  We demonstrate the reversible in situ photoreduction of molecular junctions
of phosphomolybdate [PMo12O40]3- monolayer self-assembled on flat gold
electrodes, connected by the tip of a conductive atomic force microscope. The
conductance of the one electron reduced [PMo12O40]4- molecular junction is
increased by ca. 10, this open-shell state is stable in the junction in air at
room temperature. The analysis of a large current-voltage dataset by
unsupervised machine learning and clustering algorithms reveals that the
electron transport in the pristine phosphomolybdate junctions leads to
symmetric current-voltage curves, controlled by the lowest unoccupied molecular
orbital (LUMO) at 0.6 - 0.7 eV above the Fermi energy with ca. 25% of the
junctions having a better electronic coupling to the electrodes than the main
part of the dataset. This analysis also shows that a small fraction (ca. 18% of
the dataset) of the molecules is already reduced. The UV light in situ
photoreduced phosphomolybdate junctions are systematically featuring slightly
asymmetric current - voltage behaviors, which is ascribed to electron transport
mediated by the single occupied molecular orbital (SOMO) nearly at resonance
with the Fermi energy of the electrode and by a closely located single
unoccupied molecular orbital (SUMO) at ca. 0.3 eV above the SOMO with a weak
electronic coupling to the electrodes (ca. 50% of the dataset) or at ca. 0.4 eV
but with a better electrode coupling (ca. 50% of the dataset). These results
shed lights to the electronic properties of reversible switchable redox
polyoxometalates, a key point for potential applications in nanoelectronic
devices.
