J. Phys. Chem. 1993,97, 13380-13386
13380
Distribution of Organic Molecules within Zeolites As Revealed by Aromatic Photophysical Probes: Role of Water and Other Coadsorbentsf V. Ramamurthy,. David R. Sanderson, and David F. Eaton Central Research and Development, Experimental Station, The DuPont Company,* Wilmington,Delaware 19880-0328 Received: July 1, 199P
The distribution of organic guest molecules within NaY zeolites has been examined utilizing aromatic molecules as photophysical probes. The distribution of guests within zeolites has been shown to be nonuniform. Three parameters-guest occupancy, families of sites, and distribution of sites within a family-are defined, and the results are analyzed on their basis. Coadsorbed molecules such as water and organic solventsinfluence significantly the guest distribution within zeolites.
Introduction To be able to understand and predict the photobehavior of a guest molecule within zeolites, one must know the location and distribution of the guest within these structures. Since all guest molecules undergo some motion within the channels/cages/ cavities of zeolites, X-ray structural characterizationhas not been possible.’ However, considerable literature exists on the characterization of the location of guest molecules within zeolites based on other techniques such as NMR,* infrared: and diffuse reflectance spectra? These studies, while providing a basic understanding of the location and distributionor organicmolecules within zeolites, are less helpful to predict photochemical events since most of them occur on time scales much shorter than migration of molecules on surfaces; the distribution at the time of a photochemical event can be more complex than the one observed with steady-state techniques or techniques such as NMR where time scales are much longer. It is important to have a knowledge of the location and distribution of molecules on the time scale of photochemical events to be able to reliably predict the photobehavior of adsorbed guest molecules. We attempt in this paper to focus on this problem utilizing certain wellcharacterized aromatic probes as guest molecule^.^ Fluid solutions allow reacting molecules to experience an average microenvironment by virtue of the fast relaxation time of the solvent and/or the high mobility of reactant molecules? However, in organized and very viscous media solvent relaxation and rates of guest diffusion may be slower than the time period of a photoreaction, leading to photophysical processes occurring in a variety of % i t e ~ Thus, ~ . ~ the microenvironmentaround guest molecules within zeolites could be heterogeneous. Results discussed below suggest that this is indeed the case. It is known that pyrene emissions and singlet lifetime9 are sensitive to the environment in which it is located. These established characteristics have been utilized in this study to gain information concerning the distribution of guest molecules within zeolites. A brief description of the structureof zeolite Y is appropriate.1° The topological structure of Y-type zeolite consists of an interconnected three-dimensional network of relatively large sphericalcavities,termed supercages (diameter about 13A;Figure 1). Each supercage is connected tetrahedrally to four other supercagesthrough 8-A windows or pores. The interior of zeolite Y also contains, in addition to supercages,smaller sodalitecages. The windows to the sodalite cages are too small to allow organic t Dedicated to the memory of G. L. Closs, an uncommon man who did not follow the common saying ‘The man who makes no mistakes usually does not make anything”. t Contribution No. 6613. *Abstract published in Advance ACS Absrrucrs, November 15, 1993.
Figure 1. Structure of supercages of X and Y zeolites. Cation positions are shown as type I, type 11, and type 111. In zeolites only sites I and I1 are occupied.
molecules access to these cages. Therefore, supercages are the primary sites of occupancy by adsorbed guest molecules. Charge compensating cations present in the internal structure are known to occupy two different positions (sites I and 11; Figure 1) in zeolite Y. About 30 cations occupy site 11, and roughly 27 cations occupysite1. Onlycationsofsites I1 areaccessibletotheadsorbed organic molecule.
Experimental Section Materials. Zeolite LZ-YS2 (Nay) in powder form was obtained from Aldrich. The cation of interest (Li, K,Rb,and Cs) was exchanged into these powders by contactingthe material with the appropriate nitrate solution at 90 OC. For each gram of zeolite, 10 mL of a 10% nitrate solution was used. This was repeated three times. The samples were then thoroughly washed with water and dried. Exchange loadingswere typically between 37 and 84%. Pyrene, naphthalene, and phenanthrene were obtained from Aldrich (99.9+% purity) and were recrystallized from ethanol three times to constant melting point. Activation of Zeolites. In general, ca. 150 mg of zeolite was placed in a silica crucible and heated at 500 OC for about 12 h. The freshly activated zeolites were rapidly cooled in air to ca. SO OC and added to solutions of the guests of interest. 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 keeping the activated zeolites on the pan of an analytical balance and watching the weight change.
0 1993 American Chemical Society
Distribution of Organic Molecules within Zeolites
The Journal of Physical Chemistry, Vol. 97, No.50, 1993 13381
TABLE I: Effect of Loading and Temperature on Excited-State Singlet Lifetime of Pyrene Adsorbed within NaY analysis on the basis analysison the basis loading level of of two exponentials of distribution function pyrene:moleculcsper ratio of emission intensity ratio supercage (conditions) lifetime, ns ( x 2 ) occupancy lifetime, ns XZ monomer/excimer 67:33 31 h 2 8 0.002 (dry) 91.2,29 (1.3) 1.2 no excimer 0.002 (wet) 164,59 (1.5) 83:17 138 h 49; 18 h 4.6 1.4 no excimer 0.002 (dry) at 77 K 343,109 (1 -4) 68:32 204 f 140; 7.7 h 4.7 1.1 no excimer 0.002 (wet) at 77 K 341, 103 (1.3) 8020 263 h 127; 34.8 h 14.4 1.2 no excimer 0.02 (dry) 84.7,32.5 (1.5) 53:46 25.4 & 33.7 2.2 2.5 0.02 (wet) 160.1,54.7 (1.5) 88:12 142 h 43; 17 3.8 1.7 22.3 39:61 21 h 14 0.04 (dry) 85.8,25.3 (1.6) 1.4 1.4 0.04 (wet) 161.4,55.9 (1.4) 87:13 141 h 39; 17 f 5 1.3 15 0.08 (dry) 86.5,27.6 (1.3) 4356 34 h 27; 2.5 1.8 1.3 0.8 151.9,51.5 (1.5) 88:12 134.5 h 41; 15 h 4 1.8 11 0.08 (wet) 495 1 54 f 20; 9 3 1.3 0.6 0.2 (dry) 84.8,38.4 (1.4) 0.2 (wet) 151,44 (1.5) 90: 10 137 h 41; 13 f 3 1.6 5.5 298,81 (1.5) 68:32 167 h 120; 10 h 7 1.1 0.4 0.2 (dry) at 77 K 0.2 (wet) at 77 K 307,70 (1 5 ) 83:17 261 h 107; 32 h 15 1.1 3.7
*
TABLE Ik Effect of Coadsorbents on Excited-State Lifetime Data for Pyrene Adsorbed onto NaY lifetime measured ratio of occupancies lifetime measured on the basis of two (long os short lifetime component) on the basis of coadsorbents exponentials ( x 2 ) from two-exponential analysis distribution analysis 80.7,21.1 (1.5) nil (dry sample) 51:48 32 h 21; 3.6 i 1.9 water (4 per supercage) 95.5,24.7 (1.5) 55:45 44 h 32; 3.8 h 2.1 water (10 per supercage) 109.1, 52.4 (1.4) 67:33 81 h 29; 9 2.4 water (14 per supercage) 164, 50.4 (1.5) 89:ll 148 h 44; 17 h 3.6 methanol (supercage filled) 192.5, 59.3 (1.4) 87:13 169 f 62.7; 18.8 h 4.8 diethyl ether (supercage filled) 143.7,36.2 (1.3) 91:9 136 h 31; 21 h 5 acetonitrile (supercage filled) 182.5,61.7 (1.4) 91:9 157 f 57; 19 h 5 84, 36 (1.5) hexane (supercage filled) 75:25 67 22; 9.9 h 2.4 pentane (supercage filled) 113,45.5 (1.5) 80:20 96 h 30; 14 f 6 Preparationof Zeolite-Aromatic Complexes. Known amounts of aromatics 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 0.01 mg of the guest were taken in 20 mL of hexane. Loading levels of aromatics werevaried between 0.05 and 10 mg per 250 mg of hydrated zeolite, which corresponded to an occupancy number (average number of molecules per supercage) variation of 0.0024.4. White powder collected by filtration of the solvent was washed with dry hexane several times and dried under nitrogen. Samples were taken in Pyrex cells fitted with Teflon stopcocks and degassed thoroughly (lesmm) and sealed. Complete dehydration of NaY at higher loading levels of pyrene required degassing at elevated temperatures (-100 "C). Coadsorptionof KnownAmounts of Water and Other Solvents. The complexes as prepared and dried above were placed on the pan of a Metller balance maintained under controlled humidity, and theweightincreasewasnoted. Assoonas therequiredamount of water was adsorbed, the sample was sealed and equilibrated at 50 O C for about 4 h. The amount of water contained in these samples was checked by thermogravimetric analysis (DuPont Model 95 1 thermogravimetric analyzer). About 5 mL of solvents such as acetonitrile, hexane, pentane, methanol, diethyl ether, and cyclohexane was dried thoroughly with activated NaX (2 g). To the complexes prepared and dried as described above, taken in quartz cells fitted with Teflon stopcocks, known amounts of solvents were added. Addition results in a slurry, and the absorption spectra of the solvent and the solid portions were monitored by a Varian 2400 spectrometer. Generally no aromatics were seen in the solvent layer. With dry nitrogen the solvent layer was evaporated, and the sample coadsorbed with solvents was used for further measurements. The exact amount of solvent coadsorbed is not known. Mffuae Reflectance Spectra. Diffuse reflectance spectra of thezeolitesolidsamples wererecorded in 2-mm-path lengthquartz cells using a Varian 2400 spectrometer equipped with an integrating sphere (Varian); barium sulfate (Kodak, White
~
X2
1.2 1.3
1.7 1.7 1.4 1.1 1.4 1.3
1.2
Reflectance Standard) was used as the reference. Samplepacking densitieswere not determined nor were they specifically controlled. Spectra were recorded between 220 and 800 nm. For comparison, spectra of the anhydrous zeolites were also recorded. Data were recorded digitally, and appropriate background corrections were carried out using the computer program SpectraCalc (Galactic Industries). Emission Spectra. Emission spectra were recorded a t room temperature and at 77 K in Suprasil quartz EPR tubes, under degassed conditions, with a Spex Florolog 2 12spectrofluorimeter. Emission intensity was monitored in a front face configuration. Spectra were corrected for detector sensitivity. Background scans of both as prepared and activated zeolite samples showed no detectable emission in 30&700-nm wavelength region under the excitation conditions of pyrene. SingletLifetime. Fluorescence decays were monitored a t room temperature and a t 77 K in a front face configuration using an Edinburgh FL 900 single photon counting apparatus. The decay was monitored for a duration of at least 7 lifetimes. Analysis of Lifetime Data. Deconvolution was performed by nonlinear least-squares routines minimizing x2 (software supplied with the instrument by Edinburgh), and goodness of fit was determined with plots of residuals, autocorrelation function, and reference to the Durbin-Watson statistic. The standard analysis algorithm used in this study allows curve fitting of up to four exponential terms, using the established Marquardt search method. None of our lifetime data could be analyzed as singleexponential decay. The total decay of all lifetime data (all points) could be fitted to two- and three-exponential decays, but the fits had high X* values. Noninclusion of the initial 15 ns on the analysis (two exponentials) generally gave acceptable x2 values, which are presented in Tables I and 11. For the same set of points analysis on the basis of three exponentials gave better x2 values. The lifetime for the two long components are essentially the same for both two and three exponentials, although the latter gave an additional short component (-1-5 ns). These data are not included in Tables I and 11.
13382 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993
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Figure2. Emission spectra (excitation h = 340 nm) of pyrene, at various loading levels, included within "dry" Nay. Spectra at three loading levels ((S)arc presented: (a) - -,( S ) = 0.002; (b) .-,(S) = 0.2; and (c) -, ( S ) = 0.2.
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Distribution analysisof lifetimes wascarried out with a program supplied by Edinburgh Analytical Instruments. The fitting function utilizes the Fredholm integral and uses up to 100lifetime values, and no a priori assumption about the distribution shape is made.
Results and Discussion Three parameters are utilized to present the results on the location and distribution of guest molecules within zeolites. We define these three terms (guest occupancy, families of sites, and distribution of sites within a family) below. The term "guest wupancy" is used to indicate the aggregation status (monomer, dimer, microcrystals, etc.) of guests within zeolites. The term "families of sites" indicates a group of sites which can be represented by a singleprimary characteristic. By the very term family, it is implied that there may be more than one site within this group. For example, a number of spectroscopic and X-ray studies have suggested that benzene molecules within the supercages of NaX and NaY zeolites are located near the cation and at the window interconnecting two supercages.132 We call these two sites-cationic and window-as two families of sites. Instead of calling it simply a site, the descriptor "family" is used to indicate that not all molecules at cationic or window sites need be identical. This feature is described by the term "distribution of sites within a family". This term implies that a site which is identified by a "family" (e.g., cationic, window, and wall sites) may consist of a number of sites which in their characteristics bear close resemblanceto the primary site. For example, a cationic site, which ideally can be defined by a unique arrangement between thecation and the guest molecule, may in fact consist of a number of closely related arrangements. Thus, while the principal characteristics of these closely related sites may be defined by the primary site, the individual sites belonging to the same family might show properties which are slightly different from each other. Heterogeneous 'Guest Oecupoacy". Excitation of pyrene included within dry NaY results in two emissions as shown in Figure 2, one from monomer and the other from excimer. The ratio of intensitiesof emission from monomer to that from excimer is dependent on the loading levels of pyrene (Table I). The socalled excimer emission is definitelynot an emissionfrom a typical dynamic excimer since the excitation spectra for the monomer and the excimer emissions aredifferent (Figure 3). The possibility that the excimer emission is due to microcrystals can be ruled out since both the wavelengths of emission and excitation are not identical to those for crystals reported in the literature." Also, the difference in excitation spectra between our excimer emission and microcrystals cannot be not due to any experimental artifact since the excitation spectrum was independent of dilution (up to
3io
320 Nanometers
Nanometers
Figure 3. Excitation spectra for the monomer and excimer emissions of pyrene included within "dry" Nay: (a) -, monomer, emission h = 380 nm; (b) - - -,excimer, emission X = 480 nm.
> f
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I
I
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550
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Figure 4. Emission spectra (excitation h = 340 nm) of pyrene within
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NaY at various loading levels of coadsorbed water: (a) - -,"dry", no water; (b) (S) = 2; (c) - - -,( S ) = 6; (d) -, (S) = 12.
.-,
1:lOOO)with an empty and inert silicalite.12 This suggested that the excimer results from preaggregated dimers (static excimen) present within the supercages of N a y . Consistent with this, no growing in of the excimer on a nanosecond time scale was noticed when monomer and excimer decays were monitored by timeresolved single photon counting. The absence of any negative preexponential term for excimer decay also implies that the excimer emission is not due to dynamic excimers. Further, the ratio of the excimer to monomer emission increased (Table I) slightly upon lowering the temperature. A similar effect has been reported for static excimers of pyrene on the surfaces of activated silica.11J3 Had the broad emission in the region 430550 nm been due to a dynamic excimer, lowering of temperature would result in a decrease of the excimer emission. The increase in the ratio of excimer to monomer emission intensity with the loading level of pyrene is consistent with the excimer being either a dynamic or a static one. Dynamic excimer formation would require a decrease in lifetime with the increase in loading level while the static excimer formation would not affect the lifetime at all. Varying the loading level between 0.002 and 0.2 (average number of molecules per supercage) did not alter the pyrene monomer lifetime significantly (Table I). These experimental observations support the conclusion that the excimer emission observed at various loading levels of pyrene within anhydrous NaY is essentially due to preaggregates. These species are probably dimers since the supercages are too small to accommodate more than two molecules of pyrene. HeterogeneousDistributionof Guests. Examination of Figures 2 and 3 reveals that the monomer emission is less structured than normally seen in isotropic solvents.9 This feature can be easily appreciated when one examines the emission spectra recorded for pyrene coadsorbed with water (Figure 4). With an increase
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13383
Distribution of Organic Molecules within Zeolites
Figwe 5. Decay traces of excited singlet state (monomer, emission X = 380 nm) of pyrene within NaY with respect to various loading levels of wadsorbed water. Loading levels from bottom to top: 2, 4, 9, 12, and 18 molecules water per supercage. Decay collected utilizing a single photon counting technique.
in the loading level of water (2-18 molecules of water per supercage), the ratio of emission intensities due to monomer to excimer increases, and the excimer emission nearly disappears when the water content reaches an averageof about 12 molecules/ supercage. Most importantly, thevibronicpattern in themonomer emission becomes more predominant when the zeolite is fully hydrated. This dependence on coadsorbed water is reversible,
and upon controlled removal of water from the hydrated zeolite, the vibronic structure gradually disappears and the excimer emission slowly reappears. These variations seen in steady-state emission spectra are also reflected in the excited singlet decay of the monomer as illustrated in Figure 5. When the zeolite is anhydrous the decay is nonexponential, but as the zeolite is gradually hydrated, the pyrene singlet lifetime, lengthens until in fully hydrated zeolite the pyrene excited singlet shows nearexponential decay and has a long lifetime(Table I). We interpret these two features to be an indication of a heterogeneous distribution of pyrene within the supercages of anhydrous NaY and suggest that pyrene molecules are adsorbed in a variety of environmentally closely related sites. As a consequence, there is a number of similar, but not identical, emitting species, resulting in broadening of the emission spectrumand nonexponentialdecay. The heterogeneous distribution of pyrene monomers within dry zeolite becomes even moreobviouswhen the lifetimedata are analyzed by a distribution analysisprogram supplied with the FL 900 single photon counter by Edinburgh Instruments. In recent years, the decay of excited singlet molecules, present in organized and constrained assemblies, has been analyzed on the basis of a distribution function,'' and such an analysis has been shown to provide more detail about the medium.15 Zeolites appeared to us as an appropriate medium in which analysis of singlet decay by a distributionfunctionwould be meaningful. Results obtained in thecaseof anhydrous and hydrated (todifferent extents)zeolites are presented schematically in Figure 6 and summarized in Tables I and 11. In Figure 7 the distributions obtained at two loading .
--mg water
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Figure 6. Analysis of the decay in Figure 5 by a distribution analysis program. Distribution of the lifetime of the monomer with respect to the loading level of water: (a) dry, no water; (b)-(0: 2, 4, 9, 12, and 18 molecules of water per supercage.
13384 The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 1.0
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Ramamurthy et al.
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Figure& Diffusereflectancespectraof pyrene within ‘dry” M+Yzeolites. The intensity of &I to SIband is dependent on the cation.
0.2 (a) 1OOt 50ns (b) 600f50ns ( c ) 800t50ns
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0.0
_L
0.00
‘
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Figure 7. Distribution of lifetime of the excited monomer of pyrene, at two loading levels, included within “dry” Nay.
levels of pyrene within anhydrous NaY are shown; nearly similar distribution patterns were obtained at other loading levels. It is apparent from these figures that in dry zeolites pyrene molecules are present in a wide distribution of sites. The width of distribution gets narrower as the amount of coadsorbed water increases; when the zeolite is fully hydrated essentially two sites are present. Not only water but also solvents such as hexane, pentane, acetonitrile, diethyl ether, and methanol alter the distribution pattern and enhance the excited singlet lifetime of pyrene (Table 11). A narrow distribution very similar to that in hydrated NaY is obtained in presence of coadsorbed solvents. Polar solvents such as acetonitrile, diethyl ether, and methanol exhibit influence greater than nonpolar solvents, hexane and pentane. Nature of Sites: Identification of Two “Families of Sites”. Studies14 based on N M R and steady-state techniques have identified at least two sites for benzene molecules at low loading levels-cationic and window sites. Based on micropolarity measurements, the sites in which organic photophysical probes (pyrene, pyrenealdehyde, and 4-(dimethylamino)benzonitrile) are located, at low loading levels,within X and Y zeolites are identified to be highly polar.16 *HN M R studies on phenanthrene within X and Y zeolites have identified one of the two sites in which phenanthrene is located to be a cationic site.” The binding strength of phenanthrene to this site is shown to be directly related to the charge density of the cation. On the basis of the above studies, we suggest that one of the sites in which pyrene is located is close to the cation. In this site, pyrene is stabilized by interaction with the cation through the molecular *-cloud. Since the windows of supercages are too small for pyrene to fit in such a way that it can interact with the 12-ring oxygens, we believe that theseare not the preferredsites. However, pyrene may adsorb on the walls of supercages, and such an adsorption may be stabilized through forces such as electrostatic interaction and dispersion forces. (These sites are termed wall sites.) Diffuse reflectance spectra recorded for pyrene a t a low loading level (Figure 8) within a number of cation-exchanged X and Y zeolites suggest that the interaction between the cation and pyrene is strong and that the strength of interaction depends on the polarizability of the cation. In Figure 8, the intensity of the 0-0 transition of the SOto SIband is dependent on the charge density of thecation present within supercages. Theabovespectral features recorded for pyrene included in zeolites should be
370
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Figure 9. Time-resolvedemission spectra of pyreneincludedwithin “dry” Nay. The intensity of the first band (X, = 380 nm) depends on the time window. compared to those in solution, where a relationship between the intensity of 0-0 transition of the SOto SIband and the polarity/ dielectric constant of the medium has been established.9JO This suggests that pyrene is present at a site(s) which is highly polar. Close examination of the lifetime data analyzed on the basis of two exponentials reveals that there are at least two types of pyrene within the anhydrous supercages: one with lifetime in the range 25-60 ns and the other with 85-160 ns. We are tempted to propose that the ones with shorter lifetime are the ones closer to the cation and the other with longer lifetime are the ones adsorbed on the walls. This is indeed consistent with the relationship established in solution between excited singlet state lifetime and the polarity of the medium,lO where the lifetime is fairly long in a nonpolar medium (e.g., cyclohexane, 430 ns) and is relatively short in a polar medium (e.g., dimethylformamide, 280 ns). Strong support for the above proposition comes from timeresolved emission spectra recorded in the time range 1-900 ns (Figure 9). It is known that the intensity of the first band at 373 nm is dependent on the polarity of the medium. Perusal of Figure 9 indicates that the intensity of this band is time dependent; a t early time this is intense, and a t later times its intensity is weakened. This intensityvariation is consistent with the proposal that it is pyrene near the cations which has a shorter lifetime and is present near a much polar environment than the ones near the walls having a longer lifetime. Even when the zeolite is fully hydrated, occupation of both sites persists although both with considerably longer lifetime and with different occupancies. This is in fact consistent with the argument that water lowers the micropolarity of the sites by coordinating to the cation and adsorbing to the walls of the cavity.
The Journal of Physical Chemistry, Vol. 97, No. 50, 1993 13385
Distribution of Organic Molecules within Zeolites
260
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’
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300 ’ Nanometers
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’
Si0
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Figure 10. Diffuse reflectance spectra of pyrene included within ‘dry” (-) and “wet” (- -) N a y .
Figure 11. Excitation spectra (emission X = 380 nm) of pyrene included within “dry” (-) and “wet” (- -) N a y .
FactorsThatInfluence the Distribution. Excited-state lifetime, spectral characteristics of monomer emission, and the ratio of monomer to excimer emissions are all influenced by the coadsorbents. To understand the origin of these variations, we need to probe what influences the formation of ground-state dimers within supercages. We believe that it is the strong interaction between the cation and pyrene. The interaction between the cation and pyrene probably polarizes the ground-state pyrene; such polarized pyrenes are the ones which form the ground-state complex. Such a hypothesis is supported by following observations: (1) For the same loading level, the relative intensity of excimer emission is proportional to the polarizing ability of the cation. For example, in the series LiY, N a y , KY, RbY, and CsY, both the relative intensity of excimer emission and the polarizing ability of the cation follow the same trend (Li > Na > K > Rb > Cs). Cations with higher charge density and polarizing power are expected to induce the formation of groundstate aggregates more readily than the ones with lower charge density and polarizing power. (2) An inverse linear relationship between the number of water molecules present within the supercages and the relative intensity of excimer emission was observed (Figure 4). This can be understood on the basis that cations whose polarizing power has been reduced by coordinated water molecules are less capable of inducing aggregate formation. Indeed, when the zeolite is fully hydrated no excimer emission is observed. Also, the ratio of monomer to excimer emission intensities is enhanced in presence of coadsorbed solvents. Apparently, even organic solvents can pacify the cations and thus inhibit the formation of ground-state aggregation. Thus, it is clear that the occupancy distribution (single us double occupancy per super cage) can be controlled by the cation and by the coadsorbents in addition to the loading level of pyrene (Figure 2). The extent of occupation of the two families of sites-cationic and wall-by pyrene is subjected to external factors. As pointed out in an earlier section, the adsorption of pyrene to the cationic and wall sites occur through intermolecular interactions. Coadsorption of water or other solvents, which would decrease the strength of interaction between pyrene and these sites, is expected to have consequences on the distribution of pyrene between the above two sites. Indeed, the analysis of lifetime data on the basis of two exponential terms gives some idea of the percentage distribution of pyrene between cationic and wall sites. These numbers, although only approximate,indicate that thedistribution of pyrene between the two sites has changed upon coadsorption of water and other solvents. The distribution of pyrene between cationic and wall sites is -1:l at all loading levels of pyrene investigated. However, upon inclusion of water this changes to 1:9. Such a change in distribution is also seen in the diffuse reflectance spectra and in the excitation spectra for the monomer emission of pyrene between wet and dry samples (Figures 10 and
11). With controlled amount of water a gradual red shift in the diffuse reflectance and excitation spectra was noticed.
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Conclusions Pyrene distribution within the supercages of anhydrous NaY is nonuniform. There are cages which are singly occupied, and there are those with doubly occupied pyrene molecules. Excimerlike emissioncomes from these doubly occupied cages. In addition to the nonuniform distribution of pyrene between supercages, yet another inhomogeneity is revealed by monomer emission and lifetimedata. Pyrene monomers are present in a number of closely related sites within a dry zeolite. Two families of sites are identified for pyrene molecules within the supercages-cationic and cavity wall sites. On the time scale of the excited singlet state, no exchange of pyrene molecules occurs between the singly and the doubly occupied cages and between a variety of closely related wall and cationic sites. The distribution pattern described above is influenced by coadsorbents such as water and organic solvents. Lifetime, monomer to excimer emission ratio, and vibronic content of the monomer emission are all influenced by coadsorbed water or organic solvents. The changes are reversible. Conclusions drawn with pyrene as a probe are expected to be general although such dramatic changes in photophysical prop erties may not be observed in all cases.18
Acknowledgment. We thank J. V Caspar for help with lifetime and TRES measurements. References and Notes (1) (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. I 1987,83,3199. (c) Parise, J. B.; Hriljac, J. A,; Cox, D. E.; Corbin, D. R.; Ramamurthy, V. J . Chem. Soc., Chem. Commun. 1993, 226. (2) 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. (3) OMalley, P. J. Chem. Phys. Lett. 1990, 166, 340. (4) Unland, M. L.; Freeman, J. J. J . Phys. Chem. 1978,82, 1036. ( 5 ) (a) Kalyanasundaram, K. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; pp 3977. (b) Bohne, C.; Redmond, R. W.; Scaiano, J. C. In Photochemistry in Organized and Constrained Media; Ramamurthy, V. Ed.; VCH: New York, 1991; pp 79-132. (61 , , (a) . , Declemv. A.: Rulliere. C.:Kottis. Ph. Loser Chem. 1990,10.413. (b) Weaver, M. J.;-McManis,G..E.fJarzeba, W.; Barbara, P. F:J. Phys. Chem. 1990,94,1715. (c) Su, S.-G.;Simon, J. D. J . Phys. Chem. 1989,93, 753. (7) (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. (8) (a) Nakajima, A. Bull. Chem. Soc. Jpn. 1971, 44, 3272. (b) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977,99,2039. (c) Dong, D. C.; Winnik, M. A. Photmhem. Photobiol. 1982, 35, 17.
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Ramamurthy et al. Am. Chem. Soc. 1990,112,1343. (c) Brochon, J. C.; Livemy, A. K. Chem. Phys. Lerr. 1990,174,517. (d) Krasnansky, R.; Koike, K.; Thomas, J. K. J. Phys. Chem. 1990,944521. (e) Verbeck, G.; Vaes, A,; Van der Auweraer, M.; De Scryver, F. C.; Geelen, C.; Terrell, D.; de Meutter, S.Macromolecules 1993, 26, 472. (16). (a) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. Photochem. Phntobiol. 1992,56,297. (b) Ramamurthy, V. Mol. Cryst. Liq. Cryst. 1992, 211, 211. (c) Ramamurthy, V.; Eaton, D. F. In Proceedings of the 9th
International Zeolite Conference; von Ballmoos e t al., Eds.; Butterworth-Heinamann: New York, 1993; pp 587-594. (17) Hepp, M.; Ramamurthy, V.; Corbin, D. R.; Dybowrki, C. J . Phys. Chem. 1992,96, 2629. (18) We have also utilized naphthalene and phenanthrene as probes to investigate the interior of Nay. Changes observed in lifetime emission characteresiticsfollowedthesame patternbutweresmaller than thoseobserved with pyrene.
