Hexabenzocoronene Research Articles

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.

Nanographene Coupled with Interfacial Pyrene Derivatives for Thermally Stable Perovskite Solar Cells

Although high-efficiency perovskite solar cells (PSCs) have been achieved using a hole-extracting material, spiro-MeOTAD, thermal stability has been unattainable due to the low glass transition temperature of spiro-MeOTAD and additives therein. Here, we report on the use of nanographene-based hole-transporting materials coupled with a pyrene derivative as an interface modifier for thermally stable and high efficiency PSCs. Asymmetric methyl and methoxy groups are introduced in the diphenylamino group that is attached to the hexa-peri-hexabenzocoronene (HBC) nanographene core, coded HBC-DPAMeOMe. 1-Pyrenemethylammonium iodide is coupled to enhance the chemical interaction between perovskite and HBC-DPAMeOMe, which leads to a power conversion efficiency over 23%. A thermal stability test at 85 °C for 1000 h reveals that 83.6% of the initial efficiency (23.04% → 19.25%) is maintained for the device with HBC-DPAMeOMe, while a significant degradation from 20.69% to 5.08% is observed for the device with spiro-MeOTAD. Nanographene-based hole conductors shed light on the thermal stability issue in PSCs.

Carpet Beater Molecules: Synthesis and Characterization of Functional Hexa‐peri‐benzocoronene–Alkyne Coupling Products

AbstractWithin the scope of this paper, nine π‐expanded mono‐substituted 5,8,11,14,17‐pentakis‐(tert‐butyl)‐hexa‐peri‐hexabenzocoronenes (HBC) are introduced. 2‐Iodo‐5,8,11,14,17‐pentakis‐(tert‐butyl)‐hexa‐peri‐hexabenzocoronene served as precursor and was reacted with ethynyltrimethylsilane in a Sonogashira coupling reaction. The acetylene unit is used as a linker and can undergo another Sonogashira coupling reaction combining different phenyl coupling partners with the HBC core. The electron‐withdrawing groups such as nitrile, pyridine and carbonyl species (aldehyde, methylester, carboxylic acid) as well as the three quinoxaline based species (diphenylquinoxaline, dibenzo[a,c]phenazine, phenanthro[4’,5’‐a,b,c]phenazine) serve as substitution moieties. Their influence on the optoelectronic properties were investigated by UV/Vis absorption spectroscopy demonstrating a maximum redshift of 7 nm compared to starting compound 2‐Iodo‐5,8,11,14,17‐pentakis‐(tert‐butyl) HBC. As for the phenanthro[4’,5’‐a,b,c]phenazine substituted HBC a dramatic decrease in the intensity of the absorption of the UV/Vis spectrum was observed. The fluorescence spectroscopy pointed out that the dibenzo[a,c]phenazine and phenanthro[4’,5’‐a,b,c]phenazine substitution changed the spectra to one broad peak departing from the characteristic HBC‐like emission pattern.

Investigation of the Influence of Hexabenzocoronene in Polyacrylonitrile-Based Precursors for Carbon Fibers

For several decades, carbon fibers have been used for lightweight engineering in aircraft automotive and sports industries, mostly based on high-quality polyacrylonitrile (PAN). We investigated a novel PAN-based precursor fiber (PF) modified with a polycyclic aromatic hydrocarbon, namely hexabenzocoronene (HBC), which is expected to improve the thermal conversion process and to create a carbon fiber (CF) with enhanced mechanical properties. For this purpose, the novel PF and a spun-like homopolymeric PAN-based PF were thermally stabilized and carbonized in continuous lab-scale plants. The effect of the additive HBC on the conversion processes, fiber diameter and shape, density, and mechanical properties were investigated. The results showed that HBC seems to support stabilization reactions, and HBC/PAN-based PF show potentially higher stretchability of PF and stabilized fiber. The modified CF showed an improvement in Young’s modulus of about 25% at the same tensile strength compared to the unmodified PAN-based CF, resulting from enhanced crystalline orientation. The results showed a high potential of the HBC/PAN for energy-efficient production. In particular, the influence on tensile strength and modulus under optimized process conditions, as well as the possibility to use low quality PAN, need to be further investigated.

Azepine-Embedded Seco-Hexabenzocoronene-Based Helix Nanographenes: Access to Modification of the Core by N–H Functionalization

Contorted polycyclic aromatic hydrocarbons (PAHs) or nanographenes (NGs) have received increasing attention and are mostly prepared by "bottom-up" strategies. Apparently, systematically tuning the properties of NGs for application is important but challenging. Here, a new type of helix, azepine-embedded NGs, were designed and synthesized by the introduction of NH into the hexa-peri-hexabenzocoronene (HBC) core. We demonstrate that this nitrogen-doped NG can be functionalized via N-H derivatization. Through modifications to the NH site with a chiral auxiliary reagent, optical resolution of the chiral NG was achieved. Meanwhile, it was found that by introducing various aryl groups with electron-donating or electron-withdrawing substituents, the emission intensity and the fluorescence mechanism can be modulated. Compared to the original NH-containing NG, the modified derivative exhibited improved fluorescence efficiency and tunable emission wavelength. A functionalized structure of benzoic acid with considerably improved fluorescence efficiency, hydrophilicity, and membrane permeability to stain the live cells was proved.

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.

Chalcogen-doped, (seco)-hexabenzocoronene-based nanographenes: synthesis, properties, and chalcogen extrusion conversion.

A series of chalcogen-doped nanographenes (NGs) and their oxides are described. Their molecular design is conceptually based on the insertion of different chalcogens into the hexa-peri-hexabenzocoronene (HBC) backbone. All the NGs adopt nonplanar conformations, which would show better solubility compared to planar HBC. Except for the oxygen-doped, saddle-shaped NG, the insertion of large chalcogens like sulfur and selenium leads to a seco-HBC-based, helical geometry. All the three-dimensional structures are unambiguously confirmed by single-crystal X-ray diffractometry. Their photophysical properties including UV-vis absorption, fluorescence, chiroptical, charge distribution, and orbital gaps are investigated experimentally or theoretically. The properties of each structure are significantly affected by the doped chalcogen and its related oxidative state. Notably, upon heating or adding an acid, the selenium-doped NG or its oxide undergoes a selenium extrusion reaction to afford seco-HBC or HBC quantitatively, which can be treated as precursors of hydrocarbon HBCs.

BINOL-like atropisomeric chiral nanographene.

Interest in making chiral polycyclic aromatic hydrocarbons (PAHs) or nanographenes (NGs) has greatly increased recently. To date, a majority of chiral nanocarbons have been designed based on helical chirality. Here, we describe a novel atropisomeric chiral oxa-NG 1 by the selective dimerization of naphthalene-containing, hexa-peri-hexabenzocoronene (HBC)-based PAH 6. The photophysical properties of the oxa-NG 1 and monomer 6 were investigated, including UV-vis absorption (λ max = 358 nm for 1 and 6), fluorescence emission (λ em = 475 nm for 1 and 6), fluorescence decay (15 vs. 16 ns), and fluorescence quantum yield, and it was found that the photophysical properties of the monomer are nearly maintained in the NG dimer due to its perpendicular conformation. Single-crystal X-ray diffraction analysis shows that both enantiomers cocrystallize in a single crystal, and the racemic mixture can be resolved by chiral high-performance liquid chromatography (HPLC). The circular dichroism (CD) spectra and circularly polarized luminescence (CPL) of the enantiomers of 1-S and 1-R were studied and the CD and CPL spectra exhibited opposite Cotton effects and fluorescence signals. Density functional theory (DFT) calculations and HPLC-based thermal isomerization results showed that the racemic barrier is as high as 35 kcal mol-1, suggesting a rigid chiral nanographene structure. Meanwhile, in vitro studies indicated that oxa-NG 1 is an efficient photosensitizer for white-light-induced singlet oxygen generation.

A graphyne spoked wheel

<h2>Summary</h2> γ-Graphyne is an emerging carbon allotrope, and synthesis of large-size atomically precise γ-graphyne fragments would be crucial to understand its distinctive properties. Herein, a long-sought-after graphyne spoked wheel with 6-fold symmetry was synthesized mainly through cobalt(0)-catalyzed cyclotrimerization of a triyne intermediate followed by 12-fold intramolecular Stille coupling reactions. Its structure was unambiguously confirmed by X-ray crystallographic analysis, which revealed a closely stacked dimer structure. NMR and theoretical analysis disclosed that fusion of six anti-aromatic hexadehydro[12]annulene units led to a decrease in local aromaticity of all seven benzenoid rings (particularly, the central one), which is quite different from a graphene analog, hexa-<i>peri</i>-hexabenzocoronene (HBC). Its optical and electrochemical properties are also different from those of HBC, showing allowed HOMO → LUMO electronic transition and facile electrochemical reduction. This study provided a new method for the synthesis of even larger γ-graphyne fragments and demonstrated the unique electronic properties and packing structure of γ-graphyne as compared with those of graphene.

Regiodivergent Synthesis and π‐Stacking‐Induced Chiral Self‐Recognition of Hexabenzocoronene‐Based [6]Helicenes

AbstractThe lateral π‐extension of helicenes would yield interesting physical properties derived from the enhanced π‐stacking ability as well as intrinsic helical chirality. Here we report the regiodivergent synthesis of hexabenzocoronene (HBC)‐based [6]helicenes via cationic rhodium(I)/segphos complex‐mediated intramolecular [2+2+2] and [2+1+2+1] cycloadditions of triynes followed by Scholl reaction. The resulting HBC‐based [6]helicenes possess stable helical chirality but more planar structure than previously reported HBC‐based [7] and [9]helicenes. Therefore, these HBC‐based [6]helicenes exhibit π‐stacking‐induced chiral self‐recognition in solution, allowing the measurement of enantiomeric ratios by1H NMR spectroscopy without any chiral additive.

Low-Dimensional Porous Carbon Networks Using Single-/Triple-Coupling Polycyclic Hydrocarbon Precursors.

Polycyclic hydrocarbons (PHs) share the same hexagonal structure of sp2 carbons as graphene but possess an energy gap due to quantum confinement effect. PHs can be synthesized by a bottom-up strategy starting from small building blocks covalently bonded into large 2D organic sheets. Further investigation of the role of the covalent bonding/coupling ways on their electronic properties is needed. Here, we demonstrate a surface-mediated synthesis of hexa-peri-hexabenzocoronene (HBC) and its extended HBC oligomers (dimers, trimers, and tetramers) via single- and triple-coupling ways and reveal the implication of different covalent bonding on their electronic properties. High-resolution low-temperature scanning tunneling microscopy and noncontact atomic force microscopy are employed to in situ determine the atomic structures of as-synthesized HBC oligomers. Scanning tunneling spectroscopy measurements show that the length extension of HBC oligomers narrows the energy gap between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Furthermore, the energy gaps of triple-coupling HBC oligomers are smaller and decrease more significantly than that of the single-coupling ones. We hypothesize that the triple coupling promotes a more effective delocalization of π-electrons than the single coupling, according to density functional theory calculations. We also demonstrate that the HBC oligomers can further extend across the substrate steps to achieve conjugated polymers and large-area porous carbon networks.

Probing Charge Management across the π-Systems of Nanographenes in Regioisomeric Electron Donor-Acceptor Architectures.

Inspired by light-induced processes in nature to mimic the primary events in the photosynthetic reaction centers, novel functional materials combine electron donors and acceptors, i.e., (metallo)porphyrins (ZnP) and fullerenes (C60), respectively, with emerging materials, i.e., nanographenes. We utilized hexa-peri-hexabenzocoronene (HBC) due to its versatility regarding functionalization and physicochemical properties, to construct three regioisomeric ZnP-HBC-C60 conjugates, which foster geometrical diversity by arranging ZnP and C60 in ortho-, meta-, and para-positions to each other. The corresponding hexaarylbenzene (HAB) motifs, with an interrupted π-system, were also prepared. Transient absorption measurements disclosed the fast population of charge transfer as well as singlet and triplet charge-separated states. With the help of density functional theory (DFT) calculations, we further conceive the communication across the HBCs and HABs. This work reveals the impact of both the geometrical arrangement with respect to through-space versus through-bond interactions and the structural rigidity/flexibility on the charge management across the different π-systems.

Fluorescence Enhancement by Supramolecular Sequestration of a C54-Nanographene Trisimide by Hexabenzocoronene.

A supramolecular trilayer nanographene complex consisting of a newly synthesized D3h-symmetric C54-nanographene trisimide (NTI 1) and two hexabenzocoronenes (HBC) has been obtained by self-assembly. This 1:2 complex is structurally well-defined according to UV/vis and single crystal X-ray studies and exhibits high thermodynamic stability even in polar halogenated solvents. Complexation of NTI 1 by two HBC molecules protects the NTI 1 π-surface efficiently from oxygen quenching, thereby leading to a sequestration-induced fluorescence enhancement under ambient conditions.

Hexabenzocoronene functionalized with antiaromatic S- and Se-core-modified porphyrins (isophlorins): comparison with the dyad with regular porphyrin

Abstract The important and perspective molecular building blocks composed of hexaphenylbenzenes (HPBs) or their oxidized derivatives, hexa-peri-hexabenzocoronenes (HBCs), and metalloporphyrins have recently received significant attention of the researchers. In this study, motivated by recent findings, we have addressed the modifications of structures and properties of HBC-porphyrin compounds by using instead of aromatic porphyrins antiaromatic 20π isophlorin derivatives of thiophene or selenophene. We have reported the first comparative computational investigation of the following systems: (i) HBC with one non-metallated aromatic porphyrin, P(N4H2), unit, HBC-P(N4H2), (ii) HBC with one S-core-modified antiaromatic porphyrin (S-isophlorin), PS4, unit, HBC-PS4, and (iii) HBC with one Se-core-modified antiaromatic porphyrin (Se-isophlorin), PSe4, unit, HBC-PSe4. The study has been done employing the B3LYP/6-31G* approach (in the gas phase and in the implicit solvents, benzene and dichloromethane), and comparison with the B3LYP/6-31G** and B3LYP/6-311G* approaches was performed, where relevant. The effects of the core-modified antiaromatic isophlorins on the structures, electronic, and other properties, potentially including reactivity, of the whole building block HBC-isophlorin have been shown to be quite pronounced and to be noticeably stronger than the effects of the original aromatic non-metallated porphyrin. Thus, we have demonstrated theoretically that the complete porphyrin core-modification with other elements, this time with S and Se leading to the formation of the antiaromatic isophlorins, should be considered as a promising way for modifying and tuning structures, electronic properties and reactivity of the hexabenzocoronene-porphyrin(s) building blocks.

Non-enzymatic glucose sensors based on Hexa-peri-hexabenzocoronene: A computational study

(a) Optimized structure of AlN-HBC:GLU complex, (b) its MEP, (c) HOMO, and (d) LUMO profiles. Distances are in Å. • HBC, BN-HBC, and AlN-HBC were studied as non-enzymatic glucose sensors using DFT method. • The E g value of HBC and BN-HBC remain almost constant upon adsorption of GLU molecule. • The E g value of AlN-HBC decreases by about −18.48% after GLU adsorption. • Electrical conductance of AlN-HBC becomes 1.2 × 10 5 times higher after GLU adsorption. • AlN-HBC can behave as a selective sensor of GLU in the presence of FRU molecule. Using density functional theory (DFT) method, hexa - peri -hexabenzocoronene (HBC) and its central ring doped analogues (BN-HBC and AlN-HBC) were studied as non-enzymatic glucose sensors. The HOMO-LUMO energy gap (E g ) of pristine HBC and BN-HBC remain almost constant upon adsorption of GLU molecule, and therefore these nanographenes do not show electronic sensitivity toward GLU molecule. The E g value of AlN-HBC decreases by about −18.48% after GLU adsorption and the electrical conductance of AlN-HBC turns out to be 1.2 × 10 5 times higher after adsorption of GLU molecule. AlN-HBC can behave as a selective sensor of GLU in the presence of FRU molecule.

Ordered structure constructed from C2-symmetric hexa-peri-hexabenzocoronene linked with oligo(dimethylsiloxane).

The preparation of sub-5-nm ordered structures is very important to the development of today's nanotechnology. Block molecules have the potential to form structures with significantly small characteristic dimensions. Herein two novel organic-inorganic block molecules composed of a hexa-peri-hexabenzocoronene (HBC) core and two oligo(dimethylsiloxane) (ODMS) tails with C2 symmetry are reported. A hierarchical lamello-columnar structure with a two-dimensional rectangular lattice where HBC cores adopt a tilted arrangement was obtained from their bulk self-assembly. The feature sizes are all below 5 nm and can be regulated via the number of ODMS chains. Sub-5-nm line structures were obtained through spin-coating of the block molecules onto silicon substrates modified with poly(dimethylsiloxane). As organic-inorganic hybrid materials, these block molecules may be further applied in sub-5-nm nanopatterning.

Azepine- or Oxepine-embedded Double Saddle-Helix Nanographenes.

The azepine- and oxepine-embedded polycyclic aromatic hydrocarbons (PAH) 1-3, as the hexa-peri-hexabenzocoronene (HBC)-based nanographenes (NG) were designed and synthesized by Diels-Alder reaction of cyclic alkene with tetrachlorothiophene-S,S-dioxide, followed by Suzuki-Miyaura cross-coupling and Scholl-type cyclodehydrogenation. Due to the strained seven-membered ring and the inherent structural pattern, heteroatom-doped NGs 1-3 show Cs symmetrical, double saddle-helix hybrid conformation, which represents a new shape for HBC based nanographenes. The calculation studies reveal the low aromaticity of the 8π heterocycles themselves and the heterocycles also decrease the electron delocalization of benzenes surrounding them. Dynamics-based calculation suggests the Cs symmetry would maintain druing the saddle-inversion process. Meanwhile, we show property perturbation by doping with different heteroatoms.

Synthesis of Azepine- or Azocine-Embedded Hexabenzocoronene-Based Nanographenes

AbstractClassical polycyclic aromatic hydrocarbons (PAHs) are a large group of π-conjugated planar structures. To explore more properties of PAHs, new structures have been designed, including some with nonplanar topologies and heteroatom-containing moieties. As important nanographene (NG) units, hexabenzocoronene (HBC) and its derivatives have been intensively researched. Many HBC-based nonplanar NGs have been developed, and heteroatom-doped HBC-based NGs have also been designed and synthesized. Here, we summarize some methods for the synthesis of HBC-based NGs containing a seven- or eight-membered ring, and we discuss our newly synthesized NGs containing embedded azepine or azocine moieties, obtained by a modified synthetic route and a conventional Müllen’s synthetic method, respectively.

Direct fluorination of nanographene molecules with fluorine gas

The transformation of graphene into sp3-bonded carbon films through ultra-high pressure, hydrogenation or fluorination has been studied for years. It is considered that only few layers of graphenes on metal surfaces can realize this transformation and the electron orbitals hybridization between metal substrate and graphene layer is thought to be the drving force and stabilize the formed sp3 carbon structure. Here, we use nanographene molecule, hexa-peri-hexabenzocoronene (HBC), as the model compound to exploit the fluorination reaction between graphene and fluorine gas. MALDI-TOF mass, XPS and XRD results show that the direct fluorination realized the intermolecular cross-linking of nanographene molecules without the assistence of mental substrate. Combining with theoretical calculation and EPR test results, we proved that the fluorination of nanographene molecules with F2 follows a free-radical mechanism, and the carbon radicals formed during fluorination caused the covalent cross-linking between graphene molecules which have a tight π-π stacking. Meanwhile, the fluorination of other small nanographene molecules, pyrene and perylene, further confirmed the universality of this free-radical cross-linking mechanism. This would provide a further understanding for the chemical transformation from graphene layers to 2D diamond structure under chemical adsorption process.