Langmuir 1996, 12, 6627-6631
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Study of Langmuir and Langmuir-Blodgett Films of Ferroelectric Liquid Crystals by Fourier Transform Infrared and Polarization Modulated Infrared Reflection Absorption Spectroscopies S. Payan* and B. Desbat Laboratoire de Spectroscopie Mole´ culaire et Cristalline (CNRS, URA 124), Universite´ de Bordeaux I, 351 Cours de la Libe´ ration, 33405 Talence, France
C. Destrade and H. T. Nguyen Centre de Recherche Paul Pascal, Avenue A. Schweitzer, 33600 Pessac, France Received March 4, 1996. In Final Form: August 13, 1996X Monolayer and multilayers of ferroelectric liquid crystals (FLC) obtained by the Langmuir-Blodgett (L-B) technique on the water subphase were characterized by surface pressure measurements and by polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS). The specific surface selection rules on the water surface and the high sensitivity of the PM-IRRAS method allow the estimation of the microscopic molecular organization of Langmuir films. Moreover, five layers of FLC were successfully transferred onto CaF2 and gold solid substrates and were studied by conventional Fourier transform infrared spectroscopy and by the PM-IRRAS method. Finally, the influence of the temperature on the structural arrangement of L-B films has been discussed.
Introduction The Langmuir-Blodgett (L-B) technique has been used extensively during the past few years to realize highly organized systems for structural studies or for technological applications (in the field of electronics, optics, and biotechnology). Owing to their amphipathic properties, some liquid crystals form stable films at the air-water interface and can be transferred onto solid substrates by the L-B technique. Although many studies were reported on Langmuir film liquid crystals,1-3 only a few of them have concerned transferred ferroelectric liquid crystal (FLC) on L-B films.4 That is the reason why we have been interested in the possibility to realize and characterize monolayers and multilayers of FLC at the air-water interface or in transferred films on solid substrates. To form such films, the molecules involved have to possess ferroelectric and amphipathic properties simultaneously. They have to be made of a hydrophilic headgroup, which is easily soluble in water, and a long alkyl chain, which provides a hydrophobic tail. Two molecules of the phenyl thiobenzoate series have been selected because they possess such properties. In this paper, we report a study of the organization and orientation of specific FLC molecules at the air-water interface and deposited on solid substrates. The films have been investigated by the recording of π-A isotherms (surface pressure/molecule area), by conventional FT-IR spectroscopy and by polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS). In the first section, the film preparation and PM-IRRAS method are briefly described. PM-IRRAS spectra of FLC films spread on water surface were recorded and provided the average orientation of molecular groups at the air-water X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Xue, J.; Jung, C. S.; Kim, M. W. Phys. Rev. Lett. 1992, 69, 474. (2) Diep-Quang, H.; Ueberreiter, K. Colloid Polym. Sci. 1980, 258, 1055. (3) Jego, C.; Agricole, B.; Vicentini, F.; Barrouillet, J.; Mauzac, M.; Mingotaud, C. J. Phys. Chem. 1994, 98, 13408. (4) Rettig, W.; Naciri, J.; Shashidar, R.; Duran, R. S. Thin Solid Films 1992, 210/211, 114.
S0743-7463(96)00195-3 CCC: $12.00
interface. Finally, in the last section PM-IRRAS and infrared spectra of L-B films at room temperature are presented and some spectra are recorded at higher temperatures in order to show changes in the organization of the FLC molecules in the various explored phases. Experimental Section Film Preparation. The phenyl thiobenzoate ferroelectric liquid crystals studied in this paper were synthesized by Nguyen et al.5 The molecular formulas of these two compounds are presented in Chart 1. These molecules are made of a hydrophilic polar head group (ester) and a hydrophobic alkyl tail group (hydrocarbon chain). The various phase transition temperatures are as follows:
10.S.ClIsoleu: 11.S.FIsoleu:
K K
54 °C 59 °C
S C* S C*
66 °C 70 °C
SA SA
74 °C
I
84.6 °C
I
A NIMA Langmuir-Blodgett trough (Model 611D) with a Wilhelmy balance was used for surface pressure (∆π)-molecular area (A) isotherms and for the deposition of the L-B films. Monolayers of the two FLC compounds were obtained by spreading a few microliters of chloroform solution with a concentration of 2.6 × 10-3 M onto ultrapure water (Milli-Q, Millipore). L-B films of 10.S.ClIsoleu were transferred by the vertical dipping method onto hydrophilic substrates, at a constant surface pressure of about 30 mN/m. Calcium fluoride plates and gold mirrors6 were used for infrared transmittance and PM-IRRAS measurements, respectively. The thickness measurements of the L-B films deposited on gold substrates were achieved using a GAERTNER L116B rotating-analyzer ellipsometer. PM-IRRAS experiments at the air-water interface were carried out using a Kel-F laboratory-made Langmuir trough (surface area of 125 cm2). FT-IR Spectroscopy Measurements. Transmittance and PM-IRRAS spectra were recorded on a Nicolet 740 FT-IR (5) Twieg, R. J.; Betterton, K.; Nguyen, H. T.; Tang, W.; Hinsberg, W. Ferroelectrics 1989, 91, 243. (6) Blaudez, D.; Buffeteau, T.; Desbat, B.; Orrit, M.; Turlet, J. M. Thin Solid Films 1992, 210/211, 648.
© 1996 American Chemical Society
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Chart 1
Figure 1. Surface selection rules linking the transition moment orientation and the sign and intensity of the absorption band at the air-water interface. spectrometer equipped with a HgCdTe detector cooled at 77 K. The PM-IRRAS method provides direct molecular information on Langmuir and L-B films due to its insensitivity to the isotropic absorptions of the sample environment.7 PM-IRRAS set up and experimental procedure in the case of metallic or absorbing substrates such as the water subphase have been described in detail elsewhere.7-9 All PM-IRRAS spectra were recorded with an incidence angle of the IR beam on the sample of about 75°. At this near grazing incidence, the surface selection rules allow the spectra to be analyzed in terms of molecular orientation and conformation. On a metallic surface, we can obtain these data from the relative intensity of the absorption bands in comparison with the absorbance spectrum of the bulk sample. In PM-IRRAS spectra, all the bands associated with transition moments along the surface will be completely extinguished. On the other hand, on the water surface, we can deduce the molecular organization from the sign and intensity of the absorption bands. Indeed, it has been determined6 that an upward oriented band indicates a transition moment preferentially in the plane of the surface, whereas a downward oriented band reveals an orientation rather perpendicular to the surface (Figure 1). Moreover, for an intermediate angle of about 52° between the transition moment and the surface, the band totally vanishes. Since the water subphase, considered like a dielectric substrate, contributes to the PM-IRRAS signal, it is necessary to make the ratio between spectra of the covered S(d) and uncovered S(0) water surface in order to extract the very weak absorption bands of monolayer spread onto the water subphase.
Results and Discussion ∏-A isotherms. The π-A isotherms of the two FLC compounds spread on the water subphase, 10.S.ClIsoleu (Figure 2a) and 11.S.FIsoleu (Figure 2b) (7) Tippmann-Krayer, P.; Kenn, R. M.; Mo¨hwald, H. Thin Solid Films 1992, 210/211, 577. (8) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc. 1991, 45, 380. (9) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J. M. Appl. Spectrosc. 1993, 47, 869.
Figure 2. ∏-A isotherms of 10.S.ClIsoleu (a) and 11.S.FIsoleu (b) at the air-water interface.
have revealed the possibility of stabilizing monolayers and multilayers of such compounds at a temperature of 22 °C. The variations of surface pressure as a function of the area per molecule are typical of most LC Langmuir films.1 At large area per molecule, the FLC molecules on the water surface should have a gas phase character.10 As the area per molecule is reduced, the film goes to a liquid phase. In regions where area per molecule is slightly greater than 20 Å2/molecule for 10.S.ClIsoleu and 21 Å2/ molecule for 11.S.FIsoleu, the surface pressure is about 0 mN/m and constant, indicating a coexistence of gas and liquid phases. The first significant increase of surface pressure occurs between 20 and 16 Å2/molecule for 10.S.ClIsoleu and between 21 and 17 Å2/molecule for 11.S.FIsoleu and indicates the formation of the homogeneous monolayer liquid phase on the water surface.11-13 An area per molecule of about 20 Å2 corresponds more or less with the occupied surface of an alkyl chain in all-trans conformation. As the compression increases, the surface pressure first slowly increases to an area per molecule of about 6-7 Å2/molecule and then sharply rises up to 41 mN/m for 10.S.ClIsoleu and 26 mN/m for 11.S.FIsoleu. The inflection points on the π-A isotherms indicate the existence of two other stable phases in which molecules are forced to go into the third dimension to form additional layers on top of the first monolayer. If (10) Rasing, Th.; Berkovic, G.; Shen, Y. R.; Grubb, S. G.; Kim, M. W. Chem. Phys. Lett. 1986, 130, 1. (11) Rasing, Th.; Shen, Y. R.; Kim, M. W.; Valint, P., Jr.; Bock, J. Phys. Rev. A 1985, 31, 537. (12) Sauer, B. B.; Yu, H.; Yazdanian, M.; Zografi, G.; Kim, M. W. Macromolecules 1988, 21, 2332. (13) Kim, M. W.; Liu, S. N.; Chung, T. C. Phys. Rev. Lett. 1988, 60, 2745.
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Figure 3. Normalized PM-IRRAS spectra of 10.S.ClIsoleu film at the air-water interface for different states of compression: at 0.1 mN/m (b), at 1.6 mN/m (c), at 7.3 mN/m (d), and at 20.2 mN/m (e); and absorbance spectrum of 10.S.ClIsoleu bulk sample recorded at room temperature (a).
Figure 4. Normalized PM-IRRAS spectra of 11.S.FIsoleu film at the air-water interface for different states of compression: at 1 mN/m (b), at 6.4 mN/m (c), at 10.9 mN/m (d), and at 30.2 mN/m (e); and absorbance spectrum of 11.S.FIsoleu bulk sample recorded at room temperature (a).
the area per molecule is reduced still further, the Langmuir films collapse. The different phase transitions appear more clearly for the fluorinated compound than for the chlorinated compound. These phase transitions take place when the value of the area per molecule is three times smaller or five times smaller than that of a molecule in a monolayer. Such properties of forming oddnumbered layers have been already observed on LC Langmuir films.1 As a conclusion, the studied FLC seem to be successively organized in one, three, and five layers on the water surface. As we will show later, the PM-IRRAS spectroscopy will confirm this result. Moreover, we have observed the reversibility of such π-A diagrams with decompression. PM-IRRAS Spectra at the Air-Water Interface. From the π-A isotherms we have obtained a macroscopic and averaged view of the molecular arrangement. In order to obtain precise orientation of the FLC molecules at the air-water interface, the PM-IRRAS technique was used. Normalized PM-IRRAS spectra of 10.S.ClIsoleu and 11.S.FIsoleu films spread on the water surface were recorded between 850 and 1850 cm-1, at different states of compression and are plotted in Figures 3 and 4, respectively. The absorbance spectra of both FLC bulk samples recorded at room temperature are reported in Figures 3a and 4a. Such spectra allow a precise assignment of each absorption band and provide also determination of the relative intensity of bands. Due to the differential detection of the PM-IRRAS technique, we can notice that the isotropic absorpions of water vapor are perfectly compensated and, consequently, the weak monolayer and multilayers bands are revealed with a good signal-to-noise ratio. By comparison with the corresponding absorbance spectra of bulk samples (Figures 3a and 4a) the most intense bands can be easily assigned. For the 10.S.ClIsoleu compound, for example, the ester
CdO stretching, the keto CdO stretching, the phenyl ring CdC stretching, the CH2 scissoring, the aromatic CCH bending and the C-S stretching vibrations are located at 1757, 1669.5, 1606, 1469, 1167, and 908 cm-1 in the normalized PM-IRRAS spectra, respectively. The intensity of the bands increases rapidly with decreasing molecular area. The observed intensity of normalized PM-IRRAS spectra depends both on the number of molecules per unit area of surface and on the orientation of the transition moments responsible for the absorption. For the two compounds, bands are either upward or downward with respect to the baseline and keep more or less their relative intensity and their sign in the whole studied surface pressure range. The upward orientation of the ester CdO, keto CdO, and C-S stretching bands indicates that their transition moments are mainly directed in the plane of the water surface. On the other hand, the downward orientation of the phenyl ring CdC stretching and the phenyl CCH bending bands indicates that their transition moments are rather perpendicular to the water surface. In the case of the chlorinated compound, if we compare the relative intensity of the keto CdO stretching band determined for the monolayer to the one measured at a surface pressure of 20.2 mN/m, we find a factor 5, which confirms the five layers observed in the π-A isotherm. Considering that the transition moment direction is independent to molecular area, the increase of band intensity with compression is then only due to the growing number of molecules at the air-water interface. The relative sharpness of most normalized PM-IRRAS bands at high surface pressure shows that the molecules are well organized in the Langmuir films. Moreover, the weak frequency shift and the broadening of the ester CdO stretching band, observed in PM-IRRAS spectra at large area per molecule (corresponding to one monolayer), can be related to particular interaction between the carbonyl ester groups and the water molecules
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by means of hydrogen bonds. We show here, that the ester CdO groups form the polar heads in this type of FLC molecules. The relative intensity ratio of the CH2 scissoring band versus the ester CdO stretching band passes from a factor of 1/2 in the absorbance spectrum of the bulk sample to a factor estimated at 1/5 in the multilayers spectra. This observation indicates a tilt of the aliphatic chain axis relative to the surface normal. Regarding the relative intensity of the band located at 1606 cm-1 in the normalized PM-IRRAS spectra of both compounds and in the corresponding bulk absorbance spectra, we can conclude that the phenyl rings that constitute the rigid core of the FLC molecules are quasiperpendicular to the water surface. The broad dip observed at 1650 cm-1 in all normalized PM-IRRAS spectra of the Langmuir films is due to the liquid water absorption. Although this feature can be mainly due to the different optical responses of covered and uncovered water surfaces, this dip probably also contains some information on the restructuring of the water molecules at the interface. All these spectra reveal that the molecules of these FLC substances have a specific orientation at the air-water interface at any states of compression. Moreover, taking into account the relative intensity of bands at weaker and higher surface pressure, we can conclude that our FLC molecules form stable monolayers and multilayers (namely three and five layers) on the water surface. These results, like for the cyanobiphenyl LC films,14 show that the PMIRRAS spectroscopy is very efficient to determine the orientation of the molecules in the monolayers at the airwater interface.
Payan et al.
Figure 5. Infrared absorbance and normalized PM-IRRAS spectra of 10.S.ClIsoleu L-B films deposited on CaF2 (b) and gold (c) substrates and absorbance spectrum of 10.S.ClIsoleu bulk sample recorded at room temperature (a).
Spectra of L-B Films The 10.S.ClIsoleu multilayer L-B films were transferred onto CaF2 and gold substrates, at a surface pressure of 30 mN/m. In the case of transmittance experiments with the CaF2 plate, only the transition moments oriented at the substrate surface are active and so we can estimate the in-plane molecular organization of L-B films. Conversely, on gold substrate, the surface selection rules of the PM-IRRAS technique lead to the complete extinction of the bands of which the transition moments are in the plane of the metallic substrate. Thus, the use of both of these solid substrates is of great physical interest as we obtain complementary information on the molecular group arrangement. In Figure 5, we present absorbance and normalized PM-IRRAS spectra, recorded at room temperature, of 10.S.ClIsoleu multilayers deposited onto CaF2 (Figure 5b) and gold (Figure 5c) substrates, respectively, in the 1850-850 cm-1 spectral range. A corresponding absorbance spectrum of bulk chlorinated sample is also reported (Figure 5a). The difference in relative intensity and the sharpness of most of the bands prove that the FLC molecules are well arranged on the solid substrates. The great intensity of the ester (1757 cm-1) and keto (1669.5 cm-1) CdO stretching bands on CaF2 and the quasi-extinction of these bands on gold show that their transition moments are in the plane of the substrate. In contrast, the phenyl ring CdC stretching band (1606 cm-1) being weak on CaF2 and being strong on gold, demonstrates that the phenyl rings are almost perpendicular to the substrate, as we have seen it at the airwater interface. The CH2 scissoring band (1469.5 cm-1) is detected with a medium intensity on both substrates. The orientation of this vibration mode in this type of molecule appears different from what is observed in L-B (14) Biensan, C.; Desbat, B.; Turlet, J. M. To be submitted for publication in Thin Solid Films.
films of fatty acids, in which its transition moment is in the plane of the substrate. In our case, the long alkyl chain must also be tilted with regards to the surface normal (as we have already shown it at the air-water interface). By comparing the normalized PM-IRRAS spectrum (Figure 5c) with the infrared absorbance spectrum of the bulk sample (Figure 5a), we see that the relative intensity of the CH3 symmetric bending band (1386 cm-1) has considerably increased on the metallic substrate relative to the phenyl ring CdC stretching band. These changes imply a well-defined orientation of CH3 molecular groups with respect to the substrate. Indeed, the transition moment associated with the CH3 symmetric bending vibration must be rather directed along the normal of the surface. To add to the preceding experiments, we have measured the thickness of the film deposited on gold substrate by ellipsometry. An average thickness of 90 ( 5 Å has been determined, which corresponds to a value of about 18 Å for each monolayer. This result, indicates that five layers of 10.S.ClIsoleu have been transferred onto the metallic surface and confirms that the molecules do not stand vertically but form a bend with their long alkyl chain tilted toward the surface normal and the aromatic groups perpendicular to the surface. All these observations indicate that it is possible to transfer FLC multilayers from the water surface onto a solid substrate without modifying the organization of the molecules. This surprising result at first sight shows that the forces that assure the cohesion of the monolayers are mainly due to the monolayers themselves and not to their interaction with the substrate. It is probably due to the ability of these molecules to adopt an organization comparable to a smectic mesophase. However, this result is consistent with similar effects observed for different compounds that are liquid crystals or not.15-17
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gets broader and their relative intensities are reduced. The longitudinal bands such as the phenyl ring CdC stretching band at 1606 cm-1 and the aromatic CCH bending band at 1167 cm-1 increase in relative intensity and gain in importance with regard to the transverse ones. This change is probably due to a tilt of the phenyl rings with respect to the substrate normal. On the other hand, we note a broadening of most absorption bands that is certainly connected with a more pronounced dynamic disorder. Around T ) 66 °C, an additional variation of the relative intensities takes place, and in this case, the longitudinal bands become stronger than the transverse ones. This new jump in relative intensity is also accompanied by a broadening of most bands. At 70 °C and above, it is more difficult to bring to the fore some differences in the spectra. To summarize, these spectra show that this multilayer system possesses at least two phase transitions, and the observed temperatures are very close to the ones determined for the SC*-K and SA-SC* transitions in the bulk sample. From these data, we cannot draw any conclusions regarding the accurate nature of these phases. Other experiments have to be done in order to reveal and confirm the ferroelectric character of the phase observed between 55 and 66 °C. Conclusion Figure 6. Infrared absorbance spectra of 10.S.ClIsoleu L-B films deposited on CaF2 recorded at different temperatures: at T ) 22 °C (a), T ) 45 °C (b), T ) 55 °C (c), T ) 56 °C (d), T ) 60 °C (e), T ) 66 °C (f), T ) 70 °C (g), T ) 76 °C (h) and T ) 93 °C (i).
Effect of the Temperature In order to know if the FLC multilayer films have the same characteristic phase transitions as a function of temperature as that of the bulk sample, we have studied the thermal behavior of the 10.S.ClIsoleu five layers deposited on a CaF2 substrate. Moreover, we have performed several increases and decreases in temperature to check the reversibility of the observed phenomena. In the Figure 6, we present the infrared absorbance spectra of 10.S.ClIsoleu five layers transferred on a CaF2 substrate, between 1850 and 1050 cm-1, recorded at different temperatures. The dashed lines represent the phase transition of the bulk sample. For temperatures up to 55 °C, no change appears in the spectrum of the L-B film, which suggests that the organization of the molecules remains the same. When T ) 55 °C, we can observe that the profile of the spectrum is modified. The width of ester (1757 cm-1) and keto (1669.5 cm-1) CdO stretching bands (15) Blaudez, D.; Buffeteau, T.; Desbat, B.; Orrit, M.; Turlet, J. M. Thin Solid Films 1992, 210/211, 648. (16) Buffeteau, T.; Desbat, B.; Devaure, J.; Salimi, A. J. Chim. Phys. 1993, 90, 1855. (17) Buffeteau, T.; Desbat, B.; Devaure, J.; Salimi, A. J. Chim. Phys. 1993, 90, 1871.
This study is the first one achieved on ferroelectric liquid crystal systems in-situ at the air-water interface and transferred onto solid substrates by the L-B technique. We have demonstrated the possibility to obtain stabilized FLC monolayers and multilayers on the water surface when the molecular area is reduced by compression. The PM-IRRAS technique with its surface selection rules is a very well adapted tool to probe structural and molecular information on monolayers and multilayers spread at the air-water interface. Our results show that the molecules have a specific orientation on the water surface. The carbonyl ester group constitutes the polar head of these FLC molecules and is oriented in the plane of the surface. At any state of compression, the core of the molecules (mainly the both phenyl rings) is quasi-perpendicular to the water surface, whereas the long alkyl chain is appreciably tilted with regard to the surface normal. Moreover, the molecular organization of the five layers deposited onto CaF2 and gold substrates remains the same as that at the air-water interface. The L-B film thickness measured by ellipsometry is in good agreement with the number of transferred layers and with the previous results. Finally, the last experiment shows two phase transitions near 55 and 66 °C. These temperatures are very close to the ones of SC*-K and SA-SC* phase transitions observed in the bulk sample. We have not been able to reveal the I-SA phase transition because no significant change appears in the spectra from a temperature of 70 °C. More experiments must be achieved to check if these phase transitions correspond well to the ones of the bulk sample. LA960195H
