Photostable ester-substituted bis-cyclometalated cationic iridium(III) complexes for continuous monitoring of oxygen

Article abstract of DOI:10.1039/c5dt02804c

Three bis-cyclometalated cationic Ir(iii) complexes Ir1, Ir2 and Ir3 with an ester substituent at the 4-position of the phenyl ring on the 2-phenylpyridine (ppy) have been synthesized and fully characterized. The emission maxima of ester-substituted Ir(ii

Full text of DOI:10.1039/c5dt02804c

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DOI: 10.1039/C5DT02804C
ARTICLE TYPE
Photostable ester-substituted bis-cyclometalated cationic iridium(III) complexes for continuous
monitoring oxygen
Chun Liu*, Hongcui Yu, Yang Xing, Zhanming Gao and Zilin Jin
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Three bis-cyclometalated cationic Ir(III) complexes Ir1, Ir2 and Ir3 with an ester substituent at the 4-
position of the phenyl ring on the 2-phenylpyridine (ppy) have been synthesized and fully characterized.
The emission maxima of ester-substituted Ir(III) complexes show notably blue-shift compared to the
+
-
parent complex Ir0 [Ir(ppy) (phen)] PF (phen = 1,10-phenanthroline). The influences of an ester group
2
6
1
0
on the photoelectric properties of the Ir(III) complexes have been investigated systematically. The oxygen
sensing films prepared from ethyl cellulose immobilized with Ir(III) complexes exhibit excellent
operational stability, high photostability and quick response to oxygen. Ir1~Ir3 demonstrate extended
luminescent lifetimes relative to Ir0, and display better sensitivity on changes of oxygen partial pressures.
1
Introduction
45 complexes could tune the emission wave-length red-shift for
1
7,18
developing efficient organic light-emitting diodes
emitting electrochemical cells. Moreover, the water-soluble
carboxylate Ir(III) complexes obtained from the hydrolysis of the
and light-
1
2
2
3
3
4
5
0
5
0
5
0
Oxygen measurements are of great importance in many
19
1
,2
environmental and biomedical applications. Compared with
traditional techniques, optical oxygen methods have many
advantages including high sensitivity, immunity to
electromagnetic interference, virtually non-invasive, no oxygen
ester-substituted complexes showed a promising application in
20,21
5
5
6
0
5
0
bioprocessing.
It is worth mentioning that, in 2012, Marín-
22
Suárez and co-workers described the use of ester-substituted
blue phosphorescent neutral Ir(III) complexes immobilized in the
AP200/19 polymer for efficient oxygen detection at a low
concentration. These reports demonstrate that the ester group is
an interesting substituent for adjusting the photoelectric
properties of the corresponding metal complexes. To the best of
our knowledge, no example of ester-substituted bis-
cyclometalated cationic Ir(III) complexes for oxygen sensing has
been reported yet.
3
consumption, and fast response times. Thus, they have become
extremely popular in the last two decades. The dynamic
quenching of luminescence of the oxygen sensitive probes by
molecular oxygen is the most widely used sensing mechanism for
the optical detection of this gas. An ideal oxygen sensitive
probe should possess several key requirements: efficient emission
4
,5
(
high quantum yield) for improved sensitivity, long luminescent
lifetime (from hundreds of nanoseconds to microseconds) to be
quenched efficiently by oxygen. Equally important is high
photostability of the probes to construct robust oxygen sensing
systems. In recent years, many heavy metal complexes as
oxygen-sensitive probes have been developed in the fabrication
of oxygen-sensing devices by dissolved in a solvent or
In the present paper, three new bis-cyclometalated cationic Ir(III)
complexes [Ir(C^N)
+
−
2
(phen)] PF
6
(Ir1~Ir3) modified with
6
−
COOEt were synthesized. The chemical structures of these
complexes are illustrated in Fig. 1.
7-11
immobilized in a polymer matrix.
Among them, Ir(III)
complexes are particularly suitable for the use in oxygen sensing,
due to their relatively long lifetimes, high emission quantum
yields, and flexible tuning of photophysical and electrochemical
1
2,13
properties by the coordinating ligands.
The substituent effect has been recognized as an important factor
for adjusting the photoelectric properties of the luminescence
metal complexes. Several groups have demonstrated that the
colours or quantum yields of metal complexes could be
effectively influenced by different substituents, such as CF , F,
3
14-16
COOR, Me, OMe, and so on.
ester substituent onto the cyclometalating ligands of Ir(III)
For example, introducing an
6
5
Fig. 1 Chemical structures of the Ir(III) complexes for this study.
[journal], [year], [vol], 00–00 | 1
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Dalton Transactions
Page 2 of 8
Their photophysical and electrochemical properties have been
fully investigated. In addition, we fabricated the ethyl cellulose
EC) films immobilized with these Ir(III) complexes to evaluate
Ir1~Ir3 were also tested in other solvents with different polarity
see Fig. S4, ESI). Emission spectra reveal that the
phosphorescence is attributed to a mixture of both m
(
V
e
ieta
w
l–Ar
l
t
i
i
g
cl
a
e
n
Online
d
(
3
3
DOI: 12 08 -. 31 00 39/C5DT02804C
charge-transfer ( MLCT) and ( π–π*) transitions.
their oxygen sensitivity and photostability.
5
2 Results and discussion
2
.1 Crystal structure of Ir2
The perspective view of Ir2 is shown in Fig. 2 and selected
crystallographic data are provided in Table S1 (see ESI). It can be
seen that the Ir(III) center in Ir2 adopts a distorted octahedral
coordination geometry with cis-metalated carbons and trans
nitrogen atoms. The distance of Ir−N (N^N ligands) is 2.143(4) Å,
which is longer than that of Ir−N (C^N ligands) (2.059(4) Å). The
N−Ir−N and C−Ir−N angles are 77.4º and 80.5º, which are similar
1
0
23-25
to the previously reported.
The cation of Ir2 is connected to
50
-
1
5
the anion PF by weak intra- and intermolecular C−H---F−P
6
hydrogen bonds with an average H---F distance of 2.62 Å and
intramolecular N−H---F−P hydrogen bonds with an average H---
F distance of 2.22 Å.
Fig. 3 Absorption spectra (up) and emission spectra (down) of the
-5
2 2
Ir0~Ir3 (1.0 × 10 M) (λex = 400 nm) in CH Cl solution, at
ambient temperature.
5
5
The onset oxidation potentials of Ir0~Ir3 are at 1.20 V, 1.36 V,
1
.43 V and 1.31 V, indicating that the introduction of the ester
group into the cyclometalating ligand stabilizes the metal center
2
0
Fig. 2 Perspective view of Ir2 (the thermal ellipsoids drawn at 30% as it is more difficult to oxidize. These complexes exhibit
probability level and hydrogen atoms are omitted for clarity).
reduction wave at similar potentials between −1.33 V and −1.38
V, which can be assigned to the reduction of the phenanthroline
ligands (see Fig. S3, ESI). Specifically, Ir2 shows the highest
6
0
2
.2 Photophysical and electrochemical properties
UV−visible absorption spectra (Fig. 3) of all the Ir(III) complexes
quantum efficiency (Ф ) and onset oxidation potential than other
p
were measured in CH Cl at room temperature (Table 1). All
2
2
three complexes, which is probably attributed to the addition of a
fluorine atom on the pyridine ring. The C−F bond of the
2
3
3
4
4
5
0
5
0
5
Ir(III) complexes exhibit two major absorption bands in their UV-
vis absorption spectra, which are primarily due to the spin-
1
allowed intraligand ( π−π*) transitions. The weak absorption 65 cyclometalating ligand can increase the phosphorescent quantum
31-34
bands in the range 385~460 nm can be assigned to both spin-
yield and stabilize the HOMO energy level.
Obviously, the
allowed and spin−forbidden metal−to−ligand charge transfer
luminescent lifetimes (τ) of Ir1~Ir3 are longer than that of Ir0 in
1
3
26,27
( MLCT and MLCT) transitions.
The four spectra have
deaerated CH Cl and CH CN, respectively. The reason for this
2
2
3
similar band positions, however, obvious differences in
absorption coefficient are observed. Ir1~Ir3 have larger molar
absorption coefficient in the visible region than that of Ir0. For
example, the second bands of Ir1~Ir3 around 270 nm are similar
might be that coordination of ester group with a Ir(III) center in
7
0
the complexes Ir1~Ir3, the electron from the triplet metal-to-
3
ligand charge transfer ( MLCT) to the short-lived and
−1
−1
3
with absorption coefficient above 76,500 M cm , they are
nonemissive triplet metal-centered ( MC) state is suppressed, and
−1
−1
3
3
stronger than Ir0 (38,000 M cm ). The excitation spectra of
Ir0~Ir3 are almost the same as those of the absorption spectra
the energy difference between MLCT and MC states increases,
resulting in reduced radiationless decay and prolonging the
(
3
see Fig. S1, ESI), illustrating that phosphorescence shown in Fig.
comes from the target Ir(III) complexes, and not from
impurities. The emission maxima of Ir1~Ir3 (λmax at 541, 506
and 547 nm) in CH Cl show notably blue-shift relative to Ir0
λmax at 570 nm). The tuning of the emission maxima in the ester-
35-37
7
5
luminescence lifetime.
2
2
(
substituted complexes can be rationalized by taking into account
the donor–acceptor characters of the substituents and their impact
on the HOMO and LUMO energy levels. The emission spectra of
8
0
2
|
Journal Name, [year], [vol], 00–00This journal is © The Royal Society of Chemistry [year]

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Dalton Transactions
Table 1. Photophysical and electrochemical data for Ir0~Ir3.
a
b
c
p
d
e
ꢋꢌꢍꢉꢔ
ꢈꢉꢊ
f
ꢋꢌꢍꢉꢔ
f
g
h
Complexes
λabs(nm)
λem(nm)
Ф
τ(μs) τ(μs)
ꢇ
(V)
ꢇ
ꢋꢕ
(V) HOMO (eV) LUMO (eV) CIEle(x,y)
View Artic Online
DOI: 10.1039/C5DT02804C
Ir0
Ir1
Ir2
Ir3
231(2.99),265(3.81)385(0.50), 417(0.22)
231(7.53),269(7.97)398(0.38),426(0.58)
231(5.33),272(7.65)400(0.70),428(0.60)
232(5.35),271(8.25)399(0.76),425(0.62)
-5
570
0.23
0.22
0.38
0.29
0.80
2.02
3.09
1.35
-1
1.13
3.81
4.57
3.21
-1.36
-1.35
-1.33
-1.38
1.20
-5.60
-5.76
-5.83
-5.71
-3.07
-3.13
-3.19
-3.12
0.50,0.49
0.34,0.60
0.31,0.62
0.36,0.59
541
1.36
1.43
1.31
506,539
547
a
4
-1
b
Measured in CH
2
Cl
2
at a concentration of 10 M and extinction coefficients (10 M cm ) are shown in parentheses. The emission maxima are the values in
c
d
bold style. The quantum yields (Ф
p
) in degassed CH
CN solution. 0.1 M [Bu
2
Cl
2
solution were measured with [Ir(ppy)
N]PF in CH Cl , scan rate 100 mV·s , measured using saturated calomel electrode (SCE) as the
HOMO (eV) = -e(4.4+ꢀꢁꢂ ). ELUMO (eV) =EHOMO + E00. All measured at ambient temperature.
2 p
(acac)] (Ф = 0.34) as a standard (λex = 400 nm). In degassed
e
f
-1
CH
2
Cl
2
solution. In degassed CH
3
4
6
2
2
g
ꢁꢃꢄꢅꢆ
h
standard.
E
Table 2. Contour plots of frontier molecular orbitals of Ir0~Ir3 in ground state.
Ir0
Ir1
Ir2
Ir3
LUMO -5.21 ev
LUMO -5.22 ev
LUMO -5.37 ev
LUMO -5.15 ev
HOMO -7.87 ev
HOMO -7.98 ev
HOMO -8.21ev
HOMO -7.88 ev
enhancing the oxygen sensing ability of the complex. The process
can be described as follows: Ir * + O →Ir + O *, where Ir
2 2
2
.3 Theoretical calculations
III
III
III
6
5
5
0
5
0
In order to understand the impacts of the substituents on the
photophysical and electrochemical behavior of these Ir(III)
complexes, density functional theory (DFT) calculations for Ir(III)
complexes were performed. As reported in Table 2, the HOMO
denotes the complex and the “*” indicates the excited state. The
relationship between emission intensity (or luminescent lifetimes)
and the concentration of oxygen is reflected by a two-site model
to fit the curved Stern–Volmer plots (SVPs) in the oxygen
3
8,39
distribution primarily resides on the iridium center and 70 sensing film, which can be described as follows (Eq 1).
1
1
2
cyclometalating ligands, the LUMO distribution is localized on
the whole phenanthroline ligand. The HOMO–LUMO gaps
calculated for Ir0~Ir3 are 2.66 eV, 2.76 eV, 2.84 eV and 2.73 eV,
respectively. The complexes Ir0~Ir3 show different HOMO–
LUMO gaps from 2.66 eV to 2.84 eV, which is attributed to the
ꢎ
ꢑ
ꢖ
ꢗ
ꢖ
ꢝ
Eq 1
ꢎ
ꢏ
ꢐ ꢑ ꢐ ꢘꢙꢚ
SVꢗ∙ꢛ
ꢞ ꢘꢙꢚ
SVꢝ∙ꢛ
ꢜ
ꢝ
ꢏ
ꢜ
ꢝ
In the equation, I0 and τ0 are the unquenched luminescent
intensity and lifetime in an inert atmosphere, I and τ are the
corresponding quantities in the presence of oxygen. f and f are
1
2
introduction of different substituents on the cyclometalated 70 the fractions of the total emission for each component (f + f = 1),
1
2
complexes. The enhanced photoelectric properties of ester-
substituted complexes may have great opportunities to improve
their oxygen sensitivity. Considering the measuring conditions
and basis sets of calculation, the difference between experimental
values and calculated values is acceptable.
KSV1 and KSV2 are the Stern−Volmer constants for the two
components. ꢒOꢓ is the partial pressure of oxygen.
2
.4 Oxygen sensing properties
Ir(III) complexes with a long-living excited state usually have a
strong dynamic collision with triplet oxygen, which leads to a
reduction of its decay time and/or emission intensity, and to the
formation of highly reactive singlet oxygen. The dynamic
quenching process of Ir(III) complex by triplet oxygen can occur
through two mechanisms: electron transfer and energy transfer. In
both cases, a long luminescent lifetime will favor the process,
2
5
Scheme 1 The simple diagrams of optical oxygen sensing in
solution and EC.
5
5
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View Article Online
DOI: 10.1039/C5DT02804C
Fig. 6 Emission spectra of Ir0~Ir3 in CH Cl solution (solid) and
2
2
0
.5 wt % doped in EC (dash) (λex = 400 nm), at ambient
3
5
temperature.
Dynamic quenching of the Ir(III) complexes (Ir0~Ir3)
immobilized in polymer films by oxygen was also investigated,
including polar rigid ethylcellulose (EC), apolar flexible
poly(cyclohexene carbonate) (PCHC) and apolar rigid
Fig. 4 Emission spectra of Ir0~Ir3 under different partial
-5
pressures of O in CH CN solutions (1.0 × 10 M) (λ = 400 nm,
2
3
ex
5
at ambient temperature). In order to compare the quenching
properties of the complexes in solution, the Y axis is set from 40 polystyrene (PS). The results of oxygen sensing tests show that
ꢢꢛꢛ
−
100.
the ꢟꢠꢡ value of EC film immobilized with Ir1 (Ir1/EC) is
higher than those of others (see Fig. S6, ESI). This is because EC
is easily penetrated by oxygen with a permeability coefficient P
First, we studied the oxygen sensing properties of Ir0~Ir3 in
solution. As expected, the emission intensity of complexes
decreased with the increase of oxygen concentration from 0 to
-12
2
-1
of 1.1 × 10
cm Pa s , and is regarded as a standard host
1
0
4
0
4
5
5
6
6
7
5
0
5
0
5
0
material for oxygen sensing. Additionally, the contents of
1
00% (Fig. 4), suggesting that oxygen has a significant effect on
oxygen-sensitive probes in EC were optimized to reach the
the emission intensity of these complexes. The luminescence is
quenched by oxygen without distortions in the shape of the
spectra.
highest sensitivity. The results indicate that the optimum content
ꢢꢛꢛ
of EC film immobilized with Ir1 is 0.50 mg/g and its ꢟ value is
ꢠꢡ
-1
up to 0.00861 Torr (see Table S4, ESI). The results show that at
this content the EC matrix protects the complexes from potential
interference and avoids self-quenching efficiently. Thus, EC and
0
.5 wt% Ir(III) complexes were chosen as standard conditions for
oxygen sensing (see Fig. S7, ESI). Compared with the emission
spectra in solution, the emission spectra of Ir0~Ir3 in EC films
exhibit blue-shifts (Fig. 6) due to the rigidochromic effect, thus,
4
1,42
the intraligand excited state dominates in the EC film.
1
5
According to Eq (1), the quenching behaviors of Ir0~Ir3 were
analyzed at various concentrations of oxygen. As the results
showed in Table 4, the complexes Ir1~Ir3 are also more sensitive
to oxygen than Ir0. The I0/I100 values of EC film sensors
immobilized with Ir0~Ir3 are 3.47, 6.20, 6.46, and 7.25,
respectively. The most sensitive film is the oxygen-sensitive
ꢢꢛꢛ
probe of Ir3 incorporated into EC, showing a ꢟ of 0.01052
ꢠꢡ
-1
-1
ꢢꢛꢛ
ꢠꢡ
torr (7.99520 bar ), which is about 2 times higher than the ꢟ
of Ir0 (0.00417 torr or 3.16920 bar ) under same conditions. A
reproducibility test of oxygen sensing film of Ir0 was conducted.
-1
-1
3
Fig. 5 Phosphorescence decay profiles of Ir0~Ir3 in CH CN
solutions at the different concentrations of oxygen (0–9.61%) at
ambient temperature.
The error of I
five Ir0/EC films (see Fig. S9, ESI). The I
films are calculated to be 3.47
0
/I100 value was determined using the mean value of
/I100 values of Ir0/EC
0.11, showing good
0
2
2
3
0
5
0
The luminescent lifetimes (τ) of Ir0~Ir3 in CH CN solution (1.0
3
±
-5
×
10 M) were obtained for different oxygen concentrations at
reproducibility.
ambient temperature, the results are illuminated in Fig. 5.
Interestingly, Fig. 5 shows markedly changed luminescent
lifetimes (τ) in dependence on oxygen partial pressure. For
example, the luminescent lifetime (τ) of Ir2 is 4.57 µs in
3
degassed CH CN solution. The τ value reduces drastically to 0.42
µs in the presence of 9.61% O . In contrast, the luminescent
2
lifetime of Ir0 reduces from 1.13 µs to 0.11 µs under the same
experimental conditions (see Table S5 and Fig. S8, ESI). Thus,
the dynamic ranges of response for Ir1~ir3 are larger than that of
Ir0 under the conditions used.
4
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Journal Name, [year], [vol], 00–00This journal is © The Royal Society of Chemistry [year]

Page 5 of 8
Dalton Transactions
Ir(ppy)
2
(acac) has an oxidation potential of 0.71 V, which is
considerably lower than these of the cationic Ir (III) complexes
View Article Online
(Eox = 1.20 ~ 1.43 V). It has been reported that the transformation
DOI: 10.1039/C5DT02804C
of a neutral Ir(III) complex into the cationic form could be a good
design strategy to appropriately tune the oxidation potential and
3
4
4
5
0
5
4
3,44
the photostability.
The photooxidation of iridium complexes
is enhanced by higher electron density and lower excited state
4
5
ꢥ
oxidation potential. The excited state ꢀ potential can be
ꢣꢤ
calculated from the electrochemically determined ground state
Eox potential and the energy gap (E ). The excited state oxidation
00
2
potential of Ir(ppy) (acac) (-1.83 V) is also lower than those of
the cationic Ir (III) complexes (-1.21 ~ -1.33 V). The higher
oxidation potentials of these cationic Ir (III) complexes make the
molecules less reactive toward photooxidation processes and
increase the photostability.
Fig. 7 Time trace curves of Ir0～Ir3 (0.5 wt%) immobilized in
EC to O /N saturation cycles(λex = 400 nm). In order to compare
the quenching properties of the films, the Y axis is set from 0.
2
2
Table 3. The excited state redox potentials for the Ir(III)
complexes.
5
ꢋꢌꢍꢉꢔ
a
b
ꢇ ꢥꢋ ꢕc
Complexes
ꢇꢋꢕ (eV)
E00 (V)
d
Ir(ppy)
2
(acac)
0.71
1.20
1.36
1.43
1.31
2.54
2.53
2.63
2.64
2.59
-1.83
-1.33
-1.27
-1.21
-1.28
Ir0
Ir1
Ir2
Ir3
a
-1
0
.1 M [Bu
4
6 2 2
N]PF in CH Cl , scan rate 100 mV·s , measured using
b
saturated calomel electrode (SCE) as the standard.
E00 were
calculated from the intersection of the normalized absorption and
c
ꢥ
d
emission spectra by equation of E00 = 1240/λedge.
ꢀ = ꢀꢁꢂ- ꢀꢦꢦ.
ꢣꢤ
Reference 49.
2 2
Fig. 8 The partial enlarged drawing of O /N saturation cycles.
2
.5 Operational stability and photostability
1
1
2
2
3
0
5
0
5
0
Fig. 7 shows an operational stability test conducted by the
emission intensity of oxygen sensing films of Ir0~Ir3. It is clear
that the emission intensity responses are fully reversible when the
atmosphere varies periodically between 100% N and 100% O in
2
2
4
000 s. The complete reversibility of the luminescence emission
of the oxygen sensitive films allows stable and continuous
monitoring the concentration of oxygen. All of the sensing films
demonstrate quick response times (< 4.5 s when changing from 0
2
to 100 vol% O ) and recovery times (< 6 s when changing from
Fig.
9
2
Photo-degradation histograms for Ir0~Ir3 and
1
00 to 0 vol% O ), respectively (Fig. 8). Thus it can be seen that
2
50 Ir(ppy) (acac) immobilized in EC under continuous illumination
for 60 min at ambient atmospheric conditions.
the oxygen sensing films of these cationic Ir(III) complexes show
complete reversibility with short response time.
In addition, the photostability of sensing films were studied upon
continuous irradiation by a WFH−204B 254 nm UV lamp. The
emission intensity of the Ir(III) complexes immobilized in EC
reduced in different degree (Fig. 9). In fact, 10.0%, 21.7%, 10.1%
and 15.7% decreases in intensity of Ir0~Ir3 are observed
5
5
2
respectively, compared to 64.3% for Ir(ppy) (acac) after 60 min
of irradiation. It is clear that these cationic Ir(III) complexes are
more stable than the neutral Ir(III) complex Ir(ppy) (acac) upon
2
continuous illumination. The results in Table 3 show that
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5

Dalton Transactions
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Table 4. Parameters for the O
2
–sensing film of the Ir(III) complexes with EC as the supporting matrix (fitting of the result to the two-site
model).
View Article Online
a
a
b
b
2 c
ꢪꢫꢫd
ꢨꢩ
e
Complexes (0.5 wt%)
ƒ
1
ƒ
2
K
SV1
K
SV2
r
ꢧ
DOI: 10.1039/ Cꢬ 5DT02804C
ꢭꢮ
Ir0
Ir1
Ir2
0.92270
0.96251
0.95945
0.96674
0.07730
0.03749
0.04055
0.03326
0.00452
0.00895
0.00983
0.01088
0.0001
0.0001
0.0001
0.99827
0.99790
0.99651
0.99778
0.00417
0.00861
0.00943
0.01052
239.81
116.14
106.04
95.06
Ir3
0.0000
a
b
c
d
Ratio of the two portions of the Ir(III) complexes. Quenching constant of the two portions. Determination coefficients. Weighted quenching constant,
ꢢꢛꢛ
e
ꢢꢛꢛ
ꢟ
ꢐ ꢯ ꢟ ꢞ ꢯ ꢟꢠꢡꢓ. The oxygen partial pressure at which the initial emission intensity of the film is quenched by 50% and calculated as 1/ꢟꢠꢡ , in Torr.
ꢠꢡ
ꢘ ꢠꢡꢘ ꢓ
Importantly, all the ester-substituted complexes are more 40 recorded on a Varian Unity Inova spectrophotometer. Mass
sensitive to oxygen than Ir0 both in solutions and doped in EC
films. The corresponding orders of oxygen sensitivity are Ir3 >
Ir1 > Ir2 > Ir0 and Ir3 > Ir2 > Ir1 > Ir0 (Fig. 10 (a) and (b)),
respectively. To sum up, Ir1~Ir3 are promising candidates for
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
HITACHI F-7000 fluorescence spectrophotometer (λex = 400 nm).
5
oxygen sensing due to several reasons: (i) these complexes with a 45 Luminescent lifetimes were measured on an Edinburgh FLS920
−
COOEt substituent exhibit good solubility in organic solvents;
Spectrometer. Cyclic voltammograms of the Ir(III) complexes
were recorded on an electrochemical workstation (BAS100B/W,
USA). Crystallographic data were collected using a Bruker
SMART CCD diffractometer and graphite−monochromated
50 Mo−Ka radiation (λ = 0.71073 Å). CCDC reference number for
Ir2 is 1045430. The starting geometry was from the single-crystal
structure of complex Ir2. DFT calculations (full geometry
optimization) were carried out by B3LYP. The LanL2DZ basis
set was used to treat the iridium atom, whereas the 6–31G* basis
1
0
(ii) the relatively larger ester-substituted complexes Ir1~Ir3 are
less sensitive to self-quenching, triplet annihilation effects or
radiationless decay than the smaller Ir0; (iii) the wide range of
4
6,47
the luminescent lifetime will favour the quenching process.
5
5
set was used to treat all other atoms. All these calculations were
performed with the Gaussian 09 software package.
4
8
4.2 Synthesis
Synthesis of C^N ligands. The different C^N ligands containing
an ester substituent were synthesized by the method developed
previously for analogous compounds (See ESI).
1
5
Fig. 10 (a) Stern–Volmer plots (intensity ratios I
partial pressure) of Ir0~Ir3 in CH CN solutions (1.0 × 10 M);
b) Stern–Volmer plots of Ir0~Ir3 (0.5 wt%) immobilized into
EC films (λex = 400 nm), at ambient temperature.
0 2
/I versus O
-5
3
6
0
(
3 2
Synthesis of the Ir(III) complexes Ir1~Ir3. IrCl ·3H O was
reacted with 2.5 equiv. cyclometalating ligand in a mixture of 2-
ethoxyethanol and water (15 mL/5 mL) at 120 °C for 24 h to
afford a cyclometalated iridium bridged-chloride dimer. Without
3
Conclusions
2
2
3
0
5
0
In conclusion, three new ester-substituted bis-cyclometalated 65 further purification the dimer was mixed with 3.0 equiv.
cationic Ir(III) complexes (Ir1~Ir3) were synthesized and their
photoelectric properties were studied in detail. Ir1~Ir3 have
larger molar absorption coefficient in the visible region and
higher oxidation potentials than Ir0. The prolonged luminescent
phenanthroline in a mixture of dichloromethane (10 mL) and
methanol (5 mL). The reaction mixture was stirred under
refluxing for 12 h. After cooling down to room temperature, 10
equiv. KPF was added and the mixture was stirred for 1 h. The
6
lifetimes of Ir1~Ir3 compared with Ir0 provide larger dynamic 70 solvent was evaporated under reduced pressure and the residue
range of response to molecular oxygen. Moreover, these cationic
Ir(III) complexes exhibit good operational stability in continuous
monitoring oxygen and excellent photostability against
continuous irradiation. This study demonstrates that the
was re-dissolved in dichloromethane. The organic layer was
washed with water and dried over Na SO . Further purification
2
4
for each complex was achieved using column chromatography.
1
Ir1. Yield 68.2%, a yellow solid. H NMR (400 MHz, DMSO-d
):
6
introduction of an ester group on the cyclometalating ligand can 75 δ 8.92 (d, J = 8.4 Hz, 2H), 8.41 (d, J = 7.0 Hz, 4H), 8.24 – 8.19
tune the luminescent lifetime and oxygen sensing property of the
corresponding bis-cyclometalated cationic Ir(III) complex. This
concept might bring a useful perspective in the development of
new phosphorescent materials with high performances.
(m, 2H), 8.10 (d, J = 8.3 Hz, 2H), 8.06 – 7.95 (m, 4H), 7.67 –
7.52 (m, 4H), 7.13 (t, J = 6.6 Hz, 2H), 6.87 (d, J = 1.6 Hz, 2H),
1
3
4.14 (q, J = 7.0 Hz, 4H), 1.17 (t, J = 7.1 Hz, 6H). C NMR (100
MHz, DMSO-d ): δ 166.5, 166.4, 150.8, 149.5, 149.2, 148.2,
148.2, 146.7, 139.3, 138.5, 132.3, 131.8, 131.6, 128.9, 126.6,
24.7, 124.4, 120.8, 60.9, 14.2. HRMS (m/z; EI): calcd. for
6
8
0
1
3
5
4 Experimental
+
40 32 4 4 6
C H N O Ir [M – PF ] 825.2053; found 825.2122.
Ir2. Yield 58.4%, a yellow-green solid. H NMR (400 MHz,
1
4
.1 Materials and instruments
DMSO-d ): δ 8.91 (dd, J = 8.3 Hz, 1.3 Hz, 2H), 8.52 (dd, J = 9.2
6
All starting materials were purchased from commercial suppliers
8
5
Hz, 5.4 Hz, 2H), 8.39 (s, 2H), 8.17 (dd, J = 5.1 Hz, 1.2 Hz, 2H),
and used without further purification. The solvents were treated
8
2
.11 (dd, J = 11.3 Hz, 5.5 Hz, 4H), 8.01 (dd, J = 8.3 Hz, 5.1 Hz,
H), 7.68 – 7.58 (m, 4H), 6.85 (d, J = 1.6 Hz, 2H), 4.16 (q, J =
1
13
as required prior to use. H NMR and C NMR spectra were
6
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Journal Name, [year], [vol], 00–00This journal is © The Royal Society of Chemistry [year]

Page 7 of 8
Dalton Transactions
1
3
7
.1 Hz, 4H), 1.19 (t, J = 7.1 Hz, 6H). C NMR (100 MHz,
): δ 165.4, 162.8, 159.3, 157.3, 151.2, 147.8, 146.3,
39.1, 138.6, 138.3, 131.4, 130.3, 128.3, 127.2, 127.0, 125.1,
23.7, 122.7, 60.4, 14.2. HRMS (m/z; EI): calcd. for
9 R. C. Evans, P. Douglas, J. G. Williams and D. L. Rochester, J.
DMSO-d
1
1
6
Fluoresc., 2006, 16, 201-206.
View Article Online
10 K. J. Morris, M. S. Roach, W. Xu, J. Demas and B. DeGraff, Anal.
DOI: 10.1039/C5DT02804C
Chem., 2007, 79, 9310-9314.
+
5
0
5
C H F N O Ir [M – PF ] 861.1864; found 861.1914.
40 30 2 4 4 6
Ir3. Yield 61.3%, a yellow solid. H NMR (400 MHz, DMSO-d ):
6
60 11 M. I. Stich, S. Nagl, O. S. Wolfbeis, U. Henne and M. Schaeferling,
Adv. Funct. Mater., 2008, 18, 1399-1406.
1
δ 8.91 (dd, J = 8.3 Hz, 1.2 Hz, 2H), 8.40 (s, 2H), 8.31 (d, J = 8.5
Hz, 2H), 8.20 (dd, J = 5.1 Hz, 1.3 Hz, 2H), 8.04 (dd, J = 8.3 Hz,
1
2 J. F. Fernández-Sánchez, R. Cannas, S. Spichiger, R. Steiger and U. E.
Spichiger-Keller, Anal. Chim. Acta, 2006, 566, 271-282.
3 N. Tian, D. Lenkeit, S. Pelz, L. H. Fischer, D. Escudero, R. Schiewek,
4
.4 Hz, 4H), 7.86 (d, J = 9.5 Hz, 2H), 7.61 (dd, J = 8.1 Hz, 1.6 Hz,
1
1
2H), 7.30 (s, 2H), 6.85 (d, J = 1.6 Hz, 2H), 4.13 (q, J = 7.1 Hz,
6
5
D. Klink, O. J. Schmitz, L. González and M. Schäferling, Eur. J. Inorg.
Chem., 2010, 2010, 4875-4885.
1
3
4
H), 2.01 (s, 6H), 1.18 (t, J = 7.1 Hz, 6H). C NMR (100 MHz,
DMSO-d ): δ 166.0, 163.5, 151.4, 149.6, 149.3, 148.8, 146.6,
6
1
1
4 I. R. Laskar and T.-M. Chen, Chem. Mater., 2004, 16, 111-117.
1
1
40.3, 139.4, 135.6, 131.9, 131.6, 130.4, 128.8, 127.7, 125.0,
5 T.-H. Kwon, H. S. Cho, M. K. Kim, J.-W. Kim, J.-J. Kim, K. H. Lee,
24.0, 121.4, 60.9, 18.0, 14.4. HRMS (m/z; EI): calcd. for
+
S. J. Park, I.-S. Shin, H. Kim and D. M. Shin, Organometallics, 2005, 24,
1
C H N O Ir [M – PF ] 853.2366; found 853.2413.
4
2
36
4
4
6
7
0
1578-1585.
4
.3 Oxygen sensing film preparation and photostability test
1
6 M. Tavasli, T. N. Moore, Y. Zheng, M. R. Bryce, M. A. Fox, G. C.
Griffiths, V. Jankus, H. A. Al-Attar and A. P. Monkman, J. Mater. Chem.,
012, 22, 6419-6428.
17 E. Baranoff, S. Suarez, P. Bugnon, H. J. Bolink, C. Klein, R. Scopelliti,
solution. The mixture was treated via ultrasonic for 10 min at 75 L. Zuppiroli, M. Gratzel and M. K. Nazeeruddin, ChemSusChem, 2009, 2,
4
9
Oxygen sensing films were prepared according to the literature:
EC (9.95 mg) was dissolved in CH Cl (0.90 mL) and then 0.10
mL Ir(III) complex (0.50 mg/mL) in CH Cl was added to the
2 2
2
2
2
2
0
room temperature. Then, 0.10 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. The samples with
different loading levels and different matrixes were prepared with
procedure similar to that described above. The photostability of
these complexes in EC films was investigated by illuminating the
films with a WFH–204B 254 nm UV lamp at a distance of 3 cm
305-308.
18 B. Tong, Q. Mei, D. Chen and M. Lu, Synth. Met., 2012, 162, 1701-
1706.
19 F. Kessler, R. D. Costa, D. Di Censo, R. Scopelliti, E. Orti, H. J.
Bolink, S. Meier, W. Sarfert, M. Gratzel, M. K. Nazeeruddin and E.
Baranoff, Dalton Trans., 2012, 41, 180-191.
2
5
8
8
9
9
0
5
0
5
2
0 R. A. Smith, E. C. Stokes, E. E. Langdon-Jones, J. A. Platts, B. M.
Kariuki, A. J. Hallett and S. J. Pope, Dalton Trans., 2013, 42, 10347-
0357.
2
in the air. The power density on the film is 23.4 W/m .
1
Acknowledgements
21 T. S.-M. Tang, K.-K. Leung, M.-W. Louie, H.-W. Liu, S. H. Cheng
and K. K.-W. Lo, Dalton Trans., 2015, 44, 4945-4956.
3
0
We thank the financial support from the National Natural Science
Foundation of China (21276043, 21421005) and the Research
Foundation of Dalian University of Technology for Retired
Professors (Z. Jin: DUTTX2015).
2
2 M. Marín-Suárez, B. F. Curchod, I. Tavernelli, U. Rothlisberger, R.
Scopelliti, I. Jung, D. Di Censo, M. Grätzel, J. F. Fernández-Sánchez and
A. Fernández-Gutiérrez, Chem. Mater., 2012, 24, 2330-2338.
23 Q. Zhao, S. Liu, M. Shi, F. Li, H. Jing, T. Yi and C. Huang,
Organometallics, 2007, 26, 5922-5930.
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