Picosecond Pulse Radiolysis of Propylene ... - ACS Publications

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Picosecond Pulse Radiolysis of Propylene Carbonate as a Solute in Water and as a Solvent Jean-Louis Marignier,*,† Fayçal Torche,† Sophie Le Caer̈ ,‡ Mehran Mostafavi,† and Jacqueline Belloni† †

Laboratoire de Chimie-Physique/ELYSE, UMR 8000 CNRS/UPS, Université Paris Sud, Université Paris-Saclay, Bât. 349, F-91405 Orsay Cedex, France ‡ LIONS, NIMBE, CEA, CNRS, Université Paris Saclay, CEA Saclay, F-91191 Gif-sur-Yvette Cedex, France S Supporting Information *

ABSTRACT: The ester propylene carbonate (PC) is a solvent with a high static dielectric constant where the charges generated by ionizing radiation are expected to be long-lived at room temperature. Time-resolved optical absorption spectroscopy after picosecond electron pulses reveals the formation of a UV band, within less than two nanoseconds, that is assigned to the radical anion PC−•, arising from a fast attachment reaction of electrons onto PC. Assignment and reactivity of PC−• in neat solvent and solutions are discussed in relation with data obtained in solutions of PC in water under reducing or oxidizing conditions and in solutions in PC of aromatic scavengers with various reduction potentials. The fate of the electrons and the ionization yield in PC are compared with those of other solvents.

The aim of this work9 is to study the cyclical ester propylene carbonate (PC), that is a solvent with a high dielectric constant (εs = 66).10 Studies on the gamma-irradiation of PC glasses at 77 K using the ESR and the optical absorption methods have identified radicals issued from the reduction or the oxidation of the matrix.11−16 One of the major conclusions is that the features of the ESR signal and of the UV absorption band observed in PC do not correspond to trapped electrons but that they should be assigned to the propylene carbonate anion PC−•,12,14 whereas another ESR signal corresponds to the oxidized radical PC(-H)•.14 The latter reference contains a detailed analysis of this literature. A comparison with the radiolysis of another cyclical carbonate, i.e. the fluoroethylene carbonate, studied by ESR and optical absorption spectroscopy, was recently published.17 In absence of any information on the reduced and oxidized species issued during the radiolysis of liquid PC at room temperature, the study of the pulse radiolysis of solutions of PC in water was first performed before the pulse radiolysis of PC as a solvent.

1. INTRODUCTION The fate of the charged species generated by the primary ionization effects of radiation on liquids strongly depends on the solvent dielectric properties and electron affinity. The mechanism of water radiolysis is the best known.1 Comparison with other solvents is the way to establish a general description.2 The radiolysis also enables the observation of electrons solvated in various liquids and the comparison of their optical and reactivity properties (alcohols, amines, ethers, alcanes).3,4 Recent radiolysis studies have been devoted to the group of ionic liquids.5 The alkyl carbonates, linear or cyclical, are solvents used in several industrial applications, such as organic synthesis, cosolvents in the lithium-ion batteries, .... They may be submitted to radiation environment (nuclear plants or space). Pulse and steady state radiolysis studies of the linear diethyl carbonate (DEC), neat6 or as solvent,7 were recently published. At the end of a picosecond pulse, the transient optical absorption spectrum in neat DEC is constituted of two bands in the near-infrared and in the visible, assigned to the solvated electron (decaying within 150 ns) and the oxidized radical of diethyl carbonate (decaying within 400 ns), respectively. From scavenging experiments, the solvated electron yield in DEC (of which the static dielectric constant is εs = 2.8) is G25 ps(eDEC−) = 1.4 × 10−7 mol J−1, and the initial ionization yield (measured in molar biphenyl solutions) is G0(ion) ≥ 3.25 × 10−7 mol J−1.6 Moreover, the processes of ionization and electron transfer that are induced by radiolysis have tight similarities with the longterm aging processes involved in Li-ion batteries.8 © XXXX American Chemical Society

2. EXPERIMENTAL METHODS All the reagents, propylene carbonate, ethanol, anthracene, biphenyl, naphtalene, and NaOH, were pure chemicals and were purchased from Sigma-Aldrich. They were used without further purification. Water was deionized using a Millipore Received: December 2, 2015 Revised: January 28, 2016

A

DOI: 10.1021/acs.jpcb.5b11793 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B device. Propylene carbonate is a transparent liquid in the range −48.8 to +242 °C at normal pressure. The molecular weight is 102.09 g mol−1, and the mass density at 20 °C is 1.205 g mL−1 (neat PC is 11.7 mol L−1). The dielectric constant is 66,10 and the dipolar moment is 1.10 D.18 The study was performed using the picosecond laser-induced electron accelerator ELYSE. The pulses were 4−6 nC, 15 ps, with an electron energy of 8 MeV, at a repetition frequency of 5 Hz.19,20 The transient absorbance was measured at the exit of the electron beam, and the detection line was equipped with a homemade repetitive flash-lamp21 and a streak camera.20,22 The probe beam is collinear with the electron beam. A short-lived absorbance centered at 590 nm due to the fused silica cell windows of the flow through cell was detected in the empty cell when irradiated,23 and was subtracted from the absorbance data of the cell containing the PC sample. The optical path was 1 cm. The wavelength range of the streak camera is 250−850 nm and the time scale from 20 ps to 1 ms. However, in the UV, the intense superimposed Cerenkov emission signal should be subtracted, and, due to this perturbation, the time resolution was 40 ps. The dose per pulse in water was derived from the absorbance of the hydrated electron at 660 nm with ε660 nm (eaq−) = 1.81 × 104 L mol−1 cm−1,24,25 and G3 ns (eaq−)= 3.45 × 10−7 mol J−1.26 The dose in water DW was currently 35−50 Gy (or J L−1) and stable within the same day at less than 2%. The absorbance values were an average of measurements over 1000 pulses. According to the higher electronic density of PC compared to that for water, the dose per pulse in PC, DPC in J L−1, is 20% higher than DW under the same irradiation conditions (DPC ≈ 42−65 J L−1). The absorbance data Aλ,t at the wavelength λ and time t correspond to the product Gt(M) × ελ(M) = Aλ,t/(D × l), where Gt(M) is the radiolytic yield in 10−7 mol J−1, ελ(M) is the molar absorption coefficient in L mol−1 cm−1 of the molecules M absorbing at λ, DPC is the dose in J L−1, and l = 1 cm is the optical path.

Figure 1. Absorbance decay of the hydrated electron at 660 nm in solutions of various PC concentrations. Inset: Dependence of the pseudo-first order decay rate constant on the concentration (Dw = 46 Gy/pulse).

Figure 2. Transient UV absorption spectra between 7 and 35 ns in aqueous solutions of PC 2 mol L−1 and tert-butanol 1 mol L−1 (Dw = 36 Gy/pulse).

3. RESULTS AND DISCUSSION Part 1. Aqueous Solutions of Propylene Carbonate. The knowledge of electron transfer processes of the hydrated electron and radical yields in the water radiolysis is helpful to assign the unknown species created in the presence of a solute such as PC. Reduction by Hydrated Electrons. In aqueous solutions containing 2-propanol at 0.2 mol L−1 (in order to scavenge OH• and H• radicals), and under argon atmosphere (to prevent reactions with oxygen), the decay rate of the hydrated electron absorbance observed at 660 nm increases with the PC concentration from 0 to 2 mol L−1 (Figure 1). No change is observed in the initial absorbance at the end-of-pulse, nor in the shape of the hydrated electron absorption band. Simultaneously, a new absorption band is formed in the UV at the same rate as the hydrated electron decay (Figure 2). We exclude that this band corresponds to an electron solvated by the mixture of water and PC, as found in certain binary mixtures, where the band maximum of such species is shifted at increasing fraction.6 Therefore, we conclude that the product of the reaction is the radical anion PC−•aq resulting from the electron attachment: e−aq + PC → PC−•aq

The radical anion was indeed observed by ESR in glassy PC.11 The process is also supported by molecular dynamics simulation in ethylene carbonate.27 According to previous observations by ESR at low temperature in the one-electron reduction of carbonates, it is possible that, after the electron attachment, the structure of the radical anion PC−• is subjected to a ring-opening due to the scission of a single C−O bond, concerted with a 1−2 shift.14 The second-order rate constant of the bimolecular reaction is obtained from the slope of the pseudo-first order rate constant plot versus the concentration (Figure 1, Inset) as k1 = 1.2 × 107 L mol−1 s−1. The value is lower than the one previously published (7.5 × 107 L mol−1 s−1).28 Acetone, that is the final product of OH• scavenging by 2-propanol and known itself as an efficient electron scavenger (k = 4.3 × 109 L mol−1 s−1),29 does not contribute to the decay, because it is diluted and its scavenging factor is negligible compared to PC (the same value of k1 is obtained in the presence of tert-butanol where no acetone is formed). Note that the rate constant k1 is low, and the reaction is very far from being diffusion controlled. Nevertheless, this suggests that the electron is easily attached to PC molecules. Considering that the fate of electrons is generally very similar whatever the solvent,4 it may be inferred that electrons may

(1) B


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The Journal of Physical Chemistry B also react by attachment in neat liquid PC and that electrons solvated in PC are not formed or very short-lived.30 During the reaction between hydrated electrons and PC, the new UV absorbance below 310 nm is increasing (Figure 2). However, in this region, several primary species of water radiolysis also absorb, including eaq−,H• atom, OH• radical and H2O2.31 In fact, the absorption spectra of H• atoms and H2O2 are negligible beyond 250 nm. Actually, in the presence of added tert-butanol in the solution without PC, H• atoms and • OH radicals are scavenged and the radical (CH3)3C•O, that is rapidly formed, also absorbs in the UV.32 Thus, the transient spectrum at 7 ns is the sum of two components, the UV component of the e−aq spectrum and the (CH3)3C•O spectrum. In the time range of 40 ns, an increase at 290 nm and a decrease at 320 nm are observed in the aqueous solution of PC (Figure 2 and Figure S1). Meanwhile, the absorbance remains constant at 310 nm as if this wavelength represented an isosbestic point implying that reaction 1 is stoichiometric and that both e−aq and PC−•aq have same molar absorption coefficients: ε310 nm (PC−•) = ε310 nm (e−aq). Note that since the contribution of the radical (CH3)3C•O is constant during the same time range, it does not change the isosbestic point behavior. However, because ε310 nm(e−aq) is not precisely known at this wavelength of 310 nm, i.e. below 340 nm where other radiolytic species also absorb and beyond the domain explored by Nielsen et al.,31 the absorption coefficient ε310 nm(e−aq) should be accurately evaluated in new experiments. First, the kinetics have been measured in water at nanosecond range without PC under Ar atmosphere, at Dw = 65 Gy and 310 nm where e−aq and OH• radicals both absorb (H• atoms and H2O2 do not absorb at this wavelength). Second, the kinetics is measured under exactly the same irradiation conditions, also in the absence of PC and under Ar atmosphere, but with HClO4 0.2 mol L−1. In that case, e−aq are scavenged at t1/2 = 150 ps and are replaced by H• radicals. Now OH• is the only absorbing radiolytic species. The absorbance difference between the signals is due to e−aq alone, and we derive G3 ns(e−aq) × ε310 nm(e−aq) = ΔA310 nm, 3 ns/Dw = 3.52 × 10−4 L J−1 cm−1. From G3 ns(e−aq) = 3.45 × 10−7 mol J−1, it comes ε310 nm(e−aq) = 1020 L mol−1 cm−1. Therefore, we conclude that, at the observed isosbestic wavelength of 310 nm, ε310 nm(PC−•) = ε310 nm(e−aq) = 1020 L mol−1 cm−1. In PC aqueous solution, the hydrated electron has completely reacted and is exhausted at time longer than 200 ns (Figure 1). However, the UV transient spectrum at that time still contains a negligible contribution of oxidized species arising from some partial OH• and H• scavenging by PC (see next section), and from a component assigned to the long-lived radical (CH3)3C•O. The shape and the intensity of the latter corresponds indeed to that calculated from the yields of OH• and H•.33 After subtraction from the spectrum at the end-of-pulse of the (CH3)3C•O component obtained at 250 ns, and after calibration using the molar absorption coefficient found above at 310 nm as ε310 nm (PC−•) = 1020 L mol−1 cm−1, the spectrum of (PC−•) is obtained and presented in Figure 3. It is observed that after its formation, PC−• is slowly decaying. In order to examine whether the decay may be assigned to a reaction of PC−• with protons, the kinetics were observed in the presence of the acid HClO4 at 2 × 10−4 mol L−1. The decay of PC−• is indeed accelerated. The rate constant

Figure 3. Calibrated absorption spectrum of PC−• in aqueous solution in molar absorption coefficient. Inset: Transient UV absorption spectrum at 200 ns in aqueous solution of PC 2 mol L−1 and tertbutanol 1 mol L−1 (Dw ≈ 36 Gy/pulse).

of the reaction is found to be kPC−• + H3O+ = 1.8 × 1010 L mol−1 s−1. Oxidation by OH• Radicals. The solution at pH 9.6 was added with N2O in order to scavenge hydrated electrons and to produce also supplementary OH• radicals. The propylene carbonate in aqueous solutions may also be oxidized by the OH• radicals, as shown in aqueous glasses,11 and in the gas phase.34 The product of the reaction is supposed to be PC(-H)•, the radical resulting from a H atom abstraction from PC, because PC(-H)• was already observed by ESR after the oxidation of liquid PC by OH• radicals generated by photolysis of H2O2.35 Two different structures of the radical were identified, after abstraction from the −CH− group or from the −CH2− group. The kinetics signal observed (Figure 4) contains the absorbance of the product of the reaction between OH• and PC formed within 10 ns, the absorbance of e−aq until 15 ns and also a weak absorbance of PC−• produced by one tenth of hydrated electrons not fully scavenged by N2O. The spectrum of the oxidized radical PC(−H)•, obtained after subtraction, from the transient spectrum observed at 35 ns, of the component due to PC−• as calculated from the 1/10 of the hydrated electron yield and the molar absorption coefficient determined above, is shown in Figure 5. From the absorbance in the absorption spectrum at 3 ns, the dose and the total yield of OH• radicals, generated in N2O solutions and producing PC(−H)• (G3 ns(OH•) = 6.5 × 10−7 mol J−1) (Figure 5, inset), we calibrated the PC(−H)• spectrum in molar absorption coefficient (Figure 5). Note that, though the anion radical PC−• (Figure 3) and the radical PC(-H)• absorb both in the UV, the molar absorption coefficient of the latter is 5 times lower: ε280 nm (PC(-H)•) = 300 L mol−1 cm−1 (Figures 3 and 5). The rate constant k3 of reaction 3 between OH• and PC was determined using the competition with the known reaction of OH• with carbonate ions which give the carbonate radical that absorbs at 600 nm (Figure 6): CO32 − + OH• → CO3−• + OH−

C

k 2 = 4.2 × 108 L mol−1 s−1(ref36) (pH = 11)

(2)

PC + OH• → PC( −H)• + H 2O

(3) DOI: 10.1021/acs.jpcb.5b11793 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

Figure 6. Kinetics observed at 600 nm of aqueous solutions of PC at various concentrations, Na2CO3 (0.2 M), and N2O at saturation, pH = 11. (Dw: 52 Gy/pulse). Inset: Variation of the reciprocal of G(CO3−•) versus the PC concentration.

The transient spectra present mainly a UV band (Figure 7), the spectrum of which is very similar to that of the anion radical

Figure 4. Top: Kinetics signals at 250 nm in aqueous solution saturated by N2O and containing PC 2 mol L−1, NaOH (pH 9.6) (Dw ≈ 54.2 Gy/pulse). Red: signal observed. Blue: component of e−aq reacting with N2O obtained from kinetics at 660 nm normalized according to ε250 nm.32 Bottom: Kinetics of (PC(-H)• alone. In blue, the exponential fit.

Figure 7. Transient spectra in neat propylene carbonate after the 20 ps electron pulse. DPC = 48 J L−1 Inset: kinetics observed at 310 nm. DPC = 66 J L−1. Dotted signal from ref 30 for comparison: kinetics observed at 1400 nm, then normalized to the same dose and optical path (right ordinate).

PC−• as identified in irradiated aqueous solutions of PC (Figure 3, Figure S2). This species is formed in neat PC within less than 1 ns, as shown in Figure 7, Inset. However, the absorbance increases in two successive steps: one is very fast and occurs within the pulse, whereas the second step is over within 1 ns. But the high uncertainty of this UV signal at short time resolution prevents separate quantification of the absorbance of each component. The most important feature of the transient spectra after 7 ns is the absence of any absorbance at wavelengths longer than 700 nm (Figure 7). No broad absorption band, typical of a solvated electron in PC, is observed in neat PC at that time, in contrast with the spectra in diethyl carbonate where a solvated electron was identified in the same time range.6 Moreover, no reaction was observed between the diethyl carbonate and electrons solvated in ethanol nor DEC. Because the dielectric constant of PC is significantly higher (εs = 66) than in DEC (εs

Figure 5. Calibrated absorption spectrum of PC(−H)• after subtraction of the PC−• component. Inset: Transient absorption spectrum at 35 ns in aqueous solution saturated by N2O and containing PC 2 mol L−1, NaOH (pH 9.6).

The CO3−• yield is derived from the CO3−• absorbance at 300 ns and from the molar absorption coefficient at 600 nm, εCO3‑• = 2 × 103 L mol−1 cm−1.37 The reciprocal of G(CO3−•) increases linearly with the PC concentration (Figure 6, Inset) and the k3 value is determined from the slope: k3 = 3.0 × 109 L mol−1 s−1. Part 2. Propylene Carbonate as Solvent. Neat Propylene Carbonate. In neat propylene carbonate, the molecule is now directly ionized and excited: PC vvv → PC*, PC+•, e− D


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The Journal of Physical Chemistry B PC+• + PC → PCH+ + PC( −H)•

= 2.8), the recombination of an electron possibly solvated in PC with the parent cation is to be excluded to explain the decay in such a short time range. Actually, recent observations30 of a broad band in neat PC in the picosecond time range and in the near-infrared concluded to the formation of a ultra short-lived solvated electron in PC. The position of the band maximum is shifted from 1360 nm at the end-of-pulse to 1310 nm at 50 ps, meaning that the solvation time is of a few tens of ps.30 Then, the very short lifetime of the fully solvated electron was assigned to a reaction of ePC− with the solvent, producing the radical anion PC−•. The high concentration of the solvent (11.7 mol L−1) explains that the reaction is fast in spite of a moderate rate constant (k5 ≈ 1.9 × 108 L mol−1 s−1 and t1/2 = 360 ps):30 ksolv

e‐ + PC → e pre ‐ solv − → e PC−

e−PC + PC → PC−•

k5

However, the absorbance in the region 400−800 nm is very low: it corresponds to a value of G × ε ≈ 5 × 10−5 L J−1 cm−1, which is significantly lower than the value observed in DEC (5 × 10−4 L J−1 cm−1). Note that, in contrast with aqueous solutions of PC where either reducing (in the presence of alcohols) or oxidizing conditions (in the presence of N2O) could be prepared, the radiolysis of neat PC leads simultaneously to reduced species (PC−• by reactions 5,6) and oxidized species (including PC+• and PC(-H)• by reaction 7 or PC(−H)• fragments). In fact, because PC(-H)• in water absorbs weakly in the UV (Figure 5), the visible absorbance in neat PC must be assigned to another species, or at least to another structure of PC(−H)• than in water. Moreover, if we assume, for PC−• and for PC(−H)•, similar absorption coefficients to that obtained in water, the transient UV absorbance in neat PC is mostly due to PC−•. Aromatic Solutions in Propylene Carbonate. The assignment of the UV absorbance to the negative radical PC−• is confirmed by its properties of electron donor toward aromatics, first anthracene. Figure 8, presents indeed the absorption spectrum at 700 ns with two maxima at 600 and 730 nm of the anthracene radical anion An−•,44,45 (top) and its formation kinetics (bottom) in a solution of anthracene (10−2 mol L−1) in PC. However, about 20% of the absorbance increase is very fast and is over, before 1 ns (Figure 8, bottom, inset). This fraction is assigned to the direct scavenging of presolvated as well as

(4) (5)

The reaction 5 is not diffusion controlled as well as the corresponding reaction in water of the hydrated electron with PC (k1 = 1.2 × 107 L mol−1 s−1). For comparison, the ePC− absorbance decays in the NIR in the same time range (1 ns)30 as the second step increase of the PC−• absorbance at 310 nm (Figure 7, Inset), accordingly to reaction 5. The fast initial step within the pulse in Figure 7, Inset should therefore be explained by a direct attachment of free or presolvated electrons onto PC prior full solvation (t < 50 ps):30 e pre ‐ solv − + PC → PC−•

k6

(7)

(6)

Because reaction 6 occurs during the pulse, we estimate that k6 ≥ 3 × 1010 L mol−1 s−1. The k6 value is higher than k5 because of the higher mobility of presolvated electrons compared to solvated electrons. Reactions of electron solvation (4) and of electron attachment (5) and (6) are in competition, as it was previously observed in dimethyl sulfide (DMS),4,38−42 and acetonitrile.43 It is worthy to note that for these molecules, DMS, PC and CH3CN, the reaction rates with hydrated electrons in water are slow and far from the diffusion-controlled limit, implying that most of the molecule-electron encountering pairs dissociate without reacting and release the electron. With similar low electron affinity of the molecules in neat solvent, part of the electrons become free for solvation.4 However, until now, this feature was scarcely observed. Hence, in the neat solvent, one part of the electrons attachs to the molecule and the other part also may solvate before further reaction (in DMS, parts are G3ns(eDMS−) = 0.6 × 10−7 mol J−1, and G3ns(CH3S)2−•) = 1.2 × 10−7 mol J−1, respectively). In contrast, no solvated electron was observed in fluoroethylene carbonate. 17 Generally, halo-compounds (chloroform, carbon tetrachloride, ...) display a dissociative electron attachment. They do not, as neat solvents, solvate the electrons, and, as solutes in water, they accept electrons from hydrated electrons with high rate constants.4 Instead, in solvents without electron affinity, solvated electrons are the only negatively charged species observed at early times, and as solutes in water, for example, these molecules are also unreactive toward hydrated electrons. In the wavelength range 350−620 nm and at the nanosecond range, the absorbance decay does not follow the same kinetics behavior as below 300 nm, indicating the presence of species other than PC−• (Figure S3), possibly the radicals PC+• or its daughter PC(−H)• resulting from an early ion−molecule reaction (homologue of DEC(−H)• formation in DEC):6

Figure 8. Top: Absorption spectrum of An−• at 700 ns. DPC = 65 J L−1. Bottom: Kinetics signal of the radical anion An−• at 730 nm, the maximum wavelength of its absorption spectrum, recorded in solution of anthracene (10−2 mol L−1) in PC within 400 ns. Inset: Within 40 ns. E


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The Journal of Physical Chemistry B solvated electrons reaction 8 and is in competition with the formation of PC−• by reaction 5. An + e pre ‐ solv −, ePC− → An−•

k8

(8)

From k5, the fraction value, and the concentrations of PC and An, we obtain k8 = 5.5 × 1010 L mol −1 s−1. Then, the anthracene accepts the electrons from PC−• at a slower rate via reaction 9, and the formation of An−• is completed within 200 ns: An + PC−• → An−• + PC

k9

(9)

By changing the concentration [An] between 0.5 × 10−2 mol L−1 and 2 × 10−2 mol L−1 (solubility limit in PC), the second order rate constant of reaction 9 has been determined as k9 = (1.9 ± 0.2) × 109 L mol −1 s−1. The absorbance of the product An−• increases with the concentration of An (Figure S4). Accounting for the molar absorption coefficient of An−•, ε725 nm = 5.0 × 104 L mol −1 cm−1,46 confirmed by a new evaluation in ethanol, the yield of negative charges scavenged would be G(An−•) = 3.6 × 10−7 mol J−1 at 100 ns (Figure 8, Top). This value is close to the yield of PC−• at 3 ns as calculated from the absorbance in neat PC (Figure 7, Inset) and from the molar absorption coefficient of PC−• assumed to be the same as in water, that is G3 ns(PC−•) = 4.0 × 10−7 mol J−1. According to reactions 5,6), which are over within less than 1 ns, the total yield of presolvated and solvated electrons, precursors of PC−•, should also have the same value. A previous study on anthracene solutions in PC had found a lower value of G(An−•),47 but the anthracene concentration was lower than in our experiments and the determination was based on an overestimated value of the molar absorption coefficient of An−•. It is worthy to note that, though the dielectric constant of PC is lower than that of water, the yield of the negative charges PC−• in PC at 3 ns, G3 ns(PC−•), is slightly higher than that of eaq− in water (G3ns(eaq−) = 3.45 × 10−7 mol J−1),26 apparently because the recombination of the negative charge PC−• (instead of eaq−) with the parent cation is slower, so stabilizing PC−•. By the way, G3 ns(PC−•) = 4.0 × 10−7 mol J−1 is the highest scavenging yield value observed of negative charges still living at that time in any solvent. A similar scavenging study has been performed in biphenyl solutions, at 0.1 mol L−1 in PC. Biphenyl absorbs strongly below 360 nm, so preventing the observation of any transient. But both maxima at 410 and 640 nm of the anion radical Ph2−• (Figure 9, Top) are observed at short time range. Then, the further decay over 100 μs is very slow. In order to investigate the possible competition between PC and Ph2 for the fast electron scavenging, the kinetics have been also observed with a lower Ph2 concentration, i.e. 10−2 mol L−1 (Figure 9, Bottom). The initial absorbance increase at 410 nm, that is specific of Ph2−•, is very fast, within the pulse. It is therefore assigned, as at 0.1 mol L−1, to the rapid scavenging of electrons by reaction 10 prior their solvation (lasting 50 ps),48 in competition with reaction 4 and 6. Ph 2 + e pre ‐ solv − → Ph 2−•

k10

Figure 9. Top: Transient spectra in a solution of 0.1 mol L−1 Ph2 in PC. Bottom: Kinetics signals within 40 ns in a solution of 0.01 mol L−1 Ph2 at 410 nm (red) and 360 nm (blue). DPC = 48 J L−1.

scavenging process of presolvated and solvated electrons was observed in THF.48 The absorbance increase is also fast at 360 nm where Ph2−• formed by (10) absorbs as well as PC−• formed by (6). But then, the absorbance at 410 nm due to Ph2−• decreases slowly while the total absorbance at 360 nm does increase (Figure 9, bottom). In fact, it means that at 360 nm, despite a decay of the Ph2−• component, the increase due to PC−• is larger. Actually, Ph2 behaves as an electron relay and an electron transfer would now occur from the aromatic anion Ph2−• to PC producing finally PC−• reaction 11), in opposite direction compared to the anthracene solution (reaction 8): Ph 2−• + PC → Ph 2 + PC−•

k11

(11)

Considering that the concentration of the solvent PC is 11.7 mol L−1, the rate constant of reaction 11 is about k11 ≈ 1.5 × 104 L mol −1 s−1 (Figure S5), meaning that this reaction is very slow. In millimolar solution of naphtalene (the solubility limit), the results are the same as in neat PC and no formation of the anion radical Naph−• may be observed. Diluted naphtalene (a mild scavenger of hydrated electrons)49 is not able to compete enough with PC for the electron scavenging via reactions 5 and 6. Also no electron transfer from PC−• to naphtalene is observed, meaning that the reduction potentials order is not favorable. According to the results on the effect of the addition of various aromatics to PC, we may conclude that the standard reduction potential E°PC(PC/PC−•) is lower than E°PC(An/

(10)

If t1/2 ≤ 20 ps for reaction 10 of Ph2 with presolvated electrons, we estimate that k10 ≥ 3 × 1012 L mol −1 s−1. Biphenyl is known to scavenge efficiently presolvated electrons as already observed in DEC where the reaction was also over within the pulse at the same concentration of Ph2.6 A two-steps F


The Journal of Physical Chemistry B

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An−•), but higher than E°PC(Naph/Naph −•) and E°PC(Ph2/ Ph2−•), all being higher than that of the solvated electron, as summarized in the scheme of Figure 10. Because the k11 value is

Article

AUTHOR INFORMATION

Corresponding Author

*(J.L.M.) Telephone: 33(0)169157569. E-mail Jean-Louis. [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors greatly acknowledge J.-P. Larbre, P. Jeunesse, F. Gobert, A. Demarque and M. Lourseau, for their technical assistance, and the Réseau Thématique de Recherches Avancées “Triangle de la Physique” for the financial support.

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(1) Buxton, G. V. An Overview of the Radiation Chemistry of Liquids, in Radiation Chemistry. From Basics to Applications in Material and Life Sciences; Spotheim-Maurizot, M., Mostafavi, M., Douki, T., Belloni, J., Eds.; EDP Sciences: 2008; pp 3−16. (2) Kieffer, F.; Billiau, F.; Cordier, P.; Delaire, J. A.; Delcourt, M. O.; Magat, M.; Klein, J.; Belloni, J.; Lapersonne-Meyer, C. Primary Trapping and Solvated Electrons Yields. Part 2: Correlation Between G Value and Neutralization Efficiency. Faraday Discuss. Chem. Soc. 1977, 63, 55−66. (3) Dorfman, L.; Jou, F. Y. Optical Absorption Spectrum of the Solvated Electron in Ethers and in Binary Liquid Systems. In Electrons in fluids; Jortner, J., Kestner, N. R., Eds.; Springer: Berlin, 1973; pp 447−459. (4) Marignier, J.-L.; Belloni, J. Electron-solvent Interaction. Attachment-Solvation Competition. Int. J. Radiat. Phys. Chem. 1989, 34, 157−171. (5) Wishart, J. F.; Funston, A. M.; Szreder, T.; Cook, A. R.; Gohdo, M. Electron Solvation Dynamics and Reactivity in Ionic Liquids Observed by Picosecond Radiolysis Techniques. Faraday Discuss. 2012, 154, 353−363. (6) Torche, F.; El Omar, A. K.; Babilotte, P.; Sorgues, S.; Schmidhammer, U.; Marignier, J. L.; Mostafavi, M.; Belloni, J. Picosecond Pulse Radiolysis of the Liquid Diethyl Carbonate. J. Phys. Chem. A 2013, 117, 10801−10810. (7) Ortiz, D.; Jiménez Gordon, I.; Baltaze, J. P.; Hernandez-Alba, O.; Legand, S.; Dauvois, V.; Si Larbi, G.; Schmidhammer, U.; Marignier, J. L.; Martin, J. F.; et al. Electrolytes Ageing in Lithium-ion Batteries: A Mechanistic Study from Picosecond to Long Timescales. ChemSusChem 2015, 8, 3605−3616. (8) Ortiz, D.; Steinmetz, V.; Durand, D.; Legand, S.; Dauvois, V.; Maître, P.; Le Caër, S. Radiolysis As a Solution for Accelerated Ageing Studies of Electrolytes in Lithium-Ion Batteries. Nat. Commun. 2015, 6, 6950. (9) Torche, F. Contribution à l’Etude des Electrons Solvatés dans l’ Eau et les Alcools et des Processus Radiolytiques dans les Carbonates organiques par Radiolyse Impulsionnelle Picoseconde. Ph.D. Thesis, Université Paris-Sud: 2012. (10) Chernyak, Y. Dielectric Constant, Dipole Moment, and Solubility Parameters of Some Cyclic Acid Esters. J. Chem. Eng. Data 2006, 51, 416−418. (11) Shaede, E. A. Trapped Electrons and Radicals in GammaIrradiated Propylene Carbonate Glasses. Can. J. Chem. 1972, 50, 782− 791. (12) Shaede, E. A.; Symons, M. C. R. Further Studies of GammaIrradiated Propylene Carbonate Glass. Can. J. Chem. 1973, 51, 2492− 2496.

Figure 10. Scheme of the reduction potential E°PC(PC/PC−•) compared to the potentials of aromatics anion radicals and of solvated electron in PC. On the left, are given the corresponding reduction potentials determined: (a) in water;52,53 (b) in DMF.50,51

very low, the E°PC(PC/PC−•) value is only slightly higher than E°PC(Ph2/Ph2−•). For the sake of comparison, the values of the corresponding potentials of aromatics in other solvents are in the same order: E° (An/An−•) = −1.7 VNHE50 > E°(Naph/ Naph−•) = −2.3 VNHE50 > E°(Ph2/Ph2−•) = −2.45 VNHE51 (Figure 10). Actually, the E°PC(PC/PC−•) value is very negative and PC−• may behave as an efficient electron donor. Hence, it is not so surprising that the electron transfer from a solvated electron to PC would not be diffusion controlled.

4. CONCLUSION The radiolysis of cyclical ester propylene carbonate presents an original feature, which has only previously been observed in dimethyl sulfide or acetonitrile, i.e. the competition between the electron solvation and the electron attachment onto the molecule. In both cases the reaction with the hydrated electron is far from being diffusion controlled. The electron donor character of the radical anion PC−•, stronger than An−•, may explain the feature. In spite of a lower static dielectric constant, the nanosecond yield of negative charges is even higher in PC than in water, mostly because the recombination reactions of the radical anion PC−•, once formed, are slower than those of the hydrated electron in water.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b11793. Figures S1−S5 of absorbance decays, absorption spectrum, rate constant variation, formation and decay kinetics, and time evolution of the absorbance (PDF). G


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