S. Huang, et al.
DyesandPigments172(2020)107830
(d, J = 7.8 Hz, 2H), 4.00 (d, J = 7.1 Hz, 2H), 3.91 (s, 3H), 3.78 (s, 3H),
1.09 (t, J = 7.0 Hz, 3H). 13C NMR (125 MHz, CDCl3) 191.70, 160.00,
158.36, 145.25, 138.44, 137.11, 135.99, 132.33, 129.96, 129.47,
129.32, 127.92, 127.03, 123.12, 114.63, 113.60, 54.96, 39.47, 29.56,
16.06. HRMS: calcd for [M+H]+: 413.1820; found: 413.1883.
BMIB (Yellow solid, 49%) 1H NMR (500 MHz, CDCl3) 10.03 (s, 1H),
7.89 (dd, J = 17.1, 8.0 Hz, 4H), 7.54 (d, J = 8.4 Hz, 2H), 7.26 (d,
J = 7.1 Hz, 3H), 7.15 (d, J = 8.2 Hz, 2H), 6.88 (d, J = 7.6 Hz, 4H), 6.80
(d, J = 8.4 Hz, 2H), 5.15 (s, 2H), 3.83 (s, 3H), 3.79 (s, 3H). 13C NMR
(125 MHz, CDCl3) 191.64, 162.51, 160.00, 158.53, 145.91, 138.49,
137.75, 137.23, 135.99, 132.34, 130.18, 129.88, 129.19, 128.82,
128.01, 127.62, 125.82, 122.49, 114.42, 113.66, 55.25, 55.18, 48.43.
HRMS: calcd for [M+H]+: 475.1977; found: 475.2047.
filtered through a 0.22 μM membrane and configured into samples of
different mercury ion concentrations. Later, 10 μL of BMIBD (1 mM)
was added to 990 μL of the samples individually. All samples were
measured after incubation at 37 °C for 30 min and then the fluorescence
spectra were recorded using 380 nm as the excitation wavelength.
2.8. Fluorescence imaging of Hg2+ in living cells
MDA-MB-231 cells were incubated at 37 °C in Dulbecco's modified
Eagle medium (DMEM) supplemented with 10% fetal bovine serum
(FBS), 100 IU/mL penicillin and 100 μg/mL streptomycin in a humi-
dified incubator containing 5% CO2. Cells were treated with mercury
chloride (5 μM or 10 μM) in DMEM for 1 h at 37 °C, and then washed
several times with PBS in order to remove the free Hg2+ ions. BMIBD
was incubated in the cell medium for 30 min, then washed the media
with PBS. The fluorescence imaging of Hg2+ in living cells on the slide
was observed by confocal fluorescence microscope following addition
of 50% glycerol.
2.5. Synthesis of CBMIB
Terephthalaldehyde (134 mg, 1 mmol) and 4-chloroaniline (127 mg,
1 mmol) were dissolved in ethanoic acid (15 mL) followed stirring for
1 h at 25 °C. Compound 2 (270 mg, 1 mmol) and ammonium acetate
(540 mg, 7 mmol) were added subsequently. The mixture was heated at
120 °C overnight. After quenching with water, the mixture was adjusted
to neutral, extracted with ethyl acetate, dried over anhydrous magne-
sium sulfate and dried in vacuo. Purify the crude product by column to
obtain 4-(1-(4-chlorophenyl)-4,5-bis(4-methoxyphenyl)-1H-imidazole-
2-yl)benzalde -hyde (CBMIB) as a pale yellow solid (207 mg, 42%). 1H
NMR (500 MHz, CDCl3) δ 9.98 (s, 1H), 7.79 (d, J = 8.3 Hz, 2H), 7.61
(d, J = 8.3 Hz, 2H), 7.56–7.52 (m, 2H), 7.29 (dd, J = 6.6, 2.0 Hz, 2H),
7.07–7.04 (m, 2H), 7.02–6.98 (m, 2H), 6.84–6.80 (m, 4H), 3.81 (s, 3H),
3.80 (s, 3H). 13C NMR (125 MHz, CDCl3) δ 191.69, 159.52, 158.68,
144.91, 138.93, 135.89, 135.52, 135.49, 134.59, 132.34, 130.87,
129.64, 129.58, 129.55, 129.07, 128.43, 126.64, 122.06, 114.14,
113.73, 55.21, 55.18. HRMS: calcd for [M+H]+: 495.1431; found:
495.1491.
3. Results and discussion
3.1. Design and synthesis
With BMI in hand, we explored how the different substituents as
conformation functional groups to affect the AIE properties of BMIs.
The synthetic routes were shown in Scheme 2. All of these products
have been confirmed by 1HNMR, 13CNMR, and BMIBD has been further
confirmed by mass spectrum.
3.2. Photophysical properties of BMI derivatives
First, the optical properties of these derivatives were investigated. It
was observed that fluorescence emission of these compounds was red-
shifted with a decreased tendency in intensity if the polarity of the
solvent increased (Fig. S1). To further verify the ICT mechanism, den-
sity functional theory was implemented to calculate the electron cloud
distributions of these derivatives ((B3LYP level of theory, 6-31G (d3,
3p) basis set). As shown in Fig. 1, the electronic clouds in the HOMO
orbitals of these molecules were largely allocated on the electron do-
nating groups, while in the LUMO orbitals, the electron clouds were
mainly distributed in the electron withdrawing groups. These results
coincided with the characteristics of the ICT effect, thus further de-
monstrating that these molecules possess ICT effect. As a comparison,
the electronic cloud distribution of the probe BMIBD appeared to be
more dispersed, thus its ICT effect was weaker than other BMI deri-
vatives. Additionally, from their most stable conformation, it could be
observed that as the steric hindrance of imidazole group increased, the
coplanarity of the entire molecule gradually weakened.
2.6. Synthesis of BMIBD
BMIB (237 mg, 0.5 mmol) was dissolved in 5 mL absolute ethanol,
p-toluene sulfonic acid and 2-mercaptoethanol (79 mg, 1 mmol) was
added followed stirring for 6 h under the protection of nitrogen at 80 °C.
Subsequently the solvent was removed and the mixture was separated
by the high-performance liquid chromatography to afford 2,2'-(((4-(1-
benzyl-4,5-bis(4-methoxyphenyl)-1H-imidazole-2-yl)phenyl)methy-
lene)bis(sulfanediyl))bis(ethan-1-ol) (BMIBD) as pale white solid
(187 mg, 61%). 1H NMR (500 MHz, DMSO‑d6) 7.78 (d, J = 7.7 Hz, 2H),
7.65 (d, J = 7.7 Hz, 2H), 7.39 (d, J = 7.9 Hz, 2H), 7.31 (d, J = 7.8 Hz,
2H), 7.20 (s, 3H), 7.03 (d, J = 7.8 Hz, 2H), 6.96 (d, J = 8.0 Hz, 2H),
6.83 (d, J = 4.1 Hz, 2H), 5.37 (s, 1H), 5.26 (s, 2H), 3.79 (s, 3H), 3.75 (s,
3H), 3.54 (t, J = 6.6 Hz, 4H), 2.71-2.66 (m, 2H), 2.57 (dd, J = 13.2,
6.6 Hz, 2H). 13C NMR (125 MHz, DMSO‑d6) 160.83, 160.01, 145.29,
144.70, 139.86, 135.52, 132.93, 131.52, 130.16, 130.09, 129.08,
128.66, 128.26, 126.75, 124.05, 119.19, 117.94, 115.21, 114.63,
Followed, the AIE effect of these derivatives were investigated. They
showed good solubility in DMSO, but not in water. Therefore, different
DMSO/water systems were selected as the measurement system of the
AIE effect. As can be seen from Fig. 2b, with the water ratio elevated
and the fluorescence was substantially quenched when the water ratio
was 50%. Obviously, BMI is a typical ACQ fluorophore. Similarly,
BMIM showed an obvious solvent effect. As the polarity of solvents
increased, fluorescence emission of BMIM red-shifted with a decreased
in intensity, as shown in Fig. S1f. Additionally, the AIE effect of BMIM
was also studied. The fluorescence intensity of BMIM decreased when
the water ratio was 10%–40% and remained unchanged between 40%
and 90%. However, when the water ratio reached 99%, the fluores-
cence intensity had no obvious change. It indicated that BMIM has a
weak AIE effect. On the other hand, we examined whether BMIE has
the AIE effect. When the water ratio was lower than 80%, the fluores-
cence intensity of BMIE was very weak (Fig. 2f). Upon the water ratio
was higher than 80%, the fluorescence intensity increased rapidly and
60.85, 55.26, 51.97, 48.70, 34.85, 31.57. HRMS: calcd for [M+H]+
613.2150; found: 613.2157.
:
2.7. General procedure for the spectra measurement
The stock solutions of Na+, Ca2+, Fe2+, Al3+, Mn2+, Ag+, Fe3+
,
Cr3+, Cd2+, K+, Mg2+, Zn2+ and Co2+ were prepared in deionized
water and were first configured as a 10 mmol. First prepared 1 mM of
BMIBD solution in DMSO, then diluted to 10 μM using PBS (10 mM, pH
7.4). Then 10 μL of different ion solution, 980 μL of PBS and 10 μL of
BMIBD were configured as a 1 mL solution. The BMIBD solution
(1 mM) of 10 μL was taken from the pipette gun and added to the PBS
buffer containing different Hg2+ concentration at 990 μL. To study the
practical application of BMIBD for detecting mercury ions, a real water
sample from Xiangjiang River was collected. The water sample was
3