title: Multiple recognition of barbiturate guests by Hamilton-receptor-functionalized dendrimers.
abstract: The well-known unsubstituted "Hamilton receptor" was monofunctionalized with an amino group and attached at the periphery of poly(propyleneamine) dendrimers through the use of an activated ester. Four generations of Hamilton-receptor-functionalized dendrimers (HR-dendrimers) were synthesized and characterized by (1)H and (13)C NMR spectroscopy and MALDI-TOF mass spectrometry. The photophysical properties of the HR-dendrimers were investigated by UV/Vis as well as with steady-state and time-resolved fluorescence spectroscopy. The dendrimers were used as multivalent hosts for the barbiturate guests Barbital (7) and [Re(Br)(CO)(3)(barbi-bpy)] (8; barbi-bpy=5-[4-(4'-methyl)-2,2'-bipyridyl]methyl-2,4,6-(1 H,3 H,5 H)-pyrimidinetrione). The stable adducts formed between the dendritic architectures (the hosts) and the barbiturate guests 7 and 8 were investigated by (1)H NMR spectroscopy and photophysical methods. The binding constants of the barbiturate guests for binding to reference compound 2 (with a single receptor unit) in chloroform were found to be 1.4 x 10(3) M(-1) and 1.5 x 10(5) M(-1) for 7 and 8, respectively. Binding of 7 to the dendrimers enhances the weak emission of the Hamilton receptor. This increase in emission is also generation dependent; it was found to be most pronounced in the case of 2 and the least in the case of the fourth-generation dendrimer 6. The unexpected increase in the quantum yield of emission from the HR-dendrimers with increasing generation could be caused by the rather rigid conformation of the Hamilton receptors in later-generation compounds, which is a result of intramolecular aggregation and steric hindrance at the periphery of the dendrimer. The photoinduced energy transfer from the excited state of the HR-dendrimers to the lower-lying excited state of the guest 8 was used to probe the formation of host-guest complexes. The rate of energy transfer was calculated to be 3.6 x 10(10) s(-1). Energy transfer in 2 subset 8 only occurred in the presence of a strong base, which shows that the basic amine core in the HR-dendrimers is crucial for this photoinduced process. The binding of 8 to the dendrimers is completely reversible: 8 can be exchanged with a competitive guest such as 7 and the emission of the HR-dendrimer is restored.
authors.affiliation: Institute of Molecular Chemistry, Universiteit van Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands.
authors.forename: Anouk
authors.initials: A
authors.lastname: Dirksen
authors.forename: Uwe
authors.initials: U
authors.lastname: Hahn
authors.forename: Frank
authors.initials: F
authors.lastname: Schwanke
authors.forename: Martin
authors.initials: M
authors.lastname: Nieger
authors.forename: Joost N H
authors.initials: JN
authors.lastname: Reek
authors.forename: Fritz
authors.initials: F
authors.lastname: Vögtle
authors.forename: Luisa
authors.initials: L
authors.lastname: De Cola
mesh_terms: D000596:Amino Acids / Q000737:chemistry; D001463:Barbiturates / Q000737:chemistry*; D004735:Energy Transfer; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D015394:Molecular Structure; D011073:Polyamines / Q000138:chemical synthesis / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet
chemical_list: D000596:Amino Acids; D001463:Barbiturates; D011073:Polyamines