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