  We investigate the electron-transport properties of ethyne-bridged diphenyl
zinc-porphyrin molecules suspended between gold (111) electrodes by
first-principles calculations within the framework of density functional
theory. It is found that the conductance of a molecular junction in which
phenyl and porphyrin rings are perpendicular is reduced by three orders of
magnitude compared with that of a junction in which the phenyl and porphyrin
rings are coplanar. In the coplanar configuration, electrons are transmitted
through $\pi$ states, which extend over the whole molecule. In the
perpendicular configuration, the conductance is suppressed because of the
reduction of electron hopping between $\pi$ states of the phenyl ring and
$\sigma$ states of the porphyrin ring.
