Martin-Luther-Universität Halle-Wittenberg

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Excited State Solvation Dynamics

Nonequilibrium, experimental techniques may be combined with ab-initio molecular dynamics. One example for this is local dielectric spectroscopy, where the nonequilibrium situation is generated by photoexcitation of probe molecules. Experimentally the evolution of the emission wavelength of a fluorescent dye can be used to monitor its excited state solvation dynamics. Indirectly this gives access to the local THz spectrum around the fluorescent probe molecule. Ab-initio molecular dynamics (AIMD) can simultaneously provide solute/solvent dynamics in the electronically excited state and provide the fluorescence wavelength via TD-DFT calculations.

The first solvation shell of MQ and its components: Water molecules in torus around N-O (yellow beads), H2O molecules hydrogen bonded to the MQ-oxygen terminus (beads), and entire first solvation shell (sticks)

The first solvation shell of MQ and its components: Water molecules in torus around N-O (yellow beads), H2O molecules hydrogen bonded to the MQ-oxygen terminus (beads), and entire first solvation shell (sticks)

The first solvation shell of MQ and its components: Water molecules in torus around N-O (yellow beads), H2O molecules hydrogen bonded to the MQ-oxygen terminus (beads), and entire first solvation shell (sticks)

As the experimental timescale is in the picosecond range, the experiment may be directly simulated by AIMD. In one example, we have simulated the relaxation of water around the probe N-methyl-6-oxyquinolinium betaine (MQ). The connection between MQ's experimental TDSS data and the local THz spectrum of water has been established by simple dipolar continuum theory. Our simulations confirm, that the dipole moment of MQ is drastically reduced upon electronic excitation. The experimental Stokes shift could be reproduced, then decomposed into molecular components (see Figure), with the water molecules in the dipole field having a significant contribution to the relaxation. We found the probe molecule to be influenced by more than the first solvation shell, thus giving experimentalists information about the range and region of their measurements.

Publications

C. Allolio, M. Sajadi, N.P. Ernsting and D. Sebastiani:
An Ab Initio Microscope: Molecular Contributions to the Femtosecond Time-Dependent Fluorescence Shift of a Reichardt-Type Dye
Angew. Chem. Int. Ed. 52, 1813-1816 (2013)

C. Allolio and D. Sebastiani:
Approaches to the solvation of the molecular probe N-methyl-6-quinolone in its excited state
Phys. Chem. Chem. Phys. 13, 16395-16403 (2011)

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