Iridium(iii) complexes with enhanced film amorphism as guests for efficient orange solution-processed single-layer PhOLEDs with low efficiency roll-off

Article abstract of DOI:10.1039/c3dt50834j

By introducing a phenyl substituent into the meta-site of the phenyl segment of the 2-phenylbenzothiazole ligand, two novel orange iridium(iii) complexes, namely, (3Phbt)₂Ir(acac) and (3OMePhbt) ₂Ir(acac), have been synthesized. Compared with their parent compound (bt)₂Ir(acac), both of them possess much enhanced thermostability and film amorphism, making them suitable candidates as guests for high performance solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). However, (4Phbt)₂Ir(acac) bearing para-phenyl possesses worse processability relative to (bt)₂Ir(acac) due to spontaneous crystallization stemming from the intense intermolecular interactions. Single-layer solution-processed PhOLEDs with (3Phbt)₂Ir(acac) and (3OMePhbt)₂Ir(acac) as guests show peak current efficiency of 17.2 cd A^-1 and 15.2 cd A^-1, and maximum brightness of 28 270 cd m^-2 and 27 900 cd m^-2, respectively. Both are greatly improved compared to the devices employing (bt)₂Ir(acac) (10.2 cd A^-1 and 14 350 cd m^-2) and (4Phbt)₂Ir(acac) (5.0 cd A^-1 and 13 790 cd m^-2) as phosphors. Moreover, quite low efficiency roll-off is acquired in these devices at high luminance. The much improved electroluminescence performances of these objective complexes could be mainly attributed to the presence of a rigid phenyl on the appropriate substitution site of the cyclometallate ligand, which leads to improved thermostability with compatible alleviated intermolecular interactions, and consequently enhanced film amorphism.

Full text of DOI:10.1039/c3dt50834j

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Iridium(III) complexes with enhanced film amorphism
as guests for efficient orange solution-processed single-
layer PhOLEDs with low efficiency roll-off†
Cite this: Dalton Trans., 2013, 42, 10559
Jun Dai,‡a Kaifeng Zhou,‡b Ming Li,a Huiqin Sun,c Yunqing Chen,a Shijian Su,*b
Xuemei Pu,a Yan Huang*a and Zhiyun Lu*a,d
By introducing a phenyl substituent into the meta-site of the phenyl segment of the 2-phenylbenzothi-
azole ligand, two novel orange iridium(III) complexes, namely, (3Phbt)2Ir(acac) and (3OMePhbt)2Ir(acac),
have been synthesized. Compared with their parent compound (bt)2Ir(acac), both of them possess much
enhanced thermostability and film amorphism, making them suitable candidates as guests for high per-
formance solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). However,
(4Phbt)2Ir(acac) bearing para-phenyl possesses worse processability relative to (bt)2Ir(acac) due to spon-
taneous crystallization stemming from the intense intermolecular interactions. Single-layer solution-pro-
cessed PhOLEDs with (3Phbt)2Ir(acac) and (3OMePhbt)2Ir(acac) as guests show peak current efficiency
of 17.2 cd A−1 and 15.2 cd A−1, and maximum brightness of 28 270 cd m−2 and 27 900 cd m−2, respecti-
vely. Both are greatly improved compared to the devices employing (bt)2Ir(acac) (10.2 cd A−1 and 14 350
cd m−2) and (4Phbt)2Ir(acac) (5.0 cd A−1 and 13 790 cd m−2) as phosphors. Moreover, quite low
efficiency roll-off is acquired in these devices at high luminance. The much improved electroluminescence
performances of these objective complexes could be mainly attributed to the presence of a rigid phenyl
on the appropriate substitution site of the cyclometallate ligand, which leads to improved thermostability
with compatible alleviated intermolecular interactions, and consequently enhanced film amorphism.
Received 28th March 2013,
Accepted 9th May 2013
DOI: 10.1039/c3dt50834j
www.rsc.org/dalton
enormous research efforts, many high performance PhOLEDs
with various emission colors have been demonstrated success-
Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) fully.2 However, to facilitate charge-injection and transport
employing iridium(III) complexes as emitter have attracted within the device, most of the high performance PhOLEDs
much interest because of their capability for harvesting both have multilayered device structures fabricated via sequential
singlet and triplet excitons efficiently,1 and based on vacuum deposition of small molecules with different func-
tions, which is time-consuming and costly.3 In contrast to
thermal evaporation method, the solution-processing tech-
nique is more promising with respect to the cut down of fabri-
aCollege of Chemistry, Sichuan University, Chengdu, 610064, P. R. China.
E-mail: luzhiyun@scu.edu.cn, huangyan@scu.edu.cn; Fax: +86-28-85410059
cation cost as well as the realization of large-area display and
bState Key Laboratory of Luminescent Materials and Devices (South China University
lighting. In particular, as the intermiscibility of the interfaces
of Technology) and Institute of Polymer Optoelectronic Materials and Devices, South
between the upper and lower layers is considered to be a big
China University of Technology, Guangzhou, 510640, P. R. China.
problem in multilayered solution-processed devices, it would
be more advantageous if both carrier-transporting materials
(i.e. electron and hole transporting ones) and emissive guests
could be integrated into only one active layer.4 Yet despite
many breakthroughs achieved in solution-processed PhOLEDs
E-mail: mssjsu@scut.edu.cn; Fax: +86-20-87110606
cAnalytical and Testing Center, Sichuan University, Chengdu, 610064, P. R. China
dKey Laboratory of Green Chemistry and Technology, Ministry of Education, Sichuan
University, Chengdu, 610064, P. R. China
†Electronic supplementary information (ESI) available: Crystallographic data of
(bt)2Ir(acac), (4Phbt)2Ir(acac) and (3Phbt)2Ir(acac) (CCDC 857633, 857631 and
818631, respectively). The crystal structures, crystal data and structure refine- with iridium complexes as emissive materials,5 high perform-
ance single-layered ones are relatively few.6
As the cyclometallate ligands (C^N ligands) of the phos-
phorescent iridium complexes generally possess conjugated
ment, TGA thermograms and POM images, calculated optimized geometry struc-
tures and parameters of the corresponding compounds, cyclic voltammograms,
1H NMR, 13C NMR and HR-ESI-MS spectra of the objective compounds are given
in Fig. S1–S16 and Tables S1–S3. For ESI and crystallographic data in CIF or
planar geometries, in condensed state, they often show strong
tendency toward developing small crystallites due to the
other electronic format see DOI: 10.1039/c3dt50834j
‡These authors contribute equally to this article.
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relatively strong intermolecular π–π interactions, which would spontaneous crystallization (evidenced by the intense crystal-
lead to adverse self-quenching, triplet–triplet annihilation lite signals in its X-ray diffractogram, see Fig. 1b). It should be
(TTA), and formation of interstices as well, hence resulting in pointed out that opposite to Li’s report8 that they could not get
deteriorated device performances.7 Consequently, to be solu- high-quality neat film of (bt)2Ir(acac) via spin-coating, a con-
tion-processable by itself with good amorphism stability and tinuous and homogeneous film of (bt)2Ir(acac) is easily
less inclination towards crystallization should be an important obtained according to our experimental results (see Fig. 1a). As
merit for emissive iridium complexes,8 since good morphology these observations suggest that the phenyl-modification on
could be maintained during the post-annealing process or C^N ligand impairs rather than enhances the film amorphism
under prolonged electric stress and high working tempera- of the chelates, which seems to contradict with the literature
tures.9 To suppress the intermolecular interactions between reports,21 to elucidate the reason, X-ray crystallographic ana-
π-conjugated segments of phosphors, the widely adopted strat- lysis has been carried out on (4Phbt)2Ir(acac) and (bt)2Ir(acac).
egies include constructing chelates with hyperbranched den- As shown in Fig. 2, although the biphenyl segment of
dritic/starbursted structures,6c,f,10 and nonplanar tetrahedral/ (4Phbt)2Ir(acac) reveals a desirable contorted geometry, there
spiro structures,11 which need elaborate and tedious synthetic exists quite close intermolecular contact between the phenyl
procedures.12 In contrast, incorporating bulky substituents substituents, whereas no significant π–π stacking interaction
with the complex ligands should be a simple yet effective could be found in (bt)2Ir(acac). Therefore, the spontaneous
method to obstruct intermolecular π–π interaction.13 Com- crystallization characteristics of (4Phbt)2Ir(acac) might be
pared to aliphatic sterically congested substituents, the intro- ascribed to the intense interactions between the phenyl substi-
duction of aromatic ones would bring multiple torsion tuents locating at the relatively spacious para-position, hence
barriers that break the molecular planarity, which is favourable moving the phenyls to the more congested meta-site might be
to the suppression of molecular π-packing.7c,14 Accordingly, beneficial for the alleviation of π–π interaction between the
decorating molecules with aromatic rather than aliphatic sub- phenyl substituents. Herein, we report two orange-emitting
stituents should result in compounds with good amorphism,15 iridium complexes bearing meta-phenyl substituents on
thermostability,16
and
carrier-mobility.17
Nevertheless, (bt)2Ir(acac), namely, bis[2-(biphenyl-3-yl)benzothiazole-N,C2′]-
although in the case of electrofluorescent materials, phenyl-
modification has been demonstrated to be an effective method
to improve both film amorphism and thermal stability of the
compounds,14b,18 correlative studies between aryl substituents
and thermal/morphological stability of electrophosphorescent
complexes are mainly focused on fluorenyl,5d,19 carbazyl20 and
diphenylamino6a,c,13a groups, while very few investigations
have been launched into whether the simplest phenyl-modifi-
cation would result in iridium chelates with improved
amorphism.21
Recently, in searching for electrophosphorescent iridium
complexes with alleviated self-quenching characteristics, we
developed a series of iridium complexes bearing a para-phenyl
substituent on the phenyl moiety of 2-phenylbenzothiazole
(bt) ligand, i.e. (4Phbt)2Ir(acac) (molecular structure shown in
Scheme 1) and its derivatives.22 However, although all these
complexes exhibit good PL quantum yields (PLQYs) in solu-
tion, (4Phbt)2Ir(acac) is found to show even worse film-for-
mation capability than (bt)2Ir(acac), and could not be spin-
coated from solution to give a high-quality neat film due to its
Fig. 1 XRD patterns for the neat films of (a) (bt)2Ir(acac), (b) (4Phbt)2Ir(acac),
(c) (3Phbt)2Ir(acac), (d) (3OMePhbt)2Ir(acac) (black line: pristine film, red line:
after being annealed at 140 °C for 30 min in vacuum).
Scheme 1 Molecular structures of the complexes.
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Fig. 2 Crystal packing diagrams of (bt)2Ir(acac) (a), (4Phbt)2Ir(acac) (b) and (3Phbt)2Ir(acac) (c).
iridium(III)(acetylacetonate) [(3Phbt)2Ir(acac)] and bis[2-(4′- at 340 nm. The morphology of the doped thin films was ana-
methoxybiphenyl-3-yl)benzothiazole-N,C2′]iridium(III)(acetyl- lyzed through atomic force microscopy (AFM) in tapping mode
acetonate) [(3OMePhbt)2Ir(acac)]. Both of them possess better under ambient conditions using a MFP 3D Asylum Research
morphological and thermal stability in film state relative to instrument. The concentration of solution samples for PL
(bt)2Ir(acac), and much enhanced amorphism than (4Phbt)2- measurements was 10−5 mol L−1 (in CH2Cl2). Most of the film
Ir(acac) and (bt)2Ir(acac). Single-layer solution-processed samples were obtained by spin-coating from corresponding
PhOLEDs using them as guests show much improved electro- chlorobenzene solution with concentration of 20 mg mL−1 at a
luminescence (EL) performance, i.e., higher luminance and speed of 2000 rpm for 40 s on quartz substrates. X-ray diffrac-
efficiency, and lower efficiency roll-off, indicating that the sub- tion (XRD) data of the film samples were obtained using the
stitution position for phenyl groups on the C^N ligand is an Philips DX-100 sealed-tube X-ray generator (Cu target; I =
essential factor that determines the amorphism of iridium 0.2 nm) with power of 40 kV and 25 mA, and the relatively
complexes.
thick film samples were spin-coated from corresponding
chloroform solution with concentration of 40 mg mL−1 at a
speed of 600 rpm for 40 s on quartz substrates. Cyclic voltam-
metry (CV) measurement was carried out in argon-purged
5 × 10−4 mol L−1 anhydrous CH2Cl2 solution with 0.1 mol L−1
tetrabutylammonium perchlorate as supporting electrolyte at a
scanning rate of 100 mV s−1 using a PARSTAT 2273 electro-
chemical workstation. The CV system was constructed using a
platinum plate, a Ag/AgNO3 (0.1 mol L−1 in acetonitrile) elec-
trode and a platinum wire as the working electrode, quasi-
reference electrode and counter electrode, respectively. Each
measurement was calibrated with a ferrocene/ferrocenium
(Fc/Fc+) redox couple as internal standard.
Experimental section
General information and materials
All the reagents involved in the synthetic procedure were com-
mercially available and used without further purification
unless otherwise stated. All the solvents were of analytical
grade and freshly distilled prior to use. Anhydrous N-methyl-
pyrrolidinone and 2-ethoxyethanol were prepared by freshly
distilling over calcium hydride and diphosphorous pentoxide,
respectively. H NMR and 13C NMR spectra were measured on
1
a Bruker AVANCE-400 spectrometer in CDCl3 or DMSO-d6
using TMS as internal standard. The purity of key intermedi-
ates and target molecules were determined by HPLC (Agilent
1100). High resolution MS spectra were obtained from a Q-TOF
Priemier ESI mass spectrometer (Micromass, Manchester, UK).
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were performed on TGA Q500 and DSC
Q100 instruments under nitrogen atmosphere at a heating rate
of 10 °C min−1, respectively. Optical analyses were made
through polarized optical microscopy (POM) with ECLIPSE LV
100 POL polarizing microscope equipped with a HCS 621V hot
stage and a digital camera, and the heating rate was 1 °C
min−1. The PL emission spectra of both solution and thin-film
samples were recorded on a PerkinElmer LS55 fluorescence
spectrophotometer at 298 K. The UV-vis absorption spectra
were measured on a Hitachi U-4100 UV-vis-NIR scanning
spectrophotometer. The absolute PL quantum yields (PLQYs)
X-Ray diffraction study
The crystallographic data for (bt)2Ir(acac), (3Phbt)2Ir(acac) and
(4Phbt)2Ir(acac) reported here have been deposited in the
Cambridge Structural Database (CCDC 857633, 857631 and
818631). Single crystal X-ray diffraction data of the complexes
were obtained on an Xcalibur E X-ray single crystal diffracto-
meter equipped with graphite monochromator Mo-Kα (λ =
0.71073 Å) radiation. The data collection was executed using
CrysAlisPro program.23 The structures were determined using
direct method and successive Fourier difference syntheses
(SHELXS-97) and refined using full-matrix least squares pro-
cedure on F2 with anisotropic thermal parameters for all non-
hydrogen atoms (SHELXL-97). Packing analysis of the crystal
cells was carried out using Mercury program.24
Computational method
of the doped films were determined with an integrating sphere For complexes (bt)2Ir(acac), (3Phbt)2Ir(acac) and (3OMePhbt)2-
(IS80 from Labsphere) together with a digital photometer Ir(acac), B3LYP25 geometry optimization were performed using
(S370 from UDT) under ambient conditions, under excitation LANL2DZ26 basis set for Ir and 6-31G(d) basis sets for C, H, S,
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N and O atoms. All the geometries were confirmed as station-
ary structures by the presence of only real frequencies at the
same level of theory. Orbital energies were calculated within
the framework of the IEF-PCM Model27 in CH2Cl2 media based
on the optimized geometries. All calculations were carried out
with Gaussian09 software.28
OLED fabrication and measurements
Indium-tin oxide (ITO) glass substrate with sheet resistance of
15–20 Ω per square was thoroughly cleaned in an ultrasonic
bath with acetone, isopropyl alcohol, detergent, de-ionized
water and isopropyl alcohol in sequence, and finally treated in
UV–ozone chamber under oxygen plasma for 20 min. Then a
40 nm layer of poly(3,4-ethylenedioxythiophene):poly(styrene-
sulfonate) (PEDOT:PSS) (CLEVIOS P VP AI 4083) was spin-
coated from water solution onto the ITO substrates under
ambient conditions with a speed of 2000 rpm for 40 s, fol-
lowed by baking at 200 °C for 10 min in nitrogen. Poly(N-vinyl-
carbazole) (PVK), 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-
phenylene (OXD-7) and the Ir(III) complexes were dissolved in
chlorobenzene with concentration of 20 mg mL−1, and the
weight percentage of them in the active layer is 67 wt%, 27 wt%,
and 6 wt%, respectively. Then the emissive layer was spin-
casted from this solution at 2000 rpm for 40 s in a glove box
(VAC Co.) with N2 circulation (with <1 ppm oxygen and water),
followed by thermo-annealing treatment at 140 °C for 30 min
in N2 atmosphere. The thickness of the active layer was
measured to be ∼70 nm. The single-layer devices were com-
pleted by thermoevaporation of metal cathode consisting of
3 nm Ba covered by 120 nm Al in vacuum under a pressure of
3 × 10−4 Pa. The thickness of the polymer layer was determined
by measuring the thickness of the reference film prepared
under very similar conditions with profilometry (Dektak 150
Scheme 2 The synthetic routes of the objective iridium(III) complexes.
Reagents and reaction conditions for the synthetic procedures: (i) CH3OH, con-
centrated H2SO4; (ii) dioxane, arylboronic acid, Pd(PPh3)4, K3PO4·3H2O, 90 °C;
(iii) (a) CH3OH, NaOH, reflux, (b) concentrated HCl; (iv) (a) SOCl2, reflux, (b) o-
aminothiophenol, NMP, 100 °C; (v) IrCl3·3H2O, 2-ethoxyethanol–H2O = 3/1,
110 °C; (vi) acetylacetone, 2-ethoxyethanol, Na2CO3, 110 °C.
removal of solvent, the crude product residue was purified by
column chromatograph over silica using petroleum ether–
dichloromethane (10/1) as eluent to yield the pure product as
pale yellow oil. Yield: 1a: 84.9%; 1b: 90.4%.
General procedure for the synthesis of 231
A solution of 1 (10.0 mmol), 50% aqueous NaOH (2.0 equiv.)
and methanol (25 mL) was refluxed for 2 h, then methanol
was removed. The residue was dissolved in 25 mL water and
acidified with concentrated hydrochloric acid to pH = 1.0. The
white precipitate was collected, dried, and used directly for the
next step. Yield: 2a: 89.4%; 2b: 72.1%.
General procedure for the synthesis of cyclometallate ligands 332
surface profiler). The EL spectra and CIE coordination charac- 10 mmol of carboxylic acid 2 was refluxed with 20 mL thionyl
teristics were recorded on a PR-705 Spectroscan spectrometer. chloride for 1.5 h, then excessive thionyl chloride was removed
The current density–voltage–luminance (J–V–L) characteristics under reduced pressure. The corresponding acid chloride was
of PhOLEDs were measured with a calibrated silicon photo- dissolved in 20 mL N-methylpyrrolidinone (NMP), then added
diode driven by Keithley 236 source.
slowly to a solution of 10 mmol of o-aminothiophenol in
20 mL NMP under inert atmosphere at 0 °C. After that, the
solution was heated to 100 °C and stirred for 3 h. After
Synthesis
The objective molecules and detailed synthetic routes for inter- cooling, the mixture was poured into water followed by neutral-
mediates are shown in Schemes 1 and 2, respectively. 3-Bromo- ization with 7 mol L−1 aqueous ammonia to pH = 8–9. The pre-
benzoic acid, 4-bromoanisole and phenylboronic acid were cipitate was collected and purified by column chromatograph
purchased from Aldrich Co. Intermediates methyl 3-bromo- over silica using petroleum ether–dichloromethane (3/1) as
benzoate29 and 4-methoxyphenylboronic acid30 were syn- eluent to yield the pure product as a white solid.
thesized according to the literature reports.
2-(Biphenyl-3-yl)benzo[d]thiazole (3a). White solid. Yield:
60.2%; m.p.: 117 °C; 1H NMR (CDCl3, 400 MHz), δ (ppm): 8.34
(s, 1H), 8.13 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 8.0 Hz, 1H), 7.94 (d,
General procedure for the synthesis of 131
Methyl 3-bromobenzoate (0.21 g, 1.0 mmol), appropriate J = 8.0 Hz, 1H), 7.74 (d, J = 8.0 Hz, 1H), 7.71 (d, J = 7.6 Hz, 2H),
boronic acid (1.1 eq.), K3PO4·3H2O (0.55 g, 3 eq.) and 7.60 (t, J = 7.6 Hz, 1H), 7.54–7.48 (m, 3H), 7.43–7.39 (m, 2H).
Pd(PPh3)4 (5.8 mg, 0.005 eq.) were added in 20 mL of argon
degassed dioxane in a flask. After stirring at 90 °C for 7 h Yield: 70.4%; m.p.: 123 °C;
2-(4′-Methoxybiphenyl-3-yl)benzo[d]thiazole (3b). White solid.
1
H NMR (CDCl3, 400 MHz),
under argon, the mixture was cooled to room temperature and δ (ppm): 8.29 (s, 1H), 8.11 (d, J = 8.4 Hz, 1H), 8.03 (d, J =
poured into 100 mL water followed by extraction with ethyl 7.6 Hz, 1H), 7.94 (d, J = 8.0 Hz, 1H), 7.69 (d, J = 7.6 Hz, 1H),
acetate (3 × 20 mL). The organic layer was washed with water 7.64 (d, J = 8.4 Hz, 2H), 7.56–7.49 (m, 2H), 7.42 (t, J = 7.6 Hz,
and brine, then dried over anhydrous Na2SO4. After the 1H), 7.03 (d, J = 8.4 Hz, 2H), 3.88 (s, 3H).
10562 | Dalton Trans., 2013, 42, 10559–10571
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General procedure for the synthesis of target iridium(III)
complexes
data summarized in Table 2). All these complexes exhibit high
decomposition temperatures (Td) of >300 °C (at 5 wt% loss).
The phenylation on bt ligand is demonstrated to be favourable
for the enhancement of the chelates’ thermostability, since
(4Phbt)2Ir(acac), (3Phbt)2Ir(acac) and (3OMePhbt)2Ir(acac) all
possess higher Tds (8–29 °C) relative to (bt)2Ir(acac). According
to DSC measurements, all these complexes show decompo-
sition prior to melting transition. As shown in Fig. 3, no dis-
tinct Tg could be identified for (4Phbt)2Ir(acac); only an
inconspicuous phase transition at 141 °C could be observed
for (bt)2Ir(acac); while (3Phbt)2Ir(acac) and (3OMePhbt)2-
Ir(acac) are found to form stable amorphous glasses with well-
defined Tg values of 209 °C and 185 °C, respectively. Further
investigations on phase transition characteristics of these com-
pounds have been conducted through polarizing optical
microscopy (shown in ESI†). Consistent with the DSC measure-
ments, the polycrystalline (4Phbt)2Ir(acac) sample obtained via
recrystallization is found to show good morphological stability
in its crystallite state, since no evident phase transition could
be observed in a wide temperature region of 10–260 °C. Never-
theless, although the Tg of (bt)2Ir(acac) is not quite distinct in
DSC thermogram, an anisotropic–isotropic transition is obser-
vable at a rather low temperature of 135 °C. As far as (3Phbt)2-
Ir(acac) and (3OMePhbt)2Ir(acac) are concerned, obvious ani-
sotropic–isotropic transitions are distinguishable when they
are heated to 210 °C and 182 °C, respectively, which might be
assigned to the formation of amorphous glass. Therefore, the
phenylation on either meta- or para-sites of phenyl moiety of
bt ligand is favourable for the morphological stability of the
complexes, yet the meta-substitution would result in better
amorphism with much enhanced Tg, which is highly desirable
for applications in OLEDs.34 The slightly lowered Tg of
(3OMePhbt)2Ir(acac) should be ascribed to the presence of
alkoxy substituents.35
Dichloro-bridged iridium(III) complexes were prepared by
refluxing IrCl3·3H2O (1 mmol) with the ligands 3 (2.4 mmol)
in a mixture of 2-ethoxyethanol and water (v/v = 3/1) under
argon for 24 h.33 The precipitate was filtered and washed with
water, methanol and hexane in sequence, and dried in vacuo
to afford intermediate 4. Then it (0.1 mmol) was mixed with
acetylacetone (0.3 mmol), anhydrous sodium carbonate
(1 mmol) and 10 mL 2-ethoxyethanol, and the reaction
mixture was refluxed under argon for 12 h. After being cooled
down, the precipitates were collected and purified by flash
chromatography over silica using petroleum ether and dichloro-
methane as mobile phase, followed by recrystallization more
than three times to render satisfied purity, then dried for 24 h
at 100 °C under vacuum of 1.5 kPa.
Bis[2-(biphenyl-3-yl)benzothiazole-N,C2′]iridium(III)(acetyl-
acetonate) (3Phbt)2Ir(acac). Red solid. Recrystallized from
benzene/methanol. Yield: 46.2%; purity: 99.56%. 1H NMR
(CDCl3, 400 MHz), δ (ppm): 8.15–8.13 (m, 2H), 7.95–7.93 (m,
2H), 7.86 (s, 2H), 7.52 (d, J = 8.0 Hz, 4H), 7.48–7.45 (m, 4H),
7.38 (t, J = 8.0 Hz, 4H), 7.27–7.24 (m, 2H), 6.92 (d, J = 8.0 Hz,
2H), 6.54 (d, J = 8.0 Hz, 2H), 5.17 (s, 1H), 1.80 (s, 6H). 13C NMR
(CDCl3, 100 MHz), δ (ppm): 185.84, 180.28, 150.86, 147.51,
142.49, 141.27, 135.36, 134.44, 131.37, 129.23, 128.64, 127.43,
126.55, 126.47, 125.26, 124.24, 122.37, 120.21, 101.74, 28.43.
ESI-MS: m/z 887.1312 (M + Na+); Calcd. for Mw + Na+: 887.1354.
Bis[2-(4′-methoxybiphenyl-3-yl)benzothiazole-N,C2′]iridium(III)
(acetylacetonate) (3OMePhbt)2Ir(acac). Red solid. Recrystallized
from dichloromethane/methanol. Yield: 40.2%; purity:
1
99.61%. H NMR (DMSO-d6, 400 MHz), δ (ppm): 8.31 (d, J =
8.0 Hz, 2H), 7.99 (d, J = 8.0 Hz, 2H), 7.94 (s, 2H), 7.61–7.56 (m,
4H), 7.55 (d, J = 8.0 Hz, 4H), 6.97 (d, J = 8.0 Hz, 4H), 6.91 (d, J =
8.0 Hz, 2H), 6.31 (d, J = 8.0 Hz, 2H), 5.23 (s, 1H), 3.76 (s, 6H),
1.74 (s, 6H). 13C NMR (CDCl3, 100 MHz), δ (ppm): 185.81,
180.33, 150.50, 150.90, 146.63, 142.41, 135.28, 134.12, 133.94,
131.38, 129.00, 127.56, 127.40, 125.21, 123.88, 122.35, 120.21,
114.07, 101.72, 55.27, 28.44. ESI-MS: m/z 947.1509 (M + Na+);
Calcd. for Mw + Na+: 947.1565.
The X-ray diffractograms (XRD) of the four complexes spin-
coated film samples are illustrated in Fig. 1. (4Phbt)2Ir(acac)
shows very poor film-forming capability, since sharp and
intense diffraction signals are discernable in its XRD diffracto-
gram. However, for the other three compounds, smooth and
Results and discussion
Synthesis
The syntheses of complexes (bt)2Ir(acac) and (4Phbt)2Ir(acac)
have been reported by us previously,22a and the two objective
complexes are prepared following a similar procedure with the
products in satisfactory yields and purity. Their molecular
1
structures have been characterized by H NMR, 13C NMR, and
HR-ESI-MS spectrometry, and (3Phbt)2Ir(acac) is further
characterized by single crystal X-ray diffraction method.
Thermal, XRD and X-ray crystallographic characterizations
The thermal properties of the four complexes have been inves-
tigated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) (TGA diagrams shown in ESI,† Fig. 3 DSC traces of the four complexes.
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continuous amorphous films could be obtained. To evaluate
the morphological stability of the phosphors, thermo-anneal-
ing treatment has been conducted on the films at 140 °C for
30 min in vacuum. As depicted in Fig. 1, newly emerged diffr-
action signals could be found in (bt)2Ir(acac) after annealing,
indicative of the thermoinduced amorphous–crystalline phase
transition; however, for (3Phbt)2Ir(acac) and (3OMePhbt)2-
Ir(acac), their XRD traces of pre- and post-thermotreated films
are very similar. Therefore, the alteration of the phenyl substi-
tuent from para- to meta-site of bt ligand should be quite
effective for the enhancement of film amorphism.
To elucidate the correlation between phenylation position
and morphological property, single crystal X-ray diffraction
analysis has been conducted on (bt)2Ir(acac), (3Phbt)2Ir(acac)
and (4Phbt)2Ir(acac). The crystallographic refinement para-
meters, selected bond distances as well as bond angles of the
three compounds are summarized in Tables S1 and S2,† and
their crystal structures are shown in Fig. S4 (ESI†). The crystal
packing diagrams of the three complexes are depicted in
Fig. 2. The biphenyl moieties of both (4Phbt)2Ir(acac) and
(3Phbt)2Ir(acac) are found to show distorted geometries, with
relative large dihedral angles of ∼30° or ∼50°. In (4Phbt)2-
Ir(acac), there should exist relatively intense intermolecular
interaction, since the shortest face-to-face ring contact and/or
edge-to-face ring contact between the two phenyl substituents
are ∼1.66 Å and ∼3.72 Å, respectively (Fig. 2b). In sharp con-
trast, no intense intermolecular interaction is discernable in
(3Phbt)2Ir(acac), since the shortest edge-to-face distance
between π-systems is calculated to be as large as 3.78 Å.
Although the single crystal sample of (3OMePhbt)2Ir(acac) has
not been obtained, similar conformation can be safely
assumed in view of its structural similarity to (3Phbt)2Ir(acac).
Therefore the better amorphism of (3Phbt)2Ir(acac) and
(3OMePhbt)2Ir(acac) relative to (4Phbt)2Ir(acac) may be attri-
buted to their noncoplanar geometries with compatible
alleviated intermolecular interactions.
Fig. 4 Normalized PL spectra of the iridium complexes in 10−5 mol L−1 CH2Cl2
solution and solid films at 298 K (under excitation at 400 nm).
In accordance with the UV-vis absorption characteristics,
upon irradiation at 400 nm at room temperature, the λPLmax of
these phenyl-modified complexes in dilute solution are red-
shifted relative to (bt)2Ir(acac), whose λPLmax is 555 nm.
(3Phbt)2Ir(acac) and (3OMePhbt)2Ir(acac) have their λPLmax
located at 561 nm and 568 nm, respectively, while (4Phbt)2-
Ir(acac) possesses a λPLmax of 578 nm. In neat films, all these
complexes show broader and more red-shifted PL emission
spectra than those acquired from solution (Fig. 4). It is note-
worthy that although in solution the PL emission of (3Phbt)2-
Ir(acac) is more red-shifted relative to (bt)2Ir(acac), its λPLmax is
shorter than that of (bt)2Ir(acac) (573 nm vs. 579 nm) in neat
films. In fact, for (bt)2Ir(acac) and (4Phbt)2Ir(acac), the differ-
ence between λPLmax in film and solution is as large as 24 nm
and 20 nm, respectively, suggesting there exists severe aggre-
gation in condensed states of these two compounds,37 while
those for the two objective compounds are only 12–14 nm,
indicative of the more alleviated intermolecular interactions in
the two objective complexes.
As the PL emission spectrum of PVK has an effective
overlap with the MLCT transition absorption bands of these
chelates (shown in Fig. S6†), PVK might be a suitable host for
these complexes. Consequently, PL emission spectra and
quantum yields (QYs) of the complex/PVK films with different
blending ratios have been measured (spectra shown in
Fig. S7,† data summarized in Table 1). The presence of
residual emission of PVK at even 6 wt% doping level, which
Photophysical properties
The optical absorption and photoluminescence (PL) spectra of
the four complexes in 10−5 mol L−1 CH2Cl2 solution are dis-
played in Fig. S6 (in ESI†) and Fig. 4, respectively, and data are
summarized in Table 1. Analogous to many Ir(III) complexes,36
two absorption bands arising from spin-allowed ligand-based
1(π–π*) transitions (below 380 nm) and metal-to-ligand charge-
transfer transitions (1MLCT and 3MLCT) (380–550 nm) are dis-
tinguishable in all these chelates. With respect to the MLCT
transition bands, the λab of (3Phbt)2Ir(acac) is slightly batho-
chromic-shifted relative to (bt)2Ir(acac), indicative of its
enlarged conjugation systems; and (3OMePhbt)2Ir(acac)
bearing electron-donating methoxy groups displays a more
red-shifted absorption band than (3Phbt)2Ir(acac). Neverthe-
less, (4Phbt)2Ir(acac) shows the most bathochromic-shifted λab
in both the two major bands, suggesting that the para-pheny-
lation is more effective with regard to the expansion of conju-
gation length.
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Table 1 Photophysical data of the complexes studied here and the PLQYs of complex/PVK blended thin films at different doping levels
PLQYd (%)
PLQYe (%)
Compound
λaba (nm) (log ε)
λPLmaxb (nm)
555
λPLmaxc (nm)
579
1 wt%
39.7
2 wt%
38.8
6 wt%
30.6
10 wt%
6 wt%
16.6
(bt)2Ir(acac)
(4Phbt)2Ir(acac)
313(4.4), 326(4.5), 356(3.9), 400(3.9),
440(3.7), 480(3.6)
311(4.7), 331(4.8), 341(4.8), 417(4.0),
464(3.9), 501(3.8)
16.4
11.8
578
598
25.3
25.1
19.0
15.2
(3Phbt)2Ir(acac)
(3OMePhbt)2Ir(acac)
304(4.8), 406(4.8), 456(3.8), 486(3.7)
305(4.9), 408(3.8), 462(3.7), 494(3.7)
561
568
573
582
38.8
33.3
39.1
34.8
30.4
27.5
16.0
15.2
24.8
22.9
a UV-vis absorbance is determined in 10−5 mol L−1 CH2Cl2 solution at 298 K. b PL emission spectra are acquired in 10−5 mol L−1 CH2Cl2 solution
at 298 K, under excitation of 400 nm. c Thin films are spin-coated from 20 mg mL−1 chlorobenzene solution, PL spectra are obtained under
irradiation of 400 nm. d Absolute PLQYs of the doped films are measured under ambient conditions (under irradiation of 340 nm). e Absolute
PLQYs of the post-annealed (140 °C for 30 min in vacuum) 6 wt% doped films are measured under ambient conditions (under irradiation of
340 nm).
could not be suppressed by addition of OXD-7 (27 wt%) As no reduction wave could be detected due to the limited
(Fig. S8†) suggests incomplete energy transfer between PVK range available in CH2Cl2, their LUMO energy levels of −3.19
and these guests. As far as PLQY is concerned, the PLQYs of to −3.07 eV are deduced from the HOMO energies and corres-
(3Phbt)2Ir(acac) and (bt)2Ir(acac)-based films are comparable ponding optical bandgaps.38 Compared to (bt)2Ir(acac), the
at identical doping levels, while (4Phbt)2Ir(acac)-based films meta-phenylation on bt ligand leads to slightly elevated HOMO
exhibit much lower PLQYs compared with the other three energy levels, and the addition of para-methoxy groups results
complex-based ones. Despite the fact that peak PLQYs are in further lowered oxidative potential; whereas these substi-
obtained at a doping level of 1 wt% for all these four guests, tuents have little effect on the LUMOs of the chelates. It
fairly good efficiencies still could be achieved at blending level should be pointed out that these observations differ signifi-
of 6 wt%. Yet when the concentration is increased to 10 wt%, cantly from those of para-phenyl modified (4Phbt)2Ir(acac),
the PLQYs drop drastically, which should be ascribed to the whose LUMO level is found to be lowered while HOMO level is
self-quenching of the iridium phosphors.
unchanged.22a
Electrochemical data
Theoretical calculation
The electrochemical properties of the four chelates have been To gain insights into the substituent effects of phenyl on
investigated by cyclic voltammetry (CV) in degassed 5 × 10−4 photophysical and electrochemical properties of the com-
mol L−1 CH2Cl2 solution with Fc/Fc+ redox couple as internal plexes, DFT calculations were performed using Gaussian09
standard, and the voltammograms are shown in the ESI,† data software for these complexes, and the calculation results of
are summarized in Table 2. During the anodic scan, each of (4Phbt)2Ir(acac) could be found in our previous work.22a The
the chelates shows a quasireversible one-electron oxidation optimized structures for (bt)2Ir(acac), (3Phbt)2Ir(acac) and
wave which is generally assigned to the IrIV/IrIII oxidation.38 (3OMePhbt)2Ir(acac) along with the numbering of important
The Eoonx set are determined to be 0.55 V, 0.52 V, 0.51 V, and 0.42 atoms, main geometrical parameters calculated, and some
V relative to Fc/Fc+ for (bt)2Ir(acac), (4Phbt)2Ir(acac), (3Phbt)2- experimental data derived from X-ray crystallography can be
Ir(acac) and (3OMePhbt)2Ir(acac) in sequence, hence by com- found in the ESI (shown in Fig. S5 and Table S3†). The close
parison with the Fc/Fc+ redox couple whose energy level is 4.80 similarity between calculated and experimental values con-
eV in vacuum, their HOMO energy levels are roughly calculated firms the reliability of our computation results. Consistent
to be −5.35 eV, −5.32 eV, −5.31 eV, and −5.22 eV, respectively. with the experimental observations for (3Phbt)2Ir(acac), the
Table 2 Electrochemical and thermal data of the iridium complexes
Compound
Eoonx seta,b (V)
Egc (eV)
HOMOd (eV)
LUMOe (eV)
HOMOf (eV)
LUMOf (eV)
Tgg (°C)
Tdh (°C)
(bt)2Ir(acac)
0.55
0.52
0.51
0.42
2.25
2.13
2.21
2.15
−5.35
−5.32
−5.31
−5.22
−3.10
−3.19
−3.10
−3.07
−5.28
−5.29
−5.20
−5.07
−1.80
−1.91
−1.85
−1.82
141
—
209
185
326
355
348
334
(4Phbt)2Ir(acac)
(3Phbt)2Ir(acac)
(3OMePhbt)2Ir(acac)
a Oxidation potential values are measured in 5 × 10−4 mol L−1 CH2Cl2 solutions. b Potential values are reported versus Fc/Fc+. c Eg are estimated
onset
from the onset wavelength of the optical absorption bands. d HOMO energies are deduced from the equation HOMO = 4.8 + E
.
e LUMO
ox
energies are obtained from the equation LUMO = HOMO + Eg. f Obtained from B3LYP calculations within the framework of the IEF-PCM model
in CH2Cl2 media. g Obtained from DSC measurements. h Temperatures with 5 wt% loss.
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biphenyl moieties in its optimized structures derived from cal- (70 nm)/Ba (3 nm)/Al (120 nm), where ITO acts as anode,
culation show twisted geometry with dihedral angles of ca. PEDOT:PSS as hole-injecting layer, PVK as hole-transporting
36.5°.
host, OXD-7 as electron-transporting material, and Ba/Al as the
The electronic densities of HOMOs and LUMOs for the two cathode. A doping level of 6 wt% for the guest compound is
objective complexes are depicted in Fig. 5, and the calculated used according to the PLQY measurements, so that a compro-
HOMO and LUMO energy level data are summarized in mise between adequate luminescence site for carrier recombi-
Table 2. Although the calculated LUMOs are significantly nation and minor concentration quenching could be reached.
higher, and the HOMOs are slightly higher than the experi- The reference device I using (bt)2Ir(acac) as guest has been fab-
mental data, the variation trends for both frontier orbital ener- ricated for comparison, and devices II, III and IV represent
gies and energy gaps of the complexes show perfect PhOLEDs with (4Phbt)2Ir(acac), (3Phbt)2Ir(acac) and
reproduction between the computation and experiment values. (3OMePhbt)2Ir(acac) as phosphorescent guests, respectively.
In accordance with the electrochemical measurements, The relative energy level alignment of the devices and the
the (methoxy)phenyl substituents are found to contribute to molecular structures of the chemicals used are shown in
the HOMOs rather than LUMOs of the complexes. Generally, Fig. 6.
the HOMO energy level of (bt)2Ir(acac) is dominated by Ir-d
The EL spectra of the devices are shown in Fig. 7. Devices I
orbitals and π-orbitals of phenyl moieties,39 thus the elevated and II exhibit orange and orange-red emission with λELmax of
HOMO of the target compounds should be attributed to the 564 nm and 584 nm, and CIE coordinates of (0.51, 0.49) and
effectively elongated conjugation system arising from the (0.57, 0.42), respectively, while the devices III and IV emit
(methoxy)phenyl substituents in the para-site of C–Ir bonds. orange light with λELmax of 570 and 578 nm, and CIE coordi-
On the contrary, as the LUMO of (bt)2Ir(acac) predominantly nates of (0.52, 0.47) and (0.55, 0.45), respectively. In each case,
localizes on the π* orbitals of benzothiazole segments,39 the the EL spectrum basically resembles the corresponding PL
phenyl substituent should contribute little to the LUMO due spectrum of the blend film except for the relative intensity of
to the poor electron-coupling at the meta-site of bt ligand. the shoulder peak (Fig. S10†), implying that the EL emission
Meanwhile in the case of (4Phbt)2Ir(acac), the para-phenyl sub- originates from radiative decay of the phosphors. Consistent
stituent is found to contribute to LUMO rather than HOMO.22a with the PL measurements, the λELmax of devices II, III and IV
are red-shifted compared to device I. As compared with PVK
and OXD-7, all the dopants possess higher lying HOMOs and
Electroluminescence properties
lower lying LUMOs, taking into account that the doping con-
Based on the CV and photophysical experimental results, PVK
centration is 6 wt%, the holes and electrons should be mainly
is selected as host material for these iridium complexes to
injected into PVK and OXD-7, respectively, yet some of the
realize exothermic energy transfer, and solution-processed
single-layered PhOLEDs using these chelates as dopants have
been fabricated. The configuration of the devices is: ITO/
PEDOT:PSS (40 nm)/PVK:OXD-7:Ir complex (67 : 27 : 6 in wt%)
charge carriers should also be injected directly into the phos-
phors, i.e., the phosphors may act as the carrier trapping sites
to form excitons. The absence of emission from either PVK or
Fig. 5 Isodensity plots of the HOMOs (top) and LUMOs (bottom) for
(3Phbt)2Ir(acac) and (3OMePhbt)2Ir(acac).
Fig. 6 Device configuration and energy band diagram of the PhOLEDs, and
the molecular structures of the compounds used.
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Fig. 8 Current density–voltage–luminance characteristics of the devices.
Fig. 7 EL spectra of the PhOLEDs at current density of 13.3 mA cm−2
.
and 5.0 cd A−1, respectively). It is noteworthy that ηLmax values
of devices III and IV are obtained under luminance of >2500
cd m−2, and furthermore, satisfied efficiency could be achieved
even at higher brightness (Fig. 9). For example, ηL of device III
remains as high as 16.3 cd A−1 at 5000 cd m−2 with roll-off of
only 5.2%, and 15.4 cd A−1 at 10 000 cd m−2 with roll-off of
only 10.5%; moreover, high ηL of 12.8 and 10.7 cd A−1 could
even be achieved at 20 000 and 25 000 cd m−2, respectively. In
the case of device IV, ηL is found to be as high as 15.1 cd A−1 at
5000 cd m−2 with negligible efficiency roll-off, and 14.2 cd A−1
at 10 000 cd m−2 with 6.6% roll-off; while high ηLs of 11.9 and
10.0 cd A−1 could be achieved at 20 000 and 25 000 cd m−2. In
comparison, device I shows analogous low ηL roll-off at J <
200 mA cm−2 with devices III and IV, yet more severe roll-off is
observed at higher driving current density. For instance, at
current density of 250 and 300 mA cm−2, device I exhibits ηL
roll-off of 43.1% and 57.8%, whereas that of device III is 39.5%
and 45.9%, and that of device IV is 35.5% and 40.8%, respecti-
vely. However, device II with (4Phbt)2Ir(acac) as phosphor
shows a more suppressed ηL roll-off than the other three
devices, which exhibits ηL roll-off of 14.0% and 20.0% at
current density of 250 and 300 mA cm−2, respectively. The
lower efficiencies roll-off of device II might be attributed to
the much better thermal stability of (4Phbt)2Ir(acac). It
should be pointed out that in despite of their inferior ηLmax
OXD-7 in these devices suggests effective energy transfer and/
or charge carrier trapping on the guests during the EL emis-
sion procedure.
The corresponding current density–voltage–luminance (J–
V–L) characteristics of the devices are shown in Fig. 8, and
some representative data are summarized in Table 3. The turn-
on voltage of devices I, II, III and IV is 4.0, 4.2, 3.7 and 4.4 V in
sequence. Compared with device I, devices II and III show
higher current density at identical driven voltage, which might
be attributed to the more efficient carrier trapping arising
from the relatively lower lying LUMO energy level of (4Phbt)2-
Ir(acac), and higher lying HOMO energy level of (3Phbt)2-
Ir(acac), whereas the slightly lower current density of
(3OMePhbt)2Ir(acac)-based device IV might be ascribed to the
presence of electrically insulating methoxy substituents that
may induce reduced carrier mobility.22b,35,40 Nevertheless,
although the current density of device II is much higher than
all the other three devices at voltages of >7 V, its luminance is
just comparable to device I and IV in 7–11 V, but inferior to
that of device III, i.e., device II exhibits much lower current
efficiency relative to the other devices, indicative of the more
severe self-quenching of the (4Phbt)2Ir(acac) phosphor, and/or
worse carrier balance in device II. Devices III and IV exhibit
orange emission with maximum brightness of 28 270 and
27 900 cd m−2, respectively, almost twice as bright as that of
device I (14 350 cd m−2) and device II (13 970 cd m−2). As
shown in Fig. 9, devices III and IV display maximum current
efficiencies (ηLmax) of 17.2 and 15.2 cd A−1 respectively, both
are drastically enhanced compared to the devices I and II (10.2
relative
to
those
high
performance
single-layered
PhOLEDs,6a,d,e,19c devices III and IV display rather low
efficiency roll-off, and high ηL of ∼12 cd A−1 could be even
achieved at 20 000 cd m−2, which is comparable to the best
PhOLEDs reported so far.5d,f,6a,6d–f,19c
Table 3 EL characteristics of the devices
Device
Vturn-on (V)
Lmaxa (cd m−2) (V)
ηpmaxb (lm W−1
)
ηLc (cd A−1
)
ηLd (cd A−1
)
CIE (x, y)
I
4.0
4.2
3.7
4.4
14 350 (12.3)
13 790 (11.5)
28 270 (11.8)
27 900 (13.3)
3.8 (960, 8.0)
1.9 (1310, 8.0)
7.6 (640, 6.8)
5.3 (2030, 8.8)
10.2, 10.0, 8.6, —, —
5.0, 4.9, 4.4, —, —
17.2, 16.3, 15.4, 12.8, 10.7
15.2, 15.1, 14.2, 11.9, 10.0
10.0, 8.9, 7.8, 6.9, 5.8, 4.3
4.9, 4.8, 4.7, 4.5, 4.2, 4.0
15.8, 14.1, 12.9, 11.7, 10.4, 9.3
14.8, 13.1, 12.2, 11.2, 9.8, 9.0
(0.51, 0.49)
(0.57, 0.42)
(0.52, 0.47)
(0.55, 0.45)
II
III
IV
a Data in parentheses are the corresponding driven voltages (V). b Data in parentheses are the corresponding EL brightness (cd m−2) and driven
voltages (V). c Order of measured value: maximum, then values at 5000, 10 000, 20 000 and 25 000 cd m−2 in sequence. d Order of measured value:
values at 50, 100, 150, 200, 250 and 300 mA cm−2 in sequence.
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treatment would influence the morphological and photo-
physical characteristics of the thin films. Using AFM in
tapping mode, the morphological stability of these pre- and
post-annealed 6 wt% doped films have been investigated
(Fig. 10, with phase images acquired concurrently with the
height mode). Clearly evidenced by these images, the as-pre-
pared films show smooth and homogenous surfaces with the
root-mean-square (RMS) of 0.23–0.31 nm, suggesting that
these complexes could be dispersed evenly in PVK. After
annealing, the surface roughness of (bt)2Ir(acac)-doped film
increases significantly with RMS varied from 0.27 to 0.60 nm,
and the newly emerged small crystalline domains are found to
have an average height reaching 4.6 nm (Fig. 10b), indicative
of the aggregation of (bt)2Ir(acac) molecules upon annealing.
In the case of (4Phbt)2Ir(acac)-based film, small crystalline
domains could even be found in the as-prepared films, and
the average height is 4.7 nm, while after thermal-annealing,
the average height increases to 8.1–9.3 nm, and the RMS of the
Fig. 9 Current efficiency–luminance and current efficiency–current density
characteristics (inset) of the devices.
Although the PLQYs of as-prepared (4Phbt)2Ir(acac)- blended films varies just slightly from 0.31 nm to 0.40 nm. In
blended films are the lowest among all these four compounds, contrast, the as-prepared (3Phbt)2Ir(acac)- and (3OMePhbt)2-
the 6 wt% doped film just displays slightly decreased PLQY Ir(acac)-blended films are smooth and homogenous, while
(20% decrease) after thermo-annealing (140 °C, 30 min), upon thermal treatment, their RMS increase only slightly (RMS
indicative of its better thermal stability. In contrast, although varied from 0.25 nm to 0.34 nm, and 0.23 nm to 0.29 nm,
the as-prepared 6 wt% (bt)2Ir(acac) blended film (in PVK) respectively). Therefore, (3Phbt)2Ir(acac) and (3OMePhbt)2-
shows higher PL efficiency, the PLQY of the sample is found to Ir(acac) bearing meta-phenyl substitutes are confirmed to show
drop dramatically upon annealing. Note that thermal treat- enhanced film morphological stability relative to (bt)2Ir(acac)
ment is a general fabrication procedure for OLEDs to ensure and (4Phbt)2Ir(acac), which is beneficial for the alleviation of
complete removal of solvents, and to improve the efficiency self-quenching and TTA under higher working temperatures or
and reduce the driving voltage as well.41 As depicted in upon thermal treatment. Taking into account that the PLQYs
Table 1, after thermal annealing, (bt)2Ir(acac)-doped film exhi- of the active layers in devices I and II are much lower than
bits steeply 45% decreased PLQY, while (3Phbt)2Ir(acac) and those of devices III and IV, the relatively higher luminance,
(3OMePhbt)2Ir(acac)-doped films show 16% and 18% decrease efficiency and reduced efficiency roll-off at high current
on PLQY, respectively. It is suspected that the thermal density for the PhOLEDs III and IV should mainly be
Fig. 10 Typical AFM images of height mode (upper row), phase mode (second row), cross section view of height modes (third row), and phase modes (bottom
row) for Ir complexes/PVK blended films (6 wt%) before and after thermo-annealing (140 °C for 30 min in vacuum). (bt)2Ir(acac)-doped samples: (a) as-prepared,
(b) post-annealed; (4Phbt)2Ir(acac)-doped samples: (c) as-prepared, (d) post-annealed; (3Phbt)2Ir(acac)-doped samples: (e) as-prepared, (f ) post-annealed;
(3OMePhbt)2Ir(acac)-doped samples: (g) as-prepared, (h) post-annealed. All scan scales for the images are 5.00 × 5.00 μm.
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It should be pointed out that compared with state-of-art
phosphorescent Ir complexes, PLQYs of the active layers based
on these complexes are not high, which may eventually limit
device efficiency. Nevertheless, we believe that their PLQYs can
be improved by utilizing other host materials instead of PVK,
or introducing a blue or green light emitting sensitizer. More-
over, the device structure, doping level, and layer thickness
used here have not been optimized for either low driven
voltage or high efficiency, thus much improved EL perform-
ance should be expected after further optimization has been
carried out on these issues.
Conclusions
By phenyl-modification of the phenyl moiety of bt ligand, the
resulting complexes are found to exhibit enhanced thermal
and morphological stability relative to their parent complex
(bt)2Ir(acac). Nevertheless, the compound bearing para-phenyl
shows a strong tendency to crystallize due to the presence of
intense intermolecular interaction; whereas those with meta-
substituted one, i.e. (3Phbt)2Ir(acac) and (3OMePhbt)2Ir(acac),
possess drastically enhanced processability with good film
amorphism. Consequently, (3Phbt)2Ir(acac) and (3OMePhbt)2-
Ir(acac) show much improved EL performances relative to
(bt)2Ir(acac) and (4Phbt)2Ir(acac). Single-layered solution-pro-
cessed PhOLEDs based on them display Lmax of 28 270 cd m−2
and ηLmax of 17.2 cd A−1, with quite low efficiency roll-off at
high luminance of even 20 000 cd m−2. All these preliminary
results reveal that the phenylation on ligands at appropriate
site is a simple yet effective means for the enhancement of
film amorphism, and the two resulting complexes are very
promising phosphorescent guests for application in large-area
PhOLEDs with low costs.
Acknowledgements
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The authors acknowledge the financial support for this work
by the National Natural Science Foundation of China (Projects
21072139, 21190031, 51073057, 91233116 and U1230121), and
the Open Fund of the State Key Laboratory of Luminescent
Materials and Devices (South China University of Technology).
We are grateful to the analytical and testing center of Sichuan
University for providing NMR and X-ray single crystal analyses
of the intermediates and objective compounds.
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