The effects of different solvents and excitation wavelength on the photophysical properties of two novel Ir(III) complexes based on phenylcinnoline ligand

Article abstract of DOI:10.1007/s10895-013-1229-3

Two novel cyclometalated iridium(III) complexes, Ir(pcl)₂(pic) and Ir(pcl)₂(fpic) (pcl: 3-phenylcinnoline, pic: picolinic acid, fpic: 5-fluoro-2-picolinic acid) were synthesized and characterized by FTIR, ¹H NMR spectroscopy, UV-vis, PL, and MALDI-TOF. These two Ir-complexes geometry were predicted using the Sparkle/PM6 model and suggested to a chemical environment of very low symmetry around the Ir ions (C ₁). The PL spectrum of Ir(pcl)₂(pic) and Ir(pcl) ₂(fpic) indicated that these complex belonged to red light emission, and maximum emission wavelength located at 647 and 641 nm, respectively. Most importantly, the effects of different solvents on their photoluminescent properties were detailed investigated. The results indicated that the polarity of solvent played an important role for their emission spectra. With introducing fluoro group to the pyridyl ring, the maximum emission wavelength of Ir(pcl)₂(fpic) was blue shifted about 6 nm, and the quantum yield was slightly higher than that of Ir(pcl)₂(pic). In addition, the thermal properties of these two Ir-complexes were measured by TGA, and results indicated that they had relative good thermal properties. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10895-013-1229-3

J Fluoresc
DOI 10.1007/s10895-013-1229-3
SHORT COMMUNICATION
The Effects of Different Solvents and Excitation Wavelength
on the Photophysical Properties of Two Novel Ir(III)
Complexes Based on Phenylcinnoline Ligand
Jing Xu & Chaolong Yang & Bihai Tong & Yunfei Zhang &
Liyan Liang & Mangeng Lu
Received: 23 January 2013 /Accepted: 30 April 2013
#
Springer Science+Business Media New York 2013
Abstract Two novel cyclometalated iridium(III) complexes,
Ir(pcl)2(pic) and Ir(pcl)2(fpic) (pcl: 3-phenylcinnoline, pic:
picolinic acid, fpic: 5-fluoro-2-picolinic acid) were synthe-
than that of Ir(pcl)2(pic). In addition, the thermal properties of
these two Ir-complexes were measured by TGA, and results
indicated that they had relative good thermal properties.
1
sized and characterized by FTIR, H NMR spectroscopy,
. .
Keywords Iridium(III) complex Phosphorescence
UV-vis, PL, and MALDI-TOF. These two Ir-complexes ge-
ometry were predicted using the Sparkle/PM6 model and
suggested to a chemical environment of very low symmetry
around the Ir ions (C1). The PL spectrum of Ir(pcl)2(pic) and
Ir(pcl)2(fpic) indicated that these complex belonged to red
light emission, and maximum emission wavelength located
at 647 and 641 nm, respectively. Most importantly, the effects
of different solvents on their photoluminescent properties
were detailed investigated. The results indicated that the po-
larity of solvent played an important role for their emission
spectra. With introducing fluoro group to the pyridyl ring, the
maximum emission wavelength of Ir(pcl)2(fpic) was blue
shifted about 6 nm, and the quantum yield was slightly higher
.
Photoluminescence Photophysical properties
Introduction
Cyclometalated iridium(III) complexes have been recently
shown to be excellent candidates for use in light-emitting
electrochemical cells (LECs) [1–3], oxygen sensing [4–7],
bioanalytical applications [8–10] and organic light-emitting
diodes (OLEDs) [11, 12], because of their high phosphores-
cence efficiency and relatively short excited state lifetime
[13]. As the phosphorescent materials, both singlet and triplet
excitons can be harvested for light emission [14]. For
cyclometalated iridium(III) complexes, the strong spin-orbit
coupling induced by the central metal ion leads to mixing of
the singlet and triplet excited states, consequently changing
the spin-forbidden nature of triplet transition to spin-allowed.
So far, though many iridium complexes with RGB (red, green,
blue) emissions have been developed, efficient red-emitting
materials are still scarce [15]. Moreover, it is previously found
that the emission color of iridium(III) complexes can be
strongly governed by the cyclometalating ligands in compar-
ison to the ancillary ligands [16]. But this standpoint has been
changed, since the emission colors of Ir(dfppy)2(LX) com-
plexes can be tuned from blue to red by modulating the
ancillary ligand [17]. The results demonstrated that the sub-
stitution of ancillary ligand could change the energy level of
highest occupied molecular orbital (HOMO) or lowest
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-013-1229-3) contains supplementary material,
which is available to authorized users.
:
:
:
:
:
J. Xu C. Yang B. Tong Y. Zhang L. Liang M. Lu (*)
Guangzhou Institute of Chemistry, Chinese Academy of Sciences,
Guangzhou 510650, People’s Republic of China
e-mail: mglu@gic.ac.cn
:
:
J. Xu C. Yang Y. Zhang
University of Chinese Academy of Sciences,
Beijing 100039, People’s Republic of China
B. Tong
College of Metallurgy and Resources, Anhui University
of Technology, Ma’anshan, Anhui 243002,
People’s Republic of China

J Fluoresc
unoccupied molecular orbital (LUMO). Therefore, using ap-
propriate ancillary ligands provide potential strategies for
phosphorescence color tuning as well as cyclometalating li-
gands [18].
counter electrode, and an SCE reference electrode immersed in
0.1 M Bu4NClO4 in dry acetonitrile purged with dried Argon.
The scanning rate was 50 mV/s, and all electrochemical poten-
tials were calibrated with the ferrocene/ferrocenium (Fc/Fc+)
standard. The photoluminescence (PL) measurements in differ-
ent solutions were conducted in a Hitachi F-4600 florescence
spectrophotometer. Fluorescence quantum yield was measured
by relative method using quinine sulfate as the standard
(0.546 in 0.5 mol/L H2SO4).
In the previous work, cyclometalated iridium(III) com-
plexes with phenylphthalazine derivatives have been proven
to be promising red emitters in OLEDs [19–21]. Tong et al.
reported one red-light materials with Ir(MPCPPZ)3 as a
dopant [19]. Fang et al. also obtained a novel ligand with
excellent performance by introducing carbazole unit into a
phenylphthalazine derivative [20]. Using quantum chemis-
try calculations, Mi et al. demonstrated that the ligands
which have the structure of an sp2-hybid N-atom adjacent
to the chelating N-atom (represented as CˆN=N) are superior
to these analogues with chelating N-atoms adjacent to sp2 C-
atoms (represented as CˆN=CH) due to the shorter bonding
length and the stronger bonding strength between the che-
lating N-atom and Ir-atom [21]. Phenylcinnoline deriva-
tives, having the resemblance of CˆN=N structure, are
expected to obtain good performances by coordinating with
iridium(III). Until now, there are limited reports about
phenylcinnoline derivatives utilized as the cyclometalated
ligand in iridium(III) complex.
In this work, we report two novel, red emissive phospho-
rescent Ir(III) complexes Ir(pcl)2(pic) and Ir(pcl)2(fpic),
composed of 3-phenylcinnoline serving as cyclometalating
ligand. Moreover, picolinic acid and 5-fluoro-2-picolinic
acid have been utilized as ancillary ligand in order to inves-
tigate the differences between Ir(pcl)2(pic) and Ir(pcl)2(fpic)
by introducing fluoro group in the pyridyl ring. The synthe-
sis, photophysical, thermal and electrochemical properties
of these complexes are studied, and the influences of solvent
and excitation wavelength are also detailed discussed.
Synthetic Procedures
1-benzylidene-2-phenylhydrazine (1)
To a solution of phenylhydrazine (5.30 g, 50 mmol) in
refluxing anhydrous ethanol (100 mL) was added a solution
of benzaldehyde (5.40 g, 50 mmol) in anhydrous ethanol
(50 mL) dropwise over a period of 20 min. The mixture was
stirred at this temperature for 0.5 h. The reaction mixture
was allowed to cool to room temperature and the resulting
precipitate was collected by filtration and washed with eth-
anol and dried in vacuum to give 1 as a colorless needle
crystal (9.15 g, 93 %). m.p. 154–155 °C. 1H-NMR (CDCl3,
400 MHz): δ 7.66 (s, 1H, HC=N), 7.64 (d, J=2.8 Hz, 2H,
phenyl), 7.35 (t, J=14.8, 2H, phenyl), 7.26–7.31 (m, 3H,
phenyl), 7.11 (d, J=8.4 Hz, 2H, phenyl), 6.86 (t, J=14.4 Hz,
1H, phenyl).
1-(benzylideneamino)indoline-2,3-dione (2) [22]
To a solution of oxalyl chloride (5.04 g, 40 mmol) in
refluxing dichloromethane (50 mL) was added dropwise in
2 h with stirring benzaldehyde phenylhydrazone (7.84 g,
40 mmol) dissolved in dichloromethane (50 mL). The solu-
tion was stirred and refluxed 2 h. The reaction mixture was
cooled in ice-water bath, and AlCl3 (9 g, 67.5 mmol) was
added portionwise but rapidly, and the mixture was refluxed
overnight. After cooling, the mixture was poured into cold
water. The organic layer was washed with dilute HCl, dried
over Na2SO4, and evaporated to dryness. The product was
purified by column chromatography using silica gel and
trichloromethane as the eluent to give 2 as a red solid
(7.53 g, 75 %). m.p. 143–144 °C. 1H-NMR (CDCl3,
400 MHz): δ 9.69 (s, 1H, HC=N), 7.80 (d, J=7.6 Hz, 2H,
indoline ring), 7.59–7.66 (m, 2H, phenyl), 7.43 (d, J=6.8 Hz,
4H, phenyl), 7.15 (t, J=15.2 Hz, 1H, indoline ring).
Materials and Methods
Materials and Characterization
The solvents were purified by routine procedures and distilled
under an atmosphere of dry nitrogen before use. All reagents
used were of analytical grade, and were purchased from
Aldrich. FT-IR spectra were carried out using a Tensor 27
(Bruker) Fourier Transform Infrared Spectrometer. NMR spec-
tra were taken on a DRX-400 MHz (Bruker) superconducting-
magnet NMR spectrometer with TMS as an internal standard.
Matrix-assisted laser desorption/ionization time-of-flight
(MALDI-TOF) mass spectra were measured on a Shimadzu
Kratos Compact MALDI 2 using shinapinic acid as a matrix.
UV-vis absorption spectrum was determined on a Shimadzu
spectrophotometer (UV 2550). Cyclic voltammetry (CV) mea-
surement was performed on a computer-controlled CHI600D
electrochemical analyzer with a Pt working electrode, a Pt plate
3-phenylcinnoline-4-carboxylic acid (3) [22]
Compound 2 (5 g, 20 mmol) from the previous experiment
was added to an aqueous solution of potassium hydroxide
[prepared by dissolving KOH (30 g, 0.536 mol) in water
(30 mL)]. The mixture was heated under reflux for 2 h, then

J Fluoresc
diluted carefully with boiling water and extracted with chlo-
roform. The aqueous solutions were combined and acidified
to pH 4 with concentrated HCl. The precipitate was filtered
off, washed with water, and recrystallized in ethanol to give
Ir(pcl)2(fpic) (fpic=5-fluoro-2-picolinic acid)
Complex Ir(pcl)2(fpic) was synthesized with the similar
method using 5-fluoro-2-picolinic acid in place of picolinic
acid as ancillary ligand, the yield of 60 % with brown
powder. FT-IR (KBr pellet, cm−1): 1,633, 1,588, 1,332,
1,258, 1,230, 1,100, 1,038, 798. 1H-NMR (CDCl3,
400 MHz) δ 8.46 (d, J=8.4 Hz, 1H, fpic ring), 8.36
(d, J=15.2 Hz, 2H, cinnoline ring), 8.28 (d, J=3.2 Hz, 1H,
fpic ring), 7.90 (d, J=6.8 Hz, 3H, cinnoline ring and fpic
ring), 7.71–7.77 (m, 3H, cinnoline ring), 7.63–7.68 (m, 3H,
cinnoline ring), 7.52 (t, J=16 Hz, 2H, phenyl), 6.98
(t, J=12 Hz, 1H, phenyl), 6.87 (t, J=16 Hz, 1H, phenyl),
6.77 (t, J=16 Hz, 1H, phenyl), 6.69 (t, J=16 Hz, 1H, phenyl),
6.22 (d, J=7.6 Hz, 1H, phenyl), 6.08 (d, J=7.6 Hz, 1H,
phenyl). MALDI-TOF MS m/z 766.639 ([Ir(pcl)2(fpic)+Na]+).
1
a yellow prism (4.32 g, 86 %). m.p. 233–234 °C. H-NMR
(CDCl3, 400 MHz) δ 14.57 (br, 1H, -COOH), 8.61
(d, J=7.6 Hz, 1H, cinnoline ring), 8.01–8.06 (m, 3H, cinnoline
ring), 7.84 (d, J=8 Hz, 2H, phenyl), 7.56–7.62 (m, 3H, phenyl).
3-phenylcinnoline (pcl) (4)
A mixture of 3 (3 g, 12 mmol), NaCl (5 g, 85.6 mmol),
DMSO (50 mL), and H2O (5 mL) was stirred at 120 °C for
6 h. After cooling, the mixture was poured into cold water
and extracted with ethyl acetate. The organic layer was
washed with saturated NaCl solution, dried over Na2SO4,
and evaporated to dryness. The residue was purified by
column chromatography on silica gel with trichloromethane
as eluent to give 4 as a yellow needle (1.98 g, 80 %). m.p.
112–113 °C. FT-IR (KBr pellet, cm−1): 1,588, 1,439, 1,326,
Results and Discussion
1
1,094, 906, 748, 702. H-NMR (CDCl3, 400 MHz) δ 8.58
Synthesis and Characterization
(d, J=8.4 Hz, 1H, cinnoline ring), 8.21–8.25 (m, 3H,
cinnoline ring), 7.88 (d, J=8 Hz, 1H, cinnoline ring), 7.81
(t, J=15.2 Hz, 1H, phenyl), 7.74 (t, J=14.8 Hz, 1H, phenyl),
7.55 (t, J=14.8 Hz, 2H, phenyl), 7.48 (t, J=14.4 Hz, 1H,
phenyl).
Scheme 1 represents a general synthetic route to prepare the
ligand and the corresponding complexes. The formation of
Ir-complexes are reasonably proved by FTIR spectra of
Ir(pcl)2(pic) and Ir(pcl)2(fpic), as shown in Fig. S1 in
ESI, in which not only the vibration absorptions at 1,588
and 1,094 cm−1 attributed to C=N and C=C in
cyclometalating ligand are recognized, but also the char-
acteristic absorption peaks at 1,633 and 1,038 cm−1 orig-
inating from C=N and C=C in ancillary ligand are
recognized. The absorptions at 1,718 and 1,263 cm−1 in
the IR spectra of complexes may be attributed to C=O
and C-O stretching vibrations of the carboxyl groups in
ancillary ligands. In addition, the broad and strong ab-
sorption peak at 3,000~2,500 cm−1 associated with O-H
stretching vibrations of the carboxyl groups in ancillary
ligands is not observed. All of the data suggested that
both cyclometalating ligands and ancillary ligands were
excellently coordinated with the metal iridium(III).
Ir(pcl)2(pic) (pic=picolinic acid)
To a round-bottomed flask (50 mL), compound 4 (0.33 g,
1.6 mmol), IrCl3·3H2O (0.2 g, 0.63 mmol), 2-ethoxyethanol
(15 mL) and H2O (5 mL) were added sequentially. The
mixture was stirred under nitrogen at 120 °C for 24 h and
cooled to room temperature. The precipitate was filtered off,
washed with ethanol and acetone and dried in vacuum to
give 5 as a dark red solid (0.2 g, 53 %). In a round-bottomed
flask (50 mL), a mixture of compound 5 (0.1 g, 0.17 mmol),
picolinic acid (0.042 g, 0.34 mmol), triethylamine (1 mL)
and dichloromethane (30 mL) was stirred at room tempera-
ture for 12 h. The solvent was evaporated, and the product
was purified by column chromatography (silica gel, eluent
dichloromethane/methanol, 20:1, v/v) to give Ir(pcl)2(pic) as
a dark red solid (0.057 g, 65 %). FT-IR (KBr pellet, cm−1):
As shown in Fig. 1, in 1H NMR spectroscopy, the
protons such as H1, H2 and H3 which localize in the
two same cyclometalating ligands respectively show dif-
ferent chemical shift, and this phenomenon can be under-
stood that the protons have different environment by
coordinating with the iridium(III) ion. On the other hand,
the protons of pcl in iridium(III) complexes show an
upfield shift compared with the same ones in free ligands.
For example, this phenomenon is observed for the proton
9 as well as the proton 1 adjacent to the metalated carbon
of the cyclometalating ligand. This can be rationalized in
terms of the reduction in electron density in the aromatic
ring that accompanies cyclometalation. Additionally, the
1
1,628, 1,588, 1,360, 1,332, 1,258, 1,095, 1,038, 798. H-
NMR (CDCl3, 400 MHz) δ 8.47 (d, J=8 Hz, 1H, pic ring),
8.35 (d, J=12 Hz, 2H, cinnoline ring), 8.25 (d, J=8 Hz, 1H,
pic ring), 7.86–7.90 (m, 3H, cinnoline ring and pic ring),
7.80 (t, J=16 Hz, 1H, pic ring), 7.71–7.75 (m, 3H, cinnoline
ring), 7.66 (d, J=4 Hz, 3H, cinnoline ring), 7.60 (d, J=8 Hz, 2H,
phenyl), 6.96 (t, J=16 Hz, 1H, phenyl), 6.87 (t, J=12 Hz, 1H,
phenyl), 6.76 (t, J=16 Hz, 1H, phenyl), 6.69 (t, J=12 Hz, 1H,
phenyl), 6.24 (d, J=8 Hz, 1H, phenyl), 6.11 (d, J=8 Hz, 1H,
phenyl). MALDI-TOF MS m/z 748.644 ([Ir(pcl)2(pic)+Na]+).

J Fluoresc
Scheme 1 Synthesis routes for
Ir(pcl)2(pic) and Ir(pcl)2(fpic)
proton of carboxyl group in ancillary ligand disappears,
suggesting that the oxygen atom coordinates with
iridium(III) ion.
UV-vis Absorption of Ir(pcl)2(pic) and Ir(pcl)2(fpic)
Figure 3 shows the UV-vis absorption spectra of
cyclometalating ligand, ancillary ligands and iridium(III)
complexes in CHCl3. Because of the similar characteristics
of ancillary ligands, the absorption spectra of the two com-
plexes are almost the same. The absorption bands at around
260–350 nm in the ultraviolet region are thought to be due
to the ligand-centered LC (π−π*) transitions, which is lo-
cated in the same region of absorption spectrum for pcl. The
absorption intensities of Ir(pcl)2(pic) and Ir(pcl)2(fpic) in
this area are weaker than that of free ligands. Moreover,
the maximum absorptions of ancillary ligands at around
260 nm also disappear in the spectra of Ir(pcl)2(pic) and
Ir(pcl)2(fpic). These observations imply the presence of
specific electron transfer processes in these iridium(III)
complexes. On the other hand, according to the
photophysical studies on related iridium(III) complexes,
the most intense absorption bands at around 360–410 nm
The formation of complexes was further supported by
MALDI-TOF mass spectrometry (as seen in Fig. S2). The
most abundant peak was 748.644 m/z, which corresponded
to the molecular mass of [Ir(pcl)2(pic)+Na]+. The most
abundant signal in mass spectroscopy of Ir(pcl)2(fpic) was
obtained at 766.639 m/z, and the corresponding molecule
might be formulated as [Ir(pcl)2(fpic)+Na]+. We tried to
obtain single crystals of these complexes, but failed. Thus,
the optimized molecular structures of Ir(pcl)2(pic) and
Ir(pcl)2(fpic) predicted by the Sparkle/PM6 model are
displayed in Fig. 2. The polyhedrons in two complexes are
both formed by two carbon atoms and two nitrogen atoms of
cyclometalating ligands, one nitrogen atom and one oxygen
atom of ancillary ligand. The Ir(III) ion localizes in the
center of a distorted octahedral environment, which is con-
sistent with literature reports [23, 24].

J Fluoresc
Fig. 1 1H NMR spectra of pcl,
Ir(pcl)2(pic) and Ir(pcl)2(fpic) at
400 MHz in CDCl3
and the relatively weak absorption at 470–500 nm can be
respectively assigned to the spin-allowed singlet metal-to-
ligand charge-transfer (1MLCT) and the spin-forbidden trip-
let metal-to-ligand charge-transfer (3MLCT) transitions,
which are not seen in the absorption spectra of free ligands.
The absorption like long tail with higher wavelength and
lower energy at around 470–500 nm is gained by mixing of
orbit coupling caused by the iridium(III) center. The results
also suggest that strong spin-orbit coupling are present to
effectively mix the singlet and triplet excited states.
Excitation and Emission Spectra of Ir(pcl)2(pic)
and Ir(pcl)2(fpic)
3
1
the MLCT level with the MLCT due to the strong spin-
Figure 4 gives the excitation spectra of Ir(pcl)2(pic) and
Ir(pcl)2(fpic) with a concentration of 1×10−5 mol/L in CHCl3
solution. These spectra show the most intense emission peaks
under different excitation wavelengths. In all measurement
range, the excitation bands for Ir(pcl)2(fpic) are relatively more
intense than that of Ir(pcl)2(pic). The reason will be discussed
later. The excitation with maximum efficiency at about 366 nm
Fig. 2 The ground state geometry of Ir(pcl)2(pic) (a) and Ir(pcl)2(fpic)
(b) calculated using the Sparkle/PM6 model
Fig. 3 UV-vis spectra of pcl, Ir(pcl)2(pic) and Ir(pcl)2(fpic) in CHCl3
solution with a concentration of 1×10−5 mol/L

J Fluoresc
is correlated to LC (π−π*) absorption of ligands. The excita-
are very close to pure red emission (x=0.67, y=0.33). All these
data confirm that Ir(pcl)2(pic) and Ir(pcl)2(fpic) are promising
candidates for red phosphorescent materials.
tion bands at 424 nm and 467 nm are likely to be assigned to
3
1MLCT and MLCT absorptions. Compared with the absorp-
tion spectra of two complexes, the excitation bands of
Ir(pcl)2(pic) and Ir(pcl)2(fpic) give a good match to their ab-
sorption spectra. Moreover, no excitation band is observed in
the region of emission, suggesting that the efficient energy
transfer can be obtained in two complexes.
The solution quantum yield of phenylcinnoline irid-
ium complexes were measured using quinine sulfate
(0.546 in 0.5 mol/L H2SO4) as a standard [25]. The
quantum yield is calculated according to the following
expression:
Room-temperature photoluminescence (PL) spectra of two
complexes are shown in Fig. 5. Luminescence measurements
are conducted on CHCl3 solution using 370 nm excitation of
the longest wavelength absorption in each case. Deep red
emission is observed for Ir(pcl)2(pic), of which PL maxima
(λPL) is observed at 647 nm. The full width at half maximum
(FWHM) of this transition is 93 nm. It should be noticed that
introducing one fluoro group in the ancillary ligand yields a
hypsochromic shift of 6 nm in the PL spectrum of Ir(pcl)2(fpic)
(1PL=641 nm), accompanied by FWHM of 90 nm. The PL
spectrum of Ir(pcl)2(fpic) shows a significant blue-shifted phos-
phorescence emission, due to the substitution of fluoro group in
the pyridyl ring which increases the HOMO-LUMO gap to
achieve the hypsochromic shift in emission color. The most
probable reason for this phenomenon is interpreted as follows.
The highly effective inter-system crossing to 3MLCT facilitated
Ar F n2
A Fr n2r
Φ ¼ Φr
where the subscript r refer to the reference substance, Φ
is the fluorescent quantum yield, A is the absorbance of
the solution at the exciting wavelength, F represents the
area of the emission fluorescence spectrum, and n is
the refractive index of the solvent.
The PL quantum yield of Ir(pcl)2(pic) and Ir(pcl)2(fpic)
in CHCl3 solution is ca. 0.80 % and 0.84 %, respectively.
This is presumably due to the less vibration of the C-F bond,
compared to C-H bond, thus suppressing the nonradiative
deactivation of the excited states in solutions [26].
Effect of Solvent on Photophysical Properties
for Ir(pcl)2(pic) and Ir(pcl)2(fpic)
1
by the strong spin-orbit coupling occurs after the MLCT
excitation from iridium to pcl. Considering the triplet energy
level of the ancillary ligand (3LX) compared to that of 3MLCT,
there are different ways to deactivate for pcl-centered 3MLCT
The photoluminescence spectra of Ir(pcl)2(pic) and Ir(pcl)2(fpic)
in different solvents are investigated when excited with dif-
ferent wavelengths (350–390 nm) and the results indicate that
the optimal excitation wavelength is 370 nm, as shown in
Figs. 6 and 7. The UV-vis absorption spectra (Fig. 8) and
photoluminescence spectra (excited with 370 nm) (Fig. 9) of
Ir(pcl)2(pic) and Ir(pcl)2(fpic) in different solvents are tested at
room temperature. Similar solvatochromic effects have been
observed in related complexes, due to the relative stability of
the ground and excited state dipoles [27]. The absorption peaks
of the complexes in different solvents are almost the same. This
suggests that the polarity of the solvent has very little influence
3
state. Because the level of LX locates higher than that of
3MLCT, the emission of Ir(pcl)2(fpic) is attributed to the phos-
phorescent decay from the 3MLCT state of pcl. For
3
Ir(pcl)2(pic), the decay from the LX state of the ancillary
ligand, which is resulted from inter-ligand energy transfer
3
3
(ILET) from MLCT state to LX state, occurs because the
3
3
level of LX is lower than that of MLCT [17]. As shown in
Fig. S3 in ESI, the Commission Internationale de L’Eclairage
(CIE) coordinates of Ir(pcl)2(pic) and Ir(pcl)2(fpic) are mea-
sured to be x=0.675, y=0.325 and x=0.671, y=0.329, which
Fig. 4 The excitation spectra of Ir(pcl)2(pic) and Ir(pcl)2(fpic) in
Fig. 5 The emission spectra of Ir(pcl)2(pic) and Ir(pcl)2(fpic) in CHCl3
solution (1×10−5 mol/L) (l ex=370 nm)
CHCl3 solution (1×10−5 mol/L)

J Fluoresc
on the energy levels of complexes ground state molecules. In
this case, however, the emission peaks of Ir(pcl)2(pic) and
Ir(pcl)2(fpic) in different solvents show clear differences. It
may be found that with the polarity of the solvent increasing
(Dimethyl sulfoxide > Dimethylformamide > Acetone >
Tetrahydrofuran > Trichloromethane), the emission maximum
wavelengths are obviously red-shifted from 647 nm in
trichloromethane to 671 nm in dimethyl sulfoxide. The peak
shift can be attributed to the stronger interaction between the
solvents and the excited molecules. Solvents with high polarity
can stabilize the excited state of complexes and thus decrease
energy content [28]. In addition, compared to other solvents,
the PL intensity for both Ir(pcl)2(pic) and Ir(pcl)2(fpic) is su-
perimposable in trichloromethane. The differences can be
Fig. 6 PL emission spectra of Ir(pcl)2(pic) under different excitation wavelengths (1×10−5 mol/L) (a Dimethyl sulfoxide, b Dimethylformamide,
c Acetone, d Tetrahydrofuran, e Trichloromethane)

J Fluoresc
clarified by comparing the polarizability of solvent, which is
described as the following equation:
polarizability of solvent. The polarizability and related pa-
rameters of different solvents are summarized in Table S1 in
ESI. Fa and Fb is the area of the emission fluorescence
spectra of Ir(pcl)2(pic) and Ir(pcl)2(fpic) respectively, which
represent the emission intensities of these complexes. The
results show that with the polarizability of solvent increas-
ing, the photoluminescence intensities of two complexes
obviously decrease. Generally, the main decay routes of
ꢀ
ꢁ
ε−1
n2−1
Δ f ¼
−
2ε þ 1 2n2 þ 1
In the above expression, ε is the dielectric constant of
solvent, n is the refractive index of the solvent, and Δf is the
Fig. 7 PL emission spectra of Ir(pcl)2(fpic) under different excitation wavelengths (1×10−5 mol/L) (a Dimethyl sulfoxide, b Dimethylformamide,
c Acetone, d Tetrahydrofuran, e Trichloromethane)

J Fluoresc
Fig. 8 UV-vis spectra of Ir(pcl)2(pic) (a) and Ir(pcl)2(fpic) (b) in different solvents (1×10−5 mol/L)
Fig. 9 The emission spectra of Ir(pcl)2(pic) (a) and Ir(pcl)2(fpic) (b) in different solvents (1×10−5 mol/L)
the excited state of iridium(III) complexes include radiative
and non-radiative. In addition, the polarizability of solvent
reflects the deformability of molecules in impressed field,
and the value of Δf is higher, the larger deformation of
molecules will be obtained. After the formation of excited
states, the strong interaction between the solvents and the
iridium(III) complexes molecules occurs, and this force
leads to the deformation of solvents molecules which is
instability. Consequently, the energy of non-radiative in-
creases because that the solvent molecules have a trend to be
deformed. It is presumably that the non-radiative deactivation
of the excited states of iridium complexes can be brought
down in the solvent with small deformation.
in ESI, all the complexes have good stability, and the 5 %
weight loss temperatures are 170 °C for Ir(pcl)2(pic) and
196 °C for Ir(pcl)2(fpic). The residue of Ir(pcl)2(pic) and
Ir(pcl)2(fpic) is 17.2 % and 53.6 % at 800 °C, respectively.
The thermal analysis demonstrate that Ir(pcl)2(fpic) is more
stable than its analogue, Ir(pcl)2(pic), suggesting that the
introduction of fluoro group into ancillary ligand slightly
improves the pyrolysis temperature of the complexes. The
formation of intermolecular hydrogen bonds with the
Table 1 Electrochemical data and energy levels of Ir(pcl)2(pic) and
Ir(pcl)2(fpic)
Complex
Ered (V) Eox (V) HOMO LUMO Egap (eV)
(eV)
(eV)
Thermal Properties of Ir(pcl)2(pic) and Ir(pcl)2(fpic)
Ir(pcl)2(pic)
Ir(pcl)2(fpic)
−1.43
−1.44
1.27
1.27
−5.65
−5.65
−2.95
−2.94
2.70
2.71
The thermal properties of the complexes are characterized
by thermal gravimetric analysis (TGA). As seen in Fig. S4

J Fluoresc
fluorine atom may be attributed to increase the decomposition
temperature [29]. All the above data suggest that complexes
Ir(pcl)2(pic) and Ir(pcl)2(fpic) are thermally stable enough to
experience device construction through vacuum deposition.
all exhibited saturated red emission. Especially, the effects of
excitation wavelength and solvents on emission spectra were
detailed discussed, the results showed that the optimal excitation
wavelength was 370 nm, and the polarity of solvent had impor-
tant influence for emission spectrum.
Electrochemical Properties of Ir(pcl)2(pic) and Ir(pcl)2(fpic)
Acknowledgments The authors appreciate the Guangdong-Hongkong
Technology Cooperation Finding (Project No. 2009A091300012).
The electrochemical properties of Ir(pcl)2(pic) and Ir(pcl)2(fpic)
are investigated by cyclic voltammetry (CV) (as shown in Fig.
S5 in ESI) using an degassed acetonitrile solution in presence
of 0.1 M Bu4NClO4 as a supporting electrolyte. A Hg/Hg2Cl2
electrode is used as a reference electrode; a platinum electrode
is used as a working electrode; a platinum plate is used as a
counter electrode.
In order to evaluate charge injection and device perfor-
mance, HOMO-LUMO energy levels of complexes are cal-
culated using cyclic voltammetry [30, 31]. The onset
reduction potential (Eonset, red) of Ir(pcl)2(pic) is estimated
to be −1.43 V, while Eonset, red of Ir(pcl)2(fpic) is −1.44 V.
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