Wise Adam J, Grey John K
Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, NM 87131, USA.
Polymers (Basel). 2016 Nov 3;8(11):388. doi: 10.3390/polym8110388.
Single molecule photoluminescence (PL) spectroscopy of conjugated polymers has shed new light on the complex structure⁻function relationships of these materials. Although extensive work has been carried out using polarization and excitation intensity modulated experiments to elucidate conformation-dependent photophysics, surprisingly little attention has been given to information contained in the PL spectral line shapes. We investigate single molecule PL spectra of the prototypical conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) which exists in at least two emissive conformers and can only be observed at dilute levels. Using a model based on the well-known "Missing Mode Effect" (MIME), we show that vibronic progression intervals for MEH-PPV conformers can be explained by relative contributions from particular skeletal vibrational modes. Here, observed progression intervals do not match any ground state Raman active vibrational frequency and instead represent a coalescence of multiple modes in the frequency domain. For example, the higher energy emitting "blue" MEH-PPV form exhibits PL maxima at 18,200 cm with characteristic MIME progression intervals of ~1200⁻1350 cm, whereas the lower energy emitting "red" form peaks at ~17,100 cm with intervals in the range of ~1350⁻1450 cm. The main differences in blue and red MEH-PPV chromophores lie in the intra-chain order, or, planarity of monomers within a chromophore segment. We demonstrate that the Raman-active out-of-plane C⁻H wag of the MEH-PPV vinylene group (966 cm) has the greatest influence in determining the observed vibronic progression MIME interval. Namely, larger displacements (intensities)-indicating lower intra-chain order-lower the effective MIME interval. This simple model provides useful insights into the conformational characteristics of the heterogeneous chromophore landscape without requiring costly and time-consuming low temperature or single molecule Raman capabilities.
共轭聚合物的单分子光致发光（PL）光谱为这些材料复杂的结构-功能关系提供了新的线索。尽管已经开展了大量工作，利用偏振和激发强度调制实验来阐明构象相关的光物理性质，但令人惊讶的是，人们对PL光谱线形中包含的信息关注甚少。我们研究了典型的共轭聚合物聚[2-甲氧基-5-(2-乙基己氧基)-1,4-亚苯基亚乙烯基]（MEH-PPV）的单分子PL光谱，该聚合物存在至少两种发光构象，且只能在稀释状态下观察到。使用基于著名的“缺失模式效应”（MIME）的模型，我们表明MEH-PPV构象的振动跃迁间隔可以通过特定骨架振动模式的相对贡献来解释。在此，观察到的跃迁间隔与任何基态拉曼活性振动频率均不匹配，而是代表了频域中多种模式的合并。例如，发射较高能量的“蓝色”MEH-PPV形式在18,200 cm处呈现PL最大值，特征MIME跃迁间隔为1200⁻1350 cm，而发射较低能量的“红色”形式在17,100 cm处达到峰值，间隔在1350⁻1450 cm范围内。蓝色和红色MEH-PPV发色团的主要差异在于链内有序性，即发色团段内单体的平面性。我们证明，MEH-PPV亚乙烯基的拉曼活性面外C⁻H摇摆（~966 cm）在确定观察到的振动跃迁MIME间隔方面影响最大。也就是说，更大的位移（强度）——表明链内有序性较低——会降低有效MIME间隔。这个简单的模型为异质发色团景观的构象特征提供了有用的见解，而无需昂贵且耗时的低温或单分子拉曼能力。
