Ion-Pair Dynamics upon Photoinduced Electron Transfer Monitored by Pump-Pump-Probe Spectroscopy.

Area of Science:

• Photochemistry
• Chemical Physics
• Spectroscopy

Background:

• Photoinduced electron transfer (PET) is a fundamental process in chemistry and biology.
• Understanding the excited-state dynamics of radical anions is crucial for controlling chemical reactions.

Purpose of the Study:

• To investigate the excited-state dynamics of the perylene radical anion (Pe•−) generated via bimolecular PET.
• To elucidate the evolution of PET products from ion pairs to free ions.
• To identify and characterize charge recombination pathways.

Main Methods:

• Broadband pump-pump-probe spectroscopy was employed.
• The study utilized a bimolecular photoinduced electron transfer (PET) system involving a perylene donor and acceptor.
• Experiments were conducted in acetonitrile and tetrahydrofuran.

Main Results:

• The excited-state dynamics of the perylene radical anion were found to be dependent on its 'age' or time since PET.
• Distinct differences in dynamics were observed in acetonitrile, indicating the transition from ion pairs to free ions.
• Two photoinduced charge recombination pathways were identified, occurring on picosecond and femtosecond timescales.
• These pathways were shown to be sensitive to interionic distances, acting as molecular rulers.

Conclusions:

• The study demonstrates that the excited-state dynamics of radical anions are influenced by their environment and aggregation state.
• The observed charge recombination pathways provide insights into the spatial separation of photogenerated charge carriers.
• The findings offer a molecular-level understanding of PET processes and their subsequent relaxation dynamics.