We investigate astraphloxine, an
industrial dye, on two metal surfaces,
Au(111) and Ag(111). Low-temperature scanning tunneling microscopy
with submolecular resolution in comparison to semiempirical calculations
reveal that only two of the nine possible conformers of this molecule
are adsorbed. The two conformers adsorb via one of their indol groups,
which serves as a platform that decouples the rest of the molecule
from the surfaces. A change from one to the other conformer is demonstrated
by injecting inelastic electrons from the tunneling tip selectively
into individual molecules.
Different conformations of organic molecules are often almost isoenergetic, giving rise to their coexistence at finite temperature. In the gas phase, pressure was used to shift the equilibrium toward desired conformers. Here we use lateral pressure, enforced by increased coverage, to change the relative abundance of conformers of astraphloxin, an industrial dye, on Ag(100). We show in a variable temperature scanning tunnelling microscopy study that higher coverages enforce the predominance of one conformer only, leading to a homoconformational superstructure. The necessary conformational changes are possible at temperatures as low as 120 K.
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