Novel imidazole-alkyl spacer-carbazole based fluorophores for deep-blue organic light emitting diodes: Experimental and theoretical investigation

Article abstract of DOI:10.1016/j.dyepig.2020.108853

A series of fluorescent emitters based on Imidazole-Alkyl spacer-Carbazole were designed and synthesized by connecting donor and acceptor moieties through alkyl spacer. In these design strategy we have chosen carbazole as electron rich and imidazole as electron deficient with deferent functional black (phenyl and phenyl substituted) at the N1 position of imidazole moiety. All the fluorophores were structurally confirmed by spectroscopic methods (NMR, MASS, and FTIR). The photophysical, electrochemical and electroluminescence properties were thoroughlyexamined. All the fluorophores showed good thermal stability and near-UV emission with high photoluminescence quantum yield (PLQY)of ~59% in the solid state. All the synthesized compounds feature Commission International de L'Eclairage (CIE) coordinates of y < 0.08 for a particular doping concentration, which is very close to the National Television Standards Committee (NTSC) standard blue (0.14, 0.08). Solution-processed organic light emitting diode (OLED) device consisting of molecule BIPOCz displays a maximum external quantum efficiency EQE of 1.9% with CIE coordinates of (0.17, 0.07). All these results reveal that this work provides novel approaches for realizing ultra-deep-blue emission with high performance.

Full text of DOI:10.1016/j.dyepig.2020.108853

Journal Pre-proof
Novel Imidazole-Alkyl Spacer-Carbazole based Fluorophores for Deep-Blue Organic
Light Emitting Diodes: Experimental and Theoretical Investigation
Jairam Tagare, Rajamouli Boddula, Rohit Ashok Kumar Yadav, Deepak Kumar
Dubey, Jwo-Huei Jou, Sabita Patel, Sivakumar Vaidyanathan
PII:
S0143-7208(20)31550-3
DOI:
https://doi.org/10.1016/j.dyepig.2020.108853
DYPI 108853
Reference:
To appear in: Dyes and Pigments
Received Date: 21 June 2020
Revised Date: 25 August 2020
Accepted Date: 11 September 2020
Please cite this article as: Tagare J, Boddula R, Kumar Yadav RA, Dubey DK, Jou J-H, Patel S,
Vaidyanathan S, Novel Imidazole-Alkyl Spacer-Carbazole based Fluorophores for Deep-Blue Organic
Light Emitting Diodes: Experimental and Theoretical Investigation, Dyes and Pigments, https://
doi.org/10.1016/j.dyepig.2020.108853.
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Author Statement:
The design, synthesis, photophysical and computational study were executed by JT and RB
under the supervision of SP and VS. The OLED device fabrication and characterization, and data
analysis were performed by RAKY and DKD under the supervision of JHJ.
All the authors read the manuscript and approved it for publications.

Highlights:
‹
A series of fluorescent emitters based on Imidazole-Alkyl spacer-Carbazole were
designed and synthesized.
‹
All the fluorophores showed good thermal stability and near-UV emission with high
PLQY of ~59 % in the solid state.
‹
‹
HOMO and LUMO separation endows them with good carrier transporting property.
Solution-processed OLED device consisting of molecule BIPOCz displays a maximum
EQE of 1.9% with CIE coordinates of (0.17, 0.07).

The authors declare that there is no conflicts of interest

Novel Imidazole-Alkyl Spacer-Carbazole based Fluorophores for Deep-Blue
Organic Light Emitting Diodes: Experimental and Theoretical Investigation
a
a
b
b
Jairam Tagare Rajamouli Boddula ,Rohit Ashok Kumar Yadav , Deepak Kumar Dubey , Jwo-
,
b
a
a
Huei Jou , Sabita Patel and Sivakumar Vaidyanathan *
a
Department of Chemistry, National Institute of Technology Rourkela, India
b
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu
0013, Taiwan R.O.C.
3
*
To whom correspondence should be addressed. Email: vsiva@nitrkl.ac.in (V. Sivakumar) Tel:
91-661-2462654
+
1

Abstract
A series of fluorescent emitters based on Imidazole-Alkyl spacer-Carbazole were designed and
synthesized by connecting donor and acceptor moieties through alkyl spacer. In these design
strategy we have chosen carbazole as electron rich and imidazole as electron deficient with
deferent functional black (phenyl and phenyl substituted) at the N1 position of imidazole moiety.
All the fluorophores were structurally confirmed by spectroscopic methods (NMR, MASS, and
FTIR). The photophysical, electrochemical and electroluminescence properties were
thoroughlyexamined. All the fluorophores showed good thermal stability and near-UV emission
with high photoluminescence quantum yield (PLQY)of ~59 % in the solid state. All the
synthesized compounds feature Commission International de L’Eclairage (CIE) coordinates of
y<0.08 for a particular doping concentration, which is very close to the National Television
Standards Committee (NTSC) standard blue (0.14, 0.08). Solution-processed organic light
emitting diode (OLED) device consisting of molecule BIPOCz displays a maximum external
quantum efficiency EQE of 1.9% with CIE coordinates of (0.17, 0.07). All these results reveal
that this work provides novel approaches for realizing ultra-deep-blue emission with high
performance.
Keywords: Deep-blue,Carbazole, Imidazole, DFT, Photoluminescence, OLED.
2

Introduction
Since first OLEDdevice reported by Tang et al. in 1987[1], OLEDs are received immense
interest due to their numerous advantages such as high efficiency, bright luminescence, low
power consumption, fast switching, ultrathin, flexibility and broad viewing angle.[2-6] To get an
efficienthigh-performancefull-color display (white OLEDs), deep-blue OLEDs urgently desired.
After the pioneering work done by Kido et al. in 1994, the development of WOLEDs has been
considerably boosted.[7-14] White emission is typically formed with two complementary orange
and blue colors [15,16] or three primary blue, green and red colors [17-19]. The study of the
essential emitting layer mainly goes through, fluorescent materials, [20] phosphorescent
materials [21-25] and thermally activated delayed fluorescent materials (TADF).[26-28]
Although both phosphorescent and TADF materials can efficiently use singlet and triplet
excitons to realize excellent device performance. However, some critical problems remain
unsolved for both the materials: The phosphorescent and TADF OLEDs hardly falls into the
deep-blue region, whose development is still a formidable challenge with respect to green and
red counterparts due totheir intrinsic wide bandgap and unbalance charge carrier mobility in blue
OLEDs. Generally, the blue OLEDs refers to the Commission Internationale de L’Eclairage
(CIE) coordinates of y ˂ 0.15 along with an (x+y) ˂ 0.30.[29]Furthermore, the production of the
full-color display (WOLEDs), saturated deep-blue emitters with CIE coordinates of National
Television Standards Committee (NTSC) (0.14, 0.08) and European Broadcasting Union (EBU)
(0.15, 0.06) arehighly necessary to fulfill the CIE coordinates of WOLEDs (0.33, 0.33).[30, 31]
The deep-blue emitters not only effectively decrease the power consumption of the OLED device
but also be useful to generate light of other colors (green, red and white).[32-34] Till date, in
3

literature phosphorescent and TADF electroluminescence with CIEy ˂ 0.08 or CIEy coordinate
0.06 are still rare [31-33,35], which is mainly attributed to the wide energy gap (˃ 2.8 eV)
˂
hampering the charge injection and charge balance in the device.[36] For the successful
commercialization of OLEDs, there is a tremendous need in the development of luminescence
stability, efficiency, device structure, and fabrication technique.
The key point to understand the above-mentioned concept is blue materials supposed to have
1
high triplet (T ˂ 3 eV) energy, high PLQY,and good bipolar transporting ability. Wang et al.
reported two imidazole-based fluorophores (mm-BPPI and mm-CN-BPPI)with the EQE of 1.7
and 1.13% and CIE coordinates of (0.158, 0.060) and (0.153, 0.090), respectively.[37]Huang et
al. synthesized 4,4 ′ -bis(1,2,2-triphenylvinyl)biphenyl (BTPE) based deep blue derivatives and
achieved an EQE of 1.9 % with CIE coordinates of (0.16, 0.11).[38] Chen, et al. synthesized two
blue fluorescent imidazole derivatives (PhImFD and PhImTD) with a D–π–A structure. The non-
doped devices shown blue emission with EQE of 1.21 and 1.63% and CIE coordinates of (0.151,
0
.115) and (0.151, 0.098), respectively.[39] Recently, Vallance et al. synthesized pyrene core
organic fluorophore (PY-II). Non-doped OLED was fabricated by using this fluorophore as the
emissive layer, the fabricated device is shown blue emission with CIE coordinates of (0.16,
0
.16), the maximum current efficiency of 0.41 cd/A and maximum luminescence of 202
2
cd/m .[40] However, device efficiency and color purity are still not satisfied with increasing
voltage.
As mentioned above, the EQE of OLEDs with CIEy ˂ 0.08 is extremely low for deep blue
OLEDs. The development of efficient materials for deep-blue OLEDs is an essential research
goal. Generally, bulky aromatic Donor-π-Acceptor (D-π-A) system give rise to the bathochromic
shift (red-shift) of the EL emission, due to aggregation occurs in the bulk aromatic systems leads
4

to red-shift in emission.[41-43] Therefore, relatively short π-conjugation, weak intermolecular
interaction and twisted steric configuration are tools for the constructionof deep-blue florescent
emitters.[44-50] In this regards, we report herein a series of novel deep-blue fluorophores, which
are constructed by imidazole, carbazole, aliphatic spacer and a different functional group at the
N1 position of the imidazole ring(Fig. 1).The incorporating aliphatic spacer between donor and
acceptor moieties, its leads to decrease the π-conjugation of the fluorophores and blue-shift of
the emission. Here we have chosen diphenyl-imidazole moiety because in literature diphenyl-
imidazole based materials are shown deep-blue electroluminescence with narrowFWHM,
compare with the phenanthroimidazole moiety due to more twisting nature of the diphenyl-
imidazole group.[45] N1 position of the imidazole can be functionalized with a different
functional group, which is capable of preventing strong π-π stacking and molecular interactions
in the solid state.[51]In addition, the carbazole moiety as the donor group is adopted frequently
to build florescent molecules in literature because carbazole has a better hole
transporting/injecting capability.[52, 53] All the fluorophores were synthesized with good yield
and structurally confirmedbyspectroscopic technique.Amongst all the fabricated doped and non-
doped deep-blue OLED devices, the doped device based on BIPOCz exhibits good device
performance as the EQE is up to 1.9% with CIE coordinates of (0.17, 0.07), which is very close
to the National Television Standards Committee (NTSC) standard blue (0.14, 0.08).
5

Fig. 1.Chemical structures of imidazole derivatives.
Experimental section
Synthesis and Characterization:
The synthetic route of the target imidazole derivatives is shown in Scheme1.The4-(4-
bromobutoxy)benzaldehyde (Intermediate-1), 4-(4-(9H-carbazol-9-yl)butoxy)benzaldehyde
(
Intermediate-2)and Fl-NH were prepared by the literature procedure.[54, 55]The
2
targetedcompoundswere synthesized bya condensation reaction between Intermediate-2, amine
derivatives, andbenzil, in the presence ofammonium acetate and acetic acid. The synthesized
compounds were structurally confirmed by spectroscopic techniques.
General procedure for the synthesis of imidazole derivatives:
To the mixture of benzil (0.36 g, 1 eq), amine (aniline derivatives)(0.17 g, 1.1 eq) and
intermediate-2 (0.60 g, 1 eq)the acetic acid (30 ml) was added, followed by added ammonium
acetate (1.35 g, 10 eq) and then the reaction mixture was refluxed for 12 hrs, under nitrogen
atmosphere in an oil bath. The completion of the reaction were monitored with Thin-Layer
6

Chromatography (TLC). After completion of reaction, the mixture was cooled and poured
minimum amount of water. The corresponding reaction mixture was neutralize with dilute
ammonium hydroxide solution. The resultant mixture was extracted with dichloromethane
(DCM) followed by purification by column chromatography on silica gel (100-200 mesh) with
ethyl acetate/petroleum ether (2:8) as the eluent.
9
7
-(4-(4-(1,4,5-triphenyl-1H-imidazol-2-yl)phenoxy)butyl)-9H-carbazole (BIPOCz): Yield:
1
0%, pale brown color solid, mp 138 °C, H-NMR (400 MHz, CDCl
3
, TMS,
δ
ppm): 8.13 (d,
J=7.6 Hz, 2H), 7.62 (d, J=7.2 Hz, 2H), 7.52-7.42 (m, 4H), 7.67-7.33 (m, 2H), 7.31-7.20 (m,
1H), 7.15-7.12 (m, 2H), 7.06-7.04 (m, 2H), 6.74 (d, J=8.8 Hz, 2H),4.41 (t, J=7.2 Hz, 2H), 3.92
1
1
3
(
t, J=6.0 Hz, 2H), 2.12-2.05 (m, 2H), 1.90-1.82 (m, 2H), C-NMR (100 MHz, CDCl
3
, TMS, δ
ppm): 158.91, 146.94, 140.35, 138.04, 137.17, 134.50, 131.13, 130.70, 130.49, 130.38, 129.09,
1
1
2
28.97, 128.48, 128.35, 128.20, 127.71, 127.46, 126.60, 125.70, 124.21, 123.09, 122.86, 120.43,
-1
19.87, 118.87, 114.04, 108.66, 67.39, 42.71, 26.94, 25.87.FTIR (KBr,
ν / cm ): 3427, 3060,
937, 1604, 1496, 1443, 1320, 1259, 1174, 1044, 837, 753, 692, 508.CHNS Elemental Analysis:
Anal. Calc. for C43
H
35
3
N O: C, 84.70; H, 5.79; N, 6.89; Found: C, 85.02; H, 6.13; N, 7.09 %.EI-
+
MS: m/z exp. (calc.). 609.76 found 610.76 [M]
9
-(4-(4-(4,5-diphenyl-1-p-tolyl-1H-imidazol-2-yl)phenoxy)butyl)-9H-carbazole (BIPTOCz):
1
Yield: 75%,pale brown color solid, mp 137 °C, H-NMR (400 MHz, CDCl , TMS,
δ ppm): 8.14
3
(d, J=8.0 Hz, 2H), 7.62 (d, J=7.2 Hz, 2H), 7.51-7.43 (m, 4H), 7.39-7.37 (m, 2H), 7.30-7.22 (m,
8
4
H), 7.17-7.14 (m 2H), 7.06 (d, J=8.0 Hz, 2H), 6.94 (d, J=6.8 Hz, 2H), 6.76 (d, J=5.2 Hz, 2H),
.41 (t, J=6.8 Hz, 2H), 3.93 (t, J=6.0 Hz, 2H), 2.33 (s 3H), 2.13-2.05 (m, 2H), 1.88-1.81 (m, 2H),
1
3
C-NMR (100 MHz, CDCl , TMS,
δ ppm):158.85, 146.96, 140.36, 138.08, 137.94, 134.61,
3
1
34.56, 131.16, 130.84, 130.55, 130.35, 129.72, 129.46, 128.32, 128.17, 127.84, 127.45, 126.53,
7

1
2
9
5
25.70, 123.26, 122.87, 120.43, 120.01, 118.87, 114.02, 108.66, 67.38, 42.71, 26.95, 25.88,
-1
1.23. FTIR (KBr,
ν / cm ): 3450, 3044, 2952, 2868, 1604, 1512, 1458, 1328, 1251, 1182, 1021,
60, 837, 753, 699, 508. CHNS Elemental Analysis: Anal. Calc. for C H N O: C, 84.72; H,
44
37
3
.98; N, 6.74; Found: C, 84.96; H, 6.37; N, 6.99 %.EI-MS: m/z exp. (calc.). 623.78 found 624.87
+
[M]
9
-(4-(4-(1-(4-tert-butylphenyl)-4,5-diphenyl-1H-imidazol-2-yl)phenoxy)butyl)-9H-carbazole
1
(
BITBOCz): Yield: 65%,white color solid, mp 130 °C, H-NMR (400 MHz, CDCl , TMS,
δ
3
ppm): 8.13 (d, J=7.6 Hz, 2H), 7.60 (d, J=8.4 Hz, 2H), 7.50-7.41 (m, 4H), 7.35 (d, J=8.8 Hz, 2H),
.29-7.22 (m, 10H), 7.13 (d,J=6.0 Hz, 2H), 6.96 (d, J=8.4 Hz, 2H), 6.74 (d, J=8.8 Hz, 2H), 4.41
7
1
3
(
t, J=7.2 Hz, 2H), 3.93 (t, J=6.0 Hz, 2H), 2.16-2.07 (m, 2H), 1.86-1.81 (m, 2H),1.29 (s 9H), C-
NMR (100 MHz, CDCl , TMS, ppm):158.80, 151.35, 146.96, 140.35, 137.96, 134.62, 134.46,
31.11, 130.78, 130.55, 130.33, 128.23, 128.16, 127.88, 127.76, 127.49, 126.50, 125.92, 125.81,
25.68, 123.28, 122.85, 120.42, 119.79, 118.86, 113.94, 108.65, 67.38, 42.72, 34.66, 31.27,
δ
3
1
1
2
1
8
-1
6.96, 25.88. FTIR (KBr,
ν / cm ): 3440, 3051, 2960, 2868, 1604, 1489, 1458, 1392, 1328,
259, 1174, 1021, 837, 753, 695, 561. CHNS Elemental Analysis: Anal. Calc. for C H N O: C,
4
7
43
3
4.78; H, 6.51; N, 6.31; Found: C, 85.22; H, 6.74; N, 6.51 %.EI-MS: m/z exp. (calc.). 665.86
+
found 666.88 [M]
9
-(4-(4-(1-(9,9-diethyl-9H-fluoren-2-yl)-4,5-diphenyl-1H-imidazol-2-yl)phenoxy)butyl)-9H-
1
carbazole (BIFOCz): Yield: 70%,white color solid, mp 146 °C, H-NMR (400 MHz, CDCl
3
,
TMS,
δ ppm): 8.10 (d, J=8.0 Hz, 2H), 7.64 (d, J=7.6 Hz, 3H), 7.56 (d, J=8.4 Hz, 1H), 7.47-7.39
(m, 6H), 7.34-7.32 (m, 2H), 7.30-7.28 (m, 2H), 7.26-7.25 (m,3H), 7.23-7.17 (m, 6H), 6.99-6.98
(m, 2H), 6.71 (d, J=8.8 Hz, 2H), 4.39(t, J=6.8 Hz, 2H), 3.90 (t, J=6.0 Hz, 2H), 2.08-2.04 (m, 2H),
1
3
1
.94-1.89 (m, 2H), 1.84-1.78 (m, 4H), 0.12 (s, 6H), C-NMR (100 MHz, CDCl , TMS,
δ
3
8

ppm):158.90, 150.76, 150.02, 146.91, 141.49, 140.32, 137.83, 135.91, 134.52, 131.15, 130.85,
1
1
2
1
8
30.61, 130.39, 128.33, 128.20, 127.88, 127.71, 127.45, 127.17, 127.06, 126.57, 125.67, 123.27,
23.13, 122.85, 120.40, 120.04, 120.0, 118.84, 114.0, 108.63, 67.38, 56.26, 42.68, 32.73, 26.91,
-1
5.84, 8.23. FTIR (KBr,
ν
/ cm ): 3442, 3051, 2960, 2921, 2875, 1604, 1480, 1453, 1384, 1328,
254, 1178, 1021, 837, 753, 699, 615.CHNS Elemental Analysis: Anal. Calc. for C54
47 3
H N
O: C,
6.02; H, 6.28; N, 5.57; Found: C, 87.32; H, 6.52; N, 5.78 %.EI-MS: m/z exp. (calc.). 753.97
+
found 754.95 [M]
9

Scheme 1.Synthetic routes for the imidazole-derivatives.
Results and Discussion
Thermal properties:
The thermal properties of imidazole-derivatives were investigated by thermogravimetric analysis
(
TGA), which was carried out from 0 to 800 °Cunder N atmosphere with a heating rate of 10
2
1
0

°
C/min. As shown in Fig. 2, the TGA curves reveal that the degradation temperature (T
weight loss of imidazole-derivatives are310°C (BIPOCz), 302°C (BIPTOCz), 290°C
BITBOCz) and 375 °C (BIFOCz), respectively (Table 1). Among, all the imidazole-
d
) of 10%
(
derivatives BIFOCz fluorophore showed slightly more degradation temperature (10% weight
loss) than the other imidazole-derivatives, due to the incorporationoffluorene moiety at N1
position of imidazole group. These results indicate that synthesized imidazole-derivatives
provide desirable thermal properties, which are favorable for use in the OLED device.
1
00
BIPOCz
BIPTOCz
BITBOCz
BIFOCz
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
0
100 200 300 400 500 600 700 800
Temperature
(
°C
)
Fig. 2.Thermogravimetric curves of imidazole-derivatives.
Photophysical properties:
The UV-vis absorption and photoluminescence (PL) properties of imidazole-derivatives were
-5
measured in dichloromethane (DCM) (c= 1×10 M) solution and solid state. As shown in Fig.
A, all newly synthesized compoundsexhibited similar absorption band around at 265 nm, which
3
1
1

may be ascribed to the * transitionof their common benzene ring.[56]The bands at 294 nm
π-π
were attributedto the π-π* transitionof the carbazole moieties in all the compounds[57] The weak
peaks at 345 nm in the absorption spectra (Fig. 3A), low intense band could be attributed to the
intramolecular charge transfer (ICT) transition from the HOMO of the electron-donating
carbazole moiety to the LUMO of the electron-accepting imidazole moiety.[47] Moreover, the
absorption bands of the compounds broad and red shifted in the solid as compared with those in
solution due to intermolecular interactions (Fig. 3C), for example, the absorption at 294 nm for
BIPOCz in DCMsolution red-shifted to 348 nm in solid. Theoptical band gaps of
thecompoundswere calculated by converting solid absorption data into the Kubelka-Munk
function.[58] The corresponding spectra are shown in Fig. 3B and obtain band gaps are3.01eV
(BIPOCz), 3.33eV (BIPTOCz), 3.14eV (BITBOCz), and 3.18eV (BIFOCz),respectively These
band gaps are wellalignedwith the electrochemical analysisdata (CV measurement)(Table 3).
As shown in Fig. 3A, all synthesized compounds display similar PL emission in DCM solution
due to their similar structures, particularly all the compounds show a strong deep-blue emission
peak centered around a similar wavenumber i.e. 392 nm (BIPOCz), 396 (BIPTOCz),
4
03(BITBOCz) and 407 (BIFOCz),respectively. The BIFOCz compounds shown red-shifted
emission (9 nm) compared with the other compounds, due to increasing of conjugation (fluorene
moiety substituted at the N1 position of the imidazole). However, all the compoundshowed red-
shift PL emission with a peaking wavelength ranging from 410 to 424 nm (strong peaks) and
solder peaks with a peaking wavelength ranging from 432 to 474 nm in the solid phase (Fig. 3D),
dueto intermolecular interactions.[59, 60]The CIE chromaticity diagram of the compounds in
solution and solid wasshown in Fig. 4 and the corresponding data is summarised in Table S2 in
the supplementaryinformation. Fig. 4 shows that all the compounds shown deep-blue emission in
1
2

solution as well as in solid. The CIE coordinates of solid PL emissions are deeper than that of
solution emission, it is clearly indicating that there is no aggregation occurs in solidcompounds.
It is beneficial to get pure deep-blue EL emission in the OLED device. The PLQYwere measured
in DCM solution and solid by using an integrating sphere.The quantum yield of the compounds
was calculated by using equation (1).[61]
ꢀꢁ
ꢂλꢃꢄꢀ ꢂλꢃ
ꢅ
Ф =
(1)
ꢀꢁ
ꢂλꢃ
E ꢂλꢃ − ꢂ1 − ФꢃE ꢂλꢃ
ꢆ
ꢇ
η =
EꢇꢂλꢃФ
The calculated quantum yield results are summarized in Table 1. The measured quantum yield
digital images are shown in the SI8.The calculated quantum yield of all the compounds are in the
range of 12.61-15.60% in DCM solution.The quantum yield of all the compoundsare in the range
of 35.20-59.26% in solid states. Among all the compounds BIFOCz fluorophore showed
highquantum yield of 59.26% in the solid phase. The high solid-state quantum yield of these
compounds beneficial for usethese compounds an emitting layer in OLEDs.
1
3

Fig. 3.A)UV and PL spectra of compoundsinDCMsolution, B)optical bandgap of the
compounds,C)UV absorptionspectra of compoundsin the solid state, and D)PL spectra of
compoundsin the solid state.
1
4

Fig. 4. The CIE chromaticity coordinates for the compoundsin solutions and solid phase.
Table 1. Key photophysical properties of Carbazole-Alkyl-Imidazolederivatives
a
T
d
Absolute quantum
Solution
Solid
(
°C)
yield (Φ)
Compounds
BIPOCz
Abs (nm) PL (nm) Abs (nm) PL (nm) Solution Solid
%) (%)
15.45 50.90
(
310
302
290
375
242,294,
45
255, 294,
45
244, 267,
94, 345
247, 265,
94, 345
392
396
403
407
285, 348 367, 410,
431, 460
3
BIPTOCz
BITBOCz
340
382, 424,
446, 474
15.28
13.81
15.60
39.90
44.90
59.26
3
285, 345 372, 414,
437, 467
2
BIFOCz
330
371, 413,
435
2
a
Thermal decomposition temperaturecorresponding to 10% weight loss
1
5

Theoretical calculations:
To further understand the optical behavior of imidazole-derivatives, we have carried out the
Density Functional Theory (DFT) calculation at the B3LYP/6-31(d) level.[62]The simulation
results including the optimized molecular configuration and electron distributions of thehighest
occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of
imidazole derivatives shown in Fig. 5 and corresponding data are summarized in Table 2. As
shown in Fig. 5b the electron density of HOMO of these compounds distributed mainly on the
electron-rich carbazole moiety. On the other hand, the LUMO of BIPOCz, BIPTOCz,and
BITBOCz compounds were mostly distributed on imidazole moiety, whereas, the LUMO of
BIFOCz compound populated between imidazole and fluorine. Almost complete separation of
HOMO/LUMO energy levels suggests that the separation of electron cloud of HOMO/LUMO
could afford the transporting channels between hole and electrons, which may endow the
imidazole derivatives with the ability of bipolar charge transport. The theoretical HOMO/LUMO
energy levels were found to be -5.03/-0.68 for BIPOCz,-5.00/-0.67 for BIPTOCz,-4.99/-0.67 for
0
BITBOCz,and -5.03/-1.15 for BIFOCz. After confirming the ground state (S ) geometry of all
the compounds, we were vertically excited the compounds to get the excited state (S1 and
T1).All the compounds showed high singlet and triplet energies,triplet energy of the compounds
are found to be in the range of 2.86-2.94 eV. The obtained results indicating that these
compounds are capable to use as host in WOLEDs. By using the optimized geometry of the
compounds, the absorption spectra (Fig. S22) were calculated with the help of Time-Dependent-
DFT calculations. The obtained absorption wavelength (nm), oscillator strength, major
configuration, and their assignment are collected in SI9.Orbital compositions of each excited
1
6

state have also been calculated. These results give complete information about the absorption
spectra.In addition, atom coordinates of imidazole-derivatives were given in SI11.
Fig. 5a.Optimized structures of imidazolederivatives
Table 2.Calculated Frontier Molecular Energy Levels of imidazolederivatives
Compounds HOMO LUMO HOMO LUMO
Eg
S1(gas) T (gas) S1(DCM)( T (DCM)(
1
1
(
eV)
(eV)
-0.68
-0.67
-0.67
-1.15
-1 (eV) +1 (eV) (eV)
(eV)
3.81
3.83
3.83
3.36
(eV)
eV)
3.38
3.97
3.98
3.56
eV)
2.98
2.97
2.97
2.90
BIPOCz
BIPTOCz
BITBOCz
BIFOCz
-5.03
-5.00
-4.99
-5.03
-5.35
-5.35
-5.35
-5.36
-0.61
-0.58
-0.56
-0.69
4.35
4.33
4.32
3.88
2.94
2.93
2.93
2.86
1
7

Fig. 5b.Electron Density Contours of frontier molecular orbitalsofimidazole-derivatives
Electrochemical Properties:
The electrochemical properties of the imidazole-derivatives were examined by cyclic
voltammetry (CV) using a standard three-electrode cell under N2 atmosphere. Glass-carbon rod
as the working electrode, Pt wire as the counter electrode, and Ag/AgCl wire as the reference
electrode. The cyclic voltammograms of imidazole derivatives in DMF in the presence of 0.1 M
Bu NClO assupporting electrolyte were shown in Fig. 6 and the corresponding electrochemical
4
4
data are tabulated in Table 3.Compounds BIPOCz, BIPTOCz, BITBOCz,and BIFOCz showed
oxidation onset potentials at 1.27 V, 1.46 V 1.39 V and 1.29 V, as well as reduction onset
potentials at -1.08 V, -1.55 V, -1.20 V and -1.34 V,respectively.These results indicating that all
the compounds possess good hole and electron transporting properties.On the basis of obtained
oxidation onset potentials and reduction onset potentials, the HOMO and LUMO energy levels
of imidazole derivatives were calculated by using the Leeuw et al. equations (2) and (3).[63]
ꢋꢍꢎꢏꢐ
Eꢈꢉꢊꢉ = −ꢂEꢋꢌ ꢑ 4.4ꢃeV
(2)
1
8

ꢋꢍꢎꢏꢐ
Eꢒꢓꢊꢉ = −ꢂEꢔꢏꢕ ꢑ 4.4ꢃeV
(3)
The HOMO and LUMO energy levels of BIPOCz, BIPTOCz, BITBOCz,and
BIFOCzcompounds were estimated as -5.67/-3.32eV, -5.86/-2.85eV,-5.79/-3.20eV and-5.69/-
3
.06eV, indicating that deep HOMO/LUMO energy levels possess good electron-transporting
and the good hole transporting properties. The comparison of HOMO-LUMO energy of all the
compounds is shown in Fig. 7 it is clearly indicating that the HOMO and LUMO energy
difference between the compounds.
BIPOCz
BIPTOCz
BITBOCz
BIFOCz
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
Voltage (V)
Fig. 6.Cyclic voltammograms of imidazole-derivatives.
1
9

Fig. 7. HOMO – LUMO energy gap diagram of imidazole-derivatives.
Table 3. Electrochemical properties of imidazole-derivatives.
a
b
c
d
Compounds
Eox
Ered (V)
HOMO
(eV)
LUMO
(eV)
Eg
Eg
(
V)
(eV)
2.35
3.01
2.59
2.63
(eV)
3.01
3.33
3.14
3.18
BIPOCz
1.27
1.46
1.39
1.29
-1.08
-1.55
-1.20
-5.67
-5.86
-5.79
-5.69
-3.32
-2.85
-3.20
BIPTOCz
BITBOCz
BIFOCz
-1.34
-3.06
a
b
c
The onset oxidation potential, the onset reduction potential, electrochemical bandgap
d
determined from cyclic voltammetry and optical energy bandgap estimated from the
solid-state absorption spectra.
2
0

Electroluminescence characteristics
To further explore the plausible applications of synthesized imidazole fluorescent emitters,
initially a series of non-doped and doped OLED devices were fabricated with the following
structures, ITO (125 nm)/ PEDOT:PSS (35 nm)/ 100 wt% of BIPOCz, BIPTOCz, BITBOCz or
BIFOCz (20 nm)/ TPBi (35 nm)/ LiF (1 nm)/ Al (100 nm). Herein, PEDOT:PSS was employed
as a hole injection material, CBP as a bipolar host material, and TPBi as an electron transporting
and hole blocking material. Fig. 8 illustrates the schematic energy level alignment diagram of the
resultant OLED devices composed of carbazole integrated emitters and molecular structure of
the materials used in device fabrication. The details of the device fabrication method are given in
the experimental section (SI1.2.).
Fig. 8(a). Schematic illustration of the energy level alignment diagram of the solution-processed
OLED device containing the four different near-UV carbazole integrated emitters BIPTOCz,
BITBOCz, BIFOCz, and BIPOCz doped in CBP host matrix and (b) molecular structures of the
hole injection/transport material (PEDOT:PSS), electron transport material (TPBi) and host
(CBP) used in the device fabrication.
2
1

The detailed electroluminescence (EL) performances of the OLED devices are summarized in
Table 4. As the non-doped devices showed poor electroluminescence performances due to
aggregates formation in the solid states, carrier quenching as well as non-radiative
recombination, we fabricated doped devices with the configurations of ITO (125 nm)/
PEDOT:PSS (35 nm)/ 0.5, 1, 3, and 5 wt% of BIPOCz, BIPTOCz, BITBOCz or BIFOCz doped
in CBP host (20 nm)/ TPBi (35 nm)/ LiF (1 nm)/ Al (100 nm), to mitigate aggregation caused
emission quenching and balancing charge carrier distribution across the emission zone, thereby
improving the electroluminescence properties. All the devices with low doping concentration
(0.5 wt%) exhibited an excellent device efficiency and performance.The best performing 0.5
2
wt% BIPOCz based OLED device shows a maximum luminance of 434 cd/m , a maximum
power efficiency (PE) of 0.34 lm/W, a maximum current efficiency (CE) of 0.5 cd/A, and
maximum external quantum efficiency (EQE) of 1.9% with Commission International de
L’Eclairage (CIE) coordinates of (0.17, 0.07) emits deep-blue light. As the doping concentration
increased to 1, 3 and 5 wt%, the device performance dramatically decreased, which may be
attributed to triplet-triplet annihilation (TTA) as well as concentration-quenching, resulted from
self-aggregation of the emitters at high concentration [64-69]. Furthermore, OLED devices
consisting of compounds BIPTOCz, BITBOCz, and BIFOCz as the emitters exhibited maximum
EQE of 1.4, 1.54, and 1.6 % with deep-blue emission CIE coordinates of (0.17, 0.06), (0.17,
0
.06), and (0.17, 0.07), respectively, for the same doping concentration.
Fig. 9 illustrates the current density-voltage, luminance-voltage, current efficiency-luminance,
and power efficiency-luminance plots of the solution-processed near-UV deep-blue emission
OLED devices with 0.5 wt% dopant concentration in CBP host. The effects of doping
2
2

concentration on device performance were studied by the current density-voltage, luminance-
voltage, current efficiency-luminance, and power efficiency-luminance plots as shown in SI.10.
Fig. 9. (a) Current density-voltage, (b) luminance-voltage, (c) current efficiency-luminance, and
(d) power efficiency-luminance plots of the solution-processed near-UV deep-blue emission
OLED devices using CBP host with 0.5 wt% dopant concentrations.
Fig. 10 displays the normalized EL spectra of 100 and 0.5 wt% doped OLED devices of the
synthesized molecules. The restoration of the electroluminescence spectrum to narrow and blue
shifted photoluminescence spectrum clearly indicates the suppression of formation of molecule-
aggregates in doped devices (Fig. 10b). All the emitters demonstrated near-UV deep-blue
emission with the CIE coordinate of x=0.17, y in the range 0.06-0.07 and maxima of EL spectra
peaking around 385-400 nm, which are well matched with those of standard deep-blue emission
2
3

(0.14, 0.08) prescribed by the National Television System Committee (NTSC 1987).
Furthermore, it is noteworthy that EL spectra of the OLED devices closely resemble with PL
spectra, which are recorded in DCM solution. It indicates the uniform dispersion of the dopant
molecules into proper host material with the annihilation of aggregation in thin films and
predominant source of emission from the emitter molecules. Fig. 11 expressed the EQE and CIE
coordinates of fabricated near-UV deep-blue OLED devices on the CIE chromatogram.
Fig. 10. The normalized EL spectra of the solution-processed near-UV emission (a) non-doped
and (b) 0.5 wt% doped in CBP host OLED devices.
Fig. 11. Representation of maximum EQE and CIE coordinates of fabricated near-UV deep-blue
OLED devices on the CIE chromatogram.
2
4

Table 4. Effect of the doping concentration of the synthesized dyes on the turn-on voltage,
power-efficiency (PE), current-efficiency (CE), external quantum efficiency (EQE), CIE
coordinates, and maximum luminance of solution-processed near-UV deep-blue emission OLED
devices consisting of CBP host matrix.
Emitter
Turn-on
PEmax/ CE
/
PE / CE /
Maximum
luminance
(cd m )
max
100
100
c
a
b
CIE
coordinates
Emitter concentration voltage
EQE
EQE
d
max
100
-
1
-1
-1
-1
-2
(wt%)
(V)
4.1
4.8
5.4
5.4
4.1
(lm W / cd A / %) (lm W / cd A / %)
0
.5
0.2/ 0.3/ 1.9
0.3/ 0.5/ 1.5
0.2/ 0.4/ 1.1
0.1/ 0.3/ 0.7
- / - / -
0.1/ 0.2/ 1.1
0.1/ 0.3/ 0.7
0.1/ 0.2/ 0.5
0.1/ 0.2/ 0.4
- / - / -
(0.17, 0.07)
(0.18, 0.10)
(0.18, 0.10)
(0.17, 0.09)
-
434
311
301
349
67
1
3
5
BIPOCz
1
00
0
.5
4.1
4.5
4.7
4.7
4.6
4.1
4.2
4.6
4.7
4.7
0.1/0.2/ 1.4
0.2/ 0.2/ 1.4
0.1/ 0.2/ 1.4
0.1/ 0.2/ 0.8
- / - / -
0.1/0.2/0.8
0.1/ 0.2/ 0.9
0.1/ 0.2/ 0.7
0.1/ 0.2/ 0.3
- / - / -
(0.17, 0.06)
(0.17, 0.06)
(0.17, 0.08)
(0.13, 0.07)
-
485
450
381
373
49
1
BIPTOCz
3
5
1
00
0
.5
0.1/ 0.2/ 1.5
0.2/ 0.3/ 1.3
0.2/ 0.3/ 1.1
0.1/ 0.3/ 1.0
- / - / -
0.1/ 0.2/ 0.9
0.1/ 0.2/ 1.0
0.1/ 0.2/ 0.7
0.1/ 0.2/ 0.5
- / - / -
(0.17, 0.06)
(0.17, 0.06)
(0.17, 0.07)
(0.17, 0.08)
-
474
454
408
374
21
1
BITBOCz
3
5
1
00
0
.5
4.1
4.6
5.0
6.5
4.2
0.2/ 0.3/ 1.6
0.2/ 0.2/ 1.4
0.1/ 0.2/ 0.8
0.1/ 0.3/ 1.0
0.1/ 0.1/ 0.1
0.1/ 0.2/ 0.9
0.1/ 0.2/ 0.6
0.1/ 0.1/ 0.4
0.1/ 0.2/ 0.4
0.1/ 0.1/ 0.1
(0.17, 0.07)
(0.18, 0.08)
(0.18, 0.08)
(0.18, 0.09)
(0.25, 0.30)
437
518
576
619
125
1
3
5
BIFOCz
1
00
a
-2 b
Turn-on voltage (the voltage at >1 cd m ); Power-efficiency (PE), current-efficiency (CE), and
c
-2 d
-2
EQE at maximum value; PE, CE, and EQE at 100 cd m ; CIE coordinates at 100 cd m .
2
5

Conclusion:
Insummary, we have designed and synthesized a series of imidazole-derivatives connecting
carbazole and imidazole moieties through alkyl spacer with different functional groups at the N1
position of imidazole. N1 substation of imidazole group influence the optical and electrical
properties, which leads to (meta-CF3 benzene) blue shift emission. Simultaneously, alkyl spacer
moiety controlled the π-π stacking and molecular aggregation, which leads to good thermal
stability and high photoluminescence quantum yield of ~ 59 % in the solid state. All the
compounds showed near-UV emission with CIE coordinate of y < 0.08 and maxima of EL
spectra centered in between 385-405 nm. The best performing monochromatic OLED device
based on BIPOCz displayed a maximum power efficiency of 0.34 lm/W, current efficiency of
0
.5 cd/A, and external quantum efficiency (EQE) of 1.9% with a maximum luminance of 434
2
cd/m . The present study provides novel approaches for realizing ultra-deep blue emission with
high-performance. Because of their outstanding optoelectronic characteristics and facile solution
processibility of imidazole-alkyl spacer-carbazole based emitters, that greatly reinforces their
usability in OLEDs.
Supporting Information:
1
13
19
Supporting Information consistsof NMR spectra ( H, C, and F NMR), mass spectra of the
compounds and FTIRspectra. As well as the coordinates for the compounds in solutions and
solid phase, Quantum yield studies of fluorophores in solution and solid state, device results and
atom coordinates of all the fluorophores.
Acknowledgment
VS acknowledgesSERB, Department of Science and Technology (DST), India
(EMR/2016/002462) for financial support.
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