title: Polyvalent scaffolds. Counting the number of seats available for eosin guest molecules in viologen-based host dendrimers.
abstract: We have prepared and investigated two dendrimers based on a 1,3,5-trisubstituted benzenoid-type core, containing 9 and 21 viologen units in their branches, respectively, and terminated with tetraarylmethane derivatives. We have shown that, in dichloromethane solution, such highly charged cationic species give rise to strong host-guest complexes with the dianionic form of the red dye eosin. Upon complexation, the absorption spectrum of eosin becomes broader and is slightly displaced toward lower energies, whereas the strong fluorescence of eosin is completely quenched. Titration experiments based on fluorescence measurements have shown that each viologen unit in the dendrimers becomes associated with an eosin molecule, so that the number of positions ("seats") available for the guest molecules in the hosting dendrimer is clearly established, e.g., 21 for the larger of the two dendrimers. The host-guest interaction can be destroyed by addition of chloride ions, a procedure which permits eosin to escape from the dendrimer's interior in a controlled way and to regain its intense fluorescence. When chloride anions are precipitated out by addition of silver cations, eosin molecules re-enter the dendrimer's interior and their fluorescence again disappears.
authors.affiliation: Dipartimento di Chimica G. Ciamician, Università di Bologna, via Selmi 2, I-40126 Bologna, Italy.
authors.forename: Filippo
authors.initials: F
authors.lastname: Marchioni
authors.forename: Margherita
authors.initials: M
authors.lastname: Venturi
authors.forename: Alberto
authors.initials: A
authors.lastname: Credi
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Balzani
authors.forename: Martin
authors.initials: M
authors.lastname: Belohradsky
authors.forename: Arkadij M
authors.initials: AM
authors.lastname: Elizarov
authors.forename: Hsian-Rong
authors.initials: HR
authors.lastname: Tseng
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart