3723
2005, 109, 3723-3726 Published on Web 02/09/2005
Long-Lived Radical Cation-Electron Pairs Generated by Anthracene Sorption in Non Brønsted Acidic Zeolites Se´ verine Marquis,† Alain Moissette,*,† Herve´ Vezin,‡ and Claude Bre´ mard† Laboratoire de Spectrochimie Infrarouge et Raman UMR-CNRS 8516, Centre d’ Etudes et de Recherches Lasers et Applications, FR-CNRS 2416, Baˆ t. C5, and Laboratoire de Chimie Organique et Macromole´ culaire, UMR-CNRS 8009, Baˆ t. C4, UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: January 24, 2005
The sorption of anthracene (ANT) in non Brønsted acidic ZSM-5 zeolite through the mere exposure at room temperature of solid ANT and dehydrated zeolite crystals with Li3.4(AlO2)n(SiO2)96-n chemical formulas per unit cell generates spontaneous ionization of ANT (IP 7.44 eV in the gas phase). In contrast, ANT was found to be sorbed as an intact molecule in M3.4ZSM-5 with M ) Na+, K+, Rb+, and Cs+. The radical cation (ANT•+) of the long-lived ANT•
[email protected]•- pair was characterized by conventional diffuse reflectance UV-visible and resonance Raman spectrometry. In contrast, the X-band continuous wave (CW) EPR signal was found to be typical of a weakly coupled spin correlated ion pair. The two-dimension hyperfine sublevel correlation (2D-HYSCORE) spectra provide a detailed description of the microenvironment of the trapped electron of the ANT•+@LinZSM-5•- pair. The trapped electron appears localized in close proximity of occluded ANT•+, Li+, and the Si-O-Al nearest group of the zeolite framework.
The ability of zeolites to generate spontaneously and to stabilize organic radical cations upon sorption of organic electron donors in the porous void space is one of their most fascinating properties.1 This feature has obvious implications in the area of catalysis in the petrochemical industry. A widely accepted explanation links the ionization ability of zeolites to their Brønsted or so-called true Lewis acidity.1 However, direct evidence for the fate of the ejected electron is available in rare cases.1,2 Here, we report the characterization of a radical cationelectron pair generated by sorption of anthracene (ANT) in non Brønsted acidic LinZSM-5 zeolites and without true Lewis sites. The course of the sorption was monitored by diffuse reflectance UV-visible (DRUVv) absorption, resonance Raman (RR), and electron paramagnetic resonance (EPR) spectrometry. Toward deeper mechanistic understanding of ANT•+-electron pair stability, we have employed X-band electron spin-echo envelope modulation (ESEEM) techniques and particularly the twodimension hyperfine, sublevel correlation spectroscopy (2DHYSCORE) to characterize the ion ANT•+-electron pair stabilized in LinZSM-5 zeolites.3,4 The as-synthesized ZSM-5 zeolites were calcined under air. The extraframework cations were completely exchanged by Li+, Na+, K+, Rb+, and Cs+.5 All the zeolite samples were dehydrated by a calcination procedure up to 773 K under argon. The chemical analyses, powder XRD patterns, 29Si, 27Al MAS NMR, IR, Raman, DRUVv, and EPR spectra of bare ZSM-5 zeolites were found to be characteristic of well-crystallized porous compounds with the following formula per unit cell: * Corresponding author. E-mail:
[email protected] † Laboratory of infrared and Raman spectrochemistry. ‡ Laboratory of organic and macromolecular chemistry
10.1021/jp0504120 CCC: $30.25
Mn(AlO2)n(SiO2)96-n (n ) 3.0, 3.4; M+ ) Li+, Na+, K+, Rb+, Cs+). Particularly, no evidence of aluminum extraframework species was found by 27Al MAS NMR and FTIR absorption diagnostic after sorption of pyridine in dehydrated MnZSM-5 zeolites did not provide any evidence of Brønsted and true Lewis sites.6 The framework topology of ZSM-5 is composed of a network of straight and sinusoidal channels.7 The entrance of the channels is controlled by 10-membered rings of oxygen atoms with 0.54 × 0.56 nm2 dimensions for the straight channels and 0.51 × 0.55 nm2 for the zigzag ones. The openings of the straight channels are sufficiently wide to allow ANT molecules to enter and to diffuse slowly into the straight pores.2,8 Weighted amount of bare MnZSM-5 zeolites as micro crystals (∼1 µm size) were exposed in the dark, under argon, and at room temperature to a weighted amount of dry solid ANT (C14H10). The ANT loading corresponds to one ANT molecule per unit cell of zeolite. DRUVv, Raman, and continuous wave CW-EPR spectra recorded during the course of the ANT sorption indicate that complete sorption takes place over more than several days with our experimental conditions and ANT was sorbed as intact molecule without ionization according to (1) in MnZSM-5 for n ) 3.0, 3.4 and M ) Na+, K+, Rb+, Cs+. The UV-visible absorption features and the off-resonance Raman scattering characteristics of ANT occluded in Na3.4ZSM-5 were shown in Figure 1a and Figure 2b, respectively.
ANT + MnZSM-5 f ANT@MnZSM-5
(1)
The modeling of the preferred location of ANT in the pores of MnZSM-5 performed using Monte Carlo simulations indicates the facial coordination of ANT to M+ in the straight channel. © 2005 American Chemical Society
3724 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Letters
Figure 2. Resonance Raman spectra recorded with the 632 nm helium-neon laser exciting line several weeks after mixing solid anthracene and dehydrated M3.4ZSM-5 zeolite crystals: (a) solid anthracene; (b) anthracene occluded in Na3.4ZSM-5 (1 molecule/unit cell, 20 days); (c) anthracene radical cation occluded in Li3.4ZSM-5 (1 molecule/unit cell, 20 days); (d) anthracene radical cation occluded in H3.4ZSM-5 (1 molecule/unit cell, 20 days).
Figure 1. Diffuse reflectance UV-visible spectra recorded during 3 days at room temperature during the course of anthracene sorption in dehydrated M3.4ZSM-5 zeolite with M3.4(AlO2)3.4(SiO2)92.6 chemical composition per unit cell after mixing solid anthracene with zeolite crystals according to 1 molecule/unit cell loading: (a) M ) Na+; (b) M ) Li+.
The results are in accurate agreement with X-ray diffraction study of aromatic molecules occluded in Cs3.8ZSM-5.9 In contrast, for M ) Li+ and n ) 3.0, 3.4 spontaneous ionization occurs according to (2) during the sorption process.
ANT + LinZSM-5 f ANT•+@LinZSM-5•-
(2)
The ANT•+ radical cation was readily identified through the visible absorption bands at 313, 351, 424, 652, and 709 nm in Figure 1b. Nevertheless, no clear evidence of the fate of the ejected electron was detected in the wavelength range of the DRUVv experiments. ANT•+ was also detected through the resonance Raman peaks at 610, 1261, 1391, and 1504 cm-1 using the 632 nm exciting laser line (Figure 2c). These ANT•+ Raman characteristics were found to be analogous to those exhibited after sorption in HnZSM-5 (Figure 2d).8 As previously reported, the ANT sorption in Brønsted acidic zeolites such as faujasite, mordenite and HnZSM-5 generates both ionization and protonation.8,10 Nevertheless, in HnZSM-5, the ANT•+ yield was found to be particularly high and the HANT+ yield was found to be particularly low. The stabilities of ANT•+ and HANT+ were found to increase and to decrease according to the narrow interstitial space of the pores and cavities, respectively.8,10 The X-band structured EPR signal was detected immediately after mere exposure of powdered ANT to dehydrated LinZSM-5 (n ) 3, 3.4) crystals. The characteristic spectral shape is caused mainly by 1H hyperfine splitting. Comparison of these hyperfine
Figure 3. X-band CW EPR spectra of radical cation-electron pair (ANT•
[email protected]•-) occluded in non Brønsted acidic ZSM-5 zeolite at (a) room temperature and (b) 4.2 K.
splitting with those of free ANT•+ in solution indicates marked broadening. This feature has also previously been observed for ANT•+ generated in HnZSM-5, on silica-alumina, and in polyoxometalate.2,11 Numerous CW-EPR spectra were recorded during the course of the sorption. In addition to peaks, the EPR spectra contain a broad overlapping feature that increases in intensity during the sorption. Figure 3a represents the poorly resolved X-band CW-EPR spectra recorded at room temperature 3 days after the mixing of solid ANT with Li3.4ZSM-5. The cooling of the sample to 4.2 K (Figure 3b) induced the disappearance of the weakly structured pattern to a featureless signal of 23 G. The broadness of the spectrum implies that the signal could be attributed to ANT•
[email protected]•- ion pair rather than to free ANT•+, because the exchange and dipolar interactions can induce the broadening effects on EPR spectra.12 The double integration of the EPR signal in the g ) 2 region indicates the ANT ionization was found to reach 20% after 3 days at 330 K with respect to occluded ANT. It should be noted that the ionization was found to be nearly complete upon ANT sorption in HnZSM-5.2,8 We performed spin-lattice T1 measurements of ANT•+@ Li3.4ZSM-5•- at 4.2 K using inversion recovery sequence. In contrast to ANT•
[email protected]•-, where both FID and echo processes were observed, an echo signal was only detected with T1 ∼ 300 µs in the present case.2 The exchange and dipolar
Letters
Figure 4. Experimental 2D-HYSCORE spectra of ANT•
[email protected]•- sample recorded at 4.2 K for τ ) 128 ns (a) and τ ) 200 ns (b). Experimental conditions: t1 × t2 ) 128 × 128 points; start values t1 ) 56 ns, t2 ) 56 ns; microwave frequency 9.64 GHz.
interactions between the two electrons of the ANT•+@ Li3.4ZSM-5•- pair are expected to have a very weak effect on the HYSCORE spectra if their strength is small compared to the difference in EPR frequencies of the two electrons.13,14 2DHYSCORE spectra were recorded at room temperature and 4.2 K using the sequence (π/2-τ-π/2-t1-π-t2-π/2-echo) where the echo is measured as a function of t1 and t2 for 4τ values (88, 128, 200, and 256 ns). Figure 4 shows 2D-HYSCORE patterns of ANT•
[email protected]•- recorded at 4.2 K for τ ) 128 and 200 ns, respectively. All HYSCORE patterns recorded at room temperature only display 1H nuclear frequency features. When recorded at 4.2 K, all HYSCORE spectra exhibit supplementary symmetric ridges in (+, +) quadrant along the diagonal of the Larmor nuclear frequency of the following isotopes: 6Li, 2.19 MHz, I ) 1; 29Si, 2.9 MHz, I ) 1/2; 27Al, 3.9 MHz, I ) 5/2; 7Li, 5.79 MHz, I ) 3/2. All patterns exhibit 7Li cross-peak ridges of the ∆m ) (1 nuclear transitions with l coordinates (8.4, 3.1 MHz) and (2.9, 8.4 MHz). For 200 and 256 ns τ values, similar cross-peaks are observed for 6Li isotopes. Cross-peaks were also observed in the negative quadrant at the corresponding symmetric positions with respect to the diagonal for 6Li, 7Li, and 27Al with AAl of 4.5 MHz. In the point-dipole approximation the principal values of hyperfine (hf) tensor ALi can be written in the form (Aiso - T, Aiso - T, Aiso + 2T) where Aiso is the isotropic hyperfine coupling constant and the dipolar hf tensor is T ) gegnβnβe/hr3 where r is the effective electron-nucleus distance.15 The hf dipolar tensor value can be roughly estimated from the maximum vertical shifts of the (∆mI ) (1, ∆mI ) (1) ridge of Figure 4b that yield to TLi ) 4.8 ( 0.1 MHz that corresponds to a distance of the unpaired electron-Li nucleus of 0.24 ( 0.02 nm.15,16 The Aiso value was found to be 1.8 MHz, indicating a 0.4% contribution
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3725 of the 2s orbital to the unpaired electron spin density. As for the Li nucleus, we can measure the maximum vertical shift of the proton ridge on the spectrum displayed in Figure 4a. The maximum vertical shift ∆ωs for a proton was found to be 0.84 MHz, which yields a 0.23 ( 0.02 nm distance between the electron and 1H. We can hypothesize that the dipolar component measured from the 1H pattern arises from the electron of Li3.4ZSM-5•- with a proton of ANT•+ whereas that of ANT•+ itself was found to be silent due to a shorter T1 at 4.2 K compared to the T1 of the trapped electron. The structural situation of the ANT•
[email protected]•- moiety was found to be analogous to the structure of ANT occluded in MnZSM-5 zeolites with the aromatic group facially coordinated to the M+ cation in close proximity of the Al atom. The ejected electron appears delocalized in a restricted space around Li+ with a 0.24 nm electron-Li+ average distance and around the H atom of ANT•+ with a 0.23 nm electron-H average distance. The Li cation has a positive charge weakly lower than +1. The weak interactions between electron and 27Al and 29Si nuclei were in agreement with the close proximity of Li+ with Al and Si atoms in the zeolite framework. The present results demonstrate once again that the tight fit between the size of rod shaped electron donor molecules and the diameter of the channel of ZSM-5 is necessary to generate and stabilize radical cationelectron pairs over long periods.1 The spontaneous ionization is not an intrinsic property of Brønsted or true Lewis acidic functions of porous material and depends both on the ionization potential of the sorbate and on the polarization energy of the host at the sorption site. For molecules with relatively low ionization potential such as anthracene, the spontaneous ionization was found to occur in non Brønsted and non true Lewis acidic Li3.4ZSM-5. Among the counterbalancing alkaline cations, only Li+ can induce sufficient polarization energy to initiate spontaneous ionization during the ANT sorption. The shorter distances between Li+, framework oxygen atoms, and ANT•+ associated with higher electrostatic field in the proximity of small Li+ ion can explain this unusual stability of the radical ion pair. Acknowledgment. The Centre d’Etudes et de Recherches Lasers et Applications (CERLA, FR-CNRS 2416) is supported by the Ministe`re charge´ de la recherche, the re´gion Nord/Pas de Calais, and the Fonds Europe´en de De´veloppement Economique des Re´gions. References and Notes (1) Garcia, H.; Roth, H. D. Chem. ReV. 2002, 102, 3947-4007 and references therein. (2) Vezin, H.; Moissette, A.; Bre´mard, C. Angew. Chem., Int. Ed. 2003, 42, 5587-5591. (3) Schweiger, A.; Jeschke, G. Principles of Pulse Electron Paramagnetic Resonance; Oxford University Press: Oxford, NY, 2001. (4) Po¨ppl, A.; Rudolf, T.; Michel, D. J. Am. Chem. Soc. 1998, 120, 4879-4880. (5) Gener, I.; Moissette, S.; Bre´mard, C. Phys. Chem. Chem. Phys. 2004, 6, 3732-3738. (6) Moissette, A.; Vezin, H.; Gener, I.; Bremard, C. J. Phys. Chem. B 2003, 107, 8935-8945. (7) Olson, D. H.; Khosrovani, N.; Peters, A. W.; Toby, B. H. J. Phys. Chem. B 2000, 104, 4844-4848. (8) Moissette, A.; Marquis, S.; Gener, I.; Bre´mard, C. Phys. Chem. Chem. Phys. 2002, 4, 5690-5696. (9) Mentzen, B. F.; Ge´lin, P. Mater. Res. Bull. 1998, 33, 109-116. (10) Liu, X.; Iu, K.-K.; Thomas, J. K.; He, H.; Klinowski, J. J. Am. Chem. Soc. 1994, 116, 11811-11818. (11) Khenkin, A. M.; Weiner, L.; Wang, Y.; Newmann, R. J. Am. Chem. Soc. 2001, 123, 8531-8542. (12) Ikoma, T.; Nakai, M.; Akiyama, K.; Tero-Kubota, S.; Ishii, T. Angew. Chem., Int. Ed. 2001, 40, 3234-3236.
3726 J. Phys. Chem. B, Vol. 109, No. 9, 2005 (13) Zwanenburg, G.; Hore, P. J. J. Magn. Reson. 1995, 114, 139146. (14) Dubinski, A. A.; Perekhodtsev, G. D.; Poluektov, O. G.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2002, 106, 938-944.
Letters (15) Astrakas, L.; Kordas, G. J. Chem. Phys. 1999, 110, 68716875. (16) Gutjarh, M.; Bo¨ttcher, R.; Po¨ppl, A. J. Phys. Chem. B 2002, 106, 1345-1349.
