A diarylethene photoswitch was covalently connected to two small triplet sensitizer moieties in a conjugated and nonconjugated fashion and the photochromic performance of the resulting compounds was investigated. In comparison with the parent diarylethene (without sensitizers) and one featuring saturated linkages, the conjugated photoswitch offers superior fatigue resistance upon visible-light excitation due to effective triplet energy transfer from the biacetyl termini to the diarylethene core. Our design makes it possible to switch diarylethenes with visible light in both directions in a highly efficient and robust fashion based on extending π-conjugation and by-product-free ring-closure via the triplet manifold.

Ein Diarylethen-Photoschalter wurde kovalent einmal konjugiert und einmal nichtkonjugiert mit zwei Triplett-Sensibilisatoreinheiten verknüpft, und die resultierenden Verbindungen wurden auf ihre photochromen Eigenschaften untersucht. Im Unterschied zum ursprünglichen Diarylethen (ohne Sensibilisator) und dem Derivat mit gesättigter Bindung zeigt der konjugierte Photoschalter eine ausgezeichnete Ermüdungsresistenz bedingt durch die Anregung mit sichtbarem Licht und den effektiven Triplettenergietransfer vom Biacetylterminus zum Diarylethenkern. Aufgrund der ausgedehnten π-Konjugation und des Ringschlusses über den Triplettzustand lassen sich unsere Diarylethene mit sichtbarem Licht in beide Richtungen effektiv und robust schalten.

A water-soluble furyl-substituted diarylethene derivative has been prepared that can undergo reversible Diels–Alder reactions with maleimides to yield photoswitchable Diels–Alder adducts. Employing bioorthogonal visible light, the release of therapeutically effective concentrations of maleimide-based reactive inhibitors or labels from these “prodrugs” or “protags” could be photoreversibly triggered in buffered, aqueous solution at body temperature. It is shown how the release properties can be fine-tuned and a thorough investigation of the release dynamics is presented. Our system should allow for spatiotemporal control over the inhibition and labeling of specific protein targets and is ready to be surveyed in living organisms.

Stimuli responsive compounds and materials are of high interest in synthetic chemistry and materials science, with light being the most intriguing stimulus due to the possibility to remote control the physicochemical properties of a molecule or a material. There is a constant quest to design photoswitches with improved switching efficiency and especially diarylethene-type switches promise photo cyclization quantum yields up to unity. However, only limited attention has been paid towards the influence of the solution conformation on the switching efficiency. Here, we describe a detailed NMR spectroscopic investigation on the conformational distribution of bridge-substituted dithienylcyclopentenes in solution. We could discriminate between several photoactive and photoinactive as well as two diastereomorphous conformations and show that the trends observed in the switching efficiency match the conformer populations obtained from state of the art NMR parameters in solution.

Two types of amphiphilic polymers composed of azobenzene repeat units in the main chain connected either via ethynylene (acetylene) or butadiynylene (diacetylene) linkages and carrying oligo(ethylene glycol) side chains were reported. Synthesis was accomplished by polycondensation involving Sonogashira–Hagihara cross coupling and Glaser coupling, respectively. Solvent titration experiments revealed that both polymers fold into stable helices in a polar environment. While the ethynylene-bridged polymer resembled the behavior of its oligomeric counterparts, introduction of the extended diacetylene unit strengthened π,π-stacking interactions in case of the butadiynylene-bridged polymer leading to a pronounced aggregation tendency and suppressing photoisomerization in the folded state. Our study demonstrates the importance of backbone connectivity to balance intra- and intermolecular forces for the successful design of photoresponsive polymers. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 313–318

In polyfluorenes it is generally accepted that (photo)degradation leads to fluorenone type defects that accept the excitation energy and emit green-to-yellow light with rather low efficiency. Although initial spectroscopic studies suggest the same to hold true for ladder-type poly(para-phenylene)s (LPPPs), kinetic studies of the degradation process are not compatible with the established mechanism. In general, the observed green emission can be caused by the introduction of carbonyl groups; however, only if associated with an additional disruption of the backbone rigidity and hence planarity of the entire π-system. This is clearly shown by comparison with synthesized model compounds, which are bearing the fluorenone motif yet possess very different optical properties as compared to the defects, which are actually formed. Degradation can be caused by solvent specific, yet substrate nonspecific aromatic formylation but mainly originates from reaction with in-situ generated singlet oxygen, both in solution as well as in thin films. Time-dependent photoluminescence measurements on thin films show that green emission is enhanced by energy transfer from intact molecules to defect centers.

A series of partially bridge-fluorinated derivatives of ladder-type quarterphenyl L4P is reported. Successive introduction of two fluorine atoms each into the two terminal and one central benzylic methylene positions enabled the synthesis of the three symmetrical derivatives L4P-F₂, L4P-F₄, and L4P-F₆. X-ray diffraction data show that fluorination does not notably affect the molecular structure of the π system; however, molecular packing in the crystalline solid is largely impacted by the introduction of strong dipoles perpendicular to the molecular axis. The effect of substitution with the present electron-withdrawing fluorine atoms is clearly reflected in significantly lowered (frontier) molecular orbital levels (the LUMO of L4P is at approximately −1.8 to −2.0 eV and shifts to −2.8 eV or lower), as evidenced by lower reduction and higher oxidation potentials in cyclic voltammetry measurements. Furthermore, the influence of fluorination on the optical properties is unexpectedly strong, in particular, with regard to the fluorescence quantum yield. Fluorination in fluorene bridge positions thus provides an interesting alternative tool to tune the electronic and optical properties, as well as self-assembly behavior, of ladder oligo(para-phenylene)s.

Improving the photochemical properties of molecular photoswitches is crucial for the development of light-responsive systems in materials and life sciences. ortho-Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ-electron-withdrawing F atoms ortho to the NN unit leads to both an effective separation of the nπ* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para-electron-withdrawing groups (EWGs) work in concert with ortho-F atoms, giving rise to enhanced separation of the nπ* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations.

The on-going need for feature miniaturization and the growing complexity of structures for use in nanotechnology demand the precise and controlled formation of covalent bonds at the molecular level. Such control requires the use of external stimuli that offer outstanding spatial, temporal, as well as energetic resolution. Thus, photoaddressable switches are excellent candidates for creating a system that allows reversible photocontrol over covalent chemical connection and disconnection. Here we show that the formation of covalent bonds between two reagents and their scission in the resulting product can be controlled exclusively by illumination with differently colored light. A furyl-substituted photoswitchable diarylethene was shown to undergo a reversible Diels–Alder reaction with maleimide to afford the corresponding Diels–Alder adduct. Our system is potentially applicable in any field already relying on the benefits of reversible Diels–Alder reactions.

Der Wunsch nach Bauteilverkleinerung und die wachsende Komplexität von Strukturen auf dem Gebiet der Nanotechnologie verlangen nach der präzisen und kontrollierten Bildung kovalenter Bindungen auf molekularer Ebene. Eine solche Kontrolle erfordert jedoch den Einsatz externer Stimuli mit herausragender räumlicher, zeitlicher und energetischer Auflösung. Mit Licht adressierbare Schalter sind deshalb exzellente Kandidaten für Systeme, die eine reversible Photokontrolle über kovalente chemische Ver- und Entknüpfungen ermöglichen. Hier stellen wir ein solches System vor, das ausschließlich durch die Bestrahlung mit Licht unterschiedlicher Farbe kontrolliert werden kann. Zu diesem Zweck wurde ein Furyl-substituiertes photoschaltbares Diarylethen konstruiert, das eine reversible Diels-Alder-Reaktion mit Maleinimid eingehen kann. Unser System verspricht Anwendungsmöglichkeiten in allen Gebieten, die schon jetzt die Vorteile von reversiblen Diels-Alder-Reaktionen nutzen.

Single-walled carbon nanotubes (SWCNTs) are functionalized with a spiropyran derivative, which is attached non-covalently to the SWCNT's sidewall via a pyrene anchor group. Using this non-covalent functionalization strategy, individual SWCNTs can be stabilized in solution without the need for additional surfactants. Bright luminescence confirms the presence of individual tubes in the thus-prepared samples. In these samples, the majority of pyrene-spiropyran molecules are attached to the walls of the SWCNTs. Upon complex formation with the SWCNT, the switching moiety retains its ability to switch, i.e., to undergo reversible transformations between the closed spiropyran and the opened merocyanine form, and is stable over many cycles of operation.

Noncovalent interactions, especially hydrogen-bonding interactions as well as electrostatic forces, confined within one macromolecule are the key to designing foldamers that adopt well-defined conformations in solution. In the context of significant recent activities in the area of triazole-connected foldamers, so-called clickamers, we present a fundamental study that compares various model compounds that bear adjacent N-, O-, or F-heteroatom substituents. The interplay of attractive and repulsive interactions leads to rotational constraints around the single bonds attached to both the 1- and 4-positions of the 1,2,3-triazole moiety and should therefore be able to induce well-defined conformational preferences in higher oligomers and polymers, that is, foldamers. Various compounds were synthesized and characterized with regard to their preferred conformations in all three aggregation states—that is, in the gas phase, in solution as well as in the solid state—by employing DFT calculations, NMR spectroscopic experiments, and X-ray crystallography, respectively. On the basis of the thus-obtained general understanding of the conformational behavior of the individual connection motifs, heterostructures were prepared from different motifs without affecting their distinct folding characteristics. Therefore, this work provides a kind of foldamer construction kit, which should enable the design of various clickamers with specific shape and incorporated functionality.

Small organic molecules, capable of undergoing efficient and reversible photochemical reactions to switch them between (at least) two (meta)stable isomers associated with markedly different properties, continue to impact the materials world. Such photoswitches are being implemented in a variety of materials for applications ranging from optical devices to “smart” polymers. All approaches exploit the photoswitching molecular entities as gates, which translate an incoming light stimulus to trigger macroscopic property changes of the materials. In this progress report, the most promising recent examples in this field are highlighted and put in perspective. Moving from supramolecular systems in solution to surfaces and finally to bulk materials, important design concepts are discussed, emphasizing both the challenges as well as the great promise of such truly advanced materials.

2,6-Bis(1,2,3-triazol-4-yl)pyridine (btp) ligands with substitution patterns ranging from strongly electron-donating to strongly electron-accepting groups, readily prepared by means of Cu-catalyzed 1,3-dipolar cycloaddition (the “click” reaction), were investigated with regard to their complexation behavior, and the properties of the resulting transition-metal compounds were compared. Metal–btp complexes of 1:1 stoichiometry, that is, [Ru(btp)Cl₂(dmso)] and [Zn(btp)Br₂], could be isolated and were crystallographically characterized: they display octahedral and trigonal-bipyramidal coordination geometries, respectively, and exhibit high aggregation tendencies due to efficient π–π stacking leading to low solubilities. Metal–btp complexes of 1:2 stoichiometry, that is, [Fe(btp)₂]²⁺ and [Ru(btp)₂]²⁺, could also be synthesized and their metal centers show the expected octahedral coordination spheres. The iron compounds exhibit quite a complex magnetic behavior in the solid state including spin crossover near room temperature, and hysteresis and locking into high-spin states on tempering at 400 K, depending on the substituents on the btp ligands. Cyclic voltammetry studies of [Ru(btp)₂]²⁺ reveal strong modulation of the oxidation potentials by more than 0.6 V and a clear linear correlation to the Hammett constant (σ_para) of the substituent at the pyridine core. Isothermal titration calorimetry was used to measure the thermodynamics of the Fe^II–btp complexation process and enabled accurate determination of the complexation enthalpies, which display a linear relationship with the σ_para values for the terminal phenyl substituents. Detailed NMR spectroscopic studies finally revealed that in the case of Fe^II complexation, dynamics are rapid for all investigated btp derivatives in acetonitrile, while replacing Fe^II by Ru^II or changing the solvent to dichloromethane effectively slows down ligand exchange. The results nicely demonstrate the utility of substituent parameters, originally developed for linear free-energy relationships to explain reactivity in organic reactions, in coordination chemistry, and to illustrate the potential to custom-design btp ligands and complexes thereof with predictable properties. The fast equilibration of the [Fe(btp)₂]²⁺ complexes together with their tunable stability and interesting magnetic properties should enable the design of dynamic metallosupramolecular materials with advantageous properties.

Kontrolle über einen Gegenstand oder einen gesamten Prozess auszuüben, ist zweifelsohne der ultimative Beweis für dessen Verständnis und ermöglicht erst die vollständige Nutzung seines Potenzials. Vor diesem Hintergrund haben Chemiker in den vergangenen Jahrzehnten für die meisten (relevanten) chemischen Reaktionen nicht nur eine Vielzahl von Katalysatoren entwickelt und optimiert, sondern vor kurzem auch damit begonnen, Steuerungselemente in das Katalysatordesign einzuführen. Diese integrierten Kontrolleinheiten werden durch Einwirkung geeigneter Stimuli reguliert, wobei Licht aus wissenschaftlicher und technologischer Sicht vermutlich am attraktivsten ist. Bestrahlung kann hierbei prinzipiell sowohl Aktivität als auch Selektivität in einem gegebenen katalytischen System mit hoher räumlicher und zeitlicher Präzision steuern, die zu einer allgemeinen Lokalisierung und Verstärkung des optischen Signals und dessen Umwandlung in chemische Aktivität führt. Während in der Natur dieses Konzept bereits erfolgreich Anwendung findet, im Speziellen beim Sehprozess und bei der Photobewegung, eröffnen derartige künstliche lichtgesteuerte katalytische Systeme einzigartige Möglichkeiten und verfügen über ein hohes Potenzial für zukünftige Anwendungen. In diesem Aufsatz beschreiben wir die grundsätzliche Idee der lichtgesteuerten Katalyse basierend auf photoaktivierbaren und photoschaltbaren Systemen und diskutieren relevante Beispiele aus der älteren und neueren Literatur.

Having control over an entity or even an entire process is arguably the ultimate demonstration of its understanding and it will enable its potential to be fully exploited. With this in mind, chemists have not only been creating and optimizing a myriad of different catalysts for most (relevant) chemical reactions over the past decades, but have recently started to implement controlling elements into the catalyst design. These incorporated gates operate upon exposure to suitable control stimuli, and light represents perhaps the scientifically and technologically most attractive stimulus. In principle, irradiation can thereby induce activity and selectivity in a given catalyst system with high spatial and temporal control, leading to an overall localization and amplification of an optical signal and translation into chemical action. While nature has developed and utilized this concept, in particular in the processes of vision and photomovement, such artificial photocontrolled catalyst systems offer unique opportunities and have high potential for future applications. In this Review, we outline the general concept of light-gated catalysis based on photocaged and also photoswitchable systems, and discuss relevant examples of the past and recent literature.