Conductance Switching in Expanded Porphyrins through Aromaticity and Topology Changes.

Expanded porphyrins are flexible enough to switch between different π-conjugation topologies, namely Möbius, Hückel and twisted-Hückel, each with distinct electronic properties and aromaticity. Since these switches can be induced by different external stimuli, expanded porphyrins represent a promising platform to develop a novel type of molecular switch for molecular electronic devices. In this work, the feasibility of conductance switches based on topology and/or aromaticity changes in expanded porphyrins is assessed for the first time. In particular, the electron transport properties of penta-, hexa- and heptaphyrins with different π-conjugation topologies and aromaticity were carefully investigated using the nonequilibrium Green's function formalism in combination with density functional theory for various configurations of the gold contacts. Our results highlight the importance of the macrocyclic aromaticity and connectivity and, to a lesser extent, the molecular topology, in determining the transmission functions and local currents. When the electrodes are connected along the longitudinal axis of the macrocycle, we found that aromaticity of Hückel expanded porphyrins increases single-molecule junction conductance, contrary to the negative relationship between conductance and aromaticity found in single five-membered rings. For this particular connectivity, antiaromatic Hückel structures with [4n] π-electrons exhibit a sharp reduction in transmission near the Fermi level due to destructive quantum interference between the HOMO and LUMO. Belt-shaped Möbius aromatic structures exhibit a lower conductance as compared to the Hückel aromatic structures, and the current flow avoids the molecular twist. Importantly, we show that expanded porphyrins, upon redox and topology interconversions, could act as efficient three-level molecular switches with high ON/OFF ratio, up to 10 at low bias voltage.