Dynamic Electronic Coupling in the Self-Exchange Charge Transfer Reaction of Benzothiadiazole Redoxmer in Acetonitrile Calculated with Constrained Density Functional Theory

In this work, we explore the use of constrained density functional theory for the calculation of the charge transfer parameters of 2,1,3-benzothiadiazole (BTZ), a promising redoxmer, in acetonitrile (MeCN) solvent. The BTZ molecule has been studied as an anolyte in redox flow batteries, where charge transfer is a crucial process. It is highly desirable to simulate ab initio charge-transfer parameters, given their accuracy in predicting electron-transfer rates and structure-activity relationships. This work explores the state of the art in charge-transfer simulation for this process. Constrained density functional theory (DFT) calculations are used to predict charge-transfer free energies and electronic couplings, which are crucial for evaluating charge transfer within the Marcus theory. Based on the simulations, we find that electronic coupling fluctuates rapidly with time and also depends on the difference between the donor and acceptor state energies (reaction gap energy). Based on our evaluation of Marcus theory, BTZ has a predicted self-exchange reaction rate constant on the order of 0.5 M^-1 s^-1 at 1 M concentration in MeCN. Our work demonstrates the utility of constrained DFT for providing physical insight into a charge-transfer process, while also highlighting current limitations in computational and algorithmic capacity in achieving desirable system sizes and levels of ergodicity in molecular dynamics simulations. A significant conclusion of this work is that time-dependent sampling of electronic coupling as a function of the reaction gap energy, as described herein, is essential for future predictions of charge and electron transfer.