Conformation and dynamics of conjugated polymers: A single-molecule study

我们使用单分子光谱学来研究分散在旋涂薄膜基质中的共轭聚合物MEH-PPV的构象和相关的光物理。通过聚合物链的吸收各向异性（吸收椭球比r），使用一种新的荧光显微镜方法测量构象。该方法是基于交替激发的样品与远场落射荧光和近场全内反射模式。从个别分子的偏振调制荧光与理论表达式拟合，以获得每个链的吸收椭球的形状和取向。一个粗粒分子动力学（MD）进一步被用来模拟对应于测量的吸收椭球的实际构象。我们发现，MEH-PPV的单个链可以达到一种形式的一个崩溃的构象在不良溶剂矩阵（Zeonex），或可以共存于两种形式的扩展构象在良好的溶剂矩阵（PS）。的 ...更多信息 当考虑溶剂、化学缺陷和旋涂条件等因素时，分子动力学模拟完全再现了实验结果。Zeonex中观察到的缺陷圆柱体构象解释了发射间歇性和荧光光谱分布的光物理性质。在PS中观察到的两种缺陷线圈型构象与两种类型的双折射行为和观察到的双峰光谱分布相对应。分子动力学模拟结果的矩分析提供了两种构象形式和不同的物理性质之间的联系。MEH-PPV的工作进一步补充了由聚噻吩主链和聚苯乙烯支链组成的接枝共聚物的研究。分支的存在防止分子形成塌陷的球状构象。具有接枝PS支链的聚噻吩（PT-PS）和不具有接枝PS支链的聚噻吩（PT-R）显示出明显不同的物理性质。由于光漂白，PT-PS分子的发射强度逐渐衰减。在PT-R中，在一步光漂白发生之前，发射强度在背景和一个强度水平之间振荡。PT-R聚合物的闪烁和漂白行为是由于链的发射从一个或几个共轭链段进行，其中能量在吸收后迅速定位。在PT-PS的情况下，发射强度的连续降低是每个片段独立发射的结果。这两种聚合物的发射特性的进一步特征在于通过测量单分子发射各向异性比，其直方图显示峰值在0各向异性为PT-PS和最小值在0为PT-R，并通过研究单个分子的荧光光谱的时间演化。少

We use single-molecule spectroscopy to study the conformation and related photophysics of the conjugated polymer MEH-PPV dispersed in spin-coated thin-film matrices. The conformation is measured via the polymer chain absorption anisotropy (absorption ellipsoid ratio r) using a novel fluorescence microscopic method. The method is based on alternative excitation of the sample with far-field epi-fluorescence and near-field total internal reflection modes. Polarization modulated fluorescence from individual molecules is fitted with theoretical expressions to obtain the shape and orientation of the absorption ellipsoid for each chain. A coarse-grain molecular dynamics (MD) is further used to simulate the actual conformations corresponding to the measured absorption ellipsoids. We found that MEH-PPV individual chains can attain a single form of a collapsed conformation in poor-solvent matrices (Zeonex), or can coexist in two forms of extended conformations in good-solvent matrices (PS). The … More MD simulations completely reproduce the experimental results when the factors of solvent, chemical defects and spin-coating conditions are included. The defect cylinder conformation observed in Zeonex explains the photophysical properties of emission intermittency and fluorescence spectra distributions. The two defect-coil-type conformations observed in PS correspond well with the two types of intermittency behavior and with the bi-modal spectral distributions observed. Moment analysis of the MD simulation results provides a link between the two conformation forms and the different photophysical properties. The work on MEH-PPV is further complemented by the study of a graft copolymer consisting of a polythiophene backbone and polystyrene branches. The presence of the branches prevents the molecule from forming a collapsed globule-like conformation. Polythiophenes with grafted PS branches (PT-PS) and without the branches (PT-R) show markedly different photophysical properties. Emission intensity of the PT-PS molecules decays gradually due to photobleaching. In PT-R the emission intensity oscillates between the background and one intensity level before one-step photobleaching takes place. The blinking and bleaching behavior of the PT-R polymer is due to the fact that emission of the chain proceeds from one or a few conjugated segments where the energy is quickly localized after absorption. The continuous decrease of emission intensity in case of PT-PS is a result of each segment emitting independently. The emission properties of the two polymers are further characterized by measuring the single molecule emission anisotropy ratio, the histogram of which shows a peak at 0 anisotropy for PT-PS and minimum at 0 for PT-R, and by studying the time-evolution of fluorescence spectra of individual molecules. Less