Imidazole-based dithienylethenes as a selective chemosensors for iron(III) ions

Article abstract of DOI:10.1016/j.dyepig.2011.08.005

Four novel imidazole-based dithienylethenes have been successfully synthesized in good yields. Their structures have been confirmed by NMR spectrometry, mass spectrometry, and elemental analyses. UV/Vis absorption spectra indicated that these dithienylethenes can easily isomerize between the open-ring and closed-ring isomers upon irradiation with UV or visible light in solution, and that the respective closed-ring isomers show decreased fluorescence properties compared with the open-ring isomers. Moreover, the open-ring and closed-ring isomers display high selectivity toward Fe ³⁺, such that the addition of Fe³⁺ obviously suppresses their fluorescence intensity.

Full text of DOI:10.1016/j.dyepig.2011.08.005

Dyes and Pigments 92 (2012) 961e966
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Imidazole-based dithienylethenes as a selective chemosensors for iron(III) ions
*
*
Shuyuan Huang, Ziyong Li, Sisi Li, Jun Yin , Shenghua Liu
Key Laboratory of Pesticide and Chemical Biology, Ministry of Education, College of Chemistry, Central China Normal University, Wuhan 430079, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel imidazole-based dithienylethenes have been successfully synthesized in good yields. Their
structures have been confirmed by NMR spectrometry, mass spectrometry, and elemental analyses.
UV/Vis absorption spectra indicated that these dithienylethenes can easily isomerize between the open-
ring and closed-ring isomers upon irradiation with UV or visible light in solution, and that the respective
closed-ring isomers show decreased fluorescence properties compared with the open-ring isomers.
Moreover, the open-ring and closed-ring isomers display high selectivity toward Fe3þ, such that the
addition of Fe3þ obviously suppresses their fluorescence intensity.
Received 23 June 2011
Received in revised form
1 August 2011
Accepted 3 August 2011
Available online 10 August 2011
Keywords:
Dithienylethene
Imidazole
Ó 2011 Elsevier Ltd. All rights reserved.
Molecular recognition
Photochromism
Fluorescence
Iron(III) ions
1. Introduction
photochromism and giving rise to some novel properties. There
have been some recent reports in the field of dithienylethenes.
Photochromic dithienylethenes are considered to be among the
most promising systems for applications in optical memory media
and switching devices due to their excellent photochromic proper-
ties coupled with thermal stability, fatigue resistance, and sensitivity
[1e7]. Cyclopentenes and derivatives bearing thiophene substitu-
ents attached via their 2- and 3-positions have been extensively
investigated [8e11]. Therefore, current research interest is focused
on the introduction of heterocycles to obtain novel photochromic
materials with excellent properties. Imidazole as a building block has
been widely used in many fields, such as biology [12], medicine [13],
ionic liquids [14] as well as electronic and optical materials [15e17]
due to its biological activity and available sites for further function-
alization. Furthermore, the imidazole moiety has also been widely
applied in supramolecular chemistry. For example, the imidazole NH
has been used as a hydrogen-bond donor in anion sensors [18e23].
Moreover, imidazole shows excellent coordinative abilities toward
metal ions [24e31], so imidazole derivatives have also been applied
in environmental monitoring, industrial process control, metal-
loneurochemistry, and biomedical diagnostics [32e34].
For example, Huang et al. and others have reported imidazole-
substituted thiophene cyclopentenes [35e37]. Recently, we also
synthesized a series of imidazole-substituted thiophene cyclo-
pentenes [26,38]. At the same time, the imidazole ring has also been
used as a bridging unit in substituting the cyclopentene ring [39]. As
mentioned in the introduction, the imidazole moiety has beenwidely
used in chemosensors. However, there have been few investigations
concerning the use of imidazole-based dithienylethenes as chemo-
sensors. In the present work, we have designed and synthesized four
dithienylethenes incorporating imidazole and N-methylated imid-
azole bridging units (Scheme 1), and have investigated the photo-
chromic and fluorescence properties of their open-ring and closed-
ring isomers. Furthermore, we have also investigated the effects of
metalions. These studies have revealed that: 1) the respective closed-
ring isomers show decreased fluorescence properties compared with
their open-ring isomers, 2) the respective open-ring and closed-ring
isomers show an obvious decrease in fluorescence when iron (III)
ions are added.
Therefore, the imidazole ring has been incorporated into the
photochromic dithienylethene unit with a view to optimizing the
2. Experimental
2.1. General procedures
All manipulations were carried out under an argon atmosphere
by using standard Schlenk techniques, unless otherwise stated. All
* Corresponding authors. Tel./fax: þ86 27 67867725.
E-mail addresses: yinj@mail.ccnu.edu.cn (J. Yin), chshliu@mail.ccnu.edu.cn (S. Liu).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.08.005

962
S. Huang et al. / Dyes and Pigments 92 (2012) 961e966
ether/DCM ¼ 1:2, v/v) afforded the compound 1 as white solid
(79 mg, 54%) and the compound 2 as yellow solid (95 mg, 60%).
Compound 1: 1H NMR (400 MHz, DMSO):
d
ppm ¼ 2.03 (s, 6H, CH3),
7.35 (d, J ¼ 4 Hz, 2H, thiophene-H), 7.48e7.62 (m, 13H, Ar-H), 8.07
(d, J ¼ 8 Hz, 2H, Ar-H), 13.01(s, 1H, NH). 13C NMR (100 MHz, DMSO):
d
ppm ¼ 14.65, 120.61, 121.23, 123.14, 124.04, 124.95, 125.25, 128.82,
129.14,131.81,132.78,149.46. MS (m/z): 488.04 (Mþ). Anal. Calcd for
C31H24N2S2: C, 76.19; H, 4.95; N, 5.73; S, 13.12. Found: C, 76.15; H,
4.83; N, 5.71, S, 13.16. Compound 2: 1H NMR (400 MHz, CDCl3):
d
ppm ¼ 1.99 (s, 6H, CH3), 3.82 (s, 6H, OCH3), 6.88e6.90 (m, 4H,
Ar-H), 6.96 (s, 2H, thiophene-H), 7.25e7.50 (m, 7H, Ar-H), 7.91
(d, J ¼ 8 Hz, 2H, Ar-H), 9.47 (s, 1H, N-H). 13C NMR (100 MHz, CDCl3):
Scheme 1. The molecular structures of dithienylethenes 1e4.
d
ppm ¼ 14.88, 55.37, 114.28, 121.47, 123.74, 125.31, 126.78, 128.88,
128.97, 129.64, 146.44, 159.01. MS (m/z): 548.09 (Mþ). Anal. Calcd
for C33H28N2O2S2: C, 72.23; H, 5.14; N, 5.11; S,11.69. Found: C, 72.03;
H, 5.02; N, 5.13; S, 11.72.
reagents and starting materials were obtained from commercial
suppliers and used without further purification. DMF was dried with
magnesium sulfate then distilled under vacuum. Column chroma-
tography was performed on silica gel (200e300 mesh). 1H and 13C
NMR spectra were collected on American Varian Mercury Plus 400
spectrometer (400 MHz) or 600 MHz. 1H and 13C NMR chemical
shifts are relative to TMS. Elemental analyses (C, H, N) were per-
formed by the Microanalytical Services, College of Chemistry, CCNU.
Electron impact ionization (EI) mass spectra were carried on Trace
MS 2000. UVeVis spectra were obtained on U-3310 UV Spectro-
photometer. Fluorescence spectra were taken on a Fluoromax-P
luminescence spectrometer (HORIBA JOBIN YVON INC.). Diketone
60 was prepared by literature methods [40]. Target compounds
1e4 were prepared according to the synthetic route presented in
Scheme 2 by modified procedures of reported methods [41,42].
2.2.2. Synthesis of 3 and 4
This synthesis was similar to the synthesis of 1 and 2, using the
compound 9 instead of 8. Compound 3: Yield, 70%. 1H NMR
(400 MHz, CDCl3):
d
ppm ¼ 2.06 (s, 3H, CH3), 2.19 (s, 3H, CH3), 3.65
(s, 3H, N-CH3), 7.05 (s, 1H, Ar-H), 7.21 (s, 2H, thiophene-H),
7.25e7.63 (m, 12H, Ar-H), 7.78 (d, J ¼ 8 Hz, 2H, Ar-H). 13C NMR
(100 MHz, CDCl3):
d
ppm ¼ 14.85, 15.13, 33.25, 109.66, 115.52,
119.39, 123.32, 124.17, 125.30, 125.50, 125.77, 126.49, 126.90, 127.74,
128.51, 128.64, 128.88, 129.12, 130.27, 130.43, 133.83, 134.52, 135.80,
140.55, 141.81, 145.48, 148.62. MS (m/z): 502.07 (Mþ). Anal. Calcd
for C32H26N2S2: C, 76.46; H, 5.21; N, 5.57; S, 12.76. Found: C, 76.25;
H, 5.09; N, 5.23, S; 12.79. Compound 4: Yield, 79%. 1H NMR
(400 MHz, CDCl3):
d
ppm ¼ 2.04 (s, 3H, CH3), 2.16 (s, 3H, CH3), 3.65
2.2. Synthesis of dithienylethene-based complexes 1e4, 7e9
(s, 3H, N-CH3), 3.80 (s, 3H, OCH3), 3.84 (s, 3H, OCH3), 6.85 (s, 1H,
thiophene-H), 6.87 (s, 1H, thiophene-H), 6.91e6.93 (t, J ¼ 4 Hz, 3H,
Ar-H), 7.09 (s, 1H, Ar-H), 7.45e7.56 (m, 7H, Ar-H), 7.78 (d, J ¼ 8 Hz,
2.2.1. Synthesis of 1 and 2
To a suspension of 8 (148 mg, 0.3 mmol), Pd(PPh3)4 (69 mg,
0.06 mmol) and K2CO3 (42 mg, 0.3 mmol) in a stirred solution of
toluene (12 mL), ethanol (6 mL) and water (2 mL), phenylboric acid
(109 mg, 0.9 mmol) or 4-methoxylphenylboric acid (136 mg,
0.9 mmol) was added under nitrogen atmosphere. The mixture was
heated to reflux for 8 h. After cooling to room temperature, cold
water (10 mL) was carefully added to the reaction mixture and the
water layer was extracted with DCM (3 ꢀ 20 mL). The combined
organic layers were dried (Na2SO4) before the solvent was removed
in vacuo. Chromatography of the residue on silica gel (petroleum
2H, Ar-H). 13C NMR (100 MHz, CDCl3):
d 14.85, 15.12, 29.66, 33.24,
55.31, 114.03, 114.23, 123.11, 123.31, 125.47, 126.57, 126.74, 126.79,
127.50, 128.48, 128.80, 128.88, 130.49, 135.68, 140.47, 141.71, 145.39,
148.47, 158.73, 159.32. MS (m/z): 562.05 (Mþ). Anal. Calcd for
C34H30N2O2S2: C, 72.57; H, 5.37; N, 4.98; S, 11.44. Found: C, 72.36; H,
5.49; N, 4.96; S, 11.47.
2.2.3. Synthesis of 7
A mixture of ammonium acetate (3.1 g, 40 mmol) and benzal-
dehyde (0.53 g, 5 mmol) was refluxed for 1 h in acetic acid (25 mL)
Scheme 2. Regents and conditions: (a) oxalyl chloride, aluminium trichloride, CS2, 53%; (b) benzaldehyde, ammonium acetate, acetic acid, 63%; (c) NBS, acetic acid, benzene, 81%;
(d) phenylboric acid or 4-methoxylphenylboric acid, Pd(PPh3)4, potassium carbonate, toluene, water, ethanol, 60%; (e) methyl iodide, potassium carbonate, DMF, 82%. (f) phenylboric
acid or 4-methoxylphenylboric acid, Pd(PPh3)4, potassium carbonate, toluene, water, ethanol, 79%.

S. Huang et al. / Dyes and Pigments 92 (2012) 961e966
963
under nitrogen atmosphere. Then the compound 6 (1.25 g, 5 mmol)
was added and the mixture was further refluxed overnight. The
mixture was poured into cold water (50 mL), neutralized with 10%
aq. Na2CO3 and extracted with DCM (3 ꢀ 20 mL). The combined
organic layers were dried (Na2SO4) and the solvent was removed in
vacuum. The compound was purified by column chromatography
(petroleum ether/ethyl acetate ¼ 20:1, v/v) to yield (1.06 g, 63%) of
methylated with CH3I in the presence of K2CO3 in DMF under
nitrogen atmosphere to obtain the dithienylethenes 3 and 4, which
showed perfect solubility due to this modification of the imidazole
ring. All of the dithienylethenes were characterized by standard
analytical techniques, such as NMR, mass spectrometry, and
elemental analysis (see the Supporting Information).
a white solid. 1H NMR (400 MHz, CDCl3):
d
ppm ¼ 1.92 (s, 6H, CH3),
3.2. Photochromic and fluorescent properties
6.80 (d, J ¼ 4 Hz, 2H, thiophene-H), 7.19 (d, J ¼ 8 Hz, 2H, thiophene-
H), 7.35e7.38 (m, 3H, Ar-H), 7.86 (d, J ¼ 4 Hz, 2H, Ar-H). 13C NMR
The absorption spectra of each of the compounds were
measured at room temperature from 1.0 ꢀ 10ꢁ5 mol/L solutions. As
shown in Fig. 1a, the absorption maxima of compounds 1o and 2o
(100 MHz, CDCl3):
d
ppm ¼ 14.39, 109.84, 124.99, 125.38, 125.92,
126.23, 128.78, 128.89, 129.48, 130.10, 136.28, 146.59. MS (m/z):
336.00 (Mþ). Anal. Calcd for C19H16N2S2: C, 67.82; H, 4.79; N, 8.33; S,
19.06. Found: C, 67.68; H, 4.68; N, 8.31; S, 19.11.
were observed at 328 nm and 326 nm
(
3
¼ 1.81 ꢀ104 and
2.48 ꢀ 104 L molꢁ1 cmꢁ1, respectively), which were considered to
correspond to a
pep* transition [43]. After N-methylation of the
2.2.4. Synthesis of 8
imidazole ring, the absorptions of compounds 3o and 4o showed
slight blue-shifts compared with compounds 1o and 2o possibly
due to the decrease of the conjugated degree after N-methylation
on the imidazole rings, which was in well agreement with our
previous report [26]. Fig. 1b shows the changes in the absorption of
compound 2 (1.0 ꢀ 10ꢁ5 mol/L in CH2Cl2) upon irradiation with UV
7 (2 g, 6 mmol) was suspended in a stirred solution of benzene
(50 mL) and acetic acid (50 mL). Subsequently NBS (2.24 g,
12.6 mmol) was added and the solid material dissolved. The reac-
tion mixture was refluxed for 4 h. After cooling, the mixture was
poured into 3 M aq. NaOH solution (100 mL) and yellow precipitate
was formed immediately. The solid was collected by filtration,
washed with water and dried under vacuum to give the compound
light (
l
¼ 302 nm). In general, the absorption of the ring-open
isomer at a shorter wavelength (326 nm) decreased, while the
absorption of the ring-closed isomer at a longer wavelength
8 in 81% yield. 1H NMR (400 MHz, CDCl3):
6.82 (s, 2H, thiophene-H), 7.39e7.45 (m,3H, Ar-H), 7.87 (d, J ¼ 4 Hz,
2H, Ar). 13C NMR (100 MHz, DMSO):
128.81, 129.63, 132.11, 133.27, 133.89, 134.33, 134.63, 135.10, 135.60,
146.67. MS (m/z): 493.99 (Mþ). Anal. Calcd for C19H14Br2N2S2: C,
46.17; H, 2.85; N, 5.67; S, 12.97. Found: C, 46.11; H, 2.79; N, 5.66; S,
12.99.
d
ppm ¼ 2.00 (s, 6H, CH3),
(505 nm) increased, since
the two thiophene rings [44]. Upon irradiation with visible
light ( > 402 nm), the 2c underwent a cycloreversion reaction to
p-electrons delocalized and extended to
d
ppm ¼ 14.34, 111.44, 125.27,
l
thering-opened isomer 2o. The corresponding N-methylated
compound 4 behaved similarly to compound 2. As shown in Fig. 1c,
a new absorption band appeared at 510 nm owing to the formation
of the ring-closed isomer upon irradiation with UV light. The
isomerization properties of the other synthesized compounds upon
irradiation with UV and visible light were also investigated and
showed similar changes (see Figures S1 and S2 in the Supporting
Information). According to these data, we found that compounds
1e4 showed similar photochromic properties. The absorption
characteristics and the quantum yields of open-ring (4c-o) and
closed-ring isomers (4o-c) of the compounds were summarized in
Table 1.
2.2.5. Synthesis of 9
A mixture of 8 (2.85 g, 4 mmol), and K2CO3 (1.6 g, 8 mmol) in dry
DMF (100 mL) was stirred for 0.5 h at room temperature. Subse-
quently CH3I (1.64 g, 8 mmol) was added and the mixture was
further stirred overnight at room temperature. Then the reaction
mixture was poured into ice water (100 mL), yellow precipitate was
formed. After filtering, washed with H2O and dry under vacuum,
the compound 9 was obtained in 82% yield. 1H NMR (400 MHz,
CDCl3):
d
ppm ¼ 1.98 (s, 3H, CH3), 2.20 (s, 3H, CH3), 3.53 (s, 3H,
Subsequently, their fluorescence spectra were obtained in
CH2Cl2 at room temperature. According to the data (Table 1),
compound 1o exhibited an obvious fluorescence emission at
456 nm. However, upon irradiation with 254 nm UV light, the
fluorescence intensity was quenched by ca. 76% as the ring-open
isomer 1o was converted into the ring-closed isomer 1c, as depic-
ted in Fig. 2a. Similar results were obtained when compounds
2o, 3o, and 4o were irradiated with UV light (Fig. 2b; Figures S3 and
S4 in the Supporting Information). Furthermore, we found
that the fluorescence intensities of the N-methylated dithienyle-
thenes 3o and 4o were higher than those of the corresponding
non-methylated dithienylethenes 1o and 2o. In addition, the fluo-
rescence quantum yields which were measured using quinoline
sulfate (4f ¼ 0.55, in 0.1 M aqueous H2SO4) as a reference of diary-
lethenes 3 and 4 were found to be higher than those of diary-
lethenes 1 and 2, which indicated that N-methylation of the
imidazole ring improves the fluorescence quantum yields. The
photochromic parameters of the synthesized compounds are
summarized in Table 1.
N-CH3), 6.74 (s, 1H, thiophene-H), 6.93 (s, 1H, thiophene-H),
7.24e7.48 (m, 3H, Ar-H), 7.70 (d, J ¼ 8 Hz, 2H, Ar-H). 13C NMR
(100 MHz, CDCl3):
d
ppm ¼ 14.49, 14.97, 32.96, 110.30, 114.07,
121.98, 125.97, 128.48, 128.71, 128.94, 130.10, 131.69, 132.68, 132.93,
135.49,135.61,140.60,148.67. MS (m/z): 507.78 (Mþ). Anal. Calcd for
C20H16Br2N2S2: C, 47.26; H, 3.17; N, 5.51; S, 12.62. Found: C, 47.12; H,
3.23; N, 5.66; S; 12.65.
3. Results and discussion
3.1. General synthetic procedure of 1e4, 7e9
The synthetic route to the target compounds (1e4) is shown in
Scheme 2. In our target compounds, two thiophene rings are
attached to the imidazole moiety via their 2-positions, and the
p-conjugation is extended by benzene or 4-methoxybenzene rings,
which were appended by Suzuki coupling reactions. The imidazole
unit was obtained in 63% yield by condensation of diketone 6 and
benzaldehyde in the presence of ammonium acetate in acetic acid
under nitrogen atmosphere. After bromination with NBS,
compound 8 was obtained in high yield. The target dithienyle-
thenes 1 and 2 were synthesized by palladium-catalyzed Suzuki
coupling reactions. However, they showed very poor solubility,
which was consistent with observations in previous literature
reports [26,38]. Therefore, the NH of the imidazole ring was
3.3. Fluorescence probe for Fe3þ
Subsequently, we investigated the changes in the fluorescence
properties of the open- and closed-ring forms in acetonitrile at
2.0 ꢀ 10ꢁ5 mol/L at room temperature when metal ions were
added. As shown in Fig. 3a, the fluorescence intensity of compound

964
S. Huang et al. / Dyes and Pigments 92 (2012) 961e966
Table 1
Absorption and fluorescence characteristics of dithienylethenes 1e4 in DCM
(1.0 ꢀ 10ꢁ5 mol/L).
Compound lAmbasx/nma
a
0.30
0.25
0.20
0.15
0.10
0.05
0.00
lAmbasx/nmb
Fc
lAmbasx/nmd
Fe
1o
2o
3o
(
3
ꢀ 104/
(
3
ꢀ 104/
(
)
L molꢁ1 cmꢁ1
(Open)
)
L molꢁ1 cmꢁ1
(PSS)
)
4o-c (l/nm) 4c-o (l/nm)
1
2
3
4
328(1.81)
326(2.48)
312(2.87)
316(2.50)
548(0.13)
504(0.72)
424(0.33)
528(0.07)
0.082(548) 0.0024(328) 456(0.04)
0.405(504) 0.0019(326) 456(0.04)
0.179(424) 0.0022(312) 458(0.06)
0.193(528) 0.0017(316) 457(0.11)
4o
a
b
c
Absorption maxima of open-ring isomers.
Absorption maxima of closed-ring isomers.
Quantum yields of open-ring (4c-o) and closed-ring isomers (4o-c), respectively.
Emission maxima of fluorescence.
d
e
Quantum yields of fluorescence.
3o decreased significantly in the presence of Fe3þ ions, while
other metal ions (such as Kþ, Ca2þ, Cu2þ, Zn2þ, Co2þ, Ni2þ, Fe2þ
,
300
400
Cd2þ and Hg2þ) did not exert such a quenching effect on the fluo-
rescence under the same conditions. This result suggests that
this compound displays remarkable “turn-off” fluorescence, and
shows high selectivity toward Fe3þ among the various metal ions
Wavelength / nm
0.28
0.24
0.20
0.16
0.12
0.08
0.04
0.00
b
vis
UV
a
140
120
100
80
0s
35s
120s
210s
360s
540s
780s
vis
UV
60
40
300
350
400
450
500
550
600
650
20
Wavelength / nm
0.28
0.24
0.20
0.16
0.12
0.08
0.04
0.00
0
c
UV
vis
350
400
450
500
550
600
650
Wavelength / nm
b
150
125
100
75
0s
35s
60s
140s
240s
390s
480s
600s
50
vis
UV
25
300
350
400
450
500
550
600
0
Wavelength / nm
350
400
450
500
550
600
Fig. 1. (a) Absorption spectra of dithienylethenes 1e4. Absorption spectral changes of
compound 2 (b) and 4 (c) by photoirradiation in CH2Cl2 (1.0 ꢀ 10ꢁ5 mol/L) at room
temperature.
Wavelength / nm
Fig. 2. Emission intensity changes of compounds
1
(
lex ¼ 328 nm) (a) and
3
(
lex ¼ 312 nm) (b) in CH2Cl2 (1.0 ꢀ 10ꢁ5 mol/L) with UV light irradiation.

S. Huang et al. / Dyes and Pigments 92 (2012) 961e966
965
120
100
80
60
40
20
0
the closed-ring forms in acetonitrile were performed under the
same conduction when metal ions were added. The results display
that these closed-ring isomers exhibit a quenching effects on the
fluorescence in the presence of Fe3þ ions. As shown in Fig. 4,
Figures S11 and S12 (see Supporting Information), they show high
selectivity toward Fe3þ among the various metal ions tested as well
as the ring-open isomers. Subsequently, we investigated the
influence of pH values on the rate of quenching of 2o in the pres-
ence of Fe3þ. As shown in Supporting Information, when the pH
values of systems were higher than 9.0, the rate of quenching
obviously decreases. Similar results were also observed when other
compounds were investigated (Figures S13eS16 in the Supporting
Information).
Fluorescence intensity changes are usually described in terms
of the decrease factor (DF), which is calculated from the fluores-
cence intensities of the chemosensor without (F0) or with metal
ions (F). According to these data (Figures S7eS10, S17eS20 in the
Supporting Information), we found that all of the open-ring and
closed-ring isomers displayed highly selective binding of Fe3D
ions.
free
1eq
2eq
3eq
4eq
6eq
8eq
10eq
a
120
100
80
60
40
20
0
350
400
450
500
550
600
Fe3+
Wavelength/nm
300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm
160
140
120
100
80
b
160
140
120
100
80
free
1eq
2eq
3eq
4eq
6eq
8eq
10eq
a
35
30
25
20
15
10
5
free, Cu2+, Ca2+
60
K+, Zn2+, Co2+
40
Ni2+, Fe2+, Cd2+, Hg2+
20
0
350
400
450
500
550
600
60
Wavelength /nm
Fe3+
40
Fe3+
20
0
300 350 400 450 500 550 600 650 700 750 800
Wavelength/nm
0
Fig. 3. Fluorescence responses of 3o (a) and 4o (b) in the presence of different metal
ions (Kþ, Ca2þ, Cu2þ, Zn2þ, Co2þ, Ni2þ, Cd2þ, Hg2þ, Fe2þ and Fe3þ) in acetonitrile/water
solution (100:1, v/v, 10 equiv.); (Inset) Fluorescence response changes by the addition
of Fe3D from 0 to 10 equiv.
300
350
400
450
500
550
600
650
Wavelength/nm
50
45
40
35
30
25
20
15
10
5
b
tested. A similar phenomenon was also observed in the fluores-
cence spectra of the other open-ring isomers 1o, 2o, and 4o (see
Fig. 3b; Figures S5 and S6 in the Supporting Information). The
results indicated that the respective open-ring isomers can be used
as highly selective chemosensors for Fe3þ ions. The fluorescence
quenching of compounds 1e4 upon the addition of Fe3þ may result
from ligand-to-metal charge transfer (LMCT), in which the elec-
tronic charge is transferred from the ligand toward the coordi-
nating metal [26]. The electron donor of compounds was the
nitrogen atoms on the imidazole rings. The coordination with Fe3þ
decreased its electron donating ability, which resulted that
the electron transfer from the nitrogen atom to the imidazole
p-system was hindered. Moreover, we found that the addition of
Fe3þ ions resulted in slight shifts of the fluorescence spectra
compared with the free open-ring isomers. Subsequently, we
investigated the effects of metal ions toward the closed-ring
isomers, in which the closed-ring isomers were obtained by irra-
diating the solution of compounds 1e4 in CH2Cl2 with UV light, till
the photostationary state was reached. After removing the solvent,
the closed-ring isomers 1ce4c were obtained by purification using
TLC silica gel plates. The changes in the fluorescence properties of
free, Cu2+, Ca2+
K+, Zn2+, Co2+
Ni2+, Fe2+, Cd2+, Hg2+
Fe3+
0
300
350
400
450
500
550
600
650
Wavelength/nm
Fig. 4. Fluorescence responses of 1c (a) and 2c (b) in the presence of different metal
ions (Kþ, Ca2þ, Cu2þ, Zn2þ, Co2þ, Ni2þ, Cd2þ, Hg2þ, Fe2þ and Fe3þ) in acetonitrile/water
solution (100:1, v/v, 10 equiv.).

966
S. Huang et al. / Dyes and Pigments 92 (2012) 961e966
spontaneous emission from a large single crystal. J Am Chem Soc 2005;127:
4. Conclusion
10070e4.
[17] Sun YF, Cui YP. The synthesis, structure and spectroscopic properties of novel
oxazolone-, pyrazolone- and pyrazoline-containing heterocycle chromo-
phores. Dyes Pigm 2009;81:27e34.
[18] Yoon J, Kim SK, Singh NJ, Kim KS. Imidazolium receptors for the recognition of
anions. Chem Soc Rev 2006;35:355e60.
[19] Xu Z, Kim SK, Yoon J. Revisit to imidazolium receptors for the recognition of
anions: highlighted research during 2006e2009. Chem Soc Rev 2010;39:
1457e66.
[20] Caltagirone C, Gale PA. Anion receptor chemistry: highlights from 2007. Chem
Soc Rev 2009;38:520e63.
[21] Singh NJ, Jun EJ, Chellappan K, Thangadurai D, Chandran RP, Hwang I, et al.
Quinoxalineeimidazolium receptors for unique sensing of pyrophosphate and
acetate by charge transfer. Org Lett 2007;9:485e8.
[22] Ihm H, Yun S, Kim HG, Kim JK, Kim KS. Tripodal nitro-imidazolium receptor
for anion binding driven by (C-H)þ—X- hydrogen bonds. Org Lett 2002;4:
2897e900.
[23] Chellappan K, Singh NJ, Hwang I-C, Lee JW, Kim KS. A Calix[4]imidazolium[2]
pyridine as an anion receptor. Angew Chem Int Ed 2005;44:2899e903.
[24] Sundberg RJ, Martin RB. Interactions of histidine and other imidazole deriv-
atives with transition metal ions in chemical and biological systems. Chem
Rev 1974;74:471e517.
[25] Liu H, Chen Y. A bifunctional metal probe with independent signal outputs
and regulable detection limits. Eur J Org Chem 2009;30:5261e5.
[26] Li ZY, Lin Y, Xia JL, Zhang H, Fan FY, Zeng QB, et al. Synthesis of novel dia-
rylethene compounds containing two imidazole bridge units and tuning of
their optical properties. Dyes Pigm 2011;90:245e52.
Four novel imidazole-based dithienylethenes have been success-
fully synthesized in good yields. UV/Vis absorption spectra indicated
that these dithienylethenes can easily isomerize between the open-
ring and closed-ring isomers upon irradiation with UV or visible
light in solution, and that the respective closed-ring isomers show
decreased fluorescence properties compared with the open-ring
isomers. Moreover, the respective open-ring and closed-ring
isomers display high selectivity toward Fe3þ, such that the addition
of Fe3þ obviously decreases their fluorescence intensities. In this
respect, our research provides a basis for the future construction
of multifunctional molecular architectures as well as switchable
chemosensors.
Acknowledgments
The authors acknowledge financial support from National
Natural Science Foundation of China (Nos. 2072039, 20931006,
21072070) and Program for Changjiang Scholars and Innovative
Research Team in University (No. IRT0953).
[27] Chen BZ, Wang MZ, Wu YQ, Tian H. Reversible near-infrared fluorescence
switch by novel photochromic unsymmetrical-phthalocyanine hybrids based
on bisthienylethene. Chem Commun; 2002:1060e1.
Appendix. Supplementary material
[28] Qin B, Yao RX, Zhao XL, Tian H. Enhanced photochromism of 1,2-
dithienylcyclopentene complexes with metal ion. Org Biomol Chem 2003;1:
2187e91.
[29] Tian H, Qin B, Yao RX, Zhao XL, Yang SJ. A single photochromic molecular
switch with four optical outputs probing four inputs. Adv Mater 2003;15:
2104e7.
Supplementary data related to this article can be found online at
doi:10.1016/j.dyepig.2011.08.005.
References
[30] Tchouka H, Meetsma A, Browne W. Spectroscopic and magnetic properties of
a series of
m
-cyano bridged bimetallic compounds of the type MIIeNCeFeIII
[1] Irie M. Diarylethenes for memories and switches. Chem Rev 2000;100:
1685e716.
[2] Kobatake S, Imao S, Yamashiro Y, Terakawa Y. Photoswitching of an alcohol-
sensitive photochromic diarylethene. Tetrahedron Lett 2011;52:1905e8.
[3] Tian H, Yang SJ. Recent progresses on diarylethene based photochromic
switches. Chem Soc Rev 2004;33:85e97.
[4] Tian H, Feng YL. Next step of photochromic switches. J Mater Chem 2008;18:
1617e22.
[5] Bianco A, Bertarelli C, Rabolt JF, Zerbi G. Diarylethenes with electroactive
substituents: a theoretical study to understand the effect on the IR spectrum
and a simple way to read optical memory in the mid-IR. Chem Mater 2005;17:
869e74.
(M ¼ Mn, Co, and Zn) using the building block [FeIII(CN)5imidazole]2ꢁ. Inorg
Chem 2010;49:10557e70.
[31] Ikuko K, Yuri M, Yukinari S, Naohide M, Toshio N, Masaaki K. Spontaneous
resolution induced by self-organization of chiral self-complementary cobal-
t(III) complexes with achiral tripod-type ligands containing three imidazole
groups. J Am Chem Soc 2002;124:629e40.
[32] Lim MH, Lippard SJ. Metal-based turn-on fluorescent probes for sensing nitric
oxide. Acc Chem Res 2007;40:41e51.
[33] Jiang P, Guo Z. Fluorescent detection of zinc in biological systems: recent
development on the design of chemosensors and biosensors. Chem Rev 2004;
248:205e29.
[34] Gao J, Gu H, Xu B. Multifunctional magnetic nanoparticles: design, synthesis,
and biomedical applications. Acc Chem Res 2009;42:1097e107.
[35] Xiao SZ, Yi T, Zhou YF, Zhao Q, Li FY, Huang CH. Multi-state molecular
switches based on dithienylperfluorocyclopentene and imidazo [4,5-f] [1,10]
phenanthroline. Tetrahedron 2006;62:10072e8.
[6] Majumdar D, Lee HM, Kim J, Kim KS. Photoswitch and nonlinear optical
switch: theoretical studies on 1,2-bis-(3-thienyl)-ethene derivatives. J Chem
Phys 1999;111:5866e72.
[7] Shim S, Joo T, Bae SC, Kim KS, Kim E. Ring opening dynamics of a photo-
chromic diarylethene derivative in solution.
J Phys Chem A 2003;107:
[36] Lee PH-M, Ko C-C, Zhu NY, Yam VW-W. Metal coordination-assisted near-
infrared photochromic behavior: a large perturbation on absorption wave-
length properties of N, N-donor ligands containing diarylethene derivatives
by coordination to the rhenium(I) metal center. J Am Chem Soc 2007;129:
6058e9.
[37] Yam VW-W, Lee JK-W, Ko C-C, Zhu NY. Photochromic diarylethene-containing
ionic liquids and N-heterocyclic carbenes. J Am Chem Soc 2009;131:912e3.
[38] Yuan JJ, Li ZY, Hu M, Li SS, Huang SY, Yin J, et al. Synthesis and photochromic
properties of imidazole-based diarylethenes. Photochem Photobiol 2011;10:
587e91.
8106e10.
[8] Nakatsuji S. Recent progress toward the exploitation of organic radical
compounds with photo-responsive magnetic properties. Chem Soc Rev 2004;
33:348e53.
[9] Pu SZ, Li H, Liu G, Liu WJ, Cui SQ, Fan CB. Synthesis and the effects of
substitution upon photochromic diarylethenes bearing an isoxazole moiety.
Tetrahedron 2011;67:1438e47.
[10] Irie M, Kobatake S, Horichi M. Reversible surface morphology changes of
a photochromic diarylethene single crystal by photoirradiation. Science 2001;
291:1769e72.
[39] Liu H, Chen Y. The photochromism and fluorescence of diarylethenes with
a imidazole bridge unit: a strategy for the design of turn-on fluorescent dia-
rylethene system. J Phys Chem A 2009;113:5550e3.
[40] Cantalupo SA, Crundwell G. 1,2-Bis(3-methyl-2-thienyl)ethane-1,2-dione.
Acta Crystallogr E Struct 2005;E61:o3079e80.
[41] Lucas LN, Jong JJ, vanEsch JH, Kellogy RM, Feringa BL. Syntheses of dithie-
nylcyclopentene optical molecular switches. Eur J Org Chem 2003;2003:
155e6.
[42] Eseda AO, Li W, Gao R, Zhang M, Hao X, Liang T, et al. Syntheses, structures,
and fluorescent properties of 2-(1h-imidazol-2-yl)phenols and their neutral
Zn(II) complexes. Inorg Chem 2009;48:9133e46.
[11] Zheng HY, Zhou WD, Yuan MJ, Yin XD, Zuo ZC, Ouyang CB, et al. Optic and
proton dual-control of the fluorescence of rhodamine based on photochromic
diarylethene: mimicking the performance of an integrated logic gate. Tetra-
hedron Lett 2009;50:1588e92.
[12] Bando T, Sugiyama H. Synthesis and biological properties of sequence-specific
DNA-alkylating pyrroleeimidazole polyamides. Acc Chem Res 2006;39:935e44.
[13] Bellina F, Cauteruccio S, Montib S, Rossi R. Novel imidazole-based com-
bretastatin A-4 analogues: evaluation of their in vitro antitumor activity and
molecular modeling study of their binding to the colchicine site of tubulin.
Bioorg Med Chem Lett 2006;16:5757e62.
[14] Peter W, Wilhelm K. Ionic liquidsdnew “solutions” for transition metal
catalysis. Angew Chem Int Ed 2000;39:3772e89.
[43] Li ZX, Liao LY, Sun W, Xu CH, Zhang C, Fang CJ. Reconfigurable cascade circuit
in
a photo- and chemical-switchable fluorescent diarylethene derivative.
[15] Bellina F, Cauteruccio S, Rossi RS. Synthesis and biological activity of vicinal
diaryl-substituted 1H-imidazoles. Tetrahedron 2007;63:4571e624.
[16] Park S, Kwon OH, Kim S, Park S, Choi MG, Cha M, et al. Imidazole-based excited-
state intramolecular proton-transfer materials: synthesis and amplified
J Phys Chem C 2008;112:5190e6.
[44] Li XC, Tian H. One-step synthesis and photochromic properties of a stable
triangle terthiophene. Tetrahedron Lett 2005;46:5409e12.