pubs.acs.org/Langmuir © 2009 American Chemical Society
Metallo-Supramolecular Gels Based on a Multitopic Cyclam Bis-Terpyridine Platform† Aurelien Gasnier,‡ Guy Royal,‡ and Pierre Terech*,§ ‡
D epartement de Chimie Mol eculaire, Universit e Joseph Fourier, CNRS, BP 53, 38041 Grenoble, France, and CEA-Grenoble, INAC-SPrAM (UMR5819)-LASSO 17, rue des Martyrs, 38054 Grenoble Cedex 9, France
§
Received January 15, 2009. Revised Manuscript Received March 2, 2009 The domain of soluble metallopolymers (i.e., polymers spontaneously obtained from the self-assembly of metal ions and polytopic bridging ligands) is promising for the construction of novel materials with perspectives in magnetic, redox, optical, electrochromic, or mechanical properties. We report the preparation and characterization of metallosupramolecular gels based on a multitopic cyclam bis-terpyridine platform (CHTT) that are representative of a second generation of molecular gels exhibiting “intelligent” properties. The systems were characterized by UV-visible spectroscopy, cyclic voltammetry, viscosimetry, rheology, and small-angle neutron scattering (SANS). A basic analysis of the viscosity taking into account the interplay of hydrodynamic versus Brownian motions of 1D species is proposed. SANS data demonstrate the formation of rod-like species whose radius R and aspect ratio f = L/R were extracted. Rheological features are typical of weak gels having storage moduli of more than 1 order of magnitude weaker than for ordinary molecular organogels. These new molecular materials exhibit interesting properties: (i) chemosensitivity, with the gelation being dependent on the type of metal, stoichiometry M/CHTT, solvent, and counterion; (ii) electrosensitivity: the Co(II)/CHTT system can be electrochemically and reversibly commuted between the gel (red) and liquid (green) states; and (iii) mechanical sensitivity, with the system being able to cycle from weakly to highly viscous states upon fast application/suppression of a shearing stress.
Introduction Molecular gels result from the immobilization on the macroscopic scale of a liquid (organic or aqueous) by a solid-like selfassembled network of molecular units.1-4 Rod-like assemblies and their connections in nodal zones of the networks are noncovalent links. Frequently, the molecular aggregation mechanism involves specific interactions such as electron transfers between molecular sites with complementary donor and acceptor properties. This vast category includes H bonding, π-π stacking, and metal coordination-complexation mechanisms. Such connection mechanisms bring into play weak energies of the order of thermal energy. Thereby, molecular gelation usually appears to be thermoreversible in ordinary temperature ranges. The range of chemical structures (from amides to steroids and fatty acids) generates gels with a variety of elasticities and time-dependent properties. Frequently, the scaling law of the storage modulus versus concentration is a square power law.5 Gels can be replicated (via mineralization, metallization, or organic polymerization reactions) either to take advantage of the monodispersity of the cross sections of the fibers in certain systems6,7 or to exploit the porosity developed in the self-assembled fibrillar network (SAFIN).8 † Part of the Molecular and Polymer Gels; Materials with Self-Assembled Fibrillar Networks special issue. *To whom correspondence should be addressed. E-mail: pierre.terech@ cea.fr.
(1) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237–1247. (2) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (3) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217. (4) Dastidar, P. Chem. Soc. Rev. 2008, 37, 2699–2715. (5) Terech, P.; Pasquier, D.; Bordas, V.; Rossat, C. Langmuir 2000, 16, 4485–4494. (6) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1998, 1477–1478. (7) Jung, J. H.; Ono, Y.; Shinkai, S. Langmuir 2000, 16, 1643–1649. (8) Gu, W.; Lu, L.; Chapman, G. B.; Weiss, R. G. J. Chem. Soc., Chem. Commun. 1997, 543–544.
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Beyond such approaches, gels generated with gelator molecules bearing additional chemical functions can give rise to innovative materials exhibiting promising added physical properties.9 Metalcomplexing ligands and their related 1D self-assembled aggregates are part of such new molecular gels. They can be considered to be representative of the second generation of molecular gels: the functional molecular gels. Metallo-supramolecular rods can be grown in organic solvents with various low-mass molecules that ultimately give gel-like materials. The introduction of metal ions can also modify the aggregation modes and the gelation ability, and stimuli-responsive materials can be obtained. To illustrate, the gelation temperature has already been tuned in a metal-responsive cholesterol-based organogelator.10 Also, cobalt (II) complexes of alkylated 1,2,4-triazoles form blue gels corresponding to a tetrahedral coordination of the metal. On cooling, a pink solution is obtained, corresponding to octahedral complexes.11 The sensitivity of 1D self-assembly to the redox state of complexed metal ions in coordination metallo-gelators is a remarkable feature of more sophisticated gelators illustrating the second generation of multiresponsive molecular gels. A first redox-responsive low-mass gelator has been investigated with a 2,2’-bipyridine derivative bearing two cholesteryl groups.12 Such a system exhibits reversible chromatic and sol-gel transitions controlled by the redox state of the metal (Cu(I)/Cu(II))-ligand complex. Recently, a redox-responsive system of poly(acrylic acid) containing Fe(III) ions showed a reversible gel-sol transition.13 Different “coordination gelators” are also available.9 (9) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680–1682. (10) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1715–1717. (11) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016–2021. (12) Kawano, S.-I.; Fujita, N.; Shinkai, S. J. Am. Chem. Soc. 2004, 126, 8592–8593. (13) Peng, F.; Li, G.; Liu, X.; Wu, S.; Tong, Z. J. Am. Chem. Soc. 2008, 130, 16166–161167.
Published on Web 04/17/2009
DOI: 10.1021/la900174e
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A gelator containing diketonato ligands can be complexed with divalent metal ions to reinforce the robustness of the gels.14 Porphyrins and phthalocyanins are also known to lead to metallogels. For example, a Zn(II) complex of a long-chain triester of meso-tetrakis(p-carboxyl)phenyl porphyrin in cyclohexane self-assembles into rods in a J-type aggregation mode with four molecules per cross-section.15 The suspensions in cyclohexane are highly shear-responsive gels with strongly thixotropic behaviors.16 The Zn(II) porphyrinic 1D suprapolymers in solution are weakly interacting and can easily be oriented under shear. By contrast, bimetallic tetracarboxylates in cyclohexane form breakable monomolecular threads and viscoelastic Maxwellian suspensions.17 The gel-like consistency of the system is sensitive to “chemical poisons” (akin to formic acid) while the light-emitting properties are modified.18 As recently reported,9,19 metallogels can be obtained by binding metal ions to a gelator (e.g., a cholesterol-based gelator) that contains an appended ligand site. An elegant but almost unexplored route to metallogels is the use of coordination polymers in which both ligand and metal ions act as complementary elements in a hierarchical self-assembly process. One-dimensional to threedimensional metallo-suprapolymers in which metal-ligand bonds form the polymeric backbone can thereby be obtained. For example, a 1D coordination polymer can be spontaneously obtained by the metalation of a polytopic ligand having two coordinating units linked by a spacer unit.20-28 The degree of “polymerization” (or aggregation number) mainly depends on the metal-ligand stoichiometry, concentration, and the metalligand binding constants. Usually, oxygen or nitrogen donor derivatives are used as coordinating units to prepare coordination polymers, and the tridentate 2,20 /6’,200 -terpyridine20-24 appears to be the most common because it can form well-defined 1:2 (Mn+/L) octahedral complexes with high binding constants with a variety of transition-metal cations. The design of the spacer (i.e., the linker between the terminal coordinating units) is also important and can introduce a variety of additional properties (viscoelasticity and solubility) into the material.20,25-28 In addition, the lability of metal-ligand interactions may be at the origin of specific dynamical and responsivity properties. Rowan9,18,29-31 reported an example of responsive metallogels using a bis-BIP derivative (BIP = 2,6-bis(benzimidazolyl)pyridine). Two types of metal ions were used in the same polymer:transition-metal (14) Hanabusa, K.; Maesaka, Y.; Suzuki, M.; Kimura, M.; Shirai, H. Chem. Lett. 2000, 1168–1169. (15) Terech, P.; Scherer, C.; Deme, B.; Ramasseul, R. Langmuir 2003, 19, 10641–10647. (16) Terech, P.; Scherer, C.; Lindner, P.; Ramasseul, R. Langmuir 2003, 19, 10648–10653. (17) Terech, P.; Coutin, P. J. Phys. Chem. B 2001, 105, 5670–5676. (18) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922–13923. (19) Camerel, F.; Ziessel, R.; Donnio, B.; Guillon, D. New J. Chem. 2006, 30, 135–139. (20) Hoogenboom, R.; Schubert, U. S. Chem. Soc. Rev. 2006, 35, 622–629. (21) Schubert, U. S.; Hofmeier, H.; Newkome, G. R. Modern Terpyridine Chemistry; Wiley-VCH: Weinheim, Germany, 2006. (22) Constable, E. C. Chem. Soc. Rev. 2007, 2, 246–253. (23) Schubert, U. S.; Eschbaumer, C. Angew. Chem., Int. Ed. 2002, 2892–2926. (24) Andres, P. R.; Schubert, U. S. Adv. Mater. 2004, 16, 1043–1068. (25) Friese, V. A.; Kurth, D. G. Coord. Chem. Rev. 2008, 252, 199-211. :: (26) Dobrawa, R.; Wurthner, F. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 4981–4995. (27) Kurth, D. G.; Higuchi, M. Soft Matter 2006, 2, 915–927. (28) Vermonden, T.; Van Der Gucht, J.; De Waard, P.; Marcelis, A. T. M.; Besseling, N. A. M.; Sudholter, E. J. R.; Fleer, G. J.; Cohen Stuart, M. A. Macromolecules 2003, 36, 7035–7044. (29) Zhao, Y.; Beck, J. B.; Rowan, S. J.; Jamieson, A. M. Macromolecules 2004, 37, 3529–3531. (30) Rowan, S. T.; Beck, J. B. Faraday Discuss. 2005, 128, 43–53. (31) Weng, W.; Beck, J. B.; Jamieson, A. M.; Rowan, S. T. J. Am. Chem. Soc. 2006, 128, 11633–11672.
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ions (CoII or ZnII) that form 1:2 (metal/BIP) complexes and extend the polymer chains and lanthanide ions (EuIII and LaIII) that give 1:3 (metal/BIP) complexes allowing cross-linking. Interesting thermochromic sol-gel transition and thixotropic properties were attributed to the disruption of the lanthanide-BIP bonds. Here, we describe a novel class of coordination polymers22 giving redox-responsive gels based on the CHTT ligand. The originality of these systems, shown in Scheme 1, relies on the insertion of a coordinating cyclam macrocycle (cyclam 1,4,8,11-tetraazacyclotetradecane) as a spacer between two terminal chelating terpyridine units. This strategy is promising because this spacer also allows the insertion of additional metal ions in the polymer-like chains. New properties can thereby be gained that are akin to redox activity, photosensitivity, catalysis, and switching features, and heterometallic suprapolymers can also be envisaged.32,33 The cyclam macrocycle is a versatile molecular platform for conceiving elaborate architectures because it can be N- and/ or C-functionalized to form stable complexes with a range of metal ions.34,35 Additionally, in metal complexes of N-substituted cyclams, the ligand can adopt five energetically distinct geometries (type I-V, Scheme 2) defined by the orientation of the grafts on each nitrogen, above or below the N4 coordination plane,36 and these geometries may be modified under an external stimulus.37,38 In the case of 1,8-N-disubstituted cyclam derivatives (i.e., the two substituents located on two nonadjacent nitrogen atoms), only two major geometries can be considered, with the two grafts being on the same side (cis position) or opposite sides (trans position) of the macrocycle plane (Scheme 2). This point is important for the preparation of the coordination polymers described herein because the length of the self-assembled polymeric chains will be shown to be dependent upon the geometry of the metal-cyclam unit. A trans position is expected to favor the growth of long 1D species whereas a cis geometry could lead to shorter polymeric chains or even to small cycles, such as those reported by Constable39 or Newkome with other spacers.40 We report here the preparation and characterization of such 1D homometallic Cu2+, Ni2+, and Co2+ suprapolymers in solutions or gels based on the CHTT polytopic ligand (which is not a gelator by itself). The routes for their formation in solution are investigated by electrochemistry, UV-visible spectroscopy, viscosimetry, rheometry, and neutron scattering experiments.
Experimental Section Reagents, Instrumentation, and Procedure. Electrochemical experiments were done with millimolar solutions of complexes in DMF containing tetra-n-butylammonium perchlorate (TBAP at 0.1 M) in a conventional three-electrode cell at 298 K (CH Instruments potentiostat CHI 660B). The reference electrode was Ag/AgNO3 (10 mM in CH3CN containing 0.1 M (32) Gasnier, A.; Barbe, J.-M.; Bucher, C.; Denat, F.; Moutet, J.-C.; SaintAman, E.; Terech, P.; Royal, G. Inorg. Chem. 2008, 47, 1862–1864. (33) Gasnier, A.; Bucher, C.; Moutet, J.-C.; Saint-Aman, E.; Royal, G.; Terech, P. Submitted for publication. (34) Meyer, M.; Dahaoui-Gindrey, V.; Lecomte, C.; Guilard, R. Coord. Chem. Rev. 1998, 178-180, 1313-1415. (35) Wainwright, K. P. Coord. Chem. Rev. 1997, 166, 35-90. (36) Bosnich, B.; Poon, C. K.; Tobe, M. L. Inorg. Chem. 1965, 4, 1102–1108. (37) Bucher, C. M.; Pecaut, J.-C.; Royal, J.; Saint-Aman, G.; Thomas, E. F. Inorg. Chem. 2004, 43, 3777–3779. (38) Bucher, C.; Moutet, J.-C.; Pecaut, J.; Royal, G.; Saint-Aman, E.; Thomas, F.; Torelli, S.; Ungureanu, M. Inorg. Chem. 2003, 42, 2242–2252. (39) Constable, E. C.; Housecroft, C. E.; Smith, C. B. Inorg. Chem. Commun. 2003, 6, 1011–1013. (40) Newkome, G. R.; Coll. Eur. J. Org. Chem. 2008, 3328-3334.
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Scheme 1. Structure of the CHTT Ligand and Schematic Representation of the Formation of the M2CHTT Metallopolymers (M = CoII, NiII, or CuII) via the Intermediate Formation of Mononuclear Complexes (Path A) or of Cyclam-Metal Free Polymers (Path B)
Scheme 2. (Left) Stereochemistry of Metal Complexes of Cyclam and (Right) Schematic Representations of the cis and trans Configurations of 1,8-N-Disubstituted Cyclam Complexes
TBAP), and the ferrocene/ferrocenium (E1/2 = +0.054 V vs Ag/10 mM AgNO3) redox couple was used as the internal reference. The counter electrode was a Pt gauze isolated from the working compartment by a glass frit containing DMF + TBAP (0.1 M). Cyclic voltammetry (CV) curves were recorded at a scan rate of 0.1 V 3 s-1 using a glassy carbon disk (2 mm in diameter) as the working electrode (polished with 1 μm diamond paste before each record). Electrolyses were performed at controlled potential using a Pt plate (∼2 cm2) as the working electrode. Because of the polyelectrolytic nature of the suprapolymers, electrolyses of the gels were done without the addition of a supporting electrolyte. UV-vis spectra were recorded on a Varian Cary 100 spectrophotometer using quartz cells (l = 1 cm). Elemental analyses were performed by the Service Central d’Analyses, CNRS, Lyon, France. NMR spectra were recorded on a 400 MHz Bruker spectrometer at 298 K. 1H and 13 C chemical shifts (ppm) were referenced to residual solvent peaks. Langmuir 2009, 25(15), 8751–8762
Viscosimetry. Viscosimetry measurements of the liquid solutions at low concentrations (TCHTT = 13 mM) at variable M/CHTT stoichiometric ratios have been performed with Ostwald viscometers (Comecta 50, 150). The flow times resulted from the average over five measurements at a temperature of 19 ( 0.1 °C. Neutron Scattering. The small-angle neutron scattering (SANS) experiments have been performed at the Institut Laue Langevin (ILL, Grenoble, France) using the D22 spectrometer.41 Deuterated solvents were used to restrict the incoherent scattering to that of protons of the ligands themselves. The isotropic 2D scattering patterns were radially averaged using standard procedures. The subtraction of the incoherent scattering assumed aggregates with sharp interfaces. The adjustment was such that the intensity decay in the large-Q range was the closest to Q-4. The range of scattering vector Q was from ca. 0.001 A˚-1 to 0.6 A˚-1 where |Q| = Q = (4π sin θ)/λ, with λ being the neutron wavelength and θ being half the scattering angle. Rheology. The rheological properties were measured with a RS600 controlled-stress rheometer (ThermoHaake). With gels, a serrated plate-plate geometry (20 mm or 35 mm diameter, gap of 0.6 mm) was used to prevent sliding of the gel that could be promoted by the occasional ejection of a thin liquid film from the gel network. Temperature was controlled at (0.1 °C. Synthesis. Reagents were commercial and used without further purification. Tetra-n-butylammonium perchlorate (TBAP) was purchased (Fluka). 1,4,8,11-Tetraazatricyclo [9.3.1.14,8]hexadecane (2) was prepared from cyclam (1) as previously reported.42 (41) www.Ill.Fr. (42) Royal, G.; Dahaoui-Gindrey, V.; Dahaoui, S.; Tabard, A.; Guilard, R.; Pullumbi, P.; Lecomte, C. Eur. J. Org. Chem. 1998, 1971–1975.
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1,8-Di(p-2,20 /60 ,200 -terpyrid-4-yl)tolyl)-4,11-diazaniatricyclo-(9,3,1,14,8)-hexadecane Dibromide (4). 1,4,8,11-Tetraazatricyclo-[9,3,1,1]-hexadecane (2) (1.86 mmol, 418 mg) was dissolved in 30 mL of acetonitrile, and 2 mol equiv of 40 -[4(bromomethyl)phenyl]-2,20 /60 ,200 -terpyridine (3; 1.50 g, 3.73 mmol)42 dissolved in 15 mL of CH2Cl2 were rapidly added. The solution was stirred at room temperature for 4 days, and the resulting white precipitate was filtered, washed with a solution of acetonitrile/dichloromethane (1:1 vol/vol), and dried under vacuum. Yield: 1.58 g (83%). FAB+-MS, m/z: 868 {M-2Br + H}+, 949 {M-Br}+. Anal. Calcd for C54H54N10, 0.66 CH2Cl2: C 62.7%, H 5.3%, N 12.9%. Found: C 62.5%, H 5.6%, N 13.2%. Because of the very poor solubility of this compound, NMR characterization could not be performed.
1,8-Di(p-2,20 /60 ,200 -terpyrid-4-yl)tolyl)-1,4,8,11-tetraazacyclotetradecane (CHTT). 4 (0.972 mmol, 1.00 g) was sus-
pended in 400 mL of ethanol, and 250 mL of NaOH (3 M in water) was rapidly added. The mixture was stirred at room temperature for 4 h. A yellow waxy solid was formed, and the majority of the ethanol was removed by concentration of the solution under reduced pressure. The mixture was extracted with CH2Cl2 (3 100 mL). The organic phases were collected, dried over Na2SO4, filtered, and evaporated under vacuum. If necessary, purification of the crude product was carried out by column chromatography (Alumina) with CH2Cl2-MeOH (98:2 v/v). Yield: 778 mg (95%).1H NMR (CDCl3) δ: 8.68 (m, 8 H, tpy 3-300 , 30 -50 ), 8.63 (d, J = 7.9 Hz, 4 H, tpy 6-600 ), 7.84 (dt, J = 1.8 and 7.6 Hz, 8 H, tpy 4-40 , -φ), 7.47 (d, J = 8.1 Hz, 4 H, -φ), 7.32 (m, 4 H, tpy 5-500 ), 3.81 (s, 4 H, -N- CH2-φ), 2.78 (tb, J = 6.3 Hz, 8 H, R-CH2), 2.68 (db, J = 6.3 Hz, 4 H, R-CH2), 2.60 (tb, J = 4.9 Hz, 4 H, R-CH2), 1.89 (sb, 4 H, β-CH2). 13C NMR (CDCl3) δ: 156.07, 155.80, 149.97, 149.03, 138.72, 137.11, 136.67, 129.96, 127.07, 123.63, 121.25, 118.76, 57.70 (N-CH2-φ), 51.66 (R-CH2), 49.61 (R-CH2), 47.67 (R-CH2), 26.04 (β-CH2). FAB+-MS m/z: 843 {M}+. Anal. Calcd for C54H54N10, 0.5 CH2Cl2: C 73.9%, H 6.3%, N 15.8%. Found: C 74.1%, H 6.3%, N 15.8%. Reference Complexes MII(terpy)2 and MII(CHB). The II M (terpy)2 reference complexes were prepared as their PF6 salt following previous reports.43 MII(CHB) complexes were prepared by mixing 0.1 mol of 1,8-dibenzyl 1,4,8,11-tetraazacyclotetradecane (CHB)42 with a stoichiometric amount of MCl2 (M = Co, Ni, or Cu) in 4 mL of MeOH under argon. The solution was stirred at room temperature (1 h), and a saturated aqueous solution of KPF6 was slowly added until precipitation was complete. The solid was collected, washed with MeOH and diethyl ether, and dried under reduced pressure. Yield: 80-90%. Preparation of the Gels. All of the gels were prepared from solutions of NiII or CoII complexes. Gelation was primarily tested according to the inverted test tube method. Method A. Spontaneous Gelation. Solutions of chloride metal (NiCl2 or CoCl2) in DMF were prepared to obtain solutions of a controlled concentration of 9.302 mM. CHTT ligand (40.0 μmol) was dissolved in 1 mL of DMF. To 100 μL (4.00 μmol) of this solution in a screw-cap culture tube was added 430 μL (4.00 μmol, 1 mol equiv) of the metal chloride solution. The solution was quickly homogenized. After 5 min, a second molar equivalent (430 μL, 4.00 μmol) of the same (homometallic suprapolymers) or a different metal solution (heterometallic suprapolymers) was added. The mixture was homogenized and allowed to rest. In about 1 h, the first germs of gelation were visible, and the viscosity increased concomitantly. This process can be highly accelerated by placing the sealed tube in an oven at 100 °C. A similar procedure was employed to obtain gels in acetonitrile using nitrate metal salts. However, for solubility reasons, the ligand has to be dissolved in tetrahydrofurane (THF) prior to the addition of metal ions in acetonitrile. (43) Constable, E. C.; Lewis, J.; Liptrot, M. C.; Raithby, P. R. Inorg. Chim. Acta 1990, 178, 47–54.
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Method B. Ether Diffusion Gelation. Chloride metal salt (NiCl2 or CoCl2) was dissolved in methanol to obtain solutions of controlled concentration (10.00 mM). 40.0 μmol of CHTT were dissolved in 1 mL THF (40.0 mM). To 100 μL (4.00 μmol) of the CHTT solution in a large-neck flask, 400 μL (4.00 μmol) of the metal ion solution were added. After 5 min, a second equivalent (400 μL, 4.00 μmol) of the same or a different metal ion solution was added, and the solution was homogenized. Gelation was induced by subjecting, for about 1 h, the flask to an environment saturated with diethyl ether vapor. Method C. Counterion Diffusion Gelation. The solution of polymer was prepared under the same conditions as for method B. Gelation was induced by sprinkling solid potassium hexafluorophosphate. Gel germs appear to be localized around the KPF6 crystals, and after about 3 h, complete gelation was observed.
Results and Analysis Synthesis of the CHTT Ligand. The cyclam-bis-terpyridine CHTT ligand was synthesized from the 1,4,8,11-tetraazatricyclo [9.3.1.14,8]hexadecane (2) derivative following a straightforward procedure (Figure 1A).42 The addition of 2 mol equiv of 40 -[4(bromomethyl)phenyl]-2,20 /60 ,200 -terpyridine (3) to a solution of 2 in CH3CN resulted in the selective formation of disubstituted macrotricycle 4 with two nonadjacent quaternary N atoms. 4 was insoluble in CH3CN and was filtered. This intermediate was then hydrolyzed with concentrated aqueous NaOH to yield the CHTT ligand. This simple synthetic route proceeds in good yield at room temperature, with the compounds being isolated by simple filtration or extraction. Complexation Studies in Solution. The preparation of soluble homometallic MII 2 CHTT metallo-polymers with M = Co, Ni, or Cu was undertaken. Depending on the affinity of the metal ions for the two coordination sites (terpyridine and cyclam), two mechanisms can be envisaged (Scheme 1) for the stepwise preparation of the M2CHTT polymers; the first metal ion is captured by the cyclam site (path A) or by the terpyridine units (path B). To determine the complexation routes between CHTT and the different metal ions, millimolar solutions of CHTT containing metal ions with a stoichiometry of S = M/ CHTT = 1 and 2 were prepared in DMF with tetrafluoroborate as the counter anion. After 2 h of equilibration time, the MIICHTT (S = 1) and MII 2 CHTT (S = 2) solutions were studied by UV-visible spectroscopy and, upon addition of TBAP (0.1M) as the supporting electrolyte, by cyclic voltammetry (CV). These complementary techniques are suitable for localizing the metallic cations in the multitopic cyclambis-terpyridine ligand molecules. For comparison, solutions of bis-terpyridine-MII (MII(terpy)2) and 1,8-bis-benzyl-cyclammetal complexes (MIICHB) were prepared and used as reference solutions (Figure 1B) that were characterized under the same conditions as for the CHTT derivatives. Results are given in Table 1 and Figure 2. Metalation of CHTT in DMF with 1 mol equiv of Cu2+ gave, in a 1:1 molar ratio, a light-purple solution (λmax = 590 nm), suggesting the formation of a Cu2+-cyclam-based complex in which the terpyridine units remain metal-free. Electrochemical experiments (Figure 2, right) confirmed this coordination mode because the related CV curve displayed a reduction wave at E1/2 = -0.97 V (vs the ferrocenium/ferrocene couple) close to that obtained at E1/2 = -0.99 V with the CuIICHB reference complex under the same conditions. The analysis of the CuII 2 CHTT solution indicated the presence of a supplementary CuII(terpy)2-based complex with an absorption band at Langmuir 2009, 25(15), 8751–8762
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Figure 1. (A) Synthesis of the CHTT ligand. (B) Reference complexes (M = Co, Cu, and Ni). Table 1. Characteristic Visible Absorption and Electrochemical Data of the Complexes and Polymers Used in This Worka electrochemical data E1/2 (ΔEP), Vb,c complex
λmax, nm (ε, L mol-1cm-1)a
ligand-centered reduction
MII/I (terpy)
MII/I(cyclam)
MIII/II(cyclam)
MIII/II (terpy)
536 (185) -0.99 (120) e CuIICHB CoIICHB 495 (26) Epc = -2.09 Epa = ∼0.60 f NiIICHB 471 (60) -1.57 (120) Epa = 0.77 687 (73) d -0.73 (80) CuII (terpy)2 513 (2700) -1.98 (78) -1.17 (75) -0.22 (70) CoII (terpy)2 794 (56) -1.83 (85) -1.59 (80) NiII(terpy)2 590 (222) -0.97 (160) CuIICHTT 521 (2220) -1.95 (90) -1.14 (80) -0.19 (170) CoIICHTT 805 (49) Epc = -1.77 Epc = -1.64 NiIICHTT II 571 (228), 700 (150) d -0.70 (180) -0.92 (265) Cu2 CHTT 521 (2500) Epc = -1.89 -1.11 (90) Epa = 0.67 -0.18 (180) CoII 2 CHTT 801 (45) Epc = -1.85 Epc = -1.67 Epa = 0.79 NiII 2 CHTT a In DMF; V vs the ferrocene/ferrocenium couple. M(Terpy): electron transfer associated with the metal ion located in the terpyridine units. M(Cyclam): electron transfer associated with the metal ion located in the cyclam group. b In DMF + 0.1 M TBAP. c E1/2 = (Epa + Epc)/2 at 0.1 V s-1; ΔEp = Epa - Epc. d The reduction of the ligand cannot be observed because of the prior electrodeposition of Cu0. e Not observed. f Poorly defined system as a result of the presence of coupled chemical reactions.
λmax = 700 nm and a second reduction wave at E1/2 = - 0.70 V similar to those observed with the CuII(terpy)2 reference complex. These data are unequivocal evidence for the formation of the CuII 2 CHTT species according to Scheme 1, path A: the mononuclear CuII complex is formed prior to the formation of the suprapolymer. Similar experiments were conducted with Co2+ 33 and Ni2+ metal ions. In both cases, solutions presented UV-visible and electrochemical features (Figure 2, left) close to those obtained with the CoII(terpy)2 and NiII(terpy)2 reference complexes (Table 1), indicating the formation of metallo-suprapolymers with unoccupied cyclam units. Consistently, comparison of the data II for solutions of CoII 2 CHTT and Ni2 CHTT and solutions of II II Co CHB and Ni CHB showed the presence of additional metal-cyclam fragments. The complexation modes adopted with Co2+ and Ni2+ ions follow the mechanisms of Scheme 1, path B: metallo-suprapolymers are first formed, followed by the incorporation of metal ions in the cyclam units. Langmuir 2009, 25(15), 8751–8762
These results demonstrate that polynuclear complexes having metal ions both in the terpyridine and cyclam sites are formed in solution and that, depending on the nature of the metal, two coordination modes can be observed in homometallic MII 2 CHTT. However, at this stage, the exact nature of these species (oligomers/polymers) must be clarified. Viscosity of the Metallosuprapolymer Solutions. Viscosity measurements are performed with liquid solutions of the metal ion-CHTT systems to prove the formation of coordination polymers and to investigate the role played by the M/CHTT stoichiometry in the self-assembly process. The ligand concentration is kept constant at [CHTT] = 13 mM in DMF, and three mother solutions of metallic salts Cu(BF4)2, Co(BF4)2, and Ni (BF4)2 at TM = 260 mM in DMF are prepared. Fractions of the metal salt solutions are added to the ligand solution, and the retention time is measured. Results are given in Figure 3 as the reduced viscosity ηR = η/ηS (with ηS being the viscosity of the solvent). Copper(II) salt addition is first considered. DOI: 10.1021/la900174e
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Figure 2. Cyclic voltammograms of the MIICHTT, MII 2 CHTT, MII(terpy)2, and MIICHB complexes with M = Co (left) and Cu (right) in DMF + TBAP (0.1 M). Scan rate: 0.1 V s-1.
Figure 3. Viscosimetry experiments. Progressive addition of Cu2+, Co2+, or Ni2+ ions as BF4 salts (0.26 M in DMF) to a CHTT solution in DMF. The vertical dotted lines point to the stoichiometry values corresponding to the viscosity maxima for each metal cation.
As shown by UV-visible absorption spectroscopy and electrochemistry experiments, the first equivalent of Cu2+ is captured by the cyclam spacer. Such a complexation reaction does not change the “monomeric” status of the CHTT molecules and consequently has no significant effect on the flow behavior of the solution. If the copper salt addition is continued, then the second Cu2+ equivalent is complexed by the terpyridine claws, and CuII(terpy)2 repeating units are formed. Such a process connects two neighboring ligands, thereby delivering rod-like species: the viscosity is therefore increasing and a maximum is reached for 2 mol equiv of added metal ion. The addition of the third copper equivalent forms CuII(terpy) repeating units that induce the fragmentation of the suprapolymer, and a decrease in the viscosity is thus observed. This behavior is in accordance with 8756 DOI: 10.1021/la900174e
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previous results32 and is due to comparable association constants of mono- and bis-terpyridine copper(II) complexes. In the case of Co2+, the first added metallic equivalent induces a significant viscosity increase that suggests the formation of CoII(terpy)2 species consistently with absorption spectroscopy and electrochemistry measurements.33 A further increase in metal salts is accompanied by a sharp viscosity increase up to M/CHTT = 1.5 before a smoother increase and subsequent stabilization. By contrast to the copper salt situation, the viscosimetric titration curve does not exhibit any apex. Previous studies have shown that the second metallic equivalent is complexed by the cyclam spacer. The subsequent increase in viscosity above 1 equiv of added metal confirms the important role played by the cyclam spacer that is assumed to be more rigid when containing a metal. As long as the macrocyclic spacer remains unoccupied, the cis conformation of the pending terpyridine arms can favor the formation of cyclic oligomers.28,40 The presence of such cyclic oligomers was corroborated by analyzing by TLC the solution composition with M/CHTT = 1 (silica gel, MeCN-saturated aqueous KNO3-H2O 7:0.5:1.5). Traces of presumably polymeric materials were observed that remained stationary on the baseline of the TLC plate, and a major product at RF = 0.3 attributed to oligomeric macrocycles was observed. Similar results were reported by Constable22,39 under the same conditions with iron(II) systems. Unfortunately, the dynamic nature of the cobalt-bis-terpyridine complexes prevents this product from being isolated for analyses. The third situation concerns Ni2+ cations with viscosity profiles similar to those with Co2+. This is in agreement with the absorption spectroscopy and electrochemistry studies showing that the terpyridine sites are preponderantly complexed for S = 1, followed by the occupation of the cyclam sites for S = 2. However, values of the relative viscosity for the nickel element are much larger than those for Co2+ or Cu2+. Such a difference cannot be explained only by the high values of the association constants of NiII(terpy)2, and part of the effect is attributed to the expected square-planar symmetry around the NiII center in the Ni(II)-cyclam complexes38 favoring a trans configuration and thereby the 1D connection of the CHTT ligands. To illustrate the relation between the increase in the reduced viscosity and the associated increase in the aspect ratio of the rod-like species, numerous theoretical models have been proposed. Kuhn44 has calculated the viscosity from the energy dissipation under conditions where rods are noninteracting (eq 1) and making explicit the hydrodynamic (first term) and Brownian contributions as a function of the volume fraction φ of the rods (second term). But if the system is actually in the semidilute regime, then a model taking into account the hindered rotational and translational motions in the entangled situation has to be considered. The one proposed by Doi and Edwards45 can be considered with eq 2. " # f2 f2 þ þ 1:6 φ ð1Þ ηR ¼ 1 þ 15ðlnð2f Þ -3=2Þ 5ðlnð2f Þ -1=2Þ ηR ¼
32f 6 φ3 ð1 -0:1f φÞ -2 15000lnðf Þπ2
ð2Þ
Figure 3 shows that the maximum increase in the reduced viscosity for the copper system is 1.94 at the apex. The value can (44) Kuhn, W.; Kuhn, H. Helv. Chim. Acta 1945, 28, 97. (45) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Clarendon: Oxford, U.K., 1986.
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Figure 4. Theoretical reduced viscosity according to the Kuhn (eq 1) and Doi-Edwards (eq 2) models vs L/D of rods (at a concentration of 13 mM). The horizontal dotted line at 1.94 corresponds to the experimental reduced viscosity at the apex of the Cu2CHTT system (Figure 3). Aspect ratios of f = 25 and 48 are respectively deduced. Table 2. Gelation Methods method spontaneous (A) spontaneous (A) spontaneous (A) ether diffusion (B) ion exchange (C)
mass fraction (%) 0.3 0.3 0.5 C* because its determination relies on a macroscopic method (inverted tube). C* is a theoretical definition based on dynamic features. Interestingly, in all cases, none of the gels could melt (up to 150 °C in DMF) whereas the kinetics of gelation was accelerated. This indicates that the characteristic energy of the SAFINs, involving basically intra- and interfiber components, is beyond the thermal energy. At this stage, it is necessary to quantify the viscoelastic properties of the gel-like systems. Rheology of the Metallogels. Under appropriate conditions of preparation (concentration of the ligand, metal/ligand stoichiometry, and nature of the counteranion), the CoII 2 CHTT and NiII 2 CHTT systems exhibit a gel-like consistency. The presence of a solid-like network whose elasticity can be characterized over a time scale of seconds to several thousands of seconds can be detected by oscillatory rheometry. If a smallamplitude stress, sinusoidally varying in time, is imposed on the system, then a quantitative evaluation of the major rheological parameters can be made. Thus, the elastic energy, its viscous dissipation, and the solid versus liquid-like characteristics of the material can be deduced from such measurements.48 The storage modulus G’, loss modulus G00 , and reciprocal of tan δ = G’/G00 are related to the phase lag δ between the applied stress and resulting strain. Figure 6A shows that, in the frequency range of 0.0005-1 Hz, G’ and G00 are horizontal lines with G’/G0 ≈ 7. The stress and strain are in-phase as expected for a gel. Figure 6B shows that for a NiII 2 CHTT system the gel has its storage modulus decreasing with increasing shear stress before a brutal failure beyond which (at σ = σ*, the yield stress) there is no measurable elasticity. The measurement informs us about the type of network that creeps at any imposed stress, thereby signing the lack of a linear viscoelastic regime. The elastic modulus is not strain-independent, and strain softening is occurring at all stresses, even weak ones (Figure 7). The highly nonlinear behavior does not prejudge of the recovery of the strain when the stress is removed, and appropriate creep-recovery experiments are needed to refine the analysis. At this stage, the behavior reveals the weak nature of the interactions between metallo-supramolecular aggregates in the gels. The absolute value of the elastic modulus is also weak (G’ (0.1 Hz) ≈ 48 Pa) at C ≈ 0.2 wt % (Figure 6A). This can be compared to a welldocumented reference system made up of a 12-hydroxystearic (48) Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: New York, 1999.
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acid in a polar organic solvent such as nitrobenzene, which forms strong, thermoreversible organogels.5 At an identical concentration, assuming that the elastic shear modulus scales as a square power law with concentration, the value G’ ≈ 1600 Pa can be considered. More than 1 order of magnitude differentiates the fatty acid organogel and the metallo-complexed cyclam bis-terpyridine system. The rheology of a SAFIN has a very complex dependence on the bending modulus of the fibers, the number density, and the type of junction zones. A molecular gel realizes a static equilibrium characterized by a specific particle-particle interaction potential whose shape controls the yield stress (function of the number of interparticle bonds that cross a unit area of gel) and the elastic modulus. The metallo-complexed cyclam bis-terpyridine systems can form gels with weak elasticities and yield values. Figure 8 shows a deformation-stress phase diagram that clearly exhibits the restricted domain of the solid-like behavior followed by the strained gel domain preceding the viscous domain of the ruptured gel. Multiresponsivity of the Gels: Redox Stimulus and Scattering Consequences. The self-assembly in the MII 2 CHTT systems proceeds through a two-step mechanism and forms thermally equilibrated species. The phenomenon is spontaneous and can be controlled by concentration and temperature. The gels are thixotropic and highly stress-sensitive. Moreover, because the CHTT ligand is sophisticated by containing a redox and ionic active spacer (the cyclam unit), the self-assembly can be further triggered by an external stimulus such as an electrical input (redox sensitivity) or the type of counterion (chemosensitivity). Temperature can play a subtle role in the configuration of the cyclam (cis vs trans) possibly promoting self-assembly.28,38 These tunable properties contribute to the “intelligence” of this second generation of molecular gels. The most striking and typical effect obtained in the control of the gelation phenomenon of metallo-supramolecular systems is that observed with an electrical stimulus. Indeed, we have recently reported that the red CoII 2 CHTT gel in DMF can be electrochemically and reversibly oxidized to produce a green solution33 corresponding to the CoIII 2 CHTT system. As mentioned earlier, solubility is a key parameter in the formation of gels. Here, the electrochemical oxidation of the Co2+ in the polymer generates additional positive charges in the polymer chains and modifies the solubility of the species that might also participate to the gel-to-liquid transition.33 Here, we investigate the flow properties of the Co2CHTT/DMF system to differentiate the aggregation status before and after the oxidative electrolysis at +1 V (Experimental Section). Figure 5B shows the two states of the cobalt(III/II) system, and Figure 9 represents the flow curves for the initial red gel CoII 2 CHTT system (curve 1) and for the green system corresponding to the oxidized state (curve 2). The profile of the viscosity versus shear rate presents an amplitude difference of ca. 300 before and after the application of the voltage. This confirms the presence of long 1D aggregates at rest whereas smaller and/or fewer interacting species are generated after the voltage application. It is known that the viscosity of a suspension of rods is dependent upon the number density of the rods and their aspect ratio L/R. Beyond C*, hydrodynamic interactions become important with respect to Brownian forces, and both rotational and translational motions become hindered. The profile of the nonoxidized specimen exhibits first a shear-thickening-like signature at low shear whereas classical shear-thinning behavior develops at higher shear rates. The dashed line indicates the dη/d(dγ/dt) = -1 slope, commonly observed with non-Newtonian liquids. Langmuir 2009, 25(15), 8751–8762
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-3 Figure 6. Dynamic rheology of M/CHTT organogels. (A) Frequency scan. CoII 2 CHTT at TCHTT = 2.15 mM (0.00181 g 3 cm ), CH3CN, II 00 00 counterion = NO, T = 15.5 °C. (•) = G’, (o) = G . (B) Stress scan; G’ and G profiles. Ni CHTT at T = 4.17 mM (0.00351 g 3 cm-3), 3 CHTT 2 DMF, counterion = Cl , T = 12.5 °C.
Figure 7. Nonlinear rheology of a NiII 2 CHTT gel (G’(0) = 159 Pa)
at TCHTT = 4.17 mM (0.00351 g 3 cm-3), DMF, counterion = Cl-, T = 12.5 °C, dσ/dt = 0.052 Pa 3 s-1.
It is worth noticing that the shear-rate dependence of the viscosity does not affect the relative viscosity measurements (Figure 3) that use a capillary viscometer operating at constant flow conditions. As shown in Figure 5B, the gravitational force applied to the volume of the CoII 2 CHTT gel in a tube is unable to induce any flow. Upon electrolytic oxidation of the Co2+ to Co3+ metal ions, a green liquid is freely flowing in the tube. It is necessary to characterize the morphology of the interacting aggregates responsible for the gel-like viscoelasticity. Because of the noticeable fragility of the system, it is crucial to use a noninvasive technique to probe the genuine self-assembled structures. In this respect, the small-angle neutron scattering (SANS) technique is particularly suitable. Swollen gels and sols can be probed with SANS on the basis of the elastic deviation of a neutron beam and the analysis of the large-scale fluctuations of the neutron scattering length density.49 SANS applied to molecular gels has to take into account the singularities of SAFINs such as the existence of plain fibers, bundles, and more or less organized domains. The theoretical form (49) Glatter, O.; Kratky, O. Small Angle X-ray Scattering; Academic Press: London, 1982.
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Figure 8. Strain behavior of a NiII 2 CHTT system (annealed for 12 h at T = 110 °C) at TCHTT = 4.17 mM (0.00351 g 3 cm-3), DMF, counterion = Cl-, T = 18.8 °C. Domain I: the gel behaves as a soft solid and is slightly deformed. Domain II: the gel is progressively deformed. Domain III: the gel creeps sharply and disrupts at yield stress σ* = 95 Pa.
factor for isolated fibers is given in eq 3. A series expansion at low Q gives the Guinier’s expression (eq 4) for rod-like species from which the structural features of 1D scatterers can be extracted. I ¼
πC 2 J1 ðQRÞ 2 Δb ML 2 Q QR "
ðQIÞQf0
RC 2 2 Q ¼ ðQIÞ0 exp 2
ð3Þ # ð4Þ
C is the rod concentration (g 3 cm-3), ML is the mass per unit length of the rod (g 3 A˚-1), Δb is its specific contrast (Δb = b2 Fsv2), b2 is the specific scattering length of the rod (cm 3 g-1), v2 is its specific volume (cm3 3 g-1), Fs is the scattering length per unit volume of solvent (cm 3 cm-3), and RC is the cross-sectional radius of √gyration (for a circular section, the geometrical radius R is RC 2). DOI: 10.1021/la900174e
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Figure 10. SANS Guinier plot for a NiII 2 CHTT system in CD3CN at TCHTT = 0.00181 g 3 cm-3. The fit of the linear part, according to 2 -5.4986 - 320.18Q √ following eq 4, supports the existence of rodlike species (R ≈ 2 320.18 = 17.9 A˚). Figure 9. Flow curves of a CoII 2 CHTT system (DMF, counterion Cl-) at TCHTT = 4.17 mM (0.00351 g 3 cm-3). (Upper Curve) Before electrolysis. (Lower Curve) After electrolysis at +1.0 V. The dotted line indicates a slope of dη/d(dγ/dt) = -1
Equation 4 is first used to search a linear part of the ln(QI) versus Q2 plot that characterizes a low-Q Q-1 intensity decay typical of rod-like species. Figure 10 shows such a linear variation from which, following eq 4, a geometrical radius of 18 A˚ is deduced. Depending on the transition metal, the radius values can be in the range of 18-25 A˚. It is premature to attempt molecular aggregation modeling, but it is useful to compare this value with the molecular length. The dimension of CHTT is 21 A˚ for the extended length (cyclam unit = 7 A˚).32 Nevertheless, several packing options could be speculated, and thereby refined studies using complementary techniques operating on the molecular length scale (e.g., NMR, EPR, and WAXS) are required that are beyond the scope of the present contribution. SANS has shown that the solid part of the gels is made up of self-assembled rod-like species, and rheological measurements have demonstrated their weak interconnections. SANS can also trace the structural modifications when the system is submitted to an electrical potential. Indeed, the absolute intensity is proportional to the number density n of the scatterers, their volume V, and the square of their contrast ΔF according to I µnVΔF2: any change in these parameters can be detected. It is reasonable to envision that the trans geometry can promote the 1D self-aggregation whereas the cis geometry can alter it. Equation 5 is the theoretical scattering form factor for finitelength rods completing eq 3 for fibers (with 2π/Lp, Lc < Qmin, with Lp, Lc being, respectively, the persistence and contour lengths). 2 Z π=2 sinðQL cos θÞ J1 ðQr sin θÞ 2 sin θ dθ IðQÞ ¼ 4πLr ΔF QL cos θ Qr sin θ 0 ð5Þ θ is the angle between the fiber axis and the vector Q. The slope of the intensity decay dI/dQ at very low Q is related to the aspect ratio 2L/D = f under concentration conditions of the single-particle approximation. Such a dilute regime can be considered to be reached if no low-Q component due to interferences between interacting scatterers has a significant amplitude. Figure 11 shows the theoretical effect in the low-Q region of the I vs Q curve if the length is reduced as calculated from eq 5. The calculation for different f values has been made for rods 8760 DOI: 10.1021/la900174e
having a radius comparable to the experimental one extracted from Figure 10 (R = 25 A˚). It shows that for long rods (f = 40, curve 1) an asymptotic decay of I = KQ-1 is observed that corresponds well to the usual situation for SAFINs under preparation conditions such that the contribution from nodal zones is rejected in the primary beam. Decreasing f (curves 2-4) leads to a more extended low-Q plateau (I vs Q representation) and a restricted range where the Q-1 decay can be observed to precede the sharp Q-4 decay due to the finite cross-sectional sizes. The plateau indicates that the interference between extremities of the same rod (single-particle approximation) is compatible with the Q range of the calculation/measurement and thus all curves merge at a constant intensity value related to the total volume of 1D scatterers. The experimental situation (Figure 12A, curve 1) for the CoII 2 CHTT system presents clear reminiscences, with such a Q-1 decay (dotted line in Figure 12A) in a profile somewhat obscured by a significant contribution Q < 0.03 A˚-1 attributable to nodal zones of the SAFIN, as classically encountered in molecular organogels. Nevertheless, the 1D character of the MII 2 CHTT aggregates has been demonstrated in the Guinier plot of Figure 10. Curve 2 in Figure 12, corresponding to the electrolyzed specimen CoII 2 CHTT, shows that the low-Q decay is significantly flattened compared to the situation for curve 1. This corresponds well to a reduction of parameter f when the comparison is made at an intermediate Q value (e.g., vertical dotted line in Figure 11) so as to avoid the innermost Q range overwhelmed by the scattering from various large-scale heterogeneities of the SAFIN. SANS data thereby demonstrate that the application of an appropriate voltage to the CoII 2 CHTT system induces the shortening of 1D species. This can be also illustrated by analyzing the cross-sectional intensities. Coming back to the theoretical modeling in a QI vs Q representation (Figure 11B), it appears that long fibers (f g 40, curve 1) show a plateau while reducing the length, restricting the Q extension of the plateau. Finally, for f e 5, the cross-sectional intensity in the low-Q range exhibits an intense bump. Such a theoretical trend is also observed for the experimental profile of the electrolyzed specimen (curve 2, Figure 12). The electrochemical modification of the metal oxidation state in the terpyridine and cyclam sites induces chromophoric, rheological, and structural modifications that may be related to changes in the configuration of the cyclam-metal complexes. It is worth commenting that the innermost low-Q scattering has a sharp decay Q-R (R > 3). Such a component cannot be described by the form-factor eqs 3 and 5. Preferably, it can be Langmuir 2009, 25(15), 8751–8762
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Figure 11. Theoretical small-angle scattering profiles following eq 5 for different L/R ratios (40 for 1; 20 for 2, 10 for 3, and 5 for 4) and
R = 25 A˚. (A) Intensity vs Q curves. (B) QI vs Q curves. The vertical dotted lines indicate the low-Q limit beyond which the comparison with the experimental curves should be made.
-3 Figure 12. Electrolysis of a CoII 2 CHTTsystem (in CD3CN, counterion = NO3 ) at CCHTT = 0.00351 g 3 cm . SANS evidence of the redox sensitivity of the Co2CHTT aggregation phenomenon. (1) System at rest. (2) After oxidation at +1.0 V of Co2+ into Co3+ in both terpyridine and cyclam sites. (A) Intensity vs Q curves. The sloped dotted line indicates the theoretical Q-1 decay. (B) QI vs Q curves. The horizontal dotted line corresponds to the theoretical behavior for rigid fibers.
attributed to randomly distributed large-scale heterogeneities with an exponential pair correlation function in a theoretical description proposed by Bueche.50 These heterogeneities in a SAFIN can be logically due to nodal zones where 1D species merge. They contribute to the connectivity of the SAFIN on large scales that can reach the macroscopic range where rheometry operates. They can also be considered to be the premise for a phase-separation process that always occurs (at variable time scales) in molecular gels. With M2CHTT liquid solutions made up of short rods, a residual van der Waals attractive component accounts for the presence of the low-Q Debye:: Bueche structure factor component whereas the free diffusivity of the species accounts for the shear-thinning rheological signature.
Conclusions We report the preparation and characterization of copper(II), cobalt(II), and nickel(II) 1D coordination polymers and gels based on the use of a polytopic bis-terpyridine-cyclam ligand (CHTT). These multiredox suprapolymers incorporate metal ions into the spacer units and were characterized in solution using UV-visible spectroscopy, viscosimetry, and electrochemical experiments. Rheological measurements characterized weak gels with highly thixotropic properties. The Co2CHTT system devel(50) Debye, P.; Bueche, A. M. J. Appl. Phys. 1949, 20, 518–526.
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oped an original electrochemically controlled sol-gel transition, and SANS demonstrated the effect of the applied voltage on the aspect ratio of the rods. The 1D species are built through metal/ ligand complexation reactions (Scheme 1), and no cross-linking mechanisms are expected from such systems. The 3D network is elaborated through weak fiber-fiber interactions responsible for its highly shear-sensitive rheological behavior. The data demonstrate the crucial role played by the incorporation of metal ions in the cyclam units with respect to the 1D growth. An important phenomenological aspect of the M2CHTT systems is their sensitivity to different stimuli. Mechanical Sensitivity. Molecular gels are thixotropic systems, and rheological and structural features are interrelated in a complex shear rate or time dependence. The mismatch between the characteristic time of the applied perturbation and the relaxation time of the long-lived shear-induced metastable states is at the origin of the thixotropy. In particular, sequences of creep-recovery cycles are suitable for characterizing the shear sensitivity and ability of the system to use healing mechanisms. The relaxation of the distribution of rod-like species upon the cessation of flow reveals the diffusivity and connectivity properties of the transient SAFIN. To illustrate, Figure 13 shows that, when submitted to a shear stress in Δt = 300 s, the NiII 2 CHTT system instantaneously reacts by a large deformation (γ = 0.215) followed by a progressive creep process (γ = 0.35 at Δt = 300 s). Upon the cessation of flow, part of the deformation relaxes instantaneously (γ = 0.145, i.e., 59% recovery) whereas DOI: 10.1021/la900174e
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Figure 13. NiII 2 CHTT system (in DMF at TCHTT = 4.17 mM (0.00351 g 3 cm-3) submitted to cycles of creep-recovery segments at T = 18.8 °C. Three cycles are shown. Creep segment: Δt = 300 s and σ = 25 Pa. Relaxation segment: Δt = 300 s and σ = 0 Pa. The dotted segmented curve is the applied stress.
the reminder is more slowly recovered (91% recovery at Δt = 300 s). Such results confirm the weak connectivity of the SAFIN in the M2CHTT systems. Interestingly, the major part of the deformation is recovered during a short time delay (Δt = 300 s) that makes the M2CHTT system a “self-healing” soft material.
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Chemical Sensitivity. The 1D aggregation and ability to gelate have been shown (paragraph “Gelation Conditions”, Table 3, Figure 5A) to depend on the metal ion, stoichiometry, solvent, and counterion type. Electrosensitivity. The 1D aggregation and the ability to gelate have been shown (see paragraph “Redox Stimulus and Scattering “, Table 1 and Figures 5B, 9, and 12) to depend on the applied electrical potential switching the oxidation state of the metal ions in the cyclams. The oxidation state and the cis or trans configuration of the cyclam determine the 1D growth. The versatility and multisensitivity of this novel class of materials are interesting features for fundamental and applied perspectives. For example, following a two-step metalation procedure, heterometallic suprapolymers can be envisaged. The introduction of additional functional groups to the macrocycle unit and the use of lanthanides or Ru2+ metal ions may generate additional luminescence properties. Moreover, rods can be grown by varying the M/CHTT stoichiometry, and their liquid-like suspensions could be useful in elaborating functional multilayered solid substrates.51,52 Acknowledgment. We acknowledge the Fondation Nanosciences Grenoble (RTRA) for supporting the present work. We :: thank Maelle Riutord for her valuable help in the realization of the experiments. We acknowledge Institut Laue Langevin (ILL, Grenoble, France) for providing access to the beamline, and we thank Dr. B. Deme for his help during the measurements. (51) Schutte, M.; Kurth, D. G.; Linford, M. R.; Colfen, H.; Mohwald, H. Angew. Chem., Int. Ed. 1998, 37, 2891–2893. (52) Lvov, Y.; Essler, F.; Decher, G. J. Phys. Chem. 1993, 97, 13773–13777.
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