Direct Electron Transfer of Thiol-Derivatized Tetraphenylporphyrin

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Direct Electron Transfer of Thiol-Derivatized Tetraphenylporphyrin Assembled on Gold Electrodes in an Aqueous Solution Qin Wang,† Fupeng Zhi,† Wenting Wang,† Xinghua Xia,‡ Xiuhui Liu,† Fanfu Meng,† Yanyan Song,‡ Chen Yang,‡ and Xiaoquan Lu*,† College of Chemistry and Chemical Engineering, Northwest Normal UniVersity, Lanzhou 730070, PR China, and the Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, PR China ReceiVed: April 29, 2008; ReVised Manuscript ReceiVed: April 3, 2009

Self-assembled monolayers (SAMs) of thiol-derivatized tetraphenylporphyrin (SH-TPP) and cobalt tetraphenylporphyrin (SH-CoTPP) have been prepared to examine the structure and direct electron transfer (ET) on a gold electrode in an aqueous solution. The cyclic voltammetry peaks of the porphyrin were obviously observed in aqueous solution. An interesting phenomenon was that direct ET cannot occur in phosphatebuffered solutions when a highly oriented and tightly packed monolayer is in existence, but the relatively low-density SH-TPP SAMs exhibited good electrochemical response. The electron transfer rate constants were obtained from the impedance spectra (Cole-Cole plot), upon application of different biasing potentials to the modified electrode. We propose a new kinetic model of the ET processes of SAMs of thiol-derivatized porphyrin and apply the density functional theory (DFT) to study the ET of porphyrin molecules on Au surfaces. Introduction Porphyrins and their derivatives have attracted considerable attention during the past decade because of their unique and stable physical and chemical properties, which derives from their conjugated π-electron system.1 Their structures are similar to those of many biological molecules, which play very important roles in various biological ET processes. Especially, their electrochemical and photophysical properties make these macrocyclic compounds good candidates to mimic biological processes. SAMs gain more and more interest because of their potential applications in sensor fabrication and optoelectronic devices and as active surfaces for patterning and chemical architecture of solid support.2 They offer a more simple, stable, versatile, and less time-consuming method than that of other methods and may lead to defect-free structures. Moreover, SAMs, containing redox moieties, are widely recognized as ideal models for the investigation of ET processes.3 A variety of porphyrins assembled on gold and other materials have been studied using different analysis methods, including ultraviolet-visible (UV-vis) absorption spectroscopy,4a cyclic voltammetry (CV),4b,c scanning electrochemical microscopy (SECM),4b,c surface plasmon spectroscopy,4d infrared (IR) spectroscopy,4d,e surface-enhanced Raman scattering (SERS),4f X-ray photoelectron spectroscopy (XPS),4a,g electrochemical impedance spectroscopy (EIS),4c,h the contact angle,4i atomic force microscopy (AFM),4j–l scanning tunneling microscopy (STM),4a,m,n and ellipsometry.4i,o There is great interest in the study of the ET of SAMs in electroanalytical chemistry.5 CV and EIS are powerful tools to probe the nature of the modified electrodes and ET kinetics. The theory of ET processes in * To whom correspondence should be addressed. E-mail: luxq@ nwnu.edu.cn. Telephone/Fax: +86-931-7971276. † Northwest Normal University. ‡ Nanjing University.

aqueous solutions was discussed.6 The kinetics of charge transfer at the electrochemical interface is strongly influenced by the nature of the electrode surface and the structure of the electrical double layer.7 Therefore, ET greatly depends on the structure of the porphyrin, the orientation of the porphyrins on the electrode surface, the environment surrounding of the porphyrin, and the distance between porphyrin units.8 The photoelectronchemical characterization of porphyrin SAMs was investigated.9,10 They are used in assemblies of electron donor-acceptor materials in molecular electronics and photovoltaic devices. Several metalloporphyrins with different metal atoms and carbon-based molecular systems such as porphyrin/C60 and porphyrin/single-walled nanotubes have been synthesized, and their interesting photoinduced ET processes and electrocatalysis have been extensively studied.11,12 As stated previously, there are many studies about porphyrins in organic solvent,13 and some water soluble porphyrins have been investigated in aqueous and nonaqueous media.14 However, to our knowledge, the direct ET between the electrode and thiol-derivatized porphyrin SAMs has not been investigated much in aqueous solution. The electrochemical and spectroelectrochemical behavior of the attachment of cobalt “picket fence” porphyrin [CoIIITpivPP]+ and disulfide-linked manganese halogenated tetraphenylporphyrin derivatives on a gold electrode were demonstrated in aqueous solution.8,15 In previous reports from this laboratory, we have mainly focused on the synthesis and kinetics of charge transfer, with a particular interest in porphyrins.4b,c,h,16 In this work, we focus on the direct ET between the Au electrode and porphyrin SAMs in aqueous solution. ET processes in aqueous systems are of interest not only from a physicochemical point of view but are of considerable importance also in biological oxidation systems. It is highly significant that the dynamic properties of porphyrins were studied in natural conditions. The redox currents of SH-TPP and SH-CoTPP can be observed clearly by CV. EIS was used to obtain the electron transfer rate between the electrode and porphyrin SAMs upon

10.1021/jp803725x CCC: $40.75  2009 American Chemical Society Published on Web 04/29/2009

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SCHEME 1: Molecular Structure of SH-TPP

equipped with a liquid nitrogen-cooled mercury-cadmiumtelluride (MCT) detector. Reflection FTIR spectra were collected in the wavenumber range of 650-3200 cm-1. Electrochemical Measurements and Computational Methods. EIS and CV were carried out on an Autolab electrochemical analyzer (Eco Chemie, Utrecht, The Netherlands) equipped with a PGSTAT30 potentiostat and FRA2 frequency analyzer. CV data were recorded in the analogue mode with a fast analogue scan generator. Electrochemical characterizations were performed with a three-electrode system, comprising a platinum wire as an auxiliary electrode, a KCl-saturated Ag/AgCl electrode as reference, and gold slide modified with porphyrins (or bare gold) as a working electrode. All potentials in this paper refer to the Ag/AgCl electrode. The Faradaic impedance spectra of the porphyrin-functionalized electrodes were recorded upon application of the bias potentials in the frequency range from 1 Hz to 100 kHz, using an alternating current (ac) voltage of 0.005 V amplitude. All of the measurements were carried out at ambient temperature (22 ( 2 °C). All of the calculations reported in this work were carried out using DFT with the hybrid Becke-Lee-Yang-Parr (B3LYP) functional. For all atoms except the central metal (Au), the 6-31G(d)36 basis set was used with the geometry optimizations and analytic calculations. For the central Au, the LanL2DZ basis set was used. The visual analysis of frontier molecular orbitals of the porphyrin molecule was made possible by using the GaussView 3.07 software.

orientation change modulated by an external electric field in an aqueous solution. The film was also charactered by UV-vis and Fourier transform infrared (FTIR) spectroscopy. The DFT has also been used to predict the structures and energetics of porphyrins. Experimental Section Reagents and Materials. Thiol-derivatized tetraphenylporphyrin (scheme 1) was synthesized and purified according to the same procedure previously described.4e,17 1-Octanethiol (98%, Alfa Aesar), sodium hydrogen phosphate, potassium dihydrogen phosphate, cobalt chloride, chloroform, and other reagents were of analytical grade. Buffered solutions of 0.067 M phosphate containing 0.1 M KCl were prepared using KH2PO4 and Na2HPO4. All solutions were prepared with deionized water (>18 MΩ, Purelab Classic Co.). High-purity nitrogen was used for drying the electrode and for aeration. Substrate and SAMs Preparation. A gold slide (Nanjing noble metal factory, 99.99%) was used as a working electrode. The Au electrode was polished with alumina powder (diameter, 1.0, 0.3, and 0.05 µm) and sonicated in pure water. The wellpolished Au electrode was then electrochemically cleaned by potential cycling in 0.5 M H2SO4 in the potential range of 0.2-1.9 V versus a reversible hydrogen electrode (RHE) until a typical CV of clean gold was obtained.18 The geometric area of the Au electrode was controlled by a Viton O-ring and was determined to be 0.17 cm2. The roughness factor of the Au electrode was measured as approximately 6.6.19 Substrates were rinsed with anhydrous ethanol, deionized water, and chloroform, and were then dried under a stream of N2 prior to SAMs preparation. Porphyrins were dissolved in chloroform (1 mM) and assembled on a precleaned gold electrode for at least 70 h at 4 °C. The vials containing the substrates were protected from light to prevent photodecomposition and oxidation of the porphyrins. The cobalt porphyrins were prepared using two methods unlike the methods described previously:4e (1) the free-base porphyrin monolayer-coated gold electrode was dipped in a 1 mM CoCl2 solution and (2) the insertion of a cobalt ion was performed by potential cycling in a 1 mM CoCl2 solution until the redox peaks of cobalt were obtained in CV. The freshly prepared samples were immediately used for electrochemical measurements. The electrode was washed with water and stored in a phosphatebuffered solution at 4 °C when not in use. Ultraviolet-Visible Absorption Spectroscopy and Fourier Transfer Infrared Spectroscopy. Gold-evaporated glass substrates used for UV-vis and FTIR measurements were commercially available. UV-vis absorption spectra were taken in absorption and reflection modes with a SHIMADZU UV-3600 UV-vis spectrophotometer. The spectra were recorded in the 300-700 nm region. Bare Au for background measurements was prepared in the same manner. The Fourier transform IR spectrometer was a Bruker Tensor 27 (Bruker, Germany)

Results and Discussion Spectroscopic Studies on Thiol-Derivatized Porphyrin SAMs on Gold. 1. UltraWiolet-Wisible Absorption Spectroscopy. Figure 1A showa a UV-vis spectrum of SH-TPP in a chloroform solution and the corresponding SAMs of SH-TPP on a Au electrode. The SH-TPP solution showed a strong Soret band at 419 nm and four weaker Q bands in the region between 500 and 700 nm, which are typical of a free base porphyrin. These bands arise from the π-π* transition of the macrocycle.20 The red shift or splitting of the Soret band has been reported as an interaction among porphyrins such as side-by-side or edge-toedge aggregation. When face-to-face aggregation occurs, a blue shift in the Soret band is observed.21 In the case of the porphyrin film, the Soret band appeared at 440nm. We have observed a tiny Soret band with broadening, which shifted toward a longer wavelength (red shift) in comparison with that of the solution profile. These facts suggest the formation of “head-to-tail” type aggregates.4d The red shift by 21 nm and broadening of the Soret band were due to exciton interaction between Soret band π-π* states of the porphyrin molecules.22 However, the absorption of the monolayer is small, and the Q bands were not observed in the monolayer spectrum because of the smaller SH-TPP amount in the film. 2. Infrared Reflection-Absorption Spectroscopy. FTIR spectroscopy has been used extensively to characterize organic mono- and multilayers on Au surfaces, usually in the reflectionabsorption mode, using thick Au film to obtain effective reflection. The FTIR spectra of porphyrin SAMs in the highwavenumber region is shown in Figure 1B. The SH-TPP monolayer have characteristic antisymmetric and symmetric stretching modes of CH2 at 2915 cm-1 and 2847 cm-1, respectively, which are indicative of adsorbed SH-TPP because these two bands are often observed in the SAMs of alkanethiol and aromatic thiols derivatives.23 The band at 3011 cm-1 is ascribed to the C-H stretching vibration of ring hydrogens. As previously reported, the bands at 3064 and 3031 cm-1 are

Direct ET of Thiol-Derivatized Tetraphenylporphyrin

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Figure 1. (A) UV-vis absorption spectra of SH-TPP in a chloroform solution and reflection spectra of corresponding SAMs (inset). (B) Reflection FTIR spectrum of the SH-TPP monolayer on Au.

Figure 2. (A) CVs of the porphyrin SAMs on a gold electrode in a phosphate-buffered solution (pH 7.4); the buffer solution is at a different scan rate. (Inset) CVs of bare Au (dotted line) and SH-TPP/Au (solid line) in a phosphate-buffered solution (pH 7.4) at 10 mV/s. (B) Plot of the peak currents as a function of the scan rate from the data in (A).

assigned to the CH stretching modes of aromatics (pyrrole and phenyl),4e but the band at 3064 cm-1 in Figure 1B was not very obvious. Besides the high-wavenumber band, the bands at 1261, 1174, 900, and 721 cm-1 were due to the CH in-plane and outof-plane of benzene ring and porphyrin, respectively.24 Electrochemical Characteristics of Porphyrin SelfAssembled Monolayers. Direct electrochemical communication from the porphyrin moiety and solid electrode surfaces can serve as models to comprehend the biological electron communication processes in biological systems. A phosphate-buffered solution is chosen to perform the electrochemical measurements because it is an inert, redox inactive, and physiologically relevant electrolyte solution. Figure 2A shows the CVs of a porphyrin SAMs (SH-TPP/Au) at a different scan rate in a phosphatebuffered solution. On a gold electrode, the redox processes of porphyrins can be observed, which are assigned to the porphyrin core.4b,25 Although the SAMs were prepared from a library of porphyrinic molecules that vary in the composition and length of the linker attached to the surface as well as the nature of the redox active moiety, one noteworthy characteristic of the redox kinetics exhibited by all the porphyrin SAMs is that the electron transfer rates are generally slower than those observed for other redox active species tethered with similar linkers or at similar distances from the Au surface.26–28 The CV of SH-TPP/Au is characterized by an anodic peak showing a well-defined current maximum, but the smaller corresponding cathodic wave on the reversed scan is due to the instability of the radical cation. At a scan rate of 100 mV/s, the cathodic and anodic peaks are

located at 0.64 and 0.78 V, respectively. Figure 2B shows the influence of the scan rate on the response of the porphyrin SAMs. The anodic and cathodic peak currents are found to be proportional to the scan rate, indicating that porphyrin is a surface-confined electroactive molecule. However, the poor quality redox wave was observed in aqueous solution, indicating that porphyrins cannot easily exchange electrons with the underlying Au electrode. One plausible explanation for the slower than expected electron transfer rates of the porphyrin SAMs is that the redox center is intrinsically shielded from the solvent and counterions (thereby affecting the reorganization energy) because of the “self-insulating” character imparted by the bulky, nonpolar, nonlinking substituent groups. When cobalt ions were inserted into the monolayer, some new peaks occurred in the CV. The ET reduction of metalloporphyrins can involve either the central metal or the porphyrin ligand, both of which are redox active.29 The CVs show the electrochemical responses of SH-CoTPP/Au in different solution systems (Figure 3). The redox wave was clearly visible in the phosphate-buffered solution, corresponding to the redox process of a cobalt ion occluded in the porphyrin ring. The redox process with an oxidation peak at 0.80 V and the corresponding reduction peak at 0.69 V can be observed for SH-CoTPP at 100 mV/s, which was also ascribed to the redox of porphyrin core (Figure 3A). According to the literature,4b,30 the two peaks at 0.57 and 0.44 V probably correspond to the redox reaction between Co(II) and Co(III) ions, respectively. The reduction wave of the cobalt porphyrin was clearer than that in the SH-

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TPP/SAMs in the phosphate-buffered solution. Furthermore, the characteristic response of SH-CoTPP was also detected at 0.75 V only when a scan rate of 10 mV/s was used (Figure 3B). Compared with Figure 3B, the redox peaks of the porphyrins shifted positively in CoCl2 (Figure 3C); however, the redox potentials of porphyrins obtained in K3Fe(CN)6 have no shift (Figure 3D). When the SH-TPP/Au electrode was stored in a phosphatebuffered solution for about two months at 4 °C, the CVs and EIS remained stable, suggesting electrochemical stability of the porphyrin-coated film. Electrochemical Impedance Spectroscopy Measurements. EIS is a powerful method for characterizing the electrical interfacial properties because of the precise surface sensitive analytical information that electrochemical methods provide and because of the small (∼10 mV) sinusoidal probe voltages used in EIS, which make it a less perturbing method than CV.31 It was applied in many sensing and biosensing systems as a transduction technique32 and in other fields.33 Commonly, EIS reported in the literature has been presented in the form of Nyquist plots or Cole-Cole plots.34 When applying impedance spectroscopy to a surface confined redox system, the results can be usually displayed on the Cole-Cole plots, particularly in the form of Cim versus Cre (where Cim and Cre are the imaginary and real parts of the interfacial complex capacitance, respectively).31 The equivalent circuit is shown in Figure 4A. It includes the ohmic resistance of the electrolyte solution, Rs, the double-layer capacitance, Cdl, the pseudocapacitance, Cpc, which corresponds to the electrochemical charging and discharging process of the surface-confined redox species,35,36 and the charge transfer resistance Rct. The imaginary and real parts of the capacitance can be described in terms of imaginary and real parts of the impedance Z in eqs 1 and 235

Wang et al.

Cim ) -Im[(jωZ)-1] -1

Cre ) Re[(jωZ) ]

(1) (2)

The focus of this work is the measurement of the rate constant of the ET process, which can be calculated by the frequency, f 0, corresponding to the maximum value of the imaginary capacitance, Cimmax, using eq 336

k0 ) πf 0

(3)

In our case, we used EIS technology to study the influence of the interfacial potential on the direct ET of porphyrin SAMs on gold in a phosphate-buffered solution. The environment is advantageous to pursuing biological or biomedical applications. Figure 4B shows the impedance spectra obtained upon application of different biasing potentials to the porphyrin-modified electrode in a frequency from 1 Hz to 100 kHz, where ET can occur. When Eapp (applied potential) > Eox (oxidation potential), the porphyrin molecule exists in an oxidatized state. Otherwise, when Eapp < Ered (reduction potential), the porphyrin molecule exists in the reduced state. The impedance spectra of SH-CoTPP SAMs upon application of different biasing potentials is shown in Figure 4C. The calculated k0 are listed in Table 1. However, cobalt ions were inserted to porphyrin rings, which resulted in small electron transfer rates. The result is primarily due to the larger degree of porphyrin orbital overlap with the metalcentered orbital or higher C-C, C-N, and N-M vibrational frequencies.37 Furthermore, the calculated results by DFT showed cobalt ions inserted to porphyrin rings make the density of the electron cloud around the porphyrin rings unsymmetrical, which induce smaller electron transfer rates than those of the porphyrin without cobalt ions. The difference of k0 at different biasing potentials could imply that the external electric field considerably induced the orientation of porphyrin SAMs on the gold electrode.38

Figure 3. CVs of the cobalt porphyrin SAMs on a gold electrode in different solutions. (A) Phosphate-buffered solution (pH 7.4) at 100 mV/s; porphyrin SAMs (dotted line) and cobalt porphyrin SAMs (solid line). (B) Phosphate-buffered solution (pH 7.4) at 10 mV/s. (C) 1.0 mM CoCl2 solution at 50 mV/s. (D) 1.0 mM K3Fe(CN)6 solution with 0.1 M KCl at 50 mV/s.



Figure 4. Impedance spectra corresponding to thiol-derivatived porphyrin SAMs on a gold disk electrode in the form of Cole-Cole plots obtained at different biasing potentials: (A) equivalent circuit for a Faradaic impedance spectrum, (B) free base porphyrin (SH-TPP) SAMs, and (C) cobalt porphyrin (SH-CoTPP) SAMs. All data were recorded in a 0.67 M phosphate-buffered solution at pH 7.4.

TABLE 1: Electron Transfer Rate Constants Derived from Impedance Spectra (Figure 4B,C) of SH-TPP and SH-CoTPP SAMs in a Phosphate-Buffered Solution (pH 7.4) SH-TPP SH-CoTPP a

Eapp (V) k0(s-1) Eapp (V) k0(s-1)

0.6 498 0.55 249

0.63 498 0.60 249

0.65 498 0.63 249

0.67 498 0.65 249

0.8 198 0.67 198

0.82 395 0.82

0.85 395 0.85

0.87 498 0.87

a

a

a

The electron transfer rate constant cannot be obtained.

Figure 5. (A) Influence of solution pH on the anodic peak current: (a) from pH 4.49 to 8.04 and (b) from pH 8.04 to 4.49. Data were obtained from the CVs of a SH-TPP/Au in a phosphate-buffered solution containing KCl (0.1 M) with a scan rate of 100 mVs-1. (B) Impedance spectra corresponding to the SH-TPP SAMs on a Au electrode in the form of Cole-Cole plots at a biasing potential of 0.55 V in a phosphate-buffered solution: (a) pH 4.49 and (b) pH 8.04. (Inset) CVs of SH-TPP/Au in a phosphate-buffered solution at 100 mV/s: (a) pH 4.49 and (b) pH 8.04.

For further insight into the ET of porphyrin SAMs, we characterized the function of the system at different pH values using CV and EIS. Figure 5A shows the influence of pH values on the anodic peak currents. First, we used different pH values from pH 4.49 to 8.04. The peak currents gradually decrease with pH values increasing. Contrarily, the peak currents increase again from pH 8.04 to 4.49. One can see that the electrochemical responses of the monolayers exposed to the acidic solution are clearer than those from the basic solution. The interfacial properties of the modified electrode were probed at pH 4.49 and 8.04 using EIS by applying a potential equal to 0.55V (Figure 5B). The

calculated values for the k0 in acidic and basic solutions are 314 and 249 s-1, respectively. The result indicated the ET is quicker in acidic conditions than in basic conditions. This is consistent with the fact that was observed in CV (inset of Figure 5B). It should be explained that pyrrole sites are protonated in acidic conditions and lose their π-donor properties, so relatively fast ET kinetics are observed. The direct ET of porphyrin SAMs with different ionic strengths was investigated by linear sweep voltammetry (LSP) (Figure 6). As the ionic strength increases, the redox potentials have a negative shift. As noted earlier, the dependence is due

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Figure 6. (A) Dependence of the redox potentials on ionic strength on the direct electron transfer of porphyrin in a phosphate-buffered solution containing KCl (pH 7.4). KCl concentration varied from 50 to 300 mM. (B) Plot of the anodic peak potential as a function of the concentration of KCl from the data in panel A.

Figure 7. (A) Faradaic impedance spectra (Nyquist plots) for the SH-TPP SAMs on a Au electrode in the presence of 1 mM K3[Fe(CN)6]/ K4[Fe(CN)6] and 0.1 M KCl as the supporting electrolyte at different surface coverage values (θ). The surface coverage values are gradually decreased from 1 to 6. The inset is the Randles and Ershler equivalent circuit. (B) CVs for the SH-TPP SAMs on a Au electrode in a solution of 1 mM K3[Fe(CN)6] containing 0.1 M KCl, corresponding to different surface coverage values (θ) at 50 mV/s. (C) CV for the SH-TPP SAMs on a Au electrode in a phosphate-buffered solution (pH 7.4) corresponding to different surface coverage values (θ) at 50 mV/s. Inset is bare Au in a phosphate-buffered solution at 50 mV/s.

TABLE 2: Charge Transfer Resistance (Rct) and Surface Coverage (θ) of Thiol-Porphyrin on the Surface of a Gold Electrode sample

1

2

3

4

5

6 (bare Au)

Rct(Ω) θ (%)

14880 98.7

3990 95.1

1511 87.1

511 61.7

280.6 30.3

195.5 s

to changes of the activity coefficients of the redox couple and reflects the charged environment of the redox species.39 Specifically, the effect of the surface coverage of the SAMs36 was studied by several electrochemical techniques. EIS is an

effective method for anticipating the coverage of the SAMs or pinholes on the Au electrode. The interfacial properties of the electrode modified with the porphyrin were probed at various surface coverages (θ) by EIS (Nyquist plots) in the presence of [Fe(CN)6]3-/4- as a redox label. The surface coverage can be derived from eq 45b,40

θ)1-(R0 /Rct)

(4)

where R0 denotes the charge transfer resistance of a bare Au electrode and Rct is the charge transfer resistance of the



SCHEME 2: Assumed Kinetic Model of Electron Transfer Processes of SAMs of Thiol-Derivatized Porphyrina

a (A) Perfect SAMs of porphyrin: No electrochemical response. (B) Interim state: Electron transfer from the gold electrode through the alkyl chain to the porphyrin macro ring and from the gold electrode to the lying macro ring of the porphyrin SAMs. (C) Steady state: Direct electron transfer from the gold electrode to the lying macro ring of the porphyrin SAMs. If there is a center M of metalloporphyrin, the electrons transfer from the gold electrode to the center M.

SCHEME 3: Electron Density Distributions and Energy of Four Frontier Molecular Orbitals (LUMO, LUMO + 1, HOMO, and HOMO - 1) Obtained from the DFT Calculation

porphyrin modified electrode. Figure 7 shows the impedance spectrum (Figure 7A) and CV (Figure 7B,C), corresponding to the different surface coverages of the monolayer-functionalized electrodes, respectively. One can observe that the θ values

gradually decrease from state 1 to 6. Figure 7A (inset) shows the fitting of the experimental impedance data using the Randles and Ershler equivalent circuit. Where RS is the solution resistance, Cdl is the double-layer capacitance, Rct is the charge

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transfer resistance of the corresponding SAMs electrode, and W is the Warburg impedance, resulting from the diffusion of ions from the bulk solution to the electrode surface. The surface coverage values are listed in Table 2. The result shows that the dense SAMs almost totally blocked the redox process of the [Fe(CN)6]3-/4- species (Figure 7B, curve 1). At the same time, the CVs were determined in a phosphate-buffered solution corresponding to various surface coverages (Figure 7C). It is interesting to note that direct ET cannot occur in a phosphatebuffered solution when a highly oriented and densely packed monolayer is in existence, but the peak current of porphyrins appeared and increased with a decrease in surface coverage to some extent. In fact, densely packed monolayers affect the direct ET between the porphyrin and the Au electrode or between porphyrins and porphyrins in a phosphate-buffered solution. Therefore, ET becomes difficult, and there is no peak of porphyrins in an aqueous solution. We found that oxidative desorption of SH-TPP can occur in a potential range of 0.1-0.9 V versus Ag/AgCl in aqueous solution; thus, it is possible to study the attractive phenomenon by oxidative desorption in the potential range of CV. These results indicate that the highly ordered and tightly packed porphyrin SAMs can be formed from sulfur-gold bonds (S-Au) on a gold surface and showed no electrochemical response in an aqueous solution. However, the relatively lowdensity SH-TPP SAMs exhibited unique, good electrochemical response and allowed direct ET of porphyrins on a Au electrode in an aqueous buffer solution. Therefore, we surmise that direct ET occurred between the porphyrin and gold in an aqueous solution in two ways (scheme 2): (1) A tiny part of porphyrin existed in some angle (state I) because the S-Au bonds were not completely cleaved. The electrons are transported from the Au electrode tunnel through the alkyl bridge and then through the macro ring. (2) Another possibility is the interaction of the macro ring (nitrogen atom) with the gold in a side (π-type) interaction (state II). Electrons are directly transported from the Au through the macro ring. If this was the case, we hypothesized that the porphyrin molecules are in an almost flat-lying orientation with respect to the Au surface in the SAMs. Finally, to confirm our assumption, the modified electrode, which showed an electrochemical response in an aqueous solution, was dipped in an octanethiol (C8H18S) solution for a period of time in order to make the long spacer molecules (C8H18S) fill in the space of the SH-TPP on the Au surface. In contrast, the electrochemical response of the porphyrins disappeared in the phosphate-buffered solution, when the long spacer molecules were inserted into these porphyrin molecules. It is further shown that the amount of the adsorbed molecules increases, and that van der Waals interactions among the methylene spacers strongly contribute to the formation of the monolayers. The mixed SAMs tend to form highly ordered structures on the gold electrodes, as evidenced by extremely poor quality cyclic voltammograms relative to those exhibited by neat porphyrin SAMs, which made ET difficult again. Previous studies of alkylferrocene SAMs have also shown that, when the redox center is buried within an insulating matrix of longer alkane chains, the redox kinetics are dramatically slower.41,42 In addition, we applied DFT to study the ET of porphyrin molecules on Au surfaces and confirmed the assumed kinetic model of ET processes once again. The resulting frontier orbitals of the geometry-optimized molecules are shown in Scheme 3. The changes in electron density distributions and energies of the lowest unoccupied molecular orbital (LUMO), LUMO + 1, highest occupied molecular orbital (HOMO), and HOMO -

Wang et al. 1 orbitals are presented, which reveal important differences in the macro ring of the porphyrin interacted with the Au atom on the electrode surface (AuTPP-SH, state II) relative to the orientation of the porphyrins connected with the S-Au bonds in some angle (TPP-SAu, state I). The eigenvalues of the HOMO and LUMO and their energy gap reflect the chemical activity of the molecule.43 HOMO, as an electron donor, represents the ability to donate an electron, while LUMO, as an electron acceptor, represents the ability to obtain an electron. The higher the energies of HOMO, the easier it is for HOMO to donate electrons; the lower the energies of LUMO, the easier it is for LUMO to accept electrons. From Scheme 3 and a comparison of state I and state II, the results show that the HOMO energies state II (-3.707 eV) are higher than those of state I (-4.866 eV). In other words, state II loses electrons easily, and ET occurs easily. The results are in agreement with that of our experiment and assumption. Conclusions In summary, we demonstrated that the direct ET of porphyrins in a biological environment helps to explain their role in organisms. An interesting phenomenon observed is that direct ET cannot occur in a phosphate-buffered solution when a highly oriented and tightly packed monolayer is in existence, but the relatively low-density SH-TPP SAMs exhibit good electrochemical responses and allow direct ET of porphyrins on a Au electrode in an aqueous buffer solution. This behavior can be explained in two different ways. Moreover, the calculated results of the DFT are in agreement with those of the experiment and further support our assumptions. The direct electron transfer rate constant of the porphyrin SAMs is influenced by the surface coverage (θ), orientation, pH values, ionic strength, and even the biasing potentials during the EIS measurement process. The porphyrin monolayers attract much interest because of their potential use as biosensors and active biointerfaces for cell studies. Acknowledgment. This work was supported by the Natural Science Foundation of China (20775060 and 20875077), the Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE P.R.C., and the Key Laboratory of Polymer and Material Science of Gansu Province. We also gratefully acknowledge the very helpful discussions with Professor Jianping Lei (Nanjing University). References and Notes (1) (a) Zhu, Y. Q.; Silverman, R. B. J. Org. Chem. 2007, 72, 233. (b) Auger, A.; Swarts, J. C. Organometallics 2007, 26, 102. (c) Komatsu, T.; Wang, R.-M.; Zunszain, P. A.; Curry, S.; Tsuchida, E. J. Am. Chem. Soc. 2006, 128, 16297. (d) Suslick, K. S.; Rakow, N. A.; Kosal, M. E.; Chou, J.-H. J. Porphyrins Phthalocyanines 2000, 4, 407. (e) Biesaga, M.; Pyrzyn´ska, K.; Trojanowicz, M. Talanta 2000, 51, 209. (2) (a) Chen, W.; Huang, C.; Gao, X. Y.; Wang, L.; Zhen, C. G.; Qi, D.; Chen, S.; Zhang, H. L.; Loh, K. P.; Chen, Z. K.; Wee, A. T. S. J. Phys. Chem.B 2006, 110, 26075. (b) Wang, Y. L.; Gan, L. F.; Chen, H. J.; Dong, S. J.; Wang, J. J. Phys. Chem. B 2006, 110, 20418. (c) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222. (3) (a) Lambert, C.; Kriegisch, V. Langmuir 2006, 22, 8807. (b) Saravanan, G.; Ozeki, S. J. Phys. Chem. B 2008, 112, 3. (c) Newton, M. D.; Smalley, J. F. Phys. Chem. Chem. Phys. 2007, 9, 555. (4) (a) Boeckl, M. S.; Bramblett, A. L.; Hauch, K. D.; Sasaki, T.; Ratner, B. D.; Rogers, J. W. J. Langmuir 2000, 16, 5644. (b) Lu, X. Q.; Zhang, L. M.; Li, M. R.; Wang, X. Q.; Zhang, Y.; Liu, X. H.; Zuo, G. F. ChemPhysChem 2006, 7, 854. (c) Lu, X. Q.; Li, M. R.; Yang, C. H.; Zhang, L. M.; Li, Y. F.; Jiang, L.; Li, H. X.; Jiang, L.; Liu, C. M.; Hu, W. P. Langmuir 2006, 22, 3035. (d) Zhang, Z. J.; Yoshida, N.; Imae, T.; Xue, Q. B.; Bai, M.; Jiang, J. Z.; Liu, Z. F. J. Colloid Interface Sci. 2001, 243, 382. (e) Nishimura, N.; Ooi, M.; Shimazu, K.; Fujii, H.; Uosaki, K. J. Electroanal. Chem. 1999, 473, 75. (f) Zhang, Z. J.; Hou, S. F.; Zhu, Z. H.;

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JP803725X

Direct Electron Transfer of Thiol-Derivatized Tetraphenylporphyrin

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