High on-off conductance switching ratio in optically-driven self-assembled conjugated molecular systems

TL;DR

This study reports a novel azobenzene-thiophene molecular switch with a high on-off conductance ratio, achieved through optical control of molecular configuration, supported by experimental measurements and first-principles calculations.

Contribution

It introduces a new azobenzene-thiophene molecular system with record high conductance switching ratios and elucidates the underlying mechanisms via theoretical and experimental analysis.

Findings

Achieved an on/off conductance ratio up to 7,000.

Identified the cis isomer as the high conductance state.

Demonstrated the role of molecular orbitals and interface energetics in switching.

Abstract

A new azobenzene-thiophene molecular switch is designed, synthesized and used to form self-assembled monolayers (SAM) on gold. An "on/off" conductance ratio up to 7x1E3 (with an average value of 1.5x1E3) is reported. The "on" conductance state is clearly identified to the cis isomer of the azobenzene moiety. The high "on/off" ratio is explained in terms of photo-induced, configuration-related, changes in the electrode-molecule interface energetics (changes in the energy position of the molecular orbitals with respect to the Fermi energy of electrodes) in addition to changes in the tunnel barrier length (length of the molecules). First principles DFT calculations demonstrate a better delocalization of the frontier orbitals, as well as a stronger electronic coupling between the azobenzene moiety and the electrode for the cis configuration over the trans one. Measured photoionization cross-sections for the molecules in the SAM are close to the known values for azobenzene derivatives in solution.