Spin-dependent electronic transport through a porphyrin ring ligating an $\mathrm{Fe}\left(\mathrm{II}\right)$ atom: An ab initio study

Conductance calculations employing density functional theory methodology and Landauer formalism predict that a ligated iron atom can be used as a switching device. The iron atom is ligated in our models by a porphyrin molecule. The iron-porphyrin molecular device is shown to lose more than 66% of its conductance by shifting from the low spin coupling state to excited spin states. Further reduction is also correlated with a mechanical distortion of the porphyrin plane. Both the distortions and spin transitions are fast processes that can be invoked by manipulating the iron’s ligation scheme through the axial ligands.

Figure 1

The modeled molecular device consists of a porphyrin molecule bonded to two gold atoms through thiolate atoms. The lower inset describes the molecular plane of the ligated porphyrin species. In the upper part the porphyrin is thiol-bonded to two gold tips, in a tablelike conformation.

Figure 2

(Color online) The different transmission functions calculated by employing the WBL approximation for representing the bulk (a) and a first-principles TB model (b) at the different spin coupling states of the iron-porphyrin system (four ligation scheme). The conductance includes different contributions from the spin $\alpha$ and $\beta$ channels for the excited spin coupling states.

Figure 3

(Color online) The different conductance curves of the four ligated system calculated at the WBL (left side) and TB (right) levels for representing the bulk.

Figure 4

(Color online) The origin of the transmission dependence on the spin coupling state is analyzed. The effect on transmission is negligible due to (a) the spin coupling scheme in the absence of the iron center and (b) the geometry relaxation induced due to the spin coupling scheme in the presence of the iron center. Therefore, the electronic relaxation induced by the spin coupling state induced by the iron center is the major factor responsible for the strong dependence of the transmission on the spin state.

Figure 5

(Color online) Porphyrin orbital diagram for the free molecule and $\mathrm{FeP}$ at the different spin coupling states with the gold tip functionalization. Also included in the first two columns are, respectively, the nonthiol functionalized porphyrin and singlet $\mathrm{FeP}$ orbitals. Of high interest is the apparent changing contribution of the Fe AOs to the $e_g$ porphyrin and $\mathrm{AuS}$ orbitals. All unlabeled (black lines) states are of $\mathrm{Au}[+\mathrm{Fe}]$ character. Pure Fe orbitals have been omitted.

Figure 6

The considered change in the axial ligation from a five ligated system to a six ligated system, where an addition of a $\mathrm{CO}$ ligand as the second axial ligand is illustrated.

Figure 7

(Color online) Effects of axial ligation on the transmission (a) and conductivity (b). The quintet state of the five ligated system $\left(\mathrm{FePIm}\right)$ and the singlet six ligated system $\left(\mathrm{FePImCO}\right)$ are compared.