Cationic iridium complexes with intramolecular π-π Interaction and enhanced steric hindrance for solid-state light-emitting electrochemical cells

Article abstract of DOI:10.1021/ic300673k

Cationic iridium complexes incorporated 4,5-diaza-9,9'-spirobifluorene as NWN ancillary ligands, in which one (2) or two (3) phenyl groups were introduced onto 4,5-diazafluorene to afford intraligand π-π interactions. The X-ray crystal structures of complexes 2 and 3 show that the pendant phenyl ring forms strong intramolecular face-to-face π-stacking with the difluorophenyl ring of the cyclometalated ligand with distances of 3.38 for complex 2 and 3.40 and 3.46 for complex 3. This π-π stacking interaction minimizes the expansion of the metal-ligand bonds in the excited state, resulting in a longer device lifetime in the light-emitting electrochemical cell (LEC) devices.

Full text of DOI:10.1021/ic300673k

Article
pubs.acs.org/IC
Cationic Iridium Complexes with Intramolecular π−π Interaction and
Enhanced Steric Hindrance for Solid-State Light-Emitting
Electrochemical Cells
Hsiao-Fan Chen,† Wen-Yi Hung,*,‡ Shou-Wei Chen,‡ Ting-Chih Wang,† Shih-Wei Lin,† Shu-Hua Chou,†
Chih-Teng Liao,§ Hai-Ching Su,*,§ Hsiao-An Pan,† Pi-Tai Chou,*,† Yi-Hung Liu,†
and Ken-Tsung Wong*,†
†Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
‡Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 20224, Taiwan
§Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan 71150, Taiwan
S
* Supporting Information
ABSTRACT: Cationic iridium complexes incorporated 4,5-diaza-9,9′-spirobifluorene as
N∧N ancillary ligands, in which one (2) or two (3) phenyl groups were introduced onto 4,5-
diazafluorene to afford intraligand π−π interactions. The X-ray crystal structures of
complexes 2 and 3 show that the pendant phenyl ring forms strong intramolecular face-to-
face π-stacking with the difluorophenyl ring of the cyclometalated ligand with distances of
3.38 Å for complex 2 and 3.40 and 3.46 Å for complex 3. This π−π stacking interaction
minimizes the expansion of the metal−ligand bonds in the excited state, resulting in a longer
device lifetime in the light-emitting electrochemical cell (LEC) devices.
device lifetime can be significantly improved by increasing the
hydrophobicity of the complex by introducing peripheral bulky
aromatic groups. The pendant bulky group forms a face-to-face
π−π stacking with the metalated ring of the ligand and thus
minimizes the expansion of the metal−ligand bonds in the
excited state, as well as precludes the ligand exchange by foreign
substances such as water. With such a molecular design, the
device lifetime can significantly reach to thousands of hours.5
Excited state self-quenching and triplet−triplet annihilation
within the solid film are well-known for devastating photo-
luminescent efficiency and the device lifetime due to close
intermolecular packing. An efficient way to circumvent the
close packing motif is to introduce steric bulky ligands.
Therefore, the nonradiative pathways can be largely suppressed
with increased intermolecular distance.6 We have realized
highly efficient LECs based on cationic iridium complexes with
enhanced steric hindrance provided by bulky 4,5-diaza-9,9′-
spirobifluorene (sp) ligand.7 The spiro-configured structure of
sp ligand has a substantial effect in preventing the
intermolecular interaction, as indicated by the closest Ir-to-Ir
distance larger than 8.62 Å.8 To achieve robust cationic iridium
complexes constructed by intramolecular π−π interaction with
enhanced steric hindrance, in this contribution we introduced
INTRODUCTION
■
LECs (light-emitting electrochemical cells) possess several
advantages over conventional OLEDs, such as compatibility
with air-stable electrodes (e.g., Ag and Au) and single emissive
layer which can be easily conducted from a cost-effective
solution process.1 The turn-on voltages of LEC devices are
generally low and approximately equal to the measured optical
band gaps of the emissive materials (close to Eg). The low
operating voltages are beneficial for the device power
efficiencies. In addition to polymer-based LECs, the use of
small-molecule ionic transition metal complexes (iTMCs) in
LECs have attracted much attention in recent years.2,3 In such
devices, no ion-conducting material is needed because these
metal complexes are intrinsically ionic. Furthermore, higher
electroluminescence efficiencies are expected owing to the
phosphorescent nature of the metal complexes.
Although iTMC LECs possess several promising advantages,
there are still two important barriers that limit their practical
application, that is, poor device stability (expressed as t1/2, the
time for the brightness of the device to decay from the
maximum to half of the maximum) and long turn-on time
(defined as the time required to reach a brightness of 1 cd
cm−2). The intrinsic instability of iTMC is attributed to a water-
assisted ligand exchange reaction during the device operation,
subsequently forming a new complex for efficient luminance
quenching.4 Recently, Bolink et al. demonstrated that the
Received: March 31, 2012
Published: October 26, 2012
© 2012 American Chemical Society
12114
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121

Inorganic Chemistry
Article
Scheme 1. Synthetic Pathways and Structures of phsp, diphsp, and Complexes 1−3
spectrometer. Redox potentials of all complexs were determined by
cyclic voltammetry (CV) at a scan rate of 100 mV/s in acetonitrile
solutions (1.0 mM). A glassy carbon electrode and a platinum wire
were used as the working electrode and the counter electrode,
respectively. All potentials were recorded versus the Ag/AgCl
(saturated) reference electrode and calibrated with the ferrocene/
ferrocenium redox couple. Oxidation CV was performed using 0.1 M
tetra-n-butylammonium hexafluorophosphate (TBAPF6) in acetoni-
trile as the supporting electrolyte. For reduction CV, 0.1 M of tetra-n-
butylammonium perchlorate (TBAP) in acetonitrile was used as the
supporting electrolyte. Cyclometalated Ir(III) dichlorobridged dimer,
bis-(μ)-chlorotetrakis(1-(4,6-difluorophenyl)-pyrazolato-C2,N)-
diiridium(III), was synthesized by a literature procedure.9 4,5-Diaza-
9,9′-spirobifluorene (sp)7 and complex 12c were obtained according to
our previous publication.
one or two pendant phenyl group at 3 and 6 positions of sp to
give 3-phenyl-4,5-diaza-9,9′-spirobifluorene (phsp) and 3,6-
diphenyl-4,5-diaza-9,9′-spirobifluorene (diphsp), respectively.
Furthermore, in order to obtain better intramolecular cofacial
π−π interaction, the electron-deficient cyclometalated C∧N
ligand, 1-(2,4-difluorophenyl)pyrazole (dfppz), was selected for
π-acceptor. The cationic iridium complexes incorporating phsp
([Ir(dfppz)2(phsp)]+(PF6−), 2) and diphsp ([Ir-
−
(dfppz)2(diphsp)]+(PF6 ), 3) ligands were synthesized accord-
ing to the reported synthetic route.8 Complex without pendant
phenyl group ([Ir(dfppz)2(sp)]+(PF6 ), 1) was also synthe-
−
sized for comparison (Scheme 1). The distances of the face-to-
face π−π interactions were measured to be 3.38 Å for complex
2 and 3.40 and 3.46 Å for complex 3. This π−π stacking
interaction minimizes the expansion of the metal−ligand bonds
in the excited state, resulting in a longer device lifetime in the
LEC devices, wherein the device based on 2 showed a 4-fold
lifetime enhancement as compared to that of the device based
on 1.
Synthesis of 3-Phenyl-4,5-diaza-9,9′-spirobifluorene (phsp).
A mixture of 4,5-diaza-9,9′-spirobifluorene (2.5 g, 7.86 mmol) and
diethyl ether (400 mL) was stirred at room temperature, and
phenyllithium (9.83 mL, 19.65 mmol, 2 M) was added in one portion.
The reaction was stirred for 16 h, quenched with water, and extracted
with diethyl ether. The organic layer was then concentrated and
dissolved in CH2Cl2 (300 mL). Activated MnO2 (4.78 g, 55 mmol)
was added to the solution and stirred for 3 h. The mixture was filtered
off through and successively washed with CH2Cl2 (3 × 100 mL),
CHCl3 (3 × 100 mL), and EtOAc (3 × 100 mL) to afford ligand phsp
EXPERIMENTAL SECTION
■
1
General Experiments. H and 13C NMR spectra of compounds
were collected on a 400 MHz spectrometer at room temperature.
UV−vis absorption spectra were recorded on a spectrophotometer
(Hitachi U2800A). PL spectra were measured with a fluorescence
spectrophotometer (Hitachi F4500). Photoluminescence quantum
yields (PLQYs) for solution and thin-film samples were determined
with a calibrated integrating sphere system (Hamamatsu C9920).
Phosphorescence lifetime measurements were performed with an
Edinburgh FL 900 photon-counting system. Elemental analyses for
diphsp were carried out on a Heraeus Vario EL III elemental analyzer
at NSC Regional Advanced Instrument Center, National Taiwan
University. Mass spectra were recorded on a JEOL JMS-700 mass
1
(2.88 g, 93%) as a pale yellow solid. Mp 229 °C (DSC). H NMR
(CDCl3, 400 MHz) δ 8.79 (d, J = 2.8 Hz, 1H), 8.22 (d, J = 7.6 Hz,
2H), 7.88 (d, J = 7.6 Hz, 2H), 7.60 (d, J = 7.6 Hz, 1H), 7.49 (t, J = 7.6
Hz, 2H), 7.45−7.40 (m, 3H), 7.20−7.13 (m, 5H), 6.78 (d, J = 7.6 Hz,
2H). 13C NMR (CDCl3, 100 MHz) δ 159.1, 158.9, 158.3, 150.2, 146.2,
144.2, 142.2, 141.9, 138.9, 132.4, 131.9, 129.1, 128.6, 128.4, 128.2,
127.4, 123.9, 123.6, 120.8, 120.4, 61.4. HRMS (m/z, ESI+) calcd for
C29H18N2 394.1470, found 394.1473.
Synthesis of 3,6-Diphenyl-4,5-diaza-9,9′-spirobifluorene
(diphsp). A mixture of 4,5-diaza-9,9′-spirobifluorene (500 mg, 1.57
12115
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121

Inorganic Chemistry
Article
Figure 1. Crystal structures of complex 2 (left) and 3 (right). Counteranion PF6− and hydrogen atoms are omitted for clarity. Thermal ellipsoids are
drawn at the 50% probability.
mmol) and diethyl ether (80 mL) was stirred at room temperature,
and phenyllithium (2 mL, 3.93 mmol, 1.92 M) was added in one
portion. After 3 h, the reaction was quenched with water and extracted
with diethyl ether. The organic layer was stirred with activated MnO2
(1.36 g) for 6 h. The mixture was dried over MgSO4 and then filtered
to afford 3-phenyl-4,5-diazaspirobifluorene. The target molecule was
obtained by repeating the same procedure. The crude product was
purified by column chromatography on silica gel (EtOAc/hexane = 1/
2) to afford diphsp (650 mg, 88%) as a pale yellow solid. Mp 335 °C
(DSC). 1H NMR (CDCl3, 400 MHz) δ 8.22 (d, J = 7.6 Hz, 4H), 7.89
(d, J = 7.6 Hz, 2H), 7.60 (d, J = 7.6 Hz, 2H), 7.50 (t, J = 7.6 Hz, 4H),
7.46−7.42 (m, 4H), 7.22−7.15 (m, 4H), 6.83 (d, J = 7.2 Hz, 2H). 13C
NMR (CDCl3, 100 MHz) δ 159.2, 158.4, 146.4, 142.8, 141.9, 139.1,
132.4, 129.1, 128.6, 128.4, 128.2, 127.4, 123.9, 120.8, 120.4, 61.1.
HRMS (m/z, ESI+) calcd for C35H22N2 470.1783, found 470.1780;
Anal. Calcd for C35H22N2: C, 89.33; H, 4.71; N, 5.95. Found: C, 89.21;
2H), 7.19−7.08 (m, 4H), 7.02 (t, J = 8.0, 4H), 6.88 (t, J = 2.4 Hz, 2H),
6.84 (d, J = 7.2, 4H), 6.11 (td, J = 8.0, 2.4 Hz, 2H), 4.68 (dd, J = 8.0,
2.4 Hz, 2H). MS (m/z, ESI+) 551 (100), 577 (95), 1021 (80). HRMS
(m/z, FAB+) calcd for C53H32F4IrN6 1021.2248, found 1021.2236.
Crystal Structure Determinations. Crystallographic data were
collected at 295(2) K on a NONIUS Kappa CCD diffractometer using
graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å). Cell
parameters were retrieved and refined using HKL Denzo and
Scalepack software on all observed reflections.10 Data reduction was
performed with the HKL Denzo and Scalepack software. The
structures were solved and refined with SHELX programs.11 The
hydrogen atoms were included in calculated positions and refined
using a riding mode.
Fabrication and Characterization of LEC Devices. ITO-coated
glass substrates were cleaned and treated with UV/ozone prior to use.
A PEDOT:PSS layer was spin-coated onto the ITO substrate, and
then, the structure was baked at 150 °C for 30 min. All solution
preparing and spin-coating processes were carried out in glovebox.
The emissive layer (∼400 nm) was spin-coated from the CH3CN
solutions of the compound (150 mg/mL). To reduce the turn-on time
H, 4.59; N, 5.75.
−
Synthesis of [Ir(dfppz)2(phsp)]+(PF6
) (2). Bis-(μ)-
chlorotetrakis(1-(4,6-difluorophenyl)-pyrazolato-C2,N)diiridium(III)
(585 mg, 0.5 mmol) and 3-phenyl-4,5-diaza-9,9′-spirobifluorene (433
mg, 1.1 mmol) were dissolved in 1,2-ethanediol (35 mL) under Ar,
and the mixture was kept at 150 °C for 16 h. The solution was cooled
to room temperature, and an aqueous solution of NH4PF6 (1.2 g in 10
mL deionic water) was added to yield yellow suspension. The solid
was then filtered and dried in an oven (80 °C) for 12 h. The crude
product was purified by column chromatography on silica gel
(CH2Cl2/MeCN = 10/1) to give complex 2 (817 mg, 75%) as an
yellow solid. 1H NMR (CDCl3, 400 MHz) δ 8.43 (d, J = 2.8 Hz, 1H),
8.31 (d, J = 2.8 Hz, 1H), 7.88 (dd, J = 8.0, 3.2 Hz, 2H), 7.69 (d, J = 5.2
Hz, 1H), 7.54−7.39 (m, 6H), 7.23−7.24 (m, 4H), 7.12 (t, J = 8.0,
1H), 7.02−6.99 (m, 3H), 6.86−6.78 (m, 5H), 6.63 (td, J = 8.0, 2.4 Hz,
1H), 6.28 (td, J = 8.0, 2.4 Hz, 1H), 5.58 (dd, J = 8.0, 2.4 Hz, 1H), 4.97
(dd, J = 8.0, 2.4 Hz, 1H). MS (m/z, ESI+) 551 (50), 945 (100). HRMS
(m/z, FAB+) calcd for C47H28F4IrN6 945.1935, found: 945.1942.
Synthesis of [Ir(dfppz)2(diphsp)]+(PF6−) (3). Bis-(μ)-
chlorotetrakis(1-(4,6-difluorophenyl)-pyrazolato-C2,N)diiridium(III)
(585 mg, 0.5 mmol) and 3,6-diphenyl-4,5-diaza-9,9′-spirobifluorene
(517 mg, 1.1 mmol) were dissolved in 1,2-ethanediol (35 mL) under
Ar, and the mixture was kept at 150 °C for 16 h. The solution was
cooled to room temperature, and an aqueous solution of NH4PF6 (1.2
g in 10 mL deionic water) was added to yield a yellow suspension. The
solid was then filtered and dried in an oven (80 °C) for 12 h. The
crude product was purified by column chromatography on silica gel
(CH2Cl2/MeCN = 10/1) to give complex 3 (758 mg, 65%) as a
yellow solid. 1H NMR (CDCl3, 400 MHz) δ 8.32 (d, J = 2.4 Hz, 2H),
7.90 (d, J = 8.0 Hz, 2H), 7.65 (d, J = 1.6 Hz, 2H), 7.50 (t, J = 7.2 Hz,
2H), 7.42 (d, J = 8.0, 2H), 7.32 (t, J = 7.2 Hz, 2H), 7.20 (d, J = 8.0,
−
of the LEC device, the ionic liquid [BMIM+(PF6 ) ] (20 wt %) was
added to enhance the ionic conductivity of thin films.12 After spin-
coating the emissive layer, the samples were baked at 70 °C for 10 h in
a nitrogen glovebox (oxygen and moisture levels below 1 ppm) and
then subjected to thermal evaporation of a 100-nm-thick Al top
contact in a vacuum chamber (ca. 10−6 Torr). The electrical and
emission characteristics of the LEC devices were measured using a
sourcemeter (Keithley 6430) and a picoammeter (Keithley 6487)
equipped with a calibration Si-photodiode. All device measurements
were performed under a constant bias voltage in a nitrogen glovebox.
Computational Methodology. All the calculations were using
the density functional theory (DFT) with B3LYP13 hybrid functional.
Restricted and unrestricted formalisms were adopted in the singlet and
triplet geometry optimization, respectively. To investigate the
photophysical properties, time-dependent DFT (TDDFT) calcula-
tions were performed on the basis of the optimized structures at the
ground state. A “double-ζ” quality basis set consisting of Hay and
Wadt’s effective core potentials (LANL2DZ)14 was employed for the
Ir(III) metal atom, and a 6-31G* basis set15 was employed for the rest
of the atoms. The relativistic effective core potential (ECP) replaced
the inner core electrons of Ir(III) metal atom, leaving only the outer
core valence electrons (5s25p65d6) to be concerned. The electronic
configurations of 3MC (metal-centered triplet) dd states were
calculated following the literature methodology.16 In brief, the
3MLCT state geometry was obtained by performing geometry
optimization along the triplet state potential energy surface (PES),
using the ground state optimized structure as the initial geometry. As
12116
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121

Inorganic Chemistry
Article
for the 3MC state, because the electron densities are mainly distributed
on the central metal atom, the calculation is starting with a distorted
geometry, for which the metal−ligand bondings are largely elongated,
such that its associated energy is expected to be far away from the
global minimum along the PES, and then the geometry optimization is
performed. Accordingly, the optimization is able to fall into the
presumably shallow local minimum associated with the 3MC dd
excited state. Moreover, considering the solvation effect, the
calculations were then combined with an integral equation formal-
ism-polarizable continuum model (in dichloromethane), IEFPCM.17
All calculations were carried out using Gaussian 09.
and phsp or diphsp are estimated to be Ir(1)−N(5)
[2.282(4)Å], Ir(1)−N(6) [2.158(4)Å] and Ir(1)−N(1)
[2.240(4)Å], Ir(1)−N(2) [2.249(4)Å], respectively, which are
significantly longer than those between the Ir center and 1-(2,4-
difluorophenyl)pyrazole [Ir(1)−N(1) = 2.008(4)Å, Ir(1)−
N(3) = 2.010(4)Å in complex 2, Ir(1)−N(4) = 2.028(4)Å,
Ir(1)−N(5) = 2.024(4)Å in complex 3]. This can be possibly
attributed to the anionic nature of the cyclometalated 1-(2,4-
difluorophenyl)pyrazole ligands, which have a stronger
interaction with the cationic Ir(III) ion. The Ir−N bond
distances between the Ir center and phsp or diphsp are affected
by the pendant phenyl ring. The Ir−N bond is longer where the
phenyl group is located. For example, the distance of Ir(1)−
N(5) [2.282(4)Å] is longer than that of Ir(1)−N(6)
[2.158(4)Å] in complex 2, which is due to steric repulsion of
the pendant phenyl ring. For complex 3, two pendant phenyl
rings give a comparable steric effect on both sides of Ir−N
bonds, as seen with the comparable distance of Ir(1)−N(1)
[2.240(4)Å] and Ir(1)−N(2) [2.249(4)Å]. The pendant
phenyl rings exhibit very strong intramolecular face-to-face
π−π stacking with the cyclometalated difluorophenyl ring of
the dfppy ligand as shown in Figure 2. The distances between
RESULT AND DISCUSSION
■
Synthesis. Scheme 1 depicts the synthesis of the N∧N
ancillary ligands, phsp and diphsp. Starting from sp,7 phsp and
diphsp were synthesized in 93% and 88% yields, respectively, by
nucleophilic addition (once or twice) of phenyllithium onto
4,5-diazaspirobifluorene, followed by oxidative aromatization
with manganese oxide. Complexes 2 and 3 were synthesized
according to the literature procedures in 75% and 65% yields,
respectively.
Crystal Structures of 2 and 3. Crystals of complexes 2
and 3 suitable for the X-ray diffraction analysis were obtained
by slow evaporation of the solvent from the acetone/hexane
cosolvent system. The structures of complexes 2 and 3 are
depicted in Figure 1, and the crystal data are summarized in
Table 1. Complexes 2 and 3 both exhibit a distorted octahedral
Table 1. Crystal Data of Complexes 2 and 3
2
3
empirical formula
fw
C48H28F10IrN6P
1174.85
C53H32F10IrN6P
1208.48
cryst dimensions (mm3)
0.25 × 0.15 × 0.10
triclinic
0.25 × 0.20 × 0.10
monoclinic
P21/c
cryst syst
space group
a (Å)
P1
̅
12.4320(2)
13.5430(2)
13.9288(3)
74.532(2)
85.7760(10)
79.5890(10)
2222.19(7)
2
16.3211(7)
16.8506(5)
19.4315(10)
90
b (Å)
Figure 2. Face-to-face π-stacking of the pendant phenyl ring with the
cylometalated difluorophenyl ring of complex 2 (left) and 3 (right).
c (Å)
−
α (deg)
Counteranion PF6 and hydrogen atoms are omitted for clarity.
β (deg)
114.738(6)
90
γ (deg)
cell volume (Å)3
two phenyl rings (the pendant phenyl ring and the cyclo-
metalated difluorophenyl ring) are about 3.38 Å for complex 2
and 3.40 and 3.46 Å for complex 3, indicating strong aromatic
π−π interaction. This interaction diminishes the possibility of
foreign substances to react with the metal complex and hence
reduces the possible formation of degraded products capable of
quenching the luminescence. The closest Ir-to-Ir distances in
complex 2 and 3 are as large as 9.47 and 9.97 Å, respectively,
indicating sufficient prevention of intermolecular interaction
provided by bulky 4,5-diaza-9,9′-spirobifluorene ligand.
4853.6(4)
4
Z
density (calcd) g/cm3
F(000)
1.756
1.654
1152
2380
T (K)
295(2)
295(2)
wavelength (Å)
no. reflns collected
no. indep reflns (Rint)
R(F), wR2 [I > 2σ(I)]
R(F), wR2 (all data)
0.710 73
0.710 73
31 548
21 629
10 191(0.0415)
0.0404, 0.1089
0.0536, 0.0626
9698(0.0331)
0.0350, 0.0973
0.0536, 0.1077
Photophysical Properties. Figure 3 shows the UV−vis
and PL spectra of 1, 2, and 3 in CH2Cl2 (10−5 M) and in neat
films. The photophysical properties of complexes 1, 2, and 3 in
solutions (CH2Cl2, 10−5 M) or in thin films are summarized in
Table 2. Similar absorption features were observed for all
complexes. The intense absorption bands in the UV region of
the spectra (240−350 nm) are associated with the ligand-
centered (LC) transitions of the ligands. These LC bands are
accompanied by weaker and broad bands extending from ∼300
nm to the visible region, which are related to the metal-to-
ligand-charge-transfer (MLCT) transitions (both allowed and
spin-forbidden ones) mediated by the strong spin−orbit
coupling of the Ir(III) center. In the PL spectra, all the
geometry around the Ir center as indicated by the small bite
angles of C(9)−Ir(1)−N(1) [80.52(18)o], C(18)−Ir(1)−N(3)
[79.81(17)o], and N(5)−Ir(1)−N(6) [79.87(14)o] and twisted
bond angles of N(1)−Ir(1)−N(3) [175.76(14)o], C(9)−
Ir(1)−N(5) [172.79(15)o], and C(18)−Ir(1)−N(6)
[176.18(15)o] in complex 2, and small bite angles of C(9)−
Ir(1)−N(5) [79.87(19)o], C(18)−Ir(1)−N(4) [80.4(2)o], and
N(1)−Ir(1)−N(2) [79.64(14)o] and twisted bond angles of
N(4)−Ir(1)−N(5) [172.80(16)o], C(9)−Ir(1)−N(1)
[170.05(17)o], and C(18)−Ir(1)−N(2) [176.70(18)o] in
complex 3. The Ir−N bond distances between the Ir center
12117
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121

Inorganic Chemistry
Article
Figure 3. Absorption (left axis) and PL (right axis) spectra of (a) 1, (b) 2, and (c) 3 in dichloromethane solutions (10−5 M) and in neat films.
a
Table 2. Summary of Physical Properties of Complexes 1−3
λPL (nm)
PL QY
b
c
d
red
complex
solution
neat film
solution
neat film
τ (μs)
kr
knr
E1/2ox/E1/2 (V)
1
2
3
500
504
505
512
510
510
0.46
0.47
0.38
0.20
0.10
0.11
0.54
0.35
0.38
8.5 × 105
1.3 × 106
1.0 × 106
1.0 × 106
1.5 × 106
+1.26/−1.71
+1.25/−1.69
+1.26/−1.68
1.6 × 106
a
b
c
d
At room temperature. Measured in CH2Cl2 (10−5 M). Measured in degassed CH2Cl2 solution (10−5 M). Redox potential vs ferrocene/
ferrocenium redox couple.
complexes exhibited emission maxima in the range of 500 and
505 nm. The HOMO orbital of a cationic iridium complex was
reported to be a mixture of the orbitals of iridium and the
phenyl group of the cyclometalated C∧N ligand, whereas the
LUMO orbital is generally attributed to the ancillary N∧N
ligand.18 The slightly red-shifted emissions of 2 and 3 are due
to larger conjugation of the N∧N ligands. Intriguingly, 3
emitted with a shoulder at ∼485 nm which might be attributed
to the component with longer Ir−N distance between the
iridium center and the ancillary N∧N ligand due to the
introduction of two pendant phenyl groups, causing less
electron donating effect of the ancillary ligand toward the
HOMO orbital. In thin films, the intermolecular interaction is
dominant to the emission properties, as observed for red-
shifted emission spectra of 1, 2, and 3 as compared with those
of the PL in solutions. The slightly lower photoluminescent
quantum yield (PLQY) of 3 in solution as compared to those of
1 and 2 was possibly attributed to the intraligand interactions
which may facilitate the deactivation processes. By calculating
the radiative and nonradiative rate constants, the introduction
of one phenyl ring increases both constants by a factor of 1.5.
The increase of rate constants indicates that the formation of
one intramolecular π−π stacking efficiently facilitates the
electronic transitions between the excited states and the
ground state due to enhanced molecular rigidity. However,
doubling the intramolecular π−π stacking significantly restricts
the molecular reorganization in the excited state and thus gives
less enhancement for the radiative process. In films, it is noted
that both 2 and 3 exhibited much lower PLQYs than 1,
indicating that the presence of intramolecular π−π stacking
leads to a densely extended π-stacking in the lattice by the
formation of solid-state supramolecular-caged structure.5a
Electrochemical Properties. Figure 4 depicts the electro-
chemical properties of complexes 1−3 probed by cyclic
Figure 4. Cyclic voltammograms of complexes 1−3 in acetonitrile. 0.1
M Tetrabutylammonium hexafluorophosphate (TBAPF6) and tetra-
butylammonium perchlorate (TBAP) were used as supporting
electrolytes for the oxidation and reduction CV, respectively. A glassy
carbon electrode was used as the working electrode; scan rate 100
mV/s.
voltammetry (CV, see the Experimental Section for details),
and the measured redox potentials versus ferrocene/ferroce-
nium redox couple are listed in Table 2. Notably, all complexes
exhibited nearly identical redox behavior in terms of redox
potentials and reversibility. A quasireversible oxidation potential
for each complex was observed at around +1.25 V. The highest
12118
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121

Inorganic Chemistry
Article
occupied molecular orbital (HOMO) of cationic complexes
with a general formula as [Ir(C∧N)2(N∧N)] has been reported
to be a mixture of the d orbitals of iridium and the p orbitals of
the C∧N ligand.18 The similar oxidation potentials of 1−3
suggest that the intramolecular π−π stacking does not
significantly influence the electronic structure of the HOMO
levels. For reduction, all complexes possess one reversible
reduction peak at potentials in the range of −1.71 and −1.68 V.
The reductions were unambiguously assigned to be located at
the 4,5-diazafluorene moiety of the ancillary ligands of the
complexes.18 The slight difference in the reduction potentials
was attributed to the different effective conjugation of the 4,5-
diazafluorene upon the introduction of phenyl ring(s).
defined as the time to achieve a brightness of 1 cd cm−2, were
0.5, 106, and 47 min for devices incorporated with complexes 1,
2, and 3 at 3.4 V, respectively. The brightnesses increased with
current and reached the maxima of 25.4, 5.8, and 10.6 cd m−2 in
about 54, 390, and 96 min with peak EQE value of 0.82%,
1.13%, and 1.43% for devices incorporated with complexes 1, 2,
and 3 at 3.4 V, respectively (Figure 5b). The device lifetimes
(t1/2, defined as the time for the brightness of the device to
decay from the maximum to half of the maximum under a
constant bias voltage) of complexes 1, 2, and 3 were 141, 585,
102 min, respectively. The device based on 2 and 3 exhibited
longer ton and smaller current densities as compared with those
of the device incorporating 1, indicating less efficient carrier
inject and/or transport (accumulation) when 2 or 3 is used as
the active layer. We suggest the suppressed carrier inject and/or
transport properties of 2 and 3 were attributed to the formation
of solid-state supramolecular-caged structure, which results in a
densely extended π-stacking in the lattice and leads to low ionic
mobility.5a Such molecular packing motif diminished the
current flux in the device and, as a result, substantially retarded
the degradation of the complex and hence increased the device
stability as well as EQE. Moreover, the complexes with
preferable intramolecular π−π interaction can “lock” the
comparable labile ancillary ligand which is prone to dissociation
upon electron injecting to the antibonding (π*) orbital. As a
consequence, we observed a 4-fold enhancement on the device
lifetime (t1/2) of 2 as compared to that of 1, stemming from less
current flux and enhanced molecular stability. Nevertheless,
doubling the intramolecular π−π interaction by two pendant
groups in 3 did not render a superior device lifetime, as
indicated by the shorter t1/2 of 3 as compared to that of 1.
Costa et al. have depicted the pendant group effect of LEC
performance in the 2,2′-bipyridyl system with DFT calcu-
lations.19 The complex with two phenyl groups significantly
lowers its metal-centered triplet state (3MC), rendering less
luminance yield for radiative decay and more probability on
degradation process (vide infra). Therefore, although the device
based on 3 possesses suppressed current flux, the unfavorable
Electroluminescent Properties. Device characteristics
based on the structure of the glass substrate/PEDOT:PSS/
indium tin oxide (ITO)/1 or 2 or 3 (400 nm) with 20 wt %
ionic liquid/Al are discussed below and summarized in Table 3.
Table 3. Characteristics of LEC Devices Based on
Complexes 1−3
a
b
c
d
ton
3.4 V [min]
tmax
t1/2
Lmax
ηmax
CIE
[min]
[min] [cd/m2] [%, cd/A, lm/W] (x, y)
1
2
3
0.5
106
47
54
141
585
102
25.4
5.76
10.6
0.82, 2.13, 1.97 0.23,
0.47
390
96
1.13, 3.37, 3.12 0.28,
0.50
1.43, 4.25, 3.92 0.28,
0.54
a
b
Time required to reach a brightness of 1 cd/m2. Time required to
c
reach the maximum brightness. The time for the brightness of the
device to decay from the maximum to half of the maximum under a
d
constant bias voltage. Maximum brightness achieved at 3.4 voltage.
The time-dependent brightnesses of the LEC devices based on
complexes 1, 2, and 3 operating at 3.4 V are shown in Figure
5a. The current in the device increased slowly with time after
the bias was applied. The times for turning on the device (ton),
3
excited state process (thermal population of MC state) still
accelerated the degradation of the complex even with two
intramolecular π−π interactions. By inspecting the ton and t1/2
for the devices based on 1 and 2, it is suggested that the two
device properties are in a trade-off. One cannot extend a device
lifetime without sacrificing the response time (i.e., ton) of the
device and vice versa. The stability of the ionic complex should
be the major obstacle for obtaining high device lifetime with
fast response.
Computational Studies. In order to understand the
electronic structure of the complexes with the effect of the
pendant phenyl group, we performed density functional theory
(DFT) calculations at the B3LYP/(6-31G*/LANL2DZ) level
on 1, 2, and 3 cations. The features of the frontier HOMO − 1,
HOMO, and LUMO orbitals mainly involved in the lower lying
electronic transitions are depicted in Figure 6. T1 states of 1, 2,
and 3 were calculated to lie 2.86, 2.84, and 2.80 eV above their
S0 states, respectively, which involve different contributions of
electronic transitions. The dominant excitation for the T1 states
in 1, 2, and 3 are HOMO → LUMO (95%), HOMO →
LUMO (23%) with 21% LLCT (fluorene to 4,5-diazafluorene,
HOMO − 1 → LUMO), and HOMO − 1 → LUMO (34%;
LLCT), respectively. We observed that the percentage of LLCT
transition increases along with the number of pendant phenyl
ring(s). For example, the T1 state of 1 is mainly contributed to
Figure 5. Dependence of (a) brightness (filled symbols) and current
density (empty symbols); (b) EQE (filled symbols) and correspond-
ing power efficiency (empty symbols).
12119
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121

Inorganic Chemistry
Article
CONCLUSION
■
In summary, we have successfully synthesized two cationic
iridium complexes with one or two pendant phenyl groups on
the spiro-configured N∧N ancillary ligand. The photophysical
data revealed that the longer Ir−N distance between iridium
and the ancillary ligand resulted in a blue-shifted emission
spectrum due to less interaction between the ancillary ligand
and the HOMO orbital (complex 3). The X-ray crystallography
shows substantial intramolecular π−π interaction between
pendant phenyl group and the dfppz ligand. The plane to
plane distances indicating sufficient π−π interaction are about
3.38 Å for complex 2 and 3.40 and 3.46 Å for complex 3. This
π−π interaction minimizes the expansion of the metal−ligand
bonds in the excited state and capability of preventing foreign
substances to access the metal center. The LEC devices based
on 2 and 3 showed much suppressed current densities which
resulted from densely extended π-stacking structures. The
device lifetime of the devices based on 1, 2, and 3 were 141,
585, 102 min, respectively. The 4-fold enhancement on the
device lifetime of 2 as compared to that of 1 is benefited by less
current flux and enhanced molecular stability. However, the
trade-off of device response and device lifetime limits the
practical use of LEC devices. The stability of the ionic complex
under the electric field is considered to be the major obstacle
for realizing practical LEC devices. Some possible solutions are
foreseen for searching for a new type of stable molecular
architecture with ionic character to allow higher durability
under the electric field and developing stable neutral complexes
compatible with ionic liquids to give LEC response without
using degradable ionic complexes. Work related to improving
LEC lifetime without sacrificing device response is under
investigation and will be reported in due course.
Figure 6. Frontier orbitals involved in the lower-lying electronic
transitions for 1, 2, and 3.
the HOMO → LUMO excitation with 26.8% MLCT (metal dπ
to the 4,5-diazafluorene moiety) character, whereas the T1
states of 2 and 3 involve higher percentage of LLCT transitions
(21% and 34%, respectively). This result indicates that the
dominating electronic transitions in 1−3 are fairly influenced
by the presence of intramolecular π−π stacking, restricting the
molecular reorganization process in the excited state for MLCT
transitions. Figure 7 illustrates the schematic energy diagram
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional tables, figure, and crystallographic data in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
Figure 7. Schematic energy diagram showing the relative energy of the
*E-mail: kenwong@ntu.edu.tw (K-T.W.), wenhung@mail.
ntou.edu.tw (W.-Y.H.), haichingsu@mail.nctu.edu.tw (H.-
C.S.), chop@ntu.edu.tw (P.-T.C.).
3
3MLCT/ππ* and MC excited states.
Notes
The authors declare no competing financial interest.
showing the relative energy of the T1 and 3MC excited states. It
is noted that the T1 and 3MC excited states for 3 possess almost
ACKNOWLEDGMENTS
3
■
the same energies while the MC excited states are higher by
The work was supported by the financial aid of the National
Science Council of Taiwan (NSC-98-2119-M-002-007-MY3
and 100-2112-M-019-002-MY3).
1.6 and 1.9 kcal mol−1 for 1 and 2, respectively. The unexpected
3
small energy difference between T1 and MC excited states for
3 decreases the molecular stability through population of the
3MC excited state (Table 2). Moreover, the longer Ir−N bonds
REFERENCES
■
3
between the iridium and the N∧N ligand in the MC excited
(1) (a) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Science
1995, 269, 1086. (b) Pei, Q.; Yang, Y.; Yu, G.; Zhang, C.; Heeger, A. J.
J. Am. Chem. Soc. 1996, 118, 3922.
states are calculated as 2.571, 2.403, and 2.707 Å for 1, 2, and 3,
respectively, indicating that 3 has a higher tendency to lose the
3
(2) (a) Lee, J. K.; Yoo, D. S.; Handy, E. S.; Rubner, M. F. Appl. Phys.
Lett. 1996, 69, 1686. (b) He, L.; Duan, L.; Qiao, J.; Wang, R.; Wei, P.;
Wang, L. D.; Qiu, Y. Adv. Funct. Mater. 2008, 18, 2123. (c) Colman, E.
Z.; Slinker, J. D.; Parker, J. B.; Malliaras, G. G.; Bernhard, S. Chem.
Mater. 2008, 20, 388. (d) Su, H.-C.; Chen, H.-F.; Fang, F.-C.; Liu, C.-
C.; Wu, C.-C.; Wong, K.-T.; Liu, Y.-H.; Peng, S.-M. J. Am. Chem. Soc.
ancillary ligand in the MC excited states, in line with the
shorter device lifetime of 3 as compared to that of 2. These Ir−
N bonds involve spin density on the nitrogen atoms (Figure
3
S1). The alteration of electron distribution in the MC excited
states makes the bond lengths not simply increase with the
increase of steric effect for 1 and 2.
́
2008, 130, 3413. (e) Bolink, H. J.; Coronado, E.; Costa, R. D.; Lardies,
12120
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121

Inorganic Chemistry
Article
N.; Ortí, E. Inorg. Chem. 2008, 47, 9149. (f) Su, H.-C.; Chen, H.-F.;
Wu, C.-C.; Wong, K.-T. Chem.Asian J. 2008, 3, 1922. (g) Kwon, T.-
H.; Oh, Y. H.; Shin, I.-S.; Hong, J.-I. Adv. Funct. Mater. 2009, 19, 711.
(h) He, L.; Qiao, J.; Duan, L.; Dong, G. F.; Zhang, D. Q.; Wang, L. D.;
Qiu, Y. Adv. Funct. Mater. 2009, 19, 2950. (i) Costa, R. D.; Ortí, E.;
Bolink, H. J.; Graber, S.; Schaffner, S.; Neuburger, M.; Housecroft, C.
E.; Constable, E. C. Adv. Funct. Mater. 2009, 19, 3456. (j) Costa, R. D.;
(17) Cances, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997,
107, 3032.
(18) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal, R.
A., Jr.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712.
(19) Costa, R. D.; Ortí, E.; Bolink, H. J.; Graber, S.; Housecroft, C.
E.; Neuburger, M.; Schaffner, S.; Constable, E. C. Chem. Commun.
2009, 2029.
Ces
Fernan
́
pedes-Guirao, F. J.; Ortí, E.; Bolink, H. J.; Gierschner, J.;
dez-Lazaro, F.; Sastre-Santos, A. Chem. Commun. 2009, 3886.
́
́
(k) He, L.; Duan, L.; Qiao, J.; Dong, G.; Wang, L.; Qiu, Y. Chem.
Mater. 2010, 22, 3535. (l) Costa, R. D.; Ortí, E.; Bolink, H. J.; Graber,
S.; Housecroft, C. E.; Constable, E. C. J. Am. Chem. Soc. 2010, 132,
5978. (m) Costa, R. D.; Ortí, E.; Bolink, H. J.; Graber, S.; Housecroft,
C. E.; Constable, E. C. Adv. Funct. Mater. 2010, 20, 1511. (n) Mydlak,
M.; Bizzarri, C.; Hartmann, D.; Sarfert, W.; Schmid, G.; De Cola, L.
Adv. Funct. Mater. 2010, 20, 1812. (o) Su, H.-C.; Lin, Y.-H.; Chang,
C.-H.; Lin, H.-W.; Wu, C.-C.; Fang, F.-C.; Chen, H.-F.; Wong, K.-T. J.
Mater. Chem. 2010, 20, 5521. (p) Yang, C.-H.; Beltran, J.; Lemaur, V.;
Cornil, J.; Hartmann, D.; Sarfert, W.; Frohlich, R.; Bizzarri, C.; De
̈
Cola, L. Inorg. Chem. 2010, 49, 9891. (q) Chen, H.-F.; Wong, K.-T.;
Liu, Y.-H.; Wang, Y.; Cheng, Y.-M.; Chung, M.-W.; Chou, P.-T.; Su,
H.-C. J. Mater. Chem. 2011, 21, 768.
(3) (a) Slinker, J. D.; Gorodetsky, A. A.; Lowry, M. S.; Wang, J.;
Parker, S.; Rohl, R.; Bernhard, S.; Malliaras, G. G. J. Am. Chem. Soc.
2004, 126, 2763. (b) Hu, T.; He, L.; Duan, L.; Qui, Y. J. Mater. Chem.
2012, 22, 4206.
(4) Kalyuzhny, G.; Buda, M.; McNeill, J.; Barbara, P.; Bard, A. J. J.
Am. Chem. Soc. 2003, 125, 6272.
(5) (a) Bolink, H. J.; Coronado, E.; Costa, R. D.; Ortí, E.; Sessolo,
M.; Graber, S.; Doyle, K.; Neuburger, M.; Housecroft, C. E.;
Constable, E. C. Adv. Mater. 2008, 20, 3910. (b) Graber, S.; Doyle,
K.; Neuburger, M.; Housecroft, C. E.; Constable, E. C.; Costa, R. D.;
Ortí, E.; Repetto, D.; Bolink, H. J. J. Am. Chem. Soc. 2008, 130, 14944.
́
(c) Lenes, M.; Garcia-Belmonte, G.; Tordera, D.; Pertegas, A.;
Bisquert, J.; Bolink, H. J. Adv. Funct. Mater. 2011, 21, 1581.
(d) Tordera, D.; Meier, S.; Lenes, M.; Costa, R. D.; Ortí, E.; Sarfert,
W.; Bolink, H. J. Adv. Mater. 2012, 24, 897.
(6) (a) Sun, L.; Galan, A.; Ladouceur, S.; Slinker, J. D.; Zysman-
Colman, E. J. Mater. Chem. 2011, 21, 18083. (b) He, L.; Duan, L.;
Qiao, J.; Zhang, D.; Wang, L.; Qiu, Y. Chem. Commun. 2011, 47, 6467.
(c) Rothe, C.; Chiang, C.-J.; Jankus, V.; Abdullah, K.; Zeng, X.;
Jitchati, R.; Batsanov, A. S.; Bryce, M. R.; Monkman, A. P. Adv. Funct.
Mater. 2009, 19, 2038.
(7) Wong, K.-T.; Chen, R.-T.; Fang, F.-C.; Wu, C.-C.; Lin, Y.-T. Org.
Lett. 2005, 7, 1979.
(8) Su, H.-C.; Fang, F.-C.; Hwu, T.-Y.; Hsieh, H.-H.; Chen, H.-F.;
Lee, G.-H.; Peng, S.-M.; Wong, K.-T.; Wu, C.-C. Adv. Funct. Mater.
2007, 17, 1019.
(9) (a) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am.
Chem. Soc. 1984, 106, 6647. (b) Lohse, O.; Thevenin, P.; Waldvogel,
E. Synlett 1999, 1, 45. (c) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974,
47, 767.
(10) Otwinowski, Z.; Minor, W. In Methods in Enzymology:
Macromolecular Crystallography, Part A; Carter, C. W., Jr., Sweet, R.
M., Eds; Academic Press: San Diego; Vol. 276, 2005.
(11) Sheldrick, G. M. SHELXL-97, Program for the Solution of Crystal
Structures; University of Gottingen: Gottingen, Germany, 1997.
̈
̈
(12) Parker, S. T.; Slinker, J. D.; Lowry, M. S.; Cox, M. P.; Bernhard,
S.; Malliaras, G. G. Chem. Mater. 2005, 17, 3187.
(13) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(14) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.;
Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(15) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209.
(16) Abrahamsson, M.; Lundqvist, M. J.; Wolpher, H.; Johansson, O.;
Eriksson, L.; Bergquist, J.; Rasmussen, T.; Becker, H.-C.;
Hammarstroem, L.; Norrby, P.-O.; Akermark, B.; Persson, P. Inorg.
Chem. 2008, 47, 3540.
12121
dx.doi.org/10.1021/ic300673k | Inorg. Chem. 2012, 51, 12114−12121