Publications

The ability to create organized ultrathin films using organic molecules provides systems whose chemical, mechanical, and optical properties can be controlled for specific applications. In particular, polymerization of oriented mono- and multi-layer films containing the diacetylene group has produced a variety of robust, highly oriented, and environmentally responsive films with unique chromatic properties. These two-dimensional poly(diacetylene) (PDA) films, where the conjugation runs parallel to the film surface, have previously been prepared in a variety of forms. Of particular interest is the optical absorption of PDA due to its {pi}-conjugated backbone. A wide variety of PDA materials, including bulk crystals, thin films, and solutions, exhibit a chromatic transition involving a significant shift in absorption from low to high energy bands of the visible spectrum, thus the PDA appears to transform from a blue to a red color. In addition, the red form is highly fluorescent, while the blue form is not. This transition can be brought about by heat binding of specific biological targets and applied stress (mechanochromism), among others. In this paper, the authors discuss the Langmuir deposition of ultrathin PDA films and the subsequent measurement of their structural, optical, and mechanical properties at the nanometer scale. By altering the head group functionality, the authors can choose between mono- and tri-layer PDA film structures. Measurements with the atomic force microscope (AFM) reveal strongly anisotropic friction properties that are correlated with the orientation of the conjugated polymer backbone orientation. Furthermore, the authors can use the AFM tip or a near field scanning optical microscope (NSOM) tip to locally convert the PDA from the blue form to the red form via applied stress. This represents the first time that mechanochromism has been observed at the nanometer scale. Dramatic structural changes are associated with this mechanochromic transition.

Atomically flat monolayer and trilayer films of polydiacetylenes have been prepared on mica and silicon using a horizontal deposition technique from a pure water subphase. Langmuir films of 10,12-pentacosadiynoic acid (I) and N-(2-ethanol)-10,12-pentacosadiynamide (II) were compressed to 20 mN/m and subsequently polymerized by UV irradiation at the air-water interface. Blue and red forms of the films were prepared by varying exposure times and incident power. Polymerization to the blue-phase films produced slight contractions in the film of 2 and 5% for the films of II and I, respectively. Longer UV exposures yielded red-phase films with dramatic film contraction of 15 and 32% for II and I, respectively. The horizontal deposition technique provided transfer ratios of unity with minimal film stress or structure modification. Atomic force microscopy images revealed nearly complete coverage of the substrate with atomically flat films. Crystalline domains of up to 100 microns of highly oriented polydiacetylene molecules were observed. The results reported herein provided insight into the roles of molecular packing and chain orientations in converting the monomeric film to the polymerized blue- and red-phases.

Phosphonate binding sites in guanidine and ammonium surface-functionalized silica xerogels were prepared via the molecular imprinting technique and characterized using solid state {sup 31}P MAS NMR. One-point, two-point, and non-specific host-guest interactions between phenylphosphonic acid (PPA) and the functionalized gels were distinguished by characteristic chemical shifts of the observed absorption peaks. Using solid state as well as solution phase NMR analyses, absorptions observed at 15.5 ppm and 6.5 ppm were identified as resulting from the 1:1 (one-point) and 2:1 (two-point) guanidine to phosphonate interactions, respectively. Similar absorptions were observed with the ammonium functionalized gels. By examining the host-guest interactions within the gels, the efficiency of the molecular imprinting procedure with regard to the functional monomer-to-template interaction could be readily assessed. Template removal followed by substrate adsorption studies conducted on the guanidine functionalized gels provided a method to evaluate the binding characteristics of the receptor sites to a phosphonate substrate. During these experiments, {sup 29}Si and {sup 31}P MAS NMR acted as diagnostic monitors to identify structural changes occurring in the gel matrix and at the receptor site from solvent mediated processes.