  Obtaining highly spin-polarized currents in molecular junctions is crucial
and desirable for nanoscale spintronics devices. Motivated by our recent
symmetry-based theoretical argument for complete blocking of one spin
conductance channel in atomic-scale junctions [A. Smogunov and Y. J. Dappe,
Nano Lett. 15, 3552 (2015)], we explore the generality of the proposed
mechanism and the degree of achieved spin-polarized current for various
ferromagnetic electrodes (Ni, Co, Fe) and two different molecules,
quaterthiophene and p-quaterphenyl. A simple analysis of the spin-resolved
local density of states of a free electrode allowed us to identify the Fe(110)
as the most optimal electrode, providing perfect spin filtering and high
conductance at the same time. These results are confirmed by $ab$ $initio$
quantum transport calculations and are similar to those reported previously for
model junctions. It is found, moreover, that the distortion of the
p-quaterphenyl molecule plays an important role, reducing significantly the
overall conductance.
