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We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.

respectively (20). I11 the case of two-photon absorption, the line-shape f~~n c t i o r l(21 ), where n is the transition frequency, v is the laser frequency, and r is the molec-SCIENCE VOL. 271 22 MARCH 1996 22. C. SchGnenberger, J. Jonitsma, J. A M. Sondag-Huethorst. L G. J. Fokkink, J. Phys. CChem. 99.3259(1 995). 23. We thank W. Emst for the loan of a microwave source.

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Taken together, the data show that closest packed, oriented monolayers of n-alkanoic acids can be formed by adsorption from dilute solution. The formation of these quasi-two-dimensional organic surface phases is characterized by complicated kinetics in which surface and/or monolayer defects, as well as impurities, seemingly play important roles. The structures so obtained are dynamic in nature in that they undergo rapid exchange with comparable ligands in solution. These results, and those presented in the following paper, further suggest an important limitation to closest-packed, self-assembly on this substrate which is related directly to the length of the alkyl chain. The data further show that oleophobicity is an insufficiently sensitive probe to determine the formation of equilibrium closest packed structures. In a companion paper we describe these structures and their relationship to many of the above observations in detail. Registry No. n-Infrared reflection spectroscopy has been applied to the determination of the structures of adsorbed monolayer films of n-alkanoic acids on oxidized aluminum substrates. Acids of 16-22 carbons, terminated by methyl, vinyl, or propargyl groups, were absorbed from hexadecane solution at 25 O C , using immersion times up to several days. These results, together with those for perdeuterated acids, indicate that close-packed assemblies are formed with extended alkyl tails oriented with their chain axes tilted away from normal to the surface with a value of -10" for the longer chains. Frequency shifts to higher values are observed for CH stretching modes of the vinyl and propargyl terminal groups measured in monolayer films compared to the bulk acids. These shifts are of similar magnitude to gas-liquid phase shifts for these groups and imply the environment of the ambient-monolayer interface exhibits very diminished intermolecular interactions for these terminal groups compared to the bulk. The spectra clearly show that chemisorption occurs by proton dissociation to form carboxylate species. Examination of both line width and peak positions of the carboxylate stretching modes suggests a variety of binding geometries of the carboxylate groups to the surface exists. These results lend support to the general concept of forming stable, oriented and ordered two-dimensional organic films by solution adsorption and show the utility of applying surface vibrational spectroscopy to structural determination in these films.