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 (Ps). In this work, we investigate Ps 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 Ps 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 Ps (%) using density functional theory with SOC. The method is used to predict Ps (%) for the recently synthesized indeno[2,1-b]fluorene diradicals and helicene diradicals, specifically (dicyanomethylidenyl)[5]helicene, double bis[5]diazahelicene, and the larger π-extended thio[7]helicene. We demonstrate large Ps (%) 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.