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

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• 作者：Michael M. Bishop ; Jerome D. Roscioli ; Soumen Ghosh ; Jenny Jo Mueller ; Nolan C. Shepherd ; Warren F. Beck
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：June 11, 2015
• 年：2015
• 卷：119
• 期：23
• 页码：6905-6915
• 全文大小：455K
• ISSN：1520-5207

文摘

Femtosecond pump鈥揷ontinuum 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^鈥? mode of mixed carbon鈥揷arbon 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鈥揅ondon region of the potential energy surface. Motion in the 273 cm^鈥? 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鈥揅 and C鈺怌 bond length alternation coordinates that principally contribute to the initial displacement from the Franck鈥揅ondon 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.