Synthesis of new fluorene compounds for highly selective sensing of picric acid, Fe3+ and L-arginine

Article abstract of DOI:10.1016/j.molstruc.2020.128395

Two compounds, 4-(4-(7-bromo-9, 9-dimethyl-fluoren-2-yl) phenyl) pyridine (4) and 2, 7-Bis-(4-tert-butyl-phenyl)-9,9-dimethyl-9H-fluorene (5), were synthesized by bromination, methylation and Suzuki coupling reaction with fluorene as the starting material. The compounds were used as fluorescent sensors to detect some nitro compounds (NACs), metal cations and amino acids. The test results showed that the 4-(4-(7-bromo-9, 9-dimethyl-fluoren-2-yl) phenyl) pyridine (4) as fluorescence sensor 1 detected 2, 4, 6-Trinitrophenol (TNP) at the magnitude of 10 μM, with a sensitivity of K_sv was 4.6 × 10⁵ M^?1 and the detection limit was 6.9 × 10^?7 M. The sensitivity detection for Fe³⁺ with K_sv value was 1.4 × 10⁵ M^?1, and the detection limit was 3.6 × 10^?7 M. At the same time, it was found that 2, 7-Bis-(4-tert-butyl-phenyl)-9,9-dimethyl-9H-fluorene (5) as fluorescence sensor 2 also showed excellent selective recognition ability toward the arginine (L-arg) in biomolecules with K_sv values of 3.9 × 10 ⁴ M^?1, and the detection limit was 1.15 × 10^?6 M for arginine (L-arg).

Full text of DOI:10.1016/j.molstruc.2020.128395

Journal of Molecular Structure 1217 (2020) 128395
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis of new fluorene compounds for highly selective sensing of
picric acid, Fe3 and
-arginine
þ
L
Yingying Han , Jinxin Zhao , Haicheng Yang , Xintong Huang , Xinyue Zhou , Tianqi Hui ,
*
Jie Yan
College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian, 116029, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 December 2019
Received in revised form
Two compounds, 4-(4-(7-bromo-9, 9-dimethyl-fluoren-2-yl) phenyl) pyridine (4) and 2, 7-Bis-(4-tert-
butyl-phenyl)-9,9-dimethyl-9H-fluorene (5), were synthesized by bromination, methylation and Suzuki
coupling reaction with fluorene as the starting material. The compounds were used as fluorescent
sensors to detect some nitro compounds (NACs), metal cations and amino acids. The test results showed
that the 4-(4-(7-bromo-9, 9-dimethyl-fluoren-2-yl) phenyl) pyridine (4) as fluorescence sensor 1
18 March 2020
Accepted 5 May 2020
Available online 10 May 2020
detected 2, 4, 6-Trinitrophenol (TNP) at the magnitude of 10
m
M, with a sensitivity of Ksv was
5
ꢁ1
ꢁ7
3þ
4
.6 ꢀ 10
M
and the detection limit was 6.9 ꢀ 10 M. The sensitivity detection for Fe with Ksv value
Keywords:
Fluorene compounds
Synthesis
5
ꢁ1
ꢁ7
was 1.4 ꢀ 10 M , and the detection limit was 3.6 ꢀ 10 M. At the same time, it was found that 2, 7-Bis-
(
4-tert-butyl-phenyl)-9,9-dimethyl-9H-fluorene (5) as fluorescence sensor 2 also showed excellent se-
4
ꢁ1
,
Fluorescence sensor
lective recognition ability toward the arginine (L-arg) in biomolecules with Ksv values of 3.9 ꢀ 10
M
ꢁ
6
L-argnine
and the detection limit was 1.15 ꢀ 10 M for arginine (L-arg).
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
equipment technology, such as gas chromatography, mass spec-
trometry, surface enhanced Raman spectroscopy, cyclic voltam-
metry and energy dispersive X-ray diffraction, neutron activation
analysis and nuclear quadruple resonance are also available, but the
required equipment is not easy to transport and operate, as a result,
these technologies are not suitable for real-time detection [12e16].
The advantages of fluorescence detection for nitro compounds are
As a new type of luminescent material in recent years, fluores-
cent organic compounds have been used as fluorescent probes to
detect various organic substances, such as nitro aromatic explo-
sives, anions, cations, and biological molecules [1,2]. Because of the
sensitivity, selectivity, portability, short response time and conve-
nience of visual detection, fluorescence detection has important
potential applications in analysis, biology and clinical biochemistry
3
þ
3þ
portability and ease of use. Metal cations (such as Cr , Fe and
3
þ
Al ) play an important role in the environment and human health
[17], among which iron is the most abundant metal element in cells
and is widely found in many biological enzymes and proteins. The
[
3e5]. Because fluorene polymers have many advantages, such as
easy purification, clear structural characteristics and excellent hole
transport capacity, and is a promising photoelectric material [6,7].
Therefore, more and more attention has been paid to the devel-
opment and study of the fluorescent properties of fluorene com-
pounds [8,9]. Nitro aromatic compounds are common components
in explosives and are therefore widely used in industrial develop-
ment [10]. Most nitro aromatic compounds are classified as
dangerous drugs. Among them, 2, 4, 6-trinitrophenol (TNP) is
considered as an extremely dangerous chemical due to its strong
explosive capacity and low safety factor [11]. Some advanced
3
þ
imbalance of Fe involved in the metabolism of human cells can
cause kidney failure, anemia and other diseases [18,19]. Because
traditional detection methods usually have shortcomings such as
low sensitivity, long analysis time and complicated operation [20].
3
þ
It is necessary to develop fluorescent sensors that can detect Fe
.
Amino acids are the building blocks of proteins, and they have a
very important relationship with the life activities of organism [21].
Different kinds of amino acids have their own functions in organ-
isms. Arginine (Arg) is particularly abundant in protamine and
histones, and a lack of Arg can lead to a recessive genetic disorder
called the urea cycle disorder [22e25]. Therefore, Arg has received
extensive attention in the fields of biochemistry and medicine. At
present, researchers generally synthesize different kinds of amino
*
Corresponding author.
E-mail address: yhc1994@live.cn (J. Yan).
https://doi.org/10.1016/j.molstruc.2020.128395
022-2860/© 2020 Elsevier B.V. All rights reserved.
0

2
Y. Han et al. / Journal of Molecular Structure 1217 (2020) 128395
acid derivatives by reacting amino acid molecules with other
compounds, thereby enhancing the sensitivity and selectivity of
amino acids [26].
of methyl iodide (12.5 g, 88 mmol) and dimethyl sulfoxide (80 mL)
was added by the constant pressure drop funnel for 8 h, and then
the reaction was continued for 12 h. Finally, the reaction was cooled
to room temperature. The reaction solution was poured into a
beaker containing 250 mL water, and the purple solid was precip-
itated, then filtered and dried. The solid was added to 80e100 mL
anhydrous ethanol and reflux dissolves it completely. Then the
crude product was filtered and recrystallized by the ethanol to
obtain 12.38 g yellow needle-like crystal with the yield of 86.3%.
2
. Experimental
2.1. Instruments and reagents
Infrared spectra were obtained on a Bruker AXS TENSOR-27 FT-
ꢁ1
IR spectrophotometer with pressed KBr tablet in the range of
2, 7-Dibromo-9, 9-dimethyl-fluorene: IR (KBr), n/cm : 3028
(¼CHd stretching); 2966, 2923, 2866, (CeH stretching); 1595,
1576, 1447 (Ph skeleton vibration); 1397 (CeH bending).
ꢁ
1
4
000e400 cm . A JASCO-V-570UV/VIS/VIS/NIR spectrophotom-
eter was used to record UVevis absorption. Also, F-7000 fluores-
cence spectrometer (Hitachi, Japan) was used to record
fluorescence emission studies. The mixed solvent VC2H5OH/VH2O
3
1H NMR (500 MHz, CDCl , d/ppm): 7.53e7.51 (m, 4H, Flu-H1,
Flu-H1’, Flu-H3 and Flu-H3’), 7.45e7.43 (dd, J1 ¼ 8.1 Hz,
ꢁ
5
(
3:1) was selected as the solvent and a 1 ꢀ 10 M solution of the
J2 ¼ 1.2 Hz, 2 H, Flu-H2 and Flu-H2’), 1.45 (s, 6 H, CH3).
sensor 1 and sensor 2. We take 3 mL and put them in quartz
cuvette. A Bruker AV 500 OFT-NMR spectrometer can record
1
H
2.4. Synthesis of 4-(4-(7-bromo-9, 9-dimethyl-fluoren-2-yl)
NMR and 13C NMR in CDCl
-d at 500 and 125 MHz respectively.
3
6
phenyl) pyridine (4)
Dichloromethane, trichloromethane, absolute ethyl ethanol,
dimethyl sulfoxide, dioxane, petroleum ether and ethyl acetate
were all analytically pure. Terakis(triphenylphosphine)palladium
was purchased from Shanghai Sinopharm Company, and other
chemicals and reagents were purchased from Tianjin Komiou
Chemical Reagent Co., Ltd.
9, 9-dimethyl-2, 7-dibromofluorene (5.6 g, 16 mmol) and po-
tassium phosphate (5.1 g, 96 mmol) were added to a three-necked
flask which containing 100 mL dioxane, and terakis(-
triphenylphosphine)palladium (1.1 g, 0.45 mmol) were added for
nitrogen protection. In addition, 4-(4-pyridinyl)phenoboric acid
(
5.73 g, 32 mmol) was dissolved with 80 mL dioxane in constant
2
.2. Synthesis of 2, 7-dibromo-9H-fluorene (2)
pressure drop funnel and added it into three-necked flask drop by
drop. Then the system was heated up to 85 C, and slowly heated up
to 110 C with refluxing for 24 h. Finally, the reaction solution was
extracted with ethyl acetate and saturated salt water before being
ꢂ
Fluorene (10.0 g, 60 mmol) in 150 mL chloroform was added into
ꢂ
a 250 mL three-necked flask and was stirred at room temperature
to dissolve all fluorene. After being dissolved for 0.5 h, anhydrous
ferric chloride (150 mg, 0.92 mmol) was added to the system, and
the whole reaction system was wrapped in tin foil. The whole re-
4
dried over MgSO . After drying under reduced pressure, the inter-
mediate product was purified by silica gel column chromatography
using petroleum ether and ethyl acetate with a volume ratio of 1:2.
The pale yellow flake crystal was obtained with a yield of 56%.
ꢂ
action system was then cooled to 0 C in an ice water bath after
stirring and avoiding light. The constant pressure drop funnel was
used to slowly add bromine (6.4 mL, 123 mmol) into the system.
When the drop was completed for 1.5e2 h, the ice water bath was
removed, and the temperature of the system was slowly raised to
room temperature continuing 3 h. After the reaction, 200 mL
saturated sodium thiosulfate solution was gradually added into the
system with stirring, and the red color of the system completely
disappeared. The bottom layer was extracted with dichloro-
methane for 3e4 times, and the organic phase was combined. The
organic phase was dried with anhydrous magnesium sulfate, and
the rotary evaporation was used to remove dichloromethane to
obtain a pale yellow solid. The solid was added with 120 mL
anhydrous ethanol and refluxed to be dissolved completely. After
cooling and filtering, the crude product was recrystallized in
ethanol to obtain 17.24 g white needle-like crystal with the yield of
4
-(4-(7-bromo-9,9-dimethyl-fluoren-2-yl) phenyl) pyridine (4):
ꢁ
1
IR (KBr),
n
/cm : 3432 (OeH stretching); 3019 (¼CHd stretching);
2
1
927, 2849 (CeH stretching); 1598, 1454 (Ph skeleton vibration);
398 (CeH bending); 1264 (CeN asymmetric stretching).
1
9
9’
3
H NMR (500 MHz, CDCl , d/ppm): 8.72 (s, 2H, Py-H , Py-H ),
8
8’
4
5
6
7
1
.81e7.75 (m, 5H, Py-H , Py-H , Flu-H , Flu-H and Flu-H ), 7.69 (s,
H, Flu-H )7.69e7.60 (m, 5H, Py-H , Py-H and Flu-H ), 7.51 (d,
1
7
7
0
3
2
J ¼ 8.1 Hz, 1 H, Flu-H ))1.45 (s, 6 H, CH
3
).
/ppm):157.4, 155.5, 151.2, 149.7, 143.8,
41.2, 139.4, 139.1, 138.1, 131.7, 129.3, 128.9, 127.8, 127.6, 122.8, 122.7,
21.9, 48.7, 28.5.
13
3
C NMR (125 MHz,CDCl ,d
1
1
2.5. Synthesis of 2, 7-bis(4-(tert-butyl)phenyl)-9, 9-dimethyl-9h-
fluorene (5)
8
8.4%.
, 7-Dibromo-9H-fluorene (2): IR (KBr),
ꢁ
1
2
n
/cm : 3056 (¼CHd
Weighing 9, 9-dimethyl-2, 7-dibromofluoren (5.6 g, 16 mmol),
potassium phosphate (5.1 g, 96 mmol), 4-tert-butylbenzeneboronic
acid (5.73 g, 32 mmol), terakis(triphenylphosphine)palladium
(1.1 g, 0.45 mmol), and 100 mL dioxane were added to a three-
necked flask and protected with nitrogen. The system was
stretching); 2924, 2855 (CeH stretching); 1598, 1575, 1455 (Ph
skeleton vibration); 1397 (CeH bending).
1
1
3
H NMR (500 MHz, CDCl , d/ppm): 7.61 (s, 2H, Flu-H and Flu-
1
’
3
3’
H ), 7.54e7.52 (d, J ¼ 8.1 Hz, 2H, Flu-H and Flu-H ), 7.47e7.45 (dd,
2
2’
ꢂ
J
1
¼ 8.1 Hz, J
2
¼ 1.5 Hz, 2H, Flu-H and Flu-H ), 3.79 (s, 2H, CH
2
).
refluxed at 110 C for 24 h. After that, the reaction solution was
extracted with ethyl acetate and brine for 3 to 4 times. The organic
layer was dried over anhydrous magnesium sulfate, and then
reduced pressure rotary evaporation left a small amount of solu-
tion. The mixed solvent was subjected to column chromatography
using petroleum ether and dichloromethane with a volume ratio of
20:1 to obtain 5.46 g pale yellow needle crystals and the yield is
57.6%.
2
.3. Synthesis of 2, 7-dibromo-9, 9-dimethyl-fluorene (3)
2
, 7-Dibromo-9H-fluorene (13.2 g, 40 mmol) was poured into a
2
50 mL three-necked flask. Then measuring 70 mL dimethyl sulf-
oxide and adding it to the three-necked flask. Stirring at room
temperature until the solid is completely dissolved. Potassium
hydroxide (10.0 g, 179 mmol) was crushed in a mortar and added to
a three-necked flask to turn the solution into purple black.
2, 7-bis(4-(tert-butyl)phenyl)-9, 9-dimethyl-9h-fluorene.(IR:
ꢁ
1
(KBr),
s
/cm : 3451 (
n
-OH), 3032 ((
2
n AreH), 2960、2875 (n eCH -),
ꢂ
Continuing to stirring and heating the oil bath to 35 C.The mixture
1625、1513、1461 (
n
Ar), 1363 ( -C-H).
d

Y. Han et al. / Journal of Molecular Structure 1217 (2020) 128395
3
Scheme 1. Synthetic routes.
1
H NMR (500 MHz, CDCl
3
,
d
/ppm): 7.79 (d,J ¼ 7.9Hz,2 H,Ph-
and 2, 7-bis (4-(tert-butyl)phenyl)-9, 9-dimethyl-9h-fluorene (5).
In conclusion, the systematic synthesis of new fluorene materials
and the efficient detection of TNP, ferric ion and amino acid are the
focus of this paper.
Compounds 2, 3 and 4 were confirmed by single crystal struc-
ture. Fig. 1 shows the structure unit diagram of compound 4. X-ray
single crystal study shows that compound 4 CCDC (1,581,412) is
crystallized in a monoclinic system with P2 (1) 1/c space group. The
crystallographic data of compounds 2, 3 and 4 (Figure S1A) and
main bond length (Figure S1B) were also obtained.
H3,3 ), 7.66 (s,2 H,Ph-H ), 7.63 (d,J ¼ 7.8Hz,2 H, Ph-H ), 7.59
0
2,2
0
4,4
0
5
,5
0
6,6
0
7,7
0
8,8
0
), 1.58
(
(
d,J ¼ 7.8Hz,4 H, Ph-H
), 7.51 (d,J ¼ 7.7Hz,4H,Ph-H
1
,1
0
s,6 H,CH
3
),1.39 (s,18 H,CH
C NMR (125 MHz,CDCl
26.3, 125.6, 125.2, 120.8, 119.7, 46.5, 34.0, 30.9, 26.7.
The synthetic routes and detailed procedures are shown in
3
).
1
3
3
,
d
/ppm):153.9,149.7,139.7,138.2,137.4,
1
Scheme 1. 2, 7-Dibromo-9H-fluorene (2) and 2, 7-Dibromo-9, 9-
dimethyl-fluorene (3) were synthesized by methods reported in
the literature [2728]. The synthesis of the compound 2 is improved
,
by the original literature method, by adding KOH first, followed by
mixing with 2, 7-Dibromo-9H-fluorene, and then adding a mixture
of methyl iodide and the solvent of DMSO. This can make 2, 7-
Dibromo-9H-fluorene fully form carbon anions under alkaline ac-
tion, which is more favorable for nucleophilic substitution reaction
3
. Result and discussion
3
.1. Emission spectra study of compounds
The emission spectra of fluorene and compounds 2, 3, 4 and 5, as
with CH
tallization, the solvent was firstly stirred at room temperature for
h and then refluxed by heating. Which raised the yield to 86.3%
literature yield: 76%). Two rare new multifunctional fluorescent
fluorene derivatives were synthesized by Suzuki coupling reaction:
-[4-(7-bromo-9,9-dimethyl-9H-fluoren-2-yl)phenyl]pyridine (4)
3
I. In addition, when using anhydrous ethanol for recrys-
shown in Fig. 2, were measured at a voltage of 500 V, a slit width of
4
(
5:5, and an excitation wavelength of 280 nm. For compounds 2 and
3, we can see that almost no fluorescence intensity is detected. The
maximum emission wavelength of fluorene was measured by
13 nm, and for compound 4, the maximum emission wavelength
4
3
Fig. 1. The crystal structure of compound 4 (hydrogen atoms are omitted for clarity).

4
Y. Han et al. / Journal of Molecular Structure 1217 (2020) 128395
2 5
Fig. 2. The fluorescence spectra of fluorene 1 and compounds 2、3、4、5 in C H OH.
Fig. 3. The column graph of sensor 1 reduction in fluorescence intensity (plotted as
quenching efficiency) upon the addition of different analytes.
was 400 nm, and the compound 5 was 367 nm. Compared with
fluorene, compound 1 and compound 3 exhibited significant fluo-
rescence quenching, and the maximum emission wavelength of
compound 4 appeared red-shifted. Compounds 2 and 3 are con-
taining bromine atoms. Under the condition of containing a
halogen atom, the excited state of the compound and the heavy
atom forms a composite material in a ratio of 1:1, and the degree of
spin orbital coupling in the composite is much greater than that of
the normal compound, and fluorescence quenching occurs [29,30].
Although compound 4 also contains a bromine atom, compound 4
has a benzene ring and a pyridine, which increases the conjugated
chain, resulting in a significant red-shift during fluorescence
detection. The benzene and the tert-butyl group are added to the
compound 5, and the maximum emission wavelength is blue-
shifted compared with the compound 4. It is presumed that the
presence of the tert-butyl group causes the molecular skeleton of
the original conjugated system to be distorted, and the conjugation
effect was weakened.
I
= I ¼ KSV ½Qꢃ þ 1
Where KSV represents the Stern-Volmer constant and is used to
(1)
O
indicate the sensitivity of the sensor. A liner relationship for Stern-
Volmer plot was obtained for TNP with correlation coefficient (R )
equal to 0.97 as shown in Fig. 4(B). I and I represent the fluores-
0
2
cence intensity before and after the analyte is added, [Q] is the
molar concentration of the analyte. As shown in Fig. 4(A), the KSV
curve began to curve upward as the concentration of the analyte
increased. As shown in Fig. 4(C), it can be seen more intuitively that
the linear relationship exhibited at lower concentrations is mainly
due to static quenching, while the linear deviation at higher con-
centrations may be due to dynamic quenching. Static quenching is
usually due to the interaction of the ground state between the
analyte and the sensor. Dynamic quenching is mainly attributed to
energy and electron transfer between analyte and sensor. Fitting of
liner parts allows the quenching constants of sensor 1 to be
ꢁ1
ꢁ1
ꢁ1
8
910 M for 2-NP, 7555 M for 1, 2-DNB, 464546 M for TNP,
ꢁ ꢁ1 ꢁ1
1
3.2. Fluorescence detection of partial nitro compounds
9677 M for p-DNB, 2328 M for 4-cl-NB, 29903 M for DNP,
1
4691 M for 3-NP. In addition, the sensor’s detection limit (LOD)
for TNP is calculated by equation (2) [32],
ꢁ ꢁ1 ꢁ1
1
3535 M for 1,3,5-phe, 1696 M for p-NA, 43912 M for 4-NP,
ꢁ1
When the mixed solvent VC2H5OH: VH2O ¼ 3:1 was selected as
the solvent and different nitro aromatic compounds were added to
ꢁ
5
ꢁ1
the sensor with 10
M
concentration under the excitation
LOD ¼ 3
Where
s=k
(2)
wavelength of 320 nm, the fluorescence intensity was significantly
decreased. We selected TNP (2,4,6-trinitrophenol), 4-NP (4-
nitrophenol), DNP (2,4-dinitrophenol), 1,3,5-phe (phloroglucin),
p-DNB (p-dinitrobenzene), 2-NP (2-nitrophenol), 1,2-DNB (o-dini-
s
is the standard deviation of the measured blank and k is
the slope between the fluorescence intensity and the TNP con-
centration. After calculation, sensor 1 has a detection limit of
0
below the values of detection limits recently reported methods in
Table 1.
trobenzene),
3-NP
(3-nitrophenol),
4-Cl-NB
(4-
.69 mM for TNP. Present sensor 1 for TNP detection is considerably
chloronitrobenzene), p-NA (p-nitroacetophenone). As shown in
Fig. 3, by calculating the quenching percentage of several analytes
(
Q
P
¼(I
0
eI)/I
0
ꢀ 100%, I
0
and I are the luminescence intensity before
and after the addition of the nitro compound to the sensor). The
quenching percentage of sensor 1 for several analytes was as fol-
lows:97.86%, 81.25%,75.61%, 57.44%, 49.73%,46.75%, 42.95%, 32.03%,
3.2.1. Fluorescence detection of cations
The compound 2,7-bis(4-(tert-butyl)phenyl)-9,9-dimethyl-9H-
fluorene was used as a sensor which emitted blue fluorescence
with a maximum excitation wavelength of 320 nm. Under the same
conditions, we added Fe , Pb , Al , Mg , Fe , Cr , Mn , Ni ,
Zn , Cu , Co and Cd ions to the sensor with a concentration
19.83%, and 14.81%. The order of quenching efficiency is TNP>4-
NP DNP>1,3,5-phe p-DNB>2-NP>1,2-DNB>3-NP>4-Cl-
>
>
3
þ
2þ
3þ
2þ
2þ
3þ
2þ
2þ
NB > p-NA. The quenching efficiency revealed that TNP has faster
fluorescence quenching over the other nitro aromatic compounds.
We need to calculate the Stern-Volmer constants of the quenching
process.
2
þ
2þ
2þ
2þ
ꢁ5
of 10 M to detect the fluorescence intensity of the sensor. As
3
þ
shown in Fig. 5, it can be seen that Fe has the best quenching
effect on the sensor under the same concentration conditions. The
experimental results showed that the sensor has higher selectivity
We evaluated the sensitivity by the Stern-Volmer equation (1)
[
31],

Y. Han et al. / Journal of Molecular Structure 1217 (2020) 128395
5
Table 1
Comparative study of present sensor with previously reported sensors.
Material
LOD.(
m
M)
Solvent Condition
Heterocyclic aromatic chlorides
Hydrazide Schiff
Antipyrine Derivatives
Tris-Imidazolium Derivatives
Flourene Derivatives
0.78([33])
0.99([34])
1.74([35])
2.04([36])
Aqueous DMSO
Pure Aquatic
H2O:THF(7:3)
DMSO
0.69(present work)
C2H5OH:H2O(3:1)
Fig. 5. The fluorescent response of sensor 1 (10 mM) to selected metal ions (100 mM).
to Fe3
þ
.
3
.2.2. Micro-detection of ferric ions
In order to further investigate the effect of Fe concentration on
3þ
3þ
the fluorescence intensity of sensor 1, increase the Fe concen-
tration to 200 M, and the fluorescence spectrum is shown in Fig. 6.
As the concentration of Fe is increased, the luminous intensity of
m
3
þ
the sensor 1 decreased remarkably. When the concentration is
increased to 160
peak intensity reached the minimum with the 3.1-fold (I
decreasing when 160
the concentration of Fe was increased from 160 to 200
fluorescence intensity of the sensor did not change significantly.
The sensitivity constant of sensor 1 for Fe is 142098.8 M , and the
detection limit for Fe is 0.36
formula detection limit ¼ 3 /k.
m
M, the peak value reaches a minimum value. The
/I)
M equiv of Fe ions were added in. When
0
3
þ
m
3
þ
mM, the
3
þ
-1
3
þ
mM by formula I
0
/I ¼ KSV[Q] þ 1 and
s
3.3. Detection of biological small molecules
4-(4-(7-bromo-9, 9-dimethyl-fluoren-2-yl) phenyl) pyridine (4)
as sensor 2 emitted light green fluorescence with a maximum
excitation wavelength of 367 nm. The sensor 2 was used as a
fluorescent probe to investigate the effect of different kinds of
amino acids on the fluorescence intensity. Eight common amino
acids were selected: L-methylthiol (L-met), DL-valine (DL-val),
phenylalanine (D-phe), (L-arg) -leucine (L-leu), -glutamic acid (L-
glu), -aspartic acid (L-asp), glycine (gly). We added eight different
amino acids (50 equivalents) to a 10 M sensor and examined their
D-
L
L
L
ꢁ5
Fig. 4. (A) Plot of I
tration of analytes in lower concentration ranges of analytes (up to 50
SterneVolmer plots in different concentration ranges of TNP of sensor 1(up to 50
C) The 3D bars of reduction in fluorescene intensity of analytes in concentration range
of analytes for sensor 1 (up to 100 M).
0
/I of sensor 1 (
l
ex ¼ 325 nm and
l
em ¼ 400 nm) versusconcen-
M). (B)
M).
m
m
(
m

6
Y. Han et al. / Journal of Molecular Structure 1217 (2020) 128395
4
ꢁ1
The sensitivity constant of sensor 2 to L-arg is 3.9 ꢀ 10 M by the
formula I /k,
/I]KSV [Q] þ1 and the formula of detection limit ¼ 3
and the detection limit for L-arg is 1.15 M.
0
s
m
3.4. Fluorescence quenching mechanism
To better understand sensors 1 have better quenching effects.
We explain this by using an electronic donor-receptor system be-
tween the sensor and TNP. Since TNP is an electron-deficient
molecule, when the electron-deficient LUMO is located between
the HOMO and LUMO of electron-fluorene molecules, the mutual
conversion between electrons can cause sensing during excitation.
In the excitation process, the charge is transferred from the LUMO
of the sensor to the LUMO of TNP, resulting in fluorescence
quenching.
The HOMO and LUMO energies of the sensor and TNP are
calculated by density functional theory (DFT), as shown in Fig. 8,
showing that the LUMO of electron-deficient TNP is located be-
tween the HOMO and LUMO of the sensor. The energy difference
between LUMO and TNP indicates that TNP has a good fluorescence
quenching ability.
Fig. 6. Fluorescence quenching spectra of sensor 1 (10 mM) with different concentra-
tion of Fe (0e200 mM).
3
þ
In addition, to demonstrate the charge transfer mechanism
between the sensor and TNP, we performed a series of calculations
using the DFT. In the presence of TNP, the geometric optimization of
the sensor proves the most favorable position (in terms of energy)
for the interaction between TNP and the sensor. If TNP and the
sensor form a charge transfer complex, the LUMO energy of the
newly formed complex should be lower than that of the sensor
itself, while the HOMO energy is almost unchanged.
effect on the luminescence intensity behavior of the sensor. The
results are shown in Fig. 7 (A). As can be seen from the figure, the
sample showed higher selectivity for
intensity of other amino acids did not change much. In order to
further explore the quenching response of -arginine to the sensor,
we changed the concentration of -arginine added. The experi-
mental results are shown in Fig. 7 (B). From the experimental re-
sults, it can be seen that with the increase of -arginine
L-arginine. The fluorescence
L
L
L
As shown in Fig. 8, the energy diagrams of sensor 1, [sensor
concentration, the fluorescence intensity decreased gradually, and
the cause of fluorescence quenching may be related to the change
of free energy. When the sensor donor interacts with Arg, the
1
þ TNP] and TNP show that in the presence of TNP, the initial
HOMO and LUMO energy difference of sensor 1 falls from 4.13 to
2
2
.55 eV. During this process, the LUMO energy (ꢁ1.61 eV) of sensor
electron transfer is from the large
p bond to eC]N-. Therefore,
decreased by 1.62 eV compared to the LUMO energy (ꢁ3.23 eV) of
when the concentration of -arginine is continuously increased, the
L
the complex [sensor 1 þ TNP], while the HOMO energy did not
fluorescence intensity is gradually decreased. Moreover to under-
stand the extent and nature of fluorescence quenching in sensor 2
by L-arg, plotting the normalized fluorescence intensity at 367 nm
as a function of [L-arg] resulted in a nice liner relationship. A linear
change significantly.
4. Conclusions
2
relationship for L-arg with correlation coefficient (R ) equal to 0.99.
This paper takes fluorene as the starting material. Two new
Fig. 7. (A) The fluorescent response of sensor (10
mM) in the presence of different species amino acids (50 equiv.).（B）The fluorescence spectra of sensor (10 mM) in the presence of
L-arg (050 equiv.). Inset shows the change in fluorescene intensity at 367 nm against various equiv. of L-arg (050 equiv.).

Y. Han et al. / Journal of Molecular Structure 1217 (2020) 128395
7
Fig. 8. Calculated energy level diagram of sensor 1, TNP and [sensor 1 þ TNP].
fluorene compounds, 4-(4-(7-bromo-9,9-dimethyl-9h-fluorene-2-)
phenyl)pyridine(4) and 2,7-bis(4-(tert-butyl)phenyl)-9,9-
Department(Grant No. LJ2019008).
dimethyl-9h-fluorene(5)were synthesized by bromine reaction,
Appendix A. Supplementary data
methylation reaction and Suzuki coupling reaction. The synthe-
1
sized products were characterized by infrared spectroscopy,
H
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2020.128395.
NMR, 13C NMR and high resolution mass spectrometry. Some nitro
aromatic compounds (NACs) and metal cation were detected by
using
4-(4-(7-bromo-9,9-dimethyl-9h-fluorene-2-)phenyl)pyri-
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CRediT authorship contribution statement
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