up to 80°. The

Figure 1. Schematic representation of the (a) Laponite clay and (b) LDS 722 dye atomic structures. In Laponite, Si atoms are represented in yellow, Na in orange, Mg in blue, Li in green, O in red, and H in white. In LDS 722 dyes, the C atoms are in black, the N atoms are in blue, and the H atoms are in white.

centrosymmetric fashion, and it shows poor fluorescence in solution due to the flexibility around the polymethene chains connecting donor−acceptor aromatic rings.31,32 In addition, LDS 722 dye has a very large Stokes shift (≥3000 cm−1), with fluorescence emission in the NIR and a low tendency to aggregate.33 It minimizes inner filter effects, especially in highoptical-density samples required for optical applications. With the incorporation of this dye into a Lap film a fluorescent solid material with an anisotropic response to the linear polarized light was desired. The hybrid material was prepared, and its structure and photophysical properties were characterized by experimental methods. Molecular simulations were performed as well to gain insight into the host/guest interactions and help in the interpretation of the experimental results. The inclusive approach to studying the hybrid material provided us very valuable information, essential for the future development of dye/clay films with potential in NLO applications.

Table 1. Data of LDS 722/Lap Films with Different Dye Loadingsa d001 (Å) time

% CEC

water (%)

exp

0 2 h1 9 h1 24 h1 2 h2 5 h2

0 4 10 23 29 45 v/v %

16 12.5

12.9 13.3 13.6 14.3 14.9 15.0

10 5

sim 13.2 14.1 15.2

0/100 50/50 100/0

λab (A) 494 479 479 479 475 λab 446 487 499

(0.1) (0.27) (0.61) (0.80) (1.23)

λfl (% ϕ)

ψ

685 (6.4) 723 (1.3) 727 (1.3) 733 (0.5) 737 (0.3) λfl (%ϕ)

64 62 61 59 58

711 (1.2) 713 (8.0) 714 (14)

Time, immersion time in hours (1, into dye solution 10−5 M and 2, into dye solution 10−4 M); %CEC, reached cation exchange capacity; water content in weight % obtained by TG analysis; d001, interlayer distance in angstroms (exp, obtained by XRD and sim, obtained by molecular dynamics simulations); λab(A), absorption wavelength (absorbance value); λfl(%ϕ), fluorescence wavelength (% fluorescence quantum yield); ψ (deg), tilt angle between the long molecular axis of the dye and the normal to the Lap layer. The photophysical data of LDS 722 in a water/ethanol mixture is also included (v/v %). a

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intrinsic responses of the optical components of the spectrophotometer were corrected by recording the dichroic response of the instrument for an isotropic sample (10−5 M LDS 722 solution in a 1 mm path length cuvette) at every scanned δ angle. Computational Simulations. The structure of synthetic Laponite was constructed by modifying its natural analogue Hectorite36 following the steps presented in ref 37. The initial configuration of the LDS 722 dye was relaxed using density functional theory simulations implemented in Gaussian,38 with a B3LYP exchange correlation functional39 and a 6-311+G(d,p) basis set. Appropriate numbers of dye and water molecules to match the experimental compositions were randomly placed in the interlaminar space using Packmol.40 Three dye loadings were simulated (Table 1), together with the dye in vacuum for a comparison of structural quantities. The molecular dynamics simulations were carried out using the LAMMPS simulation package.41 The CLAYFF42 and CHARMM43 force fields were employed to describe bonding interactions in Laponite and the LDS 722 dye respectively, while the inorganic and organic parts interact via nonbonding Coulombic and van der Waals forces. The Ewald summation method44 was used to compute the long-range Coulombic energy, and the van der Waals parameters were combined using geometric mixing rules. An initial equilibration period of 10 ns was performed in the isobaric−isothermal ensemble, followed by a production period of 10 ns in the canonical ensemble. A Nose-Hoover-style thermostat and barostat were applied, with coupling constants of 0.1 and 1 ps, respectively. The equations of motion were integrated with a time step of 1 fs using the Verlet algorithm.45

achieved cationic exchange (%CEC) was characterized by the absorbance of the film and by elemental analysis. By increasing the immersion time of the clay film into a 10−5 M LDS 722 dye solution, a linear increase in dye loading was observed. After 1 day, which corresponds to 23% CEC (Figure 2), the dye loading tends toward asymptotic values due to the chemical potential equilibration between the solution and the film. To achieve larger loadings, a greater concentration gradient was induced using more concentrated LDS 722 solution (10−4 M). The saturated state is reached now for an immersion time of 5 h, with a maximum %CEC of 45. Longer immersion times and higher concentrated solutions did not increase the absorbance value. The incorporation of the dye on the Lap film shifts the main diffraction band, which corresponds to the basal distance, to shorter 2θ values. From the XRD results, the interlayer space gradually increases as the dye loading increases up to 2.1 Å for the saturated dye film of CEC 45% (Table 1). The Laponite swelling is needed to accommodate the dye molecules between clay layers as the CEC increases. The exchange of sodium ions with LDS 722 molecules also entails a decrease in the water content. It is noteworthy that the interlayer spaces obtained from molecular dynamics match perfectly with the experimental results, which indicates a proper description of the hybrid system in our simulations. Absorption and Emission Spectral Shifts. The absorption spectra of the prepared LDS 722/Lap films show a main band centered at 494 nm that is gradually blue shifted as the dye loading increases, as shown in Table 1 and Figure 4. This shift could be originated by the torsion of the molecular structure due to the confinement, reducing the electronic delocalization of the system. However, molecular dynamics does not predict significant torsion of the dye, ruling it out as the origin of the emission shift. The deviation of the dye from planarity will be discussed in detail in the next section. Another possible reason for the blue shift is the interaction of LDS 722 with water molecules confined in the interlayer space.46,47 On the one hand, the donor amine group can establish hydrogen bonds with water molecules, stabilizing the ground state and increasing the HOMO−LUMO energy gap.32 On the other hand, there can be partial protonation of the amine nitrogen, reducing the resonance with the π system.48 We use the molecular dynamics trajectories to analyze the water−dye interactions. Figure 5 presents the radial distribution function of water molecules around the nitrogen atoms of the donor and acceptor sites, considering the N−Ow and N−Hw distances or the amine donor and pyridine acceptor groups, respectively. It is clear that the distance between water and nitrogen atoms decreases as the dye loading increases, i.e., when there are fewer water molecules. Under these conditions, the probability of a water−dye interaction is higher because of the increasing difficulty of establishing water−water hydrogen bonds.49 The solvation increases with the %CEC at both molecular ends, yet it is considerably larger in the donor group than in the acceptor group, which is consistent with the blue shift in the absorption spectra. For a better visualization of the solvation around the amine and pyridine nitrogen atoms, we include in Figure 5 the spatial distribution function of water around them. The blue isosurfaces represent the most probable sites of water oxygen and hydrogen atoms around the amine and pyridine nitrogen atoms, respectively. While there are more water molecules in the interlaminar space at low dye loadings, they tend to self-associate due to the hydrophobic character of

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RESULTS AND DISCUSSION Characterization of the Hybrid Material. Very transparent Lap thin films were obtained by a spin-coating procedure. The thickness at the center of the film, measured by laser profilometry, was found to be ∼420 nm (Figure 2) and easily reproducible from sample to sample under the conditions detailed in the methodology section. Then, the dye was loaded by immersion of the clay film into dye solutions, and the

Figure 2. (a) LDS 722/Lap transparent thin films over the Molecular Spectroscopy Laboratory logo. (b) Thickness of the thin films measured by laser profilometry. The measurement was made in the center of the sample carrier, and the average thickness is about 420 nm. (c) Maximum absorbance of the material as a function of the immersion time in 10−5 M LDS 722 solution and (d) as a function of the cationic exchange percentage. The corresponding wavelengths for which each immersion time and %CEC are shown in Table 1 10114

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Figure 3. Snapshots from molecular dynamics simulations of the LDS 722/Lap hybrid material at the simulated %CEC: (a) 4, (b) 23, and (c) 45%. The basal spaces (in angstroms) obtained from the X-ray diffraction experiments and simulations are indicated in black and blue, respectively. The code color is the same as in Figure 1

Figure 5. Radial distribution functions of the N−Ow and N−Hw distances for the (a) amine donor and (b) pyridine acceptor groups, respectively, at different dye loadings. Spatial distribution functions of the same atomic pairs at (c) 4% CEC and (d) 45% CEC. Figure 4. Absorption (top) and emission (bottom) spectra of the LDS 722/Lap thin films as a function of the %CEC. The emission was measured after exciting each sample at its maximum absorbance wavelength.

change. Note that the LDS 722 emission process involves an intramolecular charge-transfer mechanism, where the donor and acceptor moieties are the amine and pyridine groups, respectively. In this sense, the red shift could be originated by a switch from intramolecular to intermolecular charge transfer due to the proximity of the dye molecules at high concentrations. In order to clarify this point, the intramolecular and intermolecular distances between donor and acceptor groups were evaluated from the molecular dynamics simulations. In an intramolecular process, the distance is 14 Å, and it is constant at any loading since the dye maintains the planar structure. As the %CEC increases, the minimum distance between donor and acceptor groups of different molecules decreases from 18 to 5 Å. Such small distances, considerably lower than the intramolecular one, can increase the probability of intermolecular charge transfer, originating the red shift in the emission. Efficiency of the Emission. The emission efficiency recorded for the LDS 722 diluted film is similar to that recorded in solution. We expected enhanced fluorescence efficiency, due to a decrease in internal rotations around single

the organic molecule. The solvation of LDS 722 increases when water molecules are scarce, especially on the donor group, and causes the absorption hypsochromic shift. Contrary to the absorption spectra, the fluorescence band suffers a bathochromic shift as the dye concentration increases. Such an effect is very interesting, since the Stokes shift of the hybrid material can be modulated just by tuning the dye loading. Frequently, reabsorption−reemission phenomena induce a red shift in the emission band together with a decrease in the fluorescence efficiency as the dye concentration increase. However, this is not the case due to the large Stokes shift (≥3000 cm−1) of this dye as it was checked in solution at different concentrations (data not shown). Another alternative could be the change in the polarity of the environment as the loading increases, but then again this possibility is also discarded since the position of the emission band of LDS 722 registered in different media (Table 1) does not practically 10115

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and double bonds induced by the dye confinement in the interlayer space of the Laponite film. In order to analyze the effect of confinement on thermal motions, the methyl rotations and the molecular twisting, as depicted in Figure 6, were computed.

Figure 7. (a) Evolution of the dichroic ratio of a 45% CEC LDS 722/ Lap thin film for different orientations of the sample with respect to the incident beam. The inset shows the linear relationship between the dichroic ratio with the twisted δ angle at the absorption maximum (475 nm). (b) X-polarized absorption spectra of the 45% CEC LDS 722/Lap hybrid material. The spectra were recorded from 0 to 80° every 10°. Figure 6. Molecular rotations and torsions in LDS 722. (a, b) Most probable angles of rotation around the amine methyl groups and pyridine ethyl group, respectively (indicated with a green arrow in the molecular representation). Schematic representation of the dye out of the plane and torsion movements, with the maximum angle values at 45% CEC. The hexagons represent the aromatic rings on each side of the LDS 722.

absorption band of LDS 722 in Lap films to the X-polarized light reveals a preferential orientation of the dye molecules adsorbed in the Lap films toward the normal to the film (Z axis). As is shown in Figure 7, the DX,Y value is linearly correlated against the sin 2δ value, which, according to eq 1,35 corresponds to a tilted angle ψ of 60° between the transition moment (along the long molecular axis of the dye molecule) and the normal to the clay layers:

The rotation of the methyl groups at both extremes of the dye is not hindered by the confinement in the interlayer space of the clay film. As shown in Figure 6, the hydrogen atoms can complete full rotations around the carbon atoms. The hydrogen atoms in the ethyl group (right side, acceptor) have very well defined positions in the staggered conformation, while the hydrogen atoms in the methyl groups (left side, donor) have less well defined positions because their movement is coupled. In vacuum, the characteristic rotational times (τ), defined as the necessary time to achieve a full rotation, are 39.2 and 76.6 ps for the hydrogen atoms at the donor and acceptor extremes. While τ increases with confinement up to 83.2 and 97 ps at the highest dye loading, the rotations are still considerably faster than the excited-state lifetime and therefore can be responsible for nonradiant deactivation processes. The out-of-plane bending and the torsion angles deviate only ±30 and ±7° from planarity, respectively; these values are similar under confinement and isolation in vacuum. Possibly, the swelling capacity of the clay is not providing the necessary rigid environment to enhance the fluorescence.30,33,50 In addition, it is observed that the fluorescence quantum yield decreases with the %CEC. In accordance with the red-shifted emission, this is attributed to an intermolecular charge transfer, as these processes are usually less emissive than intramolecular ones.51 Anisotropic Response of the Hybrid Material. LDS 722/Lap-supported films present an anisotropy effect with respect to the linearly polarized light. Figure 7 shows the evolution of the absorption band for X-polarized light at different twisting angles (δ), from 0 to 80°. The response of the

DH , V =

AH 2 − 3 sin 2 ψ 2 =1+ sin δ AV sin 2 ψ

(1)

Note that a slightly variation in the tilted ψ angles is registered with the relative amount of dye in the clay surface. Thus, the LDS 722 dye is placed in a relatively planar distribution with respect to the plane of the clay in all samples, in accordance with the small increase in the interlayer space found by XRD and the molecular dynamics results.

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CONCLUSIONS In this work we produced a novel hybrid dye−clay material formed by LDS 722 molecules confined in Laponite clay. The material shows promising properties for the development of nonlinear optical devices due to the synergy between the guest dye and the host clay. As the dye loading increases, the hybrid material presents a hypsochromic shift in absorption and a bathochromic shift in emission. Therefore, the Stokes shift of the material can be easily tuned by adjusting the CEC during preparation. The blue shift of the absorption spectra was attributed to the stabilization of the ground-state structure by hydrogen bonds between the donor aniline group of the dye and water molecules, whereas the red shift of the emission spectra is assigned to a switch from an intramolecular to intermolecular charge-transfer process as the dye loading increases The LDS 722/Lap thin films show a response to linearly polarized light. The dye molecules self-organize with a tilt angle 10116

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(16) Shichi, T.; Takagi, K. J. Photochem. Photobiol. C 2000, 1, 113− 130. (17) Suzuki, Y.; Tenma, Y.; Nishioka, Y.; Kamada, K.; Ohta, K.; Kawamata, J. J. Chem. Phys. C 2011, 115, 20653−20661. (18) Ogawa, M.; Takahashi, M.; Kuroda, K. Chem. Mater. 1994, 6, 715−717. (19) Suzuki, Y.; Tenma, Y.; Nishioka, Y.; Kawamata, J. Chem. Asian J. 2012, 7, 1170−1179. (20) Kamada, K.; Tanamura, Y.; Ueno, K.; Ohta, K.; Misawa, H. J. Chem. Phys. C 2007, 111, 11193−11198. (21) Suzuki, Y.; Matsunaga, R.; Sato, H.; Kogure, T.; Yamagishi, A.; Kawamata, J. Chem. Commun. 2009, 6964−6966. (22) Umemura, Y.; Yamagishi, A.; Schoonheydt, R.; Persoons, A.; De Schryver, F. J. Am. Chem. Soc. 2002, 124, 992−997. (23) Takenawa, R.; Komori, Y.; Hayashi, S.; Kawamata, J.; Kuroda, K. Chem. Mater. 2001, 13, 3741−3746. (24) Shim, T.; Lee, M. H.; Kim, D.; Kim, H. S.; Yoon, K. B. J. Chem. Phys. B 2009, 113, 966−969. (25) Antonov, L.; Gergov, G.; Petrov, V.; Kubista, M.; Nygren, J. Talanta 1999, 49, 99−106. (26) Meral, K.; Yılmaz, N.; Kaya, M.; Tabak, A.; Onganer, Y. J. Lumin. 2011, 131, 2121−2127. (27) Van der Auweraer, M.; Verschuere, B.; De Schryver, F. Langmuir 1988, 4, 583−588. (28) Arık, M.; Onganer, Y. Chem. Phys. Lett. 2003, 375, 126−133. (29) Tsukanova, V.; Lavoie, H.; Harata, A.; Ogawa, T.; Salesse, C. J. Chem. Phys. B 2002, 106, 4203−4213. (30) Kim, J.; Lee, M.; Yang, J.-H.; Choy, J.-H. J. Chem. Phys. A 2000, 104, 1388−1392. (31) Fery-Forgues, S.; Le Bris, M.; Mialocq, J.; Pouget, J.; Rettig, W.; Valeur, B. J. Phys. Chem. 1992, 96, 701−710. (32) Seth, D.; Sarkar, S.; Pramanik, R.; Ghatak, C.; Setua, P.; Sarkar, N. J. Chem. Phys. B 2009, 113, 6826−6833. (33) Cerdán, L.; Costela, A.; García-Moreno, I.; Bañuelos, J.; LópezArbeloa, I. Laser Phys. Lett. 2012, 9, 426−433. (34) Van Olphen, H.; Fripiat, J. Data Handbook for Clay Materials and Other Non-Metallic Minerals; Pergamon: New York, 1979. (35) Martínez Martínez, V.; López Arbeloa, F.; Banuelos Prieto, J.; López Arbeloa, I. Chem. Mater. 2005, 17, 4134−4141. (36) Neumann, B.; Sansom, K. Clay Miner. 1970, 8, 389−404. (37) Duque-Redondo, E.; Manzano, H.; Epelde-Elezcano, N.; Martinez-Martinez, V.; Lopez-Arbeloa, I. Chem. Mater. 2014, DOI: 10.1021/cm500661d. (38) Frisch, M.; Trucks, G.; Schlegel, H. B.; Scuseria, G.; Robb, M.; Cheeseman, J.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (39) Kim, K.; Jordan, K. J. Phys. Chem. 1994, 98, 10089−10094. (40) Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. J. Comput. Chem. 2009, 30, 2157−2164. (41) Plimpton, S. J. Comput. Phys. 1995, 117, 1−19. (42) Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G. J. Chem. Phys. B 2004, 108, 1255−1266. (43) Brooks, B. R.; Bruccoleri, R. E.; Olafson, B. D.; Swaminathan, S.; Karplus, M. J. Comput. Chem. 1983, 4, 187−217. (44) Ewald, P. P. Ann. Phys. 1921, 369, 253−287. (45) Grubmüller, H.; Heller, H.; Windemuth, A.; Schulten, K. Mol. Simul. 1991, 6, 121−142. (46) Sasai, R.; Iyi, N.; Kusumoto, H. Bull. Chem. Soc. Jpn. 2011, 84, 562−568. (47) Ogawa, M.; Nakamura, T.; Mori, J.-i.; Kuroda, K. J. Chem. Phys. B 2000, 104, 8554−8556. (48) Krishnamoorthy, G.; Dogra, S. Spectrochim. Acta, Part A 1999, 55, 2647−2658. (49) Bakker, H. J.; Gilijamse, J. J.; Lock, A. J. ChemPhysChem 2005, 6, 1146−1156. (50) Martínez-Martínez, V.; Corcóstegui, C.; Prieto, J. B.; Gartzia, L.; Salleres, S.; Arbeloa, I. L. J. Mater. Chem. 2011, 21, 269−276. (51) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Chem. Rev. 2003, 103, 3899−4032.

of 60° between their transition dipole moment and the normal direction with respect to the clay sheets. This angle is almost independent of the dye loading and makes the material promising for NLO applications. Overall, the presented hybrid material has a simple preparation method, a tunable Stokes shift that avoids inner filter effects, and an anisotropic response to linearly polarizable light. Furthermore, we do not find any evidence of dye aggregation that could affect the absorption and emission spectra even at the maximum %CEC. Unfortunately, the fluorescence emission intensity does not improve with respect to that of the dye in solution. The swelling capacity of the clay does not impose a rigid environment to avoid nonradiant deactivation due to the vibrational motion of the dye. Future work in this direction will be carried out using strategies to increase the dye confinement.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Spanish government through the MAT2010-20646-204-04 project. H.M. and V.M.-M. acknowledge, respectively, Juan de la Cierva and Ramón y Cajal (RYC-2011-09505) postdoctoral contracts from the Spanish Ministerio de Industria y Competitividad. E.D.-R. acknowledges the IT339-10 contract from the Basque Country Department of Education, Research, and Universities and a Ph.D. fellow from the University of the Basque Country UPV/ EHU. N.E.-E. acknowledges a UPPA-UPV/EHU coadvised Ph.D. grant. Resources from the SGIker (UPV/EHU) and i2Basque projects are gratefully acknowledged.

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REFERENCES

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