Synthesis and Reactivity of an X-Shaped Molecule: Reversible Formation and Cleavage of a Four-Membered Ring in Response to External Stimuli.

We herein report the synthesis and reactivity of an X-shaped molecule featuring three four-membered rings (4MRs) arranged in a ladder configuration. This molecule exhibits a reversible opening and closure of the central 4MR upon exposure to light irradiation and thermal treatment. The central 4MR of this molecule is also cleaved via electrochemical and chemical reductions. The stimuli-responsiveness of the X-shaped molecule is attributed to the small energy gap difference between its open and closed states, stemming from the antiaromatic character of its precursor.

Heptagonal Molecular Tiling via Self-Assembly of Heptagonal Phenylene-Ethynylene Macrocycle at the Liquid-Solid Interface.

The molecular-level scrutinization of on-surface tiling garners considerable interest among scientists. Herein, we demonstrate molecular-level heptagonal tiling using the self-assembly of a heptagonal meta-phenylene-ethynylene macrocycle featuring 14 long alkoxy substituents at the liquid-graphite interface using scanning tunneling microscopy. This heptagonal macrocycle produces an antiparallel pattern at the 1-phenyloctane-graphite interface through van der Waals interactions between the alkoxy chains. This pattern resembles the densely packed pattern of heptagonal tiles, albeit with variations in the orientations and spacing of heptagonal cores owing to intermolecular interactions between the alkoxy chains. Conversely, at the 1,2,4-trichlorobenzene-graphite interface, the heptagonal molecule forms an oblique pattern composed of four independent molecular orientations. This phenomenon arises from core distortion induced by the coadsorption of the solvent molecules within the intrinsic macrocyclic pores. This study elucidates the design strategy - specifically, the choice of heptagonal molecular building block - for heptagonal tiling and fills a crucial gap in the field of two-dimensional crystal engineering.

Formation of Supramolecular Heterostacks at the Liquid-Solid Interface: Impact of Symmetry Mismatching on Structural Growth.

The construction of intricate three-dimensional (3D) nanoarchitectures on surfaces through molecular self-assembly attracts attention not only from a crystal engineering viewpoint but also because of its potential in a range of applications, given the current interest in van der Waals heterostructures. We herein report the formation of porous structures on alkane buffer layers on graphite. A dehydrobenzo[12]annulene (DBA) derivative having six decyloxy chains forms hexagonal structures on n-pentacontane and n-hexacontane buffer layers through van der Waals interactions at the 1-octanoic acid/graphite interface. The structural features are very similar to those on the graphite surface, except for the slight structural distortion, which is attributed to the p2 symmetry of the buffer layer underneath. Moreover, based on the observation of small clusters of the DBA molecules, we discussed the nucleation and structural growth of the DBA network on a buffer layer. Finally, a hierarchical multicomponent structure was formed through the coadsorption of a heteromolecular cluster formed by a hydrogen-bonded isophthalic acid cyclic hexamer hosting a coronene molecule on the buffer layer. This study on supramolecular heterostacks provides insights into the construction of intricate 3D nanoarchitectures using self-assembly at interfaces.

Homochiral hierarchical molecular assemblies through dynamic combination of conformational states of a single chiral building block at the liquid/solid interface.

We herein report the construction of homochiral, hierarchical self-assembled molecular networks (SAMNs) at the liquid/graphite interface using a single molecular building block, a chiral dehydrobenzo[12]annulene (cDBA) derivative with three chiral alkoxy and three hydroxy groups positioned in an alternating manner on the DBA core. The cDBA molecules form homochiral hierarchical SAMNs consisting of triangular clusters of several sizes, the size of which can be tuned by solvent polarity and solute concentration, reaching periodicities as large as 9.3 nm. We demonstrate the successful transmission of chirality information from the single molecular level to the hierarchical SAMN level, in a process that is mediated by dynamic self-sorting.

Influence of core size on self-assembled molecular networks composed of C3h-symmetric building blocks through hydrogen bonding interactions: structural features and chirality.

The effect of the core size on the structure and chirality of self-assembled molecular networks was investigated using two aromatic carboxylic acid derivatives with frameworks displaying C3h symmetry, triphenylene derivative H3TTCA and dehydrobenzo[12]annulene (DBA) derivative DBACOOH, each having three carboxy groups per molecule. Scanning tunneling microscopy observations at the 1-heptanoic acid/graphite interface revealed H3TTCA exclusively forming a chiral honeycomb structure, and DBACOOH forming three structures (type I, II, and III structures) depending on its concentration and whether the system is subjected to annealing treatment. Hydrogen bonding interaction patterns and chirality were carefully analyzed based on a modeling study using molecular mechanics simulations. Moreover, DBACOOH forms chiral honeycomb structures through the co-adsorption of guest molecules. Structural diversity observed for DBACOOH is attributed to its relatively large core size, with this feature modulating the balance between molecule-molecule and molecule-substrate interactions.

On the role of functional groups in the formation of diazonium based covalent attachments: dendritic vs. layer-by-layer growth.

Multilayered growth is often observed upon electrografting aryl diazonium derivatives on graphitic substrates due to the reactive nature of aryl radicals. The mechanism of the multilayer formation has been investigated either by measuring the thickness of the grafted layer, the charge transfer, or via simulations. Spectroscopy and in particular microscopy approaches are underrepresented. Herein, we demonstrate a comparative characterization of the multilayer growth of two diazonium derivatives on highly oriented pyrolytic graphite using a combination of cyclic voltammetry, atomic force microscopy, and scanning tunneling microscopy. While dendritic growth is observed for 4-nitro phenyl diazonium (4-NBD), 4-carboxy phenyl diazonium (4-CBD) shows layer-by-layer growth upon increasing the molecular concentration, revealing the impact of the functional groups on the growth mechanism.

Nanoscale chemical patterning of graphite at different length scales.

Chemical patterning surfaces is relevant in several different domains of science and technology with exciting possibilities in electronics, catalysis, sensing, and photonics. Here, we present a novel strategy for chemical patterning of graphite using a combination of covalent and non-covalent approaches. Building on our previous work, where self-assembled monolayers of linear alkanes were used as sacrificial masks for directing the covalent anchoring of aryl groups to the graphite surface in sub-10 nm arrays, we present a modified design of a template alkane with alkoxy terminal groups which allowed better pattern transfer fidelity in comparison to simple linear alkanes. We also explored the use of chronoamperometry (CA) instead of previously used cyclic voltammetry (CV) for the functionalization process, which enabled patterning of the graphite surface at two-different length scales: few hundred nanometer circular patterns interspersed with sub-10 nm linear arrays. The covalent chemical patterning process has been studied in detail using CV and CA measurements whereas the patterned substrates have been thoroughly characterized using Raman spectroscopy, scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). Based on the comparison between the pattern transfer fidelity of previously studied alkanes and newly synthesized alkoxy alkane, we discuss plausible molecular mechanism of pattern transfer.

Spatially Controlled Aryl Radical Grafting of Graphite Surfaces Guided by Self-Assembled Molecular Networks of Linear Alkane Derivatives: The Importance of Conformational Dynamics.

The covalent functionalization of carbon surfaces with nanometer-scale precision is of interest because of its potential in a range of applications. We herein report the controlled grafting of graphite surfaces using electrochemically generated aryl radicals templated by self-assembled molecular networks (SAMNs) of bisalkylurea derivatives. A bisalkylurea derivative having two butoxy units acts as a template for the covalent functionalization of aryl groups in between self-assembled rows of this molecule. In contrast, grafting occurs without a spatial order when an SAMN of bis(tetradecyl)urea was used as a template. This indicates that a degree of dynamics at the alkyl termini is required to favor controlled covalent attachment, a situation that is suppressed by strong intrarow intermolecular interactions resulting from the hydrogen bonding of the urea groups, but favored by terminal short alkoxy groups. The present information is useful for understanding the mechanism of the template-guided aryl radical grafting and the molecular design of new generations of template molecules.

Spontaneous and scanning-assisted desorption-adsorption dynamics in porous supramolecular networks at the solution-solid interface.

Self-assembled molecular networks (SAMNs) are formed by the spontaneous assembly of molecules on surfaces. On conductive atomically flat surfaces, and also at the liquid-solid interface, scanning tunneling microscopy (STM) can follow their growth dynamics. Desorption and adsorption dynamics are difficult to probe through the liquid-solid interface. Porous molecular networks are of particular interest because they may act as platforms for sensing and host-guest chemistry. Very little is known though about their stability, particularly in a liquid environment. To this end, we have investigated the desorption/adsorption dynamics of supramolecular porous monolayers of alkoxylated dehydrobenzo[12]annulene (DBA) derivatives at the interface between highly oriented pyrolytic graphite, the substrate, and 1-phenyloctane, the liquid. To trace the dynamics, structurally analogous chiral DBA derivatives were used as marker molecules, which co-assemble with the achiral ones forming the supramolecular network. This approach reveals the time scales of the adsorption/desorption dynamics, the significance of temperature, and the important role of the STM tip in inducing dynamics.

Linear π-conjugated polycyclic compounds consisting of four-, five-, and six-membered rings: benzo[1'',2'':3,4;4'',5'':3',4']bis(cyclobuta[1,2-c]thiophene).

Linear π-conjugated polycyclic compounds, BBCTs, containing linearly annulated 5-, 4-, 6-, 4-, and 5-membered rings were produced via copper-mediated double intramolecular coupling reactions. The absorption spectra and electrochemical results confirmed their moderate optical energy gaps and high HOMO energy levels, respectively. In a crystalline state, the BBCT molecules adopt a herringbone structure, while the methylated molecules form slipped one-dimensional columns. The local and global aromaticity of the new polycyclic compounds is discussed based on the experimental results and theoretical predictions. The present fundamental findings are useful for the further design and synthesis of novel π-conjugated polycyclic compounds containing four-membered rings with potential applications in electronic materials.

Solvent Mediated Nanoscale Quasi-Periodic Chirality Reversal in Self-Assembled Molecular Networks Featuring Mirror Twin Boundaries.

Grain boundaries in polycrystals have a prominent impact on the properties of a material, therefore stimulating the research on grain boundary engineering. Structure determination of grain boundaries of molecule-based polycrystals with submolecular resolution remains elusive. Reducing the complexity to monolayers has the potential to simplify grain boundary engineering and may offer real-space imaging with submolecular resolution using scanning tunneling microscopy (STM). Herein, the authors report the observation of quasi-periodic nanoscale chirality switching in self-assembled molecular networks, in combination with twinning, as revealed by STM at the liquid/solid interface. The width of the chiral domain structure peaks at 12-19 nm. Adjacent domains having opposite chirality are connected continuously through interdigitated alkoxy chains forming a 1D defect-free domain border, reflecting a mirror twin boundary. Solvent co-adsorption and the inherent conformational adaptability of the alkoxy chains turn out to be crucial factors in shaping grain boundaries. Moreover, the epitaxial interaction with the substrate plays a role in the nanoscale chirality reversal as well.

Symmetry and spacing controls in periodic covalent functionalization of graphite surfaces templated by self-assembled molecular networks.

We herein present the periodic covalent functionalization of graphite surfaces, creating a range of patterns of different symmetries and pitches at the nanoscale. Self-assembled molecular networks (SAMNs) of rhombic-shaped bis(dehydrobenzo[12]annulene) (bisDBA) derivatives having alkyl chain substituents of different lengths were used as templates for covalent grafting of electrochemically generated aryl radicals. Scanning tunneling microscopy (STM) observations at the 1,2,4-trichlorobenzene/graphite interface revealed that these molecules form a variety of networks that contain pores of different shapes and sizes. The covalently functionalized surfaces show hexagonal, oblique, and quasi-rectangular periodicities. This is attributed to the favorable aryl radical addition at the pore(s). We also confirmed the successful transmission of chirality information from the SAMNs to the alignment of the grafted aryls. In one case, the addition of a guest molecule was used to switch the SAMN symmetry and periodicity, leading to a change in the functionalized surface periodicity from oblique to hexagonal in the presence of the guest molecule. This contribution highlights the potential of SAMNs as templates for the controlled formation of nanopatterned carbon materials.

Revisiting Dehydrothiopheno[12]annulenes: Synthesis, Electronic Properties, and Aromaticity.

The aromaticity and electronic properties of acetylene-bridged hexadehydrotristhiopheno[12]annulenes (HDTAs) were revisited using a combined experimental and theoretical approach. Moreover, we attempted the synthesis of the butadiyne-bridged octadehydrobisthiopheno[12]annulenes (ODTAs). While the formation of ODTAs was indicated by NMR spectroscopy, mass spectrometry, and UV-vis absorption measurements, our attempts to isolate ODTAs were unsuccessful on account of its instability. Instead, their structure and energetic properties were predicted using DFT calculations. HDTA isomers in which the position where the thiophene rings are fused to the 12-membered ring differs (b- vs c-position) show distinct differences in their HOMO-LUMO energy gaps (EGap). ODTAs also show large EGap differences depending on the fusion position of the thiophene rings. The diene character of the thiophene ring significantly changes the electronic properties; i.e., EGap differences of >1 eV were observed between the isomers of both HDTAs and ODTAs. A theoretical evaluation of HDTAs and ODTAs revealed significant variation in the local aromaticity/antiaromaticity between the b- and c-isomers. The antiaromatic character of the 12-membered ring is attenuated for the b-isomers, whereas it is decreased substantially for the c-isomers. The results of this study are useful for a detailed understanding of the fundamental aspects of dehydrothiopheno[12]annulenes.

Chirality in porous self-assembled monolayer networks at liquid/solid interfaces: induction, reversion, recognition and transfer.

Chirality in two dimensions (2D) has attracted increasing attention with regard to interesting fundamental aspects as well as potential applications. This article reports several aspects of supramolecular chirality control as exemplified by self-assembled monolayer networks (SAMNs) formed by a class of chiral building blocks consisting of a triangular conjugated core and alkoxy chains on the periphery. It highlights 2D chirality induction phenomena through a classic "sergeants-and-soldiers" mechanism, in which the inducer is incorporated into a network component, as well as through a "supramolecular host-guest" mechanism, in which the inducer is entrapped in the porous space, leading to counterintuitive chirality reversal. Stereochemical control can be extended to three dimensions too, based on interlayer hydrogen bonding of the same class of building blocks bearing hydroxy groups, exhibiting diastereospecific bilayer formation at both single molecule level and supramolecular level arising from orientation between the top and bottom layers. Finally, we showcase that homochiral SAMNs can also be used as templates for the grafting of in situ generated aryl radicals, by covalent bond formation to the basal graphitic surface, thereby yielding topologically chiral functionalized graphite, and thus extending the potential of chiral SAMNs.

Stereospecific Epitaxial Growth of Bilayered Porous Molecular Networks.

Stereocontrolled multilayer growth of supramolecular porous networks at the interface between graphite and a solution was investigated. For this study, we designed a chiral dehydrobenzo[12]annulene (DBA) building block bearing alkoxy chains substituted at the 2 position with hydroxy groups, which enable van der Waals stabilization in a layer and potential hydrogen-bonding interactions between the layers. Bias voltage-dependent scanning tunneling microscopy (STM) experiments revealed the diastereospecificity of the bilayer with respect to both the intrinsic chirality of the building blocks and the supramolecular chirality of the self-assembled networks. Top and bottom layers within the same crystalline domain were composed of the same enantiomers but displayed opposite supramolecular chiralities.

Trapping a pentagonal molecule in a self-assembled molecular network: an alkoxylated isosceles triangular molecule does the job.

We herein report a unique example of on-surface adaptive self-assembly. A pentagon-shaped macrocycle, cyclic [5]meta-phenyleneacetylene [5]CMPA, is trapped by the adaptive supramolecular network formed by an isosceles triangular molecule, alkoxy substituted dehydrobenzo[14]annulene [14]ISODBA at the liquid/graphite interface, leading to a highly ordered and large-area bicomponent self-assembled molecular network (SAMN), as revealed by scanning tunneling microscopy (STM).

Porous Self-Assembled Molecular Networks as Templates for Chiral-Position-Controlled Chemical Functionalization of Graphitic Surfaces.

Controlled covalent functionalization of graphitic surfaces with molecular scale precision is crucial for tailored modulation of the chemical and physical properties of carbon materials. We herein present that porous self-assembled molecular networks (SAMNs) act as nanometer scale template for the covalent electrochemical functionalization of graphite using an aryldiazonium salt. Hexagonally aligned achiral grafted species with lateral periodicity of 2.3, 2.7, and 3.0 nm were achieved utilizing SAMNs having different pore-to-pore distances. The unit cell vectors of the grafted pattern match those of the SAMN. After the covalent grafting, the template SAMNs can be removed by simple washing with a common organic solvent. We briefly discuss the mechanism of the observed pattern transfer. The unit cell vectors of the grafted pattern align along nonsymmetry axes of graphite, leading to mirror image grafted domains, in accordance with the domain-specific chirality of the template. In the case in which a homochiral building block is used for SAMN formation, one of the 2D mirror image grafted patterns is canceled. This is the first example of a nearly crystalline one-sided or supratopic covalent chemical functionalization. In addition, the positional control imposed by the SAMN renders the functionalized surface (homo)chiral reaching a novel level of control for the functionalization of carbon surfaces, including surface-supported graphene.

Hierarchical two-dimensional molecular assembly through dynamic combination of conformational states at the liquid/solid interface.

Self-sorting of multiple building blocks for correctly positioning molecules through orthogonal recognition is a promising strategy for construction of a hierarchical self-assembled molecular network (SAMN) on a surface. Herein we report that a trigonal molecule, dehydrobenzo[12]annulene (DBA) derivative having three tetradecyloxy chains and three hydroxy groups in an alternating manner, forms hierarchical triangular clusters of different sizes ranging from 2.4 to 16.4 nm, consisting of 3 to 78 molecules, respectively, at the liquid/graphite interface. The key is the dynamic combination of three different conformational states, which is solvent and concentration dependent. The present knowledge extends design strategies for production of sophisticated hierarchical SAMNs using a single component at the liquid/solid interface.

Electrostatically Driven Guest Binding in Self-Assembled Molecular Network of Hexagonal Pyridine Macrocycle at the Liquid/Solid Interface: Symmetry Breaking Induced by Coadsorbed Solvent Molecules.

We present here the construction of a self-assembled two-dimensional network at the liquid/solid interface using a hexagonal pyridine macrocycle which binds an organic cation in its intrinsic porous space by electrostatic interactions. For this purpose, a hexagonal pyridinylene-butadiynylene macrocycle (PyBM) having six octyloxymethyl groups, PyBM-C8, was synthesized. As guests, tropylium (Tr) tetrafluoroborate and trioxatriangulenium (TOTA) hexafluorophosphate were used. In this study, we focused on (i) the network patterns of PyBM-C8 which change in response to its concentration and (ii) the position of the guest immobilized in the porous space of the macrocycle. Scanning tunneling microscopy (STM) observations at the interface of 1,2,4-trichlorobenzene (TCB) and highly oriented pyrolytic graphite (HOPG) revealed that PyBM-C8 formed four different polymorphs, oblique, loose hexagonal, linear, and rectangular, depending on the solute concentration and annealing treatment. Solvent TCB molecules are likely coadsorbed to not only the intrinsically porous space of PyBM-C8 (internal TCB) but also the space outside of the macrocycle between its alkyl chains (external TCB) in most of the cases. Upon adding the guest cation, whereas small Tr was not visualized in the pore due to size mismatching, larger TOTA was clearly observed in each pore. In addition, based on high-resolution STM images of the rhombus packing pattern of PyBM-C8, we revealed experimentally that TOTA was placed at an off-center position of the deformed hexagonal macrocyclic core in the rhombus pattern. On the basis of the molecular mechanics calculations, we hypothesize that the off-center location of TOTA is due to deformation of the hexagonal macrocycle through interaction with two external TCB molecules located at opposite edges of the macrocyclic core. Symmetry breaking of the macrocyclic host framework induced by coadsorbed surrounding solvent molecules thus plays a significant role in host-guest complexation at the liquid/solid interface.

Theoretical Study on the Geometry, Aromaticity, and Electronic Properties of Benzo[3,4]cyclobutathiophenes and Their Homologues.

The geometry, aromaticity, and electronic properties of benzo[3,4]cyclobutathiophenes (BCTs) and their homologues have been examined theoretically using density functional theory calculations. The harmonic oscillator measure of aromaticity and nucleus-independent chemical shift analyses revealed the aromaticity characteristics of the two regioisomers benzo[3,4]cyclobuta[1,2-b]thiophene and benzo[3,4]cyclobuta[1,2-c]thiophene. When the aromaticity of one of the six-π-electron rings increases, it concomitantly decreases in the other ring. The anti-aromaticity of the four-membered ring varies depending on the π-electron density of the shared bond with the thiophene ring. This leads to a large difference of the highest occupied molecular orbital-lowest unoccupied molecular orbital gap between the isomers. Linear BCT homologues show medium diradical characters and the smallest EGap values. In the angular and branched homologues, the π-electrons of central benzene rings are localized avoiding the shared bonds, which results in a nonaromatic character. These data were compared to those of the parent hydrocarbons. Because of the diene character of the thiophene ring, the number and position of annulated thiophenocyclobutadieno moieties significantly influence the aromaticity and EGap values of BCT homologues. The present study does not only provide insight into the aromaticity and the properties of organic compounds containing four-membered rings but also affords helpful design guidelines of novel organic semiconductors.