Enhancing the luminescence properties and stability of cationic iridium(iii) complexes based on phenylbenzoimidazole ligand: A combined experimental and theoretical study

Article abstract of DOI:10.1039/c3dt50358e

Herein we designed and synthesized a series of cationic iridium(iii) complexes with a phenylbenzoimidazole-based cyclometalated ligand, containing different numbers of carbazole moieties from zero to three (complexes 1-4). The photophysical and electroche

Full text of DOI:10.1039/c3dt50358e

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An international journal of inorganic chemistry
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Volume 42 | Number 31 | 21 August 2013 | Pages 11023–11328
ISSN 1477-9226
COVER ARTICLE
Zhu, Su et al.
Enhancing the luminescence properties and stability of cationic iridium(III) complexes
based on phenylbenzoimidazole ligand: a combined experimental and theoretical
study

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PAPER
Enhancing the luminescence properties and stability
of cationic iridium(III) complexes based on
phenylbenzoimidazole ligand: a combined
experimental and theoretical study†
Cite this: Dalton Trans., 2013, 42, 11056
Guo-Gang Shan, Hai-Bin Li, Hai-Zhu Sun, Hong-Tao Cao, Dong-Xia Zhu* and
Zhong-Min Su*
Herein we designed and synthesized a series of cationic iridium(III) complexes with a phenylbenzoimida-
zole-based cyclometalated ligand, containing different numbers of carbazole moieties from zero to three
(complexes 1–4). The photophysical and electrochemical properties of this series have been systematically
investigated. The complexes exhibit strong luminescence in both solution and in neat films, as well as
excellent redox reversibility. Introducing carbazole groups into the complexes is found to lead to substan-
tially enhanced photoluminescence quantum efficiency in the neat film, but has little effect on the emit-
ting color and excited-state characteristics as supported by density functional theory (DFT) results. DFT
calculations also suggest that functionalized complexes 2–4 reveal better hole-transporting properties
than 1. More importantly, all complexes effectively reduce the degradation reaction to some extent in
metal-centered (3MC) excited-states, demonstrating their stability. Further studies indicate that restriction
of opening of the structures in the 3MC state is caused by the unique molecular conformation of the
phenylbenzoimidazole ligand, which is first demonstrated here in cationic iridium(III) complexes without
intramolecular π–π stacking. These results presented here would provide valuable information for design-
ing and synthesizing highly efficient and stable cationic iridium(III) complexes suitable for the optical
devices.
Received 5th February 2013,
Accepted 19th March 2013
DOI: 10.1039/c3dt50358e
www.rsc.org/dalton
electrochemical cells (LECs).6 As a new type of organic electro-
luminescent devices, LECs offer several advantages (e.g.,
Introduction
Phosphorescent ionic transition-metal complexes (iTMCs) are simple device architecture, low turn-on voltage and indepen-
receiving numerous interests due to their great potential in dence of the work function) over conventional multilayered
numerous fields, such as organic optical devices,1 chemosen- organic light-emitting diodes (OLEDs). Since the first report of
sors,2 biological probes3 and photosensitizers for photo- Ir(III)-based LECs,7 extensive research has been conducted on
induced hydrogen production.4 Among the reported iTMCs, the design of cationic Ir(III) complexes and the fabrication of
cationic Ir(III) complexes have especially attracted a great deal devices.8 Up to now, green, yellow, orange and red emitting
of interest because they exhibit high phosphorescence efficien- devices have been developed through ingenious ligand
cies close to unity endowed by the strong spin–orbit coupling control. Besides, white LECs are realized by combining blue–
effect of the Ir(III) atom.5 In addition, they also show green and red emitting cationic Ir(III) complexes in a host–
good photo- and thermal stabilities, relatively short excited- guest system.6a,b,9 All these promising characteristics and
state lifetimes, as well as easy tunability of the emission color, developments make cationic Ir(III) complexes highly appealing
which makes them an attractive alternative for optoelectronic as candidates for potential flat-panel displays and low-cost
applications, in particular in solid-state light-emitting solid-state lighting sources.
Despite these advances, the intrinsic excited-state self-
quenching associated with triplet–triplet annihilation always
Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal
occurs because of the strong interactions between closely
packed molecules, which remarkably hampers their appli-
cations in highly efficient and stable devices. Ir(III)-based LECs
University, Changchun, 130024 Jilin, People’s Republic of China.
E-mail: zhudx047@nenu.edu.cn, zmsu@nenu.edu.cn
†Electronic supplementary information (ESI) available: Synthesis and character-
as single-layered electroluminescent devices, only consist
of cationic Ir(III) complexes as the active component.
ization of the ligands used in this work and calculation data. See DOI:
10.1039/c3dt50358e
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The performance of the devices in principle is determined by
the photophysical and excited-state properties of the Ir(III)
complexes.10 Therefore, the exploitation of novel cationic Ir(III)
complexes for efficient LECs is attracting significant interest in
recent years. From the molecular-design point of view, introdu-
cing steric hindrance or bulky side groups into ligands of the
complexes is an effective strategy to suppress the emission
quenching and enhance the device performance.11 In
addition, such functional groups can make the complexes
more hydrophobic, which reduces the possibility of ligand-
exchange reaction caused by nucleophiles, leading to more
stable complexes.12 In this regard, several cationic Ir(III) com-
plexes with functionalized ancillary ligands have been syn-
thesized, which exhibit enhanced photophysical and
electroluminescent properties.9,13 However, to our best of
knowledge, only few examples of cationic Ir(III) complexes that
are functionalized via modifying cyclometalated ligands, have
been reported up to now. Moreover, the effect of functional
groups on the intrinsic properties of the complexes, such as
photophysical, electrochemical and charge transporting prop-
erties, has not been well addressed. Further development of
multifunctional cationic Ir(III) complexes and investigation of
the structure–property relationship is an important issue for
achieving highly efficient optical devices. In addition to the
experimental studies, theoretical investigations have been
proven to be a very useful approach to describe the ground
and excited-state electronic structures of phosphorescent com-
plexes and to predict the intrinsic properties of unsynthesized
ones.14 In particular, density functional theory (DFT) and time
dependent DFT (TD-DFT) calculations have been successfully
applied to investigate the nature of the emitting excited state
and to analyze the charge transporting properties of organic
luminescent materials.15
In this paper, we describe the preparation and theoretical
investigation of a series of cationic Ir(III) complexes by attach-
ing different numbers of carbazole groups into cyclometalated
and/or ancillary ligands (see Scheme 1). The carbazole moiety
is chosen as a functional unit. Not only is carbazole a good
charge-transporting group, which is beneficial for the improve-
ment of device performance, but it is also considered as an
efficient hydrophobic group to decrease nucleophilic attack,
leading to more stable iTMCs.16 Complex 1 is used as a refer-
ence to compare with the other three cationic Ir(III) complexes.
Their synthesis, photophysical and electrochemical properties
have been investigated in detail. In addition, their photo-
physical and charge-transporting properties are evaluated with
the help of theoretical calculations. The obtained results reveal
that the introduction of the rigid carbazole group into cationic
Ir(III) complexes peripheries lead to much higher quantum
efficiency in neat films as well as a significant increase in the
charge-transporting ability, but have little effect on emission
color. Furthermore, our theoretical calculations indicate
that the unique molecular structure is conducive to restricting
the opening of the structure of the complexes in metal-
centered (3MC) states, demonstrating their stability when used
in LECs.
Scheme 1 Chemical structures of cationic Ir(III) complexes 1–4.
Experimental section
Materials and measurements
All reagents and solvents employed were commercially avail-
able and used as received without further purification. The sol-
vents for syntheses were freshly distilled over appropriate
drying reagents. All experiments were performed under a nitro-
gen atmosphere by using standard Schlenk techniques. 1H
NMR spectra were measured on a Bruker Avance 500 MHz
instrument with tetramethylsilane as the internal standard.
The molecular weights of ligands and complexes were
tested by using electrospray-ionization mass spectroscopy
and matrix-assisted laser desorption-ionization time-of-flight
(MALDI-TOF) mass spectrometry, respectively. UV-vis absorp-
tion spectra were recorded on a Hitachi U3030 spectrometer.
The emission spectra were recorded using an F-7000 FL
spectrophotometer. The excited-state lifetime were measured
on a transient spectrofluorimeter (Edinburgh FLS920) with
time-correlated single-photon counting technique. The photo-
luminescence quantum yields (PLQYs) of the neat film were
measured in an integrating sphere. Cyclic voltammetry was
performed on a BAS 100 W instrument with a scan rate of
100 mV s−1 in CH3CN solutions with the three-electrode con-
figuration: a glassy-carbon electrode as the working electrode,
an aqueous saturated calomel electrode as the pseudo-refer-
ence electrode, and a platinum wire as the counter electrode,
respectively. A 0.1 M solution of tetrabutylammonium per-
chlorate (TBAP) in CH3CN was used as the supporting electro-
lyte, and the ferrocene–ferrocenium (Fc+/Fc) potential was
measured and selected as the internal standard.
Synthesis
Synthesis of the cyclometalated and ancillary ligands. Syn-
thesis of the cyclometalated ligands and ancillary ligands as
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(m, 1H), 8.12 (d, J = 8.5 Hz, 2H), 7.94 (t, J = 7.5 Hz, 1H),
7.70–7.84 (m, 13H), 7.51–7.57 (m, 5H), 7.45 (d, J = 3 Hz, 1H),
7.36–7.39 (m, 2H), 7.25–7.29 (m, 3H), 7.03–7.13 (m, 4H),
6.85–6.91 (m, 3H), 6.80 (t, J = 2.5 Hz, 1H), 6.62–6.67 (m, 3H),
6.50 (d, J = 7.5 Hz, 1H), 6.43 (d, J = 8.5 Hz, 1H), 5.99 (d, J =
8.0 Hz, 1H), 5.82 (d, J = 8 Hz, 1H). ESI-MS: m/z 1167.37
(M − PF6).
Synthesis of [Ir(Crbd)2(L1)]PF6 (3). Yield 62%. 1H NMR
(500 MHz, DMSO-d6, δ [ppm]): 8.31–8.32 (m, 5H), 8.05–8.17
(m, 7H), 7.91–8.01 (m, 1H), 7.78–7.86 (m, 5H), 7.65–7.67 (m,
5H), 7.59 (d, J = 6.5 Hz, 1H), 7.51–7.56 (m, 5H), 7.32–7.41 (m,
9H), 7.28 (t, J = 8.5 Hz, 1H), 7.21 (d, J = 8.5 Hz, 1H), 7.16 (t, J =
8.5 Hz, 1H), 7.10 (t, J = 7.5 Hz, 1H), 6.92–7.01 (m, 6H), 6.76 (d,
J = 7.5 Hz, 1H), 6.57 (d, J = 8.5 Hz, 1H), 6.47 (d, J = 8 Hz, 1H),
5.98 (d, J = 8 Hz, 1H), 5.83 (d, J = 8.5 Hz, 1H). ESI-MS: m/z
1332.42 (M − PF6).
Scheme 2 Chemical structures of the cyclometalated and ancillary ligands
used in this work.
Synthesis of [Ir(Crbd)2(L2)]PF6 (4). Yield 51%. 1H NMR
(500 MHz, CDCl3, δ [ppm]): 8.33 (d, J = 5.5 Hz, 1H), 8.22 (d, J =
shown in Scheme 2 (Phbd, Crbd, L1 and L2) were prepared 3 Hz, 2H), 8.21 (d, J = 3 Hz, 2H), 8.14–8.17 (m, 3H), 8.01–8.06
using the previously reported procedures.17 The detailed (m, 3H), 7.92–7.97 (m, 4H), 7.82–7.86 (m, 2H), 7.73–7.75 (m,
synthetic procedures and corresponding 1H NMR data for all 1H), 7.58–7.69 (m, 10H), 7.50–7.55 (m, 7H), 7.44 (d, J = 8.5 Hz,
ligands can be found in the ESI.†
1H), 7.30–7.40 (m, 9H), 7.26–7.27(m, 1H), 7.19–7.25 (m, 2H),
Synthesis of [Ir(Phbd)2Cl]2. The organometallated dimer 7.00–7.03 (m, 1H), 6.88–6.97 (m, 6H), 6.79–6.80 (m, 1H), 6.66
[Ir(Phbd)2Cl]2 was synthesized from reaction of IrCl3·3H2O (d, J = 7 Hz, 1H), 6.59 (d, J = 8.5 Hz, 1H), 6.17 (d, J = 8.5 Hz,
(0.81 g, 2.32 mmol) with 1,2-diphenyl-1H-benzo[d]imidazole 1H), 5.91 (d, J = 8.5 Hz, 1H). ESI-MS: m/z 1497.48 (M − PF6).
(Phbd, 1.39 g, 5.13 mmol) in 2-ethoxyethanol–water (3 : 1 v/v,
Theoretical calculations
40 mL) for 24 h. The mixture was treated with water (30 mL) to
induce precipitation of a yellow solid. The product was filtered
out and washed with diethyl ether, followed by ethanol, and
dried (1.12 g. 0.92 mmol, 79%). Mass spectrometry and
1H NMR were performed to verify the resulting produces con-
taining the corresponding chloro-bridged dimers (ESI, Fig. S1
and S2†). Other chloride-bridged complexes, [Ir(Crbd)2Cl]2,
All calculations on the ground and excited electronic state of
the complexes were carried out at B3LYP level18 within the
Gaussian 09 software package19 together with 6-31G* basis
sets for C, H, N atoms and the LANL2DZ for Ir atom. An
effective core potential (ECP) replaces the inner core electrons
of iridium leaving the outer core (5s)2(5p)6 electrons and the
(5d)6 valence electrons of Ir(III). The geometry of the metal-
centered triplet (3MC) was fully optimized and was calculated
at the spin-unrestricted UB3LYP level with a spin multiplicity
of 3. All expectation values calculated for S2 were smaller
than 2.05. The geometry optimization for all complexes in the
different states was carried out without the counterions. The
ionization potentials and the electron affinities were obtained
using B3LYP/6-31G*/LANL2DZ method with charge of 2 and 0,
respectively.
was synthesized using
a method similar to that for
[Ir(Phbd)2Cl]2. The chloro-bridged dimers were used in the
subsequent reactions without further purification.
Synthesis of [Ir(Phbd)2(L1)]PF6 (1). A solution of ligand L1
(0.30 g, 1.1 mmol) and the dichloro-bridged [Ir(Phbd)2Cl]2
(0.77 g, 0.5 mmol) in dichloromethane (20 mL) and methanol
(10 mL) was heated to reflux for 24 h in the dark. After cooling
to room temperature, the mixture was filtered, and then an
excess of solid KPF6 was added and stirred for another 0.5 h at
room temperature. The solvent was removed under reduced
pressure and the residue was purified by silica gel column
chromatography to yield 1 as an orange powder (71%). 1H
NMR (500 MHz, DMSO-d6, ppm): δ 8.26 (d, J = 6 Hz, 1H), 8.13
(t, J = 8 Hz, 1H), 7.72–7.75 (m, 12H), 7.67–7.69 (m, 1H), 7.61 (d,
Results and discussion
Photophysical properties
J = 8 Hz, 1H), 7.57 (d, J = 7.5 Hz, 1H), 7.51 (t, J = 8 Hz, 1H), The UV-visible absorption and photoluminescent (PL) spectra
7.42–7.44 (m, 1H), 7.20–7.30 (m, 4H), 7.16 (d, J = 8 Hz, 1H), for all complexes in CH2Cl2 solution are depicted in Fig. 1 and
7.06–7.13 (m, 3H), 7.03 (t, J = 7.5 Hz, 1H), 6.83–6.91 (m, 3H), detailed photophysical characteristics are summarized in
6.78 (t, J = 7.5 Hz, 1H), 6.60–6.65 (m, 3H), 6.48 (d, J = 7.5 Hz, Table 1. All complexes exhibit similar absorption patterns. The
1H), 6.39 (d, J = 8.5 Hz, 1H), 5.93 (d, J = 8.5 Hz, 1H), 5.78 (d, J = dominant absorption bands in the ultraviolet region below
8.5 Hz, 1H). ESI-MS: m/z 1002.30 (M − PF6). The related com- 350 nm are mainly attributed to spin-allowed π–π* transitions
plexes (2, 3 and 4) were prepared using similar procedures.
from the ligands. These results are confirmed by the spectral
Synthesis of [Ir(Phbd)2(L2)]PF6 (2). Yield 65%. 1H NMR profile of the π–π* transitions for the corresponding free
(500 MHz, DMSO-d6, δ [ppm]): 8.30–8.32 (m, 3H), 8.23–8.24 ligands (see Fig. S3, ESI†). The relatively weak absorption
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bands from 350 nm extending to the visible region are results (Table S1, ESI†), the absorption bands in the region of
ascribed to both spin-allowed, spin-forbidden metal-to-ligand 270–350 nm originate predominantly from the excitation of
charge transfer (MLCT) and ligand-to-ligand charge transfer HOMO−4 → LUMO+1, HOMO−3 → LUMO+2 and HOMO−7 →
(LLCT) characters mixed with some ligand-centered (LC) LUMO. Accordingly, the high-energy absorption bands (λabs
<
content, by reference to reported Ir(III) complexes.20
350 nm) are mainly attributed to the π–π* transitions of the
To assist in assigning the nature of the excited states free ligands. For 1, the calculated absorption bands from
involved in the experimental absorption spectra, 40 singlet 380 nm extending to the visible region result from the exci-
excited-states have been calculated based on TD-DFT calcu- tation of HOMO → LUMO, HOMO−1 → LUMO and HOMO →
lations (vide supra). The results shown in Fig. S4–S7, ESI† LUMO+1. The HOMO of 1 localizes on Ir atom with some dis-
reveal that the simulated absorption spectra agree well with tribution on cyclometalated ligands. Thus, these absorption
the experiment. The calculated excited energies, configur- bands are assigned to the mixture of MLCT and LLCT, with
ations, oscillator strength, along with corresponding orbital additional LC excited-state characters, which is in line with
distributions of the four complexes, are summarized in Tables our experimental data and previous reports.6a,8g,20 The corres-
S1–S4, ESI† and shown in Fig. S8–S11 (ESI†). Here, taking ponding assignments for the absorption bands of complexes
complex 1 as an example, as shown in Fig. S8, ESI† the occu- 2–4 are achieved according to the TD-DFT results.
pied molecular orbitals involved in main excitations, HOMO
It is also known that the frontier molecular orbitals are very
−1, HOMO−3 and HOMO−4, mainly reside on the cyclometa- important since they are directly linked to the photophysical
lated ligands, while HOMO−7 locates on the ancillary ligand properties, such as absorption and emission. Herein, further
with no distribution on the cyclometalated ligands. Except for insight into their photophysical properties are gained via the
the lowest-unoccupied molecular orbital (LUMO) of complex 1 DFT approach. The frontier molecular orbitals of them, i.e. the
that resides on ancillary ligand, LUMO+1 and LUMO+2 still highest-occupied molecular orbital (HOMO) and the lowest-
delocalize over the cyclometalated ligands. Based on TD-DFT unoccupied molecular orbital (LUMO), and the energy evolu-
tion for complexes 1–4 are illustrated in Fig. 2. Similar to
reported cationic Ir(III) complexes,6a,8e,21 the HOMOs of them
reside on the cyclometalated ligands (mainly on π orbitals of
the benzimidazole group) and the iridium ions, while the
LUMOs mainly delocalize over the pyridinylbenzoimidazole
moieties of the ancillary ligands. Clearly, the peripheral carba-
zole groups in complexes 2–4 do not contribute to both the
HOMOs and LUMOs. In addition, the calculated HOMO
energy levels (−5.68 eV for 1, −5.69 eV for 2, −5.71 eV for 3
and −5.70 eV for 4) and LUMO energy levels (−2.52 eV for 1,
−2.55 eV for 2, −2.54 eV for 3 and −2.56 eV for 4) are similar.
As a result, the calculated HOMO–LUMO gap for complex 1 is
3.16 eV. Complexes 2–4 also show almost identical HOMO–
LUMO gaps as calculated for 1, with the values of 3.14, 3.17
and 3.16 eV for complexes 2, 3 and 4, respectively. The theo-
retical calculation results suggest that attaching the carbazole
groups to the cyclometalated and/or the ancillary ligands of
Fig. 1 Absorption and emission spectra of complexes 1–4 in CH2Cl2 solutions
with concentration of 1 × 10−5. The inset shows the photoluminescent images
of complexes 1–4 in CH2Cl2 solutions.
complexes 2–4 does not significantly influence the HOMO and
Table 1 Photophysical and electrochemical properties of complexes 1–4
Emission at room temperature
Electrochemical datac
Complex
λmaxa/nm
Фb (τ/μs)
10−5κr/s−1
8.7
10−5κnr/s−1
E1o/x2/V
E1re/2d/V
−1.82
−1.81
−1.81
−1.81
1
2
3
4
569 (CH2Cl2)
571 (neat film)
572 (CH2Cl2)
574 (neat film)
569 (CH2Cl2)
575 (neat film)
570 (CH2Cl2)
577 (neat film)
0.60 (0.70)
0.18 (0.25)
0.43 (0.70)
0.25 (0.62)
0.46 (0.75)
0.23 (0.63)
0.44 (0.63)
0.31 (0.66)
5.7
8.1
7.2
8.8
0.86
6.1
0.85, 1.02d
0.85, 1.00d
0.79, 1.06d
6.1
7.0
a Emission spectra of complexes 1–4 were measured in CH2Cl2 solution. b Quantum yields were measured in an integrating sphere system.
c Collected in CH3CN solutions; vs. Fc+/Fc (Fc is ferrocene). d Irreversible.
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Fig. 3 Spin density and electron density distribution calculated for the opti-
mized lowest lying triplet states (T1) of complexes 1 and 4. The energy differ-
ence E computed between the S0 and T1 states of complexes 1 and 4 is 2.23
and 2.23 eV, respectively.
Fig. 2 Schematic diagram showing the electron density distribution and the
energy values calculated for the HOMO and LUMO of complexes 1–4.
multiplicity of 3. The energy difference between T1 and S0 is
calculated to be similar for 1 (2.23 eV, 556 nm), 2 (2.23 eV,
556 nm), 3 (2.24 eV, 554 nm) and 4 (2.23 eV, 556 nm), respect-
ively, which is consistent with the trends of their emission
energy. Fig. 3 depicts the HOMO and LUMO orbitals of com-
plexes 1 and 4 that are mainly involved in the S0–T1 transitions
along with the spin density contour for the T1 excited states.
The corresponding data for complexes 2 and 3 can be found in
Fig. S13 (see ESI†). The four complexes exhibit a similar spin
density distribution in the T1 state, that is, Ir: 0.43, cyclometa-
lated ligand (C^N): 1.37, ancillary ligand (N^N): 0.20 for 1; Ir:
0.41, C^N: 1.39, N^N: 0.20 for 2; Ir: 0.43, C^N: 1.37, N^N: 0.20
for 3; Ir: 0.43, C^N: 1.39, N^N: 0.20 for 4, respectively. It is
noted that the unpaired-electron spin density distribution for
complexes 1–4 perfectly matches the topology of the HOMO →
LUMO excitation from which the T1 excited-states originate
(see Fig. 3 and Fig. S13, ESI†). These results further confirmed
LUMO energy levels as well as HOMO–LUMO energy gaps com-
pared with those of 1. Similar electrochemical gaps and emis-
sion energies are thus expected for these complexes.
Upon photoexcitation, each complex displays broad, almost
featureless and strong emission spectra in both CH2Cl2 solu-
tions and neat films at room temperature. In solution, the
emission peaks maxima of complexes 1–4 are at 569, 572, 569,
570 nm, with quantum efficiency (Ф) of 0.60, 0.43, 0.46 and
0.44 for 1, 2, 3 and 4 respectively. Compared with complex 1,
the decrease in Ф in CH2Cl2 solution for complexes 2–4 is pre-
sumably attributed to more efficient non-radiative decay
caused by intramolecular rotation of the carbazole group.22
The calculated nonradiative rates (κnr) in solution also exhibit
the same trend. Compared with 1, complexes 2–4 show rela-
tively large κnr values as presented in Table 1. In sharp con-
trast, functionalized complexes 2–4 show much higher Ф in
neat films than that of complex 1 (see Table 1), indicating that
the incorporation of a carbazole group at the periphery of
the complexes can effectively reduce the self-quenching. The
excited-state lifetimes for them in neat films have also been
measured and the results show that excited-state lifetime
increases with increasing the number of carbazole groups
(Table 1). This results further support that the bulky moieties
attached into the Ir(III) complexes decrease the intermolecular
interaction, leading to much higher Ф in neat films.11a More-
over, upon cooling the CH2Cl2 solution to 77 K, the emission
spectra of complexes 1–4 are largely blue-shifted, but the
shape of peaks remains broad and featureless (Fig. S12, ESI†).
The broad and unstructured shapes of emission spectra
in both the solution and films as well as the rigid-chromism
indicate that their excited-states show predominant 3MLCT
character.8f,23
3
3
that the mixed MLCT and LLCT excited-state characters are
attributed to their lowest triplet states. The introduction of car-
bazole groups to both ligands has little effect on the excited-
state characters as well as emission color, which is in good
agreement with our experimental data (see emission spectra,
Fig. 1).
Electrochemical properties
The electrochemical behaviors of complexes 1–4 have been
determined by cyclic voltammetry (CV) and the redox poten-
tials are listed in Table 1. According to previous reports, the
reduction of cationic Ir(III) complexes usually occurs on the
ancillary ligands.11b,24 It is proposed that all complexes exhibit
similar reduction processes in CV curves. As expected, the elec-
trochemical data show that each complex exhibits a reversible
reduction process in CH3CN solution at ∼−1.81 V vs. Fc/Fc+.
The almost identical reduction potentials for all complexes
also suggest that the carbazole group introduced into both
ligands exerts little influence on the LUMO level of the com-
plexes. For oxidation processes, all complexes show one revers-
ible oxidation peak at ∼+0.79–0.86 V vs. Fc/Fc+. Moreover, an
additional oxidation peak at ∼+1.00–1.06 V has been observed
To understand the nature of the emissive excited state
involved in the emission process, quantum chemical calcu-
lations have been performed. The electronic and molecular
structures of the lowest triplet state (T1) for complexes 1–4
were optimized at the unrestricted UB3LYP level with a spin
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in complexes 2–4, which might be attributed to the attachment
of the carbazole groups. Their redox reversibility indicates that
both the holes (upon oxidation) and electrons (upon
reduction) can be easily transported and is beneficial for the
application of the complexes in LECs. The electrochemical gap
for complexes 1–4 is 2.68, 2.64, 2.66, 2.60 eV, respectively,
further indicating that the attachment of carbazole group does
not significantly affect their emission energy as well as electro-
chemical properties. The trends are in good agreement with
their photophysical data (see Section 2.1) and the theoretical
values calculated for HOMO–LUMO energy gaps (3.16 for 1,
3.14 for 2, 3.17 for 3, 3.16 eV for 4, respectively).
Metal-centered states (3MC)
Cationic Ir(III) complexes as emitter layer in LECs exhibit
several advantages compared with OLEDs, and much effort
has been devoted to the development of novel charged Ir(III)
complexes and optimization of LECs recently. However, the
relatively short lifetime of devices is observed in iTMC-based
LECs and mainly originates from the degradation process of
iTMC in metal-centered (3MC) states during the device oper-
ation;25 the 3MC state is essentially defined as the excitation of
an electron from the occupied t2g(dπ) HOMO to the unoccu-
Fig. 4 Selected bond lengths at optimized 3MC excited-states of complexes
1–4.
3
pied e2g(dσ*). The rupture of metal–ligand bonds would open 2.08 Å in the ground state (S0) to 2.25 Å in the MC state. In
the structures of the complexes and enhance the reactivity of contrast, electron promotion results in the elongation of
complexes in the 3MC state, resulting in detrimental degra- another Ir–Ncyclometalated ligand length (BL1), from 2.08 Å in the
dation processes and unstable LECs.6d,26 To evaluate the stabi- S0 state to 2.74 Å in 3MC state for 1. Similar trends for the
lity of the complexes, the 3MC states of them have been changes in bond lengths are also observed in complexes 2–4.
studied following the methodology illustrated in the reported As a consequence, relative high stability of complexes 1–4 in
works.8e,25,27 The metal-centered characters of the triplet state LECs might be expected. Herein, the question remains as to
for complexes 1–4 were confirmed by the spin densities calcu- what results in restriction of the rupture of metal–ligand bond
lated for the optimized 3MC state geometries. The spin den- in 3MC states for these complexes. Is it caused by the cyclome-
sities are mainly concentrated on the iridium atom, and have talated ligand or by the ancillary ligand? To address this ques-
1.53, 1.54, 1.54 and 1.54 unpaired electrons for complexes 1, 2, tion, detailed analysis of 3MC states for another two cationic
3
3 and 4 in MC excited-states, respectively (ESI, Fig. S14†). In Ir(III) complexes as shown in Fig. 5, have been performed. Two
3
addition, MC states are calculated to lie 0.63 eV (1), 0.66 eV complexes, C1 and C2, contain the same cyclometalated
(2), 0.64 eV (3) and 0.66 eV (4), respectively, above the lowest ligand and ancillary ligand as our studied complexes 1–4,
triplet states. The similar energy difference between the 3MC respectively, but a different third ligand. The geometry opti-
excited-state and T1 state indicates that the probability of mization of 3MC state for C1 and C2 were carried out using the
3
populating the MC states should be approximately similar for same method as complexes 1–4 (Fig. S15, ESI†).
the four complexes.
As shown in Fig. 5, it is clear that both bond lengths of Ir–
In light of the experimental and theoretical results, Bolink Ncyclometalated ligand in C1 are lengthened from 2.08 and 2.08 Å
3
et al. have demonstrated that the metal–ligand bond length in in S0 to 2.50 and 2.51 Å in MC state, respectively, indicating
the 3MC state played a key role for the stability of the Ir(III)- that the cationic Ir(III) complexes with pyridinylbenzoimida-
based LECs. The selected key bond lengths for complexes 1–4 zole-based ancillary ligands cannot effectively restrict the
3
in MC states have been determined and are shown in Fig. 4. opening of the structure. However, we find that the bond
As found in previous studies, construction of intramolecular length (BL2) of C2 only lengthens from 2.08 Å in S0 to 2.25 Å
3
π–π interactions in cationic Ir(III) complexes is an effective in the MC state, which is very similar to those found in com-
approach to reduce the opening of structures in the 3MC plexes 1–4. Based on these results, it is suggested that the cat-
state.11e,26,28 Interestingly, the opening of the structure of the ionic Ir(III) complexes with phenylbenzoimidazole-based
complexes in the 3MC state for complexes 1–4 is effectively cyclometalated ligand would aid in reducing the degradation
3
restricted, leading to the virtual decoordination of only one of reaction in MC in some extent and precluding the attack by
the Ncyclometalated
atoms, although they do not possess nucleophiles.6d,11e This finding would be helpful to further
ligand
intramolecular π-stacking. For example, the Ir–N bonds (BL2) designing stable cationic Ir(III) complexes for use in LECs
of the cyclometalated ligands in complex 1 close to the benzo- through carefully functionalizing phenylbenzoimidazole
imidazole moiety of the ancillary ligand only lengthen from ligand.
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According to the Marcus electron transfer regime,30 two
important parameters, i.e. reorganization energy (λ) and elec-
tronic coupling (t) are considered to evaluate the charge-
transfer rate and balance. However, the electronic coupling
cannot be determined accurately without the packing struc-
ture. Thus, only the reorganization energy for all complexes
was calculated to preliminarily evaluate the charge-transfer
rate in this work. According to the semiclassical Marcus–Hush
model,30 the charge transfer rate k can be calculated as follow-
ing eqn (1), in which λ is the reorganization energy, kB is
the Boltzmann constant, A is a factor related to the electronic
coupling between adjacent molecules and
temperature.
T
is the
ꢀ
ꢁ
λ
k ¼ A exp ꢀ
ð1Þ
4kBT
As seen in eqn (1), it can be found that the charge transport
rate strongly depends on the reorganization energy λ. It is
accepted that λ is determined by fast changes in molecular
geometry when a charge is added or removed and slow vari-
ations in the surrounding medium, i.e. the inner reorganiz-
ation energy (λi) and the external reorganization energy (λe). As
presented in the previous report, λe is very small and λi is
dominant in λ. Hence, we only discuss λi to investigate their
charge-transfer rate in this work. From Fig. S16, the reorgani-
zation energy for the hole and electron transfer can be evalu-
ated according to the following expressions (eqn (2) and (3)),
respectively.31
Fig. 5 (a) Chemical structures of complexes C1 and C2. (b) Minimum-energy
structures calculated for the 3MC states of C1 and C2; BL1 and BL2 represent
the bond lengths of Ir–Ncyclometalated ligand of both complexes.
Charge carrier injection and transporting properties
It is well known that IP and EA are usually the main factors in
evaluating the efficiency of holes and electrons injection; lower
IP and higher EA are beneficial to injection of holes and elec-
trons, respectively.29 To investigate the effects of the functional
group (carbazole moiety) on the charge-injection and transport
properties of complexes, some parameters such as ionization
potentials (IP), electron affinities (EA) and the reorganization
energy (λ) have been calculated through theoretical calcu-
lations. For clarity, a schematic diagram of the calculated para-
meters is presented in Fig. S16, ESI.† The values of the IP and
EA together with hole extraction potential (HEP) and electron
extraction potential (EEP) for complexes 1–4, which are
obtained by using reported methods, are listed in Table 2.
Compared with complexes 1–3, complex 4 that contains much
more carbazole moieties shows a significant decrease in IP,
indicating that the introduction of carbazole group endows
corresponding complexes with better hole-injection ability.
Note that, all complexes exhibit nearly identical EA potential,
thus implying that they have comparable electron-injection
abilities.
λhole ¼ λþ þ λ0
¼ ½EþðM0Þ ꢀ EþðMþÞꢁ þ ½E0ðMþÞ ꢀ E0ðM0Þꢁ
¼ IP ꢀ HEP
ð2Þ
ð3Þ
λelectron ¼ λꢀ þ λ0
¼ ½EꢀðM0Þ ꢀ EꢀðMꢀÞꢁ þ ½E0ðMꢀÞ ꢀ E0ðM0Þꢁ
¼ EEP ꢀ EA
The calculated λhole and λelectron values are summarized in
Table 2. Clearly, the λhole values for complexes 2–4 are
obviously smaller than that of complex 1, indicating that com-
plexes 2–4 reveal better hole-transporting properties with
respect to 1. In contrast to the obvious changes in λhole, the
λelectron for all complexes is almost the same. The obtained
results suggest that the attachment of the carbazole moiety to
the complexes slightly affects the electron-transporting proper-
ties, but effectively improve the hole-transporting properties of
the complexes.29 The enhanced quantum efficiency, excellent
redox reversibility, better stability and hole-transporting of
functionalized complexes 2, 3 and 4 make them good candi-
dates for optical devices.
Table 2 Vertical ionization potential (IP), vertical electron affinity (EA), extrac-
tion potential (HEP and EEP) and intramolecular reorganization energy (λhole
and λelectron). All units in eV
Complex
IP/V
HEP
EA/V
EEP
λhole
λelectron
Conclusion
1
2
3
4
8.35
8.12
7.79
7.70
8.11
8.03
7.72
7.65
3.21
3.23
3.23
3.28
3.67
3.74
3.72
3.79
0.24
0.09
0.07
0.05
0.46
0.51
0.49
0.51
In summary, a new family of cationic Ir(III) complexes using
phenylbenzoimidazole-based ligand as main ligands has been
11062 | Dalton Trans., 2013, 42, 11056–11065
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Paper
designed and prepared. Different numbers of carbazole moi-
eties are attached into complexes with a focus on investigating
their structure–property relationship. The photophysical and
electrochemical results suggest that they exhibit strong lumi-
nescent with main 3MLCT/3LLCT excited-state characters and
good redox reversibility. Importantly, attaching carbazole
moiety at the peripheral of the complexes leads to much
higher quantum efficiency in neat films due to increase in
intermolecular separation, but has neglectable influence on
emission spectra as well as excited-state properties. Compared
with 1, significant increase in the charge-transporting ability
has been shown for complexes 2–4, as demonstrated by DFT
calculation. Furthermore, the unique molecular structure of
the phenylbenzoimidazole ligands can restrict the opening of
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3
the structure of them in MC states effectively, demonstrating
their stability when used in LECs, which is first found in cat-
ionic Ir(III) complexes without intramolecular π-stacking. It is
believed that the results obtained in this work would provide
useful information for the design and preparation of highly
efficient and stable cationic Ir(III) complexes as well as LECs.
Work on these aspects are underway in our laboratory.
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Acknowledgements
The authors gratefully acknowledge the financial support
from NSFC (20971020, 20903020, 21273030, 51203017
and 21131001), 973 Program (2009CB623605), the Science
and Technology Development Planning of Jilin Province
(201201062, 20100540 and 201101008), and the Fundamental
Research Funds for the Central Universities (12QNJJ012).
7 J. D. Slinker, A. A. Gorodetsky, M. S. Lowry, J. Wang,
S. Parker, R. Rohl, S. Bernhard and G. G. Malliaras, J. Am.
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