# Understanding effects of packing and chemical terminations on the optical excitations of azobenzene-functionalized self-assembled monolayers

## TL;DR

This study uses first-principles calculations to explore how packing density and chemical terminations affect the optical excitations and photo-isomerization efficiency of azobenzene-functionalized self-assembled monolayers, revealing ways to optimize their design.

## Contribution

It provides new insights into how intermolecular coupling and functionalization influence optical properties and photo-switching in azobenzene SAMs, guiding improved molecular design.

## Key findings

- Intermolecular coupling increases with packing density.
- Intense $ ext{π→π}^*$ resonance is suppressed in tightly packed SAMs.
- Mixing functional groups can partially preserve photo-isomerization excitations.

## Abstract

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.