Covalent grafting of m -phenylene-ethynylene oligomers to oxide surfaces

Oligomeric phenylene ethynylene is potentially a useful molecule for organic photovoltaics, light emitting diodes, sensor materials, or as intrinsically porous polymers for gas storage and separation. In the former two applications, the organic molecules are supported on an inorganic material for structural support or for charge collection at an electrode. Here, a 12 repeat unit oligomer of m-phenylene ethylene is covalently attached to high surface area and planar oxide supports as an alternate route to materials development, which displays novel properties and applications due to the covalent connection to the surface. Oligomers and model dimers are attached to alkyl-azide decorated surfaces via Cu-catalyzed 1,3-dipolar Huisgen cycloaddition or covalently linked with silane-bearing surfaces via Rh-catalyzed hydrosilylation. Surface structures and densities are verified by FTIR, TGA, transmission UV-visible spectroscopy, N₂ physisorption, and transmission electron microscopy. On high-surface-area materials, hydrosilylation routes yield loadings up to 0.09 molecules nm^-2, whereas on planar surfaces, densities reach 0.22 molecules nm^-2, comparable to the footprint of the extended Tg side groups. Grafted molecules retain the solvation-dependent "folding" behavior of soluble oligomers as determined by solvent- and temperature- dependent photoluminescence spectra of grafted oligomers suspensions. Grafted oligomers can be suspended in liquids in which the oligomers are not intrinsically soluble, demonstrating, for example, that dodecane and decalin have opposite association behavior, in spite of both their very low polarity parameters. Photoluminescence spectra are consistent with coexisting folded and unfolded oligomers when grafted, in contrast with highly cooperative changes in solution. Highly loaded surfaces show significant hysteresis upon heating, suggesting a kinetic as well as thermodynamic solvent effect. Pinene guest molecules enhance folding of the grafted oligomers, demonstrating a responsive adsorbent surface. Having demonstrated proof of concept, covalently grafted oligo(PE) materials are applicable as a new class of tunable adsorbent materials and optoelectronic materials.