Photoinduced Electron Transfer at Liquid|Liquid Interfaces: Dynamics of the Heterogeneous Photoreduction of Quinones by Self-Assembled Porphyrin Ion Pairs Nicolas Eugster, David J. Fermı ´n,* and Hubert H. Girault Contribution from the Laboratoire d’Electrochimie Physique et Analytique, Institut de Chimie Mole ´ culaire et Biologique, Ecole Polytechnique Fe ´ de ´ rale de Lausanne, CH-1015 Lausanne, Switzerland Received December 4, 2002; E-mail: david.fermin@epfl.ch Abstract: The initial stages of the heterogeneous photoreduction of quinone species by self-assembled porphyrin ion pairs at the water|1,2-dichloroethane (DCE) interface have been studied by ultrafast time- resolved spectroscopy and dynamic photoelectrochemical measurements. Photoexcitation of the water- soluble ion pair formed by zinc meso-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS 4- ) and zinc meso- tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP 4+ ) leads to a charge-separated state of the form ZnTPPS 3- - ZnTMPyP 3+ within 40 ps. This charge-separated state is involved in the heterogeneous electron injection to acceptors in the organic phase in the microsecond time scale. The heterogeneous electron transfer manifests itself as photocurrent responses under potentiostatic conditions. In the case of electron acceptors such as 1,4-benzoquinone (BQ), 2,6-dichloro-1,4-benzoquinone (DCBQ), and tetrachloro-1,4-benzoquinone (TCBQ), the photocurrent responses exhibit a strong decay due to back electron transfer to the oxidized porphyrin ion pair. Interfacial protonation of the radical semiquinone also contributes to the photocurrent relaxation in the millisecond time scale. The photocurrent responses are modeled by a series of linear elementary steps, allowing estimations of the flux of heterogeneous electron injection to the acceptor species. The rate of electron transfer was studied as a function of the thermodynamic driving force, confirming that the activation energy is controlled by the solvent reorganization energy. This analysis also suggests that the effective redox potential of BQ at the liquid|liquid boundary is shifted by 0.6 V toward positive potentials with respect to the value in bulk DCE. The change of the redox potential of BQ is associated with the formation of hydrogen bonds at the liquid|liquid boundary. The relevance of this approach toward modeling the initial processes in natural photosynthetic reaction centers is briefly discussed. 1. Introduction Porphyrins and quinone species are key components in the initial stages of photosynthetic reactions in bacteria and green plants. For instance, the photosynthetic reaction center of Rhodopseudomonas Viridis and Rhodobacter sphaeroides fea- tures four bacteriochlorophylls, two bacteriopheophytins, and two quinones organized in a pseudo-C 2 symmetry as depicted in Scheme 1. 1-3 Electron transfer from the photoexcited special pair to the pheophytin takes places within 4 ps. 4 The electron is subsequently transferred to the quinone Q A in approximately 200 ps. The reaction proceeds via regeneration of the special pair by the cytochrome (20 ns) and electron transfer from Q A to Q B in 0.2 ms. The quinone Q B is initially located at the interface between the plasmatic membrane and the cytosol, where it receives two electrons and two protons before diffusing to the cytochrome bc 1 . Theoretical studies indicate that the transmembrane potential and the spatial distribution of the prostheic groups determined by the intrinsic proteins are (1) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. Nature 1985, 618. (2) Chambron, J.-C.; Chardon-Noblat, S.; Harriman, A.; Heitz, V.; Sauvage, J.-P. Pure Appl. Chem. 1993, 65, 2343-2349. (3) Hutter, M. C.; Hughes, J. M.; Reimers, J. R.; Hush, N. S. J. Phys. Chem. B 1999, 103, 4906-4915. (4) Holzapfel, W.; Finkele, U.; Kaiser, W.; Oesterhelt, D.; Scheer, H.; Stilz, H. U.; Zinth, W. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 5168-5172. Scheme 1. Representation of the Photosynthetic Reaction Center of Purple Bacteria a a DM and DL denote the bacteriochlorophyll molecules of the special pair and BA and BB denote accessory chlorophylls assisting the electron transfer to the bacteriopheophytins ΦA and ΦB, while QA and QB correspond to the ubiquinone molecules. The arrows indicate the electron pathway after the initial photon capture by the special pair. The phytyl chains of the bacteriochlorophylls and the side chain of the ubiquinones were omitted for clarity. Published on Web 04/01/2003 4862 9 J. AM. CHEM. SOC. 2003, 125, 4862-4869 10.1021/ja029589n CCC: $25.00 © 2003 American Chemical Society