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Photophysical properties of dibenzotropylium cation incorporated within acidic ZSM-5 zeolite
21 September 2001
Chemical Physics Letters 345 (2001) 409±414
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Photophysical properties of dibenzotropylium cation incorporated within acidic ZSM-5 zeolite Marõa Luz Cano a, Michelle N. Chretien b, Hermenegildo Garcõa a,*, J.C. Scaiano b,* a
Departamento de Quimica, Instituto de Tecnologõa Quõmica, Universidad Polit ecnica de Valencia, CSIC-UPV, Apartado 22012, 46071 Valencia, Spain b Department of Chemistry, Centre for Catalysis, Research and Innovation, University of Ottawa, Ottawa, Canada, K1N 6N5 Received 11 June 2001; in ®nal form 2 August 2001
Abstract Dibenzotropylium ion (DT ) has been generated as an inde®nitely persistent species within the channels of ZSM-5 in its H -form. Diuse re¯ectance laser ¯ash photolysis has allowed detection of a transient (two bands: 300 nm, sharp and 440 nm broad) decaying in the ls time-scale that has been assigned to the corresponding triplet excited state. The 2 ) of ZSM-5 and the presence of coadsorbed water explain why tight ®t of DT within the straight channels (5.2´5.7 A the DT triplet excited state is not quenched by oxygen but interacts with triethylamine, which is highly water-soluble. In the latter case, formation of a new transient compatible with DT (kmax 270 and 350 nm) through electron transfer from the amine to DT triplet is observed. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Zeolites are a large family of microporous aluminosilicates diering in their crystal structure [1± 3]. The pore system of the zeolite particles are open to the external surface allowing mass transfer from the exterior to the interior of the grains. Zeolite intracrystalline void spaces are available in a variety of sizes and geometries and are able to host organic species provided that their molecular size is smaller than the pore dimensions. Zeolites are also convenient solid matrices for the generation and stabilization of organic carbocations [4]. This
*
Corresponding author. Fax: +34-96-387-9349. E-mail address: [email protected] (H. Garcõa).
has enabled the study of the photophysical and photochemical properties of some organic reaction intermediates that cannot be examined in solution [4±9]. In other cases, encapsulation of organic carbocations has allowed control to be gained over the photochemical properties with respect to the system in solution. For example, 9-(4-methoxyphenyl)xanthyl cation, which does not ¯uoresce in solution, exhibits a strong emission when included in large pore zeolites [8], and 2,4,6-triphenylpyrylium incorporated within Y zeolite acts as a photocatalyst in aqueous media [10] while in solution it undergoes hydrolytic ring opening. In a study of the electron-transfer ability of acidic zeolites dibenzotropylium ion (DT ) included in medium- and large- pore zeolites was observed to form by adsorption of dibenzosube-
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 1 3 - 7
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M. Luz Cano et al. / Chemical Physics Letters 345 (2001) 409±414
Scheme 1.
rene onto the acidic H -form of the host through a formal hydride loss mechanism [11]. Some of the samples containing DT incorporated within medium-pore sized or monodirectional, large-pore sized zeolites do not undergo any detectable variation in their spectroscopic properties for periods longer than years. The ¯uorescence of DT has been previously observed in the adiabatic photodehydroxylation of DTOH (Scheme 1) [12]. In this case, upon light absorption DTOH is promoted to its singlet excited state and subsequently undergoes dehydroxylation to form DT in the singlet excited state. The consequence is the simultaneous observation of ¯uorescence from DTOH and DT . Although DT has occasionally been used as a photosensitizer in electron transfer reactions [13] and photooxidations [14], the triplet excited state has never been characterized in solution, probably due to the diculty in obtaining DT free from the interference of quenchers. In the present work, the persistence of DT incorporated inside HZSM-5 has allowed us to study the photophysical properties of this aromatic cation in this solid matrix. Thus, we report the photophysics of DT included in the H -form of ZSM-5 zeolite, these results are markedly dierent from those obtained by excitation of DTOH in 1,1,1-tri¯uoroethanol or in aqueous acetonitrile solutions [15±17].
Diuse re¯ectance UV±Vis (DR) spectrum of the samples (Fig. 1) matches well with the spectrum of an authentic sample of DT obtained by dissolving DTOH in sulfuric acid [18], and is coincident with the DR spectra of DT adsorbed on HY previously reported by us [11]. The IR spectrum of cationic DT has not been previously reported. Given that the aluminosilicate framework of the zeolites has spectral windows in the IR, it has been possible to record the IR spectrum of DT incorporated inside HZSM-5 (Fig. 2). In Fig. 2 we have compared the aromatic region of the IR spectrum of DTOH, with the spectra of DT ±HZSM-5 and an authentic DT cation obtained by dissolving DTOH in H2 SO4 . The remarkable coincidence between the spectra of the two cation samples indicates that DT must be the predominant species present in the zeolite. Importantly, the aromatic region of the IR spectrum of the precursor (DTOH) is remarkably dierent (the central cycloheptatriene ring of DTOH is not aromatic) from that of DT . Therefore, it can be concluded that any residual DTOH presents in the zeolite must be below the detection limit of this technique. This point is relevant for subsequent photochemical studies since in a previous report on the photochemical generation of DT in tri¯uorethanol a large excess of DTOH was present and DT was detected only
2. Results and discussion The samples of DT incorporated in ZSM-5 used in this work were prepared by stirring a suspension DTOH in CH2 Cl2 at re¯ux temperature in the presence of thermally dehydrated HZSM-5. Formation of DT results in the appearance of an orange/red color in the zeolite.
Fig. 1. Diuse re¯ectance UV±Vis spectrum (plotted as the inverse of the re¯ectance, R) of DT in HZSM-5.
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Fig. 3. Emission spectrum of DT in HZSM-5 upon excitation at 355 nm using an ns YAG laser. The inset shows the emission decay at 580 nm. Fig. 2. Aromatic region of the IR spectra of DT in HZSM-5 (a), DT obtained by dissolving DTOH in H2 SO4 (b), and DTOH (c).
as a transient [19]. Thermogravimetric analyses of the prepared solids indicated that the cation content of the solids is about 3 wt% and calorimetry gives the decomposition peak of this cation at 510 °C. This is similar to, although lower than, the decomposition temperature of the related xanthyl cation adsorbed in zeolites [6]. This gives an indication of the high stability of DT adsorbed in ZSM-5. In fact, a DT ±HZSM-5 sample stored in a capped vial will persist for longer than ®ve years without any spectroscopic variation. The above data show that ZSM-5 in its H form is a solid host suitable for generating and stabilizing DT with suciently high purity in an `inert' environment free from electron donors. This fact, combined with the transparency of the zeolite aluminosilicate framework to wavelengths longer than 220 nm, makes it possible to study the photophysics of DT included in HZSM-5. After sample characterization the photophysics of DT incorporated in HZSM-5 were studied. The emission spectrum of DT incorporated within HZSM-5 (Fig. 3) exhibits a ¯uorescence band centred at 550 nm which is coincident with that previously reported for DT in H2 SO4 solution [12]. The excitation spectrum matches fairly well with the absorption spectra of DT proving that DT is the emitting species.
The ¯uorescence lifetime of DT in HZSM-5 was measured using an ns laser as the excitation source. The ¯uorescence decay can be adequately ®tted to mono-exponential ®rst-order kinetics and the estimated lifetime is 55 ns (inset in Fig. 3). This value is similar to the 40 ns previously reported for DT in 98% H2 SO4 solution [12] or 34 ns in tri¯uoroethanol [15]. Although, in general, incorporation within zeolites causes an increase of several orders of magnitude in the triplet excited state lifetime of organic guests, this is not the case for singlet excited states which typically exhibit a lifetime similar to that found in solution [4]. The fact that the lifetime of DT (S1 ) is not greatly in¯uenced by incorporation within zeolites is not unprecedented. Time resolved diuse±re¯ectance laser ¯ash photolysis of DT adsorbed within ZSM-5 shows the generation of a transient species that exhibits a narrow band at 300 nm and a broader one at kmax 440 nm accompanied by a weak positive absorption extending towards the infrared (Fig. 4). These spectroscopic features are most likely due to a single species since identical decay pro®les are recorded over the entire spectrum. This transient has a lifetime of less than 50 ls. Previous assignments of photochemically generated transient species within zeolites have been supported by the coincidence of the UV±Vis spectrum with that of the same species in solution. However, in the case of DT previous laser ¯ash
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Fig. 4. Transient diuse re¯ectance spectrum of DT in HZSM-5 recorded 0.96 ls (d), 2.56 ls (s), and 9.60 ls (h) after the 355 nm laser pulse.
photolysis studies have failed to observe any transient [19]. This failure is largely due to the fact that in the case of DT the transient spectrum is complicated at short time-scales by the intense bleaching of the highly colored ground state and the occurrence of long-lived ¯uorescence. Nevertheless, we have recorded the time-resolved UV± Vis spectra upon laser excitation of DT generated by dissolving DTOH in tri¯uorethanol containing a few drops of tri¯uoroacetic acid (TFAA). The use of the second harmonic of an Nd:YAG laser (532 nm) ensures the selective excitation of DT (kmax 500 and 540 nm, see Fig. 1), since in a control experiment no transients were recorded for DTOH using this laser wavelength. The transient spectrum recorded 0.16 ls after laser excitation is dominated by the intense negative absorption due to the bleaching of the ground state DT spectrum, but there is de®nitely a short-lived transient absorbing at 330 and 420 nm. A reasonable assumption is that this transient species, generated upon excitation of DT , in tri¯uorethanol is the same as that observed inside ZSM-5 and corresponds to the triplet excited state of the cation. This would be consistent with the photochemical behavior observed for structurally related aromatic carbenium ions, particularly xanthylium and pyrylium ions, for which triplet excited states have been observed [20]. The shift in position of kmax between tri¯uoroethanol and ZSM-5 probably re¯ects a solvatochromic eect,
while the remarkable increase in the lifetime of the triplet excited state upon zeolite incorporation is a common pattern also observed previously for zeolite-bound xanthylium and pyrylium ions [4]. The decay of the transient species generated upon excitation of DT ±ZSM-5 is unaected by oxygen purging. Triplet excited states are sometimes not readily quenched by oxygen when they are generated inside the voids of zeolite, particularly if there is a tight ®t of the organic guest inside the straight channels as is the case with ZSM2 ) [21]. Lack of reactivity with oxygen 5 (5.2´5.7 A is also caused by the impeded diusion of oxygen through the channels of fully hydrated ZSM-5 due to the poor solubility of oxygen in water. Accordingly, water-soluble molecules should be able to quench the triplet excited state of DT . To test this possibility, quenching of the DT triplet by triethylamine was carried out. It is known that triplet excited states of aromatic carbenium ions are exceedingly good electron acceptors and, therefore, should be readily quenched by amines as electron donors. In solution, this study is not easily done since amines are strong nucleophiles and react readily with any carbenium ion in its ground state. However, we reasoned that thermal reaction of aliphatic amines with DT incorporated within ZSM-5 should be hindered by spatial restrictions, for the same reasons that DT ±ZSM-5 is inde®nitely persistent even in the presence of coadsorbed water. In fact, after adsorption of triethylamine the transient spectrum upon 532 nm excitation changes and the bands at kmax 300 and 450 nm of the triplet decay with the same kinetics as the growth of two new bands at 270 and 350 nm (Fig. 5). The later can be assigned to DT radical, since the same transient is observed in the photolysis of DTCl in hexane, accompanied by the short-lived triplet excited state of DTCl absorbing at 420 nm. The triplet excited state of DTCl is not observed in the photolysis under oxygen and under these conditions the lifetime of DT is signi®cantly shorter. In conclusion, medium-pore sized ZSM-5 is a very convenient zeolite for the generation of DT as a persistent species in high purity. This persistence has enabled the recording of a transient spectrum attributable to the DT triplet excited
M. Luz Cano et al. / Chemical Physics Letters 345 (2001) 409±414
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and placed in sealed greaseless cells with CaF2 windows. The cells were outgassed at 200 °C under 10 2 Pa for 1 h before recording the spectrum at room temperature. DTCl, used in this work for the comparative generation of DT radical in solution, was prepared by the reaction of DTOH with an excess of thionyl chloride. The product chloride was puri®ed by sublimation and characterized by UV, IR, 1 HNMR and GC±MS. 3.2. Laser ¯ash photolysis Fig. 5. Transient diuse re¯ectance spectrum of DT with triethylamine in HZSM-5 recorded 9.60 ls after the 355 nm laser pulse.
state and demonstrated its electron transfer quenching with amines. In solution, the photochemistry of DT is complicated by the short lifetime of the triplet excited state and the reactivity of DT with nucleophiles. 3. Experimental 3.1. Sample preparation ZSM-5 (Si/Al 34) was prepared using tetrapropylammonium as a structure directing agent according to the method described in the literature [22] and the crystallinity was checked by XRD. DT incorporated in ZSM-5 was prepared by stirring a suspension of 50 mg of DTOH in CH2 Cl2 (20 ml) at re¯ux temperature in the presence of thermally dehydrated (500 °C, overnight) HZSM-5 (1 g). After 1 h the solid was ®ltered and Soxhlet extracted until no more organic material could be recovered. Extraction solutions were analyzed by GC±MS and only unreacted DTOH was observed. Diuse re¯ectance spectra were recorded on a Cary 5G spectrophtometer using a praying mantis attachment with BaSO4 as a standard. FT±IR spectra were obtained using a Nicolet 710 FT spectrophotometer. Self-supported zeolite wafers (10 mg) were compressed at 1 ton/cm2 for 3 min
Nanosecond laser ¯ash photolysis experiments with zeolitic samples were carried out using a time resolved diuse re¯ectance setup. Samples were excited with either the third (355 nm, 6 ns pulse width, 15 mJ/pulse) or second (532 nm, 6 ns pulse width, 25 mJ/pulse) harmonic of a Continuum Surelite Nd-YAG laser. Signals from the monochromator/photomultiplier were captured by a Tektronix 2440 digitizer and transferred to a PowerMacintosh computer programmed in the LabVIEW 4.1 environment from National Instruments. Detailed descriptions of this system can be found elsewhere [23±25]. Laser ¯ash photolysis experiments involving DTCl and DTOH/acid (to generate the cation in solution) were carried out in spectroscopic grade hexane and tri¯uoroethanol respectively. Solutions were purged with O2 or N2 for 30 min prior to each experiment. Samples were excited with 308 nm pulses (6 ns pulse width, 85 mJ/pulse) from a Lumonics EX-530 excimer laser using a Xe/HCl/ Ne mixture. Signal capture and processing were accomplished as described above. Time resolved photoluminescence studies were carried out using the third (355 nm) harmonic pulse of the Nd-YAG laser as the excitation source. Samples were irradiated in 3´7 mm2 Suprasil quartz cells and were purged with nitrogen for at least 30 min prior to each experiment. Acknowledgements Financial support by the Spanish DGICYT (HG Grant MAT97-1016-CO2) and the NSERC
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(JCS) is gratefully acknowledged. MLC also thanks the Spanish Ministry of Education for a postgraduate scholarship. References [1] D.W. Breck, Zeolite Molecular Sieves, Structure, Chemistry and Use, Wiley, New York, 1974. [2] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London, 1978. [3] A. Dyer, An Introduction to Zeolite Molecular Sieves, Wiley, Bath, UK, 1988. [4] J.C. Scaiano, H. Garcõa, Acc. Chem. Res. 32 (1999) 783. [5] A. Corma, V. Fornes, H. Garcõa, M.A. Miranda, M.J. Sabater, J. Am. Chem. Soc. 116 (1994) 9767. [6] F.L. Cozens, H. Garcõa, J.C. Scaiano, Langmuir 10 (1994) 2246. [7] M.L. Cano, F.L. Cozens, H. Garcõa, V. Martõ, J.C. Scaiano, J. Phys. Chem. 100 (1996) 18152. [8] M.L. Cano, F.L. Cozens, V. Fornes, H. Garcõa, J.C. Scaiano, J. Phys. Chem. 100 (1996) 18145. [9] M.L. Cano, A. Corma, V. Fornes, H. Garcõa, M. Miranda, C. Baerlocher, C. Lengauer, J. Am. Chem. Soc. 118 (1996) 11006.
[10] A. Sanjuan, M. Alvaro, G. Aguirre, H. Garcõa, J.C. Scaiano, J. Am. Chem. Soc. 120 (1998) 7351. [11] M.L. Cano, A. Corma, V. Fornes, H. Garcõa, J. Phys. Chem. 99 (1995) 4241. [12] M.R. Feldman, N.G. Thame, J. Org. Chem. 44 (1979) 1863. [13] V. Fornes, H. Garcõa, M.A. Miranda, F. Mojarrad, M.-J. Sabater, N.N.E. Suliman, Tetrahedron 52 (1996) 7755. [14] D. Bethell, P.N. Clare, J. Chem. Soc. Perkin II (1972) 1464. [15] L.J. Johnston, J. Lobaugh, V. Wintgens, J. Phys. Chem. 93 (1989) 7370. [16] P. Wan, J. Org. Chem. 50 (1985) 2583. [17] E. Krogh, P. Wan, Tett. Lett. 27 (1986) 823. [18] G. Berti, Gazz. Chim. Ital. 87 (1957) 293. [19] A. Azarani, A.B. Berinstain, L.J. Johnston, S. Kazanis, J. Photochem. Photobiol. A 57 (1991) 175. [20] P.K. Das, Chem. Rev. 93 (1993) 119. [21] J.K. Thomas, Chem. Rev. 93 (1993) 301. [22] R.J. Argauer, D.H. Olson, G.R. Landlolt, Brit. Pat., 1161974, 1969. [23] F. Wilkinson, C. Willsher, J. Appl. Spectrosc. 38 (1984) 897. [24] J.C. Scaiano, M. Tanner, D. Weir, J. Am. Chem. Soc. 107 (1985) 4396. [25] F. Wilkinson, G. Kelly, in: J.C. Scaiano (Ed.), Handbook of Organic Photochemistry, CRC Press, Boca Raton, FL, 1989, p. 293.
Chemical Physics Letters 345 (2001) 409±414
www.elsevier.com/locate/cplett
Photophysical properties of dibenzotropylium cation incorporated within acidic ZSM-5 zeolite Marõa Luz Cano a, Michelle N. Chretien b, Hermenegildo Garcõa a,*, J.C. Scaiano b,* a
Departamento de Quimica, Instituto de Tecnologõa Quõmica, Universidad Polit ecnica de Valencia, CSIC-UPV, Apartado 22012, 46071 Valencia, Spain b Department of Chemistry, Centre for Catalysis, Research and Innovation, University of Ottawa, Ottawa, Canada, K1N 6N5 Received 11 June 2001; in ®nal form 2 August 2001
Abstract Dibenzotropylium ion (DT ) has been generated as an inde®nitely persistent species within the channels of ZSM-5 in its H -form. Diuse re¯ectance laser ¯ash photolysis has allowed detection of a transient (two bands: 300 nm, sharp and 440 nm broad) decaying in the ls time-scale that has been assigned to the corresponding triplet excited state. The 2 ) of ZSM-5 and the presence of coadsorbed water explain why tight ®t of DT within the straight channels (5.2´5.7 A the DT triplet excited state is not quenched by oxygen but interacts with triethylamine, which is highly water-soluble. In the latter case, formation of a new transient compatible with DT (kmax 270 and 350 nm) through electron transfer from the amine to DT triplet is observed. Ó 2001 Published by Elsevier Science B.V.
1. Introduction Zeolites are a large family of microporous aluminosilicates diering in their crystal structure [1± 3]. The pore system of the zeolite particles are open to the external surface allowing mass transfer from the exterior to the interior of the grains. Zeolite intracrystalline void spaces are available in a variety of sizes and geometries and are able to host organic species provided that their molecular size is smaller than the pore dimensions. Zeolites are also convenient solid matrices for the generation and stabilization of organic carbocations [4]. This
*
Corresponding author. Fax: +34-96-387-9349. E-mail address: [email protected] (H. Garcõa).
has enabled the study of the photophysical and photochemical properties of some organic reaction intermediates that cannot be examined in solution [4±9]. In other cases, encapsulation of organic carbocations has allowed control to be gained over the photochemical properties with respect to the system in solution. For example, 9-(4-methoxyphenyl)xanthyl cation, which does not ¯uoresce in solution, exhibits a strong emission when included in large pore zeolites [8], and 2,4,6-triphenylpyrylium incorporated within Y zeolite acts as a photocatalyst in aqueous media [10] while in solution it undergoes hydrolytic ring opening. In a study of the electron-transfer ability of acidic zeolites dibenzotropylium ion (DT ) included in medium- and large- pore zeolites was observed to form by adsorption of dibenzosube-
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 9 1 3 - 7
410
M. Luz Cano et al. / Chemical Physics Letters 345 (2001) 409±414
Scheme 1.
rene onto the acidic H -form of the host through a formal hydride loss mechanism [11]. Some of the samples containing DT incorporated within medium-pore sized or monodirectional, large-pore sized zeolites do not undergo any detectable variation in their spectroscopic properties for periods longer than years. The ¯uorescence of DT has been previously observed in the adiabatic photodehydroxylation of DTOH (Scheme 1) [12]. In this case, upon light absorption DTOH is promoted to its singlet excited state and subsequently undergoes dehydroxylation to form DT in the singlet excited state. The consequence is the simultaneous observation of ¯uorescence from DTOH and DT . Although DT has occasionally been used as a photosensitizer in electron transfer reactions [13] and photooxidations [14], the triplet excited state has never been characterized in solution, probably due to the diculty in obtaining DT free from the interference of quenchers. In the present work, the persistence of DT incorporated inside HZSM-5 has allowed us to study the photophysical properties of this aromatic cation in this solid matrix. Thus, we report the photophysics of DT included in the H -form of ZSM-5 zeolite, these results are markedly dierent from those obtained by excitation of DTOH in 1,1,1-tri¯uoroethanol or in aqueous acetonitrile solutions [15±17].
Diuse re¯ectance UV±Vis (DR) spectrum of the samples (Fig. 1) matches well with the spectrum of an authentic sample of DT obtained by dissolving DTOH in sulfuric acid [18], and is coincident with the DR spectra of DT adsorbed on HY previously reported by us [11]. The IR spectrum of cationic DT has not been previously reported. Given that the aluminosilicate framework of the zeolites has spectral windows in the IR, it has been possible to record the IR spectrum of DT incorporated inside HZSM-5 (Fig. 2). In Fig. 2 we have compared the aromatic region of the IR spectrum of DTOH, with the spectra of DT ±HZSM-5 and an authentic DT cation obtained by dissolving DTOH in H2 SO4 . The remarkable coincidence between the spectra of the two cation samples indicates that DT must be the predominant species present in the zeolite. Importantly, the aromatic region of the IR spectrum of the precursor (DTOH) is remarkably dierent (the central cycloheptatriene ring of DTOH is not aromatic) from that of DT . Therefore, it can be concluded that any residual DTOH presents in the zeolite must be below the detection limit of this technique. This point is relevant for subsequent photochemical studies since in a previous report on the photochemical generation of DT in tri¯uorethanol a large excess of DTOH was present and DT was detected only
2. Results and discussion The samples of DT incorporated in ZSM-5 used in this work were prepared by stirring a suspension DTOH in CH2 Cl2 at re¯ux temperature in the presence of thermally dehydrated HZSM-5. Formation of DT results in the appearance of an orange/red color in the zeolite.
Fig. 1. Diuse re¯ectance UV±Vis spectrum (plotted as the inverse of the re¯ectance, R) of DT in HZSM-5.
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Fig. 3. Emission spectrum of DT in HZSM-5 upon excitation at 355 nm using an ns YAG laser. The inset shows the emission decay at 580 nm. Fig. 2. Aromatic region of the IR spectra of DT in HZSM-5 (a), DT obtained by dissolving DTOH in H2 SO4 (b), and DTOH (c).
as a transient [19]. Thermogravimetric analyses of the prepared solids indicated that the cation content of the solids is about 3 wt% and calorimetry gives the decomposition peak of this cation at 510 °C. This is similar to, although lower than, the decomposition temperature of the related xanthyl cation adsorbed in zeolites [6]. This gives an indication of the high stability of DT adsorbed in ZSM-5. In fact, a DT ±HZSM-5 sample stored in a capped vial will persist for longer than ®ve years without any spectroscopic variation. The above data show that ZSM-5 in its H form is a solid host suitable for generating and stabilizing DT with suciently high purity in an `inert' environment free from electron donors. This fact, combined with the transparency of the zeolite aluminosilicate framework to wavelengths longer than 220 nm, makes it possible to study the photophysics of DT included in HZSM-5. After sample characterization the photophysics of DT incorporated in HZSM-5 were studied. The emission spectrum of DT incorporated within HZSM-5 (Fig. 3) exhibits a ¯uorescence band centred at 550 nm which is coincident with that previously reported for DT in H2 SO4 solution [12]. The excitation spectrum matches fairly well with the absorption spectra of DT proving that DT is the emitting species.
The ¯uorescence lifetime of DT in HZSM-5 was measured using an ns laser as the excitation source. The ¯uorescence decay can be adequately ®tted to mono-exponential ®rst-order kinetics and the estimated lifetime is 55 ns (inset in Fig. 3). This value is similar to the 40 ns previously reported for DT in 98% H2 SO4 solution [12] or 34 ns in tri¯uoroethanol [15]. Although, in general, incorporation within zeolites causes an increase of several orders of magnitude in the triplet excited state lifetime of organic guests, this is not the case for singlet excited states which typically exhibit a lifetime similar to that found in solution [4]. The fact that the lifetime of DT (S1 ) is not greatly in¯uenced by incorporation within zeolites is not unprecedented. Time resolved diuse±re¯ectance laser ¯ash photolysis of DT adsorbed within ZSM-5 shows the generation of a transient species that exhibits a narrow band at 300 nm and a broader one at kmax 440 nm accompanied by a weak positive absorption extending towards the infrared (Fig. 4). These spectroscopic features are most likely due to a single species since identical decay pro®les are recorded over the entire spectrum. This transient has a lifetime of less than 50 ls. Previous assignments of photochemically generated transient species within zeolites have been supported by the coincidence of the UV±Vis spectrum with that of the same species in solution. However, in the case of DT previous laser ¯ash
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Fig. 4. Transient diuse re¯ectance spectrum of DT in HZSM-5 recorded 0.96 ls (d), 2.56 ls (s), and 9.60 ls (h) after the 355 nm laser pulse.
photolysis studies have failed to observe any transient [19]. This failure is largely due to the fact that in the case of DT the transient spectrum is complicated at short time-scales by the intense bleaching of the highly colored ground state and the occurrence of long-lived ¯uorescence. Nevertheless, we have recorded the time-resolved UV± Vis spectra upon laser excitation of DT generated by dissolving DTOH in tri¯uorethanol containing a few drops of tri¯uoroacetic acid (TFAA). The use of the second harmonic of an Nd:YAG laser (532 nm) ensures the selective excitation of DT (kmax 500 and 540 nm, see Fig. 1), since in a control experiment no transients were recorded for DTOH using this laser wavelength. The transient spectrum recorded 0.16 ls after laser excitation is dominated by the intense negative absorption due to the bleaching of the ground state DT spectrum, but there is de®nitely a short-lived transient absorbing at 330 and 420 nm. A reasonable assumption is that this transient species, generated upon excitation of DT , in tri¯uorethanol is the same as that observed inside ZSM-5 and corresponds to the triplet excited state of the cation. This would be consistent with the photochemical behavior observed for structurally related aromatic carbenium ions, particularly xanthylium and pyrylium ions, for which triplet excited states have been observed [20]. The shift in position of kmax between tri¯uoroethanol and ZSM-5 probably re¯ects a solvatochromic eect,
while the remarkable increase in the lifetime of the triplet excited state upon zeolite incorporation is a common pattern also observed previously for zeolite-bound xanthylium and pyrylium ions [4]. The decay of the transient species generated upon excitation of DT ±ZSM-5 is unaected by oxygen purging. Triplet excited states are sometimes not readily quenched by oxygen when they are generated inside the voids of zeolite, particularly if there is a tight ®t of the organic guest inside the straight channels as is the case with ZSM2 ) [21]. Lack of reactivity with oxygen 5 (5.2´5.7 A is also caused by the impeded diusion of oxygen through the channels of fully hydrated ZSM-5 due to the poor solubility of oxygen in water. Accordingly, water-soluble molecules should be able to quench the triplet excited state of DT . To test this possibility, quenching of the DT triplet by triethylamine was carried out. It is known that triplet excited states of aromatic carbenium ions are exceedingly good electron acceptors and, therefore, should be readily quenched by amines as electron donors. In solution, this study is not easily done since amines are strong nucleophiles and react readily with any carbenium ion in its ground state. However, we reasoned that thermal reaction of aliphatic amines with DT incorporated within ZSM-5 should be hindered by spatial restrictions, for the same reasons that DT ±ZSM-5 is inde®nitely persistent even in the presence of coadsorbed water. In fact, after adsorption of triethylamine the transient spectrum upon 532 nm excitation changes and the bands at kmax 300 and 450 nm of the triplet decay with the same kinetics as the growth of two new bands at 270 and 350 nm (Fig. 5). The later can be assigned to DT radical, since the same transient is observed in the photolysis of DTCl in hexane, accompanied by the short-lived triplet excited state of DTCl absorbing at 420 nm. The triplet excited state of DTCl is not observed in the photolysis under oxygen and under these conditions the lifetime of DT is signi®cantly shorter. In conclusion, medium-pore sized ZSM-5 is a very convenient zeolite for the generation of DT as a persistent species in high purity. This persistence has enabled the recording of a transient spectrum attributable to the DT triplet excited
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and placed in sealed greaseless cells with CaF2 windows. The cells were outgassed at 200 °C under 10 2 Pa for 1 h before recording the spectrum at room temperature. DTCl, used in this work for the comparative generation of DT radical in solution, was prepared by the reaction of DTOH with an excess of thionyl chloride. The product chloride was puri®ed by sublimation and characterized by UV, IR, 1 HNMR and GC±MS. 3.2. Laser ¯ash photolysis Fig. 5. Transient diuse re¯ectance spectrum of DT with triethylamine in HZSM-5 recorded 9.60 ls after the 355 nm laser pulse.
state and demonstrated its electron transfer quenching with amines. In solution, the photochemistry of DT is complicated by the short lifetime of the triplet excited state and the reactivity of DT with nucleophiles. 3. Experimental 3.1. Sample preparation ZSM-5 (Si/Al 34) was prepared using tetrapropylammonium as a structure directing agent according to the method described in the literature [22] and the crystallinity was checked by XRD. DT incorporated in ZSM-5 was prepared by stirring a suspension of 50 mg of DTOH in CH2 Cl2 (20 ml) at re¯ux temperature in the presence of thermally dehydrated (500 °C, overnight) HZSM-5 (1 g). After 1 h the solid was ®ltered and Soxhlet extracted until no more organic material could be recovered. Extraction solutions were analyzed by GC±MS and only unreacted DTOH was observed. Diuse re¯ectance spectra were recorded on a Cary 5G spectrophtometer using a praying mantis attachment with BaSO4 as a standard. FT±IR spectra were obtained using a Nicolet 710 FT spectrophotometer. Self-supported zeolite wafers (10 mg) were compressed at 1 ton/cm2 for 3 min
Nanosecond laser ¯ash photolysis experiments with zeolitic samples were carried out using a time resolved diuse re¯ectance setup. Samples were excited with either the third (355 nm, 6 ns pulse width, 15 mJ/pulse) or second (532 nm, 6 ns pulse width, 25 mJ/pulse) harmonic of a Continuum Surelite Nd-YAG laser. Signals from the monochromator/photomultiplier were captured by a Tektronix 2440 digitizer and transferred to a PowerMacintosh computer programmed in the LabVIEW 4.1 environment from National Instruments. Detailed descriptions of this system can be found elsewhere [23±25]. Laser ¯ash photolysis experiments involving DTCl and DTOH/acid (to generate the cation in solution) were carried out in spectroscopic grade hexane and tri¯uoroethanol respectively. Solutions were purged with O2 or N2 for 30 min prior to each experiment. Samples were excited with 308 nm pulses (6 ns pulse width, 85 mJ/pulse) from a Lumonics EX-530 excimer laser using a Xe/HCl/ Ne mixture. Signal capture and processing were accomplished as described above. Time resolved photoluminescence studies were carried out using the third (355 nm) harmonic pulse of the Nd-YAG laser as the excitation source. Samples were irradiated in 3´7 mm2 Suprasil quartz cells and were purged with nitrogen for at least 30 min prior to each experiment. Acknowledgements Financial support by the Spanish DGICYT (HG Grant MAT97-1016-CO2) and the NSERC
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