Light-emitting analogues based on triphenylamine modified quinoxaline and pyridine[2,3-b]pyrazine exhibiting different mechanochromic luminescence

Article abstract of DOI:10.1039/d1nj01350e

A series of light-emitting analogues, [4-(2,3-diphenyl-quinoxalin-6-yl)-phenyl]-diphenyl-amine (TPA-DPQ) and [4-(2,3-diphenyl-pyrido[2,3-b]pyrazin-7-yl)-phenyl]-diphenyl-amine (TPA-DPP) are synthesized and characterized.TPA-DPQandTPA-DPPpossess similar do

Full text of DOI:10.1039/d1nj01350e

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Light-emitting analogues based on triphenylamine
modified quinoxaline and pyridine[2,3-b]pyrazine
exhibiting different mechanochromic
luminescence†
Cite this: New J. Chem., 2021,
45, 11304
ab
Yuxi Gea and Bin Huang
*
A series of light-emitting analogues, [4-(2,3-diphenyl-quinoxalin-6-yl)-phenyl]-diphenyl-amine (TPA–DPQ)
and [4-(2,3-diphenyl-pyrido[2,3-b]pyrazin-7-yl)-phenyl]-diphenyl-amine (TPA–DPP) are synthesized and
characterized. TPA–DPQ and TPA–DPP possess similar donor–acceptor structures, in which triphenylamine
is used as an electron donor while 2,3-diphenyl-quinoxaline (DPQ) and its analogue 2,3-diphenyl-
pyridine[2,3-b]pyrazine (DPP) serve as electron acceptors. Due to the stronger electron-withdrawing ability
of DPP compared with DPQ, TPA–DPP exhibits longer but weaker emission than TPA–DPQ as expected.
However, TPA–DPQ and TPA–DPP show different mechanochromic luminescence (MCL) properties with
emission color changes of 42 and 10 nm, respectively. Crystallographic analysis demonstrates that changes
of CHÁÁÁp and CHÁÁÁN intermolecular interactions are responsible for the significantly different MCL
behaviors of the two compounds. This work will provide some guidance for the rational design of
MCL-active fluorophores.
Received 22nd March 2021,
Accepted 19th May 2021
DOI: 10.1039/d1nj01350e
rsc.li/njc
of MCL emitters. For example, tetraphenylethene (TPE) is a
well-known building block, which has been successfully
1. Introduction
Mechanochromic luminescent (MCL) emitters are smart fluoro- introduced to construct most MCL emitters.11–15 Other
phores that show a reversible change in photoluminescence luminogens such as the b-diketone boron complex,16 pyrene,17
(PL) color upon external stimuli such as mechanical stress (e.g. phenothiazine,18 anthracene19 etc. can also serve as key units of
grinding, pressing, crushing, scratching, and shearing), heat some MCL materials. Besides, some donor–acceptor (D–A) type
treatment, and solvent vapor fuming.1–4 Recently, MCL materials compounds have been demonstrated to be MCL
have become materials of interest for their potential applications fluorophores.20–30 The D–A structure endows enriched structural
in solid-state sensors, optoelectronic devices, optical storage and deformation and abundant intermolecular interaction variations
security inks.5–8 Most reports show that the change of molecular upon external stimuli, which may result in tunable color of the
conformations, packing and intermolecular interactions under solid-state emission and be responsible for MCL properties.
an external stimuli are considered to be key factors for MCL Moreover, exploration of MCL emitters from many fluorophores
properties.9,10 Therefore, the development of novel MCL emitters featuring D–A structures may bring a new perspective for the
with well-defined crystal structures is indispensable for the development of novel MCL emitters.
elaboration of the MCL mechanism.
In recent years, quinoxaline derivative compounds have
Compared with conventional fluorophores, due to the lack attracted considerable attention because of their promising
of a clear design strategy, MCL fluorophores remain limited in applications in some fields such as perovskite solar cells,31
number. Considerable efforts were devoted to enrich the library organic light-emitting diodes32 and chemosensors.33 Meanwhile,
some quinoxaline-based fluorophores (summarized in Fig. S1,
ESI†) with D–A structures show MCL properties.34–39 It is found
a College of Life Sciences and Chemistry, Jiangsu Key Laboratory of Biofunctional
that MCL properties of these compounds can be tuned by
changing the electron donors while fixing the electron
acceptors.34–38 Alternatively, modifications on the electron
donors may provide another strategy for fine-tuning MCL
properties of fluorophores. In this work, a quinoxaline derivative
[4-(2,3-diphenyl-quinoxalin-6-yl)-phenyl]-diphenyl-amine40 (TPA–
DPQ, Fig. 1) is found to be MCL-active. To investigate the
Molecule, Institute of New Materials for Vehicles, Jiangsu Second Normal
University, Nanjing, 210013, P. R. China. E-mail: huangbinhb31@sina.com
b State Key Laboratory of Bioelectronics, Jiangsu Key Laboratory for Biomaterials
and Devices, School of Biological Science and Medical Engineering,
Southeast University, Nanjing 210096, P. R. China
† Electronic supplementary information (ESI) available. CCDC 2065097, 2065098,
2075146 and 2080880. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d1nj01350e
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Fig. 1 Synthetic routes of TPA–DPQ and TPA–DPP.
influence of the electron acceptors on the MCL properties, we The powder XRD curves are obtained using a diffractometer
designed and synthesized an analogue of TPA–DPQ, namely (Ultima IV) with an X-ray source of Cu Ka at 40 kV and 40 mA,
[4-(2,3-diphenyl-pyrido[2,3-b]pyrazin-7-yl)-phenyl]-diphenyl-amine at a scan rate of 21 (2y) per 1 min. CCDC 2065097 (TPA–DPQ at
(TPA–DPP) (Fig. 1), in which triphenylamine is used as an 150 K), 2075146 (TPA–DPQ at 296 K), 2065098 (TPA–DPP at 193 K)
electron donor while 2,3-diphenyl-pyridine[2,3-b]pyrazine (DPP) and 2080880 (TPA–DPP at 296 K) contain the supplementary
serves as an electron acceptor. TPA–DPP exhibits longer but crystallographic data for this work.41
weaker emission than TPA–DPQ, which can be attributed to the
Density functional theory (DFT) calculations of TPA–DPQ
stronger electron-withdrawing ability of DPP compared to 2,3- and TPA–DPP are performed using the Gaussian 09 program
diphenyl-quinoxaline (DPQ). However, TPA–DPQ and TPA–DPP package.42 Based on the optimized molecular structures at
show different MCL properties with emission color changes of 42 ground state, the excitation energies of TPA–DPQ and TPA–DPP
and 10 nm, respectively. Crystallographic analysis demonstrates are calculated using the time-dependent density functional theory
that TPA–DPQ and TPA–DPP show similar highly twisted (TD-DFT) method.43
structures but minor differences in weak CHÁ ÁÁp and CHÁ Á ÁN
2.2. Synthesis of TPA–DPQ and TPA–DPP
intermolecular interactions. The totally different MCL properties
of TPA–DPQ and TPA–DPP under external stimuli can be
attributed to the differences in intermolecular interactions.
This work will provide some guidance for the rational design of
MCL-active fluorophores.
Synthesis of 6-bromo-2,3-diphenylquinoxaline (BrDPQ) and
TPA–DPQ is carried out according to the literature.40
Synthesis of 7-bromo-2,3-diphenyl-pyrido[2,3-b]pyrazine (BrDPP).
1,2-Diphenyl-ethane-1,2-dione (5.0 mmol) and 5-bromo-pyridine-
2,3-diamine (5.0 mmol) are dissolved in 50 mL glacial acetic acid
and refluxed at 110 1C for 6 h. After cooling to room temperature
and adding 50 mL of water, a gray-white precipitate is filtered and
dried to produce BrDPP as a gray-white powder with a yield of
2. Experimental
2.1. General information
All reagents are purchased from commercial sources and used 94.2%. The crude BrDPP is directly used for the next reaction
as received without further purification. 1H NMR spectra of the without purification.
products are recorded on a Bruker 400 NMR spectrometer with
Synthesis of TPA–DPP. To a mixture of BrDPP (4 mmol) and
Si(CH3)4 as the internal standard. Mass spectra (MS) are 4-(diphenylamino)phenyl boronic acid (4 mmol) in 50 mL of
recorded on a Thermo Electron Corporation Finnigan LTQ toluene and 20 mL of ethanol, K2CO3 aqueous solution (2.0 M,
mass spectrometer. UV-vis absorption spectra of TPA–DPQ 4 mL) is added. After purging with N2 for 10 min, Pd(PPh3)4
and TPA–DPP are obtained on a UV-vis spectrophotometer (0.4 mmol) is added. The resulting mixture is refluxed under N2
(Agilent 8453). The PL spectra, FPL and transient PL lifetimes for 24 h. After cooling to room temperature, the reaction
are investigated on an Edinburgh FLS920 spectrophotometer. mixture is poured into 50 mL water, and extracted with dichloro-
Single crystal XRD measurements are carried out on a single methane (DCM) (2 Â 50 mL). The combined organic layer is
crystal X-ray diffractometer (Oxford Gemini S Ultra) with Mo Ka washed with saturated aqueous NaCl solution (2 Â 20 mL), and
radiation. Differential scanning calorimetry (DSC) curves are dried over anhydrous Na2SO4. After removing the solvent under a
recorded on a Netzsch DSC 2910 modulated calorimeter vacuum, the residue is purified by silica gel column chromato-
under a dry nitrogen atmosphere at a heating rate of 10 1C minÀ1
.
graphy with petroleum ether/DCM (v : v = 10: 1–5 : 1) as an eluent
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1
Table 1 Calculated triplet and singlet excitation energies (vertical transition),
oscillator strength (f), and transition configurations of TPA–DPQ and TPA–DPP
to afford TPA–DPP as an orange solid with a yield of 54.2%. H
NMR(400 MHz, CDCl3) d (ppm): 9.50 (d, J = 4.0 Hz, 1H), 8.67
(d, J = 4.0 Hz, 1H), 7.78–7.67 (m, 4H), 7.67–7.59 (m, 2H), 7.53–
7.12 (m, 18H). MS (MALDI-TOF) [m/z]: calcd. for C37H26N4,
526.62; found: 526.85.
E
DEST
(eV)
Compound State (eV)
F
Main configuration
p
TPA–DPQ
S1
S2
T1
T2
S1
S2
T1
T2
2.75 0.33
3.41 0.36
2.32 0.00
2.77 0.00
H - L
H - L+1
H - L
HÀ2 - L
0.70 0.43
0.69
0.55
0.51
2.3. Single crystals of TPA–DPQ and TPA–DPP
The block-shaped yellow crystals of TPA–DPQ are grown from
DCM/hexane (v : v = 1 : 1) solution by slow evaporation at room
temperature. The needle-like orange crystals of TPA–DPP are
grown from DCM/ethanol (v : v = 1 : 1) solution by slow evaporation
at room temperature.
TPA–DPP
2.47 0.343 H - L
0.70 0.33
0.68
2.94 0.216 HÀ2 - L
1.92
1.95
0
0
H -L
HÀ2 - L
0.62
0.65
are localized on the electron-withdrawing DPQ or DPP units.
TPA–DPP has deeper HOMO and LUMO levels than TPA–DPQ,
which can be attributed to the stronger electron-withdrawing
ability of the DPP moiety compared to the DPP unit. Accordingly,
the optical bandgap (Eg) values of TPA–DPQ and TPA–DPP are
estimated to be 3.10 and 2.96 eV, respectively. Based on the
theoretically optimized geometries of TPA–DPQ and TPA–DPP,
TD-DFT calculations are performed to estimate the energy levels
of the excited-state. TPA–DPQ and TPA–DPP show S1 values of
2.75 and 2.61 eV, and T1 values of 2.32 and 2.28 eV (Table 1),
respectively. Correspondingly, the energy splits (DESTs) of
TPA–DPQ and TPA–DPP are calculated to be 0.43 and 0.33 eV,
respectively.
3. Results and discussion
3.1 Synthesis
Synthetic routes of TPA–DPQ and TPA–DPP are shown in Fig. 1.
First, the key intermediates BrDPQ and BrDPP are prepared by
condensation reactions of the starting material 1,2-diphenyl-
ethane-1,2-dione with 4-bromo-benzene-1,2-diamine and 5-bromo-
pyridine-2,3-diamine, respectively. Then BrDPQ and BrDPP are
converted to target compounds TPA–DPQ and TPA–DPP via Suzuki
cross-coupling reactions with 4-(diphenylamino)phenyl boronic
acid. The structures of TPA–DPQ and TPA–DPP are confirmed by
1H NMR, MS and single-crystal analysis.
3.3 Single crystallographic analysis
3.2. Theoretical calculations
The single-crystal structures of TPA–DPQ and TPA–DPP are
To predict the geometric and electronic structures of TPA–DPQ measured at 150 and 193 K, respectively. As shown in Fig. 3
and TPA–DPP, DFT calculations are carried out. As shown in and Table 2, TPA–DPQ and TPA–DPP are both monoclinic with
Fig. 2, TPA–DPQ and TPA–DPP possess twisted structures. space groups P21/c. For TPA–DPQ, the compound exhibits
The highest occupied molecular orbitals (HOMOs) of TPA–DPQ twisted molecular configuration with a D–A dihedral angle of
and TPA–DPP are mainly dispersed over the electron-donating 38.731 between the TPA moiety and the adjacent phenyl plane
TPA moiety and the adjacent phenyl group or pyridine ring. of the DPQ unit. The dihedral angles between the substituted
In contrast, the lowest unoccupied molecular orbitals (LUMO) phenyl rings and quinoxaline plane are 46.35 and 48.811,
Fig. 2 Calculated energy levels for TPA–DPQ and TPA–DPP.
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Fig. 3 The single crystal structures and molecular interactions of TPA–DPQ (at 150 K) and TPA–DPP (at 193 K).
respectively. Similar to TPA–DPQ, TPA–DPP also shows a p–p stacking arrangement in the crystals. For TPA–DPQ, the two
twisted structure with a slightly smaller dihedral angle of neighboring molecules adopt a face-to-tail stacking mode along
34.031 between the TPA moiety and the adjacent pyridine plane the b-axis. It is found that a phenyl ring of the TPA unit forms a
of the DPP unit. Similarly, the dihedral angles between the close CHÁ Á Áp interaction with a substituted phenyl ring of the
substituted phenyl rings and the pyridine[2,3-b]pyrazine plane DPQ group at a distance of 2.974 Å. Meanwhile, the phenyl
are 48.91 and 46.151, respectively.
ring of the TPA unit also forms a CHÁ Á ÁN interaction with the
As above mentioned, TPA–DPQ and TPA–DPP both show DPQ unit at a distance of 2.739 Å. In the case of TPA–DPP, the
highly twisted structures, which may inhibit the formation of two adjacent molecules also exhibit a face-to-tail stacking
Table 2 Crystal data and structure refinement for TPA–DPQ and TPA–DPP
TPA–DPQ
38H27N3
2065097
525.62
TPA–DPQ
TPA–DPP
TPA–DPP
Formula
CCDC No.
Formula weight
C
C38H27N3
2075146
525.62
C37H26N4
2065098
526.62
C37H26N4
2080880
526.62
Space group
Temperature (K)
P21/c
150
P21/c
296
P21/c
193
P21/c
296
Wavelength/Å
Crystal system
a/Å
b/Å
c/Å
a/1
0.71073
Monoclinic
19.240(13)
8.892(6)
18.043(9)
90.00
0.71073
Monoclinic
19.248(18)
8.936(8)
18.090(19)
90.00
0.71073
Monoclinic
18.920(11)
8.897(7)
17.812(10)
90.00
0.71073
Monoclinic
19.065(14)
8.994(7)
17.864(14)
90.00
b/1
116.87(2)
90.00
2754(3)
4
1.353
0.074
117.00(3)
90.00
2773(5)
4
1.259
0.074
116.82(2)
90.00
2676(3)
4
1.360
0.078
116.75(13)
90.00
2735(4)
4
1.279
0.076
g/1
Volume/Å3
Z
Calculated density/Mg mÀ3
M(mmÀ1
)
F(000)
1072
1104
1104
1104
Crystal size/mm3
0.20 Â 0.15 Â 0.10
0.20 Â 0.15 Â 0.10
0.10 Â 0.10 Â 0.05
0.10 Â 0.08 Â 0.05
No. of reflns collected
No. of unique reflns
Goodness-of-fit on F2
R1, wR2 [I 4 2s(I)]
R1, wR2 (all data)
5629
6337
4743
5982
3551
1.027
0.0492, 0.0971
0.0598, 0.1305
4049
1.022
0.0528, 0.1179
0.0921, 0.1370
2465
1.043
0.0794, 0.1518
0.2049, 0.2763
2892
0.970
0.0591, 0.1393
0.1389, 0.1814
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arrangement with CHÁ Á Áp interactions of 2.959 Å and CHÁ Á ÁN CHÁ Á ÁN interactions are formed, which can be attributed to the
interactions of 2.626 and 2.986 Å. Interestingly, two kinds of added N atom of the pyridine[2,3-b]pyrazine ring compared to
Fig. 4 (a) The digital photographs of TPA–DPQ in different solvents under 365 nm UV excitation; (b) the digital photographs of TPA–DPP in different
solvents under 365 nm UV excitation; (c) UV-vis absorption spectra of TPA–DPQ in different solvents; (d) UV-vis absorption spectra of TPA–DPP in
different solvents; (e) PL spectra of TPA–DPQ in different solvents; (f) PL spectra of TPA–DPP in different solvents; (g) Lippert–Mataga plots of TPA–DPQ
and TPA–DPP; (h) fluorescence lifetimes of TPA–DPQ and TPA–DPP in DCM.
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the quinoxaline ring. Apparently, TPA–DPP with shorter CHÁ Á Áp From the absorption edge, the Eg values of the two compounds
and CHÁ Á ÁN interactions exhibits a more compact packing than are estimated to be 2.54 and 2.32 eV, which are lower than
TPA–DPQ. It is believed that the differences in weak CHÁ Á Áp and those obtained in the theoretically calculated values by ca.
CHÁ Á ÁN intermolecular interactions in crystals may have effects 0.6 eV, which can be attributed to the calculations that are
on the emitting behaviors of the two compounds.
employed under the gas-phase conditions. With the increase of
To investigate the effect of temperature on molecular solvent polarity from n-hexane to N,N-dimethylformamide, the
conformations and interactions, the single crystal structures absorption spectra of TPA–DPQ and TPA–DPP are slightly
of TPA–DPQ and TPA–DPP are also measured at 296 K (Table 2). altered, indicating the insignificant effects of solvent polarity
As shown in Fig. S1 (ESI†), the molecular conformations of both on the ground-state electron distribution.
compounds show negligible changes with the increase of
As shown in Fig. 4e and Table 3, TPA–DPQ in hexane
temperature. In contrast, all the distances of CHÁ Á Áp and (nonpolar solvent) shows deep blue emission peaking at
CHÁ Á ÁN interactions are increased, indicating the decrease of 448 nm with a small Stokes shift (Dn) of 2678 cmÀ1, while
intermolecular interactions in both crystals with increasing the emission peak of TPA–DPQ undergoes a red-shift to
temperature. Note that TPA–DPP also shows a more compact 618 nm in N,N-dimethylformamide (polar solvent) with a
packing than TPA–DPQ at 293 K, which may govern the large Dn of 8819 cmÀ1, whereas TPA–DPP shows a sky blue
fluorescence emission.
(lem = 465 nm, Dn = 2024 cmÀ1) and deep red (lem = 723 nm,
Dn = 9809 cmÀ1) color in hexane and N,N-dimethylformamide,
respectively. (Fig. 4f) These results indicate that the excited
states of both compounds can be stabilized in polar solvents,
implying typical ICT characteristics. The solvent-dependent
Stokes shifts for TPA–DPQ and TPA–DPP are then analyzed
from the Lippert–Mataga eqn (1):44,45
3.4 UV-vis absorption and emission spectra of TPA–DPQ and
TPA–DPP in different solvents
Fig. 4a and b show the photographs of TPA–DPQ and TPA–DPP
in several solvents with various polarities under 365 nm UV
excitation. UV-vis absorption spectra of TPA–DPQ in n-hexane
show two major absorption bands at 310 and 400 nm (Fig. 2c
and Table 3), corresponding to the p–p* transition and
intramolecular charge transfer (ICT) transition, respectively.
TPA–DPP also exhibits a similar absorption pattern with two
bands at 350 and 425 nm (Fig. 2d and Table 3) in n-hexane.
Dv = va À vf = (2Dm2/hca3)Df (e, n) + A;
where Df (e, n) is calculated from eqn (2):
(1)
Df (e, n) = [(e À 1)/(2e + 1)] À [(n2 À 1)/(2n2 + 1)].
(2)
Here, Dm is the difference between the excited- and ground-
state dipole moments upon excitation, h is Plank’s constant
(6.6 Â 10À34 J sÀ1), c is the velocity of light in a vacuum (3.0 Â
108 m sÀ1), a is the Onsagar cavity radius, and n and e are the
reflective index and the static dielectric constant of the solvents,
respectively. As shown in Fig. 4g and Table 3, the slope values
of TPA–DPQ and TPA–DPP are calculated to be 23398 and
28516 cmÀ1, respectively. From the single crystal structures
(Fig. 3), the a values of TPA–DPQ and TPA–DPP are both
estimated to be 0.7157 nm. Accordingly, the Dm values of
Table 3 Spectral properties of TPA–DPQ and TPA–DPP in different
solvents
TPA–DPQ
labs lem
TPA–DPP
labs lem
Df
(e, n)
Dn
(nm) (nm) (cmÀ1
Dn
(nm) (nm) (cmÀ1
)
Solvent
)
n-Hexane 0.001 400
448
483
532
570
618
2678
4048
5534
7732
8819
425
427
438
426
423
465
520
585
628
723
2024
4188
5737
7551
9809
Toluene
THF
0.013 404
0.210 411
0.217 402
0.274 400
DCM
DMF
Fig. 5 (a) The time-resolved PL and PH spectra of TPA–DPQ in 2-MeTHF with 1 ms delay at 77 K; (b) the time–resolved PL and PH spectra of TPA–DPP
in 2-MeTHF with 1 ms delay at 77 K.
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Fig. 6 (a) Fluorescence images of TPA–DPQ crystals; (b) fluorescence images of TPA–DPP crystals; (c) fluorescence images of TPA–DPQ solid
response to grinding and heating under UV light (365 nm); (d) fluorescence images of TPA–DPQ solid response to grinding and heating under UV light
(365 nm); (e) PL spectra of TPA–DPQ and TPA–DPP crystals; (f) PL decay curves of TPA–DPQ and TPA–DPP crystals.
TPA–DPQ and TPA–DPP are calculated to be 27.8 and 30.7 D, 537 and 551 nm, respectively. Absolute FPL values and transient
respectively. TPA–DPP has a larger Dm than TPA–DPQ, suggesting PL decays of the two compounds are also measured (Table 4).
more efficient ICT between the donor TPA and the acceptor DPP Both crystals exhibit short-lived nanosecond-scaled lifetimes.
than the electronic communication between TPA and DPQ.
The radiation rate constants of TPA–DPQ and TPA–DPP are
Transient PL decays of TPA–DPQ and TPA–DPP are investigated. estimated to be 5.22 Â 107 and 5.56 Â 107 sÀ1. Compared with
In DCM, both compounds exhibit nanosecond-scaled decay proper- TPA–DPP, TPA–DPQ exhibits a higher FPL, which may be
ties with lifetimes of 8.12 and 5.12 ns for TPA–DPQ and TPA–DPP attributed to the suppressed non-radiative transition associated
(Fig. 4h), respectively, indicating typical fluorescence emission.
with weaker intermolecular interactions in the crystals (Fig. 3).
As shown in Fig. 6c, similar to the single crystals of
To estimate the energy levels of TPA–DPQ and TPA–DPP, the
time-resolved PL and phosporescence (PH, with 1 ms delay) TPA–DPQ, the as-prepared solid also emits green fluorescence
spectra are examined in 2-MeTHF at 77 K. As shown in Fig. 5a with a maximum at 539 nm and a FPL of 0.386. When the solid
and b, TPA–DPQ and TPA–DPP exhibit finely structured PL is heated at 120 1C for 10 s, the emission color of the powder is
spectra with peaks at 482 and 514 nm. The corresponding S1
values are calculated to be 2.57 and 2.41 eV, respectively. From
the PH spectra of TPA–DPQ and TPA–DPP with peaks at 571
and 603 nm, their T1 values are obtained to be 2.17 and 2.06 eV.
Table 4 PL characteristics of TPA–DPQ and TPA–DPP in the solid state
TPA–DPQ
TPA–DPP
Thus, the DEST values of TPA–DPQ and TPA–DPP are estimated
to be 0.40 and 0.35 eV, respectively, which agree well with the
theoretically calculated values.
Heated
solid
Heated
solid
Crystal Solid
Crystal Solid
lPL (nm)
FPL
537
539
0.386 0.265
7.24
5.33
8.48
497
551
552
0.315 0.287
7.28
4.33
9.41
542
3.5 MCL of TPA–DPQ and TPA–DPP
0.361
6.92
5.22
8.80
0.337
6.06
5.56
10.94
t (ns)
7.66
3.46
9.60
7.75
3.70
9.20
kr (107 sÀ1
)
As shown in Fig. 6a–e and Table 4, the single crystals of TPA–
DPQ and TPA–DPP show broad fluorescence with PL peaks at knr (107 sÀ1
)
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Fig. 7 (a) DSC curves of the as-prepared TPA-DPQ and TPA-DPP solids; (b) XRD spectra of TPA–DPQ in different states; (c) XRD spectra of TPA–DPP in
different states.
switched to blue with a maximum at 497 nm and a FPL of 0.265. suggesting that the as-prepared solids have the same crystal
Interestingly, the emission of the powder is transformed back systems as the corresponding single crystals for the two
to green after being ground. The reversible transformation in compounds. The results are in agreement with the similar
emission color in response to external stimuli indicates emission properties of the as-prepared solids and the corres-
obvious MCL property of TPA–DPQ. Note that TPA–DPQ ponding single crystals. In contrast, after being heated at 120 1C
shows a large difference in emission color change (Dlem) of for 10 s, some strong and evident diffraction peaks of the
42 nm upon external stimuli, which is comparable with as-prepared solids disappear, indicating the crystal phase
other MCL emitters.15 In contrast, the solid of TPA–DPP also transformations of TPA–DPQ and TPA–DPP during the heat
shows reversible emission color between yellow (FPL = 0.315, treatment.38 Upon grinding the heated solid, some strong and
lPL = 552 nm) and yellowish green (FPL = 0.287, lPL = 542 nm) evident diffraction peaks appear again, which are similar to
upon external stimuli, while the Dlem is decreased to 10 nm. those of the corresponding as-prepared solid. The results
Obviously, TPA–DPQ and TPA–DPP exhibit different MCL coincide with the reversible PL color changes of the two
features. During heat treatment, intermolecular interactions compounds during the heating and grinding treatments. It is
decrease as mentioned above, resulting in red-shifted PL demonstrated that the phase transformations upon external
emission.6 In this case, due to the more compact packing stimuli should be responsible for the MCL behaviors of
associated with stronger CHÁ Á Áp and CHÁ Á ÁN intermolecular TPA–DPQ and TPA–DPP.
interactions, TPA–DPP is likely to be more stable upon
heat treatment, inducing a small Dlem. In contrast, the large
Dlem of TPA–DPQ might be attributed to the loose packing
4. Conclusions
associated with weaker CHÁ Á Áp and CHÁ Á ÁN intermolecular
In conclusion, we synthesized a series of light-emitting analogues
interactions.
TPA–DPQ and TPA–DPP, which exhibit strong fluorescence
To gain insight on the origin of the MCL properties of
emission in solutions and in solid states. Interestingly, TPA–DPQ
TPA–DPQ and TPA–DPP, DSC experiments are performed.
and TPA–DPP show different MCL properties with Dlem of 42 and
As can be seen from the DSC curves (Fig. 7a), during the
10 nm, respectively. Crystallographic analysis demonstrates that
heating process, the as-prepared solids of TPA–DPQ and
the significantly different MCL behaviors of the two compounds
TPA–DPP show similar endothermic peaks at 100.61 and
can be attributed to the subtle changes of CHÁÁÁp and CHÁÁÁN
100.56 1C, respectively, indicating the phase transitions of both
intermolecular interactions. This work will provide a guideline for
compounds. To further investigate the MCL behaviors of
further design of MCL emitters.
TPA–DPQ and TPA–DPP, PXRD measurements are carried out
for respective samples. As expected, the diffraction curves of the
as-prepared solids of TPA–DPQ and TPA–DPP exhibit sharp and
strong diffraction peaks, which are similar to the simulated
Conflicts of interest
data of the single crystals of the corresponding compounds, There are no conflicts to declare.
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2021
New J. Chem., 2021, 45, 11304–11312 | 11311

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NJC
Paper
21 Y. Gong, Y. Zhang, W. Z. Yuan, J. Z. Sun and Y. Zhang,
J. Phys. Chem. C, 2014, 118, 10998.
22 S. Xu, T. Liu, Y. Mu, Y. F. Wang, Z. Chi, C. C. Lo, S. Liu,
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23 B. Huang, Z. Li, H. Yang, D. Hu, W. Wu, Y. Feng, W. Jiang,
B. Lin and Y. Sun, J. Mater. Chem. C, 2017, 5, 12031.
24 J. Guo, X.-L. Li, H. Nie, W. Luo, S. Gan, S. Hu, R. Hu, A. Qin,
Z. Zhao, S.-J. Su and B. Z. Tang, Adv. Funct. Mater., 2017,
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Acknowledgements
This work was supported by the China Postdoctoral Science
Foundation Funded Project (No. 2020M681464), the Qing
Lan Project of Jiangsu Province, and the Natural Science
Foundation of the Jiangsu Higher Education Institutions of
China (No. 19KJB150006).
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