Resolving Dual Photoreaction Channels of All-Trans-Retinal Using Femtosecond Stimulated Raman Spectroscopy.

All--retinal (ATR) plays a critical role in vision and light-sensing biological processes, serving as the retinyl chromophore in photoreceptor proteins. The excited-state dynamics of ATR include several singlet electronic states such as S(1), S (2), π*, and the intramolecular charge transfer (ICT) state, which are pivotal in its isomerization and photoinduced processes. However, spectral overlaps in transient absorption spectra have rendered the differentiation of S and ICT lifetimes challenging, resulting in two competing hypotheses regarding the ICT state: strongly coupled to S or existing as a distinct electronic state. This study employs femtosecond stimulated Raman spectroscopy to investigate ATR's structural dynamics across solvents with varying polarities and viscosities. Two separate photochemical pathways are identified: Channel 1: S (1) → S (2)→ π* → T → all- S and Channel 2: S (1) → ICT → ICT' → S. Results show that solvent viscosity strongly influences isomerization in Channel 2, while Channel 1 remains unaffected. Furthermore, the ATR's isomerization in Channel 2 involves large-scale one-bond flip torsional motions, distinct from the space-conserving bicycle-pedal isomerization observed in protein environments.