Molecular Switch Controlled by Pulsed Bias Voltages

Recent experiments have shown that the current–voltage characteristics (I–V) of BPDN‐DT (bipyridyl‐dinitro oligophenylene‐ethynylene dithiol) can be switched in a very controlled manner between “on” and “off” traces by applying a pulse in a bias voltage, V_bias. Here, the polaron formation energies are calculated to check a frequently held belief, namely, that the polaron formation can explain the observed bistability. These results are not consistent with such a mechanism. Instead, a conformational reorientation is proposed. The molecule carries an intrinsic dipole moment, which couples to V_bias. Ramping V_bias exerts a force on the dipole that can reorient (“rotate”) the molecule from the ground state (“off”) into a metastable configuration (“on”) and back. By elaborated electronic structure calculations, a specific path for this rotation is identified through the molecule's conformational phase space. It is shown that this path has sufficiently high barriers to inhibit thermal instability but that the molecule can still be switched in the voltage range of the junction stability. The theoretical I–Vs qualitatively reproduce the key experimental observations. A proposal for the experimental verification of the alternative mechanism of conductance switching is presented.