Density Functional Theory Calculations of Spin Polarization for Diradicals in Single-Molecule Junctions

Spin-dependent electron transport through chiral molecules, namely the so-called chirality-induced spin selectivity (CISS) effect, emerged as promising for quantum applications or spintronics, among other applications, but mostly considered for closed-shell systems that exhibit spin polarization (<i>P</i> _s). In this work, we investigate <i>P</i> _s in diradicals, which is analogous but distinct from CISS in closed-shell systems that are driven, in part, by spin-orbit coupling (SOC). We report on the analysis of <i>P</i> _s for open-shell singlet and triplet organic molecular junctions with finite Au(111) electrodes in extended and compact binding configurations to probe the sensitivity to the molecule-electrode interface. We describe our developed open-source nonequilibrium Green's function transport software package based on the Landauer-Büttiker approach to compute the percent spin polarization <i>P</i> _s (%) using density functional theory with SOC. The method is used to predict <i>P</i> _s (%) for the recently synthesized indeno-[2,1-<i>b</i>]-fluorene diradicals and helicene diradicals, specifically (dicyanomethylidenyl)[5]-helicene, double bis[5]-diazahelicene, and the larger π-extended thio[7]-helicene. We demonstrate large <i>P</i> _s (%) values that correlate with large diradical character, driven by the inherent spin asymmetry in the electronic structurea mechanism that is fundamentally distinct from and more effective than SOC in closed-shell chiral systems. These unique open-shell systems were found to be prime candidates for improved spin-polarized electron transport.