Efficient Method for Achieving Electronic Coupling in TEMPO/TEMPO+ System

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 (ORBs). The study employs a combination of classical molecular dynamics (MD) and various electronic coupling calculation schemes. Within the con- text of the generalized Mulliken-Hush (GMH) method, the electronic couplings are investigated via the complete active space self-consistent field (CASSCF) approach, in combination with n-eletron valence state perturbation theory (NEVPT2), to provide an accurate description of both static and dynamic electron correlation as well as using (time-dependent) density functional theory (TD–DFT) simulations. Furthermore, the electronic communication between redox-active sites is studied using the cost-efficient DFT–based frontier molecular orbital (FMO) approach. Our study reveals the de- pendence of the electronic coupling on the distance and the relative orientation of the redox pairs (TEMPO and TEMPO+ ). Apart from the expected exponential dis- tance dependence, we found a pronounced orientation dependence with coupling values varying up to 0.2 eV, especially at short distances. Our findings underline the signifi- cance of including dynamic correlation in the electronic coupling method, which, unlike CASSCF method, can be included via TDDFT–based GMH methods. Additionally, 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 coupling in TEMPO–TEMPO+ systems.