  We present a density-functional theory trend study addressing the binding of
the trans-cis conformational switch azobenzene (C6H5-N=N-C6H5) at three coinage
metal surfaces. From the reported detailed energetic, geometric, and electronic
structure data we conclude that the governing factor for the molecule-surface
interaction is a competition between covalent bonding of the central azo
(-N=N-) bridge on the one hand and the surface interaction of the two
closed-shell phenyl (-C6H5) rings on the other. With respect to this factor the
cis conformer exhibits a more favorable gas-phase geometric structure and is
thus more stabilized at the studied surfaces. With the overall binding still
rather weak the relative stability of the two isomers is thereby reduced at
Ag(111) and Au(111). This is significantly different at Cu(111), where the cis
bonding is strong enough to even reverse the gas-phase energetic order at the
level of the employed semi-local electronic exchange and correlation (xc)
functional. While this actual reversal may well be affected by the deficiencies
due to the approximate xc treatment, we critically discuss that the
rationalization of the general effect of the surface on the meta-stable
molecular states is quite robust. This should equally hold for the presented
analysis of recent tip-manipulation and photo-excitation isomerization
experiments from the view point of the derived bonding mechanism.
