Exploring a Route to Cyclic Acenes by On-Surface Synthesis

Two-Dimensional Nonbenzenoid Heteroacene Crystals Synthesized via In-Situ Embedding of Ladder Bipyrazinylenes on Au(111)

Precisely introducing topological defects is an important strategy in nanographene crystal engineering because defects can tune π-electronic structures and control molecular assemblies. The synergistic control of the synthesis and assembly of nanographenes by embedding the topological defects to afford two-dimensional (2D) crystals on surfaces is still a great challenge. By in-situ embedding ladder bipyrazinylene (LBPy) into acene, the narrowest nanographene with zigzag edges, we have achieved the precise preparation of 2D nonbenzenoid heteroacene crystals on Au(111). Through intramolecular electrocyclization of o-diisocyanides and Au adatom-directed [2+2] cycloaddition, the nonbenzenoid heteroacene products are produced with high chemoselectivity, and lead to the molecular 2D assembly via LBPy-derived interlocking hydrogen bonds. Using bond-resolved scanning tunneling microscopy, we determined the atomic structures of the nonbenzenoid heteroacene product and diverse organometallic intermediates. The tunneling spectroscopy measurements revealed the electronic structure of the nonbenzenoid heteroacene, which is supported by density functional theory (DFT) calculations. The observed distinct organometallic intermediates during progression annealing combined with DFT calculations demonstrated that LBPy formation proceeds via electrocyclization of o-diisocyanides, trapping of heteroarynes by Au adatoms, and stepwise elimination of Au adatoms.

Synthesis of Tridecacene by Multistep Single-Molecule Manipulation

Acenes represent a unique class of polycyclic aromatic hydrocarbons that have fascinated chemists and physicists due to their exceptional potential for use in organic electronics. While recent advances in on-surface synthesis have resulted in higher acenes up to dodecacene, a comprehensive understanding of their fundamental properties necessitates their expansion toward even longer homologues. Here, we demonstrate the on-surface synthesis of tridecacene via atom-manipulation-induced conformational preparation and dissociation of a trietheno-bridged precursor on a Au(111) surface. The generated tridecacene has been investigated by scanning tunneling microscopy and spectroscopy (STM/STS), combined with first-principles calculations. We observe that the STS transport gap (1.09 eV) shrinks again following the gap reopening of dodecacene (1.4 eV). Spin-polarized density functional theory calculations confirm an antiferromagnetic open-shell ground-state electronic configuration for tridecacene in the gas phase. Interestingly, tridecacene's open-shell character is significantly reduced upon interaction with the Au(111) surface despite being only physisorbed. The interaction with the surface leads to a lowering of the magnetization of tridecacene, a reduced gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), compared to the gas phase, and a reduced relative energy to the nonmagnetic state, making it nearly isoenergetic. These observations show qualitatively that the influence of the Au(111) substrate on the properties of long acenes is significant, which is important for interpreting the measured STS transport gaps. Our work contributes to a fundamental understanding of the electronic properties of long acenes, confirming a nonmonotonous length-dependent HOMO-LUMO gap, and to the development of multistep tip-assisted synthesis of elusive compounds.

Innovations in nanosynthesis: emerging techniques for precision, scalability, and spatial control in reactions of organic molecules on solid surfaces

The surface science-based approach to synthesising new organic materials on surfaces has gained considerable attention in recent years, owing to its success in facilitating the formation of novel 0D, 1D and 2D architectures. The primary mechanism used to date has been the catalytic transformation of small organic molecules through substrate-enabled reactions. In this Topical Review, we provide an overview of alternate approaches to controlling molecular reactions on surfaces. These approaches include light, electron and ion-initiated reactions, electrospray ionisation deposition-based techniques, collisions of neutral atoms and molecules, and superhydrogenation. We focus on the opportunities afforded by these alternative approaches, in particular where they may offer advantages in terms of selectivity, spatial control or scalability.

Synthesis, Aromatization and Cavitates of an Oxanorbornene-Fused Dibenzo[de, qr]tetracene Nanobox

Oxanorbornene-fused double-stranded macrocycles, represented by kohnkene, are not only synthetic precursors toward short segments of zigzag carbon nanotubes but also typical cavitands processing an intrinsic cavity. However, their capability to bind guest molecules in solution remained unexplored. Herein we report a new member of oxanorbornene-fused double-stranded macrocycles, which is named a nanobox herein because of its shape. Reductive aromatization of this oxanorbornene-fused nanobox leads to observation of a new zigzag carbon nanobelt by high resolution mass spectroscopy. The fluorescence titration and NMR experiments indicate that this nanobox encapsulates C₇₀ in solution with a binding constant of (3.2 ± 0.1) × 10⁶ M⁻¹ in toluene and a high selectivity against C₆₀ and its derivatives. As found from the X-ray crystallographic analysis, this nanobox changes the shape of its cross-section from a rhombus to nearly a square upon accommodating C₆₀.

A new oxanorbornene-fused nanobox encapsulated C₇₀ selectively in solution with a binding constant of (3.2 ± 0.1) × 10⁶ M⁻¹. Reductive aromatization of this nanobox led to observation of a new zigzag carbon nanobelt by mass spectroscopy.

Selective Oxidation of Belt[4]arene[4]tropilidene and Its Application to Construct Hydrocarbon Belts of Truncated Cone Structure with Expand Cavity

Despite their unique structures, tantalizing properties, and potential applications in carbon nanoscience and technology, the synthesis and functionalization of zigzag hydrocarbon nanobelts have remained largely unexplored until recently. Reported herein is the selective transformations of belt[4]arene[4]tropilidenes and their application in the construction of novel belts. The oxidation of belt[4]arene[4]tropilidene with benzeneseleninic anhydride under controlled conditions selectively afforded mono- to tetrakis(α-diketone)-functionalized belt intermediates. A subsequent condensation reaction with 1,2-phenylenediacetonitrile and 1,2-phenylenediamine produced a diversity of unprecedented belts with various macrocyclic cavities.

Sulphur-Embedded Hydrocarbon Belts: Synthesis, Structure and Redox Chemistry of Cyclothianthrenes

Cyclothianthrenes, a series of sulphur-embedded hydrocarbon belts proposed a decade ago, were successfully constructed through a stepwise bottom-up synthesis. The belt [6]cyclothianthrene ([6]CT) is the smallest and most strained member of the family yet reported. Both [6]CT and [8]CT are the first examples of cyclothianthrene characterized by single crystal X-ray diffraction. An unprecedented chiral belt [7]CT and a Möbius-shaped [9]CT were also achieved via modular synthesis. Crystallographic and computational studies show that belts [6]CT-[8]CT have prism-like conformations with well-defined tubular cavities which have potential for guest molecule inclusion. Cyclic voltammograms further revealed that these belts are redox-active. The success of constructing sulphur-embedded hydrocarbon belts, that is, cyclothianthrenes, greatly enriches the chemistry of heteroatom-doped molecular belts and tubes.

Synthesis of the [11]Cyclacene Framework by Repetitive Diels-Alder Cycloadditions

The Diels–Alder cycloaddition between bisdienes and bisdienophile incorporating the 7-oxa-bicyclo[2.2.1]heptane unit are well known to show high diastereoselectivity that can be exploited for the synthesis of molecular belts. The related bisdiene 5,6,7,8-tetramethylidene-2-bicyclo[2.2.2]octene is a valuable building block for the synthesis of photoprecursors for acenes, but it has not been employed for the synthesis of molecular belts. The present work investigates by computational means the Diels–Alder reaction between these bisdiene building blocks with syn-1,4,5,8-tetrahydro-1,4:5,8-diepoxyanthracene, which shows that the diastereoselectivity of the Diels–Alder reaction of the etheno-bridged bisdiene is lower than that of the epoxy-bridged bisdiene. The reaction of the etheno-bridged bisdiene and syn-1,4,5,8-tetrahydro-1,4:5,8-diepoxyanthracene in 2:1 ratio yields two diastereomers that differ in the orientation of the oxa and etheno bridges based on NMR and X-ray crystallography. The all-syn diastereomer can be transformed into a molecular belt by inter- and intramolecular Diels–Alder reactions with a bifunctional building block. The molecular belt could function as a synthetic intermediate en route to a [11]cyclacene photoprecursor.

An IMDAF approach to annellated 1,4:5,8-diepoxynaphthalenes and their metathesis reaction leading to novel scaffolds displaying an antiproliferative activity toward cancer cells

A 3,5a-epoxyfuro[2,3,4-de]isoquinoline scaffold, the product of ROCM of 1,4:5,8-diepoxynaphthalenes, is a promising antiproliferative agent toward breast and prostate human cancer cell lines.

Synthesis and ethylene-promoted metathesis of adducts of tandem [4+2]/[4+2] cycloaddition between bis-furyl dienes and maleic acid derivatives

A series of 1,4:5,8-diepoxynaphthalenes, annellated with carbo- and heterocycles, was synthesized based on the tandem intermolecular/intramolecular [4+2] cycloaddition of bis-furyl dienes with moderately to highly reactive cyclic dienophiles (maleic anhydride and maleinimides).

Radially Oriented [n]Cyclo-meta-phenylenes

Molecular compounds with zigzag carbon nanotube geometries are exceedingly rare. Here we report the synthesis and characterization of carbon-based nanotubes with zigzag geometry, best described as radially oriented [n]cyclo-meta-phenylenes, extending the tubularene family of compounds. By the incorporation of edge-sharing benzene rings into the tubularene's radial π-surface, we have uncovered the first step to give rise to the emergence of radial orbital distribution in zigzag nanorings.

Synthesis of a Sidewall Fragment of a (12,0) Carbon Nanotube

Synthesis of a carbon nanobelt (CNB) is a very challenging task in organic chemistry. Herein, we report the successful synthesis of an octabenzo[12]cyclacene based CNB (6), which can be regarded as a sidewall fragment of a (12,0) carbon nanotube. The key intermediate compound, a tetraepoxy nanobelt (5), was first synthesized by Diels-Alder reaction, and subsequent reductive aromatization gave the fully conjugated CNB 6. X-ray crystallographic analysis unambiguously confirmed the belt-shaped structure of 6. ¹ H NMR spectrum and theoretical calculations (ACID, NICS, and 2D/3D ICSS) revealed localized aromaticity and stronger shielding chemical environment in the inner region of the belt. The optical properties (absorption and emission) of 6 were studied and correlated to its electronic structure. Strain analysis indicates that the phenyl substituents at the zigzag edges are crucial to the successful synthesis of 6. This report presents a new strategy towards highly strained CNBs.

Synthetic Strategies of Carbon Nanobelts and Related Belt-Shaped Polycyclic Aromatic Hydrocarbons

The development of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons has gained momentum in recent years. This Minireview focuses on the synthetic strategies used in constructing these aesthetically appealing molecular nanocarbons. Examples of carbon nanobelts and related belt-shaped polycyclic aromatic hydrocarbons reported in recent years as well as some representative synthetic attempts in earlier times are discussed.

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.

Möbius and Hückel Cyclacenes with Dewar and Ladenburg Defects

Cyclacene nanobelts have not been synthesized in over 60 years and remain one of the last unsynthesized building blocks of carbon nanotubes. Recent work has predicted that Hückel-cyclacenes containing Dewar benzenoid ring isomers are the most stable isomeric forms for several of the smaller sizes of cyclacene belts. Here, we give a more complete picture of the isomers that are possible within these nanobelt systems by simulating embedded Ladenburg (prismane) benzenoid rings in Hückel-[n]cyclacenes (n = 5-14) and embedded Dewar benzenoid rings in twisted Möbius-[n]cyclacenes (n = 9-14). The Möbius-[9]cyclacene isomer containing one Dewar benzenoid defect and the Hückel-[5]cyclacene isomer containing two maximally spaced Ladenburg benzenoid defects are found to be more stable than their conventional Kekulé benzenoid ring counterparts. The isomers that contain Dewar and Ladenburg benzenoid rings have larger electronic singlet-triplet energy gaps and lower polyradical character when compared with the conventional isomers.

Dewar Benzenoids Discovered In Carbon Nanobelts

The synthesis of cyclacene nanobelts remains an elusive goal dating back over 60 years. These molecules represent the last unsynthesized building block of carbon nanotubes and may be useful both as seed molecules for the preparation of structurally well-defined carbon nanotubes and for understanding the behavior and formation of zigzag nanotubes more broadly. Here we report the discovery that isomers containing two Dewar benzenoid rings are the preferred form for several sizes of cyclacene. The predicted lower polyradical character and higher singlet-triplet stability that these isomers possess compared with their pure benzenoid counterparts suggest that they may be more stable synthetic targets than the structures that have previously been identified. Our findings should facilitate the exploration of new routes to cyclacene synthesis through Dewar benzene chemistry.

Enantioselective Synthesis of Planar Chiral Zigzag-Type Cyclophenylene Belts by Rhodium-Catalyzed Alkyne Cyclotrimerization

Planar chiral zigzag-type [8]- and [12]cyclophenylene (CP) belts have been synthesized in good yields with high ee values of 98% and 83%, respectively, by the rhodium-catalyzed enantioselective intramolecular sequential cyclotrimerizations of the corresponding cyclic polyynes. The observed high enantioselectivity arises from the regioselective formation of a rhodacycle intermediate from an unsymmetric triyne unit. The X-ray crystal structural analysis of the racemic planar chiral zigzag-type [8]CP belt revealed that the uneven molecules mesh with each other to form a one-dimensional columnar packing structure, in which one column contains single enantiomers, giving two types of chiral columns [(S)- and (R)-form columns] arranged alternately. The ring strain of the zigzag-type [8]CP belt was smaller than that of the armchair-type [8]CPP belt despite its smaller ring size, due to the presence of the strain-relieving m-terphenyl moieties. The effect of the number of the benzene rings of the zigzag-type CP belts on absorption and emission peaks was small due to interruption of π-conjugation at the m-phenylene moieties. However, the bending effect on the absolute fluorescence quantum yield as well as absorption and emission peaks was significant. Concerning chiroptical properties, the modest anisotropy dissymmetry factors of ECD and CPL were observed in the [8]CP belt.

Toward the Synthesis of a Highly Strained Hydrocarbon Belt

Hydrocarbon belts including fully conjugated beltarenes and their (partially) saturated analogs have fascinated chemists for decades due to their aesthetic structures, tantalizing properties, and potential applications in supramolecular chemistry and carbon nanoscience and nanotechnology. However, synthesis of hydrocarbon belts still remains a formidable challenge. We report in this communication a general approach to hydrocarbon belts and their derivatives. Closing up all four fjords of resorcin[4]arene derivatives through multiple intramolecular Friedel-Crafts alkylation reactions in an operationally simple one-pot reaction manner enabled efficient construction of octohydrobelt[8]arenes. Synthesis of belt[8]arene from DDQ-oxidized aromatization of octohydrobelt[8]arene under different conditions resulted in aromatization and simultaneous [4 + 2] cycloaddition reactions with DDQ or TCNE to produce selectively tetrahydrobelt[8]arene-DDQ₂, tetrahydrobelt[8]arene-TCNE₂, and belt[8]arene-DDQ₄ adducts. Formation of belt[8]arene, a fully conjugated hydrocarbon belt, was observed from retro-Diels-Alder reaction of a belt[8]arene-DDQ₄ adduct with laser irradiation under MALDI conditions. The new and practical synthetic method established would open an avenue to create belt-shaped molecules from easily available starting materials.

Revisiting Kekulene: Synthesis and Single-Molecule Imaging

Four decades after the first (and only) reported synthesis of kekulene, this emblematic cycloarene has been obtained again through an improved route involving the construction of a key synthetic intermediate, 5,6,8,9-tetrahydrobenzo[m]tetraphene, by means of a double Diels-Alder reaction between styrene and a versatile benzodiyne synthon. Ultra-high-resolution AFM imaging of single molecules of kekulene and computational calculations provide additional support for a molecular structure with a significant degree of bond localization in accordance with the resonance structure predicted by the Clar model.

Synthesis of Nanographenes, Starphenes, and Sterically Congested Polyarenes by Aryne Cyclotrimerization

In recent years, synthetic transformations based on aryne chemistry have become particularly popular, mostly due to the spread of methods to generate these highly reactive intermediates in a controlled manner under mild reaction conditions. In fact, aryne cycloadditions such as the Diels-Alder reaction are nowadays widely used for the efficient preparation of polycyclic aromatic compounds. In 1998, our group discovered that arynes can undergo transition metal-catalyzed reactions, a finding that opened new perspectives in aryne chemistry. In particular, Pd-catalyzed [2 + 2 + 2] cycloaddition of arynes allowed the straightforward synthesis of triphenylene derivatives such as starphenes or cloverphenes. We found that this reaction is compatible with different substituents and sterically demanding arynes as starting materials. This transformation is especially useful to increase the molecular complexity in one single step, transforming molecules formed by n cycles in structures with 3n + 1 cycles. In fact, we took advantage of this protocol to prepare a large variety of sterically congested polycyclic aromatic hydrocarbons such as helicenes or twisted polyarenes. Soon after the discovery of the reaction, the co-cyclotrimerization of arynes with other reaction partners, such as electron deficient alkynes, significantly expanded the potential of this transformation. Also the use of catalysts based on alternative metals besides Pd (e.g., Ni, Cu, Au) or the use of other strained intermediates such as cycloalkynes or cycloallenes added value to this reaction. In addition, we realized that the Pd-catalyzed aryne cyclotrimerization reaction is particularly useful for the bottom-up preparation of well-defined nanographenes by chemical methods. Although the extreme insolubility of these planar nanographenes hampered their manipulation and characterization by conventional methods, recent advances in single molecule on-surface characterization by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) with functionalized tips under ultrahigh vacuum (UHV) conditions, permitted the impressive visualization of these nanographenes with submolecular resolution, together with the examination of the corresponding molecular orbital densities. Moreover, arynes have been shown to possess a rich on-surface chemistry. In particular, arynes have been generated and studied on-surface, showing that the reactivity is preserved even at cryogenic temperatures. On-surface aryne cyclotrimerization was also demonstrated to obtain large starphene derivatives. Therefore, it is expected that the combination of aryne cycloadditions and on-surface synthesis will provide notable findings in the near future, including the "à la carte" preparation of graphene materials or the synthesis of elusive molecules with unique properties.