Highly efficient green organic light-emitting diodes containing luminescent three-coordinate copper(I) complexes

Article abstract of DOI:10.1021/ja202965y

A series of highly emissive three-coordinate copper(I) complexes, (dtpb)Cu^IX [X = Cl (1), Br (2), I (3); dtpb =1,2-bis(o- ditolylphosphino)benzene], were synthesized and investigated in prototype organic light-emitting diodes (OLEDs). 1-3 showe

Full text of DOI:10.1021/ja202965y

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Highly Efficient Green Organic Light-Emitting Diodes Containing
Luminescent Three-Coordinate Copper(I) Complexes
Masashi Hashimoto,†,‡ Satoshi Igawa,†,‡ Masataka Yashima,†,‡ Isao Kawata,†,§ Mikio Hoshino,‡ and
Masahisa Osawa*,†
†Luminescent Materials Laboratory, RIKEN (The Institute of Physical & Chemical Research), Hirosawa 2-1, Wako-Shi 351-0198, Japan
‡Device Technology Development Headquarters and §Analysis Technology Center, Canon Incorporated, Ohta-ku, Tokyo 146-8501,
Japan
S Supporting Information
b
copper(I) system containing a chelating bisphosphine ligand [dtpb =1,
ABSTRACT: A series of highly emissive three-coordinate
copper(I) complexes, (dtpb)CuIX [X = Cl (1), Br (2), I (3);
dtpb =1,2-bis(o-ditolylphosphino)benzene], were synthe-
sized and investigated in prototype organic light-emitting
diodes (OLEDs). 1ꢀ3 showed excellent photoluminescent
performance in both degassed dichloromethane solutions
[quantum yield (Φ) = 0.43ꢀ0.60; lifetime (τ) = 4.9ꢀ6.5 μs]
and amorphous films (Φ = 0.57ꢀ0.71; τ = 3.2ꢀ6.1 μs).
Conventional OLEDs containing 2 in the emitting layer
exhibited bright green luminescence with a current efficiency
of 65.3 cd/A and a maximum external quantum efficiency
of 21.3%.
2-bis(o-ditolylphosphino)benzene], and the use of these com-
plexes as dopants in OLEDs. A vapor-deposited OLED doped
with 2 exhibited a maximum EQE of 21.3%.
The bidentate bisphosphine ligand dtpb was synthesized by
addition of 2-methylphenylmagnesium bromide to 1,2-bis-
(dichlorophosphino)benzene in THF at 0 °C under argon.5
Complexes 1ꢀ3 were prepared in 75ꢀ95% yield by mixing a
suspension of CuX (X = Cl for 1, Br for 2, I for 3) in
dichloromethane with 1 equiv of dtpb. Single crystals of 1ꢀ3
suitable for X-ray analysis were obtained by layering of Et2O on
the surface of solutions of the complexes in CHCl3 (for 1) or
CH2Cl2 (for 2 and 3).
Single-crystal X-ray diffraction studies of 1ꢀ3 revealed mon-
omeric three-coordinate structures. The molecular structure of 2
is shown in Figure 1 as an example. The coordination geometries
of the copper centers in 1ꢀ3 are trigonal-planar; the sum of the
angles around the Cu(I) center are 359.66, 359.37, and 359.43°,
respectively. The o-methyl groups of dtpb are required for the
formation of three-coordinate complexes because 1,2-bis-
(diphenylphosphino)benzene (dppb), which lacks methyl
groups, forms only the halogen-bridged binuclear copper com-
plexes [Cu(μ-X)dppb]2 with CuX.2h This is probably because
the Cu2X2 diamond core in [Cu(μ-X)dtpb]2 would be highly
unstable as a result of steric hindrance of the o-methyl groups
located on the sides of the metal centers, causing the unusual
monomeric three-coordinate structures to be produced. The
introduction of a bulky substituent on the ligand appears to be
necessary to produce three-coordinate copper complexes. Re-
cently, emissive three-coordinate cooper(I) complexes with
bulky anions (monodentate amide ligands or N-heterocyclic
carbene ligands with bulky substituents) have been reported.6
The CuꢀP distances (2.2442ꢀ2.2768 Å) in 1ꢀ3 are almost
equal to those in [Cu(μ-X)dppb]2, indicating that the chelating
ability of dtpb is similar to that of dppb.
uch effort has been devoted to improving the external
M
quantum efficiency (EQE, photons per electron) of organic
light-emitting diodes (OLEDs). OLEDs doped with phosphorescent
metal complexes, such as cyclometalated iridium(III) complexes, can
exhibit high EQEs of >20%.1 This value is much higher than the
theoretical limit of 5% for fluorescent OLEDs. Over the past two
decades, phosphorescent and delayed-fluorescent tetrahedral copper-
(I) complexes containing two bidentate ligands (bisimine and/or
bisphosphine ligands) have received increasing attention as dopants
because of the low cost and stable supply of copper metal.2,4b
However, tetrahedral copper(I) complexes tend to display weak
emission because of excited-state distortion, which accelerates non-
radiative decay of the emissive excited state. According to earlier
studies,3 the use of sterically congested ligands affords a rigid
environment around the copper center in the complexes, leading to
effective suppression of the nonradiative processes caused by (i)
JahnꢀTeller distortion of emissive excited states with metal-to-ligand
charge transfer (MLCT) character, (ii) ligand dissociation in the
excited state, and (iii) solvent-induced exciplex formation. Recently,
Harkins and Peters reported {(PNP)CuI}2, a highly emissive bime-
tallic copper(I) complex wherein two tetrahedral CuI units are
bridged by the amido groups of the rigid, bulky PNP ligand
[PNPꢀ = bis(2-diisobutylphosphinophenyl)amido].4a An optimized
EQE of 16.1% was achieved for vapor-deposited OLEDs containing
the analogous complex {(PNP-tBu)CuI}2 [PNP-tBuꢀ = bis(2-
diisobutylphosphino-4-tert-butylphenyl)amido].4b
The absorption and emission spectra of dtpb and copper(I)
complexes 1ꢀ3 are shown in Figure 2. The absorption spectrum
of dtpb exhibits a broad, intense band at 289 nm (ε = 1.92 ꢁ 104
Mꢀ1 cmꢀ1), which is characteristic of an arylphosphine com-
pound. This band is assigned to a mixed transition of n f π* and
π f π*; the former is the transition of an electron from the lone-
pair orbital on phosphorus to an empty antibonding π* orbital on
Herein we describe the synthesis and photophysical properties of
Received: March 31, 2011
Published: May 17, 2011
(dtpb)CuIX [X = Cl (1), Br (2), I(3)], a simple three-coordinate
r
2011 American Chemical Society
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a phenyl ring, and the latter are transitions localized on a tolyl or
phenylene ring or those from a tolyl ring to a phenylene ring.7
Complexes 1ꢀ3 show more complicated spectra than dtpb; the
spectra possess a broad band with a maximum at 300ꢀ303 nm
[ε = (1.13ꢀ1.28) ꢁ 104 Mꢀ1 cmꢀ1] and a shoulder at ∼370 nm.
These two absorption bands are assigned to a mixed transition of
σ f π* and π f π* and an X f π*(dtpb) transition (X = Cl, Br, I),
respectively, analogous to those in related tetrahedral copper
complexes.2h,8
The photoluminescence properties of complexes 1ꢀ3 are
presented in Table 1. While solutions of dtpb do not luminesce at
293 K, complexes 1ꢀ3 emit intense green phosphorescence in
degassed CH2Cl2 solutions at 293 K with phosphorescence
quantum yields (Φ) of 0.43, 0.47, and 0.60, respectively.
The phosphorescence lifetimes (τ) of the three complexes were
measured after laser excitation at 355 nm; the phosphorescence
decay profiles are illustrated in the inset of Figure 2, from which τ
values of 4.9, 5.4, and 6.5 μs were calculated for 1ꢀ3, respec-
tively. The emission maxima (λmax) of 1ꢀ3 show the order 3 < 2
< 1, suggesting that λmax is affected by the ligand-field strength
(Iꢀ < Brꢀ < Clꢀ). Presumably the electronic nature of the triplet
excited state of 1ꢀ3 is influenced to some extent by Xꢀ
π*(dtpb) charge-transfer transitions.
f
The electrochemical properties of these complexes were
investigated by cyclic voltammetry (CV). Irreversible oxidation
waves at 407, 432, 468, and 443 mV vs Fcþ/Fc for dtpb and 1ꢀ3,
respectively, were observed. This result suggests that orbitals of
the Cu(I) atom and those of the halogen atom partially con-
tribute to the highest occupied molecular orbitals (HOMOs) of
1ꢀ3.
Natural transition orbital (NTO) analysis revealed that these
transitions can be described as single hole/electron pairs and
reproduce over 95% of the change in electron density upon
excitation. The hole (approximately HOMO) and electron
[approximately the lowest unoccupied molecular orbital
(LUMO)] distributions for the lowest triplet state of 2 at the
T1 optimized geometry are shown in Figure 3A. The hole
distribution of 2 is essentially halogen-based but also includes
the orbitals with σ character between the Cu(I) and P atoms; the
contributions from the Br, Cu, and P atoms to the hole distribu-
tion are 16, 28, and 31%, respectively. The electron distribution is
largely confined to the o-phenylene group in the dtpb ligand.
Excited-state calculations carried out for 1ꢀ3 indicated that
phosphorescence results from the transitions (σ þ X) f π*.
Another notable feature of 2 is that there is little structural change
between the ground and excited states. Furthermore, the single-
tꢀtriplet energy gaps are very small (2.63ꢀ4.51 kcal/mol) in all
of the optimized structures, as shown in Figure 3B.
Figure 1. (left) Molecular structure of 2. (center) ORTEP view of 2.
Thermal ellipsoids are drawn at the 50% probability level, and H atoms
have been omitted for clarity. (right) Core structure of 2.
Figure 2. Absorption and corrected emission spectra of dtpb (black), 1
(green), 2 (red), and 3 (purple). The inset shows the observed decay
profiles of the phosphorescence from 1ꢀ3 after excitation at 355 nm
at 293 K.
Figure 3. (A) NTO pairs for the lowest triplet excited state of 2 at the
T1 optimized geometry. (B) Optimized core structures of 2 in the
ground state (S0) and the singlet (S1) and triplet (T1) excited states and
the energy gap between the S1 and T1 levels in each optimized structure.
Table 1. Photophysical Data for 1ꢀ3 at 293 K
emission in CH2Cl2
emission in mCP (containing 10% complexes)
b
complex
λmax (nm)a
ΦPL
τ (μs)c
λmax (nm)
ΦPL
τ (μs)d
1
2
3
534
527
517
0.43
0.47
0.60
4.9
5.4
6.5
520
514
504
0.68
0.71
0.57
6.1 (4.6)
5.5 (3.9)
3.2 (1.4)
a PL maximum. b Absolute PL quantum yield. c PL lifetime. d PL lifetime is composed of two components. The faster component is shown in parentheses.
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containing 2 indicate a turn-on voltage of 2.7 V (Figure 4C).
A maximum current efficiency of 65.3 cd/A was achieved at 0.01
mA/cm2, which corresponds to a maximum EQE of 21.3%
(Figure 4B,D). This efficiency is comparable to that of cyclome-
talated iridium(III)-based devices, which currently set the stan-
dard for efficiency. The present work suggests that three-
coordinate copper(I) complexes are promising EL materials in
terms of efficiency and thermal stability. Detailed studies of the
photophysics of these three-coordinate copper complexes are in
progress.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data
b
(CIF), synthetic details, time-dependent density functional
theory calculations, and a table of experimental data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
osawa@postman.riken.jp
’ ACKNOWLEDGMENT
Figure 4. Properties of an OLED containing 2 as a dopant: (A) EL
spectrum; (B) dependence of EQE on current density; (C) IꢀV
characteristics; (D) dependence of current efficiency on current density.
The authors acknowledge Dr. Daisuke Hashizume for tech-
nical assistance with X-ray structural analysis.
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