Study of Configuration Differentia and Highly Efficient, Deep-Blue, Organic Light-Emitting Diodes Based on Novel Naphtho[1,2-d]imidazole Derivatives

Article abstract of DOI:10.1002/adfm.201502163

Two novel naphtho[1,2-d]imidazole derivatives are developed as deep-blue, light-emitting materials for organic light-emitting diodes (OLEDs). The 1H-naphtho[1,2-d]imidazole based compounds exhibit a significantly superior performance than the 3H-naphtho[1

Full text of DOI:10.1002/adfm.201502163

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Study of Configuration Differentia and Highly Efficient,
Deep-Blue, Organic Light-Emitting Diodes Based
on Novel Naphtho[1,2-d]imidazole Derivatives
Ming Liu, Xiang-Long Li, Dong Cheng Chen, Zhongzhi Xie, Xinyi Cai, Gaozhan Xie,
Kunkun Liu, Jianxin Tang, Shi-Jian Su,* and Yong Cao
in obtaining wide energy-gap phosphors,
Two novel naphtho[1,2-d]imidazole derivatives are developed as deep-blue,
light-emitting materials for organic light-emitting diodes (OLEDs). The
1H-naphtho[1,2-d]imidazole based compounds exhibit a significantly superior
performance than the 3H-naphtho[1,2-d]imidazole analogues in the single-
layer devices. This is because they have a much higher capacity for direct
electron-injection from the cathode compared to their isomeric counterparts
resulting in a ground-breaking EQE (external quantum efficiency) of 4.37%
and a low turn-on voltage of 2.7 V, and this is hitherto the best performance
for a non-doped single-layer fluorescent OLED. Multi-layer devices consisting
of both hole- and electron-transporting layers, result in identically excellent
performances with EQE values of 4.12–6.08% and deep-blue light emission
(Commission Internationale de l’Eclairage (CIE) y values of 0.077–0.115) is
obtained for both isomers due to the improved carrier injection and con-
finement within the emissive layer. In addition, they showed a significantly
better blue-color purity than analogous molecules based on benzimidazole or
phenanthro[9,10-d]imidazole segments.
reports regarding highly efficient blue
phosphors that exhibit deep-blue emis-
sion with Commission Internationale
de l’Eclairage (CIE) y values smaller than
0.15 are exiguous, and their short device
lifetimes are not suitable for commercial
applications. Adachi et al. developed deep-
blue OLEDs based on thermally activated
delayed fluorescence (TADF) with CIE
coordinates of (0.15, 0.07) and a high EQE
of 9.9% at low current density.[3] However,
in this type of device, the serious roll-off
in efficiency will have to be solved. More-
over, deep-blue phosphorescent and TADF
devices require host and periphery mate-
rials with a high enough triplet energy to
confine the triplet excitons on the emitter,
which remains a challenge.[4] To address
these issues, investigators keep focusing
their work on conventional blue fluoro-
phores, in which stability and high color
purity can be readily realized.[3,5] Furthermore, non-doped fluo-
rescent devices can play a crucial role in reducing the cost of
production and simplifying the manufacturing process of full-
color displays and white-light sources. Yet, most of the reported
devices with non-doped emitting layers have been shown to
have an unimpressive external quantum efficiency (EQE) or
insufficient deep-blue emission.[6] Particularly, for simplified
single-layer devices, which are greatly in favor to limit the
overall cost, reports in which the devices have an EQE of 3% or
higher are rare.[7] Therefore, developing highly efficient, blue,
fluorescent materials remain a significant challenge.
There are several prerequisites for high-performance fluores-
cent OLEDs with a non-doped light-emitting layer: 1) fluores-
cent molecules with high fluorescent quantum yields; 2) a good
balance between electron and hole injection as well as localizing
the carrier-recombination region in the light-emitting layer;
3) appropriate energy levels of light-emitting molecules, which
match to that of the periphery layers or electrodes; 4) good film-
forming property and morphological stability of the organic
materials. Generally, unbalanced carrier injection and transpor-
tation induces an increase in the driving voltage and a decrease
in the device efficiency. Therefore, ambipolar molecules with
a donor–acceptor (D–A) structure that encourages the balance
of carrier transport are desirable candidates for high-efficiency
OLEDs with non-doped emitting layers. On the other hand, D–A
1. Introduction
Organic light-emitting diodes (OLEDs) have attracted consid-
erable attention owing to their potential applications in both
new generation full-color flat-panel displays and solid-state
lighting.[1] During the past two decades, numerous efforts have
been devoted to developing new OLED emitters, and great suc-
cess has been achieved, particularly for heavy metal complex
phosphorescent emitters.[2] However, because of the difficulty
M. Liu, X. L. Li, D. C. Chen, X. Y. Cai, G. Z. Xie,
K. K. Liu, Prof. S.-J. Su, Prof. Y. Cao
State Key Laboratory of Luminescent
Materials and Devices
Institute of Polymer Optoelectronic
Materials and Devices
South China University of Technology
Guangzhou 510640, P. R. China
E-mail: mssjsu@scut.edu.cn
Z. Z. Xie, Prof. J. X. Tang
Jiangsu Key Laboratory for Carbon-Based
Functional Materials & Devices
Institute of Functional Nano & Soft Materials (FUNSOM)
Soochow University
Suzhou 215123, P. R. China
DOI: 10.1002/adfm.201502163
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different numbers of benzene rings between
the NI unit and TPA could play a role in
tuning the conjugation length. The molecular
structures of our new compounds, NI-1-TPA,
NI-2-TPA, NI-1-PhTPA, and NI-2-PhTPA, are
shown in Figure 1, and their synthetic routes
are outlined in Scheme 1. The pivotal precur-
sors NI-1-Br and NI-2-Br were synthesized
from 1-nitro-2-naphthol and 2-nitro-1-naph-
thol, respectively, through five steps with
considerable yields. Finally, reactions of NI-
1-Br or NI-2-Br with N,N-diphenyl-4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)aniline
or N,N-diphenyl-4′-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-[1,1′-biphenyl]-4-amine
were carried out by a palladium-catalyzed
Figure 1. Molecular structures of the naphtho[1,2-d]imidazole derived compounds NI-1-TPA,
NI-2-TPA, NI-1-PhTPA, and NI-2-PhTPA.
molecular systems are liable to cause extended π-conjugation
and to generate intramolecular charge transfer (ICT), which
gives rise to a bathochromic shift and color impurity of the
emission. Therefore, it is clear that controlling the ICT effect to
an extent is a prerequisite for deep-blue emission.[5c] Thus, intro-
ducing weak electron-deficient as well as electron-rich segments
to prevent bathochromic shift of the emission and simultane-
ously achieving appropriate energy levels for balanced electron
and hole injection has been regarded as an advisable strategy
for designing D–A type, deep-blue, light-emitting compounds.
In recent years, an increasing amount of blue light-emitting
compounds containing imidazole derivatives, for instance,
benzimidazole (BI) and phenanthro[9,10-d]imidazole (PI), as
the electron-deficient component has been reported with high
device efficiency.[5b,8] The predominant result of these investi-
gations demonstrates that imidazole derivatives can serve as
preeminent electron-withdrawing components for the construc-
tion of promising blue-light-emitting molecules. The electron-
deficiency properties of imidazole derivatives are usually weak
to avoid problems with ICT and the resulting red shift of the
emission spectra. These advantages have made imidazole, BI,
and PI some of the most commonly used segments for organic
electroluminescent materials in recent years.[9]
Herein, we report on two naphtho[1,2-d]imidazole (NI) isomers,
namely, 3H-naphtho[1,2-d]imidazole (NI-1) and 1H-naphtho[1,2-
d]imidazole (NI-2), and a series of their derivatives, which were
designed and synthesized as highly efficient, blue-fluorescent
emitters for non-doped single- and multi-layer electrolumines-
cent devices. Interestingly, our single-layer devices based on NI-1
molecules displayed an inferior efficiency and distinctly higher
driving voltage than the devices based on NI-2. The reasons for
this phenomenon will be discussed hereafter. However, in our
multi-layer devices, all of the blue fluorophores exhibited a high
EQE, indicating their efficacy as non-doped blue emitters. More-
over, our study also discovered that the NI compounds have an
advantage over the BI and PI analogues in their blue-color purity.
Suzuki cross-coupling reaction to give NI-1-TPA, NI-2-TPA,
NI-1-PhTPA, and NI-2-PhTPA. All products were fully charac-
1
terized by H, 13C NMR spectrometry, mass spectrometry, and
elemental analysis. To evaluate their frontier molecular orbitals
and energy bandgaps, density functional theory (DFT) calcula-
tions were performed with a B3LYP/6–31G(d,p) basis set using
the Gaussian 03W program (Figure S1 and Table S1 in the
Supporting Information).[11] Although these four compounds
are different in conjugation length and/or configuration of the
naphtho[1,2-d]imidazole units, their calculated lowest unoccu-
pied molecular orbitals (LUMOs) and highest occupied molec-
ular orbitals (HOMOs) were quite similar. But on the basis of
theoretical calculations, the NI-1 based compounds were pre-
dicted to exhibit a slightly narrower energy bandgap (Eg) and
smaller singlet excited energy (S1) than their NI-2 isomeric
counterparts.
2.2. Thermal, Electrochemical, and Optical Properties
All compounds showed good thermal stability, which is indi-
cated by their high decomposition temperatures (Td, cor-
responding to 5% weight loss) ranging between 372.2 and
400.5 °C, as obtained from thermogravimetric analyses (TGA)
and their glass-transition temperatures (Tg) of 113.8, 110.2,
127.1, and 124.2 °C for NI-1-TPA, NI-2-TPA, NI-1-PhTPA, and
NI-2-PhTPA, respectively, as determined from differential scan-
ning calorimetry (DSC) (Figure S2, Supporting Information).
The NI-1 compounds exhibited a slightly higher Tg than their
NI-2 analogues, but the thermal properties of all four com-
pounds were good enough for application in electrolumines-
cent devices.
Cyclic voltammetry (CV) was used to measure their ioniza-
tion potentials (IPs). Each compound displayed a reversible
oxidation wave, which could be attributed to the oxidation of
the arylamine units (Figure S3, Supporting Information). The
IPs of these NI compounds were calculated from the onset
oxidation potential (Eox vs ferrocene/ferricenium) and found
to be almost the same (about –5.22 0.01 eV), despite their
differences in configuration and/or conjugation length.[12]
The photophysical data are collected in Table 1, and represent-
ative UV-vis absorption and photoluminescence (PL) spectra are
shown in Figure S4 (Supporting Information). The absorption
2. Results and Discussions
2.1. Molecular Design, Synthesis, and Theoretical Calculations
These fluorophores adopt triphenylamine (TPA), a generally
used hole-transporting unit, as the electron-rich group;[10] and
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Scheme 1. Synthetic routes for the naphtho[1,2-d]imidazole derived compounds NI-1-TPA, NI-2-TPA, NI-1-PhTPA, and NI-2-PhTPA.
spectra of all four NI molecules in the thin-film state show an
obviously broader peak and slight red-shift of the peak compared
to the absorption spectra of the NI molecules in DCM solu-
tion. These phenomena imply that the emitters reported herein
form a compact stacking structure in the thin film that benefits
charge transport. The NI-2 based compounds NI-2-TPA and
NI-2-PhTPA display a subtle hypsochromic shift compared to
their isomeric counterparts, which is consistent with theoretical
calculations. All of our compounds were highly emissive in the
blue region with peaks in the range of 445–448 nm. According
to the onset of the absorption in the thin film, the optical energy
bandgaps (Egopt) of NI-1-TPA, NI-2-TPA, NI-1-PhTPA, and NI-
2-PhTPA were determined to be 2.89, 2.92, 2.86, and 2.89 eV,
respectively. We failed to obtain the reduction potential of these
compounds from the CV measurements, and thus their electron
moment from the ground state to the excited state. We also
plotted the Stokes shift versus the orientation polarizability (f ),
as shown in Figure S6 (Supporting Information). Two distin-
guishable excited states in high- and low-polarity solvents were
found. According to the investigations by Ma and colleagues, the
current naphtho[1,2-d]imidazole compounds may also possess
a totally intercrossed excited state of the local exciton (LE) and
charge transfer (CT) (namely hybridized local charge transfer,
HLCT) in moderately polar solvents (0.1 ≤ f ≤ 0.2), such as
chloroform.[8c,13] The spectral characteristics of our compounds
in the thin-film state were similar to those of their solutions in
chloroform. From this it can be inferred that the solid state (the
main form for applications) has a similar polarity effect on these
NI molecules as that of chloroform, in which they have been
assumed to possess an intercrossed excited state. This will be
helpful for achieving fluorophores with higher photolumines-
cence quantum yields (PLQYs) and standard blue-light emis-
sion. Consequently, all the NI molecules reported here showed
high PLQY values of above 90% in CH2Cl2 and of 57–73% in the
neat film state. These characteristics show the potential of these
compounds as highly efficient electroluminescent materials.
opt
affinities (EAs) were estimated from their IP and Eg values to
be –2.32, –2.30, –2.36, and –2.34 eV, respectively.
Their PL and UV-vis absorption spectra were also studied
in various solvents, as shown in Figure S5, and the data are
summarized in Table S2, both in the Supporting Information.
Insignificant changes were found in their absorption spectra
with increasing solvent polarity, indicating a negligible dipolar
change at the ground state in different solvents. However,
their fluorescence spectra were rather broadened and gradu-
ally shifted bathochromically by increasing the polarity of the
solvent, which implies a large increased transition of the dipole
2.3. Electroluminescent Performances
In order to probe their electroluminescent properties, we fab-
ricated single-layer OLEDs as they are easy to process and
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Table 1. Physical properties and calculated energy levels of the compounds.
opt e)
cal g)
b)
c)
b)
c)
Compound
Tg/Tda)
[°C]
PLQYd)
Eg
IP/EAf)
[eV]
HOMO/LUMOg)
Eg
λabs
λabs
λem
λem
[eV]
2.89
2.92
2.86
2.89
[eV]
[eV]
[nm]
[nm]
[nm]
452
448
466
462
[nm]
445
446
448
445
NI-1-TPA
113.8/373.0
110.2/372.2
127.1/392.5
124.2//400.5
298, 362
285, 359
302, 359
290, 357
302, 368
289, 363
303, 366
294, 363
0.92 (0.57)
0.93 (0.61)
0.92 (0.73)
0.93 (0.70)
–5.21/–2.32
–5.22/–2.30
–5.22/–2.36
–5.23/–2.34
–4.91/–1.28
–4.90/–1.22
–4.92/–1.39
–4.92/–1.33
3.63
3.68
3.53
3.59
NI-2-TPA
NI-1-PhTPA
NI-2-PhTPA
a)Glass-transition temperature (Tg)/decomposition temperature (Td) (5% weight loss); b)UV–vis absorption and PL bands in dichloromethane solutions at room tempera-
ture; c)UV-vis absorption and PL peaks in neat thin films; d)PL quantum yield (PLQY) of the dichloromethane solutions at room temperature. The values in parentheses are
obtained from the neat thin films; e)Optical energy bandgap (Egopt) estimated from the absorption edge in thin films; f)Ionization potentials (IPs) determined from cyclic
voltammetry and electron affinities (EAs) estimated from IPs and Egopts; g)HOMO and LUMO energy levels and energy bandgaps (Egcal) obtained from the DFT calcula-
tions with B3LYP/6–31G(d,p) basis set by using Gaussian 03W program.
conducive to low-cost production and we investigated them
in a configuration of ITO/ HATCN (5 nm)/ NI compound
(80 nm)/ LiF (1 nm)/ Al (devices SA to SD), in which HATCN
(hexaazatriphenylenehexacabonitrile) and LiF were used as
buffer layers for the anode and cathode, respectively. The device
characteristics are shown in Table 2 and Figure 2. Deep-blue
electroluminescence was obtained from these single-layer
devices with CIEy values in the range of 0.068–0.113. But, unex-
pectedly, the devices based on the NI-1 compounds exhibited
an unsatisfactory performance with current efficiencies (CE) of
0.04 and 0.54 cd A−1, corresponding to EQE values of 0.05%
and 0.52% for the devices SA and SC, respectively. On the
contrary, the NI-2 compounds delivered superior device effi-
ciencies. Especially, the NI-2-PhTPA based device, SD, demon-
strated a groundbreaking EQE of 4.37%, a CE of 4.28 cd A–1,
and a very low turn-on voltage of 2.7 V. This is hitherto the best
performance of a non-doped single-layer fluorescent OLED.
The carrier injection and transport properties of these mate-
rials, which can be deduced from single-carrier devices, were
essential to understand the great disparity between the efficien-
cies of the single-layer devices. Devices with a configuration
of ITO/ HATCN (5 nm)/ NI compound (80 nm)/ Au for the
hole-only devices and ITO/ TmPyPB (1,3,5-tri[(3-pyridyl)-phen-
3-yl]benzene) (10 nm)/ NI compound (80 nm)/ LiF (1 nm)/ Al
for the electron-only devices were fabricated and characterized.
The key factor in balancing the carriers in single-layer devices
was the difference in electron injection ability from the cathode
into the emitting layers, which was illustrated by the current
density of the electron-only devices, as shown in Figure 3a, and
depends on the operating voltage of the single-layer electrolu-
minescent devices. It seems that the direct injection of elec-
trons from LiF/Al was much easier into the NI-2 compounds
than into the NI-1 ones, even though their LUMO levels were
alike. On the other hand, the hole-injection from the anode
and the transport in each material appeared to be similar for
all four compounds, except for NI-1-TPA, which gave a sig-
nificantly higher current density in the hole-only device than
NI-2-TPA, NI-1-PhTPA, and NI-2-PhTPA (Figure 3b). This
explains why NI-1-TPA had the worst efficiency in a single-
layer OLED, where hole transport is the inherently dominant
carrier-transport.
We studied the reason why electrons could be much easier
injected from LiF/Al into the NI-2 compounds than into the
NI-1 ones by using ultraviolet photoelectron spectroscopy
(UPS, Figure 4) and X-ray photoelectron spectroscopy (XPS,
Figure S7, Supporting Information). Information from the in
situ UPS measurements revealed that incremental deposition
of LiF overlayers caused a shift (0.5 eV) in the vacuum level
(VL, determined from the onset of the high binding energy
(BE) side) of NI-2-PhTPA, but had almost no effect on that of
NI-1-PhTPA. XPS measurements were also performed after
each deposition step. Figure S7 displays the N 1s core level
Table 2. Summary of the electroluminescent data of the single-layer (SA-SD) and multi-layer (MA-MD) devices.
CIEa)
(x,y)
Von
CEc)
[cd A−1
Power efficiency (PE)c)
[lm W−1
EQEc)
[%]
b)
Device
EML
FWHM
[nm]
λEL
[nm]
[V]
]
]
SA
NI-1-TPA
NI-2-TPA
444
442
458
452
450
448
458
452
58
56
62
62
61
59
66
62
(0.151, 0.075)
(0.151, 0.068)
(0.144, 0.113)
(0.146, 0.105)
(0.145, 0.087)
(0.146, 0.077)
(0.143, 0.115)
(0.145, 0.100)
3.5
2.8
3.0
2.7
2.8
2.7
2.7
2.6
0.04/0.04/–
0.02/0.02/–
1.44/0.98/0.49
0.26/0.22/0.24
4.32/4.07/2.90
3.48/3.19/2.16
3.93/3.52/2.20
6.96/6.37/4.91
6.12/5.74/4.45
0.05/0.05/–
SB
1.38/1.18/0.90
0.54/0.32/0.54
4.28/4.26/4.00
3.33/3.28/3.08
3.76/3.58/3.01
6.35/6.22/5.70
5.46/5.41/5.09
1.89/1.62/1.24
0.52/0.31/0.52
4.37/4.35/4.08
4.12/4.06/3.81
5.16/4.92/4.13
6.08/5.96/5.46
5.95/5.89/5.54
SC
NI-1-PhTPA
NI-2-PhTPA
NI-1-TPA
SD
MA
MB
MC
MD
NI-2-TPA
NI-1-PhTPA
NI-2-PhTPA
Abbreviations: λEL: peak wavelength of the EL spectrum, Von: turn-on voltage, and FWHM: full width at half-maximum; a)CIE coordinates measured at 100 cd m−2
b)Driving voltage at 1 cd m−2 c)Current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) in the order of maximum, then the values at 100 and
1000 cd m−2
;
;
.
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measurements, we suspected that a coordination effect between
the exposed nitrogen atom of the NI-2 components and LiF
may reduce the energy difference between the interface of the
NI-2 compounds and the cathode, resulting in better electron
injection and thus a reduced operating voltage. In contrast, the
configuration in the NI-1 components may hinder such a coor-
dination which results in a lower electron flux and thus reduced
carrier balance for the NI-1 compounds.
In addition, we believe that reducing the charge-injection
barrier from the electrodes to the emitting layer will bring
about notable progress for the devices based on the NI-1 com-
pounds. To achieve this, OLEDs with multi-layer structures of
ITO/ HATCN (5 nm)/ NPB (40 nm)/ TCTA (5 nm)/ NI emitter
(20 nm) / TPBI (40 nm)/ LiF (1 nm)/ Al (devices MA-MD)
were also fabricated. In these devices, N,N′-di-1-naphthyl-
N,N′-diphenylbenzidine (NPB) and TCTA (4,4′,4′′-tri(N-
carbazolyl)-triphenylamine) served as the hole-transporting
and electron-blocking layer, respectively, and 1,3,5-tri(phenyl-
2-benzimidazolyl)-benzene (TPBI) served as the electron-trans-
porting and hole-blocking layer. The current density–voltage–
luminance (J–V–L), EQE, and power efficiency versus lumi-
nance characteristics of the multi-layer devices are shown in
Figure 5. Because of the improvements in charge injection into
the emissive layer and in carrier confinement, the multi-layer
devices based on the NI-1 compounds reached the same level
of performance as those based on the NI-2 ones. All devices dis-
played a similarly low turn-on voltages in the range of 2.6–2.8 V
thanks to the small carrier injection barrier. The devices MA
and MB, based on the molecules NI-1-TPA and NI-2-TPA that
both have a shorter conjugation length, exhibit deep blue emis-
sions with CIE coordinates of (0.145, 0.087) and (0.146, 0.077),
respectively. This is very close to the NTSC (National Television
Standards Committee) blue standard (CIE: 0.14, 0.08) and the
emission does almost not change over a wide range of driving
voltages (Figure S8, Supporting Information). Moreover, the
electro-generated emissive light from the NI-1-TPA and NI-
2-TPA devices is evidently more pure than that of an analogous
molecule based on PI (PI-TPA), which was reported by Ma’s
group in a similar device configuration and displayed CIE coor-
dinates of (0.15, 0.11).[8c] In addition, their CIEy values were also
smaller than that of a BI-based fluorophore (BI-TPA), which
also gave CIE coordinates of (0.15, 0.11).[15] The more ‘ideal’
blue-light emission from the devices based on the NI com-
pounds might benefit from the asymmetric structures, which
lead to their molecular arrangement differed from the BI and
PI molecules. As expected, both the MA and
Figure 2. a) Luminance and current density versus voltage and b) EQE
versus luminance characteristics for the single-layer devices in a configu-
ration of ITO/ HATCN (5 nm)/ EML (80 nm)/ LiF (1 nm)/ Al. EML: NI-
1-TPA, NI-2-TPA, NI-1-PhTPA, or NI-2-PhTPA. Inset: The configuration
and energy diagram of the single-layer devices.
spectra of NI-1-PhTPA and NI-2-PhTPA, in which the original
peaks around 399.2 eV could be assigned to the chemical state
of nitrogen with sp2 hybridization in accordance to previous
research.[14] The N 1s core level spectra of NI-2-PhTPA shifted
slightly toward a higher BE region and became broader, which
suggests that a delicate change in the chemical environment
has taken place with the gradual deposition of LiF layers. Such
changes could not be detected in similar measurements for
NI-1-PhTPA. Considering the results from the UPS and XPS
MB devices showed favorable maximum CE
values of 3.33 and 3.76 cd A−1 (corresponding
to EQE values of 4.12% and 5.16%), and the
superior performance of the MB device could
be mainly attributed to the higher PLQY of
NI-2-TPA compared to that of NI-1-TPA.
The devices based on NI-1-PhTPA and NI-
2-PhTPA attained an amazing efficiency
with EQE values of 6.08% and 5.95% (cor-
responding to CE values of 6.35 and 5.46 cd
A−1) and their light emission was located in
the deep-blue region with CIE coordinates of
(0.143, 0.115) and (0.145, 0.100), respectively.
Figure 3. Current density versus voltage characteristics for a) the electron-only devices and
b) hole-only devices.
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Figure 4. HeI UPS spectra obtained upon the formation of a) NI-
1-PhTPA/LiF and b) NI-2-PhTPA/LiF interfaces.
From the transient PL decay curves we can deduce that the neat
films of the NI compounds do not present a delayed lifetime
component of emitting light (only during a short portion of the
lifetime of about 2.2 ns, Figure S9, Supporting Information),
showing that these emitters are totally different from TADF
materials. The multi-layer devices displayed only a slight effi-
ciency roll-off, indicating a good charge balance thanks to the
felicitous D–A type molecular design. These results are among
the best performances of non-doped blue-fluorescent devices
and prove that both the NI-1 and NI-2 compounds may serve
as excellent deep-blue emitters, whereby the latter ones exhib-
ited an even better blue electroluminescence compared to their
isomeric counterparts. We also studied the electroluminescent
properties of analogous blue emitters based on BI and PI (BI-
PhTPA and PI-PhTPA) with exactly the same device struc-
ture, which gave a bathochromic-shifted emission with CIE
coordinates of (0.148, 0.140) and (0.151, 0.149), respectively
(Figure S10, S11, and Table S3, Supporting Information). These
results further prove that the current NI molecules attained
an outstanding blue color purity compared to their BI and PI
counterparts.
Figure 5. a) Luminance and current density versus voltage, b) EQE and
PE versus luminance, and c) EL spectrum characteristics of the multi-
layer devices in a structure of ITO/ HATCN (5 nm)/ NPB (40 nm)/ TCTA
(5 nm)/ EML (20 nm) / TPBI (40 nm)/ LiF (1 nm)/ Al. EML: NI-1-TPA, NI-
2-TPA, NI-1-PhTPA, or NI-2-PhTPA. Inset: The configuration and energy
diagram of the multi-layer devices.
3. Conclusion
A series of D–A molecules based on naphtho[1,2-d]imida-
zole isomers was synthesized and adopted as blue emit-
ters for OLEDs. The 1H-naphtho[1,2-d]imidazole based
compounds exhibited slightly blue-shifted absorption and
emission spectra compared to the 3H-naphtho[1,2-d]imidazole
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2015, 25, 5190–5198
2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
5195

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8.01–7.99 (m, 2H), 7.83–7.77 (m, 2H). 13C NMR (150 MHz, CDCl3, δ
[ppm]): 139.09, 138.03, 136.42, 130.49, 129.56, 129.25, 128.32, 126.59,
123.15, 120.71, 118.50.
based ones. In addition, a remarkable efficiency with EQEs
exceeding 4% was achieved for a single-layer device using a
1H-naphtho[1,2-d]imidazole based molecule, NI-2-PhTPA, as a
non-doped emitting layer, whereas the 3H-naphtho[1,2-d]imida-
zole compounds exhibited a poor performance in single-layer
devices owing to their inferior direct electron injection from
the cathode. This is the best result so far for non-doped fluo-
rescent OLEDs without additional carrier-transport materials.
Introducing both hole- and electron-transporting layers to
compensate for the carrier injection and to confine the car-
riers within the emissive layer, excellent performances with
EQE values of 4.12–6.08% and deep-blue light emission with
CIEy values between 0.077 and 0.115 were obtained for both
types of compound. The current findings open new avenues to
designing highly efficient fluorophores for OLED applications
with extraordinarily simple structures, especially for blue fluo-
rophores, which is significant for commercial applications in
flat-panel displays and lighting sources.
Synthesis of 1-nitro-N-phenylnaphthalen-2-amine (3–1) : 2–1 (4.8 g,
15 mmol), aniline (2.8 g, 30 mmol), Pd(PPh3)4 (230 mg, 0.2 mmol),
and anhydrous potassium carbonate (4.1 g, 30 mmol) were added into
80 mL of toluene and refluxed for 24 h. Then the reaction mixture was
poured into water and extracted with dichloromethane. The organic
phases were combined and dried over magnesium sulfate. After the
solvent was removed, the product was recrystallized from ethanol
to yield a red product (3.3 g, yield 82%). MS (ESI+, m/Z): calcd. for
C16H12N2O2, 264.1; found, 264.0.1H NMR (600 MHz, CDCl3, δ [ppm]):
9.58 (s, 1H), 8.55 (dd, J = 8.7, 0.6 Hz, 1H), 7.72 (d, J = 9.3 Hz, 1H),
7.71–7.67 (m, 1H), 7.62–7.60 (m, 1H), 7.44–7.40 (m, 2H), 7.39–7.37 (m,
1H), 7.32 (d, J = 9.2 Hz, 1H), 7.28–7.25 (m, 2H), 7.24 (dd, J = 4.8, 3.7
Hz, 1H).13C NMR (150 MHz, CDCl3, δ [ppm]): 142.24, 138.91, 135.64,
129.94, 129.80, 129.29, 128.54, 128.00, 127.73, 125.73, 124.60, 123.98,
122.81, 116.65.
Synthesis of 2-nitro-N-phenylnaphthalen-1-amine (3–2) : 3–2 was
synthesized according to the procedure described above for the synthesis
of 3–1, using 2–2 as a starting material yielding an orange solid in 85%
yield. MS (ESI+, m/Z): calcd. for C16H12N2O2, 264.1; found, 264.0. 1H
NMR (600 MHz, CDCl3, δ [ppm]): 9.60 (s, 1H), 8.12 (d, J = 9.2 Hz, 1H),
7.94 (d, J = 8.6 Hz, 1H), 7.80 (d, J = 8.2 Hz, 1H), 7.57–7.55 (m, 1H),
7.45 (d, J = 9.2 Hz, 1H), 7.29–7.28 (m, 1H), 7.23–7.18 (m, 2H), 7.02
(t, J = 7.4 Hz, 1H), 6.88 (dd, J = 7.7, 0.6 Hz, 2H). 13C NMR (150 MHz,
CDCl3, δ [ppm]): 144.02, 139.62, 136.90, 136.08, 129.74, 129.29, 128.49,
128.20, 126.57, 126.03, 123.20, 122.22, 121.12, 120.12.
Synthesis of N′-phenylnaphthalene-1,2-diamine (4–1) : 3–1 (2.6 g,
10 mmol) was combined with anhydrous tin(II) chloride (7.6 g,
40 mmol) and 100 mL of ethanol. The mixture was refluxed for 24 h
and was then poured into water and extracted with dichloromethane.
The organic phases were combined and dried over magnesium sulfate.
After the solvent was removed, the crude product was purified by
column chromatography on silica gel to yield a white product (1.9 g,
yield 81%). MS (ESI+, m/Z): calcd. for C16H14N2, 234.1; found, 234.0.
1H NMR (600 MHz, CDCl3, δ [ppm]): 7.83 (s, 1H), 7.79 (d, J = 8.9 Hz,
1H), 7.51–7.39 (m, 2H), 7.28 (d, J = 6.7 Hz, 2H), 7.19 (t, J = 7.6 Hz, 2H),
6.81 (s, 1H), 6.70 (d, J = 5.3 Hz, 2H), 5.33 (s, 1H), 4.09 (s, 2H).13C NMR
(150 MHz, CDCl3, δ [ppm]): 146.02, 137.83, 132.52, 129.32, 128.50,
125.66, 125.25, 125.11, 124.06, 122.29, 120.97, 119.02, 118.93, 114.67.
Synthesis of N-phenylnaphthalene-1,2-diamine (4–2) : 4–2 was
synthesized according to the procedure as described above for the
synthesis of 4–1, starting from compound 3–2, and yielding a white
solid in 79% yield. MS (ESI+, m/Z): calcd. for C16H14N2, 234.1; found,
234.0. 1H NMR (600 MHz, CDCl3, δ [ppm]): 7.72 (m, 2H), 7.64 (d,
J = 8.7 Hz, 1H), 7.34 (m, 1H), 7.23 (m, 1H), 7.14 (t, J = 8.2 Hz, 2H),
7.05 (d, J = 8.7 Hz, 1H), 6.74 (t, J = 7.3 Hz, 1H), 6.57 (d, J = 7.7 Hz, 2H),
5.26 (s, 1H), 3.87 (s, 2H). 13C NMR (150 MHz, CDCl3, δ [ppm]): 146.59,
141.94, 132.86, 129.45, 128.71, 128.19, 128.01, 126.81, 122.43, 121.18,
118.35, 118.20, 117.22, 113.35.
Synthesis of 2-(4-bromophenyl)-3-phenyl-3H-naphtho[1,2-d]imidazole
(NI-1-Br) : To a solution of 4-bromobenzoyl chloride (2.2 g, 10 mmol)
in dichloromethane (30 mL) was added slowly 4–1 (2.1 g, 9 mmol) and
triethylamine (5.7 mL, 24 mmol). The reaction mixture was stirred at
room temperature for 12 h. The precipitate was collected by filtration
and washed with ethanol (3 × 5 mL) and dried in vacuum to give the
rude product of 5–1. This solid, which could be used directly in the next
step without further purification, was added into 100 mL of acetic acid.
The mixture was refluxed for 24 h and then poured into 250 mL of water
and extracted with dichloromethane. The organic phase was combined
and dried over magnesium sulfate. After the solvent was removed, the
product was further purified by column chromatography on silica gel
to get a white solid (2.78 g, total yield of two steps: 76%). MS (ESI+,
m/Z): calcd. for C23H15BrN2, 398.0; found, 398.0. 1H NMR (600 MHz,
CDCl3, δ [ppm]): 8.78 (d, J = 8.2 Hz, 1H), 7.94 (d, J = 8.1 Hz, 1H),
7.67 (dd, J = 12.3, 4.9 Hz, 2H), 7.57–7.51 (m, 4H), 7.517.47 (m, 2H),
7.47–7.43 (m, 2H), 7.37–7.34 (m, 2H), 7.33 (d, J = 8.8 Hz, 1H). 13C NMR
4. Experimental Section
General Methods: The NMR data were collected on a Bruker AVANCE
Digital 600 MHz NMR workstation at room temperature. Matrix-
assisted laser desorption ionization time-of-flight (MALDI-TOF) mass
spectra were recorded on a Bruker Autoflex III Smartbeam. Mass
spectrometry (MS) data were obtained on a Waters TQD. Differential
scanning calorimetry (DSC) measurements were performed on
a
Netzsch DSC 209 under N2 fl ow at a heating and cooling rate of
10 °C min−1. Thermogravimetric analyses (TGA) were performed on a
Netzsch TG 209 under a N2 fl ow at a heating rate of 10 °C min−1. UV-vis
absorption spectra were recorded on a HP 8453 spectrophotometer.
Photoluminescence (PL) spectra were measured using a Jobin-Yvon
spectrofluorometer. Cyclic voltammetry (CV) was performed on
a
CHI600D electrochemical workstation with a Pt working electrode and
a Pt wire counter electrode at a scanning rate of 100 mV s−1 against
a Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) reference electrode in a
nitrogen-saturated anhydrous acetonitrile and dichloromethane (DCM)
solution of 0.1 mol L−1 tetrabutylammoniumhexafluorophosphate as
the electrolyte. PL quantum yields (PLQY) of the solution and film were
measured by using an integrating sphere on a Hamamatsu absolute PL
quantum-yield spectrometer C11347. Transient PL was measured with
an Edinburgh FL920 fluorescence spectrophotometer. All reagents for
synthesis were obtained from Alfa Aesar or Sigma-Aldrich and were used
without further purification.
Synthesis
of
1-nitronaphthalen-2-trifluoro
methanesulfonate
(2–1) : Triethylamine was added into
a
solution of 1-nitro-2-
naphthol (1–1, 3.8 g, 20 mmol) and 100 mL of dichloromethane.
Trifluoromethanesulfonic anhydride (7.1 g, 25 mmol) was then dropped
into this mixture at 0 ºC. The mixture was stirred for 10 h at room
temperature and was then poured into 500 mL of ice-water and extracted
by dichloromethane. The combined organic phase was washed with
brine and dried over anhydrous magnesium sulfate. After the solvent was
removed, the product was further purified by column chromatography
on silica gel to obtain a white solid (5.9 g, yield 92%). MS (ESI+, m/Z):
calcd. for C11H6F3NO5S, 321.0; found, 321.1.1H NMR (600 MHz, CDCl3,
δ [ppm]): 8.13 (d, J = 9.1 Hz, 1H), 8.01 (d, J = 8.2 Hz, 1H), 7.93 (d, J =
8.5 Hz, 1H), 7.78 (t, J = 7.7 Hz, 1H), 7.73 (t, J = 7.6 Hz, 1H), 7.57 (d,
J = 9.1 Hz, 1H). 13C NMR (150 MHz, CDCl3, δ [ppm]): 140.15, 137.92,
133.22, 132.58, 130.25, 128.68, 128.40, 125.03, 122.28, 119.06, 118.50.
Synthesis of 2-nitronaphthalen-1-trifluoromethanesulfonate (2–2) : 2–2
was synthesized according to the procedure described above for the
synthesis of 2–1, yielding a white solid in 88% yield. MS (ESI+, m/Z):
calcd. for C11H6F3NO5S, 321.0; found, 321.1. 1H NMR (600 MHz,
CDCl3, δ [ppm]): 8.28 (dd, J = 6.4, 2.9 Hz, 1H), 8.08 (d, J = 9.0 Hz, 1H),
©
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7.57–7.52 (m, 4H), 7.52–7.47 (m, 2H), 7.40–7.37 (m, 1H), 7.30–7.25
(m, 4H), 7.22–7.19 (m, 1H), 7.17–7.11 (m, 7H), 7.06–7.01 (m, 2H).13C
NMR (150 MHz, CDCl3, δ [ppm]): 151.11, 147.63, 147.38, 141.05, 140.39,
139.96, 138.87, 138.41, 134.28, 131.65, 130.72, 130.27, 129.90, 129.70,
129.41, 129.32, 129.16, 129.06, 127.64, 127.33, 126.98, 126.63, 125.62,
124.57, 124.51, 124.25, 123.82, 123.04, 122.11, 120.20, 119.76.
(150 MHz, CDCl3, δ [ppm]): 149.27, 138.51, 136.75, 133.47, 131.61,
130.82, 130.75, 130.08, 129.18, 128.96, 128.47, 127.47, 126.92, 126.72,
124.91, 124.67, 123.63, 122.10, 110.91.
Synthesis of 2-(4-bromophenyl)-1-phenyl-1H-naphtho[1,2-d]imidazole
(NI-2-Br): NI-2-Br was synthesized according to the same procedure
as described above for the synthesis of NI-1-Br, starting from 4–2, and
yielding giving a white solid in 77% yield (two steps). MS (ESI+, m/Z):
Device Fabrication and Characterization: The devices were fabricated
by vacuum deposition of the materials at 5 × 10−4 Pa or below onto the
indium tin oxide (ITO) coated glass substrates with a sheet resistance
of 20 Ω per square. Before the fabrication of the devices, the NI
compounds were further purified by train sublimation. All the organic
layers were deposited at a rate of 1.0–2.0 Å s−1. The cathode was
deposited with LiF (1 nm) at a deposition rate of 0.1 Å s−1 and then
capping with Al metal (100 nm) through thermal evaporation at a rate
of 4.0 Å s−1. The electroluminescence (EL) spectra were measured using
a PR705 spectra scan spectrometer. The luminance and current density
versus driving-voltage characteristics were recorded simultaneously
with measuring of the Commission Internationale de L’Eclairage (CIE)
coordinates by combining the spectrometer CS200 with a Keithley model
2420 programmable voltage–current source meter. All measurements
were carried out at room temperature and under ambient conditions.
Ultraviolet and X-Ray Photoelectron Spectroscopy: Experiments
were carried out in a Kratos AXIS UltraDLD ultrahigh vacuum (UHV)
surface analysis system, consisting of a multiport carousel chamber, a
deposition chamber, and an analysis chamber. The base pressures in
the three chambers were better than 5 × 10−9, 2 × 10−9, and 3 × 10−9
Torr, respectively. All organic materials were carefully sublimed twice
and entirely out-gassed to ensure a good material purity and to be able
to study the intrinsic behavior. The film thickness was monitored by a
quartz-crystal microbalance. Prior to film deposition, the ITO-coated
glass substrates were subjected to a routine cleaning process and ex
situ treated by UV-ozone exposure. The organic materials, including
NI-1-PhTPA, NI-2-PhTPA, and LiF were evaporated in situ in steps
onto the ITO substrates from resistively heated tantalum boats in the
deposition chamber at a rate of 1–2 Å s−1. The samples were transferred
to the analysis chamber without breaking vacuum for the UPS and XPS
measurements after each deposition step. The UPS measurements
were carried out using an unfiltered He I (21.2 eV) gas discharge lamp
to characterize the valence states and the vacuum level (VL). There
was no sign of sample charging in all measurements, even when the
thickness of the organic films reached several hundreds of Angstroms.
A monochromatic aluminum Kα source (1486.6 eV) was used in the
XPS measurements to study the interfacial chemical reactions and the
development of possible molecular level bending across the interface.
For the collection of secondary electrons, the samples were negatively
biased at 4 V. All measurements were performed at room temperature.
The photoelectrons were collected by a hemispherical analyzer with a
total instrumental energy resolution of 0.1 eV for the UPS measurements
and 0.5 eV for the XPS measurements. In all the UPS and XPS spectra,
the Fermi level (EF) is referred to as the zero binding energy (BE).
1
calcd. for C23H15BrN2, 398.0; found, 398.0. H NMR (600 MHz, CDCl3,
δ [ppm]): 7.95 (t, J = 7.7 Hz, 2H), 7.75 (d, J = 8.8 Hz, 1H), 7.67–7.63
(m, 1H), 7.63–7.58 (m, 2H), 7.50–7.46 (m, 2H), 7.41 (m, 4H), 7.38 (m,
1H), 7.20 (m, 1H), 7.13 (d, J = 8.5 Hz, 1H).13C NMR (150 MHz, CDCl3,
δ [ppm]): 150.30, 140.31, 138.55, 131.70, 131.48 130.75, 130.70, 130.31,
130.02, 129.40, 129.26, 129.00, 125.69, 124.70, 124.39, 123.63, 122.06,
120.15, 119.71.
Synthesis of NI-1-TPA:
tetramethyl-1,3,2-dioxaborolan-2-yl)aniline (2.3 g,
A
mixture of N,N-diphenyl-4-(4,4,5,5-
mmol), NI-1-Br
6
(2.0 g, 5 mmol), 100 mg of Pd(PPh3)4, potassium carbonate (2.8 g,
20 mmol), THF (15 mL), toluene (30 mL), and distilled water (20 mL)
was refluxed for 24 h under argon protection. Then the mixture was
extracted with dichloromethane. The organic phases were combined
and dried over magnesium sulfate. After the solvent was removed, the
product was further purified by column chromatography on silica gel
to afford a yellow powder (2.2 g, 78%). MALDI-TOF (m/Z): calcd. for
C41H29N3, 563.24; found, 563.19. Anal. calcd. for C41H29N3: C 87.36,
H 5.19, N 7.45; found: C 87.52, H 5.12, N 7.31. 1H NMR (600 MHz,
CDCl3, δ [ppm]): 8.82 (d, J = 7.7 Hz, 1H), 7.93 (d, J = 7.8 Hz, 1H),
7.66 (d, J = 7.8 Hz, 4H), 7.52–7.47 (m, 8H), 7.39 (d, J = 6.8 Hz, 2H),
7.34 (d, J = 8.6 Hz, 1H), 7.26–7.23 (m, 4H), 7.14–7.11 (m, 6H), 7.03 (t,
J = 6.8 Hz, 2H). 13C NMR (150 MHz, CDCl3, δ [ppm]): 150.32, 147.62,
147.58, 141.03, 138.64, 137.10, 133.86, 133.47, 130.73, 129.98, 129.77,
129.34, 128.75, 128.55, 128.44, 127.63, 127.59, 126.96, 126.60, 126.36,
124.78, 124.56, 124.32, 123.72, 123.12, 122.21, 110.96.
Synthesis of NI-2-TPA: NI-2-TPA was synthesized according to the
same procedure as described above for the synthesis of NI-1-TPA, but
using NI-2-Br, which yielded a yellow solid in 79% yield. MALDI-TOF
(m/Z): calcd. for C41H29N3, 563.24; found, 563.18. Anal. calcd. for
1
C41H29N3: C 87.36, H 5.19, N 7.45; found: C 87.48, H 5.04, N 7.37. H
NMR (600 MHz, CDCl3, δ [ppm]): 7.99 (d, J = 8.8 Hz, 1H), 7.95 (d,
J = 8.1 Hz, 1H), 7.75 (d, J = 8.7 Hz, 1H), 7.67–7.57 (m, 5H), 7.55–7.51
(m, 2H), 7.51–7.47 (m, 2H), 7.47–7.42 (m, 2H), 7.39–7.36 (m, 1H),
7.28–7.23 (m, 4H), 7.21–7.19 (m, 1H), 7.15 (d, J = 8.1 Hz, 1H), 7.13–
7.08 (m, 6H), 7.05–7.01 (m, 2H). 13C NMR (150 MHz, CDCl3, δ [ppm]):
151.27, 147.62, 147.57, 140.95, 140.44, 138.92, 133.79, 131.64, 130.69,
130.20, 129.81, 129.66, 129.38, 129.31, 129.16, 128.60, 127.64, 126.20,
125.56, 124.54, 124.50, 124.19, 123.69, 123.09, 122.12, 120.18, 119.77.
Synthesis of NI-1-PhTPA: NI-1-PhTPA was synthesized according
to the procedure as described above for the synthesis of NI-1-TPA by
combining NI-1-Br and N,N-diphenyl-4′-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-[1,1'-biphenyl]-4-amine, giving a green-yellow solid
in 79% yield. MALDI-TOF (m/Z): calcd. for C47H33N3, 639.27; found,
639.22. Anal. calcd. for C47H33N3: C 88.23, H 5.20, N 6.57; found:
1
C 88.14, H 5.34, N 6.39. H NMR (600 MHz, CDCl3, δ [ppm]): 8.83 (d,
Supporting Information
J = 8.2 Hz, 1H), 7.94 (d, J = 8.1 Hz, 1H), 7.70 (d, J = 8.4 Hz, 2H), 7.68–
7.65 (m, 2H), 7.64–7.60 (m, 4H), 7.58 (d, J = 8.4 Hz, 2H), 7.56–7.47 (m,
6H), 7.39 (dd, J = 5.2, 3.2 Hz, 2H), 7.34 (d, J = 8.8 Hz, 1H), 7.26 (dd,
J = 11.2, 4.6 Hz, 4H), 7.13 (d, J = 8.5 Hz, 6H), 7.03 (t, J = 7.3 Hz, 2H).
13C NMR (150 MHz, CDCl3, δ [ppm]): 150.20, 147.65, 147.40, 141.11,
139.96, 138.68, 138.51, 137.13, 137.09, 134.29, 133.52, 130.75, 130.01,
129.79, 129.34, 129.08, 128.79, 128.45, 127.65, 127.61, 127.35, 127.00,
126.77, 126.63, 124.80, 124.53, 124.38, 123.83, 123.05, 122.21, 110.97.
Synthesis of NI-2-PhTPA: NI-2-PhTPA was synthesized according to
the same procedure as described above for the synthesis of NI-1-TPA
by combining NI-2-Br and N,N-diphenyl-4′-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-[1,1'-biphenyl]-4-amine, giving a green-yellow solid
in 79% yield. MALDI-TOF (m/Z): calcd. for C47H33N3, 639.27; found,
639.22. Anal. calcd. for C47H33N3: C 88.23, H 5.20, N 6.57; found: C 88.29,
H 5.07, N 6.43.1H NMR (600 MHz, CDCl3, δ [ppm]): 8.00 (d, J = 8.7 Hz,
1H), 7.96 (d, J = 8.1 Hz, 1H), 7.77 (d, J = 8.7 Hz, 1H), 7.68–7.59 (m, 9H),
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
M.L. and X.L.L. contributed equally to this work. The authors greatly
appreciate the financial support from the Ministry of Science and
Technology (2015CB655003 and 2014DFA52030), the National Natural
Science Foundation of China (91233116 and 51073057), the Ministry
of Education (NCET-11–0159), and the Guangdong Natural Science
Foundation (S2012030006232).
Received: May 27, 2015
Revised: June 18, 2015
Published online: July 14, 2015
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2015, 25, 5190–5198
2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
5197

www.afm-journal.de
www.MaterialsViews.com
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