Hexa-peri-hexabenzocoronene Derivatives Research Articles

(Invited) Photoinduced Donor-Acceptor Interaction in Nanocarbon-Based Systems

Nanographenes, characterized by well-defined planar structures of sp2 carbon network with sizes over 1 nm, have attracted tremendous attention, as their extended π-conjugation offers intriguing characteristics in optical and electronic properties, such as large extinction coefficients, high photoluminescence quantum yields, and excellent charge transport abilities. These properties suggest their potential utility as basic components in various electronic devices including light-emitting diodes, solar cells, and field-effect transistors. Along this line, nanographenes with different sizes and edge configurations have been prepared, demonstrating the possibility of fine-tuning their electronic and optoelectronic properties through the structural modulation. Among them, hexa-peri-hexabenzocoronenes (HBCs) consisting of 42 sp2 carbons in the π-conjugated core can be regarded as the minimum unit of nanographenes. In contrast with nanographene molecules, other two-dimensional (2D) nanocarbons such as graphenes, graphene oxides (GOs), chemically-converted graphenes (CCGs), and graphene quantum dots (GQDs) are structurally inhomogeneous at the molecular level, containing different π-conjugation sizes, edge structures, and defect sites. Nanographenes can be regarded as a part of the larger 2D nanocarbons, and therefore, π-extended HBC derivatives with well-defined structures are ideal model units to understand the intrinsic structure–property correlations of the 2D nanocarbons.Covalent linking of nanographenes and photoactive molecules is an effective strategy to endow them with new functions, as well as to examine their optical and photophysical properties, without changing the nanographene core structures. Various HBC linked systems with photoactive molecules including porphyrins, perylene diimides, and fullerenes were prepared and their basic optical properties such as steady-state absorption and fluorescence spectra were investigated. Nevertheless, so far such covalent functionalization of nanographenes has been limited to HBCs solely, and, to the best of our knowledge, covalently linked systems of photoactive molecule–nanographenes larger than HBC have never been reported. Therefore, the size and shape effects of nanographenes on the optical and photophysical properties of nanographene–photoactive molecule linked systems have yet to be studied systematically.To this end, we employed two nanographenes as the model: the symmetrical HBC and its π-extended rectangular strip-shaped nanographene consisting of 114 sp2 carbons, which is a short graphene nanoribbon (GNR). They were linked with two zincporphyrin (ZnP) units through a p-phenylene bridge at the periphery positions for the better understanding of their intrinsic properties. The short, rigid phenylene spacer ensures a well-defined geometry between the nanographene and the attached porphyrins. The HBC core was selected as a typical, basic example of nanographenes. Although various linked systems of HBC with photoactive molecules including porphyrins have already been reported, ZnP–HBC linked systems with the p-phenylene spacer have yet to be synthesized. Meanwhile the GNR core was chosen because of its closely related structure to HBC. GNR possesses the same size on the short axis, but the large size on the long axis compared to HBC. Furthermore, GNR and HBC have the same edge structures, which provides an opportunity to focus on the size effect of nanographenes.In this talk, I will give an overview of photoinduced donor-acceptor interaction in nanocarbon-based systems. For instance, photoexcitation of the porphyrin–HBC linked system led to exclusive energy transfer (EnT) from the first singlet excited state (S1) of the nanographene to the porphyrin, whereas opposite selective EnT occurred from the first and second singlet excited states (S1 and S2) of the porphyrin to the nanographene in the porphyrin–GNR linked system. In particular, ultrafast efficient EnTs from both the S2 and S1 states of the porphyrin to GNR mimic the corresponding ultrafast EnTs from the S2 and S1 states of carotenoids to chlorophylls in light-harvesting systems of natural photosynthesis. Such unique photophysical properties will be useful for the rational design of carbon-based photofunctional nanomaterials in optoelectronics and solar energy conversion devices.[1] X. Liu, M. Kozlowska,T. Okkali, D. Wagner,T. Higashino, G. Brenner-Weiß, S. Marschner, Z. Fu, Q. Zhang, H. Imahori, S. Bräse, W. Wenzel,C. Wöll, L. Heinke, Angew. Chem. Int. Ed., 58, 9590 (2019).[2] T. Umeyama, T. Hanaoka, H. Yamada, Y. Namura, S. Mizuno, T. Ohara, J. Baek, J. Park, Y. Takano, K. Stranius, N. V. Tkachenko, H. Imahori, Chem. Sci., 10, 6642 (2019).

Porphyrin-rylene-diimide-hexabenzocoronene triads

Hexa-peri-hexabenzocoronenes (HBCs), substituted by two different chromophores in an ortho arrangement, were synthesized for the first time and investigated towards their photophysical properties. Rylene-diimide dyes, in particular naphthalene-diimides (NDIs) and perylene-diimides (PDIs), were attached to the HBC as the first, and porphyrins as the second chromophore. Therefore, porphyrin-NDI-HPB as well as porphyrin-PDI-HPB precursors were accessed and converted into the respective HBC derivatives applying oxidative Scholl conditions. UV-vis absorption and fluorescence emission spectroscopy showed interesting photophysical behavior such as energy transfer processes. Particularly, the porphyrin-PDI-HBC displays pronounced absorption features between 330–650 nm, which cover a wide range of the visible spectrum.

Multiple-Porphyrin Functionalized Hexabenzocoronenes.

Porphyrin–hexabenzocoronene architectures serve as good model compounds to study light‐harvesting systems. Herein, the synthesis of porphyrin functionalized hexa‐peri‐hexabenzocoronenes (HBCs), in which one or more porphyrins are covalently linked to a central HBC core, is presented. A series of hexaphenylbenzenes (HPBs) was prepared and reacted under oxidative coupling conditions. The transformation to the respective HBC derivatives worked well with mono‐ and tri‐porphyrin‐substituted HPBs. However, if more porphyrins are attached to the HPB core, Scholl oxidations are hampered or completely suppressed. Hence, a change of the synthetic strategy was necessary to first preform the HBC core, followed by the introduction of the porphyrins. All products were fully characterized, including, if possible, single‐crystal XRD. UV/Vis absorption spectra of porphyrin‐HBCs showed, depending on the number of porphyrins as well as with respect to the substitution pattern, variations in their spectral features with strong distortions of the porphyrins’ B‐band.

Molecularly Aligned Hexa-peri-hexabenzocoronene Films by Brush-Coating and Their Application in Thin-Film Transistors

The facile Chinese brush-coating method was used to prepare oriented thin films of hexa- peri-hexabenzocoronene (HBC) derivatives on the silicon substrate. As a result of the directional solution-coating, the D3 h-symmetry (HBC-1,3,5-Ph-C12) and the C1-symmetry (HBC-1,2,4-Ph-C12) derivatives displayed an anisotropic alignment, with mostly edge-on orientation on SiO2 surfaces modified with various silane-based monolayers. On these silane-modified surfaces, the higher symmetry molecule HBC-1,3,5-Ph-C12 developed a hexagonally packed superstructure, which provided greater π orbital overlap and presumably the electronic coupling between neighboring molecules. In particular, the use of an octyltrichlorosilane (OTS)-modified surface enabled brush-coated thin films to have higher anisotropic orientation, crystallinity, and favorable molecular arrangement. In contrast, the growth of the hexagonal packing of low-symmetry derivative HBC-1,2,4-Ph-C12 was only achieved on the phenyltrichlorosilane and OTS surfaces. Thin-film transistors based on these brush-coated films gave a maximum mobility of 0.1 and 0.056 cm2 V-1 s-1, which are 2 orders of magnitude improvement over the devices with unoriented films prepared by spin-coating. The results indicate that the molecular packing of discotic liquid crystals on the silane-modified surface is sensitively influenced by the molecular symmetry, which affects intermolecular interactions as well as molecule/surface interactions. This study provides a simple way to fabricate aligned films for HBC derivatives for transistor application.

Hydrogen-Bonded Donor-Acceptor Arrays at the Solution-Graphite Interface.

Controlling the nanoscale morphology of organic thin films represents a critical challenge in the fabrication of organic (opto)electronic devices. The morphology of the (multicomponent) thin films in turn depends on the mutual orientation of the molecular components and their supramolecular packing on the surface. Here, it is shown how the surface co-assembly of electron-donating and -accepting building blocks can be controlled via (supra)molecular design. Hexa-peri-hexabenzocoronene (HBC) derivatives with multiple hydrogen-bonding (H-bonding) sites were synthesized and their co-assembly with alkyl-substituted perylene tetracarboxy diimide (PDI) was studied using scanning tunneling microscopy (STM) at the solution-graphite interface. STM data shows that electron-rich HBCs co-assemble laterally with electron deficient PDIs via preprogrammed H-bonding sites with high fidelity. The surface stoichiometry of the two components could be readily tuned by changing the number of H-bonding sites on the HBC derivatives via organic synthesis. This model study highlights the utility of (supra)molecular design in co-assembly of building blocks relevant for organic electronics.

A Phenylene-Bridged Cyclohexa-meta-phenylene as Hexa-peri-hexabenzocoronene Precursor.

A phenylene-bridged cyclohexa-meta-phenylene was synthesized via intramolecular Yamamoto coupling of an appropriate p-quinquephenyl derivative carrying four m-chlorophenyl substituents. The structural proof could be obtained by single-crystal X-ray diffraction analysis, which also revealed a slightly strained structure with an internal phenylene bridge. Notably, this cyclo-meta-phenylene served as a novel precursor for hexa-peri-hexabenzocoronene (HBC). The cyclodehydrogenation proceeded smoothly, providing the corresponding HBC derivative as confirmed by MALDI-TOF-MS, and UV/Vis spectroscopy.

Hexa-peri-hexabenzocoronene and diketopyrrolopyrrole based D-A conjugated copolymers for organic field effect transistor and polymer solar cells

Hexa-peri-hexabenzocoronene (HBC) is a disc-shaped conjugated molecule with strong π-π stacking property, high intrinsic charge mobility and good self-assembly property. But for a long time, the organic photovoltaic (OPV) solar cells based on HBC small organic molecules demonstrated low power conversion efficiencies (PCEs). In this study, a series of polymers named as PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT were designed and synthesized through copolymerization of HBC with bulky mesityl substituents and strong electron-withdrawing diketopyrrolopyrrole (DPP) with different alkyl side chains and various π-bridges. Introduction of DPP unit into the HBC derivatives broadened the absorption spectra and lowered the band gaps. Bulky mesityl substituents attached to periphery of HBC prevented polymers from self-aggregating into too large domain size in the blend films of photovoltaic devices. The different π-bridges have significant effect on the structure conformation of the polymers. The polymer PHBCDPPDT with bithiophene π-bridges demonstrated the broadest absorption for its extensive π-conjugation and more coplanar conformation compared with the thiophene π-bridge one. PHBCDPPC20, PHBCDPPC8, PHBCDPPF and PHBCDPPDT gave field-effect hole mobilities of 1.35 × 10−3, 2.31 × 10−4, 2.79 × 10−4 and 8.60 × 10−3 cm2 V−1 s−1, respectively. The solar cells based on these polymers displayed PCEs of 2.12%, 2.85%, 1.89% and 2.74%. To our knowledge, 2.85% is the highest PCE for the HBC-based photovoltaic materials till now.

Adding Four Extra K-Regions to Hexa-peri-hexabenzocoronene.

A multistep synthesis of hexa-peri-hexabenzocoronene (HBC) with four additional K-regions was developed through a precursor based on two benzotetraphene units bridged with p-phenylene, featuring preinstalled zigzag moieties. Characterization by laser desorption/ionization time-of-flight mass spectrometry, Raman and IR spectroscopy, and scanning tunneling microscopy unambiguously validated the successful formation of this novel zigzag edge-rich HBC derivative. STM imaging of its monolayers revealed large-area, defect-free adlayers. The optical properties of the modified HBC were investigated by UV/visible absorption spectroscopy.

Unsymmetrically substituted room temperature discotic liquid crystals based on hexa–peri–hexabenzocoronene core

AbstractThe first discotic system based‐on alkoxy (tri‐ & di‐) substituted hexa‐peri‐hexabenzocoronene (HBC) that self‐organizes into room temperature columnar structure is reported. Tri‐ and di‐ substituted HBC derivatives showed reddish and yellowish green fluorescence under long wavelength (365 nm) UV light illumination in solution as well as in thin films in the liquid crystalline (LC) state at room temperature, respectively. Thermotropic LC behavior of these compounds was studied by differential scanning calorimetry, polarized optical microscopy and small‐ and wide‐angle X‐ray scattering. Tri‐alkoxy derivative was found to self‐organize into a highly ordered columnar rectangular mesophase, while the other (di‐) possessed a columnar hexagonal mesophase, indicating that in spite of their non‐planarity, the aromatic cores in the mesophase are tightly packed to give highly ordered phases. This system is a promising candidate as an active component of organic light emitting devices, due to its facile processability, low band gap, luminescence and exceptionally higher ordered self‐organization behavior in the mesophase derived from X‐ray scattering studies.

Influence of linear and branched perfluoroalkylated side chains on the π–π stacking behaviour of hexa-peri-hexabenzocoronene and thermotropic properties

The thermotropic properties and self-assembly of two different series of hexa-peri-hexabenzocoronenes (HBC) bearing either linear or branched perfluoroalkylated side chains, each with a wide range of alkyl spacer and perfluorinated tail lengths, have been studied. Correlations between thermogravimetric analysis, differential scanning calorimetry, polarised optical microscopy and small-angle X-ray scattering experiments revealed, as expected, that the transition temperatures and phase stability are influenced by the determining roles of the aliphatic spacer length and odd/even nature, as well as the size and structure (linear vs. bifurcation) of the perfluorinated sections. Most of the investigated HBC derivatives showed a single-column hexagonal columnar phase, where the columns are structured by a double segregation process between HBC aromatic cores and aliphatic spacers, on the one hand, and between aliphatic spacers and fluorinated tails, on the other hand. For the derivatives with long linear spacers, cores of untilted or quasi-untilted HBC stacks are surrounded by a cylindrical aliphatic envelope segregated from the fluorinated periphery, whereas for those with shorter linear spacers, the same structure is kept, except that the aliphatic envelope deviates from cylindricality and causes a symmetry break to rectangular envelope for the first term of the series. Of the four HBC with branched spacers, two are amorphous, whereas a columnar phase is maintained for the other two derivatives, but with tilted HBC stacks. Consequently, the evolution of the polymorphism in the series could be correlated with the variation of both interface areas.

Attogram Detection of Picric Acid by Hexa-peri-Hexabenzocoronene-Based Chemosensors by Controlled Aggregation-Induced Emission Enhancement

Hexa-peri-hexabenzocoronene (HBC) based molecules 5 and 6 have been designed and synthesized. These planar coronenes are appended with rotors to invoke aggregation induced emission enhancement (AIEE) phenomenon by controlling the ratio of H2O in solutions of aggregates. These aggregates of HBC derivatives serve as highly selective chemosensors for picric acid (PA) in mixed aqueous solution. These aggregates are also able to detect PA in vapor phase. In addition, fluorescent test strips have been prepared by dip-coating the Whatman paper with aggregates of both compounds for trace detection of PA in contact mode with detection limits in attograms.

Hexa-peri-hexabenzocoronene in organic electronics

Polycyclic aromatic hydrocarbons (PAHs) are in a class of functional organic compounds with increasing importance in organic electronics. Their tunable photophysical properties and typically strong intermolecular associations make them ideal materials in applications where control of charge mobility is essential. Hexa-peri-hexabenzocoronene (HBC) is a disc-shaped PAH that self-associates into columnar stacks through strong π–π interactions. By decorating the periphery of the HBC molecule with various substituents, a range of properties and functions can be obtained including solution processability, liquid crystallinity, and semiconductivity. In this review article, the synthesis, properties, and functions of HBC derivatives are presented with focus on work published in the last five years.

Nonlinear optical properties for a class of hexa-peri-hexabenzocoronene chromophores: a computational investigation

The systematic investigation of the linear and nonlinear optical properties on such a class of hexa-peri-hexabenzocoronene (HBC) chromophores is of significance for rationally designing two-photon absorption (TPA) materials. The results indicate that increasing the strength of electron-donating or accepting terminal groups leads to bathochromic-shift of the absorption band and enhancement of the TPA cross section (δ(max)). For the molecules with fluorinated methylene and cyano substituents, replacement of a double bond by a triple bond in the conjugated linker produces the increase of δ(max), owing to the lower bond-length alternation and better rigidity of phenylene-ethynylene. In contrast, for the molecules with nitro groups, the similar replacement results in a slight decrease of δ(max) because N-HBC-E-NO2 has excellent planarity architecture and effective electronic coupling. The TPA spectra are red-shifted and the δ(max) values are enhanced as the number of branches increases. Thus, a solvatochromism effect has a positive influence on the TPA response of the nitryl compounds due to larger polarization of the nitro moiety. We have shed light on the linear relationship between the first hyperpolarizability and δ(max). These HBC derivatives hold potential as high-performance nonlinear optical materials.

Effect of Intermolecular Dipole−Dipole Interactions on Interfacial Supramolecular Structures of C3-Symmetric Hexa-peri-hexabenzocoronene Derivatives

Two-dimensional (2D) supramolecular assemblies of a series of novel C(3)-symmetric hexa-peri-hexabenzocoronene (HBC) derivatives bearing different substituents adsorbed on highly oriented pyrolytic graphite were studied by using scanning tunneling microscopy at a solid-liquid interface. It was found that the intermolecular dipole-dipole interactions play a critical role in controlling the interfacial supramolecular assembly of these C(3)-symmetric HBC derivatives at the solid-liquid interface. The HBC molecule bearing three -CF(3) groups could form 2D honeycomb structures because of antiparallel dipole-dipole interactions, whereas HBC molecules bearing three -CN or -NO(2) groups could form hexagonal superstructures because of a special trimeric arrangement induced by dipole-dipole interactions and weak hydrogen bonding interactions ([C-H···NC-] or [C-H···O(2)N-]). Molecular mechanics and dynamics simulations were performed to reveal the physics behind the 2D structures as well as detailed functional group interactions. This work provides an example of how intermolecular dipole-dipole interactions could enable fine control over the self-assembly of disklike π-conjugated molecules.

Superaromatic Terpyridines: Hexa-peri-hexabenzocoronenes with Tridentate Functionality

Two superaromatic terpyridine ligands (1 and 2) incorporating a hexa-peri-hexabenzocoronene (HBC) unit at the 4'-position have been prepared. In 1, the terpyridine and the HBC domains are directly fused, while in 2, they are separated by an acetylene linker. Also presented is the synthesis and characterization of several novel HBC derivatives, including 2-iodo-5,8,11,14,17-penta-tert-butylhexa-peri-hexabenzocoronene (7), a valuable synthetic intermediate. Different synthetic routes to 1 and 2 are employed, and two alternative methods resulted in excellent yields of 2. The optical properties of both novel terpyridines are examined using UV-visible absorption and emission spectroscopy. The single-crystal X-ray structures of 7 and a precursor, 4-iodo-4'-tert-butylphenylacetylene (5), are discussed.

Unusual Side‐Chain Effects on Charge‐Carrier Lifetime in Discotic Liquid Crystals

When hexa-peri-hexabenzocoronene (HBC) carries paraffinic side chains with ester-ether termini (1a-1c), a hexagonal columnar liquid-crystalline (LC) mesophase forms, which is characterized by a very wide temperature range (ca. 0-300 degrees C). The LC materials from these HBC derivatives show characteristic electronic properties with an extremely long photocarrier lifetime. For example, by means of a flash-photolysis time-resolved microwave conductivity technique, the photocarrier lifetime, observed for liquid-crystalline 1a, is as long as 14 ms, which is four-orders of magnitude longer than that of fully paraffinic HBC 4 (0.0011 ms). The maximum transient photoconductivities of liquid-crystalline 1a-1c are comparable to one another and also to those of HBCs without ester-ether terminals. Liquid-crystalline 1a exhibits an obvious photocurrent generation in response to light illumination.

Controlled Self-Assembly of C3-Symmetric Hexa-peri-hexabenzocoronenes with Alternating Hydrophilic and Hydrophobic Substituents in Solution, in the Bulk, and on a Surface

In this work, we introduce a class of C(3)-symmetric hexa-peri-hexabenzocoronenes (HBCs) 1 with alternating hydrophilic and hydrophobic substituents to achieve control over the self-assembly of discotic nanographene molecules. Our studies show that the following structural parameters and experimental conditions are essential for tailoring the formation of the liquid-crystalline phase in the bulk as well as the self-assembly in solution and on surfaces: (1) steric demand of alkyl and alkylphenyl substituents; (2) noncovalent hydrophilic-hydrophobic interactions of the substituents; and (3) processing conditions, such as the type and mixture of solvents of different polarities along with the nature of the surface. The substitution of HBC with linear alkyl side chains possessing less steric demand (1b) leads to high crystallinity in the bulk solid state and at the liquid-solid interface, and the additional feature of alternating hydrophilic and hydrophobic substituents promotes a high aggregation tendency in polar/apolar solvent mixtures. In contrast, bulky branched alkyl chains (1a) and alkylphenyl substituents (1c) induce liquid crystallinity over the whole temperature range measured. While 1a does not show pronounced self-assembly in solution, compound 1c displays, even at high temperatures, aggregation in polar/apolar solution due to the intermolecular "locking" of peripheral phenyl groups. After solution deposition on a surface, distinct fiber formation is observed for the HBC derivatives, which is related to the solution self-assembly behavior. The present work provides further insight into the molecular design and self-assembly of discotic nanographene materials.

分子グラフェンの自己組織化によるグラファイトナノチューブ

This review article gives an overview of our recent discovery that a Gemini-shaped hexa-peri-hexabenzocoronene (HBC) amphiphile (1) and its derivatives undergo highly controlled self-assembly to form a new family of nanotubes that may be called ‘graphite’ nanotubes. Compound 1 possesses hydrophilic triethylene glycol (TEG) chains on one side of the HBC core and hydrophobic dodecyl side chains on the other. The nanotubes are characterized by uniform diameter and wall thickness, a large aspect ratio, and designable inner and outer surfaces. Furthermore, they possess a helical chirality. In fact, under certain conditions, a coiled assembly, which is a kinetically preferred product, is formed temporarily and then transforms, on annealing, into a tubular assembly. Furthermore, HBC derivatives bearing a stereogenic center can form coiled and tubular assemblies with a one-handed helical chirality. Analogous to compound 1, HBC amphiphiles carrying functional groups at the TEG termini self-assemble in a highly controlled manner, affording surface-functional nanotubes. As one of the ideal designs for photovoltaic materials with a nanometric size regime, photoconductive nanotubes with a coaxial configuration of the donor/acceptor heterojunction is highlighted.

Hexa-peri-hexabenzocoronenes – Controlling their Self-Assembly by Engineering the Lateral Substituents

Polycondensed aromatic hydrocarbons (PAH), which can be regarded as two-dimensional subsections of graphite, have begun to attract increasing interest in supramolecular chemistry. Substituted hexa-peri-hexabenzocoronenes (HBC), an outstanding class of PAH, are well-known to self-organize in solution into highly ordered columnar molecular stacks. The formed structures are very sensitive to any variation of the medium as well as the lateral substituents of the HBC derivatives. Various perfluoroalkylated HBC compounds have been prepared and investigated by powder XRD, DSC, fluorescence and cryo-SEM in order to gain certain control over the self-assembling behavior of this class of compounds.

Helical Packing of Discotic Hexaphenyl Hexa-peri-hexabenzocoronenes: Theory and Experiment

The arrangement of discotic hexa-peri-hexabenzocoronenes (HBCs) into columnar helical superstructures has been investigated in relation to their molecular architecture. The supramolecular structure of two hexaphenyl-substituted HBC derivatives, differing only in the chiral/achiral nature of the attached alkyl side chains, was studied by circular dichroism and temperature-dependent wide-angle X-ray diffraction on oriented filaments. A structural model in agreement with the experimental observations was developed on the basis of accompanying quantum-chemical calculations. The helical organization along the self-assembled columnar structures was induced by the steric requirements of the bulky phenyl rings near the aromatic core, i.e., by their rotation out-of-plane with respect to the aromatic core. On the other hand, a uniform handedness of the twist was generated by chiral alkyl substituents. At higher temperatures the degree of helical organization decreases due to lateral and longitudinal dynamics of the discotic molecules. Annealing at ambient conditions improved the long-range arrangement of the discs along the columnar structures. This reorganization indicated a self-healing of the plastic material which is desirable for application of discotics as active layers in electronic devices. The helical packing resulted in a considerable stability of the mesophase up to 500 degrees C, which has not been reported for a discotic so far.