The adsorption structure and orientation of a metal-free phthalocyanine $\left({\mathrm{H}}_2\mathrm{Pc}\right)$ and a magnesium phthalocyanine (MgPc) on a bilayer of NaCl films were investigated both theoretically and experimentally by means of first-principles calculations based on density functional theory and by scanning tunneling microscopy. ${\mathrm{H}}_2\mathrm{Pc}$ is adsorbed with its center over the sodium cation, and H-N bonds in the molecule are aligned with the [100] or [010] surface direction of a bilayer (001)-terminated NaCl film. The most stable structures of MgPc on the NaCl film show two kinds of orientations corresponding to the molecule rotated by $\pm 7^{\circ }$ relative to the [110] surface direction, with the Mg cation positioned over the chlorine anion in both cases. The energetic barrier for switching between these orientations is as low as 9.0 meV, and during an STM measurement, an orientational change of MgPc can be observed. The interaction between the adsorbed molecule and the NaCl film were analyzed in terms of dispersion interaction, Mg-Cl chemical bonding, and electrostatic interaction. It is found that the small electrostatic interaction between the molecule and the film gives a dominant contribution to determining the molecular orientation. Our detailed and comprehensive studies of the molecule-insulator interaction will provide knowledge to understand and control the properties of molecules on an insulating material.

• Received 26 February 2016
• Revised 4 April 2016

DOI:https://doi.org/10.1103/PhysRevB.93.165419