Langmuir Monolayers of Bent-Core Molecules - personal.kent.edu

Mar 3, 2004 - Introduction ... At minimum, a rank-three order parameter as well as the ... core liquid crystals in two ways: directly, by what can be ...
0 downloads 0 Views 261KB Size
Download PDF
2772

Langmuir 2004, 20, 2772-2780

Langmuir Monolayers of Bent-Core Molecules Lu Zou,† Ji Wang,† Violeta J. Beleva,† Edgar E. Kooijman,†,⊥ Svetlana V. Primak,†,| Jens Risse,‡ Wolfgang Weissflog,‡ Antal Ja´kli,§ and Elizabeth K. Mann*,† Department of Physics, Kent State University, Kent, Ohio 44242-0001, University Halle Wittenberg, Institute of Physical Chemistry, D-06108 Halle Saale, Germany, and Liquid Crystal Institute, Kent State University, Kent, Ohio 44242-0001 Received November 21, 2003. In Final Form: January 20, 2004 A systematic study of five different, symmetric bent-core liquid crystals in Langmuir thin films at the air/water interface is presented. Both the end chains (siloxane vs hydrocarbon) and the core (more or less amphiphilic) are varied, to allow an exploration of different possible layer structures at the interface. The characterization includes systematic surface pressure isotherms, Brewster angle microscopy, and surface potential measurements. The properties of these layers are strongly dependent on the individual type of molecule: the molecules with amphiphilic end chains lie quite flat on the surface, while the molecules with hydrophobic end chains construct multilayer structures. In both cases, the three-dimensional collapse structure is reversible.

Introduction Bent-core or banana-shaped molecules exhibit a rich variety of phases,including many liquid-crystalline ones.1 At minimum, a rank-three order parameter as well as the vector and the nematic tensor order parameters are necessary to completely describe this web of phases.2 A dozen different liquid phases2 and five smectic phases3 have been suggested. Eight phases have been identified, but most have not been fully characterized.1 Several of these phases are smectic phases in which packing of the bent molecules leads to polar ordering. Because of the ordering, achiral bent-core molecules can demonstrate chirality and (anti)ferroelectricity.4 The usefulness of bentcore molecules in scattering switching and in storage devices has been demonstrated.5 It has also been suggested that the unique properties of these molecules can make them useful for electromechanical devices.6 The order in Langmuir monolayers, which are molecular layers self-confined at the air/water interface, shows a one-to-one correspondence to that in liquid crystal phases.7 Several molecules that form liquid crystals in bulk have been shown to form stable Langmuir monolayers.8-11 The richness of the phase diagram of bent-core molecules is * Corresponding author: E-mail: [email protected]. Telephone: 330-672-9750. Fax: 330-672-2959. † Department of Physics, Kent State University. ‡ University Halle Wittenberg. § Liquid Crystal Institute, Kent State University. ⊥ Present Address: Utrecht University, CBLE, Biochemistry of Membranes, H.R. Kruytgebouw, Padualaan 8, 3584 CH Utrecht, Netherlands. | Present Address: Pacific Northwest National Laboratory, EMSL, K8-98, Battelle Boulevard, P.O. Box 999, Richland, WA 99352. (1) Pelzl, G.; Diele, S.; Weissflog, W. Adv. Mater 1999, 11, 707. (2) Lubensky, T. C.; Radzihovsky, L. Phys. Rev. E. 2002, 66, 031704. (3) Brand, H. R.; Cladis, P. E.; Pleiner, H. Eur. Phys J. B 1998, 6, 347. Roy, A.; Madhusudana, N. V.; Toledano, P.; Figueiredo Neto, A. M. Phys. Rev. Lett. 1999, 82, 1466. (4) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater. Chem 1996, 6, 1231. Sekine, T.; Niori, T.; Sone, M.; Watanabe, J.; Choi, S. W.; Takanishi, Y.; Takezoe, H. Jpn. J. Appl. Phys. 1997, 36, 6455. Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.; Korblova, E.; Walba, D. M. Science 1997, 278, 1924. (5) Ja´kli, A.; Kru¨erke, D.; Sawade, H.; Chien, L. C.; Heppke, G. Liq. Cryst. 2002, 29, 377. (6) Ja´kli, A.; Kru¨erke, D.; Nair, G. G. Phys. Rev. E 2003, 67, 051702. (7) Knobler, C. M. Mol. Cryst. Liq. Cryst. 2001, 364, 133.

expected to carry over into the Langmuir monolayer. The Langmuir layer can give insight into the molecular packing within layers, particularly in the presence of an interface. Bent-core molecules that show smectic ordering, even if only at higher temperatures, may be expected to form reversible collapsed layers. A stable Langmuir layer, transferred to a solid interface, may form a natural alignment layer for bent-core liquid crystals. It has been very difficult to align bent-core molecules, and many of the zero-field characteristics have had to be deduced from quite inhomogeneous films. Studies of Langmuir layers may thus help clear up structural questions about bentcore liquid crystals in two ways: directly, by what can be deduced from the structure of the Langmuir layers themselves, and indirectly, through the possibility of providing a suitable alignment layer to produce more homogeneous, thicker films. The purpose of this work is the characterization of the Langmuir mono- and multilayers for a range of bent-core molecules, varying both the core and the end chains but maintaining molecular symmetry, with identical end chains on either end of the core. The characterization includes systematic surface pressure isotherms, Brewster angle microscopy, and surface potential measurements. To our knowledge, the only work on Langmuir monolayers of such molecules in the literature considers two distinct cases. The first considers a single bent-core molecule, similar to one of those we consider here but with longer end chains.10 That work demonstrated that Langmuir and Langmuir-Blodgett films could be formed from this molecule. Surface pressure isotherms suggested films more than one molecule thick at even the lowest measurable pressures. Details of the molecular distribution within the layer were unclear. However, the authors demonstrated that the molecular orientation distribution function could be well-characterized by second harmonic generation.10 The other work on Langmuir monolayers of (8) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (9) De Mul, M. N. G.; Mann, J. A. Jr. Langmuir 1994, 10, 2311. De Mul, M. N. G.; Mann, J. A. Jr. Langmuir 1998, 14, 2455. (10) Kinoshita, Y.; Park, B.; Takezoe, H.; Niori, T.; Watanabe, J. Langmuir 1998, 14, 6256. (11) Xue, Q.; Yang, K.; Xiao, C.; Zhang, Q. Thin Solid Films 1999, 347, 263.

10.1021/la0361924 CCC: $27.50 © 2004 American Chemical Society Published on Web 03/03/2004

Langmuir Monolayers of Bent-Core Molecules

Langmuir, Vol. 20, No. 7, 2004 2773

Figure 1. Molecular formulas and space-filling models for the different bent-core molecules used in this work. (a) The bent-core molecules with hydrocarbon end chains R1 and core substituent R2, as given in Table 1. (b, c) Formulas for the siloxane end-chain molecules, Bc2-SiO and Bc3-SiO, respectively. (d-f) Space-filling models for Bc-H, Bc2-SiO, and Bc3-SiO, respectively. Black atoms are oxygen. The models were produced with Spartan ′02, Wavefunction, Inc.; the minimum energy configurations are determined for single isolated molecules with the semiempirical module.

bent-core molecules considers two different cores with very short hydrophobic side chains.12 Langmuir-Blodgett film of these molecules formed simple, well-aligned monolayers, as demonstrated through X-ray reflectivity, secondharmonic generation, and molecular rectification. A molecule which forms thermotropic liquid crystals typically consists of two types of regions: (i) a core that is sufficiently rigid that entropy minimization will align molecules when dense packed and (ii) end chains that are sufficiently long and flexible to fluidize dense-packed states. In the bent-core case, the core may take a number of different nonplanar chiral configurations, but when isolated it is on average planar and bent.2 The interaction of such molecules with a water surface depends on the details of the molecule. Benzene itself is amphiphilic, spreading on water;13 any additional carboxyl and amide groups in the core will make it more hydrophilic. Groups that are more or less hydrophobic or hydrophilic may be substituted at different positions on the core, which may change the preferred orientation of the core on the water surface. The hydrophobicity of the fluidizing chains may also be varied. In this work, we vary the hydrophobicity of both the core and the end chains. Two molecules in which the fluidizing chains are terminated with amphiphilic siloxane groups and three molecules with pure hydrocarbon end chains are considered. The siloxane molecules have cores with different numbers of benzene rings. In both cases, the amphiphilic siloxane chains might be expected to help tether the molecule quite flat on the surface. The hydrophobic hydrocarbon chains, on the other hand, should lead to more upright molecules. These molecules have different groups substituted at the inner angle of the central benzene ring of the core, which can affect both configuration and stability of the molecule at the surface, as well as the way in which these molecules stack to form a multilayer. (12) Ashwell, G. J.; Amiri, M. A.; Mater, J. J. Mater. Chem. 2002, 10, 2181. Baldwin, J. W.; Amaresh, R. R.; Peterson, I. R.; Shumate, W. J.; Cava, M. P.; Amiri, M. A.; Hamilton, R.; Ashwell, G. J.; Metzger, R. M. J. Phys. Chem. B 2002, 106, 12158. (13) Harkins, W. D. The Physical Chemistry of Surface Films; Reinhold Publishing Corp.: New York, 1952.

Table 1. Identification and Bulk Phase Properties of Bent Core Molecules Used Figurea identifier

R1O

R2 H

phase sequence in bulk

1a,d

Bc-Hb

C8H17O

1a 1a 1b, e

Bc-NO2c Bc-CH3d Bc2-SiOe

1c, f

Bc3-SiOf

C9H19O NO2 C8H17O CH3 (CH3)3SiO(CH3)2Si(CH2)3O (CH3)3SiO(CH3)2Cr 69 Ih Si(CH2)3O

B4 139.7 B3 151.9 B2 173.9 Ig Cr 116 B2 177 Ig Cr 157 B5 163 B2 168 Ig Cr 93 Ih

a Figures showing molecule structures. b Chemical name according to IUPAC nomenclature: 1,3-phenylene bis[4-(4-n-octyloxyphenyliminomethyl)benzoate]. c 2-Nitro-1,3-phenylene bis[4-(4n-nonyloxyphenyliminomethyl)benzoate]. d 2-Methyl-1,3-phenylene bis[4-(4-n-octyloxyphenyliminomethyl)benzoate]. e 4,4′-Bis[3(pentamethyldisiloxanyl)propoxy]benzophenone. f 1,3-Phenylene bis{4-n-[3-(pentamethyldisiloxanyl)propoxy]benzoate}. g From ref 1. h From ref 14.

Experimental Methods Five different molecules were used in this study, three with hydrocarbon end chains1 (Figure 1a) and two with siloxane end chains (Figure 1b, c).14 The bulk properties for these molecules, with the nomenclature used to identify the different molecules, are summarized in Table 1. Chemical structures are shown in Figure 1. The preparation of the compounds Bc2-SiO and Bc3-SiO was performed by a hydrosilylation reaction of the terminal unsaturated starting materials 1,3-phenylene bis(4-allyloxybenzoate) and 4,4′-diallyloxybenzophenone, respectively, with pentamethyldisiloxane in the presence of the platinum catalyst (PtCl2(C5H5)2).14 The estimated purity of all the bent-core molecules was >98%. The molecules were deposited onto a pure water surface with spreading solutions. Water was purified with a Purelab Plus UV system (US Filter, resistivity 18.2 MΩ/ cm). Hexane (Fisher, OPTIMA grade) was the spreading solution solvent for the molecules with siloxane end chains (Bc2-SiO and Bc3-SiO), while chloroform (Aldrich, A.C.S, HPLC grade) was the solvent for the molecules with hydrocarbon end chains (Bc-H, Bc-NO2, and Bc-CH3), (14) Risse, J. Ph.D. Thesis, Martin Luther University Halle-Wittenberg, 1999.

2774

Langmuir, Vol. 20, No. 7, 2004

Zou et al.

which did not dissolve in hexane. Typical concentrations in the solvent were ∼1 mg/mL. Surface tension isotherms were measured with the KSV (KSV, Finland) minitrough system. The original commercial Teflon trough was replaced with a homemade one of the same size, which performed well in tests against leakage around the KSV hydrophilic barriers.15 The surface pressure measurements were performed via the Wilhelmy method. Surface potential measurements were performed with the KSV SPOT1 surface potential meter, which works via the vibrating plate capacitor method.16 The probe head diameter was 17 mm. NaCl, 1 mM (Fisher, certified ACS grade), was added to increase the conductivity of the pure water solution, for the surface potential measurements only. After 5 min for stabilization, the surface potential drifted less than 3 mV over times comparable to the experiment, under good conditions. All surface potential increments ∆V are given with respect to the baseline on pure water. The layers were imaged with a Brewster angle microscope17 (BAM), which was assembled in our laboratory with the standard design.17 Incident light at 668 nm (SDL 7470-P6), polarized (Glan-Tayler, Lambrecht MGTYE15) in the plane of incidence, was reflected off the monolayer at the Brewster Angle to a biconvex lens that focused the image on a CCD camera (Panasonic GP-MF602). The field of view was either 11 mm × 13 mm or 3.6 mm × 4.3 mm. A diachroic sheet analyzer (Melles-Griot) before the final lens checked for any in-plane optical anisotropy in the monolayer. During the experiments, images were captured directly from the camera output by a frame grabber. Surface pressure measurements were recorded simultaneously using the KSV system. The gray level in the CCD image depends on the reflectivity R of the surface and with care can be used to quantitatively compare R for different domains. Grey levels G ranged from 0 for saturation to 255 for complete darkness. We verified that Gr ) Gb - G was linear with intensity over most of that range by checking that Gr ∝ (cos2(θp - θA)), as the angle of the second polarizer (the analyzer, with angle θA) varies from parallel to perpendicular to the first polarizer (angle θp). Gb is the gray level associated with the background light level. Since Gr is linearly related to the light onto the deflector

Gr Iref ∝ )R Iinc Iinc

(1)

In analyzing the reflectivity, we took Gb as the gray level observed with a pure water surface and the same Iinc, to maintain consistent experimental conditions. In principle, pure water is rough and thus also reflects light at the Brewster angle, which here is included in Gb. However, typical values for Gr were much greater than the variation in Gb, so this complication was ignored. Reflectivities varied by a factor of 600 for different layers, so that Iinc was adjusted during the different experiments to maintain reasonable contrast without saturating the camera. (15) The tests used poly(dimethylsiloxane) (Polymer source # P530DMS, Mw 33, 500), which gives a stable, reproducible, reversible monolayer with a strong tendency to leak around barriers. (Mann, E. K. Langmuir 1991, 7, 1112. Mann, E. K. Doctoral Thesis, Paris VI, 1992.) (16) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; Wiley-Interscience: New York, 1997. (17) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590.

Figure 2. Thinnest layers observed on the water surface, at very high average molecular areas σ, for three bent core molecules: (a) Bc-H, σ ∼ 5.5 nm2/molecule; (b) Bc-NO2, σ ∼ 12 nm2/molecule; (c) Bc3-SiO, σ ∼ 4 nm2/molecule. Bars correspond to 1 mm.

To check for reproducibility and to find any accessible equilibrium or metastable states, all layers went through at least two compression/decompression cycles, which began from 10 min to 1 h after depositing a spreading solution of the molecule to be studied. The waiting time had no obvious effect, either on the isotherms or on the texture of the monolayer. Typical compression speeds were 5 mm/min, which corresponded to molecular area changes of ∼0.05 nm2/ (mol‚min) for the siloxane molecules and ∼ 0.005 nm2/(mol‚min) for the hydrocarbon molecules. The fully compressed state was held between 2 min and 2 h, as was the decompressed state before recompression. In some cases the compression was stopped at different values of the surface pressure for ∼15 min each, to check for the stability of the surface pressure. The stability varied and will be discussed below. All isotherms presented were done with fresh solutions; aged solutions showed similar behavior but different thicknesses after the compression/decompression cycling. All the experiments were carried out at 17 °C and 50% humidity. Experimental Results Brewster angle microscopy (BAM) allows the visualization of the film and in particular any phase separation or threedimensional collapse. All the studied molecules phase separate into different uniform layers at very low pressure. However, the character of those layers is very different for the hydrocarbon end-chain molecules compared to the siloxane end-chain molecules. Hydrocarbon end-chain molecules give solid phases at zero surface pressure. With very small amounts of material on the surface (Figure 2a,b), BAM shows islands of uniform thickness with sharp corners and jagged breaks, although the long edges tend to curve. These solidlike islands move rapidly on the surface: the surroundings are probably gaseous. As σ is decreased, but the pressure remains unmeasurably small, we see regions of at least three different reflectivities, again bounded by irregular edges and showing cracks (Figure 3a); these have reflectivity about 4 times greater than the islands observed at high molecular areas σ (Figure 2a,b). In fact, the darkest regions in Figure 3a are close in reflectivity to the bright regions seen in Figure 2b. Further, the film at lower σ, very unlike that in Figure 2, is nearly stationary, which also suggests that all phases are very viscous. In contrast, at high molecular areas σ, the siloxane end-chain molecules form fluid domains with very low contrast and smooth edges that relax quickly (Figure 2c). These fluid domains are much less dense than the solids obtained with the hydrocarbon molecule, as is evident both by the low contrast and because the pressure increases at much higher areas per molecule than with the hydrocarbon end chain. To quantify this, we can compare coareas, determined in the usual way by extrapolating from the Π-A curve to the baseline pressure.18 Under the assumption that the pressure rises significantly when the molecules touch, (18) Adam, N. K. The Physics and Chemistry of Surfaces, 3rd ed.; Oxford University Press: London, 1941; p 47ff.

Langmuir Monolayers of Bent-Core Molecules

Langmuir, Vol. 20, No. 7, 2004 2775

Figure 3. Representative BAM images of Bc-NO2 layers on water at different molecular areas σ during successive compressions. (a) First compression, σ ) 0.41 nm2/molecule. (b) First compression, σ ) 0.28 nm2/molecule. (c, d) First compression, σ ) 0.10 nm2/molecule. (e) Second decompression, σ ) 0.24 nm2/molecule. (f) Second decompression, σ ) 0.41 nm2/molecule. Bar corresponds to 1 mm. Compression is symmetric from top and bottom in all images. Table 2. Summary of Isotherm Properties coareaa, σ0, nm2/molecule molecule

1st compression

2nd compression

Bc-H Bc-NO2 Bc-CH3 Bc2-SiO Bc3-SiO

0.22 ( 0.01 0.3 ( 0.05 0.29 ( 0.03 1.1 ( 0.1 1.0 ( 0.1

0.12 ( 0.01 0.13 ( 0.01 0.14 ( 0.02 1.1 ( 0.1 0.9 ( 0.1

a Coareas are estimated as usual18 by extrapolating the isotherm to the baseline; this provides a first estimate of the area/molecule. The “error” value indicated the range of values found in different experiments.

this gives a first estimation of the surface area taken up by the molecule. The coareas of the siloxane end-chain molecules (given in Table 2) are about 3-10 times the coareas of the hydrocarbon end-chain molecules. The BAM images of the three banana molecules with hydrocarbon end chains (Bc-H, Bc-NO2, and Bc-CH3) are similar. Figure 3 presents typical images of Bc-NO2, as examples. At the beginning of the first compression (Figure 3a), we see reflectivities (and thus phases) which are uniform over macroscopic areas, ∼0.5 mm and larger. However, at least three different types of layer are seen, with the brightest as a minority phase. As we compress, the darkest phase is reduced to thin lines. When the islands of the brighter two phases are jammed together, the pressure begins rising and the reflectivity increases. The laser intensity must be decreased to avoid saturating the camera. A semiquantitative analysis of image gray levels, normalized by incident intensity (see above, with the experimental methods), suggests that the reflectivities of the majority increase by up to

a factor of 10 during compression. At the end of the first compression, a much brighter threadlike structure appears, while the dark lines fade, as shown in Figure 3c,d). Finally, the other two uniform phases become one uniform layer. When decompression begins, the whole layer immediately breaks along the thread structure into several uniform sections, as shown in Figure 3e. The surface pressure drops very quickly to zero. As we decompress, the fraction of darker phase increases, but the reflectivity of the brighter layer changes little (Figure 3f). None of these layers show evident in-plane optical anisotropy: the whole surface dims in unison as the analyzer is turned from parallel to perpendicular to the polarizer. Relative proportions of the two brighter phases after deposition are different for each different experiment but remain approximately constant both as the layer sits for up to an hour and as the compression begins, until the pressure begins to increase significantly. Naturally enough, the pressure rises at different molecular areas for each experiment (Figure 4a,b gives representative isotherms for the three molecules), depending on the relative proportion of the two brighter phases. In contrast, after the first compression/decompression cycle, the brighter phase is uniform; the second compression is much more reproducible (as indicated in the spread of coareas given in Table 2), and a third compression reproduces the second. However, if the compression is stopped at any point, the pressure decreases slowly. Further, during the decompression, the pressure decreases much faster than the previous increase. Clearly the layer undergoes slow rearrangement, as well as some three-dimensional collapse into threadlike structures that are approximately parallel to the compression barriers, but both rearrangement and collapse are reversible, since the third compressions reproduce the second.

2776

Langmuir, Vol. 20, No. 7, 2004

Figure 4. Representative isotherms for Bc-H (solid line, 9), Bc-NO2 (dash-line, b), and Bc-CH3 (dash-dot line, 2). (a) Surface pressure π vs molecular area σ for the first compression. (b) π versus σ for the second compression. (c) Surface potential increment ∆V versus σ for the first compression. (d) ∆V versus σ for the second compression. All three hydrocarbon end-chain molecules follow this same pattern, with one major difference: the reflectivities of the Bc-H layers are about 4 times smaller than those of theBc-NO2 and Bc-CH3 layers, which are alike within experimental uncertainty. Figure 4c,d shows the surface potential isotherms for the first and second compressions. In the first compression, all relative

Zou et al. surface potentials reach values between 0.3 and 0.5 V as the pressure starts to increase. As the surface pressure increases, the surface potential increases further, by up to 30%. For the second compression, the surface potential is 0.52 V for Bc-H, 0.6 V for Bc-NO2, and 0.4 V for Bc-CH3 in the nearly uniform layer at nearly zero pressure, just before the surface pressure increases. Note that in one case, the ∆V after decompression has decreased to almost zero, with large fluctuations, before recompression. The fluctuations correspond to dark and bright phase in coexistence and the low value suggests that ∆V is about zero for the dark phase. The reflectivity of the dark phase is also less than before compression for all three hydrocarbon molecules, becoming indistinguishable from the reflectivity of the pure water surface. Unlike for the three hydrocarbon-end-chains molecules, films of the two siloxane end-chain molecules (Bc2-SiO and Bc3-SiO) look different under the BAM as the pressure increases to the point that the film collapses. In both cases, the film increases uniformly in brightness as the pressure increases above ∼5 mN/ m. For Bc2-SiO, nonuniform bright domains appear, with different shapes and sizes (Figure 5a). These domains are unstable. After being left about 3 h, the size of each domain enlarges as the domains break, as shown in Figure 5b. At decompression, the domains break into snowflakelike images (Figure 5c), which finally disappear. Bc3-SiO shows different collapse morphology in different experiments for the first compression. Parts a-c of Figure 6 are presented as examples. Note the fairly compact domain in Figure 6c, which is a network that explodes upon decompression (Figure 6d). In the second compression, bow-shape domains, as shown in Figure 6e, always appear. The angle of the bow is 122° ( 5°. In another contrast with the behavior of the hydrocarbon endchain molecules, the isotherms (Figure 7a) for Bc2-SiO-3 and Bc3-SiO are both stable and reproducible, at least up to about 5 mN/m. If we stop the compression in this range, the surface pressure remains constant for at least 15 min. At higher pressures, this is no longer true, as the isotherm difference for the first and second compression, as well as the difference in the images, suggests. For the second compression in both cases, the pressure reaches a plateau after a typical nucleation hump. Much higher pressures are reached in the first compression. With Bc3SiO on several experiments, the pressure reached a high plateau of ∼15 mN/m, before suddenly decreasing, several minutes later (even if compression was stopped); Figure 6b,c has examples of domains that appear during such a plateau. In other experiments, no such plateau is observed; Figure 6a is an example of domains observed under these circumstances. Apparently, layers such as those in Figure 6b,c are quite unstable with respect to other collapsed structures. Both the isotherms and the images demonstrate that more than one metastable collapse structure is available to the Bc3-SiO molecule. These structures are somewhat different from those seen with the other siloxane molecule, but with the apparent availability of several different metastable collapse structures, even slight changes in the molecule may well select a different structure. During decompression, the surface pressure shows a pronounced shoulder as it decreases by about 50% (Figure 7b). This shoulder is reproducible between the first and second decompression for Bc2-SiO. For Bc3-SiO, the shoulder smoothes out with the second decompression. During the decompression, the

Figure 5. Representative BAM images of Bc2-SiO on water at different molecular areas σ during successive compressions. (a, b) First compression, σ ) 0.58 nm2/molecule. (c) Second compression, σ ) 1.0 nm2/molecule. The dark backgrounds are uniform dense layers. Bar corresponding to 1 mm. Compression is symmetric from top and bottom in all images.

Langmuir Monolayers of Bent-Core Molecules

Langmuir, Vol. 20, No. 7, 2004 2777

Figure 6. Representative BAM images of Bc3-SiO on water at different molecular areas σ during successive compressions. (a-c) First compression, σ ) 0.35 nm2/molecule. (a) corresponds to a case with no high plateau in the isotherm; (b) and (c) correspond to a high plateau; (d) decompression of (c); (e) second compression, σ ) 0.35 nm2/molecule. The dark backgrounds are uniform dense layers. Bar corresponding to 1 mm. Compression is symmetric from top and bottom in all images.

Figure 7. Representative isotherms for Bc2-SiO (left side) and Bc3-SiO (right side) for the first (solid line) and second (dash line) compression. (a, d) Surface pressure π versus molecular area σ, compression for Bc2-SiO and Bc3-SiO. (b, e) π versus σ, decompression. (c, f) Surface potential increment ∆V vs σ, compression. The isotherms for Bc2-Si and Bc3-SiO are both stable for pauses >15 min, as long as π < 5 mN/m. islands both dispersed and left the field of view. Because the field of view is so limited compared to the total area (151 mm2 compared to 8541 mm2 when decompression starts and 10 105 mm2 at the beginning of the shoulder), the absence of an image is not definitive; that any remaining islands disappear at pressures corresponding to the shoulder remains a reasonable guess.

The surface potential isotherms are presented in Figure 7c. For both compressions, the relative surface potential reaches values of ∼0.3 for Bc2-SiO and ∼0.36 for Bc3-SiO when the surface pressure just starts to increase. With the surface pressure increases, the surface potential increases by up to 50%. The surface potential increment returns to zero at the end of the decompression.

2778

Langmuir, Vol. 20, No. 7, 2004

Figure 8. Top view: Schemata of possible close-packings of a projection of Bc3-SiO onto a surface. (a) Layered structure, showing the projections; (b) herringbone structure, with the projections slightly reduced in size for clarity.

Discussion Brewster angle microscopy immediately tells us that all the molecules can form thin layers of constant thickness, where this thickness can have more than one value. The smallest such regions observed were ∼100 µm across and the largest more than the maximum field of view of the microscope, several millimeters, and probably extending over much of the trough. The surface pressure isotherms can give a first idea as to the molecular configurations in these films. The molecules with the hydrocarbon end groups and those with the siloxane end groups behave very differently, so we will consider these separately. Dilute monolayers of the molecules with siloxane end groups, Bc2-SiO and Bc3-SiO, phase separate into fluid islands. Below σ ∼ 1.2 nm2/molecule, the islands become continuous and the surface pressure starts, rising as the layer is compressed. Because phase segregation corresponds to attractive interactions, such monolayers are generally quite close-packed. Since both core and end chains are amphiphilic, a first guess for the configuration would be that the molecule lies flat on the surface, with both parts in the immediate interfacial region. The collapse pressure ∼10 mN/ m, near that observed for polysiloxanes,19 also suggests that the siloxane end groups remain in contact with the water until collapse. We can crudely estimate the area per molecule in such a close-packed array by considering different projections (see Figure 8a for a schematic example) of a space-filling molecular model (as in Figure 1e,f) onto a surface. Copies of a projection can then be packed in different ways to find the close-packed molecular area σ0.18 In what follows, we will compare the molecular area from different projections and different packings to the experimentally observed σ0 (Table 2) derived from the σ where the pressure rises in the isotherm. For both the siloxane molecules, the layer is uniform as the pressure rises, so that this represents a true molecular area. Using the projections will yield an upper limit to a closed pack structure, since a threedimensional structure can always pack more tightly than its projection. However, this limit will put constraints on possible layer configurations. Many configurations with respect to the surface are possible, even keeping both core and end-groups quite flat on the surface. Such flat configurations could be packed as tightly as ∼1.6 nm2/molecule for both siloxane molecules. This is about 30% less dense than observed from the isotherms, but quite reasonable considering the limitations of the model, especially given the flexibility of the molecules. Generally herringbone patterns (Figure 8b), with the two end-chains fitting into the bow-shaped core, formed the tightest packings, with layers 20-30% denser than a two-dimensional layered structure (Figure 8a), mainly because the end-chains are quite bulky in (19) Fox, W.; Taylor, P. W.; Zisman, W. A. Ind. Eng. Chem. 1947, 39, 1401. Granick, S. Macromolecules 1985, 18, 1597. Mann, E. K.; Henon, S.; Langevin, D.; Meunier, J. J. Phys. II France 1992, 2, 1683.

Zou et al.

Figure 9. Side-view schemata of possible layer structures of the bent-core molecules with hydrocarbon end chains.

cross-section compared with the core. This is also why the two different molecules gave similar close-packed areas in these simplified models: the bulky siloxane groups fit more compactly into the bend of the three-ring core than the two-ring core. Given the limitations of these models, the better agreement with the data of herringbone structures is suggestive rather than definitive. However, the very close agreement in coareas for the two molecules with different core structure suggests that perhaps the core is mostly off the surface. With both siloxanes flat on the surface, projections give the molecular area as low as ∼1.4 nm2; this is certainly also consistent with the isotherm results, within the limitations of the model. These numbers should be compared with other possible configurations. For an end-on configuration, with only one of the siloxanes on the water and the rest of the molecule in the air, close-packing corresponds to ∼0.6 nm2/ molecule. With both the siloxane ends either plunged into the water (short siloxane oligomers are water-soluble) or into the air, but the core flat on the surface, close packing corresponds to ∼0.7 nm2/molecule. Since these values should correspond to an upper limit for a true close-packed layer, they are all too small to correspond to the observations. The isotherm data thus strongly suggest that the molecules pack quite flat on the surface, with both the end groups and possibly the core in close contact with the water. There are many detailed configurations of each molecule that corresponds to this, including some with the bend in the core nearly perpendicular to the surface and others with the bend essentially parallel to the surface. The isotherm data alone are not enough to distinguish between these different configurations. However, the images of collapsed structures in the Bc3-SiO show distinct bow-like shapes, with an exterior angle of ∼122°, very close to the expected angle at the bend in the core: these images suggest, for the collapsed structures at least, a distinct bend, parallel to the surface, in the core. The hydrophobic hydrocarbon chains, on the other hand, should lead to more upright configurations for that series of bent-core molecules. Indeed, we find that the surface pressure rises for these molecules only at much smaller molecular areas: in fact, as much as 10 times smaller, although the cores are more than two benzene rings larger. All the cores are clearly not flat on the surface. An upper limit for the monolayer closed-packed area can be found as before, from projections of space-filling models (Figure 1d). The most likely monolayer configurations, with only cores in contact with the water as schematized by the lower rows of molecules in Figure 9, suggest close-packed molecular areas σ ∼ 2 nm2/molecule, almost 20 times those which are observed. End-on projections of these molecules (as in the upper row of molecules in Figure 9c; here, the molecules would very improbably have only one end of one hydrocarbon end-chain in contact with the water) give molecular areas σ ∼ 0.5 nm2/molecule. Even the first compression suggests molecular areas half that value.20

Langmuir Monolayers of Bent-Core Molecules

Using projections of the three-dimensional molecules represents an upper limit for the molecular area, although one which is surprisingly close to the observed molecular areas in simple cases.18 A lower limit for the molecular area σ in a monolayer configuration can be determined assuming that the molecules can somehow pack to densely fill all of space. From space-filling models, the approximate volume of a molecule can be taken as ∼1.36 nm3. The longest dimension of the molecule is 4 nm, so that the upright configuration, which would have a single hydrocarbon chain end in contact with the water, leads to a molecular area of σ ∼ 0.33 nm2/molecule. A more likely configuration with the core in contact with the water and the hydrocarbon chains in the air would give a thickness ∼1.3 nm or a molecular area σ ∼ 1 nm2. All the layers as the pressure begins to rise, with the possible exception of the very darkest one (similar to that observed in coexistence with a gas at very large molecular areas), are certainly more than one molecule thick. The observed molecular areas of ∼0.12 nm2 (at the second compression, where the film was very uniform and the coarea was a good estimate of the molecular area for that film) would, in this simplified model, correspond to a film thickness ∼11 nm, or at least three molecular lengths. Indeed, because the cores are much bulkier than the chains, one might expect some sort of interdigitated, multilayer structure in order to form a dense hydrocarbon layer while allowing some part of the amphiphilic core to remain at the surface. Such structures have been observed in Langmuir monolayers of the nCB series.9 The single chain on one end of the nCBs also allowed a single layer structure, with the cores tilted at the interface and a dense, tilted hydrocarbon layer above it. It is difficult to imagine such a structure for the double-chained molecules considered here. However, a more complicated layer structure, with a first layer lying with the core flat on the water surface and further layers with interdigitated hydrocarbon chains and nearly upright configurations, is quite conceivable, Figure 9b,c, for example. The images in Figure 2a,b and Figure 3a,f suggest that in fact several such configurations are possible even at zero pressure. An example of the relative reflectivity, as estimated from the gray levels at different incident intensities, of these different layers are given in Figure 10, for Bc-NO2. All reflectivities are given relative to that of the layer observed at the smallest concentrations. If the optical density of the layers remains constant, the reflectivity is proportional to the square of the layer thickness. If we further argue that the thicker layers after a compression/decompression cycle are ∼11 nm, as argued from the surface isotherms above, we would deduce that the three different layers just before compression are ∼3.5, ∼4.8, and ∼5.5 nm thick. For this molecule, the layer observed at very high molecular areas (Figure 2b) appears similar to the thinnest of these three layers. These configurations could correspond to a double layer, of the general sort shown in Figure 9c, but the 11-nm configuration must be at least a triple layer. Also note that fresh solutions and solutions more than 2 weeks old gave different thicknesses after 2 compression/decompression cycles, even small amount of impurities may allow the system to explore different metastable layer structures. (20) The layer is quite inhomogeneous in this first compression; the dominant layer covers only ∼85% of the surface. However, the reflectivity of the brighter minority layer is only ∼30% greater, and the darker background covers ∼2% of the surface as the pressure rises, so that the determined coarea should be within ∼20% of the molecular area for the dominant phase. The layer is very homogeneous during the second compression, so that the determined coarea should be within 10% of the molecular area.

Langmuir, Vol. 20, No. 7, 2004 2779

Figure 10. Relative reflectivities, calculated from eq 1, of the different layers observed at zero pressure for Bc-NO2. Thin Layer: σ ∼ 5 nm2/molecule. 0 Cycle: σ ∼ 0.4 nm2/molecule before 1st compression. 2 Cycles: σ ∼ 0.4 nm2/molecule, after 2 compression/decompression cycles. Fresh solutions showed different thicknesses than aged solutions after 2 cycles.

The three hydrocarbon molecules have different groups substituted at the inner angle of the central benzene ring of the core, which can affect both configuration and stability of the molecule at the surface, as well as the way in which these molecules stack to form a multilayer. Indeed, subtle differences appear. With the second compression, the Bc-H molecule shows the smallest area/ molecule, while the Bc-CH3 coarea is about 10% higher, although these differences may be due to some residual nonuniformity in the films rather than to true differences in the uniform films. The surface potential increment of the final uniform layer at zero pressure varies from ∆V ) 0.4 V for the molecule with the methyl group (Bc-CH3) to 0.5 V for the unsubstituted molecule (Bc-H) to 0.6 for the molecule with the nitro group (Bc-NO2). This suggests that the average dipole density of Bc-NO2 is about 50% greater than that of Bc-CH3. The average dipole moment of the molecules are also different, estimated at 1.7 D for Bc-H, 1.9 D for Bc-CH3, and 2.6 D for Bc-NO2,21 all in the direction perpendicular to the long axis. The simplest model gives the relation between molecular dipole moment µ and surface potential drop ∆V as 22

∆V )

µ . σ0

(2)

A more complicated layered model, which can take into account the variation of dielectric constant through the interface from  ) 80 in the water to  ) 1 in air, would be more realistic,22 but here the films are so thick that most of the material should be in the interior of the film, away from the water, and the simple model is a reasonable place to start. Note that if all the molecules were aligned with the molecular dipole moment facing up, as in Figure 9a, and ignoring the contribution from the water molecules at the interface, we would expect a voltage drop of ∆V ∼ 1.5 V. Our much lower values suggest a more complicated layer, as in Figure 9c, where the bulk of the film is not aligned with the long axis parallel to the surface; this is consistent with the argument above on more physical grounds. However, the ratio of the dipole moments for Bc-NO2 and Bc-H is 1.5, which is also the ratio of the corresponding surface potential drops for the dense uniform layer. This agreement in the ratios would be (21) As determined by Spartan ‘02, Wavefunction, Inc., with the spacefilling models as given in Figure 1d, which were determined with the semiempirical module with the equilibrium geometry option. (22) Taylor, D. M. Adv. Colloid Interface Sci. 2000, 87, 183.

2780

Langmuir, Vol. 20, No. 7, 2004

Zou et al.

expected from eq 2, given the very similar values for molecular area σ, provided that the packing of the different layers is quite similar. These surface potential results, along with the very similar values for σ, suggest that the packing is indeed quite similar for the different molecules. There is only one major difference between the different hydrocarbon end-chain layers: the reflectivities of the Bc-H layers are about 4 times smaller than the corresponding layers of Bc-NO2 and Bc-CH3. If the optical densities of the layers were the same, this would mean that the Bc-H layer was half as thick as the other, which would imply about twice the area per molecule σ. More generally, a first approximation gives the Brewster angle reflectivity of such a layer as proportional to the square of the layer density 1/σ, rather than depending on the thickness of the layer and the optical density separately: this holds exactly if the optical density of the layer is sufficiently close to that of water.23 However, from the isotherms, the corresponding layers for the different molecules clearly have roughly the same σ. Some other explanation must hold for the large difference in reflectivity. Optical anisotropy can also affect the reflectivity substantially.23,24 However, we observed no in-layer optical anisotropy, so that any significant anisotropy is perpendicular to the layer. It would take completely unrealistic anisotropies, of the order of the refractive index of the layer, to yield a factor of 4 differences in reflectivity for otherwise similar layers. We also note that, whatever the dynamic response of the layer polarization to light, the average static dipole density is quite similar for the three molecules, as revealed in the surface potential differences. A final possibility is that the optical density is significantly decreased in the Bc-H films by the inclusion of enough air, while maintaining roughly the same σ. It is easy to see that the inclusion of air in a film can make a critical difference in reflectivity where inclusion of water does not by considering an extreme case: if enough air were included in a layer so that its average optical density were that of water, the surface would not reflect p-polarized light at the Brewster angle at all, however thick the film became. We can estimate the amount of air needed in the film to reduce the reflectivity of the film by a factor of 4, without changing the amount of material in the film, with the simple Maxwell-Garnett model,25 in which

 - a f -  )φ f + 2  + 2a

(3)

where f is the effective dielectric constant of the film,  is dielectric constant of a dense packed film, a ) 1 is the dielectric constant of air inclusions, and φ is the volume fraction of those inclusions (assumed in this model to be spherical and much smaller than the wavelength of light). We also assume that L(1 - φ) ) constant, to maintain the total amount of material in the film a constant. If the average refractive index of the Bc-NO2 and Bc-CH3 films were near the bulk value of 1.5, the Bc-H would have to contain 17% more air (and be about 17% thicker) to give a reflectivity a quarter as large with the same amount of (23) Mann, E. K.; Heinrich, L.; Voegel, J. C.; Schaaf, P. J. Chem. Phys. 1996, 105, 6082. Mann, E. K.; Heinrich, L.; Voegel, J. C.; Schaaf, P. Prog. Colloid Polym. Sci. 1832, 110, 296. Mann, E. K. Langmuir 2001, 17, 5872. (24) Meunier, J. In Light Scattering by Liquid Surfaces and Complementary Techniques; Langevin, D. Eds.; Marcel Dekker: New York, 1992; p 333. (25) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland, 1977; p 359.

material. This is certainly quite conceivable, but more direct evidence is necessary for any firm conclusions about the origin of the much smaller reflectivity of Bc-H films compared to the corresponding Bc-NO2 and Bc-CH3 films. Conclusions We have demonstrated that it is possible to make stable Langmuir monolayers of a variety of different bent-core molecules. The properties of these layers depend on the molecule. The most obvious differences are between molecules with end chains of different character: amphiphilic siloxane as compared to hydrophobic hydrocarbon chains. With amphiphilic chains, the molecules lie quite flat on the surface, with both core and end chains in direct contact with the air/water interface. With hydrophobic chains, the molecules form a complex multilayer structure; surface potential and other results suggest that these structures are quite similar with the different cores. The length of the hydrophobic chains probably plays a role in such structures. Other work,12 with somewhat different cores and very short hydrophobic side chains, found a single layer, perhaps quite similar to what we observed for the amphiphilic side chains. Different techniques would be required to determine the exact structure of these layers, but we expect a first layer with the core in contact with the surface and other layers with more upright molecules to lead to dense packing of the hydrocarbon chains. We note that at least three layers of different discrete thickness are possible, two of them of quite comparable thickness and one significantly thicker. This final layer appears to be the most stable, in that it forms irreversibly, at the expense of thinner layers, and remains even at zero pressure. As the pressure on this layer decreases, it simply breaks into pieces, with the opening cracks indistinguishable from the bare water surface. Upon being recompressed, the pressure rises, and the film brightens by a factor of 2. Threads appear at very high pressure. These may be similar in structure to those observed in the bulk, where free-standing two-dimensional films are not stable while free-standing threads are.6 Preliminary results suggest that the bent core molecules with the siloxane side chains can form good alignment layers for bent-core liquid crystals, which have been difficult to align over macroscopic regions. The configuration of the cores on the surface may result in a periodic modulation of the substrate that is compatible with the bent-core liquid crystals. Effective alignment of bent-core liquid crystals is important to studying their macroscopic properties, including viscoelasticity, piezoelectric constant, and electrostriction. Such alignment is crucial to their eventual use in devices such as sensors, actuators, or artificial muscles. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. 9984304. The surface potential work was supported by the Petroleum Research Fund, under grant ACS PRF# 35293-G 7. We thank Julie Kim for verification of the Bc2-SiO and Bc3-SiO structures by NMR and acknowledge very helpful discussions with Mary Neubert and with J. Adin Mann, Jr. LA0361924