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Multiple Recognition of Barbiturate Guests by “Hamilton‐Receptor”‐Functionalized Dendrimers
Author(s) -
Dirksen Anouk,
Hahn Uwe,
Schwanke Frank,
Nieger Martin,
Reek Joost N. H.,
Vögtle Fritz,
De Cola Luisa
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200305461
Subject(s) - dendrimer , chemistry , spectroscopy , nuclear magnetic resonance spectroscopy , proton nmr , quantum yield , adduct , fluorescence , fluorescence spectroscopy , photochemistry , stereochemistry , polymer chemistry , organic chemistry , physics , quantum mechanics
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 1.4×10 3 M −1 and 1.5×10 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 3.6×10 10 s −1 . Energy transfer in 2 ⊂ 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.