The effect of water on gold supported chiral graphene nanoribbons: rupture of conjugation by an alternating hydrogenation pattern.

In the last few years we have observed a breakpoint in the development of graphene-derived technologies, such as liquid phase filtering and their application to electronics. In most of these cases, they imply exposure of the material to solvents and ambient moisture, either in the fabrication of the material or the final device. The present study demonstrates the sensitivity of graphene nanoribbon (GNR) zigzag edges to water, even in extremely low concentrations. We have addressed the unique reactivity of (3,1)-chiral GNR with moisture on Au(111). Water shows a reductive behaviour, hydrogenating the central carbon of the zigzag segments. By combining scanning tunnelling microscopy (STM) with simulations, we demonstrate how their reactivity reaches a thermodynamic limit when half of the unit cells are reduced, resulting in an alternating pattern of hydrogenated and pristine unit cells starting from the terminal segments. Once a quasi-perfect alternation is reached, the reaction stops regardless of the water concentration. The hydrogenated segments limit the electronic conjugation of the GNR, but the reduction can be reversed both by tip manipulation and annealing. Selective tip-induced dehydrogenation allowed the stabilization of radical states at the edges of the ribbons, while the annealing of the sample completely recovered the original, pristine GNR.

On-surface synthesis of non-benzenoid conjugated polymers by selective atomic rearrangement of ethynylarenes.

Here, we report a new on-surface synthetic strategy to precisely introduce five-membered units into conjugated polymers from specifically designed precursor molecules that give rise to low-bandgap fulvalene-bridged bisanthene polymers. The selective formation of non-benzenoid units is finely controlled by the annealing parameters, which govern the initiation of atomic rearrangements that efficiently transform previously formed diethynyl bridges into fulvalene moieties. The atomically precise structures and electronic properties have been unmistakably characterized by STM, nc-AFM, and STS and the results are supported by DFT theoretical calculations. Interestingly, the fulvalene-bridged bisanthene polymers exhibit experimental narrow frontier electronic gaps of 1.2 eV on Au(111) with fully conjugated units. This on-surface synthetic strategy can potentially be extended to other conjugated polymers to tune their optoelectronic properties by integrating five-membered rings at precise sites.

Unusual Scaffold Rearrangement in Polyaromatic Hydrocarbons Driven by Concerted Action of Single Gold Atoms on a Gold Surface.

Chemical transformation of polyaromatic hydrocarbon (PAH) molecules following different reaction strategies has always been the focus of organic synthesis. In this work, we report the synthesis of a PAH molecule, formation of which consists of an unusual C-C bond cleavage accompanied by a complex π-conjugated molecular scaffold rearrangement. We demonstrate that the complex chemical transformation is steered by concerted motion of individual Au0 gold atoms on a supporting Au(111) surface. This observation underpins the importance of single-atom catalysis mediated by adatoms in on-surface synthesis as well as catalytic activity of single Au0 atoms facilitating cleavage of covalent carbon bonds.

Resolving Atomic-Scale Defects in Conjugated Polymers On-Surfaces.

Atomic scale defects significantly affect the mechanical, electronic, and optical properties of π-conjugated polymers. Here, isolated atomic-scale defects are deliberately introduced into a prototypical anthracene-ethynylene π-conjugated polymer, and its local density of states is carefully examined on the atomic scale to show how individual defects modify the inherent electronic and magnetic properties of this one-dimensional systems. Scanning tunneling and atomic force microscopy experiments, supplemented with density functional theory calculations, reveal the existence of a sharp electronic resonance at the Fermi energy around certain defects, which is associated with the formation of a local magnetic moment accompanied by substantial mitigation of the mobility of charge carriers. While defects in traditionally synthesized polymers lead to arbitrary conformations, the presented results clearly reflect the preferential formation of low dimensional defects at specific polymer sites, which may introduce the possibility of engineering macroscopic defects in surface-synthesized conjugated polymers.

On-Surface Synthesis of a Dicationic Diazahexabenzocoronene Derivative on the Au(111) Surface.

The atomically precise control over the size, shape and structure of nanographenes (NGs) or the introduction of heteroatom dopants into their sp2 -carbon lattice confer them valuable electronic, optical and magnetic properties. Herein, we report on the design and synthesis of a hexabenzocoronene derivative embedded with graphitic nitrogen in its honeycomb lattice, achieved via on-surface assisted cyclodehydrogenation on the Au(111) surface. Combined scanning tunnelling microscopy/spectroscopy and non-contact atomic force microscopy investigations unveil the chemical and electronic structures of the obtained dicationic NG. Kelvin probe force microscopy measurements reveal a considerable variation of the local contact potential difference toward lower values with respect to the gold surface, indicative of its positive net charge. Altogether, we introduce the concept of cationic nitrogen doping of NGs on surfaces, opening new avenues for the design of novel carbon nanostructures.

On-Surface Strain-Driven Synthesis of Nonalternant Non-Benzenoid Aromatic Compounds Containing Four- to Eight-Membered Rings.

The synthesis of polycyclic aromatic hydrocarbons containing various non-benzenoid rings remains a big challenge facing contemporary organic chemistry despite a considerable effort made over the last decades. Herein, we present a novel route, employing on-surface chemistry, to synthesize nonalternant polycyclic aromatic hydrocarbons containing up to four distinct kinds of non-benzenoid rings. We show that the surface-induced mechanical constraints imposed on strained helical reactants play a decisive role leading to the formation of products, energetically unfavorable in solution, with a peculiar ring current stabilizing the aromatic character of the π-conjugated system. Determination of the chemical and electronic structures of the most frequent product reveals its closed-shell character and low band gap. The present study renders a new route for the synthesis of novel nonalternant polycyclic aromatic hydrocarbons or other hydrocarbons driven by internal stress imposed by the surface not available by traditional approaches of organic chemistry in solution.

On-Surface Synthesis of One-Dimensional Coordination Polymers with Tailored Magnetic Anisotropy.

One-dimensional (1D) metalloporphyrin polymers can exhibit magnetism, depending on the central metal ion and the surrounding ligand field. The possibility of tailoring the magnetic signal in such nanostructures is highly desirable for potential spintronic devices. We present low-temperature (4.2 K) scanning tunneling microscopy and spectroscopy (LT-STM/STS) in combination with high-resolution atomic force microscopy (AFM) and a density functional theory (DFT) study of a two-step synthetic protocol to grow a robust Fe-porphyrin-based 1D polymer on-surface and to tune its magnetic properties. A thermally assisted Ullmann-like coupling reaction of Fe(III)diphenyl-bromine-porphyrin (2BrFeDPP-Cl) on Au(111) in ultra-high vacuum results in long (up to 50 nm) 1D metal-organic wires with regularly distributed magnetic and (electronically) independent porphyrins units, as confirmed by STM images. Thermally controlled C-H bond activation leads to conformational changes in the porphyrin units, which results in molecular planarization steered by 2D surface confinement, as confirmed by high-resolution AFM images. Spin-flip STS images in combination with DFT self-consistent spin-orbit coupling calculations of porphyrin units with different structural conformations reveal that the magnetic anisotropy of the triplet ground state of the central Fe ion units drops down substantially upon intramolecular rearrangements. These results point out to new opportunities for realizing and studying well-defined 1D organic magnets on surfaces and demonstrate the feasibility of tailoring their magnetic properties.

Tailoring π-conjugation and vibrational modes to steer on-surface synthesis of pentalene-bridged ladder polymers.

The development of synthetic strategies to engineer π-conjugated polymers is of paramount importance in modern chemistry and materials science. Here we introduce a synthetic protocol based on the search for specific vibrational modes through an appropriate tailoring of the π-conjugation of the precursors, in order to increase the attempt frequency of a chemical reaction. First, we design a 1D π-conjugated polymer on Au(111), which is based on bisanthene monomers linked by cumulene bridges that tune specific vibrational modes. In a second step, upon further annealing, such vibrational modes steer the twofold cyclization reaction between adjacent bisanthene moieties, which gives rise to a long pentalene-bridged conjugated ladder polymer featuring a low bandgap. In addition, high resolution atomic force microscopy allows us to identify by atomistic insights the resonance form of the polymer, thus confirming the validity of the Glidewell and Lloyd´s rules for aromaticity. This on-surface synthetic strategy may stimulate exploiting previously precluded reactions towards π-conjugated polymers with specific structures and properties.

Tailoring topological order and π-conjugation to engineer quasi-metallic polymers.

Topological band theory predicts that a topological electronic phase transition between two insulators must proceed via closure of the electronic gap. Here, we use this transition to circumvent the instability of metallic phases in π-conjugated one-dimensional (1D) polymers. By means of density functional theory, tight-binding and GW calculations, we predict polymers near the topological transition from a trivial to a non-trivial quantum phase. We then use on-surface synthesis with custom-designed precursors to make polymers consisting of 1D linearly bridged acene moieties, which feature narrow bandgaps and in-gap zero-energy edge states when in the topologically non-trivial phase close to the topological transition point. We also reveal the fundamental connection between topological classes and resonant forms of 1D π-conjugated polymers.