Summary And Conclusions

Topological properties of the Laplacian of the electron density in momentum space were calculated from ab initio electronic wavefunctions for molecules that are potential components of nanoelectronic assemblies. In particular, molecules that have been proposed to function as wires and diodes were investigated with a novel computational procedure that does not, by necessity, rely on an orbital-based description of the electronic structure of the molecules, nor does it arbitrarily subtract or select data. Instead, the salient electronic flow properties were related to the locations and magnitudes of local concentrations and depletions in the total electronic charge distribution of the molecule in momentum-space. We regard the molecular electronic charge distribution as a quantum fluid that obeys uncertainty relations. Thus specific flow features cannot be ascribed to particular locations, or functional groups, within the molecule simply by refering to the atomic orbital expansion in a "chosen" molecular orbital. Nevertheless, as functional group substitutions were made, the resulting changes in the electronic flow properties of the molecule were highlighted by the Laplacian analysis, and interpreted with chemical reasoning. The important effects of external electric fields on the flow properties of the molecules were also investigated.

Specifically, we have found that an organic molecule, 1, which in the absence of an external electric field displays nonlaminar slow electron dynamics, can undergo a transition to laminar slow electron dynamics when an external electric field is turned "on". That is, this molecule behaves as a semiconducting wire. We have also found that with similar electric field strengths that induced a transition to laminar slow electron flow in the semiconducting wire above, we observe qualitatively different behavior, "on" versus "off", for the "molecular diode" 2, depending on the direction of the applied field. Thus, this molecule behaves as a rectifying diode. Interesting new features in the structure of motion are observed for 3, which is fundamentally different in that it has a saturated, hence insulating, bridging group. Further application of this methodology seems promising.

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