Femtosecond Heterodyne Transient-Grating Studies of Nonradiative Decay of the S2 (1(1)Bu(+)) State of ß-Carotene: Contributions from Dark Intermediates and Double-Quantum Coherences.

Femtosecond transient-grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in ß-carotene from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast time scale prior to nonradiative decay to the S1 state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient-grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the Sx state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S2 state. The finding that the excited-state absorption spectrum from the Sx state is significantly red-shifted from that of S2 and S1 leads to a new assignment for the spectroscopic origin of the Sx state. Rather than assigning Sx to a discrete electronic state, such as the (1)Bu(-) state suggested in previous work, it is proposed that the Sx state corresponds to a transition-state-like structure on the S2 potential surface. In this hypothesis, the 12 fs time constant for the decay of the S2 state corresponds to a vibrational displacement of the C-C and C═C bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared Franck-Condon structure to a potential energy barrier on the S2 surface that divides planar and torsionally displaced structures. The lifetime of the Sx state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient toward a conical intersection with the S1 state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S2 torsional gradient are active in excitation energy-transfer mechanisms to (bacterio)chlorophyll acceptors.

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.

Dynamic solvation and coupling of the hydration shell of Zn(II)-substituted cytochrome c in the presence of guanidinium ions.

The fluorescence Stokes shift (FSS) response of Zn(II)-substituted cytochrome c (ZnCytc) is transformed from a monotonic red-shifting response in water to a bidirectional response with much slower time constants in the presence of low concentrations of guanidinium (Gdm(+)) ions. The FSS response in water observed over the 100 ps to 10 ns range has two exponential components with time constants of 135 ps and 1.6 ns that account for a total shift of 30 cm(-1), about one-half of the solvation reorganization energy. In contrast, in the presence of only 0.25 M Gdm(+), the FSS response initially shifts 21 cm(-1) to the blue with a 820 ps time constant and then shifts 60 cm(-1) back to the red with a 3.5 ns time constant. The effect of Gdm(+) on the FSS response effectively saturates at 1.0 M, well below the 1.75 M midpoint of the two-state unfolding transition. These results establish that the FSS response in ZnCytc includes a significant contribution from the surrounding hydration shell, which assumes a perturbed hydrogen-bonding network owing to the binding of Gdm(+) ions to the protein surface. The blue-shifting part of the FSS response arises from a light-induced conformational change that expands the protein- and solvent-derived cavity around the excited-state Zn(II) porphyrin. This non-polar part of the solvation response is enhanced in the presence of Gdm(+) because the protein/solvent surroundings of the Zn(II) porphyrin are effectively more flexible than in water. The enhanced flexibility in the presence of Gdm(+) increases the amplitudes and accordingly lengthens the correlation time scales for the protein and hydration-shell fluctuations that contribute to the FSS response.