Electron Transfers in DonorâAcceptor Supramolecular Systems

Article pubs.acs.org/JPCC

Electron Transfers in Donor−Acceptor Supramolecular Systems: Highlighting the Dual Donor and Acceptor Role of ZSM‑5 Zeolite Alain Moissette,*,† Matthieu Hureau,† Perrine Col,† and Hervé Vezin† †

Laboratoire de Spectrochimie Infrarouge et Raman, UMR-CNRS 8516, Université de Lille 1, Bât C5, 59655 Villeneuve d’Ascq, France S Supporting Information *

ABSTRACT: After coadsorption of electron-donor (p-terphenyl, PTP) and electron-acceptor (1,4-dicyanobenzene, DCB) molecules within the channels of silicalite-1 and MZSM-5 (M = Na+, H+) zeolites, photoinduced or spontaneous electron transfers were investigated. In aluminum-free silicalite-1, the reaction mechanisms after PTP ionization are similar in the presence and in the absence of the acceptor molecule. Photoionization leads to a PTP•+ radical cation, which recombines directly. In NaZSM-5, pterphenyl photoexcitation induces PTP•+ formation evolving to an electron−hole pair through capture of another electron of zeolite. This behavior is observed with and without DCB. However, when DCB is coadsorbed with PTP, recombination decays for PTP•+ and for the electron−hole pair are significantly slower. Pulsed EPR experiments show strong electron density close to DCB, through a coupling of unpaired electrons with 14N nuclei. Nevertheless, the electron transfer remains insufficient to allow DCB•− radical anion formation. High confinement within ZSM-5 and intrinsic strength of zeolite acceptor sites might be put forward to explain the nonformation of the anion. The acceptor properties of DCB and of the zeolite might then be competitive. The zeolite electron acceptor character is even more marked when PTP is coadsorbed with DCB in acidic HZSM-5. Ionization occurs spontaneously, and transient species are stabilized for months. No electronic coupling with nitrogen atoms of DCB could be observed, indicating no partial transfer to the acceptor molecule and electron trapping in acidic zeolite.

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INTRODUCTION The ability of supramolecular assemblies to induce and to stabilize charge-separated states is of particular interest for the development of potential applications in various fields like energy conversion,1−4 photovoltaics,5−17 or catalysis.18−25 However, the main obstacle of these systems to wider use remains the too fast charge recombination reactions. Maintaining a state of separated charges for a sufficiently long time is, indeed, a challenge necessary to face in order to facilitate electron transfer and subsequent harvesting after ionization. Various strategies have been investigated and have been reported in the literature. Some groups work for example on the synthesis of complex molecular assemblies that involve electron donor and acceptor chromophores. Many variations of these assemblies in solution or involving interfacial systems have been published.4,7,12,13,26−34 Nevertheless, their performance in terms of charge separation does not exceed some microseconds. Another approach is to use the internal volume of porous materials. Among these materials, zeolites are crystalline aluminosilicates whose structure, based on a threedimensional framework of AlO4 and SiO4 tetrahedra, comprises cavities and channels with well-defined shapes and sizes, suitable for the insertion of molecules. To date, it is indeed well-known that the ionization of aromatic molecules adsorbed within the porous volume can generate charge-separated states © XXXX American Chemical Society

having very long lifetimes which can exceed days, weeks, and even months.35−40 Due to their variable cage or channel dimensions and cation-exchange capacity, the internal properties of zeolites can be easily adjusted, and these materials represent suitable supports to investigate numerous electron transfer reactions. Thus, the confinement effect and the high internal electrostatic field were shown to be the main factors responsible for the electron trapping and, consequently, for the slow decay of transient species. The zeolite acts like a solid solvent, and the channels or cavities can be considered as microreactors in which the reaction intermediates are stabilized due to slower recombination reactions. In this context, even if the formation of persistent charge-separated states is not a sufficient guarantee for the effective use of these systems, this approach, through the use of microelectrodes, could actually represent an interesting and nonpolluting alternative for the conversion of solar energy.7 In addition, note that the combination of high internal volume, strong trapping and active sites, selective sorption, and molecular sieving ability make zeolites also very interesting supports for heterogeneous catalysis. Received: April 23, 2016 Revised: July 5, 2016

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DOI: 10.1021/acs.jpcc.6b04103 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

kinetics. This feature might then be used in future systems to take advantage of separated charge states stabilized in zeolites by facilitating a gradual charge transfer by electron hopping to outer semiconductor molecules. First, we describe the adsorption of PTP only, and then the incorporation of DCB only, and finally the coadsorption of PTP and of DCB in silicalite-1 and in NaZSM-5 zeolites, respectively. After mixing under argon the guest molecules with dehydrated host structure, we investigate the reaction mechanisms induced by photoexcitation of these three systems using complementary spectroscopic techniques. In particular, the effect of the presence of the charge-balancing cation is highlighted when the reaction takes place in NaZSM-5 compared to silicalite-1. For comparison, we report the electron transfers occurring between the PTP donor and the DCB acceptor after PTP spontaneous ionization in acidic HZSM-5. Finally, we emphasize the data associated with the fate of the ejected electron after ionization in acid and nonacid zeolites and put forward two competitive pathways associated with the electron acceptor characters of zeolite and of DCB.

To take advantage of these persistent charge-separated states, the following stage would be, for instance, to transfer the unpaired electrons to semiconductor materials present nearby. In a first approach, TiO2 nanoclusters were synthesized directly within the porous network and electron transfer mechanisms were investigated after photoionization of the adsorbed molecule.41 The results showed that the presence of the semiconductor within the channels slowed considerably the electron transfers after ionization. However, even if this result is consistent with a possible transfer of electrons to the conduction band of TiO2, the yield of this reaction was found to be limited because of the diffusion of TiO 2 nanoclusters to the pore entry and aggregate formation. To facilitate the electron transfer from the donor in the zeolite to the semiconductor, another method would consist in maintaining the semiconductor at the pore entry and incorporating an electron-relay acceptor molecule into the zeolite. This approach would constitute the penultimate stage before plugging the acceptor on the semiconductor. However, due to the presence of both basic and acidic Lewis sites, zeolites behave as electron donor and as electron acceptor with respect to the guest molecule. The acceptor properties of the zeolite are clearly highlighted by the stabilization of the charge-separated states after ionization and the trapping of unpaired electrons by the framework. The charge separation can take place either spontaneously after the mere mixing of the guest with the zeolite if the internal electrostatic field created by charge-compensating cations and if the confinement are high enough.39,42−49 These phenomena were frequently reported upon sorption of polyaromatics in channel-type acid zeolites43−46 with highly polarizing surroundings. In a less polarizing environment, the guest is occluded as a neutral molecule in close proximity of the charge-balancing cation and ionization has to be photoinduced.50−59 Note that the transient species created within the internal volume after spontaneous or photoionization are identical. Nevertheless, the lifetimes were found to be much longer after spontaneous ionization.44,48,52,59,60 In contrast, even if this property is lessdocumented, the electron-donating ability of zeolite was firmly demonstrated upon photoexcitation of strong electron acceptors like 1,2,4,5-tetracyanobenzene (TCNB)54,61 or methylviologen dication (MV2+)38,62 incorporated in zeolite which lead to the formation of the TCNB radical anion and of MV+, respectively. The 3-coordinated aluminum sites on the framework as well as the charge-balancing cations act as Lewis acids, whereas the framework oxygens, especially the oxygens adjacent to Al are considered to be the basic sites. In the present work, we study the electron transfers occurring in the porous void of silicalite-1 and of MZSM-5 (M = Na+, H+) type zeolites between an electron donor molecule (p-terphenyl) and an acceptor (dicyanobenzene) either after photoexcitation of the sample or simple mixing. The p-terphenyl (PTP) donor molecule belongs to the poly(pphenylene) molecule family which is well-known for interesting optoelectronic properties.63−66 The behavior of p-terphenyl48,59 as well as of biphenyl39 and of p-quaterphenyl60 parent molecules upon sorption in MFI type zeolites have been already reported in previous studies. Dicyanobenzene (DCB) is an organic molecule known as being a good electron acceptor.67,68 This molecule has two cyano groups that allow them to attract electrons by a mesomeric effect. Using this molecule, we hope to favor the capture of the electrons ejected after PTP ionization and then to slow down the recombination

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EXPERIMENTAL SECTION Materials. M-ZSM-5 samples (M = H+, Na+; Si/Al = 13.5; average particle size ∼1 μm) were obtained from VAW aluminum (Schwandorf, Germany). The purely siliceous MFI (silicalite-1) sample synthesized in fluoride medium was a gift from Dr. Patarin (Institut de Sciences des Matériaux de Mulhouse UMR CNRS 7228, France). p-terphenyl (PTP, C18H14, Sigma-Aldrich 99.5%) and 1,4-dicyanobenzene (DCB, C8H4N2, Sigma-Aldrich 98%) were purified by sublimation. Pure and dry Ar gas were 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 guest molecules corresponding to 0.5 molecule per unit cell (UC) were introduced into the cell without any solvent 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. Instrumentation. Diffuse Reflectance UV−Visible-Near IR (DRUVv) Absorption Spectroscopy. The UV−visible−NIR 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 samples stocked under an inert atmosphere in quartz cells; the corresponding bare zeolite was used as the reference. The 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 the 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 of 3500− 150 cm−1 with a 2 cm−1 resolution using 600 scans. DRIFTS. The FTIR spectrometer was a Thermo-Nicolet Magna 860 Instrument equipped with a liquid nitrogen cooled MCT detector. The DRIFT spectra were recorded with 2 cm−1 resolution. The key part of the in situ DRIFTS apparatus is a Harrick Scientific Diffuse reflectance attachment “Praying Mantis” combined with a cell equipped with CaF2 windows B


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The Journal of Physical Chemistry C and operating at 700 K under a controlled atmosphere. The DRIFTS spectra were plotted as the Kubelka−Munk function. Time-Resolved Diffuse Reflectance UV−visible (TRDRUV) Absorption Spectroscopy. The experimental setup for nanosecond diffuse reflectance spectroscopy was previously described.69 Excitation pulses at 300 nm (7−8 ns, 10 mJ) were 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 Jobin-Yvon, 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 to 400 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 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 580FT spectrometer. The EPR spectra were double integrated using Bruker software. The hyperfine sublevel correlation spectroscopy (2D-HYSCORE) measurements were carried out at 4.2 K 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, 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 16 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.

Figure 1. Diffuse reflectance UV−visible spectra obtained for (a) DCB@NaZSM-5, (b) PTP@NaZSM-5, (c) PTP-DCB@NaZSM-5. The spectra were recorded when the sorption process is completed several months after mixing the guest molecules and the host structure.

completely occluded in NaZSM-5 zeolites. The equilibrium is considered to be reached when the spectral features of the guest do not evolve anymore. During the sorption process, the samples were kept at 40 °C and protected from light in order to proceed to the irradiation of neutral molecules adsorbed within the porous material. No color change was observed during this time. Then, PTP and DCB molecules were coadsorbed in silicalite-1 and in NaZSM-5 to investigate the behavior of the donor/acceptor couple in such an environment. Samples were prepared by mixing under argon solid PTP (0.5 PTP/UC) and solid DCB (0.5 DCB/UC) with the zeolite dehydrated at 450 °C. The DRUVv spectra show a significant increase of a band centered at 300 nm, in which we find contributions from both PTP and DCB (Figure 1c). After complete adsorption and equilibration, the spectrum presented in Figure 1 corresponding to PTP/DCB system adsorbed in NaZSM-5 is also characteristic of the donor/acceptor system adsorbed in silicalite-1. The Raman spectra recorded after several months, when the adsorption process is finalized, are characteristic of the different adsorbed molecules. The spectra corresponding to the donor/ acceptor system are presented in Figure 2 and are compared to those recorded for PTP and DCB adsorbed as unique species in NaZSM-5. In particular, the spectrum of DCB exhibits lines at 1176, 1610, and especially at 2250 cm−1, characteristic of the C≡N triple bond stretching. The spectral features of PTP occluded in NaZSM-5 were already reported in previous works.59 The sorption process takes place without any chemical change and can be described by the following reactions:

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RESULTS 1. PTP and DCB Sorption in MFI Zeolites. First, PTP and DCB were incorporated as unique species in the internal volume of silicalite-1 and of NaZSM-5. After mixing under argon PTP or DCB (0.5 molecule/UC) with white solid zeolite powder previously dehydrated at 450 °C, the sorption process was followed continuously by diffuse reflectance UV−visible− NIR spectroscopy during the early hours and then periodically for several months. This monitoring has shown the gradual adsorption of guest molecule within the pore volume of the zeolite. Indeed, immediately after mixing, the molecules are under the form of microcrystals, and sorption occurs in the gas phase after sublimation of the molecules. Sorption on the external surface leads only to the spectral response of these “aggregates”. In contrast, after sublimation, internal sorption and diffusion in the void space concern isolated molecules. Then, the spectral response corresponds to the sum of all individual signals that is much more intense than the signal of the aggregates. Thus, we can observe the presence of a broad band centered at about 300 nm for PTP and 292 nm for DCB (Figure 1), with increasing intensity characteristic of neutral molecule occluded in the porous volume. The DRUVv spectrum evolution showed that the guest molecules were incorporated into the framework of the silicalite-1 after a few weeks, while several months were required for it to be

PTP + DCB + silicalite‐1 → PTP‐DCB@silicalite‐1

(1)

and PTP + DCB + MZSM‐5 → PTP‐DCB@NaZSM‐5

(2)

The Raman spectrum obtained for PTP/DCB incorporated in silicalite-1 shows sharper lines than the spectrum recorded for this same electron/donor couple adsorbed in NaZSM-5. Relative intensity differences are also noticed between both samples, in particular around 1600 cm−1. Furthermore, there is C


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to the absorption maximum of PTP as to an absorption band of zeolite. We present successively the electron transfers occurring in aluminum free silicalite-1 and in NaZSM-5 zeolites for PTP and DCB occluded as unique species and then for the PTP and DCB coadsorbed in the host framework. 2.1. Silicalite-1. The irradiation of the PTP@silicalite-1 at 295−300 nm did not lead to any signal observable by conventional spectroscopies, and consequently, the sample could only be analyzed using time-resolved DRUVv on the μs− ms time scale. The spectrum obtained by this method shows two main bands at 425 and 460 nm corresponding to the radical cation PTP•+.48,59 The recombination kinetic was computed by the Albery model, and the best fit between theoretical and experimental values is obtained for k = 0.0476 μs−1, corresponding to an apparent lifetime (τ = 1/k) for PTP•+ equal to 21 μs. In contrast, the photoexcitation of DCB@silicalite-1 did not allow the observation of any signal even using a transient spectroscopy technique. This feature reflects the absence of reactivity for DCB in this aluminum-free environment for the studied time scale (μs to ms). After irradiation of PTP-DCB@silicalite-1 sample, the TRDRUVv spectrum is identical to that reported above for PTP only (Figure 3A) and also shows the PTP•+ bands at 425 and 460 nm. Figure 3B shows the experimental decay recorded at 460 nm after excitation. The recombination kinetic values (k = 0.0366 μs−1 and τ = 27 μs) are very close to those obtained for the sample containing only PTP. Then, it is not possible to show evidence of any electron transfer from the zeolite or from the radical cation to the electron acceptor in the internal void space of silicalite-1. 2.2. NaZSM-5. The irradiation at 300 nm of PTP incorporated in NaZSM-5 zeolites causes a slightly green tint in the sample changing to pink over time. These spectra were already reported in a previous work.59 Immediately after irradiation, new absorption bands were observed at 418, 455, 514, and 551 nm. The bands at 418 and 455 nm are assigned to the PTP•+ radical cation created by PTP ionization according to the following reaction mechanism:

Figure 2. FT-Raman spectra (λ = 1064 nm) for (a) DCB@NaZSM-5, (b) PTP-DCB@NaZSM-5, (c) PTP-DCB@silicalite-1, and (d) PTP@ NaZSM-5. The spectra were recorded when the sorption process is completed several months after mixing the guest molecules and the host structure.

a significant Raman shift of the ν(C ≡ N) vibrational mode which is observed at 2245 cm−1 for the PTP-DCB system and for DCB only in aluminum-rich zeolites, whereas it is observed at 2235 cm−1 in silicalite-1. These various spectral features can probably be explained by the interactions between the extra framework cations and the adsorbed molecule within NaZSM-5 zeolite. In contrast, in silicalite-1, interactions with the channel walls are negligible, and the movements of the adsorbed molecule are not affected by the environment and remain in the range of the isotropic limit of a liquid. Nevertheless, given the different Raman spectra recorded for PTP and DCB adsorbed as unique species and for the PTP-DCB couple, no assumption can be made on a possible interaction between the donor molecule and the electron acceptor. To complete this vibrational study and to get a better understanding of the behavior of adsorbed molecules, the NaZSM-5 zeolite containing the PTP-DCB couple was also characterized by DRIFTS when the adsorption process is completed. The spectra obtained are shown in Figure S1. The spectra of dehydrated NaZSM-5 zeolite and of PTP and DCB molecules are also presented for comparison. These spectra are much less interesting than the Raman spectra because the spectral region extending from 800 to 1400 cm−1 displays huge absorption bands due to zeolite lattice modes70 and the very intense and broad band centered at 1630 cm−1, due to an overtone of a symmetric T-O-T stretching mode of the framework covers most of the vibrations in this spectral range.71 Nevertheless, for both samples, the spectra are characteristic of the adsorbed molecules and show the PTP lines at 844, 1400, 1450, and 1485 cm−1, while those corresponding to the DCB are observed at 875, 1400, 1500, and 2245 cm−1. Finally, the EPR spectra recorded for all the samples confirm the absence of paramagnetic species and, therefore, of any phenomenon of ionization at this stage of the study. 2. Photoinduced Electron Transfers. The photoinduced electron transfers were investigated several months after mixing the guest molecules and the host structure in order to study only well-equilibrated occluded molecules. The photoexcitation was carried out using an exciting line in the 290−300 nm spectral range. The excitation wavelength corresponds as well

hv

PTP@NaZSM‐5 → PTP•+@NaZSM‐5•−

(3)

Then, the radical cation disappearance can occur either directly as described by eq 4 or through the capture of another electron of zeolite leading to the formation of an electron−hole pair48,59 associated with a charge transfer complex characterized by the bands at 514 and 551 nm (eq 5). PTP•+@NaZSM‐5•− → PTP@NaZSM‐5

(4)

PTP•+@NaZSM‐5•− → PTP@NaZSM‐5•+•−

(5)

Note that during the first minutes (36 min), the contribution of the electron−hole pair is growing slightly before decreasing. The decays of the spectral concentrations for the radical cation and for the electron−hole pair were computed using the Albery model, and the calculated kinetic values are listed in Table 1. Then, the behavior of DCB occluded in NaZSM-5 was also investigated. The UV irradiation of the sample did not induce any new durable species at room temperature and requires the application of time-resolved DRUVv. Thus, the TRDRUVv spectra obtained after 290 nm excitation show a contribution maximizing at 340 nm and a broad and weak band that extends between 400 and 600 nm (Figure 4A). The 340 nm band is D


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Figure 3. (A) Time-resolved diffuse reflectance UV−vis (TRDRUVv) spectra collected at specified times following 295 nm laser excitation of PTP-DCB@silicalite-1 from 3 to 490 μs. (B) Normalized kinetic traces for the decay of the radical cation PTP•+ formed in silicalite-1 monitored at 460 nm following 295 nm laser excitation. The solid line represents the best-fit calculated decay using the Albery function; ■ represent the experimental points.

Figure 4. (A) Time-resolved diffuse reflectance UV−vis (TRDRUVv) spectra collected at specified times following 290 nm laser excitation of DCB@NaZSM-5 from 3 to 195 μs. (B) Normalized kinetic traces for the decay of the radical anion DCB•− monitored at 340 nm following 290 nm laser excitation. The solid line represents the best-fit calculated decay using the Albery function; ■ represent the experimental points.

Table 1. Kinetic Results for PTP•+@NaZSM-5•‑ and PTP•+DCB@NaZSM−5•− Radical Cation Decays and for PTP@ NaZSM-5•+•− and PTP-DCB@NaZSM-5•+•− Electron/Hole Pair Decays Following 300 nm Photolysis of PTP@NaZSM5 and PTP-DCB@NaZSM-5 Samples radical cation PTP

the numerous spectra obtained after electron/hole formation. This absorption band is also very similar to that reported after photoexcitation of DCB incorporated in NaY faujasite, which was assumed to be due to the trapped electron in the form of Na43+ clusters.54,74 Note that both bands disappear concomitantly. In addition, it should be noted that the anion is no more observed after 200 μs, the recombination rate estimated using the Albery model being k = 0.09429 μs−1 (Figure 4B). This short lifetime points out the low stability of this species in such an NaZSM-5 environment contrary to what is observed for the stable PTP•+. In this experiment, the electron transfers occur from the zeolite framework to the guest molecules occluded and confined in the channels. Thus, to explain the radical anion formation, we have to consider the electron donor ability of the zeolite in addition to the electron acceptor character of the molecule. The donor behavior is associated with the basic character of the environment. In zeolites, it is usually accepted that the electronegative oxygen atoms act as basic sites and that basicity depends on the chemical content and increases with the aluminum amount of the framework. In addition, the basicity depends on the extraframework cation type and

electron−hole pair

•+

PTP(-DCB)@NaZSM-5•+•− −1

sample

τ (min)

γ

k (min )

τ (min)

γ

k (min−1)

PTP@ NaZSM-5 PTPDCB@ NaZSM-5

20.7

2.84

0.0483

653.59

2.16

0.00153

83.33

3.39

0.012

3333.33

1.32

0.0003

assigned to the DCB•− radical anion which is probably formed by capturing an electron of zeolite according to eq 6. hv

DCB@NaZSM‐5 → DCB•−@NaZSM‐5•+

(6) 54,57,58,72,73

The attribution is based on literature data. The broad band in the visible region might correspond to the spectral signature of the positive electronic hole by analogy with E


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The Journal of Physical Chemistry C increases with the alkaline cation size from Na+ to Cs+. Consequently, the electron donor character of the aluminumfree silicalite-1 remains very limited compared to NaZSM-5. Figure 5 shows the diffuse reflectance UV−vis−NIR spectra recorded after the photoexcitation at 300 nm of the PTP-

overlap the possible contributions of radical species with weaker intensities appearing in the same region. Figure 6 (panels A and B) show the evolutions of spectral concentrations for PTP•+ and for the electron/hole pair in the

Figure 5. Diffuse reflectance UV−vis−NIR spectra recorded as a function of time after irradiation at 300 nm of PTP-DCB@NaZSM-5 (a) before irradiation, (b) 5 min, (c) 6 h, and (d) 4 days after photoexcitation. RC stands for radical cation and e/hole for the charge transfer complex associated with the electron/hole pair.

DCB@NaZSM-5 sample. Immediately after the irradiation, new bands are observed at 418, 454, 514, and 552 nm in the visible domain and at 911 and 1150 nm in the near-infrared spectral range. The intensity of the bands at 418, 454, 911, and 1150 nm decreases progressively after excitation, while the intensity of the bands at 514 and 552 nm increases for about 6 h before decreasing. Another very broad band centered at 1450 nm develops continuously with time. This evolution is very similar to that observed for the PTP@NaZSM-5 system in the absence of the acceptor molecule. On the basis of the reaction mechanism reported for this previous sample without DCB, the bands at 418 and 454 nm are attributed to the radical cation PTP•+, as well as those observed at 911 and 1151 nm which evolve similarly. The bands at 514 and 552 nm and the weaker ones at 614 and 670 nm are assigned to the electron/hole pair formation. Note that the development of the bands associated with the electron/hole pair takes place concomitantly with the decrease of the radical cation spectral signature. This feature shows evidence of the nondirect recombination of the radical cation which occurs mainly by capture of another electron of zeolite. The presence of an isosbestic point at 467 nm for the spectra recorded during 6 h after irradiation confirmed this reaction mechanism (Figure S2). By analogy with the spectral signature reported in the near-infrared region after t-stilbene in aluminated BEA and GaZSM-5 zeolites, the broad band observed between 1000 and 1800 nm might correspond to the spectral signature of electron trapped within the framework.46,49 Note that no absorption characteristic of the radical anion DCB•− could be detected in the UV and visible domains. Moreover, the spectral region around 300 nm includes intense absorptions of nonionized PTP and DCB molecules which

Figure 6. (A) Normalized decay profiles of Ct/C0 relative to spectral concentration of radical cations for (a) PTP•+@NaZSM-5•− and (b) PTP•+-DCB@NaZSM-5•− monitored at 454 nm after the 300 nm excitation of PTP@NaZSM-5 and PTP-DCB@NaZSM-5 samples. The solid line represents the best fit calculated decays using the Albery function; the squares represent the experimental points. (B) Normalized decay profiles of Ct/C0 relative to spectral concentration of charge transfer complex (electron/hole pair) for (a) PTP@NaZSM5•+•− and (b) PTP-DCB@NaZSM-5•+•− monitored at 552 nm after the 300 nm excitation of PTP@NaZSM-5 and PTP-DCB@NaZSM-5 samples. The solid line represents the best fit calculated decays using the Albery function; ■ represent the experimental points.

presence of DCB (PTP-DCB@NaZSM-5) and in its absence (PTP@NaZSM-5). It is worth noting that both experiments were carried out using the same procedure and that the radical species have been created in the same conditions at the same temperature. The calculated kinetic values deduced from the Albery model are listed in Table 1. The decays and the rate constants clearly show evidence of the slow-down of the charge recombination in the presence of DCB. The electron transfer leading to the formation of the electron/hole pair is still F


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the lines involving PTP vibrations, a weak band is observed at 1175 cm−1 and is assigned to DCB. The band expected at 1607 cm−1 for DCB is probably present but cannot be clearly distinguished between the contributions of PTP and of electron/hole pair. A low-intensity line is also observed at 1635 cm−1 after excitation and does not correspond to any spectral features known for PTP. In addition, no data is available in the literature to assign this band to a vibrational mode of DCB or to any subsequent species like the DCB•− radical anion. Note also that after irradiation, the symmetric ν(C≡N) stretching vibrational mode seems to disappear partly before to reincrease gradually. This feature might be explained by a fluorescence process occurring after ionization and involving a huge increase of the baseline especially in the spectral region above 2000 cm−1. This phenomenon could also be explained by the formation of the radical anion that is supposed to lead to a weakening of the CN bond because of antibonding character of the orbital in which the single electron is located.75 Even if, in the case of the neutral molecule, the interaction between the two CN groups is supposed to be low with two groups vibrating almost independently of one another,76 coupling exists and the intense symmetrical stretching Raman mode may not be visible during the formation of the anion because of the presence of an unpaired electron preferably located on one of the CN bonds. Figure 8 shows the vibration spectra obtained by DRIFTS. The spectra of the PTP-DCB@NaZSM-5 sample are shown

observed after about 6 h (339 min) in the sample containing the electron acceptor, whereas the spectral concentration of this species is maximum after only 36 min when PTP is adsorbed as a unique species. The lifetimes are multiplied by 4 for the radical cation and by 5 for the electron/hole pair in the presence of DCB. The longer lifetimes of the transient species could be due to a lower number of available electrons when PTP is coadsorbed with DCB. Thus, the electrons are assumed to be trapped in close proximity of DCB that behaves as new acceptor sites. Therefore, despite the supposed low stability of the anion radical (approximately 200 μs after photoexcitation of the DCB@NaZSM-5 sample) and the nonobservation of its spectral signature due to high overlapping with neutral and radical cation species, the formation of the radical anion DCB•− has to be considered. The ionization process was also followed by Raman scattering. The FT-Raman spectra recorded over time before and after irradiation are shown in Figure 7. The first spectrum

Figure 7. FT-Raman spectra (λ= 1064 nm) recorded as a function of time before and after photoexcitation of PTP-DCB@NaZSM-5 at 300 nm (a) before irradiation, (b) 15 min after irradiation, and (c) 17 h after irradiation.

recorded after photoexcitation shows the formation of the radical cation by the disappearance of bands characteristic of the PTP molecule at 1170 and 1274 cm−1 and by observing a weak line at 1353 cm−1 corresponding to the PTP+• radical.48,59 The latter line gradually disappears in favor of the bands at 1000, 1236, 1296, 1336, and 1600 cm−1 characteristic of the electron−hole pair.48,59 The assignment is based on the spectra obtained after sorption and ionization of PTP in FER and in ZSM-5.48 In the narrow pore ferrierite, the radical cation is stabilized after ionization, while in ZSM-5, the radical cation formation is followed by an electron transfer leading to longlived electron/hole pair formation. In the present study, all these new bands disappear progressively after ionization, but two successive recombination processes are clearly identified. This result is in agreement with data obtained by UV−visible− NIR absorption. The Raman features characteristic of the electron−hole pair are certainly due to the PTP molecule in close interaction with the unpaired electron. The electrostatic field created by the electron−hole pair around the neutral molecule can indeed induce small changes in terms of positions and relative intensities of the spectrum of PTP. In addition to

Figure 8. DRIFT spectra recorded for (a) DCB in the solid state, (b) PTP-DCB@NaZSM-5 before photoexcitation at 300 nm; (c) PTPDCB@NaZSM-5 after irradiation ; (d) PTP in the solid state and (e) free NaZSM-5 (Si/Al = 13,5).

before and after irradiation and are compared with the spectra of molecules in the solid state and of NaZSM-5 zeolite recorded under the same conditions. After photoexcitation, we notice a strong decrease in the intensity of the line at 1451 cm−1 assigned to the PTP molecule. The decrease in this line which then appears at 1454 cm−1 is certainly linked to the formation of the radical cation. New low-intensity lines are also observed at 1563 and 1592 cm−1 and probably correspond to the radical cation. The line initially centered at 1484 cm−1 and which displays two contributions at 1482 and 1485 cm−1 shows an intensification of the component of higher energy (1485 cm−1) after irradiation. In addition, according to the results reported in the literature, the possible formation of the radical G


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nuclei coupling with a very large number of nuclei with nonzero nuclear spin. Figure 10 shows the experimental spectrum

anion should result in a significant shift in the order of 130 cm−1 of the asymmetric ν(C≡N) stretching mode observed at 2235 cm−1 for the DCB molecule.77−79 However, only a very small displacement is observed for this band of about 5 cm−1, which we attribute to a change of environment. Thus, the spectra recorded before and after irradiation do not show this type of behavior and do not allow this transfer of electrons to be highlighted. Thus, as no evidence could be found by vibrational spectroscopy to demonstrate the radical anion formation, the PTP-DCB@NaZSM-5 sample was characterized by using the EPR technique, much more sensitive and perfectly adapted to the observation of radicals. It is important to note that the EPR spectrum of dehydrated NaZSM-5 zeolite does not show any signal of paramagnetic species even after irradiation. After photoexcitation of the PTP-DCB@NaZSM-5 sample, the EPR spectra show a continuous wave signal centered at g = 2 (Figure 9). The signal is not structured but displays anisotropy of the g

Figure 10. 2D-HYSCORE pattern recorded at 4.2 K 30 min after photoexcitation at 300 nm of PTP-DCB@NaZSM-5. The HYSCORE spectrum was recorded at 4.2 K, with τ = 200 ns and a pulse length of 16 ns for the π/2 pulse and 32 ns for the π pulse.

obtained at a temperature of 4.2 K recorded after photoexcitation of PTP-DCB@NaZSM-5. The spectrum shows three ridges in the quadrant (+, +) in which appear weak couplings. First, on the diagonal, we observe an intense signal centered at 3.9 MHz, which is characteristic of the nuclear Larmor frequency of 27Al (I = 5/2). The fact that the peak of 27Al is located on the diagonal and there is no cross-peak indicates that the coupling between the unpaired electron and Al atom is low and its coupling mechanism takes place through space via the dipolar interaction. The large contribution of 27Al masks a possible contribution of 29Si (I = 1/2), the nuclear resonance frequency of which is 2.9 MHz. On the diagonal, we also observe a low-intensity signal at 1.07 MHz corresponding to the nuclear Larmor frequency of 14N. The spectrum also shows a pair of ridges in the quadrant (+, +) whose coordinates are (3.9, 8.5) and (8.5, 3.9) MHz. These patterns are centered on the ν = 6 MHz value and probably reflect interaction with nitrogen atoms. Indeed, with nitrogen atom, the nuclear spin being I = 1, the experimental spectra become more complex. In the case of paramagnetic electronic spins interacting magnetically with 14N nuclei, the electron spin states are split into substates corresponding to different possible quantum states of 14 N nucleus. In the HYSCORE spectrum, the position of the cross peaks associated with the simple quantum nuclear transition shows a strong dependence on orientation. This results in a significant broadening of these peaks and very low intensity of these bands. On the contrary, the cross peaks that correspond to the double quantum transition are very localized and readily detected on the HYSCORE spectrum because of their higher intensity.80 Thus, in the presence of strong electron−nuclear hyperfine coupling (A ≫ 2 νN), double quantum peaks can easily be identified because the separation between the cross peaks is on the order of 4 ν or ≈ 4 MHz. In the case of a weak hyperfine coupling (A ≪ νN), the peaks are located in the spectral region near the diagonal in the quadrant (+, +). The cross peaks observed around the central position at 6 MHz are assigned to the double quantum contribution related to a relatively strong interaction between the unpaired electrons and atoms of 14N. In addition, in the quadrant (−, + ) in which strong coupling are observed, two cross-peaks of correlation with ridge coordinates at (−3.9; 0.4) and (−0.4; 3.9) MHz centered on the 2 MHz value are detected

Figure 9. CW EPR spectra recorded at room temperature after photoirradiation at 300 nm of PTP-DCB@NaZSM-5. (a) 5 min after irradiation and (b) 10 days after irradiation. Inset: enlargement of the magnetic field scale for the (a) spectrum.

factor. On the basis of spectral data obtained using diffuse reflectance UV−visible−NIR absorption, this signal is assumed to include contributions from the radical cation created by photoionization and electron−hole pairs formed by electron transfer. Note that the signal slowly decreases over time while maintaining this anisotropic structure. The formation of the radical anion by electron transfer to the acceptor molecule DCB is not highlighted. Indeed, the formation of the radical anion would have resulted in the observation of hyperfine coupling with the nuclei of the molecule. In addition, the recorded signal shows weak lines on each side of the main signal. This could indicate the presence of another paramagnetic contribution. Unfortunately, too low signal/noise ratio did not allow us to confirm this. Figure S3 shows the evolution of the doubly integrated intensity of the EPR signal with time for 11 days. The signal decreases gradually immediately after excitation and ionization of the PTP molecule, but one or more persistent paramagnetic species are observed for several days. This result is in agreement with the changes obtained by UV−visible−NIR. In order to reveal the environment of unpaired electrons, we performed pulsed EPR experiments using 2D-Hyperfine Sublevel Correlation Spectroscopy (2D-HYSCORE). These sequences allow appropriate assignment of different electronH


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indirectly to form electron−hole pairs associated with very stable charge transfer complexes observable for several years. On the basis of this work and to identify the role of zeolite in the electronic transfer, we present in this part the experimental data obtained after mixing PTP and DCB molecules with HZSM-5 zeolite. Although the ionization process is different, the reaction mechanisms induced spontaneously or by photoexcitation are assumed to be similar and cause identical transient species but with very different lifetimes. The diffuse reflectance UV−visible−NIR absorption spectra recorded as a function of time after mixing PTP (0.5 molecule/ U.C.) and DCB (0.5 molecule/U.C.) with HZSM-5 are shown in Figure 11. The evolution of the absorptions reflects the

perpendicular to the diagonal direction. These signals are related to a splitting due to hyperfine and quadrupole interaction. The extent of this ridge is 1.7 MHz on each side of the diagonal. This value represents the 3Pzz interaction with nitrogen 14N atom, that is a Pzz value of 0.6 MHz. Moreover, note that no coupling with the hydrogen atoms is observed at 14.5 MHz, 1H Larmor frequency. This lack of coupling could be explained by the absence of proton in the electron environment. The results of the experiments carried out by pulsed EPR clearly show an interaction between the unpaired electrons and the nitrogen atoms (hyperfine interaction of the order of 2 MHz or 0.8 G). This interaction is a superhyperfine interaction which appears to arise from an interaction between the ejected electron and the organic molecule. However, the spectra recorded in continuous wave do not show the characteristics of the anion radical. Indeed an anion radical containing a nitrogen atom would give a coupling of several Gauss on the EPR spectrum, and the HYSCORE pattern indicates that the density of electrons carried by the nitrogen is not sufficient to lead to a radical anion, the coupling with nitrogen being too low. It should be noted that the observation of a coupling between 14N atom and unpaired electron can only reflect an interaction with DCB. The coupling between the unpaired electron and Al displayed in Figure 10 arises from dipolar interaction with a vertical shift of 2 MHz. Such coupling can only occur from spin polarization coupling between the electron located on DCB moieties with 27Al. These results should be compared with those reported in the literature for similar donor−acceptor systems adsorbed in faujasite Y zeolites57 having pore openings and cavity dimensions wider than those offered by ZSM-5. Although in this study the authors adsorbed molecules of trans-anethole as electron donors, the acceptor molecule is also the DCB, which allows us to establish a comparison between the reaction mechanisms observed for both systems. In faujasite, the authors have clearly demonstrated the formation of a radical anion DCB•−. After photoexcitation and ionization of the transanethole molecule, the ejected electron, perhaps transiently trapped in the form of clusters Na43+, is not stabilized by the structure and can be captured by the DCB molecule to give the radical anion. In contrast, after photoionization of the PTP in the NaZSM-5 zeolite, the electron is not available even if there is a polarization mechanism which transfers part of the charge on the nitrogen atoms. This transfer is indeed only partial as the radical anion is not demonstrated experimentally by EPR. This result could be explained by the attracting power of the NaZSM-5 zeolite, which seems more predominant than that of the acceptor DCB molecule. Thus, the electron trapped by the structure is then no longer available to allow the formation of the radical anion in this environment. 3. Spontaneous Electron Transfers in HZSM-5. Processes of intrazeolite electron transfer can be initiated either by photon irradiation or spontaneously if the chemical composition and the polarization of the internal environment of the channels lower the ionization potential of the guest donor molecule enough to induce the formation of radical cations. Thus, in a previous study,48 it was shown that the mere mixing of acid HZSM-5 zeolite with PTP molecules having a sufficiently low ionization potential (IP = 7.8 eV) induced spontaneous ionization of the molecule and the formation of radical cation. UV−visible−NIR absorption and Raman scattering showed that the radicals preferably recombine

Figure 11. Diffuse reflectance UV−vis−NIR spectra recorded as a function of time over ten months after mixing PTP and DCB with dehydrated HZSM-5.

charge transfer processes leading to the electron−hole pair formation after spontaneous ionization and radical cation formation. The spectral features of the transient species are analogous to those reported above after PTP photoionization. These spectra are also similar to those obtained after the mere mixing of PTP as unique species with HZSM-5,48 and therefore, it is not possible to identify any possible contributions related to the formation of a radical anion DCB•−. The HYSCORE spectrum presented in Figure 12 was obtained at 4.2 K one year after mixing PTP and DCB with acid HZSM-5 zeolite. In accordance with the diffuse reflectance UV−vis−NIR and Raman spectra recorded after this period of time, this pattern is characteristic of charge-separated states stabilized as electron/hole pairs. The spectrum exhibits three different signals in the quadrant (+,+). The first, centered at 14.5 MHz, is characteristic of the nuclear Larmor frequency of the 1H proton. The second, at 3.9 MHz, corresponds to aluminum (27Al), while the last one at 2.9 MHz is representative of silicon (29Si). The transverse contributions of peaks centered at 14.5 and 3.9 MHz indicate the presence of superhyperfine coupling between the electron and a nucleus of one of the atoms of the system (1H and 27Al). Note that no definitive assignment can be done concerning the interaction between protons and electrons since the coupling can come both from protons of the zeolite and those of the organic molecule. The electron/nucleus coupling (about 8 I


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hole never formed due to too fast charge recombination and to the very weak electron donating character of Al-free zeolite. This last feature is also put forward to explain why the radical anion is never obtained after photoexcitation of DCB incorporated as unique species in silicalite-1. In NaZSM-5, the lifetime of the PTP radical cation and electron/hole was found to be 4−5 times longer in the presence of DCB, suggesting that the electron necessary for charge recombination is not as available as in the absence of DCB. Because of the donor property of NaZSM-5, the formation of DCB•− might be put forward to explain this feature as this species is formed after photoexcitation of DCB@ NaZSM-5 when DCB is incorporated without PTP. Nevertheless, due to very short lifetime, this species was only observed using time-resolved spectroscopy. When PTP and DCB are coadsorbed in this Al-rich zeolite, the radical anion was never observed and photoexcitation induces radical cation formation in high yield. However, as mentioned above, the excitation wavelength (300 nm) corresponds as well to the absorption maximum of PTP as to an absorption band of zeolite and then, irradiation is expected to induce PTP ionization as well as electron abstraction from zeolite. The signal obtained for PTP•+ is very intense and persistent, and consequently, does not allow the observation of anion formation on the short time scale (microseconds) as for DCB only. Then, the direct formation of radical anion by electron transfer from zeolite cannot be observed. Thus, the excess of charge density on DCB observed by HYSCORE is probably due to an electron transfer, even partial, occurring after PTP ionization. It should be noted that to enhance the possible electron capture by DCB, the experiment was reproduced by increasing the local distribution of DCB using a higher DCB loading with a DCB/PTP ratio of 0.8/0.2. However, the spectral data were identical to those reported above with the 0.5/0.5 DCB/PTP ratio without anion formation. The reaction mechanisms observed after (photo)ionization of PTP are similar to those described above for the biphenyl39 and for p-quaterphenyl,60 two other molecules belonging to the family of poly(p-phenylene) but also for many other polyaromatic molecules such as naphthalene43 or t-stilbene.44,52 The electron transfer from the zeolite to PTP•+ and the formation of the electron−hole pair are closely associated with the oxidizing power of the radical cation [Eox(PTP•+) = 1.78 V/ SCE]. This is consistent with the results previously obtained with, for example, biphenyl Eox(BP•+) = 1.9 V/SCE) and tstilbene [Eox(t-St•+) = 1.75 V/SCE], whereas this transfer reaction is not observed in the case of anthracene whose potential is significantly lower [Eox(Ant•+) = 1.1 V/SCE] and where the stabilized form is the radical cation.40 Stabilizing these transient species depends on the confinement imposed by the channels of the zeolite to the adsorbed molecule, but it also depends on the electrostatic field prevailing within the pore space. Indeed, if we look at the results obtained with the silicalite-1, it is clear that the internal volume of the dealuminated zeolite does not stabilize these reaction intermediates for periods accessible by conventional spectroscopic techniques. However, in the presence of aluminum, the charge compensating cations induce a strong intrazeolite electrostatic field and an internal environment polar enough to stabilize the separated charge states formed. As the electrostatic field gradient strongly depends on the van der Waals radius of charge-balancing cations, small cations such as

Figure 12. 2D-HYSCORE pattern recorded one year after mixing PTP and DCB with dehydrated HZSM-5. The HYSCORE spectrum was recorded at 4.2 K, with τ = 200 ns and a pulse length of 16 ns for the π/2 pulse and 32 ns for the π pulse.

MHz), evidenced by the peaks on each side of the diagonal at 14.5 MHz and with coordinates (11; 18) (14; 15) and (15; 14) (18; 11) MHz, is similar to those reported for various systems describing the formation of the electron−hole pair44 and, therefore, is consistent with stabilization of the species in the sample. The electron/proton coupling represents the major interaction in this system as evidenced by the higher intensity of the corresponding ridges (red concentric circle). For aluminum, the electron/nucleus coupling highlighted by the presence of peaks on each side of the diagonal also show the existence of interactions between the electrons and aluminum but at levels much lower than that observed for the proton. The interaction is dominated by an interaction of contact Fermi type (5.5 MHz), indicating 0.2% of electron in Al p-orbital. Such results are characteristic of connectivity between electron and Al with unpaired electron trapped on oxygen center. Note that a more diffuse interaction at a greater distance is also observed (1.2 MHz). In addition, it is interesting to note that unlike the systems studied after photoionization in a nonacidic NaZSM-5 zeolite, the signature of nitrogen is never observed. Therefore, the electron transfer to the acceptor molecule never occurs in such system, even after months or years. This tends to indicate that the acceptor character of acid HZSM-5 zeolite is much stronger than that of the molecule of DCB but also more important than the nonacidic NaZSM-5 zeolites for which we observed a partial charge transfer on nitrogen.

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DISCUSSION In the absence of DCB, PTP•+ decays either by charge recombination with the electron added to the zeolite framework upon photoexcitation or by capturing another electron from the zeolite framework inducing electron/hole pair formation. In the presence of DCB, in addition to these two previous charge recombination ways, a third path has to be considered through the transfer of the ejected electron to DCB and radical anion formation. In that context, the radical cation and electron/hole pair charge recombinations were investigated for both the PTP and PTP/DCB systems occluded in silicalite1 and in NaZSM-5 using diffuse reflectance UV−vis−NIR experiments. The presence of DCB within the pore volume is proven by the observation of DCB Raman characteristic lines. In silicalite-1, no obvious difference could be established between both systems. Charge separation could be observed only using TRDRUVv, and it is noteworthy that the electron/ J


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The Journal of Physical Chemistry C H+ and Na+ induce a local field gradient significantly higher than large cations such as, for instance, Cs+, thus favoring stabilization. Note also that the creation of a charge-separated state induced immediately a new electric potential which leads to an even stronger local field gradient. In the absence of electron acceptor molecules within zeolite, the cation radical disappears in two distinct reaction pathways. The first is a direct charge recombination of the radical cation with the electron ejected during photoionization. The second reaction path involves the oxidizing power of the radical cation and the ability of the zeolite to yield an electron (electron donor properties demonstrated for DCB@NaZSM-5). It consists of a charge transfer between the radical cation and the zeolite, leading to the formation of an electron−hole pair. In the presence of DCB, a new reaction path should be considered through the possible transfer of the ejected electron to the DCB acceptor molecule, with formation of a radical anion. This transfer can take place directly following the formation of the radical cation and the immediate capture of the ejected electron, or through the zeolite which could act as an intermediary. To ensure electron transfer, even partial, from the ionization site of the donor to the acceptor, the distance between PTP and DCB has to be sufficiently short. Even if direct transfer could be possible, the role of the zeolite framework allowing electron migration by electron hopping is more probable and has to be considered. The ejected electron is assumed to be initially trapped on oxygen atoms nearby Al atoms and then to pass from on site to another one to finally be located in close proximity of the acceptor DCB. These systems are very complex, and it is difficult to propose a reaction mechanism. However, as a first approximation, the direct electron transfer of PTP to DCB without prior stabilization by zeolite can be neglected. Indeed, if we analyze the results obtained after irradiation of the PTP-DCB system adsorbed in silicalite-1, only the radical cation is observed on a few microseconds time scale. No electron transfer to form the anion or the electron−hole pair has been demonstrated by techniques, including time-resolved spectroscopy. The formation of the radical anion DCB•− has already been observed on a time scale of the order of nanomicroseconds, after irradiation of an electron donor molecule incorporated into an aluminated faujasite-type zeolite.57 In the cavities of this type of zeolite, the influence of the local field on the adsorbed molecule is significantly smaller than in an aluminated ZSM-5 in which the molecule is confined. Nevertheless, the direct transfer of the electron to form the anion takes place. This previous study therefore demonstrates that, contrary to what is observed for the electron−hole pair, the strength of the field gradient is not decisive to allow the direct transfer of the electron to the acceptor. Thus, if the electron ejected during photoexcitation was transferred directly from PTP to DCB, this process would likely be observable. Therefore, it seems reasonable to exclude such direct transfer and favoring a twostep reaction first with a transfer of the ejected electron from PTP to the zeolite and then transfer the electron to the DCB. On the other hand, despite an analytical approach based on the use of complementary techniques, we have never been able to demonstrate the formation of the radical anion even for higher DCB loading. The results of the EPR, however, indicate a strong coupling between the unpaired electrons and 14N nuclei of the DCB molecule, reflecting a partial transfer of the charge on the acceptor after photoionization of the donor. Although the spectral characteristics of the anion are not

detected by EPR, the slight displacement of the C≡N stretching at 2240 cm−1 observed by Raman scattering in the case of NaZSM-5 system shows a change in the DCB environment. This perturbation attributed to the presence of the ejected electron near the molecule confirms the EPR spectra. The trapping of the electron, even partially, thus decreases its availability and influences all the charge transfer processes. Thus, as the electron transfer to DCB does not actually take place, the equations of reactions associated with PTP-DCB system describe a reaction mechanism similar to the one reported for the sole PTP. However, the kinetics of transfer mechanisms are strongly affected by the very different internal environments in the PTP-alone and PTP-DCB systems. The electron ejected in the case of PTP adsorbed as unique species is located in close proximity of an aluminum atom of the framework and thus remains relatively available, whereas in the presence of an electron acceptor such as DCB, the electron is likely more stabilized since it is in direct interaction with DCB.

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CONCLUSIONS To investigate and understand the charge transfer mechanisms taking place after ionization of molecules adsorbed in MFI zeolites, we conducted a comparative study between systems composed for the firsts, of electron donor or electron acceptor molecules adsorbed as unique species into the porous framework and for the other, the donor/acceptor couple coadsorbed in the same host environment. This dual approach has been followed by various complementary spectroscopic techniques to study the reaction mechanisms. In nonacidic NaZSM-5 zeolites, the photoinduced ionization of p-terphenyl molecule (PTP) leads to the formation of the PTP•+ radical cation evolving toward an electron−hole pair by electron transfer. This behavior is observed both in the absence of an electron acceptor and in the presence of dicyanobenzene (DCB), an acceptor molecule. However, when DCB is coadsorbed with PTP, the recombination kinetics of the radical cation and of the electron−hole pair are slowed significantly compared to the case for which PTP is adsorbed alone. This result demonstrates that unpaired electrons present in the network are less available in the presence of the acceptor. In this context, the logical assumption of capture by the DCB of the ejected electron and formation of a radical anion DCB•− was investigated by numerous experiments that led to the conclusion that this species is not actually formed. However, analysis by pulsed EPR showed a significant increase in the electron charge density close to the DCB, through a strong coupling of the electron with the 14N nuclei. This may reflect a partial transfer of electrons to the nitrogen but which is still insufficient to allow the formation of a radical anion. Trapping of electrons in close proximity of DCB may explain the decline of the charge recombination process compared with the system having no electron acceptor. The lack of formation of the radical anion remains, however, surprising if one refers to previous work for which the anion presence is clearly demonstrated with the same acceptor in faujasite zeolites with larger pore openings and cavities in which the adsorbed molecules are not confined. The explanation for this difference might reside in the electron acceptor character of the ZSM-5, if we consider that the zeolite acts as well as an electron donor as it does as an acceptor. This behavior is more especially pronounced within ZSM-5 zeolites, in which the influence of the confinement is important and that can stabilize the states of separated charges for several days after photoexcitation, while K


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the lifetime of the transient species is of the order of a few microseconds in faujasite. This electron acceptor character is even more marked when PTP is coadsorbed with DCB in acid HZSM-5 zeolite. Indeed, in this case, ionization occurs spontaneously and the same transient species formed are stabilized for months. However, unlike what is observed after photoionization of the same system in NaZSM-5 zeolite, no electronic coupling with nitrogen atoms, so with DCB, is observed, indicating that there is no partial transfer to the acceptor and the electrons remain trapped in the zeolite.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04103. DRIFT spectra of the p-terphenyl and 1,4-dicyanobenzene (PTP-DCB system) coadsorbed in NaZSM-5 zeolite by comparison with free PTP and DCB; highlighting the conversion of radical cation to the charge transfer complex by diffuse reflectance UV−vis spectroscopy; and electronic spin decay after photoexcitation at 300 nm of PTP and DCB coadsorbed in NaZSM-5 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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