Chirality Transfer in 1D Self-Assemblies: Influence of H-Bonding vs Metal Coordination between Dicyano[7]helicene Enantiomers

From Short- to Long-Range Chiral Recognition on Surfaces: Chiral Assembly and Synthesis

Research on chiral behaviors of small organic molecules at solid surfaces, including chiral assembly and synthesis, can not only help unravel the origin of the chiral phenomenon in biological/chemical systems but also provide promising strategies to build up unprecedented chiral surfaces or nanoarchitectures with advanced applications in novel nanomaterials/nanodevices. Understanding how molecular chirality is recognized is considered to be a mandatory basis for such studies. In this review, a series of recent studies in chiral assembly and synthesis at well-defined metal surfaces under ultra-high vacuum conditions are outlined. More importantly, the intrinsic mechanisms of chiral recognition are highlighted, including short/long-range chiral recognition in chiral assembly and two main strategies to steer the reaction pathways and modulate selective synthesis of specific chiral products on surfaces.

Self-Assembly of N-Rich Triimidazoles on Ag(111): Mixing the Pleasures and Pains of Epitaxy and Strain

In the present report, homochiral hydrogen-bonded assemblies of heavily N-doped (C9H6N6) heterocyclic triimidazole (TT) molecules on an Ag(111) substrate were investigated using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) techniques. The planar and prochiral TT molecules, which exhibit a threefold rotation symmetry and lack mirror symmetry when assembled on the substrate, carry multiple hydrogen-bonding donor and acceptor functionalities, inevitably leading to the formation of hexameric two-dimensionally extended assemblies that can be either homo- (RR/SS) or heterochiral (RS). Experimental STM data showing well-ordered homochiral domains and experimental LEED data are consistent with simulations assuming the R19.1° overlayer on the Ag(111) lattice. Importantly, we report the unexpected coincidence of spontaneous resolution with the condensation of neighboring islands in adjacent "Janus pairs". The islands are connected by a characteristic fault zone, an observation that we discuss in the context of the fairly symmetric molecule and its propensity to compromise and benefit from interisland bonding at the expense of lattice mismatches and strain in the defect zone. We relate this to the close to triangular shape and the substantial but weak bonding scheme beyond van der Waals (vdW) of the TT molecules, which is due to the three N-containing five-membered imidazole rings. Density functional theory (DFT) calculations show clear energetic differences between homochiral and heterochiral pairwise interactions, clearly supporting the experimental results.

Spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on Cu(111) surfaces

It has been demonstrated previously that electrons interact differently with chiral molecules depending on their polarization. For enantiomeric pure monolayers of heptahelicene, opposite asymmetries in spin polarization were reported and attributed to the so-called chirality-induced spin selectivity effect. However, these promising proof-of-concept photoemission experiments lack the angular and energy resolution that could provide the necessary insights into the mechanism of this phenomenon. In order to fill in the missing gaps, we provide a detailed spin- and angle-resolved photoemission spectroscopy study of heptahelicene layers on a Cu(111) substrate. Throughout the large accessible energy and angle range, no chirality induced spin asymmetry in photoemission could be observed. Possible reasons for the absence of signatures of the spin-dependent electron transmission through the chiral molecular layer are briefly discussed.

Enantioselective assembly and recognition of heterochiral porous organic cages deduced from binary chiral components

Chiral recognition and discrimination is not only of significance in biological processes but also a powerful method to fabricate functional supramolecular materials. Herein, a pair of heterochiral porous organic cages (HPOC-1), out of four possible enantiomeric products, with mirror stereoisomeric crystal structures were cleanly prepared by condensation occurring in the exclusive combination of cyclohexanediamine and binaphthol-based tetraaldehyde enantiomers. Nuclear magnetic resonance and luminescence spectroscopy have been employed to monitor the assembly process of HPOC-1, revealing the clean formation of heterochiral organic cages due to the enantioselective recognition of (S,S)-binaphthol towards (R,R)-cyclohexanediamine derivatives and vice versa. Interestingly, HPOC-1 exhibits circularly polarized luminescence and enantioselective recognition of chiral substrates according to the circular dichroism spectral change. Theoretical simulations have been carried out, rationalizing both the enantioselective assembly and recognition of HPOC-1.

Bacterial Antifouling Characteristics of Helicene-Graphene Films

Herein, we describe interfacially-assembled [7]helicene films that were deposited on graphene monolayer using the Langmuir-Schaefer deposition by utilizing the interactions of nonplanar (helicene) and planar (graphene) π–π interactions as functional antifouling coatings. Bacterial adhesion of Staphylococcus aureus on helicene—graphene films was noticeably lower than that on bare graphene, up to 96.8% reductions in bacterial adhesion. The promising bacterial antifouling characteristics of helicene films was attributed to the unique molecular geometry of helicene, i.e., nano-helix, which can hinder the nanoscale bacterial docking processes on a surface. We envision that helicene—graphene films may eventually be used as protective coatings against bacterial antifouling on the electronic components of clinical and biomedical devices.

Tailoring atomic layer growth at the liquid-metal interface

Engineering atomic structures at metal surfaces represents an important step in the development of novel nanomaterials and nanodevices, but relies predominantly on atomic/molecular beam epitaxy under ultrahigh vacuum conditions, where controlling the deposition processes remains challenging. By using solution-borne nanosized gold clusters as a precursor, here we develop a wet deposition protocol to the fabrication of atomically flat gold nanoislands, so as to utilize the dynamic exchange of surface-active molecules at the liquid-metal interface for manipulating the growth kinetics of ultrathin metallic nanostructures. While remarkable shape and size selection of gold nanoislands is observed, our experimental and theoretical investigations provide compelling evidences that organic adsorbates can impart a bias to the island orientation by preferred adsorption and alignment and intervene in the assembly and disassembly of adatom islands by complexing with Au adatoms. This approach offers a simple solution to regulate atomic layer growth of metals at ambient conditions.

Transoid-to-Cisoid Conformation Changes of Single Molecules on Surfaces Triggered by Metal Coordination

Conformational isomers are stereoisomers that can interconvert over low potential barriers by rotation around a single bond. However, such bond rotation is hampered by geometrical constraints when molecules are adsorbed on surfaces. Here, we show that the adsorption of 4,4'-bis(4-carboxyphenyl)-6,6'-dimethyl-2,2'-bipyridine molecules on surfaces leads to the appearance of prochiral single molecules on NiO(001) and to enantiopure supramolecular domains on Au(111) surfaces containing the transoid-molecule conformation. Upon additional Fe adatom deposition, molecules undergo a controlled interconversion from a transoid-to-cisoid conformation as a result of coordination of the Fe atoms to the 2,2'-bipyridine moieties. As confirmed by atomic force microscopy images and X-ray photoelectron spectroscopy measurements, the resulting molecular structures become irreversibly achiral.

Hidden Order and Haldane-Like Phase in Molecular Chains Assembled from Conformation-Switchable Molecules

Topological properties of matters have attracted tremendous interest in the past years due to the scientific and technological importance. It is of great interest to discover the analogs of topological phases in molecular architectures, if the key constituents of the phases are properly mimicked. Using scanning tunneling microscopy, we demonstrate that quasi-1D molecular chains assembled from conformation-switchable dibenzo[ g, p]chrysene molecules show hidden antiparallel order analogous to the hidden antiferromagnetic order in the Haldane phase, a known topological phase of quantum spin-1 chains. This is realized by mimicking the spin degree of freedom with the intramolecular helicene chiral switches and by emulating the interspin antiferromagnetic coupling with intermolecular homochiral coupling. The discovery of the molecular analog of topological matters may inspire the search of molecular architectures with nontrivial topological properties.

Hierarchical self-assembly of enantiopure and racemic helicenes at the liquid/solid interface: from 2D to 3D

The performance of organic nanostructures is closely related to the organization of the functional molecules. Frequently, molecular chirality plays a central role in the way molecules assemble at the supramolecular level. Herein we report the hierarchical self-assembly of benzo-fused tetrathia[7]helicenes on solid surfaces, from a single surface-bound molecule to well-defined microstructures, using a combination of various characterization techniques assisted by molecular modeling simulations. Similarities as well as discrepancies are revealed between homochiral and heterochiral aggregations by monitoring the hierarchical nucleation of helicenes on surfaces, where the impact of enantiopurity, concentration and adsorbate-substrate interaction on molecular organization are disclosed.

Pt-dipyridyl tetrazine metal-organic network on the Au(100) surface: insights from first principles calculations

Metal-organic coordination networks with active metal centers are a promising class of materials for next-generation catalysts. Motivated by experimental observations of the formation of a Pt-Dipyridyl Tetrazine (DT) metal-organic network on the Au(100) surface [D. Skomski et al., J. Am. Chem. Soc., 2014, 136, 9862], we carried out density functional theory based calculations on the same system. In this discussion, we demonstrate that the strong interaction between DT ligands and Pt metal centers makes the network stable and that the Pt centers become positively charged by donating their electrons to the DT ligands, resulting in +2 oxidation states for the Pt centers. We further show that the Au substrate withdraws electrons from and hybridizes with the d_z² orbital of the Pt centers, altering their electronic structure and related properties. Furthermore, we find that the Pt centers can absorb SO₂via donor-acceptor interactions, leading to the formation of σ-bonds in which Pt d_z² orbitals act as electron donors, and that the strength of the resultant σ-bond depends on the registry of the Pt centers with the Au(100) surface. Finally, we identify factors, such as the specificity of the ligands and the substrate, and the fullness of the outer shell of the metal centers, that may affect the chemical properties of the metal centers. We suggest modifications (and replacement) of these factors as one of the ways to tune and design metal-organic coordination networks for next-generation catalysts.

Chiral-Selective Formation of 1D Polymers Based on Ullmann-Type Coupling: The Role of the Metallic Substrate

The chiral-selective formation of 1D polymers from a prochiral molecule, namely, 6,12-dibromochrysene in dependence of the type of metal surface is demonstrated by a combined scanning tunneling microscopy and density functional theory study. Deposition of the chosen molecule on Au(111) held at room temperature leads to the formation of a 2D porous molecular network. Upon annealing at 200 °C, an achiral covalently linked polymer is formed on Au(111). On the other hand, a chiral Cu-coordinated polymer is spontaneously formed upon deposition of the molecules on Cu(111) held at room temperature. Importantly, it is found that the chiral-selectivity determines the possibility of obtaining graphene nanoribbons (GNRs). On Au(111), upon annealing at 350 °C or higher cyclo-dehydrogenation occurs transforming the achiral polymer into a GNR. In contrast, the chiral coordination polymer on Cu(111) cannot be converted into a GNR.

Supramolecular chiral host-guest nanoarchitecture induced by the selective assembly of barbituric acid derivative enantiomers

Barbituric acid derivatives are prochiral molecules, i.e. they are chiral upon adsorption on surfaces. Scanning tunneling microscopy reveals that barbituric acid derivatives self-assemble into a chiral guest-host supramolecular architecture at the solid-liquid interface on graphite. The host nanoarchitecture has a sophisticated wavy shape pattern and paired guest molecules are nested insides the cavities of the host structure. Each unit cell of the host structure is composed of both enantiomers with a ratio of 1:1. Furthermore, the wavy patterns of the nanoarchitecture are formed from alternative appearance of left- and right-handed chiral building blocks, which makes the network heterochiral. The functional guest-host nanoarchitecture is the result of two-dimensional chiral amplification from single enantiomers to organizational heterochiral supramolecular self-assembly.

Stereochemical Recognition of Helicenes on Metal Surfaces

The chiral recognition among biomolecules is fundamentally important for many processes of life, including the stereochemistry of evolution. Of special interest is chiral recognition during crystallization of racemates, when either homochiral recognition leads to a conglomerate of homochiral crystals or heterochiral recognition dominates resulting in a racemic compound. The complex nature of molecular recognition at the level of nucleation and crystal growth renders it difficult to understand and calls for manageable model systems. Notably, the approach of studying aggregation of molecules at surfaces under well-defined conditions includes the benefit of the availability of a multitude of highly sensitive investigation methods, of which scanning tunneling microscopy (STM) with its submolecular resolution is tremendously valuable. Heterogeneous nucleation at surfaces is strongly favored over homogeneous nucleation in solution; hence, surfaces are significantly involved in stereochemical recognition during crystallization. Helicenes are a fascinating class of chiral compounds with outstanding optical activity. These π-conjugated, ortho-fused, aromatic hydrocarbons are promising candidates for organic electronic devices such as sensors, circular dichroic photonics, liquid crystal displays or spin filters. But in particular the defined footprint of their terminal benzo rings on a surface makes them interesting for studying stereochemical recognition with different single crystalline surfaces and the impact this has, in turn, on intermolecular recognition. In this Account, we describe the self-assembly of helicenes on metal surfaces with the focus on stereochemical recognition in two-dimensional structures. Using the isomeric all-carbon helicenes, heptahelicene and dibenzohelicene as examples, different aggregation phenomena on different surfaces of single crystalline copper, silver, and gold are investigated. By means of STM different modes of transmission of molecular handedness from single molecules into extended two-dimensional supramolecular structures are identified. For the problem of racemate versus conglomerate crystallization, the impact of surface and molecular structure and their interplay are analyzed. This leads to detailed conclusions about the importance of the match of molecular and surface binding sites for long-range self-assembly. The absence of polar groups puts emphasis on van der Waals interaction and their maximization by steric overlap of molecular parts in enantiomeric and diastereomeric interactions. With STM as a manipulation tool, dimers are manually separated in order to analyze their chiral composition. And finally, new nonlinear cooperative effects induced by small enantiospecific bias are discovered that lead to single enantiomorphism in two-dimensional racemate crystals as well as in racemic multilayered films. By means of these model studies many details that govern chiral recognition at surfaces are rationalized.

Electronic Structures and Chiroptical Properties of Post-functionalized Helicene Quinones

The synthesis of redox-active p- and o-quinones 2-phenylamino-4-phenylimino[6]helicene-1-one 1, 2-phenylamino[6]-helicene-1,4-dione 2, and 4-phenyl[6]helicene-1,2-dione 3 in their enantiopure forms by post-functionalization of (P)- and (M)-1,2-dimethoxy[6]helicene is presented. Structural characterization in solution and in the solid state was accomplished by 2D NMR spectroscopy methods and X-ray diffraction analysis, respectively. Interpretation of electrochemical redox data was accompanied by a detailed orbital picture, derived from DFT calculations. The electronic structures of compounds 1-3 were investigated by UV/Vis and electronic circular dichroism (ECD) spectroscopy, complemented by TD-DFT calculations. Quinones 1-3 were chemically reduced to study the EPR signatures of their respective radical anions. DFT methods were used for the atom assignment of the hyperfine coupling constants. The results are discussed within the context of electrochromic chiral switches and molecular recognition.

Cyano-Functionalized Triarylamines on Coinage Metal Surfaces: Interplay of Intermolecular and Molecule-Substrate Interactions

The self-assembly of cyano-functionalized triarylamine derivatives on Cu(111), Ag(111) and Au(111) was studied by means of scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoelectron spectroscopy and density functional theory calculations. Different bonding motifs, such as antiparallel dipolar coupling, hydrogen bonding and metal coordination, were observed. Whereas on Ag(111) only one hexagonally close-packed pattern stabilized by hydrogen bonding is observed, on Au(111) two different partially porous phases are present at submonolayer coverage, stabilized by dipolar coupling, hydrogen bonding and metal coordination. In contrast to the self-assembly on Ag(111) and Au(111), for which large islands are formed, on Cu(111), only small patches of hexagonally close-packed networks stabilized by metal coordination and areas of disordered molecules are found. The significant variety in the molecular self-assembly of the cyano-functionalized triarylamine derivatives on these coinage metal surfaces is explained by differences in molecular mobility and the subtle interplay between intermolecular and molecule-substrate interactions.

Mechanical tuning of conductance and thermopower in helicene molecular junctions

Helicenes are inherently chiral polyaromatic molecules composed of all-ortho fused benzene rings possessing a spring-like structure. Here, using a combination of density functional theory and tight-binding calculations, it is demonstrated that controlling the length of the helicene molecule by mechanically stretching or compressing the molecular junction can dramatically change the electronic properties of the helicene, leading to a tunable switching behavior of the conductance and thermopower of the junction with on/off ratios of several orders of magnitude. Furthermore, control over the helicene length and number of rings is shown to lead to more than an order of magnitude increase in the thermopower and thermoelectric figure-of-merit over typical molecular junctions, presenting new possibilities of making efficient thermoelectric molecular devices. The physical origin of the strong dependence of the transport properties of the junction is investigated, and found to be related to a shift in the position of the molecular orbitals.

A protocol of self-assembled monolayers of fluorescent block molecules for trace Zn(ii) sensing: structures and mechanisms

The fluorescent behavior of the SAM of TPE-C4-L2 block molecules on HOPG surface has been revealed using STM.

Photophysical properties of quinoxaline-fused [7]carbohelicene derivatives

The electronic circular dichroism, UV-Vis absorption, nonlinear optics, and charge transport properties of novel quinoxaline-fused 7-carbohelicene derivatives have been investigated for the first time.

Programmed assembly of 4,2′:6′,4′′-terpyridine derivatives into porous, on-surface networks

4,2′:6′,4′′-Terpyridine building blocks self-assemble into hydrogen-bonded domains; addition of copper atoms results in an on-surface transformation into a coordination network.

Formation of one-dimensional helical columns and excimerlike excited states by racemic quinoxaline-fused [7]carbohelicenes in the crystal

A series of quinoxaline-fused [7]carbohelicenes (HeQu derivatives) was designed and synthesized to evaluate their structural and photophysical properties in the crystal state. The quinoxaline units were expected to enhance the light-emitting properties and to control the packing structures in the crystal. The electrochemical and spectroscopic properties and excited-state dynamics of these compounds were investigated in detail. The first oxidation potentials of HeQu derivatives are approximately the same as that of unsubstituted reference [7]carbohelicene (Heli), whereas their first reduction potentials are shifted to the positive by about 0.7 V. The steady-state absorption, fluorescence, and circular dichroism spectra also became redshifted compared to those of Heli. The molecular orbitals and energy levels of the HOMO and LUMO states, calculated by DFT methods, support these trends. Moreover, the absolute fluorescence quantum yields of HeQu derivatives are about four times larger than that of Heli. The structural properties of the aggregated states were analyzed by single-crystal analysis. Introduction of appropriate substituents (i.e., 4-methoxyphenyl) in the HeQu unit enabled the construction of one-dimensional helical columns of racemic HeQu derivatives in the crystal state. Helix formation is based on intracolumn π-stacking between two neighboring [7]carbohelicenes and intercolumn CH⋅⋅⋅N interaction between a nitrogen atom of a quinoxaline unit and a hydrogen atom of a helicene unit. The time-resolved fluorescence spectra of single crystals clearly showed an excimerlike delocalized excited state owing to the short distance between neighboring [7]carbohelicene units.

Redox-active on-surface assembly of metal-organic chains with single-site Pt(II)

The formation and stabilization of well-defined transition-metal single sites at surfaces may open new routes to achieve higher selectivity in heterogeneous catalysts. Organic ligand coordination to produce a well-defined oxidation state in weakly reducing metal sites at surfaces, desirable for selective catalysis, has not been achieved. Here, we address this using metallic platinum interacting with a dipyridyl tetrazine ligand on a single crystal gold surface. X-ray photoelectron spectroscopy measurements demonstrate the metal-ligand redox activity and are paired with molecular-resolution scanning probe microscopy to elucidate the structure of the metal-organic network. Comparison to the redox-inactive diphenyl tetrazine ligand as a control experiment illustrates that the redox activity and molecular-level ordering at the surface rely on two key elements of the metal complexes: (i) bidentate binding sites providing a suitable square-planar coordination geometry when paired around each Pt, and (ii) redox-active functional groups to enable charge transfer to a well-defined Pt(II) oxidation state. Ligand-mediated control over the oxidation state and structure of single-site metal centers that are in contact with a metal surface may enable advances in higher selectivity for next generation heterogeneous catalysts.

Van der Waals interactions in the self-assembly of 5-amino[6]helicene on Cu(100) and Au(111)

No H bonding for the amino group: van der Waals interactions guide the self-assembly of an aminohelicene on Cu(100) and Au(111).