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Probing Zeolites with Organic Molecules: Supercages of X and Y Zeolites Are Superpolar† Sundararajan Uppili, K. J. Thomas, Elizabeth M. Crompton, and V. Ramamurthy* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 Received April 2, 1999. In Final Form: July 4, 1999 Supercages of Li+- and Na+-exchanged X and Y zeolites are much more polar than even water. The extent of polarity depends on the nature and the number of cations present within a supercage. The polarity of Li+- and Na+-exchanged X and Y zeolites decreases in the presence of water. In presence of water the contribution of cations toward polarity is much smaller than water itself. In this study polarity has been monitored with organic probe molecules, Nile red, pyrene 1-carboxaldehyde and coumarin-500. A connection between “polarity” and “electric field” within a cage has also been established. Since the supercages are much more polar than all organic solvents, they can be characterized as “superpolar”. Because of this one may be able to achieve excited-state switching of carbonyl compounds within a zeolite while such may not be possible in organic solvents. The nπ*-ππ* state switching of acetophenones is easily achieved within a zeolite while such does not occur in polar solvent methanol-ethanol mixture.
Introduction Selectivity in organic phototransformations continues to be one of the main topics of current research activity. Of the various approaches, the use of constrained and organized media has shown considerable promise.1 Organized media allow one to design a system and carry out photochemical and photophysical studies within these assemblies in a more temporally and structurally quantifiable fashion than possible in isotropic media. Differences in product selectivity obtained for a particular reaction in various media are attributable, although not solely, to the differences in size, shape, and nature of reaction cavity they provide.2 However, when the medium is “active” as is zeolites,3 predictions concerning the excited state behavior of guest molecules can be made only by taking into consideration possible interactions that may exist between the cavity and the guest molecules. The common analogy of guests within zeolites to balls in boxes is very deficient. To be able to plan and rationalize selectivity within a zeolite, one needs to understand the internal characteristics of the zeolite itself. Recently, we developed a simple method to identify and quantify acidic sites within zeolites.4 By this approach we showed that even alkali cation exchanged Y zeolites are acidic and contain one H+ per 16 supercages. This finding helped us understand the side reactions that occur during singlet oxygen oxidation of olefins within zeolites.5 We present below the results of our studies directed toward characterizing yet another property, namely, the “micropolarity” of X and Y zeolites. † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”.
(1) Ramamurthy, V., Ed. Photochemistry in Organized and Constrained Media; VCH: New York, 1991. (2) (a) Weiss, R. G.; Ramamurthy, V.; Hammond, G. S. Acc. Chem. Res. 1993, 26, 530. (b) Ramamurthy, V.; Weiss, R. G.; Hammond, G. S. Adv. Photochem. 1993, 18, 67. (3) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; John Wiley and Sons: New York, 1974. (4) (a) Jayathirtha Rao, V.; Perlstein, D. L.; Robbins, R. J.; Lakshminarasimhan, P. H.; Kao, H.-M.; Grey, C. P.; Ramamurthy, V. J. Chem. Soc., Chem. Commun. 1998, 269. (b) Thomas, K. J.; Ramamurthy, V. Langmuir 1998, 14, 6687.
A zeolite may be viewed as a powerful “solid-state ionizing solvent”.6 Such a property would imbue on a zeolite an ability to stabilize ground state donor-acceptor charge-transfer complexes. Formation of CT complexes between oxygen and hydrocarbons, aromatics and DMV2+, and aromatics and tetracyanobenzene within zeolites has indeed been established by Frei et al., Kochi and Yoon, and Hashimoto, respectively.7 While such pioneering studies indicate that the supercages of X and Y zeolites must be highly polar, no definite information is available on the micropolarity of zeolites. Although, efforts have been made in the past to probe the polarity of zeolites, the reported micropolarity of zeolites vary depending on the water content and on the nature of the probe.8 Therefore, we felt that it is important to determine the micropolarity of carefully prepared dry and wet X and Y zeolites with organic probes. The meaning of “micropolarity” of a zeolite interior is somewhat nebulous. To capture the meaning of “micropolarity” within a zeolite one needs to have an understanding of the structure of a supercage.3 As illustrated in Figure 1, the supercage of X and Y zeolites is built of SisOsAl and SisOsSi networks. The SisOsAl unit carries a negative charge, and the charge-compensating cations present in X and Y zeolites occupy three positions: The first type, site I, with 16 cations per unit cell is located on the hexagonal prism faces between the sodalite units. The (5) (a) Li, X.; Ramamurthy, V. J. Am. Chem. Soc. 1996, 118, 10666.(b) Robbins, R.; Ramamurthy, V. J. Chem. Soc., Chem. Commun. 1997, 1071.(c) Several alcohols, the products of oxidation, have been found to be unstable within Na+ Y zeolite due to the presence of Brønstead acid sites: Shailaja, S.; Ramamurthy, V. Unpublished results. (6) (a) Ward, J. W. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976; p 118. (b) Kasai, P. H.; Bishop, R. J. J. Am. Chem. Soc. 1972, 94, 5560. (c) Kasai, P. H.; Bishop, R. J. J. Phys. Chem. 1973, 77, 2308. (7) (a) Yoon, K. B.; Huh, T. J.; Kochi, J. K. J. Phys. Chem. 1995, 99, 7042. (b) Frei, H.; Blatter, F.; Sun, H. Chemtech. 1996, 26, 24. (c) Hashimoto, S. J. Chem. Soc., Faraday Trans. 1997, 93, 4401. (8) (a) Baretz, B.; Turro, N. J. J. Photochem. 1984, 24, 201. (b) Yoon, K. B.; Kochi, J. K. J. Phys. Chem. 1991, 95, 1348. (c) Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1988, 110, 6586. (d) Dutta, P. K.; Tubeville, W. J. Phys. Chem. 1991, 95, 4087. (e) Iu, K. K.; Thomas, J. K. Langmuir 1990, 6, 471. (f) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. Photochem. Photobiol. 1992, 56, 297. (g) Sarkar, N.; Das, K.; Natha, D. N.; Bhattacharyya, K. Langmuir 1994, 10, 326.
10.1021/la990392r CCC: $19.00 © 2000 American Chemical Society Published on Web 09/30/1999
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Figure 1. The basic structural unit of X and Y zeolites. Entrance diameter of a supercage is shown on the left with the dimensions of a guest molecule, pyrene. Cation locations within a supercage are shown as shaded circles. Chart 1
second type, site II, with 32 per unit cell, is located in the open hexagonal faces. The third type, site III, with 38 per unit cell in the case of X and only 8 per unit cell in the case of Y type, is located on the walls of the larger cavity. Only cations at sites II and III are expected to be readily accessible to the organic molecules adsorbed within a supercage. In a dry zeolite, both site II and III cations are shielded on one side only; the side exposed to the center of the supercage remaining unshielded. This leads to a partial shielding of the anionic framework of the zeolite as well. These partially shielded charge centers generate large electric fields extending into the supercage.9 When a guest molecule is introduced into this medium, the zeolite will polarize the guest such that the negative end is stabilized by the cation and the positive end by the anionic framework. Such a phenomenon, in a qualitative sense, is similar to a reactant molecule being dissolved in a highly polar salt solution. Normally the term polarity is reserved to define the ability of a solvent to polarize a solute. On the other hand, in the field of zeolites both termsspolarity and electric fieldsare in use. It is our opinion that the micropolarity and the electric field in zeolites are manifestations of the presence of cations within them. One (9) (a) Rabo, J. A. Catal. Rev. -Sci. Eng. 1981, 23, 293. (b) Dempsey, E. Molecular Sieves: Society of Chemical Industry: London, 1968; p 293.
would therefore expect the “micropolarity” of a zeolite to depend on the cation. In this report we show that there is a relationship between the “micropolarity” and the number and nature of cations present within a supercage. All the probes employed in this study contain a carbonyl chromophore, and cations present within a zeolite are likely to interact with the lone pair of the CdO group. Under such conditions the polarity monitored is also an indication of the Lewis acidity of the cation. It is important to recognize that any probes used will have their own limitations, and the results are to be viewed within the context of their limits. Thus the probes used in this investigation are likely to show specific interactions with the cations and they probably sense the polarity very close to the cation. Since our ultimate aim is to use a zeolite to switch the excited-state ordering of carbonyls, the use of probes with carbonyl chromophore seemed appropriate. In this study, three organic molecules, Nile red (1), coumarin-500 (2), and pyrene 1-carboxaldehyde (3) (Chart 1) have been used as probes to monitor the polarity of the interior of the supercages of alkali cation exchanged (Li+, Na+, K+, Rb+, and Cs+) X and Y zeolites. The absorption and emission maxima and the excited singlet lifetime of these probes depend on the polarity of the medium in which they are present.10-12 The mechanism by which the medium influences the above spectral properties is not
Probing Zeolites with Organic Molecules
part of the presentation. Detailed discussions of such topics are available in the literature.10-12 The current study demonstrates that the micropolarity (as defined by the probes used in this investigation) of a zeolite supercage is far higher than any organic solvent in which most organic molecules can be dissolved. We therefore characterize the zeolite supercage to be “superpolar”. Owing to this specific environment, organic molecules would be expected to exhibit properties that may not be normally seen in any organic solvent. For example, we show here that unachievable alteration of electronic character of the lowest excited triplet state of a carbonyl compound in an organic solvent is possible within a zeolite. In this context we use three arylalkyl ketones 4-6 as examples (Chart 1). Experimental Section Materials. Nile red, 1 (Kodak), coumarin-500, 2 (Exciton) and pyrene-1-carboxaldehyde, 3 (Aldrich), were used as received. Acetophenone (Aldrich) and p-fluoroacetophenone (Aldrich) were distilled at reduced pressure and p-methoxyacetophenone (Aldrich) was recrystallized from hexane prior to use. Distilled dry hexane was used for loading the acetophenones 4-6 and the probe molecules 1-3 onto zeolites. Methylcyclohexane (spectroscopic grade), absolute ethanol (Florida distilleries), and methanol (Fischer Chemicals) were purified by passing through neutral alumina column after distillation. Zeolite Na+ Y (Zeolyst) and Na+ X (Aldrich) were used as received. Other zeolites of interest (Li+, K+, Rb+, and Cs+ Y and X) were prepared by repeated cation exchange as described previously.13 Sample Preparation. The quantities 1.33 µM Nile red (one molecule per 450 supercages), 6 µM coumarin-500 (one molecule per 100 supercages), and 6.67 µM pyrene-1-carboxaldehyde (one molecule per 90 supercages) per gram of zeolite were used throughout the experiments. In a typical run, 300 mg of a zeolite activated at 500 °C in an air oven (for 12 h) was stirred in a 5 mL solution of a probe molecule in hexane for 3-5 h. The slurry was filtered, washed with hexane, and dried on a vacuum line (∼10-4 Torr) for 24 h with slight heating (∼80 °C). The sample was transferred to a quartz cuvette (2 mm) in a drybox under a nitrogen atmosphere and sealed with Teflon tape. All samples used for monitoring the micropolarity were prepared paying particular attention to water content within a zeolite. On the other hand, acetophenones 4-6 were loaded onto zeolite by stirring an activated zeolite sample (300 mg) with a hexane solution of acetophenone on a laboratory bench. Samples were washed with excess hexane and air-dried on a Buchner funnel and used as such. In a typical run the loading level was one molecule in five supercages. Instrumentation. Absorption spectra were recorded on a Shimadzu 2101PC UV-vis spectrophotometer attached with a diffuse reflectance accessory. The dried zeolite samples were packed into a 2 mm quartz cuvette (in a drybox) and sealed with Teflon tape. The reflectance spectra were converted to absorption by using a Kubelka-Munk program supplied with the instrument. The solution spectra were recorded in 10 mm quartz cells using solvent as the reference. Emission spectra were recorded on an Edinburgh FS-900 CDT spectroflourimeter. Solid zeolite samples were taken in quartz electron spin resonance (ESR) tubes placed in a quartz cylindrical Dewar, and the emission collected at right angles was recorded. Phosphorescence spectra were recorded at 77 K using liquid nitrogen as the coolant. 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 emitted photons were collected at the peak maximum of the fluorescence (10) (a) Sacket, D. L.; Wolff, J. Anal. Biochem. 1987, 167, 228. (b) Deye, J. F.; Berger, T. A.; Anderson, A. G. Anal. Chem. 1990, 62, 615. (11) Chu, G.; Yangbo, F. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2533. (12) (a) Lianos, P.; Cremel, G. Photochem. Photobiol. 1980, 31, 429. (b) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
Langmuir, Vol. 16, No. 1, 2000 267 Table 1. Absorption and Fluorescence Properties of Probe Molecules in Solventsa solvent
abs max
em max
τ1 (ns)
Nile Red benzene CH2Cl2 MeOH 40% H2O/MeOH 60% H2O/MeOH
524 538 552 575 584
575 605 642 655 660
6.84 4.21 2.61 1.29
benzene CH2Cl2 MeOH 40% H2O/MeOH 60% H2O/MeOH
Coumarin-500 375 380 392 398 398
442 456 498 507 509
3.69 4.71 4.67 4.69 4.71
Pyrene-1-carboxaldehyde 396, 376, 364 392, 372, 362 417 393, 372, 362 453 398, 372, 365 460 399, 372, 365 462
1.76 4.40 5.40
benzene CH2Cl2 MeOH 40% H2O/MeOH 60% H2O/MeOH a
For extensive listing see refs 10-12.
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. Solution state lifetimes were fitted to exponential fit. The suitability of the fit was ascertained by a χ2 value close to unity. The decays are multicomponent and the predominant component (>85%) taken as the lifetime of the probe. The contribution from additional components was less than 10% in most cases. Water Adsorption Studies. Known amounts of the dried zeolite sample loaded with probe molecules 1-3 were allowed to adsorb water as monitored by increasing zeolite weight. Typically, 300 mg of a zeolite sample was found to adsorb up to ∼40 mg of water. When the weight of the zeolite stabilized, the sample was transferred to a quartz cuvette for measurements. The amount of water introduced is represented as the number of water molecules per zeolite supercage. Solvent Adsorption Studies. Solvent vapor was introduced into the zeolite sample under dry conditions. All manipulations were performed on a vacuum line. The zeolite loaded with the probe molecule was taken in a cell with two arms. In one arm the zeolite was placed and in the other a few milliliters of the required solvent was taken. 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 Teflon stopcock.
Results The results are presented below in four parts. In the first part absorption, emission, and lifetime data obtained with three probes 1-3 in “dry” zeolites are presented. This is followed by observations in “wet” zeolites and zeolites loaded with organic solvents. In the last part the data relating to the influence of zeolites on the electronic nature (nπ* or ππ*) of the lowest excited state of acetophenones 4-6 are presented. Nile Red, 1, as a Probe. The dye Nile red 1 displays solvent-dependent absorption and emission characteristics.10 The dependence of the absorption and emission maxima and the excited-state singlet lifetime on solvent polarity is highlighted in Table 1. For an extensive list of absorption maxima in various solvents, readers should refer to ref 10a. For example, the absorption and emission maxima of 1 in benzene are at 524 and 575 nm, respectively, and the singlet lifetime is 6.84 ns. Increasing the polarity of a solvent resulted in a red shift of both
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Figure 2. Absorption spectra of Nile red in alkali cation exchanged zeolites.
Figure 3. Absorption spectra of coumarin-500 in alkali cation exchanged zeolites.
Table 2. Absorption and Fluorescence Properties of Nile Red in Zeolites
Table 3. Absorption and Fluorescence Properties of Coumarin-500 in Zeolites
dry
wet
dry
wet
zeolite
abs max
em max
abs max
em max
zeolite
abs max
em max
abs max
em max
LiY NaY KY RbY CsY LiX NaX KX RbX CsX
623 615 611 607 602 619 606 594 589 606
664 666 666 670 667 662 669 665 668 674
608 600 593 595 595 610 597 590 569 585
667 667 666 667 667 666 670 672 659 670
LiY NaY KY RbY CsY
432 425 407 397 394
521 514 508 500 498
403 401 400 398 400
520 520 512 512 510
absorption and emission maxima and a decrease in singlet lifetime (Table 1). For example, in 40% water-methanol mixture, the absorption and emission maxima of the dye are centered at 584 and 660 nm, respectively, and the lifetime is reduced to 1.29 ns. Thus Nile red with the above medium-dependent characteristics seems to be an ideal probe to monitor the micropolarity of the environment in which it is present. The absorption and emission characteristics of 1 within M+ Y and M+ X zeolites are summarized in Table 2. The absorption spectra of Nile red in Li+, Na+, K+, Rb+, and Cs+ Y zeolites are given in Figure 1. Examination of Table 2 reveals that the emission maxima are not useful indicators of the polarity of the zeolite interior. In all cationexchanged Y zeolites the emission maximum is higher than 660 nm, suggesting that the micropolarity of all cation-exchanged zeolites must be higher than 60% water-methanol mixture. There is very little change in the emission maxima among the various cation-exchanged zeolites. Most likely the emission maximum has reached its peak in 60% water-methanol mixture and is not sensitive to any further changes in polarity. The possibility that the micropolarity of all cation-exchanged Y zeolites is higher than 60% water-methanol mixture is further supported by the fact that the absorption maxima are higher than 584 nm in all cation-exchanged Y zeolites (Table 2 and Figure 2). The dependence of polarity of the zeolite interior on the cation is indicated by the variation in absorption maxima with the exchanged cation. Proceeding down the alkali cation series, the absorption spectra show a uniform blue shift. The lifetime data are complicated by multiexponential decay, and we believe that they cannot be used reliably as polarity indicators. In all X and Y zeolites a component with a singlet lifetime varying between 1.1 and 1.7 ns is observed. This lifetime is close to that of water.
Data for X zeolites are included in Table 2. Once again emission maxima did not show any reliable variation with cation and in all cases were higher than 660 nm. The absorption maxima showed a clear trend; a steady blue shift in the absorption maxima with the cation series (Li+, Na+, K+, Rb+, and Cs+) is to be noted. Coumarin-500, 2, as a Probe. The absorption and emission spectra of coumarin-500 are sensitive functions of polarity of the medium in which it is present.11 The absorption and emission maxima for coumarin-500 in various solvents are presented in Table 1. Unlike Nile red, the excited singlet state lifetime of coumarin-500 is not dependent on the medium (Table 1). We have therefore utilized the polarity-dependent absorption and emission spectral features of coumarin-500 to probe the micropolarity of cation-exchanged Y zeolites. The spectral properties of coumarin-500 in various cation-exchanged Y zeolites are summarized in Table 3 and the absorption and emission spectra are displayed in Figures 3 and 4. A clear and consistent trend is observed between the absorption and emission maxima and the cations. Further, the fact that the trend is the same as the one observed with Nile red provides confidence in our approach. The lifetime (not listed in the Table) as expected did not vary with the cation. Pyrene-1-carboxaldehyde, 3, as a Probe. Similar to probes 1 and 2, pyrene-1-carboxaldehyde showed polarity-dependent spectral properties (Table 1).12 However, unlike 1 and 2, the absorption maximum of 3 is independent of the medium. The data summarized in Table 1 show that the emission maximum and the excited-state singlet lifetime are sensitive to the polarity of the medium. For example, the probe molecule is fluorescent in a relatively polar solvent such as methanol with a singlet lifetime of 1.76 ns. If the polarity of the solvent is further increased by mixing water, the lifetime shows a corresponding increase. In 40% water-methanol mixtures, the lifetime is significantly increased to 4.40 ns. The emission maximum is also significantly red shifted from methylene
Probing Zeolites with Organic Molecules
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Figure 4. Emission spectra of coumarin-500 in alkali cation exchanged zeolites. Table 4. Absorption and Fluorescence Properties of Pyrene-1-carboxaldehyde in Zeolites dry
wet
zeolite
abs max
em max
τ1 (ns)
abs max
em max
τ1 (ns)
LiY NaY KY RbY CsY LiX NaX KX RbX CsX
393 381 377 377 378 393 380 371 368 374
496 475 449 434 439 470 470 454 438 437
7.66 4.50 1.59 1.36 1.16 3.23 4.82 2.12 1.00 1.07
393 393 384 378 380 389 392 385 380 380
477 468 461 455 456 475 470 458 450 448
4.66 2.62 1.43 1.31 1.41 4.24 4.36 1.34 1.10 1.26
chloride to methanol-water mixture. We have made use of these medium-dependent features to probe the interior of a zeolite. The spectral data obtained for 3 in Y and X zeolites are summarized in Table 4. The cation-dependent emission spectra within M+ Y zeolites are reproduced in Figure 5. Consistent with the observations made with the other two probes, Li+ Y and Na+ Y is sensed to be much more polar than the methanol-water mixture by probe 3. For example, the emission maxima in Li+ Y and Na+ Y at 496 and 475 nm, respectively, are much higher than those in the 60% water-methanol mixture (462 nm). Similarly the excited singlet lifetime is much longer in Li+ Y and Na+ Y than in the 60% water-methanol mixture. The data obtained in X zeolites, also summarized in Table 4, clearly indicate that X zeolites are less polar than Y zeolites. Effect of Hydration. Zeolites X and Y when “dry” should not contain any water. The results reported in the previous section refer to zeolites that are expected to be dry. Most studies carried out thus far in our as well as a few other laboratories have adopted procedures wherein the zeolites may not be “fully” dry.8 With this fact in view, one needs to exercise caution in comparing results from different laboratories. We felt that it is important to compare the micropolarity of “dry” and “wet” zeolites. Probes 1-3 were used for this purpose. Zeolites prepared under dry conditions placed on the pan of a Mettler balance were moistened by exposure to water. The weight change was monitored and water content estimated from the initial and final weights of the zeolite sample. X and Y zeolites were generally found to adsorb about 15-18 molecules per supercage. In the tables “wet” refers to zeolites saturated with water as indicated above. The following observations suggest that the probe molecules
Figure 5. Emission spectra of pyrene 1-carboxaldehyde in alkali cation exchanged zeolites.
1-3 that were introduced into the dry zeolite were retained within the interior of a zeolite. (a) The probe molecules could not be adsorbed on an unactivated (wet) zeolite. This suggests that the probe has no affinity for the wet surface. (b) Washing of wet probe-zeolite samples (dry samples wetted intentionally) with hexane and benzene did not result in the loss of the probe from zeolite samples. The UV-visible absorption spectra of the solvent washings did not indicate the presence of the probes, suggesting that the probes are not leached out during the washing. The ground- and excited-state properties of probes 1-3 within “wet” X and Y zeolites are summarized in Tables 2-4. A comparison of the behavior of the three probes within wet and dry M+ Y zeolites indicates that the wet zeolites are less polar than the dry ones. For example, Nile red’s absorption maximum in 60% water-methanol mixture is 584 nm, whereas the maxima in wet M+ Y zeolites vary, depending on the cation, between 595 and 608 nm (Table 2). It is important to note that the cation dependence of absorption maxima is much smaller within wet zeolites than in dry zeolites. Such a trend is also observed with probes 2 and 3. The absorption and emission maxima of coumarin-500 are close to that of the watermethanol mixture and are only slightly dependent on the cation within M+ Y zeolites (Table 3). The data presented in Table 4 show that the behavior of pyrene-1-carboxaldehyde is similar to that of 1 and 2. All three probes tested suggest that the water present within a zeolite essentially controls the micropolarity of a wet zeolite. Effect of Coadsorbed Solvents. Since most of our product studies are carried out in a zeolite-hexane slurry, we were interested in probing how the solvent modifies the internal character of a zeolite. The influence of solvents on zeolite polarity has been studied by coadsorbing various solvents of different polarities on NaY loaded with the probe molecules Nile red and coumarin-500. Solvents were found to profoundly influence the micropolarity of zeolite supercages. Results obtained are tabulated in Table 5. The absorption maximum of Nile red and the emission maximum of coumarin-500 are blue shifted in the presence of polar solvents indicating a decrease in zeolite polarity. Hydrocarbon solvents lead to changes consistent with the conclusion that the internal polarity is enhanced by nonpolar solvents. In the presence of hexane and cyclohexane, the polarity of the zeolite increases. For example, the absorption maximum of Nile red in NaY under dry conditions is at 615 nm. Upon coadsorption of hexane it shifts to 619 nm, which indicates an increased polarity. Excited-State Switching with Cations. Chromophores with n and π electrons (e.g., carbonyl, azo, enone,
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Table 5. Photophysical Properties of Nile Red and Coumarin-500 in NaY in Presence of Coadsorbed Solvents
coadsorbant
absorption max (nm) Nile red
emission max (nm) Nile red
emission max (nm) coumarin-500
none water methanol acetonitrile acetone THF diethyl ether hexane cyclohexane benzene methylene chloride
615 600 603 599 607 610 607 619 626 618 617
666 667 656 658 659 660 657 671 666 668 667
514 519 505 510 509 512 512 524 522 529 525
thiocarbonyl, imine, etc.) have two types of low-lying excited states, nπ* and ππ*. When the two states are nearby ( 63), pyrene 1-carboxaldehyde senses the polarity to vary between ET(30) ∼ 50 and 75. Excepting for this qualitative difference, other conclusions drawn from pyrene 1-carboxaldehyde as a probe are the same as with Nile red: (a) X zeolites are less polar than Y, (b) wet X and Y zeolites are less polar than the corresponding dry zeolites, and (c) the polarity of a wet zeolite is close to that of methanol-water mixture (compare Tables 1 and 4). To further support our conclusions concerning cationdependent polarity within a zeolite, we have employed yet another probe, coumarin-500.11 Although the versatility of this probe has not been fully explored, solvent dependence of emission maxima has been established. Perusal of Table 1 indicates that the emission maxima shift to longer wavelength with increasing solvent polarity. The polarity trend observed with this probe is very close to that seen in the case of pyrene 1-carboxaldehyde. The emission maxima observed for Li+ and Na+ Y are at longer wavelengths than that obtained in 40% methanol-water mixture (compare the maxima 521, 514 nm with 509 nm) while that for Cs+ Y at 498 nm is identical to that in methanol. The discrepancy between Nile red and the other two probes (pyrene 1-carboxaldehyde and coumarin-500) arises only with K+, Rb+, and Cs+ Y. While Nile red reports that all three of them are more polar than water, the latter
Probing Zeolites with Organic Molecules
Langmuir, Vol. 16, No. 1, 2000 273 Chart 2
two probes suggest K+, Rb+, and Cs+ Y to be less polar or close to that of water. Such differences could arise from the differences in location of the probes within a zeolite. Presently we have no knowledge of the exact location of the probes within a supercage. Important conclusions drawn from this investigation are the following: (a) Dry alkali cation exchanged X and Y zeolites are highly polar, and the extent of polarity is dependent on the cation size (charge density). (b) When water is introduced into the zeolite (wet zeolite), the contribution of cations toward the polarity of a zeolite is diminished. These observations suggest that zeolite can be used as a superpolar medium to “dissolve” organic molecules. Li+- and Na+-exchanged Y zeolites are more polar than any organic solvent (than water even), which would allow one to observe phenomena that are normally not anticipated in polar organic solvent. One such use of alkali cation exchanged zeolite is presented below. Acetophenone both in the singlet and triplet manifolds possesses close lying nπ* and ππ* excited states.16 Both in polar and nonpolar solvents nπ* triplet is the lowest excited state (Figure 5). Rauh and Leermakers and Lamola independently established that the lowest excited state of acetophenone (and butyrophenone) changes to ππ* triplet when it is adsorbed onto silica gel.15 Altering the excited state ordering can have significant influence on photochemical reactions. For example enones upon photoreduction give different products from nπ* and ππ* triplets (Chart 2).19 We have shown recently that the product distribution during photoreduction of steroids within a zeolite depends on the cation.20 This we believe is due to differences in the nature of the lowest triplet (nπ* and ππ*) for the reactant steroid within Na+- and Cs+exchanged Y zeolite. Details of this study will be presented elsewhere. It is important to recognize that the micropolarity of a zeolite interior can have an important impact on the observed photochemistry within a zeolite. We illustrate here that the influence of cations on the ordering of excited state can be easily inferred from the emission spectrum of the adsorbed ketone. Three ac(19) (a) Schuster, D. I.; Woning, J.; Kaprinidis, N. A.; Pan, Y.; Cai, B.; Barra, M.; Rhodes, C. A. J. Am. Chem. Soc. 1992, 114, 7029. (b) Chan, A. C.; Schuster, D. I. J. Am. Chem. Soc. 1986, 108, 4561.(c) Wehrli, H.; Schaffner, K. Hel. Chim. Acta 1962, 45, 385. (20) Jayathirtha Rao, V.; Uppili, S.; Corbin, D. R.; Schwarz, S.; Lustig, S. R.; Ramamurthy, V. J. Am. Chem. Soc. 1998, 120, 2480.
etophenones 4-6 are chosen as examples. On the basis of the knowledge that a highly polar medium would be expected to increase the energy of the nπ* state and lower the energy of the ππ* state, one would predict that the lowest excited state of acetophenone and p-fluoroacetophenone could be altered within a zeolite while that of p-methoxyacetophenone will remain in the ππ* state in all solvents and in zeolites. Indeed the phosphorescence emission of p-methoxyacetophenone in Na+ Y and Cs+ Y was structureless, characteristic of the ππ* state (Figure 8). On the other hand, the structural resolution of the phosphorescence emission from acetophenone was dependent on the cation. In Na+ Y the emission was structureless, typical of ππ* emission, and in Cs+ Y it was structured similar to that in the methanol-ethanol mixture. Observations made with p-fluoroacetophenone were similar (Figure 7). On the basis of the appearance of the phosphorescence spectra we believe that both acetophenone and p-fluoroacetophenone possess ππ* excited states within Na+ Y and a nπ* state within Cs+ Y. Considering that these two ketones have the nπ* state as their lowest excited triplet in the most polar solvent mixture, methanol-ethanol, the ability to switch the states within a zeolite using cations is novel and important. We plan to exploit this feature to control the chemistry of organic molecules with close-lying nπ* and ππ* excited states. Limitations and Conclusions As per the three probes employed, the supercages of zeolites are highly polar and the extent of polarity depends on the number and nature of the cations present within. The polarity as measured by organic probes and the electric field as monitored by physical techniques and estimated by theoretical calculations depend on the charge density of the cation, suggesting that both these parameters are manifestations of the presence of cations within a supercage. The polarity of a supercage also depends on the presence of water and organic solvents within a cage. Presence of water and polar solvents can completely eliminate the contribution of cations to polarity of a supercage. It is essential therefore to dry the guest included zeolite samples before use. Water can be desorbed either by heating the zeolite at 500 °C or evacuating (10-4 Torr) the zeolite at lower temperatures (∼120 °C). With
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the help of cation-exchanged zeolites, one can switch the electronic character of a reactive excited state. Such switching is generally not possible in polar organic solvents. The probes used in this investigation contain a carbonyl chromophore, and the monitored polarity is an indication of the binding of the cation to the oxygen lone pair. The polarity as monitored by the probes that contain carbonyl chromophore results from the specific interaction of the oxygen lone pair with the cation. It would be important to employ another set of probes that do not contain a lone
Uppili et al.
pair. To achieve this end we have been probing zeolites with aromatic molecules and the results will be published shortly. Acknowledgment. The authors thank the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy for support of this program. LA990392R
