Tetraphenylpyrimidine-based AIEgens: Facile preparation, theoretical investigation and practical application

Article abstract of DOI:10.3390/molecules22101679

Aggregation-induced emission (AIE) has become a hot research area and tremendous amounts of AIE-active luminogens (AIEgens) have been generated. To further promote the development of AIE, new AIEgens are highly desirable. Herein, new AIEgens based on tetraphenylpyrimidine (TPPM) are rationally designed according to the AIE mechanism of restriction of intramolecular motion, and facilely prepared under mild reaction conditions. The photophysical property of the generated TPPM, TPPM-4M and TPPM-4P are systematically investigated and the results show that they feature the aggregation-enhanced emission (AEE) characteristics. Theoretical study shows the high-frequency bending vibrations in the central pyrimidine ring of TPPM derivatives dominate the nonradiative decay channels. Thanks to theAEE feature, their aggregates can be used to detect explosives with super-amplification quenching effects, and the sensing ability is higher than typical AIE-active tetraphenylethene. It is anticipated that TPPM derivatives could serve as a new type of widely used AIEgen based on their facile preparation and good thermo-, photo- and chemostabilities.

Full text of DOI:10.3390/molecules22101679

molecules
Article
Tetraphenylpyrimidine-Based AIEgens:
Facile Preparation, Theoretical Investigation
and Practical Application
Junkai Liu 1,2, Lingxiang Pan 1, Qian Peng 2,* and Anjun Qin 1,
*
ID
1
State Key Laboratory of Luminescent Materials and Devices, South China University of Technology,
Guangzhou 510640, China; ipowerjkl@outlook.com (J.L.); panlx2014@hotmail.com (L.P.)
Key Laboratory of Organic Solids, Beijing National Laboratory for Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
Correspondence: qpeng@iccas.ac.cn (Q.P.); msqinaj@scut.edu.cn (A.Q.);
2
*
Tel.: +86-10-8261-6830 (Q.P.); +86-20-2223-7065 (A.Q.)
Received: 20 August 2017; Accepted: 3 October 2017; Published: 10 October 2017
Abstract: Aggregation-induced emission (AIE) has become a hot research area and tremendous amounts
of AIE-active luminogens (AIEgens) have been generated. To further promote the development
of AIE, new AIEgens are highly desirable. Herein, new AIEgens based on tetraphenylpyrimidine
(TPPM) are rationally designed according to the AIE mechanism of restriction of intramolecular
motion, and facilely prepared under mild reaction conditions. The photophysical property of the
generated TPPM, TPPM-4M and TPPM-4P are systematically investigated and the results show that
they feature the aggregation-enhanced emission (AEE) characteristics. Theoretical study shows the
high-frequency bending vibrations in the central pyrimidine ring of TPPM derivatives dominate the
nonradiative decay channels. Thanks to the AEE feature, their aggregates can be used to detect explosives
with super-amplification quenching effects, and the sensing ability is higher than typical AIE-active
tetraphenylethene. It is anticipated that TPPM derivatives could serve as a new type of widely used
AIEgen based on their facile preparation and good thermo-, photo- and chemostabilities.
Keywords: tetraphenylpyrimidine; aggregation-enhanced emission; restriction of intramolecular
motion; structure-property relationship; explosive detection
1. Introduction
The research on aggregation-induced emission (AIE) is drawing increasing interest since it
can solve the aggregation-caused quenching (ACQ) problem encountered by traditional organic
luminophores. AIE-active luminogens (AIEgens) possess relatively high emission efficiency in
the aggregate or solid states, which meets the requirements for practical applications in organic
light-emitting diodes, optical waveguides, biosensors and bioimaging, etc. [1–8].
AIEgens exhibit remarkable advantages over traditional ACQ luminophores in aggregate and
solid states, so constructing new AIEgens is of great importance for promoting the development of
AIE research. Although enthusiastic efforts made by researchers have generated hundreds of AIEgens,
many AIE cores contain cyclic or linear multiple vinyl groups in their structures. Ethylene derivatives
may potentially encounter photo-oxidation and photobleaching problems. One effective strategy to
improve stability is to construct AIEgens with sole aryl moieties. Meanwhile, the introduction of
aromatic nitrogen heterocycles could help produce electron donor-acceptor structures to fine-tune the
light emission in a wide range. Along these lines, tetraphenylpyrazine (TPP)-based AIEgens were
designed based on the restriction of the intramolecular rotation mechanism and facilely synthesized
Molecules 2017, 22, 1679; doi:10.3390/molecules22101679
www.mdpi.com/journal/molecules

Molecules 2017, 22, 1679
2 of 13
under mild reaction conditions [9]. The TPP-based AIEgens exhibit remarkable advantages, such as
facile preparation, efficient emission, good thermo-, photo- and chemostability [10,11].
Inspired by the excellent performance of TPP derivatives, we designed and synthesized
a new type of AIEgen based on the rarely explored tetraphenylpyrimidine (TPPM) (Chart 1).
The central pyrimidine is anticipated to possess versatile functions such as protonation, hydrogen
bond formation and chelation of the nitrogen atoms, which make the generated AIEgens applicable
in developing advanced supramolecular assemblies and chemosensors [12]. Besides following the
working mechanism of typical AIEgens, the TPPM undergoes intense high-frequency intramolecular
bending motion in the pyrimidine ring, which contributes largely to the nonradiative decay channels.
Accordingly, attaching electron-donating groups to the electron-withdrawing pyrimidine rings could
extend the conjugation and rigidify the skeleton of the molecule, and suppress the high-frequency
bending vibrations. As expected, the generated TPPM-4M and TPPM-4P attain higher luminescent
efficiency in both solution and solid states than that of TPPM. Nanoaggregates based on TPPM
derivatives show higher performance for detecting the picric acid with larger quenching constants
than the typical AIEgen of tetraphenylethene (TPE). This is attributed to more efficient electron transfer
processes originating from the formation of hydrogen bonds between pyrimidine rings of TPPM
derivatives and the explosive molecules [13].
Chart 1. Molecular structures of AIEgens of tetraphenylpyrazine (TPP) and tetraphenylpyrimidine
(TPPM) derivatives.
2. Results and Discussion
2.1. General Routes for Synthesis of TPPM Derivatives
The general routes to synthesize TPPM and its derivatives are shown in Scheme 1.
Commercially available starting materials of tetrachloropyrimidine (1) and aromatic boronic acids (2–4)
could be readily converted to TPPM or its derivatives of TPPM-4M and TPPM-4P in satisfactory
yields through Suzuki coupling [14]. Satisfactory data corresponding to the target TPPM derivatives
1
were obtained from the H- and 13C-NMR, high-resolution mass measurements (Figures S1–S9).
These compounds are constructed by aromatic moieties, which greatly enhance their thermal stability.
The rapid drops on the thermal-gravity analysis (TGA) curves can be ascribed to the sublimation of
samples (Figure S10). Therefore, we define the temperature of 5% loss of weight as Tsm, measured to
be 258 ◦C, 345 ◦C, and 466 ◦C for TPPM, TPPM-4M and TPPM-4P, respectively.

Molecules 2017, 22, 1679
3 of 13
Scheme 1. Synthetic routes to tetraphenylpyrimidine (TPPM) and its derivatives.
2.2. Photophysical Property of TPPM-Based AIEgens
After confirming their structures, we investigated the photophysical properties of TPPM
derivatives. We first measured their UV-vis absorption, shown in Figure 1. From TPPM to TPPM-4M
and TPPM-4P, the maximum absorption peaks (λabs) red shift from 259 to 301 nm because decorating
TPPM with electron-donating methoxyl groups or additional phenyl rings readily extends the
conjugation and facilitates the charge transfer from methoxyl groups or phenyl rings to the central
pyrimidine core upon excitation. Meanwhile, the maximum molar absorbance is also improved owing
to the extension of conjugation in TPPM-4M and TPPM-4P. These conclusions are supported by the
theoretical calculation shown in Tables S1–S3. Furthermore, we measured the absorbance spectra in
different solvents. The results show that the λabs exhibit negligible shifts (Figures S11–S13).
18
TPPM
TPPM-4M
TPPM-4P
16
14
12
10
8
6
4
2
0
220
270
320
370
420
470
Wavelength (nm)
Figure 1. UV-vis absorption of TPPM and its derivatives in THF.
Next, we investigated the photoluminescence (PL) behaviors of TPPM and its derivatives in
the THF/water mixture with different water fraction (fw). Taking TPPM as an example (Figure 2a),
its emission intensity enhances gradually with increasing fw from 0 to 70%. Afterwards, it rises
markedly. The other derivatives show similar phenomena (Figures S14 and S15). These indicate that
TPPM and its derivatives feature the unique aggregation-enhanced emission (AEE) characteristics.
The fluorescence quantum yields (ΦF) measurement further confirmed their AEE feature. As shown in
Figure 2b, the ΦF values of three compounds increase in the THF/water mixtures with fw changing from
0 to 99%, although negligible fluctuation appears in the curves. Furthermore, the ΦF values of TPPM,
TPPM-4M and TPPM-4P powders are recorded to be 3.7%, 8.8% and 6.2%, respectively (Table S4).
Interestingly, although negligible shift was observed in the absorbance spectra, the maximum emission
peaks (λem) of TPPM-4M and TPPM-4P exhibit an obvious bathochromic shift with an increase in the
solvent polarity, while that of TPPM remains unchanged, which accounts for the enhanced charge
transfer in TPPM derivatives by attaching the electron-donating groups.

Molecules 2017, 22, 1679
4 of 13
Water fraction
(vol %)
5
4
3
2
1
0
90
0
TPPM
TPPM-4M
TPPM-4P
350
380
410
440
470
500
0
30
60
90
Water fraction (vol %)
Wavelength (nm)
(b)
(a)
Figure 2.
Concentration: 10 M,
(
a
) PL spectra of TPPM in THF/water mixtures with different water fraction.
−5
λ
= 300 nm; (b) Fluorescence quantum yields of TPPM and its derivatives vs.
ex
water fraction in THF/water mixtures.
2.3. Working Mechanism
We conducted a systematical theoretical investigation on the structure-property relationship
of the TPPM derivatives aiming at illuminating the photo-physical mechanism. It is found that
the HOMO of TPPM is mainly localized over the ϕ2, ϕ3, ϕ4 and the pyrimidine ring (Chart 1 and
Figure 3a). After decorating with electron-donating methoxyl and phenyl groups, the charge density in
HOMO is extended over these substituents and the energy increases in the order of TPPM, TPPM-4M
and TPPM-4P. However, LUMO is mainly distributed on the ϕ3, ϕ4 and the central rings in both
methoxyl and phenyl counterparts. The delocalization of LUMO increases in the order of TPPM-4M,
TPPM and TPPM-4P, resulting in the decrease of energy in the same order. Overall, the energy gaps
between HOMOs and LUMOs decrease in the order of TPPM, TPPM-4M and TPPM-4P. As expected,
the calculated electric transition dipole moments increase from TPPM (1.17 Debye) to TPPM-4M
(3.63 Debye) to TPPM-4P (5.54 Debye). As a consequence, the radiative rate constants of the three
compounds increase in the order of TPPM, TPPM-4M and TPPM-4P (Figure 3b), which would
contribute largely to the enhancement of the ΦF of TPPM derivatives in solution after decorating with
methoxyl and phenyl groups. These results are in good agreement with the measurement of
in Figure 2b.
ΦF shown
12
TPPM
TPPM-4M
TPPM-4P
8
4
0
(a)
(b)
Figure 3. (a) Molecular orbital amplitude plots and energy levels of the HOMOs and the LUMOs of
TPPM derivatives at the S1-geometry calculated at the B3LYP/6-31G(d, p) level; ( ) Comparison of
b
radiative rate constants of TPPM, TPPM-4M and TPPM-4P calculated in the isolated state.

Molecules 2017, 22, 1679
5 of 13
The optimized molecular geometries of these three compounds are then examined and the data
are listed in Table 1 and Table S5. It is obvious that the three compounds consist of a central stator and
peripheral rotors, which are connected through single bonds. Relative to the central core, the peripheral
phenyl rings ϕ2, ϕ3, and ϕ4 are hugely twisted due to the high steric congestion while the ϕ5 is almost
planar without repulsion from adjacent groups at the S0 state. The highly twisted conformation of the
TPPM derivatives could prevent from close packing in the condensed phase and reduce the possibility
of forming emission quenching species. Upon excitation, the largest modifications appear in dihedral
angles between ϕ2
approximate C2 symmetry. While the dihedral angles between ϕ3
/
ϕ4 and ϕ1 rings with the same decrease values of 13◦ in TPPM, keeping the
ϕ4 and ϕ1 rings in the methoxyl
/
and phenyl counterparts largely decrease with the values ranging from 11◦ to 20◦, thus breaking the
molecular C2 symmetry. The ϕ5 rings are still well conjugated with the central rings and the dihedral
angles between ϕ5 and ϕ1 experience smallest variation upon excitation for all the three compounds.
It is interesting that the bond angles in the pyrimidine ring experience intense modification upon
excitation in TPPM, which has not been found in the reported AIEgens. The bond angles of central
pyrimidine rings in three molecules are quite similar at S0 states. Nevertheless, the bond angles
A(C1-N1-C2), A(C1-N2-C4), and A(N1-C1-N2) of TPPM change drastically with absolute values larger
than 10◦ from S0 to S1 states, which would activate some non-radiative decay channels. All bond
angles of pyrimidine rings of TPPM-4M and TPPM-4P show slight variation.
Table 1. Selected dihedral angles and bond angles (in degree) for isolated TPPM, TPPM-4M, TPPM-4P
molecules in the S0 and the S1 state.
TPPM
S1
TPPM-4M
TPPM-4P
S0
∆(S1 − S0)
S0
S1
∆(S1 − S0)
S0
S1
∆(S1 − S0)
D(ϕ –ϕ )
40
63
40
4
27
65
27
7
−13
37
62
37
5
42
49
26
7
5
40
62
40
4
47
42
25
7
7
1
2
D(ϕ –ϕ )
2
−13
−11
2
−20
−15
3
1
3
D(ϕ –ϕ )
−13
3
1
4
D(ϕ –ϕ )
1
5
A(C1-N1-C2) 118
A(C1-N2-C4) 118
A(N1-C2-C3) 122
A(N2-C4-C3) 122
A(N1-C1-N2) 125
128
128
121
121
111
112
10
10
−1
−1
−14
−4
118
118
122
121
125
116
117
121
123
118
125
116
−1
118
118
122
122
125
116
117
121
123
118
125
116
−1
3
3
1
1
−3
−4
0
0
A(C2-C3-C4)
116
0
0
The reorganization energy in the S1/S0 geometrical relaxations characterizes the ability to produce
internal conversion and provides essential details of the contribution from multiple intramolecular
motions to the excited-state deactivation. Figure 4 illustrates the calculated reorganization energy
versus the corresponding normal modes. It can be seen in TPPM that the high frequency modes
contribute significantly to the total reorganization energy, which are predominantly ascribed to the
stretching vibrations of C=C and C=N and the bending vibrations of bond angles in pyrimidine ring
(Figure S16). Relative to that in TPPM, the total reorganization energy is found to decrease significantly,
by about half in TPPM-4M and TPPM-4P, which implies the non-radiative decay would become much
slower. Moreover, the contribution of bending motions turns weak and that of the low-frequency
normal modes become dominant in TPPM-4M and TPPM-4P. The low-frequency modes are assigned
as the rotational motions of the periphery phenyl rings as illustrated in Figures S17 and S18.
Further casting the reorganization energy onto the internal coordination depicts a clear picture of
the specific structure-property relationship. As shown in Figure 4d, the reorganization energy from
bond angles takes the largest proportion at 78% in TPPM, in which the bond angles in the pyrimidine
core contributes as highly as 59% with the value of 3189 cm−1. This further confirms that the bending
motions of the bond angles in the pyrimidine ring act as nonradiative decay channels and thus lead to
low fluorescence quantum yields of TPPM in the dilute solution. Meanwhile, the contribution from

Molecules 2017, 22, 1679
6 of 13
bond angles to the reorganization energy are 12 and 9%, in which those from pyrimidine rings are
5 and 4% with values of 145 and 108 cm−1, in TPPM-4M and TPPM-4P, respectively. This indicates
that attaching the methoxyl and phenyl groups to TPPM mitigates the bending motions of the central
core by extending the conjugation degree and rigidifying the conformation of the whole molecule,
thus improving the emission efficiency in both solution and aggregates.
350
300
250
200
150
100
50
1200
1000
800
600
400
200
0
Total: 2815 cm-1
Total: 5375 cm-1
0
0
500 1000 1500 2000 2500 3000 3500
Normal mode wavenumber (cm-1)
(b)
0
500 1000 1500 2000 2500 3000 3500
Normal mode wavenumber (cm-1)
(a)
350
300
250
200
150
100
50
Total: 2992 cm-1
4000
3000
2000
1000
0
Bond length
Bond angle
Dihedral angle
Bond angle in φ1
0
0
500 1000 1500 2000 2500 3000 3500
Normal mode wavenumber (cm-1)
(c)
TPPM
TPPM-4M
TPPM-4P
(d)
Figure 4. Presentation of the calculated reorganization energy versus the normal mode wavenumbers
for ( ) TPPM, ( ) TPPM-4M, ( ) TPPM-4P, and ( ) contribution to the total reorganization energy from
a
b
c
d
the bond length, the bond angle, the dihedral angle and the bond angle in the pyrimidine ring (ϕ1).
All three compounds present AEE phenomena as observed above. The light emission in the
solid states of AIEgens is closely associated with the molecular packing modes [15–17]. We here
take TPPM-4M as an example to figure out its mechanism and to establish the structure–property
relationship. Figure 5 and Tables S6 and S7 provide the comparison of the molecular geometries at
S0 and S1 states calculated in the gas and crystal phases, respectively. The calculated ground-state
geometry in the solid phase is in good agreement with that in a single crystal, which indicates the
rationality and reliability of the computational method adopted here. TPPM-4M adopts a twisted
conformation with the dihedral angles between the peripheral phenyl rings and the central pyrimidine
core ranging from 28 to 62◦. Inspection of the single crystal packing manners shows that the highly
twisted structure prevents molecules from close packing and multiple intermolecular C–H · · · π
´
interactions exist among the molecules. The C–H · · · π interactions with the distances of 2.854 Å
´
between the ϕ4 ring and the pyrimidine ring and of 3.029 Å between ϕ3 and ϕ4 generate effective
constraint, and then the variation of dihedral angles of D(ϕ1–ϕ2), D(ϕ1–ϕ3) and D(ϕ1–ϕ4) are as
expected to be reduced to 0◦, 8◦ and 4◦, respectively (Figure 5a,b). Hence, TPPM-4M in solid phase
experiences less change compared with those in the gas phase, which would correspondingly cripple
the non-radiative deactivation in the form of heat.

Molecules 2017, 22, 1679
7 of 13
(a)
(b)
(c)
Figure 5.
(a) Overlap diagram of optimized structures of S0 and S1 state of TPPM-4M in the gas
phase and (
interactions (in angstrom) of TPPM-4M.
b
) in the crystal phase; (
c
) Single crystal packing diagram and intermolecular C–H · · · π
The features of the geometrical modification are sure to produce different properties from the
gas to the solid phase. It is most obvious that the total reorganization energy is reduced from 2815
in the gas phase to 2569 cm−1 in the crystal phase for TPPM-4M. The reorganization energy from
bond lengths and bond angles in the gas phase almost equals to that in the crystal phase due to the
similar variation pattern of these structural parameters. However, the reorganization energy from
dihedral angles decreases from 1162 to 839 cm−1 (Figure 6), indicating the low-frequency rotational
motion of peripheral phenyl groups is blocked. Hence, the calculated nonradiative decay rate constants
decline sharply by five orders of magnitude in the condensed state, compared to the isolated state.
The radiative rate constant of the condensed state is in the same magnitude as that of the isolated one
because their electric transition dipole moments are close (Table 2). These evidently reveal the restriction
of the non-radiative decay pathways and the enhancement of light emission in the aggregate state.
1800
Bond angle
Bond length
Dihedral angle
Total: 2569 cm-1
1500
1200
900
600
300
0
Total: 2815 cm-1
Gas phase
Crystal phase
Figure 6. Contribution to the total reorganization energy in the gas and the crystal phase from the
bond length, the bond angle, and the dihedral angle.

Molecules 2017, 22, 1679
8 of 13
Table 2. The calculated electric transition dipole moments (
µ), radiative rate constants (kr) and
nonradiative rate constants (kic) of TPPM-4M in the gas phase and the crystal phase (T = 300 K).
−1
−1
)
300 K
µ (Debye)
kr (s
)
kic (s
5.12 × 107
3.30 × 1011
Gas
Crystal
3.63
3.21
3.54 × 107
2.34 × 106
2.4. Application in Explosive Detection
Nanoaggregates of TPPM and its derivatives can serve as efficient fluorescent probes for
explosive detection owning to their three-dimensional topological structures, which can generate
numerous internal voids for interacting with analytes and migration pathways for excitons [18–20].
Herein, we chose the commercially available picric acid (PA) as a model explosive to monitor the
variation of PL intensity of aggregates of TPPM derivatives in THF/water mixture (fw = 99%). As shown
in Figure 7a, the PL of TPPM-4M is progressively quenched with increasing PA concentration ([PA]).
Obvious light annihilation was observed even at a [PA] as low as 0.5 ppm. At a [PA] of 0.39 mM,
the emission is completely extinguished. The PL quenching spectra of the other two compounds are
given in Figures S20–S22. Upward-bending variation curves were obtained in the Stern-Volmer plots of
PL intensity ratio (I0/I) versus the PA concentration instead of straight lines. The quenching constants
−1
of 58,734, 41,703 and 32,269 M of TPPM, TPPM-4M and TPPM-4P are deduced from the plots when
PA concentration is below 0.1 mM, respectively, which are much higher than that of TPE of 9299 M−1
(Inset in Figure 7b). The upward-bending curves in the Stern-Volmer plots after 0.1 mM show the
super-amplification quenching effect.
300
[PA] (mM)
0
TPPM
TPPM-4M
TPPM-4P
250
TPE
7
200
6
0.39
5
4
3
2
150
100
50
1
0.00 0.02 0.04 0.06 0.08 0.10
PA
0
0.0
0.1
0.2
0.3
0.4
360
400
440
480
520
560
PA concentration (mM)
Wavelength (nm)
(b)
(a)
Figure 7.
(
a
) PL spectra of TPPM-4M with addition of different amounts of PA in THF/water mixture
−5
(fw = 99%). Concentration of TPPM-4M = 10 M. Excitation wavelength = 340 nm. (
b) Stern-Volmer
plots of I0/I versus [PA] in THF/water mixture (fw = 99%). I = peak intensity. I0 = peak intensity at
[PA] = 0 mM.
The fluorescence quenching could be generally ascribed to (i) the resonance energy transfer
from the excited state of probe molecules to the ground state of analytes due to spectra overlap
between emission bands of the former and absorption bands of the latter, and (ii) the photo-induced
electron transfer (PET) from the probe to PA [21–24]. As can be seen in Figure 8a, all the emission
bands of TPPM derivatives and TPE overlap with the absorption bands of PA, indicating that the
resonance energy transfer could contribute largely to the PL quenching process. However, the more
remarkable sensitivity of TPPM derivatives to PA, compared to TPE, is found to originate from the
hydrogen-bond facilitated electron transfer process. The calculated relative positions between the
probe and the PA are shown in Figure S23. The pyrimidine rings are able to form N · · · H hydrogen

Molecules 2017, 22, 1679
9 of 13
´
bonds with PA molecules with distances ranging from 1.666 to 2.058 Å. The lower complexation
energy between TPPM derivatives and the PA molecule than TPE reveals higher stability of such
complexation patterns through hydrogen bonds between TPPM derivatives and the PA molecule
(Table S8). The N · · · H hydrogen bonds can shorten the distance between fluorophores and PA
molecules and facilitate electron transfer driven by the LUMO-LUMO offsets from probes to PA
molecules (Figure 8b). Hence, TPPM derivatives attain larger quenching constants than TPE and show
higher sensing performance for detecting explosives.
PA-Abs
TPPM
TPPM-4M
TPPM-4P
TPE
220
310
400
490
580
670
Wavelength (nm)
(b)
(a)
Figure 8.
(a) Normalized UV-Vis absorption spectra of PA and PL spectra of TPPM, TPPM-4M,
TPPM-4P and TPE in THF/water mixture (fw = 99%); (b) Calculated energy level diagrams of TPPM
derivatives and the PA.
3. Materials and Methods
3.1. Synthesis of TPPM
Tetrachloropyrimidine
1 (0.22 g, 1 mmol), phenylboronic acid 2 (0.54 g, 4.4 mmol), Pd(PPh3)2Cl2
(0.035 g, 0.05 mmol), and K2CO3 (0.70 g, 5 mmol) were added to an oven-dried round bottom flask.
The flask was filled with nitrogen. The 1,4-dioxane (7.5 mL) and distilled water (2.5 mL) were injected
by syringe. The reaction mixture was stirred at 150 ◦C for 10 h. After cooling to room temperature,
the products were extracted with dichloromethane (3
×
25 mL), and the organic layer was washed
with water, dried over magnesium sulfate and filtration. Then the TPPM was isolated in a 81.6% yield
(0.31 g) as a white solid through the silica-gel chromatographic column separation using a mixture of
petroleum ether/dichloromethane as eluent and rotary evaporation under reduced pressure.
1H-NMR (500 MHz, DMSO-d6):
7.12–7.04 (m, 2H). 13C-NMR (125 MHz, DMSO-d6):
δ
(TMS, ppm) 8.57–8.47 (m, 2H), 7.64–7.51 (m, 3H), 7.42–7.16 (m, 13H)
,
δ
(TMS, ppm) 165.65, 162.15, 138.90, 137.55, 136.48,
131.38, 131.31, 129.97, 129.72, 129.20, 129.16, 128.62, 128.35, 128.21, 127.93. HRMS (MALDI-TOF):
m/z 385.1700 ([M + H]+). Calcd for C28H21N2 385.1705.
3.2. Synthesis of TPPM-4M
Tetrachloropyrimidine 1 (0.22 g, 1 mmol), 4-methoxyl-phenylboronic acid 3 (0.67 g, 4.4 mmol),
Pd(PPh3)2Cl2 (0.035 g, 0.05 mmol), and K2CO3 (0.70 g, 5 mmol) were added to an oven-dried round
bottom flask. The flask was filled with nitrogen. The 1,4-dioxane (7.5 mL) and distilled water (2.5 mL)
were injected by syringe. The reaction mixture was stirred at 150 ◦C for 10 h. After cooling to room
temperature, the products were extracted with dichloromethane (3
×
50 mL), and the organic layer
was washed with water, dried over magnesium sulfate and filtration. Then the TPPM-4M was isolated
in a 78.0% yield (0.39 g) as a white solid through the silica-gel chromatographic column separation
using dichloromethane as eluent and rotary evaporation under reduced pressure.

Molecules 2017, 22, 1679
10 of 13
1H-NMR (500 MHz, DMSO-d6):
δ (TMS, ppm) 8.47–8.40 (m, 2H), 7.34–7.28 (m, 4H), 7.13–7.05 (m, 2H),
7.00–6.90 (m, 2H), 6.89–6.75 (m, 6H), 3.85 (s, 3H), 3.75 (s, 6H), 3.72 (s, 3H). 13C-NMR (125 MHz,
DMSO-d6):
δ (TMS, ppm) 164.89, 161.91, 161.58, 159.96, 158.77, 132.42, 131.56, 131.45, 130.34, 129.88,
127.71, 114.46, 114.39, 113.63, 55.78, 55.61, 55.45. HRMS (MALDI-TOF): m/z 504.2036 ([M]+). Calcd for
C32H28N2O4 504.2049.
3.3. Synthesis of TPPM-4P
Tetrachloropyrimidine 1 (0.22 g, 1 mmol), biphenylboronic acid 4 (0.87 g, 4.4 mmol), Pd(PPh3)2Cl2
(0.035 g, 0.05 mmol), and K2CO3 (0.70 g, 5 mmol) were added to an oven-dried round bottom flask.
The flask was filled with nitrogen. The 1,4-dioxane (7.5 mL) and distilled water (2.5 mL) were injected
◦
by syringe. The reaction mixture was stirred at 150 C for 10 h. After cooling to room temperature, the
products were extracted with dichloromethane (3
×
25 mL), and the organic layer was washed with
water, dried over magnesium sulfate and filtration. Then the TPPM-4P was isolated in a 72.5% yield
(0.50 g) as a white solid through the silica-gel chromatographic column separation using a mixture of
petroleum ether/dichloromethane and rotary evaporation under reduced pressure.
1H-NMR (500 MHz, CD2Cl2):
δ (TMS, ppm) 8.76 (d, J = 8.5 Hz, 2H), 7.80 (d, J = 8.5 Hz, 2H),
7.76–7.71 (m, 2H), 7.65–7.29 (m, 28H), 7.21 (d, J = 8.3 Hz, 2H). 13C-NMR (125 MHz, CD2Cl2):
δ (TMS, ppm) 166.42, 163.68, 144.58, 142.65, 141.79, 141.51, 141.39, 141.26, 139.08, 137.97, 137.00, 132.89,
131.82, 130.16, 130.15, 130.11, 130.08, 130.06, 129.03, 128.94, 128.82, 128.45, 128.40, 128.30, 128.21, 128.11,
127.67. HRMS (MALDI-TOF): m/z 689.2876 ([M]+). Calcd for C52H36N2 689.2957.
3.4. Methods for Theoretical Calculation
All the geometrical optimizations and frequency calculations were carried out using the
(TD)B3LYP/6-31G(d, p) at the S0 (S1) for the studied compounds in the gas phase in the
Gaussian 09 package [25]. The corresponding calculations for them in the solid phase were performed
using the combined quantum chemistry and molecular chemistry (QM/MM) approach. The QM/MM
model was set up by cutting a cluster containing 64 TPPM-4M molecules from the single crystal
structure, the central molecule was treated as the QM part and the surrounding ones acted as the MM
part. (Figure S19). The QM/MM calculation was performed by using the Chemshell 3.5 package [26],
interfacing the Turbomole 6.5 [27] for QM at the B3LYP/6-31G(d) level and the DL_POLY [28] for MM
with the general Amber force field (GAFF) [29]. The analytical frequencies for the S0 state at the DFT
level and numerical frequencies for the S1 state at the TD-DFT level. The calculation of reorganization
energy, radiative and nonradiative rate constants were carried out in the MOMAP package developed
by Shuai group [30–35]. The formation of hydrogen bonds was evaluated by optimizing the relative
position between the sensor molecule and the PA molecule at the
Gaussian 09 package.
ωB97XD/6-31G(d, p) level in the
4. Conclusions
A new type of AIEgen based on tetraphenylpyrimidine (TPPM) was rationally designed
and facilely prepared. Theoretical investigation indicates that high-frequency bending motions
of bond angles in the pyrimidine ring dominate the nonradiative decay process in TPPM.
Attaching electron-donating methoxyl and phenyl groups to TPPM extends the conjugation and
rigidifies the molecule skeleton and thus mitigates bending motion of the central core. As a result,
relative to the TPPM, the reorganization energies of TPPM-4M and TPPM-4P are decreased sharply
by more than half, greatly constraining the nonradiative decay. Meanwhile, the electric transition
dipole moments of TPPM-4M and TPPM-4P are greatly enhanced, inducing faster radiative decay.
Thus, the emission efficiency is promoted in both the solution and the solid state. Furthermore, multiple
intermolecular interaction and steric hindrance existing in the aggregates block the low-frequency
rotational motions of peripheral rotors and thus intensify the solid-state emission of TPPM derivatives.

Molecules 2017, 22, 1679
11 of 13
TPPM derivatives attain higher performance in sensing PA than the typical TPE, which is ascribed to
the electron transfer process facilitated by the formation of hydrogen bonds between the pyrimidine
and PA. Hence, nanoaggregates based on TPPM derivatives are fairly applicable in practical detection
of explosive dissolved in water. Thus, these TPPM derivatives not only enrich the family of AIEgens
but could also potentially be applied in optoelectronic and biological areas.
Supplementary Materials: The supplementary materials are available online.
Acknowledgments: This work was financially supported by the key project of the Ministry of Science and
Technology of China (No. 2013CB834702), the National Natural Science Foundation of China (No. 21525417 and
No. 21490571), the Natural Science Foundation of Guangdong Province (No. 2016A030312002), the National
Program for Support of Top-Notch Young Professionals, the Fundamental Research Funds for the Central
Universities (No. 2015ZY013). A.J.Q. and B.Z.T. thank the support from the Guangdong Innovative Research
Team Program (No. 201101C0105067115).
Author Contributions: J.L. conducted the experiments and calculation; L.P. analyzed the data and discussed the
findings; Q.P. guided the theoretical calculation and analyzed the data. A.Q. designed and guided the research
work. All the authors wrote and revised the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Mei, J.; Leung, N.L.C.; Kwok, R.T.K.; Lam, J.W.Y.; Tang, B.Z. Aggregation-Induced Emission: Together We
Shine, United We Soar! Chem. Rev. 2015, 115, 11718–11940. [CrossRef] [PubMed]
2.
Hu, F.; Zhang, G.X.; Zhan, C.; Zhang, W.; Yan, Y.L.; Zhao, Y.S.; Fu, H.B.; Zhang, D.Q. Highly Solid-State
Emissive Pyridinium-Substituted Tetraphenylethylene Salts: Emission Color-Tuning with Counter Anions
and Application for Optical Waveguides. Small 2015, 11, 1335–1344. [CrossRef] [PubMed]
Zhao, Q.L.; Li, K.; Chen, S.J.; Qin, A.J.; Ding, D.; Zhang, S.; Liu, Y.; Liu, B.; Sun, J.Z.; Tang, B.
Aggregation-induced red-NIR emission organic nanoparticles as effective and photostable fluorescent
probes for bioimaging. J. Mater. Chem. 2012, 22, 15128–15135. [CrossRef]
Dong, Y.Q.; Lam, J.W.Y.; Qin, A.J.; Liu, J.Z.; Li, Z.; Tang, B.Z. Aggregation-induced emissions of
tetraphenylethene derivatives and their utilities as chemical vapor sensors and in organic light-emitting
diodes. Appl. Phys. Lett. 2007, 91, 3. [CrossRef]
3.
4.
5.
6.
Li, Z. Fabrication of high-performance non-doped OLEDs by combining aggregation-induced emission and
thermally activated delayed fluorescence. Sci. China Chem. 2017, 60, 1107–1108. [CrossRef]
Gao, Y.T.; Feng, G.X.; Jiang, T.; Goh, C.C.; Ng, L.G.; Liu, B.; Li, B.; Yang, L.; Hua, J.L.; Tian, H.
Biocompatible Nanoparticles Based on Diketo-Pyrrolo-Pyrrole (DPP) with Aggregation-Induced Red/NIR
Emission for In Vivo Two-Photon Fluorescence Imaging. Adv. Funct. Mater. 2015, 25, 2857–2866. [CrossRef]
Shi, X.Y.; Wang, H.; Han, T.Y.; Feng, X.; Tong, B.; Shi, J.B.; Zhi, J.G.; Dong, Y.P. A highly sensitive,
single selective, real-time and “turn-on” fluorescent sensor for Al3+ detection in aqueous media.
J. Mater. Chem. 2012, 22, 19296–19302. [CrossRef]
Liu, Z.T.; Xue, W.X.; Cai, Z.X.; Zhang, G.X.; Zhang, D.Q. A facile and convenient fluorescence detection of
gamma-ray radiation based on the aggregation-induced emission. J. Mater. Chem. 2011, 21, 14487–14491.
[CrossRef]
Mei, J.; Hong, Y.; Lam, J.W.Y.; Qin, A.; Tang, Y.; Tang, B.Z. Aggregation-Induced Emission: The Whole Is
More Brilliant than the Parts. Adv. Mater. 2014, 26, 5429–5479. [CrossRef] [PubMed]
7.
8.
9.
10. Chen, M.; Li, L.Z.; Nie, H.; Tong, J.Q.; Yan, L.L.; Xu, B.; Sun, J.Z.; Tian, W.J.; Zhao, Z.J.; Qin, A.J.; et al.
Tetraphenylpyrazine-based AIEgens: Facile preparation and tunable light emission. Chem. Sci. 2015, 6,
1932–1937. [CrossRef] [PubMed]
11. Pan, L.X.; Luo, W.W.; Chen, M.; Liu, J.K.; Xu, L.; Hu, R.R.; Zhao, Z.J.; Qin, A.J.; Tang, B.Z.
Tetraphenylpyrazine-Based Luminogens with Aggregation-Enhanced Emission Characteristics: Preparation
and Property. Chin. J. Org. Chem. 2016, 36, 1316–1324. [CrossRef]
12. Achelle, S.; Plé, N. Pyrimidine Ring as Building Block for the Synthesis of Functionalized
Materials. Curr. Org. Synth. 2012, 9, 163–187. [CrossRef]
π-Conjugated
13. Li, D.D.; Yu, X. Mesoporous aluminium organophosphonates: A reusable chemsensor for the detection of
explosives. J. Solid State Chem. 2016, 239, 17–22. [CrossRef]

Molecules 2017, 22, 1679
12 of 13
14. Hussain, M.; Hung, N.T.; Khera, R.A.; Malik, I.; Zinad, D.S.; Langer, P. Synthesis of Aryl-Substituted
Pyrimidines by Site-Selective Suzuki-Miyura Cross-Coupling Reactions of 2,4,5,6-Tetrachloropyrimidine.
Adv. Synth. Catal. 2010, 352, 1429–1433. [CrossRef]
15. Wang, C.; Li, Z. Molecular conformation and packing: Their critical roles in the emission performance of
mechanochromic fluorescence materials. Mater. Chem. Front. 2017. [CrossRef]
16. Wang, C.; Xu, B.J.; Li, M.S.; Chi, Z.G.; Xie, Y.J.; Li, Q.Q.; Li, Z. A stable tetraphenylethene derivative:
Aggregation-induced emission, different crystalline polymorphs, and totally different mechanoluminescence
properties. Mater. Horizons 2016, 3, 220–225. [CrossRef]
17. Li, H.Y.; Chi, Z.G.; Xu, B.J.; Zhang, X.Q.; Li, X.F.; Liu, S.W.; Zhang, Y.; Xu, J.R. Aggregation-induced emission
enhancement compounds containing triphenylamine-anthrylenevinylene and tetraphenylethene moieties.
J. Mater. Chem. 2011, 21, 3760–3767. [CrossRef]
18. Wu, Y.; Qin, A.; Tang, B.Z. AIE-active polymers for explosive detection. Chin. J. Polym. Sci. 2016, 35, 141–154.
[CrossRef]
19. Meher, N.; Iyer, P.K. Pendant chain engineering to fine-tune the nanomorphologies and solid state
luminescence of naphthalimide AIEEgens: Application to phenolic nitro-explosive detection in water.
Nanoscale 2017, 9, 7674–7685. [CrossRef] [PubMed]
20. Maity, S.; Shyamal, M.; Das, D.; Mazumdar, P.; Sahoo, G.P.; Misra, A. Aggregation induced
emission enhancement from antipyrine-based schiff base and its selective sensing towards picric acid.
Sens. Actuator B Chem. 2017, 248, 223–233. [CrossRef]
21. Dong, W.Y.; Pan, Y.Y.; Fritsch, M.; Scherf, U. High sensitivity sensing of nitroaromatic explosive vapors based
on polytriphenylamines with AIE-active tetraphenylethylene side groups. J. Polym. Sci. Pol. Chem. 2015, 53,
1753–1761. [CrossRef]
22. Thomas, S.W.; Joly, G.D.; Swager, T.M. Chemical sensors based on amplifying fluorescent conjugated
polymers. Chem. Rev. 2007, 107, 1339–1386. [CrossRef] [PubMed]
23. He, G.; Peng, H.N.; Liu, T.H.; Yang, M.N.; Zhang, Y.; Fang, Y. A novel picric acid film sensor via combination
of the surface enrichment effect of chitosan films and the aggregation-induced emission effect of siloles.
J. Mater. Chem. 2009, 19, 7347–7353. [CrossRef]
24. Feng, H.T.; Zheng, Y.S. Highly Sensitive and Selective Detection of Nitrophenolic Explosives by Using
Nanospheres of a Tetraphenylethylene Macrocycle Displaying Aggregation-Induced Emission. Chem. Eur. J.
2014, 20, 195–201. [CrossRef] [PubMed]
25. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.;
Mennucci, B.; Petersson, G.A.; et al. Gaussian09, Revision D.01; Gaussian Inc.: Wallingford, CT, USA, 2010.
26. Sherwood, P.; de Vries, A.H.; Guest, M.F.; Schreckenbach, G.; Catlow, R.C.A.; French, S.A.; Sokol, A.A.;
Bromley, S.T.; Thiel, W.; Turner, A.J.; et al. QUASI: A General Purpose Implementation of the QM/MM
Approach and Its Application to Problems in Catalysis. J. Mol. Struct. THEOCHEM 2003, 632, 1–28.
[CrossRef]
27. Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Electronic Structure Calculations on Workstation
Computers: The Program System Turbomole. Chem. Phys. Lett. 1989, 162, 165–169. [CrossRef]
28. Smith, W.; Forester, T.R. DL_POLY_2.0: A General-Purpose Parallel Molecular Dynamics Simulation Package.
J. Mol. Graph. 1996, 14, 136–141. [CrossRef]
29. Wang, J.; Wolf, R.M.; Caldwell, J.W.; Kollman, P.A.; Case, D.A. Development and Testing of a General Amber
Force Field. J. Comput. Chem. 2004, 25, 1157–1174. [CrossRef] [PubMed]
30. Shuai, Z.G.; Peng, Q.; Niu, Y.L.; Geng, H. MOMAP (Molecular Materials Property Prediction Package) is A
Suite of Codes to Study the Spectroscopy; Revision 0.2.004; Shuai Group: Beijing, China, 2017; Available online:
http://www.momap.net.cn/ (accessed on 9 October 2017).
31. Shuai, Z.G.; Peng, Q. Excited states structure and processes: Understanding organic light-emitting diodes at
the molecular level. Phys. Rep. Rev. Sec. Phys. Lett. 2014, 537, 123–156. [CrossRef]
32. Peng, Q.; Yi, Y.P.; Shuai, Z.G.; Shao, J.S. Toward quantitative prediction of molecular fluorescence quantum
efficiency: Role of Duschinsky rotation. J. Am. Chem. Soc. 2007, 129, 9333–9339. [CrossRef] [PubMed]
33. Peng, Q.; Yi, Y.P.; Shuai, Z.G.; Shao, J.S. Excited state radiationless decay process with Duschinsky rotation
effect: Formalism and implementation. J. Chem. Phys. 2007, 126, 114302. [CrossRef] [PubMed]

Molecules 2017, 22, 1679
13 of 13
34. Niu, Y.L.; Peng, Q.; Deng, C.M.; Gao, X.; Shuai, Z.G. Theory of Excited State Decays and Optical Spectra:
Application to Polyatomic Molecules. J. Phys. Chem. A 2010, 114, 7817–7831. [CrossRef] [PubMed]
35. Niu, Y.; Peng, Q.; Shuai, Z. Promoting-mode free formalism for excited state radiationless decay process
with Duschinsky rotation effect. Sci. China Ser. B Chem. 2008, 51, 1153–1158. [CrossRef]
Sample Availability: Samples of the compounds are not available from the authors.
©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).