Effect of the bipyridine ligand substituents on the emission properties of phosphorescent Ir(III) compounds

Article abstract of DOI:10.1016/j.dyepig.2017.07.034

To investigate the effect of bipyridine ligand substituents on the emission properties of Ir(III) compounds, four homoleptic iridium(III) compounds, specifically, mer-Ir(Mepypy)₃ (1), mer-Ir(Me₂pypy)₃ (2), fac-Ir(OMe₂pypy)₃ (3) and Ir(OMe₂Bupypy)₃ (4), where Mepypy = 2'-methyl-2,3'-bipyridine, Me₂pypy = 2′,6'-dimethyl-2,3'-bipyridine, OMe₂pypy = 2′,6'-dimethoxy-2,3'-bipyridine and OMe₂Bupypy = 2′,6'-dimethoxy-4-tert-butyl-2,3'-bipyridine, were prepared via a one-pot reaction of the corresponding methyl- or methoxy-substituted ligand with Ir[(COD)]BF₄ as the starting material. Under the same reaction conditions, the major isolated products differed depending on the substituent used. The methyl substituents resulted in Ir(III) compounds with a meridional geometry, while methoxy-substituted Ir(III) compounds with a facial geometry were isolated in high yields when the methoxy substituent was used. Compounds 1 and 2 emit a green phosphorescence (Φ_PL = 0.3) with a λ_max = 459–463 nm, while 3 and 4 show a bright, sky-blue emission (Φ_PL = 0.5). The dimethoxy-substituted bipyridine ligand was advantageous in terms of yield, thermal stability and quantum efficiency, and its iridium compound is a good candidate for triplet emitters in phosphorescence organic light-emitting diodes (PHOLEDs).

Full text of DOI:10.1016/j.dyepig.2017.07.034

Dyes and Pigments 146 (2017) 386e391
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Effect of the bipyridine ligand substituents on the emission properties
of phosphorescent Ir(III) compounds
,
, *
Myeongbee Kim a, Sihyun Oh a, Jinho Kim a, Ki-Min Park b **, Youngjin Kang a
a Division of Science Education & Department of Chemistry, Kangwon National University, Chuncheon 24341, Republic of Korea
b Research Institute of Natural Science, Gyeongsang National University, Jinju 52828, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
To investigate the effect of bipyridine ligand substituents on the emission properties of Ir(III) compounds,
four homoleptic iridium(III) compounds, specifically, mer-Ir(Mepypy)3 (1), mer-Ir(Me2pypy)3 (2), fac-
Received 13 June 2017
Received in revised form
14 July 2017
Accepted 16 July 2017
Available online 17 July 2017
Ir(OMe2pypy)3 (3) and Ir(OMe2Bupypy)3 (4), where Mepypy
¼
2'-methyl-2,3'-bipyridine,
Me2pypy
¼
20,6'-dimethyl-2,3'-bipyridine, OMe2pypy 20,6'-dimethoxy-2,3'-bipyridine and OMe2
¼
Bupypy ¼ 20,6'-dimethoxy-4-tert-butyl-2,3'-bipyridine, were prepared via a one-pot reaction of the
corresponding methyl- or methoxy-substituted ligand with Ir[(COD)]BF4 as the starting material. Under
the same reaction conditions, the major isolated products differed depending on the substituent used.
The methyl substituents resulted in Ir(III) compounds with a meridional geometry, while methoxy-
substituted Ir(III) compounds with a facial geometry were isolated in high yields when the methoxy
substituent was used. Compounds 1 and 2 emit a green phosphorescence (FPL ¼ 0.3) with a lmax ¼ 459
e463 nm, while 3 and 4 show a bright, sky-blue emission (FPL ¼ 0.5). The dimethoxy-substituted
bipyridine ligand was advantageous in terms of yield, thermal stability and quantum efficiency, and
its iridium compound is a good candidate for triplet emitters in phosphorescence organic light-emitting
diodes (PHOLEDs).
Keywords:
Homoleptic Ir(III) compounds
Bipyridine ligand
Phosphorescence
Organic light-emitting diodes (OLEDs)
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
on the chelate ligand is necessary to control the phosphorescent
emission of a compound. Although tuning the emission energy for
Bipyridine ligands, such as 2,3'-bipyridine, have C,N chelating
modes with heavy transition metals, and they have recently
attracted attention because of their high triplet energy (T1) [1e6].
In particular, iridium(III) compounds bearing the 2,3'-bipyridine
ligand have been shown to be suitable blue emitting-phosphors in
phosphorescence organic light-emitting diodes (PHOLEDs) due to
their excellent external quantum efficiency (EQE ~20%) [7]. The
phosphorescent emission for bipyridine(pypy)-based iridium
compounds is thought to originate from both the ligand-centered
triplet (3LC) transition and the metal-to-ligand charge transfer
transition (MLCT) [8]. Moreover, the emission energy can be tuned
via the introduction of substituents, such as electron-donating or
withdrawing groups, into the main ligand [9]. Therefore, the judi-
cious selection of both the C,N chelate ligand and the substituents
bipyridine(pypy)-based iridium compounds can be achieved by
incorporating an ancillary ligand into the ligand framework, the
effect of the bipyridine ligand substituent in homoleptic iridium(III)
has not been investigated thus far [10]. We recently developed blue
triplet emitters, Ir(dfpypy)3 [8] and Ir(OMe3pypy)3 [11], using the
fluorine- and methoxy-functionalized bipyridine ligands, 20,60-
difluoro-2,30-bipyridine (dfpypy) and 20,60,4-trimethoxy-2,30-
bipyridine (OMe3pypy), respectively. Based on our observations,
the triplet energies of the bipyridine derivatives with substituents
appeared in the region of 2.70e2.82 eV, as shown in Fig. 1. The
bipyridine ligand has a greater triplet energy than phenylpyridine
(ppy), thus, it is a very promising ligand candidate for the devel-
opment of blue phosphorescent iridium(III) compounds. However,
because of the high reactivity and low selectivity of the pyridine
ring, incorporating the proper substituents into a bipyridine ligand
is usually problematic in regard to the yields and reaction condi-
tions [10].
* Corresponding author.
Therefore, we performed a systematic investigation on the effect
of bipyridine ligand substituents on iridium(III) compounds. In our
** Corresponding author.
E-mail addresses: kmpark@gnu.ac.kr (K.-M. Park), kangy@kangwon.ac.kr
(Y. Kang).
http://dx.doi.org/10.1016/j.dyepig.2017.07.034
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

M. Kim et al. / Dyes and Pigments 146 (2017) 386e391
387
2. Result and discussion
The synthetic details for compounds (1e4) are shown in
Scheme 1. The four different ligands were prepared via typical
cross-coupling reactions using the corresponding boronic acid or
tin reagent. The procedure to produce the Ir(C^N)3 compounds uses
reactive Ir(I) and reacts it with the substituted bipyridine ligand in
1,2-propandiol at a high temperature (~180 ꢀC) [11]. Based on our
previous reports, the use of an Ir(I) compound as the starting ma-
terial and this reaction condition usually result in iridium com-
pounds with facial geometry. Compounds 3 and 4, bearing the
dimethoxy and tert-butyl substituents on the bipyridine ligand,
were obtained in the facial form in moderate yields. However,
when using the dimethyl-substituted bipyridine ligand, the iridium
compounds 1 and 2 were mainly isolated in the meridional form
[12]. Moreover, the yields (<20%) for both 1 and 2 were much lower
than those for 3 and 4. The molecular structures of 1e4 were
confirmed using various spectroscopic methods, including X-ray
analysis on 2. The molecular structure and crystal data of 2 are
shown in Fig. 2 and Table 1, respectively. Based on the NMR and
crystal structure data, both 1 and 2 have meridional geometries.
The facial geometry was observed in 3 and 4, as shown in Figure S8
and S10, respectively.
The normalized emission spectra of 1e4 in CH2Cl2 at ambient
temperature are shown in Fig. 3. The emission energy can be tuned
from blue (462 nm) to green (511 nm) depending on the ligand
substituents. Compounds 3 and 4 with the dimethoxy substituents
have a shoulder peak (vibronic structure) in the emission spectra,
indicating a mostly MLCT emission [13], whereas 1 and 2 exhibit
broad emission bands without the vibronic structure. In addition,
the emission maxima of 1 and 2 appear at 509 and 511 nm,
respectively, which are remarkably redshifted compared to the
emission maxima of 3 and 4. These results indicate that the ligand
Fig. 1. Phosphorescence of dfpypy (black, T1: 2.82 eV) and OMe2pypy (red, T1: 2.70 eV).
(For interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
ongoing investigation of homoleptic Ir(III) compounds using
[Ir(COD)2]BF4 as the starting material, we observed that homoleptic
fac-Ir(C^N)3 derivatives were obtained with a high selectivity [11].
For example, fac-Ir(OMe3pypy)3, without the meridional isomer,
was obtained via a one-pot reaction of [Ir(COD)2]BF4 and 20,60,4-
trimethoxy-2,30-bipyridine (OMe3pypy) under mild conditions.
This provides a potentially simple and selective synthetic pathway
for blue phosphorescent fac-Ir(III) compounds with substituted
bipyridine ligands. Herein, we report an efficient synthesis of
homoleptic Ir(III) compounds possessing dimethyl- and
dimethoxy-substituted bipyridine (pypy) ligands and their photo-
physical and electrochemical properties.
Scheme 1. Synthetic routes and structures of 1e4.

388
M. Kim et al. / Dyes and Pigments 146 (2017) 386e391
Fig. 2. (a) Molecular structure of 2 with a meridional geometry. (b) Packing structure
of without solvent molecules for clarity. Selected bond lengths(Å) and bond
2
angles(ꢀ): Ir1-C1 2.077(5), Ir1-C13 1.973(5), Ir1-C25 2.056(5), Ir1-N1 2.140(4), Ir1-N3
2.040(4), Ir1-N5 2.049(4); C1-Ir1-N1 77.65(17), C1-Ir1-C13 96.84(18), C1-Ir1-N3
90.27(17), C1-Ir1-C25 171.06(18), C1-Ir1-N5 95.51(17), N1-Ir1-C13 173.68(16), N1-Ir1-
N3 96.89(16), N1-Ir1-C25 94.85(16), N1-Ir1-N5 89.05(15), C13-Ir1-N3 79.89(19), C13-
Ir1-C25 90.89(18), C13-Ir1-N5 94.63(19), N3-Ir1-C25 95.52(17), N3-Ir1-N5 172.49(14),
C25-Ir1-N5 79.34(17).
Fig. 3. Emission spectra of 1e4 in CH2Cl2 at ambient temperature.
Table 1
Crystallographic data of 2.
Empirical formula
C38 H41 Ir N7 O2.50
[Ir(C12H11N2)3]$CH3CN$2.5H2O
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
827.98
173(2) K
Monoclinic
P21/n
13.9880(10)
b (Å)
14.1739(10)
c (Å)
18.0320(14)
90
90.339(3)
90
3575.0(5)
4
1.538
3.780
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
Volume (Å3)
Z
Dcalc (g/cm3)
Fig. 4. Emission spectra of 1e4 at 77 K.
m
(mmꢁ1
)
2
qmax (deg)
54.00
Reflections collected
Independent reflections
Rint
Absorption correction
Max./min. transmission
Goodness-of-fit on F2
31811
7791
0.0261
As shown in Fig. 4, the emission spectra of 1 and 2 at 77 K are
similar to those of the corresponding compounds at ambient
temperature, while the emission spectra of 3 and 4 at 77 K dis-
played a well-resolved vibronic structure along with a hyp-
sochromic shift of a few nanometers. The structure and shift are
mainly attributed to the ligand-centered triplet emission, which is
typical of phosphorescent Ir(III) compounds [17]. The emission
spectra at a low temperature also revealed the significant impact of
the substituents on the emission energy. The photophysical data for
1e4 are summarized in Table 2.
The electrochemical behaviors of 1e4 were examined using
cyclic voltammetry (CV), and their oxidations are shown in Fig. 5.
The electrochemical characteristics and HOMO/LUMO levels of 1e4
are summarized in Table 2. The HOMO energy level was determined
using the oxidation onsets (Eox) of 1e4, and the LUMO energy was
determined using the energy gap obtained from the emission
spectra at 77 K [18]. The HOMO energy levels of 1 and 2 were ꢁ5.98
and ꢁ5.88 eV, respectively, and the HOMO energy levels of 3 and 4
with the alkoxy substituents were ꢁ5.48 and ꢁ5.45 eV, respec-
tively. The much higher HOMO energies of compounds 3 and 4 than
those of the methyl-substituted compounds 1 and 2 indicated that
the electron-donating alkoxy groups significantly destabilize the
HOMO level. Based on the electrochemical data, we concluded that
the alkoxy groups cause shallower HOMO levels than the alkyl
groups, as shown in Table 2. The electrochemical data also showed
Semi-empirical from equivalents
0.7457 and 0.6086
1.047
0.0333, 0.0838
0.0465, 0.0977
R1, wR2 [I > 2
s(I)]
R1, wR2 (all data)
substituents likely have significant contributions to the energy gap
and phosphorescence of the compounds [14]. In particular, the
dimethoxy substituents in one of the bipyridine rings, which is
covalently bound to iridium, resulted in much wider energy gaps in
the bipyridine-based iridium compounds than in the methyl or
dimethyl substituents. The quantum efficiencies (FPL) of 1e4 were
estimated using FIrpic as a standard (FPL ¼ 0.6) [15], and the effi-
ciencies were moderate to excellent. The quantum efficiency fol-
lowed the order of 4 > 3 > 2 > 1. In general, as the number of
substituents on a ligand in a triplet emitter increases, the vibronic
modes in the excited state also increase, resulting in a quantum
efficiency decrease. This fact is well-known based on previous re-
ports [16]. However, the quantum efficiencies of 2 and 4 are better
than those of their counterparts, 1 and 3, and even comparable to
that of FIrpic. To understand this observation, further studies on the
radiative/non-radiative decay processes and lifetime measure-
ments are needed.

M. Kim et al. / Dyes and Pigments 146 (2017) 386e391
389
Table 2
series of four alkoxo- or alkyl-functionalized homoleptic Ir(III)
compounds with emissions from blue to green was successfully
developed using [Ir(COD)2]BF4 as the starting material and methyl-
or methoxy-substituted C,N chelate ligands. Upon changing the
substituents of bipyridine, the major isolated products were
significantly different. Using a methyl-substituted bipyridine ligand
resulted in the isolation of the meridional forms of the Ir(III)
compounds as the major products. When a dimethoxy-substituted
bipyridine ligand was used, the dimethoxy-functionalized Ir(III)
compounds with a facial form were isolated in high yields. The
Ir(pypy)3 derivatives with alkoxy groups showed a bright blue
emission color and enhanced quantum efficiency. The bipyridine
methyl-substituent redshifted the emission to the green region.
Regarding the thermal stability, the dimethoxy-functionalized
Ir(III) compounds (3 and 4) have higher decomposition tempera-
tures than 1 and 2. These results indicate that alkoxo substitution is
a viable approach for achieving highly efficient and stable blue
phosphorescent Ir(III) compounds. It is noteworthy that in spite of
the same nature of electron donors(MeO-, CH3-) as ligand sub-
stituents, the difference of emission maximum exceeds 50 nm. The
HOMO levels of 3 and 4 are much lower than those of 1 and 2 due to
the introduction of the electron-donating methoxy unit on the
main ligand. Compounds 3 and 4 in particular exhibit intense blue
phosphorescent with a high quantum efficiency (FPL ¼ 0.53) and
high thermal stability, making them good triplet emitter candidates
for PHOLEDs; this application is being investigated in our
laboratory.
Photophysical and electrochemical data of 1e4.
Emission lmax, (nm)
dF(PL) eEox (onset) fT1
gHOMO LUMO
(V)
(eV) (eV)
(eV)
aSolution bFilm c77 K
1
2
3
4
509
511
464
462
489
497
462
459
520
500
461
457
0.29
0.41
0.53
0.57
1.18
1.08
0.68
0.65
2.38 ꢁ5.98
2.48 ꢁ5.88
2.69 ꢁ5.48
2.71 ꢁ5.45
ꢁ3.60
ꢁ3.40
ꢁ2.79
ꢁ2.74
a
b
c
CH2Cl2.
Doped into PMMA at 10 wt%.
In frozen CH2Cl2 or Me-THF glass.
Phosphorescence quantum efficiency measured in CH2Cl2, relative
d
FIrpic(FPL ¼ 0.6).
e
The redox potentials of all compounds were measured in CH3CN with 0.1 M
Bu4NPF6 at a scan rate of either 50 mV sꢁ1 or 100 mV sꢁ1 (vs. Cp2Fe/Cp2Feþ).
f
The triplet energy value (T1) was estimated by using the emission spectra at
77 K.
g
Cp2Fe (4.8 eV) below the vacuum level.
4. Experimental section
General consideration: All experiments were performed under
N2 atmosphere using standard Schlenk technique. Solvents used
were dried over suitable drying agents prior to use. All starting
materials, 3-bromo-2,6-dimethylpyridine, 3-bromo-2-methyl-
pyridine, 2,6-dimethoxypyridine and 2-bromo-pyridine were pur-
chased from either Aldrich or TCI and used without further puri-
fication. The spectra of NMR were recorded on a 400 or 600 MHz
Bruker avance FT-NMR spectrometer. The absorption and photo-
luminescence spectra were obtained using UV/Vis spectrometer
Lambda 900 and Perkin Elmer Luminescence spectrometer LS 50B,
respectively. All solvents were degassed with nitrogen before use.
The cyclic voltammetry measurements were performed using BAS
100 series electrochemical analyzer under nitrogen with a scan rate
of 100 mVsꢁ1 or 50 mVsꢁ1. The measurements were carried out in
three-electrode electrochemical cell using platinum wire as a
counter electrode, Ag/AgCl as a reference electrode and 0.1 M so-
lution of NBu4PF6 in degassed CH3CN as electrolyte. Ferrocene and
ferrocium redox couple was used as an internal reference. Two
starting materials, (2,6-dimethoxypyridin-3-yl)boronic acid [19]
and 2-chloro-4-tert-butylpyridine [20] were synthesized accord-
ing to the literature procedure.
Fig. 5. CV data of 3 and 4, showing quasi-reversible oxidation.
that the tert-butyl group in the dative pyridine ring causes a slight
destabilization of the HOMO level compared with the HOMO level
for a dative pyridine ring without substituents. In addition, the
energy gap of 4 (T1 ¼ 2.71 eV) is wider than that of 3 (T1 ¼ 2.69 eV).
The substituents in the bipyridine ring that are covalently bonded
to the metal also have a notable effect on the energy levels, espe-
cially the HOMO level. This could be related to the donor strength of
the substituents since the electron density of the HOMO level is
dominated by the
p-orbitals of the C,N chelate ligand and the d-
orbital of Ir(III), according to our previous report [8].
The thermal stabilities of 1e4 were examined using TGA (ther-
mal gravimetric analysis). In the first stage (up to 250e300 ꢀC), a
small weight loss (ca. 10%) occurs in all the compounds, and the loss
is likely due to the release of solvent molecules. Under a N2 at-
mosphere and at a scan rate of 10 ꢀC/min, a 5% weight loss
(decomposition temperature, Td) appears at 247 ꢀC for 1, 310 ꢀC for
2, 370 ꢀC for 3 and 365 ꢀC for 4, respectively(See Figure S12).
Compound 1 has less thermal stability than the other compounds,
further indicating that the substituents influence the thermal sta-
bility of the Ir(III) compounds. The introduction of methoxy sub-
stituents improved the thermal stability of the Ir(III) compounds
relative to the methyl substituents.
X-ray analysis: The X-ray data were collected on a Bruker SMART
APEX II ULTRA diffractometer equipped with a graphite mono-
chromated Mo K
a
(
l
¼ 0.71073 Å) radiation generated by a rotating
anode and a CCD detector. The cell parameters for the compounds
were obtained from a least-squares refinement of the spots (from
36 collected frames). Data collection, data reduction, and semi-
empirical absorption correction (SADABS) [21] were carried out
using the software package of APEX2 [22].
The structures were determined with direct methods using the
SHELXS97 program [23], and refinement was performed against F2
using SHELXL2014 program [24].
3. Conclusions
In 2, one water molecule (O1W) was located on general position
but it was refined with the site occupancy factor of 0.5. Another
water molecule was disordered over two positions (O3W and O4W)
In summary, a simple one-pot procedure for the synthesis of a

390
M. Kim et al. / Dyes and Pigments 146 (2017) 386e391
with equal site occupancy factors of 0.5. The DFIX restraint in the
bond distance (N7-C37) in acetonitrile molecule was applied during
the refinement processes due to the large variation of bond ge-
ometry. Displacement parameters of the lattice solvent molecules
were restrained to be approximately isotropic (ISOR). The non-
hydrogen atoms were refined anisotropically. All hydrogen atoms
except the hydrogens of the lattice water molecules were placed in
calculated positions and refined isotropically in a riding manner
along with their respective parent atoms. All hydrogen atoms of the
lattice water molecules were not included in the model. The figures
were prepared using DIAMOND program [25]. Relevant crystal
collection data, refinement data for the crystal structures, selected
bond distances and angles, and inter- and intra-molecular C-H/O,
ether to give 15% yield of solid product. 1H NMR (600 MHz, CD2Cl2,
d
): 8.16 (d, J ¼ 8.4 Hz, 1H), 8.10 (d, J ¼ 1.2 Hz, 2H), 8.02 (d, J ¼ 5.4 Hz,
1H), 7.83 (d, J ¼ 5.4 Hz, 1H), 7.77e7.68 (m, 5H), 7.55 (t, J ¼ 54.2 Hz,
2H), 6.99 (t, J ¼ 4.0 Hz,1H), 6.88 (m, 2H), 6.61 (d, J ¼ 4.2 Hz,1H), 6.25
(d, J ¼ 4.2 Hz, 1H), 6.18 (d, J ¼ 4.2 Hz, 1H), 2.94 (s, 3H). 2.89 (d, J ¼
3 Hz, 6H). 13C{1H}-NMR (125 MHz, CD2Cl2,
d): 168.0, 165.9, 165.1,
163.8, 153.8, 151.5, 148.7, 147.6, 145.5, 142.0, 141.6, 137.8, 137.7, 137.3,
136.2, 130.9, 126.3, 125.3, 124.2, 124.0, 123.7, 123.4, 123.4, 123.0,
29.7, 26.7, 26.1. Anal. calcd for C33H27N6Ir; C, 56.64; H, 3.89; N,
12.01; found: C 56.59, H 3.98, N 12.05.
Synthesis of Ir(III) (20,6'-dimethyl-2,3'-bipyridinato)-N,C4) (2):
[Ir(COD)2]BF4, 206'-dimethyl-2,3'-bipyridine and degassed 1,3-
propandiol were added to a Schlenk flask under nitrogen. The so-
lution was heated to 180 ꢀC and then refluxed for 24 h. After the
reaction was complete, 1,3-propandiol was removed under reduced
pressure and the residue was washed with Et2O, then washed with
hexane and dried under reduced pressure. The resulting solid was
transferred to 150 mL of round bottom flask and was dissolved in
THF(100 mL). The solution was filtered through Celite, concentrated
in vacuo. The crude product was recrystallized from EtOAc/Et2O to
C-H/N, and C-H$$$p interactions are summarized in Tables S1-S3.
Synthesis of 2'-methyl-2,3'-bipyridine: Under nitrogen atmo-
sphere, 3-bromo-2-methylpyridine (0.69 mL, 5.37 mmol), 2-(trib-
uthylstannyl)pyridine(3.07
mL,
8.05
mmol),
LiCl(1.14g,
26.85 mmol), and Pd(PPh3)2Cl2 (0.19g, 0.27 mmol) were charged
into a a Schlenk flask and then toluene(50 ml) was added to this
reaction mixture. This mixture was refluxed at 120 ꢀC for 3 days.
After the reaction was completed, solvent was removed under
reduced pressure. The residue was dissolved in CH2Cl2 and washed
with. H2O. The organic layer was dried over anhydrous MgSO4 and
concentrated. The product was purified by column chromatography
(ethylacetate/dichloromethane ¼ 2/1, Rf ¼ 0.2) to give 60% yield of
give over 20% yield of solid product. 1H NMR (600 MHz, CD2Cl2,
d):
8.06e7.98 (m, 4H), 7.84 (d, J ¼ 4.8 Hz, 1H), 7.64 (td, J ¼ 7.8, 1.2 Hz,
1H), 7.59e7.52 (m, 3H), 6.84 (t, J ¼ 6.6 Hz, 1H), 6.71 (dt,
J ¼ 7.2, ¼ 1.4 Hz, 2H), 6.46 (s, 1H), 6.05 (s, 1H), 5.90 (s, 1H), 2.76 (s,
3H), 2.74 (d, J ¼ 1.2 Hz, 6H), 2.14 (s, 3H), 2.13 (s, 3H). 2.12 (s, 3H) 13
C
white solid. 1H NMR (400 MHz, CD2Cl2,
d): 8.62 (m, 1H), 8.45 (dd,
{1H}-NMR (125 MHz, CD2Cl2,
d): 189.2, 186.7, 168.9, 166.9, 166.5,
J ¼ 4.8, 1.6 Hz, 1H), 7.71 (td, J ¼ 8, 1.6 Hz, 1H), 7.65(dd, J ¼ 7.6, 1.6 Hz,
1H),7.34(d, J ¼ 8 Hz, 1H), 7.21 (m, 1H), 7.14 (m, 2H) 2.52 (s, 3H).
Synthesis of 20,6'-dimethyl-2,3'-bipyridine: The product was pre-
pared using the same procedure of 2'-methyl-2,3'-bipyridine except
that 3-bromo-2,6-dimethylpyridine was used instead of 3-bromo-
2-methylpyridine. Yield: 62% (ethylacetate/dichloromethane ¼ 2/1,
155.3,154.9,154.8,153.7,153.1,153.0,151.5,148.4,139.3,136.8,136.1,
134.9,129.8,125.3,123.7, 123.5,122.9,122.5,122.2,122.1, 121.3, 53.8,
53.6, 53.4, 53.2, 53.1, 27.0, 26.7, 26.4, 23.7, 23.5. Anal. calcd for
C
36H33N6Ir; C, 58.28; H, 4.48; N, 11.33; found: C 58.25, H 4.51, N
11.28.
Synthesis of Ir(III) (20,6'-dimethoxy-2,3'-bipyridinato)-N,C4) (3):
Rf ¼ 0.2, white solid). 1H NMR (400 MHz, CD2Cl2,
d): 8.67(m, 1H),
[Ir(COD)2]BF4, 206'-dimethoxy-2,3'-bipyridine and degassed 1,3-
propandiol were added to a Schlenk flask under nitrogen. The so-
lution was refluxed for 24 h. After the reaction was complete, the
solid product was produced by adding water(50 mL) in the Schlenk
flask. The filtered solid was purified by column chromatography
(CH2Cl2, Rf ¼ 0.6) to give 45% yield of solid product. 1H NMR
7.68 (td, J ¼ 7.6, 1.9 Hz, 1H), 7.53 (d, J ¼ 7.6 Hz, 1H), 7.33 (dt, J ¼ 8,
0.8 Hz,1H), 7.18 (m,1H) 7.00(d, J ¼ 8 Hz,1H), 2.45(s. 3H) 2.44 (s, 3H).
Synthesis of 20,6'-dimethoxy-2,3'-bipyridine: Under nitrogen at-
mosphere, (2,6-dimethoxypyridin-3-yl)boronic acid (1.4 g,
7.65 mmol), 2-bromopyridine(0.62 mL, 6.38 mmol), and Pd(PPh3)4
(0.44 g, 0.38 mmol) in THF (20 mL), and degassed 2 M K2CO3 were
subsequently added to a Schlenk flask. The solution was slowly
heated to 80 ꢀC and then refluxed for 12 h. The resulting mixture
was extracted with CH2Cl2. The organic layer was washed with
water and dried over MgSO4. All volatiles were removed in vacuo.
The crude mixture was purified by column chromatography and
yielding a product in 65% yields. Spectroscopic data was identical to
that reported previously [10].
(600 MHz, CD2Cl2,
7.41 (d, J ¼ 6.5 Hz, 1H), 6.81 (d, J ¼ 6.2 Hz, 1H), 5.84 (s, 1H), 4.04(s,
3H), 3.77(s, 3H). 13C{1H}-NMR (125 MHz, CD2Cl2,
): 178.8, 164.2,
d): 8.57(d,J ¼ 8.3 Hz, 1H), 7.63 (d, J ¼ 4.3 Hz, 1H),
d
162.9, 160.6, 147.2, 137.0, 123.3, 120.9, 120.7, 108.6, 52.4. Anal. calcd
for C36H33N6O6Ir; C, 51.60; H, 3.97; N, 10.03; found: C 56.65, H 3.98,
N 10.08.
Synthesis of Ir (III) (20,6'-dimethoxy-4-tert-butyl-2,3'-bipyr-
idinato)-N,C4) (4): Synthetic procedure of 4 is almost the same as
Synthesis of 20,6'-dimethoxy-4-tert-butyl-2,3'-bipyridine: The
product was prepared using the same procedure of 20,6'-dimethoxy-
2,3'-bipyridine except that 3-2-chloro-4-tert-butylpyridine was
used instead of 2-bromopyridine. Yield: 65% (ethylacetate/
that of 3. Yield: 40%. 1H NMR (600 MHz, CD2Cl2,
d): 8.64(d,
J ¼ 1.7 Hz, 1H), 7.28 (d, J ¼ 45.9 Hz, 1H), 6.84 (dd, J ¼ 6.5, 7.1 Hz, 1H),
5.86 (s, 1H), 4.04 (s, 3H), 3.77(s, 3H), 1.33(s, 9H). 13C{1H}-NMR
(125 MHz, CD2Cl2,
119.5, 117.8, 108.7, 52.6, 52.5, 34.8, 30.6, 30.2, 29.6. Anal. calcd for
48H57N6O6Ir; C, 57.29; H, 5.71; N, 8.35; found: C 57.26, H 5.75, N
d): 178.8, 163.1, 162.0, 160.4, 159.6, 145.9, 120.5,
hexane ¼ 1/3, Rf ¼ 0.5, white solid). 1H NMR (400 MHz, CD2Cl2,
d):
8.49 (d, J ¼ 5.4 Hz,1H), 8.13 (d, J ¼ 8.2 Hz,1H), 7.87 (d, J ¼ 1.8 Hz,1H),
7.11 (dd, J ¼ 5.3, 1.9 Hz, 1H), 6.38 (d, J ¼ 8.3 Hz, 1H), 3.97 (s, 3H), 3.90
(s, 3H), 1.28 (s, 9H).
C
8.41.
Synthesis of Ir(III) (2'-methyl-2,3'-bipyridinato)-N,C4) (1):
[Ir(COD)2]BF4, and 2'-methyl-2,3'-bipyridine were charged into a
Schlenk flask and then degassed 1,3-propandiol (5 ml) was added
to this reaction mixture under nitrogen atmosphere. The solution
was heated to 180 ꢀC and then refluxed for 24 h. After the reaction
was completed, 1,3-propandiol was removed under reduced pres-
sure and the residue was washed with Et2O, then washed with
hexane and dried under reduced pressure. The resulting solid was
transferred to 150 mL of round bottom flask and was dissolved in
THF(100 mL). The solution was filtered through Celite, concentrated
in vacuo. The crude product was recrystallized from ethylacetate/
Acknowledgement
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF-
2016R1D1A1B01012630 and NRF-2015R1D1A3A01020410) funded
by the Ministry of Education.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.07.034.

M. Kim et al. / Dyes and Pigments 146 (2017) 386e391
391
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