Electrochemical and Adsorption Properties of PAMAM Dendrimers

2404

J. Phys. Chem. B 2001, 105, 2404-2411

Electrochemical and Adsorption Properties of PAMAM Dendrimers Surface-Functionalized with Polypyridyl Cobalt Complexes Kazutake Takada, Gregory D. Storrier, Jonas I. Goldsmith, and He´ ctor D. Abrun˜ a* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853-1301 ReceiVed: August 10, 2000; In Final Form: December 4, 2000

Polyamidoamine dendrimers (generations 1 and 3) surface-modified with terpyridyl cobalt complexes have been prepared and characterized by electrochemical and EQCM (electrochemical quartz crystal microbalance) techniques. EQCM studies of the dendrimer complexes show that they deposit onto a Pt electrode following the Co(II/I) reduction process and that the resulting films prevent further deposition upon successive potential scanning. The effects of electron self-exchange were observed for Co(I/II) redox processes for adsorbed layers of the dendrimers. Further, the thermodynamics and kinetics of adsorption of the dendrimers have been studied using electrochemical methods. These metallodendrimers adsorb (up to a monolayer equivalent) onto Pt electrodes at -0.20 V vs Ag/AgCl, where the Co sites have +2 charges, and the adsorption thermodynamics is well characterized by the Langmuir adsorption isotherm. The kinetics of adsorption is activation-controlled and the rate constant is larger for the higher generation (i.e., generation 3) metallodendrimer.

Introduction The study of dendrimers has recently moved from the development of synthetic protocols to the application of specific functionalities.1-4 We are interested in the electrochemical and photophysical properties of systems containing an organic core surrounded by redox-active transition metal complexes, especially those of polypyridyl ligands. We have recently reported on the spectroscopic and electrochemical properties5 as well as the adsorption thermodynamics and kinetics6 of tris- and bis(bipyridyl)ruthenium(II) pendant polyamidoamine (PAMAM) dendrimers (dend-n-[Ru(bpy)3] and dend-n-[Ru(tpy)2], where n ) 4, 8, 16, 32, 64 for generations 0, 1, 2, 3, and 4, respectively). We herein report on the electrochemical properties and thermodynamics and kinetics of adsorption onto a Pt electrode of PAMAM dendrimers surface-functionalized with polypyridyl transition metal complexes of Co [dend-n-[Co(tpy)2], where n ) 8 and 32 (Figure 1) for generations 1 and 3, respectively]. Experimental Section Materials and Apparatus. Gel permeation and cation exchange chromatography were performed using SP Sephadex LH20 or C-25 media, respectively. Mass spectra were recorded at the Mass Spectrometry Laboratory, University of Illinois at UrbanasChampaign and at the BioResource Center at Cornell University. Electrospray ionization mass spectrometry (ESI-MS) and matrix-assisted laser-induced desorption ionization timeof-flight mass spectrometry (MALDI-TOF MS) were used. The ESI-MS experiments were performed on a Bruker Esquire-LC ion trap mass spectrometer, and both the PAMAM dendrimers and the Co(tpy)2-functionalized dendrimers were examined. The MALDI-TOF MS experiments were done on a Bruker Biflex III mass spectrometer, and only dend-8-[Co(tpy)2(PF6)2] was examined. Elemental (C, H, N) analyses were obtained from Quantitative Technologies Inc., Whitehouse, NJ. Metal (Co) and P analyses were obtained using inductively coupled plasma atomic emission spectrometry (ICP-AES) on a Thermo Jarrell

Ash ICAP 61E Trace Analyzer in the Department of Crop and Soil Sciences at Cornell University. Electronic spectra were recorded on a Hewlett-Packard 8451A diode array spectrophotometer. EQCM and electrochemical apparatus and procedures for adsorption studies have been previously described.7 Acetonitrile (AN; Burdick and Jackson distilled in glass) for electrochemical experiments was dried over 4 Å molecular sieves. Tetra-nbutylammonium hexafluorophosphate (TBAH; Aldrich) was recrystallized three times from ethyl acetate and dried under vacuum for 96 h. All potentials are referenced to a Ag/AgCl electrode without regard for the liquid junction potential. Synthesis. Terpyridyl-Pendant Dendrimers (dend-n-tpy). dend-n-tpy (n ) 8 and 32) ligands were prepared, as described previously, through the peptide coupling of a carboxylic acid pendant terpyridine ligand with the appropriate PAMAM starburst dendrimer.5 dend-8-[Co(tpy)2(PF6)2]. Dend-8-tpy (53 mg, 13.8 µmol) and Co(tpy)Cl2 (60 mg, 165 µmol) were heated at reflux in ethanol (40 mL) and water (5 mL) for 48 h. After cooling, excess aqueous NH4PF6 (40 mL) was added. The resulting solid was filtered, dissolved in AN, and purified by gel permeation chromatography (Sephadex LH20 and C-25, AN as eluent). After recrystallization from aqueous AN (1:1) containing excess NH4PF6, the solid was filtered, dissolved in AN, and precipitated by addition to diethyl ether to afford a light maroon powder (70 mg, 58%). ES-MS: m/z 544.2, [Co(tpy)2F]+; 670.2, [Co(tpy)2(PF6)]+. MALDI-TOF MS: m/z 1001.0 [C39H38CoN8O2(PF6)2 - H]+ (see Table 1); 2177 [C110H136Co2N26O10F4 - H]+ (see Table 1). Anal. Calcd for C334H336N74O20Co8P16F96: C, 47.20; H, 3.99; N, 12.20; P, 5.83; Co, 5.55. Found: C, 44.2; H, 3.17; N, 10.64; P, 7.39; Co, 5.69. UV/vis (CH3CN) λmax/nm (10-3/L mol-1 cm-1): 274 (328), 282 (333), 318 (294), 446 (12.6), 506 (11.3), 550 (4.9). dend-32-[Co(tpy)2(PF6)]. dend-32-tpy (50 mg, 3 µmol) and Co(tpy)Cl2 (60 mg, 165 µmol) were heated at reflux in ethanol (40 mL) and water (5 mL) for 80 h. After cooling, excess

10.1021/jp002930a CCC: $20.00 © 2001 American Chemical Society Published on Web 03/06/2001

Surface-Functionalized PAMAM Dendrimers

J. Phys. Chem. B, Vol. 105, No. 12, 2001 2405

Figure 1. Structure of dend-32-[Co(tpy)2].

aqueous NH4PF6 (40 mL) was added. The resulting solid was filtered, dissolved in AN, and purified by gel permeation chromatography (Sephadex LH20 and C-25, AN as eluent). After recrystallization from aqueous AN (1:1) containing excess NH4PF6, the solid was filtered, dissolved in AN, and precipitated by addition to diethyl ether to afford a light maroon powder (85 mg, 80%). Anal. Calcd for C1390H1440N314O92Co32P64F384: C, 47.45; H, 4.12; N, 12.50; P, 5.63; Co, 5.36%. Found: C, 44.06; H, 3.42; N, 10.88; P, 8.30; Co, 6.55. UV/vis (CH3CN) λmax/nm (10-3/L mol-1 cm-1): 274 (1258), 282 (1281), 318 (1225), 446 (53), 506 (49), 550 (19). The difficulties faced in the characterization of hyperbranched molecules have been noted,8,9 while characterization of highly charged cationic dendrimers has proven to be an even more onerous task.10 Nevertheless, a variety of methods as described in the Experimental Section was employed in an effort to better characterize these materials.

Using the ion trap technique, the PAMAM dendrimer generation 1 (dend-8-) molecular ion was seen at m/z ) 1431.0 and MS/MS of that peak yielded 8 sequential losses of m/z ) 114, corresponding to the loss of each of the eight branches of the dendrimer. ESI analysis of dend-8-[Co(tpy)2(PF6)2] showed two ions at m/z ) 545.4 and 671.0, corresponding to [Co(tpy)2F]+ and [Co(tpy)2(PF6)]+ , respectively. A peak at m/z ) 1485.1, with an intensity several times greater than the previously mentioned peaks, was seen, although its identification is unclear. Using the ion trap, the PAMAM dendrimer generation 3 (dend-32-) gave peaks at m/z ) 1728 and 1383, corresponding to the 4+ and 5+ ions, respectively. ESI-MS on dend-32-[Co(tpy)2(PF6)2] gave two major peaks at m/z ) 1484.6 and 2766.3; however, those peaks have not been satisfactorialy identified. Various matrixes were used for the MALDI-TOF analysis, but the most consistent results were achieved using one of the following three matrixes: indoleacrylic acid (IAA), dihyroxy-

TABLE 1: Fragments of dend-8-[Co(tpy)2(PF6)2] Observed by MALDI-TOF MS

2406 J. Phys. Chem. B, Vol. 105, No. 12, 2001

Takada et al. benzoic acid (DHB), or R-cyano-4-hydroxycinnamic acid. In the spectra taken of dend-8-[Co(tpy)2(PF6)2], major peaks were seen at m/z ) 1001.0, 2177, 2291, 2539, and 2955. The peak at m/z ) 1001.0 is attributable to a fragement consisting of [Co(tpy)2(PF6)2] as well as the branch of the PAMAM dendrimer to which it is attached; the cleavage occurs in the same place as was previously seen in the generation 1 PAMAM dendrimer (see Table 1). The structure of the fragment giving the peak at m/z ) 2177 is also shown in Table 1. As mentioned above, the identification of dendritic macromolecules by mass spectrometry is not a trivial task, even under optimal conditions. However, these cobalt terpyridine pendant dendrimers have attributes which make their characterization especially difficult. Even the smaller dendrimer has, due to its transition metal groups, the ability to have a charge of up to 16+. Having such a large charge around a relatively small molecule leads to immense electrostatic repulsive forces between the charged centers, potentially tearing the molecule apart. While there are counterions present in the material, their behavior in the gas phase is unclear. Additionally, the nature of the hexafluorphosphate counterion can lead to very complex mass spectra. The PF6- anion can lose a neutral PF5 molecule, leaving just a fluoride anion as the counterion. In fact, we have previously observed and documented such behavior in cobalt complexes with terdentate ligands.11 With anywhere from 16 to 64 PF6- anions present in a single molecule, a great number of counterion combinations are possible. While complete characterization of these dendrimers by mass spectrometry is not possible, those peaks that were assignable in conjunction with the presence of other peaks at high values of m/z give us confidence that our compounds are indeed what we believe them to be. As a complement to the mass spectrometric data on these dendrimers, elemental analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES) was carried out on dend-8-[Co(tpy)2(PF6)2] and on dend-32-[Co(tpy)2(PF6)2]. The amounts of cobalt and phosphorus in each compounds were determined. dend-8 contained 5.69% cobalt compared to the expected value of 5.55% and 7.39% phosphorus compared with the expected value of 5.83%. dend-32 contained 6.55% cobalt compared with the expected value of 5.36% and 8.30% phosphorus compared with the expected value of 5.63%. The anomalous amounts of phosphorus present in both of the dendrimers can be attributed to excess PF6- that was not removed during purification. It is also possible that a small amount of uncomplexed cobalt containing material remains in the dendrimers, especially in dend-32, which would account for the higher than expected precentage of cobalt found. The formation of octahedral bis(terpyridyl)cobalt(II) complexes at the surface of the dendrimers was confirmed by the UV-vis spectra, which show weak transitions in the visible region consistent with the spectra of [Co(tpy)2].12 Many authors have noted that dendrimers show discrepancies between the calculated and observed analyses due to trace impurities and the tendency toward solvent inclusion.13-15 Partial elemental analyses were obtained for some dendrimers, but the presence of water or other solvents perhaps contained within the dendritic cavities leads to significant errors in the analyses. We recognize that some discrepancies remain among the results from various techniques so that one cannot be absolutely certain that there is complete functionalization. However, taken as a whole, we feel that the results are consistent with complete or nearly complete functionalization. Moreover, and perhaps



Figure 2. Typical (A) current (CV) and (B) frequency responses as a function of applied potential between -1.2 and +0.60 V vs Ag/AgCl at 50 mV s-1 for a Pt EQCM electrode in contact with a 0.10 M TBAH/ AN solution containing 25 µM (0.2 mM Co site) dend-8-[Co(tpy)2].

more important, even in the case of incomplete fucntionalization, we believe that our observations and assertions remain valid. Results and Discussion EQCM Studies. In this study, the EQCM technique was employed to investigate the electrochemical properties of the dendrimers in detail. Concentrations of the dendrimer complex solutions were adjusted so that the concentrations of cobalt(II) redox sites were equivalent (0.2 mM of Co(II) centers) in all solutions. Figure 2 shows (A) current (cyclic voltammogram, CV) and (B) frequency changes as a function of applied potential between +0.60 and -1.20 V vs Ag/AgCl for dend-8-[Co(tpy)2] in 0.1 M TBAH/ AN solution. As can be seen in Figure 2A, the CV profile exhibits redox couples with formal potential values of +0.30 and -0.75 V which we ascribe to the Co(II/III) and Co(I/II) processes, respectively. The presence of a single wave (with a one-electron waveshape) for both Co(II/III) and Co(I/II) processes implies a simultaneous (within the time scale of the experiment) electron exchange for all the Co centers at the dendrimer’s periphery. We have previously reported qualitatively similar behavior for the diaminobutane-based ferrocenyl dendrimers7,16 as well as for dend-n-[Ru(bpy)3] and dend-n[Ru(tpy)2] (n ) 4, 8, 16, 32, 64).5 The peak heights of the Co(I/II) redox processes appeared to be much larger than those of the Co(II/III). This may be a manifestation of the faster electron self-exchange of the Co(I/ II) redox couple relative to Co(II/III), similar to the phenomenon seen for [Co(bpy)3] in a Nafion film reported by Buttry and Anson.17 The electron self-exchange has been discussed in terms of the Dahms-Ruff equation,18-20

Dexptl ) D0 + (π/6)kexδ2C

(1)

where Dexptl is the experimentally observed diffusion coefficient, D0 is the diffusion coefficient without electron self-exchange (i.e., physical movement), kex is the self-exchange constant, δ is the distance between the redox centers, and C is the total

concentration. The effects of electron self-exchange have only been observed for films in which the redox active moieties are electrostatically bound,17,21 or anchored,22,23 since in those cases the physical diffusion term of the redox species in the DahmsRuff equation18,19,23 is not dominant. It should be noted that, in this study, dend-n-[Co(tpy)2]2+ (n ) 8 and 32) are not initially present, as an adsorbed layer, on the electrode surface. However, when a clean (i.e., freshly polished) electrode is brought into contact, at open circuit, with a dend-n-[Co(tpy)2]2+ (n ) 8 and 32) dendrimer solution, the dendrimers will adsorb to a coverage of up to a monolayer equivalent. Moreover, these materials can be electrodeposited to surface coverages of multilayer equivalents onto Pt electrodes (see below) upon reduction of Co(II) to Co(I). This is a situation where the effects of electron selfexchange might be observed. Indeed, when cyclic voltammetry was performed right after immersion of a Pt electrode into a solution containing dend-n-[Co(tpy)2]2+, the peak current of the Co(I/II) redox process initially appeared to be the same as that of Co(II/III), but that due to Co(I/II) grew [relative to the Co(II/III) process] with successive potential scanning, indicating the contribution of self-exchange to the measured response. The fact that the magnitudes of the current responses for the Co(II/III) and Co(I/II) couples are significantly different for adsorbed layers of dend-n-[Co(tpy)2]2+ indicates that the rotation (or similar motions) of the adsorbed dendrimers on the surface is slow, relative to the time-scale of the voltammetric experiment. In EQCM experiments, the increase in peak currents for both Co(II/III) and Co(I/II) redox reactions [and in particular for the Co(I) to Co(II) oxidation process] and the overall decrease in frequency (Figure 2B) with continuous potential scanning indicate accumulation of the dendrimer on the electrode surface. This may be caused by deposition of dend-8-[CoI(tpy)2] as a PF6- salt, likely due to the lower solubility of the dendrimer resulting from the decrease in charge of the cobalt centers upon reduction. However, upon further successive potential scans (ca. 10 scans), the current and frequency responses reached steady states (Figure 3A,B), indicating that there is no further deposition of the dendrimer. This is likely due to physical and/or electrochemical blocking to further deposition by the deposited dendrimers, which would appear to have a lower charge propagation rate as the film thickness increases. After the steady state was achieved, frequency changes accompanying the Co(I/II) redox process appeared to be typical of anion exchange (anions being the mobile species which would compensate for the changes in charges accompanying the redox reactions). In this case the reduction of Co(II) to Co(I) is accompanied by the expulsion of anions so as to maintain electrical neutrality, and this, in turn, gives rise to an increase in frequency (Figure 3). This response is different than that observed over the same potential range during deposition of the dendrimer (Figure 2B). While the dendrimer is depositing, the increase in frequency that would result from anion expulsion upon reduction of the Co(II) centers (of dendrimer molecules already adsorbed) is masked by the large decrease in the frequency due to dendrimer deposition. On the other hand, the changes in frequency accompanying the Co(II/III) process appeared to be typical of anion exchange with anion incorporation upon oxidation and expulsion upon reduction, through the continuous potential cycling. In the case of dend-32-[Co(tpy)2], cyclic voltammteric and frequency-potential curves similar to those of dend-8-[Co(tpy)2] were obtained for both the deposition stage and after reaching


Figure 3. Steady state (A) CV and (B) frequency responses as a function of applied potential for a 0.10 M TBAH/AN solution containing 0.2 mM Co site of dend-8-[Co(tpy)2]. Experimental conditions are as described in Figure 2.

the steady state (the steady-state cyclic voltammogram and frequency curves are shown in parts A and B of Figure 4, respectively). However, an additional small peak was observed at -0.52 V vs Ag/AgCl, which was not observed for dend-8[Co(tpy)2]. This peak could arise from “charge trapping”, a phenomenon originally found in bilayer films24 and since then also found in monolayer films containing two redox active monomers25 or even a single monomer.23,26-28 Charge-trapping peaks similar to the one observed in the current study have been reported for poly-[Co(v-tpy)2] (v-tpy ) 4′-vinyl-2,2′:6′,2′′terpyridine) films, and a charge-trapping mechanism based on electrochemical and EQCM studies has also been proposed.23 Further, charge-trapping peaks have also been observed in dendn-[Ru(tpy)2] and dend-n-[Ru(bpy)3] (n ) 4, 8, 16, 32, 64).5 We believe that the charge-trapping peak observed for the dend32-[Co(tpy)2] (Figure 4A) arises, at least in part, from redox centers that are electronically isolated from the surface so that their redox reactions are mediated by adjacent redox sites in a manner that is qualitatively similar to that in bilayer films.5 The fact that such a charge-trapping peak was not observed for dend8-[Co(tpy)2] might be due to the fact that it is not large enough to form the electronically isolated redox centers necessary for charge trapping. Although the overall value (and change) of the frequency of QCM electrodes in contact with a dend-n-[Co(tpy)2] (n ) 8 or 32) solution gradually decreased upon continuous potential scans (Figure 2B), as described above, it was difficult to precisely calculate the changes in mass due to the deposition and transfer of ions and/or solvent through the film/solution interface, arising from frequency changes. This difficulty was caused by changes in film properties, including rigidity, roughness, thickness, and solvophilicity, due to changes in its structure, which affect changes in the frequency.29 Changes in film properties were studied using admittance measurements of the quartz crystal resonator on the basis of its electrical equivalent circuit, especially in terms of its resistance parameter. Theoretical aspects of admittance measurements (resistance parameter) have been described previously.30

Takada et al.

Figure 4. Steady state (A) CV and (B) frequency responses as a function of applied potential for a 0.10 M TBAH/AN solution containing 6.3 µM (0.2 mM Co site) dend-32-[Co(tpy)2]. Other experimental conditions are as described in Figure 2.

Figure 5. Changes of the resistance parameter vs time for the equivalent circuit of the resonator upon applied potentials of -0.30 (b), +0.60 (Ο), and -1.20 V (]) vs Ag/AgCl in a 0.10 M TBAH/AN solution containing 0.2 mM Co site of dend-n-[Co(tpy)2]. (A) n ) 8 and (B) n ) 32.

Figure 5 shows changes of the resistance parameter for the equivalent circuit of the resonator versus time in a 0.10 M TBAH/AN solution containing 0.2 mM Co(II) sites of dendn-[Co(tpy)2] (n ) 8 and 32), at applied potentials of -1.2, -0.30, and +0.60 V, where the charge on each of the metal complexes within the dendrimer is +1, +2, and +3, respectively. For dend-32-[Co(tpy)2] (Figure 5B), the resistance remained almost constant upon first stepping the potential from -0.30 to +0.60 V [Co(II/III) oxidation], whereas it sharply increased ca. 18 Ω upon stepping the potential to -1.20 V [Co(II/I) reduction]. This large increase in the resistance is believed to arise from an increase in the thickness, roughness, and



viscoelasticity of the film due to the deposition of the dendrimer. The resistance remained almost constant upon stepping the potential back to -0.30 V [Co(I/II) oxidation], then slightly decreased upon stepping the potential to +0.60 V [Co(II/III) oxidation]. These results indicate that once the dendrimer molecules have deposited, most of them remain on the electrode surface even after oxidation of the Co centers to Co(III). These results are in good accordance with those from the EQCM measurements mentioned above. The slight decrease in the resistance following the second potential step from -0.30 to +0.60 V [Co(II/III) oxidation] might be caused by incorporation of anions and/or solvent, which make the film more rigid. On the other hand, the slight increase in the resistance upon stepping the potential from +0.60 to -0.3 V [Co(III/II) reduction], might be caused by expulsion of those species (i.e., anions and/or solvent). For dend-8-[Co(tpy)2] (Figure 5A) qualitatively similar but quantitatively smaller changes in the resistance were observed, likely due to its smaller molecular size relative to dend-32-[Co(tpy)2]. Further, the slight decrease and increase in the resistance upon stepping the potential from -0.30 to +0.60 V and from +0.60 to -0.3 V, respectively, were not observed. This is also reasonable, since films of dend-8-[Co(tpy)2] are expected to be thinner and/or smoother than those of dend-32-[Co(tpy)2]. This might be due, at least in part, to the difference in their 3D structures. For example, for the related Ru-containing dendrimer, the molecular structure (as ascertained from calculations) of dend-8-[Ru(bpy)3] was significantly flatter than that of dend16-[Ru(bpy)3].6 Adsorption Thermodynamics. The theory of adsorption thermodynamics has been described previously, especially for the self-assembly of redox-active units.31 Briefly, there are two general models that can be used to explain the thermodynamics of adsorption of the dendrimers under study. The first model is the Langmuir isotherm, which describes the adsorption process when the only adsorbate interaction is due to size, assuming that no other interactions are present. It can be expressed as

θ βC* ) 1-θ

(2)

where β is the adsorption coefficient, C* is the concentration of an adsorbate in solution, and θ is the fractional coverage defined as Γ/Γs, where Γs is the saturation surface coverage. The second model employed is the Frumkin isotherm, which is the one of the simpler isotherms taking into account interactions such as attraction or repulsion between adsorbates,32 expressed as

βC* )

θ exp(-2aθ) 1-θ

(3)

where a represents an interaction parameter. When no interactions exist between adsorbates, this equation reduces to the Langmuir isotherm (eq 2), while positive and negative values of a indicate repulsive and attractive interactions, respectively. Figure 6A,B shows the adsorption isotherms of the dend-n[Co(tpy)2] (n ) 8 and 32, respectively) adsorbed to a Pt electrode at -0.20 V vs Ag/AgCl in a 0.10 M TBAH/AN solution, where the Co sites of the dendrimers have +2 charges. It should be noted that the electrochemical deposition mentioned in the previous (EQCM) section was likely due to the lower solubility of the Co(I) species generated at potentials negative of ca. -0.80 V. On the other hand, the driving force for the adsorption of

TABLE 2: Values of ΓS, β, and ∆G°′ads for Dend-n-[Co(tpy)2]2+ (n ) 8 and 32) Adsorbed at -0.2 V vs. Ag/AgCl in 0.10 M TBAH/AN Γs (mol cm-2)

β (L mol-1)

∆G°′ads (kJ mol-1)

dend-8-[Co(tpy)2]2+ (1.8 ( 0.1) × 10-11 (3.1 ( 0.7) × 10-6 -44 ( 1 dend-32-[Co(tpy)2]2+ (7.1 ( 0.5) × 10-12 (9.7 ( 0.8) × 10-6 -47 ( 1

the dendrimer at -0.20 V (where, as mentioned above, the cobalt centers are present as Co(II), so the complexes carry a net cherge of +2) would be surface/dendrimer interactions (see below). Thus, dendrimer adsorption onto the electrode surface takes place even at potentials where the metal centers are not in the Co(I) form. From these isotherms, parameters characterizing the adsorption thermodynamics including Γs and β were determined. The values for each dendrimer were obtained using a least-squares best fit of the experimental data to the parameters of the Langmuir equation (eq 2). The calculated values of Γs and β for the two dendrimers are summarized in Table 2. The Γs value of dend-32-[Co(tpy)2] appeared to be smaller than that of dend-8-[Co(tpy)2], as anticipated from the relative sizes of the dendrimers. Further, these Γs values are in good agreement with those of dend-n-[Ru(bpy)3] (n ) 8 and 32) [(2.0 ( 0.2) × 10-11 and (6.5 ( 0.7) × 10-12, respectively].6 As reported previously, the Γs of the dend-n-[Ru(tpy)2] (n ) 4, 8, 16, 32, 64) were found to be about 1.5 times larger than those of the [Ru(bpy)3]-pendant dendrimers in a TBAP/AN solution, likely due to deposition (precipitation) of the perchlorate salt of the dendrimer. [Note that, in the present study, the supporting electrolyte anion was hexafluorophosphate (PF6-) and not perchlorate.] Further, the sizes of dend-n-[Co(tpy)2], dend-n[Ru(tpy)2], and dend-n-[Ru(bpy)3] appear to be the same for the same generations as would be anticipated given the size similarity of [M(bpy)3]+n and [M(tpy)2]+n complexes. Therefore, comparison of the Γs of the dend-n-[Co(tpy)2] to those of dendn-[Ru(bpy)3] is justified. From the adsorption coefficient β, the adsorption free energy, ∆G°′ads, was also determined, using

∆G°′ads ) -RT ln(18.9β)

(4)

The calculated ∆G°′ads values are also summarized in Table 2. Although these dendrimers ostensibly do not have groups with pendant adsorption sites, such as pyridyl, isocyanyl, or thiol groups, they nonetheless have relatively large adsorption free energies. Anson et al.33 have shown that transition metal complexes of osmium with phenyl and 1-n-butylpentyl pendant groups, which also do not have adsorption sites, also adsorb, albeit not as strongly as complexes with pendant pyridyl groups, suggesting the presence of significant van der Waals interactions. Van Duyne and co-workers34 have observed, via SERS, that [Os(bpy)3]2+ adsorbs onto Pt. They suggested that in this (and presumably in related cases) the C-5 and C-6 carbons in the pyridine ring act, at least to some extent, as an isolated olefin and that their interaction with the Pt surface is responsible, at least in part, for their adsorption. We have also reported similar large adsorption free energies for diaminobutane-based ferrocenyl dendrimers7 and dend-n-[Ru(tpy)2] and -[Ru(bpy)3] which, as in the present case, also do not have pendant adsorption sites/ groups.6 In the present case, as well as the cases of dend-n[Ru(tpy)2] and -[Ru(bpy)3], we believe that the adsorption is governed by surface/dendrimer interactions, as well as by van der Waals interactions, likely due to the relatively large molecular size of the dendrimers. In the present case it is difficult to estimate values of the interaction parameter, a, in the Frumkin equation (eq 3). As


Takada et al.

Figure 6. Langmuir isotherms fitted to experimental points for (A) dend-8-[Co(tpy)2] and (B) dend-32-[Co(tpy)2] adsorbed to a Pt electrode at -0.20 V vs Ag/AgCl in a 0.10 M TBAH/AN solution.

can be seen in Figure 6, fits to the Langmuir model were very close to the experimental data, suggesting that, at best, a would have a small magnitude. Since experimental errors in the equilibrium coverages were not negligible, we could not, based on our data, unambiguously distinguish between the Langmuir and Frumkin models. However, the obtained values of the thermodynamic parameters such as Γs and ∆G°′ads are still reliable, because the Frumkin isotherm uses the same Γs and ∆G°′ads values in the Langmuir isotherm (i.e., the interactions between adsorbate molecules are much smaller than those between the adsorbate and the substrate) and the Frumkin isotherm reduces to the Langmuir isotherm as the interaction parameter approaches zero. Adsorption Kinetics. We also investigated the kinetics of adsorption. Parts A and B of Figure 7 present plots of the surface coverage (Γt) vs time (t) for the dend-8-[Co(tpy)2] and dend-32-[Co(tpy)2], respectively, at different concentrations. All the curves in the figures were obtained by fitting the data to the kinetic (activation) control model,35

Γt ) Γe (1 - exp(-k′C*t))

(5)

where Γe is the equilibrium surface concentration at a given bulk concentration and k′ is the rate constant which contains the activity coefficient of the adsorbate in solution. As can be seen in Figure 7, for virtually all concentrations examined, calculated curves were in very good accord with experimental data for both dendrimers. These results indicate that the kinetics of adsorption of the dendrimers is well-described by the kinetic (activation) control model. The effect of the solution concentration of the adsorbates on the kinetics of adsorption was also investigated. Table 3 lists the rate constants, k′, at different solution concentrations, for dend-8-[Co(tpy)2] and dend-32-[Co(tpy)2] calculated using the activation control equation (eq 5). If the values at the lowest concentrations, for which there exists a large experimental error due to the small volume injected, are neglected, the values of the kinetic parameters of the dend-32-[Co(tpy)2] appeared to

Figure 7. Adsorption kinetics at various concentrations of (A) dend8-[Co(tpy)2] and (B) dend-32-[Co(tpy)2] adsorbed to a Pt electrode at -0.20 V vs Ag/AgCl in a 0.10 M TBAH/AN solution.

TABLE 3: Kinetic Parameter for Dend-n-[Co(tpy)2]2+ (n ) 8 and 32) Adsorbed at - 0.2 V vs Ag/AgCl in 0.10 M TBAH /AN Solution dend-8-[Co(tpy)2] dend concn (µM) 1.8 1.3 1.0 0.75 0.50 0.25 0.13 a

dend-32-[Co(tpy)2]

k′ (M-1 s-1)

dend concn (µM)

k′ (M-1 s-1)

(1.6 ( 0.1) × 104 (1.1 ( 0.2) × 104 (4.1 ( 0.5) × 103 (3.3 ( 0.5) × 103 (2.6 ( 0.2) × 103 (1.1a ( 0.1) × 104 (9.8a ( 1.3) × 103

0.44 0.31 0.25 0.19 0.13 0.063 0.031

(3.2 ( 0.2) × 104 (3.6 ( 0.4) × 104 (1.3 ( 0.2) × 104 (7.9 ( 0.1) × 103 (8.7 ( 0.6) × 103 (9.0a ( 0.1) × 103 (7.5a ( 1.6) × 104

Not reliable.

be larger than those of dend-8-[Co(tpy)2] for comparable dendrimer concentrations. For each dendrimer, the adsorption kinetics seems to be dependent upon the identity of the dendrimer, but largely independent of concentration (with some exceptions). Similar tendencies were previously observed for the diaminobutane-based ferrocenyl dendrimers7 and dend-n[Ru(bpy)3]) and dend-n-[Ru(tpy)2].6 Conclusions We have prepared polyamidoamine dendrimers (generations 1 and 3) modified with bis-terpyridyl cobalt complexes and have characterized them by electrochemical and EQCM techniques. EQCM studies of the dendrimer complexes revealed that the dendrimers deposit onto a Pt electrode following the Co(II/I) reduction process and that the resulting films prevent further deposition with successive potential scanning. At steady state, the EQCM responses for both Co(I/II) and Co(II/III) redox reactions appear to be typical of the anion exchange type. Further, for deposited dendrimer films, the peak heights of the

Surface-Functionalized PAMAM Dendrimers Co(I/II) redox processes appear to be much larger than those of the Co(II/III), likely due to the faster electron self-exchange of the Co(I/II) couple. We have also studied the thermodynamics and kinetics of adsorption of the metallodendrimers onto a Pt electrode. In a 0.10 M TBAH/AN solution, the adsorption thermodynamics of the dendrimers which have a 2+ charge is well-described by the Langmuir adsorption isotherm. The kinetics of adsorption appears to be activation-controlled rather than diffusioncontrolled. Acknowledgment. This work was supported by the Office of Naval Research and the Cornell Center for Materials Research (CCMR), a Materials Research Science and Engineering Center of the National Science Foundation (DMR-0079992). We would like to thank Jim Kerwin for his great efforts and considerable patience in helping us with the mass spectrometric analyses. We would also like to thank Michael Rutzke for analyzing several samples by ICP-AES for us. References and Notes (1) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog. Polym. Sci. 1998, 23, 1. (2) Smith, D. K.; Diederich, F. Chem. Eur. J. 1998, 4, 1353. (3) Zeng, F.; Zimmerman, S. C. Chem. ReV. 1997, 97, 1681. (4) Issberner, J.; Moors, R.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl. 1994, 33, 2413. (5) Storrier, G. D.; Takada, K.; Abrunã, H. D. Langmuir 1999, 15, 872. (6) Takada, K.; Storrier, G. D.; Morań, M.; Abrunã, H. D. Langmuir 1999, 15, 7333. (7) Takada, K.; Dıáz, D. J.; Abrunã, H. D.; Cuadrado, I.; Casado, C.; Alonso, B.; Morań, M.; Jose´ Losada. J. Am. Chem. Soc. 1997, 119, 10763. (8) Frećhet, J. M. Science 1994, 263, 1710. (9) Newkome, G. E.; Moorefield, C. N.; Baker, G. R. Aldrichim. Acta 1992, 25, 31. (10) Achar, S.; Immoos, C. E.; Hill, M. G.; Catalano, V. J. Inorg. Chem. 1997, 36, 2314. (11) Arana, R. C.; Abrunã, H. D. Inorg. Chim. 1993, 32, 194. (12) Hogg, R.; Wilkins, R. G. J. Chem. Soc. 1962, 341. (13) Issberner, J.; Vo¨gtle, F.; Cola, L. D.; Balzani, V. Chem. Eur. J. 1997, 3, 706.

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Electrochemical and Adsorption Properties of PAMAM Dendrimers

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