Intramolecular π stacking in cationic iridium(III) complexes with phenyl-functionalized cyclometalated ligands: Synthesis, structure, photophysical properties, and theoretical studies

Article abstract of DOI:10.1002/ejic.201400007

The syntheses of two new heteroleptic cationic iridium complexes containing 2,6-diphenylpyridine (Hdppy) and 2,4,6-triphenylpyridine (Htppy) as the cyclometalated ligands, namely, [Ir(dppy)₂phen]PF₆ (1, phen = 1,10-phenanthroline) and [Ir(tppy)₂phen]PF₆ (2), are described. The X-ray crystal structure of 2 reveals a distorted octahedral geometry around the Ir center and close intramolecular face-to-face π-π stacking interactions between the pendant phenyl rings at the 2-position of the cyclometalated ligands and the NN ancillary ligand. This represents a new π-π stacking mode in charged Ir complexes. Complexes 1 and 2 are green photoemitters: their photophysical and electrochemical properties are interpreted with the assistance of density functional theory (DFT) calculations. These calculations also establish that the observed intramolecular interactions cannot effectively prevent the lengthening of the Ir-N bonds of the complexes in their metal-centered (³MC) states. Complexes 1 and 2 do not emit light in light-emitting electrochemical cells (LECs) under conditions in which the model compound [Ir(ppy)₂phen]PF₆ (3) emits strongly. This is explained by degradation reactions of the ³MC state of 1 and 2 under the applied bias during LEC operation facilitated by the enhanced distortions in the geometry of the complexes. These observations have important implications for the future design of complexes for LEC applications. A combined experimental and theoretical study of the cationic iridium complexes [Ir(dppy)₂phen]PF₆ (1) and [Ir(tppy)₂phen]PF₆ (2) is described. The complexes are green photoemitters. Strong intramolecular π-π stacking leads to enhanced distortions in the geometry of the complexes, which is detrimental to their use in light-emitting electrochemical cells (LECs).

Full text of DOI:10.1002/ejic.201400007

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DOI:10.1002/ejic.201400007
Intramolecular π Stacking in Cationic Iridium(III)
Complexes with Phenyl-Functionalized Cyclometalated
Ligands: Synthesis, Structure, Photophysical Properties,
and Theoretical Studies
Peng Li,[a,b] Guo-Gang Shan,[b] Hong-Tao Cao,[b] Dong-Xia Zhu,*[b]
Zhong-Min Su,*[a,b] Rukkiat Jitchati,[c,d] and Martin R. Bryce*[c]
Keywords: Iridium / Luminescence / Photophysics / Density functional calculations / Light-emitting cells
The syntheses of two new heteroleptic cationic iridium com- calculations. These calculations also establish that the ob-
plexes containing 2,6-diphenylpyridine (Hdppy) and 2,4,6-
triphenylpyridine (Htppy) as the cyclometalated ligands,
namely, [Ir(dppy)2phen]PF6 (1, phen = 1,10-phenanthroline)
and [Ir(tppy)2phen]PF6 (2), are described. The X-ray crystal
structure of 2 reveals a distorted octahedral geometry around
served intramolecular interactions cannot effectively prevent
the lengthening of the Ir–N bonds of the complexes in their
metal-centered (3MC) states. Complexes 1 and 2 do not emit
light in light-emitting electrochemical cells (LECs) under
conditions in which the model compound [Ir(ppy)2phen]PF6
the Ir center and close intramolecular face-to-face π–π stack- (3) emits strongly. This is explained by degradation reactions
ing interactions between the pendant phenyl rings at the 2-
position of the cyclometalated ligands and the N N ancillary
ligand. This represents a new π–π stacking mode in charged
Ir complexes. Complexes 1 and 2 are green photoemitters:
their photophysical and electrochemical properties are inter-
preted with the assistance of density functional theory (DFT)
of the 3MC state of 1 and 2 under the applied bias during
LEC operation facilitated by the enhanced distortions in the
geometry of the complexes. These observations have impor-
tant implications for the future design of complexes for LEC
applications.
∧
niques in their fabrication, and the use of air-stable elec-
trodes without the need for rigorous encapsulation, all of
which are applicable to large-area emission and cheap pro-
cessing. In LECs, ionic transition metal complexes (iTMCs)
of the generic formula [(C N)2Ir(N N)][PF6] perform all
the roles needed to generate light.[6] When an electrical bias
is applied to the LEC, the iTMCs serve to (1) decrease the
injection barriers through the displacement of the counter-
ion, (2) transport the electrons and holes through consecu-
tive reduction and oxidation processes, and (3) generate the
photons. The devices can operate at very low voltages and
yield high brightness and power efficiency with tunable
emission color.
Introduction
Cyclometalated IrIII complexes have attracted consider-
able attention during the last decade owing to their syn-
thetic versatility, high thermal stability, relatively short ex-
cited-state lifetimes, high photoluminescence efficiency, and
good emission wavelength tunability.[1] They have been
widely exploited as emitters in phosphorescent organic
light-emitting diodes (PhOLEDs),[2,3] solid-state lighting,[4]
and light-emitting electrochemical cells (LECs).[5] LECs of-
fer advantages over conventional OLEDs owing to their
simpler device architecture, the use of spin-coating tech-
∧
∧
The practical applications of iTMC-based LECs are
hampered by the current limitations of their stability.[7] Nu-
cleophile-assisted ligand-exchange reactions at the metal
center can occur, and the new complex that is formed can
quench the luminescence. A strategy initiated by Bolink,
Constable et al., which involves shielding the Ir atom of the
iTMC by intramolecular π–π stacking (to form “an intra-
molecular cage”),[6a] has led to dramatic improvements in
stability and enhanced lifetimes of the LECs.[8] For this pur-
pose, phenyl groups have been attached at the α positions
[a] College of Chemistry, Jilin University,
Qianjin Road 2699, Changchun 130023, P. R. China
[b] Institute of Functional Material Chemistry, Faculty of
Chemistry, Northeast Normal University,
Renmin Road 5268, Changchun 130024, P. R. China
E-mail: zhudx047@nenu.edu.cn
zmsu@nenu.edu.cn
http://en.nenu.edu.cn
[c] Department of Chemistry, Durham University,
Durham DH1 3LE, U.K.
E-mail: m.r.bryce@durham.ac.uk
http://www.dur.ac.uk
[d] Chemistry Department, Ubon Ratchathani University,
Warinchumrap, Ubon Ratchathani 34190, Thailand
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201400007.
∧
to the nitrogen atoms of the N N ancillary ligands. For
example, pendant phenyl group(s) on 2,2Ј-bipyridine (bpy),
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1,1-phenanthroline (phen), and 2-(1H-pyrazol-1-yl)pyridine gands.[12] The complexes were unambiguously characterized
1
(pzpy) ligands engage in face-to-face intramolecular π–π by H NMR spectroscopy, mass spectrometry, and elemen-
∧
stacking interactions with the cyclometalating C N li- tal analysis.
gands.[8,9] These interligand interactions effectively close the
complexes in the ground states (S0), the emitting (T1) states,
and the metal-centered (3MC) triplet excited states and,
X-ray Crystal Structure of 2
thereby, protect the complexes from attack by nucleophiles
such as water.[8b]
The solid-state structure of 2 was determined by single-
Similarly, our group has reported cationic IrIII complexes crystal X-ray diffraction. As depicted in Figure 2, complex
based on the 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3-yl)- 2 adopts a distorted octahedral geometry around the Ir cen-
pyridine (Phtz) ancillary ligand, which also show intramo- ter.
lecular π–π interactions. Density functional theory (DFT)
calculations clearly indicated that these interactions reduce
the possibility of ligand-exchange degradation reactions.[10]
We note that Duan et al. reported that phenyl substitution
of the N-heterocyclic carbene ancillary ligand of [Ir(ppy)2-
(pyphmi)]PF6 [Hppy = 2-phenylpyridine, pyphmi = 3-
phenyl-1-(2-pyridyl)imidazolin-2-ylidene-C,C2Ј] leads to
weak π–π interactions and increased torsion angles, which
do not enhance the stability of the LECs.[5g]
To the best of our knowledge, the π-stacking strategy has
exclusively involved pendant phenyl substitution of the an-
cillary ligand.[8–11] The aim of the present work is to address
a fundamental question: what is the effect of attaching pen-
∧
dant phenyl substituents to the cyclometalating (C N) li-
gands? We now present the synthesis and characterization
of two new complexes [Ir(dppy)2(phen)][PF6] (1) and
[Ir(tppy)2(phen)][PF6] (2) (Hdppy = 2,6-diphenylpyridine,
Htppy = 2,4,6-triphenylpyridine) (Figure 1). An X-ray crys-
tal structure analysis of 2 shows strong intramolecular π–π
stacking interactions. The photophysical and electrochemi-
cal properties of 1 and 2 are reported, along with DFT/
time-dependent DFT (TD-DFT) calculations. LECs have
Figure 2. The X-ray molecular structure of the cation in 2; the
centroid-to-centroid distances are included.
The structure is characterized by the small C(3)–Ir(1)–
been fabricated from 1 and 2, and the data are compared N(1) (80.48°), C(3A)–Ir(1)–N(1) (80.48°), and N–Ir(1)–
with those of the archetypal parent complex [Ir(ppy)2- N(OA) (75.57°) bite angles and twisted C(3)–Ir(1)–N
(phen)][PF6] (3), which serves as a reference.
(169.59°), C(3A)–Ir(1)–N(OA) (169.59°), and N–Ir(1)–
N(OA) (172.19°) bond angles. The Ir–N(phen) (2.209 Å),
Ir–C(tppy) (2.005 Å), and Ir–N(tppy) (2.074 Å) distances of
2 closely resemble those previously reported for the parent
complex 3: Ir–N(phen) 2.137 and 2.150 Å, Ir–C(ppy) 2.003
and 2.017 Å, Ir–N(ppy) 2.043 and 2.048 Å.[8c] However,
there are small differences in some of the bond lengths be-
tween structures 2 and 3 as a consequence of the steric in-
teractions of the pendent phenyl groups in 2. Specifically,
the Ir–N(tppy) (ca. 2.074 Å) and Ir–N(phen) (ca. 2.209 Å)
bonds of 2 are slightly longer than the comparable bonds in
3 [Ir–N(ppy) ca. 2.05 Å, Ir–N(phen) ca. 2.14 Å].[8c] Figure 2
illustrates the double face-to-face π-stacking between the
pendant phenyl ring of both tppy ligands and the ancillary
phen ligand of 2; the interaction is in an optimal offset ar-
Figure 1. Chemical structures of 1 and 2 and the parent complex
3, which is included for comparison.
Results and Discussion
rangement at
a separation (centroid-to–centroid) of
3.276 Å. This stacking distance is similar to those observed
Synthesis
in the [Ir(ppy)2(pbpy)]+ and [Ir(ppy)2(dpbpy)]+ cations
The synthesis of 1 and 2 (Figure 1) followed the standard (pbpy and dpbpy are 6-phenyl-2,2Ј-bipyridine and 6,6Ј-di-
routes for complexes of the generic formula [(C N)2Ir- phenyl-2,2Ј-bipyridine, respectively).[8b] This observation
∧
∧
(N N)][PF6]. The low yield for the formation of 1 and 2 is confirms that an intramolecular caged structure is formed
due to the steric hindrance from the o-phenyl substituents, by introducing a pendant phenyl group at C(2) of the cyclo-
∧
as observed previously with sterically hindered C N li- metalating ppy unit.
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Table 1. Photophysical and electrochemical data for 1 and 2.
PL at room temperature
PL at 77 K[a]
λem [nm]
Electrochemical data[a]
[a]
[b]
[a,c]
[d]
[d]
λem [nm] ΦPL (τ, μs)[a] kr[a,c] [ϫ105]
knr
[ϫ106] λem [nm] Φfilm
Eox [V]
Ered [V]
1
2
3
552
546
583
0.15 (0.23)
0.06 (0.31)
0.39 (0.23)
6.6
2.0
17.3
3.71
3.06
2.71
595
552
591
0.06
0.10
0.11
515, 552 (sh)
517, 545 (sh)
532
0.80
0.80
0.79
–1.58
–1.59
–1.58
[a] Data obtained in acetonitrile solution; λexc = 355 nm. [b] Estimated error of Ϯ10%. [c] kr = ΦPL ϫτ–1. [d] Data obtained from a neat
thin film.
Photophysical Properties
donating phenyl group at C(4) of the pyridyl ring.[16] The
emission for 3 (λmax = 583 nm in MeCN; Figure 3) is con-
sistent with previously reported data for this complex
(579 nm in MeCN;[17] 575 nm in CH2Cl2).[18] The emission
spectra for 1–3 have the same trend in a non-coordinating
solvent such as CH2Cl2 (see Figure S5 in the Supporting
Information).
For 1 and 2 in CH3CN, the photoluminescence quantum
yields (PLQYs) are 0.15 and 0.06, respectively. Complex 1
shows a significantly higher radiative rate constant than
that of 2; this may be the reason that 1 has a higher PLQY
in solution, as the nonradiative rate constants for both 1
and 2 are quite similar. Conversely, in thin films, the PLQY
of 2 (Φ = 0.10) is higher than that for 1 (Φ = 0.06), as
the steric bulk of the extra phenyl substituent now has the
beneficial effect of reducing self-quenching. The excited-
state lifetimes of 1 and 2 in solution are 0.23 and 0.31 μs,
The absorption and emission spectra of 1 and 2 (Fig-
ure 3) at room temperature were recorded in degassed ace-
tonitrile solutions. The photophysical characteristics are re-
ported in Table 1. The strong absorption bands in the range
200–350 nm are assigned to spin-allowed π–π* transitions
from the ligands. The relatively weak absorption bands that
occur in the lower-energy region (350–500 nm) correspond
to 1MLCT (metal-to-ligand charge-transfer), 3MLCT,
1LLCT (ligand-to-ligand charge-transfer), 3LLCT, and li-
gand-centered (LC) 3π–π* transitions with reference to
those reported for other IrIII complexes.[13] This observation
implies that the spin-forbidden 3MLCT, 3LLCT, and LC
3π–π* transitions have gained considerable intensity by mix-
1
ing with the higher-lying spin-allowed MLCT transitions
because of the strong spin–orbit coupling endowed by the
iridium atom. Generally, for cationic IrIII complexes, three
excited states, namely, 3MLCT, 3LLCT, and LC 3π–π*, con-
tribute to light emission.[14] At room temperature, 1 and 2
exhibit intense green emission with peak values at 546 and
552 nm, respectively, in CH3CN solutions. The broad and
featureless bands indicate that the emissive excited states of
respectively, which are typical for phosphorescent emission
[8]
∧
∧
in [(C N)2Ir(N N)][PF6] complexes. The radiative decay
rates (kr) of 1–3 in CH3CN solution were calculated as
6.6ϫ105 for 1, 2.0ϫ105 for 2, and 17.3ϫ105 for 3.
3
3
these two complexes are predominantly MLCT or LLCT
Electrochemical Properties
3
in character, rather than LC π–π* transitions, which typi-
The electrochemical behavior of 1 and 2 in CH3CN solu-
tion was investigated by cyclic voltammetry, and the data
are reported versus ferrocene/ferrocenium in Table 1. Com-
plexes 1 and 2 exhibit quasireversible oxidation and re-
duction peaks at Eox = 0.80 V and Ered = –1.58/–1.59 V,
respectively, which are very similar to the data obtained un-
der the same conditions for 3 [0.79 and –1.58 V this work
(cf. 0.85 and –1.54 V in CH2Cl2)].[19] These data are consis-
tent with oxidation primarily at the Ir center and the phenyl
ring of the cyclometalated ligand, whereas the reduction is
localized on the ancillary ligand.
cally show vibronic structure in the emission spectra.[15] The
emission spectra of 1 and 2 in CH3CN solutions at 77 K
remain broad and are blueshifted, which indicates that their
3
3
excited states retain MLCT and LLCT character at low
temperature.[14]
Quantum Chemical Calculations
The geometries and electronic structures of 1 and 2 were
calculated by DFT/TD-DFT methods at the B3LYP/(6-
31G*+LANL2DZ) level to provide additional insights into
the structures and nature of the emissive excited states. Fig-
ure 4 displays the atomic orbital compositions of the lowest
unoccupied and highest occupied molecular orbitals
Figure 3. Absorption and normalized emission spectra of 1, 2, and
3 in CH3CN solutions at room temperature.
As shown in Figure 3, the 2-phenyl substituents cause a (LUMO and HOMO, respectively) of the cations of 1 and
significant blueshift in the emission wavelengths of 1 and 2 2. The LUMO of both complexes is almost the same and
(31 and 37 nm, respectively) compared to that of 3. The resides on the phenanthroline group. The HOMO is com-
larger blueshift for 2 is ascribed to the additional electron- posed of a mixture of π orbitals of the phenyl group at C(2)
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3
of the cyclometalated ligands and iridium d orbitals: the 1.47 eV unpaired electrons for both 1 and 2 in their MC
phenyl groups at C(4) or C(6) make no contribution to the states. The key bond lengths that affect the stabilities of 1
3
HOMOs. These data are consistent with studies on analo- and 2 in the MC states are presented in Figure 5. The Ir–
3
gous complexes.[8] To further understand the emission pro- Nphen bond lengths of 2.23 Å for 1 and 2 in the MC state
cesses of 1 and 2, TD-DFT methods were used to calculate are similar to the Ir–Nphen bond lengths (2.26 Å) in T1. The
the low-lying triplet states (T1) at the optimized geometry structures show that the face-to-face π stacking, as observed
of the ground state (S0). The orbital diagrams show that in the X-ray crystal structure of 2 (Figure 2), is retained in
the T1 state for 1 mainly originates from HOMOǞLUMO the 3MC states with centroid-to-centroid distances of
(74%), and the T1 state for 2 mainly originates from 4.006 and 4.022 Å for 1 and 2, respectively.
HOMOǞLUMO+1 (56%) and HOMOǞLUMO+3
(20%) transitions (Figure S6). These data suggest that the
3
lowest excited states are induced by MLCT (iridiumǞan-
3
cillary ligand) with some LLCT character (cyclometalated
ligandsǞphen). In addition, the unpaired-electron spin
density distribution calculated for 1 and 2 perfectly matches
the topology of the HOMOǞLUMO excitation in which
the T1 state originates and confirms the mixed 3MLCT/
3LLCT character of the lowest triplet state (Figure S7).[8d]
The photophysical properties and the calculated results il-
lustrate that the emission of 1 and 2 mainly originates from
the T1 states and agree with the experimentally observed
Figure 5. Minimum–energy structures calculated for the 3MC
states of 1 and 2. Distances R1 and R2 are the optimized
Ir–Ncyclometalated ligand bond lengths [Å].
broad unstructured emission spectra of both complexes
(Figure 3).
However, a crucial point is that the intramolecular π–π
interactions in 1 and 2 do not prevent opening of the struc-
ture in the 3MC state, unlike iridium complexes with a pen-
dant phenyl group on the ancillary ligand such as [Ir(ppy)2-
(pbpy)]+.[8b] For example, as shown in Figure 5, the calcu-
lated Ir–N bond lengths (R1 and R2) of 1 increased from
3
2.13 Å in the ground state (S0) to 2.61 Å in the MC state,
and the changes are the same in 2. Complex 3 exhibits sim-
ilar changes to the corresponding bond lengths (Figure S8).
Light-Emitting Cells (LECs)
Figure 4. HOMO and LUMO distributions of 1 and 2.
To investigate the electroluminescent properties of the
complexes, LECs were prepared with a structure of ITO/
The metal-centered (3MC) states result from the exci- PEDOT:PSS (50 nm)/iridium complex–IL (molar ratio 4:1
tation of an electron from the occupied t2g (dπ) HOMO to w/w; 75 nm)/Al (120 nm). ITO is indium tin oxide,
the unoccupied e2g (dσ*) level, which is regarded as the ori- PEDOT:PSS is poly(3,4-ethylenedioxythiophene)/poly(styr-
gin of the degradation process for RuII- and IrIII-based ene sulfonate), and IL is the ionic liquid 1-butyl-3-methyl-
LECs.[7] In the 3MC states, the rupture of metal–ligand imidazolium hexafluorophosphate (BMIMPF6), which re-
bonds can induce opening of the structure and, thereby, duces drastically the turn-on time of LECs and enhances
enhance the reactivity of complex; thus, photodegradation the ionic conductivity of the thin film.[20] This is the stan-
is facilitated, and the device becomes unstable. The robust dard LEC architecture that our groups have used pre-
intramolecular π-stacking observed in the complexes re- viously.[21] As a benchmark, model complex 3 was studied
ported by Bolink et al. minimizes the expansion of the in the present work. As expected, upon applying a bias of
metal–ligand bonds in the excited state, and this prevents 3 V to the device with complex 3, light emission was ob-
the unwanted ligand-exchange reactions.[8] To evaluate the served within a few minutes, as reported previously by us[19]
3
stability of 1 and 2, the molecular structures of the MC and by Bolink et al.[8c] for this complex. However, for 1 and
states were fully optimized from the minimum-energy struc- 2, under identical conditions no light emission was ob-
ture of S0 with Ir–Nppy bond lengths lengthened to served even after the application of a bias of 3 V for as long
2.70 Å.[8h] The metal-centered character of the triplet states as 24 h. Moreover, no light emission was observed at a
for 1 and 2 was confirmed by the spin densities, which were higher bias (8 V) for 24 h. These studies demonstrate that
calculated for the optimized 3MC state geometries. The spin although 1 and 2 show efficient photoluminescence they are
densities are mainly concentrated on the iridium atom with not suitable for LECs. This can be explained by the dis-
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quantum yields (PLQYs) in solution and in neat films were mea-
sured with an integrating sphere in a fluorospectrophotometer. Cy-
clic voltammetry was performed with a BAS 100 W instrument at
a scan rate of 100 mVs–1 in CH3CN with a three-electrode configu-
ration: a glassy-carbon electrode as the working electrode, an aque-
ous saturated calomel electrode as the pseudo-reference electrode,
and a platinum wire as the counter electrode. A 0.1 m solution of
tetra-n-butylammonium perchlorate (TBAP) in CH3CN was used
as the supporting electrolyte, and ferrocene was selected as the in-
ternal standard.
torted molecular structure induced by the double π stacking
(as revealed by X-ray analysis) and by the theoretical calcu-
lations that show that the intramolecular π–π interactions
in 1 and 2 do not effectively prevent the transition to a
more open structure with an expanded Ir–N coordination
sphere in the excited states. Consequently, we conclude that
the double π stacking in 1 and 2 is detrimental to stability
and it is likely that the complexes degrade in the excited
state through reactions with adventitious nucleophiles and
so luminescence is quenched.
Synthesis: Ligands Hdppy[12] and Htppy[12] and complex 3[17] were
synthesized as described in the literature. The μ-dichloro-bridged
∧
diiridium C N ligand complexes, which are precursors to com-
Conclusions
plexes 1 and 2, were synthesized from Hdppy and Htppy, respec-
tively, by following standard literature procedures for analogs.[22]
This combined experimental and theoretical study has
provided new insights into the established strategy of using
intramolecular π–π stacking in cationic IrIII complexes to
enhance LEC performance. Complexes 1 and 2 possess the
new feature of pendant phenyl rings at the α position to the
nitrogen atom of the cyclometalating units. X-ray analysis
of 2 shows that it has a distorted octahedral geometry with
strong intramolecular face-to-face π–π stacking interactions
between the pendant phenyl units and the ancillary ligands.
DFT calculations establish that the intramolecular interac-
tions are retained in the excited triplet states and that this
mode of π–π stacking does not prevent the opening of the
Ir–N coordination sphere in the excited states. Conse-
quently, although the complexes are photoluminescent, they
do not emit light in LECs under conditions in which the
model compound [Ir(ppy)2phen]PF6, emits strongly. This is
[Ir(dppy)2(phen)][PF6] (1):
A mixture of 2,6-diphenylpyridine
(508 mg, 2.2 mmol), IrCl3·3H2O (352 mg, 1.0 mmol), 2-ethoxy-
ethanol (12 mL), and water (4 mL) was heated at 120 °C. After
12 h, the mixture was cooled to 20 °C, and the precipitate was col-
lected by filtration, washed with water, and then dissolved in
CH2Cl2. The organic solution was separated, dried with MgSO4,
filtered, and evaporated to give a pale green solid, presumed to be
the bis-μ-chloro-bridged complex, which was used directly in the
next step. A mixture of this complex (138 mg, 0.1 mmol) and phen-
anthroline (36 mg, 0.2 mmol) in dichloromethane (30 mL) and
methanol (15 mL) was heated under reflux for 24 h in the dark.
After cooling to room temperature, the mixture was filtered; an
excess of solid KPF6 was then added, and the mixture was stirred
for 1 h at room temperature. The solvent was removed under re-
duced pressure, and the residue was purified by silica gel column
chromatography with a mixture of dichloromethane/ethyl acetate
(4:1 ν/ν) as eluent to yield 1 (8 mg) as a yellow solid. 1H NMR
(500 MHz, CDCl3): δ = 8.20 (d, J = 7.5 Hz, 2 H), 8.02 (d, J =
7.5 Hz, 2 H), 7.98 (s, 2 H), 7.88 (d, J = 7.5 Hz, 2 H), 7.63–7.67 (m,
4 H), 7.20–7.24 (m, 6 H), 7.02 (t, J = 7.5 Hz, 2 H), 6.81 (d, J =
8 Hz, 2 H), 6.62 (t, J = 3.5 Hz, 4 H), 6.48 (d, J = 8 Hz, 2 H), 6.11
(s, 2 H), 5.01 (d, J = 7 Hz, 2 H) ppm. C48H36F6IrN4P (1006.02):
calcd. C 57.31, H 3.61, N 5.57; found C 57.36, H 3.65, N 5.61.
ESI-MS: m/z = 833.2 [M – PF6]+.
3
presumably because of degradation reactions of the MC
state of 1 and 2 under the applied bias during LEC opera-
tion. This combined experimental and theoretical study
provides new insights into structure–property relationships
in ionic Ir complexes and demonstrates convincingly that
intramolecular π–π stacking can be detrimental to LEC
performance in specific cases owing to enhanced distortions
in the geometry of the complex. This is valuable infor-
mation for the future design of complexes for LEC applica-
tions.
[Ir(tppy)2(phen)][PF6] (2): By following the same procedure as that
for 1, 2,4,6-triphenylpyridine (676 mg, 2.2 mmol) gave a pale green
solid, presumed to be the bis-μ-chloro-bridged complex, which was
used directly in the next step. A mixture of this complex (168 mg,
0.1 mmol) and phenanthroline (36 mg, 0.2 mmol) in dichlorometh-
ane (30 mL) and methanol (15 mL) was heated under reflux for
24 h in the dark. Workup and purification as described for 1
yielded 2 (15 mg). 1H NMR (500 MHz, [D6]dimethyl sulfoxide: δ
= 8.69 (d, J = 1.5 Hz, 2 H), 8.37–8.42 (m, 4 H), 8.07 (s, 2 H), 7.98–
8.00 (m, 4 H), 7.67 (d, J = 4 Hz, 2 H), 7.48–7.50 (m, 8 H), 7.17–
7.22 (m, 4 H), 7.03–7.06 (m, 2 H), 6.96 (d, J = 2 Hz, 2 H), 6.84 (d,
J = 7.5 Hz, 2 H), 6.64 (t, J = 7.5 Hz, 2 H), 6.56 (t, J = 7.5 Hz, 2
H), 6.06 (t, J = 7.5 Hz, 2 H), 5.11 (d, J = 7.5 Hz, 2 H) ppm.
C60H44F6IrN4P (1158.22): calcd. C 62.22, H 3.83, N 4.84; found C
62.27, H 3.91, N 4.88. ESI-MS: m/z = 983.2 [M – PF6]+. Single
crystals of 2 were obtained by slow evaporation of a dilute CH2Cl2
solution of the complex.
Experimental Section
Materials, Synthesis, and Characterization: All reagents and sol-
vents employed were commercially available and used as received
without further purification. The solvents for syntheses were freshly
distilled from appropriate drying reagents. All experiments were
performed under a nitrogen atmosphere by using standard Schlenk
techniques. 1H NMR spectra were measured with a Bruker Avance
500 MHz spectrometer with tetramethylsilane as the internal stan-
dard. Mass spectra were recorded by using matrix-assisted laser
desorption-ionization time-of-flight (MALDI–TOF) mass spec-
trometry. Elemental analyses (C, H, and N) were obtained by using
a Perkin–Elmer 240C elemental analyzer. UV/Vis absorption spec-
tra were recorded with a Hitachi U3030 spectrometer. The emission
spectra were recorded with a Hitachi F-7000 fluorescence spectro-
photometer. The excited-state lifetimes were measured with a tran-
sient spectrofluorimeter (Edinburgh FLS920) with a time-corre-
lated single-photon-counting technique. The photoluminescence
X-ray Crystallography: The data collection for 2 was performed
with a Bruker Smart Apex II CCD diffractometer with graphite-
monochromated Mo-Kα radiation (λ = 0.71069 Å) at 293 K. Ab-
sorption corrections were performed by using the multiscan tech-
nique. The crystal structure was solved by direct methods with
SHELXTL-97[23] and refined by full-matrix least-squares tech-
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atoms except for some of the nitrogen and carbon atoms. Structural
data in CIF format is available as Supporting Information. CCDC-
956631 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
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under ambient conditions by applying a constant voltage with a
Keithley 2400 source meter.
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Acknowledgments
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (NSFC) (grant
numbers 51203017, 20971020, 20903020, 21273030, 21131001, and
21303012), the 973 Program (2009CB623605), the Science and
Technology Development Planning of Jilin Province (grant num-
bers 20100540, 201101008, and 20130522167JH), and the Funda-
mental Research Funds for the Central Universities (grant number
12QNJJ012). R. J. thanks the Royal Thai Government for a schol-
arship (grant number MRG5480005).
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Published Online: ᭿
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Iridium Complexes
P. Li, G. Shan, H. Cao, D. Zhu,* Z. Su,*
R. Jitchati, M. R. Bryce* .................. 1–8
Intramolecular
π Stacking in Cationic
Iridium(III) Complexes with Phenyl-Func-
tionalized Cyclometalated Ligands: Syn-
thesis, Structure, Photophysical Properties,
and Theoretical Studies
Keywords: Iridium / Luminescence / Photo-
physics / Density functional calculations /
Light-emitting cells
A combined experimental and theoretical
study of the cationic iridium complexes
[Ir(dppy)2phen]PF6 (1) and [Ir(tppy)2phen]-
PF6 (2) is described. The complexes are
green photoemitters. Strong intramolecular
π–π stacking leads to enhanced distortions
in the geometry of the complexes, which is
detrimental to their use in light-emitting
electrochemical cells (LECs).
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim