Influence of Structural Fluctuations, Proton Transfer, and Electric Field on Polarization Switching of Supported Two-Dimensional Hydrogen-Bonded Oxocarbon Monolayers

The structural alignment, proton transfer, and molecular dipole under an electric field and as a function of simulation time have been investigated computationally for experimentally observed two-dimensional sheets of croconic acid (CA) on Ag(111) surface and rhodizonic acid (RA) molecules on Au(111) surface at room temperature. Depending on their local environment, some of the OH···O bonds in the CA monolayer exhibit spontaneous proton transfer especially for those bonds that are part of a trimer unit within the hydrogen-bonding network. In stark contrast, the RA molecules exhibit little proton transfer. It is found that thermal structural fluctuations of the molecular layers translate into considerable fluctuations of the polarization vector within the film plane, and even polarization reversal, at room temperature, which even can mask additional contributions to the polarization from the spontaneous and electric field induced proton transfer in CA monolayer. A common feature for both supported CA and RA monolayers is their constant polarization normal to the film plane. (Figure Presented).