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

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 <i>trans</i> <i>-cis</i> isomerization upon UV irradiation at 365 nm, which affected both the absorption and fluorescence spectra of the coumarin part. The study demonstrated that the <i>trans</i>-to-<i>cis</i> 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 <i>trans</i> azobenzene, and the decrease in fluorescence is due to the emitted photons absorbed by the azobenzene's n<i>-</i>π* region.