Varying the Electrochemical Potential and Thickness of Porphyrazine

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Varying the Electrochemical Potential and Thickness of Porphyrazine SAMs by Molecular Design Hong Zong,† Peng Sun,† Chad A. Mirkin,*,† Anthony G. M. Barrett,‡ and Brian M. Hoffman*,† Department of Chemistry and International Institute for Nanotechnology, Northwestern UniVersity, EVanston, Illinois 60208, and Imperial College of Science, Technology and Medicine, London, U.K. SW7 2AY ReceiVed: June 18, 2009; ReVised Manuscript ReceiVed: October 2, 2009

A series of multithiol-functionalized free-base and Zn-coordinated porphyrazines (pz’s) have been prepared and characterized as self-assembled monolayers (SAMs) on Au. The synthetic flexibility of the pz’s provides a unique opportunity to tune their electronic and chemical characteristics and to control the distance of the redox-active pz macrocycle from the Au surface. This allows us to study the reduction potentials of these surface-bound pz’s as a function of film thickness and molecular charge distribution using angle-resolved X-ray photoelectron spectroscopy and cyclic voltammetry. Upon SAM formation, the reduction potentials of all pz’s show a significant positive shift from their formal potentials when free in solution (up to ∼ +1 V), with the magnitude of the shift inversely related to the Au-pz distance as determined from the film thickness of the pz SAM (thicknesses ranging from 3.5 to 11.8 Å). When the pz lies down on the surface, in a SAM of thickness ∼3.5 Å, the charge distribution within a pz macrocycle also plays a role in determining the potential shift. These observations are consistent with our originally proposed mechanism for potential shifts upon binding to a metal surface based on image charge effects and with the analysis of Liu and Newton (J. Phys. Chem. 1994, 98, 7162). Introduction The synthesis and fabrication of nanoscale and molecular electronic devices have been extensively investigated in recent years.2-5 Major efforts in this field have been made to synthesize functional materials that might lead to the components of molecular and nanoscale electronic and photonic devices,6-9 as well as to develop techniques that allow these molecular building blocks to be assembled into higher-ordered functional structures.10,11 Selforganized films illustrate how the combination of molecular design with surface adsorption and subsequent adsorbate assembly can create sophisticated structures with unusual electron-transfer properties.12,13 However, it is still very difficult to accurately predict the surface properties for a given molecular building block.14 Further progress not only calls for detailed knowledge of how the functional properties of films depend on molecular features but also demands a detailed understanding of the role of molecular orientation, molecule-electrode distance, as well as molecular charge distribution in determining the electron transport properties of electrode-adsorbed monolayers. We recently reported the synthesis of three novel porphyrazines (pz’s) 1, 2, and 3, which were designed to “stand up”, “crouch”, or “lie down” when forming self-assembled monolayers (SAMs) on a Au surface (Scheme 1).15 Such variability in structure provides a unique opportunity to probe the role of the film thickness and molecular orientation in determining the electron transport properties of such structures. In contrast with previously studied systems, it was found that the reduction potential of the surface-bound pz macrocycle shows very large shifts, approaching ∆E ∼ 1 V for the lying-down pz, from the formal potential of the molecule when free in solution, with * Corresponding authors. E-mails: [email protected]; chadnano@ northwestern.edu. † Northwestern University. ‡ Imperial College of Science, Technology and Medicine.

the magnitude of the shift dependent on the film thickness of the pz SAMs. This study indicated that the reduction potential shift is not a discontinuous function of film thickness, as would be the case if covalent interaction of the pz core with the gold surface was controlling. We originally proposed, and supported by simple model calculations, that image charge interactions were responsible for the large potential shifts.15,16 This idea is consistent with the assumptions that underlie early treatments of heterogeneous electron transfer based on a “two-zone” (aqueous solution/metal electrode) model.17 More recently, Liu and Newton proposed a more complicated continuum dielectric three-zone model (aqueous solution/dielectric film/metal electrode) that includes the image charge in the metal when computing the solvent reorganization energy for film-modified electrodes, such as our pz SAM system.1 They found that for a three-zone interface, image-charge stabilization varies inversely with the distance between electrode and redox agent, but with a magnitude that depends on the dielectric constants of the different zones. Liu and Newton applied their model to the variation of solvent reorganization energy with film thickness for surface-attached ferrocenyl derivatives with pendant alkanethiol groups that are adsorbed to a gold electrode, one of the most extensively studied surface redox systems.18-21 However, this system has limitations. It is known that the structural order of alkanethiol SAMs decreases with decreasing monolayer thickness.22 Detailed calculations suggest that the solvent reorganization energy may be expected to vary by e0.1 eV over the range of ferrocenylterminated alkanethiol monolayers currently accessible to experiment (film thickness >15 Å),18,23-30 and this predicted variation may be just at the edge of present experimental sensitivity. Indeed, a system is required that can probe at a much shorter range (4 equiv) in water was added to a solution of 21 (80 mg, 0.058 mmol) in THF (10 mL). The solution was stirred at room temperature for 5 days. The aqueous layer containing the lithium salts of 22 was washed with CH2Cl2 and acidified with dilute HCl to precipitate the carboxylic acid derivative 22. The solid was collected by centrifugation and dried under vacuum to yield 22 in nearly quantitative yield (65 mg, 92%): ESI-MS m/z 1219.6 (M + H+) calcd for C60H67N8O12S4 1219.4. H2Pz[A2 ) ((SC4O2NHC2S2PhC)2)2; C2] (9). Macrocycle 22 (35 mg, 0.029 mmol) and N-hydroxysuccinimide (NHS) (66 mg, 0.57 mmol) were dissolved in 5 mL of THF and cooled in an ice bath. N,N′-Dicyclohexylcarbodiimide (DCC) (119 mg, 0.57 mmol) in 5 mL of THF was added with stirring. After 15 min, the mixture was taken out of the ice bath and stirred at room temperature overnight. The mixture was concentrated to dryness and dissolved in CH2Cl2. The mixture was filtered, and the solid was washed with CH2Cl2. The filtrate and washings were combined, evaporated to dryness under reduced pressure, and dissolved in 5 mL of THF. 2-[(4-Methylphenyl)dithio]ethylamine 13 (114 mg, 0.57 mmol), dissolved in 5 mL of THF, was added, and the mixture was stirred at room temperature overnight. The mixture was concentrated and purified by column chromatography on silica gel (eluent AcOEt/CH2Cl2 ) 5/95, then CH3OH/CH2Cl2 ) 1.25/98.75) to give 9 as a green solid (13 mg, 23%): 1H NMR (500 MHz, CDCl3) δ 1.68 (d, J ) 6.5 Hz, 24H), 1.95 (tt, J ) 7.0 Hz, 8H), 2.20 (s, 12H), 2.54 (m, 16H), 3.28 (m, 8H), 4.01 (m, 8H), 6.09 (sept, J ) 6.0 Hz, 4H), 6.41 (t, J ) 5.5 Hz, 4H), 6.93 (d, J ) 8.0 Hz, 8H), 7.13 (d, J ) 8.0 Hz, 8H), 7.84 (m, 4H), 8.75 (m, 4H); MALDI-TOF-MS m/z 1943.6 (M + H+) calcd for C96H111N12O8S12 1943.5. H2Pz[A,A′ ) (SC4O2Pr)2, (SC4O2Pr)(SC4O2H); B2] (24). An amount of 30 mL of 0.4 M LiOH solution (aq) was added to a solution of 23 (260 mg, 0.20 mmol) in 20 mL THF. The mixed solution was stirred at room temperature for 1.5 h. The THF was removed by rotary evaporation, and 0.1 M HCl was added dropwise until the solution became acidic. The mixture was extracted with CH2Cl2. The organic phase was dried (Na2SO4) and rotary evaporated. The resulting residue was purified by column chromatography on silica gel (eluent AcOEt/ CH2Cl2 ) 5:95, then MeOH/CH2Cl2 ) 1/99) to give 24 as a green solid (50 mg, 20%). An amount of 145 mg of 23 original

Electrochemical Potential of Surface-Bound pz’s material was recovered from column chromatography: 1H NMR (400 MHz, CDCl3) δ -0.52 (br s, 2H), 0.73 (m, 9H), 1.40 (m, 6H), 1.79 (m, 24H), 1.98 (m, 8H), 2.57 (m, 8H), 3.83 (m, 6H), 4.21 (m, 8H), 5.26 (sept, J ) 5.6 Hz, 4H), 7.71 (d, J ) 5.6 Hz, 4H); APCI-MS m/z 1245.5 (M + H+) calcd for C61H81N8O12S4 1245.5. H2Pz[A,A′ ) (SC4O2Pr)2, (SC4O2Pr)(SC4O2NHC2S2PhC); B2] (10). Macrocycle 24 (33 mg, 0.027 mmol) and NHS (31 mg, 0.27 mmol) were dissolved in 5 mL of THF and cooled in an ice bath. DCC (55 mg, 0.27 mmol) in 5 mL of THF was added with stirring. After 15 min, the mixture was taken out of the ice bath and stirred at room temperature overnight. The mixture was concentrated to dryness and dissolved in CH2Cl2. The mixture was filtered, and the solid was washed with CH2Cl2. The filtrate and washings were combined, evaporated to dryness under reduced pressure, and dissolved in 5 mL of THF. 2-[(4Methylphenyl)dithio]-ethylamine 13 (53 mg, 0.27 mmol), dissolved in 5 mL of THF, was added, and the mixture was stirred at room temperature overnight. The mixture was concentrated and purified by column chromatography on silica gel (eluent AcOEt/CH2Cl2 ) 5/95). Subsequent TLC purification gave 10 as a green solid (20 mg, 53%): 1H NMR (500 MHz, CDCl3) δ -0.52 (br s, 2H), 0.75 (t, J ) 7.0 Hz, 9H), 1.44 (m, 6H), 1.81 (m, 24H), 2.01 (m, 8H), 2.20 (s, 3H), 2.53 (t, J ) 6.0 Hz, 2H), 2.58 (m, 8H), 3.26 (q, J ) 6.0 Hz, 2H), 3.85 (t, J ) 7.0 Hz, 6H), 4.04 (t, J ) 6.0 Hz, 2H), 4.22 (t, J ) 7.0 Hz, 4H), 4.28 (t, J ) 7.0 Hz, 2H), 5.30 (m, 4H), 6.34 (m, 1H), 6.93 (d, J ) 8.0 Hz, 2H), 7.13 (d, J ) 7.5 Hz, 2H), 7.57 (m, 4H); ESI-MS m/z 1426.7 (M + H+) calcd for C70H92N9O11S6 1426.5. Porphyrazine Hybrids A2A′2 and A3A′ H2Pz[((SC4O2Pr)2)n; ((SMe)2)4-n] (25, 26, and 27). Magnesium turnings (100 mg, 4.17 mmol) and I2 (0.01 g) were added to PrOH (100 mL), and the suspension was heated under reflux for 24 h under N2 to prepare Mg(OPr)2. MNT(Me)2 (14, 0.800 g, 4.70 mmol) and MNT(C4O2Me)2 (20, 0.805 g, 2.35 mmol) were added, and the suspension was heated under reflux for 10 h, during which time the solution turned purple-black. The solvent was removed under reduced pressure, and the residue was dissolved in a mixture of CH2Cl2 (125 mL) and MeOH (5 mL). Acetic acid (15 mL) was added, and the solution was stirred overnight at room temperature. The mixture was neutralized with NH4OH and washed with water. The organic layers were dried over Na2SO4 and concentrated. The resulting residue was chromatographed on silica gel (eluent CH2Cl2, then AcOEt/CH2Cl2 ) 2/98) to yield 25-27. Each compound was further purified using the conditions below. trans- and cis-H2Pz[((SC4O2Pr)2)2((SMe)2)2]. Chromatography over silica gel (eluent AcOEt/CH2Cl2 ) 1.5/98.5) was performed to yield A2B2 25 and 26 (128 mg, 9.6%). Further purification by chromatography (eluent AcOEt/CH2Cl2 ) 1/99) gave trans-A2B2 25 (46 mg, 3.5%) as a purple solid: 1H NMR (400 MHz, CDCl3) δ 0.83 (t, J ) 7.6 Hz, 12H), 1.54 (m, 8H), 2.17 (m, 8H), 2.66 (t, J ) 7.2 Hz, 8H), 3.43 (s, 12H), 3.94 (t, J ) 6.8 Hz, 8H), 4.10 (t, J ) 6.8 Hz, 8H); MALDI-TOF-MS m/z 1140.8 (M + H+) calcd for C48H67N8O8S8 1140.6. H2Pz[((SC4O2Pr)2)3; ((SMe)2)1] (27). Chromatography over silica gel (eluent AcOEt/CH2Cl2 ) 2/98) was performed to yield purple solid 5 (17 mg, 3.2%): 1H NMR (400 MHz, CDCl3) δ 0.85 (m, 18H), 1.55 (m, 12H), 2.14 (m, 4H), 2.20 (m, 8H), 2.67 (m, 12H), 3.39 (s, 6H), 3.96 (m, 12H), 4.06 (t, J ) 7.2 Hz, 4H), 4.13 (q, J ) 7.2 Hz, 8H); MALDI-TOF-MS m/z 1368.9 (M + H+) calcd for C60H87N8O12S8 1368.9. An alternate procedure, which changes the ratio of MNT(Me)2 14 and

J. Phys. Chem. B, Vol. 113, No. 45, 2009 14895 MNT(C4O2Me)2 20 to 1:1, allowed us to achieve a higher yield of 27 (15%). Hydrolysis of Porphyrazies 25 and 27 to the Carboxylic Acid Derivatives 28 and 29. Excess LiOH (>4 equiv) in water was added to a solution of pz’s 25 or 27 in THF. The mixed solution was stirred at room temperature for 4-5 days. The aqueous layer containing the lithium salts 28 and 29, respectively, was washed with CH2Cl2 and acidified with dilute HCl to precipitate the carboxylic acid derivatives 28 and 29. The precipitated products 28 and 29 were collected by filtration and washed with a small amount of water to remove traces of HCl. In each case, the hydrolysis proceeded in quantitative yield without further purification. trans-H2Pz[(SC4O2H)2; ((SMe)2)2] (28). MALDI-TOF-MS m/z 972.0 (M + H+) calcd for C36H43N8O8S8 972.3. H2Pz[(SC4O2H)3; ((SMe)2)1] (29). MALDI-TOF-MS m/z 1116.4 (M + H+) calcd for C42H51N8O12S8 1116.4. trans-H2Pz[((SC4O2NHC2S2PhC)2)2; ((SMe)2)2] (11). This compound was synthesized from 28 by a method similar to the one described for 9. 11 as a purple solid (19% yield): 1H NMR (500 MHz, CDCl3) δ -2.08 (br s, 2H), 2.05 (s, 12H), 2.17 (m, 8H), 2.66 (t, J ) 7.0 Hz, 8H), 3.43 (s, 12H), 3.94 (t, J ) 6.5 Hz, 8H), 4.12 (m, 16H), 7.54 (m, 8H), 7.71 (m, 8H); MALDITOF-MS m/z 1697.6 (M + H+) calcd for C72H87N12O4S16 1697.5. H2Pz[((SC4O2NHC2S2PhC)2)3; ((SMe)2)1] (12). This compound was synthesized from 29 by a method similar to the one described for 9. 12 as a purple solid (10% yield): 1H NMR (500 MHz, CDCl3) δ -2.33 (br s, 2H), 1.94 (m, 8H), 2.19 (m, 12H), 2.25 (m, 10H), 2.48-2.62 (m, 16H), 2.70 (m, 8H), 3.33 (m, 8H), 3.42 (s, 6H), 3.46 (m, 4H), 3.91-4.13 (m, 12H), 6.43 (m, 6H), 6.84 (m, 6H), 6.98 (m, 12H), 7.20 (m, 6H); MALDI-TOFMS m/z 2204.3 (M + H+) calcd for C96H117N14O6S20 2204.3. Metalation of Porphyrazines. The zinc(II) complexes 32-35 were prepared readily by a reaction of the corresponding freebase pz’s 16, 23, 30, and 31 with 10 equiv of zinc chloride in THF at 70 °C. The metalation process was monitored by UV-vis spectroscopy and stopped when the optical spectrum showed the completion of the reaction (24-36 h). ZnPz[((SC4O2Pr)2)3; ((SMe)2)1] (32). Chromatography was performed over silica gel (eluent AcOEt/CH2Cl2 ) 5/95) to yield blue solid 32 (51% yield): 1H NMR (500 MHz, CDCl3) δ 0.81 (t, J ) 7.0 Hz, 6H), 1.52 (m, 4H), 2.02 (m, 4H), 2.56 (t, J ) 7.5 Hz, 4H), 3.05 (s, 6H), 3.08 (s, 6H), 3.20 (s, 6H), 3.86 (t, J ) 7.0 Hz, 4H), 3.88 (m, 4H); MALDI-TOF-MS m/z 975.8 (M + H+) calcd for C36H45N8O4S8Zn 975.7. ZnPz[((SC4O2Pr)2)2; B2] (33). Chromatography was performed over silica gel (eluent AcOEt/CH2Cl2 ) 5/95) to yield green solid 33 (42% yield): 1H NMR (500 MHz, CDCl3) δ 0.69 (t, J ) 7.5 Hz, 12H), 1.35 (m, 8H), 1.84 (d, J ) 6.0 Hz, 24H), 1.98 (m, 8H), 2.51 (t, J ) 7.0 Hz, 8H), 3.60 (t, J ) 6.5 Hz, 8H), 4.14 (t, J ) 6.5 Hz, 8H), 5.34 (sept, J ) 6.0 Hz, 4H), 7.54 (s, 4H); MALDI-TOF-MS m/z 1352.2 (M + H+) calcd for C64H85N8O12S4Zn 1352.0. ZnPz[((SC4O2Pr)2)3; B1] (34). Chromatography was performed over silica gel (eluent AcOEt/CH2Cl2 ) 5/95) to yield green solid 34 (77% yield): 1H NMR (500 MHz, CDCl3) δ 0.68 (t, J ) 7.5 Hz, 6H), 0.81 (t, J ) 7.5 Hz, 12H), 1.33 (m, 4H), 1.51 (m, 8H), 1.79 (d, J ) 6.5 Hz, 12H), 1.95 (m, 4H), 2.14 (m, 8H), 2.49 (t, J ) 7.0 Hz, 4H), 2.62 (m, 8H), 3.52 (m, 4H), 3.78 (t, J ) 6.0 Hz, 8H), 4.07 (m, 8H), 4.17 (t, J ) 7.0 Hz, 4H), 5.29 (sept, J ) 6.0 Hz, 2H), 7.57 (s, 2H); MALDI-TOFMS m/z 1506.3 (M + H+) calcd for C68H95N8O14S6Zn 1506.3.

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ZnPz[((SC11OH)2)1; ((SMe)2)3] (35). Chromatography was performed over silica gel (eluent AcOEt/CH2Cl2 ) 5/95) to yield green solid 35 (63% yield): 1H NMR (500 MHz, CDCl3) δ 0.89-1.94 (m, 20H), 1.07 (m, 4H), 1.60 (m, 12H), 2.86 (m, 4H), 3.17 (s, 6H), 3.20 (s, 6H), 3.26 (s, 6H), 3.57 (m, 4H); MALDI-TOF-MS m/z 902.1 (M - (CH2)10OH + H+) calcd for C34H44N8OS8Zn 902.6. Surface Chemistry. Gold substrates (50 nm gold on a precleaned silicon wafer with 6-10 nm titanium adhesion layer) were prepared according to literature methods.33,34 The freshly evaporated gold slides were used immediately for the preparation of pz SAMs. All glass vials used for incubating gold substrates were cleaned by immersion in a fresh piranha solution (3:1 of 98% H2SO4 and 30% H2O2) for 30 min and then rinsed thoroughly with water. Caution: piranha solution reacts Violently with organic solVents and must be handled with extreme care. Solution Electrochemistry. Cyclic voltammetry (CV) experiments were done at room temperature under nitrogen in dry, deaerated methylene chloride (HPLC grade) with 0.1 M n-Bu4NPF6 as the supporting electrolyte using a Cypress Systems 2000 electroanalytical system. A conventional threeelectrode system was used with a Pt disk working electrode, an Ag wire counter electrode, and an Ag/AgCl (3 M NaCl) electrode as reference. Measurements were calibrated by addition of ferrocene as an internal reference. All E1/2 values were calculated from (Epa + Epc)/2 at a scan rate of 110 mV s-1. Potentials were reported versus a ferrocenium/ferrocene (Fc+/ Fc) couple. Surface Electrochemistry. A fresh-prepared pz SAM on Au was used as the working electrode in a three-electrode electrochemistry cell (Bioanalytical System potentiostat, BAS 100B). Platinum wire was the counter electrode, and all potentials were referenced versus Ag/AgCl (3 M NaCl). Aqueous solutions of NaPF6 (0.25 M) and NaClO4 (0.5 M) were used as supporting electrolyte. All of the measurements were carried out at a scan rate of 100 mV/s, and the E1/2 values were calculated from (Epa + Epc)/2. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were conducted using an Omicrometer ESCA Probe equipped with monochromated Al KR radiation (hv ) 1486.6 eV) and a hemispherical analyzer in the Northwestern University Keck Interdisciplinary Surface Science Center. The positive charge resulting from X-ray radiation of the samples was neutralized by using an electron gun. All measurements were performed at a vacuum below 2 × 10-9 mBar. For single-sweep survey scans, an analyzer pass energy of 70 eV and steps of 500 meV were used, respectively. In the case of high-resolution spectra, 3-15 scans were averaged with an analyzer pass energy of 26 eV and a step of 40 meV. Adventitious C1s at 284.6 eV was used as a reference to calibrate all of the XPS spectra. The high-resolution spectra were subsequently analyzed using commercial software (XPSPeak 4.0), in which the spectra were fitted with a Gaussian-Lorentzian sum function after a Shirley background subtraction.35 In the case of the S2p region of the high-resolution spectra of the pz SAMs, curving-fitting was performed while maintaining a 1.18 eV energy difference and 1:2 area ratio between 2p1/2 and 2p3/2. For angle-resolved measurements, the X-ray photoelectron takeoff angle (the angle between the substrate parallel and the analyzer) was varied from 15 to 80°. The average thicknesses of pz SAMs were determined by measuring the attenuation of X-ray photoelectron from the substrate, i.e., Au4f, according to the following equation35

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ln(I/I0) ) -d/λ sin θ where I is the photoelectron intensity from the substrate covered by a thin film; I0 is the photoelectron intensity from a clear substrate; θ is the photoelectron takeoff angle; d is the thickness of the thin film; and λ is the attenuation length of photoelectrons escaping from the substrate. The value of 42 Å for λ is used here following the empirical equation36,37

λ (Å) ) 9.0 + 0.022 × KE (eV) where KE is kinetic energy. Results and Discussion Porphyrazine Synthesis. To prepare pz’s 1-3 and 9-11 for surface attachment, 2-[(4-methylphenyl)dithio]-ethylamine 13 was coupled to corresponding pz precursors that have terminal carboxylic acids (Schemes 6-8). Disulfides are known to adsorb onto Au surface to form SAMs.38 The A4 pz 16 with two of the longer undecanol “arms” was synthesized by cyclization of the corresponding MNTs, then tosylated to generate pz 17. This was treated with thiocarbamide followed by the reduction with sodium metabisulfite to give the desired A4 pz 7 with undecanethiol groups after silica-gel column chromatography (Scheme 5). SAM Preparation. Au substrates were prepared according to literature methods.33,34 In a typical experiment, freshly prepared Au substrates were immersed for 24 h at room temperature in a 1 mM dichloromethane solution of the adsorbate molecule of interest. The substrates were removed from solution, vigorously rinsed with dichloromethane and ethanol, and blown dry with prepurified grade N2 prior to use. Zn-pz SAMs were prepared from the corresponding free-base pz SAMs by treating them in situ with ZnCl2 aqueous solution (∼2 mM in H2O, 8 h immersion) and subsequent washing with H2O. Complete reaction was confirmed by measuring the stoichiometric ratio between the nitrogen and zinc calculated based upon a qualitative analysis of the XPS experiment (see below). XPS Studies of Porphyrazine SAMs. XPS was used to characterize the pz SAMs formed by solution deposition on gold substrates. The XP spectra for all freebase and Zn pz SAMs share the following common features. First, the signature signals of key elements, including gold (Au4f), sulfur (S2p), carbon (C1s), nitrogen (N1s), and oxygen (O1s), appear in survey scans (Figure 1). Second, the S2p region of the high-resolution XP spectra of all pz SAMs indicates the existence of two types of sulfur species (Figure 1, inset B). The doublet centered at 161.9(1) eV is generally consistent with the analogous data for alkanethiols and thiol-modified porphyrin monolayers chemisorbed on Au(111),37,39-43 which indicates that the pz molecules bind to Au through sulfur. On the other hand, the doublet at 163.3(1) eV has a value very similar to those of bulk samples (identical within experimental error).15 This signal can be assigned to the sulfur atoms attached to the pz macrocycles. Third, the N1s signal appears as a broadband centered at 399 eV (data not shown). These results are generally consistent with the assignments by others for surface-bound porphyrins and pz’s reported previously.44 In the case of Zn-pz SAMs, the high-resolution Zn2p3/2 signal was observed at 1021.5(1) eV (Figure 1, inset A). The stoichiometric ratio between zinc and nitrogen is calculated through a qualitative analysis of the spectra, in which the signal

Electrochemical Potential of Surface-Bound pz’s

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intensities of the elements are normalized using known atomic cross-section sensitivity factors (Table 1). The calculated atomic ratio of N/Zn is very close to that expected for the Zn-pz’s; the uncertainties of less than 10% likely are due to the effects of signal attenuation. We conclude that the coordination of Zn ion in the pz SAMs is complete. To determine the orientation of the surface-bound pz’s in the SAMs, the average thickness of the SAMs was measured by quantitative analysis of the attenuation of the X-ray photoelectron from the Au substrate in angle-resolved X-ray photoelectron spectroscopy measurements.15,37 The film thicknesses for all of the pz SAMs are presented in Table 1. We previously showed that 3 lies flat, attributable to surface bonds from the transbisthiolate arms. The XPS measurements reported here show that pz 9 lies flat, as expected, but surprisingly so does “onelegged” 10. The measured film thicknesses for A2B2 SAM 3 and A2C2 SAM 9 are in the ∼3.5-3.9 Å range (Table 1, Figure 2) and match well with what one would expect for a planar aromatic lying at van der Waals distance from a metal surface. As expected, 7 stands up, but surprisingly, the SAMs that have A′ ) (SMe)2 peripheral substituent groups, 11 and 12, also all stand up on the Au surface, regardless of the number of attached potential surface linkers. The SAMs from 1, 11, and 12 are comparable in film thickness, 8.5-9.0 Å; as expected, the SAM of 7, with its long (CH2)11SH linker, has the largest

film thickness, ∼11.8 Å (Table 1, Figure 2). Although thicknesses derived for pz’s that do not lie down are not precisely defined, the long “arms” on the edge of a pz remote from the edge that binds to Au are expected to show disorder, so we may take the thicknesses to represent the distance from the Au surface to the remote edge of the pz core (Figure 2). This finding shows that the availability of multiple surface attachment groups does not necessarily mean that all of these groups will bind to the surface upon SAM formation, in good agreement with a report on porphyrins.40 It also leads to the question, why does pz 3 lie down on the Au surface when 2, 11, and 12 do not? A comparison of two “four-legged” pz’s 3 and 11 reveals that the two B ) fused benzo groups on 3 account for the difference. This conclusion is supported by the fact that the “one-legged” pz 10, which has two benzo groups like 3 but only one surface linker also lies down, resulting in films of comparable thickness (3.7-3.9 Å). It is obvious that the number and placement of the fused peripheral moieties that are not involved in bonding to Au play a large role in determining the pz SAM orientation. We propose these pz’s lie down to maximize the van der Waals and/or electronic interactions between the π-systems of the two B ) benzo moieties fused to the pz macrocycle. Such interactions also can explain why pz 2 takes an intermediate structure “crouching” on the surface. With only one B ) benzo group,

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the interaction is not as strong as with 3 but greater than for the A4 type pz 1. Thus, these measurements suggest that the form of a SAM generated from these pz’s is determined by a complex interplay of intra- and intermolecular forces. Electrochemistry. The redox chemistry of a pz macrocycle is primarily determined by the peripheral substitution pattern: the number of A ) (S-R)2 substituents, independent of R; the number of B ) fused diisopropoxy benzo moieties or fused C ) diisopropoxy-naphtho moieties. The SAMs studied here then can be categorized in subclasses, as A4, A3B, A2B2, and A2C2. As references for the surface measurement, cyclic voltammetric measurements were carried out on corresponding pz’s in

solution: 16 (A4), 23 (A2B2), and 30-35 (AnB4-n, n ) 2-4), and 21 (A2C2); representative cyclic voltammograms for AnB4-n, n ) 2-4, pz’s dissolved in CH2Cl2 are shown in Figure 3. All pz’s show two sequential reversible one-electron ring reductions, pz/pz- and pz-/pz2-, as well as a single reversible one-electron ring oxidation with the exception of A2B2, which exhibited two sequential one-electron oxidations, pz/pz+ and pz+/pz2+. The potentials for members of each pz subclass, collected in Table 2, are comparable to those reported earlier.45 The A2C2 pz 21 behaves similarly, and its redox potentials also are in Table 2. Pz SAMs immersed in an aqueous medium and subjected to cyclic voltammetry exhibit robust and reversible electrochemical



SCHEME 8

behavior over multiple cycles (Figure 4). Despite the different orientations and film thicknesses adopted by the several pz’s in the SAMs they form when adsorbed on a gold surface, the CV measurements for all the pz monolayers show a single reversible wave, corresponding to a one-electron ring reduction (pz/pz-). The separation between reduction and oxidation peaks is ∼30-50 mV, and all of the CVs are symmetric with ∼90 mV full width at half-maximum (fwhm) values. The shapes of the CVs for all of the pz’s are independent of the scan rate over the 25-1000 mV s-1 range, while the peak current scales linearly with scan rate. These results further confirm that the pz’s are chemically bound to the underlying Au substrate and

Figure 1. Representative XPS survey spectrum of a Zn-coordinated pz SAM on Au indicating the signature signals of key elements, including Au4f, S2p, C1s, N1s, O1s, and Zn2p. Inset: (A) A highresolution XP spectrum of the Zn2p region. (B) A high-resolution XP spectrum of the S2p region with curve fit, indicating the existence of two types of sulfur species.

are generally consistent with those for other surface-adsorbed systems. The first reduction potential values (pz/pz-) obtained for pz’s in solution and as a SAM on a gold surface are included in Table 2. Upon SAM formation, this couple shows a large shift in potential for all pz’s. The fourth column of Table 2 summarizes the shifts (∆E) between surface and solution potentials. Figure 5 presents the measured potential shift upon SAM formation, ∆E, as a function of film thickness (L) for both freebase and Zn AnB4-n, n ) 2-4, 4-6 and 8 pz SAMs. The data reveal the following behavior of the pz SAMs: (i) the potential shifts decrease strongly with increasing film thickness. (ii) As the film thickness increases, ∆E decreases and appears to level off at a film thickness of ∼8 Å. We emphasize that the flattening shown in Figure 5 is independent of either the presence or absence of metal ion (freebase vs Zn pz’s) or the nature of the linkage C4O2NHC2S vs (CH2)11SH. (iii) This finding is in agreement with the shift predicted by a simple image charge model: as the film thickness increases, the image charge effect becomes insignificant,1 with ∆E ∝ 1/L (Figure 5, inset). (iv) ∆E is different for M ) “2H” and M ) Zn, but the dependence of shift on thickness is the same. (v) The reduction potential shift does not change with the number of the surface attachment groups. The comparison between 3 and 10 shows that attachment via four thiols does not alter the reduction potential shift from the single-thiol attached species 10, within experimental error. (vi) Although the magnitudes of the shifts vary with film thickness, L, it is also the case that the thicknesses, and thus the values for ∆E, fall into three subranges, with each one corresponding to a pz subclass, AnB4-n, n ) 2, 3, 4. We argue in the discussion below that L is the controlling parameter and that the correspondence between ∆E and pz core structure (n) is largely incidental. Indeed, we argue that differences in ∆E associated with the differences in macrocycle core structure not only are of secondary importance but also actually tend to minimize, not enhance, the effects of varying film thickness. We considered the possibility that this excellent correlation between ∆E and L may be coincidental and that the surface

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TABLE 1: Binding Energies (eV),a Atomic Ratio for Signature Elements, and Average Film Thickness (Å) of the pz SAMs S2p3/2b pz SAM lying down

crouching standing up

A 2B 2 A 2B 2 A2C2 A 2B 2 A 3B A 3B A4 A4 A4 A4 A4 A4

Au-S 3 6 (Zn) 9 10 2 5 (Zn) 1 4 (Zn) 7 8 (Zn) 11 12

c

161.9 161.9 161.9 161.9 161.9c 161.9 161.9c 161.9 161.9 161.9 161.9 161.9

Zn2p3/2b

R-S

1021.5 1021.4 1021.5 1021.5 -

c

163.3 163.3 163.3 163.3 163.3c 163.3 163.3c 163.3 163.3 163.3 163.3 163.3

a All binding energies are referred to the adventitious C1s at 298.4 eV. values from molecular structures.

Figure 2. Schematic showing orientations of pz molecules on Au surfaces.

reduction potential might be primarily varying with differences in the electrochemical accessibilities of the pz macrocycles in the different SAMs toward the charge-compensating cations in the aqueous environment.20,46 SAM formation in general partially shields the redox centers from solvent/counterions. In the case of “lying-down” pz SAMs, the pz macrocycles lie “flat” on the surface, which makes it easy for counterions in the solution phase to solvate them and therefore favors the pz reduction. On the other hand, for “standing-up” pz SAMs, the pz macrocycles might be packed tightly enough to inhibit the penetration of ions to enter the SAMs. In such a case, this would make it more difficult to reduce the pz. “Crouching” pz’s also adopt an intermediate orientation in within a SAM, but due to the bulky structure of the molecule, the pz macrocycles likely do not pack as closely as standing-up pz’s do, which would allow better ion penetration and therefore make it easier for the crouching pz SAMs to be reduced than the standing-up pz’s, yet still more difficult to reduce than the freely accessible lyingdown pz’s. To test for possible effects of counterion screening on the redox potential shift of a SAM, as suggested previously,46,47 control experiments were conducted in which electroactive molecules were added to the electrolyte solution to probe counterion accessibility to the gold electrodes bearing the pz

b

N/Zn 11.8 14.6 9.1 7.8 -

average film thickness 12d 14d 10d 8d -

Standard deviation: ( 0.1 eV.

3.9(2)c 3.7(3) 3.5(2) 3.7(2) 6.7(3)c 6.9(3) 8.9(6)c 9.1(4) 11.8(5) 12.0(4) 8.5(5) 9.0(4) c

Data from ref 16.

d

Estimated

Figure 3. Cyclic voltammograms for A4 (31), A3B (30), and A2B2 (23) pz’s in CH2Cl2 solutions with 0.1 M TBAPF6 supporting electrolyte, referenced versus Fc+/Fc; scan rate ) 110 mV s-1.

SAMs in aqueous media. K4Fe(CN)6 was used as the electroactive probe with NaClO4 as the supporting electrolyte. This probe gives a strong reduction wave around 240 mV on a bare gold electrode, corresponding to the [Fe(CN)6]3-/[Fe(CN)6]4redox couple, but there was no detectable signal when any of the pz SAMs covered the gold electrode, as shown for the SAM prepared from 1-3 (Figure 6A and C). This result strongly suggests that the pz’s form highly organized films when in contact with an aqueous solution and as a result prevent the ferrocyanide probes from accessing the gold electrode. This therefore rules out the possible contribution of counterion screening to the reduction potential shift observed. Interestingly, when the pz SAMs were immersed in a THF solution with ferrocene as the molecular probe and TBAPF6 as supporting electrolyte, a strong ferrocene one-electron oxidation reduction wave appears around 600 mV, which is at almost the same voltage observed when a bare gold electrode is used (Figure 6B and D). This result is generally consistent with our previous observations,46,47 indicating that when the SAM is exposed to an organic solvent the pz molecules cannot block the approach of the ferrocene probes to the gold electrode, presumably because the pz molecules are solvated in THF and do not pack tightly enough to block the electrode. Perhaps most significantly, when the same pz SAM is put back into the



TABLE 2: First Reduction Potentials (pz/pz-, in volts vs Fc+/Fc) and Average Film Thickness (Å) of the pz SAMs pz type lying down

crouching standing up

a

A 2B 2 A 2B 2 A 2C 2 A 2B 2 A 3B A 3B A4 A4 A4 A4 A4 A4

solution 23 33 21 23 30 34 31 35 31 35 16 32

SAM

Esa (V)

Egb (V)

3 6 (Zn) 9 10 2 5 (Zn) 1 4 (Zn) 7 8 (Zn) 11 12

c

c

–1.060 –1.235 –0.977 –1.060 –1.008c –1.172 –0.957c –1.121 –0.957 –1.121 –0.949 –0.943

–0.258 –0.294 –0.308 –0.256 –0.371c 0.415 –0.495c –0.506 –0.501 –0.500 –0.516 –0.494

∆E (V) c

0.802 0.941 0.669 0.804 0.637c 0.757 0.462c 0.615 0.456 0.621 0.433 0.449

average film thickness 3.9(2)c 3.7(3) 3.5(2) 3.7(2) 6.7(3)c 6.9(3) 8.9(6)c 9.1(4) 11.8(5) 12.0(4) 8.5(5) 9.0(4)

Es, reduction potential for pz in solution (CH2Cl2). b Eg, reduction potential for pz as a SAM on a gold surface. c Data from ref 16.

Figure 4. Cyclic voltammograms for surface-bound pz’s 1, 2, 3, 11, and 12 in H2O solutions with 0.25 M NaPF6 and 0.5 M NaClO4 supporting electrolyte, referenced versus Fc+/Fc; scan rate ) 100 mV s-1.

Figure 5. Shift of reduction potential of AnB4-n, n ) 2-4, pz’s upon surface adsorption plotted versus average thickness of pz film (Table 2): line to guide the eye.

aqueous solution after the measurement in THF, it again acts as a perfect blocking film, showing a CV response close to Figure 6C. This dependence of the results on solvent is consistent with what has been observed with related ferrocenyl azobenzenealkanethiol SAMs.46-48 ∆E also was measured for reduction of the A2C2 SAM 9. The shift in potential upon forming a SAM with 9, ∆E ) 0.669

V, is 0.133 V smaller than that for the A2B2 3 (0.802 V). As discussed below, this can be attributed to greater charge delocalization in the anion of 9. Mechanism. The strong decrease in ∆E with increasing film thickness, followed by an apparent saturation at long distances, is precisely the behavior expected for the stabilization of the pz anion by its image charge in the Au electrode. This is shown by the inset to Figure 5, which presents the ∆E values as a function of 1/L for both freebase and Zn pz SAMs. The results support the intuitive physical picture that a charge trapped on the molecule is strongly attracted to its image in the electrode, leading to a localization of the charge close to the electrode. Marcus first suggested that for systems involving a metal electrode and a dissolved redox species image-charge effects would stabilize the charged form of this species, with the stabilization energy decreasing as its distance from the electrode increases.17 Liu and Newton extended Marcus’ treatment by introducing a “three-zone” model, with a metal electrode (III), a dielectric film (II), and a redox agent dissolved in solvent (I), as illustrated in Figure 7.1 They showed that when the redox agent is near the film the image charge contributes a charge stabilization energy with the simple form for a charge in a vacuum that is near a metal surface, ∆E ∝ (∆q)2/L, with a linear dependence of the charge stabilization energy on 1/L. However, the slope is a function of the dielectric constants of zones I and II. Such a model should apply rather well to the lying-down pz’s but be less accurate with the other SAMs, where the redoxactive pz is embedded in the film (zone II) rather than immersed in solvent (III). Nonetheless, it provides a compelling foundation both for our interpretation and for further theoretical analysis of the data presented here. We observe a bigger ∆E for Mpz’s with M ) Zn than for M ) 2H, which likely reflects differences in the charge distribution on the Mpz anion. Support for the idea that ∆E is influenced by the charge distribution, as expected for image-charge effects, is provided by comparing the results for lying-down pz’s, A2B2 3 and A2C2 9. The latter has two fused C ) naphtho groups, instead of the fused B ) benzo groups of 3, and was expected to have a smaller ∆E because the charge on the anion would be more distributed than for 3 (Figure 8). Because the van der Waals area of 3 (200 Å2)49 is not that much different from that of 9, it is expected that 3 and 9 will have essentially the same microenvironment of the SAM/electrolyte interface, so any differences in ∆E for these two pz’s can be attributed to differences in imaging charge energy. According to simple imaging theory and in keeping with the model of Liu and Newton, for these two lying-down pz’s with essentially the same

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Zong et al.

Figure 6. Use of electroactive molecules to probe counterion accessibility to the gold electrodes bearing the pz SAMs in aqueous and organic media. Representative cyclic voltammograms for (A) a bare gold electrode and (C) a pz SAM covered gold electrode (pz 1, 2, and 3), respectively, in aqueous solution with K4Fe(CN)6 as the electroactive probe and NaClO4 as the supporting electrolyte. Representative cyclic voltammograms for (B) a bare gold electrode and (D) a pz SAM covered gold electrode (pz 1, 2, and 3), respectively, in THF solution with ferrocene as the molecular probe and TBAPF6 as the supporting electrolyte.

Figure 7. Schematic representation of surface-bound pz’s in a threezone model of Liu and Newton (ref 17).

value of L, the delocalized charges of their anions lead to the prediction that ∆Ej ∝ ∑i∆qij2, where j ) 3, 9 and the sum runs over carbon/nitrogens bearing charge. Use of the Voronoi Deformation Density (VDD) method50 for computing atomic charges (data not shown) shows that 12% of the charge of the anion of 9 is distributed to the two naphthyl groups, while only 3% is distributed to the two benzo groups of 3. Thus, a smaller ∆E for 9 is expected to be smaller than that for 3, in keeping with the experimental ∆E ) 0.669 V for 9, which indeed is 0.133 V smaller than that for 3 (0.802 V). Said simply, the change in potential for 3 is greater than that for 9 because the charge on the anionic pz is less delocalized, exactly as expected from the image charge picture. An extension of this analysis of the difference in ∆E for lyingdown A2B2 and A2C2 pz’s shows the structure of the pz coresn

Figure 8. Comparison of the first reduction potentials (pz/pz-, in volts vs Fc+/Fc) for 3 and 9 in solution (CH2Cl2) and as a SAM on a gold surface (H2O).

for the AnBn pz’ssactually has only a second-order influence on the plot of ∆E versus L in Figure 5 and that L indeed is the variable of consequence, not n. Clearly, the farther away a pz is from the surface, the smaller will be the magnitude of the differences in ∆E caused by different charge distributions (different n). As a result, if one imagines correcting the plots in Figure 5 for differences in charge distribution, say by adjusting for a common A4 core (n ) 1), each point would be expected to shift upward by an amount that decreased as L increased. In short, the slope of the line from ∼3-10 Å would be expected to increase in magnitude, accentuating the distance dependence. An equivalent change in slope would occur if one considered correcting to a different common AnBn core (n ) 2

Electrochemical Potential of Surface-Bound pz’s or 3). Thus, the real influence of structure (n) is to slightly mask the dependence of ∆E on L. Conclusion The formation of characteristic surface structures is of great importance in building nanoscale and molecular electronic devices. Although there are many examples of macrocycles patterned on Au with different modes, to our knowledge the dependency of the orientation/proximity of the surface structure upon surface-bound molecules has not yet been clarified.40-42,51-53 For example, a study on porphyrin SAMs shows all molecules stand up on the surface via a single thiol regardless of the number of linkers.40 In this report, we have shown that study of pz SAMs selfassembled on Au by cyclic votammetry/X-ray photoelectron spectroscopy allows us to systematically probe the electrochemical potentials of surface molecules as a function of their surface geometry. The reduction potential of a pz macrocycle shifts significantly from its solution value upon attachment to a Au electrode, and the strong dependence of this shift on the pz SAM film thickness over a thickness range of 3.5-11.8 Å can be understood, in part, in terms of image-charge effects. Extension of the bottom-up strategy for designing thin redox films through the geometrical control demonstrated here is expected to provide novel means for achieving functional specificity of such systems. Acknowledgment. This work was supported by the National Science Foundation through a Nanoscale Science and Engineering Center (NSEC) and through grant CHE 0500796 (BMH). References and Notes (1) Liu, Y. P.; Newton, M. D. J. Phys. Chem. 1994, 98, 7162. (2) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (3) Mantooth, B. A.; Weiss, P. S. Proc. IEEE 2003, 91, 1785. (4) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (5) Heath, J. R.; Ratner, M. A. Phys. Today 2003, 56, 43. (6) Oh, M.; Mirkin, C. A. Nature 2005, 438, 651. (7) Jin, R. C.; Cao, Y. C.; Hao, E. C.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487. (8) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (9) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res. 2001, 34, 433. (10) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661. (11) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. ReV. 1999, 99, 1823. (12) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (13) Lehn, J. M. Science 2002, 295, 2400. (14) Yerushalmi, R.; Scherz, A.; van der Boom, M. E. J. Am. Chem. Soc. 2004, 126, 2700. (15) Sun, P.; Zong, H.; Salaita, K.; Ketter, J. B.; Barrett, A. G. M.; Hoffman, B. M.; Mirkin, C. A. J. Phys. Chem. B 2006, 110, 18151. (16) Vesper, B. J.; Salaita, K.; Zong, H.; Mirkin, C. A.; Barrett, A. G. M.; Hoffman, B. M. J. Am. Chem. Soc. 2004, 126, 16653. (17) Marcus, R. A. J. Chem. Phys. 1965, 43, 679.

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