Stangl Thomas, Wilhelm Philipp, Remmerssen Klaas, Höger Sigurd, Vogelsang Jan, Lupton John M
Institut für Experimentelle und Angewandte Physik, Universität Regensburg, 93053 Regensburg, Germany;
Kekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, 53121 Bonn, Germany.
Proc Natl Acad Sci U S A. 2015 Oct 13;112(41):E5560-6. doi: 10.1073/pnas.1512582112. Epub 2015 Sep 28.
An appealing definition of the term "molecule" arises from consideration of the nature of fluorescence, with discrete molecular entities emitting a stream of single photons. We address the question of how large a molecular object may become by growing deterministic aggregates from single conjugated polymer chains. Even particles containing dozens of individual chains still behave as single quantum emitters due to efficient excitation energy transfer, whereas the brightness is raised due to the increased absorption cross-section of the suprastructure. Excitation energy can delocalize between individual polymer chromophores in these aggregates by both coherent and incoherent coupling, which are differentiated by their distinct spectroscopic fingerprints. Coherent coupling is identified by a 10-fold increase in excited-state lifetime and a corresponding spectral red shift. Exciton quenching due to incoherent FRET becomes more significant as aggregate size increases, resulting in single-aggregate emission characterized by strong blinking. This mesoscale approach allows us to identify intermolecular interactions which do not exist in isolated chains and are inaccessible in bulk films where they are present but masked by disorder.
“分子”一词的一个引人注目的定义源于对荧光本质的思考，离散的分子实体发射出单光子流。我们通过从单个共轭聚合物链生长确定性聚集体来探讨分子物体可以变得多大的问题。由于有效的激发能量转移，即使包含数十条单链的粒子仍表现为单量子发射器，而超结构吸收截面的增加导致亮度提高。激发能量可以通过相干和非相干耦合在这些聚集体中的单个聚合物发色团之间离域，这两者通过其独特的光谱指纹来区分。相干耦合通过激发态寿命增加10倍和相应的光谱红移来识别。随着聚集体尺寸的增加，由于非相干荧光共振能量转移导致的激子猝灭变得更加显著，导致单聚集体发射具有强烈闪烁的特征。这种中尺度方法使我们能够识别孤立链中不存在且在本体薄膜中无法获取的分子间相互作用，在本体薄膜中它们存在但被无序掩盖。
