A new series of N-substituted tetraphenylethene-based benzimidazoles: Aggregation-induced emission, fast-reversible mechanochromism and blue electroluminescence

Article abstract of DOI:10.1016/j.dyepig.2017.09.018

Four new N-substituted tetraphenylethene-based benzimidazoles N-R-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-benzo[d]imidazoles (R = phenyl (3a), R = 4-(tert-butyl)phenyl (3b), R = n-butyl (3c), R = n-hexyl (3d)) were synthesized conveniently by cyclization reaction of 4-tetraphenylenthenealdehyde with N-substituted ortho-nitroaniline. The four compounds exhibit typical aggregation-induced emission (AIE) property with relatively high solid state absolute fluorescence quantum yields (38.1–65.7%) and fast-recoverable mechanochromism property with solid-state fluorescence change between blue and yellow-green. They are thermally stable with decomposition temperatures above 319 °C. Both of multilayer electroluminescence devices fabricated with compounds 3b and 3c as emitters are blue emission. The turn-on voltage of device based on compound 3b is 3.3 V with maximum luminance and current efficiency of 2470 cd/m² and 1.48 cd/A, respectively.

Full text of DOI:10.1016/j.dyepig.2017.09.018

Accepted Manuscript
A new series of N-substituted tetraphenylethene-based benzimidazoles: Aggregation-
induced emission, fast-reversible mechanochromism and blue electroluminescence
Tengfei Zhang, Ran Zhang, Yun Zhao, Zhonghai Ni
PII:
S0143-7208(17)31313-X
10.1016/j.dyepig.2017.09.018
DYPI 6242
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 June 2017
Revised Date: 6 September 2017
Accepted Date: 8 September 2017
Please cite this article as: Zhang T, Zhang R, Zhao Y, Ni Z, A new series of N-substituted
tetraphenylethene-based benzimidazoles: Aggregation-induced emission, fast-reversible
mechanochromism and blue electroluminescence, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2017.09.018.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT
A new series of N-substituted tetraphenylethene-based benzimidazoles were designed and synthesized.
These compounds are blue fluorescence in crystalline state and show fast-reversible mechanochromism.
Furthermore, both of non-doped OLED devices fabricated with compounds 3b and 3c as emitters are
blue emission.
3b1)
3c(2)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Grinding
Fuming
Pristine
Ground
Fumed
400
450
500
550
600
650
700
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)

ACCEPTED MANUSCRIPT
A new series of N-substituted tetraphenylethene-based
benzimidazoles: aggregation-induced emission, fast-reversible
mechanochromism and blue electroluminescence
Tengfei Zhang, Ran Zhang, Yun Zhao, Zhonghai Ni*
School of Chemical Engineering and Technology, China University of Mining and Technology,
Xuzhou 221116, Jiangsu Province, P. R. China
Tel: +86-516-83883927
Corresponding author E-mail: nizhonghai@cumt.edu.cn
Abstract
Four
new
N-substituted
tetraphenylethene-based
benzimidazoles
N-R-2-(4-(1,2,2-
triphenylvinyl)phenyl)-1H-benzo[d]imidazoles (R = phenyl (3a), R = 4-(tert-butyl)phenyl (3b), R = n-
butyl (3c), R = n-hexyl (3d)) were synthesized conveniently by cyclization reaction of 4-
tetraphenylenthenealdehyde with N-substituted ortho-nitroaniline. The four compounds exhibit typical
aggregation-induced emission (AIE) property with relatively high solid state absolute fluorescence
quantum yields (38.1–65.7%) and fast-recoverable mechanochromism property with solid-state
fluorescence change between blue and yellow-green. They are thermally stable with decomposition
temperatures above 319 oC. Both of multilayer electroluminescence devices fabricated with compounds
3b and 3c as emitters are blue emission. The turn-on voltage of device based on compound 3b is 3.3 V
with maximum luminance and current efficiency of 2470 cd/m2 and 1.48 cd/A, respectively.
Keywords: N-substituted tetraphenylethene-based benzimidazoles; aggregation-induced emission;
mechanochromism; electroluminescence
1. Introduction
In recent years, organic solid luminescent materials have been developed explosively due to their wide
applications in organic field-effect transistors (OFETs) [1,2], organic photovoltaics (OPVs) [3-5],
organic lasers [6-8] and sensors [9,10], especially in organic light-emitting diodes (OLEDs) [11-13]. In
early period, aggregation-caused quenching (ACQ) effect originating from the strong π-π interaction
had prevented the electronic devices application to some degree [14-17]. Consequently, the ACQ-active
materials are utilized necessarily as dopants at low concentration in host materials and the devices
1

ACCEPTED MANUSCRIPT
performance parameters depend on suitable host/dopant energy combination [18,19]. Furthermore, full-
color display is closely related to red, green, and blue (RGB) tricolor emission and the performance
parameters of red and green devices are excellent and constantly refreshing record. Nevertheless, there
are great challenges in blue emitting device due to the defect in color purity, efficiency and life time.
Hence, the synthesis of novel effective solid-state blue luminogens is still a hot topic for the researchers.
Intriguingly, some solid luminogens exhibit mechanochromism property with emission change under
external physical stimuli and possess potential applications in optical recording, sensors and security
inks [20-22]. Mechanochromism behavior can be achieved by changing the molecular structures or
solid-state molecular packing modes. The morphologic change between crystalline state and
amorphous state is usually easier and reversible, which arises much attention recently.
Tetraphenylethylene (TPE) is one representative of the anti-ACQ effect molecules and the
modifications of TPE have been proved an efficient way to construct new aggregation-induced
emission (AIE) luminogens with high solid-state efficiencies [23-25]. As one of the commonly
important modification strategies for organic optoelectronic materials, the introduction of aryl or alkyl
can improve the solubility in solvent, regulate molecular packing structure, facilitate the film
processing of the device and bring hypsochromic shift of emission [26-29]. Based on the above
considerations, we designed and synthesized a new series of phenyl, p-(tert-butyl)phenyl and alkyl-
substituted TPE-based benzimidazoles in the nitrogen position (Scheme 1). Excepting AIE-active and
fast-recoverable mechanochromism bifunctional property, the introduction of substituent group in N
position bring bluer emitting, better solubility and good film-forming ability compared with non-
substituted tetraphenylethene-based benzimidazoles [30]. Then, the OLED devices using two
representative compounds were fabricated. Herein, we report the syntheses, structures and properties of
the new series of N-substituted tetraphenylethene-based benzimidazoles in detail.
Insert Scheme 1
2. Experimental
2.1 Materials and instrumentation
Tetrahydrofuran was purified by distillation from sodium and benzophenone ketyl under nitrogen
condition prior to use. All other reagents and solvents were purchased commercially (AR grade) and
used without further purification unless otherwise noted. The THF/water mixtures with different water
2

ACCEPTED MANUSCRIPT
fractions were prepared by slowly adding distilled water into the THF solution of samples under
1
ultrasound at room temperature. H NMR and 13C NMR spectra were collected on a Bruker-400 MHz
spectrometer in CDCl3 or DMSO solutions with TMS as an internal standard. Mass spectra were
obtained on a Bruker Ultraflextreme MALDI TOF/TOF mass spectrometer and a JMS-Q1000GC MK
ⅡUltra Quad GC/MS. Concentrated solutions of the four compounds were drop-casted on quartz plate
to prepare films, and ΦF values of the amorphous films were determined by FM-4P-TCSPC Transient
State Fluorescence Spectrometer using an integrating sphere. UV-vis spectra were recorded on
Shimadzu UV-3600 with a UV-VIS-NIR spectrophotometer. Emission spectra were performed by a
HITACHI fluorescence spectrometer (F-4600). Single crystal data of compounds were selected on a
Bruker APEX II CCDC diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å)
using the ω-scan technique. The structure was solved by direct method with the SHELXS-97 computer
program, and refined by full-matrix least-squares methods (SHELXL-97) on F2. Hydrogen atoms were
added geometrically and refined using a riding model. Images were created by using DIAMOND
program. CCDC numbers for complexes 3a–3d are 1525826, 1535658, 1525827 and 1525828,
respectively, and data can be obtained free of charge from the Cambridge Crystallographic Data Centre
(CCDC) or at www.ccdc.cam.ac.uk/conts/retrieving.html. Elemental analysis (C, H, N) of the dried
solid samples were carried out using an Elementary Vario El analyzer. The ground state geometries of
all molecules were fully optimized using density functional theory (DFT) at the B3LYP/6-31G(d,p)
level, as implemented in Gaussian 09W software package. PXRD measurements were performed by
using a Bruker X-ray diffractometer (D8 Advance, Germany) with an X-ray source of Cu-Kα (λ =
0.15418 Å) at 40 kV and 40 mA at a scan rate of 4°(2θ)/min. Cyclic voltammetry experiments were
performed with a CHI660A electrochemical work station. All measurements were carried out at room
temperature with a conventional three-electrode configuration consisting of a glassy carbon working
electrode, a platinum auxiliary electrode and a calomel reference electrode. The solvent in all
experiments was dry tetrahydrofuran and the supporting electrolyte was 0.1 M tetrabutylammonium
hexa-fluorophosphate. The E1/2 values were determined as 1/2 (Eap + Epc ), where Eap and Epc are the anodic
and cathodic peak potentials, respectively. All potentials reported were referenced to ferrocene which
was used as internal standard toward the end of each experiment. The glass-transition temperatures (Tg)
of the compounds were determined with differential scanning calorimetry (DSC) under a nitrogen
3

ACCEPTED MANUSCRIPT
atmosphere by using a DSC6000 (PerkinElmer). Samples were heated to 300 oC at a rate of 10 oC min-1
o
and cooled at 20 C min–1 then heated again under the same heating conditions as used in the initial
heating process. Decomposition temperatures (Td) were determined with thermogravimetric analysis
(TGA) under a nitrogen atmosphere by using a STA 449 F5 (NETZSCH). Samples were heated to 600
oC at a rate of 10 oC min–1.
The devices were fabricated by vacuum deposition of the materials at 10–6 Torr onto indium tin
oxide (ITO) glass with a sheet resistance of 25 ꢀ per square. Respecting to their appropriate energy
levels, the OLEDs were fabricated with a typical three-layer structure of ITO/NPB (40 nm)/3b or 3c
(20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm). The current density–voltage–luminance (I–V–L)
characteristics were measured using a Keithley source measurement unit (Keithley 2400) with a
calibrated silicon photodiode. The EL spectra of the devices were measured using a SpectraScan
PR650 spectrophotometer. All measurements were carried out under air at room temperature without
device encapsulation.
2.2 Synthesis
2.2.1 General procedure for synthesis of compounds 1a–1d
Under the atmosphere of nitrogen, a mixture of ortho-nitroaniline (5.52 g, 40 mmol), brominated
compounds a–b (40 mmol), sodium tert-butoxide (3.84 g, 40 mmol), Pd(dba)2 (0.23 g, 0.4 mmol), P(t-
o
Bu)3 (0.1 g mL-1 in toluene, 1.2 mL, 0.6 mmol) and toluene (150 mL) were heated at 80 C for 5-6 h.
After it cooled, water and dichloromethane were added and the organic layer was separated. Then, the
combined organic layer was dried over MgSO4. Finally, the crude products were purified by silica gel
column chromatography using hexane/ethyl acetate as eluent.
1
Compound 1a was obtained as a crimson oily liquid. Yield: 6.25 g, 73.0%. H-NMR (600 MHz,
d6-DMSO) δ (ppm): 9.33 (s, 1H), 8.06 (dd, J = 8.5 Hz, 1.5 Hz, 1H), 7.47 – 7.42 (m, 1H), 7.37 (t, J =
7.9 Hz, 2H), 7.27 (d, J = 7.6 Hz, 2H), 7.15 (t, J = 7.7 Hz, 2H), 6.82 (ddd, J = 8.3 Hz, 7.0 Hz, 1.1 Hz,
1H). 13C-NMR (151 MHz, d6-DMSO) δ (ppm): 142.44 (s), 139.69 (s), 136.43 (s), 133.97 (s), 130.00 (s),
126.69 (s), 125.33 (s), 124.14 (s), 118.49 (s), 117.15 (s). MALDI TOF-MS: m/z 214.26 [M]+.
Compound 1b was obtained as a crimson oily liquid. Yield: 8.32 g, 77.0%.1H-NMR (400 MHz,
d6-CDCl3) δ (ppm): 9.48 (s, 1H), 8.19 (d, J = 8.6 Hz, 1H), 7.43 (d, J = 8.4 Hz, 2H), 7.34 (t, J = 7.8 Hz,
1H), 7.20 (d, J = 8.5 Hz, 3H), 6.74 (dd, J = 8.2 Hz, 7.3 Hz, 1H), 1.35 (s, 9H). 13C-NMR (101 MHz, d6-
4

ACCEPTED MANUSCRIPT
CDCl3) δ (ppm): 148.92 (s), 143.56 (s), 135.88 (s), 135.66 (s), 132.87 (s), 131.03 (s), 127.21 (s),
126.62 (d, J = 3.7 Hz), 124.28 (s), 117.15 (s), 116.07 (s), 34.58 (s), 31.39 (s). MALDI TOF-MS: m/z
270.21 [M]+.
Compound 1c was obtained as a crimson oily liquid. Yield: 5.82 g, 75.1%.1H-NMR (400 MHz,
d6-CDCl3) δ (ppm): 8.17 (d, J = 8.6 Hz, 1H), 8.06 (s, 1H), 7.43 ( t, J = 7.8 Hz, 1H), 6.85 (d, J = 8.7 Hz,
1 H), 6.63 (dd, J = 8.2 Hz, 7.3 Hz, 1H), 3.30 (dd, J = 12.6 Hz, 6.7 Hz, 2H), 1.81 – 1.64 (m, 2H), 1.54 –
1.42 (m, 2H), 0.99 (t, J = 7.3 Hz, 3H). 13C-NMR (101 MHz, d6-CDCl3) δ (ppm):145.65 (s), 136.18 (s),
131.68 (s), 126.90 (s), 115.01 (s), 113.77 (s), 42.73 (s), 31.00 (s), 20.25 (s), 13.79 (s). MALDI TOF-
MS: m/z 194.15 [M]+.
Compound 1d was obtained as a crimson oily liquid. Yield: 6.53g, 73.5%.1H-NMR (400 MHz, d6-
CDCl3) δ (ppm): 8.17 (d, J = 8.6 Hz, 1H), 8.06 (s, 1H), 7.43 (t, J = 7.8 Hz, 1H), 6.84 (d, J = 8.7 Hz,
1H), 6.65 – 6.58 (m, 1H), 3.29 (dd, J = 12.5 Hz, 6.9 Hz, 2H), 1.78 – 1.68 (m, 2 H), 1.45 (dt, J = 14.1
Hz, 6.9 Hz, 2H), 1.35 (dt, J = 7.1 Hz, 3.4 Hz, 4H), 0.91 (t, J = 6.7 Hz, 3H). 13C-NMR (101 MHz, d6-
CDCl3) δ (ppm): 145.65 (s), 136.18 (s), 131.68 (s), 126.90 (s), 115.00 (s), 113.77 (s), 77.34 (s), 77.02
(s), 76.70 (s), 43.05 (s), 31.47 (s), 28.92 (s), 26.75 (s), 22.56 (s), 14.01 (s). MALDI TOF-MS: m/z
222.19 [M+1]+.
2.2.2 Synthesis of compound 2
Compound 2 was prepared according to the reported synthetic route and obtained as a yellowish
1
powder solid [31]. H-NMR (400 MHz, d6-DMSO) δ (ppm): 9.86 (s, 1H), 7.66 (d, J = 8.2 Hz, 2H),
7.20-7.06 (m, 11H), 7.00 (ddd, J = 7.4 Hz, 3.9 Hz, 1.9 Hz, 6H). 13C-NMR (101 MHz, d6-DMSO) δ
(ppm): 150.12 (s), 142.95 (dd, J = 25.1 Hz, 11.1 Hz), 140.05 (s), 134.59 (s), 131.86 (s), 131.07 (d, J =
8.6 Hz), 129.52 (s), 128.77-128.02 (m), 127.41 (d, J = 14.1 Hz). GC/MSD: m/z 360 [M]+.
3.2.3 General procedure for syntheses of compounds 3a–3d
Under room temperature, Na2S2O4 (1.4 g, 8 mmol) was added in batches to a solution of compounds 1
(2 mmol) and 2 (0.72 g, 2 mmol) in MeCN/water (50/10 mL), then the mixed solution was heated to
reflux for 12 h. After completion of the reaction, the reaction mixture was quenched by pouring into ice
water. The organic layer was extracted with dichloromethane, and the combined organic layers were
washed with a saturated brine solution and water, and dried over MgSO4. After filtration and solvent
evaporation, the crude product was purified by silica-gel column chromatography using hexane/ethyl
5

ACCEPTED MANUSCRIPT
acetate as eluent.
Compound 3a was obtained as a white powder solid. Yield: 0.70 g, 67.0%.1H-NMR (400 MHz,
d6-DMSO) δ (ppm): 7.75 (d, J = 7.9 Hz, 1H), 7.54 (m, 3H), 7.35 (d, J = 7.0 Hz, 2H), 7.28 (m, 4H),
7.18 – 7.09 (m, 10H), 7.00 – 6.90 (m, 8H). 13C-NMR (101 MHz, d6-CDCl3) δ (ppm): 152.30 (s),
145.06 (s), 143.22 (dd, J = 30.5 Hz, 11.2 Hz), 141.83 (s), 140.12 (s), 137.03 (d, J = 14.9 Hz), 131.61 –
130.61 (m), 129.72 (s), 128.74 (s), 128.41 (s), 127.70 (dd, J = 6.1 Hz, 4.3 Hz), 127.32 (s), 126.62 (d, J
= 6.3 Hz), 123.07 (d, J = 29.7 Hz), 119.73 (s), 110.34 (s). MALDI TOF-HRMS: m/z 524.1883 [M]+.
Anal. Calcd for C39H28N2: C, 89.28; H, 5.38; N, 5.34. Found: C, 89.31; H, 5.37; N, 5.32.
Compound 3b was obtained as a white powder solid. Yield: 0.85 g, 73.4%.1H-NMR (400 MHz,
d6-DMSO) δ (ppm): 7.74 (d, J = 7.8 Hz, 1H), 7.55 (d, J = 8.4 Hz, 2H), 7.32 – 7.20 (m, 6H), 7.19 – 7.09
(m, 10H), 7.01 – 6.88 (m, 8H), 1.35 (s, 9H). 13C-NMR (101 MHz, d6-CDCl3) δ (ppm): 152.29 (s),
151.50 (s), 144.94 (s), 143.26 (dd, J = 35.1, 16.6 Hz), 141.70 (s), 140.17 (s), 137.26 (s), 134.17 (s),
131.31 (dd, J = 11.6 Hz, 5.8 Hz), 128.72 (s), 127.78 (d, J = 26.5 Hz), 126.62 (dd, J = 11.5 Hz, 4.8 Hz),
122.93 (d, J = 27.3 Hz), 119.64 (s), 110.53 (s), 34.79 (s), 31.36 (s). MALDI TOF-HRMS: m/z
581.3379 [M+1]+. Anal. Calcd for C43H36N2: C, 88.93; H, 6.25; N, 4.82. Found: C, 88.92; H, 6.28; N, 4.
73.
Compound 3c was obtained as a white powder solid. Yield: 0.75 g, 74.8%.1H-NMR (400 MHz,
d6-DMSO) δ (ppm): 7.63 (t, J = 7.3 Hz, 2H), 7.54 (d, J = 8.1 Hz, 2H), 7.29 – 7.12 (m, 13H), 7.03 (dd, J
= 12.3 Hz, 6.4 Hz, 6H), 4.24 (t, J = 7.2 Hz, 2H), 1.62 – 1.51 (m, 2H), 1.07 (dd, J = 14.5 Hz, 7.2 Hz,
2H), 0.72 (t, J = 7.2 Hz, 3H). 13C-NMR (101 MHz, d6-DMSO) δ (ppm): 153.13 (s), 145.01 (s), 143.90
– 142.77 (m), 141.93 (s), 140.38 (s), 136.04 (s), 131.21 (dd, J = 17.9 Hz, 14.9 Hz), 129.05 (s), 128.39
(d, J = 9.0 Hz), 127.22 (s), 122.58 (d, J = 47.3 Hz), 119.52 (s), 111.27 (s), 44.01 (s), 31.45 (s), 19.55 (s),
13.74 (s). MALDI TOF-HRMS: m/z 505.2631 [M+1]+. Anal. Calcd for C37H32N2: C, 88.06; H, 6.39; N,
5.55. Found: C, 88.10; H, 6.37; N, 5.53.
Compound 3d was obtained as a white powder solid. Yield: 0.76 g, 71.5%.1H-NMR (400 MHz,
d6-DMSO) δ (ppm): 7.66 – 7.58 (m, 2H), 7.53 (d, J = 8.0 Hz, 2H), 7.30 – 7.11 (m, 13H), 7.03 (dd, J =
9.2 Hz, 5.2 Hz, 6H), 4.23 (t, J = 7.3 Hz, 2H), 1.58 (dd, J = 14.2 Hz, 7.1 Hz, 2H), 1.10 (ddd, J = 23.1 Hz,
13.9 Hz, 7.6 Hz, 6H), 0.78 (t, J = 6.9 Hz, 3H). 13C-NMR (101 MHz, d6-DMSO) δ (ppm): 153.11 (s),
144.97 (s), 143.26 (dd, J = 27.4 Hz, 14.2 Hz), 141.93 (s), 140.37 (s), 136.07 (s), 131.21 (dd, J = 17.7
6

ACCEPTED MANUSCRIPT
Hz, 13.3 Hz), 129.05 (d, J = 9.9 Hz), 128.37 (d, J = 8.4 Hz), 127.22 (s), 122.57 (d, J = 48.1 Hz), 119.53
(s), 111.25 (s), 44.34 (s), 30.87 (s), 29.27 (s), 25.94 (s), 22.33 (s), 14.21 (s). MALDI TOF-HRMS: m/z
533.2947 [M+1]+. Anal. Calcd for C39H36N2: C, 87.93; H, 6.81; N, 5.26. Found: C, 87.91; H, 6.82; N,
5.27.
3. Results and discussion
3.1 Synthesis and characterization
The synthetic procedures of the four compounds are shown in Scheme 2. Compounds 1a–1d were
synthesized by N-substituted reaction with starting materials ortho-nitroaniline [32]. Another key
reactant 4-tetraphenylenthenealdehyde (2) was obtained according to the conventional method of
literature [31]. Then, target compounds 3a–3d were one-pot synthesized by the cyclization reaction of
the intermediate 2 with ortho-nitroaniline derivatives in high yields. These compounds were purified
by column chromatography followed by recrystallization then characterized by standard spectroscopic
methods, from which satisfactory analysis data corresponding to these compounds were obtained.
Moreover, the final compounds 3a–3d were also characterized by single crystal X-ray diffraction
analysis. Generally, four new N-substituted TPE-based benzimidazoles have been synthesized
conveniently based on 4-tetraphenylenthenealdehyde.
Insert Scheme 2
3.2 Crystal structures of compounds 3a–3d
Insert Fig. 1
Single X-ray diffraction analysis is a good way to better understand the relationship between the
properties and structures. Successfully, the single crystals of compounds 3a–3d were obtained by the
solvent evaporation method in tetrahydrofuran/water (3a–3b) and ethyl acetate/n-hexane (3c–3d)
mixture. As depicted in Fig. 1, the crystal structures of the four compounds agree well with the
expected molecular structures. The four compounds all show highly twisted paddle-shaped
conformations, which are similar to the reported TPE-based compounds [33,34]. In compounds 3a–3d,
the benzimidazole segments are connected to the benzene rings of TPE unit with the torsion angles of
44.20°, 34.62°, –41.87° and 39.25°, respectively.
As expected, there are not relatively strong π-π and N-H---N intermolecular interactions in the
7

ACCEPTED MANUSCRIPT
crystal structures of the four compounds. There are affluent weak C-H---π and C-H---N intermolecular
interactions which link the molecules of compounds 3a-3d to form two-dimensional (2D) layer-like
supramolecular structures (Fig. S1). Then, all the four compounds adopt a fashion stacking by 2D
layers to form three-dimensional (3D) structures (Fig. S2). As a result, there exist interface gaps
without any intersections between adjacent two layers. Therefore, under external pressure, the layers
can slip readily and the highly ordered crystal structures are destroyed, which can be responsible for
the mechanochromism phenomenon of the four compounds.
Insert Fig. 2
As shown in Fig. 2 and Fig. S1, the packing diagrams of these compounds show affluent weakly
intermolecular interactions. For compound 3a, there are five C-H---π interactions between hydrogen
atoms and the π clouds of phenyl rings in adjacent molecules with distances of 2.722–2.949 Å and one
C-H---N(2) interactions between N(2) atom of benzimidazole and hydrogen atom on phenyl ring
attaching to the N(1) atom of benzimidazole in adjacent molecule with a distance of 2.636 Å. For
compound 3b, there exists three C-H---π interactions between hydrogen atoms and the π clouds of
phenyl rings in adjacent molecules with distances of 2.779–3.272 Å and one C-H---N(2) interactions
between N(2) atom of benzimidazole and hydrogen atom on phenyl ring of TPE unit in adjacent
molecule with a distance of 2.378 Å. In packing diagram of compound 3c, there exist six C-H---π
interactions between hydrogen atoms on phenyl rings or n-butyl and the π clouds of phenyl rings in
adjacent molecules with distances of 2.701–3.117 Å. For compound 3d, there exists five C-H---π
interactions between hydrogen atoms on phenyl rings or n-hexyl and the π clouds of phenyl rings in
adjacent molecules with a distance of 2.806–3.011 Å, one C-H---N(2) interaction between N(2) atom
of benzimidazole and hydrogen atom on phenyl ring of TPE segment in adjacent molecules with a
distance of 2.638 Å and the only π-π intermolecular interactions between phenyl rings of TPE in
adjacent molecules with distances of 3.273 Å. The affluent intermolecular interactions in the crystal
structures of compounds 3a–3d contribute to restrict the molecular rotations of C-C single bond in the
solid state, reduce the non-radiative transition consumption and further facilitate the fluorescence
emission.
3.3 Photophysical properties
8

ACCEPTED MANUSCRIPT
The UV-visible spectra and solid emission spectra of the four compounds are demonstrated in Fig. 3a,
3b and Fig.S4, respectively. Compounds 3a–3d exhibit similar absorption behavior with two major
absorption band at 270–310 nm and 310–400 nm, respectively (Fig. 3a). The former absorption band
corresponds to the π-π* electronic transition of the molecule skeleton. TPE and imidazole segment are
weak electron donor and acceptor, respectively, the latter long region absorption band is attributed to
charge transfer (CT) from the TPE core to imidazole segment, which can be compared with other some
N-containing compounds [35-37]. To study the solvent-dependent properties of the four compounds,
we choose compound 3d as an representative and measure the absorption spectra in different polar
solvents such as n-hexane (HEX), dichloromethane (DCM), acetonitrile (MeCN), methanol (MeOH)
and dimethyl sulfoxide (DMSO). As displayed in Fig. S3, slight red-shift is found for compound 3d,
which is in accordance with theory calculations, suggesting weak CT characteristics and poor CT
efficiency due to large torsion angles between benzimidazole segments and the conjoint benzene rings
of TPE unit. The absorption maxima of compounds 3a and 3b show a slight bathochromic shift
compared with those of compounds 3c and 3d, which was attributed to the enlargement of π-π
conjugation with introduction of phenyl group. As shown in Fig. 3b, the corresponding emission peaks
of compounds 3a–3d appear at 448, 454, 440 and 446 nm, respectively. The solid photoluminescence
(PL) spectra of these four compounds display relatively blue shift (in the range of 11 nm to 25 nm)
compared to the compounds without substituent group in the benzimidazole unit [30]. This may be
attributed to twisted molecular conformation disclosing their shorter π-conjugation lengths and
weakening intermolecular interactions especially π-π interactions [38]. The measurements of the
absolute fluorescence quantum yields (ΦF) in crystalline state reveal that all of the compounds are
strong solid state emitters with the ΦF values of 38.1% for 3a, 54.6% for 3b, 60.4% for 3c and 65.7%
for 3d, respectively. The fluorescence quantum yields of compounds 3c and 3d are significantly larger
than those of 3a and 3b, which can be due to that the phenyl ring attached to benzimidazole segment is
favorable for the formation of C-H---N and C-H---π intermolecular interactions, which can reduce the
fluorescence emission. In addition, the fluorescence quantum yield of 3b is higher than that of
compound 3a and the same for compound 3d is higher than that of compound 3c, which may be
attributed to the tert-butyl group and the longer hexyl chain increase the steric hindrance in solid state.
Insert Fig. 3
9

ACCEPTED MANUSCRIPT
Insert Table 1
The AIE behaviors of 3a−3d were studied by PL spectra in THF–water mixtures with different
water fractions (fw). Compounds 3a−3d are highly soluble in THF and weakly emissive, invisible
particulates formed by increasing the water fraction in THF/water mixtures, leading to the change in PL
spectra. Compounds 3a−3d are weakly fluorescent when the fw value is lower than 75%. After that,
with increasing fw, all the PL intensity of compounds 3a–3d fast increases monotonously until the
measured fw up to 95% (Fig. 3c and Fig. S4). The results of PL spectra demonstrate that all these
compounds display weak emission in THF, but exhibit intense emission in their aggregate state. This
phenomenon demonstrates that these four compounds are AIE-active and the invisible particulates fast
assemble in a constant fashion with the increase of fw. The restriction of intramolecular rotation (RIR)
may account for the different emission behaviors between their solution and aggregate state [39]. The
faint fluorescence in THF is enhanced in the aggregated suspension (fw = 95%) by 403-, 410-, 314- and
393-fold for compounds 3a−3d, respectively (Fig. 3d).
3.4 Mechanochromic properties
The mechanochromic properties of compounds 3a−3d were studied by solid state emission
spectroscopy and the data are summarized in Table 1. Compounds 3a−3d show blue emission with
wavelengths ranging from 440 nm to 454 nm in their crystalline states. Upon grinding using a spatula
or a pestle, the blue emitting solids can convert to yellowish green emitting solids and the emission
wavelength is red-shifted to a range from 463 nm to 481 nm (Fig. 4a and Fig. S5). The compounds 3a–
3d show grinding-induced spectral red shift (ꢁλ) of 21 nm, 29 nm, 23 nm and 26 nm, respectively.
After the ground samples of these four compounds are fumed with DCM vapor, the PL wavelengths
rapidly blue-shift to their original state completely within a few seconds and keep stable, indicating that
the four compounds have perfect restorability with high sensitivity comparing to other reported
mechanochromic compounds [40-42]. The absence of hydrogen bonding interaction leads to better
solubility in organic solvent such as DCM resulted from the introduction of peripheral aryl and alkyl
substituents in the nitrogen position. This may be the main reason for these compounds sensitive to
warm solvent vapor. After several grinding-fuming cycles, the PL wavelength has no obvious
abnormal changes demonstrating excellent stability (Fig. S6). Given the sensitivity and stability, these
10

ACCEPTED MANUSCRIPT
compounds have the potential application in organic vapor sensor.
Compounds 3a–3d at different solid states were studied by powder X-ray diffraction (PXRD) to
investigate mechanochromism phenomenon (Fig. 4b and Fig. S5). The diffraction patterns of
compounds 3a–3d in pristine solid state show sharp peaks, illustrating the crystalline state of
morphology. After grinding, the PXRD results of these compounds show very weak diffraction signal
intensity or a broad diffuse band, indicating that the morphology change from crystalline state to
amorphous state. The PXRD spectra of ground compounds 3a–3d return to display sharp peaks after
fumed with DCM vapor and these peaks coincide with those of as-prepared crystals, demonstrating
restorability of morphology from amorphous state to crystalline state. The PXRD study illustrates that
the morphological change from the crystalline state to the amorphous state and vice versa is related to
the mechanochromism in compounds 3a–3d.
The single crystal structures and PXRD of the compounds 3a–3d demonstrate the relationship
between solid packing structure and mechanochromic property. Due to interface gaps existing between
two adjacent layers, the crystal ordered packing structures can be destroyed under external pressure,
which can bring to the planarization of molecular conformation [43,44], results in the increase of the
effective molecular conjugation effects and further leads to a red-shift of emissions.
Insert Fig. 4
Insert Table 2
3.5 Theoretical calculation and electrochemical studies
We performed quantum mechanical computations by density functional theory (DFT) calculations
using the Gaussian 09W software to further understand the correlation between structure and the
physical properties. The DFT calculations were performed at the B3LYP/6-31G(d,p) level of theory.
The optimized molecular structures of those compounds are shown in Fig. S7. The benzimidazole
segments are connected to the benzene rings of TPE unit with the torsion angles of 27.4°, 28.6°, –40.7°
and 41.6°, respectively. Torsion angles of compounds 3c and 3d are similar to that of crystal structure.
However, for compounds 3a and 3b, torsion angles calculated by DFT method are smaller compared
with that of their crystal structure. DFT calculations illustrates that benzimidazole segment and
conjoint benzene rings of compounds 3a and 3b in ideal condition is close to coplanarity and benefit to
11

ACCEPTED MANUSCRIPT
electronic cloud movement from TPE unit to benzimidazole segment. The contours of the highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of 3a–3d are
shown in Fig. S7. Electronic distributions observed from the frontier orbitals of compounds 3a–3d are
approximately identical. The electron clouds of HOMO levels are located on the whole molecule
except the N-substituted groups, which indicates peripheral substituents have a little influence on
HOMO levels. LUMO levels of the compounds mainly distributed at the imidazole segment and the
conjoint benzene ring of TPE, demonstrating weak intramolecular charge transfer. The theoretical band
gaps for 3a–3d are 3.81 eV, 3.81 eV, 3.90 eV and 3.90 eV, respectively (Table. 2).
The cyclic voltammetry (CV) measurement was performed to investigate electrochemical
properties of the four compounds in anhydrous THF with the concentration of 1.0×10−4 M (Fig. 5) and
the corresponding data are summarized in Table 2. On the basis of the onset potentials of oxidation ( E
ox
onset), the HOMO energy levels of these compounds were calculated with the values of 5.47, 5.42, 5.43
and 5.44 eV, respectively, using the equation HOMO = −eEonset − 4.31 eV. From the edge in absorption
spectra, the band gaps were acquired with the values of 3.17 eV for 3a, 3.17 eV for 3b, 3.22 eV for 3c
and 3.22 eV for 3d. The LUMO values of the four compounds can be obtained by subtraction of the
optical band gap energies from the HOMO energy levels with the values of 2.30, 2.25, 2.21 and 2.22
eV, respectively.
Insert Fig. 5
3.6 Thermal properties
As shown in Fig. 6, the thermal properties of compounds 3a−3d were investigated by
thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses. These compounds
o
show high thermal decomposition (Td) temperatures with the values of 348, 351, 319 and 340 C for
compounds 3a−3d, respectively. As shown in Fig. 6b, glass transition temperatures (Tg) of compounds
3a and 3b are relatively high with Tg values of 87.6 and 91.1 oC, respectively. However, compounds 3c
o
and 3d have poor performance with the Tg values of 53.8 and 40.5 C. The Td and Tg values of
compounds 3a and 3b are higher than those of compounds 3c and 3d, indicating that the introduction
of phenyl and p-(tert-butyl)phenyl group is better than alkyl group to improve thermostability. The Td
value of compound 3d is higher than that of compound 3c, while the Tg value of compound 3d is lower
12

ACCEPTED MANUSCRIPT
than that of compound 3c, which illustrates that increasing the length of alkyl chain enhances thermal
property but decreases glass transition temperature. In addition, compound 3a shows obvious cold-
crystallization peak and corresponding crystal melting peak, which indicates that compounds 3b–3d
changing from glassy state to crystalline state are difficult. As shown in Fig. S8, different from pristine
crystal samples, the DSC curve of the ground powder of the four compounds show a broad exothermic
peak before melting, revealing that the ground samples are in a metastable amorphous state and can
crystallize in the solid state upon heating. It illustrates that the molecular packing patterns of
compounds 3a−3d can transform from the crystalline to the amorphous state by grinding.
Insert Fig. 6
3.7 Electroluminescence
Owing to their relatively high absolute fluorescence quantum yields and glass transition temperatures,
non-doped OLED devices based on compounds 3b and 3c were fabricated with a typical configuration
of ITO/NPB (40 nm)/3b or 3c (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) without optimized,
where the new luminogens serve as emitters, NPB was used as hole-transporting layer and TPBi was
used as electron-transporting layer. The devices performances were summarized in Table 3. Fig. 7a and
7b plot the current density–voltage–luminance (I–V–L) characteristics of the OLED devices I (3b) and
II (3c), respectively. The turn-on voltage (Von) of device I was at 3.3 V, by contrast, the Von of device II
was higher with value of 4.8 V. The lower Von value suggests the less dissipation of energy associated
with carrier injection or transportation. For device I, it has the maximum luminance efficiency and
power efficiency of 1.48 cd/A and 1.05 lm/W, respectively, yielding a maximum luminance of 2470 cd
m-2 at 6.8 V. Device II performed dissatisfactory with a maximum luminance of 843 cd cm-2 at 7.8 V
and the maximum luminance efficiency and power efficiency of the device are 2.28 cd/A and 1.12
lm/W, respectively. Both devices exhibit blue emission with maximum emission peaks at 436 nm, 432
nm and the CIE coordinates of the devices are (0.180, 0.171) and (0.179, 0.149), respectively (Fig. S9).
The EL spectra of compounds 3b and 3c are blue lighting, which are close to their crystal solid PL
spectra. Besides, the PL spectra of the vacuum deposited films of compounds 3b and 3c are sky blue
emission (Fig. S10). Therefore, the molecule stacking of compounds 3b and 3c in device I and II may
be nearly to their crystal state. As the literature reported, crowded and twisted conformation will
13

ACCEPTED MANUSCRIPT
encourage the formation of the crystalline state in the thin films in OLED devices [45]. However,
plenty of luminescent materials in OLED devices exist as amorphous state and the EL spectra are
bathochromic-shifted compared to PL spectra of crystal solid [23,45-47]. Of course, the actual
molecule stacking of luminescent materials in OLED devices depends on many factors such as the
structural nature of materials and the fabricating conditions of devices. The results herein indicate that
N-substituted tetraphenylethene-based benzimidazoles herein are suitable for OLEDs compared with
their non-substituted precursors as expected in molecular design of Scheme 1. Therefore, the
reasonable modification based on TPE and its derivatives is still one effective strategy for new OLED
materials.
Insert Table 3
Insert Fig. 7
4 Conclusions
In summary, we have designed and synthesized a new series of blue-emitting N-substituted
tetraphenylethene-based benzimidazoles (3a–3d). All the compounds are typical AIE-active
compounds with relatively excellent absolute fluorescence quantum yields (38.1-65.7%) and high
thermal stability. These compounds exhibit reversible mechanochromism behaviors under external
pressure and DCM vapor with emission changes between blue and yellow-green. Their single crystal
structures demonstrate that crystal packing structures can be destroyed due to interfaces gaps between
layers exists in unconsolidated molecular packing structures. The PXRD results confirm that the
transformation of the twisted crystalline state to the planar amorphous state is related to
mechanochromism behaviors in the compounds 3a–3d. Multilayer OLEDs are fabricated using
compounds 3b and 3c as emitting layer, which emit blue light. The OLED based on compound 3b
shows a better performance with the lower turn-on voltage of 3.3 V and maximum luminance of 2470
cd m–2. Both OLEDs are blue emission which have the potential application in flat panel display as
blue-light emitting materials and provide a reference for designing excellent blue-emitting molecules in
solid state.
Acknowledgements
This work was supported by the Fundamental Research Funds for the Central Universities
14

ACCEPTED MANUSCRIPT
(2015XKZD08) and the Priority Academic Program Development of Jiangsu Higher Education
Institutions.
References
[1] Yuen J D, Fan J, Seifter J, Lim B, Hufschmid R, Heeger A J, Wudl F. High performance weak
donor–acceptor polymers in thin film transistors: effect of the acceptor on electronic properties,
ambipolar conductivity, mobility, and thermal stability. J Am Chem Soc 2011; 133: 20799-807.
[2] O Neill M, Kelly S M. Ordered materials for organic electronics and photonics. Adv Mater 2011;
23: 566-84.
[3] Wang N N, Yu J S, Zheng Y F, Guan Z Q, Jiang Y D. Organic photovoltaic cells based on a
medium-bandgap phosphorescent material and c60. J Phys Chem C 2012; 116: 5887-91.
[4] Ma S Y, Fu Y Y, Ni D B, Mao J, Xie Z Y, Tu G L. Spiro-fluorene based 3d donor towards
efficient organic photovoltaics. Chem Commun 2012; 48: 11847-49.
[5] Thompson B C, Fréchet J M J. Polymer–fullerene composite solar cells. Angew Chem Int Edit
2008; 47: 58-77.
[6] Fan H H, Guo L, Li K F, Wong M S, Cheah K W. Exceptionally strong multiphoton-excited blue
photoluminescence and lasing from ladder-type oligo(p-phenylene)s. J Am Chem Soc 2012; 134:
7297-300.
[7] Görrn P, Lehnhardt M, Kowalsky W, Riedl T, Wagner S. Elastically tunable self-organized organic
lasers. Adv Mater 2011; 23: 869-72.
[8] Kim H, Schulte N, Zhou G, Müllen K, Laquai F. A high gain and high charge carrier mobility
indenofluorene-phenanthrene copolymer for light amplification and organic lasing. Adv Mater
2011; 23: 894-97.
[9] Zhu L N, Qin J G, Yang C L. New metal-coordination-inhibited charge transfer emission for
terfluorenes: highly sensitive and selective detection for Hg2+ with ratiometric “turn-on”
fluorescence response. Chem Commun 2009; 46: 8755-7.
[10] Nie J, Li N, Ni Z H, Zhao Y, Zhang L F. A sensitive tetraphenylethene-based fluorescent probe
for Zn2+ ion involving ESIPT and CHEF processes. Tetrahedron Lett 2017; 58: 1980-4.
[11] Chua M H, Huang K W, Xu J W, Wu J S. Unusual intramolecular hydrogen transfer in 3,5-
di(triphenylethylenyl) BODIPY synthesis and 1,2-migratory shift in subsequent scholl type
15

ACCEPTED MANUSCRIPT
reaction. Org Lett 2015; 17: 4168-71.
[12] Qin W, Lam J W Y, Yang Z Y, Chen S M, Liang G D, Zhao W J, Kwok H S, Tang B Z. Red
emissive AIE luminogens with high hole-transporting properties for efficient non-doped oleds.
Chem Commun 2015; 51: 7321-4.
[13] Kim J Y, Yasuda T, Yang Y S, Adachi C. Bifunctional star-burst amorphous molecular materials
for OLEDs: achieving highly efficient solid-state luminescence and carrier transport induced by
spontaneous molecular orientation. Adv Mater 2013; 25: 2666-71.
[14] Wu K C, Ku P J, Lin C S, Shih H T, Wu F I, Huang M J, Lin J J, Chen I C, Cheng C H. The
photophysical properties of dipyrenylbenzenes and their application as exceedingly efficient blue
emitters for electroluminescent devices. Adv Funct Mater 2008; 18: 67-75.
[15] Moorthy J N, Natarajan P, Venkatakrishnan P, Huang D, Chow T J. Steric inhibition of π-stacking:
1,3,6,8-tetraarylpyrenes as efficient blue emitters in organic light emitting diodes (OLEDs). Org
Lett 2007; 9: 5215-8.
[16] Jakubiak R, Collison C J, Wan W C, Rothberg L J, Hsieh B R. Aggregation quenching of
luminescence in electroluminescent conjugated polymers. J Phys Chem A 1999; 103: 2394-8.
[17] Berlman I B. On an empirical correlation between nuclear conformation and certain fluorescence
and absorption characteristics of aromatic compounds. J Phys Chem 1970; 74: 3085-93.
[18] Zhang R, Zhang T F, Xu L, Han F F, Zhao Y, Ni Z H. A new series of short axially symmetrically
and asymmetrically 1,3,6,8-tetrasubstituted pyrenes with two types of substituents: syntheses,
structures, photophysical properties and electroluminescence. J Mol Struct 2017; 1127: 237-46.
[19] Wee K R, Ahn H C, Son H J, Han W S, Kim J E, Cho D W, Kang S O. Emission color tuning and
deep blue dopant materials based on 1,6-bis(N-phenyl-p-(R)-phenylamino)pyrene. J Org Chem
2009; 74: 8472-5.
[20] Wang X Q, Liu Q S, Yan H, Liu Z P, Yao M G, Zhang Q F, Gong S W, He W J. Piezochromic
luminescence behaviors of two new benzothiazole-enamido boron difluoride complexes: intra-
and inter-molecular effects induced by hydrostatic compression. Chem Commun 2015; 51: 7497-
500.
[21] Luo X, Zhao W, Shi J, Li C, Liu Z, Bo Z, Dong Y Q, Tang B Z. Reversible switching emissions of
tetraphenylethene derivatives among multiple colors with solvent vapor, mechanical, and thermal
16

ACCEPTED MANUSCRIPT
stimuli. J Phys Chem C 2012; 116: 21967-72.
[22] Sase M, Yamaguchi S, Sagara Y, Yoshikawa I, Mutai T, Araki K. Piezochromic luminescence of
amide and ester derivatives of tetraphenylpyrene—role of amide hydrogen bonds in sensitive
piezochromic response. J Mater Chem 2011; 21: 8347-54.
[23] Chan C Y K, Lam J W Y, Zhao Z J, Chen S M, Lu P, Sung H H Y, Kwok H S, Ma Y G, Williams
I D, Tang B Z. Aggregation-induced emission, mechanochromism and blue electroluminescence
of carbazole and triphenylamine-substituted ethenes. J Mater Chem C 2014; 2: 4320-7.
[24] Jadhav T, Choi J M, Lee J Y, Dhokale B, Misra R. Non-doped blue organic light emitting devices
based on tetraphenylethylene-π-imidazole derivatives. Org Electron 2016; 37: 448-52.
[25] Jadhav T, Dhokale B, Patil Y, Mobin S M, Misra R. Multi-stimuli responsive donor–acceptor
tetraphenylethylene substituted benzothiadiazoles. J Phys Chem C 2016; 120: 24030-40.
[26] Zhang Y, Lai S L, Tong Q X, Lo M F, Ng T W, Chan M Y, Wen Z C, He J, Jeff K S, Tang X L,
Liu W M, Ko C C, Wang P F, Lee C S. High efficiency nondoped deep-blue organic light
emitting devices based on imidazole-π-triphenylamine derivatives. Chem Mater 2012; 24: 61-70.
[27] Kumar N S S, Varghese S, Suresh C H, Rath N P, Das S. Correlation between solid-state
photophysical properties and molecular packing in a series of indane-1,3-dione containing
butadiene derivatives. J Phys Chem C 2009; 113: 11927-35.
[28] Ho M H, Wu Y S, Wen S W, Lee M T, Chen T M. Highly efficient deep blue organic
electroluminescent device based on 1-methyl-9,10-di(1-naphthyl)anthracene. Appl Phys Lett
2006; 89: 252903.
[29] Thiery S, Tondelier D, Declairieux C, Seo G, Geffroy B, Jeannin O, Rault-Berthelot J, Métivier R,
Poriel C. 9,9′-spirobifluorene and 4-phenyl-9,9′-spirobifluorene: pure hydrocarbon small
molecules as hosts for efficient green and blue PhOLEDs. J Mater Chem C 2014; 2: 4156-66.
[30] Zhang T F, Zhang R, Zhang Z M, Ni Z H. A series of tetraphenylethene-based benzimidazoles:
syntheses, structures, aggregation-induced emission and reversible mechanochromism. Rsc Adv
2016; 6: 79871-8.
[31] Zhao N, Yang Z Y, Lam J W Y, Sung H H Y, Xie N, Chen S J, Su H M, Gao M, Williams I D,
Wong K S, Tang B Z. Benzothiazolium-functionalized tetraphenylethene: an AIE luminogen with
tunable solid-state emission. Chem Commun 2012; 48: 8637-9.
17

ACCEPTED MANUSCRIPT
[32] Zhang R, Zhao Y, Zhang T F, Xu L, Ni Z H. A series of short axially symmetrically 1,3,6,8-
tetrasubstituted pyrene-based green and blue emitters with 4-tert-butylphenyl and arylamine
attachments. Dyes Pigments 2016; 130: 106-15.
[33] Zhang Z M, Zhao Y, Zhang R, Zhang L F, Cheng W Q, Ni Z H. Design and synthesis of a new
series of tetra(polycyclic aryl)ethenes: achieving aggregation-induced emission and efficient
solid-state photoluminescence. Dyes Pigments 2015; 118: 95-101.
[34] Chen L, Jiang Y B, Nie H, Hu R R, Kwok H S, Huang F, Qin A J, Zhao Z J, Tang B Z. Rational
design of aggregation-induced emission luminogen with weak electron donor-acceptor interaction
to achieve highly efficient undoped bilayer OLEDs. Acs Appl Mater Inter 2014; 6: 17215-25.
[35] Chen L, Lin G W, Peng H R, Nie H, Zhuang Z Y, Shen P C, Ding S Y, Huang D J, Hu R R, Chen
S M, Huang F, Qin A J, Zhao Z J, Tang B Z. Dimesitylboryl-functionalized tetraphenylethene
derivatives: efficient solid-state luminescent materials with enhanced electron-transporting ability
for nondoped OLEDs. J Mater Chem C 2016; 4: 5241-7.
[36] Shi H P, Li M, Fang L, Dong X Q, Zhang X L, Peng H R, Chen S M, Tang B Z. Synthesis,
aggregation-induced emission, and electroluminescence properties of a novel emitter comprising
tetraphenylethene and carbazole moieties. Synthetic Met 2016; 220: 356-61.
[37] Li R H, Xiao S Z, Li Y, Lin Q F, Zhang R H, Zhao J, Yang C Y, Zou K, Li D S, Yi T.
Polymorphism-dependent and piezochromic luminescence based on molecular packing of a
conjugated molecule. Chem Sci 2014; 5: 3922-8.
[38] Huang J, Sun N, Chen P Y, Tang R L, Li Q Q, Ma D G, Li Z. Largely blue-shifted emission
through minor structural modifications: molecular design, synthesis, aggregation-induced
emission and deep-blue OLED application. Chem Commun 2014; 50: 2136-8.
[39] Li Z, Dong Y Q, Mi B X, Tang Y H, Häussler M, Tong H, Dong Y P, Lam J W Y, Ren Y, Sung H
H Y, Wong K S, Gao P, Williams I D, Kwok H S, Tang B Z. Structural control of the
photoluminescence of silole regioisomers and their utility as sensitive regiodiscriminating
chemosensors and efficient electroluminescent materials. J Phys Chem B 2005; 109: 10061-6.
[40] Jadhav T, Dhokale B, Patil Y, Misra R. Aggregation induced emission and mechanochromism in
tetraphenylethene substituted pyrazabole. Rsc Adv 2015; 5: 68187-91.
[41] Jadhav T, Dhokale B, Mobin S M, Misra R. Aggregation induced emission and mechanochromism
18

ACCEPTED MANUSCRIPT
in pyrenoimidazoles. J Mater Chem C 2015; 3: 9981-8.
[42] Gao H Z, Xu D F, Liu X L, Han A X, Zhou L, Zhang C, Li Z, Dang J. Tetraphenylethene-based β-
diketonate boron complex: efficient aggregation-induced emission and high contrast
mechanofluorochromism. Dyes Pigments 2017; 139: 157-65.
[43] Gong Y Y, Tan Y Q, Liu J, Lu P, Feng C F, Yuan W Z, Lu Y W, Sun J Z, He G F, Zhang Y M.
Twisted D-π-A solid emitters: efficient emission and high contrast mechanochromism. Chem
Commun 2013; 49: 4009-11.
[44] Xu B J, Chi Z G, Zhang J Y, Zhang X Q, Li H Y, Li X F, Liu S W, Zhang Y, Xu J R.
Piezofluorochromic and aggregation-induced-emission compounds containing triphenylethylene
and tetraphenylethylene moieties. Chem-Asian J 2011; 6: 1470-8.
[45] Huang J, Sun N, Yang J, Tang R L, Li Q Q, Ma D G, Qin J G, Li Z. Benzene-cored fluorophors
with tpe peripheries: facile synthesis, crystallization-induced blue-shifted emission, and efficient
blue luminogens for non-doped OLEDs. J Mater Chem 2012; 22: 12001-7.
[46] Cui Y, Yin Y M, Cao H T, Zhang M, Shan G G, Sun H Z, Wu Y, Su Z M, Xie W F. Efficient
piezochromic luminescence from tetraphenylethene functionalized pyridine-azole derivatives
exhibiting aggregation-induced emission. Dyes Pigments 2015; 119: 62-9.
[47] Yuan W Z, Tan Y Q, Gong Y Y, Lu P, Lam J W Y, Shen X Y, Feng C F, Sung H H, Lu Y W,
Williams I D, Sun J Z, Zhang Y M, Tang B Z. Synergy between twisted conformation and
effective intermolecular interactions: strategy for efficient mechanochromic luminogens with high
contrast. Adv Mater 2013; 25: 2837-43.
19

ACCEPTED MANUSCRIPT
Table 1 Optical properties of compounds 3a−3d and emission wavelengths under various external
stimuli
Compounds λabs(nm)/ε (104 M-1 cm-1)a
λpristine(nm)c λgrinded(nm)c λfuming(nm)c ꢀλ(nm)d λeme/λemf(nm)
Φ
F (%)b
38.1
54.6
60.4
65.7
3a
3b
3c
3d
301(2.47)/333(2.63)
301(2.43)/333(2.60)
448
454
440
446
469
481
463
472
449
453
442
445
21
29
23
26
475/481
479/475
473/468
476/471
299(2.62)/324(2.58)
299(2.59)/324(2.59)
a
b
c
Measured in pure THF;
Measured in crystalline solid;
Solid emission wavelength;
dꢀλ=λgrinded − λpristine; e Measured in THF/water mixtures (fw = 95%); f Measured in film.
Table 2 Electrochemical properties and thermostability of compounds 3a–3d
HOMO (eV)
LUMO (eV)
Eg (eV)
Exptlb
ox
onset
Compounds E
(V)
Tg/Td (oC)
Exptla
Calc
-5.28
-5.27
-5.32
-5.35
Exptl
Calc
-1.47
-1.46
-1.42
-1.45
Calc
3.81
3.81
3.90
3.90
3a
3b
3c
3d
1.16
-5.47
-5.42
-5.43
-5.44
-2.30
-2.25
-2.21
-2.22
3.17
3.17
3.22
3.22
87.6/348
91.1/351
53.8/319
40.5/340
1.11
1.12
1.13
a
HOMO levels were calculated using the following equations: HOMO = −e(Eonset−0.49 V) −4.8 eV,
where the value 0.49 V is for FOC versus SCE electrode. b Estimated from the onset of the absorption
spectra: 1240/λonest
.
Table 3 Electroluminescence properties of the OLED devices
Device Von(V)a EL (nm)b FWHMc Lmax (cd/m2)d ηC (cd/A)e ηP (lm/w)f
CIE (x,y)
(0.180,0.171)
(0.179,0.149)
I (3b)
3.3
4.8
436
432
85
66
2470
1.48
2.28
1.05
1.12
II (3c)
843
b
c
a Turn-on voltage (V) at a luminance of 1 cd/m2; Peak wavelength EL spectra; Full-width at half
d
e
f
maximum; Maximum luminance (cd/m2); Maximum of luminance efficiency (cd/A); Maximum
power efficiency.

ACCEPTED MANUSCRIPT
Fig. 1 Crystal structures of compounds 3a–3d.
Fig. 2 The schematic intermolecular interactions in the crystal of compounds 3a–3d, the red, blue and
yellow numbers represent C-H---π, C-H---N and π-π interactions, respectively.

ACCEPTED MANUSCRIPT
Fig. 3 (a) UV-visible spectra of the dilute solutions of compounds 3a−3d in THF (concentration: 50 ꢀM).
(b) Normalized emission spectra of the compounds 3a–3d recorded in pristine solid. (c) Fluorescence
spectra of 3a in THF-water mixtures with different fw, inset photograph: compound 3a in fw =0 and 95%
solvents under UV illumination. (d) Plots of (I/I0) values versus THF/H2O mixtures of compounds 3a–
3d, where I0 is the PL intensity in pure THF solution. Concentration: 50 ꢀM; EX wavelength: 350 nm;
intensity calculated at λmax
.
1.0
Grinding
Fuming
a
0.8
0.6
0.4
0.2
0.0
Pristine
Ground
Fumed
400
450
500
550
600
650
Wavelength (nm)
Fig. 4 (a) Emission spectra of 3a as pristine, grinded and fumed solids, inset photographs: compound 3a
in pristine (left) and ground (right) forms under UV illumination. (b) Powder X-ray diffraction patterns
of 3a in pristine, ground and fumed forms.
Fig. 5 Cyclic voltammograms of compounds 3a-3d.

ACCEPTED MANUSCRIPT
3a
3b
3c
3d
100
80
60
40
20
0
b
a
o
Tg = 40.5
C
3a
3b
3c
3d
Tm = 147 oC
o
Tg = 53.8
C
Nd
o
Tg = 91.1
Tg = 87.6
C
Tm = 219 oC
Tm = 257 oC
o
C
50
100
150
200
250
300
50 100 150 200 250 300 350 400 450 500
Temperature (oC)
Temperature ( )
oC
Fig. 6 (a) Thermogravimetric analysis (measured in crystal solid) and (b) DSC curves (second heating
scan) of compounds 3a–3d recorded under nitrogen at a heating rate of 10 oC min–1.
10000
1000
100
10
500
400
300
200
100
0
1000
100
10
200
150
100
50
a
b
1
1
0
0.1
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Voltage (V)
Voltage (V)
1.2
1.0
0.8
0.6
0.4
0.2
1.2
1.0
0.8
0.6
0.4
0.2
0.0
2.5
1.4
1.2
1.0
0.8
0.6
c
d
2.0
1.5
1.0
0.5
0.0
0
100
200
300
400
0
40
80
120
160
200
Current Density (mA/cm2 )
Current Density (mA/cm2 )
Fig. 7 The current density-voltage-luminance curves of the compounds 3b (a) and 3c (b); Luminance
efficiency and Power efficiency of the compounds 3b (c) and 3c (d).

ACCEPTED MANUSCRIPT
Scheme 1 Molecular modification strategy for tetraphenylethene-based benzimidazoles.
Scheme 2 Synthetic procedures for compounds 3a–3d.

ACCEPTED MANUSCRIPT
Highlights
ꢀ
A new series of blue luminogens N-substituted tetraphenylethene-based benzimidazoles were
synthesized.
ꢀ
Crystal structures, photophysical properties and theoretical calculation of compounds 3a-3d have
been studied.
ꢀ
ꢀ
Compounds 3a-3d show fast-reversible mechanochromism under DCM vapor condition.
Blue OLEDs employing compounds 3b and 3c as non-doped emitters were fabricated.