Dynamics within a Single Molecular Layer. Aggregation, Relaxation, and the Absence of Motion J. C. Horne and G. J. Blanchard* Contribution from the Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824 ReceiVed June 17, 1996 X Abstract: We report on the transient and steady-state optical responses of the chromophore 2,2′-bithiophene-5,5′- diylbis(phosphonic acid) (BDP) incorporated within a single zirconium-phosphonate layer as a function of chromophore density. While the dilute solution optical response of BDP reveals no anomalous behavior, its characteristics are substantially different when confined within a monolayer. We vary the concentrations of layer constituents to determine the extent of interaction between BDP moieties within a single monolayer. We observe limited initial aggregation of BDP, the extent of which is determined largely by the conditions under which the monolayer is formed. Over time, the fractional contribution of BDP aggregates to the total optical response decreases to a limiting value, implicating surface adsorption site density as the dominant factor in determining the morphology of the organobis(phosphonate) layer. Motional relaxation measurements of BDP within the layer show that the chromophores are immobile on the hundreds-of-picoseconds time scale of our experiments. Introduction Organized molecular assemblies have found use in numerous chemical and physical applications, such as device patterning, nonlinear optics, and tribology. 1-7 This class of interfacial materials have characteristically well-defined structures of controlled composition and, with a small amount of material, they offer a widely tunable range of chemical, electrical, and optical properties. Metal-phosphonate (MP) organic multilayer structures have been investigated extensively 8-24 and, like other self-assembling monolayers, show potential for use in surface modification, 25-28 electronic device, 29-31 nonlinear optics, 7,19,20 and molecular recognition applications. 32-37 Phosphonic acids form strong, sparingly soluble complexes with metal ions, giving them significant advantages over many self-assembled mono- layer (SAM) systems, such as the thiol/gold SAMs, which have been shown to be labile. 38-40 MP structures are comparable to SAMs in ease of synthesis, which generally involves immersion of the functionalized substrate into a solution of the appropriate (R,ω)-organobis(phosphonate). MP multilayers are versatile in a chemical sense because the identity of individual layers can be controlled selectively as the structure is assembled and, in this way, chemical or electrical potential, as well as optical properties, can be built into the system in three dimensions rather than two. Langmuir-Blodgett films can also be assembled as * Author to whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 1, 1996. (1) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62. (2) Wilbur, J. L.; Biebuyck, H. A.; MacDonald, J. C.; Whitesides, G. M. Langmuir 1995, 11, 825. (3) Drawhorn, R. A.; Abbott, N. L. J. Phys. Chem. 1995, 99, 16511. (4) Xia, Y.; Zhao, X.-M.; Kim, E.; Whitesides, G. M. Chem. Mater. 1995, 7, 2332. (5) Ford, J. F.; Vickers, T. M.; Mann, C. K.; Schlenoff, J. B. Langmuir 1992, 12, 1944. (6) Kim, E.; Whitesides, G. M.; Lee, L. K.; Smith, S. P.; Prentiss, M. AdV. Mater. 1996, 8, 139. (7) Katz, H. E.; Wilson, W. L.; Scheller, G. 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