Collective chirality flipping of dibenzopentacene molecules induced by an electric field

We report a scanning tunnelling microscopy (STM) study on the collective chirality flipping of dibenzopentacene (DBPen) molecules on Cd(0001). It is observed that the DBPen monolayer formed on Cd(0001) is composed of parallel molecular rows, which exhibit two enantiomeric lattices (chiral domains). The flat-lying molecules in the parallel rows exhibit an S-like shape with clockwise or anticlockwise handedness (single-molecule chirality). In particular, the pulse voltages from the STM tip lead to the simultaneous rotations of molecular rows and long-molecular-axes. When the two rotations have the same angle, pure lattice rotation takes place in the DBPen monolayer. When the two rotational angles are different, collective chirality flipping takes place in the homochiral domains, accompanied by the reversal of lattice chirality. Statistical analysis demonstrates that the chirality flipping takes place only at large negative pulse voltage, and there is a weak dependence of the chiral reversal probability on the tunnelling current, suggesting that the electric field of the STM tip is the primary driving force for chirality reversal.