Comparison between Spontaneous and Photoinduced Ionization

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Comparison between Spontaneous and Photoinduced Ionization Mechanisms for P-Quaterphenyl in M-ZSM-5 (M = H+, Na+) Zeolites Alain Moissette, Matthieu Hureau, Sonia Carré, Hervé Vezin, and Perrine Col J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405644t • Publication Date (Web): 13 Sep 2013 Downloaded from http://pubs.acs.org on September 15, 2013

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The Journal of Physical Chemistry

Comparison

between

Spontaneous

and

Photoinduced

Ionization

Mechanisms for p-Quaterphenyl in M-ZSM-5 (M = H+, Na+) Zeolites

Alain Moissette *,1 ; Matthieu Hureau 1 ; Sonia Carré 1 ; Hervé Vezin 1 ; Perrine Col 1 1

Laboratoire de Spectrochimie Infrarouge et Raman, UMR-CNRS 8516, Université de Lille 1,

Bât C5, 9655 Villeneuve d’Ascq France

Abstract The electron transfers following the initial ionization of a probe molecule (pquaterphenyl) adsorbed in the channels of M-ZSM-5 (M = H+, Na+) zeolites are investigated using various complementary spectroscopic techniques. Under the same reaction conditions, ionization occurs spontaneously during molecule diffusion in the acid H-ZSM-5 whereas charge separation needs to be photoinduced within the pores of Na-ZSM-5. The electron transfer processes are found to be identical for both the cases in terms of transient species formed before the final charge recombination. The initial ionization leads to the formation of a radical cation which evolves gradually toward an electron/hole pair associated with a charge transfer complex. However, the reaction kinetics depend dramatically on the ionization way. As spontaneous ionization is associated with the sorption process, it is closely correlated with the molecule diffusion and thus, is very slow due to the bulky size of the molecule. In this case, radical cations and subsequent charge transfer complexes are stabilized for months in high yield and are clearly characterized by diffuse reflectance UV-visible absorption, EPR and by Raman scattering in resonance and in off-resonance conditions. After photoionization, the evolution is followed as a function of time by using time resolved UV-visible absorption spectroscopy on a large scale of time extending from a few microseconds to several days.

Keywords: Electron Transfer, Charge Separated State, Radical Cation, Diffuse Reflectance UV-vis, EPR, Raman.

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Introduction The capacity of molecular assemblages to generate and to maintain charge separation for sufficiently long times is of major importance for the development of new systems in the field of energy storage or light energy recovery.

1-9

In that context, porous materials like

zeolites have already widely proven their ability to allow electron transfers and stabilize long lived separated charge states after ionization of guest molecules incorporated in their porous framework.

10-17

In case of high internal electrostatic field and if the confinement is high

enough, ionization can occur spontaneously.

18-25

This feature was often observed with

sorption of polyaromatics in the channels of medium pore acid zeolites 20-24 or in frameworks containing cations like Li+ small enough to generate highly polarizing environment.

26

Thus,

the ionizing capacity of the host was shown to depend significantly on the zeolite structure and on the Si/Al ratio but also on the charge balancing cation nature. Nevertheless, note also that the ionization can occur only if the ionization potential of the guest in gas phase is low enough, (i.e. I.P. < 8.2 eV). In the absence of spontaneous ionization, the molecules are incorporated in preferential sorption sites and require photoexcitation to be ionized.

27-38

However, previous works have demonstrated the analogy between the reaction mechanisms occurring after the photo or spontaneous ionization including radical stabilization, charge shifting reactions and final charge recombination.

22,28,32,39,40,41

In both cases, the combined

effects of the confinement and of the internal electrostatic field play a major role in the electron transfer kinetics but the development of applicative systems require thorough control of such parameters and clear understanding of the observed reactivity. Thus, the purpose of the present work is to compare the electron transfers following the initial ionization of an identical probe molecule adsorbed in two zeolites of M-ZSM-5 type with the same morphology and same Si/Al ratio but differing only by the extraframework cation type (M = H+, Na+). For this study, we have chosen p-quaterphenyl (p-QPh) as probe molecule because i) it belongs to the poly-p-phenylene oligomer family, the molecules of which display interesting opto-electronic properties as they can be active layers in organic light emitting 42,43

devices and field effect transistors; doping with acceptors and donors; value in gas phase (I.P.= 8.08 eV)

44,45

46

they also gain high electrical conductivity upon

ii) it presents a relatively low ionization potential

and iii) it has the suitable dimensions to penetrate into

the pores of dehydrated medium pore M-ZSM-5. Note that we have already reported the sorption and ionization reactions of biphenyl

19,40,41

and of p-terphenyl

28,39

in ZSM-5,

corresponding to the two first molecules of poly p-phenylene family, but the higher


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constraints imposed by the longer chain of the p-QPh are expected to slow down significantly the reaction mechanism and thus, allow the stabilization of specific transient species before reaching an equilibrium state. This work describes the sorption processes observed after mixing solid p-quaterphenyl crystals and dehydrated M-ZSM-5 (M = H+, Na+) zeolite powder under inert atmosphere and without any solvent. Using complementary techniques we show that, under similar confinement conditions, the mixing leads to the guest spontaneous ionization in the presence of Brønsted acid sites whereas only sorption takes place within the non acidic Na-ZSM-5. Thus, we compare the reaction mechanisms occurring after spontaneous ionization in HZSM-5 and after photoinduced p-quaterphenyl ionization in Na+-containing zeolite. The evolution is followed as a function of time by using time resolved UV-visible absorption spectroscopy on a large scale of time extending from a few microseconds to several days. The charge separated states are also studied by both continuous and pulsed wave electronic paramagnetic resonance (EPR) spectroscopy to characterize the possible interactions between the unpaired electrons with the surrounding nuclei. The identification of the transient species is investigated by Raman spectroscopy either in resonance or in off-resonance conditions as this technique allows the characterization of the electronic and molecular structure changes.

Experimental Materials. M-ZSM-5 samples (M = NH4+, Na+; Si/Al = 27; average particle size ~1 µm) were obtained from VAW aluminum (Schwandorf, Germany). The NH4+ counterbalancing cations of NH4-ZSM-5 zeolite were completely converted to H+ cations by calcination procedure in flowing air by increasing the temperature up to 450°C and holding for 6 h. p-quaterphenyl (pQPh, C24H18 99.5% Sigma-Aldrich) was purified by sublimation. Pure and dry Ar gas was used.

Sorption procedure. Weighed amounts (~ 1.4 g) of zeolite are introduced into an evacuable, heatable silica cell placed in a vertical oven connected to a piping network. The sample is heated stepwise up to 450°C under flowing dry Ar for 12 h. Then, the sample is cooled to room temperature under dry argon. Weighed amounts of p-QPh corresponding to 1 molecule per unit cell (UC) 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. 3 ACS Paragon Plus Environment


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Radical generation. In the absence of spontaneous ionization, the generation of radicals was carried out by photoexcitation of the sample at room temperature using a nanosecond pulse width 300 nm 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 20 Hz. After ionization, the evolution of the transient species is followed using the techniques listed below.

Instrumentation Diffuse Reflectance UV-visible (DRUVv) Absorption Spectroscopy. The UV-visible absorption spectra of the samples were recorded in the 200-1800 nm range using a Cary 6000 spectrometer. The instrument was equipped with an integrating sphere to study the diffuse reflectance of powdered p-QPh/zeolite samples stocked under an inert atmosphere in quartz cells; the corresponding bare zeolite was used as the reference. The DRUVv spectra were plotted as the Kubelka-Munk function. Raman Spectroscopy. A Bruker RFS 100/S instrument was used as a near-IR FT-Raman spectrometer with a CW Nd:YAG laser (λ0 = 1064 nm) as excitation source. A laser power of 100-200 mW was used. The spectra of samples stocked under argon in the quartz cells were recorded in the region 3500-150 cm-1 with a 2 cm-1 resolution using 600 scans. Resonance Raman scattering spectra were collected on a LabRAM spectrometer (Jobin Yvon Horiba Gr.) equipped with a liquid nitrogen cooled charge-coupled device detector. The 473 and 532 nm excitation wavelengths are supplied by solid lasers. Time-Resolved Diffuse Reflectance UV-visible (TRDRUV) Absorption Spectroscopy. The experimental set-up for nanosecond diffuse reflectance spectroscopy was previously described.47,48 Excitation pulses at 300 nm (7-8 ns, 10 mJ) was provided by a 20 Hz Panther EX OPO tunable laser (Continuum, GSI group). The probe light was provided by a Xe lamp (XBO 150W/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 JobinYvon, iHR320) and analyzed with a photomultiplier (R1477-06, Hamamatsu) coupled to a digital oscilloscope (LeCroy 454, 500 MHz). For such 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 intensity of the diffuse reflected white-light probe with and without excitation, respectively. Kinetic traces from 10 µs were detected in the 400-700 nm spectral domain every 5 nm from which transient spectra were reconstructed. To maintain sample integrity during the experiment, the sample was 4 ACS Paragon Plus Environment

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moved and/or shaken throughout the experience to ensure that a fresh region of the sample was probed by each laser pulse. EPR. The CW X-band EPR spectra of the powders were obtained as a function of time by using a Bruker ELEXYS 580-FT spectrometer. The EPR spectra were double integrated using Bruker software. The hyperfine sublevel correlation spectroscopy (2D-HYSCORE) measurements were carried out at room temperature with the four pulse sequence π/2-τ-π/2t1-π-t2-π/2-τ echo, and a four-step phase cycle where the echo is measured as a function of t1 and t2 with t1 and t2 being incremented by steps of 16 ns from their initial value. The pulse lengths of the π/2 and π pulses in these experiments were 12 and 32 ns, respectively. Prior to Fourier transformation of the HYSCORE data the background decay was removed using a polynomial fit and apodized with a Hamming function.

Data Processing. The Multivariate Curve Resolution (MCR) data processing of DRUVv spectral set F(λ, t) was carried out by using the SIMPLISMA (SIMPle-to-use Interactive Selfmodeling Mixture Analysis) approach. This method was applied to extract pure UVv absorption and respective concentration C(t) from spectral data recorded as a function of time after powder mixing without any prior information. The method used is detailed in the original papers. 49 The concentration decay C(t) was accurately fitted using the Albery function.50 The Albery function takes into account the non-homogeneity of the material. According to the model, the time dependent absorption profile of species can be represented by Eq.(1): +∞

C(t) = C/C0 =

+∞

2 ∫ exp(− x 2). exp(−kt. exp( γx )).dx / ∫ exp(− x ).dx

−∞ +∞

where

∫ exp(− x

2

(1)

−∞

).dx =

π and C(t) is the normalized concentration, k is the average rate

−∞

constant, and γ is the width of the distribution. If γ = 0 (no dispersion), this latter equation is reduced to first order kinetics: C(t) = exp(- k.t). The decay kinetic fitting was carried out by using the MicrocalTM Origin software. The best fitted k and γ values with respective errors are outputs of kinetic data processing.

Results and discussion 1. Sorption and spontaneous ionization of p-QPh in H-ZSM-5 (Si/Al = 27)



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DRUVv absorption spectroscopy. Figure 1 shows some of the DRUVv spectra recorded as a function of time for 2 years after the mixing of solid p-QPh with dehydrated Brønsted acidic H-ZSM-5. The spectra recorded immediately after the mixing and for 3 days did not exhibit significant changes. From one week, the white powder turned light pink and the color intensified gradually with time. Parallel to the color change, new bands develop slowly at 470, 505 and 1160 nm and are clearly observed after 10 days. For times longer than 1 month, the 1160 nm band seems to develop concomitantly with a weaker additional band observed at 990 nm. From 2 weeks, a new absorption band with two major peaks centered at 540 and 560 nm appeared in the visible region. This new band increased progressively over months while the intensity of the contributions observed at 470 and 505 nm seemed to have reached a maximum. After about 8 months, the 470 and 505 nm bands as well as the one observed at 1160 nm begun to decrease. After one year, the 470-505 nm bands appeared as a shoulder of the 540 nm one whereas the contribution in the near infrared region was still observed but weak. This pattern is still observed after two years even if the 1160 nm band is hardly detected. Note that a weak contribution is also observed at 730 nm several months after the mixing. To clearly identify the various pure species formed after powder mixing, data processing of the DRUVv spectra set recorded over one year was carried out using MCR methods. Two pure spectra were resolved by the MCR treatment with residuals between experimental and calculated values less than 5 % (figure 2A). The first spectrum shows two well defined absorption bands observed firstly in the visible spectral range with two maxima at 470 and 505 nm and secondly in the near infrared region with also two contributions at 990 and 1160 nm. The bands observed at 470 and 505 nm closely resemble to the band previously reported by Khanna

51,52

and by Banerjee

53,54

and are assigned to the radical cation p-QPh●+. The p-

QPh●+ formation is concomitant with the adsorption process and can be represented according to equation (2). p-QPh + H-ZSM-5 p-QPh●+@H-ZSM-5●- (2) where p-QPh●+@H-ZSM-5●- corresponds to a separated charge state formed by a radical cation and an electron trapped in the zeolite framework. The 1160 and 990 nm bands which are extracted in the same time as the 470 and 775 nm bands are assumed to belong to the same species and then, should also correspond to the radical cation. However, these bands are significantly blue shifted compared with the bands reported by Banerjee

53,54

for the radical cation formed in dichloromethane which were

observed at 1302 nm and at about 1120 nm. Nevertheless, their relative intensities and the gap 6 ACS Paragon Plus Environment

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between the two contributions (ca. 170-180 nm) are similar. To explain this shift, we might consider the influence of the solvent effect through various interaction energies in dichloromethane and in zeolite which can be considered as a solid solvent. Note that using a theoretical approach, Sun et al.

55

clearly demonstrated the solvent effect on the optical

absorption spectrum of the p-QPh●+ radical cation by comparing the calculated electronic state transition energies in the gas phase and in a solvent. The second spectrum exhibits mainly a band with two major peaks at 540 and 570 nm (figure 2A, spectrum b). To our knowledge, this spectrum does not correspond to any previously reported spectrum for p-QPh related compounds. Based on our own experience in such systems and by analogy with previous works related to electron donor ability of ZSM-5 zeolite framework towards electron deficient radical cations of biphenyl, p-terphenyl, naphthalene, tetracene, and t-stilbene,21,22,23,24,28,29,39,40,41 this spectrum is tentatively assigned to a charge transfer complex (AlO4●+- p-QPh) associated with the formation of an electronhole pair due to subsequent electron transfer occurring after the radical formation according to equation 3. p-QPh●+@H-ZSM-5●- p-QPh@H-ZSM-5●+●- (3) where H-ZSM-5●+●- includes the contributions of both the initially ejected electron (AlO4●-) and of the subsequent positive electronic hole (AlO4●+). Indeed, the tight fit of p-QPh size in the pores of ZSM-5 coupled with the high aluminum content and with the highly polarizing H+ charge balancing cation increase considerably the lifetime of the radical cation/electron pair in the H-ZSM-5 channels. Even if radical cations recombine partially via a direct way with the ejected electron, we assume that radical cations decay mainly through hole transfer from the radical to an electron donating oxygen atom of the framework in close proximity. The oxidizing power of the electron deficient radical cation is expected to be high enough to catch an electron of an available AlO4H group of the zeolite framework to create a long-lived electron-hole pair at room temperature. Creation of such hole on an AlO4H●+ center in HZSM-5 was reported previously by hybrid quantum mechanics calculations.

56

Formation of

the electron hole was shown to produce a significant geometry relaxation of the Al-O distance to the oxygen atom with the unpaired electron whereas the zeolite framework stabilizes the positive charge by long-range effects. However, even if the electron abstraction from the oxygen atom bridging Si and Al atoms appears to be energetically favored, it should be noted that the electron abstraction from oxygen bridging Si atoms in Si-O-Si fragment was also clearly demonstrated in zeolites in the particular case of acid zeolite catalysts with various alkyl halides. 57 7 ACS Paragon Plus Environment


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The observed signal is assumed to be due to electronic transitions between the zeolite electronic hole and the neutral molecule in close proximity. Note that this spectrum shows also the presence of a band at 315 nm which might be due to a contribution of the neutral molecule involved in the charge transfer complex. The emergence of a band in the UV region associated with the appearance of an electron-hole pair moiety was already reported in the case of t-stilbene.

22

In addition, a weak band centered at 730 nm is also observed on this

spectrum. As reported previously by Khanna

52

, it might correspond to the formation in low

yield of a dication that cannot be extracted separately. No further experiment could confirm this hypothesis.

No clear evidence of trapped electron absorption was found although the sample was prepared under strictly anhydrous conditions. Only the observation of a broad signal in the near infrared region around 1400-1600 nm could be associated with such trapped electron as already reported for analogous zeolite systems after spontaneous ionization. 24,27 The evolutions of the spectral concentrations C k(t) as function of time of both the extracted species involved in the p-QPh sorption in H-ZSM-5 zeolite were also calculated by the MCR method using the spectra obtained for one year and are presented on figure 2B. Curve a shows that the spectral concentration of the radical cation remained very low during the first week following the mixing. This feature demonstrates the very weak spontaneous ionization process occurring at the early stages of the sorption course. For longer times, the p-QPh●+ spectral concentration increased progressively to reach a maximum after about 120 days and start to decrease after approximately 10 months. The curve b of figure 2B shows clearly that the formation of the second species (i.e. charge transfer complex, denoted hereafter CTC) is delayed with respect to the radical cation formation. Contrary to the radical behavior, the CTC spectral concentration is found to increase continuously for one year. In the absence of former data, note that the assignment of the charge transfer complex associated with an electron/hole pair remains hypothetic. Nevertheless, the evolution observed here after mixing p-QPh and H-ZSM-5 is very similar in terms of spectral features and of spectral concentrations to our previous works performed with other polyaromatic molecules in various channel type zeolites. 21,22,23,24,28,29,39,40,41 For all these studies, we have shown that the initial radical cation formed after ionization evolved more or less rapidly toward very long lived charge separated states due to electron transfer from zeolite to the radical cation. Even if the development of the DRUVv bands clearly shows that this second species develops at the 8 ACS Paragon Plus Environment

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expense of the radical cation, the assignment has to be done by coupling the UV-visible data with the Raman scattering and EPR experiments (see below) which demonstrate also the formation of two different species all along the time evolution.

Raman and Resonance Raman scattering spectroscopies. The sorption and ionization processes of p-QPh in H-ZSM-5 were also monitored as a function of time after the mixing of the powders using FT-Raman (λ = 1064 nm) spectroscopy. The spectra are presented on Figure 3 and the spectrum of solid p-QPh is given as reference (Figure 3, spectrum a). The Raman spectrum of the solid is mainly characterized by lines at 784 cm-1 (in phase in plane ring deformation), 995 cm-1 (ring breathing), 1220 cm-1 (in phase CH in plane bending), 1275 cm-1 (in phase CC inter-ring stretching), 1593 cm-1 (CC ring stretching) and 1604 cm-1 (ring stretching). 58,59 The FT Raman spectra recorded for several months clearly show the slow intensity decrease of the peaks characteristic of pure p-QPh. In particular, the intense 1274 cm-1 line as well as the 1220 and 784 cm-1 bands weren’t observed anymore after 4 months. Parallel to the total disappearance of neutral p-QPh, new lines emerged gradually in the 400-1700 cm-1 spectral range. The evolution is found to occur according to two stages. Firstly (stage 1), after mixing and concomitantly to the disappearance of the solid lines, new bands appeared at 1614, 1530, 1332, 1239 and 468 cm-1. These bands seem to have reached a maximum of intensity after about 78 days (figure 3, spectrum d). Secondly (stage 2), from times longer than 80 days, the 1593 cm-1 band noticeably broadened and lead to the formation of a band maximizing at 1588 cm-1. In the course of the sorption process, the 1604 cm-1 line did not shift and after 6 months, the line is still present but its weaker relative intensity makes it appear as a shoulder of the 1614 and of the 1588 cm-1 bands (spectrum h). Note that the intensity of all the new lines which appeared after mixing (stage 1) did not decrease during stage 2. After two years, the system evolution is reversed and the characteristic lines of the neutral molecule at 1274, 1220 and 784 cm-1 are observed again whereas the relative intensities of all the new lines have decreased (figure 3, spectrum j). The band observed at 1588 cm-1 became also narrower and maximized again at 1592 cm-1.

To get further information on the species formed in the course of the sorption process, resonance Raman investigations were also performed using various exciting lines within the contour of the visible absorption bands observed in the DRUVv spectrum. Spectrum b on Figure 4 shows the Raman spectrum obtained by exciting the sample at 473 nm, in the radical 9 ACS Paragon Plus Environment


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cation absorption band, 1.5 year after the mixing. This spectrum exhibits principally a very strong line at 1614 cm-1 and two lines at 1329 and 1237 cm-1. It should be noted that none of the lines of the neutral molecule (e.g. 784, 1220, 1275, 1593 cm-1 and 1604 cm-1) are observed. Due to high resonance enhancement, the resulting spectrum is expected to show the specific resonance Raman bands of p-QPh●+. Thus, the 1614, 1329 and 1237 cm-1 lines are attributed to the radical cation. This assignment is in perfect agreement with the behavior observed all along the time evolution by DRUVv and FT-Raman spectroscopy in offresonance condition. Indeed, DRUVv revealed that the first stage of evolution after mixing is constituted by the formation of radical cation and the first lines which emerged in the FTRaman spectra after mixing were observed at 1614, 1530, 1332 and 1239 cm-1. Note that the 1530 cm-1 line is also observed in the resonance spectrum but is not enhanced by resonance effect. By using the 532 nm exciting line, within the contour of the CTC band, the spectrum exhibits a broad band centered at 1600 cm-1 and two other lines at 1322 and 1230 cm-1 (figure 4, spectrum c). The 1600 cm-1 band might correspond to the overlapping of two contributions at 1588 and 1604 cm-1. Thus, this result is coherent with the broadening of the 1588 cm-1 band during stage 2 that can be associated with the CTC formation. The 1322 and 1230 cm-1 might be as well assigned to specific lines characteristic of CTC as they appeared in the FT-Raman spectra as shoulders of the bands observed at 1332 and 1239 cm-1, respectively. Moreover, it might be interesting to compare the present results with the data previously observed with tstilbene

60

for which the reaction mechanism of electron hole pair formation from radical

cations was found to be reversed by excitation at the maximum of absorption of the CTC. Consequently, in the present case the excitation of the sample at 532 nm is also expected to induce partial reformation of radical cation. Thus, the presence of a broad band in the 16001620 region might also include a contribution of the radical cation reformed upon excitation. This species is indeed expected around 1614 cm-1 according to the resonance Raman experiments reported above. Thus, contrary to what was observed for other parent molecules (biphenyl, p-terphenyl) 19,28,39 and other polyaromatics (naphthalene, tetracene)

21,29

, the FT Raman spectra allow the

observation of the radical cation. This result gives evidence of the high content of radicals stabilized in the zeolite framework and has to be associated with the very slow kinetic of the reaction. The electron transfers which occur upon p-QPh sorption and diffusion in the ZSM-5 channels are dramatically slowed down compared with smaller molecules. In addition, the


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total disappearance of the characteristic lines of the neutral molecule several months after mixing confirms the very high ionization rate.

CW-EPR Spectroscopy. Immediately after exposure of powdered p-QPh to dehydrated HZSM-5 crystals, poorly resolved lines superimposed on isotropic broad signal were detected at g=2.007 with a line width of 12 G (figure 5). The signal is due to the superimposition of the structured signal of the radical cation

61

with the signal of the ejected electron. The observed

signal is in agreement with the experimental data reported above using DRUVv and Raman spectroscopy which showed spontaneous p-QPh●+ formation. The whole spin concentration increased gradually with time and reached a plateau after about 3 months and remained stable for more than one year. After 4 months, the structured signal is not detected anymore and only a featureless intense isotropic signal of 7 G centered at g = 2.007 is observed and is assigned to the unpaired electrons of the electron/hole pair according to our previous studies devoted to the sorption of biphenyl, anthracene, t-stilbene using pulsed and CW-EPR techniques. After 2 years, the unresolved signal is still observed with lesser intensity.

2D-HYSCORE Spectroscopy. To reveal the structural surroundings of the unpaired electrons of the charge separated states, we performed pulsed EPR experiments at room temperature on the g = 2 signal 6 months after the mixing of solid p-QPh with the H-ZSM-5 zeolite (Figure 6). The four-pulse 2D-HYSCORE experiment displays only 1H nuclear frequency symmetric ridges with a maximum coupling of 8 MHz. At this step of the sorption, p-QPh@H-ZSM-5●+●- is the major species and the radical is not detected any more. Thus, the observed coupling likely comes from the unpaired electrons of the electron/hole pair occluded in the channels.

2. Sorption and photoionization of p-QPh in Na-ZSM-5 (Si/Al = 27) The mixing of a weighed quantity of white dry solid p-QPh with dehydrated NaZSM-5 (1 p-QPh per Na-ZSM-5 unit cell (UC)) did not induce any color change. The sorption and diffusion processes were followed for months by using diffuse reflectance UV-visible absorption spectroscopy (DRUVv) and Raman spectroscopy. The DRUVv spectra recorded throughout the diffusion process show UV band in the 250-350 nm regions which evolved very slowly. The spectral development appeared to be complete after 8 months without any thermal ionization in Na-ZSM-5. Furthermore, all the Raman spectra of p-QPh occluded in Na-ZSM-5 recorded with the, 473, 532 and 1064 nm exciting lines one year after the mixing exhibit identical lines in terms of wavenumbers and of relative intensities which are 11 ACS Paragon Plus Environment


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characteristic of the neutral p-QPh molecule. In addition it is worth noting that the EPR analyses carried out on the equilibrated sample did not permit to detect any signal. Therefore, all these complementary techniques demonstrate that p-QPh adsorption takes place into NaZSM-5 as intact molecule without any chemical modification and the reaction mechanism can be represented by equation (4): p-QPh + Na-ZSM-5 p-QPh@Na-ZSM-5 (4) where p-QPh@Na-ZSM-5 corresponds to the neutral p-QPh molecule incorporated in the channels of non acidic Na-ZSM-5. To induce charge separation, the sample was irradiated using a 300 nm exciting line which falls within the contour of the 250-350 nm electronic transition band of p-QPh adsorbed in Na-ZSM-5. The conventional DRUVv spectra recorded at room temperature as function of time in the 200-1800 nm spectral range after the 15 seconds UV irradiation are presented on figure 7A. Five minutes after the photoexcitation, the spectrum showed a broad band in the visible region centered at 530 nm and two new bands in the near infrared at ca. 1140 and between 1450 and 1800 nm. No new absorption was observed when dehydrated empty Na-ZSM-5 was irradiated under analogous conditions. The band centered at 530 nm is similar to that observed previously after p-QPh spontaneous ionization in H-ZSM-5. By analogy with this experiment, this band is assigned to a charge transfer complex formed after the initial photoinduced ionization and subsequent electron transfer. However, the examination of this band immediately after photoexcitation shows clearly evidence of asymmetry and of the presence of a shoulder centered at ca. 475 nm. This signal corresponds to the expected initial formation of the radical cation as observed after spontaneous ionization. To demonstrate the presence of two separate contributions, the normalized kinetic traces corresponding to the decays measured at 475 and 530 nm were simulated by the heterogeneous Albery kinetic model (figure 7B). The resulting rate constantsk (and corresponding lifetimes 1/k) at 475 and 530 nm are 0.045 min-1 (τ = 22 min, γ = 2.50) and k = 0.022 min-1 (τ = 45 min, γ = 2.53), respectively. Even if the poor spectral resolution makes the interpretation difficult, the shorter lifetime observed for the species absorbing at ca. 475 nm is in agreement with the electron accepting behaviour of the radical to create the charge transfer complex (electron/hole pair). The clear intensity increase observed between 1050 and 1200 nm (figure 7A, insert) can be compared to the band observed after spontaneous ionization and assigned to the radical cation. Note the presence of a band at 1138 nm which is already observable before excitation.


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This band which is probably due to the first overtone frequency of SiOH group vibrations in the zeolite framework could not be removed using the pure zeolite as a reference. Despite this feature, the intensity decay at 1135 nm was tentatively simulated and the resulting rate constant (k = 0.035 min-1; τ = 28 min) was found to be very similar to that obtained at 475 nm. This result shows that these two bands evolved according to the same way and thus confirms that they belong to the same species, i.e. the radical cation. The photoexcitation induced also the formation of a very broad band that spread over more 300 nm above 1300 nm. This behavior is also analogous to the spectral signature reported in HZSM-5. Thus, based on the data obtained in acid H-ZSM-5 after p-QPh spontaneous ionization but also on the spectra recorded after t-stilbene ionization in ZSM-5 and BEA zeolites

22,24

, we tentatively assigned

this very broad band to the spectral signature of trapped electrons. Finally, a significant intensity increase is observed at 320 nm upon excitation. This band is observed at the position expected for the neutral molecule and thus might include the contribution of the p-QPh neutral molecule

62,63

involved in the charge transfer complex superimposed to the weaker

contribution of the radical and of the non ionized molecule. No further experiment could confirm this hypothesis but the simultaneous evolution of the 320 nm band with the 530 nm one was already observed throughout spontaneous ionization after mixing p-QPh and acid HZSM-5. To highlight clearly the primary formation of the radical cation, time resolved diffuse reflectance UV visible absorption experiments were performed to investigate the microsecond-millisecond time range. Figure 8A shows the reconstructed spectra corresponding to various delay times following the 300 nm laser pulses. These spectra exhibit a band between 420 and 540 nm with two major peaks at 475 nm and 505 nm assigned straightforwardly to p-QPh+ by analogy with the spectrum obtained previously in H-ZSM-5 and with the data of the literature.

51-54

Note that no spectral evidence of ejected electron is

observed. The kinetic trace corresponding to the decay of the radical measured at 505 nm is presented in figure 8B and is accurately simulated using the heterogeneous Albery kinetic model. The corresponding p-QPh+ lifetime is 3.76×10-2 µs-1 (τ = 27 µs). Moreover, it is interesting to note that the p-QPh+ spectral concentration decreased very slowly after about 400 µs and tend to reach a plateau. According to the Albery model, about 40% of radical species are assumed to be not recombined directly over the studied time scale. Consequently, p-QPh+ lifetime is probably long enough to allow hole transfer and to induce persistent holeelectron formation. However, the hole transfer cannot be observed by time resolved spectroscopy. According to the data obtained using both conventional and time resolved UV13 ACS Paragon Plus Environment


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visible spectroscopy, the reaction mechanism after photoexcitation can be summarized by the following reaction sequence: a) Photoinduced ionization and radical cation formation:

hν p-QPh@Na-ZSM-5 p-QPh●+@Na-ZSM-5●- (5) b) Partial geminate recombination as observed by TR-DRUVv p-QPh●+@Na-ZSM-5●- p-QPh@Na-ZSM-5 (6) c) Electron transfers from remaining radical cations and charge transfer complex formation p-QPh●+@Na-ZSM-5●- p-QPh@Na-ZSM-5●+●- (7) d) Recombination of the charge transfer complex to give again the neutral molecule in a neutral environment p-QPh@Na-ZSM-5●+●- p-QPh@Na-ZSM-5 (8) The irradiation of bare Na-ZSM-5 using the 300 nm exciting line did not induce any EPR signal. In contrast, an EPR signal was detected at room temperature after photoexcitation of p-QPh adsorbed in Na-ZSM-5. The CW-EPR spectrum recorded 15 minutes after irradiation show a narrow EPR signals of 5G and centered at g=2.006 (figure 9). This signal which corresponds to the overall spin concentration decreases slowly during the first hours following the UV laser excitation before reaching a plateau after about 36 hours. A residual EPR signal was still observed after 10 days at room temperature. As no structured signal could be observed on the EPR spectrum, the signal is assumed to include predominantly the contribution of the CTC. However, the presence of possible remaining radical cation makes difficult the discrimination between the radical cation-electron pair and electron-hole pair contributions all along the reaction process: electron-hole pair formation and recombination.

Thus, from these time resolved experiments covering the microsecond-months scale, it appears clearly first that the radical is formed initially upon very fast photoinduced electron ejection, second that a proportion of the radical recombine directly but that it remains sufficient radical amount to allow the hole transfer and charge transfer complex formation. However, the hole transfer is too slow to be observed by using nanosecond time resolved spectroscopy and too fast to be detected by conventional UV-visible technique.


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Conclusion Under the same reaction conditions, the mixing of solid p-QPh with H-ZSM-5 induces slow spontaneous ionization process whereas p-QPh is incorporated as intact molecule in Na containing ZSM-5 and requires photoexcitation to be ionized. However, identical behaviors with similar reaction mechanisms can be deduced from the experimental data recorded after p-QPh ionization in acid H-ZSM-5 and in non Brønsted acid Na-ZSM-5. Thus, using diffuse reflectance UV-vis spectroscopy, perfect analogy is observed between the spectral features obtained after spontaneous and photoinduced ionizations. Indeed, in both cases, the primary formation of the radical cation which is necessary to induce the subsequent electron transfers is not only demonstrated by the observation of bands at 475-505 nm already reported in solution but also by the observation of a broader band including two contributions in the near infrared region at 1150 and 990 nm. The system is also found to evolve progressively to form a stable electron/hole pair associated with a charge transfer complex essentially characterized by a band in the visible spectral domain at 530 nm. The formation of such separated charge states all along the sorption process is perfectly confirmed by the presence of an intense EPR signal that developed in parallel with the UV bands. In addition, thanks to resonance enhancement, the Raman spectra obtained using complementary exciting wavelengths allow also the identification of the spectral features characteristic of the radical cation and of the charge transfer complex which appeared successively over the studied time scale. The total disappearance of the characteristic Raman lines of the neutral molecule several months after mixing p-QPh with H-ZSM-5 demonstrates the very high ionization rate. Moreover, whatever the ionization process (spontaneous or photoinduced), it should be noted that the separated charge states stabilized for months (spontaneous) or for days (photo) end up recombining to give again the neutral molecule occluded in a neutral environment. The re-observation of the spectral features of the isolated neutral molecule on the Raman spectra after several months or years in case of spontaneous ionization show clear evidence of the charge recombination. Nevertheless, even if the mechanisms are identical, the reaction kinetic corresponding to the species formation depends dramatically on the ionization way. Spontaneous ionization is associated with the sorption process and is closely correlated with the molecule diffusion. Consequently, this mechanism is very slow due to the size of the molecule and allows the stabilization of radical cations in high yield and for long time contrary to what was observed with molecules with low dimensions for which the electron-hole pair was formed rapidly.



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Acknowledgments: The authors wish to thank Myriam Moreau for Raman experiments and Julien Dubois for time resolved UV-vis experiments.

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(62) Maruo, K.; Wada, Y.; Yanagida, S. Photocatalysis by Perfluorinated Oligo(pphenylene)s. Bull. Chem.Soc.. Jpn. 1992, 65, 3439-3449. (63) Matsuoka, S.; Fujii, H.; Yamada, T.; Pac, C.; Ishida, A.; Takamuku, S., Kusaba, M.; Nakashima, N.; Yanagida, S.; Hashimoto, K.; Sakata, T. Photocatalysis of Oligo(pphenylenes): Photoreductive Production of Hydrogen and Ethanol in Aqueous Triethylamine. J. Phys. Chem. 1991, 95, 5802-5808.



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Figure caption Figure 1 : Diffuse reflectance UV-visible absorption (DRUVv) spectra recorded as a function of time after the mixing of solid p-Qph and H-ZSM-5 dehydrated at 723 K under argon: a) 48 hours; b) 14 days; c) 30 days; d) 40 days; e) 49 days; f) 104 days; g) 166 days; h) 208 days; i) 340 days; j) 2 years. The spectra are vertically shifted for clarity. Figure 2 : (A) Pure diffuse reflectance UV-vis spectra resolved using MCR method applied on the DRUVv data set recorded after the mixing of solid p-QPh and H-ZSM-5. a) pQPh●+@H-ZSM-5●-; b) p-QPh@H-ZSM-5●+●(B) Evolution of the Ct/C0 profiles relative to spectral concentration of p-QPh ●+@Na-ZSM5● (square) and p-QPh@Na-ZSM-5●+/● (triangle). Figure 3 : FT Raman (λ= 1064 nm) spectra recorded as a function of time after the mixing of solid p-QPh and H-ZSM-5 dehydrated at 723 K under argon. a) solid t-St; b) 8 days; c) 58 days; d) 78 days; e) 90 days; f) 104 days; g) 139 days; h) 183 days; i) 335 days; j) 2 years. Figure 4 : Resonance Raman spectra recorded 1.5 years after the mixing of of solid p-QPh and H-ZSM-5 dehydrated at 723 K under argon using various excitation line. a) solid t-St; b) λ= 473 nm; c) λ= 532 nm. The spectra recorded 335 days (spectrum d) and 2 years (spectrum e) after the mixing using the λ=1064 nm excitation line are given as references. Figure 5 : Room-temperature X-band CW-EPR spectra recorded a) 1 hour and b) 1 week after mixing p-QPh and dehydrated H-ZSM-5. Figure 6 : Two-dimensional HYSCORE recorded 6 months after the mixing of p-QPh and dehydrated H-ZSM-5. The spectrum was recorded at room temperature. Figure 7 : (A) Diffuse reflectance UV-visible (DRUVv) spectra recorded after laser photoirradiation of p-QPh@Na-ZSM-5 as a function of time (300 nm, 15 s, 30 mJ cm-2); a) before irradiation, b) 5 minutes after irradiation, c) 10 minutes, d) 15 minutes, e) 8 hours, f) 25 hours. (B) Decay profiles of Ct/C0 relative to spectral concentration of a) p-QPh ●+@Na-ZSM-5●  (square) and b) p-QPh@Na-ZSM-5●+/● (triangle). The solid lines represent the best-fit calculated decays using the Albery function, the squares and triangles represent the experimental points. Figure 8 : (A) Time Resolved diffuse reflectance UV-visible (TRDRUVv) spectra collected at specified times following 300 nm laser excitation of p-QPh@Na-ZSM-5 from 10 µs to 400 µs. (B) Normalized kinetic traces for the decay of p-QPh●+@Na-ZSM-5● monitored at 505 nm following the 300 nm laser excitation. The solid line represents the best-fit calculated decay using the Albery function, the squares represent the experimental points. Figure 9 : Room-temperature X-band CW-EPR spectra recorded a) 10 min; b) 24 h; and c) 36 h after p-QPh@Na-ZSM-5 irradiation (λ=300 nm).


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Kubelka- Munk units

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g f e d c b

a

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Figure 1 : Diffuse reflectance UV-visible absorption (DRUVv) spectra recorded as a function of time after the mixing of solid p-Qph and H-ZSM-5 dehydrated at 723 K under argon: a) 48 hours; b) 14 days; c) 30 days; d) 40 days; e) 49 days; f) 104 days; g) 166 days; h) 208 days; i) 340 days; j) 2 years. The spectra are vertically shifted for clarity.



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Figure 2 : (A) Pure diffuse reflectance UV-vis spectra resolved using MCR method applied on the DRUVv data set recorded after the mixing of solid p-QPh and H-ZSM-5. a) pQPh●+@H-ZSM-5●-; b) p-QPh@H-ZSM-5●+●(B) Evolution of the Ct/C0 profiles relative to spectral concentration of p-QPh ●+@Na-ZSM5● (square) and p-QPh@Na-ZSM-5●+/● (triangle).


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j i

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Figure 3 : FT Raman (λ= 1064 nm) spectra recorded as a function of time after the mixing of solid p-QPh and H-ZSM-5 dehydrated at 723 K under argon. a) solid t-St; b) 8 days; c) 58 days; d) 78 days; e) 90 days; f) 104 days; g) 139 days; h) 183 days; i) 335 days; j) 2 years.

e

Raman intensity

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Figure 4 : Resonance Raman spectra recorded 1.5 years after the mixing of of solid p-QPh and H-ZSM-5 dehydrated at 723 K under argon using various excitation line. a) solid t-St; b) λ= 473 nm; c) λ= 532 nm. The spectra recorded 335 days (spectrum d) and 2 years (spectrum e) after the mixing using the λ=1064 nm excitation line are given as references. 25 ACS Paragon Plus Environment


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EPR intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 5 : Room-temperature X-band CW-EPR spectra recorded a) 1 hour and b) 1 week after mixing p-QPh and dehydrated H-ZSM-5.

ω2/ 2π (MHz)

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Figure 6 : Two-dimensional HYSCORE recorded 6 months after the mixing of p-QPh and dehydrated H-ZSM-5. The spectrum was recorded at room temperature.


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Figure 7 : (A) Diffuse reflectance UV-visible (DRUVv) spectra recorded after laser photoirradiation of p-QPh@Na-ZSM-5 as a function of time (300 nm, 15 s, 30 mJ cm-2); a) before irradiation, b) 5 minutes after irradiation, c) 10 minutes, d) 15 minutes, e) 8 hours, f) 25 hours. (B) Decay profiles of Ct/C0 relative to spectral concentration of a) p-QPh ●+@Na-ZSM-5●  (square) and b) p-QPh@Na-ZSM-5●+/● (triangle). The solid lines represent the best-fit calculated decays using the Albery function, the squares and triangles represent the experimental points.



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Figure 8 : (A) Time Resolved diffuse reflectanceUV-Visible (TRDRUVv) spectra collected at specified times following 300 nm laser excitation of p-QPh@Na-ZSM-5 from 10 µs to 400 µs. (B) Normalized kinetic traces for the decay of p-QPh●+@Na-ZSM-5● monitored at 505 nm following 300 nm laser excitation. The solid line represents the best-fit calculated decay using the Albery function, the squares represent the experimental points.


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Magnetic Field (G)

Figure 9: Room-temperature X-band CW-EPR spectra recorded a) 10 min; b) 24 h; and c) 36 h after p-QPh@Na-ZSM-5 irradiation (λ=300 nm).

Figure TOC

h+ e1560

1600

e-

1640

cm -1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

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ACS Paragon Plus Environment

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Comparison between Spontaneous and Photoinduced Ionization

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