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85% after purification) and all compounds were characterized by standard analytic methods (see ESI† for full characterization and reaction details).

Downloaded by Aalto University on 28 February 2013 Published on 27 June 2012 on http://pubs.rsc.org | doi:10.1039/C2SM25259G

Coordination studies

Fig. 10 XRD patterns of (a) solid of 1 from CHCl3, (b) xerogel of 1 from pentan-1-ol, (c) xerogel of 1 + Ag(I) from pentan-1-ol, and (d) xerogel of 1 + Zn(II) from pentan-1-ol. Note: * ¼ diffraction peaks originating from an adhesive.

The Job’s method of continuous variations was used for 1 + Ag(I) and 1 + Zn(II) in CDCl3–CD3OD (5 : 1), and for 1 + Pd(II) in CDCl3–CD3CN (3 : 1), for different ligand–metal ratios for the constant total concentration of 0.016 M. 1H NMR chemical shift titrations in CDCl3–CD3OD (2 : 1) were carried out by varying the concentration of the metal ion for a fixed concentration of the ligand (0.008 M). 13C and 15N NMR spectra were recorded for compound 1, 1 + Ag(I) (1 : 1) and 1 + Zn(II) (2 : 1) in CDCl3–CD3OD (12 : 1).

Conclusions A novel low-molecular-weight gelator of the A(LS)2 type was designed and prepared. The gelation properties of the gelator and structurally related compounds were studied in various solvents. We found that a slight change in the chemical composition of the gelator backbone can significantly change the gelation abilities. Furthermore, we studied the gelation process in the absence and presence of various metal ions. We reported that not only the choice of solvent, but also the coordination of different metal ions had an influence on the visual properties as well as on the morphology and size of aggregates of the gel systems. In addition, silver metallogels formed in situ silver nanoparticles, which possess some potential in pharmacy. This work showed the possibility of using metal coordination as a powerful method for tuning gel properties. Due to this successful work, we plan to prepare more steroidal metallogels in the future and study their properties with regard to potential applications.

Gelation studies

Experimental

In a typical gelation test a weighed amount of the gelator was mixed with a measured volume of the selected solvent in a sealed test tube. The sample was sonicated for ca. 2–3 min and then the mixture was heated until the solid was completely dissolved (if soluble). The resulting solution was allowed to cool down to room temperature. Finally the test tube was inverted to observe if the content could still flow. Upon cooling down, the formation of a gel (G), precipitate (P), or solution (S) was detected. The minimum gelation concentration (MGC) was determined by scaling a minimum amount of gelator needed for formation of a stable gel. The gel-to-sol transition temperature (Tgel) was measured using an ‘‘inversion tube’’ method, two times for each sample. Gels, prepared to the sealed test tubes and stabilized overnight at rt, were placed upside-down on a water bath and slowly heated (2 C min1). The temperature at which the gel fell under gravity was recorded as the gel-to-sol transition temperature (Tgel).

Synthesis

SEM and TEM measurements

Cholesteryl glycinate – which was prepared by employing a classic peptide coupling procedure using DCC and DMAP, and a 20% solution of piperidine in DMF for removing the Fmocgroup – was used as an initial building block for the synthesis of 1 and 2. The coupling with the aromatic central unit was done by

Scanning electron micrographs of xerogels were taken on a Bruker QuantaX400 EDS microscope equipped with a digital camera. The samples of the xerogels were prepared by placing a hot, clear solution of the gelator on carbon tape over a sample stub. The samples were dried at room temperature and then sputter coated with a thin layer of gold in a JEOL Fine Coat Ion Sputter JFC-1100. Transition electron micrographs of pentan-1-ol xerogels were acquired using a JEOL JEM-1400 Electron Microscope. The samples of the xerogels were prepared by placing a gel or a hot clear solution of the gelator on a grid and were dried at room temperature. NMR measurements

Fig. 11 The asymmetric unit of a DMF solvate of 1 with marked Hbonds between the amide groups of 1 and the solvent molecules. The DMF molecules are filling two distinct infinite voids running in the directions of the a-axis.

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Variable-temperature 1H NMR spectra of gels were recorded with a Bruker Avance DRX 500 NMR spectrometer equipped with a 5 mm diameter broad band inverse probe head working at This journal is ª The Royal Society of Chemistry 2012


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500.13 MHz for 1H. Samples were prepared directly in an NMR tube; a weighed amount of the gelator was dissolved upon heating in 0.6 mL of DMF-d7, and gel samples were stabilized overnight. The VT 1H NMR experiment was conducted varying the temperature by 10 C steps. The sample was allowed to stabilize for 5 min at each temperature before acquiring the spectrum. The 13C CPMAS NMR spectra were recorded on a Bruker AV 400 spectrometer equipped with a 4 mm standard bore CPMAS probe head whose X channel was tuned to 100.62 MHz for 13C and the other channel was tuned to 400.13 MHz for broad band 1 H decoupling. The dried and finely powdered samples were packed in the ZrO2 rotor closed with Kel-F cap and spun at 10 kHz. The 13C CPMAS NMR was carried out for all samples under Hartmann–Hahn conditions with TPPM decoupling. Glycine was used as a reference standard for 13C chemical shifts. Powder and single crystal diffraction studies The X-ray powder diffraction data of xerogels were measured with a PANalytical X’Pert PRO diffractometer in Bragg–Brentano geometry using Johansson monochromatized Cu Ka1 45 kV, 30 mA). As received fine powder radiation (1.5406 A; samples were prepared on a silicon-made zero-background holder using petrolatum as an adhesive. The data acquisition was made from a spinning sample by X’Celerator detector in the 2q range of 1–50 with a step size of 0.017 , counting times of 480 s per step. The single crystal data for gelator 1 crystallized from DMF were collected at 150.0 0.1 C (Oxford Cryostream) with Agilent Supernova dual wavelength diffractometer, using a micro-focus X-ray source and multilayer optics mono 50 kV, 0.8 mA). chromatized CuKa radiation (l ¼ 1.54184 A; The data collection, reduction, multi-scan and analytical faceindex based absorption corrections were made by program CrysalisPro.15 The structure was solved with program Olex2 (ref. 16) (see ESI† for more details).

Acknowledgements A financial support by the Finnish Ministry of Education (Doctoral Program of Organic Chemistry and Chemical Biology), the Ministry of Education, Youth and Sport of the Czech Republic (MSM6046137305), and the Academy of Science of the Czech Republic (M200380901) is gratefully acknowledged. The authors are also grateful to Spec. Lab. Tech. Esa Haapaniemi for his help running NMR spectra, Spec. Lab. Tech. Mirja Lahtiper€a for running ESI-TOF mass spectra, Lab. Tech. Hannu

This journal is ª The Royal Society of Chemistry 2012

Salo for recording SEM images, and Spec. Lab. Tech. Paavo Niutanen, Department of Biological and Environmental Science, University of Jyv€ askyl€ a, for his help with recording TEM images.

References 1 (a) J. W. Steed, Chem. Commun., 2011, 47, 1379; (b) E. K. Johnson, D. J. Adams and P. J. Cameron, J. Mater. Chem., 2011, 21, 2024; (c) P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699; (d) L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201; (e) Molecular Gels: Materials with Self-Assembled Fibrillar Networks, ed. R. G. Weiss and P. Terech, Springer, Netherlands, 2006; (f) H. Svobodova, V. Noponen, E. Kolehmainen and E. Siev€anen, RSC Adv., 2012, 2, 4985. 2 X. Y. Liu, Top. Curr. Chem., 2005, 256, 1. 3 (a) A. Dawn, T. Shiraki, S. Haraguchi, S.-I. Tamaru and S. Shinkai, Chem.–Asian J., 2011, 6, 266; (b) S. Banerjee, R. K. Das and U. Maitra, J. Mater. Chem., 2009, 19, 6649; (c) A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew. Chem., Int. Ed., 2008, 47, 8002; (d) N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005, 34, 821. 4 (a) M.-O. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed, Chem. Rev., 2010, 110, 1960; (b) F. Fages, Angew. Chem., Int. Ed., 2006, 45, 1680. c, F. V€ 5 M. Zini ogtle and F. Fages, Top. Curr. Chem., 2005, 256, 39. 6 M. Xue, K. Liu, J. Peng, Q. Zhang and Y. Fang, J. Colloid Interface Sci., 2008, 327, 94. 7 Y. Liu, X.-X. Zhang, J.-M. Dou, D.-Q. Wang, D.-C. Li and S.-G. Zhang, Chin. J. Struct. Chem., 2006, 25, 95. 8 (a) P. Laine, A. Gourdon and J.-P. Launay, Inorg. Chem., 1995, 34, 5129; (b) P. Laine, A. Gourdon and J.-P. Launay, Inorg. Chem., 1995, 34, 5138; (c) P. Laine, A. Gourdon and J.-P. Launay, Inorg. Chem., 1995, 34, 5150; (d) P. Laine, A. Gourdon and J.-P. Launay, Inorg. Chem., 1995, 34, 5156. 9 I. Grenthe, J. Am. Chem. Soc., 1961, 83, 360. 10 R. E. Melendez, A. J. Carr, B. R. Linton and A. D. Hamilton, Struct. Bonding, 2000, 96, 32. 11 In a preliminary study we also used Fe(II) ions, but due to very broad signals in the NMR spectra, we were not able to obtain sufficient results from the NMR experiments, therefore we did not report on the use of Fe(II) ions in this work. 12 M.-O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft Matter, 2011, 7, 2412. 13 (a) A. R. Hirst, I. A. Coates, T. R. Boucheteau, J. F. Miravet, B. Escuder, V. Castelletto, I. W. Hamley and D. K. Smith, J. Am. Chem. Soc., 2008, 130, 9113; (b) K. Sakurai, Y. Jeong, K. Kuomoto, A. Friggeri, O. Gronwald, S. Sakurai, S. Okamoto, K. Inoue and S. Shinkai, Langmuir, 2003, 19, 8211. 14 (a) P. Conte, R. Spaccini and A. Piccolo, Prog. Nucl. Magn. Reson. Spectrosc., 2004, 44, 215; (b) S. P. Brown and H. W. Spiess, Chem. Rev., 2001, 101, 4125; (c) I. Wawer and S. Witkowski, Curr. Org. Chem., 2001, 5, 987; (d) D. L. Bryce, G. M. Bernard, M. Gee, M. D. Lumsden, K. Eichele and R. Wasylishen, Can. J. Anal. Sci. Spectrosc., 2001, 46; (e) Nonappa, M. Lahtinen, B. Behera, E. Kolehmainen and U. Maitra, Soft Matter, 2010, 6, 1748. 15 CrysalisPro, version 1.171.35.21, Agilent Technologies, Oxford, 2012. 16 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339.

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