Self-association of protected Newkome-type second-generation dendrimers at nanomolar level concentrations in aqueous solution

Abstract

Protected Newkome-type second-generation dendrimers (based on Lin's amine) were synthesized with a pyrene moiety attached to the core. The photophysical property in aqueous solution of the protected dendrimers shows self-association behavior in water. Pyrene excimer emission at 475 nm is observed in water even at very low concentrations of protected dendrimer (ca. 5 × 10^-8 M). This emission band is absent in other solvents even up to a concentration of 10^-5 M. The corresponding unprotected dendrimer does not show the pyrene excimer fluorescence. The amide of pyrene butyric acid with tert-butylamine shows the formation of excimer, albeit with very low intensity. Quenching studies on the dendrimer with hydrophilic quencher iodide anion (I^-) reveal that there is significant quenching of fluorescence intensity in the case of N-tert-butyl-4-pyren-1-ylbutyramide as compared to that of the pyrene-attached second-generation protected dendrimer. This shows that the pyrene moiety in the case of the protected dendrimer is significantly shielded from the surrounding.