Spontaneous Charge Separation and Recombination Induced by

In situ CW-EPR and diffuse reflectance UV−visible spectroscopy were used to monitor the spontaneous incorporation of ACENE-4 or tetracene (C18H12) i...
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J. Phys. Chem. C 2007, 111, 17346-17356

Spontaneous Charge Separation and Recombination Induced by Tetracene Incorporation in Pores of Acidic HnZSM-5 Zeolites Se´ verine Marquis,† Alain Moissette,†,* Matthieu Hureau,† Herve´ Vezin,‡ and Claude Bre´ mard† Laboratoire de Spectrochimie Infrarouge et Raman UMR-CNRS 8516, Centre d’ Etudes et de Recherches Lasers et Applications, FR-CNRS 2416, Baˆ t. C5 UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France, and Laboratoire de Chimie Organique et Macromole´ culaire, UMR-CNRS 8009, Baˆ t. C4 UniVersite´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: June 14, 2007; In Final Form: September 4, 2007

In situ CW-EPR and diffuse reflectance UV-visible spectroscopy were used to monitor the spontaneous incorporation of ACENE-4 or tetracene (C18H12) in the medium-pore MnZSM-5 zeolites [Mn(AlO2)n(SiO2)96-n; MdNa+, H+; n ) 3.4, 6.6] by direct exposure under dry and inert atmosphere of solid ACENE-4 to dehydrated porous materials without any solvent. The sorption of the large ACENE-4 molecule with relatively low ionization potential (6.97 eV) occurs in Brønsted acidic HnZSM-5 zeolites according to a complex and slow reaction sequence including protonation, charge separation, hole transfer, and charge recombination while ACENE-4 is incorporated as an intact molecule in nonacidic NanZSM-5. After a long organization period, ACENE-4 lies in the straight channel in front of sodium cation in close proximity of Al framework atom as simulated by Monte Carlo calculations. The multivariate curve resolution (MCR) analysis of the huge DRUVv spectra set recorded during the ACENE-4 sorption course in acidic HnZSM-5 resolved successfully the specific absorption spectra and respective concentrations of all species as function of time. HACENE-4+@Hn-1ZSM5- protonated species and ACENE-4•+@HnZSM-5•- radical pair are generated in the first steps of sorption while a long-lived ACENE-4@HnZSM-5•-•+ electron-hole pair is formed through hole transfer and recombines slowly to ACENE-4@HnZSM-5 without any protonation. Two-dimensional hyperfine-sublevel correlation (2D-HYSCORE) experiments reveal the structural surroundings of the unpaired electrons through the proper assignment of unpaired electron couplings with 1H, 29Si, and 27Al nuclei. The tight fit between the rod shape ACENE-4 and the pore size of ZSM-5 zeolites combined with the efficient polarizing effect of proton and aluminum electron trapping sites are the most important factors responsible for the stabilization of the electronhole moiety and hinder the charge recombination efficiently.

Introduction Polycyclic aromatic hydrocarbons, or simply PAHs, represent one of the most stable families of organic compounds known. PAHs are the dominant class of molecule species in the interstellar medium. PAHs are also byproducts of petroleum manufacture and combustion processes. They represent important persistent environmental pollutants. In recent years, PAHs, especially tetracene and pentacene, are of current interest for their potential applications in organic electronic devices. The design of novel hybrid materials by the incorporation of organic molecules into porous materials is a major goal in materials science, and the elucidation of the underlying host-guest interactions is a subject of basic research. Zeolites are crystalline aluminosilicates characterized by strictly regular porous structures. Zeolites are molecular sieves traditionally used especially in petrol chemistry and gas separation. The interactions of various PAHs with a large range of zeolites have been investigated previously.1-8 However, the studies concerning the sorption of large PAHs such as tetracene and pentacene onto zeolites are less documented than those of anthracene and derivatives. One of the most intriguing properties of zeolites is their ability to promote * Corresponding author. E-mail: [email protected]. † Laboratoire de Spectrochimie Infrarouge et Raman. ‡ Laboratoire de Chimie Organique et Macromole ´ culaire.

spontaneous ionization through mere incorporation of electrondonor molecules such as PAHs in their porous networks. The basis of the spontaneous ionization phenomena upon mere incorporation in zeolites was reviewed recently.5,9 Many organic radical cations have been obtained in zeolites from a wide range of substrates, including PAHs even with relatively high ionization potential. In contrast, there is limited experimental information to identify the fate of ejected electrons, which is the second part of the ionization phenomenon.9 The ability of zeolites to generate radical cations is often related to the presence of acid sites, although, spontaneous ionization was reported recently in non-Brønsted acidic zeolites.10 Protonic zeolites are important acidic and superacidic catalysts in several reactions involving carbocationic intermediates. Therefore, adsorption onto acid zeolites may convert PAHs into carbocations and radical cations concurrently.11 The entrapment within pores or cavities of zeolites can limit the tendency of free radicals to dimerize and prevents access of reagents that typically would cause their rapid decay in solution.12 The tight fit between the rod-shaped PAH’s molecular size and the ZSM-5 zeolite channel was reported to be the major feature to trap long-lived radical pairs in the porous void space.9 The acene oligomer containing four unit cells of benzene was noted tetracene, 2,3-benzanthracene, or naphthacene; it is noted hereafter as ACENE-4. ACENE-4 is a long rod-shaped molecule with relatively low ionization potential in the gas phase (6.97

10.1021/jp074607e CCC: $37.00 © 2007 American Chemical Society Published on Web 10/30/2007

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eV).13 ACENE-4 has suitable molecular dimensions to plug the 10-ring oxygen window of the ZSM-5 channels and to diffuse in the pore networks. ACENE-4 was demonstrated previously to go safely into the porous void space of zeolites with large cavities such as faujasitic zeolites.14 In contrast, ACENE-4 migration into the channels of medium-pore zeolites such as ZSM-5 is expected to be hindered by the narrowness of the pore openings with respect to previous works related to the sorption of anthracene (ACENE-3).7,15,16 A previous paper has shown that molecules with ionization potentials below 7.0 eV can be ionized through sorption in large-cavity faujasitic zeolites. Nevertheless, the ionization yields were found to be very weak.17 In this present work, we employ diffuse reflectance UVvisible absorption (DRUVv) and X-band continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy to monitor the course of ACENE-4 sorption in Brønsted acidic medium-porezeolitesHnZSM-5(n)3.4,6.6)withHn(AlO2)n(SiO2)96-n unit cell formula, under dry and inert atmosphere without any solvent. The incorporation of ACENE-4 in non-Brønsted acidic Na+ forms of NanZSM-5 was also investigated for comparison. The multivariate chemometric methods were essential to resolve the UV-visible absorption spectra and molecular concentrations of pure species involved in the sorption course. We present evidence of the detailed mechanisms of ACENE-4 spontaneous and slow incorporation in acidic and nonacidic ZSM-5 zeolites. An unusual complex and slow reaction sequence was determined during the ACENE-4 sorption in Brønsted acidic ZSM-5. It includes protonation, charge separation, electron transfer, and charge recombination. 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 (1H, 13C, 29Si, or 27Al) using the two-dimensional hyperfine-sublevel correlation experiment, termed 2D-HYSCORE. The structural situation of ACENE-4 occluded as an intact molecule in the pore of Na4ZSM-5 was simulated by Monte Carlo calculations. We compare and discuss the ACENE-4 behavior with those of ACENE-n (n ) 2, 3) upon incorporation into acidic zeolites and dissolution in acidic and oxidizing media.

ACENE-4 Sorption in MnZSM-5. Weighted amounts (∼1.4 g) of powdered hydrated zeolite Mn(AlO2)n(SiO2)96-n (M ) H+, Na+; n ) 3.4, 6.6) were introduced into an evacuable heatable silica cell. The samples were heated up to 673, 773, or 873 K under Ar.8 Then, the sample was cooled to room temperature under dry argon. Weighted amounts of ACENE-4 corresponding to 1 ACENE-4/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 DRUVv experiments or in cylindrical EPR quartz tube and sealed. The mixtures of powders were left at 313 K. The Brønsted and Lewis acidity of the calcined Hn(AlO2)n(SiO2)96-n samples were characterized by FTIR measurements using pyridine as a probe molecule.19 Molecular Modeling. The molecular modeling of the ACENE-4 preferred sorption sites in Na4(AlO2)4(SiO2)92 zeolites were performed using Material Studio Modeling package (version 4.0) from Accelrys International. The zeolite structural parameters and partial atomic charges of M4(AlO2)4(SiO2)92 (n ) 4) were taken from previous works.20,21 The structural parameters and set of partial atomic charges of ACENE-4 were derived from previous structural and theoretical works.14,22,23 The nonbonding Lennard-Jones (L-J) force field values were taken from previous works.21,24 The simulation box of ZSM-5 was a supercell that consists in 2 × 2 × 4 orthorhombic cells. In the Monte Carlo (MC) simulations, the Si, Al, O, and Na+ positions were fixed in the simulation box. Periodic boundary conditions were applied in all directions. The ACENE-4 structure was taken to be rigid. The MC simulations at 1 ACENE-4/UC loading were carried out at 300 K using the conventional Metropolis algorithm taking into account the nonbonding interactions (EZS) between the O and Na+ atoms of zeolite and the C and H atoms of ACENE-4 as well as the nonbonding interactions (ESS) between ACENE-4. The interactions inside the zeolite were modeled by L-J and Coulombic forces.

Experimental Section Materials. As-synthesized ZSM-5 samples (Si/Al ) 13.5, 27, average particle size 1 µm) were obtained according to the template procedure in alkaline medium from VAW aluminum (Schwandorf, Germany). The as-synthesized ZSM-5 zeolites were calcined under air to evacuate the template. The extraframework cations were completely exchanged by NH4+ and Na+. All of the zeolite samples were obtained by a calcination procedure up to 673 K under argon.18 The unit cell composition of the calcined MnZSM-5 samples were found to be M3.4(AlO2)3.4(SiO2)92.6 (Si/Al ) 27, M ) H+, Na+) and M6.6(AlO2)6.6(SiO2)89.4 (Si/Al ) 13.5, M ) H+, Na+) from elemental analysis. The chemical analyses, powder XRD patterns, 29Si, 27Al MAS NMR, IR, Raman, DRUVv, and EPR spectra of bare ZSM-5 zeolites were found to be characteristic of wellcrystallized diamagnetic porous compounds with the above formula. However, the 27Al NMR spectra of the hydrated H6.6ZSM-5 (Si/Al ) 13.5) sample provide evidence of small amounts of extraframework hexacoordinated Al species.19 EPR spectroscopic investigations indicate iron impurities at trace levels in the zeolite samples not detected by conventional elemental analyses. ACENE-4 (C18H12, Merck-Schuchardt) was purified by sublimation and stocked over molecular sieves. Pure and dry Ar gas was used.

A cutoff radius of 0.9 nm was applied to the L-J interactions. The long-range electrostatic interactions were calculated using the Ewald summation technique. The simulation takes a number of steps to equilibrate from its original random position. For accurate statistical results, the steps made prior to equilibration have been excluded of the analysis. One typical MC run took 1 500 000 steps. From each sorption trajectory, a histogram of the energy distribution for each sorbate was generated. In a socalled mass-cloud analysis, the center of mass of each sorbate in each configuration was displayed as a dot in the model space. In the molecular mechanics (MM) simulations, the timeconsuming Ewald summation has not been performed. The zeolite framework was taken to be rigid, and the extraframework cations Na+ and rigid ACENE-4 were taken to be mobile.

EZS + ESS )

∑ij ARβ/rij12 - BRβ/rij6 + qi qj/rij

(1)

EZS + ESS + EZM + EMM )

∑ij ARβ/rij12 - BRβ/rij6 + qi qj/rij

(2)

The electrostatic and L-J interaction cut offs are defined by two parameters: the spline-on and the spline-off distances. Within these ranges, the nonbonded interaction energy is attenuated by a spline function. Beyond the spline-off distance, nonbonded interactions were ignored. The spline-on and spline-off distances were taken to be 1.5 and 3 nm for both the electrostatic and

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L-J interactions. The energy minimization of nonbonding sorbate-zeolite energy was performed using the conjugate gradient minimization procedure. Instrumentation. The UV-visible absorption spectra of the sample were recorded between 200 and 900 nm using a Cary 3 spectrometer. The instrument was equipped with an integrating sphere to study the powdered zeolite samples through diffuse reflectance; the corresponding bare zeolite was used as the reference. The DRUVv spectra were plotted as the KubelkaMunk function : F(R) ) (1 - R)2/2R ) K/S, 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 S is the scattering coefficient of the powder. Huge spectral set F(λ, t) was recorded as function of λ (wavelength) at several t (time) during the course of the ACENE-4 sorption. The X-band CW EPR spectra and pulsed EPR experiments were recorded on a Bruker ELEXYS 580-FT spectrometer. 2D Electron-spin-echo envelope modulation measurements have employed the 2D-four-pulse sequence (HYSCORE) (π/2-τπ/2-t1-π-t2-π/2-echo) with the appropriate phase cycling (t1 ) t2). The pulsed length was, respectively, 16 and 32 ns for π/2 and π pulses. The τ values were selected to optimize the modulation from 27Al and 1H nuclei. The 2D-HYSCORE measurements were carried out with the four-pulse sequence π/2-τ-π/2-t1-π-t2-π/2-τ echo, and a four-step phase cycle where the echo is measured as a function of t1 and t2; t1 and t2 were incremented in steps of 16 ns from their initial value. The 2D-HYSCORE experiments were recorded with a τ value of 136 ns. Prior to Fourier transformation of the HYSCORE data, the background decay was removed by a polynomial fit and apodized with a Hamming function. Simulation of HYSCORE spectra was achieved with Easyspin toolbox under MatLab 7.0, with diagonalization of the spin Hamiltonian. Easyspin toolbox is a comprehensive software package for spectral simulation and analysis in EPR.25 A nonselective ideal pulse was used, and the powder spectrum was calculated over 200 orientations on the theta angle for the anisotropic dipolar part. Multivariate Curve Resolution. F(λ, t) represents the original data matrix with spectra in rows. Primarily, it is necessary to estimate the global rank of F(λ, t) to estimate the number of pure absorbing species (k) present in the whole data set of the complex mixture. The F(λ, t) matrix is then decomposed into the following form:

F(λ, t) ) C(t) × SλT + E

(3)

C(t) represents the “spectral concentration” matrix. SλT represents the transpose of Sλ (spectral matrix), and E represents the residual error. The SIMPLISMA approach was applied to resolve both the concentration matrix C(t) (k columns and t rows) and the spectral matrix SλT of pure compounds (k rows and λ columns). SλT and C(t) were calculated by standard matrix algebra according to the procedure detailed in the original publication without any prior information.26 The difference between original and reconstructed data set lower than 5% RRSSQ (relative root of sum of square differences) provides a realistic picture of the components. The relative root of sum of square differences (RRSSQ) expresses the difference between the experimental data set F(λ, t) and the calculated data set F(λ, t)calc. In a second step, we used the multivariate curve resolution-alternating least-squares (MCRALS) as a refined method. The optimization is carried out using C and ST initial estimates obtained by the SIMPLISMA

Figure 1. Diffuse reflectance UV-visible absorption (DRUVv) spectra recorded 1 day and 8 months after the mixing of solid ACENE-4 and Na3.4ZSM-5 dehydrated at 673 K and under argon (1 ACENE-4/Na3.4(AlO2)3.4(SiO2)92.6 unit cell).

approach. Convergence is achieved when the standard deviation σ of residuals with respect to experimental data is less than 3%. The molecular concentrations were estimated from the spectral concentration C(t) taking into account the molecular extinction coefficients of ACENE-4, HACENE-4+, and ACENE4•+ previously reported in solution.12,27-29 Results 1-ACENE-4 Sorption in Dehydrated Nonacidic NanZSM-5 (n ) 3.4, 6.6) Zeolites. The IR absorption spectra of NanZSM-5 (n ) 3.4, 6.6) zeolites dehydrated at 673 K under argon show weak sharp absorption bands at 3737 cm-1, which are assigned to the stretching mode of the Si-OH silanol group. No band was observed at about 3600 cm-1 corresponding to the stretching mode range of the acidic OH group of bridged Al-OH-Si moiety. No Brønsted acidy was detected in dehydrated NanZSM-5 (n ) 3.4, 6.6) zeolites. DRUVv Absorption Spectroscopy. Bulk solid ACENE-4 is orange. The weighted ACENE-4 and zeolite quantities used correspond to one ACENE-4 per unit cell loading. When the calculated quantity of solid ACENE-4 was exposed to the colorless dehydrated NanZSM-5 (3.4, 6.6) powdered sample in the dark and under gentle warming, the powdered sample became more and more orange. The broad UV absorption between 320 and 550 nm for powdered solid ACENE-4 dispersed in the zeolite powders evolved slowly to intense absorption bands at 295, 410, 470, 500, and 525 nm (Figure 1).30 Eight months after mixing the powders, the spectral evolution appeared to be complete (Figure 1). The multivariate curve (MCR) analysis of the DRUVv spectral set resolved two spectra assigned straightforwardly to solid ACENE-4 and ACENE-4 occluded in the channel of NanZSM-5. The DRUV-v absorption spectrum of occluded ACENE-4 was found to display marked analogies with ACENE-4 absorption spectrum in solution rather than with solid ACENE-4. The ACENE-4 solution spectrum exhibits absorption bands at 270, 300, 390, 420, 450, and 470 nm assigned to S0 r Sn singlet-singlet transitions and related vibronic structures.31 The combined confinement and electrostatic field effects responsible for the spectral change of occluded ACENE-4 induce weak band red shifts. The incorporation of ACENE-4 as an intact molecule in the channel of NanZSM-5 occurs very slowly according to the

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following reaction and was found to be analogous for n ) 3.4 and 6.6:

ACENE-4 (solid) + NanZSM-5 f ACENE-4 @NanZSM-5 (4) CW EPR Spectroscopy. Bare NanZSM-5 zeolites activated at 673 K under Ar gave rise to very weak EPR signals with g ) 4.26 at 100 or 300 K. This signal was assigned to Fe(III) impurities at trace levels. No signal was detected for the 2-g range as reported previously.32 No supplementary significant features were observed in the 2-g values after exposure of NanZSM-5 to solid ACENE-4. It should be noted that a weak signal was observed when the extraframework cations were not completely exchanged by Na+. Molecular Modeling. The structural situation of ACENE-4 occluded as an intact molecule in the pore of Na4ZSM-5 can be deduced from molecular modeling of ACENE-4@Na4ZSM5. The details of the modeling procedure are given in the experimental section. The framework structure of bare ZSM-5 zeolites contains two types of intersecting channels.33-35 Both are formed by rings of 10 oxygen atoms, characterizing them as a medium-pore zeolite. One channel type is straight and has a nearly circular opening (0.53 × 0.56 nm), whereas the other one is sinusoidal and has an elliptical opening (0.51 × 0.55 nm). The exact M+ position has been fully elucidated in bare CsnZSM-5 (n ) 3.8, 5.8) zeolites by X-ray diffraction.20,36 For aluminum-rich CsnZSM-5, currently no direct experimental information of the Al atom location is available. As used recently for modeling purposes, we explicitly distinguish silicon from aluminum sites with partial and random occupation according to the aluminum content.21 Because of the strong Coulombic interactions with the zeolite framework, the energetically mostfavorable positions for the Na+ cations are near the O atoms binding Al atoms. The nearest distances between Na+ and O atoms obtained after an energy minimization procedure with the zeolite framework fixed and the Na+ mobile were found to be in reasonable agreement with theoretical and experimental studies reported previously.20,37 The resulting bare Na4ZSM-5 zeolite structure was used to predict the energy and structure of the ACENE-4 sorption site in Na4ZSM-5 by MC simulations at 300 K with one ACENE-4/UC loading (see the Experimental Section). The energy distribution of individual ACENE-4 exhibits one maximum in Na4ZSM-5. The corresponding distribution of the positions occupied by the ACENE-4 center of mass indicates that the net potential surface accessible to the molecule is maximum in the vicinity of the Na+ cation with ACENE-4 lying in the straight channel along the b direction with one phenyl group facially coordinated to Na+ cation. The sorption of ACENE-4 is expected to influence the location of Na+ in the framework as well. So, the energy minimization procedure taking ACENE-4 and Na+ as mobile leads to structural situation somewhat analogous to that obtained by MC with only small displacements. The simulated structural situation of ACENE-4@Na4ZSM-5 (Figure 2) was found to be in reasonable agreement with XRD data related to ACENE-4 occluded in MnFAU zeolites.14 The ACENE-4 confinement in the straight channel was estimated through the nearest distance from the guest molecule to Na+ and was found to be 0.30 nm. The repartition of the energy in Van der Waals energy and electrostatic energy indicates the major role of the electrostatic interactions particularly through the Na+-phenyl group interactions. The openings of ZSM-5

Figure 2. Monte Carlo simulations of ACENE-4 occluded in the straight channel of Na4ZSM-5 (1 ACENE-4/unit cell). Red, yellow, and pink lines represent the O, Si, and Al atoms of the [(AlO2)4(SiO2)92]4framework, respectively. The white and shaded cylinders represent the H and C atoms of ACENE-4 (C18H12), respectively. The purple spheres represent the Na+ cation.

straight channels are sufficiently wide to allow ACENE-4 molecules to pass through them and to diffuse slowly into the channels. 2-ACENE-4 Sorption in Dehydrated Acidic HnZSM-5 (n ) 3.4, 6.6) Zeolites. The Brønsted acid sites of bare of HnZSM-5 (n ) 3.4, 6.6) zeolites dehydrated at 673 K under argon were characterized by IR absorption spectra in the -OH stretching region. Strong sharp bands were observed at 3610 and 3612 cm-1 while a weak band was observed at 3737 cm-1 for n ) 3.4. An intense band was observed at 3605 cm-1 while a weak peak was observed at 3737 cm-1 for n ) 6.6. The intense bands at about 3600 cm-1 are caused by stretching mode of the Brønsted acid sites while the weak peaks at 3737 cm-1 arise from silanol stretching mode. The OH stretching vibrations associated with extraframework aluminum species (around 3700, 3670, and 3525 cm-1) were not detected. The relative amounts of Brønsted and Lewis acidities of dehydrated HnZSM-5 (n ) 3.4, 6.6) were estimated by integration of the IR bands at 1545 and 1465 cm-1 after sorption of pyridine.19 The relative amounts of Lewis aluminum of dehydrated HnZSM-5 were estimated to be 0.1 (n ) 6.6) and 0.07 (n ) 3.4) per unit cell after dehydration at 673 K under argon. These amounts increase slightly after dehydration at higher temperature and under O2. The low amounts of extraframework aluminum species for n ) 6.6 did not generate noticeable effect upon Lewis acid site amounts.38 DRUVv Absorption Spectroscopy. Figure 3a and b show some of the numerous DRUVv spectra recorded as a function of time during 2 years after the mixing of solid ACENE-4 with dehydrated Brønsted acidic H6.6ZSM-5. Analogous results were found after mixing ACENE-4 with dehydrated H3.4ZSM-5. However, the course of reaction is markedly slower in the H3.4ZSM-5 zeolite. First, during approximately 2 days intense bands developed at 274, 350, 395, 750, and 856 nm in addition to the absorption bands of bulk ACENE-4 dispersed in the zeolite powder (Figure 3a). After one week, supplementary broad bands at 540 and 630 nm developed concomitantly with the 274, 350, 395, 440, 750, and 856 nm peaks. Progressively, the 750 and 856 bands decreased and disappeared after about 8 months (Figure 3b) concomitantly with increase of broad bands at 540 and 630 nm. After 1 year, the 540 and 630 bands decrease slowly with concomitant appearance of bands observed at 300 and 420 nm. In order to unequivocally resolve specific absorption of pure species, we carried out data processing of the DRUVv spectra set recorded over the large period (2 years) after powder mixing using MCR methods (see the Experimental Section). Five absorbing species were expected from the matrix rank analysis.

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Figure 3. Diffuse reflectance UV-visible absorption (DRUVv) spectra recorded as a function of time after the mixing of solid ACENE-4 and H6.6ZSM-5 dehydrated at 673 K and under argon (1 ACENE-4/unit cell). (a) during 48 h; (b) during 2 years.

Five pure spectra were resolved by the MCR treatment with residuals between experimental and calculated values less than 3% despite severe spectral overlap of several pure spectra. The spectrum with ill-resolved bands in the 300-500 nm range (Figure 4a) was straightforwardly assigned to bulk solid ACENE-4, while the spectrum with sharp bands at 274, 350, 395, 750, and 856 nm (Figure 4c) has accurate resemblance to monomeric ACENE-4•+ spectra (274, 349, 394, 747, and 856 nm) obtained in concentrated sulfuric acid solution and trapped in argon matrix.12,27,28 It should be noted that the resolved pure spectrum of Figure 4c includes the possible UV-visible absorption contribution of the negatively charged counterpart corresponding probably to a trapped electron. However, careful comparison between the resolved spectrum (Figure 4c) and the ACENE-4•+ spectrum in concentrated sulfuric acid did not provide any clear supplementary absorption. In addition, no evidence was found for dimer cation radical formation in the zeolite channel, whereas ACENE-4•+ dimerization was observed in concentrated sulfuric acid through UV-visible absorption band shifts.12 The one band spectrum centered at 440 nm (Figure 4b) was attributed to the protonated species HACENE-4+ by comparison with previously reported spectrum obtained in concentrated H2SO4 solution or in HF solution.29 The absorption spectrum of a fresh solution of ACENE-4 in H2SO4 exhibits a band centered at 460 nm due to protonated HACENE-4+

species. After few hours, this band vanishes spontaneously and the absorption spectrum is that of ACENE•+ only.28 The MCR analysis also resolves an unusual spectrum (Figure 4d) with intense bands at 300, 550, and 630 nm. To our knowledge, this spectrum does not correspond to any previously reported spectrum for ACENE-4 related compounds and was straightforwardly attributed to electron-hole pairs ([email protected]+•-•) by analogy with previous works related to the electron-donor ability of the ZSM-5 zeolite framework toward electron-deficient radical cations of biphenyl, naphthalene, (ACENE-2) and trans-stilbene.9,19,39,40 These characteristic intense bands were recently assigned to photoinduced hole transfer.41 No such feature was observed at room temperature for anthracene (ACENE-3) incorporation in HnZSM-5.42 This aspect will be detailed below in the discussion section. A supplementary spectrum (Figure 4e) was resolved with band maxima at 285, 400, and 500 nm. This spectrum has a typical resemblance with ACENE-4 occluded in nonacidic NanZSM-5 zeolites (see above). Identical pure spectra were extracted from the DRUVv spectral set recorded during the course of the ACENE-4 sorption in H3.4ZSM-5. The molecular concentrations of the five species involved in the ACENE-4 sorption in the H6.6ZSM-5 zeolite were estimated from the spectral concentration Ck(t) calculated by the MCR procedure taking into account the corresponding molecular extinction coefficients of ACENE-

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Figure 4. UV-visible absorption spectra of pure species resolved by MCR chemometric procedure from the spectral set recorded as a function of time after the mixing of solid ACENE-4 and dehydrated H6.6ZSM-5. (a) Solid ACENE-4, (b) [email protected], (c) ACENE-4•[email protected]•-, (d) [email protected]•-•+, (e) [email protected].

4, HACENE-4+, and ACENE-4•+ previously reported in solution.12,29 The plots of Ck(t) as function of time are exhibited in Figure 5 a and b. The molecular concentrations deduced from UV-visible absorption results will be interpreted with the spin quantities deduced of CW EPR spectra recorded during the sorption course (see below). CW-EPR Spectroscopy. Immediately after exposure of powdered ACENE-4 to dehydrated HnZSM-5 crystals, illresolved lines superimposed on isotropic broad signal were detected in the 2-g range of the weak X-band CW-EPR spectra. The EPR pattern observed during 5 days has some resemblance to the EPR spectra recorded after ACENE-4 sorption into faujasitic Y type zeolite and after adsorption onto an acidic aluminosilicate.17,43 Typical differences were observed between EPR spectra recorded after ACENE-4 adsorption onto acidic solids and after ACENE-4 dissolution in concentrated H2SO4 or in molten SbCl3.44,45 The more- or less-resolved lines due probably to 1H hyperfine coupling are superimposed to an isotropic broad signal in acidic solids, whereas 1H hyperfine coupling constants of free ACENE-4•+ were deduced from EPR spectra with high spectral resolution and the flat baseline of stable ACENE-4•+ obtained by chemical oxidation in concentrated H2SO4 or in molten SbCl3. The structured part was assigned to ACENE-4•+, and the featureless part was attributed to ejected electron according to previous works related to pulsed and CW-EPR studies of biphenyl, ACENE-2, and ACENE-3 sorption in HnZSM-5.19,39,40 After 5 days, the fine structure disappears completely and a featureless intense signal of 23 G centered at g ) 2.0053 remains (Figure 6a). After 8 months, the X-band EPR spectra still display isotropic signal of 23 G centered g ) 2.0053 (Figure 6b). After 2 years, the featureless signal of 23 G centered at g ) 2.0053 remains with lesser intensity (Figure 6c).

Figure 5. Relative contributions resolved by MCR chemometric method of pure species as function of time after the mixing of solid ACENE-4 and dehydrated H6.6ZSM-5: (9) solid ACENE-4, (b) [email protected], (4) ACENE-4•[email protected]•-, (0) [email protected]•-•+,(2) [email protected]. The molecular concentrations were estimated from the spectral concentrations C(t) taking into account the molecular extinction coefficients of ACENE4, HACENE-4+, and ACENE-4•+ previously reported in solution.12,27,28,29

Figure 6. Continuous-wave EPR spectra recorded at room temperature: (a) 5 days; (b) 8 months; (c) 2 years after the mixing of solid ACENE-4 and dehydrated H6.6ZSM-5.

The cooling of the sample to 4.2 K did not induce any structured pattern. The double integration of the EPR signals

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provided the spin quantity and ionization yield during the course of the sorption. After 5 days, the spin quantity corresponds approximately to 0.3 ( 0.1 electron per ACENE-4. The spin quantity showed a broad maximum corresponding to 1.7 electrons per ACENE-4 after 6 months in H6.6ZSM-5. After 8 months, the spin quantity decreases slowly. The species concentrations (Figure 5) and the spin quantity as function of time are in accurate agreement with the following reaction sequence occurring during the course of ACENE-4 sorption in acidic HnZSM-5 zeolites:

ACENE-4 (solid) + HnZSM-5 f HACENE-4+@Hn-1ZSM-5- (5) ACENE-4 (solid) + HnZSM-5 f ACENE-4•+@HnZSM-5•- (6) ACENE-4•+@HnZSM-5•- f ACENE-4@HnZSM-5•-•+ (7) ACENE-4@HnZSM-5•-•+ f ACENE-4@HnZSM-5 (8) Both spectroscopic results reveal the presence of two types of radical pairs during the sorption course. ACENE-4•+@HnZSM5•- is generated as an initial ionization step. The observation of the complete disappearance of ACENE-4•+ with the persistence of paramagnetic species in high yield indicates that a supplementary electron transfer between ACENE-4•+ and the zeolite framework is taking place during the migration of ACENE-4•+ and trapped an electron in the void space. The spin quantity is tentatively related to electron-hole pair ACENE4@HnZSM-5•-•+ as previously reported for spontaneous ionization of biphenyl, naphthalene, (ACENE-2) and trans-stilbene upon incorporation in the channels of HnZSM-5 zeolites.19,39,40 2D-HYSCORE Spectroscopy. To reveal the structural surroundings of unpaired electrons of the ACENE-4•+@HnZSM5•- and ACENE-4@HnZSM-5•-•+ pairs, we performed pulsed EPR experiments. Electron spin echo envelope modulation (ESEEM) is an experimental magnetic resonance effect that can be observed in pulsed EPR experiments. ESEEM is a powerful probe of the radial distribution of magnetic nuclear spins in the environment of the electron spins producing the echo. ESEEM experiments are usually sensitive to weak hyperfine interactions (