The known electron acceptor systems whereby the redox centers are linked by reversible noncovalent interactions are in most cases restricted to organic solVents. A kinetically labile coordinative bond has been designed for self-assembly of an electron donor (phenothiazine) and a photoinducible electron acceptor (riboflavin) in water at neutral pH. The pH dependent formation of the donor-acceptor complex in water was investigated by potentiometric titrations showing a binding constant of log K ) 5.9. The strong binding constant supports the observed large fluorescence deactivation of the riboflavin emission by the phenothiazine zinc complex. The riboflavin fluorescence lifetime was found to be constant (τ ) 4.7 ns) whatever the quencher concentration, clear evidence for a static quenching mechanism. A strong thermodynamical driving force and the observation of the riboflavin radical anion and phenothiazine radical cation by transient spectroscopy provide evidence for intramolecular electron transfer as the likely mechanism for the fluorescence quenching.
Keywords: Azacrown ether / DNA binding / Macrocycles / Supramolecular chemistry / UreaThe reaction between 1,4,7,10-tetraazacyclododecane (cyclen) and partially protected derivatives and isocyanates gives macrocyclic ureas in good yields. Bridged biscyclens with aliphatic or aromatic spacers were obtained from the reaction between diisocyanates and partially protected cyclen. The substituted cyclen derivatives with one or two urea moieties coordinate zinc(II) and copper(II) ions to form stable mononuclear and dinuclear complexes. Potentiometric titration revealed that the urea substitution significantly changes the basicity of the remaining secondary amino groups of the macrocycle. In comparison with that of cyclen
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