Langmuir 1994,10, 2246-2249
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Intrazeolite Photochemistry. 9. Laser Flash Photolysis of Xanthenium Ion Generated by Adsorption of 9-Xanthenol within Acid Zeolites Frances L. Cozens,+Hermenegildo Garcia,*2$and J. C. Scaiano*it Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K I N 6N5, and Instituto de Tecnologia Quimica CSIC-UPV, Universidad Politicnica de Valencia, 46071 Valencia, Spain Received January 10, 1994. I n Final Form: April 7, 1994@ The xanthenium cation has been spontaneously generated by adsorption of 9-xanthenolwithin several HMor, and HZSM5)zeolites. The resulting compositeswere characterizedby diffise reflectance acidic (HY, and luminescence spectroscopy and were found to be stable over an extremely long period of time. Timeresolved diffuse reflectance of these samples at several excitation wavelengths allowed the identification of a 540-nm band to the triplet state of tile cation in the zeolite environment. The triplet state in the zeolite was found to have a much longer lifetime than related triplet xanthenium cations in solution. The observation of the 9-xanthylium radical requires the participation of an electron donor; the question is raised as to whether the zeolite framework can play this role.
Introduction In sharp contrast to the huge volume of data available on the formation, stability, structure, and chemical reactivity of carbenium ions in their ground state,' much less is known concerning the characterization and properties of their excited states.2 This is due in part to the high reactivity of many carbenium ions and the difficulty associated with the preparation of stable solutions containing high concentrations of these species free from adventitious byproducts. In fact, the majority of the photochemical studies concerning transient carbocations have been directed to their generation and detection under conditions where a fast reaction (submillisecond)with the media occur^.^ Studies dealing specifically with the photophysical and photochemical properties of carbocation excited states have not been reported until quite recently, with xanthenium carbocations being among the best studies It has been shown that both the singlet and triplet states of the cation display excellent electron acceptor ability. Zeolites are salts of polymeric crystalline aluminosilicates and constitute a large family of natural and synthetic microporous solids differing in chemical composition as well as in the size and shape of their internal voids.g In recent years these materials have become increasingly used as constrained media to gain control over the photochemical processes of adsorbed neutral organic molecules.1° We have shown that time-resolved diffuse reflectance is an appropriate technique to study photochemical transformations of guests within the channels and cavities of zeolites.ll + University of
Ottawa.
* Universidad Politecnica de Valencia.
Abstract published in Advance A C S Abstracts, May 15,1994. (1)Olah, G. A.; Schleyer, P. v. R. Carbenium Ions; Wiley: New York,
@
1968-1976; Vol. I-V. (2) Childs, R. F.; Shaw, G. B. Org. Photochem. 1991, 11, 111. (3) Das, P. K. Chem. Rev. 1993, 93, 119. (4) Azarani, A.; Berinstain, A. B.; Johnston, L. J.; Kazanis, S. J . Photochem. Photobiol. A: Chem. 1991, 57, 175. (5) Johnston, L. J.; Wong, D. F. Can. J . Chem. 1992, 70, 280. (6) Johnston, L. J.; Wong, D. F. J . Phys. Chem. 1993, 97, 1589. (7) Samanta, A.; Gopidas, K. R.; Das, P. K. J . Phys. Chem. 1993,97, 1583. (8) Samanta, A.; Gopidas, K. R.; Das, P. K. Chem. Phys. Lett. 1993, 204, 269. (9) Introduction to Zeolite Science and Practice; van Bekkum, H., Flanigen, E. M., Jansen, J. C., Eds.; Elsevier: Amsterdam, 1991. (10)Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Acc. Chem. Res. 1992,25, 299.
Depending on the zeolite aluminum content, a variable number of charge-balancing cations is present in the internal voids to compensate for the excess of framework negative charge caused by the isomorphic substitution of sio44- by Mod5- tetrahedra. Interestingly, due to the ionic nature of the chemical bond between the associated cations and the zeolite network, it is possible to exchange these cations without altering the crystalline lattice. Therefore, it can be anticipated that zeolites constitute an excellent inert medium to study the photochemistry of carbenium ions provided that appropriate methods to incorporate or generate the organic cations are available. We have recently shown that spontaneous carbocation generation occurs by adsorbing 9-phenyl-9-xanthenol on the acidic surfaces oflayered clays.12 In the present report, we show that the parent xanthenium ion (X+) is readily formed as a stable species by adsorption of 9-xanthenol (XOH)on acidic zeolites. Further, excitation ofthe cation leads to its readily detectable and long-lived excited triplet state. Its spectrum is strongly altered as a result of the immobilization and confinement of this cation within the restricted spaces.
Experimental Section The substrates,9-xanthenol (XOH),xanthene, and xanthone (XO), were commerciallyavailable(Aldrich)and used as received. Acid HY and HZSMB zeolites were prepared by thermal decomposition of their corresponding ammonium or tetralkylammonium forms obtained by two consecutive ion exchange treatments of steam dealuminated NaY,13 or directly by previously reported synthetic procedures.14 Acid mordenite, HMor, was prepared by HCl dealumination of a commercial sample of Mor (PQIndustries,SUA1 6).15 Acidity of the sampleswas tested using the pyridine adsorption-desorption method.l6 The main physicochemicalcharacteristics of these zeolites are summarized in Table 1. Generation of X+ within the zeolites was accomplished by stirring at room temperature for 2 h a suspension of XOH (20 mg) in CHzClz (30 mL) and the corresponding zeolite (1.0 g) previously activated by baking at 773 K for 18 h. The yellow (11)Bohne, C.; Redmond, R. W.; Scaiano, J. C. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; p 79. (12) Cozens, F. L.; Gessner, F.; Scaiano, J. C.Langmuir 1993,9,874. (13)Beyer, H. K.; Belenykaja Stud. Surf. Sci. Catal. 1980, 5, 203. (14) Argauer, R. J.; Landlolt, G. R. US.Pat. 3 702 886, 1982. (15) Fajula, F.; Ibarra, R.; Figueras, F.; Gueguen, C. J . Catal. 1984, 89, 60. (16)Arribas, J.; Corma, A,; FornBs, V. J . Catal. 1984, 88, 374.
0743-7463/94/2410-2246$04.50/0 0 1994 American Chemical Society
Intrazeolite Photochemistry
Langmuir, Vol. 10, No. 7, 1994 2247
Table 1. Physicochemical Parameters of the Zeolites Used in This Work NazO XOH adsorbed crystal SUM content % site zeolites (mdg of zeolite) size h m ) ratio (wt %) occupancy ~~
~
~~~
HYa 17 0.4-0.6 HMor 11 HZSM5 2 1-3 a Unit cell size 24.28 A.
15 10 17.5
