Photoinduced Long-Lived Charge Separation in a Tetrathiafulvalene

J. Phys. Chem. B 2005, 109, 14401-14409

14401

Photoinduced Long-Lived Charge Separation in a Tetrathiafulvalene-Porphyrin-Fullerene Triad Detected by Time-Resolved Electron Paramagnetic Resonance Marilena Di Valentin,† Arianna Bisol,† Giancarlo Agostini,‡ Paul A. Liddell,§ Gerdenis Kodis,§ Ana L. Moore,§ Thomas A. Moore,§ Devens Gust,§ and Donatella Carbonera*,† Dipartimento di Scienze Chimiche, UniVersita` di PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy, CNRsIstituto di Chimica Biomolecolare, Sezione di PadoVa, Via Marzolo 1, I-35131 PadoVa, Italy, and Center for the Study of Early EVents in Photosynthesis, Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604 ReceiVed: March 15, 2005; In Final Form: May 12, 2005

Photoinduced electron transfer has been observed in a molecular triad, consisting of a porphyrin (P) covalently linked to a tetrathiafulvalene (TTF) and a fullerene derivative (C60), in the different phases of the liquid crystal E-7 and in a glass of 2-methyltetrahydrofuran (2-MeTHF) by means of time-resolved electron paramagnetic resonance (EPR) spectroscopy. In both solvents, an EPR signal observed immediately after excitation has been assigned to the radical pair TTF•+-P-C60•-, based on its magnetic interaction parameters and spin polarization pattern. In the 2-MeTHF glass and the crystalline phase of E-7, the TTF•+-P-C60•state is formed from the TTF-1P-C60 singlet state via an initial TTF-P•+-C60•- charge-separated state. Long-lived charge separation (∼8 µs) for the singlet-born radical pair is observed in the 2-MeTHF glass at cryogenic temperatures. In the nematic phase of E-7, a high degree of ordering in the liquid crystal is achieved by the molecular triad. In this phase, both singlet- and triplet-initiated electron transfer routes are concurrently active. At room temperature in the presence of the external magnetic field, the triplet-born radical pair T (TTF•+-P-C60•-) has a lifetime of ∼7 µs, while that of the singlet-born radical pair S(TTF•+-P-C60•-) is much shorter (