A Proposed Strategy To Design Molecules Predisposed to In-Plane

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A Proposed Strategy To Design Molecules Predisposed to In-Plane Orientation in Langmuir-Blodgett Films C. Jego, B. Agricole, M.-H. Li, E. Dupart, H. T. Nguyen, and C. Mingotaud* Centre de Recherche Paul PascalsCNRS, Avenue A. Schweitzer, F-33600 Pessac, France Received October 20, 1997. In Final Form: January 27, 1998 Assuming that in-plane orientation in Langmuir-Blodgett (LB) films can be obtained from compounds having an anisotropic shape within the plane of the Langmuir film, this particular alignment is expected for calamitic compounds with a side-on polar head or with a hydrophilic function linked to the end of a tilted rodlike group. According to such hypothesis, various calamitic molecules were synthesized and showed indeed in-plane orientation. Clearly, these molecules can be described as two-dimensional calamitic compounds forming a new family of molecules with a different orientation than that of the classical disklike derivatives. These experiments demonstrate that analysis of the molecular shape anisotropy enables one to predict in-plane orientation in LB films and to control the main orientation of the compounds within the plane of the multilayers.

Introduction The Langmuir-Blodgett (LB) technique1,2 has been widely used to organize molecules in a multilayer architecture. Because of the particular orientation of the molecules at the gas-water interface, a classical anisotropy is found in LB films between out-plane and in-plane properties. On the other hand, in-plane organization of the multilayers is often considered to be totally isotropic. Recently, a new phenomenon of orientation during transfer of the Langmuir film onto a solid substrate has been demonstrated for various compounds.3-5 This behavior is mainly observed with polymers having a rodlike shape6,7 or for self-aggregating molecules8-10 (e.g., phthalocyanines). Flow orientation models11-14 have been proposed to explain the orientation during the transfer which then leads to anisotropic LB films. If the orientation process is now more or less understood, the design of molecules in order to get any in-plane orientation is not so straightforward. However, a simple hypothesis is that in-plane alignment can be observed only for molecules developing * To whom correspondence should be addressed: e-mail, [email protected]. (1) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1966. (2) Ulman, A. Introduction to ultrathin organic films; Academic Press: Boston, 1991. (3) Pe´rez, J.; Vandevyver, M.; Strzelecka, H.; Veber, M.; Jallabert, C.; Barraud, A. Liq. Cryst. 1993, 14, 1627. (4) Penner, T. L.; Schildkraut, J. S.; Ringsdorf, H.; Schuster, A. Macromolecules 1991, 24, 1041. (5) Peiffer, S.; Mingotaud, C.; Garrigou-Lagrange, C.; Delhaes, P.; Sastre, A.; Torres, T. Langmuir 1995, 11, 2705. (6) Jego, C.; Leroux, N.; Agricole, B.; Mingotaud, C. Liq. Cryst. 1996, 20, 691. (7) Sauer, T.; Arndt, T.; Batchelder, D.; Kalachev, A. A.; Wegner, G. Thin Solid Films 1990, 187, 357. (8) Albouy, P.-A. J. Phys. Chem. 1994, 98, 8543. (9) Vandevyver, M.; Albouy, P.-A.; Mingotaud, C.; Pe´rez, J.; Barraud, A.; Karthaus, O.; Ringsdorf, H. Langmuir, 1993, 9, 1561. (10) Cook, M. J.; McMurdo, J.; Miles, D. A.; Poynter, R. H.; Simmons, J. M.; Haslam, S. D.; Richardson, R. M.; Welford, K. J. Mater. Chem. 1994, 4, 1205. (11) Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. Solid State Commun. 1988, 65, 1259. (12) Schwiegk, S.; Vahlenkamp, T.; Xu, Y.; Wegner, G. Macromolecules 1992, 25, 2513. (13) Mingotaud, C.; Agricole, B.; Jego, C. J. Phys. Chem. 1995, 99, 17068. (14) Ikegami, K.; Mingotaud, C.; Delhaes, P. Phys. Rev. E 1997, 56, 1987.

in-plane anisotropic interactions in the Langmuir film. Then, molecules presenting (when they are organized in a monolayer) an anisotropic shape within the plane of the interface can lead to in-plane orientation in LB films. According to such an idea, compounds can be sorted depending on their shape and their orientation at the interface (see Figure 1). Beginning with molecules having a discotic shape in bulk (such as phthalocyanines, triphenylenes, ...), a polar group can be schematically added at the edge or in the middle of such a moiety to get amphiphilic derivatives suitable for Langmuir experiments. The organization of these molecules will then be very different at the interface if the polar group is strong enough to impose the interfacial orientation to the compound. When the polar group is attached to the middle of the discotic part (or when many polar groups are linked all around such disk), the obtained derivative should lie approximately flat on the water. Within the plane, such a molecule should be considered as isotropic and will not easily develop any in-plane orientation in LB films. On the contrary, when the polar head is linked to the edge of the discotic part (see Figure 1), the molecules should adopt an edge-on orientation in the Langmuir film. The projection of these molecules on the water plane is then highly anisotropic. Such derivatives can then be regarded as two-dimensional (2D) discotic molecules. This is evidently the case for all self-aggregating molecules already described in the literature presenting a more or less edge-on configuration and/or some columnar stacking (even if the interfacial orientation of these derivatives is mainly due to strong intermolecular π-π interactions and is not imposed by a strong polar head). Starting from a calamitic group, a similar approach can be applied. A polar head can be linked either to the end or to the middle of the rodlike group. In the first case (which is the classical one), the molecules will stand up on the water surface. The top view of such an organized compound is isotropic in shape and no in-plane orientation is expected. However, if the molecule is tilted at the gaswater interface, the apparent in-plane shape is now slightly anisotropic. It can be described as pseudo 2D calamitic: its calamitic character is highly related to the tilt angle of the molecule at the interface. Finally, if the polar group is added at the middle of the calamitic compound, the obtained side-on derivative

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Figure 1. Schematic organization and anisotropy of various molecules at the gas-water interface (see text). The rigid core of the molecule is in gray and the added polar group is symbolized by a full circle.

should have the particular orientations described in Figure 1. These orientations should be similar to the one of sideon liquid crystal polymer described by C. Jego et al.6 In the plane of the monolayer, such molecule appears like a (tilted) 2D calamitic derivative. Following this schematic classification of molecules in Langmuir films, in-plane orientation can be expected from calamitic derivatives if they have a polar head fixed in the middle of their rigid core or if the polar head is linked to the end of this group and if the molecule is tilted in the Langmuir film. To validate such an approach, various compounds were synthesized (see Figure 2) and for each case, the molecular orientation within the LB films has been determined by infrared (IR) dichroism. Experimental Methods Synthesis of the derivatives used in this work will be described elsewhere. The ultimate purifications were done by highperformance liquid chromatography and the purities of the compounds were checked by 1H NMR and thin-layer chroma-

tography. The compression isotherm curves were recorded using a homemade through working at room temperature with a constant speed of typically 1.5 Å2/(molecule‚min). The accuracy in the surface pressure is close to 0.1 mN/m and in areas to 5%, except in the case of the PY5 compound for which the accuracy in the areas is evaluated to 10-15%. Indeed, because of the small amount of PY5 available, the solutions used for the spreading had a poorly accurate concentration. LangmuirBlodgett films have been obtained by the vertical lifting method using a large-area ATEMETA trough working at room temperature under a continuous dried nitrogen flow. A stepwise compression was performed to reach the transfer surface pressure (30 mN/m for MS, 20 mN/m for CY, and 12 mN/m for PY compounds). Dipping speed was set in order to get the best transfer ratio and the highest order parameter: 1 cm‚min-1 for MS, CY, and PY5; 10 cm‚min-1 for the PY3 compound and mixtures. For this last experimental condition, drying the substrate after each upper stroke was necessary to have a regular deposition. Except for the pure monolayer of PY5 and the 2/1 PY5-PY3 mixture, Y-type LB films were obtained with a transfer ratio close to 0.9-1. For the two exceptions, the transfer is somewhat poor and erratic with a transfer ratio lower or equal

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Figure 3. Compression isotherms of MS (full circles) and CY (open circles) at the gas-water interface. of few degrees. All assumptions made for these calculations are given elsewhere.16

Results and Discussion

Figure 2. Molecules used in this work. to 0.6: the corresponding order parameter given below is then clearly limited by the poor quality of the transfer. Films were deposited onto optically polished calcium fluoride. Infrared spectra were recorded on a FTIR 750 Nicolet spectrometer. Inplane dichroic ratio, F, is defined for a particular IR peak by the relation:

F ) A⊥/A| where A⊥ and A| are respectively the absorption of the LB films when the light polarization is set perpendicular or parallel to the dipping direction. Using this dichroic ratio, one can define an order parameter, P2, as

P2 ) (1 - F)/(1 + F) This parameter is equal to the average of cos(2θ) over the distribution of molecular orientation, where θ is the angle of the dipole moment associated with the analyzed peak and the dipping direction.15,16 P2 is equal to 1 if all dipole moments are aligned with the dipping direction, equal to -1 if they are all perpendicular to this direction, and null if this dipole moment has no particular orientation within the plane of the LB film. Furthermore, the out-of-plane dichroic ratio β is defined as

β ) A|(i)60°)/A|(i)0°) where A|(i) is the absorption coefficient and i is the angle between the plane of the LB film and the IR beam. This ratio β is related to the degree of anisotropy out of the substrate plane and enables the estimation of the angle φ between the normal of the substrate and the dipole moment of a particular vibration with an accuracy (15) Chollet, P.-A.; Messier, J. Thin Solid Films 1983, 99, 197. (16) Vandevyver, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571.

The first derivative, noted MS, designed according to Figure 1, is based on a rigid core of three benzene rings. A fluorine atom was added to increase slightly the interactions between molecules. The polar head (a hydroxide group) is linked to the end of the calamitic group by a spacer of 11 carbons. Such a compound forms a stable monolayer at the gas-water interface characterized by an area ca. 24-25 Å2/molecule at the transfer surface pressure of 30 mN/m (see Figure 2). This molecular area corresponds to the cross section of the mesogenic group and suggests therefore that the MS derivative stands vertically in the monolayer. In the LB film, the IR dichroism demonstrates that the mesogenic part of this molecule keeps a similar orientation. Indeed, high outof-plane dichroism of the 1606 and 1513 cm-1 peaks associated to vibration modes of benzene rings17 is recorded (see Figure 4A) and corresponds to a small tilt angle (ca 12°) of the MS mesogenic core versus the normal of the substrate (see Table 1). As expected from such vertical orientation, no in-plane dichroism (and then orientation) is observed in the MS LB films (see Figure 4B). To follow the strategy described above, in-plane anisotropy should be found only if the MS derivative adopts a tilted orientation at the interface. To induce such configuration, the CY derivative was synthesized. In this compound, three MS molecules are linked to a cycloalkane group. Because of the small size of this cycle compared to the area of the calamitic moieties, the three CY liquid-crystal groups cannot stand up all together at the interface. At least one of them should then be tilted in the monolayer. This is easily demonstrated by recording the isotherm of CY: For a given surface pressure, the areas per calamitic group are larger than those of the MS derivative (see Figure 3). Once transferred onto solid substrate, the IR out-of-plane dichroism shows an average tilt angle much larger for the cyclic compound (see Figure 4C and Table 1) than for MS. Finally, Figure 4D demonstrates that indeed CY presents in-plane dichroism. Tilting a calamitic group can then induce in-plane orientation. More precisely, the mesogenic core is found oriented slightly parallel to the dipping direction with an order parameter equal to +0.12. This orientation is then perpendicular to the one usually found for the disklike molecules presenting some in-plane organization in LB films. Some other molecules (17) Garrigou-Lagrange, C.; Lebas, J. M.; Josien, M. L. Spectrochim. Acta 1958, 12, 305.

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Figure 4. Infrared spectra of MS and CY LB films (19 layers) on CaF2 substrate: (A) out-of-plane dichroism of a MS LB film. The angle between the plane of the substrate and the electric field is either 0° (dashed line) or 60° (solid line). (B) In-plane dichroism of a MS LB film. The electrical field is set parallel (dashed line) or perpendicular (solid line) to the dipping direction. (C) Out-of-plane dichroism of a CY LB film. The angle between the plane of the substrate and the electric field is either 0° (dashed line) or 60° (solid line). (D) In-plane dichroism of a CY LB film. The electrical field is set parallel (dashed line) or perpendicular (solid line) to the dipping direction. Table 1 MS CY PY3 PY5

tilt angle ((5°) (deg)

P2 ((0.03)

12 50 48 59

0 +0.12 +0.22 -0.13

already described in the literature are expected to have similar behavior and orientation.18 On the basis of the schematic approach of Figure 1, a polar group was linked to the middle of a calamitic group. Thus, the PY3 compound is formed by a pyrrole moiety acting as hydrophilic head and a mesogenic core of three benzene rings. Its isotherm19 begins at 200 Å2/molecule and presents a phase transition at low surface pressure (2 mN/m) and for areas between 170 and 50 Å2/molecule. The collapse pressure is found at 17 mN/m for an area of ca. 45 Å2/molecule suggesting an interfacial orientation corresponding to the tilted 2D calamitic case of Figure 1. After transfer onto solid substrate, IR spectroscopy shows that, again as expected from 2D calamitic compounds, these molecules are oriented within the plane of the multilayers and their rigid core is highly tilted versus the normal of the substrate (see Table 1). More precisely, the corresponding order parameter P2 is found close to +0.2 demonstrating that the PY3 derivative has the same orientation as the pseudo 2D calamitic molecule CY. Similar behavior and orientation are expected from (18) Zhu, Y.-M.; Lu, Z.-H.; Jia, X. B.; Wei, Q. H.; Xiao, D.; Wei, Y.; Wu, Z. H.; Hu, Z. L.; Xie, M. G. Phys. Rev. Lett. 1994, 72, 2573. (19) Jego, C.; Dupart, E.; Albouy, P.-A.; Mingotaud, C. Submitted for publication in Thin Solid Films.

recently described compounds having a structure close to the PY3 derivative.20 These few examples indicate that indeed the strategy described in Figure 1 can lead to compounds presenting in-plane orientation. They suggest that the calamitic derivatives have a different in-plane orientation and behavior in Langmuir and Langmuir-Blodgett films when compared with the discotic derivatives already described for their in-plane organization in multilayers. Molecules oriented in the plane of LB films can then be organized in three groups: the polymers, the disklike molecules, and these new calamitic derivatives. Finally, the synthesis of another calamitic compound, noted PY5, was performed based on a liquid-crystal group containing five benzene rings (see Figure 3). The size of the mesogenic part was increased in order to reinforce the anisotropic character of such material. Again after transfer of the corresponding monolayers, in-plane dichroism is observed in the multilayer. However, this in-plane anisotropy corresponds to an order parameter equal to -0.13 (see Table 1). This negative value proves that the mesogenic core of the PY5 lies perpendicular to the dipping direction, i.e., with the same orientation as the 2D discotic derivatives. Then, increasing the anisotropy of the molecular shape can lead to compounds behaving like disklike derivatives. Mixing the two related molecules PY3 and PY5 in a Langmuir film induces a marked deviation in the mean molecular area of the mixed monolayer when compared to the linear relationship expected for an ideal mixture or (20) Schro¨ter, J. A.; Plehnert, R.; Tschierske, C. Langmuir 1997, 13, 796.

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Figure 5. (A) Dependence of the molecular area at 12 mN/m on the mole fraction of PY5 in the mixed monolayer PY3/PY5. (B) Order parameter evaluated from IR dichroism experiments on the 1605 and 1259 cm-1 peaks versus the mole fraction of PY5 in the mixed monolayer PY3/PY5.

for a phase segregation (see Figure 5A). The increase in area is clearly recorded for a mole fraction close to 0.25 and suggested a real mixing between the two compounds associated perhaps to changes in the molecular organiza-

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tion within the Langmuir film. After transfer onto solid substrate, interesting behavior concerning the in-plane organization is observed. Indeed, as demonstrated by infrared studies, all mesogenic groups are oriented in the same direction. When the mole fraction of PY5 is equal to or larger than 0.15, this compound induces an orientation perpendicular to the dipping direction for the liquid crystal group of the PY3 molecules (see Figure 5B). The main orientation of this derivative is found parallel to the dipping direction (like for pure PY3 Langmuir film) only for low mole fractions of the PY5 molecule. Furthermore, when 20 to 40% in mole of PY5 are added to the PY3 monolayer, the absolute value of the P2 parameter is larger than the one obtained for a pure PY5 Langmuir film. Higher in-plane organization is thus found for such mixed films. The deviation of the order parameter from a linear relationship versus the composition of the monolayer is one other proof of the real mixing between PY5 and PY3 at the gas-water interface. Finally, the large changes in the molecular areas and in the order parameter are observed for similar compositions of the layer. This may indicate a common origin to the two phenomena: mixing can perhaps release some stresses existing in the pure materials and enables then to get better ordering (than in a pure monolayer). In conclusion, control of the in-plane orientation can be achieved following one of these ways: shearing of the monolayer by a rotating disk; designing the molecules in order to have 2D calamitic or discotic derivatives; mixing molecules having different organizations. If the design of molecules and the control of the inplane orientation is now somewhat understood, the question which has raised is the relation between the molecular structure and the order of magnitude of the order parameter. LA9711387