Toward photoswitchable dendritic hosts. Interaction between azobenzene-functionalized dendrimers and eosin

Two poly(propyleneimine) dendrimers bearing up to 32 photoisomerizable azob enzene groups in the periphery have been used as potential hosts for eosin Y (2',4',5',7'-tetrabromofluorescein dianion). The all-E azobenzene dendrim ers can be reversibly switched to their Z form by light excitation. Both th e E and Z forms of the dendrimers quench the eosin fluorescence by a static mechanism. The quenching is most likely due to an electron-transfer reacti on between the singlet excited state of eosin and the tertiary amine units present along the branches of the dendrimers. Quenching by the Z form of th e dendrimers is more efficient than quenching by the E form. The E --> Z an d Z --> E photoisomerization reactions of the azobenzene units of the dendr imers are sensitized by eosin via a triplet-triplet energy transfer mechani sm. The results obtained indicate that eosin is hosted by the dendrimers an d suggest that the Z forms are more efficient hosts than the E forms.