  The soft character of organic materials leads to strong coupling between
molecular nuclear and electronic dynamics. This coupling opens the way to
control charge transport in organic electronic devices by inducing molecular
vibrational motions. However, despite encouraging theoretical predictions,
experimental realization of such control has remained elusive. Here we
demonstrate experimentally that photoconductivity in a model organic
optoelectronic device can be controlled by the selective excitation of
molecular vibrations. Using an ultrafast infrared laser source to create a
coherent superposition of vibrational motions in a pentacene/C60 photoresistor,
we observe that excitation of certain modes in the 1500-1700 cm$^{-1}$ region
leads to photocurrent enhancement. Excited vibrations affect predominantly
trapped carriers. The effect depends on the nature of the vibration and its
mode-specific character can be well described by the vibrational modulation of
intermolecular electronic couplings. Vibrational control thus presents a new
tool for studying electron-phonon coupling and charge dynamics in
(bio)molecular materials.
