The new phosphorescent Iridium complexes containing naphthylisoquinoline ligands for WOLEDs

Article abstract of DOI:10.1080/15421406.2016.1277495

For the application to white organic light-emitting diodes (WOLEDs), new orange phosphorescent iridium complexes were designed and prepared. We introduced 1-(8-naphthyl)-isoquinoline ligands (8-NAIQ) to the iridium center along with the ancillary ligands,

Full text of DOI:10.1080/15421406.2016.1277495

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
The new phosphorescent Iridium complexes
containing naphthylisoquinoline ligands for
WOLEDs
So Hyeon Cho & Yunkyoung Ha
To cite this article: So Hyeon Cho & Yunkyoung Ha (2017) The new phosphorescent Iridium
complexes containing naphthylisoquinoline ligands for WOLEDs, Molecular Crystals and Liquid
Crystals, 644:1, 234-239, DOI: 10.1080/15421406.2016.1277495
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Date: 11 May 2017, At: 05:32

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
The new phosphorescent Iridium complexes containing
naphthylisoquinoline ligands for WOLEDs
So Hyeon Cho and Yunkyoung Ha
Department of Information Display, Hongik University, Seoul, Korea
ABSTRACT
KEYWORDS
Iridium complexes; OLED;
orange phosphorescence;
naphthylisoquinoline
For the application to white organic light-emitting diodes (WOLEDs),
new orange phosphorescent iridium complexes were designed and pre-
pared. We introduced 1-(8-naphthyl)-isoquinoline ligands (8-NAIQ) to
the iridium center along with the ancillary ligands, 2-phenylpyridine
(ppy) and 2-phenylquinoline (pq). The resulting iridium complexes syn-
thesized herein are (8-NAIQ)Ir(ppy)2 and (8-NAIQ)Ir(pq)2.
The photoabsorption, photoluminescence (PL), and electrochemical
properties of the irdium complexes synthesized in this study were inves-
tigated. The PL spectra of (8-NAIQ)Ir(ppy)2 and (8-NAIQ)Ir(pq)2 exhibited
the luminescence peaks at 536 and 583 nm with the full-width at half-
Maximum of 115 and 90 nm, respectively. These complexes exhibited
yellow to orange phosphorescence, covering rather wide luminescence
wavelength range, which are desirable as orange emitters for two emit-
ting component WOLEDs.
1. Introduction
During the past decade, organic light-emitting diodes (OLEDs) have received attention in
both scientific and industrial fields because they have unique physical properties which can
lead to be applied to many kinds of displays. OLEDs exhibit several advantages such as a
wider viewing angle, faster response time, and self emission over liquid crystal display (LCD)
[1]. However, full-color OLEDs still require improvement in terms of device efficiency, color
purity, manufacturing process, and low cost.
These factors depend on the development of efficient OLED emitters with proper color
purity. In efficiency, phosphorescene exceeds fluorescence and thus the design and synthesis
of phosphorescent emitting materials are important in particular. Furthermore, development
of phosphors widening color coordinates is required to realize full color displays. The iridium
complexs have attracted much interest as highly emissive dopants in OLEDs because of their
excellent phosphorescent quantum yields and emission color tunability by ligand molecular
structures [2–5].
The orange light emitting iridium complexes are important emitters to fabricate two-
component white OLEDs (WOLEDs), which have been one of the most important directions
of OLED studies recently. In several research groups, the orange-red phosphorescence by the
iridium complexes has been reported, and the extensive luminescence wavelength ranges dis-
played by some of them are suitable for application to WOLEDs [6–7].
CONTACT Yunkyoung Ha
Mapo-gu, Seoul, Korea.
ykha@hongik.ac.kr
Dept. of Information Display, Hongik University, Wawoosan-ro ,
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
235
To design orange-red light emitting materials for WOLEDs, we prepared main ligand,
1-(8-naphthyl)-isoquinoline (8-NAIQ), and synthesized its iridium complexes with 2-
phenylpyridine (ppy) and 2-phenylquinoline (pq) as an ancillary ligand. We expect that
chelation of a 1-(8-naphthyl)-isoquinoline ligand to the iridium center might lead to red
emission color tenability and possible luminous efficiency increase.
Thus, we report and discuss the results of our investigation on the synthesis, characteriza-
tion, photophysical properties of the iridium complexes containing (8-NAIQ), and ppy or pq
ligands. We also investigated the emission properties of these new iridium complexes to find
how 8-NAIQ main ligand has influence on emission characteristics of its chelated iridium
complexes. In addition, the electrochemical properties were studied to support orange-red
phosphorescence realization of these complexes by estimating their energy gaps between the
highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO).
2. Experiment
All of the reagents were purchased from Aldrich, TCI, and Strem Co., and used without fur-
ther purification. The reactions were carried out under an argon atmosphere. The solvents
were dried by standard procedures. The column chromatography was performed with the
use of silica gel.
2.1. Synthesis of ligands
1-(8-naphthyl)-isoquinoline (8-NAIQ). 1-(8-naphthyl)-isoquinoline was obtained from the
reaction of 8-bromoisoquinoline and naphthalene-1-boronic acid by Suzuki coupling [8].
2.2. Synthesis of Iridium complexes
(8-NAIQ)Ir(L)2 (L = ppy, pq). First, the cyclometalated Ir(III) μ-chloro-bridged dimer,
(L)2Ir(μ Cl)2Ir(L)2 (1 mmol), was prepared according to the Nonoyama method [9]. In the
second step, the resulting dimer and 1-(8-naphthyl)-isoquinoline (2 mmol) were mixed with
Na2CO3 (500 mg) in 2-ethoxyethanol (30 mL). The mixture was refluxed for 3 h, and the
solid was filtered after cooling. (8-NAIQ)Ir(L)2 were purified by chromatography on sil-
ica gel column with dichloromethane followed by recrystallization with dichloromethane/
hexane.
2.3. (8-NAIQ)Ir(ppy)2: A green powder. FAB-MS: Calculated 755; found 756
1H NMR (400 MHz, CDCl3): δ 9.99 (d,1H), 8.11 (d,1H), 7.92 (d,1H), 7.78–7.65 (m,6H), 7.60–
7.52 (m,5H) 7.18 (t,1H), 6.99 (t,1H), 6.91–6.81 (m,4H), 6.75 (t,1H), 6.43 (d,1H), 6.24 (d,1H),
3.55 (t,1H).
2.4. (8-NAIQ)Ir(pq)2: A red powder. FAB-MS: Calculated 855; found 856
1H NMR (400MHz, CDCl3): δ 9.75 (d,1H), 9.48 (d,1H), 8.85 (d,1H), 8.66–8.64 (m,3H), 8.28
(d,1H), 8.21–8.02 (m,9H) 7.92–7.87 (m,5H), 7.74–7.70 (t,2H), 7.63–7.49 (m,3H), 7.35 (t,1H),
7.19 (d,1H), 6.56 (d,1H).

236
S. H. CHO AND Y. HA
2.5. Optical and electrochemical measurements
UV-Vis absorption spectra were measured on a Hewlett Packard 8425A spectrometer. Pho-
toluminescence (PL) spectra were measured on a Perkin Elmer LS 50B spectrometer. UV-Vis
and PL spectra of iridium complexes were measured in 10−5 M dilute CH2Cl2 solution. Cyclic
voltammograms were obtained at scan rate of 100 mV/s, and tetrabutylammonium hexaflu-
orophosphate was added as an electrolyte in CH2Cl2 solution.
3. Result and discussion
The synthesis of the ligands, 8-NAIQ, was straightforward, according to the procedures
reported previously [8]. The other ligand materials were purchased and used without further
purification. The iridium complexes were synthesized via two steps, as reported by Nonoyama
[9]. The overall synthetic schemes are illustrated in Fig. 1.
The UV-Vis aborption spectra of the iridium complexes in CH2Cl2 were obtained as shown
in Figure 2. The spectra of iridium complexes have the strong absorption bands, appearing in
the ultraviolet region between 250 and 300 nm. These bands were assigned to the spin-allowed
1π→ πࢩ transitions of the ligands. The 1π-πࢩ bands are accompanied by the weak and low
energy features extending into the visible region from 300 to 380 nm. These absorptions are
assigned to a spin-allowed metal charge transfer (1MLCT) band, and the weaker absorption
bands above 380 nm and longer wavelengths can be attributed to the spinforbidden 3MLCT
and spin-orbit coupling enhanced 3π-πࢩ transition. The intensities of the MLCT and 3π-πࢩ
transition bands are varied, depending on the chelating ligands. These intensities are generally
attributed to mixing of the charge-transfer transitions with high lying spin-allowed transitions
on both main and ancillary chelating ligands. For instance, the bands above 380 nm absorbed
by (8-NAIQ)Ir(pq)2 were found more intense than those bands shown by (8-NAIQ)Ir(ppy)2.
It can be explained by the fact that the ligand pq itself has a smaller energy gap than the ligand
ppy, causing its iridium complex to absorb the light at the longer wavelengths than that of ppy
ligands.
Figure . The synthesis of the main ligand(-NAIQ) and its iridium complexes.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
237
Figure . The UV-Vis absorption spectra of the complexes, (-NAIQ)Ir(ppy) and (-NAIQ)Ir(pq) in CHCl.
Figure 3 shows the PL spectra of the Ir complexes in 10−5 M CH2Cl2 solution. The emission
maxima of (8-NAIQ)Ir(ppy)2 at (8-NAIQ)Ir(pq)2 appear at 536 and 583 nm, respectively.
The luminescence maximum of (8-NAIQ)Ir(ppy)2 was confirmed to appear at a shorter
wavelength region than that of (8-NAIQ)Ir(pq)2. It can be attributed to the similar reason
discussed in the light absorption pattern, due to a larger energy gap of ppy than pq ligand in
the complexes. The interesting contribution by the ligand 8-NAIQ to the iridium complexes
was observed in the phosphorescence wavelengths. The PL peak of (8-NAIQ)Ir(ppy)2 at
536 nm is bathochromically shifted from that of Ir(ppy)2(acac) at 520 nm10, while the PL
peak of (8-NAIQ)Ir(pq)2 at 583 nm is compared to that of Ir(pq)2(acac) at 597 nm11, hyper-
chromically shifted. Therefore, the phosphorescence of the iridium complexes of 8-NAIQ
and ppy or pq ligands was affected by both the main 8-NAIQ and by the ancillary ppy or pq
ligands, depending on the mixing energy levels of the chelating ligands in the complexes.
Another important features displayed by the iridium complexes of 8-NAIQ were their full
widths at half maximum (FWHM) in PL spectra. (8-NAIQ)Ir(ppy)2 exhibited the phospho-
rescence with a wide wavelength range from 470 to 700 nm. Its FWHM was measured to
Figure . The PL spectra of the complexes, (-NAIQ)Ir(ppy) and (-NAIQ)Ir(pq) in CHCl.

238
S. H. CHO AND Y. HA
Table . Photophysical and electrochemical parameters for (-NAIQ)Ir(ppy) and (-NAIQ)Ir(pq).
Ir complex
λem/nma
Eox/Vb
HOMO/eVc
LUMO/eVd
ꢀE/eVd
(-NAIQ)Ir(ppy)
(-NAIQ)Ir(pq)


.
.
− .
− .
− .
− .
.
.
a Measured in CHCl solution.
bscan rate: mV/s, Electrolyte: tetrabutylammonium hexafluorophosphate. The potentials are quoted against the internal
ferrocene standard.
c Deduced from the equation HOMO = −. −Eox
d Calculated form the optical edge ꢀE = LUMO − HOMO.
be 115 nm, which is twice of FWHM values displayed by Ir(ppy)2(acac) [10]. Similarly, (8-
NAIQ)Ir(pq)2 showed FWHM of 90 nm, which could be compared to 75 nm of FWHH by
Ir(pq)2(acac) [11]. The phosphors which have such large FWHMs can be utilized as good
candidates for white emission because they can emit at wide range of the visible region [12].
Thus, these complexes prepared herein might be possible orange phosphors to realize excel-
lent displays with widening color scopes for WOLEDs.
We also investigated the electrochemical properties of the iridium complexes by the
cyclic voltammetry (CV) as shown in Table 1, which reveals their positions of the
HOMO/LUMO. The oxidation potentials which indicate the HOMOs of (8-NAIQ)Ir(ppy)2
and (8-NAIQ)Ir(pq)2 were reversible at 0.36 and 0.34 V relative to an internal ferroce-
nium/ferrocene reference (Fc + /Fc), respectively. The LUMOs of the complexes were esti-
mated from their respective absorption spectra, and were calculated to be−1.77 and −2.01 eV,
according to optical-edge and band gap equation (ꢀE = Eox − Ered) [13]. The resulting ꢀE
values of the complexes were consistent with the tendencies of their PL maxima. The detailed
CV data of the heteroleptic iridium complexes were summarized in Table 1.
Therefore, both optical and cyclovoltametric measurements suggest that the iridium com-
plexes investigated in this study exhibit the orange phosphorescence with rather wide lumi-
nescence ranges as promising candidates for two-component white light-emitting materials
with a blue luminescent phosphor [14].
4. Conclusions
We reported the synthesis of the new iridium complexes containing 1-(8-naphthyl)-
isoquinoline as main ligand, (8-NAIQ)Ir(ppy)2 and (8-NAIQ)Ir(pq)2. Their photoabsorption,
PL and electrochemical properties were investigated. (8-NAIQ)Ir(ppy)2 and (8-NAIQ)Ir(pq)2
exhibited the emission maxima at 536 and 583 nm with FWHM of 115 and 90 nm, respec-
tively. The complex containing pq ancillary ligands had tendencies to exhibit the phosphores-
cence at the longer wavelengths than that containing ppy. The iridium complexes of 8-NAIQ
exhibit bathochromic and hyperchromic shift from Ir(ppy)2(acac) and Ir(pq)2(acac) which
are not chelated with 8-NAIQ ligand, respectively. These results indicate that the ancillary lig-
ands also influence the phosphorescence wavelengths displayed by their iridium complexes
with 8-NAIQ. Furthermore, such wide phosphorescence range of these complexes suggests
that these materials can be promising orange phosphors for WOLEDs.
Funding
This research work was supported by 2015 Hongik University Research Fund.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
239
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