Substituent effect of ancillary ligands on the luminescence of bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′]iridium(iii) complexes

Article abstract of DOI:10.1039/c2dt30695f

Two series of (dfppy)₂Ir(L_N^O) with different substituents were designed and successfully synthesized and the effect of substitution at the ancillary ligand on the photophysical and electrochemical properties of (dfppy)₂Ir

Full text of DOI:10.1039/c2dt30695f

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PAPER
Substituent effect of ancillary ligands on the luminescence of
bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′]iridium(III) complexes†
Yuyang Zhou,a,b Wanfei Li,*a,c Yang Liu,a,c Liqiang Zeng,a Wenming Sua and Ming Zhou*a,c
Received 28th March 2012, Accepted 24th May 2012
DOI: 10.1039/c2dt30695f
Two series of (dfppy)2Ir(LN^O) with different substituents were designed and successfully synthesized and
the effect of substitution at the ancillary ligand on the photophysical and electrochemical properties of
(dfppy)2Ir(LN^O) were investigated. The results indicate that the electron-donating group of –OMe at LN^O
increases the PL quantum efficiencies of (dfppy)2Ir(LN^O) and the electron-withdrawing groups of –CF3
and –F lower the PL quantum efficiencies.
acetylacetonate,32,33 picolinate (pic),34,35 picolinamide,36 quinoline-
Introduction
2-carboxylic acid (qui),36,37 isoquinoline-1-carboxylic acid,36,37
pyrazine-2-carboxylic acid,36 quinoxaline-2-carboxylic acid,36
pyrazine-2-carboxamide,36 N-methylsalicylimine,38 diphosphine
chelates,39 bis(pyrazolyl)borate ligands,39 triazolate40 and tetra-
zolate derivatives41,42 etc.
Since the first demonstration of organic light-emitting diode
(OLEDs) with organic fluorophores by C. W. Tang in 1987,1
OLEDs have been developed into an industrially accepted tech-
nology for flat panel display and solid-state lighting. Chemists
and materials scientists are still deeply involved in discovering
and developing novel materials to improve device performance.
After the pioneering work by Forrest et al. on using PtOEP as a
phosphorescent dopant to greatly improve OLEDs efficiencies in
1998,2 a number of heavy metal complexes, such as iridium,3–6
platinum,2,7–11 ruthenium,12–15 osmium,16,17 copper18,19 etc.
have been investigated. Among them are iridium(III) complexes
most intensively studied and used as dopants in OLED
devices5,20–23 due to their high quantum efficiencies and broad
range of emission colors. While there are many green and red
emitters that meet the requirements of device design, the devel-
opment of blue phosphorescent emitters remains a challenge.
Although extensive studies have been carried out and a number
of blue emitters have been synthesized and tested, it is a little bit
amazing that the initially designed and synthesized iridium(III)-
bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′]picolinate (FIrpic) is
still the most popular blue dopant.24–31
The studies of substitution effects have been focused on
the main ligand 2-(2,4-difluorophenyl)pyridine, rather than the
ancillary ligands. In designing the ancillary ligands, various steric
and electronic effects are taken into consideration.20,40,43–46
Furthermore, You et al. have demonstrated an inter-ligand
energy transfer (ILET) to the “emitting ancillary ligand” in a
series of (dfppy)2Ir(LN^O) complexes (where LN^O is an N^O
chelating ancillary ligand) and provided a novel approach of
phosphorescence color tuning.36 The ILET occurs in some
(dfppy)2Ir(LN^O) systems, but not in the well-studied FIrpic.
Unfortunately, utilizing the ILET process has not been
continued.
It is our objective in this work to systematically compare the
effect of substituents of ancillary ligands in two different
(dfppy)2Ir(LN^O) complexes (Scheme 1), the emission of which
are governed by the MLCT3 and an ILET process, respectively.
We chose –F, –CF3 and –OMe as the electron-withdrawing and
the electron-donating groups, and synthesized two series of
(dfppy)2Ir(LN^O) complexes, i.e. FIrpic-series (MLCT3 mecha-
nism) and FIrqui-series (ILET mechanism). Reported here is
how the properties of the photoluminescence (PL), electro-
chemistry, frontier orbitals and electroluminescence (EL) are
influenced by the substituent groups.
Compared with homoleptic iridium(III) complexes, the syn-
thesis of bis-cyclometalated iridium(III) complexes with ancillary
ligands is easier. With the established popularity of FIrpic, a
number of works have been focused on the alteration of the
ancillary ligand. The ancillary ligands so far explored include
aDivision of Nanobiomedicine, Suzhou Institute of Nano-Technology and
Nano-Bionics, Chinese Academy of Sciences, 398 Ruoshui Road,
Suzhou Industrial Park, Suzhou, Jiangsu, 215123, P. R. China.
E-mail: mzhou2007@sinano.ac.cn, wfli2007@sinano.ac.cn;
Fax: +86-512-62872553; Tel: +86-512-628725558
bGraduate School of the Chinese Academy of Sciences, 19A Yuquan
Road, Beijing, 100049, P. R. China
Experimental section
Materials
Acetonitrile (spectrophotometric grade) used for measuring
UV-Vis absorption and photoluminescent emission spectra
was purchased from Sinopharm Chemical Reagent Co., Ltd.
Anhydrous acetonitrile (from Acros) and tetrabutylammonium
cSunaTech Inc., bioBAY, Suzhou Industrial Park, Jiangsu, 215123,
P. R. China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt30695f
This journal is © The Royal Society of Chemistry 2012
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Scheme 1 The synthetic route and the chemical structures of (dfppy)2Ir(LN^O) used in this work.
Scheme 2 The synthetic route of the ligand 4-methoxyquinoline-2-carboxylic acid (qui-OMe) used for synthesizing FIrqui-OMe.
hexafluorophosphate (TBAPF6, from Fluka) were used for
the electrochemical characterization without further treatment.
The dicholoro-bridged iridium(III) complex tetrakis(2,4-difluoro-
phenypyridine-C2,N)(μ-dichloro)diiridium(III) ([(dfppy)2Ir(μ-Cl)]2)
was received from SunaTech Inc. Picolinic acid (pic,
Alfa Aesar), quinoline-2-carboxylic acid (qui, Alfa Aesar),
5-methoxypicolinic acid (pic-OMe, Fastsyn Inc.), 4-(trifluoro-
methyl)quinoline-2-carboxylic acid (qui-CF3, J&K Chemical
Ltd) and 6-fluoro-4-(trifluoromethyl)quinoline-2-carboxylic
acid (qui-CF3F, J&K Chemical Ltd) were used as received.
4-methoxyquinoline-2-carboxylic acid (qui-OMe) was syn-
thesized according to Scheme 2.
characterization were all prepared in glove box under nitrogen
atmosphere.
The cyclic voltammetry (CV) was performed with a PARE-
STAT 2263 advanced electrochemical system, Princeton Applied
Research. Anhydrous acetonitrile was used as the solvent under
nitrogen atmosphere and 0.1 mol L−1 of TBAPF6 as the support-
ing electrolyte. A platinum wire (1 mm in diameter, the electrode
area 0.785 mm2) sealed in a PTFE rod was used as the working
electrode. A piece of platinum wire and a piece of silver wire
were used as counter electrode and quasi-reference electrode,
respectively. The potentials were referred to ferrocene/ferro-
cenium Fc/Fc+ redox couple. The potential scan rate in all the
experiments was kept at 100 mV s−1
.
The EL devices were fabricated by sequentially depositing
organic layers using thermal evaporation in one run under high
vacuum (4.5 × 10−4 Pa) onto indium tin oxide (ITO) glass sub-
strates. Prior to use, the substrates were degreased with acetone
and cleaned in a UV-ozone chamber for 15 min before it was
loaded into the evaporation system. The emitting area was
3 × 3 mm2. The thickness of the deposited layer and the evapor-
ation speed of the individual materials were monitored with
quartz crystal microbalance monitors. All electrical testing and
optical measurements were performed under ambient conditions
without further encapsulation. The EL spectra were measured
with a Spectra Scan PR655. The current–voltage (I–V) and lumi-
nance–voltage (L–V) relations were characterized with a compu-
ter controlled Keithley 2400 Sourcemeter.
Measurements
1H, 13C and 19F NMR spectra were acquired on a VARIAN
400 MHz magnetic resonance spectrometer. The UV-Vis absorp-
tion spectra and PL spectra were obtained with a Perkin Elmer
Lambda 25 UV/Vis spectrophotometer and a HITACHI F-4600
spectrofluorometer, respectively. Due to oxygen quenching of
the photoluminescence of iridium(III) complexes, the acetonitrile
solution of (dfppy)2Ir(LN^O
efficiency was prepared in
atmosphere.
The PL quantum efficiencies (Φ) were measured in dilute sol-
utions (10−5 M) and compared to the standard emitters according
to the equation: Φ = Φref (Is/Iref )(Aref /As), where A is the absor-
bance at the excitation wavelength, I is the integrated intensity of
the luminescence and Φ is the PL quantum efficiency. The sub-
scripts s and ref refer to the sample and standard, respectively.
Spectrophotometric acetonitrile was deaerated by nitrogen bub-
bling for 30 min and then stored in a nitrogen glove box. The
acetonitrile solutions of (dfppy)2Ir(LN^O) for spectroscopic
)
for measuring PL quantum
glove box under nitrogen
a
Synthesis and characterization
Dimethyl 2-(phenylamino)maleate (1). A solution of phenyl-
amine (50 mmol, 4.7 g) and dimethylacetylene dicarboxylate
(50 mmol, 7.1 g) in 180 mL of anhydrous methanol was
refluxed for 18 h, and the reaction mixture was roto-evaporated
9374 | Dalton Trans., 2012, 41, 9373–9381
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to give the crude product. The pure product 1 was obtained
by recrystallization from methanol. Yield: 89%. 1H NMR
(400 MHz, CDCl3) δ ppm: 9.66 (s, 1 H), 7.30 (m, 2 H), 7.09 (m,
1 H), 6.80–6.92 (m, 2 H), 5.39 (s, 1 H), 3.74 (s, 3 H), 3.69 (s,
3 H). 13C NMR (100 MHz, CDCl3) δ ppm: 169.84, 164.83,
147.96, 140.21, 129.11, 124.20, 120.64, 93.53, 52.75, 52.18.
(m, 3 H), 8.74–8.76 (m, 1 H). 19F NMR (376 MHz, CDCl3)
δ ppm: −108.4 (q, J = 9.0), −109.4 (q, J = 9.0), −111.1 (t,
J = 11.3), −111.7 (t, J = 11.3).
FIrpic. (Yield: 75%, UPLC purity: >98%). 1H NMR
(400 MHz, CDCl3) δ ppm: 8.74 (m, 1 H), 8.34 (ddd, J = 8, 1.6,
0.8 Hz, 1 H), 8.26 (m, 2 H), 7.95 (td, J = 7.6, 1.6 Hz, 1 H),
7.77 (m, 3 H), 7.42 (m, 2 H), 7.18 (m, 1 H), 6.97 (m, 1 H), 6.45
(m, 2 H), 5.83 (dd, J = 8.8, 2.4 Hz, 1 H), 5.56 (dd, J = 8.8,
2.4 Hz, 1 H). 19F NMR (376 MHz, CDCl3) δ ppm: −108.4
(q, J = 9.0), −109.3 (q, J = 9.0), −111.0 (t, J = 11.3), −111.6
(t, J = 11.3).
Methyl 4-hydroxyquinoline-2-carboxylate (2). 1 was added to
the solution of diphenyl ether (25 mL) in a 100 mL round-
bottomed flask. The reaction mixture was stirred at 250 °C for
4 h. After it was cooled to room temperature, an excess amount
of n-hexane was added to precipitate the crude solid. After
filtration, the solid was washed with ethyl ether 3 times and
dried in vacuo to provide 3.9 g of gray pale 2. Yield: 58%.
1H NMR (400 MHz, CD3OD) δ ppm: 8.20–8.24 (m, 1 H),
7.82–7.86 (m, 1 H), 7.74–7.77 (m, 1 H), 7.45 (m, 1 H), 6.93
(s, 1 H), 4.03 (s, 3 H).
FIrpic-F. (Yield: 60%, UPLC purity: >98%). 1H NMR
(400 MHz, CDCl3) δ ppm: 8.72 (ddd, J = 5.6, 1.6, 0.8 Hz, 1 H),
8.37 (m, 1 H), 8.28 (m, 2 H), 7.80 (m, 2 H), 7.65 (m, 2 H),
7.44 (ddd, J = 5.6, 1.6, 0.8 Hz, 1 H), 7.21 (m, 1 H), 7.01 (m, 1
H), 6.50 (m, 1 H), 6.40 (m, 1 H), 5.83 (dd, J = 8.8, 2.4 Hz, 1 H),
5.54 (dd, J = 8.8, 2.4 Hz, 1 H). 19F NMR (376 MHz, CDCl3)
δ ppm: −107.9 (q, J = 9.0), −109.0 (q, J = 9.0), −110.6 (t, J =
11.3), −111.4 (t, J = 11.3), −118.3 (t, J = 5.6).
Methyl 4-methoxyquinoline-2-carboxylate (3). A 250 mL,
three-neck round flask was charged with 2 (3.7 g, 18 mmol),
K2CO3 (3.7 g, 27 mmol), and 100 mL of dimethyl sulfoxide.
The reaction mixture was stirred at 70 °C for 1 h, then cooled to
35 °C. Methyl iodide (5.1 g, 36 mmol) was added and the reac-
tion was allowed to continue for another 2 h at 35 °C. The
mixture was poured into water (300 mL). The solution was
filtrated and the precipitate was washed with water. The desired
FIrqui-OMe. (Yield: 53%, UPLC purity: >98%). 1H NMR
(400 MHz, DMSO-d6) δ ppm: 8.48 (ddd, J = 6, 1.6, 0.8 Hz,
1 H), 8.3 (d, J = 8.8 Hz, 1 H), 8.19 (m, 2 H), 8.05 (td, J = 7.6,
1.6 Hz, 1 H), 7.95 (td, J = 8, 1.6 Hz, 1 H), 7.81 (s, 1 H),
7.79 (ddd, J = 5.6, 0.8 Hz, 1 H), 7.58 (m, 2 H), 7.40 (m, 2 H),
7.23 (m, 1 H), 6.85 (m, 2 H), 5.81 (dd, J = 8.4, 2.4 Hz, 1 H),
5.36 (dd, J = 8.4, 2.4 Hz, 1 H), 4.22 (s, 3 H). 19F NMR
(376 MHz, CDCl3) δ ppm: −108.6 (q, J = 9.0), −109.0 (q,
J = 9.0), −111.9 (t, J = 11.3), −111.7 (t, J = 11.3).
1
product (2.7 g) was obtained after drying. Yield: 70%. H NMR
(400 MHz, CD3OD) δ ppm: 8.23–8.26 (m, 1 H), 8.13 (dt,
J = 8.4, 0.8 Hz, 1 H), 7.81 (m, 1 H), 7.65 (m, 1 H), 7.62 (s,
1 H), 4.16 (s, 3 H), 4.06 (s, 3 H).
4-Methoxyquinoline-2-carboxylic acid (qui-OMe). To
a
50 mL anhydrous methanol solution of 3 (at ice bath temp-
erature) was added 50 mL of aqueous NaOH solution (2 wt.%).
After 2 h, the methanol in the reaction mixture was removed by
roto-evaporation. The remaining solution was diluted with
100 mL of H2O, washed with 100 mL of dichloromethane three
times, the aqueous phase was acidified with concentrated HCl to
pH = 3, then a great deal of precipitate appeared. The desired
product (1.6 g, yield: 65.5%) was obtained after filtration.
1H NMR (400 MHz, CD3OD) δ ppm: 8.38 (ddd, J = 8.4, 1.6,
0.8 Hz, 1 H), 8.29 (dt, J = 8.8, 0.8 Hz, 1 H), 8.01 (m, 1 H),
7.82 (s, 1 H), 7.79 (m, 1 H), 4.34 (s, 3 H).
FIrqui. (Yield: 65%, UPLC purity: >99%). 1H NMR
(400 MHz, CDCl3) δ ppm: 5.43 (dd, J = 8.4, 2.4 Hz, 1 H,),
5.93 (dd, J = 8.8, 2.4 Hz, 1 H), 6.42 (m, 2 H), 6.85 (m, 1 H),
7.12 (m, 1 H), 7.36 (m, 1 H), 7.58 (m, 2 H), 7.71 (m, 2 H),
7.89 (m, 2 H), 8.19 (d, J = 8 Hz, 1 H), 8.30 (m, 1 H), 8.42 (d,
J = 8 Hz, 1 H), 8.52 (d, J = 8 Hz, 1 H), 8.66 (m, 1 H). 19F NMR
(376 MHz, CDCl3) δ ppm: −108.4 (q, J = 9.0), −108.8 (q,
J = 9.4), −111.8 (t, J = 11.3), −111.6 (t, J = 11.3).
FIrqui-CF3. (Yield: 62%, UPLC purity: >98%). 1H NMR
(400 MHz, CDCl3) δ ppm: 5.40 (dd, J = 8.8, 2.4 Hz, 1 H),
5.93 (dd, J = 8.8, 2.4 Hz, 1 H), 6.50 (m, 2 H), 6.90 (m, 1 H),
7.15 (m, 1 H), 7.47 (m, 1 H), 7.60 (m, 1 H), 7.73 (m, 2 H),
7.79 (m, 1 H), 8.10 (d, J = 4.4 Hz, 1 H), 8.20 (m, 2 H), 8.32 (m,
1 H), 8.63 (m, 1 H), 8.85 (s, 1 H). 19F NMR (376 MHz, CDCl3)
δ ppm: −62.6 (s), −107.8 (q, J = 9.0), −108.4 (q, J = 9.4),
−111.4 (t, J = 11.3), −111.3 (t, J = 11.3).
General approach to (dfppy)2Ir(LN^O). The synthetic route of
(dfppy)2Ir(LN^O
) used in this work is demonstrated on
Scheme 1. A solution of dichloro-bridged iridium(III) dimer
[(dfppy)2Ir(μ-Cl)]2, 2.2 equiv. of the corresponding ancillary
ligands and 22 equiv. of Na2CO3 in 2-ethoxyethanol (for
FIrpic-OMe, FIrpic, FIrqui, FIrqui-CF3, FIrqui-CF3F) or xylenes
(for FIrpic-F and FIrqui-OMe) was refluxed for 24 h. The
solvent was roto-evaporated, the residue chromatographed on a
silica gel column with CH2Cl2/n-hexane (various ratios based on
the complex properties) eluent to give the pure product. Yield:
53–80%.
FIrqui-CF3F. (Yield: 65%, UPLC purity: >98%). 1H NMR
(400 MHz, CDCl3) δ ppm: 5.39 (dd, J = 8.8, 2.4 Hz, 1 H), 5.92
(dd, J = 8.8, 2.4 Hz, 1 H), 6.50 (m, 2 H), 6.92 (m, 1 H), 7.17 (m,
1 H), 7.25 (m, 1 H), 7.57 (m, 1 H), 7.78 (m, 3 H), 8.16 (m, 2 H),
8.34 (m, 1 H), 8.61 (m, 1 H), 8.87 (s, 1 H). 19F NMR (376 MHz,
CDCl3) δ ppm: −63.3 (s), −105.5 (m), −107.4 (q, J = 9.0),
−108.2 (q, J = 9.4), −110.0 (t, J = 11.3), −111.1 (t, J = 11.3).
FIrpic-OMe. (Yield: 80%, UPLC purity: >99%). 1H NMR
(400 MHz, CDCl3) δ ppm: 3.87 (s, 3 H), 5.54–5.57 (dd, J = 8.8,
2.4 Hz, 1 H), 5.81–5.84 (dd, J = 8.8, 2.4 Hz, 1 H), 6.36–6.50
(m, 2 H), 6.96–7.00 (m, 1 H), 7.17–7.21 (m, 1 H), 7.37–7.40
(m, 2 H), 7.44–7.47 (m, 1 H), 7.75–7.80 (m, 2 H), 8.23–8.30
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Fig. 1 Absorption and emission spectra of (dfppy)2Ir(LN^O) in deaerated acetonitrile solutions (40 μM for absorption spectra and 10 μM for emission
spectra). (A) FIrpic-OMe, FIrpic and FIrpic-F. (B) FIrqui-OMe, FIrqui, FIrqui-CF3 and FIrqui-CF3F. ε is molar extinction coefficient.
Table 1 Photophysical data of (dfppy)2Ir(LN^O) in acetonitrile
Absorption, ε (104 M−1 cm−1
max@ λ (nm)
)
Emission (365 nm excitation)
Complex/solvent
@365 nm
λmax (nm)
Is/Iref
As/Aref
Φa
FIrpic-OMe/MeCN
FIrpic/MeCN
FIrpic-F/MeCN
FIrqui-OMe/MeCN
FIrqui/MeCN
FIrqui-CF3/MeCN
FIrqui-CF3F/MeCN
fac-Ir(ppy)3/MeCN
4.66@250, 0.45@374
4.21@253, 0.49@374
4.02@253, 0.48@374
5.72@239, 0.48@378
5.90@241, 0.48@376
6.37@246, 0.78@361
5.97@246, 0.70@361
0.43
0.47
0.46
0.53
0.53
0.76
0.68
1.14
471
471
470
532
566
—
0.69
0.70
0.44
0.26
0.07
—
0.38
0.41
0.40
0.46
0.46
—
0.73
0.68
0.44
0.23
0.06
—
—
516
—
1.00
—
1.00
—
0.40
4.74@241, 4.49@281, 1.17@374
a Referred to fac-Ir(ppy)3 (Φref = 0.4) in deaerated acetonitrile.
π–π* transition and MLCT (δ–π*) transition. For FIrpic-OMe,
FIrpic and FIrpic-F, though the MLCT (δ–π*) transitions are all
located at 374 nm, the extinction coefficients vary slightly from
between 4500 M−1 cm−1 and 4900 M−1 cm−1. Compared with
relative small range of extinction coefficient at the MLCT (δ–π*)
transition, the extinction coefficient at π–π* transition exhibits
larger variation range from 40 200 M−1 cm−1 of FIrpic-F
to 46 600 M−1 cm−1 of FIrpic-OMe. Furthermore, substituting
H with the electron-donating group of –OMe increases the extinc-
tion coefficient (from 42 100 M−1 cm−1 to 46 600 M−1 cm−1) at
π–π* transition while substituting H with the electron-withdraw-
ing group of –F decreases the extinction coefficient (from
42 100 M−1 cm−1 to 40 200 M−1 cm−1), as numerically illus-
trated in Table 1. The same phenomenon was observed in the
FIrqui series, i.e. FIrqui-OMe, FIrqui, FIrqui-CF3 and FIrqui-
CF3F. The extinction coefficients at MLCT (δ–π*) transition range
from 4800 M−1 cm−1 to 7800 M−1 cm−1, while at π–π* the tran-
Results and discussion
Photophysical properties
To confirm the accuracy of our measurement system for quantum
efficiency, we firstly measured the PL efficiency of fac-Ir(ppy)3
which has been well characterized in previous studies.38,47,48
The PL quantum efficiency of fac-Ir(ppy)3 obtained is 0.42
using quinine sulfate in 0.5 M sulfuric acid as standard
(Φ = 0.546)47,49,50 by our measurement system, which is very
close to the value (0.4) reported previously.48
The absorption and emission spectra of (dfppy)2Ir(LN^O) in
acetonitrile solutions are shown in Fig. 1. Like other heteroleptic
iridium(III) complexes reported before,39,43,50–54 these iridium(III)
complexes have strong intra-ligand absorption bands (π–π*) in
the UV region (below 300 nm, ε > 15 000 M−1 cm−1) and a fea-
tureless MLCT (δ–π*) transition in the near UV and/or blue end
of visible light (350–450 nm, ε < 6000 M−1 cm−1). Except
FIrqui-CF3 and FIrqui-CF3F, all other iridium(III) complexes
have shown strong photoluminescence under the excitation of
the lights from the whole range of absorption, indicative of their
candidacy as phosphorescent dopants in OLEDs. These iridium(III)
complexes are grouped into FIrpic and FIrqui series for con-
venient comparison and their spectroscopic data are summarized
in Table 1. Though the substitutes have no influence on the
location of intra-ligand transition (π–π*) and MLCT (δ–π*) tran-
sition, there are small differences on the extinction coefficient at
sition range is from 57 200 M−1 cm−1 to 63 700 M−1 cm−1
.
In contrast to the small influence of substituents on the elec-
tronic absorption spectra, (dfppy)2Ir(LN^O) exhibited significant
variation of the photoluminescence properties, both in color and
PL quantum efficiency.
For the FIrpic series as shown in Fig. 1A and Table 1, because
the triplet energy level of the ancillary ligand is higher than that
of dfppy-centered MLCT3, the emission is mainly attributed to
the phosphorescent decay from the MLCT3 state of dfppy, which
9376 | Dalton Trans., 2012, 41, 9373–9381
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Fig. 2 Cyclic voltammograms of 5 mM of FIrpic-OMe (A), FIrpic (B)
and FIrpic-F (C) in acetonitrile solutions with 0.1 M TBAPF6.
is clearly demonstrated by You et al.,36 the emission colors of
the FIrpic series have no differences. However, the substitution
had significant influence on PL quantum efficiency. Substituting
H, meta to the nitrogen in picolinic acid with the electron-
withdrawing group –F lowered the PL quantum efficiency from
0.68 to 0.44, while incorporating electron-donating substituent
–OMe at the same position of picolinic acid enhanced the
PL quantum efficiency of FIrpic-OMe from 0.68 to 0.73. For the
FIrqui-series, the phosphorescent decay from the triplet state of
the ancillary ligand plays a significant role in emission color.
Unlike the FIrpic series, the FIrqui series shows dramatic
changes in emission color (Fig. 1B and Table 1). Substituting
H at the 4-position of qui with the electron-donating group
–OMe caused a blue shift of ca. 34 nm compared with the
unsubstituted FIrqui. Just as the negative influence of –F to the
PL quantum efficiency of FIrpic-F, the PL intensities of
FIrqui-CF3 and FIrqui-CF3F are also very weak compared with
FIrqui-OMe and FIrqui, which are shown in Fig. 1B.
Fig. 3 Cyclic voltammograms of 5 mM of FIrqui-OMe (A), FIrqui
(B), FIrqui-CF3 (C), and FIrqui-CF3F (D) in acetonitrile solutions with
0.1 M TBAPF6.
As shown in Fig. 2 and 3, the electron-withdrawing substitu-
ents of –F and –CF3 have adverse influence on the reversibility
of the first reduction wave of (dfppy)2Ir(LN^O). Furthermore, in
both FIrpic and FIrqui series, the electron-withdrawing substitu-
ents of –F and –CF3 cause an anodic shift at the first reduction
potential compared with (dfppy)2Ir(LN^O) with an unsubstituted
LN^O. However, the electron-donating substituent of –OMe led
to a cathodic shift at the first reduction potential with respect to
(dfppy)2Ir(LN^O) with an unsubstituted LN^O (see Table 2).
Accordingly, the LUMOs of (dfppy)2Ir(LN^O) with –F and –CF3
substituted LN^O are lower than that with –OMe. For FIrqui
series, the electron-donating substituent of –OMe makes the
energy gap between HOMO and LUMO (ΔE) larger. This
explains the blue shift of emission wavelength from FIrqui to
FIrqui-OMe.
Electrochemical properties
Cyclic voltammetry (CV) is a useful tool for characterizing
redox active substances, from both the kinetics of the electron
transfer reaction and the thermodynamics of the electrode–
electrolyte interface. Furthermore, CV can provide HOMO/
LUMO information of the cyclometalated iridium(III) complexes.
To further investigate the influence of the ancillary ligand on the
complexes (dfppy)2Ir(LN^O), the electrochemical properties of
(dfppy)2Ir(LN^O) were studied by cyclic voltammetry and the
voltammograms of (dfppy)2Ir(LN^O) are presented in Fig. 2 and
3. Considering the issue of electrochemical window, dried aceto-
nitrile was chosen as the solvent and all these electrochemical
experiments were completed in a glove box, in which the
oxygen and water were kept at less than 1 ppm.
Theoretical calculations
To compare the experimentally obtained HOMO/LUMO levels
with the theoretical calculation, we have used the B3LYP density
functional theory (DFT) to calculate HOMO/LUMO energy
levels and the electronic ground states for (dfppy)2Ir(LN^O).
As usually done with cyclometalated iridium(III) complexes,55–57
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Table 2 Electrochemical data and energy levels of (dfppy)2Ir(LN^O) used in this worka
Complex
Eaox(V)
Eonsetox(V)
Ecre(V)
Eonsetre(V)
HOMOb (eV)
LUMOc (eV)
ΔE (eV)
FIrpic-OMe
FIrpic
FIrpic-F
FIrqui-OMe
FIrqui
FIrqui-CF3
FIrqui-CF3F
0.97
0.94
0.97
0.96
0.98
1.04
1.05
0.73
0.79
0.85
0.80
0.84
0.88
0.87
−2.42, −2.69
−2.32, −2.75
−2.22, −2.73
−2.09, −2.61
−1.92, −2.78
−1.56, −2.85
−1.50, −2.78
−2.28
−2.19
−2.09
−1.95
−1.80
−1.38
−1.32
−5.53
−5.59
−5.65
−5.60
−5.64
−5.68
−5.67
−2.52
−2.61
−2.71
−2.85
−3.00
−3.42
−3.48
3.01
2.98
2.94
2.75
2.64
2.26
2.19
a In acetonitrile solution (0.1 M TBAPF6) and the electrochemical potentials referred to ferrocene/ferrocenium (Fc/Fc+) redox couple. b HOMO =
−(Eonsetox + 4.8). c LUMO = −(Eonsetre + 4.8).
Fig. 5 Theoretical (black) and experimental (red, determined by cyclic
Fig. 4 Contour plots of HOMO and LUMO of (dfppy)2Ir(LN^O) used
voltammetry) energy levels of (dfppy)2Ir(LN^O).
in this work.
Table 3 The electronegativity and energy levels of HOMOs and
the 6-31G+(d) basis set was chosen for C, N, H, O, the 6-31G-
LUMOs for the ligands dfppy and LN^O
(d, p) basis set for F, and the LANL2DZ basis set for iridium
atom.
HOMO
(eV)
LUMO
(eV)
ΔEa
(eV)
Electronegativity
Ligand
dfppy
(χ) (eV)b
As we can see from Fig. 4, all (dfppy)2Ir(LN^O) complexes
have similar electron distributions on HOMOs and LUMOs.
HOMOs of (dfppy)2Ir(LN^O) are assigned to an admixture of the
d-orbital of metal and π-orbitals of dfppy while LUMOs are
assigned to the π-orbitals of the ancillary ligands. Therefore, it is
not surprising that with the change of the ancillary ligands, the
LUMOs of (dfppy)2Ir(LN^O) exhibited much bigger change than
the HOMOs. Although there are discrepancies between the
theoretical data and the electrochemistry based data, the general
tendency of the theoretical series is in line with the experimental
series (Fig. 5). As already reflected in the electrochemical study,
the electron-donating substituent of –OMe increases the energy
level of LUMO, while the electron-donating substituents of –F
and –CF3 decrease the LUMO.
The reason why the pic and qui series demonstrated different
effects on the photoluminescence of (dfppy)2Ir(LN^O) can be
understood by analyzing the energy gaps ΔE of the ligands
dfppy and the ancillary LN^O. The calculation of ΔE of these
ligands showed the following order: pic-F > pic-OMe > pic >
dfppy > qui > qui-CF3F > qui-CF3 > qui-OMe. For the FIrpic
series, due to the lower ΔE of dfppy compared with that of pic-
OMe, pic and pic-F, the energy level of dfppy-centered MLCT3
state plays a major role in the emission color, which explains the
very small impact the substitution at ancillary ligands had on the
−6.63
−1.71
−1.98
−2.25
−2.38
−2.13
−2.36
−2.84
−2.96
4.92
5.27
5.21
5.54
4.37
4.49
4.40
4.44
4.17
4.62
4.86
5.15
4.32
4.61
5.04
5.18
pic-OMe −7.25
Pic
pic-F
−7.46
−7.92
qui-OMe −6.50
qui
qui-CF3
−6.85
−7.24
qui-CF3F −7.40
a ΔE is the energy gap between HOMO and LUMO. b χ = −(EHOMO
ELUMO)/2.56,58
+
emission color. However, compared with dfppy, the qui series
have lower ΔE, and thus the emission color is mainly determined
by the triplet energy level of ancillary ligand. Consequently, the
substitution at ancillary ligands has bigger influence on the emis-
sion color.
The average of HOMO and LUMO (calculated and listed also
in Table 3) represents the electronegativity of the ligand.56 It is
evident that all the ancillary ligands in this work have larger
electronegativities than the main ligand dfppy. The HOMO elec-
tron distributions of (dfppy)2Ir(LN^O) are centered on dfppy and
the LUMO electron distributions on LN^O. Such delocalization
9378 | Dalton Trans., 2012, 41, 9373–9381
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Table 4 The summary of EL data in this work
Current efficiency
Current efficiency at
Device
Dopant
PL (nm)
EL (nm)
at peak (cd A−1
)
100 cd m−2 (cd A−1
)
CIEx,y
I
II
III
IV
FIrpic-OMe
FIrpic
FIrqui-OMe
FIrqui
471
471
532
566
468
468
512, 531
548
6.00
8.66
5.21
6.21
3.62
3.01
1.39
2.51
(0.16, 0.31)
(0.16, 0.31)
(0.29, 0.55)
(0.39, 0.57)
Fig. 6 Performances evaluation of devices I and II. (A) EL spectrum at 9 V, (B) current efficiency-current density characteristics, (C) current-
voltage-luminance characteristics.
Fig. 7 The performance of the device III. (A) EL spectrum at 9 V, (B) current efficiency-current density characteristics, (C) current-voltage-lumi-
nance characteristics.
modes, i.e. HOMOs centered on the ligand with smaller electro-
negativity and LUMOs on the ligand with larger electro-
negativity, is consistent with the results of Shi et al.56
respectively. The host materials mcp and CBP represent
1,3-bis(9H-carbazol-9-yl)benzene and 4,4′-bis(9H-carbazol-9-yl)-
biphenyl, respectively. The electron-transport materials
3TPYMB22 and TPBi are tris(2,4,6-trimethyl-3-(pyridin-3-yl)-
phenyl)borane and 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-
yl)phenyl, respectively.
EL properties
Table 4 shows the numerical data of the performances of the
four devices. Between devices I and II, although device I
showed a lower maximum current efficiency than device II, at
the brightness of 100 cd m−2, the current efficiency of device I
was higher than that of device II (Fig. 6 and Table 4). Both
devices demonstrated no obvious difference on the CIE (the
1931 Commission International de L’Eclairage) and EL spectra.
To the best of our knowledge, no OLED devices using FIrqui-
OMe and FIrqui as dopants have ever been reported, although
FIrqui has been synthesized by You et al. The performances of
the devices, i.e. III and IV are summarized in Table 4 and
detailed in Fig. 7 and 8, respectively. For both dopants, we have
Due to their relatively high PL quantum efficiencies, FIrpic-
OMe, FIrqui-OMe and FIrqui were chosen as the dopants for
fabricating OLED devices. For the purpose of comparison,
FIrpic was also used as dopant in this work. Based on their emis-
sion colors, two device structures were designed, i.e. ITO/NPB
(20 nm)/TAPC (20 nm)/mcp : dopant (25 nm, 7%)/3TPYMB
(30 nm)/LiF (1 nm)/Al for FIrpic-OMe and FIrpic (devices I
and II), ITO/NPB (30 nm)/CBP : dopant (25 nm, 7%)/TPBi
(30 nm)/LiF (1 nm)/Al for FIrqui-OMe and FIrqui (devices III
and IV), where the hole-transport materials NPB and
TAPC stand for N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-
benzidine and 1,1-bis[4-[N,N′-di(p-tolyl)amino]phenyl]cyclohexane,
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Fig. 8 The performance of the device IV. (A) EL spectrum at 9 V, (B) current efficiency-current density characteristics, (C) current-voltage-lumi-
nance characteristics.
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cence to their device electroluminescence. Such blue shifts from
solution to solid state has been observed before from (dfbmb)2Ir-
(fptz)20 and were considered to be due to the aggregation effect.
The overall performances of these devices are not among the
best, though the device structures are not the optimized ones.
Nevertheless, these materials and data help enrich the OLED
materials research and their high PL efficiencies may allow them
to be used in other areas, such as bioimaging.59
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In summary, two series of (dfppy)2Ir(LN^O) complexes with
different substituents at the ancillary ligand, FIrpic- and FIrqui-
series representing MLCT3 and ILET emission mechanisms,
respectively, were synthesized and characterized. The substi-
tution of the ancillary ligand, qui, has a significant impact on the
ILET governed FIrqui-series, while the MLCT3 governed FIrpic-
series were not sensitive to the substitution in terms of the emis-
sion wavelength. This work also revealed that, in general, the
substitution at ancillary ligands has more significant influence on
the first reduction potentials and the LUMOs of (dfppy)2Ir-
(LN^O). Compared with (dfppy)2Ir(LN^O) with unsubstituted pic
and qui, the electron-withdrawing substituents of –F and –CF3
cause an anodic shift of the first reduction potential and lead to
lower LUMOs, while the electron-donating substituent of –OMe
has a contrary effect. Most importantly, the electron-donating
substituent of –OMe at LN^O increased the PL quantum
efficiency while the electron-withdrawing substituents of –CF3
and –F at LN^O lowered the PL quantum efficiency.
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The financial support of this work from National Science Foun-
dation of China (NSFC) under Project Nos. 20902066 and
21072218 is gratefully acknowledged.
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