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.

本研究探索了约束密度泛函理论（constrained density functional theory）在计算乙腈（MeCN）溶剂中2,1,3-苯并噻二唑（BTZ）——一种极具应用前景的氧化还原介质——的电荷转移参数方面的应用。BTZ分子曾被作为阳极电解液应用于氧化还原液流电池，其中电荷转移是至关重要的核心过程。鉴于从头算（ab initio）电荷转移参数在预测电子转移速率与构效关系方面的准确性，对其开展模拟具有极高的学术价值。
本研究针对该过程的电荷转移模拟展开了前沿性探索。研究采用约束密度泛函理论计算，预测了电荷转移自由能与电子耦合强度，二者是基于马库斯（Marcus）理论评估电荷转移过程的关键参数。模拟结果表明，电子耦合强度随时间快速波动，且与供体-受体态能量差（反应间隙能）密切相关。基于对马库斯理论的评估，我们预测在乙腈溶剂中浓度为1 M时，BTZ的自交换反应速率常数约为0.5 M⁻¹·s⁻¹。
本研究证实了约束密度泛函理论可用于深入解析电荷转移过程的物理机制，同时也揭示了当前在计算与算法层面的局限：难以在分子动力学模拟中实现理想的体系规模与各态历经性水平。
本研究的重要结论之一是，如本文所述，基于反应间隙能的电子耦合时间依赖性采样，对于未来电荷与电子转移过程的精准预测至关重要。