J. Phys. Chem. 1996, 100, 18173-18176
18173
Intrazeolite Photochemistry. 16. Fluorescence of Methylviologen Adsorbed within Medium- and Large-Pore Zeolites Mercedes Alvaro,† Glenn A. Facey,‡ Hermenegildo Garcı´a,† Sara Garcı´a,† and J. C. Scaiano*,‡ Departamento de Quı´mica and Instituto de Tecnologı´a Quı´mica, UniVersidad Polite´ cnica de Valencia, Apartado 22012, 46071 Valencia, Spain, and Department of Chemistry, 10 Marie Curie, UniVersity of Ottawa, Ottawa, K1N 6N5 Canada ReceiVed: March 8, 1996; In Final Form: August 2, 1996X
Methylviologen (MV2+) incorporated within medium- and large-pore zeolites (NaZSM-5, Namordenite, NaY, and CsY) emits fluorescence at 330 and 420 nm; the latter is most prominent in zeolite NaY. The intensity ratio between these two bands depends on the structure of the zeolite. On the basis of molecular modeling and 13C CP/MAS NMR using the dipolar dephasing technique, the first band has been ascribed to the emission of MV2+ in a planar conformation. The unprecedented 420 nm band has been attributed to a twisted conformer probably interacting with the basic oxygen sites of the framework.
There is considerable interest in the use of zeolites to control the outcome of organic reactions and as tools to examine specific spectroscopic properties.1-5 In the context of the photochemical reduction of water and the production of hydrogen using visible light, a large variety of heterogeneous systems have been devised many of them having in common the presence of methylviologen (MV2+) adsorbed on an inorganic support.6-8 Zeolites9-11 have been often the hosts of choice because their strictly regular microporous framework provides the opportunity to compartmentalize and organize a supramolecular assembly where the different stages of the overall process (light absorption, energy transfer, and water reduction) can take place in a well-defined spatial arrangement. However, the majority of these studies have not paid close attention to the possible influence that the rigid, restricted environment may play in modifying the molecular properties of MV2+. In the present work we have performed a study of the fluorescence of MV2+ incorporated within different zeolites encompassing different topologies. On the basis of solid-state 13C CP/MAS NMR spectroscopy and molecular modeling, we have found that our results can be rationalized in terms of a different degree of pyridinium ring mobility around the single C-C bond when MV2+ is incorporated zeolites, as well as the formation of a charge-transfer-like complex between MV2+ and the negative oxygens of the framework.
Figure 1. (A) Excitation spectra of MV2+-doped NaY obtained monitoring at 320 (a) or 420 nm (b); (B) fluorescence spectra of MV2+ incorporated within NaZSM-5 (a), NaMor (b), and NaY (c).
Experimental Section Samples of zeolites NaZSM-5 (Si/Al 17.5), NaMor (mordenite; Si/Al 10), NaY (Si/Al 2.6) and CsY (Si/Al 2.6, 70% ion exchange) doped with MV2+ were obtained by room-temperature ion exchange using aqueous solution of MV2+ hydrochloride (Aldrich, 98%). The actual loading level was determined by combustion chemical analysis (C and N) of the samples as well as by quantitative atomic absorption spectroscopy of the remaining Na+ left in the zeolite after the partial exchange. Powder X-ray diffraction of the samples showed that the crystalline structure of the MV2+-doped zeolites was preserved during the preparation procedure. Steady-state fluorescence measurements were performed with a Perkin-Elmer LS-50 spectrofluorimeter equipped with a frontface attachment for solid samples. Semiempirical calculations were carried out using the AM1 SCF-MO method as implemented in the version 3.0 of the †
Universidad Polite´cnica de Valencia. University of Ottawa. X Abstract published in AdVance ACS Abstracts, October 15, 1996. ‡
S0022-3654(96)00733-2 CCC: $12.00
Figure 2. (A) Aromatic region of the 13C CP/MAS NMR of MV2+ within NaZSM-5 at different dephasing delays. (B) Spectra of MV2+ within NaMor (a) and CsY (b) at 0 and 80 µs dephasing delays. A decrease (0.75) in the signal at 127 ppm was observed for the NaMor sample.
© 1996 American Chemical Society
18174 J. Phys. Chem., Vol. 100, No. 46, 1996
Alvaro et al.
Figure 3. Views of the docking of the planar and perpendicular conformations of MV2+ within ZSM-5. The lines connecting MV2+ with the framework indicate an overlapping of the van der Waals atomic radii.
MOPAC program supplied with a Tektronix CAChe workstation. UV-vis spectra were calculated with the ZINDO software provided with this package. Molecular modeling related to guest docking in the zeolites was performed with the Insight II Molecular Modeling package of programs using a Silicon Graphics work station. All 13C CP/MAS NMR spectra were acquired on a Brucker ASX-200 solid-state NMR spectrometer operating at 50.3 MHz for 13C. The cross polarization contact times were set to 1 ms and a 2-s relaxation delay was used. The samples were spun at 4500 Hz. Results and Discussion All the MV2+-doped zeolites studied emit fluorescence. Luminescence spectra measured at room temperature, along with the corresponding excitation spectra are presented in Figure 1. It is well-known that aqueous solutions of MV2+ do not fluoresce. A fast radiationless deactivation pathway of the singlet excited state through rotation around the single bond connecting the two pyridinium rings has been invoked as responsible for this nonemissive decay.12 When ring rotation is hindered either by nuclear substitution12 or by intercalating MV2+ within layered clays,13,14 then MV2+ fluorescence (λmax 330 nm) can be readily observed. For MV2+ adsorbed onto
clays the interlamellar space calculated from basal spacing was 3 Å and thus it was assumed that the two pyridinium rings a held in a rigid planar conformation.14 It is remarkable that besides the reported fluorescence band (λmax 330 nm), another emission peaking around 420 nm was clearly observed for MV2+-doped zeolites. To the best of our knowledge, this emission has not been reported before. Excitation spectra monitoring at 320 and 440 nm established that there were two different emitting species (Figure 1). Thus, while the emission at 320 exhibits a sharp excitation band at 282 nm that can be assigned to MV2+, the 420 nm band originates from a species showing a very broad absorption peaking around 300 nm.15 According to the data in the literature, ground-state adsorption of MV2+ undergoes a red-shift when it is adsorbed onto silica16 or layered clays.14 At the same time the spectrum becomes much broader, extending up to 320 nm. A maximum around 290 nm has been reported for smectite samples containing MV2+. Thus, the species responsible for the 420 nm emission is probably a conformer of MV2+, perhaps involved in complexation by interaction with the zeolite framework. We addressed the possibility that this emission originates from some adventitious impurity present in the samples. This explanation is highly unlikely to apply to our systems, since samples prepared using twice recrystallized MV2+ give rise
Intrazeolite Photochemistry. 16
Figure 4. Views of the docking of the planar and perpendicular conformations of MV2+ within mordenite.
essentially to the same fluorescence spectrum. In addition, FTIR, Raman, and XPS spectroscopies did not lead to the detection of any species other than MV2+. Previous studies on the doping of zeolites with MV2+ also have not reported the presence of any byproduct associated with the adsorption procedure.17,18 Furthermore, the only common impurity of MV2+ reported in the literature as responsible for detectable emission is 1,2′dihydro-1,1′-dimethyl-2′-oxo-4,4′-bipyridinium arising from the photochemical oxygenation at the 2 position of MV2+.12,19 However, this compound has a green emission at 520 nm, while the absorption maximum is at 350 nm. None of these data are consistent with the 420 nm band that we observed for MV2+ incorporated within zeolites. In addition, the fact that the intensity ratio between the 330 and 420 nm bands depends on the zeolite structure suggests that there is a relationship between the host physicochemical parameters and/or degree of confinement experienced by MV2+ and the fluorescence spectrum. If some impurity were responsible for the emission, one would not expect the fluorescence to follow such a dependence with the zeolite structure.
J. Phys. Chem., Vol. 100, No. 46, 1996 18175 To seek experimental evidence for differences in the freedom of aryl rotation through the single bond connecting the heterocyclic rings, we performed 13C CP/MAS NMR studies of these samples using the dipolar dephasing technique.20 In this technique a short delay is inserted between the spin-locking pulse and the triggering of the receiver. During the delay, the signals from fixed C-H pairs may completely decay and may not be detected. In contrast, the signals from mobile C-H pairs are only partially attenuated and can easily be detected. Several dephasing delays were used to optimize the decay of fairly rigid C-H pairs in NaZSM-5 (Figure 2). Then, spectra of MV2+ within NaMor and CsY were recorded under the same experimental conditions (Figure 2). The results show that for MV2+ within ZSM-5 the 13C signals corresponding to the heterocyclic C-H of the pyridinium rings completely vanish using an 80 µs dephasing delay and in the case of mordenite a decrease is observable. In contrast, for the large-pore tridirectional zeolite Y no differences between the normal and the dipolar dephasing conditions could be observable, thus indicating that the faujasite framework, even when larger Cs+ ions are present, imposes very little restriction on the molecular motion of MV2+. This constitutes proof that MV2+ experiences different degrees of freedom for the rotation through the single C-C bond connecting the pyridinium ring when incorporated in zeolites with different void dimensions. We performed a molecular modeling study to determine if both planar and orthogonal conformations of MV2+ can be accommodated within the channels of pentasil and mordenite zeolites. Selected visualizations showing the results of the docking are presented in Figures 3 and 4. As anticipated, unfavorable van der Waals interactions due to host-guest atomic overlapping are much more severe for the perpendicular conformation of MV2+ within ZSM-5 (Figure 3). This overlapping is partially relieved in ZSM-5 for the planar conformation and is very minor for both conformers within the wider channels of mordenite. These predictions agree with the current knowledge about shape-selectivity in medium-pore or monodirectional large-pore zeolites.21-24 We have calculated the UV ground-state absorption spectra for the planar and perpendicular conformers of MV2+. We reasoned that if the absorption spectra of these extreme conformations were different, then the emission spectra would also be likely to be different. Calculations of the spectra using ZINDO predict that planar MV2+ should have only a maximum at 260 nm, while the orthogonal conformer should present two maxima at 220 and 270 nm with molar extinction coefficients about one-half of the planar conformer. This would imply that for ZSM-5, where MV2+ must be held in a near-planar conformation, the predominant emission should be observed at shorter wavelengths than when MV2+ is in zeolite Y where the orthogonal conformer is much likely to be the preferred conformer, rotation being a much faster process. Another probable explanation would take into account that MV2+ can form charge-transfer complexes with anionic species in solution.19 These complexes are characterized by a much broader absorption band of MV2+ in the UV (expanding up to 350 nm) and emission at longer wavelength (λmax 540 nm).19 The search for these types of complexes when MV2+ is adsorbed in silica has been pursued unsuccessfully.16 It is known that negative oxygens of the zeolite lattice behave as basic sites.25 Furthermore, the basicity of the zeolites largely depends on their chemical composition and in particular on the aluminum content of the framework. Thus, it has been established that the higher the aluminum content in the framework, the higher basicity of the framework. Accordingly, the basicity increases going from ZSM-5 and mordenite to zeolite Y. Therefore, the unprec-
18176 J. Phys. Chem., Vol. 100, No. 46, 1996 edented 420 nm emission band could also be attributed to a charge-transfer-like complex between basic framework oxygens and MV2+. In conclusion, we have shown that by confining MV2+ in a restricted intrazeolite void it is possible to thwart the conformational freedom between the pyridium ring and to control the charge density on MV2+. As a result, MV2+ when incorporated within zeolites, exhibits two distinct luminescence bands. Thus, in addition to the characteristic fluorescence of the planar conformer at 320 nm, a longer emission at 420 nm arising from a conformer able to rotate and presumably forming a chargetransfer complex is also present. Acknowledgment. Financial support by the Natural Sciences and Engineering Research Council of Canada (J.C.S.) and Spanish DGICYT (HG, Grant PB93-0380) are gratefully acknowledged. J.C.S. is the recipient of a Killam fellowship awarded by the Canada Council. S.G. also thanks the Ministerio de Educacio´n y Ciencia (Spain) for a postgraduate scholarship. References and Notes (1) Turro, N. J. Pure Appl. Chem. 1986, 58, 1219. (2) Ramamurthy, V.; Turro, N. J. J. Inclusion Phenom. Mol. Recogn. Chem. 1995, 21, 239. (3) Ramamurthy, V.; Eaton, D. F. Chem. Mater. 1994, 6, 1128. (4) Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Acc. Chem. Res. 1992, 25, 299. (5) Ramamurthy, V. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; p 429. (6) Bard, A. J.; Ledwith, A.; Shine, H. J. AdV. Phys. Org. Chem. 1976, 12, 155.
Alvaro et al. (7) Energy Resources through Photochemistry and Catalysis; Gra¨tzel, M., Ed.; Academic Press: New York, 1983. (8) Photogeneration of Hydrogen; Harriman, A.; West, M. A., Eds.; Academic Press: New York, 1982. (9) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular SieVes; Academic Press: London, 1978. (10) Dutta, P. K.; Turbeville, W. J. Phys. Chem. 1992, 96, 9410. (11) Introduction to Zeolite Science and Practice; van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, 1991. (12) Mau, A. W.-H.; Overbeek, J. M.; Loder, J. W.; Sasse, W. H. F. J. Chem. Soc., Faraday Trans. 2 1986, 82, 868. (13) Villemure, G.; Detellier, C.; Szabo, A. G. J. Am. Chem. Soc. 1986, 108, 4656. (14) Villemure, G.; Detellier, C.; Szabo, A. G. Langmuir 1991, 7, 1215. (15) The 420 nm emission cannot be due to the methyl viologen radical cation. This species absorbs strongly in the 600 nm region and would not be expected to fluoresce at higher energies. (16) Mao, Y.; Breen, N. E.; Thomas, J. K. J. Phys. Chem. 1995, 99, 9909. (17) Yoon, K. B.; Huh, T. J.; Corbin, D. R.; Kochi, J. K. J. Phys. Chem. 1993, 97, 6492. (18) Yoon, K. B.; Huh, T. J.; Kochi, J. K. J. Phys. Chem. 1995, 99, 7042. (19) Kuczynski, J. P.; Milosavljevic, B. H.; Lappin, A. G.; Thomas, J. K. Chem. Phys. Lett. 1984, 104, 149. (20) Opella, S. J.; Frey, M. H. J. Am. Chem. Soc. 1979, 101, 5854. (21) Csiczery, S. M. Pure Appl. Chem. 1986, 58, 841. (22) Moreau, P.; Finiels, A.; Geneste, P.; Moreau, F.; Solofo, J. J. Org. Chem. 1992, 57, 5040. (23) Moreau, P.; Finiels, A.; Geneste, P.; Solofo, J. J. Catal. 1992, 136, 487. (24) Moreau, P.; Finiels, A.; Geneste, P.; Moreau, F.; Solofo, J. Stud. Surf. Sci. Catal. 1993, 78, 575. (25) Hattori, H. Chem. ReV. 1995, 95, 537.
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