Electrochemical and Spectroscopic Characterization of Viologen-Functionalized Poly(Amidoamine) Dendrimers

Abstract

We report the preparation and characterization of viologen-functionalized generation 2, 4, and 6 poly-(amidoamine) (PAMAM) dendrimers. An amidation reaction between the succinimide ester of 1-ethyl-1′-(3-propionic acid)-4,4′-bipyridylium dibromide and the dendrimer primary amines resulted in 13-34% end group functionalization. The water-soluble dendrimers were examined by 1 H NMR, MALDI-TOF MS, and UV-vis spectroscopy to determine the extent of functionalization. UV-vis spectra, obtained following chemical reduction of the viologenated dendrimers, indicated that the viologen radical cations were largely dimerized, which demonstrates the close proximity of the terminal viologens. Dynamic light scattering (DLS) measurements indicate that the oxidized viologenated-dendrimers are predominately unaggregated in aqueous electrolyte solutions. Diffusion coefficients determined by chronoamperometry for the three generations of viologenated PAMAM dendrimer are within 15% of values calculated using the Stokes-Einstein relationship for individual unaggregated dendrimers. Reversible voltammetry was obtained for all three generations of dendrimers for the first viologen reduction wave. The magnitude of the peak or steady-state currents indicated incomplete electrolysis of the dendrimer viologen moieties, whereas the presence of a single wave showed that all electrochemically addressable groups were equivalent. Cycling through the second reduction wave resulted in electrode passivation due to irreversible electroprecipitation. Adsorption of a stable film of oxidized viologenated-dendrimer on a Au electrode is also demonstrated by cyclic voltammetry.

Synthesis and characterization of tri(ethylene oxide)-attached poly(amidoamine) dendrimer layers on gold

“…The PAMAM dendrimer generated a peak centered at 14,215 amu. As recently observed by Baker et al [35], a series of peaks at lower m/z were found in mass spectra for both molecules. They seem to be generated by fragmentation of the molecules during ionization, based on the observation that a mass peak of around 27,500 amu can be derived from the EO 3 -dendrimer.…”

Synthesis and Catalytic Properties of Imidazole-Functionalized Poly(propylene imine)Dendrimers

“…The MALDI-MS of PPI-Im 64 (Figure 2b) is a broad ill-defined peak typical of highgeneration dendrimers. 19,20 The inset shows the complete spectrum, which displays peaks at lower m/z, which are likely due to fragmentation of the dendrimer. The center of mass of the peak at 12,565 m/z is very close to the molecular mass (12,584) of a fully functionalized dendrimer in which the original dendritic starting material was missing 4 branches.…”

“…However, we view this possibility as unlikely because all experiments were carried out in solutions containing 0.1 M salt, which is a condition known to reduce the likelihood of dendrimer aggregation. 19 An alternative explanation for the low observed catalytic activity, is that the pK a of the imidazole units attached to dendrimers is outside the range of pH 5-9. However, it seems unlikely that the dendrimer attachment chemistry would result in a shift of more than one pH unit away from the pK a of monomeric imidazole (~7).…”

Dendrimers as Multielectron Storage Devices

“…68 Electrochemical experiments performed on the two types of dendrimers in argonpurged MeCN solutions have revealed that in all cases only a fraction of the viologen units can be reduced, in agreement with previous results obtained for viologen groups appended at the periphery of poly(amido amine) dendrimers. 69 Within experimental error, this fraction corresponds to the number of the viologen units present in the outer shell (6 for A9 18 þ and B9 18 þ , and 12 for A21 42 þ and B21 42 þ ). The fact that the number of reducible viologen units is smaller than that expected cannot be attributed to lack of branches in the structures of the dendrimers because the 1 H NMR spectroscopic and mass spectral characterization show clearly that the compounds 6.5 VIOLOGEN-BASED DENDRIMERS Figure 6.18 Structural formulas of two families of dendrimers containing a 1,3,5-trisubstituted benzenoide core and 9 (A9 18 þ and B9 18 þ ) and 21 (B21 42 þ and A21 42 þ ) 4,4 0 -bipyridinium units in their branches.…”