Heteroleptic Ir(iii) complexes containing both azolate chromophoric chelate and diphenylphosphinoaryl cyclometalates; Reactivities, electronic properties and applications

Article abstract of DOI:10.1039/c0dt00966k

The synthesis of a new family of octahedral Ir(iii) complexes with dual cyclometalating phosphine chelates, namely: 1-(diphenylphosphino)naphthalene (dpnaH) and isoquinoline (dppiH), is reported. Two series of intermediate complexes, [Ir(dpna)(tht)_2<

Full text of DOI:10.1039/c0dt00966k

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PAPER
Heteroleptic Ir(III) complexes containing both azolate chromophoric chelate
and diphenylphosphinoaryl cyclometalates; Reactivities, electronic properties
and applications†
Chen-Huey Lin,a Yun Chi,*a Min-Wen Chung,b Yi-Ju Chen,b Kang-Wei Wang,b Gene-Hsiang Lee,b
Pi-Tai Chou,*b Wen-Yi Hung*c and Hao-Chih Chiuc
Received 5th August 2010, Accepted 11th November 2010
DOI: 10.1039/c0dt00966k
The synthesis of a new family of octahedral Ir(III) complexes with dual cyclometalating phosphine
chelates, namely: 1-(diphenylphosphino)naphthalene (dpnaH) and isoquinoline (dppiH), is reported.
Two series of intermediate complexes, [Ir(dpna)(tht)2Cl2] (1), [Ir(dpna)2(OAc)] (2), [Ir(dppiH)(dppi)Cl2]
(3) and [Ir(dppi)2(OAc)] (4), which can be classified by the coexistence of either a pair of cis-chlorides or
a single acetate chelate, were obtained from treatment of phosphine with [IrCl3(tht)3] (tht =
tetrahydrothiophene). The in situ generated acetate complexes 2 and 4 could react with azolate chelates,
namely: 5-(2-pyridyl)-3-trifluoromethyl pyrazole (fppzH) and 5-(1-isoquinolyl)-3-tert-butyl-
1,2,4-triazole (iqbtzH), to afford a new series of luminescent complexes [Ir(dpna)2(fppz)] (5a and 5b),
[Ir(dpna)2(iqbtz)] (6a and 6b), [Ir(dppi)2(fppz)] (7a) and [Ir(dppi)2(iqbtz)] (8a). The phosphorescence
lifetime (tobs) fell in the range of a few tens of ms, showing possession of excessive ligand-centered pp*
mixed in part with MLCT character. A density functional theory (DFT) study was also conducted in
order to shed light on the origin of the transitions in the absorption and emission spectra and to predict
emission energies for these complexes. Organic light emitting diodes (OLEDs) displaying bright orange
emission and with maximum hext up to 17.1% were fabricated employing complexes 6a and 8a as the
phosphorescent dopants.
of the triplet emitters, for which the key factor is to increase the
Introduction
singlet and triplet intersystem crossing by spin–orbit coupling
Transition-metal complexes occupy a special position in opto-
and, ultimately to achieve better emission efficiency and reduce
electronic material research, as there exist potential applications
the radiative lifetime from the lowest lying triplet excited state.
that could promote the future prospects of organic light-emitting
As for the coordination environment around the central metal
diodes (OLEDs).1In this case, improvement of performance can be
ion, the majority of emissive Ir(III) complexes are constructed
linked to the intimate molecular design of functional components,
using three identical cyclometalated chelates, giving the so-called
homoleptic metal complexes.2 While these complexes have proven
successful for OLEDs fabrication, the heteroleptic complexes
having at least one ancillary chelate are becoming more and more
popular.3 Indeed, further tuning of the photophysical properties by
modulating the ancillary chelate broadens the scope of Ir(III) phos-
phors. The ancillary ligand, or sometimes referred to as the non-
chromophoric chelate, can be selected from a traditional choice
such as b-diketonate, N-alkyl salicylimine and picolinate,4 to mod-
ern and elaborative designs; namely: pyridyl azolate,5 dithiolate,6
picolinate N-oxide,7 2-(diphenylphosphino)phenolate,8 and even
N,N ¢-dialkylbenzamidinate with greater steric encumbrance.9
In searching for better molecular design, we became interested
in new Ir(III) phosphors with ancillaries deriving from the benzyl
substituted cyclometalating chelates, such as N-benzylpyrazole,10
benzyl benzimidazolium11 and benzyldiphenylphosphine.12 Due to
the interrupted p-conjugation, the resulting ancillaries are unlikely
to participate in the radiative transitions. As a consequence, this
so that knowledge regarding their structural versus electrical
properties can be tested, recorded and, most importantly, utilized
for fabrication of better and more efficient OLED devices.
In this regard, a large series of third-row transition-metal
complexes, particularly those with d6-electronic configuration, i.e.
either Os(II) or Ir(III) metal atoms, are under intense investigation
for serving as emissive dopants in the phosphorescent OLEDs.
This urgency is largely related to the better light-harvesting effect
aDepartment of Chemistry, National Tsing Hua University, Hsinchu, 300,
Taiwan. E-mail: ychi@mx.nthu.edu.tw
bDepartment of Chemistry, National Taiwan University, Taipei, 106, Taiwan.
E-mail: chop@ntu.edu.tw
cInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung, 202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
† Electronic supplementary information (ESI) available. CCDC reference
numbers 787571–787575. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt00966k
1132 | Dalton Trans., 2011, 40, 1132–1143
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design pushes the triplet energy of the complex far greater than
that of the typical blue phosphors such as FIrpic and FIrN6.13
Unfortunately, the benzylic group of these chelates could become
more reactive prior to or even after docking to the metal ion, such
as facile, methylene-to-carbonyl oxidation occurring on several
Ru(II) chelates.14 To avoid this complication, we herein switch
the design strategy to the new cyclometalating phosphines, i.e.
1-(diphenylphosphino)naphthalene and isoquinoline, for which
the C–H activation at the 8-position of both naphthalene and
isoquinoline moiety could also induce formation of stable five-
membered metallacycles free from the aforementioned methylene
group. As a result, preparation and characterization of the emissive
Ir(III) complexes, together with monitoring of influences of these
phosphine chelates as well as the corresponding device fabrication,
are reported herein.
(d, J = 5.6 Hz, 1H), 7.80 (d, J = 8.0 Hz, 1H), 7.63 (t, J = 8.0 Hz,
1H), 7.56 (d, J = 5.6 Hz, 1H), 7.50 (t, J = 8.0 Hz, 1H), 7.36 ~ 7.44
1
(m, 4H), 7.28 ~7.34 (m, 6H). 31P-{ H} NMR (202 MHz, CDCl3,
294 K): d -7.18 (s, 1P).
Synthesis of [Ir(dpna)(tht)2Cl2] (1). A mixture of [IrCl3(tht)3]
(450 mg, 0.80 mmol) and dpnaH (274 mg, 0.88 mmol) in decalin
(12 mL) was heated to reflux for 1.5 h. After cooling to RT, solvent
was removed under vacuum. The residue was dissolved in CH2Cl2
(50 mL) and then filtered with Celite. The filtrate was collected
and concentrated on a rotavap to give a crude product. Further
crystallization from a mixture of CH2Cl2 and hexane at RT gave
light yellow crystals (1, 499 mg, 0.67 mmol, 83%).
Spectra data of 1. MS (FAB, 193Ir): 714 m/z (M–Cl)+; 1H NMR
(500 MHz, CD2Cl2, 298 K): d 8.30 (d, J = 7.5 Hz, 1H), 8.10 ~ 8.03
(m, 4H), 8.01 (d, J = 7.5 Hz, 1H), 7.95 (t, J = 7.5 Hz, 1H), 7.66
(td, J = 7.5, 2.5 Hz, 1H), 7.54 (t, J = 7.5 Hz, 1H), 7.44 ~ 7.34 (m,
6H), 7.30 (t, J = 7.5 Hz, 1H), 3.82 ~ 3.79 (m, 2H), 3.33 ~ 3.29 (m,
2H), 2.05 ~ 1.95 (m, 6H), 1.65 ~ 1.60 (m, 4H), 1.60 ~ 1.51 (m, 2H).
Experimental
General procedures
1
All reactions were performed under an argon atmosphere and
solvents were distilled from appropriate drying agents prior to
use. Commercially available reagents were used without fur-
ther purification unless otherwise stated. All reactions were
monitored using pre-coated TLC plates (0.20 mm with fluo-
rescent indicator UV254). Mass spectra were obtained on a
JEOL SX-102A instrument operating in electron impact (EI)
or fast atom bombardment (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 Elementary Analyzer. The phosphine
ligands, namely: 1-(diphenylphosphino)naphthalene (dpnaH) and
1-(diphenylphosphino)isoquinoline (dppiH), and emitting chro-
mophores such as 5-(2-pyridyl)-3-trifluoromethyl pyrazole (fp-
pzH) and 5-(1-isoquinolyl)-3-tert-butyl- 1,2,4-triazole (iqbtzH)
were all synthesized using methods reported in literature.15,16
31P–{ H} NMR (202 MHz, CD2Cl2, 298 K): d 8.12 (s, 1P). Anal.
calcd. for C30H32Cl2IrPS2: C, 47.99; H, 4.30. Found: C, 47.82; H,
4.45.
Selected crystal data of 1: C30H32Cl2IrPS2; M = 750.75; or-
˚
thorhombic; space group = P212121; a = 9.4024(1) A, b =
3
˚
˚
˚
15.6941(2) A, c = 18.7797(2) A, V = 2771.17(5) A ; Z = 4; rc =
1.799 Mg m-3; F(000) = 1480; crystal size = 0.22 ¥ 0.18 ¥ 0.16 mm3;
-1
˚
l(Mo-Ka) = 0.71073 A; T = 150(2) K; m = 5.239 mm ; 13480
reflections collected, 6254 independent reflections (Rint = 0.0293),
GOF = 1.002, final R1[I > 2s(I)] = 0.0266 and wR2(all data) =
0.0476.
Direct synthesis of [Ir(dpna)2(OAc)] (2).
A mixture of
[IrCl3(tht)3] (169 mg, 0.30 mmol), dpnaH (197 mg, 0.63 mmol) and
sodium acetate (246 mg, 3.00 mmol) in decalin (10 mL) was heated
to reflux for 2.5 h. After cooling to RT, the solvent was removed
under vacuum. The residue was dissolved in CH2Cl2 (20 mL) and
the solution washed with H2O, dried over anhydrous Na2SO4 and
filtered. The filtrate was concentrated to give crude product, the
light yellow crystals were crystallized from a mixture of CH2Cl2
and hexane at RT (2, 191 mg, 0.20 mmol, 73%).
Synthesis of 1-(diphenylphosphino)naphthalene (dpnaH) and
1-(diphenylphosphino)isoquinoline (dppiH). 1-Iodonaphthalene
(3.04 g, 12.0 mmol), copper(I) iodide (113 mg, 0.60 mmol),
caesium carbonate (7.80 g, 24.0 mmol), diphenylphosphine (2.67 g,
14.4 mmol) and toluene (40 mL) were placed in a 25 mL Schlenk
tube under nitrogen and heated to 100 ◦C. After 48 h, the solution
was cooled to room temperature and solvent evaporated under
vacuum. The residue was dissolved in ethyl acetate (40 mL) and
washed with deoxygenated water. After that, the solution was dried
over anhydrous Na2SO4 and evaporated to dryness. The residue
was purified by silica gel column chromatography eluting with a
1 : 5 mixture of ethyl acetate and hexane. The colorless solid was
obtained from recrystallization in hot CH2Cl2 (2.40 g, 7.66 mmol,
71%). The analogous 1-(diphenylphosphino)isoquinoline (dppiH)
was synthesized in 62% yield, employing 1-chloroisoquinoline and
diphenylphosphine under similar conditions.
Conversion from 1 to 2. A mixture of 1 (100 mg, 0.13 mmol),
dpnaH (46 mg, 0.15 mmol) and sodium acetate (55 mg, 0.67 mmol)
in decalin (4 mL) was heated to reflux for 1 h. After cooling to RT,
the solvent was removed under vacuum. The residue was dissolved
in CH2Cl2 (15 mL) and the solution was washed with H2O, dried
over anhydrous Na2SO4 and filtered. The filtrate was concentrated
on a rotavap to give the crude product. The light yellow crystals
were obtained from a mixture of CH2Cl2 and hexane at RT (2,
46 mg, 0.05 mmol, 40%).
Spectra data of 2. MS (FAB, 193Ir): 875 m/z (M+1)+; 1H NMR
(500 MHz, CDCl3, 298 K): d 8.07 (t, J = 6.0 Hz, 1H), 7.90 (t, J =
8.0 Hz, 2H), 7.83 (d, J = 8.0 Hz, 1H), 7.70 (d, J = 8.0 Hz, 1H),
7.65 ~ 7.52 (m, 4H), 7.49 (d, J = 8.0 Hz, 1H), 7.31 ~ 7.25 (m, 3H),
7.24 ~ 7.15 (m. 6H), 7.12 ~ 7.06 (m, 3H), 7.03 ~ 6.96 (m, 3H),
6.69 (t, J = 7.5 Hz, 1H), 6.47 (t, J = 7.5 Hz, 1H), 6.37 (td, J =
7.8, 2.0 Hz, 2H), 6.12 (d, J = 7.5 Hz, 1H), 5.95 ~ 5.90 (m, 2H),
Spectra data of dpnaH. 1H NMR (400 MHz, CDCl3, 298 K):
d 8.38 (dd, J = 8.2, 3.6 Hz, 1H), 7.84 (t, J = 8.4 Hz, 2H), 7.48 ~ 7.39
1
(m, 2H), 7.36 ~ 7.24 (m, 11H), 6.98 (t, J = 6.0 Hz, 1H). 31P-{ H}
NMR (202 MHz, CDCl3, 298 K): d -13.60 (s, 1P).
1
Spectra data of dppiH. MS (EI): m/z 313 (M+); H NMR
1
(500 MHz, CDCl3, 294 K): d 8.61 (dd, J = 4.5, 8.0 Hz, 1H), 8.58
1.51 (s, 3H). 31P-{ H} NMR (202 MHz, CDCl3, 298 K): d 27.92
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(d, JPP = 5.5 Hz, 1P), 18.04 (d, JPP = 5.5 Hz, 1P). Anal. calcd. for
C46H35IrO2P2: C, 63.22; H, 4.04. Found: C, 63.42; H, 4.23.
residue was purified by column chromatography over silica gel
using ethyl acetate and hexane (1 : 2) as eluent to give two products
5a (40 mg, 0.04 mmol, 34%) and 5b (33 mg, 0.03 mmol, 28%),
according to the order of their elution. Light yellow crystals of 5a
and 5b were obtained from a mixture of CH2Cl2 and methanol at
RT.
Synthesis of [Ir(dppiH)(dppi)Cl2] (3). A mixture of [IrCl3(tht)3]
(200 mg, 0.36 mmol) and dppiH (233 mg, 0.75 mmol) in decalin
(10 mL) was heated to reflux for 1 h. After cooling to RT, the
solvent was removed under vacuum, the residue was dissolved
in CH2Cl2 (20 mL) and filtered. The filtrate was collected and
concentrated to give the crude product. The yellow crystals were
crystallized from a mixture of CH2Cl2 and hexane at RT (3, 217 mg,
0.24 mmol, 68.7%).
One-pot synthesis of 5a and 5b. A mixture of [IrCl3(tht)3]
(169 mg, 0.30 mmol), ndpp (157 mg, 0.66 mmol) and sodium
acetate (246 mg, 3.00 mmol) in decalin (10 mL) was heated to
reflux for 2 h. After cooling to RT, fppzH (64 mg, 0.30 mmol)
was added and the mixture was heated at reflux for another 4.5 h.
Finally, the solvent was removed and the residue was purified
by column chromatography over silica gel using ethyl acetate and
hexane (1 : 2) as eluent to give two products 5a (107 mg, 0.10 mmol,
35%) and 5b (25 mg, 0.02 mmol, 8%).
1
Spectra data of 3. MS (FAB, 193Ir): 888 m/z (M)+; H NMR
(500 MHz, CD2Cl2, 298 K): d 9.36 (dd, J = 6.0, 2.0 Hz, 1H), 8.77
(d, J = 5.5 Hz, 1H), 8.46 ~ 8.41 (m, 2H), 8.39 (d, J = 6.0 Hz, 1H),
8.15 (d, J = 8.0 Hz, 1H), 8.08 ~ 8.02 (m, 2H), 7.86 (t, J = 8.0 Hz,
1H), 7.62 ~7.56 (m, 2H), 7.52 ~ 7.45 (m, 2H), 7.40 ~ 7.37 (m, 1H),
7.35 ~ 7.31 (m, 1H), 7.30 ~ 7.25 (m, 3H), 7.22 (td, J = 8.0, 2.0 Hz,
2H), 7.16 (t, J = 7.5 Hz, 1H), 7.06 ~ 7.02 (m, 1H), 6.98 ~ 6.91
(m, 3H), 6.82 (t, J = 7.0 Hz, 1H), 6.60 ~ 6.55 (m, 3H), 6.35 ~ 6.29
1
Spectra data of 5a. MS (FAB, 193Ir): m/z 1028 (M+1)+; H
NMR (500 MHz, CDCl3, 294 K): d 8.07 (br, 2H), 7.87 ~ 7.79
(m, 4H), 7.61 (d, J = 8.0 Hz, 1H), 7.52 (d, J = 8.0 Hz, 1H), 7.42
~ 7.26 (m, 5H), 7.22 ~ 7.17 (m, 3H), 7.16 ~ 7.11 (m, 2H), 7.07
~ 7.02 (m, 2H), 6.98 (t, J = 7.3 Hz, 2H), 6.87 (t, J = 7.3 Hz,
1H), 6.68 ~ 6.56 (m, 5H), 6.48 (t, J = 6.8 Hz, 1H), 6.37 ~ 6.29
1
(m, 2H). 31P-{ H} NMR (202 MHz, CD2Cl2, 298 K): d -0.93 (d,
JPP = 16.6 Hz, 1P), -61.14 (d, JPP = 16.6 Hz, 1P). Anal. calcd. for
C42H31Cl2IrN2P2: N, 3.15; C, 56.76; H, 3.52. Found: N, 3.04; C,
56.18; H, 3.91.
1
(m, 3H), 6.20 ~ 6.14 (m, 3H), 6.04 (t, J = 8.8 Hz, 2H). 19F-{ H}
Selected crystal data of 3: C89H72Cl14Ir2N4P4; M = 2202.09; mon-
1
NMR (470 MHz, CDCl3, 294 K): d -60.65 (s, 3F). 31P-{ H} NMR
˚
˚
oclinic; space group = P 21/c; a = 21.5529(4) A, b = 14.1688(3) A,
(202 MHz, CDCl3, 294 K): d 14.92 (d, JPP = 12.1 Hz, 1P), 14.18
(d, JPP = 12.1 Hz, 1P). Anal. calcd. for C53H37F3IrN3P2: N, 4.09;
C, 61.98; H, 3.63. Found: N, 4.04; C, 61.88; H, 4.02.
◦
3
˚
˚
c = 29.0378(5) A, b = 102.3146(12) , V = 8663.5(3) A ; Z = 4;
rc = 1.688 Mg m-3; F(000) = 4344; crystal size = 0.25 ¥ 0.12 ¥
3
-1
˚
0.08 mm ; l(Mo-Ka) = 0.71073 A; T = 150(2) K; m = 3.623 mm ;
55017 reflections collected, 19863 independent reflections (Rint
1
Spectra data of 5b. MS (FAB, 193Ir): m/z 1028 (M+1)+; H
=
0.0701), GOF = 1.014, final R1[I > 2s(I)] = 0.0487 and wR2(all
data) = 0.1337.
NMR (500 MHz, CDCl3, 294 K): d 7.90 (d, J = 7.5 Hz, 1H), 7.71
(d, J = 8.0 Hz, 1H), 7.53 (d, J = 8.0 Hz, 1H), 7.50 ~ 7.35 (m, 5H),
7.34 ~ 7.28 (m, 3H), 7.27 ~ 7.20 (m, 4H), 7.12 ~ 7.00 (m, 6H), 6.88
(t, J = 7.3 Hz, 1H), 6.80 (t, J = 7.8, Hz, 2H), 6.70 ~ 6.65 (m, 3H),
6.57 (t, J = 7.8 Hz, 2H), 6.49 (t, J = 8.6 Hz, 2H), 6.38 ~ 6.29 (m,
Synthesis of [Ir(dppi)2(OAc)] (4). A mixture of 3 (100 mg,
0.11 mmol) and sodium acetate (46 mg, 0.56 mmol) in decalin
(6 mL) was heated to reflux for 3.5 h. After cooling to RT, the
solvent was removed under vacuum and the residue was dissolved
in CH2Cl2 (20 mL) and the solution washed with H2O, dried over
anhydrous Na2SO4 and filtered. The filtrate was concentrated on a
rotavap to give a crude product, which was then purified by silica
gel column chromatography using a 1 : 1 mixture of ethyl acetate
and hexane as eluent. Finally, the yellow crystals were crystallized
from a mixture of CH2Cl2 and hexane at RT (4, 31 mg, 0.04 mmol,
31%).
1
5H), 6.16 (t, J = 6.0 Hz, 1H). 19F-{ H} NMR (470 MHz, CDCl3,
1
294 K): d -60.50 (s, 3F). 31P-{ H} NMR (202 MHz, CDCl3,
294 K): d 13.93 (d, JPP = 11.1 Hz, 1P), 13.61 (d, JPP = 11.1 Hz,
1P). Anal. calcd. for C53H37F3IrN3P2: N, 4.09; C, 61.98; H, 3.63.
Found: N, 3.82; C, 61.32; H, 4.03.
Synthesis of [Ir(dpna)2(iqbtz)] (6a and 6b). A similar procedure
was conducted as described for 5a and 5b using in situ generated
acetate complex 2, isomeric (6a) and (6b) were obtained in 61%
and 17%. The single crystals of 6a and 6b were obtained from a
mixture of CH2Cl2 and hexanes, and from a mixture of CH2Cl2
and methanol at RT, respectively.
Spectra data of 4. MS (FAB, 193Ir): 877 m/z (M+1)+; 1H NMR
(500 MHz, CD2Cl2, 298 K): d 8.47 (d, J = 5.5 Hz, 1H), 8.44 ~ 8.40
(m, 3H), 8.23 (t, J = 5.5 Hz, 1H), 7.82 (t, J = 10.0 Hz, 1H), 7.74
~ 7.71 (m, 1H), 7.60 ~ 7.54 (m, 3H), 7.41 ~ 7.35 (m, 2H), 7.28 ~
7.23 (m, 3H), 7.21 ~ 7.15 (m, 5H), 7.05 (d, J = 8.5 Hz, 1H), 6.79 (t,
J = 9.0 Hz, 2H), 6.73 (t, J = 7.0 Hz, 1H), 6.66 (t, J = 8.0 Hz, 1H),
6.38 (td, J = 7.5, 2.5 Hz, 2H), 6.12 (d, J = 7.0 Hz, 1H), 5.87 (t, J =
1
Spectra data of 6a. MS (FAB, 193Ir): m/z 1067 (M+1)+; H
NMR (500 MHz, CDCl3, 294 K): d 10.10 (d, J = 9.0 Hz, 1H),
8.24 (t, J = 10.0 Hz, 2H), 8.10 (t, J = 10.0 Hz, 2H), 7.84 ~ 7.78 (m,
2H), 7.62 ~ 7.54 (m, 3H). 7.52 ~ 7.46 (m, 2H), 7.40 ~ 7.32 (m, 3H),
7.29 (t, J = 7.7 Hz, 1H), 7.24 ~ 7.17 (m, 3H), 7.15 (d, J = 8.0 Hz,
1H), 7.07 (m, 1H), 6.96 (t, J = 6.5 Hz, 3H), 6.80 (d, J = 6.5 Hz,
1H), 6.65 (t, J = 8.0 Hz, 1H), 6.59 (t, J = 8.0 Hz, 1H), 6.41 (d, J =
7.0 Hz, 1H), 6.34 ~ 6.18 (m, 6H), 6.15 (t, J = 8.5 Hz, 2H), 5.83 (t,
1
10.0 Hz, 2H), 1.59 (s, 3H). 31P-{ H} NMR (202 MHz, CD2Cl2,
298 K): d 24.30 (d, JPP = 6.06 Hz, 1P), 19.75 (d, JPP = 6.06 Hz,
1P). Anal. calcd. for C44H33IrN2O2P2: N, 3.20; C, 60.33; H, 3.80.
Found: N, 2.94; C, 60.48; H, 4.11.
1
Synthesis of [Ir(dpna)2(fppz)] (5a and 5b) from 2. A mixture of
2 (100 mg, 0.11 mmol), fppzH (27 mg, 0.13 mmol) and sodium
acetate (11 mg, 0.13 mmol) in decalin (6 mL) was heated to reflux
for 13 h. After cooling to RT, the solvent was removed and the
J = 8.5 Hz, 2H), 1.49 (s, 9H). 31P–{ H} NMR (202 MHz, CDCl3,
294 K): d 16.04 (d, JPP = 12.4 Hz, 1P), 15.58 (d, JPP = 12.4 Hz, 1P).
Anal. calcd. for C59H47IrN4P2: N, 5.25; C, 66.46; H, 4.44. Found:
N, 5.11; C, 66.15; H, 4.45.
1134 | Dalton Trans., 2011, 40, 1132–1143
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Selected crystal data of 6a: C62H54IrN4P2; M = 1109.23;
complex 4 and iqbtzH. The orange crystals of 8a were then
crystallized from a mixture of CH2Cl2 and methanol at RT.
˚
orthorhombic; space group = Pna21; a = 23.3005(5) A, b =
3
˚
˚
˚
12.3608(3) A, c = 33.9148(7) A, V = 9767.9(4) A ; Z = 8; rc =
1
Spectra data of 8a. MS (FAB, 193Ir): m/z 1068 (M+2)+; H
1.509 Mg m-3; F(000) = 4488; crystal size = 0.30 ¥ 0.15 ¥ 0.05 mm3;
l(Mo-Ka) = 0.71073 A; T = 150(2) K; m = 2.846 mm ; 39338
NMR (500 MHz, CDCl3, 294 K): d 10.13 (br, 1H), 8.74 (t, J =
10.0 Hz, 2H), 8.56 (d, J = 5.0 Hz, 1H), 8.49 (d, J = 5.0 Hz, 1H),
8.18 ~ 8.08 (m, 2H), 7.62 (br, 2H), 7.50 ~ 7.42 (m, 3H), 7.44 (br,
1H), 7.38 (t, J = 7.0 Hz, 1H), 7.28 ~ 7.22 (m, 3H). 7.21 ~ 7.14 (m,
3H), 7.03 (t, J = 7.0 Hz, 2H), 6.89 ~ 6.78 (m, 2H), 6.58 (t, J =
7.0 Hz, 1H), 6.49 (br, 1H), 6.38 (d, J = 7.0 Hz, 1H), 6.32 ~ 6.22
(m, 4H), 6.19 ~ 6.13 (m, 1H), 6.12 ~ 6.04 (m, 2H), 5.65 (t, J =
-1
˚
reflections collected, 20536 independent reflections (Rint = 0.0529),
GOF = 1.007, final R1[I > 2s(I)] = 0.0522 and wR2(all data) =
0.1393.
1
Spectra data of 6b. MS (FAB, 193Ir): m/z 1068 (M+2)+; H
NMR (500 MHz, CDCl3, 294 K): d 10.08 (d, J = 6.5 Hz, 1H),
7.89 (d, J = 7.5 Hz, 1H), 7.70 (d, J = 7.5 Hz, 1H), 7.67 ~ 7.63
(m, 2H), 7.59 ~ 7.54 (m, 1H), 7.53 ~ 7.46 (m, 2H), 7.39 ~ 7.27 (m,
8H), 7.22 ~ 7.18 (m, 2H), 7.16 ~ 7.14 (m, 3H), 7.02 (t, J = 7.5 Hz,
1H), 6.96 (t, J = 7.5 Hz, 1H), 6.88 ~ 6.81 (m, 3H), 6.75 (d, J =
6.5 Hz, 1H), 6.70 ~ 6.63 (m, 3H), 6.55 (t, J = 7.0 Hz, 2H), 6.44 ~
6.36 (m, 4H), 6.32 (d, J = 7.0 Hz, 1H), 5.95 ~ 5.92 (m, 1H), 1.14
1
10.0 Hz, 2H), 1.56 (s, 9H). 31P-{ H} NMR (202 MHz, CDCl3,
294 K): d 17.55 (d, Jpp = 11.1 Hz, 1P), 13.51 (d, Jpp = 11.1 Hz, 1P).
Anal. calcd. for C57H45IrN6P2: N, 7.87; C, 64.09; H, 4.25. Found:
N, 7.93; C, 63.82; H, 4.46.
X-Ray diffraction studies. Single crystal X-ray diffraction data
were measured on a Bruker SMART Apex CCD diffractometer
1
(s, 9H), 31P-{ H} NMR (202 MHz, CDCl3, 294 K): d 14.49 (d,
˚
using (Mo-Ka) radiation (l = 0.71073 A). The data collection
JPP = 11.1 Hz, 1P), 14.40 (d, JPP = 11.1 Hz, 1P). Anal. calcd. for
C59H47IrN4P2: N, 5.25; C, 66.46; H, 4.44. Found: N, 4.91; C, 66.27;
H, 4.55.
was executed using the SMART program. Cell refinement and
data reduction were performed with the SAINT program. The
structure was determined using the SHELXTL/PC program and
refined using full-matrix least squares.
Selected crystal data of 6b: C60H49Cl2IrN4P2; M = 1151.07;
¯
˚
˚
triclinic; space group = P1; a =◦10.6136(5) A, b = 13.8180(6) A,
◦
◦
˚
c = 17.5871(8) A, a = 83.412(1) , b = 74.040(1) , g = 82.624(1) ,
V = 2450.81(19) A ; Z = 2; rc = 1.560 Mg m ; F(000) = 1156;
Photophysical measurement and OLED fabrication. Steady-
state absorption, emission, and phosphorescence lifetime measure-
ments both in solution and in the solid state have been described
in our previous reports.17 For measuring quantum yields in the
solid state, an integrating sphere (Labsphere) was used. The solid
film was prepared via a vapor deposition method and was excited
by a 514-nm Ar+ laser line. The emission was then acquired by
an intensified charge-coupled detector for subsequent analyses.18
The fabrication procedures for the OLED devices including those
for patterning and cleaning of ITO substrates followed those
described in the literature.19
3
-3
˚
3
˚
crystal size = 0.50 ¥ 0.08 ¥ 0.03 mm ; l(Mo-Ka) = 0.71073 A;
T = 150(2) K; m = 2.944 mm-1; 31716 reflections collected, 11214
independent reflections (Rint = 0.0564), GOF = 1.068, final R1[I >
2s(I)] = 0.0414 and wR2(all data) = 0.0923.
Synthesis of [Ir(dppi)2(fppz)] (7a). This complex was obtained
in 32% yield from the reaction between the in situ generated acetate
complex 4 and fppzH. The brownish crystals of (7a) suitable for
single crystal X-ray diffraction study were obtained from a mixture
of CH2Cl2 and methanol at RT.
Computational methodology. All the lowest electronic singlet
state (S0) geometries were performed using the density func-
tional theory (DFT) with B3LYP hybrid functional in restricted
formalism.20 A “double-z “quality basis set consisting of Hay and
Wadt’s effective core potentials (LANL2DZ)21 was employed for
the Ir(III) atom, and a 6-31G* basis set,22 for H, C, N, F and P
atoms. The relativistic effective core potential (ECP) replaced the
inner core electrons of the Ir(III) metal atom, leaving the outer
core valence electrons (5s25p65d6) to be concerned. In order to get
a better insight into the nature of the spectroscopic properties,
time-dependent DFT (TD-DFT) calculations23 were used based
on the optimized geometries at the lowest singlet state to probe the
absorptive and emissive characters. Typically, the 10 lower triplet
and singlet roots of the non-Hermitian eigenvalue equations were
obtained to determine the vertical excitation energies. Oscillator
strengths were deduced from the dipole transition matrix elements
(for singlet states only). All calculations were carried out using
Gaussian 03.24
Spectra data of 7a. MS (FAB, 193Ir): m/z 1029 (M+); 1H NMR
(500 MHz, CDCl3, 294 K): d 8.64 (t, J = 8.0 Hz, 2H), 8.58 (d,
J = 5.5 Hz, 1H), 8.45 (d, J = 5.5 Hz, 1H), 7.92 (d, J = 9.8 Hz,
2H), 7.51 (d, J = 8.0 Hz, 1H), 7.49 ~ 7.44 (m, 2H), 7.40 ~ 7.32
(m, 2H), 7.31 ~ 7.20 (m, 4H), 7.20 ~ 7.14 (m, 2H), 7.12 (d, J =
8.0 Hz, 1H), 7.04 (t, J = 8.0 Hz, 2H), 6.88 ~ 6.78 (m, 2H), 6.72
(s, 1H), 6.62 ~ 6.52 (m, 3H), 6.50 (t, J = 6.0 Hz, 1H), 6.41 (t, J =
6.0 Hz, 1H), 6.34 ~ 6.28 (m, 3H), 6.12 (t, J = 9.0 Hz, 2H), 5.82
1
(t, J = 8.5 Hz, 2H). 19F-{ H} NMR (470 MHz, CDCl3, 294 K): d
1
-60.80 (s, 3F). 31P-{ H} NMR (202 MHz, CDCl3, 294 K): d 16.17
(d, JPP = 12.9 Hz, 1P), 12.54 (d, JPP = 12.9 Hz, 1P). Anal. calcd.
for C51H35F3IrN5P2: N, 6.52; C, 59.22; H, 3.92. Found: N, 6.81; C,
59.53; H, 3.91.
Selected crystal data of 7a: C51H35F3IrN5P2; M = 1028.98;
˚
monoclinic; space group = P 21/n; a = 11.8887(6) A, b =
◦
3
˚
˚
˚
18.6773(10) A, c = 18.6889(11) A, b = 91.080(1) , V = 4149.1(4) A ;
Z = 4; rc = 1.647 Mg m-3; F(000) = 2040; crystal size = 0.24 ¥ 0.20 ¥
3
-1
˚
0.10 mm ; l(Mo-Ka) = 0.71073 A; T = 150(2) K; m = 3.353 mm ;
31752 reflections collected, 9539 independent reflections (Rint
Compositions of molecular orbitals in terms of the constituent
chemical fragments were calculated using the AOMix program.25
For the characterization of the HOMO-x → LUMO+y transi-
tions as partial charge transfer (CT) transitions, the following
definition of the CT character has been used:
=
0.0362), GOF = 1.041, final R1[I > 2s(I)] = 0.0279 and wR2(all
data) = 0.0634.
Synthesis of [Ir(dppi)2(iqbtz)] (8a). This complex was obtained
in 45% yield from reaction between the in situ generated acetate
CT(M) = %(M)HOMO - x - %(M)LUMO + y
Dalton Trans., 2011, 40, 1132–1143 | 1135
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Where %(M)HOMO-x and %(M)LUMO+y are electronic
densities on the metal in HOMO-x and LUMO+y. If the excited
state, e.g., S1 or T1, is formed by more than one one-electron
excitation, then the metal CT character of this excited state
is expressed as a sum of CT characters of each participating
excitation, i → j:
CTI (M) = R[CI(i - j)]2 (%(M)i - %(M)j)
where CI(i→j) are the appropriate coefficients of the I-th eigen-
vector of the CI matrix. Accordingly, one can very effectively use
the MO compositions in terms of fragment orbital contributions
to probe the nature of the electronic transitions.
Results and discussion
Synthesis and structural characterization
It has been reported that the bis-cyclometalated Ir(III) com-
plex [Ir(bdp)2(OAc)] was prepared by the in situ treatment
of tetrahydrothiophene reagent [IrCl3(tht)3] with two equiv. of
benzyldiphenylphosphine (bdpH) and sodium acetate in refluxing
decalin.26 The reaction possibly proceeded through a prior forma-
tion of intermediate with only one cyclometalated bdp chelate,
followed by second cyclometalation and concomitant acetate
coordination. This delineation is in good agreement with literature
precedence, which stated that the Ir(III) reagent IrCl3·nH2O is capa-
ble of reacting with dimethyl(1-naphthyl)phosphine (dmnpH), giv-
ing [IrCl2(dmnp)(dmnpH)2] with only one cyclometalated dmnp
phosphine, while the remaining dmnpH retained the monodentate
bonding mode.27 This observation confirms the stepwise nature of
phosphine cyclometalation. The result then prompted us to make
a further attempt to isolate possible intermediate species.
To conduct this experiment, we then switched to two
slightly distinctive phosphine ligands, namely: 1-(diphenylpho-
sphino)naphthalene (dpnaH) and 1-(diphenylphosphino)-
isoquinoline (dppiH), for their capability in forming rigid and
highly stable five-membered metallacycles around the central
transition metal element. Following the synthetic protocols
showed in Scheme 1, we first obtained [Ir(dpna)(tht)2Cl2] (1) in
good yield by treatment of [IrCl3(tht)3] with an equal amount
of dpnaH, while the respective double cyclometalated product
[Ir(dpna)2(OAc)] (2) was isolated by subsequent treatment of
1 with another equiv. of dpnaH in the presence of sodium
acetate. Both complexes were characterized by spectroscopic
methods, while complex 1 was further identified by single crystal
X-ray diffraction analysis. As a result, the coexistence of two
cis- chlorides and two trans-coordinated tht is unambiguously
confirmed (see Fig. 1). In good agreement with this assignment,
Fig. 1 ORTEP diagram of 1 with ellipsoids shown at 40% probability
level; selected bond distances: Ir–P(1) = 2.2485(10), Ir–Cl(1) = 2.4813(9),
Ir–Cl(2) = 2.4263(9), Ir–S(1) = 2.3538(9), Ir–S(2) = 2.3485(9) and Ir–C(1) =
˚
2.053(4) A.
the 1H NMR spectrum exhibited expected multiplets in the region
between d 3.82 ~ 1.51, which can be assigned to the methylene
groups of the tht ligands. This result provides indirect support
that milder reaction conditions prevent further dissociation of
chloride as well as tht ligands. On the other hand, 1H NMR
spectra of 2 show the complete removal of tht ligands and
coordination of two dpna chelates plus acetate, as shown in
the aforementioned Ir(III) intermediate [Ir(bdp)2(OAc)], bdpH =
benzyldiphenylphosphine. It is notable that we utilize [Ir(tht)3Cl3]
as the metal source to replace the conventional IrCl3·nH2O during
all attempts, simply because [Ir(tht)3Cl3] retains good solubility in
aliphatic hydrocarbons such as decalin. By doing so, we can avoid
the use of protic 2-methoxyethanol, which is known to promote
metal hydride formation28 and is undesirable for the intended
cyclometalation of naphthyl or isoquinolinyl groups.
As for the reaction with a second type of phosphine, i.e. dppiH,
due to the presence of additional nitrogen donor at the isoquino-
linyl site, simple heating of [IrCl3(tht)3] and an equal amount
of dppiH afforded only a small amount of [Ir(dppiH)(dppi)Cl2]
(3), for which its 31P NMR spectrum showed incorporation of
two dppiH fragments around the central Ir(III) metal element
(Scheme 2). Moreover, upon addition of two equiv. of dppiH
versus that of [IrCl3(tht)3], we obtained a much improved yield
Scheme 1 Synthetic route to complexes 1 and 2; reagents and conditions:
(i) 1 eq. dpnaH, decalin, 196 ◦C, 1.5 h; (ii) 1 eq. dpnaH, NaOAc, decalin,
196 ◦C, 2.5 h.
Scheme 2 Synthetic route to complexes 3 and 4; reagents and conditions:
(i) 2 eq. dppiH, decalin, 196 ◦C, 1 h; (ii) NaOAc, decalin, 196 ◦C, 2 h.
1136 | Dalton Trans., 2011, 40, 1132–1143
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for 3, for which the subsequent X-ray structural analysis showed
existence of one chelating dppiH, one cyclometalating dppi chelate
and two cis-arranged chlorides (Fig. 2). Although the exact
reaction mechanism is pending resolution, the retention of two
cis-chlorides and one dppiH as a neutral chelate seems to be
a major factor, which stabilizes the unexpected structure of 3.
Moreover, heating of 3 in refluxing decalin for 2 h afforded
a second Ir(III) product [Ir(dppi)2(OAc)] (4), which possesses
two cyclometalating dppi chelates and one chelating acetate.
Apparently, successful conversion from 3 to 4 requires a prior
dissociation of N-coordinated isoquinolinyl fragment, rendering
a temporal vacuum coordination site; the latter then induced
the second cyclometalation, together with concomitant chloride
elimination and acetate coordination.
In addition to the anticipated Ir(III) products [Ir(dpna)2(fppz)] (5a)
and [Ir(dpna)2(iqbtz)] (6a), we also isolated a small amount of the
second isomers, which are denoted as (5b) and (6b), under the
conditions of one-pot syntheses that employed the in situ generated
acetate complex 2, followed by addition of respective fppzH and
iqbtzH chelates. On the other hand, if we switched to the purified
acetate precursor 2, the yield of b is significantly increased versus
that of a, despite the fact that there is no sign of interconversion
between these isomers at the same temperature. The molecular
drawings of 5a/5b and 6a/6b are depicted in Scheme 3, for which
their identities were characterized inter alia by mass spectrometry
and NMR spectroscopy. While 1H NMR data are difficult to
extract valuable structural information from, the involvement of
two PPh2 units is unambiguously identified by the observation of
dual 31P NMR signals, for which both appeared as doublets due
to the P–P coupling of 11–12 Hz, revealing that the phosphorus
nuclei are located at the mutual cis-disposition.
Fig. 2 ORTEP diagram of 3 with ellipsoids shown at 40% probability
level; selected bond distances: Ir(1)–P(1) = 2.2530(16), Ir(1)–Cl(1) =
2.4053(16), Ir(1)–Cl(2) = 2.4552(16), Ir(1)–P(2) = 2.2729(17), Ir(1)–N(2) =
˚
2.127(5) and Ir(1)–C(1) = 2.047(6) A.
The crystal structures of 1 and 3 are depicted in Fig. 1 and
2, respectively. Naturally, one important feature is the existence
of cis-arranged terminal chlorides, for which the Ir–Cl bond
distances are akin to the relevant distances observed in other
Ir(III) complexes such as cis-[Ir(fppz)2Cl2]H.29 Moreover, the Ir–
Cl vectors located opposite to the carbon donor seem to be
slightly elongated, which we attributed to the pronounced trans-
effect exerted by the cyclometalating groups. For complex 3,
there is a neutral dppiH chelate that interacts with the central
Ir(III) metal atom via formation of a four-membered Ir–N–C–P
metallacycle, a way that is similar to that of the 2-pyridyl phosphine
chelate.30 Close inspection of all metric parameters associated with
this four-membered metallacycle shows no noticeable elongation
away from the typical metal–ligand bonding distances, except
for the much acute bond angle N(2)–Ir(1)–P(2) = 68.6(1)◦. Such
an observation confirms its thermodynamic stability and hence
temporary retardation against the second cyclometalation, which
can only be accelerated upon addition of the sodium acetate
promoter.
Scheme 3 Structural drawing of the emissive Ir(III) complexes 5, 6, 7
and 8.
Next, the X-ray structural analyses were conducted to unveil
their intrinsic differences. As shown in Fig. 3 and 4, molecular
structures of 6a and 6b show a pseudo-octahedral ligand ar-
rangement around the Ir(III) center with the expected cis-oriented
cyclometalating PPh2 fragments. It appears that the metric pa-
rameters associated with the unique trans-C–Ir–P segment are
˚
essentially identical to each other, cf. Ir(1)–C(16) = 2.084(9) A,
◦
˚
Ir(1)–P(2) = 2.336(2) A and ∠C(16)–Ir(1)–P(2) = 175.3(3) in 6a
and Ir–C(23) = 2.090(4), Ir–P(1) = 2.3261(12) A and ∠C(23)–Ir–
˚
P(1) = 173.78(12)◦ in 6b. Furthermore, the reaction of relevant
isoquinolinyl derivative 4 with pyrazole fppzH and with triazole
iqbtzH was also conducted, showing formation of only one
isomer, i.e. the more abundant isomers a, [Ir(dppi)2(fppz)] (7a)
and [Ir(dppi)2(iqbtz)] (8a), respectively. This finding confirms
the delicate balance of various factors upon forming the final
products. For the final comparison, the X-ray structure of 7a that
is coordinated by dppi and fppz chelates is also determined (Fig. 5).
Significantly, all metal–ligand distances of 7a are identical to that
of 6a within the experimental error, irrespective to the seemingly
distinctive shape of fppz and iqbtz chelates.
To obtain the anticipated emissive Ir(III) complexes, an acetate
exchanging reaction on 2 was then conducted using two selected
azolate chelates, namely 5-(2-pyridyl)-3-trifluoromethyl pyrazole
(fppzH) and 5-(1-isoquinolyl)-3-tert-butyl-1,2,4-triazole (iqbtzH).
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Fig. 3 ORTEP diagram of 6a with ellipsoids shown at 50% probability
level; selected bond distances: Ir(1)–P(1) = 2.281(2), Ir(1)–N(1) = 2.134(6),
Ir(1)–P(2) = 2.336(2), Ir(1)–C(16) = 2.084(9), Ir(1)–C(38) = 2.071(8) and
˚
Ir(1)–N(2) = 2.117(7) A.
Fig. 5 ORTEP diagram of 7a with ellipsoids shown at 40% probability
level; selected bond distances: Ir–P(1) = 2.3243(7), Ir–C(22) = 2.088(3),
Ir–P(2) = 2.2831(8), Ir–N(1) = 2.143(2), Ir–C(1) = 2.062(3) and Ir–N(2) =
˚
2.136(2) A.
Fig. 4 ORTEP diagram of 6b with ellipsoids shown at 50% probability
level; selected bond distances: Ir–P(1) = 2.3261(12), Ir–C(23) = 2.090(4),
Ir–P(2) = 2.2830(11), Ir–N(2) = 2.056(4), Ir–C(1) = 2.054(4) and Ir–N(1) =
Fig. 6 Absorption and normalized emission spectra of complexes 5a, 5b
and 7a (group A) recorded in CH2Cl2 solution at RT.
˚
2.201(4) A.
singlet manifold. Further detailed assignment will be given in the
computational approach section.
Photophysical properties. All these studied molecules could
be divided into two groups according to their different emissive
chromophores, namely: group A: 5a, 5b, 7a with fppz chelate and
group B: 6a, 6b, 8a with iqbtz chelate, respectively. The absorption
and emission spectra of all these two groups recorded in degassed
CH2Cl2 at room temperature are displayed in Fig. 6 and 7, and
the pertinent data are summarized in Tables 1 and 2. In general,
all complexes possess intense absorption bands in the UV region
of < 300 nm, for which e values at the absorption maxima were
measured to be ≥ 104 M-1 cm-1 and can thus be attributed to ligand-
centered pp* transition. The broad band at longer wavelengths of
up to 450 nm, as supported by their relatively lower extinction
coefficients, is assigned to the tail of pp* transition overlapping
with the metal-to-ligand charge transfer transition (MLCT) in the
As for the emission spectra, the significant variation of emission
intensity between aerated and degassed solution ensures the
emission originating from the triplet manifold, i.e. the phospho-
rescence. The emission profiles for these two groups consist of a
notable vibronic progression (500–800 nm), in which the emission
of group B is slightly red shifted compared with that of A (the
first vibronic peak appearing at 520 and 550 nm for A and B,
respectively) due to the extended conjugation of isoquinoline
and the tert-butyl substituent executed on the triazolate moiety,
resulting in a decrease of the pp* energy gap (see Fig. 6–7).
The poor overlap between the emission profile and the lowest
energy absorption band can be attributed to the origin of emission
possessing excessive ligand-centered pp* and relatively low MLCT
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Table 1 Photophysical data of Ir(III) complexes 5a, 5b and 7a recorded in CH2Cl2 at room temperature
Photophysical propertiesa
E
1/2/V [DEp/V]b
UV-Vis data (nm) [e ¥
PL lmax(nm)
Q.Y. (%)
tobs (ms)
kr (s-1)
knr (s-1)
Eox
Ere
10-3, M-1 cm-1]
5a
5b
7a
320 [11.9], 350 [9.0]
322 [13.3], 350 [10.3]
318 [9.7], 360 [9.6]
534, 572, 615
535, 576, 619
541, 576, 616
14
11
22
42.3
28.6
53.5
3.3 ¥ 103
3.93 ¥ 103
4.11 ¥ 103
2.03 ¥ 104
3.10 ¥ 104
1.46 ¥ 104
0.76 [irr]
0.64 [irr]
0.87 [irr]
-1.54 [irr]
-1.59 [irr]
-1.59 [irr]
a Samples were degassed and recorded in CH2Cl2 at RT with e in M-1 cm-1. b All electrochemical potentials were measured in a 0.1 M TBAPF6/CH2Cl2
and THF solution for oxidation and reduction reaction, and reported in Volts using Fc/Fc+ as reference; DEp is defined as Eap (anodic peak potential) -
Ecp (cathodic peak potential) and these data are quoted in mV. The glassy carbon and Au(Hg) alloy were selected as the working electrode of oxidation
and reduction processes, respectively.
Table 2 Photophysical data of Ir(III) complexes 6a, 6b and 8a recorded in CH2Cl2 at room temperature
Photophysical propertiesa
E
1/2/V [DEp/V]b
UV-Vis absorption (nm) [e ¥
PL lmax (nm)
Q.Y. (%)c
tobs (ms)
kr (s-1)
knr (s-1)
Eox Ere
10-3, M-1 cm-1]
d
c
c
6a 324 [14.7], 354 [15.0], 415 [4.2] 553, 593, 640, {550, 593, 638}
~ 100, {89}
59
~ 100
25.6, {34.8}
3.91 ¥ 104 __
2.72 ¥ 104 1.89 ¥ 104 0.62 [irr] -2.38 [103]
6.31 ¥ 104 __
0.83 [irr] -2.34 [146]
0.63 [irr] -2.37 [108]
6b 320 [13.8], 360 [13.6]
8a 360 [12.4], 363 [15.3]
566, 600, 639
551, 591, 643
21.7
15.8
a Samples were degassed and recorded in CH2Cl2 at RT with e in M-1 cm-1. b All electrochemical potentials were measured in a 0.1 M TBAPF6/CH2Cl2
and THF solution for oxidation and reduction reactions, and reported in Volts using Fc/Fc+ as reference; DEp is defined as Eap (anodic peak potential)
- Ecp (cathodic peak potential) and these data are quoted in mV. The glassy carbon and Au(Hg) alloy were selected as the working electrode of oxidation
and reduction processes, respectively. c All samples were excited at 400 nm. The reference dyes of quantum yield measurement for 6a and 6b (8a) are DCM
(QY = 0.44 in methanol) and Coumarin 480 (QY = 0.87 in methanol), respectively. d Data were recorded in CBP as the host material at a concentration
of 8 wt%.
vibronic features confirm their large portion of 3pp* character in
nature for the phosphorescence.
The following TDDFT calculation and the molecular orbital
analysis listed in Fig. 8–9 and Table 3 provide additional supports
for the above assignments, i.e. information on the selected
molecular orbitals involved in the lower-lying transitions of the
titled complexes. All pertinent energy gaps and corresponding
assignments of each transition are listed in Table 3. Taking
5a for example, the calculated energy gaps of the S0 → S1
(408.7 nm) and S0 → T1 (499 nm) transitions are close to the
observed onsets and the blue-edge of the absorption and emission
spectra recorded in CH2Cl2 (see Fig. 6), respectively (vice versa
for the rest of the complexes). These results indicate that the
TDDFT calculation, in a qualitative manner, can predict the
lowest Franck–Condon absorption in singlet manifold as well
as the phosphorescence energy gap based on the S0 geometries
of the studied organometallic complexes. As for the transition
characteristic, detailed frontier orbital analyses indicate that there
are distinct differences in their properties between complexes
Fig. 7 Absorption and normalized emission spectra of complexes 6a, 6b
and 8a (group B) recorded in CH2Cl2 solution at RT.
of groups A and B. For A, the main S0 → T1 transitions (in
character. It has been well established that the metal dp orbital
view of pp* transition only) is assigned to be intra-ligand charge
transfer (ILCT, see Fig. 8 and Table 3, HOMO-1 → LUMO+1),
mainly delocalized on the naphthalene/isoquinoline moiety of
the chelating phosphine ancillary. As for B, in sharp contrast,
the ILCT is mainly delocalized on the tert-butyl-isoquinolinyl
triazolate chromophore (see Fig. 9 and Table 3). The difference in
transition character may be ascribed to the extended conjugation
of isoquinoline and the tert-butyl substituent executed on the
triazolate moiety of the iqbtz chelate, the result of which decreases
the pp* energy gap effectively.
is directly coupled into the spin–orbit coupling matrix. Thus,
low MLCT percentage causes poor mixing between singlet and
triplet manifolds, resulting in a relatively large separation between
the absorption (S0–S1) and emission bands (T1–S0).31 As for the
emission intensity, the phosphorescence of series B is strong,
as indicated by their Q.E. of ~1 (6a), 0.59 (6b) and ~1 (8a) in
degassed CH2Cl2 solution at RT. Conversely, Q.E. for series A are
relatively low with 0.14 (5a), 0.11 (5b) and 0.22 (7a). Nevertheless,
for all complexes studied, the long radiative lifetime (ꢀ 10 ms,
see Tables 1 and 2) deduced from their photophysical data and
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Table 3 Calculated wavelength (l), oscillator Strength (f ), MLCT% and transition analyses
States
lcal
F
Assignments
MLCT (%)
5a
5b
T1
S1
499
408.7
0
HOMO-1 → LUMO+1 (100%)
HOMO → LUMO (97%)
HOMO → LUMO+2 (40%)
HOMO → LUMO+3 (36%)
HOMO → LUMO+5 (31%)
HOMO → LUMO (96%)
HOMO-2 → LUMO (77%)
HOMO → LUMO (18%)
HOMO-3 → LUMO (10%)
HOMO → LUMO (97%)
HOMO-2 → LUMO (79%)
HOMO-1 → LUMO (18%)
HOMO-3 → LUMO (12%)
HOMO → LUMO (68%)
HOMO-1 → LUMO (31%)
HOMO-1 → LUMO (93%)
HOMO → LUMO (7%)
15.82
14.81
0.0035
T1
S1
T1
S1
T1
S1
T1
S1
T1
501.2
399
0
11.11
10.21
8.67
0.0002
6a
6b
7a
8a
533.9
452.7
537.3
435.3
488.1
389.4
535.8
0
0.0168
15.70
11.00
13.64
20.01
19.70
17.22
0
0.0016
0
0.0025
0
HOMO → LUMO+1 (97%)
HOMO → LUMO (46%)
HOMO-2 → LUMO (30%)
HOMO-1 → LUMO (25%)
HOMO-3 → LUMO (8%)
HOMO → LUMO (94%)
S1
429.7
0.0305
19.85
Fig. 8 Selected frontier orbitals involved in the S0 → T1 transition for group A complexes (ie. 5a, 5b and 7a).
Moreover, because the lowest-lying transition is mainly located
(according to a qualitative approach for a hydrogen like atom),32
in which r is the distance between Ir(III) center and the ancillary
chromophore. Therefore, group A complexes are expected to have
weaker heavy atom-induced spin–orbit coupling (cf. group B),
giving less allowed T1–S0 transitions and hence smaller radiative
decay rate constants, kr. This viewpoint is evidenced by the kr
value for 5a, 5b and 7a (group A) of 3.3 ¥ 103 s-1, 3.93 ¥ 103 s-1
and 4.11 ¥ 103 s-1, respectively, which is smaller than that of
6a, 6b and 8a (group B) of 3.91 ¥ 104 s-1, 2.72 ¥ 104 s-1 and
6.31 ¥ 104 s-1) by more or less one order of the magnitude. Similar
types of reduced emission Q.E. and kr were also observed for the
Pt(II) complexes with peri-cyclometalated 9-phosphinoanthracene
on the iqbtz moiety of group B complexes, amid the transition, one
can thus envisage the electron is delocalized from the triazolate
moiety, transmitting through the central Ir(III) metal ion, and
extending to the isoquinolinyl moiety effectively. The net effect,
in a qualitative manner, is to enhance metal–ligand interaction,
hence suppress the nonradiative decay rate constant (knr), giving
rise to the intense phosphorescence. As for group A complexes, it
is notable that the p-orbitals of the naphthyl/isoquinolinyl chro-
mophore, with an additional phosphine anchor, reside relatively
far away from the Ir(III) metal atom. Knowing that spin–orbit
coupling is inversely proportional to distance (r) to the sixth power
1140 | Dalton Trans., 2011, 40, 1132–1143
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Fig. 9 Selected frontier orbitals involved in the S0 → T1 transition for group B complexes (ie. 6a, 6b and 8a).
and 9,10-diphosphinoanthracene chelates.33 Finally, all complexes
show the lowest lying S0–S1 or T1–S0 transition composed of ligand
pp* transition and to a lesser extent MLCT contribution (< 20%).
The pp* domination indirectly supports the vibronic emission
profiles and large separation between the absorption (S0–S1) and
emission bands (T1–S0) (vide supra).
carbazolyl)triphenylamine (TCTA) with a high triplet energy (ET
~ 2.76 eV) as an exciton-blocking layer36 is inserted at the 4,4¢-
bis(N-(1-naphthyl)-N-phenyl)biphenyldiamine (a-NPD)/emitter
interface, which can prevent the exciton diffusion, giving a higher
device efficiency. The low HOMO/LUMO energy levels of [1,3,5-
tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) can be beneficial
for blocking holes and facilitating electron injection/transport.37
LiF and Al served as the electron-injection layer and cathode,
respectively.
The respective device characteristics are compared in Fig. 10 and
Table 4. As indicated in Fig. 10(a), CSC-based devices give turn-on
voltages as low as 2.5 V, a maximum luminance of 19300 cd m-2
at 11 V (1820 mA cm-2) and 21100 cd m-2 at 12 V (1200 mA cm-2)
for devices I and II, respectively, which are significantly better
than those of the CBP-based devices III and IV. Apparently, the
HOMO energy level of CSC (-5.63 eV) is higher than that of the
CBP host (-6.0 eV), implying lower hole-injection barriers from
the hole-transporting TCTA (HOMO = -5.6 eV) to the emissive
layer. Fig. 10(b) shows external quantum efficiencies (hext) and
power efficiencies (hp) of the as-prepared devices, the maximum
efficiencies of devices I and II are hext = 17.1%, hP = 49.3 lm
W-1 and hext = 15%, hP = 37 lm W-1, respectively. Both have EL
efficiencies higher than those of the reference device III and IV
(CBP-based), which show max. hext = 8.9%, hP = 18.6 lm W-1 and
hext = 11%, hP = 19.5 lm W-1, respectively. Although a gradual
decrease in efficiency with increase in current density is observed,
Fabrication of OLEDs. We then selected 6a and 8a for
the fabrication of OLEDs mainly due to their near unity
emission yield. To evaluate the performance of 6a and 8a as
emitters for PhOLEDs, we employed typical structures con-
sisting of ITO/PEDOT:PSS (30 nm)/a-NPD (20 nm)/TCTA
(5 nm)/host: dopant 10% (25 nm)/TPBI (50 nm)/LiF (0.5 nm)/Al
(100 nm). Here, we used 4,5-diaza-2¢,7¢-bis(carbazol-9-yl)-9,9¢-
spirobifluorene (CSC) as host material with the dopants 6a
and 8a for devices I and II, respectively. The spiro-configured
bipolar molecule (CSC) that possesses high triplet energy (ET
=
2.65 eV), suitable energy levels (HOMO = -5.63 eV; LUMO =
-2.36 eV) and balanced ambipolar carrier mobilities (mh ª me ª
10-6 cm2 V s-1) has been successfully applied in highly efficient
single-layer green PhOLEDs.34 For comparison, we also pre-
pared conventional 4,4¢-N,N¢-dicarbazolyl-1,1¢-biphenyl (CBP)-
based devices (III and IV), which have been widely applied in
green and red PhOLEDs.35 The conducting polymer polyethylene
dioxythiophene: polystyrene sulfonate (PEDOT:PSS) is used
as a hole injection layer (HIL). A thin layer of 4,4¢,4¢-tri(N-
Table 4 EL characteristics of OLED devices I–IV
Host
Dopant
V
on/V
L
max/cd m-2
Imax [mA cm-2]
Max. hext [%]
Max. hl [cd A-1]
Max. hp [lm W-1]
I
CSC
CSC
CBP
CBP
6a
8a
6a
8a
2.5
2.5
3.0
3.0
19,300 (11 V)
21,100 (12 V)
12,400 (12 V)
12,900 (12.5 V)
1820
1200
1940
1680
17.1
15
8.9
11
47
49.3
37.0
18.6
19.5
II
42.6
23.7
27
III
IV
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when appropriate guest–host combinations and effective energy
confinement are identified.
Conclusions
The sequential transformation leading to the isolation of lumines-
cent complexes [Ir(dpna)2(fppz)] (5a and 5b), [Ir(dpna)2(iqbtz)]
(6a and 6b), [Ir(dppi)2(fppz)] (7a) and [Ir(dppi)2(iqbtz)] (8a)
were examined. The formation of coordination isomers 5a/5b
and 6a/6b seems to be determined by the nature of the cy-
clometalating phoshines. The isolable intermediates consist of
[Ir(dpna)(tht)2Cl2] (1), [Ir(dppiH)(dppi)Cl2] (3), [Ir(dpna)2(OAc)]
(2) and [Ir(dppi)2(OAc)] (4), which can be classified according
to the type of anionic chelate coordinated to the Ir(III) metal
center; i.e. dual chloride ligands versus singular chelating acetate.
Subsequent photophysical and frontier orbital analyses indicate
remarkable differences in electronic transition characteristics. For
the fppz containing complexes 5a, 5b and 7a, i.e. group A
complexes, the main S0 → T1 transitions involve intra-ligand
charge transfer (ILCT) over the naphthalene/isoquinoline moiety
of the cyclometalated dpna and dppiH chelates, plus a relatively
small percentage of MLCT contribution. In sharp contrast, the
ILCT transition of the iqbtz containing complexes 6a, 6b and
8a. i.e. group B, is mainly delocalized on the iqbtz chromophore.
Their distinction may be ascribed to the extended p-conjugation
executed on the iqbtz chelate, the result of which decreases the pp*
energy gap and suppresses the nonradiative rate (knr), giving rise to
significantly more efficient phosphorescence at longer wavelength.
These findings suggest that the related Ir(III) complexes could play
an important role for the future development of phosphorescent
OLED applications.
Acknowledgements
This work was supported by the National Science Council of
Taiwan. We are also grateful to the National Center for High-
performance Computing for computer time and facilities.
Fig. 10 (a) Current density–voltage–luminance (I–V–L) characteristics
of devices I–IV (see text for detailed description). (b) External quantum
(hext) and power efficiencies (hp) as a function of brightness. (c) Normalized
EL spectra of the devices.
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