Dresden 2011 – wissenschaftliches Programm

Bereiche | Tage | Auswahl | Suche | Aktualisierungen | Downloads | Hilfe

O: Fachverband Oberflächenphysik

O 75: Metal substrates: Adsorption of organic / bio molecules VIII

O 75.5: Vortrag

Donnerstag, 17. März 2011, 16:00–16:15, PHY C213

Intramolecular proton transfer: surface-confined porphyrins as four-level conductance switches — •Knud Seufert, Wili Auwärter, Felix Bischoff, David Écija, Saranyan Vijayaraghavan, and Johannes V. Barth — Physik Department E20, TUM, Germany

Free-base porphyrins accommodate two protons bound at opposing nitrogen positions in the macrocycle. The saddle-shape deformation of 2H-TPP anchored on a Ag(111) surface leads to two different configurations for every molecule, represented by a 90° rotation of the hydrogen pair. By using a low-temperature STM to apply a tunneling current, we induce a proton transfer and thus can switch the 2H-TPP reversibly between these two configurations. Controlled voltage pulses lead to a single or double deprotonation of the macrocycle resulting in 1H-TPP or TPP species. In the first case, the remaining hydrogen has four possible positions that can be clearly identified in high-resolution STM images. Again, a tunneling current applied at a bias voltage above a given threshold induces the transfer of the proton between the different nitrogens. This switching process can be directly monitored by recording I(t) traces. We present a statistical analysis of the proton transfer rate as a function of bias voltage and current. The linear current dependence of the switching events for both the 2H-TPPs and the 1H-TPPs points to a single electron process, where twice as many electrons are necessary for the case of two protons. This is related to a transition state represented by two hydrogens in a cis-like configuration that can relax either into the initial state or the rotated configuration.