Spin-State Switching of Manganese Porphyrin by Conformational Modification

Abstract

Controlled spin-state switching in small molecules is of great interest for recent molecular spintronic and spin-caloritronic applications. The 3d transition metal incorporated porphyrin molecules with stable paramagnetic states are one of the most explored classes of molecules for this purpose where adsorption and desorption of small gaseous molecules (e.g., CO, NO, O-2) on the transition metal center show efficient control over the spin states of metalloporphyrins. However, in the present study, using on-site Coulomb interaction incorporated density functional theory (DFT + U), we demonstrate reversible spin-state switching of NO-adsorbed manganese porphyrin (MnP) on top of a gold (111) surface by inducing conformational change in the molecular geometry. In this approach, mechanical manipulation by a scanning tunneling microscope (STM) tip can reversibly interchange the binding mode of the Mn-NO bond between ground-state linear and metastable bent conformations. And this modification leads to spin-state switching between the low-spin state (S = 0) of the linear geometry and intermediate spin (S = 1) of the bent conformer. Further, nonequilibrium Green's function based studies reveal that, in a two-terminal device architecture, spin polarized electronic transport through this MnP-based molecular junction can efficiently be switched off/on upon the conformational change. Thermally induced current and thermopower can also be modified distinctly when we introduce temperature bias in this nanodevice. Interestingly, precise tuning of the Fermi level of the device results in generation of pure spin thermopower, which is highly demanding for potential spin-caloritronic application.