An aggregation-induced emission enhancement fluorescent benzoxazine-derived macromolecule: Catalyst-free synthesis and its preliminary application for the determination of aqueous picric acid

Article abstract of DOI:10.1039/c6ra06942h

A tetraphenylethene-containing fluorescent macromolecular benzoxazine derivative (TPE-BOZ) was synthesized via the Mannich reaction under catalyst-free conditions using 4,4′-(1,2-diphenylethene-1,2-diyl)diphenol, poly(propylene glycol)-bis-(2-aminopropyle

Full text of DOI:10.1039/c6ra06942h

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PAPER
AIEE-type fluorescent benzoxazine-derivatived macromolecule:
catalyst-free synthesis and its preliminary application for aqueous
picric acid detection
Received 00th January 20xx,
Accepted 00th January 20xx
Yue Su a, Wei Shi b*, Xin Chen b, Shiyu Zhao a, Yonghai Hui a*, Zhengfeng Xie b
DOI: 10.1039/x0xx00000x
Tetraphenylethene-containing fluorescent macromolecular benzoxazine derivative (TPE-BOZ) was synthesized by utilizing
4, 4'-(1, 2-diphenylethene-1, 2-diyl)diphenol, poly (propylene glycol)-bis-(2-aminopropylether) and formaldehyde as
starting materials via Mannich reaction under catalyst-free condition. TPE-BOZ displays typical aggregation-induced
emission enhancement characteristic in ethanol (good solvent) and water (poor solvent), which stems from the
incorporation of tetraphenylethene moieties into its structure. The fluorescence of TPE-BOZ in high-water fraction
medium (ethanol/water =1/9, v/v) was effectively quenched by the introduction of picric acid, with the Stern-Volmer value
(ksv) of ~1.134×107 M-1. The detection limit of picric acid by TPE-BOZ reached as low as ~4.47 nM. The presence of other
common nitrocompounds and solvent residuals did not significantly interfere the recognizing of picric acid by TPE-BOZ.
TPE-BOZ can also be utilized as solid state probe to detect picric acid.
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condensation14 and thiol−ene click polymerizaꢀons15), the
exploration of other catalyst-free synthetic methods is still
desirable and can provides more flexibility for the preparation
Introduction
Stemming from their excellent film-forming capability, 1
good optical/chemical stability2 and low toxicity,3
macromolecules with fluorescent properties (FMs) have been
successfully utilized in many research fields, such as polymer
light-emitting diodes (PLEDs),4 waveguide materials,5
chem/bio-optical probes,6 and so forth. Metal-catalyzed
synthetic mythologies (Suzuki,7 Sonogashira,8 Stille9 and Click10
reactions, etc.) have been successfully adopted to prepare
various FMs. However, these protocols encountered the issues
that the employed metal-catalysts are expensive and the
corresponding catalyst-residuals were difficult to be
completely removed from obtained materials and might exert
unfavourable affect to their optical properties.11 From the view
point of synthesis, catalyst-free preparation of FMs is highly
desirable and provides significant convenience for large-scale
preparation. Despite of the great progress obtained in the
development of non-catalyst polymerization strategies
regarding the preparation of FMs (such as Schiff-base
of novel FMs.
Due to their convenient syntheses, non-catalyst thermal
curability and good mechanical performance, benzoxazine-
derivatives have gained increasing interest over the past
decade as high-performance thermoset resins.16 Benzoxazine-
based small molecules can be synthesized from phenolic
compounds, formaldehyde, and primary amines via mild-
conditioned Mannich reaction without catalyst.17 Because of
the simplicity of the preparation and the remarkable flexibility
of reactants selection, a variety of benzoxazine derivatives
bearing different functional groups have been successfully
synthesized to meet different application requirements.18-20 In
addition to the available success gained about preparation of
benzoxazine-based small molecules, a series of benzoxazine-
based macromolecules have also been prepared by utilizing
dihydric phenols and diamines as starting monomers.21 This
provides more attractive properties of resultant benzoxazines,
such as film-formability and other specific mechanical and
physical properties of macromolecules.22-25 Against form the
extensive studies about thermal and mechanical properties of
benzoxazines,26 to the best of our knowledge, the investigation
polymerizations,12
Diels−Alder
addiꢀon,13
thiol-Br
a.Key Laboratory of Petroleum and Gas Fine Chemicals, Educational Ministry of
China, School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi
830046, P. R. China.
about fluorescent benzoxazines is scarcely reported to date.27-
29
b.Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, School of
Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu,
610500, China.
Picric acid (PA), a type of famous explosive, has been
widely used for terrorist activities, military explosives,
fireworks, sterilizations of medical and dyes,30, 31 etc, whose
discharge to the environment leads the air and aquatic
systems being seriously polluted.30, 32 Many health problems
have been brought by PA, such as the damage to respiratory
* corresponding author: Tel: +86 028 83037306; fax: +86 028 83037305
E-mail address: shiwei80@swpu.edu.cn, xjuwshi@gmail.com (Wei Shi)
hyhai97@126.com (YH Hui)
Electronic Supplementary Information (ESI) available See DOI: 10.1039/x0xx00000x
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organs, skin irritation, nausea and skin allergy, etc.33 The Shimadzu RF-5301PC fluorescence spectrometer at room
exploration of discriminating methods for probing PA is an temperature.
important task and many methods, such as infrared Preparation of TPE-hexylamine-derivatived benzoxazine (M3)
spectroscopy, electrochemical workstation, X-ray diffraction
，
N-hexylamine (0.08g, 0.8 mmol) and formaldehyde (37%
thermal neutron analysis, transmission assay and fluorescence by weight in water, 0.13 g, 1.6 mmol) were dissolved in various
sensing have been successfully utilized to realize the solvents (illustrated in Table 1) (10 mL) and stirred at R.T.
recognition of PA.34-38 Among these identification methods, under nitrogen atmosphere. After half an hour, M2 (0.145g,
optical probes exhibit some attractive advantages, such as high 0.4 mmol) was added into the reaction mixture. After reacted
sensitivity, practical-simple detecting procedure and relatively for 6 h, the solution was washed with distilled water for 3
low-cost of relevant instruments. However, conventional times, collecting the organic phase and the solvent was
organosoluble fluorescent PA probes encountered their removed by rotary evaporator. The crude product was purified
inherent aggregation-induced fluorescence quenching issue, 39 on a silica-gel column using ethyl acetate/ petroleum ether
and thus were hard to be utilized to realize PA testing in (v/v=1/20) as eluent. The yields of product in different solvents
aqueous environment. One of the choices to solve this issue is were listed in Table 1
the utilization of water-insoluble, aggregation-induced
1H NMR (400 MHz, CDCl3) δ (TMS, ppm) 7.17 – 6.97 (m,
emission enhancement (AIEE)-featured agents 40, 41to act as 10H), 6.78 – 6.67 (m, 2H), 6.62 – 6.40 (m, 4H), 4.79 (d, 4H),
suitable candidates for PA detection in aqueous environment. 3.72 (d, 4H), 2.71 – 2.57 (m, 4H), 1.76 – 1.33 (m, 5H), 1.33 –
.
Herein, we report the synthesis of a type of novel 1.23 (m, 11H), 0.96 – 0.81 (m, 6H). 13C NMR (100 MHz, CDCl3) δ
macromolecular benzoxazine derivative, TPE-BOZ, via catalyst- (TMS, ppm) 152.60, 144.03, 139.69, 135.89, 131.37, 130.72,
free, click-reaction-like Mannich reaction. Optical properties of 130.47, 127.54, 126.13, 119.06, 115.31, 82.61, 51.35, 50.01,
TPE-BOZ were investigated in ethanol (good solvent)-water 31.74, 28.05, 26.94, 22.64, 14.07. FT-IR (KBr, cm−1): 921, 1490
(poor solvent) mixtures to evaluate its aggregation-induced (tri-substituted benzene ring), 1230, 1069 (C-O-C of oxazine
emission
enhancement
characteristic.
Preliminary ring), 2987-3077 (Ar-H), 2857-2960 (-C6H13).
investigation about the application of TPE-BOZ as optical Preparation of TPE-polyetheramine-derivatived
probe for PA recognizing in high-water-fraction medium macromolecular benzoxazine (TPE-BOZ)
(ethanol/water =1/9, v/v) was carried out here.
Poly(propylene glycol)bis(2-aminopropylether) (PEA)
(Mn=230, 0.092g, 0.4 mmol) and formaldehyde (37% by
weight in water, 0.13 g, 1.6 mmol) were dissolved in
chloroform (10 mL) and stirred at R.T. under nitrogen
atmosphere. After half an hour, M2 (0.146mg, 0.4 mmol) was
then added into the reaction tube. After refluxed for 6 h, the
solution was washed with distilled water for 3 times, and
chloroform was evaporated. The crude product was
redissolved in toluene and reprecipitated from petroleum
ether to afford the final TPE-BOZ product (yield 98%).
1H NMR (400 MHz, CDCl3) δ (TMS, ppm) 7.13-6.44 (16H,
Ar-H), 5.11-4.62(2H, N-CH2-O), 3.90-3.61 (2H, C-CH2-N), 3.61-
3.22 (4H), 3.18-2.97 (m, 2H), 1.32-0.77 (9H, alkyl H). FT-IR (KBr,
cm−1): 925, 1490 (tri-substituted benzene ring), 1262, 1072 (C-
O-C of oxazine ring), 3000~3083 (Ar-H). MALDI-TOF-MS (m/z):
1512.50.
Experimental
General
Unless otherwise specified, all reagents were purchased
from Adamas-beta Chemical Co. and used without further
purification. Tetrahydrofuran (THF) and toluene were distilled
from sodium in the presence of benzophenone and degassed
before use. Dichloromethane and chloroform were distilled
from CaH2 before use. In the titration experiments, all the
detected compounds (2, 4-dinitrophenol (2, 4-DNP), p-
nitrophenol (PNP), o-ntrophenol (ONP), p-nitrotoluene (PNT),
nitrobenzene (NB), nitromethane(NM), phenol (PhOH) ) were
dissolved in ethanol (10 mL) to afford 10-1 mol/L ethanol
solution, concentration of PA lower than 10-1 mol/L were
dissolved in distilled water to obtain corresponding solution.
Tetraphenylethylene (TPE) derivative containing two hydroxyl
groups (M2) has been synthesized according to the procedures
reported previously.42 Caution: PA is highly explosive and
should be handled carefully and only in very small quantities.
Instrumentation
Results and discussion
Synthesis
The synthesis of benzoxazine-derivatived macromolecule
(
TPE-BOZ) was fulfilled according to the synthetic route
1H NMR and 13C NMR spectra were recorded on VARIAN
INOVA-400 spectrometer operating at 400 MHz (for 1H) and
100 MHz (for 13C) in deuterated chloroform solution with
tetramethylsilane (TMS, δ = 0) as internal reference. FT-IR
spectra were performed on EQUINOX 55 FT-IR spectrometer
with KBr pellets. MALDI-TOF-MS was measured with Bruker
Autoflex. Scanning electron microscope (SEM) was conducted
on EVO MA 15 (ZEISS). UV-vis and fluorescence spectra were
determined by Shimadzu UV-2450 UV-Vis spectrometer and
displayed in Scheme 1. A type of small-molecular benzoxazine
model compound, M3, was initially synthesized by using
phenolic TPE-based compound (M2), n-hexylamine and
formaldehyde as starting materials (Scheme 1) to evaluate the
feasibility of the designing thought. The preparation of M3 was
conducted in a series of solvents at fixed reaction time (6
hours) in order to investigate the influence of the reaction
solvent. As shown in Table 1, the reaction in chloroform
afforded the highest yield among these systems. That may be
because nonpolar solvents are beneficial to the ring closure of
2 | J. Name., 2012, 00, 1-3
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the intermediate Mannich base, and polar solvents tend to
induce open-loop.43, 44 Moreover, the immiscibility of
chloroform with water (introduced along with aqueous
formaldehyde solution and as condensate during reaction) can
promotes the reaction, as reported in previous reports about
the preparation of benzoxazines.45
Scheme 1 Synthetic route of benzoxazines.
Table 1 Solvent effect on the synthesis of M3.
ring appear at ~ 921 cm-1 and ~ 1490 cm-1 (correspond to tri-
substituted benzene ring).47 Signals at 1230 cm-1 and 1069 cm-1
correspond to asymmetric and symmetric stretching of C-O-C
of oxazine ring, respectively.47 Absorption signals at 2987 ~
3077 cm-1 can be assigned to the stretching of aromatic
protons on TPE, and the peaks at 2857 ~ 2960 cm-1 ascribe to
the absorption signal of alkyl protons. Above-mentioned
characterizations indicate the successful synthesis of M3
through the catalyst-free synthetic protocol under mild
reaction condition.
Run
Solvent
Temperature
Yield a
(%)
(oC)
reflux
90
1
2
3
4
5
6
7
chloroform
toluene
1,4-dioxane
DMSO
93%
90%
88%
70%
67%
58%
73%
90
90
DMF
90
ethanol
THF
reflux
reflux
Given the successful synthesis of M3, we further extend
our attention to prepare macromolecular benzoxazine with
M2 (dihydric phenols), PEA (diamines), and formaldehyde as
starting monomers (Scheme 1) in refluxing chloroform without
the involvement of any catalyst. The obtained product (TPE-
BOZ) is soluble in common organic solvents, such as THF,
chloroform, DMF, DMSO and ethanol, etc, but insoluble in
water and petroleum ether. To verify the chemical structure of
a: reaction time was fixed at 6 hours.
Chemical structure of M3 was characterized by 1H NMR
Fig. S1), 13C NMR (Fig. S2) and FT-IR (Fig. S3) analyses. In Fig.
(
S1, resonance peaks at 4.79 and 3.72 ppm can be assigned to
the characteristic protons of benzoxazine ring (marked as
proton g and h). 46, 47 Signals at ~ 2.62 ppm correspond to
another set of methylene protons adjacent to N-atoms
(marked as f). Aromatic protons illustrate their signals at 7.17 ~
6.97 ppm. As displayed in 13C NMR spectrum (Fig. S2), the
characteristic carbon signals of benzoxazine ring (O-C-N and N-
C-Ar, shown as 8 and 7) appear at 82.61 ppm and 51.35 ppm,
respectively. The signal of aromatic carbon atom, which links
1
TPE-BOZ, H NMR spectra of TPE-BOZ and PEA are compared
in Fig. S4. As reflected by Fig. S4, there is significant difference
1
between H NMR spectra of TPE-BOZ and PEA. Compare with
the spectrum of pristine PEA (Fig. S4a), new resonance peaks
appear at 5.11 ~ 4.62 ppm and 3.90 ~ 3.61 ppm in Fig. S4b
,
which could be assigned to the characteristic protons locate at
benzoxazine ring (marked as proton d and e) according to
directly to the O-atom in TPE skeleton (marked as 10 in Fig. S2
)
47
previous reports.46, Besides this, aromatic proton signals
appears at ~ 152.60 ppm. The assignment of other carbons can
also be traced in Fig. S2. One can see in the FT-IR spectrum of
M3 (Fig. S3), characteristic absorption signals of benzoxazine
appear exclusively in Fig. S4b, which provides straightforward
evidence for the successful introduction of aromatic TPE
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segment into TPE-BOZ. FT-IR analysis also strongly supports
the successful preparation of TPE-BOZ. The absorption signal
at around 3367 cm-1, which can be assigned to the –NH2
groups in PEA (Fig. S5a), weakened significantly in the
a
water fraction
vol(%)
spectrum of TPE-BOZ
(
Fig. S5b), suggesting the appreciable
90
consumption of –NH2 during the coupling reaction between
M2 and PEA. In Fig. S5b, the characteristic absorption signals
of benzoxazine ring, appear at ~ 925 cm-1, ~ 1490 cm-1, ~ 1262
cm-1 and ~ 1027 cm-1, respectively. Absorption signals in the
range of 3000 ~ 3083 cm-1, which correspond to the stretching
of aromatic protons, appear distinctively in spectrum of TPE-
0
BOZ (Fig. S5b). Theses analyses strongly demonstrate that TPE-
BOZ has been successfully synthesized.
400
500
600
The molecular weight of TPE-BOZ has been characterized
by gel permeation chromatography (GPC) (using THF as eluent)
firstly. Unfortunately, we didn’t get reliable result, which might
be due to the strong adsorption of TPE-BOZ on column during
the test (might stems from the existence of polar polyether
groups in the structure of TPE-BOZ). We thus utilized matrix-
assisted laser desorption/ionization time-of-flight mass
spectrometry (MALDI-TOF-MS) to characterize the molecular
weight of TPE-BOZ. MALDI-TOF-MS indicates that TPE-BOZ is
an oligomer-like molecule, with molecular weight of ~ 1500
Wavelength (nm)
60
50
40
30
20
10
0
b
(Fig. S6), which corresponds to ~ 3 repeating units in its
backbone. Despite of the relatively low molecular weight of
TPE-BOZ, we found that it displayed good film-formability
(discussed later on).
0
20
40
60
80
100
Optical properties
As described in previous report, 48 TPE is a type of typical
AIEE agent. Considering the successful introduction TPE into
Water fraction(vol%)
TPE-BOZ
,
TPE-BOZ is expected to demonstrate AIEE
Fig. S8 and
characteristic in the same way. UV-Vis
(
)
fluorescence (Fig. 1) alteration of TPE-BOZ in ethanol (good
solvent) and water (poor solvent) mixing solvents (with
different composition ratio) was investigated to corroborate
our surmise. TPE-BOZ emits faint luminescence in pure ethanol
Fig. 1 Emission spectra of TPE-BOZ (1.0 × 10−5 M) in
EtOH/water mixtures with different fractions of water (a)
(excited by 320 nm); variation in (I/I0-1) of TPE-BOZ (1.0 × 10−5
M) with water fraction in EtOH/water mixtures (b) and visual
images of TPE-BOZ (1.0 × 10−5 M) in EtOH/water mixtures with
different solvent ratios (under a hand-held UV lamp, 365 nm)
(c).
(Fig. 1a), fluorescence intensity of TPE-BOZ remained almost
unchanged in solvent mixtures with water fraction lower than
40%, and increased steadily with the addition of more water
(
Fig. 1b). Such fluorescence alteration can be observed visually
Fig. 1c), confirmed that TPE-BOZ is a type of typical AIEE type
(
molecule. To find straightforward evidence for the poor
solvent-induced intermolecular aggregation of TPE-BOZ
,
Fluorescence of TPE-BOZ in film-state was investigated
by spin-coating its ethanol solution on glass substrate (Fig. 2).
Although the molecular weight of TPE-BOZ is not very large, it
displayed satisfactory film-forming ability. The resultant film is
uniform and fluorescent (inset in Fig. 2). Fluorescence
spectrum of TPE-BOZ film peaks at ~ 498 nm (Fig. 2), and the
whole spectrum profile is highly resemble to that in
ethanol/water (v/v=1/9), which might owing to the occurrence
of intermolecular aggregation in both cases.
scanning electron microscope (SEM) analysis of TPE-BOZ in
ethanol/water (v/v=1/9) was conducted. As reflected by Fig. S7
molecules of TPE-BOZ assembled together to form sphere-like
nanoparticles, suggests the occurrence of aggregation in this
medium.
,
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As expected, fluorescence of TPE-BOZ decreased gradually
with the addition of incremental PA (Fig. 3a). Detectable
emission quenching can be realized with PA concentration as
low as 10 nM, which quenched ~ 4% of the initial emission
intensity of TPE-BOZ. When 5×10-4 M PA was added to the
probing platform, almost all of the emission of TPE-BOZ was
quenched (inset in Fig. 3a). The quenching efficiency can be
evaluated by the slope of linear region of quenching plot (Fig.
3b) at low concentration region (with PA concentration below
0.4 μM, Ksv= 1.134×107 M-1), and with the lowest detection
limit in the range of ~ 4.47×10-9 M (according to 3σ rule51,
y=11.34x+3.90, R2=0.9866), demonstrates the high sensitivity
of TPE-BOZ toward PA. In the UV−Vis spectrum (Fig. 3c), the
subsequent addition of PA to the solution of aggregates of
TPE-BOZ results in an increase in absorption band at 368 nm.
Moreover, there is apparent colour change during the probing
process. In Fig. 3d, both TPE-BOZ and PA are colourless, while
their mixture is pale yellow. Information about the preparation
and probing performance of TPE-BOZ and some earlier
reported macromolecules-based PA probes have been
summarized (Table S2) here to make direct comparison
Film
E/W=1/9
400
450
500
550
600
Wavelength(nm)
Fig. 2 Fluorescence spectra (λex= 320 nm) of TPE-BOZ (1.0×10-5 M) in
ETOH/water mixture (v/v=1/9) and in solid film (inset is the visual
photograph of film under hand-held UV lamp, 365 nm).
Preliminary investigation about optical response of TPE-BOZ
to PA
between TPE-BOZ and other probes. As reflected by Table S2
,
The intriguing AIEE characteristic of TPE-BOZ prompts us
to explore its potential application as optical probe in aqueous
environment. Considering the presence of electron-rich ether
(C-O) and TPE segments in the structure of TPE-BOZ, we
speculate that its fluorescence might be altered by the
introduction of electron-withdrawing agent into the probing
medium. According to previous reports, macromolecule
aggregates significantly improved the sensitivity of
corresponding probes,49, 50 we thus utilized the macromolecule
aggregate of TPE-BOZ in ethanol/water mixture (with water
fraction of 90%) and choose the famous explosive, toxic and
electron-deficient agent, PA, as probing target.
TPE-BOZ displayed comparable detection limit with the
corresponding values of other probes, and the catalyst-free
procedure and shorter polymerization time is much preferable
from the synthetic view point.48, 52-56
To evaluate the selectivity of TPE-BOZ toward PA, the
emission alteration brought by other nitro compounds (2, 4-
dinitrophenol (2, 4-DNP), p-nitrophenol (PNP), o-ntrophenol
(ONP), p-nitrotoluene (PNT), nitrobenzene (NB), nitromethane
(NM)) and the referential agent, phenol (PhOH), was
investigated here. Among the compounds we studied, PA
displayed more remarkable fluorescence quenching (Fig. 4a). It
can be noted that the further addition of PA into these
100
a
TNP concn.
b
background-analytes-containing
systems
resulted
in
0
succeeding quenching of emission intensity (Fig. 4b). For
further comparison, fluorescence quenching efficiency of TPE-
BOZ by these compounds was evaluated at different
quencher’s concentration. In Fig. 4c, although the fluorescence
of TPE-BOZ could also be partially quenched by other nitro
aromatics, the quenching efficiency is much smaller than that
of PA in all cases. As displayed in Fig. 4d, the quenching degree
reaches a constant value (~ 97%) after incubating with 10 eq.
of PA for ~ 2 min.
9
8
7
6
5
4
y=11.34X+3.90
R2=0.9866
50
5X M
10-4
0.0
0.2
0.4
PA(µM)
0
400
C
500
600
0
20
40
60
80
Wavelength(nm)
PA(µM)
5
4
3
2
1
0
TNP concn.
5X M
10-4
100
0
a
b
before
after
100
80
60
40
20
0
80
60
40
20
0
300
350
400
450
500
Fig. 3 PL (a), WUVav-eVleisng(tch)(,ntmh)e corresponding relationship between (1-
I/I0) × 100 and [PA] (inset is the fitting curve with [PA] lower than
0.4μM) (b), and visual photographs (d) of TPE-BOZ (1.0×10-5 M)
toward PA in EtOH/water mixture (v/v=1/9).
PA
DNP
PNP
PhOH
DNP
PNP
ONP
PNT
NB
NM
PhOH
ONP PNT
NB
NM
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highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels of TPE-BOZ and
nitro compounds were calculated and the results were
98
97
96
95
94
d
c
0
0
1
0
0
8
0
1
summarized in Table S1. Fig. 6 graphically presents the energy
×
0
6
)
0
I
variation for the different molecules. As revealed by the DFT
calculation, electron of TPE-BOZ was mainly bared by TPE
skeleton in both HOMO and LUMO cases, and with the values
of −4.90 eV (HOMO) and −1.32 eV (LUMO), respecꢀvely. The
energy gap (0.71 eV) between TPE-BOZ’s HOMO(-4.90 eV) -
PA’s LUMO(-4.19 eV) is smaller than the corresponding value
(3.58 eV) between HOMO (-4.90 eV)-LUMO (-1.32 eV) of TPE-
BOZ, which supports the electron-transfer from TPE-BOZ to
PA.57 In addition, the energy difference between HOMO (TPE-
BOZ)-LUMO (PA) is smaller than the corresponding values
/
0
4
I
-
1
-
4
0
(
2
5
4
A
0
P
P
N
3
P
D
N
P
P
N
30
60
90
120
150
180
210
2
T
N
P
O
B
N
1
M
N
Time(s)
H
O
Fig. 4 Quenching efficiency of TPE-BOZ (1.0×10-5 M) for different
analytes (1.0×10-4 M) (a); quenching efficiency of TPE-BOZ (1.0×10-5
M) with background analytes (1.0×10-4 M) and with succeeding
addition of 1.0×10-4
M
PA (b); concentration–dependent
fluorescence quenching of TPE-BOZ (1.0×10-5 M) for different
analytes (c) and time-dependent fluorescence quenching of TPE-
BOZ for PA (1.0×10-4 M) (d) in EtOH/water (v/v=1/9).
between HOMO
(TPE-BOZ) and LUMO of other nitro
compounds, demonstrates the preference of TPE-BOZ for PA
over other nitro compounds. Moreover, UV-Vis spectra
regarding PA titration process are displayed in Fig. 3c, the
absorption intensity at ~ 350 nm increases steadily with the
introduction of incremental PA, and with the absorption onset
extending to ~ 450 nm. This is also a reflection of the electron
transfer between TPE-BOZ and PA, as was observed in
previous report about the probing of PA by using other
chromophores.56 Additionally, as reflected by the SEM image
before
after
100
80
60
40
20
0
of TPE-BOZ (Fig. S7), some small cavities have been seen
between molecule aggregates. This could enable the small-
sized PA molecules to enter and offer more diffusion channels
for the excitons to migrate, allowing them to be more
efficiently annihilated by quenchers.58-60
Fig. 5 Quenching efficiency of TPE-BOZ (1.0×10-5 M) with
different solvents (1.0×10-3 M) in EtOH/water (v/v = 1/9) mixture
before and after the addition of PA (1.0×10-4 M).
0
LUMO
-1
-2
-3
-4
-5
-6
-7
-8
-9
TPE-BOZ
Corresponding fluorescence alteration of TPE-BOZ by the
presence of common solvents was also investigated here.
Acetonitrile (MeCN), toluene (Tol), tetrahydrofuran (THF),
dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF),
dichloromethane (DCM), chlorlform (TCM), ether, methyl
alcohol (MeOH), ethyl acetate (EAC), and acetone (DMK) were
introduced as competing agents to discern PA. As shown in Fig.
PhOH
HOMO
ONP
PNP
PNT
NB
DNP
NM
PA
5, the addition of all of these solvents exert negligible
influence to the fluorescence of TPE-BOZ, and the succeeding
instilment of PA caused remarkable fluorescence quenching in
all cases. This reveals that the detection of PA by TPE-BOZ was
not significantly affected by the solvent residues in probing
medium.
Fig. 6 Graphical representation of calculated energy levels of
TPE-BOZ and other analytes.
Fluorescence Quenching Mechanism
To investigate the mechanism regarding the fluorescence
quenching of TPE-BOZ toward nitro compounds, theoretical
calculation (the density functional theory (DFT) with
B3LYP/DNP basis set using DMol3 code) was carried out. The
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dropped onto test strips. As shown in Fig. 8, the fluorescence
intensity in the spotted area decreased gradually with the
enhancement of PA’s concentration. These results support
that TPE-BOZ has the potential to be used as solid state probe
for the testing of PA.
2.5
2.0
1.5
1.0
0.5
Absorption of PA
Emission of TPE-BOZ
1500
1000
500
0
0.0
350
400
450
500
550
600
Wavelength(nm)
Fig. 8 Paper strips of TPE-BOZ for PA detection at different
concentrations [1 × 10−3 M (a), 1 × 10−4 M (b), 1 × 10−6 M (c), 1
× 10−7 M (d), water (e), none (f)] under 365 nm UV light.
Fig. 7 Spectral overlap of the absorption spectra of PA and the
emission spectrum of TPE-BOZ in EtOH/water (v/v=1/9)
mixtures.
Aforementioned analysis theoretically supports the
photo-induced electron transfer (PET) mechanism, and we will
investigate whether the other mechanism, that is, the energy
transfer between TPE-BOZ and PA also contributes for the
fluorescence quenching here. Absorption and fluorescence
spectra of PA and TPE-BOZ were thus placed here to explore
the possibility for the energy transfer between TPE-BOZ and
PA. As revealed by Fig. 7, the absorption of PA has an overlap
with the emission of TPE-BOZ, hence, there is possibility of
resonance energy transfer from the fluorescent TPE-BOZ to
the nonemissive PA.61
Conclusion
In summary, novel fluorescent benzoxazine derivatives
were successfully synthesized by catalyst-free method under
mild reaction condition. Corresponding benzoxazine-
derivatived macromolecule (TPE-BOZ) displayed good film
formability and AIEE characteristic in the mixture of ethanol
(good solvent) and water (poor solvent). Optical response of
TPE-BOZ toward picric acid (PA) was investigated preliminarily
to explore its potential application as optical probe. TPE-BOZ
displayed sensitive fluorescence quenching toward PA in
Furthermore, we carried out the fluorescence titration of
TPE-BOZ with fixed amounts of [PA] at various excitation
wavelengths (260−440 nm). The quenching degree of PA was
slightly affected by the alteration of excitation (Fig. S9). This
rules out the possibility of decrease in fluorescence intensities
of TPE-BOZ due to the masking by PA, which experimentally
supports the energy transfer mechanism proposed according
to previous reports.61, 62
aqueous
environment
with
high
water
fraction
(ethanol/water=1/9, v/v), which might be due to the photo-
induced electron transfer (PET) and resonance energy transfer
(RET) between TPE-BOZ (electron-donor) and PA (electron
acceptor). The quenching degree of TPE-BOZ by PA is much
higher than those brought by other nitro compounds and
common solvents. We think that the concept of simple
preparation method without the involvement of catalyst is of
great significance in terms of the preparation of novel
optoelectronic materials.
On the basis of above-mentioned discussion, one can
deduce that the specific selectivity and sensitivity of TPE-BOZ
toward PA might be due to the synergetic effect of PET and
energy transfer between TPE-BOZ and PA.
Acknowledgements
Visual Detection of PA in Solid State.
Authors greatly appreciate the financial support from the
National Natural Science Foundation of China (Project No.
21364013, 21262034). We acknowledge the National
Supercomputing Center in Shenzhen for providing the
computational resources and materials studio (version 7.0, DMol3
module).
PA detection was also carried out in solid state. TPE-BOZ
coating on a thin layer chromatography (TLC) plate displayed
strong emission (Fig. S10a). When a drop of pure water (Fig.
S10b) and PA (Fig. S10c) was separately placed on the coating
of TPE-BOZ, the emission of TPE-BOZ was unaltered by water
while significantly quenched by PA.
In addition, we also utilized test strips to act as the solid
support to prepare TPE-BOZ based solid probe. Filter paper
was cut into 2 cm2 pieces and dipped into the ethanol solution
(10-3M) of TPE-BOZ for 10 min, subsequently dried under
reduced pressure 40 oC for 2 h. To investigate the sensitivity of
the paper strip for PA, different concentrations of PA were
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Graphic Abstract
Aggregation-induced emission enhancement type macromolecular benzoxazine
derivative was synthesized by catalyst-free Mannich reaction and displayed
fluorescence quenching response toward nitro compounds.