Sequential electron transfer in a bis(styryl)BODIPY-aluminum(III) porphyrin - naphthalenediimide reaction center mimic

Photoinduced electron transfer properties are reported for a triad composed of a central aluminum(III) porphyrin (AlPorF3) with a secondary electron donor, bis-triphenylaminoborondipyrromethene (TPA2-BDP) attached covalently to AlPorF3, and an electron acceptor, pyridine-appended naphthalenediimide (NDI), attached by coordination of the pyridine group to the AlPor on the opposite face. Excitation of either the AlPorF3 or BDP-TPA2 in the triad results in a two-step electron transfer leading to (TPA2-BDP)•+AlPorF3-NDI•- as the final charge-separated species. The electron transfer is investigated using the steady-state fluorescence spectroscopy and time-resolved EPR spectroscopy. The behavior of the triad is compared to that of a previously reported triad in which the NDI acceptor was replaced with C60. The two triads have virtually identical energetics, but the reorganization energies of the acceptors are known to be very different. The spectroscopic data show that the final charge-separated state is formed in both triads despite the large difference in the reorganization energy associated with the acceptors. This suggests that the ability to perform sequential electron transfer does not depend strongly on the properties of the final acceptor, provided that secondary electron transfer can outcompete recombination in the initial charge-separated state.