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Langmuir 2005, 21, 890-895
Intercalation and Photophysical Characterization of 1-Pyrenemethylamine in Zirconium Phosphate Layered Materials Ricardo A. Bermu´dez, Yaitza Colo´n, Genaro A. Tejada, and Jorge L. Colo´n* Department of Chemistry, P.O. Box 23346, University of Puerto Rico, Rı´o Piedras, P.R. 00931 Received May 17, 2004. In Final Form: October 5, 2004 The ion exchange of the luminescent probe 1-pyrenemethylamine (PYMA) into zirconium phosphate (ZrP) layered materials has been accomplished. The matrices used were the hexahydrated 10.3 Å phase of ZrP (10.3 Å ZrP, where 10.3 Å represents the interlayer distance) and butylammonium-exchanged ZrP (BAZrP) with an expanded 18.6 Å interlayer distance. The XRPD patterns for the 10.3 Å ZrP after PYMA exchange (PYMA-exchanged ZrP), at high PYMA concentrations, show an increase in the interlayer distance from 10.3 Å in unexchanged 10.3 Å ZrP to 23.5 Å in PYMA-exchanged ZrP, indicating PYMA intercalation. The luminescence spectrum for the PYMA-exchanged ZrP exhibits an excimer band at 458 nm that is absent in the luminescence spectrum of PYMA in aqueous solution at low concentrations. The intensity of the excimer emission increased at low PYMA concentrations. These results are in contrast to experiments using the BAZrP matrix. The XRPD patterns for PYMA-exchanged BAZrP do not show changes in the interlayer distance, which suggests that PYMA is not being intercalated and is only surface bound. The luminescence spectrum for PYMA-exchanged BAZrP exhibits a lower emission intensity in its excimer band, at different PYMA concentrations, compared with the PYMA-exchanged ZrP excimer band. For PYMA-exchanged ZrP, we propose a process in which exchange at low PYMA concentrations occurs at external surface sites with clustering promoting excimer formation followed by exchange at high PYMA concentrations occurring at interior sites reducing excimer formation.
1. Introduction The layered zirconium phosphates and phosphonates have a wide range of applications as ion exchangers, catalysts, solid electrolytes, molecular sieves, and hosts of different intercalating compounds.1,2 Zirconium bis(monohydrogen orthophosphate) monohydrate (Zr(HPO4)2‚ H2O), also called R-zirconium phosphate (R-ZrP), is a layered ion-exchange material capable of binding a variety of metal ions and organic cations in the interlayer region.2-4 The interlayer distance of R-ZrP is 7.6 Å. The packing of the layers creates six-sided zeolitic cavities which are interconnected by openings whose maximum sizes are 2.6 Å.4 Therefore, only spherical ions of around 2.6 Å or less in diameter can be directly intercalated into the cavities.5 Exchange of larger cations can occur with the preintercalation of polar organic molecules, such as alcohols, glycols, and amines, into ZrP, which provides a means of expanding the interlayer separation.2a Expanded forms of zirconium phosphate greatly aid in the exchange of larger cations into the interlayer space.6 One example of an expanded form of ZrP is butylammonium-exchanged ZrP (BAZrP). BAZrP is produced by the direct intercalation of butylammonium cations between the layers of R-ZrP, producing an expanded inter* Corresponding author. Phone: (787) 764-0000 Ext. 2371. Fax: (787) 756-8242. E-mail:
[email protected]. (1) Clearfield, A. Prog. Inorg. Chem. 1998, 47, 371-510. (2) (a) Clearfield, A., Ed. Inorganic Ion Exchange Materials; CRC: Boca Raton, FL, 1982. (b) Alberti, G. Acc. Chem. Res. 1978, 11, 163170. (3) (a) Clearfield, A. Chem. Rev. 1988, 88, 125-148. (b) Clearfield, A.; Stynes, J. A. J. Inorg. Nucl. Chem. 1964, 26, 117-129. (c) Clearfield, A.; Smith, G. D. Inorg. Chem. 1969, 8, 431-436. (4) Troup, J. M.; Clearfield, A. Inorg. Chem. 1977, 16, 3311-3314. (5) Clearfield, A.; Duax, W. L.; Medina, A. S.; Smith, G. D.; Thomas, J. R. J. Phys. Chem. 1969, 73, 3424-3430. (6) (a) Clearfield, A.; Tindwa, R. M. J. Inorg. Nucl. Chem. 1978, 41, 871-878. (b) Rosenthal, G. L.; Caruso, J. J. Solid State Chem. 1991, 93, 128-133.
layer distance of 18.6 Å.6 The butylammonium cations can be subsequently exchanged with large transition metal complexes.6b,7 Recently, Martı´ and Colo´n reported the direct intercalation of tris(2,2′-bipyridine)ruthenium(II) into a ZrPtype framework without preintercalation of an alkylamine.8 These authors used a highly hydrated phase of ZrP with an interlayer distance of 10.3 Å. The 10.3 Å phase of ZrP (10.3 Å ZrP) has six water molecules per formula unit.9 This hexahydrated phase can easily exchange large cations, which are not exchanged into the monohydrated phase.10 Polynuclear aromatic hydrocarbons (PAHs), like pyrene and its derivatives, are well-known luminescent probes, which have been studied in several microenvironments, such as micelles,11 sol-gel,12 cyclodextrin,13 DNA,14 R-zirconium phosphate,15 polymer films,16 and argon ma(7) Clearfield, A.; Tindwa, R. M. Inorg. Nucl. Chem. Lett. 1979, 15, 251-254. (8) Martı´, A.; Colo´n, J. Inorg. Chem. 2003, 42, 2830-2832. (9) Kijima, T. Bull. Chem. Soc. Jpn. 1982, 55, 3031-3032. (10) Alberti, G.; Costantino, U.; Gill, J. S. J. Inorg. Nucl. Chem. 1976, 38, 1733-1738. (11) (a) Dorrance, R. C.; Hunter, T. F. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1312-1321. (b) Khuanga, V.; Selinger, B. K.; McDonald, R. Aust. J. Chem. 1976, 29, 1-2. (c) Atik, S. S.; Nam, M.; Singer, L. A. Chem. Phys. Lett. 1979, 67, 75-80. (d) Lianos, P.; Zana, R. J. Phys. Chem. 1980, 84, 3339-3341. (e) Ndou, T. T.; von Wandruszka, R. J. Lumin. 1990, 46, 33-38. (f) Borsarelli, C. D.; Cosa, J. J.; Previtali, C. M. Langmuir 1992, 8, 1070-1075. (g) Bales, B. L.; Almgren, M. J. Phys. Chem. 1995, 99, 15153-15162. (h) Itoh, H.; Ishido, S.; Nomura, M.; Hayakawa, T.; Mitaku, S. J. Phys. Chem. 1996, 100, 9047-9053. (i) Mizusaki, M.; Morishima, Y.; Yoshida, K.; Dubin, P. L. Langmuir 1997, 13, 6941-6946. (j) Borsarelli, C. D.; Cosa, J. J.; Previtali, C. M. Photochem. Photobiol. 1998, 68, 438-446. (k) Vasilescu, M.; Almgren, M.; Angelescu, D. J. Fluoresc. 2000, 10, 339-346. (12) (a) Matsui, K.; Nakazawa, T. Bull. Chem. Soc. Jpn. 1990, 63, 11-16. (b) Ilharco, L. M.; Santos, A. M.; Silva, M. J.; Martinho, J. M. G. Langmuir 1995, 11, 2419-2422. (c) Eremenko, A.; Smirnova, N.; Rusina, O.; Linnik, O.; Eremeko, T. B.; Spanhel, L.; Rechthaler, K. J. Mol. Struct. 2000, 553, 1-7.
10.1021/la048783f CCC: $30.25 © 2005 American Chemical Society Published on Web 12/23/2004
1-Pyrenemethylamine Intercalated into ZrP Layers
trices.17 The fluorescence spectrum of pyrene and its cationic derivatives in solution consists of two distinct components, a violet monomer emission band with a vibrational structure and a blue-green excimer emission band which is broad and structureless.18 High probe concentrations are needed to observe the excimer emission band, since the excimer is formed by an electronically excited molecule with a second molecule in its ground electronic state. The pyrene monomer emission band shows a fine structure with five peaks referred to as peaks I, II, III, IV, and V at 372, 379, 384, 388, and 394 nm, respectively.18c,d The fluorescence spectra of cationic derivatives of pyrene show bands at 380, 385, 400, and 417 nm; the peak at 380 nm is the most intense.19 Kumar et al. studied the excimer formation of a cationic derivative of pyrene, 4-(1-pyrene)butylamine hydrochloride (PBAC), exchanged into BAZrP (PBAC-exchanged BAZrP).15 Excimer formation was observed at all probe concentrations. However, no experimental evidence was provided to establish if the excimers were formed on the surfaces or in the interlayers. We have directly ion exchanged a cationic derivative of pyrene, 1-pyrenemethylamine (PYMA), into the 10.3 Å phase of ZrP and into BAZrP to elucidate the intercalation and the nature of excimer formation through X-ray powder diffraction and photophysical studies of these phases. The results of these investigations are presented here. 2. Experimental Section Materials. Zirconium oxychloride octahydrate (ZrOCl2‚8H2O, 98%) and 1-pyrenemethylamine hydrochloride (PYMA, 95%) were obtained from Aldrich and used as received. n-Butylamine from Fisher and n-butylamine hydrochloride from TCI America were also used as received. All other chemicals were reagent grade and were obtained from commercial sources. Procedures. Crystalline R-zirconium phosphate (R-ZrP) was prepared hydrothermally in aqueous HF, as described by Alberti et al.,20 and its phase was confirmed by X-ray powder diffraction (XRPD). The 10.3 Å hydrated phase of zirconium phosphate (10.3 Å ZrP) was synthesized using a previously reported procedure8 similar to the one that Kijima used to prepare θ-ZrP, a 10.4 Å hydrated phase.9 Butylammonium zirconium phosphate (BAZrP) was prepared as described by Rosenthal et al.6b A second butylammonium zirconium phosphate matrix (BAZrP2) was prepared as described by Tsuhako et al. using 2.0 mmol of butylamine/g of R-ZrP.21 The direct ion exchange of PYMA was conducted in aqueous solution at various PYMA/ZrP molar concentration ratios (10:1, (13) (a) Yang, H.; Bohne, C. J. Photochem. Photobiol., A 1995, 86, 209-217. (b) De Feyter, S.; Van Stam, J.; Boens, N.; De Schryver, F. C. Chem. Phys. Lett. 1996, 249, 46-52. (c) Pistolis, G.; Malliaris, A. Supramol. Chem. 2003, 15, 395-402. (14) (a) Schafirovich, V. Y.; Levin, P. P.; Kuzmin, V. A.; Thorgeirsson, T. E.; Kliger, D. S.; Geacintov, N. E. J. Am. Chem. Soc. 1994, 116, 63-72. (b) Smith, B. W.; Hurtubise, R. J. Anal. Chim. Acta 2004, 502, 149-159. (15) (a) Kumar, C. V.; Asuncion, E. H.; Rosenthal, G. Microporous Mater. 1993, 1, 299-308. (b) Kumar, C. V.; Chaudhari, A.; Rosenthal, G. L. J. Am. Chem. Soc. 1994, 116, 403-404. (16) (a) Ohta, N.; Umeuchi, S.; Nishimura, Y.; Yamazaki, I. Chem. Phys. Lett. 1997, 279, 215-222. (b) Pujari, S. R.; Kambale, M. D.; Bhosale, P. N.; Rao, P. M. R.; Patil, S. R. Mater. Res. Bull. 2002, 37, 1641-1649. (17) Kalb, M.; Gudipati, M. S. J. Phys. Chem. A 1998, 102, 508-510. (18) (a) Fo¨ster, T. H.; Kasper, K. Z. Elektrochem. 1955, 59, 977-980. (b) Birks, J. B.; Chistophorou, L. G. Nature 1962, 196, 33-35. (c) Birks, J. B.; Chistophorou, L. G. Spectrochim. Acta 1963, 19, 401-410. (d) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 20392044. (e) Winnik, F. M. Chem. Rev. 1993, 93, 587-614. (19) Hrdlovie`, P.; Chmela, S. J. Photochem. Photobiol., A 1998, 118, 137-142. (20) Alberti, G.; Torraca, E. J. J. Inorg. Nucl. Chem. 1968, 30, 317318. (21) Tsuhako, M.; Danjo, M.; Nakayama, H.; Eguchi, T.; Nakamura, N.; Yamaguchi, S.; Nariari, H.; Motooka, I. Bull. Chem. Soc. Jpn. 1997, 70, 1053-1060.
Langmuir, Vol. 21, No. 3, 2005 891 5:1, 1:1, 1:50, 1:100, and 1:1000) using either 10.3 Å ZrP, BAZrP, or BAZrP2, at constant stirring and ambient temperature. The mixture was equilibrated for 48 h and filtered to collect the solid. The solid was dried at room temperature for 2 weeks and powdered before characterization. The samples are identified according to the concentration ratio of the mixture used in their preparation. Elemental analyses were performed by Quantitative Technologies Inc. (QTI). Instrument. XRPD patterns were obtained using a Siemens D5000 powder diffractometer employing Cu KR radiation (λ ) 1.5406 Å) with a filtered flat LiF secondary beam monochromator. The X-ray tube was operated at 45 kV and 40 mA. All XRPD patterns were run on a 2θ range of 1.2-45°, with a 0.020° step. Ultraviolet-visible (UV-vis) spectrophotometric measurements were performed using a Cary 1E spectrophotometer. Steady-state fluorescence spectra were obtained with a PTI spectrofluorometer. The samples were excited with a 150 W xenon lamp. The excitation wavelength was 340 nm, and the emission scans were recorded between 360 and 600 nm. The emission bandpath was set to 2.5 nm. Fluorescence was detected with a Hamamatsu R1527P photomultiplier tube, which was configured for photon counting. The spectra were obtained on watersuspended samples and are not corrected for the photomultiplier efficiency and lamp output. X-ray photoelectron spectroscopy (XPS) analyses were performed at the Materials Characterization Center at the University of Puerto Rico using Physical Electronics PHI 5600 ESCA System equipment. Primary excitation was provided by a Mg anode (KR radiation, 1253.6 eV) biased at 15 kV and at a power setting of 400 W. The spectra were collected using a fixed analyzer transmission mode on a hemispherical electroanalyzer. The carbon 1s signal at 285 eV was used as the internal reference. Solid-state proton-decoupled CP/MAS 31P NMR spectra were recorded using an AVANCE Bruker 400 spectrometer. Samples were packed into a sapphire tube and spun at 4136 Hz for the 1:50 PYMA/ZrP molar ratio sample and 5 kHz for the 1:1 PYMA/ ZrP molar ratio sample. 31P spectra were acquired with broadband decoupling. The chemical shifts were measured relative to NH4H2PO4 (δ ) 0.9) used as the reference. Curve fitting was performed using the spectrometer software (LB ) -100.00 Hz and GB ) 0.25 for the 1:50 PYMA/ZrP molar ratio sample, and LB ) -150.00 Hz and GB ) 0.25 for the 1:1 PYMA/ZrP molar ratio sample).
3. Results and Discussion Figure 1a shows the XRPD patterns for the PYMAexchanged ZrP samples at 10:1, 5:1, and 1:1 PYMA/ZrP molar ratios. The XRPD patterns show the formation of a new phase with an expanded interlayer distance of 23.2-23.5 Å. The increase in the interlayer distance from 10.3 Å in the unintercalated material indicates that PYMA is being intercalated between the layers of ZrP. The XRPD pattern for the 1:20 PYMA/ZrP molar ratio sample shows a mixture of two phases (not shown), one corresponding to the new phase of PYMA-exchanged ZrP and the other one unintercalated ZrP. Subtracting 6.6 Å (the thickness of the ZrP sheet)22 from the interlayer distance obtained in the PYMA-intercalated material results in enough distance (16.9 Å) to accommodate PYMA, whose van der Waals dimensions are 13.3 × 9.9 × 4.3 Å3.23 The interlayer distance when PYMA is intercalated shows a difference of ∼3.6 Å compared with the dimensions of PYMA. This difference of 3.6 Å is probably due to nonimbricated (noninterdigitated) layers (as described below). Figure 1b shows that at low PYMA concentrations (1:50, 1:100, and 1:1000 PYMA/ZrP molar ratio samples) the XRPD patterns obtained are those of unintercalated R-ZrP with an interlayer distance of 7.6 Å. The R-ZrP phase is formed (22) Yang, C.-Y.; Clearfield, A. React. Polym., Ion Exch., Sorbents 1987, 5, 13-21. (23) As calculated using SPARTAN SGI, version 5.1.3, Wavefunction, with a HF/6-31G* basis set.
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Figure 1. XRPD patterns of (a) 1:1, 5:1, and 10:1 and (b) 1:50, 1:100, and 1:1000 PYMA-exchanged zirconium phosphate. Table 1. Nitrogen/Phosphorus Molar Ratio for PYMA-Exchanged ZrP Samples Obtained from X-ray Photoelectron Spectroscopy PYMA/ZrP ratio
N/P molar ratio
10:1 5:1 1:1 1:50 1:100 1:1000
1:2 1:2 1:2 1:15 1:30 1:99
upon dehydration of 10.3 Å ZrP without any intercalated species present.5 The results shown in Figure 1b indicate that at those low PYMA loading levels a new phase is not formed in the material and any exchanged PYMA in these samples is mainly surface bound. X-ray photoelectron spectroscopy (XPS) provides important information regarding the chemical composition of the samples. Table 1 shows the N/P molar ratio for the PYMA/ZrP samples. The 10:1, 5:1, and 1:1 PYMA/ZrP molar ratio samples, for which the XRPD results indicate intercalation, show a N/P ratio of 1:2, which indicates that for each molecule of PYMA there is one zirconium phosphate formula unit. In contrast, the N/P ratio for the 1:50, 1:100, and 1:1000 PYMA/ZrP molar ratio samples were 1:15, 1:30, and 1:99, respectively, varying proportionally with the probe concentration. Similar results were obtained from elemental analyses. These results indicate that at those low loading levels the PYMA concentration within ZrP has not reached saturation. These are the samples for which the XRPD results indicate no intercalation or an amount insufficient to form a new phase. The XPS results are consistent with the XRPD results, indicating that in these samples the PYMA molecules are mainly surface bound. The N/P ratio of 1:2 obtained for the high PYMA loading levels indicates that not all exchangeable protons of the phosphate groups of ZrP can be replaced by PYMA molecules and only 50% of the exchangeable phosphate protons have been replaced. Similar results have been reported previously by Brunet et al.24
Brunet et al. studied the covalent bonding of phosphonates derived from crown ethers to γ-zirconium phosphate.24 These authors reported materials with imbricated or nonimbricated layers depending on crown size and exchange level. Bruent et al. showed that a maximum of 50% of the exchangeable phosphates could be replaced, provided that the crowns arrange themselves perpendicular relative to one another. This arrangement would then give rise to layer nonimbrication. The nonimbricated layers have a larger interlayer distance than that expected from the molecule’s dimensions. If the exchange is
