Non-benzenoid Rings Research Articles

Exploring the Surface Stability of Non-Benzenoid and Non-Planar High-Membered Rings

The synthesis of carbon-based nanomaterials has experienced an enormous advance in the last 15 years, in part motivated by the advent of On-surface Synthesis (OSS).1 This young field, based on the synthesis of carbon-based nanostructures directly on surfaces thanks to their unique catalytic properties, has allowed the formation of otherwise unattainable extended pi-conjugated systems, like atomically precise graphene nanoribbons,which turned out to be an excellent workbench to explore new physico-chemical properties at the atomic and molecular level.2 Once it was shown the feasibility to use this bottom-up approach to induce organic chemical reactions between tailored precursor monomers, the field evolved toward adding increasing complexity to the final products, either by modifying the dimensionality, edge structure and shape or by including heteroatoms or defects. All these approaches resulted in new intriguing properties as modified band structures or topological states. Of particular recent interest is the effect of non-benzenoid rings in the emergence of pi-magnetism, so far mainly focused on pentagonal rings.3 However, if we want to increase the versatility of the field and explore new possible functionalities associated to the presence of non-benzenoid rings in the nanostructures, it is mandatory to study their stability on surfaces.In this talk, we will discuss the thermal stability of different high-membered non-benzenoid rings, specifically heptagons and octogons, when included into graphene-based nanostructures, paying special attention to the reaction mechanisms involved. References (1) Gourdon, A. On-Surface Covalent Coupling in Ultrahigh Vacuum. Angew. Chem. Int. Ed. 2008, 47 (37), 6950–6953. https://doi.org/10.1002/anie.200802229.(2) Clair, S.; de Oteyza, D. G. Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis. Chem. Rev. 2019, 119 (7), 4717–4776. https://doi.org/10.1021/acs.chemrev.8b00601.(3) Oteyza, D. G. de; Frederiksen, T. Carbon-Based Nanostructures as a Versatile Platform for Tunable π-Magnetism. J. Phys. Condens. Matter 2022, 34 (44), 443001. https://doi.org/10.1088/1361-648X/ac8a7f.

Fused Porphyrin-Graphene Nanoribbons, One-Dimensional Porphyrinoid Polymers, and Subphthalocyanine-Based 2D-Polymers: On-Surface Synthetic Approaches

On-surface synthesis offers a versatile approach to fabricate novel carbon-based nanostructures that cannot be obtained via conventional solution chemistry. Within the family of such nanomaterials, graphene nanoribbons (GNRs) hold a privileged position due to their high potential for different applications. One of the key issues for their application in molecular electronics lies in the fine-tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non-benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) represents a highly appealing strategy.

π-extended pyrenes: from an antiaromatic buckybowl to doubly curved nanocarbons with gulf architectures.

The synthesis of π-extended pyrenes keeps attracting considerable attention. In particular, frameworks containing nonbenzenoid rings might display intriguing properties. Here, we report a practical synthetic pathway to access a new buckybowl (1), which is composed of four five-membered rings externally fused to a pyrene core. The buckybowl 1 exhibits antiaromaticity involving 22 π-electrons, a rapid bowl-to-bowl interconversion, and a small band gap. Furthermore, this buckybowl could be subjected to Scholl cyclodehydrogenation to prepare the doubly curved nanocarbons (2rac and 2meso), which exist as two diastereomers, as demonstrated by X-ray crystal structure determination. Variable temperature 1H NMR measurements reveal that 2meso can isomerize into 2rac under thermal conditions, with an activation free energy of 27.1 kcal mol-1. Both the enantiomers of 2rac can be separated by chiral HPLC and their chiroptical properties are thoroughly examined. In addition, the nanocarbon 2meso with two gulf architectures facilitates host-guest chemistry with a variety of guests, including PDI, TDI, C60 and C70.

Update for Isomerization Stabilization Energies: The Fulvenization Approach.

An alternative approach for calculating aromatic stabilization energies is proposed based on transforming an (anti)aromatic ring into a fulvene isomer. This fulvenization process gives a value of 34.05 kcal·mol-1 for benzene in the singlet state and a value of -17.85 kcal·mol-1 in the triplet state. Additionally, it is possible to use experimental values (as long as they exist) for the calculation as the gas-phase formation enthalpies of benzene and fulvene, whose difference is 33.72 kcal·mol-1. On the other hand, this same approach has been evaluated on several six-membered rings, including those persubstituted, biradicals, azines, and inorganic analogues, giving results in agreement with those reported in the literature using different criteria. Additionally, it is possible to differentiate the aromaticity of the rings in polycyclic aromatic hydrocarbons according to Clar's rules. Assigning the (anti)aromatic character in various nonbenzenoid rings (neutral and charged), except for five- and seven-membered rings, is also possible. The construction of the fulvene isomers in PAHs is set such that nonaromaticity-related effects are not considered. The results show that the fulvenization approach is an effective and efficient approach that can serve as an alternative or complement to existing tools.

Bespoke Tailoring of Graphenoid Sheets: A Rippled Molecular Carbon Comprising Cyclically Fused Nonbenzenoid Rings.

The incorporation of nonbenzenoid rings into the hexagonal networks of graphenoid nanostructures is of immense importance for electronic, magnetic, and mechanical properties, but the underlying mechanisms of nonbenzenoid ring fusion are rather unexplored. Here, we report the synthesis and characterization of a rippled C84 molecular carbon, which contains 10 nonbenzenoid rings (five-, seven-, and eight-membered rings) that are contiguously fused to give a cyclic geometry. The fused nonbenzenoid rings impart high solubility, configurational stability, multiple reversible redox behaviors, unique aromaticity, and a narrow band gap to the system. Moreover, this carbon nanostructure allows for further functionalization via electrophilic substitution and metalation reactions, enabling access to finely tuned derivatives. Interestingly, both the bowl-shaped and planar conformations of the core in molecular carbon are observed in the solid state. Additionally, this molecular carbon displays ambipolar transport characteristics.

Extension of Non-alternant Nanographenes Containing Nitrogen-Doped Stone-Thrower-Wales Defects.

Non-alternant topologies have attracted considerable attention due to their unique physiochemical characteristics in recent years. Here, three novel topological nanographenes molecular models of nitrogen-doped Stone-Thrower-Wales (S-T-W) defects were achieved through intramolecular direct arylation. Their chemical structures were unambiguously elucidated by single-crystal analysis. Among them, threefold intramolecular direct arylation compound (C42 H21 N) is the largest nanographene bearing a N-doped non-alternant topology to date, in which the non-benzenoid rings account for 83 % of the total molecular skeleton. The absorption maxima of this compound was located in the near-infrared region with a long tail up to 900 nm, which was much longer than those reported for similarly sized N-doped nanographene with six-membered rings (C40 H15 N). In addition, the electronic energy gaps of these series compounds clearly decreased with the introduction of non-alternant topologies (from 2.27 eV to 1.50 eV). It is noteworthy that C42 H21 N possesses such a low energy gap (Eg opt =1.40 eV; Eg cv =1.50 eV), yet is highly stable under ambient conditions. Our work reported herein demonstrates that the non-alternant topology could significantly influence the electronic configurations of nanocarbons, where the introduction of a non-alternanting topology may be an effective way to narrow the energy gap without extending the molecular π-conjugation.

Recent progress in chemistry of non-benzenoid carbohelicenes

Helicenes fused by ortho-annulated benzene rings are one of interesting subset of chiral polycyclic aromatic hydrocarbons (PAHs). Helicenes containing non-benzenoid rings such as pentagonal and heptagonal rings have attracted increasing attentions because of their novel electronic properties, optical characteristics, dynamic behavior, and solid-state packing. Despite the disclosure of several beautiful examples in recent years, this field remains young and challenging due to the intrinsic difficulties of the formation of non-hexagonal rings. Herein an overview of non-benzenoid carbohelicenes with respect to structures, synthesis, optical properties, and stereochemical dynamics is provided. In this review, recent reports of helicenes with non-hexagons as chiral motif are surveyed with an emphasis on synthesis, molecular configurations and intriguing optical properties. Finally, a brief conclusion of non-benzenoid helicenes is achieved and an outlook for this field is provided according to these discussions.

Tuning the Diradical Character of Pentacene Derivatives via Non-Benzenoid Coupling Motifs.

The development of functional organic molecules requires structures of increasing size and complexity, which are typically obtained by the covalent coupling of smaller building blocks. Herein, with the aid of high-resolution scanning tunneling microscopy/spectroscopy and density functional theory, the coupling of a sterically demanded pentacene derivative on Au(111) into fused dimers connected by non-benzenoid rings was studied. The diradical character of the products was tuned according to the coupling section. In particular, the antiaromaticity of cyclobutadiene as the coupling motif and its position within the structure play a decisive role in shifting the natural orbital occupancies toward a stronger diradical electronic character. Understanding these structure-property relations is desirable not only for fundamental reasons but also for designing new complex and functional molecular structures.

(Invited) Porphyrinoid-Carbon Nanostructure Ensembles and Fused Porphyrin-Graphene Nanoribbons

Phthalocyanines (Pcs) have emerged as excellent light harvesting antennas for incorporation into D-A systems, mainly in connection with carbon nanostructures, like endohedral metallofullerenes, SWCNT and graphene, as acceptor or donor moieties, in which the Pc has been attached, covalently or through supramolecular interactions. They are among the few molecules that reveal an intense red and NIR absorption and therefore, constitute also promising dyes in molecular photovoltaics. Pcs have a great chemical versatility, which allows to modify their electronic character and their physicochemical properties by organic synthesis, by introducing substituents in the periphery or modifying the structure of the macrocycle. Most recently they have reached good efficiency values participating as hole transporting materials in Carbon-based Perovskite sensitized solar cells (PSSCs). Pcs are be appropriately designed to adapt well to the electronic levels of the different types of perovskites. Through a rational design, structure-property relationships will be established that will gradually improve the performance of the devices.On the other hand, on-surface synthesis offers a versatile approach to fabricate novel carbon-based nanostructures that cannot be obtained via conventional solution chemistry. Within the family of such nanomaterials, graphene nanoribbons (GNRs) hold a privileged position due to their high potential for different applications. One of the key issues for their application in molecular electronics lies in the fine-tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non-benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) represents a highly appealing strategy.

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.

Recent progress in pyrazinacenes containing nonbenzenoid rings: synthesis, properties and applications

This review focuses on the recent progress in the inclusion of a nonbenzenoid ring into the π-backbone of azaacenes, which can largely tune absorption, energy levels, and antiaromaticity, and produce exciting size-dependent properties.

Porphyrinoid-Carbon Nanostructure Systems and Fused Porphyrin-Graphene Nanoribbons

Phthalocyanines (Pcs) have emerged as excellent light harvesting antennas for incorporation into D-A systems, mainly in connection with carbon nanostructures, like endohedral metallofullerenes, SWCNT and graphene, as acceptor or donor moieties, in which the Pc has been attached, covalently or through supramolecular interactions [1]. They are among the few molecules that reveal an intense red and NIR absorption and therefore, constitute also promising dyes in molecular photovoltaics. Pcs have a great chemical versatility, which allows to modify their electronic character and their physical-chemical properties by organic synthesis, by introducing substituents in the periphery or modifying the structure of the macrocycle. Most recently they have reached good efficiency values participating as hole transporting materials in Carbon-based Perovskite sensitized solar cells (PSSCs). Pcs are be appropriately designed to adapt well to the electronic levels of the different types of perovskite. Through a rational design, structure-property relationships will be established that will gradually improve the performance of the devices.On the other hand on-surface synthesis offers a versatile approach to fabricate novel carbon-based nanostructures that cannot be obtained via conventional solution chemistry. Within the family of such nanomaterials, graphene nanoribbons (GNRs) hold a privileged position due to their high potential for different applications. One of the key issues for their application in molecular electronics lies in the fine-tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non-benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) represents a highly appealing strategy [2]. Reference s[1] G. Bottari, M. Á. Herranz, L. Wibmer, M. Volland, L. Rodríguez-Pérez, D. M. Guldi, A. Hirsch, N. Martín, F. D’Souza, T. Torres, Chem . Soc . Rev . 2017, 46 , 4464-4500; G. de la Torre, G. Bottari, T. Torres, Energy Mater. 2017, 1601700[2] L. M. Mateo, Q. Sun, S. Decurtins, S.-X. Liu, J. J. Bergkamp, K. Eimre, C. A. Pignedoli, P. Ruffieux, G. Bottari, R. Fasel, T. Torres, Angew. Chem. Int. Ed. 2020, 59, 1355-1355.

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.

Contribution of edge-carbon atoms and non-benzenoid rings in graphitic carbons to π* peak profiles in CK-XANES spectra

In order to analyze the local structure of graphitic carbons by soft X-ray absorption spectroscopy, CK-XANES spectra of polycyclic aromatic compounds having 5-membered pentagonal or 7-membered heptagonal rings were theoretically analyzed by first-principles calculations. Pentagonal rings in the bent graphitic structure take different π* orbital states. Hence, pentagonal rings split the π* peak into two peaks in CK-XANES spectra. Heptagonal rings create electrostatic potential holes in graphitic structures, and the carbon atoms in the heptagonal ring can be regarded as conjugated edge carbon atoms. Hence, heptagonal rings also split the π* peak into two peaks. Edge-carbon atoms split the π* peak into two peaks in CK- XANES spectra. Consequently, carbon atoms in non-benzenoid rings and the edge-carbon atoms in graphitic structures make the π* peak profile wider when compared to graphite.

On‐Surface Synthesis and Characterization of Triply Fused Porphyrin–Graphene Nanoribbon Hybrids

On-surface synthesis offers a versatile approach to prepare novel carbon-based nanostructures that cannot be obtained by conventional solution chemistry. Graphene nanoribbons (GNRs) have potential for a variety of applications. A key issue for their application in molecular electronics is in the fine-tuning of their electronic properties through structural modifications, such as heteroatom doping or the incorporation of non-benzenoid rings. In this context, the covalent fusion of GNRs and porphyrins (Pors) is a highly appealing strategy. Herein we present the selective on-surface synthesis of a Por-GNR hybrid, which consists of two Pors connected by a short GNR segment. The atomically precise structure of the Por-GNR hybrid has been characterized by bond-resolved scanning tunneling microscopy (STM) and noncontact atomic force microscopy (nc-AFM). The electronic properties have been investigated by scanning tunneling spectroscopy (STS), in combination with DFT calculations, which reveals a low electronic gap of 0.4 eV.

Negatively Curved Warped Nanographene Self-Assembled on Metal Surfaces.

We report the investigation of a conjugated polycyclic hydrocarbon containing multiple nonbenzenoid rings and exhibiting negative curvature-the warped nanographene C80H30-adsorbed on several noble metal surfaces in an ultrahigh vacuum environment. From a detailed analysis of the molecular self-assembly at different molecular coverages via scanning tunneling microscopy and spectroscopy measurements in combination with theoretical modeling, the nature of the intermolecular interactions is unraveled. For high molecular coverages on Cu(111), the formation of homochiral porous networks is observed, which is rationalized by (i) intermolecular π-π interactions between neighboring C80H30 molecules that promote the formation of molecular dimers and (ii) enantioselective intermolecular CH···π interactions between the dimers. Such interactions are also observed after deposition of C80H30 molecules on Au(111) and Ag(111) substrates. Our results provide perspectives for the on-surface study of negatively curved nanographenes whichopen new avenuestothe design of novel and functional chiral structures with potential use in the field of organic optoelectronics.

Overcoming Steric Hindrance in Aryl-Aryl Homocoupling via On-Surface Copolymerization.

On-surface synthesis is a unique tool for growing low-dimensional carbon nanomaterials with precise structural control down to the atomic level. This novel approach relies on carefully designed precursor molecules, which are deposited on suitable substrates and activated to ultimately form the desired nanostructures. One of the most applied reactions to covalently interlink molecular precursors is dehalogenative aryl-aryl coupling. Despite the versatility of this approach, many unsuccessful attempts are also known, most of them associated to the poor capability of the activated precursors to couple to each other. Such failure is often related to the steric hindrance between reactants, which may arise due to their coplanarity upon adsorption on a surface. Here, we propose a copolymerization approach to overcome the limitations that prevent intermolecular homocoupling. We apply the strategy of using suitable linkers as additional reactants to the formation of fully conjugated polycyclic nanowires incorporating non-benzenoid rings.

Soft X-ray absorption spectra in the CK region of carbon black and spectral analysis using the discrete variational Xα method

To investigate the local structure of carbon black (CB) crystallites, soft X-ray absorption spectra (XAS) in the CK region of CB were measured using synchrotron radiation, and the π* peak profile in the CK-XAS was analyzed by the discrete variational (DV) Xα method. Compared to highly oriented pyrolytic graphite (HOPG), CB exhibits a broader π* peak profile where the intensity of the broad portion in CB is proportional to the hydrogen occluded in the CB. However, the broad portion cannot be reduced even in H2-degassed CB by thermal desorption. These findings on the broad π* peak profile can be simulated by the unoccupied C2p density of states (DOS) of the carbon atoms that form the nonbenzenoid rings and hydrogenated edge carbon atoms. Therefore, plausible origins for the broad π* peak in the CK-XAS of CB are the nonbenzenoid structure and/or hydrogenated edge carbon atoms in the graphitic crystallites.

Time-Dependent Density Functional Study of the Electronic Excited States of Polycyclic Aromatic Hydrocarbon Radical Ions

A uniform, comprehensive theoretical interpretation of spectroscopic data is presented for 51 radical ion species of polycyclic aromatic hydrocarbons (PAHs) with the aid of (Tamm−Dancoff) time-dependent density functional theory (TDDFT). TDDFT is capable of predicting the transition energies to the low-lying excited states of PAH ions with quantitative accuracy (the standard deviation from experimental results being less than 0.3 eV) and their intensity patterns qualitatively correctly. The accuracy is hardly affected by the sizes of PAH ions (azulene through dinaphthocoronene), the types of transitions (Koopmans or satellite transitions), the types of orbitals involved (π* ← π, π* ← σ, or σ* ← π transitions), the types of ions (cations or anions), or other geometrical or electronic perturbations (nonplanarity, sp3 carbons, or heterocyclic or nonbenzenoid rings).

Multicenter bond indices as a measure of aromaticity

A MO multicenter bond index involving the σ+π electron population is proposed as a measure of aromaticity. It is related both to the energetical and to the magnetic criteria. The index is applied to linear and angular polycyclic hydrocarbons with benzenoid rings, to hydrocarbons including non-benzenoid rings, to monocyclic azines, benzoazines and other heterocyclic compounds with 5-membered rings. The index gives satisfactory values for monocyclic molecules and is shown to be quite suitable in discriminating the relative aromaticity of the different rings in polycyclic compounds.