Theoretical investigation of the origin of negative differential resistance in substituted phenylene ethynylene oligomers

In the very active area of molecular electronics, individual molecules or self-assembled molecules have been shown to behave as microscopic switches in transistor and diode architectures. In particular, it has been demonstrated that molecular wires inserted into nanopores and positioned between two metallic electrodes can be used as active elements for the fabrication of resonant tunnelling diodes (RTDs), whose I/V characteristics reveal a negative differential resistance (NDR) behaviour (i.e. a negative slope in the I/V curve). In this paper, we describe at the quantum chemical level a possible mechanism, based on conformational effects, rationalizing the experimental observation of an NDR signal in phenylene ethynylene oligomers. We will demonstrate that the origin of the peak profile in the I/V curves can be described on a qualitative basis from the evolution of the one-electron structure of the wires upon application of a static electric field aligned along the molecular axis, which simulates the driving voltage applied between the two electrodes in the RTD devices.