DOI: 10.1002/chem.200701335 Single-Step Electron Transfer on the Nanometer Scale: Ultra-Fast Charge Shift in Strongly Coupled Zinc Porphyrin–Gold Porphyrin Dyads JØrôme Fortage, [a] Julien Boixel, [a] Errol Blart, [a] Leif Hammarstrçm,* [b] Hans Christian Becker,* [b] and Fabrice Odobel* [a] Introduction The development of molecular systems for long-range charge separation has attracted considerable attention over the past decades. One driving force for the growing interest in this area is the realization that such molecular devices can be used for a variety of applications, ranging from solar energy conversion (artificial photosynthesis and photovolta- ic) to molecular electronics. Generally, the devices designed for light-induced charge separation are composed of donors and acceptors covalently linked via a molecular spacer. [1–7] The development of ap- propriate spacers to promote electronic coupling is an im- portant issue from the viewpoint of keeping a high rate of electron transfer while increasing the distance between the donor and the acceptor. Extensive theoretical analyses have been devoted to this problem, [1,2,8] but surprisingly there are few molecular devices for photoinduced charge separation Abstract: The synthesis, electrochemi- cal properties, and photoinduced elec- tron transfer processes of a series of three novel zinc(II)–goldACHTUNGTRENNUNG(III) bispor- phyrin dyads (ZnP  S  AuP + ) are de- scribed. The systems studied consist of two trisaryl porphyrins connected di- rectly in the meso position via an alkyne unit to tert-(phenylenethyny- lene) or penta(phenylenethynylene) spacers. In these dyads, the estimated center to center interporphyrin separa- tion distance varies from 32 to 45 . The absorption, emission, and electro- chemical data indicate that there are strong electronic interactions between the linked elements, thanks to the direct attachment of the spacer on the porphyrin ring through the alkyne unit. At room temperature in toluene, light excitation of the zinc porphyrin results in almost quantitative formation of the charge shifted state C + ZnP S AuPC , whose lifetime is in the order of hun- dreds of picoseconds. In this solvent, the charge-separated state decays to the ground state through the inter- mediate population of the zinc porphy- rin triplet excited state. Excitation of the gold porphyrin leads instead to rapid energy transfer to the triplet ZnP. In dichloromethane the charge shift re- actions are even faster, with time con- stants down to 2 ps, and may be in- duced also by excitation of the gold porphyrin. In this latter solvent, the longest charge-shifted lifetime (t = 2.3 ns) was obtained with the penta- (phenylenethynylene) spacer. The charge shift reactions are discussed in terms of bridge-mediated super-ex- change mechanisms as electron or hole transfer. These new bis-porphyrin arrays, with strong electronic coupling, represent interesting molecular systems in which extremely fast and efficient long-range photoinduced charge shift occurs over a long distance. The rate constants are two to three orders of magnitude larger than for correspond- ing ZnP  AuP + dyads linked via meso- phenyl groups to oligo-phenyleneethy- nylene spacers. This study demon- strates the critical impact of the attach- ment position of the spacer on the por- phyrin on the electron transfer rate, and this strategy can represent a useful approach to develop molecular photon- ic devices for long-range charge separa- tions. Keywords: electron transfer · fem- tosecond spectroscopy · gold · mo- lecular wires · porphyrinoids · zinc [a] Dr. J. Fortage, J. Boixel, Dr. E. Blart, Dr. F. Odobel UniversitØ de Nantes, Nantes Atlantique UniversitØs CNRS FacultØ des Sciences et des Techniques Laboratoire de Synthse Organique (LSO), UMR CNRS 6513 2, rue de la Houssinire - BP 92208–44322 Nantes Cedex 3 (France) Fax: (+ 33) 2-51-12-54-29 E-mail: Fabrice.Odobel@univ-nantes.fr [b] Prof. L. Hammarstrçm, Dr. H. C. Becker Department of Photochemistry and Molecular Science The ngstrçm Laboratories, Uppsala University Regementsvägen 1, 752 37 Uppsala (Sweden) Fax: (+ 46) 18-471 6844 E-mail: Leif.Hammarstrom@fotomol.uu.se hcb@fotomol.uu.se Supporting information for this article (synthesis of compounds, 1 H NMR spectra of the dyads D1–D3 and of the compounds 25, 27, 29, 31, 32 and 33, spectroelectrochemical data, and additional transi- ent absorption data) is available on the WWW under http://www.che- mistry.org or from the author. Chem. Eur. J. 2008, 14, 3467 – 3480 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3467 FULL PAPER