Sequential Electron Transfer in a BODIPY–Aluminum(III) Porphyrin–C₆₀ Triad Studied by Transient EPR Spectroscopy

Spin polarized transient EPR spectra are reported for an aluminum(III) porphyrin (AlPorF₃) triad in which the electron donor bis-triphenylaminoborondipyrromethene (BDP-TPA₂) is attached covalently to one face of the porphyrin and an imidazole-appended C₆₀ is attached by coordination of the imidazole group to the Al(III) center on the opposite face. Excitation of the porphyrin results in two sequential electron transfer steps leading to charge separation between BDP-TPA₂ and C₆₀. The transient EPR spectra measured at room temperature in the organic solvent o-dichlorobenzene consist of two emission/absorption antiphase doublets assigned to (BDP-TPA₂)^•+ and C₆₀ ^•− in the radical pair generated by the charge separation. The two antiphase doublets show opposite net polarization with net emission on (BDP-TPA₂)^•+and net absorption on C₆₀^•−. It is proposed that the net polarization develops as a result of singlet–triplet mixing during the lifetime of the initial radical pair generated by either electron transfer from ¹AlPorF₃* to C₆₀ or hole transfer from ¹AlPorF₃* to BDP-TPA₂. Simulations of the spectrum only reproduce the observed line shape if the influence of singlet–triplet mixing is included. However, because the singlet–triplet mixing that occurs in the two possible primary radical pairs is similar, determining the order of the electron transfer and hole transfer steps unambiguously is challenging. It is argued that the larger reorganization energy and electronic coupling expected for the hole transfer make it the more likely the first step.