pubs.acs.org/Langmuir © 2010 American Chemical Society
Activation Energies for Oxidation of Porphyrin Monolayers Anchored to Au(111) Jieying Jiao,† Masahiko Taniguchi,‡ Jonathan S. Lindsey,*,‡ and David F. Bocian*,† †
Department of Chemistry, University of California, Riverside, California 92521-0403, United States, and Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States
‡
Received July 13, 2010. Revised Manuscript Received September 15, 2010 The activation energy for the oxidation of porphyrin monolayers anchored to gold surfaces is determined via measurement of the temperature dependence of the electron-transfer rates. The activation energy (1) increases with increasing surface concentration of the porphyrin and (2) is significantly lower (8.1-17 versus 37-49 kJ mol-1) when smaller, more mobile counterions (Cl- versus PF6-) are used as the supporting electrolyte. Regardless, the lower activation energies do not result in radically different electron-transfer rates for the different types of counterions owing to compensating entropic effects.
Introduction The prospect that devices relying on the bulk properties of semiconductors will fail to retain their characteristic properties as sizes reach nanoscale dimensions has motivated studies of molecular materials as potential active media in electronic devices.1 Toward this goal, we have been engaged in a program aimed at constructing devices that use the properties of molecules to store information.2,3 In this approach, a collection of redox-active molecules (monolayer or polymer film) attached to an electroactive surface functions as the storage medium; information is stored in the discrete redox states of the molecules. Our studies have primarily utilized porphyrinic molecules as the chargestorage medium. The molecules have been covalently attached to both metal (Au via S anchor atoms)2 and semiconductor (Si via O, S, Se, and C anchor atoms)3 surfaces. Semiconductor substrates are of particular interest because the first molecular-based electronic devices are likely to be hybrid designs wherein molecules are integrated onto semiconductor platforms.4 One important characteristic of a memory device is its read/ write speed, with faster speeds being generally more desirable. In a molecular-based memory device that relies on charge storage, the read/write speed is directly related to the electron-transfer rate of the surface-attached molecules. In this regard, our previous studies of porphyrin monolayers on both metal and semiconductor surfaces have shown that the electron-transfer rates depend on *To whom correspondence should be addressed. E-mail: David.Bocian@ ucr.edu (D.F.B.);
[email protected] (J.S.L.). Telephone (951) 827-3660 (D. F.B.); (919) 515-6406 (J.S.L.). (1) For reviews see (a) Kwok, K. S.; Ellenbogen, J. C. Mater. Today 2002, 5, 28–37. (b) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378–4400. (c) Lindsay, S. M.; Ratner, M. A. Adv. Mater. 2007, 19, 23–31. (2) (a) Roth, K. M.; Dontha, N.; Dabke, R. B.; Gryko, D. T.; Clausen, C.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. J. Vac. Sci. Technol., B 2000, 18, 2359– 2364. (b) Roth, K. M.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. Langmuir 2002, 18, 4030–4040. (c) Roth, K. M.; Gryko, D. T.; Clausen, C.; Li, J.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F. J. Phys. Chem. B 2002, 106, 8639–8648. (3) (a) Roth, K. M.; Yasseri, A. A.; Liu, Z.; Dabke, R. B.; Malinovskii, V.; Schweikart, K.-H.; Yu, L.; Tiznado, H.; Zaera, F.; Lindsey, J. S.; Kuhr, W. G.; Bocian, D. F. J. Am. Chem. Soc. 2003, 125, 505–517. (b) Liu, Z.; Yasseri, A. A.; Lindsey, J. S.; Bocian, D. F. Science 2003, 302, 1543–1545. (c) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2004, 126, 15603–15612. (d) Wei, L.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 6323–6330. (4) Kuhr, W. G.; Gallo, A. R.; Manning, R. W.; Rhodine, C. W. Mater. Res. Soc. Bull. 2004, 838–842.
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factors such as the surface concentration (packing density) of the molecules.2c,3c,3d In particular, we have observed that the electron-transfer rates monotonically decrease as the surface concentration of the molecules increases. More recently, we have examined the effects of counterion mobility, surface morphology, and charge screening on electron-transfer rates of porphyrin monolayers.5 Such effects have previously been shown to be important determinants of the electronic properties of molecular monolayers.6-9 Our studies of the porphyrins showed that the inverse correlation between the electron-transfer rates and surface concentration of the porphyrins is due primarily to space-charge effects.5 Another important physicochemical property of molecular monolayers is the activation energy associated with the redox event. In this regard, a number of early studies examined this factor via variable temperature studies of the electron-transfer process.10-12 Such information is lacking for the porphyrin monolayers because all of our previous studies were conducted at a single (ambient) temperature.2,3,5 This deficiency in our understanding of the redox process prompted the study reported herein where we examine the temperature dependence of the electron-transfer rates for thiol-anchored porphyrin monolayers on Au(111) surfaces. The particular porphyrin chosen for study, ZnPBzSAc, is a zinc chelate that contains a thiol-terminated benzyl tether at one meso position and mesityl groups at the other meso positions (Chart 1). The ZnPBzS- monolayers were selected for the present study because we have previously examined their electron-transfer (5) Jiao, J.; Nordlund, E.; Lindsey, J. S.; Bocian, D. F. J. Phys. Chem. C 2008, 112, 6173–6180. (6) (a) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500–5507. (b) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1997, 420, 291–299. (c) Sumner, J. J.; Creager, S. E. J. Phys. Chem. B 2001, 105, 8739–8745. (7) (a) Pyati, R.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 1743–1749. (b) Ingram, R. S.; Murray, R. W. J. Chem. Soc., Faraday Trans. 1996, 92, 3941–3946. (8) Campbell, D. J.; Herr, B. R.; Hulteen, J. C.; Van Duyne, R. P.; Mirkin, C. A. J. Am. Chem. Soc. 1996, 118, 10211–10219. (9) Finklea, H. O.; Liu, L.; Ravenscroft, M. S.; Punturi, S. J. Phys. Chem. 1996, 100, 18852–18858. (10) Chidsey, C. E. D. Science 1991, 251, 919–922. (11) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton, M. R.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141–13149. (12) (a) Finklea, H. O.; Ravenscroft, M. S.; Snider, D. A. Langmuir 1993, 9, 223–227. (b) Finklea, H. O.; Ravenscroft, M. S. Isr. J. Chem. 1997, 37, 179–84.
Published on Web 09/27/2010
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Letter Chart 1
characteristics as a function of a number of parameters including surface concentration, surface type, and counterion type.2,5 Two particular counterions, PF6- and Cl-, were chosen for study. These anions were chosen because the electron-transfer rates observed with the latter counterion are much faster than with the former; in addition, the details of the rate versus surface concentration behavior differ for the two types of counterions.5 Collectively, the studies elucidate the activation energy for the oxidation of the porphyrin monolayers and how this energy varies with surface concentration and counterion type.
Experimental Section Chemicals and Materials. The porphyrin ZnPBzSAc was synthesized as previously described.13 The solvent used in the preparation of the monolayers was anhydrous CH2Cl2 (Aldrich, 99%); the solvent used in the electrochemical measurements was propylene carbonate (PC, Aldrich, 99%). The supporting electrolyte n-Bu4NPF6 (TBAH, Aldrich) was recrystallized three times from methanol and dried in a vacuum at 100 °C. The supporting electrolyte n-Bu4NCl (TBACl, Fluka) was used as received. The Au(111) substrates were prepared by e-beam vapor deposition of 20 nm Cr (99.999%) followed by 200 nm of Au (99.995%) onto the surface of a precleaned B-doped Si(100) wafer. The chemicals used in microelectrode fabrication were AZ 5214 positive photoresist (Baker), AZ 400K photoresist developer (Baker), Baker Aleg 355 resist stripper (Baker), nanostrip (Cyantek), HF dip (5:1), buffered oxide etch (10:1), and J. T. Baker clean (Baker). Ar and N2 (99.995%) were passed through Drierite (Fisher) and Oxyclear (Supelco) gas purifiers prior to use. Deionized water (from a Milli-Q system) had a F g16 MΩ 3 cm. Electrode Preparation. The electrochemical measurements on the monolayers were performed using Au(111) microelectrodes (100 100 μm), prepared as previously described.5 The Au microelectrode was defined by a silicon oxide boundary layer (thickness ∼ 300 nm) that was deposited by plasma-enhanced chemical vapor deposition. Microelectrodes were used to ensure that the RC time constant of the electrochemical cell was sufficiently short so that the electron-transfer kinetics of the porphyrin monolayers could be measured accurately. A bare Ag wire was used as the counter/reference electrode. The electrode was prepared by sonicating a section of 500 μm diameter Ag wire (Alfa Aesar) in 7.0 M NH4OH, rinsing the wire in deionized water and ethanol, and then sonicating the wire in CH2Cl2 containing 1.0 M TBAH. The Ag wire prepared in this manner was placed inside a 10 μL polypropylene pipet tip containing ∼5 μL of supporting electrolyte (1.0 M TBAH or TBACl) in PC. Monolayer Preparation. The monolayers composed of ZnPBzS- on Au(111) were prepared by dispensing 5 μL of ZnPBzSAc porphyrin solution in CH2Cl2 onto the surface of the microelectrode contained in a sparged vial sealed under Ar for ∼30 min. The S-acetyl protecting group is cleaved during the surface-attachment process.2 The surface concentration of (13) Gryko, D. T.; Clausen, C.; Roth, K. M.; Dontha, N.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7345–7355.
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ZnPBzS- was determined electrochemically and was varied in a controlled fashion from the mid 10-12 to the mid 10-11 mol cm-2 range by varying the porphyrin concentration in the deposition solvent from 2 μM to 2 mM. Additional details of the monolayer preparation on Au(111) can be found in refs 2a and 2c. Electrochemical Measurements. The electrochemical measurements on the monolayers were performed in a two-electrode configuration using the fabricated microelectrode and the Ag counter/reference electrode described above. The RC time constant for the microelectrode/electrochemical cell, measured to be ∼4 μs, is sufficiently short to preclude any significant interference with the measurement of the electron-transfer rates. The cyclic voltammograms were record using a Gamry Instruments PC4FAS1 femtostat running PHE 200 Framework and Echem Analyst software. The surface concentration of the porphyrins in the monolayer was determined by integration of the total charge in the first anodic wave and by using the geometric dimensions of the microelectrode. The standard electron-transfer rate constant, k0, of the porphyrin monolayers was obtained using the same SWAV method used previously to obtain k0 values for porphyrin monolayers on Au surfaces.2c Variable temperature studies were conducted with an Aerocool HT-102 thermoelectric cooler powered by a Tekpower HY3020 DC power supply. The temperature was controlled by varying the voltage and current, which affords temperature control of (1 C°. The substrates patterned with the microelectrode where placed in contact with the active element of the thermoelectric cooler. The temperature was measured by attaching a thermocouple to the surface of the substrate in close proximity to the microelectrode. Prior to each electrochemical measurement at a given temperature, the substrate/ microelectrode and reference electrode/supporting electrolyte were allowed to equilibrate for ∼20 min. The electrochemical setup was housed inside a plexiglass enclosure containing dry nitrogen to prevent air currents from affecting the temperature.
Results and Discussion Plots of the standard electron-transfer rate constant, k0, versus surface concentration, Γ, for the E0/þ1 oxidation process of the ZnPBzS- monolayers on Au (111) are shown in Figure 1. Data are shown for three different temperatures, 285, 295, and 307 K, and for the two different supporting electrolytes, TBAH (top panel) and TBACl (bottom panel). The error estimates were obtained from three to five data sets obtained at each surface concentration and temperature. Data were also obtained below 285 K and above 307 K (not shown); however, the quality of the raw data was poor. The poor quality of these data appeared to be due to effects such as electrolyte precipitation (lower temperatures) and solvent evaporation (higher temperatures). The trends observed for the electron-transfer rates at the different temperatures are similar to one another. In particular, the rates monotonically decrease as the surface concentration increases and the rates with TBAH as the supporting electrolyte are slower than those with TBACl as the supporting electrolyte. The origin of these effects has been discussed in detail in previous publications and will not be reiterated herein.2a,c,5 The key new observation is that the electron-transfer rates exhibit a marked temperature dependence. As the surface concentration increases, the temperature dependence becomes more pronounced. In addition, the temperature dependence of the electron-transfer rates with TBAH as the supporting electrolyte is significantly larger than with TBACl as the supporting electrolyte. Arrhenius plots for the E 0/þ1 oxidation process of the ZnPBzSmonolayers on Au(111) at the different surface concentrations are shown in Figure 2 for the two different supporting electrolytes, TBAH (top panel) and TBACl (bottom panel). Although the number of points is small, the data are fit with reasonable fidelity DOI: 10.1021/la102802n
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Figure 2. Arrhenius plots for the E0/þ1 oxidation process of 0
Figure 1. Standard electron-transfer rate constant, k , versus surface concentration, Γ, for the E 0/þ1 oxidation process of ZnPBzS- monolayers on Au (111) at various temperatures: (top panel) TBAH supporting electrolyte. (bottom panel) TBACl supporting electrolyte.
to ln-linear behavior. The values of the activation energy, Ea, and prefactor, A, extracted from fits to the data at the various surface concentrations are summarized in Table 1. Inspection of these values reveals the following trends: (1) The activation energy monotonically increases as the surface concentration increases. (2) The activation energy with TBAH as the supporting electrolyte is significantly larger (3- to 4-fold) than with TBACl as the supporting electrolyte. (3) The incremental change in the activation energy over the range of surface concentrations examined is approximately the same for the two supporting electrolytes (8-10 kJ mol-1). (4) The Arrhenius prefactors with TBAH as the supporting electrolyte are significantly larger (4-5 orders of magnitude) than with TBACl as the supporting electrolyte. The observation that the activation energy for the redox process in the ZnPBzS- monolayers increases with surface concentration is generally consistent with the view that space-charge effects play a key role in determining the rates of electron transfer in monolayers.5,6c,7b,8,9 The observation that the activation energy for the redox process with TBAH as the supporting electrolyte is larger than that with TBACl as the supporting electrolyte is further consistent with the view that PF6- is not as facile as Cl- in terms of charge compensation of the porphyrin cation formed upon porphyrin oxidation.5 Quite plausibly, this occurs because PF6is larger than Cl-14,15 and, therefore, is less mobile and/or able to (14) Xuan, X.; Wang, J.; Wang, H. Electrochim. Acta 2005, 50, 4196–4201. (15) Huheey, J. E. Inorganic Chemistry, 2nd ed.; Harper & Row: New York, 1978.
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ZnPBzS- monolayers on Au(111) at various surface concentrations. Solid lines are least-squares fits to the data: (top panel) TBAH supporting electrolyte. (bottom panel) TBACl supporting electrolyte. Table 1. Activation Energy (Ea) and Prefactor (A) for the E 0/þ1 Oxidation Process of ZnPBzS- Monolayers on Au(111) as a Function of Surface Concentration (Γ) and Supporting Electrolyte supporting electrolyte
Γ (10-11 mol cm-2)
A (s-1)
Ea (kJ mol-1)
TBAH
0.55 1.2 1.5 2.0 2.7 3.5
(3.2 ( 3) 1011 (2.9 ( 1) 1011 (1.1 ( 5) 1011 (7.9 ( 5) 1011 (2.9 ( 2) 1012 (7.2 ( 2) 1012
39 ( 4 46 ( 3 39 ( 5 44 ( 5 48 ( 4 50 ( 3
TBACl
1.1 2.1 2.7 3.1 3.5
(1.8 ( 4) 106 (1.6 ( 6) 106 (1.5 ( 4) 107 (2.4 ( 4) 107 (3.6 ( 3) 107
8.0 ( 3 8.5 ( 4 14 ( 4 16 ( 4 17 ( 3
penetrate into the monolayer and associate with the porphyrin cation.5 The effective size of PF6- versus Cl- may be further exaggerated by differences in the nature of the ion-solvent complex. Regardless of the differences in the absolute activation energies observed for the two types of counterions, the incremental change in the activation energy as the surface concentration increases is approximately the same for the two anions. This effect is likely due to the fact that the porphyrin surface concentrations that are obtainable via self-assembly on Au(111) are relatively low and correspond to relatively sparsely packed monolayers2 (relative to the densely packed porphyrin monolayers that can be obtained Langmuir 2010, 26(20), 15718–15721
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via Langmuir-Blodgett techniques16). In more densely packed monolayers, the change in activation energy as a function of surface concentration could be quite different for the two types of counterions. The observation of such different values of the Arrhenius prefactors with TBAH versus TBACl as the supporting electrolyte is unexpected. The prefactors with the former electrolyte are in the range that would be viewed as typical (1010-1012 s-1), whereas those for the latter electrolyte are relatively small (106-107 s-1). The origin of this difference in prefactors is not obvious given that both PF6- and Cl- are nominally spherical (albeit of different diameter). Accordingly, it is difficult to rationalize entropic effects associated with porphyrin-counterion interactions that would be so different for the two types of anions. Regardless, the relatively small prefactor for the TBACl versus TBAH supporting electrolyte effectively counterbalances the lower activation energy for the former electrolyte. The result is that the electron-transfer rates for oxidation of the porphyrin monolayers are not radically different for the two types of electrolytes. (16) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344–1350.
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Conclusions The studies reported herein elucidate the activation energies for the oxidation of porphyrin monolayers and demonstrate that these activation energies are dependent on both the surface concentration and type of counterion. This behavior is unlikely to be unique to porphyrinic monolayers and/or to gold substrates, but rather should be a general characteristic of redox-active species anchored to surfaces. The studies further demonstrate that the decreased activation energy associated with smaller, more mobile counterions does not necessarily lead to significantly faster electron-transfer rates owing to counterbalancing effects that arise from entropic considerations. This result suggests that devices whose functional characteristics are based on redox processes in molecular materials may be subject to constraints that limit their operational speed. Acknowledgment. This material is based on research sponsored by the Defense Microelectronics Activity (DMEA) under Agreement Number H94003-09-2-0901. The United States Government is authorized to reproduce and distribute reprints for Government purposes, notwithstanding any copyright notation thereon.
DOI: 10.1021/la102802n
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