Matthew O. Blunt

Vernier templating and synthesis of a 12-porphyrin nano-ring

Templates are widely used to arrange molecular components so they can be covalently linked into complex molecules that are not readily accessible by classical synthetic methods. Nature uses sophisticated templates such as the ribosome, whereas chemists use simple ions or small molecules. But as we tackle the synthesis of larger targets, we require larger templates—which themselves become synthetically challenging. Here we show that Vernier complexes can solve this problem: if the number of binding sites on the template, nT, is not a multiple of the number of binding sites on the molecular building blocks, nB, then small templates can direct the assembly of relatively large Vernier complexes where the number of binding sites in the product, nP, is the lowest common multiple of nB and nT (refs 8, 9). We illustrate the value of this concept for the covalent synthesis of challenging targets by using a simple six-site template to direct the synthesis of a 12-porphyrin nano-ring with a diameter of 4.7 nm, thus establishing Vernier templating as a powerful new strategy for the synthesis of large monodisperse macromolecules.

Random Tiling and Topological Defects in a Two-Dimensional Molecular Network

A molecular network that exhibits critical correlations in the spatial order that is characteristic of a random, entropically stabilized, rhombus tiling is described. Specifically, we report a random tiling formed in a two-dimensional molecular network of p-terphenyl-3,5,3′,5′-tetracarboxylic acid adsorbed on graphite. The network is stabilized by hexagonal junctions of three, four, five, or six molecules and may be mapped onto a rhombus tiling in which an ordered array of vertices is embedded within a nonperiodic framework with spatial fluctuations in a local order characteristic of an entropically stabilized phase. We identified a topological defect that can propagate through the network, giving rise to a local reordering of molecular tiles and thus to transitions between quasi-degenerate local minima of a complex energy landscape. We draw parallels between the molecular tiling and dynamically arrested systems, such as glasses.

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.

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.

Guest-induced growth of a surface-based supramolecular bilayer

Self-assembly of planar molecules on a surface can result in the formation of a wide variety of close-packed or porous structures. Two-dimensional porous arrays provide host sites for trapping guest species of suitable size. Here we show that a non-planar guest species (C(60)) can play a more complex role by promoting the growth of a second layer of host molecules (p-terphenyl-3,5,3″,5″-tetracarboxylic acid) above and parallel to the surface so that self-assembly is extended into the third dimension. The addition of guest molecules and the formation of the second layer are co-dependent. Adding a planar guest (coronene) can displace the C(60) and cause reversion to a monolayer arrangement. The system provides an example of a reversible transformation between a planar and a non-planar supramolecular network, an important step towards the controlled self-assembly of functional, three-dimensional, surface-based supramolecular architectures.

Directing two-dimensional molecular crystallization using guest templates

The use of a coronene guest template directs the formation of a 2D Kagomé network in preference to alternative close packed and parallel hydrogen-bonded structures of tetracarboxylic acid tectons self-assembled from solution on a graphite surface.

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.

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

Front instabilities in evaporatively dewetting nanofluids

Various experimental settings that involve drying solutions or suspensions of nanoparticles-often called nanofluids-have recently been used to produce structured nanoparticle layers. In addition to the formation of polygonal networks and spinodal-like patterns, the occurrence of branched structures has been reported. After reviewing the experimental results we use a modified version of the Monte Carlo model first introduced by Rabani [Nature 426, 271 (2003)] to study structure formation in evaporating films of nanoparticle solutions for the case that all structuring is driven by the interplay of evaporating solvent and diffusing nanoparticles. After introducing the model and its general behavior we focus on receding dewetting fronts which are initially straight but develop a transverse fingering instability. We analyze the dependence of the characteristics of the resulting branching patterns on the driving effective chemical potential, the mobility and concentration of the nanoparticles, and the interaction strength between liquid and nanoparticles. This allows us to understand the underlying instability mechanism.

Mesoscale DNA Structural Changes on Binding and Photoreaction with Ru[(TAP)2PHEHAT]2+

We used scanning force microscopy (SFM) to study the binding and excited state reactions of the intercalating photoreagent Ru[(TAP)(2)PHEHAT](2+) (TAP = 1,4,5,8-tetraazaphenanthrene; PHEHAT = 1,10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene) with DNA. In the ground state, this ruthenium complex combines a strong intercalative binding mode via the PHEHAT ligand, with TAP-mediated hydrogen bonding capabilities. After visible irradiation, SFM imaging of the photoproducts revealed both the structural implications of photocleavages and photoadduct formation. It is found that the rate of photocleaving is strongly increased when the complex can interact with DNA via hydrogen bonding. We demonstrated that the photoadduct increases DNA rigidity, and that the photo-biadduct can crosslink two separate DNA segments in supercoiled DNA. These mechanical and topological effects might have important implications in future therapeutic applications of this type of compounds.