Symmetry breakdown of 4,4″-diamino-p-terphenyl on a Cu(111) surface by lattice mismatch

Effect of the linking bridge type on the self-assembly behaviour of 2',3'-difluoroterphenyl derivatives

The self-assembly behaviour and the crystallization kinetics of two liquid crystal compounds containing 2',3'-difluorosubstituted terphenyl as the mesogenic core have been described. Calorimetric studies show that the linking bridge type affects the polymorphism of smectic phases. The CH2O.3 compound with the -CH2O- linking bridge connected with a longer methylene spacer possesses the chiral smectic phase with antiferroelectric properties (SmCA* phase), while the COO.6 liquid crystal with the -COO- linking bridge connected with a shorter alkyl chain forms the chiral smectic phase with ferroelectric properties (SmC* phase). Both compounds crystallize upon slow cooling, while fast cooling causes the vitrification of the conformationally disordered crystal phase. Dielectric measurements reveal the complex relaxation dynamics in the identified thermodynamic states. DFT calculations allow us to estimate the nature of relaxation processes.

Site-selective chemical reactions by on-water surface sequential assembly

Controlling site-selectivity and reactivity in chemical reactions continues to be a key challenge in modern synthetic chemistry. Here, we demonstrate the discovery of site-selective chemical reactions on the water surface via a sequential assembly approach. A negatively charged surfactant monolayer on the water surface guides the electrostatically driven, epitaxial, and aligned assembly of reagent amino-substituted porphyrin molecules, resulting in a well-defined J-aggregated structure. This constrained geometry of the porphyrin molecules prompts the subsequent directional alignment of the perylenetetracarboxylic dianhydride reagent, enabling the selective formation of a one-sided imide bond between porphyrin and reagent. Surface-specific in-situ spectroscopies reveal the underlying mechanism of the dynamic interface that promotes multilayer growth of the site-selective imide product. The site-selective reaction on the water surface is further demonstrated by three reversible and irreversible chemical reactions, such as imide-, imine-, and 1, 3-diazole (imidazole)- bonds involving porphyrin molecules. This unique sequential assembly approach enables site-selective chemical reactions that can bring on-water surface synthesis to the forefront of modern organic chemistry.

Chiral and Catalytic Effects of Site-Specific Molecular Adsorption

The changes of properties and preferential interactions based on subtle energetic differences are important characteristics of organic molecules, particularly for their functionalities in biological systems. Only slightly energetically favored interactions are important for the molecular adsorption and bonding to surfaces, which define their properties for further technological applications. Here, prochiral tetracenothiophene molecules are adsorbed on the Cu(111) surface. The chiral adsorption configurations are determined by Scanning Tunneling Microscopy studies and confirmed by first-principles calculations. Remarkably, the selection of the adsorption sites by chemically different moieties of the molecules is dictated by the arrangement of the atoms in the first and second surface layers. Furthermore, we have investigated the thermal effects on the direct desulfurization reaction that occurs under the catalytic activity of the Cu substrate. This reaction leads to a product that is covalently bound to the surface in chiral configurations.

Selective activation of four quasi-equivalent C-H bonds yields N-doped graphene nanoribbons with partial corannulene motifs

Selective C–H bond activation is one of the most challenging topics for organic reactions. The difficulties arise not only from the high C–H bond dissociation enthalpies but also the existence of multiple equivalent/quasi-equivalent reaction sites in organic molecules. Here, we successfully achieve the selective activation of four quasi-equivalent C–H bonds in a specially designed nitrogen-containing polycyclic hydrocarbon (N-PH). Density functional theory calculations reveal that the adsorption of N-PH on Ag(100) differentiates the activity of the four ortho C(sp³) atoms in the N-heterocycles into two groups, suggesting a selective dehydrogenation, which is demonstrated by sequential-annealing experiments of N-PH/Ag(100). Further annealing leads to the formation of N-doped graphene nanoribbons with partial corannulene motifs, realized by the C–H bond activation process. Our work provides a route of designing precursor molecules with ortho C(sp³) atom in an N-heterocycle to realize surface-induced selective dehydrogenation in quasi-equivalent sites.

Selective activation of C–H bonds is a key challenge in organic reactions. Here, the authors achieve the selective activation of four quasi-equivalent C–H bonds, leading to the formation of N-doped graphene nanoribbons with partial corannulene motifs.

Topological Stone-Wales Defects Enhance Bonding and Electronic Coupling at the Graphene/Metal Interface

Defects play a critical role for the functionality and performance of materials, but the understanding of the related effects is often lacking, because the typically low concentrations of defects make them difficult to study. A prominent case is the topological defects in two-dimensional materials such as graphene. The performance of graphene-based (opto-)electronic devices depends critically on the properties of the graphene/metal interfaces at the contacting electrodes. The question of how these interface properties depend on the ubiquitous topological defects in graphene is of high practical relevance, but could not be answered so far. Here, we focus on the prototypical Stone-Wales (S-W) topological defect and combine theoretical analysis with experimental investigations of molecular model systems. We show that the embedded defects undergo enhanced bonding and electron transfer with a copper surface, compared to regular graphene. These findings are experimentally corroborated using molecular models, where azupyrene mimics the S-W defect, while its isomer pyrene represents the ideal graphene structure. Experimental interaction energies, electronic-structure analysis, and adsorption distance differences confirm the defect-controlled bonding quantitatively. Our study reveals the important role of defects for the electronic coupling at graphene/metal interfaces and suggests that topological defect engineering can be used for performance control.

On-Surface Synthesis of Polypyridine: Strain Enforces Extended Linear Chains

Strain-induced on-surface transformations provide an appealing route to steer the selectivity towards desired products. Here, we demonstrate the selective on-surface synthesis of extended all-trans poly(2,6-pyridine) chains on Au(111). By combining high-resolution scanning tunneling and atomic force microscopy, we revealed the detailed chemical structure of the reaction products. Density functional theory calculations indicate that the synthesis of extended covalent structures is energetically favored over the formation of macrocycles, due to the minimization of internal strain. Our results consolidate the exploitation of internal strain relief as a driving force to promote selective on-surface reactions.

Anchoring and Reacting On-Surface to Achieve Programmability

On-surface synthesis has developed into a modern method to fabricate low-dimensional molecular nanostructures with atomic precision. It impresses the chemistry community mostly via its simplicity, selectivity, and programmability during the synthesis. However, an insufficient mechanistic understanding of on-surface reactions and the discriminations in methodologies block it out from the conventional cognition of reaction and catalysis, which inhibits the extensive implication of on-surface synthesis. In this Perspective, we summarize the empirical paradigms of conceptually appealing programmability in on-surface synthesis. We endeavor to deliver the message that the impressive programmability is related to chemical heterogeneity which can also be coded at the molecular level and deciphered by the catalytic surfaces in varying chemical environments as specific chemical selectivity. With the assistance of structure-sensitive techniques, it is possible to recognize the chemical heterogeneity on surfaces to provide insight into the programmable on-surface construction of molecular nanoarchitectures and to reshape the correlation between the mechanistic understanding in on-surface synthesis and conventional chemistry.

Chemisorption-Induced Formation of Biphenylene Dimer on Ag(111)

We report an example that demonstrates the clear interdependence between surface-supported reactions and molecular-adsorption configurations. Two biphenyl-based molecules with two and four bromine substituents, i.e., 2,2'-dibromobiphenyl (DBBP) and 2,2',6,6'-tetrabromo-1,1'-biphenyl (TBBP), show completely different reaction pathways on a Ag(111) surface, leading to the selective formation of dibenzo[e,l]pyrene and biphenylene dimer, respectively. By combining low-temperature scanning tunneling microscopy, synchrotron radiation photoemission spectroscopy, and density functional theory calculations, we unravel the underlying reaction mechanism. After debromination, a biradical biphenyl can be stabilized by surface Ag adatoms, while a four-radical biphenyl undergoes spontaneous intramolecular annulation due to its extreme instability on Ag(111). Such different chemisorption-induced precursor states between DBBP and TBBP consequently lead to different reaction pathways after further annealing. In addition, using bond-resolving scanning tunneling microscopy and scanning tunneling spectroscopy, we determine with atomic precision the bond-length alternation of the biphenylene dimer product, which contains 4-, 6-, and 8-membered rings. The 4-membered ring units turn out to be radialene structures.

Constructing covalent organic nanoarchitectures molecule by molecule via scanning probe manipulation

Constructing low-dimensional covalent assemblies with tailored size and connectivity is challenging yet often key for applications in molecular electronics where optical and electronic properties of the quantum materials are highly structure dependent. We present a versatile approach for building such structures block by block on bilayer sodium chloride (NaCl) films on Cu(111) with the tip of an atomic force microscope, while tracking the structural changes with single-bond resolution. Covalent homo-dimers in cis and trans configurations and homo-/hetero-trimers were selectively synthesized by a sequence of dehalogenation, translational manipulation and intermolecular coupling of halogenated precursors. Further demonstrations of structural build-up include complex bonding motifs, like carbon–iodine–carbon bonds and fused carbon pentagons. This work paves the way for synthesizing elusive covalent nanoarchitectures, studying structural modifications and revealing pathways of intermolecular reactions.

Tailoring the size and connectivity of organic nanostructures is challenging but is often key in molecular electronics for tuning the properties of the quantum materials. Now an approach has been developed for building low-dimensional covalent architectures block by block on a surface by highly selective tip-induced intermolecular reactions.

Diamantanethiols on Metal Surfaces: Spatial Configurations, Bond Dissociations, and Polymerization

We report the on-surface chemistry of diamantanethiols on metal surfaces by combining low-temperature STM studies with quantum mechanical density functional theory computations. First, we examined the spatial configurations of diamantanethiols on metal surfaces, in which the thiol-substrate confinement plays a key role. We then thermally desorbed the diamantanethiols from the substrate surfaces to determine whether the C-S or S-metal bonds preferentially break. Finally, we explored diamantane-4,9-dithiol and its polymerization on metal surfaces, forming linear nanodiamond disulfur chains. This work broadens the fundamental knowledge of functionalized diamondoid behavior on surfaces and provides a novel approach to link diamantane as necklace-chain nanodiamond hybrid materials.

Adsorption of 4,4″-Diamino-p-Terphenyl on Cu(001): A First-Principles Study

Single-molecular devices show remarkable potential for applications in downscale electronic devices. The adsorption behavior of a molecule on a metal surface is of great importance from both fundamental and technological points of view. Herein, based on first-principles calculations, the adsorption of a 4,4″-diamino-p-terphenyl (DAT) molecule on a Cu(001) surface has been systematically explored. The most stable configuration is the DAT molecule lying flat with a rotation angle of 13° relative to the [100] surface direction. It was found that the adsorption sites of benzene rings and nitrogen atoms in the DAT molecule have important influences on the stability of the adsorption configuration. Electron density differences analysis shows that the electrons accumulate at the DAT-Cu(001) interface. The density of states projected on a DAT molecule of DAT/Cu(001) exhibits a metallic character, while the freestanding ones are semiconducting, indicating a strong interaction between the DAT molecule and the Cu(001) surface in the most stable adsorption configuration. These results provide useful information for tuning the properties and functions of DAT molecules, and may offer useful insights for other organic molecule/surface systems.

Surface-controlled reversal of the selectivity of halogen bonds

Intermolecular halogen bonds are ideally suited for designing new molecular assemblies because of their strong directionality and the possibility of tuning the interactions by using different types of halogens or molecular moieties. Due to these unique properties of the halogen bonds, numerous areas of application have recently been identified and are still emerging. Here, we present an approach for controlling the 2D self-assembly process of organic molecules by adsorption to reactive vs. inert metal surfaces. Therewith, the order of halogen bond strengths that is known from gas phase or liquids can be reversed. Our approach relies on adjusting the molecular charge distribution, i.e., the σ-hole, by molecule-substrate interactions. The polarizability of the halogen and the reactiveness of the metal substrate are serving as control parameters. Our results establish the surface as a control knob for tuning molecular assemblies by reversing the selectivity of bonding sites, which is interesting for future applications.

Substrate induced strain for on-surface transformation and synthesis

Intermolecular strain has long been used to steer and promote chemical reactions towards desired products in wet chemical synthesis. However, similar protocols have not been adopted for the on-surface synthesis on solid substrates due to the complexity of reaction processes. Recent advances in the sub-molecular resolution with scanning probe microscopy allow us to capture on-surface reaction pathways and to gain substantial insights into the role of strain in chemical reactions. The primary focus of this review is to highlight the recent findings on strain-induced on-surface reactions. Such substrate-induced processes can be applied to alter the chemical reactivity and to drive on-surface chemical reactions in different manners, which provides a promising alternative approach for on-surface synthesis. This review aims to shed light on the utilization of substrate-induced strain for on-surface transformation and synthesis of atomically-precise novel functional nanomaterials.

Role of intermolecular interactions and conformational changes in the polymorphism and vitrification process of 2,2′′-bis-substituted para-terphenyls

Conformational diversity of 2,2′′-bis-substituted para-terphenyls leads to polymorphism and contributes to their unique vitrification abilities.

Stepwise on-surface dissymmetric reaction to construct binodal organometallic network

Dissymmetric reactions, which enable differentiated functionalization of equivalent sites within one molecule, have many potential applications in synthetic chemistry and materials science, but they are very challenging to achieve. Here, the dissymmetric reaction of 1,4-dibromo-2,5-diethynylbenzene (2Br-DEB) on Ag(111) is realized by using a stepwise activation strategy, leading to an ordered two-dimensional organometallic network containing both alkynyl-silver-alkynyl and alkynyl-silver-phenyl nodes. Scanning tunneling microscopy and density functional theory calculations are employed to explore the stepwise conversion of 2Br-DEB, which starts from the H-passivation of one Br-substituted site at 300 K in accompaniment with an intermolecular reaction to form one-dimensional organometallic chains containing alkynyl-silver-alkynyl nodes. Afterwards, the other equivalent Br-substituted site undergoes metalation reaction at 320-450 K, resulting in transformation of the chains into the binodal networks. These findings exemplify the achievement of the dissymmetric reaction and its practical application for controlled fabrications of complicated yet ordered nanostructures on a surface.

Bond-Level Imaging of the 3D Conformation of Adsorbed Organic Molecules Using Atomic Force Microscopy with Simultaneous Tunneling Feedback

The chemical structure and orientation of molecules on surfaces can be visualized using low temperature atomic force microscopy with CO-functionalized tips. Conventionally, this is done in constant-height mode by measuring the frequency shift of the oscillating force sensor. However, this method is unsuitable for analyzing 3D objects. We are using the tunneling current to track the topography while simultaneously obtaining submolecular resolution from the frequency shift signal. Thereby, the conformation of 3D molecules and the adsorption sites on the atomic lattice can be reliably determined.

Coordination-Controlled C-C Coupling Products via ortho-Site C-H Activation

The coordination-restricted ortho-site C-H bond activation and dehydrogenative homocoupling of 4,4'-(1,3-phenylene)dipyridine (1,3-BPyB) and 4,4'-(1,4-phenylene)dipyridine (1,4-BPyB) on different metal surfaces were studied by a combination of scanning tunneling microscopy, noncontact atomic force microscopy, and density functional theory calculations. The coupling products on Cu(111) exhibited certain configurations subject to the spatial restriction of robust two-fold Cu-N coordination bonds. Compared to the V-shaped 1,3-BPyB, the straight backbone of 1,4-BPyB helped to further reduce the variety of reactive products. By utilizing the three-fold coordination of Fe atoms with 1,4-BPyB molecules on Au(111), a large-scale network containing single products was constructed. Our results offer a promising protocol for controllable on-surface synthesis with the aid of robust coordination interactions.

Adsorption Structure of Mono- and Diradicals on a Cu(111) Surface: Chemoselective Dehalogenation of 4-Bromo-3″-iodo- p-terphenyl

Selectivity is a key parameter for building customized organic nanostructures via bottom-up approaches. Therefore, strategies are needed that allow connecting molecular entities at a specific stage of the assembly process in a chemoselective manner. Studying the mechanisms of such reactions is the key to apply these transformations for the buildup of organic nanostructures on surfaces. Especially, the knowledge about the precise adsorption geometry of intermediates at different stages during the reaction process and their interactions with surface atoms or adatoms is of fundamental importance, since often catalytic processes are involved. We show the selective dehalogenation of 4-bromo-3″-iodo- p-terphenyl on the Cu(111) surface using bond imaging atomic force microscopy with CO-functionalized tips. The deiodination and debromination reactions are triggered either by heating or by locally applying voltage pulses with the tip. We observed a strong hierarchical behavior of the dehalogenation with respect to temperature and voltage. In connection with first-principles simulations we can determine the orientation and position of the pristine molecules as well as adsorbed mono/diradicals and the halogens. We find that the isolated radicals are chemisorbed to Cu(111) top sites, which are lifted by 16 pm ( meta-position) and 32 pm ( para-position) from the Cu surface plane. This leads to a strongly twisted and bent 3D adsorption structure. After heating, different types of dimers are observed whose molecules are either bound to surface atoms or connected via Cu adatoms. Such knowledge about the intermediate geometry and its interaction with the surface will open the way to rationally design syntheses on surfaces.