Bond-Scission-Induced Structural Transformation from Cumulene to Diyne Moiety and Formation of Semiconducting Organometallic Polyyne

On-Surface Synthesis of Five-Membered Copper Metallacycles Using Terminal Alkynes

We present our studies on the adsorption, deprotonation, and reactions of 4,4″-diethynyl-1,1':4',1″-terphenyl on Cu(111) under ultrahigh-vacuum conditions using scanning tunneling microscopy combined with density functional theory calculations. Sequential annealing treatments induce deprotonation of pristine molecules followed by chemical reactions, resulting in branched nanostructures. Within the nanostructures, a previously unreported, double-spot linkage is observed. Our density functional theory calculations unravel that this linkage corresponds to a five-membered copper metallacycle.

On-Surface Covalent Synthesis of Carbon Nanomaterials by Harnessing Carbon gem-Polyhalides

The design of innovative carbon-based nanostructures stands at the forefront of both chemistry and materials science. In this context, π-conjugated compounds are of great interest due to their impact in a variety of fields, including optoelectronics, spintronics, energy storage, sensing and catalysis. Despite extensive research efforts, substantial knowledge gaps persist in the synthesis and characterization of new π-conjugated compounds with potential implications for science and technology. On-surface synthesis has emerged as a powerful discipline to overcome limitations associated with conventional solution chemistry methods, offering advanced tools to characterize the resulting nanomaterials. This review specifically highlights recent achievements in the utilization of molecular precursors incorporating carbon geminal (gem)-polyhalides as functional groups to guide the formation of π-conjugated 0D species, as well as 1D, quasi-1D π-conjugated polymers, and 2D nanoarchitectures. By delving into reaction pathways, novel structural designs, and the electronic, magnetic, and topological features of the resulting products, the review provides fundamental insights for a new generation of π-conjugated materials.

On-surface synthesis and characterization of polyynic carbon chains

Carbyne, an elusive sp-hybridized linear carbon allotrope, has fascinated chemists and physicists for decades. Due to its high chemical reactivity and extreme instability, carbyne was much less explored in contrast to the sp2-hybridized carbon allotropes such as graphene. Herein, we report the on-surface synthesis of polyynic carbon chains by demetallization of organometallic polyynes on the Au(111) surface; the longest one observed consists of ∼60 alkyne units (120 carbon atoms). The polyynic structure of carbon chains with alternating triple and single bonds was unambiguously revealed by bond-resolved atomic force microscopy. Moreover, an atomically precise polyyne, C14, was successfully produced via tip-induced dehalogenation and ring-opening of the decachloroanthracene molecule (C14Cl10) on a bilayer NaCl/Au(111) surface at 4.7 K, and a band gap of 5.8 eV was measured by scanning tunnelling spectroscopy, in a good agreement with the theoretical HOMO-LUMO gap (5.48 eV).

Bottom-Up Synthesis of Metalated Carbyne Ribbons via Elimination Reactions

Elimination reactions are one of the most important reactions in organic synthesis, especially in the formation of alkenes and alkynes. Herein, based on scanning tunneling microscopy, we report the bottom-up synthesis of one-dimensional carbyne-like nanostructures, metalated carbyne ribbons with the incorporation of Cu or Ag atoms, through α- and β-elimination reactions of tetrabromomethane and hexabromoethane on surfaces. Density functional theory calculations demonstrate a width-dependent band gap modulation within these ribbon structures, which is affected by interchain interactions. Moreover, mechanistic insights into the on-surface elimination reactions have also been provided in this study.

On-Surface Debromination of C6Br6: C6 Ring versus C6 Chain

Carbon allotropes comprising sp-hybridized carbon atoms have been investigated for decades for their molecular structure. One of the unsolved mysteries is whether they should take a linear or cyclic configuration in condensed phases due to the lack of atomistic characterizations. Herein, we designed a molecule with a C₆ skeleton as a model system to address this issue, which was achieved by eliminating Br atoms from hexabromobenzene (C₆Br₆) molecule on the Ag(111) substrate via thermal treatment. It is found that the C₆ ring intermediate resulting from complete debromination is energetically unstable at room temperature based on theoretical calculations. It subsequently transforms into the C₆ polyynic chain via a ring-opening process and ultimately polymerizes into the organometallic polyyne, whose triyne structural unit is revealed by bond-resolved noncontact atomic force microscopy. Theoretical calculations demonstrated an energetically favorable pathway in which the ring-opening process occurs after complete debromination of C₆Br₆. Our study provides a platform for the synthesis of elusive carbon-rich materials.

Stepwise on-surface synthesis of thiophene-based polymeric ribbons by coupling reactions and the carbon-fluorine bond cleavage

The rational synthesis of thiophene-based cross-coupled polymers on surfaces has been attracting more attention recently. Here, we report the stepwise activation of 5,5'-(2,3-difluoro-1,4-phenylene)bis(2-bromothiophene) as a precursor to synthesize thiophene-based polymeric ribbons on the Au(111) surface. Scanning tunneling microscopy studies showed that the precursor adopted different conformations in the self-assembled structure, organometallic species, and covalent polymers. On annealing the sample at a relatively low temperature (150 °C), the conversion of the organometallic structure into a covalent product with straight lines was observed, in which the Br adatoms arranged between the neighboring chains. On further annealing the sample at 270 °C, the detached Br adatoms played a key role in promoting the C-H bond activation. The cross-linked polymer was achieved by a combination of Ullmann and dehydrogenative coupling. When the annealing temperature was up to 390 °C, the C-F bond activation was triggered, which led to the formation of polymeric ribbons resulting from the cyclodehydrogenation of the fluorinated polymer. This study further supplements the reaction mechanism of thiophene-based dehalogenative, dehydrogenative and defluorinative coupling, and provides us a rational way for synthesizing cross-linked functional materials.

On-Surface Synthesis of sp-Carbon Nanostructures

The on-surface synthesis of carbon nanostructures has attracted tremendous attention owing to their unique properties and numerous applications in various fields. With the extensive development of scanning tunneling microscope (STM) and noncontact atomic force microscope (nc-AFM), the on-surface fabricated nanostructures so far can be characterized on atomic and even single-bond level. Therefore, various novel low-dimensional carbon nanostructures, challenging to traditional solution chemistry, have been widely studied on surfaces, such as polycyclic aromatic hydrocarbons, graphene nanoribbons, nanoporous graphene, and graphyne/graphdiyne-like nanostructures. In particular, nanostructures containing sp-hybridized carbons are of great advantage for their structural linearity and small steric demands as well as intriguing electronic and mechanical properties. Herein, the recent developments of low-dimensional sp-carbon nanostructures fabricated on surfaces will be summarized and discussed.

On-surface synthesis of organocopper metallacycles through activation of inner diacetylene moieties

The design of organometallic complexes is at the heart of modern organic chemistry and catalysis. Recently, on-surface synthesis has emerged as a disruptive paradigm to design previously precluded compounds and nanomaterials. Despite these advances, the field of organometallic chemistry on surfaces is still at its infancy. Here, we introduce a protocol to activate the inner diacetylene moieties of a molecular precursor by copper surface adatoms affording the formation of unprecedented organocopper metallacycles on Cu(111). The chemical structure of the resulting complexes is characterized by scanning probe microscopy and X-ray photoelectron spectroscopy, being complemented by density functional theory calculations and scanning probe microscopy simulations. Our results pave avenues to the engineering of organometallic compounds and steer the development of polyyne chemistry on surfaces.

The diacetylene skeletons of DNBD precursors are attacked on Cu(111) by copper adatoms resulting in the synthesis of organocopper metallacycles.

Chemical Identification and Bond Control of π-Skeletons in a Coupling Reaction

Highly unsaturated π-rich carbon skeletons afford versatile tuning of structural and optoelectronic properties of low-dimensional carbon nanostructures. However, methods allowing more precise chemical identification and controllable integration of target sp-/sp²-carbon skeletons during synthesis are required. Here, using the coupling of terminal alkynes as a model system, we demonstrate a methodology to visualize and identify the generated π-skeletons at the single-chemical-bond level on the surface, thus enabling further precise bond control. The characteristic electronic features together with localized vibrational modes of the carbon skeletons are resolved in real space by a combination of scanning tunneling microscopy/spectroscopy (STM/STS) and tip-enhanced Raman spectroscopy (TERS). Our approach allows single-chemical-bond understanding of unsaturated carbon skeletons, which is crucial for generating low-dimensional carbon nanostructures and nanomaterials with atomic precision.