  It was observed in recent experiments that the current-voltage
characteristics (IV) 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. We have
calculated the polaron formation energies to check a frequently held belief,
namely, that the polaron formation can explain the observed bistability. Our
results are not consistent with such a mechanism. Instead, we propose a
conformational reorientation. 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, we identify a specific path for this rotation through the
molecule's conformational phase space. We show that this path has sufficiently
high barriers to inhibit thermal instability but still the molecule can be
switched in the voltage range of the junction stability. The theoretical IVs
reproduce qualitatively the key experimental observations. We propose, how the
alternative mechanism of conductance switching can be experimentally verified.
