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

Abstract

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.

Identification of Nonradiative Decay Pathways in Cy3

“…Coherent vibrational wavepacket propagation appears as oscillatory features in the time-domain traces. These oscillations correspond to wavepackets on the ground- and excited-state potential energy surfaces and persist for several picoseconds after photoexcitation and have been observed in similar cyanine systems. , A Fourier transform along the time axis of the transient grating data ( t 2 ) recovers the mode frequencies. The resultant power spectra are shown in Figure c for three emission frequencies (18 620, 18 050, and 17 450 cm –1 ), which span the ground-state bleach and stimulated emission peaks.…”

Experimental Detection of Branching at a Conical Intersection in a Highly Fluorescent Molecule

“…To obtain chemical insight from the coherent oscillations, many researchers have used Fourier transforms to produce one-dimensional profiles of the amplitude and phase of each oscillation frequency as a function of probe wavelength. ,− As a consequence of interference effects, a coherent wavepacket that oscillates about the minimum of the ground-state (excited-state) potential will yield a node in the amplitude profile and a π shift in the phase profile at the wavelength of the absorption (fluorescence) maximum. − These nodes and phase shifts have been used as strong evidence supporting the notion that a conical intersection drives photoisomerization in rhodopsin . While these studies have yielded tremendous insights, Fourier-domain methods involve mathematical integration over all of the coherent dynamics.…”

Time-Resolved Dynamics of Stable Open- and Closed-Shell Neutral Radical and Oxidized Tripyrrindione Complexes

“…No blue shifts are observed for TD1-Pd-Ox. The spectral shift of an ESA to higher energies is generally associated with vibrational relaxation on the excited state [54][55][56][57][58] .…”