Department of Physical Chemistry, University of Geneva, 30 quai Ernest-Ansermet, CH-1211 Geneva 4.
J Phys Chem A. 2012 Jul 19;116(28):7516-22. doi: 10.1021/jp3045548. Epub 2012 Jul 3.
The excited-state dynamics of a series of Wurster's salts (p-phenylenediamine radical cations) with different subtituents on the nitrogen atoms was investigated under a variety of experimental conditions using a combination of ultrafast spectroscopic techniques. At room temperature, the lifetime of the lowest excited state of all radical cations is on the order of 200 fs, independently of the solvent, that is, water, nitriles, alcohols, and room-temperature ionic liquid. On the other hand, all cations, except that with the bulky nitrogen substituents, become fluorescent below 120 K. The observed dynamics can be accounted for by the presence of a conical intersection between the D(1) and D(0) states. For the cations with a small nitrogen substituent, this conical intersection could be accessed through a twist of one amino group, as already suggested for Wurster's Blue. However, this coordinate cannot be invoked for the cation with bulky nitrogen subtituents, and more probably, pyramidalization of the nitrogen center and/or deformation of the phenyl ring play an important role. Consequently, the excited-state dynamics of these structurally very similar Wurster's salts involves different decay mechanisms.
用一系列带有不同取代基的 Wurster 盐（对苯二亚胺自由基阳离子）作为研究对象，在不同实验条件下，采用多种超快光谱技术相结合的方法，研究了它们的激发态动力学。在室温下，所有自由基阳离子的最低激发态的寿命都在 200fs 左右，与溶剂（水、腈、醇和室温离子液体）无关。另一方面，除了具有较大取代基的氮原子的阳离子之外，所有阳离子在 120K 以下都会发生荧光。观察到的动力学可以用 D(1)和 D(0)态之间存在的锥形交叉来解释。对于取代基较小的阳离子，这个锥形交叉可以通过一个氨基的扭曲来实现，这已经被提议用于 Wurster 的蓝色。然而，对于具有较大取代基的氮原子的阳离子，这个坐标不能被调用，更可能的是，氮中心的三角锥形化和/或苯环的变形起着重要作用。因此，这些结构非常相似的 Wurster 盐的激发态动力学涉及不同的衰减机制。
