Toward robust electronic coupling predictions in redox-active TEMPO/TEMPO+ systems

This research elucidates the intricate nature of electronic coupling in the redox-active (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), commonly utilized in organic radical batteries. This study employs a combination of classical molecular dynamics and various electronic coupling calculation schemes. Within the context of the generalized Mulliken-Hush method, the electronic couplings are investigated via the complete active space self-consistent field approach, in combination with n-electron valence state perturbation theory, to provide an accurate description of both static and dynamic electron correlation as well as using (time-dependent) density functional theory simulations. Furthermore, the electronic communication between redox-active sites is studied using the cost-efficient density functional theory (DFT)-based frontier molecular orbital (FMO) approach. Our study reveals the dependence of the electronic coupling on the distance and the relative orientation of the redox pairs (TEMPO and TEMPO+). Apart from the expected exponential distance dependence, we found pronounced orientation dependence, with coupling values varying up to 0.2 eV, which is reflected by a substantial basis set dependency of the couplings, in particular at short distances. In addition, our study highlights the limitations of the DFT-based FMO method, in particular at short intermolecular distances between the redox-active sites, which may lead to a mixing of the involved molecular orbitals. This comparison will provide us with the most cost-accuracy-effective method for calculating electronic couplings in TEMPO-TEMPO+ systems.