Solvent Induced Pseudopolymorphism in a Calixarene-Based Porous

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DOI: 10.1021/cg900719f

Solvent Induced Pseudopolymorphism in a Calixarene-Based Porous Host Framework

2010, Vol. 10 1527–1533

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Consiglia Tedesco,*,†,‡ Loredana Erra,† Ivano Immediata,† Carmine Gaeta,† Michela Brunelli,§,# Marco Merlini,§ Carlo Meneghini, Philip Pattison,§,^ and Placido Neri*,†,‡ †

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Dipartimento di Chimica, Universit a di Salerno, via Ponte don Melillo, I-84084 Fisciano (Salerno), Italy, ‡NANO_MATES, Research Centre for NANOMAterials and nanoTEchnology at Salerno University, Universit a di Salerno, via Ponte don Melillo, I-84084 Fisciano (Salerno), Italy, § ESRF (European Synchrotron Radiation Facility), 6 rue Jules Horowitz, BP220, 38043 Grenoble, a di Roma 3, Roma, Italy, and CNR-TASC France, Dipartimento di Fisica ‘E. Amaldi’, Universit c/o CRG-GILDA (ESRF) Grenoble, France, and ^Laboratoire de Crystallographie, Ecole Polytechnique F ed erale de Lausanne, 1015 Lausanne, Switzerland. # Current address: ILL Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France. Received June 26, 2009; Revised Manuscript Received January 28, 2010

ABSTRACT: The self-assembly properties of the proximal p-tert-butylcalix[4]dihydroquinone compound have been studied to investigate the role played by crystallization conditions in driving the formation of a previously reported cubic porous framework. In chloroform and anhydrous ethyl acetate, the mutual inclusion of the tert-butyl groups is favored, leading to the cubic porous structure; otherwise, in the presence of a higher water amount, the OH groups provide H-bonds with bridging water molecules and a new triclinic crystal structure is obtained, in which the calixarene molecules include chloroform inside their cavities. By exposing a cubic/triclinic powder mixture to acetonitrile vapors, a new monoclinic chiral crystal structure is obtained by supramolecular assembly of calixarene, acetonitrile, and water molecules with the formation of single handed helices.

Introduction Calixarenes have been defined as “macrocycles with (almost) unlimited possibilities”1a because various types of host molecules with different three-dimensional shapes can be obtained by their appropriate functionalization.1b-f Among the large number of possible applications, calixarenes are currently being also exploited as useful building blocks2 for the construction of solid-state supramolecular systems with defined structures and functions, and with application as molecular sieves,3 sensors,4 and gas-storage devices.5 In the frame of an ongoing project concerning the supramolecular architectures of calixarenes bearing quinone/hydroquinone moieties,6 we prepared and characterized a new crystalline solid 1A based on proximal p-tert-butylcalix [4]dihydroquinone 1 (Scheme 1).7 Crystals 1A were obtained by crystallization of 1 in chloroform and ethyl acetate. They have a cubic structure (a = 36.412(4) A˚) with 48 calixarene molecules and 155 water molecules in the unit cell.7 A nanoporous architecture is generated by a fundamental [6 þ 2] supramolecular unit constituted by six calixarene and two water molecules. The [6 þ 2] supramolecular unit propagates in space by means of a CH-π interaction with six other surrounding units. This enhanced connectivity determines the presence of water channels and hydrophobic cavities (Figure 1). The simultaneous presence of networked channels, filled with easily removable water, and isolated hydrophobic cavities (988 A˚3) may be a prelude to potential applications of nanotechnological interest.

In particular, adsorption studies8 showed that the BET surface area corresponds to 230 m2 g-1 (N2 at 77 K), and this material is able to adsorb CO2 at room temperature with unusually high efficiency and exceeding absorption values previously reported for other organic compounds.9 Here we report the results of further studies aimed to exploit the self-assembly properties of compound 1 and to investigate the role played by crystallization conditions in driving the formation of the cubic porous framework 1A. The pseudopolymorphism of compound 1 is discussed, as two other structurally different solvated compounds are reported. It is noteworthy that, in one case, chiral crystals are obtained in spite of the fact that 1 is an achiral molecule. Experimental Section

*Authors to whom correspondence should be addressed. E-mail: ctedesco@ unisa.it.

Synthesis. All chemicals were reagent grade and were used without further purification. Flash chromatography was performed using silica gel (Kieselgel-60, 0.040-0.063 mm, Merck). Reactions were monitored by TLC on Merck silica gel plates (0.25 mm) and visualized by UV light and spraying with H2SO4-Ce(SO4)2. All NMR spectra were recorded at 400 (1H) and 100 (13C) MHz on a Bruker Avance-400 spectrometer. Compound 1 has been prepared according to the literature.7 Thermal Analyses. DSC measurements were run on a TA 2920 calorimeter; samples of approximately 6 mg were sealed in aluminum pans and heated at a rate of 2 C/min under a purified N2 flow. TGA measurements were performed on a Netschz TG209 F1 analyzer with a sample of 15.271 mg under a purified N2 flow by heating at 1.0 C/min. Single Crystal X-ray Diffraction. White prismatic crystals 1B suitable for single crystal X-ray diffraction were obtained at the interface by slowly adding distilled water to a solution of 1 in chloroform. Diffraction data were measured at room temperature with a Rigaku AFC7S diffractometer using Mo KR radiation. The structure

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Tedesco et al. Table 1. Crystal Data and Refinement Details for 1B and 1D asymmetric unit

Figure 1. Space-filling model of the unit cell along [110] of 1A showing the polar open channels (upper left and lower right) and two hydrophobic cavities contoured by tert-butyl groups (lower left and upper right).

Scheme 1. Proximal p-tert-Butylcalix [4]dihydroquinone 1

was solved by direct methods using the program SIR9710 and refined by means of full matrix least-squares based on F2 using the program SHELXL97.11 All non-hydrogen atoms, excluding those belonging to the water molecules, were refined anisotropically. For the water molecules, isotropic displacement parameters were considered. Hydrogen atoms were positioned geometrically and included in structure factors calculations but not refined. Very small and irregular white crystals 1D were obtained by exposing to acetonitrile vapors a sample of the two phase crystalline powder (1A þ 1B) contained in a 0.5 mm capillary. Diffraction data were collected at the Swiss-Norwegian Beamline (SNBL) at the European Synchrotron Radiation Facility (ESRF). The structure has been solved with SHELXS11 and refined with SHELXL97.11 All non-hydrogen atoms, excluding those belonging to the solvent molecules, were refined anisotropically. Hydrogen atoms were positioned geometrically and included in structure factors calculations but not refined. Crystal data and refinement details for 1B and 1D are reported in Table 1. High Resolution X-ray Powder Diffraction and Rietveld Analysis. Crystalline powder samples (1A þ 1B) were obtained from a solution of 1 in CHCl3 and ethyl acetate with high water content. 0.5 mm Lindemann capillaries were filled in air with the crystalline powder. The filled capillaries were dipped into vials containing CCl4 or CH3CN, so that the powders were exposed to the corresponding vapors. High resolution X-ray powder diffraction (XRPD) profiles were recorded at ESRF beamline ID31 (λ = 0.80168(3) A˚ for data collected after three days exposure, λ = 1.25248(3) A˚ for data collected after ten days exposure to CCl4, and λ =1.25253(4) A˚ for data collected after ten days exposure to CH3CN).12 Rietveld refinement on the two phase crystalline powder (1A þ 1B) was accomplished with the program GSAS.13 The peak profile was described by a pseudo-Voigt function, in combination

formula weight crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Z0 Dc (g/cm3) F(000) wavelength (A˚) absorption coeff (mm-1) absorption correction type 2θmax (deg) no. of reflections no. of variables R1 (F > 4σ(F)) wR2 (all data) goodness of fit min and max. diff. Fourier (e/A˚3)

1B

1D

C38H44O6, CHCl3, H2O 734.12 triclinic P1h 9.993(3) 11.259(3) 17.330(4) 85.03(2) 87.27(2) 75.26(2) 1877.9(9) 2 1 1.298 776 0.7107 0.292 none 41.6 3921 437 0.0766 (1955) 0.2727 (3921) 0.988 -0.43, 0.38

2(C38H44O6), 3(CH3CN), 2(H2O) 1352.66 monoclinic P21 12.7264(5) 19.1816(5) 15.5830(5) 91.64(2) 3802.4(5) 4 2 1.181 1452.0 0.8000 0.080 none 49.3 4585 882 0.0633 (4441) 0.1700 (4585) 0.869 -0.34, 0.71

with a special function that accounts for the asymmetry due to axial divergence.14 No absorption correction was applied to the data. The atomic parameters were taken from the structure model as obtained from the single crystal X-ray diffraction data and were not refined. The final indices are Rp = 0.087 and wRp = 0.1161. XRPD images for the two phase crystalline powder (1A þ 1B) before and after one day exposure to CH3CN vapors were also recorded at the GILDA beamline (ESRF), λ = 0.685307 A˚. The translating imaging plate (200  300 mm2) setup available at GILDA allows monitoring in situ the crystallographic transformation induced by the guest uptake and release, also as a function of the temperature.15

Results and Discussion Several crystallization conditions for compound 1 were investigated with the aim to explore the role of solvents in driving the formation of the cubic porous framework 1A and, in particular, to understand the role of water molecules. The results are summarized in Table 2. By adding water to a CHCl3 solution of 1 (Table 2, entry 1), X-ray diffraction quality single crystals 1B were obtained at the interface, which were characterized by single crystal X-ray diffraction and thermal analyses. Crystals 1B are triclinic (a = 9.993(3) A˚, b = 11.259(3) A˚, c = 17.330(4) A˚, R = 85.03(2), β = 87.27(2), γ = 75.26(2), space group P1h). The unit cell contains two equivalent calixarene molecules, two water molecules, and two chloroform molecules. The calixarene molecules display a cone conformation, which is favored by hydrogen bond formation at the lower rim (endo-OH groups).16 Calixarene molecules are connected to each other through upper rim-upper rim hydrogen bonds (between exo-exo OH groups) O2 3 3 3 O4 2.762(8) A˚ (Figure 2). The water molecules bridge the calixarene molecules through lower rim-upper rim hydrogen bonds [O2 3 3 3 O7 2.700(9) A˚, O6 3 3 3 O7 2.842(8) A˚]. The chloroform molecule is located inside the cavity of the calixarene molecule, occupying a volume of 154 A˚3 and forming CH-π interactions (C39 3 3 3 C12 3.58(2) A˚, C39 3 3 3 C13

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Table 2. Crystallization Conditions for Compound 1a entry

sample

1 2

as synthesized as synthesized

3

as synthesized

4 5

as synthesized two phase crystalline powder (1A þ 1B) two phase crystalline powder (1A þ 1B)

6 a

solvent 1

solvent 2

method

chloroform anhydrous chloroform chloroform

water anhydrous ethyl acetate

slow diffusion slow evaporation

triclinic single crystals 1B cubic single crystals 1A

ethyl acetate (high water content) anhydrous ethyl acetate

slow evaporation slow evaporation vapor diffusion

two phase crystalline powder (1A þ 1B) cubic single crystals 1A cubic crystalline powder 1C

vapor diffusion

monoclinic single crystals 1D

hexane CCl4 CH3CN

results

For conditions 5 and 6, the crystallization was accomplished into 0.5 mm capillaries.

Figure 2. H-bond linkages in the unit cell of 1B as viewed along the b axis.

3.49(1) A˚, C39 3 3 3 C14 3.57(1) A˚) (see Figure 3 for the crystal packing). The calixarene molecules interconnected by H-bonds give rise to columns. The columns interact by means of van der Waals contacts; no H-bonds occur between any molecule of two adjacent columns. In Figures 4 and 5 DSC and TGA show that crystals 1B start to release chloroform and water molecules in the temperature range between 75 and 150 C, with an observed weight loss of 20.1%, which corresponds to the loss of one chloroform and one water molecule for each calixarene molecule. The endothermic process at 230 C followed by an exothermic process correspond to the melting and subsequent decomposition of the compound.

When a 1:1 mixture of anhydrous chloroform and anhydrous ethyl acetate was used as crystallization solvent (Table 2, entry 2), cubic crystals 1A were obtained. Instead, by using ethyl acetate with a higher water content (Table 2, entry 3), invariably a two phase crystalline powder was obtained. This powder was further characterized by X-ray powder diffraction for phase identification. Measurements were performed at the high resolution XRPD beamline ID31 at ESRF (Figure 6). Rietveld analysis shows that the observed powder profile can be modeled by the previously described cubic 1A and triclinic 1B phases. These results indicate that the formation of the cubic framework 1A is favored if the water content of the crystallization solvents is kept low. This happened the first time we

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Figure 3. Crystal packing of 1B as viewed along the b axis. Adjacent H-bonded columns interact by van der Waals contacts.

Figure 5. TGA/DTG trace of 1B.

Figure 4. DSC profile of a sample of 1B.

crystallized compound 1, since crystallization solvents from freshly opened bottles were used. It is noteworthy that, in the cubic form 1A the ratio calixarene/water molecules is 3:1, without considering the channel water molecules, whose number actually is influenced by ambient humidity. In the triclinic form 1B, the ratio calixarene/water molecules is 1:1. This suggests that first the porous structure of 1A is formed by assembling of the fundamental [6 þ 2] supramolecular unit, constituted by six calixarene and two water molecules, and then additional water molecules fill the open channels. Further experiments were performed on the powder mixture of cubic and triclinic phases (1A þ 1B), by exposing it to the vapors of several volatile organic compounds and by monitoring the changes by high resolution XRPD at beamline ID31 (Figure 7). It was evidenced that by exposure to CCl4 vapors for 3 days (Table 2, entry 5) the triclinic phase slowly disappeared and

the final crystalline powder sample could be identified as a single cubic phase (a = 36.66847(15) A˚) 1C. Unfortunately, the very small dimensions of this crystalline powder did not allow any single crystal X-ray analysis. A possible explanation for the formation of 1C is that CCl4 vapors condense inside the capillary and dissolve the crystalline powder that recrystallizes in the cubic form 1C.17 The exposure to CCl4 is able to favor the cubic form with respect to the triclinic one. Moreover, the slight increase of the lattice parameter with respect to 1A indicates that CCl4 molecules could enter the channels of the cubic form and substitute the water molecules. By exposing the (1A þ 1B) powder mixture to acetonitrile vapors, the high resolution XRPD profiles showed a progressive phase change to a lower symmetry space group. The same experiment was repeated at the GILDA beamline (ESRF), which is equipped with a large area detector suitable for rapidly collecting high statistics XRPD patterns. The corresponding diffraction images clearly showed that the crystalline powder was progressively transforming into new single crystals 1D (Figure 8).

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Figure 8. (a) Diffraction image of the two phase crystalline powder (1A þ 1B) recorded at GILDA, ESRF. (b) Diffraction image of the same powder after 1 day exposure to CH3CN vapors recorded at GILDA. (c) Oscillation frame of a single crystal of 1D collected at SNBL, ESRF (Table 2, entry 6).

Figure 6. Rietveld analysis of the high resolution XRPD profile for the two phase sample (1A þ 1B) as obtained from chloroform and ethyl acetate with high water content (Table 2, entry 3). The observed and calculated profiles are reported as a function of Q = λ/(2 sin θ) and are shown respectively in red and green. The difference curve between observed and calculated profiles (magenta) and reflection markers (vertical bars, cubic phase in black, triclinic phase in blue) is also shown.

Figure 7. High resolution XRPD profiles (ID31 beamline) as a function of Q = λ/(2 sin θ) of the two phase crystalline powder (1A þ 1B): (i) as crystallized from chloroform and ethyl acetate with high water content (1A þ 1B, Table 2, entry 3); (ii) after 1 day exposure to CCl4 vapors (1C, Table 2, entry 5); (iii) after 3 days exposure to CCl4 vapors (1C, Table 2, entry 5).

In this case, acetonitrile vapors condensed inside the capillary and dissolved the crystalline powder, which then formed new single crystals 1D. These rather small and frequently twinned crystals 1D were removed from the capillary and analyzed at SNBL (ESRF). They are monoclinic and, surprisingly, belong to the P21 space group, which is a chiral one, while compound 1 is achiral, containing a molecular mirror plane. As shown in Figure 9, the unit cell contains two independent calixarene molecules, 1Da (blue) and 1Db (green), three independent acetonitrile molecules, and two water molecules in the asymmetric unit. Both calixarene molecules 1Da and 1Db display a cone conformation, although there are some slight differences in the conformation of the two molecules; see Table 3.

Two acetonitrile molecules are located in the two calixarene cavities orienting the methyl groups inside the cavities,18 so that CH-π interactions are possible (for 1Db C61 3 3 3 C25 3.71(1) A˚, C61 3 3 3 C24 3.76(1) A˚, C61 3 3 3 C23 3.78(1) A˚; for 1Da C71 3 3 3 C70 3.69(1) A˚, C71 3 3 3 C20 3.69(1) A˚) and rather weak H-bonds are formed with the two water molecules (N2 3 3 3 OW1 2.97(1) A˚ and N1 3 3 3 OW1 3.09(1) A˚). The third acetonitrile molecule (orange) occupies the interstitial space between two calixarene molecules. In the literature there are increasing examples of achiral molecules, which crystallize in chiral space groups.19 The space group P21 is the fifth favored space group for achiral molecules with Z0 = 1, being preceded by P21/c, P1, P212121, and C2/c.20 The P21 space group contains a 2-fold screw axis, which determines in this case the supramolecular assembly of a helix made by calixarene molecules, water molecules, and acetonitrile molecules (Figure 10). The mechanism beyond the homochiral packing of helices in crystals is still a matter of debate; chirality may be induced by atropoisomerism, with noncovalent supramolecular interactions as hydrogen bonding or π-π interactions. High Z0 values are often an indication of an interplay between competing factors such as close packing, hydrogen bonding, and chirality.21 In this case, chirality is determined at a supramolecular level: in particular, molecule 1Da (blue) connects through one exo-OH group (at the upper rim) to another exo-OH group of molecule 1Db (green), whose exo-OH group is H-bonded to one water molecule (yellow), which connects to another water molecule (red). This last is H-bonded to a CH3CN molecule (cyan), which is included in the cavity of the calixarene molecule 1Da (blue). The helices are connected to each other through upper rim-lower rim H-bonds between the second exo-OH group of 1Db and one endo-OH group of 1Da. In conclusion, p-tert-butylcalix[4]-1,2-dihydroquinone 1 exhibits a certain variety of assembly possibilities, which can be triggered by appropriate solvent choice. Chloroform and anhydrous ethyl acetate produce microporous cubic single crystals 1A in the Pn3hn space group; chloroform and water produce triclinic single crystals in P1h 1B; carbon tetrachloride favors the formation of cubic phase 1C; acetonitrile produces chiral crystals 1D in P21. We believe that the molecular flexibility associated with the calixarene moiety accounts for this behavior; as can be seen from Table 3, slightly distorted cone conformations can be adopted in order to maximize the intermolecular interactions. The presence of tert-butyl groups together with OH groups at the upper rim allows multiple assembly possibilities: the mutual inclusion of the tert-butyl groups determines a tight

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Figure 9. Crystal packing along the a axis. H-bond connections are indicated.

Figure 10. Supramolecular helix: (a) as viewed along the b axis, which is the helix axis; (b) view perpendicular to the b axis.

embrace of calixarene molecules in the cubic porous structure; otherwise, the OH groups provide H-bonds with bridging water molecules, in the presence of a higher water amount in the crystallization conditions. In this way the calixarene

molecules are able to include solvent molecules such as chloroform or acetonitrile inside their cavities. The 1,2-substitution of calixarene moieties allows them to have a hydrophobic part and a hydrophilic part at the upper

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Table 3. Distortion of the Cone Conformation of Compound 1 in Its Pseudopolymorphsa compound

crystal system

space group

rmsd O4

θ1 (deg)

θ2 (deg)

θ3 (deg)

θ4 (deg)

1A 1B 1Da 1Db

cubic triclinic monoclinic

Pn3hn P1h P21

0.125 0.075 0.178 0.025

58.68(9) 59.62(19) 51.8(2) 58.7(2)

44.72(8) 39.28(18) 70.95(16) 60.0(2)

65.35(9) 59.13(14) 43.38(18) 71.83(15)

52.49(13) 77.32(18) 67.33(19) 57.20(17)

a

θ1, θ2, θ3, and θ4 are the interplanar dihedral angles between each aromatic plane and the mean plane of the four endo oxygen atoms.

rim, and this possibly can explain the formation of single handed helices. Thus, the present case represents a rather interesting example of the solvent induction effects during solid state assembly of calixarene molecules. (6)

Acknowledgment. We gratefully acknowledge Prof. A. N. Fitch for stimulating discussions. This work has been supported by the Italian MIUR. We acknowledge the European Synchrotron Radiation Facility for providing beamtime (Proposal Nos. 08-02-632 and CH-2388). We wish to thank the technical staff of ESRF beamlines ID31, GILDA, and SNBL for their assistance during data collection. Laboratory X-ray diffraction instrumentation was funded by the University of Salerno “Finanziamento grandi e medie attrezzature 2004”. L.E. wishes to acknowledge ESRF for providing financial support. Supporting Information Available: X-ray crystallographic information files in CIF format for 1B and 1D, and image of crystals of 1D. This information is available free of charge via the Internet at http://pubs.acs.org/.

References (1) (a) B€ ohmer, V. Angew. Chem., Int. Ed. Engl. 1995, 34, 713–745. For additional comprehensive reviews on calixarenes see: (b) Ikeda, A.; Shinkai, S. Chem. Rev. 1997, 97, 1713–1734. (c) Gutsche, C. D. In Calixarenes Revisited; Stoddart, J. F., Ed.; The Royal Society of Chemistry: Cambridge, 1998. (d) Calixarenes 2001; Asfari, Z., B€ohmer, V., Harrowfield, J., Vicens, J., Eds.; Kluwer: Dordrecht, 2001. (e) B€ ohmer, V. In The Chemistry of Phenols; Rappoport, Z., Ed.; Wiley: Chichester, U.K., 2003; Chapter 19. (f) Calixarenes in the Nanoworld; Vicens, J., Harrowfield, J., Eds.; Springer: Dordrecht, 2007. (2) (a) For a recent review on the subject, see: Gaeta, C.; Tedesco, C.; Neri, P. In Calixarenes in the Nanoworld; Vicens, J., Harrowfield, J., Eds.; Springer: Dordrecht, 2007; Chapter 16, pp 335-354. (b) Atwood, J. L.; Barbour, J. L.; Jerga, A. Angew. Chem., Int. Ed. Engl. 2004, 43, 2948–2950. (c) Atwood, J. L.; Barbour, L. J.; Jerga, A.; Schottel, B. L. Science 2002, 298, 1000–1002. (d) Ripmeester, J. A.; Enright, G. D.; Ratcliffe, C. I.; Udachin, K. A.; Moudrakovski, I. L. Chem. Commun. 2006, 4986–4996. (e) Dalgarno, S. J.; Thallapally, P. K.; Barbour, L. J.; Atwood, J. L. Chem. Soc. Rev. 2007, 36, 236–245. (3) (a) Liu, C. Q.; Lambert, J. B.; Fu, L. J. Am. Chem. Soc. 2003, 125, 6452–6461. (b) Yin, J.; Wang, L. J.; Wei, X. Y.; Yang, G.; Wang, H. J. Chromatogr., A 2008, 1188, 199–207. (4) (a) Casnati, A.; Sansone, F.; Ungaro, R. Calixarene receptors in ion recognition and sensing. In Advances in Supramolecular Chemistry; George W. Gokel, Eds.; Cerberus Press, Inc.: 2003; Chapter 9, pp 163-218. (b) Kim, J. S.; Quang, D. T. Chem. Rev. 2007, 107, 3780– 3799. (c) Liu, C. J.; Lin, J. T.; Wamg, S. H.; Jiang, J. C.; Lin, L. G. Sens. Actuators, B;Chem 2005, 108, 521–527. (d) Rudkevich, D. M.; Xu, H. Chem. Commun. 2005, 2651–2659. (e) Lo, P. K.; Wong, M. S. Sensors 2008, 8, 5313–5335. (f) Gupta, V. K.; Jain, A. K.; Ludvig, R.; Maheshwari, G. Electrochim. Acta 2008, 53, 2362–2368. (5) (a) Thallapally, P. K.; McGrail, P. B.; Dalgarno, S. J.; Schaef, H. T.; Tian, J.; Atwood, J. L. Nat. Mater. 2008, 7, 146–150. (b) Thallapally, P. K.; Dobrzanska, L.; Gingrich, T. R.; Wirsig, T. B.; Barbour, L. J.; Atwood, J. L. Angew. Chem., Int. Ed. 2006, 45, 6506–

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