Photoinduced long-lived charge separation in a tetrathiafulvalene-porphyrin-fullerene triad detected by time-resolved electron paramagnetic resonance

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 (C-60), 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-C-60(.-), 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-C-60(.-) state is formed from the TTF-P-1-C-60 singlet state via an initial TTF-P.+-C-60(.-) charge-separated state. Long-lived charge separation (similar to 8 mu s) 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.+-C-60(.-)) has a lifetime of similar to 7 mu s, while that of the singlet-born radical pair (S)(TTF.+-P-C-60(.-)) is much shorter (< 1 mu s). The difference in lifetimes is ascribed to spin dynamic effects in the magnetic field.