We present the detailed theoretical study of single attosecond pulse production from excited molecules driven by intense laser pulse. We demonstrate the possibility for continuously tunable soft x-ray production via high-order harmonic generation from a stretched aligned molecule undergoing Coulomb-barrier suppression ionization. This frequency tuning results from the quantum interference of free-electron wave packets emanating from different nuclei. High sensitivity of the shape of ionized electron wave packet to molecular alignment and internuclear separation in this regime offers the opportunity for ultrafast dynamic imaging of excited molecular transients.