  We investigate the spin transport properties of molecules belonging to the
acenes series by using density functional theory combined with the
non-equilibrium Green's function approach to electronic transport. While short
acenes are found to be non-magnetic, molecules comprising more than nine acene
rings have a spin-polarized ground state. In their gas phase these have a
singlet total spin configuration, since the two unpaired electrons occupying
the doubly degenerate highest molecular orbital are antiferromagnetically
coupled to each other. Such an orbital degeneracy is however lifted once the
molecule is attached asymmetrically to Au electrodes via thiol linkers, leading
to a fractional magnetic moment. In this situation the system Au/n-acene/Au can
act as an efficient spin-filter with interesting applications in the emerging
field of organic spintronics.
