The reorganization energy (l), which is a sum of two terms, inner-sphere reorganization energy, l i , and outer-sphere reorganization energy, l o , imposes probably the most farreaching impact on biological electron-transfer (ET) systems. [1] In particular, the primary ET processes in photosynthesis are all characterized by small reorganization energies. [2] This situation allows, for instance, forward ET processes to proceed under nearly optimal conditions, that is, near the top region of the Marcus parabola, whereas the highly exergonic and energy-wasting back-ET process is shifted deeply into the inverted region. To achieve small reorganization energies, it is highly desirable for the construction of artificial photosynthetic systems to employ donor ± acceptor couples, which offer room for the delocalization of the charges-electrons or Chem. Commun. 1987, 951 ± 952; f) C.

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.

A homologous series of zincporphyrin (ZnP)-pyromellitimide (Im)-C 60 linked triads where the pyromellitimide (Im) moiety is incorporated as an intermediate acceptor between the above two chromophores with a linkage of different spacers, ZnP-Im-CH 2 -C 60 , ZnP-Im-C 60 , and ZnP-CH 2 -Im-C 60 as well as the reference dyads (ZnP-Im-CH 2 -ref, ZnP-Im-ref, and ZnP-CH 2 -Im-ref) have been prepared to investigate linkage dependence of photoinduced electron transfer (ET) and back ET to the ground state in the triads. Time-resolved transient absorption spectra of the triads measured by picosecond laser photolysis as well as the fluorescence lifetimes in THF reveal the occurrence of photoinduced ET from the singlet excited state of the ZnP to the Im moiety to give the initial charge-separated state, i.e., the zincporphyrin radical cation (ZnP •+ )-imide radical anion (Im •-) pair, followed by a charge shift (CSH) to produce the final charge-separated state, the ZnP •+ -C 60 •-pair. The rate constants of photoinduced ETs in ZnP-Im-C 60 (1.8 × 10 10 s -1 ) and ZnP-Im-CH 2 -C 60 (1.3 × 10 10 s -1 ) in THF are much larger than those in ZnP-CH 2 -Im-C 60 (2.9 × 10 9 s -1 ) and ZnP-CH 2 -Im-ref (1.9 × 10 9 s -1 ). The larger charge separation (CS) rates in the former case are ascribed to the relatively strong electronic coupling because of the absence of a methylene linkage between the ZnP and the Im moieties in ZnP-Im-C 60 and ZnP-Im-CH 2 -C 60 as compared to the triad and dyad with the methylene linkage. The transient absorption spectra of the final charge-separated state, the ZnP •+ -C 60 •-pair, have been also measured by nanosecond laser photolysis. It has been found that the rate constants of charge recombination (CR) of ZnP •+ -Im-C 60 •-are temperature independent, but that the CR rate constants of ZnP •+ -Im-CH 2 -C 60 •-exhibit an Arrhenius-like temperature dependence with an activation energy of 0.13 eV which corresponds to the energy difference between ZnP •+ -Im •--CH 2 -C 60 and ZnP •+ -Im-CH 2 -C 60 •-. This indicates that the relatively strong electronic coupling without methylene linkage in ZnP-Im-C 60 results in the preference of the tunneling superexchange ET over the sequential ET in the CR process which requires the thermal activation to reach the higher energy state (i.e., ZnP •+ -Im •--C 60 ), whereas the sequential ET predominates in the triads with the methylene linkage.

The reorganization energy (l), which is a sum of two terms, inner-sphere reorganization energy, l i , and outer-sphere reorganization energy, l o , imposes probably the most farreaching impact on biological electron-transfer (ET) systems. [1] In particular, the primary ET processes in photosynthesis are all characterized by small reorganization energies. [2] This situation allows, for instance, forward ET processes to proceed under nearly optimal conditions, that is, near the top region of the Marcus parabola, whereas the highly exergonic and energy-wasting back-ET process is shifted deeply into the inverted region. To achieve small reorganization energies, it is highly desirable for the construction of artificial photosynthetic systems to employ donor ± acceptor couples, which offer room for the delocalization of the charges-electrons or Chem. Commun. 1987, 951 ± 952; f) C.