Search

Crystals of trans-3,3′-dimethylazobenzene (DMAB) exhibit crawling motion on solid surface upon simultaneous exposure to ultraviolet (UV) and visible light from opposite directions. In this study, the shape and velocity during the photoinduced crawling motion of DMAB crystals are successfully controlled by irradiation light intensities. A higher intensity of the visible light than the UV light causes the crystals to be spread shape and move slower (∼1 μm min−1). In contrast, a stronger UV light allows them to be droplet-like shape and move faster (∼4 μm min−1). The shape and velocity can be varied by adjusting the light intensities. Shape transformation is effectively applied to cargo capture–carry–release tasks. In particular, spread-shaped crystals capture silica particles over a wide area, whereas droplet-like crystals gather and transport them. This result suggests that a multifunctional soft transporter composed of a single component does not require complex fabrication, thereby contributing to the fields of soft robotics and microfluidics. [ABSTRACT FROM AUTHOR]

Here we present a study on the solid state properties of trans tetra-ortho-bromo azobenzene (4Br-Azo). Two distinct crystal polymorphs were identified: the α-phase and β-phase. Notably, only the β-phase exhibited an extraordinary photosalient effect (jumping/breaking) upon exposure to a wide range of visible light. Powder X-ray diffraction and Raman spectroscopy revealed that the β-phase is metastable and can transition to the α-phase when subjected to specific stimuli like heat and light. Furthermore, single crystal X-ray diffraction and density functional theory calculations highlighted the significance of a highly strained conformer in the β-phase, showing that the metastability of the phase potentially arises from relieving this strain. This metastability leads to a light induced phase transition, which appears to be the cause of the photosalient effect in these crystals. Interestingly the polymorphism at the core of 4Br-Azo's dynamic behavior is based on different arrangements of halogen based intermolecular interactions. It is possible that continued study on combining visible light capturing chromophores with halogen interaction-based polymorphism will lead to the discovery of even more visible light controlled dynamic crystal materials. [ABSTRACT FROM AUTHOR]

The photoinduced crawling motion of crystals is a continuous motion that azobenzene molecular crystals exhibit under light irradiation. Such motion enables object manipulation at the microscale with a simple setup of fixed LED light sources. Transportation of nano-/micromaterials using photoinduced crawling motion has recently been reported. However, the details of the motion mechanism have not been revealed so far. Herein, we report visualization of the dynamics of fine particles in 4-(methylamino)azobenzene (4-MAAB) crystals under light irradiation via diffracted X-ray tracking (DXT). Continuously repeated melting and recrystallization of 4-MAAB crystals under light irradiation results in the flow of liquid 4-MAAB. Zinc oxide (ZnO) particles were introduced inside the 4-MAAB crystals to detect diffracted X-rays. The ZnO particles rotate with the flow of liquid 4-MAAB. By using white X-rays with a wide energy width, the rotation of each zinc oxide nanoparticle was detected as the movement of a bright spot in the X-ray diffraction pattern. It was clearly shown that the ZnO particles rotated increasingly as the irradiation light intensity increased. Furthermore, we also found anisotropy in the rotational direction of ZnO particles that occurred during the crawling motion of 4-MAAB crystals. It has become clear that the flow perpendicular to the supporting film of 4-MAAB crystals is enhanced inside the crystal during the crawling motion. DXT provides a unique means to elucidate the mechanism of photoinduced crawling motion of crystals. [ABSTRACT FROM AUTHOR]

The stimulus‐driven motion of microscale objects on solid surfaces is a promising process to realize the manipulation of microdroplets, which has applications in fields areas such as material accumulation and sensing at a micro level. Light is an effective external stimulus for object manipulation because it can provide contactless spatial and temporal control. In this study, a new method is devised to transport nano/micromaterials on a glass surface by exploiting the photoinduced crawling motion of crystals. 4‐(methylamino)azobenzene (4‐MAAB) crystals are used to transport nano/micromaterials via photoirradiation from a single visible‐light source without using expensive equipment such as lasers and positioning devices. Nano/micromaterials mixed with the 4‐MAAB crystals are successfully transported with the crawling 4‐MAAB crystals. The nano/micromaterial transport is tracked using the fluorescence from the nano/micromaterials embedded into the 4‐MAAB crystals. In situ time‐resolved X‐ray diffraction measurements are also conducted for mechanistic analysis. This study offers new development paths in fields such as microfluidics and microrobotics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Emission‐switchable fluorophores often include stimuli‐responsive units in their molecular structures. This strategy works well, but the applicable compounds are limited to the derivatives of several kinds of photochromic molecules such as diarylethene, azobenzene, and spiropyran. Here, this work presents a simple methodology based on a photoresponsive precipitant for achieving color‐switchable photoluminescence. A luminescent dye, cyano‐substituted oligo(phenylenevinylene) (COPV), features both twisted intramolecular charge transfer and aggregation‐induced emission shift properties, leading to the change in the luminescence color from green to red upon precipitation. The COPV, together with photoisomerizable precipitant azobenzene (C6), is doped into spherical droplets of epoxy resin (ER) in a liquid state. The photoisomerization of C6 induced by UV irradiation and heating alternatively precipitates out and dissolves COPV in ER and changes the photoluminescence color, while maintaining the optical microresonator properties. This study will open a promising way for assembling/disassembling novel emission color‐switchable systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Controlled assembly of plasmonic particles is vital for catalyst and sensor applications. Because light is considered a promising external stimulus, various methods for assembling plasmonic particles using photoswitching molecules are investigated. However, the light used to photoisomerize such molecules is often limited to the UV region. In this study, anionic ortho‐fluoroazobenzene is synthesized to reversibly control the self‐assembly of plasmonic nanoparticles under only green and blue visible light. This azobenzene alters the dispersibility of cubic gold nanorattles protected by a cationic surfactant via visible‐light irradiation, causing a visible change in the solution color. It is possible that the cationic surfactant and anionic azobenzene interact to neutralize the surface charge of the gold nanorattles. Because the interaction is changed by the photoisomerization of azobenzene, the aggregation and redispersion of the gold nanorattles are repeatable. Through this method, the dispersibility of nonspherical plasmonic particles can be controlled using only visible‐light irradiation. [ABSTRACT FROM AUTHOR]

The synthesis, X-ray crystal analysis, photochemical and thermal studies of a xanthene based cyclic azobenzene dimer and its precursor is described. The X-ray crystal structures of the trans-trans isomer (open) and the cis-cis isomer (closed) provide unambiguous proof for the hinge-like molecular motion in the class of molecules.

The two-dimensional self-assembly of rufigallol derivatives and their metal coordination were studied by scanning tunnelling microscopy. Ex situ Cu(II)-coordinated rufigallol derivatives exhibited columnar structures with some defects, whereas regular and linear structures were formed upon in situ metal coordination at solid/liquid interfaces. [ABSTRACT FROM AUTHOR]

Nematic liquid crystal elastomers (LCEs) generally show soft elasticity, masking entropic elasticity inherent in crosslinked networks. They macroscopically deform with little stress increase under straining because of the soft shear mode attained by nematic director rotations. Moreover, the nematic interaction in main‐chain LCEs can arrest the ergodic response to cycling strain. It manifests hysteresis, slow relaxation, and increased viscosity, which critically affect their mechanical applications such as actuation and adhesion. Here, it is shown that entropic elasticity fully recovers after photo‐isomerization of azobenzene units incorporated in a nematic LCE. With the light‐induced bent cis‐isomers at room temperature, the degree of soft elasticity, viscosity, and adhesion are tangibly lowered. Mechanical responses rationalize that cis isomers unlock the frozen configurational degree of freedom by reducing nematic order in the same manner observed upon temperature increase. The present results not only unveil how the nematic interaction suppresses entropic elasticity leading to the soft state, but also offer a guide to tuning viscoelasticity toward dynamic mechanical applications. [ABSTRACT FROM AUTHOR]

The embedding of photonic crystals within stimuli‐responsive hydrogels has attracted tremendous interest because it provides new applications such as optical switches, displays, and sensors. However, the production of electrically tunable photonic hydrogels with a wide range of color tunability, fast electrical response, and high stability for repeated use is still challenging. Here, electrically tunable photonic hydrogels are fabricated using ultrahigh‐water‐content polyelectrolyte layered hydrogels composed of thousands of bilayer domain structures. The layered hydrogel exhibits versatile color tunability by applying an electric field parallel or perpendicular to the direction of the gel layers. The hydrogel exhibits homogeneous color tuning when a perpendicular electric field is applied, while it shows a large rainbow‐like electric‐optic response in the visible to near‐infrared region when a parallel electric field is applied. Additionally, by a simple method using patterned electrodes, a reflective display showing the designed characters is demonstrated and maintained for hours underwater without an external source of energy. Moreover, the electrically induced optical patterns can be erased, and the responsive photonic hydrogel shows excellent stability for repeated use. We anticipate that this study will provide the foundation for the development of responsive photonic hydrogels with new and large electro‐optical effects for future chromatic applications. [ABSTRACT FROM AUTHOR]

The ability to repeatedly record information with external stimuli, in particular light, is of fundamental importance for the development of photoresponsive materials. Here, we demonstrated a photo‐patterning system by using crosslinked azopolymers incorporating meta‐substituted azobenzene (M‐azo) moieties into dodecyl glyceryl itaconate (DGI) polymer networks. M‐azo inhibited H‐aggregation and showed efficient trans‐to‐cis photoisomerization in the crystal and film states, making photopatterning of the crosslinked polymers fully reversible. Macro‐ or microscale patterning could be written and erased in a reversible manner using light on the polymer film with fast response, high resolution, and flexibility. Moreover, these polymer films exhibited good fatigue resistance, which enabled hundreds of cyclic photo‐writing and erasing tests without any decay under alternating UV/Vis irradiation. The high fatigue resistance indicates the ability of the azopolymers to repeatedly harvest and convert light energy into chemical conformation in the polymer networks by taking advantages of efficient photoisomerization. [ABSTRACT FROM AUTHOR]

Smart surfaces that reversibly change the interfacial friction coefficients in response to external stimuli enable a wide range of applications, such as grips, seals, brake pads, packaging films, and fabrics. Here, a new concept of such a smart frictional system is reported: a composite film of a plain‐weave polyester textile sheet and a thermoresponsive nematic liquid crystalline elastomer (LCE). The composite is deployed with retractable microundulations of the elastomer inside each weave mesh, enabling dramatic changes of the contact interface with the opposing surface on LCE actuation, which is induced, e.g., by a change in temperature (T). At room T, the protruding viscoelastic parts of LCE in the nematic phase make contact with the opposing flat surface, resulting in a very high friction. At an elevated T (≈50 °C, isotropic phase), the undulations of LCE surface are retracted within the thickness of the textile, and the contacts are limited to small regions around overlapping textile fibers, lowering the friction dramatically. This effect is fully reversible on heating/cooling cycles. The surface undulations are spontaneous, i.e., fabricated without any lithographic or alignment techniques. The present composite opens a new way for the application of sheets/films with switchable friction enabled by stimuli‐responsive LCEs. [ABSTRACT FROM AUTHOR]

A study conducted on azobenzenes in which two (2), three (3), and four (4) azobenzene units are connected cyclically by methylene linkers at the meta positions, is presented. The effects of ring strain on the structure, and the photochemical and the thermal isomerization of azobenzene are investigated.

The potential energy surfaces of the twist of the C=C double bond and the intramolecular hydrogen atom transfer in the excited triplet state by the UB3LYP/6-31G** level is calculated to study the reaction mechanism of the one-way i somerization of 2'-hydroxychalcone. The potential energy curve of the triplet state between the keto forms (trans-2HC and cis-2HC) is similar to that of stilbene, which exhibits mutual isomerization.

The spatial organization of polycyclic aromatic molecules during adhesion on a solid/liquid interface is the key event of a plethora of natural and industrial processes. Herein, we report the supramolecular assembly of violanthrone‐79—a model of asphaltenes, an intractable mixture of polycyclic aromatics from crude oil—at the solid‐liquid interface between highly oriented pyrolytic graphite (HOPG) and a hydrophobic medium (1‐phenyloctane). High‐resolution scanning tunneling microscopy revealed that molecules of violanthrone‐79 readily self‐assemble on the surface in supramolecular "nanoring" structures. The lattice parameters of the 2D unit cell are on the same order as those determined from a bulk single crystal. Transmission electron microscopy showed long‐range ordered patterns, and the spacing between the fringes is in agreement with the inter‐planar distances between aromatic cores of molecules that are helically arranged around the [001] axis in the 3D crystal. The results confirm that upon adsorption on solid/liquid interfaces, polycyclic aromatic molecules such as violanthrone‐79 form supramolecular assemblies by interaction with the substrate and self‐association, and this process could be the initial step of deposition of asphaltenes on carbonaceous oil reservoir walls and production tubing. [ABSTRACT FROM AUTHOR]

Phototaxis of azobenzene crystals is demonstrated for the first time by using a simple setup. The crystals on a glass move away from the light source like living organisms during visible light irradiation. The motion is driven by repeated melting and recrystallization associated with trans↑cis isomerization. [ABSTRACT FROM AUTHOR]

Microwave heating in chemical reactions was first reported in 1986. There have since been many reports employing microwave heating in organic chemistry, where microwave heating has afforded higher yields of products in shorter time periods. However, such reactions are challenging to scale in batch due to the limited penetration depth of microwaves as well as the wave propagation dependence on cavity size. Continuous flow has addressed both these issues, enabling scalability of microwave processes. As such, a host of reports employing microwave flow chemistry have emerged, employing various microwave heating and reactor configurations in the context of either custom‐built or commercial apparatus. The focus of this review is to present the benefits of microwave heating in the context of continuous flow and to characterize the different types of microwave flow apparatus by their design (oscillator, cavity type and reactor vessel). We advocate the adoption of tunable, solid‐state oscillator single‐mode microwave flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications. We present the benefits of microwave (MW) heating in the context of continuous flow and characterize different types of MW flow apparatus by their design. Various MW flow reactors and reaction examples (including our own) are presented on g/h to 100's g/h scale. We advocate for tunable, solid‐state oscillator single‐mode MW flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications. [ABSTRACT FROM AUTHOR]

The chirality found in living organisms is one of unsolved mysteries on Earth. It is crucial to understand the manner in which small achiral molecules evolve into helical superstructures in the absence of chiral components because this process can provide important insights regarding the origin of chirality in nature. 1) the uncommon helical assembly of an achiral trigonal chromophore into helical nanostructures with aggregation-induced emission enhancement (AIEE) characteristics and 2) the tunability of the helical pitch and fluorescence intensity in response to light is reported. The Rietveld refinement of X-ray diffraction (XRD) patterns and the growth process suggest that a striking transformation from an achiral to an asymmetric molecule can occur as a result of specific interactions with certain solvents, presumably leading to the unique helical assembly. More importantly, exposure to UV or visible light promoted not only the formation of irregular helical structures with a wide range of pitch lengths but also an increase in fluorescence intensity. [ABSTRACT FROM AUTHOR]

We report the synthesis of liquid crystalline materials, which repeatedly liquefied and solidified when irradiated with ultraviolet (UV) and visible light at room temperature. The materials are the derivatives of the sugar alcohols, D-mannitol, D-sorbitol, and allitol, in which all hydroxyl groups were substituted with multi-mesogenic azobenzenes. The thermal phase transition behavior and stability of the liquid crystal phase were changed according to the sugar alcohols, but photochemical phase transition behaviors were not different. In addition, when applied as an adhesive layer to glass substrates, the materials exhibited changes in the adhesion force when irradiated with light. [ABSTRACT FROM PUBLISHER]

The direct and reversible transformationof matter between thesolid and liquid phases by light at constant temperature is of greatinterest because of its potential applications in various manufacturingsettings. We report a simple molecular design strategy for the phasetransitions: azobenzenes having para-dialkoxy groupswith a methyl group at the meta-position. The photolithography processeswere demonstrated using the azobenzene as a photoresist in a singleprocess combining development and etching of a copper substrate. [ABSTRACT FROM AUTHOR]

Photoinduced phase transitions caused by photochromic reactions bring about a change in the state of matter at constant temperature. Herein, we report the photoinduced phase transitions of crystals of a photoresponsive macrocyclic compound bearing two azobenzene groups ( 1) at room temperature on irradiation with UV (365 nm) and visible (436 nm) light. The trans/ trans isomer undergoes photoinduced phase transitions (crystal-isotropic phase-crystal) on UV light irradiation. The photochemically generated crystal exhibited reversible phase transitions between the crystal and the mesophase on UV and visible light irradiation. The molecular order of the randomly oriented crystals could be increased by irradiating with linearly polarized visible light, and the value of the order parameter was determined to be −0.84. Heating enhances the thermal cis-to- trans isomerization and subsequent cooling returned crystals of the trans/ trans isomer. [ABSTRACT FROM AUTHOR]

We report a simple preparation and color-tunable fluorescence of a series of azobenzene derivatives. The introduction of an electron-withdrawing or electron-donating group at the X position of azobenzenes (1–8) containing a biphenyl unit makes it possible to modulate the fluorescence color of the UV-exposed azobenzene solutions from blue to yellow, which correlates with the electron-donating abilities of the respective substituents. Theoretical calculations suggest that changes in both the dihedral angles between two phenyl rings of the biphenyl unit and the dipole moments between the trans and cis forms depending on the substituents seem to be important factors in determining the photochemical properties of chromophores. [ABSTRACT FROM AUTHOR]

The photochromic behavior of a spiroperimidine compound, 2,3-dihydro-2-spiro-7′-(8′-imino-7′,8′-dihydronaphthalen-1′-amine)perimidine (PNI), doped in various polymers has been studied. Upon irradiation with 405 nm light, PNI exhibits photochromism, and a color change (light yellow to brown) is observed. The initial (closed) and colored (open) forms exhibit absorption maxima (λmax) around 400 and 500 nm, respectively. The open form possesses a wide absorption band, extending to ∼750 nm. Little effect of the polymer matrix on the λmaxof either the closed or open forms was observed, and these values were also similar to those in solution. The lifetime of the open form depends mainly on the glass transition temperature (Tg) of the polymer, and ranges from, for example, 11 min to 5.4 d. This indicates that the lifetime of the open form can be readily tuned by simply choosing a suitable polymer matrix. Photochromism in different polymer substrates are demonstrated herein. [ABSTRACT FROM AUTHOR]

Energy conversion of light into mechanical work is of fundamental interest in applications. In particular, diligent molecular design on nanoscale, in order to achieve efficient photomechanical effects on macroscopic scale, has become one of the most interesting study topics. Here, by incorporating a “photomelting” azobenzene monomer crosslinked into liquid crystalline (LC) networks, we generate photoresponsive polymer films that exhibit reversible photoswitchable glass transition temperatures (Tg) at room temperature (~20 °C) and photomechanical actuations under the stimulus of UV/visible light. The trans-to-cis isomerization of azo chromophores results in a change in Tg of the crosslinked LC polymers. The Tg of the polymer network is higher than room temperature in the trans-form and lower than room temperature in the cis-form. We demonstrate the photoswitchable Tg contribute to the photomechanical bending and a new mechanism for photomechanical bending that attributes the process to an inhomogeneous change in Tg of the film is proposed. Energy conversion of light into mechanical work is of fundamental interest for a wide range of applications, but the development of efficient light responsive polymers is challenging. Here the authors demonstrate photoswitchable  glass transition temperatures contribute to the photomechanical bending in a crosslinked azobenzene polymer. [ABSTRACT FROM AUTHOR]

Review: 29 refs. [ABSTRACT FROM AUTHOR]

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., (Copyright © 2023, Liu et al.)

Skeletal or concave polyhedral crystals appear in a variety of synthetic processes and natural environments. However, their morphology, size, and orientation are difficult to control because of their highly kinetic growth character. We report a methodology to achieve synchronous, uniaxial, and stepwise growth of micrometer-scale skeletal single crystals from planar-chiral double-decker molecules. Upon drop-casting of a heated ethanol solution onto a quartz substrate, the molecules spontaneously assemble into standing vessel-shaped single crystals uniaxially and synchronously over the wide area of the substrate, with small size polydispersity. The crystal edge is active even after consumption of the molecules and resumes stereoselective growth with successive feeding. The resultant morphology can be packed into polycyclic aromatic hydrocarbon-like microarchitectures and behaves as a microscopic container.