Electron Transfer Reactions and Mobile Holes in Radiolysis of

mol-1 dm3 s-1. The ion-molecule reactions of radical cations are somewhat slower than reactions of radical anions. The scavenging of the alkane hole i...
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J. Phys. Chem. 1996, 100, 14681-14687

14681

Electron Transfer Reactions and Mobile Holes in Radiolysis of Squalane. Time-Resolved FDMR Study† I. A. Shkrob and A. D. Trifunac* Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: NoVember 2, 1995; In Final Form: April 11, 1996X

Fluorescence detected magnetic resonance (FDMR) is applied to study ion-molecule reactions in squalane (2,6,10,15,19,23-hexamethyltetracosane). Several time-resolved pulsed FDMR experiments are suggested to measure rate constants of scavenging by aromatic solutes. These constants vary from 5 × 108 to 1.6 × 109 mol-1 dm3 s-1. The ion-molecule reactions of radical cations are somewhat slower than reactions of radical anions. The scavenging of the alkane hole is significantly faster than ion-molecule reactions of molecular cations. The mobile hole decays on a time scale of several tens of nanoseconds. These FDMR results are in accord with our recent work on transient absorption spectroscopy in radiolysis of squalane.

Introduction Despite rapid progress in the field, the radiation chemistry of alkanes is still poorly understood.1,2 One of the obstacles is lack of identification of short-lived reaction intermediates. While optical detection is difficult due to weak and featureless absorption, other detection techniques are insufficiently fast. The traditional approach is to use probe molecules. This method is rewarding in the studies of electron transfer (ET) reactions. In the presence of aromatic scintillator molecules (A), the primary charges, the solvent hole (RH•+) and the thermalized electron (et-), are scavenged, and the recombination of the solute ions yields the delayed fluorescence from 1A*:2,3

et- + A f A•-

(1)

RH•+ + A f RH + A•+

(2)

RH•+ + A•- f RH + 1,3A*

(3)

A•+ + A•- f A + 1,3A*

(4)

et- + A•+ f 1,3A*

(5)

A* f A + hν

(6)

1

This fluorescence and the absorption of aromatic solute ions A•( are easy to detect, which explains the popularity of the scavenging techniques. Unfortunately, the solute molecules are involved in reactions other than reactions 1-6. At low solute concentration the scavenging is slow, while at high concentration the energy transfer from the singlet alkane can be a significant source of 1A*.4 Absorption spectra of aromatic monomer and dimer cations, anions, proton adducts, radicals, and triplets frequently overlap.1 Multiple pairs and cross-recombination between the ions in different spurs make the kinetics less tractable. All these factors introduce ambiguity in the interpretation of the data. To simplify the kinetics, many researchers added a large quantity of electron (or hole) scavengers. For example, haloalkanes and † Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE, under contract number W-31-109-ENG-38. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, August 15, 1996.

S0022-3654(95)03247-3 CCC: $12.00

electrophylic gases (e.g., CO2) were added to prevent the formation of A•-. Recently, it has been realized that many commonly used electron scavengers exhibit complex chemistry of their own: they quench the excited states of alkanes,5 their anions can transfer electrons to the solute,6 and they can dissociate or lose charge. As a result, instead of the simplification of the reaction kinetics, one achieves its further complication. There is a need for alternative ways of studying ET reactions occurring in radiolysis. Here we address this problem using pulsed time-resolved fluorescence detected magnetic resonance (FDMR). With this technique, the resonance microwave (µw) field is applied to modulate the recombination yield of spin-correlated radical ion pairs. The decrease in the fluorescence yield is studied as a function of the external magnetic field and the delay time of the µw pulses.3,7 FDMR has four advantages as compared to the detection of the delayed fluorescence. First, FDMR is selective to radical ions and no other species. Energy transfer reactions may yield fluorescence, but they do not result in FDMR. Second, only geminate pairs yield FDMR. Random encounters of radical ions (including those occurring in spurs) do not contribute to the FDMR signal. Third, the involved radical ions can be characterized through their EPR signatures. Fourth, rapid phase relaxation in some radical ions (such as CO2•-) precludes their observation by FDMR.7 Within 20-30 ns after the formation of CO2•-, the spin alignment between the radical ion partners required for the formation of FDMR is lost. Thus, FDMR is “blind” to nongeminate reactions, energy transfer, and the involvement of certain radical ions. This results in considerable simplification of the observed kinetics. However, there is a drawback; FDMR is a slow technique. The detection is delayed by the 10-50 ns that is required for the spin mixing and the µw excitation of the geminate pairs.3,7 In nonviscous alkanes (η ≈ 1 cP), many fast ET reactions are complete by this time. In this work, we study FDMR in the viscous hydrocarbon squalane (η ≈ 20 cP 1) for which the diffusion-controlled reactions occur on the FDMR time scale. Several time-resolved experiments were used to examine electron and proton transfer reactions by means of pulsed FDMR. We find that the rate constants of the ET reactions fall in the range (0.5-1.6) × 109 mol-1 dm3 s-1. These rates are insufficient to explain the fast generation of the solute radical cations as observed by FDMR and transient absorption spec© 1996 American Chemical Society

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Figure 1. (a) FDMR spectra observed from a 5 × 10-4 mol dm-3 solution of perylene in N2-saturated squalane (50 ns µw pulse, 200 ns boxcar gate). The emission at λ > 460 nm was collected. The spectra were normalized at the center; the delay time t is indicated next to the traces. The broad signal is from a long-lived olefin ion of squalane, SQ•+. (b) Pulse-sweeping FDMR experiment with the same solution (50 ns µw pulse, 50 ns boxcar gate). The kinetics were collected at zero and 3.5 mT offset field; for comparison, the latter trace is normalized. The broken curve is the convolution of the zero-offset curve with exp(-kt), k ) 2.6 × 105 s-1.

troscopy.1 From the latter, the scavenging of the holes was found to proceed with rate constants ∼(5-7) × 109 mol-1 dm3 s-1. The present work supports the identification of the mobile cation species as the squalane radical cation. Experimental Section The scintillators 2,5-diphenylcarbazole (PPO), biphenyl-d10 (d10Bh), anthracene-d10 (d10An), and octafluoronaphthalene (F8Np) were used as received; naphthalene-h8 (h8Np), perylene (Pe), and rubrene were twice sublimed in Vacuo; N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) was recrystallized from ethanol and sublimed three times (all from Aldrich); perdeuterated naphthalene (d8Np), p-terphenyl, and nitrobenzene (d5NB) were supplied by Cambridge Isotope Labs; triethylamine (TEA, 99+%), tetramethylethylene (TME), and benzonitrile (BN) were used as received from Aldrich. n-Hexane and n-hexadecane were passed through 1.5 m silica gel columns. The quality of purification was tested by measuring absorption at 200-220 nm. The methods for squalane purification are given in ref 1. Radiolysis was performed with 12 or 25 ns electron beam pulses from a 3 MeV van de Graaff accelerator operated at 5-10 nC per pulse. The solutions were purged with pure nitrogen or carbon dioxide prior to and during the experiment. The details of our FDMR spectrometer may be found in ref 7. Three types of time-resolved FDMR experiments were performed. In the first, the delay time td of a 30-80 ns µw pulse was stepped in increments of 10-50 ns relative to the radiolytic pulse. The FDMR signal (the difference in the fluorescence with and without the resonant µw field) was sampled within 50-100 ns after the µw pulse using a boxcar averager (see diagram in Figure 1b; we will refer to this experiment as the pulse-sweeping FDMR). In the second, the delay time td was fixed and the FDMR kinetics (the difference between the fluorescence traces with and without the µw field) was sampled using a digital oscilloscope (10 ns/point). In the third experiment, the magnetic field was swept for a fixed delay time td of the µw pulse and the FDMR spectrum was sampled with a boxcar averager (a typical FDMR spectrum is shown in Figure 1a). If not indicated otherwise, the µw field was 0.28 mT. All experiments were performed at 23 °C.

Results and Discussion Squalane Ions. The radical cation partner of A•- (dubbed SQ•+) as it appears in our FDMR spectra resembles a Gaussian line with the second moment σ of the spectrum ∼2.52 mT (Figure 1a). This ion must be very stable since in (1-10) × 10-6 mol dm-3 perylene solutions the broad signal from SQ•+ persists for 15 µs after the electron beam pulse. For inhomogeneously broadened lines

σ2 ) (1/3) ∑kAkIk(Ik + 1)

(7)

where Ak is the hyperfine coupling constant (hfcc) and Ik is the spin of the kth nucleus.8 For aromatic ions, the spectrum width σ can be calculated from the reported EPR data.9 For example, for d10An•+ and d10An•- the second moments are 0.19 and 0.16 mT, respectively, while for Pe•+ σ ) 0.58 mT. Thus, for deuterated solutes σ(A•() , σ(SQ•+), and the [SQ•+ A•-] and [A•+ A•-] pairs can be readily distinguished spectrally. We found that SQ•+ reacts with aromatic solutes with ionization potentials (IP) less than 7.7 ( 0.1 eV (all IP used are the gas-phase values):

SQ•+ + A f SQ + A•+

(8)

Following this reaction, the fraction of the [SQ•+ A•-] pairs decreases, while the fraction of the [A•+ A•-] pairs increases. Consequently, the broad signal from SQ•+ decays faster than the FDMR signal from A•+ and A•- (the narrow line at the spectrum center, Figure 1a). When the FDMR kinetics is probed with a short µw pulse (10-50 ns), the decay of the signal at the spectrum center follows Λ(t), the geminate decay of radical ion pairs.7,10 The signal from SQ•+ obtained with a µw pulse applied at t ) td decays as Λ(t) exp(-k8[A]td), where k8 is the rate constant of reaction 8. Comparing the pulse-sweep FDMR kinetics sampled at the spectrum center and 3.5 mT off-center (where the signal is from SQ•+), one can find the rate constant of reaction 8 by deconvolution of the off-center trace (Figure 1b). For anthracene and perylene we obtained 5 × 108 mol-1 dm3 s-1. When 0.1 mol dm-3 TME was added to the solution, the broad signal changed. It is known that at this high concentration of TME the radical cations of TME aggregate, giving a broad

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J. Phys. Chem., Vol. 100, No. 35, 1996 14683

featureless FDMR signal.11 In perylene-TME solutions, this broad signal decayed with the rate constant 1.2 × 109 mol-1 dm3 s-1. If the delay time of the µw pulse is fixed rather than swept, the normalized FDMR traces obtained at the spectrum center and off-center are identical. Though SQ•+ ions are fully scavenged by aromatic solute, the signal from SQ•+ was still observed. FDMR spectra reflect the relatiVe abundances of the pairs that exist at the moment of application of a short µw pulse. Our result also implies that the diffusion coefficients of SQ•+ and A•+ are close; otherwise the trace obtained at the center would decay faster than that at an off-center position. Let us discuss the identity of SQ•+. The broad FDMR signal observed in our experiments resembles that observed by Okazaki et al., who used a continuous-wave FDMR.12 It was speculated that the broad signal is from the squalane hole involved in rapid resonant charge transfer,

RH•+ + RH f RH + RH•+

(9)

with k9 = 107 mol-1 dm3 s-1.12 Charge hopping (9) mixes nuclear subensembles. As the rate of the hopping increases, it first broadens and then narrows the FDMR signals. Tadjikov et al. suggested that even at 77 K the charge hopping is so fast that it narrows the FDMR spectra.13 This would explain the observed 20% increase in the spectrum width on dilution of squalane with 3-methylheptane.13 The dilution reduces the rate of charge hopping and broadens the FDMR signal. It was estimated that k9 ≈ 8 × 107 mol-1 dm3 s-1 at 77 K and 3 × 108 mol-1 dm3 s-1 at 290 K.13,14 However, the broadening observed on dilution (ca. 0.2 mT) could well be due to matrix effects. In the spectra reported by Okazaki et al., the FDMR signal at 290 K was 2 times broader than the signal obtained at 45-150 K.12 If, following Tadjikov et al., one tries to explain this broadening through a faster reaction (9), it is necessary to assume that even at 290 K the hopping is still in the line-broadening regime. The only way to account for this anomalous result is to suggest that the two groups observed two different radical cation species. From the study on the damping of the ∆g beats, Veselov et al. suggested that at 290 K the FDMR signal of the solvent hole in squalane was ∼0.6 mT wide.14 The signal from SQ•+ is 4 times wider. The spectrum width σ of the squalane hole observed in lowtemperature solid is 1.2 mT.13,15 Using k9 from ref 14, we calculated that the time T2 of the electron exchange relaxation due to reaction 9 would be 20-50 ns (T2-1 ) σ2τres is the residence time of the hole at room temperature, 1.7 ns). This relaxation rate is insufficient for the narrowing of the FDMR signal within the duration of short µw pulses in our timeresolved experiments. At best, such a slow hopping would introduce a line-broadening.16 Experimentally, the FDMR signal neither narrows nor widens when the length of a weak µw pulse (0.1 mT) increases from 20 ns to 2 µs. There is no indication that SQ•+ undergoes rapid resonant charge transfer (reaction 9). It seems very likely that SQ•+ is a molecular ion, probably an olefin radical cation formed on fragmentation of the parent hole RH•+.3 Addition of 0.1-0.5 mol dm-3 cis- and transdecalins, toluene, and norbornane does not result in the disappearance of the broad signal from SQ•+, although decalins are known to scavenge the squalane hole at 77 K.15 Naphthalene and biphenyl (IP ) 8 and 7.9 eV, respectively17) also do not scavenge the signal from SQ•+. Only solutes with IP < 7.8 eV

scavenge this ion. This ionization energy is too low for an alkane radical cation. Radiolysis of 0.01 mol dm-3 squalene in n-hexane (with 10-4 mol dm-3 biphenyl-d10) yields a FDMR spectrum very similar to that observed in neat squalane. This spectrum exhibits the wings of the same width as SQ•+ and a narrower signal (with fwhm ≈ 1.5 mT) superimposed on the signal from d10Bh•- (σ ≈ 0.12 mT). The scavenging of squalene•+ follows the same dependence on the solute IP as that of SQ•+. In naphthalened8 solutions, the signal from squalene•+ grows with the delay time of the µw pulse at any concentration of naphthalene-d8. Apparently, squalene scavenges naphthalenes•+. The IP of most olefins, dienes, and trienes is in the range from 8.1 to 9 eV.17 The lowest IP known for a diene (7.5 eV) was found for 2,5dimethyl-2,4-hexadiene.17 This olefin is similar to squalene in structure. Double bonds in squalene are located near the branching points and are not conjugated. The spin density in squalene•+ must be localized at the single double bond. The same applies to any olefin ion formed on fragmentation of squalane, so the similarity between the FDMR spectra for squalene•+ and SQ•+ is expected. An estimate of σ for various olefin ions of squalane using the EPR parameters reported for smaller ions of similar structure yields 2.2-2.7 mT.9 This is close to the observed spectrum width of 2.52 mT. From our transient absorption study1 we deduced that radiolysis of squalane results in the formation of two radical cation species, a mobile ion-I and a normally diffusing ion-II. The latter undergoes a transformation on the microsecond time scale and yields a stable ion-III, whose lifetime exceeds 50 µs. Ion-I and ion-II are formed in the very early time frame. Ion-I fully decays in 40 ns; our estimate of its lifetime in a free state is ∼25 ns. This time is shorter than the time of spin mixing and µw-induced spin-flip required for the development of FDMR. No ion with a lifetime that short can be observed with FDMR, continuous-wave and time-resolved alike. The scavenging of ion-I occurs with rate constants (5-7) × 109 mol-1 dm3 s-1, while typical ET reactions of solute cations are 4-5 times slower. Scavenging of normally diffusing ion-II and ionIII by aromatic solutes follows the pattern of SQ•+. We believe that these ions and SQ•+ are the same species. It does not seem likely that SQ•+ is an impurity ion. In purified squalane, the half-life of et- is 0.43 µs; aromatic solutes scavenge the electron with a rate constant of (3-5) × 1011 mol-1 dm3 s-1. Hence, the conjugated double bond impurity can account for no more than 10-5 mol dm-3. The most abundant impurity in squalane must be the products of partial hydrogenation of squalane. From the extinction coefficient of squalane at 220 nm, we estimated that the purified solvent contained no more than 7 × 10-4 mol dm-3 of the olefin impurity. Thus, the impurity can account for no more than 10% of the secondary ions. We turn now to the FDMR studies of the ion-molecule reactions of aromatic ions. In these experiments we studied electron transfer from solute ions to solute molecules. For radical anions, the kinetics measurements were complicated by an unexpected effect: the second scavenger affected the initial distribution of the solute ions. Effect of CO2 on the FDMR Kinetics. In the presence of electron scavenger CO2 the reaction scheme (1)-(6) must be extended:

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Shkrob and Trifunac

et- + CO2 f CO2•-

(10)

RH•+ + CO2•- f R• + •HCO2

(11)

A•+ + CO2•- f 1,3A* + CO2

(12)

CO2•- + A f CO2 + A•-

(13)

Reaction 13 occurs with pyrene and perylene; with anthracene and PPO it is not energetically favorable. The mobility of CO2•is ∼10 times higher than that of aromatic ions. In the presence of 0.037 mol dm-3 CO2 (saturated solution) most of the geminate pairs generated in radiolysis are [SQ•+ CO2•-]. These pairs do not contribute to the scintillator emission. The [A•+ CO2•-] pairs yield strong emission from 1A* on recombination. However, the FDMR spectra sampled in N2- and CO2-saturated solutions are identical; nothing indicates the involvement of the [A•+ CO2•-] pairs in the formation of FDMR. The EPR spectrum of CO2•- in water is indicative of the fast relaxation with T2 ≈ 10-20 ns.18 With this short T2 time the [A•+ CO2•-] pairs (and all subsequent pairs) lose their spin correlation by the time of FDMR acquisition. Therefore, the FDMR obserVed in CO2 solutions is from the [SQ•+ A•-] and [A•+ A•-] pairs. Since both the aromatic solute and CO2 compete for et-, the yield of [SQ•+ A•-] pairs decreases on addition of CO2. Taking into account the difference in the concentration of A and CO2, one could expect that virtually no FDMR signal would be observed in the saturated CO2 solutions. We compared the kinetics of fluorescence and FDMR in N2- and CO2-saturated solutions of biphenyl, PPO, and An in n-hexane, cyclohexane, n-hexadecane, and squalane. The results may be summarized in the following way: (i) The addition of CO2 increases the yield of the delayed fluorescence. (ii) In CO2-saturated solutions the decay of the FDMR signal is significantly faster than in N2-saturated solutions (Figure 2). (iii) The FDMR effect always decreases with addition of CO2 to scintillator solutions. For a given [A], the reduction factor (γ) is the same for all scintillators (cf. Figure 2a,b); it increases with the delay time of the µw pulse (Figure 2b) and correlates with the bulk viscosity of the solution. In CO2-saturated squalane ([d10An] ) 5 × 10-4 mol dm-3) the reduction in the FDMR signal sampled with the µw pulse applied at 50 ns was γ ≈ 3.7 times (compare with γ ) 12 for td ) 1.5 µs), in n-hexadecane γ ) 11 times, and in n-hexane we did not find any FDMR signal. This reduction is concentration-dependent: for example, for [d10An] ) 5 × 10-4 mol dm-3 (n-hexadecane) γ ) 11, while for [d10An] ) 5 × 10-3 mol dm-3 solution γ ) 2.2. In squalane, the FDMR signal in CO2-saturated solution can be observed even at [A] ) 10-5 mol dm-3 (γ ≈ 35). To account for the observed behavior, we used a Monte Carlo simulation program4,19 using the set of parameters from ref 1. Figure 3a shows the time dependence of the emission from the [SQ•+ A•-] pairs, which are the main contributors to the FDMR. The simulation reproduces the values γ and its time dependence quite well. It is frequently assumed that the initial distribution Π(r) of radical ions follows the distribution p(r) of thermalized electrons. For dilute solutions of electron scavengers this assumption is incorrect, since the radial distribution of the electrons widens with time.20 To illustrate this point, we calculated Π(r) for 5 × 10-4 mol dm-3 anthracene solution in squalane with and

Figure 2. Time dependences of FDMR for 5 × 10-14 mol dm-3 anthracene-d10 (a) and perylene (b) in N2- and CO2-saturated squalane ([CO2] ) 0.036 mol dm-3). The emission at 300-400 nm (a) and 460-600 nm (b) was collected; the traces were obtained on the top of the resonance lines from the aromatic radical ions. A 12 ns electron pulse was applied at t ) 0; a 30 ns µw pulse (the oscilloscope trace of the diode signal is shown next to the traces) was applied (a) at td ) 50 ns and (b) td ) 50 and 450 ns. For the latter delay time the traces were multiplied by 3. The dotted curves are the CO2 traces normalized to the N2 traces at t ) td. As seen from these traces, on the saturation of squalane with CO2 the decay kinetics becomes faster and the FDMR signal decreases several times.

without 0.037 mol dm-3 CO2 using the Monte Carlo program (Figure 3b). In low-viscosity solutions, the broadening of Π(r) relative to p(r) has a minor effect on the recombination: by the time of the FDMR observation the distribution is broadened by diffusion of the ions. In viscous solutions, the broader distribution causes a lesser number of pairs to recombine by the observation time. Following the WAS approach,21 the reduction factor γ can be estimated as fe(c) × [A]/c, where c ) [A] + [CO2] and fe(c) is the fraction of scavenged electrons fe(c) ) [Rc]0.6/(1 + [Rc]0.6). At [CO2] . [A] the factor γ ∝ c-0.6, as was observed experimentally (see below). Ion-Molecule Reactions of Aromatic Solutes. The rate constants of ET reactions measured with our time-resolved FDMR method are compiled in Tables 1 and 2. Here we consider several experimental schemes that have been suggested to determine these rate constants. The gas-phase electron affinity (EA) of benzonitrile is ∼0.1 eV higher than EA of biphenyl and ∼0.25 eV lower than EA of perylene. Thus, benzonitrile accepts an electron from Bh•-,

Bh•- + BN f Bh + BN•-

(14)

while BN•- transfers an electron to perylene:

BN•- + Pe f BN + Pe•-

(15)

Since the IP of benzonitrile, nitrobenzene, and CCl4 are more than 10 eV,17 these solutes do not scavenge any of the involved

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J. Phys. Chem., Vol. 100, No. 35, 1996 14685

γ Figure 3. (a) Monte Carlo simulation of the emission yield from recombination of geminate pairs of SQ•+ and A•- for [A] ) 5 × 10-4 mol dm-3. The mobility of et- was 0.016 cm2/(V s), p(r) ∝ r2 exp(-r/bE), bE ) 2.06 nm; the rate constant of scavenging et- by CO2 and aromatic solute was taken as 3 × 1011 mol-1 dm3 s-1; the fluorescence time of A was 1.5 ns; lifetime of the primary squalane ion is 25 ns; and the rate constants of scavenging the primary and secondary ions of squalane by A were taken as 6.5 × 109 and 1.5 × 109 mol-1 dm3 s-1, respectively. The trace for a CO2-saturated squalane (0.037 mol dm-3) was normalized at t ) 50 ns. The ascending line is the time dependence of the ratio γ of the emission yields from the N2and CO2-saturated solutions. (b) The distribution Π(r) of radical anions at the moment of electron scavenging. The traces were simulated for the CO2- and N2-saturated solutions of 5 × 10-4 mol dm-3 aromatic scintillator. In the CO2 solution, the distribution of the anions (both CO2•- and A•-) is considerably narrower.

TABLE 1: Rate Constants of Electron Transfer Reactions of Radical Anions in Squalane at 23 °C (×109 mol-1 dm3 s-1) donor•biphenyl perylene benzonitrile a

EA, eVa 0.15 ∼0.7 0.25

acceptor

EA, eV

rate constant

benzonitrile perylene CCl4 nitrobenzene-d5 perylene

0.25 ∼0.7 2.0 2.0 ∼0.7

1.6 1.8 1.6 1.5 1.6

TABLE 2: Rate Constants of Electron Transfer and Proton Transfer for Radical Cations in Squalane at 23 °C (×109 mol-1 dm3 s-1) IP, eVa

SQ•+ ∼7.8 tetramethylethylene 8.3 perylene 6.9 biphenyl 7.95 triethylamine 7.5

donor perylene perylene TMPD triethylamine triethylamine

rate IP, eV constant typeb 6.9 6.9 6.2 7.5 7.5

0.5 1.4 1.5 0.6 0.26

addition of 0.01-0.1 mol dm-3 benzonitrile, the FDMR spectra from perylene solutions undergo several changes. First, the fwhm of the FDMR spectra at td ) 50 ns decreases from 1.49 to 1.2 mT. This change indicates that most electrons were scavenged by benzonitrile, whose radical anion has a narrower line than Pe•-. For longer delay times td of the µw pulse, the line at the center of the spectrum broadens and finally becomes as broad as that of Pe•- (reaction 15). The rate of this broadening suggests that k15 ≈ 1.8 × 109 mol-1 dm3 s-1. Second, on addition of benzonitrile the decay of FDMR becomes faster. When [BN] > 0.01 mol dm-3, further addition of the scavenger does not change the decay kinetics while decreasing the FDMR signal as γ ≈ [BN]-0.6 (see above). Apparently, most of the thermalized electrons are rapidly scavenged (i.e., Π(r) ≈ p(r)) and the decay kinetics do not change upon further increase in the scavenger concentration. Addition of TMPD to the perylene solution causes a notable acceleration of the FDMR decay. TMPD scavenges A•+ and SQ•+,

TMPD + A•+ f TMPD•+ + A

Gas-phase EA is taken from ref 27.

acceptor•+

Figure 4. (a) FDMR decay traces obtained in N2-saturated 10-3 mol dm-3 squalane solution of biphenyl-d10 with (from the top to the bottom) 1.5, 3, 6, 12, and 18.5 × 10-3 mol dm-3 triethylamine. A 25 ns electron beam pulse was applied at t ) 0, the µw pulse was applied at td ) 350 ns, and the emission at 300-400 nm was collected. The FDMR kinetics were obtained on the top of the resonance line from the radical cations of biphenyl-d10. (b) The concentration dependence of the first-order rate constant k of the FDMR decay. The constant k was found by deconvolution of the FDMR traces with exp(-kt); the [TEA] ) 1.5 × 10-3 mol dm-3 trace was used as the standard.

ET ET ET ET PT

a Gas-phase IP is taken from refs 17 and 27. b Type of the reaction; ET, electron transfer; PT, proton transfer.

radical cations. To discriminate against the fluorescence from benzonitrile, we observed the emission at λ > 460 nm.22 On

(16)

without scavenging et- and A•-. Due to the low IP of TMPD, no fluorescence is formed when TMPD•+ recombines with a radical anion whose parent molecule has gas-phase EA > 0.35 ( 0.05 eV.23 In the presence of TMPD the decay of the FDMR signal from 1Pe* follows the geminate decay curve Λ(t) multiplied by exp(-k16[TMPD]t). The deconvolution yielded k16 ) (6 ( 0.5) × 108 mol-1 dm3 s-1. A similar experiment was performed with d10Bh/BN solution (10-3 and 1.5 × 10-2 mol dm-3, respectively). The emission of biphenyl-d10 was observed at 300-400 nm. On addition of benzonitrile the central line broadens from 0.7 to 1.2 mT following reaction 15; this broadening proceeds with the rate constant 1.8 × 109 mol-1 dm3 s-1. In another experiment, we applied the µw pulse at td ) 0.35 µs and collected the FDMR traces for different concentrations of triethylamine added to the biphenyl-d10 solution (Figure 4a). Triethylamine (IP ) 7.5 eV)

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scavenges both SQ•+ and Bh•+. Apparently, the recombination of the [TEA•+ Bh•-] pairs yields 1Bh* since the resonance lines from TEA•+ appear in the spectra. For [TEA] < 2 × 10-3 mol dm-3 the FDMR signal observed at td ) 0.35 µs increases with [TEA]; for higher concentrations of triethylamine the signal decreases. In the latter range the FDMR decay kinetics become considerably faster, but the spectrum shape does not change. This indicates proton transfer between TEA•+ and TEA.24 The dependence of the rate constant of the FDMR decay on [TEA] exhibits two linear ranges (Figure 4b). For the first one ( 460 nm) are transformed into [SQ•+ NB•-] pairs which fluoresce in the UV. At the same time, both the [Pe•+ Pe•-] and [Pe•+ NB•-] pairs yield emission at λ > 460 nm. This causes a fast decay of the signal in the wings of the FDMR spectrum. By comparative analysis of the pulsesweeping FDMR kinetics at the spectrum center and in the wings (e.g., Figure 5b) we found k19 ) 1.5 × 109 mol-1 dm3 s-1. Mobile Radical Cations in Squalane. Our pulse radiolysis results suggest that one of the cations formed in ionization of squalane (ion-I) is ∼7 times more mobile than other cations. It was found that the scavenging of ion-I by aromatic solutes yields solute radical cations and that the absorption spectrum of ion-I resembles that of the solvent hole stabilized at 72 K.25 Both of these observations point to the radical cation of squalane. In this section we consider the FDMR evidence that the mobile ions-I are indeed the radical cations in geminate pairs. Although the lifetime of ion-I is too short to be detected by means of magnetic resonance (∼25 ns1), the fast generation of aromatic radical cations can be observed by FDMR. A similar FDMR experiment has been performed by Lefcowitz and Trifunac,24 who tried to observe the mobile solvent radical cation in cyclohexane. Contrary to their expectations, the growth of the signal from the solute radical cations at td > 50 ns was not faster than diffusion-controlled.24 This was interpreted as an indication of the short lifetime of the mobile species. In our experiments we observed the growth of FDMR from TEA•+ as a function of [TEA] in the benzonitrile/triethylamine solution. Benzonitrile emits at 300 nm;22 its fluorescence was collected using a Corning 3-54 filter. Due to its high IP, benzonitrile does not scavenge the holes. With no TEA present,

Figure 5. (a) Normalized FDMR spectra from a N2-saturated squalane solution of 8.8 × 10-4 mol dm-3 perylene with (i-iii) 3 × 10-3 mol dm-3 and (iv) 1.5 × 10-2 mol dm-3 of nitrobenzene-d5. The bold traces i and iv were obtained at td ) 50 ns; traces ii and iii were obtained at td ) 100 ns and td ) 300 ns, respectively (50 ns µw pulse, 50 ns boxcar gate, λ > 460 nm). For higher concentration of nitrobenzene-d5 and longer delay times td the intensity of the broad signal from SQ•+ relative to the central line rapidly decreases. The shoulder to the left of the spectrum center is from nitrobenzene-d5; this signal increases with td. Both changes are due to reaction 19. (b) Pulse-sweeping FDMR experiment performed at the spectrum center and 3.5 mT off-center (shown by arrows in the spectra) for [Pe] ) 3.6 × 10-4 mol dm-3 and [NB] ) 3.5 × 10-3 mol dm-3. The dashed line is the zero-offset trace convoluted with exp(-kt), k ) 5.3 × 106 s-1.

a spectrum from the [SQ•+ BN•-] pairs was observed. On addition of TEA, a set of resonance lines from TEA•+ emerged. If the scavenging rate of the solvent hole was diffusion-limited, then only 2% of the holes would be scavenged in 50 ns. The actual conversion is an order of magnitude greater. It can be argued that the fluorescence bands of TEA and BN overlap and that TEA has greater luminescence yield per recombination. These two factors would bias FDMR toward the observation of the emission from [TEA•+ BN•-] pairs. To exclude the interference of the TEA emission, we performed FDMR experiments with solutes whose fluorescence bands do not overlap with that of TEA: perylene (λ > 460 nm) and rubrene (λ > 540 nm). It is known that the fluorescence of TEA exhibits a tail to 400 nm in moist alkane solutions.26 We found no evidence that this happens in pure squalane. No FDMR was observed at λ ) 290 nm, where 1TEA* has its emission maximum. Apparently, the recombination of TEA•+ and Pe•- does not yield 1TEA*.24 The use of the donor-acceptor solutes presents two complications: first, perylene and rubrene scavenge TEA•+; second, unlike benzonitrile, perylene scavenges SQ•+ and the solvent holes. Therefore, we used dilute (1-20) × 10-6 mol dm-3 solutions. For [Pe] ) 2 × 10-5 mol dm-3 the lifetime of the normally diffusing cation (such as TEA•+ and SQ•+) is ∼90 µs. Since the mobile cation is short-lived, its scavenging by perylene accounts for less than 0.05% of the total decay. On addition of ca. 10-3 mol dm-3 TEA the lifetime of SQ•+ is ∼3 µs, and the lifetime of TEA•+ is ∼6 µs. Although the quality of FDMR spectra is poor, the signal from TEA•+ was clearly

Mobile Holes in Radiolysis of Squalane

J. Phys. Chem., Vol. 100, No. 35, 1996 14687 cations are scavenged at slower rates ((0.5-1) × 109 mol-1 dm3 s-1). Proton transfer reactions are even slower. The scavenging of the mobile ions proceeds with a rate constant (5-7) × 109 mol-1 dm3 s-1.1 Our identification of the mobile ions in squalane as the solvent radical cations was supported by (i) the absorption spectrum and (ii) the fast generation of solute radical cations on scavenging.1 In this paper we demonstrate (iii) the participation of the mobile ions in the formation of FDMR. We believe that the high mobility of the squalane radical cations is due to fast resonant charge transfer with τres ≈ 200 ps.1 For this short τres, the FDMR spectrum of the solvent hole must be a single narrow line. However, at room temperature the hole is too short-lived to be observed by FDMR. The narrow FDMR signals observed by Tadjikov et al.13 are from impurity ions. Acknowledgment. We thank Dr. C. D. Jonah for his help with the Monte Carlo simulations, Dr. M. C. Sauer, Jr., for constructive criticism, and Dr. D. W. Werst and Mr. R. Lowers for their operation of the accelerator. References and Notes

Figure 6. FDMR spectra from N2-saturated solution of squalane with 2 × 10-5 mol dm-3 perylene and (i) no, and (ii) 5 × 10-3 and 7 × 10-4 mol dm-3 triethylamine (50 ns µw pulse, 50 ns boxcar gate, λ > 460 nm). Traces i and ii were obtained at td ) 50 ns; for other traces td is indicated in the figure. The lines from TEA•+ are marked with arrows. These lines emerge on the top of the broad signal from SQ•+ from the earliest observation times. The spectra were normalized by the signal at the spectrum center.

observed from the earliest detection time (Figure 6) and indicated 20-30% conversion of the holes. Another type of FDMR experiment was performed with perylene/nitrobenzene-d5 solutions. The emission from 1Pe* at λ > 460 nm was collected. Due to reaction 20, for [d5NB] ) 0.06 mol dm-3 the lifetime of Pe•- is 10 ns. The recombination of the [SQ•+ d5NB•-] pairs does not yield the fluorescence at λ > 460 nm. Thus, the only pair that yields FDMR at λ > 460 nm and td > 50 ns is [Pe•+ d5NB•-]. Experimentally, the FDMR signal from Pe•+ can be observed even for [Pe] ) 2.4 × 10-5 mol dm-3. At this concentration, the charge transfer from SQ•+ would take 50 µs. The FDMR signal linearly increases with [Pe] (2 × 10-5 to 10-3 mol dm-3). This result implies the rapid formation of Pe•+ competing with the decay of the squalane hole. To summarize, our FDMR data support the View that the mobile ions reacting with triethylamine and aromatic solutes are short-liVed solVent radical cations of squalane. Conclusion Radiolysis of squalane yields a mobile solvent hole and normally diffusing olefin ions. Only the latter can be observed directly by FDMR. The mobile ion is scavenged by aromatic solutes significantly faster than other cations. In squalane, the fastest ET reactions of normally diffusing ions have the rate constants ∼(1.6-1.8) × 109 mol-1 dm3 s-1. Most aromatic

(1) Shkrob, I. A.; Sauer, M. C., Jr.; Trfiunac, A. D. J. Phys. Chem. 1996, 100, 5993. (2) Willard, J. E. In Radiation Chemistry: Principles and Applications; Farhataziz, Rodgers, M. A. J. Eds.; VCH: New York, 1987; p 395. (3) Werst, D. W.; Trifunac, A. D. J. Phys. Chem. 1991, 95, 3466. (4) Sauer, M. C., Jr.; Jonah, C. D.; Naleway, C. A. J. Phys. Chem. 1991, 95, 730. (5) Sauer, M. C., Jr.; Jonah, C. D.; Le Motais, B. C.; Chernowitz, A. C. J. Phys. Chem. 1988, 92, 4099. (6) Sauer, M. C., Jr.; Jonah, C. D. J. Phys. Chem. 1992, 96, 5872. (7) Shkrob, I. A.; Trifunac, A. D. J. Chem. Phys. 1995, 103, 551. (8) Poole, C. P., Jr. ESR: A ComprehensiVe Treatise on Experimental Techniques; Wiley Interscience: New York, 1967. (9) Landolt-Bornstein. Magnetic Properties of Free Radicals; SpringerVerlag: New York, 1977. (10) Smith, J. P.; Trifunac, A. D. J. Phys. Chem. 1981, 85, 1645. (11) Desrosiers, M. F.; Trifunac, A. D. J. Phys. Chem. 1986, 90, 1560. (12) Okazaki, M.; Nunome, K.; Matsuura, K.; Toriyama, K. Bull. Chem. Soc. Jpn. 1990, 63, 1396. (13) Tadjikov, B. M.; Melekhov, V. I.; Anisimov, O. A.; Molin, Yu. N. Radiat. Phys. Chem. 1989, 34, 353. (14) Veselov, A. V.; Bizyaev, V. L.; Melekhov, V. I.; Anisimov, O. A.; Molin, Yu. N. Radiat. Phys. Chem. 1989, 34, 567. (15) Werst, D. W.; Percy, L. T.; Trifunac, A. D. Chem. Phys. Lett. 1988, 153, 45. (16) Shkrob, I. A.; Werst, D. W.; Trifunac, A. D. J. Phys. Chem. 1994, 98, 13262. (17) Levin, R. D.; Lias, S. G. Ionization Potential and Appearance Potential Measurements; 1971-1981 NRSDS-NBS 71. (18) Chawla, O. P.; Fessenden, R. W. J. Phys. Chem. 1975, 79, 2693. (19) Schmidt, K. H.; Sauer, M. C., Jr.; Lu, Y.; Liu, A. J. Phys. Chem. 1990, 94, 244. (20) Hummel, A. AdV. Radiat. Chem. 1974, 4, 1. (21) Warman, J. M.; Asmus, K.-D.; Schuler, R. H. J. Phys. Chem. 1969, 73, 931. (22) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic Molecules, 2nd ed.; Academic Press: New York, 1971. (23) Werst, D. W. Chem. Phys. Lett. 1996, 251, 315. (24) Lefkowitz, S. M.; Trifunac, A. D. J. Phys. Chem. 1984, 88, 77. (25) Teather, G. G.; Klassen, N. V. J. Phys. Chem. 1981, 85, 3044. (26) Muto, Y.; Nakato, Y.; Tsubomura, H. Chem. Phys. Lett. 1971, 15, 147. (27) Lias, S. G.; Bartmess, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, 1.

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