A series of dinuclear cuprous iodide complexes chelated with 1,2-bis(diphenylphosphino)benzene derivatives: Structural, photophysical and thermal properties

Article abstract of DOI:10.1039/c5ce02533h

Three new bisphosphine ligands, 4-phenyl-1,2-bis(diphenylphosphino)benzene (Ph-dppb), 4-pyrrolyl-1,2-bis(diphenylphosphino)benzene (Pr-dppb), and 4,5-dimethoxyl-1,2-bis(diphenylphosphino)benzene (OMe-dppb), were synthesized to coordinate with cuprous iodi

Full text of DOI:10.1039/c5ce02533h

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A series of dinuclear cuprous iodide complexes
chelated with 1,2-bisIJdiphenylphosphino)benzene
derivatives: structural, photophysical and thermal
properties†
Cite this: DOI: 10.1039/c5ce02533h
*
*
Xiaoyue Li, Juanye Zhang, Feng Wei, Xiaochen Liu, Zhiwei Liu, Zuqiang Bian
and Chunhui Huang
Three new bisphosphine ligands, 4-phenyl-1,2-bisIJdiphenylphosphino)benzene (Ph-dppb), 4-pyrrolyl-1,2-
bisIJdiphenylphosphino)benzene (Pr-dppb), and 4,5-dimethoxyl-1,2-bisIJdiphenylphosphino)benzene (OMe-
dppb), were synthesized to coordinate with cuprous iodide (CuI), and compared with 1,2-
bisIJdiphenylphosphino)benzene (dppb). Single crystal X-ray diffraction and elemental analysis experiments
indicate that three binuclear CuI complexes with formulae of [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, and
[Cu(μ2-I)OMe-dppb]2 were obtained, which are the same as the reference complex [Cu(μ2-I)dppb]2. The
three complexes showed a maximum emission band (λmax) in the range of 484–535 nm which varies in the
electron effect of the substituted groups (i.e. methoxy, pyrrolyl, phenyl) on [Cu(μ2-I)dppb]2, and in high
photoluminescence quantum yield (PLQY) from 53 to 90% in the solid state. Furthermore, the complexes
[Cu(μ2-I)Ph-dppb]2 and [Cu(μ2-I)Pr-dppb]2 exhibited a thermal decomposition temperature exceeding 380
°C. The high PLQY and excellent thermal stability of these CuI complexes imply their great potential appli-
cation as efficient emitters in organic light-emitting diodes (OLEDs).
Received 23rd December 2015,
Accepted 16th March 2016
DOI: 10.1039/c5ce02533h
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Systematic studies on mononuclear [CuIJN^N)2]+ com-
plexes, where N^N stands for a chelating bisimine ligand,
Introduction
Luminescent transition metal complexes are widely studied
because of their potential use in organic light emitting diodes
(OLEDs). Noble metal such as IrIJIII), PtIJIII), or OsIJII)-based com-
plexes can generate light from both triplet and singlet exci-
tons in OLEDs, allowing the internal quantum efficiency (IQE)
of such a device to approach 100%.1–6 As they are relatively inex-
pensive, abundant and environment-friendly luminophores,
CuIJI) complexes have been investigated widely. After the
pioneering work of McMillin, Eisenberg and Ford,7–9 it is
well known that most CuIJI) complexes frequently feature metal-
to-ligand charge-transfer (MLCT) transition bands in the spec-
trum which can be associated with a transformation between
CuIJI) in the ground state and CuIJII) in the excited state.
have shown that the substituent effects of typical bisimine
ligands, such as phenanthroline derivatives at 2,9-positions
on the MLCT luminescence intensity can be explained by
restraining the structural distortion from the tetrahedral to
the flattened structure in the excited state.10,11 The major
nonradiative process for the MLCT excited state is attributed
to the structural distortion, so the inhibition of diversifica-
tion could increase the photoluminescence quantum yield
(PLQY) of Cu(I) complexes. Thereafter, attention has been
paid to [Cu(N^N)(P^P)]+, [Cu(P^N)(P^P)]+, and [Cu(P^P)2]+
complexes, where P^N and P^P denote an amidophosphine
and bisphosphine ligand, respectively.12–14 These complexes
exhibited greatly enhanced emission intensity because the P
atom introduces rigid and bulky groups, which prevents
structural distortion and results in less nonradiative transi-
tion. By modifying the chemical and electronic structure of
the N^N, P^N, or P^P type ligand, the emission properties
of these complexes have been tuned, with emission colour
varying from blue to red and PLQY up to unit,13,14 which
shows great potential application as efficient emitter in
OLEDs.
Beijing National Laboratory for Molecular Sciences (BNLMS), State Key
Laboratory of Rare Earth Materials Chemistry and Applications, College of
Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR
China. E-mail: zwliu@pku.edu.cn, bianzq@pku.edu.cn
† Electronic supplementary information (ESI) available: X-ray crystallographic
files in CIF format are all available. CIFs have also been deposited at the Cam-
bridge Crystallographic Database Centre. CCDC 1456957, 1443647, and 1443643
for [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2, respectively.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ce02533h
Besides the mononuclear CuIJI) complexes, there have also
been several reports on dinuclear CuIJI) complexes.15–21
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Among which the one with chelation between CuI and 1,2-
bisIJdiphenylphosphino)benzene (dppb), named as [CuIJμ2-
I)dppb]2, is appealing.17 The compound showed not only high
PLQY up to 80% in the solid state but also good thermal sta-
bility that can be processed in OLEDs using a traditional vac-
uum thermal deposition technique. Recently, we have dem-
onstrated that high efficiency nondoped OLEDs can be
achieved with a neat [Cu(μ2-I)dppb]2 film as the emissive
layer, eliminating the need for a host matrix and making the
device fabrication more simplified.22 The device showed
green emission and a maximum external quantum efficiency
(EQE) of 8.3%, which is comparable to that of the doped
OLEDs with triplet exciton confinement architecture in the liter-
ature.23 Consequently, it would be interesting to tune the emis-
sion colour of this type of Cu(I) complex, hence to make high
efficiency non-doped OLEDs with various emission colours.
Based on Hartree–Fock calculation on [CuIJμ2-I)dppb]2,17
the electron density in the highest occupied molecular orbital
(HOMO) is distributed over the copper and iodine atoms,
while that in the lowest unoccupied molecular orbital
(LUMO) is localized on the dppb ligands. Theoretically, the
emission colour of this type of complex could be tuned by
attaching electron-rich or electron-poor groups on the dppb
ligand. Herein, three dinuclear Cu(I) complexes, [Cu(μ2-I)Ph-
dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2, which
are derived from [Cu(μ2-I)dppb]2 by attaching a phenyl,
pyrrolyl, or methoxy group on the dppb ligand were designed
and synthesized (Scheme 1). The structures, photophysical
and thermal properties of these complexes are discussed.
sponse. Quantum yield measurements were conducted using
a Hamamatsu C9920 system equipped with a xenon lamp,
calibrated integrating sphere and model C10027 photonic
multichannel analyser. Thermal gravimetric analysis was un-
dertaken on a Q600SDT instrument unit at a heating rate of
5 °C min−1 from room temperature to 600 °C under N2. The
glass transition temperature was determined from the third
heating scan.
X-ray crystallography
Suitable single crystals for X-ray measurement were collected
by thermal sublimation or by a diffusion method with
dichloromethane and pentane as the solvents described in
the following section. The crystals were mounted on a glass
fiber with Paratone-N oil. X-ray diffraction data were deter-
mined using an Agilent diffractometer with graphite-
monochromated Mo Kα radiation, and structures were solved
by direct methods with standard Fourier techniques with the
help of AXS software package.
Synthesis
Potassium diphenylphosphine (KPPh2). KPPh2 was pre-
pared by adding a THF solution (20 mL) of chlorodiphenyl
phosphine (5.50 g, 25 mmol) to drossy potassium metal
(1.95 g, 50 mmol) in THF (60 mL) dropwise under N2 at room
temperature, and the resulting mixture was stirred and
heated to reflux for 12 h. The solution of KPPh2 in THF was
brownish red, ultimately.
1,2-BisIJdiphenylphosphino)benzene (dppb). dppb was syn-
thesized in a way similar to the reported procedure.24 To a
solution of KPPh2 (25 mmol) in THF (80 mL) was added
slowly a solution of 1,2-difluorobenzene (1.25 g, 11 mmol) in
toluene (50 mL). The mixture was refluxed for 28 h and then
allowed to cool down to room temperature. A crude product
was obtained by removal of the solvent, which was
recrystallized in toluene to get 3.01 g of white solid (yield:
Experimental
Instruments and measurements
The 1H NMR and 31P NMR spectra were recorded using a
Bruker-400 MHz and 162 MHz spectrometer, respectively.
Chemical shift data for each signal were reported in ppm
units with CD3OD or CD2Cl2 as reference, where δ(CD3OD)
and δ(CD2Cl2) are 3.31 and 5.21, respectively. Elemental anal-
yses (C, H, and N) were carried out with an elemental
analyser vario MICRO cube from Elementar Co. The mass
spectra were obtained using a Bruker Apex IV Fourier Trans-
form Ion Cyclotron Resonance Mass Spectrometer. The UV-
vis spectra were recorded on a Shimadzu UV-3100. The photo-
luminescence spectra were recorded on an Edinburgh FLS980
fluorescence spectrometer and corrected for detector re-
1
60%). H NMR (400 MHz, CD2Cl2) δ 7.24–7.14 (m, 14H), 7.14–
7.05 (m, 8H), 6.99–6.92 (m, 2H). MS: m/z 447.1 (M+, 100).
3,4-Difluorobiphenyl (S1). To a round-bottom flask was
added phenylboronic acid (2.70 g, 22 mmol), sodium carbon-
ate (4.40 g, 41.5 mmol), 4-bromo-1,2-difluorobenzene (4.01 g,
20.7 mmol), PdIJPPh3)4 (0.40 g, 0.4 mmol), ethanol (100 mL)
and water (50 mL), and the mixture was stirred at 80 °C for
16 h. After cooling to room temperature, the mixture was
extracted by using DCM (3 × 50 mL). The organic phase was
washed with NaCl solution, dried over MgSO4, filtered and re-
moved off the solvent under reduced pressure. The crude
product was purified by column chromatography to obtain
3.61 g of white solid (yield: 95%). 1H NMR (400 MHz, CD3OD):
δ (ppm) 7.58 (m, 2H), 7.52 (m, 1H), 7.42 (m, 3H), 7.34 (m, 2H).
MS: m/z 191.1 (M+, 100).
4-Phenyl-1,2-bisIJdiphenylphosphino) benzene (Ph-dppb).
Ph-dppb was prepared as a white solid in 30% yield by using
the same synthesis and work up procedure to dppb, where S1
(2.09 g, 11 mmol) was used to substitute 1,2-difluorobenzene.
Scheme 1 Synthetic route and molecular structure of [CuIJμ2-I)dppb]2,
[Cu(μ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2.
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1
1H NMR (400 MHz, CD3OD): δ (ppm) 7.275 (m, 28H). MS: m/z
523.2 (M+, 100).
[CuIJμ2-I)Ph-dppb]2. H NMR (400 MHz, CD2Cl2): δ 7.49 (d,
J = 8.0 Hz, 2H), 7.42–7.34 (m, 2H), 7.34–7.17 (m, 28H), 7.12
(t, J = 7.5 Hz, 8H), 7.01 (t, J = 7.6 Hz, 16H). 31P NMR (162
MHz, CD2Cl2): δ −19.70 to −26.80 (m). Anal. calcd. for
C72H56Cu2I2P4, C, 60.64; H, 3.96%. Found: C, 60.65; H,
3.99%.
1-(3,4-Difluorophenyl)-1H-pyrrole (S2). To a 100 mL round-
bottom flask was added acetic acid (1.4 mL), 1,2-
dichloroethane (37.2 mL), water (22.4 mL), 3,4-difluoroaniline
(2.58 g, 20 mmol), and 2,5-bisIJmethyloxy)tetrahydrofuran
(2.77 g, 21 mmol). After overnight stirring at room tempera-
ture under N2, the mixture was extracted with CH2Cl2 (3 × 30
mL). The organic phase was washed with NaCl solution,
dried over MgSO4, filtered and distilled off the solvent under
a reduced pressure. The crude product was purified by col-
umn chromatography to get 2.80 g of product (yield: 77%).
1H NMR (400 MHz, CD3OD): δ (ppm) 7.46 (m, 1H), 7.30 (m,
2H), 7.15 (t, 2H), 6.28 (t, 2H). MS: m/z 180.0 (M+, 100).
4-pyrrolyl-1,2-bisIJdiphenylphosphino)benzene (Pr-dppb).
Pr-dppb was prepared as a white solid in 43% yield by using
the same synthesis and work up procedure to dppb, where S2
(1.97 g, 11 mmol) was used to substitute 1,2-difluorobenzene.
1H NMR (400 MHz, CD3OD): δ (ppm) 7.310 (m, 20H), 7.055
(m, 2H), 6.950 (m, 1H), 6.795 (t, 2H), 6.180 (t, 2H). MS: m/z
511.2 (M+, 100).
1
[CuIJμ2-I)Pr-dppb]2. H NMR (400 MHz, CD2Cl2): δ 7.39 (dd,
J = 8.3, 2.2 Hz, 2H), 7.35–7.19 (m, 28H), 7.11 (t, J = 6.9 Hz,
16H), 6.95–6.82 (t, J = 2.2 Hz, 4H), 6.37–6.12 (t, J = 2.2 Hz,
4H). 31P NMR (162 MHz, CD2Cl2): δ −23.31 (q, J = 200.3, W1/2
= 197.1 Hz). Anal. calcd. for C68H54Cu2I2N2P4, C, 58.17; H,
3.88; N, 1.99%. Found: C, 58.16; H, 3.88; N, 2.00%.
1
[CuIJμ2-I)OMe-dppb]2. H NMR (400 MHz, CD2Cl2): δ 7.31–
7.18 (m, 24H), 7.01 (t, J = 7.5 Hz, 16H), 6.60 (t, J = 4.6 Hz,
4H), 3.46 (s, 12H). 31P NMR (162 MHz, CD2Cl2): δ −21.97 (brs,
W1/2 = 162 Hz). Anal. calcd. for C65H56Cu2I2O4P4, C, 55.53; H,
4.01%. Found: C, 55.48; H, 4.19%.
Results and discussion
Synthesis and structures
1,2-BisIJdiphenylphosphino)-4,5-dimethoxybenzene (OMe-dppb).
OMe-dppb was prepared with a different method to that of
The dppb series ligands used in this study were prepared
from coupled reactions of potassium diphenylphosphine and
1,2-difluorobenzene derivatives, or chlorodiphenyl phosphine
and methoxy substituted 1,2-dibromobenzene with the aid of
n-BuLi, in a mixed solvent under N2 atmosphere. According
to the work of Ford,26 CuI complexes exhibit various coordi-
nation motifs depending on the chemical structure of the co-
ordinating ligand as well as the molar ratio of the ligand and
CuI. Choosing the stoichiometry with a molar ratio of 1 : 1 : 2
results in a [CuIJμ2-I)]2L4 (where L stands for a ligand) form
with an isolated rhombohedron Cu2I2 core. Consequently,
three new dinuclear Cu(I) iodide complexes [Cu(μ2-I)Ph-
dppb]2, [Cu(μ2-I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2, and a refer-
ence complex [Cu(μ2-I)dppb]2 were prepared by the reaction
of CuI with one equivalent of dppb series ligands in toluene
at room temperature, and purified by recrystallization or vac-
uum thermal gradient sublimation according to their thermal
stability. The characterization of intermediates, ligands, and
dppb.25
A solution of 1,2-dibromo-4,5-dimethoxybenzene
(1.48 g, 5 mmol) in 50 mL of Et2O/THF (1 : 1) was cooled to
−100 °C, and a solution of n-BuLi (2.5 M in hexane, 4.2 mL,
10.5 mmol) was added dropwise. The pale brown reaction
mixture was stirred at −100 °C for 45 min, after which a solu-
tion of chlorodiphenyl phosphine (2.32 g, 10.5 mmol, in
10 mL of THF) was slowly added. The temperature of the re-
action flask was maintained at −100 °C during the addition.
The reaction mixture was further stirred at −100 °C for 2 h
and was then allowed to warm to room temperature over-
night. The white product (0.63 g, yield: 25%) was obtained by
concentrating the solution under vacuum and recrystallizing
the precipitate in toluene. 1H NMR (400 MHz, CD3OD):
δ (ppm) 7.255 (m, 20H), 6.575 (m, 2H), 3.590 (s, 6H). MS: m/z
506.1 (M+, 100).
1
complexes were established on the basis of H/31P NMR, ESI
CuIJI) complexes
mass spectra, or elemental analysis, which is shown in the
experimental section. The three complexes [Cu(μ2-I)Ph-
dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2 were fur-
ther characterized by X-ray diffraction for comparison with
the reference complex [Cu(μ2-I)dppb]2, where the single crys-
tals of [Cu(μ2-I)Ph-dppb]2 and [Cu(μ2-I)OMe-dppb]2 were
obtained by a diffusion method with dichloromethane and
pentane as the solvents, while that of [Cu(μ2-I)Pr-dppb]2 was
formed during vacuum thermal sublimation.
The ORTEP diagrams of [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-
dppb]2, and [Cu(μ2-I)OMe-dppb]2 are displayed in Fig. 1. De-
tailed crystal parameters and structure refinements of the
three complexes are summarized in Table 1. The comparison
on key parameters, i.e. Cu–Cu, Cu–P, Cu–I distances, I–Cu–I,
Cu–I–Cu, and P–Cu–P angles among [Cu(μ2-I)Ph-dppb]2,
[Cu(μ2-I)Pr-dppb]2, Cu(μ2-I)OMe-dppb]2, and [Cu(μ2-I)dppb]2
The four dinuclear CuIJI) complexes were prepared according
to the following general procedure: the same equivalent of
CuI and dppb series ligand in a moderate amount of toluene
was stirred for 5 h at room temperature to form a pale-yellow
precipitate. After concentrating the solution, the precipitation
was filtered off, and washed with ether. Except for the
complex [CuIJμ2-I)OMe-dppb]2 which was purified by recrystal-
lization in DCM/ethanol due to a poor thermal stability, the
other three complexes were purified by thermal gradient sub-
limation around 300 °C with a base pressure of 10−4 Pa.
[CuIJμ2-I)dppb]2. 1H NMR (400 MHz, CD2Cl2): δ 7.44–7.31
(m, 4H), 7.29 (q, J = 5.9, 5.3 Hz, 20H), 7.24 (t, J = 7.6 Hz, 8H),
7.12 (t, J = 7.5 Hz, 16H). (1H). 31P NMR (162 MHz, CD2Cl2): δ
−22.74 (brs, W1/2 = 188 Hz). Anal. calcd. for C60H48Cu2I2P4, C,
56.57; H, 3.80%. Found: C, 56.77; H, 3.79%.
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tween the Cu(1)–I(1)–Cu(2) and Cu(1)–I(2)–Cu(2) faces are
158.22°, 154.57°, 156.98°, and 156.20° for [Cu(μ2-I)Ph-dppb]2,
[Cu(μ2-I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2 and [Cu(μ2-I)dppb]2,
respectively. The crystallography study indicates that a com-
pact structure was obtained by attaching a pyrrolyl group on
the dppb ligand.
Photophysical properties
Fig. 2 shows the UV-vis absorption spectra of [CuIJμ2-I)Ph-
dppb]2, [Cu(μ2-I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2 and [Cu(μ2-
I)dppb]2 in CH2Cl2 solution (10−5 M) at room temperature.
These complexes showed intense absorption bands at 260–
290 nm, which are assigned to the π–π* transition of the
bisphosphine ligand (Fig. S1†). All the complexes showed
broad shoulders around 330 nm and weak ones around 390
nm. These absorption shoulders can be attributed to the
electronic transition of the Cu(I) chromophore.17
As anticipated, the band gap (Eg) of this type of complexes
changed as an electron-rich or electron-poor group was at-
tached on the dppb ligand. For example, the complex [CuIJμ2-
I)OMe-dppb]2 having two electron-donating methoxy groups
showed the maximum Eg of 3.13 eV (396 nm), while the
complex [Cu(μ2-I)Ph-dppb]2 having a phenyl group showed
the minimum Eg of 2.84 eV (437 nm). This is consistent with
the Hartree–Fock calculation on [Cu(μ2-I)dppb]2: the electron
density in the HOMO is distributed over the copper and io-
dine atoms, while that in the LUMO is localized on the dppb
ligands. Attaching an electron-rich or electron-poor group on
the ligand results in a lower or higher LUMO energy level
and consequently enlarged or decreased Eg, respectively.
Fig. 3 shows the photoluminescence spectra of the four
complexes in the solid state as microcrystallites (Fig. S2†) at
room temperature. The maximum emission peaks (λmax) for
[CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2,
and [Cu(μ2-I)dppb]2 are at 535, 528, 484, and 512 nm, respec-
tively, which is consistent with the order of their band gap.
Obviously, the luminescent excited state is affected by the
electronic properties of the substituted group (i.e. methoxy,
pyrrolyl, or phenyl) on the centered-benzene. The complex
[Cu(μ2-I)Ph-dppb]2 modified by a phenyl group, which ex-
pands the π-conjugation of the ligand hence to decrease the
LUMO energy of the complex, showed a low transition energy
with the most red-shifted λmax. While the complex [Cu(μ2-
I)OMe-dppb]2 showed a significant blue-shifted λmax since
two electron-donating methoxy groups were attached on the
centered-benzene. Interestingly, the complex [Cu(μ2-I)Pr-
dppb]2 having an electron-donating pyrrolyl group showed a
slightly red-shifted λmax, which may be due to an enhanced
electron-conjugation of the ligand. All the complexes have ex-
cited state lifetimes on the microsecond scale, implying that
these emissions may arise from triplet excited states
(Table 3).
Fig. 1 ORTEP drawings of [CuIJμ2-I)Ph-dppb]2 (top), [Cu(μ2-I)Pr-dppb]2
(middle), and [Cu(μ2-I)OMe-dppb]2 (bottom), respectively, with
ellipsoids at the 30% probability level. Hydrogen atoms are omitted for
clarity.
are listed in Table 2. In all complexes, two Cu(I) ions are
bridged by two μ2-I ions to form a dinuclear structure with a
four-membered Cu2I2 ring and coordinated by two bidentate
P^P ligands additionally; each Cu(I) ion is in a distorted tetra-
hedral geometry. The Cu–Cu distances are 2.904(3),
2.7745(7), 2.8317(10), and 2.8238(11) Å for [Cu(μ2-I)Ph-dppb]2,
[Cu(μ2-I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2, and [Cu(μ2-I)dppb]2,
respectively. Except [Cu(μ2-I)Ph-dppb]2, all the three com-
plexes have a Cu–Cu distance shorter than those found in di-
nuclear complexes with isolated P and N ligands (2.872–3.303
Å),18 but longer than those found in dinuclear complexes
with P^N type ligands (2.51–2.63 Å).27,28 Among the four com-
plexes, it is found that the complex [Cu(μ2-I)Pr-dppb]2 showed
a Cu–Cu distance a little shorter than that in the reference
complex [Cu(μ2-I)dppb]2, while the other two complexes
showed a Cu–Cu distance longer than that in [Cu(μ2-I)dppb]2.
This phenomenon was also observed when comparing Cu–P
distances among the four complexes. The dihedral angle be-
As the temperature is decreased to 77 K, all of the com-
plexes showed a similar emission spectrum to that observed
at room temperature and less than 15 nm shifted maximum
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Table 1 Summary of X-ray data collection and refinement of [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2
[CuIJμ2-I)Ph-dppb]2
[CuIJμ2-I)Pr-dppb]2
[CuIJμ2-I)OMe-dppb]2
Formula
Mw
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C72H56Cu2I2P4
1510.85
Monoclinic
C121
24.271(2)
14.1584IJ14)
9.6410(9)
90
98.970(9)
90
3272.5(5)
2
C68H54Cu2I2N2P4
1404.00
C64H56Cu2I2O4P4
1648.63
Monoclinic
C12/c1
19.5414IJ11)
14.5730(6)
26.9535IJ14)
90.00
113.268(7)
90.00
Triclinic
¯
P1
13.6302(6)
13.8186(5)
18.0831(7)
110.055(3)
104.744(3)
97.913(3)
2997.7(2)
2
γ (°)
Volume (Å3)
Z
7051.5(6)
4
T (K)
180.01(10)
1.533
1508
100.1(2)
1.5553
1400.9
180.01(10)
1.553
3288
Dcalcd (g cm−3
F (000)
)
θ range (°)
Index range
2.927–25.022
−27 ≤ h ≤ 28
−14 ≤ k ≤ 17
−16 ≤ l ≤ 16
1.034
3.2040–28.4070
−15 ≤ h ≤ 18
−17 ≤ k ≤ 12
−22 ≤ l ≤ 22
1.074
3.17–26.02
−17 ≤ h ≤ 24
−14 ≤ k ≤ 17
−33 ≤ l ≤ 33
1.023
GOF on F2
R [I > 2σ(I)]
R (all data)
0.0558
0.0828
0.0367
0.0598
0.0429
0.0681
Table 2 The comparison on key structure parameters of [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2, and [Cu(μ2-I)dppb]2
[CuIJμ2-I)Ph-dppb]2
[CuIJμ2-I)Pr-dppb]2
[CuIJμ2-I)OMe-dppb]2
[CuIJμ2-I)dppb]2
Cu1–Cu2
Cu1–I1
Cu1–I2
Cu1–P1
Cu1–P2
Cu2–I1
Cu2–I2
Cu2–P3
Cu2–P4
P1–Cu1–P2
P1–Cu1–I1
I1–Cu1–I2
I2–Cu1–P2
P3–Cu2–P4
P3–Cu2–I1
I1–Cu2–I2
I2–Cu2–P4
2.904(3)
2.7745(7)
2.6540(5)
2.6044(6)
2.2793(11)
2.2772(10)
2.6042(6)
2.6275(5)
2.2716(11)
2.2564(10)
87.38(4)
110.10(3)
111.325IJ19)
121.09(3)
90.67(4)
121.44(3)
112.186IJ19)
112.04(3)
2.8317(10)
2.6578(6)
2.6086(6)
2.2875(12)
2.2805(12)
2.6578(6)
2.6086(6)
2.2875(12)
2.2805(12)
86.85(4)
118.43(3)
111.43(2)
107.50(3)
86.85(4)
121.00(4)
111.43(2)
108.99(4)
2.8238(11)
2.6786(10)
2.6197(9)
2.281(2)
2.6039(17)
2.6558(18)
2.291(4)
2.283(4)
2.277(3)
2.6039(17)
2.6044(6)
2.291(4)
2.5710(10)
2.6504(10)
2.273(2)
2.267(2)
89.31(8)
104.27(6)
110.08(3)
119.49(6)
86.97(8)
118.19(7)
112.53(3)
109.64(6)
2.283(4)
86.03(14)
114.40(11)
109.94(6)
105.19(11)
86.03(14)
123.47
109.94(6)
115.96(11)
emission wavelength. Obvious bathochromic shifts were ob-
served in [CuIJμ2-I)Pr-dppb]2 and [Cu(μ2-I)OMe-dppb]2. This
phenomenon may be attributed to the suppression of ther-
mally activated delayed fluorescence (TADF) and is usually ac-
companied by a marked increase in emission lifetime.29 As
expected, the decay lifetimes of these complexes increased
dramatically from less than 5 μs at room temperature to
more than 100 μs at 77 K. Based on the aforementioned
photophysical study, as well as previous work in the litera-
ture,10,17 excited states of these complexes can be explained
by the assumption that the singlet and triplet metal/halide to
ligand charge transfer (MLCT/XLCT) excited states are in
thermal equilibrium.
All the four complexes showed strong emission in the
solid state at room temperature with high PLQYs of 0.72,
0.90, 0.53, and 0.80 for [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2,
Fig. 2 The UV-vis spectra of complexes [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-
I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2, and [Cu(μ2-I)dppb]2 in CH2Cl2 solu-
tion (10−5 M) at room temperature.
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Fig.
3 PL spectra of complexes [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-
dppb]2, [Cu(μ2-I)OMe-dppb]2, and [Cu(μ2-I)dppb]2 in solid state at
room temperature.
[Cu(μ2-I)OMe-dppb]2, and [Cu(μ2-I)dppb]2, respectively. The
high PLQY may arise from the bulky bisphosphine ligand,
which can tightly wrap the Cu center and effectively suppress
the flattening distortion of the Cu(I) complex in the excited
state, leading to less non-radiative transition and hence high
PLQY. To correlate the PLQY and molecular structure, we an-
alyzed the single crystal data of the four complexes thor-
oughly, and found that the PLQY may depend on the Cu–Cu dis-
tance, i.e. the compact degree of the copper center. For
example, the Cu–Cu distances in [Cu(μ2-I)Pr-dppb]2, [Cu(μ2-
I)dppb]2, and [Cu(μ2-I)Ph-dppb]2 are 2.7745(7) 2.8238(11) and
2.904(3) respectively, which is in a reverse order with respect
to their PLQYs. Though the complex [Cu(μ2-I)OMe-dppb]2 is
out of the line, this may be attributed to a different purifica-
tion method, i.e. recrystallization, in which it is more difficult
to eliminate the negative effect of impurity and solvent as
compared with thermal gradient sublimation.
Fig. 4 (a) TGA and (b) DSC thermograms of [CuIJμ2-I)dppb]2, [Cu(μ2-
I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2 recorded at
a heating rate of 5 °C min−1
.
OLEDs).30 While the complex [Cu(μ2-I)OMe-dppb]2 showed a
decomposition temperature of 174 °C during TGA measure-
ment, which is a disadvantage since this compound emits an
efficient and nice blue emission in solid state. During the
DSC measurement, it is found that the glass-transition tem-
peratures (Tg) are 177.1 and 178.5 °C for [Cu(μ2-I)Ph-dppb]2
and [Cu(μ2-I)Pr-dppb]2, respectively, while that for [Cu(μ2-
I)OMe-dppb]2 was not measured due to its poor thermostabil-
ity. As for the reference complex [Cu(μ2-I)dppb]2, no Tg was
observed even at the third ramp heating, which arises from
an easy crystallization property that can be observed in the
heat flow curve. Such good thermal properties provide advan-
tages for [Cu(μ2-I)Ph-dppb]2 and [Cu(μ2-I)Pr-dppb]2 to be
processed in OLEDs using a traditional vacuum thermal de-
position technique, as well as may guarantee the stability of
the corresponding OLEDs, since it has been reported that the
Thermal properties
To evaluate whether and how these complexes could be uti-
lized in OLEDs, the four complexes were investigated by
thermo gravimetric analysis (TGA, Fig. 4a) and differential
scanning calorimetry (DSC, Fig. 4b). TGA measurement re-
veals high decomposition temperature values (Td, corre-
sponding to 5% weight loss) of 388, 396, and 387 °C for
[CuIJμ2-I)dppb]2, [Cu(μ2-I)Ph-dppb]2, and [Cu(μ2-I)Pr-dppb]2, re-
spectively, which are higher than 373 °C for 4,4′-N,N′-
dicarbazole-biphenyl (CBP, a widely used host material in
Table 3 Photophysical and thermal properties of [CuIJμ2-I)Ph-dppb]2, [Cu(μ2-I)Pr-dppb]2, [Cu(μ2-I)OMe-dppb]2, and [Cu(μ2-I)dppb]2
[CuIJμ2-I)Ph-dppb]2
[CuIJμ2-I)Pr-dppb]2
[CuIJμ2-I)OMe-dppb]2
[CuIJμ2-I)dppb]2
PLQY (%)
72
90
53
80
λmax (nm, 298 K)
λmax (nm, 77 K)
Lifetime (μs, 298 K)
Lifetime (μs, 77 K)
Eg (eV)
535
536
3.6/6.7
191.6
2.84
396
528
534
3.2/1.0
145.8
2.91
387
484
496
2.6/1.0
42.0/101.1
3.13
174
512
507
4.1/1.7
165.74
3.01
388
Td (°C)
Tg (°C)
177.1
178.5
—
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temperature inside an actual OLED can be higher than
10 D. R. McMillin, J. R. Kirchhoff and K. V. Goodwin, Exciplex
quenching of photo-excited copper complexes, Coord. Chem.
Rev., 1985, 64, 83–92.
86 °C.31
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14 A. J. Miller, J. L. Dempsey and J. C. Peters, Inorg. Chem.,
2007, 46, 7244.
15 D. M. Zink, M. Bachle, T. Baumann, M. Nieger, M. Kuhn, C.
Wang, W. Klopper, U. Monkowius, T. Hofbeck, H. Yersin
and S. Brase, Inorg. Chem., 2013, 52, 2292.
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CrystEngComm, 2014, 16, 5338.
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and K. Ueno, Inorg. Chem., 2007, 46, 1992.
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Conclusions
Based on 1,2-bisIJdiphenylphosphino)benzene (dppb), three
P^P-type ligands with a methoxy, pyrrolyl, or phenyl group at-
tached on the central benzene ring were designed and syn-
thesized for coordination with CuI. Consequently, three
dinuclear CuIJI) complexes with formulae of [CuIJμ2-I)Ph-
dppb]2, [Cu(μ2-I)Pr-dppb]2, and [Cu(μ2-I)OMe-dppb]2 were
obtained and characterized by single crystal X-ray diffraction.
Photophysical property comparison of the three complexes
and a reference complex [Cu(μ2-I)dppb]2 indicates that all
complexes exhibited strong emission in the solid state with
maximum PLQY up to 90%, as well as the emission color of
this type of complex could be tuned by attaching an electron-
rich or electron-poor group to the dppb ligand, which arises
from a varied LUMO with unchanged HOMO energy levels.
Through thermo gravimetric analysis, it is found that com-
plexes [Cu(μ2-I)Ph-dppb]2 and [Cu(μ2-I)Pr-dppb]2 showed excel-
lent thermal stability that can be processed in OLEDs using a
traditional vacuum thermal deposition technique. Applica-
tion of the two complexes in high efficiency non-doped
OLEDs is in progress.
20 D. M. Zink, D. Volz, T. Baumann, M. Mydlak, H. Flügge, J.
Friedrichs, M. Nieger and S. Bräse, Chem. Mater., 2013, 25,
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Z. H. Lu and C. H. Huang, Adv. Funct. Mater., 2014, 24, 5385.
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Bian, Y. L. Zhao, Z. H. Lu and C. H. Huang, Org. Electron.,
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23 Q. S. Zhang, T. Komino, S. P. Huang, S. Matsunami, K.
Goushi and C. Adachi, Adv. Funct. Mater., 2012, 22, 2327.
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Acknowledgements
We gratefully acknowledge the financial support from the Na-
tional Basic Research Program of China (No. 2014CB643802)
and the National Natural Science Foundation of China
(NNSFC, No. 21201011).
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