The influence of molecular structure on collision radius for optical sensing of molecular oxygen based on cyclometalated Ir(iii) complexes

Article abstract of DOI:10.1039/C8RA07115B

Three triphenylamine (TPA) substituted cyclometalated Ir(iii) complexes IrA1, IrA2, and IrA3 based on Ir(ppy)₃ were synthesized and applied as phosphorescent probes for the monitoring of molecular oxygen. The phosphorescence intensity of all the Ir(iii) complexes in tetrahydrofuran (THF) was gradually quenched with an increase of oxygen concentration. The increase of TPA substituents on the meta-position of 2-phenylpyridine (IrA1-IrA3) gradually improved the oxygen sensitivity of cyclometalated Ir(iii) complexes. IrA3 showed the highest oxygen sensitivity in THF with a KappSV of 204.8 bar^?1 and a limit of detection (LOD) of 0.27 mbar. The relationship between molecular structure and the collision radiuses (σ) of all the Ir(iii) complexes has been investigated on the basis of the Demas model and the fundamental expression of luminescence quenching systems by oxygen. The ratio of collision radiuses are σ_IrA1/σ_Ir(ppy)3 = 1.27 ± 0.05, σ_IrA2/σ_Ir(ppy)3 = 1.72 ± 0.10, and σ_IrA3/σ_Ir(ppy)3 = 2.13 ± 0.07, respectively. The introduction and increase of TPA substituents can obviously increase the collision radiuses of cyclometalated Ir(iii) complexes which leading to potential oxygen sensitivity. And the incremental effect of collision radiuses caused by the introduction of TPA substituents resulted in outstanding oxygen sensitivity of IrA3. The results demonstrate for the first time evidence between molecular structure and oxygen sensitivity of the emitters for optical sensing.

Full text of DOI:10.1039/C8RA07115B

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PAPER
The influence of molecular structure on collision
radius for optical sensing of molecular oxygen
based on cyclometalated Ir(III) complexes†
Cite this: RSC Adv., 2018, 8, 41040
a
b
Ling Di,‡a Yang Xing,
‡
Xiaoning Wang,a Daoyuan Zheng,b Yang Yang
*
a
*
and Fayun Li
Three triphenylamine (TPA) substituted cyclometalated Ir(III) complexes IrA1, IrA2, and IrA3 based on Ir(ppy)3
were synthesized and applied as phosphorescent probes for the monitoring of molecular oxygen. The
phosphorescence intensity of all the Ir(III) complexes in tetrahydrofuran (THF) was gradually quenched
with an increase of oxygen concentration. The increase of TPA substituents on the meta-position of 2-
phenylpyridine (IrA1-IrA3) gradually improved the oxygen sensitivity of cyclometalated Ir(III) complexes.
app
SV
IrA3 showed the highest oxygen sensitivity in THF with a K
of 204.8 barꢀ1 and a limit of detection
(LOD) of 0.27 mbar. The relationship between molecular structure and the collision radiuses (s) of all the
Ir(III) complexes has been investigated on the basis of the Demas model and the fundamental expression
of luminescence quenching systems by oxygen. The ratio of collision radiuses are sIrA1/sIr(ppy) ¼ 1.27 ꢁ
3
0.05, sIrA2/sIr(ppy) ¼ 1.72 ꢁ 0.10, and sIrA3/sIr(ppy) ¼ 2.13 ꢁ 0.07, respectively. The introduction and
3
3
increase of TPA substituents can obviously increase the collision radiuses of cyclometalated Ir(III)
complexes which leading to potential oxygen sensitivity. And the incremental effect of collision radiuses
caused by the introduction of TPA substituents resulted in outstanding oxygen sensitivity of IrA3. The
results demonstrate for the first time evidence between molecular structure and oxygen sensitivity of the
emitters for optical sensing.
Received 26th August 2018
Accepted 3rd December 2018
DOI: 10.1039/c8ra07115b
rsc.li/rsc-advances
absence of oxygen. kq means bimolecular quenching constant
and [O2] expresses the molar concentration of oxygen. I0/I (or s0/
1. Introduction
s) versus [O2] are linear with slopes of kqs0. In principle, oxygen
sensitivity is roughly in proportion to phosphorescent decay
time of the emitter.11–14 State of the art optical oxygen sensors
are mainly made up of phosphorescent iridium, platinum
complexes, BF2, and aluminum chelates, which possess decay
time varying in long range of microseconds to
milliseconds.15–19
To prevent potential interference and loss of emitters, poly-
mer matrices are oen used as permeation-selective barrier.20–24
In eqn (2), the oxygen concentration in the polymer matrix is
Optical sensors have attracted a great deal of interest for oxygen
quantication due to the advantages of excellent reversibility,
quick response, minimal invasiveness and suitability for
monitoring of oxygen in gaseous condition and in solution
(dissolved oxygen).1–4 Various formats of optical oxygen sensors
have been developed including bre-optic sensors, micro- or
nanoparticles, paints and so on.5–9 Optical oxygen sensors
depended on the dynamic (diffusion-controlled) quenching of
emitters by molecular oxygen which is characterized by the
Stern–Volmer equation (eqn (1))10
proportional to the oxygen partial pressure (pO ) just above the
2
I0
I
s0
s
¼
¼ 1 þ kqs0½O2ꢂ
(1)
lm. And the proportionality constant SO is the oxygen solu-
2
bility in organic solvents and polymer matrices dened by
Henry's law.25
where I and s are the intensity and decay time of the emitter
respectively. I0 and s0 are the corresponding values in the
[O2] ¼ SO pO
(2)
2
2
aCollege of Chemistry, Chemical Engineering, and Environmental Engineering,
Liaoning Shihua University, Fushun 113001, China. E-mail: xingyang@lnpu.edu.cn;
ilnpu@163.com; Tel: +86-24-56860865
Because quenching of phosphorescence is caused by diffu-
sion between emitters and molecular oxygen, the bimolecular
quenching constant kq by the expression (eqn (3))26
bState Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, Dalian 116023, China
4pNA
† Electronic supplementary information (ESI) available: Phosphorescent emission
decay curves and cyclic voltammogram. See DOI: 10.1039/c8ra07115b
‡ These authors contributed to this work equally.
kq ¼
saDO
(3)
2
1000
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DO is the diffusion coefficient of oxygen in solvents or matrices,
2
and NA is Avogadro's constant. a represents the probability that
a collision leads to quenching. Many authors set a equal to
unity for phosphorescence quenching by oxygen in analyzing
data.27–31 s is the collision radius of an emitter molecule to
molecular oxygen. Upon substitution of eqn (2) and (3) into (1),
the fundamental expression (eqn (4)) governing luminescence
quenching by oxygen for systems in equilibrium with oxygen
was obtained.26
I0
I
s0
s
4paNA
¼
¼ 1 þ
ðDO SO Þðss0ÞpO
(4)
2
2
2
1000
On one hand, oxygen sensitivity could be effectively
enhanced by using of high oxygen permeable matrix. The extent
of quenching markedly depends on the oxygen permeability
(PO ) of the matrix, where PO is dened as the product of DO
2
2
2
Fig. 1 Chemical structure of phosphorescent complexes Ir-1, Ir-2,
Cu-1, and Cu-2. Structure variations of intraligands of Ir-1 and Ir-2,
Cu-1 and Cu-2 are showed in red circles.
and SO (PO ¼ DO SO ). Much work so far has focused on the
2
2
2
2
modication of the matrices with uorinated substituents,
etc.32–35 On the other hand, decay time (s0) of the emitters also
determined oxygen sensitivity of the sensors. Considerable
research efforts have been devoted to the design and synthesis
of emitters with long triplet decay time.3,11,36,37 Borisov and co-
workers reported an aluminium chelate Al(HBANPF)3 with
ultra-long decaying phosphorescence (350 ms). Al(HBANPF)3
which were immobilized in peruorinated polymers can
monitor oxygen in nearly anoxic system.3 Zhao et al. investi-
gated the oxygen sensitivity of two dual-emissive Pt(II)
complexes Pt-1 and Pt-2. And Pt-2 with long phosphorescent
decay time (3.75 ms) exhibited higher oxygen sensitivity (136.0
barꢀ1) compared with short-decay Pt-1 (1.53 ms, 45.1 barꢀ1).11
phosphorescent decay time showed lower oxygen sensitivity
compared to short-decay Ir-1 (Fig. 1).40 Phosphorescent decay
time of Cu(I) complex Cu-2 is 17-fold longer than Cu-1, however
the oxygen sensitivity of Cu-2 is 72% of Cu-1 (Fig. 1).44 Obvi-
ously, changes of molecular structure of the emitter lead to the
changing of the collision radius, and then affect oxygen sensi-
tivity. For quantication of low concentrations or low partial
pressures of oxygen, researchers would like to design phos-
phorescent emitters with long decay time and then immobilized
them in the matrices with extremely high oxygen permeability.
To the best of our knowledge, no systematic study concerning
how change of molecular structure affect the collision radius (s)
of a phosphorescent emitter for optical oxygen sensing.
Triphenylamine (TPA) substituent is an important structural
motif in numerous dye-sensitized solar cells (DSSCs) and
organic electroluminescence materials because of features,
such as non-co-planarity of the three phenyl substituents,
strong electron-donating nature and high light-to-electrical
energy.45,46 Herein, we report a systematic investigation of the
Besides PO and s0, the collision radius (s) of the emitter is the
2
third factor to oxygen sensitivity of emitters which was oen
ignored by researchers. In some cases, oxygen sensitivity and
decay time of emitters are not proportional in the same
microenvironment (emitters were immobilized in the same
matrix with the same preparation method).7,10,22,36–43 For
instance, neutral blue emitting cyclometalated Ir(III) complexes
Ir-1 and Ir-2 were immobilized in a nanostructured aluminum
oxide–hydroxide matrix named AP200/19. Ir-2 with long
Scheme 1 Structures of the cyclometalated Ir(III) complexes.
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Table 1 Photophysical data of all the Ir(III) complexes
Emission maxima
lem (nm) (293 K)
s c(ms)
b
Complexes
Absorption (293 K) labs a(nm)
Fp
Ir(ppy)3
IrA1
IrA2
250 (1.87), 284 (3.75), 379 (1.12), 411 (0.71), 456 (0.27), 490 (0.12)
249 (1.71), 287 (4.56), 340 (3.37), 412 (0.78), 444 (0.32), 488 (0.09)
254 (1.27), 287 (4.12), 334 (5.07), 405 (0.60), 480 (0.10)
511
520
522
522
0.40
0.48
0.49
0.53
2.38
2.93
2.51
2.42
IrA3
262 (4.18), 296 (8.42), 332 (9.48), 404 (1.32), 472 (0.27)
a
Measured in THF at a concentration of 10ꢀ5 M and log 3 values are shown in parentheses and extinction coefficients (104 Mꢀ1 cmꢀ1) are shown in
b
c
parentheses. In degassed THF relative to [Ir(ppy)3] (Fp ¼ 0.40), (lex ¼ 400 nm). Measured in degassed THF at a sample concentration of ca.
10ꢀ5 M and the excitation wavelength was set at 355 nm for all the samples at 293 K.
relationship between the molecular structure and the collision when increasing TPA substituents at the meta-position on
radius (s) of the emitter by modication of cyclometalated Ir(III) benzene ring of 2-phenylpyridine ligands. The change in
complexes Ir(ppy)3 with incremental introduction of TPA emission color of all the Ir(III) complexes were quantied with
substituents. Chemical structures of cyclometalated Ir(III) CIE coordinates of Ir(ppy)3 (0.2750, 0.6268), IrA1 (0.2977,
complexes IrA1, IrA2, IrA3 and Ir(ppy)3 are shown in Scheme 1. 0.6354), IrA2 (0.3184, 0.6228), and IrA3 (0.3164, 0.6341) in
Fig. S2 (see ESI†). The electrochemical properties of IrA1–IrA3
and Ir(ppy)3 were examined using cyclic voltammetry (Fig. S3,
see ESI†). All the cyclometalated Ir(III) complexes showed
2. Result and discussion
2.1. Photophysical and electrochemical property
reversible oxidation waves. The oxidation potentials of IrA1-IrA3
and Ir(ppy)3 are 0.78 V, 0.77 V, 0.78 V, and 0.84 V, respectively,
which reected the donor character of TPA groups. The frontier
orbital energy levels of all the Ir(III) complexes are given in Table
S1 (see ESI†).
The photophysical properties of IrA1–IrA3 and Ir(ppy)3 are
exhibited Table 1. The absorptions of 250–375 nm which belong
to p–p* transitions in intraligands enhanced with the incre-
mental introduction of TPA substituents on meta-position of 2-
phenylpyridine ligands (Fig. 2). The phenomenon also caused
the increase of phosphorescence quantum efficiency of Ir(ppy)3
(0.40), IrA1 (0.48), IrA2 (0.49), and IrA3 (0.53) (Table 1). The
transient phosphorescence decay for each Ir(III) complex in THF
was mono-exponential, and the lifetimes ranged from 2.38 ms to
2.93 ms. Compared to Ir(ppy)3 (2.38 ms), phosphorescent lifetime
of IrA1 (2.93 ms) became longer with introducing a TPA
substituent on 2-phenylpyridine. Notably, the incremental
introduction of TPA substituents on 2-phenylpyridines of
Ir(ppy)3 gradually shortened the lifetimes of cyclometalated
Ir(III) complexes. Phosphorescent lifetimes of all the Ir(III)
complexes are in the sequence of IrA1 > IrA2 > IrA3 > Ir(ppy)3
(Fig. S1, see ESI†). The emission maxima for all the Ir(III)
complexes are in region from 511 to 522 nm. Based on Ir(ppy)3,
the emission maxima of IrA1–IrA3 shis to a longer wavelength
2.2. Theoretical calculations
Density functional theory (DFT) calculations were performed in
order to estimate the ground-state energy levels and electron
density distributions of the orbitals of IrA1–IrA3 and Ir(ppy)3.47
And the ground-state geometry of were optimized under the
polarizable continuum model (PCM, solvent ¼ THF) of the self-
consistent reaction eld (SCRF). The HOMO and LUMO iso-
surfaces displayed in Fig. 3 have p symmetry with opposite
phases above and below the molecular plane. The LUMOs of
IrA1–IrA3 and Ir(ppy)3 consist of a mixture of 2-phenylpyridine
moieties. The HOMO of Ir(ppy)3 is predominantly distributed
on the benzene rings of 2-phenylpyridine ligands and Pt atom.
The TPA moieties, benzene rings of 2-phenylpyridine ligands
and Pt atoms contribute the HOMOs of IrA1–IrA3. The intro-
duction of a TPA substituent on 2-phenylpyridine caused the
delocalization of electron from 2-phenylpyridine moieties to the
TPA moiety with energy rise (0.29 eV) of HOMO of IrA1. While
the incremental introduction of TPA substituents on 2-phenyl-
pyridines have little effect on the HOMOs of IrA2 and IrA3
(Fig. 3).
The low-lying excited states of the singlet (S1) and the triplet
(T1) of IrA1–IrA3 and Ir(ppy)3 were also studied with the time-
dependent DFT calculations under PCM model (solvent ¼
THF) based on the optimized ground state geometry (Table 2).47
The calculated S1 to T1 energy gaps of IrA1–IrA3 and Ir(ppy)3 are
0.33 eV, 0.31 eV, 0.30 eV, and 0.29 eV, respectively. The orbitals
are involved in HOMOꢀ3, HOMOꢀ2, HOMOꢀ1, HOMO,
LUMO, and LUMO+1. The S1 and T1 states of all the Ir(III)
complexes were mainly assigned as HOMO to LUMO of electron
transition. Similar to Ir(ppy)3, the S1 state and T1 state of IrA3
Fig. 2 Absorption and emission spectra of IrA1–IrA3 and Ir(ppy)3;
10ꢀ5 M in degassed THF, 25 ꢃC.
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Fig. 3 HOMOs and LUMOs of IrA1–IrA3 and Ir(ppy)3 for the ground state geometry. Calculated by DFT/PCM ¼ THF at the B3LYP/6-31G(d)/
LanL2DZ level using Gaussian 16.
were rendered with considerable nature of IL, mixed with MLCT oxygen concentration. Above 83% of original intensity was
character. The S1 state and T1 state of IrA1 were rendered with quenched with 6.8% oxygen in the air volume. Notably, the
nature of LLCT, MLCT character. And the S1 state and T1 state of emission of IrA3 was almost completely quenched (above 90%
IrA2 were rendered with nature of IL, LLCT, mixed with MLCT of original intensity) at the oxygen concentration of 6.8% which
character (Table 2 and Fig. S4 see ESI†).
showed high oxygen sensitivity.
Demas et al. have investigated the quenching mechanism of
oxygen of several emitters in a lot of polymer matrices.48,49 The
results showed that heterogeneous microenvironment exists in
2.3. Oxygen sensing
Puried tetrahydrofuran (THF) was selected as dispersion phase the emitter-contained matrix. The Demas' model48 was intro-
for the bimolecular quenching process (Fig. 4). The emission of duced to t the Stern–Volmer plots (SVPs) of all the Ir(III)
all the Ir(III) complexes obviously weaken with the increase of complexes in THF (Fig. 4) which reads as follows in eqn (5)
Table 2 Electronic excitation energies, oscillator strengths (f), configuration-interaction coefficients (CI), and orbital transition contribution
percentages (Co) of the low-lying electronically excited states of IrA1–IrA3 and Ir(ppy)3
TDDFT//B3LYP/6-31G(d)/LanL2DZ
Elec. Trans.
Energya
fb
Compositionc
CId
0.69705
Coe
Characterf
Ir(ppy)3
IrA1
S0 / S1
S0 / T1
S0 / S1
3.00 eV, 413 nm
2.71 eV, 457 nm
2.92 eV, 425 nm
0.0109
0.0000
0.0238
H / L
H / L
H / L
H-1 / L
H / L
97.18%
58.80%
66.71%
19.19%
33.32%
25.38%
73.52%
12.87%
35.34%
16.10%
78.21%
17.69%
36.97%
13.64%
IL, MLCT
IL, MLCT
0.54221
0.57752
0.30978
0.40814
ꢀ0.35624
0.60632
0.25367
0.42034
0.28375
0.62533
0.29739
0.42993
0.26116
LLCT, MLCT
LLCT, MLCT
LLCT, MLCT
LLCT, MLCT
IL, LLCT, MLCT
LLCT, MLCT
IL, LLCT, MLCT
IL
IL, MLCT
IL, MLCT
IL, MLCT
IL, MLCT
S0 / T1
S0 / S1
S0 / T1
S0/S1
2.59 eV, 479 nm
2.89 eV, 429 nm
2.58 eV, 481 nm
2.88 eV, 431 nm
2.58 eV, 481 nm
0.0000
0.0133
0.0000
0.0059
0.0000
H / L+1
H / L
IrA2
IrA3
Hꢀ2 / L
H / L
Hꢀ1 / L+1
H / L
Hꢀ3 / L
S0 / T1
H / L
Hꢀ3 / L
Only the S1 and T1 states are presented. b Oscillator strengths. And the oscillator strengths of the T1 states are zero because of the neglect of spin–
orbit coupling in time-dependent DFT calculations. c Only the main congurations are presented. H and L denote HOMO and LUMO, respectively.
a
The conguration-interaction coefficients (CI) are in absolute values. e The contribution percentages of orbital transitions. Only the orbital pairs
d
f
whose Co values above 10% are presented. IL: intraligand, LLCT: ligand-to-ligand charge transfer, MLCT: metal-to-ligand charge transfer.
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higher than Ir(ppy)3 (0.985) which exhibited remarkable line-
arity of SVPs (Table S2, see ESI†). With the introduction of TPA
substituents at meta-position on 2-phenylpyridine ligands, f1
values were steadily improved which caused more homoge-
neous microenvironment (high oxygen sensitivity) of cyclo-
metalated Ir(III) complexes. The limit of detection (LOD) of all
the cyclometalated Ir(III) complexes in THF was calculated of
IrA1 (0.39 mbar), IrA2 (0.30 mbar), IrA3 (0.27 mbar) compared to
Ir(ppy)3 (0.43 mbar) (see ESI†).
From eqn (4), the ratio of collision radiuses (s1/s2) of two
emitters with different molecular structure under homoge-
neous microenvironment and uniform oxygen partial pressure
is obtains by the expression, reads as (eqn (6))
ÂÀ Á
Ã
I0
ꢀ 1 s2
s1
s2
I
ÂÀ Á
I0
1 ꢀ 1 s1
Ã
¼
(6)
I
2
The relationship between molecular structure and the colli-
sion radius have been investigated by parallelly measuring data
of I0 and I100 ve times. And results of bimolecular quenching
process in THF and phosphorescent decay time of IrA1–IrA3
and Ir(ppy)3 were employed for analyzing the variation of
collision radiuses (Fig. 5 and Table S3, see ESI†). The ratio of
collision radiuses are sIrA1/sIr(ppy) ¼ 1.27 ꢁ 0.05, sIrA2/sIr(ppy)
¼
3
3
1.72 ꢁ 0.10, and sIrA3/sIr(ppy) ¼ 2.13 ꢁ 0.07, respectively.
3
Comparision with Ir(ppy)3, the collision radius of IrA1 can be
obviously increased with the introduction of a TPA substituent
at meta-position on benzene ring of 2-phenylpyridine ligand.
And compared to IrA1, the progressive increase of TPA
substituents on the rest of 2-phenylpyridine ligands strongly
increased the collision radius of cyclometalated Ir(III) complexes
(5) (IrA2 and IrA3) as well which leading to higher collision prob-
ability between cyclometalated Ir(III) complexes and molecular
oxygen, and provided potential oxygen sensitivity.
Fig. 4 Dependence of emission spectra and Stern–Volmer plots of all
the Ir(III) complexes; 10ꢀ5 M in THF on oxygen concentration of 0–
100%, 25 ꢃC.
I
s
f1
f2
¼
¼
þ
O2
I0
s0
1 þ KSV1
p
1 þ KSV2
p
O2
As a practical performance, IrA3 was composited with a thin
lm of silicon gel supported on a glass slide to construct
a simple oxygen sensor. The phosphorescence of IrA3 oxygen
sensor was totally quenched in air atmosphere and emitted
bright yellow-green light under N2 stream. The quenching-
emission response of IrA3 oxygen sensor is ultrafast and fully
f1 and f2 are fractions of two microenvironments of the
heterogeneous optical sensing system, respectively, which are
dened as f1 + f2 being 1. f1 is the quenchable fraction with
quenching rate constant KSV1, and f2 is the fraction which is not
quenched at all with quenching rate constant KSV2. Weighted
app
app
quenching constant K
(K
SV
¼ f1 ꢄ KSV1 + f2 ꢄ KSV2) is the
SV
reversible that a ame-like light spot can be seen (Fig. 6).
guide of oxygen sensitivity of an optical oxygen sensor. The
Demas' model is well-suited to describe homogeneity of oxygen
sensing system. The Stern–Volmer constants for all the Ir(III)
complexes in THF are summarized in Table S2 (see ESI†). The I0/
I100 of IrA1-IrA3 and Ir(ppy)3 are 122.9, 143.2, 170.8, and 79.4.
app
SV
And K
are 138.6 barꢀ1, 171.4 barꢀ1, 204.8 barꢀ1, and 119.6
barꢀ1, respectively. IrA3 showed outstanding oxygen sensitivity
in THF. Contrary to the sequence of phosphorescent lifetimes,
the oxygen sensitivities are in the following order: IrA3 > IrA2 >
IrA1 > Ir(ppy)3. Compared to Ir(ppy)3, the introduction of TPA
substituents on cyclometalating ligand 2-phenylpyridine
improved oxygen sensitivity of cyclometalated Ir(III) complexes.
On the other hand, the increase of TPA substituents on meta-
position of 2-phenylpyridine ligands (IrA1–IrA3) gradually
improved the oxygen sensitivity (Fig. 4). Large quenchable
fractions (f1) of IrA1 (0.999), IrA2 (0.998), and IrA3 (0.998) are
Fig. 5 The variation of collision radiuses of IrA1–IrA3 via Ir(ppy)3 for
bimolecular quenching process.
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were recorded by cary eclipse G9800A uorescence spectro-
photometer (Agilent Technologies). Cyclic voltammograms
were recorded on an electrochemical workstation PARSTAT
2273 (Princeton Applied Research) at room temperature.
4.2. Synthesis section
Synthetic procedure of 2-(3-bromophenyl)pyridine.
A
mixture of 2-bromopyridine (2.0 mmol, 191 mL), 1.5 equiv. of (3-
bromophenyl)boronic acid (3.0 mmol, 482 mg), 2 equiv. of
K2CO3 (4.0 mmol, 548 mg), Pd(OAc)2 (1.5 mol%, 0.03 mmol,
6.74 mg), ethanol/water (6 mL/2 mL) was stirred at 80 ꢃC in air
for indicated time. The reaction mixture was added to brine (15
mL) and extracted four times with ethyl acetate (4 ꢄ 15 mL). The
solvent was concentrated and the product 2-(3-bromophenyl)
pyridine was isolated by short-column chromatography on
silica gel (200–300 mesh).
Fig. 6 (A) Photographs of a IrA3 oxygen sensor excited at 365 nm
under air (B) with a nitrogen stream in copper pipe.
3. Conclusions
In conclusion, three TPA substituted cyclometalated Ir(III)
complexes IrA1, IrA2, and IrA3 based on Ir(ppy)3 were synthe-
sized and applied as phosphorescent probes for analysis of
molecular oxygen. The phosphorescent intensity of Ir(III)
complexes in THF was drastically changed by the oxygen
concentration. IrA3 demonstrated the highest oxygen sensitivity
with I0/I100 of 170.8, KSapVp of 204.8 barꢀ1, and LOD of 0.27 mbar.
With the incremental introduction of TPA substituents at meta-
position on 2-phenylpyridine ligands of Ir(ppy)3, the oxygen
sensitivities of IrA1–IrA3 were obviously improved, and the f1
values were gradually increased which caused more homoge-
neous oxygen sensing system and high oxygen sensitivity. The
relationship between molecular structure and the collision
radius of the emitter for optical oxygen sensing have been
investigated and quantized through the cooperation of the
Demas' model and the fundamental expression of lumines-
cence quenching systems by oxygen. The incremental intro-
duction of TPA substituents could effectively increase the
collision radiuses of cyclometalated Ir(III) complexes which
leading to more likely collision between the emitter and
molecular oxygen, and provided high oxygen sensitivity. This
research could potentially be applied as new prospect for
molecular structure design of the emitter with high oxygen
sensitivity for optical monitoring of molecular oxygen.
2-(3-Bromophenyl)pyridine.12 Yield: 53.2%; colorless oil. 1H
NMR (500 MHz, CDCl3): d 8.70 (d, 1H), 8.17 (s, 1H), 7.91 (d, 1H),
7.77 (td, 1H), 7.71 (d, 1H), 7.54 (dd, 1H), 7.34 (t, 1H), 7.29–7.25
(m, 1H).
Synthetic procedure of ligand N,N-diphenyl-30-(pyridin-2-yl)-
[1,10-biphenyl]-4-amine (L1). A mixture of 2-(3-bromophenyl)
pyridine (0.25 mmol, 58.25 mg), 1.5 equiv. of (4-(diphenyla-
mino)phenyl)boronic acid (0.375 mmol, 108.40 mg), 2 equiv. of
K2CO3 (0.5 mmol, 69 mg), Pd(OAc)2 (1.5 mol%, 0.00375 mmol,
0.84 mg), ethanol/water (3 mL/1 mL) was stirred at 80 ꢃC in air
for indicated time. The reaction mixture was added to brine (15
mL) and extracted four times with ethyl acetate (4 ꢄ 15 mL). The
solvent was concentrated and the product L1 was isolated by
short-column chromatography on silica gel (200–300 mesh).
L1. Yield: 84.2%; white solid. 1H NMR (500 MHz, CDCl3)
d 8.71 (s, 1H), 8.20 (d, 1H), 7.92 (d, 1H), 7.77 (d, 2H), 7.63–7.50
(m, 4H), 7.26 (d, 5H), 7.17–7.11 (m, 6H), 7.07–6.98 (m, 2H). 13
C
NMR (126 MHz, CDCl3) d 157.5, 149.7, 147.7, 147.3, 141.2, 139.9,
136.8, 135.0, 129.3, 129.2, 128.0, 127.3, 125.4, 125.4, 124.4,
123.9, 122.9, 122.2, 120.7. HRMS (EI): calc.: 398.1783 [M]+.
Found: 398.1780 [M]+.
Synthetic procedure of IrA1. The cyclometalating ligand 2-
phenylpyridine (0.4 mmol, 57 mL) was reacted with 0.6 equiv.
IrCl3$3H2O (0.24 mmol, 84.5 mg) in a mixture of 2-ethox-
yethanol and water (9 mL/3 mL) at 105 ꢃC for 24 h to afford an
iridium dimer, which was subsequently reacted with 0.6 equiv.
of the cyclometalating ligand L1 (0.24 mmol, 95.5 mg) and 5
4. Experimental
4.1. Materials and methods
Unless otherwise noted, all the cross-coupling reactions for equiv. of Na2CO3 (2 mmol, 212 mg) in glycerol (10 mL) at 200 ꢃC
preparing the ligands were carried out in air. 2-Bromopyridine, for 48 h.
(3-bromophenyl)boronic
acid,
(4-(diphenylamino)phenyl)
IrA1. Yield: 73.6%; yellow acicular microcrystal. 1H NMR (500
boronic acid, phenylboronic acid, and iridium(III) chloride tri- MHz, CDCl3) d 7.96 (td, 2H), 7.86 (d, 4H), 7.64 (d, 2H), 7.58–7.54
hydrate were purchased from Adamas. Other chemicals were (m, 2H), 7.51 (dd, 2H), 7.27–7.20 (m, 4H), 7.11 (d, 10H), 6.98 (dd,
purchased from commercial sources and used without further 3H), 6.85 (dd, 8H). 13C NMR (126 MHz, CDCl3) d 166.7, 166.6,
1
purication. H NMR and 13C NMR spectra were recorded on 161.3, 147.9, 147.1, 147.1, 145.9, 144.3, 143.7, 137.4, 137.1,
a BRUKER AVANCE III HD 500 MHz spectrophotometer. Mass 135.9, 129.9, 129.2, 127.1, 124.7, 124.0, 123.9, 122.5, 122.1,
spectra were recorded with a MALDI micro MX spectrometer. 121.9, 119.9, 119.8, 118.9, 118.8. HRMS (EI): calc.: 898.2647
Phosphorescence lifetimes were determined on a Laser Flash [M]+. Found: 898.2646 [M]+.
Photolysis (LFP) (Dalian Institute of Chemical Physics).
Synthetic procedure of IrA2. The cyclometalating ligand L1
Absorption spectra were recorded by cary series 5000 UV-Vis- (0.4 mmol, 159.2 mg) was reacted with 0.6 equiv. IrCl3$3H2O
NIR spectrometer (Agilent Technologies). Emission spectra (0.24 mmol, 84.5 mg) in a mixture of 2-ethoxyethanol and water
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(9 mL/3 mL) at 105 ꢃC for 24 h to afford an iridium dimer, which
was subsequently reacted with 0.6 equiv. of the cyclometalating
ligand 2-phenylpyridine (0.24 mmol, 34.2 mL) and 5 equiv. of
Na2CO3 (2 mmol, 212 mg) in glycerol (10 mL) at 200 ꢃC for 48 h.
IrA2. Yield: 71.0%; yellow acicular microcrystal. 1H NMR (500
MHz, CDCl3) d 7.96 (t, 4H), 7.85 (d, 4H), 7.61–7.54 (m, 6H), 7.50
(dd, 4H), 7.26–7.20 (m, 8H), 7.10 (d, 12H), 6.98 (dd, 8H), 6.85 (d,
4H). 13C NMR (126 MHz, CDCl3) d 166.8, 166.7, 148.0, 147.3,
147.2, 146.0, 144.4, 143.8, 137.5, 137.2, 137.0, 136.1, 132.2,
130.0, 129.9, 129.3, 128.6, 127.2, 124.8, 124.1, 122.6, 122.2,
120.1, 119.9, 118.9. HRMS (EI): calc.: 1141.3695 [M]+. Found:
1141.3692 [M]+.
Synthetic procedure of IrA3. The cyclometalating ligand L1
(0.4 mmol, 159.2 mg) was reacted with 0.6 equiv. IrCl3$3H2O
(0.24 mmol, 84.5 mg) in a mixture of 2-ethoxyethanol and water
(9 mL/3 mL) at 105 ꢃC for 24 h to afford an iridium dimer, which
was subsequently reacted with 0.6 equiv. of the cyclometalating
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was monitored by bubble owmeter.10
4.4. IrA3 oxygen sensor
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22 C. Liu, X. Song, X. Rao, Y. Xing, Z. Wang, J. Zhao and J. Qiu,
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23 B. Lei, L. Wang, H. Zhang, Y. Liu, H. Dong, M. Zheng and
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Conflicts of interest
´
´
´
´
24 N. de Acha, C. Elosua, D. Martınez, M. Hernaez, I. R. Matıas
and F. J. Arregui, Sens. Actuators, B, 2017, 239, 1124.
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There are no conicts to declare.
Acknowledgements
This work was supported by the funds of Liaoning Shihua
University (2016XJJ-099, 2017XJJ-001) and the fund of Depart-
ment of Education Of Liaoning Province (L2017LQN007).
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