4478
Langmuir 2008, 24, 4478-4481
Direct Time-Resolved Detection of Singlet Oxygen in Zeolite-Based Photocatalysts Bogdan Cojocaru,† Marie Laferrie`re,‡ Esther Carbonell,§ Vasile Parvulescu,*,† Hermenegildo Garcı´a,*,§ and J. C. Scaiano*,‡ Center for Catalysis Research and InnoVation, Department of Chemistry, UniVersity of Ottawa, Ottawa K1N 6N5, Canada, UniVersity of Bucharest, Faculty of Chemistry, b-dul Regina Elisabeta, 4-12, 030016 Bucharest, Romania, and Instituto de Tecnologı´a Quı´mica CSIC-UPV, UniVersidad Polite´ cnica de Valencia, AV. De los Naranjos s/n, 46022 Valencia, Spain ReceiVed February 11, 2008. In Final Form: March 12, 2008 Singlet oxygen has been characterized spectroscopically as a product of the exposure of suspensions of zeolites containing oxidation catalysts. Spectroscopic and lifetime studies show that a part of the singlet oxygen formed reacts within the zeolite porous structure, while a significant fraction escapes and becomes available for reaction in the bulk media. The liquid phase plays a key role in determining intra- and extracavity dynamics.
Zeolites have been used as hosts for photochemical reactions for over a quarter of a century.1,2 Recently, there has been increased interest in their use as hosts for photocatalytic processes, in particular, oxidation reactions.3-6 In our case, this interest relates to the development of catalysts with potential warfare chemical agent (CWA) decontamination applications in relation to NATO’s Science for Peace program. Oxidations within zeolite cavities can be initiated by the formation of oxygen-centered radicals (notably HO•), electron transfer, singlet oxygen, or direct excitation of oxygen-organic complexes (especially involving alkenes).2,3 In the case of singlet oxygen (1O2 1∆g), there are a number of examples of intracavity oxidations, in which remarkable chemo- and stereoselectivity in the product selectivity can be achieved compared to solution. Control of the singlet oxygen reactivity, particularly stereoselection, has been extremely difficult in solution due to the short lifetime of this transient. The interest in zeolite-based oxidation catalysts reflects in part the ease of separation once the oxidation has been completed. Recent work by Clennan has placed the mechanism of intrazeolite singlet oxygen reactivity on solid ground and estimated a singlet oxygen intracavity lifetime of less than 7.5 µs for zeolite NaY suspended in perfluorohexane.5 This estimation has been supported by direct measurements of singlet oxygen lifetime in zeolites by NIR phosphorescence of dry powders. Time-resolved NIR phosphorescence has shown that the singlet oxygen lifetime decreases as the aluminum content (or the presence of charge-balancing associated cations) of the zeolite increases and also that the lifetime is highly sensitive to the nature of the photosensitizer.7 For benzophenone incorporated inside carefully dehydrated NaY zeolite, a lifetime of 7.9 µs * Corresponding authors.
[email protected]; hgarcia@ qim.upv.es;
[email protected]. † University of Bucharest. ‡ University of Ottawa. § Universidad Polite ´ cnica de Valencia. (1) Chretien, M. N. Pure Appl. Chem. 2007, 79 (1), 1. Dutta, P. K.; Kim, Y. Curr. Opin. Solid State Mater. Sci. 2003, 7 (6), 483. (2) Scaiano, J. C.; Garcı´a, H. Acc. Chem. Res. 1999, 32 (9), 783. (3) Clennan, E. L. Mol. Supramol. Photochem. 2003, 9, (Photochemistry of Organic Molecules in Isotropic and Anisotropic Media), 275. Clennan, E. L.; Pace, A. Tetrahedron 2005, 61 (28), 6665. (4) Corma, A.; Garcia, H. Chem. Commun. 2004 (13), 1443. (5) Pace, A.; Clennan, E. L. J. Am. Chem. Soc. 2002, 124 (38), 11236. (6) Clennan, E. L. AdV. Phys. Org. Chem. 2008, 42, 225. (7) Jockusch, S.; Sivaguru, J.; Turro, N. J.; Ramamurthy, V. Photochem. Photobiol. Sci. 2005, 4 (5), 403.
(very close to the Clennan’s lifetime upper limit) was determined. However, other photosensitizers give remarkably shorter lifetimes.7 Thus, the time-resolved phosphorescence study emphasizes the importance of the direct measurement of singlet oxygen lifetime for each specific zeolite-based photocatalyst. In this contribution, we report the direct detection of the phosphorescence of singlet oxygen generated within zeolite cavities by photosensitizers that have proven effective in the mineralization of molecules with structures closely resembling CWA. Beyond this, we see their usefulness in numerous other decontamination strategies, such as pesticide contaminated soils and water and for air purification. Since our measurements have been made with zeolite powders suspended in a solvent, we can provide an estimation of the percentage of singlet oxygen that migrates from the zeolite pores to the liquid phase. Moreover, the data presented show a remarkable example of the positive influence of zeolite encapsulation promoting the generation of singlet oxygen in one photosensitizer that in solution does not generate detectable concentrations of singlet oxygen. Two series of zeolite-based photocatalysts containing either metal phthalocyanines or 2,4,6-triphenylpyryliyum (TP+) have shown high photocatalytic activity for the photodegradation of CWAs. The embedded photocatalysts containing Fe, Mn, Ni, Cu, and Co phthalocyanines were prepared by ship-in-a-bottle method (Scheme 1) starting from o-dicyanobenzene using the procedures already reported.8 TP@NaY was obtained by adsorption and subsequent thermal cyclization of 1,3,5-triphenyl-2penten-1,5-dione in zeolite NaY (Scheme 2).9 The UV-vis and FT-IR spectra of the samples coincide with those that have been previously reported for these materials.4 The samples were examined for singlet oxygen phosphorescence at 1270 nm using a modified Luzchem laser flash photolysis system incorporating a near-infrared (NIR) detection system with time and spectral resolution; detection in heterogeneous systems is usually challenging.10 The photocatalyst powders were suspended either in D2O or in 1,1,1,2,3,4,4,5,5,5-decafluoropentane (DFP); the former penetrates the cavities and replaces any water sites that may be present. The latter is quite inert, (8) Alvaro, M.; Carbonell, E.; Espla, M.; Garcia, H. Appl. Catal. B 2005, 57 (1), 37. (9) Amat, A. M.; Arques, A.; Bossmann, S. H.; Braun, A. M.; Gob, S.; Miranda, M. A. Angew. Chem., Int. Ed. 2003, 42 (14), 1653. (10) Zebger, I.; Poulsen, L.; Gao, Z.; Andersen, L. K.; Ogilby, P. R. Langmuir 2003, 19 (21), 8927.
10.1021/la800441n CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
Letters
Langmuir, Vol. 24, No. 9, 2008 4479 Table 1. Relative Importance of Fast and Slow Components in the Decay of 1O2 in Aerated DFP
Figure 1. Decay of singlet oxygen phosphorescence from FePc@NaY in D2O monitored at 1270 nm following 308 nm laser excitation. Inset: Emission spectrum observed, characteristic of singlet oxygen.
guest
host
% fast
total signala
FePc FePc MnPc MnPc MnPcc TP TP xanthone CoPc CuPc NiPc
MCM-41 NaY NaY NaY NaY NaY MCM-41 none NaY NaY NaY
40 43 n.d.b 6b 44 9 (52) none 60 67 29
(0.030) 0.030 0.071 0.082 0.039 0.091 (0.030) (0.110)
a Given only for experiments performed on a single series; see text. Values in parenthesis do not have zeolite NaY as host and their scattering properties are different. b These two values correspond to the same fresh sample, studied in the order shown. It is possible that sample drying was preserved on the first measurement, but some water was already acquired on the second. c Extracted following the suite of solvents (acetone, methanol, pyridine, and acetone) previously reported.15
Scheme 1. Ship-in-a-Bottle Synthesis of Metal Phthalocyanines Encapsulated Inside Zeolites
Scheme 2. Ship-in-a-Bottle Synthesis of TP+ Encapsulated Inside NaY Figure 2. Luminescence decay monitored at 1270 nm following 308 nm laser excitation of a sample of CuPc@NaY suspended in DFP. Inset: Semilog representation of the same data illustrating the biexponential nature of the decay. The short component has a lifetime of ca. 7 µs.
incapable of extracting zeolite guests, and does not affect any protic (mainly water) sites present within the cavities. Studies in both DFP and D2O for the two sets of photocatalysts reveal NIR emission following laser excitation at either 355 or 308 nm under air. The spectrum (see insert in Figure 1) is characteristic of singlet oxygen, and should be formed by energy transfer from the sensitizers.11 The phosphorescence decay in D2O (see Figure 1 for the example of FePc@NaY) was monoexponential with a lifetime of 45.6 µs, which is an acceptable value for D2O.12 The fact that reported lifetimes can be as long as 68 µs13 suggests that some quenching by either the zeolite or water released through exchange processes may take place. All the other phthalocyanines in Table 1 gave similar emissions with lifetimes around 46 µs. The long singlet oxygen lifetime as compared to the previous measurements on dry powders (maximum τ 7.9 µs for zeolite NaY)7 and the independence with the nature of the metal atom of the phthalocyanine strongly suggest that this emission corresponds to the singlet oxygen that has migrated from the zeolite micropores to the D2O liquid phase. There were some variations in the intensity of these signals, but it is hard to identify these with true quantum yield changes, (11) Schweitzer, C.; Schmidt, R. Chem. ReV. 2003, 103 (5), 1685. (12) Sanrame, C. N.; De Rossi, R. H.; Arguello, G. A. Photochem. Photobiol. 1998, 68 (4), 474. Matheson, I. B. C.; Lee, J.; King, A. D. Chem. Phys. Lett. 1978, 55 (1), 49. (13) Ogilby, P. R.; Foote, C. S. J. Am. Chem. Soc. 1983, 105 (11), 3423.
since the absorption and particularly scattering properties of each suspension are somewhat different. Measurement of absolute emission yields is difficult for scattering samples such as those used here, but a relative comparison is possible for samples recorded under identical conditions (same day, same system alignment) and with similar scattering properties (same host zeolite); these correspond to the data of Table 1. Even this comparison yields only approximate values, since singlet oxygen radiative lifetimes (and thus quantum yields) are known to show media dependence.14 In contrast with the case of D2O, suspensions in DFP systematically led to biexponential behavior, with both decay components (see Figure 2) having the emission spectrum characteristic of singlet oxygen. It was immediately clear that the long component of this decay had a lifetime of around 470 ( 30 µs, the same value we determined independently for singlet oxygen in DFP. The short lifetime was always around 7 µs, although different samples gave variability of as much as 25%. There were also differences in the relative yields of fast and short components, as the data in Table 1 illustrates. Total yields are only given for a set of samples where the emission data were collected in a single session trying to maintain all experimental conditions as constant as possible. We interpret the data in DFP to mean that singlet oxygen decays both within the zeolite and after exiting the NaY particle. (14) Schweitzer, C.; Schmidt, R. Chem. ReV. 2003, 103 (5), 1685.
4480 Langmuir, Vol. 24, No. 9, 2008
Thus, the large fraction of singlet oxygen exiting into DFP would be available for conventional oxidative processes outside the zeolite micropores. The fast component is attributed to intracavity decay, probably by interaction with water or other protic sites that lead to quenching, under our conditions most likely without chemical consequences. Interestingly, the value of 7 µs coincides with that estimated by Clennan on the basis of product studies,5 and it is about twice the lifetime in pure water, suggesting that zeolite samples exposed to the laboratory environment provide only limited protection for singlet oxygen, given their wellestablished tendency to absorb humidity. Furthermore, this shortlifetime component is within the range of those previously measured in powders of dehydrated zeolites containing aromatic ketones.7 Notably, the studies on dry powdered zeolites did not allow the detection of the long component of the singlet oxygen phosphorescence observed using DFP as solvent, and that can be used to quantify the percentage of singlet oxygen escaping from the zeolite and diffusing into the liquid phase. With respect to the relative yield of intrazeolite vs extrazeolite decay depending on the nature of photocatalyst, the variation from one photocatalyst to another can reflect the influence of the zeolitic environment on the singlet oxygen decay kinetics. It has been recently reported that changes in the composition of the zeolite cavity (such as the presence of fluorinated counterions) can lead to significant changes on the singlet oxygen lifetime and reactivity.16 When the samples are suspended in D2O, it is clear that molecular and isotopic D/H exchange leads to lifetimes that are comparable within the cavities and in the bulk solvent. Our attempts to detect a second, presumably faster component in the emission decay in D2O were unsuccessful, suggesting that at least in this time scale singlet oxygen cannot distinguish intracavity solvent from bulk solvent. It seems reasonable to assume that some singlet oxygen will still undergo intracavity and liquid-phase decays. From the data in DFP, where intracavity and extracavity yields are similar one can estimate that escape occurs with a rate constant around 105 s-1, that is, competitive with the 7 µs fast decay. Thus, given the observed lifetime of ca. 46 µs in D2O, one estimates that most of the singlet oxygen will have enough time to escape and react in the bulk solvent. One of the MnPc samples showed undetectable fast component decay when freshly prepared and only 6% shortly after. Extracting the sample following the reported procedure15 (to clean up sensitizer from the external surface) led to 44% fast decay, a value in line with those obtained for other samples, suggesting that the variability observed simply reflects environmental water capture of NaY during handling. Not surprisingly, D2O eliminates this variability and the fast component. We note that, while extraction procedures may be of advantage for photophysical studies, for photodecontamination applications, catalysts on the external surface should also be able to generate reactive oxygen species capable of reacting in the bulk media. A very interesting case was that of the TP@NaY sample. It has been reported that soluble TP+ salts in solution do not generate singlet oxygen.17 This is in contrast to cyanoaromatics and (15) Paez-Mozo, E.; Gabriunas, N.; Lucaccioni, F.; Acosta, D. D.; Patrono, P.; La Ginestra, A.; Ruiz, P.; Delmon, B. J. Phys. Chem. 1993, 97 (49), 12819. Armengol, E.; Corma, A.; Forne´s, V.; Garcı´a, H.; Primo, J. Appl. Catal. A 1999, 181, 305. (16) Pace, A.; Pierro, P.; Buscemi, S.; Vivona, N.; Clennan, E. L. J. Org. Chem. 2007, 72 (7), 2644. (17) Latour, V.; Pigot, T.; Simon, M.; Cardy, H.; Lacombe, S. Photochem. Photobiol. Sci. 2005, 4 (2), 221. Sridhar, M.; Kumar, B. A. Chem. Lett. 1998, (5), 461. Akaba, R.; Sakuragi, H.; Tokumaru, K. J. Chem. Soc. Perkin Trans. 2 1991, (3), 291. Bonesi, S. M.; Carbonell, E.; Garcia, H.; Fagnoni, M.; Albini, A. Appl. Catal. B 2008, 79 (4), 368.
Letters
Figure 3. Luminescence decay monitored at 1270 nm following 308 nm laser excitation of a sample of TP@MCM-41 suspended in DFP.
heterocyclic dyes that simultaneously act as electron transfer and singlet oxygen photosensitizers.18 This has led to TP+ being proposed as a clean electron-transfer photosensitizer, where the products arise exclusively from photoinduced electron transfer without contamination from singlet oxygen reaction.19 In agreement with these previous studies, we have been unable to detect singlet oxygen phosphorescence from a 10-4 M CH3CN solution of TPBF4. In sharp contrast with the solution measurements, when a sample of TP@NaY or TP@MCM-41 is suspended on D2O or DFP, singlet oxygen phosphorescence was clearly observed (Figure 3). Both experiments in D2O and DFP suggest that the relative quantum yield of singlet oxygen for TP@NaY is even higher than that of zeolite encapsulated metal phthalocyanines. Moreover, as expected, the lifetime of singlet oxygen obtained from the long component of the phosphorescence decay in DFP coincides with that measured for metal phthalocyanines. Observation of singlet oxygen phosphorescence from TP in zeolites (Figure 3) indicates that the molecular photochemistry of TP+ is modified by encapsulation inside the confined space of the zeolite cavity. As mentioned earlier, the composition of the intrazeolite cavity from metal phthalocyanines strongly influences singlet oxygen decay and reactivity.16 This observation is in line with previous reports showing that immobilization of a photoactive guest inside the micropores of zeolites can alter its photochemical properties.2,20 Particularly in the case of TP+ incorporated inside NaY zeolite, we have previously observed that the dye becomes indefinitely persistent in neutral aqueous solutions, and that upon photoexcitation, TP@NaY can generate hydroxyl radicals.2 This is the base of the use of TP@NaY as photocatalyst to degrade organic pollutants in water.4 The ability of TP@NaY to generate singlet oxygen in contrast to the behavior of this dye in solution can be rationalized on the basis of previous time-resolved diffuse-reflectance studies that showed that the lifetime of the TP+ triplet excited state is considerably increased from a few microseconds to longer than milliseconds upon encapsulation of TP+ inside zeolites.21 This is probably the result of conformational immobilization of TP+ (18) Bonesi, S. M.; Manet, I.; Freccero, M.; Fagnoni, M.; Albini, A. Chem.s Eur. J. 2006, 12 (18), 4844. Lacombe, S.; Cardy, H.; Simon, M.; Khoukh, A.; Soumillion, J. P.; Ayadim, M. Photochem. Photobiol. Sci. 2002, 1 (5), 347. Tung, C.-H.; Guan, J.-Q. J. Am. Chem. Soc. 1998, 120 (46), 11874. (19) Miranda, M. A.; Garcı´a, H. Chem. ReV. 1994, 94 (4), 1063. (20) Wilkinson, F.; Willsher, C. J.; Casal, H. L.; Johnston, L. J.; Scaiano, J. C. Can. Chem. J. 1986, 64, 539. (21) Cano, M. L.; Cozens, F. L.; Garcı´a, H.; Martı´, V.; Scaiano, J. C. J. Phys. Chem. 1996, 100, 18152.
Letters
inside the zeolite cages. We have observed previously unique effects as result of this long triplet lifetime such as delayed fluorescence emission arising from triplet-triplet annihilation.21,22 We propose that the ability of encaged TP+ to generate singlet oxygen is also related to this prolonged triplet lifetime and to the increased TP+ triplet concentration inside the zeolite micropores. In summary, zeolite encapsulated photocatalysts are capable of generating singlet oxygen that to a large extent can escape the zeolite particle to become available for reaction in the bulk solvent. In the case of hydrated zeolites, a considerable fraction decays in the intracavity voids with a lifetime ca. 7 µs, while the rest (22) Marquis, S.; Ferrer, B.; Alvaro, M.; Garcia, H.; Roth, H. D. J. Phys. Chem. B 2006, 110 (30), 14956.
Langmuir, Vol. 24, No. 9, 2008 4481
escapes and has a lifetime characteristic of the bulk solvent. The positive influence of the zeolite framework on the ability to generate singlet oxygen can be demonstrated by the fact that TP+, a clean single electron transfer photosensitizer that does not form singlet oxygen in solution; however, when entrapped inside the zeolite micropores, it is able to generate singlet oxygen in similar or higher apparent quantum yields as other typical singlet oxygen photosensitizers. Acknowledgment. J.C.S. and H.G. thank the Natural Sciences and Engineering Research Council of Canada and Spanish DGES for generous support. All authors are grateful to NATO’s Science for Peace program (SfP 981476). LA800441N
