Quantum Chemical Investigation of the Influence of Oriented External Electric Fields on the Mechanical Properties of Mechanophores in Polymers
Final Report Abstract
Oriented external electric fields (OEEFs) can be used to catalyze various chemical reactions, as they energetically favor zwitterionic resonance structures in the transition state. Furthermore, OEEFs change the geometries and dissociation energies of chemical bonds. The central hypothesis of the research project was that OEEFs can specifically influence the mechanical properties of molecules, which are the focus of the rapidly growing field of mechanochemistry. The aim was to show that an OEEF can reduce the force required to break a bond. To this end, the strength of the OEEF and its orientation to the mechanophore, i.e. a molecular subunit of a polymer that reacts to external force by undergoing significant geometric changes, were optimized. Quantum chemical methods were used in this project, as it is extremely difficult experimentally to precisely adjust the relative orientation of electric fields and molecules. The application of electric fields is a standard method in computational chemistry. Mechanical forces were simulated by the EFEI (External Force is Explicitly Included) method. This made it possible to calculate the force that must be used to rupture the molecule. At the beginning of the project, a quantum chemical method was identified through comparisons with exact reference values, which allowed the accurate and at the same time cost-efficient calculation of mechanical properties of molecules in strong electric fields. Subsequently, the forces required to rupture molecules in various known mechanochemical reactions were calculated in the presence of OEEFs with different strengths and orientations. The initial hypothesis that OEEFs reduce the activation force of mechanophores was confirmed by these calculations. Finally, external influences such as the system’s temperature on the mechanical properties of the mechanophore were investigated. The potential applications of combining OEEFs with mechanochemistry are intriguing: firstly, the simultaneous influence of OEEFs and mechanical force enables dual switching of the mechanical properties of the molecule, i.e., a controlled increase or decrease in the force required to break a bond in the polymer. This is particularly interesting in the context of mechanochromic materials, in which a color change occurs as soon as a characteristic minimum force is exerted. Furthermore, many self-healing polymers are based on the force-induced formation of radicals and the subsequent recombination of neighboring polymer strands. OEEFs, on the other hand, have already been used to convert typically homolytic to heterolytic bond breaks. The combination of OEEFs and mechanochemistry thus potentially opens up the possibility of switching the self-healing of polymers on or off.
Publications
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Investigating the accuracy of density functional methods for molecules in electric fields. The Journal of Chemical Physics, 159(12).
Scheele, Tarek & Neudecker, Tim
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Using oriented external electric fields to manipulate rupture forces of mechanophores. Physical Chemistry Chemical Physics, 25(41), 28070-28077.
Scheele, Tarek & Neudecker, Tim
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On the Interplay Between Force, Temperature, and Electric Fields in the Rupture Process of Mechanophores. ChemPhysChem, 25(22).
Scheele, Tarek & Neudecker, Tim
