Visible light-harvesting cyclometalated Ir(III) complexes with pyreno[4,5-d]imidazole C^N ligands as triplet photosensitizers for triplet-triplet annihilation upconversion

Article abstract of DOI:10.1016/j.dyepig.2012.07.020

Cyclometalated Ir(III) complexes with pyrenyl-fused imidazole ligands were prepared. The complexes show strong absorption of visible light and long-lived triplet excited state and were used as triplet photosensitizers for triplet-triplet annihilation (TTA) upconversion. Pyreno[4,5-d]imidazole C^N ligand was used to access the long-lived T₁ excited state (Ir-1, bpy = 2,2′-bipyridine as the N^N ligand. τ_T = 56.1 μs). In order to enhance the absorption in visible range, a coumarin derived N^N ligand was used (Ir-2, ε = 51,500 M^-1 cm^-1 at 466 nm, τ_T = 73.9 μs). The complexes show room temperature phosphorescence in the red. The T₁ excited states of Ir-1 and Ir-2 were identified as mainly intraligand (³IL) states, vs. the metal-to-ligand-charge-transfer (³MLCT) state for the model complex (Ir-0), proved by steady state emission, transient absorption, 77 K emission spectra and DFT calculations. The complexes were used as triplet photosensitizers for TTA upconversion and upconversion quantum yield up to 23.7% was observed.

Full text of DOI:10.1016/j.dyepig.2012.07.020

Dyes and Pigments 96 (2013) 104e115
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Visible light-harvesting cyclometalated Ir(III) complexes with pyreno[4,5-d]
imidazole C^N ligands as triplet photosensitizers for tripletetriplet annihilation
upconversion
*
Xiuyu Yi, Pei Yang, Dandan Huang, Jianzhang Zhao
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cyclometalated Ir(III) complexes with pyrenyl-fused imidazole ligands were prepared. The complexes
show strong absorption of visible light and long-lived triplet excited state and were used as triplet
photosensitizers for tripletetriplet annihilation (TTA) upconversion. Pyreno[4,5-d]imidazole C^N ligand
was used to access the long-lived T1 excited state (Ir-1, bpy ¼ 2,20-bipyridine as the N^N ligand.
Received 27 June 2012
Received in revised form
25 July 2012
Accepted 27 July 2012
Available online 4 August 2012
sT ¼ 56.1
ms). In order to enhance the absorption in visible range, a coumarin derived N^N ligand was
used (Ir-2,
3
¼ 51,500 Mꢀ1 cmꢀ1 at 466 nm , sT ¼ 73.9
ms). The complexes show room temperature
phosphorescence in the red. The T1 excited states of Ir-1 and Ir-2 were identified as mainly intraligand
(3IL) states, vs. the metal-to-ligand-charge-transfer (3MLCT) state for the model complex (Ir-0), proved by
steady state emission, transient absorption, 77 K emission spectra and DFT calculations. The complexes
were used as triplet photosensitizers for TTA upconversion and upconversion quantum yield up to 23.7%
was observed.
Keywords:
Iridium
Phosphorescence
Photochemistry
Pyreno[4,5-d]imidazole
Tripletetriplet annihilation
Upconversion
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
cyclometalated Ir(III) complexes to be used for luminescent bio-
imaging and TTA upconversion [17,18,20,29e32].
Cyclometalated Ir(III) complexes have attracted much attention
due to their applications in electroluminescence [1e13]. Concern-
ing this aspect, the most desired photophysical property of the
cyclometalated Ir(III) complexes is the efficient intersystem
crossing effect, by which the triplet excited states of the complexes
can be populated upon excitation. Second, short lifetime of the T1
excited state avoids the saturation effect of the electrolumines-
cence [5,6]. The conventional cyclometalated Ir(III) complexes
show strong phosphorescence and relatively short T1 state lifetime
[6,10,12e14].
In recent years, Ir(III) complexes have been used for photo-
induced charge separation [15,16], luminescent molecular probes
[17e21], photocatalysis [22e24], photooxidation [25], and more
recently the tripletetriplet annihilation (TTA) based upconversions
[26e29]. Concerning these new applications, the demanding for
the photophysical properties are different from those for the
conventional applications. For example, strong absorption of visible
light and long-lived triplet excited states are desired for the
In order to address these challenges, herein we designed new
cyclometalated Ir(III) complexes that show strong visible light-
harvesting and long-lived triplet excited states (Ir-1 and Ir-2,
Scheme 1). Our strategy is to employ the metalation of organic
chromophores to access the strong absorption of visible light and
long-lived triplet excited state [30e32]. Pyrenyl moiety was used
for preparation of the cyclometalation ligands of Pt(II) complexes
[18]. The long-lived T1 excited states with C^N ligands in Ir(III)
complexes are rarely reported. Previously pyrene was used for
preparation of Ru(II) complexes that show long-lived triplet excited
states [33e35]. However, pyrene was rarely used for preparation of
Ir(III) complexes [8]. Only coumarin was used as C^N cyclo-
metalation ligands of the cyclometalated Ir(III) complexes and T1
lifetime up to 11.3 ms was observed [36].
In order to prepare Ir(III) complexes that shows long-lived T1
excited states and at the same time, strong absorption of visible
light, coumarin was used to prepare the ancillary N^N ligand (Ir-2)
[36,37]. Ir-1 and Ir-2 show the ligand localized 3IL excited states vs.
the 3MLCT state for the model complexes, supported by the time-
resolved transient difference absorption spectra and the spin
density analysis. The absorption of the complex in visible range was
substantially enhanced, compared to the model complexes.
* Corresponding author. Tel./fax: þ86 411 3960 8007.
E-mail address: zhaojzh@dlut.edu.cn (J. Zhao).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.07.020

X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
105
Scheme 1. Synthesis of Ir-1 and Ir-2. (a) NaIO4, RuCl3$3H2O; (b) CH3COOH, Ar, reflux, 12 h; (c) IrCl3$3H2O, 2-ethoxyethanol/water ¼ 3:1 (v/v), 110 ꢁC, 24 h; (d) 2,20-bipyridine,
CH2Cl2/MeOH ¼ 2:1 (v/v), reflux, 5e6 h; (e) L2, CH2Cl2/MeOH ¼ 2:1 (v/v), reflux, 5e6 h; (f) POCl3, DMF, 0e5 ꢁC; (g) 1,10-phenanthroline-5,6-dione, CH3COONH4, CH3COOH, Ar,
reflux, 6 h; (h) IrCl3$3H2O, 2-ethoxyethanol/water ¼ 3:1 (v/v), 110 ꢁC, 24 h; (i) 2,20-bipyridine, CH2Cl2/MeOH ¼ 2:1 (v/v), reflux, 5e6 h.
Furthermore, the lifetimes of the triplet excited states of the
complexes was greatly prolonged (73.9 s) compared to the model
complex (0.3 s) [6,37].
2. Experimental section
m
m
2.1. General information
The new Ir(III) complexes were used as triplet photosensitizers
for triplet-triplet-energy-transfer (TTET) processes, herein demon-
strated by the tripletetriplet annihilation upconversion. Upcon-
version quantum yield up to 23.7% was observed, vs. a previously
reported Ir(III) complex that has to be excited in UV range [27a]. Our
study may pave the way for preparation of visible light-harvesting
Ir(III) complexes and for their application in areas such as photo-
oxidation, photodynamic therapy (PDT) and TTA upconversion, etc.
NMR spectra were taken on a 400 MHz Varian Unity Inova
spectrophotometer. Mass spectra were recorded with a Q-TOF
Micro MS spectrometer. UVeVis spectra were taken on a HP8453
UVevisible spectrophotometer. Fluorescence spectra were recor-
ded on a Shimadzu RF5301 spectrofluorometer and a Sanco 970
CRT spectrofluorometer. Luminescence quantum yields were
measured with quinine sulfate as standard (
F
¼ 54.6% in 0.05 M

106
X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
H2SO4). Luminescence lifetimes were measured on OB 920 lumi-
nescence lifetime meter (Edinburgh, U.K.).
8H); 7.31e7.40 (m, 4H). EI-HRMS: calcd (C29H18N2): m/z ¼ 394.1470,
found, m/z ¼ 394.1459.
2.4.2. 2-[7-(Diethylamino)-2-oxo-2H-chromen-3-yl]-1H-imidazo
[4,5-f] [1,10]phenanthroline (L2)
2.2. Tripletetriplet annihilation upconversion
7-diethylamino-2-oxo-2H-chromene-3-carboxyldehyde
Diode pumped solid state laser was used for the TTA upcon-
version. The samples were purged with N2 or Ar for 15 min before
measurement. The upconversion quantum yields were determined
(128 mg, 0.68 mmol), 1,10-phenanthroline-5,6-dione (104 mg,
0.5 mmol), and ammonium acetate (805 mg, 11.3 mmol) were
dissolved in glacial acetic acid (13 mL). The mixture was refluxed
for 6 h under Ar atmosphere. The color of the mixture turned to
orange. After completion of the reaction, the mixture was cooled to
room temperature, and concentrated NH3$H2O was added until the
pH of the mixture was brought to about 7.0. Yellow precipitate
appeared. The precipitate was filtered and washed with water, then
dried under vacuum overnight. The crude product was further
purified by column chromatography (silica gel, CH2Cl2/
methanol ¼ 10:1, v/v). Yield: 166.9 mg, 68.5%. 1H NMR (400 MHz,
with coumarin-6 as the quantum yield standard (
F
¼ 78.0% in
EtOH) and the quantum yields were calculated with modified
literature method (Eq. (1)), where Funk, Aunk, Iunk and hunk repre-
sents the quantum yield, absorbance, integrated photo-
luminescence intensity of the samples and the refractive index of
the solutions [26]. Note the absorption correction factor 1e10ꢀA
were used instead of the absorbance A of the samples for the
calculation of the upconversion quantum yields, due to the strong
absorption of the solutions used for the determination of the
upconversion quantum yields, which is contrary to the ordinary
practicing of using dilute solutions for measuring of the lumines-
cence quantum yields. The absorbance of the quantum yield stan-
dard (Coumarin 6) is A ¼ 0.054 (in EtOH), A(Ru-1) ¼ 0.13, A(Ir-
2) ¼ 0.56 (in CH2Cl2), respectively.
CDCl3):
d
¼ 9.16 (d, J ¼ 4.2 Hz, 2 H), 8.99 (s, 1 H), 8.78 (d, J ¼ 5.8 Hz,
2H), 7.72e7.69 (m, 2 H), 7.48 (d, J ¼ 8.0 Hz, 1 H), 6.66e6.63 (m, 1H),
6.50 (d, J ¼ 2.3 Hz, 1 H), 3.48e3.43 (m, 4 H), 1.26 (t, J ¼ 7.0 Hz, 6 H).
13C NMR (100 MHz, CDCl3):
d
¼ 162.1, 156.4, 151.7, 148.3, 146.9,
144.4, 141.5, 130.1, 129.4, 122.9, 110.0, 108.7, 107.7, 96.8, 45.1,
12.5 ppm. ESI-HRMS: calcd ([C26H22N5O2 þ H]þ): m/z ¼ 436.1774,
found, m/z ¼ 436.1767.
!
ꢀ
ꢁꢀ
ꢁ
2
ꢀA
1 ꢀ 10
Iunk
Istd
hunk
hstd
std
Funk ¼ 2Fstd
(1)
ꢀA
1 ꢀ 10
unk
2.4.3. General method for synthesis of the Ir(III) complexes
All procedures were carried out under Argon atmosphere.
Cyclometalated Ir(III) chloro-bridged dimers were synthesized by
the literature method [41]. IrCl3$3H2O and 2.5 equiv. of ligand were
heated in a 3:1 (v/v) mixture of 2-ethoxyethanol and water. This
slurry was heated at 110 ꢁC for 24 h. After cooling to room
temperature, the crude product was filtered and washed with
water, n-hexane and ether. Then the solid was dried under vacuum.
Column chromatography was applied to purify the complexes.
The photography of the upconversion were taken with Samsung
NV 5 digital camera.
The delayed fluorescence lifetime (sDF) of the upconversion was
measured with nanosecond pulsed laser (OpolettÔ 355 II nano-
second pulsed laser, pulse length: 7 ns. Pulse repetition: 20 Hz. Peak
OPO energy: 4 mJ. Wavelength is tuneable from 210 to 335 nm and
410e2200 nm. OPOTEK, USA), which is synchronized to FLS 920
spectrofluorometer (Edinburgh, U.K.). The pulsed laser is sufficient
to sensitize the TTA upconversion. The decay kinetics of the
upconverted fluorescence (delayed fluorescence) was monitored
with FLS 920 spectrofluorometer (synchronized to the OPO nano-
second pulsed laser). The prompt fluorescence lifetimes were
measured with EPL picosecond pulsed laser (405 nm) which is
synchronized to the FLS 920 spectrofluorometer.
2.4.4. Ir-0 ([Ir(ppy)2bpy][PF6])
A solution of [Ir(ppy)2Cl]2 (54 mg, 0.05 mmol) and 2,20-bipyr-
idine (20 mg, 0.125 mmol) in CH2Cl2/MeOH (12 mL, 2:1 v/v) was
refluxed under Ar atmosphere. After 5 h, the orange solution was
cooled to room temperature, and then a 10-fold excess of ammo-
nium hexafluorophosphate (NH4[PF6]) was added. The suspension
was stirred for 15 min and then was filtered to remove the insoluble
inorganic salts. The solution was evaporated to dryness under
reduced pressure to give an orange solid. The solid was dissolved in
CH2Cl2 and filtered to remove the residual inorganic salts, then
purified with column chromatography (silica gel, CH2Cl2/
MeOH ¼ 25:1) to give a light yellow solid. Yield: 80 mg, (60.9%).
2.3. DFT calculations
The density functional theory (DFT) calculations were used for
optimization of the singlet states and triplet states. The spin density
surfaces of the complexes were calculated based on the optimized
triplet state geometries. All the calculations were performed with
Gaussian 09W [38].
M.p. > 250 ꢁC. 1H NMR (400 MHz, CDCl3, CH3OH),
d
¼ 8.75 (m, 5H);
8.49 (d, 4H, J ¼ 7.8 Hz); 7.87e7.94 (m, 6H); 7.76 (t, 1H, J ¼ 7.8 Hz);
7.77e7.79 (m, 4H); 7.52 (d, 1H, J ¼ 7.8 H); 7.36e7.42 (m, 5H);
7.00e7.07 (m, 2H). ESI-HRMS: [(M-PF6)]þ, calc (C32H24N4Ir): m/
z ¼ 657.1630, found, m/z ¼ 657.1625.
2.4. Synthesis
2.4.1. 9,10-Diphenyl-9H-pyreno[4,5-d]imidazole (L1)
Benzaldehyde (69.0 mg, 0.65 mmol), aniline (74 mg, 0.67 mmol),
pyrene-4,5-dione [39] (150 mg, 0.65 mmol), and ammonium
acetate (498 mg, 6.46 mmol) were mixed in glacial acetic acid
(10 mL). The mixture was refluxed for 12 h under Ar atmosphere.
Water was added to the mixture. The pH was brought to 7 with
ammonium hydroxide. The solid was collected by filtration and
washed with water. The solid was dissolved in dichloromethane,
then concentrated under reduced pressure to give a light brown
solid. The crude product was purified by column chromatography
(silica gel, CH2Cl2/n-hexane ¼ 50:1, v/v) to give a pale yellow solid
[40]. Yield: 123.6 mg, (48.5%). M.p. > 250 ꢁC. 1H NMR (400 MHz,
2.4.5. Ir-1 ([Ir(9,10-diphenyl-9H-pyreno[4,5-d]imidazole)2(bpy)]
[PF6])
A solution of cyclometalated Ir(III) chloro-bridged dimers (Ir-
Cl2-1) (60.0 mg, 0.03 mmol) and 2,20-bipyridine (11.5 mg,
0.074 mmol) in CH2Cl2/MeOH (12 mL, 2:1 v/v) was refluxed under
Ar atmosphere. After 6 h, the brown solution was cooled to room
temperature, then a 10 eq. of NH4[PF6] was added. The suspension
was stirred for 15 min and then filtered to remove insoluble inor-
ganic salts. The solution was evaporated to dryness under reduced
pressure to obtain a crude orange solid. The solid was dissolved in
CH2Cl2 and filtered to remove the residual inorganic salts, then
purified by column chromatography (silica gel, CH2Cl2) to give
CDCl3),
d
¼ 9.13 (d, 1H, J ¼ 7.3 Hz); 8.01e8.20 (m, 5H); 7.57e7.64 (m,

X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
107
a yellow solid [10a]. Yield: 48.1 mg, (70.5%). M.p. > 250 ꢁC. 1H NMR
(400 MHz, CDCl3/CH3OH),
1.0
0.8
0.6
0.4
0.2
0.0
Ir-0
L-1
Ir-1
L-2
Ir-2
d
¼ 8.75 (d, 2H, J ¼ 4 Hz); 8.34 (d, 2H,
J ¼ 7.6 Hz); 8.06e8.10 (m, 6H); 7.93e8.00 (m, 8H); 7.86 (d, 4H,
J ¼ 7.3 Hz); 7.77e7.79 (m, 4H); 7.61e7.70 (m, 4H); 7.43 (d, 2H,
J ¼ 7 Hz); 7.33 (d, 2H, J ¼ 8.1 Hz); 7.13 (t, 2H, J ¼ 7.8 Hz); 6.87 (t, 4H,
J ¼ 7.9 Hz); 6.72 (d, 2H, J ¼ 6.4 Hz), 7.64e7.68 (t, 1H, J ¼ 6.0 Hz),
7.42e7.46 (q, 2H, J ¼ 4.8 Hz). 13C NMR (100 MHz, DMSO-d6):
d 166.9,
160.1, 153.9, 153.6, 144.8, 142.1, 141.7, 140.2, 138.4, 137.4, 137.2, 137.0,
136.9, 136.1, 134.6, 134.5, 133.7, 133.2, 131.6, 130.8, 130.3, 129.3,
128.8, 127.7, 127.5, 127.0, 126.3, 126.0, 125.1, 122.8. ESI-HRMS: [(M-
PF6)]þ, calc (C68H42N6Ir): m/z ¼ 1135.3100, found, m/z ¼ 1135.3099.
2.4.6. Ir-2 [Ir(9,10-diphenyl-9H-pyreno[4,5-d]imidazole)2(2-[7-
(diethylamino)-2-oxo-2H-chromen-3-yl]-1H-imidazo[4,5-f] [1,10]
phenanthroline)][PF6]
The synthetic procedure is the same as that for Ir-1, except that
L2 (54 mg, 0.12 mmol) was used. The product was an orange solid,
which was purified with column chromatography (silica gel,
CH2Cl2) to give a red solid. Yield: 69 mg, (40.6%). M.p. > 250 ꢁC. 1H
300 350 400 450 500
Wavelength/nm
Fig. 1. UVeVis absorption of Ir(III) complexes and the ligands. c ¼ 1.0 ꢂ 10ꢀ5 M in
toluene, 25 ꢁC.
atmosphere were studied. The emission was not completely
quenched in air or O2 saturated solution, indicating that the lifetime
of the T1 state is not very long [33,43,44]. For the new Ir(III)
complexes Ir-1 and Ir-2, however, the emission was completely
quenched in air or O2 saturated solutions. Thus we propose that the
lifetimes of the emissive T1 excited states of Ir-1 and Ir-2 are much
longer than that of Ir-0 [33,43,44]. Furthermore, the vibrational
progression of the emission band of the new complexes Ir-1 and Ir-
2 are more significant than that of Ir-0. Based on these emission
profiles, we postulate that the emission of Ir-0 is due to an emissive
excited state with significant 3MLCT feature [42]. For Ir-1 and Ir-2,
however, the 3IL feature of the emissive excited state is more
NMR (400 MHz, CDCl3),
d
¼ 9.00 (d, 2H, J ¼ 4 Hz); 8.83 (s, 2H); 8.63
(s, 1H); 8.33 (d, 2H, J ¼ 8 Hz); 7.95e8.16 (m, 12H); 7.90 (d, 2H,
J ¼ 4 Hz); 7.75e7.84 (m, 4H); 7.70 (d, 2H, J ¼ 8 Hz); 7.60 (t, 2H,
J ¼ 8 Hz); 7.52 (d, 2H, J ¼ 8 Hz); 7.27e7.41 (m, 4H); 7.06 (t, 2H,
J ¼ 8 Hz); 6.86e6.95 (m, 4H); 6.77 (d, 2H, J ¼ 4 Hz); 6.66 (d, 2H,
J ¼ 8 Hz); 6.46 (s, 1H); 3.34e3.44 (m, 4H, J ¼ 4.8 Hz); 1.18e1.22 (m,
6H, J ¼ 4.8 Hz) ppm. 13C NMR (100 MHz, CDCl3):
d 161.9, 161.7, 156.9,
152.5,149.0,148.3,143.3,137.4,136.6,134.9,133.8,132.0,131.6,131.1,
129.7, 128.9, 128.4, 127.9, 127.8, 126.0, 125.5, 125.3, 123.7, 123.1,
122.9, 122.6, 121.4, 121.0, 120.7, 120.0, 117.9, 110.5, 108.6, 106.2, 96.7,
45.1, 29.7, 12.3 ppm. ESI-HRMS: [(M-PF6)]þ, calc (C84H55N9IrO2): m/
z ¼ 1414.4108, found, m/z ¼ 1414.4077.
significant. It should be pointed out that upon increasing the
p-
conjugation of the ligand, there is chance for increasing the radia-
tive lifetime of the triplet manifold, as well as shifting of emission
from phosphorescence to fluorescence [8b]. The radiative lifetimes
3. Results and discussion
3.1. Design and synthesis of complexes
(sR) of the complexes were also calculated. The sR values of the new
complexes are much longer than the model complex Ir-0 (Table 1).
The design rationale for the Ir(III) complexes that show long-
lived T1 excited state is to introduce a ligand with appropriate T1
excited state energy level which is lower than the 3MLCT state of
the coordination center, herein a pyrenyl unit was used (Scheme 1).
Firstly pyrene was oxidized to 4,5-dione. Then condensation with
benzenaldehyde and aniline give the intermediate L1. Complex Ir-1
was prepared via the Ir(III) dimer intermediate Ir-Cl2-1. Ir-1 shows
absorption in UV range, thus Ir-2 was designed and prepared, for
which the coumarin-containing N^N ligand was used. Thus Ir-2 will
show strong absorption in the visible range due to the light-
harvesting coumarin imidazole ligand.
3.3. Nanosecond time-resolved transient difference absorption
spectroscopy
In order to study the T1 excited state of the complexes, the
nanosecond time-resolved transient difference absorption spec-
troscopy of the complexes were studied (Fig. 3). Upon pulsed laser
excitation, transient absorption in the range of 400 nme700 nm
was observed for Ir-1. Bleaching at 360 nm was observed. The
discrepancy between the bleaching and the steady state UVevis
absorption band at 403 nm may be due to the significant absorp-
tion at ca. 400 nm. For Ir-2, significant bleaching of the ground state
3.2. Steady-state UVevis absorption and emission spectra
The UVevis absorption of the complexes were studied (Fig. 1).
The model complex Ir-0 gives very weak absorption in visible
range. In comparison, Ir-1 gives stronger absorption at 403 nm
(19,100 Mꢀ1 cmꢀ1). The absorption of Ir-1 in the visible range is
enhanced compared to the Ir(III) complex with phenylimidazole
ligands [13,14a,14b]. With the coumarin ligand, Ir-2 gives intense
Table 1
Photophysical parameters of the complexes and the ligand.
Fd
sT
(
m
s)e
sR
(m
s)f
6.7
a
b
c
labs
3
lem
L1
382
405
403
466
0.72
0.26
1.91
5.15
385, 406, 429
87.3%
4.3%
7.6%
3.4%
29.5
0.3
56.1
73.9
e
Ir-0
Ir-1
Ir-2
600
569
607
absorption in visible range (
3
¼ 51,500 Mꢀ1 cmꢀ1 at 466 nm
1408.7
10,715.3
Table 1). To the best of our knowledge, Ir(III) complexes that show
such strong absorption of visible light are rarely reported
[20,31,32,37,42]. The intense absorption of Ir-2 in the visible range
a
In toluene (1.0 ꢂ 10ꢀ5 M): In nm.
b
c
Molar extinction coefficient at the absorption maxima. In 104 Mꢀ1 cmꢀ1
.
/
1MLCT
In toluene: In nm.
In toluene, with [Ru(dmb)3][PF6]2 (Ru-1) (
is due to the absorption of ligand, and it is not the S0
absorption.
d
F
¼ 0.073 in deaerated acetonitrile)
and quinine sulfate as the quantum yield standards (
F
¼ 0.546 in 0.05 M H2SO4).
The luminescence of the complexes were studied (Fig. 2). All the
complexes show room temperature phosphorescence in fluid
solution. The emission of the complexes under different
e
Triplet state lifetime, measured by time-resolved transient absorption spec-
troscopy. 1.0 ꢂ 10ꢀ5 M in deaerated toluene.
f
Radiative lifetime. sR
¼ sobs/FF.

108
X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
In order to validate the assignment of the transient absorption,
a
75
60
45
30
15
0
the time-resolved absorption of the complexes were calculated
with TDDFT methods (Fig. 3def). Generally good agreement
between the experimental results and the calculated spectra were
found (Fig. 3). For example, transient absorption bands at 425 nm
and 773 nm were predicted for Ir-0 (Fig. 3d). Experimentally the
transient absorption bands at 379 nm and 762 nm were observed
(Fig. 3a). The discrepancies between the predicted spectra and the
experimental results are within expectation between the experi-
mental spectra were complicated by the bleaching effect, this may
make the apparent absorption maxima to be different from its real
value. Similar results were found for Ir-1 and Ir-2 (Fig. 3e and f). For
example, transient absorption at 396 nm, 501 nm and 725 nm were
predicted for Ir-2. These predicted transient absorption bands are
close to the experimental results (Fig. 3).
in N2
in air
in O2
500
600
700
Wavelength/nm
b
800
600
400
200
0
in N2
3.4. 77 K emission spectra
in O2
in air
In order to study the emissive excited state of the complexes, the
emission at 77 K were compared with that at room temperature
(RT, Fig. 4). For the model complex Ir-0, the emission at 77 K shows
significant blue shift compared to that at RT, i.e. the thermally
500
600
700
800
Wavelength/nm
induced Stokes shift (
D
ES) is up to 1899 cmꢀ1. The emission band is
Fig. 2. The emission spectra of the compounds under different atmosphere. (a) Ir-
0 (lex ¼ 403 nm), (b) Ir-1 (lex ¼ 403 nm). c ¼ 1.0 ꢂ 10ꢀ5 M in toluene, 25 ꢁC.
structureless. Thus the T1 excited state of Ir-0 can be characterized
as 3MLCT state [31,42]. For the complexes Ir-1 and Ir-2, however,
the emission bands at 77 K become more structured, and the
thermally induced Stokes shift is very small (112 cmꢀ1 and
837 cmꢀ1, respectively). Thus we propose that 3MLCT feature is
significant for the emissive T1 state of Ir-0. Conversely 3IL feature is
more significant for the emissive T1 state of Ir-1 and Ir-2.
absorption at 450 nm was observed, which coincides with the
steady state absorption of the coumarin ligand (Fig. 1).
Transient absorption in the range of 500 nme750 nm was found
for Ir-2. Based on these results, we propose that the T1 excited state
of Ir-2 is localized on the coumarin ligand, i.e. the T1 state is with
significant 3IL feature. The lifetimes of the triplet excited states of
3.5. Assignment of the triplet excited state by DFT calculations
Ir-1 and Ir-2 were determined as 56.1 ms and 73.9 ms, respectively.
These values are among the longest lifetimes for light-harvesting
Ir(III) complexes [31,32,42], and much longer than the triplet
state lifetime of normal cyclometalated Ir(III) complexes [6]. The
lifetime of the Ir-1 is much longer than Ir(III) complex with phe-
nylimidazole ligands [13,14a], thus the pyreno[4,5-d]imidazole
ligand is crucial for the long-lived triplet excited state. The photo-
physical properties of the complexes were summarized in Table 1.
DFT calculations have been used for study of the photophysics of
Ir(III) complexes [45,46]. Previously we used the DFT calculations
for rationalization of the fluorescence sensing mechanisms of the
molecular sensors and transition metal complexes, especially the
assignment of the triplet excited states by using the DFT/TDDFT
calculations on the electronic transition of the T1 excited state and
the spin density surface [42].
Fig. 3. Nanosecond time-resolved transient difference absorption spectra of the complexes after pulsed excitation (lex ¼ 355 nm). (a) Ir-0, (b) Ir-1. (c) Ir-2. c ¼ 1.0 ꢂ 10ꢀ5 M. In
deaerated toluene, 25 ꢁC. The calculated transient absorption spectra were presented (d) Ir-0, (e) Ir-1 and (f) Ir-2. Toluene was used as solvent in the calculations. Calculated by DFT
at the B3LYP/6-31G/LanL2DZ level using Gaussian 09W.

X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
109
Fig. 4. Photoluminescence spectra of the complexes at RT and 77 K. (a) Ir-0 (lex ¼ 404 nm), (b) Ir-1 (lex ¼ 404 nm), (c) Ir-2 (lex ¼ 460 nm). In C2H5OH/CH3OH (4:1, v/v).
The geometry of the ground state of Ir-1 was optimized and the
electronic transitions for the singlet manifold and the triplet
manifold were calculated (Table 2). The frontier molecular orbitals
were presented in Fig. 5. The predicted absorption bands are at
393 nm and 358 nm, which are in good agreement with the
experimental results (Fig. 1a). The main component of these tran-
sitions are HOMO ꢀ 1 / LUMO þ 2 and HOMO / LUMO þ 2. Thus
these transitions can be identified as MLCT and LLCT transitions.
The molecular geometry and the electronic transitions of Ir-2
were studied with DFT/TDDFT calculations. The significant differ-
ences between the predicted absorption of Ir-2 and that of Ir-0 are
the strong absorption of Ir-2 in the visible range (the predicted
absorption bands at 468 nm and 466 nm, Table 3). The molecular
orbitals involved in these transitions suggested the involvement of
the coumarin ligand (Fig. 6). This theoretical prediction is in
agreement with the experimental results (Fig. 1).
moiety contributes to the spin density. Thus, we predict that the
transient absorption of Ir-1 and Ir-2 will be different, although both
complexes show long-lived triplet excited states.
The different origin of the triplet excited state, demonstrated by
the spin density, makes the transient absorption spectra different.
For Ir-0, the energy level of the triplet states of either bpy or phen is
much higher than the Ir(III) coordination framework, thus the
lifetime of the triplet excited state will be different from that of Ir-1
and Ir-2.
The theoretical approach to study the photophysical properties
of the complexes will be useful for design of the Ir(III) complexes
that show the predetermined photophysical properties, such as the
long-lived triplet excited states.
3.6. Tripletetriplet annihilation upconversion
The triplet states of the complex Ir-2 were also calculated
(Table 3). The predicted phosphorescence emission is at 605 nm,
which is in good agreement with the experimental results of
607 nm (Table 1). The components of the transition indicated
significant coumarin-localized intraligand character (Fig. 6). This
conclusion coincide with the time-resolved transient absorption
spectra (Fig. 3).
The spin density surfaces of the triplet state of the complexes
were analyzed (Fig. 7). For Ir-2, the spin density is localized on the
coumarin ligand, which suggested 3IL feature. For Ir-1, the imiazole
We have shown that the long-lived T1 excited states of
complexes Ir-1 and Ir-2 were populated upon photoexcitation.
Thus these complexes can be used as triplet photosensitizers for
TTA upconversion [26e29,47,48]. TTA upconversion is a new
upconversion method, and shows advantages over the other
upconversion methods in that TTA upconversion requiring non-
coherent low excitation power (terrestrial solar light is sufficient),
high upconversion quantum yield, strong absorption of excitation
light and readily tuneable upconversion wavelength. Conventional
upconversion methods, such as those with two-photon absorption
dyes or rare earth nanocrystal materials failed show such advan-
tages [49]. TTA upconversion can be used to improve the efficiency
of solar cells, hydrogen (H2) production by photocatalysis of water
and recently oxygen (O2) sensing [50].
Currently one of the challenges facing the development of TTA
upconversion is the limited availability of the triplet photosensi-
tizers [28,29,51]. Usually the Pt(II) or Pd(II) porphyrin complexes
were used as the triplet photosensitizer for the TTA upconversion
[48,52e55]. Previously one Ir(ppy)3 was used as triplet photosen-
sitizer for TTA upconversion [27a]. However, that Ir(III) complex
contains no light-harvesting moiety, thus the absorption of the
complex in the visible range is very weak [27a].
Based on the photophysical principals of the TTA upconversion
[26e29], we propose that the transition metal complexes with
strong absorption of visible light and long-lived triplet excited state
will be ideal as triplet photosensitizers [29,30]. Thus the TTA
upconversion with these complexes as triplet photosensitizers
were studied (Fig. 8). Upon 473 nm laser excitation, the complexes
show different emission intensity, which is dependent not only on
the quantum yield, but also the molar extinction coefficients of the
complexes at the excitation wavelength (Table 1). A Ru(II) complex
triplet photosensitizer Ru(dmb)3[PF6]2 (Ru-1, dmb ¼ 4,40-dime-
thylbipyridine) is used for comparison because it shows the typical
absorption wavelength and the luminescence lifetime of the
conventional transition metal complexes [51].
Table 2
Electronic excitation energies (eV) and corresponding oscillator strengths (f), main
configurations and CI coefficients of the low-lying electronically excited states of
complex Ir-1, calculated by TDDFT//B3LYP/6-31G/LanL2DZ, based on the DFT//
B3LYP/6-31G/LanL2DZ optimized ground state geometries.a
Electronic TDDFT//B3LYP/6-31G
transition
Energy (eV)a
fb
Compositionc CId
Character
Singlet
S
S
S
S
S
0 / S1 2.06 eV 603 nm 0.0005 H / L
0 / S7 3.05 eV 406 nm 0.0240 H / L þ 1
0 / S9 3.16 eV 392 nm 0.1158 H / L þ 2
0.6853 LLCT
0.6871 LLCT
0.6561 MLCT
0 / S18 3.47 eV 358 nm 0.4736 H ꢀ 1 / L þ 2 0.5445 LLCT
0 / S19 3.54 eV 351 nm 0.1962 H ꢀ 1 / L þ 5 0.6318 ILCT
H ꢀ 1 / L þ 4 0.2316 LLCT
e
Triplet
T
1 / S0 1.78 eV 695 nm 0.0000 H / L
0.6800 LLCT
0.1802 MLCT
0.7003 LLCT
H ꢀ 2 / L
e
T
T
2 / S0 2.07 eV 599 nm 0.0000 H ꢀ 1 / L
e
3 / S0 2.23 eV 557 nm 0.0000 H ꢀ 1 / L þ 4 0.4234 LLCT
H / L þ 3
0.4482 LLCT
a
Only the selected low-lying excited states are presented.
Oscillator strength.
H stands for HOMO and L stands for LUMO. Only the main configurations are
b
c
presented.
d
The CI coefficients are in absolute values.
No spin-orbital coupling effect was considered, thus the f values are zero. The
e
single/triplet state energy gaps were calculated based on the optimized ground state
geometry.

110
X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
Fig. 5. Frontier molecular orbitals of Ir-1. Calculated by DFT at the B3LYP/6-31G/LanL2DZ level using Gaussian 09W.
Next, the emission of complex solutions was studied in the
For other complexes, however, no upconversion was observed
under the same experimental conditions (Fig. 8b). The lack of the
TTA upconversion for the other complexes is due to either the weak
absorption at the excitation wavelength or the short T1 excited state
lifetime [29]. We noticed that the quenched phosphorescence peak
area of Ir-2 (integrated area: 5700) is much smaller than the
upconversion emission band area (integrated area: 49,800). This is
abnormal since the conventional transition metal complex photo-
sensitizers are all phosphorescent, and the TTA upconversion
mechanism will require that the quenched phosphorescence peak
area is at least 2-fold of the upconverted fluorescence emission
peak area. Based on the Jablonski diagram of the TTA upconversion
scheme [29], we propose that the otherwise non-emissive triplet
excited state of Ir-2 was involved in the triplet-triplet-energy-
transfer (TTET) process, which is one of the critical steps of the
TTA upconversion [26,29]. That is, the “dark” excited states were
involved in the TTA upconversion [29,31,56e63]. Therefore, we
propose that different from previous studies, weakly phosphores-
cent, or non-phosphorescent transition metal complexes can be
used as triplet photosensitizers for the TTA upconversions, as long
as the triplet excited state of the transition metal complexes can be
efficiently populated. This concept will greatly increase the avail-
ability of the triplet photosensitizers for the TTA upconversion
[28e30,57,58,63e65].
presence of the triplet acceptor 9,10-diphenylanthracene (DPA)
(Fig. 8b). For Ir-2, intense emission in the range of
400 nme550 nm was observed in the presence of DPA, which is
the characteristic fluorescence emission of DPA. Irradiation of DPA
alone could not produce the emission band, thus the upconversion
of the Ir-2/DPA system was verified [29]. The upconversion
quantum yields were determined as 23.7% with Ir-2 as the triplet
photosensitizer.
Table 3
Electronic excitation energies (eV) and corresponding oscillator strengths (f), main
configurations and CI coefficients of the low-lying electronically excited states of
complex Ir-2, calculated by TDDFT//B3LYP/6-31G/LanL2DZ, based on the DFT//
B3LYP/6-31G/LanL2DZ optimized ground state geometries.a
Electronic TDDFT//B3LYP/6-31G
transition
Energy (eV)a
fb
Compositionc
CId
Character
S0 / S1
S0 / S2
S0 / S3
2.30 eV 540 nm 0.0006 H / L
2.62 eV 474 nm 0.0729 H ꢀ 1 / L
2.65 eV 468 nm 0.3125 H ꢀ 2 / L
H / L þ 1
0.6563 LLCT
0.6430 LLCT
0.6576 ILCT
0.1505 LLCT
0.1970 ILCT
0.5548 LLCT
0.2688 LLCT
0.2094 LLCT
S0 / S4
2.66 eV 466 nm 0.1423 H ꢀ 2 / L
H / L þ 1
H / L þ 2
H ꢀ 1 / L
The TTA upconversion with Ir-2 as the triplet photosensitizer
was confirmed by the observation of the delayed fluorescence of
DPA at 410 nm upon selectively excitation of Ir-2 at 473 nm
(nanosecond pulsed laser mixed solution of Ir/DPA). The lifetime of
e
S0 / T1
2.05 eV 605 nm 0.0000 H ꢀ 2 / L þ 1 0.5223 ILCT
H ꢀ 2 / L
0.3313 ILCT
H ꢀ 1 / L þ 1 0.2392 LLCT
e
e
S0 / T2
S0 / T3
2.22 eV 558 nm 0.0000 H / L þ 3
2.23 eV 556 nm 0.0000 H / L þ 4
0.4687 ILCT
0.4234 ILCT
the upconverted emission was determined as 481.8
ms (inset of
Fig. 8b), which confirms the delayed fluorescence feature of the
upconversion [48]. In comparison, the acceptor DPA alone shows
a fluorescence lifetime of 5.4 ns (prompt fluorescence).
The TTET efficiency was studied by the phosphorescence
quenching in the presence of triplet acceptor DPA (Fig. 9). With
increasing the DPA concentration in the solutions of Ir-1 or Ir-2, the
phosphorescence of Ir-1 and Ir-2 were significantly quenched. For
a
Only the selected low-lying excited states are presented.
Oscillator strength.
H stands for HOMO and L stands for LUMO. Only the main configurations are
b
c
presented.
d
The CI coefficients are in absolute values.
No spin-orbital coupling effect was considered, thus the f values are zero. The
single/triplet state energy gaps were calculated based on the optimized ground state
geometry.
e

X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
111
Fig. 6. Frontier molecular orbitals of Ir-2. Calculated by DFT at the B3LYP/6-31G/LanL2DZ level using Gaussian 09W.
Fig. 7. Spin density of the complexes Ir-0, Ir-1 and Ir-2. Calculated based on the optimized triplet state by DFT at the B3LYP/6-31G/LanL2DZ level using Gaussian 09W.
Ir-0, however, the quenching is to a much less extent. Similar weak
quenching effect was observed for Ru(dmb)3[PF6]2 (see ESI). The
much weaker quenching effect with Ir-0 and Ru(dmb)3[PF6]2 can be
attributed to the short lifetimes of the triplet excited states of these
two complexes [29,30].
The quenching efficiency of the phosphorescence of the
complexes in the presence of DPA can be quantified by the
SterneVolmer plotting(Fig. 9d). The quenchingconstants (KSV) of the
complexes Ir-1 and Ir-2 are close to each other, the values are almost
10- to 20-fold of the model complexes Ir-0 and Ru(dmb)3[PF6]2
(Table 4). Thus our results show that the crucial TTET step can be
enhanced by using transition metal complexes that show long-lived
triplet excited state [29,31,56,58,60,62,63,65]. This finding is useful
for design of new triplet photosensitizers for TTA upconversion.
The excitation power dependency of the upconversion emission
intensity was also studied (Fig. 10). Linear relation was observed,
indicate the efficiency TTA upconversion with Ir-2 as the triplet
photosensitizer [27b,48].
Fig. 8. Emission and upconversion of the complexes with 473 nm (5 mW) laser excitation. (a) Emission of the Ir (III) complexes. (b) The upconverted DPA fluorescence and the
residual phosphorescence of the mixture of DPA and the photosensitizers. Inset in (b) is the decay trace of the upconverted emission at 410 nm with excitation at 473 nm (delayed
fluorescence lifetime sDF ¼ 481.8
m
s). c[DPA] ¼ 5.0 ꢂ 10ꢀ5 M; c[sensitizers] ¼ 1.0 ꢂ 10ꢀ5 M. In deaerated CH2Cl2. The asterisks in (a) and (b) indicate the scattered 473 nm excitation
laser, 25 ꢁC.

112
X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
Fig. 9. Quenching of the phosphorescence of the triplet photosensitizers with increasing the concentration of DPA. (a) Ir-2 (lex ¼ 470 nm), (b) Ir-1 (lex ¼ 400 nm), (c) Ir-
0 (lex ¼ 445 nm). (d) SterneVolmer plots for phosphorescence quenching. c[photosensitizers] ¼ 1.0 ꢂ 10ꢀ5 mol/L. In deaerated CH2Cl2, 25 ꢁC.
TTA upconversion with Ir-2 as triplet photosensitizer is visible to
un-aided eyes (Fig. 11). Ir-2 gives yellow emission upon 473 nm
laser excitation. Intense blue emission was observed upon addition
of DPA into the Ir-2 solution. For Ir-0, however, the yellow emission
did not change in the presence of DPA, due to the lack the upcon-
version with Ir-0 under the same experimental conditions. This
negative control also confirmed that the blue emission of the Ir-2/
DPA mixed solution is not due to the excitation of DPA by the
473 nm laser. No upconversion was observed for Ru(dmb)3[PF6]2
and Ir-1 (the blue color for the Ir-1/DPA may be due to the exci-
tation beam).
fluorescence intensity is proportional to the concentration of the
DPA molecules at the triplet excited state. Thus the light-
harvesting ability of the triplet photosensitizer and the lifetime
of T1 excited state of photosensitizer will be important for TTA
upconversion [29,30]. Long-lived T1 excited state of photosensi-
tizer will enhance the TTET process. Thus the ideal triplet photo-
sensitizer for the TTA upconversion should show strong
absorption of visible light and long-lived triplet excited states.
These criteria are useful for the selection of new triplet photo-
sensitizers for TTA upconversions.
The mechanism of the TTA upconversion with the Ir(III)
complexes as the triplet photosensitizer is outlined in Scheme 2.
Excitation into the 1MLCT or 1IL state of the complexes will
produce the 3MLCT and 3IL state. The excitation energy is trans-
ferred to the triplet acceptor DPA, via the TTET. Annihilation of two
DPA molecules at the triplet excited states will produce one
acceptor molecule at the singlet excited state and another one at
the ground state. Radiative decay of the singlet excited state
produces the upconverted fluorescence. The upconverted
1000
1.0
800
I
0.5
9.0 mW
600
0.0
2 4 6 8
Power / mW
400
Table 4
Ⅲ
200
Upconversion related photophysical properties of Ir complexes.
2.0 mW
s/
m
sa
FP
Ksv/Mꢀ1 (ꢂ103)c
FUC
b
d
0
Ir-0
Ir-1
0.29
4.3%
7.6%
3.4%
7.3%
9.2
130.6
146.7
5.0
e
e
400
500
600
700
107.06
364.32
0.84
Ir-2
23.7%
0.5%
[Ru(dmb)3]2D
a
Phosphorescence lifetime.
Phosphorescence quantum yield.
Quenching constant.
Fig. 10. Excitation power dependency of the upconverted emission intensity with Ir-2
as the triplet sensitizer and DPA as the triplet acceptor. In deaerated CH2Cl2. Inset is the
normalized integrated emission intensity plotted as a function of excitation light
power. c[Ir-2] ¼ 1.0 ꢂ 10ꢀ5 M, c[DPA] ¼ 5.0 ꢂ 10ꢀ5 M, 20 ꢁC.
b
c
d
Upconversion quantum yields, and with coumarin-6 as the standard.

X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
113
Fig. 11. (a) The photographs of the upconversion. (b) The CIE coordinates of the emission of the photosensitizers alone and (c) in the presence of DPA. Excited with 473 nm laser
(5 mW). In deaerated CH2Cl2 solution. c[photosensitizers] ¼ 1.0 ꢂ 10ꢀ5 mol/L, c[DPA] ¼ 5.0 ꢂ 10ꢀ5 mol/L, 25 ꢁC.
Ir(III) sensitizer
DPA Annihilator
Effect of light-
harvesting
Effect of and 3IL
τ
1DPA*
1IL*
3DPA*
1.77 eV
ISC
3IL*
E
TTET
TTA
3DPA*
2.66 eV
466 nm
3.10 eV
400 nm
2.04 eV 2.04 eV
607 nm 607 nm
700 nm
3DPA*
= 51500
ε
GS
Scheme 2. Qualitative Jablonski Diagram illustrates the sensitized TTA up-conversion process between Ir(III) complexes (exemplified by Ir-2) and DPA. The effect of the light-
harvesting ability and the luminescence lifetime of the Ir(III) sensitizer on the efficiency of the TTA-up-conversion are also shown. E is energy. GS is ground state (S0). 1IL* is
intraligand singlet excited state (coumarin derived N^N ligand localized). IC is inner conversion. ISC is intersystem crossing. 3IL* is intraligand triplet excited state (coumarin derived
N^N ligand localized). TTET is tripletetriplet energy transfer. 3DPA* is the triplet excited state of DPA. TTA is tripletetriplet annihilation. 1DPA* is the singlet excited state of DPA. The
emission bands observed for the sensitizers alone are the 3IL emissive excited state. The emission bands observed in the TTA experiment are the simultaneous 3IL* emission
(phosphorescence) and the 1DPA* emission (fluorescence).
4. Conclusions
steady state emission, time-resolved transient absorption, 77 K
emission spectra and DFT calculations. The complexes were used as
triplet photosensitizers for tripletetriplet energy transfer process,
i.e. tripletetriplet annihilation based upconversion. Upconversion
quantum yield up to 23.7% was observed with Ir-2. Neglectable
upconversions were observed for other complexes, due to the weak
absorption of visible light or the short lifetime of the T1 excited
states. Our results are useful for preparation of new Ir(III) complexes
that show strong absorption of visible light and long-lived triplet
excited states, and application of these complexes as triplet photo-
sensitizers for tripletetriplet annihilation based upconversion, etc.
In conclusion, cyclometalated Ir(III) complexes with pyrenyl-
fused imidazole C^N ligand were prepared, together with bpy
(Ir-1) or coumarin-fused bpy (Ir-2) N^N ligands. Ir-2 shows strong
absorption of visible light (
weak absorption of Ir-1 in visible range. Room temperature phos-
3
¼ 51,500 Mꢀ1 cmꢀ1 at 466 nm), vs. the
phorescencewas observed for both complexes. Long-livedT1 excited
states were observed for Ir-1 and Ir-2 (up to 73.9
excited state lifetime of the model complex Ir-0 (0.3
excited state of Ir-1 and Ir-2 was identified mainly as 3IL feature, by
ms), vs. the short T1
ms). The T1

114
X. Yi et al. / Dyes and Pigments 96 (2013) 104e115
electrophosphorescent properties of mononuclear platinum(II) complexes
based on 2-phenylbenzoimidazole derivatives. Organomet Chem 2009;
694(17):2777e85;
(f) Tsujimoto H, Yagi S, Asuka H, Inui Y, Ikawa S, Maeda T, et al. Pure red
electrophosphorescence from polymer light-emitting diodes doped with
highly emissive bis-cyclometalated iridium(III) complexes. J Organomet Chem
2010;695(17):1972e8.
Acknowledgments
J
We thank the NSFC (20972024 and 21073028), the Fundamental
Research Funds for the Central Universities (DUT10ZD212), the
Royal Society (UK) and NSFC (China-UK Cost-Share Science
Networks, 21011130154) and Ministry of Education (NCET-08-
0077) for financial support.
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Appendix A. Supplementary material
Supplementary material associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.dyepig.
2012.07.020.
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