Investigation on sensing mechanism of a fluorescent probe for TNP detection in aqueous solution

Article abstract of DOI:10.1016/j.tet.2018.04.034

In this paper, the synthesis and spectral properties of a fluorescent probe Nph-An for TNP detection has been introduced and its sensing mechanism has been studied with density functional theory (DFT) and time-dependent density functional theory (TD-DFT)

Full text of DOI:10.1016/j.tet.2018.04.034

Tetrahedron 74 (2018) 2684e2691
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Investigation on sensing mechanism of a fluorescent probe for TNP
detection in aqueous solution
Yinhua Ma d, Li Zhao a d, Yang Li a d, Jianyong Liu , Yanqiang Yang , Tianshu Chu
a
,
,
,
a
c
a, b, *
a
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
Institute for Computational Sciences and Engineering, Laboratory of New Fiber Material and Modern Textile, the Growing Base for State Key Laboratory,
b
Qingdao University, Qingdao 266071, PR China
c
Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621900, China
University of the Chinese Academy of Sciences, Beijing 100049, PR China
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 February 2018
Received in revised form
In this paper, the synthesis and spectral properties of a fluorescent probe Nph-An for TNP detection has
been introduced and its sensing mechanism has been studied with density functional theory (DFT) and
time-dependent density functional theory (TD-DFT) methods. For TNP detection in aqueous solution the
probe shows high sensitivity, selectivity and efficiency. The theoretical results demonstrate that the
excited-state intermolecular hydrogen bonding (HeB) plays an important role for the TNP detection. In
the excited state, the intermolecular HeB between Nph-An and TNP is strengthened, and thus strongly
facilitates the nonirradiative PET process to induce the fluorescence quenching. Whereas, the probe Nph-
An emits strong fluorescence because of the intramolecular charge transfer (ICT) properties, which is
confirmed by the solvatochromism effect. The calculated absorption spectra of Nph-An and Nph-
An þ TNP agree well with the experimental values and theoretical analysis provides a detailed and clear
explanation of the sensing mechanism.
12 April 2018
Accepted 13 April 2018
Available online 14 April 2018
Keywords:
Sensing mechanism
Density functional calculations
Photo induced electron transfer (PET)
Excited-state hydrogen bonding
©
2018 Elsevier Ltd. All rights reserved.
1
. Introduction
associated with the respiratory system and red blood cells.10e12
Considering the explosive and toxic properties, more and more
attention has been paid on efficient detection of TNP. A variety of
methods have been established to detect TNP molecule, such as
Energetic materials especially nitroaromatics (NACs) are widely
used in the military industries, dye industries, pharmaceutical in-
dustries, and chemical laboratories.1 Due to their highly explosive
nature, nitroaromatic explosives have a serious impact on home-
land security, environmental contamination and humanitarian
implications.5 There are many nitroaromatic explosives in the
world, such as 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol
e4
13
fiber optic methods, gas chromatography coupled with mass
1
4
15
spectrometry,
infrared and Raman spectroscopy,
thermal
16
17
neutron analysis, surface plasmon resonance and ion mobility
spectroscopy(IMS). However, most of these techniques are expen-
sive and time-consuming, which limit their application in the field
of TNP detection.
,6
7
e9
(
TNP), nitrobenzene (NB), and 2,4-dinitrotoluene (2,4-DNT).
Among these nitroaromatic compounds, TNP is a kind of the
acidic nitrophenol derivatives. TNP can be passed on to surround-
ing areas through the soil, water and air, leading to ecological
Fluorescence-based detection systems have attracted much in-
terest for the detection of TNP because of their high sensitivity, fast
response time, stable, and low-costing. In this background, a series
of chemosensors based on fluorescence quenching have been
8
,10
destruction and environmental issues.
On the other hand, TNP
8
,10,11,
causes some serious health effects such as muscle pain, diarrhea,
strong irritation to the eye and skin and damage to organs
constantly reported.
18e26 Nevertheless, further insights into
sensing mechanisms are worthwhile not only for understanding
the mechanisms of the current fluorescent probes, but also for
27
designing more effective fluorescent chemosensors. Based on
previous studies, various fluorescence transduction mechanisms
have been suggested for TNP chemosensors, including intra-
*
Corresponding author. State Key Laboratory of Molecular Reaction Dynamics,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
PR China.
2
8e33
molecular charge transfer (ICT),
the photo-induced electron
2
7,
E-mail addresses: tschu@dicp.ac.cn, tschu008@163.com (T. Chu).
transfer(PET), 34e38 the excited state proton transfer
https://doi.org/10.1016/j.tet.2018.04.034
0
040-4020/© 2018 Elsevier Ltd. All rights reserved.

Y. Ma et al. / Tetrahedron 74 (2018) 2684e2691
44e48
2685
(
ESPT),39e43 fluorescence resonance energy transfer (FRET)
An and Nph-An þ TNP with Gaussian 09 program. There is no re-
striction on the optimized structures. All the local minima were
confirmed by the vibrational frequencies analysis calculations at
the optimized electronic structures. We tested different basis sets
and functionals (see Tables S1 and S2). In the beginning, we
4
9
and other mechanisms.
Recently, Ma et al.5 designed and synthesized Nph-An as a new
highly selective fluorescent chemosensor for TNP detection.
Compared to DNT and TNT, TNP caused the fluorescence of Nph-An
decreased obviously. Nph-An is a donor(D)eacceptor (A) type
molecule, composed of 1,8-naphthalimide units as the acceptor and
anthracene units as the donor, respectively. Like many previously
reported fluorescent chemosensors, the Nph-An probe presents
strong fluorescence emission in aqueous solution under photo-
excitation. After addition of TNP in aqueous solution, fluorescence
is remarkably quenched. According to the experimental results, Ma
et al.50 proposed the TNP detection mechanism is based on PET
process and protonation of carbonyl group, that is, the acidic
behavior of TNP hindered the ICT emission of Nph-An. And their
work provided information about fluorescent spectra of the mol-
ecules and analysis about the ground-state geometries involved in
the sensing process.
0
6
4e66
employed the hybrid exchange-correlation functional B3LYP,
and the 6e311g(d, p) was chosen as the basis set. However, it is
generally known that the TDDFT/B3LYP method suffers severe
failure when dealing with CT states, and the hybrid functional with
long-range corrections (CAM-B3LYP) has been proved more accu-
rate when calculating the excitation energy of charge-transfer
5
5,67
excited states.
Therefore, CAM-B3LYP/6-311g (d, p) were cho-
sen to describe the properties of the molecular system. Further-
1
more, H NMR spectra of Nph-An and Nph-An þ TNP were
calculated with B3LYP function and 6-311 þ G(2d,p) basis set,
1
which has been proved suitable for calculating the H chemical
6
8
shift.
Considering that the experiments were conducted in solvent, in
all calculations the solvent effects were considered using the in-
tegral equation formalism(IEF)
Excited-state calculation is necessarily helpful to understanding
the sensing process. In addition, owing to the existence of the
intermolecular hydrogen bonding (HeB) between the Nph-An and
TNP, the importance of the electronic excited-state hydrogen-
bonding dynamics should not be ignored either, because fluores-
cence quenching mechanisms, including internal conversion (IC)
and PET, etc., can be dramatically influenced by the hydrogen-
6
9,70
version of the polarizable con-
71
50
tinuum model(PCM). In the experiment of Ma et al., the
fluorescent spectra were recorded in different polarity solvents,
and in order to understand solvent effects on fluorescence emission
and the sensing mechanism of Nph-An, we also carried out the
theoretical investigation by employing IEF-PCM for six different
solvents, that is, water with the dielectric constant ε ¼ 78.4, DCM
(ε ¼ 9.1), Cyclohexane (ε ¼ 2.1), Toluene (ε ¼ 2.38), THF (ε ¼ 7.58),
and DMSO (ε ¼ 46.8).
51e62
bonding interactions, as reported by Han and co-workers.
In
their systematic studies, they suggested that the strengthened
intermolecular hydrogen bonding can enhance the rate of IC and
facilitate the PET process, thus can influence the fluorescence
quenching.5
1,58,59
Hence, it is assumed that the HeB may also play a
3. Experimental section
significant role in the sensing mechanism of Nph-An for TNP
detection.
3.1. Synthesis procedure of the molecule probe
In order to give a reasonable and clear picture of the TNP sensing
mechanism, we conduct a theoretical study by employing the DFT/
TDDFT methods to investigate both the ground and the excited
states63 of Nph-An and Nph-An þ TNP molecules relevant to the
sensing process. The ground and excited state geometries have
been optimized, accompanying with the vertical excitation energy
calculation. Then, the frontier molecular orbitals (MOs), electronic
transition energies, and oscillator strengths are analyzed, which
indicates that the sensing mechanism of the chemosensor Nph-An
is attributable to the combination of PET process and excited-state
HeB enhancement (see Scheme 1).
Zhang and co-works50 have synthesized the novel molecular
probe Nph-An in three steps (see Fig. 1): (1) Compound 1 was
synthesized from mixture of aniline, 4-bromo-N-phenyl-1,8-
naphthalimide and acetic acid. After reflux, filtration and recrys-
tallization, a solid sample was obtained. (2) Compound 2 was
synthesized from mixture of bis(pinacolato) diboron, 9-bromo-10-
(
naphthalen-2-yl) anthracene, potassium acetate, 1,4-dioxane and
dichloropalladium. The prepare procedure is composed of four
processes of heating, extraction, drying and purification. (3) Nph-
An was obtained through the Suzuki coupling interaction be-
tween the compound 1 and compound 2. Potassium carbonate
solution and the mixture of compound 1 and compound 2 were
added into a flask. Then, toluene was added. The new mixture was
refluxed, quenched, extracted, dried, concentrated and purified,
after that, the sample Nph-An was formed.
2
. Computational details
In the present work, we performed the electronic structure
calculations for both the ground state and the excited state of Nph-
Scheme 1. The proposed reaction mechanism of the chemosensor Nph-An with TNP.

2686
Y. Ma et al. / Tetrahedron 74 (2018) 2684e2691
Fig. 1. Synthetic route of Nph-An. (Taken from ref.50).
3.2. Spectral recording
They recorded 1H NMR spectra at 298 K, where CDCl
/DMSO
3
and TMS (tetramethylsilane) were chosen as the solvent and the
internal standard respectively. UVevis spectra and fluorescence
emission spectra were recorded using UV-3100 spectrophotometer
and RF-5301PC spectrophotometer in their work. In DCM solvent,
Nph-An exhibited two main absorption peaks at 262 nm and
371 nm respectively. And the fluorescence peak of the molecule
probe is located on 558 nm. Moreover, considering that Nph-An is a
donor-acceptor type molecule, the fluorescent spectra were recor-
ded in different solvents (cyclohexane, toluene, THF and DMSO) to
study the ICT properties of the molecule probe. It showed that, with
the increase of the solvent polarity, the fluorescence peaks of Nph-
An were shifted from 460 nm to 586 nm. Different from the ag-
gregation caused quenching (ACQ) fluorescence materials, Nph-An
exhibited strong aggregation induced emission (AIE). This character
makes TNP detection possible in aqueous solution. In order to
investigate the AIE behavior, the authors recorded the photo-
luminescence spectrum of Nph-An by gradually changing the water
fractions (f
found when f
w
) in the THF solvent. The strongest fluorescence was
exceeded 50% and increased to 90%.
Fig. 2. (a) Fluorescence spectra of the mixture of Nph-An and TNP (water/THF ¼ 9/1).
Inset is the SterneVolmer plots; (b) fluorescence quenching efficiency of Nph-An in
different nitroaromatics solutions. (Taken from ref.50).
w
3.3. Test on the TNP-detection ability of the probe
The detection is executed via performing the photo-
part to the explosives detection. Firstly, a large number of fluores-
cence probes can detect explosives only in non-aqueous solutions,
while, benefiting from the properties of the AIEE, Nph-An shows a
strong emission in aqueous mixture (water/THF ¼ 9/1). Such a
capability of efficiently sensing TNP explosives in aqueous solution
is favorable for the practical detection of explosives. Secondly,
compared with other materials synthesized previously, Nph-An has
much lower limits of detection for TNP and this indicates that Nph-
An has an excellent sensitivity to TNP. Thirdly, among the three
nitroaromatics of TNP, TNT and DNT, TNP has been proven to be the
most powerful quencher, demonstrating the high selectivity of the
molecule probe for TNP. Finally, the quality of the molecule probe
demonstrates great potential towards real application. For example,
the authors have immersed the filter paper into the probe Nph-An
solution and the dry filter paper showed strong fluorescence after
exposing to UV light. When the filter paper contacted with a human
thumb that matted with TNP, the thumb's prints is seen clearly
under the ultraviolet light. This illustrates that Nph-An can detect
TNP residual products quickly and efficiently.
luminescence spectra of mixture of Nph-An and TNP in coalescing
solvent (water/THF ¼ 9/1). The fluorescence detection spectrum is
shown in Fig. 2a. Fluorescence intensity decreased remarkably
when adding TNP to the Nph-An solution. The fluorescence
quenching efficiency, fluorescent quenching coefficient Ksv and the
limits of detection (LODs) were also given in their work. The fluo-
rescence quenching efficiency reached 88.3% when the concen-
ꢁ
4
tration of Nph-An is 1 ꢀ 10 M. According to the Stern-Volmer
4
equation, the Ksv of the molecule sensor is 7.0 ꢀ 10 M. And the
ꢁ7
LODs is 4.7 ꢀ 10 M, which is lower than previously reported TNP
detection materials in aqueous solution. Besides, the selectivity of
the molecule probe was testified with the analytes of TNP, TNT and
DNT, and the corresponding spectra were shown in Fig. 2b, where I
0
and I present the fluorescence intensity in the presence and
absence of the analyte TNP. As can be seen, at the same analyte
concentration, the quenching degree of TNP is obviously higher
than that of TNT and DNT, which illustrated the outstanding
selectivity of the molecule probe for TNP.
Herein, considering the great research significance of the syn-
thesized probe Nph-An, the dynamical mechanisms underlying its
TNP sensing processes are further theoretically investigated and
shown below.
3
.4. Main experimental discovery
The synthesis of the molecule probe Nph-An plays an important

Y. Ma et al. / Tetrahedron 74 (2018) 2684e2691
2687
4
. Theoretical results and analyses
Table 1
Calculated HeB lengths (Å) and bond angles (deg) for the fully optimized structure
of Nph-An þ TNP1, Nph-An þ TNP2, and Nph-Anþ2TNP.
1
4
.1. Optimized structures and H NMR spectra
Nph-AnþTNP1 Nph-AnþTNP2 Nph-Anþ2TNP
The optimized ground-state structures of the compounds of
d(O /H )
1.95
1.99
1
1
Nph-An and Nph-An þ TNP were displayed in Fig. 3. The hydrogen
angle(O
1
/H
1
eO
eO
3
)
)
133.05
132.53
2.06
126.02
d(O
2
/H
2
)
2.06
126.44
6.33
bonding can be formed between Nph-An and TNP. Due to Nph-An
angle(O
Binding energy/(kcal mol
2
/H
2
4
contains two hydrogen bonding interaction sites, i.e., the C
1
]O
1
ꢁ1
ꢂ
)
5.39
and C ]O groups, two different HeB complexes can be formed
2
2
(
Nph-An þ TNP1, Nph-An þ TNP2) between Nph-An with one TNP
molecule. Moreover, the compound formed between Nph-An with
two TNP molecules (Nph-Anþ2TNP) were also optimized, and the
structures of Nph-An þ TNP2 and Nph-Anþ2TNP were shown in
Fig. S1.
results indicate the signals at d ¼ 9.17 and 9.01 ppm should be
assigned to Hb and Ha. Similarly, for Nph-An þ TNP, the signals at
d ¼ 9.08 and 8.97 ppm should be assigned to Hb' and Ha'. This is
because that, if considering the group electronegativity and
conjugation effects, the chemical shift of Hb must be higher than
Ha. Further, the proton signals of Hb' and Ha' shift to upfield, sug-
gesting that the HeB complex of Nph-An þ TNP has been formed.
Table 1 listed all the HeB lengths and bond angles from the
calculations. The binding energy (E ) between Nph-An and a TNP
B
molecule was calculated to determine the HeB strength based on
the following formula,
4
.2. Absorption spectra and molecular orbitals
ꢀ
ꢁ
EB ¼ ENph­An þ ETNP ꢁ Ecomplex
(1)
Based on the optimized ground state structures, the absorption
spectrum, the corresponding oscillator strengths, and CI values
have been obtained and investigated. Table 2 summarized the
vertical excitation energies and oscillator strengths as well as the
transition compositions of Nph-An and Nph-An þ TNP in water
solvent. Our theoretical calculations predict a main absorption
bands at 358 nm for Nph-An in DCM solvent, and well reproduced
the experimental results with the absorbance band of Nph-An
located at 371 nm (see Fig. 4). Seen from Table 2, S
dominant transition with the largest oscillator strength, mainly
assigned to a
In the calculation, the BSSE corrected interaction energy were
obtained by employing the counterpoise method of Boys and Ber-
nardi.72 These data were also listed in Table 1. And from Table 1, it
can be seen that the length of O
1 1
/H is close to the length of
O
2
/H , and the hydrogen bonding lengths do not change too much
2
in Nph-Anþ2TNP. Moreover, there are no significant differences
among the calculated VEEs of the three HeB complexes (Fig. S2).
Due to these, Nph-An þ TNP1 is chosen as a representative complex
to discuss the photophysical property of the chemosensor.
0 1
/S is the
*
1
p/p type transition from the highest occupied
H NMR spectra of Nph-An and Nph-An þ TNP were calculated
molecular orbital (HOMO) to the second lowest unoccupied mo-
lecular orbital (LUMOþ1). Fig. 5 exhibits that the electrons for
HOMO and LUMOþ1 are delocalized over the anthracene moiety,
and those for LUMO are located over the 1,8-naphthalimide unit.
and shown in Fig. S3, with TMS (tetramethylsilane) as a standard
substance in this calculation. For Nph-An, our calculation results
1
agree well with the experimental H NMR spectra, except for sig-
nals at d ¼ 9.17 and 9.01 ppm. The two signals have been assigned
5
0
1
This indicates that the S state of Nph-An is a locally excited (LE)
to Ha and Hb respectively by Ma et al., while our calculation
27,73
state.
For Nph-An þ TNP, we can see from Table 2 and Fig. 5 that the
/S transition with the
dominant transition for Nph-An þ TNP is S
0
2
largest oscillator strength of 0.4335, which is mainly assigned to
HOMO/LUMOþ4 transition. The excited transition for HOMO-
*
/
LUMOþ4 is the local transition of
p/p
type transition between
the anthracene moieties, which also indicates that the S
2
state of
Nph-An þ TNP is a locally excited (LE) state. The very small
oscillator strength of S /S implies that the S state is a forbidden
state (not an emissive state). Moreover, the S /S excitation cor-
74
0
1
1
0
1
responds to orbital transition from HOMO to LUMOþ2 with the
electron density localized over anthracene moiety and 1,8-
1
naphthalimide unit respectively, indicating S state is a complete
charge transfer (ICT) state. From the perspective of electronic states,
when the energy of the charge transfer state lies beneath that of the
locally excited state, the PET process is prone to happen to cause the
3
5,75,76
fluorescence quenching.
Here, based on the calculation re-
sults, our analysis about the frontier orbitals and the electronic
transitions confirmed that the PET process indeed can occur in the
HeB complex of Nph-An þ TNP to induce fluorescence quenching.
Thus, the previously proposed PET mechanism is correct and is well
supported by theory.
4.3. The emission spectra and the excited state HeB
In this section, we investigated the excited-state properties of
Nph-An and Nph-An þ TNP, particularly the excited-state HꢁBs.
Fig. 3. The optimized S
0
geometries for Nph-An and Nph-An þ TNP1.
The excited states structures were optimized at the TD-DFT/CAM-

2688
Y. Ma et al. / Tetrahedron 74 (2018) 2684e2691
Table 2
The calculated electronic transition energies and corresponding oscillator strengths of the low-lying singlet excited states of Nph-An and Nph-An þ TNP in water solvent.
Transition
Energy (nm/eV)
358 (3.47)
f a
Contrib.b
CI(%)c
Nph-An
S
0
/S
1
0.4456
HOMO/LUMOþ1
HOMO-2/LUMO
HOMO/LUMO
HOMO/LUMOþ2
HOMO/LUMOþ4
HOMO/LUMOþ2
HOMO-2/LUMOþ2
68.7
10.0
68.8
67.6
67.4
14.8
10.4
S
S
S
0
0
0
/S
/S
/S
2
1
2
352 (3.52)
361 (3.44)
358 (3.47)
0.0031
0.0274
0.4335
Nph-An þ TNP
a
Oscillator strength.
b
H, HOMO (highest occupied molecular orbital) and L, LUMO (lowest unoccupied molecular orbital).
The CI coefficients are in absolute values.
c
polar locally excited (LE) state.54,77e80 Fig. 7 shows the relevant
MOs involved in the emission of Nph-An, and the vertical excitation
energies, oscillator strengths and transition compositions are listed
1 0
in Table 3. We can see from Table 3 and Fig. 7 that the S /S
transition corresponds to LUMO/HOMO and LUMOþ1/HOMO;
the electrons for LUMO were mainly delocalized over 1,8-naph-
thalimide group and a small portion were distributed on anthra-
cene moiety; the electrons for LUMOþ1 and HOMO were
delocalized over the anthracene moiety mostly. In principle, there
are two paths when
1 0
S state relaxes to S state, namely
LUMO/HOMO and LUMOþ1/HOMO. Clearly, LUMO/HOMO
corresponds to the ICT state emission and LUMOþ1/HOMO be-
longs to the LE state emission, according to the characters of the
dominant frontier molecular orbitals involved in the emission of
Nph-An. Moreover, from the CI values, the percentage of the ICT
state is 66.9% while the LE state accounts for 18.3%. Here, again,
theory confirmed the previously proposed ICT mechanism for
fluorescence emission of Nph-An.
5
0
On the other hand, Ma et al. recorded the fluorescent spectra
in different polarity solvents for the purpose to investigate the ICT
8
1e84
properties from a viewpoint of solvatochromism effect.
They
found the peaks of emission spectra red shift as the solvent polarity
increases, providing an evidence for the ICT properties of Nph-An.
Our calculations also well reproduced this experimental observa-
tion and confirmed the existence of solvatochromism effect (see
Fig. 8). Besides, for Nph-An in DCM solvent, the calculated emission
band is located at 489 nm, agreed with the experimental result
Fig. 4. Comparison of experimental and calculated UVeVis absorption and photo-
luminescence spectra of Nph-An: (a) experimental UVeVis and photoluminescence
spectra in DCM solvent; (b) calculated absorption (black line) and photoluminescence
bands (red line), obtained at the TDDFT/CAM-B3LYP/6-311g(d,p) level.
(558 nm) (see Fig. 4). Note here, the subtle difference in value be-
tween calculation and experience may be attributed to the influ-
ence of the solvent viscosity not be considered in the calculation by
Gaussian 09 program. The viscosity is known to have effects on the
configuration distortion, therefore will influence the ICT state for-
mation and fluorescence emission.
B3LYP/6-311g(d, p) level and the optimized structures are displayed
in Fig. 6.
Turning to Fig. 6, the first excited state (S
demonstrates a partial configuration change comparing with the
1
) of Nph-An þ TNP
For Nph-An, an obvious change between the ground state
structure and the excited state structure is found. In the ground
state, the dihedral angle of C3eC4eC5eC6 and C7eC8eC9eC10
0
0
0
ground state S
0
, because the dihedral angles of C3 eC4 eC5 eC6'
ꢃ
0
0
0
ꢃ
(
ꢁ89.10 ) and C7 eC8 eC9 eC10' (92.64 ) in the S
0
state have
ꢃ
ꢃ
ꢃ
are ꢁ91.38 and 90.30 respectively, which increase to ꢁ127.45 and
ꢃ
ꢃ
changed to ꢁ125.82 and 113.03 in the S
1
state. Since such
ꢃ
1
14.68 in the excited state. This means, the donor and the acceptor
configuration change of Nph-An þ TNP is similar to that of Nph-An,
it is therefore reasonable to deduce that TNP has little effect on the
molecular configuration. As of now, we can explain the phenome-
non of fluorescence quenching as follows, with the addition of TNP
to the probe solution, the HeB complex will formed first. Then, the
are perpendicular to each other in the ground state, while the
molecular structure tends to be planar in the excited stated. This
change of the 1,8-naphthalimide unit in the excited state configu-
ration is in favor of orbital overlapping and electron transferring,
and thus causes the strong fluorescence emission herein.
Due to the instability of the excited state electronic structure
and the variability of molecular configuration of the D-A type
molecule in which donor and acceptor is linked by a single bond,
the electron transfer reaction will take place with the relaxation of
molecular structure, therefore intramolecular charge transfer
state(ICT) will be formed. It is proved that formation of ICT state
should be preferred in the polar solvents as compared to the non-
HeB complex of Nph-An þ TNP is photoexcited to the S
2
state. But
according to the Kasha's rule, the fluorescence can be emitted only
85
from the lowest singlet excited state (S
complex will return to the first excited state S
conversion (IC) process. The first excited state S
from which the HeB complex decays to the S state through the
1
state), hence, the excited
though an internal
is a dark state,
1
1
0
nonirradiative PET process. As a result, the strong fluorescence of
the probe Nph-An has been remarkably quenched after the

Y. Ma et al. / Tetrahedron 74 (2018) 2684e2691
2689
Fig. 5. The dominant molecular orbitals contributed to the transition for Nph-An and Nph-An þ TNP in water solvent.
Fig. 6. The optimized excited stated geometries for Nph-An and Nph-An þ TNP.
Fig. 7. The dominant frontier molecular orbitals involved in the emission of Nph-An.
addition of TNP.
state vibrational spectra of Nph-An þ TNP in the vibrational re-
As mentioned above, the hydrogen bonding should play an
important role in the sensing mechanism of the chemosensor Nph-
An. As labeled in Fig. 6, the length of HeB (O
gions of the O
1
/H
1
eO
2
stretching modes. In the ground state, the
ꢁ1
O
1
/H
1
eO
2
stretching modes is located at 3741 cm , while in the
1
/H
1
) is shortened
/H ) is
excited stated, the O
1
/H
1
eO
2
stretching mode strongly red-shifted
from 1.95 Å (S ) to 1.45 Å (S ), indicating that the HeB (O
0
1
1
1
ꢁ1
to 2471 cm . This indicates that the intermolecular HeB is signif-
icantly strengthened in the excited stated. Because the HeB
strengthening in the excited-state enhanced the electronic
coupling between Nph-An and TNP, therefore, the PET process is
strengthened in the excited stated. The HeB strength can be further
monitored by the spectral shifts of the hydrogen-bonded
groups.6 86e88 Fig. 9 shows the ground-state and the excited-
2,

2690
Y. Ma et al. / Tetrahedron 74 (2018) 2684e2691
Table 3
The calculated fluorescence emissions for the chemosensor Nph-An by cam-b3lyp/6-311g(d,p) in water solvent.
Electronic transition
Energy (nm/eV)
498 (2.49)
f
Contrib.
CI(%)
S
1
/S
0
0.7647
LUMO/HOMO
66.9
18.3
16.5
67.3
33.2
33.1
24.7
31.7
10.2
10.3
LUMOþ1/HOMO
S
S
S
2
/S
3
/S
4
/S
0
0
0
401 (3.09)
339(3.65)
324(3.83)
0.0224
0.3586
0.0041
LUMO/HOMO
LUMO/HOMO-2
LUMO/HOMO-4
LUMOþ1/HOMO-4
LUMOþ3/HOMO
LUMOþ5/HOMO
LUMOþ4/HOMO
LUMOþ2/HOMO-1
S
S
5
/S
/S
0
0
303(4.09)
301(4.11)
0.0352
0.1240
6
Fig. 8. Comparison of experimental (a) and calculated PL spectra (b) of Nph-An
recorded in cyclohexane, toluene, THF and DMSO, respectively.
Fig. 9. The calculated vibrational frequencies of the OeH group's stretching modes in
and S
states for Nph-An þ TNP.
S
0
1
facilitated.58,59 An illustration for such PET promotion can be found
in Table S3 and Fig. S4, by taking LUMOþ3/HOMO of Nph-
An þ TNP as an example. It showed that the hydrogen bond
and excited state geometries, 1H NMR spectra, UVevis spectra,
fluorescence emission spectra, molecular orbitals and excited state
H-Bs. In the probe Nph-An, the donor and the acceptor are
perpendicular to each other in the ground state, while the probe
structure tends to be planar in the excited state. This configuration
change in the excited state is in favor of orbital overlapping and
electron transferring which are responsible for the strong fluores-
cence emission. The ICT characters of the probe are also confirmed
through theoretical analysis on molecular orbitals and sol-
vatochromism effect. The fluorescence quenching after the TNP
1
strengthening in the S state accelerated the electron transfer be-
tween Nph-An and TNP and promoted the process of c.
5
. Conclusion
The synthesis procedure and the properties of a novel fluores-
cent probe Nph-An have been described and introduced. Then, DFT
and TDDFT methods have been carried out to investigate the
sensing mechanism of Nph-An for TNP detection. Nph-An has a
high selectivity and sensitivity for TNP detection, and compared
with TNT, the hydroxyl group in TNP can interact with the carbonyl
groups in the probe molecule by hydrogen bonding. In addition,
addition is mainly ascribed to the formation of the HeB complex of
89,90
Nph-An þ TNP and its PET process.
The theory still revealed
that the intermolecular HeB is significantly strengthened in the
excited state. And because of this, the electronic coupling between
Nph-An and TNP is enhanced to promote strongly the non-
irradiative PET process. Thus, the remarkable fluorescence
nitro groups in TNP affect the electron deficiency of the
p system,
which promotes the interaction with the probe Nph-An. Theoret-
ical calculations present the information about the ground state

Y. Ma et al. / Tetrahedron 74 (2018) 2684e2691
2008;867:64e70.
2691
quenching during the detection of TNP is caused by the non-
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Acknowledgments
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This work is supported by the National Natural Science Foun-
dation of China (Grant No. 21273234), the Science Challenging
Program (JCKY2016212A501), the National Basic Research Program
of China (Grant No. 2013CB834604) and the Open Fund of the State
Key Laboratory of Molecular Reaction Dynamics in DICP, CAS
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Appendix A. Supplementary data
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