Structural and photoluminescent properties of near-infrared emissive bis(2,3-diphenylbenzoquinoxalinato)(2-pyrazinecarboxylato) ridium(III)

Full text of DOI:10.1002/bkcs.11357

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DOI: 10.1002/bkcs.11357
BULLETIN OF THE
Y.-I. Kim et al.
KOREAN CHEMICAL SOCIETY
Structural and Photoluminescent Properties of Near-Infrared Emissive
Bis(2,3-Diphenylbenzoquinoxalinato)(2-Pyrazinecarboxylato)
ridium(III)
Young-Inn Kim,†,‡, Seong-Jae Yun,†,‡ Daeyoung Kim,§ and Sung Kwon Kang§,
*
*
†Department of Chemistry Education, Pusan National University, Busan 46241, Korea
‡Department of Chemical Materials, Graduate School, Pusan National University, Busan 46241, Korea.
*E-mail: yikim@pusan.ac.kr
§Department of Chemistry, Chungnam National University, Daejeon 34134, Korea.
*E-mail: skkang@cnu.ac.kr
Received September 28, 2017, Accepted November 14, 2017, Published online January 8, 2018
Keywords: 2,3-Diphenylbenzoquinoxaline, Octahedral iridium(III) complex, Near-infrared emissive
iridium(III) complex, Structural and photoluminescent properties
Phosphorescent iridium(III) complexes have been devel-
oped as efficient phosphors for use in organic light-
emitting devices (OLEDs) to apply for large area flat panel
displays and solid-state lighting in the next generation.1
Octahedral d6 iridium(III) complexes usually exhibit a
variety of emission colors with high device efficiencies
due to their relatively short-triplet lifetimes. Numerous
iridium(III) complexes emitted from red to blue have been
reported. However, near-infrared (NIR) emissive
iridium(III) complexes are rare,2 even though NIR-emitting
materials are expected to be applied for night-vision dis-
plays and sensors for the needs in both military and civil-
ian uses.3 Recently, we reported pure red emissive
iridium(III) complex bearing 2,3-diphenylquinoxalinato
(dpq) and 2-pyrazinecarboxylate (prz) ligands, (dpq)2Ir
(prz), which adopted an octahedral symmetry around irid-
ium cation.4 The red emission was originated from the
intra-ligand charge transfer (3ILCT) in a π-conjugated dpq
moiety. Utilizing the π-conjugated phenyl-based ligands as
metal coordination ligands usually decrease the lowest
unoccupied molecular orbital (LUMO) energy levels,
which lead red-shift emissions in iridium(III) complexes.
Therefore, in order to develop a novel NIR emissive mate-
rial, we prepared 2,3-diphenylbenzoquinoxaline (dpbqH), a
more π-extended ligand than dpqH, and its cyclometalated
iridium(III) complex, (dpbq)2Ir(prz), to investigate the
photoluminescent (PL) properties for an application in
NIR-emitting OLEDs. The structures of dpbqH (1) and
(dpbq)2Ir(prz) (2) were determined.
emission at 763 nm in solution (1 × 10−5 M in dichloro-
methane), and two maximum peaks at 755 and 833 nm in
film (poly(methyl methacrylate) [PMMA] doped with
(dpbq)2Ir(prz), 5 wt %). UV–visible and PL spectra are
shown in Figure 1 and the resulted data are summarized in
Table 1. A photoluminescence quantum yield (PLQY) was
obtained for the value of 14% in solution using Ir(ppy)3
(40%), ppy = 2-phenylpyridine, as a reference. Film PL
spectrum showed a blue-shifted emission by 8 nm when
compared to the solution spectrum, which is due to the
motional relaxation of excited-state geometry resulting from
the rigidity of the matrix in film.6 Recently, pure red-
emissive iridium(III) complex, (dpq)2Ir(prz), was reported
and the red emission was attributed to the π-extended
cyclometalated dpq ligand.4 The iridium complexes with
largely conjugated ligands have shown efficient red emis-
sion due to allowed intersystem crossing.7 A red-shift emis-
sion by 110 nm was observed in complex 2 in the film
compared to that of (dpq)2Ir(prz) [λmax = 645 nm]. This
bathochromic shift is believed to be caused by the exten-
sion of π-conjugation in dpbq and the emission was arisen
from a dpbq moiety in complex 2.
The electrochemical property was studied in dichloro-
methane by cyclic voltammetry (CV) analysis. The highest
occupied molecular orbital (HOMO) and LUMO energy
levels were calculated using EHOMO = − (E1/2ox + 4.4 eV)
and ELUMO = EHOMO − ΔEgap. The band gap (ΔEgap) was
ascertained from the optical absorption spectrum onset
value, ΔEgap =1240/λedge.8 The obtained values are listed in
Table 1.
The space group of compound 1 is monoclinic centro-
symmetric C2/c. Two phenyl (C15–C20 and C21–C26)
planes and benzoquinoxaline (N1/C2–C11/N12/C13, C14)
plane are twisted each other with dihedral angles of 47.52
(4)ꢀ and 44.25 (4)ꢀ, respectively, as shown in Figure 2. In
the crystal, weak π–π interactions between aromatic rings
In UV–visible spectrum of complex 2, the intense
absorption bands assigned to ligand centered spin-allowed
1π–π* transitions (1LC) were found at about 250–370 nm,
and the mixed singlet and triplet metal-to-ligand charge
transfers (1MLCT and 3MLCT) with ligand-to-ligand
charge transfer transitions (LLCTs) were observed as long
tails extending to 600 nm.5 PL spectrum showed an NIR
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that of Ir1–N57 (2.154(7) Å) due to a stronger trans effect
of the phenyl group compared to the pyrazine ring, result-
ing in a trans N, N configuration. The stronger trans effect
of the phenyl group caused a longer Ir1–O54 bond distance
of 2.187 (5) Å than the average Ir─O distance of
2.088 Å.11
In bidentate dpbq ligands, the dihedral angles between
the coordinated phenyl rings (C16–C21 and C42–C47) and
benzoquinoxaline rings (N2/C3–C12/N13/C14, C15 and
N28/C29–C38/N39/C40, C41) are 19.27 (41)ꢀ and 19.73
(27)ꢀ, respectively. These angles are much small compared
to those of other free phenyl rings (43.53 (22)ꢀ for C22–
C27 and 48.71 (27)ꢀ for C48–C53). There are two kinds of
hydrogen bonds in complex 2, viz. C H…N and C H…
O. These intra- and inter-molecular interactions stabilize
and link the molecules into a three-dimensional network, as
shown in Figure S2. The hydrogen bond data are shown in
Table S3.
In summary, a novel NIR emitting heteroleptic cyclome-
talated iridium(III) complex, (dpbq)2Ir(prz) (2), has been
developed through increasing π-system on 2,3-
diphenyquinoxalines (dpqH) with bathochromic shifted
emission up to 763 nm in solution. The NIR emission was
attributed from π-extended dpbq moiety in complex 2. The
molecular structures of the main ligand, (dpbqH) (1) and its
iridium complex, (dpbq)2Ir(prz) (2), were determined. The
intra- and inter-molecular hydrogen bonds stabilize a three-
dimensional network in complex 2.
Figure 1. UV–visible and photoluminescence (PL) spectra of
(dpbq)2Ir(prz).
generate dimeric aggregates, centroid–centroid distance
(Ct1–Ct4 or Ct2–Ct3) = 3.753 (5) Å as shown in Figure 3.
Two molecules of the dimer have a crystallographic inver-
sion center, and these dimers interacted with van der Waals
interactions. Figure S1 (Supporting Information) shows
three-dimensional crystal packing of compound 1. Complex
2 crystallized in monoclinic centrosymmetric P21/c space
group. As shown in Figure 4, iridium(III) cation is coordi-
nated by three N atoms, two C atoms, and one O atom of
two cyclometalated dpbq ligands and one prz ligand to
form a distorted octahedral geometry. The selected bond
lengths and bond angles are listed in Table S2. The Ir1─C,
Ir1─N and Ir─O bond lengths are compatible with those of
other iridium complexes.9 The dpbq ligands show a mutual
eclipse configuration each other. Their coordinated N atoms
(N2 and N28) and C atoms (C21 and C47) are in trans and
cis orientation, respectively. The prz ligand is positioned at
the opposite of the carbon atoms of two coordinated dpbq
ligands. The cis C, C geometry is more stable and labile
than trans C, C geometry due to the rich σ-phenyl ligand,
which is known as “transphobia.”10 For the cyclometalated
ligands, the bite angles at iridium(III) ion are expected to
be less as 80.0 (3)ꢀ and 81.2 (3)ꢀ for dpbq ligands and 75.3
(2)ꢀ for the prz ligand. The bond distances of Ir1–N2
(2.046 (6) Å) and Ir1–N28 (2.045 (6) Å) are shorter than
Experimental
Measurements. UV–visible and PL spectra were exam-
ined using a Jasco V-570 spectrophotometer and a Hitachi
F-4500 fluorescence spectrophotometer, respectively. PL
spectra were examined in a dichloromethane solution
(~10−5 M) and in a film. The film spectrum was obtained
on the doped film of (dpbq)2Ir(prz) in PMMA with the con-
centrations of 5% by weight. CV was carried out using a
Bioanalytical Systems CV-50W voltametric analyzer with a
potential scan rate of 50–100 mV/s in dichloromethane
solution of tetra(n-butyl)ammonium tetrafluoroborate
(0.1 M). X-ray data were collected on a Bruker SMART
Table 1. .Photophysical and electrochemical properties for (dpbq)2Ir(prz) (2).
Emission λmax (nm)
Absorption λmax (nm)
Compound
(ε [×10−5 M−1 cm−1])
Solutiona
763
Flima
Φa
Ered (V)
−1.02
HOMO (eV)
LUMO (eV)
Ega (eV)
(dpbq)2Ir(prz)
285 (0.66), 327 (0.52)
369 (0.14), 391 (0.13)
416 (0.22), 449 (0.10)
486 (0.08), 533 (0.05)
569 (0.04), 605 (0.02)
755, 833
0.14
−5.30
−3.25
2.05
a
All data were measured in CH2Cl2 solution (1 × 10−5 M).
PMMA film doped with 5 wt % of iridium(III) complex.
b
c
Excitation wavelength was 380 nm. Ir(ppy)3 (0.40) was used as an external reference.
Bull. Korean Chem. Soc. 2018, Vol. 39, 133–136
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using a slightly modified method reported previously.9 The
synthetic route used is outlined in Scheme 1.
C24
C23
C25
2,3-Diphenylbenzoquinoxaline (dpbqH) (1).
Benzil
C26
C21
(3 mmol), 2,3-diaminonaphthalene (4.4 mmol), and iodine
(0.37 mmol) were reacted in acetonitrile (10 mL) at room
temperature. The precipitates were extracted with ether after
adding into water. The residue evaporated in vacuum was
purified using silica gel chromatography in ethyl acetate/
hexane (1:20), and further purified by recrystallization in
dichloromethane/hexane (1:1) using a slow evaporation
method at room temperature. Pale yellow crystal. Yield:
N12
C8
C10
C7
C6
C9
C11
C2
C22
C13
C14
C20
C4
C19
C3
N1
C5
C15
C16
1
46%. H NMR (300 MHz, CDCl3): 8.76 (s, 1H), 8.14 (d,
C18
C17
1H, J = 6.3 Hz), 7.58 (d, 3H, J = 7.2 Hz), 7.39 (t, 3H,
J = 9.3 Hz).
Figure 2. The molecular structure of dpbqH showing the atom-
numbering scheme. Displacement ellipsoids are drawn at the 30%
probability level.
Bis(2,3-Diphenylbenzoquinoxalinato)(2-Pyrazinecarbox-
ylato)Iridium(III) ((dpbq)2Ir(prz)) (2). The cyclometa-
lated iridium(III) μ-chloro-bridged dimer, [(dpbq)2Ir
(μ-Cl)]2, was prepared from IrCl3Á3H2O and dpbqH, and
further reacted with 2-pyrazinecarboxylic acid (przH) to
C6
C5
Ct2
C3
Ct1
C4
C2
give
bis(2,3-diphenylbenzoquinoxalinato)(2-pyrazinecar-
C7
C8
C9
boxylato)iridium(III), (dpbq)2Ir(prz).
C10
C11
The reaction of IrCl3Á3H2O (3 mmol) with dpbqH
(7 mmol) at 403 K in a solution of 2-ethoxyethanol/water
(20 mL; 3:1 v/v) for 24 h to give μ-chloro-bridged dimer,
[(dpbq)2Ir(μ-Cl)]2. Yield: 86%. przH (0.44mmol), sodium
carbonate (2.5 mmol), and [(dpbq)2Ir(μ-Cl)]2 (0.2 mmol)
were reacted in 20 mL of 2-ethoxyethanol at 403 K for
24 h. The crude product extracted with dichloromethane
was obtained on silica gel chromatography using dichloro-
methane/methanol to yield (dpbq)2Ir(prz). The recrystallization
from a mixed solution of chloroform/hexane (1:1 v/v) at room
temperature gives single crystals suitable for X-ray analysis.
C11
C10
Ct3
C9
C8
C7
C6
Ct4
C2
C4
C3
C5
Figure 3. Dimeric structure of dpbqH showing molecules linked
by intermolecular π–π interactions [symmetry code: −x,
−y + 1, −z].
C33
C26
C19
C34
C18
C25
C24
C17
C35
C32
C31
C27
1
Yellow crystal. Yield: 44%. H NMR (300 MHz, CDCl3) δ
C20
C36
C37
9.32 (s, 1H), 9.04 (s, 1H), 8.93 (d, 1H, J = 3.0 Hz), 8.69 (d,
1H, J = 14.1 Hz), 8.54 (s, 1H), 8.16 (d, 1H, J = 9.0 Hz),
8.01–8.08 (m, 4H), 7.84–7.90 (m, 4H), 7.65–7.71 (m, 6H),
7.46–7.55 (m, 4H), 7.36 (t, 1H, J = 7.4 Hz), 7.21 (t, 1H,
J = 6.5 Hz), 7.11 (d, 1H, J = 7.5 Hz), 6.86 (d, 1H,
J = 7.2 Hz), 6.74 (t, 1H, J = 7.1 Hz), 6.58–6.64 (m, 2H), 6.50
(t, 1H, J = 6.9 Hz), 6.20 (d, 1H, J = 7.2 Hz).
C16
C58
C23
C59
C15
C22
C30
C29
C21
N57
C38
C14
Ir1
N28
N39
N60
C41
N2
C3
N13
C47
C42
C40
C48
C49
C12
C46
C61
C56
C55
C50
C51
C43
C45
O54
C44
Cl2
C4
Refinement. Crystallographic data for compound 1 and
complex 2 are shown in Table S1. In two title compounds,
all hydrogen atoms were geometric positioned and refined
using a riding model, with C H = 0.93–0.98 Å and
Uiso(H) = 1.2Ueq(C) for aromatic and chloroform H atoms.
In complex 2, C and Cl atoms in chloroform solvent mole-
cule were heavily disordered and refined isotropically.
Bond-distance restraints were also applied for C–Cl. The
maximum and minimum residual electron density peaks for
complex 2 were found at 2.39 and 0.67 Å, respectively,
from atom Cl2.
C11
C10
C53
O62
C5
C6
C52
Cl3
C9
C63
Cl1
C7
C8
Figure 4. The molecular structure of (dpbq)2Ir(prz) showing the
atom-numbering scheme. Displacement ellipsoids are drawn at the
30% probability level.
APEX-II CCD diffractometer. The physical measurements
were carried as reported previously.9
The deposit numbers of molecular structures of com-
pound 1 and complex 2 in the Cambridge Crystallographic
Data Center (www.ccdc.cam.ac.uk/deposit) are compound
1, CCDC-1572681, and complex 2, CCDC-1572682.
Synthesis and Crystallization. The main ligand, dpbqH,
and its iridium complex, (dpbq)2Ir(prz), were synthesized
Bull. Korean Chem. Soc. 2018, Vol. 39, 133–136
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BULLETIN OF THE
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KOREAN CHEMICAL SOCIETY
O
NH2
NH2
N
N
Iondine, acetonitrile
RT
+
O
N
N
N
N
N
N
O
N
N
OH
N
N
N
N
Cl
Cl
IrCl33H2O
Ir
Ir
N
Ir
EtO-EtOH : H2O = 3 : 1
Na2CO3, EtO-EtOH
O
O
N
N
N
N
N
Scheme 1. Synthetic route for the preparation of dpbqH (1) and (dpbq)2Ir(prz) (2).
4. M. K. Song, S. J. Yun, K. S. Nam, H. Liu, Y. S. Gal,
J. W. Lee, S. H. Jin, J. Y. Lee, S. K. Kang, Y. I. Kim,
J. Organomet. Chem. 2015, 794, 197.
Acknowledgments. This work was supported by research
fund of Chungnam National University.
5. (a) C. L. Ho, W. Y. Wong, G. J. Zhou, B. Yao, Z. Xie,
L. Wang, Adv. Funct. Mater. 2007, 17, 2925. (b) S. C. Lo,
C. P. Shipley, R. N. Bera, R. E. Harding, A. R. Cowley,
P. L. Burn, D. W. Samuel, Chem. Mater. 2006, 18, 5119.
(c) Y. M. You, K. S. Kim, T. K. Ahn, D. H. Kim, S. Y. Park,
J. Phys. Chem. C 2007, 111, 4052.
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
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