Spectra and photorelaxation of tris-biphenyl-triazine-type UV absorbers: from monomers to nanoparticles.

Abstract

Water-insoluble organic UV filters like tris-biphenyl-triazine (TBPT) can be prepared as aqueous dispersions of nanoparticles. The particles consist of the respective UV absorber molecules and show strong UV absorbance. Since there is a certain solubility of such UV absorbers in organic solvents, it is possible to measure the absorbance spectrum also in solution, for instance in ethanol or dioxane. The UV spectrum of the aqueous dispersion shows a slight hypsochromic shift of the original band with an additional shoulder at longer wavelengths. For the understanding of the observed changes of UV-Vis spectra of this UV absorber, either dissolved in an organic solvent or dispersed as nanoparticles in water, DFT calculations were carried out with the respective monomer and aggregates of TBPT molecules in the different media. The calculated UV-Vis spectra of isolated, that means dissolved, TBPT molecules in ethanol and in dioxane agree well with experimentally observed ones. The observed changes in the shape of experimental UV-Vis spectra in aqueous dispersion cannot be explained with a solvent effect only. It was found that the studied molecules could form stable energetically favorable π-stacked aggregates, which show UV-Vis spectra in reasonable agreement with those experimentally observed in aqueous dispersion. Such aggregates of TBPT are most likely the reason for the observed additional shoulder in the UV/vis absorbance spectrum. In addition, the mechanism of the photochemical deactivation of excited TBPT molecules was studied in detail with TD DFT in dioxane and in water.