Polymer mechanochemistry with force-modulated excited state mechanochromism

• Applicants: Professor Dr. Robert Göstl; Professor Dr. Alexander Kühne
• Subject Area: Preparatory and Physical Chemistry of Polymers ; Analytical Chemistry ; Organic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 527252926

The mechanical deformation of a polymer material typically starts with the macroscopic application of force and leads to conformational, configurational, and constitutional covalent and non-covalent bond rearrangements on the molecular level. The entire process of deformation thus covers around seven to ten orders of magnitude in length scale, rendering its analysis and understanding a conceptual and instrumental challenge. A comprehensive insight into the mechanical properties of a polymer material requires suitable analytical tools and techniques that allow collecting and mapping of mechanical information across these scales. In the context of polymer mechanochemistry, the force-induced activation of a latent molecular motif can be tailored to cause a detectable change in the optical properties, rendering the motif an optical force probe (OFP). There is a strong need for new OFPs with precisely designed optical responses, to overcome fundamental detection limitations, such as optical orthogonality and complementarity that are currently still in place. For example, further shifting of the excitation and emission wavelengths into the red and IR region is a standing challenge. In addition, a rational approach toward fluorescence lifetime-based OFPs is unknown. More generally, the fundamental question how force affects the excited state landscape in OFPs in the context of polymer mechanochemistry is unanswered, yet will be a prerequisite for future rational OFP developments. The goal of this project is to develop spectrally modulated annihilators and sensitizers for triplet-triplet annihilation photon-upconversion (TTA UC) as well as hot exciplex emitters for lifetime modifications that respond to the force-induced alteration of the energy levels of triplet and charge transfer (CT) excited states. First, this will modularize the available annihilators and sensitizers for force-induced TTA UC and shifted locally excited (LE) to CT emission. Secondly, the unprecedented mechanochemical activation of triplet sensitizers and CT exciplex to LE bridge emitters will be demonstrated. Together, these individual force-activated photophysically active moieties will enable their combination in polymer materials to understand fracture in complex, non-uniform environments. Specifically, the spectral features of TTA UC will allow the implementation of conjunctive fracture analysis, which would be difficult to achieve using regular OFPs. Concomitantly, lifetime-based OFP analysis will allow the use of non-intensity-based microscopy (FLIM) and allow additional insight into the local microenvironment of the fracture zone.

• DFG Programme: Research Grants