Orbital Inversion Limits Conductance in Molecular Electronics

One of the major appeals of using organic molecules is that their electronic properties can be easily tuned and improved by making designer modifications to the molecular structure. Organic radicals have garnered significant interest as potential electronic components as they offer the promise of overcoming fermi-level pinning, as the energy levels at the molecule-metal interface are more closely aligned, as well as the possibility of spin-dependent transport. When strongly electron-donating or -withdrawing groups are added to the radical, the energy separation between the singly occupied orbital characteristic of radicals and the spin-degenerate closed-shell orbitals collapses and can eventually lead to orbital inversion. Though such inversion leads to greater stability in radicals, which is desirable for device usage, we show that this also limits conductance and propose a mechanism for this phenomenon, whereby orbital inversion leads to increased hybridization with the metal electrodes, and consequently, a larger injection barrier. By means of mechanically controlled break junction experiments and non-equilibrium Greens function calculations, we demonstrate this for the case of verdazyl radicals. Our findings indicate a fundamental limit for designing interfaces for efficient charge- and spin-transport in organic electronic devices.