Charge Transport and Conductance Switching of Redox-Active Azulene Derivatives

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 center, the influence of the redox functionality on charge transport is evaluated. The conductance of the 1,3Az derivative is at least one order of magnitude lower than those of the 2,6Az and 4,7Az derivatives, in agreement with density functional theory (DFT) calculations. In addition, only 1,3Az 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 4,7Az derivatives, whereas the 1,3Az derivative uses the HOMO at low and the LUMO+1 at high bias. In return, the localized, weakly coupled LUMO of 1,3Az creates a slow electron-hopping channel responsible for the voltage-induced switching due to the occupation of a single molecular orbital (MO).