Impact of Di- and Poly-Radical Characters on the Relative Energy of the Doubly Excited and La States of Linear Acenes and Cyclacenes

Linear and cyclic acenes are polycyclic aromatic hydrocarbons that can be viewed as building blocks of graphene nanoribbons and carbon nanotubes, respectively. While short linear acenes demonstrated remarkable efficiency in several optoelectronic applications, the longer members are unstable and difficult to synthesize as their cyclic counterparts. Recent progress in on-surface synthesis, a powerful tool to prepare highly reactive species, opens promising perspectives and motivates the computational investigations of these potentially functional molecules. Owing to their di- and poly-radical character, low-lying excited states dominated by doubly excited configurations are expected to become more important for longer members of both linear and cyclic molecules. In this work, we investigate the lowest-lying La and the doubly excited (DE) state of linear acenes and cyclacenes, with different computational approaches, to assess the influence of the di-/poly-radical characters (increasing with the molecular dimensions) on their relative order. We show that DFT/MRCI calculations correctly reproduce the crossing of the two states for longer linear acenes, while TDUDFT calculations fail to predict the correct excitation energy trend of the DE state. The study suggests a similarity in the excited electronic state pattern of long linear and cyclic acenes leading ultimately to a lowest lying dark DE state for both.

π-Extended Pyrrole-Fused Heteropine: Synthesis, Properties, and Application in Organic Field-Effect Transistors

Sulfur-embedded polycyclic aromatic compounds have been used as building blocks for numerous organic semiconductors over the past few decades. While the success is based on thiophene-containing compounds, aromatic compounds that contain thiepine, a sulfur-containing seven-membered-ring arene, has been less well investigated. Here we report the synthesis and properties of π-extended pyrrole-fused heteropine compounds such as thiepine and oxepine. A π-extended pyrrole-fused thiepine exhibited a "pitched π-stacking" structure in the crystal, and exhibited a high charge carrier mobility of up to 1.0 cm²  V^-1  s^-1 in single-crystal field-effect transistors.

Synthesis and Characterization of peri-Heptacene on a Metallic Surface

The synthesis of long n-peri-acenes (n-PAs) is challenging as a result of their inherent open-shell radical character, which arises from the presence of parallel zigzag edges beyond a certain n value. They are considered as π-electron model systems to study magnetism in graphene nanostructures; being potential candidates in the fabrication of optoelectronic and spintronic devices. Here, we report the on-surface formation of the largest pristine member of the n-PA family, i.e. peri-heptacene (n=7, 7-PA), obtained on an Au(111) substrate under ultra-high vacuum conditions. Our high-resolution scanning tunneling microscopy investigations, complemented by theoretical simulations, provide insight into the chemical structure of this previously elusive compound. In addition, scanning tunneling spectroscopy reveals the antiferromagnetic open-shell singlet ground state of 7-PA, exhibiting singlet-triplet spin-flip inelastic excitations with an effective exchange coupling (J_eff ) of 49 meV.

Tandem Desulfurization/C-C Coupling Reaction of Tetrathienylbenzenes on Cu(111): Synthesis of Pentacene and an Exotic Ladder Polymer

Surface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C-C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C-C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces.

On-surface synthesis and characterization of nitrogen-substituted undecacenes

Heteroatom substitution in acenes allows tailoring of their remarkable electronic properties, expected to include spin-polarization and magnetism for larger members of the acene family. Here, we present a strategy for the on-surface synthesis of three undecacene analogs substituted with four nitrogen atoms on an Au(111) substrate, by employing specifically designed diethano-bridged precursors. A similarly designed precursor is used to synthesize the pristine undecacene molecule. By comparing experimental features of scanning probe microscopy with ab initio simulations, we demonstrate that the ground state of the synthesized tetraazaundecacene has considerable open-shell character on Au(111). Additionally, we demonstrate that the electronegative nitrogen atoms induce a considerable shift in energy level alignment compared to the pristine undecacene, and that the introduction of hydro-aza groups causes local anti-aromaticity in the synthesized compounds. Our work provides access to the precise fabrication of nitrogen-substituted acenes and their analogs, potential building-blocks of organic electronics and spintronics, and a rich playground to explore π-electron correlation.

Defect-Induced π-Magnetism into Non-Benzenoid Nanographenes

The synthesis of nanographenes (NGs) with open-shell ground states have recently attained increasing attention in view of their interesting physicochemical properties and great prospects in manifold applications as suitable materials within the rising field of carbon-based magnetism. A potential route to induce magnetism in NGs is the introduction of structural defects, for instance non-benzenoid rings, in their honeycomb lattice. Here, we report the on-surface synthesis of three open-shell non-benzenoid NGs (A1, A2 and A3) on the Au(111) surface. A1 and A2 contain two five- and one seven-membered rings within their benzenoid backbone, while A3 incorporates one five-membered ring. Their structures and electronic properties have been investigated by means of scanning tunneling microscopy, noncontact atomic force microscopy and scanning tunneling spectroscopy complemented with theoretical calculations. Our results provide access to open-shell NGs with a combination of non-benzenoid topologies previously precluded by conventional synthetic procedures.

Efficient photogeneration of nonacene on nanostructured graphene

The on-surface photogeneration of nonacene from α-bisdiketone precursors deposited on nanostructured epitaxial graphene grown on Ru(0001) has been studied by means of low temperature scanning tunneling microscopy and spectroscopy. The presence of an unoccupied surface state, spatially localized in the regions where the precursors are adsorbed, and energetically accessible in the region of the electromagnetic spectrum where n-π* transitions take place, allows for a 100% conversion of the precursors into nonacenes. With the help of state-of-the-art theoretical calculations, we show that such a high yield is due to the effective population of the surface state by the incoming light and the ensuing electron transfer to the unoccupied states of the precursors through an inelastic scattering mechanism. Our findings are the experimental confirmation that surface states can play a prominent role in the surface photochemistry of complex molecular systems, in accordance with early theoretical predictions made on small molecules.

Synthesis and Absorption Properties of Long Acenoacenes

Acenes, polyaromatic hydrocarbons composed of linearly fused benzene rings have received immense attention due to their performance as semiconductors in organic optoelectronic applications. Their appealing physicochemical properties, such as extended delocalization, high charge carrier mobilities, narrow HOMO-LOMO gaps and partially radical character in the ground state make them very attractive targets for many potential applications. However, the intrinsic synthetic challenges of unsubstituted members such as high reactivity and poor solubility are still limiting factors for their wider exploitation. Herein, we report a simple general synthesis of a new family of angularly fused acenoacenes with improved stability compared to their isoelectronic linear counterparts. The synthesis and comprehensive characterization of pentacenopentacene, pentacenohexacene and hexacenohexacene, with lengths between decacene and dodecacene, are disclosed.

Persistent peri-Heptacene: Synthesis and In Situ Characterization

n‐peri‐Acenes (n‐PAs) have gained interest as model systems of zigzag‐edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n‐PAs larger than peri‐tetracene remains challenging because of their intrinsic open‐shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n‐PA, that is, peri‐heptacene (7‐PA), in which the reactive zigzag edges are kinetically protected with eight 4‐tBu‐C₆H₄ groups. The formation of 7‐PA is validated by high‐resolution mass spectrometry and in situ FT‐Raman spectroscopy. 7‐PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t_1/2≈25 min) under inert conditions. Moreover, electron‐spin resonance measurements and theoretical studies reveal that 7‐PA exhibits an open‐shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next‐generation (3 N+1)‐PAs (where N≥3).

Pushing the Limits of Acene Chemistry: The Recent Surge of Large Acenes

Acenes, consisting of linearly fused benzene rings, are an important fundamental class of organic compounds with various applications. Hexacene is the largest acene that was synthesized and isolated in the 20th century. The next largest member of the acene family, heptacene, was observed in 2007 and since then significant progress in preparing acenes has been reported. Significantly larger acenes, up to undecacene, could be studied by means of low-temperature matrix isolation spectroscopy with in situ photolytic generation, and up to dodecacene by means of on-surface synthesis employing innovative precursors and highly defined crystalline metal surfaces under ultrahigh vacuum conditions. The review summarizes recent experimental and theoretical advances in the area of acenes that give a significantly deeper insight into the fundamental properties and nature of the electronic structure of this fascinating class of organic compounds.

Unravelling the Open-Shell Character of Peripentacene on Au(111)

Polycyclic aromatic hydrocarbons (PAHs) are a family of organic compounds comprising two or more fused aromatic rings which feature manifold applications in modern technology. Among these species, those presenting an open-shell magnetic ground state are of particular interest for organic electronic, spintronic, and non-linear optics and energy storage devices. Within PAHs, special attention has been devoted in recent years to the synthesis and study of the acene and fused acene (periacene) families, steered by their decreasing HOMO-LUMO gap with length and predicted open-shell character above some size. However, an experimental fingerprint of such magnetic ground state has remained elusive. Here, we report on the in-depth electronic characterization of isolated peripentacene molecules on a Au(111) surface. Scanning tunnelling spectroscopy, complemented by computational investigations, reveals an antiferromagnetic singlet ground state, characterized by singlet-triplet inelastic excitations with an experimental effective exchange coupling (J_eff) of 40.5 meV. Our results deepen the fundamental understanding of organic compounds with magnetic ground states, featuring perspectives in carbon-based spintronic devices.

Combinatorial Selection Among Geometrical Isomers of Discrete Long-Carbon-Chain Naphthalenediimides Induces Local Order at the Liquid/Solid Interface

We report two families of naphthalenediimides (NDIs) symmetrically functionalized with discrete carbon chains comprising up to 55 carbon atoms (C_n-NDI-C_n, n = 39, 44, 50, and 55) and their self-assembly at the 1-phenyloctane/highly oriented pyrolytic graphite interface (1-PO/HOPG interface). The compounds differ by the presence or absence of two or three internal double bonds in the carbon chains (unsaturated and saturated C_n-NDI-C_n, respectively). Combinatorial distributions of geometrical isomers displaying either the E- or Z-configuration at each double bond are obtained for the unsaturated compounds. Analysis of the self-assembled monolayers of equally long unsaturated and saturated C_n-NDI-C_n by scanning tunneling microscopy (STM) reveal that all C_n-NDI-C_n tend to form lamellar systems featuring alternating areas of aromatic cores and carbon chains. Extended chain lengths are found to significantly increase disorder in the self-assembled monolayers due to misalignments and enhanced strength of interchain interactions. This phenomenon is antagonized by the local order-inducing effect of the internal double bonds: unsaturated C_n-NDI-C_n give qualitatively more ordered self-assembled monolayers compared to their saturated counterparts. The use of combinatorial distributions of unsaturated C_n-NDI-C_n geometrical isomers does not represent a limitation to achieve local order in the self-assembled monolayers. The self-assembly process operates a combinatorial search and selects the geometrical isomer(s) affording the most thermodynamically stable pattern, highlighting the adaptive character of the system. Finally, the antagonistic interplay between the extended carbon chain lengths and the presence of internal double bonds brings to the discovery of the lamellar "phase C" morphology for unsaturated C_n-NDI-C_n with n ≥ 50.

Synthetic Tailoring of Graphene Nanostructures with Zigzag-Edged Topologies: Progress and Perspectives

Experimental and theoretical investigations have revealed that the chemical and physical properties of graphene are crucially determined by their topological structures. Therefore, the atomically precise synthesis of graphene nanostructures is essential. A particular example is graphene nanostructures with zigzag-edged structures, which exhibit unique (opto)electronic and magnetic properties owing to their spin-polarized edge state. Recent progress in the development of synthetic methods and strategies as well as characterization methods has given access to this class of unprecedented graphene nanostructures, which used to be purely molecular objectives in theoretical chemistry. Thus, clear insight into the structure-property relationships has become possible as well as new applications in organic carbon-based electronic and spintronic devices. In this Minireview, we discuss the recent progress in the controlled synthesis of zigzag-edged graphene nanostructures with different topologies through a bottom-up synthetic strategy.

A Free-Radical Prompted Barrierless Gas-Phase Synthesis of Pentacene

A representative, low-temperature gas-phase reaction mechanism synthesizing polyacenes via ring annulation exemplified by the formation of pentacene (C22 H14 ) along with its benzo[a]tetracene isomer (C22 H14 ) is unraveled by probing the elementary reaction of the 2-tetracenyl radical (C18 H11 . ) with vinylacetylene (C4 H4 ). The pathway to pentacene-a prototype polyacene and a fundamental molecular building block in graphenes, fullerenes, and carbon nanotubes-is facilitated by a barrierless, vinylacetylene mediated gas-phase process thus disputing conventional hypotheses that synthesis of polycyclic aromatic hydrocarbons (PAHs) solely proceeds at elevated temperatures. This low-temperature pathway can launch isomer-selective routes to aromatic structures through submerged reaction barriers, resonantly stabilized free-radical intermediates, and methodical ring annulation in deep space eventually changing our perception about the chemistry of carbon in our universe.

Synthetic Applications of Oxidative Aromatic Coupling-From Biphenols to Nanographenes

Oxidative aromatic coupling occupies a fundamental place in the modern chemistry of aromatic compounds. It is a method of choice for the assembly of large and bewildering architectures. Considerable effort was also devoted to applications of the Scholl reaction for the synthesis of chiral biphenols and natural products. The ability to form biaryl linkages without any prefunctionalization provides an efficient pathway to many complex structures. Although the chemistry of this process is only now becoming fully understood, this reaction continues to both fascinate and challenge researchers. This is especially true for heterocoupling, that is, oxidative aromatic coupling with the chemoselective formation of a C-C bond between two different arenes. Analysis of the progress achieved in this field since 2013 reveals that many groups have contributed by pushing the boundary of structural possibilities, expanding into surface-assisted (cyclo)dehydrogenation, and developing new reagents.

Tetrabenzononacene: "Butterfly Wings" Stabilize the Core

In combination with bulky substituents at the core, fourfold benzannulation at the cata‐positions stabilizes a nonacene sufficiently to allow its isolation and characterization by ¹H NMR and X‐ray analysis. The four benzo units blueshift the absorption spectrum in comparison to a solely linear nonacene, but significantly increase the stability in the solid state.

On-surface synthesis of doubly-linked one-dimensional pentacene ladder polymers

On-surface investigations of pentacene molecules functionalized with four dibromomethylene groups reveal, after an annealing step, the formation of π-conjugated ladder polymers doubly-linked by ethynylene bridges on a Au(111) surface.

Exploring a Route to Cyclic Acenes by On-Surface Synthesis

A route to generate cyclacenes by on-surface synthesis is explored. We started by synthesizing two tetraepoxycyclacenes by sequences of Diels-Alder cycloadditions. Subsequently, these molecules were deposited onto Cu(111) and scanning-tunneling-microscopy(STM)-based atom manipulation was employed to dissociate the oxygen atoms. Atomic force microscopy (AFM) with CO-functionalized tips enabled the detailed characterization of the reaction products and revealed that, at most, two oxygens per molecule could be removed. Importantly, our experimental results suggest that the generation of cyclacenes by the described route might be possible for larger epoxycyclacenes.

On-Surface Synthesis of Ethynylene-Bridged Anthracene Polymers

Engineering low‐band‐gap π‐conjugated polymers is a growing area in basic and applied research. The main synthetic challenge lies in the solubility of the starting materials, which precludes advancements in the field. Here, we report an on‐surface synthesis protocol to overcome such difficulties and produce poly(p‐anthracene ethynylene) molecular wires on Au(111). To this aim, a quinoid anthracene precursor with =CBr₂ moieties is deposited and annealed to 400 K, resulting in anthracene‐based polymers. High‐resolution nc‐AFM measurements confirm the nature of the ethynylene‐bridge bond between the anthracene moieties. Theoretical simulations illustrate the mechanism of the chemical reaction, highlighting three major steps: dehalogenation, diffusion of surface‐stabilized carbenes, and homocoupling, which enables the formation of an ethynylene bridge. Our results introduce a novel chemical protocol to design π‐conjugated polymers based on oligoacene precursors and pave new avenues for advancing the emerging field of on‐surface synthesis.

On-surface light-induced generation of higher acenes and elucidation of their open-shell character

Acenes are an important class of polycyclic aromatic hydrocarbons which have recently gained exceptional attention due to their potential as functional organic semiconductors. Fundamentally, they are important systems to study the convergence of physico-chemical properties of all-carbon sp2-frameworks in the one-dimensional limit; and by virtue of having a zigzag edge topology they also provide a fertile playground to explore magnetism in graphenic nanostructures. The study of larger acenes is thus imperative from both a fundamental and applied perspective, but their synthesis via traditional solution-chemistry route is hindered by their poor solubility and high reactivity. Here, we demonstrate the on-surface formation of heptacene and nonacene, via visible-light-induced photo-dissociation of α-bisdiketone precursors on an Au(111) substrate under ultra-high vacuum conditions. Through combined scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy investigations, together with state-of-the-art first principles calculations, we provide insight into the chemical and electronic structure of these elusive compounds.

A Practical General Method for the Preparation of Long Acenes

The field of long acenes, the narrowest of the zig-zag graphene nanoribbons, has been an area of significant interest in the past decade because of its potential applications in organic electronics, spintronics and plasmonics. However the low solubility and high reactivity of these compounds has so far hindered their preparation on large scales. We report here a concise strategy for the synthesis of higher acenes through Diels-Alder condensation of arynes with a protected tetraene ketone. After deprotection by cleavage of the ketal, the obtained monoketone precursors cleanly yield the corresponding acenes through quantitative cheletropic thermal decarbonylation in the solid state, at moderate temperatures of 155 to 205 °C. This approach allows the preparation of heptacene, benzo[a]hexacene, cis- and trans-dibenzopentacene and offers a valuable new method for the synthesis of even larger acenes.

Reaction Selectivity in On-Surface Chemistry by Surface Coverage Control-Alkyne Dimerization versus Alkyne Trimerization

This work reports the influence of molecular coverage in on-surface C-C-bond formation on reaction outcome. 6-Ethynyl-2-naphthoic acid (ENA) was chosen as organic component and Ag(111) as substrate. The alkyne moiety in ENA can either react by dimerization to ENA dimers (Glaser coupling or hydroalkynylation) or cyclotrimerization to generate a benzene core as connecting moiety. Dimer formation is preferred at high surface coverage whereas trimerization is the major reaction pathway at low coverage. Mechanistic studies are provided.

Higher Acenes by On-Surface Dehydrogenation: From Heptacene to Undecacene

A unified approach to the synthesis of the series of higher acenes up to previously unreported undecacene has been developed through the on-surface dehydrogenation of partially saturated precursors. These molecules could be converted into the parent acenes by both atomic manipulation with the tip of a scanning tunneling and atomic force microscope (STM/AFM) as well as by on-surface annealing. The structure of the generated acenes has been visualized by high-resolution non-contact AFM imaging and the evolution of the transport gap with the increase of the number of fused benzene rings has been determined on the basis of scanning tunneling spectroscopy (STS) measurements.