Phosphorescent cationic iridium(III) complexes with cyclometalating 1H-indazole and 2H-[1,2,3]-triazole ligands

Article abstract of DOI:10.1016/j.ica.2012.03.008

Two new cationic iridium(III) complexes with cyclometalating 1-phenylindazole or 2-phenyl-1,2,3-triazole ligands, [(C^N)₂Ir(4, 4′-di-tert-butyl-2,2′-dipyridyl)](PF₆), exhibit yellow or green phosphorescence with quantum yields and ex

Full text of DOI:10.1016/j.ica.2012.03.008

Inorganica Chimica Acta 388 (2012) 84–87
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Note
Phosphorescent cationic iridium(III) complexes with cyclometalating
1H-indazole and 2H-[1,2,3]-triazole ligands
⇑
⇑
Nail M. Shavaleev , Rosario Scopelliti, Michael Grätzel, Mohammad K. Nazeeruddin
Laboratory of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new cationic iridium(III) complexes with cyclometalating 1-phenylindazole or 2-phenyl-1,2,3-tria-
zole ligands, [(C^N) Ir(4,4 -di-tert-butyl-2,2 -dipyridyl)](PF ), exhibit yellow or green phosphorescence
2 6
with quantum yields and excited state lifetimes of up to 45% and 840 ns in argon-saturated dichloro-
methane solution at room temperature.
Received 23 January 2012
Received in revised form 2 March 2012
Accepted 4 March 2012
0
0
Available online 12 March 2012
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Iridium
Indazole
[
1,2,3]-Triazole
Cyclometalation
Complex
Phosphorescence
1
. Introduction
The reaction of L1 or L2 with IrCl
water gave cyclometalated complexes [(C^N)
3
ꢀ3H
2
O in 2-ethoxyethanol/
Ir( -Cl)] (Scheme
2
l
2
Phosphorescent Ir(III) complexes are actively developed for
1) [19]. However, this reaction failed for L3 probably because its
N3-atom coordinates to Ir(III) to give a mixture of complexes
[12,13].
next-generation lighting, display, and sensor applications [1–4].
The electronic and photophysical properties of these complexes
can be tuned by variation of the organic ligands [1–3]. Pyridine
derivatives are commonly used as ligands in Ir(III) coordination
chemistry [1–4]. Azoles are versatile and easy-to-make alterna-
tives to pyridines [5,6], and several research groups are developing
Ir–azole complexes [7–15]. Here, we report phosphorescent cat-
0
0
Reaction of [(C^N)
dyl (bpy ) gave two new cationic Ir(III) complexes 1 and 2
2
Ir(
l
-Cl)]
2
with 4,4 -di-tert-butyl-2,2 -dipyri-
⁄
(Scheme 1) as air- and moisture-stable solids that are soluble in
⁄
polar organic solvents. We chose a bulky neutral bpy ligand be-
cause it reduces solid state interaction and increases solubility of
metal complexes. 1 and 2 were purified by column chromatogra-
phy and re-crystallization, and were characterized by C, H, N ele-
0
0
ionic Ir(III) complexes [(C^N)
dyl)](PF ) (1 and 2, Scheme 1) with less-common cyclometalating
ligands that have 1H-indazole or 2H-[1,2,3]-triazole heterocycles
16,17], and we discuss the influence of aryl ring fusion (1) or het-
eroatom addition (2) on the properties of the known analogs 3
Chart 1) [8].
2
Ir(4,4 -di-tert-butyl-2,2 -dipyri-
6
1
13
mental analysis. H and C NMR spectra of 1 and 2 show a
[
single set of signals for the constituent ligands and indicate that
1
the complexes have C
2
-symmetry in solution. Integration of
H
⁄
19
(
peaks confirms the 2:1 ratio of the C^N to bpy ; F NMR confirms
ꢁ
+
the presence of PF
6
anion; and ESI TOF mass-spectra show the
⁄
+
2
. Results and discussion
peak of a cation [(C^N) Ir(bpy )] .
2
Figs. 1 and 2 display the X-ray structures of 1 and 2. The Ir(III)
+
2
Three ligands, L1–L3, were prepared by Cu O-catalyzed C–N
ion is in a distorted octahedral [(C^N) Ir(N^N)] coordination
2
coupling of aryl halides with azoles (Scheme 2) [18]. For indazole,
only the N1-derivative (L1), which is the major product, was
isolated. For 1H-[1,2,3]-triazole, the reaction gave N2- and
N1-derivatives L2 and L3 in equal yields after separation by col-
umn chromatography.
environment with the two nitrogen atoms of the C^N ligands in
trans-position to each other. The Ir–N bonds to the anionic C^N
⁄
ligand are shorter than those to the neutral bpy by ꢂ0.1 Å
(Table 1). The ligands are nearly planar: the dihedral angle be-
⁄
tween their rings is 3–5° (bpy ) or 8–13° (C^N) (Table 1). The larg-
est angle is observed for the C^N ligands of 1: it probably prevents
0
⇑
Corresponding authors. Tel.: +41 21 693 6124; fax: +41 21 693 4111.
E-mail addresses: nail.shavaleev@epfl.ch (N.M. Shavaleev), mdkhaja.nazeerudd
steric clash between the hydrogens of phenyl (2 -H) and indazole
(7-H).
in@epfl.ch (M.K. Nazeeruddin).
0
020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.008

N.M. Shavaleev et al. / Inorganica Chimica Acta 388 (2012) 84–87
85
R
R
N
2
N
Ir
PF6
N
N
3: R = H (3H); F (3F)
Chart 1. Previously reported Ir(III) complexes [8].
a
b
IrCl 3H O
[(C^N) Ir(μ-Cl)]
3
2
2
2
N
N
N
2
2
N
N
Fig. 1. Structure of 1 (CCDC 863360; 50% probability ellipsoids; H atoms, PF
6
anion,
Ir
PF6
Ir
PF6
co-crystallized acetone molecule, and the disorder within the tert-butyl group are
omitted; ORTEP). Heteroatoms are shown as octant ellipsoids: Ir, black; N, clear.
N
N
N
N
1
2
Scheme 1. Synthesis of new Ir(III) complexes: (a) cyclometalating ligand, 2-
⁄
ethoxyethanol/water (3/1), under argon, 120 °C; (b) bpy , under argon, 40 °C,
CH
2
2
Cl /methanol.
a
a
HN
N
N
N
L1
N
HN
N
N
N
Fig. 2. Structure of 2 (CCDC 863361; 50% probability ellipsoids; H atoms and PF
6
anion are omitted; ORTEP). Heteroatoms are shown as octant ellipsoids: Ir, black; N,
clear.
N
+
N
N
N
L2
L3
Table 1
Scheme 2. Synthesis of the ligands: (a) iodobenzene, Cs
DMF, under argon, 120 °C.
2 3 2
CO , Cu O (catalyst), dry
a
Selected structural parameters of Ir(III) complexes.
b
Ir–Ir (Å)c
Complex
Bond lengths (Å)
C^N
Angles (°)
⁄
⁄
bpy
bpy
C^N
2
The complex 1 contains a C -symmetry axis and the two C^N li-
Ir–Nd
Ir–C
Ir–N
gands are equivalent as are the two 4-tert-butylpyridine rings of
bpy . In 1, the ‘large surface’ indazole rings of both C^N ligands
1e
2
2.011(7)
2.025(3)
2.042(3)
2.026(6)
2.026(2)
2.026(2)
2.131(7)
2.125(2)
2.138(2)
13.00
8.94
7.88
4.98
2.76
8.050(2)
8.236(1)
⁄
in each complex participate in a face-to-face
p–p stacking with
indazole rings of their neighbors at inter-planar distance of 3.344
Å. The net result is the formation of a column of molecules of 1 that
a
b
c
Each row corresponds to one ligand in the complex.
The dihedral angle between the constituent rings of the ligands.
The shortest Ir–Ir distance in the structure.
are connected through
ligands in each complex takes part in a weak
p
–
p
interactions. In 2, only one of the C^N
stacking with a
d
e
⁄
p
–p
N-atom of bpy trans to C-atom of C^N in the same row.
1
2
contains a C -symmetry axis.
C^N ligand of its neighbor at inter-planar distance of 3.503 Å. The
⁄
bulky bpy in 1 or 2 is not involved in
p–p
stacking. Inter-metallic
communication is likely to be negligible because of the long Ir–Ir
distances (>8 Å, Table 1). The main structural parameters of 1
and 2 are similar to those of 3H [8].
second one, at ꢁ2.5 V, is irreversible. Both processes are likely to
⁄
be bpy -centered [21] because their potentials show less than
Redox potentials of 1 and 2 were measured by cyclic voltamme-
try relative to Fc /Fc [20] in acetonitrile and DMF (Fig. 3, Table 2,
and Figs. S1 and S2, Supporting information). 1 and 2 show two
reduction processes: the first one, at ꢁ1.8 V, is reversible; the
40 mV variation with the change of the C^N ligand.
+
The complexes display a quasi-reversible or irreversible oxida-
tion process that probably takes place on Ir–aryl fragment because
its potential is sensitive to the nature of the cyclometalating ligand

8
6
N.M. Shavaleev et al. / Inorganica Chimica Acta 388 (2012) 84–87
Table 3
2
a
Optical absorption of Ir(III) complexes.
/103
M
ꢁ1
cm
ꢁ1
)
k
onset/nmb
Complex
k
abs/nm (
e
1
2
271 (46), 297 (35), 309 (36), 365 (9.6, sh), 429 (0.7, sh) 530
261 (41), 296 (24), 306 (22, sh), 346 (6.8, sh) 490
I
a
1
In CH
2 2
Cl , at room temperature, in the range 250–600 nm. Estimated errors:
±
2 nm for kabs; ±5 nm for konset; ±5% for e.
b
Defined as a wavelength above which
e
max
< 0.01% e .
-
2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
+
E/V vs Fc /Fc in DMF
2
1
4 6
Fig. 3. Cyclic voltammograms of 1 and 2 in DMF (0.1 M NBu PF , 100 mV/s). The
potential range is limited to the first reduction and oxidation processes. The unit on
the vertical axis is 5
lA. Additional CVs are shown in the Supporting information.
Table 2
a
Redox properties of Ir(III) complexes.
ox
red
D
E (V)b
Complex
Solvent
DMF
E
1/2 (V)
E
1/2
(V)
1c
2
0.83 (112) d
e
e
e
400
500
600
700
800
ꢁ1.85 (78), ꢁ2.54
ꢁ1.82 (78), ꢁ2.48
ꢁ1.81 (88), ꢁ2.51
ꢁ1.89
2.68
3.05
e
CH
3
CN
1.23
Wavelength / nm
f
DMF
g
ꢁ5
3
3
H
F
CH
CH
3
CN
CN
0.95
1.25
2.84
3.08
Fig. 5. Corrected and normalized emission spectra of 1 and 2 at 10 M in argon-
g
3
ꢁ1.83
saturated dichloromethane at room temperature (kexc = 360 nm; Dkem = 1 nm).
a
+
Relative to Fc /Fc. On glassy carbon working electrode, in the presence of 0.1 M
(
4 6
NBu )PF , at scan rate 100 mV/s. Estimated error: ±50 mV. The anodic/cathodic
Table 4
peak separation for the reversible processes is given in brackets (for the standard,
Fc /Fc couple, it was 73–83 mV).
a
+
Photophysical properties of Ir(III) complexes.
b
c
ox
red
D
1
E = E1/2 ꢁ E1/2
.
Complex
k
em/nm
U
/%
s
/ns
rad
s /ls
3
has low solubility in CH CN (CV not recorded).
Quasi-reversible process; the anodic/cathodic peak separation is given in
d
1
2
575
520
24
45
640
840
2.7
1.9
brackets.
e
Irreversible process; the oxidation or reduction peak potential is reported.
Oxidation was outside of the electrochemical window of DMF (>1.0 V relative to
a
ꢁ5
f
In argon-saturated CH
2 2
Cl at 10 M at room temperature. Estimated errors:
+
±2 nm for kem; ±15% for ; ±20 ns for s.
U
Fc /Fc).
g
Data from the literature [8].
1
and 2 are pale yellow solids; in solution they exhibit a broad
band in the visible range at k > 400 nm with molar absorption coef-
ficients (e) of <10 M cm (Fig. 4, Table 3, and Figs. S3 and S4,
Supporting information). The absorption onset of 1 and 2 is ob-
served at 530 and 490 nm, respectively (Table 3): the lower konset
and increases by 400 mV in the order 1 < 2 (Table 2). The variation
of oxidation potentials in 1–3 shows that the C^N ligands become
less electron-rich and more electron-withdrawing in the order
3
ꢁ1
ꢁ1
0
0
L1 < 1-phenylpyrazole [8] < L2 ꢂ 1-(2 ,4 -difluorophenyl)pyrazole
corresponds to the higher value of the electrochemical gap (Ta-
[
8] (Table 2).
Electrochemical gap,
.05 V, respectively. It is of note that the same effect on the redox
ox
ble 2). We assign the lowest energy transition in 1 and 2 to (Ir-phe-
D
E = E1/2 ꢁ E1/2red, in 1 and 2 is 2.68 and
⁄
nyl)-to-bpy charge transfer, and we explain its weak intensity by
3
the poor overlap of the participating molecular orbitals [8].
properties of 3H [8] is achieved by replacing either pyrazole with
0
0
The intense near-UV bands of 1 and 2 at k < 400 nm with e = (7–
2
[
H-[1,2,3]-triazole (2) or phenyl with 2 ,4 -difluorophenyl (3F,
8]): in both cases, the changes in potentials are almost identical
and
E increases by ꢂ0.2 V (Table 2).
3
ꢁ1
ꢁ1
⁄
4
6) ꢃ 10 M cm likely originate from Ir-to-bpy charge-trans-
⁄
fer and ligand
information).
p
–
p
transitions (Table 3; Figs. S3 and S4, Supporting
D
1
and 2 are phosphorescent at ambient conditions and exhibit
8
6
4
2
00
00
00
00
0
ꢁ5
broad emission band in 10 M deoxygenated (argon-saturated)
dichloromethane solutions at room temperature (Fig. 5): the phos-
phorescence color is yellow (1) or pale green (2). The excitation
spectra of 1 and 2 match the absorption spectra (Fig. S5, Supporting
information). The luminescence decays are single exponential
functions suggesting the presence of one emissive center in solu-
tion (Fig. S6, Supporting information). The emission maxima
2
1
ε
(kem), luminescence quantum yields (
lifetimes ( ), and calculated radiative lifetimes of the excited states
) of 1 and 2 are reported in Table 4.
With respect to 3H (kem = 538 nm in CH Cl
sion in 1 red-shifts the phosphorescence spectrum, while hetero-
atom addition in blue-shifts it (Table 4). The quantum
U), observed excited-state
s
rad
(s = s/U
2
2
, [8]), aryl ring fu-
4
20 440 460 480 500 520 540
Wavelength / nm
2
ꢁ
5
ꢁ5
efficiencies and lifetimes of 1 and 2 are comparable to those of
the reported analogs [8,15]. The broad phosphorescence spectra
Fig. 4. Absorption spectra of 1 (7.26 ꢃ 10 M) and 2 (8.46 ꢃ 10 M) in CH
2 2
Cl .
Additional absorption spectra are shown in the Supporting information.

N.M. Shavaleev et al. / Inorganica Chimica Acta 388 (2012) 84–87
87
and relatively short radiative lifetimes suggest that 1 and 2 emit
30.01 (CH
710 Hz, PF
2: [(L2)
3
) ppm. 19F NMR (376 MHz, CD
2
Cl
2
): d = ꢁ73.43 (d, JP–F
=
+
+
from a charge-transfer excited state, probably, of (Ir–phenyl)-to-
6
) ppm. ESI TOF MS: m/z 847.3 ({MꢁPF
6
} , 100%).
⁄
⁄
bpy nature [8].
2
Ir(
l-Cl)]
2
(100 mg, 0.097 mmol) and bpy (65 mg,
In conclusion, cyclometalating ligands with 1H-indazole or 2H-
1,2,3]-triazole heterocycles provide Ir(III) complexes that are
0.24 mmol) gave pale yellow-green solid: 89 mg (0.10 mmol,
51%). It emits pale green phosphorescence in solution; the emis-
[
brightly-phosphorescent in solutions. The large electrochemical
gap and blue-shift of phosphorescence spectrum of 2 with respect
to 3H [8] suggest that 2H-[1,2,3]-triazole can be used as a building
block to develop high-energy emitting Ir(III) complexes.
sion in the powder is very weak. Anal. Calc. for C34
36 6 8
H F IrN P
(MW 893.88): C, 45.68; H, 4.06; N, 12.54. Found: C, 45.70; H,
1
4.38; N, 12.35%. H NMR (400 MHz, CD
(d, J = 6.0 Hz, 2H), 8.00 (s, 2H), 7.79 (dd, J = 8.0, 1.2 Hz, 2H), 7.52
dd, J = 6.0, 1.6 Hz, 2H), 7.23 (s, 2H), 7.20 (dd, J = 8.0, 1.2 Hz, 2H),
2 2
Cl ): d = 8.31 (s, 2H), 8.06
(
7
1
.05 (td, J = 7.2, 1.2 Hz, 2H), 6.34 (dd, J = 7.6, 0.8 Hz, 2H), 1.46 (s,
8H, tert-butyl) ppm. C NMR (100 MHz, CD Cl ): all of the ex-
2 2
3
. Experimental
1
3
pected signals in aromatic (13C) and aliphatic (2C) regions were
observed, d = 165.05, 156.14, 150.99, 141.22, 136.12, 133.42,
The following data are provided in the Supporting information:
general methods, equipment, and chemicals used; synthesis and
1
13
19
1
32.34, 129.07, 128.90, 125.58, 124.12, 121.15, 114.83, 35.72
characterization of L1–L3 and [(C^N)
NMR spectra; experimental details for the X-ray, electrochemical,
and spectroscopic measurements; crystallographic data
Table S1); cyclic voltammograms (Figs. S1 and S2); electronic
2
Ir(
l
-Cl)]
2
;
H, C, and
F
1
9
[C(CH
3
)
3
], 29.95 (CH
3
)
ppm.
F
NMR (376 MHz, CD
2
Cl
2
):
+
d = ꢁ73.30 (d, JP–F = 710 Hz, PF
6
) ppm. ESI TOF MS: m/z 749.3
+
(
{MꢁPF
6
} , 100%).
(
absorption spectra (Table S2 and Figs. S3 and S4); phosphorescence
excitation spectra (Fig. S5); and luminescence decays (Fig. S6).
Purification, crystal growth, and handling of all compounds
were carried out under air. All products were stored in the dark.
Chemicals from commercial suppliers were used without purifica-
tion. Chromatography was performed on a column with an i.d. of
Acknowledgments
European Union (CELLO, STRP 248043; https://www.cello-
project.eu/). Md.K.N. thanks World Class University Program,
funded by the Ministry of Education, Science and Technology
through the National Research Foundation of Korea (No. R31-
3
0 mm on silica gel 60 (Fluka, Nr 60752). The progress of reactions
and the elution of products were followed on TLC plates (silica gel
0 F254 on aluminum sheets, Merck).
2
008-000-10035-0), Department of Material Chemistry, Korea
6
University, Chungnam 339-700, Korea.
3.1. Synthesis of the complexes
Appendix A. Supplementary material
The structures of 1 and 2 are shown in Scheme 1. The reactions
were performed under argon and in the absence of light. The sol-
vents were de-oxygenated by bubbling with Ar, but they were
CCDC 863360 and 863361 contain the supplementary crystallo-
graphic data for 1 and 2, respectively. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.ica.2012.03.008.
not dried. [(C^N)
2
Ir(
2 2 2
l-Cl)] was dissolved in a mixture of CH Cl
0
0
(
40 mL) and CH
3
OH (5 mL) at RT, and 4,4 -di-tert-butyl-2,2 -dipyr-
⁄
idyl (bpy ; excess, Aldrich) was added. The reaction mixture was
stirred at 40 °C overnight to give yellow solution. It was evaporated
to dryness. The impurities were removed by chromatography (sil-
ica, 15 g) by eluting with 2% (1) or 3–5% (2) CH
product was recovered as yellow fraction by eluting with 4–6% (1)
or 5–7% (2) CH OH in CH Cl . The fractions containing the product
were evaporated. Dry residue was dissolved in CH OH (5 mL) and
added drop-wise to a stirred aqueous solution of KPF (670 mg in
0 mL of water, 3.64 mmol, excess, Alfa Aesar). The resulting sus-
pension was stirred for 10 min and filtered. The complex was
washed with water and either ether (1) or hexane (2). This work-
up gave the pure 1. In contrast, 2 had to be additionally re-crystal-
3 2 2
OH in CH Cl . The
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CD Cl ): d = 8.34 (d, J = 1.6 Hz, 2H), 8.28 (d, J = 8.8 Hz, 2H), 8.13
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(
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[
[
[
17] M. Nonoyama, C. Hayata, Trans. Met. Chem. 3 (1978) 366.
18] A. Correa, C. Bolm, Adv. Synth. Catal. 349 (2007) 2673.
19] M. Nonoyama, J. Organomet. Chem. 86 (1975) 263.
CD
2C) regions were observed, d = 164.55, 156.04, 150.57, 144.62,
2 2
Cl ): all of the expected signals in aromatic (18C) and aliphatic
(
1
1
38.03, 134.36, 132.66, 131.79, 130.07, 125.40, 125.37, 124.72,
23.64, 123.08, 121.78, 120.94, 111.70, 110.58, 35.68 [C(CH ],
[20] R.R. Gagné, C.A. Koval, G.C. Lisensky, Inorg. Chem. 19 (1980) 2854.
21] M. Krej cˇ ik, A.A. Vl cˇ ek, J. Electroanal. Chem. 313 (1991) 243.
[
3 3
)