Reactions of Alkyl Peroxyl Radicals with Metal Nanoparticles in

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Reactions of Alkyl Peroxyl Radicals with Metal Nanoparticles in Aqueous Solutions Ronen Bar-Ziv,†,‡ Israel Zilbermann,*,†,‡ Tomer Zidki,† Haim Cohen,†,§ and Dan Meyerstein*,†,§ Chemistry Department, Ben-Gurion UniVersity of the NegeV, Beer-SheVa, Israel, Nuclear Research Centre NegeV, Beer-SheVa, Israel, and Biological Chemistry Department, Ariel UniVersity Center of Samaria, Ariel, Israel ReceiVed: October 31, 2008; ReVised Manuscript ReceiVed: December 29, 2008

Methyl-peroxyl radicals react with Au0 and Ag0 nanoparticles (NPs) in aqueous suspensions in fast reaction with rate constants which approach the diffusion-controlled limit. The transients thus formed decompose via heterolysis of the O-O bond resulting in the oxidation of the NPs and formation of methanol. Introduction Recently, it was shown that alkyl radicals react, in relatively fast reactions, with metal powders and with metal nanoparticles (NPs) immersed in aqueous solutions.1-3 The results point out that in these reaction intermediates, I, with metal-carbon σ bonds are formed.1-3

The mechanisms of decomposition of these transients depend on the nature of M, on the substitutes on R · , on the solution composition, e.g., on pH, on the temperature, etc. These findings are of importance as alkyl radicals are formed in catalytic processes,4 electrocatalytic processes,5 electrochemical processes,6 and photochemical processess,7 by ionizing radiation and by a variety of thermal reactions, e.g., reactions with peroxides.8 Often these processes occur near a metal surface, and therefore the reactions of alkyl radicals with metal surfaces are of major importance. However, whenever dioxygen is present, e.g., in all catalytic oxidations,9 in many industrial processes,10 in the environment,11,12 and in biological systems,13-15 the alkyl radicals are transformed into alkyl-peroxyl radicals via k2

R · + O2 98 RO2 ·

(2)

k2 for most alkyl radicals approaches the diffusion-controlled limit, i.e., k2 g 109 M-1 s-1.16 Though many reactions of alkylperoxyl radicals in homogeneous solutions were studied,12,14,16-18 their reactions with NPs or metal surfaces were not reported up to date. Materials in the nanosize regime have size-dependent optical, electronic, and chemical properties which might be useful in many applications, e.g., in photochemistry, biosensors, nanoelectronics, or optics.19-26 As the size of the crystallites decreases * Corresponding author. E-mail: [email protected]. † Ben-Gurion University of the Negev. ‡ Nuclear Research Centre Negev. § Ariel University Center of Samaria.

to the nanometer range, a gradual transition from the bulk properties of the material to its molecular components is observed.27-29 Investigations have shown that the redox chemistry of these particles is enhanced and that they could be useful as photochemical sensitizers30 and catalysts for radical processes31,32 and photocatalyzed decomposition of organic substrates.33,34 Despite extensive studies of NPs and the emphasis on redox processes occurring on these particles, data on the reactions of the particles with radicals are rather rare. It is suggested by various groups35 that metal NPs are very efficient catalysts because of their large surface-to-volume ratio resulting in a large percent of surface atoms, which are available for binding and activating of a variety of substrates. In radicals involving catalytic processes induced by metal NPsinitiatedbylight,ionizationradiation,orthermalprocesses,33,36-38 electrons are often transferred across the particle-solution interface similar to electrode reactions in electrochemistry.39,40 Thesmallparticlesarethereforeoftenreferredtoas“microelectrodes”41-43 and can even react as catalysts for multielectron processes.41,44 However, the reactions of radicals with NPs have been studied almost solely under anaerobic conditions.3,31,44-46 It seemed therefore of interest to study the reactions of alkyl-peroxyl radicals with metal surfaces and especially with noble metal NPs. In the present study, only reaction of methyl-peroxyl radicals, CH3OO · , was studied. Experimental Section Materials. All chemicals were of analytical reagent (A.R.) grade and were used without further purification. The water used was deionized and was further purified by a Millipore Milli-Q setup with a final resistivity of >10 MΩ/cm. Prior to the irradiation, the solutions were saturated with a gas stream of N2O/O2 (75:25 v/v) for 15 min in septum-sealed glass reactors. Instrumentation and Analysis. UV-vis measurements were carried out using a Hewlett-Packard Diode Array spectrophotometer model 8453A, which enables measurements in the range of 190-1100 nm and resolution of (1 nm. For TEM analyses, the NP solutions were dried on TEM grids. The grids were Lacey Formvar/Carbon, 300 mesh, copper from Tepdella. TEM analyses were performed using: Tecnai 12 G2 TWIN TEM (FEI) acc. Volt. 200 kV. The solutions were irradiated using a 60Co gamma source of Noratom Gamma cell, which emits γ-rays of 1.1 MeV at a dose rate of 20 Gy/min. The resultant formaldehyde was measured spectrophotometrically, using the acetylacetone/ammonium acetate method47 in which the maximum absorbance at λ ) 412 nm is measured.

10.1021/jp809631v CCC: $40.75  2009 American Chemical Society Published on Web 01/28/2009

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The methanol was analyzed using a Varian CP-3800 gas chromatograph with a flame ionization detector (FID) equipped with a carbowax-peg capillary column, 0.53 mm ID. Preparation of NPs. Silver and gold sols were prepared according to Creighton’s procedure48 as modified by Zidki et al.3 Briefly, 30 mL of ice-cold solution of NaBH4 (2 × 10-3 M in water) was added at once, with vigorous stirring, to 10 mL of Ag2SO4 or HAuCl4 solutions (1 × 10-3 M of Ag+/Au3+ in water). Since the solution of silver NPs was somewhat unstable (the UV-vis spectrum of the solution was broadened and redshifted after a while), 0.4 mL of NaCl (0.01 M) was added after the solution turned yellow, in order to stabilize the silver NPs.3 The resultant pH of the NP solutions was 9.5 ( 0.1 for the silver and 8.0 ( 0.1 for the gold. The maximum plasmon absorption wavelengths of the silver and gold sols were 394 ( 2 and 518 ( 2 nm, respectively. In order to calculate the concentrations of the NPs in the solutions (which are required to derive the rate constants, see below), the mean size of the NPs is needed. The particle mean radii of the gold and silver sols were measured from the TEM micrographs,3 1.8 and 5.1 nm, respectively. From the initial Ag(I) or Au(III) ion concentrations and the size of the NPs measured, one calculates that [Au]NP ) (1.7 ( 0.4) × 10-7 M and [Ag]NP ) (8 ( 4) × 10-9 M. It should be noted that the size distribution of the Au0 NPs is much narrower than that of the Ag0 NPs.3 Radiation Induced Production of Methyl Peroxyl Radicals. When ionizing radiation (γ-radiation, 20 Gy/min) is absorbed by dilute aqueous solutions, the following initial products are formed49 γ, e-

H2O 98 · H (0.60), · OH (2.65), e-aq (2.65),

(3)

H2O2 (0.75), H2 (0.45)

where the numbers given in parentheses are G values (G values are defined as the number of molecules of each product per 100 eV of radiation absorbed by the solution). In concentrated solutions, the yields of · OH and e-aq are somewhat higher, and those of H2O2, H2, and H · are somewhat lower. In N2O-saturated solutions, the hydrated electron is converted into the hydroxyl radical via50

e-aq + N2O f N2 +

-

· OH + OH

-1 -1

k4 ) 8.7 × 10 M 9

s

(4) Thus, at pH > 3, the hydrated electrons react with N2O yielding · OH as the major radical. The · OH radicals are converted into methyl radicals upon the reaction with dimethylsulfoxide via the following mechanism51

· OH + (CH3)2S d O f (CH3)2 · S(O)OH

k5 ) 7.0 ×

109 M-1 s-1 (5) (CH3)2 · S(O)OH f (CH3)S(O)OH + · CH3 k6 ) 1.5 × 9 -1

10 s

(6)

In solution containing N2O, (CH3)2SdO, and dioxygen, the methyl radicals are converted into methyl peroxyl radicals (reaction 7) while the H atoms which are produced via reaction 3 react with dioxygen. The HO2 · radical, thus formed, is in equilibrium with its deprotonated form (pKa ) 4.7)52

· CH3 + O2 f CH3OO ·

(7)

· H + O2 f HO2 · a H+ + O2 · -

(8)

It should be pointed out that due to the much higher solubility of N2O (2.4 × 10-2 M) compared to that of O2 (1.3 × 10-3 M) and the volume ratio of these gases under the experimental conditions (N2O:O2 ) 75:25 v/v), the reaction of the hydrated electron with the dioxygen is negligible, and almost complete conversion of e-(aq) into · OH radicals is achieved. Sample Preparation. The samples were prepared as follows. Small glass bulbs (15 mL) containing 3 mL of solution of the NPs (one day after preparation, thus all excess of NaBH4 has been decomposed) and 0.05 M (CH3)2SdO were sealed with a rubber septum and were deaerated by bubbling a gas mixture of N2O and O2 (75:25 v/v) for 15 min. The blank solution was the same medium but without NPs, i.e., an aqueous solution of (CH3)2SdO with the same amount of the reducing agent as was added to the NPs solutions (2 mM NaBH4 solution). The solution was also kept overnight so that decomposition into borate was completed (BH4- + 4H2O f H3BO3 + OH- + 4H2). These bulbs were irradiated to the appropriate dose using the gamma source. After the irradiation, 30 µL of concentrated MgCl2 solution was added to precipitate the NPs (high ionic strength causes precipitation of the NPs). The precipitation was done as the color of the NPs affects the colorimetric analysis of the formaldehyde. The solutions were analyzed for formaldehyde and methanol using the UV-vis spectrophotometer and the GC. Results and Discussion Reaction between the Metal NPs and the Methyl Peroxyl Radicals. The chemistry of organic peroxyl radicals in aqueous solution has been intensively studied by Von Sonntag and coworkers (for reviews on peroxyl radicals in aqueous solution, see refs 12 and 53). The alkyl-peroxyl radicals decompose via bimolecular reactions in which short-lived tetraoxide transients are formed, e.g., for CH3OO · radicals54

2CH3OO · f CH3OOOOCH3 f products 2k55 9 )8× 108M-1s-1

(9)

The short-lived intermediate CH3OOOOCH3 thus formed decomposes to yield several products, the yields of which are TABLE 1: Yields of Formaldehyde and Methanol from the Reaction between the Methyl-Peroxyl Radicals and the Metal NPsa sampleb

G(CH2O)

G(CH3OH)c

blank 0.05 M (CH3)2SO, pH 8.5 Ag0 NPsd Ag0 NPs/5 Ag0 NPs/10 Au0 NPse Au0 NPs/5 Au0 NPs/10

3.5 2.0 2.8 3.1 1.1 1.8 2.2

0.8 2.7 (2.3) 1.7 (1.1) 1.3 (0.6) 2.9 (2.7) 2.6 (2.2) 2.0 (1.5)

a Dose: 440 Gy (44 000 rad). b All solutions contained 0.05 M (CH3)2SO at pH 8 (for gold solutions) or 9.5 (for silver solutions) and were N2O/O2 saturated. Experimental error (15%. c Subtraction of the amount of methanol produced via reaction 9 from the measured value gives the number in parentheses which is the methanol formed in the reaction of CH3OO · with the NPs and used for calculating the rate constant (see below). d All silver NPs contain 1.0 × 10-4 M NaCl before the dilution. [Ag]NP ) (8 ( 4) × 10-9 M. e [Au]NP ) (1.7 ( 0.5) × 10-7 M.

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SCHEME 1: Mechanism of the Reaction between Methyl-Peroxyl Radicals and NPs

slightly pH dependent. The major final product is CH2O with a yield ca. 50%-60% at pH 4-8.53,54 Indeed, after N2O/O2 (3:1) saturated solution containing 0.05 M (CH3)2SO is irradiated, CH2O is formed as one of the final products with G ) 3.5 (Table 1), i.e., a yield corresponding to 60% of the · OH radical yield under the experimental conditions in accord with expectations. When Ag0 NPs or Au0 NPs are added to the samples, the yield of CH2O decreases and CH3OH is formed as a significant product (Table 1). It should be noted that the concentrations of CH2O and CH3OH formed are linear with the dose, in the range 340-500 Gy; i.e., G(CH2O) and G(CH3OH) are dose independent. Furthermore, the results clearly point out that the decrease in G(CH2O) correlates, within the experimental error, to the increase in the yield of CH3OH. In addition, the results point out that the decrease in G(CH2O) and the increase in G(CH3OH) depends on the concentration of the NPs added to the sample solution. When lower concentrations of metal NPs are irradiated, the ratio G(CH2O)/ G(CH3OH) increases, as a smaller percentage of the radicals are trapped by the NPs. These results indicate that while methanol is a product of the reaction between the methyl-peroxyl radicals and the NPs the formaldehyde is formed only, or mainly, via the radical-radical reaction 9 and not in the reaction of methyl-peroxyl radicals with the NPs. A plausible mechanism based on all the observed results is outlined in Scheme 1. (One of the reviewers pointed out that the results could also be interpreted via the reaction of CH3OOOOCH3, formed in reaction 9, with the NPs. However, the observation that these NPs react via fast reactions with alkyl radicals3 suggests that Scheme 1 is the most probable mechanism.) The mechanism proposed involves the formation of transients in which the NPs are oxidized by the radicals via an inner sphere mechanism, i.e., via the formation of a metal-oxygen σ bond. This mechanism is analogous to that proposed for the reaction of alkyl radicals with metal surfaces. The alternative mechanism that would involve an outer sphere reaction of the radical with the NPs would form either CH3O2-

or CH3O2+. If CH3O2- is formed, this would yield CH3O2H which decomposes into CH2O; therefore, this mechanism can be ruled out. If CH3O2+ is the product, then methanol would be the final product as observed. Thus, the outer sphere path has to involve the reduction of the NPs via

This mechanism seems less reasonable as Au0 and Ag0 NPs are not powerful oxidants. Furthermore, the (NPs)- formed via

Figure 1. Plasmon absorption of gold NPs after different doses of irradiation.

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Figure 2. Plots of the ratios G(CH2O)/G(CH3OH) · [CH3OO · ]ss vs [NP]-1 in order to derive k10 for the reaction between methyl-peroxyl radicals and (a) silver NPs and (b) gold NPs.

this mechanism are known to react with N2O to form additional · OH radicals,44 a reaction which would increase the total CH3O2 · radical yield. The results (Table 1) point out that this is not the case. The suggestion that the methyl-peroxyl radicals react with the NPs via an inner sphere mechanism is also in accord with the mechanism of reaction of alkyl-peroxyl radicals with low valent transition metal complexes, MnLm+1, in which transients of the type LmMn+1-OOR14,15,17,18,56 are formed. The intermediates formed (Scheme 1) might decompose via three alternative pathways: (a) heterolysis of the M-O bond (path (a) in Scheme 1) or, in other words, one-electron reduction of the radical forming formaldehyde. This pathway is negligible as formaldehyde is not formed or is a minor product of the reaction of the radicals with the NPs. (b) Homolysis of the O-O bond in the transient (NP)-OOCH3 (path (c) in Scheme 1). This would yield the radical CH3O · which is known to rearrange very rapidly into · CH2OH12 which in the presence of oxygen is converted into the corresponding peroxyl radicals · O2CH2OH which decompose to yield O2 · - + CH2O. Clearly, this does

not occur. (c) Heterolysis of the O-O bond (path (b) in Scheme 1), i.e., two-electron reduction forming methanol. As methanol is the final product, it is concluded that reaction 10 followed by reaction 11b fits best the experimental observations. The surface plasmon absorption band of metallic NPs is very sensitive to modifications of the surface.57-62 The wavelength and shape of this band depend on the solvent, NPs size and charge, stabilizing polymer, and in particular chemisorbed solutes which might change the electron density on the particles and modify its Fermi level. Thus, it was of interest to check the difference in the Plasmon absorption of the NPs before and after the reaction with the radicals. The results are presented in Figure 1. It can be seen that upon irradiation the plasmon band of the gold nanoparticles increases and is red-shifted in a dosedependent way. The results support the proposed mechanism, Scheme 1, which assumes oxidation of the NPs. A similar effect was reported by Henglein63 when he studied the interaction between ultrafine gold nanoparticles with air which increases the intensity of the absorption band and shifts the plasmon

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shoulder toward longer wavelengths. The color of the solution acquired a rose tinge, which may indicate an increase in particle size. Henglein suggested that the changes in the spectrum were due to oxide formations on the surface of the particles or the accumulation of positive charges (or removal of excess electrons).63 In addition, the particles seem to grow, i.e., to agglomerate, as a consequence of destabilization by those oxidation processes. Our results are in accord with this proposal. Calculation of the Rate Constants of the Reactions. The rate constants of the reaction between the NPs and the methylperoxyl radicals, reaction 10, can be evaluated under the following assumptions: (a) Almost all the methanol is formed via reaction 11b. The yield of methanol produced via 11b can be calculated by subtraction of the methanol that is produced via reaction 9 (which is known from the ratio between methanol and formaldehyde in the blank solution (first sample in Table 1)) from the total amount of methanol measured. (b) No other products are formed via the reaction of NPs with the CH3O2 · radicals. (c) All the CH2O is formed via reaction 9. The yield of CH2O in this reaction is 60%. Admittedly, these assumptions, based on the experimental results, are reasonable but might be only a good approximation. Thus, the calculated rate constants are only an approximation. The dose rate of the 60Co γ source used is 20 Gy/min; therefore, as G(CH3O2 · ) ) G( · OH) ) 6, the rate of formation of CH3O2 · radicals is 20 × 100 × 6 × 10-9/60 ) 2.0 × 10-7 M/s. Therefore, applying the steady-state approximation, one derives

2 × 10-7 ) 2k9[CH3OO · ]ss2 + k10[CH3OO · ]ss[NP]

(14)

where [CH3O2 · ]ss is the steady-state concentration of the CH3O2 · radicals. One can calculate the [CH3O2 · ]ss for any NP concentration in Table 1, by the fraction of the methyl-peroxyl radicals lost via the bimolecular reaction 9 as measured by G(CH2O), i.e., [(G(CH2O) × 100/60)/G(CH3OO · )total] × 2 · 10-7 ) 2k9[CH3OO · ]ss2, where G(CH3OO · )total ) 6 /100 eV is the total yield of methyl-peroxyl radicals. To derive the rate constant k10, eq 15 should be applied.

G(CH2O) × 100 ⁄ 60 2k9[ · OOCH3]ss2 ) w G(CH3OH) k10[ · OOCH3]ss[NP] 60 × 2k9 G(CH2O) ) G(CH3OH) · [CH3OO · ]ss 100k10[NP]

(15)

Thus, plotting the ratio (G(CH2O)/(G(CH3OH) · [CH3OO · ]ss) vs [NP]-1 using the results in Table 1, one can derive k10 (the slope is equal to: 60 · 2 · k9/100 · k10). Thus, the rate constants k10 of the reaction of · OOCH3 radicals with the silver and the gold NPs at room temperature were derived from the slopes in Figure 2 using the results from Table 1 and the value of the rate constant k9. The rate constants k10 for silver and gold NPs thus evaluated are (1.9 ( 1.2) × 109 M-1 s-1 and (3.5 ( 1.5) × 108 M-1 s-1, respectively. These results are in accord with the observation that alkyl radicals react with these NPs via very fast reactions; i.e., these results suggest that radicals react with these NPs via reactions which approach the diffusion-controlled limit. It should be noted that as the Ag0 NPs are significantly larger than the Au0 NPs and have therefore a larger surface area per particle, the difference in the rate constants does not reflect an intrinsic difference in the chemical properties.

Conclusions The results obtained in this study demonstrate that CH3O2 · and therefore probably also other alkyl-peroxyl radicals and HO2 · radicals react in fast reactions with a variety of metal surfaces. The results point out that the mechanisms of decomposition of the transients M0-OOR, for M ) Ag or Au, involve the heterolysis of the O-O bond. Acknowledgment. This study was supported in part by a grant from the Budgeting and Planning Committee of The Council of Higher Education and the Israel Atomic Energy Commission. D.M. wishes to express his thanks to Mrs. Irene Evens for her ongoing interest and support. References and Notes (1) Rusonik, I.; Cohen, H.; Meyerstein, D. Inorg. Chem. 2006, 45, 7389. (2) Rusonik, I.; Polat, H.; Cohen, H.; Meyerstein, D. Eur. J. Inorg. Chem. 2003, 4227. (3) Zidki, T.; Cohen, H.; Meyerstein, D. Phys. Chem. Chem. Phys. 2006, 8, 3552. (4) Kamigaito, M.; Ando, T.; Sawamoto, M. Chem. ReV. 2001, 101, 3689. (5) Ardizzone, S.; Cappelletti, G.; Doubova, L. M.; Mussini, P. R.; Passeri, S. M.; Rondinini, S. Electrochim. Acta 2003, 48, 3789. (6) Kjaersbo, T.; Daasbjerg, K.; Pedersen, S. U. Electrochim. Acta 2003, 48, 1807. (7) Hoffmann, N. Pure Appl. Chem. 2007, 79, 1949. (8) Fossey, J.; Lefort, D.; Sorba, J. Production of Free Radicals. In Free Radicals in Organic Chemistry; Wiley: New York, 1995; p 108. (9) Masarwa, A.; Meyerstein, D. In AdVances in Inorganic Chemistry: Including Bioinorganic Studies; Elsevier Academic, 2004; Vol. 55, p 271. (10) Getoff, N. Peroxyl Radicals. In The Chemistry of Free Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, 1997; p 483. (11) Getoff, N. Radiat. Phys. Chem. 1999, 54, 377. (12) Von Sonntag, C.; Schuchmann, H.-P. Peroxyl Radicals In the Chemistry of Free Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, 1997; p 173. (13) Spiteller, G. Free Radical Biol. Med. 2006, 41, 362. (14) Meyerstein, D. Metal Ions in Biological Systems; Marcel Dekker: New York, 1999; Vol. 36, p 41. (15) Mansano-Weiss, C.; Masarwa, A.; Cohen, H.; Meyerstein, D. Inorg. Chim. Acta 2005, 358, 2199. (16) Neta, P.; Huie, R. E.; Ross, A. B. J. Phys. Chem. Ref. Data 1990, 19, 413. (17) Solomon-Rapaport, E.; Masarwa, A.; Cohen, H.; Meyerstein, D. Inorg. Chim. Acta 2000, 299, 41. (18) Solomon-Rapaport, E.; Masarwa, A.; Cohen, H.; Valentine, J. S.; Meyerstein, D. Eur. J. Inorg. Chem. 2002, 2427. (19) El-Sayed, I. H.; Huang, X. H.; El-Sayed, M. A. Nano Lett. 2005, 5, 829. (20) Bradley, J. S. Clusters and Colloids: From Theory to Applications; Schmid, G., Ed.; VCH: Weinheim, 1994; p 459. (21) Henglein, A. Chem. ReV. 1989, 89, 1861. (22) Francois, L.; Mostafavi, M.; Belloni, J.; Delouis, J. F.; Delaire, J.; Feneyrou, P. J. Phys. Chem. B 2000, 104, 6133. (23) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (24) Schmid, G. Chem. ReV. 1992, 92, 1709. (25) Brongersma, M. L.; Zia, R.; Schuller, J. A. Appl. Phys. A 2007, 89, 221. (26) Somorjai, G. A.; Tao, F.; Park, J. Y. Top. Catal. 2008, 47, 1. (27) Wilcoxon, J. P.; Abrams, B. L. Chem. Soc. ReV. 2006, 35, 1162. (28) Henglein, A.; Tauschtreml, R. J. Colloid Interface Sci. 1981, 80, 84. (29) Henglein, A.; Fojtik, A.; Weller, H. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1987, 91, 441. (30) Nedeljkovic, J. M.; Nenadovic, M. T.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. 1986, 90, 12. (31) Henglein, A. J. Phys. Chem. 1979, 83, 2209. (32) Zidki, T.; Cohen, H.; Meyerstein, D.; Meisel, D. J. Phys. Chem. C 2007, 111, 10461. (33) Obare, S. O.; Meyer, G. J. J. EnViron. Sci. Health, Part A 2004, 39, 2549. (34) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Chem. Mater. 2002, 14, 3808. (35) Roucoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (36) Kraeutler, B.; Jaeger, C. D.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4903.

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