Phosphorescent Ir(iii) complexes with both cyclometalate chromophores and phosphine-silanolate ancillary: Concurrent conversion of organosilane to silanolate

Article abstract of DOI:10.1039/c3dt32408g

Ir(iii) metal complexes with general formula [(C^N)₂Ir(P^SiO)], where (C^N)H is 2-phenylisoquinoline (1), 2-phenylpyridine (2) or 2-(2,4-difluorophenyl)pyridine (3), and (P^SiO)H is an organosilanolate ancillary chelate with either diphenylsilyl (a) or dimethylsilyl (b) substituent, were synthesized, among which the structure of 3a was also confirmed using single-crystal X-ray diffraction analyses. These complexes exhibit bright phosphorescence in the region of 489-632 nm in solution at room temperature, showing the first successful example of using organosilanolate as an ancillary chelate. For application, organic light emitting diodes (OLEDs) using phosphors 3a and 3b doped in N,N′-dicarbazolyl-3,5-benzene (mCP) exhibited a maximum brightness of 50 800 cd m^-2 at 800 mA cm ^-2 (12 V) with η_ext of 10.1% and a brightness of 45 900 cd m^-2 at 700 mA cm^-2 (14.5 V) with η_ext of 10.2%, respectively. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt32408g

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Phosphorescent Ir(III) complexes with both
cyclometalate chromophores and phosphine-silanolate
ancillary: concurrent conversion of organosilane to
silanolate†
Cite this: Dalton Trans., 2013, 42, 7111
Fushi Zhang,*a Liduo Wang,a Shih-Han Chang,b Kuan-Lin Huang,b Yun Chi,*b
Wen-Yi Hung,*c Chih-Ming Chen,c Gene-Hsiang Leed and Pi-Tai Chou*d
Ir(III) metal complexes with general formula [(C^N)2Ir(P^SiO)], where (C^N)H is 2-phenylisoquinoline (1),
2-phenylpyridine (2) or 2-(2,4-difluorophenyl)pyridine (3), and (P^SiO)H is an organosilanolate ancillary
chelate with either diphenylsilyl (a) or dimethylsilyl (b) substituent, were synthesized, among which the
structure of 3a was also confirmed using single-crystal X-ray diffraction analyses. These complexes exhibit
bright phosphorescence in the region of 489–632 nm in solution at room temperature, showing the first
successful example of using organosilanolate as an ancillary chelate. For application, organic light emit-
ting diodes (OLEDs) using phosphors 3a and 3b doped in N,N’-dicarbazolyl-3,5-benzene (mCP) exhibited
a maximum brightness of 50 800 cd m−2 at 800 mA cm−2 (12 V) with ηext of 10.1% and a brightness of
45 900 cd m−2 at 700 mA cm−2 (14.5 V) with ηext of 10.2%, respectively.
Received 10th October 2012,
Accepted 22nd February 2013
DOI: 10.1039/c3dt32408g
www.rsc.org/dalton
development of these Ir(III) metal based emitting materials is
essential for the realization of highly efficient OLEDs.
1. Introduction
Over the past two decades, Ir(III) metal-based phosphors have
Conventionally, the aforementioned homoleptic Ir(ppy)3 or
been subject to extensive investigation owing to their potential analogues have been synthesized by heating of Ir(acac)3
applications in flat-panel displays and lighting applications.1–8 (acacH = acetylacetone) with three equiv. of 2-phenylpyridine
This has led to the successful design of a number of red, green or chelating heterocyclic aromatics (C^N)H at high tempera-
and blue-emitting materials for use in phosphorescent organic ture.13 Alternatively, they can be prepared via a stepwise
light-emitting diodes (OLEDs). These Ir(III) phosphors such as manner, by the formation of dicyclometalated dimer with
Ir(ppy)3, ppyH = 2-phenylpyridine, in which strong spin–orbit formula [(C^N)2Ir(μ-Cl)]2 via treatment of IrCl3·nH2O with two
coupling is induced by the heavy Ir(III) metal atom, effectively equiv. of (C^N)H, followed by addition of the third equiv. of
promote intersystem crossing and afford OLEDs with unprece- (C^N)H at elevated temperature.12 In such investigations,
dented luminescence efficiencies.9,10 Moreover, tuning the scientists also realized that the same dicyclometalated
emission across the whole visible spectrum can be achieved by dimer [(C^N)2Ir(μ-Cl)]2 readily reacts with chelating anions
replacement of ppy cyclometalates with phenyl isoquinoline11 (L^X)H to afford heteroleptic metal complexes [(C^N)2Ir(L^X)],
or 2,4-difluorophenylpyridine12 counterparts for generation of with retention of almost all desired photophysical
red and blue emission, respectively. The success in the characteristics.14,15
In this respect, suitable anionic L^X chelates included
acetylacetonate (acac),16–18 picolinate (pic) and analogues,19–22
carbamate,23 amidinate,24,25 dithiophosphate26 and even
aDepartment of Chemistry, Tsinghua University, Beijing 100084, China.
2-pyridyl azolate (pyaz) ligands.27,28 Furthermore, these hetero-
E-mail: zhangfs@mail.tsinghua.edu.cn
leptic complexes retain the major emission characteristics of
the parent dicyclometalated fragment [Ir(C^N)2]. Minor alter-
nation on the peak wavelength was also noted, which varied
according to the electronic characteristics of the L^X ancillary
ligand. For instance, the observed red-shifting of emission
λmax follows an order of picolinate < N-methylsalicyliminate
∼ acetylacetonate, which is proportional to their relative
bDepartment of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan.
E-mail: ychi@mx.nthu.edu.tw; Tel: +886 3 5712956
cInstitute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung
202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
dDepartment of Chemistry, National Taiwan University, Taipei 10617, Taiwan.
E-mail: chop@ntu.edu.tw
†CCDC reference number 905261. For crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c3dt32408g
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electron donor strengths, resulting in a reduction of the emis- 7.7 mmol) was added. The resulting mixture was maintained
sion energy gap.29 Moreover, electron deficient and/or higher at −78 °C for another 30 min, followed by slowly warming up
field strength L^X ancillaries, such as imidodiphosphinate30 to RT and then stirred for another 12 h. The solution was con-
and pyridyl azolate,31,32 and phosphorus-containing chelates, centrated and the precipitate was washed with deionized
such as benzyl phosphine33–37 and phosphine substituted pyr- water, and then recrystallized from CH2Cl2–hexanes, giving a
azole,38 are particularly suitable for construction of blue- or colorless solid of (2-diphenylphosphinophenyl)diphenylsilane
even bluish-green-emitting Ir(III) metal phosphors.39
(L1, 2.0 g, 4.5 mmol, 70%). The analogous (2-diphenylphosphino-
In this contribution, we report a systematic synthesis and phenyl)dimethylsilane (L2) was similarly prepared in 90%
characterization of heteroleptic Ir(III) complexes possessing an yield using (2-bromophenyl)diphenylphosphine and chloro-
ancillary chelate (P^SiO) that is derived from the hypothetical dimethylsilane as the starting materials.
(2-diphenylphosphinophenyl)dialkylsilanol. We believe that
Selected spectral data for L1. 1H NMR (400 MHz, CD2Cl2,
this new class of P^SiO chelate, due to the synergy of π-accept- 298 K): δ 7.51–7.48 (m, 5 H), 7.36–7.22 (m, 15 H), 7.10 (t, JHH
=
ing diphenylphosphino unit and electron donating silanolate 6.0 Hz, 4 H), 5.82 (d, JPH = 6.8 Hz, Si–H, 1 H). 31P NMR
fragment, should render significantly different properties from (202 MHz, acetone-d6, 298 K): δ −9.72 (s, 1 P).
those of previously discussed ancillary ligands. For example,
Selected spectral data for L2. 1H NMR (400 MHz, CD2Cl2,
its dual accepting/donating character and six-membered 298 K): δ 7.66–7.63 (m, 1 H), 7.35–7.30 (m, 8 H), 7.26–7.22
ligand–metal bonding capability are expected to exhibit (m, 4 H), 7.10–7.07 (m, 1 H), 4.71–4.65 (m, Si–H), 0.36 (dd,
enhanced emission efficiencies and intramolecular π⋯π stack- JHH = 3.6 Hz, JPH = 1.2 Hz, 2 CH3, 6 H). 31P NMR (202 MHz,
ing, details of which are elaborated in the following sections.
acetone-d6, 298 K): δ −9.94 (s, 1 P).
Synthesis of [(piq)2Ir(L1)] (1a)
2. Experimental
General procedures
All cyclometalated Ir(III) complexes were synthesized by the
generalized protocol as indicated in the literature.40,41
typical procedure is given below.
A
All reactions were performed under nitrogen and solvents were
distilled from appropriate drying agents prior to use. Commer-
cially available reagents were used without further purification
unless otherwise stated. All reactions were monitored using
pre-coated TLC plates (0.20 mm with fluorescent indicator
UV254). Mass spectra were obtained on a JEOL SX-102A instru-
ment operating in electron impact (EI) or fast atom bombard-
ment (FAB) mode. 1H and 13C NMR spectra were recorded on a
Varian Mercury-400 or an INOVA-500 instrument. Elemental
analysis was carried out with a Heraeus CHN-O Rapid Elemen-
tal Analyzer. Cyclic voltammetry (CV) measurements were per-
formed using a BAS 100 B/W electrochemical analyzer.
A solution of [(piq)2Ir(μ-Cl)]2 (500 mg, 0.39 mmol), L1
(380 mg, 0.86 mmol), Na2CO3 (410 mg, 3.9 mmol) and 2-meth-
oxyethanol (10 mL) was heated at 120 °C for 2 h. After cooling
to RT, the solvent was removed under reduced pressure and
the residue was purified by silica-gel column chromatography
eluting with a 1 : 1 mixture of ethyl acetate and hexanes.
Recrystallization from ethyl acetate–hexanes gave a red crystal-
line solid (1a, 530 mg, 0.50 mmol, 64%).
Selected spectral data for 1a. 1H NMR (400 MHz, CD2Cl2,
298 K): δ 9.31 (d, JHH = 6.8 Hz, 1 H), 8.82 (d, JHH = 8.4 Hz, 1 H),
8.74 (d, JHH = 6.4 Hz, 1 H), 8.63 (d, JHH = 8.4 Hz, 1 H), 8.11 (d,
JHH = 7.6 Hz, 1 H), 7.88 (t, JHH = 7.6 Hz, 2 H), 7.72–7.64 (m,
5 H), 7.61–7.55 (m, 3 H), 7.48 (t, JHH = 8.4, 2 H), 7.40 (t, JHH
=
Preparation of (2-bromophenyl)diphenylphosphine
7.6 Hz, 1 H), 7.33–7.24 (m, 5 H), 7.19–7.08 (m, 4 H), 6.95–6.87
(m, 2 H), 6.80–6.69 (m, 5 H), 6.62–6.56 (m, 2 H), 6.49–6.37 (m,
6 H), 6.30 (d, JHH = 7.6 Hz, 1 H), 6.03 (m, 1 H); 31P NMR
(202 MHz, CD2Cl2, 298 K): δ −0.57 (s, 1 P); MS (FAB, 193Ir): m/z:
1060 [M]+. Anal. Calc. for C60H44IrN2OPSi: N, 2.64; C, 67.96;
H, 2.68. Found: N, 2.84; C, 68.15; H, 2.52%.
A solution of Pd(PPh3)4 (0.10 g, 0.087 mmol), diphenylpho-
sphine (3.7 g, 20 mmol), 1-bromo-2-iodobenzene (2.6 mL,
20 mmol), Et3N (3.1 mL, 22 mmol) and 50 mL of anhydrous
toluene was heated at 80 °C for 12 h under nitrogen. After
cooling to RT, the solvent was removed under reduced pressure
and washed with deionized water. Further purification was
carried out by silica-gel column chromatography eluting with a
1 : 20 mixture of ethyl acetate–hexane to give a white solid
product 6.7 g (17 mmol, 98%).
Synthesis of [(ppy)2Ir(L1)] (2a)
Yield: 31%. Selected spectral data: 1H NMR (400 MHz,
Selected spectral data. 1H NMR (400 MHz, CDCl3, 298 K):
δ 7.66–7.63 (m, 1H), 7.38–7.36 (m, 6H), 7.31–7.20 (m, 4H),
7.19–7.21 (m, 2H), 6.75–6.77 (m, 1H).
acetone-d6, 298 K): δ 9.19 (d, JHH = 5.6 Hz, 1 H), 8.86 (d, JHH
=
6 Hz, 1 H), 7.89 (d, JHH = 8 Hz, 1 H), 7.78–7.72 (m, 2 H),
7.56–7.55 (m, 4 H), 7.49–7.45 (m, 4 H), 7.40–7.29 (m, 3 H),
7.22–7.14 (m, 6 H), 7.08–7.01 (m, 2 H), 6.88–6.72 (m, 8 H),
Preparation of L1 and L2
6.65–6.58 (m, 4 H), 6.45 (t, JHH = 8.4 Hz, 2 H), 6.33 (d, JHH
=
n-BuLi (2.5 M in hexane, 2.9 mL, 7.3 mmol) was slowly added 6.4 Hz, 1 H), 5.88 (m, 1 H); 31P NMR (202 MHz, CD2Cl2,
to a solution of (2-bromophenyl)diphenylphosphine (2.3 g, 298 K): δ 0.69 (s, 1 P); MS (FAB, 193Ir): m/z: 962 [M]+. Anal. Calc.
6.6 mmol) in 50 mL of THF at −78 °C. The solution was stirred for C52H42IrN2OPSi: N, 2.91; C, 64.91; H, 4.40. Found: N, 2.78;
for 30 min at −78 °C before chlorodiphenylsilane (1.5 mL, C, 65.03; H, 4.14%.
7112 | Dalton Trans., 2013, 42, 7111–7119
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Synthesis of [(dfppy)2Ir(L1)] (3a)
Single-crystal X-ray diffraction studies
1
Yield: 70%. Selected spectral data: H NMR (400 MHz, CD2Cl2, Single-crystal X-ray diffraction data were acquired with a
298 K): δ 9.06 (d, JHH = 5.2 Hz, 1 H), 8.81 (d, JHH = 6.0 Hz, 1 H), Bruker SMART Apex CCD diffractometer using Mo-Kα radiation
8.07 (d, JHH = 6.4 Hz, 1 H), 7.74–7.64 (m, 3 H), 7.49–7.35 (m, (λ = 0.71073 Å). The data collection was executed using the
7 H), 7.29–7.23 (m, 8 H), 7.16–7.05 (m, 2 H), 6.92–6.76 (m, SMART program. Cell refinement and data reduction were per-
7 H), 6.54 (t, JHH = 6.4 Hz, 1 H), 6.46 (t, JHH = 8.4 Hz, 2 H), 6.33 formed with the SAINT program. The structure was deter-
(m, 1 H), 6.21 (m, 1 H), 5.71 (dd, JHH = 9.2, 2.0 Hz, 1 H); 19F mined using the SHELXTL/PC program and refined using full-
{1H} NMR (376 MHz, CD2Cl2, 298 K): δ −106.79 (t, J = 8.8 Hz, matrix least squares.
1 F), −108.44 (d, J = 9.8 Hz, 1 F), −108.81 (t, J = 9.4 Hz, 1 F),
Selected crystal data of 3a. C52.5H37ClF4IrN2OPSi; M =
−109.02 (d, J = 10.2 Hz, 1 F); 31P NMR (202 MHz, CD2Cl2, 1074.55; monoclinic, space group = P21/n; a = 14.2505(10), b =
298 K): δ −0.01 (t, JPF = 8.1 Hz, 1 P); MS (FAB, 193Ir): m/z: 14.8900(11), c = 21.0030(16) Å, β = 107.351(2)°, V = 4253.8(5)
1032 [M]+. Anal. Calc. for C52H36F4IrN2OPSi·0.5CH2Cl2: Å3; Z = 4; Dc = 1.678 Mg m−3; F(000) = 2132; crystal size = 0.32
N, 2.59; C, 58.91; H, 3.45. Found: N, 2.92; C, 58.76; H, 3.87%.
× 0.25 × 0.20 mm3; λ(Mo-Kα) = 0.71073 Å; T = 150(2) K; μ =
3.329 mm−1; 32 447 reflections collected, 9764 independent
reflections (Rint = 0.0410), GOF = 1.065, final R1 [I > 2σ(I)] =
0.0301 and wR2(all data) = 0.0647.
Synthesis of [(piq)2Ir(L2)] (1b)
1
Yield: 61%. Selected spectral data: H NMR (400 MHz, CD2Cl2,
298 K): δ 8.90 (q, JHH = 8.0 Hz, 3 H), 8.79 (d, JHH = 6.4 Hz, 1 H),
8.16 (t, JHH = 6.8 Hz, 2 H), 7.85–7.82 (m, 2 H), 7.74–7.68 (m,
4 H), 7.51–7.47 (m, 3 H), 7.38 (m, 1 H), 7.31 (m, 1 H), 7.20–7.14
(m, 3 H), 7.00–6.96 (m, 4 H), 6.89 (t, JHH = 8 Hz, 1 H), 6.79 (t,
JHH = 7.6 Hz, 1 H), 6.67 (q, JHH = 6.8, 2 H), 6.60–6.52 (m, 3 H),
6.45 (t, JHH = 7.2 Hz, 2 H), 5.91–5.89 (m, 1 H), 0.32 (s, 3 H),
−0.79 (s, 3 H); 31P NMR (202 MHz, CD2Cl2, 298 K): δ −0.99
(s, 1P); MS (FAB, 193Ir): m/z: 937 [M]+. Anal. Calc. for C50H40-
IrN2OPSi·0.5CH2Cl2: N, 2.86; C, 61.98; H, 4.22. Found: N, 2.85;
C, 61.86; H, 4.52%.
OLED device fabrication
All chemicals were purified through vacuum sublimation prior
to use. The OLEDs were fabricated through vacuum deposition
of the materials at 10−6 torr onto ITO-coated glass substrates
having a sheet resistance of 15 Ω sq−1. The ITO surface was
sequentially cleaned with acetone, methanol and deionized
water, followed by UV-ozone treatment. The deposition rate of
each organic material was ca. 1–2 Å s−1. Subsequently, LiF was
deposited at 0.1 Å s−1 and then capped with Al (ca. 5 Å s−1
)
through shadow masking without breaking the vacuum. The
J–V–L characteristics of the devices were measured simul-
taneously in a glove-box using a Keithley 6430 source meter
and a Keithley 6487 picoammeter equipped with a calibration
Si-photodiode. EL spectra were measured using a photodiode
array (Ocean Optics USB2000).
Synthesis of [(ppy)2Ir(L2)] (2b)
1
Yield: 34%. Selected spectral data: H NMR (400 MHz, CD2Cl2,
298 K): δ 8.82 (d, JHH = 5.2 Hz, 1 H), 8.65 (d, JHH = 5.6 Hz, 1 H),
7.93 (d, JHH = 7.6 Hz, 1 H), 7.73–7.63 (m, 3 H), 7.56 (t, JHH
=
7.6 Hz, 1 H), 7.51–7.43 (m, 4 H), 7.37 (t, JHH = 6 Hz, 1 H), 7.30
(t, JHH = 7.2 Hz, 1 H), 7.16 (t, JHH = 7.6 Hz, 4 H), 6.94–6.81
(m, 4 H), 6.73–6.50 (m, 8 H), 5.75–5.72 (m, 1 H), 0.28 (s, 3 H),
−0.69 (s, 3 H); 31P NMR (202 MHz, CD2Cl2, 298 K): δ 0.64 (s,
1 P); MS (FAB, 193Ir): m/z: 837 [M]+. Anal. Calc. for C52H40-
IrN2OPSi: N, 3.35; C, 60.34; H, 4.34. Found: N, 3.65; C, 60.12;
H, 4.63%.
Results and discussion
Synthesis and characterization
First, (2-diphenylphosphinophenyl)diphenylsilane (L1) and
(2-diphenylphosphinophenyl)dimethylsilane (L2) were syn-
thesized and employed as precursors for the ancillary (P^SiO)
chelate. These silane reagents were easily obtained from initial
Synthesis of [(dfppy)2Ir(L2)] (3b)
1
Yield: 75%. Selected spectral data: H NMR (400 MHz, CD2Cl2, lithiation of (2-bromophenyl)diphenylphosphine at −78 °C,
298 K): δ 8.82 (d, JHH = 5.2 Hz, 1 H), 8.65 (d, JHH = 6 Hz, 1 H), followed by addition of either chlorodiphenylsilane or chloro-
8.33 m, 1 H), 8.09 (d, JHH = 8.4 Hz, 1 H), 7.74 (t, JHH = 8 Hz, dimethylsilane for inducing the formation of the C–Si bond.
1 H), 7.62 (t, JHH = 7.2 Hz, 1 H), 7.51–7.48 (m, 1 H), 7.44–7.35 In the meantime, the chloride bridged dimers [(C^N)2Ir-
(m, 4 H), 7.25–7.16 (m, 4 H), 6.93 (td, JHH = 7.6, 2.0 Hz, 2 H), (μ-Cl)]2, where (C^N)H is 1-phenylisoquinoline (piqH), 2-phe-
6.84 (t, JHH = 8.4 Hz, 1 H), 6.70 (m, 1 H), 6.64 (m, 1 H), 6.54 (t, nylpyridine (ppyH) or 4,6-difluorophenylpyridine (dfppyH),
JHH = 9.2 Hz, 2 H), 6.44–6.38 (m, 1 H), 6.26–6.20 (m, 1 H), 6.00 were routinely synthesized from condensation of IrCl3·hydrate
(dd, JHH = 9.2, 2.4 Hz, 1 H), 5.21–5.17 (m, 1 H), 0.29 (s, 3 H), with two equiv. of (C^N)H at elevated temperature. Subsequent
−0.68 (s, 3 H); 19F{1H} NMR (376 MHz, CD2Cl2, 298 K): treatment of [(C^N)2Ir(μ-Cl)]2 with the aforementioned organo-
δ −107.86 (dd, J = 9.8 and 7.9 Hz, 1 F), −109.80 (t, J = 9.8 Hz, silane precursors in the presence of Na2CO3 and in refluxing
1 F), −110.07 (d, J = 9.8 Hz, 1 F), −110.62 (d, J = 10.2 Hz, 1 F); methoxyethanol gave the heteroleptic Ir(III) complexes
31P NMR (202 MHz, CD2Cl2, 298 K): δ −0.25 (t, JPF = 8.1 Hz, [(C^N)2Ir(P^SiO)] (1–3) in moderate yields. Structural drawings
1 P); MS (FAB, 193Ir): m/z: 909 [M]+. Anal. Calc. for C42H32F4- of 1–3 are depicted in Scheme 1.
IrN2OPSi: N, 3.09; C, 55.56; H, 3.55. Found: N, 3.35; C, 55.32;
H, 3.64%.
It is notable that, all reactions between [C^N)2Ir(μ-Cl)]2 and
the dialkylsilane L1 (or L2) have never afforded the Ir(III)
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Despite the fact that we could not obtain the intended Ir(III)
complexes with a chelating Ir–Si^P unit, complexes with a
direct Ir–Si linkage have been documented in literature.
Among these investigations, Eisenberg prepared the mono-
and bis-silyl complexes of the formula IrH2(SiR3)(CO)(dppe)
and IrH(SiR3)2(CO)(dppe) (dppe = 1,2-bis(diphenylphosphino)-
ethane) via reaction of IrH3(CO)(dppe) with silane reagents
(HSiR3);44 Tilley and co-workers reported the isolation of Ir(III)
complexes Cp*(PMe3)Ir(SiR2OTf)Me (Cp* = η5-C5Me5, OTf =
OSO2CF3) via activation of silanes (HSiR3) with Cp*(PMe3)Ir-
(Me)OTf;45 also, Milstein reported the formation of the transi-
ent silane derivative (PMe3)3Ir(SiMe2Ph) and subsequent Ir(III)
iridasilacycle from treatment of Ir(PMe3)4Cl and PhMe2SiLi at
ambient temperature.46 However, all these reactions were con-
ducted under strictly anaerobic and anhydrous conditions to
prevent the possible oxygenation of the silane product, which
may explain our failure in obtaining the originally intended
Ir–Si complexes. On the other hand, the catalytically active
[{Ir(cod)(μ-OSiMe3)}2] has been synthesized by halide–silan-
oxide metathesis,47 but there is no precedent for the respective
Ir(III) silanolate complexes, making our report the first success-
ful case to show their existence and structural characteristics.
Moreover, the photophysical properties of this class of
[(C^N)2Ir(P^SiO)] complexes, to a certain extent, are analogous
to the Ir(III) complexes that possess similar C^N chromo-
phores, despite the possession of the third, non-emissive sila-
nolate ancillaries. These properties are elaborated in the
following section of photophysical measurement. In addition,
all P^SiO complexes 1–3 are highly soluble in chlorinated sol-
vents and show negligible decomposition upon raising temp-
erature for both SiPh2 and SiMe2 derivatives, i.e. either
complexes a or b. Full characterizations were executed using
FAB MS spectrometry, NMR techniques and elemental ana-
lyses (see Experimental section), while dfppy complex 3a was
further identified by single-crystal X-ray analysis to establish
their structure and packing in solid state.
Scheme 1 Structural drawings of the heteroleptic Ir(III) complexes [(C^N)2Ir-
(P^SiO)] (1–3).
complexes with the proposed P–Si chelate via direct Si–H bond
activation, as shown in the proposed structure of [(C^N)2Ir-
(P^Si)] depicted in transformation (a) of Scheme 2. Instead,
the [(C^N)2Ir(P^Si)] intermediate may be extremely unstable
and undergo instant oxidation under the reaction conditions
to afford the silanolate complexes of formula [(C^N)2Ir-
(P^SiO)]. Such a possibility is demonstrated by the recent reac-
tivity study of a dimethylplatinum(II) complex with the bis-
(2-pyridyl)dimethylsilane ligand, for which the cleavage of
Si–Me bond and formation of chelating Si–O–Pt linage are
induced by both metal oxidation and hydroxide attack at the
silicon atom.42 Alternatively, the added organosilane may first
convert to free organosilanol during the reaction, for which
similar hydrolytic oxidation of organosilanes to the organosila-
nols had been documented in high yields in the presence of
iridium catalysts and under mild conditions.43 The organosila-
nol then coupled with the added Ir(III) metal reagent [C^N)2Ir-
(μ-Cl)]2 to afford the isolated silanolate complexes [(C^N)2Ir-
(P^SiO)], see transformation (b) shown in Scheme 2. To
demonstrate the wide scope of the present method, we also
performed syntheses under several distinctive conditions, and
the corresponding silanolate products were all obtained in
moderate to good yields, particularly for those utilizing “wet”
methoxyethanol as the reaction medium. Thus, the current
procedure represents an efficient method for the formation of
silanolate metal complexes from organosilane reagents.
According to the X-ray structural determination, complex 3a
reveals a typical heteroleptic arrangement, with two dfppy
cyclometalates and one PPh2 substituted silanolate ancillary
(Fig. 1, also see Scheme 1 for the drawing). The dfppy chelates
adopt a mutually eclipsed configuration with their coordinated
nitrogen atoms N(1) and N(2) and carbon atoms C(1) and
C(12) being located in trans- and cis-orientation, respectively.
Moreover, the P^SiO ancillary resides opposite to the carbon
atoms of both dfppy chelates. The overall ligand arrangement
around the Ir(III) metal center is akin to that of Ir(III) metal
complexes with relevant oxygenate ancillaries such as 2-(2-
hydroxyphenyl)-substituted
benzothiazole
and
benzox-
azole.48,49 This suggests that the employed P^SiO chelate is
capable of coordinating to the Ir(III) metal atom via a stereo-
selective replacement of chloride ligands in its dinuclear pre-
cursor [(dfppy)2Ir(μ-Cl)]2.50 Moreover, the elongated Ir–C(1)
distance (2.038(3) Å) vs. Ir–C(12) bond (1.995(3) Å) in 3a also
shows a notable trans-effect imposed by the strong π-accepting
PPh2 fragment. In fact, the Ir(III) metal complexes with the ana-
logous PPh2 substituted phenolate chelate also show the
Scheme 2 Two proposed transformation pathways leading to the Ir(III) com-
plexes [(C^N)2Ir(P^SiO)]; employed conditions: Na2CO3, methoxyethanol,
120 °C, 2 h.
7114 | Dalton Trans., 2013, 42, 7111–7119
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Fig. 1 ORTEP diagram of 3a with ellipsoids shown at 40% probability level;
selected bond distances (Å): Ir–N(1) 2.058(2), Ir–N(2) 2.040(2), Ir–C(1) 2.038(3),
Ir–C(12) 1.995(3), Ir–P(1) 2.4045(8), Ir–O(1) 2.126(2); dashed lines show the cen-
troid–centroid contacts between adjacent phenyl substituents.
expected variation of Ir–C distances at the cyclometalate che-
lates, confirming the generalized structural behavior of this
class of Ir(III) metal complexes.51
Moreover, the X-ray structural analysis also revealed the
presence of two sets of intramolecular π⋯π stacking inter-
actions (Fig. 1). The first is ascribed to the close contact
between one phenyl group of the Ph2P fragment and the
pyridyl group of adjacent dfppy chelate, for which the cen-
troid–centroid contact is calculated to be 3.609 Å. The other is
observed between the second pyridyl group of dfppy chelate
and the diphenylsilyl substituent, which is documented by a
comparable centroid–centroid contact of 3.693 Å. In good
agreement with this finding, a large variation of centroid–cen-
troid π–π separation of 3.52–3.74 Å was detected among cat-
ionic Ir(III) complexes, for which the π–π stacking interaction
was found to be associated with the degree of fluorination at
the phenyl pendant of ancillary chelate.52
Fig. 2 UV-Vis absorption (a) and normalized emission spectra (b) of the
studied Ir(III) metal complexes 1a/1b–3a/3b in CH2Cl2 solution at RT.
Highly intensive luminescence was observed for 1a/1b,
2a/2b and 3a/3b in degassed CH2Cl2 with λmax located at
624/632 nm, 510/520 nm and 489/499 nm, respectively, among
which the SiPh2 substituted isomers a always showed a much
blue-shifted emission vs. that of the SiMe2 substituted counter-
parts b due to the greater electron withdrawing character of
the SiPh2 substituent. Moreover, the entire emission band
originating from a triplet state manifold was ascertained by
the O2 quenching rate constant of as high as 1.5–2.0 × 109 M−1
s−1 for all samples in CH2Cl2. The significant overlap of the
0–0 onsets between emission signal and the lowest energy
absorption band, in combination with a broad, structureless
spectral feature even for the blue-emitting 3a and 3b, leads us
to conclude that the phosphorescence originates primarily
from the 3MLCT state, together perhaps with a lesser contri-
bution from the 3ππ excited states. In comparison to 1a and 1b
with the piq cyclometalates, complexes 2a and 2b and 3a and
3b bearing either ppy or dfppy group reveal a ∼110 nm and
additional ∼20 nm hypsochromic shift in the emission peak
wavelength, the results of which can qualitatively be
Photophysical data
The absorption and luminescence spectra of complexes 1–3 in
CH2Cl2 are depicted in Fig. 2, and their metrical parameters
are summarized in Table 1. The strong absorption bands in
the UV region with distinct vibronic features are assigned to
the spin-allowed 1ππ* transition of the C^N cyclometalates.
The lowest energy absorption with peak wavelengths in the
region of 382–466 nm can be ascribed to a spin-allowed metal-
to-ligand charge-transfer transition (1MLCT), while their
extinction coefficients at the peak wavelength are in the range
of 3000–5000 M−1 cm−1. Finally, the weak shoulder that
further extends into the longer wavelength region is believed
to be associated with the spin-forbidden 3ππ* and 3MLCT
transitions.
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Table 1 Photophysical and electrochemical data of the studied Ir(III) metal complexes in CH2Cl2 solution
em
max
ox
1/2
UV/Vis λmax/nm (10−3εa /M−1 cm−1
)
λ
/nm
Φ (%)
τobs/μs
10−5kr (s−1
)
10−5knr (s−1
)
E
(ΔEp)b
Er1e/2d (ΔEp)b
1a
2a
3a
1b
2b
3b
335 (18), 400 (6.5), 462 (5.5)
259 (47), 358 (5.0), 394 (3.9)
345 (5.1), 382 (4.0)
347 (18), 395 (9.7), 466 (3.6)
260 (41), 361 (5.0), 402 (3.2)
350 (5.3), 385 (4.3)
624
510
489
632
520
499
0.26
0.73
0.62
0.36
0.90
0.59
1.57
1.66
0.92
1.36
1.72
0.66
1.63
4.38
6.79
2.63
5.23
8.98
4.73
1.64
4.09
4.72
0.58
6.26
0.35 (90)
0.37 (90)
0.64 (80)
0.33 (110)
0.34 (90)
0.59 (90)
−2.33 (110), −2.55 (110)
−2.8 (irr)
−2.8 (irr)
−2.34 (130), −2.59 (140)
−2.8 (irr)
−2.7 (irr)
a The systematic error of the absorption coefficient measurement is ∼ 20%. b E1/2 (V) refers to [(Epa + Epc)/2] where Epa and Epc are the anodic and
cathodic peak potentials referenced to the Fc+/Fc couple; ΔEp = |Epa − Epc| are reported in mV, the reduction experiments were conducted in THF
solution; irr indicates irreversible process.
rationalized by a reduction of ligand-centered ππ* energy gap the less conjugated pyridyl group present on either ppy or
due to the diminished π-conjugation on the ppy vs. piq cyclo- dfppy chelates vs. the isoquinolinyl group of piq chelate.
metalate, as well as the electron-withdrawing 4,6-difluoro sub-
stituents of the dfppy units. This observation is in accordance
Electroluminescent devices
with the general color-tuning principle of transition-metal
To test the applicability of the Ir(III) complexes as emitters in
based phosphors.2,14,53 Table 1 lists the corresponding photo-
electroluminescence (EL), we selected 3a and 3b as the
dopants. Selection of dopants was based on the fact that the
physical data for the studied complexes in degassed CH2Cl2
solution at room temperature. The reddest emitting 1a and 1b
tend to possess the lowest emission quantum yield, which
could be attributed to the effect of energy-gap law;54 on the
other hand, the reduced emission quantum yield for the blue-
emitting 3a and 3b vs. that of the green-emitting 2a and 2b is
probably caused by the thermally induced thermal population
to the upper-lying metal-centered dd excited state, which led
to the faster radiationless deactivation to the ground state,
giving the less efficient emission quantum yield.55,56 Both of
the physical characteristics such as Φ and τobs are comparable
to those of Ir(III) metal phosphors reported in literature.53,57–65
Electrochemistry
The electrochemical behavior of complexes 1–3 was investi-
gated by cyclic voltammetry using ferrocene as the internal
standard. The respective redox data are listed in Table 1.
During the anodic scan in CH2Cl2, all Ir(III) metal complexes
exhibited an irreversible oxidation peak in the region of
0.33–0.64 V, which is assigned as the metal-centered oxidation
process. Moreover, the anodic potentials showed a systematic
trend of 3a/3b ≫ 2a/2b > 1a/1b, which is also in agreement
with the estimated electron density at the Ir(III) metal center.
On the other hand, despite of their large variation in color, the
small variation of oxidation potential between the pair of com-
plexes 2a/2b and 1a/1b was attributed to the much smaller
influence of the overall electron density at the Ir(III) center vs.
the π-conjugation of the chelating ligand.
Upon switching to the cathodic sweep in THF, two rever-
sible peaks ranging from −2.33 to −2.59 V were also detected
for the red-emitting 1a and 1b, while the remainder of green
and blue-emitting complexes showed only a single irreversible
peak at ca. −2.8 V. Apparently, the extended π-conjugation of
the piq chelate would afford the lower lying empty orbital for
Fig. 3 (a) Current density–voltage–luminance (J–V–L) characteristics. (b) Exter-
the incoming electron. In contrast, complexes 2a/2b and 3a/3b
nal quantum and power efficiencies as a function of brightness. (c) EL spectra of
would afford much more negative reduction potentials due to devices of PhOLEDs with different dopants.
7116 | Dalton Trans., 2013, 42, 7111–7119
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Table 2 EL performances of devices fabricated with blue–green phosphors 3a and 3b
Dopant
Vonb/V
Lmax/cd m−2
Imax/mA cm−2
ηext (%)
ηc/cd A−1
ηp/lm W−1
(V, %)c at 1000 cd m−2
CIE [x, y]
Aa
B
C
3a
3b
FIrpic
3
3
3
50 800 (12.0 V)
45 900 (14.5 V)
62 000 (13.5 V)
800
700
1010
10.1
10.2
14.5
23.7
26.5
30.3
15.7
15.3
19.7
(6.7, 9.7)
(8, 9.5)
(8.1, 11.2)
0.19, 0.43
0.20, 0.48
0.16, 0.36
a The notations A, B and C indicate the devices fabricated with employment of dopants 3a, 3b and FIrpic, respectively. b Turn-on voltage at which
emission became detectable. c The values of driving voltage and ηext of device at 1000 cd m−2
.
blue–green emitting complexes 3a and 3b are the materials corresponding CIE coordinates x = 0.19, y = 0.43 for 3a and x =
with the highest thermal stability upon sublimation and hence 0.20, y = 0.48 for 3b, respectively, which are similar to that
give the least amount of tarry material during the evaporation observed in the PL spectra recorded in dilute solution (Fig. 2).
processes. A optimized device consists of the multi-layer Overall, our device performance data are judged to be moder-
architecture: ITO|PEDOT:PSS (30 nm)|TAPC (20 nm)|TCTA ate, in comparison to the recent documented reports on the
(5 nm)|mCP: 3a or 3b (10 wt%, 25 nm)|TAZ (50 nm)|LiF blue-emitting PhOLEDs.71,72 The reduced performance could
(0.5 nm)|Al (100 nm). We used N,N′-dicarbazolyl-3,5-benzene be in part attributed by the inherent nature of the silanolate
(mCP) as host material as it possesses a high triplet energy of ancillary.
2.9 eV and has served as the host for blue PhOLEDs in many
reports.66 Here, the conducting polymer, poly(ethylene dioxy-
thiophene)/poly(styrene sulfonate) (PEDOT:PSS) is used as the
Conclusion
hole-injection and passivation layer, while both 1,1′-bis[4-[N,N-
di(p-tolyl)amino]phenyl]cyclohexane (TAPC)67 and 4,4′,4′-tri(N-
In summary, a series of phosphine-substituted organosilanes
carbazolyl)triphenylamine (TCTA)68,69 are employed as
were synthesized, which were then utilized as the source for
the in situ generation of an P^SiO ancillary for assembling of
hole-transport layers, 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butyl-
phenyl)-1,2,4-triazole (TAZ)70 as an electron-transport and
Ir(III) based multi-colored phosphors. The transformation from
silane to silanolate has been discussed and is associated with
hole-blocking layer, LiF as an electron-injection layer and Al as
metal-catalyzed oxidative hydrolysis. The as-synthesized Ir(III)
a cathode, respectively. For comparison, the control device C
complexes showed bright luminescence that could be tuned
from blue–green to red depending on the cyclometalate
chromophores. Although these Ir(III) complexes are stable
against moisture during work-up, they tended to exhibit
insufficient stability during sublimation at higher temperature.
Despite this potential weakness, blue-emitters for PhOLEDs
were successfully synthesized, showing the potential for such
an ancillary chelate in preparation of new Ir(III) phosphors.
was also fabricated using FIrpic as dopant under the same
device structure.
Fig. 3 shows the current density–voltage–brightness (J–V–L)
characteristics and efficiency vs. brightness curves for the
devices, and the electroluminescence (EL) data are summar-
ized in Table 2. As indicated in Fig. 3a, all of the devices exhi-
bited turn-on voltages of 3 V and the difference in the J–V
performances is presumably because of the different emitting
Fabrication of PhOLEDs using a solution process, which does
not require thermal deposition of emitting layer, is deemed to
be more suitable for such type of phosphors toward appli-
dopants that influence the charge carrier transport of EML.
The current density and brightness of 3a-based device A exhi-
bited a maximum brightness (Lmax) as high as 50 800 cd m−2
at 12.0 V (800 mA cm−2), which is distinctly higher than that of
cations as either displays or lighting applications.
the 3b-based device B (45 900 cd m−2 at 14.5 V at 700 mA
cm−2). The operating voltage at 1000 cd m−2 of device A (6.7 V)
Acknowledgements
is slightly lower than that of the device B (8.0 V). In Fig. 3b, 3a-
based device A achieves maximum external quantum efficiency
This work was gratefully supported by the Hsinchu-Beijing
Tsing Hua Collaboration Fund.
(ηext) of 10.1% corresponding to a current efficiency (ηc) of 23.7
cd A−1 and power efficiency (ηp) of 15.7 lm W−1, which are
similar to those of the 3b-based device B (10.2%, 26.5 cd A−1
,
15.3 lm W−1). It is worth mentioning that the maximum
efficiencies in the devices appeared to occur in the more
useful brightness range of 100–500 cd m−2, and exhibited low
efficiency roll-off at high brightness; the ηext of devices A and B
at 1000 cd m−2 were 9.7 and 9.5%, respectively. For compari-
son, device C was also fabricated using FIrpic as emitter under
the same device structure, for which the ηext of 14.5% is higher
than the device using 3a or 3b as dopant. As depicted
in Fig. 3c, devices exhibited sky-blue emission with the
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