Four new cyclometalated phenylisoquinoline-based Ir(III) complexes: Syntheses, structures, properties and DFT calculations

Article abstract of DOI:10.1016/j.inoche.2014.02.031

Four new cyclometalated phenylisoquinoline-based iridium(III) complexes [Ir(?N)₂(bipy)]PF₆ (5a-5d) (bipy = 2,2′-bipyridine) have been synthesized and fully characterized, where the ?N ligands are 1-(4-(trifluoromethyl)phenyl)isoquino

Full text of DOI:10.1016/j.inoche.2014.02.031

Inorganic Chemistry Communications 43 (2014) 146–150
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Four new cyclometalated phenylisoquinoline-based Ir(III) complexes:
Syntheses, structures, properties and DFT calculations
Zhi-Gang Niu a, Dong Liu a, Da-Chao Li a, Jing Zuo a, Jian-Ming Yang a, You-Hui Su a,
Yi-Ding Yang a, Gao-Nan Li a,b,
⁎
a
College of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, PR China
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
b
a r t i c l e i n f o
a b s t r a c t
Four new cyclometalated phenylisoquinoline-based iridium(III) complexes [Ir(C∧N)2(bipy)]PF6 (5a–5d) (bipy =
2,2′-bipyridine) have been synthesized and fully characterized, where the C∧N ligands are 1-(4-(trifluoromethyl)
phenyl)isoquinoline, 4-(isoquinolin-1-yl)benzaldehyde, 4-(isoquinolin-1-yl)benzonitrile and 1-(3-fluoro-4-
methylphenyl)isoquinoline, respectively. The crystal structures of 5a and 5c have been determined. The
photophysical and electrochemical properties of these new complexes 5a–5d have been studied. All Ir(III) com-
plexes exhibit orange phosphorescence in dichloromethane solution at room temperature with a maximum at
593–618 nm, quantum yield of 0.046–0.16. The frontier molecular orbital diagrams and the lowest-energy
electronic transitions of 5a–5d have been calculated with density functional theory (DFT) and time-dependent
DFT (TD-DFT).
Article history:
Received 8 October 2013
Accepted 20 February 2014
Available online 1 March 2014
Keywords:
Iridium(III) complex
Phenylisoquinoline
Syntheses
Crystal structures
Photoluminescence
DFT calculation
© 2014 Elsevier B.V. All rights reserved.
Currently, a considerable amount of interests have been drawn to
the application of iridium(III) complexes in organic light-emitting de-
vices (OLEDs), as they have high phosphorescence quantum efficiency,
long excited-state lifetime and excellent color tunability [1–3]. In gener-
al, the structure and even the position of the substituents on the ligand
framework play important roles in tuning the photophysical properties
of Ir(III) complexes [4,5]. Meanwhile, substantial researches on iridium
complexes have showed that the introduction of a strongly electron-
withdrawing group in the cyclometalating ligand could eventually
cause a great increase of the luminescence efficiency [6,7]. These results
prompted us to systematically study the effects on emissive colors and
quantum efficiencies of different electron-withdrawing substituents
on cyclometalating ligand. Therefore, we chose 1-phenylisoquinoline
(piq) as the cyclometalated ligand with electron-withdrawing substitu-
ents (\CF3, \CHO, \CN and \F) to synthesize a series of ionic
iridium(III) complexes 5a–5d. The photophysical and electrochemical
properties of these complexes were investigated, and the lowest-
energy electronic transitions were analyzed based on density functional
theory (DFT) and time-dependent DFT (TDDFT).
The C^N ligands 4-(isoquinolin-1-yl)benzaldehyde (3b) and 1-(3-
fluoro-4-methylphenyl)isoquinoline (3d) were both synthesized by
the suzuki reaction between 1-chloroisoquinoline and 1.1 equivalents
of substituted phenylboronic acids [8]. The other two C^N ligands
1-(4-(trifluoromethyl)phenyl)isoquinoline (3a) and 4-(isoquinolin-1-
yl)benzonitrile (3c) were prepared according to the literature proce-
dure [9,10]. The reaction of 3a–3d with iridium trichloride hydrate
yielded four new precursors [(C^N)2Ir(μ-Cl)]2 (4a–4d). The correspond-
ing iridium(III) complexes (5a–5d, Scheme 1) were then produced by
the reaction of 4a–4d with the N^N ligand 2,2′-bipyridine [11]. Charac-
terization of all these new compounds has been accomplished by 1H
NMR and mass spectroscopies. In 1H NMR spectra of 5a–5d, the ratio
of the C^N to N^N ligands is 2:1, which suggests that the chemical
formula of each Ir(III) complex is [(C^N)2Ir(N^N)](PF6).
The structures of complexes 5a and 5c were determined at room
temperature [12]. As shown in Fig. 1, the Ir(III) ion in each cationic com-
plex is in a distorted octahedral [(C∧N)2Ir(N∧N)]+ coordination geome-
try with the C and N atoms of two phenylisoquinoline-based (C∧N)
ligands and the N atoms of bipyridine (N∧N) ligand. The average dis-
tances of Ir\N (2.108 Å for 5a, 2.097 Å for 5c) and Ir\C (2.003 Å for
5a, 2.000 Å for 5c) are typical for this type of complexes [13,14]. In par-
ticular, the Ir\N bond lengths from N∧N ligands are shorter than those
from C∧N ligands. Also, the two N atoms of the C∧N ligands are trans-
position to each other. The X-ray data show a large torsion angle for
complex 5a [C8–C9–C10–C16, −14.70°], which suggest that the \CF3
⁎
Corresponding author at: College of Chemistry and Chemical Engineering,
Hainan Normal University, Haikou 571158, PR China. Tel.: +86 898 31381637;
fax: +86 898 65889422.
E-mail address: ligaonan2008@163.com (G.-N. Li).
http://dx.doi.org/10.1016/j.inoche.2014.02.031
1387-7003/© 2014 Elsevier B.V. All rights reserved.

Z.-G. Niu et al. / Inorganic Chemistry Communications 43 (2014) 146–150
147
Scheme 1. Synthetic routes of C^N ligands 3a–3d and Ir(III) complexes 5a–5d.
group can cause larger molecular deformation. However, the torsion
angle [C8–C9–C11–C12] of complex 5c with \CN group is only 0.63°,
indicating that the 4-(isoquinolin-1-yl)benzonitrile ligand is
almost coplanar.
spectra of 5a–5d display the vibronically structured emission bands
3
3
3
3
⁎
arose from a mixture of LC ( π–π ) and CT ( MLCT) excited states.
Phosphorescence relative quantum yields (Φ) of 5a–5d in dichloro-
methane solution were measured to be 0.046–0.16 (Table 1) at
room temperature by using typical phosphorescent fac-Ir(ppy)3 as a
standard (Φ = 0.40) [4,23]. Compared with the parent complex
[Ir(piq)2(bpy)](PF6) (Φ = 0.089) [24], the introduction of the C\F
bond or \CF3 group in complexes 5b and 5d results in the increase of
the luminescence quantum yield dramatically. This is probably due to
the presence of the C\F bonds that could reduce the radiationless deac-
tivation rate in comparison with C\H bonds [6,25]. Additionally, the
electron-withdrawing substituents (\CHO and \CN) in the same site
of the ligand lead to a decrease in the phosphorescence quantum yield
for complexes 5a and 5c.
The electrochemical properties of the complexes 5a–5d were
studied by cyclic voltammetry (Fig. 5). All complexes show a pair of
reversible redox peaks, with potentials of 1.27–1.65 V. The positive
oxidation potential is attributed to the metal-centered Ir3+/Ir4+ oxida-
tion couple [26,27], because the higher oxidation potentials are ascribed
to the stronger electron-withdrawing ability of the substituent on
phenylisoquinoline ligands. Based on the electrochemical data, the
complexes can be arranged in the increasing order as follows:
5c N 5a N 5b N 5d, which is consistent with the actual electron-with-
drawing ability of \CN N \CF3 N \CHO N \F. The HOMO energy
deduced by the equation EHOMO = −(Eox + 4.8 eV) [28] is listed in
Table 1, as well as compared with the theoretical calculation results.
From the results, it can be seen that the HOMO orbital energy also
reveals the order as 5c N 5a N 5b N 5d.
The absorption spectra of complexes 5a–5d in CH2Cl2 solution are
presented in Fig. 2 and the spectral data are summarized in Table 1. In
common with most Ir(III) complexes [15–17], the absorption spectra
of 5a–5d are dominated by multiple bands originating from ligand-
⁎
centered π–π transitions and MLCT transitions. The strong absorption
bands of the higher energies (below 350 nm) are mainly assigned to
the spin-allowed intraligand 1(π–π ) transitions [18]. The moderate
⁎
absorption peaks of lower-energies (350–550 nm) are likely due to
charge-transfer (CT) transitions, with their nature being both spin-
allowed (singlet-to-singlet metal-to-ligand charge transfer, 1MLCT)
[19] and spin-forbidden (singlet-to-triplet metal-to-ligand charge
3
3
⁎
transfer, MLCT and ligand-centered π–π transition) [20].
Density functional theory (DFT) and time-dependent DFT (TDDFT)
calculations have been performed for complexes 5a–5d to gain insights
into the lowest-energy electronic transition (435 nm for 5a, 456 nm for
5b, 442 nm for 5c, 434 nm for 5d). The representative molecular frontier
orbital diagrams have been presented in Fig. 3 and the calculated spin-
allowed electronic transitions and electron density distributions are
listed in Tables S1 and S2. The electron density in the HOMO/HOMO-1
of all the complexes 5a–5d resides significantly on the cyclometalating
ligands and the core Ir(III) atom. The electron density in LUMO + 1
(5a) and LUMO + 2 (5c) locates mainly on the cyclometalating ligand
and ancillary ligand, while the electron density in LUMO (5b) and
LUMO + 2 (5d) locates primarily on the cyclometalating ligand.
⁎
All these electronic transitions are ascribed as LLCT and MLCT π → π
In conclusion, four new cyclometalated phenylisoquinoline-based
iridium(III) complexes [Ir(C∧N)2(N∧N)]PF6 (5a–5d) with the N∧N li-
gand 2,2′-bipyridine and four different C∧N cyclometalating ligands
have been synthesized. The photophysical properties, electrochemical
behaviors and theoretical calculations have been investigated. X-ray dif-
fraction studies of complexes 5a and 5c indicate that the coordinations
of the iridium atoms are both distorted octahedral. Analyses of DFT and
TDDFT calculations for 5a–5d indicate that the lowest-energy electronic
transition, which are consistent with the actual absorptions.
All complexes are luminescent at room temperature in CH2Cl2 solu-
tion (Fig. 4). Upon photoexcitation at ca. 360 nm, the complexes 5a–5d
show intense orange luminescence with the emission peak appeared at
593–618 nm, and display the wavelength order as 5b N 5c N 5a N 5d
(Table 1). The results are in good agreement with those of the lowest-
energy electronic transition in UV–Vis spectra and DFT calculations
(Table S1). As compared with 5b and 5c, 5a and 5d exhibit the blue-
shifted emission spectra, which can be explained by steric hindrance
of 4-CF3, 4-Me and 3-F group. According to analogous iridium(III)
cyclometalates reported in references [21,22], the photoluminescence
⁎
transitions are attributed to MLCT and LLCT π → π transition. Each
Ir(III) complex shows orange emission with a maximum main peak
and a shoulder peak, which is suggested to be a mixture of 3LC
3
( π–π ) and 3CT (3MLCT) excited states. The results will facilitate the
⁎

148
Z.-G. Niu et al. / Inorganic Chemistry Communications 43 (2014) 146–150
Table 1
Photophysical and electrochemical data for complexes 5a–5d.
a
Complex Absorption
abs (nm)
Emission Φem
em (nm)
Eox
(eV) (eV)
HOMOb HOMOc
λ
λ
(eV)
5a
5b
5c
5d
285, 341, 356, 379, 435 594
294, 345, 360, 398, 456 618
288, 343, 359, 384, 442 604
293, 345, 357, 374, 434 593
0.14
1.63 −6.43
−6.044
−6.043
−6.177
−5.266
0.046 1.54 −6.34
0.074 1.65 −6.45
0.16
1.37 −6.17
a
fac-Ir(ppy)3 as referenced standard (0.4) [4,23].
HOMO = −(Eox + 4.8 eV).
Obtained from theoretical calculations.
b
c
design of new phenylisoquinoline-based ligands for light-emitting
iridium complexes.
Acknowledgments
This work was supported by the Science and Research Project of
Education Department of Hainan Province (No. Hjkj2013-25), the
Natural Science Foundation of Hainan Province (No. 21401), (The
research of novel ionic iridium(III) complexes: syntheses, struc-
tures and optoelectronic properties) and the National Innovation
Experiment Program for University Students (Nos. 201311658021,
201311658023).
Appendix A. Supplementary material
CCDC 962809 and 962810 contain the supplementary crystallo-
graphic data of 5a and 5c. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif. Other supplementary data associated with this
article: Crystallographic data for 5a and 5c in CIF format. Calculated
spin-allowed electronic transitions and electron density distributions
for 5a–5d. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.inoche.
2014.02.031.
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Z.-G. Niu et al. / Inorganic Chemistry Communications 43 (2014) 146–150
149
5a
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Z.-G. Niu et al. / Inorganic Chemistry Communications 43 (2014) 146–150
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(d, J = 8.0 Hz, 2H), 8.77 (d, J = 8.0 Hz, 2H), 8.38 (d, J = 7.6 Hz, 2H), 8.19
(t, J = 7.6 Hz, 2H), 7.95 ~ 7.98 (m, 2H), 7.82 ~ 7.86 (m, 4H), 7.76 (d, J =
5.6 Hz, 2H), 7.51 (d, J = 7.2 Hz, 2H), 7.37 ~ 7.45 (m, 6H), 6.43 (s, 2H). MS-ESI:
m/z calculated: 893.2; found: 893.3 (M+); Synthesis of 5b: 5b was obtained by a
method similar to the preparation of 5a, Yield 62%. 1H NMR (400 MHz, CDCl3) δ
9.64 (s, 2H), 8.93 (d, J = 7.2 Hz, 2H), 8.71 (d, J = 7.6 Hz, 2H), 8.46 (d, J =
8.4 Hz, 2H), 8.14 ~ 8.18 (m, 2H), 7.97 (t, J = 6.4 Hz, 2H), 7.80 ~ 7.86 (m, 6H), 7.62
(t, J = 8.0 Hz, 2H), 7.39 ~ 7.54 (m, 6H), 6.75 (s, 2H). MS-ESI: m/z calculated:
813.2; found: 813.3 (M+); Synthesis of 5c: 5c was obtained by a method similar
to the preparation of 5a, Yield 78%. 1H NMR (400 MHz, CDCl3) δ 8.89 (d, J =
5.6 Hz, 2H), 8.76 (d, J = 8.8 Hz, 2H), 8.41 (d, J = 7.2 Hz, 2H), 8.20 (t, J =
8.0 Hz, 2H), 7.98 ~ 8.01 (m, 2H), 7.87 ~ 7.89 (m, 4H), 7.76 (d, J = 4.8 Hz, 2H),
7.54 (d, J = 6.4 Hz, 2H), 7.42 ~ 7.46 (m, 6H), 6.51 (s, 2H). MS-ESI: m/z calculated:
807.2; found: 807.3 (M+); Synthesis of 5d: 5d was obtained by a method similar
to the preparation of 5a, Yield: 54%. 1H NMR (400 MHz, CDCl3) δ 8.85 ~ 8.92
(m, 2H), 8.71 (t, J = 8.0 Hz, 1H), 8.12 ~ 8.18 (m, 2H), 8.08 (d, J = 9.2 Hz, 1H),
8.00 ~ 8.04 (m, 1H), 7.89 ~ 7.94 (m, 2H), 7.73 ~ 7.83 (m, 6H), 7.33 ~ 7.43 (m, 4H),
7.20 ~ 7.28 (m, 3H), 6.99 ~ 7.03 (m, 1H), 5.99 ~ 6.01 (m, 1H), 1.99 ~ 2.08 (m, 6H).
MS-ESI: m/z calculated: 821.2; found: 821.3 (M+).