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Langmuir 2006, 22, 10506-10509
Attenuating Electron-Transfer Rates via Dendrimer Encapsulation: The Case of Metal Tris(bipyridine) Core Dendrimers† Young-Rae Hong and Christopher B. Gorman* Department of Chemistry, North Carolina State UniVersity, Box 8204, Raleigh, North Carolina 27695-8204 ReceiVed March 31, 2006. In Final Form: May 3, 2006 Heterogeneous electron-transfer rates in metal tris(bipyridine) core dendrimers were measured using Osteryoung square-wave voltammetry. Rates decreased with generation, and this decrease could be correlated with the molecular weight increase. These results indicate that the coordination number around the redox center did not play any special role in sterically encapsulating the redox center.
Introduction Dendrimers have emerged as good molecular architectures for encapsulation.1-3 This behavior can include small molecule (host-guest) encapsulation, encapsulation and passivation of nanometer-scale particles, and electronic encapsulation of redox centers. To this latter end, several dendrimers with electroactive (e.g., easily oxidizable or reducible) cores have been synthesized and examined. In general, the rate of heterogeneous electron transfer (that between the electroactive unit and an electrode) decreases with increasing dendrimer generation.3-17 This decrease, however, can be modest or negligible18,19 in some cases, begging the question as to what features of the dendrimer result in efficient encapsulation and thus large decreases in the electron transfer rate with generation. It has been shown, for example, that backfolded linkages in the dendrimer repeat unit lead to more sterically encumbered core redox units.8 Another parameter †
Part of the Electrochemistry special issue. * Corresponding author. E-mail:
[email protected].
(1) Cameron, C. S.; Gorman, C. B. AdV. Funct. Mater. 2002, 12, 17-20. (2) Gorman, C. B.; Smith, J. C. Acc. Chem. Res. 2001, 34, 60-71. (3) Hecht, S.; Fre´chet, J. M. J. Angew. Chem., Int. Ed. 2001, 40, 74-91. (4) Dandliker, P. J.; Diederich, F.; Gisselbrecht, J.-P.; Louati, A.; Gross, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 2725-2728. (5) Dandliker, P. J.; Diederich, F.; Gross, M.; Knobler, C. B.; Louati, A.; Sanford, E. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1739-1741. (6) Dandliker, P. J.; Diederich, F.; Zingg, A.; Gisselbrecht, J. P.; Gross, M.; Louati, A.; Sanford, E. HelV. Chim. Acta 1997, 80, 1773-1801. (7) Vinogradov, S. A.; Lo, L. W.; Wilson, D. F. Chem.sEur. J. 1999, 5, 1338-1347. (8) Chasse, T. L.; Sachdeva, R.; Li, Q.; Li, Z.; Petrie, R. J.; Gorman, C. B. J. Am. Chem. Soc. 2003, 125, 8250-8254. (9) Gorman, C. B.; Smith, J. C.; Hager, M. W.; Parkhurst, B. L.; SierzputowskaGracz, H.; Haney, C. A. J. Am. Chem. Soc. 1999, 121, 9958-9966. (10) Pollak, K. W.; Leon, J. W.; Fre´chet, J. M. J.; Maskus, M.; Abrun˜a, H. D. Chem. Mater. 1998, 10, 30-38. (11) Ong, W.; Grindstaff, J.; Sobransingh, D.; Toba, R.; Quintela, J. M.; Peinador, C.; Kaifer, A. E. J. Am. Chem. Soc. 2005, 127, 3353-3361. (12) Ong, W.; Gomez-Kaifer, M.; Kaifer, A. E. Chem. Commun. 2004, 16771683. (13) Toba, R.; Quintela, J. M.; Peinador, C.; Roma´n, E.; Kaifer, A. E. Chem. Commun. 2002, 1768-1769. (14) Wang, Y.; Cardona, C. M.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 9756-9757. (15) Stone, D. L.; Smith, D. K.; McGrail, P. T. J. Am. Chem. Soc. 2002, 124, 856-864. (16) Vo¨gtle, F.; Plevoets, M.; Nieger, M.; Azzellini, G. C.; Credi, A.; De Cola, L.; De Marchis, V.; Venturi, M.; Balzani, V. J. Am. Chem. Soc. 1999, 121, 6290-6298. (17) Takada, K.; Dı´az, D. J.; Abrun˜a, H. D.; Cuadrado, I.; Casado, C.; Alonso, B.; Mora´n, M.; Losada, J. J. Am. Chem. Soc. 1997, 119, 10763-10773. (18) Ong, W.; Kaifer, A. E. J. Am. Chem. Soc. 2002, 124, 9358-9359. (19) Ceroni, P.; Vincinelli, V.; Maestri, M.; Balzani, V.; Mu¨ller, W. M.; Mu¨ller, U.; Hahn, U.; Osswald, F.; Vo¨gtle, F. New J. Chem. 2001, 25, 989-993.
that should affect encapsulation is the coordination number around the coreshow many arms emanate from the redox core unit. To date, all quantitative studies of electron-transfer rate attenuation in redox-active core dendrimers have focused on monovalent (e.g., ferrocenyl),15,20 divalent (e.g., viologen),18,19 and tetravalent (e.g., Fe4S42,8,9 and porphyrin3-7,10) core redox units. Here, we study electron-transfer rate attenuation in hexavalent, metal tris(bipyridine) core dendrimers. Using electrochemical methods, it is shown that increasing the number of dendritic arms around the redox unit does indeed attenuate electron transfer more rapidly with generation as one would expect. This attenuation, however, might not be best rationalized by considering the number of dendrons attached to the core.
Results and Discussion A series of metal tris(bipyridine) core dendrimers were synthesized as described previously.21,22 Figure 1 illustrates the molecular structures. The central metal was either iron or ruthenium, and molecules of generations 0 through 2 were specifically studied by electrochemistry. The analogous thirdgeneration dendrimers were also synthesized. However, as described below, the rate of heterogeneous electron transfer was too low to measure for these molecules. Cyclic voltammograms (CV) and Osteryoung square-wave voltammograms (OSWV) are shown for the ruthenium core (Figure 2) and iron core (Figure 3) dendrimers, respectively. The somewhat unusual solvent combination of 1:1 v/v acetonitrile/ dichloroethane was used. All of the molecules were soluble in this solvent system. Whereas it would have been more desirable to use 100% acetonitrile, a solvent with the higher dielectric constant, the second-generation dendrimers were insufficiently soluble in this solvent. Moreover, attempts to perform electrochemistry in polar aprotic solvents (e.g., DMF) failed because the electrochemical window of these solvents was not wide enough to allow observation of all of the redox processes. The addition of a chlorinated cosolvent to acetonitrile solved these problems. Dichloroethane was chosen instead of the more commonly employed dichloromethane because it has a similar dielectric constant and is much less volatile than dichloromethane. In our (20) Cardona, C. M.; Kaifer, A. E. J. Am. Chem. Soc. 1998, 120, 4023-4024. (21) Hong, Y.-R.; Gorman, C. B. J. Org. Chem. 2003, 68, 9019-9025. (22) Hong, Y.-R.; Gorman, C. B. In Metal-Containing and Metallosupramolecular Polymers and Materials; Schubert, U. S, Newkome, G. R., Manners, I., Ed.; ACS Symposium Series 928; American Chemical Society: Washington, DC, 2006; pp 205-214.
10.1021/la060867w CCC: $33.50 © 2006 American Chemical Society Published on Web 06/29/2006
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Figure 1. Dendrimers explored in this study. M ) Fe and Ru; counterions were PF6-.
Figure 2. Cyclic voltammograms (left) and Osteryoung squarewave voltammograms (right) for ruthenium core dendrimers. Ru(G0Bpy)32+ (upper), Ru(G1Bpy)32+ (middle), and Ru(G2Bpy)32+ (bottom). Scan rate 100 mV/s for CV and 60 mV/s for OSWV; argon-purged acetonitrile/dichloroethane (1:1 v/v) solution; 300 mM tetrabutylammonium hexafluoroborate supporting electrolyte; platinum as the working electrode. Counterions were PF6-.
hands, the evaporation of dichloromethane and subsequent increases in the electrolyte and dendrimer concentrations were sources of error in quantitative measurements of the electrontransfer kinetics. A fairly high (300 mM) concentration of supporting electrolyte was used to minimize solution resistance. At lower concentrations (e.g., 100-200 mM), the solution resistance was insufficiently high to permit confident fitting of the data to obtain heterogeneous electron-transfer rates. The redox kinetics for the ruthenium core dendrimers (Figure 2) were sufficiently fast to observe a well-defined voltammetric
Figure 3. Cyclic voltammograms (left) and Osteryoung squarewave voltammograms (right) for iron core dendrimers. Fe(G0Bpy)32+ (upper), Fe(G1Bpy)32+ (middle), and Fe(G2Bpy)32+ (bottom). Scan rate 100 mV/s for CV and 60 mV/s for OSWV; argon-purged acetonitrile/dichloroethane (1:1 v/v) solution; 300 mM tetrabutylammonium hexafluoroborate supporting electrolyte; platinum as the working electrode; counterions were PF6-. The inset in the plot for OSWV of Fe(G2Bpy)32+ shows an expansion of the current axis to illustrate the voltammetric response.
wave for all of these molecules. All CVs show quasi-reversible behavior in this solvent system. As expected, as the generation of the molecule increased, the peaks grew smaller and broader, indicative of a decreasing rate of heterogeneous electron transfer. In the case of the iron core dendrimers (Figure 3), the same trend was observed, but the second-generation dendrimer initially appeared to show no faradaic response. The use of Osteryoung
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Hong and Gorman
Table 1. Diffusion Coefficients, Do, Obtained from Pulsed-Field Gradient Spin-Echo NMR Spectroscopy (PFG-SE) and Chronoamperometry (CA), Heterogeneous Electron-Transfer Rate Constant (ko), Oxidation Potential (E1/2), and Transfer Coefficient (r) for Ruthenium and Iron Core Dendrimers CA 6
Ru(G0Bpy)32+ Ru(G1Bpy)32+ Ru(G2Bpy)32+ Fe(G0Bpy)32+ Fe(G1Bpy)32+ Fe(G2Bpy)32+
PFG-SE 2
6
CV 2
3
OSWV
Do (× 10 cm /s)
Do (× 10 cm /s)
ko (× 10 cm/s)
E1/2 (mV)
∆E (mV)
8.35 3.64 2.10 8.37 3.17 NAa
7.60 3.55 2.56 7.79 3.57 2.43
10.3 3.24 1.74 10.0 3.51 NAa
865 887 908 639 683 NAa
90 115 127 90 109 NAa
c
ko (× 10 cm/s)
E1/2 (mV)
R
18.8 4.23 3.03 15.6 4.95 2.26b
866 890 910 640 691 704b
0.44 0.46 0.64 0.74 0.46 0.49b
3
a Irreversible electrochemistry prevented this value from being determined. b Calculated from a fit of the OSWV using the diffusion coefficient measured by PFG-SE. c At a 100 mV/s scan rate.
molecule increases. In the case of the molecule Fe(G2Bpy)32+, pulsed field gradient spin-echo (PFG-SE) NMR was the only method available to measure the diffusion coefficient of this molecule. PFG-SE measurements were obtained in the absence of the supporting electrolyte, whereas voltammetry was performed in the presence of supporting electrolyte. Thus, there is some unknown error in this determination. To determine if we could improve the detection limit in the case of slow electron-transfer kinetics for Fe(G2Bpy)32+ and perhaps extend our investigation to Fe(G3Bpy)32+, we explored the use of thin-layer voltammetry (TLV).23,24 Previously, this technique was shown to be useful in observing electron transfer in dendrimer-encapsulated bis(phenanthroline) copper(I) core units.25 We constructed a thin-layer cell and measured voltammograms for the six molecules. The results are shown in Figure 4. In the case of these molecules, an enhanced faradaic response was indeed observed. However, the magnitude of the background current was high. Attempts to subtract this background current and obtain quantitative rate measurements were not successful. Calculated apparent rate constants were much lower than those obtained for the molecules in solution. We concluded that, under these conditions, TLV was not a good method to obtain rate data.
Conclusions Figure 4. Thin-layer cyclic voltammograms for ruthenium (left) and iron (right) core dendrimers. M(G0Bpy)32+ (upper), M(G1Bpy)32+ (middle), and M(G2Bpy)32+ (bottom). Scan rate 1 mV/s, argonpurged acetonitrile/dichloroethane (1:1 v/v) solution, 300 mM tetrabutylammonium hexafluoroborate supporting electrolyte. Counterions were PF6-.
square-wave voltammetry (inset of plot for Fe(G2Bpy)32+) afforded a measurable response. These data highlight the utility of square-wave methods to examine slow, heterogeneous electrontransfer kinetics. We have previously described the use of voltammetry in conjunction with chronoamperometry to obtain heterogeneous electron-transfer rates for dendrimers.8,9 A similar analysis was performed on these electrochemical data, and the results are shown in Table 1. The variability of OSWV measurements is lower than that of CV, so those values are preferred. In each case (iron core and ruthenium core dendrimers), the diffusion coefficient decreases with generation, indicative of increasing molecular size. The heterogeneous electron-transfer rate (ko) decreases with generation, indicating a larger effective distance of electron transfer. The redox potential (E1/2) increases with generation, indicating that the redox center is becoming more difficult to oxidize. This result is consistent with the redox unit becoming surrounded by an effectively less polar medium (the dendrimer compared to the solvent) as the generation of the
Do six dendrons around a redox core serve to encapsulate it more than an analogous, four-armed molecule? If one compares the rate constants reported here with those obtained previously on Fe4S4(GnS)42- cluster core dendrimers,9 then one does find slower electron transfer for molecules of the same generation. However, a six-armed dendrimer also has the virtue of collecting more mass around the core than an analogous four-armed dendrimer, so a comparison of electron-transfer rates as a function of generation is arguably not fair. Thus, these molecules are compared by molecular weight. Because the cores are different, the molecular weight of the zeroth-generation molecule (e.g., M(G0Bpy)32+) is subtracted from the total molecular weight of the molecule. This value is intended to represent the molecular weight of the encapsulating dendrons. In Figure 5, the natural logarithm of this rate constant versus this weight is graphed. The molecules (except for G0) show a roughly linear decrease in rate versus molecular weight of the dendrons. Although the metal tris(bipyridine) core dendrimers have slower rates versus generation, when viewed in this way, all of the molecules follow the same trend. Thus, the six-arm geometry does not encapsulate a cluster better than a four-arm geometry. It merely has a higher molecular weight for a given generation. (23) Deangelis, T. P.; Heineman, W. R. J. Chem. Educ. 1976, 53, 594-597. (24) Murray, R. W.; Heineman, W. R.; Odom, G. W. Anal. Chem. 1967, 39, 1666-1668. (25) Rio, Y.; Accorsi, G.; Armaroli, N.; Felder, D.; Levillain, E.; Nierengarten, J. F. Chem. Commun. 2002, 2830-2831.
Attenuating Electron-Transfer Rates
Figure 5. Graph of the heterogeneous rate constant versus molecular weight of the encapsulating dendrons for the molecules studied here and for Fe4S4(GnS)42- reported previously.9
Experimental Details All chemicals were purchased from Aldrich or Acros. Acetonitrile was dried by distillation over CaH2. Anhydrous dichloroethane in a sure seal bottle was purchased from Acros and used without further drying. Tetrabuthylammonium hexafluorophosphate (TBAPF6) was recrystallized three times from ethanol and dried in a vacuum oven for 24 h. All dendrimers were synthesized as described previously.21,22 Pt-disk working electrodes were polished with a 0.25 µm diamond suspension (from BAS) and further cleaned electrochemically in 0.2 M H2SO4 by cycling the potential between 1200 and -200 mV (vs Ag/AgCl).
Langmuir, Vol. 22, No. 25, 2006 10509 A thin-layer electrode was fabricated as follows. A Pt-coated glass slide was prepared via electron beam evaporation of the metal and cleaned in freshly prepared piranha solution. Scotch tape (60 µm thick) was applied as a spacer between the Pt and glass slides. Scotch tape also blocks solution contact with the unused platinum area, giving a 0.2 cm2 effective electrode area on the platinum slide. Before bonding Pt and the glass slide with Teflon tape, the Scotchtaped Pt slide was further electrochemically cleaned as described above. PFG-SE NMR studies were carried out in a mixture of CD3CN/ C2D4Cl2 (1:1 v/v, both from Cambridge Isotope Laboratories). The gradient strength (g) was calibrated using literature values for HOD diffusion in D2O.26 Throughout the experiments, each sample solution volume (0.4 mL) was fixed to give the same gradient field throughout the samples. The duration of the gradient pulse (δ) was 4 ms. More details of the electrochemical and PFG-SE NMR experiments have been published elsewhere.9
Acknowledgment. We thank the NSF (grant CHE- 0315311) for financial support of this work, Dr. Hanna Gracz for helpful discussions about PFG-SE NMR, Professor Ed. Bowden for advice on performing the thin-layer voltammetry experiments, and Professor Gregory Parsons’ group for preparation of the platinum-covered glass slide. LA060867W (26) Mills, R. J. Phys. Chem. 1973, 77, 685-688.
