MATERIALS SCIENCE AND CHEMISTRY

Introduction

Within the project “Ab-initio investigations of properties and reactions at interfaces” numerous scientific investigations based on density functional theory (DFT) could be realized over many years with the support of the John von Neumann Institute for Computing (NIC) by providing computing time at the Jülich Supercomputing Centre (JSC). As an example, we here summarize a work about reversible opening of chemical bonds by mechanical forces which was conducted in the final period of the project.

Anthracene molecules have the interesting property that they can reversibly form dimers. Two anthracene molecules bond chemically upon exposure to UV-light. The two chemical carbon-carbon bonds that form at the centers of the anthracene molecules break upon thermal activation, i.e., at elevated temperature. This property can be used to synthesize polymers with controllable rheological properties. By functionalizing polymers with anthracene, the degree of cross-linking between the polymer macromolecules can be varied. Anthracene molecules attached at different polymer chains can form dimers upon stimulation by UV-light and thus cross-link two adjacent polymer chains. More cross-linked polymer sites lead to a stiffening of the material. Experimental partners from the Fraunhofer Cluster of Excellence Programmable Materials CPM could demonstrate this working principle with the example of anthracene- functionalized polydimethylsiloxane (PDMS). PDMS is a silicone oil and by the addition of anthracene functional groups the oil viscosity could be successfully varied by UV-light. Moreover, a decline of viscosity was observed after the application of mechanical load in the form of shearing in rheological experiments. Because this behavior could be reversibly reproduced for many cycles, it can be assumed that reversible bond breaking of the anthracene dimers are responsible for the mechanically induced decrease in viscosity.

Results and Methods

To test this hypothesis DFT simulations of ester-functionalized anthracene dimers exposed to an external mechanical force were performed. The ester functionalization was chosen because the addition of anthracene molecules to the PDMS chains was achieved via ester functional groups in the experiments. The mechanical load acting on the molecule dimer was gradually increased by geometric constraints on the anthracene functional groups, the so-called constrained geometry simulates forces (COGEF) method. Surprisingly, these simulations predicted a very high stability of the two bonds holding the anthracene molecules together. Instead, an irreversible breaking of a carbon-oxygen bond of the ester group was found at odds with the experimental findings. A main shortcoming of the COGEF method is that it does not account for temperature, i.e., it predicts the behavior of molecular systems at a temperature of 0 K (-273.15°C). In order to take into account temperature effects, the energy barriers for the transition between the dimer state and the state with individual anthracene molecules were determined when the dimer was strained by external forces to mimic the mechanical load in the shearing experiments. The energy barrier of chemical reactions is the quantity that determines how likely the reaction will happen in a given period of time and for a given temperature. The smaller the barrier and the higher the temperature, the more likely and thus the faster the reaction occurs. Our simulations showed that the breaking of the anthracene dimer is a complex process with the subsequent breaking of the two involved carbon-carbon bonds. But most importantly, our simulations revealed that the energy barrier for the reversibly breaking of the two carbon-carbon bonds holding the anthracene dimer together is considerably lower than for the irreversible breaking of the carbon-oxygen bond, except at extremely high external forces. This indicates that at finite temperatures the reversible dissociation of the anthracene dimer will dominate.

Overall, the simulations could successfully explain the reversible mechanical switching of the anthracene functionalized PDMS silicone oil by exploring the underlying mechanism of the mechanochemical anthracene dimer dissociation. Moreover, this work demonstrates that care is required when extrapolating results obtained under quasistatic conditions, i.e., for temperatures of 0 K to thermally activated processes.

The quantum mechanical simulations based on DFT required computing resources that exceeded internal capacities. Therefore, computing time provided at the supercomputer Juwels by the Jülich Supercomputing Centre (JSC) were crucial to enable this work with implications for the fundamental study of mechanical properties and the design of programmable materials.

Publications

The scientific details of this project were published in the project's final report, as well as in The journal of Physical Chemistry Letters:

Michael Walter, Dominic Linsler, Tobias König, Chris Gäbert, Stefan Reinicke, Michael Moseler,

Leonhard Mayrhofer, Mechanochemical Activation of Anthracene [4+4] Cycloadducts, J. Phys. Chem. Lett. 14, 1445-1451 (2023).

For further information please click here: https://pubs.acs.org/doi/abs/10.1021/acs.jpclett.2c03493