Important role of ancillary ligand in the emission behaviours of blue-emitting heteroleptic Ir(III) complexes

Article abstract of DOI:10.1039/c7tc00844a

A series of heteroleptic Ir(iii) complexes composed of 2-(2,4-difluoro-3-(trifluoromethyl)phenyl)-4-methylpyridine (dfCF₃) as the main ligand and such ancillary ligands as acetylacetonate [Ir(dfCF₃)₂(acac)] (acac), picolin

Full text of DOI:10.1039/c7tc00844a

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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Important Role of Ancillary Ligand in the Emission Behaviours of
Blue-emitting Heteroleptic Ir(III) Complexes
Yang-Jin Cho,a So-Yoen Kim,a Jin-Hyoung Kim,a Douglas W. Crandell,b Mu-Hyun Baik,*,b,c Jiwon
Lee,d Chul Hoon Kim,a Ho-Jin Son,*,a Won-Sik Han,*,d and Sang Ook Kang*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A series of heteroleptic Ir(III) complexes composed of 2-(2,4-difluoro-3-(trifluoromethyl)phenyl)-4-methylpyridine (dfCF3)
as the main ligand and such ancillary ligands as acetylacetonate [Ir(dfCF3)2(acac)] (acac), picolinate [Ir(dfCF3)2(pic)] (pic),
and tetrakis-pyrazolyl borate [Ir(dfCF3)2(bor)] (bor) were prepared, and their emission behaviors depending on the
ancillary ligands were systematically investigated. It was found that the Huang–Rhys factors (SMs) of the emission decrease
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1
in the order bor (0.97) > acac (0.87) > pic (0.76), while the nonradiative rate constants (knr/105 s− ) calculated from the
quantum yields and lifetimes of emission were in the order acac (4.89) > pic (1.17) > bor (0.28). It was assumed that the
large difference of knr for the complexes arose from important contributions of the ancillary ligands to the crossing from an
–
emissive state (3MLCT) to a nonemissive metal centered state (3MC). The activation energies for the crossing from 3MLCT
to 3MC were estimated from the temperature dependencies of the emission lifetime and were found to be 46 meV for
acac, 61 meV for pic, and > 100 meV for bor. The experimental results were in line with the theoretical calculations based
on integrating quantum chemical modeling methods. By the excellent emission behavior, bor was applied as a dopant to
prototype deep-blue phosphorescent organic light-emitting diode devices, which revealed high emission efficiency and
colour purity.
key issues in the commercialization of phosphorescent OLEDs.
Therefore, developing highly robust phosphorescent transition
metal complexes is essential for their practical applications. In
Introduction
Phosphorescent transition metal complexes are distinct from
pure organic luminophores due to their characteristic long
emission lifetimes, large absorption–emission Stokes shifts,
and tunable excited states.1,2 Transition metal complexes emit
phosphorescence at room temperature due to the heavy atom
effect that induces strong spin–orbit coupling (SOC),
facilitating both fast intersystem crossing (ISC) from the
excited singlet state to a triplet state and the spin-forbidden
radiative process from the triplet state.3,4 There are numerous
reports on the photophysical and photochemical properties of
transition metal complexes, particularly for Ru(II),5,6 Ir(III),7-10
and Pt(II) complexes.11-13
this regard, a primary requirement is that triplet emitters
should be resistant to structural rearrangements as
encountered in some Ir(III)-OLED emitters during thermal
treatment20,21 and should be stable against chemical
degradation during operational device aging.22,23 Indeed, the
robustness of a phosphorescent metal complex is directly
related to chemical changes, which are considered to occur, in
many cases, by crossing from the phosphorescent state to a
metal–centered d-d state.24-26 In this state, an electron is
populated in the metal–ligand (M–L) anti-bonding dσ orbitals,
and the electronic configuration associated with weakening of
the M–L bond result in its rupture.27 The crossing to a d-d state
is believed to be a major non-radiative process.
Among them, phosphorescent OLEDs based on
cyclometalated Ir(III) emitters have exhibited high efficiencies
with an external quantum efficiency (EQE) > 20%.14-19 However,
stabilities of the various materials used in the OLED devices are
The rate constant of non-radiative decay (knr) should provide
an important measure for the crossing to a d-d state. It is
estimated from the phosphorescence quantum yield (ΦP) and
lifetime (τP): ΦP = kr• τP and τP = 1/ (kr + knr); kr = radiative rate
constant. Therefore, one possible avenue to diminish knr is to
design suitable ligands that can diminish excited-state
structural distortions. Specifically, the so-called Huang–Rhys
factor S serves to quantify the structural distortion ΔQ of the
excited state on the ground state. If S = 0 (i.e. minimum ΔQ),
the equilibrium geometries of the excited and ground states
should be identical, and only a sharp peak corresponding to
the 0–0 transition should be observed. With an increase of S
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(i.e. an increase of ΔQ) and hence an increase in S, vibronic prepared according to a previously reported method.31 The
progressions can be observed; the intensity ratios of the 0–0 syntheses of all complexes were summarized in Scheme S1:
and 0–1 vibrational peaks give S: S = (I0–1/I0–0). Since the overall Suzuki–Miyaura coupling between 2,4-difluorophenylboronic
emission bandwidth is associated with the vibronic acid and 2-bromo-4-methylpyridine gave product 2-(2,4-
progressions, a smaller S implies a spectrum of narrower difluorophenyl)-4-methylpyridine (
bandwidth and higher colour purity of the emission.28,29 to the iodo derivative 2-(2,4-difluoro-3-iodophenyl)-4-
Consequently, it is assumed that complexes with highly rigid methylpyridine ( ) by treatment with lithium diisopropylamide
scaffolds would be advantageous for developing highly (LDA) and iodine. Trifluoromethylation of ( ) with copper(I)
1), which was transformed
2
2
luminescent materials. Since the magnitude of the Huang– iodide in the presence of potassium fluoride and
Rhys factor is proportional to the square of the displacement trifluoromethyl-trimethylsilane at room temperature for 24
in the excited state, the Huang–Rhys factor has frequently hours afforded the cyclometalating ligand (3) in 20% yield.
been used to estimate the degree of participation in a non- This ligand was used to prepare the cyclometalated, chloride-
radiative process.30 However, a more important factor that has bridged diiridium(III)-complex, [Ir(N^C)2Cl]2 (N^C = 2-(2,4-
been overlooked in this estimation is the activation barrier difluoro-3-(trifluoromethyl)-phenyl)-4-methylpyridine
between the radiative triplet state (normally 3MLCT and/or 3LC) IrCl3∙nH2O and excess amounts of ligand (
(
4
),31
3) were dissolved in a
3
and a non-radiative state (usually MC). Namely, even though 2-ethoxyethanol:H2O (3:1) mixture and refluxed for 18 h.
3
the Huang–Rhys factor is small, the crossing to MC would be Finally, acac and pic were prepared according to a previously
facilitated if the activation energy is small. An opposite case reported standard procedure,32,33 in which the ligand exchange
can be true if the activation energy is large enough to suppress with corresponding ancillary ligands was successfully
the crossing to 3MC.
accomplished to yield the bis-cyclometalated Ir(III) complexes.
In this work, we report the excited-state behavior of a series bor was prepared by treating the cyclometalated, chloride-
of heteroleptic cyclometalated complexes composed of 2-(2,4- bridged diiridium(III)-complex with CF3SO3Ag to produce
difluoro-3-(trifluoromethyl)phenyl)-4-methylpyridine as the chloride-free,
main ligands and acetylacetonate (acac), picolinate (pic) and (dfCF3)2Ir(H2O)2(CF3SO3), which was subsequently reacted with
tetrakis-pyrazolyl borate bor as ancillary ligands. The the potassium tetrakis(1-pyrazolyl)borate salt in acetonitrile
synthesis and characterization of the complexes, as well as solution to give the desired product in modest yields.38
their photophysical and electrochemical properties, were The solid-state molecular structures of acac pic and bor were
monometallic
Ir–complex
(
)
,
systematically investigated. The effects of the different determined by single-crystal X-ray diffraction using crystals
ancillary ligands on the excited-state behavior associated with grown in dichloromethane solution, as illustrated in Fig. 1.
3
the crossing to MC are discussed in the results section. More Details of the X-ray diffraction studies are given in Table S1−S4.
precisely, its effects on photophysical properties are presented acac, pic and bor were fallen in the orthorhombic space group
together with theoretical calculations. The developed Pccn, a monoclinic space group P21/c and a triclinic space
heteroleptic Ir(III) complexes were tested as dopants in group P-1 respectively. In all complexes, the iridium(III) center
prototype phosphorescent organic light-emitting diode devices, adopted a distorted octahedral coordination geometry with
and it was found that bor showed a deep-blue electro- cis-metallated carbons and trans-pyridine nitrogen atoms. The
luminescence with a high emission efficiency and colour purity.
Results and Discussion
Synthesis and Crystal Structures
All complexes depicted in Chart 1 were prepared under an
inert gas atmosphere, isolated by flash column
chromatography and were further purified by train
sublimation in moderate yields. The formation of all
compounds was confirmed by high-resolution mass
spectrometry, elemental analysis and 1H and 13C NMR
spectrometry. 2-(2,4-difluorophenyl)-4-methylpyridine was
Fig. 1 Single crystal structures of acac (a), pic (b), and bor (c). Hydrogen atoms
were omitted for clarity.
Bond distances (Å)
acac
pic
bor
Ir–CPh
Ir–CPh
Ir–CPh
Ir–CPh
Ir–NPy
Ir–NPy
Ir–Opic
Ir–Npic
1.983
2.006
2.038
2.056
2.139
2.142
Ir–CPh
Ir–CPh
Ir–NPy
Ir–NPy
Ir–Nbor
Ir–Nbor
2.002
2.008
2.044
2.059
2.122
2.169
1.971
2.034
2.125
Ir–NPy
Ir–NPy
Ir–Oacac
Ir–Oacac
Chart 1. Chemical structures of heteroleptic Ir(III) complexes in this work.
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N-Ir-N angles for the two trans-N, N atoms in acac
ARTICLE
,
pic and bor
were 175.28(17)°, 176.19(17)° and 175.19(19)°, respectively.
Ir-CPh bond lengths of acac (1.971(3) Å), pic (2.006(5) and
1.983(5) Å) and bor (2.002(5) and 2.008(4) Å) were similar to
the reported values for Ir(dfppy)2(pic), Ir(tpy)2(bor), and
Ir(dfppy)2(acac), respectively.33-38 For heteroleptic compounds,
the ancillary ligands were less tightly bound to the iridium(III)
center compared to the main ligand, with acac (Ir−O: 2.125(3)
Å), pic (Ir−N: 2.143(3) Å and Ir−O: 2.139(3) Å), bor (Ir−N:
2.122(4) and 2.169(5) Å) distances. The distances between the
iridium centre and the coordinating atoms of the ancillary
ligands acac, pic and bor were shown in Table 1. The distances
between iridium and the main ligand (N^C) were the longest in
bor. Within the series of compounds, the two phenyl–pyridine
rings were roughly coplanar, but the dihedral angle showed
that pic was the smallest with the value of 0.22°, followed by
acac with the value of 1.46°, and bor was the most distorted
with the value of 5.49°. These structural differences were
expected to affect the photophysical properties of each
compound as seen below.
Fig. 2 Oxidation waves for 1 mM DCM solution of acac, pic, and bor containing 0.1
M TBAP taken at a scan rate of 0.1 V/s by cyclic voltammograms (inset: reduction
wave of pic). Platinum disk electrode is the working electrode, and a platinum
wire and Ag/AgNO3 is the counter and reference electrodes, respectively.
energy level compare with pic
.
Electrochemistry
The electrochemical properties of acac, pic, and bor were
Photophysical Properties
investigated by using cyclic voltammetry (CV), and their redox
potentials were summarized in Table 2. CV was performed by
utilizing a three-electrode cell system: platinum disk electrode
was used as the working electrode, whereas a platinum wire
and Ag/AgNO3 were used as the counter and reference
electrodes, respectively. All of the electrochemical data were
measured relative to an internal ferrocenium/ferrocene
reference (Fc+/Fc). As shown in Fig. 2, the oxidation potentials
Fig.
3
showed steady-state absorption spectra of the
pic and bor in
heteroleptic Ir(III)-complexes acac
,
,
dichloromethane (DCM) solution. The absorption spectra at
298 K featured characteristic transitions that have been
observed in other similar systems, and could tentatively be
assigned to: (i) strong ligand–centered (LC) spin-allowed π →
π* transitions between 243 and 264 nm and confirmed by
absorption spectra of the ligand as shown in the Fig. S2, (ii)
spin-allowed singlet metal to ligand charge transfer (1MLCT) Ir-
dπ → ppy-π* transitions between 320 and 380 nm, and (iii) a
weak absorption band between 435 and 470 nm that was
likely associated with a spin-forbidden 3MLCT transition. In
addition, even though it was not observed in absorption
spectra, an energetically close-lying ligand-based excited state
(3LC) might be mixed together other states. As suggested and
previously reported in the literature,39 the lowest triplet
excited state, T1, would presumably be mixed in terms of
orbital characteristics with a wave function description of ΨT1
of acac, pic, and bor were significantly different which showed
oxidation at 0.90, 1.12, and 1.18 V, respectively. Increasing the
ligand field strength of the ancillary ligand clearly leads to a
higher oxidation potential which indicates ancillary ligands
with stronger ligand field strength stabilize the HOMO (highest
occupied molecular orbital) energy level. On the other hand,
due to the limitation of the CV window, the reduction
potentials of acac and bor were not obtained, while the
reduction potential for pic was measured as −2.34 V, as shown
in the Fig. 2 inset. Therefore, LUMO (Lowest unoccupied
molecular orbital) levels of acac and bor would be at higher
= aΨ3MLCT + bΨ3LC
.
.
HOMOc/
LUMOd
(eV)
emission at 298 K
kr
105 s-1 105 s-1
emission at 77 K
abs, λmax
Eox/
Complex
knr
λ (nm) [ε, 103 M-1cm-1
Ered (eV)b
a
]
λ (nm) τ (μs) ФPL
λ (nm) τ (μs)
459,
2.5
0.88/
-
-5.68/
-2.98
-5.92/
-3.16
-5.98/
-3.19
acac
pic
246 (54.9), 263(49.2), 323 (13.61), 379 (6.3), 416 (2.5), 446 (1.1) 470, 494 0.9 0.56 6.22
4.89
1.17
0.28
491, 524
449,
2.6
1.12/
-2.33
1.18/
-
260 (38.0), 279 (32.6), 372 (4.9), 448 (0.2)
455, 484 1.8 0.79 4.39
450, 478 4.6 0.87 1.89
481, 514
446,
4.9
bor
239 (6.8), 316 (11.4), 341 (6.8), 361 (5.4), 413 (0.4)
478, 510
aMeasured in Ar-saturated DCM solution (10 μM) at 298 K. bRedox potentials were measured in DCM and their values are reported relative to Fc/Fc+. The HOMO and
LUMO levels were determined using the following equations: cEHOMO(eV)= -e(Eox+4.8), dELUMO(eV)= e(EHOMO+Egopt).
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Fig. 3 (a) Steady-state absorption spectra for acac, pic, and bor in DCM solution at 298 K (inset: enlarged spectra from 400 to 500 nm). Emission spectra in DCM solution at 298 K
(b) and mTHF solution at 77 K (c) for Ir(III) complexes.
The emission properties were investigated using steady-state overlapping satellites belonging to different vibronic
photoluminescence spectroscopy at 298 K and 77 K, as shown transitions. Namely, cooling to 77
K resulted in more
in Fig. 3b and 3c, respectively. At 298 K, heteroleptic Ir(III) populated in 3LC excited state as evidenced by the highly
complexes exhibited different phosphorescence emission structured emission. Furthermore, it was reported that the
peaks with distinctive vibronic fine structures depending on intensity ratio of this first major vibrational transition to the
the ancillary ligand at 470, 455, and 450 nm for acac, pic, and highest energy peak (Eem(0−0)) was a measure of vibronic
bor, respectively, which indicated that the emitting excited coupling between the ground and excited state (Huang–Rhys
state was a mixed MLCT/LC state. At 77 K, acac exhibited two factor, SM) and was proportional to the degree of structural
emission peaks at 459 and 492 nm, with vibronic progressions distortion that occurs in the excited state relative to the
of 1461 cm–1. pic, and bor showed similar phosphorescence ground state.40 The dominant vibrational mode associated
emissions at 77 K with two emission peaks at 449 and 480 nm with the excited-state distortion (M) could be obtained from
(vibronic progressions = 1439 cm–1) for pic and two peaks at the energy difference (in cm−1) of these vibronic transitions at
446 and 478 nm (vibronic progressions = 1501 cm–1) for bor
The vibronic mode at 1440 ~ 1500 cm-1 was known as the heights. As shown by the energy curves in Fig. 4, the
stretching vibration of the Cph–Cpy bond (A mode) of the main corresponding SM values for acac pic, and bor were 0.76, 0.87,
ligand, which increased in the order of pic acac and bor. Table and 0.97, respectively. This result indicated that the structure
3 shows the calculated dihedral angles for acac pic, and bor of bor in the excited state would be mostly changed,
.
77 K, whereas the SM value could be estimated from the peak
,
,
,
,
indicating that the dihedral angle of bor was the largest and therefore, it was expected that its lifetime would be shortest
bor was the most distorted. This suggested that the excited among the series of Ir(III) complex in this work.
states of acac
,
pic, and bor may be affected by the distortion
As shown in Fig. 5a for the emission decay profiles, the
of the phenyl–pyridine. Furthermore, all Ir(III) complexes emission lifetime of bor (4.6 μs) at room temperature was
showed rigidochromic shift with a decrease in temperature however substantially longer than that of either acac (0.9 μs)
from 298 K to 77 K. Therefore, spectral changes between 298 K or pic (1.8 μs) in dichloromethane solution. This was again true
and 77 K indicate that the emitting state involved a 3MLCT even at 77 K as shown in Fig. 5b; the emission lifetime of bor
state. In addition, the vibrational features became more (4.9 μs) was remarkably longer than that of acac (2.5 μs) or pic
defined at low temperature, as highlighted by a direct (2.6 μs). From the observed values of Φem and τem, the
comparison of Fig. 3b and 3c. The vibrational fine structure radiative (kr) and nonradiative (knr) rate constants were
observed in emission spectra was often the result of several calculated by using the following equations: τrad = τem/Φem, kr =
1/τrad, Φem = kr/(kr + knr), and τem = 1/(kr + knr). Therefore, there
Dihedral Angle (°)
acac
0.63
2.17
5.92
8.53
pic
bor
1.77
S0
T1
0.07
0.28
7.50
8.23
2.01
TS
20.44
16.26
MC
Fig. 4 Energy potential curves of acac, pic and bor.
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distortions along the vibrational vectors of the Ir–N bond
stretching mode, which may lead to ligand dissociation. Due to
the high oxidation state of Ir(IV) and the electron-richness of
the photoreduced ligand, the M–L bond will likely cleave in a
homolytic fashion, resulting in a five-coordinate, triplet Ir(III)
complex. These intermediates were labeled T3 and T4 which
emerged from T1 by traversing the bond cleavage transition
states TS1 and TS2, as illustrated in Fig. 6. Rapid intersystem
crossing to the singlet surface gave the metastable five-
coordinate singlet state S1, which was a classical, coordinately
Fig.
5 Phosphorescence decay lifetime of Ir(III) complexes at 298 K in
unsaturated Ir(III)-low spin complex. Reattachment of the
dangling ancillary ligand completed the reaction cycle and
would be another factor that may determine the emission regenerated the reactant molecule S0 in what constituted a
dichloromethane (a) and at 77 K in mTHF (b) with excitation at 355 nm.
lifetime, such as an activation energy.
non-radiative decay of the
The intersystem crossing from T3
expected to be particularly efficient, as the spin–orbit coupling
T1 state.
→
S1 and T4→S2 was
Mechanism of Non-Radiative Decay.
Fig. 6 conceptualized the most plausible non-radiative decay of the electrons so close to the Ir–center would be large and
promote rapid electron spin interconversion.3 Other
3
pathways expected for the MLCT excited states of acac
,
pic
,
and bor. In heteroleptic complexes that carry an ancillary temperature independent pathways for non-radiative decay,
ligand with low lying π*-orbitals, two MLCT states must be such as direct coupling between the triplet and ground states
considered: one in which the electron is excited into the and vibrational coupling to the ground state,43,44 were not
cyclometalating ligand
be excited into the ancillary ligand
responsible for phosphorescence was T1 and emerged from T3 and T4 states, governed by the activation barrier associated
the singlet excited state S* by rapid intersystem crossing. It with TS1 and/or TS2
may be major contributor to
could decay back to the ground state S0 through a number of determining the lifetime of the emissive
1 intermediate.
L
1 and a second where the electron may explicitly considered in this work. Namely, this model plausibly
L
2. The 3MLCT state suggested that the thermal accessibility of the metal–centered
,
a
T
competitive radiative and non-radiative processes, which all
contributed to the overall lifetime of the excited state. The
For pic, the T2 state was 8.8 kcal mol–1 higher in energy than
T
1, which was not surprising, since the π*-orbital of the dfCF3
non-radiative decay rate should be related to the energy gap ligand was much more delocalized and lower in energy
law, as demonstrated in analogous Ru(II) and Os(II) compared to that of the ancillary ligand, picolinate. In addition,
complexes:41,42 The non-radiative decay becomes more
our calculations suggested that the non-radiative intermediate
with picolinate ancillary ligand,
energy than 1. This T4 state was thermodynamically more
favorable than the T3 state which was non-radiative
T
4, was 1.1 kcal mol–1 higher in
important when the energy gap between the emissive excited
state and the ground state is increased by e.g. varying the
ligand composition, as the radiative decay rate is inversely
proportional to the energy gap. For the Ir complexes, the
T
3MLCT state 1 formally consisted of a Ir(IV)–(L)-1 fragment and
the low-spin Ir(IV)-d5 center necessarily has an asymmetrically
occupied d-π orbitals among the t2g orbitals of the pseudo-
octahedral coordination environment. Therefore, these
excited states were susceptible to first-order Jahn–Teller
T
a
intermediate with a main ligand. The kinetic barrier to reach T4
state from T1 state was only 2.9 kcal mol–1 which was
substantially lower than the T3 state, 10.4 kcal mol–1. This
finding was alarming, as it suggested that employing picolinate
as an ancillary ligand was counterproductive and did not
enhance the photoluminescence of the Ir-complex. We found
Fig. 6 Reaction profiles for Ir–N bond cleavage to form non-radiative T3 and T4 states for heteroleptic complexes (a) pic and (b) bor (Reaction profile acac and explanations
are in supporting information, Fig. S5).
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that acac displayed a very similar behavior. The computed
reaction profiles and supplemental discussions are given in the
Supporting Information.
The detrimental impact of the picolinate ligand on the stability
3
of the MLCT state raises the question whether the opposite
effect can be engineered by employing an ancillary ligand that
cannot access a T4 state. The computed reaction energy
profiles for bor are shown in Fig. 6b. Interestingly, the T3 state
was found to be 0.8 kcal mol–1 lower in energy with respect to
the T1 state, but the activation energy required to reach the T3
state was dramatically increased to 7.9 kcal mol–1. In addition
to the aforementioned electronic effect, the steric demands of
the borate ligand made reorganization into the trigonal
bipyramidal geometry of T3 difficult. The bulkiness of the
borate ligand also contributed to a T4 state that was 14.4 kcal
mol–1 higher in energy than the
T1 state as the pyrazolyl moiety
was unable to rotate away from the Ir center. The pyrazolyl
fragments of the anionic borate ligand did not possess any
low-lying vacant π*-orbitals and consequently the ancillary
ligand could not participate in the MLCT process to generate a
T2 state as was the case for pic. Even though the calculated
Fig. 7 Temperature-dependent excited-state deactivation rates (kin = 1/τ) of acac,
pic, and bor. Excited-state lifetimes were determined from the luminescence
decays with the time-correlated single photon counting technique.
absolute values could be different compared with the (kin) as a function of temperature from 293 to 343 K. acac
experimental values, we believed that the trends of the clearly showed a reduced emission lifetime at a higher
calculated results could support the experimental data.
temperature, while bor showed an almost invariant emission
lifetime at different temperatures. This indicated that the
activation energy from T1 to the 3MC for bor, was much higher
than for acac. The activation energies of each one of the
Temperature Dependent Emission Properties
To support our hypothesis, we investigated temperature complexes were estimated by using the intrinsic deactivation
dependent emission properties of acac pic, and bor. Fig. 7 rate constant, kin (T) = 1/τ (T) where τ was the excited-state
,
showed the evolution of intrinsic deactivation rate constant lifetime at a certain temperature T which could be estimated
Fig. 8 Performances of devices acac, pic and bor (a) J−V−L characteristics and (b) Current efficiency–Current density–Power efficiency (c) External quantum efficiency and EL
spectra for devices acac, pic and bor.
Max
Max power
current
efficiency
(cd/A)
34.8
Turn-on
efficiency
(lm/W)
CIE (x, y)
Max EQE (%)
voltage (V)
acac
pic
21.9
16.4
25.4
4.6
4.0
3.7
(0.14, 0.26)
(0.14, 0.18)
(0.14, 0.20)
19.9
15.5
22.6
22.5
bor
32.9
Device structure: [ITO (150 nm)/HAT-CN (10 nm)/TAPC (85 nm)/mCBP:acac, pic or bor (30 nm,
8
wt%)/TmPyPB (30 nm)/Liq/Al (150 nm)]
6 | J. Name., 2012, 00, 1-3
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by the following equation:45-47
d’Echairage (CIE) coordinates value [0.14, 0.20] was slightly
higher than that of pic [0.14, 0.18] and the roll-off
1/τ(T) = k0 + A1 exp(–ꢀE1 / kBT) + A2 exp(–ꢀE2 / kBT)
phenomenon was prominently accelerated.
A
notable
difference on the device performance may be arisen due to
the HOMO energies of the dopants. In particular, the HOMO
energy of bor was low enough to get closer to that of the host,
mCBP; there seemed to be hardly any trap sites in bor
resulting in a low turn-on voltage. However, this in turn gives
rise to an adverse effect in the efficiency measurements in Fig.
8b because a p-type host, mCBP, required to have a certain
amounts of trap sites to achieve charge valance for higher
device performance. In additions, as shown below, the EL
spectra are almost similar to the PL spectra of the dopants,
suggesting that the energy transfer from mCBP to the dopant
is well facilitated. These data are now updated in the
supporting information (see Fig. S5).
In this equation, k0 was a temperature independent term, and
the second and third terms contained frequency factors (A1
and A2) with activation energy barriers (ꢀE1 and ꢀE2) which
could be expressed by an Arrhenius–type equation. The
second term represented thermal redistribution between the
triplet sublevels which depended on the zero field splitting
(zfs). Since this second term could be treated as a constant
above 293 K and the first term was the temperature-
independent term as indicated above, the third term which
represented the thermal population of non-radiative 3MC state
played a predominant role to determining activation energy
barriers of acac, pic, and bor. The estimated activation
energies for acac and pic were 46 and 61 meV, respectively.
The activation energy for bor could not be assumed but it
could be assumed to be higher than 100 meV. Therefore, when
combining the quantum chemical modelling method and
temperature dependent emission lifetime measurements, we
could conclude that the activation energy from T1 to 3MC could
be controlled by using different ancillary ligand in Ir(III)
heteroleptic complexes.
Conclusions
In this work, a series of heteroleptic Ir(III) complexes, acac
, pic,
and bor, were prepared and their electrochemical and
photophysical properties as well as different activation
energies depending on ancillary ligand were systematically
investigated. Even though the Huang–Rhys factors indicate the
structure of bor in the excited state would be mostly changed,
result shows that the emission lifetime of bor is longest among
the series. We carried out theoretical quantum chemical
calculations and measured experimental temperature
dependent emission lifetime to explain the importance of
activation energy barriers from T1 state to 3MC state in
heteroleptic Ir(III) complexes. As a result, both theoretical and
experimental investigations showed that the activation energy
barrier for bor is significantly higher than acac and pic, which
was estimated by temperature dependent emission lifetime
experiments with the values of 46, 61, and higher than 100
meV, respectively. Therefore, we can conclude and proved
that the activation energy from T1 to 3MC can be controlled by
using different ancillary ligand in Ir(III) heteroleptic complexes.
Finally, based blue phosphorescent OLED device was
fabricated and the device shows a high current efficiency of
32.9 cd/A, which corresponds to an external quantum
efficiency of 21.5%.
Device Performance
The blue phosphorescent OLED device was fabricated using pic
and bor as dopants with the following device structure:
ITO/HAT-CN (10 nm)/TAPC (85 nm)/mCBP:bor or pic (30 nm, 8
wt%)/TmPyPB (30 nm)/Liq (1 nm)/Al (150 nm) where HAT-CN
stood for 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile,
TAPC
4,4′-cyclohexylidene-bis[N,N-bis(4-
methylphenyl)benzeneamine], and TmPyPB 1,3,5-tri(m-
pyridin-3-ylphenyl)benzene. HAT-CN was used as the hole
injection layer (HIL) material with
a deep-lying lowest
unoccupied molecular orbital (LUMO) to reduce the driving
voltage and improve voltage stability and power efficiencies.48–
50
The triplet energy levels of TAPC (2.87 eV), mCBP (2.90 eV),
and TmPyPB (2.80 eV) were higher than those of pic and bor
;
thus, effective confinement of the triplet excitons within the
emission layer was expected. Fig. 8 and Table 4 showed the
device characteristics. Blue emitting heteroleptic Ir(III)
complexes were employed as triplet-emitting dopants. Among
the heteroleptic Ir(III) complexes, bor outperformed over
other heteroleptic Ir(III) complexes, acac or pic achieving the
highest blue emission (acac: 459 nm, pic: 449 nm and bor: 446
Acknowledgements
This work was supported by the MOTIE (Ministry of Trade, Industry
&
Energy (10051379) and KDRC (Korea Display Research
nm) and recording a high Q.Y.(acac: 56% < pic: 79% < bor
:
Corporation) support program for the development of future
devices technology for display industry, the International Science
and Business Belt Program through the Ministry of Science, ICT and
Future Planning (2015K000287), the Basic Science Research
Program through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education (NRF-2014R1A6A1030732).
87%). The unique properties of bor have also been observed in
the devices consistent to our understanding of its inherent
HOMO energy. Triplet energy confinement of bor by the common
layer materials provides a high current efficiency of 32.9 cd/A,
which corresponds to an external quantum efficiency of 21.5%. The
bor device exhibited lower turn-on voltage at 3.7 V with
a
maximum power efficiency up to 25.4 lm/W superior to acac (4.6
V) or pic (4.0 V), whereas the Commission Internationale
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