  We report on the fabrication and transport characterization of
atomically-precise single molecule devices consisting of a magnetic porphyrin
covalently wired by graphene nanoribbon electrodes. The tip of a scanning
tunneling microscope was utilized to contact the end of a GNR-porphyrin-GNR
hybrid system and create a molecular bridge between tip and sample for
transport measurements. Electrons tunneling through the suspended molecular
heterostructure excited the spin multiplet of the magnetic porphyrin. The
detachment of certain spin-centers from the surface shifted their spin-carrying
orbitals away from an on-surface mixed-valence configuration, recovering its
original spin state. The existence of spin-polarized resonances in the
free-standing systems and their electrical addressability is the fundamental
step for utilization of carbon-based materials as functional molecular
spintronics systems.
