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.

Controlling On-Surface Photoactivity: The Impact of π-Conjugation in Anhydride-Functionalized Molecules on a Semiconductor Surface.

On-surface synthesis has become a prominent method for growing low-dimensional carbon-based nanomaterials on metal surfaces. However, the necessity of decoupling organic nanostructures from metal substrates to exploit their properties requires either transfer methods or new strategies to perform reactions directly on inert surfaces. The use of on-surface light-induced reactions directly on semiconductor/insulating surfaces represents an alternative approach to address these challenges. Here, exploring the photochemical activity of different organic molecules on a SnSe semiconductor surface under ultra-high vacuum, we present a novel on-surface light-induced reaction. The selective photodissociation of the anhydride group is observed, releasing CO and CO2. Moreover, we rationalize the relationship between the photochemical activity and the π-conjugation of the molecular core. The different experimental behaviour of two model anhydrides was elucidated by theoretical calculations, showing how the molecular structure influences the distribution of the excited states. Our findings open new pathways for on-surface synthesis directly on technologically relevant substrates.

Highly Stereospecific On-Surface Dimerization into Bishelicenes: Topochemical Ullmann Coupling of Bromohelicene on Au(111).

The on-surface dimerization into bis(hexahelicene) on a gold(111) surface has been studied by means of scanning tunneling microscopy and time-of-flight secondary mass spectrometry. C-C Ullmann coupling of (rac)-2-bromo-hexahelicene leads to formation of the (M,M)- and (P,P)-diastereomers of 2,2'-bis(hexahelicene), whilst formation of the (M,P)-diastereomer is not observed. Upon cooling, the bis(hexahelicene) aggregates into an ordered two-dimensional lattice with partly randomly distributed enantiomers. The highly specific diastereomeric coupling is explained by the surface alignment of educt in combination with the strong steric overcrowding in a possible surface-confined (M,P)-product.

Correlation between Electronic Configuration and Magnetic Stability in Dysprosium Single Atom Magnets.

Single atom magnets offer the possibility of magnetic information storage in the most fundamental unit of matter. Identifying the parameters that control the stability of their magnetic states is crucial to design novel quantum magnets with tailored properties. Here, we use X-ray absorption spectroscopy to show that the electronic configuration of dysprosium atoms on MgO(100) thin films can be tuned by the proximity of the metal Ag(100) substrate onto which the MgO films are grown. Increasing the MgO thickness from 2.5 to 9 monolayers induces a change in the dysprosium electronic configuration from 4f9 to 4f10. Hysteresis loops indicate long magnetic lifetimes for both configurations, however, with a different field-dependent magnetic stability. Combining these measurements with scanning tunneling microscopy, density functional theory, and multiplet calculations unveils the role of the adsorption site and charge transfer to the substrate in determining the stability of quantum states in dysprosium single atom magnets.

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.

On-Surface Hydrogen/Deuterium Isotope Exchange in Polycyclic Aromatic Hydrocarbons.

Hydrogen plays important roles in the on-surface synthesis of carbon-based materials in ultra-high vacuum. The complex interplay between hydrogen and surface-adsorbed polycyclic aromatic hydrocarbons (PAHs) is tracked by in situ time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with isotope labeling. In situ deuterium labeling of prototypical PAHs, coronene (CR) and 7-armchair graphene nanoribbons (GNRs), on Au(111) is achieved by annealing either in D2 gas or in the vapor of perdeuterio-acenaphthene. By following the mass spectra of in situ deuterated CR mixed with hydrogen-CR, it is demonstrated that PAHs adsorbed at hot Au(111) surfaces continuously exchange hydrogen atoms. Also, D2 present during the Ullmann coupling step leads to incorporation of deuterium and to shorter GNRs.

1D Coordination π-d Conjugated Polymers with Distinct Structures Defined by the Choice of the Transition Metal: Towards a New Class of Antiaromatic Macrocycles.

Recently π-d conjugated coordination polymers have received a lot of attention owing to their unique material properties, although synthesis of long and defect-free polymers remains challenging. Herein we introduce a novel on-surface synthesis of coordination polymers with quinoidal ligands under ultra-high vacuum conditions, which enables formation of flexible coordination polymers with lengths up to hundreds of nanometers. Moreover, this procedure allows the incorporation of different transition-metal atoms with four- or two-fold coordination. Remarkably, the two-fold coordination mode revealed the formation of wires constituted by (electronically) independent 12-membered antiaromatic macrocycles linked together through two C-C single bonds.

Graphene Grown from Flat and Bowl Shaped Polycyclic Aromatic Hydrocarbons on Cu(111).

The growth of carbon layers, defective graphene, and graphene by deposition of polycyclic aromatic hydrocarbons (PAHs) on Cu(111) is studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Two different PAHs are used as starting materials: the buckybowl pentaindenocorannulene (PIC) which contains pentagonal rings and planar coronene (CR). For both precursors, with increasing sample temperature during deposition, porous carbon aggregates (350 °C), dense carbon layers (400-450 °C), disordered defective graphene (500 °C-550 °C), and extended graphene (≥600 °C) are obtained. No significant differences for defective graphene grown from PIC and CR are observed. C 1s X-ray photoelectron spectra of PIC and CR derived samples grown at 350-550 °C exhibit a characteristic C-Cu low binding energy component. Preparation at ≥600 °C eliminates this C-Cu species and only C-C bonded carbon remains.

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.

Stereospecific Autocatalytic Surface Explosion Chemistry of Polycyclic Aromatic Hydrocarbons.

Autocatalytic processes are important in many fields of science, including surface chemistry. A better understanding of its mechanisms may improve the current knowledge on heterogeneous catalysis. The thermally induced decomposition of eight different polycyclic aromatic hydrocarbons (PAHs) on a saturated monolayer of atomic oxygen on a Cu(100) surface is studied using temperature-programmed reaction spectroscopy (TPRS), X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). 9-Bromo-heptahelicene decomposes autocatalytically in a narrow temperature range into CO2 and H2O, while non-halogenated heptahelicene decomposes into the same products but does not show autocatalytic behavior. Fixation of the hydrocarbon to the surface via the organometallic bond after elimination of the bromine is identified as a prerequisite for the autocatalytic reaction mechanism. Of all the hydrocarbons studied, only those being sterically overcrowded decompose autocatalytically. Such an observation can be explained by facile dehydrogenation of the overcrowded PAHs. The reaction of such hydrogen with oxygen creates vacancies in the oxygen layer which act as active sites and catalyze further decomposition.

Chiral molecules adsorbed on a solid surface: Tartaric acid diastereomers and their surface explosion on Cu(111).

The adsorption of diastereoisomers of tartaric acid, namely, meso (R,S)-tartaric acid, (R,R)-tartaric acid, and the racemic mixture of (R,R) and (S,S) tartaric acid on the (111) surface of a copper single crystal has been studied by means of reflection-absorption IR Spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction, and thermal desorption spectroscopy. Two distinct adsorption modes are identified for all three adsorbate systems. All molecules undergo an identical thermally induced autocatalytic decomposition reaction above 510 K. The pure enantiomers show 2D chiral long-range ordered structures of opposite handedness.

Molecular Chessboard Assemblies Sorted by Site-Specific Interactions of Out-of-Plane d-Orbitals with a Semimetal Template.

We show that highly ordered two-dimensional (2D) chessboard arrays consisting of a periodic arrangement of two different molecules can be obtained by self-assembly of unsubstituted metal-phthalocyanines (metal-Pcs) on a suitable substrate serving as the template. Specifically, CuPc + MnPc and CuPc + CoPc mixtures sort into highly ordered Cu/Mn and Cu/Co chessboard arrays on the square p(10 × 10) reconstruction of bismuth on Cu(100). Such created bimolecular chessboard assemblies emerge from the site-specific interactions between the central transition-metal ions and the periodically reconstructed substrate. This work provides a conceptually new approach to induce 2D chessboard patterns in that no functionalization of the molecules is needed.

Magnetic Hysteresis in Er Trimers on Cu(111).

We report magnetic hysteresis in Er clusters on Cu(111) starting from the size of three atoms. Combining X-ray magnetic circular dichroism, scanning tunneling microscopy, and mean-field nucleation theory, we determine the size-dependent magnetic properties of the Er clusters. Er atoms and dimers are paramagnetic, and their easy magnetization axes are oriented in-plane. In contrast, trimers and bigger clusters exhibit magnetic hysteresis at 2.5 K with a relaxation time of 2 min at 0.1 T and out-of-plane easy axis. This appearance of magnetic stability for trimers coincides with their enhanced structural stability.

Superlattice of Single Atom Magnets on Graphene.

Regular arrays of single atoms with stable magnetization represent the ultimate limit of ultrahigh density storage media. Here we report a self-assembled superlattice of individual and noninteracting Dy atoms on graphene grown on Ir(111), with magnetic hysteresis up to 5.6 T and spin lifetime of 1000 s at 2.5 K. The observed magnetic stability is a consequence of the intrinsic low electron and phonon densities of graphene and the 6-fold symmetry of the adsorption site. Our array of single atom magnets has a density of 115 Tbit/inch2, defined by the periodicity of the graphene moiré pattern.

Configuring Electronic States in an Atomically Precise Array of Quantum Boxes.

A 2D array of electronically coupled quantum boxes is fabricated by means of on-surface self-assembly assuring ultimate precision of each box. The quantum states embedded in the boxes are configured by adsorbates, whose occupancy is controlled with atomic precision. The electronic interbox coupling can be maintained or significantly reduced by proper arrangement of empty and filled boxes.

Controlling the dimensionality of on-surface coordination polymers via endo- or exoligation.

The formation of on-surface coordination polymers is controlled by the interplay of chemical reactivity and structure of the building blocks, as well as by the orientating role of the substrate registry. Beyond the predetermined patterns of structural assembly, the chemical reactivity of the reactants involved may provide alternative pathways in their aggregation. Organic molecules, which are transformed in a surface reaction, may be subsequently trapped via coordination of homo- or heterometal adatoms, which may also play a role in the molecular transformation. The amino-functionalized perylene derivative, 4,9-diaminoperylene quinone-3,10-diimine (DPDI), undergoes specific levels of dehydrogenation (-1 H2 or -3 H2) depending on the nature of the present adatoms (Fe, Co, Ni or Cu). In this way, the molecule is converted to an endo- or an exoligand, possessing a concave or convex arrangement of ligating atoms, which is decisive for the formation of either 1D or 2D coordination polymers.

Light-Controlled Multiconfigurational Conductance Switching in a Single 1D Metal-Organic Wire.

Precise control of multiple spin states on the atomic scale presents a promising avenue for designing and realizing magnetic switches. Despite substantial progress in recent decades, the challenge of achieving control over multiconfigurational reversible switches in low-dimensional nanostructures persists. Our work demonstrates multiple, fully reversible plasmon-driven spin-crossover switches in a single π-d metal-organic chain suspended between two electrodes. The plasmonic nanocavity stimulated by external visible light allows for reversible spin crossover between low- and high-spin states of different cobalt centers within the chain. We show that the distinct spin configurations remain stable for minutes under cryogenic conditions and can be nonperturbatively detected by conductance measurements. This multiconfigurational plasmon-driven spin-crossover demonstration extends the available toolset for designing optoelectrical molecular devices based on SCO compounds.

Chirality transfer in 1D self-assemblies: influence of H-bonding vs metal coordination between dicyano[7]helicene enantiomers.

Chiral recognition as well as chirality transfer in supramolecular self-assembly and on-surface coordination is studied for the enantiopure 6,13-dicyano[7]helicene building block. It is remarkable that, with this helical molecule, both H-bonded chains and metal-coordinated chains can be formed on the same substrate, thereby allowing for a direct comparison of the chain bonding motifs and their effects on the self-assembly in experiment and theory. Conformational flexure and both adsorbate/adsorbent and intermolecular interactions can be identified as factors influencing the chiral recognition at the binding site. The observed H-bonded chains are chiral, however, the overall appearance of Cu-coordinated chains is no longer chiral. The study was performed via scanning tunneling microscopy, X-ray-photoelectron spectroscopy and density functional theory calculations. We show a significant influence of the molecular flexibility and the type of bonding motif on the chirality transfer in the 1D self-assembly.

On-surface coordination chemistry: direct imaging of the conformational freedom of an axial ligand at room temperature.

The on-surface ligation of nitric oxide (NO) with Co-tetraphenylporphyrin (CoTPP) sublimed onto oxygen-reconstructed Ni(001) is studied using room-temperature scanning tunneling microscopy (STM) and complementary photoemission spectroscopies. On the oxygen-reconstructed substrates, the porphyrins are observed to form well-ordered, self-assembled layers. STM directly images the NO ligand as a characteristic feature in the center of the molecule. Under certain STM imaging conditions the dynamicity of this feature can be related to the temperature-activated conformational flexibility of the NO ligand. This provides an indirect confirmation of the bending of the Co-NO bond, as predicted from classical coordination chemistry.

Ammonia coordination introducing a magnetic moment in an on-surface low-spin porphyrin.

Amazing ammonia: The molecular spin state of Ni(II) porphyrin, supported on a ferromagnetic Co surface, can be reversibly switched between spin-off (S = 0) and spin-on (S = 1) states upon coordination and decoordination of the gaseous ligand NH3, respectively (see picture). This finding clearly indicates the possible use of the system as a single-molecule-based magnetochemical sensor and in spintronics.