Inclusion Compound Formation with a New Columnar Cyclodextrin Host

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Inclusion Compound Formation with a New Columnar Cyclodextrin Host Cristian C. Rusa,†,⊥ Todd A. Bullions,‡ Justin Fox,† Francis E. Porbeni,† Xingwu Wang,§ and Alan E. Tonelli*,† Fiber and Polymer Science Program, Campus Box 8301, North Carolina State University, Raleigh, North Carolina 27695-8301; Center for High Performance Manufacturing, College of Engineering, Virginia Polytechnic Institute & State University, 132 Norris Hall, Blacksburg, Virginia 24061-0219; and Department of Chemistry, Campus Box 8204, North Carolina State University, Raleigh, North Carolina 27695-7907 Received July 15, 2002. In Final Form: October 2, 2002 R- and γ-cyclodextrin in columnar structures with only water molecules included were successfully obtained by appropriate recrystallization from their aqueous solutions. These crystals were found to adopt a channel-type structure similar to the cyclodextrin inclusion compounds formed with guest polymers. Experimental investigations of their inclusion properties demonstrate that only R-cyclodextrin in the columnar structure (R-CDcs) is able to include both small molecules and polymers. Thermal measurements reveal that columnar structure R-CDcs contains three different types of water molecules. The most strongly held water molecules are located outside of the cyclodextrin cavity, likely hydrogen-bonded between the rims of neighboring cyclodextrins in the columnar R-CD stacks. X-ray analyses confirm that the channel structure is preserved in the dehydrated R-CDcs and its inclusion compounds formed with various guests. In contrast, a completely different behavior was observed for γ-CDcs in the columnar structure. It appears that R-CDcs, at least, can function as a nanoscopic filter for separating both small molecules and polymers on the basis of their abilities to be included, or not, in the narrow (∼0.5 nm) channels of the R-CDcs crystals.

Introduction Cyclodextrins (CDs) are cyclic oligosaccharides consisting of six (R-CD), seven (β-CD), or eight (γ-CD) glucose units linked by 1,4-R-glucosidic bonds. Because of their shallow truncated cone shapes, they can act as host molecules including a great variety of small or long molecule guests. Cyclodextrins provide a “hydrophobic matrix in a hydrophilic environment” due to the fact that their inner cavities are nonpolar relative to their outer surfaces, which are coated with hydroxyl groups and are fairly polar. These compounds have therefore been studied as hosts for guest molecules capable of entering (in whole or in part) into the CD cavities and forming noncovalent host-guest inclusion compounds (ICs). In the past decade, CD ICs formed with macromolecular guests have gained considerable interest because of their multiple applications.1-12 Formation of polymer-CD ICs † Fiber and Polymer Science Program, North Carolina State University. ‡ Virginia Polytechnic Institute & State University. § Department of Chemistry, North Carolina State University. ⊥ Permanent address: Department of General Chemistry, “Gh. Asachi” Technical University, Iasi, Romania. * To whom correspondence should be addressed: Tel +1-919515-6588; Fax +1-919-515-6532; e-mail [email protected].

(1) Herrmann, W.; Keller, B.; Wenz, G. Macromolecules 1997, 30, 4966. (2) Harada, A.; Li, J.; Kamachi, M. Macromolecules 1993, 26, 5698. (3) Shigekawa, H.; Miyake, K.; Sumaoka, J.; Harada, A.; Komiyama, M. J. Am. Chem. Soc. 2000, 122, 5411. (4) Harada, A. Acc. Chem. Res. 2001, 34, 456. (5) Huang, L.; Gerber, M.; Taylor, H.; Lu, J.; Tapaszi, E.; Wutkowski, M.; Hill, M.; Lewis, C.; Harvey, A.; Herndon, A.; Wei, M.; Rusa, C. C.; Tonelli, A. E. Macromol. Symp. 2001, 176, 129. (6) Jiao, H.; Goh, S. H.; Valiyaveettil, S. Macromolecules 2002, 35, 1980. (7) Lu, J.; Hill, M. A.; Greeson, D. F.; Horton, J. R.; Orndorff, P. E.; Herndon, A. S.; Tonelli, A. E. J. Appl. Polym. Sci. 2001, 82, 300. (8) Lu, J.; Mirau, P.; Tonelli, A. E. Macromolecules 2001, 34, 3276. (9) Rusa, C. C.; Tonelli, A. E. Macromolecules 2000, 33, 1813.

consists of CD molecules threading onto a polymer chain from the end toward the center of the macromolecule.13 According to Harada,14 the formation of a polymer-CD IC (also called a “molecular necklace” or “supramolecular adduct”) is entropically unfavorable, since the enthalpic effect of the whole threading process is not balanced by that derived from the intermolecular interactions between the included polymer and the CDs threaded upon it, as the entropic gain related to the release of water molecules initially included in the CD cavity at least partially compensates the loss of conformational entropy of the polymer threaded by CDs. Consequently, only the hydrogen bonds between the hydroxyl groups on neighboring CDs in a columnar stack and any strong interactions established between the polymer repeating units and the internal hydrophobic CD cavities may justify the stability of the final supramolecular structure. Packing of the CD molecules within the crystal lattice occurs in one of two principal modes, described as cage and channel structures (see Figure 1).15 In the cage structure, CD molecules are arranged in a “herringbone” fashion, and both ends of the cavity are blocked by adjacent CDs to form isolated cages. CDs in crystal hydrates usually adopt a cage structure (form I). In the channel-type structure, the “dimer units” formed by two neighboring CDs (head-to-head or head-to-tail orientation) are stacked to form an endless column in the crystal. In polymer-CD ICs, CD molecules are stacked into columns, and the guest (10) Rusa, C. C.; Tonelli, A. E. Macromolecules 2000, 33, 5321. (11) Shuai, X.; Porbeni, F. E.; Wei, M.; Shin, I. D.; Tonelli, A. E. Macromolecules 2001, 34, 7355. (12) Shuai, X.; Porbeni, F. E.; Wei, M.; Bullions, T.; Tonelli, A. E. Macromolecules 2002, 35, 3126, 3778. (13) Ceccato, M.; Lo Nostro, P.; Baglioni, P. Langmuir 1997, 13, 2436. (14) Harada, A. Coord. Chem. Rev. 1996, 148, 115. (15) Harata, K. In Crystallographic Studies; Atwood, J. L., Davies, J. E., MacNicol, D. D., Vogtle, F., Lehn, J. M., Eds.; Pergamon: Oxford, 1996; Vol. 3.

10.1021/la0262452 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/06/2002

Inclusion Compound Formation

Figure 1. Schematic representation of the packing of cyclodextrin molecules within their crystal In the cage structure, CD molecules are lattices: channel structure (a), cage herringbone type (b), and cage brick type (c).

polymer chains are included in the narrow channels that run continuously down each CD column. Recently, several groups reported the formation of a threaded molecular structure, called a “molecular tube” (MT), a rodlike, rigid molecule with an empty hydrophobic cavity that can behave as a host for ions or small organic molecules.16-19 MTs were obtained upon cross-linking of the threaded R-CDs with epichlorohydrin and removal of the linear guest PEG chains. Topchieva et al. obtained a new kind of crystal hydrate for R-, β-, and γ-CDs (so-called form II) by removing the threading polymers from the initial polymer-CD IC crystals under the action of a selective organic solvent.20,21 These products consist of extended cylindrical nanopores with diameters of ∼5, ∼7 and ∼10 Å for R-, β-, and γ-CD, respectively, which contained fewer water molecules and were thermally less stable compared to the crystal hydrates of the initial cyclodextrins (form I, cage structure) and the related polymer-CD ICs. Unfortunately, none of these cyclodextrins in columnar structure have been reported to be capable of reversible inclusion of polymers into the channels formed by the CD macrocycles. Related to the above-mentioned CD structures, Antonietti and co-workers have demonstrated the existence of CD columnar aggregates in solution.22 They assumed that the driving force for the CD self-assembly in water is given by the minimization of the mutual contact areas of their individual hydrophobic and hydrophilic domains. The aim of the present study was to develop a new and easier method for obtaining R- and γ-CD in columnar or channel structures (form II). An appropriate recrystallization process from a suitable solvent under certain conditions leads to the new crystalline form of CDs with extended channels (form II). R-CD in columnar structure (R-CDcs) has proven to be an open structure, with the potential to be a nanoscopic molecular sieve, which can include both small molecules and polymers. It is worth noting that this new crystalline structure for R-CDcs, containing 6.7 molecules of water per molecule of R-CDcs, keeps the columnar structure even after more than half of the water molecules were removed by vacuum-drying. The same features were not observed, however, for γ-CD in columnar structure (γ-CDcs) that was also obtained through an appropriate recrystallization. Many attempts (16) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821. (17) Harada, A.; Li, J.; Kamachi, M. Nature (London) 1993, 364, 516. (18) Harada, A.; Li, J.; Kamachi, M. Nature (London) 1992, 356, 325. (19) Ceccato, M.; Lo Nostro, P.; Rossi, C.; Bonechi, C.; Donati, A.; Baglioni, P. J. Phys. Chem. B 1997, 101, 5094. (20) Topchieva, I. N.; Panova, I. G.; Popova, E. I.; Matukhina, E. V.; Gerasimov, V. I. Dokl. Chem. 2001, 380, 242. (21) Topchieva, I. N.; Panova, I. G.; Popova, E. I.; Matukhina, E. V.; Grokhovskaya, T. E.; Spiridonov, V. V.; Gerasimov, V. I. J. Polym. Sci., Part A 2002, 44, 352. (22) Polarz, S.; Smarsly, B.; Bronstein, L.; Antonietti, M. Angew. Chem., Int. Ed. 2001, 40, 4417.

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were made to include different small and long guest molecules in the preexistent channels of the host γ-CDcs lattice, but all of these experiments failed. Furthermore, γ-CDcs underwent a phase transition during the vacuumdrying process, in which the channel structure was transformed to an amorphous γ-CD. R-CDcs and γ-CDcs in columnar structure, before and after vacuum-drying, as well as the inclusion compounds formed by exposure to neat liquids or solutions of guests, were investigated by wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared (FTIR) and solid-state CP/MAS 13C NMR spectroscopies. Experimental Section Materials. R- and γ-CDs were obtained from Cerestar (Hammond, IN). Acetone (99.9+ %), chloroform (99.8%), dioxane (99+%), valeric acid (99+%), propionic acid (99+%), and hexanoic acid (99.5+%) were purchased from Aldrich and were used without any further purification. Poly(-caprolactone) (PCL) (Mw ) 65 000), poly(ethylene glycol) (PEO) (Mw ) 600), and poly(dimethylsiloxane) (PDMS) (Mw ) 5970) were obtained from Aldrich, Sigma, and United Chemical Technology, respectively. Atactic poly(R,S-hydroxybutyrate) (a-PHB) (MW ) 16 000) was synthesized previously.12 For all recrystallizations, we used deionized water purified with a US Filter system. CD Recrystallization. R-CD (1.825 g) was dissolved in 12.5 mL of deionized water, while continuously stirring at 50 °C for 1 h. The clear solution of R-CD (50 °C) was then quickly poured into 50 mL of cold (room temperature) chloroform, while moderately stirring. The white precipitate was immediately vacuum-filtered and allowed to dry overnight directly in the Buchner funnel under vacuum draft. CDcss dried in this manner are subsequently referred to as “air-dried”. Attempts to scale up the above method have not as yet been successful. A similar recrystallization procedure was applied to γ-CD. This time, 1.8 g (or 11.6 g) of γ-CD was dissolved in 8 mL (or 50 mL) of deionized water, while stirring at 50 °C for 15 h. The γ-CD solution was then added dropwise into 50 mL (or 300 mL) of stirred acetone at room temperature. After vacuum filtration, the white powder was air-dried directly in the Buchner funnel. Recrystallization of R-CD by precipitation into stirred acetone did not result in R-CDCS. Both recrystallized R-CDcs and γ-CDcs were further dried in a vacuum oven at 90 °C for 15 h and are referred to as “vacuum-dried” CDcss. We have not as yet attempted to produce columnar structure β-CDCS by a similar recrystallization procedure, because it is difficult to distinguish between the columnar (form II) and cage (form I) crystals structures with X-ray diffraction from powdered samples. Inclusion of Guest Molecules into the Channels of the r-CDcs. Transfer of Small Molecules. 0.4 g of recrystallized air- or vacuum-dried R-CDcs was added to 10 mL of valeric acid (VA) or 10 mL of propionic acid (PA). The suspension was allowed to stir at 40 °C for 2 h. The white powder was separated by vacuum filtration and then washed several times with cold acetone to remove the uncomplexed acid. The final precipitate was air-dried overnight. The reason for choosing these two acids is because PA-R-CD IC adopts a cage structure, whereas VAR-CD IC prefers a channel structure, as demonstrated by detailed single-crystal X-ray analyses,23,24 when their R-CD-ICs are formed in solution. Transfer of Guest Polymer. 0.1 g of poly(-caprolactone) (PCL) was dissolved in 10 mL of acetone at 50 °C with moderate stirring. The recrystallized and vacuum-dried R-CDcs powder was added to the clear PCL solution, and the suspension was kept at 50 °C for 3 h, while continuously stirring. At the end of this period the white powder was vacuum-filtered, washed with cold acetone, and dried at 70 °C for 16 h. (23) McMullan, R. K.; Saenger, W.; Fayos, J.; Mootz, D. Carbohydr. Res. 1973, 31, 37. (24) Takeo, K.; Kuge, T. Agric. Biol. Chem. 1970, 34, 1787.

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Figure 2. X-ray diffraction diffractograms of as-received R-CD (a) and recrystallized, air- (b) and vacuum-dried R-CDcss (c).

Figure 3. X-ray diffraction diffractograms of as-received γ-CD (a) and recrystallized air- (b) and vacuum-dried γ-CDcss (c).

Competition between Hexanoic Acid and PCL Inclusion in r-CDcs. PCL and hexanoic acid (HA) were dissolved together (1:1 molar ratio) in 10 mL of acetone at 50 °C. The recrystallized, vacuum-dried R-CDcs (1 mol) was added to the clear solution containing both HA and PCL guests. The resulting suspension was allowed to stir for 6 h at 50 °C. The white precipitate was then filtered out, washed with cold acetone, and air-dried overnight. Characterization. Wide-Angle X-ray Diffraction. Wideangle X-ray diffraction (WAXD) measurements were performed on a Siemens type-F X-ray diffractometer with a Ni-filtered Cu KR radiation source (λ ) 1.54 Å). The supplied voltage and current were 30 kV and 20 mA, respectively. The diffraction intensities were measured every 0.1° from 2θ ) 5 to 30° at a rate of (2θ ) 3°)/min. All displayed X-ray patterns have been normalized. Differential Scanning Calorimetry. DSC measurements were carried out on 3-5 mg samples with a Perkin-Elmer DSC-7 thermal analyzer equipped with a cooler system. A heating rate of 10 °C/min was employed, and an indium or tin standard was used for calibration. Nitrogen was used as the purge gas at 35 mL/min. Thermogravimetric Measurements. TGA scans were obtained with a Perkin-Elmer Pyris 1 thermogravimetric analyzer on 5-10 mg samples. Samples were placed in an open platinum pan that was hung in the furnace. The weight percentage of remaining material in the pan was recorded during heating from 25 to 550 °C at a heating rate of 20 °C/min. Nitrogen was used as the purge gas. Fourier Transform Infrared Spectroscopy. A Nicolet 510P FT IR spectrometer was utilized to obtain the infrared spectra of samples mixed into potassium bromide and pressed into pellets. The spectra were recorded over the range of 4000-400 cm-1, with a resolution of 2 cm-1 using 64 scans. Solid-State 13C NMR Spectroscopy. Solid-state 13C NMR data were recorded at 75 MHz, using a Bruker DSX wide-bore system with MAS speeds of 4-5 kHz and a CP contact time of 1 ms.

in our R-CD samplessspectra not shown), we concluded that this new columnar structure of R-CDcs contains only water molecules inside. Comparing this crystal structure with that of as-received R-CD crystal hydrate (R-CD‚ 6H2O), which crystallizes to form a “herringbone” cage structure,26 WAXD patterns reveal noticeable differences between the angular positions of reflections characterizing the two lattices. Moreover, the vacuum-drying of R-CDcs eliminates only a few water molecules from the crystals (see next section on thermal behavior) and does not cause any phase change in the resulting dried product. This result indicates that the water molecules escaping in the first stages of vacuumdrying at 90 °C for 15 h are primarily included in the hydrophobic cavity of R-CDcs and are less tightly bound than the remaining water molecules that presumably are located outside of the R-CDcs channels and may be involved in hydrogen bonds with OH groups from both sides of the R-CD torus. These X-ray data recorded for R-CDcs are in good agreement with those obtained by Topchieva and co-workers,21 who reported a similar structure upon removal of the threaded polymer from PEO-R-CD IC by treatment with various selective organic solvents for relatively long periods (up to 54 h). The X-ray diffractogram of recrystallized, air-dried γ-CDcs displayed in Figure 3b shows a strong peak at 2θ ) 7.6°, which has been suggested as an indicator for the channel structure.27 The diffraction patterns of as-received γ-CD‚7H2O and recrystallized, vacuum-dried γ-CDcs are also shown for comparison. It is well-known that the diffraction pattern of γ-CD‚7H2O is typically that of a cage herringbone structure.26 In contrast to the recrystallized R-CDcs, almost half of the water molecules from the recrystallized γ-CDcs were lost by vacuum-drying (see thermal behavior section). As can be observed, the well dried γ-CD no longer has a channel or a cage structure but appears amorphous. To our knowledge, this is the first report that amorphous γ-CD can be obtained by vacuum-drying. When water was added to the powderlike, amorphous γ-CD in an attempt to recrystallize the macrocycles, the crystalline channel structure of γ-CDcs was regenerated (pattern not shown). Thus, a reversible transition between columnar and amorphous γ-CD is possible upon dehydration or hydration, respectively. This behavior may have applications as a moisture indicator or as a nanoswitch or valve. Knowing that CD molecules adopt a more symmetric conformation in their IC channel crystal structures than in their hydrated cage structures, the CP/MAS 13C NMR

Results and Discussion Evidence for the Columnar Structure of the Recrystallized r- and γ-CDs. For this purpose, the CDcs powders obtained after recrystallization from a suitable solvent were studied by X-ray crystallography and 13C NMR spectroscopy. Figures 2 and 3 show the X-ray diffraction patterns of pure, as-received R- and γ-CDs and air- and vacuum-dried recrystallized R-and γ-CDcss, respectively. As is well-known,25 the peak at 2θ ∼ 20° in the WAXD of R-CD ICs is characteristic for the channel structure of R-CD when including long guest molecules and polymers in particular. It was previously thought that this structure may be formed only when an appropriate long molecule or polymer guest is included. Since chloroform is not able to form an inclusion compound with R-CD (FTIR proved that there is no trace of chloroform (25) Huang, L.; Tonelli, A. E. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1998, 38, 781.

(26) Szejtli, J. Cyclodextrin and Their Inclusion Compounds; Academiai Kiado: Budapest, 1982. (27) Harada, A.; Suzuki, S.; Okada, M.; Kamachi, M. Macromolecules 1996, 29, 5611.

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Table 1. Characteristic Temperatures and the Calculated Water Contents of As-Received r- and γ-CDs and Their Analogous Columnar Structures, As Observed by Thermal Analysis no. of water moleculesa/CD compound as-received R-CD

air-dried R-CDcs vacuum-dried R-CDcs as-received γ-CD air-dried γ-CDcs vacuum-dried γ-CDcs a

DSC event

DSC peak (°C)

A B C D A B C D broad shallow peak

87 98 159 187 86 107 140 198

A

168

A B C D broad shallow peak

77 98 127 157

released

total

Tdecompa (°C)

24.4 1.9

2.4 3.3

6.1

306

61 20.5 4.9

0.4 b

6.7

256

b

2.4

256

b

6.9

317

b

12.0

319

b

6.4

312

∆H (J/g)

51.8

5.2 12.7 (exo) 10.6

b

From corresponding TGA curves. One dehydration step in TGA.

spectra of R-CDcs and γ-CDcs, both air-dried and vacuumdried samples, were recorded (not shown here, but available in the Supporting Information). The spectra of as received R-CD‚6H2O and γ-CD‚7H2O cage structures show strong splitting for all C1-6 resonances, indicating that their CD molecules are in rigid, less symmetric cyclic conformations.2,28 However, when channel-type structure CD ICs are formed, their 13C NMR spectra display wellresolved, single, rather than split or multiple, resonances for each carbon of all glucose units. Also, the peaks at ∼98 and ∼80 ppm, corresponding to C1 and C4, respectively, adjacent to a conformationally strained glucosidic linkage, disappeared in the spectra of CDcss in the columnar structure. All of these results suggest that the CD molecules adopt a symmetrical conformation when they are packed in a columnar structure and are indifferent to how many molecules of water reside in the channels. A more careful examination of the spectra of R-CDcss reveals a slight downfield shift of C1 and C4 resonances for the air-dried sample in comparison with the vacuum-dried R-CDcs, likely reflecting their different contents of water of hydration. Moreover, in the case of the spectrum of vacuum-dried γ-CDcs, broader peaks are noted, probably a consequence of the larger variety of different environments within the amorphous structure. All of the NMR data are consistent with the conclusions drawn from the X-ray diffractograms recorded for both CDcss. Thermal Behavior. Despite the fact that thermoanalytical techniques are considered reliable and are relatively straightforward methods, there are only a few reports concerning the general thermal properties of CDs.29,30 The well-known thermoanalytical techniques DSC (differential scanning calorimetry), TGA (thermogravimetric analysis), and DTG (differential thermogravimetric analysis) are used for the measurement of thermally dependent heat flow and mass changes, as well as the water contents, of materials. The general thermal behaviors of natural CDs are similar, though differences can be found in water content, onset temperatures of thermal degradation, and the mass loss values at given temperatures. The DSC thermograms of as-received R-CD, recrystallized air-dried, and vacuum-dried R-CDcs are presented (28) Gidley, M. J.; Bociek, S. M. J. Am. Chem. Soc. 1988, 110, 3820. (29) Szejtli, J.; Szente, L.; Banki-Elod, L. Acta Chim. Acad. Sci. Hung. 1979, 101, 27. (30) Kurozumi, M.; Nambu, N.; Nagai, T. Chem. Pharm. Bull. 1975, 23, 3062.

Figure 4. DSC thermograms of the as-received R-CD (a) and recrystallized air- (b) and vacuum-dried R-CDcss (c).

in Figure 4. The first stage in the decomposition of the as-received and the recrystallized R-CDcs is dehydration. Characteristic observed temperatures and the calculated water contents are listed in Table 1. Dehydration of the as-received R-CD‚6H2O occurs in three steps as indicated by the three peaks in the DTG trace (not shown) and as distinct endotherms (A, B, and D) in the DSC curve. These three steps indicate the as-received R-CD contains three types of water molecules. The 2.4 molecules of water per CD molecule that escaped between 50 and 107 °C (DTG peak 87 °C) are likely included in the CD cavity, and the other 3.3 molecules of water per CD molecule liberated up to 150 °C are located in the interstices31 external to the cavity. Thus, the included water molecules in the R-CD hydrophobic cavity are considered to be interacting less strongly with the CD ring than water molecules in the outer interstices. The 0.4 water molecules per CD molecule retained to a higher temperature are the third type of water molecules, entrapped in the R-CD crystals by strong interactions. The same conclusions can be drawn for the recrystallized, air-dried R-CDcs, when its DSC thermogram is analyzed. However, the TGA curve of air-dried R-CDcs exhibits a one-step liberation of the entire water content (6.7 molecules of water per CD molecule). The corresponding DTG peak centered at ∼84 °C is very broad and extends from 25 to 230 °C. The event C observed in the DSC scans discussed above for as-received R-CD and (31) Kohata, S.; Jyodoi, K.; Ohyoshi, A. Thermochim. Acta 1993, 217, 187.

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Figure 6. DSC scans recorded for as-received γ-CD (a) and recrystallized air- (b) and vacuum-dried γ-CDcss (c). Figure 5. X-ray powder diffractograms of the as-received R-CD (a) and air-dried R-CDcs columnar structure (b), heated at 160 °C for 30, 60, and 180 min.

recrystallized, air-dried R-CDcs is attributed to a rearrangement of their hydrated structures. To confirm the nature of this transition, X-ray powder diffractograms were also recorded for these samples after heating at 160 °C for 30, 60, and 180 min (Figure 5). The diffraction peaks due to the dehydrated form of the asreceived R-CD (2θ ) 13.8, 14.6, and 20.5°) appeared immediately at 160 °C, and the peak intensities increased with the drying period, whereas the characteristic peaks of the hydrated form (2θ ) 11.9, 15.8, 18.1, and 21.6°) are diminished. Similar behavior was observed by Nakai et al. when they studied the water vapor sorption isotherms for R-, β-, and γ-CD.32 In contrast, the recrystallized R-CDcs appears to be a more stable hydrate structure, although the new diffraction peaks (2θ ) 7.8, 12.5, 16.5°), owing to a less stable hydrated form, increased in intensity with storage time at 160 °C. Therefore, we suggest that the R-CDcs columnar structure retains molecules of water even after it was dried at 160 °C for 180 min, which are eventually liberated at the same time and temperature as the melting-decomposition of R-CDcs takes place. It seems likely that at least some of these water molecules are located outside of the R-CD cavity, where they are hydrogen-bonded to hydroxyl groups on both sides of the neighboring R-CDs that form a strong hydrogen-bond network in the crystal. The recrystallized, vacuum-dried R-CDcs, which has a much lower initial water content (2.4 water molecules per CD molecule) than the as-received R-CD (6.1 water molecules per CD molecule) and recrystallized, air-dried R-CDcs (6.7 water molecules per CD molecule), appears by TGA to lose its water continuously over a broad temperature range and without significant structural reorganization as indicated by its unremarkable DSC response in Figure 4c. The DSC results obtained for as-received γ-CD, recrystallized, air-dried γ-CDcs and vacuum-dried γ-CD are displayed in Figure 6. When γ-CD is recrystallized from water and stored at 93.6% relative humidity, crystals containing 17 molecules of water per molecule of γ-CD are obtained.32 Thermogravimetric analysis there showed two distinct weight losses up to 150 °C. An intermediate solid phase containing 7 molecules of water per molecule of γ-CD was evidenced during dehydration and rehydration. We have also observed such a structure for our as-received γ-CD (see Table 1). The entire amount of water (6.9 molecules (32) Nakai, Y.; Yamamoto, K.; Terada, K.; Kajiyama, A.; Sasaki, I. Chem. Pharm. Bull. 1986, 34, 2178.

Figure 7. X-ray diffractograms of the as-received γ-CD heated at 170 °C (a) and recrystallized, air-dried γ-CDcs samples (b) heated to 100 and 160 °C for 10 min.

of water per CD molecule) was released in one dehydration step according to the TGA scan. After precipitation from deionized water a new γ-CDcs structure containing 12 molecules of water was obtained. Dehydration of this structure occurs in two steps as indicated by the peaks A and C in the DSC trace. After water molecules, possibly included in the cavity, have been liberated at 77 °C, the structure underwent an exothermic recrystallization at 98 °C (peak B). The sharp endothermic peak (event D) at 157 °C corresponds to a phase transition. Our assumptions for the events B and D were confirmed by X-ray studies. X-ray diffractograms in Figure 7 were recorded for asreceived (a) and recrystallized, air-dried γ-CDcs (b) samples heated to 100 and 170 and 160 °C, respectively, for 10 min. The sample heated at 100 °C yielded a new X-ray powder pattern, which demonstrates that the γ-CDcs first collapses into a cage structure and then immediately passes to its amorphous state. The final powder obtained after γ-CDcs in columnar structure was dried at 160 °C for 10 min was essentially amorphous. Also, the X-ray pattern of the as-received γ-CD dried at 170 °C for 10 min shows new diffraction peaks at 2θ ) 12.7, 17, and 22.2°, characteristic for the dehydrated form of γ-CD. This evidence proves that the endothermic peak at 167 °C in the DSC curve of the as-received γ-CD is due to the phase rearrangement into the dehydrated form. The DSC thermogram of the recrystallized, vacuum-dried γ-CD indicates just one broad endothermic peak due to the liberation of 6.4 water molecules per molecule of CD. It is also worth noting that the decomposition temperature of γ-CD does not depend so much on the water content or the type of structure adopted. The DSC thermograms of recrystallized, vacuum-dried R-CDsc (Figure 4c) and γ-CD (Figure 6c) are somewhat reminiscent of glass transitions. However, in both cases their second heating scans (not shown) are essentially

Inclusion Compound Formation

featureless, and so the broad endotherms observed in their first heating scans most likely result from dehydration and not glass transitions. Recrystallized, air-dried, columnar R- and γ-CDcs contain 6.7 and 12.0 water molecules per CD, respectively, which are reduced to 2.4 and 6.4 water molecules per CD upon vacuum-drying, with retention and loss of the columnar structure for R- and γ-CDcs. It can be estimated25,33 that both the channel and intersticial volumes inside and between CD stacks in γ-CDcs are at least 2.5 times greater than in R-CDcs, because their channel diameters are ∼8 and 5 Å, respectively. In addition, the ratio of channel to intersticial volumes is ∼3.7 for both Rand γ-CDcs. Thus, while a larger proportion of the water of hydration is lost upon vacuum-drying of recrystallized R-CDcs than is the case for recrystallized γ-CDcs, it is likely that not all of the water present in the channels of recrystallized, air-dried R-CDcs is removed during vacuumdrying, even though vacuum-dried R-CDcs remains columnar. On the other hand, vacuum-drying of the recrystallized, air-dried γ-CDcs removes a smaller proportion of the water of hydration and yet leads to an amorphous structure. This behavior suggests important roles for both included and intersticial water in the stabilization of columnar CDcss containing no guests other than water of hydration. Inclusion Properties of r- and γ-CDcs in Columnar Structure. It was of interest to verify whether these new columnar structures of R-CDcs and γ-CDcs are capable of including either small molecules or macromolecules without melting or dissolution. Therefore, we did several experiments by mixing solutions containing the guests/or the pure liquid guests with the powders of CDcss in the columnar structure. Propionic acid (PA) and valeric acid (VA) were chosen as small molecule guests, since they are able to form cage and channel structure ICs, respectively, with R-CD23,24 in solution. The X-ray diffractograms of the as-received R-CD, R-CDcs, VA-R-CDcs IC, PA-R-CDcs IC, and PCL-R-CDcs IC were recorded and are available in the Supporting Information. The channel-type structure of the R-CDcs ICs was maintained, having identical WAXD patterns with that of the starting recrystallized, vacuum-dried R-CDcs structure. The PA-R-CDcs IC also retains the channel structure, even though when PA-R-CD IC is formed in solution a cage herringbone structure results.23 The most distinct differences have been observed when the recrystallized, air-dried R-CDcs powder was used to obtain an IC with PA. The X-ray diffractogram of this structure (not shown, but available in the Supporting Information) is characteristic for R-CD ICs in the cage structure. This result indicates that the recrystallized, air-dried R-CDcs structure has a weaker hydrogen bond network than recrystallized, vacuum-dried R-CDcs crystals and may be converted to a cage structure in the presence of guests that form cage structure R-CD ICs. Thus, by controlling the state of hydration of R-CDcs the PA-R-CD IC resulting from exposure to PA, but without dissolution of the R-CD, can be altered to produce either channel or cage type crystals. This observation has important implications for the mechanism of formation and stabilities of CD ICs and the role(s) played in the process by the attendant water molecules. FTIR measurements have been used to demonstrate that the IC samples contain both guest and host components. FTIR spectra (available in the Supporting Infor(33) Szejtli, J. In Inclusion Compounds; Atwood, J., Davies, J., MacNicol, D., Eds.; Academic Press: London, 1984; Vol. 3.

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Figure 8. Comparison of the FTIR spectra recorded between 1550 and 1850 cm-1 for HA/PCL-R-CDcs IC (1:1:2 molar ratio)35 (a), HA/PCL-R-CDcs IC (1:1:1 molar ratio) (b), and HA/PCLR-CD IC (1:1:1 molar ratio formed in solution)35 (c).

mation) of as-received R-CD, R-CDcs, VA-R-CDcs IC, PAR-CDcs IC, and PCL-R-CDcs IC were recorded. The characteristic absorption bands of R-CD at 1026 and 1079 cm-1 due to coupled C-C/C-O stretching vibrations and the band at 1158 cm-1 attributed to the antisymmetric stretching vibration of the C-O-C glucosidic bridge34,35 are found to be unmodified in the R-CDcs. All the guests in the above ICs are characterized by intensive absorption bands at 1710 cm-1 (VA), 1715 cm-1 (PA), and 1732 cm-1 (PCL), which are assigned to the stretching of their carbonyl groups. These bands appear in the FTIR spectra of their R-CDcs ICs but are shifted to higher frequencies. These vibrational shifts may result from interactions occurring between the CdO groups of the guest acids with the R-CD hydroxyl groups. Also, the PCL carbonyl peak at 1736 cm-1 in the FTIR spectrum of its R-CDcs IC corresponds to the carbonyl absorption frequency observed in the amorphous regions of bulk PCL samples. According to our previous observations on PCL-b-PLLA-R-CD IC,11 this result indicates that no uncomplexed crystalline PCL phase exists and that all the polymer chains have been included in the IC channels. When R-CDcs is suspended in the PCL solution for a longer time (6 h), a larger amount of PCL was found to be included, according to the relative intensity of the PCL carbonyl band in the FTIR spectrum of PCL-R-CDcs IC. We also considered it important to understand if the R-CDcs columnar structure possesses the same preferential inclusion properties as R-CD‚6H2O when IC formation takes place in the presence of a mixture of guests, such as HA and PCL. In a previous investigation36 we were able to demonstrate that R-CD in solution prefers the inclusion of the longer molecular chain guest, when the amount of CD used in the IC formation was sufficient for complexing either all of one or just half of each guest. Repeating this experiment with the vacuum-dried R-CDcs columnar structure suspended in a solution containing both HA and PCL, we observed (see Figure 8) a different behavior. Comparison of the intensities of PCL and HA carbonyl bands at 1734 and 1714 cm-1, respectively, illustrates that comparable amounts of PCL and HA are included. We intend to further study the inclusion properties of R-CDcs columnar structure, because of its (34) Nells, G.; Weisser, M.; Back, R.; Wohlfart, P.; Wentz, G.; MitllerNeher, S. J. Am. Chem. Soc. 1996, 118, 5039. (35) Casu, B.; Reggiani, M. J. Polym. Sci., Polym. Symp. 1964, 7, 171. (36) Rusa, C. C.; Tonelli, A. E. Macromolecules 2001, 34, 1318.

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Figure 9. TGA scans of R-CDcs (a), PA-R-CDcs IC (b), VAR-CDcs IC (c), and PCL-R-CDcs IC (d). The inset represents DTG curves of the above-mentioned ICs.

potential to serve as a nanoscopic filter to separate closely related small-molecules and polymers, based solely on their abilities to be included, or not, in the narrow (∼0.5 nm) channels formed by the stacked R-CDs. FTIR data do not always necessarily indicate whether the guest molecules are included in the channel or are just adsorbed onto the outer surface of the R-CD molecules. Therefore, DSC and TGA measurements were performed on all of the IC powders. If the guest component is not included, then an endothermic peak for fusion of free crystalline guest will be observed in the DSC scans. Since the melting points of pure PA and VA are -24 and -20 °C, respectively, DSC thermograms of PA-R-CDcs IC and VA-R-CDcs IC were recorded between -40 and 250 °C and are available in the Supporting Information. The absence of any endothermic peaks in the heating runs demonstrates that PA and VA are fully included in the R-CDcs channels. Also, no fusion peak was observed for free PCL (Tm ) 60 °C) in the DSC thermogram of PCLR-CDcs IC. The decomposition temperatures of the complexes are considerably higher than that of the R-CDcs. Figure 9 presents the thermal decomposition behaviors of R-CDcs, PA-R-CDcs IC, VA-R-CDcs IC, and PCL-R-CDcs IC. All the R-CDcs ICs have higher initial decomposition temperatures than the starting R-CDcs crystals (256 °C), which indicates that the inclusion of the guest molecules enhances the thermal stability of the columnar structure, possibly because of the hydrogen bonds between the two components. Moreover, TGA scans of PA-R-CDcs IC and VA-R-CDcs IC exhibit three mass-loss stages. The first stage is related to dehydration in the temperature range 3-165 °C, with 4.65% water weight loss (2.8 molecules of water per CD molecule), and 30-174 °C, with 6.0% of water weight loss (3.6 molecules of water per CD molecule) for PA-R-CDcs IC and VA-R-CDcs IC, respectively. Therefore, the preexistent water content in the starting R-CDcs structure (2.4 mol) was not entirely replaced by the guest acid molecules. This information supports the fact that not all of the water molecules in the recrystallized, vacuum-dried R-CDcs reside in the channels, but some must be located outside of the cavities between the R-CD columns. Furthermore, a higher content of water molecules is expected in the R-CDcs ICs, because of the initial amount of water present in the acids used (99% pure). The second stage in the TGA curves of PA-R-CDcs IC and VA-R-CDcs IC consists of acid volatilization. The release of part of the guest acid from their R-CDcs ICs occurred at 166-280 and 180-275 °C, with 5.92% (0.83

Rusa et al.

mol) of PA weight loss and 3.39% (0.46 mol) of VA weight loss, instead of 2 mol of PA and 1 mol of VA, which were estimated from their expected IC stoichiometries. Partial volatilization of the acids above 280 °C may accompany R-CD decomposition, suggesting an increase in the R-CD thermal stability with IC formation. The above results are in agreement with those obtained by Soldi and coworkers for β- and γ-CD inclusion complexes with capric and caprilic acids.37 The third mass-loss stage represents only the R-CD decomposition, which occurs at 302 °C for PA-R-CDcs IC and 300 °C for VA-R-CDcs IC. The TGA curve of PCLR-CDcs IC shows an even higher decomposition temperature at 315 °C, suggesting that the guest polymer apparently consolidates the IC structure better than a small guest molecule. In this case, the second decomposition step (400 °C) corresponds to the decomposition of PCL. CP/MAS 13C NMR measurements, available in the Supporting Information, confirm once again the formation of ICs and the presence of the guests inside of the R-CDcs channels. Resonance peaks belonging to PCL (26.3, 30.5, and 34.5 ppm) and PA (9.3, 28.5, and 178 ppm) are clearly visible. Many attempts were made to include both small molecule (propanol, hexanoic acid, and dioxane) and polymer (PEG, PCL, a-PHB, PDMS) guests in the preexistent channels of γ-CDcs in the columnar structure. To date, none of the guest molecules were found to reside in the channels of γ-CDcs. During these experiments we noted that when a high stirring rate, long mixing time, or high temperature was applied to the suspension of γ-CDcs powder in the guest solution, destruction of the initial columnar structure of γ-CDcs occurred. Further investigations to elucidate the inclusion properties of γ-CDcs are underway. Conclusions A new solid-state packing mode of R-CDcs and γ-CDcs columnar crystals with just water molecules included inside the narrow channels was obtained by appropriate recrystallization for the first time. These structures were found to have channels like those of CD ICs including long guest molecules. The inclusion properties of R-CDcs are completely distinct from those of γ-CDcs. While R-CDcs is able to include both small molecules and polymers, γ-CDcs seems to be a more rigid structure that inhibits the initiation of an inclusion process. Furthermore, R-CDcs contains three types of water molecules as evidenced by DSC and loses more than a half of the included water molecules upon vacuum-drying. This does not induce any phase transition, thereby suggesting that the retained water molecules are very strongly hydrogen bonded possibly between the rims of the neighboring stacked R-CDs. However, when γ-CDcs is vacuum-dried, its columnar structure collapses to yield amorphous γ-CD, though it loses less than half its water of hydration. In addition, through control of the hydration state of R-CDcs, the crystal structure of the R-CDcs IC resulting from exposure to a guest can be either channel or cage type, as demonstrated here for the guest PA. Discovery of these readily obtained solid-state columnar CDcs structures significantly extends their possible applications, because of their nanoporous natures. Molecular filtration and confined catalysis may be possible in the accessible, narrow channels of the columnar CDcss. In (37) Meier, M. M.; Marilde, T. B.; Szpoganicz, B.; Soldi, V. Thermochim. Acta 2001, 375, 153.

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addition, beginning with solid, columnar structure CDcs eliminates utilization of water as a CD solvent, which many times is not miscible with the solvent used to dissolve the guest component during the usual formation of crystalline guest-CD ICs. Acknowledgment. The authors thank the Army Research Office, the National Textile Center (U.S. De-

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partment of Commerce), and North Carolina State University for their financial support. Supporting Information Available: Additional solidstate 13C NMR, FTIR, and X-ray data. This material is available free of charge via the Internet at http://pubs.acs.org. LA0262452