Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, SE-10691 Stockholm, Sweden.
Proc Natl Acad Sci U S A. 2010 Sep 21;107(38):16453-8. doi: 10.1073/pnas.1006572107. Epub 2010 Sep 7.
We have investigated the dependence of the static first hyperpolarizability on the bond-length alternation (BLA) parameter. Our analysis indicates that the validity of the first hyperpolarizability/BLA parameter relationship is restricted to the no-field, vacuum, limit, while it successively breaks down along with increasing polarity of a surrounding medium, becoming invalid, for instance, in an aqueous solution. This contention is based on a series of TD-DFT, TD-DFT/PCM and hybrid TD-DFT/MM calculations of the first hyperpolarizability for a set of molecular configurations generated from Car-Parrinello hybrid QM/MM simulations of the stilbazolium merocyanine chromophore in chloroform and water solvents, and on a rationalization by means of the two-state model for the first hyperpolarizability. The BLA dependence of the three parameters entering the two-state model is shown to be qualitatively different in vacuum and in solvents. Particularly, in the vacuum case, the difference between ground and excited state dipole moments goes to zero for a vanishing BLA, which is not true in the presence of an aqueous medium. In the aqueous medium, an opposing behavior of the parameters involved in the two-state model results in an almost constant first hyperpolarizability with varying BLA parameter.
我们研究了静态第一超极化率与键长交替（BLA）参数的关系。我们的分析表明，第一超极化率/BLA 参数关系的有效性仅限于无场、真空极限，而随着周围介质极性的增加，它会逐渐失效，例如在水溶液中。这一论点是基于一系列对 stilbazolium 甲川菁发色团在氯仿和水溶剂中的 Car-Parrinello 杂化 QM/MM 模拟生成的分子构象的第一超极化率的 TD-DFT、TD-DFT/PCM 和混合 TD-DFT/MM 计算，以及通过第一超极化率的两态模型进行合理化。两态模型中三个参数的 BLA 依赖性在真空和溶剂中表现出定性的差异。特别是在真空情况下，对于零 BLA，基态和激发态偶极矩之间的差异趋于零，而在存在水介质的情况下则不是这样。在水介质中，两态模型中涉及的参数的相反行为导致第一超极化率随 BLA 参数的变化几乎保持不变。
