Synthesis, Characterization, Thermal Analyses, and Spectroscopic Properties of Novel Naphthyl-Functionalized Imidazolium Ionic Liquids

Article abstract of DOI:10.1134/S0036024418030196

A series of novel ionic liquids based on naphthyl-functionalized imidazolium cation have been prepared. Their structure was characterized by NMR. The thermal stabilities of the prepared liquids were studied by thermal gravimetric analysis. The new ionic liquids containing NTf^- ₂ anion display significantly higher thermal stabilities (CloseSPigtSPi400°C). Anion exchange to PF^- ₆, BF^- ₄, and Br^– decreases the thermal stabilities of such ionic liquids. Fluorescence and UV–Vis absorption spectroscopy were used to study the spectroscopic properties of the ionic liquids. Compared with common ionic liquids, the described ionic liquids provide robust fluorescence properties and remarkably increased UV–Vis absorption. This research may enrich the field of functionalized ionic liquids and provide a platform for extension of ionic liquid applications.

Full text of DOI:10.1134/S0036024418030196

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2018, Vol. 92, No. 3, pp. 502–507. © Pleiades Publishing, Ltd., 2018.
STRUCTURE OF MATTER
AND QUANTUM CHEMISTRY
Synthesis, Characterization, Thermal Analyses,
and Spectroscopic Properties of Novel Naphthyl-Functionalized
1
Imidazolium Ionic Liquids
a,b,
a
c
b
Meihuan Yao *, Qing Li , Yanqiu Xia , and Yongmin Liang
School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, 453007 China
a
b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, 730000 China
c
School of Energy and Mechanical Engineering, North China Electric Power University, Beijing, 102206 China
*
e-mail: gtymh@163.com
Received November 10, 2016
Abstract—A series of novel ionic liquids based on naphthyl-functionalized imidazolium cation have been pre-
pared. Their structure was characterized by NMR. The thermal stabilities of the prepared liquids were studied
−
by thermal gravimetric analysis. The new ionic liquids containing NTf anion display significantly higher
2
−
6
−
4
–
thermal stabilities (>400°C). Anion exchange to PF , BF , and Br decreases the thermal stabilities of such
ionic liquids. Fluorescence and UV–Vis absorption spectroscopy were used to study the spectroscopic prop-
erties of the ionic liquids. Compared with common ionic liquids, the described ionic liquids provide robust
fluorescence properties and remarkably increased UV–Vis absorption. This research may enrich the field of
functionalized ionic liquids and provide a platform for extension of ionic liquid applications.
Keywords: ionic liquids, spectroscopy, naphtyl-functionalized, thermal analyses, fluorescence, UV–Vis
absorption
DOI: 10.1134/S0036024418030196
INTRODUCTION
modified by functional groups have similar electron-
donating properties with that of the major classes of
organic molecules [28–30], which plays important
role in exploring the potential applications of ILs in
more fields.
In the past years, ionic liquids (ILs) have attracted
numerous attention in many fields of chemistry and
industry [1–7], due to their unique properties such as
low melting point, negligible volatility, non-flamma-
bility and high thermal stability [8–13]. Recently, the
It has been a hot topic to develop new kinds of ILs
appearance of functionalized ILs endows the ILs with with different properties in the field of current IL
huge diversity in properties, which makes the research research. In this paper, we reported the preparation
of ILs has a greater space for further development. and properties of a series of ILs based on naphthyl-
Generally, the chemical or physical characteristics of functionalized imidazolium with different alkyl chain
−
4
−
6
−
2
–
the ILs are modified by adding functional groups such
as ethylene glycol [14, 15], nitrile [16, 17], sulfo acid
18, 19], amine [20, 21], aryl [22, 23], etc. Carlisle et
al. [24] discovered that nitrile-functionalized ILs show
good performance in gas separation. Wang et al. [25]
reported that ethanolamine-based quaternary ammo-
nium ILs display high activity and recyclability in pal-
ladium-catalyzed Heck reaction. Chaugule et al. [26]
found that glycerol functionalized ILs exhibit excel-
lengths and anions (Br , BF , PF , and NTf ), which
adding a new family of functionalized ILs and provid-
ing a platform for extension of IL applications.
[
EXPERIMENTAL
Materials
2-(Bromomethyl)naphthalene 96% (Alfa Aesar)
lent catalytic properties for 2-azidoalcohol synthesis. and lithium bis(trifluoromethylsulfonyl)imide 99%
Besides, mixtures of ether-functionalized ILs and (Fluka) were used as received. 1-Bromobutane and
methyl perfluorobutyl ether as new safe electrolytes 1-bromooctane were obtained from Shanghai Guang-
were also reported by Fang et al. [27]. Moreover, ILs ming Reagent Company. These reagents were first
treated with concentrated sulfuric acid and then redis-
tilled before use. 1-Methylimidazole was obtained
1
The article is published in the original.
502

SYNTHESIS, CHARACTERIZATION, THERMAL ANALYSES
503
N
N
X−
R
[
[
[
[
[
[
NMMIm]Br: R=CH , X=Br; [NMMIm]BF : R=CH , X=BF ;
3 4 3 4
NMMIm]PF : R=CH , X=Br; [NMMIm]NTf : R=CH , X=N(CF SO ) ;
6
3
2
3
3
2 2
NMBIm]Br: R=C H , X=Br; [NMBIm]BF : R=C H , X=BF ;
4
9
4
4
9
4
NMBIm]PF : R=C H , X=Br; [NMBIm]NTf : R=C H , X=N(CF SO ) ;
6
4
9
2
4
9
3
2 2
NMOIm]Br: R=C H , X=Br; [NMOIm]BF : R=C H , X=BF ;
8
17
4
8
17
4
NMOIm]PF : R=C H , X=Br; [NMOIm]NTf : R=C H , X=N(CF SO )
6
8
17
2
8
17
3
2 2
Fig. 1. Molecular structures and abbreviations of the naphthyl-functionalized ILs.
from Shanghai Weite Regent Company and redistilled the upper organic phase, residual solvent was removed
before use. All the other chemicals used in this work under reduced pressure. The bromide salt was dis-
were of analytical grade.
solved in water and then washed with petroleum ether
and ethyl acetate. After removal of the water under
reduced pressure, the residue was purified by recrys-
tallizing from acetone. 1-Naphthylmethyl-3-butylim-
Synthesis
1-naphthylmethyl-3-alkylimidazolium
1
idazolium bromide at 80% yield (16.57 g). H NMR
Twelve
(
DMSO-d , 400 MHz): 9.52 (1H, N=CH), 8.00–7.52
ILs were prepared according to the similar approach.
6
Figure 1 gives the chemical structures and abbrevia- (9H), 5.64 (2H, NCH ), 4.20 (2H), 1.77 (2H), 1.23
2
tions of them. The synthetic process of 1-naphthyl- (2H), 0.87 (3H).
methyl-3-butylimidazolium bis(trifluoromethylsul-
fonyl)imide ([NMBIm]NTf ) as an example is given
2
in the following and other naphthyl-functionalized
ILs can be obtained analogously. Conventional IL
Synthesis of 1-Naphthylmethyl-3-butylimidazolium
Bis(trifluoromethylsulfonyl)imide Ionic Liquid
1
(
-methyl-3-butylimidazolium hexafluorophosphate
([NMBIm]NTf )
2
[MBIm]PF ) was also synthesized for comparison.
6
Under vigorous stirring, 0.01 mol (3.45 g)
NMBIm]Br was dissolved in water, to the aqueous
[
solution was added lithium bis(trifluoromethylsulfo-
nyl)imide (3.45 g, 0.012 mol). The ion exchange pro-
ceeded for 12 h at room temperature. Two phases were
formed in the mixture solution. The resulting solution
was extracted with ethyl acetate and then washed with
water. The organic phase was dried over anhydrous
Synthesis of 1-Butylimidazole [2]
Under Ar atmosphere, 0.55 mol (12.65 g) Na was
added in one portion to 250 mL absolute ethanol. The
mixture was stirred until Na disappeared and the reac-
tion temperature got back to room temperature. Then
0
.5 mol (34.04 g) imidazole was added and the mix-
MgSO , filtered. Removal of solvent under reduced
4
ture was stirred for 1 h, followed by dropwise addition
of freshly distilled 1-bromobutane (75.36 g, 0.55 mol)
at room temperature. Then the mixture was refluxed
for 3 h. The resulting the mixture was filtered to
remove the precipitated NaBr after cooling down to
room temperature. After removal of solvent by rotary
evaporation, the sample was refiltered to remove
NaBr. The residual liquid was distilled under reduced
pressure afforded 1-naphthylmethyl-3-butylimidaz-
olium bis(trifluoromethylsulfonyl)imide (yield 90%,
1
4
.91 g). H NMR (DMSO-d , 400MHz): 9.35 (1H,
6
N=CH), 8.00–7.53 (9H), 5.62 (2H, NCH ), 4.19
2H), 1.78 (2H), 1.26 (2H), 0.90 (3H).
2
(
1
Instruments
pressure. Yield 45%, 27.94 g. H NMR (CDCl ,
3
4
00 MHz): 7.36 (1H, N=CH), 6.95 (1H, C=CH),
1
H NMR spectra were recorded on a Bruker AMX
6
.81(1H, C=CH), 3.82 (2H, NCH ), 1.67 (2H), 1.23
2
FT 400 MHz NMR spectrometer. Thermal gravimet-
ric analysis (TGA) was carried out on a STA 449 C
Jupiter®–Simultaneous TG-DSC at a heating rate of
(
2H), 0.85 (3H).
1
0°C/min in air atmosphere over temperature range
Synthesis of 1-Naphthylmethyl-3-butylimidazolium
Bromide Ionic Liquid ([NMBIm]Br)
from ambient to about 600°C. UV–Vis absorption
spectra were conducted on an Agilent 8453 UV–Vis
Under vigorous stirring, a solution of 2-(bro- absorption spectrophotometer. Photoluminescence
momethyl)naphthalene (13.27 g, 0.06 mol) and measurement was performed at room temperature
1
(
-butylimidazole (8.87 g, 0.072 mol) in 2-propanol using Hitachi Model F-4500 fluorescence spectro-
180 mL) was heated at 95°C for 72 h. After removal of photometer at a scan speed of 1200 nm/min.
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5
04
MEIHUAN YAO et al.
100
8
0
0
0
6
4
2
4
2
3
1
0
0
1
00
200
300
400
500
600
T, °C
–
−
4
−
6
−
2
Fig. 2. TGA curves of 1-naphthylmethyl-3-butylimidazolium ILs with different anions: (1) Br , (2) BF , (3) BF , (4) NTf .
RESULTS AND DISCUSSION
chain on the effect of T , ILs [NMOIm]Br,
d
[NMBIm]Br, and [NMOIm]Br with the same anion
Thermal Properties
was studied. We can see that T of the ILs slightly
d
Generally, thermal properties of ILs are affected by reduced as the chain length increasing. The same
−
−
salt structure [3]. The effect of both anion and alkyl
chain length on the thermal stabilities of the newly
synthesized ILs were investigated. Figure 2 gives the
TGA curves of 1-naphthylmethyl-3-butylimidaz-
trend was also found as for ILs with NTf , PF , and
2
6
−
4
BF anion.
−
4
−
6
–
olium ILs with different anions (Br , BF , PF ,
Spectroscopic Properties
−
2
NTf ). We can see that anion greatly influences ther-
The UV–Vis absorption spectra of the ILs with and
without naphthyl group were investigated. Conven-
tional IL 1-methyl-3-butylimidazolium hexafluoro-
mal stabilities of the ILs. A general trend of the ther-
mal stabilities for the ILs is observed as follows:
−
2
−
6
−
4
−
2
–
NTf > PF ~ BF > Br . IL with NTf anion is phosphate ([MBIm]PF ) without naphthyl group was
6
shown to be stable up to 400°C, whereas the one with selected for comparison. Absorption spectra in the
–
Br anion decomposes at a temperature lower than range 200–500 nm were recorded using a 1 cm path
3
00°C. In order to study the effect of alkyl chain length length quartz cuvette. All the ILs were diluted with
on the thermal stabilities of the ILs, TGA curves of ethanol in 1 : 12 000 weight ratio because of high
-naphthylmethyl-3-alkylimidazolium tetrafluorobo- absorbance of the naphthyl-functionalized ILs.
rate ILs with methyl ([NMMIm]BF ), butyl Figure 4 gives UV–Vis absorption spectra of 1-naphth-
1
4
ylmethyl-3-butylimidazolium ILs with different
anions and IL [MBIm]PF without naphthyl group.
(
[NMBIm]BF ), and octyl group ([NMOIm]BF ) on
4 4
imidazolium cation are shown in Fig. 3. TGA curves
of the three kind of ILs with different alkyl chain
6
From the picture we can seen that [MBIm]PF only
6
length are similar and almost flat up to nearly about has a very weak and broad absorption band at about
3
00°C. As the temperature increased, a slightly
decrease of thermal stabilities has been detected by
increasing length of the side alkyl chain.
Table 1. The onset decomposition temperature (T ) of the
d
naphthyl-functionalized ILs
The onset decomposition temperature (T ) of the
d
Td, °C
ILs
Td, °C
–
ILs
ILs is listed in Table 1. The T of the ILs with Br
d
−
4
−
6
anion is in the range of 267–284°C. When BF or PF
[NMMIm]Br
[NMMIm]BF4
284
338
332
417
275
335
[NMBIm]PF6
[NMBIm]NTf2
[NMOIm]Br
[NMOIm]BF4
[NMOIm]PF6
331
413
267
332
329
405
–
replaces Br anion, T of the ILs increases to 329–
d
3
38°C ranges. It is worthwhile to note that T of the
d
[NMMIm]PF6
−
2
ILs containing NTf anion is in the range of 405–
[NMMIm]NTf2
4
17°C, which is significantly higher than others.
[NMBIm]Br
Therefore anion plays a crucial role in determining the
[
NMBIm]BF4
[NMOIm]NTf2
T of the ILs. In order to investigate length of alkyl
d
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SYNTHESIS, CHARACTERIZATION, THERMAL ANALYSES
00
505
1
8
0
0
0
6
4
2
1
2
3
0
0
1
00
200
300
400
500
600
T, °C
Fig. 3. TGA curves of 1-naphthylmethyl-3-alkylimidazolium tetrafluoroborate ILs with different length of the side alkyl chain:
methyl (1), butyl (2), and octyl (3) group.
2.0
3
1.5
1, 2
1.0
4
0.5
0
5
−0.5
200
250
300
350
400
450
500
Wavelength, nm
Fig. 4. UV–Vis absorption spectra of 1-naphthylmethyl-3-butylimidazolium ILs with different anions (1–4, see Fig. 2) and ionic
liquid [MBIm]PF (5) in ethanol.
6
2
11 nm. But for the 1-naphthylmethyl-3-butylimidaz- trated in Fig. 5. As can be seen, when excited at wave-
olium ILs, a strong peak at about 225 nm and a weak lengths from 220 to 320 nm, a two-component emis-
absorption band in the region 250–300 nm were sion spectrum is observed: a short-wavelength band
observed. Besides, similar absorption characteristics around 335 nm and a long-wavelength band around
were found in the spectra of 1-naphthylmethyl-3- 351 nm. With increase in the excitation wavelength,
butylimidazolium ILs with different anions, which the emission gains intensity with no shift of the maxi-
indicates that anion has no obvious effect on the UV mum. Then, with further increase in the excitation
properties of ILs.
wavelength to 340 nm, the fluorescence disappeared
immediately. In contrast, Fig. 6 depicts the fluores-
A comparative study was conducted by fluores-
cence excitation spectra of [MBIm]PF . No fluores-
6
cence
between
naphthyl-functionalized
IL
cence could be observed at excitation wavelengths
below 250 nm. When excited between 250 and 350 nm,
[
NMBIm]PF and the common IL [MBIm]PF ,
6
6
where the anion is same and the cation is similar. The it exhibits weakly fluorescence and the spectral behav-
fluorescence behavior of neat [NMBIm]PF is illus- ior dependent on the excitation wavelength. As the
6
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06
MEIHUAN YAO et al.
6
100
1
220 nm
2
3
4
5
6
7
240 nm
260 nm
280 nm
300 nm
320 nm
340 nm
8
0
0
0
0
0
6
4
2
3
2
1
7
4
5
200
300
400
500
600
λ, nm
Fig. 5. Excitation wavelength-dependent emission behavior of neat [NMBIm]PF6.
7
6
5
4
3
2
1
250 nm
270 nm
290 nm
3
10 nm
3
30 nm
50 nm
3
300
400
500
600
λ, nm
Fig. 6. Excitation wavelength-dependent emission behavior of neat [MBIm]PF6.
excitation wavelength shifts from 250 to 350 nm, the tion and a remarkably increased luminescence inten-
fluorescence maximum shifts toward longer wave- sity. The unusual spectroscopic properties of ILs may
length with decrease of the overall intensity. The fluo- attributed to the absorption of naphthyl group or the
rescence disappeared when excitation wavelength is conformational changes in the imidazolium groups
above 350 nm. The unusual wavelength-dependent which caused by the interaction between the naphthyl
emission behaviors of [MBIm]PF is attributed to the group with other neighboring cations or anions.
6
presence of different imidazolium associated forms
[
31, 32], whose position and intensity vary with exci-
CONCLUSIONS
tation wavelength.
Compared with naphthyl-functionalized ILs and
A series of novel naphthyl-functionalized ILs have
the common IL [MBIm]PF , we can seen that the been synthesized and characterized in terms of their
6
UV–Vis absorption spectra and fluorescence behavior thermal and spectroscopic properties. Results of ther-
−
of the naphthyl-functionalized ILs are quite different mal gravimetric analysis suggested that ILs with NTf2
from that of the common one. The introduction of anion have good thermal stability, so they may be used
naphthyl group makes both a change in the peak posi- in high temperature reactions or gas chromatography.
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SYNTHESIS, CHARACTERIZATION, THERMAL ANALYSES
507
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The authors gratefully acknowledge the financial
support of this work by Key Scientific Research Proj-
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