Pronounced luminescence efficiency and thermal stability of small imidazole architect 2-(1, 4, 5-triphenyl-1H-imidazol-2-yl)phenol for efficient non-doped blue OLEDs

Article abstract of DOI:10.1016/j.jphotochem.2018.08.009

A remarkable attention have been devoted to fabricate blue OLEDs, since it plays a vital role in the field of organic electronics particularly in full colour flat panel displays. Though, notable research outputs were emerged, it is still being a challenging task by the researchers to fulfil its essential criteria such as highly efficient, colour stable and long life-span materials. With the view to address the issues, herein we report an easy synthetic strategy under catalyst free condition for a single imidazole moiety, 2-(1, 4, 5-triphenyl-1H-imidazol-2-yl)phenol, (TSPI-1) with excellent emissive behavior and high thermal stability. OLED device utilizing TSPI-1 as an active layer with the configuration of ITO/PEDOT: PSS (40 nm) /TSPI-1 (60 nm)/ TPBi (30 nm) / LiF (1 nm)/Al (100 nm) displayed blue emission with the maximum brightness of 1.60 Cd/m², CIE coordinates (0.17, 0.18) and the turn on voltage of 3.77 V. The synthesized imidazole derivative, TSPI-1 was well characterized by FT-IR, UV–vis, photoluminescence (PL), ¹H & ¹³C NMR, TGA-DTA, mass spectrometry and cyclic voltammetric (CV) studies. The results revealed the existence of localized electron density through non-planar geometry, specifically in solid state, good decay time, better morphological stability, (T_g = 111.93 °C) and high decomposition temperature (T_d = 344 °C), therefore rendering it as a highly desirable material to fabricate OLED device under vacuum evaporation.

Full text of DOI:10.1016/j.jphotochem.2018.08.009

JournalofPhotochemistry&PhotobiologyA:Chemistry365(2018)232–237
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Pronounced luminescence efficiency and thermal stability of small
imidazole architect 2-(1, 4, 5-triphenyl-1H-imidazol-2-yl)phenol for efficient
non-doped blue OLEDs
Sarala La, Ramesh Babu Yathirajulab, Peddaboodi Gopikrishnab, Elanthamilan Elaiyappillaia,
Bella Aa, Sundar Manoharan Sd, Parameswar Krishnan Iyerb,c, Princy Merlin Johnsona,⁎
a
Department of Chemistry, Bishop Heber College, Tiruchirappalli- 620017, Tamil Nadu, India
Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati- 781039, Assam, India
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati- 781039, Assam, India
Vice Chancellor, Sathyabama Institute of Science and Technology, Chennai- 600119, Tamil Nadu, India
b
c
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
Imidazole
Photoluminescence
Blue OLED
Electrochemical behavior
Organic electronics
A remarkable attention have been devoted to fabricate blue OLEDs, since it plays a vital role in the field of
organic electronics particularly in full colour flat panel displays. Though, notable research outputs were
emerged, it is still being a challenging task by the researchers to fulfil its essential criteria such as highly effi-
cient, colour stable and long life-span materials. With the view to address the issues, herein we report an easy
synthetic strategy under catalyst free condition for a single imidazole moiety, 2-(1, 4, 5-triphenyl-1H-imidazol-2-
yl)phenol, (TSPI-1) with excellent emissive behavior and high thermal stability. OLED device utilizing TSPI-1 as
an active layer with the configuration of ITO/PEDOT: PSS (40 nm) /TSPI-1 (60 nm)/ TPBi (30 nm) / LiF (1 nm)/
Al (100 nm) displayed blue emission with the maximum brightness of 1.60 Cd/m2, CIE coordinates (0.17, 0.18)
and the turn on voltage of 3.77 V. The synthesized imidazole derivative, TSPI-1 was well characterized by FT-IR,
UV–vis, photoluminescence (PL), 1H & 13C NMR, TGA-DTA, mass spectrometry and cyclic voltammetric (CV)
studies. The results revealed the existence of localized electron density through non-planar geometry, specifi-
cally in solid state, good decay time, better morphological stability, (Tg = 111.93 °C) and high decomposition
temperature (Td = 344 °C), therefore rendering it as a highly desirable material to fabricate OLED device under
vacuum evaporation.
1. Introduction
Quite enduring focus is made on a wide variety of novel organic
molecules to improve the efficiency of OLEDs, in particular, at low
Organic molecules in electroluminescent devices have received re-
cently a lot of attention by many researchers because of their specific
optoelectronic properties, which contribute several applications in
various fields, pioneered by Tang et al [1,2]. Particularly, its con-
tribution towards organic light emitting diodes (OLEDs) is exemplary
for display devices, with a view of its low cost, easy processability, fast
response, low power consumption, flexible and wide viewing angle,
compared to liquid crystal displays (LCDs). Moreover their high lumi-
nous efficiency and their ability to emit multiple colors throughout the
visible spectrum make them prominent in various display applications
[3–11]. Further significant efforts are employed to design versatile
fluorescent materials with improved device efficiency, chromaticity and
life time [12,13].
driving voltages with viable device fabrication method. One of the
notable hurdles in device fabrication is the lack of amorphous nature of
the materials in thin films, since the amorphous materials provide pin
hole free thin films, an essential criteria for device fabrication. On
comparing multi emitting single components, specifically for WOLEDs,
single emitters show several benefits such as high stability, good re-
producibility and easy fabrication processes.12 However, only few ma-
terials have been reported to emit pure light. Additionally, the main
drawback in the solid state fabrication includes the concentration
quenching effect caused by the strong intermolecular π-π interaction or
intermolecular hydrogen bonding with neighboring fluorophores,
which debases the emission [14].
To satisfy the requirements, till now, lot of organic heterocyclic
⁎
Corresponding author.
E-mail address: pmej_68@yahoo.co.in (P.M. Johnson).
https://doi.org/10.1016/j.jphotochem.2018.08.009
Received 20 April 2018; Received in revised form 4 August 2018; Accepted 5 August 2018
Availableonline07August2018
1010-6030/©2018ElsevierB.V.Allrightsreserved.

S. L et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry365(2018)232–237
moieties with conjugated units such as porphyrins, thiophene, fluorene,
quinolone, oxidazole etc. have been prepared and utilized for optoe-
lectronic applications [15,16]. However, these materials were found to
be less promising in OLEDs due to their own limitations, namely low
luminescence. Notably the heterocyclic imidazole based chromophores
have received attention due to their significant linear and non-linear
optical properties. Moreover, the imidazole ring can be tailored easily
with different chromophores, resulting in new conjugated systems,
which allow the fine tuning of important physical and photophysical
properties [17,18]. Imidazole rings, which act as useful n-type building
blocks, with high electron affinity and good thermal stability, have
been successfully incorporated in small molecular frameworks and also
in polymers as electron transport component in OLEDs [19–21].
For the real time application, especially in full color displays, it is
more important to have the three primary colors, Red, Blue and Green.
Moreover, Red and Green emissive materials with high efficiency and
long life time have been successfully prepared, while the fabrication of
efficient blue OLEDs is still the challenge of an era due to its good CIE
coordinates [21–23]. With the view to address this challenge, in the
present piece of work, significant efforts have been made to design a
versatile blue fluorescent material from a single small organic archi-
tecture namely a tetra substituted imidazole, 2-(1,4,5-triphenyl-1H-
imidazol-2-yl)phenol, (TSPI-1) to overcome some of the existing lim-
itations and to improve the performance of the blue OLEDs.
133.12, 135.21, 137.18, 144.95, 158.46 ppm; mass m/z value: 387.171
[24–26]. Anal. Calcd for (C27H20N2O): C, 83.48; H, 5.19; N, 7.21; O,
4.12. Found: C, 83.42; H, 5.16; N, 7.25; O, 4.17. (Please refer sup-
porting information Fig. S1 to S4 for FT-IR, 1H-NMR, 13C-NMR and
Mass respectively)
3. Characterization
3.1. Thermal stability of TSPI-1
The thermal properties of the as-synthesized TSPI-1 was examined
by thermogravimetric analysis. TG-DTA/DSC analyses were carried out
to determine the complete decomposition temperature Td, melting
temperature Tm and glass transition temperature Tg of TSPI-1. The
sample under study was heated from 35 °C to 500 °C at a heating rate of
5 °C/min. under nitrogen atmosphere. Under these conditions, a pro-
minent melting temperature was observed for the derivative TSPI-1
(Fig. 1), which is clearly indicative of its amorphous nature. TSPI-1
exhibited good thermal stability with Td of 344 °C, Tg of 111.93 °C and
Tm of 244 °C. The high values of Tg and Tm demonstrate the presence of
highly rigid substituent at N1 position of imidazole moiety, which
greatly improve its thermal stability [27]. In general, high Tg value of a
material implies that it could form a stable amorphous film upon
thermal evaporation, the most important criterion for fabricating
OLEDs [28]. In particular, the high thermal stability renders the com-
pound to withstand the inevitable joule heat encountered during device
operation [29].
2. Experimental
2.1. Materials and methods
TG-DTA and DSC curves of TSPI-1 are depicted in Fig. 1. The first
endothermic peak attributed to the melting point of the sample, was
observed to commence at 244 °C, which is also evident from the DSC
curve (Fig. 1b). Another important observation is that, there was no
phase transition until the material melts and this enhances the potential
applicability of the material for nonlinear optics (NLO) applications.
The weight loss started at 244 °C and continued till 344 °C. The com-
plete decomposition of the synthesized compound, TSPI-1 was observed
only at 344 °C [29]. Since the material possesses Tg above 100 °C, it
suggests that TSPI-1 has better stability than the commonly used host
materials, therefore the molecule TSPI-1 can be used as a stable organic
material for OLEDs under vacuum evaporation [30].
All the chemicals were purchased from Merck and used without
further purification. Solvents used were of analytical grade. FT-IR, 1H-
NMR (CDCl3), 13C-NMR (CDCl3) and TGA were recorded using Perkin
Elmer Infra-Red spectrometer, Bruker FT-NMR 500 MHz (TMS as in-
ternal reference) and TGA 4000 Perkin Elmer in nitrogen atmosphere at
a heating rate of 5 °C per minute, respectively, for the organic ligand
TSPI-1. Elemental analysis was carried out in a CHNO/S elemental
analyzer interfaced with a LECO Truspec Micro Analyzer. The glass
transition temperature of TSPI-1 was taken from the second heating
scan after quick cooling from 350 °C. Solution state absorption mea-
surements were carried out using Jasco, UV–vis-NIR V-670
Spectrophotometer and the solid state UV–vis diffuse reflectance
spectra were obtained using Jasco V- 650 spectrophotometer, the re-
ference used for solid state was barium sulphate and the emission
measurements were analyzed by Flurolog-3 photoluminescence. Cyclic
Voltammogram (CV) was recorded using Princeton Applied Research
Electrochemical Workstation using a three electrode system with glassy
carbon as working electrode, Pt as counter electrode and saturated Ag/
AgCl as reference electrode, using TBAP as supporting electrolyte under
nitrogen atmosphere.
3.2. Photophysical properties of TSPI-1
In order to observe the photophysical property of the derivative
TSPI-1, both solution (DMSO, 10−3 M) as well as solid state UV- spectra
were recorded. In the solution spectra, both spin allowed π- π* transi-
tion and forbidden n-π* transitions were noted at 284 nm and 319 nm,
respectively. Hence, it is evident that the synthesized compound TSPI-1
is rich in π-electron density. The optical band gap calculated using the
respective UV–vis spectrum is discussed below.
The UV–vis spectrum of TSPI-1 recorded in solution (Fig. 2.b) dis-
played two strong absorption bands at 284 nm and 319 nm, corre-
sponding to π-π* and n-π* transitions [7]. The particular compound
does not show any characteristic absorption below 260 nm in solid
state. However, the absorption maximum is red shifted as compared to
the solution spectrum. It indicates the presence of strong inter-mole-
cular interaction in solid state. (Fig. 2a), a bathochromic shift was ob-
served in solid state, annotates the steric hindrance of the –O-H group
attached to the phenyl ring, confirming the non-planar structure [17].
The absorption edge in solid state was found to be 482 nm, which re-
sults in the optical band gap of 2.57 eV calculated using the formula
2.2. Synthesis of tetrasubstituted imidazole (TSPI-1)
Stoichiometric amount of 1, 2 -diphenylethane-1,2-dione, 2-hydro-
xybenzaldehyde, ammonium acetate and phenylamine in ethanol were
refluxed using water condenser in an oil bath maintained at 160 °C for
6 h and the progress of the reaction was monitored by TLC using to-
luene : ethylacetate as eluent (7:3). The schematic representation for
the synthesis of TSPI-1 is depicted in Scheme 1. After the completion of
the reaction, the yellow solid separated out was washed with water,
ethanol, acetone and dried under vacuum and further purified by
column chromatography and then by vacuum sublimation. (Yield: 85%)
Structural Characterization of TSPI-1: FT-IR (KBr) νmax: 3426
(OH), 3056 (CAH aromatic), 1588 (C‚C aromatic), 1482 (C‚N)cm−1; UV
(DMSO) λmax: 284, 319 nm; 1H NMR (400 MHz, CDCl3): 13.47 (s, 1H,
OH) ppm, ≈7-8 ppm (phenyl protons); 13C NMR (100 MHz, CDCl3): C
113.02, 117.67, 126.03, 127.02, 128.29, 129.19, 129.92, 130.47,
[31] Eg = 1240 / λonset
.
Photophysical properties of the molecule TSPI-1 was examined by
measuring PL spectra in solid state as well as in solution state (DMSO- X
10−3M). From the Fig. 3, It is clearly depicted that the molecule
strongly emits blue light with the λmax at 444 nm in solid state (FWHM-
68 nm) and 461 nm in DMSO (FWHM- 112 nm). The maximum
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JournalofPhotochemistry&PhotobiologyA:Chemistry365(2018)232–237
Scheme 1. Schematic Representation for the Synthesis of TSPI-1.
Fig. 1. Thermo Analytical Behaviour of TSPI-1 a) Merged TGA and DTA Curves b) DSC Curve with Tg as inset.
emission peak of TSPI-1 in solution state exhibited bathochromic shift
in comparison with PL emission in solid state. This shift accounts for the
characteristic intermolecular π-π stacking interaction of the organic
material in the solid state [32].
emissive materials in OLEDs can be estimated directly by CV, using the
information about the oxidation and reduction potentials, since the
oxidation process matches with the removal of electrons from the
HOMO energy level and the reduction process corresponds to the ad-
dition of electron in the LUMO energy level of the material. The current
that flows will arise by the transfer of electrons between the energy
level of the working electrode and the molecular energy level of the
materials under study. The onset oxidation and reduction potentials of a
material can be correlated to the ionization potential (Ip) and electron
3.3. Electrochemical properties
The energy levels of the relative highest occupied molecular orbitals
(HOMO) and lowest unoccupied molecular orbitals (LUMO) of the
Fig. 2. UV–vis spectrum of TSPI-1 a) in solid state b) in solution (DMSO).
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JournalofPhotochemistry&PhotobiologyA:Chemistry365(2018)232–237
Fig. 3. PL spectrum of TSPI-1 a) in Solid State b) in DMSO.
affinity (Ea), in accordance with the empirical relationship proposed by
Bredas and co-workers, on the basis of the comparison between valence
effective Hamiltonian calculations and experimental electrochemical
measurements. The correlation can be expressed by the following
equations 1, 2 & 3.
Ip = Eoxdn + 4.8 eV
(1)
Ea = Eredn + 4.8 eV
(2)
Eg = Ip + Ea
(3)
where Eox and Ered are the onset potentials of oxidation and reduction,
respectively, while Eg is the optical band gap of the material, 4.8 eV is a
constant in the relation between Ip and Ea. The redox potential arises
from the difference in gas-phase ionization potential and electro-
chemical oxidation potential of solid films and the solid-state polar-
ization energy. The onset potentials are determined from the intersec-
tion of the two tangents drawn at the rising current and baseline
charging current of the CV traces. Using the onset potentials, rather
than the oxidation (anodic) and reduction (cathodic) peak potentials,
minimum Ip and maximum Ea values can be calculated for the typical
sample. The ionization potential (Ip), electron affinity (Ea) (HOMO and
LUMO energy levels) and Eg were determined from the CV and ab-
sorption measurements [33]. The calculated HOMO and LUMO energy
values using the electro chemical analysis are -4.99 eV and -2.42 eV,
respectively (Fig. 4).
Fig. 5. PL Decay Pattern of TSPI-1.
3.4. PL decay pattern of TSPI-1
To interpret the photophysical properties in a more intuitive
manner, the fluorescence life time of the TSPI-1 was measured and thus
fluorescence decay traces of the compound (Fig. 5) were collected as a
function of emission wavelength, λem, using the single photon counting
technique. Simultaneous curve fitting of the time resolved fluorescence
histograms of the compounds as a function of emission wavelength with
τ
linked over λem revealed that the decays are tri-exponential
(τ1 = 2.391974E-09 s, τ2 = 0.1519506E-09 s, τ3 = 3.237576E-09 s).
The transient fluorescent decay curve is fitted Tri exponentially. The
average life time of the excited state is 2.75 ns. It indicates the presence
of single excited state and thus identified as a better candidate for OLED
application [31].
4. OLED device fabrication using TSPI-1
This section outlines the device fabrication using the tetra sub-
stituted imidazole derivative, 2-(1,4,5-triphenyl-1H-imidazol-2-yl)
phenol (TSPI-1) as an active layer in a device consisting of ITO/
PEDOT:PSS (40 nm) /TSPI-1(60 nm)/ TPBI (30 nm)/LiF (1 nm)/Al
(100 nm) (Fig. 6). The compound displayed EL spectrum in blue region
with a driving voltage of 4 V.
All the organic layers were deposited by vacuum sublimation onto
the patterned, pre-cleaned and ozone treated indium tin oxide (ITO)
glass substrate. A hole injecting PEDOT: PSS layer was spin coated to a
thickness of 40 nm and vacuum dried. The title compound TSPI-1 was
Fig. 4. CV of TSPI-1 in DMSO, at the Scan Rate of 100 mV/s (inset = Eoxdn
onset).
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JournalofPhotochemistry&PhotobiologyA:Chemistry365(2018)232–237
emission in blue region, (blue emitter). The maximum brightness was
found to be very low and found to be 1. 60 cd/m2 (Fig. 7c). The current
density vs. voltage characteristic of the device is shown in Fig. 7d. The
CIE co-ordinates of emission color of TSPI-1 show the color chromati-
city values of (0.17, 0.18), which corresponds to blue color. On com-
paring the results with the existed literature, the advantage of TSPI-1 is
that it possess low onset voltage whereas the isoquinoline imidazole
and phenanthroimidazole derivatives has high turn on voltage
[30,34–36]. Though the material TSPI-1 results are inferior to the other
reported literature, efficient single emitting component blue OLEDs are
expected after optimizing the device configuration [21,22].
5. Conclusion
Fig. 6. Device Structure Fabricated using TSPI-1 as an Active Layer.
In summary, we document the emissive feature of the tetra sub-
stituted imidazole, TSPI-1, wherein a significant and selective blue light
emission was observed. The optical studies demonstrated the conclusive
evidences for the existence of localized electron density through non-
planar geometry of the derivative TSPI-1, especially in solid state. TGA
of the compound indicated a one-step decomposition, substantiating
the presence of intermolecular H-bonding. The salient features like
amorphous nature, high Tg (111.93 °C), observed band gap (2. 57 eV),
blue emissive behavior with low FWHM values and non-planarity of
TSPI-1in solid state render it as a highly desirable versatile organic
material to fabricate blue OLED device under vacuum evaporation.
Hence the strategy in the development of OLED with blue emission via
a single layer structure in a host can be easily achieved by the syn-
thesized TSPI-1 molecule. Based on the findings, the performance of the
derivative TSPI-1 as a device was further examined. The EL spectrum of
the device fabricated using TSPI-1 as an active layer showed a single
strong broad emission maximum at 457 nm (FWHM 84 nm) which also
deposited to a thickness of 60 nm and was sandwiched between the
PEDOT: PSS and the electron transport material TPBi (30 nm). A thin
layer of lithium fluoride (LiF) was topped onto the TPBi layer.
Aluminium (Al) layer was finally deposited as the cathode material.
The EL characteristics of the device with active layer TSPI-1 is
shown in Fig. 7. EL spectrum of the device showed a sharp peak at
457 nm with a FWHM of 84 nm at a turn on voltage of 3.77 V. It is
noteworthy to mention that the λmax of the solid state PL spectrum of
TSPI-1 agrees well with the EL spectrum, which further supports the
electrochemical stability of TSPI-1. The EL spectrum of the derivative
TSPI-1 was recorded with increasing applied voltage at an interval of
1 V. The graph is plotted between EL intensity and wavelength at var-
ious voltages (6 V to 10 V) and is shown in Fig. 7b. No shift in the EL
emission is observed in the device with the increase in the applied
voltage, which substantiates the fact that the compound is stable even
under bias stress. This shows the effectiveness of the TSPI-1 with the
Fig. 7. (a) EL spectra, (b) EL spectra at different voltages, (c) Brightness vs. Voltage and (d) Current Density vs. Voltage of the TSPI-1.
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JournalofPhotochemistry&PhotobiologyA:Chemistry365(2018)232–237
substantiated for blue emitter. No shift in the EL emission was observed
in the device with the increased applied voltage, which further con-
firmed the fact that the compound is stable even under bias stress. Thus
the study portrayed the effectiveness of TSPI-1 as blue emitter.
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