Two poly(propyleneimine) dendrimers bearing up to 32 photoisomerizable azobenzene groups in the periphery have been used as potential hosts for eosin Y (2＇,4＇,5＇,7＇-tetrabromofluorescein dianion). The all-E azobenzene dendrimers can be reversibly switched to their Z form by light excitation. Both the 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 reaction 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 the dendrimers is more efficient than quenching by the E form. The E --> Z and Z --> E photoisomerization reactions of the azobenzene units of the dendrimers are sensitized by eosin via a triplet-triplet energy transfer mechanism. The results obtained indicate that eosin is hosted by the dendrimers and suggest that the Z forms are more efficient hosts than the E forms.