Though s-indacene is an intriguing antiaromatic hydrocarbon of 12 π-electrons, it has been underrepresented due to the lack of efficient and versatile methods to prepare stable derivatives. Herein we report a concise and modular synthetic method for hexaaryl-s-indacene derivatives bearing electron-donating/-accepting groups at specific positions to furnish C2h-, D2h-, and C2v-symmetric substitution patterns. We also report the effects of substituents on their molecular structures, frontier molecular orbital (MO) levels, and magnetically induced ring current tropicities. Both theoretical calculations and X-ray structure analyses indicate that the derivatives of the C2h-substitution pattern adopt different C2h structures with significant bond length alternation depending on the electronic property of the substituents. Due to the nonuniform distribution of the frontier MOs, their energy levels are selectively modulated by the electron-donating substituents. This leads to the inversion of the HOMO and HOMO-1 sequences with respect to those of the intrinsic s-indacene as theoretically predicted and experimentally proven by the absorption spectra at visible and near-infrared regions. The NICS values and the 1H NMR chemical shifts of the s-indacene derivatives indicate their weak antiaromaticity. The different tropicities are explained by the modulation of the HOMO and HOMO-1 levels. In addition, for the hexaxylyl derivative, weak fluorescence from the S2 excited state was detected due to the large energy gap between the S1 and S2 states. Notably, an organic field-effect transistor (OFET) fabricated using the hexaxylyl derivative exhibited moderate hole carrier mobility, a result which opens the door for optoelectronic applications of s-indacene derivatives.
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.
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.
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.
The dianion and dication of tetramesityl-substituted tetracyclopentatetraphenylene, a circulene consisting of alternating five- and six-membered rings, have been generated by reduction with alkali metals and oxidation with antimony(V) halides, respectively. They are theoretically predicted to adopt double annulenoid structures called annulene-within-an-annulene models in which the outer and inner conjugation circuits are significantly decoupled. The theoretical structures were experimentally proven by X-ray crystallographic analyses and the electronic configurations were supported by MCD spectra. Based on the 13 C NMR chemical shifts, negative and positive charges are shown to be located mainly at the outer periphery, indicating that the dianion and dication have delocalized 22-π and 18-π electron outer perimeters, respectively, and 8-π electron structure at the inner ring. Notably, the dianion has an open-shell character, whereas the dication has a closed-shell ground state.
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.
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.
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).
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.
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.
The synthesis of a new triaminoguanidinium-based ligand with three tris-chelating [NNO]-binding pockets and C3 symmetry is described. The reaction of tris-(2-pyridinylene-N-oxide)triaminoguanidinium salts with zinc(II) formate leads to the formation of cyclic supramolecular coordination compounds which in solution bind fullerenes in their spherical cavities. The rapid encapsulation of C60 can be observed by NMR spectroscopy and single-crystal X-ray diffraction and is verified using computation.
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.
Chiral structures created through the adsorption of molecules onto achiral surfaces play pivotal roles in many fields of science and engineering. Here, we present a systematic study of a novel chiral phenomenon on a surface in terms of organizational chirality, that is, meso-isomerism, through coverage-driven hierarchical polymorphic transitions of supramolecular assemblies of highly symmetric π-conjugated molecules. Four coverage-dependent phases of dehydrobenzo[12]annulene were uniformly fabricated on Ag(111), exhibiting unique chiral characteristics from the single-molecule level to two-dimensional supramolecular assemblies. All coverage-driven phase transitions stem from adsorption-induced pseudo-diastereomerism, and our observation of a lemniscate-type (∞) supramolecular configuration clearly reveals a drastic chiral phase transition from an enantiomeric chiral domain to a meso-isomeric achiral domain. These findings provide new insights into controlling two-dimensional chiral architectures on surfaces.
Scanning tunnelling microscope observations at the 1-phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self-assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.
Two isomers of 9,10-dihydro- as-indacenodithiophenes (DIDTs) and the corresponding diketones having an as-indacene core were synthesized. Their thermal, photophysical, and electrochemical properties were investigated, revealing that they depend on the direction of the fusion of the thiophene rings. For the DIDTs, the effect of the mode of ring fusion on the physical properties is discussed by comparison with the previously reported derivatives of DIDT isomers with an s-indacene core. The observed difference between the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels of the DIDT isomers is ascribed to the efficiency of π-conjugation, which depends on α- or ß-linkage between the terminal thiophenes with the central benzene ring. In addition, the effect of the peripheral aromatic ring (thiophene or benzene) is elucidated by comparison with indeno[2,1- a]fluorene (DIF) bearing an as-indacene core. The HOMO levels of DIDTs are significantly raised compared to that of structurally related DIF because of electron-donating character of the thiophene rings. For the DIDT diketones, structural effect due to the proximate carbonyl groups is discussed by comparison with the isomers with remote carbonyl groups. In diketones bearing proximate carbonyl groups, the LUMO levels are destabilized owing to antibonding interaction between the carbonyl oxygen atoms, resulting in approach of the LUMO and LUMO+1 energy levels.
By using a novel protocol to spatially confine molecules in well-defined small 2D areas, the so-called nanocorrals, we show using scanning tunneling microscopy (STM) how this kind of confinement affects self-assembled molecular network (SAMN) formation at a liquid-solid interface. The 2D lateral confinement, imposed by the size of the nanocorrals, has a clear impact on the phase selectivity of a molecule that can form both low-density and high-density SAMNs, the high-density phase being promoted by the confinement.
Grafting of aryl radicals generated by electrochemical reduction of aryldiazonium salts has been extensively studied on various surfaces. However, there exists two unclear aspects; the first one is the generality of the blocking ability of simple functional groups toward multilayer growth, and the second one is the electronic impact of substituent groups of aryl radicals on grafting efficiency. To address these aspects, we have studied the electrochemical functionalization of graphite using aryldiazonium salts having electron-donating or electron-withdrawing groups at the 3,4,5-positions. Atomic force microscopy investigation of the functionalized surfaces revealed the formation of monolayers for all aryldiazonium salts, and thus, nitro, carboxy, ester, methyl, and methoxy groups at the 3,4,5-positions of the benzene ring suppress polyaryl growth. The degree of grafting estimated by scanning tunneling microscopy imaging and Raman spectroscopy of the functionalized surfaces depends on the electronic state of the aryl radicals, in which the radicals with electron-donating groups show a high degree of functionalization, whereas those with electron-withdrawing groups exhibit a low degree of functionalization. We discuss several possibilities that affect grafting density. Though there are several factors, we hypothesize that one factor to explain the observed reactivity trend is the electronic property of the aryl radicals, namely, the relative position of the singly occupied molecular orbital energy levels of the aryl radicals with respect to the graphite Fermi energy level.
An approach for nanoscale covalent functionalization of graphite surfaces employing self-assembled molecular monolayers of n-alkanes as templating masks is presented. Linearly aligned aryl groups with a lateral periodicity of 5 or 7 nm are demonstrated utilizing molecular templates of different lengths. The key feature of this approach is the use of a phase separated solution double layer consisting of a thin organic layer containing template molecules topped by an aqueous layer containing aryldiazonium molecules capable of electrochemical reduction to generate aryl radicals which bring about surface grafting. Upon sweeping of the potential, lateral displacement dynamics at the n-alkane terminal edges acts in conjunction with electrochemical diffusion to result in templated covalent bond formation in a linear fashion. This protocol was demonstrated to be applicable to linear grafting of graphene. The present processing described herein is useful for the realization of rationally designed nanoscale materials.
We present here the construction of a self-assembled two-dimensional (2D) porous monolayer bearing a highly polar 2D space to study guest co-adsorption through electrostatic interactions at the liquid/solid interface. For this purpose, a dehydrobenzo[12]annulene (DBA) derivative, DBA-TeEG, having tetraethylene glycol (TeEG) groups at the end of the three alternating alkoxy chains connected by p-phenylene linkers was synthesized. As a reference host molecule, DBA-C10, having nonpolar C10 alkyl chains at three alternating terminals, was employed. As guest molecules, hexagonal phenylene-ethynylene macrocycles (PEMs) attached by triethylene glycol (TEG) ester and hexyl ester groups, PEM-TEG and PEM-C6, respectively, at each vertex of the macrocyclic periphery were used. Scanning tunneling microscopy observations at the 1,2,4-trichlorobenzene/highly oriented pyrolytic graphite interface revealed that PEM-TEG was immobilized in the pores formed by DBA-TeEG at higher probability because of electrostatic interactions such as dipole-dipole and hydrogen bonding interactions between oligoether units of the host and guest, in comparison to PEM-C6 with nonpolar groups. These observations are discussed based on molecular mechanics simulations to investigate the role of the polar functional groups. When a nonpolar host matrix formed by DBA-C10 was used, however, only phase separation and preferential adsorption were observed; virtually no host-guest complexation was discernible. This is ascribed to the strong affinity between the guest molecules which form by themselves densely packed van der Waals networks on the surface.
