A Cyclam Core Dendrimer Containing Dansyl and Oligoethylene Glycol Chains in the Branches: Protonation and Metal Coordination

Abstract

We have synthesized a dendrimer (1) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core, appended with four benzyl substituents that carry, in the 3- and 5-positions, a dansyl amide derivative (of type 2), in which the amide hydrogen is replaced by a benzyl unit that carries an oligoethylene glycol chain in the 3- and 5-positions. All together, the dendrimer contains 16 potentially luminescent moieties (eight dansyl- and eight dimethoxybenzene-type units) and three distinct types of multivalent sites that, in principle, can be protonated or coordinated to metal ions (the cyclam nitrogen atoms, the amine moieties of the eight dansyl units, and the 16 oligoethylene glycol chains). We have studied the absorption and luminescence properties of 1, 2, and 3 in acetonitrile and the changes taking place upon titration with acid and a variety of divalent (Co2+, Ni2+, Cu2+, Zn2+), and trivalent (Nd3+, Eu3+, Gd3+) metal ions as triflate and/or nitrate salts. The results obtained show that: 1) double protonation of the cyclam ring takes place before protonation of the dansyl units; 2) the oligoethylene glycol chains do not interfere with protonation of the cyclam core and the dansyl units in the ground state, but affect the luminescence of the protonated dansyl units; 3) the first equivalent of metal ion is coordinated by the cyclam core; 4) the interaction of the resulting cyclam complex with the appended dansyl units depends on the nature of the metal ion; 5) coordination of metal ions by the dansyl units follows at high metal-ion concentrations; 6) the effect of the metal ion depends on the nature of the counterion. This example demonstrates that dendrimers may exhibit complete functionality resulting from the integration of the specific properties of their component units.

Dendrimers as Nd3+ ligands: Effect of Generation on the Efficiency of the Sensitized Lanthanide Emission

“…This effect is due to the charge-transfer nature of the lowest-energy electronic transition of the dansyl chromophore. The same effect has been reported in the coordination of different metal ions [11] as well as in protonation. [15] Figure 2 shows the changes in the absorption spectra and the titration plot (inset) of 3 .…”

“…These results can be attributed to the initial coordination of Nd 3 + ions by the cyclam core and subsequent coordination to the dansylamine units at the dendrimer periphery. Indeed, three quenching mechanisms can be expected for metal ions coordinated to the cyclam core, namely 1) an effect of the positive charge of the metal ion on the emission transition moment, as observed for similar cyclam-cored dendrimers, [11] 2) an energy-transfer quenching when the metal ion has low-lying excited states, and 3) an electrontransfer quenching for metal ions that can be easily reduced (the oxidation potential of excited dansyl is ca. À1.9 V vs. SCE).…”

“…Semi-preparative RP-HPLC gave Dansyl 8 in 11 % yield with high purity ( Dansyl as reference compound (FA C H T U N G T R E N N U N G (Dansyl) = 0.28 in CH 3 CN). [11] The excited-state lifetime measured upon excitation at 350 nm is approximately 15 ns for Dansyl 4 , Dansyl 8 and the model compound Dansyl. These results demonstrate that no electronic interaction takes place in dendrimers among their chromophoric units.…”

“…units is able to bind a variety of lanthanide ions, but no sensitized emission has been detected. [11] On the contrary, efficient energy transfer from the same dansyl donor has been reported when the ions are linked by amide units of the dendritic branching points. [12] Herein we report the synthesis, characterization, photophysical behavior and sensitization of Nd 3 + emission of a novel class of dendrimers (Dansyl 4 and Dansyl 8 in Scheme 1) consisting of a cyclam core, with appended poly(amido amine) (PAMAM) dendrons, decorated at the periphery with four and eight dansyl chromophores, respectively.…”

Adducts between Dansylated Poly(propylene amine) Dendrimers and Anthracene Clips Mediated by ZnII Ions: Highly Efficient Photoinduced Energy Transfer

“…[3] This feature, coupled with the presence of coordinating moieties, has been exploited to host metal ions in the interior of dendrimers. [4,5] Molecular clips [6,7] are preorganized synthetic receptors in which the nature of the sidewalls and/or the substituents appended to the bridging units allow the tuning of their chemical properties, such as solubility, luminescence, redox potentials, and their affinity and selectivity towards specific substrates. They can mimic natural receptors and take advantage of multivalent weak interactions to exhibit strong and specific recognition of biologically relevant substrates.…”