Fine tuning of the photophysical and electroluminescent properties of DCM-type dyes by changing the structure of the electron-donating group

Article abstract of DOI:10.1039/b604563d

Using 2H-indene-1,3-dione as an electron-withdrawing group and aromatic amines of different structures as electron-donating groups, four new DCM-type dyes, IN-1, IN-2, IN-3, and IN-4, were designed and synthesized for application in organic light-emitting diodes (OLEDs) as red light-emitting materials. IN-2 exhibits a red shift in fluorescence but a blue shift in absorption with respect to IN-1. Similarly, IN-4 exhibits a red shift in fluorescence but a blue shift in absorption with respect to IN-3. These unique photophysical properties were utilized to optimize the electroluminescence color purity and efficiency of these red emitters in OLEDs. In EL devices with the configuration of ITO/NPB (60 nm)/Alq₃ (or Gaq₃): red dopant (2.0 wt%) (7 nm)/BCP (12 nm)/Alq₃ (45 nm)/LiF (0.3 nm)/Al (300 nm), IN-3 in Alq₃ and IN-1 in Gaq₃ show CIE coordinates of (0.64, 0.36) and (0.65, 0.34), and current efficiencies of 3.24 cd A^-1 and 3.02 cd A ^-1, respectively, much better than the other guest/host combinations. This can be ascribed to the good spectrum overlap either between IN-3 and Alq₃ or between IN-1 and Gaq₃, indicating that the absorption spectrum has the same importance as the fluorescence spectrum in the improvement of color purity of red emitters. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b604563d

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www.rsc.org/materials | Journal of Materials Chemistry
Fine tuning of the photophysical and electroluminescent properties of
DCM-type dyes by changing the structure of the electron-donating group{
Yi-Shan Yao,a Qian-Xiong Zhou,a Xue-Song Wang,*a Yue Wang*b and Bao-Wen Zhang*a
Received 29th March 2006, Accepted 19th July 2006
First published as an Advance Article on the web 7th August 2006
DOI: 10.1039/b604563d
Using 2H-indene-1,3-dione as an electron-withdrawing group and aromatic amines of different
structures as electron-donating groups, four new DCM-type dyes, IN-1, IN-2, IN-3, and IN-4,
were designed and synthesized for application in organic light-emitting diodes (OLEDs) as red
light-emitting materials. IN-2 exhibits a red shift in fluorescence but a blue shift in absorption
with respect to IN-1. Similarly, IN-4 exhibits a red shift in fluorescence but a blue shift in
absorption with respect to IN-3. These unique photophysical properties were utilized to optimize
the electroluminescence color purity and efficiency of these red emitters in OLEDs. In EL devices
with the configuration of ITO/NPB (60 nm)/Alq3 (or Gaq3) : red dopant (2.0 wt%) (7 nm)/BCP
(12 nm)/Alq3 (45 nm)/LiF (0.3 nm)/Al (300 nm), IN-3 in Alq3 and IN-1 in Gaq3 show CIE
coordinates of (0.64, 0.36) and (0.65, 0.34), and current efficiencies of 3.24 cd A21 and
3.02 cd A21, respectively, much better than the other guest/host combinations. This can be
ascribed to the good spectrum overlap either between IN-3 and Alq3 or between IN-1 and Gaq3,
indicating that the absorption spectrum has the same importance as the fluorescence spectrum in
the improvement of color purity of red emitters.
from ideal. At low doping levels in tris(8-hydroxyquinolinato)
1. Introduction
aluminium (Alq3), a well known host material for emitters of
Among the blue-, green-, and red-emitting materials required
different colors, part of the DCJTB emission falls in the region
for full color organic light-emitting diodes (OLEDs), red-
below 600 nm. Although the emission of DCJTB undergoes a
emitting materials remain one of the greatest challenges in
red shift upon increasing the doping level, the luminance and
terms of both color purity and efficiency for dynamic
efficiency are greatly compromised due to the concentration
research.1–4 To realize red light emission, two strategies are
quenching effect. One solution to this problem is to shift the
commonly applied in material design, i.e. utilizing either
emission bands of DCM-type dyes to longer wavelengths by
elongated conjugation structures or intramolecular charge
transfer characters.5 Thus, most organic red emitters can
of the resultant dyes generally shift to longer wavelengths
structure modification, nevertheless the absorption bands
also,14–16 which impairs the spectrum overlap between the
be classified into two categories, polycyclic aromatic hydro-
carbon (PAH) and electron donor–p bridge–electron acceptor
emission of Alq3 and the absorption of DCM-type dyes, and
(D–p–A) systems. Compared with PAH, D–p–A systems
have a remarkable advantage: the ease in fine tuning of
their photophysical and electrochemical properties including
absorption band, emission color, emission efficiency, as well as
redox potentials. This advantage was fully taken over the past
decade to develop lots of promising red emitters, such as
4-dicyanomethylene-4H-pyran based dyes (the so called
DCM-type dyes), of which 4-(dicyanomethylene)-2-tert-butyl-
6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) still
represents one of the most efficient red-emitting materials for
OLED applications and has received extensive studies.2,4,6–13
Considering its emission color purity, however, DCJTB is far
thus decreases the energy transfer efficiency between host and
guest. Recently, we designed and synthesized a new congener
of DCJTB, DCQTB (2-(2-tert-butyl-6-((E)-2-(2,6,6-trimethyl-
2,4,5,6-tetrahydro-1H-pyrrolo[3,2,1-ij]quinolin-8-yl)vinyl)-4H-
pyran-4-ylidene)malononitrile, Fig. 1), which emits at longer
wavelengths but absorbs at shorter wavelengths with respect to
DCJTB.17 This desirable property was obtained by only
changing the connecting mode of the nitrogen atom with the
N-phenyl ring in the aniline moiety. In DCJTB the nitrogen
atom is locked by two six-membered rings, while one six-
membered ring is replaced by a five-membered ring in
DCQTB. Thus, it is necessary to investigate further the role
of the D structure in DCM-type dyes. While the D structure is
receiving increasing attention, the A moiety in DCM-type
dyes was little changed for property optimization. Using
2H-indene-1,3-dione to serve as A and 1-phenylpiperidine or
4-phenylmorpholine as D, Kim and co-workers18 once pre-
pared two new DCM-type dyes, IAP-1 (2-(2-(4-(piperidin-1-
yl)styryl)-6-methyl-4H-pyran-4-ylidene)-2H-indene-1,3-dione)
and IAP-2 (2-(2-(4-morpholinostyryl)-6-methyl-4H-pyran-4-
ylidene)-2H-indene-1,3-dione). Though it was claimed that
aTechnical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing, 100101, P. R. China. E-mail: xswang@mail.ipc.ac.cn;
g203@mail.ipc.ac.cn; Fax: +86-010-6487-9375; Tel: +86-010-6488-8103
bKey Laboratory for Supramolecular Structure and Materials of
Ministry of Education, College of Chemistry, Jilin University,
Chang Chun, 130012, P. R. China. E-mail: yuewang@jlu.edu.cn;
Fax: +86-431-5193421; Tel: +86-431-5168-494
{ Electronic supplementary information (ESI) available: absorption
and fluorescence spectra; cyclic voltammograms; HOMO and LUMO
orbitals. See DOI: 10.1039/b604563d
3512 | J. Mater. Chem., 2006, 16, 3512–3520
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Fig. 1 Chemical structures of IN-1, IN-2, IN-3, IN-4, DCJTB, DCQTB, and the other materials used in the preparation of EL devices.
IAP-1 had improved color purity compared with DCJTB, the
Commission Internationale de l’Eclairage (CIE) coordinates of
IAP-1 were x = 0.61, y = 0.38, far from the saturated red
emission. With the above-mentioned efforts in mind, four new
pyran-containing D–p–A structures (Fig. 1), IN-1, IN-2, IN-3,
and IN-4, were prepared to examine the effects of D and A
structures on their photoluminescence (PL) and electro-
luminescence (EL) properties. In solutions, the fluorescence
maxima of these four dyes are all located at longer wavelengths
than DCJTB, while the absorption maxima cover a wide range
with that of DCJTB just in the middle of them. These features
make it possible to optimize the color purity of red OLEDs
without impairing the energy transfer efficiency between host
and guest. As expected, the four dyes exhibit promising
electroluminescence behaviors, and these behaviors correlate
very well with their photophysical properties in solutions.
are also included (see ESI{ for their absorption and
fluorescence spectra).
Compared with DCJTB, both the absorption and fluores-
cence spectra of IN-1 undergo bathochromic shifts. Consider-
ing the fact that the oxidation peak potential of IN-1 is less
positive than that of DCJTB (Table 2), the electron-with-
drawing ability of the 2H-indene-1,3-dione moiety is not likely
to be stronger than that of the dicyanomethylene group. As a
result, the red shift is more probably due to the larger
conjugation system of IN-1 (see ESI{ Fig. 4S). To better
understand the role of the electron-donating moiety in tuning
the photophysical properties, the four new DCM-type dyes can
be classified into two groups based on their structures, i.e. IN-1
vs. IN-2, and IN-3 vs. IN-4. Very similar to the case of DCJTB
vs. DCQTB,17 substitution of one five-membered locking ring
for a six-membered locking ring gives rise to a red shift in the
fluorescence but a blue shift in the absorption of IN-2 relative
to IN-1. As a result, IN-2 exhibits a bigger Stokes shift than
IN-1. The same phenomena can be found in a spectral
2. Results and discussion
comparison between IN-3 and IN-4 (lab
of IN-4 , lab
2.1. Synthesis
max
max
of IN-3; however, lem
of IN-4 . lem
of IN-3) as the
max
max
Scheme 1 shows the synthetic routes for IN-1, IN-2, IN-3,
and IN-4. As an example, IN-2 was prepared as follows.
Conventional N-alkylation of 1 (Scheme 1) with 1 equiv. of
1-bromo-3-methyl-2-butene gave 2. After annulation in con-
centrated H2SO4, 2 was transformed into 3. Then, 4 was
prepared from 3 by Vilsmeier reaction using DMF/POCl3 as
condensing agent. Finally, the target molecule IN-2 was
obtained by the condensation between 4 and 15. IN-1, IN-3,
and IN-4 were synthesized in a similar way. As shown in
Scheme 1, all reactions have high collection yields (over 70%),
which means that the four new DCM-type dyes can be
synthesized easily and cost-effectively.
result of the structure variation of the aromatic amine moiety.
Fig. 2 shows the plots of the Stokes shifts of DCJTB, IN-1,
IN-2, IN-3, and IN-4 as a function of the solvent polarity
parameter f (e, n), which is defined as eqn (1) where e is the
static dielectric constant and n is the optical refractive index of
the solvent.19 From the slopes, the dipole moment differences
(Dm) between the fluorescent singlet excited state (me) and the
ground state (mg) were calculated (Table 2), based on the
Lippert and Mataga theory (eqn (2)).20 In eqn (2), a0 repre-
sents the radius of the Onsager cavity, which was deduced from
their energetically optimized ground state geometries obtained
with the B3LYP/6-31G(d) basis set (see below). In terms of the
energetically optimized ground state geometries, mg can also be
achieved, thus, me can be estimated as well (Table 2).
2.2. Photophysical properties
Table 1 shows the absorption and fluorescence maxima of the
four new DCM-type dyes in solutions with varied polarity,
along with their relative fluorescence quantum yields in
THF. For comparison, the corresponding data for DCJTB
f(e, n) = (e 2 1)/(2e + 1) 2 (n2 2 1)/(2n2 + 1)
(1)
(2)
3
na 2 nf = 2(me 2 mg)2?f(e, n)/(hca0 ) + const
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Scheme 1 Synthetic routes of IN-1, IN-2, IN-3 and IN-4.
The dipole moment of a D–p–A molecule directly depends
on the extent of charge separation and the distance across
which charge transfer occurs.21 The latter factor is similar
in both IN-1 and IN-2, assuming that the center of the
positive charge lies on the nitrogen atom of the aromatic amine
moiety and the center of the negative charge lies on the
Table 1 Absorption and emission properties of IN-1, IN-2, IN-3, IN-4, and DCJTB in solvents of varied polarity
DCJTB IN-1 IN-2 IN-3
labmax/nma lemmax/nmb labmax/nma lemmax/nmb labmax/nma lemmax/nmb labmax/nma lemmax/nmb labmax/nma lemmax/nmb
IN-4
Solvent
Cyclohexane 476.5
Tetrachloromethane 484.5
530.4
548.4
607.4
617.0
618.0
653.8
655.0
661.0
668.8
495.2
504.6
542.0
529.0
534.0
535.0
552.2
547.0
557.4
0.85
555.6
574.2
643.0
656.2
660.8
717.2
718.6
720.8
726.8
461.4
465.6
493.2
489.2
493.4
487.6
493.6
496.2
499.0
0.36
566.0
584.0
653.0
665.0
668.6
721.4
722.4
723.8
729.2
482.8
489.8
520.2
506.0
510.0
509.8
521.6
521.4
524.8
1.06
542.2
555.4
616.2
623.4
628.0
667.6
677.0
669.8
680.4
468.5
474.5
483.5
474.0
477.8
474.8
482.8
482.4
488.2
0.50
578.6
591.8
654.2
656.4
661.0
718.4
722.6
721.2
724.0
Chloroform
Ethyl acetate
Tetrahydrofuran
Acetonitrile
Methanol
508.5
493.0
498.5
503.5
510.0
Dimethylformamide 515.5
Dimethylsulfoxide 521.5
Wfc
1.00
a
b
Absorption maximum. Fluorescence maximum. Relative fluorescence quantum yield in THF assuming that of DCJTB is unity.
c
3514 | J. Mater. Chem., 2006, 16, 3512–3520
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ground state to the singlet excited state, is observed at higher
energy, but the fluorescence of IN-2, i.e., the jump from the
lowest vibrational level of the singlet excited state to the
ground state, is at lower energy. The greater distortion extent
of the excited state with respect to the ground state also
increases the vibrational overlap and favors non-radiative
decay,22 which is in line with the fact that the fluorescence
quantum yield of IN-2 is lower than that of IN-1 (Table 1).
Recently, Ishitani et al. observed that the intramolecular
interactions between ligands in Re(I) complexes red shifted
absorption but blue shifted emission, and a similar explanation
was applied to these discrepant phenomena.23 The red shift in
fluorescence and blue shift in absorption of IN-4 with respect
to IN-3 can be understood similarly. The possible structure–
property relationships underlying these interesting findings are
discussed below.
Table 2 The dipole moments and energy levels of DCJTB, IN-1,
IN-2, IN-3, and IN-4
d
Dm/Da
mg/Db
me/Dc
Epox/eVe
˚
a0/A
DCJTB
IN-1
IN-2
IN-3
IN-4
a
13.1
16.3
17.1
16.4
16.9
5.9
3.8
3.7
3.5
2.8
19.0
20.1
20.8
19.9
19.7
6.10
6.05
6.23
6.08
6.61
1.08
0.94
1.03
1.01
1.38
Difference between the dipole moments (in Debye) of the excited
state and the ground state, obtained from Fig. 2 and eqn (2).
Dipole moment of the ground state obtained from quantum-
b
c
chemical calculation. Dipole moment of the excited state obtained
from Dm and mg. Onsager cavity obtained from quantum-chemical
d
e
calculation. The oxidation peak potential vs. SCE.
It is worthwhile to note that (1) all the four new DCM-type
dyes emit at longer wavelengths than DCJTB, and (2) their
absorption maxima fall in a wide range with that of DCJTB
just located between them (Table 1). The former feature
may help the improvement of the color purity of EL devices
based on these new red emitters, while the latter one may
facilitate the choice of adequate host material to get better
spectral overlap and thus energy transfer efficiency between
host and guest.
2.3. Redox properties
The redox behaviors of these DCM derivatives were deter-
mined by cyclic voltammetry (CV)24 in dry chloroform, and
the oxidation peak potentials are shown in Table 2 (see ESI{
for their cyclic voltammograms). While IN-1, IN-2, and IN-3
have similar oxidation peak potentials, the oxidation peak
potential of IN-4 shifts to the positive direction remarkably,
implying its stronger resistance to oxidation. The sharp
contrast of IN-4 with respect to its three congeners may be
the result of its distorted structure and therefore impaired
conjugation extent (see below).
Fig. 2 Plots of the Stokes shifts of DCJTB, IN-1, IN-2, IN-3, and
IN-4 as a function of the solvent polarity parameter f(e, n).
2H-indene-1,3-dione moiety. Thus, the larger me implies that
the structure variation of IN-2 from IN-1 facilitates the
intramolecular charge transfer in the excited states. Moreover,
the bigger Stokes shift and Dm of IN-2 suggest that the excited
state equilibrium nuclear geometry distorts from the ground
state equilibrium nuclear geometry more greatly in IN-2 than
in IN-1, as shown in Fig. 3. In this situation, the absorption of
IN-2, i.e., the transition from the lowest vibrational level in the
2.4. Quantum-chemical calculation of electronic structure
In an effort to understand at the molecular level the nature
behind the different photophysical properties of these new
DCM-type dyes, density functional theory (DFT) was used
to calculate their electronic structures. Fig. 4 shows their
optimized ground-state geometries obtained with the B3LYP/
6-31G(d) basis set, while the main dihedral angles, by which
the structures can be defined, are tabulated in Table 3.
Moreover, the sum of bond angles (h) about the N atom in the
aromatic amine moiety is also included in Table 3. One would
expect a larger orbital overlap between the amine nitrogen
and the N-phenyl when the h value is closer to 360u.25
Thus, the replacement of one six-membered locking ring by a
five-membered one decreases the electronic communication
between the amine nitrogen and the N-phenyl in IN-2,
reflected by the smaller h (354.1u in IN-2 vs. 360u in IN-1)
and larger dihedral angels (|C53–N17–C2–C3| = 14.1u and
|C13–N17–C2–C1| = 212.0u in IN-2 vs. |C62–N15–C2–C3| =
6.9u and |C12–N15–C2–C1| = 6.9u in IN-1). The decreased
orbital overlap between the amine nitrogen and the N-phenyl
Fig. 3 Pictorial illustration of the potential energy curves of the
ground state (GS) and singlet excited state (ES) of IN-1 and IN-2. The
vertical arrows denote the absorption and emission transitions.
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Fig. 4 Optimized ground-state geometries of IN-1, IN-2, IN-3, and IN-4 obtained with the B3LYP/6-31G(d) basis set.
Table 3 Structure parameters for IN-1, IN-2, IN-3, and IN-4 calculated using the DFT/B3LYP/6-31G(d) method
IN-1
IN-2
IN-3
IN-4
ha/u
ab/u
360.0
354.1
357.5
357.9
|62–15–2–3| = 6.9
|12–15–2–1| = 6.9
|4–5–16–18| = 20.2
|16–18–20–21| = 2180.0
|21–40–41–43| = 0.0
|53–17–2–3| = 14.1
|13–17–2–1| = 212.0
|4–5–26–28| = 20.8
|26–28–30–31| = 179.5
|31–50–59–61| = 20.1
|12–74–2–1| = 22.6
|63–74–2–75| = 18.8
|3–4–23–25| = 20.8
|23–25–27–28| = 179.2
|28–47–48–50| = 0.2
|11–73–2–1| = 33.3
|62–73–2–74| = 54.4
|3–4–22–24| = 227.8
|22–24–26–27| = 177.8
|27–46–47–49| = 0.1
a
b
The sum of bond angles about the N atom. Some dihedral angels associated with the discussion in the text. The numbers within the two
vertical lines denote the atoms involved in the corresponding dihedral angles (see Fig. 4).
Fig. 4S (ESI{) shows the plots of the molecular orbitals in
the ground states of IN-1, IN-2, IN-3, IN-4, and DCJTB. In
these molecules, the HOMOs are mainly localized within the
electron-donating fraction. In contrast, the LUMOs are
mainly localized within the electron-withdrawing fraction,
indicative of the great extent of intramolecular charge transfer
upon excitation.
will weaken the electron-donating ability of the corresponding
aromatic amine, as a result, IN-2 exhibits a somewhat smaller
dipole moment in the ground state (mg = 3.7 D) with respect to
that of IN-1 (mg = 3.8 D). Upon excitation, IN-2 experiences a
larger dipole moment change and thus exhibits a greater
excited state dipole moment (me = 20.8 D) than IN-1 (me =
20.1 D), suggesting that the aromatic amine moiety of IN-2
may adopt a more planar conformation in the excited state
than in the ground state, while that of IN-1 may deviate from
the original planarity to adapt the varied electron density
distribution in the excited state. In the case of IN-3 and IN-4,
the presence of an extra phenyl structure (naphthalene group)
2.5. Electroluminescent properties
Generally, the EL color and efficiency of the DCM-type dyes
depend remarkably on their doping level in host materials.
Thus, EL devices, having the configuration of ITO/NPB
(60 nm)/Alq3 : IN-1 (x wt%) (7 nm)/BCP (12 nm)/Alq3 (45 nm)/
LiF (0.3 nm)/Al (300 nm), were first fabricated and examined
(Fig. 5). In these devices, NPB serves as the hole-transporting
material, BCP as the hole-blocking material, and Alq3 as both
the host material and electron-transporting material. At the
doping concentration of 0.5 wt%, the EL emission from Alq3,
which is located at around 520 nm, is obvious (Fig. 5b),
indicating the inefficient energy transfer from Alq3 to IN-1.
With the increase of the doping level, EL emissions shifted
to longer wavelengths gradually, however, the luminance
efficiency decreased simultaneously (Fig. 5a and Table 4).
The spectrum shift is due to the polarization effect26,27 and the
efficiency decrease is the result of concentration quenching.
Considering the trade-off between efficiency and color purity,
the EL property comparison of our four new DCM-type dyes
were all based on the devices at 2 wt% doping concentration.
in IN-4 introduces
a remarkable steric hindrance, and
accordingly, distorts the ground state structure greatly, which
is evidenced by the larger dihedral angle of |C3–C4–C22–C24|
(227.8u) in IN-4 than that of |C3–C4–C23–C25| (20.8u) in
IN-3. Thus, the charge transfer is restricted in IN-4 due to the
weakened conjugation between the aromatic amine and the
other part of the molecule, though the h angles are very similar
in both IN-3 and IN-4. The smaller ground state dipole
moment of IN-4 (mg = 2.8 D) than that of IN-3 (mg = 3.5 D)
agrees with the distorted conjugation system of IN-4. In the
excited state, the difference between the dipole moments of
IN-4 and IN-3 becomes smaller, implying the steric hindrance
caused by the naphthalene group is alleviated to some extent,
probably due to the shift of the electron density from the
aromatic amine moiety to the electron-withdrawing moiety
upon excitation.
3516 | J. Mater. Chem., 2006, 16, 3512–3520
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Fig. 5 Electroluminescence characteristics of IN-1 doped in Alq3 at different concentrations. (a) Luminance vs. current density; (b) EL spectra at
the applied voltage of 9 V.
Table 4 EL characteristics of IN-1, IN-2, IN-3, IN-4, along with those of DCJTB cited from references for comparison
(J, V) at Lmax
mA cm22, Vg
/
(gc, L) at 20 mA cm22/cd A21
,
Lmax/cd m222f
cd m22h
EL peak/nm
CIE (x, y)i
Reference
IN-1 (0.5)a
IN-1 (1.0)a
IN-1 (2.0)a
IN-1 (3.5)a
IN-1 (2.0)b
IN-2 (2.0)a
IN-3 (2.0)a
IN-4 (2.0)a
DCJTBc
12270
7900
5654
4892
7842
6100
8387
4472
—
(357.7, 13)
(373, 15)
(376.6, 14)
(388.3, 12)
(404.4, 15.5)
(305.8, 15)
(393, 16.5)
(499, 15)
—
(3.86, 770.7)
(3.65, 718)
(2.22, 471)
(1.24, 260)
(3.02, 630.6)
(2.47, 508.2)
(3.24, 642.6)
(1.47, 288)
(1.89, 378)j
(—, 1200)j
(4.44, 966)j
632
644
652
660
652
668
628
654
620
628
630
(0.52, 0.42)
(0.62, 0.37)
(0.63, 0.36)
(0.65, 0.34)
(0.65, 0.34)
(0.66, 0.33)
(0.64, 0.36)
(0.62, 0.37)
(0.63, 0.37)
(0.62, 0.38)
(0.65, 0.35)
This work
This work
This work
This work
This work
This work
This work
This work
8
DCJTBd
15000
—
—
—
10
7
DCJTBe
a
ITO/NPB (60 nm)/Alq3 : dopant (x wt%) (7 nm)/BCP (12 nm)/Alq3 (45 nm)/LiF (0.3 nm)/Al (300 nm), x is shown in the bracket following
the compound name. ITO/NPB (60 nm)/Gaq3 : dopant (x wt%) (7 nm)/BCP (12 nm)/Alq3 (45 nm)/LiF (0.3 nm)/Al (300 nm). ITO/CFx/
b
c
d
NPB/Alq3 : DCJTB (1%)/Alq3/Mg : Ag. ITO/NPB/Alq3 : QAD (0.5%) : DCJTB (1%)/Alq3/LiF/Al. ITO/CFx/NPB/Alq3 : rubrene (60%) :
e
f
DCJTB (2%)/Alq3/LiF/Al. Maximum luminance. Current density (J) and voltage (V) at Lmax
g
h
.
Current efficiency (gc) and luminance (L) at
.
i
j
20 mA cm22
.
CIE coordinates. Maximum current efficiency (gc) and luminance (L) at 20 mA cm22
At this doping level, the device based on IN-1 : Alq3 still
showed residual Alq3 emission, suggesting poor spectral
overlap between host and guest. When Gaq3 was used as the
host material instead of Alq3, whose emission spectrum has
a red shift over that of Alq3,26 significant improvements
in both color purity and current efficiency were achieved.
At the driving voltage of 9 V, the CIE coordinates of IN-1
(2 wt%) : Gaq3 and IN-1 (2 wt%) : Alq3 are (0.65, 0.34) and
(0.63, 0.36), respectively. The current efficiency of IN-1 doped
in Gaq3 (2 wt%) is 3.02 cd A21, about 40% higher than
that doped in Alq3 at the same doping concentration. The
improved performance can be attributed to the better spectral
overlap between IN-1 and Gaq3. Another way to strengthen
the spectrum overlap of the dopant with Alq3 is to use red
emitters with blue shifted absorption compared with that in
IN-1. IN-2 has a blue shifted absorption spectrum compared
with that of IN-1 in solution, this character is expected to be
still valid in the doped film. On the other hand, IN-2 emits
at longer wavelengths than IN-1. Thus, the combination of
IN-2 with Alq3 might give promising EL properties. As
expected, the device based on IN-2 (2 wt%) : Alq3 indeed
exhibited improved color purity (CIE coordinates of 0.66, 0.33
vs. 0.63, 0.36, Table 4) and even enhanced current efficiency
(2.47 cd A21 vs. 2.22 cd A21 at 20 mA cm22, Table 4 and
Fig. 6a) compared with those of the device based on IN-1
(2 wt%) : Alq3, though the fluorescence quantum yield of IN-2
in THF is only about half that of IN-1 (Table 1). Among the
four new DCM-type dyes, IN-3 showed the highest current
efficiency (Table 4 and Fig. 6c). Considering the facts that its
absorption maximum (510 nm in THF) is just between those of
IN-1 (534 nm) and IN-2 (493.4 nm), and its fluorescence
quantum yield in THF is the highest (Table 1), its EL behavior
is not difficult to understand. In contrast, IN-4 behaves the
worst in both color purity and current efficiency. The CIE
coordinates of the IN-4 (2 wt%) : Alq3 based device are (0.62,
0.37), much inferior to its three congeners. The over-blue-
shifted absorption spectrum of IN-4 (lab
= 477.8 nm) may
max
impair the spectrum overlap with Alq3 emission, and thus be
responsible for its poor EL color. Additionally, this point
combined with its low fluorescence quantum yield results in
the decreased current efficiency.
Table 4 also includes the data from the red OLEDs based on
DCJTB and assistant dopants. Though the device structure
was not fully optimized, IN-3 in Alq3 and IN-1 in Gaq3
show comparable electroluminescent behaviors to the tertiary
emitting systems of DCJTB–rubrene or quinacridone–
Alq3,7,10,25 showing the importance of the fine tuning of the
photophysical properties of DCM-type dyes.
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Fig. 6 Electroluminescence characteristics of IN-1, IN-2, IN-3, and IN-4 dopants. (a) Luminance vs. current density; (b) luminance vs. voltage; (c)
current efficiency vs. current density; (d) EL spectra at the applied voltage of 9 V.
procedures as shown in Scheme 1 (see below for the
3. Experimental
details). N,N9-Bis(1-naphthyl)-N,N9-diphenyl-1,19-biphenyl-
General information
4,49-diamine (NPB), tris(8-hydroxyquinolinato)aluminium
lH NMR spectra were recorded on a Varian Gemini-300
(Alq3), and tris(8-hydroxyquinolinato)gallium (Gaq3) were
prepared via published methods. All these materials were
(300 MHz) or Bruker-400 (400 MHz) spectrometer. Mass
subjected to train sublimation before device fabrication.
spectra (MS) were measured on a Finnigan GC-MS 4021 C
spectrophotometer. Elemental analyses were carried out on
Synthesis of IN-2
a Carlo Erba 1106 using the Flash EA 1112 method.
Absorption and fluorescence spectra were run on
a
To a refluxed suspended solution of 2.8 g of (0.07 mol, 60 wt%)
sodium hydride in 100 ml of 1,2-dimethoxyethane (mono-
glyme), was added under a dry nitrogen atmosphere a solution
of 6.66 g (0.05 mol) of 2-methylindoline (1 in Scheme 1) in
50 ml of monoglyme, followed, after 45 min, by a solution of
7.45 g (0.05 mol) of 1-bromo-3-methyl-2-butene in 50 ml of
monoglyme. The reaction mixture was refluxed for 6 h. Most
of the solvent was then removed under reduced pressure and
the pasty residue was cooled in an ice-water bath. Diethyl ether
(150 ml) and cold water (100 ml) were added, and the formed
two layers were separated. The water layer was extracted
with two 100 ml portions of ether. The ether extracts were
combined and dried over anhydrous sodium sulfate. The
solvent was evaporated to leave 10.74 g of oily residue (crude
2-methyl-1-(3-methylbut-2-enyl) indoline, 2, in Scheme 1).
The oily residue was dissolved slowly in 50 ml of cold
concentrated sulfuric acid (0 uC), and then was poured into
ice water. After being neutralized by saturated NH4OH
solution, the resulting slurry was extracted with four 150 ml
portions of ether. The extracts were combined and dried over
anhydrous sodium sulfate. Removal of ether left an oily
residue, which, after crystallization from 5 ml of 95% ethanol,
Shimadzu UV-1601PC UV-Vis spectrophotometer and a
Hitachi F-4500 fluorescence spectrophotometer, respectively.
The redox potentials were measured by cyclic voltammetry
on an EG&G Instruments Potentiostat/Galvanostat
(Model 283) with a scanning rate of 100 mV s21, in which a
three-electrode single-compartment electrochemical cell was
used with two platinum wires as working and counter
electrodes, and a saturated calomel electrode (SCE) as
reference. A 0.1 M solution of Bu4NClO4 in CHCl3 was used
as the electrolyte and was flushed with N2 prior to the
measurements to avoid oxygen contamination. 2,9-Dimethyl-
4,7-diphenyl-1,10-phenanthroline (BCP) was purchased
from Aldrich. 4-(2H-Indene-1,3-dione)-2-tert-butyl-6-(juloli-
dyl-9-enyl)-4H-pyran (IN-1), (2-(2-tert-butyl-6-((E)-2-(2,6,6-
trimethyl-2,4,5,6-tetrahydro-1H-pyrrolo[3,2,1-ij]quinolin-8-yl)
vinyl)-4H-pyran-4-ylidene)-2H-indene-1,3-dione (IN-2), 2-(2-
tert-butyl-6-((E)-2-(1,2,3,4-tetrahydro-4,4-dimethyl-1-phenyl-
quinolin-6-yl)vinyl)-4H-pyran-4-ylidene)-2H-indene-1,3-dione
(IN-3), and 2-(2-tert-butyl-6-((E)-2-(1,2,3,4-tetrahydro-4,4-
dimethyl-1-phenylbenzo[h]quinolin-6-yl)vinyl)-4H-pyran-4-
ylidene)-2H-indene-1,3-dione (IN-4) were synthesized through
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gave 8.66 g (86% yield based on 1) of 2,6,6-trimethyl-2,4,5,6-
tetrahydro-1H-pyrrolo[3,2,1-ij]quinoline (3 in Scheme 1).
monitor. The deposition rates for the organic layers and the
cathode were 0.1 and 6 nm s21, respectively. The emissive area
of the device was 2 mm 6 2.5 mm. The EL devices fabricated
in this work have a configuration of ITO/NPB (60 nm)/Alq3
(or Gaq3) : red dopant (x wt%, 7 nm)/BCP (12 nm)/Alq3
(45 nm)/LiF (0.3 nm)/Al (300 nm), in which NPB was used as
the hole-transporting layer, doped Alq3 (or doped Gaq3) as the
emissive layer, BCP as the hole-blocking layer, and undoped
Alq3 as the electron-transporting layer. The emissive layer was
prepared by co-evaporating the red dopant and Alq3 (or Gaq3)
simultaneously from two sources at calibrated rates to achieve
the desired mass ratio. The EL spectra and brightness–current
density–voltage characteristics of the devices were measured
with a PR650 Spectra Scan in air at room temperature.
Then
2,6,6-trimethyl-2,4,5,6-tetrahydro-1H-pyrrolo[3,2,1-
ij]quinoline-8-carbaldehyde (4 in Scheme 1) was prepared
from 3 via the Vilsmeier reaction using 3.0 equiv of POCl3/
DMF as condensation agent (75% yield based on 3). Finally,
the synthesis was completed by condensation of 4 with 2-(2-
tert-butyl-6-methyl-4H-pyran-4-ylidene)-2H-indene-1,3-dione
(15) to give the target molecule IN-2 in 80% yield (based on 4).
The intermediate 15 was synthesized using the approach
reported by us recently.28 The chemical structure of IN-2 was
characterized as follows. 1H NMR (CD3COCD3, 300 MHz): d
(ppm) = 8.40–8.32 (m, 2 H), 7.79–7.73 (d, J = 15.99 Hz, 1 H),
7.70–7.67 (m, 2 H), 7.58 (s, 1 H), 7.53 (s, 1 H), 7.46 (s, 1 H),
7.34 (s, 1 H), 6.96–6.90 (d, J = 15.87 Hz, 1 H), 3.68–3.63 (m,
1 H), 3.36–3.28 (m, 1 H), 3.17–3.09 (m, 1 H), 2.96–2.85 (m,
1 H), 2.63–2.54 (m, 1 H), 2.0–1.7 (m, 2 H), 1.5–1.4 (m, 18 H);
MS m/z 505 (M+); Anal. Calc. for C34H35NO3 C, 80.76; H,
6.98; N, 2.77; Found. C, 80.81; H, 6.94; N, 2.73%.
Theoretical methods
All calculations were done on the Gaussian03 program
package29 by using density functional theory (DFT). Becke’s
three-parameter functional30 combined with Lee, Yang, and
Parr’s correlation functional31 (B3LYP), along with the
6-31G(d) basis set, was used. Geometry optimizations attempt
to locate the minima on the potential energy surface to predict
equilibrium structures of a given molecule. Before performing
optimization of ground-state geometries by B3LYP/6-31G*,
the molecular structures were initially optimized in semi-
empirical PM332,33 calculations as recommended in the
Gaussian03 program package. Single-point energy calculations
of the electronic properties of the DCM-type dyes at their
optimized ground-state geometries were carried out by
utilizing DFT at the B3LYP/6-31G(d) level. All the geometries
and electronic properties were calculated by assuming the
target molecules to be isolated molecules.
IN-1, IN-3, and IN-4 were prepared similarly (Scheme 1),
and their structure characterization data are as follows.
IN-1: dH (400 MHz; CDCl3; Me4Si) 8.41–8.40 (1 H, J =
1.7 Hz, d), 8.32–8.33 (1 H, J = 1.7 Hz, d), 7.60–7.73 (4 H, m),
7.4–7.5 (1 H, J = 15.8 Hz, d), 7.18 (2 H, s), 6.9–6.8 (1 H, J =
15.8 Hz, d), 3.3–3.2 (4 H, J = 5.6 Hz, t), 2.76–2.73 (4 H, J =
5.5 Hz, t), 1.98–1.84 (4 H, m), 1.44 (9 H, s); MS m/z 477 (M+);
Found. C, 80.53; H, 6.52; N, 2.90. Calc. for C32H31NO3 C,
80.47; H, 6.54; N, 2.93%.
IN-3: dH (400 MHz; CDCl3; Me4Si) 8.42 (2 H, s), 7.72–7.76
(2 H, m), 7.56–7.61 (2 H, m), 7.45–7.46 (1 H, J = 2 Hz, d),
7.40–7.44 (2 H, J = 7.6 Hz, t), 7.33–7.37 (1 H, J = 15.9 Hz, d),
7.25–7.28 (2 H, m), 7.19–7.24 (1 H, J = 7.4 Hz, t), 7.13–7.15
(1 H, J1 = 8.6 Hz, J2 = 2 Hz, m), 6.65–6.69 (1 H, J = 15.9 Hz,
d), 6.62–6.64 (1 H, J = 8.6 Hz, d), 3.68–3.71 (2 H, J = 6 Hz, t),
1.89–1.92 (2 H, J = 6 Hz, t), 1.43 (6 H, s), 1.44 (9 H, s); MS m/z
541 (M+); Found. C, 82.10; H, 6.46; N, 2.62. Calc. for
C37H35NO3, C, 82.04; H, 6.51; N, 2.59%.
4. Conclusions
Using 2H-indene-1,3-dione as an electron-withdrawing group
and aromatic amines of different structures as electron-
donating groups, four new DCM-type dyes, IN-1, IN-2,
IN-3, and IN-4, were designed and synthesized for application
in red OLEDs. Due to the large dipole moment difference
which occurs in the transition from the ground state to the
excited state, IN-2 exhibits a red shift in fluorescence but a
blue shift in absorption with respect to IN-1. Similar
phenomena are also found in spectral comparisons between
IN-3 and IN-4. These findings indicate the important role of
the structure of the aromatic amine in the photophysical
properties of DCM-type dyes. In EL devices with the con-
figuration of ITO/NPB (60 nm)/Alq3 (or Gaq3) : red dopant
(2.0 wt%) (7 nm)/BCP (12 nm)/Alq3 (45 nm)/LiF (0.3 nm)/Al
(300 nm), IN-3 in Alq3 and IN-1 in Gaq3 show comparable
properties to the DCJTB–assistant dopant (rubreen or
quinacridone)–Alq3 tertiary system. This can be ascribed to
the good spectral overlap either between IN-3 and Alq3 or
between IN-1 and Gaq3, indicating that the absorption
spectrum has the same importance as the fluorescence
spectrum in the improvement of color purity of red emitters.
Following the guidelines provided in this work, it may be
possible to achieve new DCM-type red light-emitting mate-
rials, even better than DCJTB in the applications of OLEDs.
IN-4: dH (400 MHz; CDCl3; Me4Si) 8.55–8.56 (1 H, J =
1.7 Hz, d), 8.48–8.49 (1H, J = 1.7 Hz, d), 8.25–8.29 (1 H, J =
15.7 Hz, d), 8.04–8.07 (1 H, J = 8.4 Hz, d), 7.92 (1 H, s), 7.76–
7.78 (1 H, J = 8.4 Hz, d), 7.76–7.80 (2 H, m), 7.61–7.63 (2 H,
m), 7.43–7.45 (1 H, m), 7.24–7.27 (1 H, m), 7.17–7.21 (2 H, J =
7.5 Hz, m), 6.97–7.01 (1 H, J = 15.7 Hz, d), 6.91–6.94 (1 H, J =
7.34 Hz, t), 6.83–6.85 (2 H, J = 7.6 Hz, d), 3.87–3.90 (2 H, m),
1.80–1.84 (2 H, m), 1.50 (9 H, s), 1.48 (6 H, s); MS m/z 591
(M+); Found. C, 83.17; H, 6.35; N, 2.42. Calc. for C41H37NO3,
C, 83.22; H, 6.30; N, 2.37%.
Preparation and testing of electroluminescent (EL) devices
OLED devices were fabricated by thermal vacuum deposition.
The substrate was indium tin oxide (ITO)-coated glass with a
sheet resistance of about 50 ohms per square. The pre-
treatment of ITO conducting glass included a routine chemical
cleaning using detergent and alcohol in sequence, followed by
oxygen plasma cleaning. All organic layers were successively
deposited onto the ITO conducting glass substrate by thermal
evaporation at high vacuum (5 6 1026 Torr), and the
thickness of each layer was determined by a quartz thickness
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Acknowledgements
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This work was supported by NNSFC (20272065, 50225313)
and the Ministry of Science and Technology of China (973
project, No. 2002CB613401).
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