From 2D functional nanoarchitectures to novel carbon-based materials ... and back!
The Klappenberger Group targets the atom-precise construction of novel 2D functional nanoarchitectures and carbon-based materials employing the bottom-up strategy and utilizing molecular self-assembly and self-organized growth. For a detailed and comprehensive understanding the nanosystems are characterized with the STM+XS approach complementing the advantages of scanning tunneling microscopy (STM) with space-averaging X-ray spectroscopy like X-ray photoemission spectroscopy (XPS) and near-edge X-ray absorption-fine-structure (NEXAFS) spectroscopy. Currently, our main target is the fabrication and characterization of novel graphdiyne-related nanomaterials by on-surface synthesis from terminal alkyne precursors as depicted in the Figure below. For reaching this goal excellent knowledge is required on molecule-molecule and molecule-substrate interaction, the underlying surface chemistry and the influence on the functionalities of the interface.
Recent achievement (accepted in Nat. Chem.):
Complex interfacial architectures such as semi-regular Archimedean tilings (AT) promise extraordinary properties, but their fabrication remains challenging. By combining on-surface chemical conversions with supramolecular self-assembly, our work demonstrates that a simple organic precursor can be converted through a multistep convergent reaction to a more complex building block self-assembling a fascinating (188.8.131.52) AT. Intriguingly, despite the numerous covalent transformations the intricate architecture is synthesized in high yield with unexpected atom economy achieved through catalytic activity of the in-situ generated reaction intermediates of the tecton conversion.
Our research addresses the following topics:
Novel carbon-based materials.
The homo-coupling of terminal alkynes provides the basis for the fabrication of novel carbon-based materials. Recently, the Klappenberger Group succeeded in synthesizing a new type of organic polymer by employing the covalent reaction on a vicinal surface, namely Ag(877). The stepped surface imposes chemo- and regioselectivity onto the reaction by templating the positioning of the monomers along the step edges. As a result terphenylene-butadiynylene polymer chains can be fabricated (10.1021/nl4046747) with the length of the single strands reaching 270 Å, thus breaking the record length of similar polymers synthesized with solution methods by a factor of two. The picture shows a polymer chain (white with dark surroundings) at a step edge between the upper terrace (brighter yellow) and the lower terrace (darker red) and a sketch of the corresponding chemical structure of the polymer displayed in the red reagion.
Surface-confined organometallic chemistry
Organocobalt complexes represent a versatile tool in organic synthesis being an important intermediate in Pauson-Khand, Friedel-Crafts and Nicholas reactions. We address the formation of an organocobalt complex at a solid-vacuum interface. Deposition of 4,4'-(ethyne-1,2-diyl) dibenzonitrile and Co atoms on the Ag(111) surface followed by annealing resulted in genuine complexes where single Co atoms laterally coordinated to two carbonitrile groups undergo organometallic bonding with the internal alkyne moiety of adjacent molecules. Alternative complex scenarios involving fragmentation of the precursor were ruled out by complementary X-ray photoemission spectroscopy. According to our density functional theory analysis the complexation with the alkyne moiety follows the Dewar-Chatt-Duncanson model for a two electron-donor ligand where an alkyne-to-Co donation occurs together with a strong metal-to-alkyne back-donation.
The aggregation of (pro)chiral/achiral molecules into crystalline structures at interfaces forms conglomerates, racemates and solid solutions, comparable to known bulk phases. Here, we employ scanning tunneling microscopy and Monte Carlo simulations to unveil a distinct racemic phase, expressing one-dimensionally (1D)-disordered chiral sorting through random tiling, in surface-confined supramolecular assembly of achiral 4,4"-diethynyl-1,1':4',1"-terphenyl (DETP) molecules. Through analytical modeling, we verify that the configurational entropy (firstly introduced by Pauling to explain the residual entropy of ice) of the 1D-disordered racemic tiling phase lies between that of a perfectly-ordered 2D racemate and a racemic solid-solution.
The construction of covalent nanoarchitectures by on-surface synthesis is an exciting new development and currently establishing itself as an independent research field. The Klappenberger Group contributed significantly to this field by introducing terminal alkyne moieties as reactive functional groups allowing covalent carbon-carbon coupling at significantly lower reaction temperatures compared to other more established covalent reactions. The homo-coupling of terminal alkynes thus allows the controlled synthesis of novel organic species on a nobel metal surface (Ag111) and at moderate temperatures (300-330 K) with only volatile hydrogen as only by-product. The picture depicts a mixture of original Ext-TEB species (one highlighted in green) and the newly created dimerized species (in yellow, with ball-and-stick model superposed).
Hexaphenysilole (HPS) on Cu(111) forms extended hierarchically organized networks stabilized by π–π interaction between exocyclic phenyl groups. After 430 Kthe compound is chemically altered as evidenced by Si 2p core level shift. This change is accompanied by a different adsorption configuration and a transformation of the assembled structures to uniformly sized chiral clusters dispersively distributed on the surface. Monte Carlo simulations signal that this new phase can be understood as a result of competing long-range repulsions and short-range attractions. After annealing at 520 K, the exocyclic phenyl moieties undergo a cyclodehydrogenation reaction to form a polycyclic aromatic compound, a nanographene flake, incorporating a silole moiety.
Basic understanding of interactions at the atomic scale
The principles of supramolecular chemistry are highly relevant for the construction of nanoarchitectures. Accordingly, a profound understanding of the various interactions at the atomic scale, like hydrogen bonding, metal-organic coordination, electrostatic interaction, van der Waals forces, is important and needs to be constantly developed further. For example, the often-appearing binding between carbonitrile groups and neighboring phenyl units is usually categorized as a weak H bond. However, our thorough study of the nature of this interaction provides good arguments that the dominant contribution to the stabilizing force is better described as Proton acceptor ring interaction (PARI).
Ionic hydrogen bonds (IHB) exhibit increased bonding strength due to the charged nature of the proton donor or acceptor groups. They play a key role in different fields covering diverse topics such as molecular crystal engineering, protein folding, proton-coupled electron transfer, and biomolecular recognition. Recently, we found an unusual deprotonated alkynyl hydrogen bonding motif, which drives the formation of room-temperature stable assemblies of hydrocarbon species on the Cu(111) surface. Through DFT calculations, the stabilizing interaction is identified as a trifurcated ionic C–H•••π- hydrogen bonding between the π-system of the ionic alkynyl groups and methine moieties of nearby benzene rings, providing an energy gain of 0.63 eV/molecule upon network formation
Functionalities of Nanoarchitectures
Molecular orbital engineering is a key ingredient for the design of organic devices. Intermolecular hybridization promises efficient charge carrier transport but usually requires dense packing for significant wave function overlap. Here we use scanning tunneling spectroscopy to spatially resolve the electronic structure of a surface-confined nanoporous supramolecular sheet of a prototypical hydrocarbon compound featuring terminal alkyne (−CCH) groups. Surprisingly, localized nanopore orbitals are observed, with their electron density centered in the cavities surrounded by the functional moieties. Density functional theory calculations reveal that these new electronic states originate from the intermolecular hybridization of six in-plane π-orbitals of the carbon−carbon triple bonds, exhibiting significant electronic splitting and an energy downshift of approximately 1 eV. Importantly, these nanopore states are distinct from previously reported interfacial states. We unravel the underlying connection between the formation of nanopore orbital and geometric arrangements of functional groups, thus demonstrating the generality of applying related orbital engineering concepts in various types of porous organic structures.
Since nanoarchitectures are supposed to be key ingredient for future devices it is important to understand their functionalities. One important application of organic networks is to tune the electronic properties of surfaces. The Klappenberger Group was among the first to investigate the influence of a purely organic network on the two-dimensional electron gas (2DEG) provided by a 111-facet of fcc-metals (10.1021/nl901700b) and the first to demonstrate the formation of tunable quantum dot arrays from self-assembled metal-organic networks (10.1103/PhysRevLett.106.026802). The picture is a 3D representation of the molecular network topography (in yellow) superposed with the standing wave pattern of the surface state electrons at an energy of 225 meV.
Allene molecules are known for the outstanding chiroptical activity. We employed the compound depicted in a) which has extreme degrees of conformational freedom for the fabrication of surface-confined nanostructures. Morphological complementarity between the homochiral units (b) triggered the self-assembly of in two diastereomers (c) of a highly ordered, upstanding chiral architecture (UCA) visualized in d). The novel, intertwined self-assembled monolayers feature reactive terminal alkynes for further functionalization and carry potential for widespread applications exploiting chiroptical amplification