Fluorescent guests hosted in fluorescent dendrimers

We have investigated the formation of host-guest complexes between dendrimers, of the poly(propylene amine) family functionalized with dansyl units at the periphery (hosts) with dye molecules (guests). Each dendrimer nD, where the generation number n goes from I to 5, comprises 2(n+1) (i.e. 64 for 5D) dansyl functions in the periphery and 2(n+1)-2 (i.e. 62 for 5D) tertiary amine units in the interior. The most thoroughly investigated systems were those with eosin as a guest. The results obtained show that: (i) the nD dendrimers dissolved in dichloromethane solution extract eosin from aqueous solutions; (ii) the maximum number of eosin molecules hosted in the dendrimers increases with increasing dendrimer generation, up to a maximum of 12 for the 5D dendrimer; (iii) the fluorescence of the peripheral dansyl units of the dendrimers is completely quenched via energy transfer by the hosted eosin molecules; (iv) the fluorescence of the hosted eosin molecules is partially quenched; (v) the eosin molecules can occupy two different sites (or two families of substantially different sites) in the interior of the dendritic structure; (vi) excitation of eosin hosted in the dendrimers causes sensitization of the dioxygen emission via eosin triplet excited state. The behavior of fluorescein and rose bengal is qualitatively similar to that of eosin, whereas naphthofluorescein is not extracted. The maximum number of dye molecules extracted by the 4D dendrimer is 25 for rose bengal and ca. 1 for fluorescein, showing that the formation of host-guest species is related to the electronic properties rather than to the size of the dye molecules. (C) 2002 Elsevier Science Ltd. All rights reserved.