title: Charge Transport and Conductance Switching of Redox-Active Azulene Derivatives.
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 center, 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 molecular orbital (MO).
authors.affiliation: Science and Technology Department, IBM Research - Zürich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland.
authors.forename: Florian
authors.initials: F
authors.lastname: Schwarz
authors.affiliation: Chemie Departement, University of Zurich, Winterthurerstrasse 190, 8057, Zürich, Switzerland.
authors.forename: Michael
authors.initials: M
authors.lastname: Koch
authors.affiliation: Institut für Theoretische Physik, TU Wien - Vienna University of Technology, Wiedner Haupstrasse 8-10, Vienna, 1040, Austria.
authors.forename: Georg
authors.initials: G
authors.lastname: Kastlunger
authors.affiliation: Chemie Departement, University of Zurich, Winterthurerstrasse 190, 8057, Zürich, Switzerland. hberke@chem.uzh.ch.
authors.forename: Heinz
authors.initials: H
authors.lastname: Berke
authors.affiliation: Institut für Theoretische Physik, TU Wien - Vienna University of Technology, Wiedner Haupstrasse 8-10, Vienna, 1040, Austria. robert.stadler@tuwien.ac.at.
authors.forename: Robert
authors.initials: R
authors.lastname: Stadler
authors.affiliation: Chemie Departement, University of Zurich, Winterthurerstrasse 190, 8057, Zürich, Switzerland. venkatesan.koushik@chem.uzh.ch.
authors.forename: Koushik
authors.initials: K
authors.lastname: Venkatesan
authors.affiliation: Science and Technology Department, IBM Research - Zürich, Säumerstrasse 4, 8803, Rüschlikon, Switzerland. eml@zurich.ibm.ch.
authors.forename: Emanuel
authors.initials: E
authors.lastname: Lörtscher
keywords: azulene; charge transport; hopping; molecular switch; redox-activity