Molecular Symmetry Sets Surface Disco Dance Moves

The symmetry of a molecule can determine how it moves after it’s adsorbed on a surface, according to researchers at the University of Regensburg, in Germany (Phys. Rev. Lett., DOI: 10.1103
/physrevlett.107.186103). Ingmar Swart, Jascha Repp, and colleagues studied the vibronically induced movement of copper(II) tetraazaphthalocyanine molecules deposited on a NaCl film. Varying the position of the nitrogen atoms on the outer edges of the phthalocyanine ring system produces isomers of varying symmetry that behave differently on the NaCl surface. The C₄ symmetry isomer has a fourfold rotational axis and moves across the NaCl film in two dimensions, landing mostly on chlorine atoms without rotating. The C_s isomer has a mirror plane bisecting the molecule and also moves in two dimensions, but it lands on sodium atoms and can rotate by 90° so that its mirror plane aligns with those of the surface. The asymmetric C₁ isomer moves in two dimensions as well, but it has no apparent preferred absorption site or orientation. And the C₂ isomer, which has a twofold rotational axis, moves back and forth along a straight line above NaCl bridge sites, without rotating. The researchers suggest that the different diffusion patterns arise from polar interactions between the molecule’s nitrogen atoms and the NaCl substrate.