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Negative Differential Resistance in Conjugated Molecular Wires: A Quantum-Chemical Insight Jérôme Cornil Laboratory for Chemistry of Novel Materials Center for Research on Molecular Electronics and Photonics University of Mons-Hainaut (Belgium) Conjugated molecular wires inserted into nanopores can be used as active elements for the fabrication of resonant tunneling diodes (RTDs), whose I/V characteristics reveal a Negative Differential Resistance (NDR) behavior, i.e., an initial raise of the current followed by a sharp decrease as the voltage is amplified [1]. Several mechanisms have been proposed to explain the origin of the NDR signal and are still hotly debated. In this contribution, we demonstrate on the basis of quantum-chemical calculations that the incorporation of saturated spacers [2] or the introduction of ring twists [3] along conjugated backbones can promote a NDR behavior in molecular wires; this is simply illustrated by analyzing the evolution of the one-electron structure of the wires as a function of a static electric field applied along the chain axis (which simulates the driving voltage applied in the experiments). [1] M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, J. M. Tour, Science 1997, 278, 252; M. A. Reed, IEEE Proc. 1999, 87, 652. [2] Y. Karzazi, J. Cornil, J.L. Brédas, J. Am. Chem. Soc. 2001, 123, 10076. [3] J. Cornil, Y. Karzazi, J.L. Brédas, J. Am. Chem. Soc., in press.