Coordination of Co2+ ions in the interior of poly(propylene amine) dendrimers containing fluorescent dansyl units in the periphery

We have investigated the interaction of Co2+ ions (as nitrate salt) with dendrimers of the poly(propylene amine) family functionalized in the periphery with fluorescent dansyl units. Each dendrimer nD, where the generation number n goes from 1 to 5, comprises 2((n+1)) dansyl functions in the periphery and 2((n+1)) - 2 tertiary amine units in the interior. For comparison purposes, the behavior of a monodansyl reference compound (I) has also been investigated. The results obtained have shown that: (i) the absorption and fluorescence spectra of the reference compound I are not affected by addition of Co2+ ions; (ii) in the case of the dendrimers, the absorption spectra are unaffected, but a strong quenching of the fluorescence of the peripheral dansyl units is observed; (iii) the fluorescence quenching takes place by a static mechanism involving coordination of metal ions in the interior of the dendrimers; (iv) metal ion coordination by the dendrimers is a fully reversible process; (v) a strong amplification of the fluorescence quenching signal is observed with increasing dendrimer generation. For the larger dendrimers (n = 3, 4, and 5), at very low metal ion concentration 1:1 metal/dendrimer species are formed, in which the Co2+ ion guest quenches each one of the dansyl units that becomes excited after light absorption.