Determination of the energetic position of electronically excited states, their structures and life times

• Applicant: Professor Dr. Michael Schmitt
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2010 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 171864351

Permanent dipole moments of isolated molecular systems in their ground and electronically excited states will be determined using electronic Stark spectroscopy. We aim to shed light on various problems. First, we seek to understand the influence of the substitution position of different ligands with varying electronic effects on the chromophore. Absorption of a photon may lead to large changes of the dipole moments, depending on the excited state. The dipole moments of the lowest excited singlet states differ considerably due to their differing electron distribution. Thus, the electronic nature of these states can be elucidated from the size and direction of the dipole moments.Apart from permanent dipole moments the size and directions of the transition dipole moment are important for the classification of electronically excited states. The direction of the transition dipole moment is important for Förster energy transfer, in cases, in which the orientations of the donor and acceptors are not distributed statistically through flexible linkers. Spectroscopic techniques, which are capable of determining the direction of the transition dipole moment in general work with oriented molecules. The orientation does not have to be macroscopic, as quantum interference effects of torsions about molecule fixed axes with the transition dipole moment axis show. We propose an experiment in which the oriented reference system are the axes of the permanent dipole moments in both electronic states. Simulations of the spectra with filed strength of up to 10 kV/cm as can be reached in the new set-up show that the effect is large enough to be observed.Utilization of linearly polarized light in a perpendicular arrangement enables us to switch between different sets of selection rules. We could show, how this enhances the accuracy of dipole moment determinations. Exchange of the half wave plate used to rotate the linearly polarized light by a quarter wave plate we are able to switch between right and left circular polarized light. They show different selection rules in the Stark spectra and allow for a differentiation of enantiomers in the Stark spectra.Finally, electronic Stark spectroscopy will be used to determine the dipole moments of different push-pull systems. We will put the main attention to systems in which donor and acceptor are located in different covalently bridged chromophores. These systems may act as molecular switches through their large dipole moment changes upon absorption of a photon.

• DFG Programme: Research Grants
• International Connection: Colombia, Mexico, Netherlands
• Cooperation Partners: Professor Dr. W. Leo Meerts; Dr. Jörg Tatchen; Professor Dr. Leonardo Álvarez-Valtierra