Atomistic Modeling of PEDOT:PSS Complexes II: Force Field Parameterization

The conductive polymer complex poly­(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is integral to many next-generation polymeric devices. All-atom molecular dynamics (MD) is increasingly employed to probe film microstructure, but the commonly used Generalized AMBER Force Field (GAFF) parameters remain unvalidated for this system. Here, we develop all-atom force fields for both undoped and highly doped PEDOT compatible with GAFF. Molecular geometries, vibrational energies, torsional profiles, and conformational energetics calculated with density functional theory comprise the training data. The optimized force fields improve upon GAFF for all the training data, including notable improvements in torsional barriers, and capture the finite size of charge carriers in doped PEDOT. Although GAFF predicts similar behavior between undoped and doped polymers, our models identify meaningful differences in ground-state geometry, vibrational spectra, and torsional barriers between the two. Furthermore, vacuum MD simulations show that the neutral polymer is more flexible than the highly doped polymer, primarily due to the weaker inter-monomer dihedral potentials. The foregoing trends are directly attributable to the aromatic-to-quinoid transition PEDOT experiences upon doping. This coupling between doping and molecular flexibility highlights the importance of charge–geometry interactions in atomistic modeling and should be considered while modeling conjugated polymers beyond PEDOT.聚(3,4-乙烯二氧噻吩)-聚苯乙烯磺酸（poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, PEDOT:PSS）作为一种导电高分子复合物，是众多下一代高分子器件的核心组成部分。全原子分子动力学（all-atom molecular dynamics, MD）正愈发广泛地用于探究薄膜微观结构，但目前常用的通用AMBER力场（Generalized AMBER Force Field, GAFF）在该体系下的参数尚未得到验证。本研究开发了两套适配通用AMBER力场的全原子力场，分别对应未掺杂与高掺杂态的PEDOT。本研究以密度泛函理论（density functional theory, DFT）计算得到的分子几何结构、振动能量、扭转势能面与构象能作为训练数据集。经优化后的力场在全部训练数据集上均优于通用AMBER力场，其中在扭转势垒方面的提升尤为显著，同时能够准确捕捉掺杂态PEDOT中载流子的有限尺寸效应。尽管通用AMBER力场对掺杂与未掺杂态高分子的预测行为相近，但本研究的模型能够识别出二者在基态几何结构、振动光谱以及扭转势垒上的显著差异。此外，真空环境下的分子动力学模拟结果表明，中性态PEDOT相较于高掺杂态具有更高的分子柔性，这主要源于单体间二面角势能更弱。上述趋势可直接归因于PEDOT在掺杂过程中发生的芳香环-醌式结构转变。掺杂与分子柔性之间的这种耦合效应，凸显了电荷-几何相互作用在原子级建模中的重要性，该结论对于除PEDOT之外的共轭高分子建模同样具有参考价值。