High-Potential Hypervalent Antimony(V) Porphyrin-C Conjugates: Excitation Energy Transfer Dominates over Reductive Electron Transfer.

High-potential hypervalent antimony(V) porphyrins have been covalently linked to the well-known electron acceptor C molecule, resulting in the formation of antimony(V) porphyrin-fullerene conjugates: SbP-C and SbPF-C. The two porphyrins, SbP and SbPF contain four -phenyl and -3,4,5-trifluorophenyl units, respectively. These systems are designed to leverage the exceptionally high redox potentials of SbP and SbPF to create an unusual pattern of excited state energies in a porphyrin-fullerene conjugate, in which the energy of the (porphyrin)-C state lies between the porphyrin and fullerene excited singlet states. Time-resolved spectral data show that ultrafast singlet-singlet energy transfer from the porphyrin to the C unit occurs. The estimated energetics suggest that the C* state could be populated from porphyrin excited singlet state by either the usual Förster mechanism or by electron transfer from the C unit to the excited SbP/SbPF moiety followed by charge recombination. However, spectral features associated with the charge-separated state are not observed, and the energy transfer rates calculated for the Förster mechanism are in reasonable agreement with the experimental values. Thus, direct energy transfer appears to be the dominant process in these novel dyads derived from high-potential antimony(V) porphyrins.