Enhanced Quenching of Anthracene Fluorescence by Nitroalkanes in

Spectroscopic Study of a Cinchona Alkaloid-Catalyzed Henry Reaction. Tatu Kumpulainen , Junhong Qian , and Albert M. Brouwer. ACS Omega 2018 3 (2), ...
2 downloads 0 Views 113KB Size
Download PDF
2446

Langmuir 2001, 17, 2446-2454

Enhanced Quenching of Anthracene Fluorescence by Nitroalkanes in Zeolite X and Y E. H. Ellison and J. K. Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received November 22, 2000. In Final Form: February 15, 2001 Quenching of the first excited singlet state of anthracene (1An*) by nitromethane (NM) and 2-methyl2-nitropropane (2M2NP) has been evaluated by picosecond, fluorescence-based methodologies in alkalimetal, ion-exchanged zeolite X and Y. Three forms of quenching were observed in NaY including static quenching, dynamic quenching on time scales of 1 ns. Static quenching of 1An* by NM was extensive. For example, a 50% drop in the initial intensity of 1An* decay profiles was observed at NM loadings equivalent to 0.10 M in the zeolite (or 0.13 NM/sc, where sc ) supercage). This level of quenching was not predicted by a random or Poisson distribution of NM among the An-occupied cages. In this case, only 13% of the An-occupied supercages should be occupied by NM at loadings of 0.13 NM/sc. To explain the enhanced static quenching, the action of conjugate acid-base sites has been invoked whereby An adsorbs to acidic, cationic sites and induces the adsorption of NM to nearby negatively charged oxygen atoms or basic sites. In NaX, ionization of NM to aci-NM was observed. Thus, to test quenching in NaX, the less acidic nitro compound 2M2NP was used. In NaY, static quenching of 1An* by 2M2NP was indistinguishable from quenching by NM and in NaX was much less extensive relative to NaY. The latter result is explained by a higher number of basic adsorption sites (or negative charges) as well as a higher degree of intrinsic basicity of adsorption sites in NaX, relative to NaY. Through competitive effects, these sites lower the extent of adsorption of 2M2NP to the An-induced basic sites. The dynamic quenching of 1An* on time scales of 1 ns is explained by electron tunneling to quenchers located outside the An-occupied supercages. The effects of temperature and added solvent on all three forms of quenching as well as comparisons of the quenching of 1An* with that of the first excited singlet state of pyrene are examined and discussed.

Introduction In the previous two decades, zeolites have been evaluated as media for producing molecular photochemistry1 and other photoinduced molecular processes including phosphorescence,2 electron transfer,3-8 and energy * Corresponding author. Professor J. Kerry Thomas, Department of Chemistry & Biochemistry, 251 Nieuwland Science Hall, University of Notre Dame, Notre Dame, IN 46556. Tel: (219) 6317589. Fax: (219) 631-6652. E-mail: [email protected]. (1) Studies of photochemistry in zeolites include: (a) Turro, N. J. Acc. Chem. Res. 2000, 33, 637. (b) Turro, N. J. In Molecular Dynamics in Restricted Geometries; Klafter, J., Drake, J. M., Eds.; John Wiley and Sons: New York, 1989; Chapter 14. (c) Ramamurthy, V. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH Publishers: New York, 1991; Chapter 10. (d) Thomas, J. K. Chem. Rev. 1993, 93, 301. (e) Scaiano, J. C.; Garcia, H. Acc. Chem. Res. 1999, 32, 783. (f) Blatter, F.; Sun, H.; Vasenkov, S.; Frei, H. Catal. Today 1998, 41, 297. (g) Frei, H.; Blatter, F.; Hai, S. CHEMTECH 1996, 26, 24. (2) (a) Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. Coord. Chem. Rev. 1990, 97, 225. (b) Ramamurthy, V.; Caspar, J. V.; Eaton, D. F.; Kuo, E. W.; Corbin, D. R. J. Am. Chem. Soc. 1992, 114, 3882. (3) (a) Yoon, K. B. Chem. Rev. 1993, 93, 321. (b) Yoon, K. B.; Hubig, S. M.; Kochi, J. K. J. Phys. Chem. 1994, 98, 3865. (4) (a) Hashimoto, S. Chem. Phys. Lett. 1996, 252, 236. (b) Hashimoto, S.; Mutoho, T.; Fukumura, H.; Masuhara, H. J. Chem. Soc., Faraday Trans. 1996, 92, 3653. (c) Hashimoto, S. J. Chem. Soc., Faraday Trans. 1997, 93, 4401. (5) (a) Iu, K. K.; Thomas, J. K. Colloids Surf. 1992, 63, 39. (b) Iu, K. K.; Thomas, J. K. J. Phys. Chem. 1991, 95, 506. (c) Liu, X.; Thomas, J. K. Chem. Mater. 1994, 6, 2303. (d) Liu, X.; Iu, K. K.; Thomas, J. K. J. Phys. Chem. 1994, 98, 7877. (e) Iu, K. K.; Liu, X.; Thomas, J. K. J. Photochem. Photobiol., A 1994, 79, 103. (f) Liu, X.; Thomas, J. K. Langmuir 1993, 9, 727. (6) (a) Ramamurthy, V.; Lakshminarasimhan, P.; Grey, C. P.; Johnston, L. J. Chem. Commun. 1998, 22, 2411. (b) Brancaleon, L.; Brousmiche, D.; Rao, V. J.; Johnston, L. J.; Ramamurthy, V. J. Am. Chem. Soc. 1998, 120, 4926.

transfer.8a,9 The objectives of these studies have been to establish the advantages of using zeolites as a photochemical medium and to develop theories of reactivity leading to more predictable chemistry and molecular behavior in zeolites. One advantage of the photochemical approach is that reactions can be carried out at room temperature, often with high product selectivity. This is important for the application of zeolites in fine chemical synthesis, where high-temperature activation can lead to thermal decomposition of reactants and products. The photochemical approach also provides a means to examine the kinetics of early events in the reaction mechanism, by use of pulsed excitation sources and rapid detection electronics. Thus, a detailed description of the reaction mechanism is possible. One of the more popular tools used in studies of photophysics and photochemistry in zeolites has been fluorescence spectroscopy. This approach involves adsorbing a molecular probe to the internal surface of the zeolite and monitoring its fluorescence emission by steadystate and time-resolved approaches. From the characterization of emission spectra and quenching dynamics, the polarity10,11 and heterogeneity12 of the zeolite medium (7) (a) Cozens, F. L.; Garcia, H.; Scaiano, J. C. Langmuir 1994, 10, 2246. (b) Alvaro, M.; Garcia, H.; Garcia, S.; Marquez, F.; Scaiano, J. C. J. Phys. Chem. B 1997, 101, 3043. (8) (a) Iu, K. K.; Thomas, J. K. Langmuir 1990, 6, 471. (b) Liu, X.; Iu, K. K.; Thomas, J. K. J. Phys. Chem. 1989, 93, 4120. (9) Calzaferri, G. Chimia 1998, 52, 525. (10) Uppili, S.; Thomas, K. J.; Crompton, E.; Ramamurthy, V. Langmuir 2000, 16, 265. (11) Iu, K. K.; Liu, X.; Thomas, J. K. Mater. Res. Soc. Symp. Proc. 1991, 233, 119. (12) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. J. Phys. Chem. 1993, 97, 13380.

10.1021/la0016219 CCC: $20.00 © 2001 American Chemical Society Published on Web 03/23/2001

Fluorescence Quenching in Zeolite X and Y

have been characterized as well as molecular processes including dimerization,4a,5f,8b,13-15 access to probes by molecular oxygen (O2),8b,11 and the formation of chargetransfer complexes.3,16 In the present study, quenching of the first excited singlet state of anthracene (1An*) by the two nitro compounds nitromethane (NM) and 2-methyl-2-nitropropane (2M2NP) is examined in zeolite X and Y. Because the zeolite is a polar medium, it is useful to comment on the rate of quenching for these systems in polar solvents. In ethanol, quenching by NM occurs by oxidative electron transfer at a nearly diffusion-controlled rate, specifically, 8 × 109 s-1 M-1.17 Given this efficient rate of quenching, it is expected that quenching in the zeolite will reflect the mobility of the reactants and any distribution that places the reactants in close proximity, rather than their intrinsic reactivities. To examine the quenching of 1An* in zeolites, the fluorescence approach has been strategically chosen to gain dynamic information on subnanosecond time scales. On this time scale, quenching by contact electron transfer should originate from a condition where anthracene and the quencher are located in very close proximity (or in the same supercage) at the time of excitation. This statement is supported by the following data. The mean square molecular displacement is given by the Einstein equation, 〈x2〉 ) 6Dt, from which the time scale for diffusion can be calculated. In a previous study of triplet quenching in dehydrated, potassium-exchanged zeolite Y (or KY), diffusion coefficients of arenes larger than naphthalene were estimated to be 103-106 times less than in fluid solution, corresponding to D values of 10-8-10-11 cm2 s-1.18 NMR pulsed field gradient studies have indicated D values of 1-2 × 10-7 cm2 s-1 for self-diffusion of benzene and acetonitrile in similar systems at room temperature.19 As the spacing between supercages in zeolite X and Y is about 13 Å, the estimated time required for migration of small molecules from one supercage to another is 30-3000 ns. Given that the fluoresence lifetime of anthracene in NaY is about 3 ns, then the anthracene fluorescence time scale is too short to sense cage-to-cage motion of the quenchers. Thus, quenching of 1An* by quenchers in supercages adjacent to the one occupied by 1An* should occur only by an electron tunneling mechanism, occurring in the nanosecond time regime. For quenchers inside the supercage, diffusional contact transfer or electron tunneling can occur, but the time scale is unknown. Given the constraints on molecular mobility in zeolites, quenching of 1An* by electron transfer (excluding longrange electron tunneling) to coadsorbed quenchers should be observed only at high loadings of the quencher, such that An and the quencher are in close proximity or occupying the same supercage at the time of excitation. The high loadings necessitate the usage of quenchers which do not strongly absorb the excitation light, especially when initial intensities of fluorescence decay profiles (or I0’s) need to be compared. For this purpose, nitromethane is particularly useful, as the molar extinction coefficient of nitromethane at the excitation wavelength employed (13) Hashimoto, S.; Ikuta, S.; Ashai, T.; Masuhara, H. Langmuir 1998, 14, 4284. (14) Cozens, F. L.; Regimbald, M.; Garcia, H.; Scaiano, J. C. J. Phys. Chem. 1996, 100, 18165. (15) Thomas, K. J.; Sunoj, R. B.; Chandrasekhar, J.; Ramamurthy, V. Langmuir 2000, 16, 4912. (16) Hashimoto, S.; Hagiwara, N.; Asahi, T.; Masuhara, H. Langmuir 1999, 15, 3123. (17) Thomas, J. K.; Hashimoto, S. New J. Chem. 1987, 11, 145. (18) Ellison, E. H. J. Phys. Chem. B 1999, 103, 9314. (19) Karger, J.; Pfeifer, H. Zeolites 1987, 7, 90.

Langmuir, Vol. 17, No. 8, 2001 2447

to generate 1An* (i.e., 355 nm) is only 0.4 (i.e., 355(NM) ) 0.4 M-1 cm-1). By use of the Beer-Lambert law, the optical density (OD) due to 1.0 M NM loaded into a 0.01 cm thick zeolite pellet is calculated to be 0.004, whereas that of 0.004 M An (the An concentration used in this study) is 0.4. Thus, high concentrations of NM can be loaded into the zeolite without inner filter effects. The results of this study illustrate how static and dynamic quenching in the zeolite are affected by temperature, added solvents, the silica-alumina ratio of the zeolite, and the probe (anthracene or pyrene) used. The results are used to describe various quenching mechanisms and how the quenchers are distributed among the probe-occupied supercages. Experimental Section Chemicals. Anthracene (An) was purified by recrystallization from ethanol. Pyrene (Py) was purified by column chromatography over silica gel. NM and 2M2NP from Fluka Chemicals were used as received. Benzene (Bz) and cyclohexane (Ch) were dried over activated molecular sieves (Linde 3A). Sodiumexchanged zeolite X and Y (NaX and NaY) were stirred in 1 M NaCl and rinsed thoroughly with ultrapure water. Procedures. Zeolite powders were pressed into thin (100 µm thick) pellets using a Wilks Uni-die (13 mm barrel diameter) and an applied pressure (Carver laboratory press) of 4 tons. The sample dry weight in the die was 12 mg and was carefully controlled to achieve uniform light scattering and collection of fluorescence. The pellets were dehydrated in air at 500 °C for at least 2 h. After the pellets were cooled under a flowing stream of dry N2, they were quickly transferred to 1 mL cyclohexane solutions in 20 mL glass scintillation vials. Anthracene or pyrene and NM were loaded simultaneously into the zeolite from cyclohexane. To achieve uniform loadings in the pellets, the samples were agitated at 150 rpm for at least 4 h on an orbital shaker and then allowed to equilibrate for at least 12 h. From a quantitative analysis of the supernatant, the probe and quencher were found to be >97% adsorbed from cyclohexane solutions, even when the NM loading was as high as 1.0 M in the zeolite. Loaded samples were removed from the cyclohexane solution, quickly transferred to 1 mm quartz cuvettes, and evacuated to milliTorr pressures at 80 °C for 15 min to remove the cyclohexane and small amounts of H2O. It was determined by a gravimetric-based procedure that NM was not removed by this drying procedure. All data were collected under vacuum conditions (2-5 mTorr pressures) with the obvious exception of samples that were bathed in solvent. For samples bathed in cyclohexane, the cyclohexane was quickly added under dry nitrogen following the post-loading drying procedure. Instrumentation. Fluorescence was measured by steadystate and time-resolved approaches, employing 45° front-face excitation and collection. The steady-state instrument was an SLM SPF-500C spectrofluorometer. A detailed description of the picosecond time-resolved setup used for collecting and comparing fluorescence decay profiles from a single laser pulse has been described in a recent report.20 Briefly, the excitation source employs a passively mode-locked Nd:YAG laser (35 ps pulses from the third harmonic of a Continuum PY-61). The detection system employs a Hamamatsu R1328U-02 phototube and a Tektronix SCD5000 single transient digitizer (5 GHz bandwidth). The total rise time of the measurement system is 95 ps. Transient absorption spectra in the pellets were measured in the transmittance mode following excitation by nanosecond pulses from a Continuum Surelite-I Nd:YAG laser. The detector was a CCD array spectrometer coupled to fiber optic cables. The analyzing light source was a 2 µs pulse from a Xenon flashlamp operated by a model 457 micropulser unit manufactured by the Xenon Corp. (Woburn, MA). The flashlamp and CCD array spectrometer were also used to collect the ground-state absorption spectrum of various adsorbates in the zeolite pellets. Optical density decay profiles were measured in the transmittance mode following either nanosecond pulsed laser excitation (20) Ellison, E. H.; Thomas, J. K. J. Phys. Chem. B, in press.

2448

Langmuir, Vol. 17, No. 8, 2001

Figure 1. The influence of added O2 and nitromethane on the decay profile of anthracene fluorescence when the anthracene is loaded into the zeolite powder prior to preparing the pellet. From top to bottom: no quencher, 1 atm O2, 3 NM/sc. [An] ) 20 mM. or pulsed radiolysis (0.4 MeV electron output from a Febetron linear accelerator). The analyzing light source was either the steady-state or pulsed output of a 450 W Xe arc lamp. An Oriel model 77250 was used to select the analyzing wavelength. A five-stage 1P28 photomultiplier tube was used as the detector, and a Tektronix TDS-620B digitizing oscilloscope (500 MHz bandwidth) was used to capture and store the decay profiles.

Results The interpretation of data from this study depends critically on how the loading procedure affects the distribution of adsorbed reactants in the zeolite. Normally, adsorbate concentration in the zeolite is calculated as follows. The unit cell volume of NaY is 15 200 Å3 (a ) 24.78 ( 0.02 Å), and each unit cell contains 8 supercages. By use of these values, the supercage concentration is calculated to be 0.87 M. Thus, the adsorbate concentration at loadings of 1/supercage (1/sc) is 0.87 M. This assumes that the adsorbate exhibits no preference for particular regions of the zeolite particle and that all the supercages are physically accessible to the adsorbate. Considerations of these assumptions are given below. These include the possible effects of inaccessible void spaces and loading inhomogeneities. Inaccessible Voids. In this study, the zeolite powder is pressed under high pressure in a die cell to form thin pellets. The use of pellets offers many advantages related to sample preparation, handling, and spectroscopic analysis including OD measurements in the transmittance mode. However, one potential side effect of this approach is damage to the zeolite. The high pressure used (4 tons) to form the pellets crushes the particles, and this could produce inaccessible voids in the samples. To gauge the extent of the damage, the zeolite powder was loaded with An, dried, and pressed in the usual way to form a pellet. With An already loaded in the zeolite, some of the An should be trapped in the inaccessible voids. The concentration of trapped An was ascertained from measurements of quenching of the An fluorescence by either O2 or NM. This is illustrated in Figure 1 where NM loaded from the vapor phase statically quenched 90% of the fluorescence. The NM loading was high (3/sc), and that is probably why a dynamic component was not observed. At 1 atm O2, both static and dynamic quenching is observed. The dynamic component is from quenching of An fluorescence in the accessible voids. This was confirmed from measurements of O2 quenching when An was loaded into the zeolite in

Ellison and Thomas

the usual way, after the pellet was prepared. In this case, only the dynamic component was observed; no unquenched components were present in the data. The signal which cannot be quenched is evidently from An fluorescence in void spaces which are inaccessible to O2. Because the level of static quenching is the same for O2 and NM, both of these quenchers appear to be equally excluded from the inaccessible voids containing An. Extraction of An from the pellet with ethanol leaves only the trapped An. It was determined that the trapped An was not quenched by O2, indicating that O2 does not penetrate into regions where An is trapped. The fluorescence from the trapped An corresponds to about 10% of the signal prior to adding the quencher. Thus, damage to the zeolite occurs from pressing it in the die cell, but this is not severe. Further details about the actual number of accessible supercages are given below. Loading Inhomogeneities. Another factor which could affect quenching is an inhomogeneous distribution of quencher in the samples. For example, there could be a higher concentration of quencher on the exterior region of the pellets, relative to the interior region. This same consideration applies to the internal and external regions of the particles which constitute the pellet. We have accounted for the former by prolonged agitation of the pellets in cyclohexane solutions on an orbital shaker. This should produce a uniform distribution of reactants in the solvent-filled void spaces between the zeolite particles throughout the pellet during loading. However, once the reactants adsorb to the particle, they may not diffuse in a reasonable period of time (i.e., hours) to achieve a uniform distribution throughout the particles. To examine this potential problem, pulsed radiolysis was used to produce trapped electrons in the zeolite pellet and to assess how NM reacts with the trapped electrons. It is important to note that in pulsed radiolysis a high-energy electron beam produces a uniform distribution of trapped electrons in the thin (100 µm thick) zeolite pellet. Under normal conditions, the electrons absorbed by NaY travel about 30 Å (a much smaller distance compared to the diameter of the zeolite particles, i.e., 1 µm) after which they become trapped in Na44+ clusters.21 This occurs by the reaction Na44+ + e- f Na43+. In dehydrated NaY, the Na43+ trapped electrons are sufficiently long-lived such that they can be easily observed by microsecond, transient absorption spectroscopy. The Na43+ decay rates in the presence and absence of NM are illustrated in Figure 2. In the absence of NM and in vacuo, the Na43+ lives for minutes and its broad visible absorption spectrum imparts a pinkish hue to the zeolite that is equally intense on both sides of the pellet. In the presence of NM, both static and dynamic quenching of Na43+ were observed. The static quenching indicates that NM can capture some of the electrons prior to their becoming trapped in Na44+ clusters. The decay rate of the Na43+ trapped electron depends on the NM concentration. However, at 1.0 M NM there is essentially no dynamic component observed (see Figure 2, lower plot), indicating that the rate of decay is beyond the speed of the measurement system or that some of the electrons have been captured by NM. A slowly decaying component, corresponding to the decay rate at 0.01 M NM, was not observed at 0.1 and 1.0 M NM. Because these slowly decaying components were not observed at the higher NM concentrations, there appears to be no region of the pellet or particle where NM is in low concentration or where (21) Zhang, G.; Liu, X.; Thomas, J. K. Radiat. Phys. Chem. 1998, 51, 135.

Fluorescence Quenching in Zeolite X and Y

Figure 2. The influence of nitromethane concentration on the decay profile of the Na43+ trapped electron in NaY. In the upper graph (5 ms time scale) from top to bottom: 0, 0.01, and 0.1 M NM. In the lower graph (0.25 ms time scale) from top to bottom: 0.1 and 1.0 M NM.

NM has not significantly penetrated. Thus, NM appears to be evenly distributed among the accessible voids in the pellet. The unquenched Na43+ signal at 1.0 M NM (Figure 2) is about 10% of that in the absence of NM and is not quenched by O2. These unquenched trapped electrons most likely occupy the inaccessible void spaces, consistent with the fluorescence quenching studies. Thus, it appears that the native zeolite powders from which the pellets are made do not contain excessive numbers of inaccessible voids which could possibly originate from manfacturing processes such as grinding or milling. The only loss of void space appears to be from pelletizing the zeolite. Therefore, the theoretical estimate of 0.87 M supercages in the powder is acceptable but should be adjusted to 0.78 M in the pellets to account for the inaccessible voids. With the result that NM is uniformly loaded into the sample, the fluorescence quenching can be analyzed on the basis of a random or statistical (Poisson) loading of NM into supercages. Fluorescence Quenching by NM in NaY. Figure 3 illustrates the variation with NM concentration of the fluorescence spectrum and decay profile of An in dehydrated, solvent-free NaY (or NaY). Static quenching (or a large drop in the initial intensity of the decay profile) is the primary quenching mechanism at [NM] e 0.1 M, whereas a dynamically quenched component begins to appear at [NM] g 0.2 M. This dynamic component dominates the decay profile at 1.0 M NM. The steadystate quenching data in Figure 3B confirm the extensive quenching observed in the time-resolved analysis. The

Langmuir, Vol. 17, No. 8, 2001 2449

Figure 3. The influence of nitromethane concentration on the decay profiles and steady-state intensity of anthracene fluorescence in dehydrated NaY. In graph A from top to bottom: 0, 0.05, 0.1, 0.2, 0.4, and 1.0 M NM; λex ) 355 nm (35 ps laser pulse). A high-pass cutoff filter (λcut ) 400 nm) was used to collect decays using a fast photodiode. In graph B from top to bottom: 0, 0.05, 0.10, 0.2, and 0.4 M NM; λex ) 355 nm (10 nm bandwidth); emission was collected using a 1 nm bandwidth and no filters. Scattered light is present in the data and is more significant at λ < 380 nm and at higher NM concentration. The spikes in the data in the region of 470 nm are from scatter of the arc lamp emission profile and do not represent An fluorescence. [An] ) 4 mM. A 10% error is associated with predicting the intensities.

data in Figure 3 were invariant upon increasing the An concentration by a factor of 10. No significant change in OD or peak position of the An ground-state absorption spectrum was observed in a NaY sample containing 1.0 M NM relative to a sample not containing NM. This result implies that An and NM do not form a ground-state chargetransfer complex. An expanded view of the data at 1.0 M NM is shown in Figure 4. At room temperature, there are essentially two components: one decaying inside 300 ps and another decaying on a time scale of >1 ns. Application of a previously developed procedure allows the decay time of the fast component to be predicted from the full width at half-maximum of the decay profile.20 Although the slowly decaying component obscures the data analysis somewhat, the decay time of the faster component is estimated to be 30-50 ps. The data at 77 K in Figure 4 are described in more detail below. Effects of Low Temperature. The effect of lowering the temperature to 77 K on the static quenching by NM in NaY was examined. This was accomplished by monitoring the decay profiles at 0.10 M NM where dynamic quenching inside 0.3 ns was not significant at room temperature. As illustrated in Figure 5, the relative initial

2450

Langmuir, Vol. 17, No. 8, 2001

Figure 4. Expanded view of the anthracene fluorescence decay profile in NaY containing 1.0 M NM and its dependence on temperature.

Figure 5. The dependence of static quenching of anthracene fluorescence by NM on lowering the temperature to 77 K. From top to bottom: 0 NM (77 K), 0 NM (293 K), 0.10 M NM (77 K), and 0.10 M NM (293 K).

intensities at 0 and 0.1 M NM were essentially unchanged upon lowering the temperature to 77 K. The absence of a dynamic component at 77 K indicates that the quenching mechanism remains static at this temperature, despite a decrease in molecular motion relative to room temperature. In the presence and absence of NM, the observed increase in initial intensity at 77 K is due to an increase in fluorescence quantum yield (φfl) that results from lowering the rate of intersystem crossing to the triplet state. This is also observed in condensed polymer systems.22 In solution and in NaY, φfl values of An are 0.323 and 0.16,24 respectively. In solution and at room temperature, the triplet quantum yield of An is 0.7.23 Given this high triplet yield at room temperature, the φfl of An should increase significantly on lowering the temperature to 77 K, whereas that of the triplet state should decrease. Although lowering the temperature did not affect the static quenching, it had a large influence on the dynamic (22) (a) Bennett, R. G.; McCartin, P. J. J. Chem. Phys. 1966, 44, 1969. (b) Stevens, B.; Thomaz, M. F.; Jones, J. J. Chem. Phys. 1967, 46, 405. (23) Birks, J. B. Photophysics of Aromatic Molecules; John Wiley and Sons: New York, 1970; p 251. (24) In this study, φfl (An) was determined by using an approach described in ref 5d.

Ellison and Thomas

quenching that occurred in the region of 97%) of the probe and quencher are adsorbed to the surface. Thus, the fluorescence is a reflection of adsorbed probes surrounded by cyclohexane. The differences in quenching in NaY and NaY/Ch are related to the differences in average fluorescence lifetime (τavg) of 1An* in the two systems, and these values are listed in Table 1. The higher value in NaY/Ch, which approaches that of 1An* in simple systems,

A description of the Gaussian fitting model is given in ref 1d.

indicates that cyclohexane weakens the association of An with the surface. Whereas the static quenching in NaY was affected by bathing the sample in cyclohexane (i.e., 5-6 cyclohexane/ sc), the addition of smaller amounts of either cyclohexane or benzene from the vapor phase had little effect on the static quenching. For example, the addition of either 2.5 cyclohexane/sc or 2.4 benzene/sc (or about one-half the maximum possible loading of these solvents) to NaY did not significantly alter the static quenching of 1An* at 0.1 M NM (data not shown). Also, these small amounts of solvent did not strongly affect τavg of 1An* (see Table 1). The results using An as a fluorescent probe were not unique. Similar results were found using Py and are illustrated in Figure 6B. For 1Py* in NaY, there was extensive static quenching at low NM loadings and rapid dynamic components were observed at higher NM loadings (decay profiles not shown). One major difference between Py and An was that quenching of 1Py* in NaY/Ch was more extensive than that of 1An* and did not conform very closely to a Poisson distribution of NM. This is related to the fact that τavg of 1Py* (Table 1) in NaY/Ch was not largely different from that in NaY, whereas the opposite was true for 1An*. Hence, cyclohexane does not alter the adsorption character of pyrene in the zeolite. A legitimate concern is whether the NM reacts with either Py or An to produce a nonfluorescent nitro derivative. Nitroanthracene has been shown to exhibit φfl close to zero.29 To determine if this was the case, the NM was extracted from a NaY sample containing Py and 1.0 M NM by bathing the sample in methanol. The use of pyrene was particularly advantageous for this analysis, because it was established that once adsorbed to the intraparticle surface of NaY, Py cannot be extracted with methanol. This was not the case for An which was easily extracted with MeOH. This difference between Py and An may reflect a number of things including size, solvent interactions, and the nature of adsorption. After extraction of the NM with excess methanol followed by heating and evacuation of the sample to remove the methanol, the Py fluorescence increased to the levels observed in the absence of NM. Thus, it is evident that the process leading to the static quenching is reversible. Quenching in NaX. In dehydrated, solvent-free NaX (or NaX) it was found that NM thermally reacts in the dark at room temperature to yield a visibly colored product. Identification of this product was made by transmission spectroscopy of the zeolite pellet. The absorption band of the zeolite-derived product was similar to that of an alkaline solution of NM revealing the deprotonated (and conjugated) aci-anion, that is, CH2dNO3-. This is supported by the fact that the methyl protons in NM are moderately acidic (pKa ) 10.0) and that aci-NM has been observed on other basic catalysts, including MgO and CsX.30 Also, according to the Sanderson electronegativity scale,31,5d NaX is more basic than NaY and is thus more (29) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice Hall: Englewood Cliffs, NJ, 1969; p 253. (30) Kheir, A. A.; Haw, J. F. J. Am. Chem. Soc. 1994, 116, 817.

2452

Langmuir, Vol. 17, No. 8, 2001

Ellison and Thomas

quenching, it is appropriate to assign an electron tunneling mechanism to quenching on this time domain. Rates of electron tunneling are strongly dependent on the distance (r) between the donor and acceptor.32 In the absence of diffusion, the electron tunneling quenching rate is given by k(r) ) ka exp(-(r - a)/re), where a is the distance of closest approach, ka is the rate constant at this distance, and re is a scaling parameter. Quenching rates in the nanosecond range have been measured in zeolites for electron tunneling between 1Py* and cupric or thallous ions at loadings of 0.2 M8 and on silica gel surfaces between pyrene and nitroalkanes.33 The earlier quenching data and those now obtained fall into the same time regime, and it is appropriate to assign an identical mechanism in these studies. Discussion

Figure 8. The decay profiles of 1Py* and their dependence on NM concentration and temperature in NaY. In both graphs from top to bottom: 0, 0.1, 0.15, and 0.2 M NM.

likely to accept a proton. These factors indicate that aciNM should be observed only in NaX and not in NaY. To compare nitroalkane quenching of 1An* in NaX and NaY, a less acidic nitro compound (2M2NP) was employed as the quencher. This molecule is less acidic than NM and therefore did not react to yield visibly colored products in NaX. With respect to quenching of 1An* by 2M2NP, one major difference between NaX and NaY was the level of static quenching, being much less in NaX than in NaY, and the data approach a Poisson distribution of 2M2NP in the zeolite as shown in Figure 6A. It is important to point out that in NaY the results using either NM or 2M2NP were indistinguishable, and this is shown for both Py and An in Figure 6. Thus, differences in size of the two quenchers have no influence on the quenching. Static quenching of 1 Py* by 2M2NP in NaX was less extensive than in NaY (Figure 6B). This result is similar to that observed for 1 An*. However, relative to 1An*, quenching of 1Py* by 2M2NP in NaX was more significant. Both 1Py* and 1An* exhibited significantly longer τavg in NaX relative to NaY (see Table 1). “Slow” Dynamic Quenching by NM. In addition to the static quenching and the dynamic quenching inside 300 ps, there was also quenching by NM on slower time scales (τ > 1 ns). The data in Figure 8 illustrate the quenching of 1Py* in NaY on these time scales and include an analysis at 77 K. The decay rates are comparable at both temperatures, indicating that the quenching mechanism does not require movement of the reactants. As there is little effect of temperature on the slow fluorescence (31) (a) Sanderson, R. T. In Chemical Bonds and Bond Energy; Academic Press: New York, 1976. (b) Sanderson, R. T. Polar Covalence; Academic Press: New York, 1983.

Presently, in three time domains, three mechanisms of quenching of anthracene and pyrene fluorescence by NM and 2M2NP have been observed in zeolites X and Y. One of these mechanisms involves electron tunneling, which explains the quenching on nanosecond time scales. Nothing more will be discussed concerning this mechanism. A second mechanism involves dynamic, motion-dependent quenching on subnanosecond time scales. This quenching must originate from quenchers located in probe-occupied supercages, because cage-to-cage motion is too slow to account for the rate. A third mechanism involves static quenching, which was enhanced in NaY compared to NaX. On the basis of a random or Poisson distribution of quenchers among the supercages, simultaneous occupancy of the probe and quencher in the same supercage (or double occupancy) is expected to be no more than 13% at NM loadings of 0.1 M. If the static quenching occurs by double occupancy, then it is intriguing that this quenching was equivalent to as much as 50% double occupancy at [NM] ) 0.1 M in NaY. Thus, it can be considered that NaY brings the two reactants together or that the quenching reaction is catalyzed in NaY. Given the potential for describing a form of catalysis in the zeolite, most of the discussion will be devoted to the origin of the static quenching in NaY. Origin of the Static Fluorescence Quenching. In the zeolite, there are three possibilities which account for the probe and quencher being in close contact, thus leading to static quenching. One possibility is that the reactants are concentrated in certain regions of the zeolite through selective adsorption. One form of selective adsorption can originate from the manner in which the reactants are loaded into the zeolite pellets. This can produce an inhomogeneous distribution of reactants in the pellets or particles which constitute the pellets. However, this possibility has been eliminated by showing that there were no regions of low nitromethane concentration in NaY, other than the inaccessible void spaces produced from pelletizing the zeolite. Another form of selective adsorption could originate from defect sites. Because the probe concentration is small (i.e., 2-4 mM), even small numbers of defect sites could be important. Possible defect sites in alkali metal ion exchanged zeolite X and Y include certain Lewis acid structures27 and residual Bro¨nsted acid sites.25,26 An attempt to eliminate these problems was carried out by the addition of small amounts of pyridine or ammonia vapor to the samples, consistent with previous studies.5d,26 (32) Miller, J. R.; Beitz, J. V.; Heddleston, R. K. J. Am. Chem. Soc. 1984, 106, 5057. (33) Krasnansky, R.; Thomas, J. K. J. Photochem. Photobiol., A 1991, 57, 81.

Fluorescence Quenching in Zeolite X and Y

However, this had no effect on the quenching. Also, the concentration of An was increased by a factor of 10 (from 4 to 40 mM), but this too had no effect on the quenching. Thus, the commonly suggested defect sites do not participate in the static quenching mechanism. Another possibility to account for close contact between the probe and quencher involves the formation of groundstate charge-transfer (or CT) complexes. In homogeneous solution, static quenching of fluorescence is usually explained by the formation of CT complexes. A similar situation could exist in zeolites. Evans’ studies in solution show the formation of CT complexes between aromatic hydrocarbons and tetranitromethane.34,35 However, no such complexes were observed for NM. Although the propensity for CT complex formation with NM is low in solution, the zeolite could be a special case, providing an efficient medium for complex formation. However, it is expected that an electron acceptor such as NM would be less likely to form a CT complex with an aromatic species adsorbed to the zeolite. This is due to a strong interaction of the aromatic molecule with unshielded cations on the zeolite. For example,23Na magic angle spinning NMR studies have shown that benzene prefers site II cations.36 There is also agreement that the site II cation is a preferred adsorption site for polyaromatics, including pyrene and anthracene.13,15 The interaction of arenes with site II cations eliminates the symmetry of their π electron cloud through shifts of electron density to the cation. This is the nature of the so-called π-cation effect,13,15 which is revealed by the very low III/I ratio of the pyrene fluorescence spectrum in dehydrated NaY and NaX. The resulting decrease in electron donating ability of the adsorbed aromatic should lower the extent of any complex formation. This parallels similar data in solution on the effect of electron withdrawing groups substituted on the aromatic ring. For example, the association constant of dimethylaminobenzene with 1,3,5-trinitrobenzene is 2.5 times that of benzene.37 It is concluded that relative to solution, nitromethane is less likely to form a CT complex in the zeolite. This is also supported by the lack of any spectroscopic evidence for such a complex. A third and most likely possibility to account for the static quenching involves close contact through the action of conjugate acid-base sites. Conjugate Acid-Base Sites. The presence of conjugate acid-base sites in zeolite X and Y was shown by Barthomeuf38 in studies using pyrrole as a probe of zeolite basicity. The basic sites in these zeolites are the negatively charged oxygen atoms to which the cations are electrostatically bonded; the acid sites are the exchangeable cations. With decreasing electronegativity of the cation, there is increased negative charge on the oxygen and the basicity of the zeolite increases. For example, CsY is more basic than NaY. These conjugate acid-base sites provide a basis for describing the static quenching of the fluorescence. As stated above, there is a transfer of electron density from arenes such as Py or An to cationic adsorption sites in the zeolite. It is possible that this transfer of electron density lowers the electronegativity of the cation thus increasing the basicity of the negatively charged (34) Evans, D. F. J. Chem. Soc. 1957, 4229. (35) Tamres, M.; Strong, R. L. In Molecular Association; Foster, R., Ed.; Academic Press: New York, 1979; Chapter 5. (36) Hu, K.; Hwang, L. Solid State Nucl. Magn. Reson. 1998, 12, 211. (37) Foster, R. Organic Charge-Transfer Complexes; Academic Press: London, 1969; p 197. (38) (a) Barthomeuf, D. J. Phys. Chem. 1984, 88, 42. (b) Barthomeuf, D. Catal. Rev.sSci. Eng. 1996, 38, 613.

Langmuir, Vol. 17, No. 8, 2001 2453

oxygen atoms. As these sites are more basic, an electrophilic species such as nitromethane should prefer them, relative to the normal basic adsorption sites which are not affected by the probe. The resulting close proximity of the probe and quencher leads to an ultrafast rate of electron transfer either by a tunneling mechanism or by the conjugate acid-base pair behaving essentially as a wire. It may be possible to achieve the ultrafast quenching, through the action of conjugate acid-base sites, by location of quenchers in adjacent supercages. The only stipulation is that the quencher must be adsorbed to the sodalite cage to which the probe is adsorbed. In NaX and NaY, each sodalite cage forms a union between four supercages and there are four sodium cations in each sodalite cage. When adsorbed to a site II cation, the probe lowers the electronegativity of all four cations in the sodalite cage. This effectively increases the basicity of all the negatively charged oxygens on the sodalite cage. This mechanism could explain why no difference was observed in NaY with respect to static quenching by the more bulky 2M2NP relative to NM. There should be no steric limitation to static quenching from quenchers located in adjacent supercages. The more extensive static quenching by 2M2NP in NaY relative to NaX can be explained by differences in basicity of the two zeolites. It is without question that NaX is more basic than NaY, and this is clearly revealed in the present study by the formation of aci-NM in NaX. In the zeolites containing adsorbed probes, there should be a distribution of NM among the stronger (probe-induced) basic sites (Sib) and the normal basic sites (Snb). In NaX containing An or Py, adsorption of NM to Sib is less favorable than in NaY presumably because of a higher number and strength of Snb. The differences in quenching of Py and An fluorescence may be related to the way in which the two probes interact with the zeolite. Apparently, Py is more strongly associated with the zeolite than An, as revealed by the insensitivity of its fluorescence lifetime to added cyclohexane and the fact that Py could not be extracted from the zeolite with methanol. The difference in adsorption of Py and An is unclear but could arise from the effects of either geometry or energetics. In NaY/Ch, measurements of τavg revealed that the association of An with the surface is weakened when An is surrounded by cyclohexane. This effectively lowers the strength of Sib to an extent where the conjugate acid-base mechanism is no longer significant. This was not the case when the supercages were only half-filled with cyclohexane. With cyclohexane filling the cage, the intensity lowering at 0.3 ns appears to originate entirely from dynamic quenching in the cage and from double occupancy predicted by a random, cage-filling process. One reviewer suggested that the increase in fluorescence lifetime of anthracene in NaY/Ch may be the result of small amounts of water in the samples introduced during sample preparation. However, the fluorescence spectrum and lifetime of pyrene have been shown to be very sensitive to the addition of H2O to the zeolite, and from the pyrene data in cyclohexane there was no indication that significant amounts of water were present in the zeolite. Thus, the results in NaY/Ch represent anhydrous conditions. In NaY, there was rapid dynamic quenching of 1An* in the region t < 0.3 ns. Because this was prevalent only at higher loadings (see Figure 3A), a fraction of the 1An* must be quenched by NM randomly loaded into the Anoccupied supercages. This is consistent with the data in NaY/Ch (see Figure 7), where dynamic quenching inside 0.3 ns was observed only at higher loadings of NM. For

2454

Langmuir, Vol. 17, No. 8, 2001

this to occur, a fraction of the An population could be adsorbed to sites which do not induce the adsorption of NM into the cage. These could be site III cations. It could also be that the NM is loading randomly into probeoccupied cages with the probe adsorbed to site II cations and the NM to Snb in the cage. The convolution of static and dynamic quenching makes it difficult to determine if Sib is saturable because the dynamic quenching eventually leads to complete loss of the fluorescence. Summary In NaY, the extent of the static quenching of anthracene and pyrene fluorescence by nitromethane was much higher than expected based on a random distribution of quencher in the zeolite. This is explained by the action of conjugate acid-base pairs in the zeolite that provide adsorption sites for close contact between the aromatic probe and the electrophilic quencher. The close proximity of adsorption leads to an ultrafast rate of electron transfer. It is possible that this could occur by electron transfer across the sodalite cage. The results of this study are important because they point out that in zeolite X and Y the distribution of a particular molecular species among supercages occupied by a different molecular species is not necessarily a random one. For example, in NaY the different molecules (one an electron donor and the other an electron acceptor) prefer a common location in the zeolite and are poised to react upon excitation. This could be an advantage or a disadvantage depending on the system but may provide a mechanism to explain certain types of chemistry in zeolites.

Ellison and Thomas

The fact that a Poisson distribution of quencher in NaY/ Ch was observed is important. It points out the homogeneous nature of the zeolite in that no preferred spaces or defect sites exist in the zeolite where all the chemistry takes place. A previous study of quenching of triplet xanthone in NaY on submicrosecond time scales also gave evidence for a modified statistical (or Poisson) distribution of quencher.39 In this case, quenching by triplet energy transfer to 1-methylnaphthalene was predicted by consideration of molecules located not only in probe-occupied supercages but also in the four adjacent supercages surrounding the probe. Although on microsecond time scales diffusional quenching of xanthone may be possible, the results of the triplet study add credence to the description of random loading in the zeolite that has been presented here. The different results for pyrene and anthracene imply that generalizations concerning solvent effects in the zeolite cannot be made as in solution. This is due to differences in the way the two probes interact with the zeolite surface. It appears that the approach developed here is useful for describing bimolecular encounters in the zeolite. As each system exhibits its own unique behavior, more studies of this nature will aid in developing a unified model of molecular behavior in zeolites. Acknowledgment. This work was supported by the National Science Foundation. LA0016219 (39) Scaiano, J. C.; de Lucas, N. C.; Andraos, J.; Garcia, H. Chem. Phys. Lett. 1995, 233, 5.