pubs.acs.org/Langmuir © 2010 American Chemical Society
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Synergism in Multicomponent Self-Propagating Molecular Assemblies† Leila Motiei,‡ Mauro Sassi,‡ Revital Kaminker,‡ Guennadi Evmenenko, Pulak Dutta, Mark A. Iron,§ and Milko E. van der Boom*,‡
Department of Organic Chemistry, and §Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel, and Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3113, United States )
‡
Received September 30, 2010. Revised Manuscript Received November 8, 2010 Multicomponent self-propagating molecular assemblies (SPMAs) have been generated from an organic chromophore, a redox-active polypyridyl complex, and PdCl2. The structure of the multicomponent SPMA is not a linear combination of two assemblies generated with a single molecular constituent. Surface-confined assemblies formed from only the organic chromophore and PdCl2 are known to follow linear growth, whereas the combination of polypyridyl complexes and PdCl2 results in exponential growth. The present study demonstrates that an iterative deposition of both molecular building blocks with PdCl2 results in an exponentially growing assembly. The nature of the assembly mechanism is dictated by the polypyridyl complex and overrides the linear growth process of the organic component. Relatively smooth, multicomponent SPMAs have been obtained with a thickness of ∼20 nm on silicon, glass, and indium-tin oxide (ITO) coated glass. Detailed information of the structure and of the surface-assembly chemistry were obtained using transmission optical (UV/Vis) spectroscopy, ellipsometry, atomic force microscopy (AFM), synchrotron X-ray reflectivity (XRR), and electrochemistry.
Introduction Surface-confined supramolecular architectures are an intriguing class of materials that can be designed at the molecular level.1-7 The structure and properties of such materials are often difficult to predict and control a priori due to several factors, including intermolecular interactions, packing, and molecular orientation. Solution-based deposition of molecular building blocks offers the possibility to create multicomponent materials in an organized manner with precise control of their structure and properties not readily accessible by other means.8 For instance, stepwise formation of metal-organic frameworks (MOFs) allows the formation of crystalline structures that are ordered differently than materials obtained with solvothermal synthesis.2,3,9,10 Layerby-layer deposition has also been used for the formation of noncentrosymmetric materials without the need for postdeposition chromophore alignment.11-14 An elegant and versatile method based on a combination of click-chemistry and π-π interactions was recently introduced by Palomaki and Dinolfo who generated † Part of the Supramolecular Chemistry at Interfaces special issue *To whom correspondence should be addressed. E-mail: milko.vanderboom@ weizmann.ac.il.
(1) Ariga, K.; Hill, J. P.; Ji, Q. Phys. Chem. Chem. Phys. 2007, 9, 2319. (2) Zacher, D.; Shekhah, O.; W€oll, C.; Fischer, R. A. Chem. Soc. Rev. 2009, 38, 1418. (3) Makiura, R.; Kitagawa, H. Eur. J. Inorg. Chem. 2010, 24, 3715. (4) Maspoch, D.; Ruiz-Molina, D.; Veciana, J. Chem. Soc. Rev. 2007, 36, 770. (5) Ariga, K.; Ji, Q.; Hill, J. P.; Vinu, A. Soft Matter 2009, 5, 3562. (6) Yamanoi, Y.; Nishihara, H. Chem. Commun. 2007, 3983. (7) Decher, G. Science 2007, 277, 1232. (8) Zhang, X.; Chen, H.; Zhang, H. Chem. Commun. 2007, 1395. (9) Shekhah, O.; Wang, H.; Paradinas, M.; Ocal, C.; Schupbach, B.; Terfort, A.; Zacher, D.; Fischer, R. A.; W€oll, C. Nat. Mater. 2009, 8, 481. (10) Long, J. R.; Yaghi, O. M. Chem. Soc. Rev. 2009, 38, 1213. (11) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197. (12) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Lee, P. A.; Armstrong, N. R.; Dutta, P.; Marks, T. J. Chem. Mater. 2001, 13, 15. (13) Evmenenko, G.; van der Boom, M. E.; Dugan, S. W.; Kmetko, J.; Marks, T. J.; Dutta, P. J. Chem. Phys. 2001, 115, 6722. (14) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485.
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structurally ordered, multicomponent, hierarchical architectures composed of different chromophores.15 The vast majority of assemblies show linear correlations between physicochemical properties (e.g., thickness, electro-optical response, absorption intensity) and the number of deposition steps.11-36 Self-propagating (15) Palomaki, P. K. B.; Dinolfo, P. H. Langmuir 2010, 26, 9677. (16) Altman, M.; Shukla, A. D.; Zubkov, T.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2006, 128, 7374. (17) Altman, M.; Zenkina, O.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2008, 130, 5040. (18) Altman, M.; Zenkina, O. V.; Ichiki, T.; Iron, M. A.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. Chem. Mater. 2009, 21, 4676. (19) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (20) Doron-Mor, I.; Hatzor, A.; Vaskevich, A.; van der Boom-Moav, T.; Shanzer, A.; Rubinstein, I.; Cohen, H. Nature 2000, 406, 382. (21) Hatzor, A.; Moav, T.; Cohen, H.; Matlis, S.; Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13469. (22) Lee, H.; Kepley, L. J.; Hong, H. G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (23) Hong, H. G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, 3, 521. (24) Altman, M.; Rachamim, M.; Ichiki, R.; Iron, M. A.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. Chem.;Eur. J. 2010, 16, 6744. (25) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (26) Kanaizuka, K.; Haruki, R.; Sakata, O.; Yoshimoto, M.; Akita, Y.; Kitagawa, H. J. Am. Chem. Soc. 2008, 130, 15778. (27) DiBenedetto, S. A.; Frattarelli, D. L.; Facchetti, A.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 11080. (28) Pan, Y.; Tong, B.; Shi, J.; Zhao, W.; Shen, J.; Zhi, J.; Dong, Y. J. Phys. Chem. C 2010, 114, 8040. (29) Kurita, T.; Nishimori, Y.; Toshimitsu, F.; Muratsugu, S.; Kume, S.; Nishihara, H. J. Am. Chem. Soc. 2010, 132, 4524. (30) Tuccitto, N.; Ferri, V.; Cavazzini, M.; Quici, S.; Zhavnerko, G.; Licciardello, A.; Rampi, M. A. Nat. Mater. 2009, 8, 41. (31) Greenstein, M.; Ben Ishay, R.; Maoz, B. M.; Leader, H.; Vaskevich, A.; Rubinstein, I. Langmuir 2010, 26, 7277. (32) Kaminker, R.; Motiei, L.; Gulino, A.; Fragala, I.; Shimon, L. J. W.; Evmenenko, G.; Dutta, P.; Iron, M. A.; van der Boom, M. E. J. Am. Chem. Soc. 2010, 132, 14554. (33) Li, D.-Q.; Smith, D. C.; Swanson, B. I.; Farr, J. D.; Paffett, M. T.; Hawley, M. E. Chem. Mater. 1992, 4, 1047. (34) Motreff, A.; Raffy, G.; Del Guerzo, A.; Belin, C.; Dussauze, M.; Rodriguez, V.; Vincent, J.-M. Chem. Commun. 2010, 46, 2617. (35) Zhao, W.; Tong, B.; Shi, J.; Pan, Y.; Shen, J.; Zhi, J.; Chan, W. K.; Dong, Y. Langmuir 2010, 26, 16084. (36) Zhao, W.; Tong, B.; Pan, Y. X.; Shen, J. B.; Zhi, J. G.; Shi, J. B.; Dong, Y. P. Langmuir 2009, 25, 11796.
Published on Web 12/03/2010
DOI: 10.1021/la103936t
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Article Scheme 1. Compounds Used in This Study to Generate Multicomponent Self-Propagating Molecular Assemblies (SPMAs) with PdCl246,50,52
growth, where the film thickness increases exponentially with the number of deposition steps, is known for polyelectrolyte multilayers formed by electrostatic self-assembly and is a result of diffusion of the polymers through the film during its formation.37-43 Multicomponent films consisting of polyelectrolyte multilayers with exponential and linear growth have been reported.42,43 However, exponential growth and/or self-propagating processes with molecular systems where surface-bound assemblies actively participate rather than being a static platform with reactive end groups for the incoming molecular layers are rare.44-48 The integration of different molecular components in one exponentially growing molecular-based assembly has not yet been demonstrated. We have recently shown that coordination-based oligomers and three-dimensional (3D) ordered assemblies are formed by a stepwise solution deposition approach from vinylpyridyl-based organic chromophores and PdCl2.16,17,24,32 Metal-organic networks (MONs) were obtained with chromophore 1 and PdCl2 (Scheme 1).32 These molecular-based assemblies are formed on solid-substrates via a linear growth process. In contrast, assemblies consisting of polypyridyl ruthenium(II) complexes, such as 2, and PdCl2 exhibit exponential growth under identical reaction conditions.44-46,49 The growth mechanism in this latter case might be similar to that in the formation of polyelectrolyte-based films.37-41 Our self-propagating molecular assemblies (SPMAs) store excess palladium salt that is used to bind more than one molecular layer during subsequent chromophore-deposition steps.44,46 The nature of the growth (linear or exponential) can be controlled by varying deposition times and concentrations.46 These SPMAs (37) Cini, N.; Tulun, T.; Decher, G.; Ball, V. J. Am. Chem. Soc. 2010, 132, 8264. (38) Decher, G., Schlenoff, J. B., Eds. Multilayer Thin Films; Wiley-VCH: Weinheim, Germany, 2003. (39) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J.-C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (40) Salomaki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232. (41) Porcel, C.; Lavalle, P.; Ball, V.; Decher, G.; Senger, B.; Voegel, J.-C.; Schaaf, P. Langmuir 2006, 22, 4376. (42) Wood, K. C.; Chuang, H. F.; Batten, R. D.; Lynn, D. M.; Hammond, P. T. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10207. (43) Garza, J. M.; Schaaf, P.; Muller, S.; Ball, V.; Stoltz, J.-F.; Voegel, J.-C.; Lavalle, P. Langmuir 2004, 20, 7298. (44) Motiei, L.; Altman, M.; Gupta, T.; Lupo, F.; Gulino, A.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2008, 130, 8913. (45) Motiei, L.; Lahav, M.; Gulino, A.; Iron, M. A.; van der Boom, M. E. J. Phys. Chem. B 2010, 114, 14283. (46) Choudhury, J.; Kaminker, R.; Motiei, L.; de Ruiter, G.; Morozov, M.; Lupo, F.; Gulino, A.; van der Boom, M. E. J. Am. Chem. Soc. 2010, 132, 9295. (47) Maoz, R.; Matlis, S.; DiMasi, E.; Ocko, B. M.; Sagiv, J. Nature 1996, 384, 150–153. (48) Maoz, R.; Cohen, S. R.; Sagiv, J. Adv. Mater. 1999, 11, 55. (49) Motiei, L.; Lahav, M.; Freeman, D.; van der Boom, M. E. J. Am. Chem. Soc. 2009, 131, 3468.
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are electrochromic49 and have been used for (i) electron-transfer studies,45 (ii) inverted bulk-heterojunction solar cells,50 and (iii) nonvolatile memory.51 Moreover, the electrochemical characteristics of SPMAs can be represented by flip-flops, which form the heart of sequential digital circuits.51 In this study, we combine the two different molecular building blocks 1 and 2 with PdCl2 to form a multicomponent SPMA. Although the number of vinylpyridyl groups of 1 and 2 and their coordination chemistry are identical, the geometry, size, symmetry, and charge distribution of these tritopic molecules are completely different. Despite these dissimilarities, structurally regular assemblies composed of both components are obtained. However, the functionality imposed on the assembly by the complexes (2) clearly dominates as the exponential growth enforced by these polypyridyl salts still prevails over the linear growth of the organic chromophore (1).46 Moreover, the stepwise combination of the two molecular compounds in a single assembly leads to an enhancement of the total incorporation of each component during SPMA deposition, relative to separate deposition under similar conditions.
Results and Discussion The multicomponent molecular-based assemblies were formed at room temperature in air by stepwise immersion of 1-based template layers (Figure 1, supported on silicon, glass or indiumtin oxide (ITO) coated glass) in a tetrahydrofuran (THF) solution of PdCl2(PhCN)2 and THF/DMF (9:1, v/v; DMF = dimethylformamide) solutions of the vinylpyridyl-based chromophores (1 and 2, Scheme 1).32,46,52 The samples were sonicated twice in THF and once in acetone for 3 min each after each deposition step to remove any physisorbed material. Pyridyl groups readily react with the palladium precursor, forming trans-(RC5H4N)2PdCl2 complexes with release of the benzonitrile ligands.18,32 The structure of these new assemblies (i.e., film thickness, microstructural regularity, redox properties, interfacial roughness) and details regarding the assembly chemistry were elucidated by means of ex situ transmission optical (UV/Vis) spectroscopy, ellipsometry (thickness, index of refraction), atomic force microscopy (AFM), synchrotron X-ray reflectivity (XRR), and electrochemistry. The transmission-mode UV/Vis measurements of the SPMAs on glass show, with each deposition step, exponential increase in the absorption intensities of the characteristic metal-to-ligand charge transfer (MLCT) band at λ = 494 nm of the ruthenium complex (2) and the combined absorption band at λ = 340 nm (Figure 2, exponential fits have R2 > 0.99). These bands do not shift with each subsequent deposition step, indicating that the conjugation of the surface-confined chromophores is not extended nor are there any strong π-π interactions.17,32,53,54 Furthermore, the absorption intensities of the MLCT bands of complex 2 do not noticeably change upon deposition of the organic constituent (Figure 2A,B). Although the SPMAs are exposed to an excess of chromophore 1 or 2 in each deposition cycle, it seems that surface-solution chromophore exchange does not take place. (50) Motiei, L.; Yao, Y.; Choudhury, J.; Yan, H.; Marks, T. J.; van der Boom, M. E.; Facchetti, A. J. Am. Chem. Soc. 2010, 132, 12528. (51) de Ruiter, G.; Motiei, L.; Choudhury, J.; Oded, N.; van der Boom, M. E. Angew. Chem., Int. Ed. 2010, 49, 4780. (52) Kaminker, R.; Lahav, M.; Motiei, L.; Vartanian, M.; Popovitz-Biro, R.; Iron, M. E.; van der Boom, M. E. Angew. Chem., Int. Ed. 2010, 49, 1218. (53) Yerushalmi, R.; Scherz, A.; van der Boom, M. E. J. Am. Chem. Soc. 2004, 126, 2700. (54) Shukla, A. D.; Strawser, D.; Lucassen, A. C. B.; Freeman, D.; Cohen, H.; Jose, D. A.; Das, A.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Phys. Chem. B 2004, 108, 17505.
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Figure 1. Schematic structure of the 1-based template layer on glass, silicon, or indium-tin oxide (ITO) coated glass for the iterative generation of multicomponent self-propagating molecular assemblies (SPMAs). The relative thicknesses of the layers is based on ellipsometric data (see text). The 1-based template layer was prepared by reacting p-chlorobenzyl-functionalized glass and silicon substrates with chromophore 1 in a dry toluene/acetonitrile 2:1 solution at 95 °C for 3 days under an inert atmosphere in a glass pressure tube.32 This 1-based template layer is defined as step 1 for the SPMA formation by iterative deposition of PdCl2 and 2 (steps 3, 7, 11, 15, 19) or 1 (steps 5, 9, 13, 17); the even steps are the PdCl2 depositions.
Figure 2. UV/Vis absorption data as a function of the number of deposition steps. (A) Transmission UV/Vis spectra of the self-propagating molecular assemblies (SPMAs) on glass. The blue and red spectra are chromophores 1- and 2-terminated assemblies, respectively. (B) Absorption intensities at λ = 494 nm versus the number of deposition steps. The blue and red markers indicate the 1- (solid blue square) and 2- (solid red circle) terminated SPMAs, respectively. For a comparison of the optical data of three SPMAs, see Figure S1 in the Supporting Information. (C) Absorption intensities at λ = 340 nm versus the number of deposition steps. For (B) and (C), the traces are exponential fits with R2 > 0.99. For (C), the absorption intensities for both the 1- and 2-terminated SPMAs have been modeled together with one exponential fit. The 1-based template layer is defined as step 1.
The nonlinear film growth and the formation of a multicomponent structure are further confirmed by the plot of ellipsometric-derived thickness as a function of the number of deposition steps (Figure 3A). The increase in the SPMA thickness (ΔT ) is greater for the deposition of 2 than for 1 (Figure 3B). This trend is also confirmed by synchrotron XRR measurements (vide infra). UV/Vis and ellipsometry measurements show the same stepwise and exponential trends as is evident from the good correlation (R2 > 0.99) between the absorption intensities and the film thickness during buildup independent of the substrate used (glass and silicon, Figure 3C). The index of refraction (n) of the assemblies is within the range of 1.6-1.7. The film thickness is ∼20 nm after 19 deposition steps. Synchrotron XRR measurements were used to obtain detailed structure information regarding the thickness, electron density, and roughness of the SPMAs. In Figure 4 are shown the reflectivity curves for the 1-based template layer (black trace; solid black circle) and deposition steps 9 (red trace; empty red circle) and 17 (blue trace; blue times sign). Surface and interface roughness as well as fluctuations of electron density of thin films are Langmuir 2011, 27(4), 1319–1325
known to reduce the specular intensities of Kiessig fringes. Nevertheless, all reflectivity spectra show distinct features. The traces are fits using a model for a uniform structure.13 The XRR data indicate that there are not any significant changes of the molecular density during the exponential film growth. The XRR-derived thickness estimations from the Kiessig fringes and the uniform fitting model13 are shown in Figure 5A. These two sets of values are in good agreement, and the exponential fits provide additional evidence for a self-propagating assembly. These thickness values are very similar to those obtained by ellipsometry (Figure 3A), with a linear correlation (R2 > 0.99, Figure 5B) between them. As observed in the ellipsometry data (vide supra), ΔT is larger for 2 than for 1 (Figure 5C). We have previously shown that SPMAs consisting of polypyridyl complexes, such as 2, contain an excess of palladium that can be used in subsequent chromophore deposition steps, thereby leading to exponential growth of the assembly.44,46 Chromophore 1-based MONs contain only the amount of palladium needed to generate a fully formed network and thus grow linearly.32,46 Assuming that in this new multicomponent SPMA the amount of accessible DOI: 10.1021/la103936t
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Figure 3. (A) Ellipsometry-derived thickness versus the number of deposition steps for the self-propagating molecular assemblies (SPMAs) on silicon (1 = solid blue square; 2 = solid red circle). The traces are exponential fits with R2 > 0.99. (B) SPMA thickness increase (ΔT ) after deposition of PdCl2 and 1 (solid blue square) or 2 (solid red circle). The black line is a guide to the eye. (C) Absorption intensities at λ = 340 nm of the SPMA on glass vs the ellipsometry-derived thickness on silicon. The lines are linear fits with R2 > 0.99. The 1-based template layer is defined as step 1.
Figure 4. Representative synchrotron X-ray reflectivity (XRR) spectra of the 1-based template layer (solid black circle) and self-propagating molecular assemblies (SPMAs) after 9 (empty red circle) and 17 (blue times sign) deposition steps. The traces are fits to the experimental data.13
PdCl2 is identical for both chromophores (1, 2) and that their coordination chemistry is similar, the difference in ΔT might reflect the molecular dimensions and their packing efficiency. The ellipsometry and XRR derived ΔT for the deposition of 2 is 1.5-2 larger than that of 1. The ΣΔT1 of ∼7.5 nm and ΣΔT2 of ∼12.6 nm are about twice the thicknesses observed for assemblies grown only with one of these components (1 or 2) for the same number of deposition steps.32,46 The combination of both compounds in one assembly results in nonadditivity of certain properties, and both deposition steps are affected by the presence of the other component. Complex 2 imposes self-propagating molecular growth on the other component (1), and both ΔT1 and ΔT2 are enhanced. The XRR-derived roughness increases from 4 A˚ for the 1-based template layer to ∼10 A˚ for deposition steps 11-17. Such effects are not uncommon;13 however, the values here are relatively low, indicating the presence of a smooth surface even for the SPMAs thicker than 100 A˚. After an initial increase, the roughness seems to level off at ∼9.5 A˚ after deposition step 10 while the film growth continues to ∼200 A˚. Apparently, the exponential growth is not strongly correlated with the surface roughness, which increases with a factor of ∼2.5 while the thickness increases by a factor of ∼9. The relative roughness, defined as the roughness divided by 1322 DOI: 10.1021/la103936t
the SPMA thickness, decreases by ∼70% from the 1-based template to the fully formed SPMA (Figure 5D). Representative semicontact AFM images of the 1-based template layer and the multicomponent SPMAs on silicon substrates after deposition steps 9 and 17 are shown in Figure 6. The surface morphology and the root-mean-square roughness (Rrms = 0.2-0.3 nm for 500 500 nm2 scan areas) are consistent with other assemblies grown by the same methodology on silicon substrates.16-18,24,44-46,49 It seems that the nature of the film growth (i.e., linear or exponential) does not lead to pronounced AFM-observable structural differences.46 Moreover, the roughnesses of these coordination-based assemblies are lower or comparable to values reported for molecular-based assemblies formed by other layer-by-layer methods.12,13,21,55,56 Island-type domains were not observed in the mixed assemblies. The surface morphology/roughness is in agreement with the XRR data (vide supra) demonstrating that the interface is similar regardless of the dimensions of the analyzed areas. An exponential growth mechanism due to the film roughness is unlikely.44,46 Exponentially growing polyelectrolyte-based films have been reported with smooth surfaces.57 We have reported that SPMAs exclusively based on polypyridyl complexes linked via PdCl2 are redox-active and exhibit electrochromic properties comparable with commercially available polymeric materials such as poly(3,4-ethylenedioxythiophene) (PEDOT) and derivatives thereof.45,49,51,58,59 The new, multicomponent SPMAs formed on ITO coated glass are also redoxactive. Cyclic voltammetry (CV) measurements showed a reversible redox process characteristic of a Ru2þ/3þ couple with a half-wave redox potential (E1/2) of 1.3 V (vs Ag/AgCl) at a scan rate of 100 mV s-1 (Figure 7, inset).50 The total reduction charge (Q), estimated by integration of the voltammetric peaks, increases exponentially with the number of deposition steps (Figure 7). This good correlation (R2 = 0.99) is fully in agreement with the optical (UV/Vis, ellipsometry) measurements and the XRR data and is apparently not significantly affected by the substrate (i.e., glass, (55) Wanunu, M.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 2006, 128, 8341. (56) van der Boom, M. E.; Evmenenko, G.; Dutta, P.; Marks, T. J. Adv. Funct. Mater 2001, 11, 393. (57) Picart, C.; Lavalle, P.; Hubert, P.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C. Langmuir 2001, 17, 7414. (58) Patra, A.; Wijsboom, Y. H.; Zade, S. S.; Li, M.; Sheynin, Y.; Leitus, G.; Bendikov, M. J. Am. Chem. Soc. 2008, 130, 6734. (59) Yen, H.-J.; Liou, G.-S. Chem. Mater. 2009, 21, 4062.
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Figure 5. (A) Synchrotron X-ray reflectivity (XRR) derived thickness of the self-propagating molecular assemblies (SPMAs) as a function of the number of deposition steps. Step 1 is the 1-based template layer. Step 3 is a 2-terminated SPMA, step 5 is a 1-terminated SPMA, and so forth (see Figure 1). The SPMA thickness is obtained from the Kiessig fringes (solid green triangle) and by fitting the experimental data (solid black circle) shown in Figure 4.13 The black trace is an exponential fit with R2 = 0.98 through the modeled thicknesses (solid black circle). (B) Ellipsometry-derived thickness versus the synchrotron X-ray reflectivity (XRR) thickness data of the SPMAs. The XRR data in this plot are obtained from the Kiessig fringes; the thickness obtained by fitting the experimental XRR data gives a very similar linear correlation. The black trace is a linear fit with R2 > 0.99. (C) XRR-derived thickness increase (ΔT ) of the SPMAs after each deposition of PdCl2 and a chromophore (1 = solid blue square; 2 = solid red circle). The black line is a guide to the eye. (D) Relative roughness (roughness/SPMA thickness) as a function of the number of deposition steps. See Figure 1 for a description of the SPMA structure.
Figure 6. Representative atomic force microscopy (AFM) images of (A) the 1-based template layer (step 1) and the multicomponent self-
propagating molecular assemblies (SPMAs) after deposition steps 9 (B) and 17 (C). The scan areas are 500 nm 500 nm with a root-meansquare roughness (Rrms) of 0.2-0.3 nm. The values shown in the vertical scales are in nm.
silicon, ITO). It also shows that the ruthenium complexes (2) are electronically addressable despite the presence of a large amount of the organic constituent (1).
Summary and Conclusions The combination of complementary analytical methods shows the exponential growth of hybrid assemblies while the structural regularity (i.e., molecular density) is not seemingly affected. It is interesting that the iterative deposition of two molecular components induces self-propagating (exponential) growth whereas the use of either tritopic compound separately under the same reaction conditions results in linear (1)32,46 or exponential (2) growth.46 Our results imply that synergistic effects play an important role in the formation of surface-bound molecular assemblies. The structural properties generated by the polypyridyl complexes (2) Langmuir 2011, 27(4), 1319–1325
dominate the overall process, which is also beneficial for the deposition of the organic chromophore (1). Furthermore, the combination of the compounds in one assembly leads to an enhancement of the amount of both materials that is added to the SPMA during buildup. Its symbiotic nature causes more material to bind to the surface than is obtainable with a linear growth process and/or by a linear combination of the properties of monomolecular assemblies. There is a clear relationship between complexity and molecular diversity in our materials. Enlarging the complexity of the SPMA by increasing the diversity of the molecular components resulted in several advancements over single molecular component assemblies in terms of growth, thickness, and light absorption while maintaining the low surface roughness and redox properties. Such materials might be useful for the design of electrochromic-based sensors for the selective detection of small DOI: 10.1021/la103936t
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Figure 7. Exponential increase of the charge, Q, versus the number of deposition steps for a series of 2-terminated self-propagating molecular assembly (SPMAs). The black trace is an exponential fit with R2 = 0.99. These electrochemical experiments were carried out at room temperature in 0.1 M TBABF4/CH3CN. ITO-coated glass, Pt wire, and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. Inset: Representative cyclic voltammogram of a SPMA after 19 deposition steps at a scan rate of 100 mVs-1.
molecules in a matrix of other components having similar redox properties.60-63 The multicomponent SPMAs probably do not undergo reversible self-assembly during their formation. Excessive surfacesolution exchange of the two molecular components would not have resulted in the multicomponent SPMAs. The Pd-pyridine coordination does not only create an excellent structural directing unit,16-18,24,44-46,49-51,64,65 but it also is apparently stable enough to allow the incorporation of two different molecular building blocks having nearly identical binding motifs. This coordination chemistry has been studied extensively in solution to generate series of supramolecular structures with defined geometries and shapes,66-69 some of which show interesting properties related to host-guest chemistry, sensing, storage, and catalysis. Further utilization of self-assembled molecular architectures requires the formation of surface-confined assemblies.70,71 The multicomponent SPMAs introduced here also demonstrate the versatility of the deposition process. The use of two chromophores that are completely different structurally and functionally might allow one to generate multifunctional stimuli-responsive SPMAs.
Experimental Section Materials and Methods. Pentane and toluene were dried using an M. Braun solvent purification system and degassed before being introduced into an M. Braun glovebox. Acetonitrile (60) Williams, M. E.; Benkstein, K. D.; Abel, C.; Dinolfo, P. H.; Hupp, J. T. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5177. (61) Motesharei, K.; Ghadiri, M. R. J. Am. Chem. Soc. 1997, 119, 11306. (62) de Ruiter, G.; Gupta, T.; van der Boom, M. E. J. Am. Chem. Soc. 2008, 130, 2744. (63) Kikkeri, R.; Grunstein, D.; Seeberger, P. H. J. Am. Chem. Soc. 2010, 132, 10230. (64) South, C. R.; Weck, M. Langmuir 2008, 24, 7506. (65) South, C.; Pinon, V.; Weck, M. Angew. Chem., Int. Ed. 2008, 47, 1425. (66) Northrop, B. H.; Zheng, Y.-R.; Chi, K.-W.; Stang, P. J. Acc. Chem. Res. 2009, 42, 1554. (67) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853. (68) Oliveri, C. G.; Ulmann, P. A.; Wiester, M. J.; Mirkin, C. A. Acc. Chem. Res. 2008, 41, 1618. (69) Lehn, J. M. Science 2002, 295, 2400. (70) Sergeyev, S.; Pisulab, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902. (71) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9, 014109.
1324 DOI: 10.1021/la103936t
(anhydrous, 99.98%) was purchased from Aldrich. Compounds 1 and 2,32,44,46,52,72 PdCl2(PhCN)2,73 and the 1-based templates were prepared and characterized as reported. Note that the procedure used to prepare compound 2 has been shown, in related systems, to result in mixtures of mer and fac isomers that are exceedingly challenging to separate.74,75 Reaction tubes were washed with deionized (DI) water followed by acetone and then oven-dried overnight at 130 °C. All glassware and Teflon holders for film formation were cleaned by immersion in a piranha solution (7:3 v/v H2SO4/30% H2O2) for 10 min and then DI water. Caution! Piranha is an extremely dangerous oxidizing agent and should be handled with care using appropriate personal protection. Single-crystal silicon (100) substrates, purchased from Wafernet (San Jose, CA), were cleaned by sonication for 8 min sequentially in n-hexane, acetone, and ethanol and then dried under a stream of N2. Subsequently, the slides were treated with a UVOCS cleaning system (Montgomery, PA), washed with isopropanol, dried under a stream of N2, and heated overnight at 130 °C in an oven. Corning glass slides (2.5 cm 0.8 cm 0.1 cm, Chase Scientific Glass, Rockwood, TN) were rinsed with DI water and cleaned by immersion in a piranha solution for 1 h. The substrates were then rinsed with DI water followed by the RCA cleaning protocol (1:5:1 (v/v) solution of aqueous NH3 (33%)/H2O/30% H2O2 at room temperature for 1 h). The substrates were subsequently washed with DI water and isopropanol, dried under a N2 stream, and heated overnight at 130 °C. UV/Vis spectra were recorded on glass slides with a Cary 100 spectrophotometer in the double beam mode. Atomic force microscopy (AFM) images were recorded using a Solver P47 (NT-MDT, Russia) instrument operated in the semicontact/tapping scanning mode. Film thicknesses were estimated on silicon using a J.A. Woollam (Lincoln, NB) model M-2000 V variable angle spectroscopic ellipsometer with the VASE32 software. XRR measurements were performed at Beamline X6B of the National Synchrotron Light Source, Brookhaven (Upton, NY) using a four-circle Huber diffractometer in the specular reflection mode (i.e., incident angle I^ was equal to the exit angle). X-rays with an energy of E = 10.0 keV (λ = 1.240 A˚) were used. The beam size was 0.30 mm vertically and 0.50 mm horizontally. The samples were held under a helium atmosphere during the measurements to reduce radiation damage and background scattering from the ambient gas. The off-specular background was measured and subtracted from the specular counts. Electrochemical measurements were performed using a potentiostat (CHI660A) and a three-electrode cell configuration consisting of (i) an ITOmodified substrate (working electrode), (ii) a Pt wire (counter electrode), and (iii) Ag/AgCl (a reference electrode). The experiments were performed at room temperature using 0.1 M solutions of TBABF4 in dry CH3CN. ITO-coated glass slides (0.7 cm 5 cm 0.7 cm, Rs = 5-15 Ω/0) were obtained from Delta Technologies. Tetrabutylammonium tetrafluoroborate (TBABF4) and anhydrous CH3CN (H2O < 0.001% v/v) were purchased from Aldrich. The experimental data were fitted to the expression y = y0 þ c1 exp(c2x), where y is the measured absorption or thickness after x deposition steps, and c1 and c2 are fitting parameters. Similar exponential fitting models have been used.39,44,76
Stepwise Formation of the Self-Propagating Molecular Assemblies. Substrates functionalized with the 1-based template layer were placed in a Teflon holder and immersed in a 1 mM THF solution of PdCl2(PhCN)2. The samples were subsequently immersed in a 0.25 mM solution of complex 2 in THF/DMF (9:1, v/v), followed by another reaction with PdCl2(PhCN)2 in (72) Amoroso, A. J.; Cargill Thompson, A. M. W.; Maher, J. P.; McCleverty, J. A.; Ward, M. D. Inorg. Chem. 1995, 34, 4828. (73) Anderson, G. K.; Lin, M. Inorg. Synth. 1990, 28, 60. (74) Fletcher, N. C.; Nieuwenhuyzen, M.; Rainey, S. J. Chem. Soc., Dalton Trans. 2001, 2641. (75) Kyakuno, M.; Oishi, S.; Ishida, H. Inorg. Chem. 2006, 45, 3756. (76) Lavalle, P.; Picart, C.; Mutterer, J.; Gergely, C.; Reiss, H.; Voegel, J.-C.; Senger, B.; Schaaf, P. J. Phys. Chem. B 2004, 108, 635.
Langmuir 2011, 27(4), 1319–1325
Motiei et al. THF and deposition of chromophore 1 (0.25 mM) in THF. The immersion time for each deposition step is 15 min. The samples were then sonicated in THF (2) and in acetone (1) for 3 min between each deposition step. This four-step procedure can be repeated at least four times at room temperature. Finally, the samples were rinsed in ethanol and dried under a stream of N2.
Acknowledgment. This research was supported by the Helen and Martin Kimmel Center for Molecular Design, the Gerhardt M. J. Schmidt Minerva Center on Supramolecular Architectures,
Langmuir 2011, 27(4), 1319–1325
Article the U.S. Department of Energy (Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, Grant No. DEFGO2-84ER45125). The X-ray reflectivity measurements were performed at Beamline X6B of the National Synchrotron Light Source. We thank Dr. Arkady Bitler (WIS) for his assistance with the AFM measurements.
Supporting Information Available: Figure S1: Comparison of the transmission UV/Vis absorption data for three different self-propagating molecular assemblies (SPMAs). This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la103936t
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