  In a joint theoretical and experimental work the optical properties of
azobenzene-functionalized self-assembled monolayers (SAMs) are studied at
different molecular packing densities. Our results, based on density-functional
and many-body perturbation theory, as well as on differential reflectance (DR)
spectroscopy, shed light on the microscopic mechanisms ruling photo-absorption
in these systems. While the optical excitations are intrinsically excitonic in
nature, regardless of the molecular concentration, in densely-packed SAMs
intermolecular coupling and local-field effects are responsible for a sizable
weakening of the exciton binding strength. Through a detailed analysis of the
character of the electron-hole pairs, we show that distinct excitations
involved in the photo-isomerization at low molecular concentrations are
dramatically broadened by intermolecular interactions. Spectral shifts in the
calculated DR spectra are in good agreement with the experimental results. Our
findings represent an important step forward to rationalize the excited-state
properties of these complex materials.
