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Resonant hot charge-transfer excitations in fullerene-porphyrin complexes: a many-body Bethe-Salpeter study

We study within the many-body Green's function GW and Bethe-Salpeter approaches the neutral singlet excitations of the zinctetraphenylporphyrin and C70 fullerene donor-acceptor complex. The lowest transition is a charge-transfer excitation between the donor and the acceptor with an energy in excellent agreement with recent constrained density functional theory calculations. Beyond the lowest charge-transfer state, of which the energy can be determined with simple electrostatic models that we validate, the Bethe-Salpeter approach provides the full excitation spectrum. We evidence the existence of hot electron-hole states which are resonant in energy with the lowest donor intramolecular excitation and show an hybrid intramolecular and charge-transfer character, favouring the transition towards charge separation. These findings support the recently proposed scenario for charge separation at donor-acceptor interfaces through delocalized hot charge-transfer states.