Photo- and electroluminescence from deep-red- and near-infrared- phosphorescent tris-cyclometalated iridium(III) complexes bearing largely π-extended ligands

Article abstract of DOI:10.1016/j.inoche.2013.09.075

Deep-red- and near-infrared-phosphorescent tris-cyclometalated iridium(III) complexes bearing largely π-extended cyclometalated (C^N) ligands were newly synthesized, and their photo- and electroluminescence properties were investigated. When 2-(benzo[b]furan-2-yl)quinoline and 2-(benzo[b]thiophen-2-yl) quinoline were employed as C^N ligands, deep-red photoluminescence was obtained (Ir-1a and Ir-1b; λ_PL in CH₂Cl₂, 647 and 652 nm, respectively). In the case of the isoquinoline analogues of Ir-1a and Ir-1b, emission maxima were further red-shifted, ranging from deep-red to near-infrared regions (Ir-2a and Ir-2b; λ_PL in CH ₂Cl₂, 696 and 690 nm, respectively). Especially, Ir-2b showed an excellent photoluminescence quantum yield (Φ_PL = 0.15), and a polymer light-emitting diode doped with Ir-2b exhibited deep-red-near-infrared electroluminescence with a high external quantum efficiency (λ_EL = 694 nm, η_{ext max} = 1.41%).

Full text of DOI:10.1016/j.inoche.2013.09.075

Inorganic Chemistry Communications 38 (2013) 14–19
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Photo- and electroluminescence from deep-red- and
near-infrared-phosphorescent tris-cyclometalated iridium(III)
complexes bearing largely π-extended ligands
a
a
b
Shigeru Ikawa a, Shigeyuki Yagi a, , Takeshi Maeda , Hiroyuki Nakazumi , Hideki Fujiwara ,
⁎
Shiro Koseki b, Yoshiaki Sakurai c
a
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1–1 Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan
Department of Chemistry, Graduate School of Science, Osaka Prefecture University, 1–1 Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan
Textile and Polymer Section, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594–1157, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Deep-red- and near-infrared-phosphorescent tris-cyclometalated iridium(III) complexes bearing largely
π-extended cyclometalated (C^N) ligands were newly synthesized, and their photo- and electroluminescence
properties were investigated. When 2-(benzo[b]furan-2-yl)quinoline and 2-(benzo[b]thiophen-2-yl)quinoline
were employed as C^N ligands, deep-red photoluminescence was obtained (Ir-1a and Ir-1b; λPL in CH2Cl2, 647
and 652 nm, respectively). In the case of the isoquinoline analogues of Ir-1a and Ir-1b, emission maxima were
further red-shifted, ranging from deep-red to near-infrared regions (Ir-2a and Ir-2b; λPL in CH2Cl2, 696 and
690 nm, respectively). Especially, Ir-2b showed an excellent photoluminescence quantum yield (ΦPL = 0.15),
and a polymer light-emitting diode doped with Ir-2b exhibited deep-red–near-infrared electroluminescence
with a high external quantum efficiency (λEL = 694 nm, ηext max = 1.41%).
Received 8 August 2013
Accepted 30 September 2013
Available online 11 October 2013
Keywords:
Iridium complex
Cyclometalated ligand
Phosphorescence
Near-infrared emission
Organic light-emitting diode
© 2013 Elsevier B.V. All rights reserved.
Organic light-emitting diodes (OLEDs) have attracted much atten-
tion for the last two decades because their technological advantages,
such as self-emission, fast response, simple device construction, and so
on, make them applicable to flat panel displays and lighting apparatuses
in next generations [1–6]. Recently OLEDs emitting in near-infrared
(NIR) regions have been also receiving growing interest [7–10] because
they are expected to be applied to night-vision readable displays and se-
curity sensors [11]. Therefore, NIR-emitting materials have been eagerly
required for such purposes. In general, device efficiencies of OLEDs
are markedly improved when fluorescent dopants are replaced with
phosphorescent ones such as platinum(II) [12,13] and iridium(III)
complexes [14–16] because they can effectively form triplet excitons
upon electric excitation. With this respect, a number of bis- and tris-
cyclometalated iridium(III) complexes (Ir(C^N)2LX and Ir(C^N)3, respec-
tively [17]) emitting RGB phosphorescence have been developed
[18–20]. Nevertheless, few examples of excellent NIR-phosphorescent
iridium(III) complexes that allow us to fabricate high-efficiency NIR-
OLEDs have so far been reported [11,21,22] since the photoluminescence
(PL) quantum yield of an emitting material intrinsically tends to de-
crease as the emission wavelength is red-shifted (i.e., so-called energy
gap law) [23–25]. Recently, Qiao and coworkers reported that an OLED
doped with bis[(2-phenylbenzo[g]quinoline)-C2,N]iridium(III) acetyl-
acetonate as a phosphorescent dopant gave NIR emission at 708 nm
[22]. Although this device exhibited relatively efficient device perfor-
mance for NIR-OLED, the maximum external quantum efficiency was
still low (ηext max = 1.07%). In the present paper, we report synthesis
and PL properties of phosphorescent tris-cyclometalated iridium(III)
complexes Ir-1 and Ir-2 emitting from deep-red to NIR regions. Also,
we discuss about the electroluminescence (EL) properties of poly(N-
vinylcarbazole)-based polymer light-emitting diodes (PLEDs) doped
with those iridium(III) complexes.
To obtain NIR phosphorescence, a quite low T1 level is required. So
we designed tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2
(Scheme 1), the C^N ligands of which are based on 2-(benzo[b]furan-
2-yl)- and 2-(benzo[b]thiophen-2-yl)quinolines and their isoquinoline
analogues. Until now, theoretical studies about a variety of Ir(C^N)2LX-
and Ir(C^N)3-type complexes were reported, revealing that the excited
states are mainly based on metal-to-ligand charge transfer (MLCT) tran-
sitions. That is, from density functional theory (DFT) calculations of the
most basic complexes such as Ir(ppy)2(acac) and Ir(ppy)3 (ppy and
acac; 2-phenylpyridinato-C2,N and acetylacetonato-O,O ligands, respec-
tively), the HOMO consists of a mixture of phenyl-pπ and Ir-dπ orbitals,
and the LUMO receives a considerable contribution from the C^N
ligands, especially the pyridyl-π orbitals [26–29]. With this respect,
replacement of the phenyl moieties of the C^N ligands by electron-
enriched benzo[b]furan and benzo[b]thiophene components gives rise
to destabilization of the HOMO [30,31]. On the other hand, employment
of the benzologues such as quinoline and isoquinoline in place of the
pyridine moiety is effective on stabilization of the LUMO [32,33]. Thus,
⁎
Corresponding author. Tel.: +81 72 254 9324; fax: +81 72 254 9910.
E-mail address: yagi@chem.osakafu-u.ac.jp (S. Yagi).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.09.075

S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
15
Scheme 1. Reagents and Conditions: (i) IrCl3 · 3H2O, 2-ethoxyethanol, H2O, 100 °C; (ii) acetylacetone, Na2CO3, 2-ethoxyethanol, 90 °C; (iii) C^N ligand, glycerol, 230 °C; (iv) C^N ligand,
glycerol, 280 °C, microwave irradiation.
the present molecular design based on the HOMO–LUMO tuning should
allow us to obtain NIR-emitting cyclometalated iridium(III) complexes.
The synthesis of the iridium(III) complexes is shown in Scheme 1.
The preparation of HC^N-2b was previously reported [34]. Similarly,
HC^N-1a and HC^N-1b were also prepared by the Suzuki–Miyaura
cross-coupling reaction of (benzo[b]furan-2-yl)- and (benzo[b]
thiophen-2-yl)boronic acid with 2-chloroquinoline, respectively.
HC^N-2a was newly synthesized by the Suzuki–Miyaura cross-
coupling method in 92% yield (see the supplementary materials
section). In general, Ir(C^N)3-type complexes are efficiently synthesized
by the reaction of a μ-chloro-bridged iridium(III) dimer [(C^N)2Ir(μ-Cl)]2
or a bis-cyclometalated iridium(III) complex (C^N)2Ir(acac) with the
corresponding C^N ligand [16]. These procedures yield a facial (fac) iso-
mer exclusively when the reactions proceed at high temperatures
(N200°C). In the present case, we examined the ligand replacement of
(C^N-1a)2Ir(acac) and (C^N-1b)2Ir(acac) at 230 °C and yielded Ir-1a
and Ir-1b in 11 and 14%, respectively (overall yields from HC^N-1a
and HC^N-1b; 2.4 and 8.9%, respectively). For the synthesis of Ir-2,
microwave-assisted reactions (2450 30MHz, 300 W, 280 °C, 45min)
brought about better results, where [(C^N-2a)2Ir(μ-Cl)]2 and [(C^N-
2b)2Ir(μ-Cl)]2 were reacted with the corresponding C^N ligands to
yield Ir-2a and Ir-2b in 41 and 88%, respectively (overall yields from
HC^N-2a and HC^N-2b; 31 and 86%, respectively). Unfortunately, in
the case of the preparation of Ir-1a and Ir-1b, the microwave synthesis
was unsuccessful, and trace amounts of the target complexes were
obtained.
consideration that these three products were obtained at more than
230°C, they should be fac isomers [37,38].
Fig. 2 shows the UV–vis absorption spectra of Ir-1 and Ir-2 in dichlo-
romethane at rt. The data are also listed in Table 1. All complexes exhibit
similar absorption spectral profiles. The absorption bands at b390nm are
⁎
assigned to the spin-allowed π–π transitions at the C^N ligands, and the
next absorption bands up to ca. 570nm are assigned to the spin-allowed
1MLCT transitions from the Ir(III) centers to the C^N ligands. As repre-
sented by the magnified spectra (Fig. 2, dashed lines), the lower-
energy absorption bands in the region of N590 nm are assignable to
the spin-forbidden 3MLCT transitions. In this region, the molar absorp-
tion coefficients of Ir-1b and Ir-2b are larger than those of the corre-
sponding benzo[b]furan analogues (i.e., Ir-1a and Ir-2a, respectively),
According to 1H NMR spectral analysis, each complex of Ir-1 and Ir-2
was obtained as a single isomer. As shown in Fig. 1, the molecular struc-
ture of Ir-2b was determined by X-ray crystallographic analysis [35].
The molecule of Ir-2b adopts distorted octahedral coordination geome-
try around the metal center, and three C^N ligands are arranged in a
fac manner, affording a racemic mixture of enantiomeric fac isomers.
Although we attempted X-ray crystallographic analysis of Ir-1a, Ir-1b
and Ir-2a, any suitable single crystals have not been obtained at this
point. According to the previous reports [16,36], the fac isomer is
thermodynamically more stable than the meridional one. Taking into
Fig. 1. ORTEP drawing of Ir-2b. The hydrogen atoms are omitted for clarity.

16
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
Fig. 2. UV–vis absorption spectra of Ir-1 and Ir-2 in dichloromethane at rt. The solid
lines are the spectra at concentrations of 10 μM. The dashed lines are the ones 20 times
expanded in intensity in the region from 540 to 750 nm.
indicating that the benzo[b]thiophene-derived complexes exhibit
more effective spin–orbit coupling. Focusing on the N-heterocycle in
the C^N ligand, the 3MLCT transition bands of Ir-2a and Ir-2b are
bathochromically shifted in comparison with those of Ir-1a and Ir-1b.
This indicates that the isoquinoline-based complexes exhibit more red-
shifted phosphorescence than the quinoline-based ones.
Fig. 3a shows the PL spectra of Ir-1 and Ir-2 in dichloromethane at rt,
and the spectral and photophysical profiles are summarized in Table 1.
The PL lifetimes (τPLs) range from 1.13 to 2.09μs, each of which is well
fitted to single-exponential decay (χ2; 1.03–1.07), indicating that the
obtained emission are exclusively phosphorescence. In the PL spectra
of Ir-1a and Ir-1b, the emission maxima (λPL) are observed at 652
and 647 nm, respectively, and their emission colors are deep-red. The
PL quantum yield (ΦPL) of Ir-1a is 0.12, whereas Ir-1b emits more
efficiently in solution, affording ΦPL of 0.21. So, one can see that the im-
proved ΦPL of Ir-1b rather than Ir-1a is attributed to the enhanced spin–
orbit interaction as indicated in the UV–vis spectra. The isoquinoline-
based C^N ligands gave rise to further red shifts of PL spectra: the λPLs
of Ir-2a and Ir-2b are observed at 696 and 690 nm, respectively, and
the shoulders of the spectra reached NIR regions. The ΦPLs of Ir-2a
and Ir-2b are 0.068 and 0.15, respectively, and as discussed in Ir-1b,
the stronger spin–orbit interaction of Ir-2b should bring about a larger
Fig. 3. PL spectra of Ir-1 and Ir-2 (a) in dichloromethane ([Ir(C^N)3] = 10 μM) and (b) in
PMMA thin films (PMMA/Ir(C^N)3 = 100/4, wt/wt).
excellent NIR-PL properties make us expect the possibility of a solid-
state NIR electroluminescent device.
The electrochemical behavior of Ir-1 and Ir-2 was examined using
cyclic voltammetry. The cathodic and anodic scans were carried out in
anhydrous DMF solutions at rt. The cyclic voltammograms of Ir-1 and
Ir-2 are shown in Fig. 4, and the redox potentials determined in compar-
ison with the ferrocene/ferrocenium redox (Fc/Fc+) are listed in Table 2.
Reversible one-electron oxidation and reduction couples were obtained
for Ir-1b and Ir-2a, whereas Ir-1a and Ir-2b showed reversible one-
electron oxidation potentials as well as one- and two-electron reduction
ΦPL. The PL properties in polymer thin films were also investigated.
ones. Oxidation potentials of Ir-1a and Ir-1b (Eo1x/2; 0.54 and 0.51 V,
ox
Poly(methyl methacrylate) (PMMA) was chosen as a matrix polymer
because the absence of absorption bands from near-UV to visible re-
gions allows us to estimate the intrinsic PL properties of the iridium(III)
complexes [39]. The PL spectra of Ir-1 and Ir-2 in PMMA films are
almost identical to those in solutions (Fig. 3b). As summarized in
Table 1, the ΦPLs in PMMA films are also similar to those in dichloro-
methane. It is worthy to note that Ir-2b is still emissive even in the
solid matrix (ΦPL = 0.11) in spite of its low-lying triplet level. Such
respectively) are slightly higher than those of Ir-2a and Ir-2b (E
;
1/2
0.42 V), although the first reduction potentials of all complexes are al-
most similar (Er1e/d2; ‒2.07 to −2.11 V). It was found that the oxidation
and reduction potentials are not so different between benzo[b]fran-
and benzo[b]thiophene-derived complexes, i.e., between Ir-1a and
Ir-1b and between Ir-2a and Ir-2b. The HOMO/LUMO energy levels
(EHOMO/ELUMO) of Ir-1 and Ir-2 are also estimated in comparison
with the energy level of ferrocene (EHOMO = −4.80 eV below vacuum
Table 1
UV–vis absorption and photoluminescence (PL) spectral data of Ir-1 and Ir-2a.
Compd
CH2Cl2 solution
abs/nm (10−4 ε/M−1 cm−1
PMMA thin film
b
c
d
λ
)
λ
PL/nm (λex/nm)c
ΦPL
τ
PL/μs (χ2)c
λ
PL/nm (λex/nm)d
ΦPL
Ir-1a
Ir-1b
Ir-2a
Ir-2b
318 (4.6), 432 (2.4), 520 (0.83) [sh],
639 (0.012) [sh]
276 (4.7), 314 (4.6), 435 (2.1),
529 (0.57) [sh], 623 (0.022)
328 (3.8), 450 (2.6), 518 (1.1) [sh],
624 (0.031) [sh], 683 (0.015) [sh]
291 (4.2), 317 (4.3), 456 (3.0),
535 (1.1), 612 (0.046) [sh], 665 (0.024)
652 (454)
647 (453)
696 (468)
690 (468)
0.12
0.21
0.068
0.15
1.34 (1.07)
2.09 (1.03)
1.13 (1.05)
1.77 (1.05)
648 (454)
644 (453)
692 (467)
694 (467)
0.11
0.22
0.065
0.11
a
λ
abs, UV–vis absorption maximum; λPL, PL maximum; λex, excited wavelength for PL; ΦPL, PL quantum yield; τPL, PL lifetime.
b
c
[sh]; shoulder peak.
Obtained for deaerated samples at rt.
Obtained under N2 atmosphere at rt. The ratio of PMMA/Ir(C^N)3 was adjusted to 100/4 (wt/wt).
d

S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
17
Fig. 5. (a) EL spectra and (b) external quantum efficiency (ηext)–current density curves of
PLED-1 and PLED-2. The spectra were taken at the voltages where the maximum values of
luminance were obtained.
Fig. 4. Cyclic voltammograms of (a) Ir-1 and (b) Ir-2 in anhydrous DMF at a scan rate of
100 mV s-1
.
[40]) (Table 2). These levels are placed between HOMO and LUMO of
poly(N-vinylcarbazole) (PVCz, ‒5.80 to −2.20eV) and 2-(4-biphenylyl)-
5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, ‒6.20 to −2.40 eV) [41],
and thus Ir-1 and Ir-2 have the EHOMO and ELUMO values suitable for emit-
ting dopants in PVCz-based PLEDs.
and benzo[b]thiophene units on the HOMO/LUMO energy levels are
hardly observed.
Employing Ir-1 and Ir-2 as emitting dopants, PLEDs were fabricated;
namely, PLED-1 and PLED-2, respectively. The device structure is
as follows; ITO (anode, 150 nm)/PEDOT:PSS (40 nm)/emitting layer
(100 nm)/CsF (1.0 nm)/Al (cathode, 250 nm). The emitting layer con-
sists of PVCz doped with the iridium(III) complex and PBD (electron
transporting material), where the weight ratio of PVCz:PBD:Ir-1 (or
Ir-2) is adjusted to 1.0:0.30:0.040 (wt/wt/wt).
The obtained EL spectra of the PLEDs are shown in Fig. 5a, each
of which is comparable to the PL spectrum of the emitting dopant.
Thus, PLED-1 exhibits deep-red EL (λEL; 654 and 648 nm for PLED-1a
and PLED-1b, respectively), whereas EL spectra of PLED-2 range
from deep-red to NIR regions (λEL; 697 and 694 nm for PLED-2a and
PLED-2b, respectively). In Fig. 5b are shown the external quantum
efficiency (ηext)–current density curves for the PLEDs. The device
DFT calculations were also carried out for Ir-1 and Ir-2. To obtain
their optimized structures, the GAMESS suite of program codes was
used [42]. In these calculations, the B3LYP functional [43] with the
SBKJC effective core potentials and the associated basis sets [44,45]
were used. Calculated EHOMOs and ELUMOs of Ir-1 and Ir-2 as well as
the electron density distributions of the orbitals were listed in the sup-
plementary materials section (Fig. S1 and Table S1). The HOMO distri-
butions of electronic density primarily reside on the Ir-dπ orbital, and
the LUMO distributions mainly reside on the quinoline (or isoquinoline)
moieties of the C^N ligands, as is seen in typical cyclometalated
iridium(III) complexes [26–29]. The EHOMOs of Ir-1a and Ir-1b are
the same (−5.08 eV) as each other, and their ELUMOs are very close
(−2.01 and −2.05 eV for Ir-1a and Ir-1b, respectively). In Ir-2a and
Ir-2b, similar values are also found for the EHOMOs (−5.01 and −5.00
eV, respectively) and the ELUMOs (−2.06 and −2.11 eV, respectively).
Therefore, the heteroatom substituent effects of the benzo[b]furan
Table 3
Device performance of PLED-1 and PLED-2.
PLED-1a
PLED-1b
PLED-2a
PLED-2b
Vturn-on (V)a
6.0
6.0
12.5
8.5
b
Lmax (cd m−2
)
187 (13.5)
0.25 (7.0)
0.11 (7.0)
0.85 (7.0)
(0.63, 0.28)
415 (14.5)
0.63 (7.5)
0.28 (7.0)
1.83 (7.0)
(0.64, 0.27)
9.04 (17.5)
0.0077 (13.0)
0.0019 (13.0)
0.15 (12.5)
46.5 (16.5)
0.046 (11.0)
0.013 (11.0)
1.41 (8.5)
(0.65, 0.26)e
b
η
η
η
j max (cd A−1
)
p max (lm W−1
ext max (%)b
)
b
Table 2
Electrochemical properties of Ir-1 and Ir-2.
CIE (x, y)c
‒
d
ox
Complex
E
(V)a
Er1e/d2 (V)a
EHOMO (eV)b
ELUMO (eV)c
Eg (eV)d
1/2
Turn-on voltage is the one at which the luminance over 1 cd m−2 was obtained.
Maximum values of the luminance (L), current efficiency (ηj), power efficiency (ηp),
a
Ir-1a
Ir-1b
Ir-2a
Ir-2b
0.54
0.51
0.42
0.42
−2.36, ‒2.10
−2.11
−5.34
−5.31
−5.22
−5.22
−2.70
−2.69
−2.70
−2.73
2.64
2.62
2.52
2.49
b
and external quantum efficiency (ηext). The values in the parentheses represent the
−2.10
voltages (V) at which they were obtained.
−2.28, ‒2.07
c
The Commission Internationale de L'Eclairage chromaticity coordinate obtained at the
b
a Potential values vs. Fc+/Fc were obtained in anhydrous DMF. Calculated from the
voltage where the Lmax was observed.
ox
c
red
d
equation EHOMO = −4.8 ‒ E
.
Calculated from the equation ELUMO = −4.8 ‒ E
.
1/2
Visible EL is too weak to detect the correct value of CIE(x, y).
The value of CIE (x, y) is saturated because only deep-red EL was detected.
1/2
d Calculated from the relation Eg = Eo1x/2 ‒ E
.
red
e
1/2

18
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
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PLED-1a (or PLED-2a). This result reflects the magnitude of ΦPL
,
and it is indicated that the benzo[b]thiophene-derived complexes are
more suitable for OLED application than the benzo[b]furan analogues.
Especially, PLED-2b shows the highest ηext max of 1.41% at a current
density of 15.8 mA cm−2
.
In conclusions, we demonstrated PL properties of phosphorescent
tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2 emitting
from deep-red to NIR regions. The PLEDs doped with these complexes
were also fabricated, and their device performance was investigated.
Especially, adopting largely π-extended 1-(benzo[b]thiophen-2-yl)
isoquinoline as a C^N ligand, relatively efficient PL ranging from deep-
red to NIR was obtained in dichloromethane as well as in PMMA film.
Furthermore, the PVCz-based PLED doped with this complex afforded
efficient EL in a similar region to PL, showing the excellent device per-
formance for an NIR-OLED (ηext max = 1.41%).
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This work was partially supported by JSPS Grant-in-Aid for Scientific
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The supplementary crystallographic data for Ir-2b (except structure
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Centre as supplementary publications CCDC 954294. Copies of the data
can be obtained free of charge by application to CCDC, 12 Union Road,
Cambridge CB21EZ, UK (e-mail: deposit@ccdc.cam.ac.uk). Supplemen-
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materials syntheses, X-ray crystallographic analysis, UV-vis and PL
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