Topological Radical Pairs Produce Ultrahigh Conductance in Long Molecular Wires

In the field of molecular electronics, it has been a challenge to make highly conductive long molecular wires. Typically, the conductive wires made to date are mostly designed and built from conjugated units, which conduct through a coherent and off-resonant mechanism. Within this mechanism, the conductance decays exponentially with increasing length. Therefore the conductance becomes low when more units are added to lengthen the wires. To overcome this problem, a new class of molecular wires with topological radical states at the edge sites (so-called edge states) can be created following the Su-Schrieffer-Heeger (SSH) model. These wires are one-dimensional topological insulators (1D TIs) and conduct through the energetically low-lying edge states that are localized to the wire ends. As a result, such 1D TI wires feature a reversed conductance decay, that is, the conductance increases exponentially with increasing wire length (<i>Nat. Chem. </i><b>2022</b>, <i>14</i>, 1061-1067.). However, the reversed conductance decay only appears for short wires, as when the wire length increases beyond a threshold, the localized edge states become decoupled and no longer support electron transport. To extend the wire length at which these properties can be observed, we designed topological oligo[<i>n</i>]emeraldine wires, using short 1D TIs as building blocks (<i>J. Am. Chem. Soc</i>. <b>2023</b>, <i>145</i>, 2492-2498.). As the wire length increases, so does the number of topological states, resulting in an enhanced electronic transmission along the wire with increasing length. In our experiments, we successfully drove a current of over a microampere through a single ∼5 nm molecular wire, significantly surpassing the performance of previously reported long wires. With DFT calculations, we demonstrated that the longest topological oligo[7]emeraldine exhibits over 10⁶ enhancements in the transmission compared to its isostructural non-topological molecular wire. Furthermore, there is no theoretical limit to the number of 1D SSH TI repeat units that can be incorporated with a single chain. Therefore, it is reasonable to expect that longer analogues of topological oligo[<i>n</i>]emeraldine (<i>n</i> &gt; 7) will achieve higher currents and conductances. This discovery represents a significant breakthrough in implementing molecules into complex, nanoscale circuitry, as it overcomes a fundamental hurdle that their structures become too insulating at practical lengths for designing nanoscale circuits.