Vibrationally Coherent Preparation of the Transition State for Photoisomerization of the Cyanine Dye Cy5 in Water.

Femtosecond pump-continuum probe spectroscopy with impulsive excitation was employed to observe coherent wavepacket motions of the cyanine dye Cy5 in water that promote photoisomerization after optical preparation of the first excited singlet state, S1. The chief component in the excited-state vibrational coherence is a resonance Raman-inactive, 273 cm(-1) mode of mixed carbon-carbon bond length alternation and out-of-plane or twisting character. The ultrafast (30 fs) damping of these motions arises from trajectories that irreversibly cross the transition state barrier; after several recurrences to the transition state region, vibrational cooling traps a significant fraction of the excited-state molecules in the planar, Franck-Condon region of the potential energy surface. Motion in the 273 cm(-1) promoting mode is apparently launched by a change in conformation of the conjugated polyene backbone during the first few vibrations of the high-frequency C-C and C═C bond length alternation coordinates that principally contribute to the initial displacement from the Franck-Condon structure. To our knowledge, this work provides the first direct observations of the intramolecular redistribution of excited-state potential energy into reactive motions that are rapidly damped by transition state barrier-crossing events leading to photoisomerization in a conjugated polyene. These results provide insight into the vibrational dynamics that contribute to the photoisomerization of retinal protonated Schiff bases in the rhodopsins and to the formation of intramolecular charge transfer character in carotenoids in photosynthetic light-harvesting proteins.