Magnetic Alignment of Self-Assembled Anthracene Organogel Fibers

The aligned organogel is stable after removal of the magnetic field at room temperature and consists of fibers ... The Past, Present, and Future of Mo...
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Magnetic Alignment of Self-Assembled Anthracene Organogel Fibers Igor O. Shklyarevskiy,† Pascal Jonkheijm,‡ Peter C. M. Christianen,*,† Albertus P. H. J. Schenning,‡ Andre´ Del Guerzo,§ Jean-Pierre Desvergne,§ E. W. Meijer,‡ and J. C. Maan† High Field Magnet Laboratory HFML, Radboud University Nijmegen, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands, Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Laboratoire de Chimie Organique et Organome´ tallique, CNRS UMR 5802, Universite´ Bordeaux I, 33405 Talence ce´ dex, France Received November 18, 2004. In Final Form: January 21, 2005 High magnetic fields are shown to be remarkably effective to orient self-assembled 2,3-bis-ndecyloxyanthracene (DDOA) fibers during organogel preparation. Magnetic orientation of DDOA results in a highly organized material displaying a fiber-orientation order parameter of 0.85, a large linear birefringence, and fluorescence dichroism. The aligned organogel is stable after removal of the magnetic field at room temperature and consists of fibers oriented perpendicular to the magnetic field direction, as shown by scanning electron microscopy. Models for the molecular organization within the gel fibers are discussed upon quantitative analysis of the birefringence. Prospectively, magnetic alignment can be used to improve specific properties of organogel materials.

1. Introduction Recently the interest in organogels based on low molecular mass organic gelators (LMOGs) has increased considerably, as they are promising for a wide range of industrial applications, such as cosmetics and chiral chromatography, and for developments in nanotechnology (optoelectronics, photovoltaics, analyte sensing).1-4 Organogels are thermally reversible soft materials consisting of an organic liquid trapped by an interweaved threedimensional continuous network of nanometric fibers that are formed by self-assembled LMOGs (≈0.1-1 wt %).5-7 The precise nature of the noncovalent interactions stabilizing the architecture and molecular organization is often not well understood, as in the case of LMOG-based gels they cannot thoroughly be revealed by many conventional techniques such as X-ray crystallography.8-10 The knowledge of the mutual molecular organization within a gel framework contributes to the understanding * To whom correspondence may be addressed. E-mail: [email protected]. † High Field Magnet Laboratory HFML, Radboud University Nijmegen. ‡ Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology. § Laboratoire de Chimie Organique et Organome ´ tallique, CNRS UMR 5802, Universite´ Bordeaux I. (1) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133-3159. (2) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237-1247. (3) Terech, P.; Weiss, R. G. In Surface Characterization Methods: Principles, Techniques, and Applications; Milling, A. J., Ed.; Marcel Dekker: New York, 1999. (4) van Esch, J.; Schoonbeek, F.; de Loos, M.; Veen, E. M.; Kellogg, R. M.; Feringa, B. L. Supramolecular Science: Where It Is and Where It Is Going; NATO ASI Series C, Mathematical and Physical Sciences; Kluwer Academic: Boston, MA, 1999; Vol. 527, p 233. (5) de Vries, E. J.; Kellogg, R. M. J. Chem. Soc., Chem. Commun. 1993, 238-240. (6) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352-355. (7) Terech, P.; Countin, A.; Giroud, A. M. J. Phys. Chem. B 1997, 101, 6810-6818. (8) Terech, P.; Bouas-Laurent, H.; Desvergne, J.-P. J. Colloid Interface Sci. 1995, 174, 258-263. (9) Terech, P.; Allegraud, J. J.; Garner, C. M. Langmuir 1998, 14, 3991-3998. (10) Liu, X. Y. J. Chem. Phys. 2000, 112, 9949-9955.

of the properties of the soft material and provides important information for the design of other gelators belonging to the same family. Intensive investigations are therefore necessary to gain insight in the molecular structures of organogels. The first aim of this paper is to address this lack of knowledge by the analysis of the optical properties, i.e., linear birefringence and fluorescence dichroism, of organogels consisting of unidirectionally aligned fibers, providing novel information on the degree of alignment and molecular arrangement inside an organogel fiber. Our second aim is to demonstrate that this homogeneous orientation is efficiently achieved by the use of high magnetic fields, an extremely versatile technique that exploits the anisotropy in diamagnetic susceptibility of the LMOG. This procedure is very attractive as it is contact free, homogeneously effective over the whole sample, and can be used to produce structured thin films as well as bulk material. The interest in strongly organized fiber networks resides in its potential to induce or to enhance specific physical properties of the LMOG, like the polarization of absorbers and emitters,11 or the charge carrier mobility of conjugated materials.12 It was also recently shown that LMOG can be used as a host matrix to immobilize magnetically deformed nanocapsules selfassembled in alcohols from conjugated molecules.13 The studies presented in this paper relate to organogels of 2,3-bis-n-decyloxyanthracene (DDOA), an LMOG that has been extensively studied due to its outstanding capacity to form gels, its simple molecular structure, and its remarkable optical properties. As we will see, its elongated aromatic core provides the handle for the orientation by a magnetic field. Previous studies on DDOA have demonstrated that DDOA fibers can be oriented by (11) Grell, M.; Bradley, D. D. C. Adv. Mater. 1999, 11, 895-905. (12) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; MacKay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539-541. (13) Shklyarevskiy, I. O.; Jonkheim, P.; Christianen, P. C. M.; Schenning, A. P. H. J.; Meijer, E. W.; Henze, O.; Kilbinger, A. F. M.; Feast, W. J.; Del Guerzo, A.; Desvergne, J.-P.; Maan, J. C. J. Am. Chem. Soc. 2005, 127, 1112-1113.

10.1021/la047166o CCC: $30.25 © 2005 American Chemical Society Published on Web 02/10/2005

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mechanical shear stress (in organogels14) or manually (in aerogels15), affording in the latter case unprecedented spectroscopic data for the determination of fiber organization at the molecular level, palliating thus the lack of X-ray diffraction pattern on pure DDOA gel fibers. Our investigation has also been encouraged by the reports on steroidbased gels16 and polymer gels17,18 prepared under magnetic fields limited to 5 T. Nevertheless, these previous reports are not a sufficient guidance for the study of self-assembly of gelators under applied external fields. The present study provides a contemporary and prospective analysis of the method, paving the way for the application of the magnetic orientation technique on other LMOGs, like phenylenevinylene-,19 thiophene-,20 or tetracene-based gels. Magnetic Field Induced Orientation. The principle of magnetic field alignment originates from the diamagnetism of organic molecules, that is when a molecule is placed in a uniform magnetic field B B , a magnetic moment m b is induced.21 This magnetic moment interacts with the magnetic field and the molecule will acquire extra energy E

E ) -m b ‚B B)-

χmB2 µ0NA

(1)

where χm is the diamagnetic susceptibility of the molecule, NA is Avogadro’s number, and µ0 is the permeability of free space (µ0 ) 4 × 10-7 H/m). The diamagnetic susceptibility is closely related to the molecular structure and therefore generally anisotropic. As a result the magnetic energy depends on the orientation of the molecule with respect to the field, resulting in an orientational force that drives the molecule to align with its axis of smallest susceptibility along the field direction. The strength of this magnetic force depends on the magnitude of the anisotropy in χm, normally expressed as a difference of susceptibilities ∆χm ) χ|,m - χ⊥,m between two orthogonal molecular axes. For single molecules the field-induced magnetic energy is small compared to the thermal energy at the fields that are currently available (∆χm‚B2/µ0NA , kT), and therefore, the Brownian motion prevents a high degree of alignment of the molecules. However, the collective molecular behavior in the course of a self-organization process can provide a sufficiently high magnetic energy in order to overcome this randomizing thermal motion. Indeed, for example in gel phases, the magnetic energy of all the molecules within a fiber (N) adds up (N‚∆χm‚B2/µ0NA . kT), thus allowing magnetic alignment. Magnetic Field Induced Linear Birefringence. The effect of linear birefringence manifests itself in a difference between the refractive indices for light propagating in orthogonal directions through the medium and originates from the anisotropy in optical polarizability of the (14) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Heuze´, K.; Fages, F.; Pozzo, J.-L. Langmuir 2002, 18, 7151-7153. (15) Placin, F.; Desvergne, J.-P.; Belin, C.; Buffeteau, T.; Desbat, B.; Ducasse, L.; Lasse`gues, J. C. Langmuir 2003, 19, 4563-4572. (16) Terech, P.; Berthet, C. J. Phys. Chem. 1988, 92, 4269-4272. (17) Matsumoto, Y.; Yamamoto, I.; Yamaguchi, M.; Shimazu, Y.; Ishikawa, F. Jpn. J. Appl. Phys. 1997, 36, L1397-L1399. (18) Matsumoto, Y.; Yamamoto, I.; Yamaguchi, M.; Shimazu, Y.; Ishikawa, F. Physica B 1998, 246-247, 408-411. (19) Ajayaghosh, A.; George, Subi J. J. Am. Chem. Soc. 2001, 123, 5148-5149. (20) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Rep, D. B. A.; de Haas, M. P.; Klapwijk, T. M.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 1999, 38, 1393-1397. (21) Maret, G.; Dransfeld, K. In Strong and Ultrastrong Magnetic Fields and Their Applications; Springer-Verlag: Berlin, 1985, Chapter 4 and references therein.

molecules.22 The induced dipole moment in the molecule is given by b p ) RE B , where R is the polarizability tensor of the molecule and E B is the electric field vector of the light. In the cylindrical approximation the polarizability tensor can be characterized by two components R| and R⊥ corresponding to the polarizability along the long and short molecular axes, respectively. For light propagating through the solution in the x direction, the difference in refractive indices in directions parallel and perpendicular to the magnetic field direction is given by

∆n ) nz - ny ≈

1 n C(R| - R⊥) × 2 1

∫ ∫(cos2 θ - sin2 θ sin2 φ) sin(θ) f(θ) dθ dφ ∫ ∫ sin(θ) f(θ) dθ dφ

(2)

where θ and φ are the usual polar and azimuthal angles in polar coordinates, describing the orientation of the long molecular axis with respect to the magnetic field direction, C is the number of molecules per unit volume (concentration) and n1 is the refractive index of the suspending medium. The angular distribution function f(θ) describing the molecular orientation in a magnetic field is given by a Maxwell-Boltzmann distribution

(

)

∆χmB2 cos2 θ f(θ) ) exp µ0NAkT

(3)

The sign of the measured birefringence indicates the orientation of the long molecular axis and is negative in the case of orientation along the field and positive for the opposite case. Strictly speaking, the birefringence signal consists of two terms: the intrinsic birefringence (described by eq 2) and the form birefringence resulting from the shape anisotropy of the fibers. We have estimated the form birefringence in the cylindrical shell approximation using shells of length L and diameter b using Mie scattering theory (L . b).23 Taking realistic experimental values (L ∼ 100 µm, b ∼ 50 nm, vide infra), we have found that the maximum form birefringence is always smaller than 10-6 and can be neglected. 2. Materials and Methods The synthesis of the DDOA molecules (top panel Figure 1) has been published elsewhere.24 The experiments were performed for a concentration of 8 g/L in 1-butanol. Prior to the gelation experiments the DDOA solution was first heated to 70 °C until no powder remnants were visible for a total DDOA dissolution. Subsequently, the sample was loaded in an optical cell (2 mm thick, 10 mm wide, 45 mm high) inside a 20 T Bitter magnet and allowed to cool to room temperature. The cell was heated to 60 °C and in a background magnetic field cooled to 30 °C to gelate the solution. The temperature of the sample was controlled by a water-based temperature controller with a precision of (0.1 °C. High-resolution magnetic birefringence measurements were carried out using equal components of sinusoidally phasemodulated laser light (He-Ne laser, 632.85 nm) polarized at 45° to the magnetic field direction. The intensity of the transmitted light was guided through an analyzer at -45° and detected by a Si photodiode. The ac phase shift between the original (22) Fowles, G. R. Introduction to modern optics; Dower: New York, 1989. (23) van Hulst, H. C. Light scattering by small particles; Dover: New York, 1981. (24) Brotin, T.; Utermo¨hlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J.-P. J. Chem. Soc., Chem. Commun. 1991, 416-418.

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Figure 1. (top) Chemical structure of the 2,3-bis-n-decyloxyanthracene. (bottom) Magnetic field induced birefringence at different magnetic field strengths, recorded during the gelation of DDOA (reproducible within 3% error). Inset: Magnetic field dependence of the maximum birefringence, obtained after complete gelation. polarizations was recovered via lock-in amplifiers, referenced to the frequency of the photoelastic modulator (PEM-90). The dc part of the signal was used for monitoring the transmission through the sample. The optical configuration of this birefringence setup was such that a positive signal corresponds to molecules aligning with their fast optical axis (i.e., the axis with the highest polarizability) perpendicular to the magnetic field direction, whereas a negative birefringence signal reflects the opposite situation (fast axis along the field direction). The detection limit of the measured phase shift was 10-5 rad. Polarized fluorescence experiments were performed using a Perkin-Elmer luminescence spectrometer LS50B with an excitation wavelength of 340 nm. To record scanning electron microscopy (SEM) images, gels were carefully transferred from the cell to a metal sample holder and allowed to dry out. The obtained xerogel was coated by a 2 nm Pt layer using a BALZERS BAE 121 multicoating system and subsequently studied by a JEOL JSM T300 scanning microscope at 3 kV using a 1.2 nA probe current.

3. Results and Discussion Magnetic Alignment. Figure 1 shows the variation of the birefringence during the gelation of butanol by DDOA (cooling from 60 to 30 °C) at different magnetic field strengths. The data reveal that the degree of orientation, which is directly related to the absolute value of the birefringence, increases strongly with the applied field. Moreover, the negative sign of the birefringence indicates that the DDOA molecules are oriented with their axes of largest polarizability along the magnetic field direction (0 e θ < 45°), as expected for aromatic substrates.21 This likely corresponds to the long molecular axes, by analogy with DDA (2,7-diamino-3,6-dinitroanthracene) for which the calculated values of the polarizability are equal to R| ) 48.5 × 10-30 m3 and R⊥ ) 30.5 × 10-30 m3.25 The inset of Figure 1 shows the magnetic field dependence of the birefringence obtained after completion of the gelation (symbols), fitted by a procedure that will be discussed later (solid line). The characteristic inverted S-shape of this field dependence is typical for an alignment process of diamagnetic substances and indicates an almost complete orientation at the highest fields.

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SEM and Fluorescence Dichroism. The scanning electron microscopy images of DDOA xerogels show the contrast between the unaltered entwined fiber network (Figure 2a) and the strongly aligned fibers obtained in the 20 T field (Figure 2b). At all tested locations of the sample, the SEM reveals that the fibers were oriented perpendicularly to the applied field direction, even when they were entangled in larger bundles. The uniaxial orientation evidenced by SEM was achieved by exploiting the anisotropic dimensions of the cell (2 × 10 mm) in order to guide the fiber formation along the width of the cell. This so-called container effect is required since the magnetic field can only impose an orientation in the plane perpendicular to the field. To further characterize the oriented optical material, we have performed fluorescence dichroism measurements (Figure 3). The spectra were measured using an excitation wavelength of 340 nm with both the excitation and detection polarizations either parallel or perpendicular to the direction of the fibers. The fluorescence intensity at parallel polarization, thus perpendicular to the original magnetic field, is ∼1.7 times larger than that at perpendicular polarization. These results are similar to those observed for physically aligned aerogel fibers.15 An estimation of the angle of the optical transition dipole moment with respect to the fiber axis depends on the model used to interpret the data, but the fluorescence anisotropy shows that it is on average smaller than the magic angle (0 e ω < 54.7°). Knowing that the optical transition dipole moment is perpendicular to the long molecular axis and in the plane of the aromatic ring, this experiment shows that the molecules align with an angle (0 e θ < 54.7°) relative to the original magnetic field. This is in agreement with the birefringence data (where we estimated 0 e θ < 45°), albeit by fluorescence, due to the high concentration of chromophores and complex excited state dynamics, it is possible that only a fraction of the molecules are probed.26 Quantitative Analysis of the Birefringence. In the literature, several models for the stacking of DDOA in gel fibers have been presented, evolving from a simple unidirectional molecular π-π stacking perpendicular to the fiber axis27 to a more elaborate hollow cylindrical structure with molecular planes displaying average inclination and tilting with respect to the fiber axis.15 As those models still have uncertainties associated to them, we use the SEM data and a quantitative analysis of the birefringence to discriminate between different structural models describing the molecular arrangements in organogels. In a first analysis, by eq 2 we can estimate the maximum value of the birefringence ∆n, which corresponds to the ideal case with all molecules oriented with their long axes along the magnetic field direction. Using C ) 16NA m-3 (for 8 g/L concentration, molecular weight of DDOA ) 490 g/mol), n1 ) 1.4 (for butanol), R| ) 48.5 × 10-30 m3, R⊥ ) 30.5 × 10-30 m3, and for a magnetic field of 20 T, the value of ∆n ) -1.2 × 10-4 is obtained. This is nevertheless larger than the experimental value of -8.8 × 10-5 observed at 20 T, prompting thus the necessity to refine this quantitative analysis. Therefore, we propose several simple models for the molecular arrangement in fiber (see models I-VI in the top panel of Figure 4). Even though we have not extensively (25) Tomonari, M.; Ookubo, N. J. Mol. Struct. (THEOCHEM) 1998, 451, 179-188. (26) Desvergne, J.-P.; Brotin, T.; Meerschaut, D.; Clavier, G.; Placin, F.; Pozzo, J.-L.; Bouas-Laurent, H. New J. Chem. 2004, 28, 234-243. (27) Kato, T.; Kutsuna, T.; Yabuuchi, K.; Mizoshita, N. Langmuir 2002, 18, 7086-7088.

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Figure 2. SEM photographs of the DDOA xerogel obtained at (a) 0 T and (b) 20 T. The fibers are oriented perpendicular to the magnetic field direction.

Figure 3. Polarized fluorescence spectra of a magnetically aligned DDOA gel obtained for excitation and detection parallel to the fiber direction (solid line) and perpendicular to it (dotted line). Excitation wavelength was 340 nm. No changes in the unpolarized fluorescence spectra were detected before and after application of magnetic field.

studied the very large number of possible molecular arrangements, the calculations on these basic and realistic models afford the necessary qualitative and quantitative information for the discussion. For the calculations of the field dependence of the birefringence, we have taken a maximum birefringence value ∆n ) -1.2 × 10-4 (see above), ∆χm ) 2.29 × 10-9 m3/mol,28 T ) 303 K and the number of cohesive molecules N ) 6 × 104 molecules (see below). First we consider structures I and II (see Figure 4 for a depiction of a single fiber), in which the long axes of the individual DDOA molecules (rectangles in Figure 4) are perpendicular to the fiber axis. We can rule out this stacking geometry for two major reasons: (i) the fibers would align parallel to the magnetic field direction (as shown), with aromatic rings parallel to the field; (ii) the field-induced birefringence (curve a in Figure 4) would have a positive value, because the individual molecules are perpendicular to the field. These two features are in contradiction with the experimental findings. The field-induced birefringence of fibers III and IV show the expected negative sign (curve c in Figure 4), because the DDOA molecules are aligned along the field. However, to exhibit aromatic rings parallel to the magnetic field, the fibers in structure III need to be oriented parallel to the field, which is in disagreement with the SEM data. In structure IV, the fibers are aligned perpendicular to the field leading to a negative birefringence value. This model does not totally fit with the experimental data (symbols,

Figure 4. Results of the model calculations (solid lines) to describe the experimental data points (symbols). The different fitting curves correspond to different stacking geometries of the DDOA fiber structure (top panel), which are explained in the text.

Figure 4) as the birefringence calculated using eq 2 is ca. 20% higher for each applied field value. Nevertheless, this model can act as a basis for other models when implemented with an additional orientation parameter. This leads to arrangements V and VI, where the variable orientation of DDOA molecules along the fiber induces an overall reduction of the birefringence of a fully aligned fiber as compared to the case of model IV. A more accurate description of the field dependence of the birefringence is thus obtained by modifying eq 2 as follows (see curve b in Figure 4) ∆n ≈ -

1 n C(R| - R⊥) × 2 1

∫ ∫ cos

2

θ(cos2 θ - sin2 θ sin2 φ) sin θ exp

∫ ∫ sin θ exp( µ N kT cos N∆χmB 0

(28) Lonsdale, K. Proc. R. Soc. London, Ser. A 1937, 156, 597.

A

2

2

(

)

N∆χmB2 cos2 θ dθ dφ µ0NAkT

)

θ dθ dφ

(4)

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The additional degree of freedom for the orientation within the fiber structure is accounted for by the extra cos2 θ term. Therefore the integration is performed over all possible radial molecular arrangements perpendicular to the cylinder (fiber) axis. The calculated birefringence values for structures V and VI are identical and coincide with the experimental data. This analysis also predicts that the alignment is incomplete at 20 T with a related order parameter reaching 0.85 and, by extrapolation, a total alignment at 50 T (order parameter 1). Another important parameter is the number of cohesive molecules N in a fiber. We obtain the best description of the field dependence of the birefringence using a value of 6 × 104. Assuming a dense packing of DDOA molecules similar to that of DHOA crystals29 and a fiber diameter of 100 nm, this would correspond to a cylinder of about 6 nm in length, thus surprisingly short. This rather suggests that alignment occurs before the complete formation of fibers, when they have not reached yet a diameter as high as 100 nm but are >6 nm in length. This observation suggests that the growth mechanism of the gel fibers could involve two steps: (i) spontaneous formation of a fiber fragment at elevated temperatures, which is mobile and is oriented by the field, followed by, (ii) upon cooling, a subsequent lengthening of the fragment leading to a long fiber (as proposed for DDOA by M. Lescanne et al.30) that would be oriented in the same direction as the starting fragment. The combination of SEM, quantitative determination of the birefringence and fluorescence dichroism is very valuable to acquire an insight on the molecular structure of DDOA gel fibers. It allows ruling out several molecular arrangements (structures I-IV) and restricts the type of geometries consistent with the experimental data (types V and VI). It is probable that other more sophisticated molecular arrangements would better characterize the (29) Pozzo, J.-L.; Desvergne, J.-P.; Clavier, G. M.; Bouas-Laurent, H.; Jones, P. G.; Perlstein, J. J. Chem. Soc., Perkin Trans. 2001, 2, 824-826. (30) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Fages, F.; Pozzo, J.-L. Langmuir 2003, 19, 2013-2020.

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actual fiber structure and describe more precisely the birefringence and the fluorescence anisotropy. However, our analysis clearly shows that such a quantitative description has to incorporate a certain degree of molecular disorder, which directly affects the birefringence and fluorescence anisotropy.31 Therefore these models are compatible with the one proposed for DDOA in aerogels, taking also into account that slightly different structures might appear depending on the fabrication process of the gels (e.g., magnetically aligned vs supercritical process32). 4. Conclusions Magnetic field orientation has proven to be a successful contact-free technique for the preparation of highly organized self-assembled material like organogels, as shown by an orientational order parameter of 0.85 obtained at 20 T, displaying particular optical properties such as a strong birefringence and fluorescence dichroism. The quantitative analysis of the birefringence is decisive for the elaboration of models of the molecular structure in DDOA organogel fibers. Prospectively, magnetic alignment can be used to improve specific properties of organogel materials, such as polarized absorbance and fluorescence or an enhanced charge carrier mobility. Acknowledgment. We thank Huub Geurts for his invaluable help with the SEM experiments, and Tom Verbogt for valuable discussions. This work is part of the research programs of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM)”, financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)”, the Council for the Chemical Sciences of The Netherlands Organization for Scientific Research (CW-NWO), the CNRS, and the French Ministry of Research, Re´gion Aquitaine. LA047166O (31) Jeukens, C. R. L. P. N.; Lensen, M. C.; Wijnen, F. J. P.; Elemans, J. A. A. W.; Christianen, P. C. M.; Rowan, A. E.; Gerritsen, J. W.; Nolte, R. J. M.; Maan, J. C. Nano Lett. 2004, 4, 1401-1406. (32) Placin, F.; Desvergne, J.-P.; Cansell, F. J. Mater. Chem. 2000, 10, 2147-2149.