Excited State Vibrational Spectra of All- trans Retinal Derivatives in Solution Revealed By Pump-DFWM Experiments

Area of Science:

• Photochemistry
• Molecular Spectroscopy
• Structural Dynamics

Background:

• The photoinduced isomerization of retinal-protonated Schiff base (RPSB) is crucial for vision but its ultrafast structural dynamics remain unclear.
• Understanding these changes is key to elucidating photochemical mechanisms.

Purpose of the Study:

• To investigate the ultrafast structural dynamics during photoinduced isomerization in retinal Schiff bases.
• To resolve vibrational signatures from excited electronic states using advanced spectroscopic methods.

Main Methods:

• Application of pump-degenerate four-wave mixing (pump-DFWM) spectroscopy.
• Analysis of vibrational spectra of all-trans retinal (ATR) and retinal Schiff bases (RSB) in excited electronic states.

Main Results:

• Excited state vibrational spectra of ATR and RSB exhibit significant differences from the ground state.
• Predominantly low-frequency modes (100-500 cm⁻¹) characterize excited state spectra.
• Distinct spectral features arise from excitation in specific electronic transition regions.
• Low-frequency modes persist throughout the excited state lifetime, suggesting a stable transient structure.

Conclusions:

• Pump-DFWM successfully resolved ultrafast vibrational dynamics in retinal photoisomerization.
• The observed low-frequency modes and their persistence indicate a transient molecular structure that stabilizes within the excited state lifetime.