Highly fluorescence emissive 5, 5′-distyryl-3, 3′-bithiophenes: Synthesis, crystal structure, optoelectronic and thermal properties

Article abstract of DOI:10.1016/j.dyepig.2020.108396

Seven 5,5′-distyryl-3,3′-bithiophene derivatives (1–7) with A-π-A or D-π-D structures were synthesized and their photophysical, electrochemical, and thermal properties were investigated. Photoluminescence experiments showed that the compounds gave a high fluorescence emission. In photophysical studies, compound 2, which has a nitro substituent, showed significant solvatochromism, thermochromism and aggregation induced emission (AIE) characteristics. Cyclic voltammetry and thermogravimetric analysis indicated that 2 had good electron affinity and thermal stability. The molecular structures and packing arrangements were identified by single crystal X-ray diffraction. Theoretical calculations for the electronic transitions explained the observed special optical behaviors of 2 well. The excellent optical properties of 2 made it a promising emitter for optoelectronics.

Full text of DOI:10.1016/j.dyepig.2020.108396

Dyes and Pigments 179 (2020) 108396
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Highly fluorescence emissive 5, 50-distyryl-3, 30-bithiophenes: Synthesis,
crystal structure, optoelectronic and thermal properties
,
Keke Peia, Huiting Zhoub, Yan Yina *, Guozhen Zhangb, Wanyong Pana, Qinglin Zhanga,
Huifeng Guoa
a Department of Chemical and Environmental Engineering, Shanghai Institute of Technology, 201418, Shanghai, PR China
b Hefei National Laboratory for Physical Sciences at the Microscale, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei,
230026, Anhui, PR China
A R T I C L E I N F O
A B S T R A C T
Seven 5,50-distyryl-3,30-bithiophene derivatives (1–7) with A-
π-A or D-π-D structures were synthesized and their
Keywords:
3, 30-bithiophenes
Fluorescence emission
Solvatochromism
Thermochromism
Crystal structure
photophysical, electrochemical, and thermal properties were investigated. Photoluminescence experiments
showed that the compounds gave a high fluorescence emission. In photophysical studies, compound 2, which has
a nitro substituent, showed significant solvatochromism, thermochromism and aggregation induced emission
(AIE) characteristics. Cyclic voltammetry and thermogravimetric analysis indicated that 2 had good electron
affinity and thermal stability. The molecular structures and packing arrangements were identified by single
crystal X-ray diffraction. Theoretical calculations for the electronic transitions explained the observed special
optical behaviors of 2 well. The excellent optical properties of 2 made it a promising emitter for optoelectronics.
1. Introduction
ketone
and
acrylates
[7,8].
In
2017,
several
highly
fluorescence-emissive 2,20-distyryl-3,30-bithiophenes were developed in
our laboratory [9]. Although a large amount of thiophene-based con-
jugated organic compounds have been synthesized, and their properties
have been widely investigated [10–15], the potential applications of
symmetric 3,30-bithiophene derivatives have rarely been studied
[16–19]. Different substitutions can cause the electron cloud density to
Highly fluorescence-emissive materials are widely used in organic
light-emitting diodes (OLEDs) [1,2], organic photovoltaic cells (OPVs)
[3,4], fuel-sensitized solar cells (DSSCs) [5], and organic field-effect
transistors
(OFETs)
[6].
The
development
of
highly
fluorescence-emissive materials is therefore crucial for fabrication of
optoelectronic and photonic devices and has become a hot topic in sci-
entific research in recent decades. Highly fluorescence-emissive mate-
change across
a molecule, therefore π
-conjugated 5,50-distyryl-3,
30-bithiophene derivatives (1–7), which have different small sub-
stitutions at the terminal phenyl rings, were prepared from 4-bromo-2--
formylthiophene as a continuation of our research on 3,30-bithiophenes.
Photophysical experiments indicated that these symmetrical compounds
gave broad UV–vis light absorption and clear fluorescence emission. In
addition, 2 showed strong solvatochromism, thermochromism and
aggregation-induced emission (AIE) behavior. The crystal structure
confirmed high coplanar and conjugation in this series of compounds.
Theoretical studies with Gaussian 16 program explained the observed
special optoelectronic properties of 2 well. Excellent UV–vis absorption
and high fluorescence emission, good electron affinity and thermal
stability, as well as clear solvatochromism, thermochromism and an AIE
effect made 2 a promising fluorescence-emissive material. Herein, the
identification of 2 as a promising emitter for optoelectronics, its
rials are usually organic molecules with a large π-conjugated system, and
can include small molecules and polymers. Organic small molecules
have been more widely studied because they have various advantages
over polymers, e.g., well-definite structures, easy of structural modifi-
cation, synthesis, and purification, better batch-to-batch reproduction,
good chemical stability, high absorption coefficients, and flexible device
preparation.
Molecules with a 3,30-bithiophene backbone are pi-conjugated and
rotation of the C–C single bond between two thiophene rings is restricted
if 2-, 4-, or 5-positions of the 3,30-bithiophene are substituted. A chiral
3,30-bithiophene derivative that was prepared by our group showed a
moderated asymmetric induction effect in the enatioselective
Chalcogeno-Baylis-Hillman reaction of benzaldehydes with methyl vinyl
* Corresponding author.
E-mail address: yinyan@sit.edu.cn (Y. Yin).
https://doi.org/10.1016/j.dyepig.2020.108396
Received 12 November 2019; Received in revised form 14 February 2020; Accepted 25 March 2020
Available online 30 March 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

K. Pei et al.
Dyes and Pigments 179 (2020) 108396
synthesis, crystal structure, and evaluation of its optoelectronic and
thermal properties are reported in detail.
determinations were available in the Cambridge Crystal Data Center
with reference number 1952356.
2. Experimental section
2.4. Synthesis of compounds 1–7
2.1. Measurements and instruments
A mixture of 4-bromothiophene-2-carbaldehyde A, D, or F (1 mmol),
bis(pinacolato)diboron (1.2 mmol), KOAc (2 mmol), and Pd(dppf)Cl2
(0.02 mmol) in dioxane (1 mL) was stirred at 80 �C until the complete
conversation of A. The dioxane was concentrated under reduced pres-
sure and the obtained residue was extracted with EtOAc (3 � 30 mL).
The combined organic phases were washed successively with saturated
NaCl, dried over anhydrous sodium sulfate, and concentrated under
reduced pressure. Finally, the purification over column chromatography
gave B or G.
Reagents and chemicals purchased from commercial suppliers were
used without further purification except anhydrous diethyl ether and
tetrahydrofuran were pre-dried and distilled to remove water prior to
use. Column chromatography used a silica gel position of 200–300
mesh. Thin layer chromatography (TLC) analysis was performed on
commercially available 1 mm thick silica gel plates. NMR spectra were
obtained by Bruker 500 MHz with chloroform-d (CDCl3) as the solvent
and tetramethylsilane (TMS) as the internal standard. HRMS spectra
were obtained by a Solari X 7.0 FT-MS. The IR spectra were performed
The mixture of B or G (1 mmol), A, D, or F (1.1 mmol), K2CO3 (2
mmol), and Pd(PPh3)4 (0.02 mmol) in THF/H2O (1 mL, V:V ¼ 5:1) was
heated until the complete conversation of B. Then THF was concentrated
under reduced pressure and the obtained residue was extracted with
EtOAc (3 � 30 mL). The combined organic phases were washed suc-
cessively with saturated NaCl, dried over anhydrous sodium sulfate, and
concentrated under reduced pressure. Finally, the purification over
column chromatography gave 3, 30-bithiophene-5,50-dicarbaldehyde C,
6, or 7 with Z conformation.
with a Nicolet 6700 FT-IR spectrometer in the rage of 500–4000 cmÀ 1
.
The melting points were measured on a SGW melting point instrument.
UV–vis absorption spectra were measured on a Phenix UV1902PC
spectrophotometer. Fluorescence spectra of 1–7 in CHCl3 were per-
formed on F97pro15053. Fluorescence spectra of 2 in aprotic solvents,
at various fw and in film were performed on Hitachi F-4600. Fluores-
cence spectra of 2 at various temperature were performed on Horiba FL-
3000FM4 fluorescence spectrometer. Thermogravimetric (TG) and dif-
ferential scanning calorimetry (DSC) analysis were performed on a
Netzsch STA 449F3 analyzer at a heating rate of 10 �C minÀ 1 under a
nitrogen atmosphere.
A or C (1 mmol), triphenylphosphonium bromide (2.2 mmol), and
DBU (2.2 mmol) were sequentially added to CHCl3 (18 mL) solution and
the reaction mixture was refluxed. After the complete conversation of C,
1 mol/L HCl (30 mL) was added to quench the reaction. The organic
phase was extracted with CH2Cl2 (3 � 30 mL), and the combined organic
phases were washed successively with saturated Na2CO3 solution and
NaCl, dried over anhydrous sodium sulfate, and concentrated under
reduced pressure. Finally, the purification over column chromatography
gave the targeted compounds 1–5, D, or E with Z conformation.
A mixture of E (1 mmol), NH4Cl (1.2 mmol), iron powder (6 mmol) in
EtOH/THF/H2O (v:v:v ¼ 3:3:2, 2 mL) was reacted at 85 �C for 1 h. Then
the reaction mixture was concentrated under reduced pressure and
extracted with EtOAc (3 � 30 mL). The combined organic phases were
washed successively with saturated NaCl, dried over anhydrous sodium
sulfate, and concentrated under reduced pressure. Finally, the purifi-
cation over column chromatography gave aniline F.
Cyclic voltammetry (CV) was tested in dichloromethane at room
temperature with 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) as supporting electrolyte. Pt disk, platinum electrode, and
saturated calomel electrode (SCE) were used as working electrode,
counter electrode and reference electrode, respectively. The scan rate
was 50 mV sÀ 1. The internal standard was a ferrocene-ferrocene (Fc/Fþc )
redox pair (E ¼ 0.52 V vs. SCE). The fluorescence quantum yield (Фx)
was calculated from the formula: Фx ¼ Фst * (Ast/Ax) * (Ix/Ist) * (nx/nst)2.
Ast and Ax were the absorbance of the reference sample and the sample
to be tested, respectively. Ist and Ix were the integrated areas of the PL
emission spectra of the reference sample and the sample to be tested,
respectively. nst and nx were the refractive indices of the solvent used for
the reference sample and the sample to be tested, respectively. Фst were
the fluorescence quantum yield of the reference sample (9, 10-
diphenylanthracene).
2.4.1. 5,50-Di((Z)-styryl)-3,30-bithiophene (1)
White solid. 57% yield after three steps. m. p. 112–113 OC. 1H NMR
(500 MHz, CDCl3, ppm) δ 7.361–7.348 (m, 10H), 7.083 (s, 2H), 7.002 (s,
2H), 6.664 (d, J ¼ 12.0 Hz, 2H), 6.607 (d, J ¼ 12.0 Hz, 2H). 13C NMR
(125 MHz, CDCl3, ppm) δ 140.280, 137.128, 129.551, 128.834,
128.562, 127.661, 126.993, 126.381, 123.215, 119.932. IR (KBr, cmÀ 1):
2954, 2923, 2853, 1688, 1456, 1071, 947, 701, 595. HRMS calcd for
2.2. Theoretical calculations
The quantum chemistry calculations were finished through the
Gaussian 16 program [20]. In the presence of the CHCl3 solvent at the
B3LYP/6-31G (d) level was used for geometric optimization of mole-
cules [20]. Theoretical calculations of vibration frequency at the same
level confirmed that the structure of these molecules was truly minima
on the potential energy surface (PES). The time-density functional the-
ory (TD-DFT) [21] calculation was used to calculate and simulate the
excited state characteristics on the basis of optimized structures. The
solvation effect of trichloromethane which is used in the experiment was
treated implicitly using polarizable continuum model (PCM) [22,23].
C
24H19S2 [MþH]þ: 371.0928, found: 371.0929.
2.4.2. 5,50-Bis((Z)-4-nitrostyryl)-3,30-bithiophene (2)
Dark red solid. 41% yield after three steps. m. p. 193–194 OC. 1H
NMR (500 MHz, CDCl3, ppm) δ 8.198 (d, J ¼ 8.5 Hz, 4H), 7.542 (d, J ¼
8.5 Hz, 4H), 7.115 (s, 2H), 7.095 (s, 2H), 6.803 (d, J ¼ 12.0 Hz, 2H),
6.585 (d, J ¼ 12.0 Hz, 2H). 13C NMR (125 MHz, CDCl3, ppm) δ 144.028,
139.288, 136.251, 129.867, 127.914, 127.115, 125.620, 124.227,
123.863, 121.001. IR (KBr, cmÀ 1): 3001, 2929, 2853, 1733, 1593, 1515,
1371, 1340, 1108, 856, 735, 689. HRMS calcd for C24H17N2O4S2
[MþH]þ: 461.0630, found: 461.0631.
2.3. Single-crystal structure determination
The single crystal of compound 4 was grown from petroleum ether
solution. The determination of unit cell parameters and data collection
were performed at 293 K on a D8 VENTURE single crystal diffractometer
equipped with Mo or Cu radiation (λ ¼ 0.71073 Å). The cell size was
obtained by refinement by least squares method, and all structures were
resolved by direct method using SHELXS-97. The anisotropic thermal
parameters of non-hydrogen atoms were finally optimized by full matrix
least squares method [24]. More data on crystal structure
2.4.3. 5,50-Bis((Z)-4-bromostyryl)-3,30-bithiophene (3)
Light green solid. 48% yield after three steps. m. p. 76–77 OC. 1H
NMR (500 MHz, CDCl3, ppm) δ 7.470 (d, J ¼ 8.0 Hz, 4H), 7.235–7.054
(m, 6H), 7.054 (s, 2H), 6.682 (d, J ¼ 12.0 Hz, 2H), 6.501(d, J ¼ 12.0 Hz,
2H). 13C NMR (125 MHz, CDCl3, ppm) δ 139.936, 136.246, 135.924,
131.855, 131.727, 130.574, 128.239, 127.219, 123.799, 120.227. IR
(KBr, cmÀ 1): 2954, 2852, 1683, 1485, 1260, 1069, 951, 858, 796, 725,
2

K. Pei et al.
Dyes and Pigments 179 (2020) 108396
Scheme 1. Synthesis of compounds 1–7 with 3,30-bithiophene core.
560. HRMS calcd for C24H17Br2S2 [MþH]þ: 526.9138, found: 526.9138.
2.4.7. 5,50-Bis((Z)-4-aminostyryl)-3,30-bithiophene (7)
Red solid. Yield: 30% after four steps. m. p. 96-97OC.1H NMR (500
MHz, CDCl3, ppm) δ 7.195 (d, J ¼ 8.0 Hz, 4H), 7.149 (s, 2H), 7.039 (s,
2H),6.648 (d, J ¼ 8.0 Hz, 4H), 6.524 (d, J ¼ 12.0 Hz, 2H), 6.487 (d, J ¼
12 Hz, 2H), 3.892 (s, 4H)⋅13C NMR (125 MHz, CDCl3, ppm) δ 146.281,
141.074, 136.634, 132.425, 130.340, 130.244, 128.748, 121.525,
119.579, 115.219. IR (KBr, cmÀ 1): 3418, 3342, 2920, 2850, 1620, 1605,
1518, 1280, 1178, 835, 733, 532, 497. HRMS called for C24H21N2S2
[MþH]þ: 401.1118, found: 401.1119.
2.4.4. 5,50-Bis((Z)-4-(tert-butyl)styryl)-3,30-bithiophene (4)
White solid. 51% yield after three steps. m. p. 87–88 OC. 1H NMR
(500 MHz, CDCl3, ppm) δ 7.359 (d, J ¼ 8.0 Hz, 4H), 7.308 (d, J ¼ 8.0 Hz,
4H), 7.083 (d, J ¼ 1.5 Hz, 2H), 7.022 (d,J ¼ 1.5 Hz, 2H), 6.614 (d, J ¼
12.0 Hz, 2H), 6.567 (d, J ¼ 12.0 Hz, 2H), 1.336 (s, 18H). 13C NMR (125
MHz, CDCl3, ppm) δ 150.837, 140.475, 136.332, 134.048, 129.724,
128.484, 126.777, 125.399, 122.653, 119.635, 34.644 31.350. IR (KBr,
cmÀ 1): 2956, 2867, 1508, 1463, 1362, 1268, 1186, 1122, 1018, 863,
830, 730. HRMS calcd for C32H35S2 [MþH]þ: 483.2180, found:
483.2181.
3. Results and discussion
3.1. Synthesis
2.4.5. 5,50-Bis((Z)-4-(trifluoromethyl)styryl)-3,30-bithiophene (5)
Light grey solid. 50% yield after three steps. m. p. 105-106OC. 1H
NMR (500 MHz, CDCl3, ppm) δ 7.595 (d, J ¼ 8.0 Hz, 4H), 7.475 (d, J ¼
8.0 Hz, 4H), 7.070 (s, 2H), 7.050, (s, 2H), 6.736 (d, J ¼ 12.0 Hz, 2H),
6.579 (d, J ¼ 12.0 Hz, 2H). 13C NMR (125 MHz, CDCl3, ppm) δ 140.812,
139.673, 136.217, 129.226, 127.925, 127.493, 125.502, 125.471,
125.439, 124.592, 120.492. IR (KBr, cmÀ 1): 2925, 2854, 1614, 1322,
1168, 1183, 1107, 868, 828, 720. HRMS calcd for C26H17F6S2 [MþH]þ:
507.0676, found: 507.0677.
The synthetic route of the targeted compounds 1–7 was described in
Scheme 1. 1–7 were readily synthesized by a short sequence (Scheme 1).
Palladium catalyzed borylation of 4-bromothiophene-2-carbaldehyde A
gave aryl boronic acid B. Then Ph(PPh3)4 catalyzed Suzuki coupling
between A and B afforded 3, 30-bithiophene-5,50-dicarbaldehyde C [25,
26]. Finally, 1–5 were obtained from intermediate C and the corre-
sponding phosphonium salts by Wittig reaction. 6 with OCH3 and 7 with
NH2 were obtained after Witting reaction, borylation, and Suzuki
coupling. 1–7 were characterized by 1H NMR,13C NMR, IR, and HRMS.
The corresponding triphenylphosphonium bromide salt was obtained
from benzyl bromide and triphenylphosphine according to Krohnke
procedure [27]. Although compounds 1–7 shared the same 5, 50-dis-
2.4.6. 5,50-Bis((Z)-4-methoxystyryl)-3,30-bithiophene (6)
White solid. Yield: 39% after three steps. m. p. 161-162OC.1H NMR
(500 MHz, CDCl3, ppm) δ 7.298 (d, J ¼ 8.0 Hz, 4H), 6.864 (d, J ¼ 8.0 Hz,
4H), 6.816 (s, 2H), 6.677 (s, 2H), 6.566 (d, J ¼ 12.0 Hz, 2H), 6.464 (d, J
¼ 12.0 Hz, 2H), 3.818 (s, 6H)⋅13C NMR (125 MHz, CDCl3, ppm) δ
159.280, 142.284, 139.926, 130.415, 129.887, 129.638, 128.614,
123.044, 120.384, 114.077, 55.514. IR (KBr, cmÀ 1): 2923, 2851, 1604,
1504, 1459, 1243, 1178, 1107, 1028, 948, 859, 831, 706, 595, 535.
HRMS called for C26H21O2S2 [MþH]þ: 431.1139, found: 431.1140.
tyryl-3, 30-bithiophene
π-conjugated central core, they had different
substitutions on the terminal benzene ring. High purity products 1–7
were then applied to the following photoelectronic and thermal studies.
3.2. Photophysical properties
The UV–vis absorption spectra and the corresponding optophysical
3

K. Pei et al.
Dyes and Pigments 179 (2020) 108396
Fig. 1. (a) Normalized absorption and (b) Emission spectra of 1–7 in CHCl3 at room temperature (λex ¼ 365, 400, 362, 363, 372, 370, 368 nm for 1–7 in emission
spectra, respectively; λex ¼ 365 nm in fluorescent photograph).
Table 1
Photophysical data for 1–7.a.
Stokes shiftsd,
cmÀ 1
c
Eogpt
eV
,
b
Compd
UV–vis λmax, nm
λem
nm
,
ΦF
(
ε
, mMÀ 1 cmÀ 1
)
1
2
3
4
5
6
7
274(36.9),328
(39.5)
3.28
2.74
3.13
3.16
3.22
3.05
3.07
423
558
428
429
431
450
428
0.03
0.38
0.06
0.16
0.06
0.21
0.15
6847
7786
6666
6102
6384
7629
5875
261(30.0),389
(35.7)
281(30.5),333
(31.3)
288(36.4),340
(44.3)
281(33.1),338
(37.9)
282(39.2),335
(45.2)
280(12.6),342
(10.9)
a
All spectra were recorded in CHCl3 solution (1.0 � 10À 5 M) at room
Fig. 2. Emission spectra (λex ¼ 400 nm) and Fluorescent photograph (λex
365 nm) of 2 in various aprotic solvents at room temperature (10À 5 M).
¼
temperature.
b
c
Calculated from the onset of the absorption spectra in CHCl3 solution.
Determined relative to 9,10-bis-phenylethynyl-anthracene in cyclohexane
they were irradiated with an ultraviolet lamp (λex ¼ 365 nm) at room
temperature; the λem values were 423, 558, 428, 429, and 431, 450, and
428 nm for 1–7, respectively (λex ¼ 365, 400, 362, 363, 372, 370, and
368 nm for 1–7, respectively). Compound 2 gave a strong fluorescence
intensity and showed a clear red shift. Fluorescence experiments also
indicated that introduction of strongly electron-withdrawing NO2 group
(ΦF ¼ 1.00).
d
Calculated from less energetic absorption band.
data for 1–7 in CHCl3 were shown in Fig. 1a and Table 1. Two bands can
be observed. The weak one at around 270–290 nm corresponding to the
π-π* transition of the entire π-conjugated system, and the strong one at
improved the ф value (ф ¼ 0.38 for 2 vs 0.03 for 1, 0.06 for 3, 0.16 for
longer wavelengths (320–390 nm) arises from intramolecular charge
transfer (ICT). It was worth that compound 2 gives a high and broad
absorption peak with the absorption edge even reaching 450 nm, which
is advantageous in photosensitizer applications. The larger bath-
ochromic shift of 2 relative to those of compounds 1 and 3–7 are
attributed to the enhanced ICT and lower band gap of the 5,50-distyryl-
3,30-bithiophene moiety (further information on the ICT behavior of 2
was obtained by theoretical calculations; the results are shown in
F
F
4, 0.06 for 5, 0.21 for 6, and 0.15 for 7), probably because the NO2
group acts as a chromophore via ICT and expands the conjugate length,
which causes a red shift.
The UV absorption and fluorescence emission spectra show that 1–7
have large Stokes shifts (Table 1, 6847, 7786, 6666, 6102, 6384, 7629,
and 5875 cmÀ 1 for 1–7, respectively), which might be a large difference
(geometry, electron and vibration) between the Franck-Condon state
and the excited state of emission. Fluorescent dyes with large Stokes
shift have higher signal-to-noise ratio in imaging applications, therefore
1–7 have potential applications in chemical biology and clinical
diagnosis.
Table 4). The molar extinction coefficients at λmax (ε) show that 1–7
have good light-harvesting abilities (
ε
¼ 12.6–45.2 mMÀ 1cmÀ 1). The
optical band gaps (Eogpt) in the CHCl3 solution were calculated from the
onset of the absorption spectra. Both electron-donating (t-Bu, OCH3, and
NH2) and electron-withdrawing (Br and CF3) substitutions at the ter-
minal phenyl rings had no obvious effect on the Eopt g values compared
3.3. Solvatochromism
with that of 1, which has no substitutions at the π-conjugated system
(Eogpt ¼ 3.13 eV for 3, 3.16 eV for 4, 3.22 eV for 5, 3.05 eV for 6, and 3.07
eV for 7 vs 3.28 eV for 1). However, a strongly electron-withdrawing
substituent (NO2) caused significant decrease in the optical band gap
Solvatochromic behavior is classified as negative, positive, or
reversed. In negative behavior, an increase in the solvent polarity leads
to a hypsochromic shift (blue shift) of the solvatochromic band. Positive
behavior causes a bathochromic shift (red shift) is observed. Reversed
behavior involves a change from negative to positive behavior. Among
compounds 1–7, 2 shows good optical properties such as strong UV–vis
absorption and fluorescence emission, a significant red shift, and the
highest Stokes shift and narrowest optical band gap, the absorption and
(Eogpt ¼ 2.74 eV).
Fig. 1b and Table 1 show that 1–7 have photoluminescence prop-
erties. Solutions of 1–7 in the CHCl3 (1.0 � 10À 5 M) gave fluorescence
emissions of different colors (yellow for 2, and blue for 1 and 3–7) when
4

K. Pei et al.
Dyes and Pigments 179 (2020) 108396
Table 2
Photophysical data for 2 in various aprotic solvents.a.
Solvent
Toluene
Tetrahydrofuran
Dichloromethane
Chloroform
CH3CN
DMSO
λab, nm
λemb,nm
381
517
0.05
382
541
0.28
380
559
0.24
389
572
0.39
385
600
0.06
383
616
0.05
ф
F
a
All spectra were recorded at 1.0 � 10À 5 M at room temperature.
b
Determined relative to 9,10-bis-phenylethynyl-anthracene in cyclohexane (ΦF ¼ 1.00).
Fig. 3. (a) PL spectra (λex ¼ 400 nm) and (b) fluorescence photograph (λex ¼ 365 nm) of 2 in THF/water mixture with various water fractions (10À 5 M).
emission behaviors of 2 in various aprotic solvents were further inves-
tigated to get a better understanding of its behavior (Fig. 2 and Table 2).
No obvious changes in the solvent-dependent UV–vis absorption and the
λab values were observed between 380 and 389 nm. This indicates that
the coordination ground state of the solvent molecule did not change
significantly, because of the relatively small dipole moment [28].
However, the fluorescence emission of 2 showed solvatochromism at an
excitation wavelength of 365 nm. The emission was green in toluene and
tetrahydrofuran, yellow in dichloromethane and chloroform, and red in
acetonitrile and dimethyl sulfoxide (DMSO). The λem values were 517
nm in toluene, 541 nm in THF, 569 nm in CH2Cl2, 572 nm in CHCl3, 600
nm in CH3CN, and 616 nm in DMSO. The FL intensity first increased and
then decreased with increasing polarity, and almost no significant
fluorescence emission was observed in the DMSO solution. The observed
solvatochromic behavior is probably caused by dual matrix charge
transfer, proximity effects, and conformational changes [29].
3.5. Thermochromism
The thermochromic properties of 2 in CHCl3 solution (10À 5 M) and in
a solid powder state were investigated (Fig. 5). The fluorescence in-
tensity of the solution increased gradually with increasing temperature
from 223 to 313 K, which might be the reduced viscosity of solution,
thereby reducing the loss of energy transfer and increasing the FL in-
tensity. However, the fluorescence intensity of the solid powder
decreased gradually with increasing temperature from 153 to 393 K.
This could be because the thermal motion of 2 increases with increasing
temperature, which results in increased energy loss and therefore a
decreased Fl intensity.
For the solid powder, changes in the emission color with temperature
changes were not observed, which indicates that the configuration of 2
did not change with changing temperature. However, the emission color
of the solution gradually changes from yellow-green to yellow and then
red with decreasing temperature from 293 to 273 and then 253 K (λ
¼
3.4. AIE behavior
641, 627, 607, 578, and 568 nm at 223, 233, 263, 293, and 31e3mK,
respectively). The color change was consistent during many thermal
cycles, which indicates that 2 is color reliable [36]. The fluorescence
emission blue shifts with increasing solution temperature, probably
Increasing the water fraction (fw) in a THF/H2O mixed solvent can
change the existing form of the molecule from a homogeneous solvent in
pure THF to an aggregated fluorescent small particle in a water mixture,
which can result in a change in the PL intensity [30]. Because 2 had a
large Stokes shift, it can be assumed that various aggregations are
formed in solution and this leads to variations in the emission spectra.
The fluorescence behavior of 2 was investigated in THF/H2O mixture
with various fw values (Fig. 3). The fluorescence intensity decreased
sharply with increasing fw from 0% to 50%, and the fluorescence color
changed from yellow-green to blue. When fw was increased from 50% to
80%, the PL intensity increased and the fluorescence color changed from
blue to red. Finally, the PL intensity dropped again and the color did not
change from red when fw was 80%–95%. The addition of water clearly
affects the form of 2, and AIE was observed with the increasing fw from
50% to 80% [31–34].
Because 2 shows AIE, the fluorescence-emissive properties of 2
coated on a glass [35] where studied. Fig. 4 shows that PL intensities of 2
in a film and CHCl3 solution weresimilar, but λem for the film showed a
43 nm red shift compared with that in CHCl3 solution. This is probably
because aggregation and molecular structure differ from those in film.
Fig. 4. Emission spectra of 2 at room temperature (λex ¼ 400 nm).
5

K. Pei et al.
Dyes and Pigments 179 (2020) 108396
Fig. 5. PL emission of 2 in (a) CHCl3 (10À 5 M) and (b) solid powder state at various temperatures.
because the coplanarity and conjugate length decrease as a results of
intermolecular interactions and solvent effects.
subsequent high temperatures encountered during material processing.
Compound 2 showed good thermal stability without significant weight
losses below 200 �C, therefore it is suitable for use in photoelectric de-
vices. The melting temperatures (Tm) of 1 and 5 were 122.5�C, and 99.5
�C, respectively, as shown by the endothermic peaks during the first
heating cycle. However, no other significant endothermic or exothermic
peaks were observed for 2 in the heating or cooling runs.
3.6. Electrochemical properties
Despite interference from random errors caused by intermolecular
interactions and solvent effects, CV experimental data is still essential
for characterizing optoelectronic properties and provides important
guidance for a better understanding of structural properties [37–39].
The redox behaviors of 1–7 were studies by CV to investigate their
abilities to transfer electrons excited molecules to ground-state mole-
cules. CV was performed in a solution of 0.1 M TBAPF6 in dichloro-
methane with a CHI660E electrochemical workstation. The electronic
states (HOMOs, LUMOs) and band gaps of 1–7 were summarized in
Table 3. The measured irreversible oxidation potentials (Eox vs SCE)
were 0.79, 1.4, 1.1, 0.76, 0.92, 1.18, and 0.85 eV for 1–7, respectively.
Based on the onset oxidative potentials and the corresponding optical
band gaps, the HOMO energy levels were estimated to be À 5.53, À 6.14,
À 5.84, À 5.50, À 5.66, À 5.92, and À 5.59 eV, and the LUMO energy levels
were estimated to be À 2.25, À 3.40, À 2.71, À 2.34, À 2.44, À 2.87, and
À 2.52 eV for 1–7, respectively. It can reasonably be concluded that
strongly electron-withdrawing substituents on the 5,50-distyryl-3,
30-bithiophene moiety lower the HOMO level; these results are consis-
tent with those of previous studies on 2, 20-distyryl-3,30-bithiophenes
[40]. The electrochemical properties of 1–7 indicated that they can act
as electron acceptors and therefore have potential applications as
charge-transport materials.
3.8. Single-crystal structure
Crystal structure is a good method to understand the relationship
between molecular structure and optoelectronic properties. Fortunately,
single crystal of compound 4 (CCDC 1952356) that were suitable for X-
ray analysis were obtained by slow volatilization at room temperature of
a solution in petroleum ether. Although the molecular torsion is affected
by the terminal substituents, we analyzed the molecular conformations
and packing structures of 4 to clarify the optical behaviors of compounds
1–7 (Fig. 7). Compound 4 belongs to the monoclinic crystal system and
has the space group of p 21/c. The terminal phenyls are connected to the
middle carbon carbon double bonds with a dihedral of 71.2�, and the
central thiophene rings and double bonds are highly coplanar. These
results explain the fluorescence-emissive properties of 5,50-distyryl-3,30-
bithiophene derivatives well. The crystals are packed in a crisscross
structure with only weak short-range interactions between H8 in phenyl
ring and C3 and C4 in thiophene ring.
3.7. Thermal properties
The thermal properties of 1, 2, and 5 were investigated by TG
analysis and DSC (Fig. 6). The decomposition temperatures (Td, 5%
weight loss) were 282, 207, and 263 �C for 1, 2, and 5, respectively. For
practical applications, photoelectric material must have sufficient
thermal stability to withstand the electric-field polarization and
Table 3
Electrochemical properties.
HOMOcv (eV)a
LUMOcv (eV)b
Eogpt (eV)
Compd.
Eox (vs SCE)
(eV)
1
2
3
4
5
6
7
0.79
1.40
1.10
0.76
0.92
1.18
0.85
À 5.53
À 6.14
À 5.84
À 5.50
À 5.66
À 5.92
À 5.59
À 2.25
À 3.40
À 2.71
À 2.34
À 2.44
À 2.87
À 2.52
3.28
2.74
3.13
3.16
3.22
3.05
3.07
a
b
HOMOCV ¼ -(Eox þ 4.74) eV.
LUMOcv ¼ HOMOcv þ Egopt
.
Fig. 6. TG curves for 1, 2, and 5.
6

K. Pei et al.
Dyes and Pigments 179 (2020) 108396
Fig. 7. (a) Crystals under UV light. (b) molecular structure, and (c) packing arrangement of 4. Weak short-range interactions are denoted by dotted lines.
Table 4
Electron-hole patterns for S0→S1.
Compd.
Sra
Db (Å)
0.035
Δ
σc (Å)
Hd (Å)
4.76
Te (Å)
Ecoulf (eV)
3.88
Electron hole distribution (level set ¼ 0.003)
1
0.86
0.22
À 3.94
2
3
0.74
0.83
0.342
0.071
2.09
0.50
6.02
4.72
À 0.56
À 0.77
3.23
3.73
4
5
0.86
0.82
0.033
0.100
0.002
0.82
4.96
4.69
À 0.92
À 0.76
3.58
3.73
a
Sr index represents the overlap degree of holes and electrons. The larger Sr index indicated a higher overlap of holes and electrons, and a smaller Sr value indicates
more significant separation of holes and electrons.
b
D index represents the distance between the hole and the electronic centroid.
c
Δσ index represents the difference in the overall spatial distributions of electrons and holes.
d
e
H index represents the overall average distribution breadths of electrons and holes.
T Index represents the separation degree between holes and electrons.
f
Ecoul (exciton binding energy) represents the coulomb attractive energy between holes and electrons.
3.9. Computational calculations
Declaration of competing interest
The ICT properties of 1–5 were explored by electron-hole analysis.
The calculations were performed with Multiwfn [41] (Table 4). Com-
pound 2 had the largest D index and Δσ index among 1–5, which indi-
cated that 2 has more remarkable characteristics of central symmetric
charge transfer excitation. From the picture of electron hole distribution,
it was clearly seen that the regions of electrons and holes were clearly
separated, which indicates that 2 has significant ICT properties in
S0→S1 transition. This well explains the special red shifts displayed by
2.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgement
The work was supported by Shanghai Gaofeng&Gaoyuan Project for
University Academic Program Development and Shanghai Key Labora-
tory of Chemical Assessment and Sustainability. The National Super-
computing Center in Changsha was acknowledged for the computing
time. We thank Helen McPherson, PhD, from Liwen Bianji, Edanz
Editing China (www.liwenbianji.cn/ac), for editing the English text of a
draft of this manuscript. Support from Prof. Guanjun Wang and Prof.
Gang Zhao was also greatly appreciated.
4. Conclusions
Seven novel small organic fluorescent molecules with a π-conjugated
5,50-distyryl-3,30-bithiophene structure were designed and synthesized.
Photophysical experiments showed that these symmetric compounds
had broad light absorption and fluorescence emissions and relatively
Appendix A. Supplementary data
large ф values. In particular, the luminescence intensity of 2 gradually
F
increased with increasing temperature in chloroform solution. In addi-
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108396.
tion, 2 showed the lowest Eogpt and AIE behavior with increasing fw from
50% to 80%. Electrochemical and thermal experiments further demon-
strate the high electron affinity and good thermal stability of 2. Theo-
retical studies by Gaussian 16 explained the special red shift of 2. These
results indicated that 2 has various potential applications in optoelec-
tronic devices.
References
[1] Uoyama H, Goushi K, Shizu K, et al. Highly efficient organic light-emitting diodes
from delayed fluorescence. Nature 2012;492:234–8.
7

K. Pei et al.
Dyes and Pigments 179 (2020) 108396
[2] Kim HU, Jang JH, Park HJ, et al. Highly efficient and spectrally stable white
organic light-emitting diodes using new red heteroleptic Iridium(III) complexes.
Dyes Pigments 2018;149:363–72.
[22] Kulinich AV, Derevyanko NA, Ishchenko AA, et al. Structure and redox properties
of polymethine dyes: electrochemical and DFT/TD-DFT study. Dyes Pigments
2019;161:24–33.
[3] Wang Y, Kan B, Ke X, et al. Two thieno[3,2-b]thiophene-based small molecules as
bifunctional photoactive materials for organic solar cells. Solar Rrl 2017;2:
1700179.
[23] Xu Z, Li Y, Zhang W, et al. DFT/TD-DFT study of novel T shaped phenothiazine-
based organic dyes for dye-sensitized solar cells applications. Spectrochim Acta A:
Mol Biomol Spectrosc 2019;212:272–80.
[4] Raju TB, Vaghasiya JV, Afroz MA, et al. Twisted donor substituted simple
thiophene dyes retard the dye aggregation and charge recombination in dye-
sensitized solar cells. Org Electron 2017;50:25–32.
[24] Liu B, Zhang Q, Ding H, et al. Synthesis, crystal structures and two-photon
absorption properties of a series of terpyridine-based chromophores. Dyes
Pigments 2012;95:149–60.
[5] Dhar A, Kumar NS, Paul PK, et al. Influence of tagging thiophene bridge unit on
optical and electrochemical properties of coumarin based dyes for DSSCs with
theoretical insight. Org Electron 2018;53:280–6.
[25] Wang LH, Li JY, Cui XL, et al. Yangjie WuCyclopalladated ferrocenylimine as
efficient catalyst for the syntheses of arylboronate esters. Adv Synth Catal 2010;
352:2002–10.
[6] Zen A, Bilge A, Galbrecht F, et al. Solution processable organic field-effect
[26] Wang Y, Song J, Xu L, et al. Synthesis and characterization of
cyclooctatetrathiophenes with different connection sequences. J Org Chem 2014;
79:2255–62.
transistors utilizing an α,α’-dihexylpentathiophene-based swivel cruciform. J Am
Chem Soc 2006;128:3914–5.
[7] Yin Y, Sun GF, Zhang H, Zhou H, Wu FH. Enativeselective Chalcogeno-Baylis-
Hillman reaction of arylaldehydes with MVK and acrylates catalyzed by chiral
thiepin-TiCl4 complex. Chin J Chem 2014;32:365–9.
[27] Fritz K. Carbeniat-Zwitter-Ionen III Krohnke F. Mitteil.: tieffarbige, mesomere
phosphonium-betaine. Chem Ber 1950;83:291–6.
[28] Bondarev SL, Tikhomirov SA, Buganov OV, et al. Fluorescence and picosecond
induced absorption from the lowest singlet excited states of quercetin in solutions
and polymer films. Optic Spectrosc 2017;122:452–61.
[8] Yin Y, Liu XC, Zhang H. Asymmetric synthesis of 3, 3’-bithiophene derivatives
catalyzed by lipase. J Shanghai Inst Technol 2013;13:30–3.
[9] Cheng F, Yin Y, Zhang G, et al. Optoelectronic and thermal properties of highly
fluorescence emissive 2,2’-distyryl-3,3’-bithiophenes. Dyes Pigments 2017;140:
222–8.
[29] Chung PH, Tregidgo C, Suhling K. Determining a fluorophore’s transition dipole
moment from fluorescence lifetime measurements in solvents of varying refractive
index. Methods Appl Fluoresc 2016;4:045001.
[10] Zhu L, Wang R, Tan L, et al. Aggregation-induced emission and aggregation
promoted photo-oxidation in thiophene-substituted tetraphenylethylene
derivative. Chem Asian J 2016;11:2932–7.
[30] Sales T, Melo C. Charge transfer versus isomerization effects on the polarizabilities
of retinal analogs. Chem Phys Lett 1991;180:105–8.
[31] Shao A, Xie Y, Zhu S, Guo Z, et al. Far-red and near-IR AIE-active fluorescent
organic nanoprobes with enhanced tumor-targeting efficacy: shape-specific effects.
Angew Chem Int Ed 2015;54(25):7275–80.
[11] Ai Q, Pang S, Ahn KH. Photoswitchable “turn-on” fluorescence diarylethenes:
substituent effects on photochemical properties and electrochromism. Chem Eur J
2016;22:656–62.
[32] Zhang XH, Wang B, Xia YQ, et al. In vivo and in situ activated aggregation-induced
emission probes for sensitive tumor imaging using tetraphenylethene-
functionalized trimethincyanines-encapsulated liposomes. J ACS Appl Mater Inter
2018;10:25146–53.
[12] Mehenni H, Dao LH. Synthesis and characterization of novel conducting
homopolymers based on amino β-styryl terthioohene. Can J Chem 2008;86:
1010–8.
[13] Oliveira E, Baptista RM, Costa SP, et al. Synthesis and solvatochromism studies of
novel bis(indolyl)methanes bearing functionalized arylthiophene groups as new
colored materials. Photochem Photobiol Sci 2014;13:492–8.
[33] Yao S, Zhang X, Wang H, et al. Schiff base derivatives containing thiodiazole:
twisted molecular conformation avoiding π… π stacking promotion aggregation-
induced emission enhancement. Dyes Pigments 2017;145:152–9.
[34] Yue H, Jiaorui D, Jingdan H, et al. Rewritable optical data storage based on
mechanochromic fluorescence materials with aggregation-induced emission. Dyes
Pigments 2019;160:830–8.
[14] Azoulay JD, Koretz ZA, Wong BM, Bazan GC. Bridgehead imine substituted
cyclopentadithiophene derivatives:An effective strategy for band Gap control in
donor_acceptor polymers. Macromelecules 2013;46:1337–42.
[15] Zhao J, Yang X, Cheng M, Li S, Sun L. Molecular design and performance of
hydroxylpyridium sensitizers for dye-sensitized solar cells. ACS Appl Mater
Interfaces 2013;5:5227–31.
[35] Chatterjee A, Banerjee M, Khandare DG, et al. Aggregation-induced emission-based
chemodosimeter approach for selective sensing and imaging of Hg(II) and
methylmercury species. Anal Chem 2017;89:12698–704.
[16] Annemarie M, Eva S, Friedhelm L, et al. Syntheses and properties of thienyl-
substituted dithienophenazines. Beilstein J Org Chem 2010;6:1180–7.
[17] Li B, Zhang S, Li L, et al. All-thiophene-based double helix: synthesis, crystal
structure, chiroptical property and arylation. ACS Omega 2018;3:16014–20.
[18] Giri D, Bankura A, Patra SK. Poly(Benzodithieno-imidazole-alt-carbazole) based
[36] Qi ZJ, Wei B, Wang XM, Kang F, Hong MX, Sun YM. Synthesis, optical and
electrochemical properties of novel 3,4-dialkyloxythiophene-based D-A-D organic
conjugated molecules. Acta Phys Chim Sin 2010;26:3310–6.
[37] Nishino Kenta, Morisaki Yasuhiro, Tanaka Kazuo, et al. Design of thermochromic
luminescent dyes based on the bis(o-carborane)-substituted benzobithiophene
structure. Chem Asian J 2019;14:789–95.
π
-conjugated copolymers: highly selective and sensitive turn-off fluorescent probes
for Hg2þ. Polymer 2018;158:338–53.
[38] Zhao J, Yang X, Cheng M, Li S, Sun L. Molecular design and performance of
hydroxylpyridium sensitizers for dye-sensitized solar cells. ACS Appl Mater
Interfaces 2013;5:5227–31.
[19] Satapathy R, Wu YH, Lin HC. Novel thieno-imidazole based probe for colorimetric
detection of Hg2þ and fluorescence turn-on response of Zn2þ. J Org Lett 2012;14:
2564–7.
[39] Gauthiera S, Guena F, Wojcikb L, et al. Synthesis and properties of novel
pyranylidene-based organic sensitizers for dye-sensitized solar cells. Dyes Pigments
2019;171:107747.
[20] Lipparini F, Egidi F, Goings J, et al. Gaussian 16. Wallingford CT: Gaussian, Inc;
2016.
[21] Ding Y, Li XC, Ferreira D. Theoretical calculation of electronic circular dichroism of
the rotationally restricted 3,8-biflavonoid morelloflavone. J Org Chem 2007;72:
9010–7.
[40] Qi ZJ, Wei B, Wang XM, Kang F, Hong MX, Sun YM. Synthesis, optical and
electrochemical properties of novel 3,4-dialkyloxythiophene-based D-A-D organic
conjugated molecules. Acta Phys Chim Sin 2010;26:3310–6.
[41] Lu T, Chen FW. Multiwfn: a multifunctional wavefunction analyzer. J Comput
Chem 2012;33:580–92.
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