Tuning the photophysical properties of cyclometalated Ir(III) complexes by a trifluoroacetyl group

Article abstract of DOI:10.1515/znb-2012-0306

Four cationic Ir(III) complexes, [Ir(dpq)₂(bpy)]PF₆ (1), [Ir(dpq)₂(phen)]PF₆ (2), [Ir(tfapq) ₂-(bpy)]PF₆ (3), and [Ir(tfapq)₂(phen)]PF ₆ (4) (dpqH = 2,4-diphenylquinol

Full text of DOI:10.1515/znb-2012-0306

Tuning the Photophysical Properties of Cyclometalated Ir(III) Complexes
by a Trifluoroacetyl Group
Bihai Tonga, Jiayan Qianga, Qunbo Meib, Hengshan Wangc, Qianfeng Zhanga,
and Zhao Hana
a College of Metallurgy and Resources Institute of Molecular Engineering and Applied Chemistry,
Anhui University of Technology, Ma’anshan, Anhui 243002, P. R. China
b Jiangsu Key Lab of Organic Electronics & Information Displays, Institute of Advanced Materials
(IAM), Nanjing University of Posts & Telecommunications (NUPT), Nanjing 210046, P. R. China
c Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources, College of
Chemistry & Chemical Engineering, Guangxi Normal University, Guilin 541004, P. R. China
Reprint requests to Dr. Qunbo Mei. Fax.: +86-25-85866396. E-mail: iamqbmei@njupt.edu.cn
Z. Naturforsch. 2012, 67b, 213 – 218; received January 3, 2012
Four cationic Ir(III) complexes, [Ir(dpq)2(bpy)]PF6 (1), [Ir(dpq)2(phen)]PF6 (2), [Ir(tfapq)2-
(bpy)]PF6 (3), and [Ir(tfapq)2(phen)]PF6 (4) (dpqH = 2,4-diphenylquinoline, tfapqH = 2-(4ꢀ-trifluoro-
acetylphenyl)-4-phenylquinoline, bpy = 2,2ꢀ-bipyridine, phen = 1,10-phenanthroline) have been syn-
thesized and fully characterized. The structure of 4 was also confirmed by single-crystal X-ray diffrac-
tion. The electron-acceptor character of the trifluoroacetyl unit leads to a reduced HOMO-LUMO gap
and consequently a red-shift of the UV/Vis absorption and luminescence spectra. The solvophobic
character of the trifluoroacetyl unit gives rise to a molecule assembly in solution.
Key words: Iridium(III) Complex, Crystal Structure, Photoluminescence
Introduction
In the present paper, trifluoroacetyl groups were in-
troduced into diphenylquinoline ligands of iridium(III)
complexes in order to tune their photophysical proper-
ties. The synthesis and photophysical properties of the
complexes were investigated to study the influence of
the trifluoroacetyl group.
Due to their relatively short excited state lifetime,
high photoluminescence efficiency and excellent color
tuning, luminescent cyclometalated iridium(III) com-
plexes have been widely used in organic light-emitting
diodes (OLEDs), luminescence sensitizers, and biolo-
gical imaging [1]. By changing the structure of the
ligand one can modulate the HOMO and LUMO en-
ergies of Ir(III) complexes in order to tune the emis-
sion color and luminous efficiency [2 – 5]. For exam-
ple, the emission wavelength of the iridium(III) com-
plexes can cover the whole visible region by modi-
fication or variation of the cyclometalated 2-arylpy-
ridine primary and the ancillary ligands [6]. The in-
corporation of fluorine into molecules results in pro-
found changes in the physical and chemical properties
of these compounds [7]. There are several effects of
the fluorine in Ir-cyclometalated complexes. The first
one is the strong electron-withdrawing effect on the
ligand π system, and the second is the increase of the
optical and thermal stability. The most important ob-
servation is that the fluorine can reduce the lumines-
cence quenching. However, so far trifluoroacetyl iri-
dium complexes have rarely been studied [8].
Experimental Section
General
The solvents were purified by routine procedures and
distilled under an atmosphere of dry nitrogen before
use. All reagents, unless otherwise specified, were pur-
chased from Aldrich and were used as received. 2,4-di-
phenylquinoline (dpqH) and 2-(4ꢀ-bromophenyl)-4-phenyl-
quinoline (bppqH) were obtained via Friedla¨nder reac-
tions [9, 10]. UV/Vis absorption spectra were obtained on
a Shimadzu UV-2501 PC spectrophotometer. Positive-ion
ESI mass spectra were obtained on a Perkin Elmer Sciex
API 365 mass spectrometer. NMR spectra were recorded
on a Bruker AV400 spectrometer using CDCl3 as sol-
vent. Photoluminescence (PL) spectra were measured with a
Shimadzu RF-5301PC fluorescence spectrophotometer. Lu-
minescence lifetimes were determined on an Edinburgh
FL920 time-correlated pulsed single-photon-counting instru-
ment.
c
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B. Tong et al. · Tuning the Photophysical Properties of Cyclometalated Ir(III) Complexes
130.4, 129.54, 128.74, 128.71, 128.10, 127.88, 127.29,
126.24, 125.79, 121.07, 119.22, 118.18, 115.28. – 19F NMR
(CDCl3, 376 MHz): δ = −71.35 (s). – MS ((+)-ESI): m/z =
378 (calcd. 378 for C23H15NOF3, [M+H]+).
Synthesis of the iridium complexes 1 – 4
To a round-bottomed flask (25 mL), 2-ethoxyethanol
(9 mL), 2,4-diphenylquinoline (0.56 g, 2.0 mmol),
IrCl3·3H2O (0.20 g, 0.56 mmol) and water (3 mL) were
added s◦equentially. The mixture was stirred under nitrogen
at 120 C for 24 h and cooled to r. t. The precipitate was
collected and washed with ethanol and acetone, and then
dried in vacuo to give the red dimer ([{Ir(µ-Cl)(dpq)2}2])
(0.32 g, 0.22 mmol).
In a round-bottomed flask, 0.08 g (0.05 mmol) of [{Ir(µ-
Cl)(dpq)2}2] and 0.03 g (0.20 mmol) of 2,2ꢀ-bipyridine were
mixed together in 10 mL of CH2Cl2. The solution was then
stirred at r. t. for 6 h. To the solution was added a methanol
solution with an excess of potassium hexafluorophosphate
for the ion exchange. The product was filtered, washed with
absolute ethanol, dried and purified by chromatography on
a silica gel column using dichloromethane-ethyl acetate
(v/v = 2 : 1) as eluent. An orange solid (88 mg, 83 %) of
[Ir(dpq)2(bpy)]PF6 (1) was obtained. – 1H NMR (CDCl3,
400 MHz): δ = 6.67 (d, J = 7.9 Hz, 2H), 6.86 (t, J = 7.6 Hz,
2H), 6.97 (t, J = 8.8 Hz, 2H), 7.16 (t, J = 8.1 Hz, 2H),
7.28–7.33 (m, 4H), 7.38 (d, J = 8.8 Hz, 2H), 7.42 (t, J =
6.6 Hz, 2H), 7.56–7.64 (m, 6H), 7.76 (d, J = 8.7 Hz, 2H),
7.83 (t, J = 8.0 Hz, 2H), 8.03 (t, J = 6.3 Hz, 2H), 8.08 (s,
2H), 8.21 (d, J = 7.3 Hz, 2H), 8.36 (d, J = 8.4 Hz, 2H),
8.40 (d, J = 8.1 Hz, 2H), 8.69 (d, J = 7.8 Hz, 2H). – 13C
NMR (CDCl3, 100 MHz): δ = 122.26, 125.52, 128.08,
129.07, 129.39, 129.41, 129.55, 129.59, 129.69, 129.86,
130.05, 130.22, 130.25, 130.27, 130.43, 130.46, 130.54,
131.63, 132.17, 133.28, 134.22, 135.77, 141.37, 148.79,
151.61, 162.00. – MS ((+)-ESI): m/z = 909 (calcd. 909 for
C52H36N4Ir, [M]+).
Scheme 1. Synthetic routes to iridium(III) complexes [Ir-
(dpq)2(bpy)]PF6 (1), [Ir(dpq)2(phen)]PF6 (2), [Ir(tfapq)2-
(bpy)]PF6 (3), and [Ir(tfapq)2(phen)]PF6 (4).
Synthesis of 2-(4ꢀ-trifluoroacetylphenyl)-4-phenylquinoline
(tfapqH)
The synthetic routes to the ligands and the iridium(III)
complexes are given in Scheme 1. To a stirred solution of
diisopropylamine (1.2 g, 0.012 mol) in THF (10 mL) was
added n-BuLi (2.66 M, 4 mL) dropwise at −40 ◦C, and
the resulting mixture was stirred at −40 ◦C for 1 h and
◦
cooled to −78 C. To the mixture was added bppqH (3.6 g,
The Ir(III) complexes [Ir(dpq)2(phen)]PF6 (2), [Ir-
(tfapq)2(bpy)]PF6 (3) and [Ir(tfapq)2(phen)]PF6 (4) (bpy =
2,2ꢀ-bipyridine, phen = 1,10-phenanthroline) were prepared
from the corresponding ligands by similar procedures.
0.010 mol) in THF (5 mL) while keeping the internal temper-
◦
◦
ature below −70 C, and the mixture was stirred at −78 C
for 1 h. After addition of ethyl trifluoroacetate (1.4 mL,
◦
0.012 mol) in THF (2 mL) at −78 C, the mixture was al-
◦
lowed to warm to 0 C, quenched by addition of saturated
[Ir(dpq)2(phen)]PF6 (2)
aqueous NH4Cl, and partitioned between EtOAc and wa-
ter. The layers were separated, and the organic layer was
1H NMR (CDCl3, 400 MHz): δ = 9.20 (d, J = 6.5 Hz, 3H),
washed with brine, dried over MgSO4, and concentrated. The 8.59 (d, J = 5.5 Hz, 2H), 8.55 (d, J = 8.6 Hz, 1H), 8.27 (d,
residue was purified by flash silica gel column chromatog- J = 8.4 Hz, 3H), 8.13 (s, 2H), 8.09 (d, J = 8.6 Hz, 1H), 7.90
raphy (EtOAc-hexanes = 1 : 4) to give tfapqH (2.9 g, 77 %) (t, J = 5.8 Hz, 3H), 7.81 (s, 3H), 7.64 (dd, J = 4.5, 8.4 Hz,
as a colorless solid. – 1H NMR (CDCl3, 400 MHz): δ = 4H), 7.58 (d, J = 7.4 Hz, 3H), 7.53 (d, J = 10 Hz, 3H), 7.32
7.53 – 7.58 (m, 6H), 7.79 (t, J = 7.2 Hz, 1H), 7.88 (s, 1H), (d, J = 8.8 Hz, 1H), 7.21 (t, J = 8.2 Hz, 1H), 7.12 (t, J =
7.95 (d, J = 8.4 Hz, 1H), 8.24 (d, J = 8.4 Hz, 2H), 8.28 8.0 Hz, 2H), 6.90 (t, J = 8.2 Hz, 1H), 6.73 – 6.79 (m, 3H). –
(d, J = 8.4 Hz, 1H), 8.41 (d, J = 8.4 Hz, 2H). – 13C NMR 13C NMR (CDCl3, 100 MHz): δ = 169.19, 152.00, 150.52,
(CDCl3, 100 MHz): δ = 154.55, 149.85, 148.83, 146.05, 150.28, 148.07, 148.00, 146.76, 146.32, 145.87, 138.89,
137.99, 130.70, 130.68, 130.66, 130.64, 130.34, 130.13, 136.47, 135.96, 135.15, 130.90, 130.52, 129.53, 129.42,
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B. Tong et al. · Tuning the Photophysical Properties of Cyclometalated Ir(III) Complexes
215
Table 1. Crystal data, data collection parameters and details
128.91, 128.67, 128.20, 127.25, 126.99, 126.72, 126.53,
126.32, 126.06, 124.54, 123.22, 123.03, 117.81. – MS ((+)-
ESI): m/z = 933 (calcd. 933 for C54H36N4Ir, [M]+).
of the structure refinement for compound 4.
Formula
Mr
C60H42F12IrN4O4P
1334.15
Crystal size, mm3
Crystal system
Space group
0.17×0.17×0.08
[Ir(tfapq)2(bpy)]PF6 (3)
triclinic
1H NMR (CDCl3, 400 MHz): δ = 7.11 (t, J = 7.0 Hz,
2H), 7.34 – 7.40 (m, 2H), 7.41 – 7.53 (m, 2H), 7.61 – 7.72
(m, 8H), 7.86 (t, J = 7.0 Hz, 2H), 7.91 (d, J = 9.7 Hz,
2H), 7.96 – 8.06 (m, 2H), 8.09 – 8.20 (m, 4H), 8.21 (s, 2H),
8.30 (d, J = 9.0 Hz, 1H), 8.42 (d, J = 6.8 Hz, 1H), 8.54 (d,
J = 9.7 Hz, 2H), 8.58 (d, J = 7.1 Hz, 2H), 8.78 (s, 2H). –
¯
P1
˚
a, A
13.4991(17)
13.9630(18)
17.011(2)
76.367(2)
72.050(2)
63.592(2)
2713.7(6)
2
˚
b, A
˚
c, A
α, deg
β, deg
γ, deg
13C NMR (CDCl3, 100 MHz): δ = 171.47, 169.33, 165.94, V, A
3
˚
Z
165.67, 163.01, 157.93, 157.07, 151.49, 151.30, 148.59,
148.25, 146.89, 146.57, 145.06, 144.99, 139.82, 139.42,
136.05, 135.01, 132.65, 132.09, 131.93, 131.39, 130.91,
130.44, 130.05, 129.73, 128.62, 128.47, 128.30, 127.15,
127.07, 126.55, 126.10, 125.30, 125.27, 124.16, 123.88,
123.85, 123.73, 121.92, 121.38, 119.31, 118.45. – MS ((+)-
ESI): m/z = 1101 (calcd. 1101 for C56H34N4O2F6Ir, [M]+).
Dcalcd, g cm−3
F(000), e
1.60
1324
θ range, deg
2.53 – 26.00
16, −16 → 17, −15 → 20
15142 / 10407 / 0.0348
693
0.0811 / 0.1521
0.943
hkl range
Refl. measured / unique / Rint
Param. refined
R(F) / wR(F2)a (all refs.)
GoF (F2)b
−3
˚
Largest diff. peak / hole, e A
1.70 / −1.22
[Ir(tfapq)2(phen)]PF6 (4)
a
2
R1 = ΣꢁFo| − |Fcꢁ/Σ|Fo|; wR2 = [Σw(Fo − Fc2)2/Σw(Fo2)2]1/2
,
1H NMR (CDCl3, 400 MHz): δ = 6.78 – 6.98 (m, 2H),
7.12 (d, J = 9.7 Hz, 2H), 7.22 (d, J = 9.1 Hz, 2H), 7.25 –
7.34 (m, 2H), 7.39 (t, J = 7.4 Hz, 1H), 7.47 (t, J = 7.2 Hz,
1H), 7.58 – 7.71 (m, 4H), 7.75 (d, J = 8.6 Hz, 1H), 7.81 (d,
J = 9.6 Hz, 1H), 8.04 – 8.12 (m, 2H), 8.16 – 8.21 (m, 2H),
8.41 (d, J = 6.5 Hz, 1H), 8.45 (d, J = 6.1 Hz, 1H), 8.50 –
8.55 (m, 2H), 8.58 (d, J = 5.8 Hz, 2H), 8.61 (s, 1H), 8.65
(s, 1H), 8.75 (d, J = 6.0 Hz, 2H), 8.80 (d, J = 8.6 Hz, 2H),
w = [σ2(Fo2)+(AP)2 +BP]−1, where P = (Max(Fo2,0)+2Fc2)/3;
b
2
GoF = [Σw(Fo −Fc2)2/(nobs −nparam)]1/2
.
CCDC 821882 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data request/cif.
8.86 (t, J = 8.8 Hz, 2H). – 13C NMR (CDCl3, 100 MHz): Results and Discussion
δ = 169.358, 167.877, 167.742, 154.412, 154.018, 152.986,
Synthesis and characterization of compounds 1 – 4
152.327, 147.852, 147.822, 147.792, 147.111, 146.520,
146.192, 145.987, 140.851, 136.329, 136.067, 136.058,
136.036, 131.089, 131.028, 130.854, 130.383, 130.372,
130.346, 130.338, 130.327, 130.302, 130.274, 130.247,
130.199, 130.187, 130.178, 130.172, 130.164, 130.158,
130.075, 129.979, 129.973, 129.735, 129.488, 129.482,
129.477, 129.455, 129.420, 129.375, 129.330, 129.307,
128.715, 128.677, 128.668, 128.654, 128.409, 128.405,
127.593, 126.652, 126.330, 126.049. – 19F NMR (CDCl3,
376 MHz): δ = −69.17, −70.60, −70.64, −71.06. – MS
((+)-ESI): m/z = 1125 (calcd. 1125 for C58H34N4O2F6Ir,
[M]+).
The cyclometalating ligands dpqH and bppqH were
prepared by Friedla¨nder reactions. The trifluoroacetyl
unit was introduced into bppqH at low temperatures us-
ing n-BuLi as metalating agent. The iridium complexes
were synthesized in two steps using standard methods.
The cyclometalated chloride-bridged dimers were pre-
pared according to the reported procedures and were
readily dissociated by NˆN type ligands in high yield
(Scheme 1). In general, the final cationic iridium com-
plexes have lower solubilities in commonly used sol-
vents than the corresponding complexes without a tri-
fluoroacetyl group. There are two sets of NMR sig-
nals for the CF3 units, thus the complexes deviate from
ideal C2 symmetry in solution as expected. This phe-
nomenon is commonly encountered in cyclometalated
iridium(III) complexes by our experience.
Crystal structure determination
A suitable single crystals of 4 was obtained by recrystal-
lization from a mixture of CH2Cl2, ethyl acetate and hexane
at r. t. and mounted on a glass fiber. Diffraction data were col-
lected on a Bruker SMART Apex CCD diffractometer with
MoKα radiation at T = 296 K using an ω scan mode. Table 1
summarizes the cyrstal data and numbers pertinent to data
collection and structure refinement.
In order to confirm the 3-dimensional structure of
these complexes, the crystal structure of 4 was deter-
mined. Red crystals of 4 were obtained from recrystal-
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216
B. Tong et al. · Tuning the Photophysical Properties of Cyclometalated Ir(III) Complexes
value of 90◦. Furthermore, the C–C and C–N bond
lengths and angles are within normal ranges and are
in agreement with corresponding parameters described
for similarly constituted complexes [16].
Photophysical properties of compounds 1 – 4
Fig. 2 compares the UV/Vis absorption spectra of
the new Ir(III) complexes in CH2Cl2 solution at r. t.
Complexes 1 and 2 have similar spectra. The strong
absorption bands between 250 nm and 320 nm in the
ultraviolet region are assigned to the spin-allowed π-
Fig. 1 (color online). Perspective view of the [Ir(tfapq)2-
(phen)]+ cation in 4 with selected displacement ellipsoids
drawn at the 30 % probability level. H atoms omitted.
π∗ transition of the ligands. The moderately intense
˚
Selected distances (A) and angles (deg): C(15)–Ir(1)
1.987(8), C(38)–Ir(1) 1.985(8), Ir(1)–N(3) 2.166(6),
Ir(1)–N(4) 2.181(7), Ir(1)–N(1) 2.086(6), Ir(1)–N(2)
2.092(6); C(38)–Ir(1)–C(15) 94.7(3), C(15)–Ir(1)–N(1)
79.8(3), C(15)–Ir(1)–N(2) 95.4(3), C(38)–Ir(1)–N(3)
171.5(3), N(1)–Ir(1)–N(3) 83.9(2), C(38)–Ir(1)–N(4)
96.1(3), N(1)–Ir(1)–N(4) 105.0(2), N(3)–Ir(1)–N(4) 76.1(3),
C(38)–Ir(1)–N(1) 95.0(3), C(38)–Ir(1)–N(2) 79.1(3),
N(1)–Ir(1)–N(2) 172.2(2), C(15)–Ir(1)–N(3) 93.4(3),
N(2)–Ir(1)–N(3) 102.6(2), C(15)–Ir(1)–N(4) 167.7(3),
N(2)–Ir(1)–N(4) 80.9(2).
lization using a mixture of CH2Cl2, ethyl acetate and
hexane at r. t. The molecular structure of complex 4 is
depicted in Fig. 1, including selected bond lengths and
angles. Complex 4 consists of a cationic distorted octa-
hedrally coordinated iridium(III) center ligated by two
tfapq ligands and a phen ligand with [PF6]− as counter
anion. The two tfapq ligands chelate the iridium center
with N-N in the trans and C-C in the cis configuration.
The cis C-C chelate arrangement implies that there is
a stronger trans influence of the phenyl group over
that of the pyridyl group [11]. The Ir–C bond lengths
Fig. 2 (color online). UV/Vis absorption spectra of the new
iridium(III) complexes 1 – 4 in CH2Cl2.
˚
ranging from 1.985(8) to 1.987(8) A are within the
range reported for closely related complexes [12– 14].
The Ir–N bond lengths of the tfapq ligands spanning
˚
from 2.086(6) to 2.092(6) A are within the range re-
ported for other mononuclear complexes containing
analoguous Ir[CˆN] moieties [15]. For the same rea-
son of trans influence, the Ir–C bonds are shorter
than the Ir–N bonds. Moreover, the Ir–N bond lengths
between the Ir center and the phen ancillary ligand
˚
(from 2.166(6) to 2.181(7) A) are longer than those be-
tween the Ir center and the tfapq ligands because of
stronger donating and back-bonding interactions be-
tween the aryl groups of tfapq and the iridium atom.
The phen chelation results in an N(3)–Ir(1)–N(4) bond
angle of 76.1(3)◦, appreciably smaller than the ideal
Fig. 3 (color online). The room temperature photolumines-
cence spectra of the new complexes 1 – 4 in CH2Cl2 solution
(λex = 420 nm).
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B. Tong et al. · Tuning the Photophysical Properties of Cyclometalated Ir(III) Complexes
217
Table 2. Photoluminnescence performances of the new com-
(τ1) of 3 is 237 ns, and its contribution is 54 %, while
the long short lifetime (τ2) is 577 ns and its contri-
bution 46 %. Complex 4 showed smaller values of τ1
(219 ns, 69%) and τ2 (575 ns, 31 %) as compared to
those of 3. In general, the radiative lifetime of the cy-
plexes 1 – 4.
Complex
λem (nm)
564
ϕ
ꢂτꢃ (ns)
1
2
3
4
0.95
0.79
0.28
0.53
705
564
597
609
693
τ1 = 237, τ2 = 577
τ1 = 219, τ2 = 575 clometalated iridium complexes falls in the microsec-
ond and submicrosecond range [19], and the experi-
bands at 345 nm probably correspond to a spin-allowed
singlet metal-to-ligand charge-transfer (1MLCT) [17].
The weak absorption bands at ca. 450 nm can be
assigned to a spin-forbidden triplet metal-to-ligand
charge-transfer (3MLCT). Compared to 1 and 2, the
corresponding absorption bands of complexes 3 and 4
have an obvious red-shift after the introduce of the tri-
fluoroacetyl unit. The results indicated that 3 and 4
have a reduced energy band gap (Eg) because the ab-
sorption edge reflects the magnitude of Eg.
mental decay times are also in this range. The phospho-
rescence quantum efficiency (ϕ) of 1 and 2 in degassed
CH2Cl2 solution is ca. 0.95 and 0.79, respectively, with
an aqueous solution of [Ru(bpy)3]Cl2 (ϕ = 0.042) as
the standard solution [20]. The efficiency of 3 and 4 is
ca. 0.28 and 0.53, respectively. It is lower than that of 1
and 2, which may also be related to the strong inter-
molecular interactions. Anyway, these complexes are
remarkably bright compared to similar cationic iridium
complexes found in the literature [21].
The r. t. photoluminescence spectra of the new com-
plexes in CH2Cl2 solution are shown in Fig. 3. Com-
plexes 1 and 2 have almost the same spectra. They
emit intense luminescence with an emission wave-
length of 562 nm. The full width at half maximum
(FWHM) of this transition is 45 nm. The emission
wavelength of 3 is red-shifted to 597 nm compared
with that of 1, and the FWHM is 42 nm, which is con-
sistent with the UV/Vis absorption result. The red-shift
of the emission spectra is related to the influence of an
electron-acceptor substituent [18]. The emission wave-
length of 4 is 609 nm with a FWHM of 81 nm. It also
has a red shift as compared to that of 2.
In summary, four new cationic iridium complexes
have been prepared, and the photophysical influence
of the trifluoroacetyl unit has been investigated. The
electron-acceptor character of the trifluoroacetyl group
led to a reduced HOMO-LUMO gap and consequently
to a red-shift of the UV/Vis absorption and lumines-
cence spectra. At the same time, the solvophobic char-
acter of the trifluoroacetyl unit led to a molecule as-
sembly in solution. These new iridium complexes are
also found to be efficient emitters. Introduction of a tri-
fluoroacetyl unit into iridium complexes can thus serve
for the design of efficient electroluminescent materials.
Table 2 presents the luminescence lifetimes of the
four iridium complexes in CH2Cl2 at r. t. Complexes 1
and 2 show a single exponential decay, and the lifetime
is 705 and 693 ns, respectively. However, complexes 3
and 4 show a double-exponential decay which is re-
lated to strong intermolecular interactions and may be
ascribed to the incorporation of the solvophobic triflu-
oroacetyl groups into the ligands. The short lifetime
Acknowledgement
This project was supported by the National Natural
Science Foundations of China (grant nos. 50903001 and
50803027), project 973 (2011CB512005), the Guangxi Na-
tural Science Foundation of China (2011GXNSFD018010)
and the Program for New Century Excellent Talents in Uni-
versity of China (NCET-06-0556).
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