Strain-induced skeletal rearrangement of a polycyclic aromatic hydrocarbon on a copper surface

Molecular Symmetry and Art: Visualizing the Near-Symmetry of Molecules in Piet Mondrian's De Stijl

Symmetry and shape are essential aspects of molecular structure and how we interpret molecules and their properties. We, as chemists, are comfortable with pictorial representations of structure, in which some nuance is lost-investigating molecular shape numerically by looking at how closely it fits a reference, such as a plane, or a set of vectors or coordinates, is informative, though far from engaging. Often relationships between chemical structure and derived values are obscured. Taking our inspiration from Piet Mondrian's Compositions, we have depicted the symmetry information encoded within 3D data as blocks of color, to show clearly how chemical arguments and resultant molecular distortion may contribute to symmetry. Great art gives us a new perspective on the world; as a pastiche, this art may allow us to look at familiar molecules, such as porphyrins, in a new light, understanding how their shape and properties are intertwined.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

Structural Transformation of Azonia[5]helicene Photoproduct via Reaction Field Function of Layered Inorganic Material

In an attempt to generalize "on surface synthesis", which has unique potential in the area of organic synthesis, the focus was placed on layered silicates having a highly flat surface. The photoreaction of (±)-13-bromo-6a-azonia[5]helicene (AHHBr) and (±)-2-bromo-13-methyl-6a-azonia[5]helicene (AHBrMe) in solution and within the layers was examined. In the case of AHBrMe, the photoproduct was different from that in solution. 1H nuclear magnetic resonance (NMR), Fourier transform-infrared spectroscopy (FT-IR), and electrospray ionization-mass spectrometry (ESI-MS) measurements revealed that the photoproduct obtained within the layers was a benzo-perylene molecule with a completely flat lactone structure (AL). This study is the first example of the successful conversion of a chemical reaction path due to the steric effect of the flat surface of layered silicate.

Highly Selective On-Surface Ring-Opening of Aromatic Azulene Moiety

Controllable ring-opening of polycyclic aromatic hydrocarbons plays a crucial role in various chemical and biological processes. However, breaking down aromatic covalent C-C bonds is exceptionally challenging due to their high stability and strong aromaticity. This study presents a seminal report on the precise and highly selective on-surface ring-opening of the seven-membered ring within the aromatic azulene moieties under mild conditions. The chemical structures of the resulting products were identified using bond-resolved scanning probe microscopy. Furthermore, through density functional theory calculations, we uncovered the mechanism behind the ring-opening process and elucidated its chemical driving force. The key to achieving this ring-opening process lies in manipulating the local aromaticity of the aromatic azulene moiety through strain-induced internal ring rearrangement and cyclodehydrogenation. By precisely controlling these factors, we successfully triggered the desired ring-opening reaction. Our findings not only provide valuable insights into the ring-opening process of polycyclic aromatic hydrocarbons but also open up new possibilities for the manipulation and reconstruction of these important chemical structures.

Stone-Wales Defect in Graphene

2D graphene the most investigated structures from nanocarbon family studied in the last three decades. It is projected as an excellent material useful for quantum computing, artificial intelligence, and next generation advanced technologies. Graphene exists in several forms and its extraordinary thermal, mechanical, and electronic properties, principally depend on the kind of perfection of the hexagonal atomic lattice. Defects are always considered as undesired components but certain defects in graphene could be an asset for electrochemistry and quantum electronics due to the engineered electronclouds and quantum tunnelling. The authors carefully discuss the Stone-Wales imperfections in graphene and its derivatives comprehensively. A specific emphasis is focused on the experimental and theoretical aspects of the Stone-Wales defects in graphene with respect to structure-property relationships. The corroboration of extrinsic defects like external atomic doping, functionalization, edge distortion in the graphene consisting of Stone-Wales imperfections, which are very significant in designing graphene-based electronic devices, are summarized.

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.

Manipulation of C-C coupling pathways using different annealing procedures

From scanning tunnelling microscopy and density functional theory calculations, we demonstrate that different annealing mechanisms could modulate distinct reaction pathways, where in a stepwise annealing procedure the detached Br atoms may reduce the activation barrier of CH activation resulting in hierarchical cross dehydrogenative coupling, while in a one-step annealing procedure only Ullmann coupling products are observed.

Step-Assisted On-Surface Synthesis of Graphene Nanoribbons Embedded with Periodic Divacancies

The bottom-up approach through on-surface synthesis of porous graphene nanoribbons (GNRs) presents a controllable manner for implanting periodic nanostructures to tune the electronic properties of GNRs in addition to bandgap engineering by width and edge configurations. However, owing to the existing steric hindrance in small pores like divacancies, it is still difficult to embed periodic divacancies with a nonplanar configuration into GNRs. Here, we demonstrate the on-surface synthesis of atomically precise eight-carbon-wide armchair GNRs embedded with periodic divacancies (DV8-aGNRs) by utilizing the monatomic step edges on the Au(111) surface. From a single molecular precursor correspondingly following a trans- and cis-coupling, the DV8-aGNR and another porous nanographene are respectively formed at step edges and on terraces at 720 and 570 K. Combining scanning tunneling microscopy/spectroscopy, atomic force microscopy, and first-principles calculations, we determine the out-of-plane conformation, wide bandgap (∼3.36 eV), and wiggly shaped frontier orbitals of the DV8-aGNR. Nudged elastic band calculations further quantitatively reveal that the additional steric hindrance effect in the cyclodehydrogenative reactions has a higher barrier of 1.3 eV than that in the planar porous nanographene, which also unveils the important role played by the monatomic Au step and adatoms in reducing the energy barriers and enhancing the thermodynamic preference of the oxidative cyclodehydrogenation. Our results provide the first case of GNRs containing periodic pores as small as divacancies with a nonplanar configuration and demonstrate the strategy by utilizing the chemical heterogeneity of a substrate to promote the formation of novel carbon nanomaterials.

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.

"In-water" Dehydration Reaction of an Aromatic Diol on an Inorganic Surface

The effect of a synthetic saponite surface on the "in-water" dehydration reaction of diol was examined using 4-formyl-1-methylquinolinium salt (MQu⁺) as a substrate. The equilibrium between aldehyde (MQu⁺-Aldehyde) and diol (MQu⁺-Diol) was affected by the surrounding environment. The equilibrium behavior was observed by ¹H nuclear magnetic resonance (NMR) and UV-vis absorption measurements. Although MQu⁺ was completely in the form of MQu⁺-Diol in water, the equilibrium almost shifted to the MQu⁺-Aldehyde side when MQu⁺ was adsorbed on the saponite surface in water. In addition, the MQu⁺-Aldehyde ratio depended on the negative charge density of saponite. The factors that determine MQu⁺-Aldehyde: MQu⁺-Diol ratio were discussed from the thermodynamic analysis of the system. These data indicate that the electrostatic interaction between the charged saponite surface and MQu⁺ stabilized the aldehyde side enthalpically and destabilized it entropically. The major reason for these results is considered to be the difference in adsorption stabilization between MQu⁺-Aldehyde and MQu⁺-Diol on saponite surfaces.

Surface-Mediated Ring-Opening and Porphyrin Deconstruction via Conformational Distortion

The breakdown of macrocyclic compounds is of utmost importance in manifold biological and chemical processes, usually proceeding via oxygenation-induced ring-opening reactions. Here, we introduce a surface chemical route to selectively break a prototypical porphyrin species, cleaving off one pyrrole unit and affording a tripyrrin derivative. This pathway, operational in an ultrahigh vacuum environment at moderate temperature is enabled by a distinct molecular conformation achieved via the specific interaction between the porphyrin and its copper support. We provide an atomic-level characterization of the surface-anchored tripyrrin, its reaction intermediates, and byproducts by bond-resolved atomic force microscopy, unequivocally identifying the molecular skeletons. The ring-opening is rationalized by the distortion reducing the macrocycle's stability. Our findings open a route to steer ring-opening reactions by conformational design and to study intriguing tetrapyrrole catabolite analogues on surfaces.

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.

Oxidative cyclo-rearrangement of helicenes into chiral nanographenes

Nanographenes are emerging as a distinctive class of functional materials for electronic and optical devices. It is of remarkable significance to enrich the precise synthetic chemistry for these molecules. Herein, we develop a facile strategy to recompose helicenes into chiral nanographenes through a unique oxidative cyclo-rearrangement reaction. Helicenes with 7~9 ortho-fused aromatic rings are firstly oxidized and cyclized, and subsequently rearranged into nanographenes with an unsymmetrical helicoid shape through sequential 1,2-migrations. Such skeletal reconstruction is virtually driven by the gradual release of the strain of the highly distorted helicene skeleton. Importantly, the chirality of the helicene precursor can be integrally inherited by the resulting nanographene. Thus, a series of chiral nanographenes are prepared from a variety of carbohelicenes and heterohelicenes. Moreover, such cyclo-rearrangement reaction can be sequentially or simultaneously associated with conventional oxidative cyclization reactions to ulteriorly enrich the geometry diversity of nanographenes, aiming at innovative properties.

Nanographenes are emerging as a distinctive class of functional materials for electronic and optical devices. Here, the authors develop a facile strategy to recompose helicenes into a variety of chiral nanographenes through an oxidative cyclo-rearrangement reaction.

A Fundamental Role of the Molecular Length in Forming Metal-Organic Hybrids of Phenol Derivatives on Silver Surfaces

In on-surface chemistry, the efficient preparation of metal-organic hybrids is regarded as a primary path to mediate controlled synthesis of well-ordered low-dimensional organic nanostructures. The fundamental mechanisms in forming these hybrid structures, however, are so far insufficiently explored. Here, with scanning tunneling microscopy, we studied the bonding behavior of the adsorbed phenol derivatives with different molecular lengths. We reveal that shorter molecules favor bonding with extracted metal adatoms and result in metal-organic hybrids, whereas longer molecules prefer to bond with lattice metal atoms. The conclusions are further confirmed by density functional theory calculations.

Defective Nanographenes Containing Seven-Five-Seven (7-5-7)-Membered Rings

Defects have been observed in graphene and are expected to play a key role in its optical, electronic, and magnetic properties. However, because most of the studies focused on the structural characterization, the implications of topological defects on the physicochemical properties of graphene remain poorly understood. Here, we demonstrate a bottom-up synthesis of three novel nanographenes (1-3) with well-defined defects in which seven-five-seven (7-5-7)-membered rings were introduced to their sp² carbon frameworks. From the X-ray crystallographic analysis, compound 1 adopts a nearly planar structure. Compound 2, with an additional five-membered ring compared to 1, possesses a slightly saddle-shaped geometry. Compound 3, which can be regarded as the "head-to-head" fusion of 1 with two bonds, features two saddles connected together. The resultant defective nanographenes 1-3 were well-investigated by UV-vis absorption, cyclic voltammetry, and time-resolved absorption spectra and further corroborated by density functional theory (DFT) calculations. Detailed experimental and theoretical investigations elucidate that these three nanographenes 1-3 exhibit an anti-aromatic character in their ground states and display a high stability under ambient conditions, which contrast with the reported unstable biradicaloid nanographenes that contain heptagons. Our work reported herein offers insights into the understanding of structure-related properties and enables the control of the electronic structures of expanded nanographenes with atomically precise defects.

On-surface synthesis of graphene nanostructures with π-magnetism

Graphene nanostructures (GNs) including graphene nanoribbons and nanoflakes have attracted tremendous interest in the field of chemistry and materials science due to their fascinating electronic, optical and magnetic properties. Among them, zigzag-edged GNs (ZGNs) with precisely-tunable π-magnetism hold great potential for applications in spintronics and quantum devices. To improve the stability and processability of ZGNs, substitutional groups are often introduced to protect the reactive edges in organic synthesis, which renders the study of their intrinsic properties difficult. In contrast to the conventional wet-chemistry method, on-surface bottom-up synthesis presents a promising approach for the fabrication of both unsubstituted ZGNs and functionalized ZGNs with atomic precision via surface-catalyzed transformation of rationally-designed precursors. The structural and spin-polarized electronic properties of these ZGNs can then be characterized with sub-molecular resolution by means of scanning probe microscopy techniques. This review aims to highlight recent advances in the on-surface synthesis and characterization of a diversity of ZGNs with π-magnetism. We also discuss the important role of precursor design and reaction stimuli in the on-surface synthesis of ZGNs and their π-magnetism origin. Finally, we will highlight the existing challenges and future perspective surrounding the synthesis of novel open-shell ZGNs towards next-generation quantum technology.

Manipulable Metal Catalyst for Nanographene Synthesis

Performing bottom-up synthesis by using molecules adsorbed on a surface is an effective method to yield functional polycyclic aromatic hydrocarbons (PAHs) and nanocarbon materials. The intramolecular cyclodehydrogenation of hydrocarbons is a critical process in this synthesis; however, thus far, its elementary steps have not been elucidated thoroughly. In this study, we utilize the metal tip of a low-temperature noncontact atomic force microscope as a manipulable metal surface to locally activate dehydrogenation for PAH-forming cyclodehydrogenation. This method leads to the dissociation of a H atom of an intermediate to yield the cyclodehydrogenated product in a target-selective and reproducible manner. We demonstrate the metal-tip-catalyzed dehydrogenation for both benzenoid and nonbenzonoid PAHs, suggesting its universal applicability as a catalyst for nanographene synthesis.

On-Surface Intramolecular Reactions

In the past decade, on-surface chemistry has provided fascinating concepts for the construction of covalently bonded molecular nanostructures and the exploration of new synthetic pathways that may be different from chemical synthesis in solution. Although the intermolecular reaction of precursor molecules may lead to the formation of the desired low-dimensional molecular architectures, it remains challenging to realize defect-free syntheses over large areas. Recently, intramolecular on-surface reactions have attracted increasing attention because they offer promising ways to synthesize functional organic molecules, especially those with extended conjugated π-systems. In this Perspective, we summarize the recent achievements in the field of on-surface intramolecular reactions and discuss future prospects.

The Macrocycle versus Chain Competition in On-Surface Polymerization: Insights from Reactions of 1,3-Dibromoazulene on Cu(111)

Ring/chain competition in oligomerization reactions represents a long‐standing topic of synthetic chemistry and was treated extensively for solution reactions but is not well‐understood for the two‐dimensional confinement of surface reactions. Here, the kinetic and thermodynamic principles of ring/chain competition in on‐surface synthesis are addressed by scanning tunneling microscopy, X‐ray photoelectron spectroscopy, and Monte Carlo simulations applied to azulene‐based organometallic oligomers on Cu(111). Analysis of experiments and simulations reveals how the ring/chain ratio can be controlled through variation of coverage and temperature. At room temperature, non‐equilibrium conditions prevail and kinetic control leads to preferential formation of the entropically favored chains. In contrast, high‐temperature equilibrium conditions are associated with thermodynamic control, resulting in increased yields of the energetically favored rings. The optimum conditions for ring formation include the lowest possible temperature within the regime of thermodynamic control and a low coverage. The general implications are discussed and compared to the solution case.

On-Surface Synthesis of Unsaturated Carbon Nanostructures with Regularly Fused Pentagon-Heptagon Pairs

Multiple fused pentagon–heptagon pairs are frequently found as defects at the grain boundaries of the hexagonal graphene lattice and are suggested to have a fundamental influence on graphene-related materials. However, the construction of sp²-carbon skeletons with multiple regularly fused pentagon–heptagon pairs is challenging. In this work, we found that the pentagon–heptagon skeleton of azulene was rearranged during the thermal reaction of an azulene-incorporated organometallic polymer on Au(111). The resulting sp²-carbon frameworks were characterized by high-resolution scanning probe microscopy techniques and feature novel polycyclic architectures composed of multiple regularly fused pentagon–heptagon pairs. Moreover, the calculated analysis of its aromaticity revealed a peculiar polar electronic structure.

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.

Quality control of on-surface-synthesised seven-atom wide armchair graphene nanoribbons

On-surface synthesis is a powerful method for fabricating atomically precise graphene nanoribbons (GNRs), but the products always include defective structures. In this study, scanning tunnelling microscopy and atomic force microscopy were used to determine the length distribution of armchair-edge GNRs with a width of seven carbon atoms (7-AGNRs) synthesised on Au(111) and to characterise defective structures. The product quality was improved by increasing the precursor deposition amount because of a preference for intermolecular polymerisation over intramolecular cyclodehydrogenation at a high coverage. However, the annealing rate had a complex effect on the quality, with a low rate elongating 7-AGNRs but degenerating the length uniformity. These insights advance the understanding of the critical parameters for obtaining high-quality products in high yield by on-surface synthesis.

Proving hydrogen addition mechanism from manure to coal surface obtained by GC-MS and 1H-NMR analysis

In this study, to explain the possibility of hydrogen transfer paths from manure to coal, Elbistan lignite (EL) combined with manure liquefaction of oil + gas products were analysed with Gas Chromatography-Mass Spectroscopy (GC-MS) and Nuclear Magnetic Resonance Spectroscopy (¹H-NMR) technique. In the same way, it is observed that oils which as they fragment to an alkane-alkene mixture, serve as a hydrogen “sponge” and put a serious hydrogen need on the parts of the free radicals and molecules that are currently hydrogen poor. Concerning Elbistan lignite and manure do not have any aromatic hydrogen. Moreover, when the aromatic compounds were hydrogenated, their aromatic hydrogen was transformed to naphthenic hydrogen. Hydrogen transfer was due to isomerization of heptane from 3-methylhexane obtained in test oil where only manure was present as hydrogen donor in the liquefaction environment despite hydrogenation of isomerization from naphthalene to azulene.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Nitrous oxide as an effective AFM tip functionalization: a comparative study

We investigate the possibility of functionalizing Au tips by N2O molecules deposited on a Au(111) surface and their further use for imaging with submolecular resolution. First, we characterize the adsorption of the N2O species on Au(111) by means of atomic force microscopy with CO-functionalized tips and density functional theory (DFT) simulations. Subsequently we devise a method of attaching a single N2O to a metal tip apex and benchmark its high-resolution imaging and spectroscopic capabilities using FePc molecules. Our results demonstrate the feasibility of high-resolution imaging. However, we find an inherent asymmetry of the N2O probe-particle adsorption on the tip apex, in contrast to a CO tip reference. These findings are consistent with DFT calculations of the N2O- and CO tip apexes.

Single spin localization and manipulation in graphene open-shell nanostructures

Turning graphene magnetic is a promising challenge to make it an active material for spintronics. Predictions state that graphene structures with specific shapes can spontaneously develop magnetism driven by Coulomb repulsion of π-electrons, but its experimental verification is demanding. Here, we report on the observation and manipulation of individual magnetic moments in graphene open-shell nanostructures on a gold surface. Using scanning tunneling spectroscopy, we detect the presence of single electron spins localized around certain zigzag sites of the carbon backbone via the Kondo effect. We find near-by spins coupled into a singlet ground state and quantify their exchange interaction via singlet-triplet inelastic electron excitations. Theoretical simulations picture how electron correlations result in spin-polarized radical states with the experimentally observed spatial distributions. Extra hydrogen atoms bound to radical sites quench their magnetic moment and switch the spin of the nanostructure in half-integer amounts. Our work demonstrates the intrinsic π-paramagnetism of graphene nanostructures.

Polyyne formation via skeletal rearrangement induced by atomic manipulation

Rearrangements that change the connectivity of a carbon skeleton are often useful in synthesis, but it can be difficult to follow their mechanisms. Scanning probe microscopy can be used to manipulate a skeletal rearrangement at the single-molecule level, while monitoring the geometry of reactants, intermediates and final products with atomic resolution. We studied the reductive rearrangement of 1,1-dibromo alkenes to polyynes on a NaCl surface at 5 K, a reaction that resembles the Fritsch–Buttenberg–Wiechell (FBW) rearrangement. Voltage pulses were used to cleave one C–Br bond, forming a radical, then to cleave the remaining C^•–Br bond triggering the rearrangement. These experiments provide structural insight into the bromo-vinyl radical intermediates, showing that the C=C^•–Br unit is nonlinear. Long polyynes, up to the octayne Ph–(C≡C)₈–Ph, have been prepared in this way. The control of skeletal rearrangements opens a new window on carbon-rich materials and extends the toolbox for molecular synthesis by atom manipulation.