Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces
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260 Charged Particle and Photon Interactions with Matter
These few examples demonstrate that the radical cations produced directly from functional neu-
tral molecules by ionization are highly reactive. Under selected experimental conditions (absence of
reaction partners and choice of a matrix preventing bimolecular reaction), they can undergo various
intramolecular transformations, leading to more stable radical cation structures, which may not
exist as parent ground-state molecules. The rearrangement can occur through an intramolecular
H-shift and ring closing. The calculations of the thermochemical parameters of possible reaction
pathways and electronic parameters of different structures are often helpful for a better understand-
ing
of the studied mechanism.
O
O
O
OH
OH
HC
H
H
C
O
O
H
C
H
2
C
C
OH
OH
H
H
2
C
CH
3
CH
2
O
H
2
C CH
3
+
+
+
+
+
ΔH = –29
E
a
= 50
ΔH = –8
ΔH = –122
ΔH = –80
E
a
= 38
E
a
= 50
E
a
= 46
•
•
•
•
•
Figure 10.18 Experimentally observed intramolecular transformations of an ethyl acrylate radical cation
via an intramolecular H-shift to carbonyl oxygen and ring closing. ΔH and E
a
are the calculated reaction
enthalpy
and the activation energy (kJ mol
−1
), respectively.
ε-caprolactone
δ-hexanolactone
δ-valerolactone
γ-valerolactone
γ-butyrolactone
77 K
O
O
O
OH
145 K
OH
O
O
O
5 mT
OH
OH
OH
O
O
O
O
5 mT
CH
3
CH
3
CH
3
CH
3
+
+
+
+
+
ΔH(2–3) = –85
E
a
(2–3) = 220
L(H–O) = 0.267
ΔH(2–3) = –75
E
a
(2–3) = 216
L(H–O) = 0.267
ΔH(2–3) = –82
E
a
(2–3) = 160
L(H–O) = 0.243
ΔH(2–3) = –80
E
a
(2–3) = 162
L(H–O) = 0.244
ΔH(2–3) = –62
E
a
(2–3) = 118
L(H–O) = 0.233
O•+
•
•
•
•
•
O•+
O•+
O
•
+
O•+
Figure 10.17 EPR spectra and intramolecular transformations of different lactone radical cations through
the H-shift from C
1
in the α-position on the ring to carbonyl oxygen. ΔH and E
a
are the calculated reaction
enthalpy and the activation energy (kJ mol
−1
), respectively, for the intramolecular H-shift; L(H–O) is the
distance
(nm) between a H-atom on C
1
and carbonyl oxygen in a primary radical cation.
Reactivity of Radical Cations in Nonpolar Condensed Matter 261
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265
11
Radiation Chemistry
and
Photochemistry
of
Ionic Liquids
Kenji Takahashi
Kanazawa University
Kanazawa,
Japan
James F. Wishart
Brookhaven National Laboratory
Upton,
New York
11.1 introduCtion
Ionic liquids are commonly dened as molten salts with melting points below 100°C. Typically,
they are combinations of organic and inorganic ions selected to have poor packing or weakened
interionic interactions in the solid state, and hence, low melting points. Ionic liquids have been
considered as an ideal medium for a wide range of applications in the areas of energy, synthesis,
electrochemistry, and biocatalysis, among others, and the eld of ionic liquids continues to grow
as new types of ionic liquids continue to be developed (Welton, 2004; MacFarlane etal., 2007;
van Rantwijk and Sheldon, 2007; Plechkova and Seddon, 2008; Wishart, 2009). The potential
safety and environmental benets of ionic liquids, as compared to conventional solvents, have
drawn interest in their use as processing media for the nuclear fuel cycle (Earle and Seddon,
2000; Allen etal., 2002; Jensen etal., 2003; Chaumont and Wipff, 2004; Nikitenko etal., 2005;
Cocalia etal., 2006; Dietz, 2006; Stepinski etal., 2006; Luo etal., 2006; Binnemans, 2007;
Han and Armstrong, 2007). Therefore, an understanding of the interactions of ionizing radiation
and photons with ionic liquids is strongly needed.
Contents
11.1 Introduction ..........................................................................................................................265
11.2 Physicochemical
Properties of Ionic Liquids .......................................................................266
11.3
Formation
of Excess Electrons in Ionic Liquids: Pulse Radiolysis and Electron
Photodetachment ..................................................................................................................268
11.4 Solvation
Dynamics and Reactions of Pre-Solvated (Dry) Electrons ..................................270
11.5
Photoexcitation of Solvated Electrons .................................................................................. 273
11.6 Radical
Reactions and Kinetic Salt Effects.......................................................................... 275
11.6.1
Reactions
between Diiodide Anion Radicals in Ionic Liquids................................. 275
11.6.2
Kinetic
Salt Effects in Ionic Liquid/Methanol Mixtures ......................................... 279
11.6.3
Reactions
of Excited-State Radicals in Ionic Liquids .............................................. 281
11.7
Summary ..............................................................................................................................283
Acknowledgments..........................................................................................................................284
References......................................................................................................................................284
266 Charged Particle and Photon Interactions with Matter
In 2001, Neta and coworkers began to publish a series of investigations on ionic liquid radia-
tion chemistry and radiation-induced chemical kinetics in ionic liquids (Behar et al., 2001; 2002;
Grodkowski and Neta, 2002a,b,c). Contemporaneously, groups in Poland and the United Kingdom
studied the radiolysis of ionic liquids containing the imidazolium cation (Marcinek etal., 2001;
Allen et al., 2002). Since then, picosecond pulse radiolysis on ionic liquids and static electron
paramagnetic resonance (EPR) measurements on irradiated ionic liquid glasses have been used to
characterize the initial radiolysis products of ionic liquids and their reactivity (Grodkowski etal.,
2003; Wishart, 2003; Wishart and Neta, 2003; Funston and Wishart, 2005; Wishart etal., 2005,
2006; Shkrob et al., 2007; Shkrob and Wishart, 2009; Wishart and Shkrob, 2009). Electron
photodetachment studies have also been performed using UV lasers to study solvated electrons in
ionic liquids (Katoh et al., 2007; Takahashi etal., 2008). Through these studies, it has been revealed
that the reactivities of pre-solvated and solvated electrons, and the kinetics of radical reactions, are
quite different from those found in molecular solvents. In this chapter, we introduce recent advances
in radiation chemistry and photochemistry of ionic liquids. We also include studies related to the
physicochemical properties, structure, and solvation dynamics of ionic liquids that are important to
understand the radiation chemistry of ionic liquids.
11.2 physiCoChemiCal properties oF ioniC liQuids
Because ionic liquids are binary ionic compounds, their physicochemical properties can vary signif-
icantly from those of molecular solvents. Ionic liquids are typically characterized by their melting
point, viscosity, conductivity, and ionic self-diffusion coefcients. Conventional salts like sodium
chloride (NaCl, melting point 801°C) form high-melting solids because of the very strong Coulombic
attractions between ions in the close-packed lattice of the solid. Weakening the Coulombic interac-
tions makes it easier for the solid to melt. Replacing sodium with the larger cesium cation reduces
the melting point of CsCl to 645°C, and substituting the large organic tetrapropylammonium cation
[(Pr)
4
N]
+
lowers it to 241°C. Going further, the melting point of ethylmethylimidazolium chloride
(Emim
+
Cl
−
) is 87°C, allowing it to qualify as an ionic liquid.
Melting points may be inuenced not only by the size and shape of the cation but also by the anion.
The melting point of Emim
+
NO
3
−
is 38°C, while those of Emim
+
BF
4
−
and Emim
+
N(SO CF )
2 3 2
−
are 15°C and −18°C, respectively, making them both room-temperature ionic liquids (RTILs). It is
important to clarify, however, that ion size is only one of the many factors that determine the melt-
ing point of a specic combination of ions. Another important factor is the distribution of charges
on cations and anions. If the charges are localized on specic parts of the cation and the anion, the
electrostatic interaction between them can be strong, leading to a higher melting point. On the other
hand, if the charges are delocalized widely over the cation and the anion, the attraction between the
ions
would be weaker.
The
presence of aliphatic, aromatic, or uorous side chains, as well as the addition of functional
groups, also inuences the lattice-packing properties and interionic interactions that control melt-
ing points. These features also control the solubility of solutes in ionic liquids and the miscibility
of ionic liquids with other solvents. Frequently, the hydrophobicity or hydrophilicity of the ionic
liquid is determined by the afnity of the anion for water. For example, Bmim
+
Cl and Bmim
+
BF
4
are water-miscible, as are most ionic liquids containing Cl
−
,
BF
4
−
, Br
−
, I
−
,
CF CO
3 2
−
, and
CH OSO
3 3
−
,
unless the cation contains a lot of organic side chains. On the other hand, Bmim
+
N(SO CF )
2 3 2
−
and
Bmim
+
PF
6
−
contain hydrophobic anions and are water-immiscible.
It is well known that viscosities of ionic liquids are much higher than those of normal molecular
solvents. For example, the viscosity of Bmim
+
N(SO CF )
2 3 2
−
at 303K is 40mPa s (Jacquemin etal.,
2006), almost a factor of 40 higher than that of water. The viscosities of three ionic liquids as a func-
tion of temperature are shown in Figure 11.1 (Jacquemin etal., 2006). Ionic liquids generally show
glassy behavior and their viscosities decrease in a “super-Arrhenius” manner with temperature,
that is, their viscosities decrease faster with temperature than predicted by a linear Arrhenius plot.
Radiation Chemistry and Photochemistry of Ionic Liquids 267
Thus, at moderately elevated temperatures, many ionic liquids are uid enough for applications in
replacing
conventional solvents.
Recently,
the heterogeneous nature of ionic liquids has begun to be examined in detail. There are
in fact two types of heterogeneities, static and dynamic. The static heterogeneity is manifested by
the existence of structurally distinct domains of polarity and nonpolarity occupied by the charged
and uncharged parts of the constituent ions and their side chains, respectively. The volumes and
interconnectivity of these heterogeneous domains can depend on the lengths of the aliphatic side
chains (Del Popolo and Voth, 2004; Wang and Voth, 2005; Lopes and Padua, 2006; Triolo etal.,
2007). Various solutes will distribute themselves differently among the domains depending on their
characteristics. Consequently, attempts to characterize the polarity of ionic liquids using solvato-
chromic probes resulted in disparate answers depending on the probe molecule used (Weingartner,
2008). Hamaguchi etal. showed through Raman spectral analysis that two distinct conformational
structures of the Bmim
+
ion exist simultaneously in the ionic liquid state (Hayashi etal., 2003;
Ozawa etal., 2003). They also have obtained evidence of the mesoscopic local structure in ionic
liquids
by coherent anti-Stokes Raman scattering measurements (Shigeto and Hamaguchi, 2006).
The
second type of heterogeneity exhibited in ionic liquids is the dispersive nature of their
dynamics and kinetics (Giraud etal., 2003; Holbrey etal., 2004; Castriota etal., 2005; Shirota and
Castner, 2005; Shirota etal., 2005, 2007). For example, the Maroncelli group observed that the
time-dependent response to a solute electronic perturbation extends over three or more decades in
time in ionic liquids (Arzhantsev etal., 2007; Jin etal., 2007). In the case of conventional super-
cooled liquids, it is generally agreed that dispersive kinetics are the results of heterogeneity (Ediger,
2000). In such situations, molecules in different local environments relax at signicantly differ-
ent rates. The presence of such heterogeneity in ionic liquids has been clearly demonstrated both
experimentally and in computational studies (Del Popolo and Voth, 2004; Mandal et al., 2004;
Wang and Voth, 2005; Hu and Margulis, 2006; Lopes and Padua, 2006; Triolo etal., 2007). Such
inhomogeneous environments may have signicant effects on the formation and reactions of solvated
electrons
in ionic liquids.
5
6
7
10
2
3
4
5
6
7
100
2
3
Viscosity (mPa s)
380360340320300
T (K)
Bmim-PF
6
Bmim-NTf
2
Bmim-BF
4
Figure 11.1 Viscosities of the ionic liquids Bmim
+
PF
6
−
,
Bmim N(SO CF )
+
2 3 2
−
(NTf
2
), and
Bmim BF
+ −
4
as
a
function of temperature. (Data from Jacquemin, J. etal., Green Chem., 8, 172, 2006.)
268 Charged Particle and Photon Interactions with Matter
11.3 Formation oF exCess eleCtrons in ioniC liQuids:
pulse
r
adiolysis
and e
leCtron
p
hotodetaChment
The earliest pulse radiolysis experiments on ionic liquids were performed on imidazolium cation-
based ionic liquids (Allen etal., 2002; Behar etal., 2001, 2002; Grodkowski and Neta, 2002a,b,c;
Marcinek etal., 2001). Pulse radiolysis transient absorption spectroscopy of neat, deoxygenated
Bmim
+
PF
6
−
revealed a transient product peak at 325nm and a shoulder around 375nm (Behar
etal., 2001; Marcinek etal., 2001). The identities of the species responsible for the observed tran-
sients were determined by pulse radiolysis on aqueous solutions of the Bmim
+
salt. The hydrated
electron reacts with Bmim
+
very rapidly (k = 1.9 × 10
10
M
−1
s
−1
) (Behar etal., 2001). The product is
the neutral Bmim
•
radical, which has an absorption peak at 323nm (e = 5900M
−1
cm
−1
) and a reduc-
tion potential of −2V vs. normal hydrogen electrode (NHE) in aqueous solution. Reactions of the
Bmim
+
cation with OH and H radicals produce the corresponding radical adducts, and oxidation
of Bmim
+
by
SO
4
−i
produces the ring-centered Bmim
2+•
radical dication. All three of these prod-
ucts have similar absorption spectra to the Bmim
•
radical. Thus, in the neat Bmim
+
PF
6
−
liquid, the
transient absorption seen at 325 nm contains contributions from both the reduction and oxidation
products
of the Bmim
+
cation (Behar etal., 2001). Because the excess electrons produced by ioniz-
ing radiation are rapidly scavenged by imidazolium cations, solvated electrons in such imidazolium-
based
ionic liquids have not yet been conclusively observed.
On
the other hand, if the cations (and anions) of the ionic liquid are sufciently unreactive toward
reduction, solvated electrons can exist for microseconds as stable transient species. Quaternary
ammonium and phosphonium cations meet this criterion. The electrochemical window of ionic liq-
uid methyltributylammonium bis(triuoromethylsulfonyl)imide (MeBu
3
N
+
N
(SO CF )
2 3 2
−
) is almost
6V at a Pt electrode, with a cathodic limit of about −3.2V vs. ferrocene/ferrocenium. The rst
observation of the solvated electron in a neat ionic liquid was made by pulse radiolysis transient
absorption measurements on
MeBu N N(SO CF )
3
+
2 3 2
−
(Wishart and Neta, 2003). The absorption
spectrum of the solvated electron in this ionic liquid is shown in Figure 11.2. The spectrum is
very broad and the absorption peak is located around 1400 nm (e = 22,000M
−1
cm
−1
) with a long
tail toward the visible and UV regions. Spectra of the solvated electron in ionic liquids without
functional groups tend to peak in the region between 1000 and 1500nm (Wishart and Neta, 2003;
Funston
and Wishart, 2005; Wishart etal., 2005; Takahashi etal., 2008, 2009b).
14
12
10
8
6
4
2
0
e
solv
600400 800 1000
Wavelength (nm)
Gε×10
–3
1200 1400 1600
–
Figure 11.2 Transientspectraobservedafter electron pulseradiolysisofMeBu N N(SO CF )
+
2 3 23
−
.(Reproduced
from
Wishart, J.F. and Neta, P., J. Phys. Chem. B, 107, 7261, 2003. With permission.)
Radiation Chemistry and Photochemistry of Ionic Liquids 269
The reactivity of solvated electrons with some aromatics, such as benzophenone, pyrene, and
phenanthrene, was examined in the same study (Wishart and Neta, 2003). Figure 11.3 shows
examples of the decay of the solvated electron absorption at 900 nm with increasing scavenger
concentration. The reaction rate constants are in a range of 1.3−1.7 × 10
8
M
−1
s
−1
. The theoretical
diffusion-controlled rate limit is estimated to be 1.5 × 10
7
M
−1
s
−1
from the measured viscosity
and the equation k
diff
= 8000RT/3η. The calculated rate limit is one order of magnitude slower
than the experimental values. A similar difference was observed for diffusion-limited electron-
transfer reactions from pyridinyl radical to duroquinone in several ionic liquids (Behar etal.,
2002; Skrzypczak and Neta, 2003), and in numerous other cases of diffusion-controlled reactions
in ionic liquids. This trend indicates that reactant solutes may experience different microviscosi-
ties that are lower than the viscosity of the medium, the latter being determined by the movement
of the anions and cations subject to the constraints of their Coulombic interactions. It is also worth
noting that the diffusion-controlled reactions of the solvated electron with the aromatic scaven-
gers are approximately two orders of magnitude slower than in common molecular solvents and
about ten times slower than the reaction of the hydrogen atom with the same reactants in the same
ionic liquid (Grodkowski etal., 2003).
Solvated electrons can also be produced in ionic liquids by electron photodetachment by the
excitation of the charge-transfer-to-solvent (CTTS) band of iodide (Katoh etal., 2007). In trimethyl-
n-propylammonium bis(triuoromethanesulfonyl)imide (TMPA
+
N
(SO CF )
2 3 2
−
), the CTTS absorp-
tion maximum is located around 225nm. Upon photoexcitation in this wavelength range, electron
photodetachment
occurs efciently:
I
− −
→ +
h
e
ν
I
hot
where
e
hot
−
is the electron before it has become thermalized by losing its excess kinetic energy
through interaction with the ionic liquid medium. (The thermalized electron subsequently under-
goes a solvation process that is discussed later in this chapter.) The quantum yield of the photode-
tachment from iodide to
TMPA N(SO CF )
+
2 3 2
−
(F
ion
) is estimated to be 0.34 (Katoh etal., 2007). This
high quantum yield of the photodetachment may be due to the high mobility of the pre-solvated
electron (see below) and/or effective screening of the geminate electron–hole pair by the ionic liquid.
30
25
20
15
10
5
0
0 200 400
Time (ns)
Absorbance×10
3
600 800
Figure 11.3 Transient absorbance at 900nm for electron capture by (top to bottom) 0, 24, 48, 64, and 90mM
benzophenone in
MeBu N N(SO CF )
+
2 3 23
−
. (Reproduced from Wishart, J.F. and Neta, P., J. Phys. Chem. B, 107,
7261,
2003. With permission.)