Synthesis, Characterization, Electrochemistry, and EQCM Studies of

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Synthesis, Characterization, Electrochemistry, and EQCM Studies of Polyamidoamine Dendrimers Surface-Functionalized with Polypyridyl Metal Complexes Gregory D. Storrier, Kazutake Takada, and He´ctor D. Abrun˜a* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301 Received July 27, 1998. In Final Form: November 24, 1998 Polyamidoamine dendrimers have been surface-modified via peptide coupling with pyridyl, bipyridyl, and terpyridyl ligands to give the analogous polypyridyl dendrimer ligands in high yield. Complexation of the pendant chelating groups with appropriate ruthenium(II) precursor complexes yielded dendrimers surface-functionalized with tris(bipyridyl)ruthenium(II) or bis(terpyridyl)ruthenium(II) pendant complexes. Electrochemical studies of the dendrimer complexes show metal-centered and ligand-centered redox couples. These molecules also adsorb onto platinum electrodes, and the deposition process and the properties of the resulting films have been investigated with the electrochemical quartz crystal microbalance. The resulting films exhibit morphological changes with potential that can be attributed to the deposition or dissolution of the dendrimer and/or to ejection or incorporation of counterions and/or solvent into the film. A number of the electrodeposited films exhibited charge trapping peaks. Dendrimers containing terminal tris(bipyridyl)ruthenium(II) complexes exhibited room-temperature luminescence, while these and dendrimers with terminal bis(terpyridyl)ruthenium(II) complexes exhibited luminescence in a rigid butyronitrile matrix at 77 K.

Introduction Recently, much attention has focused on the assembly of molecular components into supramolecular systems capable of performing complex functions such as light harvesting and information storage.1 Polypyridyl transition metal complexes, especially those of ruthenium(II), have been extensively applied in these areas.2 The breadth of interest in polypyridyl transition metal complexes for such applications arises, in part, because they exhibit a wide range of photophysical and electrochemical properties,3-5 plus the fact that they can be used to induce a specific stereochemistry within the molecule.6 Polynuclear macromolecules containing the D3-symmetric [M(diimine)3]2+ unit (where diimine ) 2,2′-bipyridine or 1,10-phenanthroline) encounter stereochemical complexity because of the chirality (∆ or Λ) induced at each of the metal centers. In contrast, terpyridine derivatives, containing the D2-symmetric [M(tpy)2]2+ unit, avoid this stereochemical complexity, but interest in these complexes is tempered by the absence of room-temperature luminescence.5,7 Dendrimers are highly branched molecules that form in a well-defined pattern that allows control over molecular * To whom correspondence should be addressed. (1) (a) Balzani, V.; Scandola, S. Supramolecular Photochemistry; Horwood: Chichester, U.K., 1991. (b) Wasielewski, M. R. Chem. Rev. 1992, 92, 435. (c) Lehn, J.-M. Supramolecular Chemistry, Concepts and Perspectives; VCH: New York, 1995. (2) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (3) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, U.K., 1991. (4) Sauvage, J.-P.; Collin, J.-P.; Chambron, J. C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigeletti, F.; DeCola, L.; Flamingi, L. Chem. Rev. 1994, 94, 993. (5) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759. (6) Newkome, G. E.; Gu¨ther, R.; Moorefield, C. N.; Cardullo, F.; Echegoyen, L.; Pe´rez-Cordero, E.; Luftmann, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 2023. (7) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev. 1988, 85.

weight, topology, cavity size, and surface functionality.8-10 Metal centers have been incorporated into dendritic molecules for applications such as catalysis,11,12 lightharvesting devices,13 and others. Metal-containing dendrimers can be broadly classified into three groups: the metal centers can be incorporated in the branching unit,14-16 inserted at the dendrimer core,6,11,17 or attached to the surface of the molecule.18-20 The latter two systems offer molecules containing two widely different chemical environmentssan organic core surrounded by a metallo(8) (a) Tomalia, D. A. Adv. Mater. 1994, 6, 529. (b) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Syntheses, Perspectives; VCH: Weinheim, 1996. (c) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (d) Moore, J. S. Acc. Chem. Res. 1997, 30, 402. (e) Matthews, O. A.; Shipway, A. N.; Stoddart, J. F. Prog. Polym. Sci. 1998, 23, 1. (f) Frey, H.; Lach, C.; Lorenz, K. Adv. Mater. 1998, 10, 279. (g) Gorman, C. Adv. Mater. 1998, 10, 295. (9) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (10) Fre´chet, J. M. J. Science 1994, 263, 1710. (11) (a) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am. Chem. Soc. 1996, 118, 5708. (b) Dandliker, P. J.; Diederich, F.; Gross, M.; Knobler, C. B.; Louati, A.; Sanford, E. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1739. (12) (a) Knapen, J. W. J.; van der Made, A. W.; de Wilde, J. C.; van Leeuwen, P. W. N. M.; Wijkens, P.; Grove, D. M.; van Koten, G. Nature 1994, 372, 659. (b) Miedaner, A.; Curtis, C. J.; Barkley, R. M.; DuBois, D. L. Inorg. Chem. 1994, 33, 5482. (13) For example, see: (a) Balzani, V.; Campagna, S.; Denti, G.; Juris, A.; Serroni, S.; Venturi, M. Acc. Chem. Res. 1998, 31, 26. (b) Jiang, D.-L.; Aida, T. Nature 1997, 388, 454. (c) Newkome, G. E.; Gross, J.; Moorefield, C. N.; Woosley, B. D. J. Chem. Soc., Chem. Commun. 1997, 515. (d) Bryce, M. R.; Davenport, W. Adv. Dend. Macromol. 1996, 3, 115. (e) Issberner, J.; Moors, R.; Vo¨gtle, F. Angew. Chem., Int. Ed. Engl. 1994, 33, 2413. (14) Issberner, J.; Vo¨gtle, F.; De Cola, L.; Balzani, V. Chem. Eur. J. 1997, 3, 706. (15) (a) Constable, E. C. J. Chem. Soc., Chem. Commun. 1997, 1073. (b) Haga, M.; Ali, M. M.; Arakawa, R. Angew. Chem., Int. Ed. Engl. 1996, 35, 76. (c) Achar, S.; Puddephatt, R. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 847. (d) Achar, S.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1994, 1895. (e) Newkome, G. E.; Cardullo, F.; Constable, E. C.; Moorefield, C. N.; Cargill Thompson, A. M. W. J. Chem. Soc., Chem. Commun. 1993, 925. (f) Campagna, S.; Denti, G.; Serroni, S.; Ciano, M.; Juris, A.; Balzani, V. Inorg. Chem. 1992, 31, 2982. (16) Bodige, S.; Torres, A. S.; Maloney, D. J.; Tate, D.; Kinsel, G. R., Walker, A. K.; MacDonnell, F. M. J. Am. Chem. Soc. 1997, 119, 10364.

10.1021/la980939m CCC: $18.00 © 1999 American Chemical Society Published on Web 01/13/1999

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organic shell or a metal complex encompassed within an organic framework. We are interested in the electrochemical and photophysical properties of systems containing an organic core surrounded by redox-active transition metal complexes, especially polypyridyl complexes. Recently, Astruc and co-workers21 have prepared dendrimers containing an Fe(η5-C5H5)+ core linked via an alkyl ether chain to six surrounding tris(bipyridyl)ruthenium(II) or bis(terpyridyl)ruthenium(II) complexes. Hong and co-workers22 prepared a series of terpyridine-pendant dendrimers linked through phosphonium groups to a polyhedral silsesquioxane core but have yet to report the corresponding dendrimers containing terminal metal complexes. Cuadrado, Mora´n, and co-workers23,24 have prepared and characterized silane and diaminobutane-based ferrocenylcontaining dendrimers. We recently reported on the thermodynamics and kinetics of adsorption of diaminobutane-based ferrocenyl dendrimers using electrochemical and electrochemical quartz crystal microbalance (EQCM) techniques.20 We were also able to image, with tapping mode AFM, a layer of the dendrimer containing 64 ferrocenyl groups adsorbed onto a Pt(111) surface. A number of recent studies have focused on the preparation and characterization of polyamidoamine (PAMAM) dendrimers and their derivatives for a variety of uses.25-27 In the present study, we have employed PAMAM dendrimers28 generations 0-4 (G0-G4 dendrimers) containing 4, 8, 16, 32, and 64 surface amino groups, respectively. The G0 and G1 dendrimers were chosen to facilitate characterization, since lower molecular weight oligomers should aid in the determination of the reactivity of the terminal amine groups to the reaction conditions used. The larger generation dendrimers should allow the determination of the effects that the increase in both core size and the number of surface metal centers induces upon (17) (a) Cardona, C. M.; Kaifer, A. E. J. Am. Chem. Soc. 1998, 120, 4023. (b) Gorman, C. B.; Parkhurst, B. L.; Su, W. Y.; Chen, K.-Y. J. Am. Chem. Soc. 1997, 119, 1141. (c) Kimura, M.; Nakada, K.; Yamaguchi, Y.; Hanabusa, K.; Shirai, H.; Kobayashi, N. J. Chem. Soc., Chem. Commun. 1997, 1215. (d) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996, 118, 3978. (e) Dandliker, P. J.; Diederich, F.; Gisselbrecht, J.-P.; Louati, A.; Gross, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 2725. (18) (a) Alonso, E.; Valerio, C.; Ruiz, J.; Astruc, D. New J. Chem. 1997, 21, 1139. (b) Cuadrado, I.; Casado, C. M.; Alonso, B.; Mora´n, M.; Losada, J.; Belsky, V. J. Am. Chem. Soc. 1997, 119, 7613. (c) Shu, C.-F.; Shen, H.-M. J. Mater. Chem. 1997, 7, 47. (d) Fillaut, J.-L.; Linares, J.; Astruc, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 2460. (e) Moulines, F.; Djakovitch, L.; Boese, R.; Gloaguen, B.; Thiel, W.; Fillaut, J.-L.; Delville, M.-H.; Astruc, D. Angew. Chem., Int. Ed. Engl. 1993, 32, 1075. (19) Lange, P.; Schier, A.; Schmidbaur, H. Inorg. Chem. 1996, 35, 637. (20) Takada, K.; Dı´az, D.; Abrun˜a, H. D.; Cuadrado, I.; Casado, C.; Mora´n, M. Mora´n, Losada, J. J. Am. Chem. Soc. 1997, 119, 10763. (21) (a) Marvaud, V.; Astruc, D. J. Chem. Soc., Chem. Commun. 1997, 773. (b) Marvaud, V.; Astruc, D.; Leize, E.; Van Dorsselaer, A.; Guittard, J.; Blais, J.-C. New J. Chem. 1997, 21, 1309. (22) Hong, B.; Thoms, T. P. S.; Murfee, H. J.; Lebrun, M. J. Inorg. Chem. 1997, 36, 6146. (23) (a) Alonso, B.; Cuadrado, I.; Mora´n, M.; Losada, J. J. Chem. Soc., Chem. Commun. 1994, 2575. (b) Alonso, B.; Mora´n, M.; Casado, C. M.; Lobete, F.; Losada, J.; Cuadrado, I. Chem. Mater. 1995, 7, 1440. (24) Cuadrado, I.; Mora´n, M.; Casado, C. M.; Alonso, B.; Lobete, F.; Garcı´a, B.; Ibisate, M.; Losada, J. Organometallics 1996, 15, 5278. (25) (a) ben-Avraham, D.; Schulman, L. S.; Turro, C.; Turro, N. J. J. Phys. Chem. B 1998, 102, 5088. (b) Miller, L. L.; Duan, R. G.; Tully, D. C.; Tomalia, D. A. J. Am. Chem. Soc. 1997, 119, 1005. (c) M. Zhao, H. Tokuhisa, R. M. Crooks, Angew. Chem., Int. Ed. Engl. 1997, 36, 2596. (d) M. Wells, R. M. Crooks, J. Am. Chem. Soc. 1996, 118, 3988. (e) Miller, L. L.; Hashimoto, T.; Tabakovic, I.; Swanson, D. R.; Tomalia, D. A. Chem. Mater. 1995, 7, 9. (26) Aoi, K.; Itoh, K.; Okada, M. Macromolecules 1995, 28, 5391. (27) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (28) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117.

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the larger molecules when compared with the properties of the smaller G0 and G1 dendrimers. Herein, we report the synthesis, spectroscopic, and electrochemical properties of PAMAM dendrimers surface-functionalized with polypyridyl transition metal complexes. Experimental Section Materials and Apparatus. All reactions were carried out under an atmosphere of dry nitrogen unless otherwise stated. Solvents were routinely distilled from the appropriate drying agent immediately prior to use. Anhydrous N,N-dimethylacetamide (DMAc) stored under an inert atmosphere was purchased from Aldrich Chemical Co. and used as received. All other solvents (analytical grade) were used without further purification. 4-(3Carboxypropyl)-4′-methyl-2,2′-bipyridine,29 4′-methyl-2,2′:6′,2′′terpyridine,30 cis-Ru(bpy)2Cl2‚2H2O,31 and Ru(tpy)Cl332 were prepared according to literature methods. PAMAM dendrimers were obtained from Aldrich as 20 wt % methanolic solutions. All other reagents were purchased from Aldrich and used as received. DOWEX A-XZ anion-exchange resin was used. Column chromatography silica gel was purchased from ICN Biomedicals; gel permeation and cation exchange chromatography were performed using SP Sephadex LH20 or C-25 media, respectively. 1H and 13C NMR spectra were obtained in the designated solvents on a Varian 200 (200 MHz) or a Varian 400 (400 MHz) spectrometer. Mass spectra were recorded at the Mass Spectrometry Laboratory, University of Illinois at Urbana-Champaign. Electronic spectra were recorded on a Hewlett-Packard 8451A diode array spectrophotometer. Emission spectra were recorded with a Spex Fluorolog spectrometer. Room-temperature absorption and emission spectra were recorded in acetonitrile solution. Samples for emission spectra in butyronitrile at 77 K were prepared by at least three freeze-pump-thaw cycles. Gel permeation chromatography using DMAc as eluent was also carried out using a Milton Roy differential refractometer as a detector. Thermal analyses were determined using a Seiko 5200 thermal analysis system coupled with a DSC 220C accessory under a nitrogen atmosphere. Electrochemical experiments were carried out with a BAS CV27 potentiostat. Three-compartment electrochemical cells (separated by medium porosity sintered glass disks) were employed. All joints were standard taper so that all compartments could be hermetically sealed with Teflon adapters. Acetonitrile (AN, Burdick and Jackson distilled in glass) was dried over 4 Å molecular sieves. Tetra-n-butylammonium perchlorate (TBAP, GFS Chemicals) was recrystallized three times from ethyl acetate and dried under vacuum for 96 h. A platinum disk (area ) 0.006 cm2) electrode was used as the working electrode. Prior to use the electrodes were polished with 1 µm diamond paste (Buehler) and rinsed thoroughly with water and acetone. A coiled platinum wire was used as a counter electrode. All potentials are referenced to a Ag/AgCl electrode without regard for the liquid junction potential. The EQCM apparatus and instrumentation for resistance parameter measurements of a resonator have been previously described.20 Synthesis. 4′-(3-Carboxypropyl)-2,2′:6′,2′′-terpyridine. 4′Methyl-2,2′:6′,2′′-terpyridine (2.3 g, 9.3 mmol) in tetrahydrofuran (100 mL) was cooled to -78 °C in a dry ice/acetone bath. n-BuLi (6.25 mL, 10 mmol, 1.6 M solution in hexane), diisopropylamine (1.4 mL, 10 mmol), and tetrahydrofuran (20 mL) were added dropwise, and the solution was stirred for 1 h. 1-Bromoethyl2,3-dioxolane (1.18 mL, 9.3 mmol) was added, and the reaction mixture was slowly allowed to rise to room temperature where it was stirred for 16 h. Aqueous saturated NaCl (40 mL) was added to quench the reaction, and then the reaction mixture was extracted into CH2Cl2 and concentrated in vacuo. The residue (29) (a) Della Ciana, L.; Hamachi, I.; Meyer, T. J. J. Org. Chem. 1989, 54, 1731. (b) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, T. J. Inorg. Chem. 1996, 35, 5319. (30) Potts, K. T.; Usifer, D. A.; Guadalupe, A.; Abrun˜a, H. D. J. Am. Chem. Soc. 1987, 109, 3961. (31) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17, 3334. (32) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19, 1405.

874 Langmuir, Vol. 15, No. 3, 1999 was dissolved in 1 M HCl (100 mL) and heated at 60 °C for 3 h. After cooling, the solution was neutralized with aqueous Na2CO3 and then extracted into CH2Cl2 and concentrated in vacuo. The residue was dissolved in acetone (50 mL) and oxidized by dropwise addition of an acetone solution of KMnO4 until the purple color persisted in solution. The solution was concentrated in vacuo, and water (10 mL) was added to the residue and subsequently heated to boiling. The mixture was then cooled at 4 °C for 16 h and filtered, and the solid was washed with aqueous NaHCO3. The filtrate was extracted into CH2Cl2, the pH of the aqueous phase was adjusted to pH 4.8, and the aqueous phase was then further extracted. The extracts were combined, concentrated in vacuo, and then purified by chromatography (silica, CH2Cl2/CH3OH 95:5 as eluent). A light yellow oil was obtained which was recrystallized from CH2Cl2 to give a white powder (1.63 g, 55%). Mp ) 190-192 °C. EI-MS: 319 (9%) [M+], 274 (100%) [M+ - COOH], 260 (100%) [M+ - CH2COOH], 246 (100%) [M+ - (CH2)2COOH], 232 (18%) [M+ - (CH2)3COOH]. 1H NMR (DMSO): δ 8.75 (d, 2H, J ) 4.8 Hz, H6), 8.64 (d, 2H, J ) 8.0 Hz, H3), 8.02 (td, 2H, J ) 8.0,1.8 Hz, H4), 7.52 (ddd, 2H, J ) 8.0,4.6,1.6 Hz, H5), 2.58 (d, 2H, J ) 7.0 Hz, CH2), 2.33 (d, 2H, J ) 7.0 Hz, CH2), 1.95 (q, 2H, J ) 7.0 Hz, CH2). 13C NMR (DMSO): δ 174.32, 155.30, 155.10, 153.16, 149.45, 137.58, 124.55, 121.00, 120.85, 34.24, 33.14, 25.55. General Synthesis for the Polypyridyl-Pendant PAMAM Dendrimers. A molar excess of the carboxylic acid-pendant pyridyl ligand (either isonicotinic acid, 4-(3-carboxypropyl)-4′-methyl2,2′-bipyridine, or 4′-(3-carboxypropyl)-2,2′:6′,2′′-terpyridine), N,N′-dicyclohexylcarbodiimide (DCC), and 1-hydroxybenzotriazole (HOBt) were dissolved in N,N-dimethylacetamide. The PAMAM dendrimer (20 wt % solution in methanol) was added, and the solution was stirred at room temperature. After a suitable reaction period, the reaction mixture was filtered to remove precipitated N,N′-dicyclohexylurea, and the filtrate was added dropwise to diethyl ether (400 mL). The solid was filtered and washed well with diethyl ether and acetone. The solid was then dissolved in methanol, and anion-exchange resin was added and the flask swirled. The resin was removed by filtration, the filtrate concentrated in vacuo, and the solution purified by repeated gel permeation chromatography using CH2Cl2/CH3OH (5:1) as eluent. The dendrimer was collected, concentrated, and then added dropwise to diethyl ether, and the mixture filtered and dried in vacuo. Yields were in the range 65-90%. Preparation of Tris(bipyridyl)ruthenium(II) Pendant PAMAM Dendrimers. Acetone was bubbled with prepurified nitrogen for 15 min. cis-Ru(bpy)2Cl2‚2H2O and AgPF6 (2 equiv) were added, and the mixture was stirred at room temperature for 2 h. The precipitated AgCl was removed by filtration, and the filtrate containing [Ru(bpy)2(acetone)2]2+ was added to dend-n-bpy in methanol under nitrogen and then heated at reflux. After cooling, excess aqueous NH4PF6 was added and the solution concentrated in vacuo until precipitation occurred. The solid was filtered and then suspended in hot methanol, boiled for 10 min, and then filtered. The solid was then dissolved in CH3CN and purified by gel permeation and cation exchange chromatography. Concentration and precipitation by slow evaporation of acetone from a methanolic solution gave a red solid, dend-n-[Ru(bpy)3] (n ) 4, 8, 16, 32, 64). Yields were in the range 65-80%. Preparation of Bis(terpyridyl)ruthenium(II) Pendant PAMAM Dendrimers. Dend-n-bpy and Ru(tpy)Cl3 were heated at reflux in ethanol/water (10:1) containing triethylamine for 20 h. The mixture was then filtered hot to remove unreacted Ru(tpy)Cl3. After cooling, excess aqueous NH4PF6 was added to precipitate the products. The resulting solid was then filtered, suspended in hot methanol, boiled for 10 min, and then filtered. The solid was then dissolved in CH3CN and purified by gel permeation and cation exchange chromatography. Concentration and precipitation by slow evaporation of acetone from methanol gave a red solid, dend-n-[Ru(tpy)2] (n ) 4, 8, 16, 32, 64). Yields were in the range 55-80%.

Results and Discussion Syntheses. The terminal amine groups of the PAMAM dendrimers can be reacted with a variety of reagents containing functional groups such as carboxylic acids, acid

Storrier et al.

halides, and acid anhydrides. To optimize the modification of the terminal amine groups to quantitatively produce dendrimers fully functionalized with polypyridine ligands, a simple high yielding reaction utilizing mild reaction conditions and leading to byproducts that are easily separated from the dendritic products is required. This divergent synthetic strategy (i.e., where dendrimer growth takes place outward from the central core) relies on the complete reactivity of the terminal amine groups to the peptide coupling procedure in order to produce the desired dendrimer ligands. It is widely recognized that for higher generation dendrimers produced via a divergent synthesis complete functionalization of the surface groups is extremely difficult.33 Less than ideal and complete reactivity results in branch defects which leads to a decrease in the number of metal centers incorporated in the dendrimers after complexation. Added to the inherent defects in larger generation dendrimers,9 this will lead to less than ideal purity for characterization purposes. Hence, we assume that complete amidation of the terminal groups has occurred but will address this point during discussion of the characterization of the dendrimers. A general reaction scheme is shown in Scheme 1 using PAMAM G1 for illustrative purposes. We chose to use the well-known peptide coupling agents N,N′-dicyclohexylcarbodiimide (DCC) and 1-hydroxybenzotriazole (HOBt). In a typical synthesis, a methanolic solution of PAMAM dendrimer was added to a carboxylic acid-pendant pyridyl ligand, DCC, and HOBt in DMAc solution and stirred under nitrogen at room temperature. To facilitate optimal surface functionalization of the terminal amino groups, all reagents were used in a large molar excess relative to the PAMAM dendrimer, and the time of reaction was also increased with the increase in the dendrimer generation size. After a suitable reaction period (16 h for G0 to 80 h for G4) the reaction mixture was filtered to remove precipitated N,N′-dicyclohexylurea and the products were precipitated by dropwise addition of the filtrate to diethyl ether. HOBt was then removed using anion-exchange resin. The dendrimer ligands could not be purified by adsorption chromatography, and so further purification was achieved by repeated gel permeation chromatography. Only the compact main band was collected, and the tailings were separated and removed from the bulk material. Nevertheless, the yields of the dendrimer ligands were generally high. This reflects the high yields expected for the peptide coupling reaction, which was assisted by the use of excess reagents. Further purification was achieved using solubility differences since the carboxylic-pendant bipyridyl and terpyridyl ligands are soluble in acetone while the dendrimers, dend-n-bpy and dend-n-tpy, are insoluble in this solvent and so any unreacted ligand was separated by repeated washing of the products. The use of GPC linked to a differential refractometer detector to monitor the purity of the dendrimer products was attempted but was unsuccessful because only an extremely low refracting signal was observed. The purity of the final product could be determined by other techniques such as 1H NMR spectroscopy where none of the reactants or byproducts were detectable in spectra of the samples after purification. The terpyridyl-pendant dendrimer ligands were all precipitated as white solids (except for dend-4-tpy which was a colorless oil). The pyridyl- and bipyridyl-pendant dendrimers were precipitated as white or light yellow solids by dropwise addition of a methanolic solution to (33) Ashton, P. R.; Boyd, S. E.; Brown, C. L.; Nepogodiev, S. A.; Meijer, E. W.; Peerlings, H. W. I.; Stoddart, J. F. Chem. Eur. J. 1997, 3, 974.

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Langmuir, Vol. 15, No. 3, 1999 875 Scheme 1

Scheme 2

diethyl ether. These samples turned oily over a period of weeks, perhaps due to the absorption of moisture. Measurements of the melting point for the dendrimer ligands were complicated by the tendency of the samples to show a number of transitions (generally in the range 60-90 °C) before decomposition at higher temperatures above 100 °C. The presence of a number of thermal transitions was confirmed for a number of samples by differential scanning calorimetry measurements. These multiple transitions could arise from solvent trapped within the dendrimer; behavior that is often found especially for the higher generation dendrimers. Complexation of the dendrimer ligands was achieved by standard synthetic procedures. Scheme 2 shows the dendrimer complexes prepared. The tris(bipyridyl)ruthenium(II) pendant dendrimers, dend-n-[Ru(bpy)3] (n ) 4, 8, 16, 32, 64), were prepared by the initial formation of [Ru(bpy)2(acetone)2]2+ from cis-Ru(bpy)2Cl2‚2H2O and AgPF6 (2 equiv) in acetone, followed by complexation of dend-n-bpy in acetone/methanol at reflux. The bis(terpyridyl)ruthenium(II) dendrimer complexes, dend-n[Ru(tpy)2] (n ) 4, 8, 16, 32, 64), were prepared by the reaction of dend-n-tpy and excess Ru(tpy)Cl3 in aqueous ethanol containing the reducing agent triethylamine. All metal precursors were added in excess, and the reaction times were increased for the larger generations to ensure full complexation of the coordination sites. The ruthenium(II) complexes were obtained as hexafluorophosphate salts

by metathesis with NH4PF6. The raw products were washed well with methanol to remove soluble monomeric material from the insoluble dendrimer complexes and were obtained in essentially quantitative yield. The dendrimer complexes were insoluble in alcoholic solutions but were soluble in polar aprotic solvents such as acetone, acetonitrile, DMSO, and DMF. Each of the dendrimer complexes was purified by gel permeation and cation exchange chromatography to separate the precursor metal complexes and complexes formed by ligand scrambling (i.e., [Ru(bpy)3]2+ and [Ru(tpy)2]2+). Chromatographic purification of the dendrimer complexes was continued until the thin-layer chromatography (silica, CH3CN/H2O/saturated aqueous KNO3 20:4:1 as eluent) showed only one spot. Further, any tailings of the compact main band were discarded to improve the purity of the product. Evidence for complexation was provided by NMR and UV spectrometry, electrochemical measurements, and elemental analysis. Results from each of these techniques will be discussed in detail. Characterization of the Dendrimers. The difficulties faced in the characterization of hyperbranched molecules have been noted,10,34 while characterization of highly charged cationic dendrimers has proven to be an even more onerous task.35 Nevertheless, a variety of methods were employed to characterize the dendrimers. Full analytical and spectroscopic data for the new compounds are listed in the Supporting Information. Many authors have noted that dendrimers show discrepancies between the calculated and observed analyses due to trace impurities and the tendency toward solvent inclusion.14,19,36 Partial elemental analyses were obtained for some dendrimers, but the presence of water or other solventssperhaps contained within the dendritic cavitys leads to significant errors in the analyses (see Supporting Information). Furthermore, the difficulties associated with microanalyses of ruthenium(II) oligopyridine complexes (34) Newkome, G. E.; Moorefield, C. N.; Baker, G. R. Aldrichim. Acta 1992, 25, 31. (35) (a) Achar, S.; Immoos, C. E.; Hill, M. G.; Catalano, V. J. Inorg. Chem. 1997, 36, 2314. (b) Ashton, P. R.; Shibata, K.; Shipway, A. N.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1997, 36, 2781. (c) Haga, M.; Ali, M. M.; Arakawa, R. Angew. Chem., Int. Ed. Engl. 1996, 35, 76. (d) Campagna, S.; Denti, G.; Serroni, S.; Juris, A.; Venturi, M.; Ricevuto, V.; Balzani, V. Chem. Eur. J. 1995, 1 (4), 211. (36) Archut, A.; Vogtle, F.; De Cola, l.; Azzellini, G. C.; Balzani, V.; Ramanujam, P. S.; Berg, R. H. Chem. Eur. J. 1998, 4, 699.

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Storrier et al.

Figure 2. (A) FAB mass spectrum for dend-4-[Ru(tpy)2]. (B) MALDI-TOF mass spectrum for dend-8-[Ru(tpy)2]. Figure 1. FAB mass spectra for (A) dend-4-bpy and (B) dend8-tpy.

due to the formation of ruthenium carbides, especially in polynuclear species, have been noted.37 Although, mass and NMR spectral analyses have been obtained for some of the G0 and G1 dendrimers, for the higher molecular weight dendrimers the spectra became very complicated. Large excesses of all the reagents for the syntheses, in comparison with the dendrimer, were used. However, given the inherent statistical errors in the dendrimer, which increase with generation size, some errors in the reactivity are inevitable during the surface functionalization reaction. Mass Spectrometry Studies. Fast atom bombardment (FAB), matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF), and electrospray ionization (ESI) mass spectrometry (MS) techniques were used to characterize the dendrimers. Generally, the results were dependent on the generation of the dendrimer, with useful mass spectra being obtained for the lower generation dendrimers. However, none of the pyridyl-pendant dendrimers exhibited MS from which any information could be deduced and essentially gave no peaks in the spectra of any of the aforementioned mass spectrometric techniques. For the dendrimer ligands, peaks in the FAB MS were obtained for dend-4-bpy, dend-4-tpy, and dend-8-tpy at m/z 1470.6 ((M + H)+, calcd 1470.8), 1724.0 (M+, calcd 1726.1), and 3841.2 ((M + H)+, calcd 3841.6), respectively. Figure 1 shows FAB MS for the dendrimer ligands dend4-bpy and dend-8-tpy. The molecular masses of the G2G4 dendrimers (dend-16-bpy and dend-16-tpy (G2) are 7068.8 and 8077.7, dend-32-bpy and dend-32-tpy (G3) are 14 534.1 and 16 552.0, and dend-64-bpy and dend-64-tpy (G4) are 29 464.8 and 33 725.0, respectively) are too high for determination by FAB MS. The MALDI-TOF spectrum (37) Constable, E. C.; Housecroft, C. E.; Cattalini, M.; Phillips, D. New J. Chem. 1998, 193.

of dend-8-bpy exhibited a peak at m/z 3341.8 (M+, calcd 3336.0), while dend-8-tpy showed a peak at m/z 3864.0 ((M + Na)+, calcd 3863.6). Reliable molecular weight determinations could not be obtained for the other dendrimer ligands samples. In many cases, the matrix peaks in the MALDI-TOF spectra were suppressed by the sample or a broad distribution of peaks was observed. For the dendrimer complexes, reliable molecular weight determinations were obtained for the G0 and G1 [Ru(tpy)2]pendant dendritic complexes but not for the larger generation dendrimer complexes. Figure 2 shows the FAB MS obtained for dend-4-[Ru(tpy)2] and the MALDI-TOF MS for dend-8-[Ru(tpy)2]. For example, dend-4-[Ru(tpy)2] gave excellent results, while dend-8-[Ru(tpy)2] gave poor results for all three ionization techniques. For dend-4[Ru(tpy)2], a peak in its FAB mass spectrum was observed for the molecular ion at m/z 4219.6 (calcd 4222.0), plus peaks for the successive loss of a PF6- anion at 4075.1 (calcd 4074.2), 3927.6 (calcd 3929.2), and 3782.9 (calcd 3784.3). In the MALDI-TOF MS, peaks that corresponded to the loss of one, two, and three PF6- counterions were observed at m/z 4072.8, 3926.0, and 3782.9. For the G1 dendrimer complex, dend-8-[Ru(tpy)2], a peak in the MALDI-TOF MS was observed at m/z 8546.1, which could be assigned to (M - 2PF6)+ (calcd 8545.0). Surprisingly, poor FAB and MALDI-TOF MS data were obtained from samples of dend-n-[Ru(bpy)3] (n ) 4, 8). ESI-MS has become a widely used technique for supramolecular systems in recent years and has also been applied to dendritic systems.38,39 As anticipated, the G1G4 dendrimer complexes afforded extremely complex ESI mass spectra due to the high total charge of the molecules. Moucheron et al.39 have reported that the ESI spectra for similar polynuclear pyridyl complexes are complicated by (38) Brady, P. A.; Sanders, J. K. M. New J. Chem. 1998, 411. (39) Moucheron, C.; Kirsch-De Mesmaeker, A.; Dupont-Gervais, A.; Leize, E.; Van Dorselaer, A. J. Am. Chem. Soc. 1996, 118, 12834 and references therein.

Polyamidoamine Dendrimers Table 1.

1H

Langmuir, Vol. 15, No. 3, 1999 877

NMR Data (δ) for Aryl Region for the Dendrimer Ligandsa

compound

H2

H3

N-H

dend-4-py dend-8-py dend-16-py dend-32-py dend-64-pyb

8.66 8.72 8.72 8.73 8.68

7.76 7.71 7.73 7.75 7.71

5.3 9.0, 8.3, 8.0 9.0, 8.2 8.9, 8.1, 7.8

compound

H3,3′

H5,5′

H6,6′

N-H

dend-4-bpy dend-8-bpy dend-16-bpy dend-32-bpy dend-64-bpy

8.15 8.20 8.20 8.21 8.18

7.26 7.24 7.24 7.24 7.2

8.50 8.51 8.51 8.51 8.45

8.0, 7.9 7.9 8.0-7.7 8.0-7.7 8.0-7.7

compound

H3′

H3

H4

H5

H6

N-H

tpyc dend-4-tpy dend-8-tpy dend-16-tpy dend-32-tpy dend-64-tpy

8.31 8.29 8.26 8.26 8.26 8.26

8.64 8.59 8.56 8.58 8.58 8.56

8.02 7.97 7.96 7.96 7.96 7.95

7.52 7.49 7.46 7.45 7.45 7.44

8.75 8.70 8.69 8.70 8.68 8.64

7.9 8.0-7.8 8.0-7.7 8.0-7.7 8.0-7.7

a Recorded in (CD ) SO. b Recorded in (CD ) SO/D O. c tpy ) 4′3 2 3 2 2 (3-carboxypropyl)-2,2′;6′,2′′-terpyridine.

the prevalence of ions such as POF4- and PO2F2- resulting from the hydrolysis of PF6-. The G0 dendrimer complexes gave some peaks which could be easily assigned. For example, peaks for dend-4-[Ru(tpy)2] were observed at m/z 910.2, 698.9, 558.3, 457.8, and 382.6, which could be assigned to (M - 4PF6)4+ (calcd 909.8), (M - 5PF6)5+ (calcd 698.9), (M - 6PF6)6+ (calcd 558.2), (M - 7PF6)7+ (calcd 457.8), and (M - 8PF6)8+ (calcd 382.8), respectively. In contrast, the ESI-MS of the other G0 dendrimer complex, dend-4-[Ru(bpy)3], was more complicated, though the molecular ion peak at m/z 570.8 can be assigned to (M 6PF6)6+ (calcd 568.9). Nuclear Magnetic Resonance Studies. The NMR spectra of the dendrimers became more complex as the generation size increased. The lower generation dendrimers gave relatively simple and easily assignable 1H NMR spectra, as observed for the mass spectra. The 1H NMR spectra of the dendrimers all exhibited signal broadening, with the level of broadening increasing with the corresponding increase in generation. For each series of polypyridyl-pendant ligands the solubility of the dendrimers decreased with increase in generation size. For example, dend-64-py and dend-64-bpy (G4) were insoluble in D2O and CDCl3, respectively, while the corresponding G0-G3 dendrimers were soluble in these solvents. On the other hand, the higher generation dendrimers were soluble in (CD3)2SO so that NMR spectra could be obtained in it. The aryl regions of the 1H NMR spectra of the dendrimer ligand were relatively simple and were assigned by comparison with the spectrum of the parent pyridyl ligand. The pyridyl-, bipyridyl-, and terpyridyl-pendant dendrimers exhibited two, three,40 and five sets of aromatic signals characteristic of 4-substituted pyridine, 4-substituted 4′methylbipyridine, and 4′-substituted terpyridine ligands, respectively. 1H NMR data for the aryl regions for the dendrimer ligands are summarized in Table 1. The signals showed very little deviation in the chemical shift of the corresponding protons with change in generation size. In addition, the dendrimer core had little effect on the position of the signals for the pyridyl/bipyridyl/terpyridyl protons. (40) The corresponding protons on the separate pyridine rings of the bipyridyl moiety (3 and 3′, 5 and 5′, 6 and 6′) are chemically nonequivalent but are observed as overlapping multiplets.

The alkyl regions exhibited greater complexity, especially for the G4 species, due to the broad nature of the methylene peaks of the dendrimer core, the overlap of some signals, and the presence of the broad multiplet for the residual (CH3)2SO protons. However, the use of elevated temperatures and comparison with similar surface-functionalized PAMAM dendrimers allowed assignments consistent with the proposed structures. Metal complexation of the polypyridyl-pendant groups leads to the expected changes in the aryl regions of the spectra which were complicated by the increase in the number of protons and the overlap of some of these signals. The aryl regions of the 1H NMR spectra for dend-n-[Ru(bpy)3] were more complicated since the formation of the D3-symmetric [Ru(bpy)3]2+ units results in complex stereochemistry due to the chirality of the ruthenium(II) centers (∆ or Λ). Recent studies have utilized stereospecific syntheses to incorporate chiral structural elements into dendritic structures for applications in chiral molecular recognition.16 The alkyl regions for the bipyridyl complexes were similar to the spectra of the corresponding dendn-bpy dendrimer ligand. The aryl regions of the 1H NMR spectra for dend-n[Ru(tpy)2] were easily assigned by comparison with spectra of other heteroleptic bis(terpyridyl)ruthenium(II) complexes.41 The signals were observed at similar chemical shifts for each generation, though they broadened as the generation size of the dendrimer increased. The peaks associated with the CH2 groups of the dendrimer core were complicated by downfield shifting and intense broadening of the signals associated with the coordination of the ruthenium(II) center. The 13C NMR spectra of the dendrimers showed the expected similarities in each of the series of dendrimers (i.e., pyridyl-, bipyridyl-, and terpyridyl-functionalized dendrimers, respectively). The aryl region of the spectra for the dendrimer ligands showed the expected number of signals for the magnetically inequivalent carbons, i.e., 3, 10, and 8 signals for the pyridyl-, bipyridyl-, and terpyridyl-pendant dendrimers, respectively. The 13C NMR spectra are valuable in determining the degree of reactivity of the terminal amine groups of the PAMAM dendrimers to the peptide coupling reaction. In D2O, peaks for the R- and β-methylene carbons, from unreacted amino groups, are found at δ 41.9 and 41.2.26 No peaks were observed at these chemical shifts in the spectra of dend-n-py (n ) 4, 8, 16, 32), suggesting that essentially quantitative functionalization occurred in these dendrimers. Unfortunately the bipyridyl- and terpyridyl-pendant dendrimer ligands were not soluble in D2O, and the presence of the multiplet for the solvent precluded the easy identification of the R- and β-methylene carbons in (CD3)2SO solution. Since similar reaction conditions were used for the preparation of all three families of polypyridyl-pendant dendrimers, we anticipate that similar reactivity to that exhibited by the pyridylpendant dendrimers should be exhibited by all the dendrimer species. The signals associated with the carbonyl groups are also particularly helpful in assessing reaction completion. The G0 dendrimers show two carbonyl signals for the dendrimer core and the carbonyl of the pyridyl ligand, respectively. The G1 dendrimer ligands each show three carbonyl signals, two of which are associated with the dendrimer core, plus one signal from the carbonyl of the pyridyl ligand. For the larger generation of the dendrimers (41) For example, see: Constable, E. C.; Cargill Thompson, A. M. W. New J. Chem. 1992, 16, 855.

878 Langmuir, Vol. 15, No. 3, 1999

Figure 3. Absorption spectra recorded in acetonitrile (298 K) for the dendritic complexes, dend-n-[Ru(bpy)3] (n ) 4, 16, 64). Inset: emission spectra recorded (A) in acetonitrile (298 K) and (B) in rigid butyronitrile matrix (77 K), λex (excitation wavelength) ) 460 nm.

Figure 4. Absorption spectra recorded in acetonitrile (298 K) for the dendritic complexes, dend-n-[Ru(tpy)2] (n ) 4, 16, 64). Inset: emission spectra recorded in rigid butyronitrile matrix (77 K), λex ) 465 nm.

some signal broadening was observed for the carbonyl signals and the other signals. Such behavior is prevalent in the spectra of high molecular weight polymer samples. Absorption and Emission Spectroscopies. Absorption and room-temperature emission spectra of the dendrimer complexes were recorded in acetonitrile solution, while low-temperature emission spectra were obtained at 77 K in a butyronitrile glass matrix. Representative absorption and emission spectra for dend-n-[Ru(bpy)3] (n ) 4, 16, 64) and dend-n-[Ru(tpy)2] (n ) 4, 16, 64) are shown in Figures 3 and 4, respectively. Data are summarized in Table 2 where values for [Ru(bpy)3]2+ and [Ru(tpy)2]2+ are also included for comparison. Molar absorptivity values for the dendrimer complexes per metal center are also given for the metal-to-ligand charge transfer (MLCT) band. As expected, the absorption spectra for each of the series of dendrimer complexes are very similar. The UV region is dominated by intense ligand-centered π f π* absorptions. The dendritic polypyridylruthenium(II) complexes each exhibited an intense, broad absorption in the visible region which is ascribed to the dπ(Ru) f π* MLCT transition. For the G0 and G1 dendrimers the molar absorptivity values for the MLCT bands suggest that each of the metal centers is acting independentlysi.e., there are no metal-metal interactions. Hence, the molar absorptivity of the MLCT band is the sum of the individual

Storrier et al.

metal centers. For the higher dendrimer generations, these values are significantly lower than the sum of the absorptions of the independent metal centers. A number of factors could be responsible, at least in principle, for such behavior. These could include, among others, an inefficient peptide coupling reaction leading to generation defects and inefficient ruthenium(II) complexation by the pendant pyridyl ligands, plus the effect of the increase of dendrimer core size and the resulting molecular weight increase on the calculation of the absorption coefficients. The tris(bipyridyl)ruthenium(II) dendrimers exhibited room-temperature luminescence at ca. 615 nm in acetonitrile solution as shown in Figure 3 (inset). This value shows some deviation from the reported value of 630 nm for [Ru(bpy)3]2+.5 In a rigid butyronitrile matrix at 77 K, the dend-n-[Ru(bpy)3] dendrimer complexes show a strong emission at ca. 590 nm with shoulders to lower energy, which is similar to the reported behavior of [Ru(bpy)3]2+. In contrast, the bis(terpyridyl)ruthenium(II) dendrimers exhibited no fluorescence at room temperature, but a strong band centered at 610 nm with a shoulder at ca. 655 nm when excited at 465 nm in a rigid butyronitrile matrix at 77 K (see Figure 4, inset). For comparison, [Ru(tpy)2]2+ exhibits bands at 598 and 650 nm.42 Electrochemical and EQCM Studies. Dend-n-[Ru(tpy)2] (n ) 4, 8, 16, 32, 64). In this study, concentrations of the dendrimer complex solutions were adjusted so that the concentrations of ruthenium(II) redox sites were equivalent (0.1 mM of Ru(II) centers) in all solutions. Figure 5 shows (A) current (cyclic voltammogram) and (B) frequency changes as a function of applied potential between +0.60 and +1.50 V for dend-64-[Ru(tpy)2] in AN solution. Similar cyclic voltammograms and frequencypotential curves were obtained for dend-n-[Ru(tpy)2] (n ) 4, 8, 16, 32). Figure 5A shows a redox couple with a formal potential value of +1.31 V and which we ascribe to the Ru(II/III) process. The wave shape of this process shows contributions from freely diffusing and surface-immobilized redox species. The presence of a single Ru(II/III) wave implies a simultaneous redox process for all the Ru(II) centers at the dendrimer periphery. Similarly, a single wave has been previously observed for the ferrocenyl redox process of a variety of polynuclear ferrocenyl dendrimers.20,23,24,43 As shown in Figure 5B, the frequency decreased (indicating an increase in mass) upon oxidation of the metal center to Ru(III) (at +1.31 V) and continued to decrease after reversal of the potential scan until the metal center was reduced back to Ru(II). Such frequency changes are characteristic of film deposition and stripping from the electrode surface and are similar to those which we previously reported for diaminobutane-ferrocenyl dendrimers.20 The frequency decrease could arise, at least in part, as a result of anions and/or solvent being incorporated into the film to maintain charge neutrality (an adsorbed or deposited layer must be present on the electrode surface for this to occur). The magnitude of the frequency changes decreased with generation size (i.e., from G4 to G0). For dend-n-[Ru(tpy)2] (n ) 16, 32), similar stripping-type frequency changes to those observed for dend-64-[(Ru(tpy)2] were obtained, while for dend-n-[Ru(tpy)2] (n ) 4, 8), the changes in the frequency appear to be simple anion exchange type. (42) Stone, M. L.; Crosby, G. A. Chem. Phys. Lett. 1981, 79, 169. (43) Alonso, B.; Mora´n, M.; Casado, C. M.; Lobete, F.; Losada, J.; Cuadrado, I. Chem. Mater. 1995, 7, 1440.

Polyamidoamine Dendrimers

Langmuir, Vol. 15, No. 3, 1999 879

Table 2. Absorption and Luminescence Data for the Dendrimer Complexes absorptiona λmax/nm

complex [Ru(tpy)2]2+ d,e dend-4-[Ru(tpy)2] dend-8-[Ru(tpy)2] dend-16-[Ru(tpy)2] dend-32-[Ru(tpy)2] dend-64-[Ru(tpy)2] [Ru(bpy)3]2+ f,g dend-4-[Ru(bpy)3] dend-8-[Ru(bpy)3] dend-16-[Ru(bpy)3] dend-32-[Ru(bpy)3] dend-64-[Ru(bpy)3] a

234 (201) 230c (372) 232 (696) 232 (1418) 230c (2500) 243 (49) 246 (97) 248 (172) 246 (379) 244 (697) 246 (1069)

(/10-3

270 (32) 272 (248) 272 (400) 272 (771) 272 (1713) 272 (2560)

L

mol-1

emission (λem/nm)

cm-1)

307 (52) 306 (269) 310 (569) 310 (1034) 310 (2105) 310 (3550)

286 (91.3) 288 (271) 288 (532) 288 (1017) 288 (1535) 288 (3360)

Recorded in acetonitrile. b Recorded in butyronitrile. c Shoulder.

 per 475 (12) 478 (78) 478 (147) 478 (273) 478 (518) 478 (900) 450 (16) 454 (44) 454 (93) 454 (174) 454 (248) 454 (533)

d

RuII

298 Ka

11.6 19.5 18.3 17.1 16.2 14.1 15.6 11.0 11.6 10.9 7.8 8.3

77 Kb 598, 650 610, 656c 610, 662c 608, 655c 608, 650c 612

630 615 614 616 618 614

582 588, 632, 692c 594, 636, 694c 592, 630 590, 628c 594, 636, 695c

Reference 50. e Reference 43. f Reference 51. g Reference 5.

Figure 5. Typical (A) current (cyclic voltammogram) and (B) frequency responses as a function of applied potential between +0.60 and +1.50 V vs Ag/AgCl at 50 mV s-1 for a Pt electrode in contact with a 0.10 M TBAP/AN solution containing 1.56 µM (0.1 mM Ru site) dend-64-[Ru(tpy)2].

Figure 5A also shows a small voltammetric wave with a formal potential of +0.97 V in addition to the Ru(II/III) couple at +1.31 V mentioned above. Although we have not been able to unambiguously assign/identify the origin of this couple, potential scan rate dependence studies indicate that it corresponds to a surface-confined redox species; possibly a microdomain in the dend-64-[Ru(tpy)2] film. This couple appears to have a smaller charge-transfer rate constant than the Ru(II/III) processsalthough the peak currents were directly proportional to the potential scan rate (up to 500 mV s-1), the peak potential separation (∆Ep) increased more rapidly than for the Ru(II/III) process as the scan rate increased. In addition, there appears to be some contribution to the anodic peak from “chargetrapping”.44 The anodic peak height was enhanced on the initial positive-going scan, but on the subsequent potential cycle (between +0.60 and +1.50 V) such an enhancement of the peak current was not observed. This behavior is (44) (a) Abrun˜a, H. D.; Denisevich, P.; Uman˜a, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1. (b) Denisevich, P.; Willman, K. W.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 4727. (c) Willman, K. W.; Murray, R. W. J. Electroanal. Chem. 1982, 133, 211. (d) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. Soc. 1984, 106, 1991.

Figure 6. (A) Cyclic voltammogram and (B) frequency responses as a function of applied potential between -1.70 and -0.50 V vs Ag/AgCl at 50 mV s-1 for 1.56 µM of dend-64-[Ru(tpy)2] in 0.10 M TBAP/AN.

typical of charge-trapping processes and will be discussed in more detail later. A similar set of small peaks at a potential prior to the Ru(II/III) peaks were also observed for dend-n-[Ru(tpy)2] (n ) 16, 32). The peak height of this couple relative to the Ru(II/III) couple increased with increasing dendrimer generation, again suggesting the presence of microdomains whose relative effect increases with increasing generation size. Figure 6 shows the (A) cyclic voltammogram and (B) frequency changes as a function of applied potential between -1.70 and -0.50 V for dend-64-[Ru(tpy)2]. The two sets of waves present at formal potential values of -1.18 and -1.40 V are ascribed to terpyridine-localized reduction processes. These redox waves suggest that the

880 Langmuir, Vol. 15, No. 3, 1999

Figure 7. Typical (A) current (cyclic voltammogram) and (B) frequency responses as a function of applied potential between -1.80 and +1.60 V vs Ag/AgCl for 1.56 µM dend-64-[Ru(tpy)2] in 0.10 M TBAP/AN.

electrochemical processes of the equivalent terpyridyl ligands on the peripheral [Ru(tpy)2]2+ complexes take place almost simultaneously suggesting a lack of significant interactions between the [Ru(tpy)2]2+ groups, although there appears to be some broadening relative to the Ru(II/III) wave. The increase in current and overall decrease in frequency (due mainly to a mass increase) with continuous potential scanning indicate the accumulation of the dendrimer on the electrode surface. This may be caused by deposition of electrically neutral dend-64-[Ru(tpy)2], since the dendrimer has zero charge after reduction of the second terpyridine ligand. The increase in frequency (due mainly to a mass decrease) from -1.10 V on the negative-going scans may be a manifestation of release of anions upon reduction of the ligands to maintain charge neutrality in the deposited dendrimer film. For the [Ru(tpy)2]-pendant dendrimers, the magnitude of the decrease in frequency (i.e., mainly increase in mass) upon continuous potential scanning over the potential range -1.70 to -0.50 V increased as the generation of the dendrimer increased (e.g., after five consecutive potential scans for dend-4-[Ru(tpy)2] and dend-64-[Ru(tpy)2] and under otherwise identical conditions, the frequency changes were 84 and 165 Hz, respectively). Also consistent with this is the fact that the area under the Ru(II/III) peak for the electrode modified with dend-4-[Ru(tpy)2] was smaller than the corresponding one for dend-64-[Ru(tpy)2], indicating a lower coverage in the former. This, in turn, would mean that fewer dendrimers are depositing upon reduction of the terpyridyl ligands. The cyclic voltammogram of dend-64-[Ru(tpy)2] (Figure 7A) over the potential range of -1.80 to +1.60 V shows “charge trapping peaks” at potentials of +1.20 and -0.95 V on the anodic and cathodic scans, respectively. Charge

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trapping is a phenomenon that was initially observed for spatially segregated bilayer films44 on electrode surfaces where electronic communication of the outer film with the electrode is inhibited (due to the distance separating the electrode and outer film) so that the outer film’s redox reactions are mediated by the inner film to produce very sharp current peaks that typically occur at the leading edge of voltammetric waves for the inner film. The current flow is unidirectional or rectifying, similar to that at interfaces between semiconductor materials. A similar charge trapping effect has also been reported for various films formed from a single type of monomer.45,46 Charge trapping in monocomponent films 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.45 For the present single-component dendritic systems, the charge-trapping mechanism could be explained as follows. The charge trapped in an isolated microdomain upon oxidation of Ru centers (+1.31 V) is discharged by mediation through the (nonisolated) ligand-based reductions of the metal complex giving rise to the sharp prepeak at about -0.95 V, whereas charge trapped upon reduction of the ligands is discharged by mediation through the nonisolated metal-centered oxidation giving a sharp prepeak at +1.2 V. If the potential scan is reversed between the Ru(II/III) and ligand-centered redox processes, the charge trapping peaks are absent since the isolated domains are not able to be reduced or oxidized (and thus “trapped”) until they are mediated by the redox processes of the nonisolated domains. Similar charge trapping peaks were observed for the dend-n-[Ru(tpy)2] (n ) 4, 8, 16, 32) dendrimers. The peak current of the charge trapping peaks decreased with decreasing dendrimer generation, becoming barely observable for dend-4-[Ru(tpy)2]. In contrast, in previous work no charge trapping peaks were observed for the ferrocenyl-based dendrimers.20 This is likely due to the fact that such films only have a single redox wave thus precluding the discharging processes described above. It is also worth noting that although the overall value of the frequency for electrodes in contact with a dend64-[Ru(tpy)2] solution gradually decreased upon successive potential cycling between -1.80 and +1.60 V (Figure 7B), the peak currents in the cyclic voltammogram did not exhibit a concomitant increase. This behavior can be rationalized by taking into account changes in film morphology. Film properties such as viscoelasticity and roughness may change upon potential scanning possibly due to changes of the film’s structure, whereas the coverage of the dendrimer may remain constant. Conversely, increases in viscoelasticity and roughness may also cause a decrease in frequency (vide infra).47 Decreases in overall frequency upon successive potential scanning diminished as the generation size of the dendrimer decreased. Changes in film morphology 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 parmeter) have (45) Takada, K.; Storrier, G. D.; Pariente, F.; Abrun˜a, H. D. J. Phys. Chem. B 1998, 102, 1387. (46) Gottesfeld, S.; Redondo, A.; Rubinstein, I.; Feldberg, S. W. J. Electroanal. Chem. 1989, 265, 15. (47) (a) Borjas, R.; Buttry, D. A. J. Electroanal. Chem. 1990, 280, 73. (b) Muramatsu, H.; Ye, X.; Suda, M.; Sakuhara, T.; Ataka, T. J. Electroanal. Chem. 1992, 332, 311. (c) Beck, R.; Pittermann, U.; Weil, K. G. J. Electrochem. Soc. 1992, 139, 453. (d) Yang, M.; Thompson, M.; Duncan-Hawitt, W. C. Langmuir 1993, 9, 802. (e) Yang, M.; Thompson, M. Langmuir 1993, 9, 1990.

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been described previously.48 Figure 8 shows changes of the resistance parameter for the equivalent circuit of the resonator versus time in a 0.1 M TBAP/AN solution containing 0.1 mM Ru(II) sites of dend-n-[Ru(tpy)2] (n ) 4, 8, 16, 32, 64), at applied potentials of +0.40, +1.50, and -1.60 V, where the total charge on each metal complex is +2, +3, and 0, respectively. For dend-n-[Ru(tpy)2] (n ) 32, 64) a sharp increase in the resistance of ca. 35 and 7 Ω, respectively, was observed upon stepping the potential from +0.40 to +1.50 V (Figure 8D,E). The resistance rapidly decreased to the original value when the potential was stepped back to +0.40 V. These increases in resistance are believed to arise from an increase in the thickness, roughness, and viscoelasticity of the film due to the continuous deposition of the dendrimer at +1.50 V. Conversely, the decreases in the resistance are believed to arise from the stripping of the dendrimer film from the electrode surface at an applied potential of +0.40 V. In contrast, for dend-n-[Ru(tpy)2] (n ) 4, 8, 16) (Figure 8AC) the resistance remained essentially constant following a similar potential sequence indicating that there were no significant morphological changes in the films. These resistivity results are in agreement with the frequency changes mentioned previously, except for dend-16-[Ru(tpy)2]. From EQCM measurements, dend-16-[Ru(tpy)2] would be expected to exhibit resistance behavior similar to dend-n-[Ru(tpy)2] (n ) 32, 64), since as mentioned above, deposition and stripping-like frequency behavior was observed. This behavior could be rationalized if the rigidity and/or roughness of the dend-16-[Ru(tpy)2] film remained invariant as the dendrimer was deposited. Upon stepping the potential from +0.40 to -1.60 V (ligand-based reductions), the resistance parameter for electrodes modified with [Ru(tpy)2]-pendant dendrimers increased sharply and then reached a steady state. This is likely due to deposition of the dendrimer and is in good agreement with the EQCM results mentioned above. Upon stepping the potential back to +0.40 V, dendrimergeneration-dependent behavior was observed. For dend4-[Ru(tpy)2], the resistance decreased to its original value, suggesting simple dissolution (stripping) of the dendrimer film. For dend-n-[Ru(tpy)2] (n ) 8, 16), the resistance parameter initially decreased upon stepping the potential to +0.40 V, followed by a transient response where the resistance increased and subsequently decreased to reach a steady state. The initial abrupt decrease in the resistance parameter may arise from the dissolution of the dendrimer film, while the transient changes may be caused by a temporary increase in the roughness or other properties of the film. For dend-n-[Ru(tpy)2] (n ) 32, 64), the resistance increased upon stepping the potential to +0.40 V, which suggests much larger changes in the film properties than those for dend-n-[Ru(tpy)2] (n ) 8, 16), likely due to the increase in molecular size. In addition, overall increases in the resistance parameter for dendn-[Ru(tpy)2] (n ) 4, 8, 16, 32, 64) upon each series of potential steps reflect changes in roughness caused by repeated deposition and stripping of the dendrimer plus ejection or injection of anions and/or solvent. Dend-n-[Ru(bpy)3] (n ) 4, 8, 16, 32, 64). Electrochemical properties of dend-n-[Ru(bpy)3] were similar to those of dend-n-[Ru(tpy)2], though the magnitude of the currents and changes in frequency of dend-n-[Ru(bpy)3] were smaller (given that all measurements were obtained at 0.1 mM Ru concentration). (48) (a) Beck, R.; Pittermann, U.; Weil, K. G. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1363. (b) Martin, S.; Granstaff, V. E.; Frye, G. Anal. Chem. 1991, 63, 2272. (c) Yang, M.; Thompson, M. Anal. Chem. 1993, 65, 1158. (d) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355.

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Figure 8. Changes of the resistance parameter vs time for the equivalent circuit of the resonator upon applied potentials of +0.40 (b), +1.50 (O), and -1.60 V (]) vs Ag/AgCl in a 0.1 mM TBAP/AN solution containing 0.1 mM Ru site of dend-n-[Ru(tpy)2]: (A) n ) 4; (B) n ) 8; (C) n ) 16; (D) n ) 32; (E) n ) 64.

Figure 9 shows the (A) cyclic voltammogram and (B) frequency changes as a function of potential between +0.40 and +1.50 V for dend-64-[Ru(bpy)2] in AN solution. The Ru(II/III) redox couple was observed at a potential of +1.30

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Figure 9. (A) cyclic voltammogram and (B) frequency as a function of a potential between +0.40 and +1.50 V vs Ag/AgCl at 50 mV s-1 for 1.56 µM (0.1 mM Ru site) of dend-64-[Ru(bpy)3] in 0.10 M TBAP/AN.

V, and the concurrent changes in frequency appeared to be of the anion-exchange type. Instead of the small voltammetric wave observed prior to the Ru(II/III) process for dend-64-[Ru(tpy)2], a broader anodic charge trapping peak was observed at ca. +1.00 V in the first anodic scan. This difference may result from the lower coverage49 of dend-64-[Ru(bpy)3] (relative to dend-64-[Ru(tpy)2]), which makes the formation of microdomains within the film less likely. Similar cyclic voltammograms and frequency changes were obtained for dend-n-[Ru(bpy)3] (n ) 4, 8, 16, 32). At negative potentials, three one-electron redox couples (one for each of the bipyridyl ligands) are anticipated by analogy to [Ru(bpy)3]2+ which exhibits them at -1.28, -1.46, and -1.71 V.50 On potential scanning between -1.60 and -0.60 V for dend-64-[Ru(bpy)3] (Figure 10A), only two sets of waves were observed at -1.24 and -1.38 V. Upon extension of the negative potential limit to -2.00 V, a third poorly defined redox process was observed at ca. -1.90 V whose electrochemical response was not reproducible. Similar behavior was observed for all the [Ru(bpy)3]-pendant dendrimers. Upon reduction of the third bipyridyl ligand, an overall charge of -1 exists on each metal center in the dendrimer complex, and this may effect the complicated behavior observed. The overall frequency decreased (due mainly to an increase in mass) upon continuous potential scanning (Figure 10B), indicating deposition of a film onto the electrode surface. On the negative-going scans, the frequency increased over the potential range of -1.17 to -1.25 V (potentials that correspond to the first ligandcentered reduction peak) and then decreased during continuous cathodic scanning. The origin of this initial increase in frequency, which was much smaller for dend64-[Ru(tpy)2] and was not observed for lower generations of the [Ru(tpy)2] dendrimers, might arise from displacement of anions and/or solvent molecules from the electrode surface since film deposition onto the electrode should (49) An electrochemical investigation of the adsorption kinetics and thermodynamics of these dendrimer complexes is currently in progress. Results will be reported elsewhere. (50) Tokel-Takvoryan, N. E.; Hemingway, R. E.; Bard, A. J. J. Am. Chem. Soc. 1973, 95, 6582.

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Figure 10. (A) Current and (B) frequency responses as a function of applied potential between -1.60 and -0.60 V vs Ag/AgCl at 50 mV s-1 for 1.56 µM (0.1 mM Ru site) dend-64[Ru(bpy)3] in 0.10 M TBAP/AN.

Figure 11. (A) Current and (B) frequency responses as a function of applied potential between -1.60 and +1.55 V vs Ag/AgCl at 50 mV s-1 for 1.56 µM (0.1 mM Ru site) dend-64[Ru(bpy)3] in 0.10 M TBAP/AN.

result in a decrease in frequency. Similar behavior was observed for dend-n-[Ru(bpy)3] (n ) 8, 16, 32). The difference between the [Ru(bpy)3]- and the [Ru(tpy)2]-pendant dendrimers may result from the flexibility of the [Ru(bpy)3]-pendant dendrimers resulting in the release of anions from the film instead of incorporation of cations. For dend-4-[Ru(bpy)3], only a thin film was formed. Thus, the small increase in the frequency, due to expulsion of a small number of anions, was ostensibly counterbalanced by the decrease in frequency resulting from deposition of the dendrimer. In addition to the Ru(II/III) and ligand-based redox processes, the cyclic voltammogram of dend-64-[Ru(bpy)3] between -1.60 and +1.55 V (Figure 11A) exhibited an anodic peak at +1.05 V (+1.00 V on the first scan) and a broad cathodic peak at -0.60 V with a small shoulder at -0.90 V. These peaks are ascribed to charge trapping analogous to those observed for dend-64-[Ru(tpy)2]. (It should be mentioned that no charge trapping peaks were

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evident if the potential was continuously cycled between -0.20 and -1.60 V.) The broadness of the peak at -0.60 V might be caused by “charge leaking” from less isolated microdomains in the film to the electrode surface perhaps due to easier motion of redox sites. The trapped charge would be gradually released by means of movement of the redox sites. The more flexible [Ru(bpy)3]2+ moieties compared with the 4′-substituted terpyridyl moieties in the [Ru(tpy)2]-pendant dendrimers may result in less steric hindrance in the former relative to the latter making charge propagation (charge leakage in the present context) easier and thus yielding a less intense charge trapping peak. The changes in frequency on potential scanning between -1.60 and +1.55 V are shown in Figure 11B. On the cathodic scan from ca. +0.20 to -0.90 V the frequency gradually increased corresponding to the broad peak at -0.60 V. This behavior can be attributed to the slow ejection of anions and/or solvent from the deposited film to compensate for changes in the film’s charge. A much sharper increase in the frequency, corresponding to the charge trapping peak, was observed for dend-64-[Ru(tpy)2] as previously shown in Figure 7B. For the small anodic charge trapping peak at +1.05 V, the corresponding frequency decrease was less sharp than that observed for dend-64-[Ru(tpy)2]. The difference in the amount of trapped charge in the anodic charge trapping peak compared with that on the cathodic one may be the result of either a lower total amount of trapped charge or a higher degree of leakage of the trapped charge during the anodic potential scan. Similar broad charge trapping peaks were observed for other dend-n-[Ru(bpy)3]. The frequency changes upon oxidation of Ru(II) to Ru(III) for dend-64-[Ru(bpy)3] appear to be of the cationexchange type. This behavior was not observed for any of the other dend-n-[Ru(bpy)3] or dend-n-[Ru(tpy)2] examined and may be rationalized if the kinetics of the release of the trapped cations upon Ru(II) oxidation (charge trapping peak) is taken into account. As mentioned above, the charge trapping peak for dend-64-[Ru(bpy)3] is not as sharp as that observed for dend-64-[Ru(tpy)2]. In addition, there is no abrupt decrease in frequency at +1.05 V corresponding to the charge trapping peak (see Figure 11B). These results indicate that dend-64-[Ru(bpy)3] shows slower release of trapped charge, accompanied by slower anion injection, than dend-64-[Ru(tpy)2]. Therefore, cations contained within the film are more likely to be ejected upon metal-centered oxidation and, hence, cation exchange may occur. Changes in the resistance parameter of electrodes in contact with solutions of dend-n-[Ru(bpy)3] (n ) 4, 8, 16, 32, 64) upon the application of various potentials are depicted in Figure 12. The overall changes in resistance appear smaller than those for the corresponding [Ru(tpy)2]pendant dendrimers, indicating less deposition of the dendrimers and/or smaller potential-induced changes in film morphology, which is consistent with the EQCM data. The increases in the resistance observed for dend-n-[Ru(tpy)2] (n ) 32, 64) upon stepping the potential from +0.40 to +1.50 V (oxidation of Ru(II) centers) were generally not observed for the [Ru(bpy)3]-pendant dendrimers (an exception was the second positive potential step of dend64-[Ru(bpy)3]). This might indicate that dend-n-[Ru(bpy)3] do not accumulate onto the electrode in the oxidized form (presumably as a salt). The decrease in frequency upon oxidation of the Ru(II) center observed in Figure 9A is likely caused by insertion of anions and/or solvent. Furthermore, the transient responses observed for dend-

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Figure 12. Changes of the resistance parameter vs time for the equivalent circuit of the resonator upon applied potentials of +0.40 (b), +1.50 (O), and -1.60 V (]) vs Ag/AgCl in a 0.1 mM TBAP/AN solution containing 0.1 mM Ru site of dend-n[Ru(bpy)3]: (A) n ) 4; (B) n ) 8; (C) n ) 16; (D) n ) 32; (E) n ) 64.

n-[Ru(tpy)2] (n ) 8, 16, 32) were not evident in the resistance measurements for dend-n-[Ru(bpy)3] (n ) 4, 8, 16, 32, 64). Again, this could also be due to having less material deposited on the electrode surface.

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Conclusions We have shown that amino-terminated PAMAM dendrimers can be surface-modified with pyridyl, bipyridyl, and terpyridyl ligands in high yield via a peptide coupling reaction. Complexation of these dendrimer-ligands yielded dendrimers that contained pendant tris(bipyridyl)ruthenium(II) (dend-n-[Ru(bpy)3]) or bis(terpyridyl)ruthenium(II) (dend-n-[Ru(tpy)2]) complexes. These dendrimer complexes were electroactive and adsorbed onto platinum electrodes. Dendrimers containing terminal tris(bipyridyl)ruthenium(II) complexes exhibited room temperature luminescence, while those with terminal bis(terpyridyl)ruthenium(II) complexes exhibited luminescence at 77 K. EQCM measurements confirmed the deposition of the dendrimers onto electrode surface. The adsorbed films exhibited charge trapping peaks. The films

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exhibit morphological changes with potential that can be attributed to the deposition or dissolution of the dendrimer and/or to ejection or absorption of counterions and/or solvent into the film. With the current interest in obtaining new compounds for use in electron and energy transfer, particularly for surface-adsorbed films, these results show that dendrimers containing pendant polypyridyl metal complexes have the potential to be applied in such areas. Acknowledgment. This work was supported by the Office of Naval Research. Supporting Information Available: Analytical and spectroscopic data for the dendrimers. This material is available free of charge via the Internet at http://pubs.acs.org. LA980939M