Molecular scale rectifier: Theoretical study

The geometric and electronic structures of polyphenyl-based conjugated mole cules (donor-spacer-acceptor),. potential candidates for molecular rectifyi ng devices, have been investigated theoretically using ab initio quantum me chanical calculations. The individual donor (n-type) and acceptor (p-type) molecular devices have been designed by substituting one or two of the hydr ogen atoms of benzene with -NH2 and -NO2 functional groups, respectively. T he molecular rectifier has been modeled by combining donor and acceptor mol ecules with methylene or dimethylene functional groups. The electronic rect ification behavior in these molecules has been analyzed from the molecular orbital energy levels and the spatial orientations of the unoccupied molecu lar orbitals. The results suggest that in such donor-acceptor molecular com plexes, while the lowest unoccupied orbital is localized on the acceptor ri ng, the highest occupied molecular orbital is localized on the donor ring. The approximate potential differences for mono substituted donor-acceptor c omplexes have been estimated to be 1.56 and 2.05 eV for the -CH2- and -CH2- CH2- spacers, respectively. It is found that for the disubstituted rectifie r complex, the potential difference increases to 2.76 eV.