Charge Transport and Conductance Switching of Redox-active Azulene Derivatives

TL;DR

This study investigates how the position of substituents on azulene derivatives affects their charge transport and conductance switching behavior, revealing position-dependent electronic properties and nonlinear current-voltage characteristics.

Contribution

It demonstrates the impact of substitution points on azulene's conductance and switching, combining experimental measurements with density functional theory calculations.

Findings

1,3 Az derivative shows lower conductance and hysteresis.

Position of substituents influences the active transport channel.

1,3 Az exhibits bias-dependent conductance switching.

Abstract

Azulene (Az) is a non-alternating, aromatic hydrocarbon composed of a five-membered, electron-rich and a seven-membered, electron-poor ring; an electron distribution that provides intrinsic redox activity. By varying the attachment points of the two electrode-bridging substituents to the Az centre, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3 Az derivative is at least one order of magnitude lower than those of the 2,6 Az and 4,7 Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3 Az exhibits pronounced nonlinear current-voltage characteristics with hysteresis, indicating a bias-dependent conductance switching. DFT identifies the LUMO to be nearest to the Fermi energy of the electrodes, but to be an active transport channel only in the case of the 2,6 and the 4,7 Az derivatives, whereas the 1,3 Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3 Az creates a slow electron-hopping channel responsible for the voltage-induced switching due to the occupation of a single MO.