Assembly of amphiphilic phenylacetylene macrocycles at the air-water interface and on solid surfaces

The assembly of amphiphilic phenylacetylene macrocycles (PAMs), with molecular structures that vary in terms of the nature and orientation of their pendant functional groups, has been studied on a Langmuir-Blodgett trough and after transfer onto solid substrates. These monolayer films are of interest as two-dimensional host matrices and shape selective membranes whose two-dimensional organization should bring together shape selective compartments. The disk-like PAMs can, in principle, adopt orientations in which the plane of the macrocycle can range from perpendicular (edge-on) to parallel (face-on) at the interface. PAMs functionalized with six hydrophilic groups around the periphery do not prefer the face-on orientation and are most likely tilted, perhaps in a poorly organized state. PAMs that have spatially segregated hydrophilic and hydrophobic groups adopt the edge-on orientation when the hydrophilic moieties are carboxylate groups. In contrast, PAMs appended with acid moieties do not lead to stable monolayers, most likely because they engage in strong intermolecular hydrogen bonding interactions as evidenced by H-1 NMR and vapor pressure osmometry of the solutions. Monolayers of the dicarboxylate PAMs were transferred onto fused silica and Si(100) surfaces and these were in turn characterized by contact angle, ellipsometry, absorption FTIR, and angle-resolved X-ray photoelectron spectroscopies. Taken together these characterization experiments strongly support the hypothesis that the dicarboxylate PAMs form a well-ordered and stable two-dimensional array and that they adopt the edge-on configuration with near-vertical orientation of the macrocycle plane.