Chemically Functionalized Two‐Dimensional Carbon Materials

Nanographenes (NGs), also known as graphene quantum dots, have recently been developed as nanoscale graphene fragments. These nanocarbon species can be excited with UV light and emit light from the UV‐to‐visible region. This photoemission has received great attraction across multiple scientific fields. NGs can be produced by cutting off carbon sources or fusing small organic molecules to grow graphitic structures. Furthermore, the organic synthesis of NGs has been intensely studied. Recently, the number of research papers on postsynthetic modification of NGs has gradually increased. Installed organic groups can tune the properties of NGs and provide new functionalities, opening the door for the development of sophisticated carbon‐based functional materials. This review sheds light on recent progress in the postsynthetic modification of NGs and provides a brief summary of their production methods.     

Revealing the Preferred Interlayer Orientations and Stackings of Two‐Dimensional Bilayer Gallium Selenide Crystals

Characterizing and controlling the interlayer orientations and stacking orders of two‐dimensional (2D) bilayer crystals and van der Waals (vdW) heterostructures is crucial to optimize their electrical and optoelectronic properties. The four polymorphs of layered gallium selenide (GaSe) crystals that result from different layer stackings provide an ideal platform to study the stacking configurations in 2D bilayer crystals. Through a controllable vapor‐phase deposition method, bilayer GaSe crystals were selectively grown and their two preferred 0° or 60° interlayer rotations were investigated. The commensurate stacking configurations (AA′ and AB stacking) in as‐grown bilayer GaSe crystals are clearly observed at the atomic scale, and the Ga‐terminated edge structure was identified using scanning transmission electron microscopy. Theoretical analysis reveals that the energies of the interlayer coupling are responsible for the preferred orientations among the bilayer GaSe crystals.     

Tailoring Magnetic Features in Zigzag-Edged Nanographenes by Controlled Diels–Alder Reactions

Nanographenes (NGs) with tunable electronic and magnetic properties have attracted enormous attention in the realm of carbon-based nanoelectronics. In particular, NGs with biradical character at the ground state are promising building units for molecular spintronics. However, most of the biradicaloids are susceptible to oxidation under ambient conditions and photolytic degradation, which hamper their further applications. Herein, we demonstrated the feasibility of tuning the magnetic properties of zigzag-edged NGs in order to enhance their stability via the controlled Diels–Alder reactions of peri-tetracene ( 4-PA ). The unstable 4-PA (y₀=0.72; half-life, t_1/2=3 h) was transformed into the unprecedented benzo-peri-tetracenes ( BPTs ) by a one-side Diels–Alder reaction, which featured a biradical character at the ground state (y₀=0.60) and exhibited remarkable stability under ambient conditions for several months. In addition, the fully zigzag-edged circumanthracenes ( CAs ) were achieved by two-fold or stepwise Diels–Alder reactions of 4-PA , in which the magnetic properties could be controlled by employing the corresponding dienophiles. Our work reported herein opens avenues for the synthesis of novel zigzag-edged NGs with tailor-made magnetic properties.     

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.     

Effects of Edge Functionalization of Nanographenes with Small Aromatic Systems

Regulation of the physical properties of nanographenes (NGs) by edge functionalization is an active research area. We conducted a computational study of the effects of edge functionalization on the physical properties of NGs. The computed NGs were models of experimentally obtained NGs and composed of a C₁₇₄ carbon framework with one to four 3,5-dimethylnaphthalene units on the edge. The effects were assessed structurally, magnetically, and electronically by the least square planarity index, harmonic oscillator model of aromaticity, nucleus-independent chemical shift, and HOMO–LUMO (H–L) gaps. Density functional theory calculations indicate that although the structures of the model NGs are not very sensitive to edge functionalization, but the magnetic and electronic properties are. The installed substituents narrowed the H−L gap and induced a redshift of the photoluminescence (PL) band by the π conjugation between NG and the substituent. These results are consistent with the extension of the absorption band and the redshift of the PL bands of the experimentally modified NGs. Furthermore, the calculations confirmed the contribution of the charge transfer character to the absorption spectra.     

Intercalation of a nonionic surfactant (C10E3) bilayer into a Na-montmorillonite clay

A nonionic surfactant, triethylene glycol mono-n-decyl ether (C(10)E(3)), characterized by its lamellar phase state, was introduced in the interlayer of a Na-montmorillonite clay at several concentrations. The synthesized organoclays were characterized by small-angle X-ray scattering in conjunction with Fourier transform infrared spectroscopy and adsorption isotherms. Experiments showed that a bilayer of C(10)E(3) was intercalated into the interlayer space of the naturally exchanged Na-montmorillonite, resulting in the aggregation of the lyotropic liquid crystal state in the lamellar phase. This behavior strongly differs from previous observations of confinement of nonionic surfactants in clays where the expansion of the interlayer space was limited to two monolayers parallel to the silicate surface and cationic surfactants in clays where the intercalation of organic compounds is introduced into the clay galleries through ion exchange. The confinement of a bilayer of C(10)E(3) nonionic surfactant in clays offers new perspectives for the realization of hybrid nanomaterials, since the synthesized organoclays preserve the electrostatic characteristics of the clays, thus allowing further ion exchange while presenting at the same time a hydrophobic surface and a maximum opening of the interlayer space for the adsorption of neutral organic molecules of important size with functional properties.     

Semi-Rigid Nitroxide Spin Label for Long-Range EPR Distance Measurements of Lipid Bilayer Embedded β-Peptides

β-Peptides are an interesting new class of transmembrane model peptides based on their conformationally stable and well-defined secondary structures. Herein, we present the synthesis of the paramagnetic β-amino acid β³-hTOPP (4-(3,3,5,5-tetramethyl-2,6-dioxo-4-oxylpiperazin-1-yl)-d -β³-homophenylglycine) that enables investigations of β-peptides by EPR spectroscopy. This amino acid adds to the, to date, sparse number of β-peptide spin labels. Its performance was evaluated by investigating the helical turn of a 3₁₄-helical transmembrane model β-peptide. Nanometer distances between two incorporated β³-hTOPP labels in different environments were measured by using pulsed electron/electron double resonance (PELDOR/DEER) spectroscopy. Due to the semi-rigid conformational design, the label delivers reliable distances and sharp (one-peak) distance distributions even in the lipid bilayer. The results indicate that the investigated β-peptide folds into a 3.25₁₄ helix and maintains this conformation in the lipid bilayer.     

Influence of Neighboring Layers on Interfacial Energy of Adjacent Layers

The binding energy and generalized stacking-fault energy (GSFE) are two critical interface properties of two dimensional layered materials, and it is still unclear how neighboring layers affect the interface energy of adjacent layers. Here, we investigate the effect of neighboring layers by comparing the differences of binding energy and GSFE between trilayer heterostructures (graphene/graphene/graphene, graphene/graphene/boron nitride, boron nitride/graphene/boron nitride) and bilayer heterostructures (graphene/graphene, graphene/boron nitride) using density functional theory. The binding energy of the adjacent layers changes from -2.3% to 22.55% due to the effect of neighboring layer, with a very small change of the interlayer distance. Neighboring layers also make a change from -2% to 10% change the GSFE, depending on the property of the interface between adjacent layers. In addition, a new simple expression is proven to describe the GSFE landscape of graphene-like structure with high accuracy.     

Vectorial photoinduced electron transfer in alternating Langmuir–Blodgett films of phytochlorin–[60]fullerene dyad and regioregular poly(3-hexylthiophene)

Alternating Langmuir–Blodgett (LB) bilayer structures, consisting of a donor–acceptor (DA) layer of a phytochlorin–fullerene (PF) dyad and a layer of a regioregular poly(3-hexylthiophene) (PHT) polymer were used to study interlayer vectorial photoinduced electron transfer (VPET). As the dyad PF undergoes, under light illumination, an intramolecular ET from the phytochlorin to the fullerene moiety an intralayer VPET takes place in the LB monolayer. When PF was deposited on the PHT layer and excited the second ET took place from the PHT layer to the phytochlorin cation. Thus the PHT layer can act as a secondary electron donor and accompany the primary photoinduced electron transfer in the PF layer by a spontaneous interlayer electron transfer. Important characteristic properties of the VPET bilayer are the longer distance of charge separation (CS) and the longer lifetime of the charge separated state (lifetime from microsecond to second) as compared to VPET of the PF monolayer alone (where the lifetime of CS state was ≈30 ns). The CT measurements were carried out for different molecular orientations and film structures. Models for the multistep photochemical reactions are discussed.     

Large Hexagonal Bi‐ and Trilayer Graphene Single Crystals with Varied Interlayer Rotations

Bi‐ and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the synthesis of high‐quality large‐size bi‐ and trilayer graphene single crystals still remains a challenge. Here, the synthesis of 100 μm pyramid‐like hexagonal bi‐ and trilayer graphene single‐crystal domains on Cu foils using chemical vapor deposition is reported. The as‐produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier‐transformed infrared spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first‐principle calculations, it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the experimental observations.     

How far can a sodium ion travel within a lipid bilayer?

Analogues of a synthetic ion channel made from a helical peptide were used to study the mechanism of cation translocation within bilayer membranes. Derivatives bearing two, three, four, and six crown ethers used as ion relays were synthesized, and their transport abilities across lipid bilayers were measured. The results showed that the maximum distance a sodium ion is permitted to travel between two binding sites within a lipid bilayer environment is 11 ?.     

Towards Nonalternant Nanographenes through Self-Promoted Intramolecular Indenoannulation Cascade by C−F Bond Activation

Large polycyclic aromatic hydrocarbons (PAHs) containing pentagons represent an important class of compounds that are considered to be superior materials in future nano-electronic applications. From this perspective, the development of synthetic approaches to large PAHs and nanographenes (NGs) is a matter of great importance. In this context indenoannulation appears to be the most practical way to introduce pentagons into NGs. Here we report that alumina-mediated C−F bond activation is an attractive tool for the synthesis of non-alternant NGs bearing several pentagons. The unique nature of the reaction leads to a rather counter-intuitive outcome and allows considering each previous aryl–aryl coupling as a promoter of the following one, despite the continuous increase in the strain energy. Thus, the presented strategy combines both facile synthesis and significant yields for large nonalternant PAHs and NGs.     

Interface geometry of Ag thin films supported at the MgO(1 0 0) surface: an ab-initio periodic study

In this Letter, the interface geometry of silver thin films of thickness T between 1 and 3 ML epitaxially deposited at the MgO surface has been accurately characterized employing DFT periodic calculations. The Ag–Ag out-of-plane interlayer spacing is considerably shorter than the bulk values because of: (i) the reduced dimension; (ii) the perfect epitaxy constraint; (iii) the interaction with the substrate. The interface distance between the silver monolayer and the MgO substrate, d(Ag–O) = 2.60 Å, is considerably longer than the estimates computed for the bilayer and the trilayer, d(Ag–O) = 2.47 and 2.48 Å, respectively. The difference between the values of the interface distance computed for the monolayer and the results obtained for thicker films, lies in the peculiar electronic properties of the silver monolayer.     

水溶性阳离子型卟啉对层状磷酸锆插层行为的研究

比较了水溶性卟啉meso-四(4-N-甲基吡啶基)卟啉(TMPyP)对具有不同层间距和结构的层状磷酸锆[α-磷酸锆(α-ZrP)和γ-磷酸锆(γ-ZrP)]的插层行为. 研究发现: 相比α-磷酸锆, γ-磷酸锆虽然具有相对较大的层间距, 但同α-磷酸锆一样, TMPyP不能直接嵌入其中. 为嵌入TMPyP, 用预撑剂正丁胺(BA)处理磷酸锆. TMPyP可以嵌入α-ZrP•BA和α-ZrP•2BA(分别为单层丁胺和双层丁胺嵌入α-磷酸锆而形成的插层化合物), 其中, TMPyP以较短的时间与单层排列的丁胺交换而嵌入磷酸锆; 而卟啉却不能嵌入具有较大层间距的γ-ZrP•2BA(双层丁胺嵌入γ-磷酸锆而形成的一种很稳定的形式), 表明预撑剂在磷酸锆层板间的流动性是影响卟啉嵌入的一个重要因素. 另外, 结合XRD、红外、可见吸收等实验数据和α-磷酸锆层板高电荷密度的特性, 我们可推算出: TMPyP以自由碱的形式呈单层倾斜方式紧密堆积在α-磷酸锆层板间.     

Size effect in the titanium/diamond‐like carbon bilayer films: effect of relative thickness on their structure and mechanical properties

Titanium/diamond‐like carbon (Ti/DLC) bilayer films with different relative thickness were fabricated by direct‐current and pulsed cathode arc plasma method. Microstructure, morphological characteristics, and mechanical properties of the films were investigated in dependence of the thickness of Ti and DLC layers by Raman spectroscopy, atomic force microscopy, Knoop sclerometer, and surface profilometer. Raman spectra of Ti/DLC bilayers show the microstructure evolution (the size and ordering degree of sp²‐hybridized carbon clusters) with varying the thicknesses of Ti interlayer and DLC layer. Nano‐scaled Ti interlayer of 12–20 nm thickness presents the largest size effect. The catalytic effect of the sublayer is most pronounced in the carbon layer of less than 106 nm. In these thickness ranges, the bilayer films possessed the highest micro‐hardness and reactivity between atoms at interface. Internal stress in the bilayer monotonically decreases, with the thickness of Ti interlayer increasing to 30 nm and then becomes stable with the thickness. These results are associated with the occurrence of atomic diffusion process at Ti/C interface, and they are of cardinal significance to optimize the structure and mechanical properties of carbon‐based multilayer films. Copyright © 2016 John Wiley & Sons, Ltd.     

Carbon Origami via an Alumina-Assisted Cyclodehydrofluorination Strategy

The synthesis of pristine non-planar nanographenes (NGs) via a cyclodehydrofluorination strategy is reported and the creation of highly strained systems via alumina-assisted C−F bond activation is shown. Steric hindrance could execute an alternative coupling program leading to rare octagon formation offering access to elusive non-classical NGs. The combination of two alternative ways of folding could lead to the formation of various 3D NG objects, resembling the Japanese art of origami. The power of the presented “origami” approach is proved by the assembly of 12 challenging nanographenes that are π-isoelectronic to planar hexabenzocoronene but forced out of planarity.     

Edge-Functionalized Nanographenes

Nanographenes (NGs) have recently emerged as new carbon materials. Their nanoscale size results in a size-dependent quantum confinement effect, opening the band gap by a few eV. This energy gap allows NGs to be applied as optical materials. This property has attracted researchers across multiple scientific fields. The photophysical properties of NGs can be manipulated by introducing organic groups onto their basal planes and/or into their edges. In addition, the integration of organic functional groups into NGs results in NG-based hybrid materials. These features make the post-synthetic modification of NGs an active research area. As obtainable information on chemically functionalized NGs is limited owing to their nonstoichiometry and structural uncertainty, their structural characterization requires a combination of multiple spectroscopic methods. Therefore, information on the characterization procedures of recently published chemically functionalized NGs is of value for advancing the field of NG-based hybrid materials. The present review focuses on the structural characterization of chemically functionalized NGs. It is hoped that this review will help to advance this field.     

Cove-Edged Nanographenes with Localized Double Bonds

The efficient synthesis and electronic properties of two large-size cove-edged nanographenes (NGs), CN1 and CN2 , are presented. X-ray crystallographic analysis reveals a contorted backbone for both molecules owing to the steric repulsion at the inner cove position. Noticeably, the dominant structures of these molecules contain four (for CN1 ) or six (for CN2 ) localized C=C double bonds embedded in nine (for CN1 ) or twelve (for CN2 ) aromatic sextet rings according to Clar's formula, which is supported by bond length analysis and theoretical (NICS, ACID) calculations. Furthermore, Raman spectra exhibit a band associated with the longitudinal CC stretching mode of olefinic double bonds. Owing to the existence of the additional olefinic bonds, both compounds show a small band gap (1.84 eV for CN1 and 1.37 eV for CN2 ). They also display moderate fluorescence quantum yield (35 % for CN1 and 50 % for CN2 ) owing to the contorted geometry, which can suppress aggregation in solution.     

Cove‐Edged Nanographenes with Localized Double Bonds

The efficient synthesis and electronic properties of two large‐size cove‐edged nanographenes (NGs), CN1 and CN2 , are presented. X‐ray crystallographic analysis reveals a contorted backbone for both molecules owing to the steric repulsion at the inner cove position. Noticeably, the dominant structures of these molecules contain four (for CN1 ) or six (for CN2 ) localized C=C double bonds embedded in nine (for CN1 ) or twelve (for CN2 ) aromatic sextet rings according to Clar's formula, which is supported by bond length analysis and theoretical (NICS, ACID) calculations. Furthermore, Raman spectra exhibit a band associated with the longitudinal CC stretching mode of olefinic double bonds. Owing to the existence of the additional olefinic bonds, both compounds show a small band gap (1.84 eV for CN1 and 1.37 eV for CN2 ). They also display moderate fluorescence quantum yield (35 % for CN1 and 50 % for CN2 ) owing to the contorted geometry, which can suppress aggregation in solution.     

Computational Studies on the Structures of Nanographenes with Various Edge Functionalities

Computational studies have often been carried out on hydrogen-terminated nanographenes (NGs). These structures are, however, far from those deduced from experimental observations, which have suggested armchair edges with two carboxy groups on the edges as dominant. We conducted computational studies on NGs consisting of C₄₂, C₆₀, C₇₈, C₉₆, C₁₄₂, and C₁₇₄ carbon atoms with hydrogen, carboxy, and N-methyl imide-terminated armchair edges. DFT calculations inform distorted basal planes and similar HOMO-LUMO gaps, indicating that the edge oxidation and functionalization do not very influence the electronic structure. Comparison of observed UV-vis spectra of carboxy- and N-octadecyl chain terminated NGs with calculated spectra of model NGs informs the contribution of π-π* transitions on the basal plane to the absorptions in the visible region. A dimeric structure of NG and octadecyl-installed NG demonstrate that both the distorted basal planes and the steric contacts among the functional groups widen the surface-to-surface distance thereby allowing the invasion of solvent molecules between the surfaces. This picture is consistent with the improved solubility of edge-modified NGs.