Effect of Fluorine and Molecular Charge-State on the Aggregation

Article pubs.acs.org/JPCC

Effect of Fluorine and Molecular Charge-State on the Aggregation Behavior of (S)‑(−)‑N‑Benzylpyrrolidine-2-methanol Confined within the AFI Nanoporous Structure Luis Gómez-Hortigüela,*,† Fernando López-Arbeloa,‡ Carlos Márquez-Á lvarez,† and Joaquín Pérez-Pariente† †

Instituto de Catálisis y Petroleoquímica-CSIC, C/Marie Curie 2, 28049 Cantoblanco, Madrid, Spain Departamento de Química Física, Universidad del País Vasco-EHU, Apartado 644, 48080 Bilbao, Spain

‡

S Supporting Information *

ABSTRACT: The effect of the molecular charge-state and of the presence of fluorine atoms on the aggregation behavior of (S)-(−)-N-pyrrolidine-2-methanol has been studied by fluorescence and FTIR spectroscopies in aqueous solution and when confined within the nanoporous AFI structure during crystallization or after gas-phase adsorption. Results show that a higher aggregation is achieved via inclusion of the guest molecules in postsynthetic gas-phase adsorption treatments compared to what occurs during crystallization. In the former case, neutral species are incorporated within the aluminophosphate AFI framework, as evidenced by FTIR, which results in a high aggregation because of the lack of electrostatic repulsions. In contrast, a lower aggregation is observed when the organics are occluded during crystallization, which in turn is dependent on the presence of heteroatoms. In the undoped aluminophosphate systems, FTIR results show that the molecules are incorporated mainly as neutral species, which leads to a higher aggregation. On the contrary, on Mg- and Zn-doped systems, FTIR shows that the molecules are protonated; interestingly, this results in a lower aggregation on Mg-doped systems. However, a higher aggregation is observed for Zn-doped systems despite the charged molecular state, suggesting that the chemical nature of the dopant does influence the aggregation behavior. Fluorine in ortho- or meta-positions of the aromatic ring leads to a partial deaggregation of the molecules in solution and within the nanoporous structures, leading to a preferential incorporation of monomers within the AFI framework. This is probably due to an inductive effect caused by fluorine, which withdraws electron density from the aromatic ring, this being the driving-force for this type of aggregation.

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INTRODUCTION Nanoporous materials represent an important class of materials with a great impact in the industry because of their molecular sieving, ion-exchanging, and catalytic properties,1 which exploit the molecular dimensions and the crystalline nature of the nanoporous structure to discriminate between molecules with very subtle steric differences. Nanoporous aluminophosphates (AlPOs) were first discovered by Wilson et al. in 19822 and soon provided a diversity of structural types comparable to that of the previously known aluminosilicate-based zeolites.3 In AlPO materials, the network is composed of AlO4 and PO4 tetrahedral units arranged in a strict alternation, giving place to neutral frameworks. Both Al and P ions can be isomorphically replaced by heteroatoms (dopants) through different substitution mechanisms. The most common one is the replacement of Al3+ by a divalent metal, such as Mg2+ or Zn2+, that imparts a negative charge to the framework, usually charge-balanced by cationic organic molecules occluded in the nanoporous structure. Nanoporous materials are synthesized through hydrothermal methods, which usually requires the addition of an organic molecule to direct the crystallization of a certain nanoporous © XXXX American Chemical Society

structure, and so they are called structure directing agents (SDAs).4−6 Recently, a new concept in structure direction of nanoporous materials has been proposed by us7−9 and by Corma et al.,10 consisting in the use of supramolecular selfassembled molecules as SDAs in order to get larger entities able to direct the crystallization of more-open frameworks. This type of supramolecular assembly can be achieved using aromatic molecules, which tend to self-assemble with their aromatic rings parallel to each other stabilized through π−π type interactions, being this molecular association the actual SDA of the nanoporous framework. One molecule we have been recently studying that provides a rich supramolecular chemistry when structure-directing the crystallization of a nanoporous material is (S)-(−)-N-benzylpyrrolidine-2-methanol (bpm), applied to the synthesis of the AFI structure. This is a nanoporous aluminophosphate whose structure is composed of onedimensional 12 membered-ring (MR) channels with a diameter of 7.3 Å.3 Received: January 31, 2013 Revised: April 2, 2013

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dx.doi.org/10.1021/jp401135f | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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B. Hydrothermal Synthesis of AFI Materials. The AFI nanoporous materials were prepared in the same way as in our previous works.16,20 AlPO-5 was prepared from gels having a molar composition of 1:1:1:40 SDA/P2O5/Al2O3/H2O, whereas MeAPO-5 materials (where Me stands for Zn or Mg) were prepared from gels with compositions of 1:1:0.89:0.22:40 SDA/P2O5/Al2O3/MeO/H2O. The amount of MeO in the synthesis gel was set to the maximum amount of negative charges that can be balanced by cationic SDA molecules, assuming the maximum loading of 1.33 SDAs per AFI u.c. when they arrange as dimers, as proposed in our previous computational study.9 Detailed preparation of the synthesis gels can be found in our previous publication.20 The respective gels were introduced into 60 mL Teflon lined stainless steel autoclaves and heated statically at 150 °C for 24 (MeAPO-5) or 72 (AlPO-5) h. The resulting solids were separated by filtration, washed with ethanol and water, and dried at 60 °C overnight. For the experiments involving postsynthetic gas-phase adsorption of the guest molecules, an (undoped) AlPO-5 sample was obtained by using triethylamine (TEA) as SDA (AlPO-5-TEA), from a gel with the same molar composition as indicated above. Calcination of this sample was carried out by heating at 550 °C under an inert atmosphere of N2 for 1 h, followed by 5 h at the same temperature under O2 atmosphere. Complete removal of the organic molecule was assessed by TGA. C. Adsorption of the Organic Molecules within AlPO5-TEA. Adsorption of bpm and its fluorinated derivatives was carried out in vapor phase, after an activation stage. The calcined AlPO-5-TEA sample was preheated at 200 °C for 2 h under a N2 stream (N2 flow ∼80 mL/min) to eliminate all the adsorbed water. During the adsorption experiments, the temperature of the AFI sample was maintained at 200 °C in order to prevent adsorption of water. Then, for the adsorption of the organic molecules, the N2 stream was bubbled through a vessel containing a liquid sample of the corresponding molecule (bpm, mFbpm or oFbpm) preheated at 80 °C; the organic molecules were thus carried to the AlPO-5 sample in the N2 stream. The amount of organic molecule adsorbed within the AFI structure was controlled by adjusting the time of adsorption (between 1 and 24 h). After adsorption of the organic molecule for the corresponding time, the AFI samples were kept at 200 °C under the N2 stream (without passing through the organic-containing vessel) for 2 h in order to remove the organic that could be adsorbed on the external surfaces of the AlPO-5 material. D. Characterization of the Solid Samples. The nature of the crystalline phases obtained was determined by X-ray diffraction (Seifert XRD 3000P diffractometer, Cu Kα radiation). The organic content of the samples was studied by chemical CHN analysis (Perkin-Elmer 2400 CHN analyzer) and thermogravimetric analysis (TGA) (Perkin-Elmer TGA7 instrument). The solid materials were also studied by solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR). These MAS NMR spectra were recorded with a Bruker AV 400 WB spectrometer, using a BL7 probe for 13C and a BL4 probe for 31P. 1H to 13C Cross-polarization spectra were recorded using π/2 rad pulses of 4.5 μs for 1H, a contact time of 5 ms and a recycle delay of 3 s. For the acquisition of the 13C spectra, the samples were span at the magic angle (MAS) at a rate of 5−5.5 kHz. For 31P, π/2 rad pulses of 4.25 μs and recycle delays of 80 s were used; these spectra were recorded while spinning the samples at ca. 11 kHz.

In a recent work we demonstrated the efficiency of fluorescence spectroscopy in studying the aggregation behavior of aromatic SDAs within nanoporous structures.11−13 This study allowed us to understand the supramolecular chemistry involved in the structure direction of aromatic molecules during the synthesis of the AFI structure (AlPO-5). In the present work we further explore the supramolecular behavior associated with this type of molecule by analyzing (i) the influence of the molecular charge-state and (ii) the effect of introducing fluorine atoms in the aromatic ring in ortho or meta positions. In recent years, our group has developed a research line in which fluorine atoms have been introduced in the SDA molecules in order to study the effect of the modification of the chemical nature of the host−guest interactions between SDAs and the nanoporous frameworks they direct.7,14−19 We have observed several effects of these F atoms on the structure-directing efficiency of the SDA molecules: (i) F can improve the structure-directing ability by increasing the nonbonding electrostatic interactions due to the presence of polarized C−F bonds, (ii) F can influence the ability to form supramolecular aggregates, and (iii) F can favor the interaction with other ions, giving place to cooperative structure-directing effects. Therefore, it seems that F atoms also develop a rich supramolecular chemistry. Due to the important effects that these F atoms can impart on the mode of structure-direction of the organic SDA molecules, we now want to study if they can also affect their aggregation behavior when occluded within nanoporous materials. For this purpose, we have succeeded in synthesizing AlPO-5, ZnAPO-5, and MgAPO-5 with the fluorinated derivatives of (S)-(−)-N-benzylpyrrolidine-2-methanol (bpm) in ortho and meta positions (para-fluorinated derivatives were not studied since previous results showed that these isomers do not tend to give AlPO-5);7 we also managed to load by gas-phase postsynthetic adsorption treatments different amounts of the guest molecules in the neutral state within the calcined AFI structure. We then study by fluorescence spectroscopy the formation of aggregates as a function of the presence of fluorine and as a function of the charge-state (studied by FTIR spectroscopy).

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EXPERIMENTAL DETAILS A. Synthesis of the Organic Molecules. Synthesis of pure (S)-(−)-N-benzylpyrrolidine-2-methanol (bpm) and the meta- and ortho-fluorinated derivatives, (S)-(−)-N-metafluorobenzylpyrrolidine-2-methanol (mFbpm) and (S)-(−)-N-orthofluorobenzylpyrrolidine-2-methanol (oFbpm), was carried out starting from the commercial amino acid L-proline (Sigma− Aldrich).20 Its acid function was reduced with LiAlH4 (Sigma− Aldrich) in refluxing THF to give the corresponding amino alcohol, L-prolinol (1 proline: 1.5 LiAlH4). L-Prolinol was then alkylated with benzyl chloride (Sigma−Aldrich), or the corresponding meta- or ortho-fluorobenzyl chloride (Avocado), in ethanol (90 °C, 24 h) in the presence of potassium carbonate. The tertiary amines were extracted with chloroform and purified by vacuum distillation. The purity of the amines was assessed by thin layer chromatography (hexane/ethyl acetate as solvent) and chemical analysis (Table 1, Supporting Information). Aqueous solutions of the molecules were prepared by adding equimolar amounts of the corresponding organic amine and HCl. In this way, 10−3, 10−2, 10−1, and 1 M aqueous solutions of the corresponding chlorides were obtained and studied by fluorescence spectroscopy. B



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Figure 1. Height-normalized fluorescence emission spectra of bpm-H+ chloride (top-left), mFbpm-H+ chloride (top-right), and oFbpm-H+ chloride (bottom) aqueous solutions at different concentrations. The excitation wavelength was 260 nm.

282 nm, which is assigned to the emission from the aromatic system of the monomers due to the low concentration at which this band predominates. An increase of the concentration to 10−1 M in the bpm-H+ solution results in the appearance of a shoulder at longer wavelengths (around 325 nm), which becomes the predominant band as the concentration rises to 1 M. The occurrence of this new band with the increase in the concentration of the molecules led us to assign it to the fluorescent emission from molecules in an aggregated state. At 1 M concentration, most of the bpm-H+ cations are aggregated. A similar behavior is observed for the meta- and orthofluorinated derivatives, but in this case the extent of aggregation is different. The wavelengths of the maximal emissions for the monomer and aggregates are practically independent of the presence of F atoms. At 1 M concentration, both the monomer and the dimer coexist in the mFbpm-H+ case, whereas even at this high concentration the monomer is still predominant for the oFbpm-H+ molecule. Considering that the presence of F atoms does not affect the fluorescence efficiency of monomer over aggregates and the ratio of monomer/aggregates excited at 260 nm, these results indicate that the aggregation trend of the protonated molecules in aqueous solution follows the decreasing order bpm-H+ ≫ mFbpm-H+ > oFbpm-H+ and hence clearly evidence a notable effect of the presence of fluorine atoms on the supramolecular behavior of this type of molecules. B. Aggregation of Molecules Occluded by Adsorption. We then analyzed the aggregation of the molecules when they were occluded by adsorption of the molecules in gasphase. In this case, neutral molecules are incorporated within the AlPO-5 structure, as AlPO-5 has no acidity to protonate the

The aggregation state of the molecules in solution and in the solid samples was studied by fluorescence spectroscopy. Liquid and solid state UV−visible fluorescence excitation and emission spectra were recorded in a RF-5300 Shimadzu fluorimeter. The fluorescence spectra were registered in the front-face configuration by a solid sample holder in which the samples were oriented 30 and 60° with respect to the excitation and emission beams, respectively. Liquid solutions of the SDAs samples were placed in 1-mm pathway quartz cells, whereas the fluorescecence spectra of the solid samples were recorded by means of thin films supported on glass slides ellaborated by solvent evaporation from a dichloromethane suspension of the solid AFI samples. Fourier-transform infrared spectra were recorded in the transmission mode using a Thermo Nicolet Nexus 670 FTIR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. The powder samples were pressed into thin selfsupporting wafers with a thickness in the range 5−8 mg/cm2. The pellets were mounted on a stainless steel sample holder within a glass cell with calcium fluoride windows and dehydrated at 150 °C for 4 h under dynamic vacuum (10−3 Pa) prior to analysis. The spectra were taken at room temperature with a resolution of 4 cm−1 coadding 250 scans and using Happ-Genzel apodization.

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RESULTS A. Aggregation of Protonated Molecules in Aqueous Solution. Figure 1 displays the fluorescence spectra of aqueous solutions of the hydrochlorides of the three molecules at increasing concentrations. At low concentrations (lower than 10−2 M), a unique fluorescence band is observed centered at C



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Figure 2. Height-normalized solid state fluorescence emission spectra of AlPO-5 samples after adsorption of oFbpm (left) or mFbpm (right). The excitation wavelength was 260 nm. The dashed region was manually removed from the spectra since a low-intense emission from the sample-holder is observed.

Table 1. CHN Analyses of the Different Samples (Given in Weight Percentage) sample

C

H

N

A-bpm A-mFbpm A-oFbpm Zn-bpm Zn-mFbpm Zn-oFbpm

9.93 9.56 9.44 10.53 10.17 10.08

1.56 1.48 1.43 1.88 1.69 1.71

1.09 1.04 1.14 1.11 1.10 1.08

C/Na 10.7 10.8 9.7 11.1 10.8 10.9

(12) (12) (12) (12) (12) (12)

% organicb

Me2+/u.c.c

13.2 (1.20) 13.9 (1.16) 13.7 (1.14) 14.0 14.8 14.6

1.25 1.20 1.20

a Theoretical C/N ratio in brackets. bSDA molecules per unit cell in brackets, calculated with the residue observed in the TGA. A: AlPO-5; Zn: ZnAPO-5; note: organics were not completely removed in the TGA of Zn-APO samples even at 1000 °C; hence, the organic content per unit cell for these samples could not be estimated. c: atoms per unit cell, as calculated from 31P NMR.

Worth is noting the stronger presence of dimers at this low concentration compared to the oFbpm case. An increase in the concentration (AlPO-mFbpm-ads-high, red line) involves a decrease of the monomer in favor of the dimer incorporation, hence confirming the assignment of the fluorescence bands. In contrast, only the emission band in the range between 400 and 500 nm was observed for AlPO samples loaded with bpm regardless of the concentration, even at very low concentrations (where just traces of organic are observed), suggesting that only dimers are present in the AFI samples.11 Following the trend observed in solution, results of this section clearly indicate that the aggregation trend of the molecules within the AFI channels incorporated by adsorption follows the same decreasing order than protonated molecules in solution: bpm ≫ mFbpm > oFbpm, evidencing again the effect of the presence of fluorine in the aromatic ring. C. Aggregation of Molecules during Crystallization. We then analyzed the incorporation of the molecules within the AFI framework during crystallization, i.e., when they act as structure-directing agents. C.1. Synthesis of AlPO-5. The XRD patterns of the AlPO materials obtained with the three SDA molecules indicate that pure AlPO-5 was obtained in all cases, regardless of the presence of the fluorine atoms in the SDA (Figure 1, Supporting Information). The integrity of the SDA molecules occluded within the AlPO-5 materials was evidenced by 13C NMR (Figure 2, Supporting Information). CHN elemental analyses data are listed in Table 1; the C/N ratios observed were very similar to those of the isolated molecules (12), providing further evidence for the integral incorporation of the SDA molecules. TG analyses of the samples (Figure 3) show a strong desorption at temperatures below 200 °C, correspond-

molecules (see FTIR results, section D below). Different amounts of the guest molecules were loaded by adjusting the adsorption time (between 1 and 24 h) to load 1−2 wt % (samples referred to as low) or 7−9 wt % (samples referred to as high), as determined by TGA. Fluorescence emission spectra of AlPO-5 samples loaded with different amounts of mFbpm and oFbpm after adsorption are shown in Figure 2 (the corresponding spectra for bpm-AFI systems were reported in a previous publication).11 It can be clearly observed that for samples loaded with oFbpm (left) at low concentrations (AlPO-oFbpm-ads-low, ∼1.5% of organic matter, blue line) the emission band centered at 288 nm predominates, which can be directly assigned to oFbpm monomers occluded within the AFI structure. However, the monomer emission is accompanied by another broader band in the range between 400 and 500 nm; such a band becomes predominant when the concentration is increased (AlPOoFbpm-ads-high, ∼7.5% of organic, red line). As previously demonstrated for bpm, the high wavelength of this band and its concentration dependence allowed us to assign this band to oFbpm aggregates within the AFI structure. The confinement effect might be responsible for the red-shift observed for the emission band of the aggregated species occluded within the AFI framework compared to those in solution, due to the interaction of the aromatic rings with the channel walls but also to a stronger π−π interaction between the aromatic rings in the dimers due to a closer distance when confined within the AFI channels. Likewise, the same behavior is observed for the AFI samples loaded with mFbpm (Figure 2, right). At low concentrations (AlPO-mFbpm-ads-low, blue line), both emission bands at 288 nm (monomers) and at 400−500 nm (dimers) are observed. D



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sample obtained with the nonfluorinated SDA (1.20 bpm/u.c.); the presence of F in meta or ortho positions led to a decrease of the organic incorporation (1.16 and 1.14 molecules/u.c., respectively). This different organic content is in line with the higher water content observed in the TG analyses for the samples obtained with mFbpm and oFbpm, evidencing the incorporation of water molecules to fill the void space left over by the occlusion of a smaller amount of organic molecules. C.2. Synthesis of ZnAPO-5. We then studied the incorporation of Zn in the AFI structure. XRD patterns of these samples evidenced the crystallization of ZnAPO-5 as a pure phase in all cases (Figure 3, Supporting Information). CHN analyses of the samples (Table 1) showed the integral incorporation of the molecules within the Zn-doped solids since the C/N ratios were close to the theoretical value (12); this was further evidenced by 13C NMR (Figure 2, Supporting Information). In this case, TG studies did not allow for an accurate estimation of the number of molecules per unit cell since the weight loss did not finish even at temperatures as high as 1000 °C. 31 P NMR was employed to analyze the incorporation of Zn atoms within the AFI network (Figure 4, Supporting Information). The spectra of the ZnAPO-5 samples display two signals at −30 and −22 ppm, assigned to P(4Al) and P(3Al,1Zn) environments, respectively,21 evidencing the incorporation of the Zn cations into the tetrahedral oxide network of the ZnAPO-5 structure, substituting Al cations. Deconvolution of the spectra allowed us to determine the Zn content of the samples embedded in the network (Table 1).20

Figure 3. TGA (solid lines) and DTA (dashed lines) of AlPO-5 samples obtained with bpm (black), mFbpm (blue), and oFbpm (red).

ing to water molecules, which is more intense in the samples obtained with the fluorinated SDAs, indicating a higher water content for these samples. Desorption/combustion of the organic molecules takes place in the 200−800 °C temperature range. The amount of organic molecules occluded during the synthesis of the materials (Table 1) was obtained from the C content determined by chemical CHN analyses and related to the inorganic unit cell (using the TGA residue of the asprepared samples). A higher organic content was found for the

Figure 4. Height-normalized fluorescence emission spectra of the different AFI frameworks. The excitation wavelength was 260 nm. E



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A higher Zn content was observed for the sample obtained with the nonfluorinated molecule (1.25 Zn/u.c.) than those obtained with the fluorinated SDAs (1.20 Zn/u.c. for both samples). Such higher Zn content for the sample obtained with bpm indirectly suggests a higher organic content for this sample, since protonated bpm cations compensate for the negative charge generated by the substitution of Al3+ by Zn2+. C.3. Synthesis of MgAPO-5. Synthesis of MgAPO-5 materials with the three SDAs has been reported elsewhere.22 As in previous cases, the three SDAs led to the crystallization of MgAPO-5 as a pure phase. 13C NMR results demonstrated the resistance of the molecules to the hydrothermal treatment. The organic content of the samples could not be determined by CHN elemental analysis due to an incomplete combustion of the organic matter, possibly associated with the high strength of Brönsted acid sites created by decomposition of the template cations when Mg cations embedded in AlPO networks. The incorporation of Mg into the AFI tetrahedral network (substituting Al) was evidenced by 31P NMR in our previous report:22 deconvolution of the spectra gave in this case Mg contents of 1.09, 1.03, and 0.95 Mg/u.c. for the MgAPO-5 samples obtained with bpm, mFbpm, and oFbpm, respectively, lower than that of the ZnAPO-5 solids. Due to charge-balance considerations, we estimate a similar organic content for these samples, and so a lower organic content than the ZnAPO-5 and AlPO-5 materials, and lower in the samples obtained with the fluorinated SDAs (the organic content should decrease in the order Mg-bpm > Mg-mFbpm > Mg-oFbpm). C.4. Fluorescence Spectroscopy. The aggregation behavior of the SDA molecules occluded within the different AFI materials during crystallization was studied by UV−visible fluorescence spectroscopy. Figure 4 shows the fluorescence emission spectra of the different samples obtained with the three SDA molecules (the excitation wavelength was 260 nm). In general terms, we observe that for all samples, regardless of the dopant present in the structure, the presence of F atoms, both in meta or ortho positions, involves a decrease of the intensity of the fluorescence band corresponding to the dimer, suggesting that F atoms enhance the SDA incorporation as monomers. This observation is repeated in the undoped (AlPO), Zn-doped (ZnAPO), and Mg-doped (MgAPO) materials. The lower dimer incorporation of the samples obtained with the fluorinated SDAs is in line with the lower organic content found for AlPO samples (and associated higher water content), as previously mentioned, possibly due to a higher space-filling efficiency of dimers compared to monomers. If we now compare the samples obtained with the same fluorinated SDA (mFbpm or oFbpm), we observe that the dimer incorporation follows the decreasing order: AlPO > ZnAPO > MgAPO, similar to the results observed for the nonfluorinated SDA.13 D. FTIR Spectroscopy. The FTIR spectra of dehydrated, as-synthesized AlPO-5, MgAPO-5, and ZnAPO-5 obtained with bpm as SDA are compared in Figure 5 with that of calcined AlPO-5 subjected to adsorption of bpm. The spectrum of bpm adsorbed on calcined AlPO-5 (spectrum a) shows the characteristic bands of aromatic and aliphatic C−H bond stretching in the 3100−2850 cm−1 wavenumber range, as well as three abnormally low frequency C−H stretching bands at 2805, 2815 (shoulder), and 2765 cm−1. The latter bands, known as Bohlmann bands, are unambiguous evidence of the presence of the neutral amine in the sample. It has been well

Figure 5. FTIR spectra of adsorbed bpm on calcined AlPO-5 (a) and as-synthesized AlPO-5 (b), MgAPO-5 (c), and ZnAPO-5 (d). Dotted lines indicate the location of Bohlmann bands near 2800 cm−1 and the N−H stretching band around 3150 cm−1.

established that this downshift in frequency of C−H stretching bands is caused by the trans-lone-pair effect, which leads to a lengthening of the C−H bonds trans-periplanar to the N lonepair of electrons23 (Figure 5, Supporting Information). The reported infrared spectra of pyrrolidine and their derivatives show these Bohlmann bands near 2800 cm−1.24,25 Accordingly, we assign bands at 2815, 2085, and 2765 cm−1 to C−H bonds of the methine and the two α-methylene groups of adsorbed neutral bpm. The FTIR spectrum of sample AlPO-5 as-synthesized (Figure 5, spectrum b) shows essentially the same bands as that of bpm adsorbed on calcined AlPO-5 in the C−H stretching region. This result indicates that the as-synthesized undoped material contains the neutral amine trapped within its porous network in a state similar to that obtained by adsorption of the organic compound on the previously calcined solid. However, the spectra of as-synthesized samples MgAPO-5 and ZnAPO-5 (Figure 5, spectra c and d, respectively) show significant differences respect to the former samples. In particular, the spectra of samples MgAPO-5 and ZnAPO-5 present no Bohlmann bands around 2800 cm−1. It has been shown for N-methylpyrrolidine that Bohlmann bands disappear when the lone-pairs become involved in bond formation, either when the amine is adsorbed on a Si surface through the lone pair25 or by protonation when adsorbed on solid acids.26,27 Therefore, lack of Bohlmann bands in spectra c and d would suggest that the amine molecules present in the as-synthesized samples MgAPO5 and ZnAPO-5 are predominantly protonated. This interpretation is supported by the observation that both spectra exhibit an additional broad band in the 3250− 3050 cm−1 range, overlapping with the aromatic C−H stretching bands, and with maximum at ca. 3160 cm−1, which can be attributed to the N−H stretching of the bpm-H+ cation.

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DISCUSSION Our results show the effect of fluorine on the structuredirecting role of these molecules toward the crystallization of the AFI framework. No important effect has been observed on the structure-directing ability toward the undoped (AlPO) nor F



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displays a higher dimer incorporation, although so far it is not clear whether this relationship is in this way or the other way around. In any case, this difference might be related to a different stability and/or distribution of these divalent dopants within the AFI framework, suggesting that Zn-dopants are more susceptible to being incorporated within the AFI framework than Mg.

the Mg- and Zn-containing materials: the three SDAs led to pure-phase AFI structures. The main aim of our work was to study the effect of the presence of F atoms in meta or ortho positions of the aromatic ring on the aggregation behavior of (S)-(−)-N-benzylpyrrolidine-2-methanol in different media and charge-states. We have observed that at the concentration at which crystallization takes places (around 1 M), bpm protonated cations in solution are mostly aggregated, while mFbpm-H+ and oFbpm-H+ cations show a mixture of monomers and aggregates. When occluded within the AFI materials, with the confinement effect provided by the channel walls, a clear difference is observed as a function of the charge-state. During crystallization of Me2+-containing AFI materials, due to the need for charge-compensation of the dopant incorporated, the molecules are predominantly incorporated as protonated ammonium cations, as evidenced by FTIR in Zn- and Mg-doped materials; in contrast, molecules loaded by adsorption on the AlPO-5 materials, which has no strongly acidic protons, are incorporated as neutral species. Besides, FTIR also showed that neutral bpm is occluded during crystallization of the undoped material since in this case there are not negatively charged defects. In consequence, we have observed a strong difference in the aggregation behavior as a function of the charge-state: in all cases (bpm, mFbpm, and oFbpm), neutral molecules show a higher trend to aggregate within the AFI channels than protonated cations included during crystallization. This might be due to the electrostatic repulsion generated either by the positive charge of the species composing the dimer or the negative-charge introduced by the dopants, located close to each other, when protonated cations are occluded during crystallization. On the other hand, the effect of fluorine in meta or ortho positions of the aromatic ring on the aggregation behavior shows a consequent trend regardless of the media (solution or occluded in AFI channels) and charge-state (neutral amine or protonated ammonium cation): fluorine in meta and, especially, in ortho positions clearly leads to a reduction of the incorporation of the molecules as dimers with respect to the nonfluorinated derivative (bpm). This might be due to a rearrangement of the electron density caused by the inductive effect of the F atoms that withdraws electron density from the aromatic ring, decreasing the π−π type interactions which represent the driving force for the formation of dimers; such an effect seems to be more prominent when F locates in the ortho position. A similar electron-withdrawing effect is observed in the particular crystallization pattern of per-fluorinated aromatic molecules.28 In line with our observations for the nonfluorinated SDA,13 we have observed that the undoped AFI materials show a higher incorporation of dimers, which is a consequence of the incorporation of the molecules in the neutral state, as shown by FTIR. In Me2+-containing materials, due to charge-balance considerations, the organic content is related to the incorporation of divalent cations (Mg and Zn). The Zncontaining materials lead to higher dimer incorporation, whereas MgAPO-5 involves a major incorporation of monomers, despite in both cases bpm molecules incorporate predominantly as protonated species, as evidenced by FTIR, to compensate for the negatively charged dopant incorporation. These results evidence that the aggregation behavior is not only a consequence of the molecular-charge state and/or the presence of F but that the chemical nature of the dopant plays a determinant role as well. The Zn-containing material

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CONCLUSIONS The effect of the presence of F atoms in ortho or meta positions on the aggregation behavior of (S)-(−)-N-benzylpyrrolidine-2-methanol has been studied in different media (in solution and confined within the AFI nanoporous structure) and in different charge-states (neutral amine or protonated ammonium cation) by UV−visible fluorescence and FTIR spectroscopies. Our study indicates that neutral molecules, incorporated within the AFI channels by gas-phase adsorption, show a higher trend to form dimers within the confined space of the AFI channels. In contrast, when positively charged molecules are incorporated (after protonation during crystallization of the Me2+-containing materials), the tendency to aggregate is severely hindered, possibly due to the electrostatic repulsion generated between the charged species composing the dimer, which locate very close to each other, and the associated presence of negatively charged defects in the inorganic network. On the other hand, our results show that F in meta and, specially, in ortho positions leads to a reduction of the aggregation of the molecules; this occurs regardless of the media and charge-state. This particular supramolecular behavior caused by the presence of fluorine, the molecular charge-state, and the chemical nature of the dopant might provide a powerful tool for rationally designing and synthesizing host−guest systems with a specific aggregation behavior, which would be useful for instance in preparation of dye-containing systems.

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ASSOCIATED CONTENT

S Supporting Information *

Chemical analysis of the molecules, XRD patterns, 13C and 31P MAS NMR of the solids, and an explanation of the trans-lonepair effect. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +34-915854785. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has recevided funding from the Spanish Ministry of Science and Innovation MICINN (projects MAT2009-13569 and MAT2012-31127) and the European Research Council, under the Marie Curie Career Integration Grant program (FP7-PEOPLE-2011-CIG), Grant Agreement PCIG09-GA-2011-291877. L.G.H. acknowledges the Spanish Ministry of Education and Science for a Juan de la Cierva contract. T. Blasco is acknowledged for the acquisition of the NMR spectra. G



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Effect of Fluorine and Molecular Charge-State on the Aggregation

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