The effect of PAMAM dendrimers on the excited states of little-explored xanthene dyes. In search of eco-friendly photoinitiating systems

Abstract

A complete photophysical and photochemical study on the partially halogenated xanthene dyes 2’,7’-dichlorofluorescein; 4, 5, 6, 7- tetrachlorofluorescein; 4’, 5’-dibromofluorescein and 4’, 5’-diiodofluorescein was carried out in aqueous solution. The effect of three PAMAM dendrimers of low generations (G0, G1 and G3) on their excited states was also evaluated. Absorption and fluorescence measurements showed that although the ground and singlet-excited states of the dyes interact weakly with the dendrimers, the binding constants obtained correlated with dendrimer size. The triplet-excited states of the dyes were studied by laser flash photolysis including those for the chlorinated fluoresceins; which, to the best of our knowledge, had not been characterized until now. Iodo- and bromo-fluoresceins showed high triplet quantum yields, whereas chlorinated ones showed values in the order of 0.10, indicating that, although low, the intersystem crossing is not a negligible process. Efficient electron transfer reactions were observed between amino moieties of PAMAM and the triplet states of the dyes. The triplet quenching rate constants also correlated with dendrimer size and the semi-reduced species of the dyes were characterized by their transient spectra. Radical formation efficiencies similar to those obtained with a typical tertiary amine were estimated, so the present results suggest that these dye/dendrimer couples might be promising as photoinitiating systems for polymerizations with low ecological impact.