Electrostatic Layer-by-Layer Deposition of Photoactive Dendrimers

The number of assembled dendrimers was found to increase as we go to higher-generation dendrimers. This dendrimer generation effect was evident from t...
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Langmuir 2005, 21, 8844-8851

Electrostatic Layer-by-Layer Deposition of Photoactive Dendrimers with Triviologen-Like Cores on Their Surfaces. Synthesis and Electrochemical and Photocurrent Generation Measurements Kheireddine H. Boubbou and Tarek H. Ghaddar* Department of Chemistry, American University of Beirut, Beirut 110236, Lebanon Received April 25, 2005. In Final Form: July 12, 2005 The stepwise assembly of Fre´chet-type dendrimers with naphthalene peripheral groups and positively charged viologen-like cores on quartz and ITO surfaces utilizing the layer-by-layer approach was investigated. We were able to deposit only the (+6) charged dendrimers series on ITO. The number of assembled dendrimers was found to increase as we go to higher-generation dendrimers. This dendrimer generation effect was evident from the UV-vis and electrochemical measurements of the assembled dendrimers. The half-wave potentials (E1/2) of the dendrimers shift to less negative values as the dendrimer generation increases in acetonitrile and to more negative values when assembled on ITO. Anodic photocurrent generation was seen upon light irradiation of the second- and third-generation dendrimers, NB1V3+6 and NB2V3+6, assembled on ITO but not for the zero-generation one, NV3+6. This observation was attributed to a fast charge recombination process in NV3+6 when compared to that of NB1V3+6 and NB2V3+6 dendrimers.

Introduction Dendrimers containing high concentrations of redox and photoactive chromophores have been extensively studied for their charge- and energy-transfer properties.1-25 * Corresponding author. E-mail: [email protected]. Fax: +9611-365217. (1) Ghaddar, T. H.; Wishart, J. F.; Thompson, D. W.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2002, 124, 8285-8289. (2) Ghaddar, T. H.; Wishart, J. F.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 12832-12836. (3) Cotlet, M.; Gronheid, R.; Habuchi, S.; Stefan, A.; Barbafina, A.; Mullen, K.; Hofkens, J.; De Schryver, F. C. J. Am. Chem. Soc. 2003, 125, 13609-13617. (4) Jordens, S.; De Belder, G.; Lor, M.; Schweitzer, G.; Van der Auweraer, M.; Weil, T.; Reuther, E.; Mullen, K.; De Schryver, F. C. Photochem. Photobiol. 2003, 2, 177-186. (5) Maus, M.; De, R.; Lor, M.; Weil, T.; Mitra, S.; Wiesler, U. M.; Herrmann, A.; Hofkens, J.; Vosch, T.; Mullen, K.; De Schryver, F. C. J. Am. Chem. Soc. 2001, 123, 7668-7676. (6) Liu, D. J.; De Feyter, S.; Cotlet, M.; Stefan, A.; Wiesler, U. M.; Herrmann, A.; Grebel-Koehler, D.; Qu, J. Q.; Mullen, K.; De Schryver, F. C. Macromolecules 2003, 36, 5918-5925. (7) Kunieda, R.; Fujitsuka, M.; Ito, O.; Ito, M.; Murata, Y.; Komatsu, K. J. Phys. Chem. B 2002, 106, 7193-7199. (8) Schweitzer, G.; Gronheid, R.; Jordens, S.; Lor, M.; De Belder, G.; Weil, T.; Reuther, E.; Mullen, M.; De Schryver, F. C. J. Phys. Chem. A 2003, 107, 3199-3207. (9) Schenning, A.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 4489-4495. (10) Bar-Haim, A.; Klafter, J. J. Lumin. 1998, 76-77, 197-200. (11) Neuwahl, F. V. R.; Righini, R.; Adronov, A.; Malenfant, P. R. L.; Frechet, J. M. J. J. Phys. Chem. B 2001, 105, 1307-1312. (12) Kimura, M.; Shiba, T.; Muto, T.; Hanabusa, K.; Shirai, H. Tetrahedron Lett. 2000, 41, 6809-6813. (13) Lor, M.; Jordens, S.; De Belder, G.; Schweitzer, G.; Fron, E.; Viaene, L.; Cotlet, M.; Weil, T.; Mullen, K.; Verhoeven, J. W.; Van der Auweraer, M.; De Schryver, F. C. Photochem. Photobiol. 2003, 2, 501510. (14) Lor, M.; Thielemans, J.; Viaene, L.; Cotlet, M.; Hofkens, J.; Weil, T.; Hampel, C.; Mullen, K.; Verhoeven, J. W.; Van Der Auweraer, M.; De Schryver, F. C. J. Am. Chem. Soc. 2002, 124, 9918-9925. (15) Yamanaka, K.; Fujitsuka, M.; Ito, O.; Aoshima, T.; Fukushima, T.; Miyashi, T. Bull. Chem. Soc. Jpn. 2003, 76, 1341-1349. (16) Vicinelli, V.; Maestri, M.; Balzani, V.; Muller, W. M.; Muller, U.; Hahn, U.; Osswald, F.; Vogtle, F. New J. Chem. 2001, 25, 989-993. (17) Sadamoto, R.; Tomioka, N.; Aida, T. J. Am. Chem. Soc. 1996, 118, 3978-3979. (18) Janssen, R. A. J.; Jansen, J.; vanHaare, J.; Meijer, E. W. Adv. Mater. 1996, 8, 494-&.

Because of the unique architecture of these synthetic 3D hyperbranched polymers, dendrimers can be synthesized in such a way as to create an energy or redox gradient.26-29 As such, they have the potential to act as light-,10,30,31 hole-,32 or even electron-harvesting antennae.1 With such properties, dendrimers are very attractive synthetic polymers that can be used in the area of photon-to-current conversion33,34 because dendrimers can be synthesized with a high number of light-absorbing or photoactive peripheral groups that can transfer the harvested light energy through an energy-gradient framework by multistep electron- or energy-transfer reactions to a redox-active core. However, utilizing the funneled energy at a dendrimer’s core can be achieved only if the charge recom(19) Haga, M. A.; Ali, M.; Arakawa, R. Angew. Chem., Int. Ed. Engl. 1996, 35, 76-78. (20) Tyson, D. S.; Luman, C. R.; Castellano, F. N. Inorg. Chem. 2002, 41, 3578-3586. (21) Capitosti, G. J.; Cramer, S. J.; Rajesh, C. S.; Modarelli, D. A. Org. Lett. 2001, 3, 1645-1648. (22) Toba, R.; Quintela, J. M.; Peinador, C.; Roman, E.; Kaifer, A. E. Chem. Commun. 2001, 857-858. (23) Devadoss, C.; Bharathi, P.; Moore, J. S. Macromolecules 1998, 31, 8091-8099. (24) Gorman, C. B.; Smith, J. C.; Hager, M. W.; Parkhurst, B. L.; Sierzputowska-Garcz, H.; Haney, C. A. J. Am. Chem. Soc. 1999, 121, 9958-9966. (25) Choi, M. S.; Aida, T.; Luo, H.; Araki, Y.; Ito, O. Angew. Chem., Int. Ed. 2003, 42, 4060-4063. (26) Selby, T. D.; Blackstock, S. C. J. Am. Chem. Soc. 1998, 120, 12155-12156. (27) Heinen, S.; Meyer, W.; Walder, L. J. Electroanal. Chem. 2001, 498, 34-43. (28) Shortreed, M. R.; Swallen, S. F.; Shi, Z. Y.; Tan, W. H.; Xu, Z. F.; Devadoss, C.; Moore, J. S.; Kopelman, R. J. Phys. Chem. B 1997, 101, 6318-6322. (29) Devadoss, C.; Bharathi, P.; Moore, J. S. J. Am. Chem. Soc. 1996, 118, 9635-9644. (30) Serroni, S.; Campagna, S.; Puntoriero, F.; Loiseau, F.; Ricevuto, V.; Passalacqua, R.; Galletta, M. C. R. Chim. 2003, 6, 883-893. (31) BarHaim, A.; Klafter, J.; Kopelman, R. J. Am. Chem. Soc. 1997, 119, 6197-6198. (32) Hara, M.; Samori, S.; Cai, X.; Tojo, S.; Arai, T.; Momotake, A.; Hayakawa, J.; Uda, M.; Kawai, K.; Endo, M.; Fujitsuka, M.; Majima, T. J. Am. Chem. Soc. 2004, 126, 14217-14223. (33) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 7, 3693-3723. (34) Nelson, J. M. Curr. Opin. Solid Mater. Sci. 2002, 6, 87-95.

10.1021/la051100r CCC: $30.25 © 2005 American Chemical Society Published on Web 08/11/2005

Layer-by-Layer Deposition of Photoactive Dendrimers

bination process (back ET) is slow and if a charge flow to an electrode can occur only on the nanometer scale.35,36 Recently, dendrimers have been incorporated in the preparation of mono- and multilayer thin films because of their various potential applications in the area of material science.37-49 Multilayered dendrimer-based assemblies have been constructed by many groups;40,50-61 however, very few reports are found in the literature about dendrimer-based multilayer films assembled by the LbL method that incorporate redox- or photoactive sites.38 Herein, we report for the first time the preparation of multilayer films electrostatically assembled by utilizing the layer-by-layer (LbL) approach between Fre´chet-type dendrimers incorporating a triviologen-like core and poly(styrenesulfonate) (PSS) as the corresponding anionic polyelectrolyte. The step-by-step formation of these films was studied by UV-vis spectrophotometry and electrochemistry. The photocurrent generation of these assembled dendrimers on ITO was also investigated. Experimental Section Materials and Instrumentation. All chemicals were purchased from Aldrich and used as supplied. Methylene chloride (CH2Cl2) and tetrahydrofuran (THF) were distilled from calcium hydride (CaH2) and sodium/benzophenone prior to use, respectively. All aqueous solutions were prepared with doubly distilled or deionized water (resistivity 18 MΩ cm). The NMR spectra were recorded on a Bruker AM 300 MHz spectrometer. Absorption (35) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (36) Ohkita, H.; Ishii, H.; Ogi, T.; Ito, S.; Yamamoto, M. Radiat. Phys. Chem. 2001, 60, 427-432. (37) Khopade, A. J.; Caruso, F. Biomacromolecules 2002, 3, 11541162. (38) Yoon, H. C.; Hong, M. Y.; Kim, H. S. Anal. Chem. 2000, 72, 4420-4427. (39) Yoon, H. C.; Hong, M. Y.; Kim, H. S. Anal. Biochem. 2000, 282, 121-128. (40) Yoon, H. C.; Kim, H. S. Anal. Chem. 2000, 72, 922-926. (41) Wang, J.; Rivas, G.; Fernandes, J. R.; Jiang, M.; Paz, J. L. L.; Waymire, R.; Nielsen, T. W.; Getts, R. C. Electroanalysis 1998, 10, 553-556. (42) Krasteva, N.; Besnard, I.; Guse, B.; Bauer, R. E.; Mullen, K.; Yasuda, A.; Vossmeyer, T. Nano Lett. 2002, 2, 551-555. (43) Khopade, A. J.; Caruso, F. Nano Lett. 2002, 2, 415-418. (44) Weener, J. W.; Meijer, E. W. Adv. Mater. 2000, 12, 741-+. (45) Tully, D. C.; Wilder, K.; Frechet, J. M. J.; Trimble, A. R.; Quate, C. F. Adv. Mater. 1999, 11, 314-318. (46) Gooding, J. J.; Mearns, F.; Yang, W. R.; Liu, J. Q. Electroanalysis 2003, 15, 81-96. (47) Rio, Y.; Accorsi, G.; Armaroli, N.; Felder, D.; Levillain, E.; Nierengarten, J. F. Chem. Commun. 2002, 2830-2831. (48) Zhong, H.; Wang, J. F.; Jia, X. F. R.; Li, Y.; Qin, Y.; Chen, J. Y.; Zhao, X. S.; Cao, W. X.; Li, M. Q.; Wei, Y. Macromol. Rapid Commun. 2001, 22, 583-586. (49) Wang, J. F.; Jia, X. R.; Zhong, H.; Luo, Y. F.; Zhao, X. S.; Cao, W. X.; Li, M. Q. Chem. Mater. 2002, 14, 2854-2858. (50) Liu, Y. L.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114-2116. (51) Anzai, J.; Kobayashi, Y.; Nakamura, N.; Nishimura, M.; Hoshi, T. Langmuir 1999, 15, 221-226. (52) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171-2176. (53) Casson, J. L.; Wang, H. L.; Roberts, J. B.; Parikh, A. N.; Robinson, J. M.; Johal, M. S. J. Phys. Chem. B 2002, 106, 1697-1702. (54) Esumi, K.; Akiyama, S.; Yoshimura, T. Langmuir 2003, 19, 7679-7681. (55) He, J. A.; Valluzzi, R.; Yang, K.; Dolukhanyan, T.; Sung, C. M.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268-3274. (56) Khopade, A. J.; Caruso, F. Langmuir 2002, 18, 7669-7676. (57) Luo, Y. F.; Li, Y.; Jia, X. R.; Yang, H. Q.; Yang, L.; Zhou, Q. F.; Wei, Y. J. Appl. Polym. Sci. 2003, 89, 1515-1519. (58) Tully, D. C.; Frechet, J. M. J. Chem. Commun. 2001, 12291239. (59) Tsukruk, V. V. Adv. Mater. 1998, 10, 253-+. (60) Kim, B. Y.; Bruening, M. L. Langmuir 2003, 19, 94-99. (61) Ito, M.; Imae, T.; Aoi, K.; Tsutsumiuchi, K.; Noda, H.; Okada, M. Langmuir 2002, 18, 9757-9764.

Langmuir, Vol. 21, No. 19, 2005 8845 spectra were recorded on a Jasco V-570 UV/VIS/NIR spectrophotometer. Fluorescence spectra were measured on a JobinYvon Horiba Fluorolog-3 spectrofluorometer. The electrochemical setup consisted of a three-electrode cell, with a homemade glassy carbon electrode as the working electrode, a Pt wire ∼2 mm diameter as the counter electrode, and Ag/AgCl as the reference electrode. All solution phase electrochemical measurements were done in acetonitrile (CH3CN) and 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) as the corresponding electrolyte. The glassy carbon electrode was first polished with 5µm and 0.05µm alumina slurries consecutively, rinsed with doubly distilled water then dried with N2 prior to each electrochemical measurement. The photoelectrochemical setup consisted of a three-electrode quartz cell, with (dendrimer+6/PSS)n on ITO as the working electrode, a Pt wire ∼2 mm diameter as the counter electrode, and Ag/AgCl as the reference electrode. The cell was irradiated with a Hg lamp (Oriel Instruments) operated at 300 W through a homemade light chopper. An identical quartz cell ∼3.4 cm in diameter filled with double-distilled water was used to filter the IR radiation. Films were deposited on both sides of the ITO surface, but the conductive side was oriented facing the incident light. NaCl(aq) (0.1 M) was used as the supporting electrolyte solution. To deoxygenate the solution, N2 was purged through the solution for 30-40 min, and a N2 blanket was maintained over the solution during measurements. Current-voltage measurements were collected with a home-interfaced bipotentiostat (model AFCBP1, Pine Instrument Company and Pine 2.7 software). The MALDI-TOF measurements were done at the Georgia Tech. Research Corporation (Atlanta, GA). The spectra were recorder using positive-ion mode with a sample concentration of 10 mg/mL mixed with R-cyanohydroxycinnamic acid (CHCA, 10 mg/mL) as the corresponding matrix. Film Preparation. Multilayer films were prepared on (1 × 0.5 cm2) quartz slides and on (2 × 5 cm2) ITO slides (Delta Technologies, Ltd.). The quartz slides were immersed in piranha solution for 1 h at 80 °C (3:1 v/v mixture of concentrated H2SO4 and 30% H2O2) (caution: piranha reacts violently with organic compounds and should not be stored in closed containers), rinsed thoroughly with double-distilled water, and then dried with N2 prior to film preparation. The ITO slides were cleaned in boiling ethanol for 30 min, immersed in a 3:1 v/v mixture of ammonia and 30% H2O2 for 10 min, rinsed with double-distilled water and ethanol, and then dried with N2 prior to use. The deposition of one dendrimer layer occurred by immersion of the above cleaned substrates in a solution of the respective dendrimer (2 mg/mL) in acetonitrile for 20 min, followed by washing with acetonitrile and then water and drying with N2. The dendrimer-coated substrate was then immersed in an aqueous solution of poly(styrenelsulfonate) (PSS, 50 mM) containing 0.5 M NaCl for 20 min, followed by washing with water and drying with N2. This procedure was repeated until the desired number of (dendrimer+6/ PSS-NaCl)n bilayers was deposited. Preparation of 1,3,5-Tris[(4,4′)bipyridinyl-1-ium]benzene. 4,4′-Bipyridine (3.0 g, 19.2 mmol) and 1,3,5-tris(bromomethyl)benzene (0.36 g, 1.0 mmol) were refluxed in acetonitrile for 24 h under a nitrogen atmosphere. After cooling, a saturated aqueous solution of NH4PF6 was added with vigorous stirring until the entire product precipitated down. The off-white solid was filtered off and washed with water, methanol, CH2Cl2, and petroleum ether, respectively, to afford a quantitative yield of the pure product. 1H NMR (CD3CN): δ 9.01 (s, 6H), 8.95 (d, J ) 6.9 Hz, 6H), 8.50 (d, J ) 6.9 Hz, 6H), 7.97 (d, J ) 5.8 Hz, 6H), 7.73 (s, 3H), 5.93 (s, 6H). 13C NMR (CD3CN): δ 154.4, 150.6, 144.9, 141.0, 134.9, 130.8, 125.9, 121.7, 117.0, 62.7. MALDITOF MS (CHCA): m/z ) 876.2 [M-PF6] and 1020.3 [M]; calculated for C39H33F18N6P3, 1020.3. General Procedure for the Preparation of Dendrimers NV3+6 and NV1+2. 2-Bromomethyl naphthalene (4 equiv) and 1,3,5-tris[(4,4′)bipyridinyl-1-ium]benzene (1 equiv) or 4,4′-bipyridine (1 equiv) were placed in a minimum amount of acetonitrile and stirred at room temperature for 3 days, respectively, after which the desired compound was precipitated down upon the addition of aqueous NH4PF6. The off-white solid was then filtered and washed with water, ethanol, and hexane, respectively. Both solids were recrystallized from acetonitrile and methylene chloride.

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NV3+6: 1H NMR (CD3CN): δ 8.97 (d, J ) 6.5 Hz, 6H), 8.86 (d, J ) 6.6 Hz, 6H), 8.31-8.35 (m, 12H), 7.91-7.98 (m, 12H), 7.557.60 (m, 12H), 5.94 (s, 6H), 5.77 (s, 6H). 13C NMR (CD3CN): δ 150.28, 149.94, 145.45, 145.40, 134.53, 133.26, 132.85, 131.51, 129.60, 129.23, 129.20, 127.86, 127.50, 127.27, 127.16, 127.10, 126.92, 125.37, 64.60, 63.28. FAB MS: m/z ) 1443.8 [M-3PF6], 1588.8 [M-2PF6], and 1733.6 [M-PF6]; calculated for C72H60N6F36P6, 1878.3. NV1+2: 1H NMR (CD3CN): δ 9.04 (d, J ) 6.9 Hz, 4H), 8.39 (d, J ) 6.9 Hz, 4H), 7.96-8.07 (m, 8H), 7.57-7.65 (m, 6H), 6.00 (s, 4H). 13C NMR (CD3CN): δ 150.02, 145.36, 133.25, 132.85, 129.65, 127.86, 127.50, 127.25, 127.13, 126.91, 126.32, 125.38, 118.03, 64.58. FAB MS: m/z ) 583.2 [M-PF6]; calculated for C32H26F12N2P2, 728.1. Preparation of Dendrimers NB1V1+2 and NB2V1+2. These dendrimers were synthesized using the same method as reported in the literature.1 General Procedure for the Preparation of Dendrimers NB1V3+6, NB2V3+6, and BB2V3+6. 1,3,5-Tris[(4,4′)bipyridinyl1-ium]benzene (1 equiv) and the corresponding brominated dendron1 (3.5 equiv) in a minimum amount of acetonitrile were refluxed for 24 h under nitrogen in the dark. A saturated solution of NH4PF6 in water was then added slowly to the solution while stirring. The resulting yellow solid was filtered, washed with water, and then air dried. The solid was then purified by precipitating it from CH2Cl2 by the addition of toluene. The fine solid was filtered and washed with toluene and hexane, respectively. NB1V3+6: 1H NMR (CD3CN): δ 8.83 (d, J ) 2.2 Hz, 6H), 8.68 (d, J ) 2.2 Hz, 6H), 8.19 (d, J ) 2.2 Hz, 6H), 8.01 (d, J ) 2.2 Hz, 6H), 7.69-7.75 (m, 24H), 7.64 (s, 3H), 7.31-7.40 (m, 18H), 6.71 (d, J ) 2.1 Hz, 3H), 6.58 (d, J ) 2.1 Hz, 6H), 5.75 (s, 6H), 5.53 (s, 6H), 5.16 (s, 12H). 13C NMR (CD3CN): δ 160.26, 149.86, 149.48, 145.45, 145.01, 134.66, 134.23, 134.07, 132.77, 132.59, 131.78, 127.94, 127.45, 127.32, 126.71, 126.32, 126.12, 126.05, 125.98, 125.11, 108.24, 103.21, 69.75, 64.24, 63.39. MALDI-TOF MS (CHCA): m/z ) 2229.7 [M-3PF6], 2374.7 [M-2PF6], and 2519.7 [M + H-PF6]; calculated for C126H102F36N6O6P6, 2664.7. NB2V3+6: 1H NMR (CD3CN): δ 8.67 (d, J ) 6.9 Hz, 6H), 8.44 (d, J ) 6.9 Hz, 6H), 7.96 (d, J ) 6.9 Hz, 6H), 7.79 (d, J ) 6.9 Hz, 6H), 7.58-7.68 (m, 51H), 7.28-7.36 (m, 36H), 6.57 (d, J ) 2.2 Hz, 12H), 6.55 (d, J ) 2.2 Hz, 6H), 6.51 (t, 6H), 6.45 (d, J ) 2.1 Hz, 3H), 5.68 (s, 6H), 5.31 (s, 6H), 4.96 (s, 24H), 4.84 (s, 12H). 13C NMR (CD CN): δ 160.08, 159.71, 149.19, 149.01, 145.13, 3 144.59, 139.00, 134.62, 134.29, 133.77, 132.69, 132.49, 132.00, 127.81, 127.38, 127.29, 126.68, 126.29, 126.07, 125.94, 125.90, 125.18, 108.29, 106.39, 102.95, 101.19, 69.45, 69.36, 64.32, 63.35. MALDI-TOF MS (CHCA): m/z ) 3802.3 [M-3PF6] and 3947.3 [M-2PF6]; calculated for C234H186F36N6O18P6, 4237.2. BB2V3+6: 1H NMR (CD3CN): δ 8.74-8.83 (m, 12H), 8.138.22 (m, 12H), 7.61 (s, 3H), 7.20-7.29 (m, 60H), 6.58 (s, 12H), 6.56 (s, 6H), 6.48 (s, 6H), 6.47 (s, 3H), 5.74 (s, 6H), 5.55-5.58 (m, 6H), 4.91-4.95 (m, 36H). 13C NMR (CD3CN): δ 160.1, 159.7, 149.9, 149.7, 145.4, 145.1, 139.0, 136.8, 134.6, 134.1, 131.7, 128.2, 127.7, 127.4, 127.1, 117.0, 108.2, 106.2, 102.9, 101.1, 69.4, 64.3, 63.3. MALDI-TOF MS (CHCA): m/z ) 3059.0 [M-4PF6] and 3203.9 [M-3PF6]; calculated for C186H162F36N6O18P6, 3638.9.

Results and Discussion Dendrimer/PSS Multilayer Film Formation. The dendrimers studied here (Scheme 1) are two series of zero-, first-, and second-generation Fre´chet-type dendrimers. The first series (NV3+6, NB1V3+6, and NB2V3+6) is composed of naphthalene peripheral groups (N), benzylether spacers (B), and a triviologen-like core (V3+6), whereas the second series (NV1+2, NB1V1+2, and NB2V1+2) has one viologen-like moiety at the core (V1+2). We were able to deposit multilayer films of the (+6) positively charged dendrimers (NV3+6, NB1V3+6, and NB2V3+6) on ITO and quartz surfaces using the LbL approach and PSS as the corresponding anionic polyelectrolyte. However, we were not successful in depositing

Boubbou and Ghaddar

films made up of the second series (NV1+2, NB1V1+2, and NB2V1+2) and PSS on either ITO or on quartz most probably because of fewer positive charges at the core (+2). The important finding that was studied during the course of this work was the ability to deposit such hydrophobic dendrimers with only a (+6) charge at the core and the successful use of dendrimer solutions in acetonitrile to deposit these films, whereas water was used as the solvent for the PSS-NaCl solution. This new finding of the ability to use such hydrophobic and positively charged dendrimers widens the use of the LbL method to incorporate similar dendrimers. Figure 1 shows the UVvis spectra of (NB2V3+6/PSS-0.5 M NaCl)n multilayers deposited on a quartz substrate, where the first layer is NB2V3+6 and the outmost layer is PSS. The bilayer deposition was monitored at 280 nm, where the triviologen core has the most significant absorption.1,62 The multilayers grew linearly after each sequential deposition of the dendrimer+6/PSS-NaCl bilayer, which suggested a successful film deposition by the electrostatic LbL approach (Figure 1 inset). In a control experiment, the PSS0.5 M NaCl solution was replaced by a 0.5 M NaCl(aq) solution after the deposition of the first bilayer (dendrimer+6/PSS-0.5 NaCl)1 in order to investigate the possibility of forming a continuous layer of dendrimer that does not incorporate any PSS upon multiple successive dipping in the dendrimer and NaCl solutions. However, this was not the case because no change in absorbance was detected after the first cycle of deposition of the first dendrimer layer above the (dendrimer+6/PSS-0.5 NaCl)1 bilayer. In another control experiment that strengthened the above observation, we monitored the amount of dendrimer that was deposited over the first bilayer (dendrimer+6/PSS-0.5 M NaCl)1 by following the absorbance change at 280 nm versus time (Figure 2). It was clear that the amount of dendrimer that gets deposited during the formation of the bilayer somehow follows an exponential curve, where the amount of deposited dendrimer does not increase significantly after 20 min of continuous dipping in the dendrimer-CH3CN solution and then reaches a plateau after 40 min. No desorption of the three different deposited dendrimers at each bilayer stage was seen upon dipping in the PSS-NaCl solution. This is may be due to the poor solubility of the dendrimers in water, which renders the extraction of the adsorbed dendrimer by the PSS solution. The absorbance of the 10 bilayer films of NB2V3+6, NB1V3+6, and NV3+6/PSS0.5 M NaCl at 280 nm was 0.26, 0.23, and 0.18 ( 0.01, respectively. This difference in absorption intensity at 280 nm is mainly due to the increase in the number of incorporated dendrimer molecules within each layer and is not due to the increase in the absorption extinction coefficient as we go higher in dendrimer generation because at 280 nm the major absorption is from the viologen core and a very small contribution is from the dendrons.1,62 This dendrimer generation effect on the amount of deposited dendrimer was also evident from the electrochemical measurements that are discussed later in the article. The number of assembled dendrimers on PSS was also found to depend on the ionic strength of the PSS solution during the multilayer deposition. Figure 3 shows plots of (NB2V3+6/PSS-NaCl)n film absorbance at 280 nm as a function of the number of bilayers assembled from PSSNaCl solutions of different ionic strengths while keeping the dendrimer and PSS concentrations the same. Whereas (62) Ceroni, P.; Vicinelli, V.; Maestri, M.; Balzani, V.; Muller, W. M.; Muller, U.; Osswald, F.; Vogtle, F. New J. Chem. 2001, 989-993.

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Scheme 1. Molecular Structures of the Two Dendrimer Series as Their PF6 Salts

in the absence of any salt the (NB2V3+6/PSS)10 film has an absorbance of 0.09 at 280 nm, increasing the salt content to only 0.05 M resulted in an increase in dendrimer absorbance to 0.13 at 280 nm. The dendrimer absorbance per layer increased significantly (by ∼100% in reference to 0.05 M) upon increasing the ionic strength to 0.5 M but did not increase by much upon increasing the salt concentration further to 1.0 M. The same effect was observed for dendrimers NV3+6 and NB1V3+6. It is well known that the density and microenvironment of deposited polyelectrolytes, such as PSS, depend on the

repulsion forces of the ionic sites (SO3- in the case of PSS) distributed along the chain, which can be easily manipulated by changing the ionic strength of the polyelectrolyte solution.63-66 Upon increasing the ionic strength of the PSS solution, more of the anionic sites get shielded, and (63) Ghannoum, S.; Xin, Y.; Jaber, J.; Haloui, L. Langmuir 2003, 19, 4804-4811. (64) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 8153. (65) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (66) Korneev, D.; Lvov, Y.; Decher, G.; Schmidt, J.; Yaradaikin, S. Physica B 1995, 213/214, 954.

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Figure 1. UV-vis absorption spectra of (NB2V3+6/PSS-0.5 M NaCl)n bilayers on quartz, where n ) 1-10. The inset is a plot of the absorbance at 280 nm vs the number of bilayers.

Figure 4. Cyclic voltammetry of (NB2V3+6/PSS-0.5 M NaCl)n bilayers on ITO, where n ) 2 (‚ ‚ ‚), 5 (- - -), and 8 (-). The scan rate is 100 mV s-1, and 0.1 M NaCl(aq) is the supporting electrolyte.

Figure 2. Plot of the absorbance at 280 nm vs the total dipping time of one (NB2V3+6/PSS-0.5 M NaCl)1 bilayer in the bulk solution of NB2V3+6 in CH3CN.

Figure 5. Cyclic voltammetry of (- - -) NB2V1+2 and (-) NB2V3+6 in CH3CN. The scan rate is 100 mV s-1, and 0.1 M TBAPF6 is the supporting electrolyte.

Figure 3. Absorbance at 280 nm vs the number of the (NB2V3+6/PSS-NaCl)n bilayers (n ) 1-10) deposited from a PSS(aq) solution with NaCl concentrations of 0, 0.05, 0.5, and 1.0 M.

a more compact PSS layer gets deposited. This seems to increase the amount of dendrimer that gets deposited per layer most probably because of denser anionic sites along the PSS chain that electrostatically hold the cationic core of the dendrimer and also because of an increase in the hydrophobic interactions between the dendrimer’s aromatic groups and the PSS phenyl groups. Electrochemical Measurements in Solution and on ITO. The dendrimer generation effect on the electrochemical behavior of the viologen core was investigated by cyclic voltammetry in acetonitrile for the two dendrimer series and on ITO as bilayer films for the (+6) charged dendrimers (NV3+6, NB1V3+6, and NB2V3+6). Figure 4 shows the cyclic voltammetry of (NB2V3+6/PSS-0.5 M

NaCl)n bilayers on ITO with n ) 2, 5, and 8, which clearly indicates that the formation of the bilayers takes place in a similar fashion as on quartz surfaces where a steady increase in the current density is seen upon sequential deposition of the NB2V3+6/PSS-NaCl bilayers. Figure 5 shows the cyclic voltammetry of the two dendrimers, NB2V3+6 and NB2V1+2, in acetonitrile for the first reduction process of the viologen moieties. In the first reduction process, the (+6) dendrimer series would exchange three electrons per dendrimer, whereas the (+2) dendrimer series would exchange only one.67 In addition, in the (+6) dendrimer series the three viologen moieties seem to be independent and lack any communication as evident from the cyclic voltammetry at different scan rates. Similar behavior has been reported by Walder et al.67 with dendrimers consisting of a viologen skeleton made up of 3, 9, 21, and 45 viologen moieties. The viologens lacked any electronic communication during the first reduction process, and the number of electrons transferred per dendrimer molecule, as determined by electrolysis coulometry, during the reduction process is the same as the theoretical value (i.e., one electron transfer per one viologen molecule for the first reduction process). Scanning the potential toward more negative values than the first reduction process resulted in the precipitation of the totally reduced dendrimers on the electrode surface. This was evident from the appearance of distorted peaks in the voltammograms when the applied potential is scanned to more negative potentials than -0.8 V. The half-wave potentials (E1/2) for the six dendrimers on ITO and in (67) Heinen, S.; Walder, L. Angew. Chem., Int. Ed. 2000, 39, 806809.

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Table 1. Half-Wave Potentials (E1/2 in V vs Ag/AgCl) for the Two Dendrimer Series in CH3CN and as Bilayers on ITO electrochemical parameters

NV3+6

(solution)a

-0.293 -0.484

E1/2 E1/2 (film)b

first series NB1V3+6 -0.292 -0.532

NB2V3+6

NV1+2

-0.214 -0.566

-0.323

second series NB1V1+2 -0.316

NB2V1+2 -0.277

a Measured at 100 mV/s with a glassy carbon working electrode in 0.1 M TBAPF solutions in acetonitrile. b Measured at 100 mV/s in 6 0.1 M NaCl(aq) solution.

Figure 6. Cyclic voltammetry of eight bilayers of (- - -) NV3+6, (‚ ‚ ‚) NB1V3+6, and (-) (NB2V3+6/PSS-0.5 M NaCl)8 on ITO. The scan rate 100 mV s-1, and 0.1 M NaCl(aq) is the supporting electrolyte.

Figure 7. Steady-state fluorescence spectra of (a) NB2, (b) NB2V3+6, (c) NB1V3+6, and (d) NV3+6 in degassed CH3CN, with λex ) 266 nm and absorbance ) 0.1 at 266 nm.

acetonitrile are summarized in Table 1. An analysis of the E1/2 of the (dendrimer+6/PSS-0.5 M NaCl)8 bilayers on ITO showed an inverse trend when compared to the solution phase. The half-wave potentials of the bilayers shift to more negative values as the dendrimer generation increases, whereas the E1/2 values of all of the dendrimers in acetonitrile shift to less negative values with increasing dendrimer generation. The latter observation is consistent with the electrochemical measurements in acetonitrile reported by different groups on similar Fre´chet-type dendrimers with one viologen moiety at the core.22,62 This is attributed to the fact that the reduction process of the viologen core becomes thermodynamically more favored in acetonitrile as the dendrimer size increases because with a higher dendrimer generation an increase in the hydrophobic environment results around the viologen core, which in turn destabilizes the positive charges of the viologen core. On ITO, the positive charges of the triviologen core are stabilized by the water shell around it and by the electrostatically held PSS, thus shifting the redox potentials to more negative values (E1/2 ) -0.566 V on ITO compared to -0.214 V in acetonitrile for NB2V3+6). However, as we go to higher dendrimer generation, the E1/2 values shift even more toward negative values for the dendrimer+6/PSS bilayers (E1/2 ) -0.532 and -0.566 V for NB1V3+6 and NB2V3+6, respectively). This is due to the increase of the viologen charge stabilization due to denser PSS layers that get deposited. This is deduced from the current densities of the three (dendrimer+6/PSS-0.5 M NaCl)8 bilayers on ITO (Figure 6), where an increase in the current is seen as we go to a higher dendrimer generation that represents denser dendrimer+6/PSS bilayers. This dendrimer generation effect on the amount of dendrimer that gets deposited is consistent with the UV-vis data of the bilayers on quartz. This increase in the number of deposited dendrimers is most probably due to the hydrophobic interactions between the aromatic rings of the dendrimer and PSS, which gets stronger as we go to higher-generation dendrimers. It is worth mentioning here that Kaifer et al.68 recently

reported on the electrochemical behavior of Newkometype dendrons with a viologen group at the core, where E1/2 shifts to more negative values in less polar solvents such as CH3CN and to less negative values in more polar solvents such as DMSO as the dendrimer size increases. The authors attributed this trend change to the microenvironment that gets created by the dendrimer around the viologen core, which presents an average polarity that is intermediate between that of CH3CN and DMSO compared to the dendrimer’s. Similarly, in the case of the dendrimers studied here, the microenvironment around the triviologen core depends on the concentration of PSS molecules around it, which seems to increase as we go to higher dendrimer generation, thus creating a more hydrophilic microenvironment around the triviologen core. Photocurrent Generation in the Dendrimer Films. The three dendrimers (NV3+6, NB1V3+6, and NB2V3+6) studied here are expected to initiate forward electrontransfer reactions (ET) from the excited donor groups (N and/or B) to the triviologen electron acceptor (V3+6) upon UV-light irradiation. Indeed, steady-state fluorescence measurements of the three dendrimers show strong fluorescence quenching of both the excited naphthyl and benzyl-ether groups in acetonitrile (Figure 7) compared with that observed for the NB2 dendron (Scheme 2), which lacks the electron-acceptor core (V3+6). These results are consistent with our previous extensive work on the (+2) charged dendrimers (NV1+2, NB1V1+2, and NB2V1+2),1 where we have found that upon laser irradiation (266 nm) a very fast forward electron-transfer reaction takes place from the dendron’s excited benzyl and/or naphthalene groups to the viologen core in the first- and secondgeneration dendrimers with rate constants higher than 3 × 109 s-1, whereas the charge recombination process was found to be more than 4 orders of magnitude slower than the forward ET rates. We are currently studying this observation using pulse radiolysis and transient absorption techniques in order to understand this large difference between the forward and back electron-transfer reaction rates for the two dendrimer series studied here. Bearing in mind this difference between the forward and back ET reaction rates, we decided to study the photo-

(68) Ong, W.; Kaifer, A. E. J. Am. Chem. Soc. 2002, 124, 9358-9359.

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Scheme 2. Molecular Structures of Second-Generation Dendrimers NB2 and BB2V3+6

current generation of the dendrimer+6/PSS-NaCl bilayers on ITO. Figure 8 shows the anodic photocurrent generation of the three dendrimer multilayers on ITO, (NV3+6, NB1V3+6, and NB2V3+6/PSS-0.5 M NaCl)8, in 0.1 M NaCl(aq) solution with 1 mM ascorbic acid (AA) as a sacrificial electron donor. For the (NV3+6/PSS-0.5 M NaCl)8 multilayer film, no anodic photocurrent was detected upon light irradiation, whereas for dendrimers NB1V3+6 and NB2V3+6 a steady anodic photocurrent was generated upon light irradiation. Both dendrimer films had similar photocurrent densities after correcting the NB1V3+6/PSS-NaCl film optical density to match that of NB2V3+6 (Figure 8). However, no cathodic photocurrent generation was detected for the three dendrimer films regardless of the applied potential. The anodic photocurrent gradually diminished as we went to more negative potentials (less than -0.1 V) until it disappeared at potentials lower than -0.4 V, which suggested that the photocurrent at potentials less negative than -0.4 V is indeed due to the flow of electrons from the reduced triviologen core to ITO. Scheme 3 illustrates the interfacial electron-transfer reactions that take place in the (NV3+6, NB1V3+6, and NB2V3+6/PSS-NaCl)8 films on ITO. In the case of the NV3+6/PSS-NaCl film, it can be deduced that the charge recombination process (back ET) between the reduced viologen core and the oxidized naphthyl moieties is faster than the ET reaction between the reduced viologen core and ITO, whereas for NB1V3+6 and NB2V3+6

Figure 8. Photocurrent response of (- - -) NV3+6, (‚ ‚ ‚) corrected NB1V3+6, and (-) (NB2V3+6/PSS-0.5 M NaCl)8 on ITO in 0.1 M NaCl(aq) solution with 1 mM ascorbic acid as the sacrificial electron donor and an applied potential of -0.1 V.

Scheme 3. Redox Potentials and Electron-Transfer Reactions in the ITO/Dendrimer+6/PSS-NaCl Filmsa

a Solid arrows indicate the dominant ET reaction path in the case of dendrimers NB1V3+6 and NB2V3+6. Dashed arrows indicate the ET reaction path in dendrimer NV3+6.

the ET reaction rates between the reduced triviologen core and ITO are faster than the charge recombination process. And because the thermodynamic driving force (∆G°) of the ET reaction between the reduced viologen and ITO (applied potential ) -0.1V) is the same for all three dendrimer+6/PSS-NaCl films (∆G° ≈ -0.43 eV), we expected that both dendrimer films NB1V3+6/PSSNaCl and NB2V3+6/PSS-NaCl would show similar photocurrent density, which indeed is the case after correcting for the optical densities of the films (Figure 8). In a control experiment, we deposited an eight-bilayer film of BB2V3+6 (Scheme 2) (BB2V3+6/PSS-0.5 M NaCl)8 in which the naphthyl groups are substituted by benzyl groups at the periphery. The anodic photocurrent intensity was about half of that seen for films incorporating NB1V3+6 and NB2V3+6 (Figure 9). This suggested that the ET reactions taking place in all of the studied dendrimers are from the excited benzyl ether and/or the naphthyl groups to the triviologen core with a higher contribution from the naphthyl moieties. The thermodynamic driving force of the forward ET reactions ∆G° between the excited naphthyl or benzyl-ether and the viologen moiety is derived from electrochemical excited-state potentials to be -1.8 and -2.0 eV, respectively.1,62 The oxidation of a naphthyl group in the flexible first- and second-generation dendrimers (NB1V3+6 and NB2V3+6) likely results in charge transfer between the naphthyl groups along the

Layer-by-Layer Deposition of Photoactive Dendrimers

Figure 9. Photocurrent response of (- - -) BB2V3+6 and (-) (NB1V3+6/PSS-0.5 M NaCl)8 on ITO in 0.1 M NaCl(aq) solution with 1 mM ascorbic acid as the sacrificial electron donor and an applied potential of -0.1 V.

perimeter69 and/or the formation of a naphthalene dimer cation. Therefore, a slower charge recombination process would result in both cases when compared to the less flexible dendrimer NV3+6. However, in the BB2V3+6 case the lower photocurrent intensity can be attributed to a weaker light antennae effect due to the lack of photoactive naphthyl groups on the periphery, which also in turn do not form any naphthalene dimer cations. Conclusions Using the polyionic self-assembly technique (LbL), it was possible to incorporate redox- and photoactive dendrimers with a triviologen-like core into multilayer structures. The amount of incorporated dendrimer within these films was found to be dependent on the dendrimer’s generation, where more of the dendrimer was incorporated into the multilayer films as we went to higher dendrimer (69) Leventis, N.; Yang, J.; Fabrizio, E. F.; Rawashdeh, A. M. M.; Oh, W. S.; Sotiriou-Leventis, C. J. Am. Chem. Soc. 2004, 126, 4094-4095.

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generation. The half-wave potentials (E1/2) of the dendrimers shift to less negative values as the dendrimer generation increase in acetonitrile but to more negative values when assembled on ITO. This was attributed to the fact that the reduction process of the viologen core becomes thermodynamically more favored in acetonitrile as the dendrimer size increases because of the increase in hydrophobicity that in turn destabilizes the positive charges of the viologen core. On ITO, the positive charges of the triviologen core are stabilized by the increase in the hydrophilic microenvironment around the dendrimer core as we go to higher dendrimer generation, thus shifting the redox potentials to more negative values. An anodic photocurrent was observed in the ITO/dendrimer+6/PSSNaCl films for the first- and second-generation dendrimers (NB1V3+6 and NB2V3+6) but not for the zero-generation dendrimer (NV3+6). The photocurrent was reduced by 50% when a second-generation dendrimer was used that lacks the naphthyl peripheral groups (BB2V3+6). These observations were attributed to the slower charge recombination processes that take place in the NB1V3+6 and NB2V3+6 cases when compared to NV3+6. These results demonstrate that incorporating redox- and photoactive dendrimers in layer-by-layer films is an attractive and effective system for photon-to-current energy conversion. We are currently working on synthesizing similar dendrimers loaded with visible-light-absorbing peripheral groups that can be incorporated into multilayer films and the study of the photocurrent generation of such films. Acknowledgment. This work was supported by the University Research Board (URB) at the American University of Beirut (AUB), the Lebanese National Council for Scientific Research (LNCSR), and the Third World Academy of Science (TWAS). We thank Dr. Lara Halaoui at AUB for assistance with the electrochemical measurements. LA051100R