By means of scanning tunneling microscopy (STM), the self-assembly of two organic donor-acceptor-donor triads (donor=oligo(p-phenylene vinylene) (OPV); acceptor=perylene diimide (PDI)) and their mixtures has been investigated at the liquid/solid interface. Both triads differ in the nature of the substituents and, therefore, in the redox properties of the central perylene diimide unit (H or Cl). Thanks to the submolecular resolution, the distinct electronic properties of the units, within a triad and between the two triads, are reflected by the relative STM contrast in the bias-dependent imaging experiments. Moreover, scanning tunneling spectroscopy reveals an inverse rectifying behavior of the OPV and H-substituted PDI units, which is discussed in the framework of quasi-resonant tunneling. A striking difference is observed for the Cl-substituted triad.