Oligomers of hexa-peri-hexabenzocoronenes as "super-oligophenylenes": synthesis, electronic properties, and self-assembly

Hexa-peri-hexabenzocoronene (HBC) is a remarkable polycyclic aromatic hydrocarbon and is often called "superbenzene" because of its similarity to benzene. In this article we present the facile synthesis of oligomers of HBC, up to trimers (3, 4, 5a-c) with different modes of connection. UV-vis and fluorescence spectroscopy studies reveal that the oligomers are electronically decoupled. This arises from the small atomic orbital coefficients of the bridge-head carbon atoms, the large torsion angle between the HBC units, and the large distance of interacting transition dipoles due to the size of the HBC chromophore. For comparison, a methylene-bridged HBC dimer 6, so-called "superfluorene", was prepared. The induced planarity improves pi-conjugation and suppresses the geometrical relaxation of the backbone upon electronic excitation, leading to a prominent 0-0 transition band in the fluorescence spectra. The self-assembly of the oligomers and of superfluorene 6 was studied by wide-angle X-ray diffraction (WAXD) in the bulk state, and ordered columnar stacking occurs in the HBC dimer 3, p-HBC trimer 4, and superfluorene 6. Measurements of shear-aligned samples show that, despite increasing aspects ratio by linear entrainment of disks, the anitropic element that is subject to alignment by shear is the supramolecular columns.     

Alpha-helical-within-discotic columnar structures of a complex between poly(ethylene oxide)-block-poly(l-lysine) and a hexa-peri-hexabenzocoronene

Poly(ethylene oxide)-block-poly(l-lysine) (PEO-PLL) was complexed with an amphiphilic hexa-peri-hexabenzocoronene (HBC). This produced a thermotropic liquid crystalline material (PEO-PLL-HBC), which was investigated by FTIR spectroscopy and differential scanning calorimetry as well as by wide- and small-angle X-ray scattering. It was found that the poly(l-lysine) blocks form an alpha-helical secondary structure. Each helix is surrounded symmetrically by six discotic columns of HBC, which gives an alpha-helical-within-discotic column structural entity. The dense packing of these entities produces hexagonal sublattices (formed by the columns) in the frame of a two-dimensional hexagonal lattice (formed by the helices). An order-order transition from a columnar structure Col1 to Col2 was found at 54 degrees C. The unit cell constants are 5.75 nm (Col1) and 6.60 nm (Col2). The larger unit cell size of Col2 was explained by a higher intracolumnar order of the latter in which the packing distance of the disklike HBC cores is well-defined (0.353 nm). PEO-PLL-HBC combines essential features of liquid crystals with a basic structural element of proteins into a single material.     

Macroscopic alignment of graphene stacks by Langmuir-Blodgett deposition of amphiphilic hexabenzocoronenes

We present structural studies of Langmuir (L) and Langmuir-Blodgett (LB) films of new amphiphilic hexa-peri-hexabenzocoronene (HBC) discotics, carrying five branched alkyl side chains and one polar group. The polar group is either a carboxylic acid moiety or an electron acceptor moiety (anthraquinone). Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity, both utilizing synchrotron radiation, show that these amphiphilic HBCs form well-defined Langmuir monolayers at the air-water interface, with a pi-stacked columnar structure where the HBC cores are rotated around the surface normal and tilted relative to the water surface. The intercolumnar distance is 20 A. The HBCs are confined to a layer lying on top of the layer of polar groups that are in contact with the water subphase. Efficient transfer of the monolayer of the anthraquinone-substituted HBC derivative to hydrophobic quartz substrates by vertical dipping gave well-defined multilayer Y-type LB films. Polarized optical spectroscopy, GIXD, and X-ray reflectivity measurements show that the LB films consist of at least two phases. Heating the films results in an irreversible rearrangement to a single macroscopically aligned phase of hexagonally packed columns oriented along the dipping direction with disk planes perpendicular to the columnar axes and stacked in a cofacial manner. This phase transition is analogous to the reversible transition observed in the bulk material.     

Synthesis and self-assembly of functionalized hexa-peri-hexabenzocoronenes

Monolayers of hexa-alkyl substituted derivatives of hexa-peri-hexabenzocoronene (HBC) 1b have previously been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It is expected that different functional groups (electron donating or withdrawing) connected to the aromatic core will influence the packing pattern and possibly the current-voltage characteristics as well. In order to provide suitable model systems, a new synthetic approach to synthesize functionalized HBC derivatives has been developed. This was accomplished by [4 + 2]-cycloaddition of suitably bromo-substituted diphenylacetylenes and 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones followed by an oxidative cyclodehydrogenation with iron(III) chloride/nitromethane. Using this strategy three different substitution patterns were synthesized: 2-bromo-5,8,11.14,17-pentadodecylhexa-pecri-hexabenzocoronene (2a), 2,5-dibromo-8,11,14,17-pentadodecylhexa-peri-hexabenzocoronene (2b), and 2,11-dibromo5,8,14,17-pentadodecylhexa-peri-hexa-benzocoronene (2c). These bromo-substituted HBC derivatives were subjected to palladium catalyzed coupling reactions to give donor (alkoxy, amino) as well as acceptor (ester, cyano) substituted derivatives. The self-assembly of these new HBC derivatives was studied in the bulk as well as at an interface. DSC, optical microscopy, and X-ray diffraction revealed the existence of columnar mesophases. The bulk structure in the mesophase is largely insensitive to changes of the substitution pattern; however, in situ scanning tunneling microscopy at the solid-fluid interface between an organic solution of the HBC derivative and highly oriented pyrolytic graphite reveals different packing patterns of the first adsorbed monolayer.     

Alternating Donor–Acceptor Arrays from Hexa‐peri‐hexabenzocoronene and Benzothiadiazole: Synthesis,Optical Properties,and Self‐Assembly

Donor–acceptor (D–A) structures were obtained by alternating arrays of hexa‐peri‐hexabenzocoronene (HBC) and benzo[c][1,2,5]thiadiazole (BTZ). Optoelectronic investigations revealed a charge transfer due to strong push–pull interactions. 2 D wide‐angle X‐ray scattering (WAXS) data indicated an arrangement in liquid‐crystalline columnar assemblies, in which the π‐stacking distances and molecular orientation depend on the number of HBC units in the molecules.     

Synthesis and controlled self-assembly of covalently linked hexa-peri-hexabenzocoronene/perylene diimide dyads as models to study fundamental energy and electron transfer processes

We report the synthesis and photophysical characterization of a series of hexa-peri-hexabenzocoronene (HBC)/perylenetetracarboxy diimide (PDI) dyads that are covalently linked with a rigid bridge. Both the ratio of the two components and the conjugation of the bridging element are systematically modified to study the influence on self-assembly and energy and electron transfer between electron donor HBC and acceptor PDI. STM and 2D-WAXS experiments reveal that both in solution and in bulk solid state the dyads assemble into well-ordered two-dimensional supramolecular structures with controllable mutual orientations and distances between donor and acceptor at a nanoscopic scale. Depending on the symmetry of the dyads, either columns with nanosegregated stacks of HBC and PDI or interdigitating networks with alternating HBC and PDI moieties are observed. UV-vis, photoluminescence, transient photoluminescence, and transient absorption spectroscopy confirm that after photoexcitation of the donor HBC a photoinduced electron transfer between HBC and PDI can only compete with the dominant F?rster resonance energy transfer, if facilitated by an intimate stacking of HBC and PDI with sufficient orbital overlap. However, while the alternating stacks allow efficient electron transfer, only the nanosegregated stacks provide charge transport channels in bulk state that are a prerequisite for application as active components in thin film electronic devices. These results have important implications for the further design of functional donor-acceptor dyads, being promising materials for organic bulk heterojunction solar cells and field-effect transistors.     

Trace analysis of impurities in bulk gases by gas chromatography-pulsed discharge helium ionization detection with "heart-cutting" technique

A method has been developed for the detection of low-nL/L-level impurities in bulk gases such as H(2), O(2), Ar, N(2), He, methane, ethylene and propylene, respectively. The solution presented here is based upon gas chromatography-pulsed discharge helium ionization detection (GC-PDHID) coupled with three two-position valves, one two-way solenoid valve and four packed columns. During the operation, the moisture and heavy compounds are first back-flushed via a pre-column. Then the trace impurities (except CO(2) which is diverted to a separate analytical column for separation and detection) together with the matrix enter onto a main column, followed by the heart-cut of the impurities onto a longer analytical column for complete separation. Finally the detection is performed by PDHID. This method has been applied to different bulk gases and the applicability of detecting impurities in H(2), Ar, and N(2) are herewith demonstrated. As an example, the resulting detection limit of 100 nL/L and a dynamic range of 100-1000 nL/L have been obtained using an Ar sample containing methane.     

Corrosion inhibitor by a polymerizable columnar mesogen based on hexabenzocoronene derivative

A hexa-peri-hexabenzocoronene derivative (HBC-1,2,4-Ph-DA-C₁₂) with the central HBC core tethered with three diacetylene (DA)-containing chains and three saturated alkyl chains was applied for the anticorrosion of an iron surface. This compound, which exhibits a columnar liquid crystalline phase, could be polymerized by ultraviolet irradiation through reactions between the DA units. This polymer coating exhibited a good anticorrosion property with good hydrophobicity (contact angle [CA] ~ 84°–105°) and excellent mechanical property. It showed a high resistance of 149 kΩ/cm², much higher than that of known anticorrosion materials.     

Transfer, amplification, and inversion of helical chirality mediated by concerted interactions of C3-supramolecular dendrimers

The synthesis, structural, and retrostructural analysis of two libraries containing 16 first and second generation C(3)-symmetric self-assembling dendrimers based on dendrons connected at their apex via trisesters and trisamides of 1,3,5-benzenetricarboxylic acid is reported. A combination of X-ray diffraction and CD/UV analysis methods demonstrated that their C(3)-symmetry modulates different degrees of packing on the periphery of supramolecular structures that are responsible for the formation of chiral helical supramolecular columns and spheres self-organizable in a diversity of three-dimensional (3D) columnar, tetragonal, and cubic lattices. Two of these periodic arrays, a 3D columnar hexagonal superlattice and a 3D columnar simple orthorhombic chiral lattice with P222(1) symmetry, are unprecedented for supramolecular dendrimers. A thermal-reversible inversion of chirality was discovered in helical supramolecular columns. This inversion is induced, on heating, by the change in symmetry from a 3D columnar simple orthorhombic chiral lattice to a 3D columnar hexagonal array and, on cooling, by the change in symmetry from a 2D hexagonal to a 2D centered rectangular lattice, both exhibiting intracolumnar order. A first-order transition from coupled columns with long helical pitch, to weakly or uncorrelated columns with short helical pitch that generates a molecular rotator, was also discovered. The torsion angles of the molecular rotator are proportional to the change in temperature, and this effect is amplified in the case of the C(3)-symmetric trisamide supramolecular dendrimers forming H-bonds along their column. The structural changes reported here can be used to design complex functions based on helical supramolecular dendrimers with different degree of packing on their periphery.     

Ordered architectures of a soluble hexa-peri-hexabenzocoronene-pyrene dyad: thermotropic bulk properties and nanoscale phase segregation at surfaces

An alkylated hexa-peri-hexabenzocoronene with a covalently tethered pyrene unit serves as a model to study self-assembling discotic pi-system dyads both in the bulk and at a surface. Wide-angle X-ray scattering, polarized light microscopy, and differential scanning calorimetry revealed bulk self-assembly into columnar structures. Relative to a control without a tethered pyrene, the new dyad exhibits a more ordered columnar phase at room temperature but with dramatically lowered isotropization temperature, facilitating homeotropic alignment. These two features are important for processing such materials into molecular electronic devices, e.g., photovoltaic diodes. Scanning tunneling microscopy at a solution-solid interface revealed uniform nanoscale segregation of the large from the small pi-systems, leading to a well-defined two-dimensional crystalline monolayer, the likes of which may be employed in the future to study intramolecular electron transfer processes at surfaces, on the molecular scale.     

Homochiral supramolecular polymerization of an "S"-shaped chiral monomer: translation of optical purity into molecular weight distribution

An "S"-shaped chiral motif of a p-xylylene-bridged bis(cyclic dipeptide) (1), having four hydrogen-bonding amide functionalities, formed a homochiral supramolecular polymer in solution. X-ray crystallography of a slightly modified version of 1 for an enhanced crystallinity showed one-dimensional columnar assemblies via four double hydrogen-bonding interactions. Model studies with half-protected analogues of 1 indicated a nearly perfect enantioselectivity in hydrogen-bonding dimerization. When 1 was not racemic but enriched in either of the enantiomers, a supramolecular polymer with a bimodal molecular weight distribution resulted, due to the formation of two homochiral polymers with different molecular weights. By taking advantage of this, separation of optically pure 1 from an enantiomerically unbalanced mixture was possible by means of size-exclusion chromatography.     

5 nm Ordered Structures Self-Assembled by C2-Symmetric Hybrids with Polyhedral Oligomeric Silsesquioxane and Hexa-peri-Hexabenzocoronene.

Hybrids consisting of polyhedral oligomeric silsesquioxane (POSS) and hexa-peri-hexabenzocoronene (HBC) with a dumbbell topology and C₂ symmetry were designed and synthesized. They self-assemble into 5 nm ordered structures. In particular, the increased steric effect with increasing POSS units stabilizes a square columnar phase (Col_squ) which is important in nanotemplating. These hybrids containing discotic liquid crystal HBC and POSS units have an excellent etching contrast and present an approach to obtain 5 nm nanopatterns.     

Gemini-induced columnar jointing in vitreous ice. Cryo-HRSEM as a tool for discovering new colloidal morphologies

A gemini surfactant is able to promote columnar jointing in vitreous ice where long pillars, often of hexagonal cross section, are formed. This jointing is visible by cryo-high-resolution scanning electron microscopy (cryo-HRSEM), in which colloidal suspensions in bulk water are cooled rapidly in liquid ethane, thereby avoiding the potential artifacts with other types of EM. The jointing is proposed to arise from a new type of colloidal morphology where the surfactant self-assembles into hexagonal columns. Evidence for this mechanism comes from a cryo-HRSEM photo of an ice-free hexagonal "skeleton" composed of surfactant. Cryo-HRSEM, a method that is just beginning to realize its potential, would seem to have a promising future in the discovery of additional and as yet unimagined colloidal structures.     

Star-shaped hexa-peri-hexabenzocoronene "heptamer": synthesis and self-assembly

[structure: see text] A star-shaped hexa-peri-hexabenzocoronene "heptamer" was prepared, which showed a strong tendency to aggregate in solution and in the bulk states. Higher order was found in a high-temperature, columnar liquid crystalline phase due to the higher mobility of the molecules in comparison with the low-temperature solid. In addition, physical gel formation was observed due to the presence of covalent intercolumnar interactions.     

含树枝化以及线性侧链的超分子共聚物的液晶相结构调控

设计了一类由聚4-乙烯基吡啶(P4VP)、含弯曲树枝化分子12CBP以及线性分子PDP组成的三元共混体系,其中12CBP和PDP端基均为酚羟基,可与P4VP侧基的吡啶环上的N原子进行氢键复合.傅立叶变换红外光谱、示差扫描量热法、偏光显微镜、小角X射线散射和原子力显微镜等多种研究表明该共混体系为均相体系,三组分自组装形成以P4VP为主链,12CBP和PDP为超分子侧链的"无规共聚物"P4VP(12CBP)x(PDP)y(x+y=1).该超分子侧链共聚物的聚集态结构与小分子12CBP和PDP相对含量有关.当12CBP含量x≥0.5时,体系组装形成六方柱状相结构,柱子的直径随着体系中12CBP含量减少而逐渐减小.当12CBP含量0.5时,即使少量12CBP的引入也会引起体系层状相结构的破坏,体系表现为无定形状态.     

Natural anthraquinones probed as Type I and Type II photosensitizers: singlet oxygen and superoxide anion production

The photosensitizing properties of six anthraquinones (AQs): soranjidiol (1), soranjidiol-1-methyl ether (2), rubiadin (3), rubiadin-1-methyl ether (4), damnacanthal (5) and damnacanthol (6), isolated from leaves and stems of Heterophyllaea pustulata Hook. f. (Rubiaceae) were studied. By means of photobiological and photophysical methods in vitro, the type of photosensitization that these metabolites are capable of producing was determined. Whereas the photosensitized generation of superoxide anion radical (O(2)(-)) (Type I) was evaluated in leukocyte suspensions, singlet molecular oxygen ((1)O(2)) production (Type II) was examined in organic solution. In addition, the quantum yield of (1)O(2) (Phi) in chloroform was measured for those AQs that generate it. It was established that 4 behaves exclusively as a Type I photosensitizer. By contrast, the others AQs act by both types of mechanisms, among which 5 showed the largest Phi of (1)O(2).     

Incorporation of Hexa‐peri‐hexabenzocoronene (HBC) into Carbazole–Benzo‐2,1,3‐thiadiazole Copolymers to Improve Hole Mobility and Photovoltaic Performance

Hexa‐peri‐hexabenzocoronene (HBC) is a discotic‐shaped conjugated molecule with strong π–π stacking property, high intrinsic charge mobility, and good self‐assembly properties. For a long time, however, organic photovoltaic (OPV) solar cells based on HBC demonstrated low power conversion efficiencies (PCEs). In this study, two conjugated terpolymers, poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5′‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT)‐ 5 HBC and PCDTBT‐ 10 HBC, were synthesized by incorporating different amounts of HBC as the third component into poly[N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5′‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) through Suzuki coupling polymerization. For comparison, the donor–acceptor (D –A) conjugated dipolymer PCDTBT was also synthesized to investigate the effect of HBC units on conjugated polymers. The HBC‐containing polymers exhibited higher thermal stabilities, broader absorption spectra, and lower highest‐occupied molecular orbital (HOMO) energy levels. In particular, the field‐effect mobilities were enhanced by more than one order of magnitude after the incorporation of HBC into the conjugated polymer backbone on account of increased interchain π–π stacking interactions. The bulk heterojunction (BHJ) polymer solar cells (PSCs) fabricated with the polymers as donor and PC₇₁BM as acceptor demonstrated gradual improvement of open‐circuit voltage (V_OC) and short‐circuit current (J_SC) with the increase in HBC content. As a result, the PCEs were improved from 3.21 % for PCDTBT to 3.78 % for PCDTBT‐ 5 HBC and then to 4.20 % for PCDTBT‐ 10 HBC.     

Peralkylated coronenes via regiospecific hydrogenation of hexa-peri-hexabenzocoronenes

A remarkable, regiospecific hydrogenation zips around the approximately 4 nm perimeter of hexa-peri-hexabenzocoronenes (HBC) adding 18 hydrogen atoms, leading to the first peralkylated coronenes, in quantitative yields in some cases. Increasing steric bulk of side chains was found to hinder the reaction, while unsubstituted HBC could be converted to a modest extent despite its vanishing solubility. The structures of the novel coronenes are unequivocally confirmed by MALDI-TOF, 1H, 13C, and heteronuclear correlation NMR, and UV-vis absorption spectroscopy. The puckered-ring periphery of these discotics does not prohibit self-assembly to columnar structures in a fashion similar to that of the planar precursors, as determined from wide-angle X-ray diffraction, but decreases the isotropization temperature by approximately 300 degrees C relative to the latter. Branching in the alkyl chains frustrates nucleation from the melt, resulting in clear polymorphism depending on the thermal treatment. Nonetheless, preliminary measurements indicate high charge-carrier mobilities and lifetimes within the bulk material, on the same order as those previously found for HBCs.     

"Single sample concept": theoretical model for a combinatorial approach to solid-state inorganic materials

A theoretical model for a "single sample concept" (SSC) applied to the combinatorial chemistry of solid-state inorganic compounds is presented. The SSC is performed by reacting N starting materials (randomly mixed) in a single sample of approximately 1 cm(3). Combinatorial calculations demonstrate that the number of reasonably estimated phases to be found in the space of N components grouped into compounds (e.g. oxides, sulfides, or halogenides) containing up to q < or = 6 metallic elements is smaller than combinations set up by starting conditions for local reactions within the bulk of a single ceramic sample. Recently, the SSC proved to work in the case of synthesizing libraries of 3d metal oxides, from which magnetic particulate matter was extracted by a magnetic separation technique. The SSC may be applied in a first screening cycle followed by 2D approaches of combinatorial chemistry.     

Thermal and optical behaviour of octa-alkoxy substituted phthalocyaninatovanadyl complexes

This paper deals with the synthesis of vanadyl phthalocyanines substituted with eight alkoxy chains in the peripheral (2, 3, 9, 10, 16, 17, 23, 24) positions. The alkoxy chain length was varied, and octa-octyloxy (C₈H₁₇O), octa-dodecyloxy (C₁₂H₂₅O) and octa-hexadecyloxy (C₁₆H₃₃O) substituted vanadyl phthalocyanine complexes were prepared. Studies by polarizing optical microscopy and small angle X-ray diffraction (XRD) revealed that all the complexes are liquid crystalline and that these metallomesogens exhibit a columnar phase. The symmetry of the 2D lattice is rectangular, with a ^c2^mm space group, as determined by the indexation of the XRD reflections; hence a rectangular columnar phase (Colr) was assigned. A double periodicity, although weak, along the axis of the columns was found, which indicates some degree of pairing or dimerization. A tentative explanation based on an antiferroelectric stacking is given. Transition enthalpies were determined by differential scanning calorimetry. The compounds start to decompose above 250°C before reaching the clearing temperatures. A significant bathochromic shift of the Q-band in the UV/Vis spectra of the vanadyl complexes compared with the metal-free ligands and other metallophthalocyanines (M=Co^II, Ni^II, Cu^II, Zn^II) was also observed.