Amthor Stephan, Lambert Christoph, Dümmler Stefan, Fischer Ingo, Schelter Jürgen
Institut für Organische Chemie, Bayerische Julius-Maximilans-Universität Würzburg, Am Hubland, D-97074 Würzburg.
J Phys Chem A. 2006 Apr 20;110(15):5204-14. doi: 10.1021/jp056728p.
We investigated the spectroscopic properties of a series of four bistriarylamine donor-pi-bridge-donor D-pi-D compounds (dimers), composed of two asymmetric triarylamine chromophores (monomers). UV/vis, fluorescence, and transient absorption spectra were recorded and compared with those of the corresponding D-pi monomers. Bilinear Lippert-Mataga plots indicate a major molecular reorganization of the excited state in polar media for all compounds. The excited states of the dimers are described as mixed-valence states that show, depending on the chemical nature of the pi bridge, a varying amount of interactions (couplings). We found that superradiant emission, that is, an enhancement of the fluorescence rate in the dimer, is observed only in the case of weak and medium coupling. Whether the first excited-state potential energy surface of the dimers is described by single minimum or a double minimum potential depends on the solvent polarity and the electronic coupling. In the latter case, the dimer relaxes in a symmetry broken CT state with partial positive charge at the triarylamine donor and negative charge at the pi bridge. The [2.2]paracyclophane bridged dimer is an example of a weakly coupled system because the spectroscopic behavior is very similar to the corresponding p-xylene monomer. In contrast, anthracene as well as p-xylene bridges mediate a stronger coupling and reveal a significant cooperative influence on the optical properties.
我们研究了一系列由两个不对称三芳基胺发色团（单体）组成的四个双三芳基胺供体-π-桥-供体D-π-D化合物（二聚体）的光谱性质。记录了紫外/可见光谱、荧光光谱和瞬态吸收光谱，并与相应的D-π单体的光谱进行了比较。双线性Lippert-Mataga图表明，在极性介质中，所有化合物的激发态都发生了主要的分子重排。二聚体的激发态被描述为混合价态，根据π桥的化学性质，其相互作用（耦合）量不同。我们发现，只有在弱耦合和中等耦合的情况下才会观察到超辐射发射，即二聚体中荧光速率的增强。二聚体的第一激发态势能面是由单极小值还是双极小值势能来描述，取决于溶剂极性和电子耦合。在后一种情况下，二聚体在对称破缺的电荷转移（CT）态中弛豫，三芳基胺供体带有部分正电荷，π桥带有负电荷。[2.2]对环芳烷桥连的二聚体是弱耦合体系的一个例子，因为其光谱行为与相应的对二甲苯单体非常相似。相比之下，蒽以及对二甲苯桥介导更强的耦合，并对光学性质显示出显著的协同影响。
