Heptagonal Rings Research Articles

Two-Step Synthesis of B2 N2 -Doped Polycyclic Aromatic Hydrocarbon Containing Pentagonal and Heptagonal Rings with Long-Lived Delayed Fluorescence.

Pentagon-heptagon embedded polycyclic aromatic hydrocarbons (PAHs) have aroused increasing attention in recent years due to their unique physicochemical properties. Here, for the first time, this report demonstrates a facile method for the synthesis of a novel B2 N2 -doped PAH (BN-2) containing two pairs of pentagonal and heptagonal rings in only two steps. In the solid state of BN-2, two different conformations, including saddle-shaped and up-down geometries, are observed. Through a combined spectroscopic and calculation study, the excited-state dynamics of BN-2 is well-investigated in this current work. The resultant pentagon-heptagon embedded B2 N2 -doped BN-2 displays both prompt fluorescence and long-lived delayed fluorescence components at room temperature, with the triplet excited-state lifetime in the microsecond time region (τ= 19 µs). The triplet-triplet annihilation is assigned as the mechanism for the observed long-lived delayed fluorescence. Computational analyses attributed this observation to the small energy separation between the singlet and triplet excited states, facilitating the intersystem crossing (ISC) process which is further validated by the ultrafast spectroscopic measurements.

Facile Synthesis of Nitrogen‐Doped Nanographenes with Joined Nonhexagons via a Ring Expansion Strategy

AbstractSynthetic methodology is considered a holy grail in both organic chemistry and materials science. Over the past few decades, researchers have explored graphene‐type molecules (or nanographenes) through classic Scholl oxidative cyclodehydrogenation. Despite the successes achieved with various nanographenes, the development of new methods to synthesize these highly desired molecules lags behind. Herein, we developed a facile and effective method to produce a series of nanographenes bearing nitrogen (N)‐doped pentagon‐heptagon pairs in acceptable yields. Modification of the heptagonal ring endowed the resultant nanographenes with tunable physicochemical properties; for instance, M9 exhibited both aggregation‐caused quenching and aggregation‐induced emission behavior. Most strikingly, novel nanographenes containing N‐doped pentagon‐octagon pairs were also obtained using the same synthetic strategy, demonstrating the superior versatility and efficiency of the proposed ring expansion method.

Facile Synthesis of Nitrogen-Doped Nanographenes with Joined Nonhexagons via a Ring Expansion Strategy.

Synthetic methodology is considered a holy grail in both organic chemistry and materials science. Over the past few decades, researchers have explored graphene-type molecules (or nanographenes) through classic Scholl oxidative cyclodehydrogenation. Despite the successes achieved with various nanographenes, the development of new methods to synthesize these highly desired molecules lags behind. Herein, we developed a facile and effective method to produce a series of nanographenes bearing nitrogen (N)-doped pentagon-heptagon pairs in acceptable yields. Modification of the heptagonal ring endowed the resultant nanographenes with tunable physicochemical properties; for instance, M9 exhibited both aggregation-caused quenching and aggregation-induced emission behavior. Most strikingly, novel nanographenes containing N-doped pentagon-octagon pairs were also obtained using the same synthetic strategy, demonstrating the superior versatility and efficiency of the proposed ring expansion method.

The photophysics of distorted nanographenes: Ultra-slow relaxation dynamics, memory effects, and delayed fluorescence

The controlled deformation and engineering of the sp2 carbon network in atomically-precise nanographenes, and their substantially larger size as compared to typical optical dyes, opens new opportunities for the modulation of optical and electronic properties, but the peculiar photophysics of these systems is still poorly understood. Here, through a detailed comparative study of two well-defined distorted nanographenes, we show that they can exhibit interesting photophysical features, such as triplet-triplet annihilation delayed fluorescence, ultra-slow excited state dynamics, and excitation-wavelength memory effects on the nanosecond and sub-nanosecond relaxation cascades. Some of these behaviours are highly unexpected and strongly deviate from the archetypal behaviour of typical optical dyes. The distortion of the sp2 network was obtained by means of a non-hexagonal ring and a helicene unit, introduced by taking advantage of the high control provided by specific bottom-up organic synthesis. We believe the rich conformational landscape provided by the combination of the saddle-shaped curvature due to the heptagon ring and the helicene edge is at the heart of the uncommon photophysics.

Azatriseptanes: Strained Framework Analogs of [7,7,7]Circulenes.

The synthesis and characterization of heptagon-embedded polycyclic aromatic compounds are essential for understanding the effect of negative curvature on carbon allotropes such as fullerenes and graphenes that have applications in functional organic materials. However, owing to the synthetic difficulties in functionalizing and embedding seven-membered rings, these strain-challenged structures are relatively unexplored. We report here the synthesis, characterization, and properties of a triarylamine core bridged with ethano chains at the 2,2'-positions. In doing so, we provide access to the first heterocycle containing three fused heptagon rings with a nitrogen at its core (BATA-NHAc). X-ray crystallographic analysis and DFT calculations revealed a remarkably strained structure wherein two of the bridged aryl units approach coplanarity, while the third ring is twisted out of plane at 70°. UV-vis and emission spectroscopies identify red-shifted absorption and concentration-dependent emission profiles, respectively, as a result of the unique conformation and self-assembly properties of BATA-NHAc. Furthermore, cyclic voltammetry shows a decrease in the oxidation potential for BATA-NHAc in comparison to the non-bridged analog. This study opens new avenues in understanding the structure-property relationships of curved π-aromatics and the construction of π-frameworks of increasing complexity.

Atomically Precise Distorted Nanographenes: The Effect of Different Edge Functionalization on the Photophysical Properties down to the Femtosecond Scale.

Nanographenes (NGs) have been attracting widespread interest since they combine peculiar properties of graphene with molecular features, such as bright visible photoluminescence. However, our understanding of the fundamental properties of NGs is still hampered by the high degree of heterogeneity usually characterizing most of these materials. In this context, NGs obtained by atomically precise synthesis routes represent optimal benchmarks to unambiguously relate their properties to well-defined structures. Here we investigate in deep detail the optical response of three curved hexa-peri-hexabenzocoronene (HBC) derivatives obtained by atomically precise synthesis routes. They are constituted by the same graphenic core, characterized by the presence of a heptagon ring determining a saddle distortion of their sp2 network, and differ from each other for slightly different edge functionalization. The quite similar structure allows for performing a direct comparison of their spectroscopic features, from steady-state down to the femtosecond scale, and precisely disentangling the role played by the different edge chemistry.

Pyrene bridged double[7]helicene embedded with a heptagonal ring

The article explores the intricate relationship between chiroptical properties and molecular symmetry of pyrene-conjugated single and double [7]helicene.

Electronic properties and carrier mobilities of nanocarbons formed by non-benzoidal building blocks.

Exotic 1D and 2D carbon nanostructures have been grown in the laboratory in the last few years by means of surface-assisted chemical routes. In these processes, the strategical choice of a molecular precursor plays a dominant role in the determination of the synthesized nanocarbon. Further variations of these techniques are able to produce non-benzoidal carbon quantum-dots (QDs). Considering this experimental scenario as motivation, we propose a series of nanoribbon systems based on concatenating recently synthesized carbon QDs containing pentagonal, hexagonal, and heptagonal rings. We use density functional theory (DFT) simulations to reveal their properties can range from metallic to semiconducting depending on the concatenation hierarchy used to form the nanoribbons. This DFT implementation is based on a LCAO approach to describe valence wavefunctions and most of the simulations employ the PBE-GGA functional. Since this functional is known to underestimate band gaps, we also use the B3LYP functional in a plane-wave DFT approach for a selected case for comparison purposes. These systems show a different gap versus width relationship compared to conventional graphene nanoribbons setups and a particular set of carrier mobility values. We further discuss the interplay between the QD's frontier states and the electronic properties of the nanoribbons in light of their structural details.

Doping Heptagon‐Containing π‐Conjugated Systems with Boron

AbstractAs the models for studying defected graphene, π‐conjugated systems containing heptagonal rings have attracted increasing interest due to their dynamic behaviors, electronic properties, aromaticity, and solid‐state packing. Doping heptagon‐containing π‐conjugated systems with boron not only makes π‐conjugated systems with the boron‐doped heptagonal ring have structural similarity to their all‐carbon analogues, but also could significantly change their electronic properties and intermolecular interactions. This review summarizes research on the different synthetic routes to access boron‐doped heptagons including borepins, azaborepins, and oxaborepins, with the emphasis on the structure‐property relationships at the molecular level.

Strength criterion of graphene GBs combining discrete bond strength and varied bond stretch

The strength criterion of graphene grain boundaries (GBs) under complex stress states plays an important role in guiding the design of high-performance graphene-related materials and devices under severe mechanical environments like ultrastrong structural materials or flexible electronics. However, finding a unified strength criterion that can adapt to different GB angles and loading states is still a challenge. In this work, we proposed a general method to construct the strength criterion of graphene GBs that includes the atomic details of defect. Firstly, systematic molecular dynamics (MD) simulations were carried out to explore the fracture behaviors of graphene GBs with different GB angles and loading states. Then, bond strength of heptagon ring σ t h R was obtained by projecting the applied stress field to the fractured bond direction, which gives the same value for the same type of C-C bond of same GB angle under different loading states. Besides, to further unify the bond strength of different GB angles, the residual stress caused by other pentagon–heptagon defects was considered by using the disclination dipole theory, in which the intrinsic bond strength σ t h 0 for different GB angles collapses to the same value. With this scenario, the strength criterion of graphene GBs is presented as the bond stretch stress generated by external load exceeds any bond strength of the hexagon–heptagon defect. Furthermore, a unified bond strength is found, linearly decreasing with the equilibrium bond length, which can further be applied to the strength criterion of other defective structures like two types of SW defects (without analytical solution of residual stress) and even pristine graphene. The present results provide a comprehensive understanding of the failure of graphene by considering the balance of bond strength of defective structure and bond stretch stress generated by external load, which may pave the way to construct the atomic structure based strength criterion of other two-dimensional materials.

Phosphorus and nitrogen codoped entangled carbon nanotubes forming spongy materials: Synthesis and characterization

Carbon sponges are of great interest of the scientific community given their potential in advanced applications such as water cleaning, catalysis, energy storage, and mechanical reinforcement. These carbon nanomaterials improve their attributes for unexpected properties and applications by doping with foreign atoms. Nitrogen‑phosphorus-codoped and functionalized carbon nanotube sponges (NP-CNTSs) were synthesized, using the aerosol-assisted chemical vapor deposition (AACVD) method. We used acetonitrile and triphenylphosphine (TPP) as nitrogen and phosphorus precursors. According to our results, the NP-CNTSs were formed by entangled multi-walled carbon nanotubes (MWCNTs) of diameters ranging between 60 and 300 nm with Fe2P and Fe3C nanoparticles in the walls and inside the carbon nanotube sponges (CNTSs) respectively. The Fourier transform infrared (FTIR) characterization revealed the vibrational modes of aromatic and aliphatic CH, PO, PH, and OCN bonds. The NP-CNTSs undergo a quasi-reversible hydroquinone/quinone redox process as revealed by cyclic voltammetry studies. They also efficiently retain organic solvents, absorbing up to 40 times their weight for dichlorobenzene. The XPS and Raman characterizations show the presence of pyridinic nitrogen, which might be responsible for the formation of cavities in the wall of the CNTSs and the entanglement of carbon nanotubes, which promotes the growth of the NP-CNTSs. To study the role of nitrogen and phosphorus in the electronic properties of carbon materials, we carried out calculations proposed in the density functional theory (DFT) for phosphorus‑nitrogen codoping bilayer graphene and carbon nanotube junctions containing pentagonal, hexagonal, and heptagonal carbon rings.

Study of fullerenes C126 and C156 with seven heptagon rings

We consider fullerenes C126 and C156, both of them with seven heptagon rings. The first of them is C126 with 19 pentagons, and 39 hexagons. It is also considered C156 with 19 pentagons, and 54 hexagons. From previous work, a couple of fullerenes, C130 and C134, are mentioned for comparative reasons. These two fullerenes also contain seven heptagon rings. Fullerene C130 is constituted of 19 pentagons, and 41 hexagons. On the other hand, fullerene C134 has 19 pentagons, and 43 hexagons. Therefore, the four fullerenes considered, with seven heptagon rings, also share the property that, the number of pentagons in each of them, is 19. This fact is a consequence of a couple of equations that relates the number of pentagons, hexagons, and heptagons contained in a fullerene. Classical fullerenes contain only pentagons and hexagons, and the more familiar example is C60 with 20 hexagons, and 12 pentagons. Actually, all classical fullerenes have 12 pentagons, regardless of how many hexagons are involved. Besides this type of molecules, there exists nonclassical fullerenes, where another type of ring is presented, like squares, or heptagons. In this paper, fullerenes with 126, 130, 134, and 156 carbons are considered, with seven heptagons, and as a consequence, all them contain 19 pentagons, and they have 39, 41, 43, and 54 hexagons, respectively. Future work would focus on finding fullerenes with a different number of carbons, and hexagons, but with 7 heptagons, and 19 pentagons. Thus, fullerene C128 of this type, should contain 40 hexagons.

Planar Tetraindolodipleiadiene via Zirconium-Promoted Intramolecular Indolyl C4-H Homocoupling.

A novel N-rimmed PAH molecule containing a dipleiadiene core (TIDP) was designed and synthesized from indole, wherein a ZrCl4-promoted intramolecular C4-H homocoupling reaction of the indole moieties was the key approach. TIDP exhibited a nearly full planar structure and antiaromaticity of the two embedded heptagonal rings. The extremely stable radical cation TIDP•+·PF6- was isolated quantitatively by oxidation with AgPF6.

Structural, electronic, and optical properties of the PAI-BN monolayer: A first-principles study

We have applied first-principles calculations to investigate the structural, electronic, and optical properties of the boron nitride structure, PAI-BN, which is composed of pentagonal, hexagonal, and heptagonal rings. We have found that this structure exhibits dynamic and thermal stability and its energy is slightly higher than that obtained for hexagonal boron nitride (h-BN) monolayer. Our calculations for the electronic structure have shown that PAI-BN presents a bandgap of 2.50 eV, which is approximately half that of the h-BN bandgap, thus presenting a semiconductor behavior. Furthermore, we have found, from optical absorption results, that PAI-BN is transparent in the infrared and visible regions, absorbing only in the ultraviolet region. Therefore, based on these results, we can consider the possibility of applications of this nanostructure in electronic and optoelectronic devices.

First-principles study on haeckelite hexagonal monolayer with high specific capacity for sodium-ion battery

Sodium ion batteries are considered to be promising energy storage devices due to the vast earth abundance and low cost of sodium-containing precursors. The suitable anode for sodium ion batteries is required to have a large pore size to facilitate the insertion of sodium ions. We investigate the properties of haeckelite hexagonal monolayer as anode in sodium ion batteries by first-principles calculations. The existence of large radius heptagon ring induces the sodium ions to be adsorbed on the monolayer easily. Low diffusion barrier (0.30 eV) and small open circuit voltage ranges (0.37–0.16 V) indicate that the haeckelite hexagonal monolayer has excellent mobility properties and a large driving force for Na-ion diffusion in the charge/discharge process. Most remarkably, the storage capacity of hexagonal monolayer can be up to 1116.71 mA h g−1. The above properties demonstrate that the haeckelite hexagonal monolayer is an excellent anode material for rechargeable sodium ion batteries. Our research provides guidance for further application of haeckelite structure in the battery field.

Aberration-corrected transmission electron microscopy of a non-graphitizing carbon

Non-graphitizing carbons (NGCs) are an important class of solid carbons which cannot be converted into graphite by high-temperature heat treatment. They include commercially valuable materials such as activated carbon and glassy carbon. These carbons have been intensively studied for decades, but there is still no agreement about their detailed atomic structure, or the reasons for their resistance to graphitization. The first models for graphitizing and NGCs were proposed by Rosalind Franklin in the early 1950s, and while these are broadly correct, they are incomplete. Many alternative models of NGCs have been put forward since Franklin's time, but none has received universal acceptance. Diffraction and spectroscopic techniques can provide important insights into the nature of these carbons, but only direct microscopic imaging can reveal their true atomic structure. Here, we apply aberration-corrected transmission electron microscopy to an activated carbon prepared from waste biomass and present evidence for the presence of pentagonal and heptagonal carbon rings. This provides support for a model of the structure of NGC made up of curved fragments in which non-hexagonal rings are dispersed randomly throughout hexagonal networks.

Cage‐Opened C60 Isomers with Different Reactivities

AbstractWe synthesized two cage‐opened C60 isomers with different arrangement of an imine moiety on their orifices and investigated reactivities of them toward nucleophiles. Both isomers underwent the enlargement of the opening by the reaction with N‐methylmorpholine N‐oxide in comparable yields while the reaction with MeOH caused an orifice contraction via a transannular cyclization resulting in obviously different yields. The products in the latter reaction have a new hexagonal or heptagonal ring on their skeletons. The observed difference in reactivity could be explained by the greater thermodynamic stabilization in the hexagon‐containing isomer compared with the other.

Pyrene-Fused [7]Helicenes Connected Via Hexagonal and Heptagonal Rings: Stereospecific Synthesis and Chiroptical Properties.

In this manuscript, we portrayed a stereospecific synthesis of C2- and C1-symmetric pyrene-fused [7]helicene compounds 1 and 2, respectively. Compounds 1 and 2 were synthesized via a one-pot Suzuki coupling-C-H activation and two-step Suzuki coupling-Scholl reaction, respectively, with complete retention of configuration. The synthesized molecules differ in the fusing mode of [7]helicene units with pyrene via six- and seven-membered rings for 1 and 2, respectively. There was a significant difference in the functional properties and enantiomerization barrier of both compounds because of their distinct molecular symmetry as well as fusing mode to pyrene moiety. The heptagon-containing molecule 2 showed remarkable photophysical and chiroptical properties with commendable configurational stability compared to 1 and pristine [7]helicene as well as its [5]helicene congener.

Thgraphene: a novel two-dimensional carbon allotrope as a potential multifunctional material for electrochemical water splitting and potassium-ion batteries

With the crystal structure prediction package AISP, thgraphene composed of tetragonal and heptagonal carbon rings is theoretically predicted as a bifunctional HER/OER electrocatalyst and potassium-ion anode material.

PAHs Containing both Heptagon and Pentagon: Corannulene Extension by [5+2] Annulation

AbstractUtilizing Pd‐catalyzed [5+2] annulation a series of heptagon‐extended corannulenes could be synthesized from a borinic acid precursor furnished by C−H borylation strategy. Single‐crystal X‐ray analysis revealed the presence of two conformational enantiomers crystallizing in a racemic mixture. Through their embedded five‐ and seven‐membered rings these polycyclic aromatic hydrocarbons (PAHs) exhibit both negative and positive curvature and UV/Vis/NIR absorption spectroscopy as well as cyclic voltammetry experiments provided insights into the influence of larger flanking aromatic systems and electron‐donating substituents encompassing the heptagonal ring. Through [5+2] annulation of acenaphthylene an azulene‐containing PAH with intriguing optoelectronical properties including a very small bandgap and absorption over the whole visible spectrum could be obtained. Theoretical calculations were employed to elucidate the long‐wavelength absorption and aromaticity.