4770
J. Phys. Chem. B 2007, 111, 4770-4774
Electron Photodetachment from Iodide in Ionic Liquids through Charge-Transfer-to-Solvent Band Excitation† Ryuzi Katoh,*,‡ Yoichi Yoshida,§ Yosuke Katsumura,| and Kenji Takahashi*,⊥ National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, The Institute of Scientific and Industrial Research (ISIR), Osaka UniVersity, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, Department of Nuclear Engineering and Management, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and DiVision of Material Science, Graduate School of Natural Science and Technology, Kanazawa UniVersity, Kakuma-machi, Kanazawa 920-1192, Japan ReceiVed: October 30, 2006; In Final Form: December 5, 2006
Solvation of iodide and electrons in an ionic liquid (N,N,N-trimethyl-n-propylammonium bis(trifluoromethanesulfonyl)imide; TMPA-TFSI) was studied through the absorption spectra of the charge-transfer-to-solvent (CTTS) state of iodide and of solvated electrons. The interaction between the TMPA cation and iodide was strong, whereas electrons were weakly solvated in TMPA-TFSI. We followed electron photodetachment from iodide to the ionic liquid and formation of the solvated electrons by observing absorption in the visible and near-infrared regions using a nanosecond laser flash photolysis method. The quantum yield of the photodetachment in TMPA-TFSI was estimated to be 0.34, which is much higher than that in a high-concentration aqueous salt solution previously reported. We also examined a reaction of the solvated electrons with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI) as a solute in TMPA-TFSI. The reaction rate was determined to be 5.3 × 108 M-1 s-1. The electrons before full solvation (dry electrons) reacted with Bmim cations efficiently. These observations suggest that the electrons in TMPA-TFSI can move easily before solvation.
1. Introduction Room-temperature ionic liquids are receiving considerable attention because of their remarkable properties, such as extremely low vapor pressure.1,2 Thus, ionic liquids are now used in various fields as solvents in chemical processes, such as organic synthesis, separation processes, and electrochemical processes. Ionic liquids are also interesting topics for basic science, and their characteristic properties are being investigated through various experimental techniques. Solvation properties of ionic liquids are among the most fascinating issues because ionic liquids consist entirely of ionic species and are therefore much different from conventional polar solvent molecules. Solvation properties of ionic liquids have been mainly studied through spectroscopic techniques, such as static and dynamic solvatochromic shifts in the fluorescence of probe molecules.3-5 Through these studies, effective polarity and donor and acceptor numbers have been discussed. Recently, dielectric constants of ionic liquids were investigated through microwave dielectric spectroscopy, and the polarities of ionic liquids were found to be markedly lower than those expected from the results based on fluorescence spectroscopy.6 This discrepancy implies that the interaction between ionic liquid and ion is important for the solvation properties of ionic species in ionic liquids. The solvation properties of smaller ions, such as metal and halide ions, in ionic liquids are interesting because relatively †
Part of the special issue “Physical Chemistry of Ionic Liquids”. * Corresponding author. E-mail:
[email protected] (R.K.); ktkenji@ t.kanazawa-u.ac.jp (K.T.). ‡ AIST. § Osaka University. | The University of Tokyo. ⊥ Kanazawa University.
strong interactions are expected. Note that small ions in ionic liquids are key species for many chemical systems based on ionic liquids, especially for electrochemical applications. Iodidebased electrolytes (iodide/triiodide) have been widely used for dye-sensitized solar cell applications.7,8 Thus, charge transport properties have been studied in detail through conductivity measurements.9,10 The solvation of halide ions can be also investigated by spectroscopic techniques. Halide ions in polar solvents show characteristic absorption spectra in the deep ultraviolet wavelength range, called the charge-transfer-tosolvent (CTTS) absorption band. By using CTTS absorption spectra, solvation properties can be evaluated.11 Recently, this technique was applied to explore solvation properties in supercritical fluids.12 Solvation properties of the smallest negative charged species, the electron, in ionic liquids have been attracting much interest for not only fundamental interest but also practical applications. Recently, ionic liquids have been used as the solvent for nuclear fuel cycle processes13 because the extremely low vapor pressure of ionic liquids may improve the safety of nuclear fuel and waste handling processes. Thus, the chemical stability and reactivity of ionic liquids as well as solute molecules in the liquids under exposure to radiation have been studied.14-17 Under irradiation, solvated electrons play an important role in the reactions, and therefore the physical properties and reactivity of solvated electrons in ionic liquids have been investigated.14-17 Here we studied the solvation properties of two smaller negative species, an iodide and an electron, in an ionic liquid (N,N,N-trimethyl-n-propylammonium bis(trifluoromethanesulfonyl)imide; TMPA-TFSI). Absorption spectra of the CTTS state of iodide and of the solvated electrons were measured. On the basis of the correlation between these absorption bands
10.1021/jp067107e CCC: $37.00 © 2007 American Chemical Society Published on Web 01/11/2007
Iodide and Electron Solvation in an Ionic Liquid
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4771
Figure 1. Molecular structure of TMPA-TFSI.
in many polar solvents reported so far,18 we discuss the solvation properties of iodide and electrons in TMPA-TFSI. Upon photoexcitation, an electron was ejected from an iodide ion, and then the solvated electron was observed. We estimated the quantum yield of the solvated electrons. We also evaluated the reaction of the solvated electrons with an ionic liquid (1-butyl3-methylimidazolium bis(trifluoromethanesulfonyl)imide; BmimTFSI) in TMPA-TFSI.
Figure 2. Absorption spectrum of iodide in TMPA-TFSI.
2. Experiments Figure 1 shows the structure of ionic TMPA-TFSI (Kanto Chemical Co.) used in this study. No strong absorption at wavelengths longer than 210 nm was observed (data not shown), suggesting that the purity of the liquid was sufficient for optical measurements in the UV wavelength range. KI solutions in ionic liquid were prepared in a bottle and dried under vacuum at 60 °C. The concentrations of KI were adjusted to from 0.4 to 0.8 mM. For optical measurements, screw-capped, quartz cuvettes with 1 or 0.2 cm optical thickness were used. The samples in the cuvettes were dried again under vacuum at 60 °C before use. We also used ionic liquid Bmim-TFSI (Solvent Innovation GmbH) as a solute molecule for the reaction with solvated electrons in TMPA-TFSI. Absorption spectra of solutions were measured with an absorption spectrophotometer (Shimadzu, UV-3101PC). Pulse radiolysis measurements were performed using the 28 MeV L-band linac of the Institute of Scientific and Industrial Research, Osaka University. The time resolution of the instrument was about 10 ns. Electron photodetachment from iodine was performed by pulsed laser excitation. To measure the transient absorption spectra and the quantum yield of solvated electrons, we used transient absorption spectrometry based on the second harmonic of an optical parametric oscillator (Spectra Physics, MOPOSL) excited by a Nd3+:YAG laser (Spectra Physics, Pro-23010). The wavelength of the laser was set to 230 nm, and the pulse duration was about 8 ns. A Xe flash lamp (Hamamatsu, L4642, 2-µs pulse duration) was used as a probe light source. The sample was excited through a pinhole (4 mm in diameter) placed in front of the cell. The probe light was collinear with the excitation pulse and was transferred through the pinhole. The signal from the detector was amplified (NF Electronic Instruments, BX-31A) and processed with a digital oscilloscope (Tektronix, TDS680C). The intensity of the laser pulse was measured with a pyroelectric energy meter (Ophir, PE25). To measure the reaction kinetics of electrons with solute molecules, we used transient absorption spectrometry based on a KrF excimer laser (Lambda Physik, Lextra 100, λex ) 248 nm). The probe light source was a 300 W xenon arc lamp (Ushio, UXL300D). A wide band-pass filter (fwhm ) 40 nm, Opto-line) was used to select the analyzing wavelength. Transient signals were detected with a fast silicon photodiode (3 ns rise time, NewFocus, 1801) with a 500 MHz oscilloscope (Tektronix, DPO 7054). All measurements were carried out at room temperature. 3. Results and Discussion 3.1. Absorption Spectra of CTTS State and Solvated Electrons. Figure 2 shows the absorption spectrum of KI in the ionic liquid TMPA-TFSI. The peak at 225 nm can be assigned to the CTTS absorption of iodide in TMPA-TFSI. The
Figure 3. Absorption spectrum of solvated electrons in TMPA-TFSI obtained through pulse radiolysis.
peak position is similar to the positions in water (228 nm) and in methanol (220 nm), but the peak is blue-shifted compared to the peaks in acetonitrile (247 nm)19 and in ammonia (260 nm).18 This indicates that iodide is strongly solvated by TMPA molecules. The absorption coefficient at the peak wavelength was 15 800 M-1 cm-1, which is similar to the coefficients in other solvents.11 The absorption maximum appeared at 1100 nm in the transient absorption spectra in TMPA-TFSI measured by the pulse radiolysis technique recorded just after excitation (Figure 3). When aromatic hydrocarbon molecules are added as an electron scavenger, decay of the signal at 1100 nm and rise of the anion signal of the scavenger occur with the same time constant.20 Thus, the absorption peak can be assigned to the absorption due to solvated electrons in the ionic liquid. The peak is located at a substantially longer wavelength than the peaks of electrons in methanol (633 nm) and in 2-propanol (820 nm), but the peak is blue-shifted compared to the peaks in ammonia (1500 nm).18 The peak energy of the absorption spectrum due to solvated electrons reflects the solvation energy. Thus, the electrons in TMPA-TFSI can be considered to be weakly solvated, whereas iodide was strongly solvated by TMPA cation, as mentioned above. This difference implies that specific solvation occurred between iodide and TMPA cations. The decay profile of the pulse radiolysis of TMPA-TFSI observed at 700 nm (Figure 4) can be fitted by a singleexponential function with a time constant of about 200 ns. This relatively short lifetime suggests that electrons reacted with some molecules in the liquid. Although at present the origin of reactive species is not clear, some impurities and remaining oxygen molecules would be responsible for the reaction. Also, the reaction of the electrons with TMPA cations is possible. Further investigations are needed to clarify the quenching reaction in detail. Another possibility of the short lifetime is geminate recombination between a parent cation and an electron. However, geminate recombination is unlikely because the kinetics of geminate recombination does not follow first-order kinetics. The effect of ion concentration on the absorption spectrum of the solvated electrons has been studied.21 For aqueous solutions, the peak position shifts toward the higher energy
4772 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Katoh et al.
Figure 4. Decay profile of absorption due to solvated electrons in TMPA-TFSI observed at 700 nm with pulse radiolysis. Figure 5. Correlation between the absorption peaks for solvated electrons and those for the CTTS band of iodide. Closed circles and open circles represent measurements in various polar solvents [18] and in TMPA-TFSI, respectively.
region with increasing ion concentration. This effect has been tentatively attributed to the formation of a pair between an electron and a metal cation.22 In TMPA-TFSI, in contrast, the electrons seem to be weakly trapped, which suggests that pairing of electrons with TMPA cations is unlikely and that the solvation of electrons in ionic liquids is quite different from their solvation in high-concentration aqueous salt solutions. Wishart et al. reported the absorption spectrum of solvated electrons in a similar ionic liquid (methyltributylammonium bis(trifluoromethylsulfonyl)imide, R4NNTf2).14 They observed the electron peak at 1400 nm, which is at slightly longer wavelength than that obtained in the present study. This difference may be due to weak solvation of electrons because of the bulky butyl group in R4NNTf2. The 200 ns lifetime of electrons in our study was similar to the 300 ns Wishart et al. measured in R4NNTf2. They also studied the effect of functional group substitution on the spectra of solvated electrons in ionic liquids containing ether-, alcohol-, and alkyl-functionalized ammonium dications.15 The absorption peaks in the spectra measured in the present study were similar to those of ether- and alkyl-functionalized ionic liquids. In contrast, the spectra of alcohol-functionalized ionic liquids show a peak at a shorter wavelength (700 nm). They speculated that this wavelength difference is due to the electron localization at the end of the functional group. For the imidazolium salts, which are the most examined ionic liquids thus far, the cation can react with solvated electrons immediately,16,17 and therefore solvated electrons have not been observed so far. We will consider this point when we discuss the results for the Bmim cation in TMPA-TFSI. For further discussion, we compare the results obtained in TMPA-TFSI with those in polar solvents. The following correlation between the peak position of the CTTS absorption of iodide (ECTTS) and that of solvated electron (Eelectron) has been proposed previously,18
example, ion-pair formation. Alkyl-ammonium compounds are reported to form complexes with iodide in nonpolar solvents.23 Thus, iodide in TMPA-TFSI likely forms a complex with the TMPA cation. Recently, ion-pair formation between anions and cations for many ionic liquids was proposed on the basis of the fact that the observed diffusion coefficient is smaller than that expected from the conductivity measurements.24 We also studied the effect of TMPA-TFSI on CTTS absorption of iodide in acetonitrile (Figure 6). The peak position of the CTTS band of KI in acetonitrile (247 nm) shifts to shorter wavelength (242 nm) by addition of a small amount of TMPATFSI (molar fraction of TMPA-TFSI X ) 0.013). This suggests that selective solvation of iodide by TMPA cation occurs. 3.2. Electron Photodetachment from Iodide in Ionic Liquid. Upon photoexcitation of the CTTS absorption band of iodide, electron photodetachment occurs efficiently.
Eelectron ) AECTTS1.65
I- 9 8 I + ehν
(1)
where A is a proportional constant. Figure 5 shows the correlation reported previously (closed circles) together with our result (open circle) for TMPA-TFASI. Clearly the data point for TMPA-TFSI is significantly different from the correlation. From the CTTS absorption maximum of iodide in TMPA-TFSI (225 nm), the absorption maximum of solvated electrons can be expected to be at about 730 nm according to the correlation; however, the predicted absorbance peak is much shorter than the one we observed: 1100 nm. This discrepancy shows that the solvation process of iodide in TMPA-TFSI differs from that of electrons. The peak position of the CTTS absorption band demonstrates that solvation of iodide is relatively strong, whereas weak solvation can be seen for the electrons. This implies that the strong solvation is due to the specific interaction between iodide and TMPA, for
Figure 6. Absorption spectrum of iodide in acetonitrile (MeCN), in TMPA-TFSI/MeCN (molar fraction of TMPA-TFSI, X ) 0.013), and in TMPA-TFSI (dashed line).
(2)
Electron photodetachment has been widely studied in aqueous solutions. Recently, Sauer et al. reported a systematic study of the process.25 For iodide in water, the electron photodetachment upon 248 nm light excitation occurs with high quantum yield (0.286).25 This is the reason we examined photoexcitation of a TMPA-TFSI solution of KI. Figure 7 shows the transient absorption spectrum of a TMPA-TFSI solution of KI excited by 230 nm light recorded just after excitation. The spectrum is similar to that obtained by the pulse radiolysis technique (Figure 3), indicating that an electron can be ejected from iodide into TMPA-TFSI by CTTS band excitation. Figure 8 shows the absorption change observed at 1100 nm (∆Aelectron) due to the production of electrons in the ionic liquid as a function of laser intensity. The linear correlation clearly shows that electron photodetachment proceeds through a one-
Iodide and Electron Solvation in an Ionic Liquid
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4773
Figure 7. Transient absorption spectrum obtained after 230 nm excitation of 2.1 mM KI solution of TMPA-TFSI recorded just after excitation.
Figure 8. Absorption due to solvated electrons observed at 1100 nm as a function of excitation intensity (Iex) after 230 nm excitation of 2.1 mM KI solution of TMPA-TFSI.
photon process. The quantum yield (Φion) of electron photodetachment can be expressed by
Φion )
∆Aelectron electron(1 - T)N0
(3)
where electron is the absorption coefficient of the solvated electrons at the observed wavelength, T is the transmittance of excitation light, and N0 is the number of incident excitation photons per unit area. Under the present experimental conditions, the quantum yield can be expressed as follows.
Φion )
6500 electron
(4)
After an electron photodetachment, an iodine atom I is produced. Subsequently, the iodine atom reacts with iodide, and I2- ion is formed.
I• + I- f I2-
(5)
The I2- ion shows a characteristic absorption band centered at 720-750 nm.26 Accordingly, Sauer et al. have pointed out that both solvated electrons and I2- ions are observed around 700800 nm through the transient absorption measurements of aqueous solutions.25 Figure 9 shows the time profile of transient absorption of a TMPA-TFSI solution of KI after 248 nm light excitation. During the early time range, a strong absorption signal can be seen, which we assign to the solvated electrons. The decay rate is similar to that observed in the pulse radiolysis measurements (Figure 4). After decrease of the electron signal, the absorbance increases again, indicating production of I2-. We already examined a detailed study of the formation and reaction of I2- in ionic liquids through transient absorption measurements.27 The absorption coefficient of I2- at 700 nm
Figure 9. Time course of transient absorbance at 700 nm after 248 nm excitation of KI in TMPA-TFSI.
in water has been estimated to be 2400 M-1 cm-1.26 Thus, we can estimate the absorption coefficient of electrons (electron) in TMPA-TFSI to be 1.9 × 104 M-1 cm-1 at 1100 nm if all iodine atoms I• generated react with I- completely. Accordingly, we estimate the quantum yield of the photodetachment from iodide to TMPA-TFSI (Φion) to be 0.34. On the basis of the similarity between the estimated value of electron for TMPA-TFSI (1.9 × 104 M-1 cm-1) and that in R4NNTf2 (2.2 × 104 M-1 cm-1),14 we consider our estimation of electron to be reasonable. In a study of the effect of ionic strength on the photodetachment of iodide in water, Sauer et al. found that the quantum yield linearly decreases by 6-12% per each 1 M increase in sodium perchlorate concentration.21 They also studied the geminate recombination process by means of femtosecond timeresolved measurements. They observed that the initial quantum yield is not sensitive to the ion concentration, and therefore they proposed that the decrease of the quantum yield is due to efficient geminate recombination. Accordingly, they proposed that the screening of Coulomb interaction by ions for the geminate recombination plays only a minor role and that pair formation between an electron and a metal cation plays an important role in the lower quantum yield. In the present study, we found that the quantum yield of the electron photodetachment from iodide at 248 nm excitation in TMPA-TFSI (Φinj ) 0.34) was higher than that in water, which is estimated to be 0.29.25 This clearly indicates that pairing of the electron with TMPA cation is not efficient, which is consistent with the fact that the peak position of electrons is located in the near-IR wavelength range (Figure 3). The high quantum yield of the photodetachment may be due to a large thermalization distance in the geminate ion pair. If so, then presolvated electrons just after ejection from iodide have a high mobility and/or a long lifetime. This is consistent with the fact that electrons in TMPA-TFSI reacted with an electron scavenger molecule very quickly; that is, the reaction occurred before solvation. We will discuss this point in the next section. 3.3. Reaction of the Ejected Electrons with Bmim. As shown in the previous section, the solvated electrons in TMPATFSI did not react efficiently with TMPA cation. Thus, in TMPA-TFSI, the reaction of electrons with solute molecules can be studied. For ionic liquids based on the imidazolium cation, on the other hand, solvated electrons could not be observed in nanosecond time-resolved pulse radiolysis measurements.16 This is because the solvated electrons generated initially in such ionic liquid react with imidazolium cation immediately and form an unstable species. For this reason, the imidazolium cation-based ionic liquids are not suitable for the nuclear fuel cycle process application. However, the rate constant of electron solvation with the imidazolium cation in an ionic liquid has not been determined; so in this study, we investigated the
4774 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Katoh et al. We observed electron photodetachment from iodide to the ionic liquid and estimated the quantum yield of the photodetachment to be 0.34, which is much higher than that of highconcentration aqueous salt solutions. This difference suggests a large thermalization distance between iodine atoms and ejected electrons in TMPA-TFSI. We also estimated the rate constant of the reaction of the solvated electrons with Bmim-TFSI to be 5.3 × 108 M-1 s-1. The reaction of electrons before full solvation (dry electrons) with Bmim cations was efficient, which suggests that the electrons in TMPA-TFSI could diffuse before full solvation. Thus, we believe that the ionic liquids are an interesting reaction medium for examining the reaction mechanisms of dry electrons.
Figure 10. Kinetics of the reaction of electron with Bmim in TMPATFSI observed at 700 nm after 248 nm excitation. The inset is the quasi-first-order reaction rate as a function of Bmim concentration.
reaction of solvated electrons with imidazolium cation in TMPATFSI in the hope that the rate constant of the reaction would give us a strategy for designing ionic liquids that are stable under irradiation. Fitting the data from transient absorption monitored at 700 nm with several different concentrations of Bmim in TMPATFSI with a single-exponential function revealed pseudo-firstorder reaction rates (Figure 10). As shown in the inset of Figure 10, the reaction rate of Bmim with solvated electrons in TMPA was estimated to be 5.3 × 108 M-1 s-1. From the second-order rate constant, we estimate the half-life of solvated electrons in neat Bmim-TFSI to be 350 ps. Clearly this time region is not accessible with the nanosecond and microsecond pulse radiolysis measurements we performed. Hence, to study the reactions of solvated electrons in imidazolium cation-based ionic liquids, picosecond time-resolved measurements should be examined. Note that the initial absorbance decreased with increasing Bmim concentration (Figure 10), which suggests that the presolvated electrons (dry electrons) reacted efficiently with Bmim. Similar behavior has been observed for the reaction of electrons with some aromatic molecules in another ionic liquid (R4NNTf2).14 As described previously,14 the fractional yield of solvated electrons after the fast reaction of the dry electrons can be evaluated by using the relation Gc/G0 ) exp(-c/C37), where G0 is the yield of solvated electrons without scavengers, Gc is the yield of solvated electrons at a given solute concentration c, and C37 is the concentration at which only 37% ()1/e) of the electrons survive to be solvated. The C37 value for Bmim cation in TMPA is estimated to be 0.057 mol/L. The C37 values for various aromatic molecules in R4NNTf2 have been reported: 0.062 mol/L for benzophenone, 0.063 mol/L for pyrene, and 0.084 mol/L for phenanthrene.14 Hence, Bmim cations react more efficiently with dry electrons than do aromatic molecules. 4. Conclusion We studied the solvation properties of small negative species, iodide and electrons, in TMPA-TFSI. From the correlation of the absorption peaks due to the CTTS state of iodide with those of the solvated electrons, we found that the interaction between a TMPA cation and iodide was relatively strong, whereas electrons were weakly solvated in TMPA-TFSI. This specific interaction, which may be ion-pair formation, is important for understanding both the diffusion of halide ions in ionic liquids and the reactivity of halide ions.
Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research (Project 18045033, Priority Area 452 “Science of Ionic Liquids”) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. References and Notes (1) Welton, T. Chem. ReV. 1999, 99, 2071-2083. (2) Earle, M. J.; Seddon, K. R. Pure Appl. Chem. 2000, 72, 13911398. (3) Arzhantsev, S.; Ito, N.; Heitz, M.; Maroncelli, M. Chem. Phys. Lett. 2003, 381, 278-286. (4) Lang, B.; Angulo, G.; Vauthey, E. J. Phys. Chem. A 2006, 110, 7028-7034. (5) Karmakar, R.; Samanta, A. J. Phys. Chem. A 2002, 106, 44474452. (6) Wakai, C.; Oleinikova, A.; Ott, M.; Weinga¨rtner, H. J. Phys. Chem. B 2005, 109, 17028-17030. (7) Kuang, D.; Wang, P.; Ito, S.; Zakeeruddin, M.; Gra¨tzel, M. J. Am. Chem. Soc. 2006, 128, 7732-7733. (8) Yamanaka, N.; Kawano, R.; Kubo, W.; Kitamura, T.; Wada, Y.; Watanabe, M.; Yanagida, S. Chem. Commun. (Cambridge) 2005, 740742. (9) Kawano, R.; Watanabe, M. Chem. Commun. (Cambridge) 2005, 2107-2109. (10) Paulsson, H.; Kloo, L.; Hagfeldt, A.; Boschloo, G. J. Electroanal. Chem. 2006, 586, 56-61. (11) Blandamer, M.; Fox, M. K. Chem. ReV. 1970, 70, 59-93. (12) Sciaini, G.; Marceca, E.; Ferna´ndez-Prini, R. J. Phys. Chem. B 2006, 110, 8921-8923. (13) Allen, D.; Baston, G.; Bradley, A. E.; Gorman, T.; Haile, A.; Hamblett, I.; Hatter, J. E.; Healey, M. J. F.; Hodgson, B.; Lewin, R.; Lovell, K. V.; Newton, B.; Pitner, W. R.; Rooney, D. W.; Sanders, D.; Seddon, K. R.; Sims, H. E.; Thied, R. C. Green Chem. 2002, 4, 152-158. (14) Wishart, J. F.; Neta, P. J. Phys. Chem. B 2003, 107, 7261-7267. (15) Wishart, J. F.; Lall-Ramnarine, S. I.; Raju, R.; Scumpia, A.; Bellevue, S.; Ragbir, R. Engel, R. Radiat. Phys. Chem. 2005, 72, 99-104. (16) Marcinek, A.; Zielonka, J.; Ge¸ bicki, J.; Gordon, C. M.; Dunkin, I. R. J. Phys. Chem. A 2001, 115, 9305-9309. (17) Behar, D.; Gonzalez, C.; Neta, P. J. Phys. Chem. A 2001, 115, 7607-7614. (18) Fox, M. F.; Hayon, E. Chem. Phys. Lett. 1974, 25, 511-513. (19) Xia, C.; Peon, J.; Kohler, B. J. Chem. Phys. 2002, 117, 88558866. (20) Yoshida, Y. To be submitted for publication. (21) Sauer, M. C., Jr.; Shkrob, I. A.; Lian, R.; Crowell, R. A.; Bartels, D. M.; Chen, X. C.; Suffern, D.; Bradforth, S. E. J. Phys. Chem. A 2004, 108, 10414-10425. (22) Bonin, J.; Lampre, I.; Mostafavi, M. Radiat. Phys. Chem. 2005, 74, 288-296. (23) Blandamer, M. J.; Gough, T. E.; Symons, M. C. R. Trans. Faraday Soc. 1966, 62, 286-295. (24) Tokuda, H.; Ishii, K.; Susan, M. A. B. H.; Tsuzuki, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833-2839. (25) Sauer, M. C., Jr.; Crowell, R. A.; Shkrob, I. A. J. Phys. Chem. A 2004, 108, 5490-5502. (26) Elliot, A. J.; Sopchyshyn, F. C. Int. J. Chem. Kinet. 1984, 16, 12471253. (27) Takahashi, K.; Sakai, S.; Tezuka, H.; Hiejima, Y.; Katsumura, Y.; Watanabe, M. Submitted for publication in J. Phys. Chem B.
