  One of the primary objectives in molecular nano-spintronics is to manipulate
the spin states of organic molecules with a d-electron center, by suitable
external means. In this letter, we demonstrate by first principles density
functional calculations, as well as second order perturbation thoery, that a
strain induced change of the spin state, from S=1 $\to$ S=2, takes place for an
iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene
lattice. The process is reversible in a sense that the application of tensile
or compressive strains in the graphene lattice can stabilize FeP in different
spin states, each with a unique saturation moment and easy axis orientation.
The effect is brought about by a change in Fe-N bond length in FeP, which
influences the molecular level diagram as well as the interaction between the C
atoms of the graphene layer and the molecular orbitals of FeP.
