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J. Phys. Chem. B 2003, 107, 7254-7260
Effect of Charge Stabilization on Electron Transfer Reactions in Zeolites† Guohong Zhang‡ and J. Kerry Thomas* Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: NoVember 20, 2002; In Final Form: January 30, 2003
Photoinduced charge-transfer reactions between adsorbed pyrene and N,N′-dimethylaniline (DMA) in zeolites are examined by fluorescence quenching and transient absorption spectroscopy. The quenching of pyrene fluorescence by DMA is found to be limited by the diffusion of DMA that is confined in zeolite supercages. The diffusivity of DMA is measured to be 8.4 × 10-6 cm2/s in K+ ion-exchanged zeolite X. The resultant exciplex between pyrene and DMA is very short lived, and its decay leads to a large yield of ion radicals. Although no emission from the exciplex is observed in ionic zeolites within the experimental capability (i.e., fluorescence quantum yield Φf < 0.001), it exhibits a normal fluorescence spectrum in a nonionic zeolite (Ultra Stable Y: USY) with a maximum around 460 nm, indicating a very weak interaction between the adsorbate and the host in contrast to its behavior in ionic zeolites. The large yield and the long lifetime of ion radicals are attributed to the high electric field and the strong ionic interaction within the supercage, which favors charge separation and dissociation of the exciplex into a cage-confined ion pair. The slow charge recombination reaction may be understood in terms of a charge stabilization effect characterized by a lower driving force for back electron transfer (-∆G-et) and a high media reorganization energy λs. A charge stabilization effect is also observed in charge-shifting reactions where cations such as biphenyl and pyrene cation radicals are produced in zeolites by high-energy radiation. Quenching of the above cations by mobile DMA, conventionally observed as a diffusion-controlled process, is dramatically slowed. It is found that a significant amount of reorganization energy is required to promote charge-transfer reactions in ionic zeolites. Such unique properties of zeolites in charge separation and stabilization are distinct from those in other systems such as liquids and porous silica.
Introduction Photoinduced charge transfer has drawn particular attention over the last 30 years because of its fundamental importance in various electron transfer (ET) reactions in chemistry and biology. So far, extensive and systematic studies on ET processes have been conducted in homogeneous systems including both the gas and the liquid phases.1-8 In his early theoretical work, Marcus first showed that solvents, as a significant part of the reaction coordinates, play a major role in ET reactions in condensed phases.1 With the introduction of solvent reorganization energy, the relationship between the reaction rates (ln k) and the freeenergy driving force (-∆G) is well described by a parabolic energy-gap law for most ET reactions in liquid solutions.1 Subsequent studies by Miller and co-workers provided the first experimental evidence in support of the energy-gap law.3,4 It is found that the majority of the forward ET reactions (charge separation) lie in the normal region and that most of the back ET reactions (charge recombination) are in the inverted region. Experiments in low-temperature organic glasses and solid polymers also demonstrate that the same principle applies equally well to solid solutions.4,9 However, an extension of the above successful approaches developed from the homogeneous systems to multiphase systems, particularly solid surfaces, remains to be tested. †
Part of the special issue “Arnim Henglein Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Present address: Texas Instruments, Silicon Technology Development, 13560 North Central Expressway, M/S 3737, Dallas, Texas 75243.
It is well known that light-induced charge separation is the key process in heterogeneous photocatalysis. Early work in various organized and constrained media have found that micelles can be successfully used to enhance ion radical production by tuning the charge on the micellar surfaces.10 One of the motivations behind the recent photochemical studies in solid systems is to promote charge separation by making use of the unique properties of surface environments such as high polarity, rigid orientation, and geometric constraint.11 Kochi and co-workers have shown that charge-transfer ion pairs from the photoexcitation of MV2+ and arene complexes are more stable within zeolite supercages than in very polar solvents such as acetonitrile (dielectric constant ) 37.5).12 Such a stabilization effect was evident from significantly longer lifetimes of the ion pairs because of the slower back ET. This is somewhat different from what is conventionally observed in the inverted region. In contrast, recent work by Miyasaka found that the same energy-gap law is operative in a porous glass (pretreated at Ta ) 480 °C) as in liquid solutions (i.e., it exhibits typical invertedregion behavior13). The total reorganization energy for the charge recombination of the contact ion pairs was estimated to be 0.36 eV. After the deduction of the vibrational contribution, the surface reorganization energy is less than 0.10 eV. It is much smaller than that in a polar solution, indicative of a very limited environmental reorganization on the surface. The difference between the two solid systems above with regard to their influence on ET reactions calls for further investigation. Our recent study of an intermolecular quenching of the pyrene singlet excited state by N,N′-dimethylaniline (DMA) on silica gel surfaces shed more light on the surface effects on charge
10.1021/jp022454j CCC: $25.00 © 2003 American Chemical Society Published on Web 03/15/2003
Effect of Charge Stabilization on ET in Zeolites separation and recombination within the ion-pair exciplex 1(DMA+‚Py-•)*.14 It is found that charge transfer on silica surfaces can be well understood using the Marcus theory of photoassisted ET reactions. Surface hydroxyls are the major factor in controlling the energetics of the electron-transfer reaction in the adsorbed state. Different from the solvation effect commonly observed in the liquid solutions, where the formation of solvent-separated ion pairs competes favorably with the back electron transfer, charge separation through the adsorption of ionic species does not compete as effectively against the back electron transfer within the exciplex. Solid bulk does not play a major role here because of its rigidity and low dielectric constant. As a result, silica surfaces do not constitute favorable environments for charge separation because of the lack of solvation and the diffusion of ionic species. In this work, the same model system is used to characterize the electron-transfer reaction in zeolites. Charge-shifting reactions are also studied here in parallel to examine the fundamental impact of the ionic microenvironment on charge-transfer reactions in general. Experimental Section Sample Preparation. Sodium zeolites X (NaX, Si/Al ) 1.4) and Y (NaY, Si/Al ) 1.0) were obtained from Aldrich. The K+-form zeolites KX and KY were made through ion exchange twice with a 1.0 M aqueous solution of potassium chloride KCl (Breck, 1974). Ultrastable zeolite Y (USY) with nearly 100% SiO2 content was obtained from Dr. Xinsheng Liu of Engelhard Corporation. It is nonionic silica, but it has the same cage structure as ionic zeolite NaY. All zeolite samples were calcinated at 550 °C for 8 h before use. Self-supported disk samples were used in this work. They were dried overnight at 350 °C in an oven before being transferred into an anhydrous pentane solution in a 20-mL vial. The pentane solution was kept anhydrous by adding zeolite A pellets (activated at 350 °C) to the vial. Organic molecules such as pyrene, DMA, and biphenyl were adsorbed into the disk samples from their pentane solutions. The vial was sealed and kept in a desiccator for 8 h before the disk was finally transferred to another pentane solution in an irradiation cell. The irradiation cell was specially made by using an in situ concept to combine sample treatment and transfer, solvent removal, vacuum pumping, and irradiation processes in one apparatus to keep zeolite disk samples from readsorbing water. After an initial vacuum pumping and cryotrapping procedure to remove the air and pentane in the cell, the sample was pumped at a high vacuum of 10-3 Torr for 3 h to remove the residual pentane completely. No significant loss of adsorbates was observed by UV-vis absorption spectroscopy during the pumping process . Fluorescence Spectroscopy. Both steady-state and timeresolved fluorescence spectroscopy are used to examine the polarity of the inner surface environment and the possible aggregation of probe molecules in zeolites. Steady-state fluorescence spectra are taken on an SLM-5000C spectrofluorimeter. The fluorescence quantum yields of pyrene and the pyreneDMA exciplex at different DMA coverages were measured by using the pyrene emission as a standard. The fluorescence yields of pyrene on silica gel surfaces were taken as 0.51 and 0.56 for pretreatment at 150 and 500 °C, respectively.15 Fluorescence lifetimes of the exciplex and fluorescence quenching kinetics of pyrene in zeolites were measured by using a PRA nitrogen laser (0.13-ns pulse width) as the excitation source and a Hamamatsu microchannel plate PMT (R1644U) as the detector. Under the conditions used for the fluorescence measurement, no scattered light could be detected.
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Figure 1. Steady-state measurements of pyrene fluorescence quenching by DMA in zeolite KX. [Py] ) 1.3 × 10-7 mol/g; [DMA] ) 0, 1.0 × 10-5, 2.0 × 10-5, and 4.0 × 10-5 mol/g.
Transient Absorption Spectroscopy. A nitrogen laser (Photonics UV-24) with a pulse width of 8 ns and an output of 5 mJ at 337 nm and a high-energy electron pulse (0.4 MeV, pulse width ∼2 ns, and dose ∼200 krad per pulse) from a febetron 706 (field emission) were used as the excitation sources in laser photolysis and pulse radiolysis, respectively. An intense flash generated from pulsing a hot Xe arc lamp (450W, Oriel), which is arranged in a cross configuration with the laser and electron beam, was used to analyze the transient species in diffuse transmittance mode. The irradiation cell has a 180-µm-thick quartz window that allows the electron beam to penetrate and reach the zeolite samples and also provides a wide spectral range from 300 to 800 nm. The current method enables transient measurement down to the nanosecond time scale, which is much faster than what the diffuse reflectance technique can typically achieve. Transient absorption spectra are taken by assembling and normalizing the measurement at each wavelength. Details of the optical and electronic setup have been published elsewhere.16 Results and Discussion I. Quenching of Pyrene Fluorescence by DMA in the Adsorbed State. Previous studies have shown that the aggregation of pyrene occurs at a loading larger than 1.0 × 10-5 mol/g in dehydrated zeolites X and Y, which is clearly indicated by the fluorescence and excitation spectra of the pyrene excimer resulting from the formation of the ground-state dimer in a supercage. This means that less than 20% of the supercages are occupied by pyrene before aggregation occurs. DMA shows a much more efficient adsorption than pyrene because of its smaller size and stronger interaction through the electronegative nitrogen atom. Slight discoloration was observed during the adsorption of DMA from a pentane solution into the sodium forms of zeolites X (NaX) and Y (NaY). The potassium form of zeolite X (KX) was used in this work to reduce the zeolite acidity and its reactivity toward DMA in the presence of air and room light during sample preparation. Fluorescence measurements were made for a series of zeolite KX samples loaded with the same amount of pyrene (1.3 × 10-7 mol/g) and different amounts of DMA. Figure 1 shows that the pyrene fluorescence is reduced with increased loading of DMA in KX. The loss of fluorescence fine structure due to spectral broadening suggests that pyrene molecules adsorbed in the zeolite supercages experience a highly inhomogeneous
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Zhang and Thomas
environment, which leads to a distribution of energies for an electronic state. Such inhomogeneous spectral broadening has been found in other solid systems such as glasses, polymers, and surfaces. The much-enhanced band at 373 nm (band I) relative to the one at 382 nm (band III) is indicative of a much stronger interaction between pyrene and zeolite than what we conventionally see in liquids and on silica surfaces. Smaller amounts of spectral broadening and band enhancement were observed in zeolites NaX and NaY. It appears that the size of the cation plays an important role in the ion-molecule interaction between zeolites and pyrene, a point that is well illustrated by Ramamurthy and others.17a,b This type of ion-molecule interaction is different from the pure molecular interaction in liquid solutions and the hydrogen bonding in alcohols and on silica surfaces. The electric field that pyrene feels in a supercage can be as high as 104 V/cm. Coadsorption of DMA results in the suppression of the pyrene fluorescence emission. However, no exciplex emission was observed over the entire spectral range within the sensitivity of the spectrometer, even when the pyrene fluorescence was completely quenched. The quantum yield of exciplex fluorescence Φf is estimated to be less than 0.001. The I0/I versus [DMA] plot with the intensity measured at 373 nm shows upward curvature and is different from the Stern-Volmer type of linear plots observed in liquid solutions. The DMA loading at 50% quenching of pyrene fluorescence is 1.5 × 10-5 mol/g, which corresponds to a volume concentration of about 15 mM. In simple liquid solutions, the DMA concentration at half quenching is around 0.2 mM. It is found from the above steady-state results that pyrene and DMA react much less efficiently in the adsorbed and confined states than in homogeneous solutions. Figure 2a illustrates the time-resolved fluorescence decay traces of pyrene measured in the same KX samples as above. It shows that the quenching is dynamic on the nanosecond time scale as fluorescence intensity at t ) 0 (i.e., the end of the pulse is constant). No static quenching is observed within the experimental error. The absence of static quenching is indicative of no aggregation between pyrene and DMA. Since the chargetransfer reaction between 1Py* and DMA is operative within only a short distance of 7-8 Å,4a it is reasonable to assume that the diffusive motion of reactants is needed for the formation of a reactive encounter. The deleterious effect of coadsobed solvent on the diffusion of guest molecules in zeolites has been noted previously.19 This is also confirmed by the strong temperature effect on the quenching kinetics (i.e., cooling dramatically slows down the reaction). Therefore, the present system with both reactants in the adsorbed state provides a model system for the study of diffusion-controlled reactions through a tortuous channel of cages. The fluorescence quenching kinetics is nonexponential in all samples. This might be attributed to the inhomogeneous nature of the DMA adsorption sites in the zeolite, as we have seen in other solid systems. A Gaussian model was successfully used to describe this type of reaction kinetics.18
∫-∞∞ exp{-exp(γx)kht} exp(-x2) dx I(t) ) I0 exp(-k0t) ∫-∞∞ exp(-x2) dx x)
(1)
( - j) ∆
(2)
∆ kT
(3)
γ)
where k0 and kh are the average decay rates of the pyrene S1
Figure 2. Time-resolved measurements of pyrene fluorescence quenching by DMA in zeolite KX. [Py] ) 1.3 × 10-7 mol/g. (a) Decay traces at different DMA loadings: [DMA] ) 0, 5.0 × 10-6, 1.0 × 10-5, 3.0 × 10-5, 5.0 × 10-5, and 1.0 × 10-4 mol/g. (b) Gaussian decay rates plotted against the DMA loading.
state in the absence and in the presence of DMA, j and ∆ are used to characterize the distribution of adsorption sites, and γ is the normalized distribution width. Nonlinear least-squares fitting of decay traces to eq 1 is also shown in Figure 2a. An average bimolecular quenching rate constant k ) 3.18 × 1012 (mol/g)-1s-1 was obtained from the linear relationship between the average decay rates and DMA loadings (Figure 2b).
kh ) k0 + k[DMA]
(4)
The diffusivity of DMA in zeolite KX is estimated to be 8.41 × 10-6 cm2/s by treating its movement through the network of supercages as a homogeneous diffusion in 3D space. It is surprising to see that DMA is quite mobile in the adsorbed state. The adsorption of pentane to a saturated level leads to a slower quenching kinetics, which is indicative of a blocking effect. The
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deleterious effect of coadsorbed solvent on the diffusion of guest molecules in zeolites has been noted previously.19 It is also important that no exciplex emission was observed in the presence of pentane. It is well established in liquid solutions that quenching of the pyrene singlet excited state by DMA occurs in an encounter via an electron-transfer interaction that leads to the formation of a CT complex between pyrene and DMA in the excited state. Such an exciplex has very high CT character in liquid solutions and also on silica surfaces. It can be well approximated by a pure contact ion pair, which is denoted as 1(Py-•DMA+•)*. Recent studies of exciplex formation from bimolecular quenching reactions and direct excitation of the ground-state CT complex found that the efficiency (denoted as R) of exciplex formation from the encounter ion pair is unity even in polar liquids and on solid surfaces (i.e., R ) 16). The same is true for the pyrene + DMA system in zeolites.
Py* + DMA f
1
Figure 3. Pyrene fluorescence quenching by DMA in zeolite USY. [Py] ) 1.0 × 10-7 mol/g; [DMA] ) 0 and 2.0 × 10-5 mol/g.
[1Py*ΛDMA] f reaction encounter R)1
[1Py-•ΛDMA+•] 98 (1Py-•DMA+•)* (5) encounter ion pair exciplex Further deactivation of the exciplex can take place in several different wayssradiatively and nonradiatively including fluorescence, internal conversion to the ground state via back electron transfer, intersystem crossing to give the pyrene triplet excited state (another type of back electron transfer), and dissociation into ionic products. These processes are clearly shown in the following kinetic scheme.
dynamic fashion and gives rise to a strong exciplex emission at 460 nm. No pyrene excimer is observed in the presence of DMA, and the exciplex has a lifetime of 19.3 ns as shown in the time-resolved fluorescence measurement. The bimolecular quenching rate constant is estimated to be 9.5 × 1012 (mol/ g)-1 s-1, which is three times as fast as the rate in zeolite KX. The fluorescence spectra of the Py-DMA exciplex on silica surfaces were also shown in Figure 3b for comparison. The spectral shift was understood in terms of photoassisted electrontransfer theory and was related to the reorganization energy of the charge-transfer reaction in the following equation.2
hcν˜ max em ) -∆G-et - λ
(9)
The total reorganization energy λ includes contributions from a vibrational reorganization λv and a solvent or media reorganization λs.
λ ) λ v + λs
The decay rate kex and the fluorescence quantum yield Φf of the exciplex can be simply defined according to the above scheme.
kex ) kf + kisc + k-et + kion Φf )
kf kex
(7) (8)
The absence of exciplex fluorescence (i.e., Φf < 0.001) indicates that fluorescence is a very minor process in the deactivation of the exciplex (i.e., kf , kex). For comparison, a nonionic zeolite sample USY was used to examine exciplex formation in the same cage structure in the absence of the metal and framework ions and the strong electric field. At a loading of pyrene as low as 1.0 × 10-7 mol/g, the sample shows excimer formation from pyrene aggregation in the ground state (Figure 3a). The fluorescence lifetime of the excimer is measured to be 63 ns. This is in agreement with the fact that the adsorption of organic molecules in ionic zeolites occurs at ionic sites via the ion-molecule interaction. The adsorption of DMA removes the pyrene fluorescence in a
(10)
In zeolite USY of pure SiO2, the driving force for the back electron transfer was determined to be -∆G-et ) 3.10 eV, and the reorganization energy was determined to be λ ) 0.40 eV using the method we established in the previous work.14 These are compared to -∆G-et ) 2.95 eV and λ ) 0.55 eV on the fully hydroxylated surface, -∆G-et ) 3.02 eV and λ ) 0.48 eV on the partially dehydroxylated silica surface, and -∆G-et ) 3.18 eV and λ ) 0.32 eV in n-hexane. If we consider that the vibrational reorganization energy λv is around 0.28 eV for the back ET within the ion pair of Py-• and DMA+•,8 then the difference between hexane and zeolite USY might be attributed to their dielectric constants. As we have shown in the previous work, the stabilization of the exciplex by 0.27 eV and 0.20 eV on the fully hydroxylated and partially hydroxylated silica surfaces, respectively, is due to the adsorption of ionic species, particularly the cation radical DMA+•, via hydrogen bonding with the surface hydroxyl groups. A much stronger ionic interaction between the exciplex and the metal cations (K+) and the framework anions (SiAlO2-) should shift the spectrum of the exciplex much further toward the red in zeolite KX, which is indicative of a higher media reorganization energy. Unfortunately, the very weak exciplex fluorescence does not allow us to estimate the reorganization energy λ and therefore λs from the spectral measurement. Nevertheless, we expect λs in ionic zeolites to be significantly larger than the solvation of the
7258 J. Phys. Chem. B, Vol. 107, No. 30, 2003
Figure 4. Pyrene and DMA ion radicals produced from the dissociation of the exciplex. (a) Transient spectrum of ionic species at 415, 465, and 490 nm. (b) Decay traces of transient species in the absence and presence of oxygen. Oxygen pressure is 100 mbar.
exciplex in methanol (i.e., λs > 0.35 eV). In other words, such a large reorganization energy can be attributed to the stabilization of the dipolelike exciplex in a strong electric field within the supercage. This makes zeolites very different from porous silica. The strong ionic interaction is also responsible for the very fast decay of the exciplex by pulling the ion complex apart (i.e., dissociation of the exciplex into separated ion pairs). The decay rate of the exciplex kex is estimated to be around 10101011 s-1. The formation of long-lived ion pairs on the picosecond time scale after initial photoexcitation was reported for MV2+ and arene complexes in zeolites.12 II. Charge Separation in Zeolite Cages. Two of the three nonradiative relaxation processes of the exciplex, namely, intersystem crossing to a triplet excited state and dissociation into free ion radicals, can be measured directly by transient absorption spectroscopy. Figure 4a shows the transient absorption spectra of a pyrene-loaded KX samples (Ta ) 350 °C) without DMA and with [DMA] ) 40 µmol/g. To avoid the twophoton ionization of pyrene, the laser beam is only moderately focused onto the zeolite samples in the diffuse transmittance setup. A higher loading of pyrene, typically 4.0 × 10-7 mol/g, is used to increase the signal/noise ratio. Three absorption bands
Zhang and Thomas are identified in the DMA-free sample with their maxima at 410, 450, and 515 nm. The two bands at 410 and 515 nm are assigned to the T1-Tn transition of the pyrene triplet state 3Py*(T1). The other weaker band at 450 nm is from Py+• produced by two-photon ionization. In the presence of DMA, two new major bands appear at 465 and 490 nm. The strong band at 490 nm is due to Py-•, and the band at 465 nm is attributed to DMA+•. The introduction of oxygen leads to dynamic quenching of 3Py* and Py-• whereas DMA+• is left untouched (Figure 4b). The quantum yield of ion formation was estimated using a comparison method and the known values of the extinction coefficients of 3Py* and Py-• from the literature. The transient absorption of 3Py* in a methanol-saturated Py + DMA/KX sample was used as a reference (i.e., Φ(3Py*) ) 0.38). In zeolite KX free of DMA, the pyrene triplet yield is 0.15 via intersystem crossing from the pyrene S1 state. The yield of intersystem crossing from the exciplex is Φisc ) 0.20 whereas the quantum yield of ion dissociation is Φion ) 0.52. Such a large ion yield is higher than those observed in polar liquids such as methanol and acetonitrile. The mechanism of charge separation in zeolites, however, is different from that in polar liquids. The ion pair in the exciplex is strongly bound by the mutual Columbic interaction. Unlike the situation in polar liquids, there are no solvent molecules available to move between the two oppositely charged radical ions to lead to the formation of solvent-separated ion pairs. The mobilities of DMA+• and Py-• are so low due to their strong interaction with the ionic adsorption sites within the supercages that only local movement within the supercage is possible on a subnanosecond time scale. The favorable factors for charge separation in liquids such as ultrafast solvation and diffusive dissociation simply do not exist in zeolite systems. A similar situation was found in silica systems where the slow ionic dissociation rates on silica surfaces are responsible for the much less efficient charge separation from the exciplex (Φion ) 0.029-0.056). However, the high ion yield in zeolite KX suggests that there is a fundamental difference between silica and zeolites. The high electric field and the strong ionic interaction helps to pull the exciplex apart inside the supercage and makes it possible for an ion pair to be stable enough to exist in close proximity. Dissociation of the exciplex driven by the strong electric field may be followed by an ion exchange between DMA+• and Na+, leading to the formation of a cageconfined ion pair in an energetically favorable configuration. The ionic interaction as described above plays a major role in charge separation and stabilization. The long-lived ionic intermediates can be used to initiate further reactions in the microcage environment. However, their reactivity via chargetransfer reactions will be significantly reduced because of charge stabilization as illustrated in the following section. III. Positive Charge-Transfer Reactions in Zeolites. In addition to the charge separation and recombination reactions described above, another category of electron-transfer reactions (i.e., charge shifting) is also examined in this study to look into the effect of ionic interaction on charge transfer further. Radical cations of biphenyl and pyrene are produced in zeolite supercages by a high-energy electron beam, and their reactions with coadsorbed DMA are monitored and compared with those in liquids and on solid surfaces. It has been well established that the ionization of cationic zeolites by high-energy irradiation leads to electron trapping by cation clusters and positive charge transfer to organic molecules adsorbed in the zeolite cavities. Our recent work
Effect of Charge Stabilization on ET in Zeolites
Figure 5. Transient absorption spectra of radical cations produced in e-beam-irradiated zeolites. (a) Biphenyl in NaX; (b) pyrene in NaX.
shows that the oxidation of adsorbed water and the subsequent formation of hydroxyl radicals are major processes in irradiated zeolites with a high radiolytic yield of GOH• ) 5.8. It was also found that electron attachment to aromatic molecules and the formation of radical anions are significant in zeolite KX because of the lower electron affinity of the potassium ion clusters. To avoid complications from the spectral overlap and the cation reaction with the radical anions, sodium forms of zeolites X and Y are chosen as the model systems in this particular study. Figure 5a shows the transient absorption spectrum of biphenyl cations produced in NaX. Three absorption bands were observed. The bands around 380 and 700 nm are assigned to biphenyl cations, and the band at 550 nm is due to Na43+, electrons trapped at Na44+ clusters. Both species are long lived with half lives >100 µs for the cations and >2 ms for the trapped electrons. In a pyrene-loaded NaX disk, the presence of a weak band at 490 nm, in addition to the pyrene cation band at 450 nm and the trapped electron Na43+ band at 550 nm, is indicative of the formation of pyrene anions (Figure 5b), but its yield is much less than what we have observed in zeolite KX. When DMA is introduced as the only adsorbate, the characteristic spectrum of DMA radical cations (λmax ) 465 nm) was observed in zeolites NaX and KX with a concomitant enhancement of the cation cluster trapped-electron signals. The yield of DMA+• increases with DMA loading until it reaches a plateau at a level of 5 × 10-5 mol/g. By reference to the oxidation of water,20 the maximum cation yield is expected to be around G ) 5-6 when all of the positive charges are captured by DMA.
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Figure 6. Decay of biphenyl cations with and without DMA in NaX and KX. (a) KX, [Ph2] ) 1.0 × 10-4 mol/g, [DMA] ) 1 × 10-5 mol/ g; (b) NaX, [Ph2] ) 1.0 × 10-4 mol/g, [DMA] ) 5 × 10-6 mol/g
Charge transfer from biphenyl and pyrene cations to DMA occurs in nonpolar liquids with a diffusion-limited rate. +• Ph+• 2 + DMA f Ph2 + DMA
(11)
Py+• + DMA f Py + DMA+•
(12)
The driving force for reaction 11 is -∆G ) 1.4 eV, and it is -∆G ) 0.7 eV for reaction 12. The reorganization energies for the above reactions in a typical nonpolar liquid are around λ ) 1.2-1.5 eV.3 This explains the fast reaction rates in nonpolar liquids, which are essentially limited by the diffusion of reactants. The rates in polar liquids, for example, EtOH (λ ) 2.5 eV) drop with increasing λ, typical of ET reactions in the normal region. The above two reactions were examined in zeolite NaX. Figure 6a shows the decay traces of biphenyl cations in zeolites KX and NaX in the absence and in the presence of DMA. Higher loading of DMA cause complications due to precursor quenching of biphenyl cations. The rate constant is estimated to be around 3.33 × 108 (mol/g)-1 s-1 in NaX. A trace amount of pyrene is also introduced to monitor the diffusion rate of DMA by the fluorescence quenching technique. No blocking effect by biphenyl is observed. Compared to the pyrene fluorescence quenching, the charge-transfer reaction is nearly 4 orders of magnitude slower than the diffusion-controlled reaction in zeolites, although the driving force is as high as -∆G ) 1.4 eV. The reaction is inhomogeneous in nature, as evidenced by a significant number of bipenyl cations left after 0.4 ms in NaX. In the case of pyrene as the positive charge donor, virtually
7260 J. Phys. Chem. B, Vol. 107, No. 30, 2003 no enhanced decay of pyrene cations was seen in the presence of DMA. Similar charge-transfer reactions have been studied on silica surfaces with anthracene as the donor and DMA as the acceptor. It was found that the reaction is limited by the diffusion of DMA across the silica surface and that rate constant is 1.5 × 1010 (mol/g)-1 s-1. Our previous work showed that the silica surfaces have a reorganization energy of 0.48-0.55 eV and that its effect on the electron-transfer reaction is mainly due to hydrogen bonding at the hydroxyl sites.14 The situation is very different in zeolites where the high electric field in the supercage and the stabilization through ion exchange result in a much higher reorganization energy that pushes the ET reaction further toward the lower end of the normal region. As a result, biphenyl and pyrene cations are very stable and are unlikely to reach the configuration required for electron transfer to occur. Conclusions Zeolites X and Y provide an ideal medium for charge separation and stabilization in photoinduced electron transfer reactions, in particular, in the pyrene-DMA system. The strong electric field inside the zeolite supercage facilitates the dissociation of the exciplex into ion pairs while suppressing the back electron transfer. The ionic interaction including ion exchange helps to stabilize the ion radicals and dramatically slow the electron transfer in charge recombination and chargeshifting reactions. Steric factors may also contribute to the charge stabilization, but the present studies cannot separate this effect from the consideration of energetics. Acknowledgment. We thank the National Science Foundation and the University of Notre Dame for their financial support of this work. References and Notes (1) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (b) Electron Transfer in Inorganic, Organic, and Biological Systems; Boton, J. R., Mataga, N., McLendon, G., Eds.; Advances in Chemistry Series 228; American Chemical Society: Washington, DC, 1991. (2) (a) Marcus, R. A. J. Phys. Chem. 1989, 93, 3078. (b) Marcus, R. A.; Siddarth, P. In Photoprocesses in Transition Metal Complexes, Biosystems and Other Molecules: Experiment and Theory; Kochanski, E.,
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