J. Phys. Chem. B 2001, 105, 8885-8894 8885 Electrochemical and Spectroscopic Characterization of Viologen-Functionalized Poly(Amidoamine) Dendrimers† Wendy S. Baker, Buford I. Lemon, III, and Richard M. Crooks* Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842-3012 ReceiVed: June 29, 2001 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 1H 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. Introduction water-soluble and they have well-behaved electrochemical properties. Applications for viologens range from electron- In this paper, we describe the preparation and characterization transfer mediation to electrochromic devices.7,8 of poly(amidoamine) (PAMAM) dendrimers functionalized on their periphery with viologen groups (Scheme 1).1-3 We There has been significant recent interest in the synthesis and undertook this study in an effort to develop a series of characterization of electroactive dendrimers. These studies are electrochemically active, water-soluble macromolecules having motivated principally by the prospect of correlating electro- well-defined geometrical characteristics and discrete sizes in chemical properties with the geometrically well-defined mac- the range of 1-10 nm. This size range spans the gap between romolecular structure of dendrimers. In an electroactive den- molecular chemistry and state-of-the-art lithography. In the drimer the redox-active site may be: (1) incorporated as a single - future, we will report on electrode reactions in which the redox- core unit,9 16 (2) contained within the repeating branch units - active probe molecule and the electrode are on the same scale. of the dendrimer,17,18 (3) located on the dendrimer periphery,19 34 The unique structural and chemical properties of the viologe- or (4) positioned in more than one of these locations.30,31,35-37 nated dendrimers make them promising candidates for under- Electroactive dendrimers have been prepared that incorporate standing other fundamental electrochemical processes as well electroactive species such as ferrocene,10,11,25-27,32,34 anthraqui- such as charge-trapping, cooperative electrocatalysis, and the none,17,18,29 cobalt polypyridyl complexes,21 pyrrole,28 thio- dependence of heterogeneous electron-transfer kinetics on phene,12,13 tetrathiafulvalene,29-31,37, porphyrins,9,15,16 and ru- molecular geometry. thenium complexes.19,20 The preparation of an electroactive PAMAM dendrimers are synthesized by addition of branches dendrimer based on a viologen skeleton has been reported by to a molecular core (typically ammonia or ethylenediamine) by Walder et al.35,36 These dendrimers, which were prepared by sequential Michael reaction with methyl methacrylate followed linking benzyl viologen branches to a phenyl or benzyl viologen by amidation with ethylenediamine.4 Dendrimers have been used core, were soluble in DMF and exhibited charge-trapping to study light-harvesting phenomena and gene transfection, for behavior. PAMAM dendrimers in particular have been func- chemical sensing applications, as templates for preparing tionalized with electroactive ruthenium complexes19,20 and dendrimer-stabilized nanoparticles, and as vehicles for drug naphthalene diimide.22-24 In the majority of these studies, a delivery.3,5,6 PAMAM dendrimers were chosen for the studies single reduction/oxidation process was observed, and it was presented here because they have relatively well-defined sizes, reported that all electroactive groups appended to the dendrimer are water-soluble, and because they can be functionalized using were electroactive regardless of generation. Adsorption and standard peptide coupling chemistry. Viologens are likewise electroprecipitation of electroactive dendrimers is also com- mon.12,19-22,24,27-29,31,32,36,37 † Part of the special issue “Royce Murray Festschrift”. * To whom correspondence should be addressed. Voice: 979-845-5629. The specific objective of the study reported here was to Fax: 979-845-1399. E-mail: [email protected]. synthesize and characterize a series of viologen-functionalized 10.1021/jp012473d CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001 8886 J. Phys. Chem. B, Vol. 105, No. 37, 2001 Baker et al. SCHEME 1 PAMAM dendrimers prepared using the simple synthetic â-cyclodextrin hydrate, NaOD (98+%) and anhydrous dimeth- strategy shown in Scheme 1. 1H NMR, matrix-assisted laser ylformamide (99.8%) were purchased from the Aldrich Chemi- desorption-ionization time-of-flight mass spectrometry (MALDI- cal Co. (Milwaukee, WI). KNO3 (99%), HCl, CH2Cl2 (99.5%), TOF MS), and UV-vis spectroscopy verified the extent of acetonitrile (99.8%), and benzene (99%) were from EM Science. functionalization of the peripheral groups. Dynamic light Toluene (Mallinckrodt), Na2SO4 (Mallinckrodt), TRIZMA Base scattering (DLS) measurements showed that there is little (99.9%,Sigma), Ru(NH3)6Cl3 (Strem, 99%), ethanol (Aaper, interdendrimer aggregation in low ionic strength electrolytic 100%), D2O (99.9% Cambridge Isotopes Lab), and all other solutions. Cyclic voltammetry at a glassy carbon macroelectrode chemicals were used as received unless otherwise noted. indicated that the reversibility of the electrochemical response Deionized water (Milli-Q, Millipore, 18 MΩ-cm) was used depends on the identity of the supporting electrolyte. In 0.1 M throughout. K2SO4, a thermodynamically reversible response was obtained Electrochemistry. Electrochemical experiments were per- for the first reduction wave of generations 2, 4, and 6 > 2+ 2+ formed in deoxygenated aqueous solutions ( 15 min N2 purge) viologenated dendrimers (G2-(V )x(NH2)16-x, G4-(V )x- 2+ in a sealed cell using a Pt-gauze counter electrode and a Ag/ (NH2)64-x, and G6-(V )x(NH2)256-x, respectively), allowing AgCl (3M NaCl) reference electrode (Bioanalytical Systems, diffusion coefficients to be determined. UV-vis spectra of the Inc. (BAS), West Lafayette, IN). Microelectrode experiments reduced viologenated-dendrimers indicate intramolecular dimer- were performed in a Faraday cage using a BAS model 100B ization, as has been previously observed upon reduction of electrochemical analyzer coupled to a PA-1 current preamplifier dendrimers peripherally functionalized with naphthalene di- and i-E transducer (BAS). Prior to microelectrode experiments, imide.22-24 Further reduction of the viologen to its direduced the 5.2 µm-radius Pt electrode (BAS) was polished for 5 min species resulted in reduced solubility and electroprecipitation. on a polishing cloth (Microcloth, Buehler, Lake Bluff, IL) using an aqueous 1 µm γ-Al O (Buehler) slurry, rinsed with deionized Experimental Section 2 3 water, and polished again with 0.1 µm γ-Al2O3 (GAMAL, Chemicals. Generation 2, 4, and 6 amine-terminated Fisher). Polishing was followed by 2 min of sonication in a PAMAM dendrimers (G2-NH2, G4-NH2, and G6-NH2, respec- water/ethanol mixture. Microelectrode chronoamperometry ex- tively) were purchased from Dendritech, Inc. (Midland, MI). periments were performed at least five times. Background N-hydroxysuccinimide (97%), 1,3-dicyclohexylcarbodiimide voltammograms, obtained using 0.1 M K2SO4 electrolyte (99%), bromoethane (98%), 3-bromopropionic acid (97%), 4,4′- solutions adjusted to the pH of the viologenated dendrimer dipyridyl (98%), Na2S2O4 (85%), KF (99%), K2SO4 (99%), solutions (pH 6.0-6.3), were subtracted from the microelectrode Viologen-Functionalized Poly(Amidoamine) Dendrimers J. Phys. Chem. B, Vol. 105, No. 37, 2001 8887 voltammograms. A similar background subtraction procedure was performed for all chronoamperometry experiments. A Pine Instruments AFRDE4 bipotentiostat (Grove City, PA) and Kipp and Zonen XYY′ chart recorder (Bohemia, NY) were employed for all macroelectrode experiments. The Au (BAS) and glassy carbon (BAS) macroelectrodes were polished for 5 min on a polishing cloth in an aqueous 1 µm γ-Al2O3 slurry, cleaned gently with a Kimwipe, and sonicated for 2 min in a water/ethanol solution prior