Crystal Engineering in Two Dimensions:  An Approach to Molecular Nanopatterning

Abstract

We describe a surprising cooperative adsorption process observed by scanning tunneling microscopy (STM) at the liquid-solid interface. The process involves the association of a threefold hydrogen-bonding unit, trimesic acid (TMA), with straight-chain aliphatic alcohols of varying length (from C 7 to C 30 ), which coadsorb on highly oriented pyrolytic graphite (HOPG) to form linear patterns. In certain cases, the known TMA "flower pattern" can coexist temporarily with the linear TMA-alcohol patterns, but it eventually disappears. Time-lapsed STM imaging shows that the evolution of the flower pattern is a classical ripening phenomenon. The periodicity of the linear TMA-alcohol patterns can be modulated by choosing alcohols with appropriate chain lengths, and the precise structure of the patterns depends on the parity of the carbon count in the alkyl chain. Interactions that lead to this odd-even effect are analyzed in detail. The molecular components of the patterns are achiral, yet their association by hydrogen bonding leads to the formation of enantiomeric domains on the surface. The interrelation of these domains and the observation of superperiodic structures (moire ṕatterns) are rationalized by considering interactions with the underlying graphite surface and within the two-dimensional crystal of the adsorbed molecules. Comparison of the observed two-dimensional structures with the three-dimensional crystal structures of TMA-alcohol complexes determined by X-ray crystallography helps reveal the mechanism of molecular association in these two-component systems.

Two-Dimensional Nanotemplates as Surface Cues for the Controlled Assembly of Organic Molecules

“…The most stable of the two quadruple tapes is reported in Fig. 15b: it is based on the doubling of thermodynamically stable TMA dimers [10], in an eclipsed manner. In the less stable quadruple-TMA (Fig.…”

“…Recently, Nath and co-workers showed that the threefold symmetry patterns (flower or chicken-wire), which TMA forms on threefold symmetry substrates (HOPG in this case), can be broken by introducing an alcohol to the solution [10,62]. For example, when using 1-undecanol, the well-known flower structure is found to be surrounded by a linear pattern, which is a novel TMA polymorph.…”

“…30). The superstructure is constituted of discrete pores and rotated by 15 ± 4 • with respect to the [1][2][3][4][5][6][7][8][9][10] direction of the underlying Ag(111) surface and shows a periodicity of 3.3 nm. The pores have a diameter of 1.2 nm and a depth of 0.12 nm (these dimensions are underestimated because of the tip-surface convolution due to the tip finite size).…”

“…The fairly weak lateral and periodic forces exerted by the surface of choice on the physisorbed molecules guide molecule-molecule interactions in such a way as to form long-range aggregates. Once the desired ordered monolayer is formed, it may serve as the starting point to build a 3D molecular crystal [10][11][12][13][14].…”

Expanding the Scope of Molecular Self-organization Studies through Temperature Control at the Solution/Solid Interface

“…[8,9] The investigation of molecular organization at the SSI is largely conducted via scanning tunnelling microscopy (STM), which can be used to directly visualize molecular kinetics and geometries in situ. [10][11][12][13][14] The development of this technique has provided unprecedented access to the details of surface molecular self-assembly. [15,16] The Gibbs free energy for molecular systems at the SSI comprises an enthalpic contribution dominated by the interaction energies of the adsorbed species (intermolecular and molecule-substrate) and an entropic component primarily associated with the molecules in solution.…”