Anomalous Effect of Quantum Interference in Organic Spin Filters

The molecular topology in the single-molecule nanojunctions through which the de Broglie wave propagates plays a crucial role in controlling the molecular conductance. The enhancement and reduction of the conductance in para- and meta-connected molecules due to constructive and destructive quantum interference (QI), respectively, are quite well established. Herein, we investigated the effect of localized spin centers on spin transportation using organic radicals as molecular junctions. The role of the localized spins on the QI as well as on spin filtering capability is investigated employing density functional theory in combination with nonequilibrium Green’s function (NEGF-DFT) techniques. Various organic radicals including nitroxy (NO•), phenoxy (PO•), and methyl (CH2•) attached to the central benzene ring of pentacene with different terminal connections (para and meta) to gold electrodes are examined. Due to more obvious QI effects, para-connected pentacene is found to be more conductive than the meta one. Surprisingly, on incorporating a radical center, along with spin filtering, a significant reversal of QI effects is observed which manifests itself in such a way that the conductance of meta-coupled radicals is found to be more than para-coupled ones by 2 orders of magnitude. The anomaly in QI patterns induced by the radical center is analyzed and discussed in terms of orbital and structural perspectives.