Department of Physical Chemistry, University of Geneva , 30 quai Ernest-Ansermet, CH-1211 Geneva 4, Switzerland.
J Phys Chem A. 2013 Dec 12;117(49):13112-26. doi: 10.1021/jp409646g. Epub 2013 Nov 27.
The excited-state dynamics of two donor-acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck-Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from a normal to an overcritical merocyanine evidenced by steady-state spectroscopy.
我们研究了两种供体-受体联苯的激发态动力学，它们在受体的强度上有所不同，一个是吡啶鎓部分，另一个是吡喃鎓部分，使用稳态溶剂化吸收、超快荧光以及可见和红外瞬态吸收光谱的组合进行研究。吡啶鎓酚盐的负溶剂化颜色表明，在 S1 ← S0 激发时，永久偶极矩减小，这意味着基态比激发态具有更多的两性离子特征。相比之下，吡喃鎓酚盐表现出微弱的正溶剂化颜色，对应于激发时偶极矩的微小增加，这意味着激发态比基态具有更多的两性离子特征。因此，这两种化合物都位于与氰基限制结构不同的两侧，该结构具有相同极性的基态和激发态。尽管存在这些差异，这两种分子都表现出定性相似的激发态性质。它们的荧光寿命很短，从大约 1 到 20 皮秒不等，当溶剂粘度从 0.4 到 11 厘泊变化时。然而，这两种化合物之间存在特征差异：吡喃鎓染料的激发态寿命较短，并且也在一定程度上依赖于极性。光谱数据的总体可以用一个模型来解释，在该模型中，发射 Franck-Condon 激发态在围绕芳基单元之间的单键扭曲时松弛，直到激发态和基态表面非常接近或相交。在内部转换回基态后，扭曲的分子以很大程度上依赖于溶剂粘度的方式松弛回其平衡平面构型。然而，在这种情况下，吡喃鎓染料的动力学比吡啶鎓染料慢，极性溶剂诱导的加速比激发态强得多。这种两种化合物之间动力学行为的差异是由稳态光谱证明的电子结构从正常到超临界的变化直接导致的。
