Comparison of Electron-Transfer Rates for Metal-versus Ring

The faster rates for the ring- versus metal-centered redox process are ..... different electron-transfer rates, with the form wherein Cl− is bound i...
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Langmuir 2008, 24, 12047-12053

12047

Comparison of Electron-Transfer Rates for Metal- versus Ring-Centered Redox Processes of Porphyrins in Monolayers on Au(111) Jieying Jiao,† Izabela Schmidt,‡ Masahiko Taniguchi,‡ Jonathan S. Lindsey,*,‡ and David F. Bocian*,† Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403, and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed June 25, 2008. ReVised Manuscript ReceiVed August 1, 2008 The standard electron-transfer rate constants (k0) are measured for redox processes of Fe versus Zn porphyrins in monolayers on Au(111); the former undergoes a metal-centered redox process (conversion between FeIII and FeII oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the π-cation radical). Each porphyrin contains three meso-mesityl groups and a benzyl thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu4NCl) conditions, the ZnII center has a coordinated Cl- ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observedsa low-potential form (El0 ≈ -0.6 V) wherein the metal center has a coordinated Cl- ion when it is in either the FeII or FeIII state and a high-potential form (Eh0 ≈ +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the FeIII to FeII states. The k0 values observed for all of the porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k0 values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k0 values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl- exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a π molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.

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.2-5 In this approach, a collection of redoxactive molecules (monolayer or polymer film) attached to an electroactive surface functions as the storage medium; information * To whom correspondence should be addressed. E-mail: david.bocian@ ucr.edu (D.F.B.); [email protected] (J.S.L.). Phone: (951) 827-3660 (D.F.B.); (919) 515-6406 (J.S.L.). † University of California. ‡ North Carolina State University. (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) Gryko, D.; Li, J.; Diers, J. R.; Roth, K. M.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Mater. Chem. 2001, 11, 1162–1180. (c) Roth, K. M.; Lindsey, J. S.; Bocian, D. F.; Kuhr, W. G. Langmuir 2002, 18, 4030–4040. (d) 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. (e) Schweikart, K.-H.; Malinovskii, V. L.; Yasseri, A. A.; Li, J.; Lysenko, A. B.; Bocian, D. F.; Lindsey, J. S. Inorg. Chem. 2003, 42, 7431–7446. (f) Wei, L.; Padmaja, K.; Youngblood, W. J.; Lysenko, A. B.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 1461–1469. (3) (a) Yasseri, A. A.; Syomin, D.; Malinovskii, V. L.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2004, 126, 11944–11953. (b) Wei, L.; Tiznado, H.; Liu, G.; Padmaja, K.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 23963–23971. (4) (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.

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)2,3 and semiconductor (Si via O, S, Se, and C anchor atoms)4,5 surfaces. Semiconductor substrates are of particular interest because the first molecularbased electronic devices are likely to be hybrid designs wherein molecules are integrated onto semiconductor platforms.6 One important characteristic of a memory device is its read/ write speed, 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 factors such as the surface concentration (packing density) of the molecules.2d,f,3,5a-d,g In particular, we have observed that the electron-transfer rates monotonically decrease as the surface concentration of the molecules increases. More recently, we have (5) (a) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2004, 126, 15603–15612. (b) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2005, 127, 9308. (c) Wei, L.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Phys. Chem. B 2005, 109, 6323–6330. (d) Padmaja, K.; Wei, L.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2005, 70, 7972–7978. (e) Thamyongkit, P.; Yu, L.; Padmaja, K.; Jiao, J.; Bocian, D. F.; Lindsey, J. S. J. Org. Chem. 2006, 71, 1156–1171. (f) Jiao, J.; Anariba, F.; Tiznado, H.; Schmidt, I.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2006, 128, 6965– 6974. (g) Padmaja, K.; Youngblood, W. J.; Wei, L.; Bocian, D. F.; Lindsey, J. S. Inorg. Chem. 2006, 45, 5479–5492. (6) Kuhr, W. G.; Gallo, A. R.; Manning, R. W.; Rhodine, C. W. Mater. Res. Soc. Bull. 2004, 838–842. (7) Jiao, J.; Nordlund, E.; Lindsey, J. S.; Bocian, D. F. J. Phys. Chem. C 2008, 112, 6173–6180.

10.1021/la8019843 CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

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shown that the inverse correlation between the electron-transfer rates and surface concentration is due primarily to space-charge effects.7

Jiao et al. Chart 1

To date, all of our studies of electron-transfer in porphyrin monolayers have utilized molecules wherein the redox process involves formation of a ring-centered π-cation. None of our studies have examined porphyrins wherein the redox process is metalcentered. In this regard, the most studied metal-based redox process in porphyrins is the conversion between FeII and FeIII, for which extensive solution electrochemical studies have been reported.8 There have also been a number of studies of electrontransfer in heme proteins, particularly cytochrome c, immobilized on electroactive surfaces.9 On the other hand, studies of electrontransfer in Fe porphyrin monolayers directly attached to electroactive surfaces are much more limited. Pilloud et al. measured the standard electron-transfer rates for a series of Fe porphyrins tethered to Au(111) via relatively short alkyl thiol groups.10 The electron-transfer rates for the metal-centered redox process were found to be in the 102-104 s-1 range, depending on the length of the linker to the surface. These rates are slower than those we have obtained for ring-centered oxidations of porphyrin monolayers, which are typically in the 104-105 s-1 range.2d,f,3,5a-d,g,7 However, no direct comparison can be made between our studies and those by Pilloud et al. because the porphyrins and surface anchors, as well as the experimental conditions, are different. In the studies reported herein, we compare the electron-transfer rates for metal- versus ring-centered redox processes in porphyrins in monolayers on Au(111) under experimental conditions that are as closely matched as possible. The porphyrins used in the study are shown in Chart 1 and are the Fe and Zn chelates of a porphyrin containing three meso mesityl groups and a benzyl thiol for surface attachment (Cl-FeIIIPBzSAc and ZnPBzAc). Also shown in Chart 1 are the structures of the Fe and Zn chelates of tetramesitylporphyrin (Cl-FeIIIP and ZnP), which were used as benchmark molecules for Cl- binding studies. The PBzSAc porphyrin was chosen for the electron-transfer studies for two reasons. First, we have previously examined the electrontransfer characteristics of the ring-centered oxidation process in monolayers of the Zn chelate under a wide variety of conditions (solvents, electrolytes, types of surface).7 Second, the three mesomesityl groups sterically encumber the face of the porphyrin, precluding formation of µ-oxo-bridged dimers in the case of the Fe chelate. This latter point proved to be an important experimental consideration because numerous attempts to prepare monomeric Fe porphyrins that contained the surface attachment group and sterically unencumbered nonanchoring substituents (such as p-tolyl groups) uniformly failed, always yielding µ-oxo-bridged Fe dimers instead (see Experimental Section). Collectively, the studies reported herein provide new insights into the electrontransfer rates for metal- versus ring-centered redox processes in porphyrin monolayers. (8) For reviews see. (a) Kadish, K. M. In Iron Porphyrins; Lever, A. P. B; Gray, H. B, Eds.; Addison-Wesley: Reading, MA, 1983; Part II; pp 161-249. (b) Kadish, K. M.; Royal, G.; Van Caemelbecke, E., Gueletti, L. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R, Eds.; Academic Press: San Diego, 2000; Vol. 9, pp 1-219. (9) (a) Tarlov, M. J.; Bowden, E. F. J. Am. Chem. Soc. 1991, 113, 1847–1849. (b) Haas, A. S.; Pilloud, D. L.; Reddy, K. S.; Babcock, G. T.; Moser, C. C.; Blasie, J. K.; Dutton, P. L. J. Phys. Chem. B 2001, 105, 11351–11362. (c) Chen, X; Ferrigno, R.; Yang, J.; Whitesides, G. M. Langmuir 2002, 18, 7009–7015. (10) Pilloud, D. L.; Chen, X.; Dutton, P. L.; Moser, C. C. J. Phys. Chem. B 2000, 104, 2868–2877.

Experimental Section Synthesis. The porphyrins ZnP11 and Cl-FeIIIP12 were first described several decades ago. The syntheses now are facilitated by the ready availability of free base tetramesitylporphyrin.13 The procedure employed herein for the synthesis of Cl-FeIIIP is described below. The S-acetylthio-derivatized porphyrins were obtained by metalation of 5-[4-(S-acetylthiomethyl)phenyl]-10,15,20-trimesitylporphyrin14 with zinc acetate to give ZnPBzSAc14 or with ferrous chloride to give Cl-FeIIIPBzSAc as described below. The mass spectral characterization was performed by laser desorption mass spectrometry (LD-MS) without a matrix15 and by high-resolution fast atom bombardment mass spectrometry (FAB-MS) using a matrix of nitrobenzyl alcohol and polyethylene glycol. Chloro(tetramesitylporphinato)iron(III) (Cl-FeIIIP). A solution of tetramesitylporphyrin (23.5 mg, 30.0 µmol) in DMF (12 mL) was treated with FeCl2 (76.1 mg, 0.600 mmol) at 140 °C for 18 h. The reaction mixture was concentrated to dryness under reduced pressure. The resulting residue was dissolved in CH2Cl2 (60 mL) and treated with 10% aqueous hydrochloric acid (30 mL) at room temperature for 10 min. The organic layer was separated, dried (Na2SO4), filtered, and concentrated to dryness under reduced pressure. The resulting solid was chromatographed [alumina, hexanes/CH2Cl2 (1:1), CH2Cl2, (11) (a) Badger, G M.; Jones, R. A.; Laslett, R. L Aust. J. Chem. 1964, 17, 1028–1035. (b) Robinson, L. R.; Hambright, P. Inorg. Chim. Acta 1991, 185, 17–24. (c) Iseki, Y.; Watanabe, E.; Mori, A.; Inoue, S. J. Am. Chem. Soc. 1993, 115, 7313–7317. (12) (a) Cheng, R.-J.; Latos-Grazynski, L.; Balch, A. L. Inorg. Chem. 1982, 21, 2412–2418. (b) Groves, J. T.; Nemo, T. E. J. Am. Chem. Soc. 1983, 105, 6243–6248. (13) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828–836. (14) 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. (15) (a) Fenyo, D.; Chait, B. T.; Johnson, T. E.; Lindsey, J. S. J. Porphyrins Phthalocyanines 1997, 1, 93–99. (b) Srinivasan, N.; Haney, C. A.; Lindsey, J. S.; Zhang, W.; Chait, B. T. J. Porphyrins Phthalocyanines 1999, 3, 283–291.

Comparison of Electron-Transfer Rates then CH2Cl2/methanol (50:1)] to give a greenish brown solid (22.8 mg, 87%): 1H NMR (300 MHz, CDCl3) δ 3.28-4.38 (br, 12H), 4.04-4.30 (br, 12H), 5.74-7.52 (br, 12H), 14.15-14.70 (br, 4H), 15.80-16.20 (br, 4H), 79.8-82.8 (br, 8H); MALDI-MS (using a matrix of 1,4-bis(5-phenyloxazol-2-yl)benzene) obsd 870.8; calcd 871.3 (C56H52ClFeN4); λabs (relative intensity, in CH2Cl2) 377 (0.52), 419 (1.00), 510 (0.13), 656 (0.03) nm. Chloro(5-[4-(S-acetylthiomethyl)phenyl]-10,15,20-trimesitylporphinato)iron(III) (Cl-FeIIIPBzSAc). A solution of 5-[4-(Sacetylthiomethyl)phenyl]-10,15,20-trimesitylporphyrin14 (35 mg, 0.042 mmol) in THF/MeOH (20 mL, 4:1) was treated with FeCl2 (40 mg, 0.32 mmol), and the mixture was refluxed for 6 h. The mixture was concentrated, and the residue was taken up in CH2Cl2. The organic phase was washed (brine, water), dried (Na2SO4), and concentrated to a dark solid (38 mg, 98%): λabs (CH2Cl2) 377, 419, 510; LD-MS obsd 808.5 [(M - SAc - Cl)+; M ) C56H50ClFeN4OS]; FAB-MS obsd 882.3089, calcd 882.3089 [(M - Cl)+; M ) C56H50ClFeN4OS]. A portion of the isolated solid (20 mg) was chromatographed (CH2Cl2/MeOH, 9:1). Chromatography resulted in exchange of the axial ligand (as indicated by changes in the absorption spectrum). The title compound was recovered by shaking a solution of the iron porphyrin in CH2Cl2 (20 mL) with an aqueous solution of concentrated HCl (1 mL, 38%) for 2 min. The organic layer was then washed with water, separated, dried (Na2SO4), and concentrated to afford material identical with that prior to chromatography. Chemicals, Materials, and Substrate Preparation. The solvent used for solution electrochemical and absorption spectroscopic measurements, and preparation of the monolayers was anhydrous CH2Cl2 (Aldrich, 99%). The solvent used for monolayer electrochemical measurements was propylene carbonate (PC, Aldrich, 99%). The supporting electrolytes n-Bu4NPF6 (TBAH, Aldrich) and n-Bu4NClO4 (TBAP, Aldrich) were recrystallized three times from methanol and dried under vacuum at 100 °C. The supporting electrolyte n-Bu4NCl (TBACl, Fluka) was used as received. Na2S2O4 was purchased from Aldrich and used as received. 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 g 16 MΩ cm. 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 (Silicon Valley Microelectronics). Electrode Preparation. The electrochemical measurements on the monolayers were performed using Au(111) (100 × 100 µm) microelectrodes, prepared as previously described.7 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 (TPAH, TPAP, or TPACl) in PC. Monolayer Preparation. The monolayers for the electrochemical measurements were prepared by dispensing ∼15 µL of ∼2 mM ZnPBzSAc or Cl-FeIIIPBzSAc 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 surfaceattachment process.3,5a Additional details of the monolayer preparation on Au(111) can be found in refs 3a and 3b. The monolayers

Langmuir, Vol. 24, No. 20, 2008 12049 for FTIR experiments required much larger areas (∼1 cm2) and, as a result, required a much larger drop size (∼50 µL). Electrochemical Measurements. The electrochemical measurements of the porphyrins in solution were made in a standard threeelectrode cell using Pt working and counter electrodes and a Ag/ Ag+ reference electrode. The concentration of the supporting electrolyte (TPAH, TPAP, or TPACl) was 0.1 M in CH2Cl2. 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 cyclic voltammograms of the monolayers were acquired at relatively fast scan rates (100 V s-1) to afford sufficient currents for the measurements on the microelectrodes. All potentials are reported vs Fc/Fc+ ) 0.19 V. 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 recorded using a Gamry Instruments PC4-FAS1 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 electroactive surfaces.2d FTIR Spectroscopy. The FTIR spectra of the various porphyrins in either solid or monolayer forms were collected at room temperature with a spectral resolution of 4 cm-1 using a Bruker Tensor 27 spectrometer. The spectra of solid porphyrins were obtained in KBr pellets (∼1-2 wt % porphyrin) in transmission mode using a roomtemperature DTGS detector by averaging over 32 scans. The IR spectra of the monolayers were obtained using a Harrick Scientific horizontal reflection Ge attenuated total reflection accessory (GATR, 65° incidence angle). The Au substrates were placed in contact with the flat surface of a semispherical Ge crystal that serves as the optical element, and IR spectra were collected with p-polarized light using a liquid-nitrogen cooled medium-bandwidth (600-4000 cm-1) MCT detector and averaging over 256 scans. The spectra of the monolayers were referenced against a clean Au(111) surface previously subjected to the same deposition conditions as those used to obtain the monolayer but using only the neat deposition solvent. The Ge crystal was cleaned with neat 2-butanone before every experiment, and the GATR accessory was purged with dry N2 during data acquisition. UV-Visible Spectroscopy. The solution absorption spectra of ZnP, Cl-FeIIIP, and FeIIP were obtained using a Varian Cary 50 UV-vis spectrophotometer. FeIIP was prepared by reducing ClFeIIIP with excess Na2S2O4 in a two-phase reaction wherein the reducing agent in H2O was vigorously shaken with the porphyrin in CH2Cl2.

Results and Discussion As was noted in the Introduction, we have extensively characterized the ring-centered oxidation process in monolayers such as ZnPBzS- (and related porphyrins).2-5 These studies included extensive spectroscopic characterization (FTIR, XPS, etc.) of the monolayers, which interrogated the general characteristics of the monolayers, such as surface coverage, surface binding motif, and adsorption geometry of the porphyrins.3,5,7,16 We have not previously investigated any Fe porphyrin monolayers; however, there was no reason a priori to suspect that the Fe chelates would behave differently than the Zn chelates with respect to monolayer formation and general characteristics. Regardless, to ensure that the Fe porphyrins were well behaved with respect to monolayer formation, the vibrational and voltammetric characteristics of the Cl-FeIIIPBzS- monolayers were examined and compared with those of the ZnPBzS(16) Jiao, J.; Thamyongkit, P.; Schmidt, I.; Lindsey, J. S.; Bocian, D. F. J. Phys. Chem. C 2007, 111, 12693–12704.

Comparison of Electron-Transfer Rates

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Figure 2. Fast scan (100 V s-1) voltammograms of the Cl-FeIIIPBzS- monolayers on Au(111) with varying concentrations of TBACl in PC. Potentials vs Fc/Fc+ ) 0.19 V.

it is important to have a clear picture of the anion-complexation state of the metal ions in the porphyrins as this factor can affect the electron-transfer rates.8a,21 One strategy to examine ligand binding is to perform redox titrations. However, this method is problematic for the present studies because of limitations in dynamic range. In particular, the redox waves for the Fe center are not clearly observed until the Cl- concentration is quite high (∼0.5 M). This characteristic, along with the fact with the saturating Cl- concentration is ∼1 M, limits the range over which the Cl- concentration can be varied to a factor of 2. The expected redox potential shifts for concentration changes of this order are too small to be accurately measured. Owing to the limitations associated with electrochemical characterization of Cl- binding, a series of absorption spectroscopic studies were performed. These studies examined the effects of added Cl- on the spectra of both the Zn and Fe porphyrins. These studies necessarily involved porphyrins in solution because the absorption features of the monolayers are too weak to observe. Extensive absorption spectroscopic studies of Cl-FeIIIPBzSAc and ZnPBzSAc as a function of Cl- concentration were not feasible owing to the somewhat limited quantities of these materials. Thus, the majority of the absorption spectroscopic studies utilized the model complexes Cl-FeIIIP and ZnP, which were available in much larger quantities. The absorption spectroscopic studies of Cl-FeIIIP and ZnP also utilized CH2Cl2 as the solvent instead of PC, owing to the insolubility of any of the porphyrins in PC. CH2Cl2 was utilized because this solvent is known to be noncoordinating to Fe porphyrins,8a,21 which should also be the case for the relatively bulky PC solvent used for the electrochemical studies of the monolayers. The results of the absorption spectroscopic studies on ClFeIIIP and ZnP are shown in Figure 3; spectral data are also shown for the ferrous form of the Fe chelate. We first describe the spectra of ZnP for which the effects of Cl- are more apparent and readily interpreted. We then describe the spectra of the Fe porphyrins, which are somewhat more complicated.

Figure 3. (Top) Absorption spectra of ZnP with varying concentrations of TBACl. (Bottom) Absorption spectrum of Cl-FeIIIP in the absence of TBACl (solid trace); absorption spectra of FeIIP with varying concentrations of TBACl (dashed and dot-dash traces). The spectrum in the present of 1 M TBACl is expanded ∼2-fold to more clearly show the spectral feature at 443 nm. In all cases, the solvent was CH2Cl2.

The absorption spectra of ZnP in the absence of Cl- and in the presence of 0.1 and 1 M Cl- are shown in the top panel of

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Figure 3. Spectra were also obtained at a number of other Clconcentrations; however, these spectra are omitted from the figure for clarity. In the absence of Cl-, the Soret band of ZnP is observed at 420 nm; upon addition of Cl-, a second Soret feature appears at 436 nm, which grows as the Cl- concentration increases. In the presence of 1 M Cl-, the 436-nm band dominates the spectrum and only a weak 420-nm band is present. The 436 nm feature is attributed to a form wherein Cl- axially coordinates the Zn ion of ZnP, yielding [Cl-ZnP]-. Thus, under the conditions required to obtain good quality voltammetric data for Cl-FeIIIP (high Cl- concentrations) and at which the electron-transfer rates were ultimately measured (1 M Cl-), ZnP appears to be predominantly in the form [Cl-ZnP]-. [Note, however, that a quantitative measure of the amount of complexed versus uncomplexed forms is not readily extracted from the absorption spectra because the relative extinction coefficients of the two forms are not known.] It is further expected that upon oxidation of the Zn porphyrin, the Cl- ion would remain bound because of the much larger affinity of the porphyrin π-cation for the Clanion. This view is consistent with previous EPR studies of oxidized Zn porphyrins, which have shown that Cl- is complexed to the Zn ion of the oxidized porphyrin.22 The absorption spectra of Cl-FeIIIP in the absence of exogenous Cl- are shown in the bottom panel of Figure 3 (solid trace). In the absence of exogenous Cl-, the Soret band of Cl-FeIIIP is at ∼420 nm; the addition of up to 1 M of Cl- has no effect on the spectrum (not shown). The absorption features observed for ClFeIIIP in the absence (and presence) of exogenous Cl- are generally consistent with those expected for a Cl- complexed form.21 The absorption spectrum of the ferrous Fe porphyrin in the absence of exogenous Cl- is also shown in the bottom panel of Figure 3 (dashed trace). Reduction of the metal center in the Fe porphyrin results in a slight blue shift of the Soret band to about 416 nm. These features are generally consistent with those expected for an Fe porphyrin in which the anion dissociates upon formation of the ferrous state, yielding FeIIP.23 The absorption spectra of the ferrous porphyrin in the presence of 0.5 and 1 M Cl- are also shown in the bottom panel of Figure 3 (dot-dash traces). Spectra were also obtained at a number of other Cl- concentrations; however, these spectra are omitted from the figure for clarity. Upon addition of Cl-, a new Soret feature appears at 443 nm; this feature only becomes clearly apparent in the presence of 1 M Cl-. Even at this concentration, the 443 nm Soret band is weaker than the other Soret band near 420 nm. In general, the spectral response of FeIIP to the addition of Cl- is very similar to that of ZnP, indicating that the anion is axially coordinating the neutral ferrous porphyrin, yielding [Cl-FeIIP]-. However, the amount of [Cl-FeIIP]- formed in the presence of 1 M Cl- appears to be less than that for the Zn porphyrin. (Again, exact quantitation of the amount of complexed versus uncomplexed species is not certain owing to the absence of the relative extinction coefficients of the two forms.) Thus, the absorption spectroscopic studies are in qualitative agreement with the electrochemical studies, which show that substantial amounts of two different forms of the Fe porphyrin are present at high Cl- concentrations. Electron-Transfer Characteristics of the Monolayers. The standard electron-transfer rate constants, k0, were measured for the first ring-centered oxidation process of the ZnPBzSmonolayers (E0 ≈ +0.75 V) and two metal-centered redox processes of the Cl-FeIIIPBzS- monolayers (Eh0 ≈ +0.2 V and (22) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979, Vol. IV; pp 197-256. (23) Adar, F. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978, Vol. III; pp 167-209.

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Figure 4. Standard electron-transfer rate constants, k0, versus surface concentration, Γ, for the first oxidation process (E0/+1) of the ClFeIIIPBzS- and ZnPBzS- monolayers on Au(111).

El0 ≈ -0.6 V). For all of the monolayers, the electron-transfer rates were measured as a function of surface concentration, Γ. These studies were undertaken because our previous studies of Zn porphyrin monolayers on a variety of substrates have shown that the electron-transfer rates depend on surface coverage. Higher surface coverage generally results in slower rates of electron transfer;2a,f,5a-d,g although the variation of k0 with Γ is less pronounced when Cl- is the counterion compared with PF6- or ClO4-.7 These trends are also observed for Cl-FeIIIPBzSmonolayers as shown in Figure 4, which compares plots of k0 versus Γ for the Cl-FeIIIPBzS- versus ZnPBzS- monolayers. The key difference in the electron-transfer characteristics of the Cl-FeIIIPBzS- versus ZnPBzS- monolayers is that the rates for the metal-centered redox process are significantly slower (10-40fold) than those for the ring-centered process. The k0 values we observed for the Cl-FeIIIPBzS- monolayers are generally in the range of those previously reported by Pilloud et al. for monolayers of different types of Fe porphyrins on Au(111).10 The two forms of the Cl-FeIIIPBzS- monolayers also exhibit somewhat different electron-transfer rates, with the form wherein Cl- is bound in both the FeIII and FeII state exhibiting somewhat slower (2-4-fold) rates than the form wherein Cl- can exchange. This observation is counterintuitive. One might have expected that the form where no ligand exchange occurs might exhibit faster rates. In this regard, it has been proposed that the rate limiting step in the reduction of halide-bound FeIII porphyrins is formation of the FeII species followed by halide dissociation.8a,21 In the reverse process, the FeII species is oxidized to the FeIII species followed by rebinding of the halide. If ligand exchange were very slow on the time scale of electron transfer, the k0 values for the two different forms of the Cl-FeIIIPBzSmonolayers would be expected to be the same because the Clion could not dissociate during the rapid modulation of the oxidation state, which forms the basis of the SWAV experiment to measure the rate.2d,24 Accordingly, the ligand dissociation/ reassociation rate must be generally comparable to the electrontransfer rate. The key question is why the rate of the ring-centered redox process is much faster than those of the metal-centered processes. Counter ion accessibility to the redox center does not seem to be a likely candidate because the large differences in rates are present in cases where the Cl- counterion is bound in both redox (24) (a) Creager, S. E.; Wooster, R. T. Anal. Chem. 1998, 70, 4257–4263. (b) Li, J.; Schuler, K.; Creager, S. E. J. Electrochem. Soc. 2000, 147, 4584–4588.

Comparison of Electron-Transfer Rates

states of the porphyrins and when the porphyrins are at very low surface concentrations, the latter which would allow ready access to the molecules by other exogenous Cl- ions. One plausible explanation for the much faster rates of the ring- versus metalcentered redox process is that the former involves an electron exchange from an a2u-type molecular orbital that is delocalized over the porphyrin ring22 and in particular, has significant electron density at the meso-carbon atoms,25 one of which is the site of attachment of the linker to the surface anchoring thiol, whereas the latter involves electron exchange from a d-orbital that is relatively well localized on the metal center.26 Thus, the redox center for the Zn porphyrins is much closer to the surface than for the Fe porphyrins both in terms of the actual distance (because the porphyrins are relatively upright on the surface) and number of bonds through which the tunneling electron must pass to move between the molecule and the metal surface. Another factor that may contribute to the faster electron-transfer rates for the Zn versus Fe porphyrins is the different amount of outer- versus inner-sphere character for the two types of redox reactions. In this regard, previous NMR studies of heme and metalloporphyrin redox reactions in solution have shown that ring-centered redox processes are generally faster than metal-centered processes.27

Conclusions The studies reported herein demonstrate that the electrontransfer rates for redox processes of molecules in monolayers (25) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978, Vol. III; pp 1-165. (26) Loew, G. H. In Iron Porphyrins; Lever, A. P. B; Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; Part I; pp 1-87. (27) Simonneaux, G.; Bondon, A. Chem. ReV. 2005, 105, 2627–2646.

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can differ substantially depending on the location of the redox center with respect to the linker-attachment site. In the case of metalloporphyrins, localized metal-centered redox events exhibit slower electron-transfer rates than delocalized ring-centered redox events wherein the linker-attachment site includes a position of substantial electron density in the redox-active molecular orbital. These redox characteristics have implications not only in the area of molecular-based memory materials based on charge storage, but also in the area of heme-protein redox chemistry. In the former area, the results suggest that molecular architectures that undergo ring-centered redox events should be superior to those that undergo metal-centered redox events because fast electron-transfer rates are generally more desirable. In the latter area, the results indicate that an Fe center in a cytochrome undergoing reduction cannot be appropriately described as having changed redox state until the tunneling electron reaches the metal as opposed to the “edge” of a cytochrome. Collectively, the studies reported herein demonstrate the versatility of synthetic design in manipulating the electron-transfer rates for molecules in monolayers. In particular, the utilization of a metal- versus ring-centered redox process could be used in concert with other strategies, such as varying the length and/or the electronic structure of the linker, to fine-tune electron-transfer rates to desired values. Acknowledgment. This work was supported by the Center for Nanoscience Innovation for Defense and DMEA (H9400307-2-0708) and by ZettaCore, Inc. LA8019843