Self-Assembly of Ruthenium Porphyrins into Monolayer and Multilayer Architectures via Heterogeneous Coordination Chemist
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Self-Assembly of Ruthenium Porphyrins into Monolayer and Multilayer Architectures via Heterogeneous Coordination Chemistry David M. Sarno, Luis J. Matienzo,† Wayne E. Jones, Jr.* Department of Chemistry and Institute for Materials Research, State University of New York at Binghamton, Binghamton, NY 13902 and †IBM Corporation, Endicott, NY 13760
ABSTRACT Self-assembly chemistry is finding use as a means of controlling the nanoscale structures at materials interfaces. Building on the success of a previous methodology, Ruthenium porphyrins have been coordinated to the pyridine-terminated surface of a self-assembled alkylsilane coupling layer on silica glass. These arrays are built in a stepwise fashion in which the surface of each layer allows further reaction to yield the next layer. Characterization by UV-vis and XPS confirms the deposition of the alkyl chains and pyridine moieties of the coupling layer, as well as coordination of the subsequent Ru-porphyrin layer. AFM analysis indicates very smooth surfaces, with roughness measured at 0.43 nm and 1.43 nm for the pyridine and porphyrin layers, respectively. The occasional appearance of large surface features (15-30 nm diameter) suggests aggregation among π-systems, with further characterization underway. Strategies toward the construction of a multilayer porphyrin architecture are also reported. INTRODUCTION A fuller understanding of the relationship between the molecular structure of a system and its macroscopic properties is key to the development of new materials for use in LEDs, chemosensors, and light-harvesting devices. In many cases, these technologies have utilized novel conjugated oligomers and polymers composed of porphyrins and other chromophores, most often as spin-coated or vapor-deposited thin films on a variety of substrates [1]. Unfortunately, it is difficult to separate and distinguish inter- and intramolecular interactions in amorphous thin films. This not only influences electronic and optical properties, but also hinders their complete characterization. A more controlled separation might be facilitated by the systematic deposition of materials in arrays possessing long-range structural order. In this way, specific interactions could be varied and exploited. Highly organized supramolecular arrays have been constructed on silicon oxide surfaces via self-assembly of large groups of molecules, by employing coordination chemistry, hydrogen bonding, and van der Waals interactions [2, 3]. We have recently reported the successful preparation of multilayers composed of alternating Zinc-porphyrin oligomers and bipyridyl spacers anchored to silane-modified glass substrates [4]. X-ray diffraction, UV-vis absorbance, and Atomic Force Microscopy indicated an ordered multilayer system. However, Zn-porphyrins are typically limited to 5-coordinate square pyramidal complexes, which, in conjunction with inconsistencies in the silane coupling layer, lead to localized structural defects, as suggested by X-ray Photoelectron Spectroscopy (XPS). Based on these results,
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