pubs.acs.org/Langmuir © 2009 American Chemical Society
Novel Polyaromatic-Terminated Transition Metal Complexes for the Functionalization of Carbon Surfaces Hillary L. Smith, Rachel L. Usala, Eden W. McQueen, and Jonas I. Goldsmith* Department of Chemistry, Bryn Mawr College, 101 North Merion Avenue, Bryn Mawr, Pennsylvania 19010 Received August 21, 2009. Revised Manuscript Received October 22, 2009 In order to investigate the process of noncovalent adsorption on glassy carbon surfaces, two terpyridine ligands 4-pyren-1-yl-N-[5-([2,20 ;60 ,200 ]terpyridin-40 -yloxy)-pentyl]-butyramide (tpy∼py) and N-[5-([2,20 ;60 ,200 ]terpyridin-40 -yloxy)pentyl]-2-naphthamide (tpy∼nap) as well as the homoleptic cobalt(II) complexes of these ligands (Co(tpy∼py)2(PF6)2 and Co(tpy∼nap)2(PF6)2) were synthesized. Electrochemical measurements in solution were used to characterize the transport behavior of these complexes and to verify that the polyaromatic portion of each ligand did not dramatically influence the electronic properties of the transition metal complex. The adsorption of the cobalt complexes above on glassy carbon electrode surfaces was then examined using cyclic voltammetry and was found to be well described by Langmuir or Frumkin isotherms. The free energy of adsorption for Co(tpy∼py)2(PF6)2 was considerably larger than that for Co(tpy∼nap)2(PF6)2: -41 versus -30 kJ/mol.
Introduction Deliberate molecular modification of surfaces has long been a tool that researchers have used to add functionality to a wide variety of materials and to examine the fundamental nature of interfacial processes.1-13 When the surface under consideration is metallic or semiconducting, electrochemical techniques have often been employed in the investigation of the functionalization process, especially when the moiety used to modify the surface is, itself, redox active.1,2,5,11,12,14-18 Until recently, most surface *To whom correspondence should be addressed. E-mail: jigoldsmit@ brynmawr.edu. (1) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, pp 191-368 and references therein. (2) Wrighton, M. S.; Austin, R. G; Bocarlsy, A. B.; Bolts, J. M.; Haas, O.; Legg, K. D.; Nadjo, L.; Palazzotto, M. C. J. Electroanal. Chem. 1978, 87(3), 429–433. (3) Tao, F.; Bernasek, S. L.; Xu, G.-Q. Chem. Rev. 2009, 109(9), 3991–4024 and references therein. (4) Tokuhisa, H.; Zhao, M. Q.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120(18), 4492–4501. (5) Abru~na, H. D.; Denisevich, P.; Umaa, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103(1), 1–5. (6) Guadalupe, A. R.; Abru~na, H. D. Anal. Chem. 1985, 57(1), 142–149. (7) Fan, F. F.; Yang, J.; Cai, L.; Price, D. W.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y.; Rawlett, A. M.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2002, 124(19), 5550–5560. (8) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312–1319. (9) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Acc. Chem. Res. 2000, 33(9), 617– 624. (10) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120(12), 2721–2732. (11) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J. Y.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121(5), 1059–1064. (12) Chidsey, C. E. D. Science 1991, 251, 919–922. (13) Kamat, P. V. J. Phys. Chem. C 2007, 111(7), 2834–2860 and references therein. (14) Wrighton, R. S.; Palazzotto, M. C.; Bocarsly, A. B.; Bolts, J. M.; Fischer, A. B.; Nadjo, L. J. Am. Chem. Soc. 1978, 100(23), 7264–7271. (15) Smalley, J. F.; Finklea, H. O.; Chidsey, C. E. D.; Linford, M. R.; Creager, S. E.; Ferraris, J. P.; Chalfant, K.; Zawodzinsk, T.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2003, 125(7), 2004–2013. (16) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6(3), 682–691. (17) Maskus, M.; Abru~na, H. D. Langmuir 1996, 12(18), 4455–4462. (18) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105(4), 1103–1169 and references therein.
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modification work was concentrated on adding functionality to the surfaces of the materials (i.e., silicon, Al2O3, gold, and other interconnect materials) most widely used in the fabrication of integrated circuits. However, with the surge in interest in, as well as the enormous promise of, carbon nanotubes for use in advanced electronics,19 researchers have turned their attention to developing methodologies for modifying graphene surfaces.20,21 A wide range of strategies have been used to accomplish this, from functionalizing defects with diazonium salts22,23 and by amide bond formation chemistry22,24-26 to exploiting a variety of noncovalent and nonspecific interactions between carbon nanotubes and the species functionalizing them.27-29 While the specificity of covalent functionalization is appealing, such modifications necessarily change the bonding and electronic structure of an aromatic conjugated substrate. However, a significant degree of specificity can be achieved through the noncovalent π-π stacking interaction between a sp2-hybridized carbon surface and a molecule containing the appropriate planar aromatic system, and such methodologies have been shown to lead to successful functionalization of (19) (a) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49–52. (b) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787–92. (c) Avouris, P.; Appenzeller, J.; Martel, R.; Wind, S. J. Proc. IEEE 2003, 91(11), 1772– 1784 and references therein. (d) Avouris, P.; Chen, Z.; Perebeinos, V. Nat. Nanotechnol. 2007, 2, 605–615 and references therein. (20) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106(3), 1105–1136 and references therein. (21) Delgado, J. L.; Herranz, M.; Martin, N. J. Mater. Chem. 2008, 18, 1417– 1426 and references therein. (22) Dyke, C. A.; Tour, J. M. J. Phys. Chem. A 2004, 108(51), 11151–11159 and references therein. (23) Usrey, M. L.; Lippmann, E. S.; Strano, M. S. J. Am. Chem. Soc. 2005, 127 (46), 16129–16135. (24) Prato, M.; Kostarelos, K.; Bianco, A. Acc. Chem. Res. 2008, 41(1), 60–68. (25) Guo, X.; Small, J. P.; Klare, J. E.; Wang, Y.; Purewal, M. S.; Tam, I. W.; Hong, B. H.; Caldwell, R.; Huang, L.; O’Brien, S.; Yan, J.; Breslow, R.; Wind, S. J.; Hone, J.; Kim, P.; Nuckolls, C. Science 2006, 311, 356–359. (26) Delgado, J. L.; de la Cruz, P.; Urbina, A.; Navarrete, J. T. L.; Casado, J.; Langa, F. Carbon 2007, 45(11), 2250–2252. (27) Star, A.; Liu, Y.; Grant, K.; Ridvan, L.; Stoddart, J. F.; Steuerman, D. W.; Diehl, M. R.; Boukai, A.; Heath, J. R. Macromolecules 2003, 36(3), 553–560. (28) (a) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. J. Am. Chem. Soc. 2001, 123 (16), 3838–3839. (b) Zhao, Y.-L.; Stoddart, J. F. Acc. Chem. Res. 2009, 42(8), 1161– 1171 and references therein. (29) Staii, C.; Chen, M.; Gelperin, A.; Johnson, A. T. Nano Lett. 2005, 5(9), 1774–1778.
Published on Web 11/18/2009
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Figure 1. The molecules synthesized herein: (a) 4-Pyren-1-yl-N-[5-([2,20 ;60 ,200 ]terpyridin-40 -yloxy)-pentyl]-butyramide (tpy∼py); (b) N-[5([2,20 ;60 ,200 ]terpyridin-40 -yloxy)-pentyl]-2-naphthamide (tpy∼nap); (c) [Co(tpy∼py)2]2þ (synthesized as a hexafluorophosphate salt); (d) [Co(tpy∼nap)2]2þ (synthesized as a hexafluorophosphate salt).
carbon nanotubes.28 While systems such as these have been investigated for over 30 years,30 detailed thermodynamic information about the strength of these interactions is not readily available. For many applications, including the sensing of biological and chemical analytes, light harvesting, and graphene-based nanoelectronics, the ability to tune device properties via surface functionalization depends not only on the nature of the species doing the functionalizing but also on its coverage. The sensitivity of sensors could be modulated by changing the density of “receptors” on a surface; the efficiency of solar energy conversion may be impeded if the distance between photosensitizers on a surface leads to significant self-quenching; the performance characteristics of electronic devices can be changed by varying the carrier density, which may be achievable through surface functionalization of nanostructures. In all these examples, control over the coverage of surface-modifying species can lead to tunable device properties but, in order to achieve this, a deeper exploration of the thermodynamic parameters that govern these systems is necessary. The understanding gained from an investigation of the noncovalent π-π stacking interactions described above would facilitate the controllable functionalization of graphene surfaces without adding defects or traps on the carbon surface and would be critical in developing methodologies for tuning the properties of nanoscale devices based on carbon nanotubes and other graphene materials. In this work, we have designed, synthesized, and investigated molecules for the controllable noncovalent functionalization of graphene surfaces. A bifunctional ligand with a terpyridine moiety at one end and a polyaromatic functionality (pyrene or naphthalene) at the other end has been synthesized in order to effect the subsequent synthesis of terpyridyl transition metal complexes that can functionalize graphene surfaces via π-π stacking interactions. Electrochemical techniques have been utilized to examine the behavior of these transition metal complexes in solution and to investigate the adsorption process of these (30) Koval, C. A.; Anson, F. C. Anal. Chem. 1978, 50(2), 223–229.
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molecules onto a glassy carbon electrode. Though glassy carbon does not contain the large and pristine graphene surface of a carbon nanotube, its smaller domains of graphene-like areas make it a convenient proxy for the surface of a carbon nanotube.
Materials and Methods Synthesis. Unless otherwise noted, all materials were purchased from Aldrich Chemical Co. and used as received. All ligand synthesis reactions were carried out under argon gas and used anhydrous solvents (Aldrich SureSeal). The compound 5-aminopentyl 40 -(2,2’:60 ,200 -terpyridinyl) ether (tpy∼NH2) was synthesized as described in the literature.31 The compounds synthesized can be seen in Figure 1. 4-Pyren-1-yl-N-[5-([2,20 ;60 ,200 ]terpyridin-40 -yloxy)-pentyl]butyramide (tpy∼py). In a septum-capped 25 mL round-bot-
tomed flask, 100 mg of tpy∼NH2 (0.3 mmol) was stirred in 5 mL of CH2Cl2 until it dissolved. Then 120 mg of 1-pyrenebutyric acid N-hydroxysuccinimide ester (0.31 mmol) was dissolved in 5 mL of CH2Cl2 and added to the reaction flask via syringe. The contents were stirred overnight at room temperature, and the precipitate that had formed was filtered off. The organic phase was extracted with 1 M HCl (225 mL) which was subsequently basified by the addition of NaOH pellets. This basic aqueous phase was extracted with CH2Cl2 (3 75 mL), which was dried over anhydrous MgSO4 and evaporated to dryness to yield 145 mg (79% yield) of the desired product as an off-white powder. 1H NMR (CDCl3) δ 8.65(d, 2H, tpy), 8.57 (d, 2H, tpy), 8.28 (d, 1H, py), 8.16-8.04 (m, 4H, py), 8.01 (s, 2H, tpy), 7.98-7.93 (m, 3H, py), 7.85 (d, 1H, py), 7.82 (td, 2H, tpy), 7.30 (td, 2H, tpy), 4.17 (t, 2H, tpy-OCH2), 3.39 (t, 2H, py-CH2), 3.23 (m, 2H, tpy-O(CH2)4CH2), 2.35-2.2 (m, 4H, py-CH2CH2CH2), 1.81 (m, 2H, tpy-O-CH2CH2), 1.5-1.4 (br, 4H, tpy-O-CH2CH2CH2CH2). 13C NMR (CDCl3) δ 172.6 (CdO), 167.3 (tpy 40 ), 156.9 (tpy 20 , 60 ), 156.0 (tpy 2, 200 ), 148.9 (tpy 6, 600 ), 136.9 (tpy 30 , 50 ), 135.9 (py), 131.4 (py), 130.9 (py), 129.9 (py), 128.8 (py), 127.5-127.4 (3C, py), 126.7 (py), 125.8 (py), 125.1-124.8 (5C, py), 123.9 (tpy 3, 300 ), 123.4 (py), 121.4 (tpy 4, 400 ), 107.4 (tpy 5, 500 ), 67.9 (tpy-O-CH2), 39.4 (31) Newkome, G. R.; He, E. J. Mater. Chem. 1997, 7(7), 1237–1244.
DOI: 10.1021/la9031249
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Article (tpy-O-(CH2)4-CH2-NH), 36.1 (py-(CH2)2-CH2), 32.8 (pyCH2), 29.3 (tpy-O-(CH2)3-CH2-CH2-NH), 28.6 (tpy-OCH2-CH2), 27.5 (py-CH2-CH2), 23.4 (tpy-O-CH2-CH2CH2). FT-IR (KBr): 3422, 3306, 3043, 2938, 2866, 1652, 1600, 1583, 1566, 1469, 1443, 1407, 1357, 1203, 1026, 846, 795, 697 cm-1. C40H35N4O2 3 0.5H2O requires 78.41% C, 5.92% H, 9.14% N; found 78.00% C, 5.68% H, 9.36% N. ESI-MS in CH3CN gave a monocharged peak at m/z 604.2 (calcd 604.3).
N-[5-([2,20 ;60 ,200 ]Terpyridin-40 -yloxy)-pentyl]-2-naphthamide (tpy∼nap). A total of 100 mg (0.30 mmol) of tpy∼NH2 and 75 mg
of 2-naphthoyl chloride (0.39 mmol) were placed in a 50 mL round-bottomed flask equipped with a reflux condenser and drying tube. CH2Cl2 (20 mL) and triethylamine (2 mL) were added, and the mixture was brought to reflux and stirred for 1 h. Upon cooling, the reaction mixture was extracted with 1 M HCl (2 25 mL) which was subsequently basified by the addition of NaOH pellets. This basic aqueous phase was extracted with CH2Cl2 (3 50 mL), which was dried over anhydrous MgSO4 and evaporated to dryness to yield 108 mg (74% yield) of the desired product as a white powder. 1H NMR (CDCl3) δ 8.71 (d, 2H, tpy), 8.64 (d, 2H, tpy), 8.31 (s, 1H, nap), 8.06 (s, 2H, tpy), 7.83-7.95 (m, 2H tpy, 4H nap), 7.55 (m, 2H, nap), 7.35 (td, 2H, tpy), 6.41 (br s, 1H, amide), 4.31 (t, 2H, tpy-O-CH2), 3.60 (t, 2H, nap-CONH-CH2), 1.97 (m, 2H, tpy-O-CH2-CH2), 1.80 (m, 2H, nap-CONH-CH2-CH2), 1.69 (m, 2H, tpy-O-CH2CH2-CH2). 13C NMR (CDCl3) δ 167.7 (CdO), 167.3 (tpy 40 ), 157.1 (tpy 20 , 60 ), 156.1 (tpy 2, 200 ), 148.9 (tpy 6, 600 ), 136.9 (tpy 30 , 50 ), 134.7 (nap), 132.6 (nap), 132.0 (nap), 128.9 (nap), 128.5 (nap), 127.7 (nap), 127.5 (nap), 127.3 (nap), 126.7 (nap), 123.9 (tpy 3, 300 ), 123.6 (nap), 121.4 (tpy 4, 400 ), 107.4 (tpy, 5, 500 ), 68.0 (tpy-O-CH2), 40.1 29.4 (tpy-O-(CH2)3-CH2(tpy-O-(CH2)4-CH2-NH), CH2-NH), 28.7 (tpy-O-CH2-CH2), 23.6 (tpy-O-CH2-CH2CH2). FT-IR (KBr): 3328, 3056, 2940, 2862, 1637, 1626, 1584, 1561, 1467, 1409, 1359, 1204, 1043, 824, 792, 738, 622 cm-1. C31H28N4O2 3 0.5H2O requires 74.83% C, 5.87% H, 11.26% N; found 74.35% C, 5.58% H, 11.35% N. ESI-MS in CH3CN gave a monocharged peak at m/z 488.2 (calcd 488.2). Co(tpy∼py)2(PF6)2. A total of 58 mg of tpy∼py (0.096 mmol) and 10 mg of CoCl2 3 6H2O (0.043 mmol) were added to a 50 mL round-bottomed flask containing 25 mL of absolute ethanol. The contents were brought to reflux and stirred for 2 h. The yelloworange solution was cooled and added to 50 mL of DI water. This resulting aqueous mixture was washed with CH2Cl2 (250 mL) to remove any excess ligand, and then 1 g of NH4PF6 dissolved in 10 mL of DI water was added to the solution to effect precipitation. The product was collected by centrifugation, washed with copious amounts of DI water, dried under vacuum, and then recrystallized from CH3CN by diethyl ether vapor diffusion to yield 57 mg (84% yield) of a brown powder. UV-vis: λmax = 242 (ε = 1.2 105), λmax = 265 (ε = 6.0 104), λmax = 275 (ε = 8.5 104), λmax =312 (ε=3.1104), λmax =326 (ε=4.2104), λmax = 342 (ε=5.3104), λmax =375 (ε=3.2103), λmax =451 (ε=1.1 103), λmax = 501 nm (ε = 8.1 102 M-1 cm-1). C80H72CoF12N8O4P2 3 4H2O requires 58.93% C, 4.94% H, 6.87% N; found 58.99% C, 4.79% H, 6.68% N. ESI-MS in CH3CN gave a monocharged peak at m/z 1412.3 for [Co(tpy∼py)2(PF6)]þ1 (calcd 1412.5) and a doubly charged peak at m/z 633.8 for [Co(tpy∼py)2]2þ (calcd 633.8). Co(tpy∼nap)2(PF6)2. This complex was synthesized as described above from 53 mg of tpy∼nap (0.108 mmol) and 12 mg of CoCl2 3 6H2O (0.050 mmol); the synthesis yielded 51 mg (77% yield) of a brown powder. UV-vis: λmax = 229 (ε = 1.4 105), λmax=272 (ε=5.2104), λmax=306 (ε=2.5104), λmax=340 (ε= 8.2103), λmax =451 (ε=7.6102), λmax =505 nm (ε=5.1102 M-1 cm-1). C62H56CoF12N8O4P2 3 4H2O requires 53.26% C, 4.61% H, 8.01% N; found 52.97% C, 4.23% H, 7.85% N. ESIMS in CH3CN gave a monocharged peak at m/z 1180.2 for [Co(tpy∼nap)2(PF6)]þ1 (calcd 1180.3) and a doubly charged peak at m/z 517.8 for [Co(tpy∼nap)2]2þ (calcd 517.7). 3344 DOI: 10.1021/la9031249
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Electrochemistry. For electrochemical experiments, the reagents employed were 18 MΩ 3 cm water, acetone and acetonitrile (both 99.9% pesticide residue grade, from Burdick & Jackson), and tetra-n-butylammonium hexafluorophosphate (TBAH) recrystallized from ethanol. Electrochemical measurements were carried out either using a Princeton Applied Research (Oak Ridge, TN) model 263A computer controlled potentiostat or using a Pine Instruments (Grove City, PA) AFCBP1 computer controlled bipotentiostat and AFMSRCE electrode rotator. Measurements were carried out in homemade one- or threecompartment cells with 0.1 M TBAH/CH3CN as the supporting electrolyte. A coiled platinum wire was used as a counter electrode, the reference was either a home-built Ag/AgCl electrode or a silver wire. For the greatest degree of comparability, all potentials were ultimately referenced to the Fc/Fcþ redox couple; ferrocene was added as an internal standard to each sample after all other electrochemical measurements were completed. It should be noted that in our laboratory the potential of the Fc/Fcþ redox couple in 0.1 M TBAH/CH3CN has been reproducibly measured to be þ0.412 V versus Ag/AgCl. The working electrodes (3.0 mm diameter glassy carbon, 1.6 mm diameter platinum) were purchased from Bioanalytical Systems (West Lafayette, IN) and (5.0 mm diameter platinum rotating disk electrode) from Pine Instruments. Prior to use, working electrodes were mechanically polished with 1 μm diamond paste (for the Pt electrode) or 0.02 μm alumina slurry (for the glassy carbon electrode). All polishing materials were purchased from Buehler (Lake Bluff, IL). After polishing, the electrodes were rinsed with water (the glassy carbon electrode was also sonicated in water for 2 min), rinsed with acetone, and dried under a stream of dry nitrogen. All solutions used for electrochemistry were bubbled with dry nitrogen prior to use.
Results and Discussion Synthesis and Characterzation. In Figure 1, the two ligands, as well as the Co(II) complexes synthesized from those ligands, can be seen. The general technique involved in the ligand synthesis was to use amide bonds, chosen for their strength and redox stability, to link together the two subunits of the ligand: the polypyridyl portion which can bind a transition metal ion and the polyaromatic portion for attaching to graphene surfaces. These reactions produced the desired products in both high yield and high purity. The synthesis and purification of the Co(II) complexes proceeded according to standard inorganic techniques. The identity of the products was confirmed by ESI-MS; as is often the case, both the singly charged molecular ion due to the loss of one counterion and the doubly charged molecular ion due to the loss of both counterions were observed. No crystal structures were obtained, and it is suspected that the flexible aliphatic portion of the ligands led to the lack of significant crystal formation. Electrochemical Measurements in Solution. The initial electrochemical measurements made on these complexes are the cyclic voltammograms (CVs) that can be seen in Figure 2; the CV of a solution of [Co(tpy)2]2þ is included for the purpose of comparison. In all three compounds, both the Co(II/III) and the Co(II/I) redox processes can be clearly observed and all of these redox couples appear to be reversible, with ΔEpeak between 60 and 80 mV.32,33 As can be seen in Figure 2, the observed redox processes in [Co(tpy∼py)2]2þ and [Co(tpy∼nap)2]2þ are quite similar to those seen in the parent terpyridine complex. In both [Co(tpy∼py)2]2þ and in [Co(tpy∼nap)2]2þ, the Co(II/III) redox (32) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; J. Wiley & Sons: New York, 2001; Chapter 6. (33) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36(4), 706–723.
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Figure 2. Cyclic voltammograms of [Co(tpy)2]2þ, [Co(tpy∼py)2]2þ, and [Co(tpy∼nap)2]2þ in 0.1 M TBAH/CH3CN at a Pt working electrode and at scan rate of 100 mV/s. The solutions are of varying concentrations, and for the purpose of presentation the current has been scaled to make each voltammogram of approximately equal size.
process occurs at a potential approximately 70 mV more negative than that in [Co(tpy)2]2þ and the Co(II/I) redox process is observed at a potential approximately 160 mV more negative than that in [Co(tpy)2]2þ. This shift in redox potentials indicates that the two new complexes are more easily oxidized (and more difficult to reduce) than the unsubstituted [Co(tpy)2]2þ complex. This is likely the result of the oxygen substituent in the 40 position of the terpyridyl moiety, the additional electron density of which would make oxidation more favorable and reduction less so. One important property of any molecule is its diffusion coefficient, which measures its ability to move through a quiescent solution. This quantity is especially important to consider in electrochemical systems, as diffusion is one of the significant modes of mass transport that brings electroactive material into contact with the surface of an electrode, allowing electron transfer events to take place. There are various methods32,34,35 for measuring the diffusion coefficients of redox-active molecules, and voltammetry at a rotating disk electrode was employed to determine the diffusion coefficients of the complexes that were synthesized. Additionally, as a validation of experimental technique, we used the same technique to determine the diffusion coefficient of [Co(tpy)2]2þ. For voltammetry at a rotating disk electrode, the limiting current of a redox wave is given by the Levich equation:34,36,37 I l ¼ 0:62nFAD0 2=3 ω1=2 ν -1=6 C 0
ð1Þ
where n is the number of electrons transferred in a redox event, A is the surface area of the electrode, Do is the diffusion coefficient, Co* is the bulk concentration of the species under investigation, F is the Faraday constant (96 485 C/mol of electrons), ω is the rotation rate of the electrode in rad/s, and ν is the kinematic viscosity of the solution (taken to be 0.005 cm2/s for 0.1 M TBAH/ CH3CN). From a series of voltammograms obtained at various electrode rotation rates, the diffusion coefficient can be calculated using eq 1. All analyses were conducted using the 5.0 mm diameter (34) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; J. Wiley & Sons: New York, 2001; Chapter 9. (35) Goldsmith, J. I.; Takada, K.; Abru~na, H. D. J. Phys. Chem. B 2002, 106(34), 8504–8513. (36) Albery, W. J.; Hitchman, M. L. Ring Disc Electrodes; Clarendon Press: Oxford, 1971. (37) Levich, V. G. Physicochemical Hydrodynamics; Prentice Hall: Englewood Cliffs, NJ, 1962.
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platinum RDE working electrode, the electrochemically active surface of which was determined by obtaining a cyclic voltammogram in 0.1 M H2SO4 and using the commonly accepted factor for hydrogen desorption of 210 μC of charge passed/cm2 of polycrystalline platinum.38 The electrochemical experiments from which the determination of diffusion coefficients were carried out can be seen in Figure 3. In panels A1-A3 are the voltammograms of each species under investigation in 0.1 M TBAH/CH3CN at the rotating platinum disk electrode mentioned above. For all voltammograms, a scan rate of 50 mV/s and rotation rates of 300, 500, 800, 1200, 1600, and 2000 rpm were employed. In panels B1-B3 are the fits of limiting current versus square root of rotation rate (from eq 1) that were used to calculate the diffusion coefficient. As can be seen from the quality of the linear fits, the diffusion coefficients extracted from these data should be quite reliable. The calculated values of the diffusion coefficients can be found in each of the B panels. As Einstein observed, the magnitude of the diffusion coefficient scales with the diameter of a spherical molecule.39 The diffusion coefficients shown in Figure 3 indicate that [Co(tpy∼nap)2]2þ and [Co(tpy∼py)2]2þ have very similar hydrodynamic radii and are approximately twice as large as [Co(tpy)2]2þ. If one considers [Co(tpy)2]2þ to be roughly spherical, it has a diameter of ca. 1 nm.40 This suggests that in solution [Co(tpy∼nap)2]2þ and [Co(tpy∼py)2]2þ are each approximately 2 nm in diameter. If fully extended, the aliphatic linker and the polyaromatic portion of these molecules would add considerably more than 1 nm to the diameter of each molecule;40 hence, it appears that both [Co(tpy∼nap)2]2þ and [Co(tpy∼py)2]2þ adopt a relatively tightly balled conformation in solution. Electrochemical Measurements of Adsorption. These compounds were synthesized for the purpose of functionalizing graphene surfaces, and, to examine this functionalization process, electrochemical techniques were utilized to observe and characterize the adsorption behavior of these complexes on the surface of a glassy carbon electrode. Glassy carbon was chosen because, while somewhat disordered, it contains significant regions with mesoscale order and because of its ease of use compared to surfaces such as highly oriented pyrolytic graphite and carbon nanotubes. The initial examination of the adsorption process of these complexes involved the overnight soaking of a polished glassy carbon working electrode in a concentrated (ca. 0.5 mM) solution of the complex in acetonitrile. The electrode was subsequently removed, rinsed with acetonitrile and with acetone, and then placed into pure 0.1 M TBAH/CH3CN. For ease of experimentation and to avoid the possibility of reductive desorption, the potential of the working electrode was cycled between -0.45 and þ0.15 V versus Fc/Fcþ to observe only the Co (II/III) redox process. Shown below in Figure 4 are a series of voltammograms, taken at various potential sweep rates, for the working electrode that was soaked in [Co(tpy∼py)2]2þ. As there is no redox-active material present in solution, any electrochemical response seen in Figure 4 must come from material adsorbed to the surface of the electrode. There was no observable electrochemical response from a platinum working electrode that had been soaked overnight in [Co(tpy∼py)2]2þ, rinsed with acetonitrile and acetone, and placed in pure 0.1 M TBAH/CH3CN. There was also no observable electrochemical response from a glassy carbon electrode that had been soaked (38) Trasatti, S.; Petrii, O. A. Pure Appl. Chem. 1991, 63(5), 711–734. (39) Einstein, A. Investigations on the Theory of the Brownian Movement; F€urth, R., Ed.; Methuen & Co. Ltd.: London, 1926; Section II. (40) Estimations of molecular dimensions were carried out using Chem3DPro 11.0 from CambridgeSoft.
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Figure 3. (A1-A3) Voltammograms at a rotating platinum disk electrode of a 0.327 mM solution of [Co(tpy)2]2þ, a 0.074 mM solution of
[Co(tpy∼py)2]2þ, and a 0.10 mM solution of [Co(tpy∼nap)2]2þ, respectively, all in 0.1 M TBAH/CH3CN. (B1-B3) Fits to the Levich equation (eq 1) of the limiting current versus the square root of rotation rate used to extract diffusion coefficients for the data presented in A1-A3.
overnight in [Co(tpy)2]2þ, rinsed with acetonitrile and acetone, and placed in pure 0.1 M TBAH/CH3CN. As expected for an electroactive absorbate, the separation between the peak of the anodic wave and the peak of the cathodic wave is less than 58 mV.41 In these experiments, ΔEp ranges from 25 to 40 mV, increasing with increasing potential sweep rate, indicating that a kinetic (41) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; J. Wiley & Sons: New York, 2001; Chapter 14.
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barrier to electron transfer does exist.41,42 It is well-known that, for electroactive material adsorbed to a working electrode, the peak current observed for a redox process increases linearly with the rate at which the potential is being swept.41 This is in stark comparison to the peak current due to material free to diffuse in solution, which increases as the square root of the potential sweep (42) Laviron, E. J. Electroanal. Chem. 1979, 101(1), 19–28.
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Figure 4. Cyclic voltammograms of [Co(tpy∼py)2]2þ adsorbed to
Figure 5. Cyclic voltammograms of [Co(tpy∼nap)2]2þ adsorbed
the surface of a glassy carbon electrode, placed in 0.1 M TBAH/ CH3CN, taken at a variety of potential sweep rates. Inset is a plot of anodic peak current at each sweep rate versus potential sweep rate; for the linear fit, R2 > 0.99.
to the surface of a glassy carbon electrode, placed in 0.1 M TBAH/ CH3CN, taken at a variety of potential sweep rates. Inset is a plot of anodic peak current at each sweep rate versus potential sweep rate; for the linear fit, R2 > 0.99.
rate.32 In the inset of Figure 4, a plot of peak current versus potential sweep rate, extracted from the data shown in Figure 4, can be seen. The linearity of this plot further confirms that the electrochemical response observed is due only to material specifically adsorbed on the surface of the glassy carbon working electrode.41 The layer of adsorbed [Co(tpy∼py)2]2þ appears to be quite robust; after 2 h of continuous cycling in 0.1 M TBAH/ CH3CN, the peak current only diminished by 25%. A similar set of experiments to the ones described above was carried out for the [Co(tpy∼nap)2]2þ, and the results can be seen below in Figure 5. The results shown in Figure 5 are very similar to those shown in Figure 4, indicating that [Co(tpy∼nap)2]2þ also adsorbs to the surface of a glassy carbon electrode. As will be described below, the comparatively weak adsorption of [Co(tpy∼nap)2]2þ leads to rapid desorption from the glassy carbon surface. Consequently, the peak current observed in Figure 5 is not representative of full surface coverage and is somewhat smaller that what is seen in Figure 4 for the more strongly adsorbing [Co(tpy∼py)2]2þ. Further work was subsequently undertaken in order to characterize the thermodynamics of adsorption of the two compounds under investigation on the surface of glassy carbon. A freshly polished glassy carbon electrode was placed into solutions of varying concentrations of [Co(tpy∼py)2]2þ in 0.1 M TBAH/ CH3CN, and cyclic voltammograms of the Co (II/III) redox process were obtained approximately every 5 min. For each concentration of [Co(tpy∼py)2]2þ investigated, this process was continued for ca. 90 min, until the size of the Co(II/III) peak ceased increasing, that is, until the material adsorbed to the electrode surface had reached equilibrium with that in solution. Because of the extremely low concentration of analyte used (15 μM), the contribution to the electrochemical response from material freely diffusing in solution was minimal; the observed current flow was attributed wholly to material adsorbed to the electrode surface.43 The relationship between current and surface coverage (at 298 K) for an adsorbed electroactive species is given below:41
Figure 6. Coverage versus concentration data for [Co(tpy∼py)2]2þ
ip ¼ ð9:39 105 Þn2 υAΓo
ð2Þ
where n and A have the meanings described above, υ is the potential sweep rate in V/s, and Γo* is the coverage of the species of interest in (43) Acevado, D.; Abru~na, H. D. J. Phys. Chem. 1991, 95(23), 9590–9594.
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adsorbed onto a glassy carbon electrode fit to the Langmuir isotherm. R2 = 0.97 for the curve fit.
mol/cm2. For each concentration of [Co(tpy∼py)2]2þ, the equilibrium surface coverage at that concentration was obtained from the peak current of a voltammogram acquired after the surface coverage had ceased changing. The relationship between surface coverage and the amount of material present in solution can be seen in Figure 6, and the solid line is a fit of the data to the Langmuir Isotherm. This model is expressed below in eq 3:43 Γi ¼ ΓS βC i =ð1 þ βC i Þ
ð3Þ
where Γi is the equilibrium surface coverage at a solution concentration given by Ci, ΓS is the saturation coverage of the electrode, and the free energy of adsorption, ΔGads, is equal to -RT ln(18.9β). This adsorption process is well described by the Langmuir model, and a value of 4.910-11 mol/cm2 was obtained for the saturation coverage and a value of 8.8 105 M-1 was obtained for β, indicating that the free energy of adsorption for [Co(tpy∼py)2]2þ on glassy carbon is -41 kJ/mol. These results are in dramatic contrast to those obtained for [Co(tpy∼nap)2]2þ: for this species, it was impossible to conduct the adsorption experiment in the manner described above due to DOI: 10.1021/la9031249
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ð4Þ
a, indicating that there is a moderately attractive interaction between adsorbate molecules. That the free energy of adsorption for [Co(tpy∼nap)2]2þ on glassy carbon is significantly less favorable than that of [Co(tpy∼py)2]2þ is not surprising. These molecules were designed to adsorb via a π-π stacking interaction, and the smaller delocalized π system of naphthalene would be expected to lead to weaker π-π interactions than the larger π system of pyrene. Consequently, the free energy of adsorption of [Co(tpy∼nap)2]2þ was expected to be significantly less favorable than that of its counterpart [Co(tpy∼py)2]2þ. Recent calculations of the strength of the π-π-stacking interaction between aromatic molecules and carbon nanotube substrates suggest that binding energies are in the range of 10-25 kJ/mol45 per benzene ring , and this is in good agreement with our experimental results. These molecules both adsorb to glassy carbon surfaces using a polyaromatic “foot”, and the size of the molecule’s “footprint” may have an impact on how tightly it is possible to pack the molecules on a surface. The smaller “footprint” provided by the naphthalene group on [Co(tpy∼nap)2]2þ appears to allow for a higher surface coverage to be achieved. Another difference in adsorption behavior between the two molecules under investigation is the thermodynamic model that best describes the adsorption process. The adsorption of [Co(tpy∼py)2]2þ is well described by a simple Langmuir isotherm that does not include the possibility of any interactions between adsorbate molecules. However, that model fails to give an accurate description of the behavior of [Co(tpy∼nap)2]2þ. In the case of [Co(tpy∼nap)2]2þ, the Frumkin isotherm provides a much better fit to the experimental data and suggests that there are favorable interactions, on the order of 30 J/mol per pmol/cm2 (i.e., the free energy of adsorption becomes more favorable by 30 J/mol for every 110-12 mol/cm2 increase in coverage) that aid in the adsorption process. It is not entirely clear why this is the case, but we hypothesize that the interactions between pendant aromatic groups that are not involved in binding to the surface may play a part in this. As [Co(tpy∼nap)2]2þ molecules are adsorbed to the surface, the naphthalene groups dangling out into solution may interact with the naphthalene groups of molecules still in solution and may play a part in shepherding additional material to the surface. Because the binding of [Co(tpy∼py)2]2þ to glassy carbon is so much more favorable (by more than 10 kJ/mol), the effect of this process may not contribute appreciably to the adsorption of [Co(tpy∼py)2]2þ. We are currently considering further investigation into this issue by synthesizing heteroleptic complexes containing only one polyaromatic “foot” in order to better understand what role the nonadsorbing polyaromatic functionality may have in the overall adsorption process.
where θ is equal to the fractional coverage Γi/ Γs, a indicates the degree of interaction between adsorbates (a = 2gΓs/RT, where g indicates the change in free energy of adsorption as a function of surface coverage), and the other symbols have the meanings described above. While this model relies on three parameters (since Γs is necessary for the calculation of θ), the data analysis was simplified by allowing only a and β to vary for different values of ΓS (ΓS was input and incremented by hand) until the best fit was found. This fit, shown in Figure 7 as the solid line, has a value of 6.6 10-11 mol/cm2 for the saturation coverage, a value of 8.0 103 M-1 for β which results in a free energy of adsorption of -30 kJ/mol, and a value of 1.4 for the interaction parameter
This report describes the synthesis and characterization of two novel ligands that can facilitate the noncovalent functionalization of graphene surfaces with transition metal complexes as well as two cobalt-based complexes containing these ligands. Solutionphase electrochemical studies have been used to measure the diffusion coefficients of these complexes and have also demonstrated that the essential nature of the redox processes of the transition metal complex is not significantly affected by the presence of the pyrene or naphthalene functional group. Electrochemical techniques have been utilized to show that these complexes adsorb strongly to glassy carbon surfaces and the adsorption
(44) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; J. Wiley & Sons: New York, 2001; Chapter 13.
(45) Kar, T.; Bettinger, H. F.; Scheiner, S.; Roy, A. K. J. Phys. Chem. C 2008, 112(50), 20070–20075.
Figure 7. Concentration versus coverage data for [Co(tpy∼py)2]2þ adsorbed onto a glassy carbon electrode. The dashed line is the fit of these data to the Langmuir isotherm (R2 = 0.91), and the solid line is the fit of these data to the Frumkin isotherm (R2 = 0.96) using a value of 6.6 10-11 mol/cm2 for ΓS.
the relatively weak adsorption of this complex. At low (ca. 5 μM) solution concentrations, the adsorption of [Co(tpy∼nap)2]2þ was too sparse to accurately quantify, and at high solution concentrations the issue of how to accurately deconvolve the solution response from that of adsorbed material limited the reliability of the measurements. Consequently, adsorption experiments were carried out by incubating a clean glassy carbon electrode in a moderately high concentration (approx 10-100 μM) solution of [Co(tpy∼nap)2]2þ in CH3CN overnight followed by rinsing with CH3CN and placing the electrode into fresh 0.1 M TBAH/ CH3CN. The necessary cyclic voltammograms were immediately (within 2 min of placing the electrode in fresh solution) obtained to keep any desorption to a minimum, and the coverage at each solution concentration was determined using eq 2. In Figure 7, a plot of equilibrium surface coverage versus solution concentration for [Co(tpy∼nap)2]2þ can be seen. A comparison of Figure 7 to Figure 6 indicates that the adsorption of [Co(tpy∼nap)2]2þ on glassy carbon is considerably less favorable than the adsorption of [Co(tpy∼py)2]2þ and that [Co(tpy∼nap)2]2þ has a higher saturation coverage on glassy carbon than does [Co(tpy∼py)2]2þ. The dashed line in Figure 7 is a fit of the data to the Langmuir isotherm as described above, and it is clear that this model does not accurately describe what is occurring. The data were then fit using the Frumkin isotherm described below:44 ½θ=ð1 - θÞ expð-aθÞ ¼ βC i
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Conclusions
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thermodynamics are well-described by standard models. Notably, we have been able to demonstrate that rational synthetic ligand modifications can lead to significant changes in the parameters that control the process of adsorption. An increased understanding of the structure-property relationships in these types of systems may prove valuable in the rational design of functional hybrid nanomaterials, and the ability to study such systems conveniently using glassy carbon electrodes can dramatically increase the efficiency of an iterative optimization process. Work is also currently underway to examine the adsorption of these polyaromatic-terminated transition metal complexes on singlewalled carbon nanotubes (SWNTs), in order to demonstrate that
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such adsorption occurs, to quantify the parameters that govern it, and to allow accurate comparisons between glassy carbon surfaces and SWNTs to be made. Preliminary results indicate, as expected, that [Co(tpy∼py)2]2þ is capable of strongly interacting with and functionalizing SWNTs.46 Acknowledgment. This work was supported by the Bryn Mawr College Office of the Provost, the Camille and Henry Dreyfus Faculty Start-up Awards Program, and the Pittsburgh Conference National Memorial College Grants Program. (46) McQueen, E.; Goldsmith, J. I. J. Am. Chem. Soc., accepted for publication.
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