  We investigate the torsional vibrations in biphenyl-like molecular junctions
and transport properties in the presence of an external THz field. Ab-initio
calculations including external electric fields show that the torsional angle
{\phi} of a thiolated biphenyl junction exhibits virtually no response.
However, if functional groups are added to the molecule, creating a dipole
moment in each of the rings, an external field becomes more effective for
changing {\phi}. A model based on the cos2{\phi} dependence of the current is
proposed for the biphenyl-like molecular junctions in presence of an external
THz field including 2,2'-bipyridine, 3,3'-bipyridine and 2,2',4,4'-
tetramethyl-3,3'-bipyridine. The current through these molecules is shown to
change if the THz frequency gets in resonance to the torsional vibration mode.
