Manipulation of organic polyradicals in a single-molecule transistor

Inspired by cotunneling spectroscopy of spin-states in a single OPE5-based molecule, we investigate the prospects for electric control of magnetism in purely organic molecules contacted in a three-terminal geometry. Using the gate electrode, the molecule is reversibly switched between three different redox states, with magnetic spectra revealing both ferromagnetic and antiferromagnetic exchange couplings on the molecule. These observations are shown to be captured by an effective low-energy Heisenberg model, which we substantiate microscopically by a simple valence bond description of the molecule. These preliminary findings suggest an interesting route towards functionalized all-organic molecular magnetism.

Figure 1

(a) Structure of the cruciform molecule and a sketch of the measurement setup. (b) Molecular spin model of three coupled spins with spin $1/2$. ${\mathbf{S}}_1$ and ${\mathbf{S}}_2$ are ferromagnetically coupled, $J_{12}<0$, and both are antiferromagnetically coupled to ${\mathbf{S}}_3$ with $J_{13}\approx J_{23}>0$. Increasing the gate voltage removes first ${\mathbf{S}}_3$ and then ${\mathbf{S}}_2$.

Figure 2

(a) $dI/dV$ measured as a function of $V$ and $V_g$ at zero magnetic field. (b) Same as (a), but in a magnetic field $B=8$ T. The red dashed lines represent the excitation energies extracted from the cotunneling spectrum and black solid lines are the best fit to the model Hamiltonian (1). (c) Calculated excitation spectrum [eigenenergies of Eq. (1)] as a function of magnetic field in the leftmost diamond. (d) $dI/dV$ as a function of $V$, obtained by averaging $dI/dV(V,V_g)$ between $V_g=0.2$ V and $V_g=0.4$ V (leftmost diamond). Black (red) lines are taken at $B=0$ ($B=8$ ). (e) and (f) Same as (d), but obtained by averaging between $V_g=1.4$ V and $V_g=1.5$ V (leftmost diamond) and between $V_g=2.4$ V and $V_g=2.5$ V (rightmost diamond), respectively. (g) and (h) is magnetic field scan at $V_g=1.6$ V (triplet ground state) and 2.6 V (doublet ground state), respectively.

Figure 3

Triradical form of the molecular cruciform, corresponding to the condition in the leftmost diamond in Fig. 2. One charge (most likely negative), and the corresponding spin (${\mathbf{S}}_1$), is localized close to one end of the OPE5 backbone. The vertically disposed TTF unit is oxidized twice. The corresponding spins, ${\mathbf{S}}_2$ (${\mathbf{S}}_3$) are in a meta (ortho) configuration relative to ${\mathbf{S}}_1$, resulting in a ferromagnetic (antiferromagnetic) coupling. ${\mathbf{S}}_2$ and ${\mathbf{S}}_3$ are para coupled, resulting in an antiferromagnetic coupling.