Blue-green-emitting cationic iridium complexes with oxadiazole-type counter-anions and their use for highly efficient solution-processed organic light-emitting diodes

Article abstract of DOI:10.1039/d0tc01054e

Cationic iridium complexes are promising phosphorescent dopants for solution-processed organic light-emitting didoes (OLEDs) and counter-anion control has emerged as a facile approach to tailor their properties for high-performance devices. A series of oxadiazole-type anions, 3-(5-phenyl-1,3,4-oxadiazol-2-yl)benzenesulfonate (OXD-SO3-), 3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzenesulfonate (tBuOXD-SO3-) and (3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)trifluoroborate (tBuOXD-BF3-), have been prepared as counter-anions for blue-green-emitting cationic iridium complexes. The photophysical and electrochemical properties of the anions and the complexes have been comprehensively characterized. The anions do not affect the emission properties of the phosphorescent cation and efficiently transfer their energy to the cations in films. Solution-processed, double-layer OLEDs using the complexes as dopants have shown much higher (×1.4) efficiencies than the device using the reference complex with a PF6- counter-anion, owing to the improvement of carrier transport/recombination balance by the electron-trapping effect of oxadiazole-type anions. In particular, the blue-green device using the complex with the OXD-SO3- counter-anion affords a peak current efficiency of 37.6 cd A-1 and a peak external quantum efficiency (EQE) of 15.2%, which is the highest for solution-processed OLEDs based on cationic iridium complexes reported so far.

Full text of DOI:10.1039/d0tc01054e

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Materials Chemistry C
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Blue-green-emitting cationic iridium complexes
with oxadiazole-type counter-anions and their
use for highly efficient solution-processed organic
light-emitting diodes†
Cite this:DOI: 10.1039/d0tc01054e
Xianwen Meng, Pei Wang, Rubing Bai and Lei He
*
Cationic iridium complexes are promising phosphorescent dopants for solution-processed organic light-
emitting didoes (OLEDs) and counter-anion control has emerged as a facile approach to tailor their
properties for high-performance devices.
A series of oxadiazole-type anions, 3-(5-phenyl-1,3,4-
oxadiazol-2-yl)benzenesulfonate (OXD-SO3À), 3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene-
À
sulfonate (tBuOXD-SO3
) and (3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)trifluoroborate
(tBuOXD-BF3À), have been prepared as counter-anions for blue-green-emitting cationic iridium complexes.
The photophysical and electrochemical properties of the anions and the complexes have been
comprehensively characterized. The anions do not affect the emission properties of the phosphorescent
cation and efficiently transfer their energy to the cations in films. Solution-processed, double-layer OLEDs
using the complexes as dopants have shown much higher (Â1.4) efficiencies than the device using the
À
reference complex with
a
PF6 counter-anion, owing to the improvement of carrier transport/
Received 1st March 2020,
Accepted 31st March 2020
recombination balance by the electron-trapping effect of oxadiazole-type anions. In particular, the blue-
À
green device using the complex with the OXD-SO3 counter-anion affords a peak current efficiency of
DOI: 10.1039/d0tc01054e
37.6 cd AÀ1 and a peak external quantum efficiency (EQE) of 15.2%, which is the highest for solution-
processed OLEDs based on cationic iridium complexes reported so far.
rsc.li/materials-c
efficiency of up to 100% for devices, facile tuning on emission
properties, and good solubility in polar solvent.12–33 In general,
1. Introduction
Organic light-emitting diodes (OLEDs) have emerged as promising cationic iridium complexes have a form of [Ir(C^N)2(N^N)]+AÀ,
thin-film emissive devices for lighting or display applications.1,2 with [Ir(C^N)2(N^N)]+ as the phosphorescent cation and AÀ as the
Because OLEDs are composed of multiple carrier-injecting/trans- counter anion (C^N is the cyclometalating ligand and N^N is the
porting and emitting layers sandwiched between two electrodes, ancillary ligand).12,34,35 Previous researches have been focused
thin-film deposition technology plays a key role in the fabrication on modifying [Ir(C^N)2(N^N)]+ through judicious ligand control
of OLEDs. Most OLEDs are made by depositing the organic layers to tune the phosphorescent properties,12–23,25,28–31,33 but the
through thermal evaporation under high vacuum, which is cost- counter anion AÀ, which is usually PF6À, has rarely been
inefficient and unfriendly for mass production.2 OLEDs can also considered.24,27,32,36 When the complexes are used as active
À
be fabricated by solution-processing technologies such as inject components in light-emitting devices, the small PF6 anions
printing, spray coating and slot-die coating, which do not require migrate within the layers under external electrical field, which is
high-vacuum condition and thus significantly reduce the production essential for the operation of light-emitting electrochemical
cost.2,3 For this notable advantage, solution-processed OLEDs have cells (LECs)12,35,37 but is unwanted for the operation of OLEDs.
experienced a rapid development in the past decades.2,4–11
Bulky anions such as tetraphenylborate and its derivatives have
In recent years, cationic iridium complexes have emerged as been used as counter-anions, which render the complexes with
promising phosphorescent dopants for solution-processed good volatility to enable the fabrication of vacuum-deposited
OLEDs, because of their capability to afford an internal quantum OLEDs;12,36,38–42 in a special case, a small chlorine ion has been
used as the counter-anion in a cationic iridium complex for
vacuum-deposited OLED.43 However, like PF6À, these anions are
still optoelectronically inactive, which cannot largely enhance
College of Chemistry, Key Laboratory of Pesticide and Chemical Biology of Ministry
of Education, Hubei International Scientific and Technological Cooperation Base of
Pesticide and Green Synthesis, Central China Normal University, Wuhan 430079,
the performances of the complexes in OLEDs. So far, cationic
iridium complexes have shown lower performances than neutral
People’s Republic of China. E-mail: helei@mail.ccnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc01054e
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iridium complexes in OLEDs,5,7,8 which leaves large space for Solution-processed, double-layer, blue-green OLEDs using complexes
further improvements. The lower device performances for cationic 1–3 afford much higher performances than the reference device
À
iridium complexes compared to the neutral counterparts can be using the complex with a PF6 counter-anion, with high peak
ascribed to (i) unbalanced carrier-recombination and exciton current-efficiencies of up to 37.6 cd AÀ1 and external quantum
leakage to adjacent layers in solution-processed OLEDs with efficiencies (EQEs) of up to 15.2%, which is the highest for
simple device structure (usually double-layer structure), and solution-processed OLEDs using cationic iridium complexes as
(ii) lower luminescent efficiencies of cationic iridium complexes dopants reported so far (Table S1, ESI†),13–32 and even compar-
compared to well-studied neutral complexes, especially for blue- able to the highest one (EQE = 16.4% for a green-yellow device)
emitting ones.35,44 These issues need to be addressed by judicious reported for multilayer vacuum-evaporation OLEDs using catio-
molecular design and device engineering.
nic iridium complexes.40 These efficiencies are also comparable
It is noted that [Ir(C^N)2(N^N)]+ and AÀ are closely bound to those afforded by solution-processed OLEDs using neutral
together by electrostatic interactions between them yet they can iridium complexes.5,7
be independently varied. Rendering the counter-anions with
optoelectronic properties represents a facile approach to tune
the overall optoelectronic properties of the complexes.45,46 In
this regard, carrier-trapping/transporting anions are promising
2. Experimental
2.1. General information
counter-anions for cationic iridium complexes used in OLEDs,
A BRUKER 400/500 NMR spectrometer, a LTQ-ORBITRAP-ETD
because they are bulky and immobile, and more importantly,
mass spectrometer and an EA3000 elemental analyzer were
can tune carrier transport/recombination balance in the emissive
layers by carrier-trapping/transporting on the complexes.27,32
used for measurements of NMR spectra, mass spectrometry
and elemental analysis, respectively. A Macy UV-1800 UV-vis
Here we report blue-green-emitting cationic iridium complexes
spectrophotometer and a Gangdong F-320 fluorospectrophoto-
with electron-deficient oxadiazole-type counter-anions and their
meter were used to measure absorption and photoluminescence
use as dopants for highly efficient solution-processed OLEDs
(PL) spectra, respectively. A transient spectrofluorometer equipped
(Chart 1). 2,5-Diphenyl-1,3,4-oxadiazole (OXD) is used as the
with an integrating sphere (Edinburgh Instruments, FLS1000) was
core of the anion because of its electron-deficiency for electron-
used to measure PL transient lifetimes and photoluminescent
trapping/transporting and high triplet energy (B2.7 eV) for
confining triplet energy on the phosphorescent cation.47,48
efficiencies in thin films. The photoluminescent efficiencies in
degassed CH3CN solution were measured versus quinine sulfate
(Fp = 0.545 in 1 M H2SO4). A LK1100 voltammetric analyzer was
1,3,4-oxadiazole derivatives have previously been used as ancillarÀy
ligands in cationic iridium complexes.49 In our work, SO3À or BF3
used to conduct cyclic voltammetry in solution (10À3 M) at a scan
are anchored to OXD to afford three anions: 3-(5-phenyl-1,3,4-
rate of 100 mV sÀ1, with a glass-carbon working electrode, a silver-
wire pseudo-reference electrode and a platinum-wire counter
electrode. The supporting electrolyte and the internal standard
are tetrabutylammonium hexafluorophosphate (0.1 M) and ferro-
oxadiazol-2-yl)benzenesulfonate (OXD-SO3À), 3-(5-(4-(tert-butyl)-
phenyl)-1,3,4-oxadiazol-2-yl)benzenesulfonate (tBuOXD-SO3À) and
(3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)phenyl)trifluoroborate
(tBuOXD-BF3À). The photophysical and electrochemical properties
cene, respectively. The solutions were degassed with argon for
of the anions and the complexes have been comprehensively
cyclic voltammetry experiments.
investigated. The oxadiazole-type anions affect neither the emission
color nor the efficiency of the phosphorescent cation. In films,
efficient energy-transfer occur from the anions to the phosphor-
2.2. Materials
escent cation because of the close proximity between them. Unless otherwise stated, all reactants and solvents were purchased
from commercial sources and used as received. [Ir(ppy)2(pzpy)]+ClÀ
and [Ir(ppy)2(pzpy)]PF6 were prepared according to reported
procedures.27,50
Synthesis of sodium 3-(hydrazinecarbonyl)benzenesulfonate
(1). Sodium 3-carboxybenzenesulfonate (4.0 g, 17.8 mmol) and
concentrated HCl solution (1.5 mL, 18 mmol) were dissolved in
ethanol (40 mL). The mixture was stirred at 70 1C under a
nitrogen atmosphere for 5 h to produce sodium 3-(ethoxycarbonyl)-
benzenesulfonate. To the solution, hydrazine hydrate (20 mL) were
added dropwise. The solution was refluxed under a nitrogen
atmosphere for 12 h. After being cooled to room temperature,
the mixture was filtered. To the filtrate, sodium hydroxide (1.6 g,
40 mmol) dissolved in ethanol (15 mL) was added under
stirring. The mixture was stirred at room temperature for 3 h.
The precipitate was collected and dried under vacuum, affording
the pure product as a white solid (3.6 g, 15 mmol). Yield: 84%.
Chart 1 Chemical structures of cationic iridium complexes 1–3 with
oxadiazole-type counter-anions. tBu is short for tert-butyl.
1H-NMR (500 MHz, DMSO-d6, d [ppm]): 9.78 (s, 1H), 8.08 (s, 1H),
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7.79–6.69 (m, 2H), 7.36 (t, J = 8 Hz, 1H), 4.46 (s, 2H). HRMS (ESI, 1H-NMR (500 MHz, DMSO-d6, d [ppm]): 8.10–8.01 (m, 3H), 7.83
m/z): 215.01328 [M À Na]À (calc. 215.01320).
(d, J = 7.5 Hz, 1H), 7.65 (d, J = 7.8 Hz, 2H), 7.58 (d, J = 7.3 Hz, 1H),
Synthesis of sodium 3-(2-benzylidenehydrazine-1-carbonyl)- 7.35 (t, J = 7.5 Hz, 1H), 1.34 (s, 9H). 13C-NMR (126 MHz, DMSO-
benzenesulfonate (2). Sodium 3-(hydrazinecarbonyl)benzene- d6, d [ppm]): 165.60, 164.06, 155.21, 135.67, 130.05, 127.69,
sulfonate (2.4 g, 10 mmol) and benzaldehyde (1.3 g, 12 mmol) 126.89, 126.72, 124.10, 121.79, 121.39, 35.30, 31.33. HRMS
were dissolved in ethanol (100 mL). The mixture was refluxed (ESI, m/z): 345.13838 [tBuOXD-BF3]À (calc. 345.13915).
for 12 h and then cooled to room temperature. The precipitate
Synthesis of [Ir(ppy)2(pzpy)]+[OXD-SO3À] (complex 1). OXD-
was collected, affording the pure product as a white solid (2.1 g, SO3Na (130 mg, 0.4 mmol) was dissolved in methanol/deionized
6.5 mmol). Yield: 65%.1H-NMR (500 MHz, DMSO-d6, d [ppm]): water (15/15 mL). To the solution, [Ir(ppy)2pzpy]+ClÀ (272 mg,
12.09 (s, 1H), 8.55 (s, 1H), 8.22 (s, 1H), 7.93 (d, J = 7.5 Hz, 1H), 0.4 mmol) dissolved in methanol/deionized water (15/15 mL)
7.81 (d, J = 7.5 Hz, 1H), 7.74 (d, J = 6.5 Hz, 2H), 7.53–7.42 (m, was added dropwise. The mixture was stirred at room temperature
4H). HRMS (ESI, m/z): 303.04486 [M À Na]À (calc. 303.04450).
for 1 h and then concentrated. The precipitate was collected,
Synthesis of sodium 3-(2-(4-(tert-butyl)benzylidene)hydrazine- recrystallized from CH3OH/H2O and dried under vacuum at
1-carbonyl)benzenesulfonate (3). The synthesis of compound 3 80 1C, affording the pure product as a yellow powder (300 mg,
was similar to that of compound 2, except that 4-(tert-butyl)- 0.32 mmol). Yield: 80%. 1H-NMR (500 MHz, DMSO-d6, d [ppm]):
benzaldehyde replaced benzaldehyde. Yield: 65%. 1H-NMR 9.31 (d, J = 2.9 Hz, 1H), 8.53 (d, J = 8.5 Hz, 1H), 8.37–8.30 (m, 2H),
(500 MHz, DMSO-d6, d [ppm]): 11.94 (s, 1H), 8.48 (s, 1H), 8.20 8.30–8.24 (m, 2H), 8.15 (d, J = 6.0 Hz, 2H), 8.10 (d, J = 7.7 Hz, 1H),
(s, 1H), 7.88 (d, J = 7.5 Hz, 1H), 7.81 (d, J = 7.5 Hz, 1H), 7.67 7.97 (t, J = 7.8 Hz, 2H), 7.93–7.87 (m, 2H), 7.85 (d, J = 7.7 Hz, 1H),
(d, J = 8.0 Hz, 2H), 7.53–4.45 (m, 3H), 1.31 (s, 9H). HRMS (ESI, m/z): 7.77 (d, J = 5.7 Hz, 1H), 7.72 (d, J = 5.6 Hz, 1H), 7.69–7.57 (m, 5H),
359.10645 [M À Na]À (calc. 359.10710).
7.54 (t, J = 7.0 Hz, 1H), 7.29 (s, 1H), 7.24 (t, J = 6.6 Hz, 1H), 7.20
Synthesis of sodium 3-(5-phenyl-1,3,4-oxadiazol-2-yl)benzene- (t, J = 6.6 Hz, 1H), 7.05–6.96 (m, 2H), 6.94–6.82 (m, 3H), 6.19
sulfonate (OXD-SO3Na). Sodium 3-(2-benzylidenehydrazine-1- (t, J = 7.9 Hz, 2H). 13C-NMR (126 MHz, DMSO-d6, d [ppm]):
carbonyl)benzenesulfonate (2.0 g, 6.0 mmol), iodine (2.2 g, 167.32, 167.18, 164.56, 164.40, 150.10, 149.69, 149.62, 149.56,
8.7 mmol) and K2CO3 (1.7 g, 12 mmol) were suspended in dry 149.19, 148.65, 146.59, 144.58, 144.53, 143.74, 142.23, 139.40,
DMSO (10 mL). The mixture was stirred at 100 1C under a nitrogen 139.27, 133.23, 132.51, 131.77, 131.60, 130.63, 130.20, 129.92,
atmosphere for 4 h. After being cooled to room temperature, 129.56, 129.47, 127.21, 127.12, 125.78, 125.55, 125.19, 124.40,
sodium thiosulfate (1.1 g, 6.8 mmol) was added to the reaction 124.27, 124.15, 123.85, 123.34, 122.85, 122.63, 120.46, 120.41,
mixture. The mixture was stirred at room temperature for 3 h and 114.05, 112.03. HRMS (ESI, m/z): 646.15574 [Ir(ppy)2(pzpy)]+
then brine solution was added. The precipitate was collected and (calc. 646.15772); 301.02874 [OXD-SO3]À (calc. 301.02885). Anal.
purified by column chromatography on silica gel with CH2Cl2/ found: C, 55.59; H, 3.63; N, 10.31. Anal. calcd for C44H32IrN7O4S:
CH3OH (5 : 1) as the eluent, affording the pure product as a white C, 55.80; H, 3.41; N, 10.35.
solid (1.6 g, 4.9 mmol). Yield: 82%. 1H-NMR (500 MHz, DMSO-d6, d
Synthesis of [Ir(ppy)2(pzpy)]+[tBuOXD-SO3À] (complex 2). The
[ppm]): 8.34 (s, 1H), 8.15 (d, J = 7.2 Hz, 2H), 8.10 (d, J = 7.5 Hz, 1H), synthesis of complex 2 followed a procedure similar to that for
7.87 (d, J = 7.5 Hz, 1H), 7.71–7.59 (m, 4H). 13C-NMR (126 MHz, complex 1, except that tBuOXD-SO3Na replaced OXD-SO3Na.
DMSO-d6, d [ppm]): 164.56, 164.36, 149.90, 132.52, 129.92, 129.59, Yield: 82%. 1H-NMR (500 MHz, DMSO-d6, d [ppm]): 9.31
129.47, 127.20, 124.14, 123.82, 123.35. HRMS (ESI, m/z): 301.02881 (d, J = 2.3 Hz, 1H), 8.53 (d, J = 8.5 Hz, 1H), 8.38–8.30 (m, 2H),
[M À Na]À (calc. 301.02885).
8.27 (d, J = 7.8 Hz, 2H), 8.07 (d, J = 7.6 Hz, 3H), 7.97 (t, J = 7.5 Hz,
Synthesis of sodium 3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol- 2H), 7.93–7.82 (m, 3H), 7.78–7.72 (m, 2H), 7.69 À 7.64 (m, 3H),
2-yl)benzenesulfonate (tBuOXD-SO3Na). The synthesis of tBuOXD- 7.61 (t, J = 7.7 Hz, 1H), 7.54 (t, J = 6.5 Hz, 1H), 7.29 (s, 1H),
SO3Na was similar to that of OXD-SO3Na, except that sodium 7.26–7.18 (m, 2H), 7.07–6.95 (m, 2H), 6.95–6.82 (m, 3H), 6.20
3-(2-(4-(tert-butyl)benzylidene)hydrazine-1-carbonyl)benzenesulfonate (t, J = 8.0 Hz, 2H), 1.34 (s, J = 8.0 Hz, 9H). 13C-NMR (126 MHz,
replaced sodium 3-(2-benzylidenehydrazine-1-carbonyl)benzene- DMSO-d6, d [ppm]): 166.76, 166.62, 164.02, 163.64, 154.92,
sulfonate. Yield: 84%. 1H-NMR (500 MHz, DMSO-d6, d [ppm]): 149.55, 149.11, 149.04, 148.99, 148.62, 148.09, 146.01, 144.01,
8.31 (s, 1H), 8.12–8.03 (m, 3H), 7.85 (d, J = 7.7 Hz, 1H), 7.67 143.96, 143.17, 141.66, 138.83, 138.69, 132.66, 131.20, 131.03,
(d, J = 8.4 Hz, 2H), 7.61 (t, J = 7.7 Hz, 1H), 1.34 (s, 9H). 13C-NMR 130.05, 129.63, 128.98, 128.84, 126.51, 126.17, 125.21, 124.98,
(126 MHz, DMSO-d6, d [ppm]): 164.60, 164.19, 155.50, 150.01, 129.58, 124.62, 123.82, 123.69, 123.52, 122.82, 122.27, 122.05, 120.57,
129.41, 127.11, 127.08, 126.75, 124.08, 123.40, 121.13, 35.33, 31.31. 119.89, 119.83, 113.48, 111.46, 34.76, 30.74. HRMS (ESI, m/z):
HRMS (ESI, m/z): 357.08997 [M À Na]À (calc. 357.09145).
646.15521 [Ir(ppy)2(pzpy)]+ (calc. 646.15772); 357.09064 [tBuOXD-
Synthesis of potassium (3-(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol- SO3]À (calc. 357.09145). Anal. found: C, 57.24; H, 4.28; N, 9.65.
2-yl)phenyl)trifluoroborate (tBuOXD-BF3K). (3-(5-(4-(tert-butyl)- Anal. calcd for C48H40IrN7O4S: C, 57.47; H, 4.02; N, 9.77.
phenyl)-1,3,4-oxadiazol-2-yl)phenyl)boronic acid (1.1 g, 3.5 mmol)
Synthesis of [Ir(ppy)2(pzpy)]+[tBuOXD-BF3À] (complex 3).
was dissolved in methanol (100 mL). To the solution, KHF2 (0.8 g, The synthesis of complex 3 followed a procedure similar to that
10 mmol) dissolved in H2O (10 mL) was added dropwise. The for complex 1, except that tBuOXD-BF3K replaced OXD-SO3Na.
mixture was stirred at room temperature for 1 h and then 1H-NMR (500 MHz, DMSO-d6, d [ppm]): 9.31 (s, 1H), 8.53 (d, J =
concentrated and filtered. The precipitate was collected and dried 8.4 Hz, 1H), 8.33 (t, J = 7.9 Hz, 1H), 8.27 (d, J = 7.7 Hz, 2H), 8.08
under vacuum, affording a white solid (1.2 g, 3.1 mmol). Yield: 88%. (s, 1H), 8.04 (d, J = 7.8 Hz, 2H), 7.97 (t, J = 7.6 Hz, 2H), 7.93–7.85
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(m, 2H), 7.82 (d, J = 7.6 Hz, 1H), 7.77 (d, J = 5.7 Hz, 1H), 7.72 for 10 min. The emissive layer was then spin-coated on top of
(d, J = 5.7 Hz, 1H), 7.68–7.62 (m, 3H), 7.58 (d, J = 7.1 Hz, 1H), 7.54 PEDOT:PSS from 1,2-dichloroethane solution (8 mg mLÀ1) at
(t, J = 6.4 Hz, 1H), 7.35 (t, J = 7.4 Hz, 1H), 7.29 (s, 1H), 7.23 2000 rpm and baked at 80 1C for 30 min in a nitrogen-filled
(t, J = 7.0 Hz, 1H), 7.20 (t, J = 7.0 Hz, 1H), 7.05–6.95 (m, 2H), glove box. The film-coated substrate was transferred into a
6.93–6.82 (m, 3H), 6.19 (t, J = 8.0 Hz, 2H), 1.34 (s, 9H). 13C-NMR high-vacuum chamber (o5 Â 10À4 Pa) and TmPyPB, LiF and Al
(126 MHz, DMSO-d6, d [ppm]): 166.75, 166.62, 165.03, 163.49, layers were consecutively deposited by thermal evaporation at
154.63, 149.11, 149.04, 148.99, 148.62, 148.09, 146.01, 144.01, 1–2, 0.1 and 6 Å sÀ1, respectively. The devices were characterized
143.96, 143.17, 141.65, 138.82, 138.69, 135.10, 132.65, 131.20, in a nitrogen-filled glove box without exposure to air. The current
131.03, 130.06, 129.63, 129.50, 127.11, 126.32, 126.14, 125.21, density–voltage–brightness characteristics were measured by a
124.97, 124.62, 123.82, 123.69, 123.52, 122.27, 122.05, 121.22, Keithley 2400 source meter and a calibrated silicon photodiode.
120.82, 119.88, 119.83, 113.46, 111.45, 34.72, 30.75. HRMS (ESI, The EL spectra were recorded by an OPT-2000 fiber spectrometer.
m/z): 646.15607 [Ir(ppy)2(pzpy)]+ (calc. 646.15772); 345.13827
[tBuOXD-BF3]À (calc. 345.13915). Anal. found: C, 57.95; H, 4.26;
N, 9.79. Anal. calcd for C48H40BF3IrN7O: C, 58.18; H, 4.07; N, 9.89.
3. Results and discussion
3.1. Synthesis and structural characterizations
OLED fabrication and characterization
Scheme 1 depicts the synthetic routes to the oxadiazole-type
Indium-tin-oxide (ITO) coated glass substrates with a sheet anions and complexes 1–3. OXD-SO3Na and tBuOXD-SO3Na can
resistance of 17 O &À1 were sufficiently cleaned in detergent be readily prepared by starting from sodium 3-carboxybenzene-
and then in acetone and ethanol. Before use, the ITO substrates sulfonate. Sodium 3-(hydrazinecarbonyl)benzenesulfonate (com-
were exposed with UV-zone for 10 min. The PEDOT:PSS layer pound 1) was conveniently prepared by esterification reaction of
was spin-coated onto the ITO substrate and baked at 150 1C the starting compound, followed by hydrazinolysis of the ester.
Scheme 1 Synthetic routes to the oxadiazole-type anions and complexes 1–3.
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Sodium 3-(2-benzylidenehydrazine-1-carbonyl)benzenesulfonate
(compounds 2) and sodium 3-(2-(4-(tert-butyl)benzylidene)hydra-
zine-1-carbonyl)benzenesulfonate (compound 3) were then pre-
pared by condensation reaction of benzaldehyde or 4-tert-butyl
benzaldehyde with compound 1. OXD-SO3Na and tBuOXD-SO3Na
were prepared with high yields (480%) by iodine-assisted cyclization
reactions of compounds 2 and 3, respectively. tBuOXD-BF3K was
readily prepared Àfrom a boric ester precursor through a convenient
–B(OR)2 to –BF3 transformation in the presence of KHF2. The
complexes were prepared by anion-exchange reactions between
[Ir(ppy)2(pzpy)]+ClÀ and the oxadiazole-type anions in CH3OH/H2O
solution. Chemical structures of the complexes were characterized
1
by H and 13C-NMR spectroscopies (Schemes S1–S6, ESI†), high-
resolution mass spectrometry and elemental analysis.
Fig. 2 Absorption and PL spectra of complexes R and 1–3 in CH3CN
solution. PL spectra were measured under excitation at 380 nm.
3.2. Photophysical characterizations
Fig. 1 depicts the absorption and PL spectra of OXD-SO3Na,
tBuOXD-SO3Na and tBuOXD-BF3K in CH3OH solution. The
anions show strong absorption bands in the ultra-violet region
below 325 nm, which should arise from the OXD core. From the
absorption onset, the optical band gaps of OXD-SO3À, tBuOXD-
SO3À and tBuOXD-BF3À were estimated to be around 3.8 eV. In
solution, OXD-SO3Na, tBuOXD-SO3Na and tBuOXD-BF3K emit
ultra-violet light peaked at 342, 349 and 337 nm, respectively.
These emission bands overlap with the absorption of the
[Ir(ppy)2(pzpy)]+ cation (Fig. 2), which allows the occurrence of
In degassed CH3CN solution, complexes 1–3 afford blue-
green emission peaked at around 475 and 500 nm under the
excitation at 380 nm, which is almost the same as that afforded
by complex R (Fig. 2).50 Moreover, complexes 1–3 show very
similar photoluminescent quantum yields (F) (0.21–0.22) and
excited-state lifetimes (t) (1.45–1.62 ms) to complex R (F = 0.23
and t = 1.56 ms). From these F and t values, the radiative and
non-radiative decay rates of complexes 1–3 were calculated,
which are also similar to those of complex R (Table 1). These
experimental results reveal that the oxadiazole-type anions do
not disturb the emission of the [Ir(ppy)2(pzpy)]+ cation in
solution. At excitation wavelengths shorter than 320 nm, both
the oxadiazole-type anions and the [Ir(ppy)2(pzpy)]+ cation are
excited. At these excitation wavelengths, the complexes afford
ultra-violet emission from the oxadiazole-type anions as well as
blue-green emission from the [Ir(ppy)2(pzpy)]+ cation (Fig. S1,
ESI†), which indicates an incomplete energy-transfer from the
oxadiazole-type anions to the [Ir(ppy)2(pzpy)]+ cation in the
solution. In solution, the counter-anion and the [Ir(ppy)2(pzpy)]+
cation are not always in close proximity because of the fluid
nature of the medium. For this reason, the energy transfer
between them is inefficient in solution.
¨
efficient Foster resonance energy transfer from the anions to
[Ir(ppy)2(pzpy)]+.
Fig. 2 shows the absorption and PL spectra of complexes 1–3
in CH3CN solution. Detailed photophysical characteristics were
summarized in Table 1. For comparison, those of the reference
complex [Ir(ppy)2(pzpy)]PF6 (complex R) were also shown. The
complexes show strong absorption bands below 350 nm, which
should arise from the oxadiazole-type anions and the ligands in
the [Ir(ppy)2(pzpy)]+ cation. The complexes also show weak
absorption bands above 350 nm that extends to the visible
region, which are ascribed to the absorption of [Ir(ppy)2(pzpy)]+,
because similar weak absorption also appears in the absorption
spectrum of complex R (Fig. 2).50
In the 2 wt% doped poly(methylmethacrylate) (PMMA) films,
complexes 1–3 also afford blue-green emission peaked at
around 475 and 505 nm under excitation at 380 nm, which
resembles that from complex R. The complexes afford almost
the same F (0.78–0.79) and t (3.14–3.20 ms) values as those of
complex R (F = 0.80 and t = 3.2 ms), which further reveals that
the oxadiazole-type anions do not affect the emission of the
[Ir(ppy)2(pzpy)]+ cation. In the thin films, the similar F and t
values for complexes 1–3 and complex R indicate that back
triplet energy-transfer from the oxadiazole-type anions to
[Ir(ppy)2(pzpy)]+ does not occur owing to the high triplet energy
(2.7 eV) of the OXD core. At excitation wavelengths shorter than
320 nm, both [Ir(ppy)2(pzpy)]+ and oxadiazole-type anions are
excited but complexes 1–3 show predominant blue-green emission
from the [Ir(ppy)2(pzpy)]+ cation and nearly no emission from
the oxadiazole-type anions (Fig. S2, ESI†), which indicates
an almost complete energy-transfer from the oxadiazole-type
Fig. 1 Absorption and PL spectra of OXD-SO3Na, tBuOXD-SO3Na and
tBuOXD-BF3K in CH3OH solution.
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Table 1 Photophysical data of complexes 1–3 in solution and doped films. For comparison, those of [Ir(ppy)2(pzpy)]PF6 (complex R) were also listed.31,50
PL in CH3CNb PL in PMMA filmc
a
Absorption l [nm] (e [ Â 104 MÀ1 cmÀ1])
l [nm]
Fp
t [ms]
kr, knr [105 sÀ1
]
l [nm]
Fp
t [ms]
kr, knr [105 sÀ1
]
1
2
3
R
266 (6.90), 380 (0.60), 408 (0.39)
268 (6.27), 380 (0.54), 408 (0.34)
268 (5.86), 380 (0.49), 408 (0.32)
255 (5.00), 380 (0.48), 408 (0.30)
475, 500
475, 500
477, 502
475, 503
0.21
0.21
0.22
0.23
1.62
1.45
1.47
1.56
1.3, 4.9
1.4, 5.5
1.5, 5.3
1.5, 5.0
475, 504
475, 505
475, 505
474, 502
0.79
0.78
0.79
0.80
3.14
3.17
3.20
3.20
2.5, 0.7
2.5, 0.7
2.5, 0.7
2.5, 0.6
a
b
c
In CH3CN solution (10À5 M). e denotes the molar extinction coefficient. In degassed CH3CN (10À5 M) solution. In 2 wt% doped PMMA films.
anions to [Ir(ppy)2(pzpy)]+ in the PMMA films. The efficient energy- voltammetry in solution. Fig. 3 shows the Cyclic voltammograms.
transfer in the highly diluted films reveals that the oxadiazole-type In the solution, OXD-SO3Na, tBuOXD-SO3Na and tBuOXD-BF3K all
anion and the [Ir(ppy)2(pzpy)]+ cation are in close proximity in exhibit reversible reduction processes (Fig. 3a), which occur on the
films because of the strong electrostatic interaction between OXD core of the oxadiazole-type anions. The reversible reductions
them, which benefits the use of the complexes in thin-film light- indicate that the oxadiazole-type anions have good stability for
emitting devices.
acceptance of electrons, which agrees with the electron-trapping/
transporting property of the OXD core.47,48
3.3. Electrochemical measurements
As shown in Fig. 3b, complexes 1–3 exhibit irreversible
oxidation processes, which resemble the oxidation of complex
R, indicating that the oxidations of complexes 1–3 occur on the
[Ir(ppy)2(pzpy)]+ cation. In the solution, complexes 1–3 show
two reduction processes, with the first one being irreversible
and the second one being quasi-reversible. These two reduction
processes should result from the reductions of [Ir(ppy)2(pzpy)]+
cation and oxadiazole-type anion, respectively, because similar
reduction processes appear in the cyclic voltammograms of
complex R and oxadiazole-type anions (Fig. 3a and b).
The electrochemical properties of the oxadiazole-type anions
and complexes R and 1–3 were investigated through cyclic
3.4. Solution-processed OLEDs
Complexes 1–3 were tested as dopants in solution-processed
OLEDs with a structure of ITO/PEDOT:PSS (40 nm)/26DCzPPy:
x% complex (50 nm)/TmPyPB (40 nm)/LiF (1 nm)/Al (100 nm),
where 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) was
used as the host and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene
(TmPyPB) was used as the electron-transporting/hole-blocking
layer. For comparison, the device using complex R as a dopant was
also fabricated. The emissive layers were deposited by solution-
processes. The doping concentrations of complexes 1–3 and R in the
emissive layers were set at 12 wt%, 12 wt%, 12 wt% and 10 wt%,
respectively, so that the doping concentrations of [Ir(ppy)2(pzpy)]+
were all at 8 wt%.
Fig. 4 depicts the EL spectra of the OLEDs. The devices all
showed blue-green EL peaked at around 480 and 510 nm, with
Commission Internationale de L’Eclairage (CIE) coordinates at
around (0.18, 0.49). The nearly identical EL spectra indicate
that the emission of all devices arose from the [Ir(ppy)2(pzpy)]+
cation. The PL spectra of the emissive layers show dominant
emission from [Ir(ppy)2(pzpy)]+ (Fig. S3, ESI†), which indicates
efficient energy-transfer from the 26DCzPPy host and the
oxadiazole-type anions to [Ir(ppy)2(pzpy)]+ in the emissive layers.
Fig. 5 shows the current density–voltage–brightness curves
of the OLEDs. Detailed device performances were summarized
in Table 2. As shown in Fig. 5, the devices using complexes 1–3
as dopants all showed lower current-density then the device
using complex R as the dopant. From the energy level diagrams
of the devices (Scheme S7, ESI†), the oxadiazole-type anions are
Fig. 3 Cyclic voltammograms of (a) oxadiazole-type anions and (b) com-
plexes R and 1–3 in DMF solution.
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Table 2 Summary of performances for OLED devices using complexes R
and 1–3
a
b
c
d
Von
Zc,max
EQEmax
(%)
Bmax
EL l
(nm)
CIE
(x, y)
Complex (V) (cd AÀ1
)
(cd mÀ2
)
R
1
2
3
3.8
4.0
4.1
4.1
26.6
37.6
33.8
33.1
10.2
15.2
13.6
13.3
14 000
12 000
12 000
4600
480, 509 (0.19, 0.49)
481, 510 (0.18, 0.49)
480, 509 (0.18, 0.49)
480, 509 (0.18, 0.49)
a
b
c
Voltage to reach 1 cd mÀ2
.
Maximum current efficiency. Maximum
d
external quantum efficiency. Maximum brightness.
the carrier-recombination balance and thus the performances
of the devices. Similar electron-trapping effects that promote
carrier recombination balance have been observed in OLEDs using
neutral iridium complexes with electron-trapping oxadiazole or
triazole-type units.52–54 In addition, the F values of the emissive
layers for devices using complexes R and 1–3 were measured to be
0.66, 0.72, 0.71 and 0.70, respectively. The higher F values for
complexes 1–3 compared to complex R in the emissive layers could
be related to suppression of inter-complex interactions caused by
the bulky counter-anions in complexes 1–3.36 By assuming a light
out-coupling efficiency of 0.2, the theoretical EQE for the device
using complex R was estimated to be 13.2%, which is considerably
higher than the experimental one (10.2%), indicating an unbalanced
carrier recombination in the device using complex R. The theoretical
EQEs of the devices using complexes 1–3 were estimated to be
14.0–14.4%, which agree with the experimental ones (13.3–15.2%),
indicating nearly balanced carrier-recombination in the devices
using complexes 1–3. It is thus concluded that the enhanced device
performances for cationic iridium complexes using oxadiazole-type
counter-anions can be ascribed to improved carrier-recombination
balance (major effect) and enhanced F values (minor effect) in the
emissive layer.
It is worth noting that the efficiency afforded by the device
using complex 1 (EQE = 15.2%) is the highest for solution-
processed OLEDs using cationic iridium complexes reported so
far (Table S1, ESI†).13–32 This efficiency is higher than that (9.8%)
afforded by the multilayer vacuum-evaporation blue-green OLEDs
using the [Ir(ppy)2(pzpy)]+ cation,55 and even comparable to the
highest one (16.4% for a green-yellow device) for multilayer
vacuum-evaporation OLEDs using cationic iridium complexes
(Table S1, ESI†).40 It is noted that [Ir(ppy)2(pzpy)]+ is not the most
efficient blue-green phosphorescent cation.31,44 Even higher
device performances can be expected by using more efficient
phosphorescent cations in combination with the oxadiazole-
type counter-anions.
Fig. 4 EL spectra of OLED devices using complexes R and 1–3. Inset
shows the device structure.
found to behave as electron-traps in the emissive layers when
they cannot form electron-transporting channels within the
layers at low doping concentrations (4 wt%).51 The lowered
current density for the devices using complexes 1–3 agrees with
the electron-trapping effects of the oxadiazole-type counter-
anions in the emissive layers.51–53
As shown in Table 2, the reference device using complex R
afforded a peak current efficiency and a peak EQE of 26.6 cd AÀ1
and 10.2%, respectively. For comparison, the devices using
complexes 1–3 showed much higher performances, with peak
current efficiencies of 37.6, 33.8 and 33.1 cd AÀ1, respectively,
and peak EQEs of 15.2%, 13.6% and 13.2%, respectively. The
largely enhanced device performances reveal the advantages of
using oxadiazole-type counter-anions for the use of cationic iridium
complexes in light-emitting devices. The electron-trapping on the
oxadiazole-type anions would on one hand achieve better hole/
electron-transport balance in the emissive layers, and on the other
hand, facilitate direct carrier recombination on the [Ir(ppy)2(pzpy)]+
cation because of the close proximity between the counter-anion
and the [Ir(ppy)2(pzpy)]+ cation. These effects help to improve
As shown in Table 2, devices using complexes 1–3 show slightly
higher turn-on voltages than the device using complex R, which
agrees with the electron-trapping effect of the oxadiazole-type
anions in complexes 1–3.51 The device using complex 2 showed
lower efficiencies than the device using compleÀx 1, which suggests
that the bulky tert-butyl group in tBuOXD-SO3 could hinder the
carrier-transport/recombination in the emissive layer. The device
using complex 3 showed comparable high efficiencies to the
devices using complexes 1 and 2 but much lower maximum
brightness, which suggests that the phenyltrifluoroborate anion
Fig. 5 Current-density and brightness versus voltage curves for OLED
devices using complexes R and 1–3.
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À
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in tBuOXD-BF3 has lower stabilityÀthan the benzenesulfonate
À
anion in OXD-SO3 or tBuOXD-SO3 in operating OLEDs, pre-
sumably due to the low chemical stability of B–C(sp2) bonds.56 All
devices showed decreased current efficiencies and EQEs at high
brightness (Fig. S4, ESI†), owing to the triplet–triplet or triplet-
polaron quenching at high current densities.57,58 Nevertheless, the
devices using complexes 1 and 2 still showed high current
efficiencies of 35.7 and 32.5 cd AÀ1, respectively, at 1000 cd mÀ2
which indicates widened carrier recombination zones in the
emissive layers.
,
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4. Conclusion
A series of oxadiazole-type counter-anions, OXD-SO3À, tBuOXD-
SO3À and tBuOXD-BF3À, and blue-green-emitting cationic iridium
complexes using them have been prepared and characterized.
These counter-anions affect neither the emission nor the electro-
chemical properties of the phosphorescent iridium cation. In
films, the anion and the phosphorescent cation are closely bound
together which allows efficient energy-transfer between them. The
new complexes showed remarkably enhanced performances
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À
compared to the reference complex with a PF6 counter-anion
in solution-processed OLEDs, with a record-high external quantum
efficiency of 15.2% from a double-layer structure. Even higher device
performances can be expected by combining these oxadiazole-type
anions with more efficient phosphorescent cations. The work has
revealed that cationic iridium complexes with oxadiazole-type
counter-anions can achieve performances comparable to neutral
complexes in solution-processed OLEDs, which afford highly
efficient devices with a great potential for display or lighting
applications.
21 H. J. Tang, Y. H. Li, Q. L. Chen, B. Chen, Q. Q. Qiao,
W. Yang, H. B. Wu and Y. Cao, Dyes Pigm., 2014, 100, 79–86.
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Conflicts of interest
There are no conflicts to declare.
23 F. L. Zhang, D. X. Ma, L. Duan, J. Qiao, G. F. Dong,
L. D. Wang and Y. Qiu, Inorg. Chem., 2014, 53, 6596–6606.
24 D. X. Ma, L. Duan and Y. Qiu, Dalton Trans., 2015, 44,
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25 H. J. Tang, Z. Y. Chen, L. Y. Wei, J. S. Miao, G. Y. Meng,
Y. H. He and H. B. Wu, Dyes Pigm., 2016, 131, 340–348.
26 D. X. Ma, C. Zhang, Y. Qiu and L. Duan, J. Mater. Chem. C,
2016, 4, 5731–5738.
Acknowledgements
We thank the National Natural Science Foundation of China
(Grant No. 51773074), the program of introducing talents of
discipline to universities of china (111 program, B17019), and
the self-determined research funds of CCNU from the colleges’
basic research and operation of MOE for financial support.
27 N. Z. Luo, Y. Lan, R. R. Tang, L. He and L. Duan, Chem.
Commun., 2016, 52, 14466–14469.
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