  We report on the observation of Kondo and split Kondo peaks in
single-molecule transistors containing a single spin transition molecule with a
Fe2+ ion. Coulomb blockade characteristics reveal a double quantum dot behavior
in a parallel configuration, making our system a molecular equivalent to a
semiconducting double-quantum-dot system. As the gate voltage is increased the
charging of the second dot by an additional electron induces a splitting of the
Kondo peak. We discuss possible origins of this effect including a spin
transition into a high-spin state.
