Triangulenes: From Precursor Design to On‐Surface Synthesis and Characterization

Triangulene and its higher homologues are a class of zigzag‐edged triangular graphene molecules (ZTGMs) with high‐spin ground states. These open‐shell molecules are predicted to host ferromagnetically coupled edge states with net spin values scaling with molecular size and are therefore considered promising candidates for future molecular spintronics applications. Unfortunately, the synthesis of unsubstituted [n]triangulenes and the direct observation of their edge states have been a long‐standing challenge due to a high reactivity towards oxygen. However, recent advances in precursor design enabled the on‐surface synthesis and characterization of unsubstituted [3]‐, [4]‐, and [5]triangulene. In this Minireview, we will highlight key aspects of this rapidly developing field, ranging from the principles of precursor design to synthetic strategies and characterization of a homologous series of triangulene molecules synthesized on‐surface. We will also discuss challenges and future directions.     

Collective All-Carbon Magnetism in Triangulene Dimers**

Triangular zigzag nanographenes, such as triangulene and its π-extended homologues, have received widespread attention as organic nanomagnets for molecular spintronics, and may serve as building blocks for high-spin networks with long-range magnetic order, which are of immense fundamental and technological relevance. As a first step towards these lines, we present the on-surface synthesis and a proof-of-principle experimental study of magnetism in covalently bonded triangulene dimers. On-surface reactions of rationally designed precursor molecules on Au(111) lead to the selective formation of triangulene dimers in which the triangulene units are either directly connected through their minority sublattice atoms, or are separated via a 1,4-phenylene spacer. The chemical structures of the dimers have been characterized by bond-resolved scanning tunneling microscopy. Scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy measurements reveal collective singlet–triplet spin excitations in the dimers, demonstrating efficient intertriangulene magnetic coupling.     

A Short and Efficient Synthesis of the [3]Triangulene Ring System

Triangulenes are of current interest for potential applications in molecular electronics. We describe here a three step synthesis of the 4,8,12‐trihydro[3]triangulenium cation by cascade cyclization of a tetra‐benzyl alcohol precursor in triflic acid solution. This stable carbocation is easily observed by NMR and optical spectroscopy and is highly fluorescent. Quenching of the cation into basic solutions or by hydride transfer from triethylsilane provides access to stable dihydro and tetrahydro[3]triangulenes. These neutral species interconvert with cations in a complex series of proton and hydride transfers. This route provides several important [3]triangulene precursors. Preliminary experiments designed to generate [3]triangulene in the solution phase provide evidence for its formation and rapid oligomerization.     

Single-Atom Protecting Group for on-Surface Synthesis of Graphdiyne Nanowires

On-surface synthesis of semiconducting graphdiyne nanowires usually suffer severe side reactions owing to the high reactivity of the butadiynylene units at noble metal surfaces, limiting the production of isolated nanowires. In this work, we report the high-yield synthesis of branchless graphdiyne nanowires [-C≡C-Ph₂-C≡C-]_n via on-surface Ullmann coupling of 1,4-bis(4-bromophenyl)-1,3-butadiyne molecules with chemical vapor deposition method. Non-contact atomic force microscopy with single-bond resolution reveals that single gold adatoms act as effective protecting groups for butadiynylene units by forming Au-π ligand bonds, preventing unwanted branched coupling reactions and enabling the synthesis of ultralong isolated graphdiyne nanowires. This study will stimulate further investigation on the role of various surface adatoms in protecting on-surface reactions.     

Precision Graphene Nanoribbon Heterojunctions by Chain-Growth Polymerization

Graphene nanoribbons (GNRs) are considered promising candidates for next-generation nanoelectronics. In particular, GNR heterojunctions have received considerable attention due to their exotic topological electronic phases at the heterointerface. However, strategies for their precision synthesis remain at a nascent stage. Here, we report a novel chain-growth polymerization strategy that allows for constructing GNR heterojunction with N=9 armchair and chevron GNRs segments ( 9-AGNR/cGNR ). The synthesis involves a controlled Suzuki–Miyaura catalyst-transfer polymerization (SCTP) between 2-(6′-bromo-4,4′′-ditetradecyl-[1,1′:2′,1′′-terphenyl]-3′-yl) boronic ester ( M1 ) and 2-(7-bromo-9,12-diphenyl-10,11-bis(4-tetradecylphenyl)-triphenylene-2-yl) boronic ester ( M2 ), followed by the Scholl reaction of the obtained block copolymer ( poly-M1/M2 ) with controlled M_n (18 kDa) and narrow Đ (1.45). NMR and SEC analysis of poly-M1/M2 confirm the successful block copolymerization. The solution-mediated cyclodehydrogenation of poly-M1/M2 toward 9-AGNR/cGNR is unambiguously validated by FT-IR, Raman, and UV/Vis spectroscopies. Moreover, we also demonstrate the on-surface formation of pristine 9-AGNR/cGNR from the unsubstituted copolymer precursor, which is unambiguously characterized by scanning tunneling microscopy (STM).     

Collective All‐Carbon Magnetism in Triangulene Dimers

Triangular zigzag nanographenes, such as triangulene and its π‐extended homologues, have received widespread attention as organic nanomagnets for molecular spintronics, and may serve as building blocks for high‐spin networks with long‐range magnetic order, which are of immense fundamental and technological relevance. As a first step towards these lines, we present the on‐surface synthesis and a proof‐of‐principle experimental study of magnetism in covalently bonded triangulene dimers. On‐surface reactions of rationally designed precursor molecules on Au(111) lead to the selective formation of triangulene dimers in which the triangulene units are either directly connected through their minority sublattice atoms, or are separated via a 1,4‐phenylene spacer. The chemical structures of the dimers have been characterized by bond‐resolved scanning tunneling microscopy. Scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy measurements reveal collective singlet–triplet spin excitations in the dimers, demonstrating efficient intertriangulene magnetic coupling.     

Magnetism in Nonplanar Zigzag Edge Termini of Graphene Nanoribbons

Graphene nanoribbons (GNRs) and nanographenes synthesized by on-surface reactions using tailor-made molecular precursors offer an ideal playground for a study of magnetism towards nano-spintronics. Although the zigzag edge of GNRs has been known to host magnetism, the underlying metal substrates usually veil the edge-induced Kondo effect. Here, we report the on-surface synthesis of unprecedented, π-extended 7-armchair GNRs using 7-bromo-12-(10-bromoanthracen-9-yl)tetraphene as the precursor. Characterization by scanning tunneling microscopy/spectroscopy revealed unique rearrangement reactions leading to pentagon- or pentagon/heptagon-incorporated, nonplanar zigzag termini, which demonstrated Kondo resonances even on bare Au(111). Density functional theory calculations indicate that the nonplanar structure significantly reduces the interaction between the zigzag terminus and the Au(111) surface, leading to a recovery of the spin localization of the zigzag edge. Such a distortion of planar GNR structures offers a degree of freedom to control the magnetism on metal substrates.     

Manifold dynamic non-covalent interactions for steering molecular assembly and cyclization

Deciphering rich non-covalent interactions that govern many chemical and biological processes is crucial for the design of drugs and controlling molecular assemblies and their chemical transformations. However, real-space characterization of these weak interactions in complex molecular architectures at the single bond level has been a longstanding challenge. Here, we employed bond-resolved scanning probe microscopy combined with an exhaustive structural search algorithm and quantum chemistry calculations to elucidate multiple non-covalent interactions that control the cohesive molecular clustering of well-designed precursor molecules and their chemical reactions. The presence of two flexible bromo-triphenyl moieties in the precursor leads to the assembly of distinct non-planar dimer and trimer clusters by manifold non-covalent interactions, including hydrogen bonding, halogen bonding, C–H⋯π and lone pair⋯π interactions. The dynamic nature of weak interactions allows for transforming dimers into energetically more favourable trimers as molecular density increases. The formation of trimers also facilitates thermally-triggered intermolecular Ullmann coupling reactions, while the disassembly of dimers favours intramolecular cyclization, as evidenced by bond-resolved imaging of metalorganic intermediates and final products. The richness of manifold non-covalent interactions offers unprecedented opportunities for controlling the assembly of complex molecular architectures and steering on-surface synthesis of quantum nanostructures.

A real-space characterization of dynamic non-covalent interactions in molecular assemblies and chemical reactions at the atomic bond level.     

Synthesis and Structure of Functionalized Zigzag Hydrocarbon Belts

Described in this paper are the synthesis and structure of novel and edge‐functionalized zigzag hydrocarbon belts. A stepwise “fjord‐stitching” strategy featuring repetitive intramolecular acylation reactions of a resorcin[4]arene derivative as the key steps afforded a biscarbonyl‐functionalized octahydrobelt[8]arene product. Facile ketone reduction with NaBH₄ and nucleophilic addition with n‐butyllithium produced secondary and tertiary alcohol‐containing molecular belts, respectively. Selective oxidation reactions of biscarbonyl‐bearing octahydrobelt[8]arene with m‐CPBA and (PhSeO)₂O furnished the corresponding lactone‐ and 1,4‐quinone‐embedded molecular belts. Depending on the functional groups on the edges, the acquired belt molecules adopt different shapes such as square prism, truncated cone, and elliptical cylinder.     

1,2,4-triazines. 7. Regioselective n2-amination of 3-methylthio-1,2,4-triazin-5(2H)-ones.A novel synthesis of [1,2,4]triazolo[2,3-b][1,2,4]triazinones and -triazine

A regioselective synthesis of 2-aminotriazinones 6a-d is reported, by reaction of 3-methylthio-1,2,4-triazinones 5a-d with O-(2,4-dinitrophenyl)hydroxylamine (2) as an amino-transfer agent. A spectroscopic study and an unequivocal synthesis of 2-amino-4-methyl-6-phenyl-1,2,4-triazinedione ( 11a ) has shown the site of amination to be N2 of the 1,2,4-triazinone ring. Subsequent reaction of 2-amino-1,2,4-triazinones 6a-b with amines, followed by ring closure with aliphatic acids provided [1,2,4]triazolo[2,3-b][1,2,4]triazine-7(1H)ones 13a-e. Conversion of [1,2,4]triazolo[2,3-b][1,2,4]triazinone 13c to unsubstituted [1,2,4]triazolo[2,3-6][1,2,4]triazine (15) was attained.     

Synthesis of 7,8-dihydro-6H-pyrazolo[3,4-b]quinolin-5-ones and related derivatives

This paper describes the synthesis of a new series of 4-amino-1-(unsubstituted and chloro or fluoro substituted benzyl)-7,8-dihydro-6H-pyrazolo[3,4-b]quinolin-5-ones 8 and the corresponding 7,8-dihydro-6H,9H-pyrazolo[3,4-b]quinoline-4,5-diones 13 . The derivatives obtained by reaction of these compounds with sodium azide in concentrated sulfuric acid are also reported.     

Polyquinazolotriazoles

A novel high molecular weight (η_inh = 1.4 dl/g) polyquinazolotriazole (PQT) was prepared from the reaction of 3,3′-(2″,6″-pyridinediyl)bis[5-(o-aminophenyl)-1,2,4-triazole] with isophthalic acid by solution cyclopolycondensation in polyphosphoric acid, with diphenyl isophthalate by melt polycondensation, and with isophthaloyl chloride to form a precursor polyamide which subsequently underway cyclodehydration. Thermal characterization of the PQT by thermogravimetric analysis showed initial weight loss in air commencing at ～490°C. The PQT exhibited a weight loss of only 4% after aging in static air for 200 hr at 316°C and a T_g of 342°C as measured by differential scanning calorimetry. Prior to polymer synthesis, a series of model compounds was prepared as a guide to polymer synthesis and identification.     

Isomeric Dioxodihydro-1H-Benzo[b]thiophenoindole Synthesis and Properties

The synthesis and properties of new heterocyclic systems are described: isomeric 2,3-dioxo-2,3-dihydro-1H-benzo[b]thiopheno[2,3-g]-, 1,2-dioxo-1,2-dihydro-1H-benzo[b]thiopheno[3,2-e]-, and 2,3-dioxo-2,3-dihydro-1H-benzo[b]thiopheno[2,3-f]indoles. The reduction of the latter to the corresponding unsubstituted benzo[b]thiophenoindoles depends on both the nature of the reducing agent and the reaction conditions.     

On-surface synthesis of cyclic organic molecules

Creating or connecting together large organic molecules, as polycyclic aromatic hydrocarbons (PAH), by chemical reactions readily on surfaces is the first step to a true advance in the field of molecular electronics. On-surface synthesis can be regarded as an efficient means to build new molecular species by using bottom-up strategies. Recently, a collection of different reactions leading to large tailor-made organic molecules on single-crystal metal surfaces has been reported. The fundamental mechanisms controlling these reactions can be investigated from a surface science perspective. This discipline skillfully combines the use of characterization techniques at the nanoscale, with single-crystal metallic surfaces able to catalyse these reactions. We present a tutorial review that highlights the relevance of the new bottom up strategies and classifies most of the different molecular on-surface reactions involving aromatic organic molecules that have been published up to date.     

On-Surface Synthesis and Real-Space Visualization of Aromatic P3N3

On-surface synthesis is at the verge of emerging as the method of choice for the generation and visualization of unstable or unconventional molecules, which could not be obtained via traditional synthetic methods. A case in point is the on-surface synthesis of the structurally elusive cyclotriphosphazene (P₃N₃), an inorganic aromatic analogue of benzene. Here, we report the preparation of this fleetingly existing species on Cu(111) and Au(111) surfaces at 5.2 K through molecular manipulation with unprecedented precision, i.e., voltage pulse-induced sextuple dechlorination of an ultra-small (about 6 Å) hexachlorophosphazene P₃N₃Cl₆ precursor by the tip of a scanning probe microscope. Real-space atomic-level imaging of cyclotriphosphazene reveals its planar D_3h-symmetric ring structure. Furthermore, this demasking strategy has been expanded to generate cyclotriphosphazene from a hexaazide precursor P₃N₂₁ via a different stimulation method (photolysis) for complementary measurements by matrix isolation infrared and ultraviolet spectroscopy.     

Aqueous Assembly of Zwitterionic Daisy Chains

The synthesis and characterization of zwitterionic molecular [c2]- and [a2]-daisy chains are described, relying on recognition of a positively charged cyclophane and a negatively charged oligo(phenylene-ethynylene) (OPE) rod in aqueous medium. For this purpose, syntheses of an acetylene-functionalized macrocyclic receptor and a water-soluble OPE-rod as the guest component are presented, from which a heteroditopic daisy chain monomer was prepared. This monomer aggregated strongly in water/methanol 4:1 and formed molecular daisy chains, which were isolated as interlocked species from a stoppering reaction at 1 mm concentration. The cyclic dimer [c2] was the main product with an isolated yield of 30 % and consisted of a mixture of diastereomers, as evidenced by ¹H NMR spectroscopy.     

The Synthesis of Novel Iodinated Iminodiacetic Acid Analogues as Hepatobiliary Imaging Agents

The synthesis and characterization of N-[2-[[4-iodo-2,6-bis(1-methylethyl)phenyl]amino]-2-oxoethyl]-N-(carboxymethyl)glycine and N-[2-[(4-iodo-2,6-diethylphenyl)amino]-2-oxoethyl]-N-(carboxymethyl)glycine is presented, as well as a modified and improved synthesis of N-[2-[(2,4-diiodo-6-methylphenyl)amino]-2-oxoethyl]-N-(carboxymethyl)glycine. These compounds are new agents for hepatobiliary imaging.     

The Synthesis of Novel Iodinated Iminodiacetic Acid Analogues as Hepatobiliary Imaging Agents

Summary. The synthesis and characterization of N-[2-[[4-iodo-2,6-bis(1-methylethyl)phenyl]amino]-2-oxoethyl]-N-(carboxymethyl)glycine and N-[2-[(4-iodo-2,6-diethylphenyl)amino]-2-oxoethyl]-N-(carboxymethyl)glycine is presented, as well as a modified and improved synthesis of N-[2-[(2,4-diiodo-6-methylphenyl)amino]-2-oxoethyl]-N-(carboxymethyl)glycine. These compounds are new agents for hepatobiliary imaging.     

Novel Route for Obtaining Isomeric Benzo[b]thiophenoindoles

We describe a novel route for synthesis of benzo[b]thiopheno[2,3-e]- and benzo[b]thiopheno[3,2-f]indoles. As the starting compounds, we used the corresponding annelated isatins obtained by the Sandmeyer reaction. We have established that reduction of the latter to the corresponding unsubstituted benzo[b]thiophenoindoles depends both on the nature of the substituent and on the reaction conditions.