Novel π-conjugated molecules based on diimidazopyridine: Significantly improved the photophysical, thermal and electrochemical properties bearing different aryl substituents

Article abstract of DOI:10.1016/j.tetlet.2018.01.015

A series of π-conjugated molecules based on diimidazolepyridine derivatives were designed, synthesized by Suzuki coupling reaction and cyclization reaction and characterized. Diimidazolepyridine motif as the main structure could improve the thermal stabil

Full text of DOI:10.1016/j.tetlet.2018.01.015

Accepted Manuscript
Novel π-conjugated molecules based on diimidazopyridine: Significantly im-
proved the photophysical, thermal and electrochemical properties bearing dif-
ferent aryl substituents
Xin Huang, Jinchang Tian, Feng Xu, Xiaochong Liu, Yuqin Li, Yanyan Guo,
Wenyi Chu, Zhizhong Sun
PII:
S0040-4039(18)30027-3
https://doi.org/10.1016/j.tetlet.2018.01.015
TETL 49602
DOI:
Reference:
Tetrahedron Letters
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17 November 2017
25 December 2017
5 January 2018
Please cite this article as: Huang, X., Tian, J., Xu, F., Liu, X., Li, Y., Guo, Y., Chu, W., Sun, Z., Novel π-conjugated
molecules based on diimidazopyridine: Significantly improved the photophysical, thermal and electrochemical
properties bearing different aryl substituents, Tetrahedron Letters (2018), doi: https://doi.org/10.1016/j.tetlet.
2018.01.015
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Tetrahedron Letters
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Novel π-conjugated molecules based on diimidazopyridine: Significantly
improved the photophysical, thermal and electrochemical properties bearing
different aryl substituents
Xin Huanga,b, Jinchang Tiana,b, Feng Xua,b, Xiaochong Liua,b, Yuqin Lia,b, Yanyan Guoa,b, Wenyi Chua,b,∗
Zhizhong Suna,b,*
,
a School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, P. R. China
b Key Laboratory of Chemical Engineering Process & Technology for High-efficiency Conversion, College of Heilongjiang Province, Harbin 150080, P. R.
China
ARTICLE INFO
ABSTRACT
Article history:
Received
A series of π-conjugated molecules based on diimidazolepyridine derivatives were designed,
synthesized by Suzuki coupling reaction and cyclization reaction and characterized.
Diimidazolepyridine motif as the main structure could improve the thermal stability and optical
property of the materials. All of the target compounds exhibited good thermal stabilities with Td
values in the range of 416-490 °C. These compounds showed steady blue light emissions in the
range of 424-478 nm and high quantum yields (0.33-0.69) in solution. Especially, compound 4f
achieved appropriate energy gap (Eg=2.69eV) and high fluorescence quantum yields (Φf=0.69)
because of introducing electron-donating group, such energy gap was helpful to electronic
transfer and transport. The materials have great potential for good electronic-transmission
materials in OLEDs.
Received in revised form
Accepted
Available online
Keywords:
Diimidazolepyridine derivatives
Suzuki coupling reaction
Cyclization reaction
Blue light emission
1. Introduction
wide energy gaps (≥3 eV) of these materials resulted in higher
charge-injection barriers and inferior electrical properties [24,25].
Organic light-emitting diodes (OLEDs) have gained
considerable attention owing to their potential applications in
solid-state lightings, flexible panel and full-color displays [1-4].
The OLEDs based on small molecules exhibited a lot of
advantages as low-energy consumption[5], compatibility with
flexible substrates [6], high color rendering index, high contrast
and wide viewing angles. Therefore, numerous building blocks
such as: biphenyl [7,8], carbazole [9,10], triphenylamine [11-13],
anthracene [14,15], and fluorine [16-18] are usually introduced to
the structures of OLED blue lighting materials. Most of them
possessed non-planar molecular structures that made them
difficult to aggregation in the solid state due to dipole-dipole
interaction or intermolecular π–π stacking. Thence, they are not
conducive to efficient charge injection and transmission in OLED
devices. And it is still a great significance to seek excellent
performance blue-light emitting materials not only in basic
theory but also in applications of device.
As an improvement, R. Zhong [26,27] introduced a pyridine
group into the imidazole host structure, which not only improved
the thermal stability of the compound but also enhanced the
electron-transport properties. Therefore, the compounds
containing imidazole and pyridine rings can improve the
photoelectric properties and have good application prospects.
In this work, a series of novel diimidazolepyridine were
initially designed and synthesized. Wherein, diimidazolepyridine
as a condensed unit of diimidazole and pyridine was used as the
core backbone, and then different aryl-substitutes were
introduced to improve their photoelectric properties. Similarly,
two butyl groups were introduced to the N-1/5 positions of the
imidazole to increase solubility and released intramolecular π-π
1
stacking. Their structures were characterized by H NMR, 13C
NMR and mass spectrometry. Their properties were studied
which included thermal analysis, UV absorption spectra,
fluorescence emission spectra and the electron-transport ability
of the materials in experiment and theory calculation.
Recently, fluorescent materials containing imidazole ring have
attracted more attention [19-22]. Because the imidazole
derivatives are non-centrosymmetric structures and their
conjugated
systems
can
tolerate
strong
electron-
withdrawing/donating groups, imidazole derivatives become
potentially useful in electrical and optical materials with high
oxidation and reduction potential, chemical stability and thermal
decomposition temperature. H. Huang [23] demonstrated the
realization of excellent OLEDs based on a high-performance,
[a] School of Chemistry and Materials Science, Heilongjiang
University,
Harbin 150080, P. R. China
E-mail: wenyichu@hlju.edu.cn
[b] Key Laboratory of Chemical Engineering Process &
Technology for High-efficiency Conversion,
College of Heilongjiang Province, Harbin 150080, P. R.
China
stable
phenanthro[9,10-d]imidazole compounds. However, the intrinsic
and
readily
processable
electron-transporting
———
∗ Corresponding author. Tel.: +86-451-86609135; fax: +86-451-86609135; e-mail: wenyichu@hlju.edu.cn
1

ArB(OH)2
Pd(PPh3)4
PEG400
Br
CHO
Ar
CHO
1.5M K2CO3
1
2b-g
CHO
H
N
2a
N
H2N
H2N
NH2
MeOH 65℃
Ar
Ar
3HCl
·
N
H
N
conc HCl
·
N
NH2
N
Ar
CHO
3a-g
2b-g
H
r.t.
N
N
C4H9Br
N
N
N
Ar
Ar
Ar
Ar
N
H
N
N
N
N
KOH DMF
4a-g
3a-g
F
F
S
,
,
OCH3
,
,
CF3
,
Ar= H
,
g
c
d
e
a
f
b
N
N
N
N
N
N
N
N
N
N
4a
4b
N
N
N
N
N
4c
N
N
N
N
N
N
S
S
CF3
F3C
N
N
N
N
4e
4d
F
F
F
N
N
N
N
N
N
OCH3
H3CO
N
N
N
N
F
4g
4f
Scheme 1. The synthetic routes of compounds 4a-g
2

2. Experimental Section
In addition, it could be clearly observed molecular orbital
energy level changed by introducing different substituents.
Compared with the compound 4a, introduction of benzene,
naphthalene and thiophene groups to the para-position of
phenyl(4b，4c and 4d) could effectively extend the conjugate
chain. It could increase HOMO level and decrease LUMO level.
The π-π* transition energy would reduce and cause the band
bathochromic shift in the absorption and emission spectrum [28].
Simultaneously, introducing strong electronegative groups into
the compounds could reduce the HOMO and LUMO levels of the
compounds, but the effect of LUMO was more obvious, thus a
red-shift of the absorption band was caused [29]. In contrast, the
introduction of electron-donating group into the compounds
could significantly elevate the HOMO and alter the LUMO
moderately. There was considerable electronic coupling between
the electron-donating group and the diimidazolepyridine core,
thus decreasing the HOMO-LUMO energy gaps and causing red-
shift of 4f.[30] The HOMO-LUMO energy gaps of these
compounds were ranged from 2.89 to 3.23 eV. The wide energy
gap of 4f was higher than 4e and 4g, and this could be explained
that the strong electron-donating substituents reduced the
oxidation resistance of the molecule. So combining with the
electron acceptor core and electron donor substituent (-OCH3)
could allow the electronic directional movement and be helpful
to electronic transfer and transport. [31]
2.1 Materials and Measurements
All starting materials were purchased from TCI; the reagents
were obtained from J&K Chemical Company and used without
further purification. Synthetic process of materials was monitored
by thin layer chromatography (TLC), and the materials were
purified by column chromatography carried out on silica gel (200
~ 300 mesh). 1H NMR and 13C NMR spectra were recorded on a
Bruker DRX 400 MHz and 100 MHz spectrometer in CDCl3 or
DMSO. Chemical shifts were reported in ppm with
tetramethylsilane (TMS) as internal standard, and coupling
constants (J) were reported in Hertz (Hz). High resolution mass
spectra were recorded on Agilent 1100 (VL) mass spectrometer.
Thermogravimetric (TGA) measurements were performed on
Shimadzu DTG-60 A thermal analyzers at a heating rate of
10 °C/min under nitrogen atmosphere. UV-visible spectra and
photo-luminescence (PL) spectra were measured by Shimadzu
UV-2501PC UV-visible spectrophotometer and Shimadzu RF-
5301PC fluorescence spectrophotometer, respectively. Cyclic
voltammetric(CV) measurements were carried out on the Chi
1200A system in a conventional three-electrode cell with a glass
carbon working electrode, a platinum-wire counter electrode and
a
Ag/AgCl reference electrode referenced in anhydrous
dichloromethane solution of C16H36ClNO4 (0.10 M) at a
sweeping rate of 100 mV/s at room temperature. Density
functional theory (DFT) calculations were applied to characterize
the frontier molecular orbital energy levels of the compounds at
the B3LYP/6-31G(d) level by using the Gaussian 03 program.
Melting points were measured on a digital melting point
apparatus without correction. Infrared spectroscopy (IR) was
measured by SP-100 Fourier transform infrared spectroscopy.
2.2 Synthesis of materials
Main synthetic routes of diimidazolepyridine derivatives were
shown in Scheme 1. To begin our study, 4-bromobenzaldehyde
was selected as the starting material. The 4-arylbenzaldehydes
(2b-2g) were obtained by Suzuki coupling reaction.
Intermediates (3a-3g) were synthesized by refluxing cyclization
of compounds 2a-2g with 2, 3, 5, 6-tetraaminopyridine
trihydrochloride under acidic condition in methanol solution.
Finally, the target products (4a-4g) were obtained by alkylation
reaction with 1-bromobutane under basic condition.
Fig.1. Spatial distributions of frontier orbitals of the target
compounds at the B3LYP/6-31G(d) level
3.2 Thermal analysis
The thermal stabilities of the target compounds were
determined by thermogravimetric analysis (TGA) in N2
atmosphere and data were shown in Fig. 2 and Table 1.
Decomposition temperatures (Td, corresponding to 5% weight
loss) were measured to be 416, 443, 490, 458, 463, 484 and
467 °C for 4a-g, respectively. The thermodynamic data
demonstrated that the Td values of 4b-g were higher than 4a, it
was mainly related to the introduction of different aryl
substituents. The Td value of compound 4c was higher than 4b
and 4d. Thence, the Td values increased with the increase of their
rigid structure and conjugation degree[32]. In addition, compared
with 4b, the Td values of 4e, 4g and 4f were increased by 20, 24
and 41 °C, respectively. The reason is that introducing
substituents into the target compounds leaded to an increase in
molecular weights. This would increase the energy required for
molecular cleavage to result in an increase of Td values.
Moreover, the melting points(Tm) of these compounds were 181,
229, 261, 252, 244, 236 and 213 °C, respectively. The Tms of 4b-
4g were higher than 4a, this is due to the introduction of different
aryl substituents which made the molecular weight increase, then
resulted in an increase in the intramolecular π-π interactions.
Therefore, these compounds showed good thermal stability.
3. Results and Discussion
3.1. Theoretical calculations
For better understanding the structure-property relationship of
the diimidazolepyridine derivatives, the molecular configuration
and the frontier molecular orbital energy levels were determined
using DFT at the B3LYP/6-31G(d) level in the Gaussian 03
software. The results of HOMO and LUMO distribution of these
seven compounds were shown in Fig.1. Their computed frontier
orbital energies were shown in Table 1.
As shown in the Fig.1, the optimized structures of these
derivatives revealed that the diimidazolepyridine derivatives had
axial aryl substitutes. Such structural features could influence the
electrochemical and physicochemical properties. The HOMO and
LUMO of the compounds were all populated on the central
diimidazolepyridine. But it was found that the LUMO levels
were also located over the conjugate parts and electron
withdrawing groups. The HOMO levels were also located over
the electron-donating group. Obviously, by changing the
substitutes, a large difference of electronic density was found.
3

Table 1. Eox, Eg, EHOMO, ELUMO and physical properties of the target compounds
c
d
b
Tde/Tm(°C)
λ
maxAbs (nm)
λ
max PL(nm)
Solutionf Filmg
Φfh
Band
gapa
HOMO/LUMOa
Eox
EHOMO/ELUMO
(eV)
Compound
Eg
（eV）
(V)
Solutionf
306,391
271,408
251,412
279,415
274,406
281,417
268,404
Filmg
4a
4b
4c
4d
4e
4f
3.23
3.11
2.89
3.06
2.92
2.99
2.96
-5.27/-2.04
-5.20/-2.09
-5.13/-2.24
-5.17/-2.11
-5.44/-2.52
-4.99/-2.00
-5.41/-2.45
2.90
2.79
2.67
2.72
2.78
2.69
2.77
0.68
0.62
0.57
0.61
0.95
0.46
0.84
-5.48/-2.58
-5.42/-2.63
-5.37/-2.70
-5.41/-2.69
-5.75/-2.97
-5.26/-2.57
-5.64/-2.87
416/181
443/229
490/261
458/252
463/244
484/236
467/213
269,403
261,410
256,412
279,414
271,408
278,415
274,405
424
462
469
464
445
478
466
528
532
563
567
550
562
531
0.33
0.55
0.56
0.43
0.42
0.69
0.67
4g
[a] DFT/B3LYP calculated values. [b] Optical energy gaps calculated from the edge of the electronic absorption band rate of 100 mV s-1. [c] Oxidation potential
in CH2Cl2 (10-3 M) containing 0.1 M (n-C4H9)4NPF6 with a scan rate of 100 mV s-1. [d] Measured by Cyclic voltammogram. [e] Obtained from TGA
measurements with a heating rate of 10 °C/min under N2. [f] Measured in a dilute CH2Cl2 solution (10-5 M). [g] Measured in solid films. [h] Measured in CH2Cl2
with quinine sulfate as the standard.
It was obvious that the Td values of the diimidazolepyridine
derivatives were higher than the reported benzimidazole
derivatives [33] which synthesized with a single imidazole ring.
The π-conjugation and thermal stability were improved owing to
the main structure which contained two imidazole groups. So
diimidazolepyridine derivatives had great potential to fabricate
into devices by vacuum thermal evaporation technology.
evidently electron-withdrawing substituents (σ=+0.54/+0.34).
The introduction of -CF3 and -F showed the electron-
withdrawing inductive effect, which reduced the electron cloud
density of the conjugation system and increased the transition
energy of the whole system, then resulted in blue shift. Otherwise,
the compound 4f showed a pronounced bathochromic shift of the
π-π* band. The p-OMe group is evidently electron-donating
substituent (σ=-0.27). The introduction of -OCH3 showed the
electron-donating conjugated effect, which increased the electron
cloud density of the conjugation system and reduced the
transition energy of the whole system, then resulted in red shift.
Moreover, compared to the 4a, with increasing the π-conjugation
of the compounds 4b-d, the corresponding π-π* electron
transition energy level would be decreased and caused the whole
absorption spectrum red-shift [36]. The optical band gaps (Eg) of
the target compounds, which were calculated from the absorption
edges (λon) of the UV-vis absorption spectra in the dilute CH2Cl2
solution with a formula Eg=1240/λon. They were 2.90, 2.79, 2.67,
2.72, 2.78, 2.69 and 2.77 eV, respectively[37].
As shown in Fig. 3(b), significant solvatochromic behaviors
were observed with remarkable red-shifts (408-428nm) of the
maximal absorption band for the compound 4f from the toluene
to methanol. With the increasing of the polarity of the solvent,
the polarity of the excited state was greater than that of the
ground state, which resulted in a large degree of energy reduction
of the excited state. The gap between the ground state and the
excited state would be decreased. Then, the transition ability
became larger, which was beneficial to the intramolecular charge
transfer. [38,39] Meanwhile, these compounds exhibited good
solubility in different solvents.
Fig.2. TGA graphs of target compounds recorded at a scanning
rate of 10 °C /min.
3.3. Photophysical properties
The UV-visible absorption and photoluminescence (PL) of
these compounds in CH2Cl2 dilute solution and solid films were
shown in Fig. 3, and the spectral parameters were summarized in
Table 1.
As shown in Fig. 3(d), the photoluminescence (PL) spectra of
all the synthesized materials were highly emissive in CH2Cl2
solution with emission maxima focusing on the blue light region
of 424-478 nm. The PL maxima of the compounds 4b-d were
red-shifted by 38-45 nm relative to that of the compound 4a
because of extending the conjugation length. [40] Compared with
the electron-withdrawing derivatives (-CF3 and -F), the electron-
donating derivative (-OCH3) occurred red-shift obviously due to
the increase in the degree of intramolecular charge transfer. As
shown in Fig. 3(e), by comparison with the emission spectras in
the dilute solution, the solid-state emissions were red-shifted in
the range of 65-105nm. The reason is that intermolecular π-π
As shown in Fig. 3(a)(c), the similar spectral patterns of
different derivatives were observed. Due to the similar structure,
these compounds exhibited two similar UV absorption bands in
solution and solid film at wavelength about 270 and 410 nm,
corresponding
to
n-π*
and
π-π*
transitions
of
diimidazolepyridine and π-π* transition of aryl substituents.[34]
However, compared with the compound 4b, there was a slightly
blue-shift in the peak maximum of the compounds 4e and 4g.
According to the Hammets constants, the substituent constants
(σ)[35] (Fig.S29) are known that the p-CF3 and m-F groups are
4

Fig.3. (a) UV-vis absorption in CH2Cl2 solution (10-5 M) of diimidazolepyridine derivatives. (b) UV-vis absorption in different solution
of 4f. (c) UV-vis absorption in solid films. (d) PL spectra in CH2Cl2 solution (10-5 M). (e) PL spectra in solid films. (f) PL spectra in
different solution of 4f.
stacking increased intermolecular forces, reduced twist, enhanced
the coplanarity and the conjugation degree. [41] Simultaneously,
standard [45], and the results were summarized in Table 1. The
experimental results indicated these compounds exhibited
relatively high FQY in dilute CH2Cl2 solution. The FQYs of them
were 0.33, 0.55, 0.56, 0.43, 0.42, 0.69 and 0.67, respectively. The
results also indicated that the fluorinated compound and electron-
donating substituted compound in periphery exhibited the high
FQY since the twisted and rigid configuration could reduce the
nonradiative decay. [46]
these
diimidazolepyridine
derivatives
presented
the
intermolecular interactions, originating due to the specific
molecular packing of the planar molecular configuration and
making them easy to aggregate in the solid state. [42] As shown
in Fig. 3(f), significant solvatochromism behaviors were also
observed with remarkable red-shifts (474-495nm) of the PL
spectra for the compound 4f from the toluene to methanol. This
indicates the occurrence of charge transfer upon photoexcitation.
3.4. Electrochemical properties
To further evaluate the feasibility of the new materials in the
field of optoelectronic applications, the electrochemical
behaviours of the compounds 4a-g were explored by cyclic
voltammetry (Fig.5) and the data were listed in Table 1. As
shown in Fig.5, these seven materials revealed seven irreversible
anode peaks of the initial oxidation characteristic and seven
irreversible cathodic peaks of the initial reduction feature. The
onset oxidation peaks (Eox) for the compounds 4a-g were from
0.46 to 0.95 V (versus the ferrocenium/ferrocene redox couple).
Using Ag/AgCl as the reference electrode the HOMO levels were
Contrast to the reported 6,10-dihydrofluoreno[2,3-d:6,7-
of
d’]diimidazole
derivatives[43],
the
PL
spectra
diimidazolepyridine derivatives in solution and in the solid state
occurred red-shift. This indicated the fusion of electron acceptor
pyridine with high electron affinity made the intramolecular
charge-transfer greatly enhanced. [44] The emission images of
the materials in CH2Cl2 under irradiation at 365 nm under room
temperature were shown in Fig. 4. All compounds exhibited blue
light emission stably. Therefore, these compounds had the
potential to be used as fluorescent materials.
calculated according to the following formula (EHOMO = -(Eox
+
4.8) eV). The HOMOs were in the range of -5.26 to -5.75 eV,
while the LUMOs were in the range of -2.57 to -2.97 eV by the
formula (ELUMO = -(EHOMO + Eg))[47]. This trend was in good
consistent with the calculated values (the HOMOs were in the
range of -4.99 to -5.44 eV and the LUMOs were in the range of -
2.00 to -2.52 eV). The results showed that the similar
voltammograms were obtained for all target compounds.
Compared to the only phenyl-substituted compounds 4a, it
could be rationalized that the increasing conjugation might
reduce the oxidation potentials of the molecule and made the
LUMO energy level decrease by 0.05, 0.12 and 0.11 eV (4b-d),
respectively. Lower LUMO levels reduced the energy barriers
between electron-transport and emitting layers and would
facilitate electron-injection into the emission layers. [48] In
addition, the introduction of electron-donating groups in the
compound 4f made the HOMO/LUMO energy levels increase by
Fig.4. The emission images at 365 nm UV illuminations.
In order to further understand the emission property of these
diimidazolepyridine derivatives, fluorescence quantum yields
(FQY) of all of the blue materials were evaluated using quinine
sulfate in 0.01 M H2SO4 solution (Φf= 0.55) as the calibration
5

0.22/0.01 eV. And it showed the HOMO/LUMO energy levels of
4f were higher than that of 4a. The results were consistent with
the DFT calculation. And the introduction of electron-
withdrawing groups at the para-position of the aryl rings (4e, 4g)
could decrease the HOMO/LUMO energy levels by 0.27/0.39
and 0.16/0.29 eV, respectively.
Further research about these new compounds used as blue light-
emitting materials is presently under studied in our laboratory.
Acknowledgments
We are grateful for financial support from the Research Project
of the Natural Science Foundation of Heilongjiang Province of
China (No. B201207 and No. B201208).
Specifically, thermodynamic stability requires that the
electrode electrochemical potentials µA and µC (anode and
cathode electrochemical potentials µA and µC (vs Eox and Ered)) are
locating in the window of the electrolyte (µA - µC ≤ Eg). The
reduction potentials (Ered) for the compounds 4a-g from CV
curves were -1.89, -2.03, -1.85, -1.76, -1.68, -1.97 and -1.63 eV,
respectively. Taking the compound 4f as an example: µA - µC =
0.46 -(-1.97) =2.43 ≤ Eg (2.69). Therefore, an anode and a
cathode with their µA and µC values well-matched to the window
of the electrolyte (Eg) could demonstrate these compounds good
thermodynamic stability. [49]
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Seven kinds of diimidazolepyridine derivatives with donor-
accepter-donor systems were designed and synthesized. Their
structures were confirmed by 1HNMR, 13CNMR, HRMS, UV and
IR. Their thermal, optical and electrochemical properties were
investigated, and the results indicated that the photoelectric
characteristics of the target compounds showed dependence on
their molecular structure. For diimidazolepyridine motif as the
main structure, the introduction of biimidazole could improve the
thermal stability of the materials, resulting in Td values exceeding
416 °C, or even up to 490 °C (4c). Moreover, the fusion of
electron acceptor pyridine unit enhanced the intramolecular
charge-transfer and enabled the emission wavelength in the range
of 424-478nm. The introduction of the dibutyl chain also
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transport properties of the electron-acceptor core and achieved
appropriate energy gap (Eg=2.69eV) and high fluorescence
quantum yields (Φf=0.69). Therefore, these compounds have
great potential for electron-transporting materials in OLEDs.
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7

Scheme1. The synthetic routes of compounds 4a-g

Fig.1. Spatial distributions of frontier orbitals of the target compounds at the B3LYP/6-31G(d)
level
Fig.2. TGA graphs of target compounds recorded at a scanning rate of 10 °C /min.

Fig.3. (a) UV-vis absorption in CH2Cl2 solution (10-5 M) of diimidazolepyridine derivatives. (b)
UV-vis absorption in different solution of 4f. (c) UV-vis absorption in solid films. (d) PL spectra
in CH2Cl2 solution (10-5 M). (e) PL spectra in solid films. (f) PL spectra in different solution of 4f.
Fig.4.The emission images at 365 nm UV illumination
Fig.4. The CV curves of compounds 4a-g in the solution of CH2Cl2.

Graphical Abstract
Novel π-conjugated molecules based on
diimidazopyridine: Significantly improved
the photophysical, thermal and electrochemical
properties bearing different aryl substituents
Xin Huang, Jinchang Tian, Feng Xu, Xiaochong Liu, Yuqin Li, Yanyan Guo, Wenyi Chu *, Zhizhong
Sun *

1. Diimidazolepyridine derivatives were designed and successfully synthesized.
2. Diimidazolepyridine motif could improve thermal stability and optical property.
3. Introducing different aryl-substituents to balance charge transfer and prevent to form an "electron trap".
4. These new compounds showed the potential to be used as blue fluorescent materials.

Table 1. Eox, Eg, EHOMO, ELUMO and physical properties of the target compounds
c
d
b
Tde/Tm(°C)
λ
maxAbs (nm)
λmax PL(nm)
Φfh
Band
gapa
HOMO/LUMOa
Eox
EHOMO/ELUMO
(eV)
Compound
Eg
（eV）
(V)
Solutionf
306,391
271,408
251,412
279,415
274,406
281,417
268,404
Filmg
269,403
261,410
256,412
279,414
271,408
278,415
274,405
Solutionf
424
Filmg
528
532
563
567
550
562
531
4a
4b
4c
4d
4e
4f
3.23
3.11
2.89
3.06
2.92
2.99
2.96
-5.27/-2.04
-5.20/-2.09
-5.13/-2.24
-5.17/-2.11
-5.44/-2.52
-4.99/-2.00
-5.41/-2.45
2.90
2.79
2.67
2.72
2.78
2.69
2.77
0.68
0.62
0.57
0.61
0.95
0.46
0.84
-5.48/-2.58
-5.42/-2.63
-5.37/-2.70
-5.41/-2.69
-5.75/-2.97
-5.26/-2.57
-5.64/-2.87
416/181
443/229
490/261
458/252
463/244
484/236
467/213
0.33
0.55
0.56
0.43
0.42
0.69
0.67
462
469
464
445
478
4g
466
[a] DFT/B3LYP calculated values. [b] Optical energy gaps calculated from the edge of the electronic absorption band rate of 100 mV s-1. [c]
Oxidation potential in CH2Cl2 (10-3 M) containing 0.1 M (n-C4H9)4NPF6 with a scan rate of 100 mV s-1. [d] EHOMO was calculated by - (Eox + 4.4), and
ELUMO = - (EHOMO + Eg) [e] Obtained from TGA measurements with a heating rate of 10 °C/min under N2. [f] Measured in a dilute CH2Cl2 solution
(10-5 M). [g] Measured in solid films. [h] Measured in CH2Cl2 with quinine sulfate as the standard.