Formation, Structure, and Morphology of Triazole-Based Langmuir

Feb 15, 2007 - Université Bordeaux 1, CNRS, Centre de Recherche Paul Pascal, 115 .... Irene Bräunlich , Samuel Lienemann , Christiane Mair , Paul Sm...
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Langmuir 2007, 23, 3110-3117

Formation, Structure, and Morphology of Triazole-Based Langmuir-Blodgett Films O. Roubeau,*,† E. Natividad,‡,§ B. Agricole,† and S. Ravaine† UniVersite´ Bordeaux 1, CNRS, Centre de Recherche Paul Pascal, 115 aVenue du Dr. Albert Schweitzer, F-33600 Pessac France, and UniVersite´ Bordeaux 1, CNRS, ICMCB, 87 aVenue du Dr. Albert Schweitzer, F-33600 Pessac, France ReceiVed July 27, 2006. In Final Form: NoVember 3, 2006 The formation, morphology, and structure of two-dimensional Langmuir-Blodgett (LB) assemblies of octadecyltriazole (ODT)-based metal-containing oligomers presenting, in the case of iron, the spin-crossover phenomenon is studied with Brewster angle microscopy, IR dichroism, X-ray diffraction, and atomic force microscopy. Two processes occurring at the air-water interface are confirmed to dominate the mechanism of formation of these LB films, the instability of the coordination polymers at the air-water interface and recoordination of metal ions in the subphase at the interface during the LB deposition process. The Langmuir film allowing the LB film formation is mostly made of ODT. The films do present a lamellar structure in which the ODT molecules are tilted and incorporate coordinated isolated metal ions and oligomers of metal ions. The morphology of the LB films is globally flat but with a rather high roughness resulting from inhomogeneities related to phenomena occurring during the LB film formation. These observations are in agreement with the relative affinity of the metal ions with ODT and the relative stability of the coordination polymers at the air-water interface, which have been determined for the group Cu-Fe-Co-Ni.

Introduction The design and synthesis of molecule-based magnetic materials mostly rely on controlling molecular properties and assembling molecules in a bottom-up approach. Although a better understanding of the factors dictating molecular assembly has been gained through the development of supramolecular chemistry, this latter step still often relies on serendipitous assembly.1 Therefore prediction and control of cooperative properties of the solid remain difficult. This is particularly true in spin-crossover solids,2 for which cooperative hysteretic high-spin (HS) T lowspin (LS) conversion, of interest for potential applications,3 can only be observed either as a result of a phase transition or when the compound’s electronic structure is strongly coupled to the environment.4 The latter case has been ascribed to the presence of long-range elastic interactions among the molecules within the solid.5 The Langmuir-Blodgett (LB) technique, which allows the orientation and placement of molecules in thin films of controlled thickness,6 has thus been used as a possible tool to influence the electronic spin-state conversion, as well as other magnetic and conducting properties.7 Indeed, LB films of amphiphilic complexes modeled after the parent [FeII(4,4′bipyridine)2(NCS)2] mononuclear spin-crossover complex have * Corresponding author: e-mail, [email protected]; telephone, +33(0)556845684; fax, +33(0)556845600. † CNRS, Centre de Recherche Paul Pascal. ‡ CNRS, ICMCB. § Present address: Instituto de Ciencia de Materiales de Arago ´ n, Sede Campus Rı´o Ebro, Marı´a de Luna 3, 50018 Zaragoza, Spain. (1) Winpenny, R. E. P. J. Chem. Soc., Dalton Trans. 2002, 1. (2) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 2024. (3) (a) Kahn, O.; Kro¨ber, J.; Jay, C. AdV. Mater. 1992, 4, 718. (b) Kahn, O.; Jay Martinez, C. Science 1998, 279, 44. (4) Gu¨tlich, P.; Goodwin, H. A. Top. Curr. Chem. 2004, 233, 234, and 235. (5) (a) Spiering, H. Top. Curr. Chem. 2004, 235, 171. (b) Spiering, H.; Kohlhaas, T.; Romstedt, H.; Hauser, A.; Bruns-Yilmaz, C.; Kusz, J.; Gu¨tlich, P. Coord. Chem. ReV. 1999, 192, 629. (6) (a) Ulman, A. An Introduction to Ultra-Thin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (b) Petty, M. C. Langmuir-Blodgett Films. An Introduction; Cambridge University Press: Cambridge, U.K., 1996. (7) Talham, D. R. Chem. ReV. 2004, 104, 5479.

been prepared, in which a large part of the transferred complexes were trapped in the low-spin state, as a consequence of the LB packing.8 This effect was destroyed upon disordering of the alkyl chains upon heating. Although a cooperative behavior was not induced by the LB arrangement, the potential influence of the LB packing was thus demonstrated. Attempts were also made at forming LB films of cooperative one-dimensional materials based on the 1,2,4-triazole ligand, though first unsuccessful because the coordination polymers were unstable at the airwater interface.9 We have recently shown that magnetic LB films could be obtained within the system M(II)/4-octadecyl-1,2,4triazole when transition-metal cations were present in the subphase.10 When M ) Fe, spin crossover was observed, although only for a portion of the spin centers. This observation was tentatively ascribed to the presence in the LB packing of short oligomers and potentially of isolated Fe(II) ions. We present here a detailed study of the formation mechanism, the morphology, and the organization of the obtained LB films. Experimental Section Materials. All reagents were purchased from Aldrich and were used without further purification. 4-Octadecyl-1,2,4-triazole (ODT) was obtained from octadecylamine and hydrazine hydrate as described previously.11 The synthesis of the metal(II) metallo-organic polymers of ODT of formula [M(ODT)3](NO3)‚xH2O (M ) Co, 1; M ) Ni, 2; M ) Cu, 3) was achieved according to a published procedure,11 using Co(NO3)2‚6H2O, Ni(NO3)2‚6H2O, and Cu(NO3)2 as reagents, respectively. For [Fe(ODT)3](NO3)‚xH2O, 4, an iron nitrate aqueous solution containing a small amount of ascorbic acid to prevent the formation of iron(III) was prepared from FeSO4‚7H2O and Ba(NO3)2 and the same procedure was applied. Infrared spectra of 1-4 are (8) (a) Soyer, H.; Mingotaud, C.; Boillot, M.-L.; Delhae`s, P. Thin Solid Films 1998, 327-329, 435. (b) Langmuir 1998, 14, 5890. (c) Soyer, H.; Dupart, E.; Go´mez-Garcı´a, C. J.; Mingotaud, C.; Delhae`s, P. AdV. Mater. 1999, 11, 382. (d) Colloids Surf., A 2000, 171, 275. (9) Armand, F.; Badoux, C.; Bonville, P.; Ruaudel-Texier, A.; Kahn, O. Langmuir 1995, 11, 3467. (10) Roubeau, O.; Agricole, B.; Cle´rac, R.; Ravaine, S. J. Phys. Chem. B 2004, 108, 15110. (11) Roubeau, O.; Alcazar Gomez, J.; Balskus, E.; Kolnaar, J. J. A.; Haasnoot, J. G.; Reedijk, J. New J. Chem. 2001, 25, 144.

10.1021/la062207x CCC: $37.00 © 2007 American Chemical Society Published on Web 02/15/2007

Organization of Triazole LB Films

Figure 1. Schematic representation of 4-octadecyl-1,2,4-triazole (ODT) (top) and of the polymeric 1D compounds obtained by coordination of ODT to transition metal(II) ions (bottom). Key: red, M(II); blue, N; gray, C. in agreement with a one-dimensional structure with only N1, N2bridging ODT (see Figure 1) (i.e., the intensities of the ring torsion vibration modes of the triazoles around 670 cm-1 are very weak, which occurs when the local triazole ring symmetry is C2V12). The water content was confirmed to be close to x ) 1 by TGA experiments. LB Film Fabrication. For LB film preparations, chloroform (HPLC grade from Prolabo) was the spreading solvent and Millipore Q grade water with a resistivity higher than 18 MΩ‚cm was used for the subphase solutions. For the subphases containing metal ions, the concentration of the corresponding nitrate salt was fixed to 5 × 10-4 mol‚L-1.13 The step-by-step compression of the Langmuir films was performed 10 min after spreading in order to allow the spreading solvent to evaporate. LB films have been obtained by the vertical lifting method using a homemade LB trough working at room temperature under a continuous dried nitrogen flow.14 Glass plates were used as deposition substrates for X-ray diffraction, UV-vis absorption spectroscopy, and atomic force microscopy (AFM) studies, with a number of layers ranging from 2 to 100. Samples (20 layers) for infrared investigations were prepared on fluorine (CaF2) plates. Samples (400-1100 layers) for magnetic studies were elaborated on Mylar substrates (Dupont). CaF2 plates and Mylar substrates were cleaned prior to use with ethanol. When it was needed, the fluorine and glass plates were made hydrophobic by predeposition of three monolayers of arachidic acid. Physical Characterization. Infrared (IR) spectra were recorded on a FTIR 750 Nicolet spectrometer. IR linear dichroism was performed to calculate the out-of-plane dichroic ratio R ) A(i)60°)/ A(i)0°) where A(i) is the absorption coefficient and i the angle between the plane of the LB film and the IR light electric vector. This ratio R is related to the degree of anisotropy out of the substrate plane and allows one to estimate the angle φ between the normal to the substrate and the dipole moment of a particular vibration with a precision of few degrees. All the assumptions made for these calculations are given elsewhere.15 X-ray diffraction experiments at low angles were performed on a homemade experimental setup.16 Magnetic measurements were performed with a Quantum Design MPMS-7XL SQUID magnetometer between 2 and 375 K in fields (12) Haasnoot, J. G.; Groeneveld, W. L. Z. Naturforsch. 1977, 32b, 533. (13) This concentration corresponds to the ideal one obtained in previous studies with metal ions in the subphase. See: Romualdo Torres, G., Ph.D. Thesis, Bordeaux 1 University, 2002. It proved successful and although the subphase concentration is likely to have an influence on the formed LB film properties, the present study only concentrates on experiments with the same concentration to allow comparison. (14) Clemente-Leon, M.; Agricole, B.; Mingotaud, C.; Gomez-Garcia, C. J.; Coronado, E.; Delhae`s, P. Langmuir 1997, 13, 2340. (15) Vandevyver, M.; Barraud, J.; Ruaudel-Texier, A.; Maillard, P.; Gianotti, C. J. Colloid. Interface Sci. 1982, 85, 571. (16) Nguyen, H. T.; Bouchta, A.; Navailles, L.; Barois, P.; Isaert, N.; Twieg, R. J.; Maaroufi, A.; Destrade, C. J. Phys. II Fr. 1992, 2, 1889.

Langmuir, Vol. 23, No. 6, 2007 3111 up to 70 kOe. Mylar substrates of ca. 2.5 × 0.5 cm2 were cut in four pieces that were fit and packed parallel to the applied field within the plastic straw used for measurements. Raw magnetization data were corrected for the free Mylar substrate response determined experimentally. A BAM2plus from NFT was used for the Brewster angle microscopy (BAM) experiments, during which compression isotherms were performed on a NIMA trough (type 601BAM) equipped with a Wilhelmy plate and maintained at constant temperature. The compression speed of the monolayer was close to 1.4 Å2/(molecule/min). Amplitude modulation atomic force microscopy (AM-AFM) can be used to study the morphology of the LB surface, and even the surface order and stability of LB films when atomic resolution can be obtained, by sensing tip-surface atomic forces.17 The present measurements were performed in ambient conditions with a Veeco Dimension 3100 scanning probe microscope, using commercial silicon cantilevers with resonance frequency, f0, of 264-266 kHz and provided with a single-crystal silicon tetrahedral tip, with radius Fe ) Cu > Co and Cu > Ni > Fe > Co, respectively. Brewster angle microscopy (BAM) of monolayers of ODT, 1, 2, 3, and 4 over several subphases (pure water, aqueous solutions of KNO3, Ni(NO3)2, Cu(NO3)2, Co(NO3)2, and Fe(NO3)2) was performed to confirm these observations in situ. Representative BAM images illustrating the present section are shown in Figures 2 and 3, together with the corresponding compression isotherms. All images are identical in size, e.g., 0.5 mm × 0.4 mm. In all cases, at submonolayer coverage (zero pressure) the formation of expanded gas domains of various sizes and shapes is discernible (see for example Figure 2, image e). Upon compression, though, the behavior is highly dependent on the spread compound and the composition of the subphase. ODT spread over pure water (see Figure 2, images a-d) forms a stable homogeneous monolayer up to the collapse. The images are homogeneous and dark up to the collapse, except at pressures where two phases coexist and darker features are observed, corresponding to a (17) Schwartz, D. K.; Garnaes, J.; Viswenathan, R.; Zasadzinski, J. A. N. Science 1992, 27, 508.

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Figure 2. BAM typical images of monolayers of ODT over pure water (top, a-d) and aqueous solution of Co(NO3)2 (middle, e-h), taken at several points of the pressure vs area isotherm (bottom). In both cases, coexistence of two phases corresponding to a transition from liquid-expanded phases (light background) to liquidcondensed phase (darker features) is observed. All images are 0.5 × 0.4 mm.

transition from an expanded liquid phase to a condensed liquid phase (Figure 2, images b, c, f, and g). Coordination polymers 1-4 over pure water also form comparable monolayers (see for example Figure 3, image a) but with the observation of some strongly luminescent objects evidenced as darker dots, present even at low pressure. The number of these dots increases upon compression and are present up to the collapse. These features are therefore not corresponding to the occurrence of a transition from expanded liquid phase to a condensed liquid phase, as confirmed by the absence of any plateau in the pressure vs area

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isotherms, but to inhomogeneities at the surface. In the case of the Co and Cu polymers, the amount of dots is already important at low pressure and only slightly increases with the pressure. On the other hand, for Ni (see Figure 3, images a and b) and Fe polymers a significant amount of these dots only appears at medium pressure, e.g., after more time spent at the air-water interface. Varying the polarization of the incident beam had no effect on the appearance of the luminescent dots, thus excluding the possibility of different orientations of the spread objects as the origin of the observed features. These inhomogeneities are therefore likely due to the persistence at the interface of 3D multilayers of polymer. Another source of inhomogeneities may be the decoordination itself of metal ions from ODT molecules at the interface. In both cases, the relative stability of the coordination polymers at the air-water interface would be at the origin of the differences observed among the four compounds. Thus, it is likely that transferring these films will result in LB films mostly formed of ODT, with a metal ion content dependent on the stability of the polymer at the gas-water interface, and occurring as small oligomeric moieties or even sole cations. Indeed, these hypotheses are in agreement with the observation of strong variations in metal content in films from the various polymers over water.10 Over metal-containing subphases, inhomogeneities are observed in all cases, even with ODT as the spread amphiphile. Over a Co-containing subphase (Figure 2, images e-h; Figure 3, images c and d), these remain smaller and fewer, while the worse cases showing larger and many inhomogeneities are with Ni and Cu subphases (Figure 3, images e and g, respectively). Again this points at a decoordination/ recoordination process occurring at the air-water interface, dependent on the metal ion. Interestingly, the BAM observations are in agreement with the previously proposed10 sequences for the relative stability of compounds 1-4 toward decoordination and the efficiency of recoordination with ODT (see above). It is also to be noted that in all cases a monolayer of ODT and/or ODT with some metal ion coordinated in the subphase is observed and probably accounts for the fabrication of lamellar LB films, though the inhomogeneities and multiple layers observed with BAM point at a complex behavior at the air-water interface and likely have consequences on the organization of transferred LB films. The transfer ratio was found to be close to unity, although slightly inferior, except in the case of ODT over water for which a smaller ratio of ca. 0.8 was found. In both cases, values inferior to 1 indicates some tilting of the ODT molecules in the transferred films. Magnetic Measurements. In our previous work, we used magnetization measurements against the applied field at 2 K to estimate the metal ion content of LB films containing only one metal ion.10 In certain cases, e.g., for films obtained with Cu and Fe polymers over subphases containing the corresponding metal ion, very high metal ion contents were deduced, reaching the maximum value calculated by considering a perfect LB film in which all ODT molecules would bind metal ions by making triple bridges between them. Such observation, though interesting for the overall magnetic properties of the LB films (e.g., the sought spin-crossover properties), may be an indication of defects in the lamellar organization of the transferred LB films, arising from areas of multilayers or even aggregates. A partial thermal spin crossover was detected for films containing only Fe2+ ions, e.g., from 4 over water and Fe2+-containing subphase.10 Additional temperature-dependent measurements were performed for all LB films containing Fe2+ ions. These are plotted in Figure 4, confirming the absence of any spin-crossover phenomenon when various metal ions are present in the LB films.

Organization of Triazole LB Films

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Figure 3. (top) Typical BAM images of 2 spread over water (images a and b) and aqueous solutions of Co(NO3)2 (images c and d) and Ni(NO3)2 (image e), of 4 spread over an aqueous solution of Fe(NO3)2 (image f), and of 3 spread over aqueous solutions of Cu(NO3)2 (image g) and Ni(NO3)2 (image h). Dark features corresponding to inhomogeneities of the Langmuir films are observed in most cases (see text). All images are 0.5 × 0.4 mm. (bottom) Pressure vs area isotherms for 2 (left) and 3 and 4 (right) indicating the pressure at which the BAM images were taken.

Figure 4. Temperature dependence at 1 kOe of the product MT per cm2 of layer of iron-containing LB films deposited over Mylar from Fe(ODT)3(NO3)2 spread over water (empty squares) or an aqueous solution of Fe(NO3)2 (full circles), Co(NO3)2 (empty circles), Ni(NO3)2 (crosses), and Zn(NO3)2 (full squares). Only the data at high temperatures, where a spin crossover may be observed, are shown (T > 100 K).

Infrared Dichroism. IR dichroism is one of the techniques permitting characterizing the organization of the deposited molecules in the LB films, thanks to the evaluation of orientations of different parts of molecules within the films. Figure 5 gives as example two areas of the spectra of a LB film obtained from Fe(ODT)3(NO3)2 spread over an aqueous solution of Ni(NO3)2 for two orientations of the substrate relative to the IR beam. Differences in the intensities of several peaks are observed that are related to the orientations of the ODT molecules. In particular,

peaks corresponding to triazole ring stretches18 at 1370 and 1470 cm-1 allow one to deduce a tilt of ca. 40° of the triazole ring relative to the normal of the substrate. Unfortunately no information could be obtained on vibrations related to M-ligand (ODT and water) bonds. The bands at 2850 and 2920 cm-1 associated to the stretching of C-H bonds characterize the alkyl chains on the triazole, which are supposed to adopt a fully extended all-trans conformation in the LB film. Their dichroic ratios suggest that the average dipole moment of these vibrations is oriented near 55-60° with respect to the substrate normal. Since these dipoles of CH2 vibrations are perpendicular to the hydrocarbon chain axis,19 one can conclude that the hydrocarbon chain axis of ODT is tilted in the LB films relative to the surface with an average angle of about 30-35°. Furthermore one weak peak at 3000 cm-1 is associated with a C-H stretch of the triazole hydrogen atoms. The deduced average tilt angle of 41° is in agreement with that deduced from the triazole ring stretches, given that the C-H average dipole of one triazole lies in the plane of the aromatic triazole ring and is parallel to the substitution axis on the 4-position of the triazole. Comparable average orientations are found for the various metal-containing LB films studied. The common orientation of ODT deduced from these infrared measurements is illustrated in Figure 6. In the pure ODT film, the alkyl chains and the triazole rings were found to have similar average tilts with respect to the normal. In all cases, the observation of out-of-plane dichroism indicates a rather homogeneous organization of the molecules in the LB films. Nevertheless, it is obvious from the initial structure of oligomeric [M(ODT)3]2n+ units (see Figure 1) that several orientations are likely present in the formed LB (18) Bougeard, D.; Le Calve´, N.; Saint Roch, B.; Novak, A. J. Chem. Phys. 1976, 64, 5152. (19) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700.

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Figure 5. Out-of-plane linear IR dichroism of a LB film (20 layers deposited over CaF2) obtained from Fe(ODT)3(NO3)2 spread over an aqueous solution of Ni(NO3)2.

Figure 6. Average orientation of the ODT molecules in the obtained LB films deduced from out-of-plane linear IR dichroism. The thick line represents the aromatic plane of the triazole ring.

films. This is particularly true for the triazole rings, which should keep a kind of propeller-like distribution of their orientations, albeit possibly distorted, though such organization indeed yields one average orientation if all [M(ODT)3]2n+ oligomers are similarly oriented. Such homogeneity, resulting in the observation of out-of-plane dichroism, is likely to be induced by the organization of the fatty tails on the triazole rings. Further evidence for the average tilt of ODT in the LB films and of the overall two-dimensional (2D) organization is given by X-ray diffraction.

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Figure 7. X-ray diffractograms of a LB film (50 layers deposited on quartz) obtained from Ni(ODT)3(NO3) (top) and Fe(ODT)3(NO3) (bottom) spread over an aqueous solution of Ni(NO3)2. Kiessig fringes are observed at low angle. The arrows indicate the (001) Bragg reflections.

Reflectivity X-ray Measurements. X-ray diffraction experiments were performed on LB multilayers (in general 50). In most cases a reflection consistent with a 2D lamellar arrangement is observed at q ) 0.14 Å-1 (see Figure 7), although its intensity and width vary greatly depending on the spread compound and the metal ion present in the subphase. For a Y-type LB film, this reflection gives a layer thickness of ca. 25 Å. In the cases of ODT spread over water or Co(II) subphase, this Bragg peak was either extremely broad or absent. Although this may indicate poorly defined lamellar structures, it is in fact most likely due to the absence or quasi-absence of metal ions in the LB films. Indeed, for purely organic LB films, no significant Bragg peak is to be expected for a low number of layers.20 On the other hand, Kiessig fringes were observed for LB films containing Fe, Ni, and/or Cu, indicating in the corresponding LB films a well-defined thickness. The fringes period is in agreement with the position of the Bragg peak and allows, considering the number of layers deposited, a more accurate evaluation of the layer thickness at 23-26 Å, depending on the LB film (see inset in Figure 7).3 Since the length of one ODT molecule is ∼23 Å, such values are compatible with a lamellar structure of metal ions connecting two layers of tilted ODT molecules. Nevertheless various organizations are possible depending on the coordination mode of the ODT molecules. AFM Observations. Images of LB films with 4, 20, and 50 layers of ODT molecules and obtained over pure water subphase (20) Jego, C. Ph.D. Thesis, University of Bordeaux I, 1997.

Organization of Triazole LB Films

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Figure 8. (left) AFM topographical images (5 × 5 µm2) of LB films of ODT over pure water, with (a) 4, (b) 20, and (c) 50 monolayers of ODT. (right) Height histograms corresponding to the selected areas of the images shown on the left. Full lines represent the deconvolution of the histograms into Gaussian distributions (see text).

were first recorded. Figure 8 (left) shows an example of a 5 × 5 µm2 area of each case, in which lighter contrasts represent higher features. Images a and b, with 4 and 20 layers, respectively, reveal a flat surface, but quite discontinuous and with frequent defects. To estimate the depth of the observed defects, the height histograms of the framed parts of both images were analyzed. The deconvolution of such histograms into Gaussian distributions is also displayed in Figure 8 (right). Note that height is represented in abscises and that the lowest point of each framed surface is set to 0 nm, so only height differences must be considered. In the case of 4 layers (a), the histogram is the sum of three main peaks. Peaks P1 and P3, corresponding to the dark holes and the flat discontinuous surface, respectively, are quite well-defined, and the distance between their maxima is 4.1 nm, that is, the length of two slightly tilted ODT molecules. Peak P2, broader, represents an area situated at the edge of the holes, and its

maximum is situated 2.6 nm above that of peak P1. The histogram of the 20-layer film (b) has also been deconvoluted into three peaks. Peaks P1 and P2 stand for the two main heights and are 1.9 nm distant (the length of one tilted ODT molecule), while peak P3 is quite small and comes from isolated areas, 3.9 nm higher than the layer represented by P1. For the case of 50 layers (c), even if different heights can be distinguished, no histogram deconvolution was performed due to surface roughness. If we compare the mean roughness (Ra) in Figure 8, we obtain values of 1.3, 1.1, and 2.5 nm for 4, 20, and 50 layers, respectively. The study of larger areas (100 and 400 µm2, not shown) does not derive Ra changes for 4 layers, but for 20 layers, Ra increases to 1.4 and 1.9 nm, respectively, and for 50 layers, to 2.8 and 3.0 nm, respectively. Then, we can conclude that the 4-layer LB film is flat, with defects of one or two ODT molecules. The 20-layer one is similar to the former in small areas of several tenths of

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Figure 9. AFM topographical images (20 × 20 µm) of 20-layer LB films obtained from (a) ODT over Cu(II) aqueous solution, (b) Cu coordination polymer 3 over Cu(II) aqueous solution, (c) Cu coordination polymer 3 over Ni(II) aqueous solution, and (d) Fe coordination polymer 4 over Fe(II) aqueous solution.

square micrometers, but the presence of groups of multilayers with different heights makes the roughness rise when broader areas are considered. Finally, in the 50-layer film, there is a lack of flat areas but, however, Ra does not increases very much when studying larger areas, so we can consider that the LB surface is quite homogeneous and still not very rough. BAM measurements show that the formation of LB films over metal-containing subphases involves complex mechanisms, which has direct influence in the film final organization. Figure 9 shows 20 × 20 µm AFM images of the surface of four 20-layer LB films formed from different spread polymers and subphases. In all cases, the stratified structure observed in Figure 8b (left) appears to be lost and, instead, a rougher surface with a different kind of defects is found. Figure 9a shows a ODT over Cu(II) aqueous solution film. The presence of this metal ion, with the highest efficiency of recoordination with ODT, gives rise to a film of Ra ) 7.4 nm, with well-spread defects of two kinds, rounded and elongated, with heights of about 100 and 40 nm, respectively. The film (not shown) formed from ODT over Co(II) aqueous solution (lowest efficiency of recoordination) is similar to that shown in Figure 9a, but with scarce rounded defects, fewer and some lower elongated ones, and a surface roughness of 6.4 nm. In the case of Cu(ODT)3 over Cu(II) aqueous solution (Figure 9b), the rounded defects are more numerous and some of them appear agglomerated in particles of some micrometers in size, all this resulting in a surface roughness of 11.3 nm. This agglomeration is more evident in Figure 9c, which shows a LB

film formed from Cu(ODT)3 over Ni(II) aqueous solution. In this case, isolated defects are rare, and large areas of aggregated particles can be observed. In particular, that in the figure reaches about 100 nm high and 9 µm long. The roughness of the surface free of this defect is 7.8 nm, and that of the whole image is 11.3 nm. Finally, Figure 9d displays the surface of a LB film formed from [Fe(ODT)3](NO3)‚xH2O over Fe(II) aqueous solution, of interest due to its spin-crossover transition. Few, spread, and thicker elongated defects can be observed in this case, resulting in a surface roughness of 6.5 nm.

Discussion Our previous work and the present extensions demonstrate that LB films can be obtained with any variation of the system ODT/water or M2+-containing aqueous subphase and M′(ODT)3(NO3)2/water or M2+-containing aqueous subphase, M and M′ being cations of the first row of transition metals. Nevertheless, in all cases where M is different from M′ both metal ions are found in the formed LB film, which indicate that the expanded polymers are unstable at the air-water interface and that insertion of metal ions from the subphase occurs, both phenomena being dependent on the metal ion. Therefore, the identity of the species forming the LB films is unclear. The origin of the instability of the coordination polymers lies in the propeller arrangement of triazole and therefore triazole fatty substituents. These cannot remain as such once the coordination compound is spread at the air-water interface, where fatty groups will tend to point out of

Organization of Triazole LB Films

the water surface. Though the occurrence of a spin-crossover phenomenon only in the film obtained with M′ ) M ) Fe2+ points at the presence of oligomers of Fe2+ ions connected linearly by triple N1, N2 triazole bridges.21 Thus, either spread polymers are sufficiently stable (with M′ ) Fe2+) to remain at the airwater interface at least as smaller oligomers or oligomers can re-form upon LB film formation. The fact that spin crossover is also obtained when spreading the iron coordination polymer over pure water proves the former hypothesis is likely true. On the other hand, the absence of spin-crossover when M * Fe2+ and M ) Fe2+ is in favor of the latter hypothesis, since if oligomers include other metal ions in addition to Fe2+, then spin crossover is highly unlikely to occur. Aside from these oligomers forming part of the LB films, the observation of SC may also result from 3D large aggregates of the spread coordination polymer, incorporated in the lamellar LB films. Nevertheless, the absence of SC in the cases Fe/Cu and Fe/Ni indicates that even the 3D aggregates form with recoordination from the subphase. With the image of small M(ODT)3 oligomers forming the LB films and having formed during the film fabrication through exchange of metal ions at the air-water interface, the perfect 2D structure expected in a LB film is then questionable, since multilayers and objects differing in size are likely present. Indeed, BAM confirms that spread coordination polymers are unstable at the air-water interface and form multilayer areas and aggregates. This normally impedes the formation of LB films with a correct 2D organization. Nevertheless, ODT over pure water or aqueous subphases containing metal ion with poor uptake by ODT (mostly Co2+) do behave as expected with the observation of Langmuir monolayers and gas to liquid expanded phase transition in BAM experiments. Such a Langmuir monolayer is likely to exist in all cases, underlying below the multilayers and aggregates arising from the two phenomena occurring at the air-water interface. Indeed, IR dichroism and X-ray data indicate that in most cases a lamellar organization has formed in the deposited films. The similar orientation of the ODT molecules in all cases is also in favor of a similar 2D lamellar structure. Therefore, it is likely that it is the existence of underlying Langmuir monolayer of ODT and possibly poor films of oligomers that enables the deposition of LB films. Indeed, it is often that a good amphiphile forming Langmuir monolayers is used to build hybrid LB films of inorganic species or extended systems that would not organize in such a 2D way otherwise.7 Here ODT is playing the role of such an amphiphile and participates as well to the hybrid structure. The AFM studies on few layered LB films of ODT obtained over a pure water subphase indicate a 2D stratified structure, the different levels being observable because of the occurrence of many defects in the LB structure. These defects are probably arising because of the observed transfer ratio of 0.8 in this specific case. Defects in the hydrophobic layer introduced on the substrate may be an explanation of these observations. The rougher morphology of the films with several tens of layers is a consequence of the accumulation of such defects, a fact that is also consistent with the aggregate formation at the air-water interface of the films containing metal ions. The morphology of such films is in good agreement with the BAM observations. Aggregates are observed, in particular with Cu2+ and Ni2+ in the subphase. These aggregates relate to small areas of multilayers, (21) So far Fe(II) compounds with monocoordinated triazole have always shown a high-spin ground state. See: Haasnoot, J. G. Coord. Chem. ReV. 2000, 200-202, 131. For example, in Fe(II) trimers with triple-N1, N2-triazole bridges, only the central Fe(II) ions presents a spin-crossover. See: Kolnaar, J. J. A.; van Dijk, G.; Kooijman, H.; Spek, A. L.; Ksenovontov, V.; Gu¨tlich, P.; Haasnoot, J. G.; Reedijk, J. Inorg. Chem. 1997, 36, 2433.

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as observed in BAM. The largest aggregates present some flat thick structures with terraces that also imply multilayered structure and are thus 3D crystallites. In this context, it is to be mentioned that similar structures of crystallized fatty acids have been observed by BAM.22 It is thus likely that these extended structures are formed during the compression and are subsequently squeezed out of the monolayer on its surface. 3D crystallites were also observed in mixed LB films of a dye and stearic acid.23 The size of the largest 3D aggregates observed in the present study is larger and would correspond to up to 20 bilayers. Even in areas free of such aggregates, the overall roughness is much higher than in the case of ODT, resulting from tiny grainlike species that are probably small inhomogeneities in the LB film organization resulting from the presence/absence of small oligomers (see Figure S1). Indeed no detection of the different strates present in a well-organized LB films was possible, except for pure ODT. Nevertheless, the overall planarity of these films, together with IR and X-ray data, demonstrate a 2D lamellar structure, incorporating metal-containing triazole oligomers.

Conclusions Two-dimensional Langmuir-Blodgett assemblies of octadecyltriazole (ODT)-based metal containing oligomers can be obtained with first-row transition metal ions both in a spread coordination polymer, which acts as a semiamphiphile, and in the aqueous subphase. Specifically in the cases containing only Fe(II), a thermal spin-crossover phenomenon, typical for these triazole-based coordination polymers, is exhibited by the LB films. Two processes occurring at the air-water interface dominate the mechanism of formation of these LB films, the cutting of coordination polymers at the air-water interface into smaller aggregates, with liberation of metal ions into the subphase, the recoordination of metal ions in from the subphase occurs at the interface. The Langmuir film allowing the LB film formation is mostly made of ODT, and these are tilted and incorporate coordinated isolated metal ions and oligomers of metal ions in the LB films. Except for pure ODT, which gives flat films with low roughness, the morphology of the LB films presents a high roughness resulting from an inhomogeneous distribution of oligomers, multilayers, and crystallites, which are more important in the Cu and Ni cases, in agreement with the relative affinity of the metal ions with ODT and the relative stability of the coordination polymers at the air-water interface. Although successful in obtaining LB films of triazole-based compounds presenting the spin-crossover property, our studies highlight the difficulty to translate a supramolecular assembly together with its physical properties of interest, here the spin-crossover, into well-defined lamellar LB structures. Acknowledgment. The authors are grateful to Dr. Christophe Mingotaud (IMRCP, Toulouse) for allowing BAM experiments and thank Marie-France Achard for her help in X-ray diffraction measurements. This work was supported by the CNRS, the University of Bordeaux 1, and the Re´gion Aquitaine. Eva Natividad is grateful to the Spanish Ministerio de Educacio´n y Ciencia for her postdoctoral fellowship. Supporting Information Available: A figure showing a schematic projection of the organization of the LB films in areas free of large aggregates. This material is available free of charge via the Internet at http://pubs.acs.org. LA062207X (22) Siegel, S.; Honig, D.; Vollhardt, D.; Mobius, D. J. Phys. Chem. 1992, 96, 8157. (23) Dutta, A. K.; Vanoppen, P.; Jeuris, K.; Grim, P. C. M.; Pevenage, D.; Salesse, C.; De Schryver, F. C. Langmuir 1999, 15, 607.