Multifunctional Gels from Polymeric Spin-Crossover Metallo-Gelators

pubs.acs.org/Langmuir © 2010 American Chemical Society

Multifunctional Gels from Polymeric Spin-Crossover Metallo-Gelators Pauline Grondin,†,‡ Olivier Roubeau,*,†,‡,§ Miguel Castro,§ Hassan Saadaoui,†,‡ Annie Colin,^ and Rodolphe Cl
erac*,†,‡ †

CNRS, UPR 8641, Centre de Recherche Paul Pascal (CRPP), Equipe “Mat
eriaux Mol
eculaires Magn
etiques”, 115 avenue du Dr. Albert Schweitzer, Pessac, F-33600, France, ‡Universit
e de Bordeaux, UPR 8641, Pessac, F-33600, France, §Instituto de Ciencia de Materiales de Arag
on, CSIC - Universidad de Zaragoza, Facultad de Ciencias, Plaza San Francisco, s/n, Zaragoza, 50009, Spain, and ^LOF, CNRS-RhodiaUniversit
e Bordeaux 1, UMR 5258, 178 avenue du Dr. Albert Schweitzer, Pessac, F-33608, France Received September 28, 2009. Revised Manuscript Received November 7, 2009 The gelation abilities toward organic solvents of a series of triazole-based coordination polymers of formula [M(Cntrz)3]A2 (M = Fe(II) or Zn(II); Cntrz =4-n-alkyl-1,2,4-triazole with n = 13, 16, 18; A = monovalent anions, abbreviated as MCnA) have been studied to form thermally responsive multifunctional metallogels, in particular for the iron polymers that present the spin-crossover phenomenon. Indeed thermo-reversible physical gels exhibiting thermally reversible magnetic and optical crossovers are formed in decane and toluene. The FeC18ptol/decane and FeC18ptol/ toluene phase diagrams are described (ptol = p-toluene sulfonate anion), together with the rheological properties of the gels determined as a function of the solvent, the gelator concentration as well as temperature. Microscopic observations of the gel structure are correlated to the composition and rheological properties of the gels. Magnetic and thermal studies show that both the gel-liquid and spin-crossover phenomena can be adjusted through the composition of the gel mixture.

Introduction Gels are among the materials with most widespread applications that can arise from the use of both small and polymeric gelator molecules. Low-molecular weight (LMW) hydro- and organo-gels or molecular gels results from the self-assembly of small molecules in water or organic liquids through specific noncovalent interactions. This supramolecular process often takes place unidimensionally, resulting in fibrous (nano)architectures that form entangled 3D networks able to trap the solvent molecules.1 The design of gelator molecules has allowed the formation of materials with potential applications in organic light emission diodes,2 photovoltaic cells-light harvesting systems,3 drug delivery,4 or even food science.5 In some cases, the formation and/or properties of these supramolecular gels are sensible to external stimuli, often as a consequence of the incorporation of a “stimulable” unit as part of the gelator molecule.6 Although the specific coordination properties of metal *Corresponding authors. E-mail: (O.R.) [email protected]; (R.C.) [email protected]. (1) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3159. (b) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201–1217. (c) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (d) Special issue: “Low Molecular Mass Gelators” Top. Curr. Chem. 2005, 256. (e) Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2005. (f) de Loos, M.; Feringa, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2005, 3615–3631. (2) O’Neill, M.; Kelly, S. M. Adv. Mater. 2003, 15, 1135–1146. (3) See, for example: Kudo, W.; Kambe, S.; Nakade, S.; Kitamura, T.; Hanabusa, K.; Wada, Y.; Yanagida, S. J. Phys. Chem. B 2003, 107, 4374–4381. Susuki, T.; Shinkai, S.; Sada, K. Adv. Mater. 2006, 18, 1043–1046. (4) See, for example: Dai, H.; Chen, Q.; Qin, H.; Guan, Y .; Shen, D.; Hua, Y.; Tang, Y.; Xu, J. Macromolecules 2006, 39, 6584–6589. Ramanan, R. M. K.; Chellamuthu, P.; Tang, L.; Nguyen, K. T. Biotechnol. Prog. 2006, 22, 118–125. (5) Rogers, M. A.; Wright, A. J.; Marangoni, A. G. Soft Matter 2009, 5, 1594– 1596. (6) Yang, H.; Yi, T.; Zhou, Z.; Wu, J.; Xu, M.; Li, F.; Huang, C. Langmuir 2007, 23, 8224–8230. Wang, S.; Shen, W.; Feng, Y. L.; Tian, H. Chem. Commun. 2006, 1497– 1499. (7) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives: VCH: Weinheim, Germany, 1995.

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ions are major tools of supramolecular chemistry,7 metal-containing molecular gelators have been somewhat less studied. Early examples, such as metal complexes of fatty acids,8 phtalocyanines9 or porphyrines,10 are preformed coordination complexes bearing organic functions able to gelate solvents, but in which the coordination bonds have no or little effects on the gelation abilities. More recently, there has been a surge of interest in such metallo-gels.11 The binding of metal ions to gelator molecules containing appended ligands has been used to provoke, modify or tune the aggregation modes and gelation abilities of a chemical system.12,13 The formation of molecular gels from discrete molecular organic or metallo-organic components with a welldefined chemical structure thus represents an elegant bottomup approach to fabricate materials with controlled and tuned properties. On the other hand, polymeric gels14 may also be stimuliresponsive and implement designed chemical functions. The polymeric gelators are macromolecules, e.g., high molecular weight covalent compounds, which brings some practical difficulties such (8) Lopez, D.; Guenet, J.-M. Macromolecules 2001, 34, 1076–1081. MiedenGundert, G.; Klein, L.; Fischer, M.; V€ogtle, F.; Heuz
e, K.; Pozzo, J.-L.; Vallier, M.; Fages, F. Angew. Chem., Int. Ed. 2001, 40, 3164–3166. (9) Kimura, M.; Muto, T.; Takimoto, H.; Wada, K.; Ohta, K.; Hanabusa, K.; Shirai, H.; Kobayashi, N. Langmuir 2000, 16, 2078–2082 and references therein . (10) Terech, P.; Gebel, G.; Ramasseul, R. Langmuir 1996, 12, 4321–4323. Ishi-i, T.; Iguchi, R.; Snip, E.; Ikeda, M.; Shinkai, S. Langmuir 2001, 17, 5825–5833. (11) Fages, F. Angew. Chem., Int. Ed. 2006, 45, 1680–1682. (12) See, for example: (a) Hanabusa, K.; Maesaka, Y.; Suzuki, M.; Kimura, M.; Shirai, H. Chem. Lett. 2000, 1168–1169. (b) Ziessel, R.; Pickaert, G.; Camerel, F.; Donnio, B.; Guillon, D.; Cesario, M.; Prang
e, T. J. Am. Chem. Soc. 2004, 126, 12403– 12413. (c) Shirakawa, M.; Fujita, N.; Tani, T.; Kaneko, K.; Shinkai, S. Chem. Commun. 2005, 4149–4151. (d) Kishimura, A.; Yamashita, T.; Aida, T. J. Am. Chem. Soc. 2005, 127, 179–183. (e) Bull, S. R.; Guler, M. O.; Bras, R. E.; Meade, T. J.; Stupp, S. I. Nano Lett. 2005, 5, 1–4. (f) Leong, W. L.; Batabyal, S. K.; Kasapis, S.; Vittal, J. J. Chem. Eur. J. 2008, 14, 8822–8829. (13) Weng, W.; Beck, J. B.; Jamieson, A. M.; Rowan, S. J. J. Am. Chem. Soc. 2006, 128, 11663–11672. (14) Guenet, J. M. Polymer-Solvent Molecular Compounds; Elsevier: London, 2008.

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as solubility. Nevertheless, organic polymeric gels have been developed for potential uses in fields as diverse as light-harvesting or emitting materials,15a batteries,15b supercapacitors,15c drug delivery,15d,e or biomedicinal applications.15f Along this line, preformed coordination polymers have been rarely, and only recently,16-18 used as macromolecular gelators while, as for LMW metallo-gels, the incorporation of metal ions opens up the possibility of imparting their functional properties, such as catalytic, optical, magnetic, conducting properties etc., to the resulting gel materials. Indeed, we previously reported that preformed iron(II) triazole-based coordination macromolecules are able to gelate alkane solvents while maintaining their spincrossover related optical and magnetic switching properties.16 In a similar cobalt(II) system, the thermally reversible shortening of the coordination polymer chains through Co-N bond breaking was shown to be at the origin of a peculiar solution to gel thermal transition, accompanied by a color change.17 We present here a complete study of the [M(Cntrz)3]A2 (hereafter denoted as MCnA)/solvent systems (M =Fe(II) or Zn(II); Cntrz =4-nalkyl-1,2,4-triazole with n=13, 16, 18) showing that in addition to alkanes, toluene is also gelated. The phase diagrams of the FeC18ptol/decane and FeC18ptol/toluene are determined (ptol = p-toluene sulfonate anion), and the rheological properties of the gels studied in details, and correlated to their structure. Magnetic and thermal studies show that both the gel-liquid and spincrossover characteristic temperatures can be adjusted through the composition of the gel.

Experimental Section Methods. A. Infrared Spectra. IR spectra were performed on pure solids and gels with a Perkin-Elmer Spectrum 100 spectrometer equipped with the ATR option (Figure S2). B. Dynamic Light Scattering Measurements. DLS measurements were performed with a Coherent Innova 90 red laser (Kr-ion, λ = 647.1 nm) and a digital correlator Brookhaven BI9000 AT to calculate the temporal correlation function of photons on the detector. The viscosity of chloroform solutions of the coordination polymers was too low (0.5 cP), which resulted in characteristic times close to the detection limit of 2.5.10-9 s. Therefore, all studies were done on solutions in 50:50 decaneCHCl3, for which viscosities of ca. 0.8 cP were obtained. To avoid any dust particle, the pure solvents and then the decane-CHCl3 solutions were filtered through 0.2 μm polytetrafluoroethylene (PTFE) membranes prior to both the preparation of the solutions and the DLS measurements. C. Gelation and Solubility Tests. A typical procedure for qualitative gelation testing was the following: in a glass test tube the coordination polymer MCnA was mixed with the appropriate amount of solvent to reach 3 wt % of solid. The tube was sealed with a cap and the mixture heated until the solid dissolved, or up to 140 C. The tube was then cooled to 20 C and kept at this temperature for a minimum of 12 h. After this, if the test tube could be inverted without any flow of its content within 30 s, it was identified as a gel. Solubility of the solids was tested in the same manner but with concentrations down to 0.1 wt %. (15) (a) Ajayagosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644–656. (b) Nov
ak, P.; M€uller, K.; Santhaman, K. S. V.; Haas, O. Chem. Rev. 1997, 97, 207–281. (c) Hashmi, S. A.; Kumar, A.; Tripathi, S. K. Eur. Polym. J. 2005, 41, 1373–1379. (d) Kowalczyk, A.; Nowicka, A. M.; Karbarz, M.; Stojek, Z. Anal. Bioanal. Chem. 2008, 392, 463–469. (e) Jo, S.; Kim, J.; Kim, S. W. Macromol. Biosci. 2006, 6, 923–928. (f) Fei, S.-T.; Phelps, M. V. B.; Wang, Y.; Barrett, E.; Gandhi, F.; Allcock, H. R. Soft Matter 2006, 2, 397–401. (16) Roubeau, O.; Colin, A.; Schmitt, V.; Cl
erac, R. Angew. Chem., Int. Ed. 2004, 43, 3283–3286. (17) Kuroiwa, K.; Shibata, T.; Takada, A.; Nemoto, N.; Kimizuka, N. J. Am. Chem. Soc. 2004, 126, 2016–2021. (18) Fujigaya, T.; Jiang, D.-L.; Aida, T. Chem. Asian J. 2007, 2, 106–113.

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D. Determination of Tmelt and Tgel. The melting and gelling temperatures of the gels were also determined by the so-called inversed-tube method: the gels were first formed as for the gelation tests with the corresponding mass fraction of solid gelator, the test tubes being then immersed in a thermo-regulated bath (water or oil depending on the highest temperature needed). The temperature of the bath was increased by steps of 5 degrees. After 15 min at a constant temperature, the test tubes were inverted for 30 s. If no flow could be observed, the system was identified as a gel. Tmelt was defined as the lowest temperature for which flowing was observed. Once the gels had been melted, the temperature was raised a further 10 C, and the same procedure was repeated upon cooling, to determine Tgel, the highest temperature for which no flowing was observed anymore. Note that identical tubes (thickness, volume) and identical total mass (and therefore similar occupied volume in the tubes) were used both for the gelation tests and phase-diagram determination. Variation in these parameters is known to affect slightly the determination of melting or gelling temperatures with the inversed-tube method.19 E. AFM Measurements. The measurements were performed under ambient conditions with a Digital Instruments multimode Nanoscope IIIa in tapping mode. The previously formed gels were melted and dropped onto a tilted HOPG surface to form a thin layer by flowing. Measurements were performed immediately, to limit evaporation of solvents, and thus concentration variation. Silicon cantilevers with a resonance frequency of ca. 280 kHz were used. F. Electron Microscopy. Electron microscopy observations on xerogels were made at CREMEM, Universit
e Bordeaux 1. Scanning electron microscopy (SEM) was done with a JEOL JSM840A scanning electron microscope operating at 10 kV, while transmission electron microscopy (TEM) was performed with a JEOL 2000 FX microscope (accelerating voltage of 200 kV). The gel samples were melted in order to deposit a drop on the holders. G. Rheology Measurements. At 20 C, the measurements were performed on a stress-controlled rheometer (TA Instruments Advanced Rheometer 1000) with a cone (40 mm diameter, 2 cone angle, 0.056 mm truncation) and plate geometry. A solvent trapping device is above the cone to avoid evaporation. Rheological measurements are particularly sensible to sliding of the gel layer during experiments. To avoid this, a striated cone and a rough plate are used. The experiments will show in the following that this attention is sometimes not sufficient and that fractures of the gel near the wall may happen. The temperature dependent measurements were performed with homemade thermo-regulated Couette cell.20 H. Magnetization Measurements. Temperature dependent magnetic measurements have been performed on gels enclosed in Perkin-Elmer DSC aluminum pans (50 μL) using a Quantum Design MPMS-XL SQUID magnetometer. The response of the aluminum pan was determined prior to the measurement and removed from the raw data. Typical mass of gel used for these measurements was in the range of 16-25 mg. I. Differential Scanning Calorimetry. Calorimetric measurements were done with a differential scanning calorimeter Q1000 from TA Instruments with the LNCS accessory. The experiments were performed in a helium gas atmosphere. The measurements were carried out using 3-7 mg of gels sealed at room temperature in aluminum pseudohermetic pans with a mechanical crimp. Temperature and enthalpy calibrations were made with a standard sample of indium, using its melting transition (156.6 C, 3.296 kJ mol-1). The samples were weighed after each temperature cycle to ascertain potential solvent loss. Excess enthalpies were deduced by subtracting a suitable baseline. J. Thermogravimetric Experiments. Thermogravimetric analysis was performed with a Setaram TAG-16 apparatus. (19) See Raghavan, S. R.; Cipriano, B. H. pp 233-244 in ref 1e. (20) Grondin, P. Ph.D. thesis. Universit
e de Bordeaux: Bordeaux, France, 2007.

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Measurements were done under an Ar flow (15 cm3min-1) with ramps of 5 C min-1, after a 30 min stabilization isotherm at 30 C. Two identical cycles with a plateau at the highest temperature were performed to correct data of systematic variations of the weight due to dynamics of the oven. Materials. All solvents and reagents were obtained from commercial sources as analytical grade and used without further purification. A. Fe(ptol)2 and Zn(ptol)2 Hydrates. These were obtained by stirring, respectively, iron and zinc powder (0.5 mol per liter of solution) in an aqueous solution of p-toluenesulfonic acid (1.5 mol/L) at 80 C for 4 h. After filtration of remaining metal solid, the filtrate was left cooling and standing at room temperature, allowing for crystallization by slow evaporation of water of respectively Fe(ptol)2 3 6H2O and Zn(ptol)2 3 6H2O crystals that were recovered by filtration and washed with cold water. The water content of the hydrates was determined by thermogravimetric analysis. B. 4-n-Alkyl-1,2,4-triazole Ligands. These were prepared as previously described.21 C. Coordination Polymers. The synthesis of all the polymeric compounds of formula [M(4-n-1,2,4-triazole)3]A2 3 xH2O was achieved following the same procedure: a hot ethanolic solution (7 mmol, 40 mL) of the 4-n-alkyl-1,2,4-triazole was added slowly to a hot aqueous solution of the iron(II) or Zn(II) salt (2 mmol, 20 mL), containing in the iron(II) cases a small amount of ascorbic acid, to prevent oxidation to iron(III). A white precipitate formed rapidly upon the addition. After 30 min aging, the solution was filtered and the solid washed with successively 2
40 mL water, 20 mL of a 50:50 V/V water/ethanol mixture and 2
50 mL ethanol. After drying, all compounds were white solids, except the iron compounds with ptol anions that turned purple, indicating the presence of iron(II) ions in a low-spin state. The formula, and in particular the water content x (that was found to range from 0.5 to 4 depending on M, n, and A), of the coordination polymers were determined by elemental and thermogravimetric analysis. Note that this procedure is similar to an early report by Armand et al. on attempts at forming Langmuir-Blodgett films of FeC18ptol and in which the synthesis was performed in either water or ethanol,22 while being identical to that we previously reported.21

Results and Discussion Gelation Tests, Gel Formation, and Stability. The gelation abilities toward a number of organic solvents of 4-n-alkyl-1,2,4triazole iron(II) and Zn(II) coordination polymers with p-toluenesulfonate (ptol), tetrafluoroborate and chloride counterions with formula [M(CnH2nþ1trz)3](A)2 3 xH2O were studied. A systematic procedure has been used that consist in warming up to 140 C (or to lower temperatures until an homogeneous gel or solution formed) a weighed amount of solvent and the solid complexes corresponding to a mass fraction Φm of 3 wt % in sealed glass tubes (Figure 1). Observations were then made after cooling the mixtures to room temperature. The results are collected in Table 1. The ptol compound, FeC18ptol, is able to gelate, as previously reported,16 apolar n-alkanes, but also toluene, while it remains insoluble in alcohols and other common polar solvents, e.g. THF, DMF and water. Interestingly, diminishing the length of the n-alkyl substituent on the triazole ligand from n = 18 to n = 13 does not modify these gelating abilities. Moreover, replacing iron(II) by the diamagnetic zinc(II) ion also does not alter the gelation properties of the resulting ZnC18ptol coordination polymer. Using the tetrafluoroborate ion instead (21) Roubeau, O.; Alcazar Gomez, J. M.; Balskus, E.; Kolnaar, J. J. A.; Haasnoot, J. G.; Reedijk, J. New J. Chem. 2001, 25, 144–150. (22) Armand, F.; Badoux, C.; Bonville, P.; Ruaudel-Texier, A.; Kahn, O. Langmuir 1995, 11, 3467–3472.

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Figure 1. Scheme showing the conditions of gel formation from heterogeneous mixtures of solid FeCnptol in either decane (top) or toluene and decane-CHCl3 mixtures (bottom). The pictures correspond to 3 wt % FeC18ptol/decane and decane-CHCl3 (85:15) and 2 wt % FeC18ptol/toluene gels.

of ptol, e.g., FeC18BF4, seems to reduce these gelling abilities: FeC18BF4 also allows to form gels, albeit much weaker23 as low shear or simple shaking results in breaking of the gel into smaller pieces and in the release of some free alkane solvent. On the other hand, replacing the counterion ptol by the spherical chloride ion yields a drastic change, as neither FeC18Cl nor ZnC18Cl are able to gelate any of the tested solvents. The corresponding solids remain insoluble in all solvents, even in chloroform, only solvent in which the ptol compounds form stable solutions (note that solutions do also form in DMSO, but oxidation of the Fe(II) is rapidly observed). Gel formation in all alkanes only occurs after warming the heterogeneous mixture above 80 C, as previously reported for the decane gels (Figure 1). On the other hand, in chloroform/decane mixtures the gelation is spontaneous at room temperature, most likely thanks to the solubility of the coordination polymers in pure chloroform. Nevertheless, higher chloroform contents yield viscous solutions (vide infra). Most interestingly, gelation of toluene also occurs at room temperature, without the need to add chloroform (Figure 1). The iron containing gels are either purple, typical of Fe(II) ions in a S = 0 low-spin (LS) state with the ptol anion or colorless, typical of a S = 2 highspin (HS) state, with BF4-, indicating that the spin state of the Fe(II) ions of the gelators has been retained upon gelation. Moreover, this result indicates that it is possible to vary the spin-crossover temperature of the gel, as that was observed previously on the related solids. Zinc containing gels on the other hand are colorless as expected. In the following, we will mostly focus on gels formed with FeC18ptol. Some gel systems such as giant micelles or worm-like phases are true thermodynamically stable gels,24 although most organogels are in fact dispersed solid phases such as the gels formed with DDOA (2,3-di-n-decyloxyanthracene),25 and thus not thermodynamically stable. The gels formed with the metal-organic complexes here are likely falling in this second class of gels, as in the case of FeC18ptol/decane, crystallization-induced phase separation is indeed observed on a short time scale.16 However, toluene gels did not present spontaneous phase separation, even after 3 years. Therefore, to ascertain the true thermodynamic stability of the gels studied in this work, the different (23) The term weak gel is coined to systems presenting a marked increase of G00 with increasing shear frequency, resulting in breakdown of the gel into smaller bits and eventually flowing. See: Handbook of Hydrocolloids; Phillips, G. O., Williams, P. A., Eds.; Woodhead Publ. Ltd.: Cambridge, U.K., 2000; p 294. (24) (a) Terech, P.; Coutin, A. J. Phys. Chem. B 2001, 105, 5670–5676. (b) Terech, P.; Maldivi, P.; Dammer, C. J. Phys. II 1994, 4, 1799–1811. (25) Lescanne, M.; Colin, A.; Mondain-Monval, O.; Fages, F.; Pozzo, J.-L. Langmuir 2003, 19, 2013–2020.

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Table 1. Gelation Properties of 4-n-Alkyl-1,2,4-triazole Iron(II) and Zinc(II) Coordination Polymers in Various Liquidsa solvent/gelator

FeC18ptol

FeC16ptol

FeC13ptol

FeC18BF4

FeC18Cl

ZnC18ptol

ZnC18Cl

n-heptane G G G WG I G I n-decane G G G WG I G I n-dodecane G G G WG I G I n-hexadecane G G G WG I G I toluene G G G WG I G I S S S S I S I CHCl3 DMSO S+O S+O S+O S+O I S I a Letters indicate the observations done at 3 wt %: S, solution; G, clear gel; WG, weak gel;23 I, insoluble (down to 0.5 wt %); O, oxidation of iron(II) to iron(III) based on the appearance of a brown color. Solids were found to be insoluble in water, methanol, ethanol, 1-hepanol, 1-octanol, 1-decanol, 1-dodecanol, acetone, pentanone, dichloromethane, dimethylformamide, and tetrahydrofuran.

Figure 2. Left: typical biphasic gel/liquid sample obtained after centrifugation at 50000 rpm for t = 1 h. H(0) and H are respectively the initial gel height in the tube, and that after centrifugation. Right top: relative gel height as a function of centrifugation time for three FeC18ptol/toluene gels with increasing gelator mass fraction. Full lines are fits to an exponential law x(t) = exp(-t/ τ). Right bottom: Characteristic densification time of the gel, τ, as a function of the FeC18ptol gelator concentration. The experimental data seem to follow an exponential law (full line).

MCnA/solvent gels were centrifuged at 50000 rpm at 20 C. After 1 h centrifugation, all gels, including toluene ones, have transformed into a biphasic system composed of a gel phase and a supernatant liquid (see Figure 2, left). As shown by 1H NMR, this supernatant liquid is indeed pure solvent, so that the observed behavior corresponds to a densification of the gels. The present gels are thus, as expected, not thermodynamically stable, justifying the use of the term pseudophase diagram below. In order to evaluate their stability in more detail, we followed the normalized height of gel x(t) = H(t)/ H(0) as a function of centrifugation time (H(0) is the initial gel height in the centrifugation tube, and H(t) its height after a centrifugation during a time t), and for increasing mass fractions of FeC18ptol, in the case of the seemingly more stable toluene gels. For all concentrations (three are shown in Figure 2, right top), x decreases exponentially with time: the experimental data are reproduced by a decreasing exponential x(t) = exp(-t/ τ) where τ corresponds to a characteristic densification time of the gel. These τ have been determined and plotted as a function of FeC18ptol mass fraction in Figure 2 (right bottom). They show an increase with the gelator concentration that seems Langmuir 2010, 26(7), 5184–5195

to follow an exponential law. As intuitively expected, the higher concentrations in gelator yield the stronger gels. As example of stability, a 3 wt % FeC18ptol/toluene gel displays a characteristic densification time of 4 h under the centrifugation conditions used. Pseudo Phase Diagrams. To determine the FeC18ptol/decane and FeC18ptol/toluene pseudophase diagrams, mixtures of FeC18ptol and decane or toluene with mass fractions of FeC18ptol Φm ranging respectively from 0.1 to 30 wt % and 0.3 to 20 wt % were prepared and warmed, respectively to 140 and 60 C, until an homogeneous colorless liquid formed. After cooling down to room temperature, the FeC18ptol/decane samples with more than 1.8 wt % of FeC18ptol were found to be homogeneous purple gels, while below this value an heterogeneous mixture of purple gel pieces and colorless liquid was observed. On the other hand, FeC18ptol/toluene samples with less than 1 wt % of FeC18ptol were homogeneous purple viscous solutions. Nonetheless, repeating these samples, but cooling to 1 C instead of 20 C, produced homogeneous purple gels, as in all cases above 1 wt % of FeC18ptol. The melting and gelling temperatures, Tmelt and Tgel, for all the concentrations were determined respectively upon warming and cooling by the inversed tube method (see Experimental Section). The resulting pseudophase diagrams are shown in Figure 3. As often observed in organogels,1,26 the gelling temperature Tgel is in most cases inferior to the melting temperature Tmelt, leading to the observation of an hysteresis about 5 to 10 C wide, in the case of decane gels but significantly less for the toluene gels. Also, the increase of gelator concentration results in an increase of both Tgel and Tmelt, related to a change in rheological properties (vide infra), although the decane gels present much higher gelling and melting temperatures. Indeed, for mass fractions of FeC18ptol in decane higher than 30%, homogenization of the gel requires temperatures higher than 140 C, and the starting solid then suffers decomposition (oxidation of Fe(II) to Fe(III)) as immediately seen by the brown/orange color of the mixture. The FeC18ptol/decane gels are subject to phase separation with the crystallization of a white solid from hours to weeks after their formation, with the appearance of crystallites within the gel (Figure S3, Supporting Information).16 Even if this process is reversible and a homogeneous gel is recovered by heating, the FeC18ptol/decane gels are not stable from a thermodynamic viewpoint, as previously demonstrated by centrifugation experiments (vide supra). Interestingly, no such phase separation has been observed in the FeC18ptol/toluene systems, that thus form “more stable” gels. Considering that chloroform is the only good solvent of these MCnA solids,27 and that decane and chloroform (26) Placin, F.; Desvergne, J.-P.; Lassegues, J.-C. Chem. Mater. 2001, 13, 117– 121. (27) Chloroform solutions of similar coordination polymers were used to form Langmuir-Blodgett films. See: (a) Roubeau, O.; Agricole, B.; Cl
erac, R.; Ravaine, S. J. Phys. Chem. B 2004, 108, 15110–15116. (b) Roubeau, O.; Natividad, E.; Agricole, B.; Ravaine, S. Langmuir 2007, 23, 3110–3117.

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Figure 4. Pseudo phase-diagrams for Φm = 3 wt % FeC18ptol/ decane-CHCl3 mixtures as a function of the CHCl3 weight fraction. Temperatures were determined upon heating by the inversedtube method.

Figure 3. Pseudo phase-diagrams of the FeC18ptol/decane (top) and FeC18ptol/toluene (bottom) systems showing Tgel (blue squares) and Tmelt (red circles). Empty and full symbols correspond to gels prepared by cooling to respectively 20 and 1 C. The full red lines correspond to the ideal solution model, ln(c) = f(1/Tmelt) (c is the molar concentration, see text), for the decane and toluene cases, down to Φm = 1.8 and 1 wt % respectively. The spin-crossover temperatures of the gels are depicted with gray crosses (dashed gray lines are only guide to the eye). Part of the decane diagram was previously reported.16

are miscible liquids, gelation of decane-CHCl3 mixtures with increasing amounts of chloroform was attempted, in order to slow down and eventually avoid the crystallization process. As expected, the addition of chloroform increases the solubility of the solid in the liquid mixture, (i) allowing an easier homogenization of the gel formed (a lower temperature is necessary and even room temperature homogenization is observed for the higher chloroform fractions) and (ii) improving the stability of the resulting gel: no phase separation is observed up to more than 2 years after gel preparation. To study the effect of the added chloroform fraction, 3 wt % gels of FeC18ptol in decane-CHCl3 mixtures were prepared with an increasing chloroform fraction. For mixtures with more than 35% of chloroform, a viscous solution was observed, instead of a gel. The melting temperatures of the gels were determined by the inversed tube method and are reported in Figure 4. As the chloroform fraction increases, the gels become less compact and Tmelt decreases. Interestingly, by adjusting the chloroform fraction, it is possible to adjust Tmelt so that, as in the toluene case, the melting and the SC phenomena are simultaneous. At 3 wt % of FeC18ptol this simultaneity occurs for the 70:30 decane to chloroform ratio, at ca. 50 C. The optical and magnetic properties of the initial solid are retained in the formed gels. A remarkable optical change from purple at low temperatures to colorless at high temperatures occurs at TSC (reported as gray crosses in Figure 3 and as a dashed line in Figure 4) between 47 and 56 C for the decane gels, depending on the gelator concentration, and at ca. 52 and 47 C for respectively toluene and decane/chloroform gels. This color change, due to the spin-crossover (SC) from the diamagnetic LS 5188 DOI: 10.1021/la903653d

state to the paramagnetic HS state of the iron(II) ions, is reversible and occurs in a rather narrow temperature range (vide infra, DSC and magnetic characterizations). The purple color is due to the 1 A1 to 1T1 d-d transition of the iron(II) ions in the LS state (around 550 nm), while the HS state is virtually colorless, with only a weak absorption in the near-infrared corresponding to the 5 T2 to 5E d-d transition. The decane-based materials all present this opto-magnetic behavior in the gel state, since TSC is below Tmelt at all concentrations. On the other hand, the gel melting and the opto-magnetic phenomena occur simultaneously for the 3 wt % FeC18ptol/toluene gel. Thus, for concentrations below this value, the toluene gels remain purple and diamagnetic, while above this value, they show a thermal crossover from purple to colorless, accompanied by a jump in magnetization (vide infra). Since previous studies have shown that similar systems with Co(II) do form gels but are subject to coordination bond breaking,17 the exact species present in solution or gel might differ from the initial coordination polymers, at least in length. In the present systems, IR spectra (Figure S2) are very similar to those of the starting solids, thus in agreement with the conservation of the chain structure.28 The size of the species present in chloroform/decane solutions at low concentration was determined by dynamic light scattering (Figure S1). The average sizes derived for the FeC18ptol systems (see Supporting Information for details) range from 12 to 15 nm (or 35 to 45 Fe(II) ions per chain), and are in reasonable agreement with those determined on the basis of bulk magnetic properties of the starting coordination polymers solids, e.g. 1819 nm.29 Although this study was only possible in the decaneCHCl3 mixture, and at low concentrations, it seems likely that the (28) In the visible range, mainly the 1A1f1T1 band of the Fe(II) ions in the LS state is observed, centered at 550 nm, while the terminal Fe(II) ions in linear triazole-based oligomers are expected to be in the HS state, thus indicating the presence of long oligomers, e.g. chains; when the triazole ligand is in a local C2v symmetry (the case of a 1,2-bridging coordination mode), the intensity of ring torsion vibration modes are weak in IR compared to their intensities in a non-C2v case, as for example in the pure 1H-1,2,4-triazole. Here in the gel spectra, these vibration bands at ca. 670 cm-1 are very weak confirming the local C2v symmetry. (29) The length of the coordination polymers can be evaluated from the magnetic properties for the iron compounds. Considering that in linear trinuclear complexes with triple triazole bridges, the external Fe(II) ions remain HS over the whole temperature range, the Fe(II) ions at both ends of the coordination polymers will likewise remain HS at low temperature. The number of Fe(II) ions per coordination chain can then be derived from the residual paramagnetic signal at low temperature, assuming that no other paramagnetic species is present in the material. See ref 21 and: Vos, G.; Le F^ebre, R. A.; de Graaf, R. A. G.; Haasnoot, J. G.; Reedijk, J. J. Am. Chem. Soc. 1983, 105, 1682–1683.

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species at the origin of the gelation properties are those of the original material, without any chemical modification, nor significant length reduction of the coordination chains. The observed increase of Tmelt and Tgel with the gelator concentration is in agreement with most studies on both molecular and polymeric gelators. Mostly, two physical models have been considered that allow reproducing such experimental data, both based on the formation-breaking of intermolecular interactions. On one hand, Eldridge and Ferry showed first that the gel-liquid transition of biopolymers such as gelatin can be considered to be controlled by the formation-disruption of pairwise cross-links among polymeric chains.30 Under such assumption, with the cross-linking reaction being exothermic, Tmelt (and likewise Tgel) is related to the gelator molar concentration c by eq 1, in which ΔcrH represents the molar heat of crosslinking reaction and K is a constant. -Δcr H þK lnðcÞ ¼ RTmelt

ð1Þ

The transition should in such a case be considered as second-order and the derived ΔcrH should be expected to match experimental figures derived from calorimetric measurements on gel samples.31 On the other hand, the observation that most gel phases are formed by fibrillar aggregates of the gelator molecules (vide infra) leaded to analyze the gel-liquid transition in the same manner as the dissolution process of crystals.1a In the ideal solution approximation, the solubility of a solid as a function of temperature is given by the so-called Schrader or Schr€oder-van Laar equation (eq 2) in which ΔfusH and Tfus are the fusion molar enthalpy and temperature of the solid, here the gelator.32 ! Δfus H 1 1 ð2Þ lnðcÞ ¼ T Tfus R At Tmelt (and likewise at Tgel), one then derives eq 3, in which K is a constant: lnðcÞ ¼

Δfus H þK RTmelt

ð3Þ

Although resulting in an identical expression, this second model contemplates a first-order phenomenon, the dissolution of gelator crystals, and the derived enthalpy difference has to be compared with that experimentally determined for the fusion of the solid gelator. Experimental ln(c) vs 1/Tmelt for decane and toluene gels are reported in Figure 5, and are indeed nicely reproduced33 by an Arrhenius relationship such as eq 1 or 3 (also reported as full lines in the pseudo phase-diagrams of Figure 3). It is worth noting that the data corresponding to the toluene gels formed by cooling either to 1 C or to 20 C follow the same Arrhenius law, which would indicate that the gelation process is sufficiently fast to remain unaffected by variations in the thermal conditions in (30) Eldridge, J. E.; Ferry, J. D. J. Phys. Chem. 1954, 58, 992–995. (31) Guenet, J. M. Thermoreversible Gelation of Polymers and Biopolymers: Academic Press: London, 1992. (32) (a) Atkins, P. W.; de Paula, J. Physical Chemistry, 8th ed.; Oxford: Oxford University Press, 2006. (b) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664–6676. (c) George, M.; Snyder, S. L.; Terech, P.; Glinka, C. J.; Weiss, R. G. J. Am. Chem. Soc. 2003, 125, 10275–10283. (33) Note that the experimental points corresponding to concentrations inferior to c* (e.g., for 1.8 wt % of FeC18ptol) have not been taken into account for the determination of ΔH. This is obviously because for these concentrations c < c*, the system is heterogeneous and the gel phase has in fact a concentration c0 higher than c.

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Figure 5. Plot of the FeC18ptol molar concentration logarithm vs the inverse melting temperature for decane (squares) and toluene (circles) gels. Empty and full symbols correspond to gels prepared by warming (to 140 C for decane gels and 60 C for toluene gels) and cooling to 20 and 1 C respectively. The full lines are best fits to an Arrhenius expression of the data for gels obtained by cooling to 20 C (see text).

which it occurs. The derived enthalpy differences ΔH are very similar for decane and toluene gels, respectively at 60(2) and 59(1) kJ/mol, in good agreement with values found for organogels formed through interactions among alkyl chains.1a Although ΔfusH of the FeC18ptol solid could not be determined experimentally, because the solid suffered decomposition in the melting temperature range, a value of ΔfusH = 50 kJ/mol was obtained by DSC for the related ZnC18ptol solid,34 which compares well with the ΔH derived above.35 Moreover, the constant terms from eq 3 for decane or toluene gels yield a melting temperature of the solid Tfus in the range 150-190 C, in good agreement (i) with the temperature at which the solid FeC18ptol starts to melt-decompose, ca. 190-200 C, and (ii) with the melting temperature of the related ZnC18ptol, i.e., 208 C. This would agree with the dissolution of crystals assumption. Also, the fact that the likely weaker links formed in a gelation process performed at a lower temperature, and thus faster, seem to have no influence on Tmelt of the toluene gels, would indicate that the cross-linking of the coordination polymers or their aggregates does not play any substantial role. Nevertheless, DSC data for the gel-liquid transition temperature range (vide infra) show only broad anomalies with small excess enthalpies upon warming while no anomaly at all is observed upon cooling. These calorimetric observations support a second-order process and thus the assumption of Eldridge and Ferry. It is therefore difficult to discriminate the exact gel-liquid transition mechanism,36 and both the melting-dissolution processes of the solid-like aggregates of FeC18ptol gelator and the cross-linking among these aggregates may be playing a role. Structure of the Gels. Atomic force microscopy (AFM) and scanning and tunneling electron microscopy (SEM/TEM) experiments were performed to ascertain the structure of the gels formed with decane, toluene and decane-CHCl3 mixtures. The images (34) Roubeau, O.; Grondin, P.; Cl
erac, R. Unpublished results. (35) ΔH values derived from phase diagrams are often found to be higher than the experimental ΔfusH of the solid. The most likely explanation is that dissolution of the gel includes the “discrete” dispersion of the gelator molecules into the solvent, which is not the case for the simple melting of the solid gelator. See ref 31. (36) Even after using multiple characterization tools, others also reach the conclusion that the exact nature of gels remains elusive. See, for example: (a) Menger, F. M.; Caram, K. L. J. Am. Chem. Soc. 2000, 122, 11679-11691, or a very recent perspective article about the understanding gelation: (b) van Esch, J. H. Langmuir 2009, 25, 8392-8394.

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Figure 7. AFM tapping mode images (2 μm
2 μm) of toluene gels at 1 wt % (a) and 3 wt % (b) of FeC18ptol. True concentrations are likely higher due to partial evaporation of solvent (see text).

Figure 6. AFM tapping mode images (1 μm
1 μm) of 3 wt % FeC18ptol/decane-CHCl3 gels for different decane-CHCl3 weight ratios: 100:0 (a), 65:35 (b), and 50:50 (c).

described below are representative and typical of these systems, although due to slight evaporation of solvents during the preparation of samples and problems in the wetting of supports, the concentrations of the gelator and the mesh sizes of the gels are only semiquantitative. Typical AFM images of 3 wt % FeC18ptol/decane and FeC18ptol/decane-CHCl3 (35 and 50% of CHCl3) gels are given in Figure 6. The images of the pure decane and the 35% CHCl3 gels are similar and reveal a three-dimensional (3D) structure of entangled elongated aggregates with polydispersed 5190 DOI: 10.1021/la903653d

size distribution. Nevertheless, an average size of 100 nm long by 10 nm wide can be roughly estimated. These observations are in agreement with our preliminary studies on pure decane gels.16 Nevertheless, the addition of CHCl3 does modify the gel structure, as the average mesh size, ξ, increases from ca. 30 nm for the decane case to ca. 50 nm for the gel with 35% CHCl3. A possible hypothesis is that the chloroform inflates the network of coordination polymer aggregates. At 50% CHCl3, the sample is in fact a viscous liquid, and the AFM observations confirm the destruction of the 3D network seen at lower chloroform concentration. AFM images of systems at 1 and 3 wt % FeC18ptol in toluene are shown in Figure 7. In both cases, a 3D entangled structure of polydisperse fiber-like aggregates is observed, similar to the decane and decane-CHCl3 cases and characteristic of a gel phase. The fiber dimensions are on average 150 by 15 nm, but the network gets visually more compact as the fraction of FeC18ptol increases, the approximate mesh size ξ being 150 and 50 nm respectively at 1 and 3 wt %. The toluene gels were found to be sensible to evaporation of the solvent, so that concentrations are only given as indicative values. Drying the toluene gels yield xerogels in which the entangled fiber 3D structure remains, as shown by SEM and TEM observations. Figure 8 shows SEM and TEM images of two xerogels obtained by drying on the sample holders a solution of respectively 1 and 0.5 wt % of FeC18ptol in toluene. The 3D networks observed are made of larger fiber-like aggregates, varying from 100 nm to 1 μm in diameter. Such increase in the size of the solid aggregates is likely related to diffusion-crystallization phenomena occurring as a consequence of solvent evaporation and thus leads to an increase in solid concentration. Langmuir 2010, 26(7), 5184–5195

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Figure 8. SEM (top, scale bar 2 μm) and TEM (bottom, scale bar 1 μm) observations of xerogels formed from 1 (SEM) and 0.5 (TEM) wt % solutions of FeC18ptol in toluene.

As in the case of most organogelators, the microscopic observations indicate that all the gels formed from FeC18ptol solids are due to the construction of a network of entangled fiber-like aggregates. The observed sizes of these fibers indicate that the gels are not formed from individual coordination chains of the solid precursor but from more or less thick bundles of them. The most likely interaction at the origin of such chain aggregation is van der Waals interdigitation of alkyl tails on the triazole ligands. Nonetheless, H-bonding of the sulfonate group of ptol with H atoms on the triazole rings may play a role, in particular for the toluene gels in which π-stacking of toluene molecules with the ptol ring may also participate.18 Rheological Properties. The studied gels were deposited in the rheometer from their liquid state and let cool to 20 C. In all measurements, an adapted rheometer cover was used to minimize evaporation of the solvent from the gel. The linear viscoelastic (LVE) regime was determined for various FeC18ptol decane, toluene and decane-CHCl3 gels by measuring the elastic (G0 ) and viscous (G00 ) moduli at 0.1, 1, and 10 Hz oscillating stresses (see Figure 9). Both G0 and G00 remain practically constant with increasing stress amplitude up to a σLVE of 14 and 30 Pa for the 3 wt % decane and 2 wt % toluene gels and of 14 and 20 Pa for 3 wt % decane-CHCl3 gels with ratios 85:15 and 65:35 respectively. In all cases, the data demonstrate that the samples are true gels since G0 is always greater than G00 in the LVE regime. In addition, the values of the phase shifts, δ = arctan(G00 / G0 ), are very low, e.g. 0.04 to 0.06, indicating a pronounced elastic (gel) nature. Nevertheless, qualitative differences in the strength of the gels are confirmed as the toluene and the decane-CHCl3 gels are comparatively less elastic with smaller G0 values, e.g. Langmuir 2010, 26(7), 5184–5195

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Figure 9. Elastic (G0 , full symbols) and viscous (G00 , empty symbols) moduli at 20 C vs shear stress at 0.1 (circles), 1 (squares) and 10 (rhombuses) Hz frequency of respectively 3, 2, and 3 wt % FeC18ptol gels in decane, toluene and decane-CHCl3 15:85 and 35:65 mixtures, respectively from top to bottom. The linear viscoelastic (LVE) domain is defined by σLVE, the highest shear stress up to which G0 and G00 remain constant.

150-170 Pa, with respect to pure decane gel, e.g. 400 Pa. Moreover, the 50:50 decane-CHCl3 system presents a much higher δ value in the LVE (ca. 0.4, data not shown), although G0 remains above G00 . This is in agreement with the visual observation of a viscous liquid and with the disentangled network observed in the AFM image (Figure 6c). The variation of G0 with ξ is given by: G0 ¼

E ξ3

ð4Þ

where E corresponds to the energy stored in one repeating unit of the network. For the 3 wt % pure decane and decane-CHCl3 with 65:35 ratio, the rheological measurements of G0 together with the microscopic observations (from AFM, ξ35 = 50 nm and ξ0 = 30 nm) allow to evaluate the origin of the difference in viscosities:  3 E35 G035 ξ ¼ 0
35 ≈ 0:7 E0 G0 ξ0

ð5Þ

This decrease by about 30% of the energy stored per repeating unit of the network, related to an increase in the mesh size, results in the 65:35 decane/CHCl3 gel being less elastic. Because no significant differences are observed in G0 nor in ξ between the 65:35 and 85:15 decane-CHCl3 gels, it is likely that the differences DOI: 10.1021/la903653d

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frequency, this suggests the existence of a slow relaxation mode. Considering the AFM and SEM-TEM images, it may be ascribed to disentanglement or even microfractures within the interconnected network of fiber-like aggregates. Frequency dependence experiments were also performed as a function of FeC18ptol mass fraction Φm in decane and toluene, showing in both solvents an increase of the elastic modulus with increasing Φm (see Figure 10, bottom). This is expected for organogels, and can be modeled by considering that these gels are formed by interdigitated rigid fibers, as shown by microscopy. From a thermodynamic viewpoint, the entropic contribution in such systems arising from the degree of freedom of these fibers is small. To describe organogels, Terech et al.38 used a model considering a permanent network of rigid chains developed by Jones and Marques for polymer networks.39 The present gels can be considered as permanent networks for short times. In this model, a degree of freedom can be left for the fibers by allowing for a variation of the angles at the interconnections, these latter remaining fixed or permanent. In these conditions, G0 is then related to the mass fraction of gelator by: G0 ¼ CkB TΦm 3=2

ð6Þ

Without this degree of freedom, the expression of G0 is given by Figure 10. Logarithmic plots of the FeC18ptol mass fraction (Φm) dependences of σLVE (top) and elastic modulus G0 (bottom) for decane (full circles) and toluene (empty circles), showing measurements at 1 Hz of various samples for each concentration. Full lines are power-law fit of the experimental data, respectively σLVE = 4.1Φm1.3 and G0 = 24Φm2.4 for decane gels and σLVE = 5.6Φm1.5 and G0 = 25Φm1.5 for toluene ones.

observed between pure decane gels on one hand and decaneCHCl3 gels on the other hand originate in a change of solvent; the actual amount of chloroform in the mixture has, in comparison, little effect. The LVE regime was determined as a function of gelator concentration Φm for both decane and toluene gels. The corresponding σLVE are plotted in Figure 10. The LVE domain increases with increasing gelator concentration, and σLVE seems to verify a power-law in Φm. Indeed, it can be expected that for a fibrillar structure, the gel shear resistance increases with the gelator concentration. Moreover, and for all concentrations, σLVE is higher for the toluene gel than for the decane gel. This can likely be ascribed to the fact that the toluene gels are more stable, but also to the fact that the toluene gels seem to stick more efficiently to the rheometer walls (see below). Cyclic frequency sweeping experiments under an applied stress within the LVE regime were also performed (Figure S3, Supporting Information). First, a good repeatability of the measured moduli is observed, as long as the sample remains in the rheometer. On the other hand, variations of up to 30% are observed from sample to sample with the same gel composition. This observation highlights a strong influence of the sample handling, most likely through the quality of the gel-rheometer surface interaction.37 However, these measurements do confirm the viscoelastic behavior dominated by the elastic component G0 , about one decade or more superior to G00 , whatever the frequency, which is characteristic of solid gels. In the studied frequency range, G0 remains constant, while G00 slightly increases toward lower and higher frequencies. Since G0 has to be zero at zero (37) Note, nevertheless, that such variations are common in studies on organogels. See, for example: Lescanne, M.; Grondin, P.; d’Al
eo, A.; Fages, F.; Pozzo, J.-L.; Mondain Monval, O.; Reinheimer, P.; Colin, A. Langmuir 2004, 20, 3032–3041.

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G0 ¼ BΦm 2

ð7Þ

The experimental data indeed follow a power law (see Figure 10, bottom), the derived exponents being respectively 2.4 for the decane gels and 1.5 for the toluene gels. Thus, the FeC18ptol/ decane gels seem to behave as permanent networks. Indeed, Terech et al. obtained similar results on organogels that showed exponents superior to 2.38 On the other hand, the FeC18ptol/ toluene gels are more flexible, as a variation of the angle at the nodes of the network is necessary to reproduce correctly the experimental data. These conclusions are also in agreement with the smaller values of G0 obtained for toluene gels with respect to decane ones. Above σLVE, the elastic and viscous moduli become shear stress dependent (Figure 9). In particular, the viscous modulus, G00 , increases slightly for most gels. This behavior is probably induced by local rearrangements within the gel, as previously observed in emulsions.40 For shear stresses above 80 Pa for the decane gels and 40 Pa for those of toluene, both moduli drop drastically. Because a discontinuity is observed in shear stress-deformation plots (Figure S4, Supporting Information), it is likely that unhooking and further wall-slip and/or fractures of the gels are occurring. This hypothesis is confirmed by the fact that a portion of the sample is systematically ejected out of the rheometer cell in these experiments. This unhooking occurs at an unhooking shear stress that was found to be highly dependent on the sampling, varying by up to 300% from one sample to another. The toluene gels were nevertheless found to stick to the walls in a more efficient manner, as in some cases no discontinuity in shear stressdeformation plots was observed (up to 2000 Pa). Temperature dependent measurements were performed using a specially designed thermo-regulated Couette cell20 for three 3 wt % FeC18ptol/decane-CHCl3 gels, with decane-CHCl3 ratio of (38) Terech, P.; Pasquier, D.; Bordas, V.; Rossat, C. Langmuir 2000, 16, 4485– 4494. (39) Jones, J. L.; Marques, C. M. J. Phys. (Paris) 1990, 5, 1113–1127. (40) (a) Princen, H. M. J. Colloid Interface Sci. 1982, 91, 160–175. (b) Mason, T. G.; Bibette, J.; Weitz, D. A. J. Colloid Interface Sci. 1995, 179, 439–448. (c) Hebraud, P.; Lequeux, F.; Palierne, J. F. Langmuir 2000, 16, 8296–8299.

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Figure 12. Excess heat capacities measured on 18.3 (left, top), 10.2 (left, bottom), 6.4 (right, top) and 3.0 (right, bottom) wt % FeC18ptol/decane gels. Endothermic events are up, the arrows indicating the sense of temperature variations. For comparison, the vertical range of energies is identical for all curves.

Figure 11. Graphs a, b, and c: thermal dependence of the elastic G0

(empty symbols) and viscous G00 (full symbols) moduli upon cooling at ca. 1 C/min for three 3 wt % FeC18ptol/decane-CHCl3 gels with proportions (%:%) from top to bottom: (a) 100:0 (squares), (b) 85:15 (rhombuses), and (c) 65:35 (circles). A shear stress of 1 Pa at 1 Hz is applied throughout the measurements. Graph d: thermal dependence of the magnetization for a 3 wt % FeC18ptol/decaneCHCl3 gel with proportion 65:35, example of a diamagnetic to paramagnetic crossover for the gel. The vertical dashed gray line shows the coincidence of TSC and Tmelt for this specific gelator concentration and liquid mixture proportion.

100:0, 85:15 and 65:35. The gels were first melted within the cell that was set to the necessary temperature with a thermo-regulated bath. To avoid evaporation problems, this temperature was kept as low as possible (actual cell temperatures of respectively 99, 73, and 52 C). The cell was then closed and measurements of G0 and G00 were done, with a 1 Hz oscillating shear stress of 1 Pa, as a function of time upon cooling, while monitoring the cell temperature. The evolutions of both moduli for the three gels are shown in Figure 11a, b and c, respectively. The data obtained upon warming were similar to those upon cooling with Tmelt and Tgel differing in less than 2 C. In these measurements, three temperature domains are observed: (i) at low temperatures, both G0 and G00 present a plateau ; (ii) increasing the temperature, G00 first increases, while G0 remains practically constant, before starting to decrease; (iii) at higher temperatures, typically close or above Tmelt, both G0 and G00 decrease with increasing the temperature. G0 is found to be higher at 20 C than previously observed (Figure 9). Most likely, this difference is arising as a result of a different gel preparation: since for the temperature dependent measurements, the gels are formed by a slow cooling to 20 C (1 C/min), the resulting gels are likely stronger than those obtained by a faster cooling mode. The low temperature domain corresponds to the gel phase in its LVE domain, while the intermediate temperature range may be Langmuir 2010, 26(7), 5184–5195

related to the exiting of the LVE domain, where upon increasing the shear stress, G0 remains constant while G00 increases (see Figure 9). This can be due to local rearrangements or microfractures. A possible explanation would be that upon warming the gel network starts to melt locally until the melted zones percolate to form a liquid and thus both G0 and G00 drop. Such a behavior would agree with the observation of broad anomalies in DSC experiments (vide infra). These temperature-dependent measurements allow, in theory, a more accurate determination of Tmelt. From these data, the best method is to use the tangents in G0 or tan δ,41 but in order to avoid solvent evaporation, the measurements were started at the lowest possible temperature (in cooling mode) and therefore the lack of high temperature plateau in G0 or maximum in tan δ does not allow us to use this technique. As a consequence, we considered simply that Tmelt corresponds to the temperature at which G0 = G00 ,42 which is certainly more accurate than the inversed-tube method. Thus, Tmelt is found to be respectively 85, 65, and 49 C for the 100:0, 85:15 and 65:36 decane-CHCl3 3 wt % gels. These values can be compared with 90, 85, and 40 C respectively, that have been determined by the inversed-tube method. These differences and in particular the larger one observed for the 85:15 gel may have different origins. Besides the more subjective determination of these temperatures with the inversed-tube method, the different gel preparation methods might be responsible, at least partially, for these differences. Magnetism and Calorimetry. The temperature dependence of the magnetization has been studied for the different FeCnptol gels (Figure S5, Supporting Information). A rather abrupt jump, occurring in less than 10 C, is systematically observed around 45 to 55 C (see Figure 11d). As previously reported,16 these magnetization jumps correspond to the spin-crossover of the iron(II) centers within the coordination polymers acting as gelator. The derived characteristic temperatures, TSC, are (41) Winter, H. H.; Chambon, F. J. Rheol. 1986, 30, 367–382. (42) Da Silva, M. A.; Farhat, T. I. A.; Ar
eas, E. P. G.; Mitchell, J. R. Biopolymers 2006, 83, 454.

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Figure 13. Left: DSC trace (red and blue line correspond to respectively warming and cooling thermograms, endothermic events are up) of a 18.3 wt % FeC18ptol/decane gel, showing the SC anomaly around 52 C and a broad anomaly ascribed to the gel to liquid transition from 93 to 114 C. Right: Semilogarithmic plot of the FeC18ptol molar concentration c vs inverse melting temperature for FeC18ptol/decane gels derived from: the inverse tube method (red circles), offset of DSC anomaly (blue squares) and temperature-dependent rheology (black rhombus). The full lines are best-fits to an Arrhenius expression.

reported as grey crosses and dashed lines in the pseudo phasediagrams in Figures 3 and 4. The spin-crossover temperature is almost identical for all the gels independently of the solvent and gelator concentration. This observation supports a weak to inexistent influence of the solvent on the ligand-field strength around the iron(II) ions, that tunes the spin-crossover phenomenon. The actual values of magnetization above and below TSC are obviously dependent on the gelator concentration. Hence the concentration can be adjusted to obtain a crossover from a diamagnetic, and thus negative magnetization, to paramagnetic, and thus positive magnetization, response of the gel under the applied dc field. Most interestingly, it is possible to form gels for which both thermally induced phenomena, the spin-crossover and the gel melting, are simultaneous, playing on the gelator concentration or on the fraction of chloroform in decane-CHCl3 gels as they are both able to tune Tmelt (vide supra). A designed example is given in Figure 11, e.g., a FeC18ptol 3 wt % decaneCHCl3 gel with ratio 65:35. The enthalpy variation associated with the spin-crossover phenomenon is often considered to be correlated with the abruptness of the conversion, e.g. with the cooperative character of the spin-crossover. Values as large as 28 kJ/mol were found in [Fe(Htrz)2(trz)] 3 H2O43 or 10 to 20 kJ/mol in similar compounds,44 as determined from DSC. Here, an excess heat capacity anomaly is observed in the DSC trace of FeC18ptol/decane gels (Figure 12) that can be ascribed to the spin-crossover phenomenon since it occurs in the range of temperatures where the optical and magnetic responses of the gels vary as a result of the spincrossover. Indeed, the excess enthalpies associated with these (43) Kr€ober, J.; Audiere, J. P.; Claude, R.; Codjovi, E.; Kahn, O.; Haasnoot, J. G.; Groliere, F.; Jay, C.; Bousseksou, A.; Linares, J.; Varret, F.; GonthierVassal, A. Chem. Mater. 1994, 6, 1404–1412. (44) (a) Cantin, C.; Kliava, J.; Marbeuf, A.; Mikaı¨ litchenko, D. Eur. Phys. J. B. 1999, 12, 525–540. (b) Seredyuk, M.; Gaspar, A. B.; Ksenofontov, V.; Reiman, S.; Galyametdinov, Y.; Haase, W.; Rentschler, E.; G€utlich, P. Chem. Mater. 2006, 18, 2513–2519. (c) Seredyuk, M.; Gaspar, A. B.; Mu~noz, M. C.; Verdaguer, M.; Villain, F.; G€utlich, P. Eur. J. Inorg. Chem. 2007, 4481–4491. (d) Berezovski, G. A.; Bushuev, M. B.; Pishchur, D. P.; Lavrenova, L. G. J. Therm. Anal. Cal. 2008, 93, 999–1002. (e) Roubeau, O.; Castro, M.; Burriel, R.; Haasnoot, J. G.; Reedijk, J. unpublished results. (45) The starting FeC18ptol solid possesses a complex behavior with both a structural transition and SC phenomenon in the same temperature range, the detailed description of which is beyond the present study and will be published separately. Since it is likely that the structural transition is no longer observed in gels of FeC18ptol, the values of ΔHSC are best compared to those observed for similar compounds; e.g., see ref 43.

5194 DOI: 10.1021/la903653d

anomalies are proportional to the mass fraction of spin-crossover coordination polymer. Moreover, the values derived for the molar enthalpy (ranging from 15 to 30 kJ/mol) due to the spin-crossover phenomenon of this fraction are in reasonable agreement with those of the starting solid or similar compounds.45 In theory, the gel to liquid phase transition should also be detectable by DSC measurements. Nevertheless, difficulties related to the high content of the compounds in volatile solvents and to the usual broad anomalies associated with such transitions are known to potentially hamper accurate determination of the energies and transition temperatures by DSC.19,46 Indeed, attempts to perform such measurements on toluene or decaneCHCl3 gels were impeded by substantial losses of solvent upon scanning temperature. On the other hand, decane gels could be measured up to above 140 C without significant weight losses. However, only very broad and poorly energetic anomalies were observed, aside those due to the spin-crossover, and only in the first warming cycles (see Figure 13 left).47 The transition temperature corresponding to these broad anomalies should correspond to Tmelt. So far DSC has only been used to determine Tmelt when rather sharp anomalies are observed.30 Here, the melting process is covering about 20 C (e.g., from the onset temperature of the anomaly up to the temperature at which the anomaly disappears), and the gel sample can be considered to have melted only at the offset of the DSC anomaly. We have therefore considered the DSC offset temperatures as Tmelt, and compared these Tmelt (79, 95, 103, and 114 C respectively for 3, 6.8, 10.2, and 18.3 wt % gels) with data from rheological measurements (85 C for a 3 wt % gel) and inversed tube data (96, 109, 122, and 133 respectively for 3.5, 5.6, 10.3, and 19.5 wt % gels). All the data are gathered in Figure 13 right. A similar ΔH value to that obtained from Figure 5 is derived from the DSC data, e.g. 57(2) kJ/mol. It is worth mentioning though that the whole set of data from DSC experiments is being shifted about 10 C toward lower temperatures. Such discrepancies among different techniques to determine Tmelt are expected,46 although no systematic comparison of DSC with another technique is available. Overall, (46) Terech, P.; Rossat, C.; Volino, F. J. Colloid Interface Sci. 2000, 227, 363– 370. (47) The absence of the broad anomaly in the second warming cycle can likely be ascribed to the measurements being performed at 10 C/min. After the first melting of the gel, the gelation process is performed much faster than for the initial gel, resulting in weaker links in the gel network and therefore smaller excess melting enthalpy.

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Article

considering the differences in sample environment and size and their consequences on thermal equilibrium, the agreement among the three techniques used here to determine Tmelt can be considered to be good.

crossover phenomena, as opposed to a synergetic cooperative system previously reported.18 The approach used here to engineer new thermoresponsive multifunctional materials can likely be extended to metal-organic systems in which metal-metal interactions are well-defined and rigid.

Conclusions Utilizing preassembled coordination polymers as metalorganogelators, physical gels are formed with alkanes, toluene and chloroform/decane mixtures. These gels are thermoreversible but not thermodynamically stable, while they exhibit the magnetic and optical properties of the metal-organogelators: a spin-crossover of the iron(II) centers and a reversible purple to colorless optical transition. They are formed of a network of fiber-like aggregates, at the origin of the gelation process and reminiscent of organogels. The acquired knowledge of the pseudo phasediagrams of the different systems allows to adjust the melting transition, for example so as to coincide with the spin-crossover temperature. Moreover, the gelator concentration can be tuned to obtain a system presenting a transition from a dark diamagnetic gel to a colorless paramagnetic gel. Such tuning is possible thanks to the independent character of the physical gelation and spin

Langmuir 2010, 26(7), 5184–5195

Acknowledgment. The authors are grateful to Marie-France Achard for helpful comments, Marlene Soubaigne and Julien Richard for preliminary studies. This work was supported by the EU through MAGMANet (NMP3-CT-2005-515767), the French CNRS, R
egion Aquitaine, the Universit
e Bordeaux, the Spanish National Council of Sciences CSIC (PIE to OR). This work has been funded by the Spanish MICINN and FEDER, Project No. MAT2007-61621. Supporting Information Available: Text giving details about treatment of dynamic light scattering data and figures showing the correlation function, IR spectra, additional magnetic properties, frequency dependence of rheological properties, and deformation of gels. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la903653d

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