Synthesis and Photocatalytic Properties of Mixed ... - ACS Publications

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Synthesis and Photocatalytic Properties of Mixed Polyoxometalate-Porphyrin Copolymers Obtained from Anderson-Type Polyoxomolybdates Delphine Schaming,† Cl
emence Allain,†,‡ Rana Farha,§, Michel Goldmann,§,^ Sylvie Lobstein,# Alain Giraudeau,# Bernold Hasenknopf,*,‡ and Laurent Ruhlmann*,†

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† Laboratoire de Chimie Physique, Groupe TEMiC, UMR 8000 au CNRS, Universit
e Paris-Sud (XI), e Paris 06, Institut Parisien de Chimie B^ atiment 349, F-91405 Orsay Cedex, France, ‡UPMC Universit
Mol
eculaire, UMR 7201 au CNRS, case courrier 42, 4 place Jussieu, F-75252 Paris Cedex 05, France, §Institut des NanoSciences de Paris, UMR 7588 au CNRS, Universit
e Paris 6, 140 rue de Lourmel, F-75015 Paris, France, Laboratoire d’Analyse et Contr^ ole des Syst emes Complexes -LACSC- Ecole Centrale d’Electronique e Paris Descartes, 45 rue des Saint P eres, F-75006 (ECE), 37 Quai de Grenelle, F-75015 Paris, France, ^Universit
Paris, France, and #Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, Institut de Chimie, UMR 7177 au CNRS, Universit
e de Strasbourg, 4 rue Blaise Pascal, F-67070 Strasbourg Cedex, France

Received September 21, 2009. Revised Manuscript Received October 26, 2009 Hybrid polyoxometalate-porphyrin copolymeric films are obtained by the electro-oxidation of zinc octaethylporphyrin (ZnOEP) and zinc 5,15-dipyridinium octaethylporphyrin (5,15-ZnOEP(py)22þ) in the presence of the polyoxometalate [MnMo6O18{(OCH2)3CNHCO(4-C5H4N)}2]3- (Py-POM-Py). These films allow the photocatalytic reduction of AgI2SO4 under visible irradiation in air in the presence of propan-2-ol at the 2D interface between water and the copolymeric films. The formation of metallic Ag0 nanowires and triangular nanosheets is observed.

Introduction Polyoxometalates (POMs) are attractive molecular clusters not only because of their structural diversity1 but also because of their rich electronic and optical properties.2 The introduction of organic groups into POMs is an efficient way to diversify these properties for applications in heterogeneous catalysis, host-guest chemistry, biochemistry, nanotechnology, and electrical, magnetic, or photochemical materials.1a,1f In particular, the development of hybrid polymers incorporating POMs is a promising approach to elaborate new functional materials. Different strategies are currently being explored. One approach consists in the entrapment of POMs in polymeric networks2a,2d,3-5 or the sandwiching of POMs between cationic polymers in layer-by-layer assemblies.6 In a second *Author to whom correspondence should be addressed. E-mail: laurent. [email protected]. Tel: þ33 (0)1 69 15 44 38. Fax: þ33 (0)1 69 15 61 88.

(1) (a) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: New York, 1983. (b) Pope, M. T., M€uller, A., Eds. Polyoxometalates: From Platonic Solids to Anti-Retroviral Activity; Kluwer Academic: Dordrecht, The Netherlands, 1994. (c) Hill, C. L. Chem. Rev. 1998, 98, 8. (d) Borras-Almenar, J. J., Coronado, E., M€uller, A., Pope, M. T., Eds.; Polyoxometalate Molecular Science; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003. (e) Long, D.-L.; Burkholder, E.; Cronin, L. Chem. Soc. Rev. 2007, 36, 105–121. (f) Gouzerh, P.; Che, M. Actual. Chim. 2006, 298, 9–22. (g) Proust, A.; Thouvenot, R.; Gouzerh, P. Chem. Commun. 2008, 1837–1852. (2) (a) Sadakane, M.; Steckhan, E. Chem. Rev. 1998, 98, 291. (b) Yamase, T. Chem. Rev. 1998, 98, 307. (c) M€uller, A.; Shah, S. Q. N.; B€ogge, H.; Schmidtmann, M.; K€ogerler, P.; Hauptfleisch, B.; Leiding, S.; Wittler, K. Angew. Chem., Int. Ed. 2000, 39, 1614. (d) Coronado, E.; Gomez-Garca, C. J. Chem. Rev. 1998, 98, 273. (3) (a) Fabre, B.; Bidan, G.; Laplowski, M. J. Chem. Soc., Chem. Commun. 1994, 1509. (b) Gomez-Romero, P.; Lira-Cantu, M. Adv. Mater. 1997, 9, 144. (c) Otero, T. F.; Cheng, S. A.; Huerta, C. F. J. Phys. Chem. B 2000, 104, 10522. (d) Cheng, S. A.; Otero, T. F.; Coronado, E.; Gomez-Garcia, C. J.; Martinez-Ferrero, E.; Gimenez-Saiz, C. J. Phys. Chem. B 2002, 106, 7585. (4) Katsoulis, D. E. Chem. Rev. 1998, 98, 359. (5) For relevant pioneering studies, see (a) Nomiya, K.; Murasaki, H.; Miwa, M. Polyhedron 1986, 5, 1031. (b) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 255, 303. (c) Bidan, G.; Lapowski, M. Synth. Met. 1989, 28, C113. (d) Shimidzu, T.; Ohtani, A.; Aiba, M.; Honda, K. J. Chem. Soc., Faraday Trans. 1988, 84, 3941. (e) Bidan, G.; Genies, E. M.; Lapkowski, M. J. Chem. Soc., Chem. Commun. 1988, 533.

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approach, coordination polymers were formed from polyoxometalates and organic ligands to yield new oxide materials with various structures, such as 1D chains,7,8 2D networks,7d,9 and 3D frameworks.7d,10 Moreover, a large number of porphyrin copolymers exist, such as alternating porphyrin-furan,11 porphyrin-thiophene,12 and porphyrin-dithienothiophene13 copolymers, and following these models, organic polymers containing covalently bonded POMs in their backbone or as pendant groups were also developed.14,15 Conjugated 1D polymers containing hexamolybdate clusters covalently attached to side chains have been reported, and photoinduced electron transfer from the polymer backbone to the POM cluster was demonstrated.15 Further developments along these lines require diversity with respect to the light-harvesting component and the electron acceptor. We envisaged the formation of covalently bonded POM-porphyrin copolymers in order to obtain efficient photoinduced intramolecular electron transfer from the porphyrin ring to the POM cluster. The main goal of this work was to demonstrate that the bridging of POMs with chromophores allows one to activate the POMs by visible light instead of UV irradiation used for POMs alone in catalytic reductions. (6) (a) Yang, G.; Guo, H.; Wang, M.; Huang, M.; Chen, H.; Liu, B.; Dong, S. J. Electroanal. Chem. 2007, 600, 318. (b) Gao, S.; Li, X.; Yang, C.; Li, T.; Cao, R. J. Solid State Chem. 2006, 179, 1407. (c) Gao, S.; Cao, R.; Li, X. Thin Solid Films 2006, 500, 283. (d) Jiang, M.; Wang, E.; Wang, X.; Wu, A.; Kang, Z.; Lian, S.; Xu, L.; Li, Z. Appl. Surf. Sci. 2005, 242, 199. (e) Lu, M.; Xie, B.; Kang, J.; Chen, F.; Yang, Y.; Peng, Z. Chem. Mater. 2005, 17, 402. (f) Wang, L.; Wang, E.; Hao, N.; Jiang, M.; Wang, Z.; Lu, J.; Xu, L. J. Colloid Interface Sci. 2004, 274, 602. (g) Wang, L.; Li, J.; Wang, E. B.; Xu, L.; Peng, J.; Li, Z. Mater. Lett. 2004, 58, 2027. (h) Sousa, F. L.; Ferreira, A. C. A. S.; Ferreira, R. A. S.; Cavaleiro, A. M. V.; Carlos, L. D.; Nogueira, H. I. S.; Rocha, J.; Trindade, T. J. Nanosci. Nanotechnol. 2004, 4, 214. (i) Ma, H.; Peng, J.; Chen, Y.; Feng, Y.; Wang, E. J. Solid State Chem. 2004, 177, 3333. (j) Jiang, M.; Wang, E.; Xu, L.; Kang, Z.; Lian, S. J. Solid State Chem. 2004, 177, 1776. (k) Zhang, T. R.; Lu, R.; Zhang, H. Y.; Xue, P. C.; Feng, W.; Liu, X. L.; Zhao, B.; Zhao, Y. Y.; Li, T. J.; Yao, J. N. J. Mater. Chem. 2003, 13, 580. (l) Liu, S.; Volkmer, D.; Kurth, D. G. J. Cluster Sci. 2003, 14, 405. (m) Jiang, M.; Wang, E.; Wei, G.; Xu, L.; Kang, Z.; Li, Z. New J. Chem. 2003, 27, 1291. (n) Xu, L.; Zhang, H.; Wang, E.; Kurth, D. G.; Li, Z. J. Mater. Chem. 2002, 12, 654. (o) Liu, S.; Kurth, D. G.; Bredenkotter, B.; Volkmer, D. J. Am. Chem. Soc. 2002, 124, 12279.

Published on Web 12/23/2009

DOI: 10.1021/la903564d

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Schaming et al. Scheme 1. Representation of (A) ZnOEP, (B) 5,15-ZnOEP(py)22þ, and (C) Py-POM-Py

Previous studies have described 1D or 2D copolymers as porphyrin wire assemblies synthesized by the electrochemical oxidation of zinc meso-bipyridinium-porphyrin or zinc mesodibipyridinium-porphyrin monomers.16a-cApplying a similar electrochemical process,16d we report herein the electropolymerization of zinc octaethylporphyrin (ZnOEP) and zinc 5,15-dipyridinium-octaethylporphyrin (5,15-ZnOEP(py)22þ) in the presence of

a functionalized polyoxometalate (POM) bearing two pyridyl groups [MnMo6O18{(OCH2)3CNHCO(4-C5H4N)}2]3- (Py-POMPy) leading to hybrid copolymers.8c,17a,17b The photocatalytic reduction of AgI2SO4 by the copolymer film takes place under visible light and aerobic conditions in the presence of propan-2-ol. The formation of metallic Ag0 nanowires and triangular nanosheets is reported.

Experimental Section (7) (a) Hagrman, D.; Zubieta, C.; Rose, D. J.; Zubieta, J.; Haushalter, R. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 873. (b) Zapf, P. J.; Warren, C. J.; Haushalter, R. C.; Zubieta, J. Chem. Commun. 1997, 1543. (c) Bu, W.-M.; Ye, L.; Yang, G.-Y.; Gao, J.-S.; Fan, Y.-G.; Shao, M.-C.; Xu, J.-Q. Inorg. Chem. Commun. 2001, 4, 1. (d) Wu, C.-D.; Lu, C.-Z.; Zhuang, H.-H.; Huang, J.-S. Inorg. Chem. 2002, 41, 5636. (e) Lu, C.-Z.; Wu, C.-D.; Zhuang, H.-H.; Huang, J.-S. Chem. Mater. 2002, 14, 2649. (8) (a) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed. 1999, 38, 2639. (b) Yan, B.; Xu, Y.; Bu, X.; Goh, N. K.; Chia, L. S.; Stucky, G. D. J. Chem. Soc., Dalton Trans. 2001, 2009. (c) Favette, S.; Hasenknopf, B.; Vaisserman, J.; Gouzerh, P.; Roux, C. Chem. Commun. 2003, 2664–2665. (d) Song, Y.-F.; Long, D.-L.; Cronin, L. Angew. Chem., Int. Ed. 2007, 46, 3900–3904. (e) Qi, Y.; Li, Y.; Qin, C.; Wang, E.; Jin, H.; Xiao, D.; Wang, X.; Chang, S. Inorg. Chem. 2007, 46, 3217. (9) (a) Chen, J.; Lu, S.; Yu, R.; Chen, Z.; Huang, Z.; Lu, C. Chem. Commun. 2002, 2640. (b) Zhang, L.; Zhao, X.; Xu, J.; Wang, T. J. Chem. Soc., Dalton Trans. 2002, 3275. (c) Liu, C.-M.; Zhang, D.-Q.; Xiong, M.; Zhu, D.-B. Chem. Commun. 2002, 1416. (d) Lu, Y.; Xu, Y.; Wang, E.; Lu, J.; Hu, C.; Xu, L. Cryst. Growth Des. 2005, 5, 257. (e) Duan, L.-M.; Pan, C.-L.; Xu, J.-Q.; Cui, X.-B.; Xie, F.-T.; Wang, T.-G. Eur. J. Inorg. Chem. 2003, 14, 2578. (10) (a) Hagrman, D.; Zapf, P. J.; Zubieta, J. Chem. Commun. 1998, 1283. (b) Lin, B. Z.; Liu, S.-X. Chem. Commun. 2002, 2126. (c) Tian, A.-X.; Ying, J.; Peng, J.; Sha, J.-Q.; Han, Z.-G.; Ma, J.-F.; Su, Z.-M.; Hu, N.-H.; Jia, H.-Q. Inorg. Chem. 2008, 47, 3274. (d) An, H.-Y.; Wang, E.-B.; Xiao, D.-R.; Li, Y.-G.; Su, Z.-M.; Xu, L. Angew. Chem., Int. Ed. 2006, 45, 904. (11) Umeyama, T.; Takamatsu, T.; Tezuka, N.; Matano, Y.; Araki, Y.; Wada, T.; Yoshikawa, O.; Sagawa, T.; Yoshikawa, S.; Imahori, H. J. Phys. Chem. C 2009, 113, 10798–10806. (12) Yamamoto, T.; Fukushima, N.; Nakajima, H.; Maruyama, T.; Yamaguchi, I. Macromolecules 2000, 33, 5988–5994. (13) Huang, X.; Zhu, C.; Zhang, S.; Li, W.; Guo, Y.; Zhan, X.; Liu, Y.; Bo, Z. Macromolecules 2008, 41, 6895–6902. (14) (a) Judeinstein, P. Chem. Mater. 1992, 4, 4. (b) Mayer, C. R.; Cabuil, V.; Lalot, T.; Thouvenot, R. Angew. Chem., Int. Ed. Engl. 1999, 38, 3672. (c) Mayer, C. R.; Thouvenot, R.; Lalot, T. Chem. Mater. 2000, 12, 257. (d) Schroden, R. C.; Blanford, C. F.; Melde, B. J.; Johnson, B. J. S.; Stein, A. Chem. Mater. 2001, 13, 1074. (e) Mayer, C. R.; Thouvenot, R.; Lalot, T. Macromolecules 2000, 33, 4433. (f) Johnson, B. J. S.; Stein, A. Inorg. Chem. 2001, 40, 801. (g) Moore, A. R.; Kwen, H.; Beatty, A. B.; Maatta, E. A. Chem. Commun. 2000, 1793. (h) Schubert, U. Chem. Mater. 2001, 13, 3487. (i) Kang, J.; Xu, B.; Peng, Z.; Zhu, X.; Wei, Y.; Powell, D. R. Angew. Chem., Int. Ed. 2005, 44, 6902. (15) (a) Xu, B.; Lu, M.; Kang, J.; Wang, D.; Brown, J.; Peng, Z. Chem. Mater. 2005, 17, 2841. (b) Xu, L.; Lu, M.; Xu, B.; Wei, Y.; Peng, Z.; Powell, D. R. Angew. Chem., Int. Ed. 2002, 41, 4129–4132. (16) (a) Ruhlmann, L.; Schulz, A.; Giraudeau, A.; Messerschmidt, C.; Fuhrhop, J.-H. J. Am. Chem. Soc. 1999, 121, 6664. (b) Hao, J.; Giraudeau, A.; Ping, Z.; Ruhlmann, L. Langmuir 2008, 24, 1600–1603. (c) Ruhlmann, L.; Hao, J.; Ping, Z.; Giraudeau, A. J. Electroanal. Chem. 2008, 621, 22–30. (d) Giraudeau, A.; Schaming, D.; Hao, J.; Goldmann, M.; Ruhlmann, L. J. Electroanal. Chem. [Online early access]. DOI: j.jelechem.2009.10.018.

5102 DOI: 10.1021/la903564d

Materials. All solvents, ZnOEP, and pyridine were of reagentgrade quality and used without further purification. [MnMo6O18{(OCH2)3CNHCO(4-C5H4N)}2](NBu4)3 (Py-POM-Py) was synthesized as previously described.17b Electrochemistry. All electrochemical measurements were carried out under argon at 20 C on a glassy carbon disk electrode (d = 3 mm). Voltammetric data were obtained with a standard three-electrode system using a PARSTAT 2273 potentiostat. A one-side indium tin oxide (ITO, Aldrich, 8-12 Ω/square) electrode with a surface of about 1 cm2 was used as an optically transparent electrode to record the UV-visible spectra of the deposited copolymers. A platinum wire was used as an auxiliary electrode. The reference electrode was a saturated calomel electrode (SCE). It was electrically connected to the solution by a junction bridge filled with the electrolyte. Electrochemical Synthesis of Zinc 5,15-Dipyridiniumoctaethylporphyrin Hexafluorophosphate (5,15-ZnOEP(py)22þ2PF6-). ZnOEP (50 mg, 0.084 mmol) and pyridine (0.27 mL, 3.36 mmol) were dissolved in 100 mL of a CH2Cl2/ CH3CN (10:1) solution containing 0.1 mol L-1 tetraethylammonium hexafluorophosphate. Prior to electrolysis, the mixture was stirred and degassed by bubbling argon through the solution for 10 min. Then, the desired working potential was applied. During anodic oxidation, the electrolyzed solution was continuously stirred and maintained under argon. After electrolysis for 24 h at 0.95 V versus SCE, the initial red solution turned red-brown. The decrease in the oxidation current was not exponential with time. When the current value reached the residual current value measured in the absence of electroactive species, the electrolysis was stopped. The number of electrons per molecule of ZnOEP transferred was found to be equal to 4.2 (17) (a) Marcoux, P. R.; Hasenknopf, B.; Vaissermann, J.; Gouzerh, P. Eur. J. Inorg. Chem. 2003, 2406. (b) Allain, C.; Favette, S.; Chamoreau, L.-M.; Vaissermann, J.; Ruhlmann, L.; Hasenknopf, B. Eur. J. Inorg. Chem. 2008, 3433–3441. (c) Dandliker, P. J.; Diederich, F.; Zing, A.; Gisselbrecht, J. P.; Gross, M.; Louati, A.; Sanford, E. M. Helv. Chim. Acta 1997, 80, 1773–1801.

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Schaming et al. electrons, also characterizing the end of the reaction.16b,c Then the solvents were removed, and the residue was dissolved in a minimum amount of CH2Cl2. The solution was poured into water, and the organic layer was washed twice. The organic extract was concentrated (∼5 mL) and chromatographed on alumina. The first fraction (eluted with CH2Cl2/hexane (80:20)) was the unreacted ZnOEP. The second fraction (eluted with a mixture of CH2Cl2/CH3OH (99:1)) was 5,10-ZnOEP(py)22þ2PF6- (42 mg, 0.040 mmol, yield 47%). 5,15-ZnOEP(py)22þ2PF6- was eluted with a mixture of CH2Cl2/CH3OH (98:2). The solvent was removed, and 5,15-ZnOEP(py)22þ2PF6- was recrystallized from CH2Cl2/n-hexane to give violet crystals (40 mg, 0.038 mmol, yield 45%). UV-vis (CH2Cl2) λmax (nm) (ε, M-1 cm-1): 416 (131 400), 548 (13 300), 581 (17 700). 1H NMR (300 MHz, (CD3)2CO, 25 C): δ = 10.55 (s, 2H, meso-H), 10.46 (d, J = 6.1 Hz, 4H, o-H of pyþ), 9.59 (t, J = 8.1 Hz, 2H, p-H of pyþ), 8.95 (dd, J = 6.1 Hz, J = 8.1 Hz, 4H, m-H of pyþ), 4.12 (q, J = 7.5 Hz, 8H, CH2 of CH2CH3), 2.49 (q, J = 7.5 Hz, 8H, CH2 of CH2CH3), 1.90 (t, J = 7.5 Hz, 12H, CH3 of CH2CH3), 1.41 (t, J = 7.5 Hz, 12H, CH3 of CH2CH3). FAB-MS (NBA) (m/z): 1043.3 [C46H52N6Zn(PF6)2-Hþ], 100%. Anal. Calcd for ZnN6C46H52(PF6)2 (M = 1044.28 g mol-1): C, 52.91 ; H, 5.02 ; N, 8.05. Found: C, 52.98 ; H, 5.04 ; N, 8.10. Copolymer Poly-ZnOEP-POM (1). Electropolymerization was performed under an argon atmosphere in a 0.1 mol L-1 solution of tetrabutylammonium hexafluorophosphate in 1,2-C2H4Cl2/CH3CN (7:3) containing 0.25 mmol L-1 ZnOEP and 0.25 mmol L-1 Py-POM-Py using iterative scans. Cyclic scanning (0.2 V s-1) was applied at potentials between -1.50 and 1.80 V versus SCE or between 0 and 1.80 V versus SCE. The starting potential of the first scan and the ending potential value of the final scan were 0.0 and 0.5 V, respectively. Thus, at the end of the electropolymerization, the porphyrin subunits (ZnOEP) of the polymers were not oxidized (neutral form). The direction of the first scan was cathodic in order to record the eventual presence of reduction signals attributed to the pyridinium unit(s) before polymer formation. Polymers were obtained during the reverse anodic scan when the potential reached the oxidation potential value of the porphyrin ligand corresponding to the dications formation. After electrolysis, the working electrodes were washed five times with 10 mL of CH3CN to remove traces of the conducting salt present on the deposited films. Copolymer Poly-5,15-ZnOEP(py)22þ-POM (2). Electropolymerization was carried out as described above in the same medium containing 0.25 mmol L-1 5,15-ZnOEP(py)22þ2PF6and 0.25 mmol L-1 Py-POM-Py. Cyclic scanning (0.2 V s-1) was applied at potentials between -1.30 and 1.80 V versus SCE or between 0 and 1.80 V versus SCE. UV-Visible Spectroscopic Measurements. UV-vis spectra were recorded either with a Hewlett-Packard HP 8453 spectrophotometer or with a Perkin-Elmer Lambda 9 spectrophotometer. Fluorescence Measurements. Steady-state luminescence emission spectra were obtained using a Spex fluorolog 1681 spectrofluorimeter equipped with a Hamamatsu R928 photomultiplier that was cooled to -20 C. The fluorescence spectra were not corrected for the response of the detection system. All measurements were carried out at room temperature. The fluorescence spectra corresponding to copolymers poly-ZnOEP-POM (1) and poly-5,15-ZnOEP(py)22þ-POM (2) were obtained from the solution by combining several batches. Each batch was dissolved off of the electrode by DMF. The concentration of each corresponding solution was adjusted so that its UV-visible absorbance value (O.D.) at the excitation wavelength (λ = 420 nm) was equal to 0.1. Atomic Force Microscopy (AFM). AFM was performed directly on the surface of the ITO using a Nanoscope III (Digital Instruments, Santa Barbara, CA) in tapping mode under ambient Langmuir 2010, 26(7), 5101–5109

Article conditions. Silicon cantilevers (Nanosensors, Wetzlar/Germany) with a spring constant between 31 and 77 N/m and a resonance frequency in the range of 299-402 kHz were used. The scanning rate was 1.0 Hz. Transmission Electronic Microscopy (TEM). TEM observations were performed with a JEOL 100 CXII TEM instrument operated at an accelerating voltage of 100 kV. Samples for TEM analysis were prepared by solution drops deposited and dried on carbon-coated copper TEM grids. Photocatalysis. The electrochemically deposited copolymers were dissolved and removed from the electrode with dimethylformamide (DMF). The copolymer in DMF solution was deposited later on a quartz slide, and the solvent was evaporated in air. Then the slide was plunged in an optical cell containing an aqueous solution with 8  10-5 mol L-1 Ag2SO4 and 0.13 mol L-1 propan2-ol. Irradiation was performed using a 300 W Xe lamp equipped with a water cell filter to absorb near-IR radiation and a 385 nm cutoff filter to prevent POM photoexcitation. Deaerated solutions were obtained by bubbling with argon (Ar-U from Air Liquide).

Results and Discussion Synthesis of Copolymers Poly-ZnOEP-POM (1) and Poly-5,15-ZnOEP(py)22þ-POM (2). All of the electropolymerizations were carried out under the same experimental conditions by iterative scans in 0.1 mol L-1 solutions of tetrabutylammonium hexafluorophosphate in 1,2-C2H4Cl2/CH3CN (7:3) containing the studied porphyrin (0.25 mmol L-1) and Py-POMPy (0.25 mmol L-1) under an argon atmosphere. The reported electrochemical synthesis of the copolymers uses the previously reported EPOP process16d of nucleophilic substitution on porphyrins.18-20 The polarization of a working electrode at the porphyrin’s second ring-oxidation potential in the presence of pyridine induced a nucleophilic attack leading to the attachment of the pyridyl nitrogens to the meso positions of the porphyrin. This wave clearly disappeared when porphyrin was added to the solution. When the nucleophilic pyridine came from the functionalized POM (Py-POM-Py),8c,17b copolymers {POMporphyrin}n were obtained (Scheme 2). For a degree of polymerization n, the corresponding global reaction can be written as ðn þ 1ÞZnOEPðpyÞ2 2þ þ ðn þ 1ÞPy-POM-Py f ZnOEPðpyÞ2 2þ -½Pyþ -POM-Pyþ -ZnOEPðpyÞ2 2þ n -Pyþ -POM-Py þ ð2n þ 1ÞHþ þ ð4n þ 2Þe The reactions generated Hþ ions. Scanning between -1.3 or -1.5 V and þ1.8 V reduced these protons to H2, and they were accumulated in solution when the scan was limited from 0 to þ1.8 V. Even in the second case, they did not perturb the coating of the electrodes. The demetalation of the metalloporphyrins was controlled by UV-visible spectroscopy and did not occur in any case. Two types of porphyrins were used for the polymerizations. Their oxidation potentials are listed in Table 1. First, we used ZnOEP, which can make zigzag polymers (1, Scheme 2A). Indeed, the four methine positions of the porphyrin ZnOEP are free, and nucleophilic attack could occur at positions 5, 10, 15, and 20. As a result, various branched forms of (18) El Kahef, L.; Gross, M.; Giraudeau, A. J. Chem. Soc., Chem. Commun. 1989, 963. (b) Giraudeau, A.; Ruhlmann, L.; El Kahef, L.; Gross, M. J. Am. Chem. Soc. 1996, 118, 2969. (19) (a) Ruhlmann, L.; Lobstein, S.; Gross, M.; Giraudeau, A. J. Org. Chem. 1999, 64, 1352. (b) Giraudeau, A.; Lobstein, S.; Ruhlmann, L.; Melamed, D.; Barkigia, K. M.; Fajer, J. J. Porphyrins Phthalocyanines 2001, 793. (20) (a) Ruhlmann, L.; Giraudeau, A. J. Chem. Soc., Chem. Commun. 1996, 2007. (b) Ruhlmann, L.; Giraudeau, A. Eur. J. Inorg. Chem. 2001, 656.

DOI: 10.1021/la903564d

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Table 1. Electrochemical Data for Py-POM-Py and ZnOEP,5,15ZnOEP(py)22þ and Copolymers Poly-ZnOEP-POM (1) and Poly5,15-ZnOEP(py)22þ-POM (2)a compounds

oxidation

reduction pyþ

Py-POM-Pyb ZnOEPb 5,15-ZnOEP(py)22þb

0.94 1.50irr d

0.85 0.68 1.23irr d

-0.88irr d -0.98irr d -0.90irr d -0.95irr d

-0.59 -1.60 -1.44

poly-ZnOEP-POM (1)c 1.22irr d 0.79irr d poly-5,15-Zn1.49irrd OEP(py)22þ-POM (2)c a All potentials in V versus SCE were obtained from cyclic voltammetry in 1,2-C2H4Cl2/CH3CN (7:3) containing 0.1 mol L-1 (NBu4)PF6. Scan rate = 0.1 V s-1. b Working electrode: glassy carbon electrode. c ITO, S = 1 cm2 after 20 scans. Potential values in oxidation are measured on the first scan because the electrodes are coated by the further polymerization of ZnOEP and 5,15-ZnOEP(py)22þ2PF6-. d Peak potential values. The given half-wave potentials are equal to E1/2 = (Epa þ Epc)/2.

the copolymer were expected. ZnOEP was then replaced by 5,15ZnOEP(py)22þ. In this case, only positions 10 and 20 were free, leading to the formation of linear 1D copolymers (5,15-ZnOEP(py)22þ)-POM-(5,15-ZnOEP(py)22þ (2) by the above reaction (Scheme 2B). Figure 1A illustrates the evolution of the CVs during the electropolymerization of ZnOEP in the presence of equimolar amounts of Py-POM-Py. During the first scan in reduction, no signal was detected between 0 and -1.5 V versus SCE. This result was surprising because the first reduction signal of the Py-POM-Py cluster, corresponding to the couple MnIII/MnII, was expected near -0.59 V versus SCE as a reversible, rapid electron transfer on the glassy carbon electrode.17b Moreover, the signal of this redox process (MnIII/MnII) was also well defined on the ITO electrode. Under these conditions, it appeared as a kinetically slow redox system with a cathodic peak potential Epc at -0.70 V versus SCE and an anodic peak potential Epa at -0.20 V versus SCE (ΔE = 0.50 V). The unexpected disappearance of the redox signal can be explained by the coordination of the Zn porphyrin to the pyridyl groups of Py-POM-Py. To confirm the formation of this complex, which is similar to our previously reported system,17b we carried out titrations of Py-POM-Py with ZnOEP. During the titrations, both UV-visible absorption and fluorescence were measured. Αbsorbance at 409 nm was plotted as a function of XPOM for the above titration. The curve showed two sections that were fit by straight lines. The line intersection occurred at XPOM = 0.33, which corresponded to a complex between two porphyrins and one Py-POM-Py. Moreover, the fluorescence intensity at 574 nm plotted versus the mole fraction of Py-POM-Py during the titrations showed also a slight slope break at XPOM = 0.33, which also indicated axial binding. Analogous electrochemical behavior was observed after the axial coordination of Py-POM-Py to the metal ion in [Ru(CO)TPP] (TPP = tetraphenylporphyrin) and ZnTPP.17b Then it was noticed that the MnIII/MnII redox process was remarkably slowed with the quasi-complete disappearance of the electrochemical signal, even at the glassy carbon electrode. One might also consider the adsorption of ZnOEP(py)22þ or ZnOEP, which would limit the approach of Py-POM-Py to the (21) (a) Zou, Z.-Q.; Chen, F. J. Appl. Phys. 2008, 103, 094304/1–094304/4. (b) Ogunrinde, A.; Hipps, K W; Scudiero, L. Langmuir 2006, 22, 5697–5701.

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electrode.21 Then, significant electron-transfer rate constant attenuation would be expected as a consequence of long-distance electron transfer from the working electrode to the electroactive site of the molecule.17c In the second scan, an irreversible reduction peak appeared at ca. -0.95 V versus SCE (peak a, Figure 1A), which corresponded to the reduction of the pyridinium units of the dipyridinium-POM spacers of the generated polymer [(ZnOEP-Pyþ-POM-Pyþ)n].16,22 The height of this peak grew with repetitive scans. The irreversibility of the signal indicated that the generated pyridyl radicals were not stable and reacted further.23 At the reverse potential sweep, an oxidation peak appeared at ca. þ0.25 V (peak b, Figure 1A) after the reduction of the pyridiniums. This signal was attributed to the oxidation of adsorbed H2 formed upon the reduction of Hþ and/or -py-Hþ, which was generated during nucleophilic substitution onto the porphyrins (Figure S2). This process was previously well discussed.18-20 When we used meso-substituted porphyrin 5,15-ZnOEP(py)22þ instead of ZnOEP, the cyclic voltammograms showed the appearance of two separated reduction peaks (-0.88 and -0.98 V) for the first cathodic scan (Figure 1B). That was expected because the two pyridinium substituents of the porphyrin alone were reduced in two well-separated steps (Table 1) because of the existence of mutual interactions.20 This splitting apparently disappeared as the coating of the electrode progressed and could be reasonably interpreted by the large overlapping of the two peaks at -0.88 and -0.98 V and the additional signal appearing at ca. -0.95 V, which increased rapidly as the copolymers were formed and was attributed to the bridging pyridiniums. The reduction signals of the two pyridiniums belonging to the Py-POM-Py bridging units of copolymers 1 and 2 appeared at the same potential (ca. -0.95 V). This result clearly indicated that these pyridiniums were necessarily independent redox sites in the copolymers. The increase in the height of corresponding reduction peak a showed the regular growth of the conducting polymeric films at the electrode.24 In the case of the preparation of copolymer 1, toward the positive potential scale, two distinct oxidation peaks (c and d) in Figure 1A were recorded in the first anodic scan. The first oneelectron oxidation peak corresponded to the generation of the πradical cation porphyrin (peak c), and the second one might be attributed both to the second oxidation step of the porphyrin ring associated with the isoporphyrin oxidation16b,c,25,26 or/and to the MnIII oxidation of the Py-POM-Py cluster (peak d).17b That would agree with Py-POM-Py oxidation on the ITO electrode where the signal of the MnIII/MnIV process was shifted anodically to þ1.25 V versus SCE. A similar redox behavior at the positive potential scan was also observed using 5,15-ZnOEP(py)22þ instead of ZnOEP (Figure 1B). In the case of substituted porphyrin 5,15-ZnOEP(py)22þ, peaks c (22) (a) Oturan, M. A.; Dostert, P.; Strolin Benedetti, M.; Moiroux, J.; Anne, A.; Fleury, M. B. J. Electroanal. Chem. 1988, 242, 171. (b) Carelli, V.; Liberatore, F.; Casini, A.; Tortorella, S.; Scipione, L.; Di Rienzo, B. New J. Chem. 1998, 999. (c) Buston, J. E. H.; Marken, F.; Anderson, H. L. Chem. Commun. 2001, 1046. (d) Carelli, V.; Liberatore, F.; Tortorella, S.; Di Rienzo, B.; Scipione, L. J. Chem. Soc., Perkin Trans. 2002, 1, 542. (23) (a) Brisach-Wittmeyer, A.; Lobstein, S.; Gross, M.; Giraudeau, A. J. Electroanal. Chem. 2005, 576, 129. (b) Giraudeau, A.; Callot, H. J.; Gross, M. Inorg. Chem. 1979, 18, 201–206. (c) Schaming, D.; Giraudeau, A.; Lobstein, S.; Farha, R.; Goldmann, M.; Gisselbrecht, J.-P.; Ruhlmann, L. J. Electroanal. Chem. 2009, 635, 20–28. (24) Bard, A. J. Adv. Phys. Org. Chem. 1976, 13, 155. (25) El Baraka, M.; Jannot, J. M.; Ruhlmann, L.; Giraudeau, A.; Deuni
e, M.; Seta, P. Photochem. Photobiol. A: Chem. 1998, 113, 163. (26) Hinman, A. S.; Pavelich, B. J.; Kondo, A. E.; Pons, S. J. Electroanal. Chem. 1987, 234, 145.

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Figure 1. Cyclic voltammograms recorded during the electropolymerization of (A) ZnOEP and (B) 5,15-ZnOEP(py)22þ in the presence of Py-POM-Py in 1,2-C2H4Cl2/CH3CN (7:3) (NBu4)PF6 0.1 mol L-1. Working electrode: ITO. S = 1 cm2. Scan rate: 0.2 V s-1. (r) Start of the scan. Scheme 2. Copolymers 1 and 2 Obtained with (A) ZnOEP and (B) 5,15-ZnOEP(py)22þ, Respectively, in the Presence of Py-POM-Py

and d collapsed and were shifted to higher anodic potential values as a consequence of the electron-withdrawing effects of the two pyridinium substituents on the oxidation potentials of the porphyrin ring.23b,c Copolymers were also obtained by cyclic scanning (0.2 V s-1) in the potential range from 0 to þ1.80 V versus SCE in order to avoid H2 formation. As expected, the signal (peak b, Figure 1) corresponding to the oxidation of adsorbed H2 disappeared under these conditions (Figure S1 in Supporting Information). The obtained copolymers exhibited redox behavior similar to that observed when scanning from -1.50 to 1.80 V versus SCE. UV-Visible Characterization. All of the UV-visible spectra recorded on ITO electrodes coated with the copolymers obtained from ZnOEP and 5,15-ZnOEP(py)22þ presented Langmuir 2010, 26(7), 5101–5109

identical characteristics. A typical UV-visible spectrum is depicted in Figure 2. The shape of each spectrum consisted of a large absorption band where the absorption maximum was red shifted compared to the monomer. These evolutions can be interpreted by the exciton-coupling theory involving intra- or intermolecular excitonic interaction between the porphyrins subunits.27 Similar spectral effects in dimeric or trimeric porphyrins have been reported and analyzed18b,28,29 as dependent on the interporphyrins angles, the direction of the transition dipole moments in (27) Kasha, M. Rev. Mod. Phys. 1959, 31, 162–169. (28) Sessler, J. L.; Johnson, M. R.; Creager, S. E.; Fettinger, J. C.; Ibers, J. A. J. Am. Chem. Soc. 1990, 112, 9310–9329. (29) Ruhlmann, L.; Gross, M.; Giraudeau, A. Chem.;Eur. J. 2003, 9, 5085– 5096.

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Figure 2. Normalized UV-vis spectra of the modified ITO electrodes with (A) poly-ZnOEP-POM (1) and (B) poly-5,15-ZnOEP(py)22þPOM (2) obtained after 20 iterative scans. (-) ZnOEP or 5,15-ZnOEP(py)22þ in DMF. (- 3 -) Copolymer on the ITO electrode. (---) Copolymer in DMF solution.

Figure 3. Luminescence spectra of poly-ZnOEP-POM (---) and ZnOEP (-) in DMF. λexcitation = 420 nm.

the monomer subunits, and also the distances between the porphyrins.30-32 After polymerization, the films were removed from the electrodes by dissolution in DMF. The shape of the UV-vis absorption bands of the solutions became sharper in comparison to those recorded on the solid films. This evolution suggested that for the most part intramolecular interactions remain when the polymers were in solution. Steady-State Spectroscopic Measurements. The fluorescence emission (Figure 3) obtained in DMF solution from the films removed from the electrodes showed the total quenching of the luminescence both for poly-ZnOEP-POM (1) and poly-5,15ZnOEP(py)22þ-POM (2). This is probably due to an energy- or electron-transfer process from the excited state of the porphyrin to the POM cluster. Copolymer Morphology. The coated electrodes were washed with CH3CN to remove any trace of the conducting salt present on the film and were examined by scanning atomic force microscopy (AFM). Copolymer poly-ZnOEP-POM (1) appeared in the form of tightly packed coils, which did not depend on whether the coated electrodes were obtained by scanning over the potential (30) Tabushi, I.; Kugimiya, S. I.; Kimmaird, M. G.; Sasaki, T. J. Am. Chem. Soc. 1983, 107, 4192–4199. (31) Sessler, J. L.; Hugdahl, J.; Johnson, M. R. J. Org. Chem. 1986, 51, 2838– 2840. (32) Won, Y.; Friesner, R. A.; Johnson, M. R.; Sessler, J. L. Photosynth. Res. 1989, 22, 201–210.

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Figure 4. Atomic force micrograph (AFM, surface plot) images of poly-ZnOEP-POM (1) (A and A’) and poly-5,15-ZnOEP(py)22þPOM (2) (B and B’) on an ITO electrode. Poly-ZnOEP-POM (1): Cyclic scanning was applied at potentials between (A) -1.50 and þ1.80 V vs SCE and (A’) 0 and þ1.80 V vs SCE. Poly-5,15ZnOEP(py)22þ-POM (2): Cyclic scanning was applied at potentials between (B) -1.30 and þ1.80 V vs SCE and (B’) 0 and þ1.80 V vs SCE.

range from -1.5 to þ1.8 (Figure 4A) or from 0 to þ1.8 V (Figure 4A’). A quite different morphology of the films appeared for poly5,15-ZnOEP(py)22þ-POM (2) when the copolymer was obtained by cyclic scanning in the potential range from -1.30 to þ1.80 V versus SCE (Figure 4B). The coils were more “agglomerated”, which could be explained by the chemical reactivity of the two reduced pyridiniums used as protecting groups at carbon atoms C5 and C15 in the methine bridges. The electrogenerated pyridyl radicals (Py•) could react to give dimeric or oligomeric species resulting from coupling at the 2 or more likely the 4 position of the radicals. This was discussed in previous work.22,23 In the absence of pyridyl radicals (i.e., when the dipyridinium spacers were not reduced by limiting the iterative scans between 0 and þ1.8 V), the morphology of poly-5,15-ZnOEP(py)22þ-POM (2) was similar to that described for ZnOEP. The rms surface roughness of the films ranged from 9.18 nm (Figure 4A) to 7.78 and 9.16 nm (Figure 4A’,B’ respectively). The AFM studies showed an increase in the rms surface roughness Langmuir 2010, 26(7), 5101–5109

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(27.17 nm) in the case of poly-5,15-ZnOEP(py)22þ-POM (2) obtained by cyclic scanning applied from -1.30 V versus SCE to 1.80 V versus SCE (Figure 4B). These measurements are in agreement with a morphology of tightly packed coils covalently agglomerated as a “bunch of grapes”. Photocatalysis. Because our main objective was to show that covalent copolymers of POMs with chromophores allow us to activate the POMs by visible light, we wanted to test this hypothesis on a model system. We chose the reduction of metal ions because of the usefulness of the process for depollution or for the production of metal nanoparticles. Metal recovery is a topic of great interest concerning metal depollution, the environment, and resource conservation. Moreover, the synthesis of colloid metal nanoparticles is a growing research field because of their numerous possible applications, such as catalysis,33 optics, optoelectronics, information storage, photonics, and the production of batteries, by using various reagents. Metallic nanoparticles can be produced using various methods, such as electrochemical techniques,34 thermal decomposition,35 sonochemical synthesis,36 microwave irradiation,37 radiolysis,38 and photocatalysis.39 In particular, POMs can be used in photocatalytical processes for the recovery of metals or the synthesis of colloid metal nanoparticles. Several studies have described the formation of silver nanoparticles with sometimes inhomogeneous shapes during the photoreduction of silver salts with POMs.40-42 However, it was necessary to irradiate the POMs, used as a catalyst, in their O-M charge-transfer band range in the UV spectral domain. The stability of the obtained Ag0 nanoparticles against aggregation was attributed to the role of POMs as surfactants.42,43 In the depicted porphyrin-POM copolymers, the porphyrins should act as photosensitizers able under visible illumination to transfer electrons to the POMs, which are known to be good electron acceptors. Thus, the photochemical excitation of the porphyrin units would lead to their oxidation and the simultaneous reduction of the POM units. Then the reduced POM would be able, in turn, to transfer one electron to the Agþ cation to give Ag0. The porphyrin would finally be reduced by propan-2-ol, used as a sacrificial electron donor. Figure 5A illustrates the change in the UV-vis absorption spectrum recorded during the illumination of a quartz slide covered with poly-ZnOEP-POM (1) in an aerated aqueous solution containing 8  10 -5 mol L-1 Ag2SO4 and 0.13 mol L-1 propan-2-ol. The spectrum presents a very broad absorption band in the whole visible domain, which suggests that (33) Aiken, J. D., III; Finke, R. G. J. Mol. Catal. A: Chem. 1999, 145, 1–44. (34) (a) Reetz, M. T.; Helbig, W. J. Am. Chem. Soc. 1994, 116, 7401–7402. (b) Starowicz, M.; Stypula, B.; Banas, J. Electrochem. Commun. 2006, 8, 227–230. (35) Kim, Y. H.; Lee, D. K.; Kang, Y. S. Colloids Surf., A 2005, 257-258, 273– 278. (36) (a) Nagata, Y.; Watananabe, Y.; Fujita, S.; Dohmaru, T.; Taniguchi, S. Chem. Commun. 1992, 21, 1620–1622. (b) Fujimoto, T.; Terauchi, S.-Y.; Umehara, H.; Kojima, I.; Henderson, W. Chem. Mater. 2001, 13, 1057–1060. (c) Okitsu, K.; Teo, B. M.; Ashokkumar, M.; Grieser, F. Aust. J. Chem. 2005, 58, 667–670. (37) Yin, H.; Yamamoto, T.; Wada, Y.; Yanagida, S. Mater. Chem. Phys. 2004, 83, 66–70. (38) (a) Tausch-Treml, R; Henglein, A; Lilie, J. Ber. Bunsen-Ges. Phys. Chem. 1978, 82, 1335–1343. (b) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J.-L.; Delcourt, M.-O. New J. Chem. 1998, 1239–1255. (39) (a) Ohtani, B.; Kakimoto, M.; Miyadzu, H.; Nishimoto, S.-i.; Kagiya, T. J. Phys. Chem. 1988, 92, 5773–5777. (b) Ohtani, B.; Nishimoto, S.-i. J. Phys. Chem. 1993, 97, 920–926. (40) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911–1914. (41) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Appl. Catal. B 2003, 42, 305– 315. (42) Costa-Coquelard, C.; Schaming, D.; Lampre, I.; Ruhlmann, L. Appl. Catal. B 2008, 84, 835–842. (43) Zhang, G.; Keita, B.; Dolbecq, A.; Mialane, P; S
echeresse, F.; Miserque, F.; Nadjo, L. Chem. Mater. 2007, 19, 5821–5823.

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Agþ cations were reduced to give large particles that are inhomogeneous in size and shape. Thus, the well-known plasmon band at around 400 nm for silver clusters was not observed, which could be understood by the absence of any stabilizers.44 After 8 h of light exposure, the UV-visible absorption spectrum did not evolve any more. This indicated the end of the reaction. Then, the morphology of the films was scrutinized by AFM. No change in their morphology was detected, indicating that the silver particles generated on the deposited catalyst did not adsorb on its surface. When the illumination was stopped, no change in the UV-visible absorption spectrum was noticed after 5 days in aerated solution, demonstrating that the generated particles were stable. The TEM micrographs confirmed the formation of silver particles (Figure 5B,D-G) that were inhomogeneous in size and shape because the samples presented not only spherical, elongated, or bent particles but also long linear silver nanowires (thickness ca. 30-50 nm and length ca. 1-5 μm) and silver triangular nanosheets. The presence of contrast fringes in TEM images of silver nanowires and nanosheets could be due to stacking faults during crystallization. Each nanosheet was a single crystal as shown by electron diffraction analysis (Figure 5C). The 6-fold rotational symmetry displayed by the diffraction spots implied that the faces represented the {111} planes. The first set of spots could be indexed to the formally forbidden 1/3{422} reflections of face-centered cubic (fcc) silver with a corresponding lattice spacing of 2.48 A˚. The second set corresponded to Bragg diffraction from the {220} planes of fcc silver with a lattice spacing of 1.44 A˚ (1.445 A˚ in JCPDS file 04-0783). These observations were in agreement with the usual published indexes for silver nanosheets.45,46 We explain the observation of 1 /3{422} reflections that are formally forbidden for a perfect fcc structure by the presence of stacking faults.47 These stacking faults could be caused by bending, which explains the presence of contrast fringes in TEM images.48 The evolutions of the absorbance spectra during illumination were identical for the two different polymers (1 and 2) and were not dependent on the presence or absence of oxygen (Figure S6 in Supporting Information). To be able to assume that the silver nanowires and triangular nanosheets were really due to the presence of copolymers, control experiments were carried out under identical conditions. An aqueous solution containing Ag2SO4 and propan-2-ol was illuminated under visible light in the absence of copolymer. The rate of nanoparticle formation was ca. 10 times slower than the corresponding rate measured in the presence of copolymers. TEM micrographs showed that the shape of the generated particles was quite different. Only small, spherical silver particles were observed (diameter ca. 4 nm, see Figure S4 in Supporting Information). When the same experiment was carried out in the presence of dioxygen, no change in the UV-visible absorption spectrum occurred during the first 10 h. That indicated the absence of silver nanoparticle formation. We can suggest here a competition between the Agþ reduction and the reoxidation by dioxygen of the generated Ag0 nanoparticles.42 (44) Lampre, I.; Pernot, P.; Mostafavi, M. J. Phys. Chem. B 2000, 104, 6233– 6239. (45) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901–1903. (46) Sun, Y.; Xia, Y. Adv. Mater. 2003, 15, 695–699. (47) Germain, V.; Li, J.; Ingert, D.; Wang, Z. L.; Pileni, M. P. J. Phys. Chem. B 2003, 107, 8717–8720. (48) Rodrı´ guez-Gonz
alez, B.; Pastoriza-Santos, I.; Liz-Marz
an, L. M. J. Phys. Chem. B 2006, 110, 11796–11799.

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Figure 5. (A) Change in the UV-vis absorption spectrum of poly-ZnOEP-POM (1) deposited on quartz in aerated aqueous solution

containing 8  10-5 mol L-1 Ag2SO4 and 0.13 mol L-1 propan-2-ol under illumination. *Soret band of the copolymer on quartz. (B) TEM images of the silver nanostructures. (C) Selected-area electron diffraction pattern of the silver triangular nanosheets. The inner spots (circled) corresponded to the formally forbidden 1/3{422} reflections. The second spots (squared) could be indexed to the {220} reflections. (D, E) TEM images of individual silver triangular nanosheets. (F, G) TEM images of individual silver nanowires.

The photocatalytic reduction of the Agþ ions with polyoxometalates usually yielded Ag0 in the form of spheroids. Previous work reported on the formation of 1D silver nanostructure formation and included discussions of the role of polyvinylpyrrolidon (PVP) used as a shape controller, which would strongly interact with the (100) facets of Ag0 nanoparticles, thus favoring their growth into nanowires along the Æ111æ direction.43,49 We can suggest here that the solid-state dispersion of polymers deposited onto the quartz plates should play the role of a matrix during the photocatalytic processes. That would explain the unexpected growth of metallic silver nanowires and nanosheets. (49) Gou, L.; Chipara, M.; Zaleski, J. M. Chem. Mater. 2007, 19, 1755–1760.

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Conclusions Two main conclusions can be drawn from the presented results. First, new mixed porphyrin-polyoxometalate copolymers were synthesized by one-step anodic electropolymerization. This process should be feasible for other POMs functionalized with pendant pyridine groups, which are known in the literature. Therefore, a diverse family of copolymers with different properties as a function of the incorporated POM should be accessible. Second, we demonstrated the efficiency of these new copolymers in the photocatalytic reduction of Ag(I) using visible light. Under ambient conditions, AgI2SO4 was reduced at the interface between water and the copolymeric films Langmuir 2010, 26(7), 5101–5109

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deposited on quartz plates. Both 1D and 2D metallic nanostructures were obtained. We characterized silver nanowires and triangular nanosheets. The silver nanosheets were single crystals of face-centered-cubic structures of various sizes. This process verifies our starting hypothesis that the copolymerization of chromophores and POMs yields materials that are useful for photocatalytic reduction in general. Furthermore, original nanostructures are obtained in this particular case. Further optimization of the molecular components and the polymerization process are the next steps in the development of more homogeneous polymers with design properties. Acknowledgment. This work was supported by the CNRS and by the Universit
e Paris-Sud (11) (Orsay, France), the Universit
e Pierre et Marie Curie (Paris 6, France), the Universit
e Ren
e Descartes (Paris 5), the Ecole Centrale d’Electronique (ECE), and the Universit
e de Strasbourg (Strasbourg, France). This work was also supported by the ANR agency, project no. JC05-52437,

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NCPPOM. Patricia Beaunier (UPMC) is gratefully acknowledged for TEM images. Supporting Information Available: Cyclic voltammograms recorded during the electropolymerization of ZnOEP and 5,15ZnOEP(py)22þ in the presence of Py-POM-Py with cyclic scanning in the potential range of 0 to þ1.80 V versus SCE. Mechanism proposed for electropolymerization. Atomic force micrographs of poly-ZnOEP-POM (1) and poly-5,15-ZnOEP(py)22þ-POM (2). TEM micrograph of the formed silver nanoparticles obtained in aerated aqueous solution containing Ag2SO4 and propan-2-ol under illumination. Schematic representation of the proposed photocatalytic cycle. Change in the UV-vis absorption spectrum and TEM images of the silver nanostructures (triangular nanosheets and nanowires) using as a catalyst poly-5,15-ZnOEP(py)22þ-POM (2) on quartz in aerated aqueous solution containing Ag2SO4 and propan-2-ol under illumination. This material is available free of charge via the Internet at http://pubs.acs.org.

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