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Three-pulse photon-echo peak-shift (3PEPS) experiments have been performed to explore the time scales and the nature of structural relaxations relevant to solvation dynamics of a cyanine dye non-covalently anchored to phospholipid/water interfaces. For comparison, equivalent 3PEPS data are presented for the same chromophore dissolved in neat water. In the bulk liquid, solvation includes at least two distinct time scales. A fast component dominating the solvation response below 3 ps is connected to restricted intermolecular translational degrees of freedom of the hydrogen-bonded liquid network in qualitative agreement with aqueous solvation dynamics of other chromophores previously reported in the literature. However, on longer time scales above 3 ps, an additional exponential tail can be found in the 3PEPS decay which has not been observed previously. Its time constant is in good agreement with the dielectric relaxation time of bulk water indicating that a signiÐcant fraction of the solvation energy is also relaxed by single-molecule rotational di †usion of water. At the membrane interface, the 3PEPS data indicate that already on sub-picosecond time scales, solvation is considerably perturbed in comparison to bulk water. This Ðnding indicates a substantial disruption of the hydrogen-bonded network of the bulk liquid. Di †usive single-molecule reorientation of water seems to contribute to solvation at the interface in the same manner as it does in the bulk phase. These Ðndings are in qualitative agreement with recent molecular dynamics simulations on the structure and the dynamics of water at phospholipid membrane interfaces.

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Femtochemistry / Wavepacket Dynamics / Proton TransferUltrafast pump-probe experiments with a time-resolution of 30 fs have been carried out to explore the non-radiative relaxation dynamics of electronically excited 1,8-dihydroxyanthraquinone (DHAQ) in polar liquid solution. The results are discussed in terms of a Lippincott-Schroeder double-minimum potential along the proton-transfer reaction coordinate for the ground (S0) and first excited singlet states (S1) of DHAQ. The 400-nm pump-visible probe data reveal a strongly Stokes-shifted stimulated emission due to the proton-transferred 1,10-quinone configuration in the S1-state. A dominant fraction of this stimulated emission appears instantaneously implying that cross-well excitation directly from the ground-state 9,10-quinone form into the excited-state 1,10-quinone configuration takes place. A smaller fraction of the stimulated emission appears delayed with a time constant of approximately 300 fs. This component may be due to proton-transfer in the S 1 -state following optical excitation. Further, a transient absorption is observed on the red edge of the linear absorption spectrum of ground-state DHAQ due to higher-lying electronic states presumably from the 1,10-configuration. Periodic modulations of the transient absorption due to wavepacket motion in the 9,10-quinone form are partially consistent with previous fluorescence excitation spectra of the molecule taken under jetcooled conditions.