Imidazole-Pyrene Hybrid Luminescent Materials for Organic Light-Emitting Diodes: Synthesis, Characterization & Electroluminescent Properties

Article abstract of DOI:10.1016/j.molstruc.2021.130306

A series of multichromophoric, pyrene-imidazole-phenyl based hybrid luminescent small molecules (PA, PI, PnB, PtB, PoM and PnDM) with different donating groups at the para position of the phenyl attached to N1 position of imidazole moiety were designed, synthesized, and characterized, for use in blue organic light-emitting diodes (OLEDs). The photophysical, thermal and electrochemical properties of the molecules were systematically investigated. All the molecules displayed delayed fluorescence at room temperature with a lifetime ranging from 7.1 to 8.5 μs. The synthesized fragment molecules (tetraphenyl-imidazoles: IA, IoM and InDM) revealed high triplet energies of ~ 2.90eV. Glass transition temperatures determined to range from 77°C to 123°C and decomposition temperatures are found to be above 280°C. The molecules possess appropriate HOMO and LUMO energy levels for effective charge injection. The crystal structure for PI is reported. OLED devices were fabricated based on the pyrene-imidazole-phenyl hybrids as emitters and as dopants with CBP as host. OLED devices with PI doped (5% wt.) with CBP exhibits excellent device performance with a current efficiency of 9.82 cd/A, the power efficiency of 8.32 lm/W and external quantum efficiency of 4.64%.

Full text of DOI:10.1016/j.molstruc.2021.130306

Journal of Molecular Structure 1236 (2021) 130306
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Imidazole-Pyrene Hybrid Luminescent Materials for Organic
Light-Emitting Diodes: Synthesis, Characterization &
Electroluminescent Properties
Gorakala Umasankara,b,#, Hidayath Ullac,#, Chakali Madhua,b, Gontu Ramanjaneya Reddya,1
,
Balaiah Shanigaramd, Jagadeesh Babu Nanubolue, Bhanuprakash Kotamarthid,∗
,
Galla V. Karunakara,b, M.N. Satyanarayanf,∗, V. Jayathirtha Raoa,b,∗
a Fluoro-Agrochemicals Department, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
b Academy of Scientific & Innovative Research, Ghaziabad 201002, India
c National Centre for Flexible Electronics, Indian Institute of Technology Kanpur, Kanpur 208016, India
d Catalysis and Fine Chemicals Department, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
e Centre for X-ray Crystallography, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
f Optoelectronics Laboratory, Department of Physics, National Institute of Technology Karnataka, Surathkal, Mangalore 575025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of multichromophoric, pyrene-imidazole-phenyl based hybrid luminescent small molecules (PA,
PI, PnB, PtB, PoM and PnDM) with different donating groups at the para position of the phenyl at-
tached to N1 position of imidazole moiety were designed, synthesized, and characterized, for use in blue
organic light-emitting diodes (OLEDs). The photophysical, thermal and electrochemical properties of the
molecules were systematically investigated. All the molecules displayed delayed fluorescence at room
temperature with a lifetime ranging from 7.1 to 8.5 μs. The synthesized fragment molecules (tetraphenyl-
imidazoles: IA, IoM and InDM) revealed high triplet energies of ~ 2.90eV. Glass transition temperatures
determined to range from 77°C to 123°C and decomposition temperatures are found to be above 280°C.
The molecules possess appropriate HOMO and LUMO energy levels for effective charge injection. The
crystal structure for PI is reported. OLED devices were fabricated based on the pyrene-imidazole-phenyl
hybrids as emitters and as dopants with CBP as host. OLED devices with PI doped (5% wt.) with CBP
exhibits excellent device performance with a current efficiency of 9.82 cd/A, the power efficiency of 8.32
lm/W and external quantum efficiency of 4.64%.
Received 18 January 2021
Revised 10 March 2021
Accepted 12 March 2021
Available online 18 March 2021
Keywords:
Pyrene-imidazole derivatives
High quantum yield
Delayed fluorescence
Blue OLEDs
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
dent on fluorescence-based materials [9-13]. Efficient OLEDs can
be achieved by tuning the molecular architecture such as shape,
Since the emergence of organic light-emitting diodes (OLEDs)
[1], there has been an endless pursuit for high-efficient organic
emitters, which can generate light of all visible colours. Though
reliable high-efficient red and green OLEDs have been demon-
strated, blue OLEDs are lagging [2-8]. Red and green phosphores-
cent OLEDs have exhibited up to 100% internal quantum efficien-
cies (ηIQE). They have found their way into flat panel displays.
Conversely, the mainstream practical blue OLEDs are still depen-
solubility, polarity, crystallinity, and energy bandgap [12-14]. Par-
ticularly in the display technologies, enhancement of OLED de-
vice performances gradually increased from first-generation (fluo-
rescent emitters) to second-generation (phosphorescent emitters)
by reaching a higher ηIQE. These phosphorescence emitters have
some drawbacks in the sense that the device shows lower electro-
chemical efficiency even at high current density [15] and are also
less stable. Furthermore, phosphorescent OLEDs have traditionally
relied on heavy metal atoms, such as Pt, to induce an apprecia-
ble spin-orbit coupling for triplet states. The use of such met-
als is not cost-effective, besides being an environmental hazard.
Hence, the third generation OLEDs have attracted significant atten-
tion towards developing organic electroluminescent materials for
blue, green, red OLEDs with functional modification of the donor
and acceptors [16-20]. In the third generation, to achieve ηIQE~100
∗
Corresponding authors.
E-mail addresses: bprakash@iict.res.in (B. Kotamarthi),
mn.satyanarayan@gmail.com (M.N. Satyanarayan), vaidya.opv@gmail.com (V.J. Rao).
#
These authors contributed equally to this work.
1
Present Address: Center for Nanoscience & Nanotechnology, International Re-
search center, Sathyabama Institute of Science and Technology, Chennai-600119.
https://doi.org/10.1016/j.molstruc.2021.130306
0022-2860/© 2021 Elsevier B.V. All rights reserved.

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
%, the thermally activated delayed fluorescence (TADF) and non-
TADF luminescent probes have been developed [21]. White OLEDs
(WOLEDs) display immense potential in providing glare-free light-
ing. However, due to the blue component’s limited performance,
WOLEDs hardly withstand acceptable efficiency at high luminance
[22]. These factors suggest that OLEDs’ numerous applications can
be realized by possessing high-performance blue OLEDs with good
efficiencies at high brightness.
selector (3980-5S, Spectra-Physics, single shot to 8 MHz). The ex-
citation pulses at the desired wavelength (370 nm) were obtained
by frequency doubling the 740 nm Ti-sapphire output with 0.5 mm
BBO crystal using CDP2015 frequency conversion unit. These ex-
citation pulses were focused on the sample using a Fluorescence
Up-conversion set up. The time distribution data of fluorescence
intensity were recorded on a SPC-130 TCSPC module (Becker and
Hickl). The instrument response function was 250 fs.
Pyrene derivatives exhibit high photoluminescence efficiency,
excellent thermal stability, and promising charge carrier mobil-
ity, which can be exploited in organic devices [23-27]. However,
pyrene derivatives exhibit excimer formation and aggregation in
the thin film form, which hampers the device lifetime [27-29].
Many researchers have strived to overcome this issue. Pyrene is
functionalized using some compounds such as imidazole, benz-
imidazole, oxadiazole and phosphine oxide etc., to improve the
electron-transport character in OLEDs [24-36]. The OLEDs using
these derivatives as luminescent materials exhibited good device
performance and emitted in the green/blue region. The function-
alization of pyrene and extended pyrene luminescent compounds
has many advantages towards maximizing the quantum efficien-
cies and device performances [37].
Single-Crystal X-ray studies: Single-crystal X-ray data were
collected at room temperature on a Bruker D8 QUEST equipped
with a four-circle kappa diffractometer and Photon 100 detector.
˚
An Iμs microfocus Mo source (λ=0.71073A) supplied the multi-
mirror monochromated incident beam. A combination of Phi and
Omega scans were used to collect the necessary data. Unit cell
dimensions were determined using 9977 reflections for PI. Inte-
gration and scaling of intensity data were accomplished using the
SAINT program. The structures were solved by Direct Methods us-
ing SHELXS-97, and the refinement was carried out by full-matrix
least-squares technique using SHELXL-2014/7. Anisotropic displace-
ment parameters were included for all non-hydrogen atoms. All
H atoms were positioned geometrically and treated as riding on
˚
their parent C atoms with C-H distances of 0.93–0.97 A, and with
In the present work, we have synthesized multichromophoric
(benzene; imidazole and pyrene) hybrid luminescent pyrene em-
bedded imidazole fluorophores for OLEDs. 1, 3-imidazole five-
Uiso(H) = 1.2Ueq (C) or 1.5Ueq for methyl atoms. The isopropyl
group atoms were disordered over two sites, with the site occu-
pancy factor of 0.799(9) for C1, C2 and C3 atoms (major compo-
nent) and 0.201(9) for C1D, C2D and C3D atoms (minor compo-
nent). The anisotropic displacement parameters of the disordered
carbon atoms were restrained to be similar (SIMU instruction), and
the direction of motion along the axis between these atoms was
also restrained (DELU instruction). The C-C bond distances of dis-
ordered ethyl groups were restrained to their expected values with
DFIX instruction and performed the final refinement cycle. The
phenyl ring atoms were also treated as split models joining iso-
propyl group major and minor components, respectively.
numbered-ring is used as
a functional group, which acts as
electron-transporter, energy transfer agent and improves the ther-
mal property for pyrene emitting moiety in virtue of its ampho-
teric characteristics originated from the two different kinds of
sp2 hybrid nitrogen atoms. A facile and one-pot straight forward
methodology is used to synthesize six novel organic materials by
slightly modifying the electron-donating groups at the para posi-
tion of N1 attached phenyl of imidazole moiety and using pyrene
and imidazole as the key units. The photophysical, thermal and
electrochemical properties of the fluorophores are investigated.
Theoretical calculations also support molecular design. The fluo-
rophores as dopants in blue OLEDs showed superior performance
compared to their counterparts in non-doped OLEDs. Notably, the
fluorophore PI-based OLED exhibited blue emission with a current
efficiency of 9.82 cd/A, the power efficiency of 8.32 lm/W and ex-
ternal quantum efficiency (ηEQE) of 4.64%.
Thermal Studies: For thermodynamic properties, differential
scanning calorimetry measurements were performed under nitro-
gen atmosphere on a DSC 7020 (EXSTAR) modulated calorimeter at
a heating/cooling rate of 10°C min−1 over a temperature range of
30–550°C. The glass transition temperature was determined from
the second heating scan of DSC plots. Thermogravimetric analysis
was carried out on an SII TG-DTA 7200 thermogravimeter, under
nitrogen atmosphere over a temperature range of 30–500°C at a
heating rate of 10°C min−1. The thermal decomposition tempera-
tures corresponded to 5% weight loss temperatures.
2. Materials and Methods
Electrochemical Characterization: Cyclic voltammetry mea-
surements were recorded in a conventional three-electrode cell
comprising of a glassy carbon working electrode, a non-aqueous
Ag/AgCl reference electrode, and a platinum wire as a counter
electrode on a computer-controlled CH instruments model CHI
620C electrochemical workstation at room temperature. The exper-
iments were performed under a nitrogen atmosphere in anhydrous
dichloromethane (1 mM solution) using 0.1M tetrabutylammonium
perchlorate (TBAP) as a supporting electrolyte at a scan rate of
100 mV/s. The working electrode surface was first polished with
1 mm alumina slurry, followed by 0.3 mm alumina slurry on a mi-
cro cloth. It was then rinsed with Millipore water and sonicated in
water for 5 min. The polishing and sonication steps were repeated
twice.
Materials: All the reagents and raw materials were procured
from commercial sources and were used as received. The synthe-
sized materials were confirmed by NMR spectra (1H and 13C) on a
Bruker Avance (400 & 500, MHz) spectrometer in CDCl3 with TMS
as an internal standard. Mass spectra and high-resolution mass
spectra (HRMS) were recorded using electrospray ionization (ESI)
ion trap mass spectrometry.
Optical Measurements: The UV–vis absorption and steady-state
fluorescence spectra were measured using an Agilent Technolo-
gies Cary Series UV-Vis-NIR spectrometer and an Agilent Tech-
nologies Cary Eclipse spectrofluorometer, respectively. The flu-
orescence quantum yields (ꢀF) were determined by integrat-
ing the fluorescence bands and by using 9,10-diphenylanthracene
(DPA) as the standard. Delayed fluorescence, phosphorescence
and lifetime measurements were carried out using a Fluorolog-
3 spectrometer equipped with a flash lamp in a glassy solvent
1:1ethanol/methanol using 0.06 ms delay, 61 ms time per flash,
and flash count 100. Time correlated single photon counting (TC-
SPC) was used to measure the fluorescence lifetime in the nanosec-
ond time scale. The femtosecond pulses at a repetition rate of
8 MHz were generated from the fractional part of MaiTai HP
(Spectra-Physics) output passing through the femtosecond pulse
Synthetic Procedure
Synthesis of Pyrene-Imidazole Fluorophores
The pyrene-imidazole compounds were synthesized by the one-
pot methods. Here, the mixture of benzil (1 g, 4.7mmol), pyrene
aldehyde (1.2 g, 5 mmol), aniline (0.44 g, 4.7 mmol)/ 4-butyl ani-
2

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
130.89, 130.15, 128.68, 128.34, 128.14, 128.07, 128.01, 127.84,
127.66, 127.53, 127.23, 126.53, 126.0, 125.57, 125.30, 124.76, 124.44,
123.90, 33.37 and 23.57 ppm.
1-(4-butylphenyl)-4,5-diphenyl-2-(pyren-1-yl)-1H-imidazole
(PnB): Yield: 1.6
g (64 %), mp:195-197°C; ESI-MS (m/z): 553
[M+1], ESI-HRMS [M+H]+ (m/z): Calculated for C41H33N2 is
553.2565; found (m/z): 553.2646; 1H NMR (400 MHz, CDCl3): δ
8.49 (d, J = 9.2 Hz, 1H), 8.19 (d, J = 2.0 Hz, 1H), 8.17 (d, J = 2.2
Hz, 1H), 8.07 (t, J = 8.9 Hz, 2H), 8.03 – 7.96 (m, 3H), 7.83 (d,
J = 7.9 Hz, 1H), 7.72 (d, J = 1.5 Hz, 1H), 7.70 (d, J = 1.1 Hz, 1H),
7.31 – 7.20 (m, 8H), 6.81 – 6.73 (m, 4H), 2.37 – 2.33 (m, 2H),
1.40 – 1.28 (m, 2H), 1.19 – 0.99 (m, 2H), 0.75 (t, J = 7.3 Hz, 3H)
ppm; 13C NMR (100 MHz, CDCl3): δ 146.61, 142.38, 138.18, 134.69,
134.14, 131.40, 131.11, 131.05, 130.91, 130.86, 130.79, 130.12, 128.69,
128.47, 128.33, 128.13, 128.03, 127.83, 127.61, 127.51, 127.21, 126.53,
125.97, 125.56, 125.28, 124.72, 124.42, 123.90, 34.84, 32.83, 21.95
and 13.75 ppm.
1-(4-tert-butylphenyl)-4,5-diphenyl-2-(pyren-1-yl)-1H-
imidazole (PtB): Yield: 1.65 g (64%), mp:231-233°C; ESI-MS (m/z):
553 [M+1], ESI-HRMS [M+H]+ (m/z): Calculated for C41H33N2 is
553.2565; found (m/z): 553.2642; 1H NMR (500 MHz, CDCl3): δ
8.53 (d, J = 9.2 Hz, 1H), 8.19 (d, J = 7.6 Hz, 2H), 8.08 (dd, J = 9.1,
5.5 Hz, 2H), 8.03 – 8.01 (m, 1H), 8.00 – 7.97 (m, 2H), 7.80 (d,
J = 7.9 Hz, 1H), 7.71 – 7.69 (m, 1H), 7.31 – 7.26 (m, 6H), 7.25 – 7.21
(m, 3H), 6.99 – 6.96 (m, 2H), 6.82 – 6.80 (m, 2H), 1.09 (s, 9H).)
ppm; 13C NMR (100 MHz, CDCl3): δ 150.70, 146.58, 138.19, 134.69,
133.88, 131.38, 131.06, 130.89, 130.15, 128.68, 128.32, 128.13,
128.07, 128.01, 127.83, 127.54, 127.35, 127.23, 126.54, 126.00,
125.58, 125.45, 125.30, 124.76, 124.44, 123.89, 34.39 and 31.03
ppm.
Scheme 1. The synthetic procedure of PA, PI, PnB, PtB, PoM and PnDM.
1-(4-methoxyphenyl)-4,5-diphenyl-2-(pyren-1-yl)-1H-
line (0.70g, 4.7 mmol)/ 4-ter butyl aniline (0.7 g, 4.7 mmol)/ 4-iso
propyl aniline (0.64 g, 4.47 mmol)/ 4-methoxy aniline (0.58g, 4.7
mmol)/ 4-N,N-dimethylaniline (0.64 g, 4.7 mmol) and ammonium
acetate (4.4 g, 57mmol) was added to 20 ml of glacial acetic acid as
a solvent. The corresponding reaction mixtures were refluxed for
6 h. The mixture was poured in ice, and the resulting precipitate
was washed with water to remove the excess acetic acid. To obtain
the desired product, the reaction mixture was purified by column
chromatography on silica gel with hexane: ethyl acetate (9:1) as an
eluent. Extended purification was achieved by repeated recrystal-
lization in isopropanol and chloroform mixture to yield colourless
solid products. The corresponding final compounds are represented
as PA [23], PI, PnB, PtB, PoM and PnDM. A synthetic route of final
compounds is presented in Scheme 1.
imidazole (PoM): Yield: 1.7
g (70 %), mp:217-219°C; ESI-MS
(m/z): 527 [M+1], ESI-HRMS [M+H]+ (m/z): Calculated for
C38H27N2O is 527.2045; found (m/z): 527.2121; 1H NMR (400
MHz, CDCl3): δ 8.12 (d, J = 1.2 Hz, 1H), 8.10 (d, J = 1.4 Hz, 1H),
7.61 – 7.58 (m, 3H), 7.51 – 7.41 (m, 4H), 7.26 – 7.24 (m, 4H), 7.23
– 7.19 (m, 3H), 7.18 – 7.09 (m, 3H), 6.98 – 6.94 (m, 2H), 6.77 –
6.74 (m, 2H), 3.77 (s, 3H).) ppm; 13C NMR (100 MHz, CDCl3): δ
159.03, 147.02, 138.02, 134.37, 131.08, 131.0, 130.65, 130.46, 129.89,
129.76, 129.36, 128.88, 128.65, 128.51, 128.37, 128.28, 128.14,
128.07, 128.02, 127.84, 127.37, 127.17, 126.51, 114.13 and 55.29 ppm.
4-(4,5-diphenyl-2-(pyren-1-yl)-1H-imidazol-1-yl)-N,N-
dimethylaniline (PnDM): Yield: 1.65
g (67%), mp:234-236°C
ESI-MS (m/z): 540 [M+1], ESI-HRMS [M+H]+ (m/z): Calculated for
C39H30N3 is 540.2361; found (m/z):540.2482; 1H NMR (300 MHz,
CDCl3): δ 8.49 (d, J = 9.2 Hz, 1H), 8.18 (d, J = 7.6 Hz, 2H), 8.07
(dd, J = 9.1, 4.4 Hz, 2H), 8.03 – 7.97 (m, 3H), 7.85 (d, J = 7.9 Hz,
1H), 7.69 (d, J = 7.0 Hz, 2H), 7.28 (m, 6H), 7.26 (m, 2H), 6.75 (d,
J = 8.9 Hz, 2H), 6.23 (d, J = 9.0 Hz, 2H), 2.73 (s, 6H) ppm;13C NMR
(100 MHz, CDCl3): δ 149.20, 146.82, 137.87, 134.88, 131.26, 131.11,
130.92, 130.85, 130.40, 128.72, 128.40, 128.32, 128.11, 127.92,
127.67, 127.50, 127.25, 126.40, 125.91, 125.70, 125.22, 124.71,
124.47, 123.95 and 40.05 ppm.
1,4,5-triphenyl-2-(pyren-1-yl)-1H-imidazole (PA) [23]: Yield:
1.42 g, (60%), mp:225-227°C; ESI-MS (m/z): 497 [M+1], ESI-
HRMS [M+H]+ (m/z): Calculated for C37H25N2 is 497.1939; found
497.2011; 1H NMR (400 MHz, CDCl3): δ 8.49 (d, J = 9.2 Hz,
1H), 8.20 (d, J = 1.1 Hz, 1H), 8.18 (d, J = 1.6 Hz, 1H), 8.08 (dd,
J = 9.1, 5.3 Hz, 2H), 8.00 (d, J = 7.0 Hz, 2H), 7.99 (s, 1H), 7.82
(d, J = 7.9 Hz, 1H), 7.72 (d, J = 1.5 Hz, 1H), 7.70 (d, J = 1.1
Hz, 1H), 7.32 – 7.29 (m, 3H), 7.28 – 7.27 (m, 2H), 7.27 – 7.26
(m, 1H), 7.26 – 7.22 (m, 2H), 7.02 – 6.96 (m, 3H), 6.92 – 6.89
(m, 2H) ppm; 13C NMR (100 MHz, CDCl3): δ 146.57, 138.30,
136.63, 134.58, 131.48, 131.13, 131.07, 130.83, 130.07, 128.65, 128.59,
128.44, 128.17, 127.96, 127.90, 127.64, 127.52, 127.22, 126.62, 126.04,
125.45, 125.37, 124.73, 124.42 and 123.94 ppm.
Synthesis of TetraPhenyl-Imidazole Derivatives
The phenyl-imidazole compounds were synthesized by the one-
pot method [38,39]. Here the mixture of benzil (1 g, 4.7 mmol),
benzaldehyde (0.49 g, 1.1 mmol), and aniline (4-methoxyaniline
and 4-N,N Dimethyl aniline) and ammonium acetate (4.4 g, 57
mmol) was added to 10 ml of acetic acid as a solvent and re-
fluxed for 6 h. The reaction mixture was poured into the ice, and
the resulting precipitate was washed with water to remove the ex-
cess acetic acid. Thus, the obtained product was purified by the
silica gel column chromatography with hexane: ethyl acetate (9:1)
as an eluent. Extended purification was achieved by the repeated
1-(4-isopropylphenyl)-4,5-diphenyl-2-(pyren-1-yl)-1H-
imidazole (PI): Yield:1.6 g (62 %), mp:208-210°C; ESI-MS (m/z):
539 [M+1], ESI-HRMS [M+H]+ (m/z): Calculated for C40H31N2 is
539.2409; found 539.2489; 1H NMR (400 MHz, CDCl3): δ 8.52
(d, J = 9.2 Hz, 1H), 8.19 (d, J = 7.7 Hz, 2H), 8.08 (dd, J = 9.1,
5.1 Hz, 2H), 8.03 – 7.96 (m, 3H), 7.81 (d, J = 7.9 Hz, 1H), 7.71
(dd, J = 5.3, 3.2 Hz, 2H), 7.30 – 7.19 (m, 9H), 6.81 (s, 3H), 2.71 –
2.60 (m, 1H), 1.03 (s, 3H), 1.01 (s, 3H) ppm; 13C NMR (100 MHz,
CDCl3): δ 148.34, 146.59, 138.19, 134.69, 134.18, 131.40, 131.07,
3

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
glass substrates were cleaned with acetone and isopropyl alcohol
with a sonication process and then were rinsed in DI water [40].
To eliminate any organic impurities and to modify the work
function of ITO, the ITO substrates were exposed to UV-Ozone
[40]. The ITO substrates were then transferred into a deposition
chamber. Small molecules such as F4TCNQ as hole-injection ma-
terial, TAPC as hole-transport material, emissive materials, and
TPBi as electron-transport material were sequentially deposited on
the ITO substrate with thicknesses appropriate for the intended
device structure. Finally, a bilayer cathode consisting of LiF and
Al were deposited on the ETM using a shadow mask. All the
layers were sequentially deposited on the ITO substrates at a
chamber pressure of 5 × 10−6 Torr by thermal evaporation. The
deposition−r1ates of organic materials, LiF and Al, were maintained
−1
and 6 A s−1, respectively. The deposition
˚
˚
˚
at 0.5 A s , 0.1 A s
rates and thickness were monitored and controlled in situ by a
quartz crystal thickness oscillator placed near the substrate. The
light-emitting area was 1.6 mm2. The EL spectra were acquired
using a Horiba Jobin Yvon iHR320 spectrophotometer with an
attached charge-coupled device camera. Current density-voltage-
luminance (J-V-L) characteristics were measured in a customized
setup using a computer-controlled programmable Keithley 2400
SMU, Keithley 6485 Picoammeter and a calibrated Si SM1PD2A
photodiode. All the measurements were performed in dark and
ambient conditions without any encapsulation.
Scheme 2. The synthetic procedure of IA, IoM and InDM.
recrystallization in the isopropanol and chloroform mixture to get
colourless solids of pure product. These imidazole fragments are
represented as IA, IoM and InDM. A synthetic route to these frag-
ments is presented in Scheme 2.
3. Results and Discussion
1,2,4,5-tetraphenyl-1H-imidazole (IA): Yield: 1.1
g (64%),
mp:210-212°C; ESI-MS (m/z): 373 [M+1], ESI-HRMS [M+H]+ (m/z):
Calculated for C27H21N2 is 373.1626; found (m/z): 373.1706; 1H
NMR (400 MHz, CDCl3): δ 7.62 – 7.57 (m, 2H), 7.44 – 7.41 (m,
2H), 7.27 – 7.18 (m, 12H), 7.14 – 7.11 (m, 2H), 7.05 – 7.02 (m, 2H)
ppm; 13C NMR (100 MHz, CDCl3):δ 146.88, 138.23, 137.06, 134.38,
131.07. 130.80, 130.60, 130.48. 129.0, 128.91,128.38, 128.28, 128.19,
128.10,128.03, 127.90, 127.36 and 126.54 ppm.
3.1. Synthesis and Characterization
All the multichromophoric hybrid fluorescent molecules stud-
ied in this work were readily synthesized using the synthetic route
shown in Scheme 1 and Scheme 2. The explanation of the com-
prehensive synthetic procedures of the final products is described
below. The molecular structures were confirmed by 1H NMR, 13C
NMR, mass spectrometry and HRMS (Figure S1-S36, ESI†).
1-(4-methoxyphenyl)-2,4,5-triphenyl-1H-imidazole
(IoM):
Yield: 1.2 g (67 %), mp:138-140°C; ESI-MS (m/z):403 [M+1], ESI-
HRMS [M+H]+ (m/z): Calculated for C28H23N2O is 403.1732; found
(m/z): 403.1810; 1H NMR (500 MHz, CDCl3): δ 7.60 – 7.58 (m,
2H), 7.47 – 7.44 (m, 2H), 7.27 – 7.25 (m, 3H), 7.25 – 7.23 (m, 3H),
7.23 – 7.21 (m, 2H), 7.20 – 7.17 (m, 1H), 7.15 – 7.12 (m, 2H), 6.97
– 6.94 (m, 2H), 6.77 – 6.74 (m, 2H), 3.77 (s, 3H) ppm; 13C NMR
(100 MHz, CDCl3):δ 159.03, 147.01, 138.01, 134.35, 131.07, 131.00,
130.63, 130.43, 129.74, 129.36, 128.87, 128.28, 128.15, 128.07,
128.03, 127.83, 127.37, 126.51, 114.13 and 55.29 ppm.
3.2. Crystal Structure
The crystal data and structure refinement parameters of the
fluorophore PI are presented in Table 1. The fluorophore PI crys-
tallizes in monoclinic space group P21/n with cell parameters
˚
˚
˚
a = 13.068(1) A, b = 8.4857(15) A, c = 29.248(6) A, V = 3243.3(12)
3
˚
A , Z = 8. The ORTEP diagram with the labelling and packing pat-
tern of PI is shown in Fig. 1. Since the obtained crystals of the
remaining fluorophores were inferior and not suitable for X-ray
diffraction, crystal studies of other fluorophores could not be per-
formed.
N,N-dimethyl-4-(2,4,5-triphenyl-1H-imidazol-1-yl)aniline
(InDM): Yield: 1.3 g (67 %), mp:190-192°C ESI-MS (m/z): 416
[M+1], ESI-HRMS [M+H]+ (m/z): Calculated for C29H26N3 is
416.2048; found (m/z): 416.2123; 1H NMR (500 MHz, CDCl3):δ
7.61 – 7.55 (m, 2H), 7.50 – 7.47 (m, 2H), 7.24 – 7.22 (m, 8H), 7.18 –
7.14 (m, 3H), 6.87 (d, J = 8.9 Hz, 2H), 6.51 (d, J = 8.9 Hz, 2H), 2.92
(s, 6H) ppm; 13C NMR (100 MHz, CDCl3): δ149.73, 147.07, 137.88,
134.72, 131.26, 131.13, 131.0, 130.87, 128.82, 128.71, 128.18, 128.05,
127.96, 127.89, 127.81, 127.62, 127.38, 126.35, 125.64, 125.25, 111.86
and 40.26 ppm.
3.3. Theoretical Investigations
Density Functional Theory (DFT) calculations were carried out
with Gaussian 09 ab initio quantum chemical package [41]. All
the geometries of the fluorophores were fully optimized with no
symmetry constraints during geometry optimization. The minima
were further confirmed by vibrational frequency analysis with zero
imaginary frequencies. The gas-phase relaxations of atomic posi-
tions of all the molecules have been carried out by using the
hybrid Becke, three-parameter, Lee-Yang-Parr [42-44] exchange-
correlation functional (B3LYP) and a 6-311G(d,p) basis set [45-47].
These relaxed geometries were used as input to obtain frontier
molecular orbitals (FMOs) and for the TD-DFT calculations to get
electron transition details. TD-DFT calculations were performed us-
ing cam-B3LYP/6-311G(d,p) [48] level of theory in chloroform with
PCM solvent model framework [49]. Fully relaxed ground state ge-
ometry with dihedral angle values of PI is given in Fig. 2 and for
Device Fabrication and Characterization: Pre-patterned in-
dium tin oxide (ITO) coated glass substrates (sheet resistance
~
15 ꢁ/ꢀ, thickness ~120 nm) were purchased from Kintec,
Hong Kong. 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane
ꢀ
(F4TCNQ) was purchased from LUMTEC. 4,4 -cyclohexylidenebis
ꢀ
[N,N-bis(4-methylphenyl) benzenamine (TAPC), 2,2 ,2"-(1,3,5-
ꢀ
benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), 4,4 -Bis(N-
ꢀ
carbazolyl)-1,1 -biphenyl (CBP) and lithium fluoride (LiF) were
purchased from Sigma Aldrich. Aluminium (Al) wire was pur-
chased from Alfa Aesar. OLEDs were fabricated on patterned ITO
coated glass substrates which were used as anodes. ITO coated
4

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
Table 1
Crystallographic data for PI.
Empirical formula
C40H30N2.C2H6O
Formula Weight, M
584.73
˚
Radiation wavelength, λ (A)
Radiation type
0.71073
Mo Kα, PHOTON 100 area detector
Temperature (K)
293(2)
Crystal dimension (mm3)
0.44 × 0.39 × 0.28
Crystal system
monoclinic
Space group
P21/n (No. 14)
˚
a (A)
13.068(1)
˚
b (A)
8.4857(15)
˚
c (A)
29.248(6)
α(°)
β (°)
γ (°)
90
90.399(7)
90
3
˚
Volume, V (A )
3243.3(12)
Z
8
Density, ρc (g/cm3)
1.197
μ (mm−1
)
0.071
F000
1240
2θmax (°)
50
Reflections Collected
Parameters
28,904
443
Rint
0.049
Goodness-of-fit on F2
1.06
3
˚
Min. and Max. Resd. Dens. (e/A )
-0.21, 0.23
Final R indices
CCDC
R1 = 0.0605, wR2 = 0.1518
1576203
5707 unique, R indices based on 4261 reflections with I >2σ(I) (refinement on F2)
Fig. 1. (a) The molecular structure of PI with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level and H atoms are shown as small
spheres of arbitrary radius. Only major component of the disordered isopropyl group atoms is shown for clarity and (b) Crystal packing diagram.
other molecules in Figure S37 (see ESI†, Table S1), which are non-
planar. In all the molecules, the dihedral angle between pyrene
core and imidazole molecular planes is around 46°(Ø1) and be-
tween imidazole and first (Ø2), middle (Ø3) and substituted phenyl
ring (Ø4) molecular planes it is around 25°, 57° and 61° respec-
tively as shown in Fig. 2. Higher dihedral angles for substituted
phenyl ring indicate a more twisted arrangement concerning imi-
dazole ring in its ground state. From the MO analysis, the electron
density is mainly populated on pyrene moiety with a minor contri-
bution from unsubstituted phenyl rings in HOMOs for all the flu-
orophores, as shown in Fig. 3. Here again, between unsubstituted
phenyl groups, the contribution from the middle one is lower com-
pared to the last phenyl towards HOMOs. There is no contribution
to HOMOs from the substituted phenyl part. In LUMOs, the elec-
tron density is mainly localized on pyrene with a minor contri-
bution from the imidazole part. The TD-DFT data is tabulated in
Table 2.
Fig. 2. Optimized geometry with of PI with dihedral angle representations using
DFT.
5

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
Table 2
TD-DFT data obtained at cam-B3LYP/6-311G(d,p) level of theory in chloroform solvent for the fluorophores.
Cpd.
λnm
f
Major contribution
HOMO
(eV)
LUMO
(eV)
ꢂE
(eV)
PA
343
343
343
343
344
345
0.9032
0.8995
0.9022
0.8940
0.9004
0.9062
HOMO→LUMO (91%)
HOMO→LUMO (91%)
HOMO→LUMO (91%)
HOMO→LUMO (91%)
HOMO→LUMO (91%)
HOMO→LUMO (87%)
5.32
5.17
5.29
5.29
5.27
5.17
1.84
1.82
1.81
1.81
1.81
1.73
3.48
3.35
3.48
3.48
3.46
3.44
PI
PnB
PtB
PoM
PnDM
Fig. 4. Absorption and fluorescence spectra of the Pyrene-Imidazole fluorophores in
(a) solution and (b) in thin film state.
in Table 3. The electronic absorption spectra (Fig. 4a) of the syn-
thesized fluorophores in chloroform solution (c = 10−5 M) exhibit
two well-defined absorption bands corresponding to π-π∗ (longer
wavelength ~350nm) and shorter wavelength assisted for π-π∗ of
phenyl imidazole electronic transitions (Table 3). UV-visible ab-
sorption maxima obtained from DFT calculations (Table 2) are in
good agreement with the experimental values (Table 3). The main
transition is seen to be from the HOMO to the LUMO (Fig. 3).
The nearly equal absorption maxima of the fluorophores indicate
the minimal impact of the substituents on imidazole moiety. The
thin-film state absorption spectra (Fig. 4b) of the synthesized flu-
orophores are comparable with the solution-state spectra showing
almost negligible bathochromic shifts (<6nm), revealing the weak
intermolecular aggregation in their thin-film state. The correspond-
ing optical band gaps were found to be in the range of 2.71-2.99
eV (Table 3). The fluorescence spectra of these fluorophores in the
solution-state are shown in Fig. 4a. All the materials emit in the
Fig. 3. Molecular orbital isosurface plots of the fluorophores generated at B3LYP
/6-311G(d,p) level in gas phase.
3.4. Photophysical properties
The UV-visible absorption and fluorescence spectra of the syn-
thesized fluorophores at room temperature (RT) are shown in
Fig. 4, and the corresponding key photophysical data is provided
6

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
Table 3
Absorption and fluorescence data of the Pyrene-imidazole compounds.
Egopt
ꢀF
DF
(nm)f
Phos
(nm)g
ET (eV)h
τi
(μs)
τj
(ns)
a
b
a
b
d
e
Cpd.
λabs
(nm)
λabs
(nm)
λem (FWHM)c
λem (FWHM)c
(nm)
(nm)
(eV)
RT
77 K
PA
349, 294
349, 297
350, 293
348, 292
348, 296
355, 292
353, 297
353, 293
354, 297
352, 291
355, 297
-
432(66)
433 (66)
430 (69)
432 (66)
433 (66)
453 (76)
379
469 (92)
452 (81)
456 (88)
463 (80)
472 (70)
475 (88)
-
2.99
2.94
2.89
2.98
2.84
2.71
-
0.81
0.86
0.82
0.78
0.63
0.58
-
440
439
455
440
443
454
375
376
379
395
394
393
394
393
437
361
360
383
-
-
7.4
1.7
1.8
1.8
1.8
1.6
1.9
-
PI
-
-
8.3
PnB
PtB
PoM
-
-
8.5
-
-
8.1
-
-
7.4
PnDM 350, 296
-
-
7.1
IA
299
289
497
491
489
2.90
2.85
2.85
1.99
1.78
1.85
IoM
-
364
-
-
-
-
InDM 304
-
377
-
-
-
-
a
Recorded in chloroform at room temp. (10−5 M).
Recorded in thin-film state.
b
c
d
e
f
Full width at half-maximum.
Optical band gap Estimated from = 1240/λonset, λonset is the onset is intersection of absorption and emission spectra of thin films.
Fluorescence quantum yields measured in cyclohexane, relative to 9,10-diphenyl anthracene (ꢀ = 0.9 in cyclohexane).
Delayed fluorescence in nm
g
h
i
Phosphorescence λmax in nm
Triplet energy in eV (Calculated by onset process on Delayed fluorescence spectra at 77k)
Delayed fluorescence lifetime in microseconds
j
Fluorescence lifetime in nanoseconds
wavelength range of 380 to 550 nm at emission maxima of 430–
453 nm with a full width at half maxima (FWHM) of ~66 nm,
except PnDM, which shows an FWHM of 76 nm. Among the six
fluorophores, PnDM with N,N-dimethyl amine substituent showed
a bathochromic shift in emission spectra, which may be due to
the effective intramolecular charge transfer from electron-donating
N,N-dimethyl amine moiety to electron-accepting imidazole moi-
ety. Compared with the fluorescence measured in solution-state
(chloroform), the emission bands are broadened (FWHM ~ 85
nm) in the thin-film state (Fig. 4b) with bathochromic shifts in
the emission maxima. However, no characteristic pyrene excimer
emission is observed, indicating the weak intermolecular interac-
tions between the pyrene units of these fluorophores. All the flu-
orophores exhibited high quantum yields (Table 3), which is vital
to evaluate OLEDs’ device performance. The relatively low fluores-
cence quantum yields of PoM and PnDM demonstrated that the
methoxy and N,N-dimethyl amine substituents could induce in-
tramolecular charge transfer (ICT) [50,51] interaction leading to PL
quenching. With increasing solvent polarity (Figure S38, ESI†), the
emission spectra gradually broadened and showed bathochromic
shifts (Table S2, ESI†) from ~420 nm in hexane to ~460 nm in
dimethylformamide (DMF). Such solvatochromic behaviour demon-
strates the existence of a charge transfer moiety in the excited
state [52].
Materials possessing delayed fluorescence (DF) will have the
ability to convert triplet excited state to emissive singlet excited
states [24,25,53]. The DF characteristics of the synthesized flu-
orophores are measured in their solution-state (equal ratio of
ethanol and methanol) and the DF data are provided in Table 3.
All the fluorophores displayed DF property at RT (Fig. 5a). The DF
emission maxima of the fluorophores at RT were almost similar to
the fluorescence maxima. However, hypsochromic shifts were ob-
served in the DF emission maxima at 77K (Fig. 5b and Table 3).
To further understand the observed DF properties, we have mea-
sured the DF and phosphorescence of the tetra-phenyl imidazole
fragments (IA, IoM and InDM). As shown in Fig. 6, all the three
fragments show phosphorescence with emission maxima at ~ 490
nm with a DF maximum of ~ 375 nm (Table 3). The triplet energies
of the tetra-phenyl imidazole fragments estimated from the phos-
phorescence data reveal that fluorophores have high triplet ener-
gies of ~ 2.90 eV. Overlapping of the phosphorescence spectra of
the triphenyl imidazole fragments (i.e., IA, IoM and InDM) with
Fig. 5. Delayed fluorescence of fluorophores at (a) RT and (b) 77 K.
7

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
Table 4
Thermal and electrochemical data of the synthesized com-
pounds.
Cpd.
HOMO (eV)
LUMO (eV)
Tg/ Tm/Td (oC)a
PA
5.3
5.3
5.3
5.3
5.1
5.0
2.31
2.36
2.41
2.32
2.26
2.29
104/226/368
97/225/371
77/197/362
104/231/369
93/217/282
123/236/377
PI
PnB
PtB
PoM
PnDM
a
Tg: glass transition temperature; Tm: melting point; Td:
decomposition temperature (corresponding to 5% weight
loss).
3.5. Thermal properties
A large amount of Joule heat is generated from high current
density during device operation [55]. Hence, to use the material in
organic device applications, particularly OLEDs, its thermal stabil-
ity is essential, as excellent thermal stability facilitates the forma-
tion of uniform amorphous films upon evaporation, which in turn
helps improve efficiency long-term stability of OLEDs. The ther-
mal properties of the pyrene-imidazole fluorophores, such as de-
composition temperature (Td), melting temperature (Tm), and glass
transition temperatures (Tg) were evaluated using thermogravimet-
ric analysis (TGA) and differential scanning calorimetry (DSC) mea-
surements under nitrogen atmosphere. The thermal data are sum-
marised in Table 4, and TG/DSC curves are provided in Fig. 8. The
repeated DSC heating scans revealed that the fluorophores possess
high glass transition temperatures in the range of 77-123°C, sug-
gesting that the fluorophores are amorphous even at higher tem-
peratures. The fluorophores’ melting temperatures are in the range
of 197-236°C (Fig. 8a & 8b). A reduction in the melting tempera-
ture was observed by the addition of the N-alkyl substituents (i.e.,
225°C, 197°C and 217°C, respectively for PI, PnB and PoM) com-
pared to PA (226°C), demonstrating a more flexible molecular fea-
ture of PI, PnB and PoM compared to PA which may be resulting
from the substituents at the imidazole moiety. However, the ad-
dition of a tertiary butyl group (PtB) increased the Tm to 231°C.
Also, PnDM with N,N-dimetyl group substituent showed a high Tm
of 236°C, which may be due to an increase in the molecule’s rigid-
ity.
Fig. 6. Delayed fluorescence spectra of Tetra phenyl imidazole fragments at (a) RT
and (b) 77 K.
Further, as shown in Fig. 8c, all the fluorophores exhibited
excellent thermal stability with high Td (5% weight loss) in the
range of 282-377°C. This can be ascribed to the high rigid-
ity of the aromatic rings offering high resistance to thermolysis.
Among all the fluorophores, PnDM showed high thermal stabil-
ity (Td = 377°C), which can be attributed to the 4-N,N-dimethyl
moiety, which is also likely to improve the morphological sta-
bility significantly. The results are in line with that of other re-
ported pyrene-imidazole derivatives [33-36]. These values indi-
cate that the pyrene-imidazole systems have good thermal stabil-
ity over a wide range of temperature. The glassy state implies that
they can form morphologically stable films upon thermal evapora-
tion, which is highly significant to improve organic devices’ perfor-
mance.
Fig. 7. Comparative overlap of the phosphorescence spectra of the tetraphenyl im-
idazole fragments with the fluorescence spectra (solution and thin film state) of
their corresponding Pyrene-Imidazole based fluorophores.
the fluorescence spectra of their corresponding pyrene-imidazole
based synthesized fluorophores (i.e., PA, PoM & PnDM) are ar-
ranged in Fig. 7. Besides, the time-resolved DF lifetimes measure-
ments of the fluorophores were carried out at 77K (Table 3), and
the estimated DF lifetimes are in the microsecond range (7.1 to 8.5
μs) as shown in Figure S39 (ESI†) and Table 3, which are essen-
tial for display applications [24,54]. The time-resolved fluorescence
lifetime studies (Figure S40, ESI†) were carried out at emission
maxima, and the decays are fitted mono exponentially, and the re-
sults are tabulated in Table 3. The fluorophores exhibited fluores-
cence lifetimes of ~ 1.8 ns, comparable to that of reported pyrene-
imidazole molecules [23,33,34]. All the observations suggest that
the synthesized fluorophores have the potential as effective emit-
ting materials in OLED applications.
3.6. Electrochemical properties
To better understand the electron structure (HOMO and LUMO)
of the fluorophores, the electrochemical behaviours were evalu-
ated using cyclic voltammetry (CV) analysis in a standard electrode
electrochemical cell using anhydrous DCM as a solvent. The re-
sults are summarised in Table 4, and the cyclic voltammograms are
provided in Figure S41(ESI†). The fluorophores exhibit one quasi-
reversible oxidation wave with oxidative onset potentials com-
8

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
eV, which may lead to a change in the LUMO values of the mate-
rials (Table 4).
3.7. Electroluminescence properties
The visually perceptible high intense emission and favourable
HOMO−LUMO energy levels make the new fluorophores poten-
tial emitters for OLEDs. The adaptability of the fluorophores as
potential blue emitters in OLEDs was explored using two types
of vacuum-deposited multilayer device architectures A and B: ITO
(120 nm)/F4TCNQ (4 nm)/TAPC (30 nm)/ Emitting layer (35 nm)/
TPBi (30 nm)/ LiF (1 nm)/ Al (150 nm). The schematic diagram of
the different OLED architectures and the related energy level align-
ment of the fluorophores and other materials used in the OLEDs
is depicted in Fig. 9. Here, patterned ITO coated glass substrate
served as a transparent anode. F4TCNQ served as hole-injection
material as it efficiently injects holes from the ITO anode to the
hole transport-layer [24]. An optimized thickness of F4TCNQ was
used for better hole injection, as reported earlier [40]. TAPC was
chosen as the hole-transport material because of its high HOMO
energy (similar to the fluorophores). Besides, considering the en-
ergy levels (shown in Fig. 9) of materials used in the OLEDs, TAPC
with a higher LUMO level also plays a role in effective electron-
blocking. The fluorophores were either used as sole emitters (de-
vice A) or as dopants (5/10 wt.%) in the CBP host (device B). TPBi
was used as an electron-transporting and hole/exciton-blocking
material, which assists in injecting the electrons into the emitting
material and confining the remnant holes in the emitting layer that
did not recombine with the electrons in the emitting zone [40]. For
better electron injection, in all the OLEDs, we have used a bilayer
cathode comprising of LiF and Al in the device structures; here, LiF
acts as an electron injection material. The J−V−L characteristics of
the fabricated OLEDs are shown in Figure S42 (ESI†). The relevant
device performance data are summarised in Table 5. All the devices
with configuration B exhibited lower current densities with respect
to the corresponding devices with configuration A. However, de-
vices B displayed high drive voltages compared to the counterpart
devices A. This can be ascribed to the leakage of charges at the
electrodes due to the imbalance in charge carrier transport. As the
energy barrier for the hole is much lesser (<0.16 eV) compared
to the energy barrier for electron (<0.48 eV) injection into the
emitter, there is a superficial injection of holes into the emitter
layer from TAPC and low injection of electrons into the emitting
layer from TPBi. Hence, the observed current density in devices A
is mainly due to the hole-only current. However, when the flu-
orophores were introduced as dopants in CBP (device configura-
tion B), a significant reduction in the current density is observed
with an associated increase in the luminance. This is attributed to
the fact that, in the case the device B, CBP as a host constitutes
the major charge carriers and ensures a balanced charge carrier
transport. Besides, the dopants effectively apprehend the excitons
formed in the host layer (CBP), and in turn, the emission is origi-
nated from the dopant molecules [56].
Fig. 8. (a) TGA thermograms, (b) and (c) DSC thermograms Pyrene-Imidazole
derivatives measured at a heating rate of 10 °C min–1 under nitrogen atmosphere.
All the fabricated OLEDs with device configuration A and B un-
der investigation showed pure blue to sky blue stable EL spectra
when ITO was positively biased, which were consistent with their
PL counterparts, indicating the emission originated from the emit-
ting layer without emission from interface exciplex. However, the
EL spectra of devices A were bathochromic shifted when compared
to the corresponding devices B (Table 5, Fig. 10), which visibly in-
dicates that devices with configuration A with the fluorophores
as sole emitters exhibit aggregation-induced self-quenching. When
the fluorophores were used as dopants in the CBP host (devices
B), the EL spectra showed hypsochromic shifts with respect to
the corresponding devices A, which indicates that aggregation is
averted in the case of devices B. To further confirm the effect of
pared to ferrocene. The threshold of the oxidation potential in-
creases with the decreasing electron-donating strength. The HOMO
energy levels were estimated from the oxidation onset potentials
onset
ox
according to the equation of EHOMO = −(4.40 + E
) eV with ref-
erence to Ag/AgCl (4.4 eV vs. vacuum). As shown in Table 4, the
calculated HOMO energy values were in the range of 5.0-5.3eV. The
high HOMO energy levels would be beneficial to the improvement
of their hole injecting ability. The LUMO energy levels were ob-
tained through the equation of ELUMO = EHOMO + Eg and are in the
range of 2.26 to 2.41eV. The donor atoms in the imidazole moiety
could effectively decrease the HOMO energy levels from 5.3 to 5.0
9

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
Fig. 9. Schematic of the device architecture and energy levels of the materials used in the OLEDs.
Fig. 10. EL spectra of doped (device A), non-doped (device B) and PA (device C) with different doping concentrations.
dopant concentration on aggregation, a device with 10 wt% of PA
(device B) and the confinement of recombination well within the
host layer (CBP). The inferior performance of the devices with flu-
orophores as sole emitters (devices A) as well as the device with
high dopant concentration (10 wt.% of PA) can be ascribed to the
increased tendency for crystallinity which hampers the film mor-
phology of the fluorophores.
in CBP was fabricated, which showed a bathochromic shift in the
EL spectra (Fig. 10) compared to the device with 5 wt% dopant
concentration. This indicates that the aggregation is hindered at
a lower dopant concentration of the fluorophores. The devices
with fluorophores as dopants (device B) demonstrated excellent
performance compared to their counterparts with configuration A
(Table 5). This distinctly notifies the prominent role of balanced
charge carrier transport at low concentrations of the fluorophores
Interestingly, the device with PI as dopant exhibited the best
performance with high EQE, current efficiency and luminance in
the synthesized fluorophore series. The OLEDs (with device config-
Table 5
Electroluminescence characteristics of the OLEDs.
a
f
Material
Conc. [wt.%]
Vonset
[V]b
Lmax
(cd m−2
ηc
(cd A−1
ηp
(lm W−1
)
ηEQE
(%)d
λEL
(nm)e
CIE
c
d
d
)
)
PA
Neat
5
4.81
5.26
5.11
4.76
5.19
4.68
5.17
4.79
5.22
4.63
5.14
4.74
5.35
3715
10355
9458
3812
11428
3684
9884
3725
9928
3459
9174
3408
9328
4.73
9.24
7.91
4.91
9.82
4.64
8.66
4.58
8.39
4.26
7.47
4.35
7.81
3.37
8.07
6.83
3.58
8.32
3.14
7.34
3.11
7.23
2.98
6.81
3.09
6.95
1.87
4.39
3.74
1.94
4.64
1.81
3.97
1.82
4.09
1.33
3.48
1.49
3.61
465
447
454
450
436
453
434
459
438
468
443
472
459
(0.1396, 0.1497)
(0.1472, 0.0867)
(0.1435, 0.1053)
(0.1454, 0.0922)
(0.1514, 0.0596)
(0.1456, 0.0896)
(0.1526, 0.0574)
(0.1418, 0.1192)
(0.1499, 0.0671)
(0.1387, 0.1723)
(0.1489, 0.0718)
(0.1358, 0.1947)
(0.1425, 0.1202)
PA+CBP
10
PI
Neat
5
PI+CBP
PnB
Neat
5
PnB+CBP
PtB
Neat
5
PtB+CBP
PoM
Neat
5
PoM+CBP
PnDM
Neat
5
PnDM+CBP
a
Device Configuration: ITO (120 nm)/ F4TCNQ (6 nm)/ TAPC (30 nm)/ Emissive Material (35 nm)/TPBI (30 nm)/ LiF (1 nm)/ Al (150 nm)
Vonset: turn-on voltage at luminance of 1 cd m−2
b
c
d
e
f
Lmax: Maximum Luminance
current efficiency (ηc), power efficiency (ηp) and external quantum efficiency (ηEQE
EL: Emission wavelength maximum.
CIE color coordinate
)
λ
10

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
uration B) showed no change in the EL spectra’s shape and peak
with an increase in applied voltage, demonstrating excellent spec-
tral stability. The CIE chromaticity coordinates were determined
using the EL spectra at 10 V, and the results are tabulated in
Table 5. Also, it must be noted that the devices are unoptimized.
Thus, further optimizations as interlayer and anode modifications,
tuning drift mobility, charge balance, and other engineering efforts
can lead to better device performance, including brightness and ef-
ficiency. It is expected that the introduction of additional transport
layers on the cathode side to ensure a more balanced charge in-
jection may result in more efficient OLEDs. However, the devices’
performance is superior in terms of efficiencies and colour purity
compared to the OLEDs based on other reported imidazole-pyrene
hybrid molecules [30,34,36]. These observations offer the potential
use of the synthesized fluorophores in OLED applications.
[2] J.-X. Chen, K. Wang, C.-J. Zheng, M. Zhang, Y.-Z. Shi, S.-L. Tao, H. Lin, W. Liu, W.-
W. Tao, X.-M. Ou, X.-H. Zhang, Red Organic Light-Emitting Diode with External
Quantum Efficiency beyond 20% Based on a Novel Thermally Activated Delayed
Fluorescence Emitter, Adv. Sci. 5 (2018) 1800436, doi:10.1002/advs.201800436.
[3] R.K. Gupta, H. Ulla, M.N. Satyanarayan, A.A. Sudhakar,
A Perylene-Triazine-
Based Star-Shaped Green Light Emitter for Organic Light Emitting Diodes, Eur.
J. Org. Chem. 2018 (2018) 1608–1613, doi:10.1002/ejoc.201800161.
[4] Y. Wang, W. Wang, Z. Huang, H. Wang, J. Zhao, J. Yu, D. Ma, High-efficiency
red organic light-emitting diodes based on a double-emissive layer with an
external quantum efficiency over 30%, J. Mater. Chem. C. 6 (2018) 7042–7045,
doi:10.1039/C8TC01639A.
[5] J. Tagare, H. Ulla, M.N. Satyanarayan, S. Vaidyanathan, Synthesis, photophysical
and electroluminescence studies of new triphenylamine-phenanthroimidazole
based materials for organic light emitting diodes, J. Lumin. 194 (2018) 600–
609, doi:10.1016/j.jlumin.2017.09.020.
[6] Z. Huang, S. Xiang, Q. Zhang, X. Lv, S. Ye, R. Guo, L. Wang, Highly efficient green
organic light emitting diodes with phenanthroimidazole-based thermally acti-
vated delayed fluorescence emitters, J. Mater. Chem. C. 6 (2018) 2379–2386,
doi:10.1039/C7TC05576E.
[7] J. Tagare, H. Ulla, M.N. Satyanarayan, S. Vaidyanathan, Efficient non-doped
bluish-green organic light emitting devices based on N1 functionalized star-
shaped phenanthroimidazole fluorophores, J. Photochem. Photobiol. Chem. 353
(2018) 53–64, doi:10.1016/j.jphotochem.2017.11.001.
4. Conclusions
[8] C. Wang, X. Li, Y. Pan, S. Zhang, L. Yao, Q. Bai, W. Li, P. Lu, B. Yang, S. Su, Y. Ma,
Highly Efficient Nondoped Green Organic Light-Emitting Diodes with Com-
bination of High Photoluminescence and High Exciton Utilization, ACS Appl.
Mater. Interfaces. 8 (2016) 3041–3049, doi:10.1021/acsami.5b10129.
[9] H. Ulla, B. Garudachari, M.N. Satyanarayan, G. Umesh, A.M. Isloor, Blue
organic light emitting materials: Synthesis and characterization of novel
1,8-naphthalimide derivatives, Opt. Mater. 36 (2014) 704–711, doi:10.1016/j.
optmat.2013.11.017.
[10] H. Ulla, M. Raveendra Kiran, B. Garudachari, M.N. Satyanarayan, G. Umesh,
A.M. Isloor, Blue emitting halogen–phenoxy substituted 1,8-naphthalimides for
potential organic light emitting diode applications, Opt. Mater. 37 (2014) 311–
321, doi:10.1016/j.optmat.2014.06.016.
[11] Z. Xu, B.Z. Tang, Y. Wang, D. Ma, Recent advances in high performance blue
organic light-emitting diodes based on fluorescence emitters, J. Mater. Chem.
C. 8 (2020) 2614–2642, doi:10.1039/C9TC06441A.
[12] J.-H. Lee, C.-H. Chen, P.-H. Lee, H.-Y. Lin, M. Leung, T.-L. Chiu, C.-F. Lin, Blue
organic light-emitting diodes: current status, challenges, and future outlook, J.
Mater. Chem. C. 7 (2019) 5874–5888, doi:10.1039/C9TC00204A.
In summary, a series of novel pyrene-imidazole based fluo-
rophores with different donating groups at the para position of
phenyl attached to the N1 position of imidazole moiety were
synthesized and characterized. The photophysical, thermal, and
electrochemical properties were studied. The fluorophores showed
blue emission with high quantum yields. The delayed fluorescence
exhibited by the fluorophores indicated that the materials har-
vested triplet energy. The fluorophores possess excellent thermal
stability and appropriate HOMO-LUMO energy levels for efficient
charge injection. The OLEDs fabricated using the fluorophores as
dopant materials in the CBP host showed superior performance
compared to their counterpart non-doped devices. Moreover, the
EL spectra of the doped devices show hypsochromic shifts com-
pared to their non-doped counterparts. The device with PI as
dopant with 5% doping concentration showed an EQE of 4.64%
with CIE chromaticity coordinates of (0.1514, 0.0596). Hence, our
molecular strategy developed on pyrene-imidazole moieties can be
utilized when pursuing high EQE in blue OLEDs.
[13] Y. Im, S.Y. Byun, J.H. Kim, D.R. Lee, C.S. Oh, K.S. Yook, J.Y. Lee, Recent Progress
in High-Efficiency Blue-Light-Emitting Materials for Organic Light-Emitting
Diodes, Adv. Funct. Mater. 27 (2017) 1603007, doi:10.1002/adfm.201603007.
[14] J.-H. Jou, S. Kumar, A. Agrawal, T.-H. Li, S. Sahoo, Approaches for fabricating
high efficiency organic light emitting diodes, J. Mater. Chem. C. 3 (2015) 2974–
3002, doi:10.1039/C4TC02495H.
[15] C. Adachi, Third-generation organic electroluminescence materials, Jpn. J. Appl.
Phys. 53 (2014) 060101, doi:10.7567/JJAP.53.060101.
[16] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Highly efficient organic
light-emitting diodes from delayed fluorescence, Nature 492 (2012) 234–238,
doi:10.1038/nature11687.
Supplementary Information
1H NMR, 13C NMR, LCMS and HRMS spectra of the derivatives
and fragments; photophysical and DFT optimized structures are
provided. Also, CCDC 1576203 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
https://www.ccdc.cam.ac.uk/structures/
[17] T. Hosokai, H. Matsuzaki, H. Nakanotani, K. Tokumaru, T. Tsutsui, A. Furube,
K. Nasu, H. Nomura, M. Yahiro, C. Adachi, Evidence and mechanism of effi-
cient thermally activated delayed fluorescence promoted by delocalized ex-
cited states, Sci. Adv. 3 (2017) e1603282, doi:10.1126/sciadv.1603282.
[18] W. Li, D. Liu, F. Shen, D. Ma, Z. Wang, T. Feng, Y. Xu, B. Yang, Y. Ma, A Twisting
Donor-Acceptor Molecule with an Intercrossed Excited State for Highly Effi-
cient, Deep-Blue Electroluminescence, Adv. Funct. Mater. 22 (2012) 2797–2803,
doi:10.1002/adfm.201200116.
[19] Y.J. Cho, K.S. Yook, J.Y. Lee, Cool and warm hybrid white organic light-emitting
diode with blue delayed fluorescent emitter both as blue emitter and triplet
host, Sci. Rep. 5 (2015) 7859, doi:10.1038/srep07859.
[20] J. Kang, R. Zaen, K.-M. Park, K.H. Lee, J.Y. Lee, Y. Kang, Cyclometalated Plat-
inum(II) β-Diketonate Complexes as Single Dopants for High-Efficiency White
OLEDs: The Relationship between Intermolecular Interactions in the Solid State
and Electroluminescent Efficiency, Cryst. Growth Des. 20 (2020) 6129–6138,
doi:10.1021/acs.cgd.0c00838.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
[21] Q. Zhang, D. Tsang, H. Kuwabara, Y. Hatae, B. Li, T. Takahashi, S.Y. Lee, T. Ya-
suda, C. Adachi, Nearly 100% Internal Quantum Efficiency in Undoped Electro-
luminescent Devices Employing Pure Organic Emitters, Adv. Mater. 27 (2015)
2096–2100, doi:10.1002/adma.201405474.
[22] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting
diodes: Status and perspective, Rev. Mod. Phys. 85 (2013) 1245–1293, doi:10.
1103/RevModPhys.85.1245.
VJR thanks to CSIR New Delhi for Emeritus Scientist Honour. HU
acknowledge DST-SERB, India (NPDF Award No. PDF/2016/000711),
for financial support. GRR acknowledge DST-SERB, India, for fi-
nancial aid. GUS and CM thank UGC and CSIR for fellowships.
IICT/Pubs./2019/382
[23] Y. Ge, Y. Wen, H. Liu, T. Lu, Y. Yu, X. Zhang, B. Li, S.-T. Zhang, W. Li, B. Yang,
A key stacking factor for the effective formation of pyrene excimer in crystals:
degree of π–π overlap, J. Mater. Chem. C. 8 (2020) 11830–11838, doi:10.1039/
D0TC02562C.
[24] S. Chidirala, H. Ulla, A. Valaboju, M.R. Kiran, M.E. Mohanty, M.N. Satyanarayan,
G. Umesh, K. Bhanuprakash, V.Jayathirtha Rao, Pyrene–Oxadiazoles for Organic
Light-Emitting Diodes: Triplet to Singlet Energy Transfer and Role of Hole-
Injection/Hole-Blocking Materials, J. Org. Chem. 81 (2016) 603–614, doi:10.
1021/acs.joc.5b02423.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130306.
References
[25] G. Mallesham, C. Swetha, S. Niveditha, M.E. Mohanty, N.J. Babu, A. Kumar,
K. Bhanuprakash, V.Jayathirtha Rao, Phosphine oxide functionalized pyrenes as
[1] C.W. Tang, S.A. VanSlyke, Organic electroluminescent diodes, Appl. Phys. Lett.
51 (1987) 913–915, doi:10.1063/1.98799.
11

G. Umasankar, H. Ulla, C. Madhu et al.
Journal of Molecular Structure 1236 (2021) 130306
efficient blue light emitting multifunctional materials for organic light emit-
ting diodes, J. Mater. Chem. C. 3 (2015) 1208–1224, doi:10.1039/C4TC01753F.
[26] T.M. Figueira-Duarte, K. Müllen, Pyrene-Based Materials for Organic Electron-
ics, Chem. Rev. 111 (2011) 7260–7314, doi:10.1021/cr100428a.
[41] M.J. Frisch, G.W. Trucks, H.B. Schlegel, et al., Gaussian 09, Gaussian, Inc.,
Wallingford CT, 2009 https://gaussian.com/g09citation.
[42] A.D. Becke, Density-functional exchange-energy approximation with correct
asymptotic behavior, Phys. Rev. A. 38 (1988) 3098–3100, doi:10.1103/PhysRevA.
38.3098.
[43] A.D. Becke, Density-functional thermochemistry. I. The effect of the exchange-
only gradient correction, J. Chem. Phys. 96 (1992) 2155–2160, doi:10.1063/1.
462066.
[27] D. Chercka, S.-J. Yoo, M. Baumgarten, J.-J. Kim, K. Müllen, Pyrene based materi-
als for exceptionally deep blue OLEDs, J. Mater. Chem. C. 2 (2014) 9083–9086,
doi:10.1039/C4TC01801J.
[28] H. Jeong, H. Shin, J. Lee, B. Kim, Y.-I. Park, K.S. Yook, B.-K. An, J. Park, Re-
cent progress in the use of fluorescent and phosphorescent organic compounds
for organic light-emitting diode lighting, J. Photonics Energy. 5 (2015) 057608,
doi:10.1117/1.JPE.5.057608.
[44] C. Lee, W. Yang, R.G. Parr, Development of the Colle-Salvetti correlation-energy
formula into a functional of the electron density, Phys. Rev. B. 37 (1988) 785–
789, doi:10.1103/PhysRevB.37.785.
[29] Y. Hong, J.W.Y. Lam, B.Z. Tang, Aggregation-induced emission: phenomenon,
mechanism and applications, Chem. Commun. (2009) 4332–4353, doi:10.1039/
B904665H.
[45] P.C. Hariharan, J.A. Pople, The influence of polarization functions on molecular
orbital hydrogenation energies, Theor. Chim. Acta. 28 (1973) 213–222, doi:10.
1007/BF00533485.
[30] Y. Liu, Q. Bai, J. Li, S. Zhang, C. Zhang, F. Lu, B. Yang, P. Lu, Efficient pyrene-
[46] M.M. Francl, W.J. Pietro, W.J. Hehre, J.S. Binkley, M.S. Gordon, D.J. DeFrees,
J.A. Pople, Self-consistent molecular orbital methods. XXIII. A polarization-
imidazole derivatives for organic light-emitting diodes, RSC Adv
17239–17245, doi:10.1039/C5RA25424H.
6
(2016)
type basis set for second-row elements, J. Chem. Phys. 77 (1982) 3654–3665,
doi:10.1063/1.444267.
[31] G. Jones, V.I. Vullev, Ground- and Excited-State Aggregation Properties of a
Pyrene Derivative in Aqueous Media, J. Phys. Chem. A. 105 (2001) 6402–6406,
doi:10.1021/jp010087q.
[47] V.A. Rassolov, J.A. Pople, M.A. Ratner, T.L. Windus, 6-31G∗ basis set for atoms
K through Zn, J. Chem. Phys. 109 (1998) 1223–1229, doi:10.1063/1.476673.
[48] T. Yanai, D.P. Tew, N.C. Handy, A new hybrid exchange–correlation functional
using the Coulomb-attenuating method (CAM-B3LYP), Chem. Phys. Lett. 393
(2004) 51–57, doi:10.1016/j.cplett.2004.06.011.
[32] G. Mallesham, S. Balaiah, M.A. Reddy, B. Sridhar, P. Singh, R. Srivastava,
K. Bhanuprakash, V.Jayathirtha Rao, Design and synthesis of novel anthracene
derivatives as n-type emitters for electroluminescent devices:
a combined
experimental and DFT study, Photochem. Photobiol. Sci. 13 (2014) 342–357,
doi:10.1039/C3PP50284H.
[49] B. Mennucci, J. Tomasi, R. Cammi, J.R. Cheeseman, M.J. Frisch, F.J. Devlin,
S. Gabriel, P.J. Stephens, Polarizable Continuum Model (PCM) Calculations of
Solvent Effects on Optical Rotations of Chiral Molecules, J. Phys. Chem. A. 106
(2002) 6102–6113, doi:10.1021/jp020124t.
[33] Liu Yulong, Man Xiaxia, Bai Qing, Liu Hui, Liu Pengyuan, Fu Ying, Hu De-
hua, Lu Ping, Ma Yuguang, Highly Efficient Blue Organic Light-Emitting Diode
Based on
a
Pyrene[4,5-d]Imidazole-Pyrene Molecule, CCS Chem.
0
(n.d.).
[50] V.R. Gopal, A.M. Reddy, V.Jayathirtha Rao, Wavelength Dependent Trans to
Cis and Quantum Chain Isomerizations of Anthrylethylene Derivatives, J. Org.
Chem. 60 (1995) 7966–7973, doi:10.1021/jo00129a043.
https://doi.org/10.31635/ccschem.021.202000627.
[34] G. Chen, Z. Qiu, J.-H. Tan, W.-C. Chen, P. Zhou, L. Xing, S. Ji, Y. Qin,
Z. Zhao, Y. Huo, Deep-blue organic light-emitting diodes based on push-pull
π-extended imidazole-fluorene hybrids, Dyes Pigments 184 (2021) 108754,
doi:10.1016/j.dyepig.2020.108754.
[35] H. Lee, C.D. Sunesh, M.S. Subeesh, Y. Choe, Blue-light emitting electrochemi-
cal cells comprising pyrene-imidazole derivatives, Opt. Mater. 78 (2018) 44–51,
doi:10.1016/j.optmat.2018.02.018.
[51] M. Paramasivam, A. Gupta, A.M. Raynor, S.V. Bhosale, K. Bhanuprakash,
V.Jayathirtha Rao, Small band gap D-π-A-π-D benzothiadiazole derivatives
with low-lying HOMO levels as potential donors for applications in organic
photovoltaics: a combined experimental and theoretical investigation, RSC Adv
4 (2014) 35318–35331, doi:10.1039/C4RA02700K.
[52] K. Mani Bushan, G. Venugopal Rao, T. Soujanya, V. Jayathirtha Rao, S. Saha,
A. Samanta, Photochemical E (trans) → Z (cis) Isomerization in Substituted 1-
Naphthylacrylates, J. Org. Chem. 66 (2001) 681–688, doi:10.1021/jo000819v.
[53] P. Nagaraju, C. Madhu, M.E. Mohanty, V. Jayathirtha Rao, Delayed fluorescence
based phenanthroimidazoles as OLED emitters having electron transporting
properties, IJC-B Vol57B02 Febr. 2018. (2018). http://nopr.niscair.res.in/handle/
123456789/43607 (accessed February 19, 2021).
[36] Y. Liu, Z. Gao, Z. Wang, C. Feng, F. Shen, P. Lu, Y. Ma, Synthesis and Character-
ization of an Imidazole-Containing Pyrene π-System, Eur. J. Org. Chem. 2013
(2013) 7267–7271, doi:10.1002/ejoc.201301285.
[37] X. Feng, J.-Y. Hu, C. Redshaw, T. Yamato, Functionalization of Pyrene To Prepare
Luminescent Materials—Typical Examples of Synthetic Methodology, Chem. –
Eur. J 22 (2016) 11898–11916, doi:10.1002/chem.201600465.
[38] K. Ramesh, S.N. Murthy, K. Karnakar, Y.V.D. Nageswar, K. Vijayalakhshmi,
B.L.A. Prabhavathi Devi, R.B.N. Prasad, A novel bioglycerol-based recyclable car-
bon catalyst for an efficient one-pot synthesis of highly substituted imidazoles,
Tetrahedron Lett 53 (2012) 1126–1129, doi:10.1016/j.tetlet.2011.12.092.
[39] K. Buttke, H. Baumgärtel, H.-J. Niclas, M. Schneider, Synthese, Struktur und
photophysikalische Eigenschaften von polyarylierten Imidazolen und Oxa-
zolen, J. Für Prakt. ChemieChemiker-Ztg. 339 (1997) 721–728, doi:10.1002/prac.
199733901131.
[54] M.J. Sung, H. Chubachi, R. Sato, M.-K. Shin, S.-K. Kwon, Y.-J. Pu, Y.-H. Kim,
Dimethylsilyl-linked anthracene–pyrene dimers and their efficient triplet–
triplet annihilation in organic light emitting diodes, J. Mater. Chem. C. 5 (2017)
1090–1094, doi:10.1039/C6TC05308D.
[55] C. Gärditz, A. Winnacker, F. Schindler, R. Paetzold, Impact of Joule heating on
the brightness homogeneity of organic light emitting devices, Appl. Phys. Lett.
90 (2007) 103506, doi:10.1063/1.2711708.
[56] T. Chatterjee, K.-T. Wong, Perspective on Host Materials for Thermally Activated
Delayed Fluorescence Organic Light Emitting Diodes, Adv. Opt. Mater. 7 (2019)
1800565, doi:10.1002/adom.201800565.
[40] H. Ulla, M.R. Kiran, B. Garudachari, T.N. Ahipa, K. Tarafder, A.V. Adhikari,
G. Umesh, M.N. Satyanarayan, Blue emitting 1,8-naphthalimides with elec-
tron transport properties for organic light emitting diode applications, J. Mol.
Struct. 1143 (2017) 344–354, doi:10.1016/j.molstruc.2017.04.103.
12