Synthesis, photophysical and electrochemical characterization of the heteroleptic iridium complexes with modified ancillary ligand by carrier-transporting groups

Article abstract of DOI:10.1002/cjoc.201190021

Two heteroleptic iridium complexes with a general formulation of (piq) ₂Ir(G-pic) were synthesized and characterized by ¹H NMR, ¹³C NMR and element analysis, in which piq is 1-phenylisoquinoline, G-pic is picolinic acid de

Full text of DOI:10.1002/cjoc.201190021

FULL PAPER
Synthesis, Photophysical and Electrochemical Characteriza-
tion of the Heteroleptic Iridium Complexes with Modified
Ancillary Ligand by Carrier-transporting Groups
Liang, Aihui(梁爱辉)
Wang, Yafei(王亚飞)
Liu, Yu(刘煜)
Tan, Hua(谭华)
Cao, Yunbo(曹韵波)
Li, Liang(李亮)
Li, Xiaoshuang(李小双)
Zhu, Weiguo*(朱卫国)
Ma, Wenjie(马文杰)
Zhu, Meixiang(朱美香)
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education,
Xiangtan University, Xiangtan, Hunan 411105, China
Two heteroleptic iridium complexes with a general formulation of (piq)2Ir(G-pic) were synthesized and charac-
1
terized by H NMR, 13C NMR and element analysis, in which piq is 1-phenylisoquinoline, G-pic is picolinic acid
derivative containing carrier-transporting group by a non-conjugated connection of 1,6-dioxyhexane. Both
(piq)2Ir(G-pic) complexes exhibited an enhanced UV absorption band at 310—400 nm, an increased HOMO energy
level and an identical red emission peaked at 612 nm with higher fluorescence quantum efficiency (Ф ), compared
f
to (piq)2Ir(pic) in dichloromethane solution. Interestingly, this iridium complex containing both hole-transporting
triphenylamine and electron-transporting oxadiazole moieties exhibited the best Ф of 0.58 using Ru(bpy)3(PF6)2 as
f
the reference (Ф ＝0.062 in acetonitrile). This work indicates that incorporating carrier-transporting groups into an-
f
cillary ligand by a non-conjugated connection is available for improving the optophysical properties of their iridium
complexes.
Keywords Iridium complex, bipolar-transporting, 1,3,4-oxadiazole, triphenylamine, photophysics
Introduction
portating properties in OLEDs/PLEDs, respectively.
Either aryl-oxadiazole or triarylamine units were incor-
porated into cyclometalated ligands or ancillary ligands
by a conjugated connection, and the resulting heterolep-
tic iridium complexes have displayed improved opto-
electronic properties.3-8 However, few of the iridium
complexes with bipolar-transporting groups were re-
ported, in which both aryl-oxadiazole and triarylamine
units were simultaneously introduced into cyclometa-
lated iridium complexes.9
In order to further study influence of the carrier-
transporting groups on optophysical and electrochemi-
cal properties of their iridium complexes, we here re-
ported two new iridium complexes with hole-trans-
porting group of triphenylamine or bipolar-transporting
groups of triphenylamine and phenyl-oxadiazole. The
hole- or bipolar-transporting groups were incorporated
into the common ancillary ligand of picolinic acid by a
non-conjugated connection of 1,6-dioxyhexane. This
non-conjugated connection between picolinic acid and
the carrier-transporting group is expected to avoid the
shift of emission color and improve the optophysical
property of its iridium complex.
In recent decades, phosphorescent organic/polymer
light-emitting diodes (OLEDs/PLEDs) have attracted a
great deal of attention due to their promising application
in solid lighting and flat panel display. As heavy metal
complexes can utilize the electrogenerated singlet and
triplet excitons to emit with an internal quantum effi-
ciency of 100%, they have been developed as emitters
in high-efficiency OLEDs/PLEDs. In these reported
heavy metal complexes, cyclometalated iridium(III)
complexes have been considered as most valuable can-
didates owing to their higher efficient electrolumines-
cence (EL) and widely studied in the devices.1 To fur-
ther improve EL properties of iridium complexes, an
important approach was recently presented in which
some carrier-transporting functional groups with
hole-transporting or electron-transporting properties
were directly introduced into the iridium(III) com-
plexes.2
These functional groups were mostly aryl-oxadiazole
and triarylamine units because aryl-oxadiazole and tri-
arylamine possess excellent electron- and hole-trans-
*
E-mail: zhuwg18@126.com; Tel.: 0086-0731-58298280; Fax: 0086-0731-58292251
Received April 2, 2010; revised May 28, 2010; accepted June 12, 2010.
Project supported by the National Natural Science Foundation of China (Nos. 20772101 and 50973053), the Science Foundation of Hunan Province
(Nos. 2009FJ2002 and 2007FJ3017), the Postgraduate Science Foundation for Innovation in Hunan Province (Nos. S2008 yjscx09 and CX2009B124)
and the Undergraduate Innovation Experiment Plan in the Ministry of Education (No. 81053009).
Chin. J. Chem. 2010, 28, 2455— 2462
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Liang et al.
FULL PAPER
Both iridium complexes have a general formulation
of (piq)2Ir(G-pic), in which piq is 1-phenyl-isoquinoline
and G-pic is picolinic acid derivatives. The G-pic ancil-
lary ligands are 6-(6-(4'-(diphenyl-amino)biphenyl-
4-yloxy)hexyloxy) picolinic acid (TPAPh6pic) and
6-(6-(4'-((4-((6-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadi-
azol-2-yl)phenoxy)hexyloxy)methyl)phenyl)(phenyl)-
(piq)2Ir(Ph6pic) with a non carrier-transporting group
were also synthesized, in which Ph6Pic is 6-(6-pheny-
loxy hexyloxy) picolinic acid, and Pic is picolinic acid.
1
Their molecular structures were characterized by H
NMR, 13C NMR and element analysis. Their UV-vis
absorption, photo-luminescence and electrochemical
properties were studied. The results indicate that this
iridium complex with bipolar-transporting group has
exhibited better optophysical and electrochemical prop-
erties.
amino)biphenyl-4-yloxy)hexyloxy)
picolinic
acid
(OXD6TPAPh6Pic), respectively. The synthetic route
of these iridium complexes is shown in Scheme 1. For
comparison, the iridium complexes of (piq)2Ir(pic) and
Scheme 1 Synthetic route of the functionalized iridium complexes
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Chin. J. Chem. 2010, 28, 2455— 2462

Synthesis, Photophysical and Electrochemical Characterization of the Heteroleptic Iridium Complexes
Experimental
used as the counter electrode, and a calomel electrode
was used as the reference electrode. Ferrocene was em-
ployed as the reference of redox system.
Reagents and physical measurements
4-Bromo-N,N-diphenylbenzenamine (BrTPA), tetra-
kis(triphenylphosphine)palladium (Pd(PPh3)4), 1,1'-bis-
(diphenyphosphino)ferrocene-palladium(II)dichloride-
dichloromethane [Pd(dppf)Cl2•CH2Cl2], sodium hydride
(NaH), tetrahydrofuran (THF), dimethyl sulfoxide
(DMSO), toluene and ethanol were purchased from
some reagent plants in China. 2-Ethoxyethanol and
iridium trichloride hydrate were obtained from ARCOS.
DMSO and THF were dried based on standard proce-
dures. 2-(4-(6-Bromo-hexyloxy)phenyl)-5-(4-tert-
butylphenyl)-1,3,4-oxadiazole (t-BuOXD6Br) and 1-
phenylisoquinoline were made according to the litera-
tures.10,11 The ancillary ligands of Ph6Pic and BrPh6Pic,
and the (Piq)2Ir(Pic) complex were synthesized accord-
ing to our previous work.12 4-Bromo-4'-hydroxy-methyl
triphenylamine (BrTPA-CH2OH) was synthesized
according to Park et al. work.13 All other reagents were
used as received from commercial sources, unless oth-
erwise stated.
Synthesis of BrTPA6OXD
A 60% dispersion of NaH in mineral oil (0.60 g,
15.0 mmol) was added into
a
solution of
BrTPA-CH2OH (1.94 g, 5.5 mmol), t-BuOXD6Br (2.28
g, 5.0 mmol) and THF (30.0 mL) under stirring during 1
h period. The resulting mixture was then stirred for 24 h
at 60—65 ℃ under nitrogen atmosphere and filtered.
The filtrate was concentrated in vacuum and the residue
was purified by column chromatography [V(acetate)/
V(n-hexane)＝1/5] to provide BrTPA6OXD (1.35 g) as
1
a yellow solid in 37% yield, m.p. 78—80 ℃; H NMR
(CDCl3, 400 MHz, TMS) δ: 1.37 (s, 9H), 1.50—1.85 (m,
8H), 3.52 (t, J＝8.08 Hz, 2H), 4.04 (t, J＝7.42 Hz, 2H),
4.44 (s, 2H), 6.92 (d, J＝8.48 Hz, 2H), 7.02—7.06 (m,
6H), 7.21—7.31 (m, 7H), 7.53 (d, J＝8.04 Hz, 2H),
8.04 (d, J＝8.60 Hz, 4H).
Synthesis of BTPA6OXD
1
All H NMR and 13C NMR spectra were acquired at
A mixture of BrTPA6OXD (0.50 g, 0.685 mmol),
bis(pinacolato)-diboron (0.23 g, 0.89 mmol), potassium
acetate (0.21 g, 2.06 mmol) and Pd(dppf)Cl2•CH2Cl2
(0.028 g, 0.03 mmol) was dissolved in DMSO (30 mL)
and heated to 80 ℃ for 24 h under nitrogen atmosphere.
The reaction mixture was then cooled to room tempera-
ture and mixed with water (60 mL). The organic phase
was separated and aqueous phase was extracted with
diethyl ether (50 mL×3). The combined organic layers
were dried over MgSO4 and distilled to remove the sol-
vents under vacuum. The residue was purified by col-
umn chromatography [V(acetate)/V(n-hexane)＝1/4] to
provide BTPA6OXD (0.37 g) as a yellow solid in
a Bruker Dex-400NMR instrument using CDCl3 as sol-
vent. Elemental analyses were performed on a Harrios
elemental analysis instrument under nitrogen atmos-
phere. UV absorption spectra were recorded with a
HP-8453UV visible system. Photoluminescence (PL)
spectra were recorded on a fluorescence spectropho-
tometer (HITACHI-850). Cyclic voltammetry was car-
ried out on a CHI660A electrochemical workstation in a
solution of tetrabutylammonium hexafluorophosphate
－
1
(Bu4NPF6) (0.1 mol•L ) and acetonitrile at a scan rate
of 50 mV/s and room temperature under nitrogen pro-
tection. The iridium complexes were dissolved in the
above solution (1 mg/cm3). A platinum electrode was
used as the working electrode. A platinum wire was
1
68.6% yield, m.p. 80—82 ℃; H NMR (CDCl3, 400
MHz, TMS) δ: 1.28 (s, 12H), 1.37 (s, 9H), 1.50—1.85
Chin. J. Chem. 2010, 28, 2455— 2462
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Liang et al.
FULL PAPER
(m, 8H), 3.52 (t, J＝8.04 Hz, 2H), 4.03 (t, J＝7.38 Hz,
2H), 4.45 (s, 2H), 6.99—7.09 (m, 8H), 7.22—7.26 (m,
5), 7.54 (d, J＝8.44 Hz, 2H), 8.04 (d, J＝8.44 Hz, 2H),
8.05 (d, J＝2.28 Hz, 2H).
C48H39BrIrN3O4: C 58.00, H 3.95, N 4.23; found C
57.82, H 4.27, N 4.96.
Synthesis of (Piq)2Ir(Ph6Pic)
(Piq)2Ir(Ph6Pic) was prepared according to the syn-
thetic procedure of (Piq)2Ir(BrPh6Pic) with a yield of
Synthesis of BTPA
1
A
mixture of BrTPA (1.62 g, 5.0 mmol),
68.3% as a red powder. m.p. 110—112 ℃; H NMR
bis(pinacolato)diboron (1.52 g, 6.0 mmol), potassium
acetate (1.47 g, 15.1 mmol) and Pd(dppf)Cl2•CH2Cl2
(0.204 g, 0.25 mmol) were dissolved in DMSO (30 mL)
and heated to 80 ℃ for 24 h under nitrogen atmos-
phere. The reaction mixture was cooled to room tem-
perature and mixed with water (60 mL). The organic
phase was separated and aqueous phase was extracted
with diethyl ether (50 mL×3). The combined organic
layers were dried over MgSO4 and evaporated to re-
move the solvents under vacuum. The residue was puri-
fied by column chromatography [V(acetate)/V(n-hexane)
＝1/30] to offer BTPA (1.40 g) as a yellow solid in
(CDCl3, 400 MHz, TMS) δ: 1.54—1.55 (m, 4H), 1.81—
1.83 (m, 4H), 3.61 (t, J＝2.12 Hz, 2H), 3.85 (t, J＝6.38
Hz, 2H), 6.19 (d, J＝7.44 Hz, 1H), 6.40 (d, J＝7.56 Hz,
1H), 6.61—6.93 (m, 8H), 7.22—7.40 (m, 4H), 7.66—
7.97 (m, 11H), 8.65 (d, J＝6.4 Hz, 1H), 8.95 (d, J＝
9.48 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ: 25.55,
25.80, 27.27, 28.91, 67.82, 69.20, 109.44, 114.40,
119.26, 119.77, 120.37, 120.62, 120.76, 121.17, 125.98,
126.42, 126.48, 127.04, 127.23, 127.45, 127.63, 127.87,
129.14, 129.41, 129.46, 129.63, 129.87, 130.64, 130.85,
132.00, 132.82, 136.86, 140.61, 141.02, 141.53, 144.93,
145.46, 150.72, 158.97, 163.98, 169.37, 169.70, 173.09.
Anal. calcd for C48H40IrN3O4: C 63.00, H 4.41, N 4.59;
found C 63.61, H 4.32, N 4.95.
1
75.0% yield, m.p. 89—91 ℃; H NMR (CDCl3, 400
MHz, TMS) δ: 1.33 (s, 12H), 7.02 (d, J＝8.08 Hz, 2H),
7.04—7.11 (m, 6H), 7.24—7.27 (m, 4H), 7.66 (d, J＝
8.38 Hz, 2H)
Synthesis of (Piq)2Ir(TPAPh6Pic)
A mixture of Pd(PPh3)4 (13 mg, 0.01125 mmol),
(Piq)2Ir(BrPh6Pic) (149 mg, 0.15 mmol), BTPA (83.5
mg, 0.225 mmol), toluene ( 10.0 mL), ethanol ( 5.0 mL)
and 2.0 mol•L－ 1 K2CO3 solution (5.0 mL) was degassed
and heated to 80 ℃ with vigorously stirring for 24 h
under N2 flow protection. After cooled to room tem-
perature, the mixture was poured into water. The or-
ganic phase was separated and the aqueous phase was
extracted with diethyl ether (30 mL×3). The combined
organic layers were dried over MgSO4 and evaporated
to remove the solvents under vacuum. The residue was
purified by column chromatography [V(acetate)/
V(methanol)＝20/1] to provide (Piq)2Ir(TPAPh6Pic) (95
Synthesis of (Piq)2Ir(BrPh6Pic)
A mixture of iridium trichloride hydrate (0.36 g,
1.20 mmol), 1-phenylisoquinoline (0.52 g, 2.50 mmol),
2-ethoxyethanol (12 mL) and water (4 mL) was refluxed
for 24 h under nitrogen atmosphere and cooled to room
temperature. The precipitate was formed and collected,
and then washed respectively with water, ethanol and
petroleum ether to afford 580 mg chlorine-bridged irid-
ium dimer (Piq)4Ir2Cl2 as a red powder. The dimer was
directly used in the following procedure.
A mixture of the (Piq)4Ir2Cl2 dimer (580 mg, 0.455
mmol), BrPh6Pic (449 mg, 1.14 mmol), Na2CO3 (604
mg, 5.7 mmol) and 2-ethoxyethanol (20 mL) was re-
fluxed for 24 h under nitrogen atmosphere, then was
allowed to cool to room temperature and mixed with
water (40 mL). The mixture was extracted with diethyl
ether (30 mL×3). The resulting organic layers were
dried over MgSO4 and evaporated to remove the sol-
vents under vacuum. The residue was purified by col-
umn chromatography [V(acetate)/V(methanol)＝20/1] to
obtain (Piq)2Ir(BrPh6Pic) (589 mg) as a red solid in
1
mg) as red solid in 55.2% yield, m.p. 97—99 ℃; H
NMR (CDCl3, 400 MHz, TMS) δ: 1.54—1.55 (m, 4H),
1.81—1.88 (m, 4H), 3.64 (t, J＝6.18 Hz, 2H), 3.88 (t,
J＝4.04 Hz, 2H), 6.18 (d, J＝7.68 Hz, 1H), 6.40—6.41
(d, J＝1.88 Hz, 1H), 6.64—7.12 (m, 10H), 7.25—7.38
(m, 20H), 7.56—7.84 (m, 6H), 8.66 (d, J＝6.36 Hz, 1H),
8.95 (d, J＝6.36 Hz, 2H); 13C NMR (CDCl3, 100 MHz)
δ: 25.46, 25.80, 27.31, 29.71, 67.82, 69.26, 109.31,
114.45, 119.26, 119.75, 120.78, 121.19, 124.21, 124.29,
126.54, 127.04, 127.32, 127.46, 127.86, 128.46, 128.58,
129.16, 129.26, 129.48, 129.65, 129.91, 130.63, 130.84,
131.93, 131.96, 132.08, 132.18, 133.10, 137.11, 140.97,
141.56, 169.45, 173.59. Anal. calcd for C66H53IrN4O4: C
68.43, H 4.61, N 4.84; found C 68.39, H 4.75, N 4.88.
1
65.5% yield, m.p. 108—110 ℃; H NMR (CDCl3, 400
MHz, TMS) δ: 1.54—1.55 (m, 4H), 1.81—1.85 (m, 4H),
3.63 (t, J＝4.24 Hz, 2H), 3.82 (t, J＝4.22 Hz, 2H), 6.20
(d, J＝7.12 Hz, 1H), 6.40 (d, J＝7.56 Hz, 1H), 6.61—
6.89 (m, 7H), 7.22—7.40 (m, 4H), 7.66—7.97 (m, 11H),
8.66 (d, J＝5.92 Hz, 1H), 8.95 (d, J＝2.64 Hz, 2H); 13C
NMR (CDCl3, 100 MHz) δ: 25.55, 25.80, 27.27, 28.91,
67.82, 69.20, 109.26, 112.72, 116.17, 116.28, 119.19,
119.73, 120.28, 121.17, 125.98, 126.43, 126.48, 126.98,
127.29, 127.45, 127.57, 127.87, 129.10, 129.43, 129.61,
129.89, 130.61, 130.84, 132.07, 132.23, 132.87, 136.85,
136.96, 140.64, 140.96, 141.52, 144.95, 150.61, 151.76,
158.08, 163.96, 169.40, 169.76, 173.43. Anal. calcd for
Synthesis of (Piq)2Ir(OXD6TPAPh6Pic)
(Piq)2Ir(OXD6TPAPh6Pic) was prepared according
to the synthetic procedure of (Piq)2Ir (TPAPh6Pic) with
a yield of 52.9% as a red powder. m.p. 101—103 ℃;
1H NMR (CDCl3, 400 MHz, TMS) δ: 1.30 (s, 3H),
1.25—1.27 (m, 8H), 1.68—1.70 (m, 8H), 3.53 (t, J＝
4.26 Hz, 2H), 3.64 (t, J＝6.02 Hz, 2H), 3.88 (t, J＝3.98
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Synthesis, Photophysical and Electrochemical Characterization of the Heteroleptic Iridium Complexes
Hz, 2H), 4.04 (t, J＝4.02 Hz, 3H), 4.45 (s, 2H), 6.20 (d,
J＝6.92 Hz, 1H), 6.41 (d, J＝7.88 Hz, 1H), 6.64—7.12
(m, 12H), 7.25—7.38 (m, 19H), 7.56—7.84 (m, 8H),
8.03 (d, J＝3.04 Hz, 4H), 8.65 (d, J＝6.8 Hz, 1H), 8.96
(d, J＝6.48 Hz, 2H); 13C NMR (CDCl3, 100 MHz) δ:
22.79, 24.83, 27.33, 29.18, 29.26, 29.36, 29.47, 34.03,
68.27, 69.35, 70.52, 72.79, 109.50, 114.79, 115.09,
120.46, 124.20, 124.29, 124.37, 126.12, 126.79, 127.39,
127.73, 127.76, 127.98, 128.77, 129.01, 129.35, 129.85,
130.01, 130.13, 130.96, 136.98, 141.13, 141.65, 145.55,
155.25, 161.99, 164.09, 164.50, 169.86, 178.88. Anal.
calcd for C91H83IrN6O7: C 69.84, H 5.35, N 5.37; found
C 69.62, H 5.34, N 5.39.
which the former bands are assigned to spin-allowed
1-phenylisoquinoline-centered π-π* transitions and the
latter bands are assigned to the mixing transitions of the
spin-allowed singlet metal-to-ligand charge transfer
(1MLCT) and the spin-forbidden triplet metal-to-ligand
charge transfer (3MLCT).19,20 Compared to the common
(Piq)2Ir-(Pic) complex, the (Piq)2Ir(Ph6Pic) complex
showed a similar absorption spectrum. However, the
(Piq)2Ir(TPAPh6Pic) complex exhibited a new intense
absorption band at about 330 nm, which is assigned to
TPAPh6Pic-centered
π-π*
transitions.
The
(Piq)2Ir(OXD6TPAPh6Pic) complex displayed another
new intense absorption band at about 360 nm, which is
assigned to OXD6TPAPh6Pic-centered π-π* transitions.
In contrast, both iridium complexes containing car-
rier-transporting groups exhibited decreasing MLCT
absorption than (Piq)2Ir(Pic). Therefore, introduction of
a hole- or bipolar-transporting unit into the ancillary
ligand of piconilic acid by a non-conjugated connection
is available to increase the ligand-centered π-π* absorp-
tion and decrease the MLCT absorption.
Results and discussion
Synthesis and characterization
An outline of syntheses of picolinic acid derivatives
and their heteroleptic iridium(III) complexes is shown
in Scheme 1. The intermediate of BrTPA6OXD was
synthesized by a nucleophilic substituted reaction be-
tween t-BuOXD6Br and BrTPA-CH2OH based on the
method.14 The arylboronates of BTPA and BTPA6OXD
were synthesized by a Pd-catalyzed coupling reaction of
the bis(pinacolato) diboron and aryl halides BrTPA and
BrTPA6OXD, respectively.15 This synthesis of arylbo-
ronates by a direct conversion of haloaryl compounds
presented a high yield of about 70% in the presence of
these functional groups. The iridium complexes of
(Piq)2Ir(Ph6Pic) and (Piq)2Ir(BrPh6Pic) were synthe-
sized by a chlorine-bridged reaction of IrCl3 with
1-phenylisoquinoline and a debridged reaction of the
resulting chloro-bridged dimers with the ancillary
ligands of Ph6Pic and BrPh6Pic, respectively, according
to the literatures.16-18 As both ancillary ligands between
BrPh6Pic and TPA-Ph6Pic (or OXD6TPA6Ph6Pic) are
very difficult to separate, TPAPh6Pic and
OXD6TPA6Ph6Pic have not been synthesized by a Su-
zuki-Miyaura reaction between BrPh6Pic and BTPA (or
BTPA6OXD). Therefore, the iridium complexes of
(Piq)2Ir(TPAPh6Pic) and (Piq)2Ir(OXD6TPAPh6Pic)
have to be synthesized by another Suzuki-Miyaura reac-
tion between the arylboronates of BTPA (or
BTPA6OXD) and the bromo-containing iridium com-
plex of (Piq)2Ir(BrPh6Pic). The result showed that the
iridium complexes containing carrier-transporting
groups were conveniently obtained by this synthetic
method with good yield. All of these iridium complexes
Figure 1 UV-Vis absorption spectra of iridium complexes in
CH2Cl2.
Figure 2 shows the photoluminescence (PL) spectra
of theses iridium complexes in dichloromethane solu-
tion and in their neat films, respectively. Similar emis-
sion spectra peaked at 612 nm were observed for these
four iridium complexes in dichloromethane solution.
However, these iridium complexes exhibited different
PL spectra in their neat films. The PL emission peaks
were 635 nm for (Piq)2Ir-(Ph6Pic), 640 nm for
(Piq)2Ir(TPAPh6Pic) and 623 nm for (Piq)2Ir-
(OXD6TPAPh6Pic), which have some blue shift com-
pared to that from (Piq)2Ir(Pic). The blue-shifted PL
spectra from (Piq)2Ir(TPAPh6Pic) and (Piq)2Ir-
(OXD6TPAPh6Pic) are relative to the space hindrance
effect of carrier-transporting groups, which can effec-
tively inhibit intermolecular aggregation’s formation.
Accordingly, for the heteroleptic iridium complexes
with carrier-transporting groups, introduction of a hole-
or bipolar-transporting unit into the ancillary ligand of
piconilic acid by a non-conjugated connection has little
1
were confirmed by H NMR, 13C NMR and elemental
analysis and were coincident with their actual molecular
structures.
Optical properties
Figure 1 shows the UV absorption spectra of iridium
complexes in dichloromethane solution. The intense
high-energy absorption bands at about 290 nm and weak
low-energy absorption bands at about 400—500 nm
were observed for these four iridium complexes, in
Chin. J. Chem. 2010, 28, 2455— 2462
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Figure 2 Photoluminescence spectra of iridium complexes in CH2Cl2 (a) and their neat films (b).
Table 1 UV absorption and photoluminescence properties of iridium complexes
－1
－1
d
Complex
(Piq)2Ir(Pic)
UV-vis absorption,a λ/nm [εmax/(dm3•mol •cm )]
296 (43611), 340 (23977), 400 (10151), 460 (7436)
299 (42777), 344 (21944), 401 (10000), 467 (5694)
299 (44907), 338 (27824), 405 (7962), 465 (5138)
298 (7998－3), 359 (38232), 465 (2812)
λ
b/nm
λ
emc/nm
Ф
em
f
606
657
635
640
623
0.491
0.494
0.558
0.577
(Piq)2Ir(Ph6Pic)
612
612
612
(Piq)2Ir(TPAPh6Pic)
(Piq)2Ir(OXD6TPAPh6Pic)
a Measured in CH2Cl2 solution at a concentration of 10 5 mol/L; b measured in CH2Cl2 solution at 298 K; c measured in neat film at 298 K;
measured in CH2Cl2 at 298 K. Excitation wavelength was 460 nm for all complexes, Ru(bpy)3(PF6)2 (Ф ＝0.062 in acetonitrile) was
d
f
used as the reference for quantum yield (Ф ).
f
influence on solution PL spectra, but it is available to
tune maximum emission wavelength in their neat films.
In order to further understand the effect of carrier-
transporting groups in ancillary ligands on the PL per-
formances of their iridium complexes, the fluorescence
Electrochemical properties
All of these iridium complexes showed a reversible
oxidation wave (Eox) at 0.9—1.0 V, but their reduction
waves were disappeared. The reduction potential (Ered)
was estimated based on energy band gap (Egap) and Eox,
in which Egap was estimated based on their UV-Vis ab-
sorption edge.21 Thus, the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) energy levels (EHOMO and ELUMO) were
quantum yields (Ф ) of these iridium complexes were
f
measured using Ru(bpy)3(PF6)2 as the reference in di-
chloromethane at 298 K and summarized in Table 1. It
is obvious that the (Piq)2Ir(OXD6TPAPh6Pic) and
(Piq)2Ir(TPAPh6Pic) complexes with carrier-transpor-
ting groups displayed higher fluorescence quantum
yield than the (Piq)2Ir(Ph6Pic) and (piq)2Ir(Pic) com-
plexes. The (Piq)2Ir(OXD6TPAPh6Pic) complex con-
taining bipolar transporting group exhibited best PL
property with fluorescence quantum yield as high as
0.577. This implies that an introduction of carrier-
transporting groups into picolinic acid can increase
fluorescence quantum yield of their iridium complexes.
Figure 3 shows the PL spectra of these four iridium
complexes in different solvents. The solvate-chromism
and identical change rule for their maximum emission
peak are observed. The maximum emission peak of
iridium complexes with carrier-transporting groups lo-
cates at around 615 nm in toluene (weak-polar solvent),
612 nm in CHCl3 (medium-polar solvent) and 605 nm
in ethanol (strong-polar solvent). The peak shift can be
attributed to the stronger interaction between the sol-
vents and the excited molecules. The excited states of
all iridium complexes are more stabilized in non-polar
solvents than in polar solvents. This leads to a blue shift
of emission with increasing solvent polarity.
calculated based on the empirical formula: EHOMO
＝
－e(Eox＋4.38) and ELUMO＝－e(Ered＋4.38). The re-
sulting electrochemical data are listed in Table 2 for
comparison. Compared to (Piq)2Ir(Pic), the iridium
complexes with carrier-transporting groups exhibited an
increased HOMO energy level. Furthermore, the
(Piq)2Ir(OXD6TPAPh6Pic) complex containing both
triphenylamine and oxadiazole groups displayed a de-
creased LUMO energy level at the same time. Therefore,
an introduction of carrier-transporting groups into pi-
colinic acid can effectively tune HOMO and LUMO
energy levels.
Thermal properties
The thermal properties of iridium complexes were
determined by thermogravimetric analysis (TGA) under
nitrogen atmosphere. Their onset degradation tempera-
tures (Td) for 5% weight loss are also listed in Table 2.
The results show that incorporating carrier-trans-
porting group into piconolic acid by a non-conjugated
connection of 1,6-dioxyhexane has much effect on the
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© 2010 SIOC CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2010, 28, 2455— 2462

Synthesis, Photophysical and Electrochemical Characterization of the Heteroleptic Iridium Complexes
Figure 3 PL spectra of Ir complexes in different solvents.
Table 2 Electrochemical and thermal properties of iridium complexes
Complex
(Piq)2Ir(Pic)
(Piq)2Ir(Ph6Pic)
(Piq)2Ir(TPAPh6Pic)
(Piq)2Ir(OXD6TPAPh6Pic)
Eox a/V
Ered b/V
HOMOc/eV
LUMOd/eV
Egapb/eV
Td/℃
1/2
1/2
0.99
0.97
0.94
0.95
b
－1.68
－1.68
－1.71
－1.69
－5.37
－5.35
－5.32
－5.33
－2.68
－2.68
－2.67
－2.69
2.69
2.67
2.65
2.64
256
230
290
248
Potential values are reported versus Fe/Fe＋. Estimated according to the oxidative potential and the UV-vis absorption spectrum.
a
c EHOMO＝－(4.38＋Eox1/2) eV. d ELUMO＝－(4.38＋Ered1/2) eV.
thermal stability of their iridium complexes. The Td was
suggested that incorporating a bipolar-transporting
functional group into the picolinic acid with a
non-conjugated connection is available to improve the
optophysical and electrochemical properties of its irid-
ium complex.
290
℃
for (Piq)2Ir(TPAPh6Pic), 248
(Piq)2Ir(OXD6TPAPh6Pic) and 230
(Piq)2Ir(Ph6Pic). Obviously, the iridium complexes with
carrier-transporting groups presented good thermal sta-
bility.
℃
for
for
℃
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Chin. J. Chem. 2010, 28, 2455— 2462