Toward white electroluminescence by ruthenium quinoxaline light emitting diodes

Article abstract of DOI:10.1039/c4nj01938e

Ancillary ligand substitution in the new series [Ru(dicnq)(pyTz)(X)]⁺ {dicnq = 6,7-dicyanodipyrido[2,2-d:2′,3′-f]quinoxaline and X = 2,2-bipyridine (bpy), 1,10-phenanthroline (phen), 2-pyridine tetrazole (pyTz)} proves to be an effective way to create white electroluminescence with the Commission Internationale d'Eclairage Chromaticity Coordinates x = 0.40 and y = 0.39. These complexes were characterized by UV-visible and FT-IR spectroscopy, ¹H-NMR, CHN and Mass spectroscopy analysis. The absorption and emission spectra are consistent with low-lying MLCT excited states, which are typical of Ru(ii) polypyridyl complexes. All the complexes exhibiting reversible metal-centered oxidation processes undergo a one-electron oxidation within the potential range +1.30 vs. SCE. Upon reduction, each compound undergoes several reversible or quasi reversible ligand-centered reduction processes (-1.65 V to -2.40 V). It was found that white light emission can be obtained from the device ITO/PEDOT:PSS/PVK/ruthenium complex/PBD/Al by combining Forster emission from Ru(dicnq) and Exciplex emission at the PVK/Ru(dicnq) interface, although the CIE chromaticity coordinates gradually change with applied voltages. The highest luminous efficiency and luminance and lowest turn-on voltage values are obtained from the 8 wt% Ru(dicnq)(pyTz)(bpy) doped device as 1.78 cd A^-1 and 1030 cd m^-2 at 16 V and 5.2 V, respectively. As an important result, the incorporation of pyTz as the ancillary ligand into [Ru(dicnq)] complexes was found to be the most beneficial ancillary substitution in terms of creating the white electroluminescence in ruthenium quinoxaline complexes.

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Toward white electroluminescence by ruthenium quinoxaline light
emitting diodes
Hashem Shahroosvanda*, Shiva Rezaeia, Ezeddin Mohajeranib, Malek Mahmoudib
5
aChemistry Department, University of Zanjan, Zanjan, Iran.
bLaser and Plasma Research Institute, Shahid Beheshti University, Tehran, Iran.
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
10 DOI: 10.1039/b000000x
Ancillary ligand substitution in the new series [Ru(dicnq)(pyTz)(X)]+ {dicnq=6,7ꢀdicyanodipyrido[2,2ꢀd:2΄,3΄ꢀf]quinoxaline and X=2, 2ꢀ
bipyridine (bpy), 1,10ꢀphenanthroline (phen), 2ꢀ pyridine tetrazole (pyTz)}, proves to be an effective way to create white
electroluminescence with the Commission Internationale d’Eclairage Chromaticity Coordinates x=0.40 and y=0.39. These complexes
15 were characterized by UVꢀvisible and FTꢀIR spectroscopy, 1HꢀNMR, CHN and Mass spectroscopy analysis. The absorption and
emission spectra are consistent with lowꢀlying MLCT excited states, which are typical of Ru(II) polypyridyl complexes. All the
complexes exhibit reversible metalꢀcentered oxidation processes undergo a oneꢀelectron oxidation within the potential range +1.30 vs
SCE. On reduction, each compound undergoes several reversible or quasi reversible ligandꢀcentered reductions (ꢀ1.65ꢀ2.40 V). It was
found that white light emission can be obtained from the device ITO/PEDOT:PSS /PVK/ ruthenium complex / PBD/Al by combining
20 Forster emission from Ru(dicnq) and Exciplex emission at PVK/Ru(dicnq) interface, although the CIE chromaticity coordinates
gradually change with applied voltages. The highest luminous efficiency and luminance and lowest turnꢀon voltage values are obtained
from the 8 wt.% Ru(dicnq)(pyTz)(bpy) doped device as 1.78 cd/A and 1030 cd /m2 at 16 V and 5.2 V, respectively. As an important
result, the incorporation of pyTz as ancillary ligand to [Ru(dicnq] complexes was found to be the most beneficial ancillary substitution in
terms of creating the white electroluminescence in ruthenium quinoxaline complexes.
25
1. Introduction:
The emitting properties of ruthenium polypyridyl compounds
have recently been successfully applied in the development of
electroluminescent devices [1]. One of the reasons that ruthenium
30 polypyridyl compounds has expanded so enormously is that
compounds of this type have intriguing groundꢀ and excitedꢀstate
photophysical, photochemical and redox properties [2].
complexes has not received much attention so far, despite the fact
that red, green and blue are required for applications in lighting
and full color display [9].
In particular, new ruthenium complexes with different πꢀ
conjugated ligands such as dipyrido quinoxaline derivates may
overcome these drawbacks [10]. In this regard, dicnq = 6,7ꢀ
dipyrido [2,2ꢀd:2΄,3΄ꢀf] quinoxaline complexes of ruthenium (II)
were evaluated as potential for luminescence material [11]. In
55
Chemical design of organic ligands in light emitting
organometallic complexes can represent a powerful tool to
35 develop full color light emitting diodes as well as to get white 60 addition, among a great number of 1,10ꢀphenanthroline derivates,
light, thus opening the way to lighting sources with high
efficiency and low costs in alternative to inorganic LEDs,
fluorescent and incandescent bulbs [3]. Two general strategies
can be implemented in order to create electroluminescence in
dicnq as rigid πꢀconjugated systems is ideal candidate for DNA
binding because they prevent the bending along and/or rotation
around the σꢀskeleton of the molecule [12] (For more samples,
please see ESI, S2). Within this context, 2ꢀpyridylazoles are also
40 ruthenium polypyridyl complexes: (a) via molecular design, and 65 capable of using two adjacent nitrogen atoms to form a stable
(b) via energy transfer [4]. The synthesis method has the
advantage of a fine control of the electronic structure of
compounds and design of a large number of emitters with desired
properties [5, 6] (For more details, please see ESI†, S1) is better.
chelate interaction. The strong σꢀdonor property of the azolate,
together with the πꢀaccepting ability of the second pyridyl
fragment, [13] may provide
a synergism of the electron
delocalization so that the electron density is transferred from
45 In this regard, color tuning to achieve blue, green, and red is very 70 azolate to the metal ion and back to the pyridyl side of the ligand,
important because mixing of these colors can produce all colors,
especially white, which is crucial for full color display
applications [7]. Optimistic expectations predict that white OLED
efficiencies will rapidly increase in the next ten years, reaching
thus enhancing the chelate interaction. Exciplex formation at the
interface of organic emitting layer and hole transport layer
became a major subject because it changed emitting color [14].
Even though many research groups studied single layer and
50 values similar to those expected for inorganic LEDs [8]. 75 multilayer devices, the emission from Exciplex formation in
However, the demonstration of multiple colors of Ru polypyridyl
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ruthenium polypyridyl complexes has not been completely
introduced as an emitting source [15].
A specific motivation behind this study is as follows: despite
the large number of red electroluminescence emission reported in
ruthenium polypyridyl complexes in the literatures; their white
electroluminescent properties remain unexplored [16]. Beside, the
Dicnq and pyTz ligands were prepared according to the literature
65 methods [20, 21].
Synthesis of 6,7-Dicyanodipyrido[2,2-d:2',3'-f] quinoxaline
(dicnq). The dicnq ligand was synthesized by following the
reported procedures with some changes, phendione (210mg,
1mmol) and diaminomaleonitrile (162 mg, 1.5 mmol) were
5
attraction of this important class of polypyridyl complexes in 70 dissolved in ethanol, and the resulting solution was refluxed for
biological applications hustles all researchers to more depth in
photonic and optoelectronic applications. Despite these attractive
10 features, the development of dicnq and tetrazole compounds is
primarily focused on the biological aspects, and has, until now,
1h under nitrogen atmosphere. The reaction mixture was cooled
to room temperature, concentrated, and kept in an ice bath.
Brownishꢀyellow needles were formed, which were filtered,
washed with cold ethanol, and dried in vacuum.
been limited. Thus, we found that the incorporation of these 75 Yield: 80%. m. p. >250 °C. Anal. Found: C, 67.98; H, 2.19; N,
attractive ligands (dicnq and pyTz) into one emitting hybrid
structure might provide some suggestions to create the
15 electroluminescence in LED devices.
29.37. Calcd. for C16H6N6: C, 68.08; H, 2.14; N, 29.57. FABꢀ
MS: m/z 283 (M+).;1 H NMR (DMSOꢀd6, 400 MHz, TMS): 9.37
(m, 4H), 7.98 (q, 2H).
This paper systematically introduces several approaches to
Synthesis of 2- pyridine (1H-tetrazole-5-yl) (pyTz). The pyTz
design emitters based on Ru (dicnq)(pyTz)(X) (X=phen, bpy and 80 was prepared according to the literature method with some
pyTz), not only indicating the electroluminescence in ruthenium
quinoxaline compounds but also creating white
20 electroluminescence in this interesting class.
changes. A mixture of 2ꢀcyano pyridine (2.05 ml, 20 mmol), 40
ml of water, sodium azide (1.43 g, 22 mmol), and 4.5 g. (20
mmol) zinc bromide were placed in roundꢀbottomed flask with
three vertical necks. The reaction was refluxed in a hood, but
85 open to the atmosphere, for 24 hours with vigorous stirring. After
the mixture was cooled to room temperature, HCl (3 N, 30 mL)
and ethyl acetate (100 mL) were added, and vigorous stirring was
continued until no solid was present and the aqueous layer had a
pH of 1. The combined organic layers were evaporated, 200 mL
90 of 0.25 N NaOH was added, and the mixture was stirred for 1
hour, until the original precipitate was dissolved and a suspension
of zinc hydroxide was formed. The suspension was filtered, and
the solid washed with 20 mL of 1 N NaOH. To the filtrate was
added 40 mL of 3 N HCl with vigorous stirring causing the
95 tetrazole to precipitate. The tetrazole was filtered and washed
with 2 × 20 mL of 3 N HCl and dried in a drying oven to furnish
the (pyTz) as a white or slightly colored powder.
2. Experimental:
2.1 Materials and Instruments
25 All chemicals and solvents were purchased from Merck &
Aldrich and used without further purification. IR spectra were
1
recorded on a PerkinꢀElmer 597 spectrometer HꢀNMR spectra
were recorded by use of a Bruker 400 MHz, spectrometer..
Elemental analysis was performed on Elementar Vario EL CHN
30 elemental analyzer. Mass spectrometry was performed with a
Bruker BIFLEX III mass spectrometer. Electrochemical
measurements were made in DMF using model SAMA500
potentiostat.
A
conventional threeꢀelectrode configuration
consisting of a Ptꢀdisk working electrode and a Ptꢀwire auxiliary
35 electrode. The potentials are referenced to a saturated Ag/AgCl
reference electrode. In cyclic voltammetry (CV) the following
parameters and relation were used: scan rate, 100 mV sꢀ1; formal
potential Eο′ = (Epa +/Epc) / 2 where Epa and Epc are anodic and
cathodic peak potentials, respectively; ∆Ep is the peak toꢀpeak
40 separation.
The supporting electrolyte was 0.1 M [Bu4N]BF4. Ferrocene was
added as an internal standard after each set of measurements, and
all potentials reported were quoted with reference to the
ferroceneꢀ ferrocenium (Fc/Fc +) couple at a scan rate of 100
45 mV/s. The oxidation (Eox) and reduction (Ered) potentials were
used to determine the HOMO and LUMO energy levels using the
equations EHOMO =ꢀ (Eox + 4.8) eV and ELUMO = ꢀ(Ered + 4.8) eV
which were calculated using the internal standard ferrocene value
of ꢀ4.8 eV with respect to the vacuum [17]. The PL spectra were
50 recorded by Avantes spectrometer Avaspec125 during 405 nm
irradiation. The photoluminescence quantum yields (PLQYs)
were also performed in DMF solvent. Also, Ru(bpy)3 is used as
reference standard for PLQYs determination [18].
Synthesis of [Ru(dicnq)(pyTz)(phen)]BF4. A mixture of RuCl3
(0.1g, 1mmol) and pyTz (0.07g, 1mmol) dissolved in 50 ml
100 methanol was stirred at reflux temperature for 2h under a nitrogen
atmosphere. To this solution, dicnq (0.13, 1mmol) and phen (0.1,
1mmol) were add, respectively, and the solution was heated to
reflux for 3h. After cooling down to room temperature, the
solution was concentrated with a rotary evaporator followed by
105 addition of aqueous NaBF₄ saturated solution. The dark
precipitate was filtered and washed with cold water and
methanol. This product was future purified by column
chromatography using alumina as column support and
acetonitrile/ methanol (2: 1, v/v) as the eluent. The major black
110 band was collected and corresponds to the desired complex.
Yield: 40%. m. p. >250 °C. Anal. Found: C, 51.532; H, 2.319; N,
22.011. Calcd. for RuC34H11N18: C, 51.543; H, 2.308; N,
22.034. ESIꢀMS: m/z 709.741, [MꢀBF4], IR (KBr, cmꢀ1):
ʋ(C=N)pyTz 1456, ʋ(N=N)pyTz 1368, ʋ(C=N)py 1625,
1
115 ʋ(C≡N)dicnq=2230. H NMR (DMSOꢀd6, 250 MHz): δ,9.76 (dd,
2H), 9.52 (d, 1H), 9.32 (d, 1H), 9.07 (dd, 2H), 8.92 (t, 2H), 8.72
(dd, 1H), 8.53 (d, 1H), 8.39 (dd, 2H), 8.21 (m, 2H), 7.92 (d, 1H),
7.73 (d,1H), 7.57 (dd, 1H).
The molecular and electronic structure calculations were
55 performed with density functional theory (DFT) using the
Gaussian 03(G03) program package. The B3LYP functional [19]
with the LANL2DZ basis set was carried out. All geometry
optimizations were performed in either C1 or C2 symmetry with
subsequent frequency analysis to show that the structures are at
60 the local minima on the potential energy surface. The electronic
orbitals were visualized using GaussView 3.0.
Synthesis
of
[Ru(dicnq)(pyTz)(bpy)]BF4.
120 [Ru(dicnq)(pyTz)(bpy)]BF4 was prepared starting from
RuCl3.xH2O (0.1g, 1 mmol), pyTz (0.07 g, 1 mmol), dicnq (0.13
g, 1mmol) and bpy (0.09g, 1mmol) using the same procedure as
described for [Ru(dicnq)(pyTz)(phen)]BF4 to yield the product.
Yield: 44.5%. m. p. >250 °C. Anal. Found: C, 49.127; H, 2.564;
125 N,23.749. Calcd. for RuC32H8N11: C, 49.131; H, 2.579; N,
23.733. ESIꢀMS:m/z 687.545, [MꢀBF4], IR (KBr, cmꢀ1):
ʋ(C=N)pyTz 1451, ʋ(N=N)pyTz 1373, ʋ(C=N)py 1620,
2. 2. Synthesis of ligands and complexes
1
ʋ(C≡N)dicnq=2218. H NMR (DMSOꢀd6, 250 MHz): δ,9.65 (dd,
2
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2H), 9.42 (dd, 2H), 9.37 (dd, 2H), 8.88 (d, 1H), 8. 72 (d, 1H),
8.52 (m, 4H), 8.34 (dd, 1H), 8.15 (dd, 1H), 7.82 (dd, 2H), 7.61
(dd, 1H), 7.49 (t, 1H).
+
N
N
Synthesis of [Ru(dicnq)(pyTz)2]. This complex was prepared from
a mixture of RuCl3 (0.1g, 1mmol), pyTz (0.14g, 2 mmol) and
dicnq (0.13 g, 1mmol) in a manner analogous to that employed
for the synthesis of [Ru(dicnq)(pyTz)(phen)]BF4 . Yield: 41%. m.
p. >250 °C. Anal. Found: C, 49.245; H, 2.312; N, 33.202. Calcd.
for RuC28H14N16: C, 49.255; H, 2.312; N, 33.289. ESIꢀMS: m/z
N
5
N
N
N
N
NC
NC
N
N
N
10 674.157, [Mꢀ2H]. IR (KBr, cmꢀ1): ʋ(C=N)pyTz 1460, ʋ(N=N)pyTz
1374, ʋ(C=N)py 1616, ʋ(C≡N)dicnq=2223. 1H NMR(DMSOꢀd6,
250 MHz): δ,9.51 (d, 1H), 9.43 (d, 1H), 8.52 (d, 2H), 8.31 (m,
2H), 8.11 (d, 2H), 7.76 (t, 2H), 7.46 (m, 4H).
X
N
X= phen, bpy, pyTz
2.3 Preparation of EL devices and testing
15 The structure of the fabricated device is as follow:
Scheme 1. Molecular structure of [Ru(dicnq)(pyTz)(X)] X=phen,
60 bpy and pyTz
ITO/ PEDOT: PSS(55nm)/ PVK (60nm) /PBD (30nm)/Al
(130nm) and, ITO/ PEDOT: PSS(55nm) / ruthenium complex
(45nm) /Al (130 nm). PVK as a holeꢀtransporting and PBD as an
electronꢀtransporting material were uesd. Glass substrates, coated
20 with ITO (sheet resistance of 70 Ω/m^2), were used as the
This effect of ancillary ligand is also reflected in the low energy
metal to ligand charge transfer (MLCT) transitions where the λmax
of [Ru(dicnq)(pyTz)2]
65 [Ru(dicnq)(pyTz)(bpy)]BF4 emitter is 462 nm. Surprisingly, the
complex with phen moiety as ancillary ligand,
is 481 nm and that of the
conducting
anode
PEDOT:
PSS(poly(3,4ꢀethylenediꢀ
oxythiophene): poly (styrenesulfonate)) was used as a hole
injection and transporting layer. All polymeric layers were
successively deposited onto the ITO coatedꢀglass by using spinꢀ
25 coating process from the solution. A metallic cathode of Al was
deposited on the emissive layer at 8×10ꢀ5 mbar by thermal
evaporation. The PEDOT: PSS was dissolved in DMF, spin
coated on ITO and was held in an oven at 120 oC for 2 hours after
deposition. The optimized concentration of ruthenium complexes
30 was fixed at 8% in the PVK: PBD host, whereas the Ru(dicnq)
was varied from 6% to 14%. Subsequently, PVK, PBD and dicnq
with weight ratio of 100,40, x (x=6 to 14) were separately
dissolved in 8 mL of DMF, and then spin coated and baked at 80
°C for 1 hour. The thickness of the polymeric thin film was
35 determined by a Dektak 8000. The EL intensity and spectra were
measured with an ocean optic USB 2000, under ambient
conditions. In addition, Keithley 2400 source meter was used to
measure the electrical characteristics of the devices.
[Ru(dicnq)(pyTz)(phen)], exhibited a blue shift in the lower
energy MLCT band (440 nm) associated with an extinction
coefficient of 14670 Mꢀ1 cmꢀ1.
40 3. Results and Discussion
This part of the paper is divided into two sections. In section 3.1,
we present the characterization data and photophysical properties
of Ru(dicnq)(pyTz)(x),x=bpy, phen, bpy complexes. Section 3.2
examines the chemically modified compound, demonstrating that
45 EL color can be tuned through ancillary ligand substitution.
70
75
3.1. Photophysical and electrochemical properties
Figure 1. Right: absorption spectra of investigated complexes and
left, PL spectra of PVK: PBD and complexes in DMF solvent 10ꢀ
M. The emission spectra were recorded with 405 ± 2 nm
The molecular structure of investigated complexes is shown in
scheme 1. The absorption and emission spectra of complexes in
50 DMF are displayed in Fig. 1. The dicnq, pyTz are largely
responsible for the structured π – π * absorption bands between
200 and 350 nm [22]. The broad absorption band in the visible
region is due to spinꢀallowed dπ(Ru)→π *(ligands) MLCT
transition and overlapped transitions to different MLCT states,
55 with the state, or degenerate states, at higher energy localized on
the polypyridyl ligands [23].
5
excitation.
The PL spectra of ruthenium(II)–polypyridine complexes are
3
largely attributed to the MLCT states in the literature which are
populated via an ultrafast ISC process from directly photoexcited
1MLCT states [24ꢀ26]. As shown in Figure 1, the complexes emit
80 yellowishꢀred light at room temperature with an emission
maximum (lem) at region 593ꢀ611 nm. Emission maxima,
quantum yields and life time measurments are reported in Table
1. In this series, [Ru(dicnq)(pyTz)(bpy)]BF4 has the highest
quantum yield with Ф=0.01 in DMF solvent. In addition, all
85 emission maxima are shifted to higher energies relative to
[Ru(phen)3]2+, which is further indication of ancillary ligand
substitution. Regarding the lifetime in Ru(dicnq)(pyTz)(X)
complexes, it appears in the order bpy > phen> pyTz.
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Table 1. EL and PL characteristics of Ru(dicnq) complexes.a: CIE=Commission Internationale deL’Eclairage; the ideal white is x= 0.33;
y= 0.33.
EL
PL
Maximum
FWHM current
Luminous
efficiency
[cd Aꢀ1 ]
at 16 V
1.65
Turnꢀ
on
V
Luminance
EL max CIEa
[nm] (x;y)
Ф
PL max
τ(ns)
Compound
[cd m ꢀ2
at 16 V
]
[nm]
density,
at 16 V
170
[nm] [±0.001] [±5]
521 0.34; 0.44
535 0.37; 0.43
556 0.40; 0.39
183
179
197
5.5
5.2
6.3
1020
1030
865
593 0.008 330
Ru(dicnq)(pyTz)(phen)
Ru(dicnq)(pyTz)(bpy)
Ru(dicnq)(pyTz)2
601
0.01
380
117
66
1.78
0.87
611 0.005 245
5
The 1HꢀNMR spectral data of complexes are consistent with
conventional octahedral coordination geometries and diamagnetic 60 of HOMO and LUMO of investigated ruthenium complexes were
than on only one ligand [31]. As seen in table 2, the energy levels
6
behavior (t2g
)
(see ESI†, S3aꢀc). In addition, the presence of
calculated which are in agreement with DFT calculations (ESI†,
S6aꢀc).
dicnq of complexes are confirmed by FTꢀIR spectroscopy (ESI†,
10 S4aꢀe). It is well established that the redox processes in Ru(II)ꢀ
polypyridine complexes are mainly localized either on the metal
center (oxidations) or on the ligands (reductions) [ 27, 28].
Cyclic voltammograms of the investigated Ru(dicnq) complexes
are consistent with a metalꢀbased reversible oxidation and several
15 ligandꢀbased reductions. It is, therefore, of fundamental
importance to know the electrochemical behavior of the
3. 2. EL properties
65 The structure of the fabricated device is as follow:
ITO/ PEDOT: PSS (55nm)/ PVK (60nm) /PBD (30nm)/ Al
(130nm) and, ITO/ PEDOT: PSS (55nm)/ PVK (60nm)/ PBD
(30nm)/ Ru(dicnq) complex (45nm) /Al (200nm), That is shown
in Fig. 2. The EL spectra of the devices are shown in Fig 3. To
uncoordinated ligands to understand the pattern of the ligandꢀ 70 analyze the energy transfer process from PVK to ruthenium
based redox series for the complexes. The halfꢀwave potentials of
ligand and Ru complexes are summarized in Table 2 (ESI†, S5).
20 In acetonitrile containing 0.1 M (TBA)BF4, dicnq and pyTz show
a reversible oneꢀelectronꢀreduction wave at ꢀ0.66 V and two
complex and to find a relationship between the EL spectra of
ruthenium compounds and PVK / PBD, an emissive layer without
ruthenium complex was fabricated to record the PVK / PBD EL
spectrum. It should be noted that high ratio of PBD leads to lower
reversible for pyTz at ꢀ1.85 and ꢀ2.3 vs SCE. (Table 2). On the 75 stability of the device due to the decrease in the ratio of PVK.
other hand, reduction of both dicnq ligands in [Ru(phen)(dicnq)2]
2+ occurs below ꢀ1.0 V , followed by the reduction of phen or bpy
25 in region ꢀ1.3 ꢀ ꢀ1.70 V and finally the reduction of pyTz ligand
from ꢀ1.9 to ꢀ2.4 V. This means that the formal negative charge
brought by the pyTz makes their reductions more difficult [29].
Thus, the relative ease of reduction of the ligands follows the
order pyTz> phen/bpy> dicnq. It is useful to note the electron
30 addition to dicnq ligands in [Ru(dicnq)3]2+ occurs at ꢀ0.47 V and
oxidation of the ruthenium center occurs at +1.51 V, under
similar experimental conditions of solvent and supporting
electrolyte. On the other hand, a significant shift (around 300
mV) to higher negative potentials of the reduction process in
100:40 efficient ratio is suggested [32, 33], which reduces the risk
of excimer formation and increases the stability of the device.
35 [Ru(dicnq)pyTz] complexes compared to [Ru(dicnq)3]2
is
observed upon ancillary ligand substitution of the complexes. In
addition, the substitution of dicnq in [Ru(dicnq)3]2+ [21] with
ancillary ligands decreases the oxidation potential of ruthenium
steadily, resulting in an overall anodic shift of 200 mV for
40 [Ru(dicnq)(pyTz)] compared to [Ru(dicnq)3] 2+. This trend is also
seen in the known homolog ligands, when the dicnq in
[Ru(dicnq)3]2+ replaces with phen ligand, the oxidations of the
80
Figure 2. The layer arrangement of Ru(dicnq)ꢀbased LED.
ruthenium center in [Ru(phen)2(dicnq)] 2+
,
2+
[Ru(phen)(dicnq)2] , and [Ru(dicnq)3]2+ occur at +1.31, +1.41,
45 and +1.51 V, respectively [30] . Interestingly, the electron
abstraction abilities from the metal center in Ru(dicnq)(pyTz)(x)
(x=phen, bpy, pyTz) is very close to [Ru(phen)3]2+ (1.27 V).
Whereas, the first reduction wave in Ru(dicnq)(pyTz) family is
found to occur at less negative potentials than that of the
50 Ru(phen)3]2+ (ꢀ1.35V) due to presence of dicnq ligand. These
evidences confirm the key role of pyTz ligand as ancillary ligand
to control the redox behavior of Ru polypyridyl complexes. In
light of above observations, these data also suggests that the π*
orbital of dicnq lies lower than that of ancillary ligands (phen/
55 bpy and pyTz) and, probably, that the added electron is
delocalized equally on the π * levels of the dicnq ligands rather
As many reports and our analysis, the best concentration for PVK
and PBD is 100 and 40, respectively. As shown in Fig. 4, the
85 efficiency of the devices increases with increasing Ru(dicnq)
concentration from 6 to 8 wt.%, but decreases when the dopant
concentration is more than 8 wt. The device with 8 wt.% dopant
has the highest luminance and current efficiency.
90
95
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Table 2. Redox Potentials (mV) of ligands and Ru Complexes a
E
(ox)
E
(red)
½
HOMOb LUMOb Eg(eV)
½
(I)
dicnq
(II)
(III)
phen/bpy
(IV)
(V)
pyTz
pyTz
dicnq
(1)
(2)
(3)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ1.85
ꢀ
ꢀ1.90
ꢀ2.3
ꢀ
ꢀ2.25 ꢀ5.72
ꢀ2.23 ꢀ5.69
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ0.65
ꢀ1.08
ꢀ0.97
ꢀ0.82
+1.32
+1.29
+1.30
ꢀ1.37
ꢀ1.43
ꢀ
3.32
3.37
3.58
2.40
2.32
2.05
ꢀ1.70 ꢀ1.90
ꢀ1.93
ꢀ
ꢀ2.4
ꢀ5.70
aAt room temperature and in MeCN solutions containing 0.1 M tetraꢀnꢀbutyl ammonium tetrafloroborate. All E1/2 values are referred to E
5
of the ferrocenium+ /ferrocene redox couple (380 mV vs SCE in MeCN). b Energy levels estimated based on Eox and Ered
½
20 Interestingly, Ru(dicnq) complexes without pyTz ligand such as
[Ru(dicnq)2
(bpy)](BF4)2,
[Ru(dicnq)2(phen)](BF4)2
and
[Ru(dicnq)3](BF4)2 were tested as emitter layers in the same LED
cell, but the significance electroluminescence emission were not
achieved. As an important result, the pyTz ligand has a key role
25 to produce electroluminescence in Ru(dicnq) complexes. The
basic characteristics and performance data are summarized in
Table 2. As a general trend, the EL emission peak shifts to shorter
wavelengths
from
[Ru(dicnq)(pyTz)2]
through
[Ru(dicnq)(pyTz)(bpy)]BF4
to [Ru(dicnq)(pyTz)(phen)],
30 correlating with the increase in the LUMOꢀHOMO energy gap
2.01, 2.2, and 2.27 eV for [Ru(dicnq)(pyTz)(X)] where X=pyTz,
bpy and phen, respectively.
Figure 3. (a) EL spectra of PVK/PBD , [Ru(phen)3]2+ and
Ru(dicnq)(pyTz)(X)]where X=bpy, phen and pyTz.
10
As shown in Fig. 3, the effect of ancillary ligand substitution on
the spectral shift to a shorter wavelength induced a variation in
the wavelength from 557 to 520 nm.
35
Figure 5. Plot of the CIE emission coordinates [34] of the
[Ru(dicnq)(pyTz)(X)] where X=bpy, phen and pyTz .
40 At the same time, the EL characteristics of these devices are
different. As CIE chromaticity diagram (Fig. 5), the emission
from the device based on [Ru(dicnq)(pyTz)(phen)]BF4 appeared
to be close to a yellowish green hue; [Ru(dicnq)(pyTz)(bpy)]BF4
had a yellow green hue, and [Ru(dicnq)(pyTz)2] had a white hue.
45 It is useful to note, the perfect white light has CIE coordinates
(0.33, 0.33). However, there is a quite broad region of the
diagram around this point that can be considered white light
[11b]. As shown in Fig. 6, a maximum luminance of 1020 cd/m2
at 16 V was obtained from the device ITO/ PEDOT: PSS /PVK/
50 ruthenium complex/ PBD/ Al in which there was
[Ru(dicnq)(pyTz)(phen)]BF4 in emitting layer, however , the
15
Figure 4: Influence of doping ratio on electroluminescence of
[Ru(dicnq)(pyTz)2] at 16 V.
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luminance from the similar structure device in which
[Ru(dicnq)(pyTz)(bpy)]BF4 was used to increased to 1030 cd/m2
at the same voltage. When [Ru(dicnq)(pyTz)2] was used in the
above device architecture the luminance was 865 cd/m2 at 16V.
Although the LED performance of this new class is not surprising
compared to cyclometalated iridium complexes, this achievement
is the good among the great of OLED performance in ruthenium
polypyridyl complexes [35].
5
The schematic device architecture and the estimated energyꢀlevel
10 diagram are depicted in Fig. 7. The observation and
characterization of electroluminescence in this class of
Ru(dicnq)(pyTz)(x), x=bpy , phen, pyTz emitter is of interest in
the study of the fundamental electron transfer of this class of
ruthenium polypyridylꢀlight emitting diodes. Here, the concept of
15 Forster transfer in the energy transfer mechanism has been useful
one. The PVK/ PBD/ Ru emitter pair has the appropriate spectral
characteristics to act as a donor/acceptor pair in an energy
transfer mechanism [36]. To be an effective means of energy
transfer, the Forster mechanism has a number of conditions that
20 must be satisfied, but two major considerations include the spatial
separation between the donor and acceptor and the spectral
characteristics of the donor emission and the acceptor absorption.
Figure 7. Schematic showing the energy levels of device and the
40 dynamic process of emission. HOMOꢀLUMO level energy of the
investigated emitters and other materials for LED device.
It should be noted that compared with the PL spectra, there is
some red shift of the broad band in the EL one, which may be due
45 to the different nature of ELꢀ and PLꢀmechanisms. We suppose
that the new broad band emission peaking at about 565 nm in EL
spectrum of can likely be ascribed to the Exciplex under electrical
excitation [38]. However, at the interface of two components in
OLEDs, as reported in literatures [39], an Exciplex emission can
50 occurs under both photoexcitation and electric fields. In this
regard, the broadening and wavelength of PL spectrum of
PVK/Ru(dicnq) film is also similar to EL spectrum that confirms
an Exciplex transfer in the interface of PVK and Ru(dicnq)
complexes (ESI. S7).
55 The new broad emission band at longer wavelength can originate
from the intermolecular Exciplex. Generally, The Exciplex peaks
shifted to a lower energy side in comparison to the peak related to
the emitter layer because the emission energy of the Exciplex is
smaller than that of the exciton in the EML [40]. The Exciplex
60 emission can be explained from energy band theory, too.
Electrons on the LUMO of PBD must overcome 0.3 eV potential
barriers to inject onto LUMO of Ru(dicnq). Furthermore, the
HOMO of PVK is very close to the HOMO of Ru(dicnq) , so the
excited species will have a high probability to collide with PVK
65 to form excited complex, [Ru(dicnq)– PVK+]* which might result
in the accumulation of holes at the PVK/ Ru(dicnq) interface
[41]. When the voltage is increased, the electrons are injected into
the PBD layer from Al cathode and the more holes recombine at
the interface of the PVK: Ru(dicnq), which is result in the
70 increasing of Exciplex emission (Fig. 8) [42].
25 Figure 6. Current density and luminance versus applied voltage
for Ru(dicnq)ꢀdevices
In addition, both modes of energy transfer require a level of
spectral overlap of the donor emission and the acceptor
30 absorption [37]. In this study, PVK has the spectral features to act
as an energy transfer donor to Ru complexes by virtue of its 455
nm emission maxima, which overlaps the MLCT of Ru(dicnq)
complexes. Moreover, the arrangement of layers has caused the
Forster transfer of energy to result from PVK: PBD host to
35 ruthenium complexes (◌◌Fig. 1).
The Exciplex formation and emission mechanisms under an
applied electric field, including the generation of charge carriers,
can be described by Scheme 2 and Fig. 7. In the scheme 2,
Ru(dicnq)*( an excited state of Ru(dicnq)) forms an encounter
75 complex (PVK··· Ru(dicnq)*) in the presence of PVK (II), and
ꢀ
then a radical ion pair (PVK+ ··· Ru(dicnq) )* is produced by an
intermolecular electron transfer from the PVK to Ru(dicnq)*
[43].
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PVK–Ru complex interface when pyTz was incorporated as coꢀ
ligand into Ru(dicnq) complexes. We expected that the various
40 fundamental properties elaborated in this study would be
beneficial to the future development of Ru polypyridyl emitting
materials for use as luminescent WOLEDs.
Acknowledgments:
45 The authors wish to thank the University of Zanjan for financial
supports.
Scheme 2. Exciplex formation and emission mechanisms under
applied electric field.
5
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