Im sich herausbildenden Gebiet der intelligenten Materialien werden (Makro-)Moleküle über ihre bloßen Festkörpereigenschaften hinaus auf eine Ebene befördert, auf der sie sich durch den Einfluss externer Stimuli an ihre Umgebung anpassen können. Das bedeutet, dass sie z.B. chemische Reaktionen katalysieren oder an ihnen teilnehmen, Selbstheilungsprozesse initiieren oder mechanische Arbeit verrichten können. Mechanischer Stress ist einer der interessantesten dieser Stimuli, da er ubiquitär in den meisten Materialanwendungen vorkommt weswegen seine Nutzung für die Materialwissenschaften von herausragender Bedeutung ist. Jedoch erlauben es die meisten mechanisch aktivierbaren Materialien nicht gleichzeitig Funktion (d.h. das Ausführen einer Aufgabe) und Rückmeldung (d.h. die Auslesbarkeit der Funktion) zu nutzen. Dennoch ist dies hochgradig wünschenswert, da das Nachverfolgen wo und wann eine Funktion auftritt es nicht nur ermöglicht den Prozess analytisch im Festkörper oder an der Oberfläche zu beobachten, sondern auch herausragendes Potential für Anwendungen besitzt, die über die akademische Ebene hinausgehen. So könnten optische oder elektromagnetische Rückmeldungsmechanismen mit einfachen Gerätschaften, oder sogar dem bloßen Auge, präzise Auslesbarkeit gewährleisten.Während dieses Forschungsstipendiums werden neuartige mechanoresponsive Bindungsmotive identifiziert, entworfen und synthetisiert werden, die es auf einfache Art ermöglichen Rückmeldungsmechanismen in sie einzubauen. Weiterhin werden Rückmeldungsmechanismen in schon existierende mechanoresponsive Bindungsmotive implementiert werden. Zu diesem Zweck werden sowohl die reversible homolytische Bindungsspaltung von Alkoxyamin-Derivaten, wie z.B. TEMPO, als auch die reversible Diels-Alder-Reaktion zwischen Furan und Maleinimid genutzt und auf ihre Eignung für Anwendungen, z.B. in der Mechanokatalyse für lebende radikalische Polymerisationen oder mechanisch induzierten Selbstheilungsprozessen von Polymermaterialien, überprüft werden.

Living cells and tissues are constantly subjected to mechanical stress and wear stimulating biological processes, such as the signaling of pain. Researchers strive to explore these biomechanical interactions but are confronted with a lack of tools to visualize stress from within the cell up to the tissue scale. The goal of this project is to develop molecular moieties by synthetic chemistry that act as predetermined breaking points either sending out an optical signal or initiating a biochemical reaction once subjected to stress. This will allow the spatial and directional optical resolution as well as the harnessing of force effects down to the molecular level. Initially, the optical probes will be incorporated into cell-like polymer architectures that resemble certain features of cells. This will not only allow the controlled benchmarking of the probes but moreover to learn about the response of these cell-like systems to mechanical stress. Subsequently, employing the optical stress-sensors in living cells will elucidate mechanical cellular responses in a much more complex, natural environment. Beyond the sensing of stresses, the initiation of biochemical reactions in cell-like nanoreactors employing force-cleavable protecting groups will lead to bioinspired materials that can perform functions nature has not yet achieved.

Despite the sophisticated and well-established combinations of matrix and filler components, sharply defining the molecular structure – macroscopic property relationships of nanocomposites remains far from being a straightforward process even today. This complexity stems to a high degree from the intricate interactions taking place at the exact contact point of the individual materials, i.e. their interface. Yet, the mechanical properties of nanocomposites that come to define their suitability and, thus, applicability, are particularly affected by the properties of this interface.

Mechano-optical molecular force probes (mechanophores) can shine light on this matter by visualizing critical mechanical stresses and strains through changes in their color and/or luminescence. By covalently integrating mechanophores into the structure of macromolecular materials, we can track precisely where, when, and to what extent a polymer nanocomposite material is exposed to mechanical stress. This will contribute to a better understanding of the molecular causes of the mechanical properties of polymer nanocomposites, and will ultimately improve their molecular hierarchical properties and thus their resilience by reliably displaying the critical load thresholds before the material fails.

By laying the foundations for the development of a molecular-scale fractography technique, based on confocal laser scanning microscopy and super-resolution nanoscopy that probes beyond and beneath the fracture surfaces of materials, the former trial-and-error character of the optimization of high-performance polymer nanocomposites can be further redefined as results can be effectively exploited to verify the already existing computational mechanical simulations.

The damaging and failure of macroscopic polymeric materials can generally be attributed to covalent bond scission, a nanoscopic event. Hence, the observation and quantification of mechanical stress on the nanoscale is a major challenge to identify the molecular origin of polymeric materials’ macroscopic behavior under load. Recent successes in the development of novel optical force probes have proven that photochromic compounds, such as spiropyran, azobenzene, or hexaarylbiimidazole, can be employed as efficient molecular force sensors. Thus, the investigation of other photoswitches is bound to lead to improved understanding of the mechanochemical reaction pathway and promises access to superior optical force probes. With this project, I want to investigate the action of mechanical stress on diarylethene-type photoswitches incorporated into polymeric architectures. Not only will this work explore the unprecedented mechanical activation of pericyclic reactions involving a 6π-electron system thus considerably contributing to the understanding of these reactions’ pathways, but moreover try to establish diarylethenes as superior optical force probes that – contrarily to the popular spiropyran motif – are not thermally labile and as such can prospectively be used to quantify forces independent of time. Beyond their potential as superior force probes, these diarylethenes could moreover exhibit improved ring-opening efficiency under stress as compared to the generally inefficient photochemical pathway. This exploration of the interface of mechanical stress and light thus is highly likely to answer unsolved questions in each of the respective research areas and contribute considerably to their advancement.