Efficient near ultraviolet emissive (CIE: Y < 0.06) organic light-emitting diodes based on phenanthroimidazole-alkyl spacer-carbazole fluorophores: Experimental and theoretical investigation

Article abstract of DOI:10.1039/d0tc03720f

Efficient near ultraviolet light-emitting materials are of particular importance in organic light emitting devices (OLEDs) because of their potential for application in high-quality flat panel displays and white OLEDs. In the present investigation, a seri

Full text of DOI:10.1039/d0tc03720f

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Efficient near ultraviolet emissive (CIEy < 0.06) Organic Light-Emitting Diodes
based on Phenanthroimidazole-Alkyl spacer-Carbazole fluorophores:
Experimental and Theoretical investigation
a
a
b
Jairam Tagare Rajamouli Boddula , Sujith Sudheendran Swayamprabha , Deepak Kumar
,
b
b
a
a
Dubey , Jwo-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, Taiwan-30013.
*
To whom correspondence should be addressed. Email: vsiva@nitrkl.ac.in (V. Sivakumar) Tel:
+
91-661-2462654
1

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TOC
2

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Abstract:
Efficient near ultraviolet light-emitting materials are of particular importance in organic light
emitting devices (OLEDs) because of its impending application in high-quality flat panel displays
and white OLEDs. In the present investigation, a series of efficient Near-UV light emitting
fluorophores based on Donor-spacer-Acceptor (D-s-A) geometry were design and synthesized. In
this design strategy, we have linked carbazole group with phenanthroimidazole unite through alkyl
spacer (capable of hole and electron transporting in the device) with different functional groups at
the N1 position of imidazole moiety. All the fluorophores were structurally established by
spectroscopic techniques and the synthesized fluorophores are shown near-UV emission in
dichloromethane (DCM) and it indicates that the N1 substitutions of imidazole moiety are played
an imperative role in the photophysical properties of the synthesized fluorophores. Cyclic
voltammetry investigation revealed that the fluorophores have low-lying energy levels of the
LUMO ranging from -2.88 eV to -3.35 eV, and energy of the HOMO ranging from -5.55 eV to -
5
.80 eV, suggesting that they may possess good charge-transporting properties. The theoretical
calculation experiment has been executed to verify the excited state properties of newly
synthesized fluorophores and indicates that these twisting D-spacer-A fluorophores exhibit high
triplet excited state (ranging from 2.70 to 3.12 eV). The resultant OLED devices based on these
fluorophores exhibited near UV-emission with Commission International de L’Eclairage (CIE)
color coordinates of y < 0.05 (close to NTSC standard value) and maxima of EL spectra centered
in between 395-405 nm at a particle doping concentration. The device using the PIFOCz emitter
possesses a favourable maximum external quantum efficiency (EQE) of 3.3%.
Keywords: Carbazole, Phenanthroimidazole, Fluorophores, DFT computations, near UV
OLEDs.
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Introduction:
In the past two decades, owing to their potential application in large size flat panel displays
including flexible displays) and solid state lighting sources, the development of deep blue to near
(
UV organic light emitting diode materials has attracted much attention.1-3 In addition, these
fluorophores gained additional importance, due to their low power consumption and their ability
to increase the color range of full-color organic light emitting diode (White OLED) 4, 5 and also be
useful as hosts (targeting to surpass the aggregation- or concentration-induced luminescence
quenching in fluorescent as well as triplet emitter) to generate other primary color (red and green)
OLEDs.6-11 It is also worth to note that achieving high performance in the device with the near UV
emitters are still bottle neck and the external quantum efficiency (EQE) achieved so far is typically
<
2%, this restriction is attributed to the intrinsic wide band-gap that induces the unbalanced charge
injection/transport in the device. White OLED displays require the emission of red, green and blue
RGB) light (primary colors). The development of highly efficient primary color emitters is one
(
of the most important tasks for making OLEDs into commercial products.12-14 In the National
System Committee (NTSC) color system, which uses Commission Internationale del’Eclairage
(CIE) coordinates as a key parameter, for the deep blue materials CIE coordinates of (0.14, 0.08)
are required, the CIEy is further improved to 0.06 by the European Broadcasting Union (EBU).15-18
Recently, rapid improvement occurs in the efficiencies of OLED, efficient fluorescent and
phosphorescent materials have already emerged.19-25 However, there are still some vital problems
that is inevitable (i.e. wide energy band gap) in the development of highly efficient deep-blue
materials with good color purity. Unbalanced charge injection as well as transporting is a foremost
problem in deep-blue OLEDs.26-28 Recently, phenanthroimidazole (PI) has appeared as a building
block for the synthesis of highly-efficient blue luminescent materials with simple synthesis
4

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procedure, good thermal stability properties, satisfactory color purity, high fluorescent quantum
yield.29-37 After various theoretical and experimental investigation, C2 and N1 in the heterocyclic
imidazole ring were identified as a two important substitution position, and the structure-property
relationships were used as active direction for deep blue PI-based materials with good
performance.38-42 The carbazole is a widely used functional group in the construction of efficient
luminescent materials for OLED application.43-47 Carbazole has better hole transporting as well as
injection capability, due to the electron donating character of carbazole and it is commonly linked
with electron deficient moieties in various ways to construct bipolar emitters. Carbazole
derivatives, such as 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) and 1,3-Bis(N-
carbazolyl)benzene (mCP) are most widely used host for triplet emitters based phosphorescent
4
8
OLEDs because these materials have sufficient large triplet T energy.
1
4
9
In 2013, Yuguang Ma and co-workers reported two blue emitting conjugated fluorophores
M1and M2, structures are shown in Fig. S1 in SI) with carbazole and phenanthroimidazole as D
(
and A subunits, respectively. OLED based on these emitters showed deep blue EL emission with
λEL of 420 nm (M1) and 428 nm (M2) and EQE of 1.94 % and 3.02 %, respectively. Recently, R.
5
0
Butkute, et al. published blue emitting conjugated fluorophores (7, 8, 9 and 10, structures are
shown in Fig. S1 in SI) with phenanthroimidazole and carbazole, triphenylamine as the A and D
functional black, respectively. The fabricated non-doped OLED devices showed blue emission
with EQE of ~1 %. By using 10-blue emitter the white OLED device also fabricated and the white
OLED device showed white EL emission with the EQE of 2.9 %.
A bipolar molecule can be prepared by chemical connectivity between hole- and electron-transport
moieties with (conjugated or non-conjugated) and without spacer group. In which a spacer group
will be conjugated aromatic group (phenyl or biphenyl or styrene)51-55 or non-conjugated aliphatic
5

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group (single bond or an alkyl chain).56-58 Till date numerous conjugated bipolar materials have
been developed for White OLEDs,51-54 few non-conjugated bipolar materials have been reported.56-
5
8
Non-conjugated and conjugated bipolar materials without flexible linkage tend to be bulky and
rigid, thus limiting the solubility and the ability to form morphologically stable films. The
discussed design strategy context, we report non-conjugated fluorophores containing the aliphatic
linkage between the two charge transport moieties (hole-/electron- transporting). We have
synthesized a series of PI derivatives (Fig. 1.) by taking thermally stable phenanthroimidazole as
electron-transporting moiety and carbazole as hole-transporting moiety with the different
5
9
functional groups at the N1 position of the imidazole ring. All the fluorophores were structurally
confirmed by spectroscopic technique. The synthesized fluorophores are shown deep blue
photoluminescence (PL) emission in dichloromethane solution. Cyclic voltammetry investigation
revealed that these fluorophores show bipolar charge transporting properties. The theoretical
calculation also was performed to now the electronic structure of the fluorophores and it indicates
that these twisting D-s-A fluorophores exhibit high triplet excited state. The doped OLED device
based on PIFOCz radiates near UV-emission (CIEx,y = 0.17, 0.05) with a forward viewing
maximum EQE of 3.3%.
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N
N
N
N
N
N
N
N
O
O
O
O
N
N
N
N
PIPOCz
PIPTOCz
PITBOCz
PIFOCz
Fig. 1. Chemical structure of PI derivatives.
Results and Discussion
Synthesis and Characterization:
The synthetic route of the target PI derivatives is shown in Scheme 1. The Fl-NH , 4-(4-
2
bromobutoxy) benzaldehyde (Intermediate-1) and 4-(4-(9H-carbazol-9-yl)butoxy) benzaldehyde
(
Intermediate-2) were synthesized according to a modified literature procedure.60, 61 The target PI
derivatives were synthesized by a condensation reaction between phenanthrene-9,10-dione, amine
derivatives, and Intermediate-2 in the presence of ammonium acetate and acetic acid. The
synthesized fluorophores were structurally characterized by Nuclear Magnetic Resonance
Spectroscopy, Mass Spectrometry, and Infrared Spectroscopy.
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CH3
H C
KOH, KI
DMSO,
HNO3
glacial
NO2
2
Br
CH COOH
9
H-fluorene bromoethane RT, 24 Hs.
3
Fl-alkyl
Fl-NO2
NH NH H O
2
2.
2
Pd/C
EtOH.
NH2
Fl-NH2
CHO
O
CHO
H
N
CHO
OH
Na/EtOH
TBAB, KOH
reflux, 18 hrs
Br
Br
reflux, 12 hrs
O
9
H-carbazole
1
,4-dibromobutane
4
-
hydroxybenzaldeh
yde
N
Br
CHO
Intermidiate-1
Intermidiate-2
O
N
N
R
O
O
NH2
N
ACONH4, Aceticacid
reflex, under N2
R
amine derivatives
O
phenanthrene-9,10-dione
Intermidiate-2
R=
*
*
*
*
N
Fluorophores
PIPOCz PIPTOCz PITBOCz
PIFOCz
Scheme 1. Synthetic routes of PI derivatives.
Thermal properties:
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The thermal properties of PI derivatives were investigated by thermogravimetric analysis (TGA)
from 0 to 800 °C at a heating rate of 10 °C/min under a N atmosphere. As shown in Fig. 2, all the
2
fluorophores showed almost similar decomposition progress starting from the same structural
framework. TGA curves reveal that the major degradation temperature (T ) of 5% weight loss of
d
PI derivatives is 326 °C (PIPOCz), 335 °C (PIPTOCz), 275 °C (PITBOCz), and 375 °C
(
PIFOCz), respectively (Table 1), which could be attributed to the decomposition of fluorophores
fragments (N1 subunits) present in the fluorophores and final decomposition undergoes at 544 °C
PIPOCz), 573 °C (PIPTOCz), 536 °C (PITBOCz), and 525 °C (PIFOCz), respectively, which
(
could be attributed to fragments of alkyl spacer, carbazole and phenanthroimidazole groups present
in the fluorophores. Among all the fluorophores, PIFOCz fluorophore showed slightly more
degradation temperature (5% weight loss), it is due to increased molecular mass with the addition
of thermally stable fluorene moiety connected at N1 position of the imidazole ring. When the
3
7, 62
molecular weight increases the thermal decomposition temperature (T ) also increases.
These
d
results indicate that synthesized PI derivatives provide desirable thermal properties, which are
favourable for use these fluorophores in OLED device.
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Fig. 2. Thermogravimetric curves of PI derivatives.
Photophysical properties:
The photophysical properties of the PI derivatives were investigated by UV-vis and PL
spectrometry. The absorption and PL emission spectra of the fluorophores are shown in Fig. 3 and
corresponding data is tabulated in Table 1. All the fluorophores exhibit similar spectral behaviour
(in solution Fig. 3a) with three major absorption bands. The absorption bands around 260 nm can
6
3
be assigned to the π-π* transition of the phenyl moiety. The bands at 294 nm were attributed to
6
4
the π-π* transition of the carbazole moieties in all the synthesized fluorophores. The observed
higher-wavelength weak absorption bands 360 nm probably originate from the π-π interaction of
the fluorophores. The absorption of spectra of fluorophores in solid was found to be broad and red-
shifted (10 nm) due to π-π packing interactions (Fig. 3c.).64 The optical band gap of the
fluorophores was calculated by converting solid absorption data into the Kubelka-Munk
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6
5
function. The obtain spectra is shown in Fig. 3b the found band-gaps are well matching with the
experimental analysis data (CV measurement).
The PL emission spectra of fluorophores (Fig. 3a and 3d) exhibited deep-blue emission with peak
wavelength at ~400 nm in DCM solution, while the PL emission of the fluorophores is red-shifted
in the solid because of the intramolecular π-π packing interaction.66 The CIE chromaticity
coordinate image of the fluorophores in solution and solid were shown in Fig. S19 in SI and
corresponding data is summarised in Table S2. The flurescence measurement also been executed
in different polar and non-polar solvents for PIPOCz fluorophor (Fig. S20 in SI) and the
fluorophore showed the similar emission spectra (not much red-shift is obsorbed), indicating that
no strong intramolecular charge transfer (ICT) existed in PIPOCz fluorophor. The quantum yield
(PLQY) of the fluorophores was calculated in solution and solid by using an integrating sphere.
6
7
The quantum yield of the fluorophores was calculated by using equation (S1). The calculated
quantum yield results are listed in Table 1. The measured quantum yield digital images are shown
in the SI6 section in the SI. PLQYs in DCM solution was estimated to be 26.1, 24.6, 22.4, and
2
5.2 % for PIPOCz, PIPTOCz, PITBOCz, and PIFOCz. Whereas in the solid state, PLQY is
increased as 54.9, 38.7, 37.4, and 47.27 %, respectively, it is due to that intramolecular π-π packing
interaction in sold phase of synthesized molecules. The high solid-state PLQY values of these
fluorophores benefit from the twisted molecular conformation and suggest their potential to work
as the emitting layer in OLEDs.
1
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Fig. 3. a) UV absorption and PL emission spectra of fluorophores in DCM solution, b) optical
bandgap of the fluorophores, c) UV absorption spectra of fluorophores in the solid state, and d)
PL emission spectra of fluorophores in the solid state.
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Table 1. Key photophysical properties of PI derivatives
Absolute quantum
Solution
Abs (nm)
Solid
Tda
yield (Φ)
Fluorophores
(
°C)
326
335
275
375
PL
nm)
Abs
(nm)
365
PL
Solution
(%)
Solid
(%)
(
(nm)
PIPOCz
PIPTOCz
PITBOCz
PIFOCz
248, 294, 315, 363
248, 294, 315, 362
260, 294, 315, 362
260, 294, 364
405
405
402
401
413, 434,
26.1
54.9
4
76, 507
290,
411, 505
24.6
22.4
25.2
38.7
37.4
3
75
375
408, 433,
4
57, 490
310,
400, 420,
446, 475
47.27
3
70
a
Thermal decomposition temperature corresponding to 5% weight loss
Theoretical calculations:
Further insight into the molecular orbital distributions of the PI derivatives was gained by Density
Functional Theory (DFT) calculation, which was carried out at the level of B3LYP/6-31(d) basis
6
8
set to the investigative ground state and excited state energy level of the PI derivatives. The
optimized geometries and calculated HOMO/LUMO energy levels of PI derivatives are shown in
Fig. 4 and 5 and Table 2 shows the calculated results of all fluorophores. Fig. 5 shows small energy
differences in the highest occupied molecular orbitals (HOMO) of all the PI derivatives which are
mostly distributed on the phenanthroimidazole moiety. However, unlike the case of HOMOs, all
the PI derivatives have shown a great energy difference in the lowest unoccupied molecular
orbitals (LUMOs). For PIPOCz, PIPTOCz and PITBOCz fluorophores the LUMO is distributed
from phenanthroimidazole moiety to N1 substitution group, whereas in PIFOCz fluorophore
mostly on the N1 substitution groups (Fig. 5). The almost complete separation of HOMO and
LUMO energy levels (PIFOCz) suggests that the separation of electron cloud of HOMO and
LUMO could afford the transporting channels between hole and electron, which may endow the
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fluorophore with the ability of bipolar charge transport in the device. In addition, the excited
energy levels (singlet and triplet) of synthesized fluorophores were also been calculated by using
Time-Dependent-DFT (TD-DFT)/B3LYP method. The calculated singlet and triplet energy values
are tabulated in Table 2. The obtained triplet energy of the synthesized fluorophores ranging from
2
.70 to 3.12 eV (in gas phase). The obtained triplet energy values of non-conjugated fluorophores
are higher than that of conjugated 7, 8, 9 and 10 fluorophores (ranging from 2.39 to 2.65 eV in gas
5
0
69
phase), because the conjugated molecules have finite π-conjugation between D and A subunits.
These results reveal that separation of D and A subunits with non-conjugated alkyl spacer,
consequently arrest communication between D and A subunits and boost the triplet energy levels.
By using the optimized geometry of ground-state, the absorption spectra of the synthesized
fluorophores were calculated with the help of TD-DFT. The related absorption wavelength (nm),
oscillator strength, major configuration, and their assignment are collected in the SI7 section.
Orbital compositions of each excited state have also been calculated. These results give complete
information about the absorption spectra. Simulated absorption spectra of PI derivatives are shown
in Fig. S25 in SI and their corresponding data are given in the SI7 section. In addition, atomic
coordinates of PI derivatives were given in the SI9 section.
Fig. 4. Optimized structures of all the fluorophores
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Table 2. Calculated Frontier Molecular Energy Levels of fluorophores.
Fluorophores HOMO LUMO
Eg
S1(gas) T (gas) S1(DCM) T (DCM)
1
1
(
eV)
(eV)
-0.89
-0.85
-0.85
-1.24
(eV)
4.15
4.16
4.15
3.77
(eV)
3.66
3.68
3.67
3.26
(eV)
(eV)
3.65
3.64
3.64
3.53
(eV)
2.74
2.74
2.74
2.74
PIPOCz
PIPTOCz
PITBOCz
PIFOCz
-5.04
-5.01
-5.00
-5.01
2.71
2.72
3.12
2.71
Fig. 5. Electron Density Contours of frontier molecular orbitals (FMO) of the fluorophores
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Electrochemical Properties:
To better understand the electronic structure (HOMO/LUMO) of synthesized fluorophores,
electrochemical properties were evaluated by cyclic voltammetry (CV) analysis. As shown in Fig.
6
, all the PI derivatives showed one quasi-reversible redox process around 1.04-1.52 eV, as well
as one reversible one-electron oxidation process around 1.15-1.40 eV in CV measurement. These
results indicating that all the fluorophores possess good hole and electron transporting properties.
Fig. 6 shows that the HOMO energy levels are almost located in the same range and there is no
big difference between them. Whereas, the LUMO energy levels of all the fluorophores shown
distinguish difference, which would be attributed to the conjugation and deferent substitution at
the imidazole group. The HOMO and LUMO energy levels are calculated according to Leeuw et
al. equations (1) and (2).70 By using the HOMO/LUMO energy levels we can calculate the band
gap of the fluorophores (equations (3)).
표푛푠푒푡
E퐻푂푀푂 = ―
(
E
+ 4.4
)
eV
eV
(1)
(2)
(3)
표푥
표푛푠푒푡
E퐿푈푀푂 = ―
(
E
+ 4.4
)
푟푒푑
Eg = E퐿푈푀푂 ― E퐻푂푀푂 eV
The detailed electrochemical data of the synthesized PI derivatives are listed in Table 3. CV
investigation revealed that the fluorophores have low-lying energy levels of the lowest unoccupied
molecular orbitals (LUMO) ranging from -2.88 eV to -3.35 eV and energy of the highest occupied
molecular orbitals (HOMO) ranging from -5.55 eV to -5.80 eV (Fig. 7), suggesting they may
possess good electron-transporting and good hole transporting properties. The comparison of
HOMO-LUMO energy of all the fluorophores is shown in Fig. 7.
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Fig. 6. Cyclic voltammograms of PI derivatives.
Fig. 7. HOMO – LUMO energy gap diagram of all the fluorophores.
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Table 3. Electrochemical properties of all the fluorophores.
Fluorophores
Eoxa
V)
Eredb
(V)
HOMO LUMO Egc (eV)
Egd (eV)
(
(eV)
(eV)
PIPOCz
1.168
1.238
1.158
1.409
-1.047
-1.258
-1.067
-5.568
-5.638
-5.558
-5.809
-3.353
-3.142
-3.333
-2.880
2.215
2.496
2.225
2.92
2.71
2.89
3.05
PIPTOCz
PITBOCz
PIFOCz
-1.520
2.929
a
b
c
The onset oxidation potential, the onset reduction potential, electrochemical
d
bandgap determined from cyclic voltammetry and optical energy bandgap estimated
from the solid-state absorption spectra.
Electroluminescence characteristics:
In view of the first-rate photophysical, electrochemical and theoretical studies of all the
synthesized PI derivatives, they are adopted as light-emitting emitters to fabricate solution
processed doped and nondoped OLED devices. Multilayer OLED devices with and without bipolar
host material CBP were fabricated with the configuration of ITO (125 nm)/ PEDOT:PSS (35 nm)/
neat film of PI derivatives (25 nm)/TPBi (35 nm)/LiF (1 nm)/Al and ITO (125 nm)/ PEDOT:PSS
(35 nm)/ PI derivatives doped in CBP host (25 nm)/TPBi (35 nm)/LiF (1 nm)/Al, respectively, In
all the fabricated OLED devices PEDOT:PSS and TPBi are work as hole injection and electron-
transporting, hole- and exciton-blocking layers, respectively. Lithium fluoride (LiF) was used as a
cathode buffer layer for electron injection. The energy level diagram of all the fabricated doped
and nondoped OLED devices is shown in Fig. 8.
The current density-voltage-luminance (J-V-L) plots of non-doped and doped devices are shown
in Fig. 9 and the significant electroluminescent data are tabulated in Table 4. The EL spectra of
the non-doped OLED devices are broad and shifted towards low-energy emission band (higher
wavelength) when compared with PL spectra of the respective emitters recorded in DCM solvent.
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The poor colour purity and performance of the non-doped devices is credited to the unbalanced
charge transport and leakage of charge carriers at the interface of respective electrodes without
formation of radiative excitons in the desired recombination zone. It also confirmed from the
observed high current density and low brightness for the neat device compared to the doped device
(Fig. 11). The wide and red shifted EL spectra of the neat devices also suggest the aggregation of
molecules in solid state or thin film.
To prevent aggregation prompted fluorescence quenching and boost the EL performance of the
compounds, we employed them as emitting dopants in bipolar host matrix CBP and optimized the
doping concentrations from 0.5 to 5 wt %. It is interesting to note that the efficiency and luminance
of the OLED devices remarkably improved after employing the host matrix CBP. This could be
attributed to the balanced charge carriers in recombination zone, effective host to guest energy and
possibility of excitons generation on both host and guest. The electronically generated excitons on
the CBP host can be successfully captured by the dopant molecules, which also assure the emission
originating from the emitters. OLED device consisting of 0.5 wt% doping of PIFOCz as an emitter
showed a maximum of EQE 3.3% with CIE coordinates of (0.16, 0.06). For the same doping
concentration, PIPOCz based OLED devices showed a maximum EQE of 2.1% and 2.0%,
4
9
respectively. It is also notable that high doping concentration reduced the device performance
and color purity moderately, which may attribute to the crystallinity of molecules and surface
morphology of the deposited thin films. 71-74
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Fig. 8. Schematic illustration of the energy levels of the solution-processed OLED devices
The EL spectra of all the fabricated OLED devices are shown in Fig. 10. All the molecules showed
near UV-emission with low doping concentration and maxima of emission peaks are centered in
between 395-405 nm. The OLED devices consisting of 0.5 wt% of the emitters PIPOCz, PIPTOCz,
PITBOCz, and PIFOCz in CBP host matrix displayed CIE coordinates of (0.17, 0.05), (0.18, 0.12),
(0.17, 0.07), and (0.17, 0.06), respectively. Figure 11 shows the CIE chromatogram of the device
with 0.5 wt% PIPOCz doped in CBP host. The observed CIE coordinates for the developed devices
are very close to the standard blue coordinate of National Television Standard Committee (NTSC),
i.e. (0.14, 0.08). It is also remarkable that the EL of the compounds is in close resemblance to their
PL recorded in DCM solvent, indicating effective charge injection and recombination in the
emissive zone. However, the PL spectra of synthesised compounds recorded in DCM solvents and
their corresponding EL spectra exhibited multi-peak emission profile at ~400 nm. The reason
beside this may be either formation of excimer in solid-state thin film or interfacial exciplex in the
developed multi-layered OLED device structure. It is also notable that the solid-state PL spectra
of compounds are broad and red shifted as compared to spectra those recorded in solution state. It
also contains few extra peaks that may be due to formation of excimer in developed thin films and
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DOI: 10.1039/D0TC03720F
intermolecular interaction in packed molecules. The obtained solution processed OLED device
efficiency EQE of 3.3% and CIE coordinates of around (0.16, 0.06) are comparable with literature
as shown in Table S7.75-83 All the results reveal that this work provides novel approaches for
realizing deep-blue emission fluorescent and pure-white fluorescence-phosphorescence hybrid
OLEDs with high performance.
Fig. 9. The current density vs voltage (I-V) and Voltage vs luminescence (I-L) plots of the
devices a) PIPOCz b) PIPTOCz c) PITBOCz d) PIFOCz
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Fig. 10. EL Spectra of a) PIPOCz b) PIPTOCz c) PITBOCz d) PIFOCz
Fig. 11. CIE chromatogram of the device with 0.5 wt% PIPOCz doped in CBP host.
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Table 4. Summary of EL performance of solution-processed blue OLEDs-effect of doping
concentration on the operating voltage (OV), power efficiency (PE), current efficiency (CE),
external quantum efficiency (EQE), CIE coordinates, and maximum luminance of the solution-
-2
processed devices with CBP host. [PE, CE, and EQE at 100 cdm and maximum].
Solution-processed
EML
Emitter
Operation PEmax/CEmax/EQEmax PE /CE /EQE
100
CIExy
Maximum
100
100
voltage
(V)
-1
-1
-1
-1
coordinates Luminance
lmW /cdA /%
lmW /cdA /%
-2
Emitter
(cdm )
(Dopant) Concent
ration
(wt%)
PIPOCz
0.5
1
3
5.0
5.0
5.1
6.5
6.7
5.0
4.4
6.5
6.6
-
6.6
5.0
5.4
6.7
-
0.2/0.2/2.5
0.3/0.4/1.9
0.4/0.5/1.8
0.1/0.2/0.5
0.1/0.1/-
0.2/0.3/0.6
0.4/0.5/1.5
0.4/0.7/1.0
0.3/0.6/0.8
-
0.1/0.2/1.7
0.4/0.5/2.0
0.3/0.4/1.3
0.1/0.2/0.4
0.1/0.1/-
0.2/0.4/3.3
1.0/1.2/1.5
0.4/0.5/1.6
0.3/0.5/0.8
0.1/0.1/-
0.1/0.2/2.0
0.2/0.3/1.8
0.2/0.4/1.2
0.1/0.1/0.4
-/-/-
0.2/0.2/0.5
0.3/0.4/1.3
0.3/0.6/0.9
0.2/0.5/0.8
-
0.1/0.2/0.9
0.2/0.4/1.5
0.2/0.4/1.1
0.1/0.2/0.4
-/-/-
0.1/0.2/1.5
0.4/0.6/1.5
0.3/0.5/1.5
0.2/0.4/0.7
-/-/-
(0.17, 0.05)
(0.17, 0.07)
(0.17, 0.09)
(0.17, 0.08)
-
(0.18, 0.12)
(0.17, 0.08)
(0.17, 0.13)
(0.17, 0.13)
-
(0.17, 0.07)
(0.17, 0.08)
(0.17, 0.09)
(0.16, 0.09)
-
(0.17, 0.06)
(0.17, 0.10)
(0.17, 0.09)
(0.17, 0.13)
-
463
404
375
410
108
597
462
535
518
61
393
344
418
523
70
5
1
00
PIPTOCz
PITBOCz
PIFOCz
0.5
1
3
5
1
00
0.5
1
3
5
1
00
0.5
1
6.1
4.2
5.0
6.7
6.7
386
490
426
583
111
3
5
1
00
Conclusion:
In summary, a series of PI derivatives have designed and synthesized. Phenanthroimidazole has
been proved with great potential to construct efficient near UV-emission emitters because of its
bipolar nature. The different substitution at the N1 position of the imidazole group further
influence the optical and electrical properties. The synthesized fluorophores are shown good
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thermal stability, near UV-emission (DCM), high photoluminescence quantum yield in the solid
state and balance charge transporting characteristics. All the solution processed fabricated OLED
devices showed near UV-emission with maxima of EL spectra peaking in between 395-405 nm.
Devices using compounds PIFOCz and PIPOCz displayed a maximum external quantum
efficiency of 3.3% and 2.5% with CIE coordinates of (0.17, 0.06) and (0.17, 0.05), respectively. It
is believed that this work provides novel approaches for realizing near UV-emission or ultra-deep
blue fluorescent OLEDs with high performance.
Supporting Information:
Supporting Information consists of general information about the synthesis and characterisation,
1
13 19
synthesis of fluorophores, NMR spectra ( H, C F), mass spectra of the luminophores, FT-IR
spectra, the CIE chromaticity coordinates for the fluorophores in solutions and solid phase, and
Quantum yield studies of fluorophores in solution and solid state, and the calculated UV/Vis
absorption spectra and vertical excitation wavelengths, orbital contribution and oscillator strength
(f) and atom coordinates of PI derivatives.
Acknowledgment
VS acknowledges SERB, Department of Science and Technology (DST), India
(EMR/2016/002462) for financial support.
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