Langmuir 2003, 19, 1987-1989
1987
Reversible Color Change of Chromophores in Zeolites by Direct Interaction with Alkali Metal Cations Yoshihiko Komori and Shigenobu Hayashi* Institute for Materials & Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received December 24, 2002 Reversible color change of a neutral chromophore in a cationic zeolite was achieved by controlling the direct interaction between alkali-metal cations and chromophores. As a typical combination of the cationic host and the neutral chromophore, Na+-type mordenite accommodating N,N-dimethyl-p-nitroaniline (DMpNA) was prepared. The UV-vis spectra showed an extraordinarily large shift of the absorption band of DMpNA by 85 nm after the dehydration treatment of the hydrated sample. Furthermore, the absorption band was shifted reversibly by the dehydration and hydration treatments. Solid-state 23Na MAS NMR with a cross-polarization technique revealed that DMpNA molecules were present in the neighborhood of the dehydrated Na+ in the dehydrated form while the molecules were distant from 23Na in the hydrated form. These results indicated that adsorbed water molecules controlled the strength of the interaction between Na+ and DMpNA molecules.
Introduction Zeolites accommodating functional organic molecules have attracted increasing attention in recent years since the properties of the compounds are varied by not only the natures of the molecules and zeolites themselves but also host-guest interactions.1-4 For example, chromophores such as p-nitroaniline are aligned in one direction in the channels of zeolite.5-8 Applications of a unique hostguest interaction open up the way to functional materials tailor-made at a molecular level. Cations in zeolite are one of important factors in controlling the properties of the guest molecules in the micropores. In general, cations play a role of compensating the negative charges in the framework and sensitively governing the donor strength of the zeolite framework. It is reported that the donor strength of the framework affects the absorption band of chromophores of strong electron acceptors such as methyl viologen.9-12 On the other hand, direct interaction between cations and aromatic molecules has been studied extensively by using neutral aromatic molecules such as benzene, anthracene, and naphthalene.13-24 It is reported that photophysical properties of * Corresponding author. E-mail:
[email protected]. (1) Schulz-Ekloff, G.; Wo¨hrle, D.; Duffel, B. v.; Schoonheydt, R. A. Microporous Mesoporous Mater. 2002, 51, 91. (2) Schu¨th, F.; Schmidt, W. Adv. Mater. 2002, 14, 629. (3) Scho¨llhorn, R. Chem. Mater. 1996, 8, 1747. (4) Ramamurthy, V.; Eaton, D. F. Chem. Mater. 1994, 6, 1128. (5) Cox, S. D.; Gier, T. E.; Stucky, G. D.; Bierlein, J. J. Am. Chem. Soc. 1988, 110, 2986. (6) Cox, S. D.; Gier, T. E.; Stucky, G. D. Chem. Mater. 1990, 2, 609. (7) Werner, L.; Caro, J.; Finger, G.; Kornatowski, J. Zeolites 1992, 12, 658. (8) Binder, G.; Scandella, L.; Kritzenberger, J.; Gobrecht, J.; Koegler, J. H.; Prins, R. J. Phys. Chem. B 1997, 101, 483. (9) Hashimoto, S. Tetrahedron 2000, 56, 6957. (10) Park, Y. S.; Um, S. Y.; Yoon, K. B. J. Am. Chem. Soc. 1999, 121, 3193. (11) Alvaro, M.; Garcı´a, H.; Garcı´a, S.; Ma´rquez, F.; Scaiano, J. C. J. Phys. Chem. B 1997, 101, 3043. (12) McManus, H. J. D.; Finel, C.; Kevan, L. Radiat. Phys. Chem. 1995, 45, 761. (13) Hashimoto, S.; Hagiri, M.; Matsubara, N.; Tobita, S. Phys. Chem. Chem. Phys. 2001, 3, 5043. (14) Gener, I.; Buntinx, G.; Bre´mard, C. Microporous Mesoporous Mater. 2000, 41, 253. (15) Gener, I.; Ginestet, G.; Buntinx, G.; Bre´mard, C. J. Phys. Chem. B 2000, 104, 11656.
molecules depend on the kinds of cations and the presence of adsorbed water. Furthermore, NMR analysis has revealed that behaviors of molecules are affected by cations.17,24 These results have indicated the presence of a cation-π interaction. Therefore, it is expected that the optical property of chromophores can be controlled simply by the direct interaction with cations. In the present study, we have investigated the interaction between alkali-metal cations and neutral chromophores in the micropore of zeolites. We have selected Na+-type mordenite (NaMOR) and N,N-dimethyl-p-nitroaniline (DMpNA) as a typical combination of the cationic host and the neutral chromophore. NaMOR has straight channels which consist of 12-membered rings with a channel size of 0.70 × 0.65 nm. Small organic molecules such as p-nitroaniline can penetrate into the micropores of mordenite.5,6 The cation-chromophore interactions are also investigated by using solid-state 23Na and 13C MAS NMR techniques. Experimental Section NaMOR was a reference catalyst distributed by The Catalysis Society of Japan, coded JRC-Z-M20. NaMOR was dehydrated by evacuation at 473 K for 3 h before use. A weighted amount of DMpNA was introduced into the dehydrated NaMOR. The sample tube was sealed in vacuo and heated at 443 K for 72 h. After the reaction, the sample was exposed to an ambient atmosphere for preparing a hydrated form. Thermogravimetric (TG) analysis showed two steps of mass losses in the temperature ranges 300-520 and 520-770 K. The former was mainly due to (16) Thomas, K. J.; Sunoj, R. B.; Chandrasekhar, J.; Ramamurthy, V. Langmuir 2000, 16, 4912. (17) Sato, T.; Kunimori, K.; Hayashi, S. Phys. Chem. Chem. Phys. 1999, 1, 3839. (18) Hashimoto, S.; Ikuta, S.; Asahi, T.; Masuhara, H. Langmuir 1998, 14, 4284. (19) Hashimoto, S. Chem. Phys. Lett. 1996, 262, 292. (20) Ramamurthy, V.; Turro, N. J. J. Incl. Phenom. Mol. Recog. Chem. 1995, 21, 239. (21) Mellot, C.; Simonot-Grange, M.; Pilverdier, E.; Bellat, J.; Espinat, D. Langmuir 1995, 11, 1726. (22) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Phys. Chem. 1993, 97, 13380. (23) Ramamurthy, V.; Caspar, J. V.; Eaton, D. F.; Kuo, E. W.; Corbin, D. R. J. Am. Chem. Soc. 1992, 114, 3882. (24) Hepp, M. A.; Ramamurthy, V.; Corbin, D. R.; Dybowski, C. J. Phys. Chem. 1992, 96, 2629.
10.1021/la0270688 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/06/2003
1988
Langmuir, Vol. 19, No. 6, 2003
Letters
Figure 1. Reflectance UV-vis spectra of DMpNA/NaMOR. Solid and dashed lines are hydrated and dehydrated DMpNA/ NaMOR, respectively. The samples were diluted with MgO powders to concentrations of 10 mass%. desorption of water molecules and the latter was due to the loss of DMpNA molecules. The temperature of the loss of DMpNA was much higher than the melting point of crystalline DMpNA (436 K), indicating that DMpNA was confined in the micropores of NaMOR. On the basis of the TG analysis, the composition of the hydrated sample was estimated to be Na4.8Al4.3Si43.7O96.3‚ 1DMpNA‚7H2O.
Results and Discussion UV-vis spectra of DMpNA after the reaction with NaMOR show an extraordinarily large shift of the absorption band depending on the hydration (Figure 1).The UV-vis spectrum of the fully hydrated sample shows a π-π* transition band at 400 nm, which is similar to that of crystalline DMpNA. The color of the hydrated sample is yellow. In marked contrast, the intensity of 400 nm decreases and a new absorption band appears at 485 nm after dehydration by heating the sample at 393 K. The color of the sample changes to orange. The color change takes place reversibly by treatments of dehydration and hydration; the color of the sample is yellow in an ambient atmosphere while it becomes orange after the heating. Profiles of the UV-vis absorption bands are wellreproduced in a repeatable manner by dehydrationhydration treatments, indicating that DMpNA molecules are not decomposed and that water molecules are intimately related with the mechanism of the reversible color change. The mechanism of the color change is investigated by using solid-state 23Na MAS NMR with and without a crosspolarization (CP) technique (Figure 2). CP is advantageous to a selective observation of 23Na spins interacting with 1 H spins. In the hydrated sample, a small signal at -2 ppm and a broad signal at -10 ppm are observed (Figure 2a), which can be assigned to hydrated Na+ in NaMOR.25 These signals are not observed in the spectrum measured by the CP technique (Figure 2b). Because CP spectra show 23 Na spins interacting with rigid 1H spins, no detection means ineffective cross-polarization from 1H spins of H2O and DMpNA molecules. H2O has a high mobility and DMpNA molecules are distant from 23Na. On the other hand, the spectrum of the dried sample shows a broad signal at about -20 ppm, a change of the peak position to lower frequency by about 10 ppm, indicating dehydration of Na+ (Figure 2c). In the CP spectrum, a signal is detected at the similar position (Figure 2d). Because the amount of H2O is much reduced in the dried form and H2O has a high mobility, it is reasonable to assume that (25) Hannus, I.; Nagy, J. B.; Kiricsi, I.; Fonseca, A.; Fernandez, C.; Fejes, P. Z. Phys. Chem. 1995, 189, 229.
Figure 2. 23Na MAS NMR spectra of (a, b) hydrated and (c, d) dehydrated DMpNA/NaMOR. Spectra a and c were measured by the single pulse method with high power 1H decoupling, and spectra b and d, by the CP technique. Larmor frequency and the spinning rate were set at 105.84 MHz and 5 kHz, respectively. Hartmann-Hahn conditions were adjusted by using the signal of -2 ppm in borax. Chemical shifts are expressed with respect to 1 mol/L NaCl(aq).
Figure 3. 13C CP/MAS NMR spectra of (a) hydrated and (b) dehydrated DMpNA/NaMOR. Larmor frequency, the spinning rate, the contact time, and the recycle delay time are set at 100.61 MHz, 3.5 kHz, 1 ms, and 15 s, respectively. Chemical shifts are expressed with respect to neat tetramethylsilane. 1
H spins of H2O hardly contribute to the excitation of the Na spins. Consequently, the excitation of 23Na spins should arise via 1H of DMpNA molecules, indicating that DMpNA molecules are present in the neighborhood of the dehydrated Na+. Another characteristic difference is observed in 13C CP/ MAS NMR spectra before and after the dehydration treatment of DMpNA/NaMOR (Figure 3). 13C NMR 23
Letters Scheme 1. Mechanism of the Color Change
spectrum of the hydrated sample shows resolved isotropic signals and their spinning sidebands due to the DMpNA molecule. Some signals for the aromatic carbons do not consist of a single component, indicating that DMpNA molecules are adsorbed on a few sites of NaMOR. On the other hand, the spectrum of the dehydrated sample shows that signals for the aromatic carbons are significantly broadened. Because DMpNA molecules are not decomposed, it is suggested that the broadenings are caused by a strong interaction of the DMpNA molecule with Na+. The mechanism of the reversible color change is summarized in Scheme 1. In the hydrated form, DMpNA molecules are far from Na+ because of the presence of H2O around Na+. The weak interaction with Na+ provides the typical UV-vis absorption band which is similar to that of crystalline DMpNA. On the other hand, when the amount of water molecules is reduced by the heat treatment, DMpNA molecules can be present in the neighborhood of Na+. The strong interaction with Na+ provides the extraordinarily large shift of the absorption band in the UV-vis spectrum. The most plausible interaction for the large shift of the π-π* transition is suggested to be the cation-π interaction.26 The above change takes place reversibly by the treatment of dehydration and hydration. Namely, adsorbed water molecules control the strength of the interaction between Na+ and DMpNA molecules. (26) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303.
Langmuir, Vol. 19, No. 6, 2003 1989
Other Na+-containing compounds are expected to show similar phenomena. To confirm this, DMpNA was adsorbed on powders of crystalline NaCl with the similar preparation method, and its absorption spectrum was measured. The spectrum in the hydrated form showed a π-π* transition band similar to that of crystalline DMpNA, while the band in the dehydrated form was broadened at the longer-wavelength side and had a tail exceeding 550 nm. These results indicate that the shift of the π-π* transition band is caused by the interaction with Na+. However, the magnitude of the total shift is very small in the case of NaCl powders, probably because only a small part of the DMpNA molecules can contact with Na+. In general, the zeolite host is advantageous for dispersing and stabilizing chromophores in the micropore while H2O molecules can be excluded from the micropores selectively. Furthermore, cations inside the zeolite do not disappear because they must compensate for the minus charge of the framework. Consequently, the cationic zeolites provide a good working field for neutral chromophores. We are currently investigating other combinations of neutral chromophores and cationic hosts possessing various kinds of alkali-metal ions. So far, we have found that the magnitude of the π-π* band shift is smaller for K+ than for Na+. Other neutral chromophores are also being investigated. These studies will be reported in due course. Conclusion An extraordinarily large shift of the absorption band of DMpNA is observed by use of direct interaction with cations in zeolites. Reversible color change is achieved by the treatment of hydration and dehydration. Solid-state 23 Na MAS NMR with a cross-polarization technique has revealed that adsorbed water molecules control the strength of the interaction between Na+ and DMpNA molecules. LA0270688
