Green and reddish-orange light-emitting electrochemical cells using cationic iridium (III) phenanthroline complexes

Article abstract of DOI:10.1080/15421406.2013.849444

A new series of cationic heteroleptic iridium(III) complexes with phenanthroline based-ancillary ligands, namely, [Ir(ppy)₂(dmphen)] PF₆, [Ir(ppy)₂(dbphen)]PF₆ and [Ir(ppy) ₂(dpphen)]PF₆ (ppy is 2-phenylpyridine, dmphen is 2,9-dimethyl-1,10-phenanthroline, dbphen is 2,9-dibutyl-1,10-phenanthroline, dpphen is 2,9-diphenyl-1,10-phenanthroline and PF₆ is hexafluorophosphate) have been synthesized and characterized by various spectroscopic methods. These synthesized complexes were used for the fabrication of single component light-emitting electrochemical cells (LECs). The electroluminescent devices were evaluated on the basis of electroluminescence properties and the effect of substituents on the phenanthroline ligands tune the emission color of the complexes.

Full text of DOI:10.1080/15421406.2013.849444

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Green and Reddish-Orange Light-
Emitting Electrochemical Cells Using
Cationic Iridium (III) Phenanthroline
Complexes
Dan Bi Moon a & Young Son Choe a
a Department of Chemical Engineering , Pusan National University ,
Busan , 609-735 , Korea
Published online: 16 Dec 2013.
To cite this article: Dan Bi Moon & Young Son Choe (2013) Green and Reddish-Orange Light-Emitting
Electrochemical Cells Using Cationic Iridium (III) Phenanthroline Complexes, Molecular Crystals and
Liquid Crystals, 584:1, 60-68, DOI: 10.1080/15421406.2013.849444
To link to this article: http://dx.doi.org/10.1080/15421406.2013.849444
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Mol. Cryst. Liq. Cryst., Vol. 584: pp. 60–68, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.849444
Green and Reddish-Orange Light-Emitting
Electrochemical Cells Using Cationic Iridium (III)
Phenanthroline Complexes
DAN BI MOON AND YOUNG SON CHOE∗
Department of Chemical Engineering, Pusan National University, Busan
609-735, Korea
A new series of cationic heteroleptic iridium(III) complexes with phenanthro-
line based-ancillary ligands, namely, [Ir(ppy)2(dmphen)]PF6, [Ir(ppy)2(dbphen)]PF6
and [Ir(ppy)2(dpphen)]PF6 (ppy is 2-phenylpyridine, dmphen is 2,9-dimethyl-1,10-
phenanthroline, dbphen is 2,9-dibutyl-1,10-phenanthroline, dpphen is 2,9-diphenyl-
1,10-phenanthroline and PF6 is hexafluorophosphate) have been synthesized and char-
acterized by various spectroscopic methods. These synthesized complexes were used for
the fabrication of single component light-emitting electrochemical cells (LECs). The
electroluminescent devices were evaluated on the basis of electroluminescence proper-
ties and the effect of substituents on the phenanthroline ligands tune the emission color
of the complexes.
Keywords Cationic iridium complex; electroluminescence properties; light-emitting
electrochemical cells; optoelectronic devices
Introduction
Solid-state light-emitting electrochemical cells (LECs) are a promising type of electro-
luminescent device for display and lighting application [1–4]. LECs offer a number of
advantages over traditional organic light-emitting diodes(OLEDs), in particular simpler
architectures and production. LECs contain only single light-emitting layer, which can be
easily processed from solutions [1]. Furthermore, charge injection in LECs is insensitive
to the workfunction of the cathode material, thus permitting use of air-stable metals as
the cathodes. Therefore, their encapsulation does not have to be as rigorous as that with
OLEDs. These characteristics make LECs the simplest kind of electroluminescent devices.
The first solid-state LEC device was based on a polymer blend containing a light-
emitting polymer, an ion-conductiong polymer and an inorganic salt [2]. More recently,
LECs based on iTMCs have received more attention because of their several advantages
over conventional polymer-based LECs. For iTMCs-based LECs, no inorganic salt or ion-
conductiong polymer are needed because iTMCs are intrinsically ionic. Besides, iTMCs
show good thermal and photophysical stabilities.
Most of the iTMC materials reported in the literature are based on ruthenium, osmium,
copper, and iridium complexes. Among these complexes, iridium complexes are interesting
∗Address correspondence to Prof. Young Son Choe, Department of Chemical Engineering,
Pusan National University, Jang-jeon-Dong, Geumjeong-Gu, Pusan 609-735, Korea (ROK). Tel.:
(+82)51-510-3562; Fax: (+82)51-512-8634. E-mail: choe@pusan.ac.kr
[248]/60

Electrochemical Cells Using Cationic Iridium (III)
[249]/61
candidates in terms of color tuning because of the pronounced ligand-field-splitting effects.
For this reason, iridium complexes have drawn increased attention as electroluminescent
materials in Itmc-LECs in the past few years [5–9].
In this study, new iridium complexes with phenanthroline based ancillary ligands have
been synthesized. LECs based on these complexes were fabricated and investigated in terms
of their emission color and electroluminescent properties.
Experimental
General Comments
All reagents and solvents were purchased from Sigma-Aldrich, while IrCl3·H20 hydrate
was purchased from Alfa-aesar. Luminescent material was prepared from 2-phenylpyridine
as cyclometallated ligand and iridium trichloride by Nonoyama reaction to form tetrakis(2-
phenylpyridine-C2,N’)(μ-dichloro)diiridium followed by addition of ancillary ligand which
is a phenanthroline derivative, and ion exchange with ammonium hexafluorophosphate [10].
All reactions were carried out under nitrogen.
Synthesis
Synthesis of tetrakis(2-phenylpyridine-C2,N’)(μ-dichloro)diiridium. The precursor of
ionic transition metal complex was synthesized according to the literature [11,12]. Irid-
ium trichloride hydrate (0.388 g, 1.1 mmol) dissolved in distilled water (10 ml) was added
to 2-phenylpyridine (0.371 g, 2.4 mmol) in 2-ethoxyethanol (30 ml). The mixture was
refluxed for 24 h under nitrogen gas, and then cooled down to room temperature. The
yellow solid was collected on a membrane filter. The resulting precipitate was washed with
ethanol (120 ml) and dissolved in dichloromethane (97 ml) and filtered to remove residual
inorganic salts. Hexanes (13 ml) were added to the filterate, which was then reduced by
evaporation to 50 ml, and cooled down to give crystals of [Ir(ppy)2Cl]2, then filtered by
membrane filter and dried in an vacuum oven for 12 h. Yield: 0.2737 g (46%).
Synthesis of [Ir(ppy)2(dmphen)]PF6 (complex 1). [Ir(ppy)2Cl]2 (0.069 g, 0.064 mmol)
and 2,9-dimethyl-1,10-penanthroline (0.027 mg, 0.128 mmol) were heated to reflux in an
ethylene glycol solution (20 ml) for 16 h. The mixture was transferred into a separation
funnel with water (90 ml) and washed through three extractions with diethyl ether (60 ml).
A concentrated solution of ammonium hexafluorophosphate in water was slowly added into
the aqueous layer, yielding a colored suspension. The precipitate was collected by filtration
1
[13]. Yield: 0.098 g (89%). H NMR (500 MHz, CD2Cl2) δ: 8.46 (d, J = 8.28 Hz, 2H),
8.04 (s, 2H), 7.95 (d, 8.24 Hz, 2H), 7.77 (t, 7.76 Hz, 2H), 7.67 (d, 7.92 Hz, 2H), 7.61 (d,
8.26 Hz, 2H), 7.44 (d, 5.36 Hz, 2H), 6.91 (t, 8.42 Hz, 2H), 6.86 (t, 7.00 Hz, 2H), 6.80 (t,
8.15 Hz, 2H), 6.15 (d, 8.57 Hz, 2H), 2.14 (s, 6H). 13C NMR (126 MHz, CD2Cl2) δ: 168.4,
165.7, 149.9, 149.2, 148.5, 143.5, 139.6, 138.6, 132.1, 130.6, 130.4, 128.5, 127.8, 125.3,
123.3, 122.8, 120.3, 28.1.
[Ir(ppy)2(dbphen)]PF6 (complex 2) and [Ir(ppy)2(dpphen)]PF6 (complex 3) were syn-
thesized according to the similar methods with that of complex 1. Complex 2: [Ir(ppy)2Cl]2
(0.07 g, 0.065 mmol) and 2,9-dibutyl-1,10-penanthroline (0.04 g, 0.137 mmol). Yield:
0.112 g (82%).1H NMR (500 MHz, CD2Cl2) δ: 8.49 (d, 8.43 Hz, 2H), 8.23 (s, 2H), 7.94
(d, 7.95 Hz, 2H), 7.75 (t, 8.94 Hz, 2H), 7.68 (d, 8.44 Hz, 2H), 7.65 (d, 9.10 Hz, 2H), 7.42
(d, 8.03 Hz, 2H), 6.98 (t, 8.40 Hz, 2H), 6.84 (t, 7.36 Hz, 2H), 6.79 (t, 8.3 Hz, 2H), 6.14

62/[250]
D. B. Moon and Y. S. Choe
(d, 8.47 Hz, 2H), 2.73–2.6 7(m, 4H), 1.25–1.17 (m, 4H), 0.88–0.81 (m, 4H), 0.66 (t,
7.09 Hz, 6H). 13C NMR (126 MHz, CD2Cl2) δ: 169.7, 168.2, 149.8, 148.3, 147.9, 143.3,
139.4, 138.5, 131.9, 130.6, 130.1, 127.6, 126.3, 125.1, 122.9, 122.7, 120.1, 39.6, 32.7, 22.9,
13.9.
Complex 3: [Ir(ppy)2Cl]2 (0.70 g, 0.065 mmol) and 2,9-diphenyl-1,10-penanthroline
1
(0.043 g, 0.130 mmol). Yield: 0.1g (78.5%). H NMR (500 MHz, CD2Cl2) δ: 8.61 (d,
8.23 Hz, 2H), 8.23 (s, 2H), 7.74 (t, 7.53 Hz, 2H), 7.66 (d, 5.27 Hz, 4H), 7.64 (d, 3.14 Hz,
2H), 7.62 (d, 3.46 Hz, 2H), 7.09 (d, 7.79 Hz, 2H), 7.03 (t, 5.91 Hz, 2H), 6.93 (t, 6.18 Hz,
2H), 6.76 (t, 7.68 Hz, 2H), 6.58 (s, 4H), 6.53 (t, 7.23 Hz, 2H), 6.20 (t, 8.47 Hz, 2H), 5.24
(d, 7.73 Hz, 2H) 13C NMR (126 MHz, CD2Cl2) δ: 168.9, 166.7, 150.4, 147.3, 139.0, 138.4,
131.2, 130.1, 129.6, 128.8, 128.6, 128.2, 127.4, 124.6, 122.2, 121.3, 120.2.
The synthetic routes of the complexes are shown in Scheme 1.
Scheme 1. The synthetic routes and structure of iridium complexes: Complex 1: [Ir(ppy)2
(dmphen)]PF6, Complex 2: [Ir(ppy)2(dbphen)]PF6 and Complex 3: [Ir(ppy)2(dpphen)]PF6

Electrochemical Cells Using Cationic Iridium (III)
[251]/63
Figure 1. Absorption spectra for complexes in dilute CH3CN solutions (10−5M). The inset shows
the absorption spectra of the complexes in dilute CH3CN solutions (10−3M) in the range from 300 to
550 nm.
Fabrication of LEC Devices
PEDOT:PSS was purchased from HC-Starck and solvents used were obtained from Aldrich.
ITO-coated glass substrates were extensively cleaned in mixed alcohol (isopropyl alcohol,
ethyl alcohol and acetone, 1:1:1 by volume ratio) by ultrasonication 45 min and dried in an
oven for an hour. PEDOT:PSS buffer layer was spin-coate◦d on to the ITO glass at 2000 rpm
for 20 s and annealed on hot plate for an hour at 130 C under vacuum. Solid film of
Figure 2. Photoluminescence emission spectra for complex 1, 2 and 3 in CH3CN solution (10−3M)

64/[252]
D. B. Moon and Y. S. Choe
Table 1. Photophysical and electrochemical characteristics of complexes
complex
Emission λmax (nm)
Eox [V]
Ered [V]
ꢀEredox
[Ir(ppy)2(dmphen)]PF6
[Ir(ppy)2(dbphen)]PF6
[Ir(ppy)2(dpphen)]PF6
546
540
602
1.02
1.02
1.02
−1.52
−1.54
−1.44
2.54
2.56
2.46
luminous layer were obtained by spinning from acetonitrile solution using concentration
of 20 mg/ml and annealed on hot plate for 20 min at 80 ◦C under vacuum. Then, a 100 nm
thick aluminum cathode was evaporated on top under a vacuum. The deposition rate was
controlled by 3.0∼7.0Å/s.
Measurements
1H and 13C NMR spectra were collected on a Bruker 500 MHz spectrometer. UV-vis
absorption specta were recorded on 8453 UV-visible Agilent spectrophotometer. PL spectra
were measured with an F-7000 FL spectrophotometer. Cyclic voltammetry was obtained
using a potentiostat/galvanostat (Iviumstat) voltametric analyzer using 10−3 M solution
of CH3CN at a scan rate of 100 mV/s at room temperature. A glassy carbon was used
as the working electrode, a platinum wire as the counter electrode and a Ag/AgCl as
the reference electrode. The supporting electrolyte was 0.1 M of tetrabutylammonium
hexafluorophosphate (TBAPF6) and ferrocene was used as internal standard. The current
density and luminance versus voltage characteristics were recorded using a Keithley 2400
source meter and calibrated with silicon photodiode. An Avantes luminance spectrometer
was used to measure the EL spectrum and CIE coordinates.
Figure 3. Cyclic voltammogramms of the data for complex 1, 2 and 3 in dilute CH3CN solutions
containing 0.1M Bu4NPF6 as supporting electrolyte. Potentials were recorded versus Fc+/Fc (Fc is
ferrocene)

Electrochemical Cells Using Cationic Iridium (III)
[253]/65
Figure 4. Electroluminescence (EL) spectra of LECs based on the complex 1, 2, and 3
Results and Discussion
The UV-vis absorption spectra of the complexes in CH3CN solutions are shown in Fig. 1.
The intense absorption bands in the region of 250–300 nm can be attributed to spin-allowed
π-π∗ ligand-centered (LC) transitions typically involving the excitation of electrons from
filled π to vacant π∗ orbitals of the cyclometalated ppy and phen ligands. The relatively
weak absorption bands from 300 nm extending to the visible region correspond to metal-
to-ligand charge-transfer transitions (MLCT, arising from the excitation from the filled
t2g orbital of the Ir ion to the vacant π∗ orbital of the phen ligand) and ligand-to-ligand
charge-transfer transitions (LLCT) [17,18].
Figure 2 exhibits the photoluminescence (PL) spectra of complexes in CH3CN solution.
As shown from Fig. 2 and Table 1, the PL spectra of complex 1 and 2 are peaked at 546 nm
and 540 nm, located in the green region. The PL spectrum of complex 3 is significantly
red-shifted about 60 nm with respect to the ionic iridium complexes with alkyl substitution
on phenanthroline ring. All complexes have the same cyclometalated ligands, therefore,
the results demonstrate that the presence of phenyl substituents on the phenanthroline
ligand causes more stabilization of LUMO compared to the alkyl substituted phen ligand
and capability to tune the emission color of the cationic iridium complex through the
modification of ancillary ligands.
Figure 3 shows the electrochemical behaviors of the synthesized complexes with
ferrocene as the internal standard and the redox potentials are listed in Table 1. As for three
complexes, the oxidation potentials (1.02V) are nearly the same, which can be assigned to
the oxidation of Ir center and the cyclometalated ligand. Whereas the complexes exhibit
reversible reduction peaks at −1.52, −1.54 and −1.44 V for complex 1, 2 and 3, respectively,
which are mainly contributed to the reduction of ancillary ligand [14–16]. Especially, the
lower LUMO of complex 3 (−1.44 V) respect to complex 2 (−1.54 V) indicate that the
LUMO are significantly stabilized by the ancillary ligand. The HOMO-LUMO gaps of
these complexes are 2.54, 2.56 and 2.46V, respectively. A narrower energy gap observed in
complex 3 than the other two complexes results in a red-shifted light emission.

66/[254]
D. B. Moon and Y. S. Choe
Figure 5. a) Luminance and b) current density versus voltage curves of the LECs based on complex
1, 2 and 3 as pristine luminous layer.
To investigate the electroluminescence (EL) properties of the cationic iridium
complexes, LECs with device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxy-
thiophene): poly(styrene sulfonate)(PEDOT:PSS)/cationic iridium complexe/Al were
fabricated, where the PDOT:PSS layer is inserted as anode buffer layer to smoothen the
anode surface and facilitate hole injection [19]. Figure 4 shows electroluminescence (EL)
emission spectra of the LEC devices, these EL spectra are quite similar to the PL spectra of
the cationic iridium complexes (Fig. 2). LECs based on complex 1 and 2 exhibited green
light with CIE coordinates of (0.41, 0.55) and (0.37, 0.57) for complex 1 and 2, respectively.
The device based on complex 3 gave reddish-orange electroluminescence (λmax = 600 nm)
with CIE coordinates of (0.55, 0.44). The red shift of EL spectra of the device based on

Electrochemical Cells Using Cationic Iridium (III)
[255]/67
Table 2. The detailed electrical characteristics of LEC devices
ELmax
(nm)
Maximum
luminance, voltage current density
Maximum
Active material
CIE
[Ir(ppy)2(dmphen)]PF6
[Ir(ppy)2(dbphen)]PF6
[Ir(ppy)2(dpphen)]PF6
551
537
600
(0.41, 0.55)
(0.37, 0.57)
2207 cd/m2, 9 V
2397 cd/m2, 9 V
600 mA/cm2
725 mA/cm2
1150 mA/cm2
(0.55, 0.44) 3137 cd/m2, 8.5 V
complex 3 is attributed to the chemical modifications of phenanthroline ligands. The results
demonstrate that the the lowest unoccupied molecular orbital (LUMO) located on the
ancillary ligand is decreased by phenyl substituents on the phenanthroline ancillary ligand,
resulting in narrower HOMO-LUMO energy gap than devices based on complex 1 and 2.
To investigate EL properties of the devices based on these complexes, luminance and
current density were measured against voltage from 0 to 10 V. As shown in Fig. 5, both the
luminance and current density of the devices increase slowly upon applying increasing bias
to the devices, which is a typical feature of LECs. This reflects the mechanism of device
−
operation for which the PF6 counter-ions have to reach the interface of the electrodes,
which enhances the charge-injection process [2,20]. The rapid increase in current density
above 6.0 voltage is due to the fast migration of mobile ions towards the electrodes leading
to the rapid injection of electrons and holes. The luminance also increases with the voltage
due to the ion migration which lowers the injection barrier for electron and hole. Even
though high luminance is obtained with increasing voltage, reaching a maximum value
and then diminishes because of more extensive exciton quenching [21]. Detailed electrical
characteristics of the LEC devices are summarized in Table 2. The total device performances
of complex 3 are evidently better than those of complex 1 and 2. The current density for
device based on complex 3 is higher than that obtained from the devices based on other
complexes. Similarly, the maximum luminance of 3137 cd/m2 at 9 V is obtained for device
based on 3, which is significantly higher than that for device based on complex 1and 2,
and it undergoes slow decomposition with voltage after reaching a maximum. It is believed
that the luminance, current density and stability of the LEC could be further enhanced
by modifying the complex, such as by attaching bulky and hydrophobic side groups onto
the ligands. These bulky groups act as physical barriers that protect the negative charge
injected in cationic iridium complex. This shielding is a clear impediment to the entrance
of water molecules, which render the complex through water-based chemical reactions,
and it sterically hinders a close overlapping of ligands, thus reducing the possibility for
nonradiative intermolecular charge recombination [22].
Conclusions
In summary, we investigated the influence of substituents attached to phenanthro-
line based ancillary ligands on the photophysical and electroluminescent properties
of Ir(III) complexes. Three cationic iridium complexes, [Ir(ppy)2(dmphen)]PF6 (com-
plex 1), [Ir(ppy)2(dbphen)]PF6 (complex 2) and [Ir(ppy)2(dpphen)]PF6 (complex 3)
have been synthesized and fully characterized. With dpphen as the ancillary ligands,
[Ir(ppy)2(dpphen)]PF6 has significantly stabilized LUMO orbitals and, therefore, show
much red-shifted emission spectra with respect to the other complexes. These complexes
have been used as the single active component in simple double-layer LEC devices em-
ploying air-stable electrodes. With increasing size of the side groups, the luminance and

68/[256]
D. B. Moon and Y. S. Choe
current density of devices increased. LECs based on [Ir(ppy)2(dpphen)]PF6 gave a high
luminescence of 3137 cd/m2 at 9 V. Our work suggests that the light emission of cationic
iridium complexes can easily tuned by substituting attached groups on ancillary ligands,
and the complex with large bulky groups greatly improves the luminance and the stability
of the LECs employing them.
Acknowledgments
This research was supported by a 2-Years Research Grant of Pusan National University and
Brain Korea 21 Plus project.
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