Large-Size crystal based on rare earth-free Cu(I) hybrid trigger yellow light with high emissive quantum yields

Article abstract of DOI:10.1016/j.molstruc.2019.01.084

With the flourishing development of emitting materials, tremendous technological progress has been accomplished. However, they still face great challenges in convenient economical environmental-friendly large-scale commercial production. Herein we designed this hybrid lead/rare earth-free crystals with the large size of 2.7*3.0 cm, an emerging class of high-efficiency luminescent material, (PYE)[Cu₂I₃] (1), which emits intense photoluminescence with a high emissive quantum yields of 32.11%. In addition, the results demonstrate that it is possible to obtain regulatable fluorescence intensity by introducing free groups in material design through simple molecular design. The dielectric constant curve remains relatively smooth before melting point. Therefore, it will be an important exploration and effective way for new material design to achieve artificial intelligence luminescent molecules, in this way, the personal customized application can be realized in display and sensing.

Full text of DOI:10.1016/j.molstruc.2019.01.084

Journal of Molecular Structure 1183 (2019) 241e245
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Large-Size crystal based on rare earth-free Cu(I) hybrid trigger yellow
light with high emissive quantum yields
Yuan-Lan Zhang*, Gui-Bin Wang
Co-Innovation Center for Sustainable Forestry in Southern China, Nanjing Forestry University, Nanjing, 210037, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
With the flourishing development of emitting materials, tremendous technological progress has been
accomplished. However, they still face great challenges in convenient economical environmental-
friendly large-scale commercial production. Herein we designed this hybrid lead/rare earth-free crys-
tals with the large size of 2.7*3.0 cm, an emerging class of high-efficiency luminescent material, (PYE)
[Cu2I3] (1), which emits intense photoluminescence with a high emissive quantum yields of 32.11%. In
addition, the results demonstrate that it is possible to obtain regulatable fluorescence intensity by
introducing free groups in material design through simple molecular design. The dielectric constant
curve remains relatively smooth before melting point. Therefore, it will be an important exploration and
effective way for new material design to achieve artificial intelligence luminescent molecules, in this way,
the personal customized application can be realized in display and sensing.
Received 5 December 2018
Received in revised form
22 January 2019
Accepted 25 January 2019
Available online 30 January 2019
Keywords:
Organic-inorganic hybrid
Crystal structure
Luminescence
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
scientist is completed. Rather, more scientific approaches are
needed to synthesize new materials for different lighting forms and
Since ancient times, luminescent materials have existed natu-
rally, thus inspiring people's great wisdom in the application of
luminescent materials. For example, the ancient people used fluo-
rescent materials to make various types of luminous ornaments
and lighting beads [1e5]. In ancient Greece and Rome, some em-
perors put it in the palace or in the crown, and some queens and
princesses decorated it on jewelry or in the bedroom, using it as a
national treasure to promote and praise. Therefore, luminescent
materials are important materials that have been pursued for
thousands of years [6,7]. With the advancement of science and
technology, fluorescent materials no longer rely on natural forms,
nor do they fix items that are symbolic of the nobility. It is the basis
for high-tech such as electronic information and sensing. Since
luminescent materials have been used in a variety of optoelectronic
devices and luminescence sensing devices, the discovery of new
different types of luminescent materials has been an important task
for scientists, for example, the synthesis of various types of lumi-
nescent materials, such as red\orange\yellow\green\blue\purple\
white and other colors of luminescent materials, as well as various
chromaticity of luminescent materials, covering all bands of visible
light [1e3,8e15]. However, this is not to say that the task of the
devices. Traditional rare earth luminescence is too expensive, and
the natural content of rare earths is limited, which is a world
problem. Gold and silver are the same as rare earths.
So in order to find earth-abundant and fabricate easily emitting
materials, researchers have paid their attention to highly emissive
copper(I) complexes/organic-inorganic hybrid lead(II) halide com-
pound, which have low-cost preparation process and could emit
strong light under external stimulus [9e15]. Therefore, the copper
and iron with rich content in nature has become a new research
focus and breakthrough point. Many researchers have done very
interesting work and made significant progress [1e10]. Among the
many scientific research work that has been done, the copper and
iron has become a research trend. For example, the luminescence of
cuprous copper has gradually achieved very rapid progress and
improvement. However, the low thermal stability of copper(I)
compound, and the high toxicity of organic-inorganic hybrid lea-
d(II) halide compound limit their development in the field of
emitting materials [16e20]. In the context of this world voice, our
research team has done a series of work. These tasks have taken on
the translational work, which provides the important role with
multi-elementary. Herein, a stable, highly luminous copper com-
pound is synthesized by in-situ hydrothermal method, which emits
strong yellow light under external stimulus and displays high
thermal stability (>510 K). These perfect characteristics together
with the high emissive quantum yields (32.11%) enable 1 to be
* Corresponding author.
E-mail address: 247947771@qq.com (Y.-L. Zhang).
https://doi.org/10.1016/j.molstruc.2019.01.084
0022-2860/© 2019 Elsevier B.V. All rights reserved.

242
Y.-L. Zhang, G.-B. Wang / Journal of Molecular Structure 1183 (2019) 241e245
Table 1
promising candidates for the fabrication of economical environ-
mentally emitting materials [1e5,20e28]. Moreover, the single-
crystal X-ray diffraction, thermal analysis, IR spectrum and lumi-
nescence measurements have also been performed to characterize
the properties of 1, and the crystal structures and optical properties
of compound 1 were also been discussed in this paper.
Crystal data and structure refinements for 1 at 293 K.
Empirical formula
C7H10NCu2I3
Formula weight
Crystal system
Space group
a/Å
615.94
monoclinic
P21/c
8.6567(17)
14.184(3)
13.727(4)
125.37(2)
1374.4(6)
1104
b/Å
2. Experimental
c/Å
o
b/
Volume (Å3)
2.1. Preparation of crystals
F(000)
Collected reflections
Unique reflections
Absorption correction
Z, Calculated density
Goodness-of-fit on F2
3150
9210
Multi-scan
4, 2.977 Mg/m3
1.181
All reagents were purchased from commercial sources and used
as received. A mixture of CuCl2 (0.11 g, 0.6 mmol), KI (0.117 g,
0.6 mmol), H2O (8 ml), pyridine (0.5 ml) and EtOH (10 ml) in the
molar ratio of 3: 3: 224: 3:120 was stirred for 2 min [1e4]. The
resulting solution was transferred into a 25 ml Teflon-lined stain-
less steel reactor, which was heated at 100 ꢀC for 2 days. After
cooling to room temperature, yellow crystals [pye][Cu3I4] (1) as a
single phase were recovered in 54% yield based on Cu2þ. And this
compound can also be obtained by direct method as shown in
scheme of Fig. 1. The molar ratio of the starting materials for both
processes was the same except that pyridine was replaced with
ethylpyridine hydrochloride. Anal. Calc. for C7H10Cu3I4N: C, 10.42;
H, 1.24; N, 1.74. Found: C, 10.53; H, 1.23; N, 1.62. IR (KBr, cmꢁ1):
2960w, 1632s, 1566w, 1474 m, 1219 m, 1201w, 1076w, 998w and
654w. The large crystal image with the size of 3.0*2.7 cm was
provided in Fig. 1S.
R1/wR2 [ I > 2
s
(I)]
0.0672/0.1577
2.3. IR spectrum and powder X-ray diffraction measurement
IR spectrum was recorded on a Shimadzu model IR-60 spec-
trometer at room temperature in the form of KBr diluted pellets.
Powder X-ray diffraction data was collected using a Rigaku D/
max2200 X-ray diffractometer at 293 K in the 2q
range of 5e40ꢀ
with a step size of 0.02ꢀ. And the powder X-ray diffraction (PXRD)
pattern of 1 match pretty well with the pattern simulated from the
single crystal structure, which indicates the phase purity (the
powder PXRD as shown in Fig. 1b, and the single crystal structure
was shown in Fig. 2),
2.2. Single crystal X-ray crystallography
2.4. Luminescence measurements
The X-ray single crystal structure for compound 1 was deter-
mined with a Rigaku-Qxford Xlab-pro diffractometer at 293 K
The measurements of excitation and emission photo-
luminescence spectra were performed on a Pluorolog 3-TCSPC
spectrofluorometer (Horiba Jobin Yvon Inc) equipped with a
450 W xenon lamp as an exciting source, and an emission mono-
chromator, a double-excitation monochromator, and a photo-
multiplier were used for detection [1e3,25]. The crystal sample was
sealed in the quartz cuvette, and then the luminescent quantum
yields measurement was performed at room temperature by the
integrating sphere (142 mm in diameter).
employing graphite monochromaticed Mo
K
a
radiation
(l
¼ 0.71073 Å). The data processing including the absorption cor-
rections was processed with Rigaku-Qxford software package.
Crystal structure was solved with direct methods and refined on F2
by means of full-matrix least squares techniques (SHELXL-2014
software package) by using anisotropic displacement parameters.
Positions of all H atoms were fixed geometrically and refined via a
“riding” model with their Uiso values 1.2 times their rider atoms. All
the non-H atoms were refined anisotropically by means of all re-
flections with I > 2s(I). Crystallographic data and structure re-
finements at 293 K were summarized in Table 1.
2.5. Thermogravimetric (TG) analysis
Thermogravimetric (TG) analysis was undertaken on a Dazhan
TGA-101 type thermo-analyzer from 303 K to 973 K with a heating
rate of 10 K/min under nitrogen atmosphere (Fig. 4a).
Fig. 1. (a) The schematic presentation of the synthesis of compound 1 with different
methods. (b) The comparison of simulated and measured PXRD curves.
Fig. 2. The packing diagram of 1 exhibited at 293 K.

Y.-L. Zhang, G.-B. Wang / Journal of Molecular Structure 1183 (2019) 241e245
243
In the first stage (513e665 K), 1 decomposed into metal(II) iodide.
And the second stage (665e973 K) was the procedure in which
metal(II) iodide decomposed continuously [1,2]. The TG curve
shows a continuous weight loss of 79.4% finally, indicating the
removal of cations and iodine. The remaining weight percentage of
20.62% is in agreement with the theoretical value of elemental Cu.
3.2. Crystal structure of 1
In order to explore the emitting mechanism of 1, a precise
analysis to the molecular structure was performed at 293 K. Com-
pound 1 crystallized in the monoclinic space group P21/c (No 14),
and its cell parameters were shown in Table 1. And the asymmetric
unit of 1 consists of one independent cation and one Cu2Iꢁ3 anion
unit, as Fig. 2. Single-crystal X-ray structure analysis showed that
compound is a 1D inorganic Cu2I3 chain, and the cations are
anchored to the chain through crystal packing. Therefore, the
structure can be divided into two parts: PYE cation units and
(Cu2I3)n chains. As shown in Fig. 2, each PYE molecule lies along the
inorganic chain. The Cu(1) cation serve as tetrahedral coordination
site, but it adopt distortion coordination mode: IeCu(1)eI angles
are 101.200(2), 104.566(6), 118.076(6) and 114.586(6); and CueI
Fig. 3. (a) The zigzag inorganic chain of 1 shown at 293 K, all the H/C/N and part I
atoms had been omitted for charity. (b) The simplified ladder-like inorganic chain.
3. Results and discussion
3.1. Thermal properties of 1
In order to investigate the perfect thermal stability of lumines-
cent compound 1, thermo-gravimetric (TG) was carried out from
room temperature to 973 K. As displayed in Fig. 3a, the sample
shows a thermal stability up to T ¼ 513 K. What's more, the
decomposition of compound 1 mainly was divided into two steps.
distances
are
I1eCu1 ¼ 2.5466(14),
I1eCu1 ¼ 2.7206(17),
I2eCu1 ¼ 2.6531(16), I3eCu1 ¼ 2.8338(18), respectively. And for
the Cu(2), it also adopt a twisted four-coordinate pattern with
I1eCu2 ¼ 2.940(2), I2eCu2 ¼ 2.6121(17), I3eCu2 ¼ 2.5407(15),
I3eCu2 ¼ 2.7836(19), and IeCu(2)eI angles are 112.099(6),
Fig. 4. (a) The TG curve of compound 1 shown the decomposition curve depend on temperature. (b) The yellow block crystal with the size of 1 cm of 1 under sunlight and UV light
to display the yellow luminescence, respectively. (c) The excitation spectrum of 1 at room temperature, which has a maximum absorption peak at 275 nm. (d) Emission spectrum of
the powder sample at 275 nm excitation, which owns a maximum emission peak at 556 nm. And the yellow-emission intensities were associated with the temperature.

244
Y.-L. Zhang, G.-B. Wang / Journal of Molecular Structure 1183 (2019) 241e245
99.774(5), 103.595(5), 96.211(5) and 120.837(6). Obviously, the
Cu(2) atom is a more distorted tetrahedral coordination metal with
two I3 and one I1/I2 environment. Interestingly, each I1 or I3 anion
is coordinated to three copper cations, showing a less common m3-I
bridging mode, and I2 is a m2 mode [1e4,28e31]. Polynuclear
complexes, especially Cu(I), are of great interest due to their
remarkable photo-physical and catalytic properties in modern
material researches. For special d10 metal-clusters, the attractive
interactions between closed d10 shells have received our great
attention for the arrangement of metal centers, that is to say, the
photo-physical behaviors totally dependent on the number of
polynuclear metal atoms. Therefore, the controlled formation of
small clusters or low-dimension structures of the metal with
defined nuclearities and the study of their structureefunction
correlations are of great importance in designing functional ma-
terials. In this research, one 1D polynuclear Cu(I) luminescent
material was designed. The packing of the components is presented
in Fig. 2 and Fig. 2S. And the zigzag inorganic chain of 1 was shown
in Fig. 3, this simplified inorganic chain just like a ladder.
increasing or decreasing to verify the idea. The ideal results we
obtained give a solid foundation for the relationship between
fluorescence intensity and thermal motion energy loss. Although
the organic group does not participate in the coordination, there is
a weak van der Waals interaction in the structure, which acts as a
medium for energy transfer and plays a leading role in the change
of fluorescence intensity. Therefore, the results demonstrate that it
is possible to obtain regulatable fluorescence by introducing
different free groups in material design, which will enrich the
application of fluorescence sensing and intensity probes through
simple molecular design. It will be an important exploration and
effective way for new material design to achieve artificial intelli-
gence molecules, in this way, the personal customized application
can be realized. We will continue to work on this complicated
molecular regulation system. The detail information of QE mea-
surement information was submitted in experiment part.
4. Conclusions
Dielectric testing indicates that the structure does not exhibit
phase transition characteristics. The dielectric constant curve re-
mains relatively smooth before melting point. The Ueq of the room
temperature structure is very reasonable, so there is no structural
phase transition in the relatively low temperature state. High
temperature dielectric tests have shown that higher temperature
still do not trigger significant changes in structure and dielectric.
The relevant dielectric test results are presented in the supporting
information (see supporting information Fig. 3S).
The solid sample exhibits yellow emission (556 nm) under UV
radiation (275 nm), and the emission intensity shows a quantitative
linear relationship with temperature in a wide range from 200 K to
350 K, giving rise to a potential application as a temperature sensor.
This work provides a new insight into the understanding of the
interactions between Cu2I3 and cation units and the photophysical
mechanism. Further studies about the influences of different free
substituents interactions and photoluminescence properties are
ongoing. Dielectric testing indicates that the structure does not
exhibit phase transition characteristics. The dielectric constant
curve remains relatively smooth before melting point. Such perfect
performance of 1 indicates that it would be an excellent candidate
for the newly economical environmentally lead/rare earth-free
emitting materials and bring light to the development of the re-
sults demonstrate that it is possible to obtain regulatable fluores-
cence by introducing different free groups in material design,
which will enrich the application of fluorescence sensing and in-
tensity probes through simple molecular design. It will be an
important exploration and effective way for new material design to
achieve artificial intelligence molecules.
3.3. Luminescence properties
For the potential applications of luminescent materials in
modern electronic information industry, many series of fluorescent
materials have been designed and synthesized, such as rare earths,
d10 precious metals, transition metals, and so on. Among them, Cu
has been studied due to its luminescent properties and abundant
natural resources. Herein, to better characterize the luminescence
properties of compound 1, the excitation and associated emission
spectra analysis were depicted as follows. Upon excitation at
lex ¼ 275 nm, ligand PYE and its complex show strong emissions at
425 nm and 556 nm, respectively (Fig. 4). Free ligand exhibits
strong emission bands in 360e510 nm region upon excitation of
lex ¼ 275 nm. This emission may originate from its own conjugated
Acknowledgements
This project was financially supported by the Priority Academic
Program Development of Jiangsu Higher Education Institution
(PAPD).
electron transport and
p / p* transition in the packing mode of
pyridine rings, which was reported by Li and other researchers
[1e10]. The luminescence peaks are completely different for the
free ligand/CuI and complex, which told us that there is a weak
interaction between the organic and inorganic parts of the com-
plex, and that there are mechanisms for covalent and coordinate
bonds to electron transport and fluorescence excitation. Halide-
Cu(I) charge transmission here shows the contribution of its elec-
tron transport to fluorescence in compound 1. And the coordination
environment and the lmax of the emission of this complex are
similar to those of the reported complexes [1e5], which allows
enough luminous activators and a higher quantum yield [4,36]. To
our knowledge, the quantum yield of 32.11% was fairly high among
the reported luminescent molecular-based compounds.
Since the organic group/free cations in the structure have no
rigid fixation effect of coordination, its thermal movement with
temperature will be significantly higher than that of the coordi-
nated group with rigid fixation [32e36]. That is to say, the organic
group acts like a loss of energy as the temperature increases.
Therefore, there is a significant temperature effect on the fluores-
cence intensity. Based on this theoretical basis, we did relative
experiments based on the fluorescence intensity with temperature
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.01.084.
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