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Rectification in supramolecular Zinc-Porphyrin/Fullero-pyrrolidine dyads self-organized on Gold(111)
Self-assembled donor/acceptor dyads are of current interest as they are biomimetic to the natural photosynthetic conversion system. Herein, we present an ultrahigh-vacuum scanning tunneling microscopy and scanning tunneling spectroscopy. (UHV-STM/STS) study of ex-situ self-assembled supramolecular dyads consisting of fulleropyrrolidines (PyC2C60) axially ligated to zinc(II) tetraphenylprophyrin (ZnTPP), self organized on a 4-aminothiophenol (4-ATP) self-assembled monolayer on gold-(111). These dyads show both bias polarity-dependent apparent height in STM images and highly rectifying behaviour in tunneling spectroscopy. First-principles, density functional theory calculations clarify the conformational and electronic properties of the 4-ATP/ZnTPP/PyC2C60 system. Interestingly, we find easier tunneling for electrons moving from the acceptor side of the dyads to the donor side, in the inverse-rectifying sense with respect to previously reported molecular rectifiers. Such behaviour cannot be explained as an elastic resonant tunneling process, but it can by using a model based on the Aviram-Ratner mechanism.