Direct measurements of molecular motion during chemical reactions are essential to understand how molecular machines perform work. In most systems, however, the reaction rate is dictated by the probability of reaching the transition geometry by thermal fluctuations, thereby masking the underlying molecular motions. Here, we study the dynamics of rotation around the central double bond of an artificial light-driven molecular motor by femtosecond transient absorption and fluorescence spectroscopy. We observe a first rotation step, assigned to a rotation of ∼28° which occurs synchronously across the motor ensemble and without an activation barrier, such that the measurements are representative of molecular dynamics. We can thus estimate the rotation speed and the relative importance of inertia, friction and strain, and propose a simplified nanomechanical model for the molecular motor. The results suggest a new framework to investigate work at the nanoscale and provide tools to analyze the mechanics of molecular machines, both synthetic or biological.

Präorganisation durch Assemblierungs‐ oder Templatstrategien wird häufig in der Synthese von Rotaxanen eingesetzt. Während diese Ansätze die Herstellung komplexer, ineinander verzahnter Moleküle ermöglicht haben, sind sie oft durch strenge strukturelle Anforderungen an Templat und Ausgangsstoffe limitirt. Wir berichten hier über eine molekulare Maschine, die die Synthese eines Rotaxans durch aktives mechanisches Formen der Ausgangsbausteine steuert. Die lichtinduzierte Rotation eines molekularen Motors wickelt dabei gezielt einen molekularen Strang um eine Achse und erzeugt dabei diskrete, thermodynamisch ungünstige Kreuzungspunkte zwischen beiden Komponenten. Durch kovalente Fixierung werden diese kinetisch stabilen Verschlingungen chemisch konserviert, wodurch der gewundene Strang in einen Makrozyklus überführt wird. Dieser wird anschließend abgelöst und dabei mechanisch auf der Achse aufgefädelt, was zur Bildung eines Rotaxans führt. Diese maschinengesteuerte Strategie eröffnet einen neuartigen Zugang zur Synthese von Rotaxanen durch aktives mechanisches Formen molekularer Bausteine und ermöglicht den Aufbau ineinander verzahnter Architekturen, die jenseits der Möglichkeiten herkömmlicher Assemblierungs‐ und Templatmethoden liegen.

Preorganization by assembly or templating strategies is frequently used in the synthesis of rotaxanes. While these approaches have led to complex interlocked molecules, they are often limited by strict compositional requirements of templates and starting materials. Here, we use a molecular machine to direct the synthesis of a rotaxane by active shaping of starting materials through mechanical winding. Light induced rotation of a molecular motor actively winds a molecular strand around an axle, forming discrete, thermodynamically disfavored crossings between these two parts. Covalent capture preserves the kinetically stable entanglements, transforming the strand into a macrocycle that is subsequently released and mechanically trapped on the axle, yielding a rotaxane. Our machine-directed strategy pioneers a new way of synthesizing rotaxanes by active mechanical shaping of molecular building blocks, enabling access to interlocked architectures beyond the reach of traditional assembly and templating approaches.

Precise mechanical manipulation of molecules is inherently difficult owing to random thermal motion. Although directed movement on the molecular scale has been achieved, using it to impose specific—especially energetically disfavored—shapes on molecules and construct mechanically interlocked structures remains a fundamental challenge. In this study, we report the synthesis of a catenane enabled by a molecular motor that winds molecular strands into discrete entangled structures, each defined by a specific number of mechanical crossings. Light energy drives unidirectional motor rotation, enabling path-dependent control over a sequence of thermodynamically disfavored yet mechanically distinct and kinetically stable winding states, which are covalently captured and subsequently released to yield a catenane. This machine-directed approach offers a general proof-of-concept strategy for the template-free construction of mechanically interlocked molecules.

In a chemical equilibrium, the formation of high-energy species-in a closed system-is inefficient due to microscopic reversibility. Here, we demonstrate how this restriction can be circumvented by coupling a dynamic equilibrium to a light-induced E/Z isomerization of an azobenzene imine cage. The stable E-cage resists intermolecular imine exchange reactions that would "open" it. Upon switching, the strained Z-cage isomers undergo imine exchange spontaneously, thus opening the cage. Subsequent isomerization of the Z-open compounds yields a high-energy, kinetically trapped E-open species, which cannot be efficiently obtained from the initial E-cage, thus shifting an imine equilibrium energetically uphill in a closed system. Upon heating, the nucleophile is displaced back into solution and an opening/closing cycle is completed by regenerating the stable all-E-cage. Using this principle, a light-induced cage-to-cage transformation is performed by the addition of a ditopic aldehyde.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The incorporation of molecular machines into the backbone of porous framework structures will facilitate nano actuation, enhanced molecular transport, and other out-of-equilibrium host-guest phenomena in well-defined 3D solid materials. In this work, we detail the synthesis of a diamine-based light-driven molecular motor and its incorporation into a series of imine-based polymers and covalent organic frameworks (COF). We study structural and dynamic properties of the molecular building blocks and derived self-assembled solids with a series of spectroscopic, diffraction, and theoretical methods. Using an acid-catalyzed synthesis approach, we are able to obtain the first crystalline 2D COF with stacked hexagonal layers that contains 20 mol% molecular motors. The COF features a specific pore volume and surface area of up to 0.45 cm(3) g(-1) and 604 m(2) g(-1), respectively. Given the molecular structure and bulkiness of the diamine motor, we study the supramolecular assembly of the COF layers and detail stacking disorders between adjacent layers. We finally probe the motor dynamics with in situ spectroscopic techniques revealing current limitations in the analysis of these new materials and derive important analysis and design criteria as well as synthetic access to new generations of motorized porous framework materials.

Biological molecular machines enable chemical transformations, assembly, replication and motility, but most distinctively drive chemical systems out of-equilibrium to sustain life1,2. In such processes, nanometre-sized machines produce molecular energy carriers by driving endergonic equilibrium reactions. However, transforming the work performed by artificial nanomachines3-5 into chemical energy remains highly challenging. Here, we report a light-fuelled small-molecule ratchet capable of driving a coupled chemical equilibrium energetically uphill. By bridging two imine6-9 macrocycles with a molecular motor10,11, the machine forms crossings and consequently adopts several distinct topologies by either a thermal (temporary bond-dissociation) or photochemical (unidirectional rotation) pathway. While the former will relax the machine towards the global energetic minimum, the latter increases the number of crossings in the system above the equilibrium value. Our approach provides a blueprint for coupling continuous mechanical motion performed by a molecular machine with a chemical transformation to reach an out-of-equilibrium state.

In artificial small-molecule machines, molecular motors can be used to perform work on coupled systems by applying a mechanical load-such as strain-that allows for energy transduction. Here, we report how ring strain influences the rotation of a rotary molecular motor. Bridging the two halves of the motor with alkyl tethers of varying sizes yields macrocycles that constrain the motor's movement. Increasing the ring size by two methylene increments increases the mobility of the motor stepwise and allows for fine-tuning of strain in the system. Small macrocycles (8-14 methylene units) only undergo a photochemical E/Z isomerization. Larger macrocycles (16-22 methylene units) can perform a full rotational cycle, but thermal helix inversion is strongly dependent on the ring size. This study provides systematic and quantitative insight into the behavior of molecular motors under a mechanical load, paving the way for the development of complex coupled nanomachinery.

This chapter outlines how photoswitchable molecules can be used to perturb thermal equilibria and how this unique feature is applied in molecular, microscopic, and even macroscopic systems. It explains the fundamentals of photodynamic equilibria and introduces the most prominent classes of photoswitchable molecules. The chapter illustrates how photoswitches can be used to drive chemical systems with light. The photoswitching event can enhance or reduce the reactivity in a dynamic covalent reaction. Chemical transport constitutes an essential part of chemical work in living systems since reactants and products have to actively be pumped in and out of the cell, against an existing gradient. Self-assembly constitutes a very powerful mechanism to obtain elaborate functional structures and amplify the properties of the individual components. Nature developed an ingenious method to drive thermodynamically disfavored reactions: photosynthesis. The chapter highlights functional molecular systems, in which the incorporation of photoswitchable units enables light to be used as their external power supply.

Angewandte ChemieVolume 131, Issue 7 p. 1869-1883 Graphisches InhaltsverzeichnisFree Access Graphisches Inhaltsverzeichnis: Angew. Chem. 7/2019 First published: 04 February 2019 https://doi.org/10.1002/ange.201980711Citations: 2AboutPDF ToolsRequest permissionAdd to favorites ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume131, Issue7February 11, 2019Pages 1869-1883 This is the German version of Angewandte Chemie. Note for articles published since 1962: Do not cite this version alone. Take me to the International Edition version with citable page numbers, DOI, and citation export. We apologize for the inconvenience. RelatedInformation

The transfer of stereoinformation is at the heart of asymmetric reactions. By incorporating the natural monoterpene l-menthone into the backbone of a diarylethene, we achieved efficient chirality transfer upon photocyclization, resulting in the preferred formation of one major closed isomer in a diastereomeric ratio (d.r.) of 85:15. More significantly, we were able to completely reverse the diastereomeric outcome of the ring closure simply by altering the chemical environment or the irradiation conditions. As a result, we could selectively accumulate the less favored minor closed isomer, with remarkable d.r. values of >99:1 and 74:26, respectively. Computations revealed that a stability inversion after photocyclization is the basis for the observed unprecedented control over diastereoselectivity.

AbstractDer Transfer von Stereoinformation ist das Herzstück asymmetrischer Reaktionen. Durch die Integration des natürlichen Monoterpensl‐Menthon in das Rückgrat eines Diarylethens konnte ein effizienter Chiralitätstransfer bei der Photocyclisierung erreicht werden, was zur bevorzugten Bildung eines geschlossenen Hauptisomers mit einem Diastereomerenverhältnis (d.r.) von 85:15 führte. Noch stärker hervorzuheben ist allerdings, dass die Diastereoselektivität des Ringschlusses vollständig umgekehrt werden kann, indem die chemische Umgebung oder die Bestrahlungsbedingungen verändert werden. Infolgedessen konnte selektiv das weniger favorisierte geschlossene Isomer mit bemerkenswerten d.r.‐Werten von >99:1 bzw. 74:26 akkumuliert werden. Berechnungen zeigen, dass eine Stabilitätsinversion nach der Photocyclisierung die Grundlage für die beobachtete beispiellose Kontrolle der Diastereoselektivität ist.

ABSTRACTHere, we describe a “smart” polymeric material, which is able to readily detect and discriminate amine vapors. The dynamic imine‐based network can be conveniently prepared by mixing a commercially available, amino‐functionalized polysiloxane with small amounts of a diarylethene dialdehyde. The photoswitchable crosslinker allows for reversible imprinting of custom‐designed patterns on the polymer surface with (sun)light and thus enables noninvasive information storage in the material, which before, during, and after amine exposure can readily be decoded with commonly used smartphone apps. This feature along with the self‐healing nature of the dynamic polymer, an easy recycling and manufacturing procedure, and the overall low cost and toxicity render this material advantageous to develop low‐cost and practical amine sensing devices for the broad public. © 2019 The Authors. Journal of Polymer Science Part A: Polymer Chemistry published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2378–2382

The fundamental properties of a polymeric material are ultimately governed by its structure, which mainly relies on monomer composition and connection, topology, chain length, and polydispersity. Thus far, these structural characteristics are typically set ex situ by the specific polymerization procedure, eventually limiting the future design space for the creation of more sophisticated polymers. Herein, we report on a single photoswitchable catalyst system, which enables in situ remote control over the ring-opening polymerization of l -lactide and further allows regulation of the incorporation of trimethylene carbonate and δ-valerolactone monomers in copolymerizations. By implementing a phenol moiety into a diarylethene-type structure, we exploit light-induced keto–enol tautomerism to switch the hydrogen-bonding-mediated monomer activation reversibly ON and OFF. This general and versatile principle allows for exquisite external modulation of ground-state catalysis of a living polymerization process in a closed system by ultraviolet and visible light and should thereby facilitate the generation of new polymer structures.

Bond formation between two molecular entities in a closed system strictly obeys the principle of microscopic reversibility and occurs in favour of the thermodynamically more stable product. Here, we demonstrate how light can bypass this fundamental limitation by driving and controlling the reversible bimolecular reaction between an N-nucleophile and a photoswitchable carbonyl electrophile. Light-driven tautomerization cycles reverse the reactivity of the C=O/C=N-electrophiles ('umpolung') to activate substrates and remove products, respectively, solely depending on the illumination wavelength. By applying either red or blue light, selective and nearly quantitative intermolecular bond formation/scission can be achieved, even if the underlying condensation/hydrolysis equilibrium is thermodynamically disfavoured. Exploiting light-driven in situ C=N exchange, our approach can be used to externally regulate a closed dynamic covalent system by actively and reversibly removing specific components, resembling a molecular and bidirectional version of a macroscopic Dean-Stark trap.

In order to perform chemical work, molecular systems have to be operated away from thermodynamic equilibrium and therefore require the input of energy. Light is perhaps the most abundant and advantageous energy source that in combination with photoswitches allows for a reversible and hence continuous stimulation of a system. In this review, we illustrate how photoswitchable molecules can be used to escape the global thermodynamic minimum by populating metastable states, from which energy can be transferred and transformed in a controlled fashion. We emphasize the unique feature of photodynamic equilibria, in which population of the states is dictated by the excitation wavelength (and not primarily by temperature), thereby avoiding microscopic reversibility since the photoreaction involves an electronically excited state. Thus, photoswitchable molecular systems can remotely be controlled with high spatial and temporal resolution and in addition their action can be fueled by light.

AbstractMoleküle, die ihre Eigenschaften bei Lichtbestrahlung ändern, stehen zunehmend im Blickpunkt der Materialforscher. Denn Ort und Dauer der Lichteinwirkung sind präzise kontrollierbar, und die Schalt‐prozesse sind reversibel.