Highly luminescent and color-tunable salicylate ionic liquids

Article abstract of DOI:10.1002/chem.201301363

High quantum yields of up to 40.5 % can be achieved in salicylate-bearing ionic liquids. A range of these ionic liquids have been synthesized and their photoluminescent properties studied in detail. The differences noted can be related back to the structure of the ionic liquid cation and possible interionic interactions. It is found that shifts of emission, particularly in the pyridinium-based ionic liquids, can be related to cation-anion pairing interactions. Facile and controlled emission color mixing is demonstrated through combining different ILs, with emission colors ranging from blue to yellow. Brilliant liquids: Highly photoluminescent salicylate ionic liquids (ILs) were synthesized and characterized. Quantum efficiencies up to 40.5 % were achieved. The optical properties are related to cation-anion pairing interactions. Facile controlled emission color mixing was demonstrated by combining different ILs, with emission colors ranging from blue to yellow (see figure).

Full text of DOI:10.1002/chem.201301363

DOI: 10.1002/chem.201301363
Full Paper
&
Ionic Liquids
Highly Luminescent and Color-Tunable Salicylate Ionic Liquids
Paul S. Campbell,[a] Mei Yang,[a] Demian Pitz,[a] Joanna Cybinska,[a, b] and Anja-
Verena Mudring*[a, c]
In memory of John D. Corbett
Abstract: High quantum yields of up to 40.5% can be ach-
ieved in salicylate-bearing ionic liquids. A range of these
ionic liquids have been synthesized and their photolumines-
cent properties studied in detail. The differences noted can
be related back to the structure of the ionic liquid cation
and possible interionic interactions. It is found that shifts of
emission, particularly in the pyridinium-based ionic liquids,
can be related to cation–anion pairing interactions. Facile
and controlled emission color mixing is demonstrated
through combining different ILs, with emission colors rang-
ing from blue to yellow.
Introduction
uted in part to their unique and interesting physicochemical
properties such as low volatility, large liquids range, wide elec-
trochemical window, nanostructuration, and so on. Further-
more, ILs are inherently adaptable media, with countless differ-
ent anion–cation combinations available and facile modifica-
tions possible. They are therefore described as “designer sol-
vents”, and can indeed be tailored to specific tasks. As such,
they have been used in diverse fields from catalysis[16,19–24] and
separations[25–27] to electrochemistry[28,29] and nanosynthe-
sis.[30–39] An ever-increasing number of fields are witnessing the
introduction of ionic liquids.[40] For example, ionic liquids have
recently been used in the production of new luminescent ma-
terials. Introduction of transition metal or lanthanide ions is
seen to lead to highly luminescent imidazolium-based ILs,[41–46]
whereas the aptitude of ILs as media for nanochemistry has
enabled their application in the synthesis of lanthanide-based
nanophosphors.[47–54] Following on from this, our group has re-
cently reported the one-pot synthesis of luminescent polymer–
nanoparticle composites, based on a polymerizable IL.[55]
With decreasing availability and increasing cost of lanthanides,
alternative luminescent materials are highly sought after. In
comparison, luminescent organic materials are economically
very attractive; they can be synthesized from readily available
materials, and in contrast to most lanthanides, their electronic
transitions may be formally allowed, thus strong absorption
and high emission intensities can be easily envisaged. For ex-
ample, organic light-emitting diodes OLEDs currently represent
a rapidly growing area of interest based on electroluminescent
organic molecules,[1–8] with many new energy efficient and flex-
ible displays being based on this innovative technology. Fur-
thermore, organic dyes are currently used in dye-lasers,[9] and
may be applied as sensitizers to enhance lanthanide lumines-
cence.[10–12] One great set-back with organic fluorophores is
their photodegradation, meaning that after a certain time, the
dyes must be replaced.[13] However, it has already been report-
ed that incorporating such dyes into ionic liquids can enhance
their photostability.[14,15]
ILs are often based on aromatic moieties such as imidazoli-
um or pyridinium, and therefore exhibit intrinsic luminescence
thanks to low-lying p–p* transitions, as has been previously
studied and reported.[56–58] Unfortunately, the luminescence of
these common ILs is weak, inefficient and unexploitable. How-
ever, thanks to their aforementioned adaptability, ionic liquids
specifically designed to exhibit strong luminescence have re-
cently been brought into the fold. For example, Deng and co-
workers has reported turquoise-emitting quinolizinium salts,[59]
as well as functionalized imidazolium moieties,[60] whereas Boy-
dston et al. described property-tunable fluorophores based on
a benzobisimidazolium-fused ring system, displaying high
quantum yields in solution.[61,62] Furthermore, a paper by
Huang et al. describes a strongly photoluminescent ionic liquid
based on a large cationic dendrimer.[63] Herein, we report the
facile synthesis and optical characterization of highly lumines-
cent ionic liquids based on the salicylate anion, varying the
The field of ionic liquids (ILs) has experienced an explosion
in research during the last two decades.[16–18] This can be attrib-
[a] Dr. P. S. Campbell, M. Yang, D. Pitz, Dr. J. Cybinska, Prof. A.-V. Mudring
Anorganische Chemie III, Materials Engineering and Characterisation
Fakultꢀt fꢁr Chemie und Biochemie, Ruhr-Universitꢀt Bochum
Universitꢀtsstrasse 150, 44801 Bochum (Germany)
Fax: (+49)2343214951
E-mail: anja.mudring@rub.de
[b] Dr. J. Cybinska
Faculty of Chemistry, Faculty of Chemistry
University of Wrocław, Joliot-Curie 14, 50383 Wrocław (Poland)
[c] Prof. A.-V. Mudring
Materials Science & Engineering, Iowa State University
and Critical Materials Institute, Ames Laboratory, Ames, IA 50011 (USA)
E-mail: mudring@iastate.edu
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201301363.
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employed cation to control the resultant luminescent proper-
ties.
[Chol]). All measured values can be found in the Experimental
Section.
Results and Discussion
Thermal and structural characterization
In this study, a series of ionic liquids was synthesized bearing
the salicylate anion through a standard anion methathesis re-
action by using sodium salicylate and the corresponding
cation halides. The salicylate (2-hydroxybenzoate) anion, [Sal],
consists of a benzene ring bearing ortho hydroxyl and carbox-
ylate groups as shown in Scheme 1. Commonly used ionic
liquid cations were chosen, and could be divided into three
Representative differential scanning calorimetry (DSC) thermo-
grams of the as-synthesized ionic liquids are depicted in
Figure 1, with important phase transitions for all ILs tabulated
in Table 1. All other thermograms can be found Figure S1 in
the Supporting Information. It can be seen that all investigated
compounds could be classed as ionic liquids, with a liquid
phase observed below 1008C. With the exception of [C12C1Im],
all cations gave room-temperature ionic liquids, pre-
senting low glass transition temperatures (Tg) in the
DSC thermograms. During the DSC measurement,
the IL [Chol][Sal] also underwent a cool crystallization
immediately prior to melting, indicated by the ap-
pearance of consecutive broad exo- and endothermic
transitions at 8.2 and 37.78C, during the second and
subsequent heating cycles. This crystallization was
confirmed by using polarized optical microscopy
(POM) analysis, in which crystal growth was observed
at 88C upon heating. In all cases the butyl-bearing
ILs presented lower Tg values than their ethyl-bearing
counterparts. This is a common result in ILs, because
Scheme 1. Structure and abbreviations of the anion and cations used in this study.
groups based on their structures as shown in Scheme 1: 1) Ali-
phatic: choline [Chol], trihexyltetradecylphosphonium [P6,6,6,14];
2) Imidazolium: 1-ethyl-3-methylimidazolium [C2C1Im], 1-butyl-
3-methylimidazolium [C4C1Im], and 1-dodecyl-3-methylimidazo-
lium [C12C1Im]; and 3) Pyridinium: 1-ethyl-4-methylpyridinium
[C2C1Py], 1-butyl-4-methylpyridinium [C4C1Py]. Thus, it was pos-
sible to gauge the effect of varying degrees of aromaticity in
the cation on the subsequent properties as well as the effect
of the length of alkyl substituents (e.g., ethyl vs. butyl). [Chol]-
[Sal], [P6,6,6,14][Sal], [C2C1Im][Sal] and [C4C1Im][Sal] were isolated
as free-flowing pale-yellow-tinted clear liquids. [C12C1Im][Sal]
was obtained as a white solid. [C2C1Py][Sal] and [C4C1Py][Sal]
were isolated as orange/brown viscous and free-flowing liq-
uids, respectively.
Table 1. Important phase transitions of salicylate ionic liquids.
Entry Cation
Tg (heating) [8C] Tg (cooling) [8C]
Tm (heating) [8C]
(DCp [Jgꢀ1 Kꢀ1]) (DCp [Jgꢀ1 Kꢀ1]) (DH [Jgꢀ1])
1
2
3
4
5
6
7
[Chol][a]
[P6,6,6,14
[C2C1Im]
[C4C1Im]
[C12C1Im][b]
[C2C1Py]
[C4C1Py]
ꢀ54.0 (1.39)
ꢀ79.6 (1.05)
ꢀ47.7 (0.80)
ꢀ54.1 (0.84)
–
ꢀ47.0 (1.12)
ꢀ67.4 (1.00)
ꢀ39.8 (0.72)
ꢀ49.9 (0.75)
–
37.7 (93.2)
–
–
–
48.0 (116.7)
–
–
]
ꢀ39.6 (0.66)
ꢀ27.9 (0.63)
ꢀ67.7 (1.13)
ꢀ60.5 (0.73)
[a] Cold crystallization on heating onset 8.28C (ꢀ97.6 Jgꢀ1). [b] Super-
cooled SmC liquid crystal 50–19.58C.
Impurities in the ionic liquids under study cannot be ignored
as they may also play a decisive role in the luminescent prop-
erties; for example, they may be photoactive, or manifest
a quenching effect. The purities were therefore thoroughly as-
sessed through elemental analysis, Karl Fischer coulometry and
both positive and negative ion chromatography to assess
halide and sodium levels. Elemental analyses matched well
with expected values, and Karl Fischer measurements showed
that although water content in the crude products lay around
2 wt.%, this could be considerably reduced (<600 ppm) by
simply drying for three days under a dynamic vacuum at 508C.
Inorganic salt levels could be reduced by precipitation in di-
chloromethane and filtration over Celite. The resultant halide
content (bromide or chloride, depending on the starting salt)
was found to range from 0.080 ([C2C1Py]) to 1.3 wt.%
([C2C1Im]), whereas in each case the sodium content was
found to be very low (0.062 for [C2C1Py] to 0.60 wt.% for
the effect of increasing the entropic barrier to crystallization by
increasing the length of the alkyl chain tends to outweigh the
effect of increasing molecular weight until an intermediate
chain length around 8, after which the temperature of melting
transitions starts to increase, owing to increasing van der
Waals interactions between the long alkyl chains.[17]
As could be expected, [C12C1Im] exhibited the most complex
thermal behavior (thermograms for the second cooling and
heating runs are given in Figure 2a.) Upon heating, a broad
exothermic peak with onset at 488C indicates a transition from
the solid-to-liquid state. However, upon cooling over the same
range, two distinctly separate phase transition events were ob-
served. Firstly, a small exothermic peak with onset at 508C is
noted. This is followed by a large exothermic peak with onset
at 198C, presenting several local maxima. The latter can be as-
signed as crystallization, with the several maxima due to the
possibility of several solid–solid phase transitions. The phase
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the formation of mesophases.[64,65] POM and small-angle X-ray
scattering (SAXS) measurements, indeed confirm the occur-
rence of a supercooled ionic liquid crystal (Figure 2b–d). The
oily streak texture observed in the POM image taken at 298C
upon cooling is typical of a smectic phase. Multiple reflections
are visible in the X-ray scattering patterns at 308C upon heat-
ing recorded on the small- and wide-angle range, due to the
different lattice planes in the crystalline material. Upon cooling,
only one reflection can be observed in the small-angle range,
corresponding to an interlayer distance in the mesophase of
27.65 ꢁ, whereas only a broad reflection of amorphous charac-
ter is observed in the wide-angle range (note that the appar-
ent doubling of peaks is due to the SAXS experimental set-
up.). According to the SAXS data, at the same temperature in
the crystal phase, the interlayer distance is larger at 28.54 ꢁ.
This observation could be explained by the formation of
a smectic C mesophase, in which the interlayer distance short-
ens in the liquid crystal (LC) phase due to a tilt of the alkyl
chains away from the normal of the layer plane, as illustrated
in Scheme 2.
Scheme 2. Illustration of different d-spacings in the crystalline phase and the
smectic C mesophase.
Structural and environmental properties are well-known to
play an important role in the resultant luminescence of organic
molecules.[66] Intermolecular interactions have also been re-
ported to strongly influence luminescent properties. For exam-
ple, the fluorescence of cyanine molecules has previously been
shown to be highly dependent on the intermolecular arrange-
ment, with p-stacking leading to blue- or redshifted emission
depending on the orientation.[67] Furthermore, certain mole-
cules are known to form excited dimers (excimers) when excit-
ed, which have a redshifted emission compared with the mo-
nomer itself. Such behavior is commonly observed in fused
polyaromatics such as naphthalene or pyrene.[68] Another pos-
sibility is non-radiative excitation energy-transfer between
neighboring molecules. This requires Coulombic interactions
and/or intermolecular orbital overlap as well as short distances
between molecules.[66] Herein, the effect of intermolecular/in-
terionic interactions will be observed, because although the
emitting moiety in this study, the salicylate anion, is not
changed, its chemical environment varies with each different
cation used. The effects on photoluminescence will be dis-
cussed later in detail.
Figure 1. Example DSC thermograms of as-synthesized ionic liquids (top
[P6,6,6,14][Sal], center [Chol][Sal]) measured at heating/cooling rate of
10 Kminꢀ1. Only second cycles are shown; subsequent cycles were identical
in each case. Bottom: Polarizing optical micrograph of [Chol][Sal] crystals
grown at 88C on heating from ꢀ408C at 10 Kminꢀ1
.
One inherent property of ionic liquids is their high entropic
barrier to crystallization. Indeed, for this reason, crystals of suf-
ficient quality for X-ray analysis could not be obtained for the
synthesized compounds. Despite this, molecular dynamics sim-
present from 50–198C may be assigned to a supercooled
liquid-crystalline phase. Indeed, the [C12C1Im] cation has al-
ready been well-studied and has been found to be prone to
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probably proceeding through a combination of Cou-
lombic attraction and p-stacking interactions. Inter-
estingly, a cross-peak is also observed correlating the
protons at the terminal position of the ethyl group
to the d-position of the salicylate ion in the case of
[C2C1Py][Sal]. Indeed, it has previously been reported
that in the case of imidazolium ILs, the short length
of the ethyl chain does not allow for the segregation
of polar and non-polar domains,[69] which may ac-
count for the apparent proximity of the alkyl chain to
the anion in this case. The fact that no interionic
cross-peaks are observed in the spectra of the other
ILs does not necessarily mean that cation–anion in-
teractions do not exist. However, this result does sug-
gest that a notably stronger association exists in the
pyridinium-based ionic liquids. This may be due in
part to the matching symmetry of anion and cation
(6-membered aromatic rings), allowing effective orbi-
tal overlap.
Figure 2. a) DSC thermogram of [C12C1Im][Sal] recorded with a heating/cooling rate
10 Kminꢀ1; b) Polarizing optical micrograph of [C12C1Im][Sal] taken at 298C upon cooling
at 10 Kminꢀ1. X-ray scattering patterns in both small- and wide-angle range of [C12C1Im]-
[Sal] recorded at c) 308C upon heating, and d) 308C upon cooling.
Optical characterization
The ionic liquids produced were found to display
ulations, coarse-grain modelling and X-ray analysis have previ-
ously shown that a significant structuration exists on the nano-
scale even in the purely liquid state of ionic liquids.[69–71] There-
in, non-polar alkyl chains would group together in micellar-
type aggregates, segregated by ionic channels resulting from
the anion and cation head-group. 2D NMR spectroscopic tech-
niques could be employed utilizing the nuclear Overhauser
effect (NOE) to detect through-space interionic interactions.
Such techniques have previously been used to determine in-
teractions between IL solvents and organic substrates.[22] Spe-
cifically, rotating frame NOE spectroscopy (ROESY) is suitable
for obtaining positive NOEs despite the long rotational correla-
tion times possible. In 1H-1H ROESY techniques, correlations are
established from through-space cross relaxation between nu-
clear spins during the mixing period. The selective irradiation
of a proton group affects the intensities of integrals of all-
proton groups that are spatially close (max 4–5 ꢁ) but not nec-
essarily connected by chemical bonds.[72] The ROE signal
strength varies with the inverse sixth power of the correspond-
ing interatomic distance, I/1/r6. In the liquid state, this rela-
tionship is possible if the intermolecular association is close
enough to turn the intermolecular relaxation into “intramolec-
ular” within the ion pair or ion–molecule association.[73] To
obtain well-resolved spectra, solutions of the ILs were prepared
by dissolving 5 mg in 0.5 mL [D6]DMSO, which was used to
lock the NMR probe. For most ILs measured in this way, only
diagonal and intra-ionic cross-peaks were observed, due to the
dissolution in [D6]DMSO. However, in the case of both pyridini-
um-based ILs, interionic interactions could also be observed.
The 1H-1H ROESY NMR spectra and atomic assignment of
[C2C1Py][Sal] and [C4C1Py][Sal] are given in Figure 3. In spite of
the dissolution, correlation between positions on the separate
aromatic rings can be clearly detected (circled in Figure 3. This
presents clear evidence of a close anion–cation association,
varying colors depending on the cation. Whereas those based
on aliphatic and imidazolium ions were isolated as clear liquids
with a slight yellow tint, orange/brown colors were observed
for pyridinium-based ILs. These colors correspond to neither
the isolated cations nor anions (based on the observed colors
of the respective sodium or halide salts), thus must be due to
the association of ions. To investigate this, UV/Visible absorp-
tion spectra were first acquired for ethanolic solutions of each
salt prepared and compared to that of the sodium salt. For
clarity, example spectra are given in Figure 4, representative of
each for the three groups of cations under study compared to
the spectrum of sodium salicylate. Other spectra are highly
similar to their respective group and are given in Figure S2
(the Supporting Information), alongside spectra of the cation
halides in Figure S3 (the Supporting Information), for compari-
son. The UV/Visible spectrum of the sodium salicylate salt
shows three main bands, centered at 203 (e=17500), 228 (e=
3400), and 297 nm (e=2100 Lmolꢀ1 cmꢀ1), which can be attrib-
uted to p!p* and n!p* electronic transitions in the salicy-
late ion. When coupled with the aliphatic [P6,6,6,14] or [Chol] cat-
ions, the absorption spectra remain unchanged. However,
when coupled with any of the imidazolium cations, increased
absorption is noted from 200 to 230 nm, as a result of the imi-
dazolium p!p* transitions. More strikingly, the absorption of
[C2C1Py][Sal] is notably stronger from 215 nm upwards, with
a clear additional band centered at 260 nm. Furthermore, in
this case, a clear absorption tail is apparent far into the visible
range of the spectrum, hence giving an orange/brown color to
this ionic liquid. This absorption can be attributed to ion-pair
aggregates held together through Coulombic attraction and p-
stacking even in dilute solution, corroborated by the ROESY
NMR spectroscopic results. No evidence of ion-pairing could
be seen between the salicylate and imidazolium or aliphatic
cations.
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Figure 5, with corresponding data summarized in
Table 2. Excitation spectra are given in the Support-
ing Information. Emission maxima were taken as the
maxima of Gaussian distribution functions fitted to
the experimental data by using Origin software. Ex-
ample emission spectra of one IL from each cation
group are shown in Figure 5a. Despite the fact that
the same species (salicylate anion) is responsible for
emission in each case, stark differences can be seen
in the resultant spectra. Firstly, a hypsochromic shift
of emission was noted for the aliphatic cations com-
pared with the sodium salt; the emission maxima
were recorded at (399.4ꢁ0.2) and (405.3ꢁ0.2) nm
for [P6,6,6,14] and [Chol] (Figure 4b), respectively,
whereas the sodium salt emission maximum appears
at (422.5ꢁ0.2) nm. This can be rationalized in terms
of cation bulk. Compared with sodium, the ionic
liquid cations [Chol], and especially [P6,6,6,14], present
a considerable bulk and therefore inhibit the close
proximity between salicylate ions needed for the
energy-lowering p–p interactions. Because [P6,6,6,14] is
considerably bulkier than [Chol], the effect for this
cation is more pronounced.
Similarly, all ILs constituted of imidazolium cations
also displayed a hypsochromic shift compared with
the sodium salt, with emission maxima recorded at
(405.1ꢁ0.2), (404.3ꢁ0.2), and (406.8ꢁ0.4) nm for
[C2C1Im], [C4C1Im], and [C12C1Im], respectively (Fig-
ure 4c). Such values can no longer be attributed
solely to cation bulk, considering the fact that the
largest of the group presents counterintuitively the
longest wavelength emission. As previously stated,
pure imidazolium ionic liquids present a certain struc-
ture, segregated into non-polar domains, comprising
the aggregation of alkyl chains, and polar channels,
in which anion and cation head-groups are found in
close proximity. This would explain why regardless of
Figure 3. 1H-1H ROESY NMR spectrum of [C2C1Py][Sal] (top) and [C4C1Py][Sal] (bottom) re-
corded in [D6]DMSO. For clarity, interionic correlation cross-peaks are circled.
the length of the alkyl chain, the distance between neighbor-
ing salicylate anions and thus maximum emission wavelength
do not vary greatly. Furthermore, as the PL measurements
were conducted at room temperature (258C), at which accord-
ing to DSC and SAXS results [C12C1Im][Sal] exists as a crystalline
solid, closer proximity between anions can be expected, pro-
Table 2. Photoluminescence data of salicylate ILs measured.
Entry
Cation
lex
Maximum
lem [nm]
Quantum yield
(error) [%]
[nm]
1
2
3
4
5
[Chol]
[P6,6,6,14
[C2C1Im]
[C4C1Im]
[C12C1Im] (S)[a]
[C12C1Im] (LC)[b]
[C2C1Py]
340
340
340
340
340
340
420
420
405.3ꢁ0.2
399.4ꢁ0.2
405.1ꢁ0.2
404.3ꢁ0.2
406.8ꢁ0.4
403.1ꢁ0.2
599.2ꢁ0.4
573.2ꢁ0.6
40.5 (4.9)
8.1 (0.9)
13.6 (0.2)
23.3 (2.90)
35.9 (3.8)
not determined
3.95 (0.43)
2.67 (0.39)
]
Figure 4. UV/Vis absorption spectra of the salicylate ILs in ethanol solution
compared to that of the sodium salt.
6
7
Photoluminescence (PL) spectra of the pure ionic liquids
were recorded at room temperature (258C) and compared to
that of pure sodium salicylate. Normalized spectra are given in
[C4C1Py]
[a] (S): Crystalline solid. [b] (LC): Liquid-crystal phase.
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Quantum yields of the ionic liquids were measured
at 258C and results are given in Table 2. For most ILs,
relatively high quantum yields were recorded, the
highest being attributed to choline salicylate with
(40ꢁ5)%. It is highly interesting that these fluid sys-
tems, based on organic molecules whose high-
energy vibrations could easily quench emission, dis-
play quantum yields of the same order of magnitude
as the solid sodium salt. Pyridinium-based ionic liq-
uids unfortunately suffer from relatively poor emis-
sion intensities and also as seen, poor quantum
yields. This may be due to strong reabsorption of
emitted light by these colored compounds, as well as
the relative ease of quenching longer wavelength
emission through vibrational relaxation. The varying
efficiencies of the different ILs may also in part be re-
lated to the impurities. For example, the presence of
bromide may increase the photoluminescence
quenching through the external heavy atom effect,
whereby relaxation through spin-orbit coupling in-
creases with the fourth power of the atomic
Figure 5. Comparison of the normalized emission spectra of salicylate ILs. lex: excitation
wavelength; (LC): denotes that the sample was measured in the liquid-crystalline state.
voking the longer wavelength emission observed. For compari-
son, the emission spectrum was also recorded for the super-
cooled liquid-crystalline compound at room temperature. In
this state the emission maximum is shifted to (403.1ꢁ0.2) nm,
corresponding to increased disorder, as expected in the liquid-
crystalline state compared with the perfectly ordered crystal-
line state. Again, no evidence for strong imidazolium-salicylate
ion-pairs created through strong p–p and Coulombic interac-
tions is observed. This is likely due to the mismatching symme-
try (6-membered vs. 5-membered aromatic ring).
number.[66] This may explain the relatively low quantum yield
of the [C2C1Im] system in which the bromide precursor was
employed, compared with the other imidazolium systems syn-
thesized from the respective chloride salts, and in which a rela-
tively high amount of bromide remained as an impurity. Fur-
thermore, the small quantities of water may affect the efficien-
cy, because water-induced fluorescence quenching has been
reported, occurring through reorganization of hydrogen-bond-
ing interactions in the excited state.[74] Molecular oxygen is
also known to play a role in fluorescence quenching through
excitation into low-lying singlet states.[66] For this reason the
ILs were thoroughly dried and stored under inert atmosphere.
For lighting applications, white emitters are highly sought
after. In modern white LEDs, white light is achieved through
the combination of blue and yellow light; a blue-emitting
semiconductor (e.g., InGaN) is coated with a blue-to-yellow
light-converting phosphor material (e.g., YAG:Ce3+). In our
series of synthesized ILs, we observed both UV-blue and blue-
yellow fluorescence. We could therefore transpose the afore-
mentioned principle of white emission to our materials, by
mixing a UV-blue fluorophore (e.g., [Chol][Sal]) with a yellow-
blue fluorophore (e.g., [C4C1Py][Sal]), and measuring the emis-
sion of the resultant mixtures. Normalized emission spectra for
different mixtures are given in Figure 6 (left), in which x de-
notes the molar fraction of [C4C1Py][Sal]. From these spectra it
was possible to calculate the color coordinates of each mixture
in the standard Commission Internationale de l’Eclairage (CIE)
color space,[75] as shown in Figure 6 (center). Photographs of
the mixed ILs in quartz tubes under 366 nm excitation are also
given in Figure 6 (right). It can clearly be seen that by this
method, color mixing was successful by a simple mixture of
the ILs. The color coordinates are found on a pseudo-straight
line between the deep-blue emission of pure [Chol][Sal] (x=0)
and the yellow emission of pure [C4C1Py][Sal] (x=1). Amongst
the samples studied, it can be seen that the sample giving the
Interestingly, in the case of the pyridinium ionic liquids in
this study, [C4C1Py][Sal] and [C2C1Py][Sal], both excitation and
emission maxima experience large bathochromic shifts, effec-
tively converting blue to yellow light. In Figure 4a, it can clear-
ly be seen that the emission band of [C2C1Py][Sal] displays a dif-
ferent vibrational structure to [Na][Sal]. This, along with the
shifted excitation, provides a clear indication that the salicylate
anion is not alone responsible for the observed fluorescence.
Furthermore, changing the alkyl chain length has a significant
effect on emission maximum, falling at (573.2ꢁ0.6) and
(599.2ꢁ0.4) nm for [C4C1Py] and [C2C1Py], respectively (Fig-
ure 4d). This provides conclusive evidence for the involvement
of the pyridinium moiety, however, ILs based on pyridinium
coupled with optically inactive anions have been previously
shown to present emission maxima in the blue region.[58] Be-
cause no “normal” salicylate or pyridinium fluorescence is de-
tected, we can only attribute the observed photoluminescence
behavior to the strong ion-pairs, previously evidenced by
ROESY NMR spectroscopy. The strong ion-pair interactions
would effectively create moieties exhibiting a more extensive
p-system than in the individual ions. Increasing the number of
p-electrons in a molecule is well-known to provoke an increase
in absorption and emission wavelength, as can be seen in the
oligacene or oligorylene series.[66] Shortening the alkyl chain re-
duces steric hindrance, and allows the ions to bind more
strongly, and the emission is further redshifted.
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Figure 6. Normalized emission spectra of mixtures of the ILs [Chol][Sal] and [C4C1Py][Sal] excited at 340 nm (left). x is the molar fraction of [C4C1Py][Sal], that
is, x=(n[C4C1Py][Sal])/(n[Chol][Sal]+n[C4C1Py][Sal]). Calculated color coordinates in the CIE color space of each mixture excited at 340 nm (center). x=0 corre-
sponds to pure [Chol][Sal], whereas x=1 corresponds to pure [C4C1Py][Sal] excited at 410 nm. Photos of the IL mixtures in quartz tubes under 366 nm UV ex-
citation (right).
Hanau, Germany). Helium was used as the flux gas and the sam-
ples combusted at 11508C using a WO3 catalyst for conversion to
NOx, H2O, CO2 and SO2. NOx is reduced to N2 over Cu at 8508C. Re-
closest to pure white light is when x=0.67, that is, at a ratio
[C4C1Py]/[Chol] of 2:1.
sultant gases were detected individually by using thermal conduc-
tivity.
Conclusion
Karl Fischer titration: The titration was performed by using a 831
KF Karl Fischer Coulometer (Metrohm Ion Analysis, Herisau, Swit-
been synthesized and their thermal, structural, and photophys-
zerland). Water is titrated using iodine, generated directly in the
A range of ionic liquids based on the salicylate anion have
ical properties investigated. Firstly DSC confirmed all com-
pounds were synthesized as ionic liquids, with melting transi-
tions lower than 1008C. Salicylate coupled with 1,3-dodecylme-
thylimidazolium gave an IL exhibiting a supercooled ionic
liquid-crystal phase, identified as SmC by using POM and SAXS.
2D 1H-1H ROESY NMR spectroscopy identified strong ion-pair
interactions in the case of pyridinium-based cations. Photolu-
minescence studies of these different compounds showed that
the corresponding cation has a great influence on the emission
intensity and color, which could be related back to structural
aspects and interionic interactions. As a result, these com-
pounds represent high intensity soft luminescent materials,
presenting high quantum yields, and whose facile modification
could result in emission of a desired color.
electrolyte by electrochemical means. The end point is found volta-
metrically by applying an alternating current of constant strength
to a double Pt electrode. Liquid samples were injected directly into
the device, whereas for solid and viscous samples, the water con-
tent of known dichloromethane solutions were determined. The
water content of the dichloromethane was also found and the
water content of the sample could be calculated.
Ion chromatography: Ion exchange chromatography (IC) was con-
ducted by using two Metrohm 882 IC Compact Systems, one for
cations and one for anions using Metrohm IC Professional MF con-
ductivity detectors. Data were processed by using Metrohm MagIC
Net 2.4 Professional. A calibration for the quantitative analysis was
carried out by using a seven point calibration curve (0.5–10 ppm)
with FLUKA multi-ion standards and validated with a 10 ppm stan-
dard before measurements. The cation system is equipped with
a Metrohm Metrosep C4 pre-column and a Metrohm Metrosep C4
150ꢂ4 cation main column. HNO3 (1.7 mmolLꢀ1) and dipiccolinic
acid (0.7 mmolLꢀ1) in a mixture of acetonitrile (20 vol.%) and pure
water (80 vol.%) were used as the eluent for the cation column.
The anion system is equipped with a Metrohm Metrosep A Supp
4/5 pre-column, a Metrohm Metrosep A Supp 4 150ꢂ5 anion main
column and a Metrosep A trap column. Additionally, the anion is
equipped with a chemical suppressor (Metrohm MSM, regenerated
periodically with 100 mmolLꢀ1 H2SO4) and a carbon dioxide sup-
pressor (Metrohm MCS) for further signal improvement. As eluent
Experimental Section
Materials
Redistilled 1-methylimidazole (>99%, Sigma Aldrich), 4-methylpyri-
dine (98%, ABCR), quinoline (97%, ABCR), bromoethane (98%,
ABCR), 1-chlorobutane (98%, ABCR), 1-chlorododecance, (97%,
ABCR), choline chloride (99%, Acros Organics), trihexyltetradecyl-
phosphonium chloride (>95%, Io-li-tec), and sodium salicylate
(99%, ABCR) were used as received.
for the anion column, sodium carbonate (3.2 mmolLꢀ1
) and
sodium bicarbonate (1 mmolLꢀ1) were used in a mixture of aceto-
nitrile (20 vol.%) and pure water (80 vol.%).
Methods
Ionic liquid syntheses: For imidazolium and pyridinium ionic
liquids, standard quaternization reactions were undertaken to
obtain the necessary bromide (for [C2C1Im], [C2C1Py], and [C4C1Py])
or chloride (for [C4C1Im] and [C12C1Im] salts).[17] In each case, the
anion metathesis was undertaken according to the following pro-
cedure resulting in quantitative yields.
1
NMR spectroscopy: H, solution NMR data were collected at room
temperature on a Bruker AC 200 MHz spectrometer, and spectra
calibrated with respect to the residual solvent peak of [D6]DMSO.
1
2D H ROESY NMR experiments were carried out on a Bruker AC
250 MHz spectrometer.
Elemental analysis: Elemental analyses were performed using
Choline salicylate: Choline chloride (5.0 g, 35.8 mmol) and sodium
a Vario EL CHNOS analyzer (Elementar Analysensysteme GmbH,
salicylate (5.7 g, 35.8 mol) were stirred together in dry acetone
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4710
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(50 mL) for 72 h. The white precipitate that formed was filtered off
and the solvent removed in vacuo. Cold dry dichloromethane
(100 mL) was added to the resultant liquid, and the resulting pre-
cipitate filtered off over Celite. The solvent was removed and the
resulting liquid was dried in vacuo at 508C for a period of at least
72 h. The product was obtained as a yellow-tinted clear liquid in
quantitative yield. 1H NMR (200 MHz, [D6]DMSO): d=15.95 (s, 1H,
phenolic-OH), 7.74 (d, 1H, CH-CH=C-COOꢀ), 7.16 (t, 1H, CH-CH=C-
COOꢀ), 6.67 (d, 1H, CH-CH=C-OH), 6.64 (t, 1H, CH-CH=C-OH), 6.13
(s, 1H, CH2OH), 3.89 (t, 1H, N-CH2), 3.47 (t, 1H, CH2OH), 3.16 ppm
(s, 9H, N(CH3)3; elemental analysis calcd (%) for C12H19NO4: N 5.81;
C 59.73; H 7.94; found: N 5.64; C 56.34; H 8.09. Water content (Karl
Fischer): 222 ppm. Ion chromatography: Na+: 0.60 wt.%; Clꢀ:
0.28 wt.%.
Trihexyl(tetradecyl)phosphonium salicylate: 1H NMR (200 MHz,
[D6]DMSO): d=16.52 (s, 1H, H), 7.67 (d, 1H, CH-CH=C-COOꢀ), 6.59
(t, 1H, CH-CH=C-COOꢀ), 6.55 (d, 1H, CH-CH=C-OH), 2.21 (m, 8H, N-
CH2), 1.24 (m, 48H, CH2CH2CH2), 0.87 ppm (m, 12H, CH3); elemental
analysis calcd (%) for C39H73O3P: C 75.43; H 11.85; found: C 70.28;
H 11.01. Water content (Karl Fischer): 136 ppm. Ion chromatogra-
phy: Na+: 0.14 wt.%; Clꢀ: 0.67 wt.%.
CH=C-OH), 4.55 (t, 2H, N-CH2CH2), 2.59 (s, 3H, C-CH3), 1.86 (qn, 2H,
-CH2CH2CH2), 1.26 (sx, 2H, CH2CH2CH3), 0.88 ppm (t, 3H,
CH2CH2CH3); elemental analysis calcd (%) for C17H21NO3: N 4.87; C
71.1; H 7.37; found: N 4.23; C 66.89; H 6.74. Water content (Karl
Fischer): 566 ppm. Ion chromatography: Na+: 0.067 wt.%; Brꢀ:
0.27 wt.%.
Differential scanning calorimetry: DSC measurements were per-
formed on a computer-controlled Phoenix DSC 204 F1 thermal an-
alyzer (Netzsch, Selb, D) under a constant flow of argon gas. Sam-
ples of ꢂ5 mg were cold-sealed in aluminium crucibles. Experi-
mental data are displayed in such a way that exothermic peaks
occur at negative heat flow and endothermic peaks at positive
heat flow. DSC runs included heating and subsequent cooling at
108Cminꢀ1. Transition temperatures are defined as the onset of
the respective thermal processes.
Polarizing optical microscopy: POM images were acquired by
using an Axio Imager A1 microscope (Carl Zeiss MicroImaging
GmbH, Gçttingen, Germany) equipped with a hot stage, THMS600
(Linkam Scientific Instruments Ltd, Surrey, UK), and a Linkam TMS
94 (Linkam Scientific Instruments Ltd, Surrey, UK) temperature con-
troller and crossed polarizers. Images were recorded at a magnifica-
tion of 100ꢂ as video with a digital camera after initial heating
during the cooling stage. Heating and cooling rates were
5 Kminꢀ1ꢀ1. For measurement, the samples were placed under
argon between two cover slips, which were sealed with two-com-
ponent adhesive (UHU plus 300, UHU GmbH & Co. KG, Bꢃhl, Ger-
many).
1-Ethyl-3-methylimidazolium
salicylate:
1H NMR
(200 MHz,
[D6]DMSO): d=16. 11(s, 1H, OH), 9.43 (s, 1H, N-CH-N), 7.85 (s, 1H,
N-CH-CH-N), 7.76 (s, 1H, N-CH-CH-N), 7.69 (d, 1H, CH-CH=C-COOꢀ),
7.12 (t, 1H, CH-CH=C-COOꢀ), 6.61 (d, 1H, CH-CH=C-OH), 6.59 (t,
1H, CH-CH=C-OH), 4.21 (q, 2H, N-CH2-CH3), 3.87 (s, 3H, N-CH3),
1.39 ppm (t, 3H, N-CH2-CH3); elemental analysis calcd (%) for
C13H16N2O3: N 11.28; C 62.89; H 6.50; found: N 9.92; C 61.81; H
6.15. Water content (Karl Fischer): 462 ppm. Ion chromatography:
Na+: 0.16 wt.%; Brꢀ: 1.3 wt.%.
Temperature-dependent small-angle X-ray scattering: SAXS
measurements were carried out at the A2 Beamline of DORIS III,
Hasylab, DESY, Hamburg, Germany, at a fixed wavelength of 1.5 ꢁ.
The data were collected with a MarCCD detector. The detector was
calibrated with silver behenate. The sample-detector position was
fixed at 635.5 mm. For measurements, the samples were placed in
a copper sample holder between aluminium foil. The sample tem-
perature was controlled by a JUMO IMAGO 500 multi-channel pro-
cess and program controller. Data reduction and analysis, correc-
tion or background scattering and transmission were carried out
by using a2tool (Hasylab).
1-Butyl-3-methylimidazolium
salicylate:
1H NMR
(200 MHz,
[D6]DMSO): d=16. 11(s, 1H, OH), 9.43 (s, 1H, N-CH-N), 7.80 (s, 1H,
N-CH-CH-N), 7.72 (d, 1H, CH-CH=C-COOꢀ), 7.72 (s, 1H, N-CH-CH-N),
7.14 (t, 1H, CH-CH=C-COOꢀ), 6.64 (d, 1H, CH-CH=C-OH), 6.61 (t,
1H, CH-CH=C-OH), 4.16 (t, 2H, N-CH2-CH2-), 3.87 (s, 3H, N-CH3), 1.73
(qn, 2H, N-CH2-CH2-), 1.21 (sx, 2H, CH2-CH2-CH3), 0.84 ppm (t, 3H,
CH2-CH3); elemental analysis calcd (%) for C15H20N2O3: N 10.81; C
65.20; H 7.30; found: N 10.17; C 62.25; H 8.03. Water content (Karl
Fischer): 236 ppm. Ion chromatography: Na+: 0.096 wt.%; Clꢀ
0.67 wt.%.
1-Dodecyl-3-methylimidazolium salicylate: 1H NMR (200 MHz
[D6]DMSO): d=16.11 (s, 1H, OH), 9.17 (s, 1H, N-CH-N), 7.77 (s, 1H,
N-CH-CH-N), 7.70 (s, 1H, N-CH-CH-N), 7.64 (d, 1H, CH-CH=C-COOꢀ),
7.09 (t, 1H, CH-CH=C-COOꢀ), 6.57 (d, 1H, CH-CH=C-OH), 6.55 (t,
1H, CH-CH=C-OH), 4.14 (q, 2H, N-CH2-CH3), 3.85 (s, 3H, N-CH3), 1.76
(2H, N-CH2-CH2), 1.24 (m, 18H, -(CH2)9-CH3), 0.85 ppm (t, 3H, -CH2-
CH3); elemental analysis calcd (%) for C23H36N2O3: N 7.21; C 71.10;
H 9.34; found: N 6.64; C 66.74; H 9.57. Water content (Karl Fischer):
181 ppm. Ion chromatography: Na+: 0.44 wt.%; Clꢀ: 0.36 wt.%.
Powder X-ray diffraction: The powder X-ray diffraction measure-
ments were carried out on a G670 diffractometer with an image
plate detector (Huber, Rimsting, D) operating with MoKa radiation.
UV/Vis absorption spectroscopy: Visible absorption spectra were
measured at room temperature on a Cary 50 spectrometer (Varian,
Palo Alto, USA). Molar ethanolic solutions (1.0ꢂ10ꢀ5) were loaded
into quartz cuvettes (optical special-purpose (OS) glass).
Photoluminescence measurements: Fluorescence and phosphor-
escence measurements were performed on a Fluorolog FL 3–22
spectrometer (Horiba Jobin Yvon, Unterhachingen, D). A choice be-
tween a continuous xenon lamp with 450 W for fluorescence and
a pulsed xenon lamp for phosphorescence measurements is possi-
ble. Double gratings for the excitation and emission spectrometer
are applied as monochromators. The signal is detected with a pho-
tomultiplier. For measurement, powdered samples were filled into
silica tubes and carefully positioned in the incoming beam in the
sample chamber. Quantum yield measurements were achieved
with aid of an integrating sphere. For solid samples, optical stan-
dard BaSO4 was used as a reflectance standard, whereas for liquid
samples, distilled water was used. Measurements were carried out
at least three times and deviations were used to calculate the
error.
1
1-Ethyl-4-methylpyridinium salicylate: H NMR (200 MHz, [D6]DMSO):
d=16.24 (s, 1H, OH), 8.99 (d, 2H, CH=N-CH (pyridinium)), 7.97 (d,
2H, CH=C-CH (pyridinium)), 7.67 (d, 1H, CH-CH=C-COOꢀ), 7.12 (t,
1H, CH-CH=C-COOꢀ), 6.60 (d, 1H, CH-CH=C-OH), 6.58 (t, 1H, CH-
CH=C-OH), 4.58 (q, 2H, N-CH2CH3), 2.58 (s, 3H, C-CH3), 1.50 ppm (t,
3H, CH2CH2CH3); elemental analysis calcd (%) for C15H17NO3: N
5.40; C 69.48; H 6.61; found: N 5.16; C 66.34; H 5.86. Water content
(Karl Fischer): 589 ppm. Ion chromatography: Na+: 0.062 wt.%; Brꢀ:
0.080 wt.%.
1
1-Butyl-4-methylpyridinium salicylate: H NMR (200 MHz, [D6]DMSO):
d=16.24 (s, 1H, OH), 8.99 (d, 2H, CH=N-CH (pyridinium)), 7.97 (d,
2H, CH=C-CH (pyridinium)), 7.67 (d, 1H, CH-CH=C-COOꢀ), 7.12 (t,
1H, CH-CH=C-COOꢀ), 6.60 (d, 1H, CH-CH=C-OH), 6.58 (t, 1H, CH-
Chem. Eur. J. 2014, 20, 4704 – 4712
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Acknowledgements
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This work was supported by the European Research Council
through a starting grant (EMIL contract no. 200475) and the
DFG cluster of Excellence RESOLV. P.S.C. acknowledges the
Alexander von Humboldt Foundation for funding through
a Post-Doctoral fellowship. The authors thank students Rai-
mund Koerver and Kirsten Grꢃbel for their participation on this
project. Thomas Brꢃggemann and Vanessa Scherer are ac-
knowledged for the elemental analyses.
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Keywords: ionic
liquids
·
NMR
spectroscopy
·
photoluminescence · synthetic methods · UV/Vis spectroscopy
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Received: April 10, 2013
Revised: December 1, 2013
Published online on March 11, 2014
Chem. Eur. J. 2014, 20, 4704 – 4712
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