Odd−Even Effects in the Structure and Stability of Azobenzene- Substituted Alkanethiolates on Au(111) and Ag(111) Substrates

Structural properties and stability of the selfassembled
monolayers (SAMs) of two prototypical azobenzene-
based alkanethiols (C6H5−NN−C6H4−(CH2)n−SH)
on Au(111) and Ag(111) substrates were studied in detail
using a combination of complementary experimental techniques.
The azobenzene moiety in these films was linked to the
thiol headgroup via short aliphatic spacers of variable length,
i.e., (CH2)3 or (CH2)4, corresponding to a different parity of n.
For both Au(111) and Ag(111) substrates, a pronounced
dependence of the packing density and molecular inclination
on the parity of n was observed, with a higher packing density
(by ∼14%) and smaller inclination (by ∼17°) of the
azobenzene moieties for n = odd as compared to n = even
on Au(111) and reversed, but somewhat reduced, behavior on Ag(111). This dependence was related to the well-known odd−
even effects in molecular assembly on noble metal substrates, reported previously for a variety of oligophenyl-substituted
alkanethiolate SAMs and observed now for the azobenzene-substituted monolayers as well, underlining their generality. The
structural odd−even behavior was accompanied by odd−even effects in the stability of the substrate−S bond, with the latter
effects being directly correlated to the respective structure variation. The above results are of general importance for the design of
functional monomolecular films and of a particular significance as a basis for dedicated photoisomerization experiments.