Impact of tunable 2-(1H -indol-3-yl)acetonitrile based fluorophores towards optical, thermal and electroluminescence properties

Article abstract of DOI:10.1039/c8ra10448d

Herein, we have synthesized 4,5-diphenyl-1H-imidazole and 2-(1H-indol-3-yl)acetonitrile based donor-π-acceptor fluorophores and studied their optical, thermal, electroluminescence properties. Both the fluorophores exhibit high fluorescence quantum yield (Φ_f = <0.6) and good thermal stability (T_d10 = <300 °C), and could be excellent candidates for OLED applications. Moreover, the ground and excited state properties of the compounds were analysed in various solvents with different polarities. The geometric and electronic structures of the fluorophores in the ground and excited states have been studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The absorption of BIPIAN and BITIAN in various solvents corresponds to S₀ → S₁ transitions and the most intense bands with respect to the higher oscillator strengths are mainly contributed by HOMO → LUMO transition. Significantly, the vacuum deposited non-doped OLED device was fabricated using BITIAN as an emitter, and the device shows electroluminescence (EL) at 564 nm, maximum current efficiency (CE) 0.687 cd A^-1 and a maximum external quantum efficiency (EQE) of 0.24%.

Full text of DOI:10.1039/c8ra10448d

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Impact of tunable 2-(1H-indol-3-yl)acetonitrile
based fluorophores towards optical, thermal and
electroluminescence properties†
Cite this: RSC Adv., 2019, 9, 14544
b
Subramanian Muruganantham, a Gunasekaran Velmurugan, ‡a Justin Jesuraj,
b
a
Hassan Hafeez,b Seung Yoon Ryu,
Ponnambalam Venuvanalingam
*
*
a
*
and Rajalingam Renganathan
Herein, we have synthesized 4,5-diphenyl-1H-imidazole and 2-(1H-indol-3-yl)acetonitrile based donor–
p–acceptor fluorophores and studied their optical, thermal, electroluminescence properties. Both the
fluorophores exhibit high fluorescence quantum yield (Ff ¼ <0.6) and good thermal stability (Td10
¼
<300 ꢀC), and could be excellent candidates for OLED applications. Moreover, the ground and excited
state properties of the compounds were analysed in various solvents with different polarities. The
geometric and electronic structures of the fluorophores in the ground and excited states have been
studied using density functional theory (DFT) and time-dependent density functional theory (TDDFT)
methods. The absorption of BIPIAN and BITIAN in various solvents corresponds to S0 / S1 transitions
and the most intense bands with respect to the higher oscillator strengths are mainly contributed by
HOMO / LUMO transition. Significantly, the vacuum deposited non-doped OLED device was fabricated
using BITIAN as an emitter, and the device shows electroluminescence (EL) at 564 nm, maximum current
efficiency (CE) 0.687 cd Aꢁ1 and a maximum external quantum efficiency (EQE) of 0.24%.
Received 20th December 2018
Accepted 29th April 2019
DOI: 10.1039/c8ra10448d
rsc.li/rsc-advances
layered OLEDs in 1987 by Tang and Van Slyke they are
commonly used in many display applications.13 Organic uo-
Introduction
rophoric materials have been used to produce primary color
red-green-blue (RGB) and near-infrared (NIR) emissions. The
emission behaviour is usually tuned by conjugation, effective
chromophores and dopants in hole–electron transporting
materials.14 The spin statistic rule suggested that the conven-
tional15 OLEDs can harvest 25% for internal quantum efficiency
(IQE) due to the limited electron–hole conversion, and phos-
phorescence emitters can harvest both singlet–triplet excitons
for the electroluminescence process.16 Recently, Adachi and co-
workers17 developed thermally activated delayed uorescence
(TADF) technique using D–A (donor–acceptor) organic small
molecules as emitters, which could harvest 100% IQE due to
reverse intersystem crossing (RISC) process.17a,b In the p-i-n type
uorescence OLEDs the anode has injected hole through p-
doped hole transporting layer (HTL) to the emissive layer and
the cathode has injected electron via n-doped electron trans-
porting layer (ETL) to the emissive layer leading to recombina-
tion of charge in the emissive layer which emits high efficient
light.18 Interestingly, organic small molecules incorporated with
electron donor (D) and acceptor (A) by a linearly connected p-
spacer exhibit special patterns in optical and electrical proper-
ties,19 and such compounds have large electric dipoles in the
excited states which also enhance intramolecular charge
transfer (ICT).20 Many D–A type organic small molecules have
been used as luminogen in OLED applications. Among these,
Elongated organic p-conjugated molecules are extensively used
in making organic electronic materials due to their enormous
potential applications1 in two photon-absorption,2 organic
semiconductors,3 organic photovoltaics (OPVs),4 organic eld
effect transistors (OFET's),5 information storage devices,6
nonlinear optics (NLO),7 sensors,8 organic light emitting diodes
(OLEDs),9 etc., and these systems show tunable optical and
electrical properties. Owing to their promising applications in
developing modern full-colour at-panel displays and solid-
state light sources, OLEDs have received considerable atten-
tion.10 In the last few decades the vacuum thermal deposition
process,11 which is one of the important techniques, has been
used for fabricating OLEDs. Since the invention12 of multi-
aSchool of Chemistry, Bharathidasan University, Tiruchirappalli-620 024, Tamil
Nadu, India. E-mail: venuvanalingam@yahoo.com; rrengas@gmail.com; Fax: +91-
431-2407045; Tel: +91-431-2407053
bSchool of Display and Semiconductor Physics, Display Convergence, College of Science
and Technology, Korea University, Sejong Campus 2511 Sejong-ro, Sejong City, 30019,
Republic of Korea. E-mail: Justie74@korea.ac.kr
† Electronic supplementary information (ESI) available: Detailed photophysical
properties, lifetime measurement, DSC curves, cyclic voltammogram,
solvatochromism behaviour, theoretical study results and electroluminescence
performance. See DOI: 10.1039/c8ra10448d.
‡ Present address: Institute of Inorganic Chemistry, Heidelberg University, Im
Neuenheimer Feld 275, 69120 Heidelberg, Germany.
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highly substituted imidazole derivatives received signicant incorporated imidazole. Finally, Knoevenagel condensation25 of
concern in developing novel luminogen materials for opto- the corresponding aldehyde with 2-(1H-indol-3-yl)acetonitrile
electronic materials.21 Small organic molecules having highly was carried out to afford the target molecules BIPIAN and
substituted imidazole moiety as an electron donor with various BITIAN with good yield. In all the steps, the compound was
acceptors were scarcely reported.22 Based on our knowledge, puried by column chromatography (hexane/dichloromethane)
there is no literature available for 4,5-diphenyl-1H-imidazole using silica gel (100–200 mesh) and characterized using FT-IR,
linked 2-(1H-indol-3-yl)acetonitrile as luminogens. In this NMR and mass spectrometry.
context, we planned to synthesize 4,5-diphenyl-1H-imidazole
linked 2-(1H-indol-3-yl)acetonitrile and evaluate its thermal,
optical, electrochemical stability and investigate the structure–
property relationship of ambipolar p-conjugated luminogen
materials. The D–p–A compound is probed as p-i-n type non-
doped multilayer OLEDs to afford high efficient yellowish
green emission (the CIE values of 0.45, 0.52). Further, the D–p–
A interactions, optical and electrical properties are actually
elucidated with a view to explore suitable candidates to develop
multifunctional luminogen materials in the future.
Photophysical properties
The aim of this study is to understand the structure–property
relationship of newly constructed imidazole based D–p–A
organic small molecules. Generally, imidazole derivatives are
widely used to build efficient uorescent OLEDs due to their
twisted structure and charge transporting ability. Indeed, the
synthesized imidazole derivatives have better solubility in
common organic solvents like dichloromethane, toluene,
tetrahydrofuran, chloroform and N,N-dimethylformamide but
poor solubility in methanol and water due to the presence of
hydrophobic aromatic units with the concentration of 10ꢁ5 M.
Results and discussion
Synthesis and characterization
The targeted molecule BIPIAN and BITIAN were designed and The steady-state optical behaviour of the emitters was investi-
synthesized using various synthetic steps as illustrated in gated using ultraviolet-visible (UV-Vis) and photoluminescence
Scheme 1, the detailed synthetic procedure is described in the spectrometry (PL) in both solution and solid lms state on
experimental part. The highly-substituted imidazole core was quartz substrate which is better to understand the photo-
assembled via one-pot multi-component reaction23 followed by physical properties. The key optical parameters are displayed in
the Suzuki cross-coupling reaction24 to obtain the formyl group Table 1.
Scheme 1 Synthetic route for BIPIAN and BITIAN derivatives. Reaction conditions: (a) NH4OAc, AcOH, 80 ꢀC, 3 h; (b) 4-formylphenylboronic
t
acid, Pd(PPh3)4, K2CO3, THF, water, 80 ꢀC, 12 h; (c) indole-3-acetonitrile, BuOK, MeOH, 75 ꢀC, 3 h.
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Table 1 Photophysical parameters for BIPIAN and BITIAN derivatives
BIPIAN
BITIAN
Solvents
alabs
b3
clemi
dFf
eStokes shi
fET(30)
labs
3
lemi
Ff
Stokes shi
ET(30)
CyHex
Toluene
THF
DCM
CHCl3
ACN
Acetone
DMF
MeOH
EtOH
406
380
382
376
376
374
379
385
376
379
10 696
44 052
62 992
198 882
27 828
45 682
60 322
104 156
21 284
22 622
476
437
463
476/539
466
483
479
480
477
0.40
0.53
0.38
0.40
0.64
0.30
0.31
0.23
0.57
0.36
3622
3432
4579
5182
5136
5685
5063
5140
5631
5376
62.42
60.31
58.34
57.99
58.11
57.87
57.99
57.87
57.99
58.11
405
412
411
408
408
404
408
413
402
407
12 584
458
474
490
493/534
492/530
494/537
493/533
494/535
493/534
492/532
0.34
0.53
0.57
0.66
0.69
0.23
0.28
0.27
0.42
0.38
3858
3174
3922
5783
5641
6130
5748
5521
6149
5773
60.06
65.42
61.75
60.06
61.35
59.19
59.68
59.56
59.93
60.06
105 842
168 314
132 902
118 876
159 550
129 776
140 772
70 560
476
140 262
a
b
c
d
e
f
Absorption (nm). Molar absorption co-efficient (Mꢁ1 cmꢁ1). Emission. Fluorescence quantum yield (Ff). Stokes shi (Dy cmꢁ1). ET(30)
(kcal molꢁ1).
Absorption spectra
Photoluminescence studies
Fig. 1 shows UV-Visible absorption spectra of BIPIAN and The emission behavior of electronically excited molecules was
BITIAN derivatives which exhibit similar absorption pattern at investigated in dichloromethane (Fig. 1), to identify the solvent
270 nm with lower intensity than the p–p* transition of effect on the excited state. We observed most intense emission
conjugated skeleton at 376 and 408 nm respectively. Fig. S1 and lmax at 467 and 493 nm (excited at 376 and 408 nm) and a full
S3† show the absorption band in the range of 376–406 and 402– width at half maximum (FWHM) of 69 and 99 nm for emitter
413 nm in various solvents for BIPIAN and BITIAN respectively BIPIAN and BITIAN respectively. In PL peaks, red-shi was
and the longer wavelength bands can be attributed to p–p* observed in various solvents ranging from 437 to 483 and 458 to
transitions of imidazole donor and 2-(1H-indol-3-yl)acetonitrile 494 nm for BIPIAN and BITIAN respectively. Indeed, the solvent-
acceptor unit. Notably, the absorption spectra are strong and dependent excited state behaviour was observed in both
broad which covers 350 to 450 nm for BIPIAN and 330 to 470 nm compounds.27
for BITIAN. Moreover, the intensity of p–p* transitions is
The uorescence quantum yields (Ff) of the highly-
higher than that of the charge-transfer transition. In addition, substituted imidazole derivatives were investigated in various
intramolecular charge transfer (ICT) transition was observed at solvents from non-polar cyclohexane to polar N,N-dime-
408 nm which occurs between D–A unit.26 The optical energy thylformamide. The observed quantum yields (Ff) lie in the
gaps obtained from the onset of the absorption spectrum in range of 0.23–0.64 for BIPIAN and 0.23–0.69 for BITIAN (Table
dichloromethane solution are (Eg) 2.77 and 2.48 eV for BIPIAN 1). However, the external quantum efficiency of thin lm
and BITIAN respectively. Indeed, both uorophores exhibit BITIAN estimated by the integrating sphere technique provides
moderate to strong molar extinction coefficient (BIPIAN 1.90 0.24% of quantum yield. On the contrary, the uorescence
and BITIAN 1.30 ꢂ 105 Mꢁ1 cmꢁ1) of D–p–A system. This is well quantum yield decreased on increasing the solvent polarity
supported by computational predictions.
implying intramolecular charge transfer of the excited state. In
addition, the uorescence quantum yield was observed in less
polar solvents (i.e., CyHex, toluene, CHCl3 and THF), which is
higher than that of polar solvents (i.e., DCM, acetone, EtOH and
MeOH). As well the more polar solvent such as, DMF and ACN
show signicantly lower uorescence quantum yield. Obviously,
the higher Stokes shi for BIPIAN and BITIAN leads to
enhanced charge transfer at the locally excited state of the
molecule. Among these, BITIAN shows larger Stokes shi due to
the presence of thiophene p-spacer leading to a signicant
structural reorganization of the molecule. In PL emission
behaviour, both the molecules show shoulder and peak in
a polar solvent but no such thing was observed in non-polar
solvents. Hence, the emission band strongly depended on the
solvent polarity, which favours the occurrence of ICT under
light excitation. The strong bathochromic shi was observed in
both molecules due to the presence of highly conjugated units
between imidazole and 2-(1H-indol-3-yl)acetonitrile. Noticeably,
the solvatochromic study was performed to understand the
Fig. 1 Absorption/emission spectra for BIPIAN and BITIAN.
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interaction of emitters with the solvent environment on the
ground and excited state with various polarities of solvents
implying a positive solvatochromism. Moreover, the positive
solvatochromic emitters strongly induce the ICT behaviour on
the excited state. Further, the ICT effect was distinguished by
polarity of solvent and photoluminescence emission process
(Fig. S2 and S4†).
The increasing solvent polarity from non-polar cyclohexane
to polar N,N-dimethylformamide, enhanced the ICT character.
Signicantly, the absorption spectra were independent of
solvent polarity while the emission spectra dependent. When
the polarity of the solvent increases, the interaction between
solvent and solute also increases which reveal more positive
solvatochromism in emission spectrum as shown in Fig. 1.
Probably, the emission spectra exhibit higher wavelength in
polar solvents due to photoinduced intramolecular charge
transfer, structural reorganization, dipole–dipole interaction
and excimer complex formation, etc. Further, the observed
Stokes shi values are listed in Table 1. The solvatochromic
effect was investigated using Lippert–Mataga plot, Stokes shi
versus orientation polarizability (Df) of synthesized compounds,
which describe the effect of solvent in the ground and elec-
tronically excited state of the molecule (Fig. S5 and S6†).28
Normally, the solvatochromism is related to the dipole
moment of electronically excited and ground state of the
molecule. The synthesized molecules examined for Stokes shi
from the non-polar (CyHex) to polar solvent (DMF) and it
revealed that a higher charge transfer character is observed. It is
noted that the Stokes shi for BITIAN and BIPIAN falls in the
range 3858–6149 cmꢁ1 and 3622–5631 cmꢁ1 respectively. The
large Stokes shi observed for uorophores indicate a higher
dipole moment in the excited state than ground state. Inter-
estingly, the large Stokes shis for the emitters are more
advantageous and favourable for electroluminescence devices,
which may assist to avoid discarded self-absorptions. Sol-
vatochromism behaviour was described in a well-known
method by Reichardt–Dimroth polarity ET(30) parameters.29
Correlation between Stokes shi versus ET(30) parameter indi-
cates non-linearity in solvatochromism behavior (Fig. S7†). Due
to the specic solvent effect divergence from linear dependence,
may result from solute–solvent interactions and large difference
in dipole–dipole (enhance the charge transfer) interactions in
the excited state.30
Fig. 2 Fluorescence decay curves of BITIAN in various solvents.
exponential decay based on the tting ability. The uo-
rophores surrounded by solvent molecules and polarity of
solvents leads to various intensity of decay which represents the
conformational distribution of the lifetime. The steady-state
measurement of uorescence decay was observed for both
molecules in different solvents which resulted in bi-exponential
decay (lifetime-weighted quantum yield or amplitude-weighted
lifetime). Further, the average lifetime hsi was calculated and
the detailed uorescence decay measurements data are listed in
Table S1.†
Two distinct lifetimes were observed for a single uoro-
phoric compound which indicates bi-exponential decay of
electronically excited state. Bi-exponential decay may arise due
to various conformations of the excited molecule, solvent effect,
molecular environment and dissimilar spin-multiplicities of the
excited state. Subsequently, internal PL quantum yield (FPL
)
and average uorescence decay lifetime were used to calculate
radiative (kr) and non-radiative (knr) decay using corresponding
equation32 and results are summarized in Table S1.† Signi-
cantly, the FPL was obtained for both molecules which illus-
trated a higher radiative rate constant due to the presence of
electron withdrawing units thus enhancing the intersystem
crossing and internal conversion in the ground state. BIPIAN in
acetonitrile shows non-radiative decay and this may be due to
the interaction between solvent–solute. Based on these results,
both luminogen molecules are suitable for OLEDs applications.
Fluorescence lifetime measurement
Optimized geometry and frontier molecular orbital analysis
The uorescence decay time in the excited state of newly The C–C bond lengths in BIPIAN and BITIAN fall in between
synthesized molecule was investigated using time-correlated their respective single and double bond limits and this indi-
single-photon counting technique.31 Initially, the lifetime cates the C–C bond present in both molecules is no longer
histogram was well tted in single-exponential decay in a pure single bond (due to delocalization of p-electrons).21b,23,33
dichloromethane solution exhibiting lifetime (s) 1.08 and 1.12 Similarly C–N, C–O and C–S bond lengths also are lower than its
ns for BIPIAN and BITIAN, respectively and the c2 values are actual bond lengths. This also shows an enhanced delocaliza-
calculated for BIPIAN is 1.01 and BITIAN is 2. Fig. S8† and 2 tion of p-electrons due to co-planarity of the benzene and
show uorescence decay curves of BIPIAN and BITIAN, which is thiophene rings in the molecule (Fig. S9 and S10†). The highest
inuenced by emission behaviour. Further, the solvent polarity occupied molecular orbitals (HOMOs), lowest unoccupied
varied from non-polar to polar (such as cyclohexane, DCM, ACN molecular orbitals (LUMOs) and band gaps (HOMO–LUMO gap)
and MeOH), uorescence decay was tted to mono or bi- of BIPIAN and BITIAN have been explored to recognize the
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nature of electronic and optical properties.34 It is interesting to This signies that the HOMO / LUMO transition bears
note that the HOMO of BIPIAN is localized mainly on the a signicant ICT and p / p* character and the other absorp-
substituted imidazole and phenyl ring while the LUMO is tion bands have dominant contributions from HOMOꢁ1 /
mainly centered on the phenyl ring and 2-(1H-indol-3-yl) LUMO and are mainly p / p* character. The energy gaps of
acetonitrile units. On the other hand, HOMOꢁ1, HOMOꢁ2, compounds BIPIAN and BITIAN are found to be 3.18 and
HOMOꢁ3, LUMO+1 and LUMO+2 orbitals are predominantly 2.91 eV respectively (Fig. 4). The HOMO levels for BIPIAN and
localized on the substituted imidazole while the LUMO+3 of BITIAN are observed at ꢁ5.39, ꢁ5.18 eV, respectively. The
BIPIAN is localized on the substituted imidazole and 2-(1H- LUMO levels for them are in the range of ꢁ2.21 and ꢁ2.27 eV
indol-3-yl)acetonitrile and phenyl unit (Fig. 3). The HOMO of (Fig. 3) for BIPIAN and BITIAN respectively. Both compounds
BITIAN is equally localized on the highly-substituted imidazole are compared with the Mes2B[p-4,40-biphenyl-NPh(1-napthyl)]
and thiophene ring while the LUMO is centered on the entire (BNPB) due to their wider usability as HTM in OLEDs. The
molecule (highly-substituted imidazole, phenyl or thiophene HOMO of BIPIAN (ꢁ5.39 eV) and BITIAN (ꢁ5.18 eV) is compa-
ring and 2-(1H-indol-3-yl)acetonitrile). The HOMOꢁ1 and rable with BNPB (Exp. ¼ ꢁ5.30 eV;35 ꢁ5.27 eV at b3lyp/6-
HOMOꢁ2 of BITIAN are principally localized on the highly- 311g(d,p) by DFT calculations).33a The LUMO levels of BIPIAN
substituted imidazole whereas HOMOꢁ3 is primarily distrib- (ꢁ2.21 eV) and BITIAN (ꢁ2.27 eV) are comparable with that of
uted on the 2-(1H-indol-3-yl)acetonitrile. However, LUMO+1, BNPB (Exp. ¼ ꢁ2.44 eV; 1.88 eV at b3lyp/6-311g(d,p) level).
LUMO+2 is centred on the highly-substituted imidazole and Interestingly, the HOMO level of BIPIAN and BITIAN are slightly
LUMO+3 is centered on the 2-(1H-indol-3-yl)acetonitrile (Fig. 3). higher than the work function of the Indium Tin Oxides (ITO;
from ꢁ4.8 to ꢁ5.1 eV),36 and the LUMO levels are higher than
that of the tris(8-hydroxyquinoine)aluminium (Alq3, ꢁ1.81
eV),37 which is one of the widely used electron transport mate-
rial. This indicates that BIPIAN and BITIAN can act as ‘tri-
functional materials’ (emitter, hole and electron transporters)
in OLEDs.
To understand the nature of various segments of the mole-
cule and their individual contributions towards HOMOs and
LUMOs (using QMForge38), the whole molecule has been
segmented into three fragments, namely donor (imidazole), p-
spacer (phenyl or thiophene ring) and acceptor units (2-(1H-
indol-3-yl)acetonitrile). As evident from the molecular orbital
diagrams, HOMOs of BIPIAN are mainly centralized by the
donor (56%) and p-spacer (30%) and interestingly LUMOs are
majorly stabilized by p-spacer (54%) and acceptor (25%). The
donor part contributes majorly on the HOMOꢁ1 (53%),
HOMOꢁ2 (88%), HOMOꢁ3 (78%), LUMO+1 (56%), LUMO+2
(91%) and LUMO+3 (43%). In the case of BITIAN, the HOMO is
equally contributed by the donor (40%) and p-spacer (47%). The
HOMOꢁ1 (57%), HOMOꢁ2 (81%), LUMO+1 (67%) and
LUMO+2 (90%) are majorly contributed by donor part and
HOMOꢁ3 (73%) and LUMO+3 (49%) located on acceptor units
(Table 2). This conrms that the donor and acceptor parts in
BIPIAN and BITIAN are responsible for the intramolecular
charge transfer upon excitation.
To understand the nature of electronic transitions and
contributing congurations of BIPIAN and BITIAN, TD-DFT
calculations on the absorption and emission in both vacuum
and in solvent (with various solvents) were performed. The
calculated absorption spectrum of BIPIAN and BITIAN are in
good agreement with the experimental results (Table S2†). The
electronic transitions are of the p / p* character, and excita-
tion to S1 state corresponds exclusively to the promotion of an
electron from HOMO / LUMO (67–84%). The experimental
band in toluene is found at 380 nm for BIPIAN corresponds to
the transition predicted at 371 nm, and this originates from
HOMO / LUMO transition with p / p* transition character
(80%). The experimental band predicted in THF at 382 nm
corresponds to the transition calculated at 373 nm with larger
Fig. 3 Calculated frontier molecular orbitals of the BIPIAN and BITIAN
at the B3LYP/6-31G+(d,p) level.
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Fig. 4 Molecular orbital energy level graphs of BIPIAN and BITIAN at the B3LYP/6-31G+(d,p) level.
Table 2 Frontier molecular orbital compositions (%) in the ground
experimental bands found at 411 and 408 nm, corresponds to
the transition calculated at 407 nm for both solvents. This
transition has signicant p / p* character due to the exclusive
promotion of an electron from HOMO / LUMO (83%). The
same has been observed in CHCl3, ACN, acetone, DMF, MeOH
and EtOH and the excitations are to S1 state and is mainly
contributed by HOMO / LUMO transition (83–84%) and it has
signicant p / p* character. In comparison with gas phase
simulated absorption spectra, the solvent phase spectra shows
red shi due to the solute–solvent interaction.39 Overall, the
absorption of BIPIAN and BITIAN with various solvents corre-
sponds to S0 / S1transitions and the most intense bands with
higher oscillator strengths are mainly contributed by HOMO /
LUMO transition (67–84%) of signicant p / p* character.
Theoretical emission spectra for BIPIAN and BITIAN based
on optimized excited-state geometries are presented in Table 3.
The emission peaks in THF solvent for BIPIAN, with the largest
oscillator strength are due to LUMO / HOMO transition
(93%). The emission peaks in toluene, DCM, CHCl3 and ACN
with the largest oscillator strength for the BIPIAN are assigned
to p / p* character, arising from the HOMO / LUMO tran-
sition (ꢃ93%). The calculated values of uorescence wavelength
are located at 450, 452, 451 and 451 nm which correspond to the
experimental values found at 437, 476, 466 and 483 nm
respectively. The emission peaks in cyclohexane for BITIAN with
the largest oscillator strength arises from LUMO / HOMO
transition (92%). The emission peaks in toluene, THF, DCM,
CHCl3 and ACN with the largest oscillator strength for BITIAN
are assigned to p / p* transition arising from the HOMO /
LUMO transition (93–94%). The calculated values of uores-
cence wavelength are located at 467, 467, 467, 467 and 467 nm
which correspond to the experimental values obtained at 474,
490, 493, 492 and 493 nm respectively. The same trend has been
observed in acetone, DMF, MeOH and EtOH. The highest
oscillator strengths of the S1 / S0 transition for BIPIAN and
BITIAN imply that they have a large uorescent intensity and are
useful as uorescent OLED materials. The inuence of various
solvents on the emission spectra was simulated by using PCM.
state for BIPIAN and BITIAN at the B3LYP/6-31G+(d,p) level
Contribution (%)
Orbital
Donor
p-Spacer
Acceptor
BIPIAN
BITIAN
HOMOꢁ3
HOMOꢁ2
HOMOꢁ1
HOMO
78.29
88.26
52.56
55.73
21.40
55.73
90.56
42.66
2.82
80.83
56.27
40.21
50.32
67.09
90.33
37.30
14.59
7.64
7.12
4.10
24.39
29.52
53.72
33.96
4.83
23.05
14.76
24.88
10.30
4.60
50.90
73.51
6.09
21.17
12.58
20.31
21.47
3.94
LUMO
LUMO+1
LUMO+2
LUMO+3
HOMOꢁ3
HOMOꢁ2
HOMOꢁ1
HOMO
6.44
23.67
13.08
22.56
47.21
29.37
11.44
5.73
LUMO
LUMO+1
LUMO+2
LUMO+3
13.56
49.15
oscillator strengths which originate from HOMO / LUMO
(77%) with p / p* character. In DCM and CHCl3, the experi-
mental band found at 376 nm corresponds to the transition
calculated at 371 and 372 respectively. This transition has
signicant p / p* character due to the exclusive promotion of
an electron from HOMO / LUMO (74–77%). Similar trend was
observed in ACN, acetone, DMF, MeOH and EtOH and the
excitations correspond to S1 state exclusively to the promotion
of an electron from HOMO / LUMO (67–84%) with signicant
p / p* character. For BITIAN, the experimental band in
cyclohexane found at 405 nm corresponds to the transition
computed at 405 nm and it originates mainly to the excitation to
S1 states from HOMO / LUMO (81%) with signicant p / p*
character. The experimental band predicted in toluene at
412 nm, corresponds to the transition calculated 407 nm with
larger oscillator strengths which originates from HOMO /
LUMO (81%) with p / p* character. In THF and DCM, the
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Table 3 Computed emission spectra in both gas and solvent phase BIPIAN and BITIAN along with experimental data
Molecule States
Electron transition Cal. lmax (nm) Exp. lmax (nm) Oscillator strength (f) E (eV) Eb (eV) Major contribution
s (ns)
BIPIAN Gas-phase S1 / S0
425.9
447.7
450.5
451.2
452.2
451.5
450.8
451.0
453.7
450.2
451.2
447.0
465.2
467.5
466.9
467.6
467.5
465.9
466.2
468.5
465.4
466.3
—
1.613
1.658
1.658
1.620
1.619
1.633
1.599
1.604
1.608
1.597
1.602
1.863
1.976
1.986
1.968
1.971
1.975
1.955
1.958
1.969
1.952
1.958
2.91 0.27
2.77 0.41
2.75 0.43
2.75 0.43
2.74 0.44
2.75 0.43
2.75 0.43
2.75 0.43
2.73 0.45
2.75 0.43
2.75 0.43
2.78 0.13
2.67 0.24
2.65 0.26
2.66 0.25
2.65 0.26
2.65 0.26
2.66 0.25
2.66 0.25
2.65 0.26
2.66 0.25
2.66 0.25
HOMO / LUMO (93%) 1.69
HOMO / LUMO (93%) 1.81
HOMO / LUMO (93%) 1.84
HOMO / LUMO (94%) 1.88
HOMO / LUMO (94%) 1.89
HOMO / LUMO (94%) 1.86
HOMO / LUMO (94%) 1.90
HOMO / LUMO (94%) 1.90
HOMO / LUMO (94%) 1.92
HOMO / LUMO (94%) 1.91
HOMO / LUMO (94%) 1.90
HOMO / LUMO (92%) 1.60
HOMO / LUMO (91%) 1.63
HOMO / LUMO (92%) 1.65
HOMO / LUMO (92%) 1.65
HOMO / LUMO (92%) 1.66
HOMO / LUMO (92%) 1.66
HOMO / LUMO (92%) 1.66
HOMO / LUMO (92%) 1.66
HOMO / LUMO (92%) 1.67
HOMO / LUMO (92%) 1.67
HOMO / LUMO (92%) 1.66
CyHex
Toluene
THF
DCM
CHCl3
ACN
Acetone
DMF
MeOH
EtOH
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
476
437
463
476
466
483
479
480
477
476
—
458
474
490
493
492
494
493
494
493
492
BITIAN
Gas-phase S1 / S0
CyHex
Toluene
THF
DCM
CHCl3
ACN
Acetone
DMF
MeOH
EtOH
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
S1 / S0
The results reveal that the emission wavelengths, coefficients material. The redox potential is a highly essential parameter for
and congurations are nearly identical for BIPIAN and BITIAN luminescent materials accordingly the hole/electron-injecting
but a 3–20 nm red-shi has been observed and this is due to the barrier was reduced. The imidazole derivatives exhibit similar
solute–solvent interaction. The calculated absorption and reversible oxidation peak potential and detailed data was
emission bands are in good agreement with the experimental compiled in Table 4. The HOMO energy level was determined
results. Overall, the DFT and TD-DFT calculations reveal deeper using the following formula: HOMO ¼ (4.8 eV + Eox), where, Eox
insights into the electronic structures, and optical properties is onset oxidation peak potentials. Besides, the band gap energy
and the nature of the transition of BIPIAN and BITIAN are well (DEg) of the molecules were examined from the onset wave-
explored by theoretical methods.
length which is obtained from intersecting of absorption and
The radiative lifetime (s) have been computed for sponta- emission spectra, further which could be useful for calculating
neous emission.40 OLED molecules with short radiative lifetime LUMO energy levels (LUMO ¼ HOMO + DEg). The HOMO energy
have been known to have high light-emitting efficiency while levels for BIPIAN ꢁ5.75 eV, BITIAN ꢁ5.76 eV and LUMO energy
those with long radiative lifetime facilitate electron and energy level for BIPIAN ꢁ2.77 eV, BITIAN ꢁ2.48 eV. Intriguingly, the
transfer and the attack of active species.41 Both BIPIAN and electrochemical stability of D–p–A compounds is evaluated
BITIAN show short radiative lifetime (1.63–1.92 ns) and this using CV in dichloromethane by applying different scan rate
indicates that they are good light-emitting materials (Table 3). It from 100–1000 mV sꢁ1 in repeated cycles. Hence, there is no
is a key point toward the development of this type of materials
for OLEDs. It is well-known that uorescence emission is
accompanied by energy ejection. When the energy of the uo-
rescence excitation is EFlu and the energy difference between the
HOMO and LUMO are DEH–L, the exciton binding energy can be
dened as Eb ¼ DEH–L ꢁ EFlu. Therefore the exciton binding
energy (Eb) is the energy required to destroy a hole–electron
exciton. The values of Eb for BIPIAN and BITIAN indicate that
the energy required to destroy a hole–electron exciton follows
the order BITIAN < BIPIAN.
Electrochemical properties
The redox property of the new uorophores was probed using
cyclic voltammetry, further the frontier molecular orbital level
also studied. The results are shown in Fig. 5 and S11†, which
indicate their potential application in bipolar charge transport Fig. 5 Cyclic voltammogram of BITIAN.
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Table 4 Photophysical, electrochemical, thermal data for BIPIAN and BITIAN compounds
Compounds
alabs
lemi
bEox [V]
cHOMO [eV]
LUMO [eV]
Eg [eV]
dTg [ꢀC]
Td10 [ꢀC]
Tm [ꢀC]
Tc [ꢀC]
BIPIAN
BITIAN
376
408
476/539
493/534
1.45
1.46
ꢁ5.75
ꢁ5.76
ꢁ2.98
ꢁ2.98
2.77
2.48
253
384
318
312
283
n.d
185
n.d
a
Abs/emis maximum for both molecules. b Obtained from CV measurements in 1,2-dichloromethane solution. c The HOMO and LUMO energies
were determined from CV and the absorption onset. Ferrocene (Fc; 4.8 eV) was used as the internal standard in each experiment. The Fc oxidation
peak potential was located at +410 mV, relative to the saturated Ag/AgCl reference electrode. Evaluate from the onset of the absorption spectra, Eg ¼
(1240/lonset). d Measured for Tg & Td; (i.e. not detect).
change in the peak potential which revealed that both mole- ionization potentials (IPs), electronic affinities (EAs) and reor-
cules are electrochemically more stable. It indicates that the ganization energies (l) of BIPIAN and BITIAN.42 The IPs and
energy levels are benecial to assign superior electrochemical EAs, together with the hole extraction potential (HEP), which is
stability in optoelectronic device operation.
the energy difference from M+ (cationic) to M (neutral molecule)
using the M+ geometric structure and the electron extraction
potential (EEP), which is the energy difference from Mꢁ
(anionic) to M using the Mꢁ geometric structure. The IPs and
EAs can be either for vertical excitations (v, at the geometry of
the neutral molecule) or adiabatic excitations (a, at the opti-
mized structures for both the neutral and charged molecule).
The reorganization energies for electron transport (lelectron) and
hole transport (lhole) have been well claried.33,34d,43
Thermal analysis
Thermal behaviour of synthesized D–p–A compounds was
evaluated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under nitrogen atmosphere with
the heating rate of 10 ꢀC minꢁ1. The TGA shown in Fig. 6 reveals
that both compounds have good thermal stability although
small molecular weight. We observed a higher onset decom-
position temperature (Td) at 308, 312 ꢀC with 10% of weight loss
The main challenge for the application of organic molecules
in OLEDs is the achievement of high EA and low IP to improve
the electron and hole transport electronic devices.33,44 For OLED
material, lower the IP easier the entry of hole from indium tin
oxide (ITO) to hole transporting layer (HTL) and higher the EA
easier the entry of electron from cathode to electron transport
layer (ETL).45 It has been experimentally proved that BNPB is
a good trifunctional molecule.32,45 The IPs of BIPIAN (6.33 eV)
and BITIAN (6.15 eV) are close to that of BNPB (6.09 eV),
therefore they can be used as HTL materials. From EAs values,
BIPIAN (1.07 eV) and BITIAN (1.17 eV) are expected to accept the
electron easily than BNPB (0.74 eV).46 Therefore both BIPIAN
and BITIAN exhibit more excellent properties as ETL materials
than BNPB due to the nature of donor and acceptor groups. The
trends in the IPs and EAs of BIPIAN and BITIAN are similar to
those of the negative HOMO and LUMO energies (Table S3†). To
be an emitting layer material, it needs to achieve a balance
between hole injection and electron acceptance. By theoretical
concept, if l value is lower, the charge-transport rate will be
higher. The lhole for BIPIAN (0.27 eV) and BITIAN (0.28 eV) are
lower than their respective lelectron (BIPIAN ¼ 0.36 eV; BITIAN ¼
0.32 eV) and hence suggesting that the hole transfer rate is
higher than the electron transport rate. Hence, these
compounds can be used as an HTL than the ETL. However, the
energy differences between the lhole and lelectron is very small
about 0.09 and 0.04 eV for the compounds BIPIAN and BITIAN,
thus these compounds can also act as ambipolar material.
ꢀ
and onset glass transition temperature (Tg) at 253, 384 C for
BIPIAN, BITIAN respectively. For BIPIAN, crystallization
temperature observed at 185 ꢀC and melting temperature at
283 ꢀC but in the case of BITIAN unable to detect (Fig. S12† and
Table 4). The thermal properties of biphenyl compounds are
exible in nature and have lower Tg/Td of BIPIAN, when
compared with BITIAN. Moreover, thermal behaviour of
synthesized compounds was studied with better understanding
and identied which is suitable to make uniform lm
morphology and an appreciable device performance.
Ionization potential, electron affinity and reorganization
energy
To understand the nature of energy barrier for injection and
transport rates for holes and electrons, we have computed
Electroluminescence performance
The synthesized luminogen molecule BITIAN is fabricated as
emissive layer in OLED device. Computed data indicates that
these materials are suitable for both hole as well as electron
transporting layer. Further, we plan to understand the emission
properties of newly assembled D–A molecule using
Fig. 6 Thermogravimetric analysis of BIPIAN and BITIAN.
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luminescence behaviour. Fig. 7 and S13† demonstrates the performances. We speculated that with proper hole injection
current density–voltage–luminance (J–V–L) plot of OLED con- layers, this leakage problem can be reduced. The maximum
taining BITIAN as emissive layer. The OLED structure consists luminance efficiency was observed around 100 cd mꢁ2 at 20 V.
of p-i-n conguration, where p denotes N,N0-di(1-naphthyl)- Deep HOMO level of BITIAN is expected to generate electron
N,N0-diphenyl-(1,10-biphenyl)-4,40-diamine (NPB) as a HTL, n accumulation at BITIAN/TPBI interface owing to high injection
indicates
2,20,200-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benz- barrier for holes. Aer 7.5 V, the recombination process is
imidazole) (TPBi) as ETL and i depicts the BITIAN layer. The started in BITIAN in which certain leakage current is contrib-
exact conguration is as follows: ITO/NPB (40 nm)/BITIAN (40 uted by the electron accumulations. To avoid this kind of
nm)/TPBi (20 nm)/LiF/Al. The applied turn-on voltage of the leakage current, it is advisable to use hole injection layers.
device is 7.5 V indicating balanced electron to hole injection However, the emission achieved in this structure is only from
barrier available in this structure. However, the high hole BITIAN material.
injection barrier available at ITO/NPB as compared to electron
The EL spectrum recorded at 11 V for the above-discussed
injection may be a reason for this slightly higher turn-on OLED is depicted in Fig. 8. Hence the full width at half
voltage, the device working with an extended operating maximum (FWHM) for electroluminescence spectrum signies
voltage up to 20 V. The lower luminance might attribute to the the high color purity based on performance of the device.
poor hole injection in to the BITIAN molecule. Because, deeper However, the maximum emission occurred at 564 nm and there
HOMO level of BITIAN (ꢃ5.7 eV) is expected to generate hole is no additional peak observed. It indicates the charge
accumulation at BITIAN/TPBI interface owing to high injection
barrier. As a result, we expected a larger electron leakage from
the EML which subsequently reduced the luminance and device
Fig. 7 (a) Current density (J)–voltage (V)–luminance (L) characteristics Fig. 8 (a) EL spectrum of p-i-n non-doped fluorescent OLEDs with
of p-i-n fluorescent OLED with BITIAN as emitting material and (b) BITIAN as emitting material and (b) CIE color coordinates of BITIAN
schematic of the p-i-n OLED structure.
with the NTSC standard.
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Table 5 Electroluminescence performance for BITIAN
EL
lmax lmax
FWHM
Turn on
voltage
(eV)
Maximum
current
Maximum power Maximum
CIE 1931
Coordinates
(x, y)
efficiency (lm
quantum
Device conguration
(nm) (nm)
efficiency cd Aꢁ1 Wꢁ1
)
efficiency (%)
p-i-n type: (ITO/NPB/BITIAN/TPBI/ 564
LiF/Al)
116
7.5
0.687
0.125
0.243
0.45, 0.52
recombination only generated on the BITIAN layer. The EL electronic transitions are of p / p* type and excitation to S1
emission was observed at a longer wavelength (564 nm) than PL state corresponds mainly to the promotion of an electron from
emission (534 nm). This kind of Stokes shi might be attributed HOMO / LUMO in BIPIAN and BITIAN. The lhole values of the
to the charge traps presented in BITIAN due to certain oxidized BIPIAN and BITIAN are smaller than that of lelectron values and
species or due to the excimer formation.
this suggests that the hole-transfer rate is better than the elec-
Table 5 indicated the overall performance of p-i-n type non- tron transfer rate. However, the energy differences between the
doped uorescent OLEDs using BITIAN as the emission layer. lhole and lelectron is very small about 0.09 and 0.04 eV respec-
The CIE 1931 coordinates for this emission is found to be (x, y) tively for the compounds BIPIAN and BITIAN, and this indicates
0.45, 0.52 indicating the yellowish green emission47 thus that these compounds can also act as ambipolar material.
conrms the photon generation only from the BITIAN. The Indeed the results are encouraging and also in good agreement
maximum current and power efficiencies are 0.68 cd Aꢁ1 and with EL performance of p-i-n type non-doped OLEDs. The
0.12 lm Wꢁ1 respectively. The EQE of the device reordered to be emitter BITIAN molecule exhibits FWHM at 116 nm through EL
0.24%. The relatively poor performances recorded for the above spectrum of non-doped multi-layered device performance. The
OLED might be attributed to the deep HOMO of BITIAN, which device reveal yellowish green emission with maximum EQE
can contribute to leakage current by electron accumulation at 0.26% and CE 0.687 cd Aꢁ1 under vacuum deposited process.
ETL side. Even though, luminance is lower, the uniformity of BITIAN achieved a stable yellowish green EL at 564 nm and CIE
the pixel was notably good as shown in the inset of Fig. 8. coordinate (0.45, 0.52) conforming to NTSC standard.
Further, the stability of the pixels also stable for under epoxy
encapsulation for few days. However, the present work delin-
eated the possibility of light generation with BITIAN
Experimental section
General information
compounds. Despite the poor performances, BITIAN proved to
be an electroluminescence active material for OLED applica-
tions. CIE co-ordinates, emission wavelength and efficiencies
reected that the BITIAN can be processed in OLED as a uo-
rescent emitter material with a proper device conguration.
Materials and methods. All reagents were obtained from
commercial sources, unless otherwise specied. The solvents
toluene and tetrahydrofuran were puried by standard puri-
cation
techniques.
4-Methoxyaniline,
benzile,
4-for-
mylphenylboronicacid, 4-bromobenzaldehyde, 5-bromo-2-
thiophenecarboxaldehyde and 3-indoleacetonitrile were
purchased from Sigma Aldrich or Alfa Aesar and used as
Conclusion
In summary, we have developed a new series of ambipolar type received without further purication. All the reactions were
imidazole based D–p–A moiety with diverse linkage. The newly conducted in oven-dried glassware under a positive pressure of
synthesized molecules (BIPIAN and BITIAN) are well charac- argon with magnetic stirring. 1H and 13C NMR spectra were
terized by various analytical techniques. The photophysical and recorded (400 and 100 MHz, respectively) on a Bruker Avance
electrochemical studies show that the synthesized uorophores DPX 400 MHz using CDCl3 & DMSO-d6. Chemical shis for
are compatible for OLED application. Further, both molecules proton and carbon resonances are reported for the major
have good thermal, morphological stability, larger Stokes shi, isomer in parts per million (d) relative to tetramethylsilane (d
uorescence quantum yield and enhanced charge balance 0.00), chloroform (d 77.23) and DMSO-d6 (d 39.51), respectively.
abilities. In addition, Lippert–Mataga and ET(30) relationship Multiplicities are indicated by singlet (s), doublet (d), triplet (t),
suggest an increase of TCI in electronically excited molecules. multiplet (m). Coupling constants (J) were reported in hertz
We have also performed theoretical investigation to explore (Hz). Carbon types were determined from 13C NMR and DEPT
their optical, electronic properties, and device performance. experiments. High resolution mass analyses were performed
The HOMO and LUMO levels of BIPIAN and BITIAN are using electron spray ionization (ESI) technique on a Thermo
compared with those of BNPB, ITO and Alq3 and the results Exactive Orbitrap mass spectrometer. Steady-state spectro-
revealed that BIPIAN and BITIAN could be used as ‘tri- scopic techniques were analyzed to obtain solution and lm
functional materials’ (emitter, hole and electron transporters) forming (JASCO V-360 spectrophotometer) by vacuum (2 ꢂ 10ꢁ6
in OLEDs. Synthesized molecules were found to be efficient with mbar) deposition on a quartz substrate. UV/Vis absorption
the lowest band gap and high wavelength absorption and spectra were measured in various solvents such as spectro-
emission. The longest wavelength absorption corresponds to scopic or HPLC grade and photoluminescence (PL) spectra in
the excitation to the lowest singlet excited state, where the solution were obtained on a JASCO FP-8300 spectrouorometer
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in different solvents (cyclohexane to N,N-dimethylformamide) 128.40, 128.23, 128.16, 128.11, 127.3, 126.7, 122.6, 114.4,
performed at room temperature and the lm was examined by 55.3 ppm.
using a JASCO V-630 spectrouorometer respectively. Fluores-
cence quantum yield (Ff) of the molecules was determined Synthesis of BIBP-AL
using the following equation,
In a 30 ml of ame-dried Schlenk-ask BIMP-Br or BIMT-Br (1
mmol) and 4-formyphenyl boronic acid (1.1 mmol) was dis-
2
Ff ¼ FRISODRhS2/IRODShR
(1)
solved in 20 ml THF. To this solution, 10 ml of 2 N K2CO3
solution and tetrakis(triphenylphosphine)palladium (0.002
mmol) were added. The reaction mixture was stirred at room
temperature under nitrogen atmosphere for 0.5 h aer that
heated to 80 ꢀC and kept for 12 h at the same temperature. The
reaction was monitored using TLC aer completion of the
reaction cooled to room temperature and diluted with 50 ml of
water. The reaction mixture was extracted with dichloro-
methane solution (2 ꢂ 50 ml), washed with water, dried over
Na2SO4 and solvent removed under reduced pressure. The
resultant residue was puried using column chromatography
using silica gel (100–200 mesh) (hexanes/DCM, 7 : 3, v/v) to
obtain corresponding product; yellowish white solid; yield: 1.5 g
where, Ff, IS, ODS, hS are the uorescence quantum yield,
photoluminescence integrated areas, optical densities of the
excitation wavelength, refractive indexes of sample respectively,
and FR, IR, ODR, hR uorescence quantum yield (9,10-diphe-
nylanthracene; FR ¼ 0.95 in ethanol was used), photo-
luminescence integrated areas, optical densities of the
excitation wavelength, refractive indexes of standard reference
material respectively. The electrochemical stability was deter-
mined by cyclic voltammetry (CV) using a bio-logic (SP-50)
potentiostat in degassed dichloromethane solution. Tetrabuty-
lammoniumhexauorophosphate (TBAPF6: 0.1 M) was utilized
as supporting electrolyte. The redox peak potential was ob-
tained at the scan rate of 0.1 V sꢁ1. These was a three-electrode
cell setup conguration which consist of platinum wire as an
auxiliary electrode, a saturated Ag/Ag+ reference electrode,
platinum as a working electrode and ferrocene–ferrocenium ion
(Fc/Fc+) used as internal standard (4.8 eV). However, the onset
potential was investigated at the intersect point of two tangents
at the maximum slope of the photocurrent and background
current of the cyclic voltammogram. Thermal behaviour of TGA
1
(71%); mp 227–228 ꢀC; H NMR (400 MHz, CDCl3) d: 9.98 (s,
1H), 7.87 (d, 2H, J ¼ 8 Hz) 7.69–7.49 (m, 8H), 7.25–7.12 (m, 8H),
6.99 (d, 2H, J ¼ 8.8 Hz), 6.76 (d, 2H, J ¼ 12 Hz), 3.73 (s, 3H) ppm;
13C NMR (100 MHz, CDCl3) d: 191.9, 159.3, 146.3, 146.3, 139.0,
138.5, 135.3, 134.5, 131.6, 131.2, 130.8, 130.6, 130.3, 129.8,
129.5, 129.3, 128.5, 128.3, 128.1, 127.5, 127.4, 127.1, 126.7,
114.4, 55.4 ppm.
Synthesis of BIPT-AL
and DSC were evaluated (Td and Tg)ꢁ1under a nitrogen atmo-
1
ꢀ
ꢀ
sphere at a heating rate of 10 C min
.
Pale yellow solid; yield: 1.2 g (57%); mp 212–213 C; H NMR
(400 MHz, CDCl3) d: 9.94 (s, 1H), 7.82 (d, 2H, J ¼ 8.4 Hz), 7.66 (d,
2H, J ¼ 8 Hz), 7.59 (d, 2H, J ¼ 7.2 Hz) 7.26–7.13 (m, 11H), 6.87 (d,
2H, J ¼ 8.8 Hz), 6.60 (d, 1H, J ¼ 4 Hz), 3.80 (s, 3H) ppm; 13C NMR
(100 MHz, CDCl3) d: 191.4, 160.0, 142.6, 141.9, 139.7, 138.6,
135.1, 134.8, 134.1, 131.9, 131.0, 130.5, 130.1, 130.0, 129.0,
128.5, 128.2, 128.2, 127.4, 126.9, 126.8, 125.8, 125.4, 114.7,
55.5 ppm.
Synthesis of BIMP-Br
A mixture of benzil (1 mmol), p-bromobenzaldehyde or 5-
bromo-2-thiophene carboxaldehyde (1 mmol), p-anisidine (4
mmol) and NH4OAc (4 mmol) was dissolved in glacial acetic
acid (15 ml) and reuxed for 3 h under nitrogen atmosphere.
The reaction was monitored using TLC aer completion of the
reaction it was cooled to room temperature and poured into ice
water to precipitate. The precipitate was ltered and washed
with 50 ml of water (four times), dried in vacuo and the crude
product was further puried by column chromatography using
silica gel (100–200 mesh), (hexanes/ethyl acetate; 4 : 2 v/v) to
provide corresponding product as a white solid; yield: 5.2 g
(75%); mp 188–190 ꢀC; 1H NMR (400 MHz, CDCl3) d: 7.75 (d, 2H,
J ¼ 4.0 Hz), 7.38–7.11 (m, 12H), 6.94 (d, 2H, J ¼ 8.0 Hz), 6.76 (d,
2H, J ¼ 8.0 Hz), 3.76 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) d:
159.3, 145.9, 138.3, 134.3, 131.5, 131.3, 131.1, 130.5, 130.3,
129.6, 129.4, 128.4, 128.2, 128.1, 127.4, 126.7, 122.6, 114.4,
55.4 ppm.
Synthesis of BIPIAN
In oven-dried round bottom ask BIBP-AL or BIPT-AL (1 mmol),
indole-3-acetonitrile (1 mmol), and potassium tert-butoxide (2
ꢀ
mmol) were dissolved in dry methanol and reuxed at 70 C.
The reaction was monitored using TLC, aer completion of the
reaction mixture was cooled to room temperature and poured
into ice water to give yellow precipitate and that was ltered and
washed with water (2 ꢂ 25 ml) and methanol (1 ꢂ 25 ml) and
again washed with water (1 ꢂ 25 ml). Then, the crude product
was puried using column chromatography (silica gel 100–200
mesh) in hexanes : CH2Cl2 (4 : 2 v/v) as an eluent to obtain
corresponding products. Greenish yellow solid; yield: 800 mg
ꢀ
(63%); mp 253–254 C; IR (neat): nmax 2918, 2850, 1599, 1509,
Synthesis of BIMT-Br
1443, 1249, 1133, 1026, 822, 731 cmꢁ1 1H NMR (400 MHz,
;
Pale yellow solid; yield: 5 g (72%); mp 203–205 ꢀC: 1H NMR (400 CDCl3) d: 11.76 (s, 1H), 8.10–8.00 (m, 3H), 7.86–7.73 (m, 6H),
MHz, CDCl3) d: 7.46 (d, 2H, J ¼ 7.2 Hz), 7.26–7.01 (m, 11H), 6.83 7.54–7.51 (m, 5H), 7.32–7.19 (m, 12H), 6.91–6.90 (m, 2H), 3.73
(d, 2H, J ¼ 8.0 Hz), 6.65 (d, 2H, J ¼ 12 Hz), 3.63 (s, 3H) ppm; 13
C
(s, 3H) ppm; 13C NMR (100 MHz, DMSO-d6) d: 159.0, 145.6,
NMR (100 MHz, CDCl3) d: 159.3, 145.9, 138.3, 134.3, 131.6, 139.6, 138.5, 137.2, 136.9, 135.7, 134.4, 134.0, 131.8, 131.1,
131.5, 131.3, 131.1, 130.4, 130.3, 129.50, 129.46, 129.37, 128.44, 130.5, 129.90, 129.86, 129.3, 129.1, 128.6, 128.44, 128.38, 128.1,
14554 | RSC Adv., 2019, 9, 14544–14557
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126.74, 126.68, 126.4, 126.32, 126.27, 123.6, 122.5, 120.5, 119.5,
118.5, 114.3, 112.4, 110.8, 105.5, 55.3 ppm; HRMS (ESI):
C
Conflicts of interest
45H32N4O [M + H]+ calcd m/z: 645.2654 obtained 645.2645.
There are no conicts to declare.
Acknowledgements
Synthesis of BITIAN
ꢀ
Greenish yellow solid; yield: 950 mg (73%); mp 245–246 C; IR
(neat): nmax 3055, 2926, 2212, 1650, 1511, 1439, 1247, 1113, 1025,
776 cmꢁ1; 1H NMR (400 MHz, CDCl3) d: 11.78 (s, 1H), 8.09–7.76
(m, 7H), 7.52–7.19 (m, 17H), 7.0–6.98 (m, 2H), 6.35–6.34 (m,
1H), 3.78 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3) d: 159.6,
142.6, 141.2, 137.2, 136.9, 135.5, 134.0, 133.7, 133.1, 131.9,
131.0, 130.2, 130.0, 129.2, 128.53, 128.48, 128.2, 126.8, 126.6,
126.3, 126.0, 125.4, 125.0, 123.6, 122.5, 120.5, 119.5, 118.5,
114.6, 112.5, 110.8, 105.5, 55.4 ppm; HRMS (ESI): C43H30N4OS
[M + H]+ calcd m/z: 651.2219 obtained 651.2218.
S. M. thanks University Grants Commission (UGC) F1-17.1/
2013-14/RGNF-2013-14-SC-TAM-48918/(SA-III/Website), New
Delhi for Rajiv Gandhi National Fellowship (RGNF). R. R. is
thankful to DST Nanomission (SR/NM/NS/1256/2013) for the
project and UGC (UGC-2013-35169/2015) for project and
Emeritus Fellowship (F.66/2016-17/EMERITUS-2015-17-OBC-
7855/(SA-II)). We thank DST, New Delhi, for providing 400
MHz NMR & HRMS facility under FIST program. P. V. thanks
Council of Scientic and Industrial Research (CSIR) for the
award of an Emeritus Scientist scheme (21(0936)/12/EMR-II).
This research also supported by Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education (NRF-
2014R1A6A1030732 and 2017R1A2B4005583).
Device fabrication and measurements
The electroluminescence device was fabricated on a glass
substrate which is pre-coated with indium tin oxide (ITO) as
anode, 1,4-bis[(1-naphthyl-phenyl)amino]biphenyl (NPB 40 nm)
as a hole-transporting layer and 2,20,200-(1,3,5-benzinetriyl)-
tris(1-phenyl-1-H-benzimidazole) (TPBi 20 nm) as electron
transporting layer and lithium uoride (LiF/Al) and aluminium
used as bi-layer cathode, which are commercially available
materials. Before using the device, the substrate was washed
systematically and treated with oxygen plasma for 5 min and
drying with air for 20 min at 150 ꢀC. The hole transporting
material (NPB) was vacuum deposited on cleaned ITO-coated
glass substrate in nitrogen atmosphere for 20 min. Before
making device, the emitter was treated at 100 ꢀC under vacuum.
Moreover, the emissive layer was thermally evaporated under
nitrogen atmosphere for 20 min. Subsequently, TPBI and LiF/Al
were successfully evaporated on emissive layer by vacuum
thermal process. The device performance was carried out at
ambient temperature with nitrogen atmosphere under dark
place. The electrical behaviour of the device was used to analyse
current density–voltage–luminescence by Minolta CS-1000
spectrometer and a Keithley 2400 source measuring system
used to determine the OLED performance.
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