Magnetic Molecules at the Air/Water Interface D. Vaknin,* ,† L. L. Miller, † M. Eshel, ‡ and A. Bino ‡ Ames Laboratory and Department of Physics and Astronomy, Iowa State UniVersity, Ames, Iowa 50011, and Department of Inorganic and Analytical Chemistry, The Hebrew UniVersity of Jerusalem, 91904 Jerusalem, Israel ReceiVed: March 22, 2001; In Final Form: June 8, 2001 X-ray Reflectivity (XR) and grazing angles of incidence X-ray diffraction (GIXD) were conducted to determine the structure of Cr 8 O 4 (O 2 CPh) 16 Langmuir films at the air/water interface at various surface pressures π. The molecular area versus pressure, π-A isotherm, reflectivity, and GIXD variation (width and peak position) with compression, all imply the formation of a homogeneous monolayer at very low surface pressures. The film is disordered with a liquidlike or glassy structure factor at all pressures, with correlation lengths that extend over a few molecular distances. The liquidlike state of the monolayer and an incomplete formation of a second layer have implications on the quality of transferred Langmuir-Blodgett films. Introduction Magnetic molecules with strong intramolecular magnetic exchange interactions relative to intermolecular ones are of interest for both their unique fundamental magnetic properties and their potential technological applications. 1 Typically orga- nometallic based, these “molecular magnets” consist of two or more 3d or 4f magnetic ions coupled by electronic superex- change interactions, and can be over 10 nm in size. The controllable size and geometries of the magnetic ions are of fundamental importance to testing our understanding of magnetic interactions and have already lead to the discovery of the quantum tunneling effect of the magnetic spins. 2 As the number of magnetic ions is increased, one approaches the mesoscopic regime which bridges the nanoscopic scale and the macroscopic world of extended lattice solids. On the basis of their small size and customization possibilities, the materials are potentially very high-density storage medium or functional building blocks in supramolecules. Some magnetic molecules occur naturally as biological materials which makes their study appealing to an even wider audience and gives added importance to their study. Herein we present our structural study of a two-dimensional (2D) Langmuir film formed by the magnetic molecule Cr 8 O 4 (O 2 - CPh) 16 . 3,4 The antiferromagnetic Cr 8 O 4 cubane core, shown in Figure 1, is similar to the compound described by Atkinson et al. 3 containing two coupling constants (J) -3 and -5 K. This molecule is readily soluble in CHCl 3 , and possesses a hydro- phobic outer shell composed of 16 phenyl rings making it a potential monolayer former when spread on water surfaces. Such monolayers can be subsequently transferred to solid surfaces employing the Langmuir-Blodgett (LB) technique to form well- defined multilayer systems. Control of their arrangement on solid surfaces is an important step to potential applications of these molecular magnets in electronic or magnetic devices. Such control has to be achieved prior to the LB transfer while the molecules are spread as a Langmuir film at aqueous interfaces. The Langmuir trough readily allows for the manipulation of the film by the variations of the in-plane density, pH, ionic concentration, and temperature of the subphase. In addition, desired separation among magnetic molecules can be achieved by spreading them from a mixture that includes fatty acids or lipids (assuming that they are miscible among acyl chains) that can provide a supporting 2D membrane to the embedded magnetic molecules. Similar attempts to form LB films of organometallic Mn12-acetate compound were reported recently. 5 In those studies the Mn12 molecule was embedded in multilayer fatty acid (behenic acid) where the Mn12 formed an intercalated layer between the two headgroups of adjacent fatty acids. 6 More generally, the Cr8 molecule can serve as a model system to study the 2D physical properties of highly symmetric molecules that reside on an isotropic substrate, i.e., aqueous surfaces. A similar motivation to understand the effects of physical dimension on statistical physics, led to recent studies * To whom correspondence should be addressed. E-mail: vaknin@ ameslab.gov. † Ames Laboratory and Department of Physics and Astronomy. ‡ Department of Inorganic and Analytical Chemistry. Figure 1. The Cr8 molecule [Cr8O4(O2CPh)16] used in this study, and surface pressure versus molecular-area (π-A) isotherm of Cr8 on pure water. Two sets of π-A isotherms (at T ) 20 °C) taken at compression rates 4 (curves labeled I) and 30 (II) per molecule per minute are shown. The isotherm was found to be sensitive to the compression rate; however, for compression rates that are slower than approximately 10 Å 2 per molecule per minute, the isotherms are similar to curve I. The slow rate isotherm indicate that the molecules interact at relatively large molecular areas, around 300-350 Å 2 (we estimate the cross section of the molecule at 232 Å 2 ). It is argued that the fast compression rate does not allow for the formation of a single layer, driving the system into a multilayer form. All X-ray experiments were performed on slow- rate compressed samples. 8014 J. Phys. Chem. B 2001, 105, 8014-8017 10.1021/jp011100+ CCC: $20.00 © 2001 American Chemical Society Published on Web 07/21/2001