Deciphering the factors influencing electric field mediated polymerization and depolymerization at the solution-solid interface.

Strong and oriented electric fields are known to influence structure as well as reactivity. The strong electric field (EF) between the tip of a scanning tunneling microscope (STM) and graphite has been used to modulate two-dimensional (2D) polymerization of aryl boronic acids where switching the polarity of the substrate bias enabled reversible transition between self-assembled molecular networks of monomers and crystalline 2D polymer (2DP) domains. Here, we untangle the different factors influencing the EF-mediated (de)polymerization of a boroxine-based 2DP on graphite. The influence of the solvent was systematically studied by varying the nature from polar protic to polar aprotic to non-polar. The effect of monomer concentration was also investigated in detail with a special focus on the time-dependence of the transition. Our experimental observations indicate that while the nucleation of 2DP domains is not initiated by the applied electric field, their depolymerization and subsequent desorption, are a consequence of the change in the polarity of the substrate bias within the area scanned by the STM tip. We conclude that the reversible transition is intimately linked to the bias-induced adsorption and desorption of the monomers, which, in turn, could drive changes in the local concentration of the monomers.

Highly Ordered Co-Assembly of Bisurea Functionalized Molecular Switches at the Solid-Liquid Interface.

Immobilization of stimulus-responsive systems on solid surfaces is beneficial for controlled signal transmission and adaptive behavior while allowing the characterization of the functional interface with high sensitivity and high spatial resolution. Positioning of the stimuli-responsive units with nanometer-scale precision across the adaptive surface remains one of the bottlenecks in the extraction of cooperative function. Nanoscale organization, cooperativity, and amplification remain key challenges in bridging the molecular and the macroscopic worlds. Here we report on the design, synthesis, and scanning tunneling microscopy (STM) characterization of overcrowded alkene photoswitches merged in self-assembled networks physisorbed at the solid-liquid interface. A detailed anchoring strategy that ensures appropriate orientation of the switches with respect to the solid surface through the use of bis-urea groups is presented. We implement a co-assembly strategy that enables the merging of the photoswitches within physisorbed monolayers of structurally similar 'spacer' molecules. The self-assembly of the individual components and the co-assemblies was examined in detail using (sub)molecular resolution STM which confirms the robust immobilization and controlled orientation of the photoswitches within the spacer monolayers. The experimental STM data is supported by detailed molecular mechanics (MM) simulations. Different designs of the switches and the spacers were investigated which allowed us to formulate guidelines that enable the precise organization of the photoswitches in crystalline physisorbed self-assembled molecular networks.

From One-Dimensional Disordered Racemate to Ordered Racemic Conglomerates through Metal-Coordination-Driven Self-Assembly at the Liquid-Solid Interface.

In recent years, there has been significant focus on investigating and controlling chiral self-assembly, specifically in the context of enantiomeric separation. This study explores the self-assembly behavior of 4-dodecyl-3,6-di(2-pyridyl)pyridazine (DPP-C12) at the interface between heptanoic acid (HA) and highly oriented pyrolytic graphite (HOPG) using a combination of scanning tunneling microscopy (STM) and multiscale molecular modeling. The self-assembled monolayer structure formed by DPP-C12 is periodic in one direction, but aperiodic in the direction orthogonal to it. These structures resemble 1D disordered racemic compounds. Upon introducing palladium [Pd(II)] ions, complexing with DPP-C12, these 1D disordered racemic compounds spontaneously transform into 2D racemic conglomerates, which is rationalized with the assistance of force-field simulations. Our findings provide insights into the regulation of two-dimensional chirality.

Self-Assembly of Discrete Oligomers of Naphthalenediimides in Bulk and on Surfaces.

Here, we report on the synthesis of discrete oligomers of alkyl-bridged naphthalenediimides (NDIs) and study their molecular nanostructures both in bulk, in solution, and at the liquid-solid interface. Via an iterative synthesis method, multiple NDI cores were bridged with short and saturated alkyl-diamines (C3 and C12 ) or long and unsaturated alkyl-diamines (u2 C33 to u8 C100 ) at their imide termini. The strong intermolecular interaction between the NDI cores was observed by probing their photophysical properties in solution. In bulk, the discrete NDI oligomers preferentially ordered in lamellar morphologies, irrespective of whether a saturated or unsaturated spacer was employed. Moreover, both the molecular architecture as well as the crystallization conditions play a significant role in the nanoscale ordering. The long unsaturated alkyl chains lead preferably to folded-chain conformations while their saturated analogues form stretched arrangements. At the solution-solid interface, well-defined lamellar regions were observed. These results show that precision in chemical structure alone is not sufficient to reach well-defined structures of discrete oligomers, but that it must be combined with precision in processing conditions.

On the nucleation and fast reaction kinetics of 2D polymerisation with a 2-in-1 monomer.

We report on the fast reaction kinetics of an imine based 2D polymer (2DP) formed from a single monomer carrying both aldehyde and amine groups. Our results point towards a direct monomer-to-crystalline polymer transition without an amorphous intermediate.

Nanoscale chemical patterning of graphite at different length scales.

Chemical patterning surfaces is relevant in several different domains of science and technology with exciting possibilities in electronics, catalysis, sensing, and photonics. Here, we present a novel strategy for chemical patterning of graphite using a combination of covalent and non-covalent approaches. Building on our previous work, where self-assembled monolayers of linear alkanes were used as sacrificial masks for directing the covalent anchoring of aryl groups to the graphite surface in sub-10 nm arrays, we present a modified design of a template alkane with alkoxy terminal groups which allowed better pattern transfer fidelity in comparison to simple linear alkanes. We also explored the use of chronoamperometry (CA) instead of previously used cyclic voltammetry (CV) for the functionalization process, which enabled patterning of the graphite surface at two-different length scales: few hundred nanometer circular patterns interspersed with sub-10 nm linear arrays. The covalent chemical patterning process has been studied in detail using CV and CA measurements whereas the patterned substrates have been thoroughly characterized using Raman spectroscopy, scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Based on the comparison between the pattern transfer fidelity of previously studied alkanes and newly synthesized alkoxy alkane, we discuss plausible molecular mechanism of pattern transfer.

Direct C-H Arylation of Dithiophene-Tetrathiafulvalene: Tuneable Electronic Properties and 2D Self-Assembled Molecular Networks at the Solid/Liquid Interface.

Invited for the cover of this issue is the group of Manuel Souto and co-workers at the University of Aveiro and CICECO-Aveiro Institute of Materials. The image depicts the direct C-H arylation of dithiophene-tetrathiafulvalene (DT-TTF) and the self-assembly of DT-TTF-tetrabenzoic acid studied by using scanning tunnelling microscopy. Read the full text of the article at 10.1002/chem.202300572.

Spatially Controlled Aryl Radical Grafting of Graphite Surfaces Guided by Self-Assembled Molecular Networks of Linear Alkane Derivatives: The Importance of Conformational Dynamics.

The covalent functionalization of carbon surfaces with nanometer-scale precision is of interest because of its potential in a range of applications. We herein report the controlled grafting of graphite surfaces using electrochemically generated aryl radicals templated by self-assembled molecular networks (SAMNs) of bisalkylurea derivatives. A bisalkylurea derivative having two butoxy units acts as a template for the covalent functionalization of aryl groups in between self-assembled rows of this molecule. In contrast, grafting occurs without a spatial order when an SAMN of bis(tetradecyl)urea was used as a template. This indicates that a degree of dynamics at the alkyl termini is required to favor controlled covalent attachment, a situation that is suppressed by strong intrarow intermolecular interactions resulting from the hydrogen bonding of the urea groups, but favored by terminal short alkoxy groups. The present information is useful for understanding the mechanism of the template-guided aryl radical grafting and the molecular design of new generations of template molecules.

Direct C-H Arylation of Dithiophene-Tetrathiafulvalene: Tuneable Electronic Properties and 2D Self-Assembled Molecular Networks at the Solid/Liquid Interface.

Tetrathiafulvalene is among the best known building blocks in molecular electronics due to its outstanding electron-donating and redox properties. Among its derivatives, dithiophene-tetrathiafulvalene (DT-TTF) has attracted considerable interest in organic electronics, owing to its high field-effect mobility. Herein, we report the direct C-H arylation of DT-TTF to synthesise mono- and tetraarylated derivatives functionalised with electron-withdrawing and electron-donating groups in order to evaluate their influence on the electronic properties by cyclic voltammetry, UV-vis spectroscopy and theoretical calculations. Self-assembly of the DT-TTF-tetrabenzoic acid derivative was studied by using scanning tunnelling microscopy (STM) which revealed the formation of ordered, densely packed 2D hydrogen-bonded networks at the graphite/liquid interface. The tetrabenzoic acid derivative can attain a planar geometry on the graphite surface due to van der Waals interactions with the surface and H-bonding with neighbouring molecules. This study demonstrates a simple method for the synthesis of arylated DT-TTF derivatives towards the design and construction of novel π-extended electroactive frameworks.

A BN-Doped U-Shaped Heteroacene as a Molecular Floating Gate for Ambipolar Charge Trapping Memory.

Two wide-band gap U-shaped polycyclic aromatic hydrocarbons with/without boron and nitrogen (BN-) doping (BN-1 and C-1) were synthesized to tune the electronic features to suit the performance requirements for organic field-effect transistor memory (OFET-NVM). The chemical structures were characterized by scanning tunneling microscopy and single-crystal diffraction. Owing to the electron-donor effect of N and the high electron affinity of B, the BN-1-based OFET-NVM displays large ambipolar memory windows and an enhanced charge storage density compared to C-1 and most reported small molecules. A novel supramolecular system formed from BN-1 and PMMA contributes to fabricating uniform films with homogeneous microstructures, which serve as a two-in-one tunnelling dielectric and charge-trapping layer to realize long-term charge retention and reliable endurance. Our results demonstrate that both BN doping and supramolecular engineering are crucial for the charge trapping of OFET-NVM.

Spontaneous and scanning-assisted desorption-adsorption dynamics in porous supramolecular networks at the solution-solid interface.

Self-assembled molecular networks (SAMNs) are formed by the spontaneous assembly of molecules on surfaces. On conductive atomically flat surfaces, and also at the liquid-solid interface, scanning tunneling microscopy (STM) can follow their growth dynamics. Desorption and adsorption dynamics are difficult to probe through the liquid-solid interface. Porous molecular networks are of particular interest because they may act as platforms for sensing and host-guest chemistry. Very little is known though about their stability, particularly in a liquid environment. To this end, we have investigated the desorption/adsorption dynamics of supramolecular porous monolayers of alkoxylated dehydrobenzo[12]annulene (DBA) derivatives at the interface between highly oriented pyrolytic graphite, the substrate, and 1-phenyloctane, the liquid. To trace the dynamics, structurally analogous chiral DBA derivatives were used as marker molecules, which co-assemble with the achiral ones forming the supramolecular network. This approach reveals the time scales of the adsorption/desorption dynamics, the significance of temperature, and the important role of the STM tip in inducing dynamics.

Solvent Mediated Nanoscale Quasi-Periodic Chirality Reversal in Self-Assembled Molecular Networks Featuring Mirror Twin Boundaries.

Grain boundaries in polycrystals have a prominent impact on the properties of a material, therefore stimulating the research on grain boundary engineering. Structure determination of grain boundaries of molecule-based polycrystals with submolecular resolution remains elusive. Reducing the complexity to monolayers has the potential to simplify grain boundary engineering and may offer real-space imaging with submolecular resolution using scanning tunneling microscopy (STM). Herein, the authors report the observation of quasi-periodic nanoscale chirality switching in self-assembled molecular networks, in combination with twinning, as revealed by STM at the liquid/solid interface. The width of the chiral domain structure peaks at 12-19 nm. Adjacent domains having opposite chirality are connected continuously through interdigitated alkoxy chains forming a 1D defect-free domain border, reflecting a mirror twin boundary. Solvent co-adsorption and the inherent conformational adaptability of the alkoxy chains turn out to be crucial factors in shaping grain boundaries. Moreover, the epitaxial interaction with the substrate plays a role in the nanoscale chirality reversal as well.

Zigzag-Edged Polycyclic Aromatic Hydrocarbons from Benzo[m]tetraphene Precursors.

A series of zigzag-edged polycyclic aromatic hydrocarbons (PAHs) (Z1-Z3) were synthesized from 2,12-dibromo-7,14-diphenyl-benzo[m]tetraphene (9) as a versatile building block. Their structures were unambiguously confirmed by laser desorption/ionization time-of-flight mass spectrometry, 1 H NMR, Raman, and Fourier-transformed infrared (FTIR) spectroscopies as well as scanning tunneling microscopy. The fingerprint vibrational modes were elucidated with theoretical support. The edge- and size-dependent optical properties were characterized by UV-Vis absorption and fluorescence spectroscopy and DFT calculations. Moreover, ultrafast transient absorption spectroscopy revealed distinct modulation of the photophysical properties upon π-extension from Z1 to Z2, the latter having a gulf edge.

Metal Ion and Guest-Mediated Spontaneous Resolution and Solvent-Induced Chiral Symmetry Breaking in Guanine-Based Metallosupramolecular Networks.

Two-dimensional (2D) chirality has been actively studied in view of numerous applications of chiral surfaces such as in chiral resolutions and enantioselective catalysis. Here, we report on the expression and amplification of chirality in hybrid 2D metallosupramolecular networks formed by a nucleobase derivative. Self-assembly of a guanine derivative appended with a pyridyl node was studied at the solution-graphite interface in the presence and absence of coordinating metal ions. In the absence of coordinating metal ions, a monolayer that is representative of a racemic compound was obtained. This system underwent spontaneous resolution upon addition of a coordinating ion and led to the formation of a racemic conglomerate. The spontaneous resolution could also be achieved upon addition of a suitable guest molecule. The mirror symmetry observed in the formation of the metallosupramolecular networks could be broken via the use of an enantiopure solvent, which led to the formation of a globally homochiral surface.

Host-guest chemistry under confinement: peeking at early self-assembly events.

Nanoscopic lateral confinement created on a graphite surface enabled the study of embryonic stages of molecular self-assembly on solid surfaces using scanning tunneling microscopy performed at the solution/solid interface.

"Concentration-in-Control" self-assembly concept at the liquid-solid interface challenged.

Self-assembled molecular networks (SAMNs) on surfaces evoke a lot of interest, both from a fundamental as well as application point of view. When formed at the liquid-solid interface, precise control over different polymorphs can be achieved by simply adjusting the concentration of molecular building blocks in solution. Significant influence of solute concentration on self-assembly behavior has been observed, whether the self-assembly behavior is controlled by either van der Waals forces or hydrogen bonding interactions. In both cases, high- and low-density supramolecular networks have been observed at high and low solute concentrations, respectively. In contrast to this "concentration-in-control" self-assembly concept here we report an atypical concentration dependent self-assembly behavior at a solution-solid interface. At the interface between heptanoic acid (HA) and highly oriented pyrolytic graphite (HOPG), we show, using scanning tunneling microscopy (STM), the formation of a low-density porous network at high solute concentrations, and a high-density compact network at low solute concentrations. This intriguing inverse concentration dependent self-assembly behavior has been attributed to the preaggregation of solute molecules in the heptanoic acid solution as revealed by UV-vis spectroscopy. The observed results have been correlated to the molecular density of self-assembled monolayers attained at the HA/HOPG interface.

Detection and Stabilization of a Previously Unknown Two-Dimensional (Pseudo)polymorph using Lateral Nanoconfinement.

We report on the detection and stabilization of a previously unknown two-dimensional (2D) pseudopolymorph of an alkoxy isophthalic acid using lateral nanoconfinement. The self-assembled molecular networks formed by the isophthalic acid derivative were studied at the interface between covalently modified graphite and an organic solvent. When self-assembled on graphite with moderate surface coverage of covalently bound aryl groups, a previously unknown metastable pseudopolymorph was detected. This pseudopolymorph, which was presumably "trapped" in between the surface bound aryl groups, underwent a time-dependent phase transition to the stable polymorph typically observed on pristine graphite. The stabilization of the pseudopolymorph was then achieved by using an alternative nanoconfinement strategy, where the domains of the pseudopolymorph could be formed and stabilized by restricting the self-assembly in nanometer-sized shallow compartments produced by STM-based nanolithography carried out on a graphite surface with a high density of covalently bound aryl groups. These experimental results are supported by molecular mechanics and molecular dynamics simulations, which not only provide important insight into the relative stabilities of the different structures, but also shed light onto the mechanism of the formation and stabilization of the pseudopolymorph under nanoscopic lateral confinement.

2D self-assembly and electronic characterization of oxygen-boron-oxygen-doped chiral graphene nanoribbons.

Graphene nanoribbons (GNRs), quasi-one-dimensional strips of graphene, exhibit a nonzero bandgap due to quantum confinement and edge effects. In the past decade, different types of GNRs with atomically precise structures have been synthesized by a bottom-up approach and have attracted attention as a novel class of semiconducting materials for applications in electronics and optoelectronics. We report the large-scale, inexpensive growth of high-quality oxygen-boron-oxygen-doped chiral GNRs with a defined structure using chemical vapor deposition. For the first time, a regular 2D self-assembly of such GNRs has been demonstrated, which results in a unique orthogonal network of GNRs. Stable and large-area GNR films with an optical bandgap of ∼1.9 eV were successfully transferred onto insulating substrates. This ordered network structure of semiconducting GNRs holds promise for controlled device integration.

Multicomponent Covalent Chemical Patterning of Graphene.

The chemical patterning of graphene is being pursued tenaciously due to exciting possibilities in electronics, catalysis, sensing, and photonics. Despite the intense efforts, spatially controlled, multifunctional covalent patterning of graphene has not been achieved. The lack of control originates from the inherently poor reactivity of the basal plane of graphene, which necessitates the use of harsh chemistries. Here, we demonstrate spatially resolved multicomponent covalent chemical patterning of single layer graphene using a facile and efficient method. Three different functional groups could be covalently attached to the basal plane in dense, well-defined patterns using a combination of lithography and a self-limiting variant of diazonium chemistry requiring no need for graphene activation. The layer thickness of the covalent films could be controlled down to 1 nm. This work provides a solid foundation for the fabrication of chemically patterned multifunctional graphene interfaces for device applications.

Molecular dopant determines the structure of a physisorbed self-assembled molecular network.

A small percentage of an impurity was shown, via scanning tunneling microscopy, to drastically change the on-surface self-assembly behavior of an aromatic tetracarboxylic acid, by initiating the nucleation and growth of a different polymorph. Molecular modelling simulations were used to shed further light onto the dopant-controlled assembly behaviour.