Synthetic elaboration of self-assembled monolayers (SAMs) provides a powerful method for the preparation of smooth, ordered surfaces with carefully controlled functionality. Chemically rugged siloxane-based SAMs have been employed as a foundation for the preparation of a variety of functionalized surfaces. Nucleophilic displacements are described between halide-terminated SAMs and anionic nucleophiles (e.g. azide, thiocyanate, thiolate). Problems were encountered due to the steric congestion surrounding the terminal bromomethylene carbon. The concept of trajectory control is applied to these displacement reactions. In addition, reactions between ester-terminated SAMs and neutral nucleophiles (e.g. amines, hydrazine, hydroxylamine) are described. Evidence is presented suggesting that a modest amount of cross-linking (i.e. imide formation) takes place in the amidation reactions, while reaction stoichiometry indicates that cross-linking is virtually complete for the bifunctional nucleophiles. These synthetic elaborations were also carried out on mixed monolayers to create functionalized SAMs with systematically varied loading densities. Linear correlation (or lack thereof) of elemental composition to predicted functional composition is used to provide an estimation of reaction efficiency.

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A method to determine the orientation distribution of fluorescent molecules in a thin, substrate-supported film is described. Attenuated total reflection spectrometry on the surface of a planar waveguide is used to measure absorption LD, from which the mean dipole tilt angle in the film is obtained. Steady-state fluorescence anisotropy is measured in a total internal reflection geometry on a film supported on fused silica but prepared under otherwise identical conditions. The angular distribution about the mean can be recovered from the anisotropy measurement by modeling the distribution as a probability density that is specified by two adjustable parameters. The method was tested on Langmuir−Blodgett (LB) films of arachidic acid doped with the fluorescent amphiphiles DiI and BODIPY. In the DiI-doped films, the mean tilt angle was 75° from the surface normal. Assuming a Gaussian distribution, the standard deviation was 12°, indicating a high degree of macroscopic order. In the BODIPY-doped films the distribution was 59 ± 17°, which indicates a less ordered assembly. Larger angular distributions were calculated using a step function model. The results show that dipoles in the headgroup region of arachidic acid LB films are more ordered than dipoles in the alkyl chain region. The method should prove useful in studying relationships between assembly technique, structure, and function in two-dimensional molecular arrays.

Atomic force microscopy (AFM) tips and glass surfaces were modified with various organosilanes to determine magnitude and dispersion information about single-molecule bond-rupture forces. X-ray photoelectron spectroscopy (XPS) and contact-angle measurements were used to study and quantify organosilane adsorption on the glass surface and on SiO2-coated AFM tips. Hydrogen bond interactions between hydroxyl- and thiol-terminated groups on the tip and surface were detected and measured. Differentiation between the functionalities of the acetate- and thioacetate-terminated silanes and their reduced forms produced by on-surface reduction (the alcohol and thiol, respectively) was also accomplished. The experiments demonstrate the complementary information that can be obtained from AFM and XPS and illustrate how they can be used to determine the nature of the surface after an organic transformation has occurred to the functional groups present. They also represent a first step in detecting chemical reactions on a localized scale and in measuring the dispersion in the single-molecule bond-rupture force when it exists.