COMMUNICATION www.rsc.org/dalton | Dalton Transactions Extension of the charge separated-state lifetime by supramolecular association of a tetrathiafulvalene electron donor to a zinc/gold bisporphyrin† Julien Boixel, a J´ erˆ ome Fortage, a Errol Blart, a Yann Pellegrin, a Leif Hammarstr ¨ om,* b Hans-Christian Becker* b and Fabrice Odobel* a Received 2nd July 2009, Accepted 10th November 2009 First published as an Advance Article on the web 27th November 2009 DOI: 10.1039/b920079g Supramolecular triads were prepared by self-assembly of 4¢-pyridyl-2-tetrathiafulvalene axially bound on ZnP-spacer- AuP + dyads; the lifetime of the charge separated state ( + TTF- ZnP-Spacer-AuP ∑ ) formed upon light excitation of the triad is greatly increased with respect to that found in the parent dyad. Non-covalently bonded supramolecular assemblies for photoin- duced charge separation have drawn considerable attention be- cause their preparations are relatively more accessible and offer higher versatility than those of fully covalent arrays. Besides, this approach can be seen as a direct inspiration of the natural photosynthetic reaction center because the active components in the biological system are assembled via non-covalent bonds. Self-assembly involving the preferred pentacoordination of zinc porphyrins (ZnPs) with axial amine ligands is a well-known strategy to construct sophisticated architectures for light harvest- ing properties. 1 The same approach was also used to assemble an electron acceptor, such as fullerene and naphthalene or perylene diimide onto a ZnP for photoinduced electron transfer. 2-4 Strangely, the implementation of this strategy with an electron donor on a zinc porphyrin-spacer-electron acceptor dyad (ZnP-S- A) has never been explored, in spite of its potential utility. Indeed, in the literature there are many covalent assemblies involving a ZnP 5 that could be revisited with this strategy in order to extend the charge separated state lifetime. In this work we have used the 4¢-pyridyl-2-tetrathiafulvalene (abbreviated hereafter p- TTF) as a secondary electron donor with the heterometallic porphyrin dyads shown in Fig. 1. Recently, we reported on the photophysical properties of these zinc porphyrin-gold porphyrin dyads linked either with an oligophenylene ethynylene (ZnP-OPE- AuP + ) 6a or a bisethynyl quaterthiophene (ZnP-quater-AuP + ) 6b spacer. Excitation of the zinc porphyrin induces a very fast and quantitative electron transfer to the adjacent gold porphyrin leading to a charge separated state lifetime of 2.2 ns and 3.1 ns respectively in ZnP-OPE-AuP + (in dichloromethane) and ZnP- quater-AuP + (in toluene). In this work, we show that the ligand a CEISAM, Chimie Et Interdisciplinarit´ e, Synth` ese, Analyse, Mod´ elisation, CNRS, UMR CNRS 6230, UFR des Sciences et des Techniques 2, rue de la Houssini` ere - BP 92208, 44322 NANTES Cedex 3, France. E-mail: Fabrice. Odobel@univ-nantes.fr b Chemical Physics, Department of Photochemistry and Molecular Sci- ence, Uppsala University, Box 523, SE-751 20 Uppsala, Sweden. E-mail: Leif@fotomol.uu.se †Electronic supplementary information (ESI) available: Synthetic proce- dures for the preparation of p-TTF, electrochemistry, spectroelectrochem- istry and transient absorption spectra. See DOI: 10.1039/b920079g Fig. 1 Structures of the compounds described in this study. p-TTF effectively binds to the zinc porphyrin of these dyads and substantially increases the charge separated state lifetime; 10 times for ZnP-quater-AuP + and 50 times for ZnP-OPE- AuP + . The synthesis of the new ligand p-TTF is based on a Stille cross-coupling reaction between 4-bromopyridine and (tetrabutylstannyl)tetrathiafulvalene as described in the ESI.† Cyclic voltammetry indicated that the first oxidation potential of p-TTF occurs at 0.18 V vs. SCE in dichloromethane (Fig. S1, ESI†). Under the same conditions, the oxidation potential of the zinc porphyrin in the dyads is 0.76 V, therefore there is approximately a 0.58 eV driving force 7 for the hole shift reaction for the oxidized zinc porphyrin to the TTF. The characteristic absorption spectrum of the radical cation of p-TTF was recorded by spectroelectrochemistry (Fig. S2, ESI†) showing bands at 445 and 600 nm. The association constants (K a ) between p-TTF and the zinc porphyrin in the corresponding ZnP-OPE-TIPS or ZnP-quater- TIPS reference compounds were measured by UV-Vis absorption titration. The anticipated red-shift and intensification of the Q-bands of ZnP could be clearly observed upon addition of increasing amounts of p-TTF in a solution of the dyad, thus evidencing the effective coordination of the pyridine onto the zinc porphyrin (Fig. S3, ESI†). Fitting the experimental data to the Scatchard equation n/L = nK a - nK a where n is the concentration of bound ligands over total concentration of ZnP binding sites, n is the number of binding sites per ZnP (assumed to be 1 in the evaluation of the spectra), L is the concentration of free ligand, and K a is the binding constant. 8 This led to K a = 1.0 ¥ 10 4 M -1 for p-TTF ◊◊◊ ZnP-OPE-TIPS in dichloromethane, and slightly 1450 | Dalton Trans., 2010, 39, 1450–1452 This journal is © The Royal Society of Chemistry 2010