Largely Color-Tuning Prompt and Delayed Fluorescence: Dinuclear Cu(I) Halide Complexes with tert-Amines and Phosphines

Article abstract of DOI:10.1021/acs.inorgchem.0c03755

Luminescent copper(I) halide complexes with bi- and tridentate rigid ligands have gained wide research interests. In this paper, six tetracoordinate dinuclear copper(I) halide complexes, Cu2X2(ppda)2 [ppda = 2-[2-(dimethylamino)phenyl(phenyl)phosphino]-N,N-dimethylaniline, X = I (1), Br (2), Cl (3)] and Cu2X2(pfda)2 [pfda = 2-[2-(dimethylamino)-4-(trifluoromethyl)phenyl(phenyl)phosphino]-N,N-dimethyl-5-trifluoromethylaniline, X = I (4), Br (5), Cl (6)], were successfully prepared and systematically characterized on their structures and photophysical properties. Complexes 1-5 have a centrosymmetric form with a planar Cu2X2 unit, and complex 6 has a mirror symmetry form with a butterfly-shaped Cu2X2. Solid complexes 1, 4, and 5 emit delayed fluorescence at room temperature, intense blue to greenish yellow (λmax = 443-570 nm) light, and their peak wavelengths are located at 443-570 nm with microsecond lifetimes (τ = 0.4-19.2 μs, φPL = 0.05-0.48). Complexes 2, 3, and 6 show prompt fluorescence, very weak yellowish green to yellow (λmax = 534-595 nm) emission with peak wavelengths at 534-595 nm, and lifetimes in nanoseconds (τ = 4.4-9.3 ns, φPL < 0.0001). (Metal + halide) to ligand and intraligand charge transitions are the main origin of the emission of the complexes. Solution-processed, complex-4-based nondoped and doped devices emit yellow green light with CIE coordinated at (0.41, 0.51), a maximum EQE up to 0.17%, and luminance reaching 75.52 cd/m2.

Full text of DOI:10.1021/acs.inorgchem.0c03755

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Largely Color-Tuning Prompt and Delayed Fluorescence: Dinuclear
Cu(I) Halide Complexes with tert-Amines and Phosphines
Ke Xu, Bu-Lin Chen, Fei Yang, Li Liu,* Xin-Xin Zhong,* Lei Wang,* Xun-Jin Zhu, Fa-Bao Li,
Wai-Yeung Wong,* and Hai-Mei Qin
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ABSTRACT: Luminescent copper(I) halide complexes with bi- and
tridentate rigid ligands have gained wide research interests. In this paper,
six tetracoordinate dinuclear copper(I) halide complexes, Cu2X2(ppda)2
[ppda = 2-[2-(dimethylamino)phenyl(phenyl)phosphino]-N,N-dimethylani-
line, X = I (1), Br (2), Cl (3)] and Cu2X2(pfda)2 [pfda = 2-[2-
(dimethylamino)-4-(trifluoromethyl)phenyl(phenyl)phosphino]-N,N-di-
methyl-5-trifluoromethylaniline, X = I (4), Br (5), Cl (6)], were successfully
prepared and systematically characterized on their structures and photo-
physical properties. Complexes 1−5 have a centrosymmetric form with a
planar Cu2X2 unit, and complex 6 has a mirror symmetry form with a
butterfly-shaped Cu2X2. Solid complexes 1, 4, and 5 emit delayed
fluorescence at room temperature, intense blue to greenish yellow (λmax
=
443−570 nm) light, and their peak wavelengths are located at 443−570 nm
with microsecond lifetimes (τ = 0.4−19.2 μs, ΦPL = 0.05−0.48). Complexes 2, 3, and 6 show prompt fluorescence, very weak
yellowish green to yellow (λmax = 534−595 nm) emission with peak wavelengths at 534−595 nm, and lifetimes in nanoseconds (τ =
4.4−9.3 ns, ΦPL < 0.0001). (Metal + halide) to ligand and intraligand charge transitions are the main origin of the emission of the
complexes. Solution-processed, complex-4-based nondoped and doped devices emit yellow green light with CIE coordinated at
(0.41, 0.51), a maximum EQE up to 0.17%, and luminance reaching 75.52 cd/m2.
synthesize a tetracoordinate mononuclear Cu(I) iodide
complex in highly rigid coordination geometry, which showed
a EQE up to 16.3%.17
1. INTRODUCTION
Copper(I) complexes are regarded as potential options for a
realistic way to construct low-cost and high-efficiency organic
light-emitting diodes (OLEDs) due to their rich structural and
luminescent properties.1−9 These complexes usually have small
ΔE(S1 − T1) values caused by small overlapped highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO), from which efficient room-
temperature thermally activated delayed fluorescence
(TADF) can benefit. Therefore, the emission of copper(I)
complexes can combine singlet with triplet excitons by efficient
reverse intersystem crossing.10−14
Dinuclear Cu(I) cores possess a great advantage over
mononuclear core since they have a rigid structure and thus
high efficiency recorded for bulky ligand chelated com-
plexes.18−24 However, the reported dinuclear Cu(I) halide
complexes are mainly limited to bidendate diphoshines20,24−31
or imine and phosphine18,21−23,32−36 ligands, whereas
bidentate amine and phosphine ligands are rarely reported.
In comparison to the great success in green emitters, it seems
that blue emitters are still a challenge to meet the requirement
of full color displays and white light sources.37,38
Previous studies reveal that three-coordinate planar Cu(I)
complexes coordinated with rigid ligands15 or two-coordinate
linear Cu(I) complexes16 can suppress the geometric distortion
of the Cu(I) center by excitation. Osawa’s group reported
green OLEDs containing three-coordinate mononuclear Cu(I)
halide complexes chelating a rigid diphosphine ligand with an
With great interests to explore highly efficient blue
emitters39−42 and study the structure−property relationship
of complexes, we combined two electron-donating NMe2
groups into the ortho-position of two P-linked phenyl rings
external quantum efficiency (EQE) greater than 20%.15
A
Received: December 23, 2020
Published: March 12, 2021
recent paper by Thompson et al. has reported blue-emitting
two-coordinate Cu(I)-complex-based OLEDs with a EQE of
9.0%.16 Bulky or bridged ligands are used to construct a firm
framework to decrease nonradiative decay, as well. Recently, a
tridentate phosphine ligand was adopted by Xu’s group to
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Scheme 1. Synthetic Pathways to Ligands ppda, pfda, and Complexes 1−6
in ligand ppda (2-[2-(dimethylamino)phenyl(phenyl)-
phosphino]-N,N-dimethylaniline), two electron-withdrawing
CF3 groups are introduced into the meta-position of NEt2 in
ligand pfda (2-[2-(dimethylamino)-4-(trifluoromethyl)phenyl-
(phenyl)phosphino]-N,N-dimethyl-5-trifluoromethylaniline)
to change the emissive color. Complexes 1−5 have a planar
Cu2X2 core, whereas complex 6 has a butterfly-shaped Cu2X2
core. They showed very different emissions from previously
reported ones. Here, the structures, photophysical and redox
properties, and theoretical calculations of complexes 1−6 are
reported. Also the OLED devices based on complex 4 were
fabricated via a solution-processed method, and their electro-
luminescent behaviors were studied.
dimethyl-5-(trifluoromethyl)aniline and n-butyllithium with an
equal mole in THF at −78 °C under nitrogen. After the
addition of PPhCl2 to 2-dimethylamino-5-(trifluoromethyl)-
phenyllithium, pfda was prepared in 47% yield. Complexes 1−
6 were obtained in 76−84% yields by the reaction of CuX in
acetonitrile with 1 equiv of ppda and pfda. Six complexes are
stable in air and dissolved in solvents of acetonitrile and
chloroform. Their structures were identified (Figures 1 and
S1−S22).
Figure 1 presents the perspective views of complexes 1−6,
and Tables S1 and 1 give the crystallographic data and some
bond lengths and angles. One solvent CH3CN molecule is in
complex 6. Two copper(I) are tetrahedrally connected to two
halogen atoms and a N and a P from the ppda and pfda
ligands, respectively. Complexes 1−5 have a planar Cu2X2 with
centrosymmetry, whereas complex 6 has a mirror symmetric
form with a butterfly-shaped Cu2X2, where Cu and Cl atoms
are not located in one plane with a distinct dihedral angle Cl1−
Cu2−Cu1−Cl2 of 160.82°. We found that, as the X radius
increases, the bond lengths of Cu−X increase (Table 1). This
trend is also found in the Cu1···Cu2 distances, which were
widely reported to have important impacts on the emission of
the multinuclear Cu(I) halide complexes. The Cu1···Cu2
distances are in the range of 3.014 and 3.471 Å (the sum of the
van der Waals radius of copper is 2.8 Å), indicating negligible
interaction between two Cu atoms in complexes 1−6. With
careful contrast of previously reported Cu2X2(PNMe2)2
(PNMe2 = Ph2P-(o-C6H4)-NMe2) having nonplanar mirror
symmetry and a butterfly-shaped Cu2X2 core (X = I and Br),19
the introduction of two NMe2 into the ligand ppda in
complexes 2 and 3 leads to a structural change. A planar
centrosymmetry motif and a larger Cu1···Cu2 distance is
ascribed to the increased sterical requirement of ppda. The
structures of the central Cu2X2 unit of complex 3 and
Cu2Cl2(PNMe2)2 are similar, both of which are planar
centrosymmetry and have similar Cu1···Cu2 distance (3.072
Å for 3 and 2.983 Å for Cu2Cl2(PNMe2)2). When two CF3
groups are introduced into the ligand pfda, complexes 4 and 5
still have a centrosymmetric form with a planar Cu2X2, except
2. EXPERIMENTAL SECTION
2.1. Synthesis of pfda. At −78 °C, n-BuLi (2.5 M in hexane, 2.4
mL, 6 mmol) was added dropwise to a THF (10 mL) solution of 2-
bromo-N,N-dimethyl-5-(trifluoromethyl)aniline (1.6 g, 6 mmol) and
stirred for 30 min. Later, PhPCl2 (0.39 mL, 3 mmol) was added in 5
min, and the subsequent reaction mixture was allowed to slowly warm
to room temperature. After being stirred overnight, the reaction was
quenched by the addition of water (10 mL) and then treated with
aqueous NH4Cl; CH2Cl2 (3 × 10 mL) was used to extract the
mixture, and the combined extracts were dried by anhydrous Na2SO4,
filtered, and concentrated to give a residue. The residue was further
purified by chromatography and obtained as a white powder (0.68 g,
47% yield).
2.2. Synthesis of Complexes 1−6. All Cu(I) complexes were
prepared by the following method. A mixture of copper(I) halide (1
mmol) and ppda or pfda (1 mmol) in 30 mL of CH3CN was stirred
for 2 days at room temperature to form a suspension which was then
filtered off. The filtrate was concentrated to give a residue. The
residue was dissolved in CH2Cl2/CH3CN, and complexes were
obtained as colorless crystals for 1 and yellow crystals for 2−6 after
slow evaporation at room temperature.
3. RESULTS AND DISCUSSION
3.1. Syntheses and Characterizations. Scheme 1
presents the synthetic routes to ppda, pfda, and complexes
1−6. The synthesis of 2-dimethylamino-5-(trifluoromethyl)-
phenyllithium was accomplished in a mixture of 2-bromo-N,N-
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Figure 1. ORTEP diagrams of 1−6.
for complex 6, and the Cu1···Cu2 distances in complexes 4−6
are shortened. The dihedral angles between the CuX2 plane
and bridging N,P phenyl ring plane in complexes 1−6 are
shown in Table 2 and Figures S23−S28. The dihedral angles
are on the order of 1 < 5 < 4 < 6 < 2 < 3. The two planes in
complexes 2, 3, and 6 are close to orthogonal.
3.2. Photophysical, Redox Properties, and Theoreti-
cal Calculations. The absorption spectra of ligands ppda,
pfda, and complexes 1−6 in CH2Cl2 (4 × 10−5 M) at room
temperature are displayed in Figure 2. The absorption spectra
exhibit bands at 259 and 300 nm for ppda and 266 and 318 nm
for pfda, which are ascribed to n → π* and π → π* transitions
of arylphosphine and arylamine. The absorption peak wave-
length of pfda is red-shifted by about 7−18 nm compared to
that of ppda. Complexes 1−3 show absorption peaks (269−
271 nm), shoulders (305−314 nm), and tails (350−415 nm),
and red-shifted bands are found in complexes 4−6 (274−278,
331−340, and 375−435 nm, respectively). Time-dependent
density functional theory (TDDFT) calculations (Figures 3
and S44−S50 and Tables S3−S8) show that the transitions
from HOMO to LUMO contribute to the lowest excited states
for complexes 1 and 4−6, whereas for complexes 2 and 3, the
transitions are from HOMO to LUMO+1. The shapes of the
HOMO show that the electrons in the HOMO are largely
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Table 1. Some Bond Lengths (Å) and Angles (deg) of Complexes 1−6
complex
Cu2−X1
Cu1−X1
Cu1−X2
Cu2−X2
Cu1−P1
Cu2−P2
Cu1−N1
Cu2−N2
1
2
3
4
5
6·CH3CN
2.5823(5)
2.7547(5)
2.5823(5)
2.7547(5)
2.2574(8)
2.2574(8)
2.292(3)
2.292(3)
3.471
2.4506(3)
2.4721(4)
2.4506(3)
2.4721(4)
2.2012(5)
2.2012(5)
2.2899(13)
2.2899(13)
3.133
2.3255(5)
2.3240(5)
2.3255(5)
2.3240(5),
2.1782(4)
2.1782(4)
2.3692(16)
2.3692(16)
3.072
2.6937(6)
2.5425(6)
2.6937(6)
2.5425(6)
2.2355(8)
2.2355(8)
2.352(2)
2.352(2)
3.053
2.5927(4)
2.3713(4)
2.5927(4)
2.3713(4)
2.2025(6)
2.2025(6)
2.3495(19)
2.3495(19)
3.050
2.3329(9)
2.3364(9)
2.3260(10)
2.3263(9)
2.1922(9)
2.1875(9)
2.354(3)
2.345(3)
3.014
Cu1···Cu2
P1−Cu1−N1
P2−Cu2−N2
X1−Cu1−X2
X1−Cu2−X2
Cu1−X1−Cu2
Cu1−X2−Cu2
X1−Cu2−Cu1−X2
82.64(7)
82.64(7)
98.937(16)
98.937(16)
81.062(16)
81.062(16)
0
84.54(3)
84.54(3)
100.956(10)
100.956(10)
79.043(10)
79.043(10)
0
82.41(4)
82.41(4)
97.278(16)
97.278(16)
82.722(16)
82.722(16)
0
81.06(5)
81.06(5)
108.74(2)
108.74(2)
71.26(2)
71.26(2)
0
81.58(5)
81.58(5)
104.336(13)
104.336(13)
75.664(13)
75.664(13)
0
82.88(7)
83.33(7)
97.50(3)
97.61(3)
80.39(3)
80.75(3)
160.82
has the highest first anodic wave, indicating a good stabilization
of Cu+. We speculate that the electronic effect of groups linked
to the Cu+ center is not the main reason but steric hindrance
is, which suppresses the distortion of geometry from
tetrahedron to square planar, so the oxidation of Cu+ becomes
more difficult and the resulting Cu2+ is therefore more
destabilized.
Table 2. Dihedral Angles between the CuX2 Plane and
Bridging N,P Phenyl Ring in Complexes 1−6
complex
dihedral angle (deg)
complex
dihedral angle (deg)
1
2
3
67.31
85.52
88.18
4
5
6
70.81
70.23
82.53
Complexes 1, 4, and 5 show intense blue to greenish yellow
emissions (443−570 nm) with ΦPL = 0.05−0.48, and
complexes 2, 3, and 6 exhibit very weak greenish yellow to
yellow emissions (534−595 nm) with ΦPL < 0.0001 at 297 K
(Figure 4 and Table 3). The emission maxima of 1 is blue-
shifted by 31 nm compared to that of previously reported
Cu2I2(PNMe2)2;19 as we anticipated, two electron-donating
NMe2 on the ortho-position of P-linked phenyl rings in ligand
ppda can raise the LUMO level. To the best of our knowledge,
this has been the first reported efficient blue light with
emission maxima shorter than 440 nm from neutral dinuclear
Cu(I) halide complexes. Based on the emission curve in a solid
at 297 K, the CIE color coordinates of complexes 1−6 are
(0.168, 0.179), (0.415, 0.456), (0.339, 0.478), (0.401, 0.514),
(0.453, 0.510), and (0.500, 0.478), respectively (Figure 5).
Large red-shifting of 61−105 nm for the emission bands of
complexes 4 and 6 are found in comparison to those of 1 and
3, possibly ascribed to two electron-withdrawing CF3, which
lower the LUMO level. The emission maximum of 5 is blue-
shifted by 4 nm compared to 2. According to the TDDFT
calculation, the emission properties of complexes 2−6 were
also estimated (the optimized S1 geometry of complex 1 failed
even if tried by many methods). The calculated emission
wavelengths of complexes 2−5 (Table 3) are in accord with
the experimental data. The contribution to the emission of
complexes 2−6 mainly comes from the LUMO → HOMO
transition. The shapes of HOMOs and LUMOs and S1 state
geometries are presented in Figure 6. The HOMOs are mostly
confined to Cu, X, and P and N in ligands ppda and pfda, and
the LUMOs are largely concentrated on the bridging N,P
phenyl ring. Therefore, the emission primarily comes from
(metal + halide) to ligand and intraligand charge transitions.
The emission peak wavelengths of complexes 1−6 are
located at 422−626 nm at 77 K. The emission curves of 1, 2, 4,
and 5 are red-shifted compared to those at 297 K and can be
explained by the dominated thermal population of a lower
Figure 2. Absorption spectra of ppda, pfda, and complexes 1−6 in
CH2Cl2 at room temperature. The inset shows magnified absorption
edges.
concentrated on the Cu, X, P, and N atoms, while the LUMO
levels primarily distribute to the bridging N,P phenyl rings in
ligands; similar cases are found in LUMO+1. Therefore, the
lowest excited states are composed of (metal+halide) to ligand
and intraligand charge transitions. Similar red shifting (5−26
nm) is observed for the absorption bands of 4−6 when
compared to that with 1−3, which is attributed to the reduced
LUMO level by electron-withdrawing CF3.
The electrochemical properties of 1−6 were determined by
cyclic voltammetry (CV). Cyclic voltammograms of complexes
1−6 in 10−3 M CH2Cl2 and TBAPF6 (0.1 M), potential data,
energy levels, and gaps are shown in Figure S29 and Table S9.
Cathodic peak potentials of complexes 1−6 are located at
approximately −0.94 to − 1.70 V, which correspond to the
reduction of ligand. The first and second anodic waves were
attributed to the oxidation of Cu+ and ligand, respectively.
Compared with the lowest potential of complex 3, complex 1
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Figure 3. S0 geometry optimized and shapes of HOMOs and LUMOs of 1−6.
excited state (T1) at low temperature.42 For complexes 3 and
6, blue-shifting of the emission bands can be attributed to the
changes of vibrations and rotations that suppressed the energy
relaxation of the excited state.42 The lifetimes (Table 3 and
Figures S32−S43) of 1, 4, and 5 at 297 K are 19.2, 1.1, and 0.4
μs, respectively, which are 30−505 times shorter than that at
77 K, exhibiting the possible existence of a different emission
interexchangeable and thermally activated process.19 Based on
the fluorescence and phosphorescence spectra onsets, the S1
and T1 energy levels are estimated (Table 3 and Figure 4), and
ΔE(S1 − T1) values are at 0.095−0.1667 eV for complexes 1,
4, and 5, indicating they show efficient thermally activated
delayed fluorescence.13,14,19 Complexes 2, 3, and 6 do not
show efficient TADF behaviors, which are ascribed to the
much larger ΔE(S1 − T1) values (0.3373−0.4184 eV), and that
is why they display prompt fluorescence with nanosecond
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Figure 5. CIE graph of complexes 1−6.
lifetimes (4.4−9.3 ns). Additionally, the dihedral angles
between the CuX2 plane and bridging N,P phenyl ring plane
in complexes 2, 3, and 6 (82.53−88.18°) are much larger than
those in complexes 1, 4, and 5 (67.31−70.81°) (Table 2 and
Figures S23−S28). Generally, ΔE(S1 − T1) relies on the
exchange integral between S1 and T1 or approximated between
the HOMO and the LUMO.43 The less overlap between the
frontier orbitals, the less mixing of these states, thus the less
ΔE(S1 − T1) value, which also results in less distortion upon
excitation and helps to increase the photoluminescence
quantum yield.44−47 Therefore, substituted groups can play a
very important role in sterically controlling the orientation of
ligands and varying the photophysical property of complexes.
The radiative rate constants kr of complexes 1, 4, and 5 were
calculated to be 2.5 × 104 to 1.25 × 105s−1, similar to those
previously reported for Cu(I) halide complexes
Cu2X2(PNMe2).19
Figure 4. Emission and normalized emission spectra of 1−6 in solid
at 297 K [(a,b) λex = 365 for 1, 375 for 2 and 3, 378 nm for 4−6)]
and 77 K [(c) λex = 365 for 1, 420 for 2, 360 for 3 and 5, 378 nm for 4
and 6)].
To confirm the existence of TADF, complex 4 was as a
representative, and its decay times (τobs) were measured in the
temperature range between 77 and 297 K (Figure 7). With
thermally equilibrated S1 and T1 states assumed, the relation-
ship between the decay time and temperature is shown in eq 1:
Table 3. Photophysical Data of 1−6 in a Solid
a
b
c
d
λmax (nm)
297 K
443
τ (μs)
Φ
kr (105 s−1
)
e
e
e
f
77 K
297 K
77 K
297 K
297 K
E(S1) (eV)
E(T1) (eV)
ΔE (S1 − T1) (eV)
λ (nm)
1
2
3
4
454
19.2
577
0.48
0.25
3.2461
2.8182
2.9594
2.9040
3.1392
2.4125
2.5410
2.7373
0.1069
0.4047
0.4184
0.1667
0.1085
0.095
417*, 574
534
548
465*, 626
422, 552*
551
0.0057
0.0093
1.1
2.1
0.00048
112
<0.0001
<0.0001
0.11
549
584
611
1.00
1.25
g
5
6
570
595
576
582
0.4
0.0044
202
1.7
0.05
<0.0001
2.8506
2.9314
2.7556
2.5941
617
520
0.3373
a
b
c
Emission peak wavelength. Asterisks indicate that emission peaks appear as shoulders or weak bands. Average lifetime. Absolute emission
d
e
quantum. Radiative decay rate constant; kr = Φ/τave. The S1 and T1 energy levels were estimated based on the emission peak onsets at 297 and 77
K. Calculated emission wavelengths according to the S1 geometries optimized. E(S1) and E(T1) are the vertical excitation energies, ΔE(S1 − T1) =
E(S1) − E(T1).
f
g
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Figure 6. S1 geometry optimized and shapes of HOMOs and LUMOs of 2−6.
1 + 1 exp −ΔEST
fitted (T1) lifetime (108.7 μs) approximates the measured
lifetime of 112 μs of 4 at 77 K, and the ΔEST (0.103 eV) is
similar to the calculated one (0.1085 eV) (Figure 7 and Table
3). At temperatures below 100 K, complex 4 emits almost pure
phosphorescence. As the temperature increases, the emission
from the phosphorescence (T1) decreases, and accordingly, the
emission from the TADF increases.
By TDDFT calculation, the coordinative geometries of the
Cu center in the optimized S0, S1, and T1 (Table S1 and Figure
S51) are tetrahedrally distorted. P−Cu−N bond angles in S0
and S1 geometries remain nearly unchanged. This can be
attributed to the rigid ligands ppda and pfda. Larger changes in
bond angles of P−Cu−X, N−Cu−X, and X−Cu−X may lead
(
)
ST
3
KBT
τ
=
obs
exp −ΔE
1
1
+
(
)
τ(T)
3τ(S1)
KBT
(1)
1
where KB, T, τ(S1), τ(T1), and ΔEST are the Boltzmann
constant, the absolute temperature, the lifetimes of S1 and T1,
and the energy separation between S1 and T1, respectively. In
the temperature range, the lifetime is single-exponential.
Equation 1 is fitted to the lifetime measured at various
temperatures, and then lifetimes at S1 (10.32 ns), T1 (108.7
μs), and ΔEST (0.103 eV) were obtained. The short lifetime of
10.32 ns substantiates its S1 character. Therefore, the lifetime
at 297 K of τ = 1.1 μs represents a delayed fluorescence. The
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to a Jahn−Teller distortion of excited states15 and thus lower
emissive intensity.
3.3. Thermal Properties. For OLED applications, good
thermal stabilities of emissive materials are required. Thus,
thermogravimetric analysis (TGA) under nitrogen for
complexes 1−6 was conducted to determine the onset
decomposition temperatures (Tdec). It was found that 1−6
are thermally stable, with the Tdec values over 250 °C (Figure
8, 305−367 °C for complexes 1−3 and 268−281 °C for
complexes 4−6), which meets the demand for OLED
operating temperature. The introduction of two CF3 groups
in ligand pfda can make complexes 4−6 decompose more
easily, ascribed to the distinct elongation of Cu−N and Cu−P
bonds in complexes 4−6. Complex 1−3 lose approximately.
61−77% weight at 419−486 °C, and complexes 4−6 lose
approximately 69−85% weight at 307−354 °C, which
corresponds to the loss of two ligands ppda and pfpa.
3.4. Electroluminescent Properties. Complex 4 was
used as the emitting material in solution-processed devices
with the structure of ITO/PEDOT:PSS (40 nm)/complex 4
(20 nm)/TPBi (30 nm)/LiF (1 nm)/Al (100 nm): ITO,
anode; Al, cathode; PEDOT:PSS, hole injection layer; TPBi,
electron transporting layer; LiF, electron injecting layer. Figure
9a shows the energy level diagram. This device emits yellow
green light (λem = 564 nm) with CIE coordinated at (0.41,
0.51). Figure 9c,d presents the EQE up to 0.17%, and the
maximum luminance reaches 75.52 cd/m2. Additionally,
several doped devices with CBP as host materials were
fabricated. However, the doped devices did not exhibit
satisfactory results (Figure S52), which is possibly ascribed
to the mismatching of CBP and complex 4 in energy level.
Therefore, a matching host material is required to enhance
device performance.
Figure 7. Temperature versus decay lifetime of complex 4 and a fit
curve.
4. CONCLUSIONS
This is the first reported efficient blue light with emission
maxima shorter than 440 nm from dinuclear Cu(I) halide
complexes containing tert-amines and phosphines. The
Figure 8. TGA curves of complexes 1−6.
Figure 9. (a) Energy-level drawing of the device; (b) electroluminescence spectra and corresponding CIE; (c) J−V−L characteristics; (d) EQE−
luminance characteristics.
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Article
introduction of two electron-donating NMe2 into the ortho-
position of two P-linked phenyl rings in ligand ppda and two
electron-withdrawing CF3 into the meta-position of NMe2 in
ligand pfda can sterically control the orientation of the ligands
and vary the photophysical properties of the complexes. The
emission color can change from blue to yellow. Complexes 1,
4, and 5 show delayed fluorescence, and complexes 2, 3, and 6
show prompt fluorescence, due to the difference in ΔE(S1 −
T1) values (small for the former, and large for the latter). In
addition, ΔE(S1 − T1) values depend on the dihedral angles
between the CuX2 plane and the bridging N,P phenyl ring
plane, where two nearly orthogonal planes result in the large
overlap between the HOMO and the LUMO and large ΔE(S1
− T1) values. It is evident that the investigation of the
relationship between structure and property helps us to
understand the emitting process in guarding further studies
and application in OLEDs.
Collaborative Innovation Center for Advanced Organic
Chemical Materials Co-constructed by the Province and
Ministry, Hubei University, Wuhan 430062, P.R. China
Bu-Lin Chen − Ministry of Education Key Laboratory for the
Synthesis and Application of Organic Functional Molecules,
Collaborative Innovation Center for Advanced Organic
Chemical Materials Co-constructed by the Province and
Ministry, Hubei University, Wuhan 430062, P.R. China
Fei Yang − Wuhan National Laboratory for Optoelectronics,
Huazhong University of Science and Technology, Wuhan
430074, P.R. China
Xun-Jin Zhu − Department of Chemistry, Hong Kong Baptist
University, Hong Kong, P.R. China; orcid.org/0000-
0001-7929-6964
Fa-Bao Li − Ministry of Education Key Laboratory for the
Synthesis and Application of Organic Functional Molecules,
Collaborative Innovation Center for Advanced Organic
Chemical Materials Co-constructed by the Province and
Ministry, Hubei University, Wuhan 430062, P.R. China;
orcid.org/0000-0002-1873-3128
ASSOCIATED CONTENT
■
sı
* Supporting Information
Hai-Mei Qin − Department of Chemistry, Xiamen University,
Xiamen 361005, P.R. China
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03755.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03755
NMR and mass data, molecular structures, cyclic
voltammograms, photophysical data, computational
details, and device performance of the doped device
(PDF)
Author Contributions
K.X., B.-L.C., and F.Y. contributed equally to this work. The
manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
Accession Codes
CCDC 1897752−1897754 and 2051929−2051931 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
L.L. thanks the National Natural Science Foundation of China
(21671061) and application foundation frontier special project
from Wuhan Science and Technology Bureau
(2019010701011414) for financial support. Thanks to senior
engineer Mingxing Chen (Peking University) for the photo-
physical measurements.
AUTHOR INFORMATION
Corresponding Authors
■
Li Liu − Ministry of Education Key Laboratory for the
Synthesis and Application of Organic Functional Molecules,
Collaborative Innovation Center for Advanced Organic
Chemical Materials Co-constructed by the Province and
Ministry, Hubei University, Wuhan 430062, P.R. China;
orcid.org/0000-0002-3684-4032; Email: liulihubei@
hubu.edu.cn
ABBREVIATIONS
■
OLED, organic light-emitting diode; TADF, thermally
activated delayed fluorescence; EQE, external quantum
efficiency; HOMO, highest occupied molecular orbital;
LUMO, lowest unoccupied molecular orbital; ITO, indium
tin oxide; PEDOT, poly(3,4-ethylenedioxythiophene); PSS,
poly(styrenesulfonate); TPBi, 1,3,5-tris(1-phenyl-1H-benzimi-
dazol-2-yl)benzene; CBP, 1,3-bis(9-carbazolyl)benzene
Xin-Xin Zhong − Ministry of Education Key Laboratory for
the Synthesis and Application of Organic Functional
Molecules, Collaborative Innovation Center for Advanced
Organic Chemical Materials Co-constructed by the Province
and Ministry, Hubei University, Wuhan 430062, P.R. China;
Email: xxzhong@hubu.edu.cn
Lei Wang − Wuhan National Laboratory for Optoelectronics,
Huazhong University of Science and Technology, Wuhan
430074, P.R. China; orcid.org/0000-0002-2141-3256;
Email: wanglei@mail.hust.edu.cn
Wai-Yeung Wong − Department of Applied Biology and
Chemical Technology, The Hong Kong Polytechnic
University, Hong Kong, P.R. China; orcid.org/0000-
0002-9949-7525; Email: wai-yeung.wong@polyu.edu.hk
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