Rsc_cc_c4cc03523b 3..6

Crystalline structures of geometry corresponding to trihexagonal tiling are often referred to as Kagome lattices. Natural examples of Kagome lattices are rare, but have recently been found in surface supported organics. Their particular geometry of interlacing triangles, exhibiting an ordered arrangement of hexagonal and triangular pores of different size, makes Kagome lattice structures exceptional templates for host–guest chemistry. Advances in surface science and the availability of high-resolution microscopy have accelerated the discovery of Kagome-lattice-type organic layers. Reported structures are typically based on van der Waals forces, hydrogen bonds ormetal organic bonds. Herein we report the first p–p stacked organic Kagome lattice, which consists of 3-hydroxyphenalenone (3-HPLN) molecules. A unique feature of the networks formed on flat Cu(111) is its three-dimensional architecture, which emerges from the perpendicular attachment of p–p stacked 3-HPLN dimers on planar, hydrogen-bonded molecular trimers. 3-HPLN is a topological, or proton transfer ferroelectric in its bulk crystalline form, in which intermolecular hydrogen bonds couple to the molecular p electron systems and hence the molecular dipole moment, giving rise to a switchable polarization. Our team has recently discovered self-assembled chiral structures consisting of 3-HPLN trimers on Ag(111) surfaces, as well as complex 2D networks of the related molecular ferroelectrics croconic acid and rhodizonic acid. Several of these networks exhibit similar hydrogen bonds in their ferroelectric bulk phase, so there is reason to believe that ferroelectric order is possible in 2D structures as well. A few of the observed networks are porous, such as croconic acid on both Ag(111) and Au(111), and some structural phases of 3-HPLN on Ag, which can be attributed to interactions with the substrate as well as effective shape and entropy arguments. Driven by these findings, we investigated the self-assembly of 3-HPLN on Cu(111) substrates. Deposition of 3-HPLN at room temperature results in elongated islands, two or three molecules wide and several nanometers long, as seen in scanning tunneling microscopy (STM) images such as those presented in the ESI.† The molecule-to-molecule distance of 9.0 Å is consistent with molecules that are flat-lying on the surface, forming hydrogen bonds with one another. Post-annealing the sample to approximately 120 1C and higher results in the growth of the 2D islands due to Ostwald ripening. The arrangement of the molecules in those islands corresponds to hydrogen-bonded 1D chains as in the bulk, which form 2D islands likely through van der Waals attraction, as can be seen in Fig. 1a. Only if the coverage of 3-HPLN is sufficiently high, exceeding two nominal layers, then similar post-annealing of the roomtemperature-deposited molecules to 200 1C drastically changes the morphology of the film, as seen in Fig. 1b. We observe double layers of 3-HPLN consisting of two competing structures, a linear one and a honeycomb structure. Important structural details can be obtained from high-resolution STM images such as those in Fig. 1c–f. It appears that the molecules of the bottom layer, which are in direct contact with the substrate, are flat-lying on the surface, while the molecules in the top layer appear to be standing upright on the bottom layer, as is evident from their considerably smaller apparent size. While we were unable to fully resolve the molecular arrangement in the bottom layers, it is clear that it is different for the linearand the honeycomb-like layers, a Department of Physics and Astronomy, University of Nebraska–Lincoln, 855 N 16th Street, Lincoln, NE, 68588-0299, USA. E-mail: [email protected]; Fax: +1 402 472 6148 b Department of Chemistry, University of Nebraska–Lincoln, NE, 68588, USA c Nebraska Center of Materials and Nanoscience, University of Nebraska–Lincoln, NE 68588-0299, USA d Department of Theoretical Chemistry, Jagiellonion University, 30-060 Krakow, Poland e Department of Chemistry, State University of New York at Buffalo, Buffalo, NY, 14260-3000, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc03523b Received 9th May 2014, Accepted 12th June 2014