Analysis of intermolecular interactions of n-perfluoroalkanes with circumcoronene using dispersion-corrected DFT calculations: comparison with those of n-alkanes.

Understanding the interactions between the adsorbate and substrate is critical in basic and advanced scientific fields, including the formation of well-organised nanoarchitectures via self-assembly on surfaces. In this study, the interactions of n-alkanes and n-perfluoroalkanes with circumcoronene were studied using dispersion-corrected density functional theory calculations as models of their adsorption on graphite. The interactions of n-perfluoroalkanes with circumcoronene were significantly weaker than those of the corresponding n-alkanes, e.g. the calculated adsorption energies of n-perfluorohexane and n-hexane were -9.05 and -13.06 kcal mol-1, respectively. The dispersion interactions were the major source of attraction between circumcoronene and the adsorbed molecules. Larger steric repulsion of n-perfluoroalkanes compared to those of n-alkanes increased their equilibrium distances from circumcoronene and decreased the dispersion interactions, resulting in weaker interactions. The interactions between two adsorbed n-perfluorohexane molecules and those of n-hexane molecules were -2.96 and -2.98 kcal mol-1, respectively, which are not negligible in the stabilisation of adsorbed molecules. The geometries of adsorbed n-perfluoroalkane dimers revealed that the equilibrium distance between two n-perfluoroalkane molecules did not match the width of the six-membered rings in circumcoronene, in contrast to that between n-alkanes. The lattice mismatch also destabilised the adsorbed n-perfluoroalkane dimers. The difference in the adsorption energy between flat-on and edge-on orientations of n-perfluorohexane was smaller than that of corresponding n-hexane.

Tuning the odd-even effect on two-dimensional assemblies of curcumin derivatives by alkyl chain substitution: a scanning tunnelling microscopy study.

Well-ordered molecular arrangement on surfaces is fundamental for fabrication of functional molecular devices which are of particular interest in nanotechnology. In addition to nano-manufacturing, the production of useful materials from natural resources has recently attracted increasing attention. Herein, we focused on the two-dimensional (2D) self-assemblies of curcumin derivatives. The effects of the number, length, and substitution of the alkyl chains on the 2D structures of curcumin derivatives were studied by scanning tunnelling microscopy at the highly oriented pyrolytic graphite/1,2,4-trichlorobenzene interface. Curcumin derivatives containing both methoxy and alkoxy chain groups and those possessing four alkoxy chains exhibit linear structures with and without interdigitation of alkoxy chains, respectively. These 2D structure formations are independent of the alkyl chain length. However, the bisdemethoxycurcumin derivatives periodically form stair-like and linear structures depending on the alkyl chain length, which indicates the existence of the odd-even effect. These results suggest that the 2D structural modulation of curcumin derivatives caused by the odd-even effect can be tuned by the number of alkyl chain substituents. The appearance and disappearance of the odd-even effect in curcumin derivatives are discussed in terms of the balance between intermolecular and molecule-substrate interactions.

Two-dimensional self-assemblies of azobenzene derivatives: effects of methyl substitution of azobenzene core and alkyl chain length.

Elucidating the correlation between the molecular arrangement and physical properties of organic compounds is critical to facilitating the development of advanced functional materials. X-ray structural analyses are generally performed to clarify this relationship. Several attempts have been made to ascertain the links between three-dimensional (3D) crystals and their two-dimensional (2D) structures, which can be revealed by scanning tunnelling microscopy (STM) at the molecular level. Thus, 2D self-assemblies of a series of azobenzene derivatives were investigated in this study, and the effects of methyl substitution of the azobenzene core and alkyl chain length on the 2D molecular arrangements at the solid/liquid interface were revealed. Three types of azobenzene derivatives were prepared; these contained azobenzene (Az), 3-methyl azobenzene (MAz), or 3,3'-dimethyl azobenzene (DAz) as cores and alkyloxy chains of different lengths (C8-13) at their 4,4' positions. The 2D structures of the Az and DAz compounds were found to be modulated owing to the odd-even effect of the alkyl chains in a specific chain-length range; this effect was only weakly exhibited by the MAz compounds. This result suggests that only the methyl-group substitution of the azobenzene core significantly affected the 2D structures. The 2D structural features have been discussed in terms of molecular conformation, as well as their correlation with the photo-melting behaviour of the azobenzene derivatives, particularly the MAz compounds.

Halogen bond-directed self-assembly in bicomponent blends at the solid/liquid interface: effect of the alkyl chain substitution position.

The fabrication of well-organised molecular assemblies on surfaces is fundamental for the creation of functional molecular systems applicable to nanoelectronic and molecular devices. In this study, we investigated the effect of substitution positions of alkyl chains on the formation of halogen-bonded molecular networks. For this purpose, building blocks with different head groups (i.e., pyridine (Py) or tetrafluoro-iodobenzene (FI)) were substituted with hexadecyloxy chains at either the 3,4- or the 3,5-positions. The two-dimensional assembly of each compound as a single-component system was studied using scanning tunnelling microscopy (STM) at the highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface. All compounds displayed linear structures in which the alkyl chains were aligned along one of the HOPG axes. In the exceptional case of FI bearing hexadecyloxy chains at the 3,5-positions (denoted as FI-3,5), hexagonal arrays were tentatively formed owing to the triangular molecular arrangement induced by halogen bonding. A bicomponent blend of Py-3,4/FI-3,5 (1 : 1 molar ratio) enabled the formation of a honeycomb structure, whereas that of Py-3,5/FI-3,4 (1 : 1 molar ratio) produced a rectangular assembly that was periodically arranged in a zig-zag fashion. Finally, based on the observed blend ratio dependence, the formation of these different two-dimensional structures by variation in the substitution positions of the alkyl chains was discussed in terms of molecule-molecule and molecule-substrate interactions.

Photo-Induced Crawling Motion of Azobenzene Crystals on Modified Gold Surfaces.

Photo-induced crawling motion of a crystal of 3,3'-dimethylazobenzene (DMAB) on gold surfaces having different surface properties and various patterns was studied. DMAB crystals crawl continuously when exposed to UV and visible lights simultaneously from different directions. On a gold surface functionalized by a thiol having a hydroxyl group at the terminal (16-hydroxy-1-hexadecanethiol (HOC16SH)), the crystals crawled with a relatively high velocity (ca. 4 µm min-1), and they changed the crystal shape while keeping a distinct crystal face. On a gold surface functionalized by a thiol having an alkyl chain terminal (1-hexadecanethiol (C16SH)), the crawling was observed with a slower velocity (ca. 1.5 µm min-1). However, the shape of the crystals became a droplet-like shape soon after the irradiation started, and the shape persisted during the motion. Light intensity dependence of the crawling velocity of the droplet-like crystal on this surface showed that UV light has stronger dependence for the motion than the visible light. On a substrate with a stripe pattern of alternating C16SH-modified gold and hexadecyltrimethylsilane (HDTMS)-modified glass, crystals crawled only on the surface of the C16SH-modified gold, which may be due to the wettability hysteresis at the surface. On a substrate with a stripe pattern of HOC16SH-modified gold and HDTMS-modified glass, crystals were attracted to the gold side. On a gold substrate with a periodic pattern of different height (ca. 50 nm) but having a uniform treatment with C16SH, crystals crawled up and down the steps without significant disturbance at the boundary of the step. Therefore, wettability of the surface has a greater impact on controlling the motion of the crystal than the surface structure. The present results not only unveil the crawling behavior on various surfaces but also offer a guide to controlling the motion toward applications for novel carriage vehicles to transport molecules/objects on a surface.

Coordination-Driven Construction of Porphyrin Nanoribbons at a Highly Oriented Pyrolytic Graphite (HOPG)/Liquid Interface.

Nanostructures were built at the solid/liquid interface by self-assembly and/or coordination bonds. Metalloporphyrins bearing two external coordination sites and long alkyl chains allowed the self-assembly of the compounds on highly oriented pyrolitic graphite. After addition of a metal ion, long transition-metal linked porphyrin nanoribbons were obtained and visualized by scanning tunneling microscopy. In these porphyrin ribbons electronic delocalization is possible through the d orbitals of the connecting metal ions.

Synthesis and Study at a Solid/Liquid Interface of Porphyrin Dimers Linked by Metal Ions.

Several porphyrin dimers linked by metal ions were prepared. One trimeric compound was also isolated and one porphyrin dimer linked by palladium(II) could be structurally characterized. In solution, the size of the new compounds was estimated by DOSY NMR techniques. These compounds all contained long aliphatic chains (O-C12H25), which were used to assemble them at a highly oriented pyrolytic graphite (HOPG)/liquid interface. The highly ordered arrays were visualized by scanning tunneling microscopy (STM).

Odd-even effect in two dimensions induced by the bicomponent blends of isobutenyl compounds.

Scanning tunneling microscopy (STM) investigation was performed for the blends of isobutenyl compounds, in which the long alkyl chains were connected with ester or carbamoyl linkages. Each component by itself did not show the odd-even effect of alkyl chain length, whereas after blending them, the 2D structures drastically changed and modulated to exhibit odd-even effect. Star, lozenge, twist-like, and linear structures were found, dependent on the blend ratio and alkyl chain length. The blend ratio dependence of 2D structures was explained in terms of homogeneous and heterogeneous dimerization due to the interdigitation of alkyl chains.

Trapping Nanostructures on Surfaces through Weak Interactions.

The assembly of imidazole-functionalized phenanthroline-strapped zinc porphyrins (ZnPorphen) with alkyl or polyethylene glycol (PEG) side chains was studied in solution and by AFM after casting on highly oriented pyrolytic graphite (HOPG) or mica. The nature of the solvent and its evaporation time influenced the morphology of the objects observed. On HOPG, short rods of about 100 nm were observed after fast evaporation of solutions of the alkyl derivatives in CHCl3 , THF, or pyridine, whereas islands of aligned rows of longer wires were obtained from methylcyclohexane (MCH). Slow evaporation of MCH led to a three-dimensional assembly. The PEG porphyrin assembled into short wires on HOPG or fibers on mica after slow evaporation of solutions in THF. This study shows the role of surface-molecule interactions in the interfacial assembly of ZnPorphen derivatives and contributes to understanding the parameters that control their noncovalent assembly into molecular wires on a surface.

Supramolecular engineering of oligothiophene nanorods without insulators: hierarchical association of rosettes and photovoltaic properties.

Supramolecular rosettes of oligothiophenes that do not bear long aliphatic tails have been designed as semiconducting nanomaterials for solution-processable bulk heterojunction solar cells. The rosettes consist of six barbiturated thienyl[oligo(hexylthiophene)] units (Bar-T-hTn ; n=3,4,5) aggregated by multiple hydrogen bonds, which have been directly visualized by scanning tunneling microscopy (STM) at a solid-liquid interface. (1) H NMR spectroscopy in [D8 ]toluene showed that Bar-T-hTn exists as a mixture of monomers and small hydrogen-bonded aggregates. Hierarchical organization of the hydrogen-bonded aggregates took place through π-π stacking interactions upon casting their toluene solutions, resulting in the growth of highly ordered nanorods whose widths are consistent with the diameters of the rosettes. The nanorods could be generated in the presence of soluble fullerene derivatives via solution casting or the annealing of the resulting thin films. The solar cells fabricated based on these bulk heterojunction films showed power conversion efficiencies of 1-3 %, which are far higher than those of the non-hydrogen-bonded reference oligothiophene and the derivative that possesses long aliphatic tails.

Photo-controllable microcleaner: photo-induced crawling motion and particle transport of azobenzene crystals on a liquid-like surface.

Organic crystals of 3,3'-dimethylazobenzene (DMAB) exhibit photo-induced crawling motion on solid surfaces when they are simultaneously irradiated with ultraviolet and visible light from opposite directions. DMAB crystals are candidates for light-driven cargo transporters, having simple chemical compositions and material structures. However, fast crawling motion without significant shape deformation has not yet been achieved. In this study, compared with hydrophilic glass and conventional hydrophobic surfaces with alkyl chains, siloxane-based hybrid surfaces, which are "liquid-like surfaces," result in the fastest crawling motion (4.2 µm min-1) while the droplet-like shape of DMAB crystals is maintained. Additionally, we successfully demonstrate that the DMAB crystals are capable of capturing and carrying silica particles on the hybrid surface. The transport direction is changed on demand without releasing the particles by simply changing the irradiation direction. The particles can be left on the substrate by removing the DMAB crystals via sublimation at room temperature. This result showcases a new concept of "photo-controllable microcleaner" that can operate a series of cargo capture-carry-release tasks. We expect this transporter to contribute to the development of crystal actuators, microfluidics, and microscale molecular flasks/reactors.

Photo-controllable azobenzene microdroplets on an open surface and their application as transporters.

The control of droplet motion is a significant challenge, as there has been no simple method for effective manipulation. Utilizing light for the control of droplets offers a promising solution due to its non-contact nature and high degree of controllability. In this study, we present our findings on the translational motion of pre-photomelted droplets composed of azobenzene derivatives on a glass surface when exposed to UV and visible light sources from different directions. These droplets exhibited directional and continuous motion upon light irradiation and this motion was size-dependent. Only droplets with diameters less than 10 µm moved with a maximum velocity of 300 µm min-1. In addition, the direction of the movement was controllable by the direction of the light. The motion is driven by a change in contact angle, where UV or visible light switched the contact angle to approximately 50° or 35°, respectively. In addition, these droplets were also found to be capable carriers for fluorescent quantum dots. As such, droplets composed of photoresponsive molecules offer unique opportunities for designing novel light-driven open-surface microfluidic systems.

Controlled stacking and unstacking of peripheral chlorophyll units drives the spring-like contraction and expansion of a semi-artificial helical polymer.

Developing new strategies for controlling polymer conformations through precise molecular recognition can potentially generate a machine-like motion that is dependent on molecular information-an important process for the preparation of new intelligent nanomaterials (e.g., polymer-based nanomachines) in the field bordering between polymer chemistry and conventional supramolecular sciences. Herein, we propose a strategy to endow a helical polymer chain with dynamic spring-like (contraction/expansion) motion through the one-dimensional self-assembly (aggregation/disaggregation) of peripheral amphiphilic molecules. In this developing system, we employed a semi-artificial helical polysaccharide presenting peripheral amphiphilic chlorophyll units as a power device that undergoes contractive motion in aqueous media, driven by strong π-π interactions of its chlorophyll units or by cooperative molecular recognition of bipyridyl-type ligands through pairs of chlorophyll units, thereby converting molecular information into the regulated motion of a spring. In addition, this system also undergoes expansive motion through coordination of pyridine. We anticipate that this strategy will be applicable (when combined with the established wrapping chemistry of the helical polysaccharide) to the development of, for example, drug carriers (e.g., nano-syringes), actuators (stimuli-responsive films), and directional transporters (nano-railways), thereby extending the frontiers of supramolecular science.

Nanoscopic imaging of meso-tetraalkylporphyrins prepared in high yields enabled by Montmorrilonite K10 and 3A†molecular sieves.

We have developed a high-yielding synthesis of meso-tetraalkylporphyrins, which previously have been obtained only in lower yields. By employing Montmorrilonite K10 as the acid catalyst and 3 Šmolecular sieves as the dehydrating agent, yields that reached 70 % could be achieved with some aliphatic aldehydes. The free-base porphyrins with decyl (C10) or longer chains were imaged at the single-molecule level at the solvent/surface interface. Highly oriented pyrolytic graphite (HOPG) was used as a π-stacking surface, whereas 1-phenyloctane and 1-phenylnonane were used as solvents. An odd-even effect was observed from C13 to C16. For C13 a single-crystal X-ray structure allowed an unprecedented insight into how packing from two dimensions is expanded into a three-dimensional crystal lattice.

Enzymatic degradation of polyester films by a cutinase-like enzyme from Pseudozyma antarctica: surface plasmon resonance and atomic force microscopy study.

Enzymatic degradation of polyester films by a cutinase-like enzyme from Pseudozyma antarctica JCM10317 (PaE) was analyzed by surface plasmon resonance (SPR). The adsorption of PaE and the degradation rate for polyester films were quantitatively monitored by a positive and negative SPR signal shifts, respectively. The decrease in SPR signal and the erosion depth of amorphous poly(L-lactide) (a-PLLA) film measured by atomic force microscopy (AFM) had a linear relationship, and the weight loss was estimated from the AFM data combined with a density of a-PLLA film. Furthermore, SPR sensorgrams for various polyester films showed that degradation rate of poly(ε-caprolactone) and poly(butylene succinate-co-adipate) which contain C6 units was higher than that of other polyesters such as poly(butylene succinate) and a-PLLA. These results suggest that C6 is the preferred chain length as substrates for PaE.

Two-dimensional molecular networks at the solid/liquid interface and the role of alkyl chains in their building blocks.

Nanoarchitectonics has attracted increasing attention owing to its potential applications in nanomachines, nanoelectronics, catalysis, and nanopatterning, which can contribute to overcoming global problems related to energy and environment, among others. However, the fabrication of ordered nanoarchitectures remains a challenge, even in two dimensions. Therefore, a deeper understanding of the self-assembly processes and substantial factors for building ordered structures is critical for tailoring flexible and desirable nanoarchitectures. Scanning tunneling microscopy is a powerful tool for revealing the molecular conformations, arrangements, and orientations of two-dimensional (2D) networks on surfaces. The fabrication of 2D assemblies involves non-covalent interactions that play a significant role in the molecular arrangement and orientation. Among the non-covalent interactions, dispersion interactions that derive from alkyl chain units are believed to be weak. However, alkyl chains play an important role in the adsorption onto substrates, as well as in the in-plane intermolecular interactions. In this review, we focus on the role of alkyl chains in the formation of ordered 2D assemblies at the solid/liquid interface. The alkyl chain effects on the 2D assemblies are introduced together with examples documented in the past decades.

Molecular Weight-Dependent Oxidation and Optoelectronic Properties of Defect-Free Macrocyclic Poly(3-hexylthiophene).

The redox behaviors of macrocyclic molecules with an entirely π-conjugated system are of interest due to their unique optical, electronic, and magnetic properties. In this study, defect-free cyclic P3HT with a degree of polymerization (DPn) from 14 to 43 was synthesized based on our previously established method, and its unique redox behaviors arising from the cyclic topology were investigated. Cyclic voltammetry (CV) showed that the HOMO level of cyclic P3HT decreases from -4.86 eV (14 mer) to -4.89 eV (43 mer), in contrast to the linear counterparts increasing from -4.94 eV (14 mer) to -4.91 eV (43 mer). During the CV measurement, linear P3HT suffered from electro-oxidation at the chain ends, while cyclic P3HT was stable. ESR and UV-Vis-NIR spectroscopy suggested that cyclic P3HT has stronger dicationic properties due to the interactions between the polarons. On the other hand, linear P3HT showed characteristics of polaron pairs with multiple isolated polarons. Moreover, the dicationic properties of cyclic P3HT were more pronounced for the smaller macrocycles.

Control over hierarchy levels in the self-assembly of stackable nanotoroids.

We report a precise control over the hierarchy levels in the outstanding self-organization process shown by chiral azobenzene dimer 1. This compound forms uniform toroidal nanostructures that can hierarchically organize into chiral nanotubes under the control by temperature, concentration, or light. The nanotubes further organized into supercoiled fibrils, which finally intertwined to form double helices with one-handed helical sense.

Supramolecularly engineered perylene bisimide assemblies exhibiting thermal transition from columnar to multilamellar structures.

Perylene 3,4:9,10-tetracarboxylic acid bisimide (PBI) was functionalized with ditopic cyanuric acid to organize it into complex columnar architectures through the formation of hydrogen-bonded supermacrocycles (rosette) by complexing with ditopic melamines possessing solubilizing alkoxyphenyl substituents. The aggregation study in solution using UV-vis and NMR spectroscopies showed the formation of extended aggregates through hydrogen-bonding and π-π stacking interactions. The cylindrical fibrillar nanostructures were visualized by microscopic techniques (AFM, TEM), and the formation of lyotropic mesophase was confirmed by polarized optical microscopy and SEM. X-ray diffraction study revealed that a well-defined hexagonal columnar (Col(h)) structure was formed by solution-casting of fibrillar assemblies. All of these results are consistent with the formation of hydrogen-bonded PBI rosettes that spontaneously organize into the Col(h) structure. Upon heating the Col(h) structure in the bulk state, a structural transition to a highly ordered lamellar (Lam) structure was observed by variable-temperature X-ray diffraction, differential scanning calorimetry, and AFM studies. IR study showed that the rearrangement of the hydrogen-bonding motifs occurs during the structural transition. These results suggest that such a striking structural transition is aided by the reorganization in the lowest level of self-organization, i.e., the rearrangement of hydrogen-bonded motifs from rosette to linear tape. A remarkable increase in the transient photoconductivity was observed by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements upon converting the Col(h) structure to the Lam structure. Transient absorption spectroscopy revealed that electron transfer from electron-donating alkoxyphenyl groups of melamine components to electron-deficient PBI moieties takes place, resulting in a higher probability of charge carrier generation in the Lam structure compared to the Col(h) structure.

Photoreversible supramolecular polymerisation and hierarchical organization of hydrogen-bonded supramolecular co-polymers composed of diarylethenes and oligothiophenes.

Diarylethene 1 equipped with two monotopic melamine hydrogen-bonding sites and oligothiophene-functionalized ditopic cyanurate (OTCA) were mixed in a nonpolar solvent to form AA-BB-type supramolecular co-polymers (SCPs) bearing photoswitchable moieties in their main chains and extended π systems as side chains. UV/Vis, fluorescence, dynamic light scattering (DLS), TEM, and AFM studies revealed that the two functional co-monomers formed flexible quasi-one-dimensional SCPs in solution that hierarchically self-organized into helical nanofibers through H-aggregation of the oligothiophene side chains. Upon irradiating the SCPs with UV light, a transition occurred from the H-aggregated state to non-aggregated monomeric oligothiophene side chains, as shown by spectroscopic studies, which indicates the formation of small oligomeric species held together only by hydrogen-bonding interactions. TEM and AFM visualized unfolded fibrils corresponding to elongated single SCP chains formed upon removal of solvent. The helical nanofibers were regenerated upon irradiating the UV-irradiated solution with visible light. These results demonstrated that the supramolecular polymerisation followed by hierarchical organization can be effectively controlled by proper supramolecular designs using diarylethenes and π-conjugated oligomers.