Novel naphthyridine-based compounds in small molecular non-doped OLEDs: Synthesis, properties and their versatile applications for organic light-emitting diodes

Article abstract of DOI:10.1039/c2nj40279c

A series of six new n-type conjugated 1,8-naphthyridine oligomers were examined in organic light emitting diodes (OLEDs). The compounds show a high fluorescence in both solution (10^-8 M) and the solid state. The naphthyridines have high glass-t

Full text of DOI:10.1039/c2nj40279c

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Novel naphthyridine-based compounds in small molecular non-doped
OLEDs: synthesis, properties and their versatile applications for organic
light-emitting diodes†
Antonio Fernández-Mato, José M. Quintela* and Carlos Peinador*
5
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A series of six new n-type conjugated 1,8-naphthyridine oligomers were examined in organic light
emitting diodes (OLEDs). The compounds show a high fluorescence in both solution (10-8 M) and the
solid state. The naphthyridines have high glass-transition (Tg = 65-105 ºC) and decomposition (Td = 380-
10 400 ºC) temperatures, reversible electrochemical reduction, and high electron affinities (2.79-3.00 eV).
Systematic variation of the spacer linkage can modulate and fine-tuning their properties. They emit a high
blue, green and yellow photoluminescence with 0.70-1.0 quantum yields. Good-performance, single-layer
emitter OLEDs with yellow to white-pink emission has also been demonstrated using these materials.
White-pink emitter give a performance with a high brightness (400 cd·m-2 at 4 V), and 0.6 cd/A. Yellow
15 emitters give the best performance with maximum brightness of 250 cd·m-2with a maximum current
efficiency of 1.2 cd/A. These results demonstrate the potential of this new class of n-type conjugated
oligomers as emitters and electron-transport materials for developing high-performance yellow to white-
pink OLEDs.
developing the materials used in single-layer OLEDs: (1) high
emission quantum yield; (2) good and balanced electron and hole
mobilities; (3) high glass-transition temperature; (4) matched
highest occupied molecular orbital (HOMO) and lowest
50 unoccupied molecular orbital (LUMO) energy levels with the
energy levels of the anodes and cathodes. The first three criteria
have to be tackled by appropriate molecular design and the fourth
Introduction
20 Since the discovery of organic light-emitting diodes (OLEDs) at
Eastman Kodak Company by Tangand Van Slykeusing small-
molecules (sm-OLEDs),1 there has been substantial effort aimed
at developing highly efficient electroluminescent (EL) materials,
and considerable progress has been made on both small-
25 molecule- and polymer-based OLEDs.2In particular, small
organic conjugated molecules with a built-in donor-acceptor
architecture have attracted increasing attention as active materials
criterion
can
be
assisted
by
surface/interface
modification/engineering of the electrodes.
55 To be useful as an electron transporter in an OLED, a given
material must be chemically and thermally stable and have an
in organic optoelectronic devices such OLEDs,3 4 organic solar
,
electron-deficient
π system. Although a large number of small
cells (OSCs),5 field effect transistors (OFETs),6 solid-state
30 lasers,7 and the possibility of innovative applications to new
classes of displays8 and light sources.9 They provide notable
advantages, however, the strong intermolecular interaction of
common conjugated organic molecules makes it extremely
difficult to dissolve them and even then they tend to form
35 crystalline domains in the film state.10 Therefore, it is of
significant importance for future OLEDs applications to design
and subsequently synthesize amorphous molecules in high purity
that exhibit high fluorescence quantum efficiency and are highly
soluble.
molecules, oligomers, and polymers have been investigated for
applications in OLEDs, the vast majority of the structure-property
60 studies have been devoted to organic materials having dominant
p-type (electron donor, hole transport) properties.13A number of
different organic molecules have been used as electron
transporters, including oxadiazoles, triazoles, phenanthrolines,
quinolines, quinoxalines, 1,5-naphthyridines, anthrazolines, 1,8-
65 naphthalimides, pyrazinoquinoxalines, and thienopyrazine
derivatives.14 The efficiency of small organic molecules as
emitters in OLEDs, particularly for display purpose, requires
highly efficient light-emitting materials with enhanced electron
injection and transport properties.15
40 Small-molecule-based OLEDs are considered more valuable than
the polymer counterparts because of the difficulty in controlling
the film thickness with polymers and the possibility of dissolution
of the first layer during the coating of the second layer by spin-
coating techniques,11 although intensive work has been made to
45 overcome this limitation.12 Several criteria need to be fulfilled for
70 Herein, we report the synthesis, full characterization, and
investigation of the theoretical modelling, photophysics,
electrochemistry, EL, and device fabrication/characterization of
new types of 1,8-naphthyridine (napy)-based molecules,
including
4,4’-di[(6’-cyano-7’-ethoxy-5’-phenyl)-1,8-
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Scheme 1 Synthesis of oligonaphthyridines 1-6.
naphthyridin-2’-yl]1,1-biphenyl (1), 3,11-dicyano-2,10-diethoxy-
4,12-diphenyl-6,7-dihydrobenzo[1,2-b:3,4-b’]1,8-binaphthyridine
(2), 1,6-di[(6’-cyano-7’-ethoxy-5’-phenyl)-1,8-naphthyridin-2’-
data were recorded on a Jasco V-650 Spectrophotometer,
fluorescence spectra were recorded on Perkin-Elmer LS45
fluorescence spectrometer. Cyclic voltammetry experiments were
made using Autolab equipment with a PGSTAT 20 potentiostat at
yl]pyrene
(3),
1,8-di[(6’-cyano-7’-ethoxy-5’-phenyl)-1,8-
5
naphthyridin-2’-yl]pyrene (4), 1,3-di[(6’-cyano-7’-ethoxy-5’- 45 25°C. A platinum electrode was used as the working electrode
phenyl)-1,8-naphthyridin-2’-yl]benzene (5), and 3,10-di[(6’-
cyano-7’-ethoxy-5’-phenyl)-1,8-naphthyridin-2’-yl]perylene (6).
As shown in Scheme 1, this new class of n-type conjugated
oligomers has a common bis(napy) core and various aromatic
and a platinum rod as the counter electrode. A silver wire was
used as a pseudo reference electrode and calibrated with
ferrocene as internal standard. All the potentials reported in this
work are referenced to the classical Fc+–Fc standard couple. All
10 substituents as conjugated spacers. This molecular design 50 experiments were carried out in acetonitrile under dry argon and
facilitates retention of n-type characteristics due to the bis-
naphthyridine core, while allowing variation of the electronic
structure (HOMO/LUMO levels) and physical properties by
means of the aromatic-anchored spacer group. Experimental
15 results show that the compounds were synthesized by Friedländer
route and have good solubility in organic solvents, excellent
thermal properties with high glass transition temperatures (Tg),
high thermal decomposition, high quantum yield and good film
properties. The potential of napy in ELs has not been vastly
20 explored, and the emission maximum of naphthyridines can be
conveniently tuned by structural modifications.9
withtetrabutylammonium perchlorate as supporting electrolyte.
Synthesis
Synthesis of 3,10-diacetylperylene (8).
To a solution of perylene (1.0 g, 5.95 mmol) in CS2 (16 mL) was
55 added anhydrous AlCl3 (17.98 mmol). The darkness solution was
cooled to 4 ºC and acetyl chloride (13.16 mmol) was added with
stirring during 10 minutes under argon atmosphere. The solution
was raised to room temperature and after 8 hours, water-ice
mixture (100 mL) was poured. The green precipitate was filtered
60 and purified by flash chromatography on silica gel using CH2Cl2
1
as eluent to give 8 (0.79 g, 40%). H NMR (CDCl3, 300 MHz):
2.75 (s, 6H, 2CH3); 7.65 (d, J = 7.8 Hz, 2H); 7.96 (d, J = 8.4 Hz,
2H); 8.23 (d, J = 8.4 Hz, 2H); 8.29 (d, J = 8.0 Hz, 2H); 8.79 (dd,
J = 8.4 Hz, J = 8.0 Hz 2H). MS (FAB+, m/z): 337.12 (M+H)+.
65 Anal. Calcd. C24H16O2: C, 85.69; H, 4.79. Found. C, 85.77; H,
4.84.
Experimental
Material and methods
Di-1,8-naphthyridines (1-5) were prepared from 2-amino-5-
25 cyano-6-ethoxy-3-formyl-4-phenylpyridine
and
the
Synthesis
naphthyridin-2’-yl]perylene (6).
solution of 2-amino-5-cyano-6-ethoxy-3-formyl-4-phenyl-
of
3,10-bis[(6’-cyano-7’-ethoxy-5’-phenyl)-1,8-
corresponding diketone (4,4’-diacetylbiphenyl,
1,2-
cyclohexanedione, 1,3-diacetylbenzene, 1,6-diacetylpyrene, 1,8-
diacetylpyrene) according to a procedure by Quintela, Peinador et
al.16All the reagents were purchased from Sigma-Aldrich and
30 were used without further purification. Merck 60 F254 foils were
used for thin layer chromatography, and Merck 60 (230-400
mesh) silica gel was used for flash chromatography. Proton and
carbon nuclear magnetic resonance spectra were recorded on a
Bruker Avance-300 or Bruker Avance-500 equipped with a dual
35 cryoprobe for 1H and 13C, using the deuterated solvent as lock and
the residual protonated solvent as internal standard. Mass
spectrometry experiments were carried out in a LC-Q-q-TOF
Applied Biosystems QSTAR Elite spectrometer for low- and
high-resolution ESI. TGA were recorded on a SDT 2960TA
40 instruments, DSC were recorded Q 100TA instruments and UV
A
70 pyridine (7)17 (0.27 g, 1.0 mmol), 3,10-diacetylperylene (8)
(0.168 g, 0.50 mmol) and a catalytic amount of ethanolic
potassium hydroxide (10 %) in ethanol (15 mL) was refluxed
until all starting material had disappeared as checked by TLC (12
h). After cooling, the precipitate was collected by filtration and
75 purified by flash chromatography on silica gel using CH2Cl2 as
eluent to give 6 (0.23 g, 60%). 1H NMR (CDCl3, 300 MHz): 1.59
(t, J = 7.1 Hz, 6H, 2CH3); 4.85 (c, J = 7.1 Hz, 4H, 2OCH2); 7.53-
7.68 (m, 12H); 7.70 (d, J = 8.4 Hz, 2H); 7.85 (d, J = 7.8 Hz, 2H);
8.07 (d, J = 8.4 Hz, 2H); 8.12 (d, J = 8.4 Hz, 2H); 8.33 (d, J = 8.4
80 Hz, 2H); 8.36 (d, J = 8.4 Hz, 2H). 13C NMR (CDCl3, 75 MHz):
14.4 (CH3); 64.3 (OCH3); 99.0; 114.3 (CN); 116.4; 119.7; 120.3;
2
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Fig. 2 LUMO and HOMO levels calculated of 2 (figures a and b,
respectively) and 3 (figures c and d).
35 1.5Ueg(C) and were allowed to ride on their parent atoms.
Results and discussion
Synthesis and Characterization
Fig. 1 a) Crystal structure of compound 4. b) Crystal packing of 4
Scheme 1 shows the synthesis of the oligonaphthyridines (1-6).
…
…
showing [C—H···π] interactions ([H centroid] and, [C centroid]
…
Di-1,8-naphthyridines (1-5) were already
prepared and
distances and [C–H centroid] angle): a 2.68 Å, 3.60 Å, 154º; b 2.99 Å,
3.81 Å, 139º.
40 characterized from 2-amino-5-cyano-6-ethoxy-3-formyl-4-
phenylpyridine and the corresponding diketone. Compound 6 was
prepared from 7 and 3,10-diacetylperylene (8) under base-
catalyzed Friedländer giving the desired product in 60% yield.
Compound (6) was isolated by filtration and the material eluted
45 through silica-gel column via flash chromatography. The
chemical structures of 6 were confirmed by 1H-NMR, 13C-NMR,
mass spectrometry and elemental analysis as described in the
experimental section. X-ray analysis of 4 was carried out to
establish the structure of the bis-napy compounds. As shown in
50 Figure 1a, there is not coplanarity between the linker pyrene and
the end napy groups. The dihedral angle between the linker unit
and the napthyridine moiety is 123º, this twist in the structure
121.3; 123.1; 127.4; 129.1; 130.3; 131.3; 132.2; 133.1; 136.6;
137.1; 156.0; 158.4; 162.8; 164.6. MS (FAB+, m/z): 799.2
(M+H)+. Anal. Calcd. C54H34N6O2: C, 81.19; H, 4.29; N, 10.52.
Found C, 81.57; H, 4.64; N, 10.11.
5
Device Fabrication and Testing
The layers were spin-coated onto pre-patterned ITO glass plates
(prior to deposition, the ITO-coated glass substrates were
extensively cleaned, using chemical and UV-ozone methods) and
the layers were subsequently annealed at 200ꢀ°C on a hotplate for
10 30ꢀmin. Before spin-coating the solutions were filtered over a
0.20ꢀµm PTFE filter. The thicknesses of the spin-coated films
were determined using an Ambios XP1 profilometer. Current
density and luminance versus voltage were measured using a
Keithley 2400 source meter and a photodiode coupled to a
15 Keithley 6485 picoammeter using a Minolta LS100 to calibrate
the photocurrent. An Avantes luminance spectrometer was used
to measure the electroluminescence spectrum. EQE values were
obtained assuming a Lambertian emission. The fabrication
process of the devices was as follows: Firstly, PEDOT was spin-
20 coated on a precleaned and UV/ozone treated ITO substrate and
dried. Secondly, the materials for the hole-transporting layer
(HTL) were dissolved in chlorobenzene and spin-coated on top of
the PEDOT layer or evaporated.
results in the reduction of the
π conjugation between the linker
and the napy end moieties. The asymmetric unit of the triclinic P-
55 1 unit cell has one molecule of 4 and one molecule of deuterated
nitromethane (both in general positions). The molecule of 4 has
approximate mirror symmetry, with the pseudo mirror plane lying
across the pyrene ring, as can be seen in Figure 1a. The molecular
packing exhibits intermolecular C-H···π edge-to-face interactions
60 between phenyl moiety (C-H21and C-H8) ring and pyrene ring
(Figure 1b). Intermolecular π-π electronic interaction between
aromatic rings is not observed, where this features may reduce
the excimer formation in the solid state as already observed in
ladder-type structures.19
65 To have better understanding of the properties of (1-6),
calculations on its electronic ground state were carried out using a
B3LYP density functional theory20with a 6-31G(d) basis in the
Gaussian 03 program.21 The optimized geometry and electron-
density distribution of the highest occupied molecular orbital
70 (HOMO) and lowest unoccupied molecular orbital (LUMO) are
Crystallographic Data
25 CCDC-796293 contains the supplementary crystallographic data
for compound 4.18 These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. The structure was solved by direct
methods and refined with the full-matrix least-squares procedure
30 (SHELX-97) against F2. The X-ray diffraction data were
collected on a Bruker X8 ApexII diffractometer. Non-solvent
hydrogen atoms were placed in idealized positions with Ueg(H) =
1.2Ueg(C) and were allowed to ride on their parent atoms. Solvent
hydrogen atoms were placed in idealized positions with Ueg(H) =
shown in Figure
2
where illustrates calculated spatial
distributions of the energy levels of the designed molecule 3 and
naphthyridine-containing perturbed ring 2. The spatial
distributions of the LUMO and HOMO levels are
partially
75 separated in 3, where the HOMO lies at pyrene moiety and
LUMO at napy and pyrene moiety, which indicates that the
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Table 1 Physical properties of compounds 1-6.
onset
Tm/Tg/Td [ºC]a
λabs [nm] λabs [nm] λabs
λem [nm] λem (ΦF)
λem (ΦF)
[nm, %]
filmd
Ered
[V]e
HOMO/
b
CH3CNb
CH2Cl2
[nm]
CH3CNc
[nm, %]
LUMO
filmb
CH2Cl2
[eV]f
c
385/99/401
340/101/390
313/105/380
323/98/388
340/95/398
350/65/381
371, 420
378, 420
370, 428
391, 411
358, 372
430
333, 384
378, 398
328, 418
372, 416
353, 369
314, 433
387
396
430
420
n.d.f
n.d.f
470
450
530
485
490
545
450(100)
440(100)
530(100)
480(98)
495(87)
585(85)
443 (15)
479 (14)
495 (11)
500 (11)
n.d.g
-1.65 5.9/2.85
1
2
3
4
5
6
-1.6
-1.7
6.0/2.9
5.50/2.80
-1.65 5.55/2.85
-1.71 6.0/2.79
n.d.g
-1.5
5.5/3.0
a The heating and cooling rates were 15 K/min; Tm: melting point; Tg: glass-transition temperature; Td: decomposition temperature.
b λabs: absorption maximum.
c λem: emission maximum; ΦF: fluorescence quantum yield measured by calibrating against rhodamine B (ΦF = 0.65) in ethanol as standard.
d λem: emission maximum; ΦF: fluorescence quantum yield were measured using an integrated sphere.
e Measured in CH3CN. All the potentials are reported as peak potentials, which are relative to ferrocene (used as the internal standard in each
experiment). Ferrocene oxidation potential was calibrated to be + 0.3V versus silver wire used as a pseudoreference electrode.
f The HOMO and LUMO energies were determined from cyclic voltammetry and the absorption onset.
g Not determined.
of 2 is slightly segregated compared with that of 3. This is also
preferable for efficient hole- and electron-transfer properties and
the prevention of back transfer. Besides, from the optimized
geometry of 3 it can be seen that the linker are twisted, which
makes the molecular structure nonplanar, thus facilitating the
formation of an amorphous film (see Supplementary data) and
5
reducing the intermolecular
10 All the di-napy compounds (1-6) are readily soluble in common
organic solvents, such chloroform, dichloromethane,
π-π stacking and/or interaction.
tetrahydrofuran or nitromethane. The good solubility of the
compounds is desirable for the fabrication of solution-processed,
low-cost, large area OLEDs. The thermal properties were
15 determined by thermo-gravimetric analysis (TGA) and
differential scanning calorimetry (DSC) measurements. As shown
in Table 1, the oligonaphthyridines hold high Tg values in the
range of 65-105 ºC. None of the compounds exhibited a melting
point when heated to 300 ºC and cooled to room temperature by
20 DSC. TGA measurements showed that the compounds are
thermally stable up to 300 ºC with a Td in the range of 380-400 ºC
in nitrogen. The excellent thermal stability with high Tg and Td
values should improve the morphological stability of the film and
reduce the possibility of phase separation upon heating.22
25 Photophysical Properties
The photophysical properties of the oligonapthyridine derivatives
were measured using UV-vis absorption and photoluminescence
(PL) spectroscopy, and the data are summarized in Table 1 and
Figure 3. The PL studies on films were performed to examine the
30 emission in the condensed phase. The films were made by spin
casting of their solutions in dichlorobenzene onto the quartz
plates. In the UV-visible spectra, the absorption bands observed
around 350-418 nm can be attributed to π–π* transitions of the
conjugated aromatic segments. PL spectra indicate that 3 and 4
35 are red-shifted by about 40 and 80 nm in CH2Cl2, respectively,
Fig. 3 Emissionspectra of 1-6 recorded in nitromethane (10-5 M) at 25 ºC
(top) and absorption (bottom).
HOMO/LUMO transition becomes a typical charge-transfer (CT)
transition, while the distribution of the HOMO and LUMO levels
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Fig. 4 a) Current density and luminance versus voltage of OLED A (the inset shows the normalized efficacy versus voltage and the operation image of
device A). b) EL spectrum of the device A. c) Current density and luminance versus voltage of OLEDs B (green) and C (red). The inset shows the
operation image of devices B and C. d) Normalized EL spectra of the devices B (green) and C (red).
compared with 2 as a result of the extension of aromatic moiety.
Compounds (1-6) exhibit very high emission intensities at
concentrations as low as 10-8 M. The effect of an increase in
conjugation is clearly revealed in the emission maxima, which
vary from ca. 450 to 580 nm (blue to yellow). Thus, the emission
properties can be modulated by appropriate structural
modifications.
is identical.
Electrochemistry
30 Cyclic voltammetry (CV) experiments were conducted to
investigate the electrochemical properties of the compounds.
Both the oxidation and reduction cycles of the samples were
5
measured
in
acetonitrile,
using
tetrabutylammonium
hexafluorophosphate as the electrolyte and ferrocene as internal
35 reference, and the EFOC was calibrated to be 0.3 V versus a silver
wire that was used as a pseudo reference electrode. All of these
compounds exhibit one reversible, one-electron redox wave due
the process of reduction and the LUMO or EA was calculated
using the equation: LUMO = − (4.5 + Ered) eV. The onset
40 potential of the reduction process ranges from −1.5 to −1.71 V.
By using the reduction potentials and bandgap in absorption
spectra, the LUMO levels for the samples (1-6) were calculated
and found to be between −2.79 and −3.0 eV, whereas the values
for the highest occupied molecular orbital (HOMO) levels for (1-
45 6) lay between −6.0 to −5.5 eV (Table 1).
All compounds are emissive in the blue, green and yellow region,
both in the solution and in the film state (Table 1). The trend of
10 the λem values among the different compounds is parallel with that
of λabs values: (1) compounds with a more delocation linker
shown larger λabs y λem. The fluorescence quantum yields of all
the compounds in dichloromethane are in the 0.7-1.0 range and
were measured by calibrating against rhodamine B in ethanol as
15 standard (ΦF = 0.65). On the basis of the long-wavelength onset
of the absorption band for the samples, the bandgaps of (1-6)
were estimated to be 2.5-3.3 eV decreasing with increasing the
size of linker.
The electron affinity (EA) or LUMO levels of the
20 oligonaphthyridines (1-6) were determined by voltammetry cyclic
(CV), and these data, together with the absorption spectra, were
then taken to obtain the HOMO energy levels. Clearly, the
theoretical DFT results for the designed molecules agree well
with the experimental ones, which imply that the HOMO and
25 LUMO can be controlled by the incorporation of linker size. The
bandgaps estimated from the DFT calculations are similar (2.3–
3.5 eV) than those obtained from the measurements, and the trend
Electroluminescence
Because of the high PL quantum efficiencies of compounds (1-6),
the use of the materials as emitters in OLED devices was also
explored. Films were fabricated by either vacuum deposition or
50 spin coating. All films were formed with quality amorphous film
with fairly smooth surface and devices consisting of 2 or 3 as
emitting layers could be fabricated and characterized. Three
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device with ITO/PEDOT8000/TCTA (Tris[4-(carbazol-4-
yl)phenyl]amine): (spin-coated)/BCP (2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline)/BCP:Cs2CO3/Al
achieved. Using different conjugated spacers and linking
topologies allowed tuning of the HOMO and LUMO energies.
(A), 60 The experimental results indicate that (1-6) have a potential for
the use in yellow and white-pink OLEDs with a facile preparation
methods. Further study is in progress in order to optimize the
3
ITO/PEDOT8000/PVK
Poly(N,N′-diphenyl-N,N′bis(4-
5
hexylphenyl)-[1,1′-biphenyl]-4,4′-diamine)
(30 nm)/2 (vacuum deposition) (35 nm)/LiF/Al (B) and
ITO/PEDOT8000/p-TPD/TCTA(20 nm)/2 (vacuum deposition)
(35 nm)/LiF/Al (C) were fabricated. The devices architecture was
chosen according the good energy level matches between layers.
devices A-C by modifying the layers to enhance the performance.
Acknowledgment
65 This research was supported by Ministerio Ciencia e Innovación
and FEDER (CTQ2010-16484/BQU). We thank Henk Bolink and
Sonsoles García (ICMOL, Valencia, Spain) for device
fabrications.
10 BCP serves as
a good hole and exciton blocker, The
performances of the devices with different hosts are illustrated in
Figure 4. As can be seen, devices A, B and C for compounds 2
and 3 exhibited a quite low turn-on voltage, bright luminance,
and respectable external quantum efficiency. It can be seen that
15 all devices fabricated by using the napy moieties have a low turn-
on voltage (4-6 V). The turn on voltage of A was 3.5 V with a
brightness of 1cd·m-2 and maximum brightness of 400 cd·m-2 at 4
V. The maximum current efficiency is 0.6 cd/A. B device
containing evaporated film of 2, with a turn-on voltage of 7 V
20 (with a brightness of 1 cd·m-2) and maximum brightness of 15
cd·m-2 with a maximum current efficiency is 1.5 cd/A. C device
containing evaporated film of 2, shown turn-on voltage of 3 V
(with a brightness of 1 cd·m-2) and maximum brightness of 250
cd·m-2 with a maximum current efficiency of 1.2 cd/A. The EL
25 spectra of the devices A-C, shown in Figure 4, differ significantly
from the PL of 3 and 2. The device A exhibits a white-pink EL
with the spectrum covering almost the whole visible range with
band centred at 590 nm. The blue and green part of both EL and
PL spectra are originated from the same radiation-decay process
30 of singlet excitons. The emission in the red region to 600-700 nm
appears only in the EL spectrum of 3.
The spectra reveal white-pink emissions according to the CIE
chromaticity diagram (0.1706, 0.319). The devices B and C
exhibit yellow emission according CIE diagram (B 0.5204,
35 0.4893; C 0.5089, 0.4075). The white-pink and yellow emissions
in devices A, B, and C are produced only by electric excitation.
According previous results obtained in the references, the
emission in the red and yellow regions in our devices may arise
from the singlet electromer, where similar disparity in the EL and
40 PL spectra has also been reported previously and the radioactive
decay of electromer or electroplex was usually responsible for the
additional emission in EL.23 The electromer could be regarded as
an excited state of a pair of naphthyridine molecules with
opposite charges under electrical excitation, in which one carried
45 an excess electron while the other carried a hole. Note that most
of the electromers reported in the bibliography were formed at
relatively high electric fields, but here were formed at 4-5 eV.
The white-pink and yellow emission in devices A and B may
result of fluorescence and electromer emission. The devices does
50 not change the colour emission with voltage applied.
Notes and references
70 Departamento de Química Fundamental, Universidade da Coruña,
Facultad de Ciencias, A Zapateira, s/n, 15008, La Coruña, Spain. Fax:
+34 981167065; E-mail: carlos.peinador@udc.es
jose.maria.quintela@udc.es
† Electronic Supplementary Information (ESI) available: Powder X-ray
75 diffraction patterns of compound 3 in film and cyclic voltammetry data.
See DOI: 10.1039/b000000x/
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A series of new 1,8-naphthyridine (napy)-based molecules has
been synthesized from
a simple and efficient route and
characterized. A simple and systematic variation of the spacer
55 linkage can modulate and fine-tunning their properties. These
compounds show a high fluorescence in both solution (10-8 M)
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8
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18 Crystal data for 4: C51272D8N7.5O5, M = 840.91, Triclinic, a =
9.2902(2) Å, b = 13.6836(3) Å, c = 16.0690(3) Å, α = 85.5210(10)°,
β = 80.3570(10)°, γ = 88.5340(10)°, V = 2007.57(7) Å3, T = 100(2)
K, space group P–1, Z = 2, ꢀ(MoKα) = 0.263 mm-1, 25214 reflections
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A series of new 1,8-naphthyridine-based molecules has been synthesized, the
experimental results indicate that these compounds have a potential for the use in
yellow and white-pink OLEDs with a facile preparation methods.