EPR-Detected photoinduced electron transfer in three structurally related molecular triads

A comparative electron paramagnetic resonance (EPR) study has been performed on a series of structurally related molecular triads which undergo photoinduced electron transfer and differ one from the other in terms of the acceptor or donor moieties. The molecular triads, C-P-C-60, TTF-P-C-60 and C-P-P-F, share the same free-base tetraarylporphyrin (P) as the primary electron donor, which after light excitation initiates the electron transfer process, but differ either in terms of the electron acceptor (fullerene derivative, C-60, versus fluorinated free-base porphyrin, PO, or in terms of the final electron donor (carotenoid polyene, C, versus tetrathiafulvalene, TTF). All these molecular triads can be considered artificial photosynthetic reaction centers in their ability to mimic several key properties of the reaction center primary photochemistry. Photoinduced charge separation and recombination have been followed by time-resolved EPR in a glass of 2-methyltetrahydrofuran and in the nematic phase of the uniaxial liquid crystal E-7. All the triads undergo photoinduced electron transfer, with the generation of charge-separated states in both the low-dielectric environment of the 2-methyl-tetrahydroftiran glass and in anisotropic E-7 medium. Different photochemical pathways have been recognized depending on the specific donor and acceptor moieties constituting the molecular triads. In the presence of the tetrathiafulvalene electron donor singlet- and triplet-initiated electron transfer routes are concurrently active. Recombination to the low-lying carotenoid triplet state occurs in the carotene-based triads, while singlet recombination is the only active route for the TTF-P-C-60 triad, where a low-lying triplet state is lacking. Long-lived charge separation has been observed in the case of TTF-P-C60: about 8 lis for the singlet-born radical pair in the glassy isotropic matrix and about 7 Its for the triplet-born radical pair in the nematic phase of E-7. For all the molecular triads, a weak exchange interaction (J congruent to I G) between the electrons in the final spin-correlated radical pair has been evaluated by simulation of the EPR spectra, providing evidence for superexchange electronic interactions mediated by the tetraarylporphyrin bridge.