Langmuir 1999, 15, 6897-6900
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Structure of a Phenylacetylene Macrocycle at the Air-Water Interface Oksana Y. Mindyuk, MacKenzie R. Stetzer, David Gidalevitz,† and Paul A. Heiney* Department of Physics and Astronomy and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104
James C. Nelson and Jeffrey S. Moore Roger Adams Laboratory, Departments of Chemistry and Material Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received August 24, 1998. In Final Form: May 11, 1999 We have used grazing incidence X-ray diffraction and X-ray specular reflectivity to study Langmuir films of a phenylacetylene macrocycle (PAM), a ring-shaped molecule known to form a tubular liquid crystal. PAM adopts an “edge-on” molecular arrangement at the air-water interface. The local structure is quite similar to that of the corresponding bulk columnar liquid crystal, but with enhanced order in the intracolumnar direction. The columnar order is disrupted by CsCl in the subphase and strongly enhanced by KCl in the subphase.
I. Introduction The self-organization of macromolecules at the airwater interface is of considerable interest due to the possibility of producing highly ordered monolayer films with electronic or optical properties tailored at the molecular level.1 Macrocyclic molecules, which are often capable of complexation with metal and molecular ions, are particularly interesting candidates as components of supramolecular ionic devices.1,2 They also provide model systems for more complicated ionophores such as valinomycin, whose antibiotic activity is related to the complexation with and transport of monovalent cations.3,4 Numerous studies of the complexation between macrocycles and cations at the air-water interface1-6 have employed macroscopic, indirect techniques such as pressure-area isotherms. We now report the first direct observation of the structural reorganization within macromolecular Langmuir films of disk-shaped ionophoric molecules arising from interactions with K+ and Cs+ ions in the subphase. Grazing incidence X-ray diffraction (GID) and X-ray specular reflectivity (XR) measurements are powerful tools for characterization of molecular order in Langmuir films.7-9 Such techniques have most often been used to study films of amphiphilic rodlike molecules. Recently, * To whom correspondence should be addressed. † Current address: James Frank Institute, University of Chicago, Chicago, IL 60637. (1) Lednev, I. K.; Petty, M. C. Adv. Mater. 1996, 8, 615 and references therein. (2) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (3) Kemp, G.; Wenner, C. E. Biochim. Biophys. Acta 1972, 282, 1. (4) Zaitsev, S. Y.; Zubov, V. B.; Mobius, D. Biochim. Biophys. Acta 1993, 1148, 191. (5) Boguslavsky, L.; Bell, T. W. Langmuir 1994, 10, 991. (6) Mertesdorf, C.; Plesnivy, T.; Ringsdorf, H.; Suci, P. A. Langmuir 1992, 8, 2531. (7) Als-Nielsen, J.; Jacqueman, D.; Kjaer, K.; Leveiller, F.; Lahav, M.; Leiserowitz, L. Phys. Rep. 1994, 246, 251 and references therein. (8) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D. Phys. Rev. B 1990, 41, 1111.
Langmuir films of discotic liquid crystalline compounds were successfully characterized via GID and XR.10-13 It was shown that disk-shaped molecules could adopt either an “edge-on” arrangement, in which the strong π-π interactions lead to cofacial stacking of cores perpendicular to the water surface,10 or a “face-on” structure, in which the polar character of the core favors the alignment of the cores parallel to the water surface.12,14 We have now used XR and GID to study Langmuir films of a phenylacetylene macrocycle (PAM, Figure 1).15-17 PAM is a ring-shaped molecule which forms a bulk columnar liquid crystalline phase.17,18 The rigid triple bonds in the central ring ensure that the central portion of the molecule is practically planar, with a large (8-9 Å diameter) central void. This means that the liquid crystal phase of PAM in bulk incorporates open channels running along the columns, making it the first truly tubular liquid crystal.17 It is not a priori evident whether PAM should adopt an edge-on or face-on arrangement, since the hydrophilic polar groups (both ether and ester) would favor a face-on arrangement, while the intermolecular π-π interaction (9) Jacquemain, D.; Grayer Wolf, S.; Leveiller, F.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. Angew. Chem., Int. Ed. Engl. 1992, 31, 130. (10) Gidalevitz, D.; Mindyuk, O. Y.; Heiney, P. A.; Ocko, B. M.; Henderson, P.; Ringsdorf, H.; Boden, N.; Bushby, R. J.; Martin, P. S.; Strzalka, J.; McCauley, J. P., Jr.; Smith, A. B., III J. Phys. Chem. 1998, 101, 10870. (11) Gidalevitz, D.; Mindyuk, O. Y.; Heiney, P. A.; Ocko, B. M.; Kurnaz M. L.; Schwartz, D. K. Langmuir 1998, 14, 2910. (12) Gidalevitz, D.; Mindyuk, O. Y.; Stetzer, M. R.; Heiney, P. A.; Kurnaz, M. L.; Schwartz, D. K.; Ocko, B. M.; McCauley, J. P., Jr.; Smith, A. B., III J. Phys. Chem. B 1998, 102, 6688. (13) Mindyuk, O. Y.; Heiney, P. A. Adv. Mater. 1999, 11, 341. (14) Heiney, P. A.; Gidalevitz, D.; Maliszewskyj, N. C.; Satija, S.; Vaknin, D.; Pan, D.; Ford, W. T. J. Chem. Soc., Chem. Commun. 1998, 1483. (15) Zhang, J.; Pesak, D. J.; Ludwick, J. L.; Moore, J. S. J. Am. Chem. Soc. 1994, 116, 4227. (16) Zhang, J.; Moore J. S. J. Am. Chem. Soc. 1994, 116, 2655. (17) Mindyuk, O. Y.; Stetzer, M. R.; Heiney, P. A.; Nelson, J. C.; Moore, J. S. Adv. Mater. 1998, 10, 1363. (18) Chandrasekhar, S.; Ranganath, G. S. Rep. Prog. Phys. 1990, 53, 57.
10.1021/la981092x CCC: $15.00 © 1999 American Chemical Society Published on Web 09/28/1999
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Figure 2. Pressure-area isotherms of PAM on water, 0.01 M KCl, and 0.01 CsCl. All isotherms were measured at a room temperature.
Figure 1. (a) Structure of PAM. Arrows indicate positions for possible alkali ion complexation, as discussed in text. (b) Proposed schematic molecular arrangement of PAM monolayer with intercalated K+ ions. The K+ ions selectively bind to welldefined spots on the ring, enhancing the columnar structural order.
between the phenyl rings favors a cofacial, edge-on arrangement. II. Measurements PAM was synthesized and purified as described previously.15 X-ray measurements were conducted at beam line X22B (λ ) 1.545 Å) of the National Synchrotron Light Source, Brookhaven National Laboratory. The Langmuir trough and liquid surface spectrometer employed were as described previously.10-12,19 A monochromatic X-ray beam was deflected toward the water surface via Bragg reflection from a Ge(111) crystal. The instrumental resolution was determined primarily by the slits between the sample and the detector. These slits were always set to optimize the resolution in the scattering plane while increasing the signal by lowering resolution out of the scattering plane. The resolution in the direction perpendicular to the water surface during XR measurements depended on qz but was typically on the order of 0.008 Å-1 (much sharper than any feature in the reflectivity profile), while the resolution in the XY plane during GID measurements was 0.007 Å-1. All subphases employed water purified using a Millipore filtration system with a resulting resistivity F > 18 MΩ/cm; in addition to studying Langmuir films of PAM on pure water we used 0.01 M KCl and CsCl subphases. All monolayers were spread from 10-3 to 10-4 M chloroform solutions. Pressure-area isotherms were measured for each compound prior to the scattering measurements. On both water and salt subphases, X-ray (19) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Phys. Rev. 1997; E55, 3164 and references therein.
measurements were conducted at room temperature (22 ( 2 °C) at a surface pressure Π of 10-15 mN/m. The pressure never varied by more than 20% during the course of a measurement. Typical pressure-area isotherms for PAM are presented in Figure 2. The limiting areas, defined by extrapolating the rise of the isotherm down to Π ) 0, for PAM on pure water, KCl, and CsCl are 78, 74, and 79 Å2 per molecule, respectively. These areas are consistent with the 70-100 Å2 areas expected from simple models of edge-on molecules, but not with the 300-380 Å2 areas calculated for face-on molecules. The three isotherms are qualitatively very similar, although the molecular areas differ slightly and the slope of the vertical rise is larger for the KCl subphase, indicating a somewhat stiffer film. In an XR measurement, the photon momentum transfer is perpendicular to the air-water interface, and the reflected X-ray intensity is determined by the Fourier transform of the gradient of the electron density perpendicular to the water surface. We have followed the common practice7,8 of analyzing the XR data by modeling the electron density at the interface as a stack of N uniform slabs (boxes), each with a different electron density Fi and thickness Li. The effect of capillary waves on the water surface is modeled by a single Gaussian roughness σ for all interfaces. The reflectivity is then given by
R(qz) RF(qz)
| ∑i)1
N+1
)
(
)
Fi - Fi-1 F0
|
2
e-iqz′Di e-qz′ σ /2 2 2
qz′ ) xqz2 - qc2 where qz is the component of the momentum transfer perpendicular to the interface, qz′ is the refractioncorrected wavevector, qc is the critical wavevector below which the reflectivity is unity, R(qz)/RF(qz) is the measured reflectivity divided by the Fresnel reflectivity calculated for a perfect, sharp interface, F0 is the electron density of the subphase, FN+1 ∼ 0 is the density of the air or He above the sample, and Di ) ∑j)1i Lj is the distance from the water surface to the interface between layer i and layer (i + 1) (i.e., D0 ) 0). All three films had qualitatively similar reflectivity profiles. Figure 3 shows a typical XR pattern for PAM on pure water, together with the best fit to a multislab model. We found that two boxes were required to fit the data from all three subphases and that the fitted parameters20
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Table 1. Results of Fits to X-ray Reflectivity Dataa subphase
A (Å2)
L1 (Å)
F1 (e-/Å3)
L2 (Å)
F2 (e-/Å3)
σ (Å)
H2O 0.01 M KCl 0.01 M CsCl
78 ( 8 74 ( 7 79 ( 8
21.3 ( 1.3 22.4 ( 1.1 21.8 ( 1.7
0.419 ( 0.022 0.432 ( 0.035 0.421 ( 0.031
27.2 ( 1.6 27.9 ( 6.5 26.9 ( 1.9
0.041 ( 0.015 0.015 ( 0.02 0.033 ( 0.015
4.0 ( 0.5 4.0 ( 0.3 4.78 ( 0.4
a A is the molecular area at which the measurement was made (and not a fitting parameter), obtained by correlating the measured pressure with the isotherms in Figure 2. Li and Fi are the thickness and electron density of the first and second layers, σ is the Gaussian surface roughness. These parameters were obtained by least-squares fits to the reflectivity data. The error bars indicate uncertainties in the context of the two-slab model chosen; our experience has been that there may also be larger 10-20% systematic uncertainties in the fitted parameters depending on the model chosen (number of layers, etc.).
Figure 3. X-ray reflectivity profile for PAM on pure H2O. Every third data point is plotted. Solid line shows the best fit to a two-box model. R/RF is the measured reflectivity divided by the calculated Fresnel reflectivity. Inset shows the electron density profile derived from fitted parameters.
were similar in all three cases (Table 1). The fitted parameters are in good agreement with an edge-on arrangement of the PAM molecules on all subphases. The first box corresponds to a 21 Å monolayer of edge-on PAM molecules, and the second box arises from a dilute second layer of PAM molecules (≈10% occupancy21). Molecular modeling of the PAM molecule with all chains fully extended gives a diameter of roughly 38 Å, but powder X-ray diffraction measurements of bulk PAM in its columnar liquid crystal phase yield a 24.9 Å intercolumnar distance,17 consistent with either interdigitation of the chains or a certain amount of gauche bond formation. The slightly smaller thickness obtained from the box model fit might be taken as evidence for a 30-40° molecular tilt, an increased amount of gauche bond formation, or a structural rearrangement in which the side chains are roughly parallel to the water surface rather than extending radially from the molecular core. However, the 15-20% difference between the fitted monolayer thickness and the bulk molecular diameter is also within the range of model-dependent systematic uncertainties. Indeed, if we model PAM as a 25 Å diameter solid disk with a central (20) The fitting parameters consisted of a single roughness σ, a length Li and electron density Fi for each layer, and an overall amplitude I0. This last parameter was not completely free, in that we rejected fits where it did not come within 5% of the actual incident photon flux onto the sample. The value of I0 used in Figure 3, for example, differs by 3% from the measured value at qz ) 0.02 Å-1. (21) Our experience with other Langmuir films of discogenic molecules10-12 leads us to believe that there is always some irreversible second layer formation at the time the film is deposited, even if care is taken to use dilute solutions and deposit the drops as far away from each other as possible.
Figure 4. Grazing incidence X-ray diffraction profiles, semilog scale: (a) PAM/H2O; (b) PAM/0.01 M KCl; (c) PAM/0.01 M CsCl. All measurements were performed at 10-15 mN/m. The horizontal axis has been broken to emphasize the 0.15-0.60 and 1.5-2.0 Å-1 regions. No features were present in the 0.61.5 Å-1 region in any of the three GID patterns.
9 Å hole, the reflectivity obtained by numerically calculating the Fourier transform of the gradient of the electron density is virtually identical to that of a uniform 20 Å slab. Since the fitted parameters shown in Table 1 are similar for all subphases, the XR measurements by themselves do not provide any evidence for large structural changes (molecular tilt, etc.) in the Langmuir films as a function of subphase composition. We provide a more detailed discussion of the electron densities after we present the GID data. Our GID measurements corresponded to two-dimensional powder diffraction at the air-water interface. (We verified that the qz, or out-of-plane, dependence of the intensity was indeed that expected for a two-dimensional mesh of molecules roughly 20 Å thick, showing a minimum at qz ≈ 0.3 Å-1. We saw no evidence for a circular arc of intensity in q-space, as would be expected for threedimensional crystallites floating on the surface.) The GID pattern of PAM on pure H2O (Figure 4a) shows three major features, which can be indexed using a 3.47 Å × 25.4 Å rectangular unit cell. First, there is an 010 peak at qxy ) 1.81 Å-1 corresponding to an intracolumnar spacing of 3.47 Å, with a width ∆qxy ) 0.060 Å-1 full-width at halfmaximum (fwhm) corresponding to a correlation length ξ ) 2π/∆qxy ≈ 100 Å. Second, there are very weak 100 and 200 peaks at qxy ) 0.247 and 0.518 Å-1 with ∆qxy ) 0.0150.030 Å-1, corresponding to an intercolumnar spacing of 25.4 Å with a 200-400 Å correlation length in the intercolumnar direction. (The difference in peak width and calculated correlation length may result from inhomogeneities in lattice parameter. A spread in lattice parameters ∆a gives rise to a spread ∆q in peak positions,
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resulting in a net peak width ∆q/q ∝ ∆a/a. This effect is sometimes known as “paracrystalline disorder”.) These features are all consistent with a disordered columnar structure of edge-on molecules and are in excellent agreement with the 24.9 and 3.5 Å intercolumnar and intracolumnar spacings observed in the bulk PAM columnar liquid crystal.17 (In the conventional notation for diffraction from a three-dimensional discotic liquid crystal the 25 Å peak is labeled 100 and the 3.5 Å peak is labeled 001.) This is the first observation of the intracolumnar peak in discotic liquid crystals at the air-water interface. The obtained electron densities are also consistent within error bars with those calculated from the molecular structure and surface area. Assuming a columnar structure with a 25.4 Å intermolecular spacing and 3.47 Å intracolumnar spacing (from GID measurements) and thickness of 21.3 Å (from XR) we obtain a molecular volume of 1880 ( 180 Å3. Including the 0.419 e-/Å3 electron density in the first layer obtained from XR gives us 788 ( 87 e-/molecule, to be compared with the actual value of 690 e-/molecule. In fact, as discussed in Table 1 the systematic error bars in F1 and L1 are most likely somewhat larger, and we consider the 12% discrepancy between calculated and measured electron density to be reasonable. Furthermore, we can estimate from the XR data and the known electron charge per molecule that we have roughly 14% occupancy of a second layer. Applying this correction to the molecular area from the isotherm measurement, we obtain 78 × 1.14 ) 89 Å2/molecule in the first layer, in good agreement with the 88 Å2/molecule calculated from GID. The GID pattern for PAM on a 0.01 M KCl subphase (Figure 4b) is quite different. The wide-angle feature at qxy ) 1.81 Å-1 (∆qxy ) 0.06 Å-1) is essentially identical to the corresponding feature on the H2O subphase, but the low-angle feature at qxy ) 0.255 Å-1 has become sharper (∆qxy ) 0.03 Å-1) and substantially stronger, indicating an increase in the number of ordered crystallites and a modest increase in the intercolumnar correlation length. This effect was reproducibly observed on multiple film depositions and separately prepared 0.01 M KCl solutions. We have also observed that the intensity of the 1.81 Å-1 peak grows over a period of 60 min, indicating that the order induced by the KCl subphase takes time to develop, due presumably to the time required for rearrangement of the molecules on the surface. By contrast, the effect of Cs+ in the subphase is to destroy the intercolumnar correlations. As seen in Figure 4c, the low-angle 100 and 200 peaks are completely missing in the PAM/CsCl GID pattern, although the 010 “intracolumnar” peak at qxy ) 1.80 Å-1 is actually slightly sharper (∆qxy ) 0.049 Å-1). There have been previous observations of order induced in Langmuir films by ions in the subphase9,22,23 or ordering of the ions themselves in a thin film below the water surface.9,24 In other cases, subphase solutes have been seen to disrupt crystalline self-aggregation.9,23 The mechanism for order enhancement in the case of PAM is unclear. It seems likely that the ions complex with the PAM molecules. If ions preferentially attach themselves to specific sites on PAM, then they could “button” the molecules together, leading to enhanced order, as shown (22) Gidalevitz, D.; Weissbuch, I.; Kjaer, K.; Als-Nielsen, J.; Leiserowitz, L. J. Am. Chem. Soc. 1994, 116, 3271. (23) Jacquemain, D.; Grayer Wolf, S.; Leveiller, F.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. J. Am. Chem. Soc. 1990, 112, 7724. (24) Leveiller, F.; Jacquemain, D.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. Science 1991, 252, 1532.
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in schematically in Figure 1b. There are several obvious positions for complexation. The hollow center of the molecule (point “A” in Figure 1a) is initially attractive, but since the molecules are edge-on this would require the K+ cations to actually leave the subphase, which seems implausible. (Also, the interior void is much larger than the diameter of either K+ or Cs+.) Two other likely positions are adjacent to the ester or ether groups (point “B”) or adjacent to the triple bonds (point “C”). In either case, the cations could be intercalated between the molecules or could be placed in the subphase directly below the molecules. The fact that the intracolumnar distance between molecules is unaffected by complexation strongly argues for the latter interpretation, since intercalation of a K+ cation between the molecules would increase the spacing between them by several angstroms. In either case, the K+ cation is presumed to bind to two molecules, effectively locking them in place and enhancing the structural order. Unfortunately, the K+ ions do not alter the overall electron density profile sufficiently that we can directly test this hypothesis from our XR or GID data. Cs+ should also be attracted to either site “B” or site “C”. However, since the ionic radius of Cs+ is larger than that of K+, steric hindrance between the cations may disrupt columnar order even if they sit below the molecules, in the subphase. A random staggered configuration would then be manifested as disorder in the columnar direction, as observed. The differences between K+ and Cs+ may also arise in part from differences in the efficiency of complex formation. If we assume that subphase ions induce complexation, that we have less than one ion per molecule, and that Cs+ does not complex as efficiently as K+, then this would explain why the Cs+ films are less ordered than the K+ films (but not why they are less ordered than PAM films on pure H2O). III. Conclusions In summary, we have presented the first XR and GID studies of tubular columns at the air-water interface. PAM adopts an edge-on, columnar arrangement at the air-water interface. The dimensions of the molecule obtained from XR and GID data agree quantitatively with those obtained for high-resolution bulk powder X-ray diffraction measurements. Columnar order in the film is enhanced by the addition of KCl to the subphase and almost completely disrupted by the addition of CsCl. This provides additional confirmation that alkali salts may prove an effective means to tailor the properties of Langmuir and Langmuir-Blodgett films for potential applications. We have speculated on the possible mechanisms for the interaction between the PAM molecules and the subphase, but detailed confirmation of the local structure awaits further study. Acknowledgment. We thank B. Ocko, E. DiMasi, and J. Strzalka for their assistance, T. Bell for useful discussions, and D. Vaknin and J. K. Blasie for the use of their equipment. OYM, MRS, DG, and PAH were supported primarily by the MRSEC Program of the National Science Foundation (NSF) under Award Number DMR96-32598 and in part by NSF Grant DMR 93-15341. J.C.N. and J.S.M. were supported by the U.S. Department of Energy through the Materials Research Laboratory at the University of Illinois. Research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Material Science and Division of Chemical Sciences. LA981092X
