Charge Stabilization in High-Potential Zinc Porphyrin-Fullerene via Axial Ligation of Tetrathiafulvalene

Extending the lifetime of the charge-separated states generated during photoinduced electron transfer in a covalently linked high-potential zinc porphyrin-fullerene dyad, (F15P)Zn-C-60, was accomplished by metal ligand axial coordination of pyridine-functionalized tetrathiafulvalene (TTF) via a dual-electron-transfer/hole migration mechanism. The meso-aryl positions of the zinc porphyrin carried three penta-fluorophenyl substituents that made the zinc porphyrin ring harder to oxidize by 0.43 V compared with zinc porphyrin with meso-phenyl substituents. Two TTF derivatives, a first with a pyridine directly linked to TTF (Py-TTF) and a second with a phenyl spacer between the pyridine and TTF (Py-phTTF), were employed to vary the distance between the primary photosensitizer/electron donor, zinc porphyrin, and the secondary electron donor, TTF. Both Py-TTF and Py-phTTF coordinated via the pyridine entity to the Zn center with 1:1 molecular stoichiometry and moderate binding constants. The supramolecular triads were characterized by optical absorption and emission, electrochemistry, and computational studies. An energy-level diagram was established to realize the different photochemical events in the triads. Using femtosecond transient absorption spectroscopy, it was possible to show that the coordinated TTF participated in electron transfer from the (1)(F15P)Zn* in the case of the C-60-(F15P)Zn:TTF triads to produce C-60-(F15P)Zn center dot--:TTF center dot+ charge-separated state competitively with the electron transfer from the (1)(F15P)Zn* to covalently linked C-60 to produce C-60(center dot-)-(F15P)Zn center dot+:TTF charge-separated state. The two charge-separated states, C-60-(F15P)Zn center dot-:TTF center dot+ and C-60 center dot-(F15P)Zn center dot+: TTF, were further involved in electron migration in the former case and hole transfer in the latter case to produce the C-60(center dot-)-(F15P)Zn:TTF center dot+ charge separated state as the ultimate electron-transfer product. Due to distal separation of the positive and negative radical ions, long-lived charge-separated states persistent for about 0.35 mu s was possible to accomplish, as shown by nanosecond transient absorption spectral studies.