Highly efficient deep-blue phosphorescence from heptafluoropropyl-substituted iridium complexes

Article abstract of DOI:10.1039/c4cc07768g

New deep-blue iridium complexes, consisting of a heptafluoropropyl (HFP) substituent at the 3′ position of 2′,4′′-difluorophenyl, have a deep HOMO level and decreased shoulder electronic transition and inhibit self-quenching due to the sterically hindered

Full text of DOI:10.1039/c4cc07768g

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Highly efficient deep-blue phosphorescence from
heptafluoropropyl-substituted iridium complexes
(HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz) with
electron-withdrawing heptafluoropropyl groups exhibited
wide band gaps with high PL QYs and the devices showed
EQEs of 19.7, 21.4 and 14.2 and CIE coordinates of (0.147, 0.164),
(0.146, 0.165) and (0.152, 0.148), respectively.
See Soon-Ki Kwon,
Jang-Joo Kim, Yun-Hi Kim et al.,
Chem. Commun., 2015, 51, 58.
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Highly efficient deep-blue phosphorescence from
heptafluoropropyl-substituted iridium complexes†
Cite this: Chem. Commun., 2015,
51, 58
Jung-Bum Kim,‡a Seung-Hoon Han,‡b Kiyull Yang,c Soon-Ki Kwon,*d
Jang-Joo Kim*a and Yun-Hi Kim*b
Received 1st October 2014,
Accepted 31st October 2014
DOI: 10.1039/c4cc07768g
www.rsc.org/chemcomm
New deep-blue iridium complexes, consisting of a heptafluoropropyl the phenyl and pyridyl moieties, respectively, of the cyclometalated
(HFP) substituent at the 30 position of 20,400-difluorophenyl, have a ligands.20 The electron-withdrawing group in the phenyl part lowers
deep HOMO level and decreased shoulder electronic transition and the HOMO level while the electron-donating group in the pyridyl part
inhibit self-quenching due to the sterically hindered group without increases the LUMO level. We had previously reported that the
conjugation. An OLED using (HFP)2Ir(mpic) exhibited a maximum position of the substituent in dimethylated iridium complexes affects
EQE of 21.4% with a CIE of (0.146, 0.165).
the color purity and efficiency of the corresponding OLEDs.21 We also
recently reported that overly strong electron-withdrawing groups such
Red and green phosphorescent iridium complexes are used in as the heptafluorocarbonyl unit decreased both HOMO and LUMO
organic light emitting diodes (OLEDs) for displays and solid levels.22 Therefore, appropriately strong electron-withdrawing groups
state lighting systems. Recently, external quantum efficiencies are required to effectively decrease the HOMO level without lowering
(EQEs) of over 30% have been reported.1–5 However, development of the LUMO level. A second strategy is to control the vibration shoulder
highly efficient blue, especially deep blue emitting phosphorescent intensities to have larger emission oscillator strength at the shortest
dyes still remains a challenge for high performance full color wavelength than the oscillator strength of other longer wavelength
displays and high quality white OLEDs. Iridium(III)bis[(4,6-difluoro- shoulder peaks. Since most of the blue-emitting iridium complexes
phenyl)-pyridinato-N,C20] picolinate (FIrpic) has been widely have vibrational shoulder peaks at a longer wavelength, larger
used as a dopant material for sky blue PHOLEDs with high oscillator strengths of the longer wavelength shoulder peaks
external quantum efficiency.6–14 However, it is difficult to have an effect of diminishing the color purity for deep blue.
synthesize deep blue phosphorescent materials that exhibit
In this communication, we report three new deep-blue iridium
high color purity as well as high photoluminescence quantum complexes, which consist of a heptafluoropropyl (HFP) substituent
yields (PLQYs).15–17 Recently, a couple of strategies have been at the 30 position of 20,400-difluorophenyl as the main ligand and a
developed for deep-blue emitting iridium complexes, which are picolinate (pic), a picolinate with 3-methylpyridine (mpic) or a
used in high-efficiency, deep blue phosphorescent OLEDs; trifluoromethylated triazole (fptz) as the ancillary ligand. HFP was
however, the color purity was rather limited.18,19
selected as an electron-withdrawing group that does not exhibit
One strategy involves controlling the energy levels of the extended p-conjugation and a methyl group at the 4 position of the
highest occupied molecular orbital (HOMO) and the lowest pyridine group for main ligand. Perfluoro alkyl groups have several
unoccupied molecular orbital (LUMO) localized primarily on effects in Ir(III) cyclometalated complexes. One is an appropriate
electron-withdrawing effect without conjugation on the ligand
p-system. The density functional theory (DFT) calculation showed
that the heptafluoro alkyl group more strongly affects the HOMO
level than the LUMO level, resulting in a large band gap and small
a Department of Materials Science and Engineering and the Center for Organic Light
Emitting Diodes, Seoul National University, Seoul 151-742, South Korea.
E-mail: jjkim@snu.ac.kr
b Department of Chemistry and ERI, Gyeongsang National University, Jinju 66-701,
oscillator strength of long wavelength vibrational shoulder peaks
leading to deep-blue emission. Another effect is to provide steric
hindrance around the metal to increase the PLQY.23 In addition,
fluorinated substituents in the aromatic ligands of metal complexes
make them highly volatile, enabling facile, high-yielding purifica-
tion by sublimation, a feature beneficial for the mass production
of displays. A trifluoromethyl-substituted triazole or a picolinate
ancillary ligand is also utilized so that C-linked 2-pyridyl-azoles can
South Korea. E-mail: ykim@gnu.ac.kr
c Department of Chemistry Education, Gyeongsang National University,
Jinju 660-701, South Korea
d School of Materials Science and Engineering and ERI, Gyeongsang National
University, Jinju 66-701, South Korea. E-mail: skwon@gnu.ac.kr
† Electronic supplementary information (ESI) available: Experimental details, CV,
TGA and DFT calculations. See DOI: 10.1039/c4cc07768g
‡ Jung-Bum Kim and Seung-Hoon Han contributed equally.
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and ꢁ6.49 eV, respectively, while the HOMO levels of (TFM)2Ir(pic),
(TFM)2Ir(mpic), and (TFM)2Ir(fptz) with the trifluoromethyl group
were calculated to be ꢁ6.17, ꢁ6.12, and ꢁ6.45 eV, respectively (see
ESI,† Fig. S1, Tables S3–S6). These results suggest that the intro-
duction of strongly electron-withdrawing heptafluoropropyl groups
lowers the HOMO levels. The LUMO orbitals of the complexes were
mostly distributed over one of the main ligands and the ancillary
ligand. The LUMO levels of (HFP)2Ir(pic), (HFP)2Ir(mpic), and
(HFP)2Ir(fptz) with the strongly electron withdrawing heptafluoro-
propyl group were calculated as ꢁ2.35, ꢁ2.25, and ꢁ2.46 eV,
respectively, while those of (TFM)2Ir(pic), (TFM)2Ir(mpic), and
(TFM)2Ir(fptz) with the trifluoromethyl group were calculated to
be ꢁ2.33, ꢁ2.23, and ꢁ2.43 eV, respectively. Compared with the
trifluoromethyl group, the strongly electron-withdrawing hepta-
fluoropropyl group has a greater effect on the HOMO level than
it does on the LUMO level. In addition, compared to the picolinate
ancillary ligand, the strongly electron-withdrawing triazole ancillary
ligand decreased the HOMO level while the electron-donating
Fig. 1 Synthetic schemes of the iridium complexes.
form a stable chelate using two proximal nitrogen atoms. In methylated picolinate ancillary ligand increased the LUMO level.
addition, trifluoromethyl-substituted triazoles are strongly acidic, a As a result, the band gaps increase in the following order:
feature reinforced by the incorporation of the electron-withdrawing (HFP)2Ir(pic) o (HFP)2Ir(mpic) o (HFP)2Ir(fptz). Moreover, the
heptafluoropropyl substituent.24
band gaps are larger than those of (TFM)2Ir(pic), (TFM)2Ir(mpic),
Fig. 1 illustrates the synthesis of the heptafluoropropyl substituted and (TFM)2Ir(fptz), each of which contains the trifluoromethyl
ligand and iridium complexes. The ligand, 2-(2,4-difluoro-3-(perfluoro- group. Recently, we reported that the strong heptafluorocarbonyl
propyl)phenyl)-4-methylpyridine, was obtained by Ulmann coupling substituted iridium complexes, (HF)2Ir(pic) and (HF)2Ir(fptz),
of heptafluoropropyl iodide and 2-(2,4-difluoro-3-iodophenyl)-4- lowered the LUMO levels more than trifluorocarbonyl substituted
methylpyridine. 2-(2,4-Difluorophenyl)-4-methylpyridine was synthe- iridium complexes, (TF)2Ir(pic) and (TF)2Ir(fptz), increasing the
sized using iodine and under lithium diisopropylamide (LDA) base band gaps in the following order: (HF)2Ir(pic) o (TF)2Ir(pic) o
conditions. The cyclometalated iridium m-chloro-bridged dimer was (HF)2Ir(fptz) o (TF)2Ir(fptz).27 These results suggest that the
prepared by the reaction of the ligand and IrCl3ꢀH2O in a mixture of strongly electron-withdrawing heptafluoropropyl group is an effec-
2-ethoxyethanol and water by the well-known Nonoyama reaction. tive substituent for accessing the deepest blue emission.
The new iridium complexes were prepared by reacting the chloro-
Cyclic voltammetry (CV) was used to measure the HOMO levels
bridged dimer with the ancillary ligands in 2-ethoxyethanol with of the iridium complexes (Fig. S2, ESI†). The HOMO levels of
sodium bicarbonate. The new iridium complexes, (HFP)2Ir(pic), (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz) were found to be
(HFP)2Ir(mpic), and (HFP)2Ir(fptz), were characterized by 1H-NMR ꢁ5.94, ꢁ5.92, and ꢁ6.13 eV, respectively. The HOMO levels are
and 19F-NMR spectroscopy and high-resolution mass spectro- in the same order as the DFT calculation. The LUMO levels were
metry (HRMS) (Synthesis details in ESI†).
obtained from the optical band gaps and the HOMO levels.
DFT calculations of the iridium complexes were carried out by The HOMO and LUMO levels and the optical band gaps are
optimizing the geometries of the molecules using the B3LYP/ summarized in Table S1 (ESI†).
6-31G basis set for the ligands, and the relativistic effective core
Thermal properties were evaluated by thermogravimetric analysis
potential of Los Alamos and Double-basis sets (LANL2DZ)25 for Ir (TGA) and differential scanning calorimetry (DSC) under a nitrogen
(as implemented in Gaussian 03 package22). Time-dependent- atmosphere. The 5% weight loss temperatures of the iridium
DFT (TD-DFT)26 calculations were also performed to estimate the complexes were 366 1C for (HFP)2Ir(pic), 368 1C (HFP)2Ir(mpic),
energy levels, orbital electron density distributions and singlet and 355 1C for (HFP)2Ir(fptz). Glass transition was not observed up to
and triplet transition energies on the basis of the structures 250 1C. These results suggest that the new iridium complexes have
optimized in their ground states. The HOMO, HOMO ꢁ 1, LUMO, good thermal stability (Fig. S3, Table S7, ESI†).
and LUMO + 1 for (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz)
Fig. 2(a) shows the ultraviolet-visible (UV-vis) absorption and
are shown in Fig. S1 (ESI†). The HOMO levels of (HFP)2Ir(pic), PL spectra of the iridium complexes in a CHCl3 solution (10ꢁ5
(HFP)2Ir(mpic), and (HFP)2Ir(fptz) were mostly distributed over the M) at room temperature. Each of the complexes, (HFP)2Ir(pic),
phenyl ring of the main ligand, with a large contribution from the d (HFP)2Ir(mpic), and (HFP)2Ir(fptz), displayed strong absorption
atomic orbital and a slight contribution from the ancillary ligand. bands at around 255 nm, which can be attributed to intraligand
For comparison, we calculated the HOMO and LUMO levels of p–p* transitions. The absorption bands at 277 nm for (HFP)2Ir(pic),
(TFM)2Ir(pic), (TFM)2Ir(mpic), and (TFM)2Ir(fptz) with the already 278 nm for (HFP)2Ir(mpic) and 272 nm for (HFP)2Ir(fptz) represent
reported trifluoromethyl group.27 The HOMO levels of (HFP)2Ir(pic), the spin-allowed 1p–p* transition (ligand-centered absorptions).
(HFP)2Ir(mpic), and (HFP)2Ir(fptz) with the strongly electron with- The absorptions shown at 370 nm for (HFP)2Ir(pic), 372 nm for
drawing heptafluoropropyl group were calculated as ꢁ6.22, ꢁ6.17, (HFP)2Ir(mpic), and 356 nm for (HFP)2Ir(fptz) can be assigned
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and (HFP)2Ir(mpic) showed the lowest intensity of the 0–1 vibronic
transitions among the complexes, resulting in the deepest blue
Commission Internationale de l’Eclairage (CIE) coordinates calcu-
lated from the PL spectra of blue emitters. The color properties of
materials are illustrated in Fig. 2(d), which show that our iridium
complexes have the deepest CIE coordinates.
Phosphorescent blue OLEDs were fabricated using the iridium
complexes as dopants within a phosphorescent emitting layer. The
device structure of the phosphorescent blue OLEDs consisted of
the following: glass/indium tin oxide (ITO, 70 nm)/ReO3 (4 wt%)
doped N,N0-dicarbazolyl-3,5-benzene (mCP, 30 nm)/mCP (10 nm)/
mCPPO1:iridium complexes (30 nm, 10 wt%)/diphenylphosphine
oxide-4-(triphenylsilyl)phenyl (TSPO1, 10 nm)/Rb2CO3 (8 wt%)-
doped mCP (30 nm)/Al (100 nm). The hole injection layer (HIL)
was doped with a p-type dopant, ReO3 [rhenium(VI) oxide],28–30
and the electron injection layer (EIL) was doped with an n-type
dopant, Rb2CO3,31–33 to facilitate efficient hole and electron
injection from the electrodes. The mCP and TSPO1 layers func-
tioned as the hole- and electron-transporting layers, respectively.
Compound mCPPO1 was chosen as the host material owing to its
high triplet energy and the balance of electron and hole mobilities
that it offers.19 The LUMO level of mCPPO1 was 0.24 eV lower than
Fig. 2 (a) UV-vis absorption and PL spectra of the iridium complexes in
CHCl3. (b) Transient PL spectra of mCPPO1 films doped with 10 wt%
iridium complexes. (c) PL spectra of mCPPO1 films doped with 10 wt%
Firpic, FCNIr, (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz). (d) (CIE) coor-
dinates calculated from PL spectra of Firpic, FCNIr, (HFP)2Ir(pic),
(HFP)2Ir(mpic), and (HFP)2Ir(fptz).
to the singlet metal to ligand charge-transfer (1MLCT) transitions. that of mCP, and its HOMO level was 0.59 eV higher than that of
The spin-forbidden triplet 3MLCT or 3LC transitions (or both) TSPO1, leading to effective exciton blocking. Moreover, the triplet
appeared as low-energy absorption shoulders at approximately energy levels of all the constituent materials were higher than those
410 nm for the complexes with the trifluoromethyl-substituted of the iridium complexes. Therefore, it was expected that charge
triazole and 420 nm for the complexes with the picolinate and carrier and triplet exciton would be confined within the emitting
the methylpicolinate ancillary ligand, respectively. The absorption material layer (EML).
maximum of the iridium complex with the methyl picolinate
The current density–voltage–luminance ( J–V–L) characteristics of
ancillary ligand was similar to that with the picolinate ancillary the OLEDs are shown in Fig. 3(b). The J–V characteristics of all the
ligand, while the absorption maximum of the iridium complex with three devices were similar because of their similar structure, which
the trifluoromethyl-substituted triazole ancillary ligand was blue varied only by the dopants used in each. In contrast, the devices
shifted by approximately 10 nm. The complexes (HFP)2Ir(pic), based on (HFP)2Ir(pic) and (HFP)2Ir(mpic) showed higher maximum
(HFP)2Ir(mpic), and (HFP)2Ir(fptz) showed intense blue phosphor- luminance than did (HFP)2Ir(fptz). The external quantum efficien-
escence in CHCl3 solution at room temperature, with the emission cies (EQEs) of the OLEDs are shown in Fig. 3(c). The device with
peaks at 451 and 479 nm for (HFP)2Ir(pic), 452 and 480 nm for (HFP)2Ir(mpic) showed the highest EQE among the three devices
(HFP)2Ir(mpic), and 446 and 475 nm for (HFP)2Ir(fptz).
followed by (HFP)2Ir(pic) and (HFP)2Ir(fptz), an observation consis-
The PLQYs of the iridium complexes were measured using tent with the PLQYs of the dopants. The maximum EQEs were
9-(3-(9H-carbazole-9-yl)phenyl)-3-(dibromophenylphosphoryl)-9H- 19.7%, 21.4% and 14.2% for the devices containing (HFP)2Ir(pic),
carbazole (mCPPO1) films doped with 10 wt% of the iridium (HFP)2Ir(mpic), and (HFP)2Ir(fptz), respectively. The power efficien-
complexes. An integrating sphere was used for the measurements. cies of the devices were 22.3, 25.9 and 14.5 lm Wꢁ1 for the devices
The PLQYs of (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz) were containing (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz), respec-
determined to be 81 ꢂ 3%, 90 ꢂ 3%, and 53 ꢂ 3%, respectively. tively. The performances of the three OLEDs are summarized in
The heptafluoropropyl substitutents with a bulky ancillary ligand Table S2 (ESI†). The electroluminescence (EL) spectra of the three
within the iridium complexes enhanced the steric hindrance of devices at 5 mA cmꢁ2 are shown in Fig. 3(d). The emission peaks
molecules to suppress the self-quenching due to reduction of (ELmax) of the OLEDs based on (HFP)2Ir(pic) and (HFP)2Ir(mpic)
bimolecular interactions, increasing the PLQYs.22,23
were located at 452 nm, while that of the device based on
The transient PL of the doped films is shown in Fig. 2(b). The (HFP)2Ir(fptz) was located at 447 nm. The values of the full width
exciton lifetimes of the iridium complexes were 1.46, 1.47, and 1.99 at half maximum for the emission of all three devices were less
for (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz), respectively. than 50 nm. Owing to the spectral purities of the emissions from
The iridium complexes with the picolinate-based ancillary ligand the OLEDs, the CIE coordinates (x,y) of the devices containing
show relatively shorter lifetimes than does the iridium(III) complex (HFP)2Ir(pic), (HFP)2Ir(mpic), and (HFP)2Ir(fptz) at 100 cd mꢁ2
with the trifluoromethyl-substituted triazole ligand.
were (0.147, 0.164), (0.146, 0.165), and (0.152, 0.148), respectively.
Three new deep-blue iridium(III) complexes, (HFP)2Ir(pic), (HFP)2-
Fig. 2(c) shows the normalized PL spectra of mCPPO1 films
doped with 10 wt% Firpic, FCNIr, (HFP)2Ir(pic), (HFP)2Ir(mpic), Ir(mpic), and (HFP)2Ir(fptz), consisting of a heptafluoropropyl sub-
and (HFP)2Ir(fptz). It is interesting to note that (HFP)2Ir(pic) stituent at the 30 position of 20,400 -difluorophenyl-3-methylpyridine as
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