A series of model molecules [sequential quinone (Q) or hydroquinone (HQ) rings connected by triple bonds] as molecular wires have been investigated by using density functional theory combined with nonequilibrium Green's function method. The results show that the system has two discrete conductance states: a low‐conductance state with Q form, and a high‐conductance state with HQ form. The systematic investigations have suggested that more Q/HQ pairs in the system may improve the on/off ratio, though long molecule reduces the conductance of the molecular junction. The switch mechanism has been explained via molecular electronic structure as well as transmission spectra. © 2012 Wiley Periodicals, Inc. J Comput Chem, 2012