Functionalized coumarin derivatives containing aromatic-imidazole unit as organic luminescent materials

Article abstract of DOI:10.1016/j.dyepig.2019.107958

Three new coumarin derivatives containing aromatic-imidazole unit (i.e., benzo[d]imidazole, phenanthro[9,10-d]imidazole and pyreno[4,5-d]imidazole), 3-(1-phenyl-1H-benzo[d]imidazole-2-yl)-7-(diethylamino)coumarin (BI–C), 3-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole-2-yl)-7-(diethylamino)coumarin (PI–C) and 3-(9-(4-(tert-butyl)phenyl)-9H-pyreno[4,5-d]imidazole-10-yl)-7-(diethylamino)coumarin (PyI-C), were successfully prepared for organic light-emitting devices. These compounds show good thermal properties and strong emissions from greenish-blue to green lights owing to the extended π-conjugation of the rigid polyaromatic imidazole skeleton, and have high photoluminescence quantum efficiency of 94%, 97% and 98% in chloroform solutions. Among these compounds, the doped organic light-emitting device fabricated from the compound PI-C exhibited the maximum luminous efficiency of 6.07 cd/A and maximum EQE of 2.78%.

Full text of DOI:10.1016/j.dyepig.2019.107958

Journal Pre-proof
Functionalized coumarin derivatives containing aromatic-imidazole unit as organic
luminescent materials
Youjia Wang, Yanmei Li, Tianzhi Yu, Wenming Su, Hailin Ma, Yuling Zhao, Xinyu Li,
Hui Zhang
PII:
S0143-7208(19)32007-8
DOI:
https://doi.org/10.1016/j.dyepig.2019.107958
DYPI 107958
Reference:
To appear in: Dyes and Pigments
Received Date: 27 August 2019
Revised Date: 7 October 2019
Accepted Date: 7 October 2019
Please cite this article as: Wang Y, Li Y, Yu T, Su W, Ma H, Zhao Y, Li X, Zhang H, Functionalized
coumarin derivatives containing aromatic-imidazole unit as organic luminescent materials, Dyes and
Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107958.
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Graphical Abstract:
N
N
N
O
λeOm = 479 nm
λem = 492 nm
λem = 519 nm
BI-C
Φf = 94.04%
Φf = 97.35%
Φf = 97.52%
LEmax = 2.86 cd/A
EQE = 1.59%
LEmax = 6.07 cd/A
EQE = 2.78%
LEmax = 4.80 cd/A
EQE = 2.29%

Functionalized Coumarin Derivatives Containing Aromatic-imidazole
Unit as Organic Luminescent Materials
Youjia Wang‡1, Yanmei Li‡1, Tianzhi Yu1,*, Wenming Su2,*, Hailin Ma1, Yuling Zhao3, Xinyu
Li1, Hui Zhang1
1Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of Education),
Lanzhou Jiaotong University, Lanzhou 730070, China
2Printable electronics research center, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese
Academy of Sciences, Suzhou 215123, China
3School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070,
China
Abstract: Three new coumarin derivatives containing aromatic-imidazole unit (i.e.,
benzo[d]imidazole,
phenanthro[9,10-d]imidazole
and
pyreno[4,5-d]imidazole),
(BI-C),
3-(1-phenyl-1H-benzo[d]imidazol-2-yl)-7-(diethylamino)coumarin
3-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-7-(diethylamino)coumarin (PI-C)
and 3-(9-(4-(tert-butyl)phenyl)-9H-pyreno[4,5-d]imidazol-10-yl)-7-(diethylamino)coumarin
(PyI-C), were successfully prepared for organic light-emitting devices. These compounds show
good thermal properties and strong emissions from greenish-blue to green lights owing to the
extended π-conjugation of the rigid polyaromatic imidazole skeleton, and have high
photoluminescence quantum efficiency of 94%, 97% and 98% in chloroform solutions. Among
these compounds, the doped organic light-emitting device fabricated from the compound PI-C
* Corresponding Authors.
Tianzhi Yu, E-mail: yutzh@mail.lzjtu.cn; Wenming Su, E-mail: wmsu2008@sinano.ac.cn
‡
These authors are contributed equally
1

exhibited the maximum luminous efficiency of 6.07 cd/A and maximum EQE of 2.78%.
Key words: Coumarin derivative; Rigid polyaromatic imidazole derivative; Photoluminescence;
Electroluminescence
1. Introduction
Coumarin derivatives have attracted attentions owing to the relative ease of synthesis and
their outstanding optical properties, which exhibit high fluorescence quantum yields, large Stokes
shifts, excellent photostability and thermal stability. Coumarin derivatives have been often used as
fluorescent dyes [1], fluorescent probes [2-5], and optical functional materials for organic
[6-10]
[11-14]
light-emitting devices (OLEDs)
and dye-sensitized solar cells
for decades. Since
coumarin 540 (3-(2-benzothiazolyl)-7-diethylaminocoumarin) was firstly used as an efficient
organic emitting material in OLEDs [6], and many synthetic coumarin derivatives have
successfully been applied in OLEDs [7,8,15-22]. However, the photophysical properties of coumarin
derivatives strongly rely upon their molecular structures [23-26]. Coumarin itself shows a very weak
blue emission with peak at 430 nm [27], whereas its derivative attached an electron-donating
substituent (most commonly an amine group or a hydroxyl group) on 7-position of coumarin
skeleton can exhibit strong blue-green emission due to the enhancement of intramolecular charge
transfer. In particular, an electron-donating group and an electron-withdrawing group were
attached on 7- and 3-position of coumarin ring, respectively, forming a push-pull electron effect,
the fluorescent properties of coumarins are greatly enhanced [28]. However, there are the
concentration-quenching effect (CQE) or the aggregation-caused quenching (ACQ), coumarin
derivatives exhibit usually weak emission, even their fluorescence is quenched in the concentrated
2

solution and solid state. Thus, as emitting materials coumarin derivatives are usually doped in host
materials at appropriate concentration to fabricate OLEDs [15-20]. For reducing the self-aggregation
quenching of coumarin derivatives at higher concentration or in the solid state, the grafting of the
sterically hindered substituents to the coumarin skeleton is an effective method for highly efficient
OLEDs [7,21]
.
The aromatic π‒conjugated imidazole moiety is an electron-deficient group, which has been
widely employed in the electron-transporting materials and as the electron-withdrawing group of
bipolar molecular [29-31]. Recently, the aromatic π‒conjugated imidazole moieties have often been
used to construct some organic optoelectronic functional materials for OLEDs due to their
ambipolar characteristics, high photoluminescence quantum yield, high stability, high triplet
energy and pure blue emission [32-38]. In this work, three new coumarin derivatives with D‒π‒A
structure were synthesized by grafting the aromatic π‒conjugated imidazole moieties
(benzo[d]imidazole (BI), phenanthro[9,10-d]imidazole (PI) and pyreno[4,5-d]imidazole (PyI)) as
an electron-withdrawing groups onto the 3-position of coumarin ring and linking diethylamine as
an electron-donating group to the 7-position of coumarin ring (Scheme 1). By introducing the
different conjugated aromatic imidazole moieties into the coumarin derivatives, we attempt to
explore the relationship between the structure and properties of these compounds.
2. Experimental
2.1 Materials and methods
4-(Diethylamino)-2-hydroxybenzaldehyde, 2-aminodiphenylamine, phenanthrene-9,10-dione,
pyrene and 4-tert-butylaniline were obtained from Energy Chemical (China). All the other
3

reactants and solvents were obtained from commercial sources.
The preparation of the intermediate pyrene-4,5-dione was described in our previous paper [39]
.
NMR spectra were measured on Bruker 500, AVANCE NEO. Mass spectra were recorded
using a Thermo Scientific Orbitrap Elite mass spectrometer. C, H, and N analyses were obtained
using an Elemental Vario-EL automatic elemental analysis instrument. Thermogravimetric
analysis (TGA) was performed on a PerkinElmer Pyris system. UV-vis absorption spectra were
recorded on a Shimadzu UV-2550 spectrometer. The cyclic voltammograms were performed on a
electrochemical analyzer (CHI Instruments 760 B). The photoluminescence quantum yield was
measured by an absolute method using the Edinburgh Instruments FLS920 integrating sphere with
the Xe lamp at room temperature. The photoluminescence decay lifetime was measured by a
time-correlated single photon counting spectrometer using Edinburgh Instruments FLS920 with a
microsecond flashlamp as the excitation source (repetition rate 90 Hz) at room temperature.
2.2 Synthesis of the coumarin derivatives
7-Diethylamino-coumarin:
Diethyl
malonate
(5.95
g,
0.037
mol)
and
4-(diethylamino)-2-hydroxybenzaldehyde (6.00 g, 0.031 mol) were dissolved in absolute ethyl
alcohol (100 ml), and then piperidin (1 mL) was added stepwise under ice bath. Under N2, the
o
reaction mixture was refluxed at 80 C for 6 h. After evaporating solvent in vacuum, 40 mL of
concentrated HCl/glacial acetic acid (1:1, v/v) was added into the reaction mixture. The reaction
o
solution was continued to stir for 6 h at 120 C. After cooling to room temperature, the resulting
mixture was poured into 100 mL of water and neutralized with sodium hydroxide solution (40%)
until the pH to 7. The greynish-white precipitate was filtered and recrystallized from toluene to
obtain 6.25 g (93%) of 7-diethylamino-coumarin. 1H NMR (500 MHz, CDCl3, δ): 7.51 (d, J = 8.8
4

Hz, 1H), 7.22 (d, J = 8.4 Hz, 1H), 6.54 (d, J = 8.4 Hz, 1H), 6.46 (s, 1H), 6.01 (d, J = 8.8 Hz, 1H),
3.39 (q, J = 7.1 Hz, 4H), 1.19 (t, J = 7.1 Hz, 6H).
7-(Diethylamino)coumarin-3-carbaldehyde: Under N2, anhydrous DMF (13.30 mL, 172.5
mmol) was dropped into POCl3 (3.16 ml, 34.5 mmol) with stirring for 6 h in an ice bath. The
solution of 7-diethylamino-coumarin (3.00 g, 13.8 mmol) in anhydrous 1,2-dichloroethane was
o
added to the above solution, and the mixture was stirred at 60 C for 12 h. After completing, the
mixture was poured into ice water and neutralized with NaOH solution (20%) to pH 7. The formed
precipitate was filtered off and washed three times with water. The residue was chromatographed
1
on silica, eluting with petroleum ether/CH2Cl2 (2:1, v/v) to form orange solid (2.57 g, 76%).
HNMR (500 MHz, CDCl3, δ): 10.10 (s, 1H, -CHO), 8.25(s, 1H), 7.41 (d, J = 8.0 Hz, 1H), 6.64 (d,
J = 8.0 Hz, 1H), 6.49 (s, 1H), 3.48 (q, J = 8.2 Hz, 4H), 0.96 (t, J = 8.2 Hz, 6H).
BI-C: Under N2, a mixture of 7-(diethylamino)coumarin-3-carbaldehyde (0.80 g, 3.26 mmol),
o
2-aminodiphenylamine (0.60 g, 3.26 mmol) and 2-ethoxyethanol (60 mL) was refluxed at 105 C
for 12 h. After completing, the reaction mixture was poured into water and extracted with
dichloromethane, and the organic phase was dried over anhydrous Na2SO4. After removal of
solvent, the residue was purified by silica gel column chromatography using ethyl
acetate/petroleum ether (1:1, v/v) as eluent. The product as faint yellow solid was obtained (0.81 g,
61%). m.p. > 260 oC. 1H NMR (500 MHz, CDCl3, δ): 8.16 (s, 1H), 7.86 (d, J = 8.5 Hz, 1H), 7.47
(t, J = 8.8 Hz, 2H), 7.41-7.37 (m, 3H), 7.34 (t, J = 8.8 Hz, 3H), 7.28 (d, J = 7.5 Hz, 1H), 6.59 (d, J
= 8.8 Hz, 1H), 6.42 (s, 1H), 3.42 (q, J = 7.2 Hz, 4H), 1.21 (t, J = 7.2 Hz, 6H). 13C NMR (126 MHz,
CDCl3, δ): 159.11, 157.39, 151.49, 149.14, 146.34, 142.72, 136.95, 136.64, 129.85, 129.48,
128.21, 126.35, 123.49, 122.83, 119.76, 111.162, 110.45, 109.18, 108.20, 97.12, 45.01, 29.50.
5

HRMS: Calcd. for C26H23N3O2, 410.1863 [M+H]+; Found: 410.1865. Anal. Calcd. for C26H23N3O2:
C, 76.26; H, 5.66; N, 10.26. Found: C, 76.38; H, 5.63; N, 10.30.
PI-C: A mixture of 9,10-Phenanthraquinone (0.90 g, 4.32 mmol), 4-tert-butylaniline (0.80 g,
5.36 mmol), 7-(diethylamino)coumarin-3-carbaldehyde (1.06 g, 4.32 mmol) and NH4OAc (4.0 g,
o
51.89 mmol) with 120 mL of AcOH was stirred at 120 C for 12 h under nitrogen atmosphere.
After cooling down, the reaction mixture was poured into water and extracted with
dichloromethane (3 × 100 mL). The organic phase was washed with water (2 × 100 mL) and dried
over anhydrous Na2SO4. After removal of solvent, the residue was purified by silica gel column
chromatography using ethyl acetate/petroleum ether (1:1, v/v) as eluent to obtain yellow solid
(1.21 g, 50%). m.p. > 260 oC. 1H NMR (500 MHz, CDCl3, δ): 8.80 (d, J = 8.0 Hz, 1H), 8.75 (d, J
= 8.4 Hz, 1H), 8.71 (dd, J = 8.8 Hz, 2H), 7.94 (s, 1H), 7.74-7.62 (m, 4H), 7.50 (dd, J = 8.8 Hz,
3H), 7.15 (d, J = 8.5 Hz, 1H), 6.67 (d, J = 8.8 Hz, 1H), 6.57 (d, J = 8.8 Hz, 1H), 6.43 (s, 1H), 3.42
(q, J = 7.2 Hz, 4H), 1.39 (s, 9H), 1.21 (t, J = 7.2 Hz, 6H). 13C NMR (126 MHz, CDCl3, δ): 160.16,
157.24, 152.69, 151.34, 147.61, 146.31, 144.32, 137.26, 135.12, 130.39, 129.66, 129.27, 128.44,
128.21, 127.23, 127.20, 126.43, 126.23, 125.44, 124.93, 123.96, 123.66, 123.38, 123.25, 123.10,
122.65, 121.26, 121.01, 111.98, 109.66, 108.96, 108.15, 97.07, 44.87, 34.70, 31.21, 29.80. HRMS:
Calcd. for C38H35N3O2, 566.2802 [M+H]+; Found: 566.2816. Anal. Calcd. for C38H35N3O2: C,
80.68; H, 6.24; N, 7.43. Found: C, 80.81; H, 6.21; N, 7.46.
PyI-C: The synthetic procedure of the compound PyI-C was similar to that of the compound
1
PI-C. Yield: 50% (1.35 g). m.p.: > 260 . H NMR (500 MHz, CDCl3, δ): 9.04 (d, J = 7.5 Hz,
1H), 8.17 (d, J = 7.5 Hz, 1H), 8.12 (d, J = 9.0 Hz, 1H), 8.09 (d, J = 8.5 Hz, 1H), 8.03 (m, 3H),
7.64 (t, J = 8.0 Hz, 1H), 7.56 (d, J = 8.0 Hz, 4H), 7.38 (d, J = 7.5 Hz, 1H), 7.29 (d, J = 8.8 Hz, 1H),
6

6.57 (d, J = 8.5 Hz, 1H), 6.44 (s, 1H), 3.41 (q, J = 7.0 Hz, 4H), 1.42 (s, 9H), 1.21 (t, J = 6.5 Hz,
6H). 13C NMR (126 MHz, CDCl3, δ): 160.19, 157.24, 152.77, 151.33, 147.84, 146.40, 137.78,
135.11, 132.07, 131.67, 129.68, 129.10, 128.53, 127.85, 127.53, 126.45, 126.39, 126.31, 125.29,
124.37, 124.31, 123.53, 122.80, 122.42, 119.65, 118.12, 111.90, 108.96, 108.14, 97.03, 44.86,
34.99, 31.23, 12.73. HRMS: Calcd. for C40H35N3O2, 590.2803 [M+H]+; Found: 590.2778. Anal.
Calcd. for C40H35N3O2: C, 81.47; H, 5.98; N, 7.13. Found: C, 81.26; H, 6.02; N, 7.11.
3. Results and discussion
3.1 Synthesis
NH2
CHO
H
N
N
N
2-Ethoxyethan-1-ol
105 oC, 12 h
N
+
N
O
O
O
O
BI-C 61%
Scheme 1. Synthetic routes to the coumarin derivatives.
The synthetic procedures of the coumarin derivatives (BI-C, PI-C and PyI-C) are illustrated
7

in Scheme 1. The key intermediate, 7-(diethylamino)coumarin-3-carbaldehyde, were obtained
through two step reactions from 4-(diethylamino)-2-hydroxybenzaldehyde. First, the intermediate
7-diethylamino-coumarin was prepared from Knoevenagel condensation reaction of
4-(diethylamino)-2-hydroxybenzaldehyde with diethyl malonate, and then the subsequent
Vilsmeier-Haack formylation of 7-diethylamino-coumarin in 1,2-dichloroethane produced
7-(diethylamino)coumarin-3-carbaldehyde in 76% yield. The compound BI-C was synthesized
from 2-aminodiphenylamine and 7-(diethylamino)coumarin-3-carbaldehyde in 2-ethoxyethanol as
faint yellow solid with moderate yield (61%) after purification by column chromatography. The
compounds PI-C and PyI-C were synthesized by one-pot multicomponent reaction of the
corresponding diketone (phenanthrene-9,10-dione or pyrene-4,5-dione), ammonium acetate and
7-(diethylamino)coumarin-3-carbaldehyde with acetic acid as the medium, and their yields are
about 50% after purification by column chromatography. The structures of the compounds were
1
confirmed through H, 13C NMR, high resolution mass spectra and elemental analyses. All the
compounds have good solubility in common organic solvents, such as THF, dichloromethane,
chloroform and toluene. The detailed synthetic procedures and analyses were described in
experimental section.
3.2 Thermal stabilities, photophysical and electrochemical properties
The thermal stability of the functional materials is crucial to achieving highly stable device
performance. Thus, the thermal properties of BI-C, PI-C and PyI-C were characterized by
thermogravimetric analysis (TGA) under a N2 atmosphere. As shown in Fig. 1, these compounds
o
exhibited excellent thermal stability (> 330 C), and the thermal decomposition temperatures (Td)
of the compounds increase gradually with increasing the conjugation degree of the aromatic π‒
8

conjugated imidazole moiety. The thermal decomposition temperatures of BI-C, PI-C and PyI-C
o
are about 343, 384 and 412 C at 2% loss of initial weight, respectively, and a sharp weight loss
appeared in their TGA curves, indicating that the compounds BI-C, PI-C and PyI-C underwent
one large-stage decomposition process.
100
80
60
40
BI-C
PI-C
20
PyI-C
0
100
200
300
400
500
600
Temperature (oC)
Fig. 1. Thermogravimetric analyses (TGA) of BI-C, PI-C and PyI-C in
nitrogen atmosphere (heating rate: 10 °C/min).
Solution
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
BI-C (UV-vis)
PI-C (UV-vis)
PyI-C (UV-vis)
BI-C (PL)
PI-C (PL)
PyI-C (PL)
300
400
500
600
700
Wavelength (nm)
Fig. 2. UV–vis absorption and photoluminescence spectra of the compounds
BI-C, PI-C and PyI-C in dichloromethane solutions (C = 1.0 × 10−5 mol/L).
Figure 2 shows the UV-vis absorption and photoluminescence spectra of the compounds
BI-C, PI-C and PyI-C at room temperature in CH2Cl2 solutions, and the related photophysical
data are summarized in Table 1. The compounds BI-C and PI-C have similarly shaped absorption
9

spectra in the range of 300‒550 nm, and the most obvious absorption bands are at 410 nm. This
absorption band is assigned to the π‒π* transition of the characteristic vibrational structure of
7-diethylamino-coumarin moiety [17,40]. Besides, there is a very small absorption peak at 289 nm in
the absorption spectrum of the compound PI-C, which should be assigned to π‒π* transition of
phenanthro[9,10-d]imidazole (PI) unit. However, the compound PyI-C not only exhibits the
stronger absorption band at 410 nm, but also two weaker absorption bands at 289 and 351 nm, and
the absorption peaks at 289 and 351 nm could be ascribed to π‒π* transition of
pyreno[4,5-d]imidazole (PyI) unit. From their absorption spectra, the absorption bands of the
aromatic π‒conjugated imidazole moieties increase gradually increasing the conjugation degree of
the aromatic π‒conjugated imidazole moiety. Upon excitation at 410 nm, the maximum emission
peaks of BI-C, PI-C and PyI-C in CH2Cl2 solutions were observed at 479, 492 and 519 nm,
respectively. Compared with BI-C, the compounds PI-C and PyI-C exhibit bathochromic shifts of
13 and 40 nm because of increasing conjugation by the aromatic imidazole moieties.
Additionally, the fluorescence quantum yields (Φf) of BI-C, PI-C and PyI-C were measured
to be 94, 97 and 98% in CH2Cl2 solutions at room temperature, respectively, and their
fluorescence decay lifetimes (τ) were measured to be 13.2, 12.6 and 12.8 ns. The results show that
these compounds have higher Φf and belong to the fluorescent emission. Furthermore, we
approximately estimated the radiative rate constants (kr) of BI-C, PI-C and PyI-C to be 7.12 × 107,
7.73 × 107 and 7.62 × 107 s-1, and the nonradiative rate constants (knr) to be 4.52 × 106, 2.10 × 106
and 1.94 × 106 s-1, respectively, according to the equations: kr = Φf/τ and knr = (1-Φf)/τ, assuming
that ΦISC = 1 (ISC = intersystem crossing) [41]
.
The absorption and photoluminescence (PL) spectra of the compounds in the solid films are
10

similar to those in the solutions, but their absorption and PL spectra in the solid films shift red (Fig.
3). The maximum absorption bands of the compounds in solid films were at 418 nm, which were
red-shifted by about 8 nm. The emission peaks of BI-C, PI-C and PyI-C located at 520, 535 and
553 nm, which were red-shifted by 41, 43 and 34 nm, respectively. However, the fluorescence
quenching of the compounds is very serious in the solid states, and the fluorescence quantum
yields of BI-C, PI-C and PyI-C in the thin films were measured to be 6.5, 14 and 12% at room
temperature, respectively.
Thin film
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
BI-C (UV-vis)
PI-C (UV-vis)
PyI-C (UV-vis)
BI-C (PL)
PI-C (PL)
PyI-C (PL)
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. UV–vis absorption and photoluminescence spectra of the compounds
BI-C, PI-C and PyI-C in the solid films.
The electrochemical properties of these compounds were investigated through cyclic
voltammetry (CV) at 10 mV/s scan rate in an electrolyte solution of 0.1 M Bu4NPF6 dissolved in
CH2Cl2. Fig. 4 gives the cyclic voltammograms of the compounds BI-C, PI-C and PyI-C. Onset
oxidation potentials (Eonset) of the compounds are at ca. 1.47 V, thus, the HOMO energy levels of
the compounds BI-C, PI-C and PyI-C were calculated to be -5.74 eV by the equation [17]: HOMO
= -e(Eonset -0.53 + 4.8) eV, in which 0.53 eV was the Fc/Fc+ potential measured at the same
condition. The optical band edges from the absorption spectra of these compounds in CH2Cl2
11

solutions were about 490, 460 and 492 nm for BI-C, PI-C and PyI-C, respectively, which
correspond to be 2.53, 2.70 and 2.52 eV from the equation: Eopt = 1241/λonset. The LUMO energy
levels of these compounds are -3.21, -3.04 and -3.22 eV from the equation of LUMO = (HOMO +
Eopt) eV.
BI-C
PI-C
PyI-C
2
Ferrocene
1
0
-1
-2
-2
-1
0
1
2
Potential (V, vs. Ag/Ag+
)
Fig. 4. Cyclic voltammograms of ferrocene and the compounds BI-C, PI-C
and PyI-C (scan rate: 10 mV/s; solvent: dichloromethane).
Table 1. Photophysical, thermal and electrochemical properties of the compounds
Compound
UV-vis (nm)
(ε (× 105 mol-1 dm3 cm-1))
410 (3.6)
PL
Td
Φf
(%)
94
τ
kr/knr
EOX
(V)
Eopt
HOMO/LUMO
(eV)
(nm) (oC)
(ns)
13.2
12.6
(× 106 s-1)
71.24/4.52
77.26/2.10
(eV)
2.53
2.70
479
492
343
384
1.47
1.47
-5.74/-3.21
-5.74/-3.04
BI-C
PI-C
289 (1.6), 410 (5.9)
97
289 (3.2), 351 (1.8), 410 (2.9) 519
412
98
11.4
76.19/1.94
1.47
2.52
-5.74/-3.22
PyI-C
The structures of the compounds were optimized by density functional theory (DFT) using a
B3LYP/6-311G(d) basis set to calculate the frontier molecular orbitals of the compounds. The
electronic distributions of the HOMO and LUMO orbitals for the compounds obtained from
time-dependent DFT (TD-DFT) calculation are depicted in Fig. 5. In the HOMO orbital of BI-C,
the electron density completely distributes on the benzoimidazole and 7-(diethylamino)coumarin
skeletons, and there is no electron density on the phenyl ring linked to the 2-position of
12

benzoimidazole unit, whereas in its LUMO orbital, most of the electron density resides on the
coumarin unit and small part of electron density locates on the benzoimidazole skeleton. For the
compounds PI-C and PyI-C, the electron densities in the HOMO orbitals reside significantly on
the phenanthro[9,10-d]imidazole or pyreno[4,5-d]imidazole skeleton, and small part of electron
densities locate on the phenyl ring of (tert-butyl)phenyl groups, whereas there are no electron
densities on the 7-(diethylamino)coumarin skeletons. The electron density in the LUMO orbital of
PI-C completely locates on the phenanthro[9,10-d]imidazole skeleton, and most of the electron
density in the LUMO orbital of PyI-C locates on the pyreno[4,5-d]imidazole skeleton and very
small part of electron densities locate on the phenyl ring of (tert-butyl)phenyl group.
LUMO (-3.34 eV)
LUMO (-1.85 eV)
LUMO (-3.85 eV)
HOMO (-5.34 eV)
HOMO (-5.45 eV)
HOMO (-4.54 eV)
BI-C
PI-C
PyI-C
Fig. 5 Frontier orbitals of the compound A and B calculated at the DFT
CAM-B3LYP/6-31+G* level of theory.
3.3 Electroluminescence properties of the compounds
In the light of the excellent thermal stability and strong emission of these compounds in
dilute solution, we used these compounds as the emissive materials to fabricate doped OLEDs by
vacuum-processed method. The device structures were constructed as: ITO/TAPC (30
13

nm)/CBP:Dopant (x wt%, 35 nm)/TPBi (30 nm)/Liq (2 nm)/Al (100 nm), in which the
codeposited mixture of CBP and the compounds (BI-C, PI-C and PyI-C) was served as the
emitting layer, TAPC as the hole-transporting layer, TPBi as the electron transporting and the hole
blocking layer, and Liq was used as the electron injection layer. The doped device structure and
the energy level diagrams are depicted in Fig. 6.
Fig. 6. Schematic diagram of the OLED device configuration (a), the relative
HOMO/LUMO energy levels of the materials investigated in this work (b) and
structures of TAPC, CBP and TPBi (bottom).
The current density-voltage-luminance (J‒V‒L) characteristics and the EL efficiency curves
for these compounds are shown in Fig. 7, Fig. 8 and Fig. 9, and their EL performances are
summarized in Table 2. As shown in Fig. 7, at 6 wt% doping concentration of BI-C, the device
exhibited a maximum current efficiency (CE) of 2.86 cd/A, a power efficiency (PE) of 0.91 lm/W
and a maximum external quantum efficiency (EQE) of 1.59%, but a maximum brightness of 1149
cd/m2 at 12.5 V was observed for the device at 10 wt% doping concentration. As shown in Fig. 8,
when the doping concentration of PI-C was 12 wt%, the maximum luminance of the fabricated
14

device was 2421 cd/m2, and the device showed the best EL performance: a maximum CE of 6.07
cd/A, maximum PE of 4.77 lm/W and maximum EQE of 2.78%. From Fig. 9 and Table 2, it is
found that the device with 12 wt% doping concentration of PyI-C exhibited a maximum
luminance of 2598 cd/m2, maximum CE of 4.80 cd/A, maximum PE of 1.35 lm/W and maximum
EQE of 2.29%.
10
1
500
400
300
200
100
0
6 wt%
8 wt%
10 wt%
12 wt%
B
1000
100
10
100
10
0.1
0.01
1E-3
1E-4
1E-5
1
0.1
0.01
1
6 wt%
8 wt%
10 wt%
12 wt%
A
0.1
1
10
100
1000
6
8
10
12
14
16
Voltage (V)
Luminance (cd/m2)
Fig. 7. (A) Luminance-voltage-current density (L‒V‒J) characteristics and (B)
current/power efficiency versus luminance curves of the devices based on
BI-C with different dopant concentrations.
700
600
500
400
300
200
100
0
8 wt%
10
10000
1000
100
10
10 wt%
12 wt%
14 wt%
100
10
1
0.1
1
0.01
1E-3
1E-4
1
8 wt%
0.1
10 wt%
12 wt%
14 wt%
0.1
B
A
0.01
0.01
10000
6
8
10
12
14
16
1
10
100
1000
Luminance (cd/m2)
Voltage (V)
Fig. 8. (A) Luminance-voltage-current density (L‒V‒J) characteristics and (B)
current/power efficiency versus luminance curves of the devices based on PI-C
with different dopant concentrations.
15

101
100
103
102
101
100
10-1
10-2
3x102
2x102
1x102
0
8 wt%
12 wt%
16 wt%
103
102
101
100
10-1
10-1
10-2
10-3
8 wt%
12 wt%
16 wt%
A
B
8
10
12
14
16
18
20
1
10
100
(
1000
Luminance
cd/m2
)
Voltage (V)
Fig. 9. (A) Luminance-voltage-current density (L‒V‒J) characteristics and (B)
current/power efficiency versus luminance curves of the devices based on
PyI-C with different dopant concentrations.
Table 2. EL performances of the compounds BI-C, PI-C and PyI-C
Compound
Concentration ELmax
(nm)
Lmax
(cd/m2)
984
LEmax
(cd/A)
2.86
2.39
2.15
0.32
2.42
3.23
6.07
3.17
2.02
4.80
3.81
PEmax
(lm/W)
0.91
0.78
0.64
0.07
1.52
2.54
4.77
2.21
0.48
1.35
1.32
EQEmax
(%)
BI-C
6 wt%
8 wt%
479
479
480
481
492
492
494
494
519
519
521
1.59
1.29
1.15
0.16
2.20
2.17
2.78
2.52
0.75
2.29
1.67
1112
1149
738
10 wt%
12 wt%
8 wt%
PI-C
1710
2160
2421
2304
2320
2598
2109
10 wt%
12 wt%
14 wt%
8 wt%
PyI-C
12 wt%
16 wt%
Among the three compounds, PI-C showed better EL performances than the compounds
BI-C and PyI-C, which is consistent with their radiative rate constants of photoluminescence.
Furthermore, the aromatic π‒conjugated imidazole groups have bipolar characteristics, which can
be used as either electron-acceptor group or electron-donor group [34,35,42,43]. Especially,
phenanthroimidazole (PI) shows electron-donating properties when a nature or an
electron-withdrawing group is attached to the 2-position of the imidazole ring, but it exhibits
16

electron-withdrawing properties when linked with an electron donor. The order of conjugated
degree of the aromatic π‒conjugated imidazole groups involved in this paper is as follows: PyI >
PI >BI. Compared with PI, BI moiety is more likely to be used as the electron-acceptor [43,44], and
PyI unit may prefer to be as electron-donor. Thus, the devices based on the compound PI-C may
have better carrier balance, which makes the device performance better. In addition,
4-tert-butylphenyl group substituted phenyl group was attached to the 1-position of the imidazole
ring in the compounds PI-C and PyI-C, so the steric hindrance of the compounds PI-C and PyI-C
is greater than that of the compound BI-C, suggesting that the intermolecular interaction of PI-C
or PyI-C is smaller than that of BI-C. Therefore, the fluorescence quantum yield (Φf) and the
device performance of the compound BI-C are lower than those of the compounds PI-C and
PyI-C.
BI-C (6 wt%)
PI-C (12 wt%)
PyI-C (12 wt%)
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Fig. 10. EL spectra of the devices based on the compounds with maximum
luminous efficiencies.
Under the applied voltages, the devices fabricated from these compounds exhibited bright
blue-green to green emission with emission peaks at around 479‒521 nm (Fig. 10). The
electroluminescence (EL) peaks of these compounds are basically consistent with their
17

photoluminescence (PL) peaks in solutions, indicating that the EL originated from their
corresponding emitting layer and the charge recombination occurs in the emitting layer. However,
in the EL spectra of PI-C and PyI-C, there is a shoulder peak at longer wavelength resulting from
the excimer and exciplex species formed at the interfaces, which also contributed to the emission.
4. Conclusions
Three aromatic-imidazole-based coumarin derivatives (BI-C, PI-C and PyI-C) were
designed and prepared for OLEDs, in which the aromatic π‒conjugated imidazole moieties
(benzo[d]imidazole (BI), phenanthro[9,10-d]imidazole (PI) and pyreno[4,5-d]imidazole (PyI)) as
electron-withdrawing groups were grafted onto the 3-position of 7-(diethylamino)coumarin. These
compounds exhibited strong blue-green or green emissions with high Φf in CH2Cl2 solutions as
o
well as good thermal stability (Td > 340 C). Among these compounds, the compound PI-C
showed better PL and EL properties. The fabricated devices based on PI-C exhibited the best EL
performance with the maximum luminance of 2421 cd/m2, maximum CE of 6.07 cd/A and
maximum EQE of 2.78%.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant 51563014).
We also thank to the Instrument Analysis Center of Lanzhou Jiaotong University.
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24

Research Highlights
1. Three aromatic-imidazole-based coumarin derivatives were synthesized for OLEDs.
2. They exhibited strong blue-green or green emissions with high photoluminescence quantum
yields in dichloromethane solutions.
3. The devices based on PI-C exhibited the brightness of 2421 cd/m2 and luminous efficiencies
of 6.07 cd/A, and EQE of 2.78%.

Electronic Supplementary Information
Functionalized CoumarinDerivatives Containing Aromatic-imidazole
Unit as Organic Luminescent Materials
Youjia Wang‡1, Yanmei Li‡1,Tianzhi Yu1,*,Wenming Su2,*, Hailin Ma1, Yuling Zhao3,
Xinyu Li1,Hui Zhang1
1Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of
Education), Lanzhou Jiaotong University, Lanzhou 730070, China
2Printable electronics research center, Suzhou Institute of Nano-Tech and Nano-Bionics,
Chinese Academy of Sciences, Suzhou 215123, China
3School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou
730070, China
*Corresponding Authors.
Tianzhi Yu, E-mail: yutzh@mail.lzjtu.cn; Wenming Su, E-mail: wmsu2008@sinano.ac.cn
‡
These authors are contributed equally

Fig. S1. 1H NMR spectrumof BI-C
Fig. S2. 13C NMR spectrumof BI-C

Fig. S3. 1H NMR spectrumof PI-C
Fig. S4. 13C NMR spectrumof PI-C