Biomedical subjects
Publications and source records attributed to Katherine E Plass.
The composition of physisorbed monolayers formed from multicomponent solutions varies with time and selection between two phases is possible by controlled desorption of one phase and the kinetically favored adsorption of another.
A metal-organic great rhombicuboctahedron (termed MOP-18) was functionalized with extended alkyl chains and crystallized. It has 12 rigid square-shaped Cu2(CO2)4 paddle-wheel building units and 24 5-dodecoxybenzene-1,3-dicarboxylate links. MOP-18 is soluble in a variety of organic solvents, such as tetrahydrofuran, chloroform, and toluene, which enabled its assembly on graphite substrate and its observation by scanning tunneling microscopy.
Patterning surfaces with features on the low end of the nanoscale can efficiently be accomplished with physisorbed monolayers. Here, cocrystallization is revealed as a powerful approach toward dramatically increasing the periodicity of surface features and expanding the length scale on which these patterns can form. By variation of the ratio of adsorbates in solution, surface composition can be controlled such that features on the length scale of several molecules are obtained, offering a facile approach to surface nanopatterning.
The potential for relatively minor structural changes to dramatically impact materials properties is one of the primary obstacles to achieving the rational design of functional materials. For example, having an odd versus an even number of carbons between functional groups in polymers can cause large variation in melting point and mechanical properties. This odd-even effect is especially pronounced in hydrogen-bonded polymers and oligomers. To shed light on the structural basis of this phenomenon, physisorbed monolayers and single crystals of alkyl dicarbamates were investigated by scanning tunneling microscopy and X-ray diffraction, respectively. The related two- and three-dimensional crystal structures both demonstrated a clear odd-even effect in packing geometry. The differing accommodation of intermolecular interactions between odd and even packing motifs was directly related to the melting point trends and further dissected through computation. In addition, these oligomers displayed unusual competitive adsorption behavior; the relative preference for adsorption of a smaller species from a binary solution was increased compared to alkanes. These results were explained in the context of hydrogen bond density effects that arise due to competition for a limited substrate surface area. This study provides a model for understanding oligourethane surface coatings and demonstrates the importance of molecular structure and hydrogen bonding in determining adsorption behavior.
The structures of self-assembled physisorbed monolayers of alkyl carbamates were examined with atomic detail by scanning tunneling microscopy at the liquid-solid interface. Through systematic variation of molecular structure, the factors determining the two-dimensional crystal packing and dynamics of alkyl carbamate monolayers were isolated. Two different conformational pseudopolymorphs on the surface were observed and their order of stability was varied by changing the length of the alkyl groups. The relative size of the two alkyl groups in a molecule affected the frequency of orientational flipping within a column, which in turn, exerts an influence on the relative orientation of the two-dimensional crystalline domains. These phenomena were explained on the basis of the preferred hydrogen-bonding geometry of the carbamate functional group and the different degree of van der Waals interaction for each form.
The self-assembly of a series of 1,3-disubstituted benzenes has been scrutinized by scanning tunneling microscopy (STM) and computational modeling. Small changes in the functional groups (e.g., ester, thioester, ketone) resulted in dramatic changes in packing patterns. Remarkably, several of the molecules gave rise to monolayers with more than one molecule in the asymmetric unit and displayed multiple packing patterns. This constitutes the most complex behavior observed to date in this type of monolayer and illuminates several issues of importance in three-dimensional crystallization. Intermolecular interactions associated with the observation of multiple molecules in the asymmetric unit and stabilization of pseudopolymorphs were identified. The geometry and electrostatic properties of the isolated molecule and monolayer density were found to be critical in determining which packing motif was adopted.