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Intrazeolite Photochemistry. 19. Effect of the “Spectator” Pyridine on the Behavior of Carbonyl Triplet States in the Zeolite NaY J. C. Scaiano,* Monica Kaila, and Sonia Corrent Department of Chemistry, UniVersity of Ottawa, Ottawa, Ontario K1N 6N5, Canada ReceiVed: May 30, 1997; In Final Form: August 12, 1997X
“Spectator” molecules, which do not participate directly in the photochemistry of organic molecules included in zeolites, can be used to “tune” their photochemistry and photophysics. Such control can be exerted through changes in the diffusional behavior, spatial restrictions, or modification of the acid-base properties of the zeolite. In the case of the faujasite NaY, we have employed pyridine as a “spectator” to alter the behavior of xanthone, p-methoxy-β-phenylpropiophenone, and benzophenone which provide examples of the three mechanisms mentioned above. These processes were examined with laser flash photolysis techniques employing diffuse reflectance detection. For example, the lifetime of triplet xanthone is enhanced nearly 2 orders of magnitude when a small amount of pyridine is included, showing that a small number of acid sites in NaY may severely quench the excited triplet. In the case of p-methoxy-β-phenylpropiophenone, incorporation of large concentrations of pyridine restricts the motions required for β-phenyl intramolecular quenching and leads to an increase in triplet lifetime.
Zeolites have open framework structures made from crystalline aluminosilicates. They contain one negative charge in the framework for each aluminum atom; these charges are balanced by positive counterions, frequently alkali metal cations. Zeolites contain an array of well-defined cages of molecular dimensions. In the case of faujasites (to which the zeolite NaY used here belongs), the main cage, usually referred to as a “supercage”, is approximately 13 Å in diameter and is connected to neighboring supercages through windows with a diameter of ∼7.3 Å. Each supercage has four windows in a tetrahedral arrangement.1 It is generally believed that NaY has no Brønsted acid sites.2 Organic photoreactions in zeolite cages can have different consequences than those in homogeneous solution. These differences have their origin in the restricted space, medium polarity, and limited diffusion encountered by guest molecules in the cages available in the host zeolite. Mobilities in zeolites can range from very fast processes, to cases where equilibration can take weeks or months,3 to fully entrapped molecules that can only be placed in the host by “ship-in-a-bottle” synthetic methods.4,5 In this article we examine how a simple “spectator”, which does not influence directly the photochemistry or photophysics of the guest, can modify its behavior by changing the characteristics of the zeolite cage. We have selected pyridine, a molecule that is known to bind to Brønsted and Lewis acid sites in zeolites and frequently employed to quantify their abundance.6 We anticipated that pyridine would modify the acidity of the cages and limit the space available to the guests. We have selected as guests xanthone, p-methoxy-βphenylpropiophenone, and benzophenone. In one form or another, all three ketones have been examined in zeolites before.7-9 We have employed the technique of time-resolved diffuse reflectance developed by Wilkinson and co-workers in the 1980s which facilitates the study of opaque samples with laser photolysis techniques.10-12 Xanthone was selected because its triplet-triplet absorption is known to be very sensitive to environmental polarity,13 with shorter wavelengths reflecting a more polar medium; the band X
Abstract published in AdVance ACS Abstracts, September 15, 1997.
S1089-5647(97)01745-8 CCC: $14.00
maximum positions are 605 and 595 nm for the zeolites silicalite and NaY8,9 and are indicative of a polar environment. In the case of p-methoxy-β-phenylpropiophenone, the triplet lifetime is normally determined by intramolecular quenching by the β-phenyl ring.14 This type of quenching has long been established;14-17 ketones with low-lying n,π* triplet states have much shorter triplet lifetimes and are less amenable to study. β-Phenyl quenching requires a conformation that brings the π system in proximity to the excited carbonyl group, as shown in reaction 1.
The interaction of reaction 1 is believed to involve the n,π* triplet state, which in the case of p-methoxy-β-phenylpropiophenone is in thermal equilibrium with the low-lying π,π* triplet state.14 Notably, its triplet lifetime at room temperature increases by 5 orders of magnitude upon inclusion in silicalite, a zeolite where only the stretched conformation can fit within its channel structure.18 Benzophenone was selected simply as a control substrate, since its photophysics is not very sensitive to either space restriction or environmental polarity. Benzophenone phosphoresces readily in zeolites in the absence of hydrogen donors.7,19 Experimental Section Benzophenone and xanthone were Aldrich products and were recrystallized from methanol. p-Methoxy-β-phenylpropiophenone was prepared as described in earlier work.17 Pyridine (Aldrich) was used as received. © 1997 American Chemical Society
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TABLE 1: Lifetime Distribution Analysis of NaY/Xanthone Samples with Different Pyridine Exposure Times for (a) Vacuum-Sealed Samples and (b) Nitrogen-Purged Samples pyridine diffusion time, min
molecules of pyridine/ supercage
triplet lifetime, µs largest second largest component component
0 10 20 120 480
(a) Vacuum-Sealed Samples 0 0.9 0.20 115 0.46 18.7 0.82 18.8 1.14 2.3
0 10 20 120
(b) Nitrogen-Purged Samples 0 11 0.05 180 0.10 10 0.71 5
2.6 14 90 13 26
The zeolite NaY (from Aldrich, molecular sieves ZY52, Si/ Al ) 2.4) was activated by heating overnight at 550 °C. The dry zeolite was stirred for 5 h in a solution of the guest ketone in hexane. The zeolite was then separated and washed with hexane to minimize the amount of guest on the external surface of the zeolite particles. The samples were then dried under vacuum (p e 20 mTorr) or by blowing dry nitrogen. Pyridine was diffused into the dry zeolite samples by placing the zeolite in an uncapped vial which was in turn exposed to pyridine vapors by placing it in a closed container with pyridine at room temperature. Several experiments were carried out to determine the amount of pyridine absorbed by the samples by monitoring the weight increase following different exposure periods. On average it took 59, 132, and 240 min to achieve an average occupancy of 1, 2, and 3 molecules per supercage for the treatment of “empty” zeolite. Eventually the occupancy leveled off at around 3.7 pyridine molecules per cavity. Pyridine incorporation was slower when a guest had already been included. The final occupancy level was around 1.3 pyridine molecules per supercage when xanthone had already been included in the sample; some quantitative data are presented later in Table 1 (vide infra). Supercage occupancy was calculated by the ratio of the number of molecules included to the number of available supercages, assuming there is 0.6 mmol of supercages per gram of zeolite. Laser flash photolysis experiments were carried out in a timeresolved diffuse reflectance setup similar to that developed by Wilkinson and co-workers.10-12 The samples were excited with 355 nm pulses (∼6 ns, E15 mJ/pulse at the sample) from a Continuum Surelite laser. The signals from the monochromator/ photomultiplier system were captured by a Tektronix 2440 digitizer interfaced to a PowerMacintosh computer that controlled the experiment with software developed in the LabVIEW 3.1 environment from National Instruments. The basic components of our instrument are similar to those described earlier.13,20 Distribution analysis was carried out with software from PTI that had been customized to handle diffuse reflectance data. We have reported earlier on similar data analysis.21 Results Laser flash photolysis of xanthone in NaY leads to the spectrum of Figure 1, showing a maximum at 600 nm. In samples exposed to pyridine, the shape of this band changes, showing predominantly a displacement toward longer wavelengths, with λmax ∼ 615 nm, as illustrated also in Figure 1. Note that addition of pyridine also leads to a narrower spectrum,
Figure 1. Transient absorption spectra obtained by 355 nm excitation of NaY/xanthone and NaY/xanthone with pyridine exposure time of 4 h. ∆J/J is the change in reflectance signal.
Figure 2. Normalized transient decay traces of xanthone triplet in NaY with different pyridine inclusion times for the nitrogen-purged samples. The traces were recorded at 600 nm using a 5 µs time scale setting in the digitizer.
with the effect being largely on the short wavelength side. This suggests a more homogeneous site distribution in the presence of pyridine, resulting from the elimination of some of the more polar (short wavelength)13 sites. Two sets of xanthone/pyridine samples were prepared. The first set of samples were vacuum-sealed whereas the second set were purged with dry nitrogen. Although the lifetimes of the xanthone triplet varied depending on the sample preparation method, the trends observed upon addition of various amounts of pyridine were similar. The decay kinetics for xanthone triplet showed a remarkable dependence with the amount of pyridine added. Exposure to pyridine vapor, as described in the Experimental Section, leads to a dramatic increase in lifetime, on the order of 20-100-fold. The pyridine incorporation causing this effect involves one molecule every three cavities or less. It is thus interesting that the effect would be so large. Further addition of pyridine leads to a gradual but small decrease in lifetime, as illustrated in Figure 2 for the nitrogen-purged samples. The decay traces frequently do not fit well to a singleexponential decay if sufficiently long time periods are monitored. Lifetimes obtained by simple exponential or biexponential analysis are frequently dependent on the time scale monitored, with longer lifetimes recorded at longer time scales.22 We have found that a convenient way of treating the data, that also reflects the multiplicity of sites available in zeolites, is to employ a distribution analysis. To optimize this analysis, one normally records decay traces in various instrumental time scales and then combines them to produce a composite decay with closely spaced data points at short times and larger values of
8566 J. Phys. Chem. B, Vol. 101, No. 42, 1997
Figure 3. Transient decay trace in different time domains for triplet xanthone in NaY with no pyridine. The two independent traces were monitored at 600 nm. Note the logarithmic time scale.
Scaiano et al.
Figure 5. Quenching of xanthone triplet (monitored at 600 nm) in acetonitrile solution by pyridine (top scale) and by trifluoroacetic acid (bottom scale).
TABLE 2: Lifetime Distribution Analysis of NaY/ p-Methoxy-β-phenylpropiophenone Samples with Different Pyridine Exposure Times for (a) Fresh Samples and (b) Aged Samples pyridine diffusion time, min
Figure 4. Triplet lifetime distribution analysis for xanthone included in NaY (top) and xanthone included in NaY (bottom) with pyridine exposure time of 2 h.
point-to-point separation at the long time scales. Figure 3 illustrates one such trace for a sample that has not been exposed to pyridine. Figure 4 (top) shows the lifetime distribution for the same sample as in Figure 3 and for a sample that has been exposed to pyridine for 2 h, corresponding to an average of about 0.8 pyridine molecules per supercage. The former (Figure 4, top) shows predominantly a single lifetime, although with a very broad distribution, while the sample with pyridine shows a bimodal decay with longer lifetimes than in the absence of pyridine. We have summarized the lifetime data from distribution analysis in Table 1, where pyridine was incorporated into samples already containing xanthone. In light of the results above, we examined the effect of pyridine and of a representative acid on the decay of the xanthone triplet in homogeneous solution. Figure 5 shows quenching plots for pyridine and trifluoroacetic acid in acetonitrile, leading from the slopes to xanthone triplet quenching
molecules of pyridine/ supercage
triplet lifetime, µs largest second largest component component
0 10 60 120 240
(a) Fresh Samples 0 3.8 0.11 0.7 0.73 9.6 1.80 13.6 2.52 19.2
0 10 60 120 240
(b) Aged Samples 0 9.0 0.11 2.2 0.73 15.3 1.80 17.8 2.52 19.0
0.4 0.1 3 2 0.3 0.2 0.4 0.6
rate constants of 6.6 × 106 and 2.4 × 109 M-1 s-1, respectively. By comparison, similar concentrations of acid had virtually no effect on the lifetime of triplet benzophenone, while pyridine quenches benzophenone triplets with a rate constant of 7.0 × 106 M-1 s-1 in acetonitrile. The effect of included water on xanthone triplet decay was also monitored. An amount of water similar to the amount of pyridine that was absorbed in the 10 min exposure time was absorbed onto a vacuum-dried sample of xanthone in NaY. No significant changes in the triplet lifetime of xanthone were observed between the wet and dry samples. Our other guest molecule, p-methoxy-β-phenylpropiophenone, has a low-lying π,π* triplet state which is readily detectable in the 400 nm region, where triplet decay was monitored.17 The effect on the triplet lifetime upon adding various amounts of pyridine to p-methoxy-β-phenylpropiophenone included in NaY are summarized in Table 2. Representative decays with and without pyridine are shown in Figure 6, while the results of distribution analysis are given in Figure 7. The examples of Figures 6 and 7 show that the lifetime of the main component for triplet decay decreases with the first small amount of pyridine added, followed by an increase as more and more pyridine is incorporated. We find that the effect in the case of p-methoxy-β-phenylpropiophenone is opposite to the case of xanthone where a small content in pyridine causes a dramatic lengthening of the lifetime (vide supra). A study of the effect of sample aging reveals that the trends are conserved but that the triplet lifetime for the sample with no pyridine increases by over 5 µs. A similar increase in triplet lifetime was noted for an aged xanthone sample.
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J. Phys. Chem. B, Vol. 101, No. 42, 1997 8567 present in the case of benzophenone. In the case of benzophenone typical triplet lifetimes were around 2 µs. Discussion
Figure 6. Normalized triplet decay traces of p-methoxy-β-phenylpropiophenone in NaY with different pyridine inclusion times. The traces were recorded at 400 nm using a 1 µs time scale.
Figure 7. Triplet lifetime distribution analysis of NaY/p-methoxy-βphenylpropiophenone (A) and NaY/p-methoxy-β-phenylpropiophenone with pyridine diffused in for 10 min (B) and 4 h (C).
Finally, a few control experiments were also carried out with benzophenone in NaY. While pyridine caused a minor reduction of the triplet lifetime, these effects are very small compared with those reported above for xanthone and p-methoxy-βphenylpropiophenone, suggesting that in these cases the larger effects observed have their origin in specific characteristics not
Xanthone was one of the first molecules studied in zeolites using time-resolved diffuse reflectance techniques. The triplet spectrum provided clear evidence of the zeolite’s highly polar environment.8,18 Similarly, p-methoxy-β-phenylpropiophenone provided information about the limitations to conformation interconversion, particularly in the case of pentasil zeolites, such as silicalite, where their channel structure prevents the motions required for intramolecular triplet quenching by the β-phenyl ring (see reaction 1).7,18,23 We note that in early studies the zeolite samples employed probably had a much larger water content than in the work reported here.7,18,23 FT-Raman and DRIFT spectroscopies of high and low pyridine loadings adsorbed onto sodium faujasites give some indication as to the position and interaction of the pyridine molecules within the supercages.24,25 It is also known that pyridine adsorption causes a redistribution of water and sodium cations. Thermal desorption of pyridine from zeolites provides a widely employed technique used to determine the number and strength of acid sites in zeolites.6 On the basis of this work, it is clear that pyridine will preferentially neutralize first the stronger acid sites in the zeolite. We believe that the dramatic increase in lifetime for xanthone triplet observed upon brief exposure of the sample to pyridine (see Figure 2) reflects the neutralization of the more acidic sites in the zeolite that act as effective quenchers of the xanthone triplet, just as trifluoroacetic acid does in solution (Figure 5). The pKa for triplet xanthone is 3.0,26 compared with 0.3 and 1.5 for benzophenone, depending on the solvent system,27 consistent with the acid quenching observed in the case of xanthone. Thus, we believe that the short lifetime of triplet xanthone in the dry zeolite NaY is largely controlled by its interaction with the acid sites present in the framework. Small amounts of pyridine neutralize the stronger acid sites and lead to a dramatic increase in lifetime. The fact that similar amounts of water included in a dry NaY/xanthone sample cause no significant changes in the triplet lifetime is indicative that pyridine is not just simply reducing the mobility of xanthone, since water would be expected to have a similar effect. Prolonged exposure to pyridine gradually reduces the lifetime, as a result of the quenching already observed in solution. The nature of this quenching is currently under study, although there are clear indications that the process involves charge-transfer interactions between the π system of the quencher and the excited carbonyl. Similar quenchings by aromatics, including benzene (albeit less effective) and other alkylbenzenes, are wellestablished.28,29 Thus, the enhancement of the lifetime by small amounts of pyridine reflects an effect on the environment provided by the host zeolite, while the moderate quenching observed at higher pyridine loadings is analogous to that observed in solution and reflects an interaction with the guest. The differences in triplet lifetimes between the two sets of samples are a reflection of sample preparation and handling, including residual solvent content. In the case of benzophenone the effect of acid sites is less pronounced (reflecting the difference in pKa values), while pyridine quenching is less important because the lifetimes are always much shorter (and therefore less sensitive) in the case of benzophenone. In the case of p-methoxy-β-phenylpropiophenone a decrease of the triplet lifetime is observed in NaY upon initial additions of pyridine. This again shows the strong affinity that pyridine has for the acid sites in the zeolite, and binding to them in turn
8568 J. Phys. Chem. B, Vol. 101, No. 42, 1997 probably displaces p-methoxy-β-phenylpropiophenone molecules bound to the most active sites; as a result, small amounts of pyridine facilitate mobility and access to a conformation from which intramolecular quenching can occur easily. In contrast, addition of larger amounts of pyridine (see Figures 6 and 7) leads to an enhancement of the triplet lifetime, which is a clear indicator of the reduced ability to undergo conformational changes as the pyridine occupancy of the supercages increases. This lifetime is even greater than the lifetime of 8 µs reported in the solid state,30 which reveals the extent of the compact environment that the p-methoxy-β-phenylpropiophenone finds itself in upon large amounts of pyridine addition. The pyridine may also increase the local viscosity within the supercages of the zeolite, which again would slow the intramolecular quenching process. The significant increase in triplet lifetime of p-methoxy-βphenylpropiophenone in NaY upon aging reflects its “slow” mobility within the supercages, eventually leading to more molecules adopting a conformation that makes intramolecular quenching more difficult. The samples containing pyridine, although not showing as much of a change in lifetime, can still be thought of in terms of undergoing a rearrangement process, resulting in somewhat longer lifetimes compared to the fresh samples. In the case of benzophenone, the effect of pyridine is minor and rather uninteresting, as would be expected for a ketone where proton quenching is less likely and where conformational change does not play a significant role. In conclusion, our results show that spectators can control the photochemistry of guest molecules by at least three different types of mechanisms, i.e., (i) by influencing the acid-base properties of the zeolite cages, (ii) by direct interaction with the excited states of the guest, and (iii) by restricting the mobility of the excited guest. By careful choice of the spectator molecule, it should be possible to “tune” the behavior of molecules within zeolites and other supramolecular systems. Recent work has demonstrated that such “tuning” may include asymmetric induction.31 Acknowledgment. J.C.S. thanks NSERC (Canada) and the Killam Foundation for support. S.C. is the recipient of an Ontario Graduate Scholarship, and M.K. was a RISE scholar during the summer of 1996 when much of this work was carried out.
Scaiano et al. References and Notes (1) Davis, M. E. Acc. Chem. Res. 1993, 26, 111. (2) Corma, A. Chem. ReV. (Washington, D.C.) 1995, 95, 559. (3) Cozens, F. L.; Re´gimbald, M.; Garcı´a, H.; Scaiano, J. C. J. Phys. Chem. 1996, 100, 18165. (4) Corma, A.; Forne´s, V.; Garcı´a, H.; Miranda, M. A.; Primo, J.; Sabater, M.-J. J. Am. Chem. Soc. 1994, 116, 2276. (5) Cano, M. L.; Cozens, F. L.; Forne´s, V.; Garcı´a, H.; Scaiano, J. C. J. Phys. Chem. 1996, 100, 18145. (6) Ward, J. W. J. Catal. 1968, 10, 34. (7) Casal, H. L.; Scaiano, J. C. Can. J. Chem. 1985, 63, 1308. (8) Wilkinson, F.; Willsher, C. J.; Casal, H. L.; Johnston, L. J.; Scaiano, J. C. Can. J. Chem. 1986, 64, 539. (9) Scaiano, J. C.; Camara de Lucas, N.; Andraos, J.; Garcı´a, H. Chem. Phys. Lett. 1995, 233, 5. (10) Wilkinson, F.; Willsher, C. J. Appl. Spectrosc. 1984, 38, 897901. (11) Kessler, R. W.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1 1981, 77, 309. (12) Wilkinson, F.; Kelly, G. In Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. I, p 293. (13) Scaiano, J. C. J. Am. Chem. Soc. 1980, 102, 7747. (14) Encina, M. V.; Lissi, E. A.; Lemp, E.; Zanocco, A.; Scaiano, J. C. J. Am. Chem. Soc. 1983, 105, 1856. (15) Stermitz, F. R.; Nicodem, D. E.; Muralidharan, V. P.; O’Donnell, C. M. Mol. Photochem. 1970, 2, 87. (16) Leigh, W. J. J. Am. Chem. Soc. 1985, 107, 6114. (17) Netto-Ferreira, J. C.; Leigh, W. J.; Scaiano, J. C. J. Am. Chem. Soc. 1985, 107, 2617. (18) Casal, H. L.; Scaiano, J. C. Can. J. Chem. 1984, 62, 628. (19) Okamoto, S.; Nishiguchi, H.; Anpo, M. Chem. Lett. 1992, 6, 1009. (20) Scaiano, J. C.; Tanner, M.; Weir, D. J. Am. Chem. Soc. 1985, 107, 4396. (21) Barra, M.; Scaiano, J. C. Photochem. Photobiol. 1995, 62, 60. (22) Kelly, G.; Willsher, C. J.; Wilkinson, F.; Netto-Ferreira, J. C.; Olea, A.; Weir, D.; Johnston, L. J.; Scaiano, J. C. Can. J. Chem. 1990, 68, 812. (23) Scaiano, J. C.; Casal, H. L.; Netto-Ferreira, J. C. ACS Symp. Ser. 1985, 278, 211. (24) Ferwerda, R.; van der Maas, J. H.; Hendra, P. J. J. Phys. Chem. 1993, 97, 7331. (25) Ferwerda, R.; van der Maas, J. H. J. Phys. Chem. 1995, 99, 14764. (26) Ireland, J. F.; Wyatt, P. A. H. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1053. (27) Rayner, D. M.; Wyatt, P. A. H. J. Chem. Soc., Faraday Trans. 2 1974, 70, 945. (28) Wagner, P. J.; Truman, R. J.; Puchalski, A. E.; Wake, R. J. Am. Chem. Soc. 1986, 108, 7727. (29) Yamaji, M.; Okada, K.; Shizuka, H. In 54th Okazaki Conference; Dynamic Studies of Hydrogen Atom Transfer Reactions; Institute of Molecular Science: Okazaki, Japan, 1996; p 6. (30) Boch, R.; Bohne, C.; Scaiano, J. C. J. Org. Chem. 1996, 61, 1423. (31) Leibovitch, M.; Olovsson, G.; Sundarababu, G.; Ramamurthy, V.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1996, 118, 1219.
