Electrochemically Driven Generation of Manganese(IV,V)-oxo

Nov 3, 2008 - Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China, and Research Institute of Cell Engineering, AIST, ...
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Langmuir 2008, 24, 13490-13495

Electrochemically Driven Generation of Manganese(IV,V)-oxo Multiporphyrin Arrays and Their Redox Properties with Manganese(III) Species in Langmuir-Blodgett Films Chao-Feng Zhang,† Meng Chen,† Chikashi Nakamura,‡ Jun Miyake,‡ and Dong-Jin Qian*,† Department of Chemistry, Fudan UniVersity, 220 Handan Road, Shanghai 200433, China, and Research Institute of Cell Engineering, AIST, Amagazaki 661-0974, Hyogo, Japan ReceiVed August 25, 2008. ReVised Manuscript ReceiVed September 19, 2008 High-valency manganese (IV,V)-oxo porphyrins have been electrochemically generated and in situ spectrally characterized in multiporphyrin arrays, which were formed by an interfacial coordination reaction of Na2PdCl4 with manganese (III) tetrapyridylporphyrin (MnTPyP). Multilayers of the Pd-MnTPyP multiporphyrin arrays were obtained by the Langmuir-Blodgett (LB) method. The redox behaviors of manganese in the multiporphyrin arrays were pH-dependent. Spectroelectrochemical experiments revealed a reversible redox process between Pd-Mn(III)TPyP and its Mn(IV)-oxo species, but an irreversible process between Pd-Mn(III)TPyP and its Mn(V)-oxo species. The Pd-Mn(IV)TPyP multiporphyrin arrays could be spontaneously reduced to their Mn(III) complex, while the Pd-Mn(V)TPyP arrays were rather stable in basic solutions (pH > 10.5). However, when the Pd-Mn(V)TPyP multiporphyrin arrays were washed by or immersed in water, they were immediately reduced to their Mn(III) complex. Because these well-organized multiporphyrin arrays are of high thermal and chemical stability, they are potential molecular materials in the studies of natural and artificial catalytic processes as well as redox-based molecular switches.

Introduction Reactive high-valency manganese (IV,V)-oxo porphyrin derivatives have been proposed as the key intermediates in nature and many artificial catalytic processes.1,2 They can act as models for cytochrome P450 enzymes3 and as catalysts for the oxidation of alkanes, alkenes, alcohols, ethers, and amines to a variety of products.4,5 Recently, the OdMn(V) moiety has been suggested in the photosynthetic water oxidation process, and a bridged Mn(V) porphyrin dimmer been revealed to be capable of oxidizing water into dioxygen.6 Several methods have been reported to produce high-valency Mn(IV,V)-oxo porphyrins. The first and most commonly used one is via an oxidation reaction of Mn(III) porphyrins in basic solutions by using the oxidants such as sodium hypochlorite and iodosylbenzene, by which many manganese (IV)-oxo porphyrins * Corresponding author footnote. Dong-Jin Qian, E-mail: djqian@ fudan.edu.cn., Tel/Fax: +86-21-65643666. † Fudan University. ‡ AIST. (1) (a) Goldberg, D. P. Acc. Chem. Res. 2007, 40, 626–634. (b) Gross, Z. Angew. Chem., Int. Ed. 2008, 47, 2737–2739. (c) Song, W. J.; Seo, M. S.; George, S. D.; Ohta, T.; Song, R.; Kang, M.-J.; Tosha, T.; Kitagawa, T.; Solomon, E. I.; Nam, W. J. Am. Chem. Soc. 2007, 129, 1268–1277. (d) Zhang, R.; Newcomb, M. J. Am. Chem. Soc. 2003, 125, 12418–12419. Zhang, R.; Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2005, 127, 6573–6582. (e) Jin, N.; Ibrahim, M.; Spiro, T. G.; Groves, J. T. J. Am. Chem. Soc. 2007, 129, 12416–12417. (2) (a) Meunier, B. Chem. ReV. 1992, 92, 1411–1456. (b) Dolphin, D.; Traylor, T. G.; Xie, L. Y. Acc. Chem. Res. 1997, 30, 251–259. (c) Adam, W.; Stegmann, V. R.; Saha-Mo¨ller, C. R. J. Am. Chem. Soc. 1999, 121, 1879–1882. (d) Slaughter, L. M.; Collman, J. P.; Eberspacher, T. A.; Brauman, J. I. Inorg. Chem. 2004, 43, 5198–5204. (3) (a) Meunier, B.; de Visser, S. P.; Shaik, S. Chem. ReV. 2004, 104, 3947– 3980. (b) do Nascimento, E.; Silva, G. D.; Caetano, F. A.; Fernandes, M. A. M.; da Silva, D. C.; de Carvalho, M. E. M. D.; Pernaut, J. M.; Reboucas, J. S.; Idemori, Y. M. J. Inorg. Biochem. 2005, 99, 1193–1204. (c) Brule, E.; de Miguel, Y. R. Tetrahedron Lett. 2002, 43, 8555–8558. (4) (a) Murakami, Y.; Konishi, K. J. Am. Chem. Soc. 2007, 129, 14401–14407. (b) Wang, S. H.; Mandimutsira, B. S.; Todd, R.; Ramdhanie, B.; Fox, J. P.; Goldberg, D. P. J. Am. Chem. Soc. 2004, 126, 18–19. (5) Arasasingham, R. D.; He, G. X.; Bruice, T. C. J. Am. Chem. Soc. 1993, 115, 7985–7991. (6) (a) McEvoy, J. P.; Brudvig, G. W. Chem. ReV. 2006, 106, 4455–4483. (b) Shimazaki, Y.; Nagano, T.; Takesue, H.; Ye, B. H.; Tani, F.; Naruta, Y. Angew. Chem., Int. Ed. 2004, 43, 98–100.

were prepared and isolated.7 In 1997, Groves and co-workers first isolated and characterized Mn(V)-oxo porphyrin complexes under ambient catalytic conditions by stopped-flow spectrophotometry with the use of m-chloroxybenzonic acid, HSO5-, and ClO- as oxidants,8 and then some other porphyrin-Mn(V)-oxo intermediates have been then characterized.9 Furthermore, with continuous radiation or laser flash photolysis of Mn(III) porphyrins in the presence of organic compounds, Mn(IV,V)-oxo porphyrin derivatives could be generated, which has been reviewed very recently by Zhang and Newcomb who prepared many highvalency transition metal-oxo species.10 Finally, electrochemical oxidation of water-soluble Mn(III) porphyrin is also an attractive way for the high-valency metalloporphyrins,11 though examples up to now have still been rare. It has been revealed that, dependent on the oxidation potentials, either Mn(IV) or Mn(V) porphyrin derivatives could be generated in basic solutions.11 We are currently interested in the design and assembly of metal-mediated multiporphyrin arrays at interfaces, as well as in their optical, electrochemical, and catalytic properties in the organized ultrathin films.12 Because the porphyrins were connected by metal ions via coordination bond, the as-prepared multiporphyrin arrays showed highly thermal, chemical, and structural stability as well as structural regularity and control(7) (a) Camenzind, M. J.; Hollander, F. J.; Hill, C. L. Inorg. Chem. 1982, 21, 4301–4308. (b) Schappacher, M.; Weiss, R. Inorg. Chem. 1987, 26, 1189–1190. (c) Guilard, R.; Perie, K.; Barbe, J.-M.; Nurco, D. J.; Smith, K. M.; van Caemelbecke, E.; Kadish, K. M. Inorg. Chem. 1998, 37, 973–981. (8) Groves, J. T.; Lee, J.; Marla, S. S. J. Am. Chem. Soc. 1997, 119, 6269– 6273. (9) (a) Jin, N.; Groves, J. T. J. Am. Chem. Soc. 1999, 121, 2923–2924. (b) Nam, W.; Kim, I.; Lim, M. H.; Choi, H. J.; Lee, J. S.; Jang, H. G. Chem.-Eur. J. 2002, 8, 2067–2071. (10) Zhang, R.; Newcomb, M. Acc. Chem. Res. 2008, 41, 468–477. (11) Chen, F.-C.; Cheng, S.-H.; Yu, C.-H.; Liu, M.-H.; Su, Y. O. J. Electroanal. Chem. 1999, 474, 52–59. (12) (a) Qian, D. J.; Wakayama, T.; Nakamura, C.; Miyake, J. J. Phys. Chem. B 2003, 107, 3333–3335. (b) Liu, B.; Qian, D. J.; Huang, H.-X.; Wakayama, T.; Hara, S.; Huang, W.; Nakamura, C.; Miyake, J. Langmuir 2005, 21, 5079–5084. (c) Liu, B.; Qian, D. J.; Chen, M.; Wakayama, T.; Nakamura, C.; Miyake, J. Chem. Commun. 2006, 3175–3177. (d) Liu, B.; Chen, M.; Nakamura, C.; Miyake, J.; Qian, D. J. New J. Chem. 2007, 31, 1007–1011.

10.1021/la8027622 CCC: $40.75  2008 American Chemical Society Published on Web 11/04/2008

Oxomanganese (IV,V) Multiporphyrin Arrays at Interfaces

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Figure 1. Schematic drawing for the preparation of the Pd-MnTPyP multiporphyrin arrays at the air-Na2PdCl4 subphase surface.

lability,13 the features of which resulted in the multiporphyrin arrays as attractive supramolecular materials. Moreover, since the metalloporphyrins were constructed via four pyridyl groups, the center metal ions in the porphyrin macrocycles were “free” and active, which could be oxidized to generate high-valency metal-oxo species. Here, Pd-mediated Mn multiporphyrin arrays (Pd-MnTPyP, Figure 1) were prepared by an interfacial coordination reaction between manganese (III) tetrapyridylporphyrin (MnTPyP) and Na2PdCl4, which were then deposited onto substrate surfaces by the Langmuir-Blodgett (LB) method.12 Compositions of the films were checked by X-ray photoelectron spectroscopy (XPS). The immobilized LB films of the multiporphyrin arrays on the substrate surfaces could provide a convenient route for in situ optical and electrochemical characterization of the high-valency Mn(IV,V)-oxo complexes generated. Moreover, electrochemical oxidation of the Pd-Mn(III)TPyP multiporphyrin arrays to their high-valency Mn(IV,V)TPyP-oxo complexes and their reversible reduction processes could be monitored in situ by UV-vis absorption spectra, when the LB films of Pd-MnTPyP were deposited on an optically transparent index tin oxide (ITO) electrode surface. Spectroelectrochemical results revealed a reversible redox process between Pd-Mn(III)TPyP and its Mn(IV)oxo species but an irreversible process between Pd-Mn(III)TPyP and its Mn(V)-oxo species. These well-organized multiporphyrin arrays are potential molecular materials in the studies of nature and artificial catalytic processes as well as in the development of redox-based molecular switches.14

Experimental Section Materials. Manganese (III) meso-tetra(4-pyridyl)porphine acetate (Figure 1) was purchased from Frontier Scientific Porphyrin Products. Na2PdCl4 was from Aldrich Chemical Co. Chloroform and methanol were from Fisher Chemicals Co. All chemicals were used as received without further purification. Double distilled water (first deionized) was used to prepare aqueous solutions. Preparation of Pd-MnTPyP Multiporphyrin Arrays. PdMnTPyP multiporphyrin arrays were prepared by spreading a dilute MnTPyP solution of mixed methanol and chloroform (1:4, v/v) onto (13) Qian, D. J.; Nakamura, C.; Ishida, T.; Wenk, S.-O.; Wakayama, T.; Takeda, S.; Miyake, J. Langmuir 2002, 18, 10237–10242. (14) (a) Sortino, S.; Petralia, S.; Conoci, S.; Di Bella, S. J. Am. Chem. Soc. 2003, 125, 1122–1123. (b) Harmjanz, M.; Gill, H. S.; Scott, M. J. J. Am. Chem. Soc. 2000, 122, 10476–10477.

the pure water and 0.3 mM Na2PdCl4 subphase surfaces. The surface pressure-area (π-A) isotherm measurements and LB film transfer were done with the use of a KSV 5000 minitrough (KSV Instrument Co., Finland) using a continuous speed for two barriers of 10 cm2/ min at room temperature. The accuracy of the surface pressure measurement was 0.01 mN/m. Transfer of monolayers of the PdMnTPyP multiporphyrin arrays onto solid plates was done by the vertical dipping method. For every transfer, the dipping speed was 2 mm/min. Characterization. UV-vis absorption spectra were measured with a Shimadzu UV-2550 UV-vis spectrophotometer. XPS spectra for the LB films on the quartz substrate surfaces were recorded using a VGESCALAB MKII multifunction spectrometer, with nonmonochromatized Mg KR X-rays as the excitation source. The system was carefully calibrated by Fermi-edge of nickel, Au 4f2/7 and Cu 2p2/3 binding energy. Pass energy of 70 eV and step size of 1 eV were chosen when taking spectra. In the analysis chamber, pressures of (1-2) × 10-7 Pa were routinely maintained. The binding energies obtained in the XPS analysis were corrected by referencing the C1s peak to 284.60 eV. Voltammetric and Spectroelectrochemical Measurements. The cyclic voltammograms (CVs) for the ITO electrodes modified with the LB films of the Pd-MnTPyP multiporphyrin arrays were measured by using a CHI 601b electrochemical analyzer. A Pt wire and Ag/ AgCl electrode were used as the auxiliary and reference electrodes, respectively, with 50 mM sodium phosphate as the electrolyte. The pH values of the electrolyte solutions were adjusted by mixtures of Na2HPO4 and NaH2PO4 in different molar ratios. An initial potential of 1.2 V was applied for 2 s, and subsequently, cyclic scans to a final potential of -0.8 V were done for 10 cycles. Spectroelectrochemical experiments were performed by using the three-electrode system above in a transparent quartz cell for the UV-vis absorption measurements under a given applied potential (0.4 or 0.9 V, vs Ag/AgCl) in the 50 mM sodium phosphate electrolyte solution (pH ) 12). All electrochemical measurements were done under an Ar atmosphere at room temperature.

Results and Discussion Monolayer Behaviors. Figure 2 shows π-A isotherms for the MnTPyP monolayers on the pure water and 0.3 mM Na2PdCl4 subphase surfaces at room temperature. It can be seen that, when the MnTPyP was spread on the pure water surface, very low surface pressure increase (10 mN/m) was recorded with the average MnTPyP molecular area of about 0.4 nm2 (Figure 2a); that is, it was difficult to form a stable monolayer of MnTPyP on the pure water surface. This difficulty may be because (i) the

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Figure 3. Absorption spectra for the LB film of Pd-MnTPyP multiporphyrin arrays (s) and MnTPyP in dilute methanol solution (-----). Figure 2. π-A isotherms for the MnTPyP monolayers on the (a) pure water and (b) 0.3 mM Na2PdCl4 subphase surfaces at room temperature.

MnTPyP porphyrin could slightly dissolve into water and (ii) the porphyrin did not contain long alkyl chains and easily formed three-dimensional aggregates during monolayer compression.15 However, when the MnTPyP was spread over the Na2PdCl4 subphase surfaces, both the collapsed surface pressure and average MnTPyP molecular area greatly increased (Figure 2b). In detail, the average MnTPyP molecular area increased to about 1.4 nm2, and the collapsed surface pressure reached nearly 40 mN/m. These features were in agreement with those we have observed for the water-insoluble TPyP and ZnTPyP porphyrins on the K2PdCl4 or CdCl2 subphase surface,16 where the average porphyrin molecular area was in the range 1.5-2.2 nm2. This increase of porphyrin molecular area on the monolayers has been ascribed to the formation of Pd-(Zn)TPyP or Cd-(Zn)TPyP multiporphyrin arrays. With the use of specular X-ray reflectometry and polarized total reflection X-ray absorption spectroscopy, Gentle and co-workers confirmed the formation of a network-like array of similar porphyrins at the CdCl2 and CuCl2 subphase surfaces, in which the pyridyl nitrogen atoms on the periphery of the porphyrins were coordinated in a square-planar fashion to the metal ions drawn from the subphases.17 Thus, we concluded that the Pd-MnTPyP multiporphyrin arrays (Figure 1) were formed at the air-Na2PdCl4 subphase interface. A close inspection of the average molecular area of metalloporphyrins revealed that there was a smaller occupied molecular area for the present MnTPyP (1.4 nm2), compared with previously reported ZnTPyP (1.9 nm2) in similar monolayer formation conditions. A possible reason may be the solubility of MnTPyP; that is, unlike ZnTPyP, the charged Mn(III)TPyP (anionic counterion, acetate) can be slightly dissolved in the water phase, as can be seen from the π-A isotherm in Figure 2a. This solubility was further confirmed by adding a small amount of MnTPyP powders in water, resulting in a color change of the solution after about 10 min. Although concentrations of the metal ions in the subphases could influence the formation of multiporphyrin arrays, this effect became very small when the concentration of Na2PdCl4 reached 0.3 mM.18 Thus, we suggested that the smaller occupied (15) Zhang, Y.; Chen, P.; Liu, M. Chem.-Eur. J. 2008, 14, 1793–1803. (16) (a) Qian, D. J.; Nakamura, C.; Miyake, J. Langmuir 2000, 16, 9615–9619. (b) Qian, D. J.; Nakamura, C.; Miyake, J. Thin Solid Films 2001, 397, 266–275. (17) Ruggles, J. L.; Foran, G. J.; Tanida, H.; Nagatani, H.; Jimura, Y.; Watanabe, I.; Gentle, I. R. Langmuir 2006, 22, 681–686. (18) During experiments, we found that other transition metal ions, such as Cd2+ and PtCl42-, could also act as connectors to form multiporphyrin arrays. However, when CdCl2 was used a higher concentration (generally above 0.05 M) was needed; otherwise, a longer waiting time was needed for the interfacial coordination reaction. (See conference Liu, H. G.; Feng, X. S.; Jiang, J. Z.; Lee, Y. I.; Jang, K. W.; Qian, D. J.; Yang, K. Z. Mater. Lett. 2003, 57, 2156–2161.

molecular area of MnTPyP relative to that of ZnTPyP in the monolayer of multiporphyrin arrays was due to some floating MnTPyP molecules dissolved into the subphase. Langmuir-Blodgett Films. Monolayers of the Pd-MnTPyP multiporphyrin arrays were transferred onto quartz and ITO substrate surfaces by either vertical or horizontal lifting at 20 mN/m. The transfer ratios revealed that Z-type LB films were formed when the vertical transfer method was used, which was similar to those of many coordination polymers or network-like multiporphyrin arrays assembled at interfaces.16,19 Figure 3 shows UV-vis absorption spectra for the LB film of the Pd-MnTPyP multiporphyrin arrays on the quartz substrate surface together with a dilute methanol solution of MnTPyP. The most intense absorption band at about 462 nm in the methanol solution was assigned to the Soret absorption of porphyrins,20 which was red-shifted to about 473 nm in the LB film of the Pd-MnTPyP multiporphyrin arrays. This feature was in agreement with that previously reported for other porphyrin LB films and has been attributed to a tilted “deck of cards” (J-aggregate) aggregation of chromophores. Besides the Soret absorption, both curves showed several Q-bands from 350 to 450 nm and from 540 to 700 nm. These spectral features confirmed the deposition of the Pd-MnTPyP monolayers on the quartz surface.16 X-ray Photoelectron Spectroscopy. Compositions for the LB films of the Pd-MnTPyP multiporphyrin arrays were characterized by the XPS spectra, which revealed several peaks in the binding energy from 180 to 700 eV. As shown in Figure 4, except for the Si and O elements from the quartz substrate, five elements were detected from the LB films, that is, Cl(2p), C(1s), Pd(3d3/2, 3d5/2), N(1s), and Mn(2p) at the binding energies of 198.1, 284.8, 337.4/342.8, 399.9, and 642.1 eV, respectively. The C, N, and Mn elements were from the linker of MnTPyP porphyrin, while Pd and Cl were from the connector of Na2PdCl4. These data indicated that the PdCl42- ions were coordinated with the pyridyl groups of the MnTPyP porphyrin,21 coexisted in the LB films, and confirmed the formation of the Pd-MnTPyP multiporphyrin arrays.16,17 Because of the coordination of PdCl42ions, the MnTPyP molecular area was largely increased in the monolayers of the Pd-MnTPyP multiporphyrin arrays, as we have discussed in the π-A isotherm (Figure 2). By integration of the area of the peaks, it was found that the molar ratio of Mn/Pd was about 1:3.5. On the basis of the structure (19) (a) Zhang, C. F.; Liu, A.; Chen, M.; Qian, D. J. Chem. Lett. 2008, 37, 444–445. (b) Liu, B.; Huang, H. X.; Zhang, C. F.; Chen, M.; Qian, D. J. Thin Solid Films 2008, 516, 2144–2150. (20) Yamada, T.; Hashifumi, T.; Kikumoto, S.; Ohtsuka, T.; Nango, M. Langmuir 2001, 17, 4634–4640. (21) (a) Bakir, M.; Sullivan, B. P.; MacKay, S. G.; Linton, R. W.; Meyer, T. J. Chem. Mater. 1996, 8, 2461–2467. (b) Shimazaki, Y.; Yajima, T.; Tani, F.; Karasawa, S.; Fukui, K.; Naruta, Y.; Yamauchi, O. J. Am. Chem. Soc. 2007, 129, 2559–2568.

Oxomanganese (IV,V) Multiporphyrin Arrays at Interfaces

Figure 4. XPS spectra for the LB film of the Pd-MnTPyP multiporphyrin arrays on the quartz substrate surface.

of the Pd-(Mn)TPyP multiporphyrin arrays,22 one manganese porphyrin coordinated with two PdCl42- ions, resulting in the Mn/Pd molar ratio of 1:2. On the other hand, considering that the anionic counterion (acetate) of the MnTPyP was replaced by PdCl42- at the air-Na2PdCl4 subphase surface, this molar ratio of Mn/Pd became 1:3, the value of which was close to the data estimated from the XPS measurement. A little lower molar ratio of Mn/Pd may be attributed to the fact that more PdCl42- ions were adsorbed by the monolayer of the multiporphyrin arrays during the LB film transfer, just as we have recently reported for the coordination polymers prepared at similar conditions.23 Electrochemistry. Su and co-workers have revealed that the redox behavior of manganese porphyrin was closely dependent on the solution pH values,11 especially for the oxidation of Mn(III)-porphyrin to its high-valency Mn(V)-oxo complexes. By electrochemical oxidation of water-soluble Mn(III) tetrakis(Nmethyl-2-pyridyl)porphyrin in aqueous solution, they found that the Mn(V)-oxo porphyrin could be stabilized in alkaline solution, but no corresponding oxidation wave was observed in the CV curves.11 More recently, Lahaye and Groves revealed that the generation rate of oxo-Mn(V)[TDMImP] species (TDMImP: meso-dimethylimidazolium porphyrin) accelerated with increasing pH.24 In the present work, electrochemical properties of the Pd-Mn(III)TPyP multiporphyrin arrays in the LB films were examined at pH values between 7 and 12. Figure 5 shows several CV curves for the ITO electrode modified with three layers of LB films of the Pd-MnTPyP multiporphyrin arrays in the 50 mM sodium phosphate electrolyte solution at a scan rate of 50 mV/s. These curves revealed the following features. First, three couples of redox waves were recorded, corresponding to Pd(II)-MnTPyP T Pd(0)-MnTPyP,25 Pd-Mn(II)TPyP T Pd-Mn(III)TPyP and Pd-Mn(III)TPyP T Pd-Mn(IV)TPyP redox processes in the Pd-MnTPyP multiporphyrin arrays. The peaks in the potential range of -0.8 to 0 V were due to the redox processes of Pd(II)-MnTPyP T Pd(0)-MnTPyP and Pd-Mn(II)TPyP (22) (a) Milic, T.; Garno, J. C.; Batteas, J. D.; Smeureanu, G.; Drain, C. M. Langmuir 2004, 20, 3974–3983. (b) Drain, C. M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5178–5182. (c) Drain, C. M.; Batteas, J. D.; Flynn, G. W.; Milic, T.; Chi, N.; Yablon, D. G.; Sommers, H. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6498–6502. (23) Liu, B.; Huang, H. X.; Zhang, C. F.; Chen, M.; Qian, D. J. Thin Solid Films 2008, 516, 2144–2150. (24) Lahaye, D.; Groves, J. T. J. Inorg. Biochem. 2007, 101, 1786–1797. (25) (a) Diculescu, V. C.; Chiorcea-Paquim, A. M.; Corduneanu, O.; OliveiraBrett, A. M. J. Solid State Electrochem. 2007, 11, 887–898. (b) Handbook of Chemistry; Kexue Chubanshe: Beijing, 2001; p 558.

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Figure 5. Cyclic voltammograms for the ITO electrode modified by the LB film of the Pd-MnTPyP multiporphyrin arrays in the 50 mM sodium phosphate solution at a scan rate of 50 mV/s. (a) pH ) 7.0, (b) pH ) 9.0, (c) pH ) 10.5, (d) pH ) 12. (* refers to redox process of Mn(III) T Mn(IV) in the Pd-MnTPyP multiporphyrin arrays.)

T Pd-Mn(III)TPyP. Because the relative intensity of Pd(II)-MnTPyP T Pd(0)-MnTPyP to that of Pd-Mn(II)TPyP T Pd-Mn(III)TPyP was so strong, it was difficult to distinguish the potentials for the Pd-Mn(II)TPyP T Pd-Mn(III)TPyP redox process. Second, the redox peak potentials were pH-dependent and shifted to lower potentials with the increase of the solution pH, which was in agreement with those observed in literature.11 For instance, accompanied with the pH increased from 7 to 12, the oxidation potentials of Pd-Mn(III)TPyP to Pd-Mn(IV)TPyP were shifted from 0.62 to 0.28 V. Half-reactions for the couple of oxo-Mn(V)/ Mn(III) porphyrin (Mn(III)[TDMImP]) as a function of pH and corresponding Nernst half-equations have been proposed by Lahaye and Groves.24 Finally, taking into account the current intensity in the potential range from 0.8 to 1.2 V, we found that this current intensity became stronger and stronger with increasing pH values. This feature, together with the shift of the redox potentials, may suggest that high-valency Mn(V)-oxo porphyrin species could be generated at this potential range in the basic solutions. However, similar to the findings of Su and co-workers,11 we also failed to record the redox waves of Pd-Mn(IV)TPyP T PdMn(V)TPyP. That is, direct detection of Mn(V)-porphyrin-oxo generation had failed up to now by using the cyclic voltammogram method. However, because the Pd-MnTPyP multiporphyrin arrays were very strongly immobilized on the ITO electrode surface,13 which provided a spectral method to characterize the Mn(IV or V)-oxo species generated in the LB films, the porphyrins with different valencies of central manganese ions showed different Soret absorption peaks. For example, the Soret absorption of (tetra-(N-methylpyridyl)porphyrinato)manganese(III) was about 462 nm, while that of its Mn(IV)-oxo and Mn(V)-oxo species was about 428 and 443 nm, respectively.8 These spectral features provided the possibility to spectrally detect the generation of high-valency Mn(IV,V)-oxo porphyrins by the spectroelectrochemical method as discussed below. Spectroelectrochemistry. On the basis of literature11,26 and the CVs of the Pd-MnTPyP modified electrode in basic solutions (Figure 5), applied potentials of 0.4 and 0.9 V were used for the electrochemical oxidation of the Pd-Mn(III)TPyP to its highvalency species of Mn(IV)-oxo and Mn(V)-oxo, respectively. Figure 6 shows the spectroscopic change for the LB film of Pd-MnTPyP modified ITO electrode in the 50 mM sodium (26) Trofimova, N. S.; Safronov, A. Y.; Ikeda, O. Electrochim. Acta 2005, 50, 4637–4644.

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Zhang et al. Scheme 1. Schematic Summary of Electrochemical Generation of High-Valency Mn(IV,V)-Oxo Porphyrin in the Pd-MnTPyP Multiporphyrin Arrays and Their Redox Properties with the Mn(III) Species

Figure 6. Absorption spectra for (A) the Pd-Mn(III)TPyP multiporphyrin array-modified ITO electrode with the applied potential of 0.4 V after 0, 1, 4, and 15 min; (B) the Pd-Mn(IV)TPyP multiporphyrin arrays on the electrode without applied potential after 5, 20, and 35 min.

Figure 7. Plot of absorption intensity change at 466 nm vs time with and without the applied potential of 0.4 V for the Pd-MnTPyP multiporphyrin array-modified ITO electrode.

phosphate electrolyte solution with/without an applied potential of 0.4 V vs Ag/AgCl. Before the potential applied, the multiporphyrin arrays showed one strong Soret band at 466 nm together with several Q-bands around 550 and 700 nm (Q-bands not shown), which was attributed to the absorption of PdMn(III)TPyP. After the potential of 0.4 V was applied, this Soret band decreased gradually along with a new band that increased gradually at about 420 nm. Previous studies have confirmed that this band shift was due to the formation of high-valency Mn(IV)oxo porphyrins;11 that is, the Pd-Mn(IV)TPyP-oxo multiporphyrin arrays were generated. Interestingly, when the applied potential was stopped, the absorption band at 420 nm gradually decreased along with the Soret band of Pd-Mn(III)TPyP species at 466 nm increased again (Figure 6B), which indicated that the high-valency Pd-Mn(IV)TPyP could be spontaneously reduced to PdMn(III)TPyP. Repetitions of this electrochemical oxidation process could result in repeated spectral change from 466 to 420 nm or backward. By monitoring the absorption intensity at 466 nm, the dynamic redox processes of Pd-Mn(III)TPyP T Pd-Mn(IV)TPyP were recorded with or without applied potential, the curve of which was shown in Figure 7. After several cycles, this spectral change process became quite stable, indicating that the redox reaction of Pd-Mn(III)TPyP and Pd-Mn(IV)TPyP was a reversible oneelectron process (Scheme 1). On the basis of the CV curves of Pd-MnTPyP in Figure 5, it was very difficult to know whether this redox process was reversible or not. However, the spectra in Figure 7 clearly indicated that this process was indeed a reversible redox reaction; thus, the present strategy was capable of providing a possible route to reveal an intrinsic electrochemical behavior of the organized ultrathin film-modified electrodes at the molecular level. When the applied potential was increased to 0.9 V, a new band at about 428 nm gradually increased accompanied by the decrease of the Pd-Mn(III)TPyP Soret absorption at 466 nm, as

shown in Figure 8, which was ascribed to the oxidation process of Pd-Mn(III)TPyP to Pd-Mn(V)TPyP.11 However, different from the spectral features for the redox process of Pd-Mn(III)TPyP T Pd-Mn(IV)TPyP, here when the applied potential (0.9 V) was stopped, the absorption intensity at 428 nm was not quickly decreased and the band at 466 nm did not increase (curves not shown). This means that the redox process of Pd-Mn(III)TPyP T Pd-Mn(V)TPyP was irreversible; that is, Pd-Mn(V)TPyP species was quite stable in the basic solution, which was in agreement with observations by Su and co-workers, who first generated Mn(V)-oxo porphyrins electrochemically in aqueous solution.11 However, this stability was largely pH dependent as recently reported by Lahaye and Groves, who studied in detail the generation of high-valency oxo-Mn(V)-porphyrin by the reaction of manganese meso-dimethylimidazolium porphyrin complex with HOBr/OBr- and oxygen atom transfer between bromide ion and oxo-Mn(V) species in various conditions including different pH values.19 Here, when the Pd-Mn(V)TPyP-modified ITO electrode was washed by water or immersed in solutions with pH below 10.5, significant spectral change occurred; that is, the Soret absorption band at 428 nm disappeared, and that at 466 nm increased. These spectral features suggested that the Pd-Mn(V)TPyP multiporphyrin array was reduced to the corresponding Pd-Mn(III)TPyP species in solutions with lower pH (below 10.5). Finally, the dynamic redox processes of Pd-Mn(III)TPyP and its Mn(V)-oxo species were in situ monitored at 466 nm. Figure 9 shows a plot of the absorption intensity at 466 nm vs time with or without the applied potential of 0.9 V, the curve of which showed a quick intensity decrease with the applied potential but a very slow increase when the applied potential was stopped. These features indicated that Pd-Mn(III)TPyP could be quickly

Figure 8. Absorption spectra for the Pd-Mn(III)TPyP multiporphyrin array modified ITO electrode with the applied potential of 0.9 V after 0, 1, 2, 4, and 8 min.

Oxomanganese (IV,V) Multiporphyrin Arrays at Interfaces

Langmuir, Vol. 24, No. 23, 2008 13495

expected to give quicker electrochemical and spectral response, which results in potential applications in the molecular devices, such as redox-based molecular switches. The second is that each oxidation (or reduction) state of the redox species is of a typical absorption wavelength, with the difference in the maximum wavelength enough for instrumental analysis.

Conclusions

Figure 9. Plot of absorption intensity change at 466 nm vs time with and without the applied potential of 0.9 V for the Pd-MnTPyP multiporphyrin array modified ITO electrode.

oxidized to its Mn(V)-oxo species under the potential of 0.9 V and further confirmed that Pd-Mn(V)TPyP was stable in the basic solution. On the basis of the spectral change above, we provided a schematic summary for the electrochemical generation of highvalency Mn(IV,V)-oxo porphyrins and their redox properties with the Mn(III) species in the Pd-MnTPyP multiporphyrin arrays, as shown in Scheme 1. It is likely that the following two issues are important for the spectroelectrochemical investigation of the redox processes. The first is that the redox species are strongly immobilized on the optically transparent electrode surfaces with several monomolecular layers. Although the optically transparent thin-layer electrode (OTTLE) cell has been used for the spectroelectrochemical investigations,11 the immobilized molecular layers are

A convenient spectroelectrochemical strategy was reported here to generate and in situ characterize high-valency manganese porphyrins by using organized ultrathin LB films of multiporphyrin arrays on the transparent ITO electrode surfaces. (i) The present method can be easily developed for the generation and characterization of high-valency transition metal-oxo species of other metalloporphyrins and related compounds. (ii) The reversibility of a redox reaction can be revealed by spectral features instead of cyclic voltammograms, which are sometimes strongly affected by the film resistance, resulting in difficulty providing intrinsic electrochemical properties. (iii) Because the multiporphyrin arrays are of high thermal and chemical stability, as well as structural regularity and controllability, the present method can be used to mimic natural and artificial catalytic processes of metalloporphyrins and fabricate molecular switches. Acknowledgment. The authors are grateful for the National Science Foundation of China (20573025, 20421303) and Shanghai Leading Academic Discipline Project (B108) for the financial supports. LA8027622