  We demonstrate a new class of molecules for exceptional performance in
molecular electronics and spintronics. Phenalenyl-based radicals are stable
radicals whose electronic properties can be tuned readily by heteroatom
substitution. We employ density functional theory-based non-equilibrium Green's
function (NEGF-DFT) calculations to show that this class of molecules exhibits
tunable spin- and charge-transport properties in single molecule junctions. Our
simulations identify the design principles and interplay between unusually high
conductivity and strong spin-filtering: Paired with moderate conductance
($10^{-3} G_0$), two of the four radicals investigated exhibit above 80% spin
filter efficiency that is moreover tunable via bias control. Conversely, two
radicals that make modest spin filters are excellent conductors with a low bias
conductance reaching $0.48 G_0$. This is made possible by the unusually good
alignment of the singly occupied or unoccupied molecular orbital with the Fermi
level of the electrodes, overcoming the limitations of Fermi level pinning that
typically plague molecular electronics. We show that this interplay between
excellent conductance and high spin-filter efficiency is determined by the
energy alignment between the singly (un)occupied molecular orbital and the
Fermi level of the electrodes, and that for phenalenyls this can be readily
controlled with judicious heteroatom substitution.
