Stereochemical Recognition of Helicenes on Metal Surfaces

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.

Stable π-Extended Thio[7]helicene-Based Diradical with Predominant Through-Space Spin-Spin Coupling

In this work, a novel π-extended thio[7]helicene scaffold was synthesized, where the α-position of the thiophene unit could be functionalized with bulky phenoxy radicals after considerable synthetic attempts. This open-shell helical diradical, ET7H-R, possesses high stability in the air, nontrivial π conjugation, persistent chirality, and a high diradical character (y0 of 0.998). The key feature is a predominant through-space spin-spin coupling (TSC) between two radicals at the helical terminals. Variable-temperature continuous-wave electron spin resonance (cw-ESR) and superconducting quantum interference device (SQUID) magnetometry in the solid state reveal a singlet ground state with a nearly degenerate triplet state of ET7H-R. These results highlight the significance of a stable helical diradicaloid as a promising platform for investigating intramolecular TSCs.

Helicoselective Synthesis of Indolohelicenoids through Organocatalytic Central-to-Helical Chirality Conversion

We report the helicoselective and convergent construction of indolohelicenoids with excellent efficiency and stereocontrol. This reaction proceeds through a chiral-phosphoric-acid-catalyzed enantioselective cycloaddition and eliminative aromatization sequence, which can be finely controlled by adjusting the reaction temperature. Mechanistic studies reveal that the chiral phosphoric acid cooperatively serves as both a bifunctional and Brønsted acid catalyst, enabling one-pot central-to-helical chirality conversion. Additionally, the optical properties of the synthesized indolohelicenoids were characterized to explore their potential applications in organic photoelectric materials.

Switching the on-surface orientation of oxygen-functionalized helicene

Helicenes represent an important class of chiral organic material with promising optoelectronic properties. Hence, functionalization of surfaces with helicenes is a key step toward new organic materials devices. The deposition of a heterohelicene containing two furano groups and two hydroxyl groups onto copper(111) surface in ultrahigh vacuum leads to different adsorbate modifications. At low coverage and low temperature, the molecules tend to lie on the surface in order to maximize van der Waals contact with the substrate. Thermal treatment leads to deprotonation of the hydroxyl groups and in part into a reorientation from lying into a standing adsorbate mode.

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.

Two-Dimensional Crystal Transition from Radialene to Cumulene on Ag(111) via Retro-[2 + 1] Cycloaddition

Two-dimensional (2D) crystal-to-crystal transition is an important method in crystal engineering because of its ability to directly create diverse crystal materials from one crystal. However, steering a 2D single-layer crystal-to-crystal transition on surfaces with high chemo- and stereoselectivity under ultra-high vacuum conditions is a great challenge because the transition is a complex dynamic process. Here, we report a highly chemoselective 2D crystal transition from radialene to cumulene with retention of stereoselectivity on Ag(111) via retro-[2 + 1] cycloaddition of three-membered carbon rings and directly visualize the transition process involving a stepwise epitaxial growth mechanism by the combination of scanning tunneling microscopy and non-contact atomic force microscopy. Using progression annealing, we found that isocyanides on Ag(111) at a low annealing temperature underwent sequential [1 + 1 + 1] cycloaddition and enantioselective molecular recognition based on C-H···Cl hydrogen bonding interactions to form 2D triaza[3]radialene crystals. In contrast, a higher annealing temperature induced the transformation of triaza[3]radialenes to generate trans-diaza[3]cumulenes, which were further assembled into 2D cumulene-based crystals through twofold N-Ag-N coordination and C-H···Cl hydrogen bonding interactions. By combining the observed distinct transient intermediates and density functional theory calculations, we demonstrate that the retro-[2 + 1] cycloaddition reaction proceeds via the ring opening of a three-membered carbon ring, sequential dechlorination/hydrogen passivation, and deisocyanation. Our findings provide new insights into the growth mechanism and dynamics of 2D crystals and have implications for controllable crystal engineering.

A C-H activation-based enantioselective synthesis of lower carbo[n]helicenes

The three-dimensional structure of carbohelicenes has fascinated generations of molecular chemists and has been exploited in a wide range of applications. Their strong circularly polarized luminescence has attracted considerable attention in recent years due to promising applications in new optical materials. Although the enantioselective synthesis of fused carbo- and heterohelicenes has been achieved, a direct catalytic enantioselective method allowing the synthesis of lower, non-fused carbo[n]helicenes (n = 4-6) is still lacking. We report here that Pd-catalysed enantioselective C-H arylation in the presence of a unique bifunctional phosphine-carboxylate ligand provides a simple and general access to these lower carbo[n]helicenes. Computational mechanistic studies indicate that both the C-H activation and reductive elimination steps contribute to the overall enantioselectivity. The observed enantio-induction seems to arise from a combination of non-covalent interactions and steric repulsion between the substrate and ligand during the two key reductive elimination steps. The photophysical and chiroptical properties of the synthesized scalemic [n]helicenes have been systematically studied.

Light Emission from M-Type Enantiomer of 2,13-bis(hydroxymethyl)[7]-thiaheterohelicene Molecules Adsorbed on Au(111) and C60/Au(111) Surfaces Investigated by STM-LE

Light emission from the M-type enantiomer of a helicene derivative (2,13-bis(hydroxymethyl)[7]-thiaheterohelicene) adsorbed on the clean Au(111) and the C₆₀-covered Au(111) surfaces were investigated by tunneling-current-induced light-emission technique. Plasmon-originated light emission was observed on the helicence/Au(111) surface and it was strongly suppressed on the area where the helicene molecules were adsorbed at the edges of the Au(111) terraces. To avoid luminescence quenching of excited helicene molecules and to suppress strong plasmon light emission from the Au(111) surface, C₆₀ layers were used as decoupling buffer layers between helicene molecules and the Au(111) surface. Helicene molecules were adsorbed preferentially on the Au(111) surface rather than on the C₆₀ buffer layers due to the small interaction of the molecules and C₆₀ islands. This fact motivated us to deposit a multilayer of helicene molecules onto the C₆₀ layers grown on the Au(111) surface, leading to the fact that the helicene/C₆₀ multilayer showed strong luminescence with the molecules character. We consider that such strong light emission from the multilayer of helicene molecules has a plasmon origin strongly modulated by the molecular electronic states of (M)-[7]TH-diol molecules.

Controlling anisotropic properties by manipulating the orientation of chiral small molecules

Chiral π-conjugated molecules bring new functionality to technological applications and represent an exciting, rapidly expanding area of research. Their functional properties, such as the absorption and emission of circularly polarized light or the transport of spin-polarized electrons, are highly anisotropic. As a result, the orientation of chiral molecules critically determines the functionality and efficiency of chiral devices. Here we present a strategy to control the orientation of a small chiral molecule (2,2'-dicyano[6]helicene) by the use of organic and inorganic templating layers. Such templating layers can either force 2,2'-dicyano[6]helicene to adopt a face-on orientation and self-assemble into upright supramolecular columns oriented with their helical axis perpendicular to the substrate, or an edge-on orientation with parallel-lying supramolecular columns. Through such control, we show that low- and high-energy chiroptical responses can be independently 'turned on' or 'turned off'. The templating methodologies described here provide a simple way to engineer orientational control and, by association, anisotropic functional properties of chiral molecular systems for a range of emerging technologies.

Evolution of Br⋯Br contacts in enantioselective molecular recognition during chiral 2D crystallization

Halogen-mediated interactions play an important role in molecular recognition and crystallization in many chemical and biological systems, whereas their effect on homochiral versus heterochiral recognition and crystallization has rarely been explored. Here we demonstrate the evolution of Br⋯Br contacts in chiral recognition during 2D crystallization. On Ag(100), type I contacts prevail at low coverage and lead to homochiral recognition and the formation of 2D conglomerates; whereas type II contacts mediating heterochiral recognition are suppressed at medium coverage and appear in the racemates induced by structural transitions at high coverage. On Ag(111), type I contacts dominate the 2D crystallization and generate 2D conglomerates exclusively. DFT calculations suggest that the energy difference between type I and type II contacts is reversed upon adsorption due to the substrate induced mismatch energy penalty. This result provides fundamental understanding of halogen-mediated interactions in molecular recognition and crystallization on surface.

Halogen-mediated interactions control molecular recognition in many chemical and biological systems. Here, the authors demonstrate two types of Br⋯Br contacts and their importance in chiral on-surface crystallization.

Deposition of Chiral Heptahelicene Molecules on Ferromagnetic Co and Fe Thin-Film Substrates

The discovery of chirality-induced spin selectivity (CISS), resulting from an interaction between the electron spin and handedness of chiral molecules, has sparked interest in surface-adsorbed chiral molecules due to potential applications in spintronics, enantioseparation, and enantioselective chemical or biological processes. We study the deposition of chiral heptahelicene by sublimation under ultra-high vacuum onto bare Cu(111), Co bilayer nanoislands on Cu(111), and Fe bilayers on W(110) by low-temperature spin-polarized scanning tunneling microscopy/spectroscopy (STM/STS). In all cases, the molecules remain intact and adsorb with the proximal phenanthrene group aligned parallel to the surface. Three degenerate in-plane orientations on Cu(111) and Co(111), reflecting substrate symmetry, and only two on Fe(110), i.e., fewer than symmetry permits, indicate a specific adsorption site for each substrate. Heptahelicene physisorbs on Cu(111) but chemisorbs on Co(111) and Fe(110) bilayers, which nevertheless remain for the sub-monolayer coverage ferromagnetic and magnetized out-of-plane. We are able to determine the handedness of individual molecules chemisorbed on Fe(110) and Co(111), as previously reported for less reactive Cu(111). The demonstrated deposition control and STM/STS imaging capabilities for heptahelicene on Co/Cu(111) and Fe/W(110) substrate systems lay the foundation for studying CISS in ultra-high vacuum and on the microscopic level of single molecules in controlled atomic configurations.

Enhancement of electrocatalytic oxygen evolution by chiral molecular functionalization of hybrid 2D electrodes

A sustainable future requires highly efficient energy conversion and storage processes, where electrocatalysis plays a crucial role. The activity of an electrocatalyst is governed by the binding energy towards the reaction intermediates, while the scaling relationships prevent the improvement of a catalytic system over its volcano-plot limits. To overcome these limitations, unconventional methods that are not fully determined by the surface binding energy can be helpful. Here, we use organic chiral molecules, i.e., hetero-helicenes such as thiadiazole-[7]helicene and bis(thiadiazole)-[8]helicene, to boost the oxygen evolution reaction (OER) by up to ca. 130 % (at the potential of 1.65 V vs. RHE) at state-of-the-art 2D Ni- and NiFe-based catalysts via a spin-polarization mechanism. Our results show that chiral molecule-functionalization is able to increase the OER activity of catalysts beyond the volcano limits. A guideline for optimizing the catalytic activity via chiral molecular functionalization of hybrid 2D electrodes is given.

Engineering novel surface electronic states via complex supramolecular tessellations

Tailoring Shockley surface-state (SS) electrons utilizing complex interfacial supramolecular tessellations was explored by low-temperature scanning tunnelling microscopy and spectroscopy, combined with computational modelling using electron plane wave expansion (EPWE) and empirical tight-binding (TB) methods. Employing a recently introduced gas-mediated on-surface reaction protocol, three distinct types of open porous networks comprising paired organometallic species as basic tectons were selectively synthesized. In particular, these supramolecular networks feature semiregular Archimedean tilings, providing intricate quantum dots (QDs) coupling scenarios compared to hexagonal porous superlattices. Our experimental results in conjunction with modelling calculations demonstrate the possibility of realizing novel two-dimensional electronic structures such as Kagome- and Dirac-type as well as hybrid Kagome-type bands via QD coupling. Compared to constructing SS electron pathways via molecular manipulations, our studies reveal significant potential of exploiting QD coupling as a complementary and versatile route for the control of surface electronic landscapes.

Spontaneous chiral resolution of pentahelicene molecules on Cd(0001)

Chiral resolution is of fundamental importance to conglomerate or racemate crystallization. Here we demonstrate that the spontaneous chiral resolution of pentahelicene racemates occurred in the monolayer domains. When deposited on a Cd(0001) surface, pentahelicene molecules crystallize into a commensurate (6 × 6)R0° structure built mainly from homochiral trimers. Spontaneous chirality separation takes place in the form of opposite mirror domains, where 2D enantiomorphism is not expressed by the oblique adlattice, but by the supramolecular chirality of the pentahelicene trimers. Furthermore, annealing the sample or extreme close-packing lead to the presence of lattice handedness through the formation of a porous network structure or an edge-on phase. These results provide valuable insight for 2D conglomerate crystallization and stereochemical recognition.

Homochiral to heterochiral transition in a pentahelicene monolayer on Bi(111)

We report the nucleation and two dimensional (2D) crystallization of the helical aromatic hydrocarbon pentahelicene ([5]H) on the semimetallic Bi(111) surface studied via low-temperature scanning tunneling microscopy. Individual homochiral dimers and heterochiral trimers appear on the substrate at a low coverage. With an increase in the coverage, a chiral phase transition takes place from the 2D conglomerate of [5]H dimers to the 2D racemate of [5]H trimers. The heterochiral [5]H trimers reveal a wavy arrangement due to the swing of 5[H] trimer rows after every second or third trimers. The swing mechanism of the trimer rows can be attributed to the steric repulsion between the adjacent trimers with same handedness.

Mechanically induced single-molecule helicity switching of graphene-nanoribbon-fused helicene on Au(111)

Helicene is a functional material with chirality caused by its characteristic helical geometry. The inversion of its helicity by external stimuli is a challenging task in the advanced control of the molecular chirality. This study fabricated a novel helical molecule, specifically a pentahelicene-analogue twisted aromatic hydrocarbon fused with a graphene nanoribbon, via on-surface synthesis using multiple precursors. Noncontact atomic force microscopy imaging with high spatial resolution confirmed the helicity of the reaction products. The helicity was geometrically converted by pushing a CO-terminated tip into the twisted framework, which is the first demonstration of helicity switching at the single-molecule scale.

Unbalanced 2D Chiral Crystallization of Pentahelicene Propellers and Their Planarization into Nanographenes

The chiral self-assembly of trispentahelicene propellers on a gold surface has been investigated in ultrahigh vacuum by means of scanning tunneling microscopy and time-of-flight secondary ion mass spectrometry. The trispentahelicene propellers aggregate into mirror domains with an enantiomeric ratio of 2 : 1. Thermally induced cyclodehydrogenation leads to planarization into nanographenes, which self-assemble into closed-packed layers with two different azimuths. Further treatment induces in part dimerization and trimerization by intermolecular cyclodehydrogenation.

Transition from Homochiral Clusters to Racemate Monolayers during 2D Crystallization of Trioxa[11]helicene on Ag(100)

The phenomenon of chiral crystallization into homochiral crystals is known for more than 170 years, yet it is still poorly understood. Studying crystallization on surfaces under well-defined condition seems a promising approach towards better understanding the intermolecular chiral recognition mechanisms during nucleation and growth. The two-dimensional aggregation of racemic trioxaundecahelicene on the single crystalline silver(100) surface has been investigated with scanning tunneling microscopy and with non-contact atomic force microscopy, as well as molecular modeling simulations. A transition from homochiral cluster motifs to heterochiral assembly into large islands with increasing coverage is observed. Force field modelling confirms higher stability of heterochiral arrangements from twelve molecules on. Results are discussed with respect to previous findings for the all-carbon heptahelicene on the same surface.

Stepwise Adsorption of Alkoxy-Pyrene Derivatives onto a Lamellar, Non-Porous Naphthalenediimide-Template on HOPG

The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC₃₃ -NDI-uC₃₃ , a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy-pyrene (PyrOR) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC₃₃ -NDI-uC₃₃ template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC₃₃ -NDI-uC₃₃ monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.

Nanostructured surfaces from ligand-protected metal nanoparticles

Nanostructuring surfaces with metal atoms or clusters represents a promising approach to create materials with unique electronic/magnetic properties and improved chemical reactivity. By means of plasma sputtering and mass spectrometric techniques, the deposition of precisely size-selected clusters onto single-crystal surfaces has been applied to prepare surfaces with tailored properties. However, nanostructured surfaces can as well be prepared with metal nanoparticles via solution-phase methods, but the difference is that nanoparticles prepared by wet chemistry are usually coated with a layer of ligands, which are essential not only for maintaining the size and the atomic structure of metallic cores, but also for playing crucial roles in the synthesis, physicochemical properties and catalytic activities of the nanoparticles. This Frontier covers aspects of nanostructured surfaces from ligand-protected metal nanoparticles, starting with high-resolution imaging of the ligand-protected metal nanoparticles, followed by periodic patterning of metal nanoparticles on surfaces and the well-controlled atomic layer deposition with nanoclusters at the liquid/solid interface. We also highlight the potential of the surface-supported structures from ligand-protected metal nanoparticles, and the challenges remaining to be tackled.

Thermally Induced Chiral Aggregation of Dihydrobenzopyrenone on Au(111)

The realization of chiral supramolecular architectures on solid surfaces has triggered much interest due to its potential enantiospecific applications. An in-depth study of chiral aggregation on surfaces is significant for developing functional chiral surfaces. Herein, we report thermally induced chiral aggregation of dihydrobenzopyrenone on Au(111). By high-resolution low-temperature scanning tunneling microscopy, a racemate monolayer consisting of levorotatory and dextrorotatory dihydrobenzopyrenones was found to aggregate into conglomerate domains after moderate annealing treatment. Combined with first-principles calculations, we suggest that the intermolecular dipole-dipole interaction plays an important role in chiral aggregation, which takes place via molecular in-plane diffusion rather than molecular out-of-plane flipping. This work unveils one underlying mechanism of thermally induced chiral aggregation, thus enabling potential applications such as fabricating supramolecular architectures for functional chiral surfaces.

Helical nanostructures for organic electronics: the role of topological sulfur in ad hoc synthesized dithia[7]helicenes studied in the solid state and on a gold surface

As the first of a series of molecular solenoids, two classes of dithia[7]helicenes (coil-shaped molecules with sulfur atoms integrated within a helical conjugated system) have been devised and synthesized to be used in molecular electronics. We used a modular assembly of fragments using Pd catalyzed coupling reactions and a final photocyclization step for the syntheses; this strategy gave us straightforward access to helicenes bearing thiophene end rings with either exo or endo topologies. Unequivocal structural characterization was carried out by X-ray crystallography. In the solid state, their crystal architectures show little similarities; both can be considered an ensemble of heterochiral dimers (P/M) that are themselves very different in nature in light of their main pairing interactions. On a gold surface, the effect of the sulfur atom is to strengthen their binding to the electrodes, as manifested by scanning tunneling microscopy (STM) performed at room temperature. Different coating patterns were observed for each class of molecule, although the most prominent finding is that we could see resolved STM images of a single molecule, with a full display of its inherent chirality under room temperature conditions.

Double layer crystallization of heptahelicene on noble metal surfaces

Resolution of enantiomers of chiral compounds via crystallization is the dominant method in chemical industry, but chiral recognition at the molecular level during this process is still poorly understood. Using single metal surfaces in ultrahigh vacuum as model system, the enantio-related transition from the monolayer structure into a double layer of the racemic mixture of heptahelicene has been studied with scanning tunneling microscopy. Submolecular resolution reveals enantiopure second layers on Ag(111) and almost enantiopure second layers on Au(111). In analogy to previous results on Cu(111), it is concluded that transition from the 2D first layer racemate into a layered racemate occurs.

On-Surface Evolution of meso-Isomerism in Two-Dimensional Supramolecular Assemblies

Chiral structures created through the adsorption of molecules onto achiral surfaces play pivotal roles in many fields of science and engineering. Here, we present a systematic study of a novel chiral phenomenon on a surface in terms of organizational chirality, that is, meso-isomerism, through coverage-driven hierarchical polymorphic transitions of supramolecular assemblies of highly symmetric π-conjugated molecules. Four coverage-dependent phases of dehydrobenzo[12]annulene were uniformly fabricated on Ag(111), exhibiting unique chiral characteristics from the single-molecule level to two-dimensional supramolecular assemblies. All coverage-driven phase transitions stem from adsorption-induced pseudo-diastereomerism, and our observation of a lemniscate-type (∞) supramolecular configuration clearly reveals a drastic chiral phase transition from an enantiomeric chiral domain to a meso-isomeric achiral domain. These findings provide new insights into controlling two-dimensional chiral architectures on surfaces.

Understanding Enantioselective Interactions by Pulling Apart Molecular Rotor Complexes

Enantioselective interactions underpin many important phenomena from biological mechanisms to chemical catalysis. In this regard, there is great interest in understanding these effects at the molecular level. Surfaces provide a platform for these studies and aid in the long-term goal of designing heterogeneous enantiospecific interfaces. Herein we report a model system consisting of molecular rotors, one intrinsically chiral (propylene oxide) and one that becomes chiral when adsorbed on a surface (propene). Scanning tunneling microscopy (STM) measurements enable the chirality of each individual molecule to be directly visualized, and density functional theory based calculations are performed to rationalize the chiral time-averaged appearance of the molecular rotors. While there are no attractive intermolecular interactions between the molecular species themselves, when mixed together there is a strong preference for the formation of 1:1 heteromolecular pairs. We demonstrate that STM tip-induced molecular manipulations can be used to assemble these complexes, examine the chirality of each species, and thereby interrogate if their interactions are enantioselective. A statistical analysis of this data reveals that intrinsically chiral propylene oxide preferentially binds one of the enantiomers of propene with a 3:2 ratio, thereby demonstrating that the surface chirality of small nonchiral molecules can be directed with a chiral modifier. As such, this investigation sheds light onto previously reported ensemble studies in which chirally seeded layers of molecules that are achiral in the gas phase can lead to an amplification of enantioselective adsorption.

On-Surface Synthesis of a Nonplanar Porous Nanographene

On-surface synthesis provides an effective approach toward the formation of graphene nanostructures that are difficult to achieve via traditional solution chemistry. Here, we report on the design and synthesis of a nonplanar porous nanographene with 78 sp² carbon atoms, namely C78. Through a highly selective oxidative cyclodehydrogenation of 2,3,6,7,10,11-hexa(naphthalen-1-yl)triphenylene (2), propeller nanographene precursor 1 was synthesized in solution. Interestingly, although 1 could not be cyclized further in solution, porous nanographene C78 was successfully achieved from 1 by on-surface assisted cyclodehydrogenation on Au(111). The structure and electronic properties of C78 have been investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy, and scanning tunneling spectroscopy, complemented by computational investigations. Our results provide perspectives for the on-surface synthesis of porous graphene nanostructures, offering a promising strategy for the engineering of graphene materials with tailor-made properties.

Chiral Recognition of Hexahelicene on a Surface via the Forming of Asymmetric Heterochiral Trimers

Chiral recognition among helical molecules is of essential importance in many chemical and biochemical processes. The complexity necessitates investigating manageable model systems for unveiling the fundamental principles of chiral recognition at the molecular level. Here, we reported chiral recognition in the self-assembly of enantiopure and racemic hexahelicene on a Au(111) surface. Combing scanning tunneling microscopy (STM) and atomic force microscopy (AFM) measurements, the asymmetric heterochiral trimers were observed as a new type of building block in racemic helicene self-assembly on Au(111). The intermolecular recognition of the heterochiral trimer was investigated upon manual separation so that the absolute configuration of each helicene molecule was unambiguously determined one by one, thus confirming that the trimer was "2+1" in handedness. These heterochiral trimers showed strong stability upon different coverages, which was also supported by theoretical calculations. Our results provide valuable insights for understanding the intermolecular recognition of helical molecules.

Heterochiral recognition among functionalized heptahelicenes on noble metal surfaces

No matter which heptahelicene derivative, all assemble into heterochiral zigzag chains on gold and silver(111) surfaces.

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.

Diastereoselective Ullmann Coupling to Bishelicenes by Surface Topochemistry

The comparison of the self-assembly 9,9'-bisheptahelicene on the Au(111) surface, studied with scanning tunneling microscopy, with the self-assembly of the same species obtained by on-surface synthesis via Ullmann coupling from 9-bromoheptahelicene reveals a diastereomeric excess for the ( M, P)- meso-form of 50%. The stereoselectivity is explained by a topochemical effect, in which the surface-alignment of the starting material and the organometallic intermediate sterically favor the ( M, P)-transition state over the homochiral transition states.

Spontaneous separation of on-surface synthesized tris-helicenes into two-dimensional homochiral domains

The on-surface Ullmann coupling of 2,3-dibromo[4]helicene molecules is studied on Au(111) and Cu(111) surfaces. Bis-helicene and tris-helicene are identified with scanning tunnelling microscopy and X-ray photoelectron spectroscopy as reaction products. The produced star-shaped tris-helicenes self-assemble on Au(111) spontaneously into large homochiral domains.

Single-molecule insights into surface-mediated homochirality in hierarchical peptide assembly

Homochirality is very important in the formation of advanced biological structures, but the origin and evolution mechanisms of homochiral biological structures in complex hierarchical process is not clear at the single-molecule level. Here we demonstrate the single-molecule investigation of biological homochirality in the hierarchical peptide assembly, regarding symmetry break, chirality amplification, and chirality transmission. We find that homochirality can be triggered by the chirality unbalance of two adsorption configuration monomers. Co-assembly between these two adsorption configuration monomers is very critical for the formation of homochiral assemblies. The site-specific recognition is responsible for the subsequent homochirality amplification and transmission in their hierarchical assembly. These single-molecule insights open up inspired thoughts for understanding biological homochirality and have general implications for designing and fabricating artificial biomimetic hierarchical chiral materials.

Most chiral molecules and structures in living organisms exist as single enantiomers, but why? Here, the authors investigated surface-mediated homochirality on the single-molecule level and show that it can be triggered by the chirality unbalance of two adsorption configuration monomers.

Chirality-Dependent Electron Spin Filtering by Molecular Monolayers of Helicenes

The interaction of low-energy photoelectrons with well-ordered monolayers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, composed of only carbon and hydrogen atoms, exhibits only a single helical turn but shows excess in longitudinal spin polarization of about P _Z = 6 to 8% after transmission of initially balanced left- and right-handed spin polarized electrons. Insight into the electronic structure, that is, the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density functional theory calculations and a model Hamiltonian approach, respectively. Our results support the semiclassical picture of electronic transport along a helical pathway under the influence of spin-orbit coupling induced by the electrostatic molecular potential.

Enantiomer surface chemistry: conglomerate versus racemate formation on surfaces

Research on surface chirality is motivated by the need to develop functional chiral surfaces for enantiospecific applications. While molecular chirality in 3D has been the subject of study for almost two centuries, many aspects of 2D chiral surface chemistry have yet to be addressed. In 3D, racemic mixtures of chiral molecules tend to aggregate into racemate (molecularly heterochiral) crystals much more frequently than conglomerate (molecularly homochiral) crystals. Whether chiral adsorbates on surfaces preferentially aggregate into heterochiral rather than homochiral domains (2D crystals or clusters) is not known. In this review, we have made the first attempt to answer the following question based on available data: in 2D racemic mixtures adsorbed on surfaces, is there a clear preference for homochiral or heterochiral aggregation? The current hypothesis is that homochiral packing is preferred on surfaces; in contrast to 3D where heterochiral packing is more common. In this review, we present a simple hierarchical scheme to categorize the chirality of adsorbate-surface systems. We then review the body of work using scanning tunneling microscopy predominantly to study aggregation of racemic adsorbates. Our analysis of the existing literature suggests that there is no clear evidence of any preference for either homochiral or heterochiral aggregation at the molecular level by chiral and prochiral adsorbates on surfaces.

Imaging on-surface hierarchical assembly of chiral supramolecular networks

The bottom-up assembly of chiral structures usually relies on a cascade of molecular recognition interactions. A thorough description of these complex stereochemical mechanisms requires the capability of imaging multilevel coordination in real-time. Here we report the first direct observation of hierarchical expression of supramolecular chirality at work, for 10,10'-dibromo-9,9'-bianthryl (DBBA) on Cu(111). Molecular recognition first steers the growth of chiral organometallic chains and then leads to the formation of enantiopure islands. The structure of the networks was determined by noncontact atomic force microscopy (nc-AFM), while high-speed scanning tunnelling microscopy (STM) revealed details of the assembly mechanisms at the ms time-scale. The direct observation of the chirality transfer pathways allowed us to evaluate the enantioselectivity of the interchain coupling.

A computational exploration of the crystal energy and charge-carrier mobility landscapes of the chiral [6]helicene molecule

The potential of a given π-conjugated organic molecule in an organic semiconductor device is highly dependent on molecular packing, as it strongly influences the charge-carrier mobility of the material. Such solid-state packing is sensitive to subtle differences in their intermolecular interactions and is challenging to predict. Chirality of the organic molecule adds an additional element of complexity to intuitive packing prediction. Here we use crystal structure prediction to explore the lattice-energy landscape of a potential chiral organic semiconductor, [6]helicene. We reproduce the experimentally observed enantiopure crystal structure and explain the absence of an experimentally observed racemate structure. By exploring how the hole and electron-mobility varies across the energy-structure-function landscape for [6]helicene, we find that an energetically favourable and frequently occurring packing motif is particularly promising for electron-mobility, with a highest calculated mobility of 2.9 cm² V^-1 s^-1 (assuming a reorganization energy of 0.46 eV). We also calculate relatively high hole-mobility in some structures, with a highest calculated mobility of 2.0 cm² V^-1 s^-1 found for chains of helicenes packed in a herringbone fashion. Neither the energetically favourable nor high charge-carrier mobility packing motifs are intuitively obvious, and this demonstrates the utility of our approach to computationally explore the energy-structure-function landscape for organic semiconductors. Our work demonstrates a route for the use of computational simulations to aid in the design of new molecules for organic electronics, through the a priori prediction of their likely solid-state form and properties.

Building a 22-ring nanographene by combining in-solution and on-surface syntheses

A nanographene formed by the fusion of 22 benzene rings has been prepared by combining in-solution cycloaddition reactions and on-surface cyclodehydrogenations.

Diastereoselective self-assembly of bisheptahelicene on Cu(111)

Diastereospecific two-dimensional crystallization is reported for bishelicenes on a Cu(111) surface.