Synthesis, crystal structure and photophysical study of luminescent three-coordinate cuprous bromide complexes based on pyrazole derivatives

Article abstract of DOI:10.1080/00958972.2016.1139102

The 1: 2 M-ratio reaction between cuprous bromide and pyrazole derivatives in toluene results in mononuclear Cu(I) complexes [CuBr(pyrazole)₂]. The complexes have been characterized by ¹H NMR spectroscopy and elemental analysis. The molecular structure, established by single-crystal X-ray diffraction, features a trigonal planar geometry around copper, with monodentate pyrazole derivatives. All the Cu(I) complexes are luminescent in the solid state at ambient temperature. Intense blue or blue-green emission in the solid state is observed for these complexes, with the maxima ranging from 431 to 493 nm. The observed photoluminescence could be ascribed to the metal-to-ligand charge-transfer excited states, probably mixed with some halide-to-ligand character. The microsecond lifetime scale of the complexes implies that these transitions arise from the triplet excited states.

Full text of DOI:10.1080/00958972.2016.1139102

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, crystal structure and photophysical
study of luminescent three-coordinate cuprous
bromide complexes based on pyrazole derivatives
Fengshou Wu, Hongbo Tong, Kai Wang, Xiulan Zhang, Jie Zhang, Wai-Kwok
Wong & Xunjin Zhu
To cite this article: Fengshou Wu, Hongbo Tong, Kai Wang, Xiulan Zhang, Jie Zhang, Wai-Kwok
Wong & Xunjin Zhu (2016): Synthesis, crystal structure and photophysical study of luminescent
three-coordinate cuprous bromide complexes based on pyrazole derivatives, Journal of
Coordination Chemistry, DOI: 10.1080/00958972.2016.1139102
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1139102
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Date: 28 January 2016, At: 20:05

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1139102
Synthesis, crystal structure and photophysical study of luminescent
three-coordinate cuprous bromide complexes based on pyrazole derivatives
a,b
d
a
a
c
b
FENGSHOU WU , HONGBO TONG , KAI WANG* , XIULAN ZHANG , JIE ZHANG* , WAI-KWOK WONG
b
and XUNJIN ZHU*
a
Key Laboratory for Green Chemical Process of the Ministry of Education, Wuhan Institute of Technology,
Wuhan, P. R. China
b
Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Hong Kong, P. R. China
c
School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou, P. R. China
d
Institute of Applied Chemistry, Shanxi University, Taiyuan, P. R. China
The 1:2 molar-ratio reaction between cuprous bromide and pyrazole derivatives in toluene results
in mononuclear Cu(I) complexes [CuBr(pyrazole) ]. The complexes have been characterized by
2
1
H NMR spectroscopy and elemental analysis. The molecular structure, established by
single-crystal X-ray diffraction, features a trigonal planar geometry around copper, with
monodentate pyrazole derivatives. All the Cu(I) complexes are luminescent in the solid state at
ambient temperature. Intense blue or blue-green emission in the solid state is observed for these
complexes, with the maxima ranging from 431 nm to 493 nm. The observed photoluminescence
could be ascribed to the metal-to-ligand charge-transfer excited states, probably mixed with some
halide-to-ligand character. The microsecond lifetime scale of the complexes implies that these
transitions arise from the triplet excited states.
Keywords: Cuprous bromide; Luminescence; Crystal structures; Three-coordinate
1
. Introduction
*Corresponding authors. Email: kaiwang@wit.edu.cn (K. Wang); zhang_jie@tzc.edu.cn (J. Zhang); xjzhu@hkbu.edu.hk (X. Zhu)
1

Photoluminescent metal complexes have been the focus of many scientific investigations, owing
to their potential use as organic light-emitting diodes [1], solar cells [2], and chemical sensors [3].
For example, complexes of heavy metal ions, such as cyclometalated iridium(III) complexes, are
applied as particularly useful emitter materials for efficient light generation in electroluminescent
devices. Unfortunately, iridium, as well as platinum which also forms luminescent
cyclometalated complexes, is relatively rare and costly, so that an impetus exists to find suitable
emitters based on less expensive metals. Luminescent copper(I) complexes, as a new class of
electroluminescence materials for organic light-emitting diode (OLED) applications, have been
studied for many years [4]. Over the past two decades, phosphorescent and delayed-fluorescent
tetrahedral copper(I) complexes containing two bidentate ligands (bisimine and/or bisphosphine
ligands) have received attention as dopants because of the low cost and stable supply of copper
[5]. Reaction of Cu(I) halides with monodentate aliphatic and aromatic N donors led, depending
on the reaction conditions, to remarkable structural diversity, ranging from monomeric (e.g.,
CuX L) to polymeric (e.g., (CuXL) ) networks, although the most common are dinuclear
3
∞
(
Cu X L ), tetranuclear (Cu X L ), and polymeric ((CuXL) ) clusters [6]. The stereochemistry
2 2 4 4 4 4 ∞
of copper(I) complexes is dominated by four-coordination, whereas three-coordinate species are
less common [7].
Though the luminescence behavior of copper(I) complexes is rich, tetrahedral copper(I)
complexes tend to display weak emission and short-lived excited states [8]. Strong evidence
indicates a Jahn-Teller based distortion (flattening) in the excited state, and consequent formation
of a five-coordinate exciplex, that accelerates nonradiative decay [9]. The generally accepted
approach to alleviate this problem is to increase the bulk of ligands in order to block excited state
geometrical distortion and non-emissive relaxation pathways [10]. Recently, Osawa et al.
reported a new series of highly emissive three-coordinate copper(I) complexes, (dtpb)CuIX [X =
Cl (1), Br (2), I (3); dtpb = 1,2-bis(o-ditolylphosphino)benzene] [11]. The o-methyl groups of
dtpb are required for formation of three-coordinate complexes because
1
,2-bis-(diphenylphosphino)benzene (dppb), which lacks methyl groups, forms only the
2

halogen-bridged binuclear copper complexes [Cu(μ-X)dppb]
2
with CuX [12]. This is probably
because the Cu diamond core in [Cu(μ-X)dtpb] would be unstable as a result of steric
2
X
2
2
hindrance of the o-methyl groups located on the sides of the metal centers, causing the unusual
monomeric three-coordinate structures.
Herein, we describe the syntheses, crystal structures and photophysical properties of a
series of luminescent Cu(I) complexes formed upon reacting pyrazole derivatives with cuprous
bromide in toluene. The complexes are emissive in the solid state, and their emission properties
can be controlled by proper choice of different groups on the pyrazole.
2
2
. Results and discussion
.1. Synthesis and characterization
Scheme 1. Synthetic routes for the cuprous bromide complexes.
Pyrazole derivatives, as a class of important precursors, were prepared in almost quantitative
yields from volatile 1,3-diones and hydrazine hydrate according to the literature methods [13].
Then the reactions of cuprous bromide with pyrazole derivatives in dry toluene afforded the
corresponding Cu(I) complexes in good yields, as shown in scheme 1. Analytically pure Cu(I)
complexes were obtained by further purification through precipitation and washing steps under
nitrogen. The Cu(I) complexes are air-stable in the solid state but will be oxidized by several
days' air exposure in solution inducing the formation of more stable water coordinated Cu(II)
1
complexes. The Cu(I) complexes were characterized by H NMR spectroscopy and elemental
analysis.
3

2
.2. Description of structures
The single crystals of B3 and B4 suitable for X-ray diffraction analysis were obtained by slow
evaporation of the respective saturated solutions of dichloromethane-hexane (1:3, v/v) and both
were characterized by crystallographic analyses. Perspectives views of B3 and B4 are shown in
figures 1 and figure 2, respectively. Pertinent crystallographic data and other experimental details
are summarized in table 1, and selected bond lengths and angles are given in table 2. The single
crystal structures reveal that copper exhibits a monomeric three-coordinate geometry, surrounded
by two nitrogens from two pyrazole ligands, and one bromide from cuprous bromide, as shown
in figures 1 and 2. The Cu-Br distances in B3 and B4 are 2.561 Å and 2.532 Å, respectively,
similar with those of four-coordinate Cu(I) bromide complexes (2.536 Å) [14], but longer than
that of two-coordinated complex (2.201 Å) [15], probably attributed to the steric hindrance
around the Cu center. The Cu-N bond lengths of the complexes are similar to each other,
1
1
.943-1.969 Å, comparable to the literature values [16]. The N1-Cu-N3 angles in B3 and B4 are
42.97° and 140.25°, respectively, deviating significantly from the ideal trigonal value of 120°,
probably attributed to the sterically congested pyrazole ligands around Cu. Accordingly, the
N1-Cu-Br and N3-Cu-Br angles are smaller than 120°, in the range of 107.31° to 110.71°.
2
.3. Thermogravimetric analysis
Thermogravimetric analyses of the complexes were carried out in nitrogen with a heating rate of
−
1
1
5 °C min . As depicted in figure 3, all Cu(I) complexes have low decomposition temperatures,
with 10% weight loss between 154 and 160 °C. On further heating, a two-step weight loss was
observed between 160 and 900 °C, in which the first loss between 250 and 380 °C is assigned to
the removal of pyrazole ligands, and the second step in the range 380-900 °C corresponds to
gradual loss of bromide. Compared to other complexes, B4 is slightly more stable, probably due
to the more bulky group around the copper center.
4

2
.4. Photophysical properties
The photoluminescence properties of the complexes in the solid state were recorded at ambient
temperature (figure 4). Upon exposure to UV radiation, B1-B4 show a broad emission spectrum
with the maxima at 445, 434, 428 and 493 nm, respectively (table 3). The emission maximum of
the complexes is blue-shifted with increase of steric hindrance on the pyrazole unit. For instance,
the emission maximum of B3 was blue-shifted 6 nm and 17 nm related to B2 and B1,
respectively, when the substituents on pyrazole change from tertbutyl to isopropyl and methyl
group. The reason for this is that incorporation of the bulky substituents around the copper center
can sterically prevent structural relaxation in the MLCT state, which may narrow the energy gap
between excited and ground states [17]. Moreover, B4 displayed a significant red-shifted
emission relative to the other three complexes, probably due to the stronger conjugation because
of the phenyl group. With reference to the related work [18], the highest occupied molecular
orbitals (HOMO) of the complexes were believed to most likely spread over the Cu(I) ion and
the halide ligand, while their lowest unoccupied molecular orbitals (LUMO) were thought to be
basically localized on the pyrazole ligands. Therefore, the emissive excited states of the
complexes are perhaps best regarded as the metal-to-ligand charge-transfer (MLCT) excited
states, probably with some halide-to-ligand charge-transfer (XLCT) character.
The excitation spectra of the Cu(I) complexes in the solid at room temperature are shown
in figure 5. All the complexes display similar excitation bands with maxima at 289, 277, 279 and
3
58 nm, respectively. In addition, the lifetimes of the emitting species are 15-48 μs, indicating
that the emission may stem from the triplet state.
3
. Conclusion
A series of cuprous bromide complexes based on pyrazole ligands have been synthesized and
characterized. Single crystal X-ray analyses reveal that copper exhibits a monomeric
three-coordinate geometry. The Cu(I) complexes are luminescent in the solid state at ambient
temperature. The emissive excited states of the complexes are assigned to the transition from the
5

d
π
orbital of the Cu center to the unoccupied π* orbital of the pyrazole ligand, probably mixed
with some XLCT character. The blue and blue-green emitters obtained are particularly
interesting for OLED and OLEC applications. Benefiting from the cheap and nontoxic copper
meal, ease of structural variation, and the synthetic simplicity, the new approach opens a new
perspective for the development of blue phosphorescent materials.
4
4
. Experimental
.1. Materials and methods
NMR spectra were recorded on a Bruker Ultrashield 400 Plus NMR spectrometer. The NMR
1
chemical shifts for H were referenced to tetramethylsilane (δ = 0.00 ppm). Luminescence
properties of the complexes in solid were investigated at ambient temperature. Steady-state
visible fluorescence and PL-excitation spectra were measured with a Photon Technology
International (PTI) Alphascan spectrofluorimeter. Visible-decay spectra were recorded with a
pico-N laser system (PTI Time Master) with λ = 337 nm. Thermogravimetric analyses (TGA)
2
ex
were performed on a Perkin-Elmer thermal analyzer.
All experiments were carried out without special treatments. The cuprous bromide was
purchased from Aldrich Chemical Co. Solvents were freshly distilled over appropriate drying
regents under an N atmosphere.
2
4
.2. General procedure for the preparation of Cu(I) complexes
To a dry and degassed toluene (10 mL) solution of pyrazole (2 mmol) was added CuBr (143.5
mg, 1 mmol). The mixture was kept at reflux under nitrogen for 12 h. Upon cooling, a precipitate
formed. The solid was filtered off, washed with toluene, diethyl ether, and hexane. The white
product obtained was recrystallized from dichloromethane:hexane (1:3).
1
B1. Yield 90%, H NMR (400 MHz, CDCl ): δ/ppm 2.21 (s, 12H, CH ), 5.95 (s, 2H, H4-pz),
3
3
1
2.49 (s, 2H, NH). Anal. Calcd for C10
H
16BrCuN
4
: C, 35.78; H, 4.80; N, 16.69. Found: C, 35.93;
H, 4.92; N, 16.78.
6

1
B2. Yield 90%, H NMR (400 MHz, CDCl
3
): δ/ppm 1.21 (d, 24H, CH
3
-3,5-pz), 2.93 (m, 4H,
: C, 48.27; H,
CH-3,5-pz), 5.99 (s, 2H, H4-pz), 12.48 (s, 2H, NH). Anal. Calcd for C18
H
32BrCuN
4
7
.20; N, 12.51. Found: C, 49.13; H, 7.32; N, 12.75.
1
B3. Yield 90%, H NMR (400 MHz, CDCl ): δ/ppm 1.29 (s, 36H, CH -3,5-pz), 6.02 (s, 2H,
3
3
H4-pz), 12.59 (s, 2H, NH). Anal. Calcd for C H BrCuN : C, 52.42; H, 8.00; N, 11.12. Found:
2
2
40
4
C, 53.36; H, 8.78; N, 12.07.
1
B4. Yield 90%, H NMR (400 MHz, CDCl
3
): δ/ppm 7.24 (s, 2H, H4-pz), 7.48 (m, 12H, m,p-Ph),
7
9
.88 (m, 8H, o-Ph), 13.54 (s, 2H, NH). Anal. Calcd for C H BrCuN : C, 61.70; H, 4.14; N,
3
0
24
4
.59. Found: C, 62.27; H, 4.77; N, 10.48.
4
.3. X-ray crystallography
Single crystals suitable for crystal structure analysis were obtained by slow evaporation of a
dichloromethane-hexane solution of the complex at room temperature. Data were measured
using Mo-Kα radiation on a Bruker SMART 1000 CCD diffractometer. Data collection at 296 K
and reduction were performed using SMART and SAINT software. Absorption correction was
applied using the multi-scan method (SADABS). The crystal structure of B3 and B4 was solved
2
by direct methods and refined by full matrix least-squares on F using the SHELXTL program
package.
Acknowledgements
The authors are grateful for the financial support of the Scientific Research Project of Wuhan
Institute of Technology (grant no. K201542), Wuhan Science and Technology Talent Training
Program of Chenguang Project (No. 2015070404010190), National Natural Science Foundation
of China (No. 21501128) and Zhejiang Provincial Natural Science Foundation of China
(
No. LQ13B010001). X. Zhu thanks the financial support from Hong Kong Baptist University
FRG2/14-15/034 and FRG1/14-15/058).
(
7

Supplementary material
CCDC-1411709 (B3) and CCDC-1411708 (B4) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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9

Table 1. Summary of crystallographic data.
Param.
B3
B4
Formula
Fw
C H BrCuN
C H BrCuN
4
22
40
4
30 22
504.03
296(2)
Triclinic
Pī
581.97
296(2)
T (K)
Crystal system
Space group
a (Å)
Triclinic
Pī
10.748(5)
10.882(5)
13.607(6)
88.526(9)
69.546(7)
65.110(8)
1338.8(11)
2
9.8233(19)
11.112(2)
13.813(3)
109.948(2)
98.825(3)
108.938(3)
1280.1(4)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å )
3
Z
-3
ρ
calcd (g cm )
1.250
1.510
-1
μ (mm )
2.321
2.440
F(000)
528
588
3
Crystal size (mm )
0.32 × 0.30 × 0.25
2.04, 25.050
0.49 × 0.45 × 0.40
2.08, 25.05
-11 < h <11,
-13< k < 13,
-16 < l < 16
13723
θ
min, θmax (°)
Limiting indices
-12 < h <12,
-
12< k < 12,
16 < l < 16
-
No. reflns. collected
No. unique reflns.
Rint
7342
4726
4544
0.0242
0.0218
Data / restraints / parameters
4713 / 0 / 265
0.983
4544 / 0 / 326
1.387
2
GOOF on F
Final R
I > 2σ(I)]
R , wR (all data)
1
, wR
2
0.0404, 0.0824
0.0269, 0.0717
[
a]
[
0.0744, 0.0947
0.0356, 0.0745
1
2
-3
Largest diff. peak / hole (e·Å )
0.424 / -0.244
0.546 / -0.369
[
a]
2
o
2
2
2 2 1/2
R1 = Σ⏐F
o
− F
c
⏐/ ΣF
o
. wR2 = [Σw(F
 − F
c
) / ΣwF
o
 ]
10

Table 2. Selected bond lengths (Å) and angles (°).
B3
B4
Cu-Br
2.561(4)
2.5328(5)
1.9608(19)
1.9432(18)
140.25(8)
108.82(6)
110.71(6)
116.11(14)
117.17(14)
137.29(17)
136.96(16)
Cu-N(1)
1.961(4)
Cu-N(3)
1.969(4)
N(1)-Cu-N(3)
N(1)-Cu-Br
N(3)-Cu-Br
N(2)-N(1)-Cu
N(4)-N(3)-Cu
C(1)-N(1)-Cu
C(16)-N(3)-Cu
142.97(17)
108.02(16)
107.31(15)
115.62(12)
116.08(12)
138.92(11)
138.57(14)
11

Table 3. Photophysical and thermal properties of solid complexes.
a
b
b
c
λex
λem
τ
Tdec
Complex
(
nm)
289
(nm)
445
434
428
493
(μs)
(°C)
154
156
154
160
B1
B2
B3
B4
21
277
279
358
34
48
15
a
Measurements were done with λ of the complexes in the solid
em
b
at room temperature; Measured at the excitation of 300 nm in
powder; Defined as the temperature of 10% weight loss.
c
12

Figure 1. Perspective view of the structure of B3. Thermal ellipsoids are drawn at 50%
probability level. Hydrogens are omitted for clarity.
13

Figure 2. Perspective view of the structure of B4. Thermal ellipsoids are drawn at 50%
probability level. Hydrogens are omitted for clarity.
14

1
00
B1
B2
B3
B4
90
80
70
60
50
40
200
400
600
800
T(°C)
Figure 3. TGA traces of the Cu(I) complex.
15

1.0
0.8
0.6
0.4
0.2
0.0
B1
B2
B3
B4
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure 4. Emission spectra of Cu(I) complexes in the solid at room temperature.
16

1.0
0.8
0.6
0.4
0.2
0.0
B1
B2
B3
B4
200
250
300
350
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450
Wavelength (nm)
Figure 5. Excitation spectra of the Cu(I) complexes in the solid at room temperature.
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Graphical abstract
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