Bis-cyclometalated Ir(III) complexes with a diphenylamino group: design, synthesis, and application in oxygen sensing

Article abstract of DOI:10.1016/j.dyepig.2016.09.022

A series of C?N type bis-cyclometalated Ir(III) complexes (Ir1-Ir4) with a diphenylamino group at the 4-position of the phenyl ring of 2-phenylpyridine (ppy) have been prepared and fully characterized. The influences of substituents (-H, -CH₃,

Full text of DOI:10.1016/j.dyepig.2016.09.022

Accepted Manuscript
Bis-cyclometalated Ir(III) complexes with a diphenylamino group: Design, synthesis,
and application in oxygen sensing
Chun Liu, Hongcui Yu, Xiaofeng Rao, Xin Lv, Zilin Jin, Jieshan Qiu
PII:
S0143-7208(16)30469-7
10.1016/j.dyepig.2016.09.022
DYPI 5474
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 16 August 2016
Revised Date: 9 September 2016
Accepted Date: 11 September 2016
Please cite this article as: Liu C, Yu H, Rao X, Lv X, Jin Z, Qiu J, Bis-cyclometalated Ir(III) complexes
with a diphenylamino group: Design, synthesis, and application in oxygen sensing, Dyes and Pigments
(2016), doi: 10.1016/j.dyepig.2016.09.022.
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ACCEPTED MANUSCRIPT
Bis-cyclometalated Ir(III) complexes with a diphenylamino group: design,
synthesis, and application in oxygen sensing
Chun Liu*, Hongcui Yu, Xiaofeng Rao, Xin Lv, Zilin Jin and Jieshan Qiu
[*] Dr. C. Liu, Corresponding Author
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong
Road 2, Dalian 116024, China.
Tel.: +86-411-84986182.
E-mail: cliu@dlut.edu.cn

ACCEPTED MANUSCRIPT
Bis-cyclometalated Ir(III) complexes with a diphenylamino group:
design, synthesis, and application in oxygen sensing
Chun Liu*, Hongcui Yu, Xiaofeng Rao, Xin Lv, Zilin Jin and Jieshan Qiu
[*] Dr. C. Liu, Corresponding-Author
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong
Road 2, Dalian 116024, China
Tel.: +86-411-84986182.
E-mail: cliu@dlut.edu.cn

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Abstract:
A series of C^N type bis-cyclometalated Ir(III) complexes (Ir1-Ir4) with a
diphenylamino group at the 4-position of the phenyl ring of 2-phenylpyridine (ppy)
have been prepared and fully characterized. The influences of substituents (-H, -CH3,
-F, -CF3) at the pyridyl moiety of ppy on the photophysical and electrochemical
properties of these Ir(III) complexes have been investigated systematically by
comparison with the model complex Ir(ppy)2(acac). The Ir(III) complexes (Ir1-Ir4)
show phosphorescence with emission maxima in the range of 530–558 nm, which
are all red-shifted relative to Ir(ppy)2(acac) (λmax = 518 nm). These Ir(III) complexes
demonstrate longer phosphorescence lifetimes and better oxygen-sensitivity than
Ir(ppy)2(acac). It is remarkable that introduction of both diphenylamino and
trifluoromethyl groups into the cyclometalating ligand enhances the photostability of
the corresponding Ir(III) complexes efficiently. The results reveal that the bulky and
electron-donating characteristics of the diphenylamino group enhance the
performances of the Ir(III) complexes in oxygen sensing.
Keywords: Ir(III) complex, diphenylamino group, phosphorescence lifetime, oxygen sensing,
photostability

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1. Introduction
Oxygen sensors have important applications in various fields, including
clinical analysis, environmental monitoring, and food packaging [1-6]. An
optical oxygen sensor relies on the quenching of the luminescence of an
oxygen-sensitive probe (OSP) by molecular oxygen [7, 8]. To reach smooth
oxygen diffusion and minimize probe self-quenching, OSPs are usually doped
into an appropriate matrix (such as a polymer) [9, 10]. Reversibility and
long-term stability are important factors to influence the overall performances
of the sensor. However, due to the photo-bleaching nature of indicator, OSPs
immobilized in supporting matrices usually demonstrate decreased intensity
(poor photostability) during the illumination period under continuous
conditions [11, 12]. Therefore, the development of novel OSPs with enhanced
photostability is crucial to an efficient oxygen sensor. At present, most
oxygen-sensitive probes (OSPs) are based on heavy metal complexes such as
ruthenium(II), platinum(II), iridium(III), and so on [13-15]. The high
coupling of the iridium metal core (spin-orbit coupling constant ζIr = 3909
removes the spin-forbidden nature of the radiative relaxation of the triplet state
and demonstrates high phosphorescence efficiencies [16, 17]. The facile
tunability of cyclometalated Ir(III) complexes by the coordinating ligands
differentiates them from other organometallic compounds [18, 19]. The unique
properties of Ir(III) complexes make these compounds particularly suitable for
applications in oxygen sensing [20, 21]. However, it is still a challenge to

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enhance the photostability of cyclometalated Ir(III) complexes and increase
their oxygen sensitivity as luminescent O2-sensing materials.
Electron-donating diphenylamino group (-NPh2) has been often used in
numerous organic electroluminescence materials, owing to the good
hole-transporting capability as well as the bulky size for suppressing a
self-quenching process [22-24]. However, there have been only a few
complexes modified with a diphenylamino group used in oxygen sensing. In
2009, Chan and co-workers [25] reported the use of Ir(ppy-NPh2)3 for
luminescence oxygen sensing. Due to the larger dynamic range of response
and higher sensitivity, this complex represents a promising candidate for
oxygen sensing in contrast to Ir(ppy)3. By chemically manipulating the
Pt(ppy)2(acac) with a diphenylamino group on the phenyl ring of ppy, a new
family of cyclometalated Pt(II) complexes with long phosphorescence
lifetimes and good oxygen sensing properties have been reported by our group
[26]. Inspired by these reports, we envision that bis-cyclometalated Ir(III)
complexes bearing one acetylacetonate and two 2-phenylpyridine (ppy)
ligands functionalized with a bulky electron-donating diphenylamino group
could also serve as efficient OSPs for oxygen-sensing. In this paper, we
present the synthesis and characterization of a series of Ir(III) complexes
(
Ir1-Ir4) functionalized with a diphenylamino group at the cyclometalating
ligands and a acetylacetone (acac) as the ancillary ligand. The photophysical
and electrochemical properties of these bis-cyclometalated Ir(III) complexes

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have been investigated systematically, and the results demonstrate that
introduction of a diphenylamino group at the 4-position of phenyl ring in ppy
can improve the photostability of the Ir(III) complexes effectively compared
with Ir(ppy)2(acac). Introducing a -CF3 group at the 5-position of the pyridyl
ring of cyclometalating ligand could further improve the photostability. These
photostable Ir(III) complexes with prolonged phosphorescence lifetimes
exhibit efficient oxygen-sensitivity compared with Ir(ppy)2(acac).
2. Experimental
2.1. Materials and measurements
All starting materials were purchased from commercial suppliers and used
without further purification. The solvents were treated as required prior to use.
13
1H NMR and C NMR spectra were recorded on a 400 MHz Varian Unity
Inova spectrophotometer. Mass spectra were recorded with a MALDI micro
MX spectrometer. UV/Vis absorption spectra were recorded on an HP8453
UV/Vis spectrophotometer. Emission spectra were recorded with a PTI-700
spectrofluorimeter. Photoluminescence quantum yields were measured relative
to [Ir(ppy)2(acac)] (ФP = 0.34 in CH2Cl2, under degassed conditions).
Phosphorescence lifetimes were measured on an Edinburgh FLS920
Spectrometer (picosecond pulsed diode laser made in the U. K., model:
EPL-405). Cyclic voltammograms of the Ir(III) complexes were recorded on
an electrochemical workstation (BAS100B/W, USA) at room temperature in a

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0.1 M [Bu4N]PF6 solution under argon conditions. Phosphorescent intensity
response of sensing films of the Ir(III) complexes were recorded with a F-7000
spectrofluorimeter. Thickness of sensing films was analyzed on a QUANTA
450 scanning electron microscopy (SEM).
2.2. Synthesis
Here Scheme 1. Synthesis of bis-cyclometalated Ir(III) complexes for this study. (i)
Pd(OAc)2, K2CO3, EtOH/H2O 3:1(v/v), 80 ºC, in air, 30-60 min. (ii) IrCl3·3H2O,
EtOCH2CH2OH/H2O 3:1(v/v), 110 ºC, N2, 24 h. (iii) Hacac, K2CO3, EtOCH2CH2OH, 120
ºC, N2, 24 h.
Chemical structures and the detailed synthetic protocols of the Ir(III) complexes
are shown in Scheme 1. The cyclometalating ligands L1-L4 were prepared via a
palladium-catalyzed ligand-free and aerobic Suzuki reaction in aqueous ethanol
developed by our group [27]. All of the bis-cyclometalated Ir(III) complexes were
synthesized in two steps from the cyclometalation of IrCl3·3H2O with the
corresponding ligands to form the chloride-bridged dimers initially, followed by
treatment with acetylacetone (Hacac) in the presence of K2CO3 to obtain the target
Ir(III) complexes.
2.2.1. Synthetic procedure of ligands L1
A mixture of 2-pyridyl bromide, 1.5 equiv. of arylboronic acid, 2 equiv. of
K2CO3, Pd(OAc)2 (1.5 mol%), ethanol/water (3:1 v/v) was stirred at 80 C in
-L4
°
air for indicated time. The reaction mixture was added to brine (15 mL) and
extracted with ethyl acetate (4×15 mL). The solvent was concentrated under

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vacuum, and the product was isolated by short-column chromatography on
silica gel.
2.2.2. Synthetic procedure of Ir(III) complexes Ir1-Ir4
A mixture of IrCl3·3H2O (0.5 mmol) and 2.5 equiv. of cyclometalating
ligand was heated to 110 °C in a mixed solvent of 2-ethoxyethanol and water
(3:1 v/v) under nitrogen for 24 h. Upon cooling to room temperature, the
yellow precipitate was collected by filtration and washed with water. The wet
solid was completely dried to give the crude cyclometalated Ir(III)
ꢀ-chloro-bridged dimer. Without further purification, the dimeric Ir(III)
complex, subsequently reacted with 10 equiv. of the acetylacetone (Hacac) in
the presence of 10 equiv. of K2CO3 in 2-ethoxyethanol at 120 °C under
nitrogen for 24 h. After cooling to room temperature, the precipitate was
collected by filtration, washed with water and dried. The crude product was
purified by column chromatography over silica using CH2Cl2:n-hexane (1:1)
as eluent to provide the desired Ir(III) complexes.
1
Ir1: Yield 57%, a yellow solid; H NMR (400 MHz, CDCl3)
δ
= 8.24 (d,
J =
4.8 Hz, 2H), 7.42-7.36 (m, 4H), 7.30 (d,
J
= 8.0 Hz, 2H), 7.13 (t, = 8.0 Hz,
J
8H), 6.95-6.90 (m, 12H), 6.79 (t, = 6.8 Hz, 2H), 6.48 (d,
J
J
= 8.0 Hz, 2H),
5.77 (s, 2H), 5.23 (s, 1H), 1.82 (s, 6H).
Ir2: Yield 48%, a yellow solid; 1H NMR (400 MHz, CDCl3)
δ
= 8.04 (m, 2H),
7.31 (d,
J
= 8.0 Hz, 2H), 7.26-7.24 (m, 2H), 7.19 (d,
J
= 7.2 Hz, 2H), 7.12 (t,
J
= 8.0 Hz, 8H), 6.92 (d,
J
= 8.0 Hz, 12H), 6.49 (m, 2H), 5.72 (d,
J
= 2.4 Hz,

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13
2H), 5.22 (s, 1H), 2.20 (s, 6H), 1.82(s, 6H). C NMR (100 MHz, CDCl3)
δ =
184.3, 165.2, 147.6, 147.3, 147.0, 146.9, 138.9, 136.9, 129.6, 128.7, 125.9,
125.0, 123.7, 122.3, 117.4, 114.3, 100.4, 28.9, 18.4. MALDI-TOF-MS (m/z):
962.3136 [M]+, 863.2747 [M-acac]+.
Ir3: Yield 50%, a greenish-yellow solid; 1H NMR (400 MHz, CDCl3) δ = 8.10 (t, J =
2.4 Hz, 2H), 7.39-7.36 (m, 2H), 7.23 (d, J = 8.0 Hz, 2H), 7.20-7.13 (m, 10H),
6.97-6.92 (m, 12H), 6.50 (d, J = 8.0 Hz, 2H), 5.69 (d, J = 2.4 Hz, 2H), 5.26 (m, 1H),
1.84 (s, 6H). 13C NMR (100 MHz, CDCl3) δ = 184.9, 164.7, 157.8, 155.3, 147.5,
147.3, 146.5, 137.1, 135.6, 128.8, 125.3, 125.2, 124.2, 123.9, 123.7, 122.8, 118.2,
118.1, 114.1, 100.7, 28.8. MALDI-TOF-MS (m/z): 970.2690 [M]+, 871.2173
[M-acac]+.
Ir4: Yield 69%, an orange-yellow solid; 1H NMR (400 MHz, CDCl3)
(s, 2H), 7.51-7.45 (m, 4H), 7.32 (d, = 8.0 Hz, 2H), 7.18-7.14 (m, 8H),
δ = 8.42
J
7.01-6.95 (m, 12H), 6.54-6.51 (m, 2H), 5.63 (d,
J
= 2.4 Hz, 2H), 5.26 (s, 1H),
13
1.83 (s, 6H). C NMR (100 MHz, CDCl3)
δ = 185.2, 171.0, 150.0, 149.0,
146.8, 144.7, 135.5, 133.2, 133.1, 129.0, 126.1, 125.9, 124.3, 123.8, 123.6,
122.2, 121.8, 121.6, 117.0, 113.2, 100.9, 28.7. MALDI-TOF-MS (m/z):
1070.2633 [M]+, 971.2294 [M-acac]+.
2.3. Preparation of oxygen sensing films and their photostability
Oxygen sensing films were prepared according to the literature [28]: EC
(9.95 mg) was dissolved in CH2Cl2 (0.90 mL) and then 0.10 mL Ir1 (0.5
mg/mL) in CH2Cl2 was added to the solution. After thoroughly mixing, 0.1

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mL of the solution was coated on a silica glass disk (diameter 1.4 cm). The
solvent was evaporated at room temperature and a thin film was obtained as
shown in Fig. 4(A). The samples with different loading levels and different
matrices were prepared with procedure similar to that described above. The
photostability of the complexes in EC film was investigated by illuminating
the sensing film with a 254 nm 16 W ZF-7A UV lamp at a distance of 3 cm in
the air for 90 min. Emission intensities were measured before and after
exposure of the film to illumination. The power density on the films is 21.5
W/m2. General procedure for the oxygen sensing test was performed according
to the reported method [29].
3. Results and Discussion
3.1. Photophysical and electrochemical properties
UV−vis absorption spectra and emission spectra of complexes Ir1-Ir4 in CH2Cl2
(1.0 × 10-5 M) at room temperature are presented in Fig. 1 and the corresponding
data are listed in Table 1. Similarly to other previously reported cyclometalated Ir(III)
complexes [30, 31], the complexes (Ir1-Ir4) exhibit strong absorption bands in the
ultraviolet region belong to the spin-allowed intraligand (1π−π*) transitions. The
long wavelength with lower extinction coefficient can be assigned to singlet and
3
triplet metal-to-ligand charge-transfer (1MLCT and MLCT) transitions. The results
in Fig. 1 show that Ir1-Ir4 functionalized with an electron-donating diphenylamino
group have larger molar extinction coefficient than that of Ir(ppy)2(acac). Especially,
the spectrum of Ir4 is red-shifted obviously compared to other complexes under the

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same measurement conditions, due to the electron-withdrawing ability of the -CF3
substituent [32, 33].
The normalized emission spectra of these complexes are shown in Fig. 1. The
emission maxima of Ir1-Ir4 are red-shifted compared to Ir(ppy)2(acac) (λmax at 518
nm), owing to the electron-donating ability of the diphenylamino moiety.
Additionally, the substituents at 5-position of pyridyl ring also affected the emission
properties of the complexes. Introducing a -CH3 group at the 5-position of the
pyridyl ring results in a slight blue-shift of 2 nm for Ir2 (λmax at 530 nm) relative to
Ir1 (λmax at 532 nm). Instead of a -CH3 group with a fluorine, a red-shift of 4 nm for
Ir3 is reached, while introducing a -CF3 group at the same position imparts a more
substantial red-shift up to 26 nm for Ir4 (λmax at 558 nm). These results show that the
emission of the complexes could be finely tuned by the modification of the
structures of complexes. The phosphorescence quantum yields (Φp) of Ir1-Ir4 range
from 0.18 to 0.30 (Table 1). The phosphorescence lifetimes (τ) of Ir1-Ir4 in
degassed CH2Cl2 are in the range of 3.46-3.90 ꢀs at room temperature (Fig. 2),
which are longer than that of Ir(ppy)2(acac) (1.55 ꢀs).
The electrochemical properties of the Ir(III) complexes were studied by
cyclic voltammetry (CV) and the results are also listed in Table 1. The HOMO
and LUMO energies can be calculated by following equations (EHOMO (eV) =
ꢁꢃꢄꢅꢆ
-e(4.4 +
ꢀ
), ELUMO (eV)= EHOMO
Ir4 (2.47, 2.49, 2.43 and 2.36 eV) decrease
in comparison with the Ir(ppy)2(acac) (2.54 eV). At an anodic scan rate of 100
+ Eg). The energy gaps (Eg) between the
ꢁꢂ
HOMO and LUMO levels of Ir1
-

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mV/s, the CVs of Ir1-Ir4 show oxidation potentials of 0.63, 0.58, 0.68, and
0.77 V versus saturated calomel electrode (SCE), respectively (see Fig. S2,
Supplementary Information). These positive oxidation potentials are assigned
to the metal-centered Ir(III)/Ir(IV) oxidation couple, in accordance with the
reported cyclometalated Ir(III) systems [34]. As shown in Table 1, it is found
that introducing the electron-donating group -CH3 in the pyridine unit leads to
a low potential for Ir2, while introducing an electron-withdrawing group -F
(
Ir3) or -CF3 (Ir4) at the same position makes the oxidation process shift to a
more positive potential.
Here Fig. 1. Absorption (up) and emission spectra (down, λex= 400 nm) of Ir1-Ir4 and
Ir(ppy)2(acac) in CH2Cl2 solution at room temperature.
Here Fig. 2. Phosphorescence decay profiles of Ir1-Ir4 and Ir(ppy)2(acac) in CH2Cl2 solution at
room temperature.
Here Table 1. Photophysical and electrochemical data of the Ir(III) complexes.
3.2. Fitting Formula of Oxygen Sensing
Optical oxygen sensing process of the luminescence quenching involves dynamic
collision between molecular oxygen (triplet) and the excited state of the OSPs,
leading to a reduction of its intensity and decay time [10]. The process can be
described as follows: IrIII*+O2→IrIII+O2*, where IrIII denotes the complex and the “*”
indicates the excited state (Fig. 3) [35, 36]. The phosphorescence intensity of Ir(III)
complexes immobilized in a matrix is quenched by oxygen according to the

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Stern-Volmer equation [37-39]:
ꢇ
ꢉ
ꢈ
ꢈ
=
= 1 + ꢊꢋꢌ ∙ ꢍꢎ
ꢉ
(1)
ꢏ
ꢇ
In the equation, I is the phosphorescence intensity, and τ is the lifetime of an OSP. I0
and τ0 are the corresponding values in the absence of oxygen. KSV is the
Stern-Volmer quenching constant and ꢍꢎꢏ is the partial pressure of oxygen. For
heterogeneous O2-sensing films, a two-site model is required to study the quenching
effect. In the two-site model, the O2-sensitive complexes are considered as two
different portions. In the conventional form, it reads as follows (see Eqs 2 and 3) [40,
41].
ꢇ
ꢉ
ꢐ
ꢐ
ꢏ
ꢑ
=
=
+
(2)
ꢇ
ꢈ
ꢉ
ꢈ
ꢒꢓꢔ
∙ꢗ
ꢘ
ꢒꢓꢔ
∙ꢗ
ꢕꢖꢏ ꢘ
ꢏ
ꢕꢖꢑ
ꢏ
f1 + f2 = 1
(3)
where f1 and f2 denote the fractional contribution of the total luminescence
emission from the OSP due to the heterogeneity of the microenvironment. It is
assumed that there exists two distinctly different components, one (f1) being
quenchable, the other (f2) either not being quenched at all, or being quenched
ꢚꢗꢗ
ꢚꢗꢗ
ꢛꢜ
at a very different rate. The weighted quenching constant
ꢊ
(ꢊ
= ꢝ ∙
ꢒ
ꢄꢙ
ꢊꢛꢜꢒ + ꢝ ∙ ꢊꢛꢜꢞ ) is the guide of the sensitivity of an oxygen sensor, and
ꢞ
ꢚꢗꢗ
higher values of
ꢊ
indicate that the oxygen sensor is more sensitive to
ꢄꢙ
oxygen quenching.
Here Fig. 3. Illustration of simplified mechanism for oxygen sensing.

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3.3. Effects of the polymer matrices and OSP contents on oxygen sensing
An optical oxygen sensor is ususally composed of an OPS and a sensor matrix.
Generally, polymers or organically modified silica (ormosils) are mainly employed
as matrix materials in an optical oxygen sensor [25, 42-45]. The selection of a
favorable matrix is a key to a successful sensor [10]. In this work, we prepared
sensing films of OSPs immobilized in ethyl cellulose (EC), polystyrene (PS),
poly(ether-ketone-ketone) (PPEK), poly(cyclohexene carbonate) (PCHC) and
IMPES-C , respectively (see Fig. S3, Supplementary Information). The results in
ꢚꢗꢗ
ꢋꢌ
Table S1 demonstrate that the weighted quenching constant ꢊ
of Ir1
incorporated into EC is higher than those of others. This is contributed to the large
permeability coefficient of EC, providing good permeability to oxygen [25, 46]. In
addition, it is known that the OSP contents strongly affect the quenching efficiency,
the sensitivity and linearity as well as the response time of an oxygen sensor [17, 47].
The detailed oxygen sensing properties of Ir1/EC with different Ir1 loadings are
presented in Table S2. It is found that the optimum weight content of Ir1
immobilized in EC is 0.5 wt%, which is consistent with the previous result [48]. The
reason for this might be that at this content the polymer matrix protects the OSPs
from potential interference and avoids self-quenching efficiently in this sensor
system. Additionally, the thickness of the oxygen sensing film at the optimum weight
content has been characterized by a QUANTA 450 scanning electron microscopy
(SEM). The SEM micrograph shows that the thickness of the oxygen sensing film is
ca. 4.7 ꢀm (Fig. 4).

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Here Fig. 4. (A) The transparent quartz plate (r = 7.0 mm) whose inner surface is covered with
the oxygen sensing film. (B) SEM micrograph of the oxygen sensing film. (C) The diagram of
the sensing layer.
3.4. Photostability of oxygen sensing films
An ideal oxygen sensor should possess high photostability to construct robust
oxygen sensing systems, which is critical to long-term sensing reliability. The
following study was conducted to identify the sensor’s photostability for Ir(III)
complexes immobilized on EC against continuous irradiation with a 254 nm UV
lamp under air atmosphere. After 90 min of irradiation, the emission intensity of the
sensing films reduced in different degrees (Fig. 5). The photostabilities of Ir1-Ir4
are obviously higher than that of the reference complex Ir(ppy)2(acac). In fact, 55%,
58%, 52% and 43% decreases in intensity of Ir1-Ir4 are observed compared to 79%
for Ir(ppy)2(acac). The photostabilities of these Ir(III) complexes are varied due to
the differences in their chemical structures. It is clear that the diphenylamino group
has a distinct positive effect on the photostability. Furthermore, the substituent at the
pyridyl ring also affects the photostability of the Ir(III) complexes. It is observed that
the complex Ir4 with a strong electron-withdrawing -CF3 group on the 5-position of
the pyridyl ring is more photostable than other Ir(III) complexes. The experimental
data are in good agreement with related results previously reported by our group [43,
48].
Here Fig. 5. Photo-degradation histograms for the Ir(III) complexes/EC films under continuous
illumination for 90 min at ambient atmospheric conditions.
Here Table 2. Some of Ir(III) complexes used as oxygen sensing probes.
3.5. Oxygen sensing properties

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In general, an oxygen sensor with the ratio of I0/I100 (I0 and I100 represent the
detected luminescence intensities from a film exposed to 100% nitrogen and 100%
oxygen, respectively) more than 3.0 is suitable for oxygen sensing [49]. The I0/I100
values of previous reported sensors with Ir(III) complexes immobilized in polymer
matrix are shown in Table 2 and the data can be used to evaluate the O2 sensitivity.
To compare the O2-sensing properties of the complexes quantitatively, the
O2-sensing data of Ir1-Ir4 and Ir(ppy)2(acac) (see Fig. S4, Supplementary
Information) are fitted to the two-site model, and the results are summarized in Table
S3. Ir1-Ir4 exhibit better oxygen sensitivity compared to Ir(ppy)2(acac). The I0/I100
values of these Ir(III) complexes immobilized in EC film are 16.2, 16.4, 15.2, 14.1
and 5.8, respectively (Table 2). It is clear that the Ir2 immobilized in EC exhibits the
highest sensitivity to O2 (Fig. 6) at a ꢊꢚꢗꢗ value up to 0.02698 Torr-1 (1 Torr =
ꢋꢌ
133.322 Pa), which is about 3.4 times higher than that of Ir(ppy)2(acac) (0.00792
Torr–1). The reason for this might be that the long-lived excited states of Ir1-Ir4
promote the quenching pathway. This means that, per encounter with an O2
molecule, Ir1-Ir4 are more likely to be quenched than Ir(ppy)2(acac). This may be
due to the unique properties of the diphenylamino substituent, which breaks the
molecular planarity of cyclometalating ligands and reduces the aggregation tendency
of Ir(III) complexes. In addition, introducing substituents (-H, -CH3, -F, -CF3) into
the pyridyl moiety of the cyclometalating ligands has relatively little influence on the
sensitivity of the sensors (Fig. 6). Therefore, the diphenylamino group at the
4-position of phenyl ring of ppy is crucial to enhance the oxygen sensitivity.

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Furthermore, an operational stability test was conducted by reading the luminescence
intensity signals while oxygenated and deoxygenated gases were switched for 4000 s
(shown in Fig. S5, Supplementary Information). All of the oxygen sensing films
demonstrated good operational stability during the whole process. Additionally, fast
response and recovery times were obtained. The response times of the Ir1-Ir4
immobilized in EC are ca. 4.0 s for switching from nitrogen to oxygen, and ca. 5.0 s
for switching from oxygen to nitrogen. These bis-cyclometalated Ir(III) complexes
with excellent oxygen-sensing properties and stability are potential candidates for
online continuous monitoring of oxygen concentrations.
Here Fig. 6. Stern-Volmer plots for oxygen sensing films of Ir(III) complexes immobilized in
EC (intensity ratios I0/I versus O2 partial pressure).
4. Conclusions
In summary, a series of cyclometalated Ir(III) complexes bearing a bulky
electron-donating diphenylamino substituent (Ir1-Ir4) have been synthesized
and fully characterized. The results demonstrate that the diphenylamino group
is an attractive substituent for tuning the photophysical and electrochemical
properties of the corresponding Ir(III) complexes. The emission bands of
Ir1-Ir4 are red-shifted and phosphorescence lifetimes are prolonged relative to
the unsubstituted reference Ir(ppy)2(acac). Immobilized in ethyl cellulose,
Ir1-Ir4 impart favorable oxygen sensitivity and good photostability against

ACCEPTED MANUSCRIPT
irradiation. Especially, Ir4 with a trifluoromethyl group at the 5-position of the
pyridyl ring exhibits the highest photostability. These results might provide
new insights into the chemical modification of cyclometalated Ir(III)
complexes, leading to the design of oxygen sensitive probes with high
performances.
Acknowledgments
The authors thank the financial support from the National Natural Science
Foundation of China (21276043, 21421005), Dalian University of Technology
(DUTTX2015102), and Open Fund of Key Laboratory of Biotechnology and
Bioresources Utilization (Dalian Minzu University), State Ethnic Affairs
Commission & Ministry of Education.
Appendix A. Supplementary data
Supplementary data related to this article can be found in the online version, at
http://dx.doi.
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List of Tables:
Table 1. Photophysical and electrochemical data of the Ir(III) complexes.
Table 2. Some of Ir(III) complexes used as oxygen sensing probes.

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Table 1 Photophysical and electrochemical data of the Ir(III) complexes.
b
e
f
ꢁꢃꢄꢅꢆ
ꢁꢂ
(V)
λem
τ d
(ꢀs)
ꢀ
Eg
HOMO g LUMO g
CIE
(x,y)
c
Complexes
λabs a (nm)
Фp
(nm)
518
532
530
536
558
[eV]
2.54
2.47
2.49
2.43
2.36
(eV)
-5.11
-5.03
-4.98
-5.08
-5.17
(eV)h
-2.57
-2.56
-2.49
-2.65
-2.81
Ir(ppy)2(acac)h
260 (0.36), 340 (0.08), 412 (0.04), 460 (0.02)
257 (0.55), 302 (0.51), 369 (0.46), 396 (0.42), 447 (0.10)
258 (0.53), 304 (0.51), 361 (0.47), 445 (0.10)
0.34
0.18
0.29
0.21
0.30
1.55
3.84
3.90
3.89
3.46
0.71
0.63
0.58
0.68
0.77
0.25,0.67
0.31,0.66
0.30,0.66
0.33,0.64
0.42,0.57
Ir1
Ir2
Ir3
Ir4
256 (0.71), 299 (0.69), 317 (0.70), 364 (0.63), 450 (0.13)
249 (0.45), 278 (0.43), 293 (0.41), 419 (0.48), 472 (0.14)
a Measured in CH2Cl2 at a concentration of 10-5 M and extinction coefficients (105 M-1cm-1) are shown in parentheses. b The emission maximumin in degassed CH2Cl2
solution. c The quantum yields (Фp) in degassed CH2Cl2 solution were measured with [Ir(ppy)2(acac)] (Фp= 0.34) as a standard (λex= 400 nm). d Measured in degassed
CH2Cl2 at a sample concentration of 10−5 M. e 0.1 M [Bu4N]PF6 in CH2Cl2, scan rate 100 mV s−1, measured using saturated calomel electrode (SCE) as the standard. f
Eg were calculated from the intersection of the normalized absorption and the emission spectra. g EHOMO (eV) = -e(4.4 +ꢇꢈꢈꢉꢊꢋꢌꢍ), ELUMO (eV)= EHOMO + Eg. h Reference
48. All measured at ambient temperature.

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Table 2 Some of Ir(III) complexes used as oxygen sensing probes.
Probe
Support for immobilization
λex/em (nm)
I0/I100
References
[Ir(ppy)2(CN)2]
Ir(CS-Me)2(acac)
N-833
nBuPTP
Polystyrene
Polystyrene
Polystyrene
Ethyl cellulose
AlOOH
340/510
475/566
402/529
400/526
405/524
350/490
385/585
-
Chem Mater 2005;17:4765–73.
Anal Chem 2007;79:7501–9.
Talanta 2007;71:242–50.
Talanta 2007;71:242–50.
Chem Mater 2009;21,2173–5.
Talanta 2010;82:620–6.
Talanta 2010;82:620–6.
Sens Actuators B 2010;145:278–84.
Chem Mater 2012;24:2330−8.
Analyst 2013;138:1819–27.
J Mater Chem C 2015;3:8010-7.
This work
-
1.3
2.4
-
N-926
Ir(ppy-NPh2)3
N969
47.6
5.0
7.41
-
N969
Polystyrene
FIB
Ir(mebtp)3
dye 3
296/595
335/470
-
AP200/19
Ir(piq)2(acac)
Lx4
ORMOSILs
Ethyl cellulose
Ethyl cellulose
Ethyl cellulose
Ethyl cellulose
Ethyl cellulose
Ethyl cellulose
-
-
5.7
5.8
16.2
16.4
15.2
14.1
400/517
400/528
400/528
400/534
Ir(ppy)2(acac)
Ir1
This work
Ir2
This work
Ir3
This work
Ir4
This work
400/557

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List of Scheme and Figures:
Scheme 1. Synthesis of bis-cyclometalated Ir(III) complexes for this study. (i)
Pd(OAc)2, K2CO3, EtOH/H2O 3:1(v/v), 80 ºC, in air, 30-60 min. (ii) IrCl3·3H2O,
EtOCH2CH2OH/H2O 3:1(v/v), 110 ºC, N2, 24 h. (iii) Hacac, K2CO3,
EtOCH2CH2OH, 120 ºC, N2, 24 h.
Fig. 1. Absorption (up) and emission spectra (down, λex= 400 nm) of Ir1-Ir4 and
Ir(ppy)2(acac) in CH2Cl2 solution at room temperature.
Fig. 2. Phosphorescence decay profiles of Ir1-Ir4 and Ir(ppy)2(acac) in CH2Cl2
solution at room temperature.
Fig. 3. Illustration of simplified mechanism for oxygen sensing.
Fig. 4. (A) The transparent quartz plate (r = 7.0 mm) whose inner surface is covered
with the oxygen sensing film. (B) SEM micrograph of the oxygen sensing film. (C)
The diagram of the sensing layer.
Fig. 5. Photo-degradation histograms for the Ir(III) complexes/EC films under
continuous illumination for 90 min at ambient atmospheric conditions.
Fig. 6. Stern-Volmer plots for oxygen sensing films of Ir(III) complexes immobilized
in EC (intensity ratios I0/I versus O2 partial pressure).

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Scheme 1. Synthesis of bis-cyclometalated Ir(III) complexes for this study. (i) Pd(OAc)2, K2CO3,
EtOH/H2O 3:1(v/v), 80 ºC, in air, 30-60 min. (ii) IrCl3·3H2O, EtOCH2CH2OH/H2O 3:1(v/v), 110 ºC, N2,
h. (iii) Hacac, K2CO3, EtOCH2CH2OH, 120 ºC, N2, 24 h.
Fig. 1. Absorption (up) and emission spectra (down, λex= 400 nm) of Ir1-Ir4 and Ir(ppy)2(acac) in CH2Cl2 solution
at room temperature.
Fig. 2. Phosphorescence decay profiles of Ir1-Ir4 and Ir(ppy)2(acac) in CH2Cl2 solution at room temperature.

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Fig. 3. Illustration of simplified mechanism for oxygen sensing.
Fig. 4. (A) The transparent quartz plate (r = 7.0 mm) whose inner surface is covered with the oxygen sensing film.
(B) SEM micrograph of the oxygen sensing film. (C) The diagram of the sensing layer.
Fig. 5. Photo-degradation histograms for the Ir(III) complexes/EC films under continuous illumination for 90 min
at ambient atmospheric conditions.

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Fig. 6. Stern-Volmer plots for oxygen sensing films of Ir(III) complexes immobilized in EC (intensity ratios I0/I
versus O2 partial pressure).

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Research highlights
1) A series of NPh2-modified Ir(III) complexes Ir1-Ir4 have been synthesized.
2) The luminescent lifetimes of Ir1-Ir4 are prolonged due to the diphenylamino
group.
3) The photostabilities of Ir1-Ir4 are enhanced efficiently over Ir(ppy)2(acac).
4) Ir1-Ir4 demonstrated excellent oxygen sensitivity with I0/I100 > 14.