Deceptive orbital confinement at edges and pores of carbon-based 1D and 2D nanoarchitectures.

The electronic structure defines the properties of graphene-based nanomaterials. Scanning tunneling microscopy/spectroscopy (STM/STS) experiments on graphene nanoribbons (GNRs), nanographenes, and nanoporous graphene (NPG) often determine an apparent electronic orbital confinement into the edges and nanopores, leading to dubious interpretations such as image potential states or super-atom molecular orbitals. We show that these measurements are subject to a wave function decay into the vacuum that masks the undisturbed electronic orbital shape. We use Au(111)-supported semiconducting gulf-type GNRs and NPGs as model systems fostering frontier orbitals that appear confined along the edges and nanopores in STS measurements. DFT calculations confirm that these states originate from valence and conduction bands. The deceptive electronic orbital confinement observed is caused by a loss of Fourier components, corresponding to states of high momentum. This effect can be generalized to other 1D and 2D carbon-based nanoarchitectures and is important for their use in catalysis and sensing applications.

Bridging pico-to-nanonewtons with a ratiometric force probe for monitoring nanoscale polymer physics before damage.

Understanding the transmission of nanoscale forces in the pico-to-nanonewton range is important in polymer physics. While physical approaches have limitations in analyzing the local force distribution in condensed environments, chemical analysis using force probes is promising. However, there are stringent requirements for probing the local forces generated before structural damage. The magnitude of those forces corresponds to the range below covalent bond scission (from 200 pN to several nN) and above thermal fluctuation (several pN). Here, we report a conformationally flexible dual-fluorescence force probe with a theoretically estimated threshold of approximately 100 pN. This probe enables ratiometric analysis of the distribution of local forces in a stretched polymer chain network. Without changing the intrinsic properties of the polymer, the force distribution was reversibly monitored in real time. Chemical control of the probe location demonstrated that the local stress concentration is twice as biased at crosslinkers than at main chains, particularly in a strain-hardening region. Due to the high sensitivity, the percentage of the stressed force probes was estimated to be more than 1000 times higher than the activation rate of a conventional mechanophore.

Manifold dynamic non-covalent interactions for steering molecular assembly and cyclization.

Deciphering rich non-covalent interactions that govern many chemical and biological processes is crucial for the design of drugs and controlling molecular assemblies and their chemical transformations. However, real-space characterization of these weak interactions in complex molecular architectures at the single bond level has been a longstanding challenge. Here, we employed bond-resolved scanning probe microscopy combined with an exhaustive structural search algorithm and quantum chemistry calculations to elucidate multiple non-covalent interactions that control the cohesive molecular clustering of well-designed precursor molecules and their chemical reactions. The presence of two flexible bromo-triphenyl moieties in the precursor leads to the assembly of distinct non-planar dimer and trimer clusters by manifold non-covalent interactions, including hydrogen bonding, halogen bonding, C-H⋯π and lone pair⋯π interactions. The dynamic nature of weak interactions allows for transforming dimers into energetically more favourable trimers as molecular density increases. The formation of trimers also facilitates thermally-triggered intermolecular Ullmann coupling reactions, while the disassembly of dimers favours intramolecular cyclization, as evidenced by bond-resolved imaging of metalorganic intermediates and final products. The richness of manifold non-covalent interactions offers unprecedented opportunities for controlling the assembly of complex molecular architectures and steering on-surface synthesis of quantum nanostructures.

Skeletal Rearrangement of Twisted Polycyclic Aromatic Hydrocarbons under Scholl Reaction Conditions.

Treatment of a twisted polycyclic aromatic hydrocarbon containing cyclooctatetraene fused by two 9,9'-bifluorenylidene units under the Scholl reaction conditions (FeCl3 or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and scandium trifluoromethanesulfonate) led to stepwise skeletal rearrangements to afford initially a hydrocarbon with a seven-membered ring and then tetrabenzo[a,d,j,m]coronene with all six-membered rings. The course of the rearrangement was interpreted in terms of the acid-catalyzed isomerization of 9,9'-bifluorenylidene into dibenzo[g,p]chrysene moieties on the basis of theoretical investigations.

Light-melt adhesive based on dynamic carbon frameworks in a columnar liquid-crystal phase.

Liquid crystal (LC) provides a suitable platform to exploit structural motions of molecules in a condensed phase. Amplification of the structural changes enables a variety of technologies not only in LC displays but also in other applications. Until very recently, however, a practical use of LCs for removable adhesives has not been explored, although a spontaneous disorganization of LC materials can be easily triggered by light-induced isomerization of photoactive components. The difficulty of such application derives from the requirements for simultaneous implementation of sufficient bonding strength and its rapid disappearance by photoirradiation. Here we report a dynamic molecular LC material that meets these requirements. Columnar-stacked V-shaped carbon frameworks display sufficient bonding strength even during heating conditions, while its bonding ability is immediately lost by a light-induced self-melting function. The light-melt adhesive is reusable and its fluorescence colour reversibly changes during the cycle, visualizing the bonding/nonbonding phases of the adhesive.

Non-alternant non-benzenoid kekulenes: the birth of a new kekulene family.

"Kekulene" is a doughnut-like shaped polycyclic aromatic hydrocarbon consisting of cyclically arrayed benzene rings. It has attracted a great deal of theoretical interest because it is regarded as an ideal model to study conjugation circuits of π electrons, i.e. whether they delocalize locally in benzene rings or globally throughout the molecule. Though kekulene was synthesized in 1978, it was the only known compound of this class of compounds for a long time. Recently, new kekulene-related molecules, septulene, which is a non-alternant benzenoid hydrocarbon, and a tetracyclopentatetraphenylene (TCPTP) derivative belonging to non-alternant non-benzenoid hydrocarbons, were synthesized. This article presents theoretical and experimental aspects of kekulene-related molecules focusing on the viewpoint of conjugation circuits by classifying them into three types: benzenoid kekulenes including kekulene itself and septulene, yet unknown anti-kekulene and non-alternant non-benzenoid kekulenes represented by TCPTP.

Tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene: a potential tetraradicaloid hydrocarbon.

A tetramesityl derivative of hitherto unknown tetracyclopenta[def,jkl,pqr,vwx]tetraphenylene (TCPTP), which is a potential tetraradicaloid hydrocarbon, was synthesized. Theoretical calculations based on spin-flip time-dependent density functional theory predict that the closed-shell D2h form of TCPTP is more stable than the open-shell D4h form with its slightly tetraradical character. The tetramesityl derivative (Mes)4 -TCPTP exhibits remarkable antiaromaticity as a result of the peripheral 20-π-electron circuit, which causes an absorption maximum at a long wavelength and a small HOMO-LUMO gap. In solution, (Mes)4 -TCPTP most likely adopts rapidly equilibrating D2h structures that interconvert via the D4h transition state. X-ray crystallographic analysis showed (Mes)4 -TCPTP as an approximate D2h structure.

Highly bent crystals formed by restrained π-stacked columns connected via alkylene linkers with variable conformations.

A reproducible formation of strongly bent crystals was accomplished by structurally restraining macrocyclic π-conjugated molecules. The model π-units consist of two 9,10-bis(2-thienylethynyl)anthracenes with a strong propensity for stacking, which are connected in a macrocyclic fashion via two alkylene linkers. The correlation between the crystalline morphology and the macrocyclic structures restrained by a variety of flexible alkylene linker combinations was systematically studied. Bent crystals were obtained only with specific alkylene linkers of appropriate chain length. The alkylene linkers can adopt different conformations in the crystal packing, so as to fill voids within the macrocycle. The ability to form several similar molecular structures with different alkylene conformations gives rise to contaminations of different crystalline phases within a single crystal, and it is these phase contaminations which are responsible for the bending of the crystals.

[4.2](2,2')(2,2')Biphenylophanetriyne: a twisted biphenylophane with a highly distorted diacetylene bridge.

Biphenylophane 1 bridged by acetylene and diacetylene linkages was synthesized. X-ray analysis revealed its highly deformed diacetylene unit with a bond angle of 160°. Enantiomers of 1 resolved by chiral HPLC underwent facile racemization with an activation energy of 90.6 kJ mol(-1). Transannular cyclization of 1 was induced by heating to generate benzyne intermediate 8, which was intercepted by furan to give 9, and by treatment with bromine to give dibromodibenzopicene (10).

Molecular propellers that consist of dehydrobenzo[14]annulene blades.

A new class of propel- ler-shaped compound (4), which consisted of dehydrobenzo[14]annulene ([14]DBA) blades, as well as its naphtho homologues (5 and 6), have been prepared. Although NMR studies of compound 4 did not provide useful information regarding its conformation in solution, DFT calculations with different functionals and the 6-31G* basis set all indicated that the D(3)-symmetric structure was energetically more favorable than the C(2) conformer. From X-ray crystallographic analysis, it appeared that compound 4 adopted a propeller-shaped-, approximately D(3)-symmetric structure in the solid state, in which the [14]DBA blades were twisted substantially owing to steric repulsion between the neighboring benzene rings. On the contrary, in the case of compound 6, although the DFT calculations with the B3LYP functional predicted that the D(3)-symmetric conformation was more stable, calculations with the M05 and M05-2X functionals indicated that the C(2) conformer was more favorable because of π-π interactions between the naphthalene units of a pair of neighboring blades. Indeed, X-ray analysis of compound 6 showed that it adopted an approximately C(2)-symmetric conformation. Moreover, on the basis of variable-temperature (1)H NMR measurements, we found that compound 6 adopted a C(2) conformation and the barrier for interconversion between the C(2)-C(2) conformers was estimated to be 16.2 kcal mol(-1); however, no indication of the presence of the D(3) isomer was obtained. The relatively small energy barriers to interconversion, despite the large overlapping of neighboring blades, was ascribed to the flexibility of the acetylene linkages, which could be deformed substantially in the transition state of the ring-flip.