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Understanding the Influence of Active (Zeolite) and Passive (Polyethylene) Reaction Cages on Photo-Claisen Rearrangements of Aryl Benzyl Ethers Weiqiang Gu,† Manoj Warrier,‡ Brian Schoon,‡ V. Ramamurthy,*,‡ and Richard G. Weiss*,† Departments of Chemistry, Georgetown University, Washington, D.C. 20057-1227, and Tulane University, New Orleans, Louisiana 70118 Received April 17, 2000. In Final Form: May 31, 2000 Photo-Claisen rearrangements of benzyl phenyl ether and benzyl 1-naphthyl ether have been examined in cation-exchanged Y zeolites and polyethylenes of differing crystallinities. Ratios of the principal rearrangement products, benzyl arylol positional isomers, indicate that the reactions are more selective in the zeolites than in the polyethylenes. The results are explained on the basis of the active and passive walls of the reaction cages of zeolites and polyethylenes, respectively, and the limited free volume of the polyethylene cages.
Introduction Photochemically induced reactions of molecules in normal (isotropic liquid) and organized media frequently lead to different product distributions and, in some cases, totally different products.1 Host-guest interactions on the supramolecular level can alter the mechanistic course of a reaction by affecting the conformational lability and translational mobility at all points along a reaction coordinate. As a result, it is important to understand how an organized medium affects the ground state, excited state(s), and intermediates. This is a daunting task even in isotropic liquid media. The anisotropy of organized media frequently increases the complexities of photochemical mechanistic investigations. Despite this, others and we1,2 have sought to unravel the specific factors that differentiate photochemical reactions conducted in isotropic and organized media with hopes of developing a set of criteria that will allow a priori selection of media for directed syntheses. As a part of our ongoing interest in evaluating and comparing the characteristics of different types of reaction cages, we recently probed the interactions imposed by the reaction cages of zeolite Y with different cations and polyethylene media of different crystallinities3 on the electronic states and intermediates generated during the photo-Fries rearrangements of 1-naphthyl phenylacylates.4 The fates of the two key intermediates, a 1-naphthoxy/phenylacyl radical pair and a 1-naphthoxy/benzylic radical pair (via decarbonylation of a phenylacyl radical of the former pair), determine the final product distribution. This is a very attractive system since the rearrangements are † ‡
Georgetown University. Tulane University.
(1) (a) Ramamurthy, V., Ed. Photochemistry in Organized and Constrained Media; VCH: New York, 1991. (b) Weiss, R. G.; Ramamurthy, V.; Hammond, G. S. Acc. Chem. Res. 1993, 26, 530. (c) Ramamurthy, V.; Weiss, R. G.; Hammond, G. S. Adv. Photochem. 1993, 18, 67. (2) Turro, N. J.; Garcia-Garibay, M. A. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; pp 1-38. (3) The characteristics of the high-density polyethylene (HDPE) from Brazil and low-density polyethylene (LDPE), Sclair from Dupont of Canada, have been reported previously. Zimerman, O. E.; Cui, C.; Wang, X.; Atvars, T. D.; Weiss, R. G. Polymer 1998, 39, 1177.
very sensitive to the nature of the environment in which they occur and the reaction cages of the two media are intrinsically very different, but both can be modified in subtle ways. The factors controlling the fate of the 1-naphthoxy/benzylic radical pair within zeolites and polyethylenes appear to differ from their 1-naphthoxy/ phenylacyl precursors. To examine the reactions of aryloxy/ benzyl radical pairs in zeolite Y cages and to compare their reaction pathways when formed in polyethylene cages under conditions which specify their initial relative positions (vide infra), we have examined the photochemically induced Claisen rearrangements of benzyl phenyl ether (1) and benzyl 1-naphthyl ether (2) in these media.
Homolytic cleavage of electronically excited 1 or 2 leads to aryloxy/benzyl radical pairs5,6 whose relative positions within their reaction cages are known at least initially. These radical pairs are structurally identical to those obtained from irradiation of the corresponding aryl esters after decarbonylation. From the photoproduct distributions, we have compared the influence of the two anisotropic media on these radical pair recombinations and on those derived from irradiation of the analogous esters after decarbonylation of their phenylacetyl radicals. Media and the Reactions. Zeolite Y cages (supercages with ca. 12 Å diameters1b,c) are much larger than the van der Waals volumes of 1 and 2.7 Because their walls are “stiff” (but “active” due to the cations that are placed regularly within them), cages maintain their original size and shape during reactions of guest molecules. Walls of polyethylene cages are much more “flexible” than those of zeolites, and the free volumes of “holes” in native (undoped) polyethylene, estimated from positron annihilation spectroscopy,8 are smaller than the van der Waals volumes of 1 and 2.7 Consequently, the cages of (4) Gu, W.; Warrier, M.; Ramamurthy, V.; Weiss, R. G. J. Am. Chem. Soc. 1999, 121, 9467
10.1021/la000574h CCC: $19.00 © 2000 American Chemical Society Published on Web 07/18/2000
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Scheme 1. Important Mechanistic Steps during Photo-Claisen Reactions of Benzyl Phenyl Ether (1) and Benzyl 1-Naphthyl Ether (2)a
Gu et al. Table 1. Relative Photoproduct Yields (%) from Irradiationsa of 1 in Isotropic Solutions, Unstretched (u) and Stretched (s) Polyethylene Films, and Cation-Exchanged Y-Zeolites medium
POL
(Bz)2
hexane 28.6 ( 2.1 16.1 ( 0.3 acetone 51.2 ( 1.5 33.8 ( 0.7 LDPE (u) 0 0 LDPE (s) 0 0 HDPE (u) 0 0 HDPE (s) 0 0 LiY 5.0 ( 0.5 0 NaY 7.0 ( 2.0 0 KY 8.0 ( 0.3 0 RbY 7.0 ( 2.5 0 CsY 4.0 ( 0.5 0 TlY 6.0 ( 1.2 0 a
a 1S and 1T are the first excited singlet and triplet states of 1 or 2. X ) P (from 1) or N (from 2) in products.
polyethylenes are determined in large part by the structures of the occupant ethers. Although detailed mechanistic studies on photo-Claisen rearrangements of 1 and 2 have not been reported, the photo-Claisen rearrangements of structurally related molecules, allyl phenyl ether5 and dimethylallyl 1-naphthyl ether,6 have been. By analogy, we expect 1 and 2 to react as shown in Scheme 1. It is known that allyl phenyl ether reacts from both excited singlet and triplet states, yielding the same products but with different distributions.9 Furthermore, it has been established that intersystem crossing of the excited singlet state of allyl phenyl ether does not compete with other singlet decay processes, including reaction.5,9 On the basis of the reported behavior of allyl phenyl ether, we suggest that triplet radical pairs, if formed upon direct irradiation of 1, must arise from intersystem crossing of the initially formed singlet pairs in-cage or from encounters by radicals that have escaped from their initial cages. 3-BP is the only photoproduct in Scheme 1 that should be derived exclusively from triplet radical pairs.5 By contrast, a combination of sensitization, quenching, and CIDNP experiments6 indicates that both the excited singlet and triplet states of dimethylallyl 1-naphthyl ether are reactive upon direct irradiation. Irradiations in Isotropic Media and General Mechanistic Considerations. When 1 and 2 were irradiated directly in isotropic solutions, all of the photoproducts in Scheme 1 were detected. Both [2-BP]/ [4-BP] and [2-BN]/[4-BN] ratios are 1.2 in hexane, and the [2-BN]/[4-BN] ratio is slightly higher in tert-butyl alcohol. Acetone sensitization experiments demonstrate that triplet states of 1, if present, will also yield triplet aryloxy/benzyl radicals as in-cage pairs. At room temperature, the photoproduct distributions from 1 in acetone10 under conditions where only solvent molecules (5) (a) Waespe, H. R.; Heimgartner, H.; Schmid, H.; Hansen, H. J.; Paul, H.; Fischer, H. Helv. Chim. Acta 1978, 61, 401. (b) Adam, W.; Fischer, H.; Hansen, H. J. Heimgartner, H.; Schmid, H.; Waespe, H. R. Angew. Chem., Int. Ed. Engl. 1973, 12, 662. (6) (a) Pohlers, G.; Grimme, S.; Dreeskamp, H. J. Photochem. Photobiol. A: Chem. 1994, 79, 153. (b) Grimme, S.; Dreeskamp, H. J. Photochem. Photobiol. A: Chem. 1992, 65, 371. (7) Bondi, A. J. Phys. Chem. 1964, 68, 441. (8) Gu, W.; Hill, A. J.; Wang, X.; Cui, C.; Weiss, R. G. Macromolecules, submitted for publication.
2-BP
4-BP
[2-BP]/ [4-BP]
29.6 ( 0.4 8.0 ( 1.6 64.5 ( 1.4 69.2 ( 2.7 54.9 ( 0.5 60.0 ( 0.2 85.0 ( 2.5 85.0 ( 1.4 80.0 ( 1.6 76.0 ( 2.7 65.0 ( 0.2 64.0 ( 2.0
25.6 ( 1.7 7.0 ( 0.6 35.5 ( 1.5 30.8 ( 2.5 45.1 ( 1.5 40.0 ( 0.1 10.0 ( 2.3 8.0 ( 2.7 12.0 ( 1.0 17.0 ( 3.0 31.0 ( 0.5 30.0 ( 1.5
1.2 1.1 1.8 2.2 1.2 1.5 8.5 10.6 6.7 4.5 2.1 2.1
254 nm except >300 nm in acetone.
absorb (>300 nm) and upon direct irradiation (254 nm) in hexane (Table 1) differ too drastically to be attributed to simple solvent effects. Although the [2-BP]/[4-BP] ratios from both the sensitized and direct irradiations are near 1, only 15% of the photoproduct mixture is 2- or 4-BP isomers (no 3-BP was detected) in acetone, while 55% of the photoproduct mixture is BP isomers in hexane. On the basis of these results and the absence of photoproducts from triplets of allyl phenyl ether,5 the triplet component of reaction in 1 seems to be very small. Triplet energy transfer from benzophenone (ET ) 289.5 kJ/mol11) to 2 (ET ) 254 kJ/mol from phosphorescence measurements) at >300 nm was efficient, but the quantum yield for reaction was e0.03.12 Assuming the quantum yield for S1 f T1 intersystem crossing by 2 is less than the value for 1-methoxynaphthalene (0.26),13 the quantum yield for the triplet component of photo-Claisen rearrangements by 2 upon direct irradiations is 50% from sensitized reactions. In principle, it should be possible to correlate the distribution of rearrangement products with spin densities at various positions of phenoxy and 1-naphthoxy radicals if the radical pairs equilibrate in their cages before combining and the steric requirements for attachment at each of the aryloxy positions are similar. The relative spin densities at the 2- and 4-positions of the phenoxy radical are 0.7 and 1.0, respectively, from ESR measurements, (9) Shimamura, N.; Sugimori, A. Bull. Chem. Soc. Jpn. 1971, 44, 281. (10) From energetic considerations, only the triplet state of acetone (332 kJ/mol)10a is capable of transferring energy to 1 (ET = 332 kJ/mol; Figure 1); the energy of the lowest excited singlet state of acetone (ES = 362 kJ/mol)10b is much lower than the singlet energy of 1 (ES = 426 kJ/mol from fluorescence spectra). (a) Borkman, R. F.; Keam, D. R. J. Chem. Phys. 1966, 44, 945. (b) Turro, N. J. Modern Molecular Photochemistry; University Science Books: Sausalito, CA, 1991. (11) Leigh, W. T.; Arnold, D. R. J. Chem. Soc., Chem. Commun. 1980, 406. (12) When equal volumes of a 1/2 benzophenone/benzhydrol solution and a 1/2/2 benzophenone/benzhydrol/2 solution were irradiated sideby-side, 91% of benzophenone in the former and 1.3% of 2 from the latter reacted. Since no benzophenone reduction was observed after irradiation of the 1/2/2 benzophenone/benzhydrol/2 solution, energy transfer to 2 is much faster than reduction by benzhydrol; virtually all of the benzophenone triplets were intercepted by 2.12a Thus, the fraction of 2 that rearranges from its triplet state, Φr, is Φp(2 × 1.3%)/(91%), where Φp, the quantum yield for the photoreduction of benzophenone triplets by benzhydrol, is assumed to be 1.12a (a) Moore, W. M.; Hammond, G. S.; Foss, R. P. J. Am. Chem. Soc. 1961, 83, 2789. (13) Lamola, A. A.; Hammond, G. S. J. Chem. Phys. 1965, 43, 2129.
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Table 2. Relative Photoproduct Yields (%) from Irradiationsa of 2 in Isotropic Solutions, Unstretched (u) and Stretched (s) Polyethylene Films, and Cation-Exchanged Y-Zeolites NOL
hexane benzophenone/ hexane tert-butyl alcohol LDPE (u) LDPE (s) HDPE (u) HDPE (s) LiY NaY KY RbY TlY
1.3 ( 0.2 32.4 ( 1.5
8.1 ( 1.4 49.5 ( 0.2 41.1 ( 0.4 22.6 ( 1.8 17.6 ( 0.6 27.4 ( 2.6
1.2 0.6
12.8 ( 0.2
5.1 ( 0.3 52.0 ( 0.3 30.1 ( 0.4
1.7
a
7.6 ( 0.8 3.9 ( 0.5 4.8 ( 0.3 3.2 ( 0.1 1.0 ( 1.5 1 ( 1.3 0 1.0 ( 1.7 0
(Bz)2
1.7 ( 0.3 0.6 ( 0.2 < 0.5 300 nm except >340 nm in benzophenone/hexane.
and the values in the corresponding positions of the 1-naphthoxy radical are similar.14 From statistical (N.B., phenoxy has twice as many 2-positions as 4-positions but 1-naphthoxy has one of each) and spin-density considerations, more 2-BP than 4-BP should be formed from 1 and more 4-BN than 2-BN should be obtained from 2. If the product distributions are also dependent upon the proximity of a benzyl radical to a 2- or 4-position on aryloxy at the moment of radical pair formation, the 2-isomers should be favored from both ethers; benzyl radicals are much closer to the 2-positions immediately after lysis of 1 or 2. However, when the same benzyl/aryloxy radical pairs are generated by decarbonylation of phenylacetyl radicals that are paired with aryloxy radicals (i.e., from irradiations of the aryl phenylacetate analogues of 1 and 2, phenyl phenylacetate and 1-naphthyl phenylacetate4,15), the spatial preference for combination at the 2-position should be attenuated, if not completely lost. In fact, the [2-BP]/[4-BP] and [2-BN]/[4-BN] ratios are very similar when 1 and 2 are irradiated in the same medium (hexane, zeolites, or polyethylenes; Tables 1 and 2). Neither the size nor statistical site difference between phenoxy and 1-naphthoxy seems to affect the correspondence between the distribution of photo-Claisen products from 1 and 2! By contrast, the photo-Fries product distributions from phenyl phenylacetate and 1-naphthyl phenylacetate in the same polyethylene media as employed here differed significantly.4,15 Clearly, different factors are responsible for the selectivity in these mechanistically related reactions with structurally related substrates. Irradiations in Y Zeolites and Polyethylene Films. Formation of 2-BP and 2-BN within zeolites is somewhat selective. The lack of any detectable (Bz)2 indicates that radical pairs generated in different supercages never encounter each other. 2-/4- rearrangement product ratios in zeolites, 2-10 from 1 and 4-13 from 2, are much larger than the 1-2 ratios from irradiations in solutions. The higher zeolite ratios can be understood on the basis of interactions of the benzyl and aryloxy radical with the cations that are not available in solution. In our earlier study on photo-Fries rearrangements of esters analogous to the ethers 1 and 2, a similar selectivity was attributed to the interaction of phenylacyl (and aryloxy) radicals with (14) (a) Dixon, W. T.; Moghimi, M.; Murphy, D. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1713. (b) Dixon, W. T.; Foster, W. E. J.; Murphy, D. J. Chem. Soc., Perkin Trans 2 1973, 15, 2124. (15) (a) Gu, W.; Weiss, R. G. Tetrahedron, in press. (b) Gu, W.; Weiss, R. G., To be published.
the cations. The current study indicates that cation-π interactions alone are strong enough to restrict the mobility of radicals. The cations present within a zeolite cage are likely to influence the behavior of a guest and its intermediate radical pair according to the strength of cation-π interactions, the size of the cation, and its spin-orbit coupling parameter.16 The strength of cation-π interactions depends on the charge density of the cation. For example, smaller cations, such as Li+ and Na+, bind more strongly to aromatic radicals than larger ions, such as K+ and Rb+.17 On this basis alone, reaction selectivity is expected to decrease in the order Li > Na > K > Rb > Cs. The decreases in selectivity from Na+ to Cs+ (Tables 1 and 2) are consistent with this rationale. If cation size (and, therefore, the unoccupied space within a cage) were the dominant factor here, Cs+ should have exerted the greatest influence on radical pair mobility (and photoproduct selectivity) and Li+, being very small, the least. We attribute the slightly lower selectivities from Li Y zeolites than from Na Y zeolites to the smaller Li+ ions being more buried within the walls of the zeolite cages and less accessible to a benzyl or aryloxy radical. Thus, the relative influence of Li+ ions on the reactions of 1 and 2 is lower than it would be if all of the cations were equally exposed to benzyl and aryloxy radicals. Tl+, the heaviest ion, does not fit well within this rationale. Tl and Rb cations are similar in size18 and should interact to a similar extent with the π-electrons of a benzyl or aryloxy radical. Thus, on the basis of size and the cation-π interaction, one would expect a similar selectivity within Rb and Tl Y zeolites (TlY and RbY, respectively). However, the photoproduct selectivity from irradiations of 1 in TlY is lower than within RbY. We believe that this is due to stronger spin-orbital coupling effects within TlY16c that change the excited state of 1 leading to the initial radical pair. As shown in Figure 1, fluorescence from both 1 and 2 is decreased at the expense of phosphorescence within TlY; intersystem crossing of the excited singlet states is more efficient than fluorescence or lysis leading to singlet radical pairs. Similar emission spectra were obtained from the corresponding esters in cation-exchanged Y zeolites.4 As indicated above, both singlet and triplet excited states of 1 and 2 can undergo photo-Claisen rearrangements. Reaction probably occurs almost exclusively from excited singlet states within NaY and from excited triplet states within TlY. Thus, the initially formed aryloxy/benzyl radical pairs are predominantly singlets and triplets, respectively. The singlet radical pairs can recombine to form the 2- or 4-isomer of the keto precursor of the Claisen rearrangement products without any spin barrier. Spin inversion must occur before a triplet radical pair forms the same keto intermediates. We believe that the spin barrier provides sufficient time for the triplet radical pair to move more than its singlet analogue and, therefore, to react in a less selective manner. Thus, the lower selectivity with heavier ions is likely due to a combination of decreased binding between cations and the benzyl and aryloxy radicals and enhanced contributions from triplet state reactions. (16) (a) Turro, N. J.; Zhang, Z. Tetrahedron Lett. 1987, 28, 5637. (b) Thomas, K. J.; Sunoj, R. B.; Chandrasekhar, J.; Ramamurthy, V. Langmuir 2000, 16, 4912. (c) Ramamurthy, V.; Caspar, J. V.; Eaton, D. F.; Kuo, E. W.; Corbin, D. R. J. Am. Chem. Soc. 1992, 114, 3882. (17) (a) Ma, J. C., Dougherty, D. A., Chem. Rev. 1997, 97, 1303. (b) Dougherty, D. A. Science, 1996, 271, 163. (18) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed, Wiley: New York, 1988; pp 124, 209.
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Figure 2. Approximate orientations for radical additions to 1-naphthoxy by (A) a phenylacetyl radical from 1-naphthyl phenylacetate and (B) a benzyl radical from 2. The orientations of addition to phenoxy and naphthoxy are assumed to be the same. Addition to C-4 is shown. The z-axis is orthogonal to the 1-naphthoxy plane.
Figure 1. Emission spectra of 1 (A) and 2 (B) in cationexchanged zeolites at 77 K. Compare relative intensities of fluorescence (peaks to blue side) and phosphorescence (peaks to red side) within one spectrum only.
Comparisons of photoproduct distributions in Tables 1 and 2 demonstrate that formation of (Bz)2, an out-of-cage radical recombination product, is clearly suppressed by polyethylene matrixes. The traces of (Bz)2 detected during irradiations of 2 may be due to the small component of triplet reaction (vide ante). Ratios of the photo-Claisen products, [2-BP]/[4-BP] from 1 and [2-BN]/[4-BN] from 2 are only ∼2 and 1∼2, respectively, in both types of polyethylene films, and are virtually unaffected by film stretching (that is known to decrease the hole free volume in undoped films8). They are near the ratios found from direct irradiations in the isotropic media and much lower than the ratios measured in the Y zeolites. Since the ratios of photo-Fries products from aryl phenylacetates are sensitive to changes in both the crystallinity and stretching of polyethylene matrices,4,15 the constraints imposed by the polyethylene media on the radical pairs generated during photo-Fries reactions4,15 are not operative on the analogous radical pairs during photo-Claisen rearrangements! An attractive hypothesis to explain this difference focuses on the relative reactivities of benzyl and phenylacetyl radicals and how they interact with their reaction cages in polyethylenes and zeolites. Unfortunately, we have been unable to find rate data that allow us to compare the reactivities of these two radicals toward another common radical. However, the pivaloyl radical is known to react much more slowly than the tert-butyl radical with acrylonitrile in 2-propanol.19 If benzyl and phenylacetyl
radicals follow the same trend, one may argue that (1) photo-Fries rearrangements should be intrinsically more selective because they involve less reactive radical pairs or (2) the photo-Claisen rearrangements should be less selective because the more reactive benzyl radical moves less in its reaction cage before adding to an aryloxy partner. Clearly, the second explanation is inconsistent with the extensive body of data. However, these arguments assume that the radicals are able to move as they do in isotropic media. Our data also indicate that certain motions of the radicals are restricted and, as a result, arguments such as the ones above may not be appropriate. The constraints to specific radical motions in polyethylene cages rely upon the relaxation of polymer chains to create space in directions that lead to the various rearrangement products. In the case of the photo-Claisen rearrangements of 1 or 2, C-C bond formation requires that the aryloxy and benzyl radicals approach each other in vertically displaced parallel planes (Figure 2). C-C bond formation between an aryloxy and a phenylacetyl radical from photo-Fries rearrangements of esters analogous to 1 or 2 must involve the temporal creation of more space along the axis normal to the plane of aryloxy.15 On these bases, the relative position and orientation of an aryloxy radical with respect to its benzyl radical partner within a polyethylene cage at the moment of their creation are less important than the initial relative positions of an analogous aryloxy/ phenylacetyl radical pair. As found, the [2-BP]/[4-BP] and [2-BN]/[4-BN] ratios from irradiations in polyethylene films are similar, regardless of the nature of the film or whether the precursor radical pair was generated directly from an ether or indirectly from an ester (i.e., after lysis and decarbonylation). General Considerations and Conclusions. The examples provided here show that the supramolecular character of a reaction cage can be unraveled with the help of a judiciously chosen probe reaction. However, subtle changes in the ways the cages interact with their guests, as well as the product precursors derived from them, can lead to very different reaction selectivities and, therefore, descriptions of the cages in which the reactions occur. The very high selectivities of photo-Fries rearrangements and much lower selectivities of photo-Claisen rearrangements described here offer a case in point. Although the reaction cages afforded by both polyethylene and zeolites are capable of exerting significant control over the reactions of guest molecules, detailed aspects of how the guest and host interact must be analyzed to understand how the selectivity (or lack thereof) arises. (19) Jent, F.; Paul, H.; Roduner, E.; Heming, M.; Fischer, H. Int. J. Chem. Kinet. 1986, 18, 1113.
Active and Passive Reaction Cages
Experimental Section Materials. NaY zeolite was obtained from Zeolyst International, Amsterdam, The Netherlands. Other zeolites were prepared by stirring and refluxing 10 g of NaY with 100 mL of an aqueous 10% metal nitrate solution for 12 h. The zeolite was filtered out and washed thoroughly with distilled water. After repeating this procedure twice more, the zeolite was heated in an oven at 120 °C for about 6 h. Benzyl phenyl ether (Aldrich) was recrystallized from petroleum ether. Benzyl 1-naphthyl ether, 2-benzyl-1-naphthol, and 4-benzyl-1-naphthol were synthesized as described previously.15 Phenol, bibenzyl, 1-naphthol, and 2-benzylphenol were from Aldrich. 4-Benzylphenol was from Acros as well as being isolated and characterized from a reaction mixture. Emission Spectra. Zeolite samples were packed into quartz tubes in the air, sealed, and cooled with liquid N2 in a transparent quartz dewar that was placed in the sample compartment of an Edinburgh FS-900 CDT spectrofluorometer. Although the samples were visually opaque, emission signals were easily recorded using a right-angle detection geometry and a filter that protected the detector from reflected excitation radiation. Spectra are corrected for excitation wavelength intensity variations and detector wavelength response. Irradiations. All photoproducts were identified by co-injections of reaction mixtures and authentic samples in GC analyses and by comparison of fragmentation patterns for those peaks and authentic samples during GC-MS analyses. An unidentified product was formed during irradiations of 2 mM 1 in hexane. On the basis of the similarity between the GC retention time of its peak and those of 2-BP and 4-BP as well as its molecular mass and splitting pattern from GC-MS, the unidentified compound was probably 3-BP. Its yield is
