Synthesis and properties of a series of iridium complexes with imidazolo[2,1-b]thiazole derivatives as primary ligands

Article abstract of DOI:10.1039/C8NJ06295A

3-Methyl-6-phenylimidazo[2,1-b]thiazole (mpmt) and 3-methyl-6-(4-(trifluoromethyl)phenyl)imidazo[2,1-b]thiazole (mtfpmt) were easily prepared from thiourea, acetone (or trifluoroacetone), and α-bromoacetophenone as novel primary ligands. These were used to synthesize ten phosphorescent iridium complexes with picolinate (pic), isoquinoline-5-carboxylic acid (3-IQA), quinoline-2-carboxylic acid (2-QA), phenyl isoquinoline (piq), and 1,4-difluorophenyl pyrimidine (dfppy) as ancillary ligands. Their structures, photoluminescence, and electrochemistry were investigated. The introduction of trifluoromethyl groups at the phenyl ring of mpmt and to mtfpmt blue shifts the emission spectra of Ir³⁺ complexes by about 50 nm, and the corresponding emission peaks in CH₂Cl₂, which shifted from 545 to 613 nm, were observed at room temperature with an increase in the corresponding internal quantum efficiencies (IQEs) from 5.8% to 31.6%. Constructed with title complexes as emitters, LED chips based on InGaN chip excitation show good performances. Particularly, a device based on (mpmt)₂Ir(2-QA) showed the best red light emission with a CIE (0.64, 0.30), a CRI of 72.0, and a color purity that was over 80%. Also, a device based on (mpmt)₂Ir(3-IQA) provided a maximum luminescence efficiency of 3.01 lm W^?1. These results suggest that the title complexes have potential applications in LED chips and OLEDs.

Full text of DOI:10.1039/C8NJ06295A

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DOI: 10.1039/C8NJ06295A
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Synthesis and properties of a series of iridium complexes with
imidazolo[2,1-b]thiazole derivatives as primary ligands
a
b
a
a
a
a
Xin-ying Yin, Zhi-yu Yang, Guo-li Huang, Jian-jian Bian, Deng-qiang Wang, Qin Wang, Ming-yu
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
c
Teng,* Zheng-liang Wang,* Jie Zhang,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract: 3-Methyl-6-phenylimidazo[2,1-b]thiazole (mpmt) and 3-methyl-6-(4-(trifluoromethyl)phenyl)imidazo[2,1-
b]thiazole (mtfpmt) were easily prepared from thiourea, acetone (or trifluoroacetone), and α-bromoacetophenone as novel
primary ligands. These were used to synthesize ten phosphorescent iridium complexes with picolinate (pic), isoquinoline-5-
carboxylic acid (3-IQA), quinoline-2-carboxylic acid (2-QA), phenyl isoquinoline (piq), and 1,4-difluorophenyl pyrimidine
(
dfppy) as ancillary ligands. Their structures, photoluminescence, and electrochemistry were investigated. The introduction
of trifluoromethyl groups at the phenyl ring of mpmt and to mtfpmt blue shift the emission spectra of Ir3+ complexes about
0 nm, and the corresponding emission peaks in CH Cl , which shifted from 545 to 613 nm, were observed at room
5
2
2
temperature with an increase in corresponding internal quantum efficiencies (IQEs) from 5.8% to 31.6%. Constructed with
title complexes as emitters, LED chips with the structure of InGaN chip excitation shows good performances. Particularly, a
device based on (mpmt)
over 80%. Also, a device based on (mpmt)
suggest that the title complexes have potential applications in LED chips and OLEDs.
2
Ir(2-QA) had the best red light emission with CIE (0.64, 0.30), CRI of 72.0, and color purity that was
2
Ir(3-IQA) obtained a maximum luminescence efficiency of 3.01 lm/W. The results
adjusted through substituent effects to control the energy levels of
1
. Introduction
the excited states. This allows for strong adjustability of the
luminescent color.
Iridium (III) complexes play an important role in organic
phosphorescent materials because of their high quantum efficiency
and short lifetime of triple excitation1-10. Strong spin-orbit coupling
To prepare efficient Ir(III) complexes, many heteroleptic
complexes have been synthesized. These complexes are denoted
(
SOC) induced by iridium atoms can promote the radiation
2
Ir(C^N) LX, where C^N is a monoanionic cyclometalated primary
transition from the triplet to singlet state and produce exceptionally
high phosphorescent quantum yield at room temperature. On the
other hand, because the phosphorescence of iridium complexes
mainly comes from metal-ligand charge transfer (MLCT) and ligand
center (LC) transitions11-13, the energy levels of ligands can be
ligand such as 2-(2,4-difluorophenyl)pyridine (dfppy)14,15, 2-
1
6
(
benzo[b]thiophen-2-yl)pyridine (Btp) , or 1-phenylisoquinoline
piq)17,18 and LX generally is a bidentate ancillary ligand. Most of the
(
primary ligands that are currently used are obtained via Stille
coupling of an azarene-based organotin reagent with a substituted
aromatic hydrocarbon or via the coupling of a halogenated aza-
aromatic hydrocarbon with an arylboronic acid using the Suzuki-
Miyaura reaction. The reagents used in these reactions, in
particular, the organotin reagents used in the Stille coupling, are
highly toxic. With even slight carelessness, the materials used can
a. Facult of Chemistry and Chemical Engineering, Yunnan Normal University,
Kunming 650500, China, E-mail: myteng@ynnu.edu.cn.
School of Chemistry & Environment, Yunnan Minzu University, Kunming, Yunnan
50504, China, E-mail: wangzhengliang@foxmail.com.
School of Pharmmaceutical and Chemical Engineering, Taizhou University,
Taizhou 318000, Zhejiang Province, China, E-mail: zhang_jie@tzc.edu.cn.
Electronic Supplementary Information (ESI) available: [1H, 13C NMR, IR and ESI-
HRMS spectra of title compounds and mtfpmt, CV spectra of title compounds.]. See
DOI: 10.1039/x0xx00000x
b.
6
c.
This journal is © The Royal Society of Chemistry 20xx
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cause a great deal of harm to the human body and pose potential
risks. The materials needed to synthesize Btp, such as
time of flight), absorption and photoluminescenceVie(wPLA)rticslepOecntlirnae
DOI: 10.1039/C8NJ06295A
were recorded with a Mapada UV-3100 and a Hitachi F-4600
benzothiophene-2-boronic
acid
and
tetrakis
spectrometer, respectively. The luminescence quantum efficiencies
were calculated by a comparison of the emission intensities
(triphenylphosphine)palladium, are relatively expensive and greatly
increase the experimental cost. Therefore, it is particularly
important to find a new type of primary ligand that is safe, stable,
and low cost.
3
(integrated areas) of a standard sample [Ir(ppy) ] and the unknown
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sample according to Equation 1 .
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Because of the small energy level difference (ΔEST) between S1
and T1, triplet excitons in thermally activated delayed fluorescent
(TADF) material can be returned to S1 through reciprocal
intersystem crossing (RISC) via the emission of delayed
(1)
Where Φunk and Φstd are the luminescence quantum yields of
the unknown sample and Ir(ppy) , respectively. Iunk and Istd are the
integrated emission intensities of the unknown sample and Ir(ppy)
solution, respectively. Aunk and Astd are the absorbances of the
unknown sample and Ir(ppy) solution at their excitation
3
fluorescence. Also, 100% of IQE can also be achieved, and this has
attracted widespread attention from scientists19-21. However,
3
depending on the characteristics of the materials, the development
of red TADF materials has been relatively slow. If the energy level
3
22
wavelengths, respectively. The ηunk and ηstd terms represent the
difference is small, the fluorescence emission rate is low , and for
refractive indices of the corresponding solvents (pure solvents were
many red-light materials, highly conjugated systems induce strong
2
9
3
assumed). The Φstd value of Ir(ppy) has been revalued to be 0.4 .
fluorescence quenching in the solid state because of π-π
23
conjugation . In addition, for a red TADF material, which is affected
Cyclic voltammetry (CV) was performed on IM6ex (Zahner)
electrochemistry working station at room temperature (RT), with a
2
4
by the bandgap rule , the optical quantum yield (ΦPL) also rapidly
decreases with the red shift of the material emission spectrum
platinum electrode as working electrode, Ag/AgNO
KCl (aq.) as reference electrode, a platinum wire as counter
electrode, anhydrous CH Cl containing 0.1
supporting
3
in saturated
while the non-irradiation conversion rate (kIC) increases25-27
.
2
2
M
In this article, ten phosphorescent materials based on
imidazo[2,1-b]thiazole derivatives as primary ligands were
synthesized. Two imidazo[2,1-b]thiazole derivatives were easily
prepared from thiourea, acetone (or trifluoroacetone), and α-
bromoacetophenone.
tetrabutylammonium perchlorate (TBAP) as
a
+
electrolyte, Fc /Fc as the internal standard, and the scan rate was
-
1
1
00 mV·s .
2
.2 LEDs fabrication and characterization
The InGaN chip was stuck on the bracket with adhesive, baked
2
. Experimental section
in an oven at 150 °C for 1 h. The positive and negative sides of the
chip were respectively connected to the positive and negative sides
of the bracket. The gold wire is connected to connect the chip and
the bracket together to form a conductive loop; the glue and the
solid powder of the iridium complexes are evenly mixed at the rate
of 1:1 in mass and then added to the bracket cup that has been
welded with the gold wire, and then put again into the oven curing
at 150 °C for 1 h; encapsulating the solidified, solder wire and
powdered semi-finished products with epoxy resin; curing the
epoxy resin sufficiently while heat aging the LED; cutting the whole
2.1 General information
Commercially available regents were used without further
purification unless otherwise stated. H and 13C nuclear magnetic
1
resonance (NMR) spectra were recorded using Bruker
a
spectrometer at 500 MHz using TMS as an internal standard.
Infrared (IR) spectra were recorded on a Thermo Fisher Scientific
Nicolet iS10 spectrometer as KBr pellets. Melt points (Mps) were
determined with a Tektronix XT-4 binocular microscope and were
uncorrected. High resolution mass spectra (HRMS) were obtained
on a Bruker micrOTOF II with ESI-TOF (electrospray ionization and
2
| J. Name., 2012, 00, 1-3
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piece of the holder into single material, the optoelectronic
parameters are tested under a excitation light source at 395 nm .
View Article Online
DOI: 10.1039/C8NJ06295A
2
.3 Syntheses procedure
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Scheme 1. Synthetic routes of Imidazolo[2,1-b]thiazole derivatives and title complexes
2
.3.1 General synthesis procedure of imidazolo[2,1-b]thiazole
hours. After the reaction was completed, the reaction solution was
cooled to room temperature, added saturated aqueous ammonia
and the acidity of the reaction solution were neutralized. Then
derivative30 Thiourea (7.61 g, 100 mmol) and iodine (12.70 g, 50
mmol) were dissolved in acetone (10 mL) , heated to reflux and
stirred for 4 h to give a dark red liquid. After the reaction was
completed, it was cooled to RT and the solvent was removed under
reduced pressure at 50 °C. The residue was poured into ice water
(100 mL) and stirred, and NaOH (15 g, 375 mmol) was added under
ice-cooling to adjust the mixture to pH = 8-9. The aqueous phase
was extracted several times with diethyl ether, the organic phases
were combined and dried by adding an appropriate amount of solid
NaOH. The solvent was removed by filtration under reduced
2 2
extracted with CH Cl three times. The organic phases were
combined and washed with saturated NaCl solution. The washed
organic phase was added with an appropriate amount of anhydrous
4
MgSO for 6 hours. The solvent was removed by filtration under
reduced pressure, and the residue was purified by column
chromatography ( PE:EA = 6:1) to give a pure product .
1
4-Methylthiazol-2-amine: (S
500 MHz, CDCl
.9 Hz, 3H).
1
, yellow oil, yield: 70.1%.) H NMR
(
3
) δ 6.03 (d, J = 0.9 Hz, 1H), 5.30 (bs, 2H), 2.18 (d, J =
pressure, and the residue was purified by column chromatography
0
1
(PE:EA = 2:1) to give pure S
1
as yellow oil in 72% yield. H NMR (500
MHz, CDCl
Hz, 3H).
3
) δ = 6.03 (d, J = 0.9 Hz, 1H), 5.30 (bs, 2H), 2.18 (d, J = 0.9
3-Methyl-6-phenylimidazo[2,1-b]thiazole: (mpmt, yellow
1
crystal, yield: 76.0%). H NMR (500 MHz, CDCl
3
) δ = 7.85–7.83 (m,
2
H), 7.61 (s, 1H), 7.40 (d, J = 7.5 Hz, 2H), 7.30–7.25 (m, 1H), 6.39 (d,
2
1
.28 g (20 mmol) of compound S and 20 mmol of
J = 1.2 Hz, 1H), 2.41 (d, J = 1.2 Hz, 3H).
bromoacetophenone derivatives were dissolved in 40 mL of
acetone, and the reaction was stirred and heated at reflux for 3 h.
After the reaction was completed, the resulting solid was collected
by filtration, 70 mL of a 2 M HCl solution was added to the resulting
solid, and the mixture was heated under reflux and stirred for 2
3
-Methyl-6-(4-(trifluoromethyl)phenyl)imidazo[2,1-b]thiazole:
o
(
mtfpmt, yellow crystal, yield: 72%, mp 140-142 C). IR (KBr) 1616,
1493, 1471, 1409, 1322, 1266, 1163, 1126, 1101, 1056, 1011, 848,
-1
1
6
3
91, 593 cm . H NMR (500 MHz, CDCl ) δ = 7.97 (s, 1H), 7.95 (s,
This journal is © The Royal Society of Chemistry 20xx
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H), 7.71 (d, J = 3.8 Hz, 1H), 7.67 (s, 1H), 7.65 (s, 1H), 6.47 (dd, J =
8.20 (s, 1H), 8.05 (d, J = 13.4 Hz, 2H), 7.91 (d, J = 5.1 HVzi,ew1HAr)t,ic7le.6O3nli(nse,
DOI: 10.1039/C8NJ06295A
2.4, 1.1 Hz, 1H), 2.46 (d, J = 1.3 Hz, 3H). 13C NMR (126 MHz, CDCl
)
1H), 7.49–7.38 (m, 2H), 6.95 (s, 1H), 6.80 (s, 1H), 6.75 (d, J = 6.5 Hz,
3
1
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C
50.2, 146.1, 137.5, 129.2, 128.9, 127.6, 125.6, 125.3, 125.2, 123.2,
2H), 6.60–6.52 (m, 2H), 6.20 (d, J = 7.5 Hz, 1H), 6.10 (d, J = 7.5 Hz,
3.3. HRMS Found: [M+Na]+ 305.0348; molecular formula
1H), 2.47 (s, 6H). C NMR (126 MHz, DMSO) δ = 172.7, 157.1, 155.5,
13
+
13 9 3
H F N
2
S requires [M+Na] 305.0330.
153.3, 149.4, 146.3, 140.9, 140.6, 133.8, 130.4, 130.1, 128.5, 127.1,
1
26.7, 126.1, 121.1, 120.8, 120.4, 109.2, 108.0, 106.2, 12.8. HRMS
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.3.2 Synthesis of chloro-bridged dimer complex 5 mmol of
Found: [M+H]+ 742.0947; molecular formula
30 5 2 2
C H22IrN S O
imidazolo[2,1-b]thiazole derivative and 0.71 g (2 mmol) IrCl
in 60 mL of a mixed solvent of H O and 2-ethoxyethanol (v:v =
:3), were refluxed at 120°C under a nitrogen atmosphere for 24
3 2
·3H O
+
requires [M+H] 742.0915.
2
1
(Isoquinoline-3-carboxylate)bis(2-(3-methylimidazo[2,1-
hours. After the reaction was completed, the mixture was cooled
to room temperature. The solid was collected by suction filtration,
washed with dichloromethane and methanol, gathering the solid
product .
2
b]thiazol-6-yl)phenyl)iridium(III): ((mpmt) Ir(3-IQA), orange crystal,
o
yield: 39.5%, mp more than 250 C). IR (KBr) 3435, 3084, 2918,
1643, 1621, 1590, 1553, 1491, 1433, 1383, 1326, 1268, 1189, 1095,
-
1 1
1059, 1020, 920, 808, 749, 658, 616, 556 cm . H NMR (500 MHz,
DMSO) δ = 8.58 (d, J = 5.2 Hz, 1H), 8.24 (s, 1H), 8.18 (s, 1H), 7.94 (dd,
J = 26.7, 7.9 Hz, 1H), 7.74 (s, 1H), 7.52–7.40 (m, 1H), 7.24 (s, 1H),
2
.3.2 General Syntheses of Complexes 1 mmol of chloro-bridged
dimer complex and 2.5 mmol of auxiliary ligand and NaCO (1.06 g,
3
6
.93 (s, 1H), 6.83 (s, 1H), 6.75 (s, 1H), 6.66 (s, 1H), 6.56 (s, 1H), 6.26
10 mmol) were dissolved in 60 mL anhydrous N,N-
Dimethylformamide (DMF), after degassed, the mixture was
refluxed for 24 h under nitrogen. After cooling to room
temperature, the solvent was evaporated at vacuum, silica column
purification gave pure powder.
(
d, J = 8.0 Hz, 1H), 6.13 (d, J = 7.7 Hz, 1H), 5.78 (s, 2H), 5.33 (s, 2H),
1
3
2.44 (d, J = 5.4 Hz, 3H), 2.00 (d, J = 7.6 Hz, 3H). C NMR (126 MHz,
CDCl ) δ = 179.2, 161.9, 159.9, 150.8, 150.1, 147.7, 144.0, 139.1,
3
1
37.5, 135.9, 134.4, 133.6, 132.9, 132.2, 131.4, 130.7, 130.3, 125.1,
+
124.6, 124.1, 112.3, 110.1, 16.4. HRMS Found: [M+H] 792.1072;
+
molecular formula C34
H
24IrN
5
S
2
O
2
requires [M+H] 792.1072.
(3,5-Difluoro-2-(pyridin-2-yl)phenyl)bis(2-(3-methylimidazo-
2
[2,1-b]thiazol-6-yl)phenyl)iridium(III): ((mpmt) Ir(dfppy), dark
Bis(2-(3-methylimidazo[2,1-b]thiazol-6-yl)phenyl)(quinoline-
o
green crystal, yield: 14.2%, mp 116-118 C). IR (KBr) 3429, 3049,
2
2-carboxylate)iridium(III): ((mpmt) Ir(2-QA), red crystal, yield:
2
9
918, 2851, 2012, 1598, 1469, 1436, 1340, 1263, 1156, 1099, 1021,
o
4
1
6.2%, mp more than 250 C). IR (KBr) 3439, 3072, 2856, 1643,
-1
1
83, 966, 844, 815, 736, 708, 654, 613, 588, 563 cm . H NMR (500
Cl ) δ = 8.73 (d, J = 4.7 Hz, 1H), 8.07 (td, J = 8.9, 6.8 Hz, 1H),
7.89 (d, J = 1.3 Hz, 1H), 7.87 (d, J = 0.9 Hz, 1H), 7.81–7.79 (m, 1H),
632, 1583, 1506, 1491, 1436, 1395, 1328, 1275, 1233, 1058, 1036,
MHz, CD
2
2
-
1 1
989, 906, 812, 763, 619, 563, 508 cm . H NMR (500 MHz, DMSO)
δ = 8.68 (d, J = 8.5 Hz, 1H), 8.25 (s, 2H), 8.20 (d, J = 8.9 Hz, 1H), 8.14
7
8
.72 (s, 1H), 7.43 (t, J = 7.7 Hz, 4H), 7.35–7.27 (m, 4H), 7.07 (td, J =
.0, 1.7 Hz, 1H), 7.01–6.96 (m, 1H), 6.47 (t, J = 4.9 Hz, 2H), 2.46 (d, J
(
s, 1H), 8.06 (d, J = 8.2 Hz, 1H), 7.57 (t, J = 7.6 Hz, 1H), 7.44 (dd, J =
1
2
4.4, 7.5 Hz, 2H), 7.17 (s, 1H), 6.90 (s, 1H), 6.78 (dt, J = 14.6, 7.4 Hz,
H), 6.62 (d, J = 12.7 Hz, 2H), 6.54 (s, 1H), 6.31 (d, J = 7.5 Hz, 1H),
1
3
= 1.2 Hz, 6H). C NMR (126 MHz, CD
2
Cl
34.5, 132.3, 128.6, 128.1, 127.1, 125.0, 124.0, 123.9, 122.4, 111.7,
06.3, 106.1, 104.3, 104.2, 103.9, 12.8. HRMS Found: [M+H]+
2
) δ = 149.7, 147.5, 136.4,
1
1
8
8
1
3
5.99 (d, J = 7.6 Hz, 1H), 2.41 (s, 6H). C NMR (126 MHz, DMSO) δ =
1
1
74.0, 156.4, 156.1, 155.3, 148.3, 146.9, 146.0, 140.5, 139.8, 130.5,
requires [M+H]+
10.1138; molecular formula C35
2 5 2
H24IrF N S
30.2, 130.0, 128.9, 128.8, 121.4, 120.6, 120.5, 109.2, 108.0, 106.4,
10.1142.
+
106.3, 40.9, 12.8, 12.7. HRMS Found: [M+H] 792.1053; molecular
+
formula C34
H
24IrN
5
S
2
O
2
requires [M+H] 792.1072.
Bis(2-(3-methylimidazo[2,1-b]thiazol-6-yl)phenyl)(picolinate)-
2
iridium(III): ((mpmt) Ir(pic), bright yellow crystal, yield: 43.6%, mp
(
2-(Isoquinolin-1-yl)phenyl)bis(2-(3-methylimidazo[2,1-
o
more than 250 C). IR (KBr) 3432, 3104, 3042, 1630, 1593, 1555,
2
b]thiazol-6-yl)phenyl)iridium(III): ((mpmt) Ir(piq), wine red oily
1
7
490, 1433, 1399, 1345, 1309, 1266, 1184, 1061, 1020, 843, 810,
o
solid, yield: 16.3%, mp 84-86 C). IR (KBr) 3428, 3047, 2918, 2851,
-1
1
44, 689, 658, 614 cm . H NMR (500 MHz, DMSO) δ = 8.23 (s, 1H),
2
011, 1062, 1580, 1550, 1486, 1436, 1381, 1352, 1311, 1268, 1119,
4
| J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
-
1 1
9
70, 823, 734, 698, 674, 621, 574 cm . H NMR (500 MHz, CDCl
= 8.65 (s, 1H), 8.64 (s, 1H), 8.14 (d, J = 8.5 Hz, 2H), 7.92 (d, J = 8.2 Hz,
H), 7.73 (t, J = 1.8 Hz, 2H), 7.72 (s, 4H), 7.68 (d, J = 5.7 Hz, 2H), 7.58
dd, J = 3.4, 1.5 Hz, 2H), 7.56 (s, 2H), 7.55 (s, 2H), 7.54–7.52 (m, 2H),
3
) δ
(KBr) 3428, 3109, 2921, 1649, 1623, 1556, 1495, 145V8i,ew13Ar9ti1cl,e1O3n1lin7e,
DOI: 10.1039/C8NJ06295A
-
1 1
1267, 1159, 1113, 1068, 918, 887, 809, 746, 688, 648 cm . H NMR
(500 MHz, DMSO) δ = 8.62 (s, 1H), 8.52 (s, 2H), 8.48 (s, 2H), 8.42 (s,
1H), 8.27 (d, J = 8.3 Hz, 1H), 8.06 (d, J = 8.3 Hz, 1H), 7.94 (t, J = 7.6
Hz, 1H), 7.77 (t, J = 7.5 Hz, 1H), 7.70 (d, J = 7.9 Hz, 1H), 7.66 (d, J =
7.9 Hz, 1H), 7.20 (d, J = 7.6 Hz, 2H), 7.12 (d, J = 7.9 Hz, 1H), 7.00 (s,
2
(
13
2
1
.69–2.19 (m, 6H). C NMR (126 MHz, CDCl
3
) δ = 160.7, 142.1,
39.5, 136.9, 130.0, 129.9, 128.6, 128.3, 127.6, 127.1, 126.9, 126.7,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3
125.2, 119.9, 110.4, 106.6, 105.4, 103.8, 13.3. HRMS Found: [M-
1H), 6.73 (s, 1H), 6.47 (d, J = 7.4 Hz, 1H), 6.37 (s, 1H), 2.46 (s, 3H). C
NMR (126 MHz, DMSO) δ = 173.0, 155.1, 153.8, 152.5, 147.2, 147.1,
145.7, 145.3, 145.1, 141.8, 135.5, 133.1, 130.6, 130.4, 130.1, 128.7,
128.1, 125.6, 121.1, 120.4, 110.2, 109.2, 108.5, 12.7. HRMS Found:
+
5 2
H28IrN S
requires [M-piq]+
piq] 619.0603; molecular formula C39
19.0602.
6
(3,5-Difluoro-2-(pyridin-2-yl)phenyl)bis(2-(3-methylimidazo-
2,1-b]thiazol-6-yl)-4-(trifluoromethyl)phenyl)iridium(III):
(mtfpmt) Ir(dfppy), gray green crystal, yield: 9.6%, mp 88-90 C). IR
[
[
M+H]+ 928.0818; molecular formula C36
H22IrF N S O requires
6 5 2 2
[
(
+
M+H] 928.0820.
o
2
(KBr) 3406, 3089, 2920, 2036, 1598, 1499, 1466, 1437, 1441, 1386,
Bis(2-(3-methylimidazo[2,1-b]thiazol-6-yl)-4-
(trifluoromethyl)-phenyl)(quinoline-2-carboxylate)iridium(III):
((mtfpmt) Ir(2-QA), orange crystal, yield: 50.1%, mp more than 250
oC). IR (KBr) 3429, 3111, 2919, 1645, 1557, 1491, 1455, 1391, 1368,
1
6
4
317, 1264, 1160, 1109, 1065, 946, 887, 849, 819, 780, 742, 707,
-1 1
85, 618, 584, 560 cm . H NMR (500 MHz, CD
2
Cl
2
) δ = 8.73 (dt, J =
2
.8, 1.4 Hz, 1H), 8.11–8.07 (m, 1H), 8.08–8.04 (m, 1H), 8.03–7.97
-
1 1
(m, 2H), 7.88–7.75 (m, 4H), 7.75–7.66 (m, 2H), 7.61–7.57 (m, 1H),
1316, 1266, 1161, 1113, 1067, 890, 803, 771, 687 cm . H NMR (500
MHz, DMSO) δ = 8.74 (d, J = 8.4 Hz, 1H), 8.49 (s, 1H), 8.39 (s, 1H),
8.26 (d, J = 8.4 Hz, 1H), 8.10 (d, J = 8.1 Hz, 1H), 7.99 (d, J = 9.0 Hz,
1H), 7.70 (s, 1H), 7.68 (s, 1H), 7.67 (s, 1H), 7.65 (s, 1H), 7.61 (s, 1H),
7.20 (d, J = 6.8 Hz, 2H), 7.15 (d, J = 9.3 Hz, 2H), 6.99 (s, 1H), 6.69 (s,
7.44 (dd, J = 16.5, 6.3 Hz, 1H), 7.33–7.26 (m, 2H), 7.09–7.05 (m, 1H),
6
1
.98 (ddd, J = 11.4, 8.9, 2.5 Hz, 1H), 6.81 (ddd, J = 7.2, 5.7, 1.6 Hz,
1
3
H), 6.76 (dd, J = 7.4, 1.2 Hz, 1H), 2.54–2.44 (m, 3H). C NMR (126
Cl ) δ = 162.1, 149.7, 136.3, 132.2, 125.1, 124.0, 122.4,
21.1, 120.6, 119.7, 111.6, 107.0, 105.7, 104.3, 104.1, 103.9, 13.1.
HRMS Found: [M-dfppy]+ 755.0345; molecular formula
MHz, CD
2
2
1
3
1
1H), 6.57 (s, 1H), 6.21 (s, 1H), 2.44 (s, 3H). C NMR (126 MHz,
DMSO) δ = 173.9, 159.7, 155.0, 154.1, 147.9, 147.7, 146.8, 144.8,
143.7, 141.8, 140.4, 130.7, 130.1, 129.3, 128.6, 128.0, 126.7, 125.8,
+
C
37 8
H22IrF N
S
5 2
requires [M-dfppy] 755.0342.
1
23.7, 121.2, 120.5, 118.9, 110.2, 109.0, 108.6, 12.7. HRMS Found:
Bis(2-(3-methylimidazo[2,1-b]thiazol-6-yl)-4-
[M+H]+ 928.0838; molecular formula C36
H22IrF N S O requires
6 5 2 2
2
(trifluoromethyl)-phenyl)(picolinate)iridium(III): ((mtfpmt) Ir(pic),
+
[
M+H] 928.0820.
o
yellow powder, yield: 46.5%, mp more than 250 C). IR (KBr) 3429,
3
1
8
1
104, 2920, 1640, 1598, 1492, 1390, 1352, 1316, 1266, 1160, 1122,
(2-(Isoquinolin-1-yl)phenyl)bis(2-(3-methylimidazo[2,1-
b]thiazol-6-yl)-4-(trifluoromethyl)phenyl)iridium(III):
((mtfpmt) Ir(piq), wine red oily solid, yield: 15.6%, mp 74-76 C). IR
(KBr) 3428, 3114, 2922, 2853, 2023, 1721, 1596, 1493, 1402, 1318,
-
1 1
065, 889, 821, 756, 689, 638 cm . H NMR (500 MHz, DMSO) δ =
.48 (s, 1H), 8.45 (s, 1H), 8.11 (t, J = 7.2 Hz, 1H), 8.05 (d, J = 7.9 Hz,
H), 7.83 (d, J = 5.2 Hz, 2H), 7.73–7.62 (m, 4H), 7.14 (dd, J = 13.8,
o
2
-1 1
8.5 Hz, 3H), 7.03 (s, 1H), 6.88 (s, 1H), 6.43 (s, 1H), 6.33 (s, 1H), 2.51
1268, 1162, 1113, 1067, 890, 820, 736, 703, 634 cm . H NMR (500
MHz, CDCl ) δ = 8.65 (s, 2H), 8.64 (s, 2H), 8.15 (d, J = 0.7 Hz, 2H),
13
(
s, 3H). C NMR (126 MHz, DMF) δ = 172.8, 155.7, 154.5, 153.7,
3
1
49.8, 147.4, 147.3, 145.6, 145.3, 142.1, 138.9, 130.7, 130.3, 129.1,
8.13 (d, J = 0.7 Hz, 2H), 7.93 (s, 2H), 7.91 (s, 2H), 7.74 (s, 2H), 7.72
(s, 2H), 7.69 (s, 1H), 7.68 (s, 1H), 7.56 (s, 2H), 7.55 (s, 1H), 7.54 (d, J
128.6, 127.4, 120.9, 120.2, 118.2, 110.1, 108.8, 108.1, 108.0, 12.2,
2.1. HRMS Found: [M+Na]+ 900.0460; molecular formula
= 1.3 Hz, 1H), 7.52 (s, 1H), 2.94 (d, J = 34.0 Hz, 3H). C NMR (126
13
1
+
C
32 6
H20IrF N
5 2
S O
2
requires [M+Na] 900.0482.
MHz, CDCl
3
) δ = 160.7, 142.1, 139.4, 136.9, 130.0, 129.9, 128.6,
1
28.4, 127.6, 127.1, 126.9, 126.7, 119.9, 13.3. HRMS Found: [M-
(Isoquinoline-3-carboxylate)bis(2-(3-methylimidazo[2,1-
+
6 5 2
H26IrF N S
requires [M-piq]+
piq] 755.0357; molecular formula C41
755.0342.
2
b]thiazol-6-yl)-4-(trifluoromethyl)phenyl)iridium(III): ((mpmt) Ir(3-
o
IQA), yellow green crystal, yield: 45.9%, mp more than 250 C). IR
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1
1
1
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1
2
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2
2
2
2
2
2
2
3
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3
3
3
3
3
3
3
3
4
4
4
4
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5
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6
View Article Online
DOI: 10.1039/C8NJ06295A
3
. Result and discussion
3.1. Photophysical property
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
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8
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4
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
-
5
Figure 1 The UV-vis absorption (a), emission (in CH
2
Cl
2
at 1×10 mol/L (b) and solid state (c) ) and cyclic voltammograms (d) of complexes.
The UV-vis absorption spectra (in CH
2
Cl
2
at 1×10-5 mol/L,
transitions from the central metal to the ligand of the complex.
Among these peaks, the absorption peak from 320 nm to 450 nm is
Figure 1a) and the emission spectra (in solution and solid state) of
complexes are shown in Figure 1b and 1c. The related data are
listed in Table 1. In the range below 320 nm, each complex has two
or three distinct absorption peaks. These are assigned to the spin-
1
attributed to the spin-allowed MLCT ( MLCT←S0), and the
absorption peaks between 460 nm and 550 nm are attributed to
3
the spin-forbidden MLCT ( MLCT←S0). From this information, we
1
3
allowed singlet transition of the intraligand LC ( π→π*) transition
of the cyclometalated imidazolo[2,1-b]thiazole derivative and
auxiliary ligands. Several relatively weak absorption peaks appear
in the range of 320 nm-500 nm, and these correspond to MLCT
speculate that there is a strong spin-orbit coupling between the
triplet and singlet states of the complexes, and this means that the
luminescence of the complexes may be stronger.
6
| J. Name., 2012, 00, 1-3
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1
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1
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1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
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3
3
3
3
4
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4
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4
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5
5
6
Table 1
The photophysical properties and electrochemical properties of iridium complexes containing mpmt
View Article Online
DOI: 10.1039/C8NJ06295A
a
b
c
Complex
Λ
abs (nm)
λ
em, L/S (nm)
Φ
em (%)
E
ox (eV)
1.39
0.86
0.87
0.91
1.44
1.59
1.09
1.09
1.14
1.61
E
g
(eV)
HOMO/LUMO (eV)
(mpmt)
2
Ir(dfppy)
249/306/377/416
255/279/368/429
226/249/372/432
240/268/385/436
237/260/378/426
240/269/308
552/563
568/585
593/594
613/620
605/610
550/566
567/583
545/572
582/593
589/599
6.5
2.87
2.54
2.44
2.30
2.43
2.90
2.56
2.63
2.58
2.50
-5.59/-2.72
-5.06/-2.52
-5.07/-2.63
-5.11/-2.81
-5.64/-3.21
-5.81/-2.91
-5.31/-2.75
-5.31/-2.68
-5.36/-2.78
-5.83/-3.33
(mpmt)
2
Ir(pic)
17.3
15.7
31.6
16.2
5.8
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(mpmt)
mpmt)
mpmt)
2
Ir(3-IQA)
Ir(2-QA)
Ir(piq)
Ir(dfppy)
Ir(pic)
Ir(3-IQA)
Ir(2-QA)
mtfpmt) Ir(piq)
L: liquid, S: solid.
(
2
(
2
(
mtfpmt)
2
(
mtfpmt)
2
259/346/375
17.9
13.8
27.6
14.3
(mtfpmt)
2
216/255/332
(mtfpmt)
2
240/261/382
(
2
274/322/347
a
b
c
Φ
em : quantum yield reference to Ir(ppy)
3
as the standard solution.
HOMO/LUMO calculated by the CV data with ferrocene as the internal standard and UV-Vis spectra in CH
2
Cl2.
With the different ancillary ligands, these complexes
withdrawing ability; thus, these three complexes are obviously
affected.
luminesce over a wide range from 550 nm (green) to 620 nm (red).
This fully illustrates that a rational design of the complex molecules
can be used for the purpose of adjusting the emission wavelength
In addition, we also find that the photon quantum yield of
(
mpmt)
2
Ir(2-QA) is the highest of the latter three complexes (Table
of the phosphorescent material. Compared with (mpmt)
2
Ir(3-IQA),
1), and it is close to 0.4 (Ir(ppy)
3
). The increase in the quantum yield
the maximum emission wavelength of (mpmt) Ir(2-QA) red-shifts
2
may occur because of the strong structural rigidity of the complex
and to some extent because of the increase in the planarity of the
molecule.
about 20 nm, and this illustrates that the spatial structure of the
auxiliary complex has a great influence on the emission wavelength
of the complexes. Compared to the maximum emission wavelength
of (mpmt)
mpmt) Ir(piq) red-shifts about 50 nm; compared to the maximum
emission wavelength of (mpmt) Ir(pic), the maximum emission
wavelength of (mpmt) Ir(2-QA) red-shifts about 60 nm. It is noted
2
Ir(dfppy), the maximum emission wavelength of
3
.2. Electrochemical properties
(
2
To determine the HOMO/LUMO energy levels of the
2
complexes, the electrochemical properties were investigated using
cyclic voltammetry (CV) at RT (Figure 1d and Table 1). The HOMO of
the complexes were calculated from the oxidation potentials using
ferrocene as a reference (4.8 eV), and the LUMO levels were
2
that increasing the degree of conjugation and the amount of steric
hindrance of the complex can also make the emission red-shift.
Compared with the emission spectra of the Ir(III) complexes
with mpmt as the primary ligand, the emission spectra of
calculated from the HOMO and bandgap obtained from UV-vis
spectra.31-33
complexes are blue-shifted with the introduction of
trifluoromethyl group. Compared to the emission spectrum of
mpmt) Ir(3-IQA), the emission spectrum of (mtfpmt) Ir(3-IQA) in
CH Cl shifts almost 50 nm; compared to the emission spectra of
mpmt) Ir(2-QA) and (mpmt) Ir(piq), the emission spectra of
mtfpmt) Ir(2-QA) and (mtfpmt) Ir(piq) also blue-shift about 20 nm.
a
For complexes with mpmt as the primary ligand, the oxidation
potential was obtained from CV measurements, scanning over the
range of 0.87 eV to 1.44 eV. The oxidation potential is mainly caused
(
2
2
2
2
3+ 4+
by the metal-centered Ir /Ir oxidation couple. The oxidation
potentials of (mpmt) Ir(dfppy) and (mpmt) Ir(piq) are higher than
(
(
2
2
2
2
2
2
those of the other Ir(III) complexes with mpmt as the primary
ligand. This may be because the electron-withdrawing F atoms and
This behavior is consistent with a ligand that has a large amount of
conjugation, a small energy gap, and relatively strong CF
This journal is © The Royal Society of Chemistry 20xx
3
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6
benzene that are contained in these two complexes make it difficult
for the electrons to be lost. Therefore, the oxidation potentials of
For all of the complexes, the HOMO is greatly aVffieewctAertdiclebOyntlhinee
DOI: 10.1039/C8NJ06295A
d-orbital of the central metal (Ir) and primary ligand. In contrast,
(
mpmt)
reason, the HOMO and LUMO levels of the complexes
mpmt) Ir(dfppy) and (mpmt) Ir(piq) are slightly lower than those
of the other three complexes that have mpmt as the primary ligand.
In addition, the energy gap of (mpmt) Ir(dfppy) is much larger than
the energy gaps of several other complexes, whereas the energy
gap of (mpmt) Ir(2-QA) is the smallest in the complex. This may also
lead to (mpmt) Ir(dfppy) emitting green phosphorescence,
whereas (mpmt) Ir(2-QA) emits red phosphorescence.
2
Ir(dfppy) and (mpmt)
2
Ir(piq) are slightly higher. For this
the LUMO is mainly affected by the primary ligand, and the
3
4
influences of the ancillary ligand and the central metal are small .
Therefore, after the trifluoromethyl group is used to modify the
primary ligand (mmpt), the newly synthesized five complexes have
lower HOMO/LUMO energies compared to complexes with mpmt
as the primary ligand. Also, the energy level gap of each complex
increased significantly after the trifluoromethyl group was
introduced, and this increase led to the blue-shift of the maximum
emission wavelength.
(
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
2
3
.3. LEDs performance
Table 2
The photoluminescent performance of iridium complexes
Complex
Emission (λmax, nm)
540.3
CIE (x, y)
Efficienc
0.21
2.31
3.01
0.91
0.21
0.18
2.11
1.11
1.01
0.18
CRI
Purity
61.6
65.0
73.1
82.8
67.6
52.3
92.7
47.9
52.0
32.6
(
mpmt)
2
Ir(dfppy)
(0.37, 0.42)
(0.47, 0.41)
(0.50, 0.40)
(0.64, 0.30)
(0.58, 0.35)
(0.38, 0.46)
(0.45, 0.52)
(0.43, 0.38)
(0.49, 0.35)
(0.62, 0.41)
55.8
77.3
62.9
72.0
56.3
53.5
52.2
73.0
59.9
46.3
(
mpmt)
2
Ir(pic)
585.8
(
mpmt)
mpmt)
mpmt)
(mtfpmt)
2
Ir(3-IQA)
Ir(2-QA)
Ir(piq)
Ir(dfppy)
mtfpmt) Ir(pic)
Ir(3-IQA)
Ir(2-QA)
mtfpmt) Ir(piq)
588.3
(
2
627.8
(
2
610.7
2
563.1
(
2
572.9
(
mtfpmt)
2
586.2
(
mtfpmt)
2
601.9
(
2
598.6
chips based on the title complexes are showing in Figure 2. Among
the five complexes that have mpmt as the primary ligand,
2
(mpmt) Ir(2-QA) had the best red light emission (Figure 2); the CIE
coordinate is (0.64, 0.30), there is a deep red light emitting region,
the CRI is 72.0, and thee color purity is over 80%. However, the
luminescence efficiency is relatively poor. For another red material,
(
2
mpmt) Ir(piq), there is a weakening of the bonding capabilities
between the primary ligand and central metal because of the large
steric hindrance of 1-phenylisoquinoline. This weakening of the
bonding weakens the stability of the phosphorescent molecule of
the complex, and thus, the performance indicators are relatively
Figure 2 Luminescent pictures of LED chips basing on title
complexes.
2
unsatisfactory. The green material (mpmt) Ir(dfppy) may be
The emission of LED chips is very similar to that of the title
complexes in the solid state, but there is a little residual emission
from the InGaN chip at 395 nm. The luminescence pictures of LED
affected by the electron-withdrawing effect of the fluorine atom,
and its luminescence efficiency and CRI are relatively low. For the
8
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yellow phosphorescent materials (mpmt)
IQA), the performances are relatively better, especially for
mpmt) Ir(3-IQA), for which the luminescence efficiency reaches
.01 lm/W. Considering the good luminescence properties,
2
Ir(pic) and (mpmt)
2
Ir(3-
luminescent color adjustability and can achievVeiewmAruticltlei-Oconllioner
DOI: 10.1039/C8NJ06295A
luminescence via the selection of an appropriate ancillary ligand.
(
2
We speculate that this is mainly because the electron cloud of
3
imidazolo[2,1-b]thiazole, which contains sulfur and nitrogen atoms,
excellent white OLEDs can be prepared by combining them with a
suitable blue phosphorescent material.
2
is more easily deformed. Red emitters (mpmt) Ir(2-QA) had good
properties; specifically, the CIE coordinate is (0.64, 0.30), there is a
deep red-light emitting region, the CRI is 72.0, and the color purity
is over 80%. Introducing the trifluoromethyl group successfully
reduced the triplet energy level of the complexes and expanded the
energy gap, resulting in a maximum 50 nm blue shift in the emission
wavelength of the complexes. because of the strong electron-
withdrawing ability of the trifluoromethyl group, the introduction
of the trifluoromethyl group results in a decrease in the luminous
efficiency and the solubility of the complexes. Also, the
phosphorescent materials have a relatively high electrochemical
activity, and this causes instability of the material molecules.
Further work will be undertaken to improve luminous efficiency and
stability by introducing various groups on imidazolo[2,1-b]thiazole
units.
0
1
2
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4
5
6
7
8
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Compared with complexes that do not contain
trifluoromethyl, the luminescence efficiency of the LED chips based
on the complexes that have mtfpmt as the primary ligand is
relatively lower. In the five complexes, the luminescence properties
of (mtfpmt)
color purity of (mtfpmt)
(mtfpmt) Ir (3-IQA) reaches 73.0. Only (mtfpmt)
luminescence efficiency, reaching 2.11 lm/W, whereas (mpmt)
3-IQA) has a lower luminescence efficiency of 1.11 lm/W because
of the introduction of trifluoromethyl. The color of the LED chip
based on (mtfpmt) Ir(dfppy) is near-white, and the colors of other
2
Ir(pic) and (mtfpmt)
Ir (pic) reaches 92.7%, and the CRI of
Ir (pic) has a good
Ir
2
Ir(3-IQA) are relatively good. The
2
2
2
2
(
2
LED chips are yellow and orange. It is believed that the strong
electron-withdrawing ability of the trifluoromethyl group decreases
the radiative transition rate of the material molecule and increases
the nonradiative rate. This makes the nonradiative thermal process
dominant and eventually leads to a decrease in the luminous
Conflicts of interest
There are no conflicts to declare.
Acknowledgement
3
5
efficiency .
4. Conclusion
This work was supported by the National Science Foundation
of China (21461030), Applied Basic Research Projects of Yunnan
Province (2016FB025), Zhejiang Provincial Natural Science
Foundation of China (LY18B010002) and Scientific Research
Foundation of Taizhou University (2017JQ002).
Ten Ir(III) complexes based on imidazolo[2,1-b]thiazole
derivatives as primary ligands were synthesized. These complexes
show emission from green light ((mpmt) Ir(dfppy)) to red light
(mpmt) Ir(2-QA)). It is shown that the phosphorescent iridium
2
(
2
complexes synthesized with this novel primary ligand have a strong
3.
4.
5.
L. M. Quan, B. D. Stringer, M. A. Haghighatbin, J. Agugiaro, G.
J. Barbante, D. J. D. Wilson, C. F. Hogan and P. J. Barnard,
Dalt. Trans., 2019, 48, 653-663.
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Ten novel iridium complexes phosphor based on imidazolo[2,1-b]thiazole
derivatives as primary ligands with luminescent nearly full colors.