Rectification in Porphyrin/Fullerene Dyads on Au(111)*

We present an ultrahigh vacuum scanning tunneling microscopy (UHV-STM) and scanning tunneling spectroscopy study of \textit{ex-situ} self assembled supramolecular dyads, consisting of fulleropyrrolidines (PyC$_{2}$C$_{60})$ axially ligated to zinc(II) tetraphenylporphyrin (ZnTPP), self organized by axial ligation to a 4-aminothiophenol (4-ATP), self assembled monolayer on gold (111). By highly diluting the PyC$_{2}$C$_{60}$ solution, and subsequently annealing in vacuum, isolated dyads are obtained; these 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 the electronic properties of the 4-ATP/ZnTPP/PyC$_{2}$C$_{60}$ system. The rectifying behavior is explained using a model based on the Aviram-Ratner mechanism. * Work supported by the CNR-INFM, by a NSF US-Italy Cooperative Research Program {\#}OISE-0242579, by the SpiDME European project and by MIUR FIRB 2003 `SYNERGY' grant. Lab for Physical Sciences and in part by a NSF-MRSEC, DMR{\#} 0520471.