Consequences of Varying Adsorption Strength and Adding Steric Hindrance on Self-Assembly of Supramolecular Polymers on Carbon Substrates

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• 作者：Roozbeh Shokri ; Francois Vonau ; Marion Cranney ; Dominique Aubel ; Ashok Narladkar ; Benjamin Isare ; Laurent Bouteiller ; Laurent Simon ; G眉nter Reiter
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：October 11, 2012
• 年：2012
• 卷：116
• 期：40
• 页码：21594-21600
• 全文大小：492K
• 年卷期：v.116,no.40(October 11, 2012)
• ISSN：1932-7455

文摘

We investigated the consequences of changes in adsorption strength and the influence of steric hindrance with respect to ordering of supramolecular polymers on surfaces. The focus is on the kinetics of domain formation and the guidance of this self-assembly process by the substrate. To demonstrate general features, we compared two molecules, both forming supramolecular polymers, bis(hexylureido)toluene (HUT) and bis(2-ethylhexylureido)toluene (EHUT), differing only in the architecture of the alkyl side groups, on two substrates. Although highly oriented pyrolytic graphite (HOPG) and epitaxial graphene (EG) grown on silicon carbide have identical chemical composition and nearly the same crystal lattice parameters at the surface, they differ significantly in adsorption strength. Due to its higher polarizability, HOPG adsorbs molecules much more strongly than EG. Nonetheless, even on EG, the formation of supramolecular polymers was guided by the symmetry of the substrate lattice, but at a much slower rate. Accelerating ordering on EG through appropriate solvent vapor annealing, we eventually observed similar triangular patterns of HUT molecules on both substrates. This indicates that the orientation of supramolecular polymers is not controlled by the strength of substrate鈥搈olecule interactions but rather by the possibility to establish registry with the substrate. However, such guiding influence of the substrate was lost, even on strongly adsorbing HOPG, when steric hindrance, generated by adding ethyl branches to the side chains of the HUT molecule, caused weak adsorption. As a consequence, EHUT molecules did not adsorb in a parallel but in more or less perpendicular orientation to the substrate, and the resulting patterns were not guided anymore by the symmetry of the substrate. This demonstrates that small modifications of a molecule like the addition of short side branches can lead to drastic changes in the self-assembly process.