Optical switching of electric charge transfer pathways in porphyrin: A molecular current router
The fast expanding field of molecular electronics  investigates the electrical behaviour of a molecular system, where the molecule acts as a barrier for incoming electrons, as in the original idea of "conduction as scattering" by Landauer . A central question in this research area is the features of electric charge transfer through molecular junctions . In particular, molecular junctions with several stable configurations receive significant attention in recent years .
In this work we introduce a novel molecular junction based on a thiol-functionalized porphyrin derivative. This junction possesses two energetically almost degenerate equilibrium molecular structures, shown as configurations A and B in the figure below, which define two orthogonal directions of maximal electric charge transfer, respectively, through their corresponding Highest Occupied Molecular Orbital (HOMO). We further demonstrate computationally how to switch between the two equilibrium molecular structures of the compound by optical means in two steps acting in tandem, which thus enables the control of maximal current flow direction along the junction. The two-step flipping mechanism is using one of two energetically degenerate meta-stable configurations, shown as configurations C and D in the figure below, of the compound as an intermediate ``stepping-stone''. The mechanism of optical switching is developed in the relevant configuration sub-space of the compound after obtaining the corresponding potential and electric dipole surfaces by ab initio methods. We demonstrate the potential of such a porphyrin-based molecular junction as a laser-controlled molecular current router . We have also recently discussed further proposals for laser-operated molecular devices .
The molecular junction consists of Carbon (black), Nitrogen (green), Fluor (violet), Hydrogen (cyan), and Sulfur (yellow) atoms, which potentially connect the junction to Gold electrodes (not shown here). The stable configurations A and B, with the corresponding HOMO superimposed, and the degenerate meta-stable configurations C and D are shown. The arrows schematically designate the two-step optical switching mechanism for the operation of the molecular current router.