The Cover Feature illustrates the road towards the new compound class bisacyldigermanes. These derivatives are highly efficient photoinitiators (PIs) which, among others, exhibit excellent photobleaching properties. In the image, the passengers journey from their starting point (widely used acylphosphine oxides, depicted in the rear-view mirror) and drive, with the aid of a roadmap, towards their destination, bypassing toxicity and mutagenicity to reach an oasis with the new compound class bisacyldigermanes. Cover design by Ilva Leinich. More information can be found in the Research Article by Michael Haas and co-workers.

In this contribution we present a novel synthetic procedure towards a variety of bisacyldigermanes via the Corey-Seebach approach. All isolated compounds were characterized by NMR spectroscopy, mass spectrometry and X-ray crystallography. The target compounds were further analyzed via UV/Vis absorption spectroscopy and their longest wavelength absorptions were assigned by DFT calculations. Moreover, we determined the activity of our new photoinitiators by photolysis experiments (photobleaching). Additionally, CIDNP spectra confirmed the radical pathway of the investigated compounds.

Photonic crystals are solids with regular structures having periodicities comparable to the wavelength of light. Here, we showcase the photomodulation of the refractive index of a crystalline material and present a quasi-one-dimensional photonic crystal with remote-controllable optical properties. The photonic material is composed of layers of TiO2 and films of a nanoporous metal-organic framework (MOF) with azobenzene side groups. While the rigid MOF lattice is unaffected, the optical density is reversibly modified by the light-induced trans-cis-azobenzene isomerization. Spectroscopic ellipsometry and precise DFT calculations show the optical-density change results from the different orbital localizations of the azobenzene isomers and their tremendously different oscillator strengths. The photomodulation of the MOF refractive index controls the optical properties of the quasi-one-dimensional photonic crystal with Bragg reflexes reversibly shifted by more than 4 nm. This study may path the way to photoswitchable photonic materials applied in advanced, tunable optical components and lens coatings and in light-based information processing.

Molecular motors embedded in a frustrated liquid crystal film induce the formation of sustained rotating patterns under light irradiation.

Light responsive materials that are able to change their shape are becoming increasingly important. However, preconfigurable bistable or even multi-stable visible light responsive coatings have not been reported yet. Such materials will require less energy to actuate and will have a longer lifetime. Here, it is shown that fluorinated azobenzenes can be used to create rewritable and pre-configurable responsive surfaces that show multi-stable topographies. These surface structures can be formed and removed by using low intensity green and blue light, respectively. Multistable preconfigured surface topographies can also be created in the absence of a mask. The method allows for full control over the surface structures as the topographical changes are directly linked to the molecular isomerization processes. Preliminary studies reveal that these light responsive materials are suitable as adaptive biological surfaces.

Proton conducting nanoporous materials attract substantial attention with respect to applications in fuel cells, supercapacitors, chemical sensors, and information processing devices inspired by biological systems. Here, a crystalline, nanoporous material which offers dynamic remote-control over the proton conduction is presented. This is realized by using surface-mounted metal-organic frameworks (SURMOFs) with azobenzene side groups that can undergo light-induced reversible isomerization between the stable trans and cis states. The trans-cis photoisomerization results in the modulation of the interaction between MOF and guest molecules, 1,4-butanediol and 1,2,3-triazole; enabling the switching between the states with significantly increased (trans) and reduced (cis) conductivity. Quantum chemical calculations show that the trans-to-cis isomerization results in the formation of stronger hydrogen bridges of the guest molecules with the azo groups, causing stronger bonding of the guest molecules and, as a result, smaller proton conductivity. It is foreseen that photoswitchable proton-conducting materials may find its application in advanced, remote-controllable chemical sensors, and a variety of devices based on the conductivity of protons or other charged molecules, which can be interfaced with biological systems.

Biomembranes are constantly remodeled and in cells, these processes are controlled and modulated by an assortment of membrane proteins. Here, it is shown that such remodeling can also be induced by photoresponsive molecules. The morphological control of giant vesicles in the presence of a water-soluble ortho-tetrafluoroazobenzene photoswitch (F-azo) is demonstrated and it is shown that the shape transformations are based on an increase in membrane area and generation of spontaneous curvature. The vesicles exhibit budding and the buds can be retracted by using light of a different wavelength. In the presence of F-azo, the membrane area can increase by more than 5% as assessed from vesicle electrodeformation. To elucidate the underlying molecular mechanism and the partitioning of F-azo in the membrane, molecular dynamics simulations are employed. Comparison with theoretically calculated shapes reveals that the budded shapes are governed by curvature elasticity, that the spontaneous curvature can be decomposed into a local and a nonlocal contribution, and that the local spontaneous curvature is about 1/(2.5 µm). The results show that exo- and endocytotic events can be controlled by light and that these photoinduced processes provide an attractive method to change membrane area and morphology.

Long before their development by synthetic chemists, molecular machines have been operating in nature, where they perform functions critical to life. Examples of natural molecular machines include adenosine triphosphate (ATP) synthase, which enables endergonic ATP synthesis by coupling to proton concentration gradients, and linear motors, such as kinesins and myosins, whose movement along the surfaces is driven by ATP hydrolysis. Inspired by the elegance with which these and other molecular machines operate in nature, chemists have been involved in synthesizing artificial molecular machines—entities designed to perform desired tasks (such as controlling directional motion and synthesizing other molecules) in the presence of an energy source. The successful development of artificial molecular machines requires integrating switchable elements within larger objects. In that respect, polymers have attracted considerable attention: they can be prepared inexpensively and have a wide range of useful mechanical properties (depending on the structural formulae), and they can be recycled. This Special Issue highlights the diverse ways by which molecular switches can be incorporated into macromolecular architectures, as well as the emerging properties and applications of the resulting materials. Most attention over the past decade has been devoted to molecular switches operated by light, which is reflected in the contents of this Special Issue. Barrett and co-workers discuss the current status of polymers functionalized with the archetypal molecular photoswitch, azobenzene (https://doi.org/10.1002/marc.201700253). These authors review systems that can perform bending motion as well as more complex movements (such as oscillations or helical deformations) and highlight the importance of harvesting sunlight for these operations. At the same time, they point out that the light-to-mechanical energy conversion can result from not only molecular isomerization events, but can also occur because of photothermal effects. A novel example of one such “molecular robotic machine” is reported by Priimagi, Zeng, and co-workers, who incorporated a red-shifted azobenzene into a liquid crystal elastomer (https://doi.org/10.1002/marc.201700224). The authors describe how the movement's directionality of their caterpillar-shaped “robot” can be controlled by the topology of the underlying surface. In a related contribution, Yu, Wei et al. focus on some key engineering aspects of photoactuating polymers, namely, their easy fabrication and the use of low-cost polymer matrices (https://doi.org/10.1002/marc.201700237). By dispersing crosslinked azobenzene polymers within a flexible polyurethane matrix, they were able to effectively transfer light-responsiveness to the otherwise photoinert matrix. The photoinduced bending could be reversed by combining thermal treatment and mesogen realignment, conferring to the films the ability to deform reversibly. The potential of analytical methods—in particular, vibrational (infrared and Raman) spectroscopy—for probing photoswitching in azo-polymers, is highlighted by Pellerin, Vapaavuori, and Bazuin (https://doi.org/10.1002/marc.201700430), who contend that the development of such analytical tools is essential for a better mechanistic understanding of key processes, such as translation of molecular-scale photochemical events into macroscopic motion. Light-triggered changes, such as unfolding/folding of macromolecules, mass-transport, and photoinduced chirality in azo-materials can now be studied in great detail with the help of these techniques, owing to their ability to finely probe the molecular environment, orientation, and chiral order. An important direction in the optical manipulation of molecular systems lies in the development of strategies allowing one to overcome the use of UV irradiation and instead, to operate the systems using visible or near-infrared light. This has several important advantages, including decreased invasiveness towards the other components of the system, biocompatibility, selectivity, increased penetration depth, and the ability to harvest sunlight. In their Review, Wu and Weis discuss key achievements in this direction using macromolecules incorporating azobenzene derivatives (https://doi.org/10.1002/marc.201700220). The conceptual approaches used to avoid UV light can be classified into (1) red-shifting the absorption of the molecular switches (i.e., direct photoexcitation) and (2) leveraging energy/electron transfer (indirect photoexcitation), for example, by using upconverting nanoparticles or triplet-triplet annihilation systems. Following in this direction, Bléger and co-workers reported on visible-light-activated hydrogels, whose elasticity can be reversibly tuned upon exposure to green and blue light (https://doi.org/10.1002/marc.201700527). Hydrogels have found multiple applications in biomedicine and are a suitable platform for introducing molecular switches into soft materials. Modulating the elasticity of such materials “on demand” is relevant for influencing complex biological processes, such as the differentiation of stem cells. The molecular structure of the hydrogels is based on ortho-fluoroazobenzene-containing PEGylated networks, which were prepared by bioorthogonal chemistry. In addition to being fully addressable in the visible region, ortho-fluorinated azobenzenes can exhibit remarkable bistability (i.e., high thermal stability of both the E and Z isomers). This property was utilized by Katsonis and co-workers to design photochromic chiral dopants leading to long-lived helical states in nematic liquid crystals, which can be rapidly and reversibly interconverted into one another using two wavelengths of visible light (https://doi.org/10.1002/marc.201700387). Interestingly, the Z → E relaxation times were longer when the F-azobenzenes were doped in the liquid crystal than when dissolved in solution, which could be attributed to a decreased molecular degree of freedom. Nevertheless, care should be taken to avoid fully suppressing the isomerization of molecular switches when interfacing them with higher-order structures and materials. Photoactive crosslinkers are convenient tools to remotely connect larger objects via supramolecular interactions or dynamic covalent bonds, giving rise to diverse novel applications. Building on their previous studies on cyclodextrin vesicles, Ravoo et al. assembled ternary complexes comprising the vesicles, DNA, and azo-switches appended with positively charged groups (https://doi.org/10.1002/marc.201700256). The complexes were held together by means of multivalent electrostatic interactions between DNA and the vesicle-bound positively charged dyes. Upon E → Z photoisomerization, the dyes lose their ability to bind to cyclodextrin vesicles, and the complexes disassemble. The researchers contend that this strategy holds promise for applications in the controlled capture and release of DNA. It is covalent crosslinking, however, that is better suited for fabricating robust macroscopic materials. Hecht and co-workers employed diarylethene (DAE)-based crosslinkers for connecting polymer chains to generate dynamic covalent polymer networks (https://doi.org/10.1002/marc.201700376). Crosslinking was achieved by a Diels-Alder reaction between dienophile units appended to polymer chains and dienes contained within the open form of DAEs. The photoinduced ring closure of DAEs efficiently inhibits the Diels-Alder reaction and hence removes the thermal healing ability of the polymer networks. Therefore, the healability of the material could be activated reversibly (and locally) using light. Importantly, the decrosslinking barrier could be lowered—and the healing temperatures decreased—by optimizing the polymer's composition and the DAE crosslinker's design. Although usually not regarded as macromolecules, metal-organic frameworks (MOFs) represent an interesting class of supramolecular polymers that feature nanometer-sized, regularly arranged nanopores. The excellent sorption properties often associated with these nanopores give rise to many important applications in, for example, gas storage, separations, and catalysis. Installing molecular switches inside these nanopores is an attractive route towards controlling the properties of MOFs using external stimuli. In their Feature Article, Wang, Sue, and co-workers introduce the emerging field of MOFs equipped with molecular switches (https://doi.org/10.1002/marc.201700388). They describe two approaches developed to fabricate such switchable MOFs: one based on assembling MOFs from organic building blocks incorporating molecular switches, and the other relying on post-synthetic modifications of MOFs with switchable molecules. These methods have been used to prepare pH-, light-, and redox-responsive MOFs, whose diverse applications in sensing, CO2 adsorption, gas separation, drug delivery, and photodynamic therapy are discussed. In a related contribution, Heinke et al. focused on photoresponsive MOFs functionalized with azobenzene groups (https://doi.org/10.1002/marc.201700239). This Review highlights an important issue that needs to be considered when designing switchable polymers, namely, the conformational freedom that many molecular switches need to have in order to switch efficiently. Azobenzenes have attracted the most attention as far as the integration of molecular switches with macromolecules is concerned: they are chemically robust, exhibit low fatigue, can be readily functionalized, and have tunable light absorption properties. However, other light-responsive molecular switches offer other advantages: spiropyrans, for example, can be switched using several distinct types of external stimuli, including light, metal ions, and acid/base signals. This unique feature enabled the development of thermoresponsive polymers, whose phase transitions can be induced by light (rather than temperature). Building on these previous studies, Schenning, den Toonder, and co-workers demonstrated a creative application of spiropyran-functionalized hydrogels as light-responsive passive micromixers (https://doi.org/10.1002/marc.201700086). They decorated the walls of microfluidic devices with tiny pillars composed of this material, which acted as mixers for the liquids passing through the channel. Upon exposure to blue light, however, the hydrophobic closed-ring isomer of spiropyran was generated and the micropillars collapsed, resulting in laminar flow with no mixing. Interestingly, the degree of mixing could be controlled by tuning the light intensity. In a related study, Sumaru et al. considered a more fundamental problem, namely, how the light-induced collapse is affected by the presence of macrocyclic hosts. Working with α-, β-, and γ-cyclodextrin, they found that the closed-ring isomer of spiropyran could be readily encircled by the mid-sized β ring, thus preventing the dehydration and collapse of the thermoresponsive polymer (https://doi.org/10.1002/marc.201700234). It is interesting to consider the possibility of combining these two studies, with solutions of the different cyclodextrins flowing through channels decorated with spiropyran-functionalized thermoresponsive polymer micropillars. Beyond light-controlled switches, Wang, Jiang, and co-workers developed a supramolecular polymer, which could be reversibly disassembled and reassembled using a base and an acid, respectively (https://doi.org/10.1002/marc.201700218). The key component of the polymer was a newly synthesized unsymmetrical cryptand featuring cavities having two different sizes: one exhibiting strong affinity towards the viologens, the other, binding dibenzylammonium ions. In the presence of a mixture of a bis-ammonium and a bis-viologen crosslinker, the cryptand induced the formation of a one-dimensional supramolecular polymer. Interestingly, the simultaneous complexation of the ammonium and the viologen groups took place despite both of them bearing a positive charge and hence repelling each other. Treating the polymer with a base caused the deprotonation of the ammonium groups, which disassembled the polymer. The polymer could be re-formed upon subsequent addition of an acid. An intriguing question is whether the binding of the other guest (i.e., the redox-active viologen) could also be cancelled—in this case, using a reducing agent—which would render the system responsive to two kinds of orthogonal stimuli. The development of such dual-responsive systems has, in fact, attracted increasing attention: Huang, Wu, and co-workers devised a crosslinked polymer gel that could be toggled between a tough and a soft state upon modulating pH or temperature (https://doi.org/10.1002/marc.201700361). The mechanical properties of the gel were determined by the state of the mechanically interlocked, catenane-based crosslinks, which could be either rigid (stabilized by multiple hydrogen bonds) or flexible (no H-bonds present; catenane rings freely rotating with respect to each other), depending on pH and temperature. Another thermo-/pH-responsive system was proposed by Liu, Hu, et al., who synthesized double-hydrophilic block copolymers terminated with a β-cyclodextrin moiety (https://doi.org/10.1002/marc.201700225). The two blocks of the polymers could be rendered hydrophobic upon heating or increasing pH, respectively. In either case, free polymer chains spontaneously assembled into micelles containing the hydrophobic block in the center. Finally, a state-of-the-art, triple-responsive system was proposed by Theato and co-workers, who designed a triblock copolymer featuring i) a permanently hydrophilic block, ii) a block incorporating moieties sensitive to two kinds of chemical signals (O2 and CO2), and iii) a UV-responsive, azobenzene-based block (https://doi.org/10.1002/marc.201700313). When placed in water, the triblock copolymer self-assembled into vesicles, which gradually swelled when subjected to these three stimuli. Importantly, this swelling was accompanied by increasing permeability through the vesicle walls, with potential applications in controlled release systems. The “breathing” behavior of the vesicles was demonstrated by alternately adding and removing the gaseous stimuli. We would like to thank all of the contributing authors of this Special Issue and the editorial staff members for their assistance. We hope that this collection of Communications, Feature Articles, and Reviews will inspire others to think about and contribute to research on synthetic macromolecular machines, whose functional potential is great, yet remains to be fully exploited.

Hydrogels are soft materials that have found multiple applications in biomedicine and represent a good platform for the introduction of molecular switches and synthetic machines into macromolecular networks. Tuning their mechanical properties reversibly with light is appealing for a variety of advanced applications and has been demonstrated in the past; however, their activation typically requires the use of UV light, which displays several drawbacks related to its damaging character and limited penetration in tissues and materials. This study circumvents this limitation by introducing all-visible ortho-fluoroazobenzene switches into a hydrophilic network, which, as a result, can be activated with green or blue light. Photoisomerization of the photochromic moieties is accompanied by a reversible tuning of the elastic modulus. The translation of molecular isomerization within the network into macroscopic modulation of its mechanical properties is attributed to different aggregation tendencies of the E and Z isomers of the azobenzene derivatives.

Ortho-fluoroazobenzenes constitute a new family of azobenzene photoswitches that can be activated with visible light only (green light for trans...

Azobenzene multi-state switches whose isomerization can be orthogonally induced with photons and electrons are presented. Exposure to green, blue, or ultraviolet light allows toggling between three isomers, while the fourth one is formed selectively via electro-catalytic isomerization.

To improve the sensitized Z→E photoisomerization of azobenzenes, and circumvent the threshold concentration necessary for the bimolecular photoinduced electron transfer reaction to generate the rapidly isomerizing Z-azobenzene radical anion, an IrIII complex with a covalently tethered azobenzene fragment was synthesized. Selective irradiation of the 1 MLCT band of the IrIII complex induced an efficiently sensitized photoswitching of the dyad over a wide concentration range and even at high dilution.

Molecular photoswitches, which are able to reversibly interconvert between (at least) two (meta) stable isomers upon exposure to light, are key elements for the development of photo-responsive systems that offer promising perspectives in the materials and life sciences. One current limitation in the design of functional photo-responsive systems is the need to induce switching at least in one direction by UV light, which penetrates only partially through most media and instead leads to degradation. In this chapter, we provide a summary of the different conceptual strategies to operate molecular photoswitches solely in the visible and near-infrared regions of the optical spectrum. The covered visible light-activated molecular switches and the highlighted conceptual approaches will decisively advance the field of photo-switchable systems and facilitate their implementation into future applications and technologies.

Stimuli-responsive molecules change their properties when exposed to external signals, such as light, and enable the preparation of smart materials. UV light, which often destroys organic materials, is typically required for activating the desired response of photoswitchable compounds, significantly limiting the potential applications of light-operated smart materials. Herein, we present the first metal-organic framework (MOF), which enables reversible modulation of key properties upon irradiation with visible light only. The fluorinated azobenzene side groups in the MOF structure can be reversibly switched between the trans and cis state by green and violet light, avoiding UV light. It was demonstrated that the uptake of guest molecules by these MOF films can be switched in a fully remote-controlled way. The membrane separation of hydrogen/hydrocarbon mixtures was investigated. The light-induced changes of the MOF pore size result in the switching of the permeation and of the selection factor.

Functional materials that exhibit photoinduced structural phase transitions are highly interesting for applications in optomechanics and mechanochemistry. It is, however, still not fully understood how photochemical reactions, which are often accompanied by molecular motion, proceed in confined and crystalline environments. Here we show that thin films of azobenzene trimers exhibit high structural order and determine the crystallographic unit cell. We demonstrate that thin film can be switched partially reversibly between a crystalline and an amorphous phase. The time constant of the photoinduced amorphisation as measured with real-time x-ray diffraction ([Formula: see text]220 s) lies between the two time constants (120 s and 2870 s) of the ensemble photoisomerisation processes that are measured via optical spectroscopy. Our observation of a photoinduced shrinking of the crystalline domains indicates a cascading process, in which photoisomerisation starts at the surface of the thin film and propagates deeper into the crystalline layer by introducing disorder and generating free volume. This finding is important for the rapidly evolving research field of photoresponsive thin films and smart crystalline materials in general.

The ability to control the interplay of materials with low-energy photons is important as visible light offers several appealing features compared to ultraviolet radiation (less damaging, more selective, predominant in the solar spectrum, possibility to increase the penetration depth). Two different metal-organic frameworks (MOFs) were synthesized from the same linker bearing all-visible ortho-fluoroazobenzene photoswitches as pendant groups. The MOFs exhibit different architectures that strongly influence the ability of the azobenzenes to isomerize inside the voids. The framework built with Al-based nodes has congested 1D channels that preclude efficient isomerization. As a result, local light-heat conversion can be used to alter the CO2 adsorption capacity of the material on exposure to green light. The second framework, built with Zr nodes, provides enough room for the photoswitches to isomerize, which leads to a unique bistable photochromic MOF that readily responds to blue and green light. The superiority of green over UV irradiation was additionally demonstrated by reflectance spectroscopy and analysis of digested samples. This material offers promising perspectives for liquid-phase applications such as light-controlled catalysis and adsorptive separation.

Next-generation molecular devices and machines demand the integration of molecular switches into hierarchical assemblies to amplify the response of the system from the molecular level to the meso- or macro-scale. Here, we demonstrate that multi-azobenzene oligomers can assemble to form robust supramolecular nanofibers in which they can be switched repeatedly between the E- and Z-configuration. While in isolated oligomers the azobenzene units undergo reversible photoisomerization independently, in the nanofibers they are coupled via intermolecular interactions and switch cooperatively as evidenced by unusual thermal and kinetic behavior. We find that the photoisomerization rate from the Z-isomer to the E-isomer depends on the fraction of Z-azobenzene in the nanofibers, and is increased by more than a factor of 4 in Z-rich fibers when compared to E-rich fibers. This demonstrates the great potential of coupling individual photochromic units for increasing their quantum efficiency in the solid state with potential relevance for actuation and sensing.

Nature provides much inspiration for the design of materials capable of motion upon exposure to external stimuli, and many examples of such active systems have been created in the laboratory. However, to achieve continuous motion driven by an unchanging, constant stimulus has proven extremely challenging. Here we describe a liquid crystalline polymer film doped with a visible light responsive fluorinated azobenzene capable of continuous chaotic oscillatory motion when exposed to ambient sunlight in air. The presence of simultaneous illumination by blue and green light is necessary for the oscillating behaviour to occur, suggesting that the dynamics of continuous forward and backward switching are causing the observed effect. Our work constitutes an important step towards the realization of autonomous, persistently self-propelling machines and self-cleaning surfaces powered by sunlight.