Stability of the S₃ and S₂ State Intermediates in Photosystem II Directly Probed by EPR Spectroscopy

The stability of the S₃ and S₂ states of the oxygen evolving complex in photosystem II (PSII) was directly probed by EPR spectroscopy in PSII membrane preparations from spinach in the presence of the exogenous electron acceptor P<i>p</i>BQ at 1, 10, and 20 °C. The decay of the S₃ state was followed in samples exposed to two flashes by measuring the split S₃ EPR signal induced by near-infrared illumination at 5 K. The decay of the S₂ state was followed in samples exposed to one flash by measuring the S₂ state multiline EPR signal. During the decay of the S₃ state, the S₂ state multiline EPR signal first increased and then decreased in amplitude. This shows that the decay of the S₃ state to the S₁ state occurs via the S₂ state. The decay of the S₃ state was biexponential with a fast kinetic phase with a few seconds decay half-time. This occurred in 10–20% of the PSII centers. The slow kinetic phase ranged from a decay half-time of 700 s (at 1 °C) to ∼100 s (at 20 °C) in the remaining 80–90% of the centers. The decay of the S₂ state was also biphasic and showed quite similar kinetics to the decay of the S₃ state. Our experiments show that the auxiliary electron donor Y_D was oxidized during the entire experiment. Thus, the reduced form of Y_D does not participate to the fast decay of the S₂ and S₃ states we describe here. Instead, we suggest that the decay of the S₃ and S₂ states reflects electron transfer from the acceptor side of PSII to the donor side of PSII starting in the corresponding S state. It is proposed that this exists in equilibrium with Y_Z according to S₃Y_Z ⇔ S₂Y_Z^• in the case of the S₃ state decay and S₂Y_Z ⇔ S₁Y_Z^• in the case of the S₂ state decay. Two kinetic models are discussed, both developed with the assumption that the slow decay of the S₃ and S₂ states occurs in PSII centers where Y_Z is also a fast donor to P₆₈₀⁺ working in the nanosecond time regime and that the fast decay of the S₃ and S₂ states occurs in centers where Y_Z reduces P₆₈₀⁺ with slower microsecond kinetics. Our measurements also demonstrate that the split S₃ EPR signal can be used as a direct probe to the S₃ state and that it can provide important information about the redox properties of the S₃ state

Long-Range Electron Transfer in Zinc-Phthalocyanine-Oligo(Phenylene-ethynylene)-Based Donor-Bridge-Acceptor Dyads

In the context of long-range electron transfer for solar energy conversion, we present the synthesis, photophysical, and computational characterization of two new zinc­(II) phthalocyanine oligophenylene-ethynylene based donor-bride-acceptor dyads: <b>ZnPc-OPE-AuP</b>^<b>+</b> and <b>ZnPc-OPE-C</b>_<b>60</b>. A gold­(III) porphyrin and a fullerene has been used as electron accepting moieties, and the results have been compared to a previously reported dyad with a tin­(IV) dichloride porphyrin as the electron acceptor (Fortage et al. <i>Chem. Commun.</i> <b>2007</b>, 4629). The results for <b>ZnPc-OPE-AuP</b>^<b>+</b> indicate a remarkably strong electronic coupling over a distance of more than 3 nm. The electronic coupling is manifested in both the absorption spectrum and an ultrafast rate for photoinduced electron transfer (<i>k</i>_PET = 1.0 × 10¹² s^–1). The charge-shifted state in <b>ZnPc-OPE-AuP</b>^<b>+</b> recombines with a relatively low rate (<i>k</i>_BET = 1.0 × 10⁹ s^–1). In contrast, the rate for charge transfer in the other dyad, <b>ZnPc-OPE-C</b>_<b>60</b>, is relatively slow (<i>k</i>_PET = 1.1 × 10⁹ s^–1), while the recombination is very fast (<i>k</i>_BET ≈ 5 × 10¹⁰ s^–1). TD-DFT calculations support the hypothesis that the long-lived charge-shifted state of <b>ZnPc-OPE-AuP</b>^<b>+</b> is due to relaxation of the reduced gold porphyrin from a porphyrin ring based reduction to a gold centered reduction. This is in contrast to the faster recombination in the tin­(IV) porphyrin based system (<i>k</i>_BET = 1.2 × 10¹⁰ s^–1), where the excess electron is instead delocalized over the porphyrin ring