Langmuir Films of an Oligo(p-phenylene vinylene) Functionalized with

In Final Form: March 1, 2001. A detailed investigation is presented on the Langmuir film characteristics of a novel substituted oligo-. (p-phenylene v...
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Langmuir Films of an Oligo(p-phenylene vinylene) Functionalized with a Diaminotriazine Headgroup D. T. Balogh,† A. Dhanabalan,‡ P. Dynarowicz-Ła¸ tka,§ A. P. H. J. Schenning,*,‡ O. N. Oliveira, Jr.,† E. W. Meijer,‡ and R. A. J. Janssen‡ Instituto de Fisica de Sao Carlos, Universidade de Sao Paulo, C.P. 369, 13560-970, Sao Carlos, SP, Brazil, Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Faculty of Chemistry, Jagiellonian University, 30-060 Krako´ w, Poland Received September 25, 2000. In Final Form: March 1, 2001 A detailed investigation is presented on the Langmuir film characteristics of a novel substituted oligo(p-phenylene vinylene) with a diaminotriazine headgroup, namely, 6-[4-{4-(3,4,5-tridodecyloxystyryl)2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl]phenyl-2,4-diamino-1,3,5-triazine (OPVT). The films were characterized by recording surface pressure (π-A) and surface potential (∆V-A) isotherms, in addition to Brewster angle microscopy (BAM). The limiting mean molecular area indicates formation of a monomolecular Langmuir film of OPVT at the air-water interface. The dependence of a plateau transition in the π-A isotherm on experimental conditions is studied. The critical area for the increase in surface potential revealed the interaction of OPVT molecules even at a large area per molecule, supported by BAM images. On the basis of the data, we propose a model for the organization of OPVT molecules at different stages of film compression.

Introduction The Langmuir-Blodgett (LB) technique has been extended in recent times to a number of non- and semiamphiphilic materials with a rich variety of structures.1 For conjugated polymers, in particular, LB manipulation is often accomplished utilizing special molecular engineering approaches, primarily because of the poor processibility of these rigid-rod systems.2 A recent report on monomolecular Langmuir films from tailor-made substituted polythiophenes underlines the importance of molecular design to achieve a hydrophilic-hydrophobic balance to obtain well-spread monolayers at the air-water interface.2,3 Organizational control over the orientation of conjugated polymer chains within a thin film structure is important for achieving improved performances.3,4 * Corresponding author. E-mail: [email protected]. † Universidade de Sao Paulo. ‡ Eindhoven University of Technology. § Jagiellonian University. (1) For examples, see: (a) Popovitz-Biro, R.; Edgar, R.; Weissbuch, I.; Lavie, R.; Cohen, S.; Kjaer, K.; Als-Nielsen, J.; Wassermann, E.; Leiserowitz, L.; Lahav, M. Acta Polym. 1998, 49, 626-635. (b) Dhanabalan, A.; Dabke, R. B.; Talwar, S. S.; Contractor, A. Q.; Prasanth Kumar, N.; Major, S.; Lal, R. Langmuir 1997, 13, 4395-4400. (c) Tredgold, R. H. Order in Thin Organic Films; Cambridge University Press: Cambridge, 1994. (d) Dhanabalan, A.; Balogh, D. T.; Riul, A., Jr.; Giacometti, J. A.; Oliveira, O. N., Jr. Thin Solid Films 1998, 323, 257-264. (e) Ravaine, S.; Mingotaud, C.; Delhaes, P. Thin Solid Films 1996, 284-285, 76-79. (f) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D.; Philp, D.; Preece, J. A.; Ringsdorf, H.; Fraser, S. J.; Wildburg, G. Thin Solid Films 1996, 284-285, 671-677. (g) Schenning, A. P. H. J.; ElissenRoman, C.; Weener, J. W.; Baars, M. W. P. L.; van der Gaast, S. J.; Meijer, E. W. J. Am. Chem. Soc. 1998, 120, 8199-8208. (h) Petty, M. C. Langmuir-Blodgett Films - An Introduction; Cambridge University Press: Cambridge, 1996. (i) Bjørnholm, T.; Hassenkam T.; Reitzel, N. J. Mater. Chem. 1999, 9, 1975-1990. (2) Bjørnholm, T.; Greve, D. R.; Reitzel, N.; Hassenkam, T.; Kjaer, K.; Howes, P. B.; Larsen, N. B.; Bogelund, J.; Jayaraman, M.; Ewbank, P. C.; McCullough, R. D. J. Am. Chem. Soc. 1998, 120, 7643-7644. (3) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685-688. (4) Fichou, D. J. Mater. Chem. 2000, 10, 571-588 and references therein.

Because of the poor solubility of poly(p-phenylene vinylene) (PPV), LB films of PPVs are usually obtained through a postdeposition heat treatment of LB films of the precursor polymer.5 Recently, Marletta et al. observed a polarization dependent emission from highly oriented LB films from PPV prepared using a long-chain counterion in the precursor polymer.6 Well-defined conjugated oligomers, the low molecular weight counterparts of the polymers, can serve as ideal model systems to elucidate the organization of the rather complicated polymeric materials at the air-water interface, with a further advantage of improved processibility.7 In this paper, we investigate the Langmuir film forming characteristics of a novel amphiphilic oligo(p-phenylene vinylene) functionalized with diaminotriazine (6-[4-{4(3,4,5-tridodecyloxystyryl)-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl]phenyl-2,4-diamino-1,3,5-triazine, OPVT, Figure 1), using surface potential isotherms and Brewster angle microscopy, in addition to surface pressure measurements. The design of OPVT includes the functionalization of the conjugated segment, that is, the oligo(p-phenylenevinylene) unit with a diaminotriazine group which serves as a strong polar group on one end and a mesogenic tridodecyloxyphenyl group that acts as hydrophobic tail on the other end. To the best of our knowledge, this is the first systematic study on monolayer characteristics of this type of compounds. The results are compared to those of diaminotriazine headgroup functionalized amphiphiles, which have re(5) (a) Hagting, J. Langmuir-Blodgett films of poly(phenylenevinylene) precursor polymers. Ph.D. Thesis, University of Groningen, Groningen, The Netherlands, 1999. (b) Era, M.; Kamiyama, K.; Yoshiura, K.; Momii, T.; Murata, H.; Tokito, S.; Tsutsui, T.; Saito, S. Thin Solid Films 1989, 179, 1-8. (c) Nishikata, Y.; Kakimoto, M.; Imai, Y. Thin Solid Films 1989, 179, 191-197. (d) Era, M.; Shinozaki, H.; Tokito, S.; Saito, T. Chem. Lett. 1988, 1097-1099. (e) Wu, A.; Yokayama, S.; Watanabe, S.; Kakimoto, M.; Imai, Y.; Araki, T.; Iriyama, K. Thin Solid Films 1994, 244, 750-753. (6) Marletta, A.; Gonc¸ alves, D.; Oliveira, O. N., Jr.; Faria, R. M.; Guimara˜es, F. E. G. Macromolecules 2000, 33, 5886-5890. (7) Nakahara, H.; Nakayama, J.; Hoshino, M.; Fukuda, K. Thin Solid Films 1988, 160, 87-97.

10.1021/la001356r CCC: $20.00 © 2001 American Chemical Society Published on Web 05/03/2001

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Figure 1. The structure of OPVT.

ceived attention because of their ability to recognize complementary guest molecules from the water subphase via multiple hydrogen bonding interactions.8 By also comparing the results with that from simple alkyl chain derivatized as well as azobenzene chromophores containing alkyl chain derivatized diaminotriazine amphiphiles,9 we could propose a most probable scenario for the organization of OPVT molecules during film compression. Experimental Section The synthesis of OPVT has been published elsewhere.10 Monolayer experiments and multilayer LB deposition were performed with a KSV-5000 LB (Finland) instrument placed on an antivibration table in a class 10 000 clean room. Ultrapure water supplied by a Milli-Q water purification system coupled to a Nanopure (Infinity) system (resistivity ) 18.2 MΩ cm) was used as the subphase. Spectroscopic grade chloroform was used for spreading OPVT, with a typical concentration of 0.3 mg/mL, at the water surface. The subphase temperature was controlled thermostatically within (0.1 °C, and most of the monolayer experiments were performed at a subphase temperature of 10 °C, unless mentioned otherwise. Typically, film compression started 5-10 min after spreading, to allow for evaporation of the spreading solvent. The Langmuir film was compressed with different speeds and the change in surface pressure was monitored, with an accuracy of 0.1 mN/m, with a Wilhelmy plate (made from ashless Whatman Chr 1 chromatography paper) connected to a pressure sensor. Simultaneously, the surface potential was also monitored (with an accuracy of (10 mV) using a Kelvin probe located ∼2 mm above the water surface. The platinum foil reference electrode was kept immersed in the subphase during the measurements. The monolayer morphology was investigated with Brewster angle microscopy (model miniBAM, Nanofilm Technologie GmbH, Go¨ttingen, Germany), with the images being obtained at various stages of compression.

Results and Discussion 1. Surface Pressure-Area (π-A) Isotherms. Figure 2 shows a typical surface pressure-area isotherm of the Langmuir film of OPVT at a subphase temperature of 10 °C, which consists of three distinct regimes: an initial liquid condensed region (a), a plateau region which spans over the area of ∼15 Å2 (b), and finally a condensed region (c) prior to film collapse at about 47 mN/m. The isotherm is similar to that reported for an oligo(p-phenylene (8) (a) Honda, Y.; Kurihara, K.; Kunitake, T. Chem. Lett. 1991, 681684. (b) Kurihara, K.; Ohto, K.; Honda, Y.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 5077-5079. (c) Taguchi, K.; Ariga, K.; Kunitake, T. Chem Lett. 1995, 701-702. (d) Emge, A.; Ba¨uerle, P. Synth. Met. 1999, 102, 1370-1373. (9) (a) Paleos, C. M.; Tsiourvas, D. Adv. Mater. 1997, 9, 695-710. (b) Ahlers, M.; Ringsdorf, H.; Rosemeyer, H.; Seela, F. Colloid Polym. Sci. 1990, 268, 132-138. (c) Sasaki, D. Y.; Kurihara, K.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 9685-9686. (d) Sasaki, D. Y.; Kurihara, K.; Kunitake, T. J. Am. Chem. Soc. 1992, 114, 10994-10995. (10) Schenning, A. P. H. J.; Jonkheijm, P.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2001, 123, 409-416.

Figure 2. Surface pressure-area (π-A) isotherm of OPVT at the air-water interface (subphase temperature of 10 °C) consisting of a liquid condensed region (a), a plateau region (b), and a condensed region (c).

vinylene) derivative containing a pyridine headgroup, but without the 3,4,5-tridodecyloxyphenyl mesogenic group.11 For instance, for OPVT the onset area for surface pressure was ∼110 Å2 and the limiting mean molecular area obtained by extrapolation of the steep portion of the isotherm was ∼75 Å2, which is similar to ∼110 and 84 Å2, respectively, for the oligo(p-phenylene vinylene) investigated previously.11 The areas measured for OPVT are, however, much higher than those reported for simple alkyl chain derivatized diaminotriazine (∼30 Å2).8a It suggests that the observed limiting mean molecular area is associated to the oligo(p-phenylene vinylene) segment of the OPVT. There are several factors that may cause the appearance of a plateau region in surface pressure isotherms. Examples from the literature are (a) coexistence of monolayer phases such as gas-liquid and liquid expanded and liquid condensed in phospholipid monolayers,12 (b) first-order phase transitions associated with the lifting-off from the water of one polar group of a bipolar compound during film compression,13 (c) collapse of the monolayer film into multilayer film as observed for biphenyl and terphenyl derivatives,14 (d) molecular orientational changes upon film compression in physiologically active compounds,15 (e) a combination of both orientational changes and monolayer collapse in carboxylic acid substituted triphenylbenzene compounds,16 (f) dissolution of monolayer material in the water subphase upon film compression as in case of monolayers of some polypeptides,17 and (g) squeezing out of nonamphiphilic material from the matrix (11) Schenning, A. P. H. J.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 4489-4495. (12) Yu, S.; Zaitsev, V. P.; Vereschetin, V. P.; Zubov, W.; Zeiss, W.; Mobius, D. Thin Solid Films 1996, 284-285, 667-670. (13) Legre, J. P.; Albinet, G.; Caille, A. Can. J. Phys. 1982, 60, 894899. (14) (a) De Mul, M. M. G.; Mann, J. A. Langmuir 1994, 10, 23112316. (b) Schroter, J. A.; Plehnert, R.; Tschierske, C.; Katholy, S.; Janietz, D.; Penacorada, F.; Brehmer, L. Langmuir 1997, 13, 796-800. (15) Romeu, N. V.; Trillo, J. M.; Conde, O.; Casas, M.; Iribarnegaray, E. Langmuir 1997, 13, 71-75. (16) (a) Dynarowicz-Ła¸ tka, P.; Dhanabalan, A.; Oliveira, O. N., Jr. J. Phys. Chem. B 1999, 103, 5992-6000. (b) Dynarowicz-Ła¸ tka, P.; Dhanabalan, A.; Oliveira, O. N., Jr. Langmuir 2000, 16, 4245-4251. (c) Dynarowicz-Ła¸ tka, P.; Dhanabalan, A.; Cavalli, A.; Oliveira, O. N., Jr. J. Phys. Chem. B 2000, 104, 1701-1707. (17) Vila Romeu, N.; Minones Trillo, J.; Conde, O.; Casas, M.; Iribarnegaray, E. Langmuir 1997, 13, 71-75.

Langmuir Films of an Oligo(p-phenylene vinylene)

of an amphiphilic material in mixed monolayers.18 To identify the most probable origin(s), one has to perform systematic investigations on how the π-A isotherms depend on experimental parameters such as subphase temperature and compression speed. For instance, if the plateau appearance depends on temperature, its cause is probably associated to (a) and (c) above, whereas a dependence on compression speed may indicate the possibility of material dissolution. To start with, we rule out (b) and (g) as causes for the plateau transition, because a pure (not mixed) monolayer of OPVT, which contains a single diaminotriazine headgroup, is being investigated here. We investigated the origin of the plateau in the surface pressure-area isotherm of OPVT by performing a set of experiments with systematic variation of parameters. Hysteresis experiments in which several compression/ decompression cycles were performed indicated that only for the first cycle is there some shift of the isotherm to lower molecular areas. In subsequent cycles, the changes in the molecular area are insignificant, demonstrating that stable monolayers are formed. Interestingly, the plateau transition region disappeared after few compression-decompression-recompression cycles, possibly indicating that small organizational or conformational changes, which occurred during the initial compression, remain intact in subsequent cycles. The formation of a reasonably stable Langmuir film of OPVT was also confirmed in specific stability experiments where the monolayer was kept at a fixed pressure and the area changes were monitored. Therefore, the appearance of the plateau cannot be attributed to loss of material to the subphase (factor f) or to monolayer collapse (factor c). Also, the close matching of the limiting mean molecular area of OPVT with that calculated based on molecular modeling ruled out the possibility of a transformation of a monolayer to a bilayer/multilayer structure (factor c) as the origin of the observed plateau transition. Monolayer characteristics for OPVT were not affected by the compression speed in the range 50-200 mm/min, which is typical for the experiments in the present trough arrangement. This rules out the possibility of nonequilibrium compression being the reason for the observed plateau. Only when very low speeds were employed, for example, 10 mm/min, was the plateau transition found to disappear. It may be that when the monolayer is compressed at a very low speed, OPVT molecules organize slowly, without an intermediate transition state. The surface pressure-area isotherms of OPVT, obtained with a compression speed of 25 mm/min and at different subphase temperatures, are shown in Figure 3. As shown, the plateau transition does not completely disappear upon increasing the subphase temperature while the collapse pressure did not change significantly. On the basis of such behavior, which is similar to that reported for some model amphiphilic compounds such as alcohols and phospholipids,12-19 the plateau in the surface pressure isotherm of OPVT is attributed to the coexistence of liquid expanded/ liquid condensed phases related to the transition from a less ordered expanded state to a more ordered condensed state (a). To get further insight into this point, the surface compressibility modulus (Cs-1) (the reciprocal of compressibility) value derived from the slope of the surface pressure-area isotherm curve20 is plotted against mean molecular area (Figure 4). For comparison, the surface (18) Watanabe, I.; Hong, K.; Rubner, M. F. Langmuir 1990, 6, 11641172. (19) Siegel, S.; Volhardt, D. Thin Solid Films 1996, 284-285, 424427.

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Figure 3. Surface pressure-area (π-A) isotherms of OPVT obtained at different subphase temperatures [compression speed ) 25 mm/min].

Figure 4. Surface compressibility modulus-area isotherm of OPVT at the air-water interface, at a subphase temperature of 10 °C. (The surface pressure-area isotherm from Figure 2 is reproduced, for comparison.)

pressure isotherm curve from Figure 2 is also shown. Initially, upon film compression Cs-1 increased up to an area coinciding with the beginning of the plateau. Subsequent compression into the plateau region led Cs-1 to decrease to a nonzero pseudo minimum value at an area corresponding to the middle of the plateau. Upon further compression, Cs-1 raised sharply until monolayer collapse; that is, formation of a highly compressible phase is evident from the sharp decrease of Cs-1. These results clearly demonstrate that the film does not collapse in the plateau region and that the collapse occurred only at the end of the condensed region of the isotherms (∼47 mN/ m). Such a conclusion is further supported by Brewster angle microscopy, which will be discussed later. In a separate control experiment, the variation of spreading volume was found not to influence the surface pressurearea isotherm significantly. 2. Surface Potential-Area (∆V-A) Isotherm. The surface potential-area isotherm is particularly useful because it provides information at early stages of film compression when the surface pressure is still zero. For (20) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966.

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Figure 5. Surface potential-area (∆V-A) isotherm of OPVT at the air-water interface, at a subphase temperature of 10 °C.

simple aliphatic compounds, the measured surface potential can be related to molecular dipole moments using two-layer or three-layer capacitor models.21 For our OPVT amphiphile, however, this is not possible because the various contributions to the molecular dipole moments cannot easily be obtained. Therefore, only a qualitative analysis of the surface potential data is attempted here. A typical surface potential-area isotherm for OPVT is shown in Figure 5. The critical area, defined as the onset area for surface potential, is ∼150 Å2 which is about 40 Å2 higher than the onset area for the surface pressure. Upon compression, the surface potential increased rather sharply up to a maximum value of ∼270 mV at an area of 110 Å2. Interestingly, this area coincides with the onset area for surface pressure. Upon further compression, no significant change in the surface potential was observed, despite the observed changes in the surface pressure. Further information can be gained by calculating the apparent dipole moment (µn) which is the vertical component of the dipole moment of the free molecule, using the Helmholtz equation,22 that is, ∆V ) µn/A0, wherein 0 is the dielectric permittivity of free space,  corresponds to a relative permittivity, and A corresponds to molecular area. A detailed discussion on the value of  can be found in the literature,23 but it is customary to assume  ) 1 for the sake of calculation.24 µn is plotted against the mean molecular area in Figure 6, indicating an increase at the critical area, reaching a maximum value of 0.58 D [1 D ) 3336 × 10-30 C m] at the onset area for surface pressure. Upon further film compression, a drastic decrease of µn was observed, which could be attributed to a bilayer formation because such a centrosymmetric structure would cause a substantial decrease of µn.22 This hypothesis is not likely, though, because the surface potential should also decrease.25 In addition, the estimated area per molecule, obtained by extrapolating the condensed portion of the isotherm (∼75 Å2), is consistent with a true (21) (a) Demchak, R. J.; Fort, T. J., Jr. J. Colloid Interface Sci. 1974, 46, 191-200. (b) Taylor, D. M.; Oliveira, O. N., Jr.; Morgan, H. J. Colloid Interface Sci. 1990, 139, 508-518. (c) Oliveira, O. N., Jr.; Bonardi, C. Langmuir 1997, 13, 5920-5924. (22) (a) Dynarowicz-Ła¸ tka, P.; Dhanabalan, A.; Oliveira, O. N., Jr. Adv. Colloid Interface Sci., in press. (b) Davies, J. T.; Rideal, E. K. Interfacial phenomena, 2nd ed.; Academic Press: New York, 1963. (23) Oliveira, O. N., Jr.; Taylor, D. M.; Lewis, T. J.; Salvagno, S.; Stirling, C. J. M. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1009-1014. (24) Dynarowicz-Ła¸ tka, P. Adv. Colloid Interface Sci. 1993, 45, 215241. (25) Dhanabalan, A.; dos Santos, D. S., Jr.; Balogh, D. T.; Giacometti, J. A.; Oliveira, O. N., Jr. Thin Solid Films, submitted.

Figure 6. Effective dipole moment-area isotherm of OPVT at the air-water interface, at a subphase temperature of 10 °C. (The surface pressure-area isotherm from Figure 2 and the surface potential-area isotherm from Figure 6 are reproduced, for comparison.)

monolayer of OPVT. The probable cause for the decrease in dipole moment is molecular reorientation, similar to what has been suggested for monolayers of a symmetrical triphenylbenzene ring system.22 Accordingly, we propose the following scenario for the organization of OPVT molecules at the air-water interface, at the different stages of film compression. At a very large mean molecular area, the OPVT molecule lies almost horizontally at the air-water interface. Upon compression, the molecules start to interact at the critical area and the hydrophobic portion of the molecule consisting of the oligophenylenevinylene unit with terminal 3,4,5-tridodecyloxyphenyl substituents lifts off from the water surface gradually to a vertical position while the hydrophilic diaminotriazine head is anchored at the surface. Such orientational changes are supported by the significant increase of the surface potential and the effective dipole moment at this stage. At the area where the surface pressure starts to increase, both surface potential and effective dipole moment reached their maximum value, indicating a near vertical orientation of the OPVT molecules. The compression beyond this point mainly led to the tilting of the OPVT molecule as a whole in order to maximize the interdigitation of 2-methylbutoxy side chains of adjacent molecules, as seen by the increase of the surface pressure as well as the decrease of effective dipole moment, similar to that reported by DynarowiczŁa¸ tka et al.22 3. Brewster Angle Microscopy (BAM). BAM analysis has been used extensively to investigate monolayer phase changes, especially in model amphiphilic molecules.26 For the Langmuir film of OPVT, Figure 7 shows that small aggregates appear even at large mean molecular areas (a). These aggregates come together to form a homogeneous film when the surface pressure is still practically zero (b and c), supporting the interpretation made of surface potential and effective dipole moment isotherms in terms of a preorganization of OPVT molecules even before the onset of surface pressure. In the compressed state, the film is highly uniform with no defects (d), and this rules out the possibility of bilayer formation as the reason for the decrease of the effective dipole moment. (26) Knobler, C. M. Adv. Chem. Phys. 1990, 7, 397-449.

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Figure 7. BAM images of a Langmuir film of OPVT at the air-water interface, at different stages of compression: (a) at 160 Å2/molecule, (b) at 130 Å2/molecule, (c) at 110 Å2/molecule, (d) at 70 Å2/molecule, and (e) at 45 Å2/molecule.

Upon overcompression, cracks and defects start to appear (e), indicating film collapse. Conclusions The Langmuir forming characteristics of OPVT have been investigated, using surface pressure isotherms, surface potential isotherms, and Brewster angle microscopy. Detailed analysis of the results allowed us to propose a model for the organization of OPVT molecules at the air-water interface. Though the present study is akin to Langmuir films on pure water, the results discussed here have important implications on the use of Langmuir films of diaminotriazine-based amphiphiles for recognizing complementary molecules present in the water subphase via multiple hydrogen bonding interactions.8 Preliminary experiments indicated the possibility of uniform transfer of the OPVT Langmuir film onto the substrates. The

studies on the transferred LB films are in progress and will be published separately. Acknowledgment. This work is financially supported by the Dutch Ministry of Economic Affairs, the Ministry of Education, Culture and Science, and the Ministry of Housing, Spatial Planning and the Environment through the E.E.T. program (EETK97115), the Council for Chemical Sciences of the Netherlands Organization for Scientific Research (CW-NWO) and the Eindhoven University of Technology in the PIONIER program (98400), and the Royal Netherlands Academy of Art and Sciences. Financial support from Fapesp and CNPq (Brazil) is also acknowledged. LA001356R