Efficient blue light-emitting diodes based on a classical "push-pull" architecture molecule 4,4′-di-(2-(2,5- dimethoxyphenyl)ethenyl)-2,2′-bipyridine

Article abstract of DOI:10.1039/b610880f

A novel, highly blue luminescent molecule containing donor and acceptor groups, 4,4′-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,2′-bipyridine, that shows photoluminescence emission at 450 nm with 43% quantum yield is designed and synthesized. Time dependent-DF

Full text of DOI:10.1039/b610880f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Efficient blue light-emitting diodes based on a classical ‘‘push–pull’’
architecture molecule 4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-
2,29-bipyridine{
D. Berner,a C. Klein,b Md. K. Nazeeruddin,*b Filippo De Angelis,c M. Castellani,d Ph. Bugnon,d
R. Scopelliti,b L. Zuppirolid and M. Graetzelb
Received 31st July 2006, Accepted 7th September 2006
First published as an Advance Article on the web 28th September 2006
DOI: 10.1039/b610880f
A novel, highly blue luminescent molecule containing donor and acceptor groups, 4,49-di-(2-(2,5-
dimethoxyphenyl)ethenyl)-2,29-bipyridine, that shows photoluminescence emission at 450 nm
with 43% quantum yield is designed and synthesized. Time dependent-DFT calculations
show an excellent correlation between theoretical and experimental absorption spectra, thus
allowing for a detailed description of the electronic structure and assignment of the main
absorption features. An optimized organic light-emitting diode based on a 4,49-di-(2-(2,5-
dimethoxyphenyl)ethenyl)-2,29-bipyridine blue emitting layer, with a 4,49,40-tris(carbazol-9-yl)-
triphenylamine) hole transporting layer, a 2,9-dimethyl-4.7-diphenyl-phenatroline hole
blocking layer, and a tris(8-hydroxyquinoline)aluminium electron transport layer exhibited
2.1% quantum efficiency.
Introduction
2 Experimental
Organic light-emitting diodes (OLEDs) are a promising
technology for the display market and several groups have
reported significant progress in this area.1–10 The swift
advancement in OLED technology with materials emitting
in the whole of the visible range has enabled the realization
of full color displays.11–13 However, a high performance,
blue light-emitting material is still in demand, specifically
for the emission of light at the blue end of the visible
spectrum, which is crucial for many applications, including
the construction of organic color displays that can
compete with LCDs.14–17 Therefore, engineering of novel
materials at a molecular level which exhibit efficient blue
light emission is paramount. Herein, we describe the
2.1 Materials and measurements
All the solvents and chemicals unless stated otherwise, were
purchased from Fluka, puriss grade. 4,49-Dimethyl-2,29-bipyr-
idine (dmbpy) was obtained from Aldrich and used as received.
UV/Vis and fluorescence spectra were recorded in a 1 cm-path
length quartz cell on a Cary 5 spectrophotometer and Spex
Fluorolog 112 Spectrofluorimeter, respectively. Electrochemi-
cal data were obtained by using a PC-controlled AutoLab
PSTAT10 electrochemical workstation (Eco Chimie), with a Pt
disk working electrode, a Pt coil auxiliary electrode and a silver
disk quasi reference electrode were mounted in a single-
compartment cell configuration. Proton and 13C NMR spectra
were measured on a Bruker 200 MHz spectrometer. The
reported chemical shifts were in ppm against TMS.
meticulously-designed
novel
molecule,
4,49-di-(2-(2,5-
dimethoxyphenyl)ethenyl)-2,29-bipyridine, based on ‘‘push–
pull’’ architecture (methoxy pushing and the pyridine pulling
groups), which is an exceptional blue luminophore with a
quantum yield of 43 ¡ 10%, and address its application
to OLEDs.
The quantum yields were determined using the following
equation:
aCFG Microelectronic, CH-1110 Morges, Switzerland
bLaboratory for Photonics and Interfaces, Institute of Chemical Sciences
and Engineering, School of Basic Sciences, Swiss Federal Institute of
Technology, CH-1015 Lausanne, Switzerland.
where the subscript X denotes the N945L substance whose
quantum yield is determined, and r is the reference substance
quininesulfate (3
6 M in 1 M H2SO4), whose
1025
E-mail: MdKhaja.Nazeeruddin@epfl.ch
cIstituto CNR di Scienze e Tecnologie Molecolari (ISTM-CNR), c/o
Dipartimento di Chimica, Universita` di Perugia, I-06123, Perugia, Italy
dLaboratoire d’Optoe´lectronique des Mate´riaux Mole´culaires, Institut
des Mate´riaux, Ecole Polytechnique Fe´de´rale, CH-1015 Lausanne,
Switzerland
luminescence quantum yield is assumed to be 0.545.18,19 Kopt
is the optical factor referring to the higher refractive index
of the dichloromethane (nD = 1.424) compared to the water
solution (nD = 1.333); D is the integrated area under the
emission spectrum; Aexc is the absorbance at the exciting
wavelength.
{ Electronic supplementary information (ESI) available: Crystal
structures, differential scanning calorimetry. See DOI: 10.1039/
b610880f
4468 | J. Mater. Chem., 2006, 16, 4468–4474
This journal is ß The Royal Society of Chemistry 2006

View Article Online
2.2 Synthesis of 4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-
bipyridine (N945L)
The 4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-bipyridine
was obtained in a one-pot synthesis using a modified literature
procedure.20 Solid t-BuOK (7.31 g, 65.1 mmol) was added
in one portion to a mixture of 4,49-dimethyl-2,29-bipyridine
(3.00 g, 16.3 mmol) and 2,5-dimethoxybenzaldehyde ( 8.12 g,
48.8 mmol) in anhydrous DMF (200 ml). The resulting
mixture was heated to 100 uC for 6 h under nitrogen. After
being cooled to room temperature, DMF was evaporated and
methanol (200 ml) was added resulting in the formation of a
yellow precipitate. The solid was filtered and washed with
methanol (5 6 50 ml) to afford 5.7 g (73%) of the desired
compound as a slightly yellow fluorescent solid. The DSC
curve of N945L (see Fig. S1 in the ESI{) shows a sharp melting
temperature of 195 uC.
Scheme 1 Synthetic scheme for 4,49-di-(2-(2,5-dimethoxyphenyl)ethe-
nyl)-2,29-bipyridine.
Table 1 Structural data and refinement for N945L
Identification code
Empirical formula
Formula weight
Temperature
N945L
C30H28N2O4
480.54
140(2) K
0.71073 A
Monoclinic
˚
Wavelength
1H NMR (CDCl3) d 8.69 (2H, d, J = 4.8 Hz, pyridine), 8.61
(2H, s, pyridine), 7.47 (2H, J = 16.4 Hz, d, vinylic), 7.43 (2H, d,
J = 4.8 Hz, pyridine), 7.32 (2H, t, J = 7.6 Hz, methoxyphenyl),
7.18 (2H, d, methoxyphenyl), 7.14 (4H, d, J1 = 16 Hz, broad s,
vinylic + methoxyphenyl), 6.90 (1H+1H, d, J1 = 7.6 Hz,
broad s), 3.87 (s, 12H, –OCH3). 13C NMR, 49.5, 148.9, 133.3,
129.8, 126.4, 124.9, 122.2, 121.1, 119.7, 118.4, 114.5, 112.1,
55.3, 21.2. IR (KBr) 3306, 2937, 1598, 1487, 1462, 1435, 1263,
1149, 1046, 783, 700. Anal. calcd for C30H28N2O4: C, 74.98; H,
5.87; N, 5.83; found: C, 74.69; H, 6.11; N, 5.84.
Crystal system
Space group
Unit cell dimensions
P2(1)/n
A = 7.6670(8) A
˚
a = 90u
b = 99.379(7)u
c = 90u
˚
B = 13.7182(13) A
˚
C = 11.7334(8) A
3
˚
1217.60(19) A
Volume
Z
Density (calculated)
2
1.311 mg m23
Absorption coefficient 0.087 mm21
F(000)
508
0.14 6 0.12 6 0.08 mm
3.08 to 25.02u.
Crystal size
Theta range for data
collection
Index ranges
29 ,= h ,= 8,
216 ,= k ,= 16,
213 ,= l ,= 13
7148
2.3 Device architecture aspects containing the new blue emitter
4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-bipyridine
Reflections collected
Independent reflections 2091 [R(int) = 0.0426]
Completeness to
theta = 25.02u
Absorption correction Semi-empirical from
The general architecture of the complex multi-layer diodes
used in this study is as follows. The ITO (indium–tin oxide)
coated glass substrates (18 V %21, Merck) were first cleaned
in ethanol, acetone and soap ultrasonic baths. Prior to
evaporation of the organic materials, the ITO electrodes
97.4%
equivalents (MULABS)
0.9974 and 0.9588
Max. and min.
transmission
Refinement method
were oxygen-plasma cleaned for
hole injection improvement. Then a 40 nm layer of either
a-NPD (N,N9-diphenyl-N,N9-bis(1-naphthyl)-1,19biphenyl-
4 min at 10 W for
Full-matrix least-
squares on F2
2091/0/163
Data/restraints/
parameters
Goodness-of-fit on F2
Final R indices
[I . 2sigma(I)]
R indices (all data)
Largest diff. peak
and hole
0.865
R1 = 0.0349, wR2 = 0.0630
4,4diamine) (samples 1 and 2) or TCTA (4,49,40-tris(carba-
zol-9-yl)-triphenylamine) (samples 3 and 4) was evaporated as
a hole transporting layer, followed by a 15 nm layer of the new
blue emitter molecule N945L. For hole blocking and the
resulting charge confinement in the emitting layer, we used for
samples 2 and 3 a 10 nm layer of BCP (2,9-dimethyl-4.7-
diphenyl-phenatroline). To separate the emitting zone from the
cathode a 40 nm thick electron transport layer of Alq3 (tris(8-
hydroxyquinolato)aluminium) was deposited. All organics
R1 = 0.0873, wR2 = 0.0740
23
˚
0.145 and 20.133 e A
3 Results and discussion
The 4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-bipyridine
(N945L) was conveniently synthesized by reaction of solid
tert-BuOK with 4,49-dimethyl-2,29-bipyridine and 2,5-
dimethoxybenzaldehyde in anhydrous DMF yielding 73%, as
shown in Scheme 1. Single crystals of N945L were prepared by
vaporizing a mixture of chloroform and methanol (1 : 2) slowly
(see the ESI{ for structural data). The crystal packing is
determined by a weak graphitic interaction between neigh-
bouring molecules (see Table 1).{ The distance between the
were purified by gradient sublimation and thermally evapo-
21
rated at a rate of 1.0 A s at a base pressure of around
˚
5 6 1027 mbar. A 0.8 nm LiF layer was then deposited right
after the Alq3, at a rate of 0.2 A s21. The finishing Al electrode
˚
21
˚
(cathode) was deposited at a rate of 10 A s
in another
chamber without breaking the vacuum. The active area of the
diode segments was 12 mm2. The current versus voltage I–V
characteristics of each device were measured in a nitrogen
atmosphere glove box by means of a LabView controlled
Keithley 236 measuring unit. Simultaneously the light-output
was measured by a photodiode, calibrated with a Minolta
LS110 luminance meter.
{ CCDC reference number 619928. For crystallographic data in CIF
format see DOI: 10.1039/b610880f
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 4468–4474 | 4469

View Article Online
Fig. 1 Absorption (red line) and emission (blue line) spectra of N945L in dichloromethane solution, lex 350 nm, at 298 K. The calculated
absorption spectrum obtained using a Gaussian convolution with s = 0.2 eV is also reported (purple line) together with calculated vertical
excitation energies and oscillator strengths. The calculated data have been rescaled so as to match the intensity of the experimental 358 nm band.
The insert shows a photo of the N945L solution, exhibiting very strong blue emission when excited under an ultraviolet lamp (365 nm).
centroids belonging to one pyridine ring of the first molecule
and to one 2,5-dimethoxyphenyl moiety of the second is
˚
3.830 A. There are also two weak intermolecular hydrogen-
methoxy substituents on the electronic structure of the
corresponding 4,49-di-(2-(3,4-dimethoxyphenyl)ethenyl)-2,29-
bipyridine) species. For the investigated species we found a
transoid structural arrangement similar to the experimental
X-ray structure of N945L (Fig. 2a). The main bond lengths
and angles are well reproduced by our calculations, despite a
slight overestimate of the aromatic C–C bonds and of the
double C–C bonds (see Table S7 in the ESI{). Most notably,
our calculations are able to capture the out-of-plane displace-
ment of the aromatic rings bearing the methoxy substituents
with respect to the pyridines and double carbon bonds planes,
as indicated by a dihedral angle of 12.8u, to be compared to an
X-ray value of 10.4u. Substituting the 2,5-methoxy groups in
N945L in the 3,4 positions leads to negligible changes in the
bond distances and bond angles (see Table S1 in the ESI{),
however, an almost planar arrangement (dihedral angle 3.6u)
of the overall molecular structure is computed in this case,
reflecting the reduced steric hindrance exerted by the methoxy
substituents to the ethenyl-2,29-bipyridine in 2,5 rather than in
3,4 positions. Inspection of the electronic structure of the
N945L ligand, shows that the highest occupied molecular
orbital (HOMO) and the HOMO21 constitute an almost
degenerate pair, lying at 25.26 and 25.29 eV, resulting from
in-phase and out-of-phase combinations of p-bonding orbitals
localized on the aromatic moieties bearing the methoxy
substituents and on the double C–C bonds, see Fig. 2b. At
lower energy the HOMO22/HOMO23 degenerate couple,
lying at 26.05 eV, has a similar p character and localization
as the HOMO/HOMO21, while the HOMO24, found
at 26.17 eV, is a p orbital of the two pyridine moieties.
The lowest unoccupied molecular orbital (LUMO) and the
LUMO+1, lying at 21.66 and 21.52 eV, are, respectively,
in-phase and out-of-phase p* combinations mainly localized
on the double C–C bonds and on the pyridine moieties, see
Fig. 2b. An essentially identical electronic structure is
calculated for the 3,4 isomer, even though the HOMO21/
LUMO+1 set are slightly shifted at higher energies
bonds occurring between a methyl and N1 or O1. The angle
between the pyridine and the 2,5-dimethoxyphenyl group is
12.79(8)u whereas to understand the position of the double
˚
bond [C6–C7, 1.334(2) A] these two torsion angles are
reported: C2–C3–C6–C7, 2176.70(19); C6–C7–C8–C13,
2170.39(18)u. These values show that the double bond is
almost coplanar with the pyridine whereas there is some
deviation from planarity with respect to the 2,5-dimethoxy-
phenyl system. The molecule lies on a crystallographic
inversion centre that is in the middle of the C–C bond linking
the pyridine rings.
The absorption spectra of N945L in dichloromethane show
bands in the UV region at 232 nm (e = 33140), 268 nm (e =
36510), 300 nm (e = 54300), 358 nm (e = 40410) due to p–p*
charge transfer transitions.21,22 Upon excitation at 350 nm, a
dilute (5 6 1027 M) solution of N945L in dichloromethane at
22 uC exhibited intense emission centered at 450 nm (Fig. 1)
due to substitution of donor methoxy groups on the phenyl
ring, in the ortho- and meta- positions. The position of the
methoxy groups is vital for high quantum yields and the
other isomers 4,49-di-(2-(3,4-dimethoxyphenyl)ethenyl)-2,29-
bipyridine,
4,49-di-(2-(2,4,6-trimethoxyphenyl)ethenyl)-2,29-
bipyridine show significantly low quantum yields in solution
at room temperature.
To gain insight into the electronic and optical properties of
the 4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-bipyridine,
we performed density functional theory (DFT) and time-
dependent-DFT (TD-DFT) calculations on its geometry,
electronic structure and absorption spectra. By using the
B3LYP exchange–correlation functional23 together with a
6-31G* basis set, as implemented in the Gaussian 03
program,24 we optimized, without symmetry constraints, the
geometry of N945L (4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-
2,29-bipyridine) and, to check the effect of the position of the
4470 | J. Mater. Chem., 2006, 16, 4468–4474
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Fig. 2 (a) The structure of 4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-bipyridine. The letter
A
stands for the following symmetry
transformation: 2x, 2y, 2z. (b) Isodensity plots (isodensity value 0.035) of the HOMO21, HOMO, LUMO and LUMO+1 of the N945L species.
(25.24/25.21/21.61/21.45 eV), resulting in an identical
HOMO–LUMO gap. On the basis of the similar geometrical
and electronic structures of the species with methoxy
substituents in the 2,5 and 3,4 positions, one would guess
similar electronic and optical properties. This, however, does
not appear to be the case, since the species substituted in the
3,4 positions show reduced luminescence quantum yields. This
might be related to the reduced donation capability of the
methoxy substituents bound in the 3,4 positions compared to
the 2,5 positions, as reflected by the increased calculated
Mulliken charges on the dimethoxyphenyl carbons bound to
the ethenyl moieties (+0.1 vs +0.2 e, in 2,5- and 3,4-
dimethoxyphenyl species, respectively).
Inclusion of solvation effects by means of the polarizable
continuum model25 leads to a sizable downshift in the energy
of the N945L molecular orbitals. In particular, in CH2Cl2
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 4468–4474 | 4471

View Article Online
Table 2 Calculated TD-DFT wavelength (l/nm), oscillator strength
(f) and composition in terms of molecular orbital excitations for the
most relevant transitions. Experimental values of the absorption
maxima (lmax/nm) are reported for a direct comparison
Experimental
lmax/nm
l/nm
f
Character
379.6
361.9
313.2
302.8
296.6
290.2
250.8
0.817
0.337
0.516
0.353
0.148
0.353
0.227
HOMO21ALUMO
HOMOALUMO+1
HOMO24ALUMO
HOMO23ALUMO
HOMO22ALUMO+1
HOMO21ALUMO+2
HOMO23ALUMO+2
358
300
268
solution we find the HOMO21, HOMO, LUMO and
LUMO+1 at 25.42, 25.40, 21.89 and 21.72 eV, respectively.
Moreover, including diffuse functions and increasing the basis
set quality up to 6-311+G* in CH2Cl2, leads to a further shift
of the orbital energies, with the HOMO21/LUMO+1 energies
calculated at 25.75, 25.73, 22.30 and 22.13 eV, respectively.
Overall, while basis set and solvation effects do not signifi-
cantly alter the HOMO–LUMO gap, absolute orbital energies
appear to be sensitive to both factors.
Fig. 3 I–V line of devices 2–4. Shortly characterized by TCTA
without BCP, TCTA with BCP and a-NPD with BCP.
substrates using the following architecture. A 40 nm layer
of either a-N,N9-diphenyl-N,N9-bis(1-naphthyl)-1,19-biphenyl-
4,40-diamine (NPD) (samples 1 and 2) or 4,49,40-tris(carbazol-
9-yl)triphenylamine (TCTA) (samples 3 and 4) was evaporated
as a hole transporting layer on the indium–tin oxide (ITO)
coated glass substrates, followed by a 15 nm layer of the blue
emitter molecule N945L. For hole blocking and resulting
charge confinement in the emitting layer we used for samples 2
and 3, a 10 nm layer of 2,9-dimethyl-4.7-diphenyl-phenatroline
(BCP). To separate the emitting zone from the cathode a
40 nm thick electron transport layer of the tris(8-hydroxy-
quinolato)aluminium (Alq3) was deposited. Fig. 3 shows the
different current–voltage [I–V] characteristics of device
samples 2–4 (sample 1 corresponds mainly to sample 2
therefore is not discussed). The voltage region below y2.5–
3 V is characterized by a single carrier type transport. Above
this threshold voltage, we observe a steep increase in the diode
current corresponding to two-carrier injection, which leads to
light generation. For current densities higher than 1 mA cm22
the I–V lines come into the space charge density regime. We
found, that the current threshold field (Uth) here defined as the
voltage for a current density of 1 mA cm22, is lowest for sample
4 with a TCTA hole transport layer and without a hole
blocking layer. The introduction of BCP hole blocking layer
increases the threshold field of about 15%. This proportion-
ality factor is valid for the whole current range including the
space charge density regime. The replacement of TCTA by the
standard hole transport material a-NPD the hole injection
barrier increase of nearly 0.2 eV (HOMO–HOMO levels) and
the I–V line shifts once more about 10% to higher voltages.
However, the hole energy barrier from N945L to Alq3 is
negligible. (see Table 3 for electrochemical data, which are
calculated from experimental value in V vs NHE plus 4.5 eV).
Fig. 4 shows the electroluminescence spectra of samples 2–4.
For the TCTA based devices a blue-shift of the emission
spectra with the introduction of the HBL is seen. It was
necessary to introduce a hole blocking layer (HBL) into our
diode composition to confine the recombination zone inside
the blue emission layer, since without the HBL recombination
takes place mainly inside the Alq3 layer (emission maximum
at the Alq3 typical wavelength of 530 nm).11 On the other
Analysis of the TD-DFT results allows us to assign the
character of the main transitions. Calculated wavelength,
oscillator strength (f) and molecular orbital excitations for the
most relevant transitions of the UV-vis absorption bands of
N945L are reported in Table 2, together with the correspond-
ing experimental data, while a comparison of the calculated
and experimental spectra is reported in Fig. 1. The band
experimentally found at 358 nm appears to be constituted by
two transitions of relevant intensity calculated at 379.6 (f =
0.817) and 361.9 nm (f = 0.337), which originate from
the almost degenerate HOMO21/HOMO couple to the
LUMO and LUMO+1, respectively. HOMOALUMO and
HOMO21ALUMO+1 transitions have zero oscillator
strengths. The band experimentally found at 300 nm appears
to be composed by four transitions of relevant intensity (f =
0.516, 0.353, 0.148 and 0.353) involving excitations from the
HOMO24/HOMO21 to the LUMO/LUMO+2. The most
relevant transition characterizing the 268 nm shoulder (f =
0.227) involves excitation from the HOMO24 to the
LUMO+2. We therefore see that the 358 nm band has a
pAp* character, involving charge transfer from the aromatic
moieties bearing the methoxy substituents (donor groups) to
the pyridine moieties (acceptor). The 300 nm band is instead
assigned to as a pAp* transition of mixed character, involving
both charge redistribution within the pyridine moieties and
charge transfer from the lower-lying HOMOs localized on the
aromatic moieties bearing the methoxy substituents to the
pyridines. The excited state giving rise to the 268 nm shoulder
involves charge transfer from the lower-lying HOMOs
localized on the aromatic moieties bearing the methoxy
substituents to the pyridines.
The N945L shows excellent thermal stability and strong
pure blue emission in solids, when excited under an ultraviolet
lamp (365 nm), rendering it as good OLED material.
To investigate the electroluminescence properties of the
4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-bipyridine mole-
cule OLED devices were fabricated on conducting glass
4472 | J. Mater. Chem., 2006, 16, 4468–4474
This journal is ß The Royal Society of Chemistry 2006

View Article Online
Table 3 Energy levels of the used materials
Molecule
HOMO/eV
LUMO/eV
a-NPD
TCTA
N945L
BCP
25.53
25.694
y25.8
26.7
.22.6
.22.6
y22.9
23.2
Alq3
Anode: ITO
Cathode: LiF/Al
25.795
25.8
23.5
23.0
Fig. 5 External quantum efficiency as a function of the current
density.
The impact of the different device architectures on the
quantum efficiency is shown in Fig. 5. The TCTA devices with
a reduced hole injection barrier between the HTL and the
emitting layer show the highest quantum efficiency, 2.1%. The
absolute value of the quantum efficiency for the TCTA based
devices is independent of the introduction of the BCP layer
and to the best of our knowledge, 2.1% efficiency is one of the
highest reported quantum efficiencies for a bulk blue singlet
emitting material cited in the recent literature. This impressive
value of bulk quantum efficiency also necessitates well
balanced charge relations inside the emission layer, without
any leaking current neither to the anode nor to the cathode.
Prior experiments shown us the way to get two extended well
working electrodes with ITO–oxygen plasma treatment and
LiF/Al,26–29 which obviously provide a good charge balance,
taking into account the hole blocking capabilities of BCP and
the electron blocking capabilities of a-NPD or TCTA.17
The reduced quantum efficiency for the a-NPD based
devices indicates that if the hole injection barrier is high
enough, a non-radiative recombination channel becomes
relevant, most probably non-radiative exciplex deactivation.
The possibility of electron leakage current to the anode side
can be ruled out.29
Fig. 4 Emission spectra of samples 2 to 4 are presented. The
intensities are taken at 5 mA cm22
.
hand, in the diodes with HBL the emission takes place in
N945L just near the N945L/BCP interface as the electrons are
entering the blue emitting material.
In the cases of the a-NPD based devices, the increased hole
injection energy barrier slows down the hole current in N945L
and reduces the Alq3 contribution to the emission spectra. An
Alq3 contribution to the emission spectra is visible only for
current densities below 1 mA cm22 and nearly disappears for
higher current densities (results not presented here). The
emission spectra of Fig. 4 were taken at the same current
densities of 50 mA cm22. It is evident from the Figure that all
the emitted light of the diodes containing the BCP layer comes
directly from the N945L molecules, because the same emission
double structure is found from a highly concentrated solution
luminescence spectra of the N945L molecule. The photo-
luminescence emission peak around 490 nm was attributed to
excimer emission, as it is the dominant feature of higher
concentrated solutions. In high dilute solutions the monomer
emission in the blue around 450 nm is dominant. The CIE
coordinates for various device architectures are included in
Table 4.
Another characteristic of our novel singlet bulk emitting
material is that the maximum of the external quantum
efficiency (QE) is located at the technological relevant current
densities around 7 to 10 mA cm22 (4.8 V) and that for higher
current densities a strong decrease in the QE is seen. This
kind of decrease is discussed as arising from singlet–singlet
annihilation effect and for further current density increase
non-reversible destruction appears.30
Table 4 A summary of the operating conditions for the investigated devices
Sample
500 cd m22
Maxima
Device architecture
a-NPD
a-NPD with HBL
TCTA with HBL
At bias
5.8 V
6.9 V
5.3 V
gext
Brightness
gext-max (%)
g
power-max/lm W21
Color coordinates CIE 1931
(0.170, 0.294)
(0.232, 0.475)
1.1%
1.0%
1.7%
1998 cd m22
1887 cd m22
883 cd m22
1.2% (6.3 V)
1.1% (7.3 V)
2.06% (4.7 V)
1.7 (at 2.9 V)
2.0 (at 2.9 V)
3.3 (at 4.1 V)
(0.164, 0.305)
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 4468–4474 | 4473

View Article Online
9 C.-H. Yang, S.-W. Li, Y. Chi, Y.-M. Cheng, Y.-S. Yeh, P.-T. Chou
and G.-H. Lee, Inorg. Chem., 2005, 44, 7770.
10 M. R. Craig, M. M. de Kok, J. W. Hofstraat, A. P. H. J. Schenning
and E. W. Meijer, J. Mater. Chem., 2003, 13, 2861–2862.
11 V. A. Montes, G. Li, R. Pohl, J. Shinar and J. P. Anzenbacher,
Adv. Mater., 2004, 16, 2001.
12 Y. You and S. Y. Park, J. Am. Chem. Soc., 2005, 127, 12438.
13 S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu and C.-H. Chen, Adv. Mater., 2005, 17, 285.
14 H. Kanno, Y. Sun and S. R. Forrest, Appl. Phys. Lett., 2005, 86,
263502.
Therefore in the a-NPD based devices with increasing hole
injection energy barrier the emission zone confines near the
a-NPD/N945L interface as deduced from the quantum
efficiency plot.
From the Table we see that in spite of the low quantum
efficiency of the a-NPD based devices, they show the highest
brightness. This leads to a quandary for the choice of the
architecture with the best performance. Therefore, further
optimization should be conducted.
15 P. Coppo, M. Duati, V. N. Kozhevnikov, J. W. Hofstraat and
L. De Cola, Angew. Chem., Int. Ed., 2005, 44, 1806.
16 P. Coppo, E. A. Plummer and L. De Cola, Chem. Commun., 2004,
1774.
4 Conclusion
We have synthesized a novel blue luminophore molecule
17 M. K. Nazeeruddin, R. Humphry-Baker, D. Berner, B. S. Rivier,
L. Zuppiroli and M. Gra¨tzel, J. Am. Chem. Soc., 2003, 125, 8790.
18 C. A. Parker, Measurement of Fluorescence Efficiency, Elsevier
Publishing Co, 1968.
19 T. Kartens and K. Kobs, J. Phys. Chem., 1980, 84, 1871.
20 M. K. Nazeeruddin, Q. Wang, L. Cevey, V. Aranyos, P. Liska,
E. Figgemeier, C. Klein, N. Hirata, S. Koops, S. A. Haque,
J. R. Durrant, A. Hagfeldt, A. B. P. Lever and M. Gra¨tzel, Inorg.
Chem., 2006, 45, 787.
21 K. A. Walters, L. L. Premvardhn, Y. Liu, L. A. Peteanu and
K. S. Schanze, Chem. Phys. Lett., 2001, 339, 255.
22 M. K. Nazeeruddin, Q. Wang, L. Cevey, V. Aranyos, P. Liska,
E. Figgemeier, C. Klein, N. Hirata, S. Koops, S. A. Haque,
J. R. Durrant, A. Hagfeldt, A. B. P. Lever and M. Gra¨tzel, Inorg.
Chem., 2006.
4,49-di-(2-(2,5-dimethoxyphenyl)ethenyl)-2,29-bipyridine
by
engineering at the molecular level, which exhibits a photo-
luminescence quantum yield of 43%, and an OLED efficiency
of 2.1%, which is one of the highest blue OLEDs reported in
the literature for a singlet bulk emitter. Interestingly, the
4,49-di-(2-(phenyl)ethenyl)-2,29-bipyridine is highly fluorescent
only when the two donor groups are substituted on phenyl
ring in the ortho- and the meta-positions, demonstrating the
significance of the substitution effect. Our study shows that
simple design principles can lead to desired properties with
practical applications.
23 A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
24 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth,
P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz,
Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaroni, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN
03, revision B.05, Gaussian, Inc., Wallingford, CT, 2004.
25 M. Cossi, N. Rega, G. Scalmani and V. J. Barone, Comput. Chem.,
2003, 24, 669.
Acknowledgements
We acknowledge financial support of this work by the Swiss
Federal Office for Energy (OFEN) and the European
Commission (FP6 HETEROMOLMAT/Contract no.:
516982), and thank Jacques-E. Moser, Davide Di Censo,
K. Kalyanasundaram, Morii Katsuyuki, Q. Wang, C. Miliani
and Dr Robin Humphry-Baker for their helpful discussion and
kind assistance.
References
1 B. B. Jang, S. H. Lee and Z. H. Kafafi, Chem. Mater., 2006, 18, 449.
2 E. Holder, B. M. W. Langeveld and U. S. Schubert, Adv. Mater.,
2005, 17, 1109.
3 R. Sheat, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller,
R. Moon, D. Roitman and A. Stocking, Science, 1996, 273, 884.
4 S. Lamansky, P. Djurovich, D. Murray, F. Abdel-Razzaq,
H.-E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and
M. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304.
5 M. Stockman, Nat. Mater., 2005, 3, 423.
6 I. F. Perepichka, D. F. Perepichka, H. Meng and F. Wudl, Adv.
Mater., 2005, 17, 2281.
7 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. Mackay, R. H. Friend, P. L. Burn and A. B. Holmes, Nature,
1990, 347, 539.
26 B. Masenelli, D. Berner, M. N. Bussac and L. Zuppiroli, Appl.
Phys. Lett., 2001, 79, 4438.
27 D. Berner, F. Nuesch, E. Tutis, C. Ma, X. Wang, B. Zhang and
L. Zuppiroli, J. Appl. Phys., 2004, 95, 3794.
28 D. Berner, F. Nu¨esch, E. Tutis, C. Ma, X. Wang, B. Zhang and
L. Zuppiroli, Proc. SPIE—Int. Soc. Opt. Eng., 2004, 5464, 72.
29 D. Berner, H. Houili, W. Leo and L. Zuppiroli, Phys. Status Solidi
A, 2005, 9, 202.
8 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
30 H. Nakanotani, H. Sasabe and C. Adachi, Appl. Phys. Lett., 2005,
86, 213506.
4474 | J. Mater. Chem., 2006, 16, 4468–4474
This journal is ß The Royal Society of Chemistry 2006