Rapid-Scan Time-Resolved FT-IR Spectroelectrochemistry Studies on

Chemical Biotechnology, Dongguan UniVersity of Technology, Dongguan, Guangdong, P.R. of China. ReceiVed: October 8, 2006; In Final Form: December 20, ...
0 downloads 0 Views 218KB Size
Download PDF
J. Phys. Chem. B 2007, 111, 1517-1522

1517

ARTICLES Rapid-Scan Time-Resolved FT-IR Spectroelectrochemistry Studies on the Electrochemical Redox Process Baokang Jin,*,† Peng Liu,† Ye Wang,† Zipin Zhang,† Yupeng Tian,*,† Jiaxiang Yang,† Shengyi Zhang,† and Faliang Cheng‡ Department of Chemistry, Anhui UniVersity, 230039, Hefei, Anhui proVince, P.R. China, and Faculty of Chemical Biotechnology, Dongguan UniVersity of Technology, Dongguan, Guangdong, P.R. of China ReceiVed: October 8, 2006; In Final Form: December 20, 2006

The electron transfer to or from molecules containing multiple redox centers has been extensively investigated. Rapid scan time-resolved FT-IR-RAS spectroelectrochemistry was used to investigate the electron-transfer mechanism in this report. The electron transfer of two typical compounds, 1,4-benzoquinone and 1,4-bis(2ferrocenylvinyl)benzene, was examined with this method. Although the two compounds show two-electron transfer in the redox process, 1,4-benzoquinone exhibits two single electron waves while 1,4-bis(2ferrocenylvinyl)benzene exhibits a single wave in cyclic voltammetric experiments. The IR absorption of the intermediate, BQ•- and p-(Fc-CHdCH)+2-benzene, at 1506 and 1589 cm-1, respectively, appeared and disappeared on the experimental time scale in the oxidation and reduction process was observed. In the oxidation process of the p-(Fc-CHdCH)2-benzene molecule, one Fc was oxidated to Fc+ first and the electronwithdrawing ability of Fc+ was stronger than that of Fc, which resulted in the D-π-A structure and the band at 1589 cm-1 becoming visible. Then as the oxidation continues, the other Fc was oxidated to Fc+ too, which resulted in the reforming of the symmetry of the benzene ring A-π-A, so the band at 1589 cm-1 disappeared. Similar phenomenon can be elucidated in the reduction process but the configuration type changed from A-π-A to D-π-A and finally to D-π-D. Hence, not only 1,4-benzoquinone but also 1,4-bis(2-ferrocenylvinyl)benzene show two consecutive one-electron processes. In addition, it is observed that the existing time of the electrochemical reaction intermediate (BQ•- and p-(Fc-CHdCH)+2-benzene) is prolonged at low temperatures due to slow reaction kinetics.

Introduction The electron transfer to or from molecules containing multiple redox centers has been extensively investigated,1-9 primarily due to the use of the molecules in molecular electronic devices, energy conversion, electron-transfer mediators, and ion sensors. Theoretic and experimental studies of the electrochemical behavior of molecules containing multiple noninteracting or interacting redox centers have been made.10-14 Flanagan and Bard et al. carried out an equilibrium statistical treatment of molecules containing multiple noninteracting redox centers. A’ngela Molina et al. investigated the electrochemical behaviors of molecules containing multiple interacting or noninteracting redox centers in any multipotential step technique and cyclic voltammetry. The voltammetric response varies from a single wave for ∆E0 ()E01- E02) ) -100 mV (corresponding to a single transfer reaction with two electrons) to two well-defined waves for ∆E0 ) 200 mV (corresponding to two transfers of one electron). For ∆E0 ) 35.6 mV at 25 °C, one can observe the typical shape of a voltammogram for a Nernstian one* Address correspondence to this author. B.J.: phone 86 551 5107304, fax 86 551 5107342, e-mail [email protected]. Y.T.: pbone 86 551 5108151, fax 86 551 5107342, e-mail [email protected]. † Anhui University. ‡ Dongguan University of Technology.

electron-transfer reaction. On the basis of their results, we can conclude that the voltammetric curve of consecutive multielectron-transfer reactions shows a single wave or multi-separate waves which depend on ∆E0. However, several factors, as pointed out by the authors in ref 11, the slow electron transfer at the electrode, i.e., non-Nernstian behavior, structural changes in the electrode reaction, and adsorption or precipitation of reactants or products at the electrode surface may cause voltammetric curve departure from the theoretical prediction. The information obtained from voltammetric data is not enough to elucidate the electron-transfer mechanism of molecules containing multiple noninteracting or interacting redox centers. The electron-transfer mechanism is difficult to distinguish between the multiconsecutive one-electron process and the single-step multi-electron process if a single wave was observed in the voltammogram. Our knowledge and understanding at the molecular level of electrode/electrolyte interfaces benefited greatly from in situ infrared spectroscopy.15-17 Furthermore, the time-resolved Fourier Transform Infrared Spectroscopy (TRS FT-IR) allows the study of dynamics in fast reactions or processes. There are two kinds of time-resolved FT-IR spectroscopy that can be used to obtain the kinetic information on chemical reactions or physical processes. One is step-scan time-resolved FT-IR spectroscopy

10.1021/jp0666129 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/27/2007

1518 J. Phys. Chem. B, Vol. 111, No. 7, 2007 SCHEME 1: Electron-Transfer Mechanism of p-(Fc-CHdCH)2BZ

and it suits the process with time resolutions from microseconds to nanoseconds. This method has been successfully applied in the research of the photochemical process with 10 ns time resolution.18 The disadvantages of this method, long experimental time (several hours or more) and low signal/noise ratio, lead to step-scan time-resolved FT-IR spectroscopy being unsuitabile for researching the kinetics information of the electrochemical process. The other method is rapid-scan timeresolved FT-IR spectroscopy (RS-TRS FT-IR). It allows the study of kinetic processes as long as the rate of the process is under the limit of the spectrometer speed (milliseconds to seconds).19-22 Compared with the step-scan mode, the rapidscan method is a rapid (several seconds to minutes) and convenient mode for researching both reversible and irreversible kinetic processes with low time resolution. Although the spectral time resolution obtained by the rapidscan mode cannot be improved at low temperature, the rate of electrochemical reaction is slower with reducing temperature. In addition, the life-span of the electrochemical reaction intermediate is prolonged at low temperature. So the IR absorption of the intermediate is easily observed and the dissatisfactory time resolution can be partially overcome under low temperature. Herein we report the research on the mechanism of the electrochemical process using rapid-scan time-resolved FT-IR spectroelectrochemistry combined with the low-temperature technique. Two typical compounds, 1,4-bis(2-ferrocenylvinyl)benzene (p-(Fc-CHdCH)2BZ, Scheme 1A) and 1,4-benzoquinone (BQ), which show one single two-electron wave and two consecutive one-electron waves in the voltammogram, respectively, were selected for investigation by RS-TRS FT-IR spectroelectrochemistry combined with voltammetry. The result demonstrates that the RS-TRS FT-IR spectroelectrochemistry is a powerful method for understanding the multistep electrontransfer electrode reaction mechanism. The influence of the temperature on the life-span of the intermediate produced by electrochemistry is also observed. Experimental Section Materials. 1,4-Benzoquinone (Aldrich, 98%) was used after it was recrystallized from ethanol and dried overnight. 1,4-Bis(2-ferrocenylvinyl)benzene was synthesized according to Tian et al.23 The structure and purity of the complexes was confirmed by mass, IR, and 1H NMR spectroscopes. Tetrabutylammonium perchlorate (TBAP) was also recrystallized from ethanol and dried overnight under reduced pressure at 100 °C before use. Acetonitrile and methylene dichloride were dried by vacuum distillations over P2O5. All experiments were performed in anhydrous CH3CN or CH2Cl2 solution with 0.1 mol/L tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. The solutions were deoxygenated by dry nitrogen bubbling and an inert atmosphere was maintained over the solutions during each experimental run.

Jin et al. Investigation Methods. Electrochemistry. Electrochemical experiments were performed on an EG&G PAR Model 283 potentiostat/galvanostat. A platinum electrode (Φ ) 4 mm) was polished with alumina slurries and then cleaned with deionized water before being used as a working electrode. A platinum wire and a silver wire were used as counter and quasireference electrodes, respectively. Steady-state voltammetry was performed with a Pt disk ultramicroelectrode (Φ ) 10 µm). FT-IR Spectroelectrochemistry. Subtractively normalized interfacial Fourier transform infrared (SNIFTIR) spectroscopic measurements and RS-TRS spectroscopic measurements were performed by an in situ method on a Nicolet Nexus 870 spectrometer equipped with a variable-angle specular reflectance accessory (VeeMax II) and a HgCdTe/A (MCT/A) detector cooled with liquid nitrogen. All the experiments were carried out in a homemade reflection-absorption variable-temperature spectroelectrochemical cell with liquid-nitrogen cryostat. The controlled precision of temperature is (0.5 °C. The thickness of the cell is about 3-7 µm according to the results obtained from the cyclic voltammogram and bulk electrolysis experiment. The incident angle was adjusted to 30°. A total of 50 interferometric scans with a resolution of 4 cm-1 were accumulated for an averaged spectrum in SNIFTIR experiments. After a potential step, the single-beam was collected while the current returned to zero or became steady near zero. For rapid-scan time-resolved spectroscopic measurements, 5-20 interferograms were added to each spectrum, the sampling interval is 0.12-0.48 s, and the spectral resolution is 16 cm-1. Experiment results were determined with Grams/3D software. The resulting spectra were normalized as

∆R/R ) [R(ES) - R(ER)]/R(ER) where Es, ER represent sampling potential and reference potential, and R(Es) and R(ER) represent single beam spectra obtained at ER and ES, respectively. By subtracting the reflection spectrum at the sampling potential ES, R(ES), from the reflection spectrum at the reference potential ER, R(ER), the background due to the absorption of the solvent system is eliminated. Consequently, a negative-going and a positive-going sign of bands indicates the increase and the decrease in absorption intensities of the bands at ES, respectively. Results and Discussion Cyclic Voltammetry of 1,4-Benzoquinone and 1,4-Bis(2ferrocenylvinyl)benzene. Figure 1A shows the CV response of 1,4-benzoquinone in dry acetonitrile with 0.1 M TBAP used as the supporting electrolyte. Two steady couples of redox peaks at -0.56 (E1/2 ) Epa + Epc) and -1.12 V (E1/2) can be seen in Figure 1A. The first redox wave corresponds to BQ•-/BQ, and the second redox wave corresponds to BQ•-/BQ2-. The electrochemical behavior shown here is in agreement with those reported in the literature.24 Typical cyclic voltammograms of p-(Fc-CHdCH)2BZ in CH2Cl2 are shown in Figure 1B. Although two redox active centers exist in the p-(Fc-CHdCH)2BZ, the wave for p-(FcCHdCH)2BZ shows characteristics of a reversible one-electron transfer, i.e., ipa(V)-1/2 and E1/2 independent of scan rate, Epa Epc ) 65 mV, and ipc/ipa ) 1 (where ipc and ipa are the peak anodic and cathodic currents, respectively, Epa and Epc are the anodic and cathodic peak potentials, and V is the scan rate). Steady-state voltammograms in bulk solution (Figure 1C) also showed a single wave sigmoidal shape and the plot of E vs log[I/(Il - I)] showed a straight line with a slope of 72 mV (at 298 K). Controlled-potential coulometric tests presented the

Studies on the Electrochemical Redox Process

Figure 1. Cyclic voltammogram of 5 mM BQ in AN + 0.1 M TBAP with a scan rate 50 mV s-1 (A) and 1 mM p-(Fc-CHdCH)2BZ in CH2Cl2 + 0.1 M TBAP with a scan rate of 50 mV/s (B). Steady-state voltammograms of the oxidation of p-(Fc-CHdCH)2BZ on the Pt disk ultramicroelectrode (φ ) 10 µm). The inset is the plot of E vs log[I/(Il - I)] (C); scan rate 5 mV s-1. Reference electrode: Ag/AgCl.

consumption of two electrons per molecule. The results are in agreement with the literature.10 Morrison et al., who was the first person to investigated the electrochemical process, explained that the biferrocene undergoes a single-step two-electron oxidation and the unexpected large slop of the plot is due to electrochemical irreversibility.10 Flanagan and Bard et al. pointed out that electron transfer of the biferrocene is two merged oneelectron processes.11 However, slow electron-transfer kinetics may cause departure from Flanagan’s model. At present, no additional information except electrochemical results has been used to explain the electron-transfer mechanism. SNIFTIR of 1,4-Benzoquinone and 1,4-Bis(2-ferrocenylvinyl)benzene. In situ SNIFTIR spectra of BQ in the wavenumber range 2000-900 cm-1 recorded in the process of reduction and oxidation are shown in Figure 2, parts A and B, respectively. In the reduction process, when the potential was set at -0.6 V, the upward going bands at 1673, 1659, 1638, 1308 cm-1, assigned to υCdO, υCdC, and δC-H (in-plane bending) of -CdC-H, proved the decrease of the response species BQ; the downward yielding bands at 1506 cm-1 assigned to υC-O and 1346 cm-1 assigned to υC-C were proof of the semiquinone anion radical product; the negative going bands at 1476 cm-1 assigned to υCdC and 1235 cm-1 assigned to υC-O were the formation of the benzene ring skeleton vibration. Additionally, when the potential was set at -1.3 V, the downward bands at 1506 and 1346 cm-1 due to the product of semiquinone anion (Q•-) at -0.6 V became the upward bands, which represents the disappearance of Q•-. Moreover, Two pairs of dipolar bands at around 1506, 1476 cm-1 and 1346, 1308 cm-1, which were assigned to the benzene ring skeleton vibration influenced by changes of the chemical environment, were also observed at the potential of -1.3 V. We can find the same position of peaks in the oxidation process just with inversion. In a word, during both the reduction and oxidation processes, the downward band at 1506, 1346 cm-1 which was due to the intermediate semiquinone anion radical, always appeared at a certain potential (-0.6 to -1.0 V), but finally disappeared at a certain potential (-0.4 V).

J. Phys. Chem. B, Vol. 111, No. 7, 2007 1519 Parts A and B of Figure 3 show the in situ FT-IR spectroelectrochemistry difference spectra of p-(Fc-CHdCH)2BZ in the wavenumber range 2000-1300 cm-1 recorded in the processes of oxidation and reduction, respectively. In the oxidation process, the negative going bands at1589, 1470, and 1421 cm-1, which were due to benzene ring, increased in peak intensity, indicating that the intensity of the benzene skeleton vibration was enhanced. The downward band at 1622 cm-1 was assigned to the -CdC- stretch of vinyl, indicating that the -CdC- character of vinyl increased with the oxidation of Fc. The upward band at 1382 cm-1, the absorption of the cyclopentadienyl (Cp) rings,26 indicated that the double bond (-CdC-) character of Fc decreased. This may be elucidated that the oxidation of Fc-induced Cp rings donated electrons to Fe3+, the π-electron density on Cp decreasing accordingly, and as a result, an upward band at 1382 cm-1 was observed. And the change of electronic density on Fc induced extensive electron delocalization in the molecule, which enhanced the intensities of infrared absorbance of the vinyl and benzene skeleton vibration. Particularly interesting, the downward band at 1589 cm-1, which was due to the benzene ring, always appeared at a certain voltage (0.3 < E < 0.9 V), during both the oxidation and reduction processes. Then the intensity increased subsequently, but finally disappeared at a certain voltage (E > 0.9 V or E < 0.3 V). It may be suggested that p-(Fc-CHdCH)2BZ, before the oxidation begins, is symmetrical in configuration D-π-D. Therefore the bands at 1589 cm-1 are invisible.27 However, as the potential increased in the oxidation process, one Fc was oxidated to Fc+ first and the electron-withdrawing ability of Fc+ was stronger than that of Fc, which resulted in forming the D-π-A structure and the band at 1589 cm-1 becoming visible. Then as the oxidation continues, the other Fc was oxidated to Fc+ too, which resulted in reforming of the symmetry of the benzene ring A-π-A, so the band at 1589 cm-1 disappeared. A similar phenomenon can be elucidated in the reduction process with the configuration type transfer from A-π-A to D-π-A and finally to D-π-D. RS-TRS of 1,4-Benzoquinone and 1,4-Bis(2-ferrocenylvinyl)benzene. Hereby, to monitor the kinetics process in the electrochemistry reaction more lively, in situ RS-TRS FT-IR electrochemistry was employed. The background spectrum was collected at 0 V first, then the potential was stepped to -1.5 V (for 1,4-benzoquinone) or to 1.0 V (for p-(Fc-CHdCH)2BZ) simultaneously with the spectra collected in the reduction process or in the oxidation process. Conversely, the background spectrum was collected at -1.5 or 1.0 V, and the potential was stepped to 0 V simultaneously with the spectra collected. The 3D spectra of in situ RS-TRS FT-IR spectroelectrochemistry of 1,4-benzoquinone are shown in Figure 4. The background spectrum was collected at 0 V first, then the potential was stepped to -1.5 V simultaneously with the spectra collected in the reduction process; and in the oxidation process, the background spectrum was collected at -1.5 V, and potential was stepped to 0 V simultaneous with the spectra collected. The spectra of in situ rapid-scan FT-IR during the reduction process are shown in Figure 4. From the series of spectra we can also clearly observe that the peak at 1506 and 1346 cm-1 appeared and disappeared on the experimental time scale during the reduction process. With the disappearance of the bands at 1506 and 1346 cm-1, the bands at 1476, 1235 cm-1 attributed to the formation of dianion (Q2-) yielded gradually to the last steady state. In this way, a quinonoid ring is gradually converted to a benzenoid ring upon reduction from BQ to BQ•-

1520 J. Phys. Chem. B, Vol. 111, No. 7, 2007

Jin et al.

Figure 2. (A) In situ difference FT-IR spectra of BQ Reduction: (a) ES ) -0.7V (ER ) 0 V); (b) ES ) -1.5 V (ER ) -0.7 V); (c) ES ) -1.5 V (ER ) 0 V). Test solution 5 mM BQ in AN + 0.1 M TBAP. (B) In situ difference FT-IR spectra of BQ oxidation: (a) ES ) -0.7 V (ER ) -1.5 V); (b) ES ) 0 V (ER ) -1.5 V); (c) ES ) 0 V (ER ) -0.7 V). Reference electrode: Ag wire.

Figure 3. (A) FT-IR spectroelectrochemistry difference spectra of the oxidation of p-(Fc-CHdCH)2BZ at room temperature on Pt disk electrode (φ ) 4 mm). ER ) 0 V: (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7, (e) 0.9 V. Reference electrode: Ag wire. Test solution: 5 mM p-(Fc-CHd CH)2BZ + 0.1 M TBAP in CH2Cl2. (B) FT-IR spectroelectrochemistry difference spectra of the reduction of p-(Fc-CHdCH)2BZ. ER ) 1.0 V: (a) 0.7, (b) 0.6, (c) 0.5, (d) 0.3, (e) 0.1, (f) 0 V.

Figure 5. In RS-TRS FT-IR spectra of p-(Fc-CHdCH)2BZ oxidation (A) and reduction (B) on Pt disk electrode in 5 mM p-(Fc-CHdCH)2BZ + 0.1 M TBAP CH2Cl2 solution at 243 ( 1 K. Reference electrode: Ag wire. Spectral resolution: 16 cm-1. Experimental time scale: 60 s. (A) ER ) 0 V, potential stepped from 0 to 1.0 V. (B) ER ) 1.0 V, potential stepped from 1.0 to 0 V. Figure 4. In situ RS-TRS FT-IR spectra of BQ reduction in 5 mM BQ in AN + 0.1 M TBAP at 263 ( 1 K. Spectral resolution 16 cm-1. Experimental time scale: 24 s. ER ) -1.5 V. Reference electrode: Ag wire. Potential stepped from 0 to -1.5 V.

and BQ2-. It can be suggested that BQ had two consecutive one-electron steps in the redox process, which first reduced to the shortly existing intermediate semiquinone anion radical and then, at more negative potentials, quickly reduced to dianion. It belonged to the EE electrochemical reaction mechanism: BQ + e- h BQ•-; BQ•- + e- h BQ2- ; and BQ + BQ2- h 2BQ•-. Kim and Park et al. employed UV-vis spectroelectrochemistry to investigate the electroreduction of 1,4-benzoquinone.24 The in situ spectroelectrochemical data obtained during 1,4benzoquinone reduction are highly convoluted due to the low resolution of UV-vis spectra. To resolve the spectra and determine the sequence of spectral emergence, they resorted to two-dimensional spectral correlation in conjunction with the self-

modeling curve-resolution technique. On the basis of the results obtained by the RS-TRS FT-IR method, we can straightforwardly obtain the same results without using two-dimensional spectral correlation and the self-modeling curve resolution technique. The reason for this could be that the spectra resolution obtained with FT-IR is much greater than that obtained with UV-vis. Hence, the RS-TRS FT-IR spectroelectrochemical method is a convenient and efficient way to monitor the kinetics in the electrochemical process. At room temperature (298 K), the IR absorption peak at 1589 cm-1 exists for no more several seconds when the potential is stepped from 0 to 1 V or from 1 to 0 V. Hence the 3D spectra of p-(Fc-CHdCH)2BZ in situ RS-TRS FT-IR were collected under low temperature (243 K). Figures 5A (oxidation) and 5B (reduction) show the typical experimental results. From the series spectra we can find that the peak at 1589 cm-1 appeared and disappeared on the experimental time scale during the oxidation and reduction process, during which the potential was

Studies on the Electrochemical Redox Process

Figure 6. (A) The plots of infrared spectral absorbance against time of Figure 5A. 4, absorbance at 1589 cm-1; the solid line is a plot of the simulation values at 1589 cm-1.25 The solid line was simulated based on the mechanism described in Scheme 1 with k1′ ) k2′ ) 0, k1 ) k2 ) 0.030 s-1, kf ) 0.20 A-1 s-1, and kb ) 0.030 A-1 s-1. The simulation method is available in the Supporting Information. (B) The plots of infrared spectral absorbance against time of Figure 4. 9, absorbance at 1506 cm-1; O, absorbance at 1476 cm-1. The solid lines are plots of the simulation values.25 The solid line was simulated based on the electrochemical mechanism with k1′ ) k2′ ) 0, k1 ) 0.64 s-1, k2 ) 0.41 s-1, kf ) 4.0 A-1 s-1 and kb ) 2.3 × 10-7 A-1 s-1.

stepped from 0 to 1.0 V or from 1.0 to 0 V. The result indicates that the appearance of the peak at 1589 cm-1 depended on not only the electrode potential but also the experimental time scale. These experimental results support the original hypothesis that the peak at 1589 cm-1 was assigned to the intermediate p-(FcCHdCH)+2BZ, and was due to the configuration type transfer from D-π-D (or A-π-A) to D-π-A. The electron transfer of p-(Fc-CHdCH)2BZ can be expressed by Scheme 1. The results obtained spectroelectrochemistry confirm the hypothesis of Flanagan and Bard et al.11 The weak interaction of the two ferrocene centers results in two merged one-electron transfers being observed, and the electrochemical character has all of a one-electron transfer. According to the electrochemical reaction mechanism, a kinetics simulation was performed (Supporting Information). As mentioned previously, IR absorptions at 1589, 1506, and 1476 cm-1 are assigned to the intermediate of p-(Fc-CHdCH)+2BZ, BQ•-, and BQ2-, respectively. Hence, absorbencies at 1589, 1506, and 1476 cm-1 are in direct proportion to the corresponding intermediate concentrations according to the Lambert-Beer rule. Figure 6 shows that the digital simulation values are well in line with the experimental data. Temperature can prominently influence the heterogeneous and homogeneous reaction rate. The reaction rate constant will decrease and the life-span of the electrochemical reaction intermediate, p-(Fc-CHdCH)+2BZ and BQ•-, will increase with decreasing the temperature. The influence of temperature on the life-span of the intermediate can be obtained by using RS-TRS FT-IR electrochemistry. The results are given in the Supporting Information. The linear relationship between the simulation results of the heterogeneous rate constant (lnk) and the temperature (1/T) was observed and is given in Figure 7. The result is coincident with the Arrhenius equation. Conclusion The RS-TRS FT-IR spectroelectrochemistry was presented to investigate the mechanism of electrode reaction in this report. The electron transfer of 1,4-benzoquinone and p-(Fc-CHdCH)2BZ was investigated to show the efficiency of the method provided in this report. Both of the compounds show two consecutive one-electron-transfer processes, although only one redox wave of the latter is observed. In the oxidation process of the p-(Fc-CHdCH)2BZ molecule, one Fc was oxidated to Fc+ first and the electron-withdrawing ability of Fc+ was stronger than that of Fc, which resulted in forming the D-π-A

J. Phys. Chem. B, Vol. 111, No. 7, 2007 1521

Figure 7. The plots of simulative results of heterogeneous rate constant (lnk) against 1/T. 9, experimental results; solid lines are plots of the linear simulation values. (A) Electrochemical oxidation of p-(Fc-CHd CH)2BZ at 1.0 V; (B) electrochemical reduction of BQ at -1.5 V.

structure and the band at 1589 cm-1 becoming visible. Then as the oxidation continues, the other Fc was oxidated to Fc+ too, which resulted in the reforming of the symmetry of the benzene ring A-π-A, so the band at 1589 cm-1 disappeared. A similar phenomenon can be elucidated in the reduction process but the configuration type transfer was from A-π-A to D-π-A and finally to D-π-D. The difference in the standard potentials for two ferrocenyl moieties in p-(Fc-CHdCH)2BZ molecule is 40 mV, estimated through ∆E0 ) (RT/F) lnKdisp ) (RT/F) lnkb/kf.28 The value is slightly bigger than 30 mV (at 243 K), which is a characteristic value for molecules containing multiple non-interacting redox centers. Therefore, the two anodic peaks overlapped with each other because there is weak electroncommunication between the two Fc groups. The result is in good agreement with the supposition made by Flanagan et al.11 The electron-transfer mechanism of p-(Fc-CHdCH)2BZ can be represented as shown in Scheme 1. Acknowledgment. This work was supported by the National Nature Foundation of China (Grants 20475001 and 50532030), the Excellent Youth Foundation of Anhui province (Grant 06044096), the R&D Program of Guangdong province (Grant 2004B 33301024) (China), and the Team for Scientific Innovation Foundation of Anhui province (2006KJ007TD). Supporting Information Available: 3D spectra obtained at different temperature and the kinetics simulation analysis. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Morrison, W. H., Jr.; Hendrickson, D. N. Inorg. Chem. 1975, 14, 2331. (2) Rulkens, R.; Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant, C.; Geiger, W. E. J. Am. Chem. Soc. 1996, 118, 12683. (3) Barlow, S.; O’Hare, D. Chem. ReV. 1997, 97, 637. (4) Boyd, P. D. W.; Burrell, A. K.; Campbell, W. M.; Cocks, P. A.; Gorson, K. C.; Jameson, G. B.; Officer, D. L.; Zhao, Z. D. Chem. Commun. 1999, 7, 637. (5) Camire, N.; Mueller-Westerhoff, U. T.; Geiger, W. E. J. Organomet. Chem. 2001, 637, 823. (6) Barriere, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff, U. T.; Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262. (7) Peris, E. Coord. Chem. ReV. 2004, 248, 279. (8) Caballero, A.; Tarraga, A.; Velasco, M. D.; Espinosa, A.; Molina, P. Org. Lett. 2005, 7, 3171. (9) Barriere, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980. (10) Morrison, W. H., Jr.; Krogsrud, S.; Hendrickson, D. N. Inorg. Chem. 1973, 12, 1998. (11) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am. Chem. Soc. 1978, 100, 4248. (12) Yap, W. T.; Durst, R. A. J. Electroanal. Chem. 1981, 130, 3. (13) Ribou, A. C.; Launay, J. P.; Sachtleben, M. L.; Li, H.; Spangler, C. W. Inorg. Chem. 1996, 35, 3735. (14) Molina, A.; Serna, C.; Lopez-Tenes, M.; Moreno, M. M. J. Electroanal. Chem. 2005, 576, 9.

1522 J. Phys. Chem. B, Vol. 111, No. 7, 2007 (15) Ashley, K.; Pons, S. Chem. ReV. 1988, 88, 673. (16) Hoshino, Y.; Higuchi, S.; Fiedler, J.; Su, C. Y.; Knodler, A.; Schwederski, B.; Sarkar, B.; Hartmann, H.; Kaim, W. Angew. Chem., Int. Ed. 2003, 42, 674. (17) Hartl, F.; Luyten, H.; Nieuwenhuis, H. A.; Schoemaker, G. C. Appl. Spectrosc. 1994, 48, 1522. (18) Sun, X. Z.; Nikiforov, S. M.; Yang, J.; Colley, C. S.; George, M. W. Appl. Spectrosc. 1997, 56, 31. (19) Johnson, T. J.; Weil, S. A.; J. M.; Harris, G. W. Appl. Spectrosc. 1993, 47, 1376. (20) Snively, C.; Katzenberger, S.; Oskarsdottir, G.; Lauterbach, J. Opt. Lett. 1999, 24, 1841. (21) Bellec, V.; De Backer, M. G.; Levillain; E.; Sauvage, F. X.; Sombret, B.; Warelle, C. Electrochem. Commun. 2001, 3, 483. (22) Zhou, Z. Y.; Tian, N.; Chen, Y. J.; Sun, S. G. J. Electroanal. Chem. 2004, 573, 111.

Jin et al. (23) Zuo, C.; Zhou, Y.; Wu, J.; Tian, Y. Chin. J. Inorg. Chem. 2004, 20, 1018. (24) Kim, Y.; Jung, Y. M.; Kim, S. B.; Park, S. M. Anal. Chem. 2004, 76, 5236. (25) The solid line was simulated based on the mechanism described in Scheme 1 with k1′ ) k2′ ) 0, k1 ) k2 ) 0.030 s-1, kf ) 0.20 A-1 s-1, and kb ) 0.030 A-1 s-1. The simulation method is available in the Supporting Information. (26) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley & Sons, Inc.: New York, 1986. (27) Katritzky, A. R.; Simmons, P. J. Chem. Soc. 1959, 2051. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications., 2nd ed.; John Willey & Sons, Inc.: New York, 2001.