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 air- water 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 vali- nomycin, whose antibiotic activity is related to the complexation with and transport of monovalent cations. 3,4 Numerous studies of the complexation between macro- cycles and cations at the air-water interface 1-6 have employed macroscopic, indirect techniques such as pres- sure-area isotherms. We now report the first direct observation of the structural reorganization within mac- romolecular 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, 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 * To whom correspondence should be addressed. † Current address: James Frank Institute, University of Chi- cago, 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. (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. 6897 Langmuir 1999, 15, 6897-6900 10.1021/la981092x CCC: $15.00 © 1999 American Chemical Society Published on Web 09/28/1999