Determination of dipole moments in ground and excited states of molecules in solution and in the gas phase

电子激发态的分子偶极矩是由激发引起的电子分布的变化决定的，因此提供了关于激发态性质的重要信息。目前存在用于中小分子的高分辨率光谱方法，可以非常精确地确定激发态偶极矩的大小和方向。这主要包括超音速射流中分子的振动强光谱。不幸的是，这种方法不能推广到大分子。一方面，许多大分子（超过20个重原子）无法在不分解的情况下转移到气相，而这对于分子束技术来说是必不可少的。此外，大分子的转动惯量也很大，使得转动跃迁不能再完全解决。然而，这对于解释电场中的旋转分辨光谱是必要的。由于这个原因，在过去的几十年里已经发展了许多方法，能够确定溶液中分子激发态的偶极矩信息。尽管从这些实验中可以追溯到Onsager， Lippert和Mataga的原始作品的信息有限，但他们已经发现了在溶剂变色变换形式中的广泛应用。这样做的原因是直观的概念的可理解性和容易可行性使用商业荧光和吸收光谱仪。另一方面，溶剂变色法存在一些根本性的问题。溶剂的变化导致与溶解分子的不同相互作用，这导致光谱位移严重偏离了根据Lippert和Mataga预测的线性关系。此外，通过Onsager半径计算的溶剂腔的尺寸包含在评估的第三次方中，因此，腔尺寸的轻微偏差导致偶极矩的结果差异很大。这两个问题都可以通过改变溶剂的极性函数来避免，而不是通过改变溶剂，而是通过改变单一溶剂的温度。与溶剂变色相比，可以实现的光谱位移较小，但另一方面，溶剂和溶剂之间的相互作用类型总是相同的。我们通过实验直接从溶液的密度来确定溶剂腔的体积，从而避免了难以确定（理论上的）Onsager半径的问题。有了这两种变化，溶剂极性函数和光谱位移之间的关系实际上是线性的，正如理论上假设的那样。

Molecular dipole moments in electronically excited states are determined by the change of the electron distribution due to the excitation and therefore provide important information about the nature of the excited states. High-resolution spectroscopic methods exist for small and medium-sized molecules, which allow both the magnitude and the direction of the dipole moment in the excited state to be determined with great accuracy. This primarily includes rovibronic strong spectroscopy of molecules in a supersonic jet. Unfortunately, this method cannot be extended to large molecules. On the one hand, many large molecules (more than 20 heavy atoms) cannot be transferred into the gas phase without decomposition, which is essential for molecular beam technologies. In addition, the moments of inertia of large molecules are also large, so that the rotational transitions can no longer be completely resolved. However, this is essential for the interpretation of the rotationally resolved spectra in the electric field. For this reason, many methods have been developed in the past decades that are able to determine information about the dipole moment in excited states of molecules in solution. Despite the limited information from these experiments that can be traced back to the original works by Onsager, Lippert and Mataga, they have found widespread use in the form of solvatochromic shifts. The reason for this is the intuitive comprehensibility of the concept and the easy viability using commercial fluorescence and absorption spectrometers. On the other hand, the method of solvatochromic shifts has fundamental problems. The variation of the solvent leads to differing interactions with the dissolved molecule, which results in spectral shifts that deviate strongly from a linear relationship that is predicted according to Lippert and Mataga. In addition, the size of the solvent cavity via the Onsager radius is included in the third power in the evaluation, so that slight deviations in the cavity size lead to widely varying results for the dipole moments. Both problems can be avoided by changing the solvent polarity function not by varying the solvent but by varying the temperature in a single solvent. The spectral shifts that can be achieved are smaller than in solvatochromism, but on the other hand the type of interaction between solvent and solvate is always the same. We circumvent the problem of the difficult to determine (theoretical) Onsager radius by directly determining the volume of the solvent cavities experimentally from the densities of the solutions. With these two variations, the relationship between the solvent polarity function and the spectral shifts is actually linear, as is assumed in theory.