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

All-trans-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 S₂(1B_u⁺), S₁ (2A_g^–), nπ*, 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 S₁ and ICT lifetimes challenging, resulting in two competing hypotheses regarding the ICT state: strongly coupled to S₁ 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: S₂ (1B_u⁺) → S₁ (2A_g^–)→ nπ* → T → all-trans S₀ and Channel 2: S₂ (1B_u⁺) → ICT → ICT′ → cis S₀. 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.