Kazukuni Tahara

Control and induction of surface-confined homochiral porous molecular networks

Homochirality is essential to many biological systems, and plays a pivotal role in various technological applications. The generation of homochirality and an understanding of its mechanism from the single-molecule to supramolecular level have received much attention. Two-dimensional chirality is a subject of intense interest due to the unique possibilities and consequences of confining molecular self-assembly to surfaces or interfaces. Here, we report the perfect generation of two-dimensional homochirality of porous molecular networks at the liquid-solid interface in two different ways: (i) by self-assembly of homochiral building blocks and (ii) by self-assembly of achiral building blocks in the presence of a chiral modifier via a hierarchical structural recognition process, as revealed by scanning tunnelling microscopy. The present results provide important impetus for the development of two-dimensional crystal engineering and may afford opportunities for the utilization of chiral nanowells in chiral recognition processes, as nanoreactors and as data storage systems.

Molecular Clusters in Two-Dimensional Surface-Confined Nanoporous Molecular Networks: Structure, Rigidity, and Dynamics

The self-assembly of a series of hexadehydrotribenzo[12]annulene (DBA) derivatives has been investigated by scanning tunneling microscopy (STM) at the liquid/solid interface in the absence and presence of nanographene guests. In the absence of appropriate guest molecules, DBA derivatives with short alkoxy chains form two-dimensional (2D) porous honeycomb type patterns, whereas those with long alkoxy chains form predominantly dense-packed linear type patterns. Added nanographene molecules adsorb in the pores of the existing 2D porous honeycomb type patterns or, more interestingly, they even convert the guest-free dense-packed linear-type patterns into guest-containing 2D porous honeycomb type patterns. For the DBA derivative with the longest alkoxy chains (OC20H41), the pore size, which depends on the length of the alkoxy chains, reaches 5.4 nm. Up to a maximum of six nanographene molecules can be hosted in the same cavity for the DBA derivative with the OC20H41 chains. The host matrix changes its structure in order to accommodate the adsorption of the guest clusters. This flexibility arises from the weak intermolecular interactions between interdigitating alkoxy chains holding the honeycomb structure together. Diverse dynamic processes have been observed at the level of the host matrix and the coadsorbed guest molecules.

Temperature-induced structural phase transitions in a two-dimensional self-assembled network.

Two-dimensional (2D) supramolecular self-assembly at liquid-solid interfaces is a thermodynamically complex process producing a variety of structures. The formation of multiple network morphologies from the same molecular building blocks is a common occurrence. We use scanning tunnelling microscopy (STM) to investigate a structural phase transition between a densely packed and a porous phase of an alkylated dehydrobenzo[12]annulene (DBA) derivative physisorbed at a solvent-graphite interface. The influence of temperature and concentration are studied and the results combined using a thermodynamic model to measure enthalpy and entropy changes associated with the transition. These experimental results are compared to corresponding values obtained from simulations and theoretical calculations. This comparison highlights the importance of considering the solvent when modeling porous self-assembled networks. The results also demonstrate the power of using structural phase transitions to study the thermodynamics of these systems and will have implications for the development of predictive models for 2D self-assembly.

2D Networks of Rhombic-Shaped Fused Dehydrobenzo[12]annulenes: Structural Variations under Concentration Control

A series of alkyl- and alkoxy-substituted rhombic-shaped bisDBA derivatives 1a-d, 2a, and 2b were synthesized for the purpose of the formation of porous networks at the 1,2,4-trichlorobenzene (TCB)/graphite interface. Depending on the alkyl-chain length and the solute concentration, bisDBAs exhibit five network structures, three porous structures (porous A, B, and C), and two nonporous structures (nonporous D and E), which are attributed to their rhombic core shape and the position of the substituents. BisDBAs 1a and 1b with the shorter alkyl chains favorably form a porous structure, whereas bisDBAs 1c and 1d with the longer alkyl chains are prone to form nonporous structures. However, upon dilution, nonporous structures are typically transformed into porous ones, a trend that can be understood by the effect of surface coverage, molecular density, and intermolecular interactions on the systems enthalpy. Furthermore, porous structures are stabilized by the coadsorption of solvent molecules. The most intriguing porous structure, the Kagome pattern, was formed for all compounds at least to some extent, and the size of its triangular and hexagonal pores could be tuned by the alkyl-chain length. The present study proves that the concentration control is a powerful and general tool for the construction of porous networks at the liquid-solid interface.

Covalent Modification of Graphene and Graphite Using Diazonium Chemistry: Tunable Grafting and Nanomanipulation

We shine light on the covalent modification of graphite and graphene substrates using diazonium chemistry under ambient conditions. We report on the nature of the chemical modification of these graphitic substrates, the relation between molecular structure and film morphology, and the impact of the covalent modification on the properties of the substrates, as revealed by local microscopy and spectroscopy techniques and electrochemistry. By careful selection of the reagents and optimizing reaction conditions, a high density of covalently grafted molecules is obtained, a result that is demonstrated in an unprecedented way by scanning tunneling microscopy (STM) under ambient conditions. With nanomanipulation, i.e., nanoshaving using STM, surface structuring and functionalization at the nanoscale is achieved. This manipulation leads to the removal of the covalently anchored molecules, regenerating pristine sp(2) hybridized graphene or graphite patches, as proven by space-resolved Raman microscopy and molecular self-assembly studies.

One building block, two different nanoporous self-assembled monolayers: a combined STM and Monte Carlo study.

With the use of a single building block, two nanoporous patterns with nearly equal packing density can be formed upon self-assembly at a liquid-solid interface. Moreover, the formation of both of these porous networks can be selectively and homogenously induced by changing external parameters like solvent, concentration, and temperature. Finally, their porous properties are exploited to host up to three different guest molecules in a spatially resolved way.

Site-Selective Guest Inclusion in Molecular Networks of Butadiyne-Bridged Pyridino and Benzeno Square Macrocycles on a Surface

We present here the formation of a modular 2D molecular network composed of two different types of square-shaped butadiyne-bridged macrocycles, having intrinsic molecular voids, aligned alternately at the solid-liquid interface. Site-selective inclusion of a guest cation took place at every other molecular void in the molecular network with two different recognition sites.

Solvent-Induced Homochirality in Surface-Confined Low-Density Nanoporous Molecular Networks

Induction of chirality in achiral monolayers has garnered considerable attention in the recent past not only due to its importance in chiral resolutions and enantioselective heterogeneous catalysis but also because of its relevance to the origin of homochirality in life. In this contribution, we demonstrate the emergence of macroscopic chirality in multicomponent supramolecular networks formed by achiral molecules at the interface of a chiral solvent and an achiral substrate. The solvent-mediated chiral induction provides a simple, efficient, and versatile approach for the fabrication of homochiral surfaces using achiral building blocks.

Tailoring Surface‐Confined Nanopores with Photoresponsive Groups

Recently, the construction of two-dimensional (2D) porous patterns on solid surfaces using molecular self-assembly has become a subject of interest because of potential applications in nanoscience and nanotechnology, for nanoreactors and catalysts that may function cooperatively with substrate surfaces and for molecular wires and 2D polymers generated by surface-controlled reactions. Surface-confined pores within the 2D porous molecular networks can be used as a host space to immobilize guest molecules at the surface. These molecular networks are typically observed by means of scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions or at the liquid–solid interface. One of the significant challenges in the design of 2D porous system is physical (i.e. size and shape) or chemical (i.e. electrostatic properties) modification of the interior space of porous networks to construct tailored functional pores. There are only a few studies which have investigated the influence that chemical modification of the pore structure has on guest co-adsorption. However, none of them achieved specific recognition of guest molecule(s) by shape and size complementarities between the guest and modified pore. Herein, we report the construction of tailored 2D pores, which exhibit a tight stoichiometric binding selectivity toward a guest molecule. These pores are formed by self-assembly at the liquid–solid interface of designer molecular building blocks bearing photo-responsive groups. Moreover, the size of the pores is reversibly altered by irradiation with light of different wavelengths as demonstrated by a change in the number of co-adsorbed guest molecules. This is, to our knowledge, the first example of the construction of 2D pores which respond to external stimuli in a specific manner. Among the various molecular building blocks, alkoxylated dehydrobenzo[12]annulene (DBA) derivatives are chosen because of their strong tendency to form porous honeycomb patterns by the interdigitation of alkyl chains at the liquid–solid interface, tunability of pore size by varying alkyl chain length, and their synthetic versatility in chemical modification of the alkyl chains. The design to tailor pore environments is based on the introduction of functional groups at the end of three of the DBAs six alkyl chains, in an alternating fashion (Figure S1 in the Supporting Information). Molecular modeling suggests that such DBAs form a honeycomb structure in which the functional groups are located inside the pores. By selection of functional groups we can modify the physical and chemical environments of the pores. In this case, photoresponsive azobenzene is chosen as the functional group. In addition, two carboxy groups are introduced to the azobenzene units to achieve high guest selectivity by creating a confined space within a hydrogenbonded hexamer of dicarboxyazobenzene units: a cyclic hexamer of isophthalic acid can immobilize one coronene (COR) molecule on surfaces by size and shape recognition (Figure 1a,b). To locate the cyclic hexamer of the dicarboxyazobenzene units in the pore, the azobenzene units are connected by meta-phenylene linkers at the end of shorter C12 chains. Moreover, taking into account the established photoisomerization of azobenzene derivatives at surfaces and the structural difference between a planar trans-configuration and kinked 3D cis-configuration, the azobenzene units can change the pore size and shape upon photoisomerization (Figure 1c). This change in pore geometry also leads to a change in the number of adsorbed COR guest molecules. The synthesis of azobenzene-functionalized DBAs 1 and 2 is described in Supporting information (Scheme S1 and S2). DBAs 1 and 2 reveal very similar spectroscopic properties in solution. A 1-octanoic acid solution of all-trans 1 has an absorption band at 315 nm arising from p–p* absorptions of the DBA core and the azobenzene units and a weak band (435 nm) corresponding to an n–p* transition of the azobenzene chromophore (Figure S2). Upon irradiation with UV light (313 nm) of a solution of all-trans 1 in [D8]THF, a photostationary state containing 57 % of the cis-azobenzene unit was achieved within a few minutes as indicated by H NMR spectroscopy. If we assume all the azobenzene units in 1 exhibit identical photoisomerization efficiency, the distribution of 1 with one, two, and three cis-azobenzene unit(s) becomes 31.6%, 41.9 %, and 18.5 %, respectively, and [*] Dr. K. Tahara, K. Inukai, H. Yamaga, Prof. Dr. Y. Tobe Division of Frontier Materials Science Graduate School of Engineering Science, Osaka University 1-3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan) E-mail: tahara@chem.es.osaka-u.ac.jp tobe@chem.es.osaka-u.ac.jp

Self-assembled air-stable supramolecular porous networks on graphene.

Functionalization and modification of graphene at the nanometer scale is desirable for many applications. Supramolecular assembly offers an attractive approach in this regard, as many organic molecules form well-defined patterns on surfaces such as graphite via physisorption. Here we show that ordered porous supramolecular networks with different pore sizes can be readily fabricated on different graphene substrates via self-assembly of dehydrobenzo[12]annulene (DBA) derivatives at the interface between graphene and an organic liquid. Molecular resolution scanning tunneling microscopy (STM) and atomic force microscopy (AFM) investigations reveal that the extended honeycomb networks are highly flexible and that they follow the topological features of the graphene surface without any discontinuity, irrespective of the step-edges present in the substrate underneath. We also demonstrate the stability of these networks under liquid as well as ambient air conditions. The robust yet flexible DBA network adsorbed on graphene surface is a unique platform for further functionalization and modification of graphene. Identical network formation irrespective of the substrate supporting the graphene layer and the level of surface roughness illustrates the versatility of these building blocks.