Three anthracene-based bis-imidazolium salts: Synthesis, structure and recognition for 2,4-dinitrophenylhydrazine

Article abstract of DOI:10.1016/j.dyepig.2019.107983

Three anthracene-based bis-imidazolium salts 1,8-bis [2’-(N-R-imidazoliumyl) acetamido]anthracene 2× (1: R = ⁿBu, X = Cl^?; 2: R = ⁿBu, X = PF₆ ^?; 3: R = Et, X = PF₆ ^?) were prepared. The structure of 1 was demonstrated by X-ray analysis. The selective recognition of 2 (or 3) for some aromatic compounds (toluene, chlorobenzene, phenylamine, phenol, anisole, benzaldehyde, acetophenone, nitrobenzene, m-dinitrobenzene, 2,4-dinitrotoluene, 2,4-dinitrophenylhydrazine (DNP), trinitrophenol, o-nitrophenol, p-nitrotoluene) was investigated through fluorescence, ultraviolet, ¹H NMR, HRMS and IR spectra at 25 °C. Compounds 2 (or 3) showed good selective recognition ability for DNP, and they could effectively distinguish DNP from other aromatic compounds. The association constants and detection limits of 2 and 3 were similar, which displayed sizes of side chains (ⁿBu for 2 and Et for 3) had no remarkable effect on the recognition of DNP.

Full text of DOI:10.1016/j.dyepig.2019.107983

Journal Pre-proof
Three anthracene-based bis-imidazolium salts: Synthesis, structure and recognition
for 2,4-dinitrophenylhydrazine
Qingxiang Liu, Shaocong Yu, Zhixiang Zhao, Xiantao Zhang, Rui Liu
PII:
S0143-7208(19)31968-0
DOI:
https://doi.org/10.1016/j.dyepig.2019.107983
DYPI 107983
Reference:
To appear in: Dyes and Pigments
Received Date: 20 August 2019
Revised Date: 12 October 2019
Accepted Date: 18 October 2019
Please cite this article as: Liu Q, Yu S, Zhao Z, Zhang X, Liu R, Three anthracene-based bis-
imidazolium salts: Synthesis, structure and recognition for 2,4-dinitrophenylhydrazine, Dyes and
Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107983.
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Graphic abstract:
Anthracene-based bis-imidazolium salts 1-3 have been prepared. The recognition of 2
(or 3) for 2,4-dinitrophenylhydrazine was investigated.
1

Three Anthracene-based Bis-imidazolium Salts: Synthesis, Structure and
Recognition for 2,4-Dinitrophenylhydrazine
*
Qingxiang Liu, Shaocong Yu, Zhixiang Zhao, Xiantao Zhang and Rui Liu
Tianjin Key Laboratory of Structure and Performance for Functional Molecules,
MOE Key Laboratory of Inorganic-Organic Hybrid Functional Material Chemistry,
College of Chemistry, Tianjin Normal University, Tianjin 300387, P. R. China.
*
Corresponding author, E-mail: tjnulqx@163.com
Abstract
Three anthracene-based bis-imidazolium salts 1,8-bis[2’-(N-R-imidazoliumyl)
n
-
n
-
acetamido]anthracene 2X (1: R = Bu, X = Cl ; 2: R = Bu, X = PF ; 3: R = Et, X =
6
-
PF ) were prepared. The structure of 1 was demonstrated by X-ray analysis. The
6
selective recognition of 2 (or 3) for some aromatic compounds (toluene,
chlorobenzene, phenylamine, phenol, anisole, benzaldehyde, acetophenone,
nitrobenzene, m-dinitrobenzene, 2,4-dinitrotoluene, 2,4-dinitrophenylhydrazine
(DNP), trinitrophenol, o-nitrophenol, p-nitrotoluene) was investigated through
1
fluorescence, ultraviolet, H NMR, HRMS and IR spectra at 25 ˚C. Compounds 2 (or
3
) showed good selective recognition ability for DNP, and they could effectively
distinguish DNP from other aromatic compounds. The association constants and
n
detection limits of 2 and 3 were similar, which displayed sizes of side chains ( Bu for
2
and Et for 3) had no remarkable effect on the recognition of DNP.
Keywords: bis-imidazolium; fluorescence; recognition; 2,4-dinitrophenylhydrazine
1
. Introduction
Selective recognition and sensing of aromatic compounds occupy an important
position in the field of fluorescence recognition because some aromatic compounds
are serious environmental pollutants [1-5]. As is known to all, aromatic compounds
1

are poisonous to humans and animals, and they can cause some diseases, such as
cancer [6-9]. What’s more, they can also infiltrate into biological ecosystem through
direct discharge of industrial castoffs [10-12]. The detection methods of aromatic
compounds contain LC-MS, GC-MS, HPLC [13-20]. In these methods, the
fluorescence method is widely used due to its sensitivity and speediness [21-31].
Therefore, it is necessary to develop more sensitive and efficient fluorescence
chemosensors for aromatic compounds.
In order to search for suitable chemosensors toward the detection of aromatic
compounds, we became interested in anthracene-based bis-imidazolium salts. In this
work,
the
synthesis
of
three
bis-imidazolium
salts
n
-
1
,8-bis[2’-(N-R-imidazoliumyl)acetamido]anthracene 2X (1: R = Bu, X = Cl ; 2: R =
n
-
-
Bu, X = PF ; 3: R = Et, X = PF ) was reported. The structure of 1 was demonstrated
6
6
through X-ray analysis. The selective detection of 2 (or 3) for some aromatic
compounds (toluene, chlorobenzene, phenylamine, phenol, anisole, benzaldehyde,
acetophenone,
nitrobenzene,
m-dinitrobenzene,
2,4-dinitrotoluene,
2
,4-dinitrophenylhydrazine (DNP), trinitrophenol, o-nitrophenol, p-nitrotoluene) was
1
investigated through fluorescence, ultraviolet, H NMR, HRMS and IR spectra in
acetonitrile at 25 ˚C.
2
. Experimental
2
.1. General
The reagents of analytical grade were used in tests and preparations. A Varian
1
Mercury Vx 400 spectrometer was employed for the measurement of H NMR and
1
3
C NMR spectra. A Boetius Block apparatus was used for the determination of
melting points. A Perkin-Elmer 2400C Elemental Analyzer was used for the
measurement of elemental analyses. A Cary Eclipse fluorescence spectrophotometer
was employed for the measurement of fluorescence spectra. A Bruker Equinox 55
spectrometer was used for the measurement of IR spectra (KBr). A VG ZAB-HS mass
spectrometer (VG, U.K.) was employed for the determination of EI mass spectra. The
diffraction data of 1 was collected through a Bruker Apex II CCD diffractometer [32].
The SHELXS program was used to solve the structure of 1 [33]. Crystal-Maker was
2

employed to form Fig. 1 [34]. Crystallographic data of 1 were listed in Table S1.
2
.2. 1,8-Diaminoanthraquinone
Na S (3.512 g, 45.0 mmol) and NaOH (4.280 g, 107.0 mmol) were dissolved in
2
water (190 mL) with stirring for 0.5 h, and then EtOH (112 mL) solution of
,8-dinitroanthraquinone (2.952 g, 9.9 mmol) was added to above solution under
1
refluxing for 6 h. When the solution was cooled to 20 ˚C, the purple precipitate was
generated. The purple powder of 1,8-diaminoanthraquinone was collected by filtration.
1
Yield: 2.320 g (98%). M.p.: 274-275 ˚C. H NMR (400 MH , DMSO-d ): δ 7.16 (d, J
Z
6
=
9.2 Hz, 2H, AnH), 7.36 (d, J = 8.4 Hz, 2H, AnH), 7.45 (t, J = 8.0 Hz, 2H, AnH),
7
.86 (s, 4H, NH2).
2
.3. 1,8-Diaminoanthracene
1
,8-Diaminoanthraquinone (2.000 g, 8.4 mmol) and NaOH (0.072 g, 1.8 mmol)
were dissolved in i-propanol (100 mL), and then sodium borohydride (4.009 g, 106.0
mmol) was added under N . The suspension was heated to reflux for 24 h. After
2
adding 250 mL of ice water, a yellow-green powder of 1,8-diaminoanthracene was
precipitated, and the product was collected via filtration. Yield: 1.680 g (96%). M.p.:
1
1
2
2
75-176 ˚C. H NMR (400 MH , DMSO-d ): δ 5.870 (s, 4H, NH ), 6.55 (t, J = 4.1 Hz,
Z
6
2
H, AnH), 7.19 (d, J = 4.1 Hz, 4H, AnH), 8.17 (s, 1H, AnH), 8.80 (s, 1H, AnH).
.4. 1,8-Dichloroacetamidoanthracene
To the dichloromethane solution (150 mL) of triethylamine (2.337 g, 23.1 mmol)
and 1,8-diaminoanthracene (2.060 g, 9.6 mmol) was added dropwise chloroacetyl
chloride (2.608 g, 23.1 mmol) at 0 ˚C. Then the mixture was stirred overnight at 25 ˚C
to form a yellow precipitate. The precipitate was collected via filtration and purified
by washing with dichloromethane to give
a
celadon powder of
1
1
,8-dichloroacetamidoanthracene. Yield: 3.400 g (98%). M.p.: > 320 ˚C. H NMR
(
400 MH , DMSO-d ): δ 4.55 (s, 4H, CH ), 7.56 (q, J = 5.2 Hz, 2H, AnH), 7.72 (d, J
Z 6 2
=
6.8 Hz, 2H, AnH), 8.01 (d, J = 8.5 Hz, 2H, AnH), 8.68 (s, 1H, AnH), 8.87 (s, 1H,
AnH), 10.53 (s, 2H, NH).
n
2
.5. 1,8-Bis[2’-(N- butyl-imidazoliumyl)acetamido]anthracene chloride (1) and
3

n
1
,8-bis[2’-(N- butyl-imidazoliumyl)acetamido]anthracene hexafluorophosphate (2)
n
A 1,4-dioxane (100 mL) solution of N- butyl-imidazole (1.029 g, 8.3 mmol) and
1
,8-dichloroacetamidoanthracene (1.010 g, 2.8 mmol) was heated to reflux with
stirring
for
5
days
to
give
a
yellow-green
solid.
n
1
,8-Bis[2’-(N- butyl-imidazoliumyl)acetamido]anthracene chloride (1) was obtained
by filtration and washed with 1,4-dioxane (30 mL). Yield: 1.400 g (82%). M.p.:
2
6
76-280 ˚C. Anal. Calcd for C H N O Cl : C, 63.04; H, 6.28; N, 13.78%. Found: C,
32 38 6 2 2
1
3.22; H, 6.31; N, 13.62%. H NMR (400 MHz, DMSO-d ): δ 0.92 (t, J = 7.4 Hz, 6H,
6
CH ), 1.29 (m, 4H, CH ), 1.83 (m, 4H, CH ), 4.27 (t, J = 7.1 Hz, 4H, CH ), 5.80 (s,
3
2
2
2
4
H, CH ), 7.54 (t, J = 7.9 Hz, 2H, AnH), 7.87 (s, 2H, ArH), 7.97 (t, J = 9.4 Hz, 4H,
2
ArH), 8.04 (d, J = 7.3 Hz, 2H, AnH), 8.65 (s, 1H, AnH), 9.43 (s, 2H, imiH), 9.91 (s,
1
H, AnH), 11.34 (s, 2H, NH) (imi = imidazole).
The methanol solution (100 mL) of compound 1 (2.194 g, 3.6 mmol) and NH PF
4
6
(
1.760 g, 10.8 mmol) were stirred at room temperature for two days. A deep yellow
powder was formed, and the powder was collected via filtration and purified by
washing with mL of methanol to afford
,8-bis[2’-(N- butyl-imidazoliumyl)acetamido]anthracene hexafluorophosphate (2).
5
n
1
Yield: 1.998 g (67%). M.p.: 194-198 ˚C. Anal. Calcd for C H N O P F : C, 46.38;
3
2
38
6
2 2 12
1
H, 4.62; N, 10.14%. Found: C, 46.27; H, 4.58; N, 10.22%. H NMR (400 MHz,
DMSO-d ): δ 0.93 (t, J = 7.3 Hz, 6H, CH ), 1.28 (q, J = 7.4 Hz, 4H, CH ), 1.85 (q, J =
6
3
2
7
2
.2 Hz, 4H, CH ), 4.27 (t, J = 6.8 Hz, 4H, CH ), 5.80 (s, 4H, CH ), 7.54 (t, J = 7.9 Hz,
2 2 2
H, AnH), 7.87 (s, 2H, imiH), 7.93 (d, J = 8.4 Hz, 2H, AnH), 7.97 (s, 2H, imiH), 8.04
(d, J = 7.2 Hz, 2H, AnH), 8.64 (s, 1H, AnH), 9.42 (s, 2H, imiH), 9.90 (s, 1H, AnH),
1
3
1
1.02 (s, 2H, NH). C NMR (100 MHz, DMSO-d ): δ 164.77 (C=O), 139.23
6
(imi-NCN), 137.48, 137.41, 132.96, 131.65, 127.08, 125.55, 125.38, 125.20, 124.11,
1
24.00, 121.91, 119.48, 116.94 (ArC), 51.73, 48.71, 31.34, 18.74 (CH ), 13.22 (CH ).
2 3
2
.6. 1,8-Bis[2’-(N-ethyl-imidazoliumyl)acetamido]anthracene hexafluorophosphate
(3)
Compound 3 was prepared in the analogous method of 2. Yield: 2.224 g (80%).
M.p.: 238-240 ˚C. Anal. Calcd for C H N O P F : C, 43.53; H, 3.91; N, 10.87%.
2
8
30
6
2 2 12
4

1
Found: C, 43.45; H, 3.82; N, 10.76%. H NMR (400 MHz, DMSO-d ): δ 1.47 (t, J =
6
7
.1 Hz, 6H, CH ), 4.28 (q, J = 6.8 Hz, 4H, CH ), 5.55 (s, 4H, CH ), 7.57 (t, J = 7.0 Hz,
3 2 2
2
H, AnH), 7.86 (q, J = 6.5 Hz, 6H, ArH), 8.00 (d, J = 8.5 Hz, 2H, AnH), 8.69 (s, 1H,
1
3
AnH), 9.19 (s, 1H, AnH), 9.31 (s, 2H, imiH), 10.90 (s, 2H, NH). C NMR (100 MHz,
DMSO-d ): δ 164.74 (C=O), 139.26 (imi-NCN), 137.12, 137.05, 133.35, 132.79,
6
1
1
2
31.68, 127.16, 125.70, 125.52, 124.07, 123.97, 121.86, 121.68, 121.62, 120.55,
20.18, 116.84 (ArC), 51.54, 44.47, 44.37 (CH ), 15.06 (CH ).
2
3
.7. Fluorescence titrations
The stock solutions of chemosensor 2 (or 3) (1.0 × 10 M or 1.0 × 10 M) and
guests were prepared in CH CN. Test solutions were prepared via mixing the
-3
-4
3
-5
-5
solutions of 2 (or 3) (1.0 × 10 M) and guests (0-45.0 × 10 M). The sample solutions
were excited at 381 nm for 2 and 383 nm for 3, and the slits were 5 and 3 nm. And the
spectra were recorded from 390 nm to 550 nm.
2
.8. Method for Job’s Plot
The guest (DNP) and 2 (or 3) were dissolved in CH CN in the concentration of 1
3
-4
×
10 M respectively to prepare a stock solution and the overall concentration
-5
remained 1 × 10 M in the test solutions, and the molar ratios of Chost/Cguest changed
from 1:0 to 0:1. The sample solutions were excited at 381 nm for 2 and 383 nm for 3,
and the slits were 5 nm and 3 nm. And the spectra were recorded from 390 nm to 550
nm.
3
. Results and Discussion
3
.1. Preparation and characterization of compounds 1-3
As shown in Scheme 1, 1,8-dinitroanthraquinone was reduced by Na S in the
2
presence of NaOH to afford 1,8-diaminoanthraquinone, which further reacted with
sodium borohydride to form 1,8-diaminoanthracene. Chloroacetyl chloride reacted
with 1,8-diaminoanthracene in the presence of Et N in CH Cl to give
3
2
2
n
1
,8-dichloroacetamidoanthracen, followed reaction with 1- butylimidazole in
n
1
,4-dioxane to afford 1,8-bis[2’-(N- Bu-imidazoliumyl)acetylamino]anthracene
chloride (1). The reaction of NH PF with 1 in methanol generated
4
6
n
1
,8-bis[2’-(N- Bu-imidazoliumyl)acetylamino]anthracene hexafluorophosphate (2).
5

1
Compound 3 was prepared with an analogous to the method of 2. In the H NMR
spectra, the imidazolium proton signals (NCHN) appear at 9.31-9.43 ppm for 1-3
[
35-39].
Scheme 1. Preparation of compounds 1-3.
3
.2. Structure of 1
The single crystal of 1 was obtained by slow diffusion of diethyl ether into its
CH CN solution, and the structure of 1 was demonstrated by X-ray analysis. In the
3
crystal structure of 1 (Fig. 1), the angle of N-C-N in the imidazole ring was 108.3(4)˚,
which was similar to those of the known imidazolium salts [39-40]. Two imidazole
rings in 1 were parallel, and they formed the dihedral angles of 71.4(3)˚ with
anthracene ring. In Fig. S1, 1D polymeric chain of 1 was generated through C-H···Cl
hydrogen bonds [41], in which hydrogen atoms were from methylene on both flanks
of imidazole ring (Table S2).
Fig. 1. Crystal structure of 1. Selected bond lengths (Å) and angles (˚): O(1)-C(15)
1
.216(5), N(1)-C(15) 1.423(7), N(2)-C(19) 1.320(7), N(3)-C(19) 1.330(6);
N(2)-C(19)-N(3) 108.3(4). Symmetric code: i = x, 1 + y, z.
3
.3. Recognition of 2,4-dinitrophenylhydrazine (DNP) using 2 as a host
Compounds 1 and 2 have the same cationic moiety, and different anionic moieties
-
-
(
Cl for 1 and PF for 2). As shown in Fig. S2, the fluorescence intensity of 1 is
6
weaker than that of 2 due to the influence of different anions. To avoid the
interference of chloride ion, we chose 2 as a host to study the selective recognition of
some aromatic compounds (toluene, phenylamine, chlorobenzene, anisole, phenol,
acetophenone, benzaldehyde, nitrobenzene, m-dinitrobenzene, 2,4-dinitrotoluene,
2
,4-dinitrophenylhydrazine (DNP), trinitrophenol, o-nitrophenol and p-nitrotoluene)
in CH CN at 25 ˚C.
3
-5
The free 2 (1.0 × 10 M) exhibited strong triple emission peak at 390-550 nm as
6

displayed in Fig. 2 (λ = 381 nm, emission and excitation slits: 3 nm and 5 nm), and
ex
this peak was attributed to the emission of anthracene. No significant change was
observed after adding 10 equiv. of toluene, chlorobenzene, phenylamine, phenol,
anisole,
benzaldehyde,
acetophenone,
nitrobenzene,
m-dinitrobenzene,
2
,4-dinitrotoluene, trinitrophenol, o-nitrophenol, p-nitrotoluene. However, adding the
same amount of 2,4-dinitrophenylhydrazine (DNP) caused a remarkable decrease of
the emission intensity.
Fig. 3 showed the fluorescence spectra of 2 after adding different concentrations
of DNP. When the CDNP enhanced gradually, the intensity of emission of 2 at 390-550
nm decreased fastly as displayed in the inset of Fig. 3. When the ratio of CDNP/C2
exceeded 1:1, the rate of decreasing of fluorescence intensity changed slowly. Finally,
the intensity of emission peaks no longer changed when CDNP/C surpassed 30. The
2
quenching behaviors of 2,4-dinitrophenylhydrazine on the fluorescence of 2 were
found to fit well with the traditional Stern-Volmer relationship (eqn (1)) [42].
F /F = 1 + K C
(1)
0
SV DNP
in which F and F expressed the intensities of 2 with or without DNP, CDNP
0
represented the concentration of DNP, and the K was the association constant. This
SV
equation revealed that F /F enhances in direct proportion to the enhancing CDNP
.
0
4
-1
The quenching constant KSV was computed as 5.6 × 10 M (R = 0.999) for
2
·DNP by employing the equation (1), and the linear portion was in the ranges of
-4
0
-4.5 × 10 M as shown in Fig. S3. As displayed in Fig. S4, the intensity data were
normalized between the minimum intensity and the maximum intensity, and the
concentrations of DNP were in the ranges of 0-0.019 µM. A linear regression curve
was fitted to the seven points. By calculating ratio between 3σ and k, the detection
-10
-1
limit (LOD) was computed as 3.97 × 10 mol·L for 2 (LOD = 3σ/k, σ was the
blank standard deviation and k was the slope of the linear regression curve) [43,44].
-7
-9
-1
This result was even lower than the lowest value of literatures (10 -10 mol·L )
45,46]. The complexation stoichiometry between 2 and DNP was established by
[
using the Job’s plot method (Fig. S5). When molar fraction (χ) of 2 was 0.5, the χ∆I
value for 2·DNP reached maximum, which indicated the complexation stoichiometry
7

between 2 and DNP was 1:1 [47-49].
To test recognition ability of 2 for DNP in the existence of other aromatic
compounds, the competition experiments were carried out. Firstly, compound 2 (1.0 ×
-5
-1
1
0 mol·L ) was mixed with various aromatic compounds (toluene, chlorobenzene,
phenylamine, phenol, anisole, benzaldehyde, acetophenone, nitrobenzene,
m-dinitrobenzene, 2,4-dinitrotoluene, trinitrophenol, o-nitrophenol, p-nitrotoluene) in
-4
-1
the concentration of 1.0 × 10 mol·L , and then added the 10 equiv. of
,4-dinitrophenylhydrazine. Using fluorescent spectra to monitor the competition
2
experiments, no obvious interference was observed except p-nitrotoluene and
trinitrophenol (Fig. S6). After adding p-nitrotoluene and trinitrophenol, the
fluorescence intensity increased about 8-10% than that in the existence of
2
,4-dinitrophenylhydrazine alone. These results showed that compound 2 was able to
effectively discriminate 2,4-dinitrophenylhydrazine from other aromatic compounds.
-5
-1
Fig. 2. Fluorescence spectra of 2 (1 × 10 mol·L ) in the presence of guest molecules
chlorobenzene, toluene, phenylamine, anisole, phenol, acetophenone, benzaldehyde,
nitrobenzene, m-dinitrobenzene, 2,4-dinitrotoluene, 2,4-dinitrophenylhydrazine
DNP), trinitrophenol, o-nitrophenol, p-nitrotoluene) (10 equiv. for each guest) in
CH CN at 25 ˚C. (λ = 381 nm, slits: ex = 5 nm, em = 3 nm).
(
(
3
ex
-5
Fig. 3. Emission spectra of 2 (1 × 10 mol/L) with different concentration of
2
2
2
,4-dinitrophenylhydrazine added in CH CN at 25 ˚C. The concentrations of
3
,4-dinitrophenylhydrazine for curves 1 to 21 were 0, 0.11, 0.33, 0.55, 0.80, 1.0, 1.25,
-5
.0, 2.7, 3.6, 6, 10, 12, 16, 20, 23, 27, 30, 36, 42, 45 × 10 mol/L (from top to bottom).
(λex = 381 nm, slits: ex = 5 nm, em = 3 nm). Inset: the fluorescence of 2 as a function
of CDNP/C at 439 nm.
2
3
.4. Interactions of 2,4-dinitrophenylhydrazine (DNP) with 2
In the forcipate structure of 2, oxygen atom, hydrogen atom and nitrogen atom
were possible binding sites for 2,4-dinitrophenylhydrazine (DNP). To understand the
1
binding mode between 2 and DNP, H NMR titration experiments were carried out
8

(Fig. 4). The proton signals on the benzene ring of 2,4-dinitrophenylhydrazine (DNP)
in 2·DNP underwent upfield shifting (ca. 0.02 ppm for H , H and H ) (Fig. 4(vi)),
B1
B2
B3
but the signals of protons of -NH-NH on DNP didn’t change, which indicated that
2
-
NO of DNP participated in interactions between 2 and DNP, instead of -NH-NH .
2
2
Meanwhile, the HA1 on NH beside anthracene ring of 2 was upfield shifted by 0.04
ppm (Fig. 4(vi)), which should be attributed to N-H···O interactions between DNP
and NH of 2 (Scheme 2). H and HA8 were upfield shifted 0.02 ppm. These shifts
A7
should be originated from the electron-donating effect of nitro group, in which amide
was a strong electron-withdrawing group due to the high electronegativity of nitrogen
atom and the influence of carbonyl. The proton signal of H on the imidazole ring of
A3
2
was downfield shifted by 0.05 ppm (Fig. 4(vi)), which should be originated from
C-H···O interactions between DNP and imidazole rings of 2. Also, the proton signals
of H , H , H and H were downfield shifted by ca. 0.02 ppm. These shifts should
A2
A4
A5
A6
be ascribed to the electron-withdrawing effect of nitro groups, in which the
imidazolium ring is electron-donating unit because it contains one electron-rich
bond. Besides, no obvious change for other proton signals was observed, which
showed that DNP was captured by C-H···O and N-H···O interactions. In addition, the
proton signals of H -H had no notable change upon the addition of more DNP (Fig.
A1
A8
4
(vii) and Fig. 4(viii)), and this indicated that 2 and DNP have a 1:1 complexation.
In Fig. S7, m/z (367.41) of 2·DNP was observed, and it further proved the 1:1
complex between 2 and DNP. This result was consistent with the Job’s plot
experiment as shown in Fig. S5.
In the IR spectra of DNP, 2 and 2·DNP, the bending vibration of v(C=N) moved
-1
-1
from 1549 cm in free 2 to 1554 cm in 2·DNP (Fig. S8), the v(N-H) absorption band
-1
-1
moved from 740 cm in free 2 to 744 cm in 2·DNP. The two absorption bands of
-1
-1
nitro group moved from 1639 cm and 1321 cm in free 2,4-dinitrophenylhydrazine
-1
-1
to 1647 cm and 1333 cm in 2·DNP, respectively.
1
By analyzing the structure of 2, H NMR spectra, HRMS and IR spectra, the
binding force between DNP and 2 turned out to be C-H···O and N-H···O interactions.
9

Because the imidazolium moieties are electron-rich groups and the amide groups are
electron-withdrawing groups, the nitro groups have an electron-withdrawing effect on
the imidazolium moieties and have an electron-donating effect on amide groups.
When DNP was captured by 2, the photoinduced electron transfer (PET) process from
imidazolium moieties to anthracene ring was switch-on due to the influence of nitro
groups [50]. As a result, fluorescence intensity of 2 decreased remarkably.
Scheme 2. The interactions of 2,4-dinitrophenylhydrazine (DNP) and 2.
1
Fig. 4. The partial H NMR spectra (DMSO-d , 400 MHz) of DNP, 2 and 2·DNP. (i)
6
DNP only; (ii) 2 only; (iii) 0.25 equiv. of DNP and 2; (iv) 0.5 equiv. of DNP and 2; (v)
0
.75 equiv. of DNP and 2; (vi) 1 equiv. of DNP and 2; (vii) 2 equiv. of DNP and 2;
(viii) 3 equiv. of DNP and 2.
3
.5. Recognition of 2,4-dinitrophenylhydrazine (DNP) using 3 as a host
The recognition of 3 for DNP was investigated with the analogous method to 2
1
(
Fig. S9-Fig. S14). In H NMR titration experiments of 3, the proton signals of 3 and
DNP were similar to the cases of 2 (Fig. S15), which indicated that DNP was also
captured by 3 via C-H···O and N-H···O interactions (Scheme S1). The HRMS of
3
3
·DNP, and IR spectra of DNP, 3 and 3·DNP were also measured. In the HRMS of
·DNP as displayed in Fig. S16, m/z (339.36) of 3·DNP was observed, and this
further proved a 1:1 complexation stoichiometry between 3 and DNP. IR spectra of
·DNP (Fig. S17) was similar to those of 2·DNP (Fig. S8). The above experimental
3
results indicated that 3 could also effectively discriminate DNP from other aromatic
compounds through the method of fluorescence.
4
. Conclusion
In summary, three bis-imidazolium salts 1-3 have been prepared and characterized.
The structure of 1 was confirmed through X-ray analysis. The experimental results
1
0

showed that 2 (or 3) had special selectivity for 2,4-dinitrophenylhydrazine (DNP), and
they could effectively discriminate DNP from other aromatic compounds via the
4
-1
4
-1
method of fluorescence. The K value of 5.6 × 10 M for 2·DNP and 4.5 × 10 M
SV
for 3·DNP were obtained through a 1:1 association equation by using the method of
fluorescence titration. The detection of 2 (or 3) for DNP was of high sensitivity with
-10
-10
the detection limits of 3.97 × 10 mol/L for 2 and 2.58 × 10 mol/L for 3. By
comparing, we found that the association constants and detection limits of 2 and 3 for
n
DNP were similar, which displayed that the sizes of side chains ( Bu for 2 and Et for
3
) had no obvious effect on the recognition of the host toward DNP. The high
sensitivity, selectivity and great affinity for DNP indicated that 2 (or 3) could
effectively discriminate DNP from other aromatic compounds. So, 2 (or 3) might have
potential application for distinguishing 2,4-dinitrophenylhydrazine from other
aromatic compounds. The research of developing novel chemosensors for other
nitro-aromatic compounds is underway.
Supporting Information
Tables, figures, fluorescence for 2, 2·DNP (or 3, 3·DNP) with this paper can be found
in the online version.
ACKNOWLEDGMENTS
Financial supports for this work were by Tianjin Natural Science Foundation (No.
1
8JCZDJC99600), the National Natural Science Foundation of China (No. 21572159)
and the Program for Innovative Research Team in University of Tianjin (TD13-5074).
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1
6

ò Three bis-imidazolium salts 1-3 have been prepared and characterized.
ò The structure of 1 was confirmed through X-ray analysis.
ò 2 (or 3) had high sensitivity and selectivity, and great affinity for
2
,4-dinitrophenylhydrazine (DNP), and can effectively discriminate from DNP
and other aromatic compounds via the method of fluorescence.
4
-1
4
-1
ò The K value of 5.6 × 10 M for 2·DNP and 4.5 × 10 M for 3·DNP
SV
-10
-10
ò The detection limits are 3.97 × 10 mol/L for 2 and 2.58 × 10 mol/L for 3.

Conflicts of interest
There are no conflicts to declare.