Nguyen Kiet A, Pachter Ruth, Loftus Lauren M, Hong Gongyi, Day Paul N, Azoulay Jason D, Grusenmeyer Tod A
Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433, United States.
UES, Inc., Dayton, Ohio 45432, United States.
J Phys Chem A. 2024 Oct 24;128(42):9146-9158. doi: 10.1021/acs.jpca.4c04458. Epub 2024 Oct 11.
Narrow band gap donor-acceptor conjugated polymers present excellent paradigms in photonics and optoelectronics due to their chemical tunability, correlated electronic structures, and tunable open-shell electronic configurations. However, rational design for enhancing the properties of these molecular systems remains challenging. In this study, we employed density functional theory (DFT) calculations to investigate prototypical narrow band gap donor-acceptor conjugated oligomers, consisting of alternating cyclopentadithiophene (CPDT) donors paired with benzothiadiazole (BT), benzoselenadiazole (BSe), benzobisthiadiazole (BBT), and thiadiazoloquinoxaline (TQ) acceptors. Analyses of structures, singlet-triplet gaps, and absorption spectra of oligomers with up to ten repeat units have shown that when incorporating the BT, BSe, and TQ acceptors, the backbone curvature resulted in spiral structures that were energetically favored over their linear counterparts, causing differences in the calculated circular dichroism spectra. Oligomers with BBT-based acceptors preferred, however, a linear geometry, consistent with an open-shell electronic structure. Calculated singlet-triplet splittings demonstrated the importance of long chains and specific structures for consistency with the experiment, while effects of the solvent were also quantified. Based on the predicted low-energy conformations, one-photon absorption spectra for the considered oligomers have shown that using the Tamm-Dancoff approximation within time-dependent DFT for the large systems offers good agreement with the first absorption maxima in measured experimental spectra, thus validating the method for large donor-acceptor oligomers. Natural transition orbital analyses provided insights into the excited-state characteristics. Two-photon absorption maxima were accurately predicted, but the cross-sections were overestimated or underestimated, as dependent on the level of theory employed, to be addressed in future work.
窄带隙供体-受体共轭聚合物因其化学可调性、相关电子结构和可调的开壳电子构型，在光子学和光电子学领域展现出优异的范例。然而，合理设计以增强这些分子体系的性能仍然具有挑战性。在本研究中，我们采用密度泛函理论（DFT）计算来研究典型的窄带隙供体-受体共轭低聚物，这些低聚物由交替的环戊二噻吩（CPDT）供体与苯并噻二唑（BT）、苯并硒二唑（BSe）、苯并双噻二唑（BBT）和噻二唑并喹喔啉（TQ）受体配对组成。对具有多达十个重复单元的低聚物的结构、单重态-三重态能隙和吸收光谱的分析表明，当引入BT、BSe和TQ受体时，主链曲率导致螺旋结构在能量上比其线性对应结构更有利，从而导致计算的圆二色光谱出现差异。然而，具有基于BBT受体的低聚物更倾向于线性几何结构，这与开壳电子结构一致。计算的单重态-三重态分裂表明，长链和特定结构对于与实验结果一致的重要性，同时也对溶剂的影响进行了量化。基于预测的低能构象，所考虑的低聚物的单光子吸收光谱表明，对于大体系在含时DFT中使用Tamm-Dancoff近似与测量实验光谱中的第一个吸收最大值具有良好的一致性，从而验证了该方法对大供体-受体低聚物的有效性。自然跃迁轨道分析提供了对激发态特征的深入了解。准确预测了双光子吸收最大值，但截面根据所采用的理论水平被高估或低估，这将在未来的工作中加以解决。
