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Intercalation of Dyes in Layered Zirconium Phosphates. 1. Preparation and Spectroscopic Characterization of r-Zirconium Phosphate Crystal Violet Compounds Rainer Hoppe,† Giulio Alberti,*,‡ Umberto Costantino,‡ Chiara Dionigi,‡ Gu¨nter Schulz-Ekloff,† and Riccardo Vivani‡ Institut fu¨ r Angewandte und Physikalische Chemie, Universita¨ t Bremen, D-28334 Bremen, Germany, and Dipartimento di Chimica, Universita` di Perugia, via Elce di Sotto 8, I-06123 Perugia, Italy Received May 27, 1997. In Final Form: September 22, 1997X
The intercalation of crystal violet (CV+) into the ethanol form of R-zirconium phosphate has been investigated. X-ray powder diffraction patterns of samples with increasing dye loading showed that a pure phase (interlayer distance 2.2 nm) was obtained at a dye loading of 22% of the maximum ion-exchange capacity. Computer models and calculations based on dye dimensions and the structure of the host showed that this phase possesses dye loading and an interlayer distance very near to the maximum values, i.e., about 25% and 2.14 nm, respectively, that can be obtained if the crystal violet is intercalated as a monolayer of dye cations placed perpendicular to the inorganic layers of the host. The decrease of the interlayer distance to 1.8 nm with drying was attributed to a change on the inclination of dye molecules inside the interlayer space after the solvent is eliminated. The absorption spectra measured by diffuse reflectance spectroscopy showed maxima around 430 and 640 nm due to the protonation of the dye by the acidic P-OH groups of R-zirconium phosphate, exhibiting pK values in the range from ∼0 (430 nm) to ∼1 (640 nm). The dye-dye interaction in the perpendicular orientation caused absorption around 510 nm assigned to stacked (CV+)n layers.
Introduction Mineral-accommodated dyes find increasing attention with respect to potential applications as inclusion pigments or as components in optical devices. Silica-hosted chromophores are seriously considered for applications (i) as solid-state dye lasers,1,2 having advantages over liquid dye lasers by being nonvolatile, nonflammable, nontoxic, compact, and mechanically stable, or (ii) as nonlinearoptical materials,3 exhibiting large nonlinearity and high-speed response. Crystalline mineral hosts having regular open-pore and cage structures exhibit superior properties for the incorporation of chromophores, since the guest molecules can be accommodated in noncentrosymmetric arrangements, e.g., as chains of oriented dipoles, resulting in high values for the optical susceptibilities and second harmonic generation properties.4-7 An orientational order of chromophores is also achieved by anchoring or intercalation of polar dye molecules on layered minerals like clays8 or zirconium * To whom correspondence should be addressed. † Universita ¨ t Bremen. ‡ Universita ` di Perugia. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (2) Knobbe, E. T.; Bunn, B.; Fuqua, P. D.; Nishida, F.; Zink, J. I. In Ultrastructure Processing of Advanced Materials; Uhlmann, D. R., Ulrich, D. R., Eds.; Wiley: New York, 1992; p 519. (3) (a) Izawa, K.; Okamoto, N.; Sugihara, O. Jpn. J. Appl. Phys. 1993, 32, 807. (b) Riehl, D.; Chaput, F.; Le´vy, Y.; Boilot, J.-P.; Kajzar, F.; Chollet, P.-A. Chem. Phys. Lett. 1995, 245, 36. (4) Cox, S. D.; Gier, T. E.; Stucky, G. D.; Bierlein, J. J. Am. Chem. Soc. 1988, 110, 2986. (5) Cox, S. D.; Gier, T. E.; Stucky, G. D. Chem. Mater. 1990, 2, 609. (6) Caro, J.; Finger, G.; Kornatowski, J.; Richter-Mendau, J.; Werner, L.; Zibrowius, B. Adv. Mater. 1992, 4, 273. (7) Marlow, F.; Caro, J.; Werner, L.; Kornatowski, J.; Da¨hne, S. J. Phys. Chem. 1993, 97, 11286. (8) (a) Button, C.; Kauranen, M.; Persoons, A.; Keung, M. P.; Jacobs, K. Y.; Schoonheydt, R. A. Clays Clay Miner., in press. (b) Ogawa, M.; Kuroda, K. Chem. Rev. 1995, 399.
S0743-7463(97)00551-9 CCC: $14.00
phosphates/phosphonates,9 resulting in increased hyperpolarizabilities8 and in second harmonic generation.9 Zirconium phosphates and phosphonates with layered structures of the R- and γ-type are suitable inorganic or inorganic-organic hosts for the intercalation of a large variety of polar molecules.10 Moreover, the intercalation of large species such as aminophenyl- and pyridinium-substituted porphyrins,11 the intercalation of aniline with subsequent formation of polyaniline,12 previously found in montmorillonite,l3,14 and the polar orientation of dyes in layered zirconium phosphates/phosphonates built up layer by layer9 have been reported. Up to now, little information is available on (1) the diffusion of large cationic species in the interlayer region of these materials, (2) the relationship between the maximum uptake of the dyes, their cross section, and the free area around each functional group present on the layer surface of the host, (3) the orientation and conformation of the dyes in the interlayer region of the host from knowledge of the interlayer distances at various dye loadings, and (4) the thermal stability of the dyes inserted in an inorganic matrix. In the following, the process of the intercalation of crystal violet (CV+) in R-zirconium phosphate is described as studied by X-ray diffraction (XRD) and diffuse reflectance (9) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (10) Clearfield, A.; Costantino, U. Solid State-Supramolecular Chemistry: Two- and Three- dimensional Inorganic Network. In Comprehensive Supramolecular Chemistry; Lehn, G. M., Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vogtle, F., Eds.; Pergamon, Elsevier Science: Oxford, 1996; Vol. 7, Chapter 4 and references therein. (11) Kim, R. M.; Pillion, J. E.; Burwell, D. A.; Groves, J. T.; Thompson, M. E. Inorg. Chem., 1993, 21, 4509. (12) Jones, D. J.; El Mejjad, R.; Rozier, J. In Supramolecular Architecture: Synthetic Control in Thin Films and Solids; Bein, T., Ed,; ACS Symposium Series 1992, 499; American Chemical Society: Washington, DC, 1992; p 220. (13) Cloos, P.; Moreale, A.; Braers, C.; Badot, C. Clay Miner. 1979, 14, 307. (14) Ruiz-Hitzky, E. Adv. Mater. 1993, 5, 334 and references therein.
© 1997 American Chemical Society
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spectroscopy (DRS) in the UV-vis range. The combination of these methods enables the determination of the variations of the layer spacings with increasing dye loading and the simultaneous changes of the optical absorption properties. Experimental Section Characterization. X-ray powder diffraction (XRD) patterns were recorded with a computer-controlled Philips 1710 diffractometer, using Ni-filtered Cu KR radiation (40 kV, 20 mA). XRD patterns at high temperatures were taken with an A. Paar HTK attachment. UV-Vis diffuse reflectance spectroscopy (DRS) measurements of the intercalated samples were performed in the range of 200800 nm on a Cary-4 spectrometer (Varian) equipped with a diffuse reflectance attachment for solids (Praying Mantis). The accuracy of the spectrometer used was 0.003 in the absorbance mode. The values of the absolute reflectance were obtained by using diffuse reflectance standards (Spectralon, 75% reflectance; LOT). Spectra synthesis was carried out with a commercial program (Spectra cal., Galactic Ind. Corp.) using Gauss profiles exclusively. As the reference substance, dye-free R-Zr(HPO4)2‚H2O (hereafter R-ZrP) was used. The values of the Kubelka Munk function (F(R)) were calculated with regard to the range of validity.15 Due to the deep color (large extinction) of the dye-loaded solid samples, they had to be diluted with dye-free R-ZrP in the ratios 1:10 and 1:100 before UV-vis spectroscopic investigation. Preparation of r-Zirconium Phosphate in the Monosodium Form. A batch of R-ZrP was prepared by the HFmethod16 (interlayer distance 0.756 nm, maximum exchange capacity 6.64 mequiv/g). Two grams of R-ZrP was dispersed in 100 mL of 0.2 M NaCl, and the suspension was titrated very slowly with 66.4 mL of 0.1 M NaOH. After titration, the sample was centrifuged, washed with double-distilled water to eliminate the chlorides, and dried over BaCl2-saturated solution (92% relative humidity (RH)) for about 3 days. In agreement with previous literature for R-Zr[HNa(PO4)2]‚5H2O,17 the product exhibited an interlayer distance of 1.18 nm. Preparation of the Ethanol Form of r-Zirconium Phosphate. The ethanol form was prepared according to the procedure previously reported.18 The monosodium form (2.5 g) was added to 200 mL of absolute ethanol containing perchloric acid (0.84 mL of HClO4 11.7 M in 200 mL of ethanol) and stirred for 3 h to complete the intercalation. The product was centrifuged and washed twice with a small amount of pure ethanol. In agreement with previous literature,18 the ethanol form exhibited an interlayer distance of d ) 1.41 nm. The product was then dispersed in 25 mL of pure ethanol. As also confirmed experimentally, the amount of the ethanol form per milliliter of this suspension was equivalent to 0.075 g of the parent R-ZrP. Preparation of Crystal Violet Solutions. The aqueous solution of crystal violet chloride (1.5 × 10-2 M) was prepared by using C. Erba RPE reagent as supplied (color index, basic violet 3; CAS nomenclature, methanaminium, N-[4-bis[4-((dimethylamino)phenyl)methylene]-2,5-cyclohexadien-1-ylidene]N-methyl chloride; registry number, 548-62-9). The solution of crystal violet acetate was obtained by adding equivalent amounts of silver acetate. After the precipitation of silver chloride, the filtered dye solution was used without further purification. Dye Intercalation. Portions of 3 mL of the ethanol-form suspension were first added to 72 mL of absolute ethanol placed in gas-tight bottles. To each of these bottles was then added 25 mL of aqueous dye solutions containing amounts corresponding to 1-25% of the R-ZrP maximum ion-exchange capacity. The bottles were covered with aluminum foil to protect them against daylight irradiation and shaken at room temperature until the dye-uptake equilibrium was attained. (15) Yariv, S.; Nasser, A. J. Chem. Soc., Faraday Trans. 1990, 86, 1593. (16) Alberti, G.; Costantino, U.; Giulietti, R. J. Inorg Nucl. Chem. 1980, 42, 1631. (17) Alberti, G. Acc. Chem. Res. 1978, 11, 163. (18) Costantino, U. J. Chem. Dalton Trans. 1979, 402.
Figure 1. Structure formula of the crystal violet cation (molecular weight 372.54) representing one possible mesomeric limit structure.
Figure 2. Crystal violet uptake as a function of the amounts of (CV)Cl and (CV)Ac offered in solution. The samples were separated by centrifugation, washed 3 times with pure ethanol in order to remove the dye from the external surface of the microcrystals, and air dried. In order to obtain samples containing CV+ only on the external surface of the microcrystals, 0.5 g of R-ZrP was contacted for 30 min with 50 mL of an aqueous solution containing 7 × 10-7 mol of (CV)Cl. Analytical. If the solution was found to be completely colorless after the contact, the dye uptake was obtained directly from the amount of dye originally added into the solution. In the other cases, the amount that remained in the solution after filtration was determined by vis spectroscopy at 424 nm, i.e., the absorption maximum of fully protonated crystal violet in aqueous acid solution (pH ) 1). This wavelength was chosen to prevent problems due to the intensity changes of maxima at 550-600 nm caused, for example, by aggregation.
Results and Discussion Intercalation of Crystal Violet Monitored by XRD. Owing to its large interlayer distance, the ethanol form of R-ZrP is a very convenient material for intercalation processes of large cationic species.10 Uptake of crystal violet (CV+, Figure 1) from chloride (CV)Cl and acetate (CV)Ac solutions was investigated. Preliminary experiments showed that the rate of ion exchange of the ethanol form was considerably increased in the presence of some water. For this reason, as described in the Experimental Section, 3:1 ethanol/water mixtures (in volume) were used. Under these conditions, the interlayer distance of the ethanol phase was found to increase slightly from 1.41 to 1.48 nm with a strong decrease in the intensity of the peak. This intensity, due probably to recrystallization, was then found to increase again with time. Although the composition of the 1.48-nm phase was not determined, it is reasonable to suppose that the observed changes were due to solubilization of some water in the ethanol phase. Figure 2 shows the CV+ uptake as a function of the amounts of (CV)Cl or (CV)Ac added to the solution. For convenience, the CV+ uptake is reported either as the
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Figure 3. X-ray powder diffraction patterns of the ethanol form of R-ZrP: (a) wet sample; (b) sample dried at 120 °C overnight.
percentage of the maximum exchange capacity (hereafter % MEC) or in millimoles/gram of R-ZrP (right and left ordinate, respectively). If not otherwise specified, the % MEC refers to the CV exchanged as a monovalent cation. Note that all the dye added to the solution is taken up by R-ZrP, i.e., the solution becomes completely colorless, up to the values corresponding to about 10% and 20% MEC for (CV)Cl and (CV)Ac, respectively. The higher loading obtained when (CV)Ac is used is attributed to the buffering effect of the acetate groups which, neutralizing the exchanged protons, shifts the intercalation process to the right. It was also found that the dye uptake is almost immediate for dye amounts e 1% MEC. This very high rate can be explained by assuming that the uptake occurs on the external surface of the microcrystals. In agreement with this assumption, it was found that the XRD patterns of the wet and dried samples were practically identical to the corresponding wet and dried samples of the pure ethanol form of R-ZrP (Figure 3). For dye amounts corresponding to 2% MEC, the solutions become completely colorless within 2 h for crystal violet both in the acetate or in the chloride form. For amounts of dye in solution higher than 2% MEC, the times for complete uptake are strongly increased. For example, solutions containing an amount of dye corresponding to 4, 8, 10, and 20% MEC were colorless only after 5, 18, 25, and 100 days, respectively. Dye uptake was found to be faster for the (CV)Ac form than for the (CV)Cl form. This effect may also be attributed to the acetate groups that are able to decrease the activity of the exchanged protons. The XRD patterns of the wet samples are shown in Figure 4a. Note that in the range 2-6% MEC, the ethanol phase is first partially converted to a new phase with smaller interlayer distance (∼1.15 nm); then, in the range 8-20% MEC, it is completely converted into another phase with larger interlayer distance (2.2 nm). These phase transitions can be better followed in the pattern sequences of dried samples, i.e., when the solvent present in the interlayer region is eliminated (Figure 4b). Note that in dried samples, the ethanol phase is converted into R-ZrP (d ) 0.75 nm), while the interlayer distance of the CVintercalated phase decreases from 2.2 to 1.8 nm. On the basis of these results, the following mechanism of the CV+ intercalation is proposed. Small amounts of CV+ (e1% MEC) are exchanged quickly on the external surface of the ethanol form of R-ZrP. Additional CV+ (∼0.5-1% MEC) is taken up at relatively high rates in structural defects and/or in the more external part of the interlayer region. Only when the above domains are saturated further CV+ uptake can occur via diffusion in the interlayer region. Owing to the large size of the crystal violet and the strong bonding of CV+ to the acidic P-OH groups, a very small diffusion coefficient must be expected even in the relatively large interlayer region of the ethanol form of R-ZrP. At low values of intercalated dye, a phase
Figure 4. X-ray powder diffraction patterns of crystal violet intercalated in R-ZrP: (a) wet samples; (b) samples dried at 80 °C overnight.
Figure 5. Space-filling model of the crystal violet cation.
with an interlayer distance of 1.1 nm is formed; then, with increasing CV+ uptake, this phase is converted to a new phase with an interlayer distance of 2.2 nm (wet) and 1.8 nm (dried). Molecular Modeling of Crystal Violet Intercalation. Some model representations of the CV+ cation and the intercalation compounds obtained were attempted. The CV+ geometry was optimized by the Sibyl program under the Tripos force field (Figure 5). The molecule was then positioned into the R-ZrP layers with the help of the Hyperchem program, taking into account the van der Waals radii of the host and guest moieties. Two extreme models, in which CV+ cations are placed parallel or perpendicular to R-ZrP layers, are shown in Figures 6 and 7, respectively. From these models, it is possible to derive that the minimum and maximum values of the interlayer distances that can be obtained by CV+ inter-
Dyes in Layered Zirconium Phosphates
Figure 6. Computer-generated model showing the crystal violet intercalated in R-ZrP parallel to the layers. The expected interlayer distance is indicated.
Figure 7. Computer-generated model showing the crystal violet intercalated in R-ZrP perpendicular to the layers. The expected interlayer distance is indicated.
calation in R-ZrP are about 1.15 and 2.14 nm. Note that the experimental interlayer distances of the wet phases (1.15 and 2.2 nm) are in good agreement with those derived from these extreme models. On the basis of the first model, it can be deduced that the crystal violet is first intercalated with its π-electron system parallel to the R-ZrP layers, giving the phase with an interlayer distance of 1.15 nm, as experimentally found at low conversions. From the cross section of CV+ (about 1.2 nm2) and the surface area of a formula weight of completely exfoliated R-ZrP (2.9 × 105 m2), a % MEC can be calculated that can be obtained by intercalation of the dye in the flat position, being not higher than 9-10%. It follows that for higher conversions, the dye has to be accommodated with higher inclination. The interlayer distance will depend on this inclination and will reach its maximum value (2.14 nm) when the dye is accommodated perpendicular to the layers. Thus, since the experimental distance of the phase at high conversion is 2.2 nm, it can be deduced that the dye is intercalated perpendicularly to the layers in the wet samples. The decrease of the interlayer distance to 1.8 nm when this latter phase is dried could be understood considering that the void spaces, created by the elimination of the solvent, can be filled by changing the inclination of the adjacent CV+ cations. A behavior similar to that described above for crystal violet has already been reported for the
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Figure 8. UV-Vis spectrum of crystal violet dissolved in water (10-5 mol/L), exhibiting the deconvolution of the monomer (around 590-nm) and the dimer (around 550-nm) bands.
intercalation processes of amines into R-ZrP.19,20 Also in this case, the molecules are first intercalated parallel and then assume inclinations to the layers with increased loading. Since the mean cross section of CV+ in the position represented in Figure 7 is ∼0.52 nm2, it results that not more than 25% of the MEC can be reached even with the dye perpendicular to the layers. Higher values (e.g., 50% MEC) can be expected only for double film accommodation of the CV+ cations. However, in this case, very large interlayer distances (about 3.6 nm) have to be expected. Since XRD patterns do not show these high interlayer distances, it can be concluded that in the experimental conditions used here, the maximum CV+ uptake cannot be higher than about 25% MEC. The composition of this phase, in which the solvent has been completely eliminated by drying at 80 °C overnight, is therefore Zr(PO4)2H1.5(CV)0.5. Note that the higher % MEC reported in Figure 2 is very near the maximum calculated value. Thus, in agreement with the XRD pattern shown in Figure 4, the phase obtained at 22% MEC is expected to be a pure phase. The XRD pattern of this sample is in agreement with this expectation. In conclusion, although the intercalation process of CV+ in the interlayer region of the ethanol form of R-ZrP is, as expected, very slow, an appreciable amount of crystal violet can be intercalated in a reasonable time using a (CV)Ac solution. Note that the maximum intercalated amount of CV+, corresponding to about 20-25% MEC, could involve more than 50% of the acid POH groups of R-ZrP if protonated CVH2+ and CVH23+ species are also formed. Host-Guest Interactions Revealed from DRS. In aqueous solution, CV+ exhibits a principal band (x band) around 590 nm and a shoulder around 550 nm (Figure 8). The principal band around 590 nm arises from π f π* (0-1) transitions in the direction of the chromophore system from bonding HOMOs (highest occupied molecular orbitals) to antibonding LUMOs (lowest unoccupied molecular orbitals) exhibiting polarization along the 3-fold axis.15,21-23 The shoulder around 550 nm is assigned to dimers. The formation of a (CV+)2 dimer is possible, since (19) Clearfield, A.; Tindwa, R. M. J. Inorg Nucl. Chem. 1979, 41, 871 (20) Alberti, G.; Costantino, U. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Oxford University: Oxford, 1991; Vol. 5, p 145. (21) Lewis, G. N.; Magel, T. T.; Lipkin, D. J. Am. Chem. Soc. 1942, 64, 1774. (22) Seel, F.; Suchanek, L. Chem. Ber. 1950, 89, 965. (23) Theilacker, W.; Berger, W. Chem. Ber. 1956, 89, 965.
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Figure 9. DRS in the UV-vis range of crystal violet deposited on the external surface of R-ZrP from aqueous solution (1.4 × 10-6 mol/L), exhibiting CV3+ (around 430 nm).
the electrostatic repulsion between two positively charged chromophores can be compensated by the strong dispersion interaction of the π-electron system and the screening of the repulsive Coulomb field by the high dielectric constant of the water molecules.24 In principle, the absorption of dimers should exhibit two bands due to a Davydov splitting. However, the low-energy transition can be forbidden for molecules enabling symmetry operations.25 The weak band around 360 nm is assigned to an isomer of the principally symmetrical helical type molecule, i.e., a distorted helical type conformer.26 The band around 305 nm, i.e., with approximately twice the frequency of the principal band, can be assigned to a higher transition (0-2), denoted as the x′ band,26 i.e., the overtone of the electronic vibration along the longest dimension of the conjugated system, comprising the chinoid ring and a N,Ndimethylaniline ring. Finally, the band around 250 nm might be due to a dipole of a smaller auxochromic unit in the CV+ molecule,27 e.g., the N,N-dimethylaniline exhibiting absorption bands around 250 nm.28 The deposition of CV+ on the external surface of R-ZrP changes the UV/vis spectrum completely, i.e., (i) results in the disappearance of the principal band for the CV+ monomer (590 nm) and for the (CV+)2 dimer (550 nm), but (ii) results in the appearance of two new bands around 430 and 640 nm, and (iii) results in significant alteration of the high-frequency region below 400 nm (Figure 9). The new bands arising in the visible region around 640 and 430 nm are similar to the two bands of malachite green (MG+) appearing at 622 nm (x band) and 428 nm (y band).29 A red shift (ca. 50 nm) of the principal x band of CV+ has been observed repeatedly in solutions of hydrochloric acid29 or glacial acetic acid at pH values
