Surface Noncovalent Bonding for Rational Design of Hierarchical Molecular Self‐Assemblies

Abstract

Self-assembly is a promising bottom-up route towards atomically precise fabrication of functional systems.[1] Nanoporous networks [2] that can host guest molecules [3,4] were obtained on metal surfaces under ultrahigh vacuum. Various supramolecular chemistry approaches have been applied [5][6][7][8][9] to obtain thermally [10,11] or chemically controlled [12,13] polymorphs. The spontaneous formation of patterns with hexagonal, [14] porous honeycomb, [15,16] or KagomØ [17] geometries has been also observed at the solution/solid interface. [18,19] The topologies, as well as the drastic structural changes often induced by minute changes in molecular structure [20] or solvent, [21] are usually explained a posteriori on the basis of molecular symmetry, molecule-substrate interactions, and moleculemolecule interactions. Interdigitation of alkyl chains is an example of the last-named [15,16,22,23] which is of practical interest since it is specific to the surface and does not occur in the bulk of the solution. Surprisingly, close-packed epitaxy of n-alkanes on highly ordered pyrolytic graphite (HOPG) [24][25][26] has not yet inspired the design of molecular linker exploiting this behavior.Hence, we designed a new molecular unit acting as a functional linking group able to form strong surface-assisted intermolecular "clips" which, by interdigitation, strictly mimic the atomically precise organization of n-alkanes on HOPG. It forms the basis of a design strategy which parallels polymer chemistry in that mono-, bi-, and trifunctional clipbearing building blocks form noncovalent surface dimers, polymers, and two-dimensional (2D) networks, respectively. We can then chemically steer the organization of these entities themselves at a higher supramolecular level.The adsorption of n-alkanes on HOPG results in the formation of close-packed 2D lamellae of parallel-aligned rectilinear chains, oriented along the h100i direction of graphite [19,25,26] according to the Groszek model [24] (Figure 1 A). Organization of the adsorbed monolayers is driven by two main factors: The first is the correspondence between the zigzag alternation of methylene groups and the h100i direction of HOPG, with a stabilization energy of about 64 meV per methylene group.[27] The second is the parallel packing of alkane molecules, which, besides steric hindrance, results from a stabilization energy between nearest chains 4.1 apart. From theoretical estimations, [28] we can infer 2D crystallization energies on the order of 20-25 meV per pair of facing methylene groups.

Two-dimensional nanoarchitectonics based on self-assembly

“…Artificial molecular patterns of porphyrin derivatives via hydrogen bonding with cyano groups, at a Au (111) surface was also reported by Yokoyama et al [66]. Unsubstituted, monocyanophenyl-substituted, and cis-or trans-dicyanophenyl-substituted tetraphenylporphyrins demonstrated a transition from van der Waals-dominated square motifs, to isolated clusters, to organized nanowire chains of single molecules.…”

“…Different crystalline patterns are produced as the component ratio is varied; this is analogous to phase changes in mixtures of metals, prompting these systems to be described as molecular alloys. The crystalline structures produced on Cu(111) differ from those that form on Au (111), reflecting modulation of the surface-molecule interaction based on the change in substrate character. The expected hydrogen bonding pattern cannot be regenerated by a substrate-free simulation, supporting the proposed critical role of the substrate.…”

“…Nakanishi et al utilized this spincoating method to form a twodimensional array on HOPG of an adsorbed fullerene derivative with three hexadecyl chains [110]. This was extended by Bléger et al by using intermolecular non-covalent interdigitating alkyl chains to stabilize or "clip" the pattern [111].…”

“…Yang et al showed that the dimensions of the unit cell of a self-assembled monolayer on HOPG can be tuned by changing the alkyl lengths in a two component adsorption of straight-chain mono-or dicarboxylic acids with N,N′-dialkyl-substituted quinacridone derivatives [105]. Itaya et al demonstrated that metallooctaethylporphyrin forms a 2D structure on Au (111) and subsequently showed that C 60 -ferrocene dyad will bind in the same pattern to these molecules [106,107]. C 60 molecules bound in this manner produce more pronounced and repeatable oxidation and reduction peaks in cyclic voltammetry.…”

Formation of Multicomponent Star Structures at the Liquid/Solid Interface

“…16,22,43,44 The main driving forces for this phenomenon are van der Waals interactions between a guest and the solid substrate surface and van der Waals interactions between a guest and the surrounding host matrix. For the later interactions, shape and size complementarity between the guest and the pore plays a crucial role.…”