Chemical Control of Photoinduced Charges under ... - ACS Publications

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Chemical Control of Photoinduced Charges under Confinement in Zeolites Matthieu Hureau, Alain Moissette,* Alexandre Legrand, Florence Luchez, Michel Sliwa, and Claude Bremard Laboratoire de Spectrochimie Infrarouge et Raman UMR-CNRS 8516, Bâtiment C5, Université des Sciences et Technologies de Lille, 59655 Villeneuve d’Ascq cedex, France S Supporting Information *

ABSTRACT: The organized internal porous void of dehydrated zeolites provides a suitable environment to promote long-lived photoinduced charge separation. Herein we have conducted time-resolved UV−visible absorption spectroscopy experiments from nanosecond to day time scale following nanosecond UV (266 nm) pulsed laser irradiation of transstilbene (t-St) occluded in channels of nonacidic M−FER, M− MFI, and M−MOR zeolites with various pore diameters, with differing framework aluminum content, and with different extraframework cations (M = Na+, K+, Rb+, and Cs+). The cation radical of trans-stilbene (t-St•+) and trapped electron (AlO4•−) have been generated directly by means of laser-induced electron transfer within the channels of medium pore zeolites. We have highlighted that the general back electron transfer processes include direct charge recombination (CR), hole transfer (HT), and finally electron−hole recombination to re-form the occluded t-St ground state without any isomerization or oligomerization. It was demonstrated once again that zeolites can be active participants as electron acceptors and electron donors. The decays of t-St•+ are the combination of two processes: direct CR and hole transfer. The charge-separated species as t-St•+···AlO4•− and t-St-AlO4•+···AlO4•− moieties are stabilized for approximately 10 h in aluminated medium pore zeolites with small extraframework cation such as Na+. The most remarkable feature of the transient t-St−AlO4•+ entity formation in M−MFI and M−MOR is the persistent intense color due to the prominent absorption bands in the visible range. The very slow CR rates are explained both by the long distance between the separated charges and by the large difference in free energy between the electron acceptor and electron donor (driving force −ΔG0), which increases with Al content in the order Cs+ < Rb+ < K+ < Na+. The CR rates are markedly slowed by shifting them deep into the inverted region of the Marcus parabola where −ΔG0 is larger than the reorganization energy coefficient (λ), which is particularly small under high confinement. The close match between t-St molecular size and zeolite channel diameter is critical to generating long-lived charge separations (hours).

1. INTRODUCTION Photoinduced electron transfer (PET) processes are of fundamental importance in many photochemical processes of biological and chemical systems.1−6 The ability to induce and temporarily maintain the charge-separated state (CSS) is essential for the viable functioning of natural and artificial light energy harvesting systems. Extensive efforts continue toward the development of molecular systems in which a CSS populated by PET survives for a long time. Generally, PET occurs on a subpicosecond time scale, while recombination of the CSS can be a much slower process if conditions are carefully controlled.7 The utility of porous materials such as zeolites for stabilizing long-lived CSSs in their internal voids is now well-recognized.8 Zeolites are crystalline porous aluminosilicate minerals with the general formula Mx[(AlO2)x(SiO2)y]·mH2O, where M+ is the counterion balancing the negative charge of the zeolite framework. The zeolite framework forms channels and cavities of discrete size. After the removal of water from their porous voids, empty zeolites (hosts) can act as molecular sieves, © 2012 American Chemical Society

accommodating neutral molecules (guests) with suitable size into their strictly regular cages and channels through the pore openings. A number of review articles dealing with zeolites and particularly with photochemical studies of guest@zeolite systems have been published previously.8−13 Studies of PET in guest@zeolite systems, where the guest is the electron donor, show that the lifetime of the photoinduced radical cation can be increased by as much as 10 orders of magnitude compared to the lifetime in solution at room temperature (photoionization of aromatic compounds in solution generates electron−radical cation pairs within several picoseconds resulting in the formation of solvated free ions with lifetimes of a few nanoseconds). Several examples of very long-lived radical cation−electron pairs (hours or days time scale) were reported for rod-shaped polyaromatics occluded in aluminum-rich ZSMReceived: February 13, 2012 Revised: March 30, 2012 Published: March 30, 2012 9092

dx.doi.org/10.1021/jp301448w | J. Phys. Chem. C 2012, 116, 9092−9105

The Journal of Physical Chemistry C

Article

explained by inverted region effects of the Marcus theory rather than electron spin control.

5 (MFI) with commensurate channel size. The enormous reduction in the charge recombination (CR) rate under confinement has been attributed to compartmentalization of the photoelectron away from the initial site of ionization at a distant AlO4•− site. The slow long-range CR can allow one to estimate the electron donor ability of another AlO4 group in view of the nearest electron-deficient radical cation to generate an AlO4•+ hole.14 The strong electric field inside the porous voids is also thought to contribute to the slowdown in charge recombination and charge shift reactions,15 while factors such as the framework (Si/Al ratio) and the extraframework cation (M+) influence markedly the ET rate transfer.12,15 Dehydrated zeolites thus provide an ideal organized medium in which radical ions are efficiently generated and which can survive for hours or days. Nevertheless, currently, a full understanding of the energetics and dynamics of the long-lived charge separation, and slow electron and hole migration in zeolites, has not been achieved.15 The purpose of the present work is to investigate the influences of channel size, framework aluminum content, and extraframework cation on the CS and subsequent ET rates following laser photoionization of trans-stilbene occluded in various medium pore zeolites. trans-Stilbene (t-St; (E)-1,2diphenylethene) is the prototypical photosensitized rod-shaped electron donor molecule to be encapsulated in zeolite channels. trans-Stilbene has the appropriate molecular width (∼0.54 nm) to enter the channel opening of medium pore zeolites such as ferrierite (M−FER), MFI (M−MFI), and mordenite (M− MOR) with 0.42 × 0.54, 0.54 × 0.56, and 0.65 × 0.70 nm2 channel diameters, respectively. The photochemistry of t-St has been extensively investigated in gas and condensed phases, and a number of early studies relate to the photochemistry of t-St occluded in ZSM-5 (MFI) and faujasitic zeolites.16−26 Two decades ago, it was reported that the direct excitation of t-St occluded within the large supercage of faujasitic zeolites results in trans−cis isomerization while isomerization does not occur within the narrow channel of ZSM-5 (MFI).27 Preliminary studies of the slow, photoinduced interfacial hole transfer of a t-St radical cation inside channels of Na−ZSM-5 (MFI) zeolite were recently published.28 It appears that the close match between t-St size, channel diameter, and internal surface are critical to inducing strong confinement effects on photoinduced ET rates. Here, we have conducted time-resolved UV visible absorption spectroscopy experiments from the nanosecond to the day time scale following UV pulsed laser irradiation of t-St occluded in channels of M−FER, M−MFI, and M−MOR zeolites with M = Na+, K+, Rb+, and Cs+. The diffuse reflectance technique is successfully applied using both nanosecond transient spectrosocpy and conventional techniques. The kinetics of the decays are accurately reproduced using the Albery function based on dispersion of heterogeneous rate constants determined for a large range of time scales, from milliseconds to several hours. The decays of t-St•+ are the combination of two processes: direct CR and hole transfer. The ratio of these two processes depends dramatically on the channel diameter and on the aluminum content, and to a lesser extent on the extraframework cation. The close match between channel diameter and t-St width in aluminum-rich M−FER zeolite points out the unique confinement effect characterized by a t-St•+···AlO4•− pair lifetime longer than 10 h and the absence of hole transfer. The very slow ET rates are properly

2. EXPERIMENTAL SECTION 2.1. Materials. Ferrierite (NH4−FER, Si/Al = 10, average particle size ∼ 1 μm) was obtained from Zeolyst International (USA). The purely siliceous MFI (silicalite-1) sample synthesized in fluoride medium was a gift from Dr. Patarin (Institut de Science des Matériaux de Mulhouse UMR CNRS 7228, France). NH4−MFI or NH4−ZSM-5 samples (Si/Al = 27, 13.5; average particle size ∼ 1 μm) were obtained from VAW Aluminum (Schwandorf, Germany). Mordenite (NH4− MOR, Si/Al = 10) was obtained from Zeolyst International (USA). trans-Stilbene (t-St, C14H12, Merck-Schuchardt) was purified by sublimation. Pure and dry Ar gas was used. 2.2. Exchanged Zeolites. The initial ammonium extraframework cation of aluminated zeolites was exchanged by Na+, K+, Rb+, or Cs+ using the corresponding MCl chloride salts. Primarily, the exchange process is carried out by suspending zeolite powder in MCl aqueous solution under stirring. After 24 h, the solid phase is filtered off and dried at 200 °C in an oven for 12 h, then stirred again with a fresh solution of chloride salt, and then dried. The procedure is repeated 4 times. The resulting solid is washed by deionized water, isolated, dried at 200 °C for 12 h, and then calcined at 450 °C in ambient air for 6 h. The elementary analyses of the exchanged zeolites M− FER, M−MFI, and M−MOR indicate that the ammonium cations of as-provided zeolites have been completely exchanged by Na+, K+, Rb+, or Cs+ using the above procedure. The unit cell (UC) analyses of M−FER correspond to M3.3(AlO2)3.3(SiO2)32.7. The UC analyses of Mn−MFI correspond to M3.4(AlO2)3.4(SiO2)92.6 and M6.6(AlO2)6.6(SiO2)89.4. The UC analyses of M−MOR correspond to M4.4(AlO2)4.4(SiO2)43.6. The chemical analyses, powder X-ray diffraction (XRD) patterns, 29Si, 27Al MAS NMR, IR, Raman, diffuse reflectance UV−visible (DRUVv), and EPR spectra of bare exchanged zeolites were found to be characteristic of diamagnetic well-crystallized porous compounds with very low amounts of extraframework aluminum species (see Supporting Information). 2.3. t-St Loaded Zeolites. Weighed amounts (∼ 1.4 g) of exchanged zeolite samples are introduced into an evacuable, heatable silica cell placed in a vertical oven connected to a piping network. The sample is heated stepwise to 450 °C under flowing dry Ar for 3 h. Then, the sample is cooled to room temperature under dry argon. Weighed amounts of t-St corresponding to 0.4 t-St per unit cell (UC) in M−FER, 1 tSt/UC in M−MFI, or 0.5 t-St/UC in M−MOR are introduced into the cell under dry Ar, and then the powder mixture is shaken. After homogeneous mixing the powder is transferred under dry argon in a quartz glass Suprasil cell and sealed. All the M−FER, M−MFI, and M−MOR samples are stocked in sealed cells at 40 °C for 6 months in the dark. After 6 months, the t-St loaded samples exhibit the UV absorption and Raman bands of t-St occluded as t-St molecule within the porous void of zeolites (see Supporting Information).28 2.4. Molecular Modeling. The molecular modeling of the t-St preferred sorption sites at low coverage in medium pore M−FER, M−MFI, and M−MOR zeolites were performed using the Material Studio Modeling package (version 5.2) from Accelrys International as previously detailed for other molecules.29 The atomic positions for the zeolite frameworks were obtained from previous X-ray and neutron diffraction 9093



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determinations of the structures.30−33 The structural parameters and set of partial charges of t-St are derived from previous works.34 Details of the calculations are given in Supporting Information. 2.5. Radical Generation and Recombination Kinetics of Exchanged and t-St Loaded Zeolites. The generation of radicals of the bare and t-St loaded zeolites stocked in sealed quartz cells were carried out at room temperature using a nanosecond pulse width, 266 nm laser. Excitation pulses at 266 nm wavelength were provided by the fourth harmonic of a Nd:YAG laser (DIVA II, Thales laser). The diameter of the laser beam was 0.5 mm. The pulse duration was about 8 ns (1.5 mJ), and the pulse frequency was 20 Hz. First, time-resolved diffuse reflectance (TDR) absorption spectroscopy (see below) was used to detect photoinduced species with lifetimes in the nano/millisecond time scale. Second, to follow the kinetics of long-lived photogenerated species, an approximately 1 × 1 cm2 sample area was irradiated by scanning with the UV laser beam for 15 s before the recording of UV−visible absorption spectra at different times in the 3−6000 min range using a conventional diffuse reflectance spectrophotometer. 2.6. Instrumentation. 2.6.1. Time-Resolved Diffuse Reflectance (TDR) Absorption Spectroscopy. The experimental setup for nanosecond diffuse reflectance spectroscopy, applicable to the detection of transient species (TDR) in lightscattering systems, is analogous to that previously described elsewhere.35,36 Excitation pulses at 266 nm (7−8 ns, 1.5 mJ) was provided by a 20-Hz Nd:YAG laser (DIVA II, Thales laser). The probe light was provided by a Xe lamp (XBO 150 W/CR OFR, OSRAM). A UV filter was used to avoid photochemical reactions by the analyzing light. The reflected light was dispersed by a monochromator (Horiba Jobin-Yvon, iHR320) and analyzed with a photomultiplier (R1477-06, Hamamatsu) coupled to a digital oscilloscope (LeCroy 454, 500 MHz). The excitation pulse was at 90° with respect to the monochromator/photomultiplier, whereas the white probe light was at a 63° angle to the detector which collected the diffuse reflected light. In the case of powder samples, the transient absorption intensity is displayed as percentage absorption (% absorption), given by % absorption = 100(1 − R/R0), where R(λ,t) and R0(λ,t) represent the intensities of the diffuse reflected white-light probe with and without excitation, respectively. Kinetic traces from 10 ns to 800 ms were detected from 430 to 800 nm every 5 nm, from which transient spectra were reconstructed. The total instrument response time is a few tens of nanoseconds. To ensure that fresh sample was excited with each laser pulse, excitation was done at 0.3 Hz. Indeed, to maintain sample integrity during the experiment, the sample was also moved and/or shaken throughout to ensure that a fresh region of the sample was probed by each laser pulse. 2.6.2. Diffuse Reflectance UV−Visible (DRUVv) Absorption Spectroscopy. The UV−visible absorption spectra of the samples in the ground state or after irradiation were recorded between 200 and 900 nm using a Cary 6000 spectrometer. The instrument was equipped with an integrating sphere to study the powdered zeolite samples stocked under argon in quartz cells via diffuse reflectance; the corresponding bare zeolite was used as the reference. The DRUVv spectra were plotted as the Kubelka−Munk function: F(R ) = (1 − R )2 /2R = K /S

designates an absorption coefficient proportional to the concentration C of the chromophore, and S is the scattering coefficient of the powder. F(R) was registered as a function of λ (wavelength) at several t (time). 2.6.3. Data Processing. The multivariate curve resolution (MCR) data processing of the DRUVv spectral set F(λ,t) was carried out by using the SIMPLISMA (SIMPLe-to-use Interactive Self-modeling Mixture Analysis) approach. This method was applied to extract pure UV−vis absorption and respective concentration C(t) from spectral data recorded as a function of time after powder mixing without any prior information. Accurate agreements between experimental and calculated curves were observed. The method used is briefly presented in the Supporting Information and extensively detailed in the original papers. The kinetics were analyzed through the C(t) decays. Monoexponentials or two exponentials do not accurately reproduce the decays. The concentration decay C(t) was accurately fitted using the Albery function. The Albery function takes into account the nonhomogeneity of the material.37 According to the model, the time dependent absorption profile of species can be represented by eq 2: C(t ) = C /C0 +∞

=

∫−∞

+∞

∫−∞

exp( −x 2) exp( − kt̅ exp(γx)) dx / exp( −x 2) dx

(2)

where +∞

∫−∞

exp( − x 2) dx =

π

and C(t) is the normalized concentration, k̅ is the average rate constant, and γ is the width of the distribution. If γ = 0 (no dispersion), eq 2 is reduced to first order kinetics: C(t) = exp(−kt). The decay kinetic fitting was carried out by using the Microcal Origin software. The best-fitted k and γ values with respective errors are outputs of kinetic data processing.

3. RESULTS 3.1. t-St Loaded Zeolites. 3.1.1. Loading Process. Rodshaped trans-stilbene has the appropriate dimensions to enter the channels of dehydrated medium pore zeolites such as ferrierite (M−FER), MFI (M−MFI), and mordenite (M− MOR) with various channel diameters. The close match between t-St size and shape and the channel diameters of zeolites are critical to imposing constraints and to inducing strong confinement effects. The accommodation and distribution of guest molecules within the channel frameworks of zeolites are important factors in photochemical reactions. Intercrystalline diffusion and intracrystalline diffusion of t-St take time to reach homogeneous distribution and equilibrium particularly in M−FER with its narrow and elliptical 10membered-ring channel (10-MR = 0.42 nm × 0.54 nm). The mixing of weighed amounts of dry solid t-St and dehydrated zeolites corresponding to ∼1 t-St per (AlO2)x(SiO2)96−x entity at 40 °C for 6 months in the dark generates white powders for M3.3FER with M+ = K+, Rb+, and Cs+; for MnMFI with M+ = Na+, K+, Rb+, and Cs+ and n = 0.0, 3.4, and 6.6; and for M4.4MOR with M+ = Na+, K+, Rb+, and Cs+. In contrast, the mixing of t-St white powder with dehydrated M3.3FER (M = H+, Li+, Na+), MnMFI (n = 3.4, 6.6; M = H+, Li+), and

(1)

where R represents the ratio of the diffuse reflectance of the loaded zeolite to that of the dehydrated neat zeolite, K 9094



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M4.4MOR (M = H+, Li+) generates more or less colored powders under identical experimental conditions. Early studies report that the inclusion of t-St in Brønsted acidic zeolites such as H−FER, H−MFI, and H−MOR generates spontaneously species with intense visible absorption.38 It was clearly demonstrated that ionization occurs spontaneously upon mere incorporation of substrates with relatively low ionization potentials such as t-St in acidic medium pore zeolites and even in nonacidic zeolites with small size counterions such as Li+.39 3.1.2. Spectroscopic Characterization. Detailed DRUVv, Raman scattering, and fluorescence emission data of the t-St sorption mechanism in acidic and nonacidic FER, MFI, and MOR are described in another publication.38 The spectral development monitored by DRUVv appeared to be complete after several months without any thermal ionization in M3.3FER (M = K+, Rb+, Cs+), MnZSM-5 (M = Na+, K+, Rb+, Cs+; n = 0.0, 3.4, 6.6), and M4.4MOR (M = Na+, K+, Rb+, Cs+). All the DRUVv spectra recorded after 6 months show one prominent band centered around 300 nm for all the loaded zeolites. Note that the maximum and intensity of the 300 nm band depend slightly on the zeolite topology and on the extraframework cation M+ but differ markedly from the bulk t-St spectrum. Therefore, the DRUVv spectral data show that t-St adsorption takes place without any chemical modification to achieve homogeneous distribution at sufficiently low loading in the zeolites under study. The results also suggest that the 10-MR and 12-MR channels of dehydrated M−FER, M−MFI, and M− MOR zeolites hinder the geometric trans−cis isomerization of St in the ground state. 3.1.3. Molecular Modeling. Monte Carlo simulations and energy minimization procedures of the nonbonding interactions between rigid t-St molecule and the fixed zeolite framework provide a structural picture of t-St occluded in channels of FER, MFI, and MOR. Calculations take into account the atom−atom interactions between zeolite and t-St. The framework structure of FER contains two perpendicularly intersecting channels. One consists of a 10-MR with dimensions of 0.42 × 0.54 nm2, and the other is constituted by an 8-MR with dimensions of 0.35 × 0.48 nm2. The Monte Carlo simulations and subsequent energy minimization indicate that t-St is located in the straight 10-membered-ring channel with a close interaction between the occluded molecule and the charge compensating cation M+. The framework structure of MnMFI contains two types of intersecting channels. Both are formed by 10-membered rings. One 10-MR channel type is straight and has a nearly circular opening (0.53 × 0.56 nm2), while the other one is sinusoidal and has an elliptical opening (0.51 × 0.55 nm2). The openings of straight and zigzag channels are sufficiently wide to allow t-St to pass through and diffuse into the void space. Within dealuminated silicalite-1, the preferred location of t-St is the straight channel at the intersection with the zigzag channel. In aluminum-rich MnMFI, t-St is preferentially located in the straight channel in close proximity to the extraframework cation with an aromatic ring facially coordinated to the M+ cation. The framework structure of MOR is composed of two types of channels. The larger one is composed of a 12-MR (0.67 × 0.79 nm2), whereas the narrower channel is circumscribed by a 8-MR (0.26 × 0.57 nm2). The calculations show that t-St is occluded in the 12membered-ring channel in close proximity to the M+ cation. In dehydrated zeolites, t-St is bound strongly to the cationic sites within the channel and is designated as t-St@MnZeo, with Zeo = FER, MFI, and MOR. Cation−π interactions are known to be

responsible for this binding. The calculated shortest distances between C and H atoms of t-St and Oz atoms of the pore walls of the different zeolites are listed in Table 1. Table 1. Nearest Calculated Interatomic Distances between t-St and Wall of Zeolite Channels

a

zeolite

Si/Al

Oz···H (nm)

Oz···C (nm)

K3.3FER Rb3.3FER Cs3.3FER MFI Na6.6MFI K6.6MFI Rb6.6MFI Cs6.6MFI Na4.4MOR K4.4MOR Rb4.4MOR Cs4.4MOR

10 10 10 1000 13.5 13.5 13.5 13.5 10 10 10 10

0.22a 0.22a 0.22a 0.27a 0.26a 0.25a 0.26a 0.26a 0.28b 0.29b 0.30b 0.28b

0.33a 0.32a 0.32a 0.36a 0.35a 0.35a 0.34a 0.35a 0.37b 0.37b 0.38b 0.37b

In 10-MR. bIn 12-MR.

3.2. Photoexcitation of t-St Loaded Zeolites. TDR and DRUVv Spectroscopy. The 266 nm excitation wavelength falls within the contour of the 300 nm electronic transition band of t-St@MnZeo (Zeo = FER, MFI, MOR). The transient absorption diffuse reflectance spectroscopy (TDR) combined with nanosecond pulsed laser excitation is useful to follow the evolution of new species generated after excitation in the nanosecond to millisecond time domain. In the case of longlived species, the photolysis was carried out by scanning of the laser beam to irradiate a 1 cm × 1 cm area of the zeolite sample for 15 s. Then the DRUVv spectra were recorded as a function of time using the conventional DR technique. 3.2.1. [email protected]. The transient absorption diffuse reflectance spectroscopy (TDR) of [email protected] (M = K+, Rb+, Cs+) did not provide any kinetic traces from 0.5 μs to 800 ms in the 430−800 nm range, but an instantaneous absorption (within the time resolution, about 10 ns) appeared and was detected between 400 and 550 with a maximum around 480 nm. Constant absorption is typical of long-lived absorbing species with a lifetime longer than 1s. It should be noted that the possibility of t-St adsorption only on the external surface of FER zeolite particles with our experimental conditions may be definitively ruled out because no such persistent band at 481 nm was observed during the photoionization of t-St located solely on the external surface of NaA zeolite with 8-MR pores too narrow to accommodate t-St. In contrast, in NaA, prominent transient absorption was detected at 480 nm with a very short lifetime.22 The conventional DRUVv spectra recorded after the laser photolysis of [email protected] (M = K+, Rb+, Cs+) show supplementary absorption bands in the 400− 900 nm spectral range from 3 min to several days following laser irradiation. The spectra exhibit a sharp band at 481 nm and a broad band at 780 nm. No such absorptions were observed when dehydrated exchanged M3.3FER zeolites (M = K+, Rb+, Cs+) were irradiated under analogous conditions. These new spectral sets are similar for all irradiated t-St@ M3.3FER (M = K+, Rb+, Cs+) and are presented in Figure 1A for [email protected]. The two band spectra (481, 780 nm) are assigned to t-St•+ radical cation by comparison with previous works. This finding is based on some similarities in wavelength, relative intensity, and shape of bands with the known spectrum 9095



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Table 2. Spectroscopic Results of t-St•+@MnZeolite•− and tSt@MnZeolite•+•− Photoinduced by 266 nm Photolysis of tSt@MnZeolite λmax (nm) zeolite

Si/Al

t-St@ MnZeo

K3.3FER Rb3.3FER Cs3.3FER MFI Na3.4MFI K3.4MFI Rb3.4MFI Cs3.4MFI Na6.6MFI K6.6MFI Rb6.6MFI Cs6.6MFI Na4.4MOR K4.4MOR Rb4.4MOR Cs4.4MOR

10 10 10 1000 27 27 27 27 13.5 13.5 13.5 13.5 10 10 10 10

302 308 304 320 308 309 307 308 308 309 309 309 308 310 308 311

t-St•+@ MnZeo•− 481, 481, 481, 475, 474, 477, 470 474 473, 473, 474, 474, 475 475 475 475

t-St@MnZeo•+•−

780 780 780 730 750

730 730 730 730

565, 512, 565, 564, 511, 507, 514, 519, 562, 563,

627, 558, 625, 627, 555, 548, 561, 562, 626, 627,

717 621, 704 705 707 613, 708 607 622, 704 623 703 705

constraints within the narrow 10-MR channel of M3.3FER. tSt•+ is readily formed by 266 nm photoexcitation of t-St@ M3.3FER, probably by fast two photon ionization according to reaction R1 as reported in solution.40 t ‐St@M nZeo + 2hν → t ‐St•+@M nZeo•−

(R1)

with MnZeo = M3.3FER. Analysis of the spectral sets recorded following irradiation indicates that the rates of decay of the photoinduced t-St•+ are different according to M+. The data processing gives the contributions C(t) of t-St•+ as a function of time for every experiment. Figure 1B exhibits the normalized C(t)/C0 decays as a function of time following the photolysis of [email protected]. It is apparent in Figure 1B that t-St•+ disappears according to a very slow reaction. The t-St•+ decays correspond to the concomitant growth of t-St monitored by the band intensity around 306 nm to retrieve the initial concentration value observed before photoirradiation (Figure 1A). The reasonable mode of t-St•+ decay is charge recombination with the photoejected electron to re-form the neutral ground state t-St according to reaction R2 with MnZeo = M3.3FER.

Figure 1. (A) Diffuse reflectance UV−visible (DRUVv) spectra recorded after laser photoirradiation of t-St@K−FER as a function of time (266 nm, 15 s, 30 mJ cm−2): (a) before photoirradiation; (b) 2, (c) 10, (d) 50, (e) 100, (f) 200, and (g) 1700 min after photoirradiation. (B) Decay profile of Ct/C0 relative to spectral concentration of t-St•+@K−FER•−. The solid line represents the bestfit calculated decays using the Albery function; the squares represent the experimental points.

of t-St•+ in H−FER38 as well as in a matrix, solution, and other zeolites.20,28 The intensities of spectra decrease progressively as a function of time without any shape change. The data processing of every spectral set extracts two pure spectra: a spectrum with a broad absorption from 200 to 350 nm with a maximum around 306 nm and a spectrum with two sharp absorption bands with maxima at 481 and 780 nm. The main spectroscopic characteristics of the time-resolved spectra are reported in Table 2. The residuals between calculated and experimental values are less than 3% and demonstrate the accuracy of our findings without any supplementary species. No spectroscopic evidence of a trapped electron was found in the wavelength range studied (300−900 nm). The one band resolved spectrum with a prominent band centered around 306 nm is identical to the spectrum recorded before photoirradiation and corresponds to t-St occluded as an intact molecule within the channel of M3.3FER for M = K+, Rb+, and Cs+. The absorption bands at 481 and 780 nm are identical for all the irradiated M3.3FER samples with M = K+, Rb+, and Cs+ but differ somewhat to values reported after photoirradiation in solution. The differences are tentatively attributed to strong

t ‐St•+@M nZeo•− (k CR1) → t ‐St@M nZeo

(R2)

with MnZeo = M3.3FER. The concentration decays C(t)/C0 of Figure 1B were fitted using the Albery kinetic model.37 The Albery function takes into account the nonhomogeneity of the material in which there is a Gaussian distribution of the activation free energy about some mean and which introduces the width (γ) of the lifetime distribution. All the t-St•+ decays were accurately fitted in view of the randomness of the residuals between experimental and calculated values (curve). The best values of the first order k̅obs average rate constants with the corresponding γ Gaussian distribution coefficients are listed in Table 3. It is clear that the lifetime (τ = 1/k̅CR1) of photoinduced tSt•+ which reaches 3.5 × 104 s in K3.3FER is very long within the narrow 10-MR channel of M3.3FER by comparison with previously reported values in other media such as solution and 9096



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Table 3. Kinetic Results of t-St•+@MnZeolite•− Decays and t-St@MnZeolite•+/•− Decays following 266 nm Photolysis of t-St@ MnZeolite t-St•+@MnZeo•− decay zeolite

Si/Al

K3.3FER Rb3.3FER Cs3.3FER MFI Na3.4MFI K3.4MFI Rb3.4MFI Cs3.4MFI Na6.6MFI K6.6MFI Rb6.6MFI Cs6.6MFI Na4.4MOR K4.4MOR Rb4.4MOR Cs4.4MOR

10 10 10 1000 27 27 27 27 13.5 13.5 13.5 13.5 10 10 10 10

−1

t-St@MnZeo•+•− decay −1

τ1 (s)

kobs1 (s ) (γ) 2.8 1.3 2.8 8.2 1.0 1.5

× × × × × ×

10−5 (2.6) 10−4 (3.1) 10−4 (2.7) 101 (5) 10−3 (2.1) 10−3 (2)

3.5 7.8 3.6 1.2 9.6 6.6

× × × × × ×

104 103 103 10−2 102 102

7.6 8.3 9.8 1.4 2.0 2.4 6.7 1.5

× 10−4 (3) × 10−4 (3.2) × 10−4 (3.3) × 10−3 (3.2) (2.8) (2.9) (3.3) × 101 (4.7)

1.3 1.2 1.0 7.2 5.0 4.0 1.5 6.6

× × × × × × × ×

103 103 103 102 10−1 10−1 10−1 10−2

solid supports, where the t-St•+ lifetimes were found to be in the microsecond−millisecond time scale.22−25 Decrease in the lifetime of t-St•+ upon going from K+ to Rb+ and Cs+ demonstrates that the trapped electron is readily available for charge recombination in the larger cation exchanged M3.3FER zeolites. 3.2.2. t-St@MnMFI. The TDR experiments of t-St@MnMFI (n = 3.4, 6.6; M+ = Na+, K+, Rb+, Cs+) do not provide any significant kinetic trace from 0.5 μs to 800 ms in the 430−800 nm range, but an instantaneous constant absorption was detected around 480 nm for all the aluminated samples. These results are typical of t-St•+ with lifetimes greater than 1 s in the irradiated t-St@MnMFI. After 15 s irradiation (266 nm) of tSt@MnMFI, many DRUVv spectra were recorded at room temperature as a function of time over several days. All the DRUVv spectral sets recorded for each sample exhibit supplementary complex patterns in the 400−900 nm range in addition to the broad UV absorption around 306 nm. No such absorption was observed when dehydrated empty MnMFI zeolites (n = 3.3, 6.6; M+ = Na+, K+, Rb+, Cs+) were irradiated under analogous conditions. The spectral sets are typically a mixture of species evolving over several days. Two examples of spectra recorded after 15 s irradiation of [email protected] and [email protected] are shown in Figures 2A and 3A, respectively. The spectral pattern evolutions are analogous to that previously reported in a preliminary work devoted to the study of UV photolysis of [email protected] The absorptions at 474 and 730 nm evolve into broad bands at 560 and 620 nm. The spectral patterns obtained for n = 3.4 and M+ = Rb+ and Cs+ exhibit bands only at 560 and 620 nm (not shown). Using data processing, three pure spectra were extracted from each spectral set except for n = 3.4 and M = Rb+ and Cs+, where two pure spectra only were resolved. The main spectroscopic characteristics of the resolved spectra are reported in Table 2. The residuals between calculated and experimental values are less than 4% and demonstrate the accuracy of our findings without any supplementary species. The spectrum with one broad prominent band around 308 nm is detected in all the spectral sets. It is identical to the spectrum recorded before photoirradiation and corresponds to the t-St@ MnMFI ground state (Table 2). Irradiated t-St@MnMFI

kobs2 (s ) (γ)

7.2 9.5 2.0 3.3 8.3 2.1 6.0 9.2 4.5 6.9

× × × × × × × × × ×

10−5 10−5 10−4 10−4 10−6 10−5 10−5 10−5 10−4 10−4

(2.3) (1.8) (2.1) (2.3) (2.2) (2.3) (2.3) (2.8) (3.2) (3.1)

τ2 (s)

1.4 1.0 4.7 3.0 1.2 4.7 1.7 1.1 2.2 1.4

× × × × × × × × × ×

104 104 103 103 105 104 104 104 103 103

zeolites with n = 3.4 and M = Rb+ and Cs+ exhibit one supplementary spectrum with bands at 560 and 620 nm. Two supplementary spectra were detected for the following samples: n = 3.4 and M = Na+, K+; n = 6.6 and M = Na+, K+, Rb+, Cs+. A first pure spectrum displays a sharp absorption at 474 nm and broad absorption near 730 nm (Table 2). These two band spectra are readily assigned to t-St•+ occluded in the 10-MR channel of MnMFI. This finding is based on the similarities in wavelength, relative intensity, and shape of bands with the known spectrum of t-St•+ in solution and solid supports. The second resolved spectrum exhibits two prominent bands near 560 and 620 nm for all the irradiated t-St@MnMFI samples (n = 3.4, 6.6; M+ =Na+, K+, Rb+, Cs+), sometimes with the additional appearance of minor bands near 510 and 705 nm (Table 2). Some weak wavelength shifts and some relative intensity changes of the bands were observed according to the aluminum content (n) and the nature of M+ (Table 2). The spectra with bands at 560 and 620 nm do not correspond to any known species related to trans- and cis-St, t-St•− anion, or St•+ isomer and dimer.17,19,41 These intense spectral features are assigned to the vibrational structure of a partial hole back transfer band of the t-St···AlO4•+ entity.28 This phenomenon was previously observed after spontaneous ionization and photoionization of several electron donor molecules occluded within ZSM-5 (MFI).14,39 Data processing gives the spectral contributions C(t) of t-St•+ as a function of time for every experiment for n = 6.6 and M = Na+, K+, Rb+, and Cs+ and for n = 3.4 and M = Na+ and K+. For n = 3.4 and M = Rb+ and Cs+ the t-St•+ lifetime is too short to be detected by the conventional technique but is clearly observed in the TDR experiment. Figures 2B and 3B exhibit the normalized C(t)/C0 decays as a function of time obtained after the photolysis of [email protected] and [email protected]. Photoinduced t-St•+ was assumed to disappear according to two parallel competitive ways: direct charge recombination (CR1) and hole transfer (HT) following reactions R2 and R3, respectively. t ‐St•+@M nZeo•− (k CR1) → t ‐St@M nZeo

(R2)

with MnZeo = MnMFI. 9097



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Figure 3. (A) Diffuse reflectance UV−visible (DRUVv) spectra recorded after laser photoirradiation of [email protected] as a function of time (266 nm, 15 s, 30 mJ cm−2): (a) before photoirradiation; (b) 2, (c) 40, (d) 180, (e) 660, and (f) 4000 min after photoirradiation. (B) Decay profiles of Ct/C0 relative to spectral concentration of t-St•+@ Cs6.6MFI•− (squares) and [email protected]•+•− (circles). The solid lines represent the best-fit calculated decays using the Albery function; the squares and circles represent the experimental points.

Figure 2. (A) Diffuse reflectance UV−visible (DRUVv) spectra recorded after laser photoirradiation of [email protected] as a function of time (266 nm, 15 s, 30 mJ cm−2): (a) before photoirradiation; (b) 2, (c) 10, (d) 60, (e) 500, and (f) 1000 min after photoirradiation. (B) Decay profiles of Ct/C0 relative to spectral concentration of t-St•+@ K6.6MFI•− (squares) and [email protected]•+•− (circles). The solid lines represent the best-fit calculated decays using the Albery function; the squares and circles represent the experimental points.

t ‐St•+@M nZeo•− (kHT) → t ‐St@M nZeo•−•+

(R3)

with MnZeo = MnMFI. The contributions C(t) of t-St@MnMFI•−•+ as a function of time were also obtained by data processing for all the experiments. Figures 2B and 3B (black circles) exhibit the normalized C(t)/C0 decays obtained for [email protected] and [email protected]. All the decays of the electron−hole pair spectral concentrations were successfully simulated using the Albery model (blue curves). The resulting rate constants (k̅CR2) and corresponding lifetimes (1/k̅CR2) are listed in Table 3. The lifetime of the electron−hole moiety is 1.2 × 105 s in Na6.6MFI and decreases on going from Na+ to Cs+ and on going from n = 6.6 to n = 3.4 to be 3.0 × 103 s in Cs3.4MFI. The DRUVv spectra carried out after the 15 s UV irradiation (λ = 266 nm) of t-St occluded in purely siliceous silicalite-1, tSt@MFI (n = 0.0), did not provide any evidence of long-lived photoinduced species using the conventional technique. In contrast, TDR experiments provide kinetic traces from 0.5 μs to 800 ms in the 430−800 nm wavelength range following 266 nm laser pulses. Typical TDR reconstructed spectra corresponding to various delay times are presented in Figure 4A. The spectra are characterized by a prominent band

with MnZeo = MnMFI. The creation of the [email protected]•−•+ electron−hole entity from t-St•[email protected]•− was previously detected in a preliminary publication and is again observed in this work.28 All the t-St•+ decays (black squares) were successfully simulated using the Albery kinetic model in view of the randomness of the residuals between experimental data and calculated values (red curves) (Figures 2B and 3B). The best values of the first order k̅obs = k̅CR1 + k̅HT average rate constants with the corresponding γ Gaussian distribution coefficients are listed in Table 3. It is clear that the lifetime (1/k̅obs) of photoinduced t-St•+ which reaches 1.3 × 103 s in Na6.6MFI decreases on going from Na+ to Cs+ and on going from n = 6.6 to n = 3.4. The electron abstraction or hole transfer (R3) is assumed to be faster than the direct charge recombination R2 (kHT > kCR1). After the diappearance of t-St•+, the charge recombination occurs through reaction R4 to retrieve neutral t-St. t ‐St@M nZeo•−•+ (k CR2) → t ‐St@M nZeo

(R4) 9098



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Figure 5. (A) Transient diffuse reflectance (TDR) spectra collected at specified times following 266 nm laser excitation of t-St@K−MOR: (a) 0.05, (b) 2, (c) 7.5, (d) 25, (e) 50, (f) 200, and (g) 450 ms. (B) Normalized kinetic traces for the decay of t-St•+@K−MOR•− monitored at 475 nm following 266 nm laser excitation. The solid line represents the best-fit calculated decay using the Albery function; the squares represent the experimental points.

Figure 4. (A) Transient diffuse reflectance (TDR) spectra collected at specified times following 266 nm laser excitation of t-St@MFI (silicalite-1): (a) 0.002, (b) 0.2, (c) 4, and (d) 18 ms. (B) Normalized kinetic traces for the decay of t-St•+@MFI•− monitored at 475 nm following 266 nm laser excitation. The solid line represents the best-fit calculated decay using the Albery function; the squares represent the experimental points.

centered at 475 nm assigned to t-St•+.42 No spectral evidence of an ejected electron is observed. The normalized kinetic trace for the decay of t-St•+ following laser excitation of t-St@MFI is accurately simulated according to the heterogeneous Albery kinetic model (Figure 4B). The corresponding t-St•+ lifetime is 1.2 × 10−2 s in siliceous MFI (Table 3). Decrease in the lifetimes of t-St•+ and the electron−hole entity upon going from n = 6.6 to 3.4 and 0.0 and from Na+, K+, and Rb+, to Cs+ demonstrate that the trapped electron is readily available for charge recombination in the larger cation exchanged MnMFI zeolites and at low aluminum content (Table 3). 3.2.3. [email protected]. Studies of [email protected], with M+ = Na+, K+, Rb+, and Cs+ were carried out using both TDR and conventional DRUVv techniques with 266 nm excitation. TDR transient spectra were recorded in the 100 ms time range following 266 nm laser pulses, and conventional DRUVv spectra were recorded in the day time range following 15 s irradiation. Typical TDR spectra of [email protected] and t-St@ Rb4.4MOR are presented in Figures 5A and 6A, respectively. All TDR experiments provide evidence of transient spectra characterized by a prominent band centered at 475 nm assigned straightforwardly to t-St•+. Data processing of the TDR spectra

sets resolves only one spectral component (Table 2). In contrast, the conventional DRUVv spectra provide evidence of persistent bands at 562 and 627 nm following the 15 s irradiation for [email protected] with M+ = Na+ and K+, but not with M+ = Rb+ and Cs+. Figure 7A shows the DRUVv spectra recorded after 15 s irradiation of [email protected]. Data processing of DRUVv sets recorded after the irradiation of t-St@MnMOR (M = Na+, K+) resolves two types of pure spectra: spectra with one prominent band centered around 309 nm and spectra with absorption bands centered around 562, 627, and 705 nm assigned to the hole−electron entity (Table 2). The normalized kinetic traces for the decay of t-St•+ obtained by TDR of [email protected] (Figures 5B, 6B) are simulated by the heterogeneous Albery kinetic model and provide the kinetic constants (kobs = kCR1 + kHT) listed in Table 3. The normalized C(t)/C0 decays of [email protected]•−•+ (M = Na+, K+) were also successfully simulated using the Albery model (Figure 7B). The resulting rate constants (k̅CR2) and corresponding lifetimes (1/k̅CR2) are listed in Table 3. The lifetimes of t-St•+ are relatively short in M4.4MOR and decrease on going from Na+ to Cs+, but the duration of t-St•+ is sufficiently long (∼1 s) with Na+ and K+ permitting hole 9099



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Figure 7. (A) Diffuse reflectance UV−visible (DRUVv) spectra recorded after laser photoirradiation of t-St@K−MOR as a function of time (266 nm, 15 s, 30 mJ cm−2): (a) before photoirradiation; (b) 2, (c) 10, (d) 60, and (e) 600 min after photoirradiation. (B) Decay profiles of Ct/C0 relative to spectral concentration of t-St@K− MOR•+•−. The solid lines represent the best-fit calculated decays using the Albery function; the circles represent the experimental points.

Figure 6. (A) Transient diffuse reflectance (TDR) spectra collected at specified times following 266 nm laser excitation of t-St@Rb−MOR: (a) 0.1, (b) 4, (c) 20, (d) 80, (e) 200, and (f) 800 ms. (B) Normalized kinetic traces for the decay of t-St•+@Rb−MOR•− monitored at 475 nm following 266 nm laser excitation. The solid line represents the best-fit calculated decay using the Albery function; the squares represent the experimental points.

transfer to generate persistent [email protected]•−•+ hole− electron moieties which reach 2.2 × 103 s lifetime. In contrast, no hole transfer is observed in M4.4MOR (M = Rb+, Cs+).

channel. Furthermore, the dramatic decrease in the lifetime of entrapped t-St•+ on going from Si/Al = 10 to Si/Al ∼ 1000 demonstrates the important role of the framework Al(III) content in the compartmentalization of an ejected electron away from the initial ionization site. The marked decrease in the lifetime of occluded t-St•+ upon going from Na+ to Cs+ extraframework cation shows that the trapped electron is readily available for charge recombination in the larger cation exchanged zeolites. The close match between t-St width and 10MR channel diameter is also important to hindering the chargeshifting reactions which occur during the course of recombination processes. Photoionization of aromatic compounds and particularly t-St has been studied extensively in the condensed phase. In nonpolar solvents, immediately following photoionization, a photoelectron ejected from t-St via the S1 excited state scatters and becomes thermalized at a radical cation−electron distance much smaller than the average Onsager radius. Thus, the majority of the electrons geminately recombine within several picoseconds. In polar solvents, such as acetonitrile, the UV ionization of t-St results in the generation of solvated t-St•+ free ion via the S1 excited state in tens of picoseconds. From

4. DISCUSSION 4.1. Zeolite Chemical Control Parameters. The original aim of this work was to investigate the influence of the channel diameter, framework aluminum content, and extraframework cation M+ on the charge recombination rates following laser photoionization of the electron donor molecule t-St occluded in nonacidic medium pore Mnzeolites. Our hope was to detect the main chemical parameters that delay the recombination of photogenerated charges. The spectroscopic and kinetic results of the study are displayed in Tables 1−3 and detailed above. They demonstrate that the entrapment of t-St inside the 10membered-ring channel of the aluminum-rich framework is critical to efficiently stabilizing the photoinduced t-St•+− electron separation for hours and even days at room temperature. The dramatic decrease in the lifetime of occluded t-St•+ on going from 10-MR = 0.42 nm × 0.54 nm to 12-MR = 0.67 nm × 0.76 nm points out the necessity of a close match between the width of the guest and the diameter of the host 9100



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previous work, it is concluded that t-St•+ is formed via the twophoton absorption of ultraviolet pump radiation. The solvated t-St•+−electron pair formation competes with geminate recombination and exhibits a lifetime of more than nanoseconds. No trans−cis isomerization was detected in t-St•+ during the transient studies.40,43 Photoinduced t-St•+ adsorbed on external surface of solid supports (silica, alumina, NaA zeolite) have lifetimes on the order of 100 μs, while t-St•+ photogenerated within the porous voids of large supercavities of faujasitic zeolites were reported to be of a similar time scale.22−24 It should be noted that t-St isomerization and oligomerization in solution and on several suitable solid supports were reported previously upon steady state direct excitation, through triplet sensitization, electron transfer sensitization, and proton catalysis, while the influence of the cosorbed solvent on the isomerization of t-St included in zeolites was also reported.25−27,44,45 The diffusion, distribution, and location of t-St in the porous framework are important factors in photochemical reactions in zeolites.8,9,46,47 We emphasize that, in the t-St@MnZeo samples under study, t-St is located inside the channels of medium pore zeolites (FER, MFI, MOR) and not adsorbed on the external surface of zeolite microcrystals. Long periods are required following the mixing of dehydrated zeolites and solid t-St to reach equilibrium. The channel openings are large enough in dehydrated FER, MFI (10-MR, ∼0.5 nm), and MOR (12-MR, ∼0.7 nm) to accommodate t-St within the channels (Table 1). The accommodation of t-St inside the 10-MR channel is particularly slow for M−FER. In contrast, t-St remains located on the external surface of NaA zeolites even after a long equilibrium period due to exclusion by the small entry of 8-MR pores (∼0.4 nm). Upon inclusion in nonacidic zeolites, t-St is thermally stable. Following direct photonic excitation of t-St included within the channel of M−FER, M−MFI, and M− MOR (M = Na+, K+, Rb+, Cs+), the t-St ground state is eventually re-formed without any isomerization or oligomerization. 4.2. t-St•+ Lifetime. The TDR and DRUVv spectra recorded immediately after 266 nm laser excitation of aluminum-rich t-St@MnZeo (Si/Al ∼ 10) at low t-St coverage indicate that t-St•+ is formed primarily by very fast photoinduced electron ejection via reaction R1. The analysis of t-St•+ decays in Table 3 show that lifetimes of photogenerated t-St•+ can reach 10 h (τ = 3.5 × 104 s) in aluminated K3.3FER zeolites (Si/Al = 10), where the width of the guest t-St is commensurate with the channel interior diameter of the host (10-MR = 0.42 nm × 0.54 nm). The t-St•+ lifetime decreases markedly to 20 min (τ = 1.2 × 103 s) as the channel interior diameter increases a little, as in K6.6MFI (10-MR = 0.53 nm × 0.56 nm) with Si/Al ∼ 13.5, while the t-St•+ lifetime (τ = 0.4 s) decreases dramatically as the channel interior diameter increases markedly in K4.4MOR (12-MR = 0.67 nm × 0.76 nm) with Si/Al = 10. For comparison, in aluminum-rich faujasitic zeolites (Si/Al ∼ 2) with large supercage (∼1.3 nm) photoinduced t-St•+ was observed over 150 μs.22−24 For every type of zeolites MnFER, MnMFI, and MnMOR with similar Si/ Al ratios (Si/Al ∼ 10), the t-St•+ lifetime decreases with the increasing size of the alkali metal cation M+ (Table 3). The same trend applies to the Si/Al ratio. For example, the t-St•+ lifetime decreases with the increasing Si/Al ratio in KnMFI from 20 min (τ = 1.2 × 103 s) in K6.6MFI (Si/Al = 13.5) to 11 min (τ = 6.6 × 102 s) in K3.4MFI (Si/Al = 27) and reduces to 12 ms (τ = 1.2 × 10−2 s) in siliceous MFI (Si/Al ∼ 1000). Although the

photoejected electron was not characterized in the present work by TDR and DRUVv spectroscopies, prior works using NIR absorption and pulsed EPR techniques have demonstrated the presence of trapped electrons after photoionization.48 4.3. Hole Transfer. In addition to direct t-St•+−electron recombination (R2), the supplementary charge shift reaction (R3) can contribute significantly to the decay of t-St•+. HT was observed in MnMFI and MnMOR from t-St•+ to the AlO4 group in close proximity to create the t-St−AlO4•+ entity.49 HT generates persistent distant AlO4•+···AlO4•− entities abbreviated t-St@MnZeo•−/•+ (M = Na+, K+, Rb+, Cs+) for MFI and MOR zeolites. Surprisingly, HT is not observed in MnFER zeolites with similar Si/Al ratios of ∼10. In M3.3FER the mode of t-St•+ decay (kCR1) is exclusively long-range recombination R2 (tSt•+···AlO4•−). HT was previously reported in the back electron transfer sequence following photoionization of polyaromatics such as biphenyl, naphthalene, and p-terphenyl occluded in MFI.14,50,51 In contrast, HT was not observed at room temperature for anthracene under analogous experimental conditions, but reversible HT is observed upon heating to around 120 °C.39 In irradiated t-St@MnMFI, the main mode of t-St•+ decay appears to be HT (R3). However, it is difficult to determine separately the k̅HT and k̅CR1 values from experimental k̅obs1 values of t-St•+ decays (Table 3). We assume k̅HT ≫ k̅CR1 for aluminum-rich MnMFI because kCR1 is relatively weak, but this approximation is not valid for MnMOR because kCR1 is assumed to be relatively large with respect to the experimental data. Hole transfer can be explained in MnMFI through the oxidizing power of t-St•+ with respect to the electron donor ability of AlO4 groups in close proximity, independently of the higher donor ability of the more distant AlO4•− group.52 The oxidizing power can be estimated from the t-St oxidation potential in solution (E0 = 1.75 V versus SCE). The E0 = 1.75 V value explains HT (R3) in MnMFI by comparison with analogous HT of biphenyl radical cation (E0 = 1.96 V) and naphthalene radical cation (Eox = 1.54 V), while no HT of anthracene radical cation (E0 = 1.09 V) was detected. Unfortunately, the E0 = 1.75 V value alone is unable to explain at the same time HT in MnMFI and MnMOR and the absence of HT in MnFER with similar values of Si/Al ∼ 10 and similar expected electron donor properties of the AlO4 group. We will suggest below an interpretation of these striking findings using the Marcus theory. The most remarkable feature of the hole formation is the intense color due to the prominent absorption bands in the visible range of the t-St-AlO4•+ entity. The bands listed in Table 2 are assigned to photoinduced partial hole back transfer t-St-AlO4•+ + hν →t-St•+−AlO4.28 This situation requires a delicate balance between the ionization potential of the electron donor t-St and the electron affinity of AlO4. The charge transfer nature of the visible bands is reinforced by the vibrational structure of the spectra, because bands are separated by approximately 1600 cm−1 in energy for all the electron−hole moieties in MnMFI and MnMOR; these gaps correspond to internal stretching modes of t-St•+. The decays of the t-St@ MnZeo•−/•+ (M = Na+, K+, Rb+, Cs+) hole−electron entities for MFI and MOR zeolites go to completion via electron−hole recombination (R4) (AlO4•+···AlO4•−). The kinetic results (kobs2) provide evidence that the t-St@MnZeo•−/•+ lifetimes are markedly longer than those of the corresponding t-St•+@ MnZeo•− moiety (Table 3). The lifetimes of (AlO4•+···AlO4•−) can reach 33 days (τ = 1.2 × 105 s) in the 10-MR channel of Na6.6MFI, while the (AlO4•+···AlO4•−) lifetime is reduced to 37 min (τ = 2.2 × 103 s) in the larger 12-MR channel of 9101



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Na4.4MOR with similar Si/Al ratios of ∼10. For MnMFI and MnMOR with similar Si/Al ratios (Si/Al ∼ 10) the electron− hole lifetime decreases gradually with the increase of the size of the alkali metal cation M+ (Table 3). 4.4. Long-Range Electron Transfer. The extremely slow CR rate observed inside aluminated zeolites such as FER and MFI suggests the occurrence of long-range ET.53 It is now well established that the ET rate falls off exponentially when the distance (r) between the donor and acceptor moieties is increased, provided that the other parameters remain the same. The Albery model used to determine heterogeneous rate constants presents a Gaussian distribution of the kobs about the mean k̅obs value with the width of the distribution (γ). It is tempting to relate the Gaussian distribution of kobs to the distribution of r about some mean value (r). ̅ Unfortunately, we are unable to provide a clear relation between the distribution of r ̅ and the k̅obs Gaussian distribution. Some information about r ̅ comes from previous work concerning long-lived spin pairs generated by UV irradiation of naphthalene in MnMFI. The average distance between AlO4•+ and AlO4•− groups was found to be 1.3 nm corresponding to the nearest Al−Al distance in the M6.6MFI, while electrons were found to be delocalized on the oxygen atoms of an −Al−O(M+)−Si− bridge.14 In summary, we have highlighted that the general back electron transfer processes following UV photoionization (R1) include direct CR (R2), HT (R3), and electron−hole recombination (R4) with k̅CR1, k̅HT, and k̅CR2 mean rate constants, respectively. While it was not possible to determine k̅CR1 and k̅HT separately from k̅obs1 for M−MFI and M−MOR, the Marcus theory of ET provides a valuable guide to the chemical control of the nonradiative ET rates of photoinduced charges of occluded t-St using the channel diameter, Si/Al ratio, and extraframework cation of zeolites. Efforts have been made to establish the respective driving force (−ΔG0) dependence of k̅CR1, k̅HT, and k̅CR2 mean rates.54

alkali metals (Na, K, Rb, Cs) demonstrate that EF values decrease in the following order: Cs+ > Rb+ > K+ > Na+.55 The electrons released by ionization of the alkali metal are trapped within the channels, and their donor properties are assumed to be analogous to those of photoejected electrons in the systems under study. It is reasonable to assume that the E0(Z/Z•−) values exhibit trends analogous to those values of the Fermi level EF of MFI zeolites doped with alkali metals: E0(Z/Z•−) values decrease in the order Cs+ > Rb+ > K+ > Na+. Consequently, the driving force (−ΔG0) of reaction R2 increases with aluminum content in the following order: Cs+ < Rb+ < K+ < Na+. For reaction R3 (kHT), E0(D•+/D) is E0(t-St•+/t-St) and 0 E (A/A•−) refers to the E0(Z•+/Z) potential of the zeolite. Unfortunately, the electrochemical response of zeolite-modified electrodes cannot provide the E0(Z•+/Z) oxidation potential of dehydrated zeolites. It was reported that the electron-donating ability of zeolites increases with increasing aluminum content and for zeolites with larger alkali metal cations. 12,56 Consequently, E0(Z•+/Z) is assumed to decrease in the following order : Na+ < K+ < Rb+ < Cs+. Thus the driving force of reaction R3 increases in the order Na+ < K+ < Rb+ < Cs+. For reaction R4 (kCR2), E0(D•+/D) is E0(Z•+/Z) and E0(A/ A•−) refers to E0(Z/Z•−). It is deduced that the driving force of reaction R4 increases in the order Cs+ < Rb+ < K+ < Na+ and with the aluminum content. It was reported that the rigid framework which remains unchanged during the electron transfer process keeps the total reorganization energy (λ) small.57,58 It is expected that λ is largely controlled by the tight fit between sorbate size and pore dimensions and decreases with the nearest distance between tSt and the wall of the zeolite channels (Table 1). According to Marcus theory and given constant λ and H values, the ET rate first increases with increasing −ΔG0 (normal region), reaches a maximum, and then decreases for very large values of −ΔG0 (inverted region). The bell-shaped driving force dependence of average ET rate constants between several electron donors and Fe(bpy)33+ across a faujasitic NaY zeolite−solution interface was demonstrated previously, including both normal and inverted regions.58 The rates of ET can be markedly slowed by shifting them deep into the inverted region of the Marcus parabola where −ΔG0 is larger than λ. Using the reasonable hypotheses made to evaluate the driving force trends, we discuss now the chemical control of the k̅CR1, k̅HT, and k̅CR2 rate constants according to three main viewpoints: internal channel diameter, aluminum content (Si/Al), and extraframework cations (M+) through the Marcus theory. 4.4.1. Channel Diameter. It is reasonable to assume that the driving forces of reaction R2 (kCR1) charge recombination are similar for t-St•+@MnZeo•− entities (Zeo = FER, MFI, or MOR) when the aluminum contents (Si/Al ∼ 10) are similar and the extraframework cations (M+) are identical. In addition, the mean distant values r ̅ between t-St•+ and trapped electron are expected to be similar in all the systems with similar Si/Al ratios because these distances correspond approximately to the Al−Al distances in the framework.14 By taking into account these latter conditions, H2 = H02 exp[−βr] values are similar and the variation of kET in the famous Marcus relationship is reduced approximately to

kET ̅ = [4π 3/(h2λkBT )]1/2 |H |2 exp[− (ΔG 0 + λ)2 /(4λkBT )]

(3)

where H is the electronic coupling between the donor and acceptor states; H2 is given by H2 = H0 2 exp[−βr ]

(4)

2

The H term represents electron tunnelling through a potential barrier and therefore shows an exponential dependence upon spatial separation r, where β is a function of the barrier height of the media. The mean observed rate k̅obs probably corresponds to the mean separation r.̅ −ΔG0 is the driving force for the ET process with ΔG0 corresponding to the difference in free energy between electron donor and electron acceptor moieties. An accurate determination of the driving force of the ET rate of reactions R2, R3, and R4 requires knowledge of the redox potentials of electron donor D and electron acceptor A. −ΔG 0 = e[E 0(D•+ /D) − E 0(A/A•−)] 0

•+

(5) 0

•+

For reaction R2 (kCR1), E (D /D) is E (t-St /t-St) with a redox potential E0 = 1.75 V vs SCE measured in solution and E0(A/A•−) refers to the E0(Z/Z•−) redox potential of the zeolite. Unfortunately, the electrochemical response of zeolitemodified electrodes cannot provide the E0(Z/Z•−) reduction potential of dehydrated zeolites. Nevertheless, theoretical calculations of the Fermi level EF of MFI zeolites doped with

k CR1 ̅ ≈ exp[− (ΔG 0 + λ)2 /(4λkBT )] 9102

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With similar driving force (−ΔG0) values, very different k̅CR1 kinetic constants are observed in M−FER, M−MFI, and M− MOR zeolites. For example, weak k̅CR1 (2.8 × 10−5 s−1) is observed in the narrow 10-MR channel of K3.3FER (Si/Al = 10) while a relatively large k̅CR1 is observed in the 12-MR channel of K4.4MOR (Si/Al = 10) (Table 3). According to eq 6, a small λ value (λ ≪ ΔG0) is critical to induce a very slow CR rate. It is obvious that λ and channel diameters are closely linked. The confinement effect can be evaluated by the nearest interatomic distances between t-St and the wall of the zeolite channels (Table 1). It seems plausible that a small λ value induces slowing down of CR rates (k̅CR1) by shifting them deep into the inverted region of the Marcus parabola of M−FER, where the driving force (−ΔG0) is larger than the total reorganization energy (λ). Theoretically, the same simplified equation could be used to explain the large disparities in the HT hole transfer rate constants (k̅HT). However, the ambiguity about the determination of k̅HT with respect to k̅CR1 from k̅obs1 does not permit concrete conclusions to be made. Nevertheless, it is clear that HT (R3) occurs in M−MFI and M−MOR but does not occur in M−FER. For M−FER, M−MFI, and M−MOR with similar driving forces, a very small λ value in M−FER can explain the very slow HT rate by shifting it deep into the inverted region. It seems plausible that the smaller λ value in M−MFI than in M−MOR with similar driving forces induces the slowing down of CR (R4) rates (k̅CR2) by shifting them into the inverted region of the Marcus parabola for all types of cations (M+ = Na+, K+, Rb+, Cs+). 4.4.2. Framework Aluminum Content. The experimental k̅obs1 and k̅obs2 mean rate constants corresponding to CR of irradiated t-St@MnMFI (n = 0, 3.4, 6.6) with identical extraframework cation M+ are observed to decrease with the aluminum content n. Similar features are observed for all the cations (M+ = Na+, K+, Rb+, Cs+). The driving forces (−ΔG0) of reactions R2 and R4 for irradiated t-St@MnMFI (n = 0, 3.4, 6.6) are expected to increase with the aluminum content (n = 0, 3.4, 6.6). By increasing the aluminum content in MnMFI with similarly small λ values, the kCR1 and kCR2 rate constants are markedly lowered by shifting the driving forces into the inverted region of the Marcus parabola (eq 3). The same trends were reported for the effect of aluminum content on the CR rate for anthracene radical cation−electron pairs photogenerated in NanMFI.59 The experimental features can be explained in terms of an inverted region effect of the Marcus relationship. 4.4.3. Extraframework Cation. All the experimental k̅obs1 and k̅obs2 mean rate constants corresponding to CR of photogenerated t-St•+@MnZeo•− and t-St@MnZeo•−•+ are observed to decrease in the order Cs+ > Rb+ > K+ > Na+ for M−FER, M−MFI, and M−MOR with Si/Al ratio ∼ 10. Similar findings were reported after laser irradiation of electron donor molecules occluded in zeolites.14,59,60 Based on these results and on the driving force dependence of reactions R2 (kCR1) and R4 (kCR2) in the order Cs+ < Rb+ < K+ < Na+, we can conclude that ETs of these systems fall in the inverted region of the Marcus parabola in which the back ET rate constants decrease gradually with an increase of the driving force (−ΔG0). This interpretation is based on two main hypotheses: (i) λ values of energy reorganization are relatively small (λ < ΔG0) and do not depend on the cation size; (ii) H2 = H02 exp[−βr] values are assumed to be approximately constant for similar Si/Al ratios. The latter hypothesis is based on a previous work devoted to laser irradiation of naphthalene@M−MFI. EPR results indicate

that the average separation values r ̅ between photoinduced charges are around 1.3 nm for M = Li+, Na+, K+, Rb+, and Cs+. This distance corresponds approximately to the nearest Al−Al distances in the framework.14 From these results the alkali metal has a weak influence on the distance of electron transfer in medium pore zeolites (Si/Al ∼ 10) but has a marked influence on the driving force.

5. CONCLUSIONS In this work we have conducted time-resolved UV−visible absorption spectroscopy experiments from nanosecond to day time scale following nanosecond UV (266 nm) pulsed laser irradiation of trans-stilbene (t-St) occluded in channels of dehydrated nonacidic M−FER, M−MFI, and M−MOR zeolites with different pore diameters, with differing framework aluminum content, and with different extraframework cations (M = Na+, K+, Rb+, and Cs+). The diffuse reflectance technique is successfully applied using both nanosecond transient spectroscopy and conventional techniques. The cation radical of trans-stilbene (t-St•+) and a trapped electron (AlO4•−) have been generated directly by means of pulsed laser-induced electron transfer within the channels of M−FER, M−MFI, and M−MOR zeolites. In summary, we have highlighted that the general back electron transfer processes following UV photoionization include direct charge recombination (CR), hole transfer (HT), and finally electron−hole recombination to reform the occluded t-St ground state after a more or less long period without any isomerization or oligomerization. The kinetics of t-St•+ and AlO4•+ hole decays are accurately reproduced using the Albery function based on dispersion of heterogeneous rate constants determined for a large range of time scales, from milliseconds to several hours. The t-St•+ decay times depend dramatically on the type of zeolite porous structures with similar Si/Al ratios of ∼10 and identical extraframework cation. The t-St •+ lifetime is particularly long (∼10 h) within the narrow 10-memberedring channel of M−FER zeolite, while it is shorter (∼20 min) in the moderately larger (10-MR) channel of M−MFI and dramatically shorter (

Chemical Control of Photoinduced Charges under ... - ACS Publications

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