Coherent High-Frequency Vibrational Dynamics in the Excited Electronic State of All-Trans Retinal Derivatives

Coherent vibrational dynamics of retinal in excited electronic states are of primary importance in the understanding of photobiology. Using pump-DFWM, we demonstrate for the first time the existence of coherent double-bond high-frequency modulations (>1300 cm^–1) in the excited electronic state of different retinal derivatives. All-trans retinal as well as retinal Schiff bases exhibit a partial frequency downshift of the CC double-bond mode from ∼1580 cm^–1 in the ground state to 1510 cm^–1 in the excited state. In addition, a new vibrational band at ∼1700 cm^–1 assigned to the CN stretching mode in retinal Schiff bases in the excited state is detected. The newly reported bands are observed only in specific spectral regions of excited-state absorption. Implications regarding the observation of vibrational coherences in naturally occurring retinal protonated Schiff bases in rhodopsins are discussed.