A survey of heterocyclic hemithioindigo photoswitches is presented identifying a number of structural motives with outstanding property profiles. The highly sought‐after combination of pronounced ...color change, quantitative switching in both directions, exceptional high quantum yields, and tunable high thermal stability of metastable states can be realized with 4‐imidazole, 2‐pyrrole, and 3‐indole‐based derivatives. In the former, an unusual preorganization using isomer selective chalcogen‐ and hydrogen bonding allows to precisely control geometry changes and tautomerism upon switching. Heterocyclic hemithioindigos thus represent highly promising photoswitches with advanced capabilities that will be of great value to anyone interested in establishing defined and reversible control at the molecular level.
Imidazole, pyrrole, or indole substitution of heterocyclic hemithioindigo chromophores can deliver advanced photoswitching properties: combining full visible light responsiveness, high overall absorptions, pronounced absorption changes, up to quantitative isomer interconversions in both directions, tunable thermal isomerization speeds, and very high quantum yields. Precise geometry changes can be controlled by balancing isomer‐specific chalcogen and hydrogen bonding.
Molecular motors convert external energy into directional motions at the nano-scales. To date unidirectional circular rotations and linear motions have been realized but more complex directional ...trajectories remain unexplored on the molecular level. In this work we present a molecular motor powered by green light allowing to produce an eight-shaped geometry change during its unidirectional rotation around the central molecular axis. Motor motion proceeds in four different steps, which alternate between light powered double bond isomerizations and thermal hula-twist isomerizations. The result is a fixed sequence of populating four different isomers in a fully unidirectional trajectory possessing one crossing point. This motor system opens up unexplored avenues for the construction and mechanisms of molecular machines and will therefore not only significantly expand the toolbox of responsive molecular devices but also enable very different applications in the field of miniaturized technology than currently possible.
Photoswitches are indispensable tools for responsive chemical nanosystems and are used today in almost all areas of the natural sciences. Hemiindigo (HI) derivatives have recently been introduced as ...potent photoswitches, but their full applicability has been hampered by the limited possibilities of their functionalization and structural modification. Here we report on a short and easy to diversify synthesis yielding diaryl-HIs bearing one additional aromatic residue at the central double bond. The resulting chromophores offer an advantageous property profile combining red-light responsiveness, high thermal bistability, strong isomer accumulations in both switching directions, strong photochromism, tunable acid responsiveness, and acid gating. With this progress, a broader structural realm becomes accessible for HI photoswitches, which can now be synthetically tailored for advanced future applications, e.g., in research on molecular machines and switches, in studies of photoisomerization mechanisms, or in the generation of smart and addressable materials. To showcase the potential of these distinct light-responsive molecular tools, we demonstrate four-state switching, chemical fueling, and reversible inscription into transparent polymers using green and red light as well as acid/base stimuli, in addition to a comprehensive photochemical study of all compounds.
Hemiindigo is a long known chromophore that absorbs in the blue part of the spectrum but has almost completely been ignored as potential photoswitch. Herein we show how the absorption of hemiindigo ...is shifted to the red part of the visible spectrum and how nearly perfect photoswitching can be achieved using blue or green and red light. Five derivatives were investigated giving very high isomeric yields in both switching directions, i.e. >90% E isomer after irradiation with 470 to 530 nm light and 99% Z isomer with 590 up to 680 nm light. At the same time the thermal bistability is extraordinarily high leading to half-lives of the pure isomeric states of up to 83 years at 25 °C. The herein developed photoswitches show photochromism in the visible enabling the two isomeric states to be distinguished by the naked eye. Substituted hemiindigos therefore constitute extremely promising new photoswitches with excellent properties for applications in biology, chemistry, or material sciences.
Molecular motors undergo repetitive directional motions upon external energy input. A profound challenge is the defined transfer of directional motor motions to remote entities at the molecular ...scale. Herein, we present a molecular setup that allows for the transfer of the directional rotation of a light‐powered motor unit onto a remote biaryl axis via an ethylene glycol chain link. Based on a combination of X‐ray crystallographic analysis, ECD, and NMR experiments as well as a comprehensive theoretical assessment, we provide evidence for the coupled stepwise directional motions of both molecular units. With the presented setup, facile integration of molecular motor units into larger functional frameworks and complex molecular machines can be explored consciously in the future.
Set into motion: The directional rotation of a hemithioindigo‐based molecular motor is effectively transmitted to a remote asymmetric biaryl axis via an ethylene glycol chain. With the presented setup, directional motions can now be relayed to other functional entities against the equilibrating Brownian motion on the molecular scale with high precision.
Photoisomerization reactions are quintessential processes driving molecular machines and motors, govern smart materials, catalytic processes, and photopharmacology, and lie at the heart of vision, ...phototaxis, or vitamin production. Despite this plethora of applications fundamental photoisomerization mechanisms are not well understood at present. The famous hula-twist motion-a coupled single and double-bond rotation-was proposed to explain proficient photoswitching in restricted environments but fast thermal follow-up reactions hamper identification of primary photo products. Herein we describe an asymmetric chromophore possessing four geometrically distinct diastereomeric states that do not interconvert thermally and can be crystallized separately. Employing this molecular setup direct and unequivocal evidence for the hula-twist photoreaction and for photoinduced single-bond rotation is obtained. The influences of the surrounding medium and temperature are quantified and used to favor unusual photoreactions. Based on our findings molecular engineers will be able to implement photo control of complex molecular motions more consciously.
Molecular motors transform external energy input into directional motions and offer exquisite precision for nano‐scale manipulations. To make full use of molecular motor capacities, their directional ...motions need to be transmitted and used for powering downstream molecular events. Here we present a macrocyclic molecular motor structure able to perform repetitive molecular threading of a flexible tetraethylene glycol chain through the macrocycle. This mechanical threading event is actively powered by the motor and leads to a direct translation of the unidirectional motor rotation into unidirectional translation motion (chain versus ring). The mechanism of the active mechanical threading is elucidated and the actual threading step is identified as a combined helix inversion and threading event. The established molecular machine function resembles the crucial step of macroscopic weaving or sewing processes and therefore offers a first entry point to a “molecular knitting” counterpart.
A molecular machine is presented that uses the light‐powered unidirectional rotation trajectory of a molecular motor to actively thread a tetraethylene glycol chain through a macrocyclic ring. This molecular setup thus realizes the miniaturization of macroscopic threading providing a fundamental entry step into mechanical molecular weaving or sewing processes.
Typical photoswitches interconvert between two different states by simple isomerization reactions, which represents a fundamental limit for applications. To expand the switching capacity usually ...different photoswitches have to be linked together leading to strong increase in molecular weight, diminished switching function, and less precision and selectivity of switching events. Herein we present an approach for solving this essential problem with a different photoswitching concept. A basic molecular switch architecture provides precision photoswitching between eight different states via controlled rotations around three adjacent covalent bonds. All eight states can be populated one after another in an eight-step cycle by alternating between photochemical Hula-Twist isomerizations and thermal single-bond rotations. By simply changing solvent and temperature the same switch can also undergo a different cycle instead interconverting just five isomers in a selective sequence. This behavior is enabled through the discovery of an unprecedented photoreaction, a one-photon dual single-bond rotation.
Remote control of complex molecular behavior and function is one key problem in modern chemistry. Using light signaling for this purpose has many advantages, however the integration of different ...photo processes into a wholesome yet complex system is highly challenging. Here we report an alternative approach to increase complexity of light control-simultaneous complementary photoswitching-in which spectral overlap is used as an advantage to drastically reduce the signaling needed for controlling multipart supramolecular assemblies. Two photoswitchable molecular tweezers respond to the same light signals with opposite changes in their binding affinities. In this way the configuration of two host tweezers and ultimately the dynamic relocation of a guest molecule can be trigged by only one signal reversibly in the same solution. This approach should provide a powerful tool for the construction of sophisticated, integrated, and multi-responsive smart molecular systems in any application driven field of chemistry.
Red-light responsiveness of photoswitches is a highly desired property for many important application areas such as biology or material sciences. The main approach to elicit this property uses ...strategic substitution of long-known photoswitch motives such as azobenzenes or diarylethenes. Only very few photoswitches possess inherent red-light absorption of their core chromophore structures. Here, we present a strategy to convert the long-known purple indirubin dye into a prolific red-light-responsive photoswitch. In a supramolecular approach, its photochromism can be changed from a negative to a positive one, while at the same time, significantly higher yields of the metastable E-isomer are obtained upon irradiation. E- to Z-photoisomerization can then also be induced by red light of longer wavelengths. Indirubin therefore represents a unique example of reversible photoswitching using entirely red light for both switching directions.
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