The aggregation induced fluorescence effect enhanced by a reasonable length of carbon chain

Article abstract of DOI:10.1016/j.molliq.2020.112550

Six different barbituric acid derivatives using ethyl, propyl, butyl, pentyl, hexyl and heptyl groups as hydrophobic substituents were prepared. Molecular dynamics simulation was carried out to study the effect of different carbon chain lengths on the aggregation-induced emission process. Among the compounds, the CB-5 containing a hexyl substituent exhibited the strongest Aggregation induced emission (AIE) effect, not the CB-6 with longest carbon chain. The aggregates of CB-5 was used to detect the 2, 4, 6-trinitrotoluene (TNT) in aqueous media, exhibiting a maximum quenching constant of 3.1 × 10⁵ M^{? 1}. The paper sensor based on CB-5 showed a superior sensitivity toward TNT both in vapor and solution. This provided a clear strategy for designing compounds that utilize the hydrophobic interaction of long-chain alkyl chains to enhance the AIE effect.

Full text of DOI:10.1016/j.molliq.2020.112550

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The aggregation induced fluorescence effect enhanced by a
reasonable length of carbon chain
Hanjun Zhang, Furong Tao, Yuezhi Cui, Zhen Xu
PII:
S0167-7322(19)35532-1
DOI:
https://doi.org/10.1016/j.molliq.2020.112550
MOLLIQ 112550
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
7 October 2019
2 January 2020
20 January 2020
Please cite this article as: H. Zhang, F. Tao, Y. Cui, et al., The aggregation induced
fluorescence effect enhanced by a reasonable length of carbon chain, Journal of Molecular
Liquids(2020), https://doi.org/10.1016/j.molliq.2020.112550
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The Aggregation Induced Fluorescence Effect Enhanced by a
Reasonable Length of Carbon Chain
Hanjun Zhang, Furong Tao*, Yuezhi Cui, Zhen Xu
School of Chemical and Pharmaceutical Engineering, Qilu University of Technology
(Shandong Academy of Sciences), Jinan 250353, P. R. China.
Tel: +86 531 89631208; Fax: +86 531 89631760;
E-mail address: frtao2015@126.com
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Abstract
Six different barbituric acid derivatives using ethyl, propyl, butyl, pentyl, hexyl and
heptyl groups as hydrophobic substituents were prepared. Molecular dynamics
simulation was carried out to study the effect of different carbon chain lengths on the
aggregation-induced emission process. Among the compounds, the CB-5 containing a
hexyl substituent exhibited the strongest Aggregation induced emission (AIE) effect,
not the CB-6 with longest carbon chain. The aggregates of CB-5 was used to detect the
2, 4, 6-trinitrotoluene (TNT) in aqueous media, exhibiting a maximum quenching
constant of 3.1×105 M-1. The paper sensor based on CB-5 showed a superior sensitivity
toward TNT both in vapor and solution. This provided a clear strategy for designing
compounds that utilize the hydrophobic interaction of long-chain alkyl chains to
enhance the AIE effect.
Key words: Aggregation induced emission; Molecular dynamics simulation;
Hydrophobic interaction; Alkyl chain length
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1. Introduction
Materials with aggregation induced emission (AIE) effect have attracted considerable
research attention due to their enhanced fluorescence intensity at solid state [1, 2].
Several mechanisms have been proposed to explain the AIE phenomenon, such as
restricted intramolecular rotation (RIR) [3, 4]. According to the RIR mechanism, the
channel of non-radiative decay of exciton was restained due to that the intramolecular
rotation was restricted in aggregate of fluorophore molecules, thus, the emission
intensity of the fluorophore was enhanced. The intermolecular actions in the aggregate,
such as hydrophobic interaction, intermolecular electrostatic attraction, et al, have great
impact on the RIR mechanism [5, 6].
The substituent group of alkyl has complex effect on the intermolecular action, which
provides the flexibility, large steric hindrance for the molecule. In aqueous medium, the
alkyl chain exhibits hydrophobic interaction between molecules. All these features
contributes to the intermolecular action in the aggregate, which may have great
influence on the the AIE effect by affecting the RIR mechanism. Several research about
the effect of alkyl groups on the fluorescence have been reported [7-9]. Chen's team
found that, as the length of the alkyl chain increases, the steric hindrance of the
molecules increased and the π-π stacking of intermolecular was hindered, thus the
spectral shift increased and the color transition temperature decreased [10]. Wei's group
discussed the effect of alkyl length on mechanofluorochromic. As the length of the
alkyl chain increases, the degree of bending and deformation of the alkyl group is
higher, and the crystallinity of mechanofluorochromic compounds is destroyed,
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therefore the micro-morphology of aggregate remains almost unchanged before and
after the grinding, which weakens the property of mechanofluorochromic [11]. These
reports mainly discussed about the influence of the flexibility and steric hindrance of
alkyl groups on the optical properties of the AIE aggregate. However, the effect of
hydrophobic interaction of the alkyl group on the AIE activity of aggregate in aqueous
medium have seldom been reported.
In this work, a series of barbituric acid derivatives by conjugating carbazole and
barbituric acid have been synthesized, in which, various alkyl substituents were
attached on the N-atom of carbazole (Scheme 1). In these compounds, two kinds of
intermolecular actions may exist: i) the intermolecular electrostatic attraction between
the carbazole and the barbituric acid moieties is due to that the carbazole moiety is
electron-rich aromatic ring and the barbituric acid moiety is electron-deficient; ii) the
alkyl chain struction of the compounds induce the intermolecular hydrophobic action.
To study the relationship between AIE effect and molecule structure will provide useful
strategy for designing molecule with excellent AIE effect. Additionally, one of the
compounds synthesized in this work (CB-5) showed excellent sensing performance to
2,4,6-trinitrotoluene (TNT), which may be a promising candidate for TNT sensor.
2. Experimental section
2.1. Materials
1,3-dimethyl-barbituric acid, Carbazole, 1-Bromoethane, 1-Bromopropane,
1-Bromobutane, 1-Bromopentane, 1-Bromohexane and 1-Bromononane (A.R., 99%)
were purchased from Energy, and which were used without further purification. DMF
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was dried sequentially by molecular sieves (4 Å), CaH2, odium sand and distilled prior
to use.
2.2. Measurements
1
Emission spectra, UV-vis spectra, H NMR (600 MHz) spectra were recorded by
F-4600 fluorescence spectrophotometer, UV-2500 spectrometer and AVANCE II 400
spectrometer, respectively. FT-IR spectra were recorded by a Nicolet 380 spectrometer.
2.3. Synthesis
The
six
compounds
in
this
paper
1,3-dimethyl-5-((9-ethyl-9H-carbazol-3-yl)methylene)pyrimidine-2,4,6-trione (CB-1),
1,3-dimethyl-5-((9-propyl-9H-carbazol-3-yl)methylene)pyrimidine-2,4,6-trione (CB-2),
1,3-dimethyl-5-((9-butyl-9H-carbazol-3-yl)methylene)pyrimidine-2,4,6-trione (CB-3),
1,3-dimethyl-5-((9-amyl-9H-carbazol-3-yl)methylene)pyrimidine-2,4,6-trione (CB-4),
1,3-dimethyl-5-((9-hexyl-9H-carbazol-3-yl)methylene)pyrimidine-2,4,6-trione (CB-5),
1,3-dimethyl-5-((9-nonyl-9H-carbazol-3-yl)methylene)pyrimidine-2,4,6-trione (CB-6)
were successfully synthesized. The structure of all compounds was proved by 1H NMR
spectra and FT-IR spectra. The synthesis of compounds (1) and (2) in Scheme 1 were
according to ref. [12].
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Scheme 1 Synthetic routes of CB-1, -2, -3, -4, -5 and -6.
Synthesis Procedure:
A
mixture of compound (2) (3 mmol) and
1,3-dimethylbarbituric acid (3 mmol) in ethanol (10 ml) was refluxed for 4 hours, the
filtrate was concentrated under reduced pressure and further purified by column
chromatography (Ethyl acetate: petroleum ether =1:5; Yield: 90%).
CB-1: 1H NMR (400 MHz, Chloroform-d) δ 9.28 (s, 1H), 8.81 (s, 1H), 8.55 (d, J = 8.8
Hz, 1H), 8.22 (d, J = 7.8 Hz, 1H), 7.59-7.51 (m, 1H), 7.47 (dd, J = 8.5, 5.1 Hz, 2H),
7.36 (t, J = 7.5 Hz, 1H), 4.43 (q, J = 7.3 Hz, 2H), 3.47 (s, 6H), 1.50 (t, J = 7.2 Hz, 3H)
(Fig. S1). FT-IR (KBr): 3065 cm-1 (=C-H), 1667 cm-1 (C=O) (Fig. S2).
CB-2: 1H NMR (400 MHz, Chloroform-d) δ 9.29 (s, 1H), 8.81 (s, 1H), 8.54 (d, J = 8.9
Hz, 1H), 8.22 (d, J = 7.7 Hz, 1H), 7.54 (t, J = 7.7 Hz, 1H), 7.47 (dd, J = 8.6, 3.3 Hz,
2H), 7.35 (t, J = 7.5 Hz, 1H), 4.33 (t, J = 7.1 Hz, 2H), 3.47 (s, 6H), 1.96 (p, J = 7.5 Hz,
2H), 1.01 (t, J = 7.4 Hz, 3H) (Fig.S3). FT-IR (KBr): 3005 cm-1 (=C-H), 1665 cm-1
(C=O) (Fig. S4).
CB-3: 1H NMR (400 MHz, Chloroform-d) δ 9.28 (s, 1H), 8.81 (s, 1H), 8.55 (d, J = 8.8
Hz, 1H), 8.21 (d, J = 7.8 Hz, 1H), 7.61-7.33 (m, 4H), 4.36 (s, 2H), 3.47 (s, 6H), 1.90 (p,
J = 7.4 Hz, 2H), 1.43 (q, J = 7.4 Hz, 2H), 0.98 (t, J = 7.3 Hz, 3H) (Fig. S5). FT-IR
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(KBr): 2899 cm-1 (=C-H), 1671 cm-1 (C=O) (Fig. S6).
CB-4: 1H NMR (400 MHz, Chloroform-d) δ 9.28 (s, 1H), 8.81 (s, 1H), 8.55 (d, J = 8.8
Hz, 1H), 8.21 (d, J = 7.8 Hz, 1H), 7.62-7.33 (m, 4H), 4.35 (t, J = 7.2 Hz, 2H), 3.47 (s,
6H), 1.91 (d, J = 7.5 Hz, 2H), 1.44 - 1.31 (m, 4H), 0.91 (d, J = 6.8 Hz, 3H) (Fig. S7).
FT-IR (KBr): 2900 cm-1 (=C-H), 1677 cm-1 (C=O) (Fig. S8).
CB-5: 1H NMR (400 MHz, Chloroform-d) δ 9.28 (s, 1H), 8.81 (s, 1H), 8.55 (d, J = 8.9
Hz, 1H), 8.21 (d, J = 7.8 Hz, 1H), 7.62 - 7.33 (m, 4H), 4.35 (t, J = 7.3 Hz, 2H), 3.47 (s,
6H), 2.06 - 1.77 (m, 2H), 1.48 - 1.24 (m, 6H), 0.88 (t, J = 6.9 Hz, 3H) (Fig. S9). FT-IR
(KBr): 2900 cm-1 (=C-H), 1665 cm-1 (C=O) (Fig. S10).
CB-6: 1H NMR (400 MHz, Chloroform-d) δ 9.28 (s, 1H), 8.81 (s, 1H), 8.55 (d, J = 8.9
Hz, 1H), 8.21 (d, J = 7.8 Hz, 1H), 7.47 (d, J = 3.8 Hz, 4H), 4.34 (t, J = 7.2 Hz, 2H),
3.47 (s, 6H), 2.05 - 1.81 (m, 2H), 1.31 (d, J = 45.6 Hz, 12H), 0.87 (t, J = 6.7 Hz, 3H)
(Fig. S11). FT-IR (KBr): 2988 cm-1 (=C-H), 1666 cm-1 (C=O) (Fig. S12).
3. Results and discussion
3.1 Optical properties
Fig. 1 UV-visible absorption spectra of CB-1, -2, -3, -4, -5 and -6
in THF solution (10-4 M)
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The absorption spectra of the six barbituric acid derivatives are shown in Fig. 1. The
absorption peaks in the wavelength range from 400 to 500 nm was attributed to the
intramolecular charge transfer (ICT) from the carbazole unit to the barbituric acid unit,
and those in the range between 250 to 350 nm are belong to π-π* electronic transition
[13]. Additionally, the absorption peaks of the six compounds are basically similar to
each other, indicating that the length of the hydrocarbon chains on N-atom does not
change the conjugated system of the barbituric acid derivatives in pure THF. It is also
noted that the absorption intensity is affected by the length of the alkyl chain, the trend
was that as the alkyl chain grows, the absorption intensity decreases. The reason may be
that as the length of alkyl chain grows, the host molecule is entangled by alkyl chain,
which reduces the absorbance. However, it is also found that CB-5 exhibits the highest
absorption intensity, which may be due to the special structure of CB-5 itself.
3.2 AIE activity
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Fig. 2 (a): Fluorescence spectra of CB-1, -2, -3, -4, -5, and -6 in different water content
(fw) of THF/H2O mixture (10-4 M); (b): emission peak (red) and emission intensity (blue)
relative to the water fraction in the mixture.
All the compounds exhibited weak fluorescence intensity in pure THF. However,
when in the mixed THF/H2O solvents, they all emitted the enhanced fluorescence,
indicating all the compounds were AIE active. The fluorescence spectra of these
compounds were measured in THF/H2O by changing the water fraction (fw), as shown
in Fig. 2 (a). Moreover, the curves of fluorescence peak wavelength (λem) verse fw, as
well as those of fluorescence peak intensity (Iem) verse fw, were analyzed in Fig. 2 (b).
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All the six compounds are well dissolved in THF. In pure THF, the free rotation of
certain bonds of the molecule dissipates the excited state energy, making the
fluorescence weak. As water is added to the THF, the compounds gradually aggregates
in the mixed solvent, due to that water is poor solvent for these compounds. Meanwhile,
the strong interaction between the molecules in the aggregates restricted the free
rotation of the chemical bonds in the molecules, and suppressed the non-radiative
transition channel, thus the fluorescence is enhanced. As shown in Fig. 2, the
fluorescence intensity of CB-3, -4, -5 and -6 increases monotonously with the fw
increasing. Moreover, the λem of the four compounds also exhibits monotonous
increasing trend as the water fraction of THF/H2O mixture increased. These results
indicates that interaction between molecules in the aggregates gradually enhanced,
resulting in AIE enhancement and red shift of the fluorescence spectra.
However, the changing trend of fluorescence intensity of CB-1 and -2 is complicated.
When the fw increases from 0% to 70%, the fluorescence intensity increases with the fw
increasing, accompanied by the red shift of λem. Because interaction between molecules
in the aggregates gradually enhanced, resulting in AIE enhancement and red shift of the
fluorescence spectra. When fw increases from 70% to 80%, the intensity of the
fluorescence still rises, however, accompanied by a blue shift of λem. This result is due to
the molecules aggregate in larger size with the fw of 70% to 80%, and the intramolecular
micropolarity is lowered, leading to blue shift of λem [14, 15]. when fw ＞80%, the
amorphous particles forms in solution leading to red shifted of λem.
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Fig. 3 The AIE index of CB-1, -2, -3, -4, -5 and -6.
The AIE index (IA/I0) is used to quantify the AIE activity, in which, IA is the
maximum fluorescence intensity of the molecule aggregation in THF/H2O
(0<fw<100%), while I0 is the fluorescence intensity when fw=0%. As shown in Fig. 3,
the AIE index of CB-1, -2, -3, -4, -5 and -6 is 16, 20, 8, 5, 49 and 6, respectively. The
AIE index of these compounds decreases in the following sequence:
CB-5>CB-2>CB-1>CB-3>CB-6~CB-4. Obviously, the compound CB-5 shows the
strongest AIE activity.
The Fluorescence quantum yields of aggregate state are determined relative to
coumarin 307 in ethanol solution as a quantum yield standard (ΦF (Fluorescence
quantum yield) = 56%). The coumarin 307 is selected as a standard to study the ΦF
due to its maximum absorption and emission wavelength (395 nm and 500 nm,
respectively), which are close to the synthesized AIE compounds in this work. As
shown in Table 1, the fluorescence quantum yield of these compounds in the
aggregated state decreases in the following order: CB-5> CB-2> CB-1> CB-3> CB-6
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~CB-4. Obviously, the CB-5 with the strongest AIE activity exhibites the highest
fluorescence quantum yield in the aggregated state.
Table 1 The fluorescence quantum yield of all compounds in the aggregated state.
CB-1
7.9
CB-2
8.5
CB-3
3.2
CB-4
2.1
CB-5
11.2
CB-6
1.9
Aggregates ΦF (%)
As shown in Fig. S13, at aggregate state, the average diameters of the
nanoaggregates for CB-1~6 are 106.4, 103.2, 122.6, 142.5, 91.6 and 156.4 nm,
respectively. The size of nanoaggregates in the aggregated state increases in the
following order: CB-5< CB-2< CB-1< CB-3< CB-6 ~ CB-4. (Table 2). The results
indicate that the size of CB-5 in the aggregated state is the smallest and the most
tightly combined, which is confirmed by MD. The Hydrophobic and electrostatic
interactions work together to restrict the rotation of certain bonds in CB-5, limit the
non-radiative transition channels, and result in the strongest AIE activity and the
highest fluorescence quantum yield. These data indicate that the enhanced emission of
compounds is related to the formation of nanoaggregates.
Table 2 Average diameter of nanoaggregates in the aggregated state of compounds.
CB-1
106.4
CB-2
103.2
CB-3
122.6
CB-4
152.5
CB-5
91.6
CB-6
156.4
average diameters (nm)
3.3 Aggregation structures from molecular dynamics simulations
In order to better understand the relationship between the AIE index and the
molecule structure, the molecular dynamics (MD) simulations were performed for the
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six compounds through the Materials Studio software (Accelrys Inc). For each
compound (CB-1, CB-2, CB-3, CB-4, CB-5, and CB-6), four molecules of each
compound were randomly dissolved in 200 water molecules, respectively, to form 6
different initial systems with the same cubic simulation lattice (x = 20.83 Å, y= 20.83 Å,
and z= 20.83 Å). The simple point charge (SPC) model [16, 17], which can accurately
describe the water solution environment [18], is adopted for all water molecules.
The MD simulation was performed after charges and potentials were assigned to
each atom. The long-range electrostatic interactions have been accounted for using the
Ewald method. The total energy is written as a combination of valence terms including
diagonal and off-diagonal cross-coupling terms and nonbond interaction terms, the
coulombic and Lennard-Jones functions for electrostatic and van der Waals interactions
(eq. 1).
E = Ebonds  Eangles  Edihedrals  Ecross  EVDW  Eelec
(1)
Where EVDW and Eelec are given by the eq. 2:

6 


9


ij
ij
qiqj

Enon-bond  EVDW  Eelec

 2
3











ij 

r
r
r
ij
ij
ij






(2)
The parameters for each like-site interaction are given by the COMPASS force field
[19, 20]. The energies of the initial configurations for each system were minimized with
the Smart Minimizer method. After the minimization, this simulations were equilibrated
at constant temperature (298.15 K) and volume (NVT) for 10 ns. Atomic coordinates
were saved for every 200ps.
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Fig. 4 Molecular dynamics simulations were performed for CB-1, -2, -3, -4, -5 and -6
different systems through the Materials Studio software (Accelrys Inc).
The aggregation structures of the six compounds are shown in Fig. 4. For the
aggregates of the compounds CB-1 and CB-2, the alkyl chains on the N-atom are
localized in adverse ends of two neighbouring molecules, indicating there is no
hydrophobic interaction between molecules, due to that alkyl chain on the N-atom is
relatively short (C2H5 and n-C3H7 for CB-1 and CB-2, respectively). However, because
the carbazole ring is an electron-rich aromatic ring, and the barbituric acid moiety is an
electron-deficient six-membered ring, thus the two adjacent molecules are gathered
together by the strong intermolecular electrostatic attraction which limits the rotation of
the intramolecular bonds and leads to the strong AIE effect. As observed in Fig. 4, the
molecular aggregation of CB-2 is more compact than that of CB-1, indicating the
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intermolecular electrostatic attraction in the compound CB-2 is stronger. Therefore,
CB-2 exhibits larger AIE index than CB-1.
Similarly, the alkyl chain on the N-atom in CB-3 and CB-4 are also localized in
different directions, implying no strong hydrophobic interaction between molecules.
The intermolecular electrostatic attraction between the carbazole ring and the barbituric
acid ring also exists in the aggregates, resulting in the AIE effect of both CB-3 and
CB-4. However, because the space volumes of the alkyl chain on the N-atom (n-C4H9
and n-C5H11 for CB-3 and CB-4, respectively) are larger than those of CB-1 and CB-2,
which hinders the formation of closer packing between molecules CB-3 and CB-4, thus
CB-3 and CB-4 show relatively small AIE index than that of both CB-1 and CB-2.
In CB-5 aggregates, the alkyl chains (n-C6H13) of three molecules closely gather each
other, indicating strong hydrophobic interaction. Besides, there exists electrostatic
attraction between the carbazole and the barbituric acid. These two kinds of
intermolecular actions work together to limit the rotation of certain bonds in the
molecules, and result in the strongest AIE effect of the six compounds.
For compound CB-6, the aggregated form of the N-alkyl chain (n-C9H19) is relatively
lower relative to CB-5, but there is also a weaker hydrophobic interaction. The
backbones of the four molecules in CB-6 distribute in different way and are far from
each other, implying no electrostatic attraction between molecules. Therefore, the
intermolecular actions of CB-6 are much weaker relative to those of CB-5, due to that
n-C9H19 in CB-6 is a larger alkyl chain than n-C6H13 in CB-5, which may hinder the
close packing of molecules. In result, the relative loose aggregation of CB-6 explains
the lower AIE index relative to CB-5.
3.4 Detection of nitroaromatic explosives
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Fig.5 Fluorescence quenching experiment of TNT in solution of CB-5 (fw = 90%).
The aggregation of CB-5 in THF/water (fw=90%) exhibits excellent sensing
performance to TNT. As shown in Fig. 5, with the TNT concentration increasing, the
fluorescence intensity of CB-5 aggregation gradually decreases. What is noteworthy is
that λem of CB-5 unchanged during TNT addition, meaning that no other compounds
was formed.
A quenching process can be expressed by Stern-Volmer relationship I0/I = 1 + Ksv [Q]
[21], where, I0 and I are the initial and final fluorescence intensity after the addition of
TNT respectively, [Q] is the concentration of TNT, and KSV is the quenching constants.
The plots of I0/I -1 versus [Q] are shown in Fig.6. A good relationship between I0/I -1
and [Q] was obtained with [Q] increasing from 3.2 to 4.4×10-4 M, which gives the KSV
value of 3.1×105 M-1. The detection limit is an important parameter for evaluating
whether a sensor is sensitive. According to the formula LOD = 3S/Ksv, the limit of
detection (LOD) of CB-5 in fw=90% mixture solution for TNT is 0.329 ppb [22, 23].
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Fig. 6 Fluorescence quenching Stern-Volmer curve of CB-5 with TNT.
As we know, most of the sensors that detect TNT are based on gas detection. It is still
of great importance to detect NACs in wastewater, due to that the residues of TNT and
DNT in the environment will cause serious health problems in both animals and
humans [24-25]. Because the poor water solubility of most organic sensing material,
relatively few fluorescent sensors have been reported for the detection of TNT in water
solution. However, some typical AIE sensing materials, which emit strong fluorescence
as micro aggregate in aqueous media, have been reported for detecting TNT (Table 3).
As found, the Ksv of most of the sensing materials in Table 3 is at the order of 104
Ksv/M-1 [26-31]. Exceptionally, the Ksv of CB-5 is among the highest value with the
order of magnitude of 105 Ksv /M-1. This results indicate the aggregation of CB-5 takes
superior performance in detecting TNT in aqueous medium.
Table 3 Performance comparison of chemosensors reported for TNT
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The
Limit of
Detecting
system
Receptor
Analyte type of
sensor
Ksv/M-1
8.5 × 104
0.8 × 104
detection Ref.
(LOD)
THF-H2O
mixtures
TNT
TNT
TNT
TNT
TNT
TNT
solution
solution
solution
solution
solution
solution
7.4 μM
[26]
[27]
[28]
[29]
[30]
[31]
(1 : 9 v/v)
THF-H2O
mixtures
(1 : 9 v/v)
THF-H2O
mixtures
1.37 × 105 0.22 ppb
(0.5 : 9.5 v/v)
THF-H2O
mixtures
4.45 × 103 0.14 ppb
(0.5 : 9.5 v/v)
THF-H2O
mixtures
6.0 × 104
0.25 ppb
(2 : 8 v/v)
THF-H2O
mixtures
13 × 105
0.03 μM
(0.5 : 9.5 v/v)
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THF-H2O
This
TNT
solution
mixtures
3.1×105
0.33 ppb
work
(1 : 9 v/v)
Generally, the mechanisms of fluorescence quenching include Förster resonance
energy transfer (FRET) and photoinduced electron transfer (PET). To determine which
mechanism plays the main role of quenching process, both the absorption spectrum of
TNT and the emission spectrum of CB-5 are compared, as shown in Fig. 7. Observably,
there is no overlap between these two spectra, meaning no FRET mechanism to
responsible for the quenching. Therefore, the fluorescence quenching of CB-5 are
mainly attributed to PET mechanism [32-34].
Fig. 7 Absorption spectrum of TNT and emission spectrum of CB-5.
As shown in Fig. S14, the energy levels and electron cloud distributions of the
HOMO and LUMO of TNT and CB-5 were calculated by the B3LYP/6-325 31G (d)
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program. When the excited CB-5 is exposed to TNT, the excited electrons are
transferred from the LUMO of CB-5 to the LUMO of TNT. The main driving force of
PET is the difference between the LUMO value of CB-5 and TNT, which is 1.029 V.
Paper-based sensors have attracted great interest because their economics and easy of
preparation [35, 36]. Hence, the Paper-based sensor was obtained by soaking a round
Whatman filter paper in a solution of CB-5 (fw = 90 % CCB-5=10-4M) with a following
drying (Fig. 8A). A round glass bottle containing TNT powder, which provided
saturated TNT vapor, was covered with the paper-based sensor. After standing for five
minutes, the paper-based sensor were photographed, as shown in Fig. 8B. As observed,
a quenched spot corresponding to the bottle appeared, indicating the paper sensor is
sensitive to TNT vapor. Another paper sensor was soaked into a solution of TNT in
THF (CTNT=10-4 M), as observed, the whole paper sensor was quenched thoroughly (Fig.
8C). These results showed that this paper sensor base on CB-5 can be used to detect the
TNT vapor.
Fig. 8 A: Pape sensor based on CB-5 (fw = 90 %); B: paper sensor quenched by TNT
vapor; C: Paper sensor after soaked in solution of TNT in THF.
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4. Conclusion
All the six barbituric acid derivatives with different alkyl substituents show AIE
effect. The alkyl substituents play important role in forming aggregates of the
compounds. In the compounds with short alkyl substituents (from CB-1 to CB-4), the
hydrophobic interaction was negligible, while the electrostatic attraction between the
carbazole and the barbituric acid moieties made molecules closely aggregated and
resulted in the AIE activity. In the compounds with long alkyl substituents (CB-5 and
CB-6), the hydrophobic interaction was remarkable, and steric hindrance of longer alkyl
substituents hindered the closely aggregation of the CB-6. CB-5 exhibited the highest
AIE index of 49, due to its strong hydrophobic interaction by n-hexyl substituent and
the good intermolecular electrostatic attraction between the carbazole and the barbituric
acid moieties. Based on the enhanced AIE effect, CB-5 showed excellent sensing
performance in detecting TNT, which may be used as chemosensor for TNT.
Additionally, this work revealed the effect of alkyl length on the properties of AIE,
providing a reasonable strategy for the design of AIE molecule.
Acknowledgements
This work was financially supported by a Project of Shandong Province Higher
Educational Science and Technology Program (NO. J18KZ004), the Natural Science
Foundation of Shandong province (ZR2018MB025) and the Funds for Research Leader
in Jinan (2018GXRC028).
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Declaration of Interest Statement
The authors declare that there is no conflict of interest regarding the publication of
this paper.
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Author Biography
First author: Hanjun Zhang working toward a M.S. degree in School of Chemistry
Qilu University of Technology.
Yue-Zhi Cui received her PhD degree in 2004 at Shandong University, China. She is
currently a professor at Qilu University of Technology. Her research interests are in
the areas of materials chemistry and chemosensor.
Zhen Xu received her PhD degree in 2004 at Shandong University, China. She is
currently a professor at Qilu University of Technology.
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