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

• Applicant: Professor Dr. Michael Schmitt
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 456557001

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.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Thomas J. J. Müller