Chirality-Embedded Nanographenes

The development of chiral nanographenes has mostly been carried out by bottom-up methods and examples of species developed by the post-modification of nanographenes prepared by top-down methods remain limited. We show that the attachment of chiral functional groups onto the edge of nanographenes generates chirality on the surface. X-ray diffraction analysis and DFT calculations indicate that the chirality of the functional groups is transferred to the surface via steric interactions from the chiral center through the five-membered cyclic imide to the nanographene edge. The exciton coupling between the p-bromophenyl groups confirms that the functional groups are arranged on the armchair edges at distances that permit exciton coupling, which provides information about their relative orientation. These pieces of information help to elucidate the edge structure of nanographenes prepared by top-down methods.     

Intrinsic Emission from Nanographenes

Top‐down approaches have been widely used as convenient methods for the production of nanographenes. To understand the photoemission properties of nanographenes, their separation and the optical properties of the individual fractions is important. By using a combination of size‐exclusion and silica‐gel‐adsorption chromatography, we separated lipophilic nanographenes that contained para‐methoxybenzyl groups. The mixture consisted of large (average 19.8 nm) and small (average 4.9 nm) nanographenes, whilst unreacted carboxy groups remained in the latter group. Optical measurements revealed that oxygen‐containing functional groups had little influence on the photoemission of the nanographenes, thus indicating that the intrinsic emission, that is, emission from the sp² surfaces, was responsible for the photoemission. Two photoemission bands were observed for all of the fractions, which likely originated from the edge and inner parts of nanographene.     

Phase‐Transfer‐Catalyzed Asymmetric Synthesis of Axially Chiral Anilides

Catalytic asymmetric synthesis of axially chiral o‐iodoanilides and o‐tert‐butylanilides as useful chiral building blocks was achieved by means of binaphthyl‐modified chiral quaternary ammonium‐salt‐catalyzed N‐alkylations under phase‐transfer conditions. The synthetic utility of axially chiral products was demonstrated in various transformations. For example, axially chiral N‐allyl‐o‐iodoanilide was transformed to 3‐methylindoline by means of radical cyclization with high chirality transfer from axial chirality to C‐centered chirality. Furthermore, stereochemical information on axial chirality in o‐tert‐butylanilides could be used as a template to control the stereochemistry of subsequent transformations. The transition‐state structure of the present phase‐transfer reaction was discussed on the basis of the X‐ray crystal structure of ammonium anilide, which was prepared from binaphthyl‐modified chiral ammonium bromide and o‐iodoanilide. The chiral tetraalkylammonium bromide as a phase‐transfer catalyst recognized the steric difference between the ortho substituents on anilide to obtain high enantioselectivity. The size and structural effects of the ortho substituents on anilide were investigated, and a wide variety of axially chiral anilides that possess various functional groups could be synthesized with high enantioselectivities. This method is the only general way to access a variety of axially chiral anilides in a highly enantioselective fashion reported to date.     

On low‐lying excited states of extended nanographenes

Low‐lying excited states of planarly extended nanographenes are investigated using the long‐range corrected (LC) density functional theory (DFT) and the spin‐flip (SF) time‐dependent density functional theory (TDDFT) by exploring the long‐range exchange and double‐excitation correlation effects on the excitation energies, band gaps, and exciton binding energies. Optimizing the geometries of the nanographenes indicates that the long‐range exchange interaction significantly improves the C C bond lengths and amplify their bond length alternations with overall shortening the bond lengths. The calculated TDDFT excitation energies show that long‐range exchange interaction is crucial to provide accurate excitation energies of small nanographenes and dominate the exciton binding energies in the excited states of nanographenes. It is, however, also found that the present long‐range correction may cause the overestimation of the excitation energy for the infinitely wide graphene due to the discrepancy between the calculated band gaps and vertical ionization potential (IP) minus electron affinity (EA) values. Contrasting to the long‐range exchange effects, the SF‐TDDFT calculations show that the double‐excitation correlation effects are negligible in the low‐lying excitations of nanographenes, although this effect is large in the lowest excitation of benzene molecule. It is, therefore, concluded that long‐range exchange interactions should be incorporated in TDDFT calculations to quantitatively investigate the excited states of graphenes, although TDDFT using a present LC functional may provide a considerable excitation energy for the infinitely wide graphene mainly due to the discrepancy between the calculated band gaps and IP–EA values. © 2017 Wiley Periodicals, Inc.     

Synthesis of a Helical Bilayer Nanographene

A rigid, inherently chiral bilayer nanographene has been synthesized as both the racemate and enantioenriched M isomer (with 93 % ee) in three steps from established helicenes. This folded nanographene is composed of two hexa‐peri‐hexabenzocoronene layers fused to a [10]helicene, with an interlayer distance of 3.6 Å as determined by X‐ray crystallography. The rigidity of the helicene linker forces the layers to adopt a nearly aligned AA‐stacked conformation, rarely observed in few‐layer graphene. By combining the advantages of nanographenes and helicenes, we have constructed a bilayer system of 30 fused benzene rings that is also chiral, rigid, and remains soluble in common organic solvents. We present this as a molecular model system of bilayer graphene, with properties of interest in a variety of potential applications.     

Cationic Nitrogen‐Doped Helical Nanographenes

Herein, we report the design and synthesis of a series of novel cationic nitrogen‐doped nanographenes (CNDNs) with nonplanar geometry and axial chirality. Single‐crystal X‐ray analysis reveals helical and cove‐edged structures. Compared to their all‐carbon analogues, the frontier orbitals of the CNDNs are energetically lower lying, with a reduced optical energy gap and greater electron‐accepting behavior. Cyclic voltammetry shows all the derivatives to undergo quasireversible reductions. In situ spectroelectrochemical studies prove that, depending on the number of nitrogen dopants, either neutral radicals (one nitrogen dopant) or radical cations (two nitrogen dopants) are formed upon reduction. The concept of cationic nitrogen doping and introducing helicity into nanographenes paves the way for the design and synthesis of expanded nanographenes or even graphene nanoribbons with cationic nitrogen dopants.     

Near‐Infrared‐Emitting Nitrogen‐Doped Nanographenes

The quantum‐size effect, which enables nanographenes to emit photoluminescence (PL) in the UV to visible region, has inspired intense research. However, the control of the PL properties of nanographenes through manipulation of their π‐system by post‐modifications is not well developed. By utilizing a ring‐closure reaction between an aromatic 1,2‐dicarboxylic acid and a 1,8‐naphthalenediamine derivative, which produces a perimidine framework, nitrogen‐doped nanographenes were realized. Two nanographenes produced by a one‐pot reaction of edge‐oxidized nanographene (GQD‐ 2 ) with 1,8‐naphthalenediamine derivatives (GQD‐ 1 a and GQD‐ 1 b ) displayed an absorption band extending to >1000 nm; furthermore, the PL wavelength of GQD‐ 1 a was significantly red‐shifted into the near‐infrared (NIR) region in which it can be used for bioimaging. Time‐dependent DFT calculations of model nanographenes showed that the functional groups narrow the HOMO–LUMO gap, realizing the NIR‐emitting nanographenes.     

Spontaneous transmission of chirality through multiple length scales

The hierarchical transfer of chirality in nature, from the nano‐, to meso‐, to macroscopic length scales, is very complex, and as of yet, not well understood. The advent of scanning probes has allowed chirality to be monitored at the single molecule or monolayer level and has opened up the possibility to track enantiospecific interactions and chiral self‐assembly with molecular‐scale detail. This paper describes the self‐assembly of a simple, model molecule (naphtho[2,3‐a]pyrene) that is achiral in the gas phase, but becomes chiral when adsorbed on a surface. This polyaromatic hydrocarbon forms a stable and reversibly ordered system on Cu(111) in which the transmission of chirality from single surface‐bound molecules to complex 2D chiral architectures can be monitored as a function of molecular packing density and surface temperature. In addition to the point chirality of the surface‐bound molecule, the unit cell of the molecular domains was also found to be chiral due to the incommensurate alignment of the molecular rows with respect to the underlying metal lattice. These molecular domains always aggregated in groups of three, all of the same chirality, but with different rotational orientations, forming homochiral “tri‐lobe” ensembles. At a larger length scale, these tri‐lobe ensembles associated with nearest‐neighbor tri‐lobe units of opposite chirality at lower packing densities before forming an extended array of homochiral tri‐lobe ensembles at higher converges. This system displayed chirality at a variety of size scales from the molecular (≈1 nm) and domain (≈5 nm) to the tri‐lobe ensemble (≈10 nm) and extended array (>25 nm) levels. The chirality of the tri‐lobe ensembles dictated how the overall surface packing occurred and both homo‐ and heterochiral arrays could be reproducibly and reversibly formed and interchanged as a function of surface coverage. Finally, these chirally templated surfaces displayed remarkable enantiospecificity for naphtho[2,3‐a]pyrene molecules adsorbed in the second layer. Given its simplicity, reversibility, and rich degree of order, this system represents an ideal test bed for the investigation of symmetry breaking and the hierarchical transmission of chirality through multiple length scales.     

Assignment of absolute configuration of a chiral phenyl-substituted dihydrofuroangelicin

A phenyl-substituted chiral dihydrofuroangelicin, 4-methyl-8-(2-E-phenylethenyl)-8,9-dihydro-2H-furo[2,3-h]- 1-benzopyran-2-one, synthesized in racemic form, has been resolved by HPLC chiral separation, and its absolute configuration determined by the non-empirical exciton chirality method. The solution conformation has been investigated through NMR and molecular modeling methods: two minima found by molecular mechanics and DFT methods are in keeping with observed 1H-1H 3J coupling constants and NOE effects. The experimental CD spectrum for the second eluted enantiomer shows a positive couplet between 230 and 350 nm (amplitude A = + 15.7); by application of the exciton chirality method, the absolute configuration of this enantiomer at C8 is determined as (S). The experimental spectrum is in very good agreement with the one evaluated by means of DeVoe coupled-oscillator calculations, using the DFT calculated geometries.     

Hydrodynamic and Thermophoretic Effects on the Supramolecular Chirality of Pyrene‐Derived Nanosheets

Chiroptical properties of two‐dimensional (2D) supramolecular assemblies (nanosheets) of achiral, charged pyrene trimers ( Py₃ ) are rendered chiral by asymmetric physical perturbations. Chiral stimuli in a cuvette can originate either from controlled temperature gradients or by very gentle stirring. The chiroptical activity strongly depends on the degree of supramolecular order of the nanosheets, which is easily controlled by the method of preparation. The high degree of structural order ensures strong cooperative effects within the aggregates, rendering them more susceptible to external stimuli. The samples prepared by using slow thermal annealing protocols are both CD and LD active (in stagnant and stirred solutions), whereas for isothermally aged samples chiroptical activity was in all cases undetectable. In the case of temperature gradients, the optical activity of 2D assemblies could be recorded for a stagnant solution due to migration of the aggregates from the hottest to the coldest regions of the system. However, a considerably stronger exciton coupling, coinciding with the J‐band of the interacting pyrenes, is developed upon subtle vortexing (0.5 Hz, 30 rpm) of the aqueous solution of the nanosheets. The sign of the exciton coupling is inverted upon switching between clockwise and counter‐clockwise rotation. The supramolecular chirality is evidenced by the appearance of CD activity. To exclude artefacts from proper CD spectra, the contribution from LD to the observed CD was determined. The data suggest that the aggregates experience asymmetrical deformation and alignment effects because of the presence of chiral flows.     

Size-dependent exciton chirality in (R)-(+)-1,1'-bi-2-naphthol dimethyl ether nanoparticles

Organic nanoparticles from a chiral auxiliary, (R)-(+)-1,1'-bi-2-naphthol dimethyl ether (BNDE), with a range of particle sizes from 25 to 100 nm were fabricated through the reprecipitation method. It is found that BNDE nanoparticles exhibit positive exciton chirality in 200-260 nm region in circular dichroism (CD) spectra, which are completely opposite to CD spectra of the dilute solution. The exciton chirality of the particles displays size-dependent behavior; that is, the exciton chirality peaks evolve to the low-energy side with increase in particles size. CD spectra accompanied with UV, fluorescence spectra, lifetime measurements of the excited states, and quantum mechanical calculations reveal that the chirality inversion results from intermolecular exciton coupling between two adjacent BNDE molecules in the nanoparticles, and the bathochromic shift of the peaks is attributed to the increased intermolecular interaction with increasing particle size.     

Chirality in Dynamic Supramolecular Nanotubes Induced by a Chiral Solvent

Amplification of chirality has been reported in polymeric systems. It has also been shown that related effects can occur in polymer‐like dynamic supramolecular aggregates, if a subtle balance between noncovalent interactions allows the coupling between a chiral information and a cooperative aggregation process. In this context, we report a strong majority‐rules effect in the formation of chiral dynamic nanotubes from chiral bisurea monomers. Furthermore, similar helical nanotubes (with the same circular dichroism signature) can be obtained from racemic monomers in a chiral solvent. Competition experiments reveal the relative strength of the helical bias induced by the chiral monomer or by the chiral solvent. The nanotube handedness is imposed by the monomer chirality, whatever the solvent chirality. However, the chirality of the solvent has a significant effect on the degree of chiral induction.     

General approach to allenes through copper-catalyzed γ-selective and stereospecific coupling between propargylic phosphates and alkylboranes

Copper-catalyzed γ-selective coupling between propargylic phosphates and alkylboron compounds (alkyl-9-BBN, prepared by hydroboration of alkenes with 9-BBN-H) affords multisubstituted allenes with various functional groups. The reaction of enantioenriched propargylic phosphates to give axially chiral allenes proceeds with excellent point-to-axial chirality transfer with 1,3-anti stereochemistry.     

The Toxicity of Graphene Oxides: Dependence on the Oxidative Methods Used

Graphene, a class of two‐dimensional carbon nanomaterial, has attracted extensive interest in recent years, with a significant amount of research focusing on graphene oxides (GOs). They have been primed as potential candidates for biomedical applications such as cell labeling and drug delivery, thus the toxicity and behavior of graphene oxides in biological systems are fundamental issues that need urgent attention. The production of GO is generally achieved through a top‐down route, which includes the usage of concentrated H₂SO₄ along with: 1) concentrated nitric acid and KClO₃ oxidant (Hoffmann); 2) fuming nitric acid and KClO₃ oxidant (Staudenmaier); 3) concentrated phosphoric acid with KMnO₄ (Tour); or 4) sodium nitrate for in‐situ production of nitric acid in the presence of KMnO₄ (Hummers). It has been widely assumed that the properties of these four GOs produced by using the above different methods are roughly similar, so the methods have been used interchangeably. However, several studies have reported that the toxicity of graphene‐related nanomaterials in biological systems may be influenced by their physiochemical properties, such as surface functional groups and structural defects. In addition, considering how GOs are increasingly used in the field of biomedicine, we are interested to see how the oxygen content/functional groups of GOs can impact their toxicological profiles. Since in‐vitro testing is a common first step in assessing the health risks related with engineered nanomaterials, the cytotoxicity of the GOs prepared by the four different oxidative treatments was investigated by measuring the mitochondrial activity in adherent lung epithelial cells (A549) by using commercially available viability assays. The dose–response data was generated by using two assays, the methylthiazolyldiphenyl‐tetrazolium bromide (MTT) assay and the water‐soluble tetrazolium salt (WST‐8). From the viability data, it is evident that there is a strong dose‐dependent cytotoxic response resulting from the four GO nanomaterials tested after a 24 h exposure, and it is suggested that there is a correlation between the amounts of oxygen content/functional groups of GOs with their toxicological behavior towards the A549 cells.     

Plasmon‐Exciton Coupling Interaction for Surface Catalytic Reactions

In this review, we firstly reveal the physical principle of plasmon‐exciton coupling interaction with steady absorption spectroscopy, and ultrafast transition absorption spectroscopy, based on the pump‐prop technology. Secondly, we introduce the fabrication of electro‐optical device of two‐dimensional semiconductor‐nanostructure noble metals hybrid, based on the plasmon‐exciton coupling interactions. Thirdly, we introduce the applications of plasmon‐exciton coupling interaction in the field of surface catalytic reactions. Lastly, the perspective of plasmon‐exciton coupling interaction and applications closed this review.     

Methods of studies on quantitative structure–activity relationships for chiral compounds

Chiral compounds are very important in drug development, organic synthesis, materials science, toxicology, or environmental chemistry. Therefore, for creating new drugs, several methods have been suggested in recent years. In several laboratories in the world, some new methods for the derivations of the parameters were constructed and used for studies on quantitative structure–activity/property relationships of chiral molecules. The algorithms reviewed in this paper involve Zargeb group chiral indices, chiral molecular connectivity index, chiral topological charge index, chiral A_m index, chiral indices based on the matrixes, chiral indices based on chiral product, conformation‐independent chirality code, conformation‐dependent chirality code, quantitative two‐dimensional chirality degrees of benzenoids, and so on. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and Aggregation Behavior of meso‐Sulfinylporphyrins: Evaluation of S‐Chirality Effects on the Self‐Organization to S–Oxo‐Tethered Cofacial Porphyrin Dimers

The synthesis and aggregation behavior of meso‐sulfinylporphyrins are described. The copper‐catalyzed C–S cross‐coupling reaction of a meso‐iodoporphyrin with benzenethiol and n‐octanethiol has proved to be an efficient method for the synthesis of meso‐sulfanylporphyrins, which are oxygenated by m‐chloroperbenzoic acid to produce the corresponding meso‐sulfinylporphyrins. Optically active zinc meso‐sulfinylporphyrins were successfully isolated by means of optical resolution of the racemates on a chiral HPLC column. Zinc sulfinylporphyrins readily undergo self‐organization through S–oxo–zinc coordination to form cofacial porphyrin dimers in solution, in which the hetero‐ and homodimers are present as a diastereomeric mixture. The aggregation modes of the S–oxo‐tethered porphyrin dimers were fully characterized by ¹H NMR, IR, and UV/Vis spectroscopy as well as DFT calculations on their model compounds, thus revealing that the self‐aggregation behavior depends on the combination of S chirality. The absolute configurations at the sulfur center can be determined by the exciton‐coupled CD method. The observed self‐association constant for the S–oxo‐tethered dimerization of (S)‐phenylsulfinylporphyrin in toluene is larger than that in dichloromethane, which reflects the difference in dipole moments between the homodimer and the monomer. In cyclic and differential pulse voltammetry, the first oxidation process of the cofacial dimers is split into two reversible steps, which indicates that the initially produced π radical cations are delocalized efficiently between the two porphyrin rings. The present findings demonstrate the potential utility of meso‐sulfinyl groups as promising ligands for investigating the effects of peripheral chirality on the structures and optical and electrochemical properties of metal‐assisted porphyrin self‐assemblies.     

Capillary electrophoresis of covalently functionalized single‐chirality carbon nanotubes

We demonstrate the separation of chirality‐enriched single‐walled carbon nanotubes (SWCNTs) by degree of surface functionalization using high‐performance CE. Controlled amounts of negatively charged and positively charged functional groups were attached to the sidewall of chirality‐enriched SWCNTs through covalent functionalization using 4‐carboxybenzenediazonium tetrafluoroborate or 4‐diazo‐N,N‐diethylaniline tetrafluoroborate, respectively. Surfactant‐ and pH‐dependent studies confirmed that under conditions that minimized ionic screening effects, separation of these functionalized SWCNTs was strongly dependent on the surface charge density introduced through covalent surface chemistry. For both heterogeneous mixtures and single‐chirality‐enriched samples, covalently functionalized SWCNTs showed substantially increased peak width in electropherogram spectra compared to nonfunctionalized SWCNTs, which can be attributed to a distribution of surface charges along the functionalized nanotubes. Successful separation of functionalized single‐chirality SWCNTs by functional density was confirmed with UV‐Vis‐NIR absorption and Raman scattering spectroscopies of fraction collected samples. These results suggest a high degree of structural heterogeneity in covalently functionalized SWCNTs, even for chirality‐enriched samples, and show the feasibility of applying CE for high‐performance separation of nanomaterials based on differences in surface functional density.     

Helicity Induction and Memory of the Macromolecular Helicity in a Polyacetylene Bearing a Biphenyl Pendant

A novel, cis‐transoidal poly‐(phenylacetylene) bearing a carboxybiphenyl group as the pendant (poly‐ 1 ) was prepared by polymerization of (4′‐ethoxycarbonyl‐4‐biphenylyl)acetylene with a rhodium catalyst followed by hydrolysis of the ester groups. Upon complexation with various chiral amines and amino alcohols in dimethyl sulfoxide (DMSO), the polymer exhibited characteristic induced circular dichroism (ICD) in the UV/Vis region due to the predominantly one‐handed helix formation of the polymer backbone as well as an excess of a single‐handed, axially twisted conformation of the pendant biphenyl group. Poly‐ 1 complexed with (R)‐2‐amino‐1‐propanol showed unique time‐dependent inversion of the macromolecular helicity. Furthermore, the preferred‐handed helical conformation of poly‐ 1 induced by a chiral amine was further “memorized” after the chiral amine was replaced with achiral 2‐aminoethanol or n‐butylamine in DMSO. In sharp contrast to the previously reported memory in poly((4‐carboxyphenyl)acetylene), the present helicity memory of poly‐ 1 was accompanied by memory of the twisted biphenyl chirality in the pendants. Unprecedentedly, the helicity memory of poly‐ 1 with achiral 2‐aminoethanol was found to occur simultaneously with inversion of the axial chirality of the biphenyl groups followed by memory of the inverted biphenyl chirality, thus showing a significant change in the CD spectral pattern.     

A Density Functional Study of the Nonlinear Optical Properties of Edge‐Functionalized Nonplanar Nanographenes

The atomically precise edge chlorination of nanographenes has recently been reported as a crucial technology of functionalization through which the planar structure and optical properties of nanographenes can be significantly changed. To check the effects of molecular size, geometrical symmetry and edge functionalization of nanographenes on their optical properties, a series of nanographenes is studied in the framework of density functional theory with the B3LYP functional. Our results indicate that edge functionalization remarkably changes the nonlinear optical properties and increases the anisotropy of nanographenes compared to the effects of the molecular size and system geometric symmetry. Furthermore, the nonlinear optical properties of nanographenes can be tuned by precise edge functionalization, which opens a new avenue for using nanographenes as nonlinear optical materials.