Highly Quantum Efficient Phosphorescent Sky Blue Emitters Based on Diastereomeric Iridium(III) Complexes of Atropisomeric 5-Aryl-4H-1,2,4-triazole Ligands

Article abstract of DOI:10.1021/acs.organomet.5b00198

(Chemical Equation Presented) Homoleptic fac-Ir^IIIL₃ complexes of 5-aryl-4H-1,2,4-triazole ligands are sky blue emitters. When unsymmetrically substituted, the triazole ligands exhibit atropisomerism, and upon cyclometalation to Ir(I

Full text of DOI:10.1021/acs.organomet.5b00198

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Highly Quantum Efficient Phosphorescent Sky Blue Emitters Based
on Diastereomeric Iridium(III) Complexes of Atropisomeric
5‑Aryl‑4H‑1,2,4-triazole Ligands
Jerald Feldman,* Giang D. Vo,* Charles D. McLaren, Troy C. Gehret, Kyung-Ho Park, Jeffrey S. Meth,
Will J. Marshall, Joseph Buriak, Lois M. Bryman, Kerwin D. Dobbs, Thomas H. Scholz, and Steve G. Zane
DuPont Central Research & Development, Experimental Station, 200 Powder Mill Road, Wilmington, Delaware 19803, United States
S
* Supporting Information
ABSTRACT: Homoleptic fac-IrIIIL3 complexes of 5-aryl-4H-
1,2,4-triazole ligands are sky blue emitters. When unsymmetri-
cally substituted, the triazole ligands exhibit atropisomerism,
and upon cyclometalation to Ir(III) a mixture of diastereomers
is formed. We have isolated and structurally characterized all
four possible diastereomers of the fac-IrIIIL3 complex formed
upon cyclometalation of an atropisomeric 5-aryl-4H-1,2,4-
triazole ligand onto Ir(III). The phosphorescent blue emitting
materials reported herein are among the most efficient to date,
with quantum efficiencies above 95%.
we have synthesized homoleptic iridium(III) complexes
he organic light-emitting diode (OLED) is an attractive
Tnew technology for commercial and residential lighting containing sterically hindered 5-aryl-4H-1,2,4-triazole ligands.12
We have found that these ligands can display atropisomerism:
that is, depending on substitution patterns they may be chiral as
the result of hindered internal bond rotation.13 Upon cyclo-
metalation of the ligand to Ir(III) to afford homoleptic
octahedral IrL3 compounds, complex mixtures of diastereomers
are formed. We have discovered that the individual diastereomers
may be separated by liquid chromatography. To the best of our
knowledge, this report is the first in which all four atropisomeric
diastereomers of a homoleptic octahedral ML3 complex have
been separated and structurally characterized and their individual
photophysical properties have been examined. All of these
complexes exhibit high quantum efficiencies (Φ > 0.95).
applications because of its pleasing diffuse light and flexible form
factors.1 OLED solid-state lighting (SSL) is also environmentally
safe because it does not employ mercury vapor found in gas-
discharge fluorescent bulbs. However, OLED SSL faces technical
challenges to achieve the operational requirements of high power
efficiency, high color rendering index, and long device lifetime.
To achieve high power efficiency, phosphorescent emitters are
used to harvest both triplet and singlet excitons,2 in principle
allowing for 100% internal quantum efficiency versus 25% for
fluorescent counterparts.3 To obtain a high color-rendering
index (CRI), the device needs to emit light across the entire
visible spectrum. This emission spectrum can be obtained with
three primary emitters of red, green, and blue. Of these three
types of emitters, phosphorescent red and green emitters are
widely available on the market with excellent device lifetimes,1
while the development of highly efficient, long-lived blue
emitters is an ongoing challenge.1,4
Among phosphorescent blue emitters, Ir(III) complexes have
shown the most promising performance. However, they are
limited to a few classes of core structures containing cyclo-
metalated bidentate ligands. These classes include phenyl-
imidazoles,5 phenylpyridines (e.g., FIrpic),6 phenyl-1H-1,2,4-
triazoles,7 phenyl-2H-1,2,3-triazoles,8 and pyrimidine−pyridine.9
Most of these emitters, with the exception of FIrpic6a and
substituted FIrpic analogues,10 have low quantum efficiency (Φ
< 0.5).
We prepared two 5-aryl-4H-1,2,4-triazole ligands, as shown in
Scheme 1. The triazole core was constructed from the
condensation reaction of 2,6-diisopropylaniline or 2,4-diphen-
yl-6-isopropylaniline with 2-phenyl-1,3,4-oxadiazole.14 Bromina-
tion at the C5 position,15 followed by Suzuki−Miyaura coupling,
afforded ligands 3a,b in 31% and 47% overall yields, respectively.
1
The room-temperature H NMR spectra of 1a−3a show two
doublet resonances for the diastereotopic isopropyl methyl
groups (see Figures S1, S3 and S5 in the Supporting
Information). Thus, rotation about the N−aryl bond appears
to be slow in comparison with the chemical shift difference
between the two resonances on the order of 40−150 Hz at 11.75
T (500 MHz) field strength.16
As part of our development of highly efficient phosphorescent
Received: March 10, 2015
blue emitters whose color can be tuned through steric effects,11
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00198
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Communication
a
Scheme 1. Synthesis of Ligands 3a,b and Complexes 4 and 5
a
Legend: (a) 2-phenyl-1,3,4-oxadiazole, trifluoroacetic acid, o-dichlorobenzene, reflux; (b) N-bromosuccinimide, CCl4/AcOH, reflux; (c) Pd2(dba)3/
S-Phos/K3PO4 H2O, o-tolylboronic acid, toluene, reflux; (d) Ir(acac)3, 250 °C, ∼3 days.
The cyclometalation of ligand 3a with iridium(III) acetyla-
cetonate afforded complex 4 in 78% yield after purification. 1H
NMR spectroscopic and X-ray crystallographic analyses of 4
indicate the sole presence of the fac isomer. Four resonances
corresponding to the four methyl groups and two resonances for
the methine hydrogens on the isopropyl substituents of the
ligand indicate that the chiral metal center renders the two
methines and the corresponding two sets of methyl groups
diastereotopic. The photophysical properties of complex 4 were
characterized by UV−vis absorption, luminescence, photo-
luminescent quantum efficiency (PLQE), and photolumines-
cence lifetime. Complex 4 is a highly efficient sky blue emitter in
toluene solution with photoluminescence peak emission at 475
nm and a PLQE of 97%.
these four products was ∼1:3:3:1, or what would be expected
from a statistical mixture. The combined yield of the four purified
products, after silica gel column chromatography and recrystal-
lization from toluene or ethyl acetate/toluene, was 33%. The
purity of the separated diastereomers was 95−98% according to
UPLC.
The stereochemical identities of 5 were confirmed by X-ray
crystallographic analysis. The four pairs of enantiomers
crystallize in centrosymmetric space groups. We only observed
fac-IrL3 isomers with different relative stereochemistry at the
stereogenic aryl−triazole axes and/or Ir centers. The X-ray
crystal structures of the four atropisomers of 5 are shown in
Chart 1, and metrical parameters are collected in Table 1.
1
The H NMR spectra of the four atropisomers of 5 are
Like 1a−3a, NMR spectroscopic analysis of 1b−3b shows
diastereotopic methyl groups of the isopropyl substituent.17
Unlike 1a−3a, however, 1b−3b lack a plane of symmetry and are
therefore racemic mixtures of atropisomers.15 By employing the
nomenclature developed for axially chiral biaryl compounds,13
we can assign the descriptors M and P to the two atropisomers of
ligand 3b (Figure 1).
consistent with the X-ray crystal structures. Thus, 5(Λ-MMM/
Δ-PPP) and 5(Δ-MMM/Λ-PPP) have different NMR spectra
due to different relative stereochemistry between the iridium
center and its ligands, but the spectra are simple and in each all
three ligands are magnetically equivalent (Figures S13−S15 and
S22−S24 in the Supporting Information). By comparison, the 1H
NMR spectra of 5(Δ-MPP/Λ-PMM) and 5(Λ-MPP/Δ-PMM)
are relatively complex, with each of the three ligands exhibiting a
different set of resonances (Figures S16−S21 in the Supporting
Information).
The atropisomers of 5 exhibit unique photophysical proper-
ties. While they exhibit photoluminescence quantum efficiencies
approaching 100%, their peak emission wavelengths and widths
vary slightly from one another, following the trend 5(Λ-MMM/
Δ-PPP) < 5(Δ-MPP/Λ-PMM) < 5(Λ-MPP/Δ-PMM) < 5(Δ-
MMM/Λ-PPP) (Table 2 and Figure 2). 5(Λ-MMM/Δ-PPP)
has the highest energy emission (475 nm), which is identical with
that of complex 4. The peak emission width of 5(Λ-MMM/Δ-
PPP), however, is significantly narrower than that of 4 (fwhm 70
vs 85 nm). In fact, all of the atropisomers of 5 have peak
emissions narrower than those of 4. The X-ray crystal structures
of 5 indicate that, in the solid state, the C5-Ar and N4-Ar dihedral
angles increase in the same order as peak emission and width
(Table 1).19 As with complex 4, the four atropisomers of 5 all
have faster radiative vs nonradiative rates.
Color tuning of 4 and 5 is the result of the o-tolyl group
attached to the C5 carbon of the triazole ring. Previous11a and
current DFT calculations indicate that the steric effect of this
substitution is critical to the blue emission observed in these
complexes. In Table 3, the calculated electronic properties of
complex 4 are compared to an analogous complex containing an
unsubstituted phenyl group attached to the triazole ring
(complex 6). Ortho substitution twists the aryl group in 4 out
of conjugation with the triazole ring, resulting in a greater
Figure 1. Two atropisomers of ligand 3b resulting from hindered
rotation about the N−C bond.
Octahedral tris-chelate complexes are also chiral at the metal,
and the common descriptors Λ and Δ are used to describe the
two possible stereogenic configurations around the metal
center.18 In principle, cyclometalation of an atropisomeric ligand
such as 3b to afford a homoleptic octahedral fac-IrL3 complex
may result in eight diastereomers. These diastereomers would be
obtained as four pairs of enantiomers: Λ-MMM/Δ-PPP, Δ-
MPP/Λ-PMM, Λ-MPP/Δ-PMM, and Δ-MMM/Λ-PPP. Indeed,
the high-temperature cyclometalation reaction between Ir(acac)3
and 3.3 equiv of 3b produced a mixture of products 5 that could
be separated by silica gel column chromatography and
recrystallization. The four products eluted from the chromatog-
raphy column in the following order: 5(Λ-MMM/Δ-PPP) >
5(Δ-MPP/Λ-PMM) > 5(Λ-MPP/Δ-PMM) > 5(Δ-MMM/Λ-
PPP). On the basis of the UV chromatogram areas, the ratio of
B
DOI: 10.1021/acs.organomet.5b00198
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Communication
a
Chart 1. ORTEP Diagrams for 5(Λ-MMM/Λ-PPP), 5(Λ-MPP/Δ-PMM), 5(Δ-MPP/Λ-PMM), and 5(Δ-MMM/Λ-PPP)
a
CIF files are included in the Supporting Information.
Table 1. X-ray Structural Data for the Four Diastereomers of 5
5(Λ-MMM/Δ-PPP)
P21/c
5(Δ-MPP/Λ-PMM)
5(Λ-MPP/Δ-PMM)
5(Δ-MMM/Λ-PPP)
space group
P1
̅
P1
̅
P1
̅
av Ir−N bond (Å)
av Ir−C bond (Å)
C5−Ar dihedral (deg)
N4−Ar dihedral (deg)
2.09
2.08
2.07
2.09
2.02
2.02
2.02
2.01
46.57 (47.98, 48.17, 43.57)
73.50 (71.56, 75.15, 73.79)
46.62 (46.75, 46.64, 46.47)
75.83 (83.37, 68.02, 76.09)
48.57 (49.11, 35.66, 60.65)
80.36 (82.63, 70.75, 87.71)
55.47 (52.98, 54.83, 58.60)
84.33 (89.90, 78.23, 84.86)
HOMO−LUMO gap and higher triplet energy in comparison to
that in complex 6. Indeed, a report in the patent literature
indicates that iridium(III) complexes of 5-phenyl-4H-1,2,4-
triazole ligands are green emitters (λem ∼500 nm),20 whereas
the emitters of the present study exhibit blue emission. This type
of color tuning, which is not dependent on electron-withdrawing
functionalities,21 has also been reported for iridium(III)
complexes of 5-aryl-1H-1,2,4-triazole ligands.11
In conclusion, we have identified a new class of phosphor-
escent blue emitters based on homoleptic iridium(III) complexes
of 5-aryl-4H-1,2,4-triazole ligands. Complexes 4 and 5 exhibit
high quantum efficiencies. Their emission peaks are in the sky
blue range (470−480 nm), suitable for solid-state lighting. In
these respects they compare well to blue emitters based on 5-
aryl-1H-1,2,4-triazoles previously reported by Burn and Samuel;
however, their blue emission is not as deep.7b When unsymetri-
cally substituted, 5-aryl-4H-1,2,4-triazole ligands are chiral due to
hindered bond rotation, and this results in a mixture of
atropisomers upon cyclometalation. In the case of ligand 3b,
we have isolated and structurally characterized the four possible
atropisomeric octahedral fac-IrIIIL3 complexes. Two atro-
pisomers of an octahedral Ir(III) complex containing a pyrene-
functionalized pyridine−imine ligand were reported recently;
these complexes were the first examples of metal-containing
atropisomers in which the rotationally restricted bond was “not
between two chelating atoms”.22 The atropisomers of 5 would
appear to be the second such example, and we believe this is the
only system in which the four atropisomers of a homoleptic
octahedral ML3 complex (in which L exhibits axial chirality) have
been structurally characterized.23 Moreover, we believe this is the
first report wherein the photophysical properties of phosphor-
escent atropisomers have been studied.24 We will aim to
understand substituent effects on emission wavelength and
efficiency in homoleptic iridium(III) complexes containing 1,2,4-
triazole ligands and will report our findings in due course.
C
DOI: 10.1021/acs.organomet.5b00198
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Communication
for measuring the photophysical properties of 4 and the
diastereomers of 5, computational details for 4 and 6, X-ray
structure analyses, including tables of anisotropic thermal
parameters, hydrogen atom parameters, bond angles, torsion
angles, and interatomic distances for complexes 4 and 5(Λ-
MMM/Δ-PPP), 5(Δ-MPP/Λ-PMM), 5(Λ-MPP/Δ-PMM),
and 5(Δ-MMM/Λ-PPP). The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.5b00198.
Table 2. Photoluminescence Data for Complex 4 and the
Atropisomers of Complex 5
knr
λem
(nm)
PLQE fwhm
(%)
τ
a
b
c
d
complex
kr (105 s−1
)
(104 s−1
)
(nm) (10−6 s)
4
5.9 0.32
6.5 0.35
1.7 3.4
3.1 3.7
475
475
97
95
85
70
1.7
1.5
5(Λ-MMM/
Δ-PPP)
5(Δ-MPP/
Λ-PMM)
5(Λ-MPP/
Δ-PMM)
6.9 0.37
7.1 0.38
6.5 0.35
0.66 4.0
1.2 4.1
2.5 3.7
477
478
478
99
98
96
73
76
79
1.4
1.4
AUTHOR INFORMATION
5(Δ-MMM/
1.5
■
Λ-PPP)
Corresponding Authors
a
b
The error analysis is described in the Supporting Information. The
*E-mail for J.F.: jerald.feldman@dupont.com.
*E-mail for G.D.V.: giang.vo@dupont.com.
large experimental error in the nonradiative rate measurement is a
consequence of high quantum efficiency; see the Supporting
c
Notes
Information. The luminescence spectra were measured at 298 K
with excitations at 300, 320, 340, and 360 nm in toluene. Emission
spectra were independent of exciting wavelength. The lifetime of the
The authors declare no competing financial interest.
d
triplet state was measured with excitation set at 343 nm in toluene
solutions.
ACKNOWLEDGMENTS
■
This manuscript is dedicated in memory of our dear friend and
colleague Dr. Stephan J. McLain (1953−2014).
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