Blue emitting 1,8-naphthalimides with electron transport properties for organic light emitting diode applications

Article abstract of DOI:10.1016/j.molstruc.2017.04.103

In this article, the synthesis, characterization and use of two novel naphthalimides as electron-transporting emitter materials for organic light emitting diode (OLED) applications are reported. The molecules were obtained by substituting electron donating chloro-phenoxy group at the C-4 position. A detailed optical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical calculations (DFT) were performed to get a better understanding of the electronic structures. The synthesized molecules were used as electron transporters and emitters in OLEDs with three different device configurations. The devices with the molecules showed blue emission with efficiencies of 1.89?cdA^-1, 0.98?lmW^?1, 0.71% at 100?cdm^-2. The phosphorescent devices with naphthalimides as electron transport materials displayed better performance in comparison to the device without any electron transporting material and were analogous with the device using standard electron transporting material, Alq₃. The results demonstrate that the naphthalimides could play a significant part in the progress of OLEDs.

Full text of DOI:10.1016/j.molstruc.2017.04.103

Accepted Manuscript
Blue emitting 1,8-Naphthalimides with electron transport properties for organic light
emitting diode applications
Hidayath Ulla, M. Raveendra Kiran, B. Garudachari, T.N. Ahipa, Kartick Tarafder,
Airody Vasudeva Adhikari, G. Umesh, M.N. Satyanarayan
PII:
S0022-2860(17)30525-2
10.1016/j.molstruc.2017.04.103
MOLSTR 23726
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 1 March 2017
Revised Date: 18 April 2017
Accepted Date: 20 April 2017
Please cite this article as: 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 electron transport properties
for organic light emitting diode applications, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.04.103.
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ACCEPTED MANUSCRIPT
Blue Emitting 1,8-Naphthalimides with Electron Transport Properties for Organic Light
Emitting Diode Applications
Hidayath Ullaa*, M. Raveendra Kirana, B. Garudacharib, T. N. Ahipac, Kartick Tarafdera, Airody
Vasudeva Adhikarib, G. Umesha, M. N. Satyanarayana
a Department of Physics, National Institute of Technology Karnataka, Mangalore – 575025, India
b Department of Chemistry, National Institute of Technology Karnataka, Mangalore – 575025, India
c Centre for Nano and Material Sciences, Jain University, Bangalore - 562 112, India
Corresponding author: ulla.hid@gmail.com
Abstract
In this article, the synthesis, characterization and use of two novel naphthalimides as electron-
transporting emitter materials for organic light emitting diode (OLED) applications are reported.
The molecules were obtained by substituting electron donating chloro-phenoxy group at the C-4
position. A detailed optical, thermal, electrochemical and related properties were systematically
studied. Furthermore, theoretical calculations (DFT) were performed to get a better
understanding of the electronic structures. The synthesized molecules were used as electron
transporters and emitters in OLEDs with three different device configurations. The devices with
the molecules showed blue emission with efficiencies of 1.89 cdA-1, 0.98 lmW-1, 0.71 % at 100
cdm-2. The phosphorescent devices with naphthalimides as electron transport materials displayed
better performance in comparison to the device without any electron transporting material and
were analogous with the device using standard electron transporting material, Alq3. The results
demonstrate that the naphthalimides could play a significant part in the progress of OLEDs.
Keywords: Blue, OLEDs, Electron-transport, Naphthalimide, Solvatochromism, DFT/TD-DFT.

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1. Introduction
Organic π-conjugated materials have attracted considerable research interest because of their use
in modern day organic light-emitting diode (OLED) applications [1–4]. One of the key
challenges in developing efficient OLEDs is to balance charge injection and transport from both
cathode and anode to the emissive layer in the device structure [5–7]. In most of the current
organic luminescent materials, electron mobility is usually few orders of magnitude smaller than
hole mobility. Hence, by introducing an electron-transport layer (ETL) in the device architecture,
the efficiency of OLEDs can be considerably enhanced [3,8]. The potential ETL materials are
supposed to have adequately high electron affinity (> 3.0 eV) in order to facilitate electron
injection from the commonly used metal cathode (Al). Alq3 has been widely studied as ETL
material for OLEDs [7]. Recently, heterocyclic molecules have emerged as promising class of
electron-transport materials for OLEDs [3,8–10]. 1,8-naphthalimides have shown considerable
promise and are the most frequently studied candidates [11]. The molecules possess wide energy
band-gap and low reduction potentials. The excited-state properties in these molecules can be
significantly altered by the nature of the substituent present in the aromatic ring [12,13]. The
existence of electron-deficient centre usually make these molecules attain high electron affinity
which authenticates the possibility of their use as electron-transporting or hole-blocking
materials in OLEDs [11]. One option to accomplish this is by introducing electron-donating
groups at the C-4 position of 1,8-naphthalimide [12]. Electron-donating substituents at the C-4
position induce a polar charge-transfer excited state, which usually increase the PL emission
[14]. In this article, we have synthesized and characterized two novel naphthalimides [4 and 5]
by nucleophilic substitutions of electron donating chloro-phenoxy group at the C-4 position of
1,8-naphthalimide. We have investigated the naphthalimides as electron-transport as well as
electroluminescent materials for OLED applications. Our results show that the materials are
potential candidates for electron-transport and/or blue emitters and could play a significant part
in the growth of OLEDs.
2. Experimental
2.1 Materials and Methods
All the starting reagents and materials obtained from commercial sources (Sigma Aldrich) were
used without further purification. Spectroscopic grade organic solvents were used for spectral

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analysis. The reactions were performed under the blanket of Nitrogen (N2). NMR, FT-IR and
mass spectra were used to confirm the chemical structures of the molecules. NMR spectra were
acquired on a Bruker Ascend 400 MHz NMR Spectrometer at a working frequency of 400 MHz
at room temperature in DMSO-d6 as solvent using tetramethylsilane (TMS) as internal standard.
The chemical shifts are given as δ in ppm. The mass spectra were recorded using API 2000
LC/MS/MS system and LC-MS-Agilent 1100 series. FT-IR spectra were obtained using a
Thermo Nicolet AVATAR 330-FT-IR spectrophotometer. Thin layer chromatography on silica
gel coated aluminium sheets (silica gel 60 F254) were used to access and monitor the progress
and completion of the reactions. The optical properties of the molecules in solution and thin solid
film form were investigated using UV-visible (UV-vis) spectrophotometer (Ocean Optics Inc.
SD 2000) and Fluoromax-4 TCSPC spectrophotometer (Horiba Jobin Yvon) at room
temperature. A 150 W xenon lamp was used as the excitation source. The spectral data were
obtained using FluorEscence software and OrginPro software was used for data analysis. The PL
lifetime measurements were done using the same spectrophotometer attached with a NanoLED
(350 nm) pulsed excitation source using the time-correlated single-photon counting (TCSPC)
technique at room temperature. The decay data was analysed using Data Analysis Software 6
(DAS6). Based on the absorption and PL spectra of the molecules in chloroform (CHCl3), the PL
quantum yield (ΦPL) of the molecules were calculated by a comparative method using 9,10-
diphenylanthracene (Φref = 0.90 in cyclohexane) as standard [15]. Thermo gravimetric analysis
(TGA) was recorded using EXSTAR 6000, TG/DTA 6300 thermal analyser. The melting points
of the molecules were obtained from differential scanning calorimetry (DSC) using a
SHIMADZU DSC-60. DSC and TGA measurements were performed under nitrogen atmosphere.
The surface morphology of organic films prepared by thermal evaporation was characterized by
Park System XE-70 atomic force microscope (AFM) under ambient conditions in non-contact
mode. The electrochemical studies of the molecules in acetonitrile (10-3 M) were studied using
AUTOLAB PGSTAT-30 electrochemical analyzer at a constant scan rate: 10 mV s-1 at room
temperature by cyclic voltammetry (CV) measurements in a conventional three-electrode cell
system. CV of the molecules in acetonitrile (10-3 M) were performed with tetrabutyl ammonium
hexafluoro phosphate solution (0.1 M in acetonitrile) as supporting electrolyte and Ag/AgNO3
(in 0.1 M acetonitrile) as reference electrode; platinum plays a dual role as working electrode
and counter electrode [12]. The measurements were done in N2 atmosphere and before each

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experiment, all the solutions in the cell were purged with ultrahigh-pure N2 for 10-15 min.
Ferrocene/ferrocenium (Fc/Fc+) redox system was used as internal standard to calibrate each
measurements [12].
2.2 Materials synthesis
2.2.1 6-(4-chlorophenoxy)-2-(2-hydroxyethyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (4):
4-Chlorophenol (0.28 g, 0.0016 mol) and potassium carbonate (anhydrous, 0.22 g, 0.0016 mol)
were mixed into a solution of intermediate 3 (0.5 g, 0.0015 mol) in anhydrous DMF (10 ml). In
nitrogen atmosphere, the mixture was stirred for 2 h at 100 °C. The obtained reactant mixture
was cooled to reach room temperature. It was then poured into 50 ml of cool water. The
subsequent yellow precipitate was amassed and treated with charcoal (ethyl acetate 50 ml) at 40
°C for 1 h. This was the purified using column chromatography (eluent: petroleum ether: ethyl
acetate 8:2) to obtain a pale yellow solid. Yield 83%. mp 190-192 ºC. IR (KBr νmax cm-1): 777,
848 (C-Cl str); 1038, 1172, 1251, 1360 (-O- str); 1485, 1591, 1651, 1703 (C=O str); 2957 (C-H
1
str); 3297 (O-H str). H NMR (400 MHz, DMSO-d6) δ (ppm): 8.67 (d, J=8.0 Hz, 1H, Ar-H),
8.57 (d, J=6.0 Hz, 1H, Ar-H), 8.42 (d, J=8.0 Hz, 1H, Ar-H), 7.92 (t, J=7.2 Hz, 1H, Ar-H), 7.60
(d, J=7.6 Hz, 2H, Ar-H), 7.35 (d, J=7.6 Hz, 2H, Ar-H), 7.06 (d, J=8.0 Hz, 1H, Ar-H), 4.83 (s,
1H, -OH), 4.15 (s, 2H, -NCH2-), 3.62 (d, J=5.6 Hz, 2H, -NCH2-CH2-). 13C NMR: 164.06,
163.41, 158.92, 154.08, 133.05, 131.96, 130.95, 129.97, 129.51, 128.57, 127.72, 123.79, 122.86,
117.27. 112.05, 58.28, 42.25, 128.85, 127.78, 122.96, 111.98, 61.46, 21.06. m/z: 367 (M+1).
Yield: 83%.
2.2.2 2-[6-(4-chlorophenoxy)-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl]ethyl acetate (5):
The solution of 4 (0.2 g, 0.0006 mol) in acetic anhydride (2.5 ml) and acetic acid (2.5 ml) was
stirred for 2 h at 130 oC in a round bottom flask. The mixture was cooled to obtain a pale yellow
precipitate, which was filtered and purified using column chromatography [12]. Yield 90 %. mp.
196-198. IR (KBr νmax cm-1): 780, 860 (C-Cl str); 1023, 1057, 1232, 1361 (-O- str); 1462, 1489,
1
1576, 1655, 1693, 1745 (C=O str); 2969, 3098 (C-H str). H NMR (400 MHz, DMSO-d6) δ
(ppm): 8.70 (d, J=8.4 Hz, 1H, Ar-H), 8.61 (s, 1H, Ar-H), 8.45 (d, J=8.0 Hz, 1H, Ar-H), 7.94 (t,
J=7.2 Hz, 1H, Ar-H), 7.61 (d, J=6.8 Hz, 2H, Ar-H), 7.37 (d, J=6.8 Hz, 2H, Ar-H), 7.07 (d, J=6.8
Hz, 1H, Ar-H), 4.32 (s, 4H, -N(CH2)2-), 1.93 (s, 3H, -CH3). 13C NMR: 170.82, 130.97, 128.85,
127.78, 122.96, 111.98, 61.46, 21.06. m/z: 410 (M+1). Yield: 90%

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2.3 OLED fabrication and testing
The electroluminescent and electron transport properties of the synthesized naphthalimides were
studied by fabricating unoptimized OLEDs using thermal evaporation at a base pressure of 5 x
10-6 Torr. Pre-patterned indium tin oxide (ITO, Kintec, Hong Kong) coated glass substrates, with
a sheet resistance of 15 ꢀ/□ and thickness of 120 nm, were used as anode. Prior to the deposition
of organic materials, ITO substrates were cleaned and UV-Ozone treated according to the
procedure reported earlier [11]. All the organic and cathode layers were deposited on the ITO
substrates sequentially without breaking the vacuum and the devices were never exposed to air
during fabrication. F4TCNQ (2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane, Sigma
Aldrich) was used as hole-injection material (HIL), α-NPD (4,4’-bis[N-(1-naphthyl)-N-phenyl-l-
amino]-biphenyl, Sigma Aldrich) was hole-transport material (HTL), BCP (bathocuproine,
Sigma Aldrich) was hole-blocking material (HBL), Alq3 (tris-(8-hydroxyquinoline) aluminium,
Sigma Aldrich) was ETL, Ir(ppy)3 (tris[2-phenylpyridinato-C2,N]iridium(III), Sigma Aldrich)
doped CBP (4,4’-bis(9-carbazolyl)-biphenyl, Sigma Aldrich) was phosphorescent emitter, LiF
(lithium fluoride, Sigma Aldrich) was electron-injection material (EIL), and Al (aluminium, Alfa
Aesar) was cathode. The organic materials were used as received without any further
purification. All the organic materials were deposited at a rate of 0.5 Å s-1, whereas LiF and Al
were deposited at 0.1 Å s-1 and 6 Å s-1, respectively. The thickness and the deposition rates were
controlled in situ by a quartz crystal thickness oscillator placed near the substrate. As a final step
of the fabrication, cathode was deposited through a shadow mask. The light-emitting area was
1.6 mm2 as defined by the overlap of the cathode and anode. All the electro-optical properties of
the fabricated OLEDs such as current density-voltage-luminance (J-V-L) characteristics and
electroluminescence (EL) were investigated using a computer-controlled, programmable source-
meter (Keithley 2400), calibrated Si photodiode (SM1PD2A) and spectrophotometer (Horiba
Jobin Yvon iHR320). CIE coordinates were calculated from the EL spectra. Devices were driven
under dc conditions. All the device characteristics were performed under ambient and dark
conditions without any encapsulation at room temperature.
2.4 Computational Methods
Density Functional Theory (DFT) calculations were carried out in order to have a deeper
understanding of the electronic states of the naphthalimides using GAUSSIAN 09 package. A
Becke–Lee–Yang–Parr hybrid exchange-correlation (closed shell) three-parameter functional

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(B3LYP) [16–18] along with 6-311++G (d) basis set were used in the simulation to obtain the
geometry optimization of the isolated molecules. In each case, the molecular structure was
completely optimized by using Berny’s optimization algorithm, which uses redundant internal
coordinates. The convergence to the minima on the potential energy surface was confirmed by
calculating the harmonic vibrational wavenumbers using analytic second derivatives and
properly scaled down to control the systematic errors caused by incompleteness of the basis set.
Since time dependent DFT (TD-DFT) is a powerful and efficient tool to study the ground and
excited state properties, we have used this method it considering the same B3LYP exchange-
correlation function with 6-311++G (d,p) basis set to obtain the UV-Visible absorption spectra.
3. Results and Discussion
3.1 Synthesis and Molecular Geometry
Scheme 1 depicts the detailed synthetic route that was adopted for the synthesis of 4 and 5 using
nucleophilic substitution of chloro-phenoxy electron donating group at the C-4 position of
naphthalimide moiety. The synthesis of intermediates 1 (5-Bromoacenaphthene), 2 (4-Bromo-
1,8-naphthalic Anhydride) and 3 (6-bromo-2-(2-hydroxyethyl)-1H-benzo[de]isoquinoline-
1,3(2H)-dione) were done as per the reported procedure with a slight modification [19]. 5 was
prepared by the acetylation of 4 [12]. The molecular structures determined by NMR, FT-IR and
mass spectra are in accordance with the proposed structures as well as the theoretically obtained
optimized structures. Both the molecules readily dissolve in common organic solvents. The
molecules have similar structure where the bromo group at the C-4 position of intermediate 3 is
replaced by chloro-phenoxy group to obtain 4. By replacing C2H5O attached to the nitrogen atom
in 4 by C4H7O, 5 is obtained. From the energy-optimized geometry, we have not observed any
significant change in the bond length and bond angle at the central part of both the molecules.
However, the different ligand attached to the N-atom rearranges the energy labels of molecular
orbitals. The optimized structures of 4 and 5 are shown in Figure 1.
3.2 Optical Properties
Studying the optical properties is vital in understanding the functioning of organic materials in
devices. On irradiation, the optical properties of naphthalimides are normally associated with
chromophoric system’s polarization and are governed by the nature of the substituents [20].
Absorption of light in these materials lead to an internal charge transfer (CT) excited state which

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results in the donor–acceptor interactions (D-π-A) between the electron-donating substituents at
the C-4 position and the electron-accepting carbonyl groups. These interactions may be
prompted by the environmental effect of different media, especially the polarity of organic
solvents. The effect of the solvent polarity is investigated by studying basic optical properties of
4 and 5 in both protic and aprotic organic solvents (10−5 M) of different polarities and thin solid
film using UV-Vis absorption and PL spectroscopy. The optical data is tabulated in Table 1 and
Table SI1.
At room temperature, intermediate 3 which has a bromo group (electron-withdrawing) at C-4
position, shows wavelength absorption maximum (λabs) of 340 nm (shoulder peak at 354 nm) in
CHCl3 [12]. By substituting chloro-phenoxy group which is electron-donating at the C-4 position
(4 and 5) of intermediate 3 results in the bathochromic shift of ~ 20 nm in λabs, thus exhibiting
that the lowest singlet excited state has CT nature. Both the molecules show quite similar
broadband UV-Vis absorption characteristics (Figure 2a) with λabs of 363 nm (4) and 362 nm (5).
This shift may be ascribed to the π-π* electronic transitions occurring between the chloro-
phenoxy substituent (electron-donating) at the C-4 position to the carbonyl groups (electron-
accepting) in the naphthalimide moiety. The absorption of the molecules is evidently
concentrated on the naphthalimide moiety. The lowest excited state is prevalent in π-π* character
for 1,8-naphthalimides. A small bathochromic shift (~10 nm) can be seen in λabs of the thin film
(Figure 2) compared to that in CHCl3, with λabs shifting to 374 nm (4) and 371 nm (5). The shift
signifies that the π-π* interactions are weak between the molecules in the thin solid film state,
which may arise due to the intermolecular aggregation in condensed state. Using the thin film
absorption spectra, the optical energy band gaps (Egopt) of the molecules calculated using Tauc
plots (Figure 2b) considering direct allowed transitions ((αhν) 2 versus hν) are 3.04 eV and 2.99
eV, respectively for 4 and 5. The molar extinction coefficients (log ε) are high (4.3 l mol-1 cm-1)
which indicates that the long wavelength absorption band has CT nature which may be due to the
S0→S1 transitions.
Time-dependent DFT (TD-DFT) calculations in vacuum were performed on electronic
absorption spectrum in order to have a better perceptive of the absorption spectra. Only the
lowest six singlet-to-singlet spin-allowed excited states for both the molecules were considered.
The generated spectra of 4 and 5 together with experimental observation are presented in Figure

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2a. The calculated excitation energies together with oscillator strength (f) and λabs are shown in
Table 1. In general, UV spectrum of naphthalimides are investigated by the band assigned to the
π to π* transition which is also observed in our calculation. In addition, high intensity absorption
peak at 350 nm and 348 nm for 4 and 5, respectively can be observed. Figure 2a shows
reasonably good agreement between the theory and experimental absorption spectra. A slight
deviation in the peak position is expected due to the solvent used in experiment. The molecules
exhibited intense blue PL emission corresponding to S1→S0 transition in CHCl3 and in thin solid
film, which may be ascribed to the CT interactions between the substituent donor group and
carbonyl group acceptors in the moiety.
Intermediate 3 exhibits weak PL with maximum (λPL) at 406 nm in CHCl3 due to the presence of
bromo-substituent at the C-4 position which is electron-withdrawing in nature [12]. On
substituting the bromo group in intermediate 3 with the chloro-phenoxy group (4 and 5), the λPL
was bathochromic shifted (~ 20 nm) to 426 nm (4) and 422 nm (5) with full width half maximum
(FWHM) of ~ 45 nm as shown in Figure 2a. The observed shifts (Table 1) can be attributed to
the introduction of the substituent which lead to the formation of D-π-A structures. Similar to
UV-Vis absorption, the λPL of 5 is hypsochromic shifted (4 nm) compared to that of 4. Studying
the thin film PL of the molecules is important from the viewpoint of device applications. The PL
spectra in thin solid films have similar shape to those observed in CHCl3, with λPL
bathochromically shifted to 462 nm (4) and 458 nm (5) with FWHM of ~ 60 nm. The energy of
activation of the first excited state (ES1) in CHCl3obtained from the point of intersection of
absorption and PL spectra, has the values of 312 (4) and 316 (5) kJ mol-1. The Stokes’ shifts (υst)
of the molecules (Table 1) have high values of ~ 4000 cm-1 (CHCl3) and ~ 5100 cm-1 (thin films),
confirming that the absence of reabsorption of the emitted radiation. The oscillator strengths of
the molecules are 0.307 (4) and 0.298 (5). Both the υst and f values of the molecules are
comparable with reported 1,8-naphthalimides [11–13,15]. PL quantum yield (ΦPL) of the
molecules are 0.28 (4) and 0.26 (5). The calculated energy yields of PL (EPL) are 0.33 (4) and
0.21 (5), which can also be used instead of ΦPL. The values of ΦPL and EPL are higher than their
analogous derivatives [11,15]. The performance of the materials in the EL devices can be
estimated by relating the emission colour by means of CIE chromaticity coordinates. These CIE
coordinates (Table 1) calculated using the PL spectra excited at 330 nm (in CHCl3) and 360 nm
(thin films) are located in the deep blue region of the chromaticity diagram (Figure SI1).

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The absorption spectral data of the molecules in solvents of varying polarity are summarized in
Table SI1. A small effect on the λabs of the molecules was observed with change in solvent nature
suggests that in the ground state, solvent effect is very weak as reported in the case of other 1,8-
naphthalimides [13]. The absorption spectra of the molecules (Figure SI2 and SI3) in
cyclohexane and hexane are well structured. A loss in the vibrational fine structure occurs by the
use of polar solvents. A bathochromic shift in λabs was noticed for molecules from hexane (~ 338
nm) to water (~ 361 nm), signifying a red shift in λabs for polar solvents. On the other hand, λabs
peak position remains immune to the varying polarity of the polar protic and polar aprotic
solvents indicating that solvent polarity has slight impact on the absorption spectra of the
molecules. The absence of specific effects in protic solvents shows absence of intermolecular H-
bond formation in the molecules’ ground state [21]. The PL emission spectra (Figure SI2 and
SI3) demonstrate solvent dependent shifts in emission maxima. Similar to the absorption
characteristics, both the molecules showed structured emission in non-polar solvents; however,
in the case of mild and high polarity solvents, the molecules exhibited broadband structure-less
emission without any clear vibrational structure. As can be seen in Figure SI2 and SI3, an
increase in solvent polarity, i.e., from non-polar to polar solvents, resulted in bathochromic
shifts. For instance, an enormous bathochromic shift 77 nm was observed in the case of 4 when
changing the medium from hexane to water, which was not noticed in the absorption spectra.
This shift indicates that the stabilisation of the solvent is more prominent in the excited states
when compared to the ground states. As can be seen from Table SI1, there was a significant
increase in the Stokes’ shift of 4 from 3570 cm−1 in hexane and increases to 6997 cm−1 in water.
Similar kind of behaviour was also observed for 5 and other 1,8-naphthalimides.
The variations of the dipole moments in ground and excited states can be analysed to observe the
response of the molecules to solvent polarity, which can be estimated from Lippert–Mataga
(LM) equation [22]
ꢊꢋꢌ
ꢏ ꢀꢐꢊ
(1)
ꢄ
ꢉ
=
ꢀꢁꢂꢃ = ꢁꢅꢆꢂ
̅
− ꢁꢇꢈ
̅
ꢍꢎꢅ
where ꢁabs and ꢁ̅PL are respectively, the wave numbers of the absorbance and PL emission in
̅
cm−1, h is the Planck’s constant, c is the speed of light, a is the Onsagar radius of the cavity in
which the fluorophore resides [23], ꢁµ is change in the dipole moment and ꢁf is the orientation

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polarizability involving the dielectric constant (ε) and refraction index (n) of the solvents as
given by equation (2). Figure SI4 represents the LM plot of the molecules in different solvents.
ꢗ
ꢒꢓꢔ
ꢖ ꢓꢔ
ꢀꢑ = ꢊꢒꢕꢔ
−
(2)
ꢗ
ꢊꢖ ꢕꢔ
The gross solvent polarity indicator scale such as empirical parameter of solvent polarity ET30
becomes more applicable while considering the case of charge transfer in a molecule [24]. Figure
SI5 shows the variation of Stokes’ shift with the ET30 values of the solvents revealing a related
tendency for protic and aprotic solvents. Figure SI5 clearly shows the significant influence of the
solvent polarity on the spectral properties of the molecules.
The emission mechanism is studied by investigating PL decay lifetime as it decides the suitable
use of the materials in devices. The efficiency of the radiative recombination is directly
proportional to the particular transition decay time. Time resolved PL spectroscopy was used to
study PL decay spectrum of the molecules in CHCl3 at 426 nm (4) and 422 nm (5) for blue
emission by TCSPC technique with a 350 nm pulsed NanoLED as source of excitation at room
temperature. The obtained results are tabulated in Table SI2 and the decay profiles of the
molecules are plotted in Figure 3. The lifetime data obtained for the molecules were fitted to a
double exponential function using DAS6 software as presented in equation (3), where τ1 and τ2
are the decay lifetimes of the luminescence with amplitudes A1 and A2, respectively. The
observed lifetimes are τ1 ~ 0.54 ns and τ2 ~ 2.38 ns for 4; and τ1 ~ 0.91 ns and τ2 ~ 3.63 ns for 5.
For double-exponential decay, the average lifetime, τaν, is typically tailored to replace the various
parameters of the luminescence lifetime, which is established by equation (4) [25]. The
calculated average lifetime are 1.91 ns (4) and 3.19 ns (5). It can also be noted from the decay
profile that the triplet energy transfer is totally suppressed in both the molecules. These results
are extremely suitable for display applications.
ꢃ
ꢃ
ꢄ ꢉ
ꢘ ꢙ = ꢚꢔ exp ꢛ− ꢜ ꢞ + ꢚꢊexp ꢛ− ꢜ ꢞ
(3)
(4)
ꢝ
ꢗ
ꢗ
ꢗ
ꢡꢢ ꢜ ꢕꢢ ꢜ ꢣ
ꢝ
ꢗ
ꢝ
ꢗ
ꢟꢅꢠ
=
ꢄ
ꢢ ꢜ ꢕꢢ ꢜ
ꢉ
ꢝ
ꢝ
ꢗ
ꢗ
3.3 Electrochemical Properties

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The electrochemical properties of the naphthalimides were investigated by CV in order to
determine HOMO and LUMO energy-levels as well as to explore the charge injection
capabilities. The CV measurements were performed in 10-3 M acetonitrile on platinum
containing 0.1 M Bu4NPF6 as supporting electrolyte, Ag/AgNO3 electrode as reference and
ferrocene/ferrocenium as calibrant [12] at a scanning rate of 10 Vs-1. Prior to the measurements,
all the solutions were purged with N2. The electrochemical data of the molecules are summarized
in Table 2. The cathodic scans revealed quasi-reversible reduction waves with onset reduction
potentials of -1.47 V and -1.41 V versus Ag/AgNO3, respectively for 4 and 5; whereas the anodic
scans exhibited irreversible waves with onset oxidation potentials of 1.44 and 1.47 V vs.
Ag/AgNO3, respectively for 4 and 5. The energy-levels of 4 and 5 were predicted by comparing
the onset potentials of ferrocence (4.8 eV versus vacuum). The HOMO energy-levels obtained
from the onset of the oxidation wave (HOMO = - 4.8 - Eoxonset) are 6.24 eV (4) and 6.27 eV (5);
the LUMO energy-levels estimated using the onset of the reduction wave (LUMO = - 4.8 -
Eredonset) are 3.33 eV (4) and 3.39 eV (5). Both the HOMO and LUMO energy-levels of 4 and 5
are lower than their reported counterparts [11,15]. It is interesting to note that the HOMO-
LUMO energy band-gaps of 2.91 eV (4) and 2.88 eV (5) are fairly near to the calculated optical
band gaps within experimental error. The surface of the HOMO-LUMO orbitals obtained by
DFT calculations are depicted in Figure 4. The computationally calculated HOMO-LUMO
values are in agreement with the electrochemical studies (Table 2). These low-lying LUMO
energy-levels lead to high electron affinity, which reveals that the molecules have potential as
electron-transporting (n-type) materials for OLED applications.
3.4 Thermal Properties and Surface Morphology
For organic optoelectronic device applications, materials with high thermal stability is desired. In
addition, the high thermal stability also enhances the device lifetime. The thermal behaviour,
including the decomposition temperatures (Td) and melting points (Tm) were studied for 4 and 5
using TGA and DSC. The thermal stabilities were investigated by TGA in the temperature range
30 - 600 °C and melting points were evaluated by performing DSC. A constant heating rate of 10
ºC min-1 was maintained to melt and decompose the molecules in N2 atmosphere (flow rate: 30
mL min-1). The results are summarized in Table 2. The TGA thermograms (Figure 5a) and the
values in Table 2 reveal that the molecules exhibit high Td. No weight loss was observed at lower
temperatures. The molecules show single step in the thermal decomposition process starting at ~

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235 ºC followed with a rapid weight loss of ~ 60% of the mass occurring in the range 235-330
ºC. A steady weight loss was noted on further heating. The molecules possess Td values of 299
ºC (4) and 295 ºC (5) corresponding to 10% weight loss. DSC scans (Figure 5b) exhibit clear
endothermic peaks with Tm of 191 ºC (4) and 197 ºC (5). The course of melting points obtained
by DSC are well in agreement with the capillary method melting points of the molecules. The
capability of the molecules to form amorphous uniform thin films with low roughness value
upon vacuum thermal vapour deposition plays a significant role in the functioning of organic
electroluminescent devices. The surface morphology of the vacuum-deposited films (50 nm)
prepared on n-Si substrates at a deposition rate of 0.5 Å/s has been studied by performing AFM
measurements. The three-dimensional AFM images (10x10ꢂ) of the molecules are presented in
Figure 6 which shows root-mean-square roughness values of 2.6 nm (4) and 3.5 nm (5). It can be
seen from Figure 6 as well as the surface roughness values that they are capable of forming quite
smooth films. The roughness of deposited films using thermal evaporation technique typically is
in the molecular size range. Hence the molecules exhibit good film deposition characteristics and
thermal stability.
3.5 Electroluminescent Properties
As the final part of the investigation, to study the utility of the synthesized naphthalimides
(Naph), unoptimized OLEDs were fabricated and characterized. The associated HOMO-LUMO
energy-levels of the molecules along with other materials used in the electroluminescent devices
are illustrated in Figure 7. To evaluate the emissive properties of the naphthalimides as emitters,
undoped OLEDs were fabricated with the configuration I: ITO (120 nm)/ α-NPD (30 nm)/ Naph
(35 nm)/ BCP (6 nm)/ Alq3 (35 nm)/ LiF (1 nm)/ Al (150 nm). Here ITO served as a transparent
anode. α-NPD was used as the HTL because of it is high HOMO energy. In addition, considering
the energy-levels (shown in Figure 7) of the molecules, α-NPD with a higher LUMO level also
plays a role of an effective electron-blocking material. BCP was used as HBL as it assists in
confining the reductant holes in the emitting layer which didn’t recombine with the electrons in
the emitting zone [26]. Alq3 was used as an ETL. LiF was used as EIL and Al was used as the
cathode. The current (ηc), output power (ηp) and external quantum (ηEQE) efficiencies of both the
devices along with other parameters of the devices are summarized in Table 3. The J-V-L and J-
L curves of the fabricated devices are shown in Figure 8. The relation of current density with
varying voltage (Figure 8a) reveal good diode behaviour. The J-L characteristics (Figure 8b)

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show a linear relationship. Both the devices displayed good current density signifying good
charge transport capability. The devices utilizing 4 and 5 as emitter materials showed maximum
luminance (Lmax) of 889 and 1072 cdm-2 at 15 V; efficiencies of 0.89 and 0.96 cd/A (ηc), 0.21
and 0.23 lm/W (ηp), 0.41 and 0.47 % (ηEQE), respectively at 100 cdm-2. The turn-on voltage of a
device principally depends on the HOMO and LUMO energies of the constituent layers. The
HOMO energy-level of α-NPD (-5.5 eV) creates a hole-injection barrier of 0.74 eV at the α-
NPD/4 and 0.77 eV at α-NPD/5 junctions. Similarly, the electron-injection barriers at the 4/Alq3
and 5/Alq3 are 0.23 eV and 0.29 eV, respectively as the LUMO energy-level of Alq3 is 3.1 eV.
Hence, the hole-injection barriers might have accounted for slightly high turn-on voltages of 7.03
V (4) and 6.92 V (5) corresponding to 1 cd m-2. The observed slight variation in the turn-on
voltage for 4 and 5 may be due to the difference in the electron-injection band-offsets in the
devices. The steady-state EL spectra of the devices with naphthalimides at different applied
voltages are depicted in Figure 8c. The devices showed blue light emission with peaks centred at
485 (4) and 483 (5) with FWHM of ~ 70 nm. The narrow FWHM in EL spectra suggest that the
molecules are capable of forming good chromaticity devices. The EL spectra closely matches
that of the solid film PL spectra of the molecules with a slight bathochromic shifts suggesting
emission from the molecules as the major component in EL. The bathochromic shift in EL peaks
with respect to PL may be due to interplay of charge carrier versus exciton dynamics and/or due
to the result of excimer formation [26].The CIE chromaticity coordinates were determined using
the EL spectra of the devices at 15 V. The chromaticity diagram is shown in Figure 8d and the
CIE coordinates are tabulated in Table 3. Furthermore, as depicted in Figure 8c, the EL spectra
of both the devices showed the same profile without any effect on the shape and EL spectra peak
under various driving voltages. Hence, both the devices exhibited excellent spectral stability over
a wide range of operating voltages. It is noteworthy that, during the measurements, both the
devices did not damage at higher applied electrical voltages, perhaps due to the high thermal
stabilities of the molecules. This observation offers the potential use of the molecules in OLED
applications.
The role of naphthalimides as both electron transporting and emitting materials was studied by
fabricating OLEDs with the configurations II-IV: ITO (120 nm)/ F4TCNQ (4 nm)/ α-NPD (40
nm)/ Naph (50 nm)/ BCP (6 nm)/ LiF (1 nm)/ Al (150 nm). Here F4TCNQ was chosen as a HIL
as it efficiently injects holes from the ITO anode to HTL. An optimized thickness of F4TCNQ

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was used for better hole injection as reported earlier [27,28]. The device with configuration II
was fabricated with 4 as an active material without hole injection layer, F4TCNQ and hole
blocking layer, BCP; device III with F4TCNQ and without BCP; and device IV with both
F4TCNQ and BCP. The performance data along with other parameters of all the devices are
tabulated in Table 4. In assessing the device performance, it is evident from Table 4 that the
device II without F4TCNQ and BCP shows low luminance 472 cd m-2 and efficiencies of 0.45 cd
A-1 (ηc), 0.11 lm W-1 (ηp) and 0.24 % (ηEQE). With the inclusion of 4 nm of F4TCNQ layer
(device III) there is a considerable increase in luminance (1493 cd m-2) and efficiencies; 1.79 cd
A-1 (ηc), 0.51 lm W-1 (ηp) and 0.40 % (ηEQE). This can the ascribed to the efficient injection of
holes from the anode (ITO) to the hole transport layer (α-NPD) [27,28]. Furthermore, in device
IV, with the use of BCP as an HBL, there is further increase in the luminance (1580 cd m-2) and
efficiencies to 1.87 cd A-1 (ηc), 0.86 lm W-1 (ηp) and 0.69 % (ηEQE) 100 cd/m2. This increase in
efficiencies may be due to the confining of redundant holes in the emitting layer. The device
structure IV used for 5 showed efficiencies of 1.89 cd A-1 (ηc), 0.98 lm W-1 (ηp) and 0.71 %
(ηEQE) 100 cd/m2.
The electron transporting nature of naphthalimides were investigated by fabricating
phosphorescent OLED with 4 and 5 as ETL materials. Also, OLEDs without any ETL material
and with Alq3 as standard ETL material were fabricated for comparison. Here Ir(ppy)3 doped
CBP was used as phosphorescent emitter for all the devices. The device configurations V-VII of
the fabricated OLEDs are ITO (120 nm)/ α-NPD (40 nm)/ Ir(ppy)3 doped CBP (35 nm)/ BCP (6
nm)/ No ETL or Naph or Alq3 (30 nm)/ LiF (1 nm)/ Al (150 nm). In the device configurations,
V-VII only ETL was changed, whereas the rest was kept same. In device V, no ETL material
was used; in device VI Alq3 was used as standard ETL material; whereas, device VII was
fabricated with 4/5 as ETL materials. The performance data from each of these devices are
tabulated in Table 5. The J-V-L characteristics with different ETL materials are shown in Figure
9a. The J-L characteristics (Figure 9b) show a linear relationship between current density and
luminance. It is evident from Figure 9 and Table 5 that with the inclusion of 4 as ETL, there is a
drastic increase in efficiencies (ηc: 8.9 cd A-1, ηp: 1.82 lm W-1, ηEQE: 1.39 %) when compared to
the device without ETL (ηc: 5.39 cd A-1, ηp: 1.23 lm W-1, ηEQE: 0.94 %). The device structure VII
was also used for 5 which showed efficiencies of 9.04 cd A-1 (ηc), 1.98 lm W-1 (ηp) and 1.46 %
(ηEQE). The EL spectra (Figure 9c) for all the devices was similar with λmax at 516 nm which is

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due to Ir(ppy)3 with similar CIE coordinates (0.28, 0.62) (Figure 9d). The obtained results reveal
that by using naphthalimides as ETL, a significant improvement in performance is observed and
is comparable to device with Alq3 as ETL (Table 5). Our investigations reveal that the
synthesized molecules show potential as electron transporting emitter materials in OLEDs.
Conclusions
Two chloro-phenoxy 1,8-naphthalimides, 4 and 5, were synthesized and characterized. The
molecules show high electron affinity and possess wide band gap which are essential for blue
light electroluminescence. The optical properties of the molecules revealed blue PL emission
with high Stokes’ shifts. The molecules show good thermal stabilities as revealed by thermal
properties. The HOMO-LUMO energy-levels of the molecules are low which make them acquire
excellent hole-blocking and electron-transporting properties. As emissive materials, OLEDs
using naphthalimides showed efficiencies of 0.96 cd/A (V), 0.23 lm/W (ηp) and 0.47 % (ηEQE).
Naphthalimides as both EL and ETL materials showed efficiencies of 1.89 cd/A (ηc), 0.98 lm/W
(ηp) and 0.71 % (ηEQE) revealing that they can act as electron transporting emitters. Furthermore,
compared to the phosphorescent OLED without any ETL material (ηc: 5.39 cd/A, ηp: 1.23 lm/W
and ηEQE: 0.94 %), devices with naphthalimides as ETL materials showed good performance (ηc:
9.04 cd/A, ηp: 1.98 lm/W and ηEQE: 1.46 %); the performance was comparable with standard
device with Alq3 as ETL material. Our results validate that the molecules have potential as
undoped blue emitters with electron transport properties and hence can play an important role in
the progress of high performance OLED devices.
Acknowledgement
The authors acknowledge Department of Information Technology (DIT), India for the financial
support. H.U. acknowledge the Department of Physics, NITK, Surathkal, for various facilities
and financial support.
Appendix A. Supplementary data
CIE chromaticity diagram for the materials in thin film form and solvatochromism are included
in the supplementary data.
References

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List of Figures
Scheme 1. The synthetic route to naphthalimides
Figure 1. Molecular structures of naphthalimides 4 and 5 using DFT optimization.
Figure 2. (a) UV-Vis absorption and PL spectra of the naphthalimides 4 and 5 obtained in
CHCl3 solution, thin solid film form and TD-DFT calculations; (b) Optical band-gap
determination using Tauc plots
Figure 3 Time resolved PL spectra of naphthalimides 4 and 5
Figure 4 Pictorial representations of frontier molecular orbital surfaces of the HOMO and
LUMO of the naphthalimides 4 and 5 calculated using B3LYP/6-311++G (d,p).
Figure 5 (a) TGA and (b) DSC thermograms of the naphthalimides under nitrogen
Figure 6 AFM images of thermally evaporated naphthalimides 4 and 5
Figure 7 Energy-level diagram of the materials used in the devices (I) Naphthalimides as EL
materials; (II-IV) Naphthalimides as EL and ETL materials; (V-VII) Naphthalimides and Alq3
as ETL materials.
Figure 8 (a) J-V-L characteristics, (b) J-L characteristics (c) EL spectra at different applied
voltages and (d) Chromaticity diagram of the OLEDs with naphthalimides as EL materials
Figure 9 (a) J-V-L characteristics, (b) J-L characteristics (c) EL spectra at different applied
voltages and (d) Chromaticity diagram of the phosphorescent OLEDs without ETL/with
Alq3/Naph as ETL

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List of Tables
Table 1. Photo-physical data of the synthesized naphthalimides
Table 2 Electrochemical data obtained from cyclic voltammetry measurements along with DFT
obtained values for comparison and thermal data of synthesized naphthalimides
Table 3 Electroluminescent performance data of naphthalimides as emissive materials
Table 4 Electroluminescent performance data of OLEDs using naphthalimides as both EL and
ETL materials
Table 5. Electroluminescent performance data of OLEDs using of triplet emitter Ir(ppy)3 doped
with CBP (with changing ETL)

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Table 1. Photo-physical data of the synthesized naphthalimides
d
f
h
opt
Cpd.
λabs
λabs
λEm
FWHM
(nm) a,b
log εe
ES1
υst
f g
EPL
CIE coordinatesa,
ΦPL
Eg
b
(nm)c
(nm) a,b
(nm)a,b
(cm-1 )a,b
(eV)i
3.04
2.99
x
y
363
(374)
426
(462)
44
(59)
4074
(5093)
0.307
(0.279)
0.155
0.050
350
348
0.28
0.26
4.339
4.268
312
316
0.33
0.21
4
5
(0.144) (0.143)
362
(371)
422
(458)
46
(58)
3928
(5120)
0.298
(0.298)
0.155 0.044
(0.145) (0.129)
a
b
c
d
Recorded in 10-5 M CHCl3 Solution. Values in parantheses are recorded in thin solid film state. Calculated using DFT. Relative PL
e
quantum yield (ΦPL) of naphthalimide solution in 10-5 M CHCl3 using 9,10-diphenylanthracene in cyclohexane (Φref = 0.90) as standard.
f
g
Molar extinction coefficients (l mol-1 cm-1). Energy state of activation of the first excited state (kJ mol-1). Oscillator strength in CHCl3,
DFT calculated value (in parentheses). h Energy yields of PL. i Optical band gap by Tauc plots of solid state absorption spectra.
Table 2 Electrochemical data obtained from cyclic voltammetry measurements along with DFT obtained values for comparison and
thermal data of synthesized naphthalimides
Cpd.
4
Eox
versus Efc
(V)a
Ered
versus Efc
(V)a
CV measurements
DFT calculations
Td
(ºC)d
Tm
(ºC)e
onset
onset
EC
Cal
HOMO
LUMO
(eV)
Eg
HOMO
LUMO
(eV)
Eg
(eV)
-6.24
-6.27
(eV)b
2.91
2.88
(eV)
- 6.31
-6.41
(eV)c
1.44
1.47
-1.47
-1.41
-3.33
-2.44
-2.52
3.88
299
295
191
197
-3.39
3.89
5
Eox
and Eredonset are measured vs ferrocene/ferrocenium. Eg = (Eox
- Eredonset). c DFT calculations. Decomposition temperatures at
a
b
ec
d
onset
onset
10% weight loss. e Melting point of the derivatives from DSC.

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Table 3 Electroluminescent performance data of naphthalimides as emissive materials
f
Devicea
CIE (x,y)
Vonset
Lmax
(cd m-2) c
ηc
ηp
ηEQE
λem (FWHM)
(V) b
7.03
6.92
(cd A-1)d
(lm W-1)d
(%) d
0.41
0.47
(nm)e
889
0.89
0.21
485 (70)
(0.190, 0.283)
(0.181, 0.279)
4
5
1072
0.96
0.23
483 (71)
a Device Configuration I: ITO (120 nm)/ α-NPD (30 nm)/ Naph (35 nm)/BCP (6 nm)/ Alq3 (35 nm)/ LiF (1 nm)/ Al (150 nm). b Vonset: turn-
d
on voltage at luminance of 1 cd m-2.c Lmax: Maximum luminance at 15 V. Current (ηc), power (ηp) and external quantum efficiencies (ηext)
measured at 100 cdm-2. e λem: Emission wavelength maximum; FWHM: full width half maximum at 15 V. f CIE color coordinates.
Table 4 Electroluminescent performance data of OLEDs using naphthalimides as both EL and ETL materials
Device
Vonset
(V) d
Lmax
(cd m-2) e
ηc
ηp
ηEQE
(%)f
(cd A-1) f
(lm W-1) f
4 a
4 b
4 c
5 c
9.67
6.62
6.35
6.18
472
0.45
1.79
1.87
1.89
0.11
0.51
0.86
0.98
0.24
0.40
0.69
0.71
1493
1580
1681
a Device Configuration II: ITO (120 nm)/ α-NPD (40 nm)/ 4 (50 nm)/ LiF (1 nm)/ Al (150 nm). b Device Configuration III: ITO (120 nm)/
c
F4TCNQ (4 nm)/ α-NPD (40 nm)/ 4 (50 nm)/ LiF (1 nm)/ Al (150 nm). Device Configuration IV: ITO (120 nm)/ F4TCNQ (4 nm)/ α-
d
e
NPD (40 nm)/ Naph (50 nm)/ BCP (6 nm)/ LiF (1 nm)/ Al (150 nm). Vonset: turn-on voltage at luminance of 1 cd m-2. Lmax: Maximum
luminance at 15 V. f Current (ηc), power (ηp) and external quantum efficiencies (ηext) measured at 100 cdm-2.

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Table 5. Electroluminescent performance data of OLEDs using of triplet emitter Ir(ppy)3 doped with CBP (with changing ETL)
Device a
Vonset
Lmax
(cd m-2)c
ηc
ηp
ηEQE
(V) b
12.76
9.09
9.23
9.14
(cd A-1) d
(lm W-1) d
(%) d
0.94
1.70
1.39
1.46
1714
5.39
1.23
No ETL
Alq3
9650
9.79
2.71
5341
8.90
1.82
4
5
5962
9.04
1.98
a Device Configuration II-IV: ITO (120 nm)/ α-NPD (40 nm)/ Ir(ppy)3 doped CBP (35 nm)/ BCP (6 nm)/ No ETL or Alq3 or Naph (30
b
c
d
nm)/ LiF (1 nm)/ Al (150 nm). Vonset: turn-on voltage at luminance of 1 cd m-2. Lmax: Maximum luminance at 21 V. Current (ηc),
power (ηp) and external quantum efficiencies (ηext) measured at 100 cdm-2. e λem: Emission wavelength maximum at 21 V.

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