Wang Chun I, Hsu Chih H, Hua Chi C
Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 62102, Taiwan, Republic of China.
Phys Chem Chem Phys. 2017 Aug 9;19(31):20818-20828. doi: 10.1039/c7cp03415f.
For π-conjugated polymers, the notion of spectroscopic units or "chromophores" provides illuminating insights into the experimentally observed absorption/emission spectra and the mechanisms of energy/charge transfer. To date, however, no statistical analysis has revealed a direct correspondence between chromophoric and conformational properties-with the latter being fundamental to polymer semiconductors. Herein, we propose a "persistence length" calculation to re-evaluate chain conformation over a full conjugation length. The mesoscale condensed systems of MEH-PPV and MEH-PPV/C hybrid (system size ∼10 × 10 × 10 nm) are utilized as two prototypical model systems, along with a full range of segmental lengths (2-20-mer) and five lowest singlet excited states to hint at the generality of the features presented. We demonstrate, for the first time, that two properly re-defined conformational factors that characterize chain folding and planarity, respectively, capture excellently the population distribution of chromophores in both systems investigated. In contrast, the conventional strategy of utilizing two adjacent monomer units to characterize (local) chain conformation results in only an inconspicuous correlation between the two, as previously reported. It is further shown that chain folding-and not chain planarity-is more relevant in capturing the associated oscillator strength for the first excited state, where the transient dipole moments are known to align with the chain conformation, although the corresponding excitation energy and exciton size seem relatively unaffected. The observed effects of C on the MEH-PPV adsorption spectra also agree with recent experimental trends. Overall, the present findings are expected to aid future multiscale computer simulations and spectroscopy-data interpretations for polymer semiconductors and their hybrid systems.
对于π共轭聚合物，光谱单元或“发色团”的概念为实验观察到的吸收/发射光谱以及能量/电荷转移机制提供了富有启发性的见解。然而，迄今为止，尚无统计分析揭示发色团性质与构象性质之间的直接对应关系——而后者对于聚合物半导体至关重要。在此，我们提出一种“持久长度”计算方法，以在整个共轭长度上重新评估链构象。MEH-PPV和MEH-PPV/C杂化体系（体系尺寸约为10×10×10 nm）的中尺度凝聚系统被用作两个典型模型系统，同时考虑了全范围的链段长度（2 - 20聚体）和五个最低单重激发态，以暗示所呈现特征的普遍性。我们首次证明，分别表征链折叠和平面性的两个经过适当重新定义的构象因子，能够出色地捕捉所研究的两个系统中发色团的数量分布。相比之下，如先前报道的那样，利用两个相邻单体单元来表征（局部）链构象的传统策略仅导致两者之间存在不明显的相关性。进一步表明，在捕捉第一激发态的相关振子强度方面，链折叠比链平面性更重要，已知瞬态偶极矩与链构象对齐，尽管相应的激发能和激子尺寸似乎相对不受影响。观察到的C对MEH-PPV吸收光谱的影响也与最近的实验趋势一致。总体而言，本研究结果有望有助于未来对聚合物半导体及其杂化系统进行多尺度计算机模拟和光谱数据解释。
