Tuning the emission color of iridium(III) complexes with ancillary ligands: A combined experimental and theoretical study

Article abstract of DOI:10.1002/ejic.200801185

A series of iridium(III) complexes [Ir(dfppy)₂(LX)] {dfppy = 2-(2,4-difluorophenyl)pyridine, LX = acac, Flr(acac); dbm, Flr(dbm); natfac, Flr(natfac); acac = acetylacetonate, dbm = dibenzoylmethanane, natfac = l-naphthoyl-3,3,3-trifluoro-acetonej have been synthesized and characterized by elemental analysis, NMR and IR spectroscopy, and mass spec-trometry. The cyclic voltammetric, absorption, and emission properties of these complexes have been systematically investigated. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations have been performed to investigate the ground- and excited-state properties of these complexes to provide an insight into their structural, electronic, and optical properties. The results of this work provide further evidence regarding how and to what extent the sub-stituents on the ancillary ligand LX influence the emission properties of [Ir(dfppy) ₂(LX)] complexes. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejic.200801185

FULL PAPER
DOI: 10.1002/ejic.200801185
Tuning the Emission Color of Iridium(III) Complexes with Ancillary Ligands:
A Combined Experimental and Theoretical Study
Xin Gu,[a] Teng Fei,[a] Houyu Zhang,*[a] Hai Xu,[a] Bing Yang,[a] Yuguang Ma,*[a] and
Xiaodong Liu[b]
Keywords: Iridium / Ligand effects / Dyes/Pigments / Density functional calculations / Charge transfer
∧
A series of iridium(III) complexes [Ir(dfppy)2(L X)] {dfppy =
pendent DFT (TD-DFT) calculations have been performed to
investigate the ground- and excited-state properties of these
complexes to provide an insight into their structural, elec-
tronic, and optical properties. The results of this work provide
further evidence regarding how and to what extent the sub-
∧
2-(2,4-difluorophenyl)pyridine, L X = acac, FIr(acac); dbm,
FIr(dbm); natfac, FIr(natfac); acac = acetylacetonate, dbm =
dibenzoylmethanane, natfac = 1-naphthoyl-3,3,3-trifluoro-
acetone} have been synthesized and characterized by ele-
mental analysis, NMR and IR spectroscopy, and mass spec-
trometry. The cyclic voltammetric, absorption, and emission
properties of these complexes have been systematically in-
vestigated. Density functional theory (DFT) and time-de-
∧
stituents on the ancillary ligand L X influence the emission
∧
properties of [Ir(dfppy)2(L X)] complexes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
∧
clometalating ligand and L X is an ancillary ligand), al-
Introduction
though previous studies have tended to place more empha-
sis on modifying the cyclometalating ligand C N to obtain
∧
Considerable research efforts have been focused on phos-
phorescent transition metal complexes for use as room-tem-
perature dyes in the past few years.[1] These materials have
intriguing photophysical, photochemical, and excited-state
redox properties and high internal quantum efficiencies
(ηint) which, in principle, can reach almost 100%.[2] They
also possess strong spin-orbit couplings, which allow fast
intersystem crossing among the low-lying electronic states.[3]
The motivations for using those emissive complexes include
fundamental research on excited-state charge- and energy-
transfer as well as potential applications in sensors[4] and as
organic light-emitting diodes (OLEDs).[5]
different colors.[5b,12] It used to be believed that ancillary
ligands play a minor and supporting role in the coordina-
tion sphere. However, Park[13] and Chou[14] have shown that
the ancillary ligands are also able to adjust the emission
color of complexes. Likewise, spectroscopic studies have re-
vealed that ancillary ligands alter the 1MLCT energy by
changing the HOMO energy level,[15] and theoretical calcu-
∧
lations have indicated that strong-field ancillary L X li-
gands lengthen the bonds between the C N ligands and the
metal, thus altering the electronic properties of the metal
complexes.[16] Research on ancillary ligands has therefore
attracted more and more attention.
∧
Phosphorescent d6 metal complexes of IrIII [6] RuII,[7] and
,
OsII [8] have shown high efficiency in OLEDs. Among these
complexes, iridium(III) complexes are regarded as the most
effective materials in OLEDs as they exhibit a high thermal
stability, a short lifetime in the excited state,[9] and are tun-
able in the blue to red region upon modification of the li-
gand[10] or by introducing a variety of electron donors or
acceptors into the ligand.[11] To achieve wide-range color
emission for use in full color displays, many different classes
Many questions regarding how and to what extent ancil-
lary ligands affect the optical properties of metal complexes
remain unsolved, although several recent reports have in-
vestigated the effects of ancillary ligands from both an ex-
perimental and a theoretical perspective. The function of
the ancillary ligands in these complexes remains elusive, and
their role in tuning the emission color is still under debate.
It is therefore vital to investigate their role further to gain
an insight into the interaction between the ancillary ligands
and the metal and hence understand the relationship be-
tween the structures and properties. The optical properties
of cyclometalated IrIII complexes are strongly dependent on
the characteristics of their ground and low-lying excited
states. Density functional theory (DFT) provides a means
to evaluate a variety of ground-state properties with an
accuracy close to post-HF methods.[17] Similarly, time-
dependent DFT (TD-DFT) calculations make it possible
∧
∧
∧
∧
of homoleptic [Ir(C N)3] and heteroleptic [Ir(C N)2L X]
iridium(III) complexes have been developed (C N is a cy-
[a] State Key Laboratory of Supramolecular Structure and Materi-
als, Jilin University,
Changchun 130012, P. R. China
Fax: +86-431-85168480
E-mail: houyuzhang@jlu.edu.cn
ygma@jlu.edu.cn
[b] College of chemistry, Jilin University,
Changchun 130012, P. R. China
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X. Gu, T. Fei, H. Zhang, H. Xu, B. Yang, Y. Ma, X. Liu
FULL PAPER
to calculate the vertical electronic excitation for the excited complexes exhibit a large bathochromic shift from blue
states.[18]
[482 nm for FIr(acac)] to orange-red [586 nm for FIr(nat-
Herein we demonstrate color tuning by ancillary ligands fac)] as the ancillary ligand changes. FIr(acac) exhibits
in a joint experimental and theoretical study and report the strong emission at room temperature in solution, whereas
synthesis and characterization of a series of iridium(III) emission is very weak for FIr(dbm) and FIr(natfac).
∧
complexes [Ir(dfppy)2(L X)] with acac, dbm, and natfac
(acac = acetylacetonate, dbm = dibenzoylmethanane, natfac
= 1-naphthoyl-3,3,3-trifluoroacetone) as ancillary ligands
(see Figure 1). The redox behavior and optical properties
of these complexes are systemically investigated, and DFT
calculations are performed to gain a better understanding
of the role of ancillary ligands in color tuning. Although
the structures we investigated are not particularly novel
from a structural point of view, our attention was focused
on the interaction of the ancillary ligands with the metal
from a combined experimental and DFT viewpoint. Tuning
the emission color of these complexes can easily be achieved
∧
by systematically varying the substituents on L X.
Figure 2. Absorption spectra of complexes 1–3 in CH2Cl2 solution.
Figure 1. Structures of Ir complexes FIr(acac), FIr(dbm), and FIr-
(natfac).
Results and Discussion
Preparation and Photophysical Properties
All synthetic procedures were carried out under an inert
gas atmosphere. The cyclometalated chloride-bridged dimer
[(dfppy)2Ir(µ-Cl)2Ir(dfppy)2] was synthesized according to a
procedure described in the literature[19] by refluxing
IrCl3·nH2O with the cyclometalating ligand in a mixture
of 2-ethoxyethanol and water. The chloride-bridged dimer
complexes were refluxed under N2 in 2-ethoxythanol, and
the precipitate formed after cooling to room temperature
was filtered off and washed with water and diethyl ether.
The crude product was subjected to column chromatog-
Figure 3. Emission spectra of complexes 1–3 in PMMA films.
Electrochemistry
The electrochemical properties of the IrIII complexes
raphy on silica gel, eluting with dichloromethane, to provide were examined by cyclic voltammetry (CV) in a standard
the desired product in quantitative yield (see Exp. Sect.). one-compartment, three-electrode electrochemical cell
The chemical structures of FIr(acac) (1), FIr(dbm) (2), and (BAS 100B/W, Bioanalytical Systems). A summary of the
FIr(natfac) (3) are shown in Figure 1. Figure 2 shows the redox potentials of the complexes is given in Table 1. It is
absorption spectra of FIr(acac), FIr(dbm), and FIr(natfac) generally accepted that the oxidation of IrIII species involves
in dichloromethane solution. Absorptions below 370 nm the iridium moiety of the complexes and that the HOMOs
are ascribed to metal-perturbed ligand-centered absorp- are mostly due to the d orbitals of iridium with some π-
tions on the basis of relatively higher extinction coefficients orbital contributions from the ligand. The LUMO in this
of the bands. The absorptions at long wavelength (370– kind of complex is due either to dfppy or the ancillary li-
480 nm) are weak (molar absorption coefficients of less gands. This electrochemical behavior is consistent with a
than 1ϫ10–4 –1 cm–1) and broad and are assigned to description of the ligand-localized LUMO states and a

metal-to-ligand charge transfer (MLCT) absorptions. The HOMO with substantial metal character, as can be seen
emission spectra for the complexes in PMMA films are in our theoretical calculations below. The HOMO–LUMO
shown in Figure 3. The photoluminescence spectra of the energy gaps are 3.22, 2.34, and 2.04 eV for complexes 1–3,
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Eur. J. Inorg. Chem. 2009, 2407–2414

Tuning the Emission Color of Iridium(III) Complexes
respectively, which is consistent with the results from the Table 2. Main parameters of complexes 1–3 in the ground state (S0)
optimized at the B3LYP level, together with experimental values
for FIr(dbm).
maximum emission wavelength. The effect of the ancillary
ligands on changing the energy gaps of the complexes is
therefore verified by the electrochemical measurements.
FIr(acac)
FIr(dbm) FIr(natfac) FIr(dbm)exp
Bond lengths [Å]
Ir–C1
Table 1. Electrochemical and photophysical properties of Ir com-
plexes 1–3.
2.002 (1.998),[a]
(2.002)[b]
2.002
2.065
2.001
2.066
1.974
2.033
[a]
EOx
HOMO ERed
LUMO Energy gap λmax,Em T1
onset
onset
Ir–N1
2.062 (2.031),[a]
(2.062)[b]
[V]
[eV]
[V]
[eV]
[eV]
[nm]
[eV]
Ir–C2
Ir–N2
Ir–O1
2.002 (1.998)[a]
2.062 (2.031)[a]
2.194 (2.147),[a]
(2.190)[b]
2.003
2.061
2.186
2.001
2.067
2.194
2.005
2.027
2.149
1
2
3
0.82
0.32
0.41
–5.42
–4.92
–5.01
–2.40
–2.02
–1.63
–2.20
–2.58
–2.97
3.22
2.34
2.04
482
557
586
2.57
2.23
2.11
[a] The triplet energy was estimated from λmax of the phosphores-
cence spectra.
Ir–O2
2.194 (2.147)[a]
2.186
2.205
2.121
Bond angles [°]
O1–Ir–O2
C1–Ir–C2
C3–C4–C5
86.4
93.5
127.3
86.1
92.6
127.5
86.0
92.7
125.9
89.1
90.4
128.1
Geometries and Orbital Configurations
Dihedral angles [°]
C6–N1–Ir–O1
To provide a quantum chemical insight into these com-
plexes, the DFT method was employed to investigate their
ground-state electronic structures and orbital configura-
tions. The optimized ground-state structures of FIr(acac),
FIr(dbm), and FIr(natfac) are shown in Figure 4 and se-
lected geometrical parameters are listed in Table 2, together
with available experimental X-ray diffraction data for
FIr(dbm). The common feature in all complexes is that the
ancillary ligands bind to the metal center through two Ir–
O bonds despite having different substituents on atoms C3
and C5. The calculated bond lengths are in good agreement
with available crystal structural data (standard deviation of
about 0.0038 Å)[20] and are comparable with previously cal-
culated results.[16,21]
–85.9
–87.2
–87.1
–90.8
[a] Values in parentheses are from ref.[16]. [b] Values in parentheses
are from ref.[21]
.
the naphthyl group can clearly be seen in the contour plots
for the HOMO-1 of FIr(natfac) in Figure 6. The asymmet-
ric substitution by trifluoromethyl and naphthyl groups in
this complex makes Ir–O1 (2.194 Å) and Ir–O2 (2.205 Å)
distinguishable. The calculated bond angles and dihedral
angles also show only a slight change of about 1°. It is
therefore clear that the ancillary ligands do not significantly
alter the geometries, which is a reflection of the similarities
between the ancillary ligands.
The frontier molecular orbitals of the ground state (S0)
are very important because they are closely related to the
electronic transitions pertaining to the absorption and emis-
sion processes. By analyzing the frontier molecular orbitals,
we can therefore gain a better insight into the electronic
structures of the complexes. The HOMO and LUMO en-
ergy levels of dfppy, acac, dbm, and natfac are shown in
Figure 5. As we can see, the HOMO levels of the three an-
cillary ligands and the cyclometalating ligand dfppy are
similar in energy, whereas the LUMO energies of the ancil-
lary ligands are significantly influenced by the substitution
Figure 4. Optimized geometries of FIr(acac), FIr(dbm), and FIr-
(natfac) with two N atoms from dfppy in the trans positions (satu-
rated H atoms are not shown).
The calculated metal–ligand Ir–C (about 2.002 Å) and effect. Thus, the LUMO energies of dbm and natfac are
Ir–N (about 2.064 Å) bond lengths in all three complexes dramatically lowered to –2.00 and –2.48 eV, respectively,
are almost the same, whereas the Ir–O bond lengths vary which means that the energy gaps of the ancillary ligands
slightly as the ancillary ligands change. The phenyl group range from 5.35 to 3.78 eV. The electronic effects produced
in dbm exhibits a stronger conjugating effect on the C=O upon incorporation of electron-donating and electron-with-
bond than the methyl group in acac, thus leading to higher drawing groups into the ligand structures therefore have a
electron density on the oxygen atom and a shorter Ir–O remarkable influence on the orbital energies, which in turn
bond. The substituent effect of a phenyl group on the ligand results in different photophysical behaviors of their metal
has been observed previously.[22] In the case of FIr(natfac), complexes.
however, the electron-withdrawing CF3 group in natfac re-
The frontier molecular orbital energies for the three com-
moves electron density from the metal and the naphthyl plexes, along with their composition and main bonding
group delocalizes the electron density effectively, which types, are listed in Table 3, and the contour plots of the two
weakens the interaction between the central Ir atom and highest HOMOs and two lowest LUMOs of the Ir com-
the natfac ligand and means that the Ir–O bond is length- plexes are shown in Figure 6. The HOMOs of the com-
ened. The effect of the strong π-electron delocalization of plexes are mainly formed from the metallic “t2g” d orbitals
Eur. J. Inorg. Chem. 2009, 2407–2414
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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X. Gu, T. Fei, H. Zhang, H. Xu, B. Yang, Y. Ma, X. Liu
FULL PAPER
Figure 5. Comparison of the HOMO and LUMO energy levels and
the energy gaps in ligands dfppy, acac, dbm, and natfac.
Figure 6. Contour plots of the frontier orbitals of complexes 1–3
in the ground state.
and the π orbitals of the ligand dfppy, with d components
of 52.2%, 51.4%, and 50.8% for FIr(acac), FIr(dbm), and
FIrntfac, respectively. Because of the lower HOMO level
of ligand dfppy, the HOMO energies of the complexes are
determined by the percentage dfppy orbital composition.
possess different π* character. The LUMO and LUMO+1
of FIr(acac) are degenerate orbitals which are mainly lo-
cated on both the phenyl and pyridine moieties of the dfppy
ligands, whereas the LUMOs of FIr(dbm) and FIr(natfac)
are located on the ancillary ligands dbm and natfac, respec-
tively, probably due to the lower LUMO energy of dbm and
natfac. The LUMO energy levels of FIr(dbm) and FIr(nat-
fac) decrease by 0.21 and 0.44 eV, respectively, with respect
to that of FIr(acac). The trend in the calculated energy gaps
is in good agreement with the values obtained electrochemi-
cally.
Table 3. Molecular orbital energy and composition in the ground
state for complexes 1–3 at the B3LYP level.
Orbital[a] Energy MO composition (%)
Main bonding type
∧
[eV]
Ir
C N
LX
1
2
3
L+2
L+1
L
–0.93
–1.48
–1.48
–5.25
–5.58
–6.04
–6.05
–1.46
–1.49
–1.69
–5.27
–5.56
–6.03
–6.05
–1.60
–1.63
–1.92
–5.44
–5.84
2.03
5.28
4.88
52.2
38.9
97.1
93.1
93.2
41.6
11.1
0.885
1.63
1.87
6.22
50.0
5.71
4.13
4.73
1.78
89.5
8.00
56.2
9.31
7.40
3.64
2.40
91.6
6.32
55.9
π*(dfppy)
π*(dfppy)
π*(dfppy)
d(Ir) + π(dfppy)
d(Ir) + π(acac)
π(dfppy)
d(Ir) + π(dfppy)
π*(dfppy)
H
H-1
H-2
H-3
L+2
L+1
L
0.769 93.5
36.1
5.33
5.25
2.22
51.4
33.8
27.1
24.5
5.24
5.31
2.04
50.8
32.4
59.7
89.9
93.0
8.31
40.6
10.1
63.6
68.1
91.1
92.3
6.34
42.9
11.7
π*(dfppy)
π*(dbm)
H
d(Ir) + π(dfppy)
π(dbm) + d(Ir)
π(dfppy) + d(Ir)
π(dfppy) + d(Ir)
π*(dfppy)
π*(dfppy)
π*(naftac)
d(Ir) + π(dfppy)
π(natfac) + d(Ir)
π(natfac) + d(Ir) +
π(dfppy)
H-1
H-2
H-3
L+2
L+1
L
H
H-1
Figure 7. Frontier orbital energy levels and energy gaps of com-
plexes 1–3.
H-2
H-3
–6.04
–6.19
24.9
13.5
21.0
78.2
54.1
8.31
π(dfppy) + d(Ir)
TD-DFT Calculations
[a] L = LUMO; H = HOMO.
TD-DFT calculations were employed to better under-
Figure 7 shows the frontier orbital energy levels and en- stand the nature of the excited states and examine the verti-
ergy gaps for the complexes. The HOMO of FIr(natfac) has cal excitation energies for the allowed transitions between
the lowest energy (–5.44 eV) with relatively less d compo- the low-lying excited states. The calculated excited energies,
nent and more contribution from dfppy. For the HOMO-1, dominant orbital excitation, and oscillator strengths (f) for
the contributions are mainly from the metal d orbitals and all three complexes are collected in Table 4; Figure 8 dis-
π orbitals of ancillary ligands. FIr(natfac) has a lower plays the main transitions contributing to the absorptions.
HOMO-1 energy level than that of FIr(acac) and FIr(dbm), The lowest transition-allowed singlet excited state with non-
which may be due to electron-withdrawing effects and π- zero oscillator strength for FIr(acac), FIr(dbm), and FIr-
electron delocalization of the naphthyl group in FIr(natfac), (natfac) is ascribed to the HOMO to LUMO+1 transition,
as discussed above. However, the LUMOs of the complexes with an absorption wavelength of 421, 419, and 413 nm,
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Tuning the Emission Color of Iridium(III) Complexes
Table 4. Calculated excited energies, dominant orbital excitation, and oscillator strength (f) from TD-DFT calculations for FIr(acac),
FIr(dbm), and FIr(natfac).
[a]
State
Excitation
Ecal [eV]
λcal [nm]
f
λexp [nm] Character
FIr(acac)
S1
S3
S6
S8
S9
FIr(dbm)
S2
S4
S5
S6
S7
S8
S10
H Ǟ L+1(0.66)
H-1 Ǟ L(0.67)
H Ǟ L+2(0.66)
H-2 Ǟ L+1(0.61)
H-2 Ǟ L(0.45)
2.94
3.32
3.56
3.74
3.78
421
373
348
331
328
0.0339
0.0343
0.0181
0.0686
0.0125
390
390
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
acac/Ir Ǟ dfppy(LLCT/MLCT)
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
dfppy Ǟ dfppy(ILCT)
dfppy Ǟ dfppy(ILCT)
H Ǟ L+1(0.64)
H-1 Ǟ L(0.65)
H-1 Ǟ L+1(0.64)
H-1 Ǟ L+2(0.63)
H Ǟ L+3(0.69)
H-2 Ǟ L(0.61)
H-2 Ǟ L+1(0.57)
2.83
3.22
3.32
3.37
3.58
3.62
3.72
419
385
373
368
346
343
333
0.0298
0.0749
0.0128
0.0105
0.0147
0.0992
0.0536
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
dbm/Ir Ǟ dbm(ILCT/MLCT)
dbm/Ir Ǟ dfppy(LLCT/MLCT)
dbm/Ir Ǟ dfppy(LLCT/MLCT)
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
dfppy/Ir Ǟ dbm(LLCT/MLCT)
dfppy/Ir Ǟ dfppy(ILCT/MLCT)
FIr(natfac)
S2
S4
S5
S6
S7
S8
S9
H Ǟ L+1(0.61)
H-1 Ǟ L(0.62)
H-2 Ǟ L(0.57)
H-1 Ǟ L+1(0.57)
H-1 Ǟ L+2(0.55)
H-4 Ǟ L(0.42)
H Ǟ L+3(0.66)
3.00
3.21
3.43
3.46
3.53
3.63
3.64
413
386
361
358
351
341
340
0.0297
0.0665
0.1956
0.0309
0.0096
0.0127
0.0171
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
natfac/Ir Ǟ natfac(ILCT/MLCT)
natfac/Ir Ǟ natfac(ILCT/MLCT)
natfac/Ir Ǟ dfppy(LLCT/MLCT)
natfac/Ir Ǟ dfppy(LLCT/MLCT)
dfppy/Ir Ǟ dfppy(ILCT/MLCT)
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
[a] Experimental data of this work as can be seen from Figure 2.
respectively. The calculated absorption wavelengths are in
For the emission properties, we should point out that the
good agreement with the experimental measurement present calculations are based on the ground-state geometry
(around 390 nm), with a deviation of about 30 nm. The assuming that there are no significant geometry changes be-
reason for the difference between the experimental and cal- tween the excited state and ground state. The emission pro-
culated values is that our calculation is based on the gas- files are shown in Figure 9. The lowest triplet excited ener-
phase structure and ignores solvent effects. The transition gies of the three complexes are 461, 505, and 513 nm respec-
characteristics for the absorption are ascribed to a mixed tively (see Table 5). In FIr(acac), the transition at 461 nm is
metal-to-ligand charge transfer (MLCT) and intra-ligand attributed to a HOMO Ǟ LUMO+1 transition, which is
charge transfer (ILCT) state, as can be seen from the com- consistent with the lowest singlet excitation, whereas the
positions of the HOMO and LUMO+1 orbitals for the transitions for FIr(dbm) and FIr(natfac) are ascribed to
∧
three complexes. The transition occurs from the d orbitals HOMO-1 Ǟ LUMO transitions with a [d(Ir) + π(L X) Ǟ
∧
of Ir and the π orbitals of dfppy to the π* orbitals of dfppy. π*(L X)] character. We can therefore conclude that
The ancillary ligands are involved in the transitions at FIr(dbm) and FIr(natfac) have similar emission properties.
higher energy states, such as S3 for FIr(acac).
Figure 8. Transitions contributing to the absorptions at 421, 419,
and 413 nm for FIr(acac), FIr(dbm), and FIr(natfac), respectively.
Figure 9. Transitions contributing to the emissions at 461, 505, and
513 nm for FIr(acac), FIr(dbm), and FIr(natfac), respectively.
Eur. J. Inorg. Chem. 2009, 2407–2414
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X. Gu, T. Fei, H. Zhang, H. Xu, B. Yang, Y. Ma, X. Liu
FULL PAPER
Table 5. Calculated excited energies and dominant orbital excitation from TD-DFT calculations for complexes FIr(acac), FIr(dbm), and
FIr(natfac).
[a]
State
Excitation
Ecal [eV]
λcal [nm]
λexp [nm]
Character
FIr(acac)
T1
T2
T3
H Ǟ L+1(0.64)
H Ǟ L(0.64)
H-1 Ǟ L+3(0.66)
2.69
2.70
2.96
461
459
418
482
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
acac/Ir Ǟ acac(ILCT/MLCT)
FIr(dbm)
T1
T2
T3
H-1 Ǟ L(0.69)
H Ǟ L+1(0.49)
H Ǟ L(0.37)
2.45
2.71
2.76
505
457
450
557
586
dbm/Ir Ǟ dbm(ILCT/MLCT)
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
Ir/dfppy Ǟ dfppy(MLCT/ILCT)
FIr(natfac)
T1
T2
T3
H-1 Ǟ L(0.65)
H-2 Ǟ L(0.52)
H Ǟ L(0.57)
2.42
2.59
2.69
513
478
462
natfac/Ir Ǟ natfac(ILCT/MLCT)
natfac/Ir Ǟ natfac(ILCT/MLCT)
dfppy/Ir Ǟ dfppy(ILCT/MLCT)
[a] Experimental data of this work as can be seen from Figure 3 and Table 1.
Experimentally, the efficiencies of FIr(dbm) and FIr(natfac) tations calculated in this study are MLCT and ILCT in na-
are so poor that they could not be measured. For FIr(acac), ture. The properties of the complexes are affected by both
∧
∧
however, the phosphorescence efficiency in the film is the C N and L X ligands. Although the performance of
75%.[23] These results can be rationalized by considering these IrIII complexes is far from good, it is possible that
that inefficient radiation transition might occur during the tuning the emission color can be achieved by systematically
ILCT processes. All the ILCTs in dfppy occur during the employing various distinctive ancillary ligands.
absorption process. During the emission process, although
ILCTs occur in dmb and natfac, these ligands experience
structural relaxation and redistribution of electron densities
Experimental Section
and hence energy will dissipate during system crossing from
General Procedures for Synthesis of Iridium Complexes: The cyclo-
singlet to triplet excited states through the vibrational mo-
metalated chloride-bridged dimer [(dfppy)2Ir(µ-Cl)2Ir(dfppy)2] was
tions, thus leading to radiationless transitions.
synthesized according to the procedure described in the literature
by refluxing IrCl3·nH2O with 2–2.5 equiv. of ligand in a 3:1 mixture
of 2-ethoxyethanol and water. The chloride-bridged dimer complex
(0.04 mmol), 0.12 mmol of acetyl acetone (dibenzoylmethanate, 1-
The calculated emission wavelengths are obviously hyp-
sochromic with respect to the experimental measurements
in films. There are two possible reasons for this. First of all,
our calculations for the emission are based on the assump- naphthoyl-3,3,3-trifluoroacetone), and 0.36 mmol of sodium car-
bonate were then refluxed in 2-ethoxythanol for 15 h under nitro-
gen. After cooling to room temperature, the resulting precipitate
was filtered off and washed with water and diethyl ether. The crude
product was subjected to column chromatography on silica gel,
eluting with dichloromethane, to provide the desired product in
quantitative yield.
tion that the geometry does not change much between
ground and excited states. It is well accepted that excited-
state structures are quite difficult to optimize for metal
complexes. Previous calculations based on this hypothesis
were adopted to simulate the emission properties.[18a,24]
Furthermore, solvent effects should be taken into ac-
count.[25] For high accuracy, relativity effects, including
spin-orbital coupling for the metal complexes, must be care-
fully considered.[26]
FIr(acac): 1H NMR (500 MHz, [D6]DMSO): δ = 8.43 (d, J =
5.6 Hz, 2 H), 8.23 (d, J = 8.4 Hz, 2 H), 8.07 (t, J = 8.0 Hz, 2 H),
7.50 (t, J = 7.1 Hz, 2 H), 6.69 (m, 2 H), 5.51 (dd, J = 8.7, 2.2 Hz,
2 H), 5.31 (s, 1 H), 1.76 (s, 6 H) ppm. IR (KBr): ν = 3068 (m),
˜
1604 (s), 1574 (s), 1516 (m), 1479 (m), 1403 (s), 1291 (m), 1162 (m),
1102 (m) cm–1. MALDI-TOF-MS: m/z 672.7 [M
C27H19F4IrN2O2 (671.67) calcd. C 48.28, H 2.85, N 4.17; found C
48.36, H 2.77, N 4.33.
+
H]+.
Conclusions
In summary, this work has demonstrated a simple
method for tuning the emission color of metal complexes
by changing the structures of the ancillary ligands. Further-
more, we have shown how the nature of substituents with
electron-donating and electron-withdrawing groups can sig-
nificantly influence the photophysical properties of
FIr(dbm): 1H NMR (500 MHz, [D6]DMSO): δ = 8.49 (d, J =
5.6 Hz, 2 H), 8.26 (d, J = 8.4 Hz, 2 H), 8.05 (t, J = 7.7 Hz, 2 H),
7.81 (t, J = 7.6 Hz, 4 H), 7.49 (m, 4 H), 7.39 (t, J = 7.6 Hz, 4 H),
6.76 (t, J = 10.1 Hz, 2 H), 6.70 (s, 1 H), 6.65 (dd, J = 8.6, 1.9 Hz,
2 H) ppm. IR (KBr): ν = 3059 (m), 1064 (s), 1542 (s), 1521 (s),
˜
1479 (s), 1404 (s), 1292 (m), 1163 (m), 1101 (m) cm–1. MALDI-
TOF-MS: m/z 796.8 [M + H]+. C37H23F4IrN2O2 (795.82) calcd. C
55.84, H 2.91, N 3.52; found C 55.98, H 2.93, N 3.50.
∧
[Ir(dfppy)2(L X)] complexes. By combining experimental
results with DFT calculations, we have determined the de-
tailed electronic structures and complicated absorption and
emission process involved. For the emission, the transition
FIr(natfac): 1H NMR (500 MHz, [D6]DMSO): δ = 8.62 (s, 1 H),
8.44 (dd, J = 13.1, 5.0 Hz, 2 H), 8.29 (m, 2 H), 8.08 (m, 3 H), 7.92
(m, 2 H), 7.71 (dd, J = 8.6, 1.7 Hz, 1 H),7.65 (t, J = 7.6 Hz, 1 H),
7.58 (m, 2 H), 7.51 (t, J = 6.7 Hz, 1 H), 6.82 (m, 2 H), 6.66 (s, 1
∧
∧
of FIr(acac) is [d(Ir) + π (C N) Ǟ π*(C N)], whereas for
∧
FIr(dbm) and FIr(natfac) it is [d(Ir) + π (L X) Ǟ π*
(L X)]. We have also shown that all of the low-lying exci- H), 5.66 (dd, J = 8.6, 2.3 Hz, 1 H), 5.56 (dd, J = 8.6, 2.3 Hz, 1 H)
∧
2412
www.eurjic.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 2407–2414

Tuning the Emission Color of Iridium(III) Complexes
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ppm. IR (KBr): ν = 3062 (m), 1604 (s), 1574 (s), 1479 (m), 1404
˜
(m), 1296 (s), 1200 (m), 1162 (m), 1103 (m) cm–1. MALDI-TOF-
MS: m/z 838.7 [M + H]+. C36H20F7IrN2O2 (837.78): calcd. C 51.61,
H 2.41, N 3.34; found C 51.74, H 2.59, N 3.45.
Physical Measurements: UV/Vis absorption spectra were recorded
with a UV-3100 spectrophotometer. Photoluminescence measure-
ments were performed with a RF-5301PC fluorometer. The electro-
chemical properties of these complexes were studied by cyclic vol-
tammetry (CV) with ferrocene as the internal standard and an ace-
tonitrile solution of the complexes containing 0.1  tetrabutylam-
monium hexafluorophosphate as the supporting electrolyte at a
scan rate of 100 mVs–1. In all cases, potentials refer to the system
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couple. The HOMO and LUMO energy levels were determined
using the following formula: EHOMO = –(EOx onset + 4.60) eV,
ELUMO = –(ERed onset + 4.60) eV.
[6]
Computational Details: DFT calculations were performed with the
B3LYP[27] functional and the mixed “Double-ζ” quality basis sets
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N atoms from the dfppy ligands at trans positions, were considered.
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Acknowledgments
We thank the National Nature Science Foundation of China (grant
numbers 20573040, 20474024, 20603013, 20704015, 90501001, and
50303007), the Ministry of Science and Technology of China (grant
number 2002CB6134003), and the Program for Changjiang Schol-
ars and Innovative Research Team in University (PCSIRT) for fin-
ancial support. Computational resources from in the Institute of
Theoretical Chemistry at Jilin University are highly appreciated.
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[20] Standard deviations were calculated using the formula
where xi is the calculated result for each parameter, xexp is the
experimental value for the same parameter as xi, and N is the
total number of parameters selected for the calculations.
[21] N. G. Park, G. C. Choi, Y. H. Lee, Y. S. Kim, Curr. Appl. Phys.
2006, 6, 620–626.
[22] C. Dragonetti, L. Falciola, P. Mussini, S. Righetto, D. Roberto,
R. Ugo, A. Valore, Inorg. Chem. 2007, 46, 8533–8547.
[23] FIr(acac) was dispersed in the inert host poly(methyl methacry-
late) at 8.0 wt.-% and ηPL for the film was measured by an
Received: December 7, 2008
Published Online: April 28, 2009
2414
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Eur. J. Inorg. Chem. 2009, 2407–2414