Strain-Dependent Kinetics in the Cis-to-Trans Isomerization of Azobenzene in Bulk Elastomers

Stereochemistry at the Single-Molecule Level: From Monitoring to Regulation

Traditional stereochemistry analysis is crucial for understanding the molecular behavior, but relies on measurements that encompass multiple molecules and obscure individual molecular dynamics. Single-molecule techniques enable real-time tracking of stereochemical transformations. These techniques include electrical methods (such as scanning probe microscopy, single-molecule junction techniques, and nanopore technology) and non-electrical approaches (such as circular dichroism spectroscopy and surface-enhanced Raman spectroscopy). This review highlights recent advances in monitoring and regulation of stereochemical properties at the single-molecule level. Techniques that bridge macroscopic observations with molecular-scale dynamics are emphasized. Key isomerization phenomena (constitutional, configurational, and conformational isomerizations) are explored to demonstrate how light, electric field, and mechanical force regulate molecular states. The use of chiral molecules in optical tweezers, chiral-modified scanning tunneling microscopies, and graphene-based single-molecule junctions to leverage the chirality-induced spin selectivity effect for enantiomer discrimination and manipulation is highlighted. Despite progress in this field, challenges persist in resolving ultrafast isomerization pathways, understanding chiral origin mechanisms, and integrating single-molecule devices. Emerging strategies combining multimodal stimuli, machine learning, and nanofabrication are promising for advancing stereochemical research and applications in molecular electronics and nanotechnology. This work underscores the transformative potential of single-molecule techniques in unveiling fundamental chemical dynamics and designing functional molecular systems.

Sunlight-Driven Photomobile Polymer Materials Containing Push-Pull Azobenzene Moieties

Polymer materials that show macroscopic deformation upon irradiation with light are feasible as soft actuators. However, previous photomobile systems typically required artificial light sources for actuation. Herein, we develop photomobile polymer materials that can be deformed by natural sunlight. Azobenzenes functionalized with electron-donating and -withdrawing groups (push-pull azobenzenes) show trans-cis photoisomerization upon irradiation with sunlight and cis-trans thermal back isomerization after the cessation of irradiation. Push-pull azobenzenes are incorporated into crosslinked liquid-crystalline polymers, in which azobenzene moieties are uniaxially aligned. Upon irradiation with simulated sunlight, the polymer films exhibit bending toward the incident light and revert to the initial shapes after the discontinuation of irradiation. The time scales of the macroscopic bending and unbending are consistent with those of photoisomerization behaviors of azobenzene moieties. This result indicates that macroscopic deformation is induced through photochemical processes rather than photothermal processes. The development of sunlight-driven photomobile polymer materials would lead to the creation of autonomous and ecofriendly photoresponsive systems without the need for artificial optical elements, such as light sources, lenses, and filters.

Mechanistically Different Mechanochromophores Enable Calibration and Validation of Molecular Forces in Glassy Polymers and Elastomeric Networks

Sterically distorted donor-acceptor π-systems, termed DA springs, can be progressively planarized under mechanical load causing a bathochromic shift of the photoluminescence (PL) spectrum. By combining theory and experiment, we here use a simple linear force calibration for two different conformational mechanochromophores to determine molecular forces in polymers from the mechanochromic shift in PL wavelength during multiple uniaxial tensile tests. Two systems are used, i) a highly entangled linear glassy polyphenylene and ii) a covalent elastomeric polydimethylsiloxane network. The mean forces estimated by this method are validated using known threshold forces for the mechanochemical ring-opening reactions of two different spiropyran force probes. The agreement between both approaches underlines that these DA springs provide the unique opportunity for the online monitoring of local molecular forces present in diverse polymer matrices.

Azobenzene as a photoswitchable mechanophore

Azobenzene has been widely explored as a photoresponsive element in materials science. Although some studies have investigated the force-induced isomerization of azobenzene, the effect of force on the rupture of azobenzene has not been explored. Here we show that the light-induced structural change of azobenzene can also alter its rupture forces, making it an ideal light-responsive mechanophore. Using single-molecule force spectroscopy and ultrasonication, we found that cis and trans para-azobenzene isomers possess contrasting mechanical properties. Dynamic force spectroscopy experiments and quantum-chemical calculations in which azobenzene regioisomers were pulled from different directions revealed that the distinct rupture forces of the two isomers are due to the pulling direction rather than the energetic difference between the two isomers. These mechanical features of azobenzene can be used to rationally control the macroscopic fracture behaviours of polymer networks by photoillumination. The use of light-induced conformational changes to alter the mechanical response of mechanophores provides an attractive way to engineer polymer networks of light-regulatable mechanical properties.

Thermally Stable Photomechanical Molecular Hinge: Sterically Hindered Stiff-Stilbene Photoswitch Mechanically Isomerizes

Molecular photoswitches are extensively used as molecular machines because of the small structures, simple motions, and advantages of light including high spatiotemporal resolution. Applications of photoswitches depend on the mechanical responses, in other words, whether they can generate motions against mechanical forces as actuators or can be activated and controlled by mechanical forces as mechanophores. Sterically hindered stiff stilbene (HSS) is a promising photoswitch offering large hinge-like motions in the E/Z isomerization, high thermal stability of the Z isomer, which is relatively unstable compared to the E isomer, with a half-life of ca. 1000 years at room temperature, and near-quantitative two-way photoisomerization. However, its mechanical response is entirely unexplored. Here, we elucidate the mechanochemical reactivity of HSS by incorporating one Z or E isomer into the center of polymer chains, ultrasonicating the polymer solutions, and stretching the polymer films to apply elongational forces to the embedded HSS. The present study demonstrated that HSS mechanically isomerizes only in the Z to E direction and reversibly isomerizes in combination with UV light, i.e., works as a photomechanical hinge. The photomechanically inducible but thermally irreversible hinge-like motions render HSS unique and promise unconventional applications differently from existing photoswitches, mechanophores, and hinges.

New tricks and emerging applications from contemporary azobenzene research

Azobenzenes have many faces. They are well-known as dyes, but most of all, azobenzenes are versatile photoswitchable molecules with powerful photochemical properties. Azobenzene photochemistry has been extensively studied for decades, but only relatively recently research has taken a steer towards applications, ranging from photonics and robotics to photobiology. In this perspective, after an overview of the recent trends in the molecular design of azobenzenes, we highlight three research areas where the azobenzene photoswitches may bring about promising technological innovations: chemical sensing, organic transistors, and cell signaling. Ingenious molecular designs have enabled versatile control of azobenzene photochemical properties, which has in turn facilitated the development of chemical sensors and photoswitchable organic transistors. Finally, the power of azobenzenes in biology is exemplified by vision restoration and photactivation of neural signaling. Although the selected examples reveal only some of the faces of azobenzenes, we expect the fields presented to develop rapidly in the near future, and that azobenzenes will play a central role in this development.

Azo-Crosslinked Double-Network Hydrogels Enabling Highly Efficient Mechanoradical Generation

Double-network (DN) hydrogels have recently been demonstrated to generate numerous radicals by the homolytic bond scission of the brittle first network under the influence of an external force. The mechanoradicals thus generated can be utilized to trigger polymerization inside the gels, resulting in significant mechanical and functional improvements to the material. Although the concentration of mechanoradicals in DN gels is much higher than that in single-network hydrogels, a further increase in the mechanoradical concentration in DN gels will widen their application. In the present work, we incorporate an azoalkane crosslinker into the first network of DN gels. Compared with the traditional crosslinker N,N'-methylenebis(acrylamide), the azoalkane crosslinker causes a decrease in the yield stress but significantly increases the mechanoradical concentration of DN gels after stretching. In the azoalkane-crosslinked DN gels, the concentration of mechanoradicals can reach a maximum of ∼220 μM, which is 5 times that of the traditional crosslinker. In addition, DN gels with the azoalkane crosslinker show a much higher energy efficiency for mechanoradical generation. Interestingly, DN gels crosslinked by a mixture of azoalkane crosslinker and traditional crosslinker also exhibit excellent radical generation performance. The increase in the mechanoradical concentration accelerates polymerization and can broaden the application range of force-responsive DN gels to biomedical devices and soft robots.

Recent progress in polydiacetylene mechanochromism

Polydiacetylenes (PDAs) are a family of mechanochromic polymers that change color from blue to red and emit fluorescence when exposed to external stimuli, making them extremely popular materials in biosensing. Although several informative reviews on PDA biosensing have been reported in the last few years, their mechanochromism, where external forces induce the color transition, has not been reviewed for a long time. This mini review summarizes recent progress in PDA mechanochromism, with a special focus on the quantitative and nanoscopic data that have emerged in recent years.