Long Lived Charge Separated States Induced by trans-Stilbene

10280

J. Phys. Chem. C 2010, 114, 10280–10290

Long Lived Charge Separated States Induced by trans-Stilbene Incorporation in the Pores of Brønsted Acidic HZSM-5 Zeolites: Effect of Gallium on the Spontaneous Ionization Process Alain Moissette,*,† Raul F. Lobo,*,‡ Herve´ Vezin,† Khalid A. Al-Majnouni,‡ and Claude Bre´mard† Laboratoire de Spectrochimie Infrarouge et Raman, UMR-CNRS 8516, Baˆt. C5, UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq cedex, France, and Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: April 28, 2010

In situ CW-EPR, diffuse reflectance UV-visible spectroscopy and Raman scattering were used to monitor the spontaneous incorporation of trans-stilbene (t-St, C14H12) in the medium pore H2.2-GaZSM-5 zeolites [H2.2(GaO2)2.2(SiO2)93.8] by direct exposure under dry and inert atmosphere of solid t-St to dehydrated porous material without any solvent. The sorption of t-St with relatively low ionization potential (7.65 eV) occurs in Brønsted acidic H2.2-GaZSM-5 zeolites according to a complex and slow reaction sequence. First, charge separation occurs and t-St•[email protected]•- radical pair is created, while long-lived [email protected]•-•+ electron-hole pair is formed through hole transfer. The analysis of the DRUVv spectra set recorded during the t-St sorption course shows the respective concentrations of all transient species as a function of time. In particular, note that system reorganization is observed through a second type of electron-hole pair. The broad and strong bands observed in the near-IR regions over extended periods of time are tentatively assigned to the electron and/or hole spectral signatures in slightly different environments. Applying pulsed X-band EPR techniques, we were able to reveal the structural surrounding of the unpaired electrons of chargeseparated states through the proper assignment of electron couplings with a large number of nuclei such as 1 H, 29Si, 69Ga, and 71Ga using the two-dimensional hyperfine-sublevel correlation experiment (2D-HYSCORE). The distance measurements deduced from dipolar coupling experiments provide a unique picture of the long distance distribution of unpaired electrons generated by spontaneous ionization of t-St upon incorporation within H2.2-GaZSM-5 zeolite. This result demonstrates that a large fraction of the unpaired electrons are ejected away from the initial site of ionization and that this compartmentalization plus the created electrostatic field hinder dramatically the propensity of charge recombination. The results for H-GaZSM-5 are compared to similar experiments conducted on H-AlZSM-5 zeolites. 1. Introduction Electron transfer processes at interfaces are central to many important chemical processes and are vital to the operation of energy conversion and storage technologies such as photovoltaic devices, batteries, fuel cells, water splitting, photocatalysis, and many others. Detailed mechanistic investigations of electron transfer reactions at interfaces are often difficult to undertake because of the heterogeneity of the surfaces and because the time-scales for the formation and decay of radical intermediates is short. Intermediates are generally highly reactive and difficult to isolate for extended periods of time to allow detailed characterization. For this reason, electron transfer processes are often investigated in frozen rare gas or halocarbon matrices that stabilize the intermediates and slow their decay such that the samples can be readily studied using a variety of techniques. Zeolites and other porous solids are also often used as hosts for electron-transfer processes because they serve to stabilize radical cations for extended periods of time.1 In a sense, the zeolite acts as a solvent for the organic moieties not only stabilizing intermediates but also stopping detrimental recom* To whom correspondence should be addressed. E-mail: alain.moissette@ univ.lille1.fr (A.M); E-mail address: [email protected] (R.F.L). † UMR-CNRS 8516. ‡ University of Delaware.

bination and coupling reactions. Their pore intersections serve as microreactors where intermolecular reactions are blocked from proceeding to completion.2 Electron transfer from adsorbed molecules to the zeolite framework can be produced through four different processes: (1) Adsorption of molecules with high ionization potentials after heating the acid zeolites to high temperatures. This heating to high temperatures (500 °C or above in inert gas or oxygen) leads to the generation of framework electron traps that can easily remove electrons from many organic molecules.3 (2) Ionizing radiation was used on zeolite-adsorbate complexes to form radical cations and trapped electrons.2 (3) Photogeneration of radical species preadsorbed into the zeolite pores. (4) Spontaneous ionization of molecules with low ionization potentials in dehydrated zeolites that have not been preactivated using heat or an external source of radiation.4,5 All these processes have been studied to various degrees of detail to elucidate the chemistry and mechanisms of electron transfer and the structure and reactivity of intermediate species formed after the initial electron transfer process (see Garcia and Roth for a review of the literature up to 2002).6 Spontaneous ionization is the least studied of these classes of electron transfer processes being discovered recently in the acid form of zeolite H-AlZSM-5.4 Among the studies carried out using molecules

10.1021/jp103838b  2010 American Chemical Society Published on Web 05/17/2010

Charge Separated States Induced by t-Stilbene with low ionization potential (I.P.), we described trans-stilbene (t-St, I.P. ) 7.65 eV) spontaneous ionization and subsequent electron transfer within H-AlZSM-5.7 The sorption of rodshaped t-St occurs in Brønsted acidic H-AlZSM-5 according to a complex and slow reaction sequence, including charge separation and electron transfer, while it is incorporated as an intact molecule in nonacidic Na-AlZSM-5.8 The presence of the acid proton (H+) and the framework aluminum greatly promotes the spontaneous ionization, in addition to the highly polarizable cavity and tight fit offered by the pore structure of the ZSM-5 zeolite. It should be noted that other forms of M+AlZSM-5 (M+ ) alkali-metal cation) can be effective to induce spontaneous electron transfer of low I.P. molecules such as anthracene.9 A number of trivalent cations can be easily incorporated into the MFI-type framework to form acid sites that are structurally analogous to the acid sites formed by Al in ZSM-5. Examples are BIII, GaIII, and FeIII,10,11 all of which have been incorporated in the MFI as well as in a number of other zeolite frameworks. It is of scientific interest to establish if the spontaneous ionization observed in H-AlZSM-5 also occurs with other trivalent metal atom substitutions. GaZSM-5 is experimentally the most accessible system because it has acidity slightly lower (but comparable)12 to H-AlZSM-5. H-BZSM-5 is weakly acidic11 and is less likely to lead to spontaneous ionization than GaZSM5. FeZSM-5, on the other hand, although of similar acidity than GaZSM-5, brings the experimental complications of a strong EPR signal and strong absorption in the UV range of the optical spectrum. The isomorphous substitution of Ga for Si in ZSM-5 and other zeolite structures have been known for decades.11 Investigations using X-ray spectroscopy have shown that Ga is in tetrahedral coordination in the framework.13 Other investigations using microcalorimetry have shown that the heat of adsorption of strong gas-phase bases (NH3, pyridine, etc.) with the acid sites in H-GaZSM-5 is quite similar to the heat of adsorption in H-AlZSM-5.14 A few quantum chemical investigations of Ga in zeolite frameworks15 have shown that the structure of the bridging acid site (Ga-OH-Si) is quite similar to the Brønsted acid site in H-AlZSM-5 except for a slightly longer Ga-O bond length. One important difference is that gallium is less stable in the framework of the zeolite than aluminum and care must be taken not to form extra-framework gallium species that can lead, on their own, to complex redox chemistry with adsorbed molecules. Aiming toward a detailed mechanistic understanding of the redox behavior of polyaromatics upon sorption, we have employed an in situ diffuse reflectance UV-visible absorption (DRUVv), Raman spectrometry, and X-band continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy to monitor the sorption of the rod-shaped t-St molecule (t-St) in Brønsted acidic H2.2-GaZSM-5 zeolite over extended periods of time. Applying pulsed X-band EPR techniques, we were able to reveal the structural surrounding of the unpaired electrons of charge-separated states through the proper assignment of electron couplings with a large number of nuclei using the twodimensional hyperfine-sublevel correlation experiment (2DHYSCORE). t-St is used to probe the polarizing power of the H-GaZSM-5 for spontaneous ionization upon sorption and subsequent electron transfer and charge transfer complexes. The results for H-GaZSM-5 are compared to similar experiments conducted on H-AlZSM-5 zeolites.

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10281 2. Experimental Section 2.1. Materials. NH4-GaZSM5 was synthesized using a gel with the following molar oxide composition: 4.5 (TPA)2O/ Ga2O3/40 SiO2/1500 H2O. The precursors are tetraethylorthosilicate (TEOS, reagent grade 98% from Sigma-Aldrich) as the silica source, gallium nitrate hydrate (99.99% metal basis, Strem) as the gallium source, and tetrapropyl ammonium hydroxide (TPAOH 35 wt % from Alfa-Aesar) as the structure director. The silica solution is prepared by hydrolyzing the TEOS with 60% of the water and all the TPAOH in a Teflon container. A gallium solution is prepared by mixing the gallium nitrate with the remaining water in a separate container. After TEOS hydrolysis and obtaining a clear solution, the silica solution is mixed with the gallium solution. The final solution is also optically transparent. The hydrothermal synthesis was carried out at 135 °C for 9 days under static condition on Teflon-lined Parr autoclaves. The solid product is separated from the solution by centrifugation and was washed with doubly deionized water three times. The final solid is dried overnight at 80 °C. The GaZSM5 in the as-synthesized form is heated to 550 °C with a ramping rate of 3 °C/min and held at this temperature for 6 h to remove the occluded organic structure director. The calcined samples are ion-exchanged twice using 0.5 M NH4(NO3) solutions (one gram of the zeolite in 500 mL solution). The NH4-GaZSM-5 is transformed into the acid form (H-GaZSM5) by heating in argon flow (see below). A sample of silicalite-1 synthesized as before but without the addition of gallium nitrate was prepared for comparison to GaZSM5. t-St (C14H12, 99%, Merck-Schuchardt) was purified by sublimation and stored over molecular sieves. Pure and dry Ar gas was used. Prior to sorption, weighed amounts (∼1.4 g) of hydrated zeolite were dehydrated by a new calcination procedure up to 723 K under argon. 2.2. t-St Sorption in H2.2(GaO2)2.2(SiO2)93.8. The dehydrated H2.2(GaO2)2.2(SiO2)93.8 (hereafter denoted H2.2-GaZSM-5) sample was heated up to 673 K under Ar. Then, the sample was cooled to room temperature under dry argon. Weighed amounts of t-St corresponding to 1 t-St/UC were introduced into the cell under dry argon and the powder mixture was shaken. The powders were transferred under dry argon in a quartz glass Suprasil cell for Raman and DRUVv experiments and in cylindrical quartz tube for EPR experiments and sealed. 2.3. Instrumentation. The zeolite product was characterized by X-ray powder diffraction (XRD) using a Philips X-pert powder diffractometer. The radiation source is Cu KR, generated at 45 kV and 40 mA. The diffraction scans were measured from 5 to 50 °2Θ with a 0.02° step and 2 s count time per step. The dimensions of the unit cell were determined using Si as internal standard and using the program CELREF. The scanning electron micrographs (SEM) were obtained using a Jeol JSM-7400 instrument. The N2 adsorption isotherms were measured using a Micromeritics ASAP 2010 analyzer. The pore volume is calculated using the t-plot method. The FTIR spectra were collected using Nicolet Nexus 470 FTIR with an in-house-built vacuum cell equipped with calcium fluoride windows. The [NH4+]Ga-ZSM5 sample pressed into a self-supported wafer, is heated in situ under vacuum to 550 °C to remove NH3 and convert the zeolite into H+ form. The spectrum, average of 100 scans, is taken after the sample is cooled to room temperature under vacuum. The Ga K-edge X-ray absorption fine structure (XAFS) measurements were performed in Beamlines 18XB at Brookhaven National Lab. The ring energy is 2.8 GeV. The required mass of NH4+ zeolite to give absorption length of 2 was pressed into

10282

J. Phys. Chem. C, Vol. 114, No. 22, 2010

Moissette et al.

a circular disk with a diameter of 13 mm. The measurement is carried out in transmission mode in which the X-ray beam passed through three detectors. The first two detectors are to measure the incident and the attenuated X-ray from the sample and the third detector is to measure the absorption edge from Tungsten reference for energy alignment. We fit the first shell using β-Ga2O3 as a reference model because it has both octahedral and tetrahedral structures in equal proportion (1:1). Diffuse Reflectance UV-Visible Absorption (DRUVW). The UV-visible absorption spectra of the powdered samples were recorded between 200 and 1800 nm using Cary 6000i spectrometer. The instrument was equipped with an external integrating sphere (DRA-1800) to study the powder zeolite samples through diffuse reflectance. The DRUVv spectra were plotted as the Kubelka-Munk function:

F(R) ) (1 - R)2 /2R ) K/Sc

(1)

where R represents the ratio of the diffuse reflectance of the loaded zeolite to that of the dehydrated neat zeolite, K designates an absorption coefficient proportional to the concentration C of the chromophore, and Sc is the scattering coefficient of the powder. Spectral sets F(λ, t) were recorded as a function of λ (wavelength) at several t (time) during the course of the t-St sorption and after photoionization. Raman Scattering Spectroscopy. A Bruker RFS 100/S instrument was used as a near-IR FT-Raman spectrometer with a CW Nd:YAG laser at 1064 nm as the excitation source. A laser power of 100-200 mW was used. The spectra (3500-150 cm-1) were recorded with a resolution of 2 cm-1 using 600 scans. The Raman spectra obtained by using the 785 nm exciting line were collected on a Kaiser spectrometer equipped with a Peltier-cooled charge coupled device detector. The laser line was supplied by a laser diode. A LabRAM spectrometer (Jobin Yvon Horiba Gr.) equipped with a liquid-nitrogen-cooled charge-coupled device detector was also used with excitation lines at 488, 514.5, and 632.8 nm in the visible spectral domain. The laser radiations were supplied by an argon ion laser and a helium-neon laser with low laser power to avoid irreversible laser effects on the sample. A 50× Ultra Long Work Distance (Olympus ULWD) objective was used to excite and to collect the Raman scattering of the sample within the cell. The laser power delivered was 8 mW and could be monitored via a filter wheel with optical densities 0.3, 0.6, and 1-4. X-Band EPR Spectroscopy. The CW X-band EPR and pulsed EPR spectra were recorded between room temperature and 4.2 K on a Bruker ELEXYS 580-FT spectrometer (∼9 GHz). EPR echo detected spectra were achieved using two pulse spin-echo sequences. A 12 ns π/2 pulse width allowed sufficient wide band coverage. The HYSCORE measurements were carried out with the sequence π/2-τ-π/2-t1-π-t2-π/2-τ echo, and a four-step phase cycle. The pulse lengths of the π/2 and π pulses in these experiments were 16 and 32 ns, respectively. The HYSCORE experiments were recorded with τ value of 350 ns to avoid the “blind spots” effect. Prior to Fourier transformation of the HYSCORE data the background decay was removed by a second order polynomial fit and apodized with a Hamming function. Magnitude spectra calculation is performed after 2D Fourier transformation. Distance measurements were performed with the Single Frequency Technique for Refocusing Dipolar Coupling (SIFTER) experiments. The four-pulse SIFTER experiment was achieved

with π/2 and π pulse length values of 24 and 34 ns, respectively. Interpulse delays τ1 and τ2 were incremented by steps of 16 ns with τ2-τ1 ) 32 ns for dipolar measurement. A 16-step phase cycling scheme was used according to Jeschke et al.16 Signal processing for SIFTER and DQC experiments was achieved using the Deer Analysis 2008 Program.17 The unmodulated background decay due to homogeneous spin distribution was removed for SIFTER experiments using the homogeneous model according to the equation B(t) ) exp(-ktd/3), with k quantifying the number of spins and d is a dimensionality factor. A value of d ) 6, corresponding to a Gaussian background, was applied in the SIFTER experiment. The distribution of dipolar interactions was extracted from the signal by using the Tikhonov regularization method algorithm. The regularization parameter R was set to 10. This value appears a good compromise in our case as a small R parameter is generally used for samples with well-defined distances. Alternatively, higher values of R induce a large broadening of distance distributions. 3. Results The XRD patterns of the as-synthesized and ammoniumexchanged samples of Ga-ZSM-5 indicate that the samples are highly crystalline ZSM-5. No evidence of amorphous background or Ga2O3 phases was observed in the diffractograms. The unit cell volume of Ga-ZSM5 and silicalite-1 were 5370 and 5322 Å3, respectively. This unit cell volume expansion of 0.9% is as expected since the Ga-O bond length is larger than the Si-O bond length and proves that there is Ga in the framework of the GaZSM5 sample. The SEM images of the as-synthesized GaZSM-5 show that the sample is formed of 0.2-0.8 µm spherical aggregates of smaller crystals. EDS analysis of the particles reveal a Si/Ga ratio of 29 ( 2, similar to the composition of the synthesis gel. The nitrogen adsorption isotherm of the calcined Ga-ZSM5 sample is a typical type I isotherm. However, at high relative pressure (above 0.78) condensation of N2 occurs. The absence of a hysteresis loop indicates that Ga-ZSM5 has no mesopores and that the condensation occurs in the external pore space that resulted from the aggregation of small crystals as can be seen from the SEM images. The pore volume calculated from t-plot method is 0.185 cm3/g. XRD patterns, SEM images, and the N2 isotherms of the samples are provided in the Supporting Information. The infrared absorption spectrum of dehydrated H-GaZSM-5 zeolite exhibits absorption bands at 3747 and 3615 cm-1 assigned to the OH stretching modes corresponding to terminal silanol and Brønsted acidic sites. In addition, the absence of OH vibration at 3680 cm-1 indicates that there is no extra framework gallium (the IR spectrum is provided in the Supporting Information). The Fourier transform EXAFS magnitude is shown in Figure S5 of the Supporting Information. There is no evidence for the presence of microcrystalline Ga2O3 in these data. The fitted EXAFS region indicates that the Ga is in tetrahedral coordination with a Ga-O bond length of 1.8 Å in agreement with previously reported values.13,18 The detailed measurement protocol, data reduction, and fitting procedure will be reported in a forthcoming publication.19 The aggregate data from unit cell measurements, adsorption isotherms, IR spectra, and EXAFS all point to the presence of only one form of Ga in the samples, in the framework of the zeolite, and in tetrahedral coordination. EPR spectroscopic investigations do not indicate any iron impurities. Powdered dehydrated H2.2-GaZSM-5 was exposed in the dark, under argon, and at room temperature to a weighed amount of

Charge Separated States Induced by t-Stilbene dry t-St (C14H12) corresponding to one t-St/UC loading. The entrances of the 10-membered oxygen ring straight channels of ZSM-5 are sufficiently wide to allow t-St diffusion into the void space. DRUVv Absorption Spectroscopy. The t-St incorporation into dehydrated H2.2(GaO2)2.2(SiO2)93.8 (one t-St per unit cell) was followed by diffuse reflectance UV-visible spectroscopy. The DRUVv spectrum of bare H2.2-GaZSM-5 dehydrated at 723 K under argon shows two peaks at 1370 and 1396 nm. The 1370 nm (7305 cm-1) peak is assigned to the first overtone frequency of free SiOH silanol group vibrations,20-22 whereas the band observed at 1396 nm (7163 cm-1) is tentatively assigned to the overtone of Si-OH-Ga bridging zeolitic groups.23,24 A series of the spectra recorded for a period of 6 months after the t-St/H-GaZSM-5 contact are presented in Figure 1. When the calculated quantity of white solid t-St was exposed to white dehydrated H2.2-GaZSM-5 powdered sample in the dark, the sample became blue. The broad UV absorption centered at 305 nm increasing continuously in intensity, shows the gradual sorption process. In addition to the 305 nm band, new bands centered at 472 nm and around 700-800 nm were observed immediately after the contact (Figure 1A). These spectral features are assigned to t-St•+ based on previous studies relating to t-St spontaneous ionization within H3.4(AlO2)3.4(SiO2)92.6 ZSM-57 and t-St photoionization on solid supports (silica, alumina),25 occluded within silicalite-1 (aluminum free ZSM-5),26 Na-AlZSM-5,8 and Na85(AlO2)85(SiO2)107 X faujasite zeolite where trapped electron was observed as a Na43+ cluster.25 The maximum intensity for the 472 nm line was reached after one week (Figure 2). One day after mixing, additional weak bands were observed at 567 and 624 nm. The intensity of these bands increased considerably with time and reached a maximum after about 1 month, whereas the main radical cation feature at 472 nm was not observed anymore after 2 weeks (Figures 1B and 2). This spectrum is very similar to those obtained after spontaneous and photoionization of t-St and analogous compounds in ZSM-5 zeolites.4,7,8 The spectral features were assigned to a charge transfer band due to hole transfer from the radical cation to the zeolite framework after ionization.27 Despite the proximity of the ejected electron, it is highly probable that the disappearance of the radical cation is related to the electron donor ability of ZSM-5 framework toward the electron deficient radical cation rather than direct charge recombination. Therefore, the spectrum is attributed to the charge transfer band associated with the electron-hole pair [email protected]•-•+ moiety. These prominent absorption bands in the visible region at 567 and 624 nm are tentatively assigned to an interfacial charge transfer band involving the molecule and hole of zeolite. The exact nature of the charge transfer band has not been established in this case. Nevertheless, we have demonstrated previously that absorption of light within the charge transfer band of electronstilbene-hole results in interfacial hole back transfer from zeolite to sorbate to restore the radical cation/electron pair.8 Moreover, note that the photoinduced electron-hole pair in germanium containing ITQ-17 zeolite exhibits analogous absorption in the visible domain.28 Furthermore, examination of the charge transfer band provides evidence of vibrational progressions spaced by about 1610 cm-1 between the two main bands at 567 and 624 nm. This value undoubtedly involves vibration of the molecule at the excited state and demonstrates the involvement of t-St in the electron-hole moiety. The wavelength and the intensity of the 1370 nm line corresponding to silanol groups remain constant upon sorption

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10283

Figure 1. Diffuse reflectance UV-visible absorption spectra (DRUVv) recorded at room temperature as a function of time during the course of t-St sorption into dehydrated acidic H2.2(GaO2)2.2(SiO2)93.8 zeolite. Solid t-St (1 t-St/UC loading) and H2.2-GaZSM-5 dehydrated at 673 K were mixed under Ar. (A) DRUVv spectra recorded from 5 min (spectrum a) to 5 days (spectrum b). (B) DRUVv spectra recorded from 5 days (spectrum b) to 30 days (spectrum c). (C) DRUVv spectra recorded from 42 days (spectrum d) to 3 months (spectrum e) and to 6 months (spectrum f). The spectra presented in C are vertically shifted for better clarity.

and ionization whereas the 1396 nm shift slightly toward higher wavelengths with time. This feature might be due to the interaction between the occluded molecule and/or trapped

10284

J. Phys. Chem. C, Vol. 114, No. 22, 2010

Figure 2. Diffuse reflectance UV-visible intensities measured for each transient species as a function of time after the mixing of solid t-St and dehydrated H2.2-GaZSM-5 zeolite. The measures are carried out for typical wavelength characteristic of the individual species. (a; circle) t-St•[email protected]•- (λ ) 472 nm); (b; square) [email protected]•-•+ (λ ) 624 nm); (c; diamond) second transient charge separated state ([email protected]•-•+) (λ ) 730 nm).

electrons in the Si-OH-Ga environment and to vibrational perturbation of Si-OH bond motions of the Si-OH-Ga bridging groups. It might be also correlated to the band intensity increase at 305 nm and so, to the sorption process. In addition to these bands, the spectra recorded for 5 days after the contact clearly show that a broad band develops in the infrared region at about 1450-1550 nm (Figure 1A). This broad band undoubtedly grows in parallel to the radical cation feature observed at 472 nm. The radical cation formation is indeed accompanied by the ejection of an electron and therefore, the electron spectral signature is expected to increase simultaneously to the radical cation absorption. However, the ejected electron is assumed to be trapped far enough from the radical cation to avoid fast direct recombination. Moreover, this broad band centered at 1500 nm is also very similar to the band previously reported for nearly free electrons trapped in zeolites after inorganic electride formation by metal addition to pure silica zeolites.29 The experimental data reported by Wernette et al suggest partial or complete ionization within the channels to yield M+ ions and relatively free electrons; the optical spectra of these systems exhibit broad absorption in the visible and in the near IR spectral domains. The near-IR peak was attributed to nearly free electrons. By analogy with these data, the broad feature observed in the 1450-1550 nm regions is tentatively assigned to the ejected electron. After 5 days and until 30 days, the spectra presented in Figure 1B show the considerable broadening of the near-infrared band that extends over 500 nm from 1100 to 1600 nm. The increase of this large band appears to be correlated to the increase of the charge transfer band in the visible region (Figure 2). Whereas the ejected electron can migrate in the system, the positive hole is assumed to be located in the proximity of t-St and its spectral signature is probably centered at about 1200 nm. After 45 days, a new band was clearly observed at 730 nm (Figures 1C and 2). This peak developed at the expense of the 500-650 nm charge transfer band and of the broad feature centered at 1460 nm. After six months, the 567 and 624 nm bands were very weak (Figure 1C). Moreover, note that the 730 nm band increased along with another contribution in the nearinfrared region at about 1305 nm. To our knowledge, this

Moissette et al.

Figure 3. FT-Raman spectra (λex ) 1064 nm) recorded as a function of time after the mixing of solid t-St and dehydrated H2.2-GaZSM-5: (a) solid t-St; (b) 26 days after mixing; (c) 6 months after mixing.

spectrum does not correspond to any previously reported spectrum of t-St related compounds. These features might be assigned to a partial recombination and to a possible charge separated state reorganization. The 1350 nm broad band might be also due to a hole/electron but in a slightly different environment. Therefore, these new features are assigned to electron and hole migration to create another type of electronhole pair (type I f type II). In the course of the reaction, no evidence was found for (πSt)2•+ or (σ-St)2•+ dimer cation radical or cis-St•+ isomer or t-St•anion formation in the zeolite channel, contrary to what is observed after t-St radiolysis in solution and solvent matrix.30,31 The narrow channels of the ZSM-5 zeolite are expected to hinder t-St isomerization and possibly block dimer cation radical formation.32,33 Raman Spectroscopy. The off-resonance FT-Raman spectrum recorded 1 month after mixing using the 1064 nm excitation wavelength (Figure 3b) clearly shows intense bands centered at 1604, 1589, 1328, 1192, and 997 cm-1. This spectrum markedly differs from the spectra of solid t-St (Figure 3a) and of t-St occluded in silicalite-1 or in MnZSM-5 that display two prominent bands at 1639 and 1594 cm-1.8 The 1639 cm-1 t-St contribution corresponding to the CdC central stretching is not observed anymore after 15 days. Because the 1064 nm exciting line does not induce any intensity exaltation by resonance effects, the peak relative intensities are assumed to be representative of the system composition. Therefore, the absence of the 1639 cm-1 line characteristic of occluded t-St molecule indicates that the ionization fraction should be high. Nevertheless, note that possible high scattering cross section values for the Raman bands corresponding to the charge transfer complex might hide the t-St contribution. This spectrum was observed previously after t-St spontaneous ionization in H3.4(AlO2)3.4(SiO2)92.6 (ZSM-5) and t-St photoionization in Na6.6(AlO2)6.6(SiO2)89.4. The five Raman bands are assigned to the [email protected]•+•- moiety. This moiety is constituted by two unpaired electrons and by the neutral molecule. The Raman features correspond to the vibrational modes of the molecule in this specific environment. The high local electrostatic field created by the surrounding charges induces frequency shifts and intensity changes with respect to the spectrum of the nonionized occluded molecule. The proper assignment needs to be done using theoretical calculations by

Charge Separated States Induced by t-Stilbene

Figure 4. Raman spectra recorded as a function of the excitation line wavelength after the mixing of solid t-St and dehydrated H2.2-GaZSM5: (a) 26 days after mixing (λex ) 1064 nm); (b) 26 days after mixing (λex ) 632.8 nm); (c) 26 days after mixing (λex ) 785 nm); (d) 40 days after mixing (λex ) 488 nm); (e) 40 days after mixing (λex ) 514.5 nm).

simulating the correct environment although currently, such methodology is not yet available. However, all the vibrational modes that are observed likely correspond to the t-St molecule in a specific environment: the 1328, 1192, and 997 cm-1 are very close to the frequencies of the isolated molecule at 1339 cm-1 (aromatic C-C stretching + CH in plane bending), 1197 cm-1 (aromatic CCC bending), and 999 cm-1 (aromatic CC stretching).34,35 The broad 1604 cm-1 band certainly includes the contributions of both ring stretching vibrations and of the CdC central stretching. It is important to note that the t-St•+ vibrational features were never observed during the course of the sorption even in the early stages of reaction. This result shows the remarkable transient behavior of this species as it evolves rapidly into the electron-hole moiety. After 6 months, the FT Raman spectra exhibit some additional changes (Figure 3c). In particular, Raman shifts are noticed for the 1328, 1589, and 1605 cm-1 lines that are observed at 1339, 1603, and 1625 cm-1, respectively. This might indicate either possible system reorganization or partial recombination as was inferred from the UV/vis spectra. Figure 4b,c show the Raman spectra recorded after 1 month using the 632 and 785 nm exciting lines. These spectra exhibit high analogy in terms of position with the FT-Raman spectrum obtained also after one month (Figure 4a) but show some differences in terms of relative intensities. For these three exciting lines (i.e., 632.8, 785, and 1064 nm), the central CdC stretching vibration initially observed at 1639 cm-1 cannot be distinguished among the ring stretching contributions expected in the 1500-1600 cm-1 region. On the other hand, the Raman spectra recorded using the 488 and 514 nm exciting lines 40 days after the mixing present quite different features that are mainly prominent peaks at 1285 and 1603 cm-1 (Figure 4d,e). These peaks are straightforwardly assigned to t-St•+.36,37 The 488, 514, and 632 nm and to a lesser extent the 785 nm exciting line wavelengths fall within the contour of the broad charge transfer band. As discussed above, we have demonstrated that absorption of light within the charge transfer band of the electron-t-St-hole unit results in interfacial hole back transfer from zeolite to the sorbate to restore radical cation-electron

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10285

Figure 5. Two pulse echo field sweep spectrum of t-St•[email protected]•- recorded at 4.2 K, 1 day after the mixing of solid t-St and dehydrated H2.2-GaZSM-5 (one t-St per UC loading).

pairs.8 Moreover, note that because the 488 and 514 nm exciting lines fall within the contour of the electronic transition of t-St•+ (λmax ) 472 nm), significant enhancement of the Raman lines of this chromophore occurs through resonance effect. The use of the 785 nm exciting line also allows the observation of the 1285 cm-1 radical cation peak in addition to the electron-hole lines (Figure 4c). This feature can be explained by the resonance effect with respect to the t-St•+ broad absorption band observed between 700 and 800 nm. When using the 632 nm line, such resonance phenomenon does not occur, and therefore, the radical cation cannot be observed. Note that direct t-St•+ formation generated through occluded t-St molecule photoionization would require a photolysis wavelength in the 250-300 nm spectral range corresponding to the contour of the electronic transition of t-St. EPR. To determine the structural surroundings of the unpaired electrons of the radical cation-electron pair and the subsequent charge transfer during the reaction, pulse EPR experiments were performed at various times after the mixing. 2D-HYSCORE can enable the proper assignment of the various couplings with a large number of nuclei. Figure 5 shows the 2 pulse echo field sweep experiment recorded at 4 K after 1 day. This spectrum is assigned to the t-St radical cation with respect to the system advancement determined above using the DRUVv technique. The corresponding 2D HYSCORE spectrum recorded 1 day after the mixing shows only a pair of cross peak ridge centered at 14.5 MHz corresponding to the 1H Larmor nuclear frequency (Figure 6A). This pair of cross peaks appears in the (+,+) quadrant and indicates that these protons have weak hyperfine coupling Aiso < 2ν. These cross peaks coordinates are observed at ν ( A/2. The proton hyperfine coupling was obtained by the analysis of the experimental HYSCORE spectrum recorded at 4 K using 16 and 32 ns for the π/2 and π pulse values, respectively, and 350 ns for the τ value between first and second pulse. The principal values of hyperfine (hf) tensor A [Axx, Ayy, Azz] can be written in the form (Aiso - T, Aiso - T, Aiso + 2T), where Aiso is the isotropic hyperfine coupling constant and T is the anisotropic hyperfine tensor. This spectrum is assumed to display primarily the radical cation features. Proton cross-peak features are principally dominated by the proton Aiso and the ridge extent arises from the anisotropic component. The proton cross peak ridge coordinates are at (9.7,18.5; 18.5,9.7 MHz)

10286

J. Phys. Chem. C, Vol. 114, No. 22, 2010

Figure 6. 2D-HYSCORE patterns recorded as a function of time after the mixing of solid t-St and dehydrated H2.2-GaZSM-5 (one t-St per UC loading). The HYSCORE spectra were recorded at 4.2 K, with τ ) 350 ns and a pulse length of 16 ns for the π/2 pulse and 32 ns for the π pulse. (A) t-St•[email protected]•- recorded 1 day after the mixing. (B) Intermediary state between radical cation and electron-hole pair recorded 10 days after the mixing. (C) [email protected]•-•+ electron-hole pair recorded 1 month after the mixing.

Moissette et al. and is dominated by isotropic coupling of large coupling of proton radical cation species. HYSCORE experiments were also carried out 10 days and 1 month after the mixing (Figure 6B,C). These spectra exhibit clear changes with respect to the spectrum recorded after 1 day in agreement with the electron transfers observed using DRUVv absorption and Raman scattering techniques. Figure 6B displays the HYSCORE spectrum obtained after 10 days. This spectrum shows changes in the proton pattern with a more extended ridge than for the initial radical cation species. The maximum coupling Azz for the protons does not change but the appearance of 29Si at 2.9 MHz Larmor resonance frequency is clearly observed. After 1 month of evolution, the corresponding HYSCORE pattern exhibits new features (Figure 6C). In the low frequency region, new resonance frequencies appear apart from 29Si. First, the 69Ga isotope resonance frequency is detected at 3.58 MHz with peak ridge extent of 4 MHz, and second, the Larmor frequency of 71Ga at 4.5 MHz is also detected. Such a pattern with gallium frequencies detected at an A coupling of 4 MHz indicates that spin densities are close to gallium and that the 1 H pattern arises from the proton of the zeolite. The HYSCORE pattern recorded 6 months after the mixing is very similar to that obtained after 1 month, in agreement with the presence of unpaired electrons. The modifications of the pulsed EPR patterns as a function of time are then in complete agreement with the conclusions obtained using DRUVv and Raman spectroscopy. It is clearly established that the system advances through an initial spontaneous radical cation formation (1 day) and a subsequent electron transfer to create the electron-hole pair (after 1 month). There are some important differences between t-St ionization in GaZSM-5 versus AlZSM-5. For instance, the t-St spontaneous ionization within H-AlZSM-5 did not allow verification of any coupling between Al atoms that are spin carriers in this system.7 Contrary to what is observed for H-AlZSM-5, the peak ridges observed for Ga provide evidence of coupling. Therefore, electron-electron dipolar coupling simulations were carried out and allowed the determination of the distribution of Ga atoms that are electron spin carriers within the framework (Figure 7). We tried to measure the distance between electron spin S ) 1/2 by performing SIFTER experiments that allow to refocus dipolar processes using a single frequency excitation. Such experiments show mainly a large modulation due to the long distance range between the two coupled spins (Figure 7a,b). We can also see a weak contribution of the 1H nuclear frequency that still remains even with a ∆t of 16 ns that is supposed to suppress this nuclear modulation (Figure 7c). The distance distribution of Ga atoms coupled to unpaired electrons is presented in Figure 7d. The nearest distance between Ga coupled with unpaired electron is approximately 4 nm but the main feature is observed around 7.2 nm. This feature might show that formation of electron-hole pairs is not completely uniform in the material. Considering the Ga content (and so the Brønsted acid sites) within the H2.2-GaZSM-5 zeolite framework, the results obtained by the SIFTER experiments indicate that unpaired electrons are approximately located between 3 units of Ga atoms (that is, 1 Ga out of 3 is used for electron-hole pair formation). This result is compatible with the lower ionization observed using DRUVv spectroscopy for H-GaZSM-5 compared to H-AlZSM-5. Unfortunately, due to a blind zone (less than 2 nm) that cannot be measured by SIFTER but also more than 1.5 nm that should be observed as dipolar effect on the CW spectra, the shorter distance between the two Ga close to unpaired electrons, that

Charge Separated States Induced by t-Stilbene

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10287

Figure 7. SIFTER experiments with experimental spectrum (a) and corrected dipolar evolution (b). Frequency spectrum (c) and distance distribution compute with DeerAnalysis 2008 toolbox (d).

is, the distance between electron GaO4H•- and hole GaO4H•+ could not be found. This distance was determined for two Al atoms near unpaired electrons within the Al rich Li6.6-AlZSM5. This value was established from the simulation of the dipolar component measured in CW EPR data obtained after electronhole pair formation consecutively to naphthalene photoionization.38 The electron was assumed to be located in a limited space around one Li+ close to a Si-O-Al bridge. The positive hole was assumed to be located on the oxygen atom of another Si-O-Al bridge close to another Li+ cation. The separation distance of the electron and hole of 1.3 nm was reasonably in the range of Al · · · Al distances in this sample. 4. Discussion The spectroscopic results reported above demonstrate the t-St incorporation and its spontaneous ionization into the 10-MR channels of H2.2-GaZSM-5 acidic zeolite through mere exposure of t-St crystals to the empty zeolite, a sample that has not been preactivated using heat. The charge separation induced by the strong ionizing intrazeolite environment generates a complex reaction sequence that is initiated by the following reaction:

t-St + H2.2-GaZSM-5 f t-St•[email protected]•-

(2) Encapsulation in a narrow zeolite channel offers advantages in terms of separating the molecules, improving the chemical stability of radicals, and allowing the individual moiety to be addressed experimentally. The results for H2.2-GaZSM-5 are analogous to similar experiments conducted on H3.4-AlZSM-5 zeolite.7 t-St was also reported to be incorporated as an intact molecule into the pores of other forms of M+-AlZSM-5 (M+ ) alkali-metal center), but these nonacidic zeolites are not effective at producing t-St spontaneous ionization. The diffusion of sorbate in the porous void of zeolites was primarily divided into intercrystalline and intracrystalline migrations. The intracrystalline and intercrys-

talline migration of various aromatics as intact molecules in different zeolites was extensively detailed in previous experimental and theoretical works.39 Unfortunately, no such extensive results concerning diffusion of charged species such as radical cations and electrons in zeolites were reported. The present results of EPR, DRUVv and Raman spectroscopy demonstrate that the t-St incorporation in H2.2-GaZSM-5 zeolite generates radical ions in high yield and subsequent electron and charge transfers. We try to interpret the present results using a mechanism previously used to explain the chemical processes initiated by the spontaneous ionization of t-St and rod shape polyacenes in Hn-AlZSM-5.5 The structure of the Discussion section is as follows. First, we describe briefly the model used to interpret the initial step of the t-St spontaneous ionization in H2.2GaZSM-5 as t-St•+ radical cation and ejected electron. This is followed by the physical picture obtained by pulse EPR experiments of the chemical environment of the spins of t-St•[email protected]•- moieties generated during the primary ionization step. Then, the slow migration of charged species and charge transfers in the 10-membered rings (10-MR) of H2.2GaZSM-5 is discussed, and finally, the spectroscopic characterization and the stability of the electron-hole pair [email protected]•+•- is described. Spontaneous Charge Separation of t-St upon Incorporation within 10-MR Channel of H2.2-GaZSM-5. The framework structure of MFI zeolites, including GaZSM-5, contains two types of intersecting channels, both formed by 10-MR apertures. One channel is straight and shows an elliptical opening (0.53 × 0.56 nm2), while the other one is sinusoidal with a somewhat smaller opening (0.51 × 0.55 nm2). The t-St molecule gains access to the channels through the pore openings of the available faces of zeolite crystals. It was previously reported that t-St is occluded as an intact molecule in Na6.6-AlZSM-5.8 In contrast, there is no doubt that accessible protons in the inner surface 10-MR channel of H3.4-AlZSM-5 and H2.2-GaZSM-5 are essential for t-St ionization. It was previously demonstrated that the spontaneous ionization of anthracene was a unique property

10288

J. Phys. Chem. C, Vol. 114, No. 22, 2010

of the inner surface of the 10-MR channel and not of the external surface of H3.4-AlZSM-5 zeolite crystals.40 The ionization efficiency depends both on the ionization potential values of the sorbate and on the ionizing power of the host. In general, the ionization energy of molecules occluded in the condensed phase is expected to depend on Ig the ionization potential in the gas phase, here Ig(t-St) ) 7.65 eV, the energy of the antibonding orbital in a localized orbital state and the polarization energy imposed by the confinement with surrounding ions.41 Incorporation of organic molecules inside the pores of zeolites may induce enhancement of the base character and an apparent decrease of the oxidation potential of the molecule upon tight fit of the molecule in the pores. As a result, the HOMO of the occluded molecule was assumed to be more sensitive than the LUMO, and the expected effect is a reduction on the band gap of the frontier orbital. Therefore, the electron confinement effect produces an increase in the energy of the HOMO of the aromatic molecule and in comparison with the gas phase involves a larger contribution of the orbital control when the reaction occurs inside the cavities of zeolite. Then, because of this energy level increase, the tendency of a guest molecule occluded in the zeolite void space toward giving electrons to the acid sites will increase, as well as its reactivity.42-44 Moreover, note that, upon entrapping photosensitizers within inorganic supports such as zeolites, the determination of the band gap on the basis of UV-visible measurements showed that the host is not a spectator in this process and that electronic interactions occur, which result in a lowering of the band gap of the inorganic support.45 Then, the energy of the antibonding orbital of ZSM-5 cluster is expected to depend significantly on the Ga or Al content and weakly on the nature of the extraframework cation. The polarization energy is closely linked to the local electrostatic field strength and not to the average electrostatic field. The high electrostatic field strength in aluminum rich zeolites is considered to be due to the counterbalancing cations in the channel being only partially shielded. The electrostatic field is dependent both on cation size and on Si/Al ratio but also on the steric constrains in the 10-MR channel. The small size of H+ ion induces a stronger field in its proximity than the larger Li+, Na+,.. ., or Cs+ ion despite O-H bond is partially covalent.39 The combined effects of the high polarization energy in close proximity of accessible acid site Si-O(H)-Ga within 10-MR channel and favorable antibonding orbital energy of GaZSM-5 induce spontaneous ionization of t-St by lowering the ionization energy. No supplementary Lewis acid site was necessary to induce t-St spontaneous ionization in H2.2-GaZSM-5 as well as in H3.4-AlZSM-5. The prior thermal treatment of H2.2-GaZSM-5 and H-AlZSM-5 under inert atmosphere up to 700 K release water only and does not create significantly supplementary Lewis acid sites.46 In contrast, the spontaneous ionization of aromatic molecules with high ionization potential in gas phase such as biphenyl (Ig ) 8.16 eV) or benzene (Ig ) 9.24 eV) is effective in H-AlZSM-5 only after drastic thermal treatment above 873 K or under O2 with dehydrogenation of a large fraction of the Brønsted acid sites to create supplementary Lewis acid sites.3,6,47 It should be noted that the spontaneous ionization of anthracene (Ig ) 7.4 eV) is effective in nonacidic Li-AlZSM5, while N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD, Ig ) 6.6 eV) is ionized in purely siliceous silicalite-1 but the ionization rate and yield in MFI depend significantly on Al content and on the nature of counterbalancing cation.5 The t-St ionization rate and yield were found to be significantly lower in H2.2-GaZSM-5 than in H3.4-AlZSM-5 maybe because of the

Moissette et al. lower Brønsted acid site density in the 10-MR channels of H2.2GaZSM-5 as shown previously for anthracene.48 t-St•[email protected]•- Charge Separated State. The lifetime of t-St•[email protected]•- is estimated to be approximately 10 days in the 10-MR of H2.2-GaZSM-5 at room temperature, see Figure 2. This long lifetime has permitted the characterization of the occluded moiety by several spectroscopic tools. Long-lived t-St•+ occluded within H2.2-GaZSM-5 shows analogous UV-visible absorption and resonance Raman characteristics as reported previously for long-lived t-St•+ entrapped in H3.4-AlZSM-5, t-St•+ photoinduced in nonacidic Na6.6AlZSM-5 zeolite and short-lived t-St•+ photoinduced in solution. The ejected electron trapped in GaZSM-5•- exhibits broad absorption around 1500 nm, as reported for incorporation of alkali metal in zeolites.29 Unfortunately, we have no EPR evidence to support the location of trapped electron as GaO4H•entity through 71Ga and 69Ga peaks in the HYSCORE spectra shown in Figure 6A,B because of low ionization yield and poor signal/noise ratio. No 27Al peak was also reported to support the AlO4H•- entity as electron trapping site in the first step of the t-St spontaneous ionization in H3.4-AlZSM-5. Nevertheless, an intense 27Al peak was observed in the HYSCORE pattern recorded after high yield spontaneous ionization of anthracene in H3.4-AlZSM-5 and Li3.4-AlZSM-5 which demonstrates the location of unpaired electron in the close surrounding of AlO(H)-Si or Al-O(Li+)-Si bridges.9,49 It is reasonable to assume GaO4H•- as trapping site of ejected electron in t-St•[email protected]•-. t-St•+, Electron Diffusion, and Hole Transfer in H2.2GaZSM-5. t-St•+-electron pair photoinduced in solution exhibits short lifetime (∼µs) because of fast charge recombination. The lifetime of photochemically generated t-St•+-electron pair increases up to 8 orders of magnitude as compared to solution when they are photogenerated inside Na6.6-AlZSM-5.8 The compartmentalization of photogenerated electrons away from the initial site of t-St ionization was invoked to decrease dramatically the propensity of charge recombination.50 The spontaneously generated ions t-St•+ and electrons become very persistent in H2.2-GaZSM-5 and H3.4-AlZSM-5 because electron transport over long distance is coupled with t-St•+ diffusion and the diffusion of the positive and negative species are hindered by their interaction via the electrostatic field. The charged centers give rise to deep potential wells that capture mobile ions and slow down their motion for some time.51 The main channel for t-St•+ decay is not the direct charge recombination. During the course of t-St•+ migration and electron transport, the electron deficient t-St•+ captures an electron of another available GaO(H)-Si group in close proximity preferentially to charge recombination with electron trapped as GaO4H•- entity to generate electron hole as t-St GaO4H•+ moiety according to:

t-St•[email protected]•- f [email protected]•-•+

(3) The formation of hole on an AlO4H•+ center of the H-ZSM-5 has been studied previously by hybrid quantum mechanics. Generation of the electron hole produces a substantial geometry relaxation of the Al-O distance to the oxygen atom with the unpaired electron and the zeolite framework stabilizes the positive charge by long-range effects.52 The oxidizing power of t-St•+ can be characterized by the t-St oxidation potential in solution. The Eox ) 1.75 V versus SCE value explains easily the electron abstraction from GaO4H. This aspect has been extensively discussed for hole transfer of t-St•+ photoinduced

Charge Separated States Induced by t-Stilbene in Na6.6-AlZSM-5 and of tetracene radical cation spontaneously generated within H3.4-AlZSM-5 using the nonadiabatic electron transfer theory.4,8,53 The hole transfer rates are modulated by the electronic coupling between donor and acceptor sites, the reaction free energy, and the reorganization energy. The weak reorganization energy under confinement is the main parameter of the slow hole transfer rate. The t-St•+ hole transfer rates are analogous in H3.4-AlZSM-5 and H2.2-GaZSM-5 as expected with analogous confinement within 10-MR channel. [email protected]•-•+ Charge Separated State. The most striking feature of the hole transfer is the appearance of an intense color due to the prominent absorption bands in the visible range of the t-St GaO4H•+ electronic hole entity. These bands are assigned by resonance Raman spectrometry to a degree of a hole back transfer from GaO4H•+ to t-St in close proximity. This situation requires a delicate balance between the ionization potential of electron donor t-St and the electron affinity of GaO4H•+. Two types of charge-transfer complexes are determined in H2.2-GaZSM-5 and are probably characteristic of two types of interfacial π interaction between H+ and phenyl ring related to t-St GaO4H•+. One charge-transfer absorption band set (λmax ) 567, 624 nm) corresponds most likely to noncovalent interaction between H+ and t-St terminal ring and the other one (λmax ) 730 nm) to interaction between H+ and the central double bond. Electron and hole survive for several months. However, the system reorganizes slowly as the formation of a different electron-hole pair (type II) probably occurs as indicated by the observation of a new band at 730 nm associated with a persistent EPR signal (eq 4).

[email protected]•-•+(type I) f [email protected]•-•+(type II) (4) These two types of charge-transfer complexes t-St AlO4H•+ were previously reported in H3.4-AlZSM-5 with analogous spectral characteristics at λmax ) 560, 620 nm and λmax ) 685 nm, respectively.7 The pulse EPR experiments reveal the structural surroundings of the persistent unpaired electrons of both trapped electrons and holes. It should be noted that the HYSCORE and SIFTER results are not able to discern the surroundings of electron and electronic hole of the two types of charge transfer complexes. The HYSCORE pattern demonstrates clearly the prominent participation of 69Ga, 71Ga, and 29Si nuclei in the electron spin distribution within the zeolite framework, the unpaired electrons are probably localized on the nearest oxygen atoms of the GaO3O(H)-SiO3 bridges as also observed in [email protected]•-•+ as AlO3-O(H)-SiO3 moieties. The direct participation of the acidic proton is not discerned in the spectra and is overlapped by the 1H cross peaks of in the 1H cross-peak ridges of t-St GaO4H•+ entities (Figure 6C). The 1H cross-peaks were clearly pointed out in [12D][email protected]•-•+ using labeled t-St compounds.7 The 1H cross peaks provide evidence of the close proximity of the unpaired electron with the acidic proton and the protons of t-St. Despite the striking analogy between the HYSCORE patterns of [email protected]•-•+ and [email protected]•-•+, some important differences are observed. Contrary to what is observed for AlZSM-5, the peak ridges determined for Ga provide evidence of coupling. The SIFTER experiments use such long distance dipolar coupling as to permit the extraction of the distance distribution of Ga nuclei coupled with unpaired electrons. The nearest distance between Ga coupled

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10289 with an unpaired electron (GaO4H•- or GaO4H•+) is approximately 4 nm, but the main feature is observed at 7.2 nm (see Figure 7). It is tempting to compare the distance distribution of Ga nuclei coupled with unpaired electron with the Ga · · · Ga distance distribution within the zeolite framework, assuming a random distribution. Unfortunately, due to a blind zone of the SIFTER technique, no Ga · · · Ga distance shorter than 2 nm can be measured. Nevertheless, the SIFTER experiments provide a unique picture of the long distance distribution of unpaired electrons generated by spontaneous ionization of t-St upon incorporation within H2.2-GaZSM-5 zeolite 1 month after the mixture of solids. This result demonstrates that a large fraction of the unpaired electrons is ejected away from the initial site of ionization and that this compartmentalization plus the created electrostatic field hinder dramatically the propensity of charge recombination. The charge recombination can be observed clearly 1 year after the mixing of solids through the decrease of spin concentration determined for instance by the double integration of continuous wave EPR signal. A strong synergy really exists between Brønsted acidity and channel topology on the ionization efficiency of t-St and stabilization of ionized species. These results demonstrate that the tight fit of t-St in the pores of a zeolite can generate and stabilize radical ions over long periods. The confinement effect on spontaneous ionization is constrained also by the location of BAS. The confinement results in noncovalent interactions between the zeolite framework and t-St located in the channels. These interactions have short-range components (van der Waals and electrostatic) and long-range components (electrostatic). Acknowledgment. R.F.L and K.A.A acknowledge funding for this research provided by the U.S. Department of Energy Basic Energy Sciences under Grant No. DE-FG02-07ER15921 and DE-FG02-99ER14998. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, for the EXAFS experiments was supported by the US Department of Energy, Office of Basic Energy Sciences (Grant No. DE-FG0205ER15688). Supporting Information Available: Additional figures and references. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Werst, D. W.; Trifunac, A. D. Acc. Chem. Res. 1998, 31, 651– 657. (2) Werst, D. W.; Han, P.; Trifunac, A. D. Radiat. Phys. Chem. 1998, 51, 255–262. (3) Bedilo, A. F.; Volodin, A. M. Kinet. Catal. 2009, 50, 314–324. (4) Marquis, S.; Moissette, A.; Hureau, M.; Vezin, H.; Bremard, C. J. Phys. Chem. C 2007, 111, 17346–17356. (5) Marquis, S.; Moissette, A.; Vezin, H.; Bremard, C. C. R. Chim. 2005, 8, 419–440. (6) Garcia, H.; Roth, H. D. Chem. ReV. 2002, 102, 3947–4007. (7) Vezin, H.; Moissette, A.; Hureau, M.; Bremard, C. ChemPhysChem 2006, 7, 2474–2477. (8) Moissette, A.; Bremard, C.; Hureau, M.; Vezin, H. J. Phys. Chem. C 2007, 111, 2310–2317. (9) Marquis, S.; Moissette, A.; Vezin, H.; Bremard, C. J. Phys. Chem. B 2005, 109, 3723–3726. (10) Scarano, D.; Zecchina, A.; Bordiga, S.; Geobaldo, F.; Spoto, G.; Petrini, G.; Leofanti, G.; Padovan, M.; Tozzola, G. J. Chem. Soc., Faraday Trans. 1993, 89, 4123–4130. (11) Chu, C. T. W.; Chang, C. D. J. Phys. Chem. 1985, 89, 1569–1571. (12) Challoner, R.; Harris, R. K.; Barri, S. A. I.; Taylor, M. J. Zeolites 1991, 11, 827–831. (13) Lamberti, C.; Palomino, G. T.; Bordiga, S.; Zecchina, A.; Spano, G.; Arean, C. O. Catal. Let. 1999, 63, 213–216. (14) Parrillo, D. J.; Lee, C.; Gorte, R. J.; White, D.; Farneth, W. E. J. Phys. Chem. 1995, 99, 8745–8749.

10290

J. Phys. Chem. C, Vol. 114, No. 22, 2010

(15) Yuan, S. P.; Wang, J. G.; Li, Y. W.; Peng, S. Y. J. Mol. Catal. A.: Chem. 2002, 178, 267–274. (16) Jeschke, G.; Pannier, M.; Godt, A.; Spiess, H. W. Chem. Phys. Lett. 2000, 331, 243–252. (17) Jeschke, G.; Chechik, V.; Ionita, P.; Godt, A.; Zimmermann, H.; Banham, J.; Timmel, C. R.; Hilger, D.; Jung, H. Appl. Magn. Reson. 2006, 30, 473–498. (18) Lamberti, C.; Palomino, G. T.; Bordiga, S.; Arduino, D.; Zecchina, A.; Vlaic, G. Jpn. J. Appl. Phys. 1999, 38, 55–58. (19) Al-Majnouni, K.; Hould, N.; Vlachos, D. G.; Lobo, R. F. 2010, manuscript in preparation. (20) Capek, L.; Kreibich, V.; Dedecek, J.; Grygar, T.; Wichterlova, B.; Sobalik, Z.; Martens, J. A.; Brosius, R.; Tokarova, V. Microporous Mesoporous Mater. 2005, 80, 279–289. (21) Sobalik, Z.; Dedecek, J.; Ikonnikov, I.; Wichterlova, B. Microporous Mesoporous Mater. 1998, 21, 525–532. (22) Klier, K.; Shen, J. H.; Zettlemo, A. J. Phys. Chem. 1973, 77, 1458– 1465. (23) Arean, C. O.; Bonelli, B.; Palomino, G. T.; Safont, A. M. C.; Garrone, E. Phys. Chem. Chem. Phys. 2001, 3, 1223–1227. (24) Arean, C. O.; Palomino, G. T.; Geobaldo, F.; Zecchina, A. J. Phys. Chem. 1996, 100, 6678–6690. (25) Lednev, I. K.; Mathivanan, N.; Johnston, L. J. J. Phys. Chem. 1994, 98, 11444–11451. (26) Gessner, F.; Scaiano, J. C. J. Photochem. Photobiol., A 1992, 67, 91–100. (27) Moissette, A.; Belhadj, F.; Bremard, C.; Vezin, H. Phys. Chem. Chem. Phys. 2009, 11, 11022–11032. (28) Alvaro, M.; Atienzar, P.; Corma, A.; Ferrer, B.; Garcia, H.; Navarro, M. T. J. Phys. Chem. B 2005, 109, 3696–3700. (29) Wernette, D. P.; Ichimura, A. S.; Urbin, S. A.; Dye, J. L. Chem. Mater. 2003, 15, 1441–1448. (30) Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Takamuku, S. J. Org. Chem. 1995, 60, 4684–4685. (31) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S. J. Phys. Chem. 1996, 100, 13615–13623. (32) Vaughan, P. A. Acta Crystallogr. 1966, 21, 983–&. (33) Parise, J. B.; Hriljac, J. A.; Cox, D. E.; Corbin, D. R.; Ramamurthy, V. J. Chem. Soc., Chem. Commun. 1993, 226–228.

Moissette et al. (34) Baranovic, G.; Meic, Z.; Gusten, H.; Mink, J.; Keresztury, G. J. Phys. Chem. 1990, 94, 2833–2843. (35) Meic, Z.; Gusten, H. Spectrochim. Acta, Part A 1978, 34, 101– 111. (36) Hub, W.; Kluter, U.; Schneider, S.; Dorr, F.; Oxman, J. D.; Lewis, F. D. J. Phys. Chem. 1984, 88, 2308–2315. (37) Schneider, S.; Scharnagl, C.; Bug, R.; Baranovic, G.; Meic, Z. J. Phys. Chem. 1992, 96, 9748–9759. (38) Moissette, A.; Marquis, S.; Cornu, D.; Vezin, H.; Bremard, C. J. Am. Chem. Soc. 2005, 127, 15417–15428. (39) Hashimoto, S. J. Photochem. Photobiol., C 2003, 4, 19–49. (40) Moissette, A.; Marquis, S.; Gener, I.; Bremard, C. Phys. Chem. Chem. Phys. 2002, 4, 5690–5696. (41) Hirata, Y.; Mataga, N. Prog. React. Kinet. 1993, 18, 273–308. (42) Marquez, F.; Garcia, H.; Palomares, E.; Fernandez, L.; Corma, A. J. Am. Chem. Soc. 2000, 122, 6520–6521. (43) Corma, A.; Garcia, H.; Sastre, G.; Viruela, P. M. J. Phys. Chem. B 1997, 101, 4575–4582. (44) Zicovich-Wilson, C. M.; Corma, A.; Viruela, P. J. Phys. Chem. 1994, 98, 10863–10870. (45) Cojocaru, B.; Neatu, S.; Parvulescu, V. I.; Dumbuya, K.; Steinruck, H. P.; Gottfried, J. M.; Aprile, C.; Garcia, H.; Scaiano, J. C. Phys. Chem. Chem. Phys. 2009, 11, 5569–5577. (46) Nash, M. J.; Shough, A. M.; Fickel, D. W.; Doren, D. J.; Lobo, R. F. J. Am. Chem. Soc. 2008, 130, 2460–2462. (47) Hubner, G.; Roduner, E. Magn. Reson. Chem. 1999, 37, S23–S26. (48) Hureau, M.; Moissette, A.; Marquis, S.; Bremard, C.; Vezin, H. Phys. Chem. Chem. Phys. 2009, 11, 6299–6307. (49) Vezin, H.; Moissette, A.; Bremard, C. Angew. Chem., Int. Ed. 2003, 42, 5587–5591. (50) Keirstead, A. E.; Schepp, N. P.; Cozens, F. L. J. Phys. Chem. C 2007, 111, 14247–14252. (51) Krager, J.; Ruthven, D. M. Diffusion in Zeolites and Other Microporous Solids; Wiley: New York, 1992. (52) Solans-Monfort, X.; Branchadell, V.; Sodupe, M.; Sierka, M.; Sauer, J. J. Chem. Phys. 2004, 121, 6034–6041. (53) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265–322.

JP103838B