共轭聚合物单分子构象和能量转移特性研究

The role of chain conformation in energy transfer properties of single conjugated polymer molecule

利用基于宽场显微光学系统的单分子散焦成像技术测量了不同构象poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole](PFO-DBT)共轭聚合物单分子的光物理与动力学特性.通过分析共轭聚合物单分子的荧光轨迹和对应的发射偶极取向变化识别共轭聚合物单分子发光单元,发现延伸构象下的单分子呈现多发色团发光特性,而折叠构象下的单分子保持高效链间能量转移,呈现单个发色团发光特性.共轭聚合物单分子构象对能量转移效率的影响可用于研究基于共轭聚合物的光电器件和分子器件.

Study of the relationship between conformation and photophysics of individual π-conjugated polymer chain is one of the most important problems in polymer nanoscience and nanotechnology, which will facilitate the application of conjugated polymer in a range of electronic devices such as organic field-effect transistors, light-emitting diodes, and solar cells. Single-molecule spectroscopy has emerged as a powerful tool to unravel structure and dynamic heterogeneities that are hidden in ensemble average. Identification of the emitting segments through fluorescence of single conjugated polymer molecules and their dependence on the conformation can help reveal the mechanism and the extent of energy transfer process in a single polymer chain. In this paper, the photophysical properties of individual poly[2, 7-（9, 9-dioctylfluorene）-alt-4, 7-bis（thiophen-2-yl） benzo-2, 1, 3-thiadiazole]（PFO-DBT） conjugated polymer molecules are measured based on the defocused wide-field microscopy of single molecules. The single PFO-DBT molecules are prepared on cleaned glass coverslips by spin-coating solution of poly[methyl methacrylate]（PMMA） containing 1×10-9 mol/L PFO-DBT molecules in chloroform and toluene, respectively. Defocused imaging of single conjugated polymer molecule is performed based on a wide-field fluorescence microscope system. The change of defocused patterns of individual polymer chain maps the angular distribution of emitted chromophore and thus the emitting dipole orientation. Fluorescence trajectory and corresponding emission dipole moments of single conjugated polymer molecules are analyzed to identify the emitting conjugated segments. It is found that single PFO-DBT conjugated polymer molecules prepared by chloroform solvent show extended conformation. The intrachain energy transfer is dominant in the single conjugated polymer molecules that take extended conformation, which leads to photophysical properties of multiple chromophores. In contrast, single PFO-DBT conjugated polymer molecules prepared by toluene solvent hold folded conformation, which exhibit emission from single chromophore due to efficient interchain energy transfer. The emitting chromophore is not constant in a single PFO-DBT conjugated polymer molecule with folded conformation. About 35% of the single conjugated molecules prepared with toluene show only one constant emitting chromophore before photobleaching. However, about 65% of single conjugated polymer molecules prepared with toluene show two or more sequencely emitting chromophores. It can be concluded that the energy transfer properties of single PFO-DBT conjugated polymer molecule is greatly dependent on the conformation, which can be reflected in its photophysical properties. The study on the influence of single conjugated polymer conformation on energy transfer efficiency can provide the reference for the preparation and performance of optoelectronic devices and molecular devices based on conjugated polymer.