  In a first-principles study based on many-body perturbation theory, we
analyze the optical excitations of azobenzene-functionalized SAMs with
increasing packing density and different terminations, considering for
comparison the corresponding gas-phase molecules and dimers. The intermolecular
coupling increases with the density of the chromophores independently of the
functional groups. The intense $\pi\rightarrow\pi^*$ resonance that triggers
photo-isomerization is present in the spectra of isolated dimers and diluted
SAMs, but it is almost completely washed out in tightly packed architectures.
Intermolecular coupling is partially inhibited by mixing differently
functionalized azobenzene derivatives, in particular when large groups are
involved. In this way the excitation band inducing the photo-isomerization
process is partially preserved and the effects of dense packing partly
counterbalanced. Our results suggest that a tailored design of
azobenzene-functionalized SAMs, that optimizes the interplay between the
packing density of the chromophores and their termination, can lead to
significant improvements in the photo-switching efficiency of these systems.
