Molecular scale rectifier: Theoretical study

The geometric and electronic structures of polyphenyl-based conjugated molecules (donor-spacer-acceptor), potential candidates for molecular rectifying devices, have been investigated theoretically using ab initio quantum mechanical calculations. The individual donor (n-type) and acceptor (p-type) molecular devices have been designed by substituting one or two of the hydrogen atoms of benzene with -NH₂ and -NO₂ functional groups, respectively. The molecular rectifier has been modeled by combining donor and acceptor molecules with methylene or dimethylene functional groups. The electronic rectification behavior in these molecules has been analyzed from the molecular orbital energy levels and the spatial orientations of the unoccupied molecular orbitals. The results suggest that in such donor-acceptor molecular complexes, while the lowest unoccupied orbital is localized on the acceptor ring, the highest occupied molecular orbital is localized on the donor ring. The approximate potential differences for mono substituted donor-acceptor complexes 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 rectifier complex, the potential difference increases to 2.76 eV.