Dendrimers Peripherally Modified with Anion Radicals That Form π

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Dendrimers Peripherally Modified with Anion Radicals That Form π-Dimers and π-Stacks Ibro Tabakovic,† Larry L. Miller,*,† Robert G. Duan,† David C. Tully,† and Donald A. Tomalia‡ Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, and Michigan Molecular Institute, 1910 W. St. Andrews Road, Midland, Michigan 48640 Received August 12, 1996. Revised Manuscript Received December 18, 1996X

Poly(amidoamine) (PAMAM) dendrimers, generations 1-6, were peripherally modified with cationically substituted naphthalene diimide groups. Several monomeric diimides were also prepared as models. The structure and loading of the dendrimers were determined by vis, IR, and NMR spectroscopies, elemental analysis, and coulometric analysis. In general it was possible to obtain loadings greater than 70% even from generation 6 dendrimer where there are 192 amine groups to be substituted. These polymers and monomers were reduced using sodium dithionite or electrochemically with one electron per imide group converting each imide into its anion radical. Near-infrared (NIR) spectroscopy showed that in D2O or formamide solutions the anion radicals aggregated into π-dimers and π-stacks. Cyclic voltammograms are interpreted in terms of anion radical aggregation and precipitation of the reduced dendrimers. The CV and NIR results for the various dendrimer generations were quite similar. These results allow insight into the possibilities and limitations of dendrimers to provide a scaffold for intramolecular aggregation.

Introduction Dendrimers, hyperbranched polymers with strictly controlled molecular weights, are receiving considerable current attention. 1 Of interest here are peripherally substituted poly(amidoamine) (PAMAM) dendrimers.2 PAMAM dendrimers are built from an ammonia or ethylenediamine core by addition of “amidoamine” units. Generation 6, for example, has a molecular weight of 43 507 g mol-1 and 192 primary amine groups on the periphery. A variety of groups have been attached to the periphery of PAMAM dendrimers and studies of the properties of the resulting modified dendrimers, have been reported.2 Here, we explore the chemistry of PAMAM dendrimers decorated with diimide anion radicals. It is known that monomeric diimide anion radicals aggregate into π-stacks in solution,3 and we wished to explore the effect of the 3-D dendrimer scaffold on the aggregation phenomenon. Since π-stacks are essentially one-dimensional entities,4 we thought that the three-dimensionality of the dendrimer scaffold might introduce some interesting differences. In principle the higher local concentration of anion radicals on the dendrimer should †

University of Minnesota. Michigan Molecular Institute. Abstract published in Advance ACS Abstracts, February 1, 1997. (1) (a) Issberner, J.; Moors, R.; Vogtle, F. Angew. Chem., Int. Ed. Engl. 1994, 33, 2413. (b) Frechet, J. M. J. Science 1994, 263, 1710. (c) Tomalia, D. A.; Durst, H. D. Top. Curr. Chem. 1993, 193. (d) Newkome, G. R.; Moorefield, C. N.; Baker, G. R.; Saunders, M. M.; Grossman, S. H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1178. (e) Jansen, J. F. G. A.; De Brabander-van Den Berg, E. M. M.; Meijer, E. W. Science 1994, 266, 1226. (2) (a) Tomalia, D. A. Adv. Mater. 1994, 6, 529. (b) Tomalia, D. A.; Naylor, A. M.; Goddard III, W. A. Angew. Chem. 1990, 102, 119; Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (c) Tomalia, D. A.; Baker, H.; Dewald, J. P.; Hall, M.; Kalbs, C.; Martin, S.; Roeck, J.; Smith, P. Macromolecules 1986, 19, 2466. (3) Penneau, J. F.; Stallman, B. J.; Kasai, P. H.; Miller, L. L. Chem. Mater. 1991, 3, 791. ‡

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enhance aggregation, but this would require distortion of the dendrimer framework. Thus the results could cast light on aggregation within a dendrimer structure and if aggregation depends on dendrimer size. PAMAM dendrimers were especially suitable for our purposes because they are available in quantity, they are readily substituted on the periphery, and they are water soluble.2 π-aggregation of ion radicals is enhanced by polar solvents,5 and by preparing PAMAM dendrimers decorated with ionic diimided groups, we felt that we could achieve sufficient water solubility to find π-stacks. The target dendrimers are examplified by D3-A. D3 represents generation-3 PAMAM and A represents the modifier, naphthalene diimide -CH2pyridiniumCH3+,I-. As reported in a preliminary communication,6 PAMAM dendrimers modified with diimide groups can be reduced to form poly(anion radicals), which aggregate. Here we provide details of these studies for several dendrimer generations, for three diimide modifiers, and in formamide as well as water solvent. We focus attention on the near-IR spectra of the reduced dendrimers and an analysis of cyclic voltammograms. The compounds and results reported here are of special interest for materials chemistry because the reduced dendrimers can be cast into air-stable, n-type electrically conducting films.7 The conductivity of these films is rather high (up to 20 S cm-1) and unusually (4) (a) Torrance, J. B.; Scott, B.; Welber, B.; Kaufman, F. D.; Seiden, P. E. Phys. Rev. B 1979, 19, 730. (b) Extended Linear Chain Compounds; Miller, J. S., Ed.; Plenum: New York, 1983; Vol. 1-3. (c) Lower-Dimensional Systems and Molecular Electronics; Metzger, R. M.; Day, P.; Papavassiliou, G. C. Plenum Press: New York, 1990. (d) Hunig, S. J. Mater. Chem. 1995, 5, 1469. (5) Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 417 and references therein. (6) Miller, L. L.; Hashimoto, T.; Tabakovic, I.; Swanson, D. R.; Tomalia, D. A. Chem. Mater. 1995, 7, 9.

© 1997 American Chemical Society

Dendrimers Peripherally Modified with Anion Radicals

sensitive to humidity. To our knowledge the only similar study of dendrimers is one in which dendritic polyesters were prepared with two, four, or eight TTF units.8 Conductive charge-transfer salts (σ ) 2 × 10-3 S cm-1) were precipitated by addition of TCNQ. Results and Discussion Synthesis. The synthesis began with conversion of naphthalene dianhydride to the monoimide monoanhydride. The crude product contained some diacid, so it was dehydrated with SOCl2 and then isolated as the hydrochloride salt, 1. Methylation with CH3I gave 1a. Benzylation with benzyl bromide gave 1b. Compound 1c was produced by reaction of naphthalene dianhydride with 4-aminopyridine followed by methylation with methyl iodide. To investigate the synthetic process and to obtain model compounds, several amines were modified. Compounds 2a and 2b (Scheme 2) were prepared by heating 2 equiv of 4-(aminomethyl)pyridine with the naphthalene dianhydride, followed by methylation with methyl iodide or benzylation with benzyl bromide. Reaction of the naphthalene dianhydride with 2 equiv of 4-amino(7) Duan, R. G.; Miller, L. L.; Tomalia, D. A. J. Am. Chem. Soc. 1995, 117, 10783. (8) Bryce, M. R.; Devonport, W. Synth. Met. 1996, 76, 305.

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pyridine followed by methylation gave 2c. Compounds 2d,e,f were prepared from reaction of 1a,b,c with lauryl amine. Compound 3 which has three diimide units was prepared by reaction of 1a with tris(2-aminoethyl)amine. PAMAM dendrimers were similarly modified using 1a (Scheme 1) in DMA or DMF solvent leading to the iodide salts D1-A-D6-A. D-A dendrimers isolated as methyl sulfate salts were hygroscopic and too difficult to handle. The yields of iodide salts ranged from 53 to 84%. In general, the lower generations gave better yields. Higher generation reaction products D-A contained a certain amount of precipitate after the modification reaction. After filtering this precipitate, the desired product was precipitated by adding ether. If the DMA or DMF solvent was not carefully dried, the products often had an unexpected purple color. Small molecules, such as 2d, as well as dendrimers with CH2pyridinium groups, had a weak band (1-10% of the intensity of the 380 nm band of the diimide) at 570 nm which was responsible for the color. Nonalkylated, CH2-pyridine structures or structures with pyridinium directly attached to the imide nitrogen, e.g., D3-C (see below) were not colored. The purple color was bleached by treating a DMF solution with aqueous acid and intensified by heating to 125 °C in a DMF solution containing 2,6-lutidine. All this suggested that the

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Tabakovic et al. Scheme 1

acidity of the CH2 group was responsible for the color, perhaps by formation of a dihydropyridine structure 4. Fortunately the colored product from imidization reactions could be avoided by the use of DMF which was freshly dried by passage through a column of activated, acidic alumina. Studies of the contaminated and uncontaminated products in solution and as thin films showed only minor differences. The dendrimers were somewhat soluble in DMSO, DMF, DMA, and water, but higher generations were more difficult to dissolve in water. For comparative studies two other diimide groups were attached to to the generation-3 PAMAM dendrimer providing D3-B with benzylpyridinium in place of methylpyridinium groups, and D3-C with methylpyridinium groups directly attached to the diimide. The structures of model compounds were characterized by IR, vis, and NMR spectroscopies and highresolution MS. The spectra, which were compared to previously synthesized naphthalene diimides5 showed imide carbonyl absorptions (1705, 1665 cm-1), no carboxylic acid carbonyl peaks; vis bands at 360 and 380 nm; and 1H NMR spectra with appropriate chemical shifts, integration, and splitting. The pyridinium hydrogens were present as two doublets near 8.9 and 8.2 ppm and the naphthalene hydrogens appeared as a singlet (the two nitrogen substituents on the diimides are the same or very similar) at 8.7 ppm. Modified dendrimers were similarly analyzed by IR and vis spectroscopy to demonstrate clean imidization. 1H NMR spectra on dendrimers showed broadened peaks from the modifier groups as well as broad CH and NH peaks from the PAMAM core2 and peaks due to occluded solvent, i.e., water and/or DMF. D-A dendrimers, for example, showed the expected pyridinium, naphthalene, two different N-methylenes and N-methyl peaks with appropriate peak areas due to the modifier group. A number of attemps were made to remove the occluded solvents under vacuum or by dissolution in a third solvent, DMSO, followed by reprecipitation, but the product always contained some water and whatever polar solvent was employed. FAB-MS of D1-A gave a strong (M + H)+ peak supporting the formation of fully modified dendrimer. For the higher generation dendrimers size exclusion chromatography and electrospray mass spectroscopy were not successful. The difficulty of using these methods to analyze a polymeric polycation with its associated counterions can be appreciated. Loading of the dendrimers with diimide groups was evaluated using vis spectroscopy, coulometric analysis,

and elemental analysis.9 Because dendrimers have well-defined molecular weights, it is possible to write analytical equations for the loading, but because the molecular weight changes with loading, one cannot simply take the ratio of diimides found to the number expected. The appropriate equations are developed below starting from the definition of percent loading, L, where n is the average number of diimides per molecule and nth is the theoretical number, e.g., 192 for fully modified D6-A. It was assumed that the dendrimers were not degraded during imidization and that each peripheral site is either an unreacted amine or an imide group with its associated counterion. For analysis by vis spectroscopy eq 2 was used, where A is the dendrimer absorbance at 380 nm,  is the molar absorptivity for one imide group on a dendrimer, b as the cell path length, MW is the average molecular weight of modified dendrimer, m is the mass, and V is the volume of the solution analyzed.

L ) 100n/nth

(1)

A ) bmn/(MW)V

(2)

The average molecular weight is defined in eq 3 where MWD and MWim are the molecular weights of the unmodified dendrimer, and the modifier, e.g., 1b (including the counterion) minus the water lost in imidization, respectively. For D-A, MWim is 482.2 g mol-1. Combining equations and defining La as the loading determined by this absorbance method give eq 5.

MW ) MWD + (MWim - MWH2O)n n)

LA )

AV(MWD) bm - (MWim)AV AV(MWD)100

bmnth - (MWim)AVnth

(3) (4)

(5)

The  value for a diimide group at 380 nm was determined using the unsymmetrically substituted diimide 2d, which like the dendrimers has one CH2pyridinium group and one alkyl group attached as N-substituents. The measurements for determination of  and of dendrimer loading were made using DMSO solutions, sequentially diluted to give a 20-fold concentration range, subtracting background. The  value at (9) Although the experimental measurements were reliable the loadings reported in ref 6 were incorrectly calculated.

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Scheme 2

Table 1. Diimide Loading on D-A method vis spectra LA (%) element anal. I (%) LI (%) Coulometry Q/m (coulomb/mg) LQ (%)

D1-A

D2-A

D3-A

D4-A

D5-A

D6-A

87

72

69

52

45

NA

18.40 17.50 18.56 18.24 15.40 15.72 84 82 107 104 66 70 0.14 84

0.15 88

0.12 65

0.12 73

0.11 68

0.14 66

380 nm was computed to be 18 700. Loading values (LA, Table 1) are the average of loadings calculated for each of the sequential dilutions. Extensive studies of D3-A were undertaken using the absorbance method. The loading, 69%, was reproducible to within 5%. The loadings for samples synthesized with slightly different procedures and reaction times from 12 to 48 h were within this error. Since NMR analysis shows that these dendrimers tenaciously hold water and polar solvents, the actual LA values must be higher than those reported in Table 1. We are also suspicious that the assumed  value may be too large for D4-A-D6-A. The spectra of these dendrimers had a different shape (A360 > A380) than those of generations 1-3 or the model compound 2d (A360 < A380). Elemental analysis for the iodide ion associated with each terminal imide group allowed loading, LI, percentages to be calculated using eq 6, where %I is the

LI )

%I(MWD)100 nth(AWI)100 - %I(AWI + MWim)nth LQ )

Q(MWD)100 nthFm - Q(MWim)nth

(6)

(7)

experimental result and AWI is 126.9 g mol-1. The calculated loadings varied from 66 to 107% (Table 1). In several cases the identity of the counterion as iodide and its concentration were confirmed by voltammetric oxidation of the iodide in DMF/Bu4NBF4 at a platinum electrode by comparison to a working curve developed using tetraethylammonium iodide oxidation. Coulometric analyses for diimides were also performed in DMF/Bu4NBF4 solutions at a platinum electrode. It was expected based on extensive experience with model diimides and confirmed by cyclic voltammetry that each diimide group could be reduced to its anion radical at a potential near 0.8 V (SCE). In these experiments the reduction step was followed by reoxidation to make sure that the anion radicals were stable. As expected all the cathodic charge could be recovered anodically. Correction was made for the background current, which was typically 5% of the initial current. The data were analyzed using eq 7, in which Q is the number of coulombs required and F is

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Figure 1. NIR of chemically reduced 2b and 3b. 1.5 mM in diimide units, reduced by 3 mM of Na2S2O4 in D2O.

the Faraday. The calculated loadings (Table 1) were greater than 65% in each case. Dendrimers D3-B and D3-C were analyzed using the UV method. The model compounds 2b ( 17 066) and 2c ( 15 954) were used as standards leading to loading values of 80% for D3-B and 77% for D3-C. In summary the calculated loading values for all the dendrimers are greater than 65%. Because we ignored the presence of occluded solvents, the values must be generally higher than those reported in Table 1. Indeed, the loading could be nearly 100%. There is a substantial variability in the values when the three methods are compared for one compound, but we note that small errors in measurement can lead to large errors in the calculated loading. For example, a change of 1% in the measured %I leads to a change in LI of about 18%. Anion Radicals. Anion radicals were formed by oneelectron reduction and studied by NIR spectroscopy and electrochemistry. The solvents were DMF, formamide, and water. Previous studies from this laboratory have established that in DMF solution diimide anion radicals are unaggregated and do not have bands in the NIR region. In aqueous solution π-dimers are formed (two anion radicals aggregated face to face). These π-dimers absorb near 1140 nm regardless of the substituents attached to the nitrogens. With certain N-substituents such as those used here, higher aggregates are also observed. These π-stacks are signified by bands at even longer wavelengths in the NIR or even IR (>2500 nm). In the following we will first discuss the optical spectra and then the electrochemical results. NIR Spectra. Spectra were recorded for model compounds and dendrimers which had been reduced chemically using a 2-fold excess of Na2S2O4 in D2O (H2O has strong vibrational overtones in the NIR). The excess reductant ensured complete reduction and did not generate dianions. The concentration initially studied for each compound was 0.25 mM in diimide anion radical. The spectral reproducibility was 10% in absorbance and 10 nm in wavelength. As expected, the compound 2a, which has only one diimide group, produced a nearly Gaussian shaped band with λmax at 1160 nm (Figure 1). Model compound 3, with three diimide groups per molecule, gave NIR spectra with longer wavelength absorption, λmax 1300 nm (Figure 1). (In these spectra and others reported here the “noise” near 2000 nm is due to residual water.) Dendrimer D1-A (Figure 2), with

Tabakovic et al.

Figure 2. NIR of chemically reduced D1-A, D2-A, and D3-A. All 0.25 mM in A, reduced by 0.5 mM of Na2S2O4 in D2O.

Figure 3. NIR of chemically reduced D4-A, D5-A, and D6-A. All 0.25 mM in A, reduced by 0.5 mM of Na2S2O4 in D2O.

six diimides had λmax similar to that of 3. Larger dendimers D2-A-D6-A were slightly more aggregated with λmax near 1400 nm (Figures 2 and 3). These spectra often have a shoulder assignable to the π-dimer as well as the higher aggregate(s), and it seems likely that only small stacks of three or four units are present. Importantly, there was no evidence for larger π-aggregates from higher generation dendrimers. Even D6-A with 192 diimides/molecule shows no increased propensity for stacking compared to D2-A. Are these stacks intra- or intermolecular? Although we expected intramolecular aggregation from the polymers, enhanced intermolecular aggregation is also expected. Since compounds with one diimide anion radical always dimerize intermolecularly in D2O, dendrimers should also intermolecularly dimerize. The difference is that there will be an increased propensity for forming larger aggregates from dendrimers because of the high local concentration of diimides. Data at two concentrations are collected in Table 2. In each case the dendrimer λmax shifted to longer wavelength at higher concentration indicating intermolecular aggregation was occurring. Seeking evidence for mixed valence stacks in partially reduced compounds D3-A (0.5 mM) was reduced with 0.25 mM Na2S2O4. The λmax was 1380 nm, the same as a fully reduced sample at 0.25 mM. A related experiment involved the partial reduction of 3. Electrochemical reduction in D2O, 0.1 M Na2SO4 was employed. After passage of 0.33 electron/diimide the λmax was 1240

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Table 2. Concentration Dependence of λmax in D2O (nm) anion radical concn (mM) cmpd

1.5

0.25

2a 3 D1-A D2-A D3-A D4-A D5-A D6-A

1160 1300 1470 1510 1510 b b b

1160 1350a 1340 1410 1380 1410 1480 1440

a 0.3 mM of A. b Measurement was not possible due to precipitation.

nm. At 0.66 electron/diimide it was 1290 nm, and at 1 electron/molecule 1380 nm. This compares with λmax 1360 nm when the same concentration of compound was reduced chemically with excess dithionite in D2O. The results of the electrochemical reduction show that a higher concentration of anion radicals enhances the formation of stacks to a certain extent. No evidence for mixed valence stacks with different NIR absorption was found. Previous results on monomeric diimide anion radicals indicated that the extent of aggregation could be increased by changing the N-substituent on the diimide group. To explore this effect dendrimers D3-B and D3-C were prepared, characterized, and studied. The loading was again around 70% as assayed by vis spectroscopy, using model compounds 2b and 2c. Reduction with excess dithionite in D2O produced the reduced dendrimers. The NIR of D3-B and D3-C had λmax ) 1380 and 1700 nm, respectively. The reduced model compound 2c absorbs at 1700 nm. Thus, the more hydrophobic benzylpyridinium substituent and the directly attached methylpyridium group give longer wavelength absorption (better aggregation) for both monomer and dendrimer, but the dendrimer does not improve or hinder aggregation compared to the monomer. Electrochemical reduction of D-A in DMF Et4N,ClO4 gave weak π-dimer peaks at 1140 nm. π-Dimers of diimide anion radicals are usually not observed in organic solvents, but in these cases the presence of multiple diimide groups and 0.1 M electrolyte combine to allow some dimer formation. The importance of solvent polarity is emphasized by results obtained using formamide. Using “Z values”10 as the solvent polarity scale, formamide (Z ) 83.3 kcal mol-1) can be compared to water (Z ) 94.6 kcal mol-1) and to DMF (Z ) 68.5 kcal mol-1). Thus, we expected that formamide would be intermediate between water and DMF in its ability to support aggregation. In formamide (the anion radicals were more soluble in formamide than in water) the reduced diimide 2a formed π-dimers as evidenced by the NIR λmax ) 1120 nm and vis λmax ) 453 nm. Dendrimer D3-A had λmax ) 1200 nm (Figure 4). This is a shorter wavelength than found for D3-A in D2O and suggests that there were mainly dimers, present in formamide. D3-B had λmax 1300 and D3-C had λmax 1520 nm (Figure 5). These data correlate with the results found for D2O solvent in that D3-C has the longest wavelength absorption, and in each case the wavelength is slightly shorter in formamide than in D2O. In contrast to the results in (10) Kosower, E. M. J. Am. Chem. Soc. 1958, 80, 3253, 3261, 3267.

Figure 4. NIR of chemically reduced D3-A, D3-B, and D3-C in formamide. All 1 mM in A, reduced with 1.1 mM Na2S2O4.

Figure 5. CV of 2d (1.5 mM) in DMF/0.1 M Bu4NClO4; v ) 100 mV s-1; GCE.

D2O the λmax of each dendrimer was independent of concentration over the range 0.5-5 mM. This result suggests but does not prove that the aggregation in formamide is intramolecular. Comparison of the model comounds 2a, 2e, and 2f with corresponding dendrimers again shows that the presence of multiple diimide groups enhances aggregation to a certain extent, but that higher generation dendrimers give no advantage to aggregation. Partial reduction gave no new peaks, just proportionately smaller NIR absorbances at the same wavelength. Thus, as in D2O, there was no evidence for mixed valence species. Electrochemistry. Discussed above were the coulometric reductions used to form anion radicals in DMF solution. Cyclic voltammetry (CV) was employed to

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investigate the reduction process in more detail. The voltammograms were recorded using a glassy carbon electrode and either DMF, 0.1 M Et4NClO4 or aqueous 0.1 M Na2SO4 and SCE reference. In DMF the model compound 2d (Figure 5) gave two reversible couples as expected. The third cathodic peak near -1.5 V is due to the reduction of the methylpyridinium substituent. In contrast the dendrimers in either DMF or water gave complex voltammograms as shown in Figure 6 and Figure 7. Repetitive cycles gave unchanged voltammogram shapes, with no evidence for buildup of a conducting layer. Since there is evidence for precipitated material we conclude that the layer is stripped anodically. Indeed, at 5 or 10 mV/s sweep rates sharp anodic, “stripping”, peaks are observed. Striking is the similarity of the voltammograms for the various dendrimers. Consider first the results in DMF solvent. For all the dendrimers there was a very broad first “peak” between about -0.2 and -0.75 V followed by a narrower peak at more negative potentials. Coulometry performed at -0.8 V produced products with the spectra of one-electron reduced anion radicals. Reoxidation was quantitative. Normal pulse voltammetry on several of the dendrimers D-A (Figure 8 shows the result for D6-A) indicated that the -0.2 to -0.75 process and the -0.9 V process had similar plateau currents as expected if the first process formed anion radicals and the second formed dianions. Differential pulse voltammetry (Figure 8) showed a broad first peak at about -0.4 V and a narrow second peak at -0.85 V consistent with the CV results. Using the dendrimers D3-A amd D4-A in DMF, the CV peak current at 0.75 V was measured as a function of sweep rate. It increased linearly above 100 mV s-1, suggesting that the dendrimers are adsorbed on the electrode. Indeed, the second couple, near -0.9 V (Figure 6), has the shape expected for a process involving adsorbed anion radicals being reduced to adsorbed dianions. In particular the current after the peak drops more rapidly than would be expected for a diffusion controlled reaction and the anodic peak current is larger than that expected for a diffusion controlled process. Because the poly(dianions) stay on the surface they are all present to be reoxidized. In DMF the complex process of forming anion radicals on dendrimers begins at potentials positive of the potential measured for anion radical formation from model monomer 2d (compare Figures 5 and 6). This shift could be due to π-aggregation of the anion. The dendrimer anion radical/dianion couple actually occurs at potentials slightly negative of the anion radicaldianion couple for 2d. These results are consistent with the idea that π-dimerization stabilizes anion radicals but not dianions. In aqueous Na2SO4, model compound 2a shows broadening of the peaks, perhaps due to π-dimer formation (Figure 7). The formation of dianion and the reduction of the methylpyridinium group come at almost the same potential. Since the methylpyridinium reduction involves protonation, it is expected that its potential would be shifted to substantially positive potentials, giving the observed voltammogram shape. 2a gives linear plots of the peak current at -0.34 V vs v0.5, consistent with diffusion control. The dendrimers in water show linear ip vs v plots, indicating adsorption. In the dendrimer

Tabakovic et al.

Figure 6. CV of D1-A (1.5 mM of A units) and D2-A-D6-A (0.4 mM of A units) in DMF/0.1 M Bu4NClO4; v ) 100 mV s-1; GCE.

voltammograms there is evidence for two peaks within the first broad “peak”. The second (dianion) and third

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Chem. Mater., Vol. 9, No. 3, 1997 743

Figure 8. Differential pulse (top) and normal pulse (bottom) voltammograms of D6-A (0.4 mM of A units) in DMF/0.1 M Bu4NClO4; scan rate ) 2 mV s-1, pulse width ) 50 ms; GCE.

Figure 7. CV of D1-A-D5-A (0.6 mM of A units) in H2O/0.1 M Na2SO4; v ) 100 mV s-1; GCE.

(pyridinium) cathodic peaks come at very similar potentials (Figure 7). Again, anion radical formation begins at potentials somewhat positive of the potentials required for the model compound, 2a, consistent with aggregation of the adsorbed molecules. To further explain the shape of these voltammograms consider the following Coulombic argument. The broad neutral/anion radical “peak” could be due to the reduction of different sites on the polymer molecules. As more anion radicals are formed on one molecule, the Coulombic effects become more and more important, shifting the potential for the last anion radical to more negative potentials than that for the first. This would, however, suggest that dianion formation would be affected in a similar, but more extreme way giving a broad anion radical/dianion peak as well. Knowing that anion radicals, but not dianions, are stabilized by aggregation,9 we suggest that the broad neutral/anion radical “peak” is due to differently aggregated anion radicals. For example, tetrameric tetraanions might be

formed more easily than monomeric anion radicals. For the peak to be broad it would have to be true that these different size aggregates (all at the anion radical oxidation state) would not be in equilibrium during the time of the scan. Otherwise a single peak corresponding to the formation of the equilibrium mixture of aggregates would be observed. A mixture of different size aggregates is also inconsistent with the narrow peak for the second since it would be expected that the more stable aggregates would reduce more difficultly to dianions giving several peaks or a broadened anion radical/dianion “peak” not unlike that for anion radical formation. Any proper explanation of the DMF voltammograms must account for the broad first peak and the narrow second peak, which suggests that all the anion radicals are equivalent or in equilibrium at -0.75 V, when the second process forming dianions begins. This is chemically reasonable, but how to explain the broad first peak? Our studies of dendrimer anion radicals formed into films shows evidence for mixed-valence stacks.7 That is aggregation of neutrals and anion radicals into stacks. The adsorbed, reduced material on the electrode should resemble these reduced films. Thus, it is proposed that as the cathodic sweep begins, the first anion radicals formed aggregate into mixed valence stacks. As the potential becomes more negative, these are converted into anion radical aggregates. At -0.75 V all of the diimide groups are converted to anion radical aggregates which are in equilibrium with each other. At even more negative potentials these are reduced to

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unaggregated dianions. We have recently found a similar phenomenon in the electrochemical oxidation of some oligothiophenes.11 There, oxidation of unadsorbed neutrals gives precipitated cation radicals that can be reduced to a mixed-valence precipitate. Conclusions PAMAM dendrimers modified with diimide groups and reduced to form anion radicals on the diimides have been formed. The anion radicals aggregate in water or formamide or on an electrode to form π-dimers and π-stacks. The extent of aggregation as evaluated from the NIR solution spectra is slightly larger than that found for diimide anion radicals that are not attached to the polymer. The size of the dendrimer has little effect on aggregation, which can be either intra- or intermolecular. On a glassy carbon electrode, reduction of the dendrimers gave precipitated material. Cyclic voltammograms indicated that the precipitates contained mixed valence stacks. Experimental Section The following chemicals were purchased from Aldrich Chemical Inc. and were used without further purification. 1,4,5,8-Naphthalenetetracarboxylic dianhydride; thionyl chloride; iodomethane; 4-(aminomethyl)pyridine; triethylamine; tris(2-aminoethyl)amine; dodecylamine; (2-bromoethyl)amine; 2,6-lutidine, N,N-dimethylacetamide (DMA) anhydrous; dimethylformamide (DMF) anhydrous; dimethyl sulfoxide (DMSO) anhydrous (stored over activated 4 Å molecular sieves). Spectra were run on a Perkin-Elmer 1600 FT-IR, an IBMAC200 NMR, a Shimadzu 160 UV-vis, and a Carey 16 vis NIR. Fast atom bombardment (FAB) mass spectra were obtained with a VG 7070E-HF instrument using m-nitrobenzyl alcohol (MNBA) matrix. Coulometry was carried out in a twocompartment cell with a carbon rod anode and a platinum gauze cathode. A saturated calomel electrode (SCE) was used as reference. Oxygen was removed from the system by purging with argon. Voltammetry was performed using a glassy carbon electrode (GCE) (disk area 0.071 cm2). 1‚HCl. DMA (140 mL) was passed through a column of activated neutral alumina into a round-bottomed flask under dry nitrogen containing 1,4,5,8-naphthalenetetracarboxylic dianhydride (3.62, 13.5 mmol). The mixture was heated to 130 °C, at which point 4-(aminomethyl)pyridine (0.488 g, 4.5 mmol) diluted in 20 mL of DMA (also passed through a column of activated neutral alumna) was added dropwise over 45 min. After 20 h the reaction mixture was cooled to room temperature and 400 mL of chloroform (note: chloroform must not be stabilized with ethanol) was added to precipitate unreacted dianhydride. The precipitate was filtered and identified by NMR and IR as unreacted anhydride (1.10 g, 45% recovery). The filtrate was evaporated under reduced pressure to remove chloroform; 600 mL of water was added. The precipitate was filtered and washed with ethanol and ether to obtain 1.58 g of product (a mixture of monoimide-monoanhydride 1 and its hydrated form). This crude compound (1.58 g) was added to DMF (15 mL), and the mixture was heated to 55 °C. About 1.5 equiv of thionyl chloride (0.75 g, 6.30 mmol) was added. The solution was stirred for 2 h, at which point a precipitate appeared. The mixture was cooled to room temperature, filtered, and washed with ether. A cream-colored powder was obtained (1.21 g, 68% overall). IR (KBr) 2922, 1783 and 1729 (anhydride CdO), 1708 and 1665 (imide CdO), 1650, 1599 cm-1. 1H NMR (DMSO-d6) δ 8.84 (d, J ) 6.4, 2H, pyridine), 8.72 (d, J ) 7.5, 2H, naphthyl), 8.23 (d, J ) 7.5, 2H, naphthyl), 8.12 (d, J ) 6.4, 2H, pyridine), 5,51 (s, 2H, methylene). HRMS (11) Tabakovic, I.; Maki, T.; Miller, L. L. J. Electroanal. Chem., in press.

Tabakovic et al. (FAB-MNBA matrix) calcd for C20H11O5N2 (M + H)+ 359.0668, found 359.0667. 1a. Monoimide-monoanhydride 1 (910 mg, 2.31 mmol) was added to DMF (25 mL), and the mixture was heated to 75 °C, at which point iodomethane (3.27 g, 23.1 mmol) was added. After 3 h the reaction mixture was cooled to room temperature, and 125 mL of ether was added. The precipitate was filtered and washed with ether to obtain 1.05 g (91%) of a reddish brown powder. IR (KBr) 3030, 1785, 1736, 1710, 1666 cm-1. 1H NMR (DMSO-d ) δ 8.88 (d, 2H, pyridine), 8.71 (4H, 6 naphthyl), 8.22 (2H, pyridine), 5.50 (s, 2H, methylene), 4.29 (s, 3H, methyl). HRMS (FAB-MNBA matrix) calcd for C21H13O5N2 (M - I)+ 373.0824, found 373.0821 1b. Monoimide-monoanhydride 1 (1.50 g, 3.8 mmol) was added to alumina-dried DMF (35 mL), and the mixture was heated to 75 °C, at which point benzyl bromide (3.25 g, 19 mmol) was added. After 20 h, the reaction mixture was cooled to room temperature, and 150 mL of ether was added. The precipitate was filtered and washed with ether to obtain 1.72 g (86%) of a bright yellow powder. IR (KBr) 1783, 1750, 1714, 1672 cm-1. 1H NMR (DMSO-d6) δ 9.17 (d, 2H, pyridyl), 8.72 (4H, naphthyl), 8.30 (d, 2H, pyridyl), 7.54 (2H, phenyl), 7.45 (3H, phenyl), 5.83 (s, 2H, benzyl methylene), 5.53 (s, 2H, imide methylene). HRMS (FAB-MNBA matrix) calcd for C27H17O5N2 (M - Br)+ 449.1137, found 449.1133. 1c. Using the procedure for synthesis of 1‚HCl, 4-aminopyridine (0.142 g, 1.50 mmol) dissolved in 10 mL of DMA was added to 1,4,5,8-naphthalenetetracarboxylic dianhydride (1.20 g, 4.5 mmol) in 40 mL of DMA. Dehydration of crude monoimide-monoanhydride with thionyl chloride (285 mg, 2.40 mmol) in 10 mL of DMF afforded 416 mg (73%) of the hydrochloride salt as a cream-colored powder. Then, according to the same general procedure as used for 1a methylation of the pyridinium salt (620 mg, 1.63 mmol) with iodomethane (2.31 mg, 16.3 mmol) afforded 735 mg (93%) of a reddish brown powder. IR (KBr) 1780, 1719,1682 cm-1. 1H NMR (DMSOd6) δ 9.24 (d, 2H, pyridyl), 8.77 (4H, naphthyl), 8.32 (d, 2H, pyridyl), 4.47 (s, 3H, methyl). HRMS (FAB-MNBA matrix) calcd for C20H11O5N2 (M - I)+ 359.0668, found 359.0684. 2a. 1,4,5,8-Naphthalenetetracarboxylic dianhydride (1.5 mmol, 0.40 g) was dissolved in 50 mL of DMA at 110 °C under nitrogen bubbling. 4-(Aminomethyl)pyridine (3.6 mmol, 0.39 g) was added. After 10 h the reaction mixture was cooled and 200 mL of ether was added. The precipitate was filtered and washed by ether yielding 0.61 g (1.35 mmol, 90%) of product. IR (KBr) 1704 and 1661 (imide CdO), 1602, 1581 cm-1. 1H NMR (DMSO-d6) δ 8.89 (4H, pyridine), 8.71 (4H, naphthyl), 8.18 (4H, pyridine), 5.52 (4H, methylene). HRMS (FAB-MNBA matrix) calcd for C26H17O4N4 (M + H)+ 449.1250, found 449.1278. This product (1.1 mmol, 0.50 g) and iodomethane (5.6 mmol, 0.79 g) were sonicated in 50 mL of DMF for 20 h at room temperature. Ether (100 mL) was added, and the precipitate was filtered and washed with ether giving 0.79 g (96%) of purple solid. IR (KBr) 1701, 1660, 1644, 1580 cm-1. 1H NMR (D O) δ 8.82 (4H, pyridine), 8.76 (4H, naphthyl), 8.12 2 (m, 4H, pyridine), 5.64 (4H, methylene), 4.37 (s, 6H, methyl). HRMS (FAB-MNBA matrix) calcd for C28H22O4N4 (M - 2I) 478.1641, found 478.1622. 2b. Using the procedure for the synthesis of 2a, dodecylamine (53 mg, 0.286 mmol) was added to a solution of 1b (100 mg, 0.190 mmol) in 15 mL of DMF. Precipitation of the product with ether (15 mL) afforded 105 mg (79%) of a brown powder. IR (KBr) 2921, 2851, 1705, 1660 cm-1. 1H NMR (DMSO-d6 + CDCl3) δ 9.20 (d, 2H, pyridyl), 8.70 (4H, naphthyl), 8.17 (d, 2H, pyridyl), 7.48 (2H, phenyl), 7.37 (3H, phenyl), 5.85 (s, 2H, benzyl methylene), 5.54 (s, 2H, imide methylene), 4.07 (t, 2H), 1.66 (m, 2H), 1.31-1.20 (16 H), 0.81 (t, 3H). HRMS (FAB-MNBA matrix) calcd for C39H51O4N3 (M - Br)+ 616.3175, found 616.3206. 2c. Using the procedure for the synthesis of 2a, dodecylamine (57 mg, 0.308 mmol) was added to a solution of 1c (100 mg, 0.206 mmol) in 15 mL of DMF. Precipitation of the product with ether (15 mL) afforded 90 mg (67%) of an orangish brown powder. IR (KBr) 2920, 2849, 1705, 1659 cm-1. 1H NMR (DMSO-d6 + CDCl3) δ 9.27 (d, 2H, pyridyl), 8.70 (4H, naphthyl), 8.29 (d, 2H, pyridyl), 4.51 (s, 3H, methyl),

Dendrimers Peripherally Modified with Anion Radicals 4.10 (t, 2H), 1.66 (m, 2H), 1.31-1.20 (16 H), 0.83 (t, 3H). HRMS (FAB-MNBA matrix) calcd for C32H36O4N3 (M - I)+ 526.2706, found 526.2709. 2d. DMF (15 mL) was passed through a column of activated acidic alumina into a round-bottomed flask containing 1a (100 mg, 0.200 mmol) under dry nitrogen. The mixture was heated to 125 °C, and dodecylamine (56 mg, 0.300 mmol) was added. The reaction mixture was then allowed to stir at 115 °C for 18 h, at which point all but ∼3 mL of solvent was evaporated at reduced pressure, and ether (15 mL) was added. The precipitate was filtered and washed with ether to obtain 109 g (82%) of a reddish brown powder. IR (KBr) 1705, 1662, 1580 cm-1. 1H NMR (DMSO-d6) δ 8.9 (2H), 8.7 (4H), 8.2 (2H), 5.5 (2H), 4.3 (3H), 4.1 (2H), 1.65 (2H), 1.5-1.1 (18H), 0.86 (3H,). HRMS (FAB-MNBA matrix) calcd for C33H38O4N3 (M - I)+ 540.2862, found 540.2888. 3. Et3N (4.3 mmol, 0.44 g) and 1a‚HCl (2.2 mmol, 0.85 g) was dissolved in 80 mL of DMA. Tris(2-aminoethyl)amine (0.6 mmol, 0.09 g) was added. The temperature was raised to 135 °C. After 6 h the reaction mixture was cooled and 200 mL of ether was added. The precipitate was filtered and washed with water and then acetone. A gray solid (0.591 g, 84.4%) was obtained. FT-IR (KBr) 1707, 1667, 1523 cm-1. 1H NMR (DMSO-d6) δ 9.1, 8.6, 8.4, 8.2, 5.6, 4.6, 3.9. MS (FAB-MNBA matrix) calcd for C66H43O12N10 (M + H)+ 1167.3, found 1167.3. This compound (0.2 mmol, 0.23 g) and iodomethane (16 mmol, 2.27 g) were sonicated in 20 mL of DMF for 6 h at room temperature; 200 mL of ether was added. The precipitate was filtered and washed with ether giving 0.328 g (103%) of purple solid. IR (KBr) 1706,1664, 1581 cm-1. 1H NMR (DMSO-d6) δ 8.9, 8.7, 8.4, 8.2, 5.5, 4.6, 4.3, 4.0. D-A. The general procedure is illustrated for D3-A. A methanol solution (536 mg) of generation three PAMAM dendrimer (150 mg, 0.029 mmol) in a two-neck round-bottomed flask was frozen with liquid nitrogen and lyophilized for 24 h. DMSO (5 mL) was then passed through a column of activated neutral alumina into the flask containing the dendrimer film under argon, and the solution, while stirring, was heated to 80 °C. DMF or DMA (50 mL) was passed through a column

Chem. Mater., Vol. 9, No. 3, 1997 745 of activated acidic alumina into a round-bottomed flask containing 1a (402 mg, 0.80 mmol) under argon, and the mixture was heated to 120 °C. The DMSO solution containing PAMAM dendrimer was then injected via a syringe into the DMF or DMA solution of monoimide-monoanhydride. After stirring for 24 h at 120 °C, roughly half the solvent was evaporated under reduced pressure and ether (150 mL) was added. The product was then filtered and washed with ether to obtain 527 mg of a brown powder. IR (KBr) 1703,1665 cm-1. 1 H NMR (DMSO- d6) δ 9.00 (pyridyl), 8.53 (naphthyl), 8.20 (pyridyl), 8.8-7.6 (amide NH), 5.48 (imide methylene), 4.29 (methyl), 4.10 (dendrimer terminal methylene), 3.6-2.0 (dendrimer core protons). Addition of D2O to this solution removed the NH protons, leaving the pyridyl and naphthalene protons in the aromatic region. (D3-B). Using the procedure for the synthesis of D-A, 100 mg (0.019 mmol) of generation-3 PAMAM in DMSO (5 mL) was injected into a solution of 1b (308 mg, 0.582 mmol) in DMF or DMA (40 mL) under argon at 125 °C. Rotoevaporation of half the solvent volume, addition of ether (150 mL), vacuum filtration, and washing by ether afforded 370 mg of a yellowbrown powder. IR (KBr) 1705 and 1665 cm-1. 1H NMR (DMSO-d6) δ 9.21, 8.62, 8.31, 7.57, 7.46, 5.85, 5.55, 4.16, 3.62.0. (D3-C). According to the procedure for the synthesis of D-A, 96 mg (0.019 mmol) of generation-3 PAMAM in DMSO (5 mL) was injected into a solution of 1c (260 mg, 0.535 mmol) in DMF or DMA (40 mL) under argon at 125 °C. Rotoevaporation of half the solvent volume, addition of ether (150 mL), vacuum filtration, and washing by ether afforded 310 mg of a brown powder. IR (KBr) 1706 and 1667 cm-1. 1H NMR (DMSO-d6) δ 9.27, 8.71, 8.35, 8.10, 7.60, 4.50, 4.20, 3.6-2.0.

Acknowledgment. This work was supported by the National Science Foundation and the Army Research Laboratory. Some initial studies were performed by Toshihiro Hashimoto. CM960431+