# Impact of Azobenzene Side Chains on the Ultraviolet–Visible and Fluorescence Properties of Coumarin Derivatives

## TL;DR

This paper studies how adding azobenzene to coumarin affects its light absorption and fluorescence, showing changes based on the position and type of coumarin derivative.

## Contribution

The study reveals how azobenzene isomerization impacts fluorescence in coumarin derivatives, with position and functional group dependencies.

## Key findings

- Trans-to-cis isomerization of azobenzene affects UV–visible and fluorescence properties of coumarin.
- Fluorescence intensity increases in 7-hydroxycoumarin but decreases in 7-amino coumarin derivatives.
- Fluorescence changes are linked to photon absorption by azobenzene, not electronic coupling.

## Abstract

The effect of photoswitchable compounds on the light-emitting
properties
of nanoparticles has been drawing increasing attention. To investigate
the effect of photoswitchable azobenzene units on the photophysical
properties of coumarin, a biologically relevant fluorophore, photoresponsive
azobenzene-coumarin derivatives were synthesized and characterized.
The investigation of the effect of azobenzene isomerization on the
ultraviolet (UV)–visible absorption and fluorescence properties
of coumarin was explored. The azobenzene unit, attached to the coumarin
chromophore at different positions via 3 and 8 carbon linkers, exhibited
significant trans
–cis isomerization
upon UV irradiation at 365 nm, which affected both the absorption
and fluorescence spectra of the coumarin part. The study demonstrated
that the trans-to-cis transformation
of the azobenzene moiety influences fluorescence intensity, with an
increase observed in 7-hydroxycoumarin-derived compounds. However,
for derivatives of 7-amino coumarin, the fluorescence intensity decreased.
Density functional theory (DFT) and time-dependent DFT calculations
suggested that the observed fluorescence changes are unrelated to
the electronic coupling of the azobenzene and coumarin units. The
increase in fluorescence is rather related to the absorption of excitation
photons by trans azobenzene, and the decrease in
fluorescence is due to the emitted photons absorbed by the azobenzene’s
n–π* region.