Optical properties of azobenzene-functionalized self-assembled monolayers: Intermolecular coupling and many-body interactions

TL;DR

This study combines theoretical and experimental methods to analyze how molecular packing density affects the optical properties of azobenzene-functionalized SAMs, revealing the roles of excitonic effects and intermolecular interactions.

Contribution

It provides a detailed microscopic understanding of photo-absorption mechanisms and the impact of molecular density on excitonic properties in azobenzene SAMs.

Findings

Intermolecular coupling weakens exciton binding in dense SAMs

Spectral shifts match experimental differential reflectance data

Excitations involved in photo-isomerization are broadened by interactions

Abstract

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.