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 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 . A model based on the cos2 phi dependence of the current for low bias voltages has been 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. Dithiolated bipyridine between gold leads in the presence of external THz radiation.