A series of red-light-emitting ionic iridium complexes: Structures, excited state properties, and application in electroluminescent devices

Article abstract of DOI:10.1002/ejic.200701184

A series of ionic diiminoiridium complexes [Ir(piq-C∧N) ₂(L-N∧N)](PF₆) were prepared, where piq-C∧N is 1-phenylisoquinolinato and L-N∧N are bidentate N-coordinating ligands: 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (mbpym), 5,5′-bis(thiopen-2-yl)-2,2′-bipyridine (tbpyt), and 5,5′-bis(9,9-dioctylfluoren-2-yl)-2,2′-bipyridine (FbpyF). X-ray diffraction studies of [Ir(piq)₂(mbpym)](PF₆) revealed that the iridium center adopts a distorted octahedral geometry. All complexes exhibited intense and long-lived emission at room temperature. The substituents on the 2,2′-bipyridine moieties influence the photophysical and electrochemical properties. The excited states were investigated through theoretical calculations together with photophysical and electrochemical properties. It was found that the excited state of the [Ir(piq) ₂(FbpyF)](PF₆) complex can be assigned to a mixed character of ³LC (π_N∧N→π *_N∧N), ³MLCT, ³LLCT (π _C∧N→π*_N∧N), and ³LC (π_C∧N→π*_C∧N). In addition, the alkylfluorene-substituted complex, [Ir(piq)₂(FbpyF)](PF₆), had relatively high quantum efficiency and good film-forming ability, and it was expected to be a good candidate for lighting and display applications. A nondoped, single-layer device that incorporates this complex as a light-emitting layer was fabricated and red phosphorescence was obtained. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200701184

FULL PAPER
DOI: 10.1002/ejic.200701184
A Series of Red-Light-Emitting Ionic Iridium Complexes: Structures, Excited
State Properties, and Application in Electroluminescent Devices
Shu-Juan Liu,[a] Qiang Zhao,[a] Qu-Li Fan,*[a] and Wei Huang*[a]
Keywords: Excited states / Iridium / Ionic complexes / Phosphorescence
∧
A series of ionic diiminoiridium complexes [Ir(piq-C N)2(L-
electrochemical properties. It was found that the excited
state of the [Ir(piq)2(FbpyF)](PF6) complex can be assigned
∧
∧
N N)](PF6) were prepared, where piq-C N is 1-phenyliso-
quinolinato and L-N N are bidentate N-coordinating li-
∧
to a mixed character of 3LC (πN NǞπ*N N), 3MLCT, 3LLCT
∧
∧
(πC NǞπ*N N), and 3LC (πC NǞπ*C N). In addition, the alkyl-
fluorene-substituted complex, [Ir(piq)2(FbpyF)](PF6), had
relatively high quantum efficiency and good film-forming
ability, and it was expected to be a good candidate for light-
ing and display applications. A nondoped, single-layer de-
vice that incorporates this complex as a light-emitting layer
was fabricated and red phosphorescence was obtained.
∧
∧
∧
∧
gands: 2,2Ј-bipyridine (bpy), 4,4Ј-dimethyl-2,2Ј-bipyridine
(mbpym), 5,5Ј-bis(thiopen-2-yl)-2,2Ј-bipyridine (tbpyt), and
5,5Ј-bis(9,9-dioctylfluoren-2-yl)-2,2Ј-bipyridine (FbpyF). X-
ray diffraction studies of [Ir(piq)2(mbpym)](PF6) revealed that
the iridium center adopts a distorted octahedral geometry.
All complexes exhibited intense and long-lived emission at
room temperature. The substituents on the 2,2Ј-bipyridine
moieties influence the photophysical and electrochemical
properties. The excited states were investigated through
theoretical calculations together with photophysical and
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
full color application, saturated-red emission is indispens-
able. Therefore, it is especially necessary to fabricate and
investigate red-light emitting devices based on ionic Ir com-
plexes.
Introduction
Iridium (Ir) complexes are explored for a multitude of
optoelectronic applications, including lighting, display, and
light-emitting electrochemical cells (LEC), because of their
high performance.[1–6] In particular, ionic Ir complexes have
emerged as promising chromophoric materials for lighting
and display applications according to the reports of recent
studies.[7–15] Inert metal electrodes that are resistant to oxi-
dation in air can be used in these devices and the power
consumptions of these devices are low. Further improve-
ment of the stability of these devices can be expected due
to the excellent redox stability of charged Ir complexes.
Slinker et al.[7] first demonstrated efficient (luminous power
efficiency exceeds 10 lmW–1 at just –3 V) yellow electrolu-
minescence from a single-layer device based on an ionic Ir
complex. Subsequently, Plummer et al.[8] investigated the
characteristics of electrophosphorescence devices based on
ionic Ir complexes and obtained device with high efficiency
of 23 cdA–1. Recently, single-layer devices based on ionic Ir
complexes emitting green and blue-green light have also
been reported.[9,10] As one of the three primary colors for
The photophysical and electrochemical properties of Ir
complexes depend on the electronic structures of their li-
gands and can be manipulated by modifying the chemical
structures of the ligands. The excitation properties of Ir
complexes have especially experienced a blooming attrac-
tion in the past few years, because the species are extremely
promising to play the role of active components in organic
light-emitting diodes (OLEDs).[1,16–19] In ionic Ir com-
∧
∧
plexes, both N N and C N coordination are adopted and
the excited-state properties of them are very complicated an
involve ligand-centered (LC) excited state, metal-to-ligand
charge transfer (MLCT) transitions, and Ir–C σ bond-to-
ligand charge transfer (SBLCT) transitions. In recent years,
density functional theory (DFT) techniques have been re-
ceiving large acceptance for their prediction of the ground
and excited states in organometallic molecules besides pho-
tophysical and electrochemical properties. The application
of these approaches would provide the basis for the design
of ionic Ir complex light-emitting materials.
[a] Jiangsu Key Lab of Organic Electronics
& Information
In this paper, we prepared a series of red-light-emitting
ionic Ir complexes based on 2,2Ј-bipyridine (bpy) deriva-
tives and 1-phenylisoquinolinato (piq). The molecular
structure, theoretical calculation, and photophysical and
electrochemical properties were studied. The piq C N li-
gand was selected because it is an appropriate ligand for
Displays and Institute of Advanced Materials (IAM), Nanjing
University of Posts & Telecommunications (NUPT),
66 XinMoFan Road, Nanjing 210003, China
Fax: +86-25-8349 2333
E-mail: wei-huang@njupt.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
∧
Eur. J. Inorg. Chem. 2008, 2177–2185
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2177

S.-J. Liu, Q. Zhao, Q.-L. Fan, W. Huang
FULL PAPER
Ir complexes emitting red light.[3] The 4,4Ј-dimethyl-2,2Ј- plexes were synthesized according to the synthetic routes
bipyridine (mbpym) ligand contains methyl substituents, reported previously.[6,22–24] The dimeric [Ir(piq)2(µ-Cl)]2
which are weakly electron donating and are expected to in- precursor prepared as described previously[25] was used to
fluence the redox and photophysical properties of the corre- prepare all the complexes studied here. All complexes were
sponding complex. Thiophene substituents on the bpy moi- obtained by complexing the chlorido-bridged dimer with
ety also might facilitate charge transfer and increase the π- the corresponding bidentate N-coordinating ligand under
∧
conjugation length of the N N ligand, probably giving rise to mild conditions in dichloromethane/methanol (3:1). After
a redshift in the emission of the corresponding Ir complex. the reaction was complete, a fivefold excess of potassium
Alkylfluorene substituents on the bpy ligand are expected hexafluorophosphate was added into the resulting mixture.
–
to facilitate charge transfer, improve the ability of film for- The complexes were obtained as PF6 salts. For compari-
mation, and reduce triplet–triplet (T–T) and/or concentra- son, complex 1 was also prepared. All complexes obtained
tion quenching of the corresponding Ir complex. The effects were characterized by 1H NMR spectroscopy, matrix-as-
of the ligand structures on the photophysical properties of sisted laser desorption/ionization time-of-flight (MALDI-
the complexes were investigated. DFT techniques were ap- TOF) mass spectrometry, and elemental analysis. The sin-
plied as approaches to interpret the different behavior of gle-crystal X-ray analysis of complex 2 was also investi-
these substituents. The study of the photophysical proper- gated.
ties shows that [Ir(piq)2(FbpyF)](PF6) (complex 4) is a good
candidate for red-light-emitting materials with relatively
high quantum efficiency and good film-forming ability.
Crystal Structure of Complex 2
Therefore, we report the first nondoped, single-layer, red-
light-emitting OLED by using complex 4 as the active mate-
rial. Preliminary results indicate that ionic Ir complexes
offer promising opportunities in optoelectronic applica-
A single crystal of complex 2 was obtained from an ace-
tonitrile solution by slow evaporation of the solvent, and
its structure was investigated by X-ray crystallography
analysis. The complex crystallized in the monoclinic C2/c
tions.
space group with centrosymmetric cations and centrosym-
metric hexafluorophosphate anions. The perspective draw-
ing of the complex cation is depicted in Figure 2. The coor-
Results and Discussion
dination of the Ir atom is distorted octahedral and the
largest deviation is represented by the bite angle of the bi-
pyridine ligand at 75.8°, which is similar to that found in
ionic Ir complexes based on 2,2Ј-bipyridine reported pre-
viously.[26] The two Ir–C bonds are in a mutually cis ar-
rangement, and their high trans influence renders slightly
longer Ir–N bond lengths in the mbpym ligand [Ir(1)–N(2)
and Ir(1)–N(2)#1 2.146(5) Å] than in the cyclometalating
ligand [Ir(1)–N(1) and Ir(1)–N(1)#1 2.052(6) Å]. These fin-
Synthesis and Characterization of the Complexes
The formulas and abbreviations of complexes 1–4 are de-
picted in Figure 1. 5,5Ј-Dibromo-2,2Ј-bipyridine was pre-
pared by classical procedures[20,21] and oligomer diimine li-
gands 5,5Ј-bis(thiopen-2-yl)-2,2Ј-bipyridine (tbpyt) and
5,5Ј-bis(9,9-dioctylfluoren-2-yl)-2,2Ј-bipyridine
were synthesized by Suzuki coupling reactions. All com-
(FbpyF)
dings are commonly observed for related cyclometalated Ir
[27–31]
∧
∧
complex systems [Ir(C N)2(L-N N)](PF6).
Probably
Figure 1. The formulas and abbreviations of the ionic iridium com-
plexes.
Figure 2. Perspective view of the complex cation of complex 2 with
atomic numbering scheme.
2178
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2177–2185

A Series of Red-Light-Emitting Ionic Iridium Complexes
∧
∧
because of steric reasons, the two pyridine rings of 2,2Ј- and LLCT (πC NǞπ*N N). For complexes 1, 2, and 3, the
bipyridine are usually not coplanar.[28] However, the dihe- composition of HOMO-1 is similar to that of HOMO. The
dral angle was only 4.2° in complex 2, which was signifi- LUMO+1 are mainly localized on the isoquinolyl groups
cantly smaller than that of other complexes reported,[28] in- of the C N ligands and the phenyl groups of the C N li-
∧
∧
dicating that bulky groups on the bipyridine would increase gands also have some LUMO+1 distribution for complexes
3
∧
∧
the angle.
1–4, showing the possible existence of LC (πC NǞπ*C N
)
In the crystal packing, the metal complex cations and character because of the HOMO (or HOMO-1)Ǟ
PF6– are connected by weak C–H···F–P hydrogen bonds by LUMO+1 transitions for complex 1, 2, and 3. For complex
intra- and intermolecular C–H···F interactions involving 4, however, the HOMO-1 primarily resides on the fluorene
∧
aromatic or methyl C–H bonds. The average H···F distance groups of FbpyF instead of the Ir center and the C N li-
is about 2.60 Å.
3
∧
∧
gands. Therefore, the LC (πN NǞπ*N N) character, which
is infrequent in other complexes, is possible due to the
HOMO-1ǞLUMO transitions besides 3MLCT, 3LLCT,
and LC (πC NǞπ*C N) characters.
3
Theoretical Calculations
∧
∧
Time-dependent DFT (TDDFT) calculations were then
performed to estimate the corresponding transition energy
of complex 4. The calculated parameters for various lower-
lying transitions are listed in Table 2. On the basis of
Table 2 and UV/Vis absorption spectrum of complex 4 (see
Figure 4), it is reasonable to deduce that the major bands
at 400–470 nm receive contributions mainly from S2–S5,
which incorporate mainly a spin-allowed metal–ligand
charge transfer (1MLCT) and a πǞπ* intraligand transi-
tion. The lowest singlet state, that is, S1, is obscured in the
absorption spectra as a result of the overlap with other
bands and the much lower oscillator strength.[32] Moreover,
the lowest triplet state originates from HOMO Ǟ LUMO
and HOMO Ǟ LUMO+1, which apparently possesses a
To further investigate the nature of the excited state,
DFT calculations for these complexes were performed. The
calculated and experimental geometrical parameters of
complex 2 are summarized in Table 1. The overall geometri-
cal parameters of the calculated structures of the complex
are in quite good agreement with those of the experimental
values from X-ray diffraction. The dihedral angle of the two
pyridine rings was also calculated to be 3.7°, which agrees
with the corresponding experimental value of 4.2°.
Table 1. Comparison of calculated bond lengths [Å] and angles [°]
and experimental values from X-ray diffraction for complex 2.
Calculated
Experimental
3
3
3
∧
∧
mixed character of LC (πN NǞπ*N N), MLCT, LLCT
Ir(1)–C(15)
Ir(1)–C(15)#1
Ir(1)–N(1)#1
Ir(1)–N(1)
Ir(1)–N(2)
Ir(1)–N(2)#1
2.028
2.034
2.175
2.064
2.146
2.060
1.993(6)
1.993(6)
2.052(6)
2.052(6)
2.146(5)
2.146(5)
(πC NǞπ*N N), and 3LC (πC NǞπ*C N). The S0 Ǟ T1
∧
∧
∧
∧
transition is estimated to be at ca. 687 nm.
Table 2. Calculated singlet and triplet transitions for complex 4.
State
Excitation
Ecalcd. [eV] λcalcd. [nm]
(f)
C(15)–Ir(1)–C(15)#1
C(15)–Ir(1)–N(1)#1
C(15)#1-Ir(1)–N(1)#1
C(15)–Ir(1)–N(1)
C(15)#1-Ir(1)–N(1)
C(15)–Ir(1)–N(2)#1
N(1)#1-Ir(1)–N(1)
C(15)–Ir(1)–N(2)
C(15)#1-Ir(1)–N(2)
N(1)#1-Ir(1)–N(2)
N(1)–Ir(1)–N(2)
C(15)–Ir(1)–N(2)#1
C(15)#1-Ir(1)–N(2)#1
N(1)#1-Ir(1)–N(2)#1
N(1)–Ir(1)–N(2)#1
N(2)–Ir(1)–N(2)#1
86.6
96.0
78. 7
79.8
97.2
97.4
175.1
174.1
98.4
96.1
97.5
96.1
173.8
97.0
98.5
77.9
87.7(3)
93.2(2)
79.5(2)
79.5(2)
93.2(2)
98.5(2)
170.0(2)
172.1(2)
98.5(2)
92.73(19)
95.18(19)
98.5(2)
172.1(2)
95.18(19)
92.73(19)
75.8(3)
Singlet Excited State
1
2
HOMOǞLUMO (98%)
HOMO-4ǞLUMO (2%)
HOMO-1ǞLUMO (90%)
HOMO-3ǞLUMO (39%)
HOMO-2ǞLUMO (3%)
HOMOǞLUMO+1 (53%)
HOMO-3ǞLUMO (53%)
HOMOǞLUMO+1 (36%)
HOMOǞLUMO+2 (6%)
HOMO-3ǞLUMO (2%)
HOMOǞLUMO+1 (4%)
HOMOǞLUMO+2 (87%)
2.15
2.65
578
467
0.0005
0.9272
3
4
5
2.71
2.73
2.76
458
454
449
0.0147
0.0252
0.0468
Triplet Excited State
1
HOMO-1ǞLUMO (7%)
HOMOǞLUMO (89%)
1.81
687
0
The distributions of frontier molecular orbitals (HOMO-
1, HOMO, LUMO, and LUMO+1) were investigated. All
complexes have similar HOMO and LUMO distributions.
The HOMO primarily resides on the Ir center and the
Electrochemical Properties
The redox behavior of the ionic Ir complexes was investi-
∧
phenyl groups of the C N ligand. The isoquinolyl groups
∧
of the C N ligand also have some contributions to the gated by cyclic voltammetry (CV). The cyclic voltammog-
∧
HOMO. The LUMO is mainly located on the N N ligand. rams are shown in Figure 3. Complexes 1–4 exhibited an
Therefore, the excited states of the complexes can be as- irreversible process upon oxidation. Oxidation processes in
∧
signed to the mixed character of MLCT [dπ(Ir)Ǟπ*N N
]
Ir cyclometalated compounds have been assigned to metal-
Eur. J. Inorg. Chem. 2008, 2177–2185
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2179

S.-J. Liu, Q. Zhao, Q.-L. Fan, W. Huang
Energy levels [eV]
FULL PAPER
Table 3. Electrochemical properties of complexes 1–4.[a]
Ered [V]
Eox [V]
Eonset
Epc
Epa
∆Ep
Eonset
Epa
HOMO
LUMO
Eg
1
2
3
4
–1.58
–1.67
–1.44
–1.49
–1.71
–1.83
–1.48
–1.45
–1.62
–1.68
–1.35
–1.31
0.09
0.15
0.13
0.14
0.91
0.91
0.96
0.96
1.05
1.04
1.11
1.09
–5.71
–5.71
–5.76
–5.76
–3.22
–3.13
–3.36
–3.31
2.49
2.58
2.40
2.45
[a] Epa and Epc are the anodic peak potential and cathodic peak potential, respectively; Eg is the bandgap energy; ∆Ep = Epa – Epc.
centered orbitals and/or to Ir–C σ-bond orbitals.[33,34] It is smaller than that of other complexes, probably due to the
assumed that pure metal-centered oxidation is reversible, thiophene substituents, which are well-known materials
and irreversibility increases as the contribution to the with low energy gaps.
HOMO of the cyclometalating phenyl(s) increases.[35] On
such a basis, we assigned the irreversible oxidation pro-
cesses to orbitals that received a strong contribution from
the Ir–C σ-bond orbitals. Upon reduction, a reduction wave
and the corresponding reoxidation wave could be observed
for each compound. These waves were ascribed to reduction
of the diimines instead of piq, as the reduction of the latter
is well-known to occur at a more-negative potential.[28]
Therefore, SBLCT [σ(Ir–C) bondǞπ*(diimine)] transitions
Electronic Absorption
The UV/Vis absorption spectra of complexes 1–4 in air-
equilibrated dichloromethane (CH2Cl2) solution are shown
in Figure 4 and the data are listed in Table 4. For complexes
1 and 2, the absorption spectra show broad bands from 270
to 600 nm, and the most intense bands are at λ Ͻ 300 nm
and moderately intense bands are at longer wavelengths,
which extend far within the visible region. The bands be-
tween 250 and 300 nm can be assigned to spin-allowed sing-
let ligand-centered (1LC) transitions. According to the pre-
vious papers, the absorption bands around 350–470 nm are
attributed to the spin-allowed singlet charge-transfer (1CT)
transitions.[37] The absorption features at the lowest energy
(λ Ͼ 470 nm) are likely to be assigned to spin-forbidden
triplet charge-transfer (3CT) transitions. These singlet-to-
triplet transitions become partially allowed, as the presence
of the heavy Ir center renders a strong spin–orbit coupling.
Unlike complexes 1 and 2, there are intense absorption
bands around 400 nm besides the absorption bands be-
3
and MLCT [dπ(Ir)Ǟπ*(diimine)] transitions are possible
as well.
Figure 3. Cyclic voltammograms of complexes 1–4.
According to the onset potentials of the oxidation and
reduction of the CV measurements, the HOMO/LUMO en-
ergy levels of complexes 1–4 were estimated on the basis of
the reference energy level of ferrocene: HOMO/LUMO =
–(4.8 + Eonset) eV.[36] The related data are summarized in
Table 3. It could be found that the reduction of complex 2
occurred at more-negative potentials and it had a higher
LUMO than other complexes probably because of the elec-
tron-donating properties of the methyl group. We also
Figure 4. UV/Vis absorption spectra of iridium complexes in
found that the energy gap of complex 3 was evidently CH2Cl2 solution at 10–5 .
Table 4. Absorption data of complexes.
Complexes
Absorption wavelengths [nm] (logarithmic absorption coefficients, log [ε])
1
2
3
4
290(4.7), 342(4.3), 354(4.3), 378(4.1), 439(3.9), 477(3.6)
292(4.8), 340(4.4), 358(4.3), 380(4.2), 442(3.9), 478(3.7)
291(4.7), 334(4.6), 359(4.6), 384(4.7), 479(3.6)
290(4.8), 341(4.5), 359(4.5), 403(4.7), 479(3.5)
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Eur. J. Inorg. Chem. 2008, 2177–2185

A Series of Red-Light-Emitting Ionic Iridium Complexes
tween 250 and 300 nm that receive contributions mainly
from LC involving piq in the absorption spectra of com- broad structureless bands, which are similar to the charac-
The spectra measured at 298 K of complexes 1–4 are
1
3
plexes 3 and 4. They can be attributed to the intense spin- ter of CT emission state because phosphorescence spectra
allowed π–π* transitions localized on the oligo(bipyridine) from the 3LC(3π–π*) state display vibronic progressions,
3
units (tbpyt and FbpyF). The visible-region CT bands whereas those from the CT state are broad and featureless
should be contributed from even SBLCT [σ(Ir–C)Ǟπ*(di- according to the previous investigations.[41,42] In addition,
imine)] transitions, or simplified piq-to-bpy charge transfer for the phosphorescence spectra at 77 K, well-resolved
(LLCT) transitions besides Ir-to-bpy MLCT transitions be- vibronic progressions between two major peaks are not ob-
cause it has been verified by DFT calculations and electro- served for all complexes studied here. Therefore, the lowest
chemical properties that the HOMO can be significantly excited states of these complexes are, in a first approxi-
3
delocalized over the cyclometalating ligands, with a large mation, probably the CT state. All the data related to the
participation also from the Ir–C σ bond orbital for Ir com- emission spectra of the complexes are summarized in
plexes.[26,38] The absorption characteristics of these com- Table 5.
plexes are similar to those of related cyclometalated Ir com-
plexes.[39]
Table 5. Photoluminescence properties of complexes 1–4.
Complex Medium (T [K])
λPL,max [nm]
Φem
τ [µs]
1
2
3
4
CH2Cl2 (298)
CH3CN (298)
Glass (77)
CH2Cl2 (298)
CH3CN (298)
Glass (77)
CH2Cl2 (298)
CH3CN (298)
Glass (77)
CH2Cl2 (298)
CH3CN (298)
Glass (77)
587
588
580
585
594
590
604
601
594
588
591
577
0.089
0.11
Luminescent Properties
Figure 5 shows the phosphorescence spectra of the Ir
complexes observed at 298 and 77 K in different solutions.
Upon photoexcitation at 325 nm, complexes 1–4 display in-
tense and long-lived emissions in dilute deaerated CH2Cl2
and CH3CN solutions at room temperature as well as in
EtOH/MeOH (4:1) rigid matrix at 77 K. For cyclometalated
Ir complexes, in general, phosphorescence mainly originates
from 3LC (3π–π*) and 3CT (including 3MLCT and 3LLCT)
excited states, so the wavefunction of the excited triplet
state, ΦT, responsible for phosphorescence is principally ex-
pressed as [Equation (1)]:
0.097
0.035
0.114
2.14
1.69
2.23
3
3
If MLCT and SBLCT states are emitting states, one
would therefore expect that freezing of the solvent and the
increase in the solvent polarity should cause obvious blue-
shifts and redshifts, respectively, in the PL spectra. By low-
ering the temperature, the rigidity of the medium increases
ΦT = aΦ(3LC)T +bΦ(3CT)T
(1)
where a and b are the normalized coefficients and (3LC)T
and (3CT)T are the wavefunctions of the LC and the CT
triplet excited states, respectively.[3] Equation (1) implies
that the excited triplet states of the Ir complexes mainly are
a mixed character of (3LC)T and (3CT)T.
3
3
and thus the MLCT and 3 LLCT, being less stabilized by
3
the lack of solvent mobility, rise in energy. By increasing
the solvent polarity, 3MLCT and 3LLCT will be more stable
and lower in energy. If the 3MLCT or 3SBLCT state is close
3
enough to the LC state, however, a possible inversion of
the lowest excited state can be observed upon changing the
media. On such a basis, we investigated the PL spectra of
complexes reported here at 77 K for comparison. We can
find that this shift was much less pronounced (about
10 nm), and there is almost no blueshift for complex 2. The
absence of a significant blueshift in the emission from
room-temperature fluid solutions to low-temperature rigid
3
glass suggests that the LC character, which is much less
sensitive to changes in the surrounding media, contributes
greatly to the excited state of the complexes besides the 3CT
character.[32–45] A similar behavior was observed for the
[Ir(ppy)2bpy]+ complex.[46] The emission spectra of com-
plexes in CH3CN solutions at 298 K are shown in Figure 5.
The redshift in the PL spectra of complexes 1–4 was not
as conspicuous as that reported previously.[47,48] Thus, as
3
discussed above, it was further indicated that the CT state
3
of the complexes might be close enough to the LC state,
Figure 5. Normalized phosphorescence spectra of the ionic iridium
complexes in CH2Cl2 (–––) and CH3CN (– – –) solutions at 298 K
and in EtOH/MeOH (4:1) rigid matrix at 77 K (-·-·-) excited at
325 nm.
and thereby the excited states received much contribution
3
3
from the CT character mixing strongly with the LC char-
acter for complexes 1–4.[49]
Eur. J. Inorg. Chem. 2008, 2177–2185
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2181

S.-J. Liu, Q. Zhao, Q.-L. Fan, W. Huang
FULL PAPER
Substituents on the bipyridine ligand with electron-do-
nating properties may destabilize the π* orbitals of the di-
imine ligand, which thereby increases the emission energy
of the corresponding complex.[25] Therefore, we designed
and synthesized complex 2. The emission energy increased
slightly relative to that of complex 1, which was in line with
the discussion above, and its emission spectrum showed a
very slight blueshift, probably owing to the weak electron-
donating properties of the methyl group. The PL spectrum
of the Ir complex with thiophene substituents on the di-
imine ligands (complex 3) exhibited a redshift relative to
that of complex 1, probably because the thiophene rings
lowered the energy level of the π* orbitals of the diimine
ligands as a result of electronic effects. For complex 4, a
relatively high quantum efficiency (Φem) in comparison to
other complexes is realized.[50] Moreover, complex 4 also
exhibited good film-forming ability because of the alkyl-
fluorene substituents. From this, it can be seen that the
emission properties of all the complexes studied here agree
well with the electrochemical properties and the results we
expected initially.
Figure 6. Electroluminescence spectrum of complex 4. Device
structure: ITO/complex 4/Al.
able to that of related green[9] and blue-green[10] emitting
devices reported previously. In fact, the factors influencing
the performance of devices based on ionic Ir complexes are
complicated,[13] and the investigation of factors influencing
the performance of devices based on ionic Ir complexes is
an interesting field.[7–10] Therefore, further studies and opti-
mization of the devices by using these complexes as emis-
sion layers are in progress and efficient red-light-emitting
devices can be expected.
Emission lifetime (τ) monitored at the peak emission was
recorded in degassed CH2Cl2 solutions at room temperature
and analyzed by a single-exponential decay function. The
emission lifetimes observed for complexes 1–4 at room tem-
perature fall in the microsecond to submicrosecond range
(0.11–2.23 µs) and are significantly reduced in the presence
of oxygen, which indicates the phosphorescence nature of
the emission.[27]
Conclusions
A series of luminescent cyclometalated diiminoiridium
complexes with substituted bipyridine ligands were synthe-
sized, and their theoretical calculations and photophysical
Electrophosphorescence of Complex 4
It is noteworthy that the alkylfluorene substituents can and electrochemical properties were investigated. X-ray dif-
reduce T–T and/or concentration quenching, resulting in fraction studies of complex 2 revealed the coordination of
relatively high quantum efficiency of complex 4, and the the iridium atom was distorted octahedral. DFT techniques
ability of film-forming can be improved as well. In addition, can be applied successfully for the study of the excited state
the alkylfluorene substituents will improve the compatibil- of the complex. This will facilitate the design of new ligands
ity between complexes and host when the corresponding for light-emitting Ir complexes. All complexes exhibited in-
complexes are used in the host–guest system. Therefore, tense and long-lived emission. According to theoretical cal-
complex 4 may be a good candidate for use in electro- culations and electrochemical and photophysical properties,
phosphorescent devices.[51] Here, a single-layer, non-doped the origin of the emission was assigned to 3CT mixing
3
device using complex 4 as an emission layer was fabricated strongly with LC transitions. Phosphorescence shifted to
by spin-coating, and their properties were investigated. The red by introducing thiophene substituents onto the bipyr-
structure of the device was ITO/complex 4/Al. The electro- idine core. Complex 4 is a good candidate for red-light-
luminescence (EL) spectrum is shown in Figure 6, and it emitting materials with relatively high quantum efficiency
exhibits intense broad bands from 600–640 nm. The CIE and good film-forming ability. Preliminary results of a sin-
coordinates (x = 0.62 and y = 0.38) of the EL emission gle-layer, nondoped device using complex 4 as an emission
were evaluated from the EL spectra, and they were in the layer indicate that ionic Ir complexes offer promising op-
range of red. To the best of our knowledge, this is the first portunities in optoelectronic applications.
report of a red phosphorescent OLED that is based on an
ionic Ir complex.
Experimental Section
The turn-on voltage of the device was 7.8 V. The current
density and luminance at 15 V were 589.2 mAcm–2 and
94 cdm–2, respectively. The efficiencies of the devices are
not satisfying, because the structure of the fabricated device
was not optimized; however, the luminance of the red phos-
All manipulations involving air-sensitive reagents were performed
under an atmosphere of dry nitrogen. The solvents (THF, toluene)
were purified by routine procedures and distilled under an atmo-
sphere of dry nitrogen before use. All reagents, unless otherwise
phorescence for a single-layer, nondoped device is compar- specified, were purchased from Aldrich, Across, and Lancaster and
2182 Eur. J. Inorg. Chem. 2008, 2177–2185
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

A Series of Red-Light-Emitting Ionic Iridium Complexes
were used as received. 5,5Ј-Dibromo-2,2Ј-bipyridine (1) and 9,9-
dioctylfluoren-2-boronic acid (2) were synthesized as previously de-
scribed.[52]
C68H88N2 (933.44): C 87.50, H 9.50, N 3.00; found C 87.43, H
9.30, N 2.95.
General Procedure for the Synthesis of Iridium Complexes:
IrCl3·3H2O (5.52 mmol) and piq (11.04 mmol) were heated in a 2-
ethoxyethanol/water (3:1) under a nitrogen atmosphere. This slurry
was heated at 110 °C for 24 h. After cooling to room temperature,
the precipitate was filtered off and washed with water and ethanol
to get red of the solid cyclometalated Ir µ-chlorido-bridged dimer.
CH2Cl2 and methanol (2:1) were added to the mixture of the cyclo-
Phenylboronic Acid (3): To a solution of bromobenzene (15.0 g,
10 mL, 0.10 mol) in THF (50 mL) at –78 °C was dropwise added
n-butyllithium (1.6  in hexane, 65.5 mL, 0.21 mol). After 45 min,
trimethyl boronate (24.7 g, 0.24 mmol) was added by syringe. Then,
the resulting mixture was warmed to room temperature slowly and
stirred overnight. HCl (2 ) was added to the stirred solution whilst
the temperature of the solution was maintained at 0 °C for 1 h. The
organic layer was separated, and the water layer was extracted with
diethyl ether (3ϫ). The combined ether layer was washed with brine
(2ϫ) and then dried with anhydrous MgSO4. The solvent was evap-
orated under reduced pressure, and the crude product was slowly
dropped into hexane to obtain a white solid (6.51 g, 53%). 1H
NMR (400 MHz, CDCl3): δ = 8.24 (d, J = 6.8 Hz, 2 H), 7.74 (d,
J = 6.8 Hz, 1 H), 7.60 (t, J = 7.2 Hz, 1 H), 7.40–7.53 (m, 3 H)
ppm.
∧
metalated Ir µ-chlorido-bridged dimer (0.16 mmol) and N N li-
gands (0.32 mmol). The reaction mixture was heated at reflux for
4 h. After cooling to room temperature, a fivefold excess amount
of KPF6 was added, and the mixture was stirred for another 1 h.
The solvent was removed, and the solid was dissolved in CH2Cl2
again. The precipitate was filtered off and methanol was layered
on the filtrate. Red crystal of the complexes can be recrystallized
from the solution.
1
[Ir(piq)2(bpy)](PF6): Yield: 80%. H NMR (400 MHz, CDCl3): δ =
1-Phenylisoquinoline (piq) (4): Tetrakis(triphenylphosphane)palla-
dium (0.43 g, 0.37 mmol) was added to a mixture of 1-chloroisoqui-
noline (1.85 g, 11.50 mmol), 3 (1.4 g, 11.5 mmol), toluene (50 mL),
ethanol (25 mL), and an aqueous sodium carbonate solution (2 ,
25 mL) under vigorous stirring. The mixture was stirred at 70 °C
for 24 h under a nitrogen atmosphere. After cooling to room tem-
perature, the reaction mixture was poured into water and extracted
with ethyl acetate. The organic layer was washed with brine several
times, and the solvent was then evaporated. The product thus ob-
tained was purified by silica gel column chromatography (hexane/
ethyl acetate, 9:1) to yield a white solid (5.0 g, 84%). 1H NMR
(400 MHz, CDCl3): δ = 8.61 (d, J = 5.6 Hz, 1 H), 8.10 (d, J =
8.4 Hz, 1 H), 7.87 (d, J = 8.4 Hz, 1 H), 7.63–7.72 (m, 4 H), 7.49–
7.56 (m, 4 H) ppm. C15H11N (205.25): calcd. C 87.77, H 5.40, N
6.82; found C 87.72, H 5.30, N 6.69.
8.94 (m, 2 H), 8.71 (d, J = 8.0 Hz, 2 H), 8.27 (d, J = 8.0 Hz, 2 H),
8.15 (t, J = 8.0 Hz, 2 H), 7.90 (m, 2 H), 7.76 (m, 6 H), 7.37 (m, 6
H), 7.12 (t, J = 8.0 Hz, 2 H), 6.90 (t, J = 7.6 Hz, 2 H), 6.29 (d, J
= 7.6 Hz, 2 H) ppm. C40H28F6IrN4P (901.86): C 53.27, H 3.13, N
6.21; found C 53.15, H 3.24, N 5.96. HRMS (MALDI-TOF):
calcd. for C40H28IrN4 [M – PF6] 756.9; found 755.1.
1
[Ir(piq)2(mbpym)](PF6): Yield: 78%. H NMR (400 MHz, CDCl3):
δ = 8.93 (m, 2 H), 8.57 (s, 2 H), 8.25 (d, J = 8.0 Hz, 2 H), 7.88–
7.93 (m, 2 H), 7.73–7.79 (m, 4 H), 7.53 (d, J = 5.6 Hz, 2 H), 7.38
(m, 4 H), 7.10 (m, 4 H), 6.88 (t, J = 7.2 Hz, 2 H), 6.29 (d, J =
6.4 Hz, 2 H), 2.59 (s, 6 H) ppm. C42H32F6IrN4P (929.91): C 54.25,
H 3.47, N 6.02; found C 54.07, H 3.49, N 5.96. HRMS (MALDI-
TOF): calcd. for C42H32IrN4 [M – PF6] 785.2; found 785.9.
1
[Ir(piq)2(tbpyt)](PF6): Yield: 53%. H NMR (400 MHz, CDCl3): δ
= 8.94 (m, 2 H), 8.73 (d, J = 8.8 Hz, 2 H), 8.29 (m, 4 H), 7.93 (m,
2 H), 7.76–7.83 (m, 6 H), 7.48 (d, J = 6.4 Hz, 2 H), 7.40 (d, J =
6.4 Hz, 2 H), 7.34 (d, J = 4.4 Hz, 2 H), 7.21 (t, J = 7.2 Hz, 2 H),
7.12 (m, 2 H), 6.96–7.04 (m, 4 H), 6.34 (d, J = 6.8 Hz, 2 H) ppm.
C48H32S2F6IrN4P (1066.11): C 54.08, H 3.03, N 5.26; found C
5,5Ј-Bis(thiophen-2-yl)-2,2Ј-bipyridine (5): n-Butyllithium (1.6  in
hexane, 23 mL, 74 mmol) was added to a stirred solution of 2-bro-
mothiophene (11.4 g, 70 mmol) in anhydrous diethyl ether
(500 mL) at –78 °C under a nitrogen atmosphere. After 30 min, tri-
methyl boronate (10.9 g, 105 mmol) was added. The reaction was
stirred at –78 °C for 4 h and then warmed to ambient temperature.
After 15 h, the reaction was shaken with hydrochloric acid (1.2 ,
200 mL), and the ether phase was separated and extracted with
aqueous sodium hydroxide (1 , 4ϫ50 mL). The combined aque-
ous phase was then filtered to remove trace amounts of solid and
then acidified to pH = 1 at 0 °C with concentrated hydrochloric
acid. The resulting precipitate was filtered, washed with hydrochlo-
ric acid (10–2 ), and dried in vacuo to yield the thiophen-2-yl-
boronic acid as a white powder (7.0 g, 78%). The synthesis method
of 5 was similar to that of 4. 1H NMR (400 MHz, CDCl3): δ =
8.95 (m, 2 H), 8.43 (d, J = 8.0 Hz, 2 H), 8.00 (dd, J = 8.4 Hz, J =
2.4 Hz, 2 H), 7.40 (m, 4 H), 7.16 (m, 2 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 154.31, 146.24, 140.37, 133.70, 130.24,
128.38, 126.16, 124.32, 120.91 ppm. C18H12S2N2 (320.43): C 67.47,
H 3.77, N 8.74; found C 67.42, H 3.56, N 8.94.
53.91,
H 2.95, N 5.17. HRMS (MALDI-TOF): calcd. for
C48H32S2IrN4 [M – PF6] 921.1; found 921.9.
[Ir(piq)2(FbpyF)](PF6): Yield: 66%. 1H NMR (400 MHz, CDCl3):
δ = 8.99 (m, 2 H), 8.89 (m, 2 H), 8.48 (m, 2 H), 8.37 (d, J = 8.0 Hz,
2 H), 8.06 (m, 2 H), 7.93 (m, 2 H), 7.79 (m, 4 H), 7.70 (m, 4 H),
7.60 (m, 2 H), 7.32–7.44 (m, 10 H), 7.25 (m, 2 H), 7.12 (s, 2 H),
6.99 (t, J = 7.6 Hz, 2 H), 6.43 (d, J = 7.6 Hz, 2 H), 1.96 (m, 8 H),
1.02 (m, 40 H), 0.77 (t, J = 7.2 Hz, 12 H), 0.51 (m, 8 H) ppm.
C98H108F6IrN4P (1679.11): C 70.01, H 6.48, N 3.34; found C 69.79,
H 6.69, N 3.24. HRMS (MALDI-TOF): calcd. for C98H108IrN4
[M – PF6] 1534.2; found 1534.5.
UV/Vis absorption spectra were recorded with a Shimadzu 3000
UV/Vis/NIR spectrophotometer. NMR spectra were recorded with
a Mercury Plus 400 MHz NMR spectrometer. Elemental analyses
were performed with a Vario EL III O-Element Analyzer system.
MALDI-TOF experiments were carried out by using a Shimadzu
AXIMA-CFRTM plus matrix-assisted laser desorption/ionization
time-of-flight mass spectrometer (Kratos Analytical, Manchester,
U. K.). Photoluminescence spectra were measured with an Edin-
burgh LFS920 fluorescence spectrophotometer. Emission lifetime
was recorded with a single-photon counting spectrometer from
Edinburgh Instruments (FLS920) with a hydrogen-filled pulse lamp
as the excitation source. The data were analyzed by iterative convol-
ution of the luminescence decay profile with the instrument re-
5,5Ј-Bis(9,9-dioctylfluoren-2-yl)-2,2Ј-bipyridine (6): The synthesis
method was similar to that of 4. 1H NMR (400 MHz, CDCl3): δ =
9.03 (m, 2 H), 8.55 (d, J = 8.4 Hz, 2 H), 8.12 (d, J = 8.4 Hz, 2 H),
7.82 (d, J = 7.6 Hz, 2 H), 7.75 (d, J = 8.0 Hz, 2 H), 7.65 (m, 4 H),
7.36 (m, 6 H), 2.02 (m, 8 H), 1.06–1.2 (m, 40 H), 0.81 (t, J = 7.2 Hz,
12 H), 0.68 (m, 8 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
154.46, 151.76, 151.01, 147.78, 141.35, 140.39, 136.88, 136.26,
135.12, 127.38, 126.87, 125.88, 122.93, 121.31, 120.87, 120.28,
119.93, 55.23, 40.36, 31.76, 30.01, 29.21, 23.80, 22.60, 14.09 ppm.
Eur. J. Inorg. Chem. 2008, 2177–2185
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2183

S.-J. Liu, Q. Zhao, Q.-L. Fan, W. Huang
FULL PAPER
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This work was financially supported by the Nanjing University of
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2177–2185

A Series of Red-Light-Emitting Ionic Iridium Complexes
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Received: November 3, 2007
Published Online: March 27, 2008
(Since its publication in Early View,
a few minor changes have been made.)
Eur. J. Inorg. Chem. 2008, 2177–2185
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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