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Science 18 August 2006: Vol. 313. no. 5789, pp. 961 - 962
DOI: 10.1126/science.1129309



A Homomolecular Porous Network at a Cu(111) Surface

Greg Pawin, Kin L. Wong, Ki-Young Kwon, Ludwig Bartels

Abstract: Anthraquinone molecules self-assemble on a Cu(111) surface into a large two-dimensional honeycomb network (Formula)R23° with pore diameters of {approx}50 Å. The spontaneous formation of a pattern containing pores roughly five times larger than the size of the constituent molecules is unprecedented. The network originates from a delicate balance between substrate-mediated repulsion and intermolecular attraction involving an unusual chemical motif: hydrogen bonding between a carbonyl oxygen and an aromatic hydrogen atom. Substrate-mediated long-range adsorbate-adsorbate repulsion has been observed on anisotropic surfaces and in the context of the absence of pattern formation. Its applicability for the design of tailored molecular films is explored here.

Background:
The arrangement of molecules on surfaces is governed by a combination of interactions with the substrate and with neighboring molecules. For most organic molecules, simple patterns result, in which each adsorbate occupies the same kind of adsorption site and surrounds itself with neighbors just beyond its van der Waals surface. Phenomena such as dipolar (1) and quadrupolar (2) interactions can lead to more complex surface patterns, such as aggregates of a fixed number of molecules (1). Hydrogen bonding can cause the formation of molecular rows (3, 4) and well-ordered binary phases of quite appealing pattern (5). Yokoyama et al. (6) reported that similar interactions can also be found for the interaction of cyano groups with H atoms of benzene rings, where formally no hydrogen bonding is expected.


Figure 1
(Left) STM image of the honeycomb network of anthraquinone molecules on a Cu(111) surface. Bias, 1.3 V; current, 73 pA; size, 260 Å by 150 Å. (Right) A model of the (Formula)R23° unit cell. Red spheres, oxygen atoms; black spheres, carbon atoms; background, copper surface.


Figure 2
Model of the anthraquinone molecules forming a row (left) and a vertex (right). C-H-O distances are indicated. The standard numbering of carbon atoms in anthracene is also indicated.


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