Control of intramolecular π-π stacking interaction in cationic iridium complexes via fluorination of pendant phenyl rings

Article abstract of DOI:10.1021/ic2021325

Intramolecular π-π stacking interaction in one kind of phosphorescent cationic iridium complexes has been controlled through fluorination of the pendant phenyl rings on the ancillary ligands. Two blue-green-emitting cationic iridium complexes, [Ir(ppy)₂(F2phpzpy)]PF₆ (2) and [Ir(ppy)₂(F5phpzpy)]PF₆ (3), with the pendant phenyl rings on the ancillary ligands substituted with two and five fluorine atoms, respectively, have been synthesized and compared to the parent complex, [Ir(ppy)₂(phpzpy)]PF₆ (1). Here Hppy is 2-phenylpyridine, F2phpzpy is 2-(1-(3,5-difluorophenyl)-1H-pyrazol-3-yl)pyridine, F5phpzpy is 2-(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine, and phpzpy is 2-(1-phenyl-1H-pyrazol-3-yl)pyridine. Single crystal structures reveal that the pendant phenyl rings on the ancillary ligands stack to the phenyl rings of the ppy ligands, with dihedral angles of 21°, 18°, and 5.0° between least-squares planes for complexes 1, 2, and 3, respectively, and centroid-centroid distances of 3.75, 3.65, and 3.52 A for complexes 1, 2, and 3, respectively, indicating progressively reinforced intramolecular π-π stacking interactions from complexes 1 to 2 and 3. Compared to complex 1, complex 3 with a significantly reinforced intramolecular face-to-face π-π stacking interaction exhibits a significantly enhanced (by 1 order of magnitude) photoluminescent efficiency in solution. Theoretical calculations reveal that in complex 3 it is unfavorable in energy for the pentafluorophenyl ring to swing by a large degree and the intramolecular π-π stacking interaction remains on the lowest triplet state.

Full text of DOI:10.1021/ic2021325

Article
pubs.acs.org/IC
Control of Intramolecular π−π Stacking Interaction in Cationic
Iridium Complexes via Fluorination of Pendant Phenyl Rings
Lei He, Dongxin Ma, Lian Duan,* Yongge Wei, Juan Qiao, Deqiang Zhang, Guifang Dong, Liduo Wang,
and Yong Qiu*
Key Laboratory of Organic Optoelectronics and Molecular Engineering, Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing 100084, P.R. China
S
* Supporting Information
ABSTRACT: Intramolecular π−π stacking interaction in one
kind of phosphorescent cationic iridium complexes has been
controlled through fluorination of the pendant phenyl rings on
the ancillary ligands. Two blue-green-emitting cationic iridium
complexes, [Ir(ppy)2(F2phpzpy)]PF6 (2) and [Ir-
(ppy)2(F5phpzpy)]PF6 (3), with the pendant phenyl rings
on the ancillary ligands substituted with two and five fluorine
atoms, respectively, have been synthesized and compared to
the parent complex, [Ir(ppy)2(phpzpy)]PF6 (1). Here Hppy is
2-phenylpyridine, F2phpzpy is 2-(1-(3,5-difluorophenyl)-1H-
pyrazol-3-yl)pyridine, F5phpzpy is 2-(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine, and phpzpy is 2-(1-phenyl-1H-pyrazol-3-
yl)pyridine. Single crystal structures reveal that the pendant phenyl rings on the ancillary ligands stack to the phenyl rings of the
ppy ligands, with dihedral angles of 21°, 18°, and 5.0° between least-squares planes for complexes 1, 2, and 3, respectively, and
centroid-centroid distances of 3.75, 3.65, and 3.52 Å for complexes 1, 2, and 3, respectively, indicating progressively reinforced
intramolecular π−π stacking interactions from complexes 1 to 2 and 3. Compared to complex 1, complex 3 with a significantly
reinforced intramolecular face-to-face π−π stacking interaction exhibits a significantly enhanced (by 1 order of magnitude)
photoluminescent efficiency in solution. Theoretical calculations reveal that in complex 3 it is unfavorable in energy for the
pentafluorophenyl ring to swing by a large degree and the intramolecular π−π stacking interaction remains on the lowest triplet
state.
However, for these complexes, introduction of the intra-
molecular π−π stacking interaction generally results in large
configurational distortions of the bpy ancillary ligands, which
significantly decreases the photoluminescent (PL) efficiencies
of the complexes.18,24
INTRODUCTION
■
Phosphorescent cationic iridium complexes have been widely
used in biological-labeling, oxygen or ion-sensing, and organic
electroluminescent devices, including light-emitting electro-
chemical cells1−4 and organic light-emitting diodes.5−7 Typical
phosphorescent cationic iridium complexes use 2-phenyl-
pyridine (Hppy) as the cyclometalated ligands and 2,2′-
bipyridine (bpy) as the ancillary ligands, and give orange-red
light emission.3,4 To enhance the luminescent efficiency and
tune the emission color, various cationic iridium complexes
have been developed with tailored ligands.4,8−15 Through
attaching phenyl rings at the 6 positions of the bpy ancillary
ligands, intramolecular π−π stacking interactions have been
introduced into the complexes.16−21,47 Within the complexes,
the attached pendant phenyl rings stack to the phenyl rings of
the ppy ligands. For phenyl−phenyl π−π stacking interactions,
generally there exist three stacking patterns: face-to-face
(sandwich), edge-to-face (T-shaped), and offset face-to-face
(parallel-displaced).22,23 The latter two are stable in energy,
while the face-to-face stacking is unstable and rarely observed
because of the repulsion between the two stacking phenyl rings.
For reported cationic iridium complexes, the introduced
intramolecular phenyl−phenyl π−π stacking generally adopts
an energetically favorable offset face-to-face pattern.18,20
Recently, we reported a blue-green-emitting cationic iridium
complex, [Ir(ppy)2(phpzpy)]PF6 (complex 1, Scheme 1), with
2-(1-phenyl-1H-pyrazol-3-yl) pyridine (phpzpy) as the ancillary
ligand.25 Because of the pyrazole-type ancillary ligand used,9,25
complex 1 shows an enlarged energy gap and significantly blue-
shifted emission compared to the typical complex [Ir-
(ppy)2(bpy)]PF6. The pendant phenyl ring on phpzpy stacks
to the phenyl ring of the ppy ligand, with a dihedral angle of
21° between the least-squares planes and a centroid-centroid
distance of 3.75 Å.25 The phenyl−phenyl stacking in complex 1
is far from parallel and shows a relatively large centroid-centroid
distance, largely because of the repulsion between the two
stacking phenyl rings.22 In the present work, we aim to control
the intramolecular π−π stacking interaction in the archetype
complex 1. The approach is to fluorinate the pendant phenyl
ring. The basis for this molecular design is that quadrupole
Received: October 2, 2011
Published: March 30, 2012
© 2012 American Chemical Society
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Scheme 1. Molecular Structures of Complexes 1−3
(Φp = 0.545 in 1 M H2SO4).32 The PLQYs in thin films were
measured with an integrating sphere on a fluorospectrophotometer
(Jobin Yvon, FluoroMax-3) according to a reported procedure.33
Synthesis. Synthesis of 2-(1-(3,5-Difluorophenyl)-1H-pyrazol-
3-yl)pyridine (F2phpzpy). 2-(1H-pyrazol-3-yl)pyridine (0.58 g, 4
mmol), 1,3,5-trifluorobenzene (2.6 g, 19 mmol) and K2CO3
(1.1 g, 8 mmol) were dissolved in dimethylsulfoxide (20 mL).
The mixture was refluxed at 110 °C for 5 h, cooled to room
temperature, and extracted with CH2Cl2 (100 mL). The organic
layer was separated, washed with brine, dried over Na2SO4, and
purified by column chromatography on silica gel (200−300
mesh) with petroleum ether/ethyl acetate (15:1) as the eluent,
yielding a white solid (0.85 g, 3.3 mmol). Yield: 83%. 1H NMR
(chloroform-d6, 600 MHz, δ[ppm]): 8.66(d, J = 4.1 Hz, 1H),
8.12(d, J = 8.2 Hz, 1H), 7.97(d, J = 2.8 Hz, 1H), 7.78(td, J =
7.6 and 2.0 Hz, 1H), 7.41−7.35 (m, 2H),7.30−7.27(m, 1H),
moments of benzene and hexafluorobenzene are opposite in
sign because of the electronegativity of fluorine atoms.26
Benzene dimers generally stack with each other in an offset
face-to-face or edge-to-face manner to minimize the repulsion
between the two stacking benzenes,27 while in the crystal of 1:1
mixture of benzene and hexafluorobenzene, benzene and
hexafluorobenzene stacks alternately to each other in a parallel
face-to-face manner rather than an offset face-to-face or edge-
to-face manner, because of the strong electrostatic attraction
between benzene and hexafluorobenzene.27,28 By taking
advantage of this ordered parallel stacking, fluorine-substituted
aryls have been widely used as motifs in supramolecular
chemistry29 and organic semiconductors.30,31
To reinforce the intramolecular π−π stacking interaction, the
pendant phenyl ring in complex 1 is substituted with fluorine
atoms, yielding complexes 2 and 3 (Scheme 1), of which the
pendant phenyl rings are substituted with two and five fluorine
atoms, respectively. Single crystal structures reveal that, upon
fluorination, the pendant phenyl rings stack more parallel and
closer to the phenyl rings of the ppy ligands, with complex 3
exhibiting a significantly reinforced intramolecular π−π stacking
interaction. In complex 3, the pentafluorophenyl−phenyl
stacking fits better a face-to-face pattern rather than an offset
face-to-face pattern. Moreover, introduction of the intra-
molecular π−π stacking interaction in complex 3 results in
little distortion of the ligands. Compared to complex 1,
complex 3 shows a significantly enhanced (by 1 order of
magnitude) PL efficiency in solution.
7.17(d, J = 2.8 Hz, 1H), 6.75(tt, J = 8.6 and 2.4 Hz, 1H). 13C
1
NMR (chloroform-d6, 150 MHz, δ[ppm]): 163.62(dd, JCF
=
247.0 Hz, 3JCF = 14.4 Hz), 154.02, 151.47, 149.55, 142.00(t, 3JCF
2
= 12.9 Hz), 136.78, 128.29, 123.18, 120.54, 107.52, (dd, JCF
=
23.0 Hz, 4JCF = 7.2 Hz), 101.67(t, 2JCF = 25.1 Hz). ESI-MS [m/
z]: 258.1 [M+H]+.
Synthesis of 2-(1-Pentafluorophenyl-1H-pyrazol-3-yl)-pyridine
(F5phpzpy). 2-(1H-pyrazol-3-yl)pyridine (0.7 g, 4.8 mmol), perfluor-
obenzene (8.1 g, 43 mmol), and K2CO3 (1.4 g, 10 mmol) were
dissolved in dimethylsulfoxide (30 mL). The mixture was refluxed at
110 °C for 6 h, cooled to room temperature, and extracted with
CH2Cl2 (100 mL). The organic layer was separated, washed with
brine, dried over Na2SO4, and purified by column chromatography on
silica gel (200−300 mesh) with petroleum ether/ethyl acetate (15:1)
as the eluent, yielding a white solid (0.51 g, 1.6 mmol). Yield: 33%. 1H
NMR (acetone-d6, 600 MHz, δ[ppm]): 8.65(d, J = 4.8 Hz, 1H),
8.14(s, 1H), 8.07(d, J = 8.2 Hz, 1H), 7.87(td, J = 7.6 and 1.4 Hz, 1H),
7.37(td, J = 6.3 and 1.1 Hz, 1H), 7.23(d, J = 2.7 Hz, 1H). 13C NMR
(acetone-d6, 150 MHz, δ[ppm]): 155.04, 151.33, 149.58, 144.20−
143.97(m), 142.50−142.10(m), 140.70−140.40(m), 139.13−
138.84(m), 137.50−137.20(m), 136.71, 134.70, 123.33, 116.82−
116.55(m), 119.90, 106.50. ESI-MS [m/z]: 312.13 [M+H]+.
Theoretical calculations revealed that, in complex 3, it is
unfavorable in energy for the pendant pentafluorophenyl ring
to swing by a large degree. While in complexes 1 and 2, the
pendant phenyl or 3,5-difluorophenyl rings can swing by a
relatively larger degree. For complex 3, the intramolecular π−π
stacking interaction remains on the lowest triplet state. Among
the three complexes, complex 3 exhibits the smallest structural
deviations between the ground and the lowest triplet states.
Synthesis of [Ir(ppy)2(F2phpzpy)]PF6 (Complex 2). The dichloro-
bridged diiridium complex [Ir(ppy)2Cl]2 (0.46 g, 0.43 mmol) and
F2phpzpy (0.23 g, 0.9 mmol) were suspended in ethane-1,2-diol (20
mL). The mixture was refluxed at 130 °C for 13 h under Ar
atmosphere, cooled to room temperature and diluted with deionized
water (100 mL). To the solution, NH4PF6 (1.4 g, 8.6 mmol) in
deionized water (20 mL) was slowly added under stirring, resulting in
a yellow suspension. The suspension was filtered and the precipitate
was dried under vacuum at 70 °C for 5 h. The crude product was
purified by column chromatography on silica gel (200−300 mesh)
with CH2Cl2/acetone (50:1) as the eluent, yielding a yellow solid
EXPERIMENTAL SECTION
■
General Procedures. All reactants and solvents were purchased
from commercial sources and, unless otherwise stated, used as
received. Mass spectrometry was performed with a Thermo Electron
Corporation Finnigan LTQ mass spectrometer. NMR spectra were
recorded on a JEOL JNM-ECA600 NMR spectrometer. Elemental
analysis was determined with an Elementar Vario EL CHN elemental
analyzer. Absorption spectra were recorded with an Agilent 8453 UV−
vis spectrophotometer. PL spectra were recorded with a fluorospec-
trophotometer (Jobin Yvon, FluoroMax-3). The PL transient lifetimes
were measured on a transient spectrofluorimeter (Edinburgh Instru-
ments, FLSP920). The photoluminescent quantum yields (PLQYs)
were measured in degassed CH3CN solution versus quinine sulfate
1
(0.52 g, 0.55 mmol). Yield: 64%. H NMR (acetone-d6, 600 MHz,
δ[ppm]): 8.55(d, J = 8.2 Hz, 1H), 8.29(d, J = 2.8 Hz, 1H), 8.22(td, J =
7.9 and 2.1 Hz, 1H), 8.19(d, J = 8.2 Hz, 1H), 8.16(d, J = 8.3 Hz, 1H),
8.14(d, J = 5.5 Hz, 1H), 8.04(td, J = 8.0 and 1.4 Hz, 1H), 7.99−
7.92(m, 2H), 7.88(d, J = 7.6 Hz, 1H), 7.77(d, J = 7.9 Hz, 1H), 7.68(d,
J = 2,8 Hz, 1H), 7.60(d, J = 7.6 Hz, 1H), 7.54(td, J = 6.5 and 1.4 Hz,
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Scheme 2. Synthesis of the Ancillary Ligands and Complexes 2 and 3
1H), 7.29(td, J = 6.5 and 1.3 Hz, 1H), 7.23(td, J = 6.5 and 1.3 Hz,
1H), 6.97(t, J = 7.6 Hz, 1H), 6.85(t, J = 7.6 Hz, 1H), 6.82−6.76(m,
3H), 6.69(t, J = 7.6 Hz, 1H), 6.52(t, J = 8.2 Hz, 1H), 6.14(d, J = 6.8
Hz, 1H), 5.91(d, J = 7.6 Hz, 1H). 13C NMR (acetone-d6, 150 MHz,
states (T1) were optimized at the spin-unrestricted B3LYP level with a
spin multiplicity of 3 and a C1 symmetry constraint. All calculations
were carried out with the Gaussian 03 software package.39
1
3
δ[ppm]): 168.20, 167.19, 161.81(dd, JCF = 247 Hz, JCF = 14.4 Hz),
RESULTS AND DISCUSSION
■
154.87, 151.54, 150.27, 150.14, 149.87, 147.91, 146.73, 144.12, 143.94,
3
139.86, 139.00(t, JCF = 12.9 Hz), 138.76, 137.98, 131.55, 130.87,
Synthesis and Characterizations. Scheme 2 depicts the
synthetic routes toward the ancillary ligands 2-(1-(3,5-
difluorophenyl)-1H-pyrazol-3-yl)pyridine (F2phpzpy) and 2-
(1-pentafluorophenyl-1H-pyrazol-3-yl)-pyridine (F5phpzpy),
and complexes 2 and 3. F2phpzpy and F5phpzpy were directly
synthesized from 2-(1H-pyrazol-3-yl)pyridine and fluoroben-
zene by nucleophilic substitution reactions. To suppress the
formation of multisubstitution products, fluorobenzene was in
large excess over 2-(1H-pyrazol-3-yl)pyridine in the reaction
mixture. Complexes 2 and 3 were readily synthesized from the
dimeric iridium(III) intermediate [Ir(ppy)2Cl]2 and the
ancillary ligands by a conventional synthetic method.3 All
ligands and complexes were fully characterized by ESI
(electrospray ionization) mass spectrometry, 1H NMR and
13C NMR spectroscopy. The purity of the complexes was
further ensured by elemental analysis.
130.42, 129.23, 127.05, 124.82, 124.20, 124.04, 123.70, 123.36, 122.78,
2
4
121.70, 119.91, 119.64, 110.92(dd, JCF = 21.5 Hz, JCF = 7.2 Hz),
107.39, 106.39(t, JCF = 25.1 Hz). ESI-MS [m/z]: 758.3 [M − PF6]+.
Anal. Found: C, 47.94; H, 2.96; N, 7.65. Anal. Calcd. for
C36H25F8N5PIr: C, 47.90; H, 2.79; N, 7.76.
2
Synthesis of [Ir(ppy)2(F5phpzpy)]PF6 (Complex 3). The synthesis
of complex 3 is similar to that of complex 2, except that F2phpzpy was
replaced with F5phpzpy. Yield: 67%. 1H NMR (acetone-d6, 600 MHz,
δ[ppm]): 8.63(d, J = 8.3 Hz, 1H), 8.43(d, J = 3.4 Hz, 1H), 8.27(td, J =
8.2 and 1.4 Hz, 1H), 8.24(d, J = 8.2 Hz, 1H), 8.20(d, J = 8.3 Hz, 1H),
8.07(td, J = 7.9 and 1.4 Hz, 1H), 8.02−7.98(m, 2H), 7.96(d, J = 5.4
Hz, 1H), 7.90(d, J = 5.5 Hz, 1H), 7.85(d, J = 2.8 Hz, 1H), 7.81(d, J =
8.2 Hz, 1H), 7.70(d, J = 8.3 Hz, 1H), 7.60(td, J = 6.5 and 1.4 Hz, 1H),
7.34(td, J = 6.5 and 1.4 Hz, 1H), 7.25(td, J = 6.5 and 1.4 Hz, 1H),
7.00(td, J = 7.6 and 1.4 Hz, 1H), 6.86(td, J = 7.6 and 1.4 Hz, 1H),
6.83(t, J = 8.3 Hz, 1H), 6.69(t, J = 6.9 Hz, 1H), 6.14(d, J = 7.6 Hz,
1H), 6.09(d, J = 7.6 Hz, 1H). 13C NMR (DMSO-d6, 150 MHz,
δ[ppm]): 167.23, 166.74, 156.22, 150.61, 150.22, 150.05, 149.66,
147.46, 147.20, 145.30−144.70(m), 144.42, 144.25, 143.60−
143.10(m), 141.75−141.40(m), 140.78, 140.65, 139.66, 139.57,
137.75−137.00(m), 136.10−135.40(m), 131.36, 130.92, 130.79,
129.59, 128.24, 125.56, 124.98, 124.55, 123.31, 121.61, 120.75,
120.16, 113.30−113.00(m), 109.18. ESI-MS [m/z]: 812.22 [M −
PF6]+. Anal. Found: C, 45.29; H, 2.46; N, 7.16. Anal. Calcd. for
C36H22F11N5PIr: C, 45.19; H, 2.32; N, 7.32.
Crystal Structure Determination. The low temperature (153.15
K) single-crystal X-ray experiments were performed on a Rigaku CCD
Saturn 724+ diffractometer equipped with graphite monochromatized
Mo Kα radiation. Direct phase determination yielded the positions of
all non-hydrogen atoms which were subjected to anisotropic
refinement. All hydrogen atoms were generated theoretically and
rode on their parent atoms in the final refinement.
Theoretical Calculations. Density functional theory (DFT) and
time-dependent DFT (TD-DFT) at the spin-restricted B3LYP level
were adopted for calculations on the ground and excited electronic
states of the complexes.34−36 “Double-ξ” quality basis sets were
employed for the C, H, N, F (6-31G**)37 and the Ir (LANL2DZ).38
An effective core potential (ECP) replaces the inner core electrons of
Ir leaving the outer core (5s)2(5p)6 electrons and the (5d)6 valence
electrons of Ir(III).38 The geometry of the singlet ground state (S0)
was fully optimized with a C1 symmetry constraint. The lowest triplet
Single Crystal Structures. Single crystals of complexes 2
and 3 were grown from slow evaporation of acetone/methanol
solution. Figure 1 depicts single crystal structures of complexes
2 and 3. Similar to other cationic iridium complexes,9,15,25
complexes 2 and 3 exhibit distorted octahedral geometries
around the iridium centers, with two ppy ligands adopting C,C-
cis, N,N-trans configurations. Selected bond lengths and angles
for complexes 1−3 are presented in Table 1. In complex 2, Ir−
N(F2phpzpy) bonds (2.173 and 2.185 Å) are longer than Ir−
N(ppy) bonds (2.040 and 2.040 Å), because of the strong
trans-influence of Ir−C(ppy) bonds. Similarly, in complex 3,
Ir−N(F5phpzpy) bonds (2.179 and 2.140 Å) are longer than
Ir−N(ppy) bonds (2.040 and 2.046 Å). As shown in Table 1,
for complexes 1−3, the bond lengths and ligand bite angles
around the iridium centers are similar, except that, in complex
3, the Ir−N(pyrazole) bond length (2.140 Å) is shortened by
about 0.04 Å compared to those in complexes 1 and 2 (2.174
and 2.185 Å, respectively).
In typical cationic iridium complexes with intramolecular
π−π stacking interactions, the bpy ancillary ligands generally
exhibit configurational distortions, as revealed by large dihedral
angles (about 20°) of the two pyridine planes on the bpy
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planes and a centroid-centroid distance (d) of 3.65 Å; in
complex 3, the pendant pentafluorophenyl ring stacks parallel
to the phenyl ring of the ppy ligand, with a small dihedral angle
of 5.0°, a short centroid-centroid distance of 3.52 Å, and a short
centroid-to-plane distance of 3.41 Å. As previously reported, in
complex 1, the pendant phenyl ring stacks to the phenyl ring of
the ppy ligand, with a large dihedral angle of 21° between least-
squares planes and a large centroid-centroid distance of 3.75 Å.
Upon fluorinating the pendant phenyl ring, the intramolecular
phenyl−phenyl stacking indeed becomes more parallel and
closer, indicating progressively reinforced intramolecular π−π
stacking interactions. However, in complex 2, the stacking of
the phenyl and 3,5-difluorophenyl rings is still far from parallel
and exhibits a relatively large centroid-centroid distance,
indicating a reinforced but still weak intramolecular π−π
stacking interaction. This suggests that substitution of the
pendant phenyl ring with two fluorine atoms is insufficient to
remarkably change the quadrupole moment of the phenyl ring
and significantly reinforce the intramolecular π−π stacking
interaction. In complex 3, the phenyl-pentafluorophenyl
stacking is parallel and close, corresponding to a significantly
reinforced π−π stacking interaction, because of the strong
electrostatic attraction between phenyl and pentafluorophenyl
rings.27,28 As observed in the single crystal structure, the π−π
stacking in complex 3 fits better a face-to-face pattern rather
than an offset face-to-face pattern.
Photophysical Properties. Figure 2 depicts the absorption
and PL spectra of complexes 2 and 3 in CH3CN solution and as
neat films. Detailed photophysical characteristics are summar-
ized in Tables 2 and 3. As a comparison, the data of complex 1
are also listed. The absorption spectra of complexes 2 and 3
have characteristics of the absorption spectra of phosphorescent
iridium complexes, with the intense absorption bands in the
ultraviolet region (below 350 nm) assigned to 1π−π*
transitions of the ligands and the relatively weak absorption
bands extending to the visible region assigned to 1MLCT
(metal-to-ligand charge-transfer), 1LLCT (ligand-to-ligand
Figure 1. Crystal structures of (a) complex 2 and (b) complex 3.
Thermal ellipsoids are drawn at 35% probability. The PF6
counteranions and hydrogen atoms are omitted for clarity.
−
3
3
3
charge-transfer), MLCT, LLCT, and ligand-centered π−π*
transitions.9,11,13−15
Similar to complex 1,25 in CH3CN solution, complexes 2 and
3 emit blue-green light with emission maxima at 480 and 476
nm, respectively. The fluorination on the pendant phenyl ring
exerts little influence on the emission energy of the complexes.
Complexes 1−3 all show significantly blue-shifted light
emission as compared to the typical complex [Ir(bpy)2(ppy)]-
PF6 (PL = 585 nm), because of pyrazole-type ancillary ligands
used.9,25 As shown in Figure 2, at 77 K in CH3CN glass, the
emission spectra of complexes 2 and 3 are little blue-shifted as
compared to their emission spectra at room temperature. This,
ligands.18,24 In complexes 1, 2, and 3, the dihedral angles (φ) of
the pyridine and pyrazole planes on the ancillary ligands are
7.2°, 13°, and 4.3°, respectively. For complex 3, the small φ
value indicates that the pyridine-pyrazole skeleton of F5phpzpy
is little distorted. While in complex 2, the relatively large φ
value indicates a relatively large configurational distortion for
F2phpzpy.
As shown in Figure 1, in complex 2, the pendant 3,5-
difluorophenyl ring stacks to the phenyl ring of the ppy ligand,
with a dihedral angle (Φ) of 18° between the least-squares
Table 1. Selected Bond Lengths (Å) and Angles (deg) in Single Crystal Structures of Complexes 1−3
metal−ligand bond lengths
Ir−N(ppy)
ligand bite angles
b
c
d
e
Ir−C(ppy)
Ir−N(N∧N)
ppy
N∧N
φ
Φ
d
a
1
2
3
2.000(5)
2.015(5)
1.998(4)
2.010(4)
2.000(6)
2.004(6)
2.050(3)
2.052(3)
2.040(3)
2.040(3)
2.040(5)
2.046(5)
2.179(4)
2.174(3)
2.173(3)
2.185(3)
2.179(5)
2.140(4)
80.78(16)
80.04(16)
80.62(13)
80.18(13)
80.98(25)
81.09(25)
74.61(15)
7.2
13
21
18
5.0
3.75
3.65
3.52
75.46(10)
74.36(18)
4.3
a
b
c
Data of complex 1 were cited from ref 25. N∧N denotes ancillary ligand. Dihedral angle of the pyridine and pyrazole planes on the ancillary
d
e
ligand. Dihedral angle of the two stacking phenyl rings. Centroid-centroid distance of the two stacking phenyl rings.
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Table 3. Photophysical Characteristics of Complexes 1−3 in
Thin Films
5% doped PMMA films
neat films
Φp
PL λ [nm]
Φp
τ [μs] PL λ [nm]
τ [μs]
a
1
2
3
481, 510
0.84
3.0
3.1
3.5
486, 513
519, 490
516, 484
0.07
0.18 (74%),
0.43 (26%)
480, 510
477, 506
0.59
0.78
0.05
0.08
0.16 (62%),
0.56 (38%)
0.92 (41%),
0.25 (59%)
a
Data of complex 1 were cited from ref 25.
the two states.24 The low knr value of complex 3 suggests that
complex 3 should have much smaller structural changes
between the ground and the excited states as compared to
complexes 1 and 2.
As shown in Table 3, in the 5% doped poly(methyl
methacrylate) (PMMA) films, complexes 1−3 show compara-
ble high PLQYs and long excited-state lifetimes. The PLQYs
and excited-state lifetimes in the lightly doped PMMA films are
significantly increased compared to those in solution, as always
observed for phosphorescent cationic iridium complexes,9,16
because of restricted intramolecular rotations or vibrations in
solid-states.42−44 It is noted that, in the 5% doped PMMA films,
complex 2 still exhibits the lowest PLQY among the three
complexes, which suggests that a relatively large structural
change still exists between the ground and the excited states for
complex 2 in the solid state. In neat films, complexes 1−3 all
exhibit red-shifted light emission (Supporting Information,
Figure S1) and largely decreased PLQYs and excited-state
lifetimes, because of the strong intermolecular interactions in
close-packed neat films.45
Figure 2. Absorption and PL spectra of (a) complex 2 and (b)
complex 3 in CH3CN solution and as neat films.
In solution, the PLQY of complex 3 is much higher than that
of complex 1 and comparable to those of other blue-green-
emitting cationic iridium complexes reported,9,14,15 indicating
that introduction of the intramolecular π−π stacking interaction
does not decrease the PL efficiency. This differs from previous
reports where complexes with intramolecular phenyl−phenyl
π−π stacking interactions generally show decreased PL
efficiencies because of the configurational distortion of the
ancillary ligands.24 Complex 3 with a significantly reinforced
intramolecular π−π stacking interaction and a high luminescent
efficiency is an interesting candidate for applications in
phosphorescent systems.
Theoretical Calculations. To gain deeper insight into the
intramolecular π−π stacking interaction, quantum chemical
calculations on the ground and exited states were performed for
complexes 1−3. The ground-state (S0) structures of complexes
1−3 were optimized by DFT calculations. The calculated bond
lengths and angles agree well with the experimental X-ray
values (Supporting Information, Table S1), except that the Ir−
N bonds between the iridium ions and the ancillary ligands are
together with the structured emission spectra, indicates that the
emission of complexes 2 and 3 has strong ligand-centered
3π−π* character.40,41
In CH3CN solution, the PLQYs of complexes 1, 2, and 3 are
0.03, 0.008, and 0.32, respectively, and the excited-state
lifetimes are 0.18, 0.05, and 2.0 μs, respectively. Compared to
the parent complex 1, complex 3 shows significantly increased
PLQY and excited-state lifetime, while complex 2 shows
decreased PLQY and excited-state lifetime. The radiative (kr)
and nonradiative (knr) rates of complexes 1−3 in CH3CN
solution are calculated and listed in Table 2. The kr values of
complexes 1−3 are nearly the same. The knr value of complex 3
is much smaller than that of complex 1, while the knr value of
complex 2 is higher than that of complex 1. For
phosphorescent heavy-metal complexes, an important channel
for the nonradiative deactivation of the emitting excited-states
is the vibrational coupling between the ground and the excited
states, which occurs when there are structural changes between
Table 2. Photophysical Characteristics of Complexes 1−3 in CH3CN Solution
c
d
PL at room temperature
PL at 77 K
b
absorption λ [nm] (ε [×104 M−1 cm−1])
λ [nm]
Φp (τ [μs])
kr [105 s−1
]
knr [106 s−1
]
λ [nm]
a
1
2
3
256 (4.36), 385 (0.47), 411 (0.31)
257 (4.06), 380 (0.46), 412 (0.31)
253 (4.15), 380 (0.45), 410 (0.29)
480, 509
480, 507
476, 506
0.03 (0.18)
0.008 (0.05)
0.32 (2.0)
1.7
1.6
1.6
5.2
20
478, 512, 548
471, 506, 538
475, 511, 543
0.34
a
b
c
Data of complex 1 were cited from ref 25. In CH3CN solution (1 × 10−5 M). ε denotes the molar extinction coefficient. In degassed CH3CN (1
d
× 10−5 M) solution. In CH3CN glass.
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Figure 3. Optimized geometries and calculated molecular surfaces of complexes 1−3. (a−c): HOMOs of complexes 1−3. (d −f): LUMOs of
complexes 1−3. All the MO surfaces correspond to an isocontour value of |Ψ| = 0.025.
overestimated, as always observed when the B3LYP functional
is adopted.46 Figure 3 depicts the optimized geometries and
calculated molecular surfaces of complexes 1−3 on the ground
states. As shown in Figure 3, the pendant phenyl rings on the
ancillary ligands stack to the phenyl rings of the ppy ligands,
with dihedral angles of 25.4°, 24.9°, and 10.4° between least-
squares planes for complexes 1, 2, and 3, respectively, and
centroid-centroid distances of 4.06 Å, 3.92 Å and 3.68 Å for
complexes 1, 2, and 3, respectively. Theoretical calculations
reveal that, upon fluorinating the pendant phenyl ring, the
intramolecular phenyl−phenyl stacking becomes more parallel
and closer.
To compare the intramolecular π−π stacking interactions in
complexes 1−3, single-point energy calculations were per-
formed on the complexes with varied dihedral angles (Φ)
between the two stacking phenyl rings. The Φ values are varied
by rotating the pendant phenyl rings on the ancillary ligands.
The calculated single-point energies ΔE (with respect to S0) of
complexes 1−3 are plotted versus Φ in Figure 4. Detailed ΔE
geometry (where ΔE = 0), the Φ values are gradually increased
with a step of 10°. Starting from the S0 geometry, the Φ values
are reduced to the smallest values and then gradually increased
with a step of 10°. In the latter case, the Φ values are minus so
that they can be distinguished from those in the former case. In
both cases, the calculated single-point energies are increased
with respect to S0 (ΔE > 0). As shown in Figure 4, within an
energy well of the same depth, the Φ for complex 3 spans in a
much narrower range as compared to those for complexes 1
and 2. For example, within an energy well of 0.2 eV, the Φ can
vary between 45° ∼ −45° for complex 1, 35° ∼ −35° for
complex 2 and 20° ∼ −10° for complex 3. Single-point energy
calculations reveal that, compared to the pendant phenyl or 3,5-
difluorophenyl rings in complexes 1 and 2, the pendant
pentafluorophenyl ring in complex 3 cannot swing by a large
degree, which agrees with the significantly reinforced intra-
molecular π−π stacking interaction in complex 3.
As shown in Figure 3, for complexes 1−3, the highest
occupied molecular orbitals (HOMOs) reside on the iridium
ions and the phenyl rings of the ppy ligands, and the lowest
unoccupied molecular orbitals (LUMOs) reside on the
pyridine-pyrazole moieties of the ancillary ligands. The
HOMO-1, LUMO+1, and LUMO+2 orbitals of complexes
1−3 reside mainly on the ppy ligands (Supporting Information,
Figure S2).
To ascertain the character of excited states, TDDFT
calculations were conducted on the basis of the optimized S0
geometries. Table 4 summarizes the lowest three singlet and
triplet states, together with the calculated oscillator strengths
(f) for the excitations. More singlet−singlet transitions can be
found in the Supporting Information (Table S4). The
calculated f values for the triplet excitations are zero because
of the neglect of the spin−orbit coupling in TDDFT
calculations. In phosphorescent heave-metal complexes, the
spin−orbit coupling makes the spin-forbidden triplet excita-
tions partially allowable and the real oscillator strengths for the
triplet excitations should be positive.
As shown in Table 4, for the three complexes, the S1, S2, and
S3 states arise mainly from the HOMO→LUMO, HOMO→
LUMO+1, and HOMO→LUMO+2 excitations, respectively,
except that, for complex 3, the HOMO-1→LUMO excitation
also has a large contribution to the S3 state. For the three
Figure 4. Variation of calculated single-point energies (ΔE, with
respect to S0) with dihedral angles (Φ) between the two stacking
phenyl rings for complexes 1−3.
values can be found in the Supporting Information (Table S2).
It should be noted that, in complexes 1−3, the pendant phenyl
rings cannot stack absolutely parallel (Φ = 0°) to the phenyl
rings of the ppy ligands. By rotating the pendant phenyl rings,
the accessible smallest Φ values are 13°, 11°, and 4° for
complexes 1, 2, and 3, respectively. Starting from the S0
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The lowest triplet states (T1) of complexes 1−3 were
optimized at UB3LYP levels with a spin multiplicity of 3 on the
basis of the optimized S0 geometries. Figure 5 shows the
optimized geometries and spin density distributions on the T1
states. The selected bond lengths and angles on the T1 states
are summarized in Table 5. As comparisons, the data on the
singlet S0 states are also listed. As shown in Figure 5, the spin
densities of the T1 states distribute on the phenyl rings of the
ppy ligands, the iridium ions, and the ancillary ligands. The spin
density distributions of the T1 states have a good match with
the topologies of HOMO and LUMO orbitals (Figure 3). This
agrees with the TDDFT calculations which reveal that the T1
states correspond to electron promotions from HOMO to
LUMO orbitals.
As shown in Table 5, compared to the S0 geometries, the
geometries on the T1 states exhibit a little contracted
coordination, as revealed by the shortened metal−ligand
bonds. In passing from the S0 to the T1 states, small structural
changes are observed for the two ppy ligands, while significant
structural changes are observed for the ancillary ligands. For
complexes 1−3, in passing from the S0 to the T1 states, the
pendant phenyl rings on the ancillary ligands just show a small
deviation, as revealed by the small changes (2−5°) of the
dihedral angles (Φ) between the two stacking phenyl rings. For
complex 3 on the T1 state, the pendant pentafluorophenyl ring
stacks to the phenyl ring of the ppy ligand, with a dihedral angle
of 12° between least-squares planes and a centroid-to-centroid
distance of 3.69 Å, indicating that the intramolecular π−π
stacking interaction remains on the T1 state.
For complexes 1−3, the pyridine-pyrazole skeletons are more
twisted on the T1 states, as revealed by the increases (5−9°) of
the dihedral angles (φ) between the pyridine and pyrazole rings
on the ancillary ligands (Table 5). For complexes 1 and 2, in
passing from the S0 to the T1 states, a significant structural
change should be the deviation of the pyrazole moieties on the
ancillary ligands, as revealed by the large changes (10−20°) of
the dihedral angles (θ) between the pendant phenyl rings and
the pyrazole rings on the ancillary ligands (Table 5). For
complex 3, the deviation of the pyrazole moiety on the T1 state
is small, as revealed by the small change (5°) of the θ value in
passing from the S0 to the T1 states. Among the three
complexes, complex 3 shows the smallest changes in θ, φ, and
Φ values, indicating that complex 3 has the smallest structural
changes between the S0 and the T1 states.
Table 4. Selected Singlet and Triplet States of Complexes 1−
3 Calculated from a TDDFT Approach
a
b
states E [eV]
f
dominant excitations
H → L (100%)
1
2
3
S1
S2
S3
T1
T2
T3
S1
S2
S3
T1
T2
T3
S1
S2
S3
T1
T2
T3
2.604
3.038
3.182
2.594
2.700
2.772
2.524
3.036
3.165
2.512
2.704
2.769
2.518
3.106
3.216
2.515
2.737
2.791
0.0008
0.0339
H → L+1 (100%)
H → L+2 (100%)
H → L (97%)
0.0004
0
0
H →L+1 (62%), H−1→L+2 (19%)
H → L+2 (44%), H−1→L+1 (36%)
H → L (100%)
0
0.0001
0.0281
H → L+1 (100%)
0.0022
H → L+2 (95%)
0
H → L (100%)
0
H →L+1 (61%), H−1→L+2 (15%)
H →L+2 (39%), H−1→L+1 (31%),
H → L (100%)
0
0.0002
0.0361
H → L+1 (100%)
0.0035
H−1 → L (41%), H → L+2 (53%)
H → L (100%)
0
0
0
H →L+1(66%), H−1 →L+2 (14%)
H →L+2 (35%), H−1→L+1(33%)
a
b
Calculated oscillator strengths. H and L denote HOMO and
LUMO, respectively; data in parentheses are the contributions of
excitations.
complexes, the T1 and T2 states arise mainly from the HOMO
→ LUMO and HOMO → LUMO+1 excitations, respectively,
while the T3 states have large multiconfigurational character
and arise mainly from HOMO→LUMO+2 and HOMO-1→
LUMO+1 excitations. According to the distributions of the
molecular orbitals, it can be concluded that the T1 states have
3
3
mixed MLCT (Ir → F2phpzpy or F5phpzpy) and LLCT
(ppy → F2phpzpy or F5phpzpy) character; the T2 and T3
states have mixed 3MLCT (Ir → ppy) and ppy-centered 3π−π*
character. As revealed in the photophysical characterizations,
the emission of complexes 1−3 exhibits strong ligand-centered
3π−π* character. It is thus deduced that the T2 or T3 states
should have contributions to the emission for complexes 1−3,
3
that is, the ppy-centered π−π* states should contribute to the
emission for complexes 1−3. The previous research has shown
that, for a blue-green-emitting cationic iridium complex with
the lowest triplet states close-lying in energy, the emission in
solution could occur from the T4 state which had ligand-
45
3
centered π−π* character. For complexes 1−3, the lowest
three triplet states could also be close-lying in energy, as
expected from the close-lying triplet excitation energies (Table
4). For complexes 1−3, the emission could occur from the T2
or T3 states. However, quantitative analysis of the contribution
of the T2 and T3 states to the emission is difficult to achieve
because the calculated oscillator strengths for the triplet
excitations are zero in the TDDFT calculations.
It is believed that, in complex 3, the significantly reinforced
intramolecular π−π stacking interaction restricts the structure
of the complex on both the ground and the excited states,
which prevents large structural relaxations on the excited states.
For complex 3, the small structural changes between the
ground and the excited states would significantly attenuate the
nonradiative deactivation of the excited-states and increase the
PL efficiency of the complex in solution.24 For complex 2,
Figure 5. Spin density distribution (0.004 e bohr−3) on the T1 states for (a) complex 1; (b) complex 2; (c) complex 3.
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Table 5. Selected Bond Lengths (Å) and Angles (deg) Calculated for Complexes 1−3 on the S0 and T1 States
a
b
c
d
e
states
Ir−C(ppy)
Ir−N(ppy)
Ir−N(N∧N)
C−C
θ
φ
Φ
d
1
2
3
S0
T1
S0
T1
S0
T1
2.014/2.025
2.006/1.989
2.011/2.025
2.009/1.993
2.015/2.024
1.984/1.997
2.083/2.082
2.081/2.082
2.082/2.081
2.082/2.074
2.084/2.085
2.075/2.081
2.247/2.288
2.233/2.230
2.251/2.298
2.218/2.256
2.251/2.258
2.238/2.240
1.463
1.413
1.464
1.414
1.464
1.426
61
50
53
33
83
78
3.7
13
25
27
25
30
10
12
4.06
4.02
3.92
3.87
3.68
3.69
8.7
16
2.4
7.0
a
b
C−C bond connecting the pyridine and pyrazole rings on the ancillary ligand. Dihedral angle between the pendant phenyl ring and the pyrazole
c
d
ring on the ancillary ligand. Dihedral angle between the pyridine and pyrazole rings on the ancillary ligand. Dihedral angle between the two
stacking phenyl rings. Centroid-centroid distance between the two stacking phenyl rings.
e
substituting the pendant phenyl ring with two fluorine atoms is
insufficient to guarantee a strong enough intramolecular π−π
stacking interaction that can prevent large structural relaxation
on the excited states. For complex 2, the structural changes
between the ground and the excited states are still large, and
complex 2 shows a low PL efficiency in solution.
ACKNOWLEDGMENTS
■
We would like to thank the National Key Basic Research and
Development Program of China (Grant 2009CB930602), the
National High Technology Research and Development
Program of China (Grant 2011AA03A110), and the National
Natural Science Foundation of China (Grants 50990060 and
51073089) for financial support.
CONCLUSION
■
By fluorinating the pendant phenyl ring on the ancillary ligand
in [Ir(ppy)2(phpzpy)]PF6, two new complexes, [Ir-
(ppy)2(F2phpzpy)]PF6 and [Ir(ppy)2(F5phpzpy)]PF6, have
been synthesized and compared to the parent complex
[Ir(ppy)2(phpzpy)]PF6. Upon fluorination, the pendant phenyl
rings on the ancillary ligands stack more parallel and closer to
the phenyl rings of the ppy ligands, with [Ir-
(ppy)2(F5phpzpy)]PF6 exhibiting a significantly reinforced
intramolecular face-to-face π−π stacking interaction and a
high luminescent efficiency in solution. Theoretical calculations
revealed that, for [Ir(ppy)2(F5phpzpy)]PF6, the intramolecular
π−π stacking interaction remains on both the ground and the
excited triplet states. The research demontrates an efficient
approach to control the intramolecular π−π stacking
interaction in phosphorescent iridium complexes, which
opens a new avenue to modulate the structures and
photophysical properties of the complexes.
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ASSOCIATED CONTENT
■
S
* Supporting Information
PL spectra of complexes 1−3 in 5% doped PMMA films and
neat films (Figure S1). Distributions of HOMO-1, LUMO+1,
and LUMO+2 on the complexes (Figure S2). Comparisons of
the calculated and experimental absorption spectra (Figure S3).
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and T1 geometries (Table S3). The first thirty singlet−singlet
transitions from TDDFT for complexes 1−3 (Table S4). The
Crystallographic Information (CIF) Files of complexes 2 and 3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*Phone: 86-10-62779988. Fax: 86-10-62795137. E-mail: qiuy@
mail.tsinghua.edu.cn (Y.Q.), duanl@mail.tsinghua.edu.cn
(L.D.).
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