This dissertation describes two studies on the structurally related conjugated polyenes, cyanines and carotenoids. The first study aims to elucidate the nature of the nonpolar solvation-and excited-state conformational-dynamics of cyanine 5 (Cy5) in bulk water and compare them to the dynamics in the hydration layer of ZnII-substituted cytochrome c (ZnCytc). The second study adds insight into the nonradiative decay pathways from the S2 state of the carotenoids β carotene and peridinin, the latter of which was studied in solution and in the peridinin-chlorophyll a protein (PCP).In the first study Cy5 was used as a probe for nonpolar solvation and was studied using multiple nonlinear spectroscopic methods. Three-pulse photon echo peak-shift (3PEPS) experiments revealed two timescales for nonpolar solvation in bulk water corresponding to phonon induced and diffusive responses (τ=21 and 890 fs). These dynamics were significantly different in the hydration layer of ZnCytc, where the fast phonon induced component appears to decay over multiple timescales, and the diffusive component appears to decay over a timescale much greater than the 3PEPS experimental time window (τ≫2.5 ps). Extremely fast damping of excited-state motions in Cy5 was observed in bulk water and in the hydration layer using pump-probe experiments. This fast damping provides experimental evidence for coherent torsional barrier crossing over a transition state toward a twisted conical intersection in accordance with Schlegel and Olivucci's two state two mode theory.1 An increased damping rate of torsional modes observed in the hydration layer of ZnCytc along with the slowed polar solvation timescales lead to the conclusion that water in the hydration layer is likely more viscous and contains a distinct hydrogen-bonding structure as compared to bulk water.The second study utilized electric-field resolved transient grating spectroscopy to observe nonradiative decay from the S2 state of the carotenoids β carotene and peridinin. An ultrafast kinetic intermediate was observed in both carotenoids, consistent with previous experiments,30,165,24 and the differing decay rates of the absorption and dispersion signals of these carotenoids in solution suggested nonradiative decay to a structurally displaced ground state.54 The time-resolved transient grating absorption spectra of the carotenoids in solution exhibit features consistent with photo-induced excited-state twisting. The transient grating spectra from peridinin in solution revealed the state consistent with the ultrafast intermediate exists at all times when peridinin is in the PCP protein, where the dynamics are relatively static. This implicates the intermediate as a twisted state, and the two state two mode picture developed for cyanines and protonated Schiff bases1 was applied to carotenoid systems for the first time. It is suggested that these twisting dynamics enhance the intramolecular charge-transfer character of peridinin in the S1 state, enabling more efficient energy transfer to chlorophyll a. In this light, the kinetic intermediate in this and previous work is unlikely to be the distinct electronic 1"B" _"u" ^- state, but rather a twisted conformation on the S2 state potential.
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