Hexabenzocoronene Research Articles

Dibenzo-peri-Heptacene: A Stable Open-Shell Graphene Fragment With a Balanced Combination of Armchair and Zigzag Edge Structures.

Atomically precise open-shell graphene fragments, such as extended peri-acenes, hold significant interest for electronics and spintronics. However, their inherent high reactivity poses challenges for synthesis and application. In this study, a novel approach is introduced: the fusion of a zigzag-edged peri-tetracene with an all-armchair-edged hexa-peri-hexabenzocoronene (HBC) via two shared benzene rings to produce a stable open-shell hydrocarbon, named dibenzo-peri-heptacene (DBPH). The DBPH derivative 1 exhibits considerably enhanced stability, with a half-life (t1/2) of 46 days in toluene solution under ambient conditions. This improved stability is attributed to peri-benzannulation, enhanced aromatic stabilization, and kinetic protection of the reactive sites along the zigzag edges. The structure of 1 is unequivocally verified through single-crystal X-ray diffraction analysis. With a balanced combination of armchair and zigzag edge structures, derivative 1 displays a diradical character of 39.2% and a singlet-triplet gap of ≈-3.16 kcalmol-1. It features a narrow electrochemical energy gap (0.87 eV) and exhibits amphoteric redox behavior. Notably, its dication and dianion states manifest a closed-shell singlet ground state, representing doubly charged structures where a HBC unit is fused with a benzo[f]tetraphene moiety. This research paves the way for synthesizing novel open-shell graphene fragments with adjustable electronic properties and exceptional stability.

Synthesis of Hexabenzocoronene-Cored Graphdiyne Nanosheets through Dehydrogenative Coupling on Au(111) Surface.

Thermally-induced dehydrogenative coupling of polyphenylenes on metal surfaces is an important technique to synthesize -conjugated carbon nanostructures with atomic precision. However, this protocol has rarely been utilized to fabricate structurally defined carbon nanosheets composed of sp- and sp2-hybridized carbon atoms. Here, we present the synthesis of butadiyne-linked hexabenzocoronenes (HBCs) on Au(111) surfaces as core-expanded graphdiynes. The reaction started from hexa(4-ethylphenyl)benzene, which undergoes dehydrogenation toward hexa(4-vinylphenyl)benzene, followed by planarization to hexabenzocoronene, coupling between the vinyl groups, and further dehydrogenation. In addition to butadiyne linkages, benzene groups were also found as another type of linker. The reaction sequences were monitored by scanning tunneling microscopy and bond-resolved non-contact atomic force microscopy, which disclose the structures of intermediates and final products. In combination with density functional theory simulations, the key steps from ethyl substituents to butadiyne and benzene linkers were elucidated. This is a new on-surface synthesis of core-expanded graphdiynes with unprecedented electronic properties.

Synthesis of Hexabenzocoronene‐Cored Graphdiyne Nanosheets through Dehydrogenative Coupling on Au(111) Surface

AbstractThermally‐induced dehydrogenative coupling of polyphenylenes on metal surfaces is an important technique to synthesize ‐conjugated carbon nanostructures with atomic precision. However, this protocol has rarely been utilized to fabricate structurally defined carbon nanosheets composed of sp‐ and sp2‐hybridized carbon atoms. Here, we present the synthesis of butadiyne‐linked hexabenzocoronenes (HBCs) on Au(111) surfaces as core‐expanded graphdiynes. The reaction started from hexa(4‐ethylphenyl)benzene, which undergoes dehydrogenation toward hexa(4‐vinylphenyl)benzene, followed by planarization to hexabenzocoronene, coupling between the vinyl groups, and further dehydrogenation. In addition to butadiyne linkages, benzene groups were also found as another type of linker. The reaction sequences were monitored by scanning tunneling microscopy and bond‐resolved non‐contact atomic force microscopy, which disclose the structures of intermediates and final products. In combination with density functional theory simulations, the key steps from ethyl substituents to butadiyne and benzene linkers were elucidated. This is a new on‐surface synthesis of core‐expanded graphdiynes with unprecedented electronic properties.

Morphology and Alignment Transition of Hexabenzocoronene (HBC) Mesogen Films by Bar Coating: Effect of Coating Speed.

Films of the discotic liquid crystalline hexabenzocoronene (HBC) derivative, HBC-1,3,5-Ph-C12, were prepared on the quartz substrate by the bar-coating method. Depending on the coating speed, regularly spaced stripes or continuous films were observed. In the former case, columns of the HBC derivatives align more along the stripes, which are perpendicular to the coating direction, whereas in the latter case, columns of the HBC derivatives in the film align more along the coating direction. These distinctive structures are confirmed via polarized optical microscopy (POM), polarized UV-vis spectroscopy, and grazing incidence small-angle X-ray scattering measurements.

Photoexcited States Dynamics on Cu(II) Porphyrines-Nanographenes Dyads

Donor-acceptor systems have garnered significant attention in the fields of semiconducting materials and light harvesting.1 Particularly intriguing are porphyrin diads, wherein porphyrins are covalently bonded to nanographenes, forming a dyad with an expected electron transfer from the porphyrin to the nanographene. In our study, we leverage the excellent charge transport properties of Hexabenzocoronene (HBC) in combination with the superior light harvesting capabilities of Cu(II) porphyrins.The steady-state absorption spectra of the dyads exhibit a linear combination of HBC and porphyrin spectra, suggesting a low degree of electronic coupling in the ground state. Femtosecond optical transient absorption spectroscopy (TAS) has been employed to investigate the dynamics of photoexcited states. The TAS spectrum of the dyad displays a first derivative shape of the steady-state absorption, indicative of a photoinduced electron transfer within the system.2 These findings underscore the potential application of these systems as donor-acceptor systems, wherein the Cu(II) porphyrine serves as the source, transferring electrons to the HBC nanographene. This points towards promising applications of this system in photovoltaic devices. Figure 1. A) Cu(II) porphyrine B) Cu (II) porphyrine, HBC and Cu(II) porphyrine-HBC dyad absorption spectra C) Cu (II) porphyrine with C-H chains, HBC and Cu(II) porphyrinewith C-H chains-HBC dyad absorption spectra D)u (II) porphyrine with C-H chains.______________________________________________[1] H. Imahori, T. Umevama, Journal of Physical Chemistry C 2009, 113, 9029–9039.[2] Shengqiang Xiao; Mohamed E. El-Khouly; Yuliang Li; Zhenhai Gan; Huibiao Liu; Li Jiang; Yasuyuki Araki; Osamu Ito; Daoben Zhu J. Phys. Chem. B 2005, 109, 8, 3658–3667 Figure 1

(Invited) Stereoselective Synthesis of Molecular Nanographenes

Chirality is an important and fascinating concept which has not been properly addressed in carbon nanoscience [1]. We have previously reported the first inherently chiral bilayer nanographene with a helicene linker, both as the racemate and the M isomer [2,3]. Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting in two π-π stacked hexa-benzocoronenes (HBC) joined by a helicene, thus resembling van der Waals layered 2D materials. Recently, we have synthesized and compared [9]HBNG, [10]HBNG and [11]HBNG helical bilayers endowed with [9], [10] and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being of 26, 14 and 10 benzene rings, respectively, according to the X-ray analysis [4]. More interesting, this overlapping also controls the properties.Compounds constituted only by carbon atoms, such as optically active molecular nanographenes have, so far, been obtained as racemates followed by time-consuming chiral chromatographic separations. In this presentation, an enantioselective strategy of a new bilayer nanographene which uses two pivotal stereocontrolled key synthetic steps to introduce and extend stereogenic elements into a polycyclic aromatic Csp2-based structure will be discussed. The resulting helical core of six ortho-fused rings embedded in the nanographene ensures stereochemical stability and good chiroptic activity [5]. Acknowledgements : This work has been supported by the Ministerio de Educación, Cultura y Deporte of Spain (Grant PID2020-114653RB-100).

(Invited) Innovative MOF-Based Strategies for Assembling Chromophores: Perspectives for Next-Generation Organic Electronics

In the realm of organic electronics, such as photovoltaics, photodetectors, and light-emitting devices, the integration of organic chromophores into solid-state assemblies and their connection with electrodes is crucial. A notable development in this area is the use of metal-organic frameworks (MOFs), also referred to as the MOF method [1], which has been gaining traction. This approach involves using organic chromophores like anthracene and other planar aromatics, including Hexa-peri-hexabenzocoronene (HBC), equipped with coupling units, to create ditopic linkers that connect with metal-oxo nodes in MOFs. In view of the challenges of integrating MOF powders into devices, recent advancements have led to the development of layer-by-layer methods yielding high-quality, monolithic MOF thin films [2]. These films offer exceptional optical qualities and show, in many cases, a reduced density of defects.MOFs' periodic structures enable the deliberate design of chromophore assemblies, impacting optical absorption through inter-molecular interactions. This design approach has facilitated the creation of structures like J-aggregates, initially simulated in silico and later realized experimentally [3]. Additionally, MOFs enable the realization of band-structure phenomena, such as indirect band gaps [4], and chiroptical effects, including circularly polarized light emission [5] and the helical sensitivity in photodetectors [6].We've also explored lbl architectures for constructing heterostructures, useful in photon up-conversion applications. This involves leveraging MOFs' porous nature, as seen in the integration of C60 into SURMOF pores for applications like photoconductivity [7] and organic diodes [8]. Recent efforts have also focused on assembling non-centrosymmetric SURMOFs for nonlinear optical properties, demonstrating substantial second harmonic generation (SHG) activity [9].Our presentation will showcase key instances of successfully incorporating organic chromophores into SURMOFs for device fabrication, emphasizing the synergy with theoretical models. Given the vast number (> 120,000) of known MOFs, a theoretical approach is essential to guide experimental efforts efficiently.

(Invited) Towards Understanding the Competition of Electron and Energy Transfer in “Molecular” Nanographenes on the Example of Hexa-Peri-Hexabenzocoronene

Bottom-up strategies have allowed the synthesis of “molecular” nanographenes with full control over size, shape and functionality. In recent years, the progress on wet chemical approaches, oxidative cyclodehydrogenation amongst all, has been the foundation to the synthesis of an impressive number of soluble and well-defined molecular nanographenes. The level of control over nanographene syntheses has allowed a fine-tuning of the photophysical and electrochemical properties and, in turn, has a compelling potential in the field of material science. In this regard, understanding and harnessing the competition between electron transfer and energy transfer in nanographenic systems is of utmost importance. However, a comprehensive structure-property relationship remains still an open aspect. In the present review we describe a large variety of hexa-peri-hexabenzocoronene (HBC)-based nanographenes obtained through wet chemical strategies and linked – either covalently or non-covalently – to porphyrins, rylenes, fullerenes, etc. Particular attention was placed on the optical, electrochemical and excited-state properties.

Hexa-peri-hexabenzocoronene as a vector sensing and delivery of Carmustine anticancer drug: A density functional theory study

We conducted research on the adsorption of carmustine (BCNU) on the surface of pristine, N-doped, and BN-doped Hexa-peri-hexabenzocoronene (HBC) nanographene using density functional theory calculations. We used the B3LYP functional in conjunction with 6-31+G(d) and 6-311+G(d,p) basis sets to perform geometrical optimization calculations, vibrational frequencies, and natural bond orbitals (NBO) analysis. The dispersion correction term of Grimme-D3 was incorporated to account for the dispersion interactions. We found that the geometry of the nanographenes remains unchanged after the drug molecule adsorption. The calculated adsorption energies (Eads) were −17.1 kcal.mol−1 for HBC-BCNU, −17.0 kcal.mol−1 for NHBC-BCNU, and −17.9 kcal.mol−1 for BNHBC-BCNU when using the 6-31+G(d) basis set. The corresponding values calculated employing the 6-311+G(d,p) basis set were −17.9, −17.3, and −19.1 kcal.mol−1, respectively. Analysis of frontier molecular orbitals (HOMO and LUMO) showed that the electronic properties of NHBC were significantly affected by the presence of BCNU. We used NBO analysis to investigate the distribution of partial atomic charges in different systems. Finally, using the transition state equation and the smaller basis set, we found that the required time for BCNU to release from HBC, NHBC, and BNHBC is approximately 3.3, 1.8, and 13.0 s, respectively. The obtained values are 13.0, 4.6, and 96.0 s, respectively, for the higher level of calculations. The recovery time for all nanographenes is within a reasonable range, while NHBC exhibits the highest sensitivity for drug detection.

Electrochemical Determination of Imidacloprid Using an Electrosynthesized Hexa-Peri-Hexabenzocoronene Modified Glassy Carbon Electrode (GCE) by Differential Pulse Voltammetry (DPV)

Hexa-peri-hexabenzocoronene (HBC) modified glassy carbon electrodes (HBC/GCEs) was fabricated by an electrochemical synthesis, solving the difficulty of the preparation of HBC films because of its poor solubility. HBC/GCE was used as a sensing electrode for the voltammetric investigation of imidacloprid (IDP). The presence of HBC increased the conductivity of GCE, and showed good electrocatalytic activity toward the reaction of IDP. Therefore, the HBC enhanced the electrochemical signal of IDP, which increased the sensitivity of the sensor. The best performance was obtained by optimizing the electrochemical synthesis time, pH and enrichment time. The linear response range and the limit of detection were 5 to 750 μmol·L−1 and 1.5 μmol·L−1, respectively. Moreover, the proposed sensor also exhibited good repeatability, selectivity and stability, and promising applications for real sample.

Nanographene-Fused Expanded Carbaporphyrin Tweezers.

A nanographene-fused expanded carbaporphyrin (5) and its BF2 complex (6) were synthesized. Single-crystal X-ray structures revealed that 5 and 6 are connected by two hexa-peri-hexabenzocoronene (HBC) units and two dipyrromethene or BODIPY units, respectively. As prepared, 5 and 6 both show nonaromatic character with figure-of-eight carbaoctaphyrin (1.1.1.0.1.1.1.0) cores and adopt tweezers-like conformations characterized by a partially confined space between the two constituent HBC units. The distance between the HBC centers is >10 Å, while the dihedral angles between the two HBC planes are 30.5 and 35.2° for 5 and 6, respectively. The interactions between 5 and 6 and fullerene C60 were studied both in organic media and in the solid state. Proton NMR spectral titrations of 5 and 6 with C60 revealed a 1:1 binding mode for both macrocycles. In toluene-d8, the corresponding binding constants were determined to be 1141 ± 17 and 994 ± 10 M-1 for 5 and 6, respectively. Single-crystal X-ray diffraction structural analyses confirmed the formation of 1:1 fullerene inclusion complexes in the solid state. The C60 guests in both complexes are found within triangular pockets composed of two HBC units from the tweezers-like receptor most closely associated with the bound fullerene, as well as an HBC unit from an adjacent host. Femtosecond transient absorption measurements revealed subpicosecond ultrafast charge separation between 5 (and 6) and C60 in the complexes. To the best of our knowledge, the present report provides the first example wherein a nanographene building block is incorporated into the core of a porphyrinic framework.

Tetrahedraphene: A Csp3 -centered 3D Molecular Nanographene Showing Aggregation-Induced Emission.

The bottom-up synthesis of 3D tetrakis(hexa-peri-hexabenzocoronenyl)methane, "tetrahedraphene", is reported. This molecular nanographene constituted by four hexa-peri-hexabenzocoronene (HBC) units attached to a central sp3 carbon atom, shows a highly symmetric arrangement of the HBC units disposed in the apex of a tetrahedron. The X-ray crystal structure reveals a tetrahedral symmetry of the molecule and the packing in the crystal is achieved mostly by CH⋅⋅⋅π interactions since the interstitial solvent molecules prevent the π⋅⋅⋅π interactions. In solution, tetrahedraphene shows the same electrochemical and photophysical properties as the hexa-t Bu-substituted HBC (t Bu-HBC) molecule. However, upon water addition, it undergoes a fluorescence change in solution and in the precipitated solid, showing an aggregation induced emission (AIE) process, probably derived from the restriction in the rotation and/or vibration of the HBCs. Time-Dependent Density Functional Theory (TDDFT) calculations reveal that upon aggregation, the high energy region of the emission band decreases in intensity, whereas the intensity of the red edge emission signal increases and presents a smoother decay, compared to the non-aggregated molecule. All in all, the excellent correlation between our simulations and the experimental findings allows explaining the colour change observed in the different solutions upon increasing the water fraction.

Tetrahedraphene: A Csp3‐centered 3D Molecular Nanographene Showing Aggregation‐Induced Emission

AbstractThe bottom‐up synthesis of 3D tetrakis(hexa‐peri‐hexabenzocoronenyl)methane, “tetrahedraphene”, is reported. This molecular nanographene constituted by four hexa‐peri‐hexabenzocoronene (HBC) units attached to a central sp3 carbon atom, shows a highly symmetric arrangement of the HBC units disposed in the apex of a tetrahedron. The X‐ray crystal structure reveals a tetrahedral symmetry of the molecule and the packing in the crystal is achieved mostly by CH⋅⋅⋅π interactions since the interstitial solvent molecules prevent the π⋅⋅⋅π interactions. In solution, tetrahedraphene shows the same electrochemical and photophysical properties as the hexa‐tBu‐substituted HBC (tBu‐HBC) molecule. However, upon water addition, it undergoes a fluorescence change in solution and in the precipitated solid, showing an aggregation induced emission (AIE) process, probably derived from the restriction in the rotation and/or vibration of the HBCs. Time‐Dependent Density Functional Theory (TDDFT) calculations reveal that upon aggregation, the high energy region of the emission band decreases in intensity, whereas the intensity of the red edge emission signal increases and presents a smoother decay, compared to the non‐aggregated molecule. All in all, the excellent correlation between our simulations and the experimental findings allows explaining the colour change observed in the different solutions upon increasing the water fraction.

Self-Assembled Monolayers of Gemini-Type Amphiphilic Hexabenzocoronenes on Gold: Contribution of Their Triethylene Glycol Side Chains to Self-Assembly Formation.

We report on a two-dimensional self-assembled structure of a supramolecule with hydrophilic oligoethylene glycol (EG) units, which are capable of stronger electrostatic interactions than van der Waals (vdW) interactions between alkyl chains. For this purpose, hexabenzocoronene (HBC) with two hydrophobic dodecyl chains on one side of the HBC core and two hydrophilic triethylene glycol (TEG) chains on the other side of the HBC core (HBCGemini) and HBCGemini with a trinitrofluorenone (TNF) added to the end of one TEG chain (HBCTNFGemini) were employed. Scanning tunneling microscopy (STM) revealed the presence of multiple two-dimensional self-assembled structures in each of HBCGemini and HBCTNFGemini deposited on the gold substrate in vacuum. The role of polar functional groups in these observations is discussed based on semiempirical molecular orbital simulations. Two types of 2D organized structures of HBC-TEG were observed: one with rectangular and relatively dense unit cells and the other with nearly square and relatively sparse unit cells. In both organized structures, the phenyl group TEG units and alkyl chains were considered to be the main molecular interactions with each other. On the other hand, in HBCTNFGemini, three types of organized structures were observed, which could be explained by the mechanism of interdigitation of the TEG-containing side-chain moieties to form a dimeric core. The EG units are more flexible than the alkyl chains and thus can interact flexibly with the hydrophobic HBC core, and the glycol side chains facilitate the intermolecular interactions as well as the alkyl chains.

Fine-Tuning of the Electronic and Optical Properties of Dodecabenzocoronene through Boron and Nitrogen Doping: A DFT Insight

Nanographene provides a wide range of possibilities in graphene engineering for future applications due to the higher degrees of configurational freedom with the electronic parameters that may also be continuous or discrete, depending on the intended application. Therefore, the optical and electronic properties of nanographene are of substantial technological interest. Moreover, doping of graphene with heteroatoms (B, P, N, and S, etc.) alters their chemical and electronic characteristics which are suitable for the economical construction of optoelectronic devices. Herein, geometric, electronic, and optical properties of nanographene are evaluated as a function of the nature and position of dopant. Three different nanographene including coronene, hexabenzocoronene (HBC), and dodecabenzocoronene (DBC) are considered for doping (N and B as dopants) in this study with the key focus on DBC-doped systems. For any dopant number, all possible dopant sites are studied except edge position in order to avoid the edge effect. Frontier molecular orbital (FMO) analysis is performed to evaluate the perturbations in electronic characteristics of doped nanographene. A decrease in energy gap is seen for all doped systems. Natural bond orbital (NBO) analysis indicates that doping of boron (B) and nitrogen (N) results in variation in distribution of charges over the nanographene surfaces. The density of states (DOS) analysis reveals that Fermi level ([Formula: see text] is shifted for all B- and N-doped systems. The UV-visible (UV-Vis) absorption spectra are computed to evaluate the changes in the intensity and maximum adsorption wavelength ([Formula: see text] in all doped DBC. Various chemical reactivity descriptors are also evaluated which reveal the degree of stability and chemical reactivity of doped systems. The results indicate that multiple B and N atoms doping offers a new possibility for fine-tuning of electronic and optical properties of nanographene at atomic level, thus providing guidance in development of future advanced optoelectronic devices.

(Invited) Towards Understanding the Competition of Electron and Energy Transfer in Nanographene

Graphene has captured the imagination of researchers around the world due to its groundbreaking chemical and physical properties. Opening a band gap in graphene must be achieved without, however, compromising its exceptional properties as they are of paramount importance for its use in electronic devices. Notable is the fact that the band gap design in graphene is typically carried out by either chemical or physical methodologies. Chemical modification of graphene is mostly centered around “top-down” or “bottom-up” approaches. The earlier alters, nevertheless, the graphene lattice and, as a consequence, poorly defined structures emerge. The latter by means of, for example, organic synthesis offers a wide palette of tools to control sizes as well as geometries of the resulting “molecular” nanographenes with atomic precision. It allows the fabrication of uniform and well-defined molecular structures. Such “molecular” nanographenes are compelling choices for “on demand” molecular electronics, photovoltaic applications, hydrogen storage, and sensing. In recent years, two main strategies have been developed to fabricate “molecular” nanographenes of defined chemical structures. It is, on one hand, oxidative cyclodehydrogenation of custom-made polycyclic aromatic hydrocarbons (PAHs) and, on the other hand, on-surface cyclodehydrogenation, which enabled the preparation of atomically precise “molecular” nanographenes. To this end, the 13 fused-benzene rings of hexa-peri-hexabenzocoronene (HBC), which are arranged in a 2D disk-shaped fashion, render HBCs the smallest “molecular” nanographenes.

Self-Assembly of Functional Organic Chromophores

Aggregate structures of organic chromophores strongly affect their properties and so has become an active area of research. Self-assembly and supramolecular methods present important means to influence aggregation processes, and these have become active areas of research. We have studied the self-assembly of differently structured chromophore molecules with emphasis on porphyrins,1 pyrazinacenes,2 hexabenzocoronenes (HBC) and oligophenylenes (such as sexiphenyl), and the various methods of doing this including molecular design, surface assembly (under ambient or UHV conditions), and using interfacial media such as in the Langmuir and Langmuir-Blodgett techniques. For the latter, we have applied principles of amphiphilicity to generate on-interface structures,3 which can be lifted as multilayered films and exploited for the preparation of electronic devices.4 In this work, we will describe the self-assembly properties based on precipitation/gelation, assembly at solid surfaces, and interactions at an air-water interface. We will emphasize the molecular structure as a key feature including the effects of π-π stacking, conformational flexibility and amphiphilicity. Self-assembly and supramolecular techniques stand out as the most effective methods for obtaining nanomaterials having the molecule-level definition of structure important for any applications of the resulting materials.

(Invited) Chiral Bilayer Nanographenes: A New Family of Carbon Nanostructures

Chirality is an important and fascinating concept which has not been properly addressed in nanocarbons science.1 We have previously reported the first inherently chiral bilayer nanographene with a helicene linker, both as the racemate and the M isomer.2,3 By extending precedented [6]helicene starting material, we obtained an unprecedented chiral nanographene comprised of two hexa-peri-hexabenzocoronene (HBC) layers fused to a [10]helicene . Furthermore, we have increased the family of bilayers by using the respective [5] and [7]helicenes, affording bilayers with [9] and [11]helicenes, respectively. These nanographenes are appealing systems for the study of further chemical reactions including redox reactions.4 In this presentation, the recent findings on this singular family of chiral nanographenes showing remarkable CPL values, as well as their less known outstanding photophysical properties will be discussed. Acknowledgements The author acknowledge support from the European Research Council (ERC) Synergy Grant “TOMATTO” (Project 951224).

Helical Bilayer Nanographenes: Impact of the Helicene Length on the Structural, Electrochemical, Photophysical, and Chiroptical Properties.

Helical bilayer nanographenes (HBNGs) are chiral π-extended aromatic compounds consisting of two π-π stacked hexabenzocoronenes (HBCs) joined by a helicene, thus resembling van der Waals layered 2D materials. Herein, we compare [9]HBNG, [10]HBNG, and [11]HBNG helical bilayers endowed with [9], [10], and [11]helicenes embedded in their structure, respectively. Interestingly, the helicene length defines the overlapping degree between the two HBCs (number of benzene rings involved in π-π interactions between the two layers), being 26, 14, and 10 benzene rings, respectively, according to the X-ray analysis. Unexpectedly, the electrochemical study shows that the lesser π-extended system [9]HBNG shows the strongest electron donor character, in part by interlayer exchange resonance, and more red-shifted values of emission. Furthermore, [9]HBNG also shows exceptional chiroptical properties with the biggest values of gabs and glum (3.6 × 10-2) when compared to [10]HBNG and [11]HBNG owing to the fine alignment in the configuration of [9]HBNG between its electric and magnetic dipole transition moments. Furthermore, spectroelectrochemical studies as well as the fluorescence spectroscopy support the aforementioned experimental findings, thus confirming the strong impact of the helicene length on the properties of this new family of bilayer nanographenes.

Two-Step On-Surface Synthesis of One-Dimensional Nanographene Chains

Controllable fabrication of low-dimensional graphene structures is a matter of scientific and technological interest. To date, significant efforts have been devoted to the fabrication of nanographene chains as well as nanographenes and graphene nanoribbons. Here, we present the on-surface synthesis of one-dimensional (1D) nanographene chains composed of hexa-peri-hexabenzocoronene (HBC) which is a common type of nanographene. Using dibromo-substituted hexaphenylbenzene (Br2-HPB) as a precursor, the 1D HBC nanographene chains are achieved through two reaction steps, namely polymerization and cyclodehydrogenation, on a Au(111) surface, which have been investigated by low-temperature scanning tunneling microscopy. The Br2-HPB molecules deposited on Au(111) are formed into a chiral selective self-assembled layer, and the following thermal annealing at 200–300 °C induces debromination and sequential polymerization of Br2-HPB, leading to the formation of long 1D HPB chains. In particular, we find that the same length 1D HPB chains are selectively ordered in a side-by-side arrangement. Finally, by annealing at 400 °C, HBC planar nanographenes are achieved through the cyclodehydrogenation process of HPB within the long 1D chains. The maximum length of the obtained 1D HBC nanographene chains reached is about 30 nm.