Object Transportation System Mimicking the Cilia of Paramecium aurelia Making Use of the Light‐Controllable Crystal Bending Behavior of a Photochromic Diarylethene

Strategies to Diversification of the Mechanical Properties of Organic Crystals

Structurally ordered soft materials that respond to complementary stimuli are susceptible to control over their spatial and temporal morphostructural configurations by intersectional or combined effects such as gating, feedback, shape-memory, or programming. In the absence of general and robust design and prediction strategies for their mechanical properties, at present, combined chemical and crystal engineering approaches could provide useful guidelines to identify effectors that determine both the magnitude and time of their response. Here, we capitalize on the purported ability of soft intermolecular interactions to instigate mechanical compliance by using halogenation to elicit both mechanical and photochemical activity of organic crystals. Starting from (E)-1,4-diphenylbut-2-ene-1,4-dione, whose crystals are brittle and photoinert, we use double and quadruple halogenation to introduce halogen-bonded planes that become interfaces for molecular gliding, rendering the material mechanically and photochemically plastic. Fluorination diversifies the mechanical effects further, and crystals of the tetrafluoro derivative are not only elastic but also motile, displaying the rare photosalient effect.

Collective photothermal bending of flexible organic crystals modified with MXene-polymer multilayers as optical waveguide arrays

The performance of any engineering material is naturally limited by its structure, and while each material suffers from one or multiple shortcomings when considered for a particular application, these can be potentially circumvented by hybridization with other materials. By combining organic crystals with MXenes as thermal absorbers and charged polymers as adhesive counter-ionic components, we propose a simple access to flexible hybrid organic crystal materials that have the ability to mechanically respond to infrared light. The ensuing hybrid organic crystals are durable, respond fast, and can be cycled between straight and deformed state repeatedly without fatigue. The point of flexure and the curvature of the crystals can be precisely controlled by modulating the position, duration, and power of thermal excitation, and this control can be extended from individual hybrid crystals to motion of ordered two-dimensional arrays of such crystals. We also demonstrate that excitation can be achieved over very long distances (>3 m). The ability to control the shape with infrared light adds to the versatility in the anticipated applications of organic crystals, most immediately in their application as thermally controllable flexible optical waveguides for signal transmission in flexible organic electronics.

Photoresponsive Diarylethene-Containing Polymers: Recent Advances and Future Challenges

Photoresponsive polymers have attracted increasing interest owing to their potential applications in anticounterfeiting, information encryption, adhesives, etc. Among them, diarylethene (DAE)-containing polymers are one of the most promising photoresponsive polymers and have unique thermal stability and fatigue resistance compared to azobenzene- and spiropyran-containing polymers. Herein, the design of DAE-containing polymers based on different types of structures, including main chain polymers, side-chain polymers, and crosslinked polymers, is introduced. The mechanism and applications of DAE-containing polymers in anti-counterfeiting, information encryption, light-controllable adhesives, and photoinduced healable materials are reviewed. In addition, the remaining challenges of DAE-containing polymers are also discussed.

A Synthetic Minimal Beating Axoneme

Cilia and flagella are beating rod-like organelles that enable the directional movement of microorganisms in fluids and fluid transport along the surface of biological organisms or inside organs. The molecular motor axonemal dynein drives their beating by interacting with microtubules. Constructing synthetic beating systems with axonemal dynein capable of mimicking ciliary beating still represents a major challenge. Here, the bottom-up engineering of a sustained beating synthoneme consisting of a pair of microtubules connected by a series of periodic arrays of approximately eight axonemal dyneins is reported. A model leads to the understanding of the motion through the cooperative, cyclic association-dissociation of the molecular motor from the microtubules. The synthoneme represents a bottom-up self-organized bio-molecular machine at the nanoscale with cilia-like properties.

Contactless Manipulation of Write-Read-Erase Data Storage in Diarylethene Ferroelectric Crystals

The optical manipulation of polarization has gained widespread attention because it offers a promising route to new contactless memories and switches. However, the current research basically focuses on the photocontrol of data storage rather than data reading, which cannot realize the whole process of contactless write-read-erase data storage. Here, we present a pair of enantiomorphic diarylethene derivative ferroelectric crystals, showing a light-driven phase transition triggered by photoisomerization between the open and closed forms. Under the visible light, they exhibit a binary-domain state in the open form with white color and the band gap of 3.26 eV, while they show a single-domain state in the closed form with blue color and the band gap of 1.68 eV after UV irradiation of 254/365 nm. In addition to writing and erasing ferroelectric domains with light, we can also use light to read their color to determine the polarization state of domains. Moreover, diarylethene derivatives have better thermal stability, higher photoexcited conversion efficiency, and larger changes of the absorption wavelength between two isomers than those in salicylideneaniline derivatives. This work not only discovers the first diarylethene-based ferroelectric crystals but also successfully realizes completely contactless manipulation of write-read-erase data storage in the organic ferroelectric semiconductors.

Microfluidic Applications of Artificial Cilia: Recent Progress, Demonstration, and Future Perspectives

Artificial cilia-based microfluidics is a promising alternative in lab-on-a-chip applications which provides an efficient way to manipulate fluid flow in a microfluidic environment with high precision. Additionally, it can induce favorable local flows toward practical biomedical applications. The endowment of artificial cilia with their anatomy and capabilities such as mixing, pumping, transporting, and sensing lead to advance next-generation applications including precision medicine, digital nanofluidics, and lab-on-chip systems. This review summarizes the importance and significance of the artificial cilia, delineates the recent progress in artificial cilia-based microfluidics toward microfluidic application, and provides future perspectives. The presented knowledge and insights are envisaged to pave the way for innovative advances for the research communities in miniaturization.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Correlating Reaction Dynamics and Size Change during the Photomechanical Transformation of 9-Methylanthracene Single Crystals

Photomechanical molecular crystals that expand under illumination could potentially be used as photon-powered actuators. In this study, we find that the use of high-quality single crystals of 9-methylanthracene (9MA) leads to more homogeneous reaction kinetics than that previously seen for polycrystalline samples, presumably due to a lower concentration of defects. Furthermore, simultaneous observation of absorbance and shape changes in single crystals revealed that the dimensional change mirrors the reaction progress, resulting in a smooth expansion of 7 % along the c-axis that is linearly correlated with reaction progress. The same expansion dynamics are highly reproducible across different single crystal samples. Organic single crystals exhibit well-defined linear expansions during 100 % photoconversion, suggesting that this class of solid-state phase change material could be used for actuation.

Molecular crystalline capsules that release their contents by light

Here, we present single crystalline capsules of a photoresponsive molecule produced by simple recrystallization from organic solutions without direct human processing. During the crystal growth process, a movie was taken of the capsule taking in the organic solution. The capsules responded rapidly (<1 s) to the UV light stimuli and released the captured solution or solute. In principle, they can take in any substance dissolved in organic solvents, and their size can be controlled. Moreover, the capsule can be broken by multi-photon excitation using a near-infrared laser within the biological window. Furthermore, because the molecular packing in the crystal is unidirectional, the response can be controlled by the polarization of the light. This study shows the new potential of photoresponsive molecules.

Understanding the Molecular Origin of the Collective Movement in a Diarylethene-based Photo-Responsive Actuator

Remotely controlling macroscopic movement is one of the key elements to realize intelligent materials for applications ranging from sensing to robotics. Over the last few years, a number of photomechanical materials based on diarylethene derivatives have been developed. However, a detailed picture of the structural evolution within these soft actuators is often missing. In this work, an atomistic investigation uncovers how the photo-induced molecular dynamics propagates to large-scale motion and results in macroscopic deformation of the crystal. By correlating the intramolecular rearrangement within the photo-responsive switching unit with the intermolecular packing, the molecular mechanism for the photomechanical phenomena is deciphered, which is fundamental for a rational development of photo-responsive actuators.

Rational design of photochromic diarylbenzene with both high photoreactivity and fast thermal back reactivity

Utilizing the intramolecular CH–N hydrogen bonding and the bulky substituents at the reactive carbons resulted in the development of photochromic diarylbenzene with both high photoreactivity and fast thermal back reactivity.

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase IIUV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase IIItemp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase IIUV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase IIItemp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase IIUV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

Cyclization from Higher Excited States of Diarylethenes Having a Substituted Azulene Ring

The cyclization reaction of diarylethenes having an azulene ring occurs only via higher excited states. Novel diarylethenes having an azulene ring with a strong donor or acceptor were synthesized and examined in these reactions. A derivative having an electron-donating 1,3-benzodithiol-2-ylidenemethyl group at the 1-position of the azulene ring showed photochromism, whereas neither a derivative having a π-conjugated electron-donating group at the 3-position of the azulene ring nor derivatives having a π-conjugated electron-withdrawing group at the 1- or 3-position of the azulene ring showed any photochromism. The photoreactivities of these compounds were explained by calculating forces and bond orders on the excited states using density functional theory (DFT) and time-dependent (TD)-DFT.

Restriction of the conrotatory motion in photo-induced 6π electrocyclic reaction: formation of the excited state of the closed-ring isomer in the cyclization

The electrocyclic reaction dynamics of a photochromic dithiazolylarylene derivative, 2,3-dithiazolylbenzothiophene (DTA) was investigated by using time-resolved transient absorption and fluorescence spectroscopies. The closed-ring isomer of DTA undergoes cycloreversion through the conical intersection mediating the potential energy surfaces of the excited and ground states, which is in agreement with the Woodward–Hoffmann rules for the electrocyclic reactions of 6π electron systems. On the other hand, a large portion of the open-ring isomer undergoes cyclization along the distinct reaction scheme, in which the cyclization takes place in the excited state manifold leading to the formation of the excited state of the closed-ring isomer. The suppression of the geometrical motion of DTA due to the intramolecular interaction could open a new efficient reaction pathway resulting in the formation of the electronically excited state of the product.

Restriction of the molecular geometry opens up a novel pathway in the cyclization reaction of a photochromic dithiazolylarylene derivative.

Full-Dimensional Photodynamics of Bistable Proton Transfer Switches

Excited-state intramolecular proton transfers (ESIPT) are one of the fastest reactions in chemistry (<100 fs) which – among other features like high photostability – makes them an important reaction class for molecular switches. ESIPTs can be coupled with double bond rotation/isomerization, so that molecules can act as “molecular cranes”, facilitating long-range proton transfer. A versatile model system is 7-hydroxy-4-methylquinoline-8-carbaldehyde (HMQCA): it features two proton-accepting sites, two stable ground-state isomers and should allow for easy derivatization. There is also experimental and theoretical reference data available, however, only for static properties, e.g. ground-state IR spectra or potential energy surface scans. In this contribution we show the results of full-dimensional surface-hopping molecular dynamics (MD) of HMQCA after photo-excitation, employing semiempirical quantum mechanics coupled to floating-occupation configuration interaction. The results support the potential of HMQCA as prototype system for directed proton transport by ESIPT.