Polymorphous Luminescent Materials Based on ′T′-Shaped Molecules Bearing 4,7-Diphenylbenzo[c][1,2,5]thiadiazole Skeletons: Effect of Substituents on the Photophysical Properties

Article abstract of DOI:10.1002/chem.201904026

Polymorphism, the intrinsic character of one chemical compound with at least two distinct phase arrangements, plays a very key role in the photophysical properties. In this contribution, four ′T′-shaped molecules bearing the 2,1,3-benzothiadiazole (BTD) skeleton, named 5 a–5 d, were prepared and characterized. All compounds exhibited excellent thermal stability and polymorphism in the solid state, evident from thermogravimetric analysis, differential scanning calorimetry, and polarized optical microscopy results. Intense emissions with high photoluminescent quantum yields were achieved both in solution (56–97 %) and neat films (33–98 %). All compounds possessed clearly pH-dependent luminescence properties in solution. Additionally, compound 5 d showed useful mechanochromic luminescence owing to the transformation between the crystal and amorphous state. Employing compounds 5 a–5 d as the dopant, solution-processable organic light-emitting diodes (OLEDs) were fabricated and presented a highest external quantum efficiency of 6.15 %, which is higher than the theoretical value of fluorescence-based OLEDs (～5 %). This research provided a novel strategy for designing high-efficiency BTD-based polymorphic luminescent materials.

Full text of DOI:10.1002/chem.201904026

DOI: 10.1002/chem.201904026
Full Paper
&
Materials
Polymorphous Luminescent Materials Based on ’T’-Shaped
Molecules Bearing 4,7-Diphenylbenzo[c][1,2,5]thiadiazole
Skeletons: Effect of Substituents on the Photophysical Properties
Gaowei Qian,[a] Xiangbing Wang,[a] Shengyue Wang,[a] Yanqiong Zheng,[c] Song Wang,[b]
Weiguo Zhu,*[a] and Yafei Wang*[a]
Abstract: Polymorphism, the intrinsic character of one
chemical compound with at least two distinct phase ar-
rangements, plays a very key role in the photophysical prop-
erties. In this contribution, four ’T’-shaped molecules bearing
the 2,1,3-benzothiadiazole (BTD) skeleton, named 5a–5d,
were prepared and characterized. All compounds exhibited
excellent thermal stability and polymorphism in the solid
state, evident from thermogravimetric analysis, differential
scanning calorimetry, and polarized optical microscopy re-
sults. Intense emissions with high photoluminescent quan-
tum yields were achieved both in solution (56–97%) and
neat films (33–98%). All compounds possessed clearly pH-
dependent luminescence properties in solution. Additionally,
compound 5d showed useful mechanochromic lumines-
cence owing to the transformation between the crystal and
amorphous state. Employing compounds 5a–5d as the
dopant, solution-processable organic light-emitting diodes
(OLEDs) were fabricated and presented a highest external
quantum efficiency of 6.15%, which is higher than the theo-
retical value of fluorescence-based OLEDs (~5%). This re-
search provided a novel strategy for designing high-efficien-
cy BTD-based polymorphic luminescent materials.
cent switches, and optical devices.[12–19] Polymorphism is actual-
ly the intrinsic character of one chemical compound with at
least two phase arrangements, which plays a very key role in
the photophysical properties. To date, considerable research
focusing on polymorphous luminescent materials, such as me-
chanochromic and piezochromic luminescence, have been de-
veloped.[20–26]
Introduction
Over the past several decades, emissive materials, including
both fluorescent and phosphorescent materials, have been
widely applied in organic light-emitting diodes (OLEDs),[1–11]
used in commercial products, such as car screens, mobile
phone screens, and some solid-state lighting. Among these
emissive materials, polymorphous luminescent materials have
been pursued by researchers owing to their interesting emis-
sion properties, especially for the external-stimuli-responsive
emission, which has potential application in sensors, fluores-
2,1,3-Benzothiadiazole (BTD) as a star building block has re-
ceived much attention in the optoelectronic field for liquid-
crystalline, organic solar cells (OSCs), OLEDs, and organic field-
effect transistors because of its planar structure, strong
electron-withdrawing capacity, efficient fluorophores, and
good intermolecular interactions.[27–32] With this in mind, some
polymorphous luminescent materials based on the BTD moiety
have been explored.[33–35] For example, introduction of an intra-
molecular BꢀN bond into the BTD group, Murata and his co-
workers prepared the compound 4,7-bis[3-(dimesitylboryl)-
thien-2-yl]benzothiadiazole possessing reversible solvato-,
thermo-, and mechanochromic luminescence.[36] Nagai et al.
synthesized a series of highly emissive solid-state fluorophores
(phenanthroimidazolylbenzothiadiazoles) with donor–acceptor
(D–A) framework, which showed more versatile mechanochro-
mic luminescence owing to the different morphology states.[37]
Additionally, they found that the substituents on the phenyl
group of the benzothiadiazole ring can effectively control the
mechanochromic luminescence.[37] Recently, Hayashi and Yuan
and their respective co-workers revealed the crystallization-
induced emission for commercially available 2,1,3-benzothia-
diazole derivatives.[38,39] As an alternative strategy, molecules
[a] G. Qian, X. Wang, Dr. S. Wang, Prof. W. Zhu, Prof. Y. Wang
National Experimental Demonstration Center
for Materials Science and Engineering
Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials
Jiangsu Collaborative Innovation Center of Photovoltaic Science
and Engineering, Jiangsu Engineering Laboratory of
Light–Electricity–Heat Energy Converting Materials and Applications
School of Materials Science & Engineering
Changzhou University, Changzhou 213164 (P. R. China)
E-mail: zhuwg18@126.com
qiji830404@hotmail.com
[b] S. Wang
Hubei University of Arts and Science, Xiangyang, 441053 (P. R. China)
[c] Dr. Y. Zheng
Key Laboratory of Advanced Display and
System Applications of Ministry of Education, Shanghai University
149 Yanchang Rd., Shanghai 200072 (P. R. China)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under:
https://doi.org/10.1002/chem.201904026.
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with liquid-crystalline properties can also show good
polymorphous emission. Vieira and his colleagues
synthesized a series of non-symmetric BTD-based
liquid crystals and investigated the phase-dependent
photoluminescence.[40] Sagara et al. reported an
asymmetric cyclophane featuring the 4,7-bis(pheny-
lethynyl)-2,1,3-benzothidadiazole group, which ex-
hibited different emission properties in the liquid-
crystalline and crystalline states.[41] To the best of our
knowledge, most reported examples just focused on
integrating donor groups/or alkyl chains into the 4
and 7 positions of BTD to achieve polymorphous lu-
minescence. Few BTD-based polymorphous lumines-
cent materials present satisfactory device per-
formance in solution-processable OLEDs. Therefore,
there is still space to explore the structure–property
relationship of BTD-based polymorphous lumines-
cent materials.
Herein, four novel BTD-based derivatives, 5a–5d,
with ’T’-shaped geometry were designed and pre-
pared. In these molecules, 4,7-diphenylbenzo[c]-
[1,2,5]thiadiazole was used as the emissive core,
whereas the 3,4,5-tris(dodecyloxy)benzamide was in-
tegrated into the 5 position of BTD to modify the
solubility and phase state. This kind of ’T’-shaped ge-
Scheme 1. Synthetic route for compounds 5a–5d. Reaction conditions: a) (1) H2SO4,
HNO3, 08C, RT, 6 h; (2) Fe, CH3COOH, 08C, RT, 12 h; b) Pd(PPh3)4, 2m K2CO3, toluene,
CH3CH2OH, 808C, 24 h; c) K2CO3, KI, acetone, 808C, 2 d; d) THF, CH3CH2OH, NaOH solu-
tion, 808C, 4 h; e) (COCl)2, Ph3P=O, CH3CN, RT, 30 min.
Thermal properties
ometry can control the emission property through the BTD
skeleton and regulate the polymorphism through the side
chains. To further explore the structure–property relationship,
different substituents with electron-withdrawing and electron-
donating properties were introduced into 4,7-diphenylbenzo[c]
[1,2,5]thiadiazole. All compounds showed bright emission with
very high emission quantum yield in solution (56–97%) and
the solid state (33–98%). External-stimuli-responsive emission
such as pH and mechanical grinding was observed for these
novel compounds. Employing compounds 5a–5d as dopants,
OLEDs fabricated by solution processing presented a maximum
external quantum efficiency of >6%, which is among the
highest reported for BTD-based fluorescent materials.
To explore the thermal properties of these compounds, ther-
mogravimetric analysis (TGA) and differential scanning calorim-
etry (DSC) were carried out under N2 atmosphere. As shown in
Figure S1 (in the Supporting Information), all compounds ex-
hibit good thermal stability with the decomposition tempera-
tures above 3808C at 5% weight loss, implying that introduc-
tion of different substituents into the emissive core has a negli-
gible effect on the decomposition temperature. On the contra-
ry, different DSC curves were detected for compounds 5a–5d
(Figure 1). Compound 5a shows a clear phase transition at
388C in the second heating circle, suggesting a crystal–crystal
transition. Then, a very weak transition at about 488C is detect-
ed, followed by the clearing point at about 768C. On the other
hand, three exothermic peaks at 39, 32, and 188C are observed
on the cooling process for compound 5a. The presence of
multiple transition peaks implies that the compound has poly-
morphic behavior. When the fluorine atom is introduced, there
are three endothermic peaks at 79, 103, and 1098C in the first
heating cycle. However, only one endothermic peak (1098C)
and one exothermic peak (808C) were observed during the
second heating and first cooling processes, indicating the pres-
ence of highly defective and imperfect crystals or solid meso-
phases that do not crystallize by cooling from the isotropic
melt.[42] As for compound 5c, with tert-butyl substituents, the
sample displays several endothermic peaks in the first and
second heating scans owing to the complex polymorphic be-
havior, and then the compound becomes isotropic at 1358C.
Upon the cooling curve, three exothermic peaks are clearly ob-
served at 95, 32, and 198C. The DSC curves of compound 5d
show sharp peaks at 2348C and 2058C, as well as very weak
phase transition peaks at 448C and 428C upon heating and
Results and Discussion
Synthesis
As depicted in Scheme 1, starting from the commercially avail-
able 4,7-dibromobenzo[c][1,2,5]thiadiazole, compound 1 was
obtained by nitration reaction in the presence of H2SO4 and
HNO3 at room temperature, then followed by the reduction re-
action by using iron as a catalyst in CH3COOH. Typical Suzuki
coupling reactions between compound 1 and phenylboronic
acid derivatives were able to achieve compound 2 in 70%
yield. 1-Bromododecane reacted with methyl 3,4,5-trihydroxy-
benzoate to yield compound 3 in the presence of K2CO3 and
acetone, which afforded compound 4 by hydrolysis. By em-
ploying oxalyl chloride and triphenylphosphine in acetonitrile
at room temperature, condensation reaction of compound 2
with compound 4 yielded target compounds 5a–5d, which
1
were confirmed by H NMR, 13C NMR, and TOF-MS analysis.
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Figure 1. DSC thermograms of compounds 5a–5d.
Figure 3. Absorption spectra of all compounds in dichloromethane solution
(10ꢀ5 m) at room temperature.
Figure 2. POM images of compounds on cooling (5a: 548C; 5b: 948C;
5c:1028C; 5d: 1908C).
250 nm and 550 nm. The absorption bands in the range 250–
350 nm are attributed to the p–p* transitions in the aromatic
ring, whereas the absorption bands between 350 nm and
450 nm are ascribed to the intramolecular charge transfer (ICT)
transition, evident from theoretical calculations (vide infra). In
addition, almost identical absorption spectra are observed for
compounds 5a–5c, whereas compound 5c shows a small red-
shift owing to the weak electron-donating tert-butyl unit. Com-
pound 5d presents a distinct redshifted and intense absorp-
tion spectrum compared with compounds 5a–5c, which is re-
sponsible for the carbazole group possessing good electron-
donating properties, leading to strong ICT transition. Clearly,
the different substituents have significant influence on the
ground state.
cooling scans, respectively. To further investigate the thermo-
tropic behavior, polarized optical microscopy (POM) measure-
ments were carried out for all compounds, which show distinct
birefringence upon heating and cooling processes (Figure 2).
Both DSC and POM results demonstrated that these BTD deriv-
atives have polymorphous states.
Photophysical properties
As shown in Figure 3, all compounds show three absorption
bands in dichloromethane (CH2Cl2, 10ꢀ5 m) solution between
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Under the excitation wavelength of 370 nm, all compounds
display intense emission with evident ICT character both in
CH2Cl2 solution and neat films at room temperature (Figure 4),
and the photophysical data are summarized in Table 1. Com-
was detected for compound 5d in neat film. To further explore
their photophysical properties, the photoluminescent quantum
yield (PLQY, FPL) and lifetime were investigated. Satisfactory
PLQY values of 0.56, 0.69, 0.97, and 0.95 were obtained for
compounds 5a, 5b, 5c, and 5d in CH2Cl2 solution, respectively.
Introduction of electron-donating moieties (tert-butyl and car-
bazole) has a positive effect on the emission efficiency. The
10 wt% doped poly(methyl methacrylate) (PMMA) films,
except compound 5c, exhibit decreased FPL values of 0.46 for
5a, 0.33 for 5b, and 0.70 for 5d owing to the concentration
quenching. Compound 5c still shows a high FPL in the doped
film, which could be ascribed to the sterically challenging tert-
butyl moiety effectively suppressing intermolecular interac-
tions. Transient PL decay curves (Figure 5) present single expo-
nential decay with the lifetimes in the nanosecond region
(Table 1), suggesting that these emissions are from the singlet
excitons. Based on FPL and lifetime, the radiative (Kr) and non-
radiative (Knr) rate constants were evaluated according to Equa-
tions (1) and (2), and the data are listed in Table 1.
Figure 4. PL spectra of compounds 5a–5d in (a) CH2Cl2 solution and (b) neat
film at room temperature.
F
ð1Þ
ð2Þ
Kr ¼
=
t
pounds 5a–5d show maximum emission peaks at 484, 487,
502, and 545 nm in CH2Cl2 solution at room temperature. Com-
pared with 5a and 5b, compounds 5c and 5d present clear
redshifted emission profiles owing to the donor units of tert-
butyl and carbazole. It is noted that the substituents in these
molecules play a key role on the emission property caused by
the ICT effect. To explore the ICT character of these molecules,
solvent-dependent absorption and emission properties were
examined. As depicted in Figure S2 (in the Supporting Informa-
tion), all compounds exhibit almost the same absorption spec-
tra in different solvents, which means that the solvent polariza-
tion has a negligible effect on the ground state. On the contra-
ry, redshifted emissions are observed for these compounds
with the increased solvent polarization, especially for com-
pound 5d (Figure S3 in the Supporting Information), implying
that the solvent polarization plays a positive effect on the ex-
cited state.
1
Knr ¼ = ꢀ Kr
t
Clearly, the compounds with donor units have higher emis-
sion efficiency because of the very small Knr.
Compared with the photoluminescence (PL) in solution,
clearly hypsochromic emissions are observed for 5a–5d in
neat film (Figure 3b and Table 1). Compounds 5a–5c show
almost the same emission profiles with the maximum emission
peaks at about 478 nm, whereas a maximum peak at 511 nm
Figure 5. Transient photoluminescent decay curves of compounds 5a–5d in
CH2Cl2 at room temperature.
Table 1. Photophysical data for compounds 5a–5d.
[a]
[c]
[f]
[g]
[g]
labs [nm]
lem [nm]
Td [oC]
FPL [%]
t[e] [ns]
Kr[f] [ꢁ107 sꢀ1
]
Knr [ꢁ107 sꢀ1
]
Eox [V]
Ered [V]
EHOMO [eV]
ꢀ5.96
ELUMO [eV]
ꢀ3.48
5a
5b
5c
5d
273, 326, 336, 383
271, 326, 335, 381
276, 329, 338, 388
292, 326, 340, 403
484[a]
478[b]
487[a]
477[b]
502[a]
479[b]
545[a]
510[b]
392
383
393
384
56[a]
46[d]
69[a]
33[d]
97[a]
98[d]
95[a]
70[d]
8.54
6.54
5.16
3.90
0.40
1.0
1.16
1.26
1.23
0.80
ꢀ1.32
ꢀ1.34
ꢀ1.28
ꢀ1.31
7.93
6.36
5.11
8.70
15.3
18.6
ꢀ6.06
ꢀ6.03
ꢀ5.6
ꢀ3.46
ꢀ3.52
ꢀ3.49
[a] In dichloromethane solution. [b] In neat film. [c] Under N2 with 208Cminꢀ1. [d] 10 wt% compound doped PMMA films. [e] In dichloromethane. [f] Radia-
tive decay rate kr =FPL/t; nonradiative decay rate knr =1/tꢀkr. [g] The neat film of the compounds in CH3CN, EoxFc/Fc+ =0.48 V.
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DFT calculations
substituents have a significant effect on the electronic proper-
ties, which also explains the different absorption and emission
properties of compounds 5a–5d.
To investigate the relationship between molecular structure
and electronic property, DFT was performed by using the
Gaussian 09 program based on the B3LYP functional with 6-
31G basis set. The DFT results reveal that the 4,7-diphenylben-
zo[c][1,2,5]thiadiazole moiety and amide group make the
major contributions to the highest occupied molecular orbitals
(HOMO) for compounds 5a, 5b, and 5c, whilst their lowest un-
occupied molecular orbitals (LUMO) mainly localize at
benzo[c]-[1,2,5]thiadiazole (Figure 6). In the case of 5d, the
LUMO shows the contribution from both the benzo[c]
[1,2,5]thiadiazole and amide moieties, whereas the HOMO is
mainly spread over the phenylcarbazole fragment. Notedly, the
pH-Dependent luminescence
Generally speaking, an amido bond in a molecule can play an
important role for the emissive property through intermolecu-
lar interactions.[43,44] Therefore, in this context, pH-dependence
fluorescence measurements were carried out for all com-
pounds in CH2Cl2. As shown in Figure 7, significant changes are
observed for all compounds with different pH values. For com-
pounds 5a and 5b, their emission intensities decrease sharply
with increasing pH values, whereas the emissive shape and
Figure 6. Frontier orbitals of compounds 5a–5d in the ground state from DFT calculations.
Figure 7. Emission profiles of compounds 5a–5d as a function of the pH at the excitation wavelength of 370 nm.
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wavelength exhibit negligible change. This phenomenon could
be explained by the physical quenching processes such as col-
lisional deactivation and intermolecular interactions between
the emissive unit and acid solvent. On the contrary, com-
pounds 5c and 5d have different pH-dependent luminescence
compared with compounds 5a and 5b, in which the emission
intensity increases with the pH values at first and then decreas-
es. In addition, the emission spectra of compounds 5a and 5d
display a clear blueshift (ca. 13 nm for 5c and 20 nm for 5d)
with increased pH values.
emission peak at 530 nm is observed after grinding the
sample, implying destroyed p–p interactions. When fuming
the ground sample in dichloromethane vapor for 15 min, the
emissive color can recover back to 554 nm, which is consistent
with the emission in the pristine state. The photographs under
UV light (Figure 8, inset) also demonstrate that the mechano-
chromic luminescence varies from yellow to green after grind-
ing the sample, followed by turning back to yellow by fuming.
Powder X-ray diffraction (PXRD) was then carried out to further
explore the mechanism of the mechanochromic luminescence.
Definitely, the PXRD patterns display almost identical diffrac-
tion peaks for compounds 5a–5c in different states, as shown
in Figure S5 (in the Supporting Information, taking 5b and 5c
as the examples). As for compound 5d, the PXRD patterns of
the pristine sample show some sharp and intense diffraction
peaks in the range 5–458, implying that compound 5d has
micro-crystalline structure in the pristine state.[45,46] After grind-
ing the sample, broad diffraction peaks replaced the sharp and
intense ones observed, suggesting compound 5d transforms
from the crystal to amorphous phase after grinding. When the
ground sample was fumed by the solvent, the diffraction
peaks clearly reverted to the pattern of the pristine samples.
Therefore, the mechanochromic luminescence behavior of
compound 5d is attributed to the transition between different
phase states. It is noted that the mechanochromic lumines-
cence in this molecular system is concerned not only with the
intermolecular amido bond but also with the substituents.
Mechanochromic luminescence
To further explore the relationship between molecular struc-
ture and emission, stimulus-dependent emission studies, such
as grinding and fuming, were performed. Unfortunately, the
pristine samples of compounds 5a–5c present almost the
same emissive profiles as those in the ground and fumed state
(Figure S4 in the Supporting Information), implying no mecha-
nochromic luminescence. Interestingly, distinct mechanochro-
mic luminescence was observed for compound 5d (Figure 8).
The pristine sample of compound 5d exhibits a maximum
emissive peak at 562 nm, whereas a markedly hypsochromic
Electrochemical properties
The electrochemical properties of the neat films of all com-
pounds were investigated in CH3CN solution. The cyclic vol-
tammetry (CV) curves are shown in Figure S6 (in the Support-
ing Information) and the data are listed in Table 1. All com-
pounds exhibit irreversible redox potentials between ꢀ1.5–
2.0 V. Owing to the same acceptor skeleton, all compounds
show similar reduction potentials at about ꢀ1.30 V (vs. Fc/Fc+,
the same below). The oxidation potentials are evaluated to be
1.16 V (vs. Fc/Fc+, the same below) for 5a, 1.26 V for 5b,
1.23 V for 5c, and 0.80 V for 5d. It is noted that compound 5d
appears at a significantly lower oxidation potential than that of
compounds 5a–5c, which can be explained by the strong
electron-donating property of the carbazole moiety. Using the
formula of EHOMO =ꢀ(Eox +4.8) eV and ELUMO =ꢀ(Ered +4.8) eV,
the HOMO and LUMO energy levels are estimated to be
ꢀ5.96/ꢀ3.48 eV for 5a, ꢀ6.06/ꢀ3.46 eV for 5b, ꢀ6.03/
ꢀ3.52 eV for 5c, and ꢀ5.6/ꢀ3.49 eV for 5d (Table 1). The intro-
duction of stronger donor substituents into the molecule is
thus seen to decrease the electrochemical energy gap, which
is consistent with the trends in the absorption and emission
profiles.
Electroluminescent properties
Figure 8. (a) PL spectra and photographs of compound 5d in response to
external stimuli. Photographs were taken under UV irradiation with 365 nm.
P: pristine; G: grinding; F: fuming. (b) Powder X-ray diffraction curves of
compound 5d at room temperature.
To further explore the effect of the substituents on the electro-
luminescent (EL) properties, OLEDs were fabricated by solution
processing with the configuration of ITO/PEDOT:PSS (40 nm)/
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CzAcSF:dyes (10 wt%, 50 nm)/DPEPO (9 nm)/TmPyPB (40 nm)/
CsF (1.2 nm)/Al (120 nm) (see the Experimental Section for full
details). In the devices, PEDOT:PSS acts as the hole injection
layer. The emitter layer is a mixture of host CzAcSF and the
dopant dyes. DPEPO and TmPyPB are the hole-blocking and
electron-transport layers, respectively. Similar to the PL emis-
sion, the devices exhibit bluish-green emissions with the maxi-
mum emissive peaks at 490 nm for 5a, 490 nm for 5b, 492 nm
for 5c, and 498 nm for 5d, which demonstrates that the exci-
ton recombination occurs in the emissive layer (Figure 9a).
Similar to the PL profiles, it is noted that introduction of the
electron-donating group (tert-butyl and carbazole) has a clear
effect on the redshifted EL spectra (Figure 9a and Table S1 in
the Supporting Information). The Commission Internationale
de L’Eclairage (CIE) coordinates are (0.18, 0.34), (0.18, 0.35),
(0.18, 0.35), and (0.20, 0.41) for the 5a, 5b, 5c, and 5d-based
devices, respectively (Figure S7 in the Supporting Information).
On the other hand, the absence of host emission implies that
a complete energy transfer is realized between host and guest
in these devices.
Conclusion
Four novel BTD-based polymorphous luminescent materials
were synthesized and characterized. All compounds showed
very bright emission with excellent emissive efficiency in solu-
tion and the solid state. Different substituents resulted in the
compounds possessing distinct phase transitions. Increasing
with electron-donating intensity, the emissive spectra exhibited
a redshift. pH-dependent and mechanochromic luminescence
were observed for these BTD derivatives owing to the different
intermolecular interactions. By employing compounds 5a–5d
as dopants, OLEDs fabricated by solution processing showed a
maximum external quantum efficiency of >6%, which is
among the highest reported for BTD-based fluorescent materi-
als. This research proved that elaborate molecular engineering
can provide polymorphic luminescent materials with satisfacto-
ry emission properties.
Experimental Section
The current density–voltage–luminance (J–V--L) characteris-
tics of the devices are shown in Figure 9d, and the relative pa-
rameters are listed in Table S1 (in the Supporting Information).
All the devices possess low turn-on voltages (Von, 1 cdmꢀ2) of
3.6 V, and maximum brightness in the range 1012–
1717 cdmꢀ2. Owing to the superior emission properties, the
devices based on 5c and 5d present better performances with
the maximum current efficiencies (CE) of 13.88 and
15.94 cdAꢀ1 and external quantum efficiencies (EQE) of 6.07
and 6.15%, respectively, which is higher than the theoretical
value of the fluorescence materials (ca. 5%).
Materials and measurements
1,4-Dibromo-2,5-diiodobenzene, 4-(diphenylamine)phenylboronic
acid, and diphenyl disulfide were commercially sourced from
Energy Chemical Company Ltd. Other reagents were purchased
from J&K Chemical and Aladdin. All reactions were carried out
1
under N2 atmosphere. H NMR and 13C NMR spectra were acquired
by using a Bruker Dex-300/400 NMR instrument with CDCl3 as sol-
vent. Mass spectra (MS) were recorded with a Bruker Autoflex
MALDI-TOF instrument using dithranol as a matrix. UV/Vis absorp-
tion spectra were recorded by using a SHIMADZU UV-1650PC.
Steady-state photoluminescence (PL) spectra were obtained with a
PTI QuantaMaster 40 spectrofluorometer at room temperature and
absolute PL quantum yields (PLQY) were recorded with a HAMA-
Figure 9. Electroluminescent (EL) characteristics of the devices based on 5a–5d. a) EL spectra; b) EQE–current density curves; c) current efficiency–current
density characteristics; d) current density–voltage–luminance curves.
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MASTU C9920-02G. Photoluminescence decay traces were ob-
tained through the time-correlated single photon counting
(TCSPC) technique by using a FLS980 instrument. A 377 nm pulsed
laser was used as an excitation source and data analyses were per-
formed by using exponential fitting models with FluoFit software.
Thermogravimetric analysis (TGA) was carried out with a NETZSCH
STA449 from 258C to 6008C at a 208Cminꢀ1 heating rate under N2
atmosphere. Polarized optical microscopy was recorded with a
Linkam Thms600/Nikon 50I. Cyclic voltammetry measurements
were performed by using a 273A (Princeton Applied Research).
Electrochemical properties were evaluated by cyclic voltammetry
with three typical electrodes in degassed CH3CN solution with a
rate of 100 mVsꢀ1. The CV system employed Bu4NPF6 as the elec-
trolyte. A platinum disk was used as the working electrode, plati-
num wire was used as the counter electrode, and silver wire was
used as the reference electrode. Ferrocene/ferrocenium (Fc/Fc+)
was used as the external standard compound. Each oxidation po-
tential was calibrated by using ferrocene as a reference. DFT calcu-
lations were performed in the gas phase by using the Gaussian 09
quantum-chemical package. The geometry optimization for
ground state derivatives was carried out by using B3LYP function-
als with the 6-31G (d,p) basis set for all atoms. X-ray polycrystal dif-
fraction was measured by D/MAX-2500/PC (40 kV, 300 mA) using
CuKa radiation (l=1.5418 ꢂ). The scan rate is 108minꢀ1 and the
angle range is from 5 to 908.
4,7-Diphenylbenzo[c][1,2,5]thiadiazol-5-amine (2a)
A mixture of 4,7-dibromobenzo[c][1,2,5]thiadiazol-5-amine (1.0 g,
3.24 mmol), phenylboronic acid (0.94 g, 8.20 mmol), Pd(PPh3)4
(187 mg, 0.162 mmol), ethanol (10 mL), 2m K2CO3 (10 mL), and tol-
uene (30 mL) was stirred at 808C for 24 h under a nitrogen atmos-
phere. After cooling to room temperature, the mixture was poured
into water (50 mL) and extracted with dichloromethane (3ꢁ30 mL).
The combined organic layers were washed with water, dried over
anhydrous MgSO4, and evaporated to dryness. The residue was pu-
rified by chromatography on a silica gel column eluting with petro-
leum ether/CH2Cl2 (v/v=1:1) and recrystallized from n-hexane solu-
tion to afford a yellow solid (420 mg, 43%). 1H NMR (400 MHz,
CDCl3): d=7.94–7.90 (m, 2H), 7.63–7.58 (m, 3H), 7.58–7.51 (m, 3H),
7.45 (dd, J=13.2, 7.3 Hz, 2H), 7.30 (s, 1H), 4.24 ppm (s, 2H).
4,7-Bis(4-fluorophenyl)benzo[c][1,2,5]thiadiazol-5-amine (2b)
Compound 2b was prepared by using the same synthetic process
as for compound 2a. The yield of the yellow product was 39%.
1H NMR (400 MHz, CDCl3): d=7.89 (s, 2H), 7.62–7.55 (m, 2H), 7.29
(s, 1H), 7.26–7.20 (m, 4H), 4.21 ppm (s, 2H).
4,7-Bis(4-(tert-butyl)phenyl)benzo[c][1,2,5]thiadiazol-5-amine
(2c)
Compound 2c was prepared by using the same synthetic process
as for compound 2a. The yield of the yellow product was 39%.
1H NMR (300 MHz, CDCl3): d=7.86 (d, J=8.4 Hz, 2H), 7.61–7.53 (m,
6H), 7.28 (s, 1H), 4.25 (s, 2H), 1.40 ppm (s, 18H).
Device fabrication and measurement
The patterned ITO substrates were rinsed with acetone and isopro-
pyl alcohol by using sonication for 15 min, followed by 15 min UV-
ozone treatment. Organic layers, MoO3, and Al were thermally
evaporated at a deposition rate of 0.6–1 ꢂsꢀ1 for organic layers,
0.2 ꢂsꢀ1 for MoO3, and 4–5 ꢂsꢀ1 for Al electrode. The emitter was
dissolved in chlorobenzene and spin-coated onto the PEDOT:PSS
layer. The current–voltage–luminance (I–V–L) characteristics of the
devices were measured with a Keithley-236 source-measure unit, a
Keithley-2000 multimeter unit, and a calibrated Si photodiode (Ha-
mamatsu S5227-1010BQ). The luminance and efficiencies were cal-
culated from the photocurrent measurement data obtained with
the Si photodiode. The electroluminescence spectra were obtained
by using a spectroradiometer (PR735).
4,7-Bis(4-(9H-carbazol-9-yl)phenyl)benzo[c][1,2,5]thiadiazol-
5-amine (2d)
Compound 2d was prepared by using the same synthetic process
as for compound 2a. The yield of the yellow product was 79%.
1H NMR (300 MHz, CDCl3): d=8.21 (dd, J=13.7, 8.0 Hz, 6H), 7.94 (d,
J=8.4 Hz, 2H), 7.81 (dd, J=12.0, 8.4 Hz, 4H), 7.60 (dd, J=11.8,
8.1 Hz, 4H), 7.50–7.43 (m, 5H), 7.33 (t, J=7.4 Hz, 4H), 4.46 ppm (s,
2H).
Methyl 3,4,5-tris(dodecyloxy)benzoate (3)
In the devices, poly(3,4-ethylenedioxythiophene):poly(styrenesul-
fonic acid) (PEDOT:PSS) serves as the hole-injection layer, whereas
bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) and 1,3,5-
tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) act as the hole-blocking
and electron-transport layers, respectively.
A mixture of methyl 3,4,5-trihydroxybenzoate (10.0 g, 54.3 mmol),
1-bromododecane (80.0 g, 320 mmol), K2CO3 (75.0 g, 0.54 mol), KI
(1.8 g, 10.8 mmol), and acetone (250 mL) was stirred at 808C for
2 days under a nitrogen atmosphere. After cooling to room tem-
perature, the mixture was evaporated to dryness, poured into
water (200 mL), and extracted with dichloromethane (3ꢁ150 mL).
The combined organic layers were washed with water, dried over
anhydrous MgSO4, and evaporated to dryness. The residue was pu-
rified by chromatography on a silica gel column eluting with petro-
leum ether/CH2Cl2 (v/v=1:2) and a white solid was achieved
(18.5 g, 49%). 1H NMR (400 MHz, CDCl3): d=6.94 (s, 2H), 3.78 (s,
2H), 3.64 (s, 4H), 3.27 (s, 3H), 1.66 (s, 2H), 1.45 (d, J=32.8 Hz, 6H),
1.26 (s, 52H), 0.88 ppm (t, J=6.8 Hz, 9H).
4,7-Dibromobenzo[c][1,2,5]thiadiazol-5-amine
A
mixture of 4,7-dibromobenzo[c][1,2,5]thiadiazole (6.0 g,
20.5 mmol) and H2SO4 (12 mL) were stirred at 08C and then HNO3
(24 mL) was added dropwise. The reaction mixture was stirred at
room temperature for 6 h. After the mixture was poured into ice
water, the yellow precipitate (5.12 g, 73%) was collected. The solid
1
was used in the next step without any further purification. H NMR
(400 MHz, CDCl3): d=8.28 ppm (s, 1H).
3,4,5-Tris(dodecyloxy)benzoic acid (4)
After that, a mixture of 4,7-dibromo-5-nitrobenzo[c][1,2,5]thiadia-
zole (5.0 g, 14.7 mmol)and acetic acid (120 mL) were stirred at 08C,
then iron powder (4.2 g, 73.7 mmol) was added in batches to the
mixture. After stirring at room temperature for 6 h, the mixture
was poured into ice water, and the brownish yellow precipitate
was collected (4.3 g, 95%). 1H NMR (400 MHz, CDCl3): d=7.47 (s,
1H), 4.64 ppm (s, 2H).
A
mixture of methyl 3,4,5-tris(dodecyloxy)benzoate (10.0 g,
14.5 mmol), THF (90 mL), ethanol (30 mL), and aqueous solution of
NaOH (30 mL) was stirred at 808C for 6 h. After cooling to room
temperature, the mixture was poured into water (300 mL) and ad-
justed to pH 2 with hydrochloric acid (1N). After that, the precipi-
tate was collected and recrystallized from methanol solution to
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afford a white solid (8.5 g, 87%). H NMR (300 MHz, CDCl3): d=6.98
N-(4,7-Bis(4-(9H-carbazol-9-yl)phenyl)benzo[c][1,2,5]thiadia-
(s, 2H), 3.80 (t, J=6.2 Hz, 2H), 3.68 (s, 4H), 1.67 (dt, J=14.5, 7.2 Hz,
2H), 1.47 (d, J=31.6 Hz, 6H), 1.25 (d, J=2.9 Hz, 52H), 0.87 ppm (t,
J=6.7 Hz, 9H).
zol-5-yl)-3,4,5-tris(dodecyloxy)benzamide (5d)
Compound 5b was prepared by using the same synthetic process
as for compound 5a. The yield of the brown-yellow product was
1
26%. H NMR (400 MHz, CDCl3): d=9.37 (s, 1H), 8.40–8.36 (m, 3H),
8.19 (d, J=7.8 Hz, 4H), 7.98–7.92 (m, 4H), 7.81 (d, J=8.5 Hz, 2H),
7.59 (d, J=8.2 Hz, 4H), 7.47 (q, J=7.2 Hz, 4H), 7.37–7.31 (m, 4H),
6.93 (s, 2H), 3.96 (t, J=6.5 Hz, 2H), 3.82 (t, J=6.4 Hz, 4H), 1.69 (dd,
J=14.6, 6.8 Hz, 2H), 1.53–1.39 (m, 6H), 1.33–1.07 (m, 52H),
0.88 ppm (td, J=6.8, 2.4 Hz, 9H); 13C NMR (75 MHz, CDCl3): d=
165.22, 153.38, 140.75, 140.29, 132.71, 130.93, 127.53, 127.14,
126.26, 126.07, 123.80, 123.57, 120.59, 120.54, 120.39, 120.37,
120.15, 109.97, 109.66, 109.64, 105.41, 73.61, 69.39, 31.94, 29.69,
29.39, 25.93, 22.71, 14.14 ppm; (MALDI-TOF MS): m/z calcd for
C85H103N5O4S: 1289.77; found: 1290.87.
N-(4,7-Diphenylbenzo[c][1,2,5]thiadiazol-5-yl)-3,4,5-tris(dode-
cyloxy)benzamide (5a)
A mixture of Ph3P=O (423 mg, 1.524 mmol) and acetonitrile (5 mL)
was added to a 50 mL round-bottom flask, oxalyl chloride (252 mg,
1.89 mmol) was then added dropwise to the heterogeneous mix-
ture. When the mixture was homogeneous after stirring at room
temperature for 10 min, 3,4,5-tris(dodecyloxy)benzoic acid (1.03 g,
1.524 mmol) and the acetonitrile solution of 2a (600 mg,
1.89 mmol) was added slowly in order. The mixture was stirred at
room temperature for 30 min. The reaction product was diluted
with dichloromethane (30 mL), washed with saturated sodium bi-
carbonate (3ꢁ50 mL), dried over anhydrous MgSO4, and evaporat-
ed to dryness. The residue was purified by chromatography on a
silica gel column eluting with petroleum ether/CH2Cl2 (v/v=2:1)
and recrystallized from methanol solution to afford a yellow-green
Acknowledgments
Financial support was provided by the National Natural Science
Foundation of China (51773021, 51911530197, U1663229,
51473140), Six Talent Peaks Project in Jiangsu Province (XCL-
102), the Talent Project of Jiangsu Specially Appointed Profes-
sor, Natural Science Fund for Colleges and Universities in Jiang-
su Province (No. 19KJA430002), the Open Fund of Key Labora-
tory of Advanced Display and System Applications of Ministry
of Education, Shanghai University, and Natural Science Founda-
tion of Hunan Province (No. 2017JJ2245).
1
solid (135 mg, 9%). H NMR (300 MHz, CDCl3): d=9.22 (s, 1H), 8.22
(s, 1H), 8.11–8.05 (m, 2H), 7.70–7.61 (m, 4H), 7.61–7.52 (m, 3H),
7.51–7.44 (m, 1H), 6.80 (s, 2H), 3.98 (t, J=6.5 Hz, 2H), 3.87 (t, J=
6.5 Hz, 4H), 1.86–1.66 (m, 6H), 1.45 (dd, J=14.0, 6.3 Hz, 6H), 1.30
(d, J=17.1 Hz, 48H), 0.89 ppm (d, J=6.4 Hz, 9H); 13C NMR (75 MHz,
CDCl3): d=164.09, 153.30, 153.21, 150.82, 141.59, 137.07, 133.71,
130.48, 129.62, 129.48, 129.06, 128.66, 123.25, 120.02, 105.20,
73.58, 69.17, 31.95, 29.68, 29.39, 26.06, 22.71, 14.14 ppm; (MALDI-
TOF MS): m/z calcd for C61H89N3O4S: 959.66; found: 960.77.
N-(4,7-Bis(4-fluorophenyl)benzo[c][1,2,5]thiadiazol-5-yl)-
3,4,5-tris(dodecyloxy)benzamide (5b)
Conflict of interest
The authors declare no conflict of interest.
Compound 5b was prepared by using the same synthetic process
as for compound 5a. The yield of the light-yellow product was
17%. H NMR (400 MHz, CDCl3): d=9.13 (s, 1H), 8.10 (s, 1H), 8.09–
1
Keywords: 2,1,3-benzothiadiazole derivatives · organic light-
emitting diodes · photophysical properties · polymorphous
luminescent materials · structure–property relationship
8.04 (m, 2H), 7.65–7.60 (m, 2H), 7.35 (t, J=8.5 Hz, 2H), 7.27 (s, 1H),
7.23 (s, 1H), 6.81 (s, 2H), 3.99 (t, J=6.6 Hz, 2H), 3.89 (t, J=6.4 Hz,
4H), 1.85–1.76 (m, 4H), 1.72 (dd, J=14.8, 6.9 Hz, 2H), 1.51–1.41 (m,
6H), 1.36–1.21 (m, 48H), 0.88 ppm (t, J=6.8 Hz, 9H); 13C NMR
(75 MHz, CDCl3): d=165.07, 153.28, 150.69, 141.80, 132.72, 132.50,
131.15, 128.51, 123.20, 119.16, 116.88, 116.59, 115.81, 115.53,
105.21, 73.61, 69.25, 31.94, 29.64, 29.38, 26.04, 22.70, 14.13 ppm;
(MALDI-TOF MS): m/z calcd for C61H87F2N3O4S: 996.44; found:
996.55.
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1H NMR (300 MHz, CDCl3): d=9.18 (s, 1H), 8.30 (s, 1H), 8.00 (d, J=
6.7 Hz, 2H), 7.66 (d, J=8.5 Hz, 2H), 7.58 (dd, J=8.6, 2.4 Hz, 4H),
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(MALDI-TOF MS): m/z calcd for C69H105N3O5S: 1071.78; found:
1073.11.
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Manuscript received: September 3, 2019
Revised manuscript received: September 29, 2019
Version of record online: && &&, 0000
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FULL PAPER
Four ’T’-shaped molecules bearing the
2,1,3-benzothiadiazole skeleton were
prepared and characterized. All com-
pounds exhibited excellent thermal sta-
bility and polymorphism in the solid
state. The structure–property relation-
ship of these compounds was systemat-
ically explored. A highest external quan-
tum efficiency of 6.15%, beyond the
theoretical value of fluorescence-based
OLEDs (~5%), were achieved for the so-
lution-processable OLEDs.
&
Materials
G. Qian, X. Wang, S. Wang, Y. Zheng,
S. Wang, W. Zhu,* Y. Wang*
&& – &&
Polymorphous Luminescent Materials
Based on ’T’-Shaped Molecules
Bearing 4,7-Diphenylbenzo[c]
[1,2,5]thiadiazole Skeletons: Effect of
Substituents on the Photophysical
Properties
Chem. Eur. J. 2019, 25, 1 – 11
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ