10114
J. Phys. Chem. B 1999, 103, 10114-10119
The Lattice Inclusion Compound of 1,8-ANS and Cucurbituril: A Unique Fluorescent Solid Brian D. Wagner* and Andrew I. MacRae Department of Chemistry, UniVersity of Prince Edward Island,Charlottetown, P.E.I., Canada C1A 4P3 ReceiVed: July 28, 1999; In Final Form: September 22, 1999
A solid precipitate is formed immediately upon the mixing of aqueous salt solutions of the fluorescence probe 1-anilino-8-naphthalenesulfonate (1,8-ANS) and the cage compound cucurbituril. The resulting solid compound is highly fluorescent, with a maximum emission at ca. 478 nm. Single crystals of this compound were obtained and studied by X-ray crystallography. The crystal structure indicates a rather novel solid fluorescent compound, described as a lattice inclusion compound, in which the smaller 1,8-ANS molecules have cocrystallized in the lattice of the cucurbituril cages. This structure is in effect a highly ordered solid solution, in which the fluorescent 1,8-ANS molecules are well separated from each other by the larger cucurbituril molecules. This separation of the fluorescent probe molecules by the nonabsorbing (in the visible range) cucurbituril spacers prevents interaction of the 1,8-ANS fluorophores and results in a highly fluorescent solid.
Introduction Cucurbituril1-3 is an interesting cage compound consisting of a C, N σ-framework, as shown in Figure 1. It was first prepared by Behrend et al. in 1905,4 but was not characterized until the work of Mock and co-workers, who published the first structural determination of this compound in 1981.5 Its most remarkable feature is its rigid and well-defined internal cavity, which has a diameter of ca. 5.5 Å, with two opposing portals of diameter ca. 4.0 Å, lined with carbonyl groups. The presence of this internal cavity makes cucurbituril a potential host for the encapsulation of smaller molecules. It is this potential for the formation of supramolecular host-guest complexes with cucurbituril which makes this molecule interesting. The first molecules to be successfully encapsulated within cucurbituril were a series of alkylammonium ions;6 the formation of the host-guest complexes was observed and analyzed by NMR spectroscopy. The ability of cucurbituril to encapsulate substituted ammonium cations in general has now been well established, using NMR7-11 or UV-vis7,12 spectroscopy, as well as calorimetry.13 This encapsulation ability has been shown to extend to other cations, such as alkali metal cations,12,14,15 as well as to other amino compounds, such as amino acids,16 amino alcohols,16 and amino azabenzenes.17 Subsequently, it was shown that other molecules besides cations and amines could be encapsulated by cucurbituril. For example, the inclusion of THF in cucurbituril has been studied in detail, with the presence of the THF molecule inside the cucurbituril cavity being directly confirmed by X-ray crystallography.18 There have also been a number of reports on the formation of cucurbituril inclusion complexes with aromatic molecules. These include toluene, naphthalene, and aniline,19 the fluorescent probe 1,8-ANS (the probe which is also of interest in this paper) in the solid state,20 and various large aromatic dye molecules in solution.21-23 Cucurbituril has also been shown to have an excellent ability to form rotaxanes, threading onto a variety of molecules with long alkyl chains.24-28 Although cucurbituril has now been demonstrated to have the ability to form inclusion complexes with a fairly wide range of molecules, there have been relatively few studies of the effects
Figure 1. Molecular structures of cucurbituril and 1,8-ANS.
of such encapsulation by cucurbituril on the fluorescence of the guest molecule. Of all of the reports on cucurbituril hostguest inclusion complexes referred to above, only two, both by Buschmann and co-workers, involved fluorescence studies;19,20 the rest involved NMR or UV-vis spectroscopic studies or calorimetry. This is in contrast to the inclusion complexes of cyclodextrins, for example, which have been extensively investigated by fluorescence spectroscopy. This technique has proven to be an extremely sensitive and informative method for studying such host-guest inclusion phenomena. There are a number of probe molecules available whose fluorescence properties (wavelength and intensity) are extremely sensitive to the polarity of the local environment.29 The fluorescence of an aqueous solution of such a probe would be expected to change significantly upon encapsulation of the probe inside the cucurbituril cavity, since the cavity is considerably less polar than the bulk aqueous medium. The observation of significant enhancement of the fluorescence of such a probe would be a clear indication that host-guest inclusion had occurred and would provide an extremely sensitive method for studying such inclusions. Neither of the two previous fluorescence studies of cucurbituril inclusion involved the formation of host-guest complexes in solution. One involved the formation of inclusion complexes between gas-phase aromatic molecules (e.g., naphthalene) and solid cucurbituril packed in a column, with fluorescence being used to indicate the presence of naphthalene on the column.19 The other report involved the
10.1021/jp9926342 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/28/1999
Lattice Inclusion Compound of 1,8-ANS and Cucurbituril fluorescence of the solid complexes of 1,8-ANS with various macrocyclic hosts, including cucurbituril.20 In this paper, we describe results obtained from our attempts to observe host-guest inclusion complexes of cucurbituril and the fluorescence probe 1,8-ANS in solution. 1,8-ANS is an extremely polarity-sensitive fluorescence probe and has been extensively used to study host-guest complexes of cyclodextrins30-33 and modified cylcodextrins.34-36 We were unable to investigate the expected complexes in solution, however, as a solid precipitate formed immediately upon the addition of solutions of cucurbituril and 1,8-ANS. The resulting solid was easily recoverable and was discovered to be strongly fluorescent. This paper describes a detailed investigation of the structure and properties of this unique fluorescent binary solid. Although the previously mentioned study on the fluorescence of this solid by Buschmann and Wolff has very recently been reported20 (it appeared during the preparation of this manuscript), it contains no details of the most interesting aspect of this solid fluorescent compound, namely its structure. The authors of that paper refer to the compound only as a solid complex and assume the stoichiometry to be 1:1. In the following, we will show that the stoichiometry is in fact 2:1 ANS/cucurbituril and that the solid is not a host-guest inclusion compound but is in fact a lattice inclusion compound; that is, the guest does not reside within the host cavity but rather in the interstitial spaces of the host lattice. Experimental Section Materials. The following compounds were obtained from the indicated sources and used as received: 1-anilino-8-naphthalene sulfonic acid (Aldrich); sodium sulfate, potassium sulfate, cesium sulfate, sodium chloride, potassium chloride, cesium chloride, sodium monobasic phosphate, and sodium dibasic phosphate (all from Fisher). Cucurbituril was synthesized and purified as described in the literature.4,5 The solid ANS-cucurbituril complex was formed spontaneously upon the mixing of a 0.025 M solution of 1,8-ANS with a 0.17 M solution of cucurbituril, both in 0.20 M aqueous Na2SO4. The compound was also found to form using 0.20 M aqueous solutions of K2SO4, Cs2SO4, NaCl, KCl, CsCl, or a NaH2PO4/Na2HPO4 buffer (pH ) 6.80) as the solvent. The solid was suction filtered and air-dried, yielding a fine, green-gray powder. Some samples were further dried in a vacuum oven (140 °C, 10 Torr) for 4 h. Single crystals for X-ray analysis were prepared by placing solutions of cucurbituril and 1,8-ANS in 0.20 M aqueous Na2SO4 separately into the bottom halves of a pair of test tubes connected near the top by a glass tube. The paired tubes were then carefully filled to the top (i.e., filling the connecting tube) with 0.20 M aqueous Na2SO4, and the assembly was clamped securely in place. Large, pale-green, needle-shaped crystals were found to grow within the connecting tube over the period of 1-2 days. A melting point could not be obtained; the crystals were found to decompose above 215 °C. Fluorescence Spectroscopy. Fluorescence spectra were obtained on a Perkin-Elmer LS-5 luminescence spectrometer, with excitation and emission monochromator band-passes set at 3 nm and an excitation wavelength of 340 nm. Spectra of solutions were obtained in 1 cm2 quartz cuvettes. Spectra of solids were collected by front face emission, i.e., by placing the solid powder in a 1 mm thick, 10 mm wide quartz cuvette, at a 45° angle to both the excitation and emission monochromators. The position was adjusted slightly for each sample to minimize detection of scattered light. X-ray Crystallography. The X-ray crystallographic studies were performed by Dr. Stephen Loeb at the University of
J. Phys. Chem. B, Vol. 103, No. 46, 1999 10115 TABLE 1: Fluorescence Maxima (Air-Dried and Vacuum Oven Dried) and Water Content of the ANS-Cucurbituril Solid Powders Prepared from Various 0.20 M Salt Solutions λF,max (nm) salt solution
air dried
vacuum oven dried
water content (% mass)
Na2SO4 K2SO4 Cs2SO4 NaCl KCl CsCl NaH2PO4/Na2HPO4
469 473 479 471 475 470 473
477 477 478 477 477 479 479
19.5 12.5 14.7 16.3 11.2 15.0 10.0
Windsor. Data were obtained on a Siemens SMART CCD instrument, and solutions were performed using the SHELXTL 5.03 program library.37 The dimensions of the crystal used were 0.10 mm × 0.14 mm × 0.22 mm. The X-ray wavelength used was 0.71073 Å. The θ range used for data collection was 1.5321.50°. The index ranges used were -11 e h e 8, -25 e k e 21, and -16 e l e 14. The number of reflections collected was 15 036, with 4962 independent reflections (Rint ) 0.1532). The refinement method used was full-matrix least squares on F2, with 4962 data, 0 restraints, and 596 parameters. Results ANS-Cucurbituril Solid. The spontaneous formation of the solid ANS-cucurbituril complex upon mixing of the separate 0.20 M Na2SO4 solutions differs from the procedure reported by Buschmann and Wolf,20 in which this solid was prepared from an acidic aqueous solution of 1,8-ANS and cucurbituril, by slowly increasing the pH of the solution with the addition of NH4OH. The 0.20 M Na2SO4 solution was chosen as the solvent based on a report of the high solubility of cucurbituril in this solution,18 as it is a much less harsh medium than the highly acidic solutions (e.g., 1:1 formic acid/water) typically used to dissolve cucurbituril. In fact, preliminary investigations involving 1,8-ANS in 1:1 formic acid/water showed a slow degradation of the fluorescence spectrum of the probe over a period of hours. The spontaneous precipitation of the ANScucurbituril solid does not depend on the specific salt used, as the solid was obtained from seven different salt solutions, as listed in Table 1. It was found that the air-dried solids contained a significant amount of water. Mass losses ranging from 10% to 20% were observed upon heating of the solid powders in a vacuum oven for 4 h. Mass losses of ca. 25% were observed upon the same vacuum oven treatment of the crystalline ANScucurbituril solid, indicating that the crystalline form of the solid has a slightly higher water content than does the powder form. Fluorescence of ANS-Cucurbituril Solid. Figure 2 shows the front-face emission spectra of the ANS-cucurbituril solid powders, obtained from the mixing of the 1,8-ANS and cucurbituril solutions in 0.20 M Na2SO4, K2SO4, and Cs2SO4. Figure 2a shows the results for the air-dried solids; a broad, featureless fluorescence spectrum was obtained, similar to that of 1,8-ANS in solution.36 The maximum fluorescence wavelength was found to show a small but reproducible (from batch to batch) dependence on the type of sulfate salt used in the original solutions, with a range of 469-479 nm. This small difference was found to disappear, however, upon drying the samples in a vacuum oven. These spectra are shown in Figure 2b; the fluorescence maximum of all of the vacuum oven dried samples was found to be 478 ( 1 nm. This value corresponds to that of 1,8-ANS in ethanol solution,36 indicating a polarity of the 1,8-ANS environment in the solid which is similar to
10116 J. Phys. Chem. B, Vol. 103, No. 46, 1999
Wagner and MacRae molecules and carbonyl oxygens along the rim, and are arranged in stacks with the 1,8-ANS in the interstitial spaces. However, it was not possible to resolve the number, positions, and orientations of the water molecules in the structure. Thus, although the refinement factors R1 and wR2 of the final structure are rather high, this is a result of these unresolved water molecules, and the uncertainties in their numbers and positions. However, the actual structure involving the 1,8-ANS anions and the cucurbituril molecules is unambiguous. Discussion
Figure 2. Fluorescence spectra of the ANS-cucurbituril solid, prepared from various salt solutions: (s) Na2SO4; (- - -) K2SO4; (‚‚‚) Cs2SO4. (a) air-dried; (b) vacuum oven dried.
TABLE 2. Crystal Data and Structure Refinement for the ANS-Cucurbituril Solid empirical formula temperature crystal system space group unit cell dimensions volume Z goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data)
C68H60N26O18S2‚xH2O 293(2) K monoclinic P21/n a ) 11.269(1) Å b ) 24.341(2) Å β ) 92.097(2)° c ) 15.954(2) Å 4373.1 Å3 2 1.065 R1 ) 0.1323, wR2 ) 0.3071 R1 ) 0.2342, wR2 ) 0.3649
that of ethanol. These fluorescence maxima are summarized in Table 1. Slight differences in the fluorescence maxima were also observed for the solids prepared from the other salt solutions; all were within the range observed for the sulfate salts. Again, all solids had the same emission maximum after vacuum oven drying of 478 ( 1 nm. In addition, fluorescence spectra were also measured for the ANS-cucurbituril crystals, obtained from solution as described above. These showed a very similar fluorescence spectrum, with an emission maximum of 478 nm, indicating the similarity of the powder and crystalline forms. This emission maximum was found to be unchanged after the crystals were dried in the vacuum oven. Crystal Structure of ANS-Cucurbituril Solid. The crystal data obtained for this solid are listed in Table 2. The crystal structure is shown in Figures 3 and 4. The solid was found to have an overall ANS-cucurbituril stoichiometry of 2:1. Figure 3 is an ORTEP diagram showing the relative orientation between a single cucurbituril molecule and two 1,8-ANS anions. Figure 4 illustrates the extended structure, including five cucurbituril molecules and six 1,8-ANS anions, using spherical atom representations for clarity. The cucurbituril molecule has a crystallographically imposed center of symmetry, which includes the two 1,8-ANS anions on either side. The cucurbituril molecules appear to be hydrogen bonded through water
The fluorescence maximum range of 469-479 nm determined for the various air-dried samples, as well as the value of 478 ( 1 nm found for all of the vacuum oven dried samples as well as the crystals, agrees extremely well with the value of 470 nm previously reported in the literature for the solid ANScucurbituril complex.20 Thus, although the previous literature solid was prepared in a very different way (by slowly neutralizing an acidic solution of 1,8-ANS and cucurbituril) from that in the current work, the fluorescence spectrum would suggest that they have similar structures. The small but reproducible variation in observed fluorescence maxima for solid powders prepared from different salt solutions and the change in all of these maxima to the same value of 478 nm upon heating in a vacuum oven indicate that a significant role is played by the water molecules in the solid. The unit cell empirical formula obtained is C68H60N26O18S2,‚ xH2O, which corresponds to a unit cell consisting of one cucurbituril molecule and two 1,8-ANS anions, plus an undetermined number of water molecules. Unfortunately, it was not possible to resolve the number or position of the water molecules in the crystal structure from the obtained crystallographic data. This means that it was not possible to calculate the density of the solid from these data. The density would be useful to compare to that reported for other cucurbituril solids. We are currently attempting to measure the density of the ANScucurbituril crystals by physical methods. This would not only be useful for comparison to other cucurbituril solids but would also allow for an accurate determination of the number of water molecules per unit cell. However, an estimate for this number can be obtained using the average mass loss of 25% observed upon vacuum heating of the crystalline solid. If this is assumed to be the result of loss of water from the crystal structure, then based on the above unit cell empirical formula, the value of x can be estimated to be 30. Thus, each unit cell is estimated to contain the rather large number of 30 water molecules. This is of course an estimate, as it is not certain that all of the water in the solid is removed by the vacuum oven drying procedure used (underestimate) and because some of the water may not have been part of the crystal structure (overestimate). The water molecules in the crystal structure must play an important role and may in fact be essential to maintaining the structural integrity. Some water molecules may be between the ANS and cucurbituril, some may be between cucurbituril molecules, while others may be inside the cucurbituril cavities. Further investigation of the importance of this rather large number of water molecules per unit cell to the overall structure of the solid will be undertaken. The crystal structure obtained here (ANS-cucurbituril) can be compared to two structures of solid cucurbituril from the literature,18 which were also crystallized from Na2SO4 solution. One is of cucurbituril itself; the other is of cucurbituril with a single THF molecule included inside the cucurbituril cavity (THF-cucurbituril), i.e., a true host-guest solid inclusion
Lattice Inclusion Compound of 1,8-ANS and Cucurbituril
J. Phys. Chem. B, Vol. 103, No. 46, 1999 10117
Figure 3. X-ray crystal structure of the ANS-cucurbituril solid, showing the 2:1 stoichiometry and the relative orientation of a cucurbituril molecule and a pair of ANS anions.
Figure 4. Crystal packing structure of the ANS-cucurbituril solid.
compound. These three are all monoclinic crystals, with the related space groups P2l/n for ANS-cucurbituril and P21/c for cucurbituril and THF-cucurbituril. However, the ANS-cucurbituril solid has a much larger unit cell volume of 4373 Å3, as
compared with that of 3400 Å3 for both the cucurbituril and THF-cucurbituril solids. It is also interesting to note that both of the literature structures contain Na2SO4 within the cucurbituril lattice structure, whereas the ANS-cucurbituril solid does not.
10118 J. Phys. Chem. B, Vol. 103, No. 46, 1999 Presumably, each cucurbituril molecule is doubly protonated at the carbonyl rims, with the two ANS anions serving as counterions. On the basis of these analyses and comparisons, the structure of the ANS-cucurbituril solid can be described as a lattice inclusion compound38-43 (such solids are also referred to as lattice clathrates44-47) with the ANS anions occupying the interstitial spaces of the cucurbituril lattice. In this case, the cucurbituril lattice has been expanded somewhat in the presence of ANS as compared with that of the pure cucurbituril solid or the cucurbituril host-guest inclusion compound THF-cucurbituril, in which the guest molecules are inside the cucurbituril cavity as opposed to within the lattice structure. The same general type of stacked structure is observed here as reported for other cucurbituril solids,5,18 with an expansion of the cucurbituril lattice spacing to accommodate the 1,8-ANS anions. We propose that it is this structure which makes this solid highly fluorescent; the large cucurbituril molecules act as spacers to separate the fluorescent 1,8-ANS molecules and prevent them from interacting with each other. This lack of interaction prevents the occurrence of a number of intermolecular processes, such as excimer formation and energy transfer, which would otherwise compete strongly with the 1,8-ANS fluorescence. In essence, this structure can be considered to be a highly ordered solid solution of 1,8-ANS in the cucurbituril matrix (transparent above ca. 340 nm), which exhibits fluorescence properties similar to those of a 1,8-ANS ethanol solution. The absence of sodium and sulfate counterions observed in this structure, in contrast to that in other cucurbituril solids prepared from sodium sulfate solution, provides an idea as to why this solid forms spontaneously upon mixing of the sodium sulfate of the two components. It is clear that the solubility of cucurbituril in such salt solutions arises from the complexation of the two carbonyl rims by Na+ cations.18 The addition of the 1,8-ANS to the solution must somehow result in the displacement of these cations, which greatly reduces the cucurbituril solubility. When the two solutions are mixed slowly, the result is the formation of single crystals, with a well-defined ANScucurbituril geometry. It is interesting to compare the structure of this compound to that of some other host-guest fluorescent solids reported in the literature. The simplest such case is that of a low concentration of one aromatic fluorophore doped into the crystal of another aromatic compound, one with very different spectroscopic properties. An example of such a fluorescent solid is the pentacene-doped p-terphenyl crystal.48 The large difference in spectroscopic properties of the guest (pentacene) and host (p-terphenyl) are essential to prevent energy transfer quenching of the fluorescent guests by the host, and to have the host be transparent to the excitation light. A second, supramolecular type of fluorescent host-guest solid is a zeolite with an aromatic fluorescent probe included within the zeolite framework, at a low enough loading concentration to ensure that the probes are present as single, isolated molecules. For example, the fluorescence of pyrene within NaY zeolites has been used to study the nonuniform distribution of organic molecules within these supramolecular structures.49 Fluorescent solid host-guest inclusion complexes of cyclodextrins have also been reported, for example with 9,10-dimethylanthracene50 and 1,8-ANS20 as the fluorescent guest. In these solids, it is assumed that the fluorescent guests are included singly inside the cyclodextrin cavities and thus are individually isolated from each other. Finally, Yoshida et al. have prepared51 a fluorescent host-guest lattice inclusion compound in which the host lattice molecules
Wagner and MacRae themselves are the fluorescent species. This host lattice shows greatly enhanced fluorescence when small guest molecules such as ethanol are included in the lattice. In this case, the guest molecules serve as spacers to isolate the fluorescent host molecules. Their reported fluorescent crystals all have a 1:1 host-guest stoichiometry. In general, pure crystals of aromatic fluorophores which pack in infinite stacks or as associated pairs exhibit broadened, redshifted fluorescence relative to that of the monomers in solution.52 By contrast, all of the above-described fluorescent host-guest solids, as well as the ANS-cucurbituril solid, exhibit fluorescence spectra which resemble that of the respective fluorophore monomers in solution. In all cases, the host-guest structure serves to physically separate the fluorophores, which results in the observation of solution-like fluorescence. However, we feel that our ANS-cucurbituril solid has the following unique and interesting properties, which differentiates it from others reported in the literature. (1) It is, to our knowledge, the first report of a fluorescent lattice inclusion crystal with fluorophores as the guests. (2) It has the relatively large ratio of fluorescent species to spacer of 2:1, resulting in a large number of fluorophores per unit volume, especially when compared with the doped crystals and zeolites. (3) The structure of the crystal is quite interesting, involving a high degree of symmetry in the unit cell, with pairs of 1,8-ANS fluorophores related by a center of inversion in the center of the cucurbituril molecule separating them. (4) It is formed spontaneously upon addition of salt solutions of the host and guest and is insensitive to the particular salt solution used. (5) It is very insensitive to the presence of water, showing similar fluorescence properties when air-dried (when it contains a large amount of water) or vacuum oven dried (when a significant amount of water has been removed). Fluorescent solids (crystals or powders) are quite useful materials, with a wide range of practical applications, ranging from phosphors for display screens, to the study of the nocturnal activities of small mammals.53 The fluorescent solid described in this paper may also find practical applications, especially in light of its ease of preparation (although cucurbituril is not available commercially and thus must first be prepared) and its insensitivity to water. Summary 1,8-ANS and cucurbituril spontaneously precipitate from salt solutions, yielding a highly fluorescent solid with an interesting crystal structure. This structure is described as a lattice inclusion compound, with the 1,8-ANS anions included in a 2:1 ratio in the lattice of cucurbituril. The large cucurbituril molecules act as spacers, isolating the individual 1,8-ANS fluorophores, resulting in a strong fluorescence from the solid resembling 1,8ANS monomer fluorescence in solution. This compound may have potentially useful applications as a moisture-insensitive fluorescent powder. Acknowledgment. Financial support for this work was provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) and by the UPEI Senate Committee on Research. The authors gratefully acknowledge the work of Professor Stephen Loeb of the University of Windsor, who performed the X-ray crystallography studies. References and Notes (1) Mock, W. L. Top. Curr. Chem. 1995, 175, 1.
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