An Orthogonal Conductance Pathway in Spiropyrans for Well-Defined Electrosteric Switching Single-Molecule Junctions

While a multitude of studies have appeared touting the use of molecules as electronic components, the design of molecular switches is crucial for the next steps in molecular electronics. In this work, single-molecule devices incorporating spiropyrans, made using break junction techniques, are described. Linear spiropyrans with electrode-contacting groups linked by alkynyl spacers to both the indoline and chromenone moieties have previously provided very low conductance values, and removing the alkynyl spacer has resulted in a total loss of conductance. An orthogonal T-shaped approach to single-molecule junctions incorporating spiropyran moieties in which the conducting pathway lies orthogonal to the molecule backbone is described and characterized. This approach has provided singlemolecule conductance features with good correlation to molecular length. Additional higher conducting states are accessible using switching induced by UV light or protonation. Theoretical modeling demonstrates that upon (photo)chemical isomerization to the merocyanine, two cooperating phenomena increase conductance: release of steric hindrance allows the conductance pathway to become more planar (raising the mid-bandgap transmission) and a bound state introduces sharp interference near the Fermi level of the electrodes similarly responding to the change in state. This design step paves the way for future use of spiropyrans in single-molecule devices and electrosteric switches. Reimagining molecular switches where the conducting pathway lies orthogonal to the molecular backbone provides a new class of electrosteric switches. Higher conducting states are accessible when the spiropyran switched to the merocyanine by light or protonation. Torsion of the backbone leads to increased orbital overlap between the anchor group and central functional core unit of the junction giving increased conduction.image

Automated summary

The design of molecular switches is crucial for the development of molecular electronics. This study describes the use of spiropyrans in single-molecule devices and introduces an orthogonal T-shaped approach for incorporating spiropyran moieties, which leads to higher conducting states. Photochemical isomerization to merocyanine increases the conductance of the molecules.