Optical Properties of Nanostructures Obtained by Encapsulation of

May 28, 2010 - ... M. Fonseca , Olívia S.G.P. Soares , Manuel F.R. Pereira , Tao Dong , Isabel C. Neves. Sensors and Actuators B: Chemical 2018 261, ...
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J. Phys. Chem. C 2010, 114, 10719–10724

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Optical Properties of Nanostructures Obtained by Encapsulation of Cation Chromophores in Y Zeolite Isabel C. Neves,† Celina Cunha,† Ma´rio R. Pereira,‡ Manuel F. R. Pereira,§ and Anto´nio M. Fonseca*,† Departamento de Quı´mica, Centro de Quı´mica, UniVersidade do Minho, Campus de Gualtar, 4170-057 Braga, Portugal, Departamento de Fı´sica, Centro de Fı´sica, UniVersidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal, and Laborato´rio de Cata´lise e Materiais (LCM), Laborato´rio Associado LSRE/LCM, Departamento de Engenharia Quı´mica, Faculdade de Engenharia, UniVersidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal ReceiVed: February 2, 2010; ReVised Manuscript ReceiVed: April 26, 2010

Several unsaturated cation chromophores have been prepared by ship-in-the-bottle synthesis in the supercages of Y zeolite as a host. The guests were encapsulated into the zeolite in their H+ form in the liquid phase. The chromophores were assembled inside the void space of the zeolite through the reaction of the different aldehydes, 3-phenyl-2-propynal (A), trans,trans-2,4-hexadienal (B), and trans-2-pentenal (C), with the N,Ndimethylaniline. The appropriate synthesis involved using an N,N-dimethylaniline/aldehyde molar ratio of 2:1. The host-guest chromophores obtained have been fully characterized by chemical analysis, spectroscopic methods (FTIR and UV/vis), N2 adsorption isotherms, powder X-ray diffraction, and scanning electron microscopy. Fluorescence studies provided direct evidence that the cation species are located inside the zeolite supercages of these host-guest chromophore materials. Introduction The incorporation of organic dye molecules into hosts acting as solid supports has attracted much interest due to the possibility of obtaining stable structures with potential applications in different fields. These include the development of photonic devices and materials, such as waveguide laser cavities, optical memory systems, electroluminescent devices, artificial antenna systems, radiation detectors, optical switches, and the construction of optical sensors.1–12 The large variety of pore structures and morphologies provided by different types of molecular sieves offers many possibilities for the design of host-guest systems with specific properties.13 Among host-guest assemblies, zeolites and zeolitic molecular sieves have proved to be very convenient hosts.13–19 For these applications, zeolites provide a rigid structure in which some active component or components can be included. The ship-in-the-bottle approach is an especially interesting technique that makes use of host frameworks possessing cavities that are larger than the entrance openings. In the zeolite frameworks, reactants can diffuse into the cavities, but the products are prevented from exiting.20 Zeolites are also convenient solid host for the generation and stabilization of organic carbocations.12,21 In this approach, the zeolite faujasite has been used almost exclusively, and beautiful examples have been reported.13 The first ship-in-the-bottle syntheses in zeolites were used to prepare metal complexes entrapped in the framework structure to prevent their diffusion out of the solid matrix, and these host-guest materials are categorized as heterogeneous catalysts.13–19 * To whom correspondence should be addressed. E-mail: amcf@ quimica.uminho.pt. † Departamento de Quı´mica, Universidade do Minho. ‡ Departamento de Fı´sica, Universidade do Minho. § Universidade do Porto.

The first work on the synthesis of the large cation in host frameworks was reported by Garcia et al.22–27 These authors synthesized the organic cations through the formation of new C-C bonds by coupling nucleophilic and electrophilic reagents in the absence of a metal. The carbocations derived from the triarylmethylium cations are the most stable due to charge delocalization through the aryl rings, and HY zeolite is sufficiently acidic to promote the aldolic condensation. The acidity of the zeolite is provide by their Brønsted acid sites.28,29 Inspired by these new host-guest materials, we set out to utilize HY zeolite, as a support for in situ syntheses of chromophores by the reaction of the different aldehydes, 3-phenyl-2-propynal (A), trans,trans-2,4-hexadienal (B), and trans-2-pentenal (C), with the N,N-dimethylaniline. The host-guest samples obtained have been fully characterized by chemical analysis (CA), spectroscopic methods (FTIR and UV/ vis), N2 adsorption isotherms, powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). The conjugated systems chosen for this work stabilize the chromophore formed inside the zeolite. The color of the chromophore changed due to the different π-conjugated systems and to the presence of electron-donor substituents on the aryl rings. These host-guest materials exhibit intense photoluminescence, and it would be of interest to study their optical properties. The fluorescence studies are reported and the effect of the π-conjugated systems of the chromophores is discussed. Experimental Section Materials and Reagents. The HY zeolite in powder form (CBV400) was obtained from Zeolyst International. The 3-phenyl2-propynal (A), trans,trans-2,4-hexadienal (B), trans-2-pentenal (C), and N,N-dimethylaniline were purchased from Aldrich. All other chemicals and solvents used were reagent grade and were purchased from Aldrich.

10.1021/jp101001a  2010 American Chemical Society Published on Web 05/28/2010

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SCHEME 1: Structures of the Chromophores Derived from 3-Phenyl-2-propynal (A), trans,trans-2,4-Hexadienal (B), and trans-2-Pentenal (C) with the N,N-Dimethylaniline

Encapsulation of the Organic Cations in HY Zeolite. General Procedure. Zeolite-encapsulated cation chromophores with different aldehydes were prepared by ship-in-the-bottle synthesis according the following experimental procedure.22–27,30,31 Chromophores are prepared by adsorption of 3-phenyl-2propynal (A), trans,trans-2,4-hexadienal (B), and trans-2pentenal (C) in HY zeolite as a host (Si/Al ) 2.80). The host, after being thermally activated (773 K, overnight), was mixed with a solution containing the N,N-dimethylaniline and the different aldehydes with a 2:1 (mol/mol) ratio and the xylene, as a solvent. The theoretical number of cation chromophores is 2.53 × 1020 molecules/gzeolite. This mixture was carried out by stirring at the reflux temperature. The heating duration depends on the thermal stability of the different aldehydes used. During the heat treatment, the original white color of HY changed to the characteristic colors of the resulting chromophores, indicating that these species are effectively synthesized inside the host. The suspension was then filtered. The resulting colored materials were purified by continuous solid-liquid Soxhlet extraction using CH2Cl2 as a solvent to remove the excess of the reagents. Finally, the samples were dried under vacuum for 12 h. The samples obtained were denoted as [bis(N,N)dPhCA]+@HY, [bis(N,N)dPhCB]+@HY, and [bis(N,N)dPhCC]+@HY, where CA is derived from the 3-phenyl-2-propynal, CB from the trans,trans-2,4-hexadienal, and CC from the trans-2-pentenal. Synthesis of [bis(N,N)dPhCA]+@HY. The synthesis of [bis(N,N)dPhCA]+@HY was prepared according to the following experimental procedure: organic solutions of 3-phenyl-2-propynal (A) (51 µL, 0.42 mmol) and N,N-dimethylaniline (105 µL, 0.83 mmol), with a ratio of A/(N,N)dPH ) 1:2 mol/mol, in 25 mL of xylene were added to 1.0 g of HY (previously thermally activated at 773 K overnight). The resulting mixture was stirred for 12 h at the reflux temperature. The suspension was then filtered, and the violet solid was submitted to continuous solid-liquid extraction for 12 h, using micro-Soxhlet equipment and CH2Cl2 as a solvent. The sample was dried in vacuum for 12 h. The color of the samples remained violet after extraction. Synthesis of [bis(N,N)dPhCB]+@HY. An analogous procedure is followed using the trans,trans-2,4-hexadienal (B) (46 µL, 0.42 mmol) and the N,N-dimethylaniline (105 µL, 0.83 mmol), with a ratio of A/(N,N)dPH ) 1:2 mol/mol, in 25 mL of xylene and 1.0 g of HY host. After 18 h in reflux and continuous Soxhlet extraction with CH2Cl2 (12 h), a green solid sample is obtained. Synthesis of [bis(N,N)dPhCC]+@HY. To a solution of trans2-pentenal (C) (41 µL, 0.42 mmol) and the N,N-dimethylaniline (105 µL, 0.83 mmol), with a ratio of A/(N,N)dPH ) 1:2 mol/ mol, in 25 mL of xylene was added 1.0 g of HY. The suspension

was stirred for 12 h at the reflux temperature. After Soxhlet extraction for 12 h with CH2Cl2, a pale gray solid sample is obtained. Characterization Procedures. The quantitative chemical analysis (Si, Al, and Na) has been carried out by inductively coupled plasma atomic emission spectrometry (ICP-AES) using a Philips ICP PU 7000 spectrometer. Chemical analysis of C, H, and N were carried out on a Leco CHNS-932 analyzer. The textural characterization of the samples was based on the N2 adsorption isotherms, determined at 77 K using a Quantachrome Instruments Nova 4200e apparatus. The samples were previously outgassed at 423 K under vacuum. The micropore volumes (Vmicro) and mesopore surface areas (Smeso) were calculated by the t-method. Surface areas were calculated by applying the BET equation. Phase analysis was performed by XRD using a Philips PW1710 diffractometer. Scans were taken at room temperature in a 2θ range between 4 and 80°, using Cu KR radiation. Scanning electron micrographs (SEM) were collected on a LEICA Cambridge S360 scanning microscope equipped with an EDS system. To avoid surface charging, prior to the analysis, the samples were coated with gold in vacuum, using a Fisons Instruments SC502 sputter coater. Room-temperature Fourier transform infrared (FTIR) spectra of the solid samples in KBr pellets were measured using a Bomem MB104 spectrometer in the range of 4000-500 cm-1 by averaging 20 scans at a maximum resolution of 4 cm-1. The electronic UV/vis absorption spectra from the suspension of the samples in nujol were recorded using a Schimadzu UV/2501PC spectrophotometer in quartz cells at room temperature. The fluorescence studies were carried out in a SPEX Fluorolog-2 spectrophotometer with the F2121 configuration. Results and Discussion The chromophores were assembled inside the void space of the zeolite from the reaction of the different aldehydes, 3-phenyl2-propynal (A), trans,trans-2,4-hexadienal (B), and trans-2pentenal (C), with the N,N-dimethylaniline (Scheme 1). The ship-in-the-bottle synthesis was successfully achieved by condensation of the different aldehydes with electron-rich aromatic compounds, catalyzed by the acid sites of large pore zeolites.22–27 Acid zeolites are suitable catalysts for the reaction of aldehydes with arenes,10,32,33 and on the basis of this property, the chromophore syntheses were carried out in the presence of HY, which has strong acidity.29 In general, the ship-in-the-bottle approach requires that the formed product be larger than the cavity to avoid leaching. In this case, however, this is not necessary because the final cation chromophore is stabilized by the negative charge of the zeolite framework.9,10,13 As an

Encapsulation of Cation Chromophores in Y Zeolite

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SCHEME 2: Ship-in-the-Bottle Synthesis of [bis(N,N)dPhCA]+@HY

example, the ship-in-the-bottle synthesis of the [bis(N,N)dPhCA]+@HY chromophore is presented in Scheme 2. In this synthesis, when a solution of 3-phenyl-2-propynal and N,N-dimethylaniline in xylene was heated at reflux temperature in the presence of the H+ form of zeolite HY, the host developed the violet color characteristic of the corresponding cation chromophore stabilized by the negative charge of the framework zeolite.9,10,13 Moreover, the extracts from the continuous Soxhlet extraction with CH2Cl2 were colorless, whereas the solid did not lose its color. Figure 1 shows a photograph of the chromophores obtained in nujol. The suspensions are homogeneous and transparent. The obtained solid chromophores were fully characterized by powder X-ray diffraction (XRD), chemical analysis (CA), scanning electron microscopy (SEM), N2 adsorption isotherms, and spectroscopic methods (FTIR and UV/vis). Preservation of the faujasite zeolite structure of the samples was monitored by X-ray powder diffraction. The powder XRD diffraction patterns of HY and the samples were recorded at 2θ values between 5 and 60°. All samples exhibited the typical and similar pattern of a highly crystalline faujasite zeolite structure. No variation was observed in the characteristic peaks of HY zeolite after syntheses of the chromophores. The relative crystallinity of HY and the samples was estimated by comparing the intensities with NaY as a standard sample (100% crystalline). The total intensities of the six peaks assigned to (331), (511), (440), (533), (642) and (555) reflections were used for the comparison according to the ASTM D 3906-80 method. HY and all samples present approximately 70% crystallinity (Table 1).

Figure 1. Photograph of [bis(N,N)dPhCA]+@HY (A), [bis(N,N)dPhCB]+@HY (B), and [bis(N,N)dPhCC]+@HY (C) suspensions in nujol.

The chemical analyses were carried out on the chromophore samples. The bulk Si/Al ratio was determined by inductively coupled plasma emission spectrometry (ICP-AES), and the framework Si/Al ratio by XRD was obtained from the calculated unit cell parameters using the Breck and Flanigen equation.34 Table 1 summarizes the chemical analysis and XRD results of the prepared chromophores. The difference between the Si/Al ratios determined by XRD and those determined by chemical analysis indicates an irregular distribution of silicon and aluminum throughout the zeolite structure. The framework Si/Al ratio (XRD) is higher than the bulk Si/Al ratio (CA), indicating the presence of extraframework alumina species (EFAL).35 None of the Si/Al ratios changed after chromophore encapsulation, indicating that dealumination does not occur during the in situ procedures. These results confirm that the preparation method used in this work does not modify the zeolite structure.18,36 The guest C/N ratio obtained by chemical analysis is similar to the theoretically expected C/N ratio, indicating the presence of chromophores inside the zeolite. The morphology of the samples was determined by SEM analysis. Figure 2 shows the field emission scanning electron micrographs of HY and the [bis(N,N)dPhCB]+@HY chromophore. From SEM micrographs, no morphological changes were observed on the surface upon encapsulation of the chromophores, and it is clear that the chromophore sample has well-defined crystals. The SEM results confirmed that the continuous Soxhlet extraction is a suitable method for removing all residual reagents and the chromophore species physically adsorbed on the external surface of the zeolite. Energy-dispersive X-ray analysis plots support this conclusion as no nitrogen content was detected on the spotted surface of the [bis(N,N)dPhCB]+@HY sample. This is in agreement with our claim that the synthesis of the chromophore is carried out within the host cavity and is stabilized there by the negative charge of the HY. The nitrogen adsorption equilibrium isotherms at 77 K for HY and the chromophore samples are illustrated in Figure 3. The N2 adsorption isotherms for all samples are of the Type-I isotherm, according the IUPAC classification, which is typical of solids with a microporous structure.37 The shapes of both adsorption and desorption isotherms of the chromophore samples were very similar to those of the host, which means that the encapsulation process does not significantly modify the original porosity. In addition, slight hysteresis loops were also observed, suggesting some degree of mesoporosity. After encapsulation, the adsorbed amounts are lower for all chromophore samples. The micropore volumes (Vmicro) and mesopore surface areas (Smeso) were calculated by the t-method, and the total surface areas were calculated by applying the BET equation (SBET). The

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TABLE 1: Structural Characterization of HY and the Chromophore Samples a

Si/Al Si/Alb crystallinity (%)c C (%)d N (%)d C/Ne

HY

[bis(N,N)dPhCA]+@HY

[bis(N,N)dPhCB]+@HY

[bis(N,N)dPhCC]+@HY

2.80 4.05 77

2.83 4.15 65.5 6.120 0.581 10.53 (10.72)

2.80 4.19 62.5 3.516 0.322 10.92 (9.43)

2.84 4.20 63.5 1.846 0.202 9.14 (9.01)

a Total Si/Al ratio determined from ICP-AES. b Framework Si/Al ratio determined from XRD, from NAl ) 115.2 (a0 - 24.191), where NAl is the framework aluminum number and a0 is the cell parameter. c Determined from XRD analysis. d Carbon and nitrogen from chromophores obtained by elemental analysis. e Values in parentheses refer to the theoretical ratios of C/N (w/w) in the chromophore.

Figure 2. Scanning electron micrography (SEM): HY (a) and [bis(N,N)dPhCB]+@HY (b) with the same resolution (5000×).

Figure 3. Nitrogen adsorption-desorption equilibrium isotherms at 77 K of the samples: ([) HY, (9) [bis(N,N)dPhCA]+@HY, (•) [bis(N,N)dPhCB]+@HY, and (2) [bis(N,N)dPhCC]+@HY.

mesopore volume (Vmeso) was calculated as the difference between the total pore volume for P/P0 ) 0.986 (VP/P0)0.986) and the micropore volume. These values are summarized in Table 2. The isotherms from the chromophore samples are all quite similar, showing that the encapsulation of the chromophores led to a decrease of 21.5% (chromophore B), 23.0% (chromophore C), and 32.0% (chromophore A) in the adsorption capacity of the material. These results are compatible with the reduction of the microporosity as a consequence of the encapsulation of the studied chromophores due to different sizes of the chromophores (CA is a derivate from an aromatic aldehyde, whereas the others are from the aliphatic aldehydes). IR spectroscopy also confirmed the formation of chromophores within HY. All of the chromophore samples’ spectra are dominated by the strong absorption characteristic of the host. The broad band at 3700-3300 cm-1 is attributed to surface hydroxyl groups, and bands corresponding to the lattice vibrations are observed in the spectral region between 1300 and 450 cm-1.19,38,39 In addition to these strong bands originating from the host, the FTIR spectra for the samples show the bands in

the 1600-1200 cm-1 region where HY does not absorb, and they are attributed to the presence of the chromophores. This means that the zeolite framework does not interfere with the IR absorption of the incorporation guests. The intensities of the bands of the chromophores are weak because of their low concentration in the host. In the region where the zeolite framework does not absorb (1600-1200 cm-1), the band at 1370 cm-1 typical of the formation of the cation species13 was observed for all chromophores. The spectrum of [bis(N,N)dPhCA]+@HY shows bands at 1520, 1513, 1495, 1455, and 1369 cm-1 that are attributed to the presence of the chromophore A. Similar spectra are observed for the aliphatic chromophores: the bands at 1562, 1535, 1513, 1446, 1425, and 1373 cm-1 for [bis(N,N)dPhCB]+@HY and 1560, 1513, 1447, 1425, and 1371 cm-1 for [bis(N,N)dPhCC]+@HY. The UV/vis absorption spectra of HY zeolite and the chromophore samples present a tool to study the encapsulation of the chromophores in the host zeolite. These are shown in Figure 4. The UV/vis absorption spectra of all the chromophores display bands that are not shown in the spectrum of the host (Figure 4, spectrum a). The spectrum of [bis(N,N)dPhCA]+@HY (Figure 4, spectrum b) measured in nujol at room temperature exhibited three bands at 325, 544, and 613 nm. However, the spectra of [bis(N,N)dPhCB]+@HY and [bis(N,N)dPhCC]+@HY (Figure 4, spectra c and d) show two bands, respectively, at 324 and 604 nm and 322 and 598 nm. The band at lower energy is due to a delocalization of the π electrons of the carbocation species, and the 324 nm band belongs to the π delocalization in the aryl groups. The additional band observed at 544 nm in chromophore A is assigned to the π delocalization within the alkynyl group. The shift observed in the band at lower energy attributed to the carbocation species is related to increase of the conjugation system in the chromophore structures (Scheme 1). These systems have strong resemblance to 4,4′-bis(dimethylamino)diphenyl carbidol that is known to have significant color changes when complexed with macrocyclic host molecules,

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TABLE 2: Textural Properties for HY and Chromophore Samples 2

SBET (m /g) Vmicro (cm3/g)a Smeso (m2/g)a VP/P0)0.986 (cm3/g) Vmeso (cm3/g) a

HY

[bis(N,N)dPhCA]+@HY

[bis(N,N)dPhCB]+@HY

[bis(N,N)dPhCC]+@HY

665 0.302 25 0.349 0.047

411 0.199 17 0.237 0.038

497 0.200 25 0.274 0.074

490 0.208 33 0.270 0.062

From the t-plot.

Figure 4. UV/vis spectra of the samples: (a) HY, (b) [bis(N,N)dPhCA]+@HY, (c) [bis(N,N)dPhCB]+@HY, and (d) [bis(N,N)dPhCC]+@HY.

which stabilize the cation, shifting the reaction equilibrium toward the ionic species.40 The optical properties of the chromophores were studied by fluorescence spectroscopy. Excitation and emission spectra of all chromophores incorporated in HY zeolite were obtained in the form of a suspension using nujol as a solvent. Because this solvent shows some emission in the range of wavelengths used for excitation, the results were always compared with neat solvent to exclude misleading results. All chromophores present a similar spectral behavior with a strong absorption band around 300 nm and subsequent emission at 345 nm. Zeolites do not absorb light in the region studied, whereas most organic molecules do,41 so the bands result only from the emission of the guest synthesized in the zeolitic structure. For both [bis(N,N)dPhCB]+@HY and [bis(N,N)dPhCC]+@HY chromophores, an emission band was detected with a maximum around 350 nm when exciting in the 320-330 nm region. Another emission band was also be detected at 627 and 622 nm when exciting at 608 and 604 nm, respectively. These emission bands result directly from the carbocation inside in the host. Figure 5 shows the corresponding emission and excitation spectra recorded for the [bis(N,N)dPhCB]+@HY. For the chromophore [bis(N,N)dPhCA]+@HY, the fluorescence spectra obtained are complex; see Figure 6. An acceptable fit of the emission spectra (λexc ) 538 nm) is obtained using three Lorentzians with maxima at 589.6 ( 0.8, 627.3 ( 0.9, and 742.0 ( 0.3 nm. When exciting at 609 nm, different ratios between these bands are obtained (Figure 7). In this case, the band around 600 nm should correspond to the emission of the alkynyl group from the chromophore. The bands around 624 and 740 nm are attributed to the emission of the bis(dimethylaminophenyl) carbocation part and the increased π conjugation in the chromophore structure. The successful ship-in the-bottle synthesis of the chromophores inside the cavities of HY zeolite and the presence of

Figure 5. Normalized excitation (---), λem ) 630 nm, and emission (s), λexc ) 606 nm, spectra of [bis(N,N)dPhCB].

Figure 6. Emission and excitation (λexc ) 538 nm) spectra of [bis(N,N)dPhCA]+@HY. Lorentz models with maxima at 589.6 (a), 627.3 (b), and 742.0 (c) nm.

significant host-guest interactions with the carbocation species were confirmed by the fluorescence anisotropy studies. Fluorescence anisotropy measurements were performed by exciting the dyes with polarized light in the absorption region and analyzing the polarization of the emitted light.42 In the cationic emission region, these measurements reveal relatively high values for r, around 0.3, for [bis(N,N)dPhCB]+@HY and [bis(N,N)dPhCC]+@HY and, around 0.1, for [bis(N,N)dPhCA]+@HY (Figure 8), which indicates that these chromophores have a limited ability to rotate. These values can result either from the entrapment in the zeolite cavities or from the direct interaction with the zeolite. The differences in the observed anisotropy values for the chromophores are in agreement with the reduction of the microporosity as a consequence of the encapsulation, taking into account the different sizes of these chromophores (Figure 3). The different structures of the chromophores synthesized in cavities of the zeolite are the key for the limited movements of

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Figure 7. Normalized emission spectra of [bis(N,N)dPhCA]+@HY with excitation at λexc ) 538 nm (s) and λexc ) 606 nm (s).

Figure 8. Anisotropy emission spectra of [bis(N,N)dPhCA]+@HY.

the chromophores. As described in Scheme 1, CA is derived from an aromatic aldehyde and the others originate from the aliphatic aldehydes. Conclusions In conclusion, chromophores derived from different aldehydes, 3-phenyl-2-propynal (A), trans,trans-2,4-hexadienal (B), and trans-2-pentenal (C), with the N,N-dimethylaniline have been synthesized by aldolic condensation into HY zeolite. Largepore sized HY is a very convenient zeolite as a host for the generation of the stable carbocation species with high purity. The relative sizes of the guests as a nanostructure and the free space available in the host are important keys to enhancing the photochemical properties of these materials. The present results with these host-guest materials are very encouraging and warrant further research on heterogeneous photosensitizer applications. Acknowledgment. We thank Dr. A. S. Azevedo (Departamento de Cieˆncias da Terra, Minho University) for collecting the powder diffraction data. This work was supported by the Centro de Quı´mica (University of Minho, Portugal) and by Fundac¸a˜o para a Cieˆncia e Tecnologia (FCT-Portugal), under program POCTI-SFA-3-686. References and Notes (1) Muller, M.; Zentel, R.; Maka, T.; Romanov, S. G.; SotomayorTorres, C. M. Chem. Mater. 2000, 12, 2508–2512. (2) Yang, P. D.; Wirnsberger, G.; Huang, H. C.; Cordero, S. R.; McGehee, M. D.; Scott, B.; Deng, T.; Whitesides, G. M.; Chmelka, B. F. Science 2000, 287, 465–467.

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