Ionic liquids: Novel media for characterization of radical ions

Article abstract of DOI:10.1021/jp0117718

The novel application of ionic liquids as media for radiolytic generation and UV-vis-NIR spectroscopic characterization of radical ions is described. The redox properties of neat 1-butyl-3-methylimidazolium salts and their aqueous solutions have been investigated by means of pulse radiolysis. Furthermore, ionic liquids prove to be ideal media for the simultaneous generation of radical cations and anions. The radical cations generated from 1-methyl-1,4-dihydronicotinamide, a structural analogue of NADH, have been spectroscopically characterized under matrix conditions for the first time.

Full text of DOI:10.1021/jp0117718

J. Phys. Chem. A 2001, 105, 9305-9309
9305
Ionic Liquids: Novel Media for Characterization of Radical Ions
Andrzej Marcinek,* Jacek Zielonka, and Jerzy Ge¸bicki*
Institute of Applied Radiation Chemistry, Technical UniVersity, 90-924 Lodz, Poland
Charles M. Gordon* and Ian R. Dunkin
Department of Pure and Applied Chemistry, UniVersity of Strathclyde, Glasgow G1 1XL, United Kingdom
ReceiVed: May 9, 2001; In Final Form: July 9, 2001
The novel application of ionic liquids as media for radiolytic generation and UV-vis-NIR spectroscopic
characterization of radical ions is described. The redox properties of neat 1-butyl-3-methylimidazolium salts
and their aqueous solutions have been investigated by means of pulse radiolysis. Furthermore, ionic liquids
prove to be ideal media for the simultaneous generation of radical cations and anions. The radical cations
generated from 1-methyl-1,4-dihydronicotinamide, a structural analogue of NADH, have been spectroscopically
characterized under matrix conditions for the first time.
Introduction
Compounds. 1-Butyl-3-methylimidazolium bromide (BMI-
M+Br-) and 1-butyl-3-methylimidazolium hexafluorophosphate
Ionic liquid is now a commonly accepted term for low melting
salts (melting point typically <100 °C). These materials are
prepared by combining bulky organic cations, such as 1-butyl-
3-methylimidazolium, with a wide variety of anions. The great
interest in ionic liquids which has developed in recent years is
closely related to their utilization as novel solvents for organic
synthesis and their potential use in a variety of commercial
applications.1-3 In general, ionic liquids can replace conventional
media in a wide range of chemical processes. More specifically,
there are some interesting recent examples of use of these
solvents as media for photochemical reactions.4,5 We therefore
decided to test the applicability of ionic liquids in the study of
reactive species induced by ionizing radiation. The reactive
nature of radical ions and radicals requires not only special
techniques for their generation and detection but also appropriate
solvents. One of the most successful methods applied for this
purpose is combination of radiolytic generation of radical ions
with matrix-isolation techniques for their spectroscopic charac-
terization.6-8 However, many substrates are insufficiently
soluble in the solvents generally used for the formation of glassy
matrixes suitable for these studies, thus limiting the range of
systems that may be studied. Since many ionic liquids form
transparent, good-quality glasses on freezing at 77 K, they can
be used as matrixes over a wide range of temperatures in
combination with the UV-vis-NIR spectroscopy for detection
of reactive species generated. Furthermore, ionic liquids are good
solvents for a wide range of organic and inorganic materials.
Despite extensive research on ionic liquids, much of their
basic chemistry remains to be explored. Indeed, understanding
of some of their basic properties is of general importance for
applications of ionic liquids, particularly in the fields of radiation
chemistry and photochemistry.
(BMIM+[PF6]-) were prepared following the method of Hud-
1
dleston et al.9 The identity of the salts was confirmed by H
NMR. When initially formed, the bromide salt is a pale yellow
viscous oil, but on extended storage the salt tends to crystallize
to form a white solid. The oily phase may be re-formed by
heating the salt to above its melting point. 1-Butyl-3-meth-
ylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM+[N(SO2-
CF3)2]-) was prepared from BMIM+Br- and Li[N(SO2CF3)2]
following the method of Bonhoˆte et al.10
1-Methylnicotinamide, Chloride salt (MNA+Cl-). The iodide
(MNA+I-) was prepared via reaction of nicotinamide with
methyl iodide in methanol at room temperature, and then
converted to the chloride salt by shaking its aqueous solution
with freshly precipitated silver chloride. mp 243 °C.
1-Methyl-1,4-dihydronicotinamide (MNAH). To a stirred,
degassed solution of sodium dithionite (6 g) and sodium
carbonate (2 g) in 50 mL of water, 3 g (17.4 mmol) of
MNA+Cl- was added during a 20-min period. The solution was
extracted with chloroform (3 × 100 mL), the extract was dried
with anhydrous MgSO4, and the solvent was removed on a
rotary evaporator. The residual oil was pumped out and dried
over P2O5 to give a yellow solid (60% yield). 1H NMR (Bruker
250 MHz, CDCl3) δ ppm: 2.94 (s, 3H, CH3), 3.05 (m, 2H,
CH2), 4.73 (m, 1H), 5.68 (m, 1H), 7.01 (s, 1H).
Radiolysis in Solution. The pulse radiolysis of water
•
produces three highly reactive species: H2O f eaq (2.6), OH
(2.7), and H• (0.6) (numbers in parentheses are the G values,
i.e., yields of radicals per 100 eV of energy absorbed).11
To study the reaction of BMIM+ cations with eaq without
any interference from the other products of radiolysis of water,
it is necessary to scavenge the •OH radicals using tert-butyl or
isopropyl alcohols (0.1-1 M) in the N2-saturated aqueous
solutions. Isopropyl alcohol also scavenges the H• atoms
effectively.12,13
Experimental Section
Starting Materials. 1-Methylimidazole and 1-bromobutane
(Aldrich) were distilled before use. A 70% w/w hexafluoro-
phosphoric acid solution (Fluorochem), lithium bis(trifluoro-
methylsulfonyl)imide (3M), and compounds used as the solutes
(Aldrich) were used as received.
•
The radical produced from the reaction of OH with tert-
butyl alcohol is inert with regard to further reactions but the
acetone ketyl radical formed via reactions 1 and 2
10.1021/jp0117718 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/15/2001

9306 J. Phys. Chem. A, Vol. 105, No. 40, 2001
Marcinek et al.
•OH + (CH3)2CHOH f (CH3)2 COH + H2O
(1)
(2)
•
•H + (CH3)2CHOH f (CH3)2 COH + H2
•
may act also as a reducing agent. The reduction of BMIM+ by
acetone ketyl radical was tested in aqueous solutions containing
1 M isopropyl alcohol saturated with N2O which converts eaq
into •OH radicals with a rate constant on the order of 1010 M-1
s-1
.
eaq + N2O f •OH + OH- + N2
(3)
The reaction of BMIM+ cations with Br2 was carried out
in N2O-saturated aqueous solutions of BMIM+Br- (5 × 10-2
M):12,13
•-
Figure 1. Electronic absorption spectrum of BMIM• radical obtained
on reduction of BMIM+Br- (<0.005 M) by pulse radiolysis in an N2-
saturated aqueous solution containing tert-BuOH (1 M). The spectrum
was measured 1 µs after the electron pulse. The sample was 1 cm thick
and received a radiation dose of 60 Gy.
Br- + •OH f •Br + OH-
(4)
(5)
•Br + Br- f Br2
•-
SCHEME 1
The concentration of solute was kept in the range of (1-10)
× 10-3 M.
The pulse radiolysis system based on a linear electron
accelerator delivering a dose of 10-50 Gy per pulse (17 ns)
was described earlier.14
Experiments in Cryogenic Glasses. The glassy samples of
ionic liquids were prepared by immersing air-saturated room-
temperature solutions in liquid nitrogen. The samples were 1-3
mm thick and were placed in a temperature-controlled, liquid
helium or nitrogen cooled cryostats (Oxford Instruments). The
desired temperature (10-100 K) was achieved by proper helium
flow and/or automatically controlled heating. The optical
absorption spectra were measured on a Cary 5 (Varian)
spectrophotometer.
formation of this transient product correlates with the rate of
the decay of an electron absorption and therefore can be assigned
to the process of electron capture and formation of the
corresponding radical (BMIM•, Scheme 1).
The rate of this reaction was determined by monitoring the
decay kinetics of the electron absorption at 700 nm. This was
carried out in the presence of tert-butyl alcohol (1 M), which
•
scavenges the very reactive OH radicals produced on pulse
radiolysis of water (see Experimental Section for further details).
The bimolecular rate constant determined at different concentra-
tions of BMIM+ is 1.5 × 1010 M-1 s-1, slightly higher than
that obtained for the unsubstituted imidazolium cation but in
perfect agreement with the rate of reduction of the 2-meth-
ylimidazolium ring.20,21 The presence of different counterions
seems to have no effect on this process.
The samples mounted in a cryostat were irradiated with 4 µs
electron pulses from an ELU-6 linear accelerator. Details of
the pulse radiolysis system are given elsewhere.14,15
Results and Discussion
Redox Properties of Ionic Liquids. Three different salts
were the subject of our investigation. All of them contain the
1-butyl-3-methylimidazolium cation (BMIM+), but three dif-
In our recent work on the reactivity of N-substituted pyri-
dinium salts, we have found very fast reactions not only with
•
electrons, but also with acetone ketyl radicals (CH3)2 COH (k
) (0.7-1) × 1010 M-1 s-1).22 Complete reduction was achieved
in aqueous solutions containing 2-propanol (1 M) saturated with
N2O. The reaction of acetone ketyl radical with BMIM+ was
not, however, observed under these conditions. Previous work
of Bakac et al. indicated that the substituted imidazolium cations
do indeed react very slowly with acetone ketyl radicals (k )
104-106 M-1 s-1).23 This originates from the lower reducibility
of the imidazolium compared to pyridinium cations. The
ferent counterions were used: [PF6]-, [N(SO2CF3)2]-, and Br-.
All of these salts form transparent glasses on freezing at 77 K,
and therefore they can be used over a wide range of temperatures
for optical measurements.
-
reduction of these cations by eaq with close to the diffusion-
controlled rate completely masks this significant difference in
reducibility (for example, the difference in reduction potential
of 1-ethyl-3-methylimidazolium and N-butylpyridinium chlo-
rides in CH3CN is ca. 0.8 V 4).
On pulse radiolysis of pure ionic liquids under ambient
conditions, relatively strong absorption spectra of BMIM• were
also observed, as can be seen in Figure 2A,B. This indicates
that electrons ejected from the solvent molecules can be
effectively trapped by BMIM+ cations with the rather high yield
of G ) 3.5 (yield of radicals generated per 100 eV of energy
absorbed).8 Upon radiolysis of the salts, electrons can be ejected
from the BMIM+ cations or the counterions. Removal of an
The redox properties of salts depend both on the cation and
anion. The redox properties of anions are already widely
reported.12,16-19 Much less information is available on the redox
properties of BMIM+ cation, although some similarities to the
reactivity of protonated imidazole can be expected. Indeed, pulse
radiolysis of investigated salts in aqueous solution leads to the
formation of quite stable intermediate characterized by a
relatively strong transient absorption spectrum (see Figure 1)
with a maximum at 320 nm and a shoulder around 400 nm, as
well as a weaker band at 250 nm. The extinction coefficient at
320 nm was estimated as ꢀ ) 3.2 × 103 M-1 cm-1. The

Ionic Liquids for Characterization of Radical Ions
J. Phys. Chem. A, Vol. 105, No. 40, 2001 9307
Figure 3. Electronic absorption spectra of the radical anion of TCQM
(A) and the radical cation of naphthalene (B) obtained after irradiation
of TCQM (0.1 M) and naphthalene (0.1 M), respectively, in glassy
BMIM+[N(SO2CF3)2]- at 77 K. The spectrum before irradiation was
subtracted. The samples were 1 mm thick and received a radiation dose
of 2.5 kGy.
Figure 2. Transient absorption spectra measured 200 ns after the
electron pulse irradiation of pure ionic liquids under ambient condi-
tions: (A) BMIM+[PF6]-; (B) BMIM+[N(SO2CF3)2]-; (C) BMIM+Br-.
The samples were 1 cm thick and received a radiation dose of 60 Gy.
electron from BMIM+ cation results in formation of the radical
dication BMIM•2+. A similar radical species generated from the
imidazolium cation was previously characterized by Neta and
Schuler by means of ESR spectroscopy24 and by Parsons et al.
using UV-vis optical spectroscopy.25 Ionization of anions leads
to strongly oxidizing radical species which can further form
in glasses at 77 K. The BMIM+ cations do not effectively
compete in the capture of electrons in the matrix as a result of
their low reducibility.
The spectrum of the radical anion of TCQM in glassy
BMIM+[N(SO2CF3)2]- presented in Figure 3A illustrates the
excellent properties of this matrix, such as high transparency
and spectral resolution.8 At lower solute concentrations, the
radiation yield of radical anions decreases, but for a typical
concentration used in the glassy matrix techniques, i.e., 0.01-
0.02 M, the radiation yield of G ) 1 is still reasonably high.
Under the conditions of radical ion generation by radiolytic
methods, low concentrations of solute molecules of the order
of 0.01-0.1 M ensure that the direct effect of radiation on the
solute molecules can be ignored. This is one of the major
requirements for successful use of radiolytic methods for
generating radical ions. At ambient temperatures the bimolecular
rate constant for electron capture by, for example, 2-methylan-
thraquinone (2-MAQ) (0.01 M) in the very viscous BMIM+Br-
is 1.2 × 108 M-1 s-1, and the lifetime of the radical anion
formed is τ ) 8 × 10-5 s. Under cryogenic conditions the
radical anion is stable and only thermal annealing resulting in
softening of the matrix can trigger its decay via bimolecular
reactions.
Besides radical anions, radical cations can also be generated
in glassy ionic liquids. Naphthalene can be used as an example
(see Figure 3B). Under ambient conditions the radical cation
of naphthalene is formed in a very fast process and the
subsequent process of dimerization is also fast under these
conditions (k ) 1.9 × 108 M-1 s-1). Thus, dimerization can
effectively compete with the decay of radical ions due to
recombination processes (τ ) 5 × 10-5 s in BMIM+[N(SO2-
CF3)2]-). Under cryogenic conditions the dimerization can be
observed only on annealing of the glass.
•-
•-
appropriate radical anions such as Br2 and [N(SO2CF3)2]2
(by analogy to the dithiocyanate radical anion (SCN)2•-, which
displays an absorption band around 500 nm) with a rate constant
close to that of diffusion control.26,27 These radical anions are
•-
still strong oxidants (for example, the redox potential of Br2
/
2Br- is 1.6 V vs NHE, or (SCN)2•-/2SCN- is 1.31 V vs NHE)12
and are capable of oxidizing the imidazolium cation. However,
•-
the reaction of Br2 with protonated imidazole is very slow.
•-
We have also found that the BMIM+ cation reacts with Br2
very slowly. The second-order rate constant k ) 2.5 × 105 M-1
s-1 for this reaction was determined by pulse radiolysis
experiments of N2O-saturated aqueous solutions of BMIM+Br-
(0.2 M). This reaction does not take place in viscous pure
BMIM+Br-. The radical anion Br2 can be easily identified
•-
by its strong absorption at 360 nm (see Figure 2C), and in the
pure ionic liquid this decays by a second-order reaction with a
low rate constant, k ) 2.5 × 103 M-1 s-1. Interestingly, the
•-
radical anion Br2 is also formed in the glassy matrix of pure
BMIM+Br- at 77 K, which indicates that bimolecular reactions
are still possible in this rigid environment.
Ionic Liquids as Solvents. The high yield of electrons and
holes generated by radiolysis of pure ionic liquids and further
trapped by BMIM+ or anions indicates that these solvents should
be excellent media for the generation of radical ions. Moreover,
as the ionic liquids show a tendency for supercooling, resulting
in formation of more viscous liquids and finally transparent
glasses without crystalization even on slow cooling, they can
be used for the generation and spectroscopic characterization
of solute radical ions.
On pulse radiolysis of ionic liquids containing solute mol-
ecules of high electron affinity, such as tetracyanoethene
(TCNE) or tetracyanoquinodimethane (TCQM) at concentration
of 0.1-1 M, most of the electrons generated are effectively
trapped by solute molecules both under ambient conditions and
In Figure 4 the yield of formation of naphthalene radical
cation is presented as a function of solute concentration. It is
evident from this plot that the yield of radical cations formed
in glassy matrixes is lower than the yield of radical anions and
reaches a saturation value of G ≈ 0.9 at concentrations above
0.1 M. The addition of electron scavengers such as TCNE (1

9308 J. Phys. Chem. A, Vol. 105, No. 40, 2001
Marcinek et al.
SCHEME 2: Species Identified and Reactivity Observed
in Glassy Ionic Liquids on Ionization of MNAH or
Reduction of MNA+
Figure 4. Radiation yield G of naphthalene radical cation formation
in a glassy BMIM+[PF6]- at 77 K as a function of naphthalene
concentration.
Figure 5. Electronic absorption spectrum of the radical anion of
2-MAQ and the radical cation of naphthalene obtained after irradiation
of a mixture of 2-MAQ (0.01 M) and naphthalene (0.01 M) in glassy
BMIM+[PF6]- at 77 K. The spectrum before irradiation was subtracted.
The sample was 2.5 mm thick and received a radiation dose of 2 kGy.
Figure 6. Electronic absorption spectra obtained on electron impact
ionization of MNAH (saturated solution) in glassy BMIM+[PF6]- at
77 K (spectrum a). The sample was 1.5 mm thick and received a
radiation dose of 7.2 kGy. Spectra b and c show the change of spectrum
a on thermal relaxation of the sample for 1 h at 150 and 200 K,
respectively. Inset: spectrum d obtained on irradiation of MNA+
(saturated solution). The spectrum before irradiation was subtracted.
The sample was 2.5 mm thick and received a radiation dose of 2.6
kGy
M) does not increase this yield. Some of the holes are effectively
captured by the matrix and do not reach the solute molecules.
In the case of the very viscous BMIM+Br-, the radical cation
of naphthalene was not observed at ambient temperatures (even
at a naphthalene concentration of up to 1 M), although the
strongly oxidizing Br2•- was effectively formed and long-lived.
In the other two salts good-quality spectra of the naphthalene
radical cation were observed under cryogenic conditions (see
Figure 3B). An important feature is the observation that, even
at low concentrations of solute (10-3 M), the spectrum of the
radical cation remains of good quality without noticeable
interference from other byproducts of radiolysis.
Finally, in Figure 5 we present a spectrum of the radical cation
of naphthalene and the radical anion of 2-MAQ generated in
the same glassy sample containing a mixture of both these
solutes in BMIM+[PF6]- at 77 K. At first sight the possibility
of simultaneous generation of radical anions and cations in the
same matrix seems a disadvantage, since in some cases both
these species can be generated from the same solute molecules.
This, however, can be easily overcome by addition of appropri-
ate electron or hole scavengers. Undoubtedly the advantage of
this feature is that radical anions and radical cations can be
characterized in the same environment. Moreover, some inter-
esting photochemical processes, which require the presence of
both types of radical ions, can be conveniently investigated.
Ionic Liquids as Media for the Characterization of
Previously Unobserved Radical Cations. The group of ionic
liquids investigated here have already allowed spectroscopic
characterization of radicals generated from 1-methyl-1,4-dihy-
dronicotinamide (MNAH), a structural analogue of NADH. In
our previous studies, we were unable to characterize the radical
cations generated from MNAH (see Scheme 2) owing to its
poor solubility in conventional organic solvents, which form
transparent glasses on freezing.22,28-30 This was, however,
possible in BMIM+[PF6]-. For example, on ionization of
MNAH embedded in this matrix at 77 K the radical cation in
keto form is observed (keto-MNA•+, λmax ) 525 and 565 nm;
see Figure 6, spectrum a). On annealing, this species deproto-
nates and a neutral radical MNA• (λmax ) 410 nm) is formed.
Simultaneously the radical cation in enol form can be formed
(enol-MNA•+, λmax ) 435 and 465 nm), most likely via
bimolecular reactions (Figure 6b,c).22 As these solvents can be
used as media for generation of species by oxidation or
reduction, the neutral radical MNA• was also characterized in
the same matrix on reduction of the salt MNA+Cl- (Figure 6d).
We believe that an appropriate matrix for the generation of
ionization products directly from NADH could also be selected
from the large variety of ionic liquids now available. Moreover,
since the very viscous environment of glassy ionic liquids
inhibits the recombination processes, it seems realistic to

Ionic Liquids for Characterization of Radical Ions
J. Phys. Chem. A, Vol. 105, No. 40, 2001 9309
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Conclusions
The ionic liquids used here are excellent media for the
generation and spectral characterization of radical ions of solute
molecules. We have shown that radical ions of interest can be
generated under cryogenic conditions and their reactivity can
be monitored on thermal annealing of the solvent up to ambient
temperatures. Most notably, the possibility exists for the
generation of both radical cations and anions in the same
experiment. Furthermore, we have been able to identify previ-
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radiolysis of MNAH. This novel application of ionic liquids
could be of very wide use especially in characterizing biologi-
cally important compounds with low solubility in conventional
organic matrixes. As the properties of ionic liquids can be
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Acknowledgment. This work was supported by grants from
the State Committee for Scientific Research (3/T09A/037/17
and 3/T09A/122/18). C.M.G. thanks the Royal Society of
Edinburgh and EPSRC (Grant GR/M56852) for funding and
3M for the gift of Li[N(SO2CF3)2].
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