Cation-π-Interaction Promoted Aggregation of Aromatic Molecules and

4912

Langmuir 2000, 16, 4912-4921

Cation-π-Interaction Promoted Aggregation of Aromatic Molecules and Energy Transfer within Y Zeolites K. J. Thomas,† R. B. Sunoj,‡ J. Chandrasekhar,*,‡ and V. Ramamurthy*,† Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, and Department of Organic Chemistry, Indian Institute of Science, Banaglore, India Received December 20, 1999. In Final Form: February 23, 2000 Photophysical studies of naphthalene confirm that aromatic molecules tend to aggregate within cation exchanged Y zeolites. Ground-state aggregation is traced to the presence of cation-aromatic π-interaction. Solvents that can coordinate to the cation “turn off” the cation-aromatic interaction, and consequently aggregation does not occur in zeolites that are impregnated with the above solvents. The solvent that exhibits a maximum in such an effect is water. MP2 calculations on cation-benzene dimer indicate that cation-π-interaction results in stabilization of the π-stacked benzene dimer. Results of MP2 calculations are consistent with the formation of ground-state π-stacked aggregates of naphthalene molecules within Y zeolites.

Introduction A reaction cavity may be defined as the space within which a reaction is visualized to occur. The concept, originally due to Cohen, has been extensively utilized to understand solid-state reactions.1 The cavity can be either “active” or “passive”.2 A passive cavity influences the behavior of the guest/reactant exclusively through nonbonded interactions. The distribution of guest molecules within passive reaction cavities is expected to follow a simple statistical rule. An active reaction cavity, in contrast, possesses functional groups that can interact with the guest/reactant through additional weak forces such as hydrogen bonding, cation-π-interaction, etc., and consequently lead to more complex patterns in the distribution of guest molecules. Zeolites X and Y, used as reaction media in this work, contain interconnected supercages (pore diameter ) 12 Å and window to pore ) 9 Å).3 The presence of exchangeable cations and the anionic walls make the reaction cavities within these zeolites active.3 It has been established that within the active reaction cavities of a Y zeolite, instead of the statistical distribution pattern, guest molecules prefer distinct positions.4 For example, depending on the loading level, benzene prefers two locations within a Y zeolite, the cation site and the window site.5 The nature of interactions in these two sites differ. In the former, cation-π-interaction6 plays a key role, while, in the latter, the primary force that holds a guest molecule is the CH‚‚‚O interaction. † ‡

Tulane University. Indian Institute of Science.

(1) Cohen, M. D. Angew. Chem., Int. Edn. Engl. 1975, 14, 386. (2) (a) Ramamurthy, V.; Weiss, R. G.; Hammond, G. S. Adv. Photochem. 1993, 18, 67. (b) Weiss, R. G.; Ramamurthy, V.; Hammond, G. S. Acc. Chem. Res. 1993, 26, 53. (3) Breck, D. W. Zeolite Molecular Sieves; Krieger Publishing Co.: Malabar, FL, 1984. (4) For a few selected studies see: (a) Hong, S. B.; Cho, H. M.; Davis, M. E. J. Phys. Chem. 1993, 97, 1622, 1629. (b) Liu, S. B.; Ma, L. J.; Lin, M. W.; Wu, J. F.; Chen, T. L. J. Phys. Chem. 1992, 96, 8120. (c) Pearso, J. G.; Chmelka, B. F.; Shykind, D. N.; Pines, A. J. Phys. Chem. 1992, 96, 8517. (d) O’Malley, P. J. Chem. Phys. Lett. 1990, 166, 340. (e) Unland, M. L.; Freeman, J. J. J. Phys. Chem. 1978, 82, 1036. (5) (a) Fitch, A. N.; Jobic, H.; Renouprez, A. J. Phys. Chem. 1986, 90, 1311. (b) Jobic, H.; Renouprez, A; Fitch, A. N.; Lauter, H. J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3199. (6) (a) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303. (b) Dougherty, D. A. Science 1996, 271, 163.

Aromatic molecules larger than benzene, due to their size, cannot be stabilized at the window sites via CH‚‚‚O interactions. Results of photophysical studies on pyrene by Thomas’ and our groups suggested that the cation sites within Na-Y zeolite are preferred by pyrene molecules.7,8 The same cation site is also believed to be responsible for the ground-state aggregation that gives rise to excimer emission.8 These conclusions have recently been extended to anthracene by Hashimoto’s group.9 The likely role of a cation in promoting aggregation of aromatic molecules is intriguing. To investigate whether π-stacking interaction can be enhanced through polarization by a metal cation, we have carried out ab initio calculations using a benzene dimer as a model system. Although photophysical studies have been carried out on larger aromatic molecules such as naphthalene, anthracene, and pyrene, the computed results on the simpler system are of considerable significance. The nature of the benzene-benzene interaction has been the subject of intense scrutiny both experimentally10 and theoretically.11 It is generally accepted that the T-shape (herringbone geometry) is the preferred form, not only in the solid state (7) (a) Liu, X.; Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1989, 93, 4120. (b) Iu, K.-K.; Thomas, J. K. Langmuir 1990, 6, 471. (c) Liu, X.; Thomas, J. K. Langmuir 1993, 9, 727. (d) Liu, X.; Thomas, J. K. Chem. Mater. 1994, 6, 2303. (8) (a) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Phys. Chem. 1993, 97, 13380. (b) Ramamurthy, V. Mol. Cryst. Liq. Cryst. 1994, 240, 53. (c) Ramamurthy, V.; Turro, N. J. J. Inclusion Phenom. Mol. Recogit. Chem. 1995, 21, 239. (9) (a) Hashimoto, S.; Fukazawa, N.; Fukumura, H.; Masuhara, H. Chem. Phys. Lett. 1994, 219, 445. (b) Hashimoto, S.; Ikuta, S.; Asahi, T.; Masuhara, H. Langmuir 1998, 14, 4284. (c) Hashimoto, S.; Ikuta, S. J. Mol. Struct. (THEOCHEM) 1999, 468, 85. (10) (a) Arunan, E.; Gutowsky, H. S. J. Chem. Phys. 1993, 98, 4294. (b) Janda, K. C.; Hemminger, J. C.; Winn, J. S.; Novick, S. E.; Harris, S. J.; Klemperer, W. J. Chem. Phys. 1975, 63, 1419. (c) Henson, B. F.; Hartland, G. V.; Venturo, V. A.; Felker, P. M. J. Chem. Phys. 1992, 97, 2189. (d) Ebata, T.; Hamakado, M.; Moriyama, S.; Morioka, Y.; Ito, M. Chem. Phys. Lett. 1992, 199, 33. (11) (a) Spirko, V.; Engkvist, O.; Soldan, P.; Selzle, H. L.; Schlag, E. W.; Hobza, P. J. Chem. Phys. 1999, 111, 572. (b) Sun, S.; Bernstein, E. R. J. Phys. Chem. 1996, 100, 13348. (c) Hobza, P.; Selzle, H. L.; Schlag, E. W. J. Am. Chem. Soc. 1994, 116, 3500. (d) Pang, Y.-P.; Miller, J. L.; Kollman, P. A. J. Am. Chem. Soc. 1999, 121, 1717. (e) Chipot, C.; Jaffe, R.; Maigret, B.; Pearlman, D. A.; Kollman, P. A. J. Am. Chem. Soc. 1996, 118, 11217. (f) Hobza, P.; Selzle, H. L.; Schlag, E. W. J. Phys. Chem. 1996, 100, 18790. (g) Meijer, E. J.; Sprik, M. J. Chem. Phys. 1996, 105, 8684.

10.1021/la991654s CCC: $19.00 © 2000 American Chemical Society Published on Web 05/03/2000

Aromatic Molecule Aggregation within Y Zeolites

but in the gas phase and hydrated phase12 as well. A parallel displaced structure, in which the π units are not fully aligned but still interact with each other, has often been in contention as an alternative minimum in the dimer potential energy surface. The symmetrically stacked structure is at best weakly attractive. However, the interaction energy in this geometry may be influenced to a greater degree by a distal metal cation. The magnitude of enhancement is quantified using correlated levels of theory in this study. The result has direct implications for the interpretation of photophysical studies on larger and more polarizable aromatics in which arene-arene interactions are expected to be intrinsically stronger.7-9 Most of the photochemical studies within zeolites thus far have been limited to unimolecular processes.13 The exact molecular distribution and the loading levels of the reactant are expected to be less important in the unimolecular processs within a zeolite. On the other hand, these are expected to influence the outcome of bimolecular processes within a zeolite. Knowledge of the factors that control the guest distribution within a zeolite allows us to carefully plan bimolecular processes such as energy transfer, electron transfer, and reactions within zeolites. In this context we have explored the possibility of singletsinglet energy transfer between aromatic molecules within cation exchanged X and Y zeolites. The naphthaleneanthracene pair which has been chosen as a model system for our study has been extensively investigated in solution as well as in ordered media.14,15 As a prerequisite to understanding the phenomenon of energy transfer between naphthalene and anthracene, we have investigated the photophysics of naphthalene alone within M-Y and M-X zeolites. Results of these studies provide interesting insights into the role of cations in determining the aggregation of aromatics and, in turn, their photophysical behavior. Experimental Section Materials. Zeolites Na-X and Na-Y in powder form were obtained from The PQ Corp. The cation of interest (Li+, K+, Rb+, Cs+, Ca2+, Mg2+, and Sr2+) was exchanged into these powders by contacting the zeolite with the appropriate nitrate solution at 90 °C. A 10 mL aliquot of a 10% nitrate solution was used for each gram of the zeolite. The cation exchange was repeated three times. The samples were then thoroughly washed with water and dried. Exchange loading was typically between 37 and 84%. Naphthalene, anthracene, and phenanthrene were obtained from Aldrich (99.9+% purity) and recrystallized from ethanol three times to constant melting point. (12) (a) Karlstrom, G.; Linse, P.; Wallqvist, A.; Jonsson, B. J. Am. Chem. Soc. 1983, 105, 3777. (b) Jorgensen, W. L.; Severance, D. L. J. Am. Chem. Soc. 1990, 112, 4768. (c) Linse, P. J. Am. Chem. Soc. 1993, 115, 8793. (d) Hobza, P.; Selzle, H. L.; Schlag, E. W. J. Chem. Phys. 1990, 93, 5893. (13) (a) Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Acc. Chem. Res. 1992, 25, 299. (b) Ramamurthy, V.; Robbins, R. J.; Thomas, K. J.; Lakshminarasimhan, P. H. In Organised Molecular Assemblies in the Solid State; Whitsell, J. K., Ed.; John Wiley & Sons: Chichester, U.K., 1999; pp 63-140. (14) (a) Schnepp, O.; Levy, M. J. Am. Chem. Soc. 1962, 84, 172. (b) Weber, S. E. Chem. Rev. 1990, 90, 1469. (c) Kimizuka, N.; Kunitake, T. J. Am. Chem. Soc. 1989, 111, 3758. (d) Sasaki, H.; Sisido, M.; Imanishi, Y. Langmuir 1991, 7, 1944. (e) Kumar, C. V.; Chaudhari, A.; Rosenthal, G. L. J. Am. Chem. Soc. 1994, 116, 403. (f) Bai, F.; Chang, C.-H.; Weber, S. E. Macromolecules 1986, 19, 2484. (g) Aspler, J. S.; Hoyle, E. E.; Guillet, J. E. Macromolecules 1978, 11, 925. (h) Holden, D. A.; Guillet, J. E. Macromolecules 1980, 13, 280. (i) Ng, D.; Yeshiki, K.; Guillet, J. E. Macromolecules 1983, 16, 568. (j) Bigger, S. W.; Ghiggino, K. P.; Ng, S. K. Macromolecules 1989, 22, 800. (15) Exciplex formation between anthracene and naphthalene competes with the singlet-singlet energy-transfer process. Although exciplex emission has been observed in some special cases, the exciplex emission has not been observed in the parent systems. (a) Chandross, E. A.; Ferguson, J. J. Chem. Phys. 1967, 47, 2557. (b) Chandross, E. A.; Schiebel, A. H. J. Am. Chem. Soc. 1973, 95, 611. (c) Ferguson, J.; Mau, A. W.-H.; Whimp, P. O. J. Am. Chem. Soc. 1979, 101, 2370.

Langmuir, Vol. 16, No. 11, 2000 4913 Activation of Zeolites. About 250 mg of the zeolite was placed in a silica crucible and heated at 500 °C for about 12 h. The freshly activated zeolites were rapidly cooled in air to ca. 50 °C and added to hexane solutions of the guests of interest. The zeolites were used immediately after activation. In general, we have found that the time required for these activated zeolites to readsorb water to their full capacity is about 2 h under our laboratory conditions, though the duration varies with the zeolite and the laboratory humidity. The rehydration is easily monitored by the weight gained by the activated zeolites placed in an analytical balance. Preparation of Zeolite-Aromatic Complexes. Known amounts of aromatics (naphthalene or phenanthrene) and the activated zeolites were stirred together in 20 mL of hexane for about 10 h. In a typical preparation 250 mg of the zeolite and a known amount of the guest were taken in 20 mL of hexane. Loading levels of naphthalene were varied between 7 and 25 mg/250 mg of hydrated zeolite, corresponding to an occupancy number (average number of molecules per supercage) variation of 0.32-1.32 (i.e., one in three cages to one in 0.75 cages). The white powder collected by filtration of the solvent was washed several times with hexane and dried. Samples were taken in Pyrex cells fitted with Teflon stopcocks, degassed thoroughly (10-4 mm), and sealed. Complete dehydration of Na-Y at higher loading levels of naphthalene required degassing at elevated temperatures. The samples could not be heated above 60 °C as naphthalene sublimed at 60 °C. Transferring of samples to quartz cells (for diffuse reflectance spectra) and ESR tubes (for fluorescence spectra) were carried out in a drybox and sealed with a Teflon stopcock. Coadsorption of Water. The complexes prepared and dried as above were placed on the pan of a Mettler balance maintained under controlled humidity, and the weight increase was noted. On adsorption of the required amount of water the sample was sealed and equilibrated at 50 °C for about 4 h. Coadsorption of Solvents. The solvent vapor was introduced into the zeolite sample under dry conditions. All manipulations were performed on a vacuum line. A cell with two arms was used so that heat and vacuum could be applied to individual arms. The zeolite loaded with aromatic molecules was placed in one arm and in the other a few milliliters of the required solvent. The solvent arm was cooled with liquid nitrogen while the zeolite was being evacuated (10-4 Torr) for 24 h. The dried zeolite sample was exposed to solvent vapor by heating the arm containing the solvent. The zeolite sample was kept under solvent vapors for 24 h and transferred to quartz cells under nitrogen atmosphere in a drybox and sealed with a Teflon stopcock. Preparation of Zeolite-Naphthalene-Anthracene Complexes. The zeolite (300 mg) activated at 500 °C for 12 h was stirred with a solution of known amount of naphthalene and varying amounts of anthracene in 20 mL of hexane for about 10 h. Three sets of samples containing naphthalene concentrations of 0.4, 4.0, and 24 mg/300 mg of wet Na-Y were prepared. The anthracene concentration was varied between 0.08 and 0.8 mg/ 300 mg of wet Na-Y. The zeolite-aromatics complex was filtered, washed several times with hexane, and dried. The samples were then transferred to Pyrex tubes fitted with Teflon stopcocks and evacuated on a high vacuum pump (10-4 mm) for at least 24 h. Transferring of the sample to quartz cells or ESR tubes was carried out in a drybox under nitrogen atmosphere. The quartz cells and ESR tubes were sealed with Teflon tape and used for the spectral measurements. Spectral Measurements. Absorption spectra were recorded on a Shimadzu 2101PC UV vis spectrophotometer with a diffuse reflectance accessory attachment. The dried zeolite samples were packed into a 2 mm quartz cuvette (in a drybox) and sealed with Teflon tape. Conversion of the reflectance into absorption spectra was carried out using a Kubelka-Munk program supplied with the instrument. Emission spectra were recorded on an Edinburgh FS-900 CDT spectroflourimeter. Solid zeolite samples were taken in quartz ESR tubes placed in a quartz cylindrical Dewar, and the emissions collected at right angles were recorded. Lifetime measurements were carried out on an Edinburgh FL-900 CDT single photon counter using a hydrogen filled nanosecond flash lamp (40 kHz) as the light source. The samples were excited at 280 nm, and the

4914

Langmuir, Vol. 16, No. 11, 2000

emitted photons were collected at the peak maximum of the fluorescence bands of the probe molecules. The lamp profile was collected with Ludox as the scattering medium. The observed decays were fitted to optimal analysis using the distribution or exponential analysis program supplied with the instrument. The solution-state lifetimes were fitted to an exponential fit. The suitability of the fit was ascertained by a χ2 value close to unity. The decays within zeolites were multicomponent. Computational Details. The structures of benzene, benzene dimer, and complexes of alkali metal cations (M ) Li+, Na+, K+, Rb+, Cs+) with a single benzene molecule as well as with a stacked pair of benzene molecules were optimized at the second-order Møller-Plesset (MP2) level.16 For benzene and its dimer, D6h symmetry constraints were imposed, while the complexes were optimized with C6v symmetry. The polarized 6-31G(d) basis set was used for C, H, Li, and Na. For the heavier metal ions, the Hay-Wadt effective core potentials were used.17 The contraction scheme employed for the valence region was (5s5p)/[3s2p], augmented by a six-term d-polarization function. Thus, the valence basis is comparable to the 6-31G(d) basis used for the remaining atoms. The Gaussian 94 suite of programs were used in all calculations.18

Thomas et al.

Figure 1. Normalized emission spectra of naphthalene in zeolite Na-Y: (a) 0.33, (b) 0.66, and (c) 1.32 molecules/ supercage. The excitation wavelength was set at 280 nm.

Results In this investigation steady-state and time-resolved single photon counting techniques have been employed to probe the aggregation behavior of naphthalene and the energy transfer between naphthalene and anthracene within cation exchanged zeolites. “Dry” zeolites were used for all experiments unless otherwise mentioned. The procedure adopted to prepare dry zeolites is outlined in the Experimental Section. Studies were carried out in alkali metal cation exchanged X and Y zeolites. A few experiments with alkaline earth cation exchanged X zeolites were also performed. Since activated divalent alkaline earth cation exchanged Y zeolites were acidic, they were not used.19 Since the results within alkali metal and alkaline earth cation exchanged X zeolites were similar to that in alkali metal cation exchanged Y zeolites, results obtained with alkali metal cation exchanged Y zeolites alone are discussed here. We have shown earlier that at low loading levels of naphthalene in alkali metal cation exchanged X and Y zeolites (〈S〉, average number of molecules per supercage, ) 0.1) only the monomer emission is observed both at 300 and at 77 K.20 The intensities of fluorescence and phosphorescence were dependent on the nature of the cation. The external heavy atom effect due to heavier cations such as Cs+ and Tl+ significantly enhanced the phosphorescence emission from naphthalene included within M-X and M-Y zeolites. The loading levels of naphthalene were much higher (〈S〉 ) 0.32-1.32) in this study than in our earlier ones, and under present conditions both excimer and monomer emissions were observed. Since the primary aim of this investigation is (16) Hehre, W. J; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (17) Hay, J. P.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision C.2; Gaussian, Inc.: Pittsburgh, PA, 1995. (19) (a) Thomas, K. J.; Ramamurthy, V. Langmuir 1998, 14, 6687. (b) Kao, H. M.; Grey, C. P.; Pitchumani, K.; Lakshminarasimhan, P. H.; Ramamurthy, V. J. Phys. Chem. 1998, 102, 5627. (20) (a) Ramamurthy, V.; Caspar, J. V.; Eaton, D. F.; Kuo, E. W.; Corbin, D. R. J. Am. Chem. Soc. 1992, 114, 3882. (b) Ramamurthy, V.; Caspar, J. V.; Corbin, D. R.; Schlyer, B. D.; Maki, A. J. Phys. Chem. 1990, 94, 3391.

Figure 2. Normalized emission spectra of naphthalene in zeolites: (a) Li-Y, (b) Na-Y, and (c) K-Y. The loading level was 1.3 molecules/supercage, and the excitation wavelength was set at 280 nm.

to probe the phenomenon of ground-state aggregation of aromatic molecules within Y zeolites, no discussion on the heavy cation effect will be made. The recorded emission spectra of naphthalene included in dry Na-Y at three loading levels are shown in Figure 1. Two emissions, one due to monomer and the second due to excimer fluorescence are seen. The intensity of excimer emission is dependent on the loading level, the intensity being higher at higher loading level. The emission at longer wavelength centered around 410 nm is in the same region as the excimer emission observed in solution.21 An important observation relates to the dependence of the excimer emission maximum on the nature of the alkali metal cation (Figure 2). The excimer emission maxima in Li-, Na-, and K-Y zeolites are as follows: 390, 410, and 430 nm. The excitation spectra for the monomer and excimer fluorescence were not identical. The peak maxima for the monomer and excimer emissions were at 280 and 255 nm, respectively, suggesting that the two emissions originate from naphthalene molecules present in different environments. This observation suggested that the excimer emission observed within Na-Y might not be “dynamic” in nature. To explore the possibility that the excimer (21) (a) Chandross, E. A.; Dempster, C. J. J. Am. Chem. Soc. 1970, 92, 3586. (b) Selinger, B. K. Aust. J. Chem. 1966, 19, 825. (c) Castanheira, E. M. S.; Martinho, J. M. G. J. Photochem. Photobiol A: Chem. 1994, 80, 151.

Aromatic Molecule Aggregation within Y Zeolites

Langmuir, Vol. 16, No. 11, 2000 4915

Table 1. Fluorescence Lifetime of Naphthalene in Different Zeolites at Various Loading Levels

zeolite

concn (10-4 M/g)

concn molecules /supercage

Li-Y Li-Y Li-Y Na-Y Na-Y Na-Y K-Y K-Y K-Y Rb-Y Rb-Y Rb-Y

2 4 8 2 4 8 2 4 8 2 4 8

0.32 0.63 1.27 0.33 0.66 1.32 0.35 0.70 1.40 0.39 0.78 1.57

a

excited-state singlet lifetime of naphathalene (ns) τ1 τ2 χ2 38 (70%)a 28 29 35 (74%)a 31 31 14 (76%)a 17 18 5 5.5 5.0

13 (30%)a 12 11 12 (26%)a 12 11 7 (24%)a 8 8

1.12 1.02 1.19 1.05 1.14 1.09 1.14 1.24 1.11 1.11 1.13 1.10

Percentage of each component is indicated in parentheses.

emission originates from the ground-state “static” dimers, the excited-state fluorescence (S1) lifetime was monitored by the single photon counting technique. The excited-state fluorescence lifetime of the monomer was relatively independent of the loading level of naphthalene. The fluorescence decay could only be fitted into two components. The data for Li-Y, Na-Y, K-Y, and Rb-Y are provided in Table 1. The percentages of the two components that gave a better fit are included in the table. These numbers are approximate, and no meaningful conclusion can be drawn from these numbers. Clearly an increase in the loading level of naphthalene did not lead to a significant reduction in the excited-singlet-state (S1) lifetime. The dependence of the S1 lifetime of naphthalene on a cation in Table 1 is due in part to the heavy cation effect reported previously.20 The electric field generated by the cation may also influence the excited singlet lifetime of naphthalene. Further work is needed to understand this phenomenon. To ascertain the role of the cation on the ground-state dimer formation, the emission spectra of naphthalene were recorded in the presence of various coadsorbed solvents that are capable of interacting with the cations and thus disrupting the cation-aromatic interaction. As a comparison spectra in the presence of inert solvents were also recorded. The solvent molecules were adsorbed into naphthalenesNa-Y by the vacuum line technique outlined in the Experimental Section. The emission spectra in the presence of various solvents are shown in Figures 3 and 4. Solvents such as hexane, cyclohexane, benzene, and dichloromethane enhance the intensity of excimer emission, whereas solvents such as methanol, acetonitrile, acetone, and tetrahydrofuran reduce the intensity of the excimer emission. Water was found to be the best coadsorbent able to turn “off” completely the excimer emission (Figure 5). The energy transfer between naphthalene and anthracene was investigated at three loading levels of naphthalene, (a) at a low loading level where the naphthalene molecules are well-separated from each other and no aggregates are expected, (b) at a moderate loading level where naphthalene molecules are separated by less than two cages and aggregation is still not expected, and (c) at a high loading level where the aggregation is expected and most naphthalene molecules are present in adjacent cages. Details on the projected distribution of molecules under these three conditions are presented in the discussion section. The naphthalene-anthracene pair has been extensively investigated in the context of singlet-singlet energy transfer in solution and ordered media.14,15 In this

Figure 3. Normalized emission spectra of naphthalene in zeolite Na-Y in the presence of coadsorbed solvents: (a) none, (b) hexane, (c) methylene chloride, (d) benzene, and (e) cyclohexane. The loading level of naphthalene was 1.3 molecules/ supercage and the excitation wavelength was set at 280 nm.

Figure 4. Normalized emission spectra of naphthalene in zeolite Na-Y in the presence of coadsorbed solvents: (a) none, (b) acetonitrile, (c) methanol, (d) acetone, and (e) tetrahydrofuran. The loading level of naphthalene was 1.3 molecules/ supercage, and the excitation wavelength was set at 280 nm.

Figure 5. Effect of water on excimer formation: normalized emission spectra of naphthalene in zeolite Na-Y in the (b) presence and (a) absence of water. The loading level of naphthalene was 1.3 molecules/supercage, and the excitation wavelength was set at 280 nm.

pair, naphthalene serves as the singlet energy donor and anthracene as the acceptor. In one set of experiments the loading level of naphthalene was maintained at 〈S〉 ) 0.2, i.e., one in five

4916

Langmuir, Vol. 16, No. 11, 2000

Figure 6. Fluorescence spectra recorded for naphthalenes anthracenesNa-Ycomplexes. The concentration of naphthalene was 0.17 molecules/supercage, and anthracene concentrations were (a) 0, (b) 0.0024, (c) 0.0037, (d) 0.0048, (e) 0.0072, (f) 0.0120, (g) 0.0180, and (h) 0.0240 molecules/ supercage. The excitation wavelength was set at 280 nm.

Figure 7. Stern-Volmer plot for quenching of naphthalene fluorescence emission and S1 lifetime by anthracene. The anthracene concentration is shown in terms of the number of molecules per supercage.

supercages. Under such conditions the excitation of naphthalene (within Na-Y in the absence of anthracene) results only in monomer emission. The loading level of anthracene was varied between 0.002 and 0.02, (i.e., one in 500 to one in 50 supercages). As illustrated in Figure 6, when anthracene is also present, the excitation (280 nm) of naphthalene resulted in fluorescence emission from both naphthalene and anthracene. Direct excitation (280 nm) of the Na-Y sample containing the highest loading level (〈S〉 ) 0.02) of anthracene with no naphthalene did not result in fluorescence, confirming that under the excitation wavelength used excitation of anthracene does not take place. The fluorescence observed upon excitation of naphthalene in the naphthalenesanthracenesNa-Y sample is identical to the solution fluorescence of anthracene. As seen in Figure 6, with increasing loading levels of anthracene, the fluorescence intensity of naphthalene decreases while that of anthracene increases concomitantly. This observation is consistent with the occurrence of energy transfer from S1 of naphthalene to that of anthracene. While the reduction in the fluorescence of naphthalene is evident, no corresponding decrease in the S1 lifetime was observed (Figure 7). The small decrease in the S1 lifetime certainly does not match the observed large decrease in naphthalene fluorescence intensity. Similar to the observations made with the excimer emission of naphthalene, the emission due to anthracene

Thomas et al.

Figure 8. Effect of water on the energy-transfer process: Emission specta of the naphthalene-anthracene system in Na-Y in the (a) absence and (b) presence of water. The excitation wavelength was at 280 nm.

Figure 9. Emission spectra of phenanthrene, 〈S〉 ) 0.4, included within dry and wet Na-Y: (a)-- dry Na-Y, λex ) 293 nm; (b) s, wet Na-Y, λex ) 320 nm.

can be switched “on” or “off” by controlling the amount of water (Figure 8). The effects of other solvents were not investigated. No exciplex emission between naphthalene and anthracene was seen under the loading levels employed.15 On the basis of the above observations with naphthalene and naphthalene-anthracene systems, we conclude that water molecules drastically affect the distribution of aromatic molecules within a zeolite. It is not clear whether the aromatic molecules were redistributed within the internal surfaces or driven to the outer surface of the zeolite. If all the included aromatic molecules were driven to the surface, microcrystallite formation would be expected. To investigate this phenomenon, we chose phenanthrene as the probe. Phenanthrene when present as microcrystals is known to show an emission different from that of the monomer.22 Thus we speculated that if phenanthrene molecules remain as monomers within “wet” Na-Y zeolite, one should observe monomer emission and if they are driven to the exterior by the included water to crystallize on the surface, the emission would resemble that of microcrystals. The recorded spectra under wet and dry conditions are shown in Figure 9. The emission spectrum within dry Na-Y due to the monomer and under wet conditions is due to the microcrystals. Having concluded that cation-arene-arene interactions play a critical role in aggregation of aromatic (22) Birks, J. B. Photophysics of Aromatic Molecules; WileyInterscience: New York, 1970; Chapter 11.

Aromatic Molecule Aggregation within Y Zeolites Chart 1. Definition of Interaction Energies and Geometricl Parameters (Dotted Lines Separate the Interacting Fragments)

Table 2. Interaction Energies (kcal‚mol-1) for the M+‚‚‚Benzene‚‚‚Benzene Systema metal ion

M+ with benzene (∆E1)

M+ with benzene-dimer (∆E2)

benzene with benzene (∆E3)

benzene with benzene-M+ (∆E4)

Li Na K Rb Cs

-43.78 -29.66 -16.74 -14.63 -11.94

-48.62 -33.34 -19.68 -17.04 -14.04

-2.08 -2.08 -2.08 -2.08 -2.08

-6.93 -5.77 -5.03 -4.50 -4.19

a

See Chart 1 for definition of interaction energies.

molecules within a zeolite, we were interested in probing the energetics associated with these structures. From the MP2 energies of benzene, benzene dimer, and complexes of alkali metal cations with a single benzene molecule as well as with a stacked pair of benzene molecules, the magnitudes of interaction between different subunits as shown in Chart 1 were computed. The values are provided in Table 2. The symmetrically stacked benzene dimer, although not the preferred stationary point on the potential energy (PE) surface, is calculated to be bound by about 2 kcal.mol-1. The value is comparable to previous estimates at similar levels of theory for this geometry, although lower values are obtained using the superior CCSD(T) procedure (coupled cluster theory with singles, doubles, and approximate triples corrections) with larger basis sets.11 Even after making allowance for some overestimation of the interaction energy by the MP2 procedure, the effect of a cation on arene-arene interaction is indicated to be quite dramatic. A benzene ring coordinated to a metal ion is polarized such that the open face interacts substantially more strongly with another benzene ring (Table 2). The enhancement in arene-arene interaction is the maximum for lithium ion, as expected. The binding energy of a benzene molecule to a Li+-complexed benzene ring is as large as 6.9 kcal mol-1. Even with the weakly coordinating Cs+ ion, the corresponding binding energy is double that of benzene dimerization energy at the same theoretical level. Discussion Ground-State Aggregation of Naphthalene. Excitation of naphthalene included within cation (Li+, Na+, and K+) exchanged Y zeolites resulted in both monomer and excimer emissions (Figure 1). The intensity of the excimer emission was dependent on the loading level of naphthalene. Most importantly, the excitation spectra for the monomer and excimer emissions were different. This suggests that the two emissions originate from naphthalene molecules present in different environments. One possibility is that the excimer emission originates from naphthalene molecules present as ground-state aggregates. This postulate is supported by the excited-singlet-

Langmuir, Vol. 16, No. 11, 2000 4917

state lifetime data provided in Table 1. Although excimer emission intensity increased with the loading level of naphthalene, the excited-singlet-state lifetime did not change significantly. As to the question of why aggregation of naphthalene molecules is favored within a cation exchanged Y zeolite several observations suggest a critical role to the cation. As seen in Figure 2, the intensity of the naphthalene excimer emission depends on the charge density and accessibility of the cation. The maximum intensity was recorded in NaY. To understand the observed unusual trend (Na-Y >Li-Y > K-Y), one needs to recognize that cations strongly interact with the adsorbed aromatic molecules and that Li+ ions, being buried within the walls of a Y zeolite,23 are less accessible to the adsorbed aromatic molecules. Cation-aromatic molecule interaction within a zeolite has been established through 2H NMR, IR, Raman, and X-ray studies.4,5,24 The same interaction we believe is responsible for the ground-state aggregation. According to our model the interaction between the cation and naphthalene polarizes the ground-state naphthalene, which then forms the ground-state dimers. Cations with higher charge density and polarizing power are expected to induce the formation of ground-state aggregates more readily than those with lower charge density and polarizing power. Thus Na+ is expected to favor aggregation more than K+. Further discussion on this model is presented in the subsection on theoretical calculations. The role of the cation in ground-state aggregation is also indicated by the dependence of excimer emission maxima on the nature of the cation in Y zeolite. It is known that naphthalene excimer emission maximum depends on the polarity of the medium. In general the emission maximum is blue shifted in polar solvents.21c The λmax occurring at 390 nm in LiY and 430 nm in K-Y is consistent with the expected electric fields of these cations.25,26 A close proximity of the cation to the naphthalene dimer aggregate is necessary for the cation to have an influence on the emission maximum. We are aware that polarity is not an appropriate measure of the field generated by the cation within a zeolite. In the absence of any other parameters we are using the term “polarity”. As far as we are aware, there are no reports on the influence of electric field on excimer emission maxima. The importance of the cation in the ground-state aggregation of naphthalene can also account for the influence of water on the excimer emission (Figure 5). As seen in Figure 5, the presence of water completely eliminates the excimer emission from naphthalene included within Na-Y. This can be understood on the basis of a lesser capability to form aggregates under reduced polarizing power of the cations by the coordinated water molecules. Any solvent that can coordinate to a cation should reduce the aggregation. As seen in Figure 4, this is indeed the case. The excimer emission is reduced in solvents such as methanol, acetone, acetonitrile, and tetrahydrofuran that contain lone pair electrons. It is likely (23) (a) Forano, F.; Slade, R. C. T.; Anderson, E. K.; Anderson, I. G.; Prince, J. Solid State Chem. 1989, 82, 95. (b) Shepelev, Yu. F.; Anderson, A. A.; Smolin, Yu. I. Zeolites 1990, 10, 61. (24) Hepp, M. A.; Ramamurthy, V.; Corbin, D. R.; Dybowski, C. J. Phys. Chem. 1992, 96, 2629. (25) (a) Rabo, J. A. Cat. Rev.-Sci. Eng. 1981, 23, 293. (b) Dempsey, E. Molecular Sieves; Society of Chemical Industry: London, 1968; p 293. (c) Angel, C. L. J. Phys. Chem. 1966, 70, 2420. (d) Ward, J. W.; Habgood, H. W. J. Phys. Chem. 1966, 70, 1178. (e) Angel, C. L. J. Phys. Chem. 1966, 70, 2420. (f) Shimokoshi, K.; Sugihara, H.; Yasumori, I. J. Phys. Chem. 1974, 78, 1770. (g) Sugihara, H.; Shimokoshi, K.; Yasumori, I. J. Phys. Chem. 1977, 81, 669. (26) Uppili, S.; Thomas, K. J.; Crompton, E. M.; Ramamurthy, V. Langmuir 2000, 16, 265.

4918

Langmuir, Vol. 16, No. 11, 2000

Thomas et al.

that the binding energies of these solvent molecules to Na+ ions are higher than that to the π-cloud of naphthalene. A point to note is that the excimer emission can be turned on or off by coadsorbed water. When water is introduced into a dry zeolite, the excimer emission is eliminated and removal of water from the wet zeolite by drying on a vacuum line recovers the excimer emission to the same degree. On the other hand, solvents that cannot coordinate to the cation surprisingly enhance the intensity of the excimer emission (Figure 3). This suggests that solvents such as hexane, cyclohexane, and dichloromethane facilitate better aggregation of naphthalene molecules in a zeolite whose cavities are filled by a solvent rather than a dry one. The origin of the influence of nonpolar solvents on the excimer emission intensity is less obvious. The above phenomenon bears similarity to the observation we made during the measurement of the polarity of the supercage with probes such as Nile blue and coumarin500.26 Using these probes the interior of the supercage of a Y zeolite was seen to become more polar in the presence of hexane and less polar in the presence of methanol and water. We attributed this to the polarity probes being pushed closer to the cation by the nonpolar solvents. It was speculated that the solvents that can coordinate to the cation move the probe away and the ones that cannot interact with the cation move the probe closer to the cation. The same model may account for the variation in excimer emission intensity with respect to the coadsorbed solvent molecules. Clearly the cation-π-interaction is stronger than cation-hexane or aromatic-hexane interactions. If this be otherwise, the excimer emission intensity would have been reduced. The enhanced excimer emission intensity in the presence of nonpolar solvents could be due to either enhancement of cation-π-interaction or stabilization of excimer structure by the added nonpolar solvent. The former would favor ground-state aggregation of naphthalenes, and the latter would permit the radiative process to compete favorably with a dissociative pathway that may take the excimeric state to a nonemissive dissociated state. Currently we are not sure which one of these two is responsible. We have not performed MP2 calculations on the effect of solvents on the energy of the ground-state dimer. The primary thesis of the present investigation is that a cation is responsible for ground-state aggregation of aromatic molecules included within a zeolite. The overall structure of the aggregate consists of three components, a cation and two aromatic molecules. The most energetically stable arrangement of a single cation and two aromatic molecules is the sandwich structure in which the cation is present between the two aromatic molecules (like ferrocene). Such a sandwich structure is not possible within a zeolite since one face of the cation is bound to the six-membered oxygen ring of the zeolite. The alternate feasible structure within a zeolite is the one in which the two aromatic molecules are stacked one on top of each other and the cation interacts with one of the aromatic molecules. The visualized structure in the case of naphthalene is shown in Figure 10.27 The MP2 geometry optimized structure of the lithium-benzene dimer complex is included for comparison in Figure 10. The zeolite supercage is large enough to accommodate the above structure (diameter of the supercage ∼ 12 Å; length, breadth, and height of the naphthalene dimer are 7.7, 5.7, 5.3 Å, respectively). The observed excimer emission

is consistent with the cation-aromatic-aromatic arrangement rather than with the aromatic-cationaromatic structure. Calculations on Cation-Benzene Dimer Interaction. The computed results strongly support the interpretation that the metal ions in zeolites promote aggregation of aromatic rings. The computed cation-benzene interaction energies (Table 2) compare quite well with available experimental data and estimates from related levels of theory.28 The use of an incomplete basis set and the associated basis set superposition error, neglect of higher order perturbation corrections and, zero point vibrational effects lead to a marginal overestimation of binding energies, especially for the lithium complex. Similarly, the benzene dimerization energy in the symmetrically stacked form is overestimated compared to CCSD(T) data,11 although a precise experimental value is not available. Notwithstanding these caveats, the prediction of enhanced binding in the metal-arene-arene complex is quite significant. All the alkali metal cations are indicated to bind more strongly to a benzene dimer than to a single benzene molecule. More important in the context of the present study, a metal-coordinated benzene ring interacts more strongly with a second benzene molecule (Table 2). The variation in the strength of π-stacking interaction with the metal ion follows the expected trend. The superior polarization by Li+ results in a substantially greater enhancement. The binding energy of the lithium-bound benzene to another benzene molecule is as large as 6.9 kcal.mol-1. The binding energy of a Cs+-coordinated benzene with a second benzene ring is lower, ca. 4 kcal‚mol-1, but still indicative of substantial metal polarization. The improved binding energy is also associated with predictable geometry changes. The interring separation is reduced in all the complexes compared to the value in the free benzene dimer (3.76 Å). The distance is only 3.55 Å in the lithium complex (Table 3). Correspondingly, the C-C bond length in the central ring is perturbed the most in this complex. The effect of the heavier alkali metal ions is generally smaller. In all the complexes, there is negligible geometry change in the distal benzene ring. The precise magnitude of the increase in arene-arene interaction is likely to be smaller in the zeolites due to the attenuating effect of the anions coordinated to the metal ion. However, the more polarizable aromatics studied experimentally are expected to respond to the metal ion to a greater extent. Metal ion induced arene-arene binding is thus indicated to be the most reasonable interpretation for the observed photophysical behavior.

(27) The structure was generated using the CACHE program, version 3.8, on a Power Mac 7300/200.

(28) Nicholas, J. B.; Hay, B. P.; Dixon, D. A. J. Phys. Chem. A 1999, 103, 1394.

Figure 10. Right: Cation stacked naphthalene dimer structure as generated by CACHE. Cation has been arbitrarily placed on one of the aryl rings. Left: MP2 geometry optimized structure of lithium ion-benzene dimer complex.

Aromatic Molecule Aggregation within Y Zeolites

Langmuir, Vol. 16, No. 11, 2000 4919

Table 3. Optimized Geometrical Parameters (Å) for the M+‚‚‚Benzene‚‚‚Benzene Systemsa metal ion

M-benzene ring distance (R1)

benzenebenzene distance (R2)

C-C length in central ring (R3)

C-C length in distal ring (R4)

Li Na K Rb Cs

1.902 2.369 2.804 3.082 3.403

3.550 3.584 3.649 3.662 3.667

1.406 1.404 1.401 1.400 1.399

1.398 1.397 1.397 1.397 1.397

a The interring and C-C distances for benzene dimer at the same level of theory are 3.756 and 1.397 Å, respectively. See Chart 1 for definition of geometrical parameters.

Implications of Theoretical Studies. The above discussion suggests that the main force that holds aromatic molecules within a zeolite is the cation-π-interaction. Although the binding energy of a cation to the dimer of an aromatic molecule is slightly higher than to a monomer, it is certainly lower than two cations binding to two aromatic molecules. Therefore, at low loading levels of aromatic molecules, wherein the cation to aromatic molecules ratio is high, one would expect aromatic molecules to exist as monomers bound to a cation. However, at higher loading levels wherein the cation/ aromatic molecule ratio decreases, aggregation is a possibility. Even under such conditions the ratio of monomer to dimer is expected to be small. The lower number of ground-state dimers escapes detection by NMR but can be readily detected by the fluorescence technique. Thus it is clear that prediction of the distribution of aromatic molecules within a zeolite should take into consideration the presence of the attractive force between cations and aromatic molecules. Singlet-Singlet Energy Transfer from Naphthalene to Anthracene. To generate a comprehensive model for understanding how two molecules communicate within a faujasite zeolite, we need to comprehend the internal structure of such a zeolite.3 The basic unit of a faujasite zeolite is the sodalite cage. Assembly of sodalite cages gives rise to a supercage (diameter 12 Å). Each supercage is connected to four other supercages in a tetrahedral fashion through 9 Å windows. Since every supercage is connected to four others, the internal structure of a zeolite consists of three-dimensionally interconnected supercages. A two-dimensional projection of the three-dimensional arrangement is shown in Figure 11. Consider the guest molecule (spherical ball) placed in one of the supercages in Figure 11. As shown by the dotted line in Figure 11, the guest molecule will have immediate access to four other cages. This central cage is surrounded by four cages in the first shell, eight cages in the second shell, and 12 cages in the third shell. The approximate distances between the midpoint of the center cage and that of the first, the second, and the third shell cages are ∼12,