Influence of the Alkali Metal Cation on the Distance of Electron

The chosen system is the electron-transfer reaction between ... zeolite framework significantly influences the distance of electron migration and that...
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J. Phys. Chem. C 2007, 111, 14247-14252

14247

Influence of the Alkali Metal Cation on the Distance of Electron Migration in Zeolite Y: A Nanosecond Laser Photolysis Study Amy E. Keirstead, Norman P. Schepp, and Frances L. Cozens* Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada, B3H 4J3 ReceiVed: May 2, 2007; In Final Form: July 9, 2007

Zeolites have been identified as promising solid-state host materials for the design of alternative energy systems due to their ability to promote long-lived charge-separated states. Herein we investigate the ability of alkali metal cation-exchanged Y zeolites to mediate the formation of a spatially separated radical cation/radical anion pair by photoinduced electron migration. The chosen system is the electron-transfer reaction between photoexcited trans-anethole and co-incorporated 1,4-dicyanobenzene taking place within the internal cavities of the Y zeolite. The results from the investigation show that varying the alkali metal cation within the Y zeolite framework significantly influences the distance of electron migration and that the electrons travel farthest within dry NaY as compared to the other alkali metal cation-exchanged Y zeolites. The presence of intrazeolite water was also found to impede electron migration within the zeolite matrix.

Introduction

SCHEME 1

Zeolites are crystalline aluminosilicate materials comprised of an open framework of molecular-sized pores, channels, and cavities, enabling guest molecules to be readily adsorbed within their periodic array of void spaces.1-4 The presence of tetravalent aluminum in the zeolite framework gives rise to a net negative charge that is typically counterbalanced by protons or alkali metal cations, resulting in a highly polar and strongly ionizing intrazeolite environment.5,6 In the past decade, much emphasis has been placed on the use of zeolites as hosts for photoinduced electron transfer (PET) reactions, due to their ability to stabilize charged intermediates.7-11 Zeolites have also been shown to participate directly in PET reactions, and can behave as oneelectron donors12,13 or acceptors,14-16 subsequently generating a hole or electron within the aluminosilicate-cation internal structure. While a myriad of reports appear in the literature detailing the individual contributions to the study of zeolites as redox partners, much less emphasis has been placed on determining the fate of the photochemically generated holes and electrons. The decreased rate of charge recombination within zeolites compared to analogous electron-transfer studies in homogeneous solution has mainly been attributed to the strong electrostatic fields that are present within the zeolite cavity as well as the restricted intrazeolite diffusion of guest molecules.8,11 However, since long-lived charge-separated species are also observed in nonpolar, all-silica zeolites,17 compartmentalization of photogenerated electrons away from the initial site of formation may also decrease the propensity for charge recombination. In a previous report from our laboratory,18 we studied the mobility of electrons generated upon laser-induced photoionization of trans-anethole (An, trans-1-(4-methoxyphenyl)propene) in dry NaY. In that report, we showed that these electrons did not always remain localized at the site of formation, but in some cases could travel a small distance along electron-deficient sites within the zeolite matrix (such as cations, cation clusters, and extraframework aluminum) until being trapped by a * Address correspondence to this author. E-mail: [email protected].

secondary electron acceptor, 1,4-dicyanobenzene (DCB), to give the 1,4-dicyanobenzene radical anion (DCB-•), Scheme 1. The distance of electron migration was estimated by investigating the yield of the DCB-• at various loading levels of DCB, using nanosecond transient diffuse reflectance spectroscopy, and by employing two independent mathematical models (the Distribution Model and the Perrin Formulation). From these initial studies the zeolite was estimated to mediate electron transfer over a total through-bond distance (which includes the radii of the acceptor and donor molecules) of ∼20 ( 2 Å in NaY; in other words, the electron donor (excited anethole) could be spatially separated from the secondary electron acceptor (1,4dicyanobenzene) by approximately one vacant NaY supercage. In the present work we extend these studies to investigate the influence of the alkali metal charge-balancing cation on the mechanism for the formation of the anethole radical cation1,4-dicyanobenzene radical anion pair and on the distance of electron migration. Our results obtained in the alkali metal cation-exchanged Y (MY) zeolites are particularly interesting and show that varying the alkali metal cation has a dramatic influence on the distance of electron travel in zeolite Y. Moreover, we have found that the presence of intrazeolitic water also plays a significant role by impeding the electron migration within NaY. Results and Discussion Photoionization of trans-Anethole in MY Zeolites. Photoionization of An to give the anethole radical cation (An+•) is the first step in the mechanistic pathway for the electron migration reaction as outlined in Scheme 1; thus the influence of the alkali metal cation on the photoionization of An and the reactivity of the An+• needs to be established prior to examining the effect of the alkali metal cation on the electron-transfer

10.1021/jp073369k CCC: $37.00 © 2007 American Chemical Society Published on Web 09/06/2007

14248 J. Phys. Chem. C, Vol. 111, No. 38, 2007

Keirstead et al.

Figure 1. Transient diffuse reflectance spectra collected at specified times following 308 nm laser flash photolysis of An (〈S〉 ) 0.1) alone (〈S〉 DCB ) 0) (b) and in the presence of coadsorbed DCB (〈S〉 ) 1) (O) in zeolites (a) LiY (1.44 µs), (b) NaY (1.32 µs), (c) KY (1.76 µs), (d) RbY (3.96 µs), and (e) CsY (1.28 µs).

reaction and the distance of electron migration. The transient diffuse reflectance spectra obtained following 308 nm laser flash photolysis of An (loading level, 〈S〉, ) 0.1) in zeolites LiY, NaY, KY, RbY, and CsY under evacuated dry conditions are characterized by two strong absorption bands centered at 390 and 610 nm, Figure 1 (solid circles (b)). These absorption bands are readily assigned to those of the An+•, based on the close similarity of these spectra with the known absorption spectrum of this transient species in solution19,20 and zeolites NaY21 and NaX.22 In zeolites LiY and NaY, additional absorption was present between 500 and 700 nm, attributed to Li or Na cation clusters, which are well characterized within zeolites23-25 and are formed upon capture of the photoejected electron by a cluster of charge-balancing cations. The absorption bands at 390 and 610 nm were not quenched upon oxygen saturation of the samples on the time scales studied, further supporting the assignment of these bands to the An+•. The additional absorption between 500 and 700 nm observed in LiY and NaY was rapidly quenched in the presence of oxygen, consistent with the assignment of this broad absorption to trapped electrons that react rapidly with molecular oxygen. A closer examination of the transient signal for the An+• shows that while the radical cation is readily formed in all five of the MY zeolites, the yield of the An+• in CsY was notably decreased compared to that of the other cation-exchanged zeolites, Figure 1. Furthermore, the reactivity of the An+• monitored at 390 nm was not equivalent in all MY zeolites. This observation is evident qualitatively upon inspection of Figure 2a, which shows the normalized kinetic traces obtained for the decay of the An+• at 390 nm within the five cationexchanged MY zeolites. From this plot, it is clear that the reactivity of the radical cation increases upon going from LiY to CsY.

Figure 2. Normalized kinetic traces for the decay of the An+• monitored at 390 nm following 308 nm laser excitation of An (〈S〉 ) 0.1) in LiY (b), NaY (O), KY (9), RbY (0), and CsY (2) (a) in the absence of DCB (〈S〉 ) 0) and (b) in the presence of DCB (〈S〉 ) 1).

A more quantitative analysis of the decay of the An+• was carried out by using the exponential series method,26,27 which uses a fitting function containing 200 fixed exponential terms with logarithmically spaced rate constants ranging from 1 × 100 to 1 × 107 s-1, with each fixed exponential term having a variable pre-exponential that reflects the significance of that fixed term. The kinetic data fit by using the exponential series method were a summation of five decay traces collected over different time scales ranging from 20 to 500 µs. The results of these fits for the decay of the An+• in the MY zeolites are shown in Figure 3a as plots of calculated pre-exponentials (or relative abundance) versus rate constant. In each zeolite, three distributions of rate constants were observed, indicating that the An+• decays with fast (105 s-1), medium (104 s-1), and slow (103 s-1) components. These distributions were fit to a Gaussian equation to give the median rate constants and normalized areas for each of the three decay components that are summarized in Table 1. These data show that although the rate constants for decay of the An+• in the MY zeolites are all within the same order of magnitude for each of the three components, some variation is observed upon changing the alkali metal cation. In all cases, the rate constant for the decay of the An+• increases with increasing size of the alkali metal cation. The area surrounding each midpoint was also affected, with the area associated with the fast decay becoming larger upon going from NaY to CsY, and the area associated with the slow decay becoming smaller

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Figure 3. Rate constant distributions obtained by using the exponential series method for the decay of the anethole radical cation (〈S〉 An ) 0.1) in alkali metal cation-exchanged Y zeolites (a) with no DCB (〈S〉 ) 0) and (b) with DCB (〈S〉 ) 1).

upon going from NaY to CsY. These results clearly show that the overall decay of the An+• increases significantly upon going from NaY to CsY. It should be noted that an unusually large area is associated with the fast component of the decay of the An+• in LiY, Table 1a. The reasons for this are not clear, but they may be linked to difficulties associated with complete removal of water from the highly polar LiY. Since the An+• decays without the concomitant growth of a new species or any change in the transient or ground state absorption profiles, the most reasonable mode of decay within these MY zeolites is charge recombination with the photoejected electron to reform neutral ground state anethole. The reduction in transient yield and increase in the decay rate constants of the An+• with increasing size of the alkali metal cation are consistent with the known decrease in zeolite electron affinity upon going from LiY to CsY.28,29 These results demonstrate that the electron is more readily available for charge recombination in the larger cation-exchanged zeolites compared to NaY and thus the charge recombination reaction occurs with considerably faster rate constants in RbY and CsY than NaY. Similarly, the less ionizing environments5 of RbY and CsY compared to LiY and NaY are also thought to contribute to the decreased stability of this positively charged intermediate, resulting in a greater affinity for electron capture. Photoionization of trans-Anethole in MY Zeolites Containing 1,4-Dicyanobenzene as a Secondary Electron Acceptor. To probe the ability of each of the MY zeolites to mediate charge separation by electron migration, the loading

level of An was kept constant at 〈S〉 ) 0.1 and that of DCB was varied from zero (〈S〉 ) 0) to one molecule in every Y zeolite supercage (〈S〉 ) 1). No thermal decomposition of adsorbates or formation of charge-transfer complexes was observed for any of the composites studied. The transient diffuse reflectance spectra collected following 308 nm laser flash photolysis of the various cation-exchanged An/DCB samples under evacuated conditions showed, in all cases, the absorption bands at 390 and 610 nm due to the An+•. For the samples with the highest loading level of DCB (〈S〉 ) 1), an absorption band at 350 nm, which can be assigned to the DCB-•, was present for all the MY zeolites investigated,30 Figure 1 (open circles (O)). Since DCB has very little absorption at 308 nm, especially in comparison to the absorption by anethole, and 308 nm irradiation of DCB (〈S〉 ) 1) alone gave no significant DCB-•, the observation of the DCB-• upon irradiation of An with the high loading level of DCB illustrates that the electrontransfer reaction to form the An+•-DCB-• pair occurred in all the cation-exchanged zeolites. However, for the samples with lower loadings of DCB (〈S〉 ) 0.05 to 0.5), the spectral profile in the 350 nm region due to the DCB-• varied, depending on the nature of the charge-balancing cation in the MY zeolites, and either (a) showed no change from the case where DCB was absent, (b) exhibited broadening of the 390 nm band in the blue direction due to the presence of a small additional absorption at 350 nm, or (c) showed a distinct absorption band at 350 nm similar to the case for 〈S〉 DCB ) 1, where the DCB-• can be readily observed. These results show that for all the MY zeolites at a particular loading of DCB the overall signal intensity at 350 nm increased. However, for the larger charge-balancing cations (KY, RbY, CsY) high loading levels of DCB, where nearly every zeolite cavity contained a molecule of DCB, were required to achieve scenarios b or c. While increasing the loading level of DCB from 〈S〉 ) 0 to 0.5 did not appreciably change the intensity of the 390 and 610 nm An+• bands for any of the MY zeolites, this was not the case for the samples where 〈S〉 DCB ) 1, Figure 1. In particular, a decrease in the yield of the An+• was observed in LiY, NaY, KY, and RbY upon going from 〈S〉 DCB ) 0 to 1, but the magnitude of the decrease was greatest in LiY. Furthermore, no decrease in yield of the An+• was observed at high DCB content in CsY. The decays of the An+• in the MY zeolites containing coadsorbed DCB (〈S〉 ) 1) were also analyzed by using the exponential series method, giving the distribution curves shown in Figure 3b, with results summarized in Table 1b. As for the results obtained in the absence of DCB, the overall decay of the An+• increased upon going from LiY to CsY. However,

TABLE 1: Median Rate Constants (k) and Fractional Areas Obtained by Using the Exponential Series Method from the Decay of the Anethole Radical Cation in Alkali Metal-Exchanged Y Zeolites under Evacuated Conditions in the Absence ((a), 〈S〉 DCB ) 0) and Presence ((b), 〈S〉 DCB ) 1) of DCB zeolite

kslow/s-1

fractional area

kmedium/s-1

fractional area

kfast/s-1

fractional area

(a) 〈S〉 DCB ) 0 LiY NaY KY RbY CsY

2.7 × 103 2.9 × 103 6.8 × 103 12.1 × 103 11.0 × 103

0.38 0.55 0.49 0.26 0.18

1.3 × 104 2.8 × 104 5.2 × 104 7.0 × 104 6.7 × 104

0.22 0.30 0.28 0.39 0.38

1.8 × 105 3.0 × 105 3.5 × 105 4.0 × 105 4.0 × 105

0.40 0.14 0.23 0.35 0.43

(b) 〈S〉 DCB ) 1 LiY NaY KY RbY CsY

3.4 × 103 3.1 × 103 8.1 × 103 13.0 × 103 8.1 × 103

0.50 0.49 0.43 0.33 0.32

3.7 × 104 3.0 × 104 4.7 × 104 7.9 × 104 7.8 × 104

0.31 0.30 0.31 0.39 0.36

2.8 × 105 2.7 × 105 3.0 × 105 3.9 × 105 4.6 × 105

0.19 0.20 0.26 0.28 0.32

14250 J. Phys. Chem. C, Vol. 111, No. 38, 2007 SCHEME 2

when comparing the overall decay of the An+• in a particular cation-exchanged zeolite in the presence (〈S〉 ) 1) and the absence (〈S〉 ) 0) of DCB, noteworthy results were obtained. For example, in NaY the significance of the fast component of the decay of the An+• became somewhat greater as the loading level of DCB changed from 〈S〉 ) 0 to 1, while the slow component became less important. This indicates that the overall decay of the An+• in NaY increased in the presence of high loadings of DCB within the zeolite. Conversely, in CsY, the fast component became substantially less important and the slow component more important in the presence of high concentrations of DCB indicating that in this zeolite the addition of DCB reduced the overall decay of the An+•. These reactivity trends are qualitatively shown in Figure 2b where the normalized decay traces for the An+• at 390 nm in the presence of DCB (〈S〉 ) 1) are given. These results can be explained if there is a change in the mechanism for the decay of the An+• in the MY zeolites upon increasing the loading level of DCB from 〈S〉 ) 0 to 1. Thus, in the absence of DCB (〈S〉 ) 0) the An+• decays by charge recombination with the electron added to the zeolite framework upon photoionization of An. However, at 〈S〉 DCB ) 1 where there is approximately one DCB in every zeolite cavity, the electron is most likely found in the DCB-• rather than in the framework, and the An+• will most likely be reduced to An by charge recombination from the DCB-•, Scheme 2. Under these circumstances, the slower decay of An+• in CsY in the presence of DCB can be understood if the An+•/DCB-• pair has a slower rate of charge recombination compared to the situation where the electron has no secondary electron trap present in the zeolite and is embedded within the framework structure. In other words, charge recombination from the framework to the An+• appears to be faster than charge recombination from the DCB-• to the An+• for CsY. However, for NaY where the decay of the An+• is slightly accelerated at high loading levels of DCB, the opposite order in reactivity is observed. Both the Distribution Model31 and the Perrin Formulation32 were used as described in detail in our previous report18 to estimate the distance of electron migration upon photoionization of An in the presence of varying loading levels of DCB within the different MY zeolites. The data used for the model calculations were extracted from the transient spectra at 350 nm at the various loading levels of DCB and were corrected to account for overlapping absorption by the anethole radical cation at this wavelength. The graphical results obtained by using the Distribution Model for the electron-transfer reaction in the MY zeolites are given in Figure 4. These results clearly show a dramatic dependence of the charge-balancing cation on the electron-transfer reaction. Figure 4 shows that for NaY, the results are best described by a distance dependence involving

Keirstead et al. one to two cavities as the data points fall somewhat above the solid line, which represents an electron migration distance of approximately one cavity. For LiY the data points fall on the solid line indicating that the distance the electron can travel in LiY is less than that for NaY and is now about one cavity or 13 Å. For the larger cation-exchanged zeolites (KY, RbY, and CsY), the Distribution Model plots all clearly show that the data points lie close to the dashed-dotted line representing an in-cage direct electron-transfer reaction. The resulting throughbond electron migration distances (which include the molecular radii of the electron donor and acceptor) obtained from the data by using the Distribution Model, as well as the calculated distance obtained by using the Perrin Formulation, are given in Table 2. In addition to the cation-exchanged samples, the distance of electron migration was also estimated for a sample of hydrated NaY for comparison to the dry zeolite; here the electron migration distance was found to be significantly reduced upon the adsorption of water. The electron migration values obtained by using the two mathematical models agree reasonably well and illustrate that the distance of electron migration is highly dependent on the charge-balancing cation within the zeolite matrix and also on the hydration state of the sample. For zeolite NaY, the distance that the electron can travel from the point of photoionization, represented by the center of the donor molecule (An) within the zeolite cavity, to the center of the aromatic moiety of the secondary electron acceptor (DCB) was calculated to be 17 Å with the Distribution Model and 15.3 Å with the Perrin Formulation. These values indicate that the electron-transfer reaction can still occur when the donor and the acceptor are separated by slightly more than the diameter of one zeolite cavity. Though these values are somewhat smaller than those reported for NaY in our previous work,18 they are within experimental error and any differences are thought to be due to the high sensitivity of the experiments to the source of the zeolite sample. As the other cation-exchanged Y zeolites used in this work were prepared from the batch of NaY used in the present study via cation exchange, we foresee that the relative values for the migration distances will be consistent throughout these experiments and that the trends observed would not depend on the source of the NaY zeolite. The values obtained for LiY suggest that the photoejected electron can traverse approximately 13 Å, significantly less than the value obtained for NaY. Two factors are considered to be responsible for this effect. Mainly, Li-exchanged Y zeolites are extremely hygroscopic and even under rigorous drying procedures retain some intrazeolite water. Hydration of the photoejected electron will result in a decreased distance of migration, due to stabilization of the electron compared to its capture by the dry zeolite framework. This notion is supported by the results obtained for the hydrated NaY sample, which suggest that the presence of water within the zeolite matrix strongly impedes electron migration along the aluminosilicate framework. Second, the greater electron affinity of the LiY framework7 may result in the nascent electron being held so tightly (localized) at the initial active site that it is unable to migrate along the aluminosilicate framework. The distances of electron migration estimated for zeolites KY, RbY, and CsY strongly suggest that the electron photoejected from excited state An does not migrate to a neighboring supercage. As such, formation of DCB-• in these zeolites occurs primarily at high 〈S〉 of DCB as a result of a direct electron transfer, either within the supercage or between the precursors at a window site. In the case where DCB is not proximate to An, the photoejected electron presumably does not travel from

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Figure 4. Yield of DCB radical anion (b) as a function of the fractional occupancy of DCB in zeolites (a) LiY, (b) NaY, (c) KY, (d) RbY, (e) CsY, and (f) hydrated NaY compared to the predicated occupancy lines. The experimental data have been normalized by using the 〈S〉 DCB ) 1 data point. The legend for the occupancy lines is the following: in-cage electron transfer (dashed-doted line); electron migration over one cavity corresponding to a distance of ∼13 Å (solid line); electron migration over two cavity shells or a distance of ∼21 Å (short dashed line); and electron migration over three cavity shells or a distance of ∼25 Å (long dashed line).

TABLE 2: Distances of Electron Migration Obtained for the Various MY Zeolite Series by Using the Distribution Model and the Perrin Formulationa Zeolite

Distribution Model/Å

Perrin Formulation/Å

LiY NaY NaY (wet) KY RbY CsY

∼13 (1 supercage) ∼17 (1.5 supercage)